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Inorganic Chemistry Communications 10 (2007) 61–65

Doubly bridged heptacoordinated cadmium(II) compoundscontaining a tetradentate Schiff base: Variance in molecular

and crystalline architectures with pseudohalides

Rajarshi Ghosh a,*, Sk Hafijur Rahaman a, Georgina M. Rosair b, Barindra Kumar Ghosh a,*

a Department of Chemistry, The University of Burdwan, Burdwan 713104, Indiab Department of Chemistry, Heriot-Watt University, Edinburg EH14 4AS, UK

Received 30 April 2006; accepted 25 August 2006Available online 16 September 2006

Abstract

Two heptacoordinated dinuclear complexes of the type [Cd(bpbe)(X)2]2 Æ nH2O [bpbe = N,N 0-(bis(pyridin-2-yl)benzylidene)ethane-1,2-diamine; X ¼ N�3 , n = 2, 1; X = NCS�, n = 0; 2] are prepared and the structures are solved by X-ray study that reveals interestingvariation in bridging motifs of the pseudohalides from bibridged end-on (in 1) to bibridged end-to-end (in 2). The hydrated dinuclearunits in 1 pack through O–H. . .N, C–H. . .O, and C–H. . .N hydrogen bonds along with p. . .p interaction leading to an infinite 1D chainand the dinuclear units in 2 are engaged in p. . .p interactions resulting a 2D sheet structure. They display intraligand 1(p–p*) fluorescenceand 3(p–p*) phosphorescence in glassy solutions.� 2006 Elsevier B.V. All rights reserved.

Keywords: Cadmium(II) dimers; Versatile pseudohalides; Schiff base; Superstructures; Luminescence

Metallo-organic coordination networks with variedcomplex molecular and crystalline architectures are exten-sively studied for their interesting chemical and physicalproperties and potential applications as functional materi-als [1–3]. One-pot synthesis [4] of the building units is oneof the efficient approaches to construct such topologiesbased on strong metal-ligand covalent bond [5] and multipleweak non-covalent forces like hydrogen bond [6] and p. . .pinteraction [7]. Recently, we are interested [8,9] in thisapproach for construction of coordination polymers andsupramers through variation of ligand backbones and metalion coordination environments. Different transition andnon-transition metal ions are used as templates [10], Schiffbases [11] as blocking units and pseudohalides [12] asbridges. Cadmium(II) with its d10 configuration permits awide range of symmetries and coordination numbers and

1387-7003/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2006.08.022

* Corresponding authors. Tel.: +91 342 2533913; fax: +91 342 2530452.E-mail addresses: rajarshi_chem@yahoo.co.in (R. Ghosh), barin_1

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

report on hexacoordinated cadmium(II) complexes are welldocumented [13,14]. However, heptacoordinated cad-mium(II) complexes are scarce [15]. This work stems fromour interest to build new molecular and crystalline architec-tures of cadmium(II) in combination with Schiff bases andpseudohalides like azides and thiocyanates. We have suc-cessfully prepared and characterized two heptacoordinateddimeric compounds of the type [Cd(bpbe)(X)2]2 Æ nH2O[bpbe = N,N 0-(bis(pyridin-2-yl)benzylidene)ethane-1,2-diamine; X ¼ N�3 ,

Ni Ni

C C

Np Np

bpbe

Fig. 1a. ORTEP diagram and O–H. . .N hydrogen bonding of 1 with atomlabelling scheme and 30% probability ellipsoids for all non-hydrogenatoms.

62 R. Ghosh et al. / Inorganic Chemistry Communications 10 (2007) 61–65

n = 2, 1; X = NCS�, n = 0; 2]. X-ray crystallographicstudy shows interesting change of bridging loops of thepseudohalides from bibridged four-membered end-on in 1

to bibridged eight-membered end-to-end in 2. The hydrateddinuclear units in 1 self-assemble through hydrogen bondsand p. . .p interaction leading to a 1D chain, and the dinu-clear units in 2 are engaged in p. . .p interactions resultinga 2D sheet structure. Both 1 and 2 show intraligand 1(p–p*) fluorescence and 3(p–p*) phosphorescence in glassysolutions.

The preparations [16] of 1 and 2 can be achieved in goodyield by the reaction of cadmium(II) salt, bpbe and azide/thiocyanate (X) in the 1:1:2 molar ratio in aqueous metha-nol at room temperature which should result hexacoordi-nated mononuclear complex of the type [Cd(bpbe)(X)2];instead, two such mononuclear units are covalently stackedEq. (1) by pseudohalide bridges giving rise to doublybridged heptacoordinated dinuclear compound with thislarger sized 4d ion, cadmium(II).

2CdII þ 2bpbeþ 4X !MeOH�H2O

298 K½CdðbpbeÞðXÞ2�2 ð1Þ

X-ray study [17] of 1 and 2 was made to define the coor-dination spheres and the long-range structures. Theair-stable moisture-insensitive compounds are powders,soluble in a range of common organic solvents such asmethanol, ethanol, acetonitrile and dimethylformamide,but are insoluble in water. In methanol solutions they be-have as non-electrolyte reflected from their low (KM, �5X�1 cm2 mol�1) conductivity values. In IR spectra 1 showscharacteristic asymmetric m(N3) stretch at 2096 cm�1; thesymmetric stretch ms(N3) corresponding to the end-on(EO) bridging mode is observable as a band of mediumintensity at 1330 cm�1. The inactiveness of this band forsymmetrical end-to-end (EE) coordination supports thepresence of EO motif. X-ray diffraction study confirmsthis hypothesis. In 2, two strong peaks at 2120 and2104 cm�1, which are assigned to m(CN) stretching vibra-tions; the position (>2100 cm�1) and number of signalsstrongly suggest mutual cis alignment as well as N– andS– coordinated bridging modes [18] of the bound thiocya-nate. m(C@N) and m(C@C) stretching vibrations areobserved at 1630 and 1580 cm�1 and �1640 and �1590cm�1 in free Schiff base ligand and in the metal boundstates, respectively. MeOH solutions of 1 and 2 are col-ourless and exhibit absorptions at 330 nm. Reflectancespectra [1: reflectance spectrum at 328 nm and electronicspectrum at 331 nm; 2: reflectance spectrum at 332 nmand electronic spectrum at 335 nm] in nujol and electronicspectra in MeOH solutions [19] are akin in each complexreflecting similar gross geometric and electronic structuresin solid state and in solution. The complexes exhibit in-tense absorptions centered at 330 nm which is assignedto intraligand p. . .p* transition. They show emission spec-trum at 395 nm at 298 K assignable to intraligand1(p. . .p*) fluorescence [20]. The lifetimes of 1 and 2 are2.39 and 2.42 ns, respectively. In glassy solutions (77 K)

a red shift is observable near 470 nm which is presumablydue to 3(p–p*) phosphorescence.

ORTEP diagrams of neutral dinuclear units in 1 and 2

are shown in Figs. 1a and 2a, respectively. The coordinationpolyhedron around each cadmium(II) center in each com-plex is best described as distorted pentagonal bipyramidwith CdN7 and CdN6S chromophores, respectively. In 1

cadmium(II) is connected to its neighbouring symmetryrelated counterpart [Cd(1*)] via bibridged end-on azide witha four-membered Cd–(N3)2–Cd loop whereas in 2 end-to-end thiocyanates with an eight-membered Cd–(SCN)2–Cdloop in chair conformation link the two symmetry relatedmetal centers. Heptacoordination of each metal center in1 and 2 is completed through binding to one tetradentateSchiff base (bpbe) ligand forming three five-membered che-late loops and one terminal pseudohalide. The equatorialcoordination sites around each cadmium(II) in 1 are occu-pied with two imine (Ni) nitrogen atoms [N(8) and N(9)]and two pyridine (Np) nitrogen atoms [N(7) and N(10)] ofthe tetradentate Schiff base with fifth position housed byN atom [N(1*)] of one of the end-on (EO) bibridged azidesin 1. The terminal azide N atom [N(4)] and the other EObridged N atom [N(1)] occupy the axial sites in 1. The pen-tagonal molecular plane of 2 is occupied with Ni[N(40) andN(50)] and Np [N(20) and N(60)] atoms of the Schiff base;the remaining fifth position is occupied with N atom[N(1)] of the bridging thiocyanate. The epical positions ofthe pentagonal bipyramid is occupied with N atom [N(2)]of one terminal thiocyanate and sulphur atom [S(1*)] ofthe rest bridging thiocyanate. This shows that the interest-ing variation in molecular architectures [21] is attained inthe cadmium(II) Schiff base complexes just by varying thepolyatomic pseudohalide molecular ion rod from symmetricazide to unsymmetric ambidentate thiocyanate. The degreesof distortion from ideal pentagonal bipyramidal geometryin 1/2 are reflected in the equatorial [64.57(6)–160.27(5)�]/

Fig. 1b. 1D sheet in 1 formed by p. . .p interaction.

R. Ghosh et al. / Inorganic Chemistry Communications 10 (2007) 61–65 63

[65.96(7)�–146.94(7)�] and the axial [173.72(7)�]/[161.73(7)�]angles. Ethylenic part of the Schiff base in each compoundN(8)–C(13)–C(14)–N(9)/N(40)–C(41)–C(42)–N(50) ispuckered and the pendant phenyl rings in chelated bpbeare not planar with the chelate plane. The azido bridges in1 are to some extent asymmetric [Cd(1)–N(1*),2.394(17) A and Cd–N(1), 2.3762(17) A] and the bridgingunit Cd2N2 forms a rhombohedral plane. The azido ligandsare quasi-linear as N(1)–N(2)–N(3) and N(4)–N(5)–N(6)angle is 179.2(3)� and 178.5(2)�. Similarly, quasi-linearityof thiocyanato ligands in the puckered thiocyanato loopin 2 is reflected in the bond angles of N(1)–C(1)–S(1) andN(2)–C(2)–S(2) as 177.8(2)� and 179.4�, respectively.

In the crystal lattice, the hydrated dinuclear units in 1

are engaged in three types of hydrogen bonds, O–H. . .N[O–H(1W). . .N(4), 1.89 A, �O–H(1W). . .N(4), 175�; O–H(1W). . .N(5), 2.30 A, 148�], C–H. . .O [C(12)–H(12). . .O,2.49(3) A, �C(12)–H(12). . .O, 155(2)�] and C–H. . .N

Fig. 2a. Perspective view of 2 with atom labelling scheme and

[C(1)–H(1). . .N(2), 2.61(2) A, �C(1)–H(1). . .N(2),148.5(18); C(20)–H(20). . .N(3), 2.57(4) A, �C(20)–H(20). . . N(3), 156(3)�; C(24)–H(24). . .N(6), 2.57(2) A,�C(24)–H(24). . .N(6), 173(2)�; C(26)–H(26). . .N(1),2.56(2) A, �C(26)–H(26). . .N(1), 123.9(17)�] to form aninfinite 1D chain which is further stabilized by weak face-to-face p. . .p interaction from two terminal pyridine rings[ring(3)–ring(3)]: Cg(3)–Cg(3)i separation: 4.959(2) A, ver-tical displacement of Cg: 3.349 A; i1�x,�1�y,�z;Cg(3)@C(7)–C(8)–C(9)–C(10)–C(11)–C(12) (Fig. 1b). Thedinuclear units in 2 pack alongside each other to give a2D supramolecular sheet [Fig 2b] in ab plane containingonly p. . .p interactions [ring(1)–ring(1)]: Cg(1)–Cg(1)ii sep-aration: 3.788(2) A, vertical displacement of Cg: 3.387 A;ii1�x, �y, 2�z; Cg(1)@N(20)–C(21)–C(22)–C(23)–C(24)–C(25). Four terminal pyridine rings from two closest chainsare linked through face-to-face p. . .p interactions and areresponsible for the formation of the 2D architecture. These

30% probability ellipsoids for all non-hydrogen atoms.

Fig. 2b. p. . .p interaction based 2D sheet structure in 2.

64 R. Ghosh et al. / Inorganic Chemistry Communications 10 (2007) 61–65

sheets are further stabilized by another face-to-face p. . .pinteractions from two benzene rings. Thus p. . .p interac-tions are mainly responsible for the formation of solid statestructure in 2.

In conclusion, two new photoluminous materials havebeen synthesized and X-ray crystallographically character-ized where end-on and end-to-end bridging motifs of twodifferent pseudohalides are arrested in cadmium(II) Schiffbase complexes giving rise to different molecular architec-tures. The dimeric compounds with or without crystallizedwater pack in different fashions with varied weak non-covalent forces resulting solid-state structural diversities.Chelation and congregation behaviours of related tetraden-tate N-donor Schiff bases with tailoring on ethylenic arm aswell as active hydrogen(s) on ligand network towards prep-aration of new molecular and crystalline aggregates areunder our active study.

Acknowledgement

Financial support from the DST and the CSIR, NewDelhi, India is gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.inoche.2006.08.022.

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[16] Synthesis of [Cd(bpbe)(N3)2]2 Æ 2H2O (1) and [Cd(bpbe)(NCS)-

(SCN)]2 (2): Solid Cd(ClO4)2 Æ 6H2O (0.311 g, 1 mmol) was slowlydissolved with stirring in MeOH (10 cm3). To this bpbe (0.390 g,1 mmol) dissolved in MeOH (5 cm3) was added dropwise followed bythe addition of NaN3 (0.13 g, 2 mmol) in MeOH–H2O (5 cm3). Thelight yellow solution was filtered and the supernatant liquid was keptin air for slow evaporation. After two days rectangular light yellowsingle crystals of 1 were deposited which were collected by filtrationand kept in vacuo over P4O10. Yield: 0.810 g (72%). Synthesis of 2 wascarried out similarly using NH4SCN (0.152 g, 2 mmol) instead ofazide. Yield: 0.891 g (75%). Anal. found: C, 51.60; H, 3.96; N, 23.12.Calc. For C52H48N20O2Cd2 (1): C, 51.66; H, 4.00; N, 23.17%. IR(KBr, cm�1): m(C@N) 1637, 1592; m (N3) 2050, 2073. KM (MeOH,ohm�1 cm2 mol�1): 5; UV–Vis (k, nm): 328. Anal. found: C, 54.29; H,3.53; N, 13.52. Calc. For C56H44N12S4Cd2 (2): C, 54.33; H, 3.58; N,13.57%. IR (KBr, cm�1): m(C@N) 1635, 1592; m (NCS) 2096, 2105. KM

(MeOH, ohm�1 cm2 mol�1): 4; UV–Vis (k, nm): 331.[17] Crystal structure analysis: Diffraction data were measured at 213(2) K

on a Siemens SMART CCD diffractometer (for 1) and Bruker P4 (for2) using graphite monochromated Mo Ka radiation (k = 0.7107 A).The unit cell parameters were determined by least-squares method. Of5845 and 4792 collected reflections, 4490 and 4636 unique reflectionswere recorded using the u–x scan and x technique for 1 and 2,respectively. Crystal data for 1: M.F: C52H48N20O2Cd2, F.W.:1209.92, T = 295 K, k = 0.71073 A, Monoclinic, P21/n, a =12.3333(19) A, b = 11.4574(17) A, c = 19.041(3) A, a = 90�, b =96.907(3)�, c = 90�, V = 2671.1(7) A3, Z = 2, Dc = 1.504 Mg/m3,F(000) = 1224, crystal size: 0.18 · 0.20 · 0.25 mm3, Reflection col-

lected: 15435, Final R indices [I > 2r(I)]: R1 = 0.0237, WR2 = 0.0608,R indices (all data): R1 = 0.0354, WR2 = 0.0665, Index ranges: h/k/l = 15,16/�14,13/�23,13, Independent reflections: 5845 [R(int =0.022)], Completeness to theta = 28.23� (88.4%), Tmax and Tmin =0.9630 and 0.8282, Data/restraints/parameters = 5845/0/431, Good-ness-of-fit on F2 = 1.024, Largest peak and hole (e A�3) = 0.348 and�0.367, Weighting scheme: R =

PjFoj � jFcjj/RjFo, wR2 ¼

½RwðF 2o � F 2

cÞ2=RwðF 2oÞ

2�1=2, Calc:w ¼ 1=½r2ðF 2oÞ þ ð0:0301PÞ2þ

0:7272P �ð1Þ; where P ¼ ðF 2o þ 2F 2

cÞ=3. Crystal data for 2: M.F:C56H44N12S4Cd2, F.W.: 619.04, T = 293 K, k = 0.71069 A, Triclinic,P1, a = 9.638(5) A, b = 12.001(5) A, c = 12.804(5) A, a = 87.129(5)�,b = 69.365(5)�, c = 78.886(5)�, V = 1359.7(11) A3, Z = 1, Dc = 1.512Mg/m3, F(000) = 624, crystal size: 0.42 · 0.56 · 0.24 mm3, Reflectioncollected: 5721, Final R indices [I > 2r(I)]: R1 = 0.0246,wR2 = 0.0669,R indices (all data): R1 = 0.0260, WR2 = 0.0679, Index ranges: h/k/l = �11,1/�14,14/�15,14, Independent reflections: 4792 [R(int =0.027)], Completeness to theta = 25.00� (99.8%), Tmax and Tmin =0.7874 and 0.6625, Data/restraints/parameters = 4792/0/334, Good-ness-of-fit on F2 = 1.100, Largest peak and hole (e A�3) = 0.291and �0.599, Weighting scheme: R =

PjFoj � jFcjj/RjFo, wR2 ¼

½RwðF 2o � F 2

cÞ2=RwðF 2oÞ

2�1=2, Calc:w ¼ 1=½r2ðF 2oÞ þ ð0:0339PÞ2þ

0:5866P �ð2Þ where P ¼ ðF 2oþ 2F 2

cÞ=3. Further details of the structuredetermination are available on request from the the Director,Cambridge Crystallographic Data Center, 12 Union Road, Cam-bridge, CB2 IEZ, UK (fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk) by quoting the CCDCdepository numbers 279248 for 1 and 279249 for 2.

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