Influence of Aromatic Substituents on the Supramolecular

Sep 17, 2005 - All the drums show an extensive supramolecular organization mediated by ... Accordingly, drum 1 forms a one-dimensional supramolecular ...
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CRYSTAL GROWTH & DESIGN

Influence of Aromatic Substituents on the Supramolecular Architectures of Monoorganooxotin Drums Chandrasekhar,*,†

Gopal,†

Vadapalli Kandasamy Alexander Steiner,‡ and Stefano Zacchini‡

Selvarajan

2006 VOL. 6, NO. 1 267-273

Nagendran,†

Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, India, and Department of Chemistry, UniVersity of LiVerpool, LiVerpool-L69 7ZD, U.K. ReceiVed July 7, 2005

ABSTRACT: The reaction of n-BuSn(O)(OH) with various substituted benzoic acids affords hexameric organostannoxane drums, [n-BuSn(O)OC(O)R]6, where R ) 2,6-(CH3)2-C6H3 (1), 4-CH3-C6H4 (2), 4-NH2-C6H4 (3) and 2-NH2-C6H4 (4). The central stannoxane motif (Sn6O6) is similar in all these compounds and is surrounded by six substituted benzoate groups. All the drums show an extensive supramolecular organization in the solid state. Accordingly, drum 1 forms a one-dimensional supramolecular assembly mediated by noncovalent interactions such as C-H‚‚‚O and π‚‚‚π interactions. Similarly, drums 2-4 form interesting three-dimensional supramolecular assemblies mediated by C-H‚‚‚O, N-H‚‚‚N, C-H‚‚‚π, and π‚‚‚π interactions in the solid state. The role of the peripheral aromatic substituents in determining the final course of the supramolecular assembly is discussed. Introduction The assembly of ordered supramolecular structures through the application of crystal engineering principles for the realization of functional solids is an area of considerable contemporary interest.1-13 Utilizing inorganic motifs for preparing supramolecular assemblies entails the use of coordination principles and involves an appropriate design of the ligand as well as the metal precursor.14-25 Such strategies have not yet been fully developed in main-group organometallic systems, although there is considerable potential for this class of compounds to be used as building blocks in supramolecular assemblies.26-30 Among main-group organometallic compounds, organooxotin cages have been receiving considerable attention in recent years in view of their structural diversity.31-35 Apart from their interesting molecular structures many organotin compounds can organize themselves into interesting supramolecular structures by a combination of covalent and noncovalent interactions. Admittedly this field is quite nascent, but the recent results in this area are quite exciting. Recently, Ho¨pfl and co-workers utilized covalent as well as noncovalent interactions for the assembly of a diorganotin carboxylate-based three-dimensional porous supramolecular network.36-39 We have recently reported supramolecular networks of organostannoxanes containing ferrocenyl and other substituents.40-44 In these assemblies, the supramolecular formation is assisted by a combination of noncovalent interactions including O-H‚‚‚O, C-H‚‚‚O, CH‚‚‚F, C-F‚‚‚π, C-H‚‚‚π, and π-stacking interactions. To understand the influence of secondary interactions on supramolecular structures in organostannoxane compounds, we have decided to adopt a systematic approach. We wished to retain a common stannoxane structural motif and vary the nature of the substituent. Accordingly, herein, we report a detailed investigation on the synthesis and molecular and supramolecular structures of hexameric organostannoxanes, [n-BuSn(O)OC(O)R]6, where R ) 2,6-(CH3)2-C6H3 (1), 4-CH3-C6H4 (2), 4-NH2-C6H4 (3), and 2-NH2-C6H4 (4). These compounds, also known as drums, contain a structurally similar stannoxane unit, * To whom correspondence should be addressed. Tel: +91-512-2597259. Fax: +91-512-2590007/2597436. E-mail: [email protected]. † Indian Institute of Technology, Kanpur. ‡ University of Liverpool.

which is made up of a prismatic Sn6O6 core. The peripheral aromatic substituents of these compounds, on the other hand, have subtle variations allowing an assessment of various individual secondary interactions in directing the final supramolecular assembly. Experimental Section Solvents were freshly distilled over suitable drying agents. nButyltinhydroxideoxide and 2,6-dimethylbenzoic acid (Aldrich) and 4-methylbenzoic acid, 4-aminobenzoic acid, and anthranilic acid (Spectrochem, India) were purchased and used as received. Melting points were measured using a JSGW melting point apparatus and are uncorrected. Elemental analyses were carried out using a Thermoquest CE instruments model EA/110 CHNS-O elemental analyzer. 1H and 119 Sn NMR spectra were obtained on a JEOL-JNM LAMBDA 400 model spectrometer using CDCl3 solutions (unless specified) with chemical shifts referenced to tetramethylsilane and tetramethyltin for 1 H and 119Sn, respectively. 119Sn NMR was recorded under broad-band decoupled conditions. General Synthetic Procedure. The following general procedure was used for the synthesis of compounds 1-4.40-44 A mixture of n-BuSn(O)OH and the substituted benzoic acid (in a 6:6 stoichiometric ratio) was heated under reflux in toluene (70 mL) for 6 h. The water formed in the reaction was removed by using a Dean-Stark apparatus. The reaction mixture was filtered and evaporated to afford the products. The amount of reactants used, the yields of the products obtained, and their characterization data are given below. [n-BuSn(O)OC(O)C6H3-2,6-(CH3)2]6 (1). n-BuSn(O)OH (0.42 g, 2.0 mmol); 2,6-dimethylbenzoic acid (0.30 g, 2.0 mmol). Yield: 0.61 g (88.4%). Mp: 280-281 °C (decomp). Anal. Calcd for C78H108O18Sn6: C, 45.79; H, 5.32. Found: C, 45.81; H, 5.38. 1H NMR (400 MHz, ppm): δ ) 6.56-7.21 (m, 18H, aromatic); 2.38 (s, 36H, aromatic CH3); 2.05-1.10 (m, 36H, butyl CH2); 0.90 (t, 18H, butyl CH3, J ) 7.32 Hz). 119Sn NMR (150 MHz, ppm): δ ) -480.68 (s). Colorless prismlike crystals suitable for single-crystal X-ray diffraction were obtained by a slow diffusion of methanol into the chloroform solution of 1 at room temperature. [n-BuSn(O)OC(O)C6H4-4-CH3]6 (2). n-BuSn(O)OH (0.63 g, 3.0 mmol); 4-methylbenzoic acid (0.41 g, 3.0 mmol). Yield: 0.92 g (93.9%). Mp: 273-275 °C (decomp). Anal. Calcd for C72H96O18Sn6: C, 44.08; H, 4.93. Found: C, 44.15; H, 5.12. 1H NMR (400 MHz, ppm): δ ) 7.90 (d, 12H, aromatic, J ) 8.03 Hz); 7.00 (d, 12H, aromatic, J ) 8.03 Hz); 2.29 (s, 18H, aromatic CH3); 1.83-1.18 (m, 36H, butyl CH2); 0.85 (t, 18H, butyl CH3, J ) 7.31 Hz). 119Sn NMR (150 MHz, ppm): δ ) -484.30 (s). Colorless platelike crystals suitable for single-crystal X-ray diffraction were obtained by the slow diffusion of hexane into the dichloromethane solution of 2 at room temperature.

10.1021/cg050323c CCC: $33.50 © 2006 American Chemical Society Published on Web 09/17/2005

268 Crystal Growth & Design, Vol. 6, No. 1, 2006

Chandrasekhar et al.

Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1-4 compound

1

2

3

4

empirical formula formula weight temp, K wavelength, Å crystal system space group unit cell dimensions a, Å b, Å c, Å R, deg β, deg γ, deg volume, Å3 Z density (calcd), mg/m3 absorption coefficient, mm-1 F(000) crystal size, mm3 θ range for data collection, deg index ranges

C78H108O18Sn6 2045.78 150(2) 0.710 73 triclinic P1h

C74H100Cl4O18Sn6 2131.48 150(2) 0.710 73 monoclinic P21/c

C74H102N10O18Sn6 2131.80 213(2) 0.710 73 triclinic P1h

C67H91Cl3N6O18Sn6 2086.95 150(2) 0.710 73 tetragonal I41/a

12.790(5) 12.802(5) 14.324(6) 110.687(7) 111.860(6) 97.688(7) 1939.0(14) 1 1.752

16.1100(17) 11.2664(12) 22.842(3) 90 99.925(2) 90 4083.9(8) 2 1.733

10.839(2) 14.138(2) 14.616(3) 93(2) 105.18(2) 100.58(2) 2112.9(6) 1 1.675

32.336(3) 32.336(3) 15.235(2) 90 90 90 15930.0(3) 8 1.740

1.971

2.002

1.815

2.020

1020 0.5 × 0.4 × 0.3 1.70-23.32

2112 0.4 × 0.3 × 0.16 1.28-25.03

1060 0.3 × 0.3 × 0.2 2.43-24.23

8240 0.4 × 0.4 × 0.1 1.26-28.29

-12 e h e 14, -10 e k e 14, -15 e l e 15 8496 5514 [Rint ) 0.1361] 98.3 none

-19 e h e 19, -13 e k e 10, -27 e l e 27 20631 7212 [Rint ) 0.0728] 99.9 semiempirical from equivalents full-matrix least squares on F2 7212/4/465 0.989 R1 ) 0.0479, wR2 ) 0.1016 R1 ) 0.0840, wR2 ) 0.1146 1.107, -1.037

-12 e h e 12, -16 e k e 15, -16 e l e 16 13404 6263 [Rint ) 0.0289] 91.8 none

-36 e h e 40, -41 e k e 42, -17 e l e 20 48645 9423 [Rint ) 0.0321] 95.2 semiempirical from equivalents full-matrix least squares on F2 9423/39/494 1.009 R1 ) 0.0433, wR2 ) 0.1123 R1 ) 0.0664, wR2 ) 0.1254 1.004, -0.517

reflns collected independent reflns completeness to θ, % absorption correction refinement method data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)]a R indices (all data) largest diff. peak and hole, e‚Å-3 a

full-matrix least squares on F2 5514/0/469 1.046 R1 ) 0.0446, wR2 ) 0.1093 R1 ) 0.0523, wR2 ) 0.1205 1.619, -1.628

full-matrix least squares on F2 6263/230/545 1.051 R1 ) 0.0268, wR2 ) 0.0691 R1 ) 0.0312, wR2 ) 0.0709 0.921, -0.940

R ) ∑||Fo| - |Fc||/∑|Fo|; wR ) {[∑w(|Fo|2|Fc|2)2]/[∑w(|Fo|2)2]}1/2.

[n-BuSn(O)OC(O)C6H4-4-NH2]6 (3). n-BuSn(O)OH (0.63 g, 3.0 mmol); 4-aminobenzoic acid (0.41 g, 3.0 mmol). A white solid formed in the reaction was filtered by a sintered funnel (G-4 frit) and dried. Yield: 0.95 g (96.6%). Mp: 280-283 °C (decomp). Anal. Calcd for C66H90N6O18Sn6: C, 40.29; H, 4.61; N, 4.27. Found: C, 40.18; H, 4.85; N, 4.31. 1H NMR (400 MHz, CD3OD, ppm): δ ) 7.63 (d, 12H, aromatic, J ) 8.11 Hz); 6.52 (d, 12H, aromatic, J ) 8.12 Hz); 4.49 (s, broad, 12H, amine); 1.42-0.77 (m, broad, 36H, butyl CH2); 0.48 (t, broad, 18H, butyl CH3, J ) 7.07 Hz). 119Sn NMR (150 MHz, CD3OD, ppm): δ ) -481.72 (s). Colorless prismlike crystals suitable for singlecrystal X-ray diffraction were obtained by the slow diffusion of acetonitrile into the acetonitrile/methanol (1:0.5) solution of 3 at room temperature. [n-BuSn(O)OC(O)C6H4-2-NH2]6 (4). n-BuSn(O)OH (0.63 g, 3.0 mmol); anthranilic acid (0.41 g, 3.0 mmol). Yield: 0.84 g (85.4%). Mp: 215-216 °C (decomp). Anal. Calcd for C66H90N6O18Sn6: C, 40.29; H, 4.61; N, 4.27. Found: C, 40.34; H, 4.73; N, 4.25. 1H NMR (400 MHz, ppm): δ ) 7.80 (d, 6H, aromatic, J ) 8.07 Hz); 7.14 (m, 6H, aromatic); 6.54 (m, 12H, aromatic); 5.50 (s, broad, 12H, amine); 1.76-1.18 (m, 36H, butyl CH2); 0.83 (t, 18H, butyl CH3, J ) 7.31 Hz). 119Sn NMR (150 MHz, ppm): δ ) -483.05 (s). Colorless needletype crystals suitable for single-crystal X-ray diffraction were obtained by the slow diffusion of hexane into the chloroform solution of 4 at room temperature. X-ray Crystal Structure Determination. Suitable crystals of compounds 1-4 for single-crystal X-ray diffraction were loaded on a Bruker AXS Smart Apex CCD diffractometer. All the structures were

solved by direct methods using SHELXS-97 and refined by full-matrix least squares on F2 using SHELXL-97.45 All hydrogen atoms were included in idealized positions, and a riding model was used. Nonhydrogen atoms were refined with anisotropic displacement parameters. Further details of the data collection and refinement are given in Table 1. The asymmetric units of 1-4 contain half a molecule of the cage with three tin atoms, namely, Sn1, Sn2, and Sn3. Two butyl carbons (C31 and C32) on tin in 2 are disordered over two orientations, are labeled C31, C31′ and C32, C32′ with an occupancy ratio of 0.80: 0.20, and have been refined isotropically. A solvent dichloromethane molecule was also present in the asymmetric unit as a solvate. Six butyl carbons (C42, C43, C44 and C62, C63, C64) on two different tin centers (Sn1 and Sn3) in 3 are disordered over two orientations and are labeled C42, C42′, C43, C43′, C44, and C44′ (with an occupancy ratio of 0.57: 0.43) and C62, C62′, C63, C63′, C64, and C64′ (with an occupancy ratio of 0.55:0.45). These disordered atoms were refined anisotropically. Two acetonitrile molecules were also present in the asymmetric unit as a solvate. The amino groups in 4 are disordered (positional disorder) over the two ortho positions labeled N1, N1′; N2, N2′; and N3, N3′ (with an occupancy ratio of 0.80:0.20). These atoms have been refined anisotropically. Three butyl carbons (C32, C33, and C34) on tin in 4 are also disordered over two orientations and are labeled C32, C32′; C33, C33′; and C34 and C34′ (with an occupancy ratio of 0.50:0.50). These atoms have been refined isotropically. A chloroform molecule was also present in the asymmetric unit as a solvate and is disordered with butyl carbons. The CCDC numbers for the crystal structures 1-4

Supramolecular Architectures of Organotin Drums

Crystal Growth & Design, Vol. 6, No. 1, 2006 269

Table 2. Molecular Structures of Compounds 1-4 along with Their Selected Bond Parameters

a

Molecular structures shown at 50% ellipsoidal probability level.

are 270190, 270191, 270192, and 270193, respectively. These data can be obtained free of charge via the Internet, www.ccdc.cam.ac.uk/conts/ retrieving.html (or from the Cambridge Crystallographic Center, 12 Union Road, Cambridge CB21EZ, UK; Fax: (+44) 1223-336033; or [email protected]).

Results and Discussion The reaction of n-BuSn(O)(OH) with various substituted benzoic acids in toluene in 1:1 (6:6) ratio afforded hexameric organostannoxanes 1-4 in quantitative yields (Scheme 1). The

270 Crystal Growth & Design, Vol. 6, No. 1, 2006 Scheme 1.

Synthesis of Drums 1-4

Chandrasekhar et al. Table 3. Hydrogen Bonding Parameters for Compounds 1-4a

compd 1

2

3

interactions D-H‚‚‚A

distance (Å) D-H H‚‚‚A D‚‚‚A

C17H17‚‚‚O5

0.950(9) 2.917(2)

C18H18‚‚‚O5

0.950(9) 2.922(3)

C16H16a‚‚‚O3

0.979(1) 2.612(1)

0.870(9) 2.344(3)

C13H13‚‚‚O9

0.940(8) 2.996(4)

C740.971(2) 2.789(3) H74B‚‚‚π b (C31-C36) C720.970(2) 2.572(3) H72C‚‚‚O1b 4

C55H55‚‚‚O4

0.950(7) 2.793(4)

C53H53‚‚‚π (C51-C56)

0.948(6) 3.220(1)

symmetry

-1 + x, -1 + y, -1 + z 3.562(3) 125.82(5) -1 + x, 1 + y, -1 + z 3.572(2) 166.83(1) 1 - x, 0.5 + y, 0.5 - z 3.910(1) 157.11(13) 2 - x, 1 - y, 1-z 3.487(3) 162.14(4) -1 + x, 1 + y, z 3.198(3) 167.04(3) 1 - x, 2 - y, 2-z 3.914(5) 165.70(3) x, 1 + y, z 3.661(3) 149.85(5) x, y, z 3.541(5) 176.08(4) x, y, z 3.602(7) 143.71(4) 0.75 - x, -0.25 + y, -0.25 + z 4.102(6) 155.65(4) 0.25 + x, 0.75 - y, -0.75 + z 3.559(3) 126.03(5)

C710.991(3) 2.978(1) H71b‚‚‚π (C18-C23)b N10.870(9) 2.649(2) H1A‚‚‚N3 N1H1B‚‚‚N2

angle (deg) D-H‚‚‚A

2.882(5)c

D ) donor; A ) acceptor. b Hydrogen bonding involving solvent molecules present in the lattices. c Closest hydrogen‚‚‚carbon (H53‚‚‚C51) distance for C-H‚‚‚π interaction. a

Scheme 2.

Schematic Representation of the π-stacked Aryl Moieties in Drums 1-3a

Table 4. Bonding Parameters for π‚‚‚π Interactions in Compounds 1-3

a Type I, parallel displaced stacking in drum 1; type II, parallel displaced stacking in drum 2; type III, parallel displaced stacking in drum 3.

119Sn

NMR spectra of 1-4 show a single resonance with nearly the same chemical shift (480.6-484.3 ppm). These chemical shifts are consistent with the coordination environment around the tin atoms (1C, 5O).33,40,46-48 The cages 1-4 are thermally stable solids with high decomposition points (see Experimental Section). The molecular structures of 1-4, along with some of their selected bond parameters, are summarized in Table 2. The molecular structures of 1-4 are very similar and correspond to well-known drum structures.33,40,46-48 These contain a central stannoxane core (Sn6O6) that is held together by six substituted benzoate peripheries. The prismane-like cage consists of an upper and lower Sn3O3 face; the sides of the cage consist of six Sn2O2 rings. The benzoate groups bridge alternate tin atoms by an isobidentate coordination mode. In drum 1, the carboxylate group is nearly perpendicular (∼71°) to the plane of the aromatic ring, while in drums 2-4, the carboxylate group is in the same plane as the aromatic ring. The orientation of the carboxylate moiety vis-a´-vis the aromatic group indicates the strong steric influence in 1 where the aromatic group has methyl substituents in the 2,6-positions.40 The bonding parameters found in these

compd

interactions π‚‚‚π

intercentroid distance (Å)

interplanar distance (Å)

distance range (Å)

1

C14-C19, C14-C19

4.012(12)

3.718(4)

2

C18-C23, C18-C23

3.991(1)

3.700(11)

3

C21-C26, C21-C26

3.621(3)

3.459(11)

3.710(2) [C16‚‚‚C21] 3.750(2) [C14‚‚‚C18] 3.703(14) [C19‚‚‚C21] 3.996(14) [C23‚‚‚C20] 3.443(12) [C21‚‚‚C23] 3.644(3) [C22‚‚‚C25] 3.497(11) [N2‚‚‚O6]

typea I

II

III

a Type refers to the weak contacts between aromatic substituents on tin. These are summarized in Scheme 2.

drum structures are in keeping with literature precedents40,46-48 and are summarized in Table 2. A closer inspection of the drums 1-4 reveals the presence of extensive supramolecular interactions. The type of π‚‚‚π interactions encountered in these compounds is summarized in Scheme 2. Drum 1 forms a one-dimensional supramolecular assembly mediated by π‚‚‚π and C-H‚‚‚O interactions (Figure 1). The interplanar distance observed for this for π‚‚‚π interaction is 3.718(4) Å (type I, Scheme 2; Table 4). Also, two protons (H17 and H18) in the π-stacked aromatic moiety interact with the one of the carboxylate oxygens (O5) present in the neighboring molecule in a bifurcated manner (Figure 1; Table 3). In contrast to 1, in drum 2 one of the protons (H16a) in the methyl group (p-CH3) is hydrogen-bonded with the oxygen atom (O3) of the drum core (Sn6O6) of a neighboring molecule (Table 3). Out of six p-methyl benzoates present in any given molecule, only two participate in this interaction. Each drum, thus, acts

Supramolecular Architectures of Organotin Drums

Crystal Growth & Design, Vol. 6, No. 1, 2006 271

Figure 1. One-dimensional supramolecular network of drum 1 arising out of C-H‚‚‚O and π-stacking interactions.

Figure 3. N-H‚‚‚N, C-H‚‚‚O, and π-stacking interaction-assisted supramolecular assembly of 3: (a) view showing the two-dimensional sheet formed from N-H‚‚‚N, C-H‚‚‚O, and π‚‚‚π interactions; (b) view showing the three-dimensional hydrogen-bonded network (NH‚‚‚N and π‚‚‚π interactions are shown).

Figure 2. Three-dimensional supramolecular assemblies of 2 formed from C-H‚‚‚O and π-stacking interactions: (a) view showing the twodimensional sheet with dichloromethane (solvent) molecules; (b) view showing the three-dimensional hydrogen-bonded network (C-H‚‚‚O and π‚‚‚π interactions are shown) with dichloromethane (solvent) molecules, which are C-H‚‚‚π hydrogen-bonded with aryl groups.

as a two-proton donor and a two-proton acceptor. The resultant C-H‚‚‚O interactions lead to the formation of a two-dimensional supramolecular network (Figure 2a). Two such two-dimensional sheets are interconnected by means of π‚‚‚π stacking (type II, Scheme 2) to form a three-dimensional supramolecular network (Figure 2b). The interplanar distance for this π‚‚‚π interaction is 3.700(11) Å (Table 4). Solvent dichloromethane molecules are present in the cavities of these three-dimensional grids and are held in place by C-H‚‚‚π interactions (Table 3). Drum 3, which contains a p-amino substituent on the aromatic moiety,

also shows an interesting three-dimensional supramolecular network formed as a result of N-H‚‚‚N, π‚‚‚π, and C-H‚‚‚O interactions. This can be understood in the following way. An aromatic proton (H13) of one molecule interacts with the carboxylate oxygen (O9) of a neighboring molecule to form a one-dimensional supramolecular network (Figure 3a; Table 3). Out of six p-amino benzoates present in any given molecule, only two participate in this interaction. Each drum, thus, acts as a two-proton donor and a two-proton acceptor. Two such networks are interconnected by N-H‚‚‚N and π‚‚‚π interactions (Figure 3a; Tables 3 and 4). The interplanar distance for this π‚‚‚π interaction is 3.459(11) Å (type III, Scheme 2). The cumulative effect of the C-H‚‚‚O, N-H‚‚‚N, and π‚‚‚π interactions is the formation of a two-dimensional supramolecular network (Figure 3a). Two such two-dimensional networks are further interconnected by an additional N-H‚‚‚N interaction (H1B‚‚‚N2, 2.344(3) Å) to form a porous threedimensional supramolecular network (Figure 3b). Acetonitrile

272 Crystal Growth & Design, Vol. 6, No. 1, 2006

Chandrasekhar et al.

a reinforcing interaction (N‚‚‚O) on the π-stacking interaction. As we can see from drums 1-4, the group that is present in the para position (H, CH3, or NH2) in the aromatic group plays crucial role. Thus, the interplanar distances between the aromatic groups are least in compound 3 and greatest in compound 1 (Table 4), reflecting the electron-donating ability of these groups. The anomalous position of compound 3 (containing a p-amino substituent) arises from the N‚‚‚O contacts. The role of the two ortho substituents (methyl groups) in compound 1 is steric and causes the plane of the carboxylate ring to be perpendicular to the aromatic group.40 Presumably because of this orientation, although both C-H‚‚‚O and π‚‚‚π interactions are present, the supramolecular assembly is restricted to one dimension only. In compound 4, the presence of a single amino group in the ortho position is unable to prevent the formation of a threedimensional network. Acknowledgment. We are thankful to the Department of Science and Technology (DST), India, for financial support. K.G. thanks the Council of Scientific and Industrial Research (CSIR), India, for a Research Fellowship (SRF). A.S. thanks the EPSRC, U.K., for financial support. Supporting Information Available: Crystallographic information files (CIF) for compounds 1-4. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 4. Three-dimensional supramolecular assembly of 4 arising out of C-H‚‚‚O and C-H‚‚‚π interactions: (a) view showing the threedimensional hydrogen-bonded network with chloroform (solvent) molecules in the voids; (b) view showing the helix from the side.

molecules (not shown in Figure 3b) are present in these voids as guests and are involved in C-H‚‚‚O and C-H‚‚‚π interactions with the drum (Table 3). We anticipated that the presence of the amino group (in compound 4) in the ortho position of the aromatic unit would direct the supramolecular assembly formation in a different manner in comparison to compounds 3 and 1 and 2. However, the amino group is positionally disordered over two ortho positions, and therefore its influence on supramolecular assembly cannot be discerned satisfactorily. Nevertheless, drum 4 also shows an interesting three-dimensional supramolecular network, which is a result of C-H‚‚‚O and C-H‚‚‚π interactions (Figure 4a). An aromatic proton (H55) in one molecule interacts with the carboxylate oxygen (O4) of a neighboring molecule to form a one-dimensional helical supramolecular network (Figure 4b; Table 3). On the other hand, a C-H‚‚‚π interaction is also seen in this helix, but in the opposite direction (Figure 4b). These helical structures are interconnected to form a three-dimensional supramolecular network (Figure 4a). Chloroform molecules are present in the voids of this supramolecular network. The supramolecular assemblies of 1-4 reveal the presence of three types of π-stacking interactions (Scheme 2). While all of these interactions involve a parallel displaced orientation of the aromatic groups, there are individual variations. Thus, in the type I variety, the centroid of an aromatic ring is perfectly above the ortho carbon (with respect to carboxylate group) of another aromatic moiety. In type II orientation, it is the meta carbon of one aromatic moiety that is perfectly above the centroid of another aromatic ring. Type III is very similar to type I except that the presence of the p-amino substituent causes

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