Toward the Recognition of Enolates/Dicarboxylates: Syntheses and X-ray Crystal Structures of Supramolecular Architectures of Zn(II)/Cd(II) Using 2,2′-Biimidazole A. K. Ghosh,† A. D. Jana,‡ D. Ghoshal,†,# G. Mostafa,*,‡ and N. Ray Chaudhuri*,†
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 3 701-707
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, Kolkata-700 032, India, and Department of Physics, JadaVpur UniVersity, JadaVpur, Kolkata-700 032, India ReceiVed September 14, 2005; ReVised Manuscript ReceiVed December 16, 2005
ABSTRACT: Six coordination complexes, {[M(H2biim)2(H2O)2]2+X2-}‚(nH2O) [(H2biim ) biimidazole); {M ) Zn (1), Cd (2); X ) squarate; n ) 2}; {M ) Zn (3), Cd (4); X ) glutarate; n ) 4}; {M ) Zn (5), X ) succinate; n ) 4}; {M ) Cd (6), X ) succinate; n ) 2}], have been synthesized and characterized by X-ray single-crystal structure determination. The crystal packing of all the complexes reveals that cationic [M(H2biim)2(H2O)2]2+ fragments are self-assembled by recognition of enolate/dicarboxylate anions by the uncoordinated N-H motif of H2biim resulting in one-dimensional supramolecular ribbons. The parallel ribbons are stitched across by water-mediated hydrogen bonding and π-π interactions leading to two types of topology: one is a two-dimensional brick wall (1, 2, 6), and the other is a three-dimensional architecture (3-5). In two structures (1, 2) interesting “sandwiched” π-π interactions were observed. Introduction Molecular recognition1 phenomena are the basis of most of the biological processes, and as the majority of enzyme substrates and cofactors are anionic,2 scientists have made extensive efforts to design receptors for anions to understand the mechanism of such processes and to discover new examples and applications.3-6 The most useful way of producing such receptors is based on complementarity and selectivity, which are crucial for molecular recognition using noncovalent interactions such as hydrogen bonding, π-π, C-H‚‚‚π, C-H‚‚‚O, metal-π-interaction, etc.).7,8 Indeed, the vast majority of X-ray crystal structures of protein-ligand complexes,9 small molecules exhibiting host-guest10 properties and sensor complexes,3,11 reveal noncovalent interactions as major forces that stabilize supramolecular synthons,12 which in turn, lead to supramolecular assembly. There are at least three main research goals in pursuing the designed synthesis of such receptors: (a) investigation of molecular recognition studies with designed synthetic receptors; (b) the analysis of the magnitude of individual nonbonded interactions combining crystallographic and theoretical studies; and (c) conceiving the basis of the biological recognition processes in quantitative structure-activity relationships13 as well as nanoscale devices based on noncovalent interactions.14,15 Accordingly, investigations aimed at the search for new individual interactions are of paramount importance for better rational drug design as a biological extension of the supramolecular sciences. Evidently, molecular recognition studies on anion receptors in chemistry as well as in biology are a fascinating area of future research. Central to molecular recognition studies, X-ray crystallography techniques provide accurate information on the structures of molecular complexes and the * To whom correspondence should be addressed. (N.R.C.) Fax: 91-332473 2805. E-mail:
[email protected]. (G.M.) E-mail: mostafa_ju@ yahoo.co.in. † Indian Association for the Cultivation of Science. ‡ Jadavpur University. # Present Address: Department of Chemistry, Jadavpur University, Jadavpur, Kolkata-700 032, India.
nature of the nonbonded interactions between the binding partners in the solid state. Thus, achieving the selective recognition of anions constitutes an important yet difficult task for supramolecular chemists. The main strategy for accomplishing strong and selective anion binding is based on the use of suitably designed positively charged receptors and incorporation of metal centers to withdraw electrons from receptor ligands thereby making the H centers more acidic. These in turn promote cooperative noncovalent interactions.2 Among various anions, the structural complexities of dicarboxylates make them challenging substrates to recognize in a supramolecular sense. The simplest carboxylates, for example, oxalate, malonate, fumarate, maleate, etc., are do not have flexible shapes, whereas succinate, glutarate, oxaloacetate, ketoglutarate, aspartate, and glutamate have long flexible alkyl chains that separate the individual carboxylate moieties. More complex dicarboxylate anions often possess chiral centers as well as extra functional groups. This architectural diversity and complexity account for the involvement of many of them in a wide range of biological processes. Among a large number of possibilities, we have chosen two flexible dicarboxylates, glutarate and succinate, which are different in length along with a rigid and shorter enolate (squarate). The rationale behind this choice is first to investigate the size and shape effect of the anions and then to study whether the delocalization effect adds extra stabilizing interactions both in the recognition process and in the ensuing supramolecular architecture. The metal ions zinc and cadmium have been used to study the effect of the size of metals on the stabilization of the supramolecular self-assembly. We have designed a receptor with 2,2′-biimidazole (H2biim) and Zn(II)/Cd(II) as a metal glue, since 2,2′-biimidazole has a proven efficiency in building a supramolecular structure involved in directed hydrogen-bonding interactions.16 Six new supramolecular architectures having the general formula {[M(H2biim)2(H2O)2]2+X2-}‚(nH2O) [(H2biim ) 2,2′-biimidazole); {M ) Zn (1), Cd (2); X ) squarate; n ) 2}; {M ) Zn (3), Cd (4); X ) glutarate; n ) 4}; {M ) Zn (5), X ) succinate; n ) 4}; {M ) Cd (6); X ) succinate; n ) 2}] have been isolated as single crystals. X-ray structures show that [M(H2biim)2(H2O)2 ]2+ fragments are self-assembled
10.1021/cg050473n CCC: $33.50 © 2006 American Chemical Society Published on Web 02/02/2006
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Table 1. Crystallographic Data and Details of Refinements for Complexes 1-6
empirical formula Fw temp (K) crystal syst space group a, Å b, Å c, Å R (deg) β (deg) γ (deg) V (Å3) Z µ (mm-1) reflections collected independent reflns S data [I > 2σ(I)] Ra RWb θ range F(000) a
1
2
3
4
5
6
C16H20N8ZnO8 517.63 100 triclinic P1h No. 2 7.6749(8) 7.9808(8) 9.8880(10) 95.491(2) 105.737(2) 114.892(2) 513.30(9) 1 1.261 3446 2441 1.07 2286 0.0333 0.0811 2.2-28.3 266
C16H20N8CdO8 564.65 100 triclinic P1h No. 2 7.7751(18) 7.9949(18) 9.853(2) 95.115(3) 104.819(3) 114.478(2) 525.5(2) 1 1.103 3577 2507 1.03 2401 0.0414 0.0918 2.2-28.4 284
C17H30N8ZnO10 571.86 100 monoclinic C2/c No.15 14.837(2) 7.4105(12) 21.939(4) 90 90.597(2) 90 2412.1(7) 4 1.087 7848 2956 1.04 2595 0.0382 0.0914 3.1-28.3 1192
C17H30N8CdO10 618.89 100 monoclinic C2/c No. 15 15.0996(18) 7.4915(9) 21.943(3) 90 91.050(2) 90 2481.8(5) 4 0.947 7967 3030 1.05 2866 0.0259 0.0659 2.7-28.3 1264
C16H28N8ZnO10 557.83 100 monoclinic P21/n No. 14 6.4050(7) 18.0946(19) 10.2268(10) 90 100.503(2) 90 1165.4(2) 2 1.123 7757 2865 1.07 2566 0.0317 0.0842 2.3-28.3 580
C16H24N8CdO8 568.65 100 triclinic P1h No.2 7.6450(8) 7.9146(8) 9.9039(10) 95.080(2) 95.424(2) 112.680(2) 545.32(10) 1 1.063 3679 2595 1.05 2547 0.0306 0.0737 2.8-28.3 288
R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑{w(Fo2 - Fc2)2}/∑{w(Fo2)2}]1/2.
to build the higher dimensional supramolecular structures by hydrogen bonding and π-π interactions. The enolate/dicarboxylate is nicely recognized by the uncoordinated N-H motif of the H2biim. Here two structures (1, 2), where delocalized π-electron-containing squarate is used, are also stabilized by rare “sandwiched” π-π interactions15c involving π-deficient aromatic systems. Experimental Section Materials. 2,2′-Biimidazole was synthesized following a slightly modified procedure17a of Fieselmann et al.17b All other chemicals were of AR grade. Physical Measurements. Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Perkin-Elmer 240C elemental analyzer. IR spectra were measured from KBr pellets on a Nicolet 520 FTIR spectrometer. Thermogravimetric analyses were carried out using a Perkin-Elmer DIAMOND TG/DTA machine. Synthesis. [{Zn(H2biim)2(H2O)2}(squarate)‚2H2O]n (1). An aqueous solution (5 mL) of zinc nitrate hexahydrate (0.297 g, 1 mmol) was poured slowly dropwise into a hot methanolic solution (10 mL) of biimidazole (0.268 g, 2 mmol). Disodium squarate (0.158 g, 1 mmol) dissolved in water (5 mL) was added to it. The resultant mixture was refluxed for ∼12 h and was allowed to cool to ambient temperature. This was filtered, and the filtrate was kept in a CaCl2 containing desiccator. The brown-colored single crystals suitable for X-ray analysis were obtained from the filtrate after two weeks. Yield 78%. Anal. Calcd.: C16H20N8ZnO8: C, 37.08; H, 3.86; N, 21.63%. Found: C, 37.05; H, 3.81; N, 21.61%. IR data: 3468-2784 (w), 1546 (s), 1553 (s), 1502 (s), 1354 (m), 1113 (s), 990 (m), 781 (m), 692 (m) cm-1. [{Cd(H2biim)2(H2O)2}(squarate)‚2H2O]n (2). This was synthesized by following a similar procedure adopted for 1 using cadmium nitrate tetrahydrate (0.308 g, 1 mmol), instead of zinc nitrate hexahydrate. The brown-colored single crystals suitable for X-ray analysis were obtained from the filtrate after two weeks. Yield 75%. Anal. Calcd.: C16H20N8CdO8 : C, 33.99; H, 3.54; N, 19.83%. Found: C, 33.92; H, 3.52; N, 19.81%. IR data: 3462-2642 (m), 1544 (s), 1474 (s), 1420 (s), 1405 (s), 1111 (s), 988 (m), 766 (m), 686 (m) cm-1. [{Zn(H2biim)2(H2O)2}(glutarate)‚4H2O]n (3). This was also synthesized following a similar procedure adopted for 1 using disodium glutarate (0.176 g, 1 mmol) instead of disodium squarate. The browncolored single crystals suitable for X-ray analysis were obtained from the filtrate after 7-8 days. Yield 85%. Anal. Calcd.: C17H30N8ZnO10: C, 35.67; H, 5.25; N, 19.58%. Found: C, 35.65; H, 5.23; N, 19.52%. IR data: 3490-2683 (m), 1550 (s), 1423 (s), 1355 (w), 1405 (s), 1124 (m), 1112 (w), 992 (w), 770 (m), 694 (w) cm-1.
[{Cd(H2biim)2(H2O)2}(glutarate)‚4H2O]n (4). The synthetic procedure was the same as that adopted for 3, only differing in using cadmium nitrate tetrahydrate (0.308 g, 1 mmol) instead of zinc nitrate hexahydrate. The brown-colored single crystals suitable for X-ray analysis were obtained from the filtrate after 7-8 days. Yield 78%. Anal. Calcd.: C17H30N8CdO10: C, 32.96; H, 4.85; N, 18.09%. Found: C, 32.92; H, 4.81; N, 18.05%. IR data: 3491-2672 (m), 1645 (w), 1553 (s), 1525 (s), 1354 (m), 1113 (s), 990 (m), 781 (m), 692 (m) cm-1. [{Zn(H2biim)2(H2O)2}(succinate)‚4H2O]n (5). This was synthesized by following a similar procedure adopted for 1 using disodium succinate (0.162 g, 1 mmol) instead of disodium squarate. The brown-colored single crystals suitable for X-ray analysis were obtained from the filtrate after three weeks. Yield 78%. Anal. Calcd. C16H28N8ZnO10: C, 34.42; H, 5.02; N, 20.07%. Found: C, 34.40; H, 4.99; N, 20.01%. IR data: 3459-2692 (m), 1638 (w), 1559 (s), 1530 (s), 1397 (s), 1354 (s), 1121 (s), 991(m), 789 (m), 760 (m), 692 (m), 664 (m) cm-1. [{Cd(H2biim)2(H2O)2}(succinate)‚2H2O]n (6). This was synthesized by following a similar procedure adopted for 5 using cadmium nitrate tetrahydrate (0.308 g, 1 mmol) instead of zinc nitrate hexahydrate. The brown-colored single crystals suitable for X-ray analysis were obtained from the filtrate after three weeks. Yield 76%. Anal. Calcd. C16H24N8CdO8: C, 33.75; H, 4.22; N, 19.69%. Found: C, 33.65; H, 4.19; N, 19.66%. IR data: 3373-2554 (m), 1639 (m), 1568 (s), 1231 (m), 1116 (s), 990 (m), 781 (m), 688 (m) cm-1. Crystallographic Data Collection and Refinement. Single crystals of all the six complexes were mounted on a Bruker SMART CCD diffractometer equipped with a graphite monochromator and Mo-KR (λ ) 0.71073 Å) radiation. X-ray data were collected at low temperature (100 K) with the aim of determining the hydrogen atom positions from the experimental data so that the weak interaction mediated supramolecular assembly becomes an experimental one. The structures were solved by the Patterson method and followed by successive Fourier and difference Fourier synthesis. Full matrix least-squares refinements were performed on F2 using SHELXL-9718 with anisotropic displacement parameters for all non-hydrogen atoms. The hydrogen atoms were refined isotropically, and their locations were determined from a difference Fourier map. During checking of symmetry, we found an indication of a C2/m space group for complex 5, but h + k ) 2n + 1 diffraction data did not reveal any C-centering though they were very weak but not negligible. All calculations were carried out using SHELXL 97,18 SHELXS 97,19 PLATON 99,20 ORTEP-32,21and WinGX system Ver-1.64.22 Data collection and structure refinement parameters and crystallographic data for all complexes are given in Table 1. The selected bond lengths, hydrogen-bonding interactions, as well as the π-π interaction parameters are summarized in Tables 2-4.
Recognition of Enolates/Dicarboxylates
Crystal Growth & Design, Vol. 6, No. 3, 2006 703
Figure 1. Coordination environment for all six complexes.
Results and Discussion Crystal Structures Description. Coordination Environment of the Complexes. X-ray crystallographic analysis reveals that the coordination environment for all six complexes is identical with the same building block [M(H2biim)2(H2O)2]2+ (M ) zinc for 1, 3, 5 and cadmium for 2, 4, 6) as shown in Figure 1. Four nitrogen atoms from two H2biim are chelated to the metal center, and they are arranged in trans-equatorial positions. Two oxygen atoms of water molecules occupy the trans-axial positions of the metal center, and they are almost perpendicular to the equatorial plane. The coordination environment is nearly octahedral with the MN4O2 chromophore in all the cases. Selected bond lengths and angles are given in Table 2 which are comparable to that of related reported complexes.23 Complexes 1 and 2. The crystal structure of complex 1 indicates that the unit cell consists of [Zn(H2biim)2(H2O)2]2+, squarate dianion, and two molecules of lattice water. The unit cell content of complex 2 is almost identical to that of 1, differing only in the metal center, where cadmium is replaced by zinc. Both 1 and 2 crystallize in the same space group (Table 1). Analysis of crystal packing shows that in both complexes the supramolecular architectures are also very similar. In these two species, [M(H2biim)2(H2O)2]2+ (M ) Zn/Cd), squarate dianions are attached directly to the biimidazole moiety through N-H‚‚‚O hydrogen bonding exhibiting the recognition phenomenon of the anion by the coordination complex. Such hydrogen bonding leads the mononuclear fragments to form a one-dimensional (1D) supramolecular ribbon along the [021] direction (Figure 2a). The water molecules, which are coordinated to the metal center, play an important role by taking part in hydrogen bonding with another 1D ribbon. The parallel ribbons are stitched across by water-mediated hydrogen bonding and π-π interactions leading to a two-dimensional (2D) brick wall topology (Figure 2b) in the bc plane. O1 atom of the squarate dianion is involved in both N4-H4N‚‚‚O1 and O1WH1W‚‚‚O1 hydrogen bonding, whereas O2 atom of the squarate only takes part in N2-H2N‚‚‚O2 hydrogen bonding (Table 3)
Figure 2. (a) The ribbon formed by recognition of biimidazole with squarate anion. (b) The 2D sheet with brick-wall topology formed by coordinated water-mediated hydrogen bonding and π-π interaction. The yellow spheres are lattice water molecules, which are hydrogen bonded to coordinated water molecules, are not shown for clarity. (c) Sandwiched π-π interactions shown by magenta dotted lines (interstitial water molecules are omitted).
generating R22(10) synthons in Etter’s graph notation.24 The lattice water molecules remain inside the vacant space of the brick-wall and get stabilized by hydrogen bonding (Table 3). The solid-state structure of these complexes are further stabilized by two types of π-π interactions (Table 4), one between imidazole rings of adjacent parallel ribbons and the other being
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes 1-6
M-N1 M-N3 M-O1W N1-C1 C1-C2 C2-N2 N2-C3 C3-N1 C3-C4 C4-N4 N4-C5 C5-C6 C6-N3 N3-C4 N1-M-N3
1
2
3
4
5
6
2.1617(16) 2.1209(17) 2.1820(16) 1.378(3) 1.363(3) 1.372(3) 1.347(3) 1.323(3) 1.450(3) 1.349(3) 1.374(3) 1.367(3) 1.381(3) 1.327(3) 79.36(6)
2.326(3) 2.300(3) 2.370(3) 1.382(5) 1.357(5) 1.377(5) 1.345(4) 1.319(5) 1.455(5) 1.341(5) 1.371(5) 1.356(5) 1.378(5) 1.330(4) 74.69(11)
2.0992(16) 2.1743(16) 2.2053(16) 1.378(3) 1.362(3) 1.371(3) 1.341(3) 1.331(2) 1.447(3) 1.344(3) 1.373(3) 1.365(3) 1.376(3) 1.328(3) 79.62(6)
2.3358(14) 2.2792(14) 2.3957(14) 1.374(2) 1.366(2) 1.378(2) 1.345(2) 1.333(2) 1.453(2) 1.344(2) 1.375(2) 1.362(2) 1.380(2) 1.329(2) 75.06(5)
2.1565(13) 2.1395(13) 2.1490(13) 1.3793(18) 1.363(2) 1.3829(19) 1.3469(19) 1.3196(19) 1.4524(19) 1.3416(19) 1.3768(19) 1.365(2) 1.3790(18) 1.3254(19) 79.40(5)
2.319(2) 2.318(2) 2.385(2) 1.379(3) 1.363(4) 1.375(4) 1.342(3) 1.332(3) 1.451(3) 1.346(3) 1.380(3) 1.366(3) 1.374(3) 1.324(3) 75.01(8)
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Table 3. Hydrogen-Bonding Interactions (Å, deg) for Complexes 1-6 D-H‚‚‚A
