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Inorganica Chimica Acta 359 (2006) 4441–4446

Note

A novel copper(II) complex with a pendant Schiff base:An unprecedented monodentate bonding mode of the

potentially tridentate ligand

Shouvik Chattopadhyay a,*, Gabriele Bocelli b, Andrea Cantoni b, Ashutosh Ghosh a,*

a Department of Chemistry, University College of Science,University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, Indiab IMEM-CNR, Parceo Area delle scienze 37a, I-43100, Fontanini-Parma, Italy

Received 11 April 2006; received in revised form 30 May 2006; accepted 5 June 2006Available online 16 June 2006

Abstract

The 1:1 condensation of 1-benzoylacetone and 1,2-diaminopropane yields 6-amino-3-methyl-1-phenyl-4-azahept-2-en-1-one (HL).When copper(II) perchlorate is added to the methanolic solution of HL, followed by triethylamine in 1:2:1 molar ratio, an unusual cop-per(II) complex, [Cu(L)(HL)]ClO4, is separated out where the deprotonated ligand, L, is coordinated in the usual chelating tridentatemanner but HL is coordinated to Cu(II) only through the amine N, i.e. it acts as a pendant ligand. The complex is characterized byX-ray crystal structure analysis.� 2006 Elsevier B.V. All rights reserved.

Keywords: Copper(II); Schiff base; Pendant ligand; Crystal structure

1. Introduction

The term ‘pendant ligand’ is applied to a potentialpolydentate ligand that has additional donor groupsattached to its periphery. A large number of pendantligands are produced for a variety of different chemicalapplications, e.g. in enzyme simulations [1], for the purposeof carrying a radionuclide into a target cell [2], in magneticresonance imaging reagents [3], and for progressively moreelaborate investigations for their structure/activity rela-tionship [4]. A collectively non-labile set of donor atomsthat serve to immobilize the metal ion and a set of morelabile donors which can perturb the metal ion at additionalcoordination sites, in competition with external substrates(or which can be used as attachment points for other mol-ecules), probably makes this structural motif as an impor-tant one with the potential to influence the futuredevelopment in chemistry in a major way.

0020-1693/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2006.06.009

* Corresponding authors.E-mail address: ghosh_59@yahoo.com (A. Ghosh).

The Schiff bases, formed by the condensation of anamine with an aldehyde [5], usually coordinate to a metalthrough the imine nitrogen and another group, usuallyoxygen, nitrogen or sulfur situated on the original alde-hyde. Condensation of the carbonyl function of a b-ketonewith only one end of a diamine produces terdentate ligandand this can be make to undergo further condensation withanother carbonyl function to produce unsymmetrical tet-radentate Sciff base ligands. A conventional classificationof them, based on donor sites, is indicated in Scheme 1.The Schiff bases are very important tools for the inorganicchemists as these are widely used to design molecular ferro-magnets, in catalysis, in biological modeling applications,as liquid crystals and as heterogeneous catalysts [6]. Schiffbase complexes are once again topical in connection withself-assembling cluster complexes [7].

The 1:1 condensation of acetylacetone or 1-benzoylace-tone and diamines (e.g. 1,2-diaminoethane, 1,2-diamino-propane and 1,3-diaminopropane) under the condition ofhigh dilution can produce mono-condensed terdentateSchiff bases [8–15]. This group of tridentate ligands have

Unsymmetrical Schiff bases

Tridentate Tetradentate

N, N, N donor

N, N, O donor

N,N,N,Ndonor

N,N,N,Odonor

N,N,O,Odonor

(O atom may be replaced by S atom)

Scheme 1.

C6H5

O

H3CNH NH2

CH3

Scheme 2.

4442 S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4441–4446

been used exhaustively for the synthesis of mono-, di- andtri-nuclear complexes with first transition elements [8–21]and also as precursor for the synthesis of unsymmetricaltetradentate Schiff bases [9–12]. However, to the best toour knowledge, there is no report confirming the existenceof a complex where such ligands act as monodentate, leav-ing the other two binding sites pendant. In the presentpaper, we report, for the first time, the synthesis, spectro-scopic study and crystal structure of an unusual, novel cop-per(II) complex, [Cu(L)(HL)]ClO4, containing neutralmonodentate ligand, 6-amino-3-methyl-1-phenyl-4-azah-ept-2-en-1-one (HL), and anionic tridentate chelatingligand, L (Scheme 2).

2. Experimental

2.1. Materials

All chemicals were of reagent grade and used withoutfurther purification.

Caution!!! Although no problems were encountered inthis work, perchlorate salts containing organic ligandsare potentially explosive. Only a small amount of thematerial should be prepared and it should be handledwith care.

2.2. Synthesis of the tridentate ligand HL

A solution (25 ml) of 1-benzoylacetone (10 mmol,1.62 g) in chloroform was added dropwise to a solutionof 1,2-diaminopropane (10 mmol, 0.76 cm3) in chloroform.

After completion of addition, the solution was stirred foran additional 3 h and chloroform was evaporated underreduced pressure, yielding HL as a viscous liquid.

2.3. Synthesis of [Cu(L)(HL)]ClO4 (1)

HL (10 mmol) was dissolved in methanol (10 cm3) andto it a solution of Cu(ClO4)2 Æ 6H2O (1.85 g, 5 mmol) inmethanol (10 cm3) was added with stirring, followed bythe addition of triethylamine (5 mmol, 0.7 cm3). A deepbrown compound was collected after 6 h and was crystal-lized from acetonitrile to obtain dark brown single crystalssuitable for X-ray structural analysis (Yield: 1.65 g, 55%(based on copper salt)). Anal. Calc. for C26H35ClCuN4O6:C, 52.17; H, 5.89; N, 9.36; Cu, 10.62. Found: C, 52.31; H,5.37; N, 9.49; Cu, 10.38%. l = 1.76 BM. kmax (nm)[emax (dm3 mol�1 cm�1)] (acetonitrile): 563 (123); 338(72700).

2.4. Physical measurements

Elemental analysis (carbon, hydrogen and nitrogen)was performed using a Perkin–Elmer 240C elemental ana-lyzer. Copper content in the complex was estimated spec-trophotometrically [22]. Electronic spectra in acetonitrile(1200–350 nm) were recorded in a Hitachi U-3501 spectro-photometer. IR spectra in KBr (4500–500 cm�1) wererecorded using a Perkin–Elmer RXI FT-IR spectropho-tometer. The magnetic susceptibility measurements weredone with an EG & PAR vibrating sample magnetometer,model 155 at room temperature and diamagnetic correc-tions were made using Pascal’s constants.

2.5. Structure determination

Single crystals of 1 were obtained by slow evaporationof an acetonitrile solution of the complex. The data forthe complex were collected at room temperature on BrukerAXS Smart single crystal diffractometer with CCD (areadetector). The absorption correction was performed withthe method adopted in SHELXTL-NT V5.1 [23]. The structurewas solved by direct methods with the software SHELXTL-NT

V5.1 inserted in the Bruker AXS software [23]. Non-hydro-gen atoms for all complexes were refined with anisotropicthermal parameters, while the hydrogen atoms, eitherlocated from difference electron density maps or placedgeometrically, were refined with thermal isotropic parame-ters. The numbers of independent reflections for the com-plex were 6179. The structure was refined on F2 usingSHELXL-97 [24]. Reflections with I > 2r(I) were consideredto be ‘observed’. Neutral atom scattering factors weretaken from Cromer and Weber [25], and anomalous disper-sion effects were included in Fcalc [26]. The crystallographicillustrations for all the complexes were prepared usingORTEP-3 [27]. The crystallographic data are summarized inTable 1. Selected bond lengths and bond angles are shownin Table 2.

Table 1Crystal data and refinement details of complex 1

Formula C26H35ClCuN4O6

Formula weight 598.57Temperature (K) 293Space group P21/cCrystal system monoclinica (A) 10.909(2)b (A) 20.250(3)c (A) 14.059(3)b (�) 112.19(2)V (A3) 2875.7(9)Z 4DCalc (gm/cm3) 1.383l (mm�1) 0.9F(000) 1252Total reflections 17678Unique reflections 5803Observed data [I > 2r(I)] 3369Number of parameters refined 497Rint 0.029R indices [I > 2r(I)] R1 = 0.0596 , wR2 = 0.1655

Table 2Selected bond lengths (A) and angles (�) for the complex

Cu(1)–N(2) 2.005(6) Cl(34)–O(37) 1.409(5)Cu(1)–N(5) 1.949(4) Cl(34)–O(35) 1.362(6)Cu(1)–O(9) 1.885(3) Cl(34)–O(36) 1.407(6)Cu(1)–N(18) 2.034(5) Cl(34)–O(38) 1.394(4)N(2)–C(3b) 1.399(1) C(25)–C(26) 1.409(8)C(7)–C(8) 1.368(7) N(18)–C(19) 1.475(8)C(23)–C(25) 1.375(7)

N(2)–Cu(1)–N(5) 83.9(2)N(2)–Cu(1)–N(18) 91.4(2)N(5)–Cu(1)–O(9) 94.5(1)O(9)–Cu(1)–N(18) 90.1(2)N(5)–Cu(1)–N(18) 174.9(2)N(2)–Cu(1)–O(9) 177.1(2)

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3. Results and discussion

3.1. Synthesis of the complex

The tridentate Schiff base ligand is prepared by singlecondensation of 1-benzoylacetone with 1,2-diaminopro-pane. Addition of copper(II) perchlorate solution to themethanol solution of the ligand followed by triethylamine(TEA) in 1:2:1 molar ratio resulted in the formation of[Cu(L)(HL)]ClO4. The composition of the complex is note-worthy; half of the ligand molecules are deprotonated andcoordinated to Cu(II) in tridentate chelating mode,whereas the remaining half which are not deprotonated,coordinate to the metal as monodentate ligand. Severalsquare planar Cu(II) complexes of this type of ligandsare reported where the monoanionic ligand acts as triden-tate and the fourth coordination site is occupied by anothermonodentate ligand, e.g imidazole [16]. In the present com-plex, as no other co-ligands were used during synthesis, theamine nitrogen of the neutral ligand coordinates to thefourth site. For its synthesis, the required amount of added

TEA is such that (HL:TEA is 2:1) it removes the iminehydrogen from only half of the ligand molecules. If moreTEA is added to remove imine hydrogen from all theligands, instead of the formation of six-coordinated bis-chelate [CuL2], then it would result in a trinuclear Cu(II)compound with a [Cu3OH] core [17,19–21]. The reason isthat Cu(II) being a d9 system has active Jhan–Teller effect,which makes it reluctant to be coordinated by strong coor-dinating atoms in the axial positions [28]. However, withthe increased amount of TEA, the pH of the solutionincreases and the hydroxyl ion coordinates to the fourthcoordination site to bridge three Cu(II) ions. An additionalbridging of the carbonyl oxygen provides the stability ofthe trinuclear compounds in such a way that it becomesthe major product at high pH even in the presence of amonodentate ligand such as pyridine [19]. On the otherhand, if the mixture is kept at room temperature for severalhours without adding TEA, then another Cu(II) complexwould separate slowly and would be identified as a complexof di-Schiff base of 1,2-diaminopropane and 1-benzoylace-tone by IR spectra, elemental analysis and mass spectra(Scheme 3). The crystal structure of it had been reportedelsewhere [29]. Therefore, the isolation of [Cu(L)(HL)]ClO4

is delicately balanced with the amount of TEA added andprovides an insight into the intermediate species that areformed during the synthesis of several types of complexesusing such ligands.

3.2. Infrared and electronic spectra and magnetic properties

In the IR spectra, the OH stretching vibration of thependant ligand is noticed around 3400 cm�1. The remain-ing bands at 3250 and 3325 cm�1 may be assigned tom(N–H) stretching vibrations [30]. The characteristic signalfor the C@N vibration band is found at 1594 cm�1.Another signal for a C@N vibration is observed at1514 cm�1, the frequency of which is lowered probablydue to coordination to the metal centre. The sharp, strongpeak for perchlorate anion is routinely noticed at1098 cm�1, which is devoid of any splitting and is consis-tent with the IR normal modes for Td symmetry of perchlo-rate, suggesting that the anion is not coordinated to thecopper atom. The UV–Vis absorption spectrum of thecomplex shows single d–d transition band at 563 nm as isexpected for a square planar copper(II) environment withsimilar N2O donor tridentate Schiff base along with a nitro-gen donor monodentate ligand [8,16].

Room temperature magnetic susceptibility measure-ments show that the complex has magnetic moments closeto 1.73 BM as expected for discrete magnetically non-cou-pled spin-only value for copper(II) ion, which is alsoobserved in similar systems [8,16].

3.3. Structure description

The ORTEP-3 view of the complex with atom numberingscheme is shown in Fig. 1. Within the molecule, the copper

H3C

NH

O

H3C

C6H5

NH2

CH3

Cu OH

O

N

N

C6H5

Cu

O N

N

C6H5

Cu

O

NN

C6H5

2+

(ClO4)2

Cu(ClO4)2

(Methanol solution)

Cu(ClO4)2

(Methanol solution)

Cu(ClO4) 2

(Methanol solution)

TEA

TEA (excess)

(1:1)

O

N N

C6H5O

Cu

N

OC6H5

H3CNH2

CH3

CuClO4

HN

O

H3C

C6H5

NH2

H3C

+

C6H5

CH3

Scheme 3.

Fig. 1. ORTEP-3 picture excluding H atoms for complex 1 with atom numbering scheme. Thermal ellipsoids are shown at 20% probability. Perchlorate ionis not shown for clarity. Only the major fraction (C3b) of the disordered C3 atom is shown.

4444 S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4441–4446

ion displays the square planar coordination through onecarbonyl oxygen, one imine nitrogen and one amino nitro-gen from the de-protonated tridentate mono-anionic Schiff

base ligand (L) and one amino nitrogen from the mono-dentate neutral Schiff base HL. The imine nitrogen ofHL, N22 sits nearly in the axial position, but the distance

S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4441–4446 4445

from the copper atom (3.06 A) is too long to be consideredas a bond. Thus, deprotonated Schiff base, L, acts as a tri-dentate ligand, while HL behaves as a mono-dentate one.The lengths of the bonds between the copper atoms andthe donor sites (O, N, N) of the tridentate ligand are withinthe range of values normally found for such bonds [17,18].There is a disorder in C3, and only one (C3b) of the twodisordered positions (C3a, C3b) of C3 is shown in Fig. 1.The deviations of the coordinating atoms N(2), N(5),O(9) and N(18) from the least square mean plane throughN(2), N(5), O(9), N(18) and Cu(1) are 0.067(7), 0.023(3),0.013(3) and 0.051(5) A, respectively. The deviation of thecopper atom from the same plane is �0.002(1) A. The ben-zoylacetone moiety around the metal constitutes a slightlypuckered six-membered chelate ring having the puckeringamplitudes Q = 0.114(3), / = 25(3) and h = 53.6(2) [31,32].

Copper(II) has a general tendency to expand its coordi-nation number beyond tetracoordination to the penta-coordinated square pyramidal or trigonal bi-pyramid andhexa-coordinated distorted octahedral structures. Theinteresting and very unusual feature of the present complexis that the imine N, which is known to be coordinated in allthe complexes of such ligands reported so far, remainsuncoordinated leaving the geometry around copper(II)

Fig. 2. Intra- and inter-molecular h

Table 3Hydrogen bonding distances (A) and angles (�) for the complex

D–H A D� � �A A H

N22–H22 1.04(8) N22� � �O27 2.610(7) HN18–H18b 0.70(6) N18� � �O37 3.361(7) HN18–H18a 0.91(6) N18� � �O35i 3.148(8) H

Symmetry transformation: i = �X + 1, �Y, �Z.D, donor; H, hydrogen; A, acceptor.

essentially square planar. The increased steric interactionbetween the two ligand moieties probably prevents theimine N from coming closer to the metal ion and beingcoordinated to the axial position.

There are three available H atoms (H22, H18b andH18a) for hydrogen bond formation. Amongst them,H(22) is involved in strong intramolecular H-bond withthe uncoordinated oxygen (O27) of the same tridentateSchiff base. H(18b) is involved in relatively weak hydrogenbond with a perchlorate oxygen atom (O37). On the otherhand, H(18a) is involved in H-bonding with symmetryrelated (�X + 1, �Y, �Z) perchlorate oxygen (O35) toresult in a dimeric entity (Fig. 2). The details of H-bondsare given in Table 3.

4. Conclusion

The isolation and crystal structure of the complexunambiguously show that the Schiff base, which is widelyused as anionic, tridentate ligand can also act as neutral,monodentate ligand under special circumstances. Suchbonding mode was unprecedented for this type of ligands.Thus it opens up new possibilities for the synthesis of com-plexes having pendant ligands.

ydrogen bonding in complex 1.

� � �A A D–H� � �A �

22� � �O27 1.71(8) N22–H22� � �O27 143(7)18b� � �O37 2.68(6) N18–H18b� � �O37 166(7)18a� � �O35i 2.28(6) N18–H18a� � �O35i 161(6)

4446 S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4441–4446

Acknowledgment

We thank the CSIR, India, for awarding a Senior Re-search Fellowship (Sanction No. 9/28(614)/2003-EMR-I)to one of the authors (S.C.).

Appendix A. Supplementary data

Crystallographic data for the analysis have been depos-ited with the Cambridge Crystallographic data Centre,CCDC No. 297692. Copies of this information may beobtained free of charge from CCDC, 12 Union Road, Cam-bridge, CB2 1EZ, UK, fax: +44 1223 336 033, e-mail: depos-it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk.Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.ica.2006.06.009.

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