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Interplay of coordination and hydrogen bonding modes in the self-assembly of the supramolecular network in copper(II) phthalate–trimethoprim complex (0.5:1:1)
Inorganic Chemistry Communications 6 (2003) 748–751 www.elsevier.com/locate/inoche
Interplay of coordination and hydrogen bonding modes in the self-assembly of the supramolecular network in copper(II) phthalate–trimethoprim complex (0.5:1:1) S. Baskar Raj a, P. Thomas Muthiah b
a,*
, Gabriele Bocelli b, Andrea Cantoni
b
a Department of chemistry, Bharathidasan University, Tiruchirappalli 620024, India IMEM-CNR, Palazzo,Chimico-campus,Parco Area delle Scienze 17/a, I-43100 Parma, Italy
Received 14 December 2002; accepted 7 March 2003
Abstract Trimethoprim cations forming self complementary hydrogen-bonded DADA arrays interact with the copper–phthalate supramolecular frameworks through extensive hydrogen bonding. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Copper(II) complex; Crystal structure; Extensive hydrogenbond
1. Introduction The design of supramolecular architectures by a combination of coordination and weak interactions like hydrogen bonds/p–p interactions/halogen bonds is of contemporary interest [1,2]. We present here one such system. Here, copper(II) phthalate (in the ratio of 0.5:1) supramolecular anionic frameworks and trimethoprim [2,4-diamino-5-(30 , 40 , 50 -trimethoxy benzyl) pyrimidine] cations interact through extensive hydrogen bonds. In the copper(II) phthalate supramolecular motif, the copper atoms are on the 2-fold rotation axis. Each copper coordinates to four different phthalate anions – two carboxylate groups acting as bidentate asymmetric chelator and other two carboxylates acting as unidentate ligands. Copper has a distorted octahedral coordination geometry. Two adjacent copper ions (Cu Cu distance ) are bridged by two in the polymeric chain is 5.825(2) A phthalate ions on both sides leading to cavity (14 membered ring). This copper–phthalate motif (Fig. 1a) is remarkably different from other copper–phthalate motifs [3–8] reported in the literature in the sense that the present motif involves the copper(II) and phthalate
*
Corresponding author. Fax: +91-431-240-7045. E-mail address: [email protected] (P.T. Muthiah).
in the 1:2 ratio and the copper–phthalate carries an average charge of ()2). Thus, the coordination mode differs very much from those observed in the other copper–phthalate systems. There are no direct interactions between the copper–phthalate frameworks. Trimethoprim (TMP) cations are sandwiched between the copper–phthalate frameworks (Fig. 1b). TMP molecules are protonated at N1 leading to an enhancement of the internal bond angle at N1 [C2–N1– C6, 121.0(3)°] as compared with the neutral TMP [9]. A view of the extensive hydrogen bonding interactions between TMP cations and copper(II) phthalate anionic frameworks is shown in Fig. 2. The pyrimidine moieties of TMP cations are centrosymmetrically paired through a pair of N–H N hydrogen bonds involving the 2amino group and N3 atom. This is one among the 24 most frequently observed cyclic bimolecular hydrogenbonded motifs [10]. The non-coordinated oxygen (O5) of the phthalate moiety bridges the 2-amino and 4-amino groups of the paired bases through a pair of N–H O hydrogen bonds forming a ring with graph-set notation of R23 (8). This combination of base-pairing pattern and the further bridging of the bases involved in pairing by hydrogen bonds leads to the formation of a linear array of four hydrogen bonds. This is called a complementary DADA array of quadruple-hydrogen bonding pattern [11] (Scheme 1) (D stands for hydrogen bond donor and
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1387-7003(03)00097-2
S.B. Raj et al. / Inorganic Chemistry Communications 6 (2003) 748–751
749
): Cu Cu 5.825(2), Cu–O4 1.960(2), Cu–O60 Fig. 1. (a) Supramolecular framework of copper(II) phthalate. Selected atom–atom distances (A 2.706(0), Cu–O70 1.967(2). Selected bond angles (°): O60 –Cu–O70 53.77(4), O4–Cu–O60 81.20(6), O4–Cu–O4000 87.67(13), O70 –Cu–O700 88.90(12), O60 –Cu–O600 129.65(2), O4–Cu–O700 165.93(8). Symmetry transformations used to generate equivalent atoms: (0 ) x, 1 þ y, z (00 ) x, 1 þ y, 1=2 z (000 ) x, y, 1=2 z. (b) View of the TMP cations is stacked between the two copper(II) phthalate chains.
Fig. 2. View of the packing diagram showing the interactions between the copper(II) phthalate anionic frameworks and trimethoprim cations.
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S.B. Raj et al. / Inorganic Chemistry Communications 6 (2003) 748–751
tion are listed in Table 1. Extending similar work, by employing a combination of coordinated and weak interactions in the design of supramolecules will lead to further development of crystal engineering [18,19].
2. Experiment 2.1. Preparation
Scheme 1.
A stands for hydrogen bond acceptor in the linear array). This array is a readily occurring motif as evident from the fact that it has also been observed in the crystal structures of TMP–perchlorate [12], TMP–hydrogen maleate [13], TMP–salicylate monohydrate [14], TMP– salicylate methanol solvate [15], TMP–sulfate trihydrate [16] and TMP–trifluoroacetate [17]. Thus O5 acts as a trifurcated acceptor with 2-amino, 4-amino groups of the TMP moieties and C19 atom of the phthalate moiety. Strongly coordinated oxygen O7 acts as a bifurcated acceptor with 2-amino group (N2–H2A O7, 3.042(3) ) and N1 atom (N1–H1 O7, 3.020(8) A ) of the A pyrimidinium ring forming a hydrogen-bonded chelation with the graph set of R12 (6). One of the methoxy oxygen (O2) of TMP cation is hydrogen-bonded to the carbon (C21) of the phthalate moiety (C21–H21 O2, 3.376(6) ). It is interesting to note that even in the presence of A copper–phthalate infinite one-dimensional framework, TMP cations preserve a recurring hydrogen-bonded DADA array observed in several crystal structures of TMP cations. Pyrimidine rings of the TMP cations are stacked along the y-axis. The minimum slip angle (angle between the centroid vector and normal to the plane) between two neighboring pyrimidine rings is 41.29°. The interplanar and centroid-to-centroid distances are , respectively. The geometries of 3.313(2) and 4.408(9) A the various hydrogen bonds observed in this investiga-
To a warm aqueous solution containing 0.145 g (1 mmol) of trimethoprim and 0.084 g (1 mmol) of phthalic acid, an aqueous copper(II) acetate 0.100 g (1 mmol) solution was added. The resultant mixture was heated for an hour over a water-bath. Then the solution was kept for crystallization at room temperature. Blue coloured crystals (yield, 69%) appeared after 4 days. 2.2. X-ray crystallography The data were collected at room temperature on Bruker AXS Smart diffractometer with CCD (area detector). The data reduction and the absorption correction were performed with the software inserted in SHELXTL-NT V5.1. The structure was solved by direct method using the program SHELXS97 and refined by full-matrix least-squares with SHELXL97. All the nonhydrogen atoms were refined anisotropically while the hydrogen atoms, all localized in a DF map, were refined with isotropic thermal parameters. 2.3. Crystal data C44 H46 Cu N8 O14 , F :W ¼ 974:44, monoclinic, , b ¼ 5:825ð10Þ space group C2=c, a ¼ 36:735ð4Þ A 3 , A, c ¼ 22:634ð3Þ A, b ¼ 113:94ð2Þ°, V ¼ 4426:6ð11Þ A T ¼ 293 K, Z ¼ 4, Dc ¼ 1:462 g cm3 , m:p ¼ 219°, crystal size 0:11 0:19 0:27, lðMo-KaÞ ¼ 0:598 mm1 , 6427 independent measured reflections, 3419 observed reflections (I > 2rðIÞ, 2h max ¼ 30:83°), 391 parameters, F 2 refinement, R1 ¼ 0:0545 (observed), wR2 ¼ 0:1319 (all data). CCDC reference no.: 187219.
Table 1 , °) Hydrogen-bonding geometry (A D–H A i
N1–H1 O7 N2–H2A O5ii N2–H2A O7i N2–H2B N3ii N4–H4B O5 C6–H6 O4iii C7–H7A O6iv C19–H19 O5 C21–H21 O2v
D–H
HA
DA
D–H A
0.860(0) 0.780(5) 0.780(5) 0.840(9) 0.842(4) 0.924(4) 0.951(4) 0.943(6) 0.922(0)
2.240(5) 2.176(8) 2.248(5) 2.258(8) 1.970(7) 2.482(1) 2.470(6) 2.507(7) 2.492(6)
3.020(8) 2.755(8) 3.042(3) 3.098(3) 2.805(9) 3.276(7) 3.406(2) 2.839(7) 3.376(6)
150.8(5) 131.4(0) 148.6(5) 176.1(3) 171.0(2) 144.1(9) 167.6(6) 100.7(8) 160.7(0)
Symmetry codes: (i) x, 1 y, z; (ii) x, y, z; (iii) x, 1 y, 1=2 þ z; (iv) x, 2 y, 1=2 þ z; (v) 1=2 z, 1=2 þ y, 1=2 z.
S.B. Raj et al. / Inorganic Chemistry Communications 6 (2003) 748–751
2.4. IR-spectroscopy mðN–HÞ ðstrÞ¼3468:3ðsÞ;3315ðwÞ cm1 ; mðC– Haromatic Þ ðstrÞ3050ðmÞ cm1 ; mðC–HÞ str in CH3 2840 cm1 ; mðC–Caromatic Þ str1466:6ðsÞ, 1504.2(s), 1620:88ðvÞ cm1 ; mðC–NÞ str aromatic 1682:59ðsÞ cm1 ; mðC–O–CÞ str1240ðsÞ cm1 ; mðS–OÞ1359:57ðsÞ, 1391:39ðsÞ cm1 .
Acknowledgements S.B.R thanks the Council of Scientific and Industrial Research, New Delhi, India for the award of a Senior Research Fellowship [reference no. 9/475(103)2002 EMR-I].
References [1] V.G. Albano, M.C. Aragoni, M. Area, C. Castellari, F. Demartin, F.A. Devillanova, F. Isaia, V. Lippolis, L. Loddo, G. Verani, Chem. Commun. (2002) 1170. [2] M.M. Chowdhry, D.M.P. Mingos, A.J.P. White, D.J. Williams, Chem. Commun. (1996) 899. [3] H.-X. Zang, B.-S. Kang, A.W. Xu, Z.-N. Chen, Z.-Y. Zhou, A.C.S. Chan, K.B. Yu, C. Ren, J. Chem. Soc., Dalton Trans. (2001) 2559.
751
[4] M.B. Cingi, A.M.M. Lanfredi, A. Tripicchio, M.T. Camellini, Acta Cryst. B 37 (1981) 2159. [5] M.B. Cingi, Acta Cryst. B 35 (1979) 312. [6] M.B. Cingi, A.M.M. Lanfredi, A. Tripicchio, M.T. Camellini, Acta Cryst. B 34 (1978) 134. [7] M.B. Cingi, A.M.M. Lanfredi, A. Tripicchio, M.T. Camellini, Acta Cryst. B 34 (1978) 406. [8] C.K. Prout, J.R. Carruthers, F.J.C. Rossotti, J. Chem. Soc. A 21 (1971) 3350. [9] T.F. Koetzle, G.J.B. Williams, J. Am. Chem. Soc. (1976) 2074. [10] F.H. Allen, P.R. Raithby, G.P. Shields, R. Tailor, Chem. Commun. (1998) 1043. [11] R.P. Sijbesma, F.H. Beijer, L. Brunsveld, B.J.B. Folmer, J.H.K.K. Hirschberg, R.F.M. Lange, J.K.L. Lowe, E.W. Meijer, Science 278 (1997) 1601. [12] P.T. Muthiah, B. Umadevi, N. Stanley, G. Bocelli, A. Cantoni, Acta Cryst. E 58 (2002) 59. [13] P. Prabakaran, J.J. Robert, P.T. Muthiah, G. Bocelli, L. Righi, Acta Cryst. C 57 (2001) 459. [14] S. Murugesan, P.T. Muthiah, Academy Discussion Meeting on Frontiers in Structural Chemistry, IIT, Chennai, India, abstract no. 3.4, 1996. [15] P. Panneerselvam, N. Stanley, P.T. Muthiah, Acta Cryst. E 58 (2002) 180. [16] P.T. Muthiah, B. Umadevi, N. Stanley, X. Shui, D.S. Eggleston, Acta Cryst. E 57 (2001) 1179. [17] S. Francis, P.T. Muthiah, G. Bocelli, L. Righi, Acta Cryst. E 58 (2002) 717. [18] C.B. Aakeroy, A.M. Beauty, Aust. J. Chem. 54 (2001) 409. [19] G.R. Desiraju, Curr. Sci. 81 (2001) 1038.
Interplay of coordination and hydrogen bonding modes in the self-assembly of the supramolecular network in copper(II) phthalate–trimethoprim complex (0.5:1:1) S. Baskar Raj a, P. Thomas Muthiah b
a,*
, Gabriele Bocelli b, Andrea Cantoni
b
a Department of chemistry, Bharathidasan University, Tiruchirappalli 620024, India IMEM-CNR, Palazzo,Chimico-campus,Parco Area delle Scienze 17/a, I-43100 Parma, Italy
Received 14 December 2002; accepted 7 March 2003
Abstract Trimethoprim cations forming self complementary hydrogen-bonded DADA arrays interact with the copper–phthalate supramolecular frameworks through extensive hydrogen bonding. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Copper(II) complex; Crystal structure; Extensive hydrogenbond
1. Introduction The design of supramolecular architectures by a combination of coordination and weak interactions like hydrogen bonds/p–p interactions/halogen bonds is of contemporary interest [1,2]. We present here one such system. Here, copper(II) phthalate (in the ratio of 0.5:1) supramolecular anionic frameworks and trimethoprim [2,4-diamino-5-(30 , 40 , 50 -trimethoxy benzyl) pyrimidine] cations interact through extensive hydrogen bonds. In the copper(II) phthalate supramolecular motif, the copper atoms are on the 2-fold rotation axis. Each copper coordinates to four different phthalate anions – two carboxylate groups acting as bidentate asymmetric chelator and other two carboxylates acting as unidentate ligands. Copper has a distorted octahedral coordination geometry. Two adjacent copper ions (Cu Cu distance ) are bridged by two in the polymeric chain is 5.825(2) A phthalate ions on both sides leading to cavity (14 membered ring). This copper–phthalate motif (Fig. 1a) is remarkably different from other copper–phthalate motifs [3–8] reported in the literature in the sense that the present motif involves the copper(II) and phthalate
*
Corresponding author. Fax: +91-431-240-7045. E-mail address: [email protected] (P.T. Muthiah).
in the 1:2 ratio and the copper–phthalate carries an average charge of ()2). Thus, the coordination mode differs very much from those observed in the other copper–phthalate systems. There are no direct interactions between the copper–phthalate frameworks. Trimethoprim (TMP) cations are sandwiched between the copper–phthalate frameworks (Fig. 1b). TMP molecules are protonated at N1 leading to an enhancement of the internal bond angle at N1 [C2–N1– C6, 121.0(3)°] as compared with the neutral TMP [9]. A view of the extensive hydrogen bonding interactions between TMP cations and copper(II) phthalate anionic frameworks is shown in Fig. 2. The pyrimidine moieties of TMP cations are centrosymmetrically paired through a pair of N–H N hydrogen bonds involving the 2amino group and N3 atom. This is one among the 24 most frequently observed cyclic bimolecular hydrogenbonded motifs [10]. The non-coordinated oxygen (O5) of the phthalate moiety bridges the 2-amino and 4-amino groups of the paired bases through a pair of N–H O hydrogen bonds forming a ring with graph-set notation of R23 (8). This combination of base-pairing pattern and the further bridging of the bases involved in pairing by hydrogen bonds leads to the formation of a linear array of four hydrogen bonds. This is called a complementary DADA array of quadruple-hydrogen bonding pattern [11] (Scheme 1) (D stands for hydrogen bond donor and
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1387-7003(03)00097-2
S.B. Raj et al. / Inorganic Chemistry Communications 6 (2003) 748–751
749
): Cu Cu 5.825(2), Cu–O4 1.960(2), Cu–O60 Fig. 1. (a) Supramolecular framework of copper(II) phthalate. Selected atom–atom distances (A 2.706(0), Cu–O70 1.967(2). Selected bond angles (°): O60 –Cu–O70 53.77(4), O4–Cu–O60 81.20(6), O4–Cu–O4000 87.67(13), O70 –Cu–O700 88.90(12), O60 –Cu–O600 129.65(2), O4–Cu–O700 165.93(8). Symmetry transformations used to generate equivalent atoms: (0 ) x, 1 þ y, z (00 ) x, 1 þ y, 1=2 z (000 ) x, y, 1=2 z. (b) View of the TMP cations is stacked between the two copper(II) phthalate chains.
Fig. 2. View of the packing diagram showing the interactions between the copper(II) phthalate anionic frameworks and trimethoprim cations.
750
S.B. Raj et al. / Inorganic Chemistry Communications 6 (2003) 748–751
tion are listed in Table 1. Extending similar work, by employing a combination of coordinated and weak interactions in the design of supramolecules will lead to further development of crystal engineering [18,19].
2. Experiment 2.1. Preparation
Scheme 1.
A stands for hydrogen bond acceptor in the linear array). This array is a readily occurring motif as evident from the fact that it has also been observed in the crystal structures of TMP–perchlorate [12], TMP–hydrogen maleate [13], TMP–salicylate monohydrate [14], TMP– salicylate methanol solvate [15], TMP–sulfate trihydrate [16] and TMP–trifluoroacetate [17]. Thus O5 acts as a trifurcated acceptor with 2-amino, 4-amino groups of the TMP moieties and C19 atom of the phthalate moiety. Strongly coordinated oxygen O7 acts as a bifurcated acceptor with 2-amino group (N2–H2A O7, 3.042(3) ) and N1 atom (N1–H1 O7, 3.020(8) A ) of the A pyrimidinium ring forming a hydrogen-bonded chelation with the graph set of R12 (6). One of the methoxy oxygen (O2) of TMP cation is hydrogen-bonded to the carbon (C21) of the phthalate moiety (C21–H21 O2, 3.376(6) ). It is interesting to note that even in the presence of A copper–phthalate infinite one-dimensional framework, TMP cations preserve a recurring hydrogen-bonded DADA array observed in several crystal structures of TMP cations. Pyrimidine rings of the TMP cations are stacked along the y-axis. The minimum slip angle (angle between the centroid vector and normal to the plane) between two neighboring pyrimidine rings is 41.29°. The interplanar and centroid-to-centroid distances are , respectively. The geometries of 3.313(2) and 4.408(9) A the various hydrogen bonds observed in this investiga-
To a warm aqueous solution containing 0.145 g (1 mmol) of trimethoprim and 0.084 g (1 mmol) of phthalic acid, an aqueous copper(II) acetate 0.100 g (1 mmol) solution was added. The resultant mixture was heated for an hour over a water-bath. Then the solution was kept for crystallization at room temperature. Blue coloured crystals (yield, 69%) appeared after 4 days. 2.2. X-ray crystallography The data were collected at room temperature on Bruker AXS Smart diffractometer with CCD (area detector). The data reduction and the absorption correction were performed with the software inserted in SHELXTL-NT V5.1. The structure was solved by direct method using the program SHELXS97 and refined by full-matrix least-squares with SHELXL97. All the nonhydrogen atoms were refined anisotropically while the hydrogen atoms, all localized in a DF map, were refined with isotropic thermal parameters. 2.3. Crystal data C44 H46 Cu N8 O14 , F :W ¼ 974:44, monoclinic, , b ¼ 5:825ð10Þ space group C2=c, a ¼ 36:735ð4Þ A 3 , A, c ¼ 22:634ð3Þ A, b ¼ 113:94ð2Þ°, V ¼ 4426:6ð11Þ A T ¼ 293 K, Z ¼ 4, Dc ¼ 1:462 g cm3 , m:p ¼ 219°, crystal size 0:11 0:19 0:27, lðMo-KaÞ ¼ 0:598 mm1 , 6427 independent measured reflections, 3419 observed reflections (I > 2rðIÞ, 2h max ¼ 30:83°), 391 parameters, F 2 refinement, R1 ¼ 0:0545 (observed), wR2 ¼ 0:1319 (all data). CCDC reference no.: 187219.
Table 1 , °) Hydrogen-bonding geometry (A D–H A i
N1–H1 O7 N2–H2A O5ii N2–H2A O7i N2–H2B N3ii N4–H4B O5 C6–H6 O4iii C7–H7A O6iv C19–H19 O5 C21–H21 O2v
D–H
HA
DA
D–H A
0.860(0) 0.780(5) 0.780(5) 0.840(9) 0.842(4) 0.924(4) 0.951(4) 0.943(6) 0.922(0)
2.240(5) 2.176(8) 2.248(5) 2.258(8) 1.970(7) 2.482(1) 2.470(6) 2.507(7) 2.492(6)
3.020(8) 2.755(8) 3.042(3) 3.098(3) 2.805(9) 3.276(7) 3.406(2) 2.839(7) 3.376(6)
150.8(5) 131.4(0) 148.6(5) 176.1(3) 171.0(2) 144.1(9) 167.6(6) 100.7(8) 160.7(0)
Symmetry codes: (i) x, 1 y, z; (ii) x, y, z; (iii) x, 1 y, 1=2 þ z; (iv) x, 2 y, 1=2 þ z; (v) 1=2 z, 1=2 þ y, 1=2 z.
S.B. Raj et al. / Inorganic Chemistry Communications 6 (2003) 748–751
2.4. IR-spectroscopy mðN–HÞ ðstrÞ¼3468:3ðsÞ;3315ðwÞ cm1 ; mðC– Haromatic Þ ðstrÞ3050ðmÞ cm1 ; mðC–HÞ str in CH3 2840 cm1 ; mðC–Caromatic Þ str1466:6ðsÞ, 1504.2(s), 1620:88ðvÞ cm1 ; mðC–NÞ str aromatic 1682:59ðsÞ cm1 ; mðC–O–CÞ str1240ðsÞ cm1 ; mðS–OÞ1359:57ðsÞ, 1391:39ðsÞ cm1 .
Acknowledgements S.B.R thanks the Council of Scientific and Industrial Research, New Delhi, India for the award of a Senior Research Fellowship [reference no. 9/475(103)2002 EMR-I].
References [1] V.G. Albano, M.C. Aragoni, M. Area, C. Castellari, F. Demartin, F.A. Devillanova, F. Isaia, V. Lippolis, L. Loddo, G. Verani, Chem. Commun. (2002) 1170. [2] M.M. Chowdhry, D.M.P. Mingos, A.J.P. White, D.J. Williams, Chem. Commun. (1996) 899. [3] H.-X. Zang, B.-S. Kang, A.W. Xu, Z.-N. Chen, Z.-Y. Zhou, A.C.S. Chan, K.B. Yu, C. Ren, J. Chem. Soc., Dalton Trans. (2001) 2559.
751
[4] M.B. Cingi, A.M.M. Lanfredi, A. Tripicchio, M.T. Camellini, Acta Cryst. B 37 (1981) 2159. [5] M.B. Cingi, Acta Cryst. B 35 (1979) 312. [6] M.B. Cingi, A.M.M. Lanfredi, A. Tripicchio, M.T. Camellini, Acta Cryst. B 34 (1978) 134. [7] M.B. Cingi, A.M.M. Lanfredi, A. Tripicchio, M.T. Camellini, Acta Cryst. B 34 (1978) 406. [8] C.K. Prout, J.R. Carruthers, F.J.C. Rossotti, J. Chem. Soc. A 21 (1971) 3350. [9] T.F. Koetzle, G.J.B. Williams, J. Am. Chem. Soc. (1976) 2074. [10] F.H. Allen, P.R. Raithby, G.P. Shields, R. Tailor, Chem. Commun. (1998) 1043. [11] R.P. Sijbesma, F.H. Beijer, L. Brunsveld, B.J.B. Folmer, J.H.K.K. Hirschberg, R.F.M. Lange, J.K.L. Lowe, E.W. Meijer, Science 278 (1997) 1601. [12] P.T. Muthiah, B. Umadevi, N. Stanley, G. Bocelli, A. Cantoni, Acta Cryst. E 58 (2002) 59. [13] P. Prabakaran, J.J. Robert, P.T. Muthiah, G. Bocelli, L. Righi, Acta Cryst. C 57 (2001) 459. [14] S. Murugesan, P.T. Muthiah, Academy Discussion Meeting on Frontiers in Structural Chemistry, IIT, Chennai, India, abstract no. 3.4, 1996. [15] P. Panneerselvam, N. Stanley, P.T. Muthiah, Acta Cryst. E 58 (2002) 180. [16] P.T. Muthiah, B. Umadevi, N. Stanley, X. Shui, D.S. Eggleston, Acta Cryst. E 57 (2001) 1179. [17] S. Francis, P.T. Muthiah, G. Bocelli, L. Righi, Acta Cryst. E 58 (2002) 717. [18] C.B. Aakeroy, A.M. Beauty, Aust. J. Chem. 54 (2001) 409. [19] G.R. Desiraju, Curr. Sci. 81 (2001) 1038.