Using Halogen‚‚‚Halogen Interactions to Direct Noncentrosymmetric Crystal Packing in Dipolar Organic Molecules Binoy K. Saha,† Ashwini Nangia,*,† and Jean-Franc¸ ois Nicoud‡ School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India, and Groupe des Mate´ riaux Organiques, Institut de Physique et Chimie des Mate´ riaux de Strasbourg, CNRS et UniVersite´ Louis Pasteur (UMR 7504), Strasbourg Cedex 2, France
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1278-1281
ReceiVed March 3, 2006; ReVised Manuscript ReceiVed April 20, 2006
ABSTRACT: Halogen atoms Cl, Br, and I steer crystallization of 2-halo-3-hydroxypyridine 1, pyridine-N-oxides 2, and 2-halo-3aminopyridine 3 in noncentrosymmetric space groups efficient for nonlinear optical materials. Whereas the strong hydrogen-bond chains are aligned antiparallel, the L- or V-geometry of weak interhalogen interaction steers polar alignment in the solid state, even for high dipole moment (µ) halogenated N-oxides 2 (µ > 5.0 D). Six out of seven structures evaluated crystallize in polar/chiral space groups due to interhalogen interactions compared to the one in six probability of noncentrosymmetric packing for achiral molecules. Crystallization of achiral or racemic molecules in space groups that lack the inversion center continues to be a challenge since the early days of crystal engineering.1 Crystal structures with noncentrosymmetric packing of molecules are important as electrooptic and nonlinear optical materials;2 enantiomorphous crystals are useful in asymmetric synthesis and provide insight into the evolution of chirality in nature. Centrosymmetric packing is favored for typical organic molecules having hydrogen-bonding functional groups and hydrophobic alkyl/aryl substituents because of the antiparallel alignment of dipoles (charge cancellation in neighboring domains) and the universality of close packing in molecular crystals (bumps fit into hollows). Only 10-15% of achiral organic molecules crystallize in noncentrosymmetric space groups, such as P21, P212121, Pca21, Pna21, Fdd2, Cc, etc.3 Meta-substituted nitrobenzenes have a higher probability for polar/chiral crystallization (40%) because of their dissymmetric shape.4 Among the various approaches being tried to produce nonlinear optical (NLO) crystals, e.g., vanishing dipole moment (µ) (POM),1b octupolar molecules,5 and diamondoid networks,6 the only guaranteed solution to avoiding the inversion center is to use chiral molecules (e.g., NPP).1b A better understanding of the factors that promote polar organization of achiral molecules in the solid state is important for crystal engineering and supramolecular materials. Our approach to promote noncentrosymmetric crystallization of dipolar molecules is summarized in Scheme 1. The strong hydrogenbonding functional groups will usually align antiparallel (big outline arrows). If a halogen atom is placed in the molecule, roughly orthogonal to the main dipole vector (small shaded arrows), then the overall structure would be noncentrosymmetric because of the L- or V-shaped geometry in X‚‚‚X interactions;7 L-geometry typically occurs across 21 screw-axis or glide plane, whereas V-geometry is possible about 2-fold rotation symmetry. To test our hypothesis, 2-halo-3-hydroxypyridine 1 (X ) Cl, Br, I) compounds were synthesized from the parent compound. Crystals of 1Cl, 1Br and 1I were grown from EtOAc.8 Chains of O-H‚‚‚N hydrogen bonds run in opposite directions in 1H,9 a typical packing motif in the centrosymmetric P21/c space group. In contrast, 1Cl crystallizes in the polar space group Fdd2, and 1Br and 1I have isostructural packing in the Pna21 space group. These space groups are devoid of the inversion center, a necessary condition for SHG activity of optical crystals. Antiparallel chains of O-H‚‚‚N hydrogen bonds (2.12 Å, 130.1°) between glide related molecules run along the a-axis in 1I. The polar arrangement arises due to short I‚‚‚I interaction of L-geometry (3.74 Å, C-I‚‚‚I 174.4°, * To whom correspondence should be addressed.
[email protected]. Fax: +91 40 23011338. † University of Hyderabad. ‡ CNRS et Universite ´ Louis Pasteur (UMR 7504).e
E-mail:
Scheme 1. (a) Interhalogen Interaction Types. (b) Noncentrosymmetric Alignment of Hydrogen-Bond Chains Due to L- or V-geometry of X‚‚‚X Interactions. (c) 3-Pyridinol 1, Pyridine-N-Oxide 2, and 3-Aminopyridine 3 Structures
114.5°) between the polarizable iodine atoms (Bondi’s van der Waals radius: Cl 1.75 Å, Br 1.85 Å, I 1.98 Å). C-I vectors of molecules in (021) and (021h) planes generate a net dipole along the -c direction (Figure 1). 1Br has an identical polar arrangement of hydrogen-bonded molecules (2.06 Å, 150.9°) directed by short Br‚‚‚Br interaction (3.64 Å, C-Br‚‚‚Br 170. 2°, 121.2°). Cl atoms related by 2-fold axis10 (3.35 Å, 158.9°, 158.9°) connect hydrogenbond chains in 1Cl (O-H‚‚‚N: 1.92 Å, 164.9°; Figure 2). Noting the isostructurality of 1Br and 1I, we attempted synthesis of the ethynyl analogue11 but instead isolated the cyclized benzofuran (see Supporting Information). The parallel alignment of molecules in halogenated 1 compared to the centrosymmetric structure of 1Me
10.1021/cg060117k CCC: $33.50 © 2006 American Chemical Society Published on Web 05/06/2006
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Figure 1. (a) Antiparallel chains of O-H‚‚‚N hydrogen bonds in 1I. (b) Polar alignment of molecules due to the L-geometry of I‚‚‚I interactions between adjacent chains. 1 Br is isostructural.
Figure 2. V-geometry of Cl‚‚‚Cl interaction across a 2-fold rotation axis in 1Cl.
(C2/c) highlights the polarity-inducing ability of interhalogen interactions in hydrogen-bonded crystal structures (Figure 3). A view down the hydrogen-bond chain axis shows that the angle at which the molecular planes intersect is different in Cl, Br, and I derivatives. If the intersection of aromatic rings is viewed as an X junction (Figure 4), halogen‚‚‚halogen interactions serve as the supramolecular handle to modify the hinge angle, θip, which in turn increases the NLO response of the iodo derivative (Table 1). A situation similar to Scheme 1b will also prevail in 2-halo-3hydroxypyridine-N-oxide 2 and 2-halo-3-aminopyridine 3.8 Crystals of 2Br from MeOH (Figure 5, space group P212121) contain O-H‚ ‚‚O- chains (1.88 Å, 175.7°) along [001] and [001h]. Bromine atoms of antiparallel chains engage in the L-shaped interaction (3.93 Å, 171.5°, 126.1°). Similarly, the chloro derivative 2Cl is weakly SHG active (Cl‚‚‚Cl 3.51 Å, Pna21 space group). Continuing further, 3Br crystalizes (from CHCl3) in the enantiomorphous P21 space group via N-H‚‚‚N H bonds and L-shaped Br‚‚‚Br interactions (3.64 Å, 177.8°, 102.6°) between symmetry-independent molecules (Z′ ) 4); the second Br‚‚‚Br contact (3.63 Å, 148.1°, 102.5°) has borderline L/V-geometry (Table S1, Supporting Information). However, 3Cl is centrosymmetric (P21/n,) and devoid of Cl‚‚‚Cl interactions; its packing is similar to 2Me in the P21/c space group. There are several factors that permit us to implicate specific, structure-directing X‚‚‚X interactions. (1) The interplay of hydrogen bonding and interhalogen interactions shows greater emphasis on hydrogen bonding in chloro, whereas X bonding becomes significant in bromo/iodo structures, consistent with the better polarizability of heavier halogens.7,12 Accordingly, the interaction geometry is more L-shaped for iodo than bromo, whereas chloro has V-shaped interaction (Scheme 1a). (2) Iodo compound 1I shows the highest optical response, thanks to optimal polar alignment of dipoles in
Figure 3. Noncentrosymmetric crystalline solid 1I (a) and centrosymmetric structure 1Me (b).
Figure 4. Halogen‚‚‚halogen interactions modulate the (d) interplanar hinge angle (θip) between O-H‚‚‚N hydrogen-bonded chains in 1Cl, 1Br, and 1I (a-c).
its crystal (Figure 4). SHG values in Table 1 bear out these molecular and crystal structure trends. (3) Methyl derivatives 1Me and 2Me are centrosymmetric, whereas halogenated compounds are noncentrosymmetric and SHG active. When methyl and halogen crystal structures are different, specific interhalogen interactions are usually involved.13 The most significant observation is that the halogen derivatives adopt noncentrosymmetric packing even though their dipole moment is higher than that of H/Me compounds. On the basis of a recent statistical analysis of diphenyl ethers,14 molecules with a µ < 4 have a 7:3 preference for centrosymmetry, whereas molecules with µ > 4 almost always adopt centrosymmetric packing (AM1 values). We show that optimized interhalogen
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Table 1. Molecular Dipole Momenta and SHG Efficiencyb of Crystals 1-3
c
compound
space groupc
SHG activity
µ (Debye)
1Cl 1Br 1I 1Me 2Cl 2Br 2Me 2H 3Cl 3Br
Fdd2 (*) Pna21 (*) Pna21 (*) C2/c Pna21 (*) P212121 (*) P21/c P1h P21/n P21 (*)
