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Polyhedron 25 (2006) 3104–3112

Coordination behaviour of symmetrical hexadentateN-donor Schiff bases towards zinc(II)pseudohalides:

Syntheses, crystal structures and luminescence

Rajarshi Ghosh a, Sk Hafijur Rahaman a,*, Chun-Nan Lin b,Tian-Huey Lu b, Barindra K. Ghosh a,*

a Department of Chemistry, The University of Burdwan, Golapbag Campus, Rajbati Burdwan, Burdwan 713104, West Bengal, Indiab Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, ROC

Received 25 March 2006; accepted 11 May 2006Available online 15 June 2006

Abstract

Two new Schiff bases, N-(1-pyridin-2-ylformylidene)-N 0-[2-({2-[(1-pyridin-2-ylformylidene)amino]ethyl}amino)ethyl]ethane-1,2-dia-mine (L1) and N-(1-pyridin-2-ylformylidene)-N 0-[2-(4-{2-[(1-pyridin-2ylformylidene)amino]ethyl}piperazin-1-yl)ethyl]amine (L2) havebeen prepared and used to synthesize two series of zinc(II) pseudohalide complexes of the dinuclear type [Zn2(L1/L2)(X)4] (1a–1d)(L1:X ¼ N3

�; 1a and X = NCS�, 1b; L2:X ¼ N3�; 1c and X = NCS�, 1d) and the molecular ion-pair type [Zn(L1/L2)][Zn(X)4] (2a–

2d) (L1:X ¼ N3�; 2a and X = NCS�, 2b; L2:X ¼ N3

�; 2c and X = NCS�, 2d). Elemental, spectral and other physicochemical resultsare used to characterize the complexes. Single crystal X-ray diffraction studies confirm the structure of two representative members,[Zn2(L2)(N3)4] (1c) and [Zn(L1)][Zn(NCS)4] (2b). The structural analyses reveal that each zinc atom in dinuclear 1c, has a distorted tri-gonal bipyramidal geometry with a ZnN5 chromophore, whereas in molecular ion-pair 2b, the zinc(II) ion has a distorted octahedralenvironment in the cationic part with a ZnN6 chromophore and the remaining zinc(II) ion in the anioic part has a distorted tetrahedralgeometry with a ZnN4 environment. All the complexes display intraligand 1(p–p*) fluorescence and intraligand 3(p–p*) phosphorescencein glassy solutions (77 K).� 2006 Elsevier Ltd. All rights reserved.

Keywords: Zinc(II); Azide; Thiocyanate; Hexadentate Schiff base; Luminescence

1. Introduction

Recent effort is made to the self-assembly of organic andinorganic molecules in the solid state because it extends therange of new solids with desirable physical and chemicalproperties [1,2]. The range and variety of self-assembledinorganic structures that can be constructed relies on suit-able metal–ligand interactions [3] and various types of poly-meric structures which include 1D, 2D and 3D networks.Synthesis of mono- and polynuclear coordination com-pounds of Group 12 metal ions [4–6] continues unabated

0277-5387/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2006.05.037

* Corresponding authors. Tel.: +91 342 2553913; fax: +91 342 2530452.E-mail addresses: hafijur_am@yahoo.co.in (S.H. Rahaman), barin_1@

yahoo.co.uk (B.K. Ghosh).

for the preparation of functional materials [7–10]. Exploit-ing the veracity of the coordination geometry around metalion templates, different networks can be accessed usingvaried organic blockers. We are also interested in this field[11–13] through the variation of the metal ion coordinationenvironments using 3d/4d metal ion templates, organicligands and suitable bridging units. Zinc is well suited[4,14] to this study as its d10 configuration permits a widerange of symmetries and coordination numbers and it hasa significant role in bioinorganic chemistry. Schiff bases[11a,11b–13a,15] are useful blockers because of their prepa-rational accessibility, structural varieties and varied dentic-ities. Pseudohalides are efficient intermediaries; of them theflexidentate homoatomic azide [4a,11c–13a,16] and ambid-entate heteroatomic thiocyanate [11a,11b,17] ions are the

Scheme 1.

R. Ghosh et al. / Polyhedron 25 (2006) 3104–3112 3105

most important ones, which form various networks withdifferent transition and non-transition metal ions in combi-nation with organic spacers of varied denticities. In an effortto isolate and study new forms of such aggregates, we havebeen exploring the coordination behaviour of two differenthexadentate Schiff base ligands (Scheme 1) towards zinc(II)in combination with pseudohalides. In this paper, we havereported the synthetic details, spectral characterizationand different physicochemical properties of two series ofzinc(II) pseudohalide complexes, along with the X-ray crys-tal structures of representative members, [Zn2(L2)(N3)4] (1c)and [Zn(L1)][Zn(NCS)4] (2b), where the azides and thiocya-nates, established bridging ligands [16,17] in dimeric andpolymeric systems, act as terminal ligands instead of adopt-ing a bridging mode in the presence of a bis(tridentate)Schiff base.

2. Experimental

2.1. Materials and methods

High purity pyridine-2-carboxaldehyde (Lancaster,UK), triethylenetetramine (Lancaster, UK), 2-[4-(2-amino-ethyl)-piperazin-1-yl]ethylamine (Aldrich, USA), sodiumazide (Aldrich, USA), ammonium thiocyanate (Fluka, Ger-many) and zinc(II) nitrate tetrahydrate (E. Merck, India)were purchased from the respective concerns and used asreceived. Zinc(II) perchlorate hexahydrate was preparedby treatment of zinc carbonate (E. Merck, India) with per-chloric acid (E. Merck, India) followed by slow evaporationon a steam-bath, then filtration through a fine glass-frit, andit was preserved in a desiccator containing concentrated sul-furic acid for subsequent use. All other chemicals and sol-vents were AR grade and were used as received.

Caution! Azido and perchlorate compounds of metalions are potentially explosive especially in the presence oforganic ligands. Only a small amount of the materialsshould be prepared and they should be handled with care.

2.2. Physical measurements

Elemental analyses (carbon, hydrogen and nitrogen)were performed on a Perkin–Elmer 2400 CHNS/O elemen-tal analyzer. IR spectra (KBr discs, 4000–300 cm�1) were

recorded using a Perkin–Elmer FT-IR model RX1 spec-trometer. Molar conductances were measured using a Sys-tronics conductivity meter where the cell constant wascalibrated with 0.01 M KCl solution and dry MeOH wasused as the solvent. Ground state absorption and steady-state fluorescence measurements were made with a Jascomodel V-530 UV–Vis spectrophotometer and Hitachimodel F-4010 spectrofluorimeter, respectively. Time-resolved fluorescence measurements were carried out usinga time-correlated single photon counting (TCSPC) spec-trometer Edinburgh Instruments, model 199; a hydrogenfilled coaxial flash lamp with a pulse width of 1.2 ns atFWHM and a Philips XP-2020Q Photopmultiplier tubewere, respectively, used as the excitation source and thefluorescence detector.

2.3. Preparation of Schiff bases

The Schiff bases L1 and L2 were prepared followingusing reported method [15] with a little modification.Details are described below.

2.3.1. N-(1-Pyridin-2-ylformylidene)-N 0-[2-({2-[(1-

pyridin-2-ylformylidene)amino]ethyl}amino)-ethyl]ethane-

1,2-diamine (L1)Pyridine-2-carboxaldehyde (0.214 g, 2 mmol) was

refluxed with triethylenetetramine (0.146 g, 1 mmol) indehydrated alcohol (15 cm3). After 10 h the reaction solu-tion was evaporated under reduced pressure to yield agummy mass, which was dried and stored in vacuo overCaCl2 for subsequent use. Yield: 0.212 g (80%). Anal. Calc.for C18H24N6 (L1): C, 66.63; H, 7.45; N, 25.91. Found: C,66.58; H, 7.41; N, 25.86%. IR (KBr, cm�1): 1590 (mC@N).UV–Vis (kmax, nm): 266, 280.

2.3.2. N-(1-Pyridin-2-ylformylidene)-N 0-[2-(4-{2-[(1-

pyridin-2ylformylidene)amino]ethyl}-piperazin-1-yl)-

ethyl]amine (L2)

L2 was prepared similarly using 2-[4-(2-aminoethyl)-pip-erazin-1-yl]ethylamine [18] instead of triethylenetetramine.Yield: 0.212 g (80%). Anal. Calc. for C20H28N6 (L2): C,68.14; H, 8.01; N, 23.84. Found: C, 68.09; H, 7.94; N,23.79%. IR (KBr, cm�1): 1592 (mC@N). UV–Vis (kmax,nm): 242, 290.

3106 R. Ghosh et al. / Polyhedron 25 (2006) 3104–3112

2.4. Preparation of the complexes

Complexes 1a–1d were prepared from the perchloratesalt of zinc(II) using a 2:1:4 mole ratio of the metal,L1/L2 and azide/thiocyanate. Using a 1:1 mole ratio ofmetal–L1/L2 and a 1:4 mole ratio of metal-azide/thiocya-nate 2a–2d were isolated.

2.4.1. Synthesis of [Zn2(L1)(N3)4] (1a) and

[Zn2(L1)(NCS)4] (1b)

A methanolic solution (5 cm3) of L1 (0.32 g, 1 mmol) wasadded dropwise to a solution of Zn(ClO4)2 Æ 6H2O (0.74 g,2 mmol) in the same solvent (10 cm3) then an aqueous solu-tion (5 cm3) of sodium azide (0.26 g, 4 mmol) was mixedslowly. The light yellow solution was filtered and the super-natant liquid was kept in air for slow evaporation. After afew days, the fine microcrystalline compound 1a that sepa-rated out was washed with toluene and dried in vacuo oversilica gel indicator. Compound 1b was prepared by the sameprocedure using ammonium thiocyanate in place of sodiumazide. Yield: 0.91 g (70%). Anal. Calc. for C13H24N18Zn2

(1a): C, 27.72; H, 4.29; N, 44.76. Found: C, 27.67; H,4.23; N, 44.71%. IR (KBr, cm�1): 1595 (mC@N), 2042 ðmN3

Þ.UV–Vis (kmax, nm): 330. Yield: 0.49 g (72%). Anal. Calc.for C17H24N10S4Zn2 (1b): C, 32.55; H, 3.85; N, 22.33.Found: C, 32.48; H, 3.79; N, 22.281%. IR (KBr, cm�1):1595 (mC@N), 2050, 2071 (mNCS). UV–Vis (kmax, nm): 335.

2.4.2. [Zn2(L2)(N3)4] (1c) and [Zn2(L2)(NCS)4] (1d)

To zinc(II) perchlorate tetrahydrate (0.74 g, 2 mmol) inmethanol (10 cm3), faint yellow L2 (0.35 g, 1 mmol) wasadded dropwise. To this solution a methanolic solutionof sodium azide (0.26 g, 4 mmol) was added with constantstirring. The final light brown solution was filtered and leftfor slow evaporation in air. After a few days fine whitishbrown plate shaped single crystals of 1c, suitable for X-ray measurements, separated out, and these were washedwith toluene and dried in vacuo over silica gel indicator.The same route was followed to prepare 1d only usingammonium thiocyanate instead of sodium azide. Yield:0.91 g (70%). Anal. Calc. for C20H28N18Zn2 (1c): C,36.88; H, 4.33; N, 38.71. Found: C, 36.80; H, 4.25; N,38.67%. IR (KBr, cm�1): 1595 (mC@N), 2045 ðmN3

Þ. UV–Vis (kmax, nm): 330. Yield: 0.50 g (70 %). Anal. Calc. forC24H28N10S4Zn2 (1d): C, 40.29; H, 3.94; N, 19.58. Found:C, 40.20; H, 3.87; N, 19.51%. IR (KBr, cm�1): 1595 (mC@N),2050, 2071 (mNCS). UV–Vis (kmax, nm): 328.

2.4.3. Synthesis of [Zn(L1)][Zn(N3)4] (2a) and

[Zn(L1)][Zn(NCS)4] (2b)

2.4.3.1. Method A. An aqueous solution (5 cm3) of NaN3

(0.26 g, 4 mmol) was added to a solution ofZn(NO3)2 Æ 4H2O (0.24 g, 1 mmol) in the same solvent(10 cm3) with the formation of a colourless solution. In ananother container Zn(NO3)2 Æ 4H2O (0.24 g, 1 mmol) andL1 (0.32 g, 1 mmol) were mixed together in MeOH(10 cm3) producing a yellowish solution. This coloured solu-

tion was added dropwise to the above colourless solution.The final light brown mixture was filtered and left for slowevaporation in air. Light yellow compound 2a separatedout. X-ray quality cubic single crystals of 2b were preparedby the same procedure using NH4NCS in place of sodiumazide. Yield: 0.51 g (75%). Anal. Calc. for C13H24N18Zn2

(2a): C, 27.72; H, 4.29; N, 44.76. Found: C, 27.67; H, 4.21;N, 44.71%. IR (KBr, cm�1): 1601 (mC@N), 2145 ðmN3

Þ. UV–Vis (kmax, nm): 330. Yield: 0.49 g (72%). Anal. Calc. forC17H24N10S4Zn2 (2b): C, 32.55; H, 3.85; N, 22.33. Found:C, 32.50; H, 3.84; N, 22.27%. IR (KBr, cm�1): 1592 (mC@N),2065, 2071 (mNCS). UV–Vis (kmax, nm): 330.

2.4.3.2. Method B. Zn(NO3)2 Æ 4H2O (0.48 g, 2 mmol), L1

(0.32 g, 1 mmol) and NaN3 (0.26 g, 4 mmol) were mixedin a single container containing 20 cm3 methanol, whichresulted in a light brown solution. The mixture was filteredand kept for slow evaporation in air. Light yellow rectangu-lar crystals of 2a were formed after a week. NH4NCS wasused in place of NaN3 to isolate 2b by the same process.Yield: 0.51 g (75%). Anal. Calc. for C13H24N18Zn2 (2a): C,27.72; H, 4.29; N, 44.76. Found: C, 27.67; H, 4.21; N,44.71%. IR (KBr, cm�1): 1595 (mC@N), 2065, 2052 ðmN3

Þ.UV–Vis (kmax, nm): 326. Yield: 0.49 g (72%). Anal. Calc.for C17H24N10S4Zn2 (2b): C, 32.55; H, 3.85; N, 22.33.Found: C, 32.50; H, 3.84; N, 22.27%. IR (KBr, cm�1):1595 (mC@N), 2065, 2071 (mNCS). UV–Vis (kmax, nm): 330.

2.4.4. Synthesis of [Zn(L2)][Zn(N3)4] (2c) and

[Zn(L2)][Zn(NCS)4] (2d)2.4.4.1. Method A. To a methanolic solution (10 cm3) of L2

(0.35, 1 mmol) a solution of Zn(NO)3 Æ 4H2O (0.24 g,1 mmol) was added slowly in a glass beaker. In an anothercontainer, a solution of sodium azide (0.26 g, 4 mmol) wasadded to Zn(NO)3 Æ 4H2O (0.24 g, 1 mmol) in methanol(10 cm3). The former yellow solution was transferred drop-wise to the latter colourless solution yielding a whitishbrown mixture. This was filtered and left for slow evapora-tion in air. Whitish brown rectangular crystals were foundafter a few days. A similar procedure was applied to yield2d except using NH4NCS in place of NaN3. Yield: 0.51 g(75%). Anal. Calc. for C20H28N18Zn2 (2c): C, 36.87; H,4.33; N, 38.71. Found: C, 36.77; H, 4.28; N, 38.67%. IR(KBr, cm�1): 1602 (mC@N), 2040 ðmN3

Þ. UV–Vis (kmax, nm):328. Yield: 0.49 g (72%). Anal. Calc. for C24H28N10S4Zn2

(2d): C, 40.29; H, 3.95; N, 19.58. Found: C, 40.21; H,3.87; N, 19.50%. IR (KBr, cm�1): 1595 (mC@N), 2124, 2072(mNCS). UV–Vis (kmax, nm): 332.

2.4.4.2. Method B. Zn(NO)3 Æ 4H2O (0.48 g, 2 mmol), L2

(0.35 g, 1 mmol) and sodium azide (0.26 g, 4 mmol) weremixed in a single pot giving a brownish solution whichon slow evaporation in air yielded 2c. When ammoniumthiocyanate was used in place of sodium azide in the samereaction procedure, 2d was separated out. Yield: 0.51 g(75%). Anal. Calc. for C13H24N18Zn2 (2c): C, 36.87; H,4.33; N, 38.71. Found: C, 36.77; H, 4.28; N, 38.67%. IR

R. Ghosh et al. / Polyhedron 25 (2006) 3104–3112 3107

(KBr, cm�1): 1602 (mC@N), 2040 ðmN3Þ. UV–Vis (kmax, nm):

328. Yield: 0.49 g (72%). Anal. Calc. for C24H28N10S4Zn2

(2d): C, 40.29; H, 3.95; N, 19.58. Found: C, 40.21; H,3.87; N, 19.50%. IR (KBr, cm�1): 1595 (mC@N), 2124,2072 (mNCS). UV–Vis (kmax, nm): 332.

2.5. X-ray diffraction study

A plate-shaped single crystal (0.10 · 0.15 · 0.20 mm3) of1c and a cube-shaped single crystal (0.30 · 0.30 ·0.30 mm3) of 2b were obtained by slow evaporation ofMeOH solutions of the appropriate reaction mixtures. Col-ourless crystals, suitable for X-ray crystallographic analy-sis, were selected following examination under amicroscope. Diffraction data at 297 K and 296 K for 1c

and 2b, respectively, were collected on a Bruker-SMARTCCD diffractometer using Mo Ka radiation (k =0.71073 A). Systematically absent reflections led to theidentification of space group P21/n (No. 14) for 2b. Ofthe 3159 and 6968 total reflections for the two respectivecomplexes, 2349 and 4192 with [I > 2r(I)] were used forstructure solutions. The structures were solved by directmethods, and the structure solution and refinement were

Table 1Crystallographic data for 1c and 2b

Compound 1c 2b

Formula C20H28N18Zn2 C22H26N10S4Zn2

Crystal system triclinic monoclinicSpace group P�1 P21/na (A) 7.9068(5) 9.7898(6)b (A) 9.1408(6) 15.6644(9)c (A) 9.6317(6) 19.1902(11)a (�) 74.0710(10) 90b (�) 83.5010(10) 90.8610(10)c (�) 87.3770(10) 90V (A3) 665.02(7) 2942.5(3)k (A) 0.71073 0.71073qcalc (g cm�3) 1.627 1.557Z 1 4T (K) 297 296l (mm�1) 1.853 1.946F(000) 334 1408Crystal size (mm) 0.10 · 0.15 · 0.20 0.30 · 0.30 · 0.30h Ranges (�) 2.2–28.3 1.7–28.3h/k/l �10,10/�11,

12/�12,12�12,12/�19,20/�22,25

Reflections collected 7061 6968Independent

reflections (Rint)3159 (0.033) 4192 (0.033)

Tmax and Tmin 0.8309 and 0.6903 0.563 and 0.563Data/restraints/parameters 3159/0/181 6968/0/343Goodness-of-fit on F2 1.000 0.868Final R indices

[I > 2r(I)]2349 4192

R indices (all data) R1 = 0.0360,wR2 = 0.0884

R1 = 0.0560,wR2 = 0.1070

Largest peakand hole (e A�3)

�0.60 and 0.56 �0.36 and 0.37

Weighting scheme: Calculated w ¼ 1=½r2ðF 2oÞ þ ð0:0339P Þ2 þ 0:5866P � ð1cÞ;

w ¼ 1=½r2ðF 2oÞ þ ð0:0520PÞ2 þ 2:2367P � ð2dÞ where P ¼ ðF 2

o þ 2F 2cÞ=3.

based on jFj2. All non-hydrogen atoms were refined withanisotropic displacement parameters whereas hydrogenatoms were placed in calculated positions when possibleand given isotropic U values 1.2 times that of the atomto which they are bonded. At convergence the final residu-als were R1 = 0.0360; wR2 = 0.0884 for 1c and R1 = 0.0560and wR2 = 0.1070 for 2b with [I > 2r(I)], goodness-of-fit = 1.000 and 0.868 for 1c and 2b, respectively. The finaldifferences Fourier map showed the minimum and maxi-mum peak heights at �0.60 and 0.56 e A�3 for 1c and�0.36 and 0.37 e A�3 for 2b with no chemical significance.All calculations were carried out using SHELXL-97 [19] andORTEP-32 [20]. The crystal data and data collectionparameters are listed in Table 1.

3. Results and discussion

3.1. Synthesis and formulation

The ligands (L1 and L2) were synthesized by refluxingtriethylenetetramine with pyridine-2-carboxaldehyde and2-[4-(2-aminoethyl)-piperazin-1-yl]ethylamine, respectively,in a 1:2 mole ratio in boiling alcohol (Scheme 2).

Colourless powders of the pentacoordinated dinuclearcomplexes [Zn2(L1/L2)(X)4] (1a–1d) (L1:X ¼ N3

�; 1a andX = NCS�, 1b; L2:X ¼ N3

�; 1c and X = NCS�, 1d) wereproduced in good yield through the reaction of a 2:1:4molar ratio of the metal salts, organic blockers and pseudo-halides. The process is summarized in the followingequation:

ZnðClO4Þ2 � 6H2Oþ L1=L2 þ 4NaN3=NH4NCS

��!MeOH

298 K½Zn2ðL1=L2ÞðNCS=N3Þ4� ð1Þ þ 4NaClO4 þ 6H2O

ð1ÞThe homobimetallic complexes [Zn(L1/L2)][Zn(X)4] (2a–2d) (L1:X ¼ N3

�; 2a and X = NCS�, 2b; L2:X ¼ N3�; 2c

and X = NCS�, 2d) were initially synthesized by the reac-tion of 2:1:4 molar ratios of the metal salts, organic block-ers and pseudohalides at room temperature (298 K) withthe aim to obtain homopolymers [Zn2(L1/L2)(NCS)4]n.However, to our surprise ion pair complexes were obtainedin low yields. The molecular ion, [Zn(NCS)4]2�, was gener-ated in solution by addition of four moles of ammoniumthiocyanate to one mole of zinc nitrate tetrahydrate inmethanol. The complementary unit [Zn(L1/L2)]2+ was pre-pared by dropwise addition of one mole of L1/L2 to onemolar zinc nitrate tetrahydrate in methanol in a separatecontainer. Compound 2 resulted in good yield throughunion of the [Zn(L1/L2)]2+ and [Zn(NCS)4]2� buildingblocks. The details of the sequential reactions are summa-rized in the following equations:

ZnðNO3Þ2 � 4H2Oþ L1=L2

��!MeOH

298 K½ZnðL1=L2Þ�ðNO3Þ2 þ 4H2O ð2Þ

Scheme 2.

3108 R. Ghosh et al. / Polyhedron 25 (2006) 3104–3112

ZnðNO3Þ2 � 4H2Oþ 4NH4NCS=NaN3

��!MeOH

298 KðNH4Þ2½ZnðNCS=N3Þ4� þ 2NH4NO3 þ 4H2O

ð3Þ

½ZnðL1=L2Þ�ðNO3Þ2 þ ðNH4Þ2½ZnðNCS=N3Þ4�

��!MeOH

298 K½ZnðL1=L2Þ�½ZnðNCS=N3Þ4�ð2Þ þNH4NO3

ð4ÞThe complexes were characterized using microanalytical,spectroscopic and physicochemical results. The air-stablemoisture-insensitive complexes are soluble in commonsolvents like methanol, ethanol, acetonitrile, dimethyl-formamide and dimethylsulfoxide. In methanol solutioncomplex 1 behaves as neutral and complex 2 shows aconductivity value of �140 X�1 cm2 mol�1 which reflectsmolecular association of the complex ions [21].

3.2. Spectral studies

In the IR spectra, the point of interest for 1a, 1c, 2a and2c is the band due to the azide. The spectra exhibit a verystrong and sharp absorption band at 2040–2060 cm�1 dueto an asymmetric stretching vibration of the azide. Wellresolved peaks attributed to m(NCS) are observed for 1b,1d, 2b and 2d at 2124, 2075, 2048 cm�1. The presence of

Fig. 1. ORTEP plot of [Zn2(L2)(N3)4] (1c) with the atom numbering s

multiple bands in line with the different force constantsof thiocyanates are observed. A band corresponding tothe m(CS) stretching frequency appears at 769 cm�1. Thebands belonging to the deformation frequency d(NCS)are found as doublets at 468 and 472 cm�1. The bimetallicassembly shows bands around 1590–1630 cm�1 due to theC@N stretching frequency of the Schiff base and weakbands in the range 2980–2900 cm�1 are assignable to thealiphatic C–H stretching frequency.

3.3. X-ray crystal structure

3.3.1. [Zn2(L2)(N3)4] (1c)

In order to define the coordination sphere conspicu-ously, a single-crystal X-ray diffraction study was made.An ORTEP diagram with the atom numbering scheme ofthe dinuclear unit in 1c is shown in Fig. 1. Selected bonddistances and bond angles relevant to the Zn coordinationsphere are given in Table 2. The crystal lattice of 1c consistsof neutral dinuclear [Zn2(L2)(N3)4] units. The coordinationpolyhedron around zinc is best described as distorted trigo-nal bipyramidal, with a ZnN5 chromophore. The distortionfrom ideal trigonal bipyramidal geometry is due to theasymmetric nature of the bound hexadentate Schiff baseand the deviations of the angles formed at the metal center(Table 2). The metal ion is ligated by one pyridine nitrogen

cheme and 20% probability ellipsoids for all non-hydrogen atoms.

Table 2Selected bond distances (A) and bond angles (�) for 1c

Bond distances (A)

Zn–N(1) 2.0515(19) N(2)–N(5) 1.191(3)Zn–N(2) 1.960(3) N(5)–N(6) 1.143(3)Zn–N(3) 1.977(3) N(3)–N(4) 1.175(4)Zn–N(8) 2.289(2) N(4)–N(7) 1.138(5)Zn–N(9) 2.3645(18)

Bond angles (�)

N(1)–Zn–N(2) 127.60(10) N(2)–Zn–N(8) 93.97(9)N(1)–Zn–N(3) 118.76(12) N(2)–Zn–N(9) 108.31(9)N(1)–Zn–N(8) 74.68(8) N(3)–Zn–N(8) 94.20(10)N(1)–Zn–N(9) 78.57(8) N(3)–Zn–N(9) 92.17(9)N(2)–Zn–N(3) 112.90(13) N(8)–Zn–N(9) 152.24(7)

R. Ghosh et al. / Polyhedron 25 (2006) 3104–3112 3109

(N8), one amine nitrogen (N9), one imine nitrogen (N1)and two terminal azide nitrogens (N2, N3). One iminenitrogen (N1) and two terminal azide nitrogens (N2, N3)occupy the equatorial positions of the distorted trigonalbipyramid, whereas the axial positions are occupied bythe one pyridine nitrogen (N8) and the amine nitrogen(N9). Both the equatorial Zn–N distances [1.960(3)–2.3645(18) A] and bond angles [112.90–127.60�] have somesignificant variation in that plane. The axial Zn–N dis-tances are 2.289(2) and 2.3645 (18) A. The axial bond angleN8–Zn–N9 [152.24(7)�] deviates from the ideal 180�. Thesum (359.26�) of the equatorial angles N2–Zn1–N3(112.90�), N1–Zn1–N3 (118.76�) and N1–Zn1–N2(127.60�) are very close to 360.00�. So the atoms N1, N2,

Table 3Hydrogen bond distances (A) and angles (�) for 1c

D–H� � �A D–H(A)

H� � �A(A)

D� � �A(A)

\D–H� � �A(�)

C4–H4� � �N7a 0.9300 2.5000 3.430(5) 174.00C6–H6A� � �N6b 0.9700 2.3500 3.216(4) 149.00C7–H7B� � �N7c 0.9700 2.6200 3.487(5) 149.00C10–H10A� � �N3 0.9700 2.5800 3.134(4) 116.00C10–H10B� � �N5d 0.9700 2.5400 3.488(4) 165.00

Symmetry codes: (a) 1 � x, 2 � y, 1 � z; (b) 1 � x, 1 � y, 2 � z; (c) x,�1 + y, z; (d) 2 � x, 1 � y, 2 � z.

Fig. 2. C–H� � �N hydrogen bonded 1D

N3 and Zn1 are almost in a least-squares plane. In the ter-minal azide ligands, the N2–N5 [1.191(3) A] and N3–N4[1.175 (4) A] distances are longer than N5–N6[1.143(3) A] and N4–N7 [1.138(5) A], respectively, whichindicates that the nitrogen atoms (N2 and N3) of the twoterminal azides are coordinated to the zinc(II) centre.

In the crystal lattice the dinuclear units are packed withweak C–H� � �N hydrogen bonding. The nitrogen atoms(N3, N5, N6 and N7) from terminal azide ligands arehydrogen bond acceptors and C–H (H4, H6A, H7B,H10A and H10B) hydrogens of the organic ligand skeletonare donors (Table 3). An infinite 1D supramolecular chain(Fig. 2) results through C–H� � �N hydrogen bonding inter-actions [C(4)–H(4)� � �N(7), 2.5 A, \C(4)–H(4)� � �N(7),174�; \C(6)–H(6A)� � �N(6), 2.32 A, 149�; \C(7)–H(7B)� � �N(7), 2.62 A, 149�; C(10)–H(10A)� � �N(3), 2.58 A, \C(10)–H(10A)� � �N(3), 116�; C(10)–H(10B)� � �N(5), 2.54 A,\C(10)–H(10B)� � �N(5), 165�]. This chain is furtherstabilized by strong face-to-face p� � �p interactions fromtwo terminal pyridine rings [ring(3)–ring(3): Cg(3)–Cg(3)i

separation: 4.5395(19) A, vertical displacement of Cg:3.471 A; i1 � x, 2 � y, 1 � z. Ring3 = N(8)–C(1)–C(2)–C(3)–C(4)–C(5)] (Table 4).

3.3.2. [Zn(L1)][Zn(NCS)4] (2b)

An ORTEP view with the atom numbering scheme ofthe ion-pair complex 2b is shown in Fig. 3. Selected bonddistances and bond angles relevant to the Zn coordinationsphere are given in Table 5. In the solid state 2b consists of[Zn(L1)]2+ cations and [Zn(NCS)4]2� anions. The coordi-nation polyhedron around zinc(II) is best described as a

chain of 1c along the ac-plane.

Table 4p� � �p Interaction for 1c

Ring(i)! Ring(j) Dihedralangle(i,j)(�)

CentroidseparationCg–Cg (A)

Verticaldisplacementof Cg’s (A)

Ring3! Ring3i 0.00 4.5395(19) 3.471

Symmetry code: (i) 1 � x, 2 � y, 1 � z. Ring3 = N(8)–C(1)–C(2)–C(3)–C(4)–C(5).

Fig. 3. Perspective view of [Zn(L1)][Zn(NCS)4] (2b) with the atom numbering scheme and 20% probability ellipsoids for all non-hydrogen atoms.

Table 5Selected bond distances (A) and bond angles (�) for 2b

Bond distances (A)

Zn(2)–N(5) 2.219(3) N(1)–C(1) 1.149(5)Zn(2)–N(6) 2.201(2) N(2)–C(3) 1.136(6)Zn(2)–N(7) 2.163(3) N(3)–C(4) 1.137(5)Zn(2)–N(8) 2.211(3) N(4)–C(2) 1.136(6)Zn(2)–N(9) 2.077(2) S(1)–C(1) 1.614(4)Zn(2)–N(10) 2.080(3) S(2)–C(2) 1.609(5)Zn(1)–N(3) 1.958(4) S(3)–C(3) 1.589(4)Zn(1)–N(4) 1.960(4) S(4)–C(4) 1.610(4)Zn(1)–N(1) 1.935(3)Zn(1)–N(2) 1.975(4)

Bond angles (�)

N(5)–Zn(2)–N(6) 153.74(9) N(9)–Zn(2)–N(10) 172.70(11)N(5)–Zn(2)–N(7) 80.55(11) N(1)–Zn(1)–N(4) 108.35(16)N(5)–Zn(2)–N(8) 96.05(10) N(1)–Zn(1)–N(2) 110.61(15)N(5)–Zn(2)–N(9) 78.24(10) N(1)–Zn(1)–N(3) 115.12(15)N(5)–Zn(2)–N(10) 108.36(11) N(3)–Zn(1)–N(4) 109.54(17)N(6)–Zn(2)–N(7) 100.67(10) N(2)–Zn(1)–N(3) 107.37(18)N(6)–Zn(2)–N(8) 94.22(9) N(2)–Zn(1)–N(4) 105.41(17)N(6)–Zn(2)–N(9) 76.14(9) S(1)–C(1)–N(1) 178.8(3)N(6)–Zn(2)–N(10) 97.57(10) S(2)–C(2)–N(4) 179.2(5)N(7)–Zn(2)–N(8) 152.60(11) S(3)–C(3)–N(2) 179.2(4)N(7)–Zn(2)–N(9) 105.14(11) S(4)–C(4)–N(3) 179.3(4)N(7)–Zn(2)–N(10) 79.45(12)N(8)–Zn(2)–N(9) 100.67(10)N(8)–Zn(2)–N(10) 75.86(11)

Table 6Hydrogen bond distances (A) and angles (�) for 2b

D–H� � �A D–H(A)

H� � �A(A)

D� � �A(A)

\D–H� � �A(�)

N7–H7� � �S2a 0.9100 2.8100 3.596(3) 146.00C13–H13B� � �S1 0.9700 2.6100 3.449(3) 144.00C15–H15A� � �S3b 0.9700 2.8500 3.478(4) 123.00C15–H15B� � �S4 0.9700 2.6300 3.582(4) 166.00

Symmetry codes: (a) �1/2 + x, 1/2 � y, �1/2 + z; (b) 1/2 + x, 1/2 � y,�1/2 + z.

3110 R. Ghosh et al. / Polyhedron 25 (2006) 3104–3112

distorted octahedron with a ZnN6 chromophore for thecationic part and a distorted tetrahedron with a ZnN4

chromophore for the anionic part. In the octahedral part,the metal ion is ligated by two pyridine nitrogens (N6,N8), two amine nitrogens (N5, N7) and two imine nitro-gens (N9, N10). The two amine nitrogens (N5, N7) andtwo pyridine nitrogens (N6, N8) occupy the equatorialpositions of the distorted octahedron, whereas the axialpositions are occupied by the two imine nitrogens (N9,N10). The equatorial Zn–N distances lie within a closerange [2.163(3)–2.219(3) A] but there is a significant varia-tion in the bond angles [80.55(11)–153.74(9)�] in that plane.The axial Zn–N distances [2.077(2) and 2.080 (3) A] arefound to be smaller than the equatorial ones. The N–C[1.136(6)–1.149(5) A] and C–S [1.589(4)–1.614(4) A] dis-tances are as expected for N-coordination [17c]. Heretetragonal compression of the octahedral geometrymay be raised from the p-acceptance of imine nitrogenatoms. Four hydrogen bonds, two of N–H� � �S type

Fig. 4. Packing in 2b viewed down the (100) plane.

Table 7Photophysical data

Sample Absorption(k (nm))

Emission (k (nm)) Life times(ns)Fluorescence Phosphorescence

1c 330 392 470 2.722b 330 390 482 2.43

R. Ghosh et al. / Polyhedron 25 (2006) 3104–3112 3111

[N(7)–H(7)� � �S(2), 2.81 A; \N(7)–H(7)� � �S(2), 146�] andthree of C–H� � �S type [C(13)–H(13B)� � �S(1), 2.61 A,\C(13)–H(13B)� � �S(1), 144�; C(15)–H(15A)� � �S(3), 2.85 A,\C(15)–H(15A)� � �S(3), 123�; C(15)–H(15B)� � �S(4), 2.63,\C(15)–H(15B)� � �S(4), 166�] link the components into aninfinite line structure (Table 6, Fig. 4).

3.4. Luminescence properties

The spectroscopic data in methanol solutions andglasses of two representative compounds are listed in Table7. Compound 1c shows an emission spectrum at 392 nmwhile compound 2b has a band at 390 nm at 298 K. Theseare assignable to intraligand 1(p–p*) fluorescence. Com-pound 2b has very weak fluorescence; the lifetimes are2.72 ns (1c) and 2.43 ns (2b), respectively. This higher life-time in 1c may be due to the presence of stronger p� � �pinteractions, which are mainly responsible for fluorescence.In glassy solutions (77 K) a red shift is observable at�470 nm which is presumably due to 3(p–p*) phosphores-cence [22].

4. Conclusion

This work reports the synthesis, structure and photo-physical behaviour of two different Zn(II) complexes withhexa- and pentacoordination environments of the metalcentre. Tetracoordinated/pentacoordinated zinc(II) is wellrecognized [23] but hexacoordination of the metal ion isscarce. Under enforced conditions, the neutral N-donorhexacoordinated Schiff base directs such coordination ofthis 3d member of Group 12.

5. Supplementary data

Crystallographic data for the structural analysis havebeen deposited with the Cambridge Crystallographic DataCentre Nos. 267534 for 1c and 267535 for 2b. Copies ofthis information can be obtained free of charge from TheDirector, CCDC, 12 Union Road, Cambridge CB2 1EZ,UK (fax: +44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

Financial support from the Department of Science andTechnology (DST), Council of Scientific and Industrial Re-search (CSIR), New Delhi, India is gratefully acknowl-edged. Ms. Fen-Ling Liao of National Science Council in

Taiwan is acknowledged for her data collection of X-raydiffraction.

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