Solid State Conformational Preferences of a Flexible Molecular

Mar 31, 2010 - Anna C. Cunha , Vitor F. Ferreira , Alessandro K. Jordão , Maria C. B. V. de Souza , Solange M. S. V. Wardell , James L. Wardell , Pei...
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DOI: 10.1021/cg100021f

Solid State Conformational Preferences of a Flexible Molecular Backbone Derived from Acetone: Dependence on Electron Donating/Withdrawing Ability of Substitutions

2010, Vol. 10 2298–2305

Sunil Varughese† and Sylvia M. Draper* School of Chemistry, Trinity College Dublin, College Green, D2, Ireland . † Current address: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India. Received January 6, 2010; Revised Manuscript Received March 18, 2010

ABSTRACT: Conformational preferences, crystal packing, and intermolecular interactions can play a pivotal role in solid state reactions, and in particular for bis(phenyl)acetones, these factors are known to determine the rate of the photodecarbonylation process. In the present work, conformational pliability and the supramolecular synthons present in a series of variably substituted bis(phenyl)acetones have been assessed in terms of the electron donating/withdrawing ability of the substituents. While an exo-exo conformation has been observed in the case of electron donating substituents, electron withdrawing analogues preferred an endo-endo arrangement. A linear correlation has been observed between the angular shift and the Hammett constants of the functional groups. The shift in the IR spectra with respect to that of acetone has been used to ascertain the spectral and structural analogy. SEM analysis of the nano-/microparticles of the compounds reveals the direct dependence of the hydrophobic/hydrophilic character of the functional groups on the nature and morphology of the particles.

Introduction Studies pertaining to conformationally flexible molecules are interesting, as they can provide insights on the intricacies involved in various complex molecules and recognition processes.1,3 The crystallographic information obtained from simple systems can be utilized in understanding various structural motifs present in complex assemblies. For example, as universal models of protein folding and the interactions involved in the process, foldamers are well studied.2 In such cases, various oligoimides are designed, synthesized, and characterized using X-ray diffraction, NMR, and CD analysis. Recently, Fujita and co-workers reported the conformational preferences of a series of short peptide fragments in the hydrophobic constrained cavities of a porphyrin containing coordination cage.3a In evaluating the properties of a flexible molecule and in determining its conformational preferences, in addition to the hydrogen bonds, various factors such as electronic effect, steric factors, hydrophobicity, etc. can contribute significantly.4 Molecular conformation data available from crystal structures are usually affected by crystal packing forces and cause impedance in computational modeling. Although crystal structures generally give a good indication of the conformational preferences of a given molecule, intermolecular interactions with sufficient strength can presumably lead to a strained arrangement.5 In 2008, Allen and co-workers stated that “...conformational diversity increases with an increasing number of different crystal environments and with an increasing number of flexible torsion angles. Overall molecules with one or more acyclic flexible torsion angle are observed to exist in more than one conformation in ca. 40% of cases.”6 In this context, crystallographic analysis of a series of chemically related flexible molecules is engrossing and can provide insight on the conformational preferences in their crystal lattice. *Telephone: þ353-1-8962026. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 03/31/2010

In the area of crystal engineering, urea has been a target molecule in achieving networks for inclusion compounds, nonlinear optical materials and modular assemblies stabilized by hydrogen bonds.7 N,N0 -Diaryl ureas (Scheme 1a) predominantly exhibit a W-conformation with the formation of an R-network, a chain of bifurcated N-H 3 3 3 O hydrogen bonds.8 Nangia and Etter have reported strategies for effectively designing assemblies with urea, non-urea, and urea-solvent hydrogen bonding motifs by the appropriate functionalization of the aryl rings.9 Further, diphenylcarbonates and their conformational preferences have been an interesting topic over several years.10 Unlike the cases of urea derivatives, ab initio calculations on a series of carbonates (Scheme 1b) demonstrated that they can exist as cis-cis, cis-trans, and trans-trans conformations.11 An isoelectronic compound, 1,3-bis(phenyl)acetone (Scheme 1c), and its derivatives have been target molecules for studying the photodecarbonylation reactions in the solid state.12 In a series of substituted bis(phenyl)acetones, the rate of triplet R-cleavage has been reported to be in the order -OCH3>-CH3>-H. Wong and co-workers did fit the rate of decarbonylation reaction to Hammett plots.13 It has been observed that the triplet photochemistry of the bis(phenyl) acetone is in fact controlled by a conformational “switch”.14 Resendiz et al. reported that, in crystalline p,p0 -disubstituted bis(phenyl)acetones, the relative quantum yields and chemical efficiencies of photodecarbonylation reactions are a function of the Hammett constants of the substituents. The authors further stated that “... reactions in crystals can be engineered from known molecular structure parameters”.15 In 2007, Coppens and co-workers demonstrated that intermolecular hydrogen bonding can induce the quenching of photodecarbonylation reactions in the monoketones.16 Though the solid state photochemical processes of bis(phenyl)acetones are dependent on the conformations of the molecules, surprisingly, a systematic evaluation of their conformational preferences in their crystal structures has not been done to date. Herein, we r 2010 American Chemical Society

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analyze the conformational flexibility of a series of substituted 1,3-bis(phenyl)acetones as a function of the electron donating/ withdrawing ability of the substituents. For an analogy, the structures have been compared with the dimorphic forms of acetone. Acetone. Two distinct phases of acetone are known in the literature.17 The room temperature high pressure form is composed of molecules stacked along the [010] direction. In the crystal lattice, each molecule acts as a four-donor fouracceptor hydrogen bond forming species, as shown in Figure 1a. The low temperature phase consists of molecules forming corrugated tapes running in antiparallel directions. Individual tapes consist of molecules stabilized by bifurcated C-H 3 3 3 O hydrogen bonds (Figure 1b). Results and Discussion The crystal structures of a series of symmetrically substituted bis(phenyl)acetones were studied using single crystal

Figure 1. Intermolecular interactions present in the dimorphs of acetone.

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X-ray diffraction (Scheme 2). The variations in the conformational arrangements and synthons have been evaluated as a function of substituents. Further, to correlate the structural and spectral information, infrared spectral data were analyzed. Organic nano-/microparticles, prepared using a precipitation method, were analyzed using scanning electron microscopy. 1,3-Bis(phenyl)acetone, 1. 1,3-Bis(phenyl)acetone, 1, crystallizes in the P21/c space group with a molecule in the asymmetric unit and exhibits considerable variations in the intermolecular interactions with respect to the dimorphs of acetone (Figure 2). The molecule adopts an endo-exo conformation, with two arms of the compound having opposite orientations with respect to the central carbonyl group. Unlike the cases of acetone dimorphs, in which the methyl groups make intermolecular interactions, the molecules of 1 form linear chains through bifurcated (R12(6)) C-H 3 3 3 O (H 3 3 3 O, 2.71 and 2.99 A˚) hydrogen bonds involving aromatic as well as a methylene hydrogen atoms.18 Adjacent molecular chains are stabilized by C-H 3 3 3 O (H 3 3 3 O, 2.48 A˚) hydrogen bonds, making use of the methylene bridges.19 Effect of Electron Donating Substituents. The influence of the electron donating substituents on the conformational flexibility was studied using -CH3, -C(CH3)3, and -OCH3 substituted monoketones as the representative examples. 1,3-Bis(4-methylphenyl)acetone, 2, was prepared by a phase-transfer catalysis reaction of R-bromo-p-xylene with Fe(CO)5. It has been noted that the two phenyl substituents are in an exo-exo conformation with respect to the carbonyl group. This observation shows a clear distinction from the theoretically calculated model, which predicted a bent

Scheme 1. Isoelectronic Biphenyl Derivatives of Urea, Carbonate, and Acetone

Scheme 2. Bis(phenyl)acetone Derivatives Considered for the Present Study

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Figure 2. Intermolecular interactions present in the crystal structure of 1. The distances are in angstroms.

Figure 3. Intermolecular C-H 3 3 3 O hydrogen bonds existing in the crystal lattices of (a) 2, (b) 3, and (c) 4. The distances are in angstroms.

U-conformer.14 The molecules make linear chains through bifurcated C-H 3 3 3 O (H 3 3 3 O, 2.51 A˚) hydrogen bonds exclusively through aromatic H-atoms (Figure 3a). Thus, unlike the case of 1, the introduction of a methyl (-CH3) substituent led to an increase in the number of atoms (R12(10)) involved in the recognition pattern. The 4-tert-butyl substituted monoketone, 3, crystallizes in the P1h space group with two molecules in the asymmetric unit (Z0 = 2). There exists a slight conformational difference between the two symmetry independent molecules brought about by the flexibility of the central -CH2COCH2- bridge. The molecules exhibit an exo-exo conformation and make a linear assembly, stabilized by bifurcated (R12(10)) C-H 3 3 3 O (H 3 3 3 O, 2.75 and 2.80 A˚) hydrogen bonds, as observed in 2.

Varughese and Draper

The -OMe substituted compound, 4, was redetermined, as the structure was reported by Coppens and co-workers as a part of the photochemical studies on a series of ketones.16 The monoketone, 4, also formed a similar assembly, stabilized by C-H 3 3 3 O (H 3 3 3 O, 2.53 A˚) hydrogen bonds. Adjacent linear chains are held together by centrosymmetric C-H 3 3 3 O (H 3 3 3 O, 2.64 A˚) hydrogen bonds (R22(6)) formed by the methoxy substituent (Figure 3c). In the case of compounds 2-4 with electron donating substituents, the molecules prefer an exo-exo conformation with the synthon formation exclusively through aromatic Hatoms and the central carbonyl group. Hence, there exist clear variations in conformations and recognition patterns with respect to the acetone dimorphs and also with the unsubstituted compound, 1. This hints at a possible electronic communication between the central carbonyl functionality and the substituents on the periphery of the molecule. To further analyze this effect, a series of molecules with electron withdrawing substituents were studied. Effect of Electron Withdrawing Substituents. The influence of the electron withdrawing substituents was studied using -Br, -COOCH3, and -NO2 substituted bis(phenyl)acetones as representative examples. The -Br substituted compound, 5, was synthesized from 4-bromobenzyl bromide and recrystallized from a methanol-CH2Cl2 mixture. The compound crystallizes in the P21/ c (Z0 = 2) space group and exhibits an endo-endo conformation (Table 1). In the crystal, symmetry independent molecules are stacked alternately and interact through bifurcated (R12(6)) (H 3 3 3 O, 2.69 and 2.73 A˚) and linear C-H 3 3 3 O (H 3 3 3 O, 2.58 A˚) hydrogen bonds established between methylene H-atoms and carbonyl groups (Figure 4a). Adjacent linear chains are stabilized by weak Type-II Br 3 3 3 Br interactions (Br 3 3 3 Br, 3.70 and 3.82 A˚).20 A similar endo-endo conformation is observed in the ester (-COOCH3) compound, 6. But, unlike the case of the -Br derivative, the molecules are stabilized by bifurcated C-H 3 3 3 O (H 3 3 3 O, 2.47 and 2.74 A˚) hydrogen bonds comprising the methylene H-atoms and the carbonyl group of the ester functionality. This variation may be brought about by a better hydrogen bond forming environment of the ester functionality compared to the sterically demanding neighborhood of the central carbonyl group. The nitro substituted compound, 7, crystallizes in a monoclinic crystal system (Z0 = 2), and the molecules exhibit an endo-endo conformation as in 5 and 6. The molecules are stabilized by bifurcated (R12(6)) C-H 3 3 3 O (H 3 3 3 O, 2.47, 2.55 and 2.38, 2.62 A˚) hydrogen bonds, and the recognition pattern is similar to that observed in the case of the acetone dimorphs. The two symmetry independent linear chains are arranged alternately and form C-H 3 3 3 O hydrogen bonds using -NO2 functionality.21 Thus, compounds 5-7, with electron withdrawing substituents, prefer an endo-endo conformation. In the assemblies, the molecules make bifurcated C-H 3 3 3 O hydrogen bonds through methylene H-atoms, unlike in 2-4, with electron donating substituents, where they are through aromatic H-atoms. This is possibly due to a lower electron density available in the aromatic ring as a consequence of electron withdrawing groups. A CSD analysis and a structural comparison revealed that the -F derivative also exhibits an endo-endo conformation and is in good agreement with the observations made in the structural analysis of the monoketones.22

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Table 1. Crystallographic Information Table for 1-7 complex

1

2

3

4

5

6

7

formula CCDC no. formula wt crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalc (g cm-3) T (K) μ (mm-1) 2θ range (deg) total reflns unique reflns, R(int) reflns used no. of parameters GOF on F2 final R1, wR2

C15H14O CCDC 735816 210.26 monoclinic P21/c 9.471(1) 5.684(1) 21.341(2) 90 96.525(2) 90 1141.4(3) 4 1.224 123(2) 0.075 50.00 5756 2010 1801 145 0.947 0.0352; 0.0985

C17H18O CCDC 735817 238.31 monoclinic C2/c 27.038(1) 6.079(2) 8.646(3) 90 97.990(7) 90 1407.3(7) 4 1.125 123(2) 0.068 50.24 3565 1247 962 83 1.095 0.0500; 0.1575

C23H30O CCDC 735820 322.47 triclinic P1 6.205(4) 18.353(3) 19.187(3) 69.920(1) 89.150(2) 89.080(2) 2051.8(14) 4 1.044 123(2) 0.062 50.18 15924 6990 5046 433 1.056 0.0615; 0.1845

C17H18O3 CCDC 735819 270.31 monoclinic C2/c 27.805(4) 5.854(1) 9.059(1) 90 104.349(3) 90 1428.5(4) 4 1.257 123(2) 0.085 49.98 7202 1251 1097 93 1.050 0.0379; 0.1061

C15H12Br2O CCDC 735814 368.07 triclinic P1 9.039(1) 10.061(1) 16.451(2) 75.367(3) 76.554(3) 84.260(3) 1406.5(3) 4 1.738 123(2) 5.748 50.06 7638 4776 2493 325 0.848 0.0493; 0.1186

C19H18O5 CCDC 735815 326.33 monoclinic C2/c 16.480(1) 8.385(2) 12.303(1) 90 93.624(2) 90 1696.7(4) 4 1.278 123(2) 0.092 49.98 8721 1487 1290 110 1.571 0.0496; 0.1831

C15H12N2O5 CCDC 735818 300.27 monoclinic P2/c 23.489(5) 4.679(1) 34.741(5) 90 131.997(9) 90 2837.6(10) 8 1.406 123(2) 0.108 50.04 28450 5018 3089 397 1.029 0.0531; 0.1355

Figure 5. Various conformations (phenyl)acetones (1-7).

present

in

1,3-bis-

the crystal lattice depending upon the nature of substituents (Figure 5). The extent of variations in the angles between the two arms of the compounds is a function of the electron donating and withdrawing abilities of the functional groups. These angles when plotted against the corresponding Hammett constants of the functional groups exhibit a linear correlation, as shown in Figure 6b and Table 2. In the case of electron donating substituents, in order to bring about a comparative analysis, instead of θED, 180 - θED has been plotted. The unsubstituted monoketone, 1, exhibited a clear deviation from the linear correlation observed in the case of substituted compounds, and the reason for the observed shift is still elusive. Synthon Evaluation

Figure 4. Hydrogen bond patterns observed in the cases of (a) 5, (b) 6, and (c) 7. The distances are in angstroms.

Conformational Analysis The monoketones exhibit three distinct conformational arrangements;endo-exo, exo-exo, and endo-endo;in

In the bis(phenyl)acetone series, considerable variations in the recognition pattern, as a function of the substitution, have been observed (Scheme 3). While a cyclic six-membered (R12(6)) pattern (synthon I) involving aromatic and methylene H-atoms is present in the unsubstituted bis(phenyl)acetone 1, the molecules with electron donating substituents (2-4) make hydrogen bonds (R12(10), synthon II) exclusively through aromatic hydrogen atoms. For bis(phenyl)acetones with electron withdrawing substituents (5-7), the observed recognition patterns are found to be substitution dependent. In the case of the -Br substituted compound, 5, the assembly is stabilized by two distinct synthons. The symmetry related molecules are held together by bifurcated C-H 3 3 3 O hydrogen bonds (synthon III), while the symmetry independent molecules make linear interactions (synthon IV). In both the cases, the hydrogen bond is realized exclusively through the methylene hydrogen atoms. As against the central carbonyl

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Varughese and Draper Scheme 3. Synthons Present in Bis(phenyl)acetones

Figure 6. (a) Schematic representation of the arms of 1,3-bis(phenyl) acetones and the angles between the arms. (θED and θEW are the angle between the two wings in the case of electron donating and electron withdrawing substituents, respectively. In the case of electron donating derivatives, 180 - θED has been considered for graphical representation). (b) Correlation of the Hammett constants of the functional groups with the angle between the two arms of the monoketones. Table 2. Hammett Constants of the Functional Groups and the Corresponding Angles Existing between the Two Wings of the Substituted Biphenylacetones assembly

substitution

Hammett constant

angle (deg) (θEW and 180 - θED)a

1 2 3 4 5 6 7

-H -CH3 -tert-butyl -OCH3 -Br -COOCH3 -NO2

0 -0.17 -0.197 -0.268 0.232 0.45 0.778

102.07 151.55 153.65; 153.33 158.84 127.51; 120.96 117.71 113.09; 109.12

a In the case of electron donating substituents, instead of θ, (180 - θ)° has been considered.

functionality in the other bis(phenyl)acetones, the bifurcated C-H 3 3 3 O hydrogen bonds (synthon V) in 6 are established through the peripheral ester group and the methylene hydrogen atoms. The molecules of the nitro compound, 7, make hydrogen bond patterns (R12(6)) similar to those for synthon III. IR Spectral Analysis As there exists a strong correlation between υCdO and carbon-oxygen bond distances, IR spectroscopy is a very useful analytical tool for studying hydrogen bonds.23 It is

known in the literature that with the hydrogen bond formation the >CdO bond is lengthened compared to the case of the free molecule.24 In the seminal work on weak hydrogen bonds by Desiraju and Steiner, the utility of IR analysis in quantifying various interactions in crystals has been discussed.25 While Badger and Bauer made correlations between spectral parameters and energies of strong hydrogen bonds, it was observed that such correlations are difficult for weak interactions, presumably due to the diffusive nature of the interactions and the deformable nature of the bond geometries.26 The infrared spectral analysis was carried out to analyze the C-H 3 3 3 O hydrogen bonds present in bis(phenyl)acetones and the effect of these interactions in the vibrational peak shifts (Table 3). In the case of compounds 1-7, depending upon the substitutions and interactions present, the molecules exhibited different carbonyl stretching frequencies. For the unsubstituted bis(phenyl)acetone 1 and the ester-derivative 6, the >CdO stretching frequencies are 1710 and 1717 cm-1, respectively, and are more toward the values corresponding to υCdO of liquid acetone (1715 cm-1).27 The methyl (2) and methoxy (4) derivatives exhibited >CdO frequencies corresponding to higher energy (1738 and 1739 cm-1, respectively). The tert-butyl substituted compound, 3, has exhibited a lower stretching frequency (1681 cm-1) which correlates well with the longer C-O (1.227 A˚) bond length. It is interesting to note that one of the acetone crystalline forms also exhibits a similar C-O bond length. In the case of the -Br, 5, and the -NO2, 7, derivatives, two frequencies were observed;one at lower energy (1718 and 1721 cm-1, respectively) and one at higher energy (1738 cm-1). Further, the C-H stretching frequencies of the monoketones are comparable to that observed in the case of acetone (1360 cm-1) except for the case of the nitro derivative (1344 cm-1). The C-CO-C stretching and bending bands of bis(phenyl)acetones are in agreement with those of acetone except for the cases of 3 (1288 cm-1) and 6

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Table 3. Infrared Spectral Information of the Monoketones no.

group

CdO distance

IR(CdO), cm-1

IR(-CH-), cm-1

IR(C-CO-C), cm-1

1715

1360 1422

1213

liquid acetone 1 2 3

-H -Me -tert-butyl

4 5

-OMe -Br

6 7

-ester -nitro

1.207 1.195 1.227 1.232 1.205 1.204 1.204 1.190 1.202 1.211

Figure 7. SEM images of nanoparticles of (a) 2, (b) 3, (c) 4, (d) 5, (e) 6, and (f ) 7.

(1277 cm-1). This may be attributed to the bulky functionalities attached to the periphery of the aromatic rings and also due to the intermolecular interactions existing in the ester derivative. Thus, even in simple molecules such as bis(phenyl)acetone, due to the weak nature of the interactions, the spectral complexity makes it difficult to make a proper interpretation or a clear correlation between the structural and spectral information (Experimental details, ORTEP diagrams, IR data, and SEM images are provided in the Supporting Information.) and further underscores the statement of Desiraju and Steiner, “ ...the effects of weak hydrogen bonds on vibrational spectra are not always as clear as for strong bonds, and can be quite dissimilar for different kinds of weak hydrogen bond.”25 Organic Nano-/Microparticle Analysis Although there have been extensive studies on the sizedependent properties of organic nano-/microparticles, the ability to predict and control the nature and morphology of these nanoscale materials is limited.28 The different nucleation and growth processes involved are known to influence the formation of the nano-/microparticles with different

1710 1738 1681 1739 1738 1718 1717 1738 1721

1371

1218 1217 1288

1366 1366

1217 1217

1344

1277 1217

morphologies.29 Yao and co-workers have reported that substituents can in fact determine the shape of nanoparticles of some stilbazolium-like dyes.30 In this connection, we have investigated the aforesaid ability of substituents in bis(phenyl)acetones. The aggregation of molecules was induced by injecting 50 μL of concentrated dimethylformamide (DMF) solution (1  10-3 M) of 1-7 into 20 mL of water under ultrasonication. A homogenized transparent solution was obtained upon the dispersion of the compounds in water (a poor solvent). The resulting solutions were digested for 24 h at 45 °C and were kept undisturbed for a fortnight. The experiments were repeated by varying the concentration of the stock solution. It was assumed that the hydrophilic/ hydrophobic properties of functional groups can regulate the interactions established by the compounds with the water-DMF solvent system and thereby the morphologies of the particles. Diverse morphologies were obtained, depending on the substituents, as indicated by the scanning electron microscopy (SEM) images (Figure 7). The samples derived from -Br, -NO2, and -OMe substituted compounds consist of nano-/microcrystals with well-defined faces, while those of -Me and -COOMe derivatives produced amorphous spherical particles. The unsubstituted compound, 1, failed to produce nanoparticles from a range of solvent mixtures, as it is soluble in almost all solvents and solvent mixtures. The methyl substituted bis(phenyl)acetone, 2, forms amorphous spherical particles with an average size of 200 nm, and the product from 3 consists of crystalline materials embedded in an amorphous matrix. The formation of the matrix is independent of the concentration, as 10-4 M, 10-3 M, and 10-2 M stock solutions yielded particles with similar nature and morphology. Microplates with well-defined faces are obtained in the case of the methoxy (-OMe) substituted compound, 4. In addition, the surfaces of the microplates are clean and smooth. Crystalline microparticles with an average size of 3  4 μm2 have been obtained from the bromo derivative, 5, and are found to be rich in defects. The hydrophobic nature of the ester (-COOMe) substitution led to the generation of small amorphous spherical nanoparticles, as in the case of the methyl substituent. Microparticles of the monocot leaflike morphology, with an average dimension of 303 μm2, are obtained for the nitro derivative. Thus, the present series makes an account of the role of the hydrophobic/ hydrophilic nature of substituents in regulating the interactions established by the molecules with the solvent system and thereby the morphology of nano-/microparticles. Conclusions The flexible backbone of the 1,3-bis(phenyl)acetones exhibits exo-endo, exo-exo, and endo-endo conformations in

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the solid state, and the conformational preferences are found to be a function of the electron donating/withdrawing ability of the substitution. While the compounds with electron donating substituents prefer the exo-exo conformation, an endo-endo conformation has been observed in the case of bis(phenyl)acetones with electron withdrawing groups. Further, there exists a linear correlation between the Hammett constants and the angles between the two arms of the monoketones. The weak nature of the interactions existing in the assemblies makes it difficult to formulate a proper interpretation or a clear correlation between the structural and IR spectral information. The hydrophobic/hydrophilic nature of the substituents does have a significant role in determining the nature and morphology of the particles, as revealed by SEM analysis of the nano-/microparticles. This is presumably due to the variation in the nature of interactions established between the molecules and the solvent system. Thus, the present study addresses hydrogen bond forming ability, electron donating/withdrawing effect, and the hydrophilic/ hydrophobic nature of functional groups in determining the conformational preferences of a series of acetone derivatives in crystal packing and also the nature and morphology of nano-/microparticles. Experimental Section The chemicals were purchased from Aldrich and used without further purification. Reagent grade solvent was used for the reaction. 1,3-Bis(phenyl)acetone, 1, was obtained from Aldrich and was used for crystallization. The substituted bis(phenyl)acetones were prepared as per the procedure available in the literature. General Preparation Method for 2, 3, 5, and 6. An aqueous solution (100 cm3) of calcium hydroxide (2 mol) and tetra-nbutylammoniumhydrogensulfate (phase-transfer catalyst) (0.25 mol) was prepared and introduced into a 500 cm3, three-necked flask kept at room temperature. Measured quantities of respective benzylbromide (1 mol) and dichloromethane (100 cm3) were then added to the reactor. The solution was agitated at 700 rpm for 15 min, and the reactor was purged with inert nitrogen gas. A known quantity of iron pentacarbonyl (0.5 mol) was then added to the reactor. The purging was continued for a further 30 min, and the flask was tightly closed and was left for stirring overnight. The resulting reaction mixture was further supplied with 100 cm3 of dichloromethane and was purged with air for 15 min to quench the reaction. Further, a 10% HCl solution (25 cm3) was added and was continuously purged with air with vigorous stirring. The resulting reaction mixture was extracted with dichloromethane, and the volume was reduced under vacuum. The resulting material was purified using column chromatography on silica.31 In the case of the methyl derivative 2, instead of dichloromethane, benzene was used as the reaction solvent.32 General Preparation Method for 4 and 7. A solution of substituted phenylacetic acid (1 mol) in dry dichloromethane was added slowly to a solution of DCC (1 mol) and DMAP (0.25 mol) in dry dichloromethane in an inert atmosphere. The reaction mixture was kept stirring for 24 h at room temperature and then filtered. The filtrate was distilled off, and the residue was purified by column chromatography over silica gel.33 X-ray Analysis. Single crystals of 1-7 were carefully chosen after they were viewed through a microscope supported by a rotatable polarizing stage. The crystals were glued to a thin glass fiber using NIH immersion oil and mounted on a diffractometer equipped with an APEX CCD area detector. All the data were collected at 123 K. The intensity data were processed using Bruker’s suite of data processing programs (SAINT), and absorption corrections were applied using SADABS.34 The structure solution of all the complexes was carried out by direct methods, and refinements were performed by full-matrix least-squares on F2 using the SHELXTLPLUS suite of programs.35 All the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were fixed on the

Varughese and Draper calculated positions using appropriate AFIX commands and were refined isotropically. Intermolecular interactions were computed using the PLATON program.35 Nanoparticle Preparation and Analysis. The nanoparticles were prepared by the rapid injection of 50 μL of 10-3 M compound in dimethylformamide (DMF) (filtered through a nanoporous alumina membrane (Whatman, Anodisc 13)) into 20 mL of Millipore water under ultrasonication. The solution was kept under isothermal conditions (45 °C) for 24 h and was filtered through anodizc (20 nm). The morphology of the nanoparticles was analyzed using a TESCAN scanning electron microscope using a beam voltage of 5 kV.

Acknowledgment. This material is based upon works supported by Science Foundation Ireland [05PICAI819]. S.V. thanks Dr. S. Philip Anthony for fruitful discussions and SEM analysis. Supporting Information Available: ORTEP diagrams of 1-7, table of hydrogen bond distances, preparation methods for 2-7, experimental methods, IR spectra of 1-7, and SEM images of 2-7. This material is available free of charge via the Internet at http:// pubs.acs.org.

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