MOLECULAR-CRYSTAL ENGINEERING BY SHAPE MIMICRY

Based on the high statistical preference for organics in general and t rue racemates in particular to pack in one of the centrosymmetric spac e groups, formation of molecular crystals of single enantiomers of sha pe-symmetric molecules was predicted to occur with approximate crystal lographic symmetry. Thus, enantiomerically pure sulfoxide 1 was design ed with two segments of nearly identical shape but with significantly different electron-donor/acceptor properties. Packing of 1 was predict ed to occur in a near-centrosymmetric or quasicentrosymmetric fashion, with pairs of 1 arranged as would be expected for a true racemic crys tal. Such a packing motif would lack true centrosymmetry and, in certa in space groups, could result in a polar direction in the crystal and net additivity of the vectors from nitrogen to sulfur (the direction o f polarizability for the molecules). Enantiomeric sulfoxide 1 does for m molecular crystals with near centrosymmetry, mimicking P2(1)/c, wher e the vector from the dimethylamino group to sulfur of one molecule is antiparallel to that from the isopropenyl group to the sulfur of the other. Thus, there is a substantial net directionality of polarizabili ty vectors from nitrogen to sulfur in this pair. By contrast, the sulf oxides 2 and 3, wherein the two phenyl [GRAPHICS] moieties in each dif fer significantly in shape, form molecular crystals in which the vecto rs from nitrogen to sulfur are antiparallel and there is substantial n et additivity of the dipole vectors from sulfur to oxygen. Crystal dat a for 1 at -80-degrees-C: monoclinic, P2(1) (No. 4), Z = 4 (quasi-P2(1 )/c), a = 7.6716(13) angstrom, b = 8.2498(20) angstrom, c = 26.1127(49 ) angstrom, beta = 114.716(13)degrees, for data with I > 2sigma(I), R1 = 0.0505, wR2 = 0.1059. Crystal data for 2 at 25-degrees-C: monoclini c, P2, (No. 4), Z = 2, a = 7.8030(6) angstrom, b = 6.0355(6) angstrom, c = 17.037(2) angstrom, beta = 96.899(7)degrees, for data with I > 2s igma(I), R1 = 0.0542, wR2 = 0.1353. Crystal data for 3 at -80-degrees- C: monoclinic, P2(1)(No.4), Z = 2, a = 7.7523(8)angstrom, b = 5.9869(7 ) angstrom, c = 14.8133(16) angstrom, beta = 103.244(8)degrees, for da ta with I > 2sigma(I), R1 = 0.0383, wR2 = 0.1041. All three structures were refined by full-matrix least-squares methods versus Absolute val ue of \F\2.