Substituent Effects in the Crystal Structures of Isoxindigos [(E)-3,3'-Bibenzofuranylidene-2,2'-diones]

Abstract

Single-crystal X-ray structure determinations of the parent isoxindigo (1), C₁₆H₈O₄, together with those of derivatives substituted in both benzene rings with ( i ) t-butyl groups in the 5- and 7-positions [derivative (2)] and (ii) methoxy and t-butyl groups in the 5- and 7- positions respectively [two polymorphs, (3α,8)] have been determined in order to examine the possibility of distortion of molecular planarity by substituent steric effects, and the spectroscopic consequences thereof. Crystals of (1) are monoclinic, P2₁/n, a 17.69(1), b 5.690(3), c5.693(3)Ǻ, β 100.52(5)°, Z 2; after least-squares refinement, a residual R 0.067 was obtained for N_o 385 ?observed? Independent reflections. For (2), C₃₂H₄₀O₄, crystals are monoclinic, P2₁/n, a 16.402(8), b 12.313(6), c 13.913(8)Ǻ, β 96.75(4)°, Z4; R 0.088 for N_o 1261. For (3), C₂₆H₂₈O₆ (α-form), crystals are orthorhombic, Pcab , a 10.449(4), b 12.477(4), c 17.825(6)Ǻ, Z 4; R 0.058 for N_o 959; in the case of the β-form of (3), crystals are monoclinic, P2₁/n, a 6.828(2), b 15.464(3), c 10.770(3) Ǻ, β 98.58(4)°, Z 2. In (1) and (3α,β), the molecules all lie disposed about crystallographically imposed symmetry elements, with one-half of the molecule, with a substantially planar fused-ring skeleton, comprising the asymmetric unit. In (2), the full molecule is the asymmetric unit; the fused-ring skeletons are substantially planar also, but the molecule exhibits a substantial twist about the bond connecting the two halves, with an interplanar dlhedral angle of 16.8°. This is ascribed to the steric effect of the large and more numerous t-butyl substituents.