Noncovalent Syntheses of Supramolecular Organo Gelators - Crystal

CRYSTAL GROWTH & DESIGN

Noncovalent Syntheses of Supramolecular Organo Gelators

2006 VOL. 6, NO. 3 763-768

Darshak R. Trivedi, Amar Ballabh, and Parthasarathi Dastidar* Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, BhaVnagar - 364 002, Gujarat, India ReceiVed NoVember 7, 2005; ReVised Manuscript ReceiVed December 21, 2005

ABSTRACT: Four new organo gelators, namely, dicyclohexylammonium hydrogen cyclobutane-1,1-dicarboxylate (1), dicyclohexylammonium cyclobutane-1,1-dicarboxylate (2), dibenzylammonium hydrogen cyclobutane-1,1-dicarboxylate (3), and dibenzylammonium cyclobutane-1,1-dicarboxylate (4), have been noncovalently synthesized. Single-crystal structures of all the gelators were determined to make an attempt to correlate their structure with their properties (gelation). The results support the conclusion that a one-dimensional hydrogen-bonded network in the crystal structure appears to be one of the prerequisites for gelation behavior. Introduction Multistep organic syntheses of functional materials often lead to frustration. With deeper understanding of various noncovalent interactions that are responsible for molecules to self-assemble, a fairly new concept of supramolecular synthons has emerged.1 This new synthetic tool offers high throughput syntheses of supramolecular assemblies of desired architectures and specific properties.2 Gelation of various fluids is one of the most soughtafter material properties that are being investigated with high interest in recent years. Low molecular mass organic gelators (LMOGs)3-9 are an important class of compounds that are capable of gelling organic or aqueous fluids. These gels are thermoreversible, and the phase change from gel-sol and back can generally be achieved several times. The small gelator molecules self-assemble to form various supramolecular architectures such as one-dimensional (1D) fibers, rods, nanotubes, helices, ribbons, etc., which further entangle to form threedimensional (3D) networks within which the solvent molecules are immobilized to form gels or viscous liquids. Various applications10 in materials science and biology make this class of small organic molecules attractive, and the recent upsurge in research papers concerning LMOGs definitely confirm their worth in science and technology. However, designing LMOGs is a difficult task, and it is also impossible to predict which solvent will be hardened by a particular LMOG. The major impediment for designing LMOGs is the lack of understanding of the supramolecular architecture (crystal structure) of the metastable gel fiber in its native (gel) form. It is virtually impossible to determine the crystal structure of a gel fiber; only an indirect method using X-ray powder diffraction (XRPD) data may be applied.11 However, recording good quality XRPD data of the gel fibers in their native form generally suffers from the scattering contribution of the solvent molecules and the less crystalline nature of the gel fibers. Therefore, in most cases, attempts to record XRPD of gel fibers turn out to be a major disappointment. On the other hand, correlating the single-crystal structure of a molecule in its thermodynamically more stable crystalline state with its gelling/ nongelling behavior seems to be more practical, and our efforts in this regard have been quite useful in designing new LMOGs based on organic salts12 and other compounds.13 Organic salt based LMOGs are increasingly becoming popular in recent years14 since the preparation of such salts does not involve time* To whom correspondence should be addressed. Fax: +91-2782567562. E-mail: [email protected]; [email protected].

Scheme 1

consuming nontrivial organic syntheses, and in a relatively short period of time many salts can be prepared and scanned for their gelation ability. Moreover, the supramolecular self-assembly of such salts is based on strong and directional hydrogen bonding as well as stronger but less directional electrostatic interactions between the cations and the anions. Our studies12 along with those of others15 tend to suggest that a 1D hydrogen-bonded network in the crystal structure of a molecule promotes gelation, whereas 2D and 3D networks either produce weak gel or do not produce gel at all. Thus, we have been actively engaged in designing a new system wherein a 1D hydrogen-bonded network can be envisaged. Herein, we report noncovalent syntheses of four new organo gelators, namely, dicyclohexylammonium hydrogen cyclobutane-1,1-dicarboxylate (1), dicyclohexylammonium cyclobutane1,1-dicarboxylate (2), dibenzylammonium hydrogen cyclobutane1,1-dicarboxylate (3), and dibenzylammonium cyclobutane-1,1dicarboxylate (4) (Scheme 1) and their gelation behavior. All these salts are chosen because the hydrogen-bonding networks in these salts are likely to be 1D as may be argued based on a supramolecular synthon approach (see results and discussion section). The choice of cyclobutane-1,1-dicarboxylic acid is deliberate since in our previous observations this acid moiety plays an important role in gelation.12e Attempts have been made to correlate the single-crystal structures of all the gelators with their properties (gelation). Results and Discussion Gel Formation and Characterization. Table 1 lists the gelation data of salts 1-4. In a typical experiment, an organic solvent was added to a weighed sample in a test tube. The gelator was dissolved in the solvent with the aid of few drops of cosolvent MeOH and heating. The resultant solution was allowed to cool to room temperature under ambient conditions. The test tube was then inverted to examine the material’s

10.1021/cg050590i CCC: $33.50 © 2006 American Chemical Society Published on Web 02/04/2006

764 Crystal Growth & Design, Vol. 6, No. 3, 2006

Trivedi et al.

Table 1. Gelation Behavior of Salts 1-4a 1c sr. no.

solvent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

CCl4 petrol benzene toluene chlorobenzene bromobenzene o-xylene m-xylene p-xylene mesitylene 1,2-dichlorobenzene methyl salicylate ethyl acetate nitrobenzene cotton seed oilb eucalyptus oilb castor oilb 1,4-dioxane

bp of the solvent ° C 76-77 79-80 110 130-133 154-158 142-145 138 137 163-165 177 93-95 76 210

100-101

MGC (wt %) FC 0.5226 0.5433 0.4091 0.4085 0.4190 0.4431 0.3300 0.4000 0.3136 FC 0.3985 0.4815 0.3226 1.3025 1.0276 0.5451 0.3616

2c Tgel in °C 107 71 102 78 79 104 118 117 127 81 67 63 94 88 78 82

MGC (wt %) FC 1.5547 1.4093 0.2463 0.3839 0.3844 0.2388 0.2231 0.2141 0.2001 FC 0.8457 0.7589 0.8584 ppt 1.0324 ppt 0.3430

3b Tgel in °C 73 69 65 72 74 104 110 112 121 77 45 61 75 84

MGC (wt %) FC FC 4.8297 2.9241 2.8132 1.9539 2.0502 2.2498 2.6630 FC 1.4656 2.1003 FC FC S S S S

4b Tgel in °C

78 40 77 87 98 92 80 61 64

MGC (wt %)

Tgel in °C

2.9737 ppt ppt FC 2.6621 2.1668 4.0557 4.2827 12.4133 FC 2.8747 3.8053 FC FC S S S S

84

82 90 83 96 43 84 88

MGC ) minimum gelator concentration; Tgel ) gel-sol dissociation temperature; wt % ) g/100 mL of solvent; FC ) fibrous crystals, ppt ) precipitate; S ) solution. b g/100 g of solvent. c Concentrations of the gelators used for carrying out the gelation tests are 1 wt % for 1 and 2, 2 wt % for 3, and 3.5 wt % for 4. a

Figure 1. SEM micrographs of xerogels of (a) 1 in o-xylene, 2.5 wt % (bar 30 µm); (b) 2 in o-xylene 2.5 wt % (bar 10 µm); (c) 3 in m-xylene 4 wt % (bar 200 µm); (d) 4 in m-xylene 6 wt % (bar 100 µm).

deformity. If no deformation was observed, it was considered as a “gel”. It is clear from Table 1 that salts 1 and 2 are capable of gelling almost all the solvents including commercial fuels and edible oils, whereas 3 and 4 gel mainly nonpolar solvents. Minimum gelator concentration (MGC) and gel-sol dissociation temperatures TGel indicate that all the salts 1-4 can be considered as good gelators. However, salts of alicyclic amines (1-2) appear to be better gelators compared to their benzylic counterparts 3-4, the former displaying lower MGC and higher TGel values (Table 1). It is also interesting to note that both the 1:1 and the 1:2 salts in their respective groups have comparable gelation abilities. Scanning electron microscopy (SEM) pictures of the xerogels derived from salts 1-4 clearly depict the presence of an intertwined network of several micrometer long 1D fibers of the gelator molecules, within which, the solvent molecules are understandably immobilized to form gel (Figure 1). Structure-Property Correlation. Since the salts described herein were chosen based on rational approach, it was considered worthwhile to investigate their crystal structures and correlate them with their properties.

Figure 2. Crystal structure of 1; (top) ellipsoid (50% probability) representation of the ion pair in the asymmetric unit and hydrogen bonding with selected atom numbering; (bottom) parallel packing of 1D chains.

Crystal Structure of Dicyclohexylammonium Hydrogen Cyclobutane-1,1-dicarboxylate 1. Salt 1 crystallizes in a centrosymmetric triclinic space group Pıˆ, and its asymmetric unit contains one ion pair. No hydrogen atom could be located on either of the carboxylate moieties of the anion. Almost identical C-O bond distances (O(1)-C(1) ) 1.231(4); O(2)C(1) ) 1.294(4); O(3)-C(2) ) 1.293(4); O(4)-C(2) ) 1.226(4) Å) in the carboxylate moieties make it difficult to determine which one of the acid moieties has actually lost the proton. However, a short intramolecular O‚‚‚O distance (O(2)‚‚‚O(3) ) 2.390 Å) indicates that the COOH and COO- moieties are intramolecularly hydrogen bonded. The other oxygen atoms which are not involved in intramolecular hydrogen bonding form a strong hydrogen bond with the neighboring cationic species through N-H‚‚‚O interactions (N(1)‚‚‚O(1) ) 2.785(3) Å; ∠N(1)-H(2N1)‚‚‚O(1) ) 175(3)°) resulting in a 1D hydrogenbonded network. The 1D chains are further packed in the crystal lattice in a parallel fashion (Figure 2). Crystal Structure of Dicyclohexylammonium Cyclobutane-1,1-dicarboxylate 2. Salt 2 belongs to a monoclinic space group P21/c, and its asymmetric unit contains two dicyclohexy-

Syntheses of Supramolecular Organo Gelators

Crystal Growth & Design, Vol. 6, No. 3, 2006 765

Figure 4. Crystal structure of 3; (top) ellipsoid (50% probability) representation of the ion pair in the asymmetric unit and hydrogen bonding with selected atom numbering; (bottom) parallel packing of 1D chains; the cations and anions are shown in purple and orange, respectively. Figure 3. Illustration of crystal structure 2; (top) ellipsoid (50% probability) representation of the ion pair in the asymmetric unit and hydrogen bonding with selected atom numbering; (bottom) 2D hydrogenbonded network; the COO- moieties are colored in purple and orange; the ammonium cation is also shown in purple so that synthon A type network is highlighted. Cyclobutane backbone of the anion and cyclohexyl moiety of the cation are not shown for clarity.

lammonium cations, one cyclobutane-1,1-dicarboxylate anion, and one solvate water molecule, all located in general positions. Both the carboxylic moieties are found to be hydrogen bonded with the ammonium cations and solvate water molecules via N-H‚‚‚O and O-H‚‚‚O interactions (N‚‚‚O ) 2.696(2)-2.931(2) Å; ∠N-H‚‚‚O ) 142.5(13)-175.0(14)°). Propagation of such hydrogen-bonded interactions leads to the formation of 2D grooved networks, which are further packed in the crystal structure in a parallel fashion via dispersion forces (Figure 3). Crystal Structure of Dibenzyllammonium Hydrogen Cyclobutane-1,1-dicarboxylate 3. The crystal of salt 3 belongs to a centrosymmetric monoclinic space group P21/c. The asymmetric unit is comprised of one ion pair. The monoanion is found to form a 1D zigzag chain via O-H‚‚‚O hydrogen bond (O(3)‚‚‚O(2) ) 2.574(2) Å; ∠O(3)-H(1O3)‚‚‚O(2) ) 167.0(2)°), the grooves of which are occupied by ammoninum cations via N-H‚‚‚O interactions (N(1)‚‚‚O(1) ) 2.674(2) Å; (N1)‚‚‚O(2) ) 2.749(2) Å ∠N(1)-H(2N1)‚‚‚O(1) ) 170.0(2); ∠N(1)-H(1N1)‚‚‚O(2) ) 154.9(19)), resulting into the formation of a 1D hydrogen-bonded network of cations and anions. The chains are further packed in parallel fashion via van der Waals interactions (Figure 4). Crystal Structure of Dibenzyllammonium Cyclobutane1,1-dicarboxylate 4. Salt 4 also crystallizes in a centrosymmetric monoclinic space group P21/c. The asymmetric unit is comprised of two dibenzylammonium cations, one cyclobutane1,1-dicarboxylate anion, and two solvate water molecules, all sitting on general positions. In the crystal structure, both carboxylate moieties are found to be hydrogen bonded with ammonium cations and solvate water molecules via various N-H‚‚‚O and O-H‚‚‚O interactions (N‚‚‚O ) 2.691(4)-2.779(4) Å; ∠N-H‚‚‚O ) 164.0(4)-179.0(3)°; O‚‚‚O ) 2.670(4)2.753(4) Å; ∠O-H‚‚‚O ) 158.0(4)-172.0(7)°). Such interactions lead to the formation of a 2D hydrogen-bonded networks, which are further packed in the crystal lattice in a parallel manner via van der Waals interactions (Figure 5).

Figure 5. Crystal structure representation of 4; (top) ellipsoid (50% probability) representation of the ion pair in the asymmetric unit and hydrogen bonding with selected atom numbering; (bottom) 2D hydrogenbonded network; the COO- moieties are colored in purple and orange; the ammonium cation is also shown in purple so that synthon A type network is highlighted. Cyclobutane backbone of the anion and benzyl moiety of the cation are not shown for clarity.

If one considers the self-assembly of ion pairs in secondary ammonium monocarboxylate salts, the formation of a 1D network depends on preferential occurrence of synthon A out of the three plausible supramolecular synthons (Scheme 2). However, the possibility of formation of other two 0D networks (synthons B and C, Scheme 2) is greatly reduced if one considers the plausible self-assembly of ion pairs derived from the salts of secondary amine and dicarboxylic acids in both 1:2 and 1:1 (acid/amine) stoichiometry (Scheme 3). In 1:2 (acid/amine) salts, while supramolecular motif D can be formed using supramolecular synthon A, motif E can be realized if

766 Crystal Growth & Design, Vol. 6, No. 3, 2006 Scheme 2

synthon B is adopted. Motif F can be formed using bifurcated hydrogen-bonding synthon C. On the other hand, in 1:1(acid/ amine) salts, motif G can be envisaged if the ammonium cation can simply adhere to the surface of the frequently occurring COOH‚‚‚-OOC (carboxylic‚‚‚carboxylate) 1D chain through N-H‚‚‚O hydrogen-bonding interactions. It may be noted here that all the supramolecular motifs described in Scheme 3 display a 1D hydrogen-bonded network, which appears to be one of the main prerequisites for a molecule to show gelation properties.12,15 It may be mentioned here that none of the motifs in Scheme 3 is hypothetical as all of them are observed in various crystal structures recently noted by us.16 In the present study, it may be noted that both the 1:2 (acid/ amine) salts 2 and 4 display a 2D sheetlike network instead of a 1D network as envisaged in Scheme 3. In both cases, solvate water molecules are present. In 2, the solvate water molecule appears to be responsible for holding two anions in perpendicular fashion via hydrogen bonds with the adjacent caboxylate moieties (Figure 3). On the other hand, in 4, the two hydrogenbonded dimeric solvate water molecules interfer with the expected hydrogen bonding of the cations and anions (Figure 5). As a result of the participation of solvate water molecules in the overall hydrogen bonding in the crystal structures and typical angular topology and alicyclic backbone of the acid moiety, the overall network in these two solids turns out to be a 2D hydrogen-bonded network. However, careful observation of the networks in both the structures reveals the presence of synthon A in one-half of the acid moiety. The hydrogen-bonding pattern of the other half is disrupted because of the participation of solvate water molecules in the hydrogen-bonding network (Figures 3 and 5). Both the 1:1 (acid/amine) salts 1 and 3 display 1D hydrogenbonded network. While the network in 3 exactly resembles the envisaged network for 1:1 salt (Scheme 3), the network in 1 is similar to that depicted in Scheme 3. In this case, the usual COOH‚‚‚-OOC chain is absent. However, hydrogen bonding between the strongest donor (COOH) and acceptor (COO-) is satisfied due to the presence of such hydrogen bonding in an intramolecular fashion. It is remarkable to note that both 1:1 salts (1 and 3) display a 1D hydrogen-bonded network in their crystal structures as envisaged in Scheme 3, and excellent gelation properties of these two salts (Table 1) indicate once again that a 1D hydrogenbonded network indeed supports gelation. Thus, correlation between the thermodynamically more stable crystalline form of a compound with the corresponding material (gelation) property appears to be important in designing these soft materials. However, the other two 1:2 (acid/amine) salts, namely

Trivedi et al. Scheme 3

2 and 4, display a 2D network and show gelation properties. Since solvate water molecules participate in the hydrogenbonding network in these crystal structures, it is difficult to comment on the structure-property correlation in these salts. Conclusions Thus, we have successfully demonstrated the aptitude of supramolecular synthon concepts in designing new LMOGs. On the basis of the well-established supramolecular synthons, the series of salts 1-4 have been prepared, and delightfully, all of them showed excellent gelation properties. While 1:1 (acid/ amine) salts 1 and 3 display an expected 1D hydrogen-bonded network, which is one of the prerequisite for gelation behavior, the corresponding 1:2 (acid/amine) salts 2 and 4 show a 2D network presumably because of the presence of solvate water molecules participating in the hydrogen-bonding network. However, these two salts also show excellent gelation properties, indicating the fact that the hydrogen-bonding network of these two gelators in their native state (gel state) might be adopting a 1D hydrogen-bonding network, which is definitely a possibility as envisaged in Scheme 3. The facile syntheses of the series of organo gelators exploiting the concept of noncovalent synthesis (supramolecular synthesis) presented here should encourage chemists working in the area of design and synthesis of organic materials. Experimental Section Materials. All chemicals (Aldrich) and the solvents (A. R. grade, S. D. Fine Chemicals, India) for gelation were used without further purification. All the oils were procured from the local market. Microanalyses were performed on a Perkin-Elmer elemental analyzer 2400, Series II. FT-IR and NMR spectra were recorded using PerkinElmer spectrum GX and 200 MHz Bruker Avance DPX200 spectrometers, respectively. SEM was performed on a LEO 1430VP. Syntheses. Salts 1-4 were prepared by mixing acid and corresponding amine in the required stoichiometry in MeOH at room temperature. The reaction mixture was then evaporated at room temperature, and the salts were isolated as white solids in quantitative yield. Analytical Data for Dicyclohexylammonium Hydrogen Cyclobutane-1,1-dicarboxylate 1. m.p. 190 °C. Anal. Calc. for C18H31NO4: C, 66.43; H, 9.60; N, 4.30. Found: C, 66.51; H, 9.61; N, 4.29%. FTIR (KBr): 3698, 3433, 2998, 2941, 2858, 2763, 2668, 2558, 2522,

Syntheses of Supramolecular Organo Gelators

Crystal Growth & Design, Vol. 6, No. 3, 2006 767 Table 2. Crystallographic Data of Salts 1-4

empirical formula FW crystal size (mm) crystal system space group a/Å b/Å c/Å R/° β/° γ/° volume/Å3 Z Dcalc F(000) µ Mo KR (mm-1) temperature (K) observed reflns [I > 2σ(I)] parameters refined goodness of fit final R1 on observed data final wR2 on observed data

1

2

3

4

C18H30NO4 324.43 0.649 × 0.253 × 0.182 triclinic P1h 9.1651(13) 11.0434(16) 11.1107(16) 115.760(2) 94.814(3) 111.369(2) 903.0(2) 2 1.193 354 0.083 100(2) 2100 216 1.285 0.0510 0.1678

C30H56N2O5 524.77 0.968 × 0.826 × 0.813 monoclinic P2(1)/C 11.7773(8) 21.0614(14) 13.4050(9) 90.00 115.3460(10) 90.00 3005.0(3) 4 1.160 1160 0.077 100(2) 3585 358 1.041 0.0324 0.0832

C20H23NO4 341.39 0.756 × 0.427 × 0.389 monoclinic P2(1)/C 8.6185(9) 20.265(2) 10.3832(11) 90.00 97.464(2) 90.00 1798.1(3) 4 1.261 728 0.088 100(2) 2126 238 1.199 0.0346 0.1013

C34H42N2O6 574.70 0.893 × 0.728 × 0.719 monoclinic P2(1)/C 14.9159(17) 9.1832(11) 23.998(3) 90.00 107.853(2) 90.00 3128.9(6) 4 1.218 1232 0.083 100(2) 3327 411 1.074 0.0719 0.1965

2467, 1930, 1724, 1622, 1558, 1493, 1450, 1364, 1312, 1292, 1244, 1204, 1154, 1114, 1059, 982, 922, 891, 813, 788, 759, 736, 697, 635, 591, 553, 511, 484, 448, 428 cm-1. Dicyclohexylammonium Cyclobutane-1,1-dicarboxylate 2. m.p. 172 °C. Anal. Calc. for C30H54N2O4‚H2O: C, 68.66; H, 10.76; N, 5.34. Found: C, 68.44; H, 10.81; N, 5.24. FT-IR (KBr): 3737, 3567, 3424, 2936, 2856, 2810, 2756, 2724, 2694, 2631, 2596, 2520, 2421, 2359, 2165, 2056, 1936, 1597, 1457, 1410, 1387, 1365, 1333, 1239, 1215, 1156, 1121, 1052, 1037, 976, 954, 922, 887, 850, 821, 788, 756, 706, 627, 593, 556, 526, 481, 446, 421 cm-1. Dibenzylammonium Hydrogen Cyclobutane-1,1-dicarboxylate 3. m.p. 130 °C. Anal. Calc. for C20H23NO4: C, 70.36; H, 6.79; N, 4.10. Found: C, 70.12; H, 7.47, N, 4.47%. H NMR (200 MHz, CD3OD): δ ) 7.48-7.40 (10H, m); 4.17 (4H, s); 2.52-2.44 (4H, m); 2.13-2.00 (2H, m); FT-IR (KBr): 3424, 2999, 2948, 2863, 2792, 2732, 2673, 2579, 2457, 2365, 1957, 1773, 1701, 1614, 1563, 1496, 1456, 1422, 1369, 1282, 1209, 1198, 1149, 1112, 1043, 1015, 978, 952, 904, 884, 846, 826, 793, 760, 747, 700, 653, 618, 593, 575, 522, 485, 436 cm-1. Dibenzylammonium Cyclobutane-1,1-dicarboxylate 4. m.p. 58 °C. Anal. Calc. for C34H38N2O4‚2H2O: C, 71.06; H, 7.37; N, 4.87. Found: C, 72.04; H, 7.98, N, 4.90%. 1H NMR (200 MHz, CD3OD): δ ) 7.397.36 (20H, m); 3.99 (8H, s); 2.52-2.44 (4H, m); 2.10-1.98 (2H, m); FT-IR (KBr): 3274, 3061, 3033, 2983, 2943, 2865, 2825, 2793, 2729, 2626, 2450, 2357, 1955, 1893, 1820, 1721, 1597, 1558, 1495, 1456, 1408, 1329, 1213, 1154, 1112, 1090, 1076, 1052, 1030, 1002, 980, 948, 921, 877, 848, 794, 745, 698, 631, 588, 560, 487, 422 cm-1. Single-Crystal X-ray Diffraction. X-ray quality single crystals were grown in a slow evaporative condition at room temperature. Crystals of 1-3 from n-octane/few drops of MeOH, and 4 from n-Oct ane/few drops of EtOH, were obtained. Diffraction data were collected using Mo KR (λ ) 0.7107 Å) radiation on a SMART APEX diffractometer equipped with a CCD area detector. Data collection, data reduction, structure solution/ refinement were carried out using the software package of SMART APEX. Graphics were generated using MERCURY 1.4.17 All structures were solved by direct methods and refined in a routine manner. In all cases, nonhydrogen atoms were treated anisotropically and the hydrogen atoms attached to nitrogen were located on a difference Fourier map and refined. Whenever possible, the other hydrogen atoms were located on a difference Fourier map and refined. In the rest of the cases, the hydrogen atoms were geometrically fixed. The crystallographic parameters are listed in Table 2. Scanning Electron Microscopy. A hot solution of gelator (50 µL) was placed on the SEM sample holder and allowed to form gel, which was then dried under vacuum. The dried gel was then subjected to gold sputtering by using a Polaron SC 7620 sputter coater. The goldcoated sample was used for direct viewing using a LEO 1430VP SEM instrument.

Acknowledgment. The Ministry of Environment and Forest, New Delhi, India, is thanked for financial support. D.T. and A.B. thank CSIR for a SRF fellowship. Supporting Information Available: Crystallographic information file (CIF) for the salts 1-4. This material is available free of charge via the Internet in www.pubs.acs.org.

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