Inorganic Chemistry Communications 12 (2009) 686–690

Contents lists available at ScienceDirect

Inorganic Chemistry Communications

journal homepage: www.elsevier .com/locate / inoche

Synthesis, structure and calculated NLO propertiesof [(n-Bu)2Sn-l-O-l-OH-Sn(n-Bu)2(CH3CO2)]2 and its putative derivatives

Abhinav Kumar a, Rajendra Prasad a, G. Kociok-Köhn b, Kieran C. Molloy b, Nanhai Singh a,*

a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, Indiab Department of Chemistry, University of Bath, Bath BA2 7AY, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 February 2009Accepted 23 May 2009Available online 28 May 2009

Keywords:StannoxannePseudo-cageTD-DFTFirst hyperpolarizability (b)NLO

1387-7003/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.inoche.2009.05.028

* Corresponding author. Tel.: +91 542 2307321x10E-mail addresses: nsingh@bhu.ac.in, nanhai_singh@

Title compound[(n-Bu)2Sn-l-O-l-OH-Sn(n-Bu)2(CH3CO2)]2 was obtained accidentally by the reaction ofn-Bu2SnCl2 with the metalloligand K2[Ni(CDC)2] in ethanol - water mixture while synthesizing a hetero-bimetallic complex [n-Bu2Sn Ni(CDC)2]. This has been characterized by micro analysis, UV–Vis, 1H, 13Cand 119Sn NMR spectroscopy and the single crystal X-ray. The complex forms an interesting supramolec-ular architecture via (acetate)O� � �HO hydrogen bonding interactions which generates hydrophobic‘‘pseudo-cage”. The electronic absorption bands of the title complex were assigned with the help ofthe time dependent density functional theory (TD-DFT) calculations. Density functional theory (DFT) cal-culations have been performed at the optimized molecular geometry of some of its putative derivatives e.g. trans p-amino cinnamic acid derivative 2; trans p-amino cinnamic acid and trans p-nitro cinnamic acidderivative 3; trans p-amino cinnamic acid and trans p-(N,N dimethylamino) cinnamic acid derivative 4.The first static hyperpolarizability (b) of the title compound and its derivatives were calculated with dou-ble numerical differentiation of total energies. The derivatives 3 and 4 showed three times and five timesenhancement in the non-linear optical (NLO) responses than the standard p-nitroaniline (pNA).

� 2009 Elsevier B.V. All rights reserved.

Metal-directed self-assembly has in recent years become apowerful tool for the construction of systems containing cavitieswhich possess intrinsic physical and chemical properties that arepromising for the creation of new materials and new metal-baseddrugs [1–10]. More recently, non-covalent weak molecular forcescapable of connecting these metallic subunits into looser and moreintriguing supramolecular infrastructures (such as hydrogenbonds, van der Waals forces, non-bonded contacts, and pi–pi inter-actions) have been widely investigated in structural and materialchemistry, biology, and the pharmaceutical sciences [11–16].

Among organotin compounds, organostannoxanes containingSn–O units are particularly interesting [1,17,18]. Much of the inter-est in these compounds arises from the amazing structural diver-sity [19–21]. The structural plasticity of organostannoxanesappears to be unique among the corresponding organometalliccompounds of group 14 elements. In most instances these com-pounds are prepared by the reaction of an appropriate organotinprecursor such as R3SnOSnR3, R3SnOH, (R2SnO)n or RSn(O)(OH)with a carboxylic, phosphinic, phosphonic or sulfonic acid[17,18,22]. Indeed, self-assembly of a robust n-butylstannoxanesin quantitative yield by the reaction of n-butylstannonic acid andalkyltin oxides with a number of organic acids have already beenreported [19–24].

ll rights reserved.

8; fax: +91 542 2368127.yahoo.com (N. Singh).

Theoretical calculations can help to rationalize the experimen-tal results and rank the existing molecular structures accordingto their hyperpolarizabilities (b) prior to experiment and thus topropose new promising compounds to chemists. To the best ofour knowledge the non-linear optical (NLO) properties of organo-stannoxanes have not been investigated so far. Herein, we reporta new compound [(n-Bu)2Sn-l-O-l-OH-Sn(n-Bu)2(CH3CO2)]2

which was accidentally obtained while making an approach to syn-thesize a bimetallic complex [n-Bu2Sn Ni(CDC)2]. Non-linear opti-cal properties of this compound as well as some of its putativederivatives have been investigated theoretically.

The ligand dipotassium cyanodithioimidocarbonato (K2CDC)was prepared by the literature method [25,26]. Reaction of an olivegreen metalloligand K2[Ni(CDC)2] (generated in situ by treatingNi(O2CCH3)2 and the ligand K2CDC in ethanol – water mixture)with n-Bu2SnCl2 in the same solvent mixture afforded a colourlessoxo-, hydroxo-bridged n-butylstannoxane, [(n-Bu)2Sn-l-O-l-OH-Sn(n-Bu)2(CH3CO2)]2 in 57% yield [27] instead of the anticipatedheterobimetallic adduct [28] (Scheme 1). The compound is stableat room temperature (m.p 142 �C) and can be stored at room tem-perature for indefinite period because the overall molecule is heldtogether by covalent Sn–O bonds, thus providing sufficient ther-modynamic stability for existence in the solid state.

The IR spectrum of the complex shows strong absorptions at1638, 1602, 1402 and 1365 cm�1 which correspond to the sym-metric and antisymmetric carboxylate stretches. A very strong

Fig. 3. Optimised structures of the complexes. (a) acetic acid derivative (title complex) 1; (b) trans p-amino cinnamic acid derivative 2; (c) trans p-amino cinnamic acid andtrans p-nitro cinnamic acid derivative 3; (d) trans p-amino cinnamic acid and trans p-(N,N dimethylamino) cinnamic acid derivative 4.

Fig. 1. Molecular structure of the title complex. Thermal ellipsoids are drawn at30% probability level. Except H1 all other hydrogen atoms are omitted for clarity.Selected bond lengths (Å) and angles (�): Sn1–O1 = 2.145(7), Sn1–O2 = 2.033(9),Sn1–C1 = 2.130(14), Sn1–C5 = 2.099(16), Sn2–O1 = 2.190(9), Sn2–O2 = 2.030(7),Sn2–O3 = 2.183(9), Sn2–C9 = 2.099(13), Sn2–C13 = 2.135(15), C1–Sn1–C5 =126.0(6), C1–Sn1–O2 = 119.1(5), C5–Sn1–O2 = 114.9(5), O1–Sn1–O2 = 73.6(3),C9–Sn2–C13 = 128.0(6), C9–Sn2–O2 = 113.7(5), C13–Sn2–O2 = 118.1(5), O1–Sn2–O3 =154.3(3), O3–Sn2–O2 = 81.6(3).

Fig. 2. Supramolecular association leading to pseudo cage formation via O–H� � �O(Ac) interactions.

Ni(O2CCH3)2 + 2K2CDC K2[Ni(CDC)2] + 2KO2CCH3

K2[Ni(CDC)2] + 2KO2CCH3 + n-Bu2SnCl2

[(n-Bu)2Sn-µ-O-µ-OH-Sn(n-Bu)2(CH3CO2)]2

Title complex

[n-Bu2Sn Ni(CDC)2]Heterobimetallic Adduct

C2H5OH

H2O

C2H5OH

H2O

CDC2- = cyanodithioimidocarbonate

Scheme 1. Synthesis of the title complex.

A. Kumar et al. / Inorganic Chemistry Communications 12 (2009) 686–690 687

688 A. Kumar et al. / Inorganic Chemistry Communications 12 (2009) 686–690

band around 600 cm�1 characteristic of the Sn–O–Sn linkage is as-signed to the m(Sn–O). The 1H NMR spectrum of this compoundconsists of four multiplets ranging from d 0.93–1.92 ppm whichcorrespond to the characteristic peaks of the n-butyl and acetategroups. In the 13C NMR spectrum the signals in the d 13.7–19.6 ppm range are indicative of the n-Bu group. A sharp resonanceat d 179.3 ppm indicates the presence of –OAc group in the mole-

Fig. 4. Selected orbital transitions for the title compound (orbital contour value0.05).

cule. The appearance of two distinct signals at d �183.2 and d�188.4 ppm in the 119Sn NMR spectrum of this compound suggeststwo different penta-coordination around tin.

The compound crystallizes in monoclinic space group C2/c andthe molecular structure is shown in Fig. 1. There are two moleculesin the asymmetrical unit [31]. The molecular structure of the titlecomplex as illustrated in Fig. 1 reveals distorted penta-coordina-tion geometry around each tin ranging between trigonal bipyrami-dal and square pyramidal. Geometry analysis with the index ofdegree of ‘trigonality’ s = (b � a)/60 (s = 0 for perfectly squarepyramidal geometry and s = 1 for perfectly trigonal bipyramidalgeometry) [32] for the title complex gives s value of 0.57 for termi-nal tin Sn2 and 0.43 for the central tin Sn1, indicating that the coor-dination geometry around terminal Sn2 is more trigonalbipyramidal nature than Sn1. The coordination geometry aroundthe terminal tin Sn2 is distorted trigonal bipyramidal with the axialpositions being occupied by one bridging hydroxy oxygen O1 andthe carboxylate oxygen O3 while the equatorial positions are takenup by the two carbons C9 and C13 of the n-butyl substituents andthe other bridging oxo group O2. In the case of central tin Sn1 thedistorted trigonal bipyramidal geometry comprises of axial bridg-ing oxo groups O1 and O2 while the equatorial position is occupiedby the two carbon atoms C1 and C5 of the two n-butyl substituentand the oxo group O2. The overall structure is a dimer with the twooxo groups O2 acting as l-bridges between the two tin units togenerate ladder like structure.

An analysis of the X-ray crystal structure of this compound re-veals the presence of several weak intra- and intermolecular inter-actions. Thus, the intermolecular O–H� � �O hydrogen bond [13] of2.06 Å having O1–H1� � �O4 bond angle 129� is found between theuncoordinated acetate oxygen O4 and H1 atom of the bridging hy-droxyl groups O1 of the vicinal dimer. Notably this hydrogen bondgives rise to an interesting hydrophobic ‘‘pseudo-cage” in the com-plex (Fig. 2).

Quantum chemical calculations can help to interpret the exper-imental observations and predict molecular properties. Geometryoptimizations were carried out at the level of density functionaltheory (DFT) using B3LYP functional [33,34]. The electronic absorp-tion energies and oscillator strengths for the title compound wascomputed at time dependent density functional theory (TD-DFT)methods. The 6-31G** basis set was used for C, H, N and O atomswhereas LANL2DZ basis set was used for Sn atom. All calculationsfor the title complex were done using optimized geometry. How-ever, the calculations for the derivatives 2, 3 and 4 were done usingpartially optimized geometries (where molecular core was keptfrozen) as shown in Fig. 3. All calculations were performed usingGaussian 03 program [35]. Molecular orbital plots were obtainedby MOLDEN program [36].

In CHCl3 solution, the title complex exhibits two absorptionbands at 238 and 205 nm. The TD-DFT calculations indicate thatthe lower energy band (calculated at 231 nm with oscillator

Table 1Computed absorption wavelengths (k in nm), oscillator strengths (f) and transitionnature of the title complex.

k f Major contribution

231 0.0003 na ? r*(Sn–O bond)220 0.0004 na ? p*(delocalised at acetate group)216 0.0011 na ? r*(Sn–O bond)207 0.1299 na ? r*(localised at Sn–O bond)201 0.0104 na ? r*(Sn–O bond)199 0.0042 na ? r*(Sn–O bond)193 0.0328 na ? r*(Sn–O bond)

a Lone pair located at the uncoordinated acetate oxygen.

Table 2Calculated dipole moment and static second-order polarizability (10�30 esu) of thestudied systems.

System l (D) bx by bz bvec

pNA 7.53 �13.699 0.031 0.491 13.7091 0.02 0.153 �0.077 0.244 0.2982 2.37 �0.030 0.228 �1.493 1.5103 8.91 38.880 22.659 �0.364 45.0034 8.96 62.997 �24.247 3.105 67.573

A. Kumar et al. / Inorganic Chemistry Communications 12 (2009) 686–690 689

strength 0.003) is due to the charge transfer transition from oxygenatom of acetate group to r* orbital of Sn–O bond. The next bandcalculated at 207 nm with oscillator strength of 0.1299 is assignedto HOMO-2 ? LUMO transition as illustrated in Fig. 4. Calculatedexcitation energies, oscillator strengths and assignments are listedin Table 1.

In order to gain insight into the non-linear optical (NLO) prop-erties of the title complex and its putative derivatives (Fig. 3),the first static hyperpolarizability (b) values were calculated withthe double numerical differentiation of energies. Although 27 com-ponents of b can be computed, only vector component along the di-pole moment direction is sampled experimentally in electric fieldinduced second harmonic generation (EFISH) experiments. Thusthe bvec is defined [37] as

bvec ¼ ðb2x þ b2

y þ b2z Þ

1=2

where the bx, by and bz components can be described by,

bx ¼ bxxx þ bxyy þ bxzz

by ¼ byyy þ byxx þ byzz

bz ¼ bzzz þ bzxx þ bzyy

Computed value of bvec and its components are shown in Table2. Compared with bvec of p-nitro aniline (pNA) computed at thesame level of theory, 1 and 2 show weak while 3 and 4 show strongsecond-order polarizability. The values of bvec for 3 and 4 revealthat the first static hyperpolarizability (b) can be increased byenhancing the electron donating capacity of the pendant functionalgroup at one end and keeping the remaining molecular core unal-tered. In other words, varying the molecular architecture in theperipheral region can result an increase in the dipole momentand thereby drastically enhancing the second-order NLO response.

In conclusion, the title complex [(n-Bu)2Sn-l-O-l-OH-Sn(n-Bu)2(CH3CO2)]2 forms hydrophobic pseudo-cage via O1–H1� � �O4intermolecular hydrogen bonding. A theoretical approach describ-ing the influence of conjugation, push–pull effect and dipole mo-ment on the first hyperpolarizability (b) is given. Theseobservations may be used for the rational design of new electroop-tic materials by varying the donor carboxylate group at one end.

Acknowledgements

Authors are grateful to Council of Scientific and Industrial Re-search, New Delhi for the financial assistance in the form of SRF(AK) and project no. 01(2032)/06/EMR-II (NS). SAP, Departmentof Chemistry, Banaras Hindu University, Varanasi is gratefullyacknowledged for providing computational facility.

Appendix A. Supplementary material

CCDC 729855 contains the supplementary crystallographicdata for this paper. These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre via www.ccdc.cam.a-c.uk/data_request/cif. Supplementary data associated with this

article can be found, in the online version, at doi:10.1016/j.inoche.2009.05.028.

References

[1] V. Chandrasekhar, S. Nagendran, V. Baskar, Coord. Chem. Rev. 235 (2002) 1.[2] P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem. 111 (1999) 2798;

Angew Chem. Int. Ed. 38 (1999) 2638.[3] P.J. Langley, J. Hulliger, Chem. Soc. Rev. 28 (1999) 279.[4] P.J. Stang, Chem. Eur. J. 4 (1998) 19.[5] M. Fujita, Struct. Bond. (Berlin) 96 (2000) 177.[6] B.J. Holliday, C.A. Mirkin, Angew. Chem. 113 (2001) 2076;

Angew. Chem. Int. Ed. 40 (2001) 2022.[7] R. Vilar, Angew. Chem. 115 (2003) 1498;

Angew. Chem. Int. Ed. 42 (2003) 1460.[8] C. Janiak, Dalton Trans. (2003) 2781.[9] C.N.R. Rao, S. Natarajan, R. Vaidhyanathan, Angew. Chem. 116 (2004) 1490;

Angew. Chem. Int. Ed. 43 (2004) 1466.[10] M. Nath, S. Pokharia, R. Yadav, Coord. Chem. Rev. 215 (2001) 99.[11] J.-M. Lehn, Science 295 (2002) 2400.[12] J.-M. Lehn, Supramolecular Chemistry, Concepts and Perspectives, VCH,

Weinheim, 1995.[13] G. Desiraju, T. Steiner, The Weak Hydrogen Bond in Structural Chemistry and

Biology, Oxford University Press, Oxford, 1999.[14] G.R. Desiraju, Acc. Chem. Res. 29 (1996) 441.[15] M. Nishio, CrystEngComm 6 (27) (2004) 130.[16] K. Biradha, CrystEngComm 5 (2003) 374.[17] R.R. Holmes, Acc. Chem. Res. 22 (1989) 190.[18] ERT. Tiekink, Trends Organomet. Chem. 1 (1994) 71.[19] R. García-Zarracino, H. Höpfl, Angew. Chem. 116 (2004) 1533;

Angew. Chem. Int. Ed. Engl. 43 (2004) 1507.[20] R. García-Zarracino, H. Höpfl, J. Am. Chem. Soc. 127 (2005) 3120.[21] R. García-Zarracino, R. Ramos-Quiñones, H. Höpfl, Inorg. Chem. 42 (2003)

3835.[22] V. Chandrashekhar, K. Gopal, P. Thilagar, Acc. Chem. Res. 40 (2007) 420 (and

references cited therein).[23] A. Orita, J. Xiang, K. Sakamoto, J. Otera, J. Organomet. Chem. 624 (2001) 287.[24] C. Ma, Q. Zhang, R. Zhang, D. Wang, Chem. Eur. J. 12 (2006) 420.[25] N. Singh, A. Prasad, R.K. Sinha, Inorg. Chem. Commun. 9 (2006) 1058.[26] W.A. Thaler, J.R. McDivit, J. Org. Chem. 36 (1971) 14.[27] Characterization data for the title complex: m.p. 142 �C. Anal. Calc. for

C36H80O8Sn4: C 38.75, H 7.23, Sn 42.55; Found: C 39.10, H 7.44 Sn 43.04%. IR(KBr, cm�1): 1638, 1602 (mas COO); 1402, 1365 (ms COO); 598 (m Sn–O–Sn). 1HNMR (300.40 MHz, CDCl3) d/ppm 0.93 (m, 12H, CH3), 1.37 (m, 16H, –CH2–),1.63 (d, 8H, –CH2–Sn), 1.92 (s, 3H, –OCOCH3). 13C NMR (75.45 MHz, CDCl3) d/ppm 13.7 (–CH3), 26.8, 27.0, 27.5, 29.7 (n-Bu), 179.3 (O–C@O). 119Sn NMR(111.95 MHz, CDCl3) d/ppm �183.2, �188.4.

[28] N. Singh, A. Kumar, Synth. Met. 158 (2008) 442.[29] DENZO-SCALEPACK, Z. Otwinowski, W. Minor, Processing of X-ray diffraction

data collected in oscillation mode, in: C.W. Carter Jr., R.M. Sweet (Eds.),Methods in Enzymology, Macromolecular Crystallography, Part A, vol. 276,Academic Press, 1997, pp. 307–326.

[30] L.J. Farrugia, J. Appl. Cryst. 32 (1999) 837.[31] Intensity data for the title compound were collected at 150 K on a Nonius

KappaCCD diffractometer equipped with a low temperature device, usinggraphite monochromated Mo Ka radiation (k = 0.71070 Å). Data wereprocessed using the Nonius Software [29]. A symmetry-related (multi-scan)absorption correction had been applied. Structure solution, followed by full-matrix least squares refinement was performed using the WINGX-1.70 suite ofprograms throughout [30]. Due to the weak diffracting crystal intensity datacould only be obtained to 49� in 2h. The title compound forms twoindependent dimers. In each dimer one of the n-butyl ligands showeddisorder which was refined with site occupation of 1:1. Crystallographicdata: crystal colour, colourless, crystal habit, plate, empirical formula,C36H80O8Sn4, M = 1115.79, crystal system monoclinic, space group C2/c,a = 24.2336(18), b = 11.3427(7), c = 34.806(2) Å, b = 95.170(3)�,V = 9528.4(11) Å3, Z = 4, T = 150(2) K, measured reflection = 21,600, uniquereflections = 6620, Rint = 0.0941, Final R indices [I > 2r(I)]: R1 = 0.0683,wR2 = 0.1291. R indices (all data): R1 = 0.1365, wR2 = 0.1611.

[32] A.W. Addison, T.N. Rao, J. Reedijk, J.V. Rijn, G.C. Verschoor, J. Chem. Soc., DaltonTrans. (1984) 1349.

[33] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.[34] C. Lee, Y. Wang, R.G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys. 37 (1988)

785.[35] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,

J.A. Montgomery, T. Vreven Jr., K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.

690 A. Kumar et al. / Inorganic Chemistry Communications 12 (2009) 686–690

Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.Challacombe, P.M.W. Gill, B. Johnson, W. Chen, W.M. Wong, C. Gonzalez, J.A.Pople, Gaussian, Inc., Wallingford, CT, 2004.

[36] G. Schaftenaar, J.H. Noordik, Molden: a pre- and post-processing program formolecular and electronic structures, J. Comput.-Aided Mol. Des. 14 (2000)123–134.

[37] S.P. Karna, P.N. Prasad, M. Dupuis, J. Chem. Phys. 94 (1991) 1171.