Variability in Halogen Interactions: In situ Cryocrystallization of Low

Apr 4, 2007 - Scheme 1 Compound 1: X1 = -F, X2 = -H; compound 2: X1 = -Cl, X2 = -H; compound 3: X1 = -Br, X2 = -H; compound 4: X1 = -H, X2 = -Br; comp...
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Variability in Halogen Interactions: In situ Cryocrystallization of Low Melting Substituted Trifluoroacetophenones Deepak

Chopra,†

Vijay

Thiruvenkatam,†

S. G.

Manjunath,‡

and Tayur N. Guru

Row*,†

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India, and Hikal India Limited, Bannerghatta Road, Bangalore-560078, India

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 5 868-874

ReceiVed July 25, 2006; ReVised Manuscript ReceiVed February 13, 2007

ABSTRACT: Crystal and molecular structures of four low melting halogenated trifluoroacetophenones along with the corresponding nitro derivative have been determined to understand the nature of short halogen-halogen and halogen-oxygen intermolecular contacts. The first four compounds, which are liquids at room temperature, have been analyzed using the technique of in situ cryocrystallography. The basic building block in the crystal lattice is invariant because of an intramolecular C-H‚‚‚F contact, which locks the molecular conformation. The supramolecular assembly in all these structures is stabilized with additional C-H‚‚‚O, C-H‚‚‚F, and aromatic π‚‚‚π intermolecular interactions. The nitro derivative is further stabilized by intramolecular C-H‚‚‚O contacts. Introduction Inter- and intramolecular interactions involving halogens in molecular crystals have been studied extensively in recent literature, but a clear understanding of the nature of contacts such as X (halogen)‚‚‚X and C-H‚‚‚X in terms of their strength and directionality is still debated.1 Interactions involving chlorine and bromine, however, have received much attention in this context2-4 while those of fluorine have remained ubiquitous.5-7 Indeed, studies on compounds with short contacts involving organic fluorine are being well documented in the literature,8 and patterns are beginning to emerge as to the nature of such interactions. We have been evaluating short contacts (distances less than the sum of the van der Waals radii of the participating atoms) of the type C-H‚‚‚F, C-F‚‚‚Cπ,9 and C-F‚‚‚F-C10 and have drawn several leading pointers to gain insights into these contacts particularly with respect to the ubiquitious F‚‚‚F contacts. Database studies on halogen containing small molecules11 as well as proteins and peptides12 bring out the importance of C-F‚‚‚Cπ and their use as possible tools in crystal engineering.13 The role of weak C-H‚‚‚F has been further examined related to polymorphism in several substituted benzanilides14,15 and via in situ cryocrystallography in fluorinated anilines.16 Careful charge density measurements on a substituted isoquinoline and a tetrahydroindole derivative allow for the quantification of charge density distributions in C-F‚‚‚F-C interactions.17 Further, these interactions are similar to C-H‚‚‚F, C-H‚‚‚O, and C-H‚‚‚Cπ interactions based on the values derived from the topological analysis.18 Another welldocumented interaction is halogen bonding, involving energies comparable to those of weak hydrogen bonds and is a noncovalent interaction between halogen atoms and neutral or anionic Lewis bases.19,20 A detailed database study on halogen bonding brings out the similarities between these bonds and the hydrogen bonds in terms of distance/angle criteria.21 Of these X‚‚‚O interactions are well established and have been utilized in the design of supramolecular assemblies.22 In a couple of studies devoted to halogen-substituted aromatic carboxylaldehydes23 and halotriaroylbenzenes,24 the occurrence of cooperative C-H‚‚‚X and CdO‚‚‚X interactions has been analyzed in terms * To whom correspondence [email protected]. † Indian Institute of Science. ‡ Hikal India Limited.

should

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addressed.

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Scheme 1 Compound 1: X1 ) -F, X2 ) -H; compound 2: X1 ) -Cl, X2 ) -H; compound 3: X1 ) -Br, X2 ) -H; compound 4: X1 ) -H, X2 ) -Br; compound 5: X1 ) -H, X2 ) -NO2.

of crystal engineering capabilities. During the study of structural features of substituted myo-inositols, the importance of CdO‚‚‚Br to generate polymorphic modifications and in one example mediated by crystal-to-crystal phase transitions has been identified.25,26 The nonspherical charge density distribution around halogens in the structures of trihalomesitylenes27a and 2,4,6-tris(4-halophenoxy)-1,3,5-triazine27b has been observed to generate triangular inter-halogen motifs where each halogen atom acts as a donor and an acceptor simultaneously. It is noteworthy that single-crystal studies on p-Cl, p-Br, and p-I substituted acetophenones also exhibit short X‚‚‚O contacts only for the heavier halogens, Br and I,28 while Cl prefers to form C-H‚‚‚Cl contacts. In situ growth of crystals on the diffractometer has been used to study low melting compounds,29 ionic liquids,30 and other molecular liquids.31 Further, in combination with inputs from differential scanning calorimetry (DSC), which suggest pathways for the growth of single crystals, it has become possible to evaluate intermolecular interactions. In this article, we explore structures of low melting trifluoroacetophenone (TFA) derivatives via in situ cryocrystallization techniques to analyze interactions involving fluorine, chlorine, and bromine in the presence of a -CF3 group. Further, the molecules have been chosen such that the -CF3 moiety locks up the conformation via intramolecular C-H‚‚‚F contacts. Crystal and molecular structures of 2,2,2-trifluoro-1-(4-fluorophenyl)ethanone (1), 1-(4chlorophenyl)-2,2,2-trifluoroethanone (2), 1-(4-bromophenyl)2,2,2-trifluoroethanone (3), 1-(3-bromophenyl)-2,2,2-trifluoroethanone (4), and 1-(3-nitrophenyl)-2,2,2-trifluoroethanone (5) (Scheme 1) have been analyzed via single-crystal X-ray diffraction studies. Experimental Section Conditions of Crystallization and Data Collection. Initial measurements were carried out on liquids 1, 2, and 4 to determine the

10.1021/cg0604881 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/04/2007

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Crystal Growth & Design, Vol. 7, No. 5, 2007 869

Figure 1. (a-d) Images of in situ crystal growth from liquids in 1-4. Table 1. Crystallographic Data crystal data

1

2

3(p-Br)

4(m-Br)

5(m-NO2)

formula color formula wt (g mol-1) temperature (K) radiation wavelength (Å) melting point (°C) crystal system space group a/Å b/Å c/Å R/° β/° γ/° volume (Å3) Z density (g/mL) abs coeff (mm-1) F(000) θmin,max coverage hmin,max, kmin,max, lmin,max no of reflections no of unique reflns no of observed reflns no of parameters refinement method R_all, R_obs wR2_all, wR2_obs ∆Fmin,max (e Å-3) GOF

C8H4O1F4 colorless 192.1 175(2) MoKR 0.7107 4 monoclinic P21/c 10.456(3) 9.592(3) 7.806(3) 90 102.80(5) 90 763.4(9) 4 1.67 0.174 384.0 2.0, 26.0 0.99 -12,12, -11,11, -9,9 5577 1504 1340 135 FMLS on F2 0.036, 0.033 0.090, 0.087 -0.25, +0.17 1.081

C8H4O1F3Cl1 colorless 208.6 175(2) MoKR 0.7107 30 orthorhombic P212121 7.478(3) 9.884(4) 11.222(5) 90 90 90 829.4(6) 4 1.67 0.463 416.0 2.8, 27.3 0.95 -9,9, -12,12, -12,14 6422 1718 1454 134 FMLS on F2 0.048, 0.040 0.106, 0.100 -0.20, +0.21 1.070

C8H4O1F3Br1 colorless 253.0 135(2) MoKR 0.7107 28 monoclinic P21/c 11.826(10) 10.237(9) 7.298(7) 90 101.11(2) 90 869.6(3) 4 1.93 4.729 487.9 1.8, 27.2 0.99 -14,14, -12,12, -9,9 6407 1785 1145 134 FMLS on F2 0.078, 0.041 0.096, 0.084 -0.31, +0.73 0.996

C8H4O1F3Br1 colorless 253.0 175(2) MoKR 0.7107 -1.5 monoclinic P21/c 4.504(2) 14.915(6) 12.927(5) 90 90.46(6) 90 868.3(1) 4 1.94 4.737 487.9 2.1,28.2 0.99 -5,5, -19,19, -16,16 7214 2026 1546 134 FMLS on F2 0.049, 0.032 0.089, 0.082 -0.59, +0.47 1.023

C8H4O3N1F3 pale yellow 219.0 175(2) MoKR 0.7107 55 orthorhombic Pnma 11.838(3) 6.278(2) 10.692(3) 90 90 90 851.5(1) 8 1.71 0.172 439.9 2.6,27.2 0.99 -14,15, -8,8, -12,13 6255 985 794 100 FMLS on F2 0.049, 0.038 0.102, 0.095 -0.26, +0.26 1.029

freezing point and the melting temperature of the solidified phase. The DSC data were used to design the crystallization experiments on the diffractometer, for example, the conditions of solidification and the RAMP rate (the rate at which the liquid must be cooled to attain the solidification of the sample and induce crystallinity in the sample). A

Lindemann glass capillary of ∼2.5 cm length and 0.3 mm diameter was filled with the liquid, sealed by flame on one end, and mounted on the Bruker AXS X-ray diffractometer equipped with SMART APEX CCD area detector. Cooling at a rate of 30 K/h, using an OXFORD Nitrogen cryo system, it was observed that complete solidification of

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Figure 2. (a) ORTEP plot of 1 drawn at 50% ellipsoidal probability (asymmetric unit). Dotted lines indicate intramolecular C-H‚‚‚F contacts. (b) Packing of 1 in the crystal lattice. Intermolecular C-H‚‚‚O, C-F‚‚‚F-C, π‚‚‚π, and CdO‚‚‚CdO interactions shown as dotted lines. (c) Network of C-H‚‚‚F intermolecular interactions in 1. the liquid in the capillary occurs at 284 K for 1, 253 K for 2, 235 K for 3, and 230 K for 4. In the first attempt, with this cooling rate the crystallinity in the sample was ensured by crossing over the glass transition temperature to avoid possible glass formation. To obtain a single crystal inside the capillary, a temperature gradient was created by physically moving the nitrogen stream away from the goniometer and upon subsequent melting of the solid, the stream was realigned with a time delay. This process of annealing was repeated manually until a well-defined single crystal was obtained (Figure 1a-d). A rotation photograph32 is taken at this stage to ascertain the quality of the diffraction spots. The temperature was allowed to stabilize for half an hour after which 180 frames of data were collected with the 2θ fixed at -25° and a ω scan width of -1°. The frames were then processed using SMART,33 and the spots were analyzed using RLATT33 to determine the unit cell dimensions. Data were collected on three sets of 606 frames with 2θ ) -25° and with φ values of 0°, 90°, and 180°, respectively. The crystal structures were solved using SIR9234 and refined using SHELXL97.35 All hydrogen atoms were located from difference Fourier maps in each structure and were refined isotropically.

Results and Discussion Figures 2a-6a give the ORTEP diagrams of all five compounds including the intramolecular features of the C-H‚‚‚F interaction in 1-4 and C-H‚‚‚F and C-H‚‚‚O in 5. Table 1 lists the relevant crystallographic and refinement data, while Table 2 lists all intra- and intermolecular hydrogen bonds and short contacts (X‚‚‚X and X‚‚‚O), respectively. It is to be noted that all the compounds exhibit a characteristic intramolecular C-H‚‚‚F interaction and as a consequence the dihedral

angles between the plane of the phenyl ring and the trifluoroacetyl moiety are nearly planar with values of 7.4(1)°, 3.8(2)°, 5.9(2)°, 4.9(1)°, and 0.0°, respectively, in all five structures. All the compounds have been examined with respect to the propensity of generating supramolecular packing motifs particularly involving X‚‚‚X, X‚‚‚O, and C-H‚‚‚X intermolecular interactions. Compound 1 belongs to the centrosymmetric monoclinic space group P21/c and the packing in the lattice is through a network of C-H‚‚‚O, C-H‚‚‚F, and C-F‚‚‚F-C along with aromatic π‚‚‚π interactions (Figure 2b). In addition, dipolar CdO‚‚‚CdO interactions involving the carbonyl group (d ) 3.213 Å) stabilize the crystal structure. The occurrence of an intermolecular C-F‚‚‚F-C interaction (2.987(2) Å) involving the p-substituted fluorine atom (F1) with one of the fluorine atoms (F4) of the -CF3 group is a unique feature in this molecule. Even though the sum of the van der Waals radii of fluorine atoms limit to 2.94 Å,36 it is to be noted that the position of the minimum in the intermolecular potential is about 0.4 Å longer than the sum of the van der Waals radii.37 The atom F1 further connects to the atom H3 resulting in a C-H‚‚‚F contact. Several other C-H‚‚‚F contacts (Figure 2c) involving electronically different fluorine atoms F1 connected to C (aromatic carbon) and F2, F3, F4 of the trifluoromethyl group generate the connectivity motif in the supramolecular assembly (Table 2a). Atoms F2 and F3 form bifurcated intermolecular interactions involving H5, H6 with F2 and H2, H6 with F3,

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Table 2. (a) List of Intra- and Intermolecular Interactions Involving Hydrogen Atoms in Compounds 1-5 and (b) List of Short X‚‚‚X and X‚‚‚O Contacts in Compounds 1-5 (a) Intra- and Intermolecular Interactions Involving Hydrogen Atoms in Compounds 1-5 comp

D-X‚‚‚A

r(D-X)/Å

r(X‚‚‚A)/Å

r(D‚‚‚A)/Å

∠D-X‚‚‚A/deg

symmetry operator

1

C3-H3‚‚‚F4 C3-H3‚‚‚F3 C6-H6‚‚‚O1 C3-H3‚‚‚F1 C6-H6‚‚‚F2 C6-H6‚‚‚F3 C5-H5‚‚‚F2 C2-H2‚‚‚F3 C2-H2‚‚‚F4 C3-H3‚‚‚F2 C3-H3‚‚‚F3 C3-H3‚‚‚O1 C5-H5‚‚‚F1 C2-H2‚‚‚F1 C2-H2‚‚‚F2 C3-H3‚‚‚F3 C3-H3‚‚‚F2 C2-H2‚‚‚F3 C3-H3‚‚‚F3 C3-H3‚‚‚F2 C2-H2‚‚‚O1 C1-H1‚‚‚F2 C5-H5‚‚‚Br1 C1-H1‚‚‚O3 C5-H5‚‚‚O1 C5-H5‚‚‚O2 C3-H3‚‚‚F2 C3-H3‚‚‚O1

0.96(2) 0.96(2) 0.94(2) 0.96(2) 0.94(2) 0.94(2) 0.96(2) 0.90(2) 0.90(2) 0.86(3) 0.86(3) 0.86(3) 1.02(3) 0.93(3) 0.93(3) 0.96(4) 0.96(4) 1.03(5) 0.90(3) 0.90(3) 0.91(3) 0.98(4) 0.97(4) 0.95(3) 0.94(2) 0.94(2) 0.91(2) 0.91(2)

2.55(2) 2.52(2) 2.63(2) 2.69(1) 2.77(2) 2.75(2) 2.80(1) 2.82(2) 2.80(2) 2.60(3) 2.55(3) 2.81(3) 2.80(3) 2.79(3) 2.82(3) 2.56(4) 2.40(4) 2.64(5) 2.50(3) 2.58(3) 2.53(3) 2.71(4) 3.04(4) 2.39(2) 2.51(2) 2.41(2) 2.52(2) 2.39(2)

3.075(2) 2.999(2) 3.271(2) 3.419(2) 3.620(2) 3.240(2) 3.718(2) 3.694(2) 3.251(2) 3.035(3) 3.044(3) 3.507(3) 3.738(3) 3.501(3) 3.663(3) 3.045(6) 3.007(6) 3.563(6) 3.028(4) 3.075(4) 3.427(4) 3.678(4) 4.006(3) 2.712(3) 2.792(3) 2.708(3) 3.028(2) 3.208(3)

115(1) 111(1) 125(1) 133(1) 151(1) 113(1) 159(1) 162(2) 113(1) 113(2) 118(2) 139(2) 153(2) 134(2) 152(2) 112(3) 121(3) 150(4) 118(2) 115(2) 169(2) 173(3) 171(2) 99(2) 97(2) 97(2) 116(1) 149(2)

C1-H1‚‚‚O2

0.95(3)

2.69(3)

3.450(3)

137(2)

C2-H2‚‚‚O3

0.98(2)

2.55(2)

3.481(3)

158(2)

C1-H1‚‚‚F2

0.95(3)

2.81(2)

3.437(2)

125(2)

x, y, z x, y, z -x, y +1/2, -z - 1/2 -x + 1, y + 1/2, -z + 1/2 x, y - 1, z x, -y - 1/2, z - 1/2 -x, y - 1/2, -z - 1/2 -x + 1, y - 1/2, -z + 1/2 -x + 1, -y, -z x, y, z x, y, z -x + 1/2, -y + 1, z - 1/2 -x, y - 1/2, -z + 3/2 -x + 1/2, -y + 1, z - 1/2 x - 1/2, -y + 1/2, -z + 1 x, y, z x, y, z -x + 2, y - 1/2, -z + 1/2 x, y, z x, y, z x - 1, -y + 3/2, z + 1/2 -x, y - 1/2, -z + 3/2 -x + 1, -y + 1, -z + 1 x, y, z x, y, z x, y, z x, y, z x + 1/2, y, -z + 1/2 x + 1/2, -y + 1/2, -z + 1/2 x + 1/2, y, -z - 1/2 x + 1/2, -y + 1/2, -z - 1/2 x + 1/2, y, -z - 1/2 x + 1/2, -y + 1/2, -z - 1/2 -x + 1, y - 1/2, -z -x + 1, -y + 1, -z

2

3 4

5

(b) Short X‚‚‚X and X‚‚‚O Contacts in Compounds 1-5 comp

D-X‚‚‚A-D

r(X‚‚‚A)/Å

∠D-X‚‚‚A/°

∠X‚‚‚D-A/°

symmetry operator

1 3

C8-F4‚‚‚F1-C1 C1-Br1‚‚‚O1)C7 C1-Br1‚‚‚F1-C8 C8-F2‚‚‚F2-C8 C8-F1‚‚‚F1-C8 C6-Br1‚‚‚O1)C7 C8-F3‚‚‚F1-C8 C8-F3‚‚‚F2-C8 C8-F1‚‚‚O3-N1

2.987(2) 3.380(5) 3.337(4) 2.905(5) 2.941(5) 3.332(2) 2.783(3) 2.995(3) 2.874(3)

125.4 170.4 125.2 146.8 164.9 158.3 162.8 151.8 175.6

112.2 133.2 133.3 146.8 164.9 142.4 96.4 87.0 166.4

C8-F2‚‚‚F1-C8

2.982(2)

138.2

113.5

x, -y - 1/2, z + 1/2 x, y - 1, z x, y - 1, z -x + 2, -y + 1, -z + 1 -x + 2, -y + 1, -z -x + 1, -y + 1, -z + 1 x - 1, y, z x - 1, y, z x, y, z + 1 x, -y + 1/2, z + 1 -x + 1, -y + 1, -z + 1 -x + 1/2, y - 1/2, -z + 1

4 5

respectively, forming molecular chains along the b-axis, which also encompass an additional C-H‚‚‚O contact. Further, atom F4 forms dimeric units involving hydrogen H2 providing additional stability. Subsequent stability is provided by aromatic π‚‚‚π stacking interactions with a separation distance of 3.941(1) Å (Cg1: center of gravity of the aryl ring; Figure 2b). Compound 2, with the chlorine atom in the para position, crystallizes in a noncentrosymmetric space group P212121, the corresponding acetophenone derivative also having crystallized in the same space group symmetry. It was observed28 that C-H‚ ‚‚Cl contacts stabilize the packing in para-chloroacetophenone instead of the expected Cl‚‚‚O or Cl‚‚‚Cl contacts due to the weak Lewis acid character of the chlorine atom. However, in compound 2, C-H‚‚‚Cl interactions are entirely absent, and C-H‚‚‚F and C-H‚‚‚O intermolecular contacts along with the presence of aromatic stacks along the a-axis, the stacking distance being 3.841(3) Å (Cg1: center of gravity of the aryl

ring) (Figure 3b), stabilize the crystal structure. Atom F1 is involved in the formation of bifurcated interactions, involving hydrogen H2 and H5 forming molecular chains along the crystallographic c- and b-axis in conjunction with C-H‚‚‚O interactions (involving H3), providing the overall stability. Further, atom F2, involving hydrogen H2, forms molecular chains along the crystallographic a-axis. The p-bromo compound 3, crystallizing in the space group P21/c, exhibits a well-defined Br‚‚‚O [Br1‚‚‚O1 ) 3.380(5) Å; ∠C1-Br1‚‚‚O1 ) 170.4°; Figure 4b], short contact, the observed result corroborates with the fact that bromine participates in the formation of a linear and directional halogen bond.23,25,26 An exceptional Br‚‚‚F contact [Br1‚‚‚F1 ) 3.337(4) Å; Figure 4b] forming an infinite motif along the crystallographic b-axis provides further stability along with π‚‚‚π aromatic interactions with a stacking distance of 3.739(4) Å, resulting in an overall sheetlike structural motif (Figure 4b).

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Figure 4. (a) ORTEP plot of 3 drawn at 50% ellipsoidal probability (asymmetric unit). Dotted lines indicate intramolecular C-H‚‚‚F contacts. (b) Regions of intermolecular interactions depicting Br‚‚‚O, Br‚‚‚F, C-H‚‚‚F, C-F‚‚‚F-C, and π‚‚‚π interactions in 3.

Figure 3. (a) ORTEP plot of 2 drawn at 50% ellipsoidal probability (asymmetric unit). Dotted lines indicate intramolecular C-H‚‚‚F contacts. (b) Packing features of 2 involving C-H‚‚‚F, C-H‚‚‚O, and π‚‚‚π interactions.

The remaining two fluorine atoms of the -CF3 group are involved in short F‚‚‚F contacts [F2‚‚‚F2 ) 2.905(5) Å and F3‚ ‚‚F3 ) 2.941(5) Å; with ∠C8-F2‚‚‚F2 and ∠C8-F1‚‚‚F1 being equal to 146.8° and 164.8°, respectively (Type I contact,3 Figure 4b). There is an intermolecular C-H‚‚‚F interaction, involving H2, forming chains along the crystallographic screw b-axis. It is of interest to note that there are no C-H‚‚‚Br contacts in compound 3, whereas the corresponding para-bromoacetophenone28 has several additional C-H‚‚‚Br contacts. meta-Bromo-substituted compound 4 crystallizes in situ in the space group P21/c. Short dimeric Br‚‚‚O contacts [Br1‚‚‚ O1 ) 3.332(2) Å; ∠C6-Br1‚‚‚O1 ) 158.3°; Figure 5b] are observed in the crystal packing. A rare C-H‚‚‚Br interaction [H5‚‚‚Br1 ) 3.04(4) Å; ∠C5-H5‚‚‚Br1 ) 171.2°] involving the hydrogen atom H5 is also seen as a consequence. A C-H‚‚‚F interaction, involving H1 forming chains along screw b-axis, along with a C-H‚‚‚O hydrogen bond involving H2 forming chains along the c glide plane generates a twodimensional sheetlike structure. Short Type II F‚‚‚F contacts involving fluorine atoms C8-F3‚‚‚F1-C8 and C8-F3‚‚‚F2C8 forms a triangular motif with distances 2.783(3) and 2.995(3) Å; the corresponding ∠C8-F3‚‚‚F1, ∠C8-F3‚‚‚F2 angles are 162.8° and 151.8°, respectively (Figure 5b). It is

interesting to note that even though the molecules are arranged parallel to each other the distances prohibit aromatic stacking interactions. Compound 5 was investigated to get a comparison with respect to the halogen interactions, the presence of the meta nitro group bringing in additional interactions. As can be seen from the ORTEP diagram (Figure 6a) three intramolecular C-H‚‚‚O interactions allow the entire molecule to adopt a flat conformation and the space group Pnma with Z ) 4 ensures that except for fluorine atom F2 the rest of the atoms lie on the crystallographic mirror plane (y-coordinate ) 0.2500). The packing hence is controlled through intermolecular C-H‚‚‚O interactions (involving hydrogen H1, H2 and oxygens O2, O3 of the nitro group) resulting in the formation of molecular “sheets” along the crystallographic a-axis related by glide plane (Figure 6b). An additional C-H‚‚‚O interaction (involving H3 and carbonyl oxygen atom O1) generates a sheetlike structure in the ac-plane (Figure 6b). Remarkably, a short F‚‚‚O contact [C8-F1‚‚‚O3-N1 ) 2.894(3) Å], a C-H‚‚‚F intermolecular interaction (involving H1 and F2), a short C-F...F-C intermolecular contact [C8-F1‚‚‚F2-C8 ) 2.982(2) Å], and a π‚‚‚π stacking interaction [stacking distance ) 3.750(1) Å] provide interlayer stability (Figure 6c). It is noteworthy that the crystal structure of 3-nitroacetophenone38 has both the acetyl and nitro groups out of planarity with the dihedral angles 9.4(2)° and 1.8(2)°, respectively, unlike compound 5, even though the sheetlike structure in the ac-plane is generated by similar C-H‚‚‚O contacts.

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Figure 5. (a) ORTEP plot of 4 drawn at 50% ellipsoidal probability (asymmetric unit). Dotted lines indicate intramolecular C-H‚‚‚F contacts. (b) Regions of intermolecular interactions highlighting Br‚‚‚O dimers, C-H‚‚‚O, C-H‚‚‚F, and C-F‚‚‚F-C interactions in 4.

Conclusions This study clearly demonstrates that halogens generate a variety of interactions depending on their size and reactivity, given the conformationally rigid molecular architecture. The propensity of fluorine interactions is as follows: generally C-H‚‚‚F, occasionally F‚‚‚F, and rarely F‚‚‚X (X ) Br, O) in these five compounds. The two bromine derivatives show priorities toward the formation of Br‚‚‚O and Br‚‚‚F interactions in the presence of C-H‚‚‚F and C-H‚‚‚O interactions. It is of extreme significance to note that F‚‚‚F contacts play a significant role in holding molecules together in structures 1, 3, 4, and 5. Indeed, the short F‚‚‚F contacts display features similar to those observed for Cl ‚‚Cl contacts3 (for example, Type I, θ1 ≈ θ2 in compound 3 and Type II, θ1 ≈ 180°, θ2 ≈ 90° in compound 4). However, F‚‚‚F contacts in 1 and 5 deviate from the above with θ1 * θ2 > 90°. The analysis suggests weak intermolecular contacts, without the involvement of H-atoms, develop selectively depending on the size of the substituted halogen. In

Figure 6. (a) ORTEP plot of 5 drawn at 50% ellipsoidal probability. Dotted lines indicate intramolecular C-H‚‚‚F contacts. Prime denotes fluorine atom generated across the mirror plane. (b) Packing of molecules showing C-H‚‚‚O interactions in 5. (c) Regions of intermolecular interactions showing C-H‚‚‚F, C-F‚‚‚O, C-F‚‚‚F-C, and π‚‚‚π interactions in 5.

accordance with the reported literature,23,25,26 the crystal structures confirm that larger halogens (Br and I) prefer interactions

874 Crystal Growth & Design, Vol. 7, No. 5, 2007

with oxygen atoms, while smaller halogens (Cl and F) prefer interactions of the type C-H‚‚‚Cl or C-H‚‚‚F or with “likeatoms” to generate Cl‚‚‚Cl and F‚‚‚F contacts. Acknowledgment. D.C. thanks CSIR, India, for JRF and Mr. Padaikanta for DSC measurements. We thank DST-IRHPA, India, for data collection on the CCD facility at IISc, Bangalore. Supporting Information Available: Rotation photos, DSC traces, and single-crystal X-ray crystallographic information (CIFs). This material is available free of charge via the Internet at http://pubs.acs.org.

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