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Page 1: Coordination Chemistry - Hindawi Publishing Corporationdownloads.hindawi.com/journals/focusissues/136198.pdf · Coordination Chemistry International Journal of Inorganic Chemistry.

Coordination Chemistry

International Journal of Inorganic Chemistry

Page 2: Coordination Chemistry - Hindawi Publishing Corporationdownloads.hindawi.com/journals/focusissues/136198.pdf · Coordination Chemistry International Journal of Inorganic Chemistry.

Coordination Chemistry

Page 3: Coordination Chemistry - Hindawi Publishing Corporationdownloads.hindawi.com/journals/focusissues/136198.pdf · Coordination Chemistry International Journal of Inorganic Chemistry.

International Journal of Inorganic Chemistry

Coordination Chemistry

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Copyright © 2011 Hindawi Publishing Corporation. All rights reserved.

This is a focus issue published in volume 2011 of “International Journal of Inorganic Chemistry.” All articles are open access articlesdistributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in anymedium, provided the original work is properly cited.

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Editorial Board

Christopher Allen, USAHakan Arslan, TurkeyPeter Baran, USAIvano Bertini, ItalyGeorge Britovsek, UKAlfonso Castineiras, SpainStephen Colbran, AustraliaWilliam Connick, USAMaochun Hong, ChinaYining Huang, CanadaM. Ishaque Khan, USA

Karl Kirchner, AustriaGeorge Koutsantonis, AustraliaAbdessadek Lachgar, USAWolfgang Linert, AustriaJames K. McCusker, USANorbert W. Mitzel, GermanyRabindranath Mukherjee, IndiaLuis Oro, SpainAlvaro J. Pardey, VenezuelaJames E. Penner-Hahn, USAMaurizio Peruzzini, Italy

Rainer Pottgen, GermanyStephen Ralph, AustraliaDaniel L. Reger, USAHerbert W. Roesky, GermanyAxel Schulz, GermanyKonrad Seppelt, GermanyE. I. Solomon, USAAlexander Steiner, UKWei-Yin Sun, ChinaW. T. Wong, Hong KongJohn Derek Woollins, UK

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Contents

Syntheses and Crystal Structures of Two Transition Metal Complexes (M = Mn and Co) ContainingMalonate and Reduced Imino Nitroxide Radicals, Jing Chen, You-Juan Zhang, Bing-Chang Qin,Hui-Min Zhu, and Yu ZhuVolume 2011, Article ID 257521, 5 pages

Synthesis, Characterization, and Magnetic and Thermal Studies on Some Metal(II) Thiophenyl SchiffBase Complexes, Aderoju Amoke OsowoleVolume 2011, Article ID 650186, 7 pages

Synthesis and Structural Characterization of a New Tetranuclear Nickel(II) Sulfato Complex Containingthe Anionic Form of Di-2-Pyridyl Ketone Oxime, Eleni Moushi, Constantinos G. Efthymiou,Spyros P. Perlepes, and Constantina PapatriantafyllopoulouVolume 2011, Article ID 606271, 9 pages

Improvement of Aminopeptidase Activity of Dizinc(II) Complexes by Increasing Substrate Accessibility,Md. Jamil Hossain, Akinobu Wada, Yasuhiro Igarashi, Kei-ichiro Aimono, Keisuke Suzuki, Katsuya Tone,and Hiroshi SakiyamaVolume 2011, Article ID 395418, 4 pages

Synthesis and Crystal Structure Differences between Fully and Partially Fluorinated β-Diketonate Metal(Co2+, Ni2+, and Cu2+) Complexes, Akiko Hori and Masaya MizutaniVolume 2011, Article ID 291567, 8 pages

A Selective Chemosensor for Mercuric Ions Based on 4-Aminothiophenol-Ruthenium(II)Bis(bipyridine) Complex, Amer A. G. Al Abdel Hamid, Mohammad Al-Khateeb, Ziyad A. Tahat,Mahmoud Qudah, Safwan M. Obeidat, and Abdel Monem RawashdehVolume 2011, Article ID 843051, 6 pages

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Hindawi Publishing CorporationInternational Journal of Inorganic ChemistryVolume 2011, Article ID 257521, 5 pagesdoi:10.1155/2011/257521

Research Article

Syntheses and Crystal Structures of Two Transition MetalComplexes (M = Mn and Co) Containing Malonate and ReducedImino Nitroxide Radicals

Jing Chen,1 You-Juan Zhang,1 Bing-Chang Qin,1 Hui-Min Zhu,1 and Yu Zhu2

1 School of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455002, China2 Department of Chemistry, Zhengzhou University, Zhengzhou 450052, China

Correspondence should be addressed to Jing Chen, [email protected]

Received 9 August 2010; Accepted 30 October 2010

Academic Editor: Rabindranath Mukherjee

Copyright © 2011 Jing Chen et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Two novel transition metal complexes with malonate and reduced imino nitroxide radicals, [Co(mal)(Him2-py)2] (ClO4)1 and[Mn(mal)(Him2-py) 2] (H2O)2 (Him2-py = 1-hydroxy-2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline) have been synthesized andtheir crystal structures were determined by X-ray diffraction method. During the reaction, one-electron reduction of the N–Oradical moiety in IM2py has been reviewed. The structural analyses reveal that two title complexes are isostructural and crystallizein monoclinic space group C2. For the complex 1, a = 17.004(9), b = 10.753(5), c = 9.207(5) A with β = 113.856(8)◦. For thecomplex 2, a = 16.721(5), b = 10.897(5), c = 9.253(3) A with β = 120.807(6)◦. In two complexes, the coordination numberaround the metal ion is six, and the coordination sphere is a distorted octahedron. Two nitrogen atoms from Him2-py and twooxygen atoms from malonate are in the basal plane, and two nitrogen atoms from pyridyl rings of Him2-py at the axial position.

1. Introduction

The design and synthesis of transition metal complexes withorganic free radicals is one of the major challenges in thefield of molecular magnetic materials [1]. Nitronyl nitroxide(NN) radicals are normally used as spin carriers to the devel-opment of molecular-based magnetic materials. Howevernitroxides can undergo redox reactions with transition metalions under certain conditions [2, 3]. In fact, nitronyl freeradicals are in an oxidation state intermediate between thoseof the hydroxylamino anion and the nitrosonium cation.Up to now, relatively little work has been devoted to thestudy of the redox properties of metal-nitroxyl systems andonly a few complexes containing metal ions bound to thereduced monoradical have been reported [4–9]. It is knownthat nitronyl nitroxide radicals can undergo redox reactionwith transition metal ions, yielding complexes in which theIMHR reduced form of IM acts as a diamagnetic ligand[7]. In order to extend our knowledge of extremely richchemistry of such systems, it is necessary to further explorethe reactions between metal ion and nitronyl nitroxideradicals. In this paper, we will report that syntheses and

structural characterization about two novel transition metalcompounds with malonate and reduced imino nitroxide rad-icals, [Co(mal)(Him2-py)2] (ClO4) 1 and [Mn (mal)(Him2-py)2] (H2O) 2 (Him2-py = 1-hydroxy-2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline).

2. Experimental

2.1. Syntheses. 2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoli-ne-1-oxyl (im2-py) was prepared according to the methodsreported [10].

[Co(mal)(Him2-py)2] (ClO4) 1: an aqueous solution(10 mL) of Na2(mal) (0.148 g,1 mmol) was added to amixture of Co(ClO4)2·6H2O (0.365 g,1 mmol) and im2-py (0.436 g, 2 mmol) in 25 mL of methanol (pH = 6∼ 8). The mixture was stirred for 2 h and filtered. Thefiltrate was kept at room temperature for 1 month togrow well-formed orange crystals of [Co(mal)(Him2-py)2](ClO4). Yield: 45%. Anal. Cacld. (%) for: C, 47.38; H,4.96; N, 12.34. Found (%): C, 46.52; H, 4.92; N, 12.06. IR(KBr): ν(Py)1448 cm−1, 1386 cm−1, ν(NO) 1357 cm−1, νascoo−1650 cm−1, νascoo− 1455 cm−1.

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Table 1: Summary of the crystallographic data and collections for two complexes.

Complex 1 Complex 2

Empirical formula C27H34ClCoN6O10 C27H36MnN6O7

Formula weight 696.98 611.56

Crystal system, space group Monoclinic, C2 Monoclinic, C2

Unit cell dimensionsa = 17.004(9) Ab = 10.753(5) A

c = 9.207(5) A

β = 13.856(8)◦a = 16.721(5) Ab = 10.897(5) A

c = 9.253(3) A

β = 120.807(6)◦

Volume 1539.6(14) A3 1448.1(9) A3

Z, Calculated density 2, 1.504 Mg/m3 2, 1.403 Mg/m3

Absorption coefficient 0.710 mm−1 0.511 mm−1

F(000) 724 642

Crystal size 0.18 × 0.16 × 0.14 mm3 0.24 × 0.22 × 0.18 mm3

θ range for data collection 2.30 to 25.10◦ 2.35 to 25.01◦

Limiting indices−20 ≤ h ≤ 20, −12 ≤ k ≤ 11,

−9 ≤ l ≤ 10−19 ≤ h ≤ 14, −12 ≤ k ≤ 12,

−11 ≤ l ≤ 11

Reflections collected/unique 3755/2253 (R(int) = 0.0318) 3730/2519 (R(int) = 0.0154)

Completeness to θ = 25.10 95.4% 100.0%

Absorption correction Semi-empirical from equivalents Semi-empirical from equivalents

Max. and min. transmission 1.000000 and 0.711602 1.000000 and 0.754606

Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2

Data/restraints/parameters 2253/47/228 2519/2/190

Goodness-of-fit on F2 1.037 1.107

Final R indices (I > 2σ(I)) R1 = 0.0502, wR2 = 0.1257 R1 = 0.0401, wR2 = 0.1069

R indices (all data) R1 = 0 .0583, wR2 = 0.1311 R1 = 0.0418, wR2 = 0.1091

Absolute structure parameter 0.00(3) 0.0(2)

Largest diff. peak and hole 1.136 and −0.285 e.A−3 0.745 and −0.294 e.A−3

[Mn (mal)(Him2-py)2] (H2O) 2 was obtained using thesame procedure as that of the complex 1. The filtrate waskept at room temperature for 20 days to grow well-formedorange crystals of [Mn (mal)(Him2-py)2] (H2O). The yieldwas about 55%. Anal. Cacld. (%) for: C, 53.12; H, 6.01; N,14.02. Found (%): C, 52.72; H, 5.93; N, 13.74. IR (KBr):ν(Py)14 50 cm−1, 1396 cm−1, ν(NO)1365 cm−1, νascoo− 1645cm−1, νscoo− 1471 cm−1.

2.2. Crystal Structure Determination and Refinement. Allmeasurements were made on a Bruker Smart 1000 diffrac-tometer equipped with graphite-monochromated MoKαradiation (λ = 0.71073 A). The data were collected at roomtemperature. A summary of the crystallographic data is givenin Table 1. These structures were solved by direct methodsusing the SHELXS97 program [11]. Full-matrix least-squaresrefinements on F2 were carried out using SHELXL97 [12].A summary of the crystallographic data and collections arelisted in Table 1. These significant bond parameters for twocomplexes are given in Tables 2 and 3, respectively. Views

of the molecular structures for compounds 1 and 2 areshown, respectively, in Figures 1 and 2. The sketch of theintermolecular hydrog bonds of two complexes are shown inFigure 3 and 4, respectively.

3. Results and Discussion

The data of � (= νas − νs) of IR reveal that each malonatedianion binds metal ions in bidentate mode, leading to amononuclear structure. The bonds of the N–O stretchingvibration appear at ca.1357 cm−1 and 1365 cm−1 for complex1 and 2, respectively, which suggest that one Him2py arechelated metal ion by pyridyl and imino nitrogen atoms withthe five-membered ring.

The crystal structures of both complexes have severalfeatures in common. The single-crystal X-ray structuresof complexes 1 (Figure 1) and 2 (Figure 2) confirm thebidentate chelation of ligand. The complexes 1 and 2 consistof mononuclear molecule [Co(mal)(Him2-py)2] (ClO4) and[Mn (mal)(Him2-py)2] (H2O). The metal ion is located

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International Journal of Inorganic Chemistry 3

Table 2: Selected bond distances (A) and bond angles (◦) for thecomplex 1.

Co(1)–O(2) 1.879(4) C(1)–C(2) 1.360(9)

Co(1)–N(1) 1.945(4) O(2)–Co(1)–O(2)#1 95.0(3)

Co(1)–N(2) 1.947(5) O(2)–Co(1)–N(1)#1 93.8(2)

O(1)–N(3) 1.400(6) O(2)–Co(1)–N(1) 86.8(2)

N(1)–C(5) 1.336(7) N(1)#–Co(1)–N(1) 179.1(4)

N(1)–C(1) 1.354(8) O(2)–Co(1)–N(2)#1 175.0(2)

N(2)–C(6) 1.296(7) N(1)–Co(1)–N(2)#1 97.2(2)

N(2)–C(10) 1.490(8) O(2)–Co(1)–N(2) 87.67(16)

N(3)–C(6) 1.336(7) N(1)–Co(1)–N(2) 82.1(2)

N(3)–C(7) 1.469(8) N(2)#1–Co(1)–N(2) 90.0(3)

Symmetry transformations used to generate equivalent atoms: no. 1−x, y,−z.

O3

Co

O

N

C

H

Cl

C13

N1

N1

O1

N3

C6

N2

N3

N2

Co1O2

O2

a

bc

Figure 1: Diamond views of complex 1. Hydrogen atoms areomitted for clarity.

in a distorted octahedral environment, formed by the fournitrogen atoms (N(1), N(2), N(1)#, N(2)#) of the twobidentate imino nitroxide radicals and two oxygen atoms(O(2), O(2)#) of the same malonate group. The axialpositions are occupied by nitrogen atoms (N(1), N(1)#) frompyridyl rings. The pyridyl rings of the im2-py ligands arenonplanar, and that is a consequence of steric crowding fromthe im2-py ligands of the same molecule.

For complex 1, the distances of Co(1)–O(2) is 1.879(4) Aand the N–O bond distances are 1.400(6) A, which showthat the complex is consisted of Co(III) and reduced speciesIMHR [7]. For 2, the length of Mn(1)–O(2) is 2.022(3) Aand that of N–O is 1.406(4) A, clearly indicative of thereduced form of the radical [13–16]. In complex 1, the C2–N1 and C2–N2 bond lengths of the IMH2py are 1.296(7)

Table 3: Selected bond distances (A) and bond angles (◦) for thecomplex 2.

Mn(1)–O(2) 2.022(3) N(3)–C(7) 1.468(5)

Mn(1)–N(2) 2.075(4) C(1)–C(2) 1.379(7)

Mn(1)–N(1) 2.116(3) O(2)#1–Mn(1)–O(2) 88.88(19)

O(1)–N(3) 1.406(4) O(2)#1–Mn(1)–N(2) 167.69(15)

O(2)–C(13) 1.251(6) O(2)–Mn(1)–N(2) 89.92(11)

O(3)–C(13) 1.238(6) N(2)–Mn(1)–N(2)#1 93.8(2)

N(1)–C(1) 1.312(5) N(2)–Mn(1)–N(1)#1 100.87(15)

N(1)–C(5) 1.360(5) O(2)#1–Mn(1)–N(1) 90.13(15)

N(2)–C(6) 1.287(5) O(2)–Mn(1)–N(1) 91.38(16)

N(2)–C(10) 1.485(5) N(2)–Mn(1)–N(1) 77.65(15)

N(3)–C(6) 1.367(5) N(1)#1–Mn(1)–N(1) 177.9(3)

Symmetry transformations used to generate equivalent atoms: no. 1−x, y, −z.

O3

Mn

O

N

C

H

C13

N1

N1

C5

C5

Mn1

N3

C6

C6

N2

N3

N2

O1

O1

O2

O2

a

bc

Figure 2: Diamond views of complex 2. Hydrogen atoms areomitted for clarity.

and 1.336(7) A, respectively, as well as the C2–N1 and C2–N2 bond lengths of the IMH2py are 1.287(5) and 1.367(5) A in complex 2, which are rather close to those of thereduced imino nitroxide complex [6–9]. These structuralchanges result from the one-electron reduction of the N–O radical moiety in IM2py. N(1)–M–N(1)# angles donot significantly deviate from orthogonal, ranging from

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Figure 3: View of the unit of the complex 1 showing 1-D zigzag chain formed by O–O interaction alone a axis.

Figure 4: View of the unit of the complex 2, showing 1-D zigzag chain formed by O–O interaction alone a axis.

179.1(4)◦ to 177.9(3)◦. Details of two complexes can befound in supplementary material available online at doi:10.1155/2011/257521. The dihedral angles between pyridylring (N1C5C4C1) and imino nitroxide group (N2C6N3O1)are 12.8◦ (for complex 1) and 12.9◦ (for complex 2),respectively. For complex 1, noncoordinated ClO−

4 anionsinsert in the crystal spacing as well as noncoordinated H2Omolecules insert there for complex 2.

Sketch of the intermolecular hydrogen bonds of thecomplex 1 and 2 are shown in Figure 2, 3, respectively.In the crystal packing of two complexes, hydrogen bondsof O–O type have been observed between the hydrogenatom from the coordinated malonate and the oxygen atomfrom adjacent Him2-py, thus one-dimensional structures areformed. Among the 1-D chains, the oxygen atoms from the

NO groups of Him-2py and the mal form hydrogen bonds,and the distances of O(+x, +y, +z)—O(1/2 − x, 1/2 + y, −z)are 2.612 A (for complex 1) and 2.627 A (for complex 2),respectively.

As we know, there were several reports on metalcomplexes with a reduced nitroxide radical. However, noexample in which M(mal)2 converts the IMR radical intoIMHR has been reported. The mechanismic details of thereduction of nitroxide radical are not completely clear, butit is likely that the formation of Him2-py is favored by acidicimpurities and standing for a long time. The reduced radicalcan exist in two tautomeric forms [7], the amidino oxideand iminohydroxylamine. Since the nitrogen atom from theimino group here is involved in the coordinating metal ion,the reduced radical form should be the latter [6].

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International Journal of Inorganic Chemistry 5

As mentioned in the experimental part of this paper,the cobalt reactant is cobalt(II) perchlorate hexahydrate.However, the reaction product is cobalt(III) complex 1 dueto the oxidation of Co(II) to Co(III) in air. The result iscorresponding to those lectures [17–19].

Acknowledgment

This work was supported by the Program for New CenturyExcellent Talents in University of Henan Province (no.2006HANCET-14).

References

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Hindawi Publishing CorporationInternational Journal of Inorganic ChemistryVolume 2011, Article ID 650186, 7 pagesdoi:10.1155/2011/650186

Research Article

Synthesis, Characterization, and Magnetic and Thermal Studieson Some Metal(II) Thiophenyl Schiff Base Complexes

Aderoju Amoke Osowole

Inorganic Chemistry Unit, Department of Chemistry, University of Ibadan, Ibadan, Nigeria

Correspondence should be addressed to Aderoju Amoke Osowole, [email protected]

Received 7 November 2010; Revised 22 December 2010; Accepted 31 December 2010

Academic Editor: Maurizio Peruzzini

Copyright © 2011 Aderoju Amoke Osowole. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

4-(Thiophen-3-yl)-aniline undergoes condensation with o-vanillin to form an ONS donor Schiff base, 2-methoxy-6-[(4-thiophene-3-yl-phenylimino)-methyl]-phenol, which forms complexes of the type [ML2]xH2O (where M = Mn, Co, Ni, Cu, Zn,Pd). These complexes are characterized by elemental analysis, 1H nmr, electronic, mass, and IR spectroscopies and conductancemeasurements. The electronic, IR and CHN data are supportive of a 4-coordinate tetrahedral geometry for Mn(II), Co(II), Ni(II),and Zn(II) complexes and square-planar geometry for Cu(II) and Pd(II) complexes, with the chromophores N2O2. The magneticdata reveals that the complexes are magnetically dilute and mononuclear with exception of the Cu(II) complex, which exhibitssome anti-ferromagnetisms. The complexes are air-stable solids, and none is an electrolyte in nitro methane.

1. Introduction

My group has in the last five years been actively involved inthe synthesis, characterization, and thermal and biologicalproperties of various Schiff bases and their M(II) chelates(M = VO, Mn, Co, Ni, Cu, Zn, Pd), with the objectives ofderiving Schiff base chelates which can be used as precursorsin metal-organic chemical vapor depositions (MOCVD) andthose with good in vitro antimicrobial activities as surfacecleaning agents [1–5]. Thiophenyl Schiff bases are particu-larly interesting because of their wide range of activities suchas anticancer activity as shown by benzyl-N-[1-(thiophenyl-3-yl)ethylidene] hydrazine carbodithioate [6], antibacterialand antifungal activities typified by thiophenyl-azetidinones,-cephalexins and -vinyl anilines [7–16] as well as opticalexhibited by thiophene-2-aldazine [17]. Other activitiesinclude structural activity exemplified by the hexacar-bonyldiiron complex of N-(2-thienylmethylidene)aniline,which is composed of two thienyl moieties derived fromthe original thienyl imine and coupled together by a C–Cbond [18]. Furthermore, Palladium imine complexes of 2-thiophenecarboxaldehyde are used as catalysts in the Suzukicross-coupling of aryl bromides with phenyl boric acid[19]. Extensive literature reviews show that no studies are

reported on the Schiff base derived from o-vanillin and 4-(thiophene-3-yl)-aniline and its metal(II) chelates [6–19].Thus, the objective of this work is to synthesize, characterize,and investigate the magnetic and thermal properties of theSchiff base, 2-methoxy-6-[(4-thiophene-3-yl-phenylimino)-methyl]-phenol and its Mn(II), Co(II), Ni(II), Cu(II), Zn(II)and Pd(II) complexes. These metal complexes and its ligandsare new, being reported here for the first time.

2. Experimental Details

2.1. Materials and Physical Measurements. Reagent grade o-vanillin, 4-(thiophene-3-yl)-aniline, manganese(II) nitratedehydrate, cobalt(II) nitrate hexahydrate, nickel(II) nitratehexahydrate, copper(II) nitrate hexahydrate, zinc(II) nitratehexahydrate, and palladium(II) chloride are purchased fromBDH and Aldrich chemicals and are used as received. Sol-vents are dried and distilled before use according to standardprocedures. Melting points (uncorrected) are determinedusing the Stuart scientific melting point SMP1 machine,and conductivities of 10−3 M solutions of the complexes aremeasured in nitromethane at 25◦C using a MC-1, Mark Vconductivity meter with a cell constant of 1.0. The solid

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reflectance spectra are recorded on a Perkin-Elmer λ20 spec-trophotometer while infrared spectra are measured as KBrdiscs on a Perkin-Elmer FTIR paragon 1000 spectrometer inthe range 4000–400 cm−1. The elemental analyses C, H, andN are recorded on GmbH VarioEl analyser, and manganese,cobalt, nickel, copper, zinc, and palladium are determinedtitrimetrically and by atomic absorption spectroscopy [20].The 1H nmr spectra are recorded on a 300 MHz OxfordVarian NMR instrument in CDCl3 at 295 K. 1H chemicalshifts are referenced to the residual signals of the protonsof CDCl3 and are quoted in ppm. Magnetic susceptibilitiesare measured on Johnson Matthey magnetic susceptibilitybalance, and diamagnetic corrections are calculated usingPascal’s constants [21]. Thermogravimetric analyses are donein static air, using a T6 Linseis thermal analyser with a heat-ing rate of 10◦C/min in the range 30–700◦C. The MALDI-TOF mass and atomic absorption spectra are obtained usinga Bruker Daltonic Reflex TOF spectrometer with graphite asmatrix and Perkin Elmer Analyst 200 coupled to Winlab 32software assembly, respectively.

2.2. Preparation of the Schiff Base (2-methoxy-6-[(4-thio-phene-3-yl-phenylimino)-methyl]-phenol). A 20 mL solutionof 8.71 mmol (1.33 g) o-vanillin in absolute ethanol is addeddropwise to a stirring solution of 8.71 mmol (1.53 g) of 4-(thiophene-3-yl)-aniline in 30 mL of absolute ethanol. Theresulting orange-colored solution is refluxed for 4 h afteraddition of 4 drops of acetic acid. The orange productformed on cooling to room temperature is filtered andrecrystallized from ethanol. The yield of the title compoundis 1.88 g (70%). 1H nmr (ppm) δ 11.13 (s, 1H, OH), 9.93(s, 1H, HCN), 8.70 (1H, s, C2HS), 7.35–7.43 (m, 3H, C6H3);7.03–7.24 (m, 4H, C6H4); 6.91–7.01 (m, 2H, C2H2S); 3.96 (s,3H, OCH3).

2.3. Preparation of the Metal(II) Complexes. The variouscomplexes are prepared by gradual addition of 0.35 mmol(0.06–0.11 g) M(NO3)2·6H2O (M = Mn, Co, Ni, Cu, Zn)neat to a stirring 0.7 mmol (0.22 g) of the ligand in 30 mLof absolute ethanol. The resulting solutions are then bufferedwith 0.7 mmol (0.10 mL) of triethylamine and refluxed for6 h during which the products formed. The precipitatedsolids are filtered, washed with ethanol, and dried overanhydrous calcium chloride. The yields are 0.12 g (50%),0.17 g (70%), 0.17 g (70%), 0.17 g (70%), and 0.14 g (60%),respectively.

The Pd(II) complex is prepared using a similar method.0.36 mmol (0.064 g) of Pd(II) chloride, in 10 mL of abso-lute ethanol, is added dropwise to a stirring solutionof 0.72 mmol (0.23 g) of the ligand in 30 mL of abso-lute ethanol. The resulting solution is then buffered with0.72 mmol (0.11 mL) of triethylamine and refluxed for 6 hduring which the product is formed. The product is filtered,washed with ethanol, and dried over anhydrous calciumchloride. The yield is 0.18 g (70%).

Proton nmr measurements are done only for the diamag-netic Zn(II) and Pd(II) complexes. Zn(II) complex: 1H nmr(ppm) 8.70 (s, 1H, HCN), 8.34 (1H, s, C2HS), 7.28–7.43

(m, 3H, C6H3); 7.01–7.10 (m, 4H, C6H4); 6.63–6.94 (m, 2H,C2H2S); 3.94 (s, 3H, OCH3).

Pd(II) complex: 1H nmr (ppm) 8.67 (s, 1H, HCN), 8.30(1H, s, C2HS), 7.48–7.66 (m, 3H, C6H3); 7.00–7.47 (m, 4H,C6H4); 6.88–6.98 (m, 2H, C2H2S); 3.94 (s, 3H, OCH3).

3. Results and Discussion

The equations for the formation of the complexes are

M(NO3)2 + 2HL −→ [ML2] + 2HNO3 (1)

(where M = Zn(II), Cu(II), Mn(II), Co(II), Ni(II))

PdCl2 + 2HL −→ [PdL2] + 2HCl. (2)

All complexes adopt [ML2] stoichiometry, except Mn(II),Co(II) and Ni(II) complexes that form as [ML2]xH2O, wherex = 1 and 0.5, respectively. Proposed structures for theligand and the Cu(II) complex are shown in Figure 1. Theformation of this ligand is confirmed by microanalysis and1H nmr. The colors, melting points, and room temperaturemagnetic moments (μeff) of the compounds are presented inTable 1. Attempts to isolate suitable crystals for single X-raystructural determination have not been successful so far.

3.1. Infrared Spectra. The relevant infrared bands of thecompounds are presented in Table 2. The broadband at3360 cm−1 in the ligand, which is conspicuously absentin the spectra of the metal(II) Schiff base complexes, isassigned as vOH stretching frequency, and it confirmsinvolvement of the phenol O in chelation. It is broad dueto intramolecular hydrogen bonding, usually very strongin Schiff bases [1]. The new broadband at 3500 cm−1 inthe spectra of Co(II), Ni(II), and Mn(II) complexes isassigned to the vOH frequency of crystallization H2O. Theuncoordinated C=N stretching vibrations are observed asthree bands between 1614–1521 cm−1 in the ligand [2–5]and 1639–1503 cm−1 in the metal complexes with exceptionsof the Cu(II) and Pd(II) complex which have two bands.The bathochromic/hypsochromic shifts of these bands inthe complexes are attributed to the involvement of N atomof C=N in coordination to the metal ions. Moreover, ithas been documented that square planar Pd(II) and Cu(II)Schiff base complexes do exhibit geometric isomerism [22],with the trans isomer showing two vC=N bands and the cisisomer a lone vC=N band. The spectra of the Cu(II) andPd(II) complexes in this work show two vC=N bands andare consequently in the trans-isomeric form. The vPh/C–Oand δC–H vibrations of the ligand are observed at 1461–1364and 972 cm−1, respectively. These suffer bathochromic shiftsto 1298–1177 and 896–720 cm−1 in the Schiff base complexesdue to the coordination of the phenol oxgen atom andpseudoaromatic nature of the chelates [5, 6]. The observationof new bands at 480–405 and 581–542 cm−1due to v(M–O)and v(M–N) [11, 14, 22] is further evidence of coordination.

3.2. Electronic Spectra and Magnetic Moments. The electronicspectral data for the complexes are presented in Table 2.

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OH

OCH3

N

S

(a)

O

OCH3

N

S

O

OCH3

N

S

Cu

O

OCH3

N

S

O

OCH3

N

S

Cu

(b)

Figure 1: Proposed structures for the ligand (a) and its Cu(II) complex (b).

Table 1: Analytical data for the ligand and its complexes.

Compound(empirical formula)

Formulamass

Colorm/z

(100%)μeff % Yield Λm

∗ M.p(◦C)

Analysis (calculated)

% C % H % N % M

HL (C18H15NO2S) 309.38 Orange 309 — 80 — 150–15169.82

(69.88)4.53

(4.89)4.35

(4.53)—

[MnL2]H2O(MnC36H30N2S2O5)

689.73 green 671 5.70 50 10.0 338–34062.54

(62.69)4.32

(4.38)4.02

(4.06)8.02

(7.97)

[CoL2]H2O(CoC36H30N2S2O5)

693.71 Brick Red 675 4.33 70 20.0 290–29262.36

(62.33)3.64

(4.36)4.06

(4.04)8.48

(8.50)

[NiL2]1/2H2O(NiC36H30N2S2O4.5)

684.48 Yellow 675 3.10 70 15.0 193–19563.07

(63.17)3.86

(4.27)4.17

(4.09)8.55

(8.58)

[CuL2]2

(Cu2C72H56N4S4O8)680.31 Brown 680 1.56 70 9.0 205–207

64.14(63.56)

4.06(4.15)

4.01(4.12)

9.35(9.34)

[ZnL2](ZnC36H28N2S2O4)

681.76 Yellow 681 D 60 17.0 316–31863.13

(63.42)3.84

(4.14)3.81

(4.11)9.52

(9.53)

[PdL2](PdC36H28N2S2O4)

723.18 green 720 D 70 12.0 148–15060.37

(59.79)4.32

(3.90)3.86

(3.87)14.70

(14.72)∗Ω−1 cm2 mol−1, D: diamagnetic.

The Mn(II) Schiff base complex shows two absorption bandsat 15.00 and 23.11 kK, respectively, consistent with a four-coordinate, tetrahedral geometry and are assigned to 6A1 →4E1 (ν1) and 6A1 → 4A1 (ν2) transitions [23]. A roomtemperature moment of 5.92 B.M is usually observed for theMn(II) compounds, regardless of stereochemistry becausethe ground term is an 6A1, and thus, orbital contribution

is nil. This Mn(II) complex has a moment of 5.70 B.M.complementary of tetrahedral geometry [1].

The cobalt(II) Schiff base complex gives two absorptionbands at 10.82 and 18.20 kK, respectively, typical of a four-coordinate tetrahedral geometry and is assigned to 4A2 →4T1(P) (v 2) and 4A2 → 4T1(P) (v 3). The transition 4A2 →4T2 (v 1) in the range 5–7 kK is not observed as usual

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Table 2: Relevant infrared and electronic spectral data of the ligand and its complexes.

Compound νOH ν(C=N) + ν(C=C) νPh/C–O δC–H ν(M–N) ν(M–O) Electronic transitions (kK)

HL 3360b 1614s 1593s 1521s 1461s 1364s 972s — — 33.50, 36.25, 40.20

[MnL2]H2O 3500b 1612s 1589s 1551s 1298s 1194s 848s 720s 580m 561s 450m 420s 15.00, 23.11, 30.26, 33.45, 40.20

[CoL2]H2O 3500b 1609s 1557s 1503s 1231m 1180m 840s 777m 575s 537m 475m 415m 10.82, 18.20, 32.0, 37.40, 41.0

[NiL2]1/2H2O 3500b 1599s 1585s 1537s 1233s 1192s 853s 780m 581s 564s 460s 453s 14.29, 20.21, 30.72, 36.25, 40.40

[CuL2] — 1587s 1538s 1237m 1193m 856s 795s 573s 557s 465s 410s 14.09, 21.60, 30.92, 34.35, 41.20

[ZnL2] — 1609s 1582s 1504s 1231m 1197m 896s 731m 579s 562m 480m 405m 28.20, 35.00, 42.00

[PdL2] — 1592s 1540s 1240s 1177s 843m 783m 562m 542m 450m 420m 18.20, 26.32, 30.25, 39.20, 43.00

b: broad, m: medium, s: strong., 1 kK: 1000 cm−1.

since it lies in the infrared region [10]. This geometry iscorroborated by a moment of 4.33 B.M [11].

Nickel(II) complexes are known to exhibit complicatedequilibria between coordination numbers six (octahedral) tofour (square planar/tetrahedral) [24]. The Ni(II) Schiff basecomplex exhibits two absorption bands at 14.29 and 20.21 kKtypical of a 4-coordinate tetrahedral geometry, assignedto 3T1(F) → 3T2, (ν2) and 3T1(F) → 3A2, (ν3) transitions.Its moment of 3.10 B.M is complimentary of tetrahedralgeometry, since moments of 3.1–3.5 B.M. are reported fordistorted tetrahedral complexes [12].

The copper(II) complex displays two bands at 14.09and 21.60 kK, assigned to 2B1g → 2A1g and 2B1g → 2E1g

transitions of 4-coordinate, square planar geometry [13]. Amoment of 1.9–2.2 B.M. is usually observed for mononuclearcopper(II) complexes, regardless of stereochemistry [14]. Amagnetic moment of 1.56 B.M. is observed for this com-plex, indicative of the presence of some anti-ferromagneticinteractions, operating through Cu–Cu interactions [25].However, this could not be probed further due to lack offacilities for variable temperature magnetic measurementsand nonsuitable crystal for single X-ray diffraction measure-ment (Figure 1).

The Zn(II) complex expectedly shows only charge trans-fer transition from M→L and π-π∗ transitions, as nod-d transition is expected at 28.20, 35.00, and 42.0 kK,respectively. This complex is diamagnetic, confirming itstetrahedral geometry [12–14].

The Pd(II) complex shows absorption bands at 18.20 and26.32 kK, typical of square planar geometry and is assigned to1A1g → 1B1g and 1A1g → 1E2g transitions. This complex isexpectedly diamagnetic [26].

3.3. 1H nmr Spectra. The ligand shows the phenolic proton asa singlet at δ 11.13 ppm (s, 1H, OH), the imine (s, 1H, HCN),and 2-thiophenyl (s, H, C2HS) protons resonate as singletsat 9.93 and 8.70 ppm, respectively. The o-vanillin (m, 3H,C6H3) and phenyl (aniline) protons (m, 4H, C6H4) are bothobserved as multiplets at δ 7.35–7.43 and 7.03–7.24 ppm,respectively. The two protons of thiophenyl ring at 4 and 5positions come up as a multiplet at 6.91–7.01 ppm (m, 2H,–C2H2S), and the methoxy protons are observed as a singletat 3.96 ppm (s, 3H, OCH3).

In the Zn(II) complex spectrum, the phenolic protondisappears, an indication of coordination of the phenoloxygen to the Zn(II) ion, and other protons are all upfieldshifted in comparison to the ligand. The imine hydrogenresonates as a singlet at 8.70 ppm (s, 1H, HCN) while the2-thiophenyl proton is seen at 8.34 ppm as a singlet (s, H,C2HS).

The o-vanillin protons resonate as a multiplet at δ 7.28–7.43 ppm (m, 3H, C6H3), and the phenyl(aniline) protonsare seen as multiplets at 7.01–7.10 ppm (m, 4H, C6H4).The two protons of thiophenyl ring at 4 and 5 positionsresonate as a multiplet at 6.63–6.94 ppm (m, 2H, –C2H2S),and the methoxy protons are seen as a singlet at 3.94 ppm (s,3H, OCH3). This shift shows deshielding, a consequence ofcoordination of the imine nitrogen atom [27].

The Pd(II) complex spectrum reveals the absence of thephenolic proton. This is indicative of involvement of thephenol oxygen in coordination to the Pd(II) ion. The otherprotons are all upfield shifted in comparison to the ligandwith exceptions of the o-vanillin and phenyl(aniline) protonswhich are downfield shifted. The imine hydrogen (s, 1H,HCN) and 2-thiophenyl (s, H, C2HS) protons resonate assinglets at 8.67 and 8.30 ppm, respectively. The o-vanillin (m,3H, C6H3), phenyl(aniline) (m, 4H, C6H4), and two protonsof thiophenyl ring (m, 2H, –C2H2S) at 4 and 5 positions areall seen as multiplets at δ 7.48–7.66, 7.00–7.47, and 6.88–6.98 ppm, respectively. The methoxy protons resonate as asinglet at 3.94 ppm (s, 3H, OCH3). These shifts are indicativeof coordination of the imine nitrogen atom to the Pd(II) ion[26].

3.4. Mass Spectroscopy and Thermal Studies. The mass spec-tra of ligand and the complexes showed peaks attributed tothe molecular ions m/z at 309 [L]+; 671 [MnL2-2H]+; 675[CoL2-2H]+; 675 [NiL2-2H]+; 680 [CuL2-2H]+; 681 [ZnL2-2H]+ and 720 [PdL2], respectively, and are presented inTable 1.

The thermal degradation of the ligand and complexesis presented in Table 3. The ligand, HL, decomposes inthree steps. First, the loss of the fragment C2H2 and0.5 mol N2 at 30–220◦C, with mass losses of (obs. = 12.96%,calc. = 12.93%). The next step involves the loss of the organicfraction, C11H8O2S, with mass losses of (obs. = 66.32%,

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Table 3: Thermal data for the ligand and its complexes.

Compound Temperature range (◦C)TG weight loss (%)

AssignmentsCalc. Found

HL (C18H15NSO2)30–220 12.93 12.96 C2H2 + 0.5N2

220–420 65.94 66.32 C11H8O2S

420–700 22.78 21.01 C5H5

[MnL2]H2O (MnC36H30N2S2O5)

30–200 9.57 9.40 H2O + 1.5O2

210–450 44.07 44.45 C20H18NS

450–700 22.04 22.53 C8H10OS

Mn (residue)

[CoL2]H2O (CoC36H30N2S2O5)

30–200 4.90 4.77 CH4 + H2O

200–400 20.76 21.45 C6H8S2

400–700 57.08 57.50 C24H16O4N2

Co (residue)

[NiL2]1/2H2O (NiC36H30N2S2O4.5)

30–240 10.67 10.56 0.5H2O + SO2

240–420 23.08 22.73 C10H10N2

420–700 55.81 55.51 C25H18SO2

Ni (residue)

[CuL2] (CuC36H28N2S2O4)

30–200 17.93 17.86 C8H12N

200–400 33.81 33.63 C13H10O2S

400–700 38.81 42.00 C15H6NO2S

Cu (residue)

[ZnL2] (ZnC36H28N2S2O4)

30–260 5.00 5.17 H2S

260–440 27.87 27.80 C11H10OS

440–700 52.22 52.21 C22H16N2O3

Zn (residue)

[PdL2] (PdC36H28N2S2O4)30–210 3.87 3.50 C2H4

210–400 40.79 41.33 C18H15SO2

400–700 33.32 33.42 C13H9N2SO

PdO (residue)

calc. = 65.94%) at 220–420◦C. The final step is the loss ofthe fragment C5H5, with mass losses of (obs. = 22.78%,calc. = 21.01%) at 420–700◦C.

The Mn(II) complex decomposes in three phases. Thefirst phase corresponds to the loss of 1.5 moles of O2 andH2O between 30–200◦C with mass losses of (obs. = 9.40%,calc. = 9.57%). The second phase is from 210 to 450◦C andis attributed to the loss of the organic moiety C20H18NS withmass losses of (obs. = 44.37%, calc. = 44.07%). The finalphase shows the loss of the organic moiety, C8H10OS, at 450–700◦C with mass losses of (obs. = 22.53%, calc. = 22.33%)leaving Mn as the final product, and the fragment C8N is lostas 8CO2 and 0.5N2.

The decomposition of the Co(II) complex also occurredin three steps. The first step is due to the loss of amole of water and CH4 at 30–200◦C, with mass losses of(obs. = 4.77%, calc. = 4.90%). The successive decomposi-tion occurs within a temperature range of 200–400◦C andis attributed to the loss of the organic moiety C6H8S2 withmass losses of (obs. = 21.45%, calc. = 20.76%). The last stepinvolves the loss of the organic moiety, C24H16N2O4, at 400–700◦C with mass losses of (obs. = 57.50%, calc. = 57.08%).The final product is Co, and the C5 fragment is lost as 5CO2.

The TGA curve of the Ni(II) complex reveals a three-step decomposition. The first is the loss of 0.5 mole of waterand SO2 at 30–240◦C, with mass losses of (obs. = 10.56%,calc. = 10.67%). The second step ranges from 240 to 420◦Cand is assigned to the loss of the organic moiety, C10H10N2

with mass losses (obs. = 22.73%, calc. = 23.08%). The finalstep is within a temperature range of 420–700◦C and isattributed to the loss of the organic moiety C25H18SO2

(obs. = 55.51%, calc. = 55.81%). The remaining fraction isNi residue, and the fragment CH is lost as CO2 + 0.5H2.

Cu(II) complex decomposes in three steps. The first stepis attributed to the loss of the fragment C8H12N, with masslosses of (obs. = 17.86%, calc. = 17.89%) at 30–200◦C. Thesecond step ranges from 200 to 400◦C and is attributedto the loss of the fragment C13H10O2S, with mass lossesof (obs. = 33.63%, calc. = 33.74%). The final step is from400 to 700◦C corresponding to the loss of the organicmoiety C15H4O2NS, with mass losses of (obs. = 42.0%,calc. = 38.72%). The remaining residue is Cu.

The Zn(II) complex decomposes in three steps. Step oneis between 30–260◦C, which indicates the loss of H2S, withmass losses of (obs. = 5.17%, calc. = 5.00%). The secondstep involves the loss of the organic moiety C11H10OS,

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from 260 to 440◦C, with mass losses of (obs. = 27.80%,calc. = 27.87%). The final step is attributed to the loss of theorganic moiety, C22H16N2O3, at 440–700◦C, with mass lossesof (obs. = 52.21%, calc. = 52.22%), leaving behind the Znresidue, and the C3 fragment is lost as 3CO2.

The Pd(II) complex also decomposes in three phases.The first phase is between 30–210◦C and is attributedto loss of C2H4, with mass losses of (obs. = 3.50%,calc. = 3.87%). The second phase involves the loss ofthe organic moiety, C18H15O2S at 210–400◦C, with masslosses of (obs. = 41.33%, calc. = 40.79%) while the finalstage involves the loss of the organic moiety, C13H9N2SO,from 400 to 700◦C, with mass losses of (obs. = 33.42%,calc. = 33.32%), leaving behind PdO residue, and the frag-ment C3 is lost as 3CO2.

In all cases with the exceptions of the ligand and theCu(II) complex, the decomposition pattern showed theloss of carbon fragments which got oxidized to CO2, andhydrogen or nitrogen which were lost as gases. Thus, thedecomposition pattern corroborates the proposed formula-tion of the complex.

3.5. Conductance. The molar conductances of the com-plexes in nitromethane are below 20.0 ohm−1 cm2 mol−1

confirming their covalent nature. A value in the range 75–90 ohm−1 cm2 mol−1 is expected for a 1 : 1 electrolyte [12].

4. Conclusion

The Schiff-base ligand coordinates to the Mn(II), Ni(II),Co(II), Cu(II), Pd(II), and Zn(II) ions in a tetradentatemanner using the N2O2 chromophores. The assignment ofa 4-coordinate square-planar geometry to Cu(II) and Pd(II)complexes and tetrahedral geometry to Mn(II), Ni(II),Co(II), and Zn(II) complexes is corroborated by elementalanalysis, thermal, magnetic, and electronic spectral measure-ments. The Cu(II) and Pd(II) complexes exhibit geometricisomerism and are in the trans-isomeric form as confirmedby their infrared spectra. Furthermore, the Cu(II) complexexhibits some anti-ferromagnetic interactions, operatingthrough a dimeric structure while the other complexes aremononuclear.

Acknowledgment

The author thanks The Alexander von Humboldt (AvH)Foundation for a Georg Forster Fellowship.

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[3] A. A. Osowole, B. C. Ejelonu, and S. A. Balogun, “Spec-troscopic, magnetic and antibacterial properties of somemetal(II) unsymmetric Schiff-base complexes and theirmixed-ligand analogs,” Journal of Ultra Chemistry, vol. 20, no.3, pp. 549–558, 2008.

[4] A. A. Osowole and O. E. Fagade, “Synthesis, characterizationand biopotency of some metal(II) β-ketoiminates and theirmixed-ligand complexes,” Polish Journal of Chemistry, vol. 81,no. 12, pp. 2039–2048, 2007.

[5] A. A. Osowole, G. A. Kolawole, and O. E. Fagade, “Synthesis,physicochemical, and biological properties of nickel(II), cop-per(II), and zinc(II) complexes of an unsymmetrical tetraden-tate Schiff base and their adducts,” Synthesis and Reactivity inInorganic, Metal-Organic and Nano-Metal Chemistry, vol. 35,no. 10, pp. 829–836, 2005.

[6] M. H. E. Chan, K. A. Crouse, M. I. M. Tahir, R. Rosli,N. Umar-Tsafe, and A. R. Cowley, “Synthesis and char-acterization of cobalt(II), nickel(II), copper(II), zinc(II)and cadmium(II) complexes of benzyl N-[1-(thiophen-2-yl)ethylidene] hydrazine carbodithioate and benzyl N-[1-(thiophen-3-yl)ethylidene] hydrazine carbodithioate andthe X-ray crystal structure of bisbenzyl N-[1-(thiophen-2-yl)ethylidene] hydrazine carbodithioatenickel(II),” Polyhe-dron, vol. 27, no. 4, pp. 1141–1149, 2008.

[7] P. S. Kenderekar, S. V. More, P. S. Patil, S. RS. Bhusare, and R. P.Pawar, “Synthesis of 2-(2-hydroxy- 3 -iodo-5-bromo phenyl)-3 -(substituted phenyl)- 4 -thiazolidinones as antibacterialagents,” Oriental Journal of Chemistry, vol. 18, no. 3, pp. 595–597, 2002.

[8] N. Idrees, M. Siddique, A. G. Doshi, and A. W. Raut, “Synthe-sis of N-substituted phenyl-4- thiophenyl -2-azetidinones andits antimicrobial activity,” Oriental Journal of Chemistry, vol.17, no. 1, pp. 143–146, 2001.

[9] N. Idrees, M. Siddique, S. D. Patil, A. G. Doshi, and A. W. Raut,“Synthesis of Schiff bases of thiophene-2-carboxaldehyde andits antimicrobial activity,” Oriental Journal of Chemistry, vol.17, no. 1, pp. 131–133, 2001.

[10] B. N. Reddy, P. G. Avaji, P. S. Badami, and S. A. Patil, “Synthe-sis, spectral and biological studies of cobalt(II), nickel(II) andcopper(II) complexes with 1,5-bis(thiophenylidene) thiocar-bohydrazone,” Journal of Saudi Chemical Society, vol. 11, no.2, pp. 253–268, 2007.

[11] Z. H. Chohan, H. Pervez, K. M. Khan, A. Rauf, and C. T. Supu-ran, “Binding of transition metal ions [cobalt, copper, nickeland zinc] with furanyl-, thiophenyl-, pyrrolyl-, salicylyl- andpyridyl-derived cephalexins as potent antibacterial agents,”Journal of Enzyme Inhibition and Medicinal Chemistry, vol. 19,no. 1, pp. 51–56, 2004.

[12] Z. H. Chohan, H. Pervez, K. M. Khan, A. Rauf, G. M.Maharvi, and C. T. Supuran, “Antifungal cobalt(II), cop-per(II), nickel(II) and zinc(II) complexes of furanyl-thio-phenyl-, pyrrolyl-, salicylyl- and pyridyl-derived cephalexins,”Journal of Enzyme Inhibition and Medicinal Chemistry, vol. 19,no. 1, pp. 85–90, 2004.

[13] Z. H. Chohan, H. Pervez, A. Rauf, A. Scozzafava, andC. T. Supuran, “Antibacterial Co(II), Cu(II), Ni(II) andZn(II) complexes of thiadiazole derived furanyl, thiophenyland pyrrolyl schiff bases,” Journal of Enzyme Inhibition andMedicinal Chemistry, vol. 17, no. 2, pp. 117–122, 2002.

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International Journal of Inorganic Chemistry 7

[14] Z. H. Chohan and S. Kausar, “Synthesis, characterizationand biological properties of tridentate NNO, NNS and NNNdonor thiazole-derived furanyl, thiophenyl and pyrrolyl Schiffbases and their Co(II), Cu(II), Ni(II) and Zn(II) metalchelates,” Metal-Based Drugs, vol. 7, no. 1, pp. 17–22, 2000.

[15] A. I. P. Sinha and M. Bala, “Coordination behavior of herbici-dal Schiff bases derived from 2-amino-6-ethoxybenzothiazoletowards copper(II),” Asian Journal of Chemistry, vol. 3, no. 1,pp. 45–51, 1991.

[16] P. Mittal, S. Joshi, V. Panwar, U. Vatsala, and V. Uma,“Biologically active Co (II), Ni (II), Cu (II) and Mn(II)Complexes of Schiff bases derived from vinyl aniline andheterocyclic aldehydes,” International Journal of ChemTechResearch, vol. 1, no. 2, pp. 225–232, 2009.

[17] M. Ghazzali, V. Langer, C. Lopes, A. Eriksson, and L.Ohrstrom, “Syntheses, crystal structures, optical limitingproperties, and DFT calculations of three thiophene-2-aldazine Schiff base derivatives,” New Journal of Chemistry, vol.31, no. 10, pp. 1777–1784, 2007.

[18] D. L. Wang, W. S. Hwang, L. C. Liang, L. I. Wang, L. Lee,and M. Y. Chiang, “Reaction of a thienyl schiff base withdiiron nonacarbonyl: characterization and structures of [μ-N-(((2,3-η:η )-5-methyl-2-thienyl)methyl)-η:η (N)-anilino]hex-acarbonyldiiron and [μ-N-((anilino(2-thienyl)methyl) ((2,3-η:η)-2-thienyl)methyl)-η :η(N),” Organometallics, vol. 16, no.14, pp. 3109–3113, 1997.

[19] J. Wiedermann, K. Mereiter, and K. Kirchner, “Palladiumimine and amine complexes derived from 2-thiophenecar-boxaldehyde as catalysts for the Suzuki cross-coupling of arylbromides,” Journal of Molecular Catalysis A, vol. 257, no. 1-2,pp. 67–72, 2006.

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[22] A. A. Nejo, G. A. Kolawole, A. R. Opoku, C. Muller, andJ. Wolowska, “Synthesis, characterization, and insulin-en-hancing studies of unsymmetrical tetradentate Schiff-basecomplexes of oxovanadium(IV),” Journal of CoordinationChemistry, vol. 62, no. 21, pp. 3411–3424, 2009.

[23] A. B. Lever, Inorganic Electronic Spectroscopy, Elsevier, London,UK, 1980.

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[27] X. Han, Z. L. You, Y. T. Xu, and X. M. Wang, “Synthesis, char-acterization and crystal structure of a mononuclear zinc(II)complex derived from 2-methoxy- 6-[(3-cyclohexylamino-propylimino)methyl]phenol,” Journal of Chemical Crystallog-raphy, vol. 36, no. 11, pp. 743–746, 2006.

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Hindawi Publishing CorporationInternational Journal of Inorganic ChemistryVolume 2011, Article ID 606271, 9 pagesdoi:10.1155/2011/606271

Research Article

Synthesis and Structural Characterization ofa New Tetranuclear Nickel(II) Sulfato Complex Containingthe Anionic Form of Di-2-Pyridyl Ketone Oxime

Eleni Moushi,1 Constantinos G. Efthymiou,2 Spyros P. Perlepes,3

and Constantina Papatriantafyllopoulou2, 3

1 Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus2 Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA3 Department of Chemistry, University of Patras, 265 04 Patras, Greece

Correspondence should be addressed to Constantina Papatriantafyllopoulou, [email protected]

Received 7 December 2010; Accepted 26 January 2011

Academic Editor: Daniel L. Reger

Copyright © 2011 Eleni Moushi et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The preparation and crystal structure of a tetranuclear Ni(II) sulfato cluster containing the anion of di-2-pyridyl ketone oxime,(py)2CNO−, are reported. Treatment of NiSO4·6H2O with one equivalent of (py)2CNOH and one equivalent of NEt3 in MeOHleads to the compound [Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1) in moderate yield. The metal ions are linked together by two 3.2111and two 2.1110 (Harris notation) (py)2CNO− ligands, as well as two 2.1100 SO4

2− ions to create a rare metallacrown-type (12-MC-4) ring. Strong H-bond intermolecular interactions in 1 lead to the formation of a 1D chain along the c axis. CharacteristicIR bands are discussed in terms of the known structure of 1.

1. Introduction

There is currently a renewed interest in the coordinationchemistry of oximes [1–20]. 2-pyridyl oximes (Scheme 1)are popular ligands in coordination chemistry [21–36]. Theanions of these molecules are versatile ligands for a varietyof research objectives, including μ2 and μ3 behaviour [21,22]; the activation of 2-pyridyl oximes by 3d-metal centerstowards further reactions is also becoming a fruitful area ofresearch [21, 22, 26]. The majority of the metal complexesof these ligands have been prepared in the last 15 years andmuch of their chemistry remains to be explored in moredetail [22].

We have been exploring “ligand blend” reactions involv-ing carboxylates (R′CO2

−) and various 2-pyridyloximateswith (ternary “ligand blends”) or without (binary “ligandblends”) additional inorganic monoanions (Cl−, Br−, NO3

−,N3

−, SCN−) as a means to high-nuclearity species. Thepresence of a deprotonated oxime group leads to a greatcoordinative flexibility due to the well-known ability of theoximate group to bridge two or three metal ions. On theother hand, carboxylates are able to deprotonate the oxime

group of 2-pyridyloximes under mild conditions (the use ofexternal hydroxides often perplexes the reactions). Besidestheir deprotonating ability, the R′CO2

− ions are flexibleligands, a consequence of their ability to adopt a number ofdifferent ligation modes, both terminal and bridging as wellas both bidentate and tridentate. The additional inorganicmonoanions in the ternary “ligand blends” often behave asterminal ligands and help the formation of clusters (andnot coordination polymers). However, sometimes they actas bridging ligands, and this may eventually lead to clusterswith complicated structures; the formation of coordinationpolymers cannot be ruled out in such a case. Thus, avariety of Cr, Mn, Fe, Co, Ni, and Cu clusters [22–39] withnuclearities ranging from 3 to 12 have been characterizedfrom our [23–36] and other [21, 37–39] groups, some ofthem possessing interesting magnetic properties, includingsingle-molecule magnetism behaviour [33, 40].

Recently, we have begun a program which can beconsidered as a modification of the above-mentioned binary“ligand blend” approach. We have been exploring theuse of other inorganic ions, such as SO4

2−, instead ofthe carboxylato ligand, R′CO2

−, in the 3d-metal cluster

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2 International Journal of Inorganic Chemistry

N

C

R

N

OH

R = H; (py)C(H)NOH

R = Me; (py)C(Me)NOH

R = Ph; (py)C(Ph)NOH

R =

N

; (py)2CNOH

Scheme 1: General structural formula and abbreviations of simple2-pyridyl oximes.

chemistry with 2-pyridyloximate ligands. The sulfate ion[41] is a ligand with great coordinative flexibility (μ2, μ3,μ4, μ5, μ6, μ8, or μ10 potential), see Scheme 2. Metal-sulfatocomplexes have been studied for their roles in the field ofporous framework materials [42, 43], in catalysis [44], in theconstruction of luminescent molecular materials [45, 46],and in medicinal [47], environmental [48], and bioinorganic[49] chemistry. The possible advantages of using SO4

2−

instead of R′CO2− include (i) the possibility of triggering

aggregation of preformed smaller cationic species into new,higher-nuclearity products and (ii) the possible diversionof known reaction systems developed using monoanioniccarboxylates to new species as a result of the higher chargeand higher denticity/bridging capability of sulfates. Thus, theinitial employment of the sulfate ion in NiII/(py)C(R)NOH(R = Me, Ph, NH2) chemistry has led to the isolation andcharacterization of high-nuclearity NiII compounds, such asNi12 [50] and Ni6 [51, 52] clusters which possess interestingstructural properties.

In this work, we expand our efforts to a differ-ent member of 2-pyridyl oximes which is di-2-pyridylketone oxime, (py)2CNOH, and report the synthesisand characterization of the new tetranuclear compound[Ni4{(py)2CNO}4(SO4)2(MeOH)4]. The structure of thecompound has been determined by single-crystal X-raydiffraction. The IR data are discussed in terms of the natureof bonding and the structure of the complex.

2. Experimental

2.1. General and Physical Measurements. All manipulationswere performed under aerobic conditions using materials(reagent grade) and solvents as received.

Microanalyses (C, H, N) were performed by the Univer-sity of Ioannina (Greece) Microanalytical Laboratory usingan EA 1108 Carlo Erba analyzer. IR spectra (4000-400 cm−1)were recorded on a Perkin-Elmer 16 PC FT-spectrometerwith samples prepared as KBr pellets.

2.2. Compound Preparation

2.2.1. [Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1). NEt3 (0.139ml, 1.00 mmol) was added to a colourless solution of(py)2CNOH (0.199 g, 1.00 mmol) in MeOH (25 ml). Subse-quently, solid NiSO4·6H2O (0.263 g, 1.00 mmol) was added,and the resulting red solution was stirred for 1 h at roomtemperature. A small quantity of undissolved material was

Table 1: Summary of crystal data, data collection, and structurerefinement for the X-ray diffraction study of complex 1.

Complex 1

Empirical formula Ni4C48H48N24O16S2

Formula weight 1515.97

Colour, habit orange, rod

Crystal system orthorhombic

Space group Pccn

a (A) 14.2969 (10)

b (A) 20.9678 (14)

c (A) 18.5395 (13)

V (A3) 5557.7 (7)

Z 4

ρcalc (g cm−3) 1.609

Radiation, λ (A) 0.71073

μ (mm−1) 1.488

F (000) 2760

Temperature (K) 100 (2)

2θmax [◦] 61

Ranges h −18 → 18

k −21 → 27

l −27 → 23

Measured reflections 42848

Unique reflections 6377

Reflections used 3703

(I > 2σ(I))

Parameters refined 374

GoF (on F2) 0.903

R1a (I > 2σ(I)) 0.0464

w R2b (I > 2σ(I)) 0.1042

(Δρ)max/(Δρ)min (e A−3) 0.861/−0.621aR1 =

∑(|Fo| − |Fc|)/

∑(|Fo|).

bwR2 = {∑

[w(Fo2 − Fc2)

2]/∑

[w(Fo2)2]}1/2

.

removed by filtration and the dark red filtrate layeredwith Et2O (50 ml). Slow mixing gave X-ray quality, orangecrystals which were collected by filtration, washed withEt2O (2 × 3 ml), and dried in air; yield 57%. The driedsolid was analyzed satisfactorily as 1·MeOH. Anal. Calc. forC49H52Ni4N24O17S2: C, 38.02; H, 3.99; N, 21.72. Found: C,38.45; H, 3.87; N, 21.37%. IR (KBr pellet): v = 3388 sb,2902 w, 1654 w, 1598 m, 1460 m, 1430 m, 1376 w, 1340 w,1282 w, 1219 m, 1130 m, 1118 s, 1086 s, 1045 s, 1020 m,982 m, 896 w, 788 w, 748 m, 702 m, 670 m, 641 m, 618 s,591 w, 452 w cm−1.

2.3. Single-Crystal X-Ray Crystallography. A crystal of 1 withappropriate dimensions 0.08× 0.03× 0.01 mm was attachedto a glass fiber using silicone grease. Data were collected onan Oxford Diffraction Xcalibur-3 diffractometer, equippedwith a Sapphire CCD area detector, at 100 K, using a graphitemonochromated Mo Kα radiation. Complete crystal dataand parameters for data collection and processing are listedin Table 1.

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International Journal of Inorganic Chemistry 3

S

O

O

O

OM

M M

M

M

MM

M

S

O

O

O

OM

M MM

M

M M

MM

M

8.2222

5

3.1110

S

O

O

O

O

MS

O

O

OM

OS

O

O

O

O

M

M

SO

O

O

O

M

M

SO

O

O

O

M

M

M

M

O

SO O

OM

MM

M

M

O

SO

O

O

M M

O

S

OO

OM

MM M

M

M

1.1000 2.2000

4.1111

5.2210

2.2110

6.2211

O

SO O

OM

M M

3.2100

O

SO

O

OM

M

M

3.2111

O

SO O

O

M

M

M

M

M

5.2211

SO

O

O

O

M

M

M

M

4.2210

O

SO

O

O

M M

M

S

O

O

O

OM

M

2.1110

O

S

O O

OM

M MM

4.2200

S

O

O

O

M

O

M

2.1111

2.11001.1100

10.3322

Scheme 2: The up to now crystallographically established coordination modes of the sulfato ligand and the Harris notation [53] whichdescribes these modes.

The structure was solved by direct methods using SIR92[54] and refined by full-matrix least-squares techniques onF2 with SHELXL-97 [55]. Some residual electron densityin the accessible voids of the structure was too disorderedto refine as solvent molecules; therefore, the SQUEEZEprocedure [56] of PLATON was employed to remove the

contribution of the electron density in the solvent regionfrom the intensity data. The solvent-free model and intensitydata were used for the final results reported here. The non-H atoms were treated anisotropically. The H atoms of the(py)2CNOH ligands and the methyl groups of the methanolmolecules were placed in calculated, ideal positions and

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4 International Journal of Inorganic Chemistry

N4

N5

N6

N3

O1

O2Ni2O3

O4

O7

N2

N1 O8

Ni1

N6′

O2′

N5′ O8′

O1′Ni1′O4′

O3′

N3′

N2′

Ni2′

O7′

N4′

N1′

Figure 1: The molecular structure of 1. H atoms have been omittedfor clarity.

refined as riding on their respective C atoms. The H atomof the OH group of one independent methanol molecule(O(8)H) was located in difference Fourier maps and wasrefined isotropically, but the H atom of the OH group of thesecond independent methanol molecule (O(7)H) could notbe located. The programs used were CRYSALIS CCD [57]for data collection, CRYSALIS RED [57] for cell and datarefinement, WINGX [58] for crystallographic calculations,and MERCURY [59] and DIAMOND [60] for moleculargraphics.

3. Results and Discussion

3.1. Synthetic Comments. Our general synthetic approachfor the isolation of NiII/2-pyridyloximate/sulfato clusters hasbeen to treat the metal sulfate “salt” with the appropriateligand and a base in a variety of solvents. The addition ofbase is necessary for the deprotonation of the oxime ligand.

Treatment of NiSO4·6H2O with one equivalent of(py)2CNOH and one equivalent of NEt3 in MeOH gave ared solution which, upon crystallization, gave orange crystalsof the new tetranuclear cluster which can be written as[Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1). Its formation can besummarized in (1)

4NiSO4 · 6H2O + 4(py)

2CNOH + 4NEt3 + 4MeOH

MeOH−−−−→[

Ni4

{(py)

2CNO}

4(SO4)2(MeOH)4

]

+ 2(HNEt3)2SO4 + 24H2O.

(1)

As expected, the nature of the base is not crucial for theidentity of the product, and it affects only its crystallinity,and in some cases its purity; we were able to isolate 1 byusing a plethora of different bases such as NaOMe, NMe4OH,

N5´

O2

Figure 2: Top: the [Ni4(μ-SO4)2(μ2-ONR)2(μ3-ONR)2] core ofcomplex 1, where R- = -C(py)2. The metallacrown-type ring ishighlighted. Bottom: the core of 1 using only the μ3 oximate groups.

NEt4OH, and LiOH·H2O. Small changes in the molar ratioof the reactants, the crystallization method, and the presenceof counterions do not seem to affect the identity of theisolated product.

3.2. Description of Structure. Partially labeled plots ofthe complete structure and the core of the molecule[Ni4{(py)2CNO}4(SO4)2(MeOH)4] that is present in com-plex 1 are shown in Figures 1 and 2, respectively. Selectedinteratomic distances and angles are listed in Table 2.

The structure of 1 consists of tetranuclear molecules[Ni4{(py)2CNO}4(SO4)2(MeOH)4] which lie on a crystal-lographic inversion center. The metal ions are held togetherby two 3.2111 and two 2.1110 (using Harris notation, [53],Scheme 3) (py)2CNO− ligands, as well as two 2.1100 SO4

2−

ions. Four MeOH molecules act as terminal ligands andcomplete the coordination sphere of the four metal centers.The molecule has a metallacrown-type topology [61]. Apseudo 12-MC-4 ring forms; the true 12-MC-4 topology is“destroyed” by the bridging character of the oximate oxygenatoms O2 and O2′.

A distorted octahedral environment is created about eachmetal center; the chromophores are represented by the fol-lowing formulas Ni(1,1′)(Npy)(Nox)(Oox)2(Osulf)(Omet) andNi(2,2′)(Npy)2(Nox)(Oox)(Osulf)(Omet), where the abbrevia-tions “py”, “ox”, “sulf,” and “met” are for the 2-pyridyl,oximate, sulfato, and methanolic donor atoms, respectively.The average Ni–Oox, Ni–Nox, and Ni–Npy bond lengthsof 2.054(3), 2.043(3), and 2.071(3) A, respectively, agreewell with the values expected for high-spin NiII ions inoctahedral environment [34, 35, 62–64]. The Ni–Osulf bondlengths are typical [41, 51, 62, 65]. The fact that S-(O3, O4)(average 1.474 A) > S-(O5, O6) (average 1.450 A) reflects thecoordinating nature of O3 and O4 and the noncoordinating

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International Journal of Inorganic Chemistry 5

Table 2: Selected interatomic distances (A) and angles (◦) for 1.

Ni1-O1 2.046 (3) Ni2-O2 2.071 (3)

Ni1-O2 2.046 (3) Ni2-O3 2.042 (3)

Ni1-O4 2.033 (3) Ni2-O7 2.163 (3)

Ni1-O8 2.048 (3) Ni2-N1 2.090 (3)

Ni1-N5 2.090 (3) Ni2-N2 1.997 (4)

Ni1-N6 2.074 (3) Ni2-N4 2.053 (3)

O1-Ni1-O2 89.59 (11) O2-Ni2-O3 94.82 (11)

O1-Ni1-O4 101.65 (11) O2-Ni2-O7 88.69 (10)

O1-Ni1-O8 173.53 (11) O2-Ni2-N1 169.13 (12)

O1-Ni1-N5 82.89 (11) O2-Ni2-N2 90.44 (12)

O1-Ni1-N6 83.38 (12) O2-Ni2-N4 84.07 (12)

O2-Ni1-O4 84.24 (11) O3-Ni2-O7 176.29 (10)

O2-Ni1-O8 88.62 (11) O3-Ni2-N1 88.91 (12)

O2-Ni1-N5 107.12 (11) O3-Ni2-N2 95.11 (12)

O2-Ni1-N6 170.67 (12) O3-Ni2-N4 87.08 (12)

O4-Ni1-O8 84.36 (12) O7-Ni2-N1 87.86 (11)

O4-Ni1-N5 167.92 (13) O7-Ni2-N2 86.05 (12)

O4-Ni1-N6 91.16 (12) O7-Ni2-N4 92.09 (12)

O8-Ni1-N5 91.70 (12) N1-Ni2-N2 79.04 (13)

O8-Ni1-N6 99.02 (12) N1-Ni2-N4 106.36 (13)

N5-Ni1-N6 78.16 (13) N2-Ni2-N4 174.25 (13)

NC

NN

O Ni

Ni

Ni

NC

NN

O

Ni

Ni

3.2111 2.1110

Scheme 3: The two different coordination modes of the (py)2CNO−

ligands present in complex 1 and the Harris notation [53] whichdescribes them.

character of O5 and O6; as expected, the sulfur to “free”oxygen bond lengths are the shortest.

The crystal structure of 1 is stabilized by strong inter-and intramolecular hydrogen bonds. The intramolecularhydrogen bonds involve the O atom (O7 and its symmetryequivalent) belonging to a methanol ligand as donor andthe O atom (O1 and its symmetry equivalent) of thedoubly bridging organic ligand as acceptor [O1 · · · O7 =2.721(3) A]. The O atom of the remaining methanol ligand(O8 and its symmetry equivalent) is participating as donorin an intermolecular hydrogen bond with the acceptor beingthe pyridyl N atom (N3 and its symmetry equivalent) of thedoubly bridging organic ligand [O8 · · · N3 = 2.851(3) A,H(O8) · · · N3 = 1.987(3) A, and O8-H(O8)-N3 = 172.1(1)].This hydrogen bonding leads to the formation of a 1D chainalong the c axis (Figure 3).

Figure 3: Representation of a part of the 1D chain formed in 1 alongthe c axis. The dotted lines represent H bonds.

The molecule of 1 contains the [Ni4(μ-SO4)2(μ2-ONR)2(μ3-ONR)2] core, where R- = -C(py)2 (Figure 2, top).An alternative description of the core (using only the μ3

oximate groups) is [Ni4(μ3-ONR)2]6+ (Figure 2, bottom).The topology of the four NiII ions can be also describedas “saddle-like,” and it is observed for the first time in Ni4

clusters. The most common topologies of the metal ions inNiII

4 complexes are the cubanes [66–73] and the face-shareddistorted dicubanes in which one of the corners of eachcubane is missing [74–79], while there are few Ni4 clustersin which the metal ions adopt less common topologies suchas linear [80–82], rectangular [83–87], and chair-like [88, 89]as follows.

Complex 1 joins a small but growing family of struc-turally characterized Ni(II) complexes containing the neutralor anionic forms of di-2-pyridyl ketone oxime as ligands[34, 36, 88–92]. The special features of 1 compared to theother members of this family are (1) It is the first exampleof these species containing the sulfato ligand, and (2) it has aunique Ni4 clusters “saddle-like” metal topology.

3.3. IR Spectra. The medium intensity bands at 1568 and1094 cm−1 in the spectrum of the free ligand (py)2CNOH areassigned to v(C=N)oxime and v(N-O)oxime modes, respectively[51, 52, 93]. The 1094 cm−1 band is shifted to a higherwavenumber (1118 cm−1) in 1. This shift is in accordwith the concept that upon deprotonation and oximate-O coordination, there is a higher contribution of N=O tothe electronic structure of the oximate group; consequently,the v(N-O) vibration shifts to a higher wavenumber inthe complex relative to (py)2CNOH [36]. Somewhat toour surprise, the 1568 cm−1 band is shifted to a higherwavenumber in the complex (1598 cm−1), overlapping withan aromatic stretch. This shift may be indicative of the oximenitrogen coordination [94]. Extensive studies on Schiff basecomplexes (which also contain a C=N bond) have shown[95] that a change in the s character of the nitrogen lonepair occurs upon coordination such that the s character ofnitrogen orbital involved in the C=N bond increases; thischange in hybridization produces a greater C=N stretchingforce constant relative to the free neutral ligand.

The in-plane deformation band of the 2-pyridyl ringof free (py)2CNOH at 622 cm−1 shifts upwards (641 cm−1),confirming the involvement of the ring-N atom in coordina-tion [96]. The presence of the 618 cm−1 bond in the spectrumof 1 indicates that some 2-pyridyl rings are “free,” that is,

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6 International Journal of Inorganic Chemistry

uncoordinated, in accordance with the 2.1110 (py)2CNO−

ligands that are present in the complex.

The IR spectrum of the free, that is, ionic, sulfate (theion belongs to the Td point group) consists of two bandsat ∼1105 and ∼615 cm−1, assigned to the v3 (F2) stretching[νd(SO)] and v4(F2) bending [δd(OSO)] modes, respectively[41, 97]. The v1(A1) stretching [νs(SO)] and v2(E) bending[δd(OSO)] fundamentals are not IR active. The coordinationof SO4

2− to metal ions decreases the symmetry of the group,and the v3 and v4 modes are split [41, 97]. In the casewhen the SO4

2−-site symmetry is lowered from Td to C2v

(bidentate chelating or bridging coordination), which is thecase in 1, both v1 and v2 appear in the IR spectrum, whilev3 and v4 each splits into three IR-active vibrations [97].Thus, the bands at 1219, 1130, and 1020 cm−1 are attributedto the v3 modes [97], while the bands at 591, 618, and670 cm−1 are assigned to the v4 modes [13, 74–79] withthe intermediate wavenumber band being superimposed bya ligand’s vibration. The band at 982 cm−1 and the weakfeature at 452 cm−1 can be assigned [41, 97] to the v1 andv2 modes, respectively. These spectral features agree with thelow C2v symmetry for the sulfato ligand in the complex, asalso confirmed crystallographically.

4. Conclusions

The present work extends the body of results that emphasizethe ability of the sulfate ion to create unique structuraltypes in 3d-metal cluster chemistry. The study of thecoordination chemistry of the binary SO4

2−/(py)2CNOHligand system in the presence of base in MeOH hasprovided access to the novel tetranuclear Ni(II) cluster[Ni4{(py)2CNO}4(SO4)2(MeOH)4] (1). Complex 1 containsthe [Ni4(μ-SO4)2(μ2-ONR)2(μ3-ONR)2] core, where R- = -C(py)2, with a unique saddle-like topology of the NiII ions; itis thus a valuable addition to the family of tetranuclear NiII

clusters.

Analogues of 1 with 2-pyridinealdoxime [(py)C(H)NOH],methyl(2-pyridyl)ketone oxime [(py)C(Me)NOH], orphenyl(2-pyridyl)ketone oxime [(py)C(ph)NOH] (Scheme1) are not known, until to date, and further research effortsare in progress to determine the appropriate reactionconditions that could possibly favor such species. It islikely that the preparation and stability of such tetranuclearcomplexes are dependent on the particular nature of the Rsubstituent on the oximate carbon. We are currently workingon the chemistry of the NiSO4·6H2O/(py)C(R)NOH (R=H,Me, Ph) reaction systems.

Supporting Information

CCDC 802606 contains the supplementary crystallographicdata for 1. These data can be obtained free of charge viahttp://www.ccdc.cam.ac.uk/conts/retrieving.html or fromthe Cambridge Crystallographic Data Centre, 12 UnionRoad, Cambridge CB2 1EZ, UK; fax: (+44)1223-336033 ore-mail: [email protected].

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[74] M. A. Halcrow and G. Christou, “Biomimetic chemistry ofnickel,” Chemical Reviews, vol. 94, no. 8, pp. 2421–2481, 1994.

[75] A. J. Edwards, B. F. Hoskins, E. H. Kachab, A. Markiewicz, K.S. Murray, and R. Robso, “Synthesis, x-ray crystal structures,and magnetic properties of Ni complexes of a macrocyclictetranucleating ligand,” Inorganic Chemistry, vol. 31, no. 17,pp. 3585–3591, 1992.

[76] E. Jabri, M. B. Carr, R. P. Hausinger, and P. A. Karplus, “Thecrystal structure of urease from Klebsiella aerogenes,” Science,vol. 268, no. 5213, pp. 998–1004, 1995.

[77] A. Escuer, R. Vicente, S. B. Kumar, and F. A. Mautner, “Spinfrustration in the butterfly-like tetrameric [Ni(μ-CO)(aetpy)]-[ClO ] [aetpy = (2-aminoethyl)pyridine] complex. Structureand magnetic properties,” Journal of the Chemical Society—Dalton Transactions, no. 20, pp. 3473–3477, 1998.

[78] J. M. Clemente, H. Andres, J. J. Borras-Almenar et al.,“Magnetic excitations in polyoxometalate clusters observedby inelastic neutron scattering: evidence for ferromagneticexchange interactions and spin anisotropy in the tetramericnickel(II) cluster [Ni(HO)(PWO)] and comparison withthe magnetic properties,” Journal of the American ChemicalSociety, vol. 121, no. 43, pp. 10021–10027, 1999.

[79] Z. E. Serna, L. Lezama, M. K. Urtiaga et al., “A dicubane-liketetrameric nickel(II) azido complex,” Angewandte Chemie—International Edition, vol. 39, no. 2, pp. 344–347, 2000.

[80] P. Venkateswara Rao, S. Bhaduri, J. Jiang, and R. H. Holm,“Sulfur bridging interactions of cis-planar Ni-S N coor-dination units with nickel(II), copper(I,II), zinc(II), andmercury(II): a library of bridging modes, including Ni (μ-SR)M rhombs,” Inorganic Chemistry, vol. 43, no. 19, pp. 5833–5849, 2004.

[81] K. T. Szacilowski, P. Xie, A. Y. S. Malkhasian et al., “Solid-statestructures and magnetic properties of halide-bridged, face-to-face bis-nickel(II)-macrocyclic ligand complexes: ligand-mediated interchanges of electronic configuration,” InorganicChemistry, vol. 44, no. 17, pp. 6019–6033, 2005.

[82] X. Lopez, M. Y. Huang, G. C. Huang et al., “Even-numbered metal chain complexes: synthesis, characterization,and DFT analysis of [Ni(μ-Tsdpda)(H O)] (Tsdpda = N-(p-toluenesulfonyl) dipyridyldiamido), [Ni(μ-Tsdpda)] , andrelated Ni string complexes,” Inorganic Chemistry, vol. 45, no.22, pp. 9075–9084, 2006.

[83] E. Carmona, E. Gutierrez-Puebla, A. Monge, M. Paneque,and M. L. Poveda, “Unusual alkylidene-bridged complexesof nickel by α-H abstraction from a nickelacycle. Crystaland molecular structure of [Ni (CHCMe-o-CH)Cl(PMe )],”Journal of the Chemical Society, Chemical Communications, no.3, pp. 148–150, 1991.

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[85] F. Meyer, M. Konrad, and E. Kaifer, “Novel μ-coordination ofurea at a nickel(II) site: structure, reactivity and ferromagneticsuperexchange,” European Journal of Inorganic Chemistry, no.11, pp. 1851–1854, 1999.

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Hindawi Publishing CorporationInternational Journal of Inorganic ChemistryVolume 2011, Article ID 395418, 4 pagesdoi:10.1155/2011/395418

Research Article

Improvement of Aminopeptidase Activity of Dizinc(II)Complexes by Increasing Substrate Accessibility

Md. Jamil Hossain, Akinobu Wada, Yasuhiro Igarashi, Kei-ichiro Aimono, Keisuke Suzuki,Katsuya Tone, and Hiroshi Sakiyama

Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Kojirakawa, Yamagata 990-8560, Japan

Correspondence should be addressed to Hiroshi Sakiyama, [email protected]

Received 22 November 2010; Accepted 2 March 2011

Academic Editor: Rabindranath Mukherjee

Copyright © 2011 Md. Jamil Hossain et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

A new dizinc(II) complex, [Zn2(bhmp)(MeCO2)2]BPh4 [(bhmp)−: 2,6-bis[bis(2-hydroxyethyl)aminomethyl]-4-methylphenolateanion], performs aminopeptidase activity to hydrolyze L-leucine-p-nitroanilide. As compared with a related dizinc(II) complex[Zn2(bomp)(MeCO2)2]BPh4 [(bomp)−: 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenolate anion], the activity of thepresent bhmp complex was about 80 times greater than that of the bomp complex. This is mainly because the substrate accessibilitywas improved by changing the terminal methoxy groups to hydroxyl groups.

1. Introduction

Aminopeptidases are exopeptidases that remove the N-terminal amino acid from a protein [1–5]. It is interestingthat most of the well-characterized aminopeptidases containdinuclear metal cores at their active sites, while carboxypepti-dases, which remove the C-terminal amino acid, do not havedinuclear cores. The well-characterized aminopeptidases areleucine aminopeptidase (LAP, EC 3.4.11.1) [1], methionineaminopeptidase (MAP, EC 3.4.11.18) [2], aminopeptidasefrom Aeromonas proteolytica (AAP, EC 3.4.11.10) [3], Strep-tomyces griseus aminopeptidase (SGAP, EC 3.4.11.-) [4],and proline aminopeptidase (PAP, EC 3.4.11.9) [5]. LAP,AAP, and SGAP contain dizinc(II) cores at their active sites,while MAP contains a dicobalt(II) core and PAP contains adimanganese(II) core.

With the intention of finding a minimum functional unitof aminopeptidase, Sakiyama and coworkers developed adizinc(II) complex which can be represented by the following[Zn(bomp)(MeCO2)2]BPh4 as the first functional model ofaminopeptidase [6] represented by [(bomp)−: 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenolate anion];later, the aminopeptidase activity was improved by the intro-duction of stronger electron-withdrawing p-substituents [7](the aminopeptidase activity was improved 10 times for the

chloro-substituted complex represented by the following[Zn2(bocp)(MeCO2)2]BPh4 and was improved 250 timesfor the nitro-substituted complex represented by thefollowing [Zn2(bonp)(MeCO2)2]BPh4 [(bocp)−: 4-chloro-2,6-bis[bis(2-methoxyethyl)aminomethyl]phenolate anion;(bonp)−: 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-ni-trophenolate anion]). From the kinetic studies, the substratewas proved to be incorporated within the dizinc center [7].

On the other hand, we found that the substrate acces-sibility of the bomp complexes was not good because ofthe steric hindrance of the terminal methoxy groups [7].Therefore, in the present study, a new dizinc(II) complex,[Zn2(bhmp)(MeCO2)2]BPh4 (1), has been synthesized usinga dinucleating ligand, bhmp− [(bhmp)−: 2,6-bis[bis(2-hydroxyethyl)aminomethyl]-4-methylphenolate anion] [8],in which the methoxy groups of the bomp ligand aresubstituted into less-hindered hydroxyl groups, and theaminopeptidase activity of the complex was examined (seeScheme 1).

2. Experimental

2.1. Measurements. Elemental analyses were obtained at theElemental Analysis Service Centre of Kyushu University.

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2 International Journal of Inorganic Chemistry

NN

O O

O

R R

RR

O−

O

CH3

R = CH3(bomp−)

=H(bhmp−)

Scheme 1

Infrared (IR) spectra were recorded on a Hitachi 270-50 spectrometer. Electronic spectra were recorded on aShimadzu UV-240 spectrophotometer.

2.2. Materials. Na(bhmp) was prepared as previouslydescribed in [8]. All other chemicals were commercialproducts and were used as supplied.

2.3. Synthesis of [Zn2(bhmp)(MeCO2)2]BPh4·2MeCN (1).To a methanolic solution (15 mL) of Na(bhmp) (0.19 g,0.52 mmoL) was added zinc(II) acetate dihydrate (0.22 g,1.00 mmoL), and the resulting solution was refluxed for1 hour. The addition of sodium tetraphenylborate (0.17 g,0.50 mmoL) resulted in the precipitation of colorless micro-crystals, which were recrystallized from acetonitrile. Yield0.26 g (52%). (Found: C, 59.15; H, 6.20; N, 5.65; Zn, 14.1.Calc. for C49H61BN4O9Zn2: C, 59.35; H, 6.20; N, 5.65; Zn,13.2%). Selected IR data [ν/cm−1] using KBr disk: 3410,3240, 3050–2850, 1605, 1580, 1475, 1420, 1330, 1260, 1130,1030, 870, 730, 700, 605.

2.4. Aminopeptidase Activity. The aminopeptidase activity ofthe complex was estimated using l-leucine-p-nitroanilide asa substrate [6]. The substrate was dissolved in 1.5 mL of atricine buffer solution (pH 8), and to this was added 1.0 mLof a DMF solution of the complex at room temperature. Thehydrolysis of the substrate into l-leucine and p-nitroanilinewas monitored by detecting the formation of p-nitroanilineusing a spectrometer at 405 nm. In the measurement,spontaneous hydrolysis of the substrate was subtracted asa background. This measurement was examined at variouscomplex concentrations from 0 to 5.0 × 10−4 mol dm−3 andat various substrate concentrations from 0 to 5.9 × 10−4 moldm−3. This procedure was also carried out at pH values

3

2

1

00 2 4 6

[Complex]/10−4 (mol·dm−3)

(v/[

subs

trat

e])/

10−6

(s−1

)

Figure 1: A v/[substrate] versus [complex] plot in the hydrolysis ofl-leucine-p-nitroanilide by 1 at nominal pH 8.

varying from 7 to 10 using HEPES (pH 7), tricine (pH 8),and CHES (pH 9-10) buffers.

3. Results and Discussion

3.1. Aminopeptidase Activity at pH 8. Before the discussionabout the aminopeptidase activity, it should be noted thatsimple zinc salts, such as zinc(II) chloride and zinc(II)sulfate, do not show aminopeptidase activity [6]. First, theaminopeptidase activity of complex 1 was estimated in amixture of 40% DMF and 60% aqueous solution at pH 8using l-leucine-p-nitroanilide as a substrate. Hereafter, theterm “nominal pH” will be used because the experimentswere carried out in a mixture of DMF and water. The mea-surement was carried out at various complex concentrationsand at various substrate concentrations, and the initial ratev was obtained for each experiment. A plot of the initialrate over the substrate concentration v/[substrate] versusthe complex concentration [complex] showed good linearity(Figure 1), as did the previous complexes [6, 7, 9], whichindicates that the initial rate can be written as a second-orderrate equation as follows:

v = k[substrate][complex

], (1)

where k is the second-order rate constant. The k valuefor the bhmp complex 1 was calculated as 4.4(2) ×10−3 dm3 mo1−1 s−1. The previously obtained k value for thebomp complex was 2.3(1) × 10−3 dm3 mo1−1 s−1 under thesame conditions [6]. Therefore, the rate for 1 was about twotimes greater than that for the bomp complex at nominal pH8.

3.2. Effect of pH on the Aminopeptidase Function. The aboveprocedure was also carried out at nominal pH’s varying from

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International Journal of Inorganic Chemistry 3

2.5

2

1.5

1

0.5

0

k

7 8 9 10 11

pH

Figure 2: k versus nominal pH plots for the bhmp complex 1 (O)and the previous bomp complex ( ). The curves are drawn by (2)using the parameters in the text.

7 to 10. At nominal pH 7, the activity was too small todetermine the rate constant, but in the nominal pH rangefrom 8 to 10, the second-order rate equation was foundto be valid. The k versus nominal pH plot for 1 is shownin Figure 2. The plot is sigmoidal around the nominal pH9.5, and the data could be fitted using (2) with parameterskdepro = 2.55 dm3 mo1−1 s−1 and pKa = 9.44, where kdepro isthe rate constant for the deprotonated form.

k = kdeproKa

Ka + [H+]. (2)

The result indicates that the reaction was promotedby the deprotonated form of the complex, which will bediscussed in Section 3.3. In the case of the previous bompcomplex, the data was reexamined using (2), and the param-eters were determined as kdepro = 3.26×10−2 dm3 mo1−1 s−1

and pKa = 9.07. The activity (kdepro) of the present bhmpcomplex 1 was about 80 times greater than that of the bompcomplex although the k value at nominal pH 8 was aboutonly 2 times greater. This is because the pKa value of 1 isslightly larger than that of the bomp complex.

3.3. Some Considerations about the Active Species. Accord-ing to the crystal structure of related cobalt(II) complex[Co2(bhmp)(MeCO2)2]BPh4 [8], the coordination geome-try around each zinc(II) ion is saturated, and the two zinc(II)ions are bridged by two acetate ions. However, the dissocia-tion of the acetate ions occurs rather easily in an aqueoussolution, affording vacant coordination sites for substrateincorporation. Indeed, in the cases of related cobalt(II)and nickel(II) complexes, [Co2(bhmp)(MeCO2)2]ClO4 and

[Ni2(bhmp)(MeCO2)2]ClO4, the dominant species in anaqueous solution were identified as [M2(bhmp)(H2O)4]2+

and [M2(bhmp)(MeCO2)(H2O)2]+ (M = CoII, NiII) byanalyzing the electronic spectra [10]. When one or two watermolecules of [Zn2(bhmp)(H2O)4]2+ are exchanged with thesubstrate and a remaining water molecule is deprotonated, anucleophilic Zn-OH moiety is thought to attack the carbonylcarbon of the bound substrate.

4. Conclusion

For the purpose of improving the substrate accessibility, anew dizinc(II) complex, [Zn2(bhmp)(MeCO2)2]BPh4 (1),was synthesized, and its aminopeptidase activity was inves-tigated. As compared with the previous dizinc(II) complex,[Zn2(bomp)(MeCO2)2]BPh4, the activity of 1 was about 80times greater. Routine kinetic results have been deposited assupplementary material at 10.1155/2011/395418.

References

[1] S. K. Burley, P. R. David, A. Taylor, and W. N. Lipscomb,“Molecular structure of leucine aminopeptidase at 2.7-Aresolution,” Proceedings of the National Academy of Sciences ofthe United States of America, vol. 87, no. 17, pp. 6878–6882,1990.

[2] S. L. Roderick and B. W. Matthews, “Structure of the cobalt-dependent methionine aminopeptidase from escherichia coli:a new type of proteolytic enzyme,” Biochemistry, vol. 32, no.15, pp. 3907–3912, 1993.

[3] B. Chevrier, C. Schalk, H. D’Orchymont, J. M. Rondeau,D. Moras, and C. Tarnus, “Crystal structure of Aeromonasproteolytica aminopeptidase: a prototypical member of theco-catalytic zinc enzyme family,” Structure, vol. 2, no. 4, pp.283–291, 1994.

[4] H. M. Greenblatt, O. Almog, B. Maras et al., “Streptomycesgriseus aminopeptidase: X-ray crystallographic structure at1.75 A resolution,” Journal of Molecular Biology, vol. 265, no.5, pp. 620–636, 1997.

[5] M. C. J. Wilce, C. S. Bond, N. E. Dixon et al., “Structureand mechanism of a proline-specific aminopeptidase fromEscherichia coli,” Proceedings of the National Academy ofSciences of the United States of America, vol. 95, no. 7, pp. 3472–3477, 1998.

[6] H. Sakiyama, R. Mochizuki, A. Sugawara, M. Sakamoto,Y. Nishida, and M. Yamasaki, “Dinuclear zinc(II) complexof a new acyclic phenol-based dinucleating ligand withfour methoxyethyl chelating arms: first dizinc model withaminopeptidase function,” Journal of the Chemical Society,Dalton Transactions, no. 6, pp. 997–1000, 1999.

[7] H. Sakiyama, Y. Igarashi, Y. Nakayama, M. J. Hossain,K. Unoura, and Y. Nishida, “Aminopeptidase function ofdinuclear zinc(II) complexes of phenol-based dinucleatingligands: effect of p-substituents,” Inorganica Chimica Acta, vol.351, pp. 256–260, 2003.

[8] M. J. Hossain, M. Yamasaki, M. Mikuriya, A. Kuribayashi, andH. Sakiyama, “Synthesis, structure, and magnetic propertiesof dinuclear cobalt(II) complexes with a new phenol-baseddinucleating ligand with four hydroxyethyl chelating arms,”Inorganic Chemistry, vol. 41, no. 15, pp. 4058–4062, 2002.

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4 International Journal of Inorganic Chemistry

[9] H. Sakiyama, K. Ono, T. Suzuki, K. Tone, T. Ueno, andY. Nishida, “Aminopeptidase function of dinuclear zinc(II)complexes with chiral dinucleating ligands: stereoselectivity bychiral substrate recognition,” Inorganic Chemistry Communi-cations, vol. 8, no. 4, pp. 372–374, 2005.

[10] A. Kazama, A. Wada, H. Sakiyama, M. J. Hossain, and Y.Nishida, “Synthesis of water-soluble dinuclear metal com-plexes [metal = cobalt(II) and nickel(II)] and their behaviorin solution,” Inorganica Chimica Acta, vol. 361, no. 9-10, pp.2918–2922, 2008.

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Hindawi Publishing CorporationInternational Journal of Inorganic ChemistryVolume 2011, Article ID 291567, 8 pagesdoi:10.1155/2011/291567

Research Article

Synthesis and Crystal Structure Differencesbetween Fully and Partially Fluorinatedβ-Diketonate Metal (Co2+, Ni2+, and Cu2+) Complexes

Akiko Hori1, 2 and Masaya Mizutani1

1 Department of Chemistry, School of Science, Kitasato University, 1-15-1 Kitasato, Minami-ku, Sagamihara,Kanagawa 252-0373, Japan

2 PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

Correspondence should be addressed to Akiko Hori, [email protected]

Received 28 December 2010; Accepted 28 February 2011

Academic Editor: Daniel L. Reger

Copyright © 2011 A. Hori and M. Mizutani. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Coordination complexes, [Co2(1)4(H2O)2] (2), [Ni2(1)4(H2O)2] (3), and [Cu(1)2] (4), by using an asymmetric and partiallyfluorinated 3-hydroxy-3-pentafluorophenyl-1-phenyl-2-propen-1-one (H1) have been prepared, and the structures wereinvestigated to compare with the corresponding fully fluorinated complexes of [Co2(5)4(H2O)2] (6), [Ni2(5)4(H2O)2] (7),and [Cu(5)2] (8) with bis(pentafluorobenzoyl)methane (H5) and to understand the fluorine-substituted effects. While thecoordination mode of the partially fluorinated complexes was quite similar to the fully fluorinated complexes, the intra- andintermolecular π-interactions of the ligand moieties were highly influenced by the fluorination effects; the arene-perfluoroareneinteractions were observed in complexes 2 and 3 as a reason of the dinucleation. In this paper, we describe detail structures of theprotonated form of the ligand, H1, and complexes 2–4 by X-ray crystallographic studies.

1. Introduction

While π-π interactions are observed in various aromaticcompounds in chemistry and biology [1–7], the control ofthe interaction is still challenging because of their repulsionof the electron charge of the aromatic moieties. For example,a benzene molecule has a negative quadrupole moment,−29.0 × 10−40 C m2 [8] stabilizing a rectangular orientationof CH· · ·π interaction [9] and a sliding orientation ofπ-π stacking. The negative charge of aromatic center alsointeracts the cationic source through cation· · ·π inter-actions [10–12]. On the other hand, the electron chargeof aromatic compounds can be controlled by fluorination(e.g., a hexafluorobenzene molecule which shows positivequadrupole moment, 31.7 × 10−40 C m2) [8] and uniqueelectrostatic interactions, such as arene-perfluoroarene [13–18] and anion-π [19–21] interactions, which have attractedinterests in a couple of decades. We have also synthesizedseveral fluorinated compounds to control the electrostatic

interactions and to design the metal· · ·metal [22] andmetal· · ·π [23] arrangements through the interactions [24–28]. In these studies, the fluorination into the aromaticmoieties on coordination complexes has potentially shownthree unique effects: (1) fluorinated π-planes show uniquemolecular recognition through the electrostatic interactions;(2) an electron-density of a metal is controlled by the fluori-nation; (3) fluorinated π-planes of the molecules are twistedwith the whole molecular planes by the steric hindranceof the fluorine substituents in several cases [23, 24]. Sucheffects give rise to several unique guest-recognitions of thecoordination complexes based on the crystal engineering.

In this paper, we show the crystal structures of par-tially fluorinated H1 (Scheme 1) and its three coordinationcomplexes, [Co2(1)4(H2O)2] (2), [Ni2(1)4(H2O)2] (3), and[Cu(1)2] (4) (Scheme 2). The corresponding coordinationcomplexes [M(DBM)2] (M = Co2+, Ni2+, Cu2+) with diben-zoylmethanide ligand (DBM, C6H5COCHCOC6H5

−) [29–34] and [Co2(5)4(H2O)2] (6), [Ni2(5)4(H2O)2] (7), and

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2 International Journal of Inorganic Chemistry

O F

F

F

F

F

DBM 1 5

OF

F

F

F

F

F

F

F

F

FO O− O−O−

Scheme 1

O

O O

OCu

F

F

F F

F

F

F

FF

F

OOF

F

FF

F

O

O

F

FF

F

FM

O O FF

FF

F

O

O

F

FF

F

F

M

M(OAc)2 · 4H2O H2O

OH2

Cu(OAc)2 ·H2O

NaOMe

H1

4

2: M = Co, 3: M = Ni

Scheme 2

[Cu(5)2] (8) with fully fluorinated ligand 5 [24] are alsodiscussed as a comparison.

2. Experimental

2.1. General. All chemicals were of reagent grade and usedwithout further purification. Fully fluorinated compounds,H5 and 6–8, were previously reported [24]. 1H NMR spectraldata were recorded on a Bruker DRX600 spectrometer. Themelting points were determined by a Yanako MP-500Dmelting point apparatus. Infrared and electronic absorptionspectra were recorded on a Shimadzu IR 8400s and JASCOV-660 spectrometer, respectively. The results of elementalanalysis of C and H were collected by PerkinElmer PE2400analyzer.

2.2. Synthesis and Physical Properties of H1 and 2–4

3-hydroxy-3-pentafluorophenyl-1-phenyl-2-propen-1-one (H1)[35]. Dry benzene solutions of acetophenone (47 mmol)and ethyl pentafluorobenzoate (50 mmol) were slowly addedinto NaH (120 mmol), and the mixture was refluxed for10 h. Then, the mixture was added into 10% HClaq. andextracted by AcOEt, dried by MgSO4, evaporated to remove

the solvent. After purified by column chromatography (silicagel, 30% AcOEt/hexane), white powder of H1 was obtainedin 10% yield. Colorless needle crystals of H1 were obtainedfrom an ethanol solution, suitable for X-ray crystallographicstudies. Yield 10%. mp 116◦C. EI-MS: m/z 314. 1H NMR(600 MHz, CDCl3) δ 15.89 (s, 1H), 7.94 (d, J = 7.2 Hz, 2H),7.60 (t, J = 7.2 Hz, 1H), 7.50 (t, J = 7.2 Hz, 2H), 6.50 (s, 1H).IR (KBr disk, cm−1): 1653, 1576, 1520, 1490, 1261, 1193, 997,988, 781. Elemental analysis: calcd. for C15H7F5O2 (%): C57.34, H 2.25; found: C 57.42, H 2.25.

[Co2(1)4(OH2)2] (2). A solution of H1 (0.25 g, 0.80 mmol)in 1 : 1 ethanol-CH2Cl2 solution (20 mL) was added to asolution of Co(OAc)2·4H2O (0.11 g, 0.44 mmol) in ethanol(10 mL). The mixture was stirred for 2 h at r.t. and was evap-orated to give reddish brown powder of 2. The powder wasextracted with CHCl3 and crystallized in CH2Cl2/benzeneto give complex 2·2H2O·3C6H6 suitable for X-ray crys-tallographic studies. The compound was dried by vacuumconditions for measurement of physical properties. Yield92%. mp 220◦C. UV-Vis {CH2Cl2, λnm (εM−1 cm−1)}:550 (100), 330 (76800), 254 (38200). IR (KBr disk, cm−1):3472(broad), 3328(broad), 3072, 1655, 1598, 1566, 1508,1486, 1417, 1321, 1261, 1064, 988, 716. Elemental analysis:

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C4

C5

C6C7

C8

C15

C10C11

C13

C12

C14

C9C2

C3

C1

O1 F1

F2

F4

F3

F5

O2

(a)

(b)

Figure 1: (a) ORTEP drawing of the crystal structure with 50%proba-bility thermal ellipsoids and (b) a part of packing structureof H1 at 100 K (color scheme: C: gray; F: purple; O: red).

calcd. for C66H34Co2F20O10 (2·C6H6): C 53.39, H 2.31;found: C 53.02, H 2.41.

[Ni2(1)4(OH2)2] (3). This was obtained as green powderby the same procedure as 2 with Ni(OAc)2·4H2O. Thesingle crystals of 3·2H2O·3C6H6 were obtained from aCH2Cl2/benzene solution suitable for X-ray crystallographicstudies. The compound was dried by vacuum conditionsfor measurement of physical properties. Yield 85%. mp260◦C. UV-Vis {CH2Cl2, λnm (εM−1 cm−1)}: 640 (20), 339(62400), 253 sh (35200). IR (KBr disk, cm−1): 3443(broad),3336(broad), 3071, 1654, 1599, 1568, 1509, 1488, 1450, 1433,1424, 1405, 1322, 1261, 1064, 988, 715. Elemental analysis:calcd. for C60H28Ni2F20O10 (3): C 51.25, H 2.01; found: C51.38, H 2.33.

[Cu(1)2] (4). This was obtained as bluish green crystals bythe same procedure as 2 with CuCl2·2H2O and NaOMe. Thesingle crystals of 4 were obtained from a CH2Cl2/benzenesolution suitable for X-ray crystallographic studies. Yield97%. mp 319◦C. IR (KBr disk, cm−1): 1655, 1591, 1559,1514, 1485, 1451, 1410, 1329, 1269, 986754, 710. Elementalanalysis: calcd. for C30H12CuF10O4: C 52.22, H 1.75; found:C 52.17, H 1.71.

2.3. Crystal Structure Determination. Single crystal X-raystructures were determined on a Bruker SMART APEX CCDdiffractometer with graphite monochromated MoKα (λ =0.71073 A) generated at 50 kV and 35 mA. All crystals werecoated by paraton-N and were measured at 100 K.

Crystal data for H1: C15H7F5O2, Mw = 314.21, mon-oclinic, P21/c, a = 7.1675(10) A, b = 7.1980(10) A, c =

24.239(3) A, β = 93.086(2)◦, V = 1248.7(3) A3, Z = 4,ρcalcd = 1.671 g cm−3, GOF = 1.022, R((I) > 2σ(I)) =0.0354, wR(Fo2) = 0.1027, CCDC 805820; 2·2H2O·3C6H6:C78H50Co2F20O12, Mw = 1677.04, triclinic, P-1, a =9.7381(5) A, b = 13.7690(7) A, c = 14.5108(7) A, α =114.060(1)◦, β = 93.870(1)◦, γ = 96.571(1)◦, V =1750.92(15) A3, Z = 1, ρcalcd = 1.590 g cm−3, GOF = 1.097,R((I) > 2σ(I)) = 0.0285, wR(Fo

2) = 0.0852, CCDC805821; 3·2H2O·3C6H6: C78H50F20Ni2O12, Mw = 1676.60,triclinic, P-1, a = 9.7085(9) A, b = 13.7652(13) A, c =14.5171(13) A, α = 114.385(1)◦, β = 93.688(1)◦, γ =96.484(1)◦, V = 1742.4(3) A3, Z = 1, ρcalcd =1.598 g cm−3, GOF = 1.052, R((I) > 2σ(I)) = 0.0296,wR(Fo2) = 0.0773, CCDC 805822; 4: C30H12CuF10O4,Mw = 689.94, monoclinic, P21/c, a = 11.958(5) A, b =6.273(3) A, c = 17.310(8) A, β = 107.549(5)◦, V =1237.9(10) A3, Z = 2, ρcalcd = 1.851 g cm−3, GOF =0.998, R((I) > 2σ(I)) = 0.0387, wR(Fo

2) = 0.0939,CCDC 805823. These data can be obtained free of chargefrom The Cambridge Crystallographic Data Center viahttp://www.ccdc.cam.ac.uk/data request/cif/.

3. Results and Discussion

3.1. Preparation and Structure of H1. The protonated formof the ligand, H1, was prepared by previously reportedprocedure [35] and characterized by 1H NMR, EI-MS, andelemental analysis. Colorless needle crystals of H1 wereobtained from an ethanol solution, suitable for X-ray crystal-lography. The ORTEP drawing and a part of packing struc-tures of crystal H1 are shown in Figure 1. The OH proton waslocated into the side of the pentafluorophenyl group, basedon the difference Fourier density map and refined as ridingon its idealized position, O2-H = 0.84 A. The bond distancesof the O1-C7 and O2-C9 are 1.2841(18) and 1.2918(19) A,respectively. The r.m.s deviation of the β-diketonato plane,O1-C7-C8-C9-O2, is 0.008 A. The pentafluorophenyl groupis more twisted to the plane of O1-C7-C8-C9-O2 than thephenyl group; the dihedral angle between O1-C7-C8-C9-O2 and the pentafluorophenyl group of C10-C11-C12-C13-C14-C15 is 41.42(4)◦ and that between O1-C7-C8-C9-O2and the phenyl group of C1-C2-C3-C4-C5-C6 is 16.23(6)◦.Interestingly, the H1 shows head-to-tail stacking througharene-perfluoroarene interactions to give an alternate layeredstructure; the closest intermolecular distance and the cor-responding perpendicular distance of Cg〈Ph〉 · · ·Cg〈C6F5〉i

(i: −x + 1, y − 0.5, −z + 0.5) are 3.6838(10) and 3.4529(6) A,respectively, where Cg〈Ph〉 is the centroid of the phenylring and Cg〈C6F5〉 is the centroid of the pentafluorophenylring. The carbonyl moiety also closely interacts to pentaflu-orophenyl group, and the intermolecular distance of C7-O1· · ·Cg〈C6F5〉ii (ii:−x+1, y+0.5,−z+0.5) is 3.2981(14) A.This π-stacked structure including the interaction of phenyland pentafluorophenyl groups prompted us to investigate ofthe synthesis and crystallographic studies for its coordinationcomplexes.

3.2. Preparation and Structures of Co2+ and Ni2+ Complexes.Co2+ and Ni2+ complexes with the partially fluorinated

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C37

C38C28

C27

C29

C30

C25

C24

C26 C23

C22

C19

C18

C17

C16

C11

C12

C13 C14

C15

C5C10

C9C8

C4

C7 C6C3

C2

C1

C31

C32

C33

C34C36

C35

CoO4

O6O5

O1

O3

O2C20

C21

F8

F9

F10

F7

F6

F1

F5

F4F3

F2

C39

Figure 2: Numbering scheme of an asymmetric unit of the crystal structure of 2·2H2O·3C6H6 at 100 K. ORTEP drawing is shown with50% probability thermal ellipsoids.

ligand 1 were obtained as dinuclear complexes (Scheme 2),which are the same tendency of the fully fluorinatedcomplexes of 6 and 7 [24]. Typically, M(OAc)2·4H2O (M= Co, Ni) and H1 were combined in an ethanol/CH2Cl2solution to give [Co2(1)4(OH2)2] 2 and [Ni2(1)4(OH2)2] 3.These complexes were crystallized from CH2Cl2 with vapor-phase diffusion of benzene to give red block crystals of2·2H2O·3C6H6 and green block crystals of 3·2H2O·3C6H6.Fundamentally, the reaction of DBM and Co2+/Ni2+ ionsformed mononuclear complexes, [M(DBM)2(X)2] (M =Co2+ or Ni2+, X = solvent or water), and the DBM ligandswere sited in equatorial planes [30]. Thus, the dinucleationof the complexes with partially and fully fluorinated ligands,1 and 5, are quite unique motifs, which are useful to under-stand the intramolecular interaction of the coordinationcomplexes.

The crystal structure of 2·2H2O·3C6H6 is shown inFigures 2 and 3(a), and 2·2H2O·3C6H6 and 3·2H2O·3C6H6

are isomorphs. The selected bond distances and anglesof 2·2H2O·3C6H6, 3·2H2O·3C6H6, 6·2C6H6, and7·2C6H6 are summarized in Table 1. The detail structure of2·2H2O·3C6H6 is as follows. Complex 2 comprises two Co2+

ions, four ligands 1, and two water molecules to give thedinuclear complex. The complex lies across crystallographicinversion center. Both of the geometries around the metalcenters are pseudo-octahedral. The two ligands are chelatecoordinated to each metal by the O1 and O2 (x, y, z) andO1iii and O2iii (iii: −x + 2, −y + 1, −z + 1) atoms. Two metalcenters are linked through the O4 and O4iii atoms forming alozenge geometry of the dinuclear core, and the remainingO3 and O3iii atoms are coordinated to each metal. The O5

and O5iii atoms of two water molecules are coordinated toeach metal and two water solvates, O6 and O6iii, and linkedto the coordinated water through hydrogen bonds; theintermolecular distance of O5· · ·O6 is 2.6888(19) A and theangle of O5-H40B-O6 is 174(2)◦, as shown in Figure 2. Themetal· · ·metal separations are 3.2061(3) A for 2. The M–O(ligand) distances are 2.0265(10), 2.0159(10), 2.0422(10),2.0529(10), and 2.1492(10) A for 2 (av. 2.06 A). TheCo–O5(water) distances of 2 is 2.0755(11), and the averageof the O=C bond distances is 1.27 A. The r.m.s deviationsof O1-C7-C8-C9-O2 and O3-C22-C23-C24-O4 in 2 are0.0231 and 0.0112 A, respectively. The pentafluorophenylgroups, C10-C11-C12-C13-C14-C15 and C25-C26-C27-C28-C29-C30, of 2 are highly twisted to the coordinationplane; the dihedral angles between O1-C7-C8-C9-O2 andC10-C11-C12-C13-C14-C15 is 36.75(5)◦ and that betweenO1-C7-C8-C9-O2 and C25-C26-C27-C28-C29-C30 is82.47(5)◦. The dihedral angles between O1-C7-C8-C9-O2and two phenyl groups, C1-C2-C3-C4-C5-C6 and C16-C17-C18-C19-C20-C21, are 17.19(5)◦ and 18.93(9)◦, respectively.The dihedral angle for C25-C26-C27-C28-C29-C30 isremarkably twisted because of the intramolecular stacking ofthe phenyl group of C1iii-C2iii-C3iii-C4iii-C5iii-C6iii throughthe arene-perfluoroarene interaction; the intramoleculardistance and the corresponding perpendicular distance ofCg〈C6F5〉· · ·Cg〈Ph〉iii are 4.3461(10) and 3.3637(7) A,respectively. This intramolecular stacking leading tothe efficient overlapping of the ligands gives efficientstabilization of the dinuclear framework (Figure 3(a)).

The similar structure was observed in Ni2+ complex,3·2H2O·3C6H6. Complex 3 also comprises two Ni2+ ions,

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International Journal of Inorganic Chemistry 5

Table 1: Selected bond distances and angles of Co and Ni complexes.

Complex 2·2H2O·3C6H6 3·2H2O·3C6H6 6·2C6H62 [24] 7·2C6H6 [24]

M· · ·Miii/iv 3.2061(3) A 3.1638(4) 3.1822(3) 3.1385(3)

M-O1 2.0265(10) 1.9891(10) 2.0132(11) 1.9842(11)

M-O2 2.0159(10) 1.9952(10) 2.0407(11) 2.0138(12)

M-O3 2.0422(10) 2.0051(10) 2.0496(11) 2.0087(12)

M-O4 2.0529(10) 2.0348(10) 2.0465(11) 2.0231(11)

M-O4iii/iv 2.1492(10) 2.1101(10) 2.1628(11) 2.1224(12)

M-O5 2.0755(11) 2.0567(11) 2.1127(13) 2.0828(13)

O1-C7 1.2685(17) 1.2687(17) 1.2616(19) 1.266(2)

O2-C9 1.2774(18) 1.2739(18) 1.2722(19) 1.271(2)

O3-C22 1.2596(17) 1.2573(18) 1.2506(19) 1.252(2)

O4-C24 1.2850(17) 1.2831(17) 1.2931(19) 1.293(2)

O1-M-O2 89.23(4) 91.13(4) 89.61(4) 91.74(5)

O3-M-O4 88.45(4) 90.01(4) 89.27(4) 90.79(5)

O4-M-O4iii/iv 80.57(4) 80.51(4) 81.82(4) 81.61(5)

M-O4-Miii/iv 99.43(4) 99.49(4) 98.18(4) 98.39(5)

O2-M-O5 169.54(4) 171.49(4) 171.63(5) 172.70(5)

four 1, and two water molecules. Both of the geometriesaround the metal centers are pseudo-octahedral. The coor-dination bond distances between the oxygen atoms and Ni2+

are also summarized in Table 1 and the average M-O(ligand)distance of 3·2H2O·3C6H6 is slightly shorter than thatof 2·2H2O·3C6H6, as depends on the size of metal ions.The pentafluorophenyl group of 3 interacts phenyl groupthrough the same intramolecular interactions.

While the molecular structures of the complexes 2 and3 are resembled to the corresponding complexes 6 and 7(see Figure 3), the crystal solvates are different; two watermolecules and three benzene molecules are included for thepartially fluorinated complexes and two benzene moleculesare included for the fully fluorinated complexes [24]. Thestructures of 2 and 6 are discussed as follows. The averageof M–O(ligand) distances is the same, 2.06 A, for 2 and 6,and the average of the O=C bond distances is the same,1.27 A, for 2 and 6. The Co–O5(water) distances of 2[2.0755(11) A] are shorter than those of 6 [2.1127(13) A].The metal· · ·metal separation of 2 [3.2061(3) A] is slightlylonger than that of 6 [3.1822(3) A]. The Ni2+ complexes 3and 7 have also the same difference; the average O=C bonddistances and metal· · ·metal separation of 3 are shorterand longer, respectively, than those of 7. It is pointedout that the pentafluorophenyl groups of 6 (and 7) arealso highly twisted with respect to the coordination plane(the torsion angles C5-C6-C7-C8, C8-C9-C10-C15, C20-C21-C22-C23, and C23-C24-C25-C30 are 38.9(2), 63.6(2),35.7(2), and 68.2(2)◦, resp.) and the intramolecular stackingis dominant between the two rings, C25-C26-C27-C28-C29-C30 and C1iv-C2iv-C3iv-C4iv-C5iv-C6iv (iv: −x + 1,−y,−z);the intramolecular distance and the corresponding perpen-dicular distance of Cg〈C6F5〉 · · ·Cg〈C6F5〉iv are 4.4373(10)and 3.3253(7) A, respectively. The pentafluorophenyl groupshave a twisted conformation with the coordination plane,

leading to the efficient overlapping of the ligands fordinuclear complexes (Figure 3(b)). This feature is in contrastto the case of the complexes of DBM [30], where phenylgroups and the coordination plane are essentially planardue to the expanding π-conjugation, which causes a sterichindrance and hence mononucleation.

In the packing structures of 2·2H2O·3C6H6 and3·2H2O·3C6H6, three benzene molecules (two C31-C32-C33-C34-C35-C36 and one C37-38-C39-C37v-C38v-C39v

(v: −x + 2,−y + 2,−z)) are included in the crystals andthese benzenes interact with the pentafluorophenyl groupsthrough arene-perfluoroarene interactions. This capsulatedsolvate through the interactions was also observed in thefully fluorinated complexes of 6·2C6H6 and 7·2C6H6 [24].Especially, the crystals of 7 show unique pseudopolymorphsof 7·2C6H6 and 7·4C6H6 [25]. In our examinations, whenno benzene solvent was used in the crystallization, singlecrystals were not grown as suitable for X-ray crystallography.These results indicate that the crystals of the partially andfully fluorinated Co2+ and Ni2+ complexes are stabilized bybenzene molecules through arene-perfluoroarene interac-tions.

3.3. Preparation and Structure of Cu2+ Complex. The Cu2+

complex with the ligand 1 was obtained as a mononu-clear complex (Scheme 2). Typically, an MeOH (10 mL)solution of H1 (0.10 g, 0.30 mmol) and NaOMe (16 mg,0.3 mmol) was combined into an MeOH (5 mL) solutionof CuCl2·2H2O (30 mg, 0.15 mmol). Then, the mixture wasstirred for 2 h at r.t. to give a green precipitates of [Cu(1)2] 4.In this case, the structure of 4 is mononuclear complex andno influences are observed by fluorine substitutions becausethe DBM and H5 are also mononuclear complexes [24]. Thisresult is expected from the fact that the ligands are only

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6 International Journal of Inorganic Chemistry

O1O5

O2O3 O4

CoCoiii

(a)

O1

O5

O2O3O4

CoCoiv

(b)

Figure 3: ORTEP drawings of the dinuclear complexes of (a)2·2H2O·3C6H6 and (b) 6·2C6H6 at 100 K with 50% probabilitythermal ellipsoids. For (a) and (b), symmetry transformations usedto generate equivalent atoms show iii (−x + 2, −y + 1, −z + 1) andiv (−x + 1, −y, −z), respectively.

sited in equatorial planes of Cu2+ ions by Jahn-Teller effect.The Crystallization by diffusion of benzene into a CH2Cl2solution of 4 yielded the pure product. The crystallization ofthree Cu2+ complexes, the target complex 4, [Cu(DBM)2],and the fully fluorinated complex 8 in CH2Cl2/benzeneconditions gave single crystals, 4, [Cu(DBM)2], and 8·3C6H6

[24]. While crystals of the fully fluorinated Cu2+ complexcapsulated 21 w% of benzene molecules, no solvate crystalswere obtained for partially fluorinated Cu2+ complex 4, aswell as [Cu(DBM)2].

In the crystal, complex 4 comprises one Cu2+ and two lig-and 1 (Figure 4), that the composition and the packing struc-ture was similar to [Cu(DBM)2]. The complex lies acrosscrystallographic inversion center. The geometry around themetal is essentially planar, forming a square-planar. The r.m.sdeviation of O1-C7-C8-C9-O2 is 0.016 A showing the flatplane of the chelate moiety of the β-diketonate framework.The bond distances of the Cu-O1, Cu-O2, O1-C7, and O2-C9 are 1.9100(19), 1.9216(18), 1.271(3), and 1.271(3) A,

O2

F1

F5

F4

F3F2

O1

Cu

C1

C2C3

C4

C5C6

C7

C8

C9

C10C15

C14C11

C12 C13

O2vi

O1vi

Figure 4: ORTEP drawings of the crystal structure of 4 with 50%probability thermal ellipsoids. Symmetry transformation used togenerate equivalent atoms is vi (−x,−y + 1,−z).

(a)

(b)

Figure 5: Parts of packing structures of (a) 4 and (b) [Cu(DBM)2].

respectively. The phenyl and pentafluorophenyl groups areclearly different orientation in 4. The phenyl group isflat with respect to the coordination plane (the dihedralangle between O1-C7-C8-C9-O2 and C1-C2-C3-C4-C5-C6is 6.82(16)◦), and the pentafluorophenyl group is highlytwisted (that between O1-C7-C8-C9-O2 and C10-C11-C12-C13-C14-C15 is 56.76(8)◦). On the other hand, the crystalstructure of [Cu(DBM)2] at 293 K is highly flat [31, 32]. Thephenyl groups of [Cu(DBM)2] are also flat with respect to thecoordination plane, and the dihedral angles between O1-C7-C8-C9-O2 and two phenyl rings are 9.96(30) and 4.98(33)◦,

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International Journal of Inorganic Chemistry 7

showing the π-delocalization of the whole complex. Theaverages of the Cu-O and O=C bond distances are 1.90 Aand 1.27 A, respectively. The fully fluorinated Cu2+ complex8 was obtained as 8·3C6H6 under the same crystallizationprocedure of CH2Cl2/benzene [23, 24]. The averages of theCu-O(5) and O=C bond distances of 8 are 1.92 A and 1.27 A,respectively. The pentafluorophenyl groups of 8 are alsohighly twisted with respect to the coordination plane [24],giving the benzene capsulated cavities.

Parts of the crystal packing of 4 and [Cu(DBM)2] wereshown in Figures 5(a) and 5(b), respectively. A commonintermolecular electrostatic interaction is found in bothcrystals, such as a cation-π interaction. The phenyl groupsin 4 interact with Cu2+ ion and the pentafluorophenylgroups of another molecule through the cation-π andarene-perfluoroarene interactions, respectively. The closestintermolecular distances of Cg〈Ph〉 · · ·Cuvii (vii: x, y + 1, z)and Cg〈Ph〉 · · ·Cg〈C6F5〉viii (viii: −x, y + 0.5,−z + 0.5) are3.270 A and 3.631 A, respectively. This cation-π interactionis only observed in Cu2+ complexes because of the freeapical sites of the Cu2+ ions. The CF· · ·π interaction isalso observed between the two pentafluorophenyl groups,and the distance of F4 atom in the pentafluorophenyl groupand Cg〈C6F5〉ii is 3.109 A, which is a reversed version of thecharge orientation of the CH· · ·π interaction.

In crystal of 8, one benzene molecule closely interactswith the Cu2+ ion through the cation-π interaction, andtwo benzenes interact with the pentafluorophenyl groupsthrough the arene-perfluoroarene interaction. This resultindicates, that the Cu2+ ion preferably recognized π-molecu-les and the cation-π interaction is required for the molecularpacking. The crystallographic study of 4 also indicates thatthe cation-π interaction takes priority in crystal packingin the mononuclear complex and no alternately layeredpacking structure through arene-perfluoroarene interactionswas obtained, in contrast to the case of H1.

In conclusion, we show three crystal structures of thecomplexes 2–4, using the partially fluorinated ligand 1.The octahedral geometry of the Co2+ and Ni2+ ions givesdinuclear complexes 2 and 3, as well as the correspondingfully fluorinated complexes 6 and 7, which are caused byintramolecular stacking of the pentafluorophenyl groups. Onthe other hand, the square planar geometry of the Cu2+

ion gives mononuclear complexes for all of the partially,fully, and nonfluorinated ligands (1, 5, and DBM) becauseof the Jahn-Teller effect, which gives free cavity spacesabove the Cu2+ ions. Thus, the phenyl group interacts tothe Cu2+ ions through the cation-π interaction for 4, aswell as [Cu(DBM)2]. The arene-perfluoroarene and cation-πinteractions are clearly shown as key electrostatic interactionsin the complexation behaviors and crystal structures of thepartially fluorinated complexes.

Acknowledgment

This work was supported in part by a Kitasato UniversityResearch Grant for Young Researchers.

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[12] R. A. Kumpf and D. A. Dougherty, “A mechanism for ionselectivity in potassium channels: computational studies ofcation-π interactions,” Science, vol. 261, no. 5129, pp. 1708–1710, 1993.

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Hindawi Publishing CorporationInternational Journal of Inorganic ChemistryVolume 2011, Article ID 843051, 6 pagesdoi:10.1155/2011/843051

Research Article

A Selective Chemosensor for Mercuric Ions Based on4-Aminothiophenol-Ruthenium(II) Bis(bipyridine) Complex

Amer A. G. Al Abdel Hamid,1 Mohammad Al-Khateeb,2 Ziyad A. Tahat,2

Mahmoud Qudah,1 Safwan M. Obeidat,1 and Abdel Monem Rawashdeh1

1 Department of Chemistry, Yarmouk University, Irbid 21163, Jordan2 Department of Chemistry, JUST University, Irbid 22110, Jordan

Correspondence should be addressed to Amer A. G. Al Abdel Hamid, [email protected]

Received 28 November 2010; Revised 22 February 2011; Accepted 23 February 2011

Academic Editor: W. T. Wong

Copyright © 2011 Amer A. G. Al Abdel Hamid et al. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

A new ruthenium(II) complex (cis-ruthenium-bis[2,2′-bipyridine]-bis[4-aminothiophenol]-bis[hexafluorophosphate]) has beensynthesized and characterized using standard analytical and spectroscopic techniques, FTIR, 1H and 13C-NMR, UV/vis, elementalanalysis, conductivity measurements, and potentiometric titration. Investigation of the synthesized complex with metal ionsshowed that this complex has photochemical properties that are selective and sensitive toward the presence of mercuric ion inaqueous solution. The detection limit for mercuric ions using UV/vis spectroscopy was estimated to be ∼ 0.4 ppm. The resultspresented herein may have an important implication in the development of a spectroscopic selective detection for mercuric ionsin aqueous solution.

1. Introduction

Chemosensors that are selective for specific targets of metalions are continuously demanded. An especially importantcategory of chemosensors are those targeting toxic heavymetal ions. Mercury in particular is a toxic metal, whichwhen accumulates in the vital organs of human and animalscauses poisonous effects that cause serious hematologicaldestruction, such as kidney malfunctioning and brain dam-age [1–3]. Therefore, monitoring and precise determiningof mercuric ion concentration in water and thus in relevantbiological matrices are extremely beneficial for the en-vironmental and toxicological monitoring. Despite theincreasing efforts for developing low cost methodologiesfor mercuric detection in aqueous solutions [4–16], thetailored design of new mercuric-colorimetric chemosensorsthat work effectively in aqueous media remains a key chal-lenge. Up to now the great effort of researchers in thisfield is directed toward the development of new selectivechemosensors based on fluorescence property [17–33]. Thisis of course due to many advantages in characterizing this

category of chemosensors, at top of which are the high selec-tivity and sensitivity toward targeted metal ions. Althoughthese fluorometric sensors have been employed enormouslyin this field, the colorimetric sensing [8, 28, 34–36] ofmetal ions has been shown to be less laboursome andintensive alternative to fluorescence techniques. The strongthiophilicity of mercury is the most attractive propertythat is usually taken into account when designing mercurydetection systems, whether they are based on colorimet-ric or fluorometric spectral changes [35–40]. However,investing this property in fluorescent chemosensors is notalways wise and beneficial, since many metals that are lessthiophilic than mercury (like silver, cadmium, and lead)can promote reactions similar to those of mercury andthus cause problematic sensing for mercury in relevantenvironments. In addition, significant challenges still existin this field, especially those which are related to quench-ing of fluorescent signals by foreign interference presence.Therefore, optimal ratiometric chemosensors (based oncolorimetric spectral changes) for mercuric ions that possessfast response at ambient temperature, can selectively detect

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2 International Journal of Inorganic Chemistry

N

N

NN

Ru

N

SH

SHN

HH

H

H

(PF6)2

Figure 1: Structure of the Cis-Ruthenium-bis[2,2′-bipyridine]-bis[4-aminothiophenol]-bis[hexafluorophosphate], (Ru-4-ASP).

33883476

1618

839

760

Ab

s 2650 2600 2550 2500

1335

3503–3427

Wavenumber (cm−1)

4000 3000 2000 1000 400

(a)

(b)

Figure 2: IR spectra of (a) Ru(bpy)2Cl2·2H2O and (b) The complexRu-4-ASP. Enlargement of the SH bands in Ru-4-ASP in thefrequency range 2500–2650 cm−1 is shown in the inset.

Hg(II) ions, and can operate in aqueous media are still highlydemanded.

Polypyridine-Ru(II) complexes are interesting chro-mophores. The assembly of oligopyridyl groups aroundRu(II) center yields mono- or polymetallic complex ions thathave been studied extensively for their potential ability as flu-orescent chemodetectors [41–43] and energy or electrontransformers [44–47]. Furthermore, these Ru(II)-metallocomplexes, in some cases, have demonstrated good photo-physical and colorimetric sensing properties. Actually, thisfeature has attracted our attention, and Ru(II)-bipyridinederivative was employed in this study as a potential carrierfor preparation of new optical chemosensors.

In this paper, Ru(II)-bis(bipyridine) was selected to act asthe carrier for the sulfur-containing 4-aminothiophenolmoiety, which will act as the receptor for obtaininga new chemosensor for mercuric ions. The incorporationof the Hg(II) cations into thiols of the coordinated 4-aminothiophenol ligand would be expected to influence theabsorption properties of the Ru(II)-bipyridine core and thusallowing access to a new potential chemosensor based on thechromophore-spacer-receptor concept.

2. Experimental

2.1. General. All reagents were obtained from commercialsources and were used without further purification. Column

250 nm

284 nm

413 nm

Wavelength (nm)

Abs

0.110.1

0.05

0200 400 600 800

Figure 3: UV-vis absorption bands in Ru-4-ASP (1.0 μM) complex.

chromatography was performed with Silica gel 60A/35–70 μm, Merck Al2O3 90 basic (0.063–0.200 mm). 1H and13CNMR spectra were recorded on a Bruker 400 MHz. Thechemical shifts were reported using the residual solventsignal as an indirect reference to TMS: acetone-d6 2.05 ppm(1H) and 29.84 ppm (13C). UV/vis spectra were recorded for50% ethanol/water (v/v) solutions on a Shimadzu UV-1800spectrophotometer using 1 cm quartz cuvettes. IR measure-ments were collected on a JASCO FT/IR-4100. All IR spectrawere recorded as pressed disks of the sample dispersed inKBr powder. Typically, for each spectrum, 100 scans werecoadded at 4 cm−1 resolution. Microanalyses (C, H, N, S)were performed using Euro EA elemental analyzer 3000.Conductivity measurements were carried out using JENWAY4010 conductivity meter employing 0.001 M solutions ofRu-4-ASP complex. Potentiometric titration was carriedout using KHP standardized with 0.05 M NaOH solutionemploying the Russell model RL150 Potentiometer. The pHreadings were taken after adding 1 mL of 0.05 M sodiumhydroxide increments allowing 30 seconds to pass to ensurecomplete mixing before each pH measurement.

2.2. Synthesis

2.2.1. Cis-Ruthenium-bis[2,2′ -bipyridine]-dichloride dihy-drate,Ru(bpy)2Cl2·2H2O]. It was prepared and character-ized as reported, by Sullivan et al. [48]

2.2.2. Cis-Ruthenium-bis[2,2′-bipyridine]-bis[4-aminothio-phenol]-bis[hexafluorophosphate] (Ru-4-ASP). Under argon/N2 atmosphere, Ru(bpy)2Cl2·2H2O (0.35 g, 0.67 mmol) and4-aminothiophenol (4-ASP) (0.16 g, 1.4 mmol) were dis-solved separately in a 1 : 1 mixture of absolute ethanol/water(v/v). After mixing, the resulting solution was subjected toreflux overnight, after which NH4PF6 (2.5 g, 13.5 mmol) dis-solved in 2 mL water was added to precipitate the complex.The solid product was collected by filtration and washedthroughly with 10 mL portions of water followed by diethylether. The complex was then dried under reduced pressure toyield 0.36 g (81%), m.p. 205–207◦C (dec.). Found C, 42.76;H, 3.18; N, 9.31; S, 6.21 calc. for C32H30F12N6P2RuS2, C,

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International Journal of Inorganic Chemistry 3

(1) 0.33 eq

32

54

76

1

Wavelength (nm)

Abs (2) 0.66 eq

(3) 1 eq(4) 1.33 eq(5) 1.66 eq(6) 2 eq(7) 2.4 eq

0.3

0.2

0.1

0200 220 240 260 280 300

Figure 4: The change in 250 nm absorption band of Ru-4-ASP(1.0 μM) as a function of the added 1.0 μM Hg(II).

40.30; H, 3.17; N, 8.81; S, 6.72. 1H-NMR 400 MHz (acetone-d6) δ (ppm) 10.05(dd, J = 1.6, 5.6 Hz, 2H, H-6, bpy), 8.55(dd, J = 1.2, 7.6 Hz, 2H, H-3, bpy), 8.47 (dd, J = 1.6, 6.7Hz, 2H, H-3′, bpy), 8.05 (dd, J = 7.9, 8.6 Hz, 2H, H-4, bpy),7.95 (dd, J = 7.9, 8.6 Hz, 2H, H-4′, bpy), 7.76 (dd, J = 1.6,5.6 Hz, 2H, H-6′, bpy), 7.56 (dd, J = 5.0, 6.5 Hz, 2H, H-5,bpy), 7.40 (dd, J = 5.0, 6.5 Hz, 2H, H-5′, bpy), 7.25 (dd, J =1.3, 8.6 Hz, 2H, H-3′′,5′′), 6.80 (dd, J = 1.3, 9.0 Hz, 2H, H-2′′,6′′).13C NMR (acetone-d6) δ (ppm) 158.9, 157.3, 156.6,155.8, 152.4, 139.3, 139.2, 138.4, 138.0, 137.4, 132.3, 130.6,128.8, 127.4, 127.3, 127.1, 126.3, 124.5, 124.3, 124.2, 123.9,122.8; UV–Vis λmax (ε) 250 nm (3.2 × 104), 284 nm (3.7 ×104), 413 nm (6.7 × 103).

3. Results and Discussion

3.1. Synthesis of Ru-4-ASP Complex. Interaction of theruthenium complex, Ru(bpy)2Cl2·2H2O, with 4-amino-thiophenol (4-ASP) has resulted in Ru-4-ASP complex,(Figure 1). This complex, Ru-4-ASP, appears to be ideal forthis application since we anticipated that Ru-4-ASP wouldinteract efficiently with Hg(II) ions through the two thiols.This interaction would facilitate spectral changes as a resultof mercury interaction. The Ru-4-ASP complex was preparedin good yield by interacting Ru(bpy)2Cl2·2H2O with 4-ASPligand, using one-step procedure. The formed complex wasdark brown-reddish colored and was observed to precipitateas solid material immediately upon the addition of theprecipitating agent, ammonium hexafluorophosphate.

The structure of Ru-4-ASP complex depicted in Figure 1was verified by IR, H1, and 13C-NMR, elemental analysis,conductivity, and potentiometric titration. Introduction of4-ASP ligand into Ru(bpy)2Cl2·2H2O was evidenced bythe appearance of the two IR bands in the range 2560–2625 cm−1. These two bands were assigned for the twoSH groups of the attached 4-ASP ligand. Since SH isknown as a weak IR absorber [49], the portion 2500–2650 cm−1 of the spectrum in Figure 2(b) has been blownup to clearly show the two SH bands (see the inset inFigure 2). The appearance of the NH-stretching vibrationsat 3388 and 3476 cm−1 in addition to the bending band at

Wavelength (nm)

Abs

Ru-4-ASP413 nm Ru-4-ASP-HgII

505 nm

0.2

0.15

0.05

0

0.1

300 400 500 600

350 400 450 500

0.04

0.03

0.02

0.01

0600550

Figure 5: The shift in the 413 nm absorption band of Ru-4-ASP(1.0 μM) upon addition of ∼2 equivalents of Hg(II).

Wavelength (nm)

Abs

1.7

1.5

1

0

1.7

1.5

1

0.5

0

0.5

200 400 600 800

200 250 300 350 400 450

BaII

CaII

MnII

CoII

MgII

ZnIINiII

NaI

KI

Figure 6: Absorption response of Ru-4-ASP (1.0 μM) at 250 nmupon the addition of Hg(II) co existed with 1.0 μM of Ba(II), Ca(II),Co(II), Mg(II), Mn(II), Ni(II), Zn(II), Na(I), and K(I).

1618 cm−1 (Figure 2) indicates the coordination of the 01-amino group of 4-ASP to the Ru(II) metal ion. The band at1335 cm−1, which was consistent with the C-N stretching in01-amine, is another evidence for the coordination of 4-ASPchelate to the Ru(bpy)2Cl2·2H2O moiety via NH2 group.In 50% water/ethanol (v/v) solution, Ru-4-ASP turned tolight brown-reddish-colored solution and gave rise to threeabsorption bands, two strong bands in the UV-region, at 250and 284 nm, and a third weak band in the visible region at413 nm; these bands are shown in Figure 3.

1H- and 13C-NMR, in terms of the peak shift and numberof carbon atoms along with the elemental analysis of carbon,hydrogen, nitrogen, and sulfur are consistent with thestructure of Ru-4-ASP proposed in Figure 1. On the otherhand, the potentiometric titration of one equivalent of Ru-4-ASP with 0.05 M NaOH indicated that thiol groups losttheir protons completely in the pH range of 6.0 to 8.8. Thisrange corresponds to two equivalents of sodium hydroxide.Furthermore, the deprotonation intervals of both SH groupswere observed to be indistinguishable, where no well-definedsharp equivalence points were noticed; instead, the tworegions were overlapped due to the high matching and sim-ilarity between the two groups in the Ru-4-ASP structure.

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4 International Journal of Inorganic Chemistry

3.2. Interaction of Hg(II) with Ru-4-ASP Dye. A spectro-scopic titration of Hg(II) was conducted with 1.0 μMsolution of Ru-4-ASP in 50% ethanol/water (v/v) solutionat pH 6. Upon addition of an increased amount of Hg(II)ions, the area under the UV-absorption peak correspondingto 250 nm starts to decline, while the band at 284 nmstayed in place without any noticeable change, Figure 4. Thereaction responsible for these changes was observed to reachcompletion within <30 seconds and the observed reductionin the 250 nm absorption band was proportional to theadded Hg(II) concentration. Moreover, the saturation ofRu-4-ASP with Hg(II) ions was attained after adding ∼2.0equivalents of Hg(II); beyond this point, more additionsof Hg(II) brought no spectral changes in the absorptionprofiles as shown in Figure 4. On the other hand, the visibleband at 413 nm was observed to undergo a red shift (intohigher wavelength) upon adding two equivalents of Hg(II),Figure 5. At this point, the color of the Ru-4-ASP solutiontreated with Hg(II) was visually observed, at once, to changefrom light brown-reddish to orange-reddish. This change incolor was not noticed to take place in solution unless theamount of the added Hg(II) reaches the equivalent amount.In a control experiment, Ru-4-ASP was found to retain itsoriginal color when treated with only ∼1.0 equivalent ofHg(II), and this color was found to be stable over time as wasobserved when the solution was kept on the shelf for morethan three weeks.

Similar spectral changes were observed when other saltsof mercury, such as mercuric nitrate and mercuric perchlo-rate, were used. Therefore, it appears that counter anionsaccompanying Hg(II) ion have negligible effect on the sen-sation activity of Ru-4-ASP complex. Moreover, the colorchange in the presence of Hg(II) was found to be insensitiveto interferences by other metal cations. That was observedwhen the Ru-4-ASP complex was allowed to interact withsolutions containing submicromolar amounts of Hg(II) andmicromolar quantities of the metal cations: Ag(I), Zn(II),Cu(II), Pb(II), Cd(II), Ni(II), Co(II), Fe(II), Fe(III), Mn(II),Mg(II), Ca(II), Ba(II), Li(I), K(I), Na(I), Rh(III), Cr(II),Cr(III). The absorption profiles of some of these ions areshown in Figure 6.

On the other hand, when the optical measurements wererepeated for the above cationic solutions but in absence ofHg(II), no change was observed in the three absorption peaksof Ru-4-ASP, Figure 7. However, higher absorbance valueswere noticed for the 250 nm band when the Ru-4-ASPcomplex was interacted with Fe(III), Rh(III), and Cr(III).Compared to the divalent ions, the high absorbance valuesobserved with these ions are attributed to the high electrondeficiency of the three trivalent ions and consequently thestronger interaction they exhibit with the electron-rich lig-and. Fortunately, in all cases, no spectral changes (peakreduction) similar to those witnessed when Ru-4-ASP wasinteracted with Hg(II) have been induced, Figure 8.

Therefore, the outcomes of the proceeded optical mea-surements of Ru-4-ASP with Hg(II) demonstrate the lack ofinterferences by other metal cations or their accompaniedanions. This means that Ru-4-ASP complex has a remarkableselectivity toward Hg(II) ion over other metal ions. To see

(a) (a)

(b)

(b)

0.3

0.2

0.1

0

0.3

0.2

0.1

0

200 250 300 350 400 450

Wavelength (nm)

200 400 600 800

Abs

BaII

CaII

MnII

CoII

MgII

ZnIINiII

NaI

KI

Figure 7: Absorption spectra of Ru-4-ASP (1.0μM) at 250 nm, (a)in the presence of 1.0 μM of Ba(II), Ca(II), Co(II), Mg(II), Mn(II),Ni(II), Zn(II), Na(I), and K(I) and (b) Ru-4-ASP (1.0 μM) with noadditions.

CrIII

RhIIIFeIII

Divalent cations

Abs

0.5

0.4

0.3

0.2

0.1

0

0.5

0.4

0.3

0.2

0.1

0200 250 300 350 400 450

Wavelength (nm)

200 400 600 800

Figure 8: The change in absorption response of Ru-4-ASP (1.0μM)at 250 nm upon the addition of 1.0 μM of trivalent ions: Cr(III),Fe(III), and Rh(III) compared to the divalent ions: Ba(II), Ca(II),Co(II), Mg(II), Mn(II), Ni(II), Zn(II), Na(I), and K(I).

Peak

area

−0.1

−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

0.08

0.1

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7

Equivalent Hg(II)

Figure 9: Titration of the change in the area of the 250 nmabsorption peak of Ru-4-ASP versus equivalents of Hg(II).

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International Journal of Inorganic Chemistry 5

the practical applicability of this system as a detector formercuric ions, the detection limit was evaluated. The titra-tion profile of Ru-4-ASP with Hg(II), which is shown inFigure 9, demonstrates that the detection of Hg(II) is belowpart per-million level (<0.4 ppm).

In summary, this investigation described in the pro-ceeding has resulted in a development of a highly selectiveand sensitive Ru-4-ASP-based chemosensor for mercuric iondetection in aqueous medium. This system works efficientlywith remarkable high selectivity and sensitivity and underconditions similar to those encountered in relevant Hg(II)-contaminated environments. In such environments, Hg(II)coexists in a matrix of other interfering ions. The findingsof this investigation suggest that this strategy would serveas a foundation for practical, rapid detection and precisedetermination of Hg(II) ions in aqueous environments.

4. Conclusion

The results presented in this investigation demonstrate that,the interaction taking place between Hg(II) ions and Ru-4-ASP complex (through thiol groups) was responsible forthe observed spectral changes of the corresponding Ru-4-ASP dye. Interestingly, the investigation described above hasresulted in a development of a highly selective and sensitivechemosensor for Hg(II) ions in alcoholic–aqueous solutioneven in the presence of relatively high concentrations ofpotentially competing other metal cations. This includesthose cations that are identified by the U.S. EnvironmentalProtection Agency as potential environmental water pol-lutants [50] such as Zn(II), Cd(II), Pb(II), Ni(II), andFe(II). Furthermore, and in terms of sensitivity, the limit ofquantification of the system, based on UV-vis spectroscopymeasurements, was estimated to be lower than 0.4 ppm,providing a good chemosensation for Hg(II) ions.

Acknowledgments

This work was financially supported by the Faculty of Grad-uate Studies and Scientific Research Yarmouk University.The authors express great thanks for the Department ofChemistry-Jordan University of Science and Technology(JUST) for providing instrumentation.

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