BARRIERS TO ROTATION ABOUT THE B-X BONDS OF COORDINATIVELY UNSATURATED BORATES AND THIOBORATES R2BXR' (X =O, S) ARE NOT MEASURES OF THE RELATIVE STRENGTHS OF THEIR B=O AND B=S PI BONDS

The molecular structures of (2,4,6-C6H2(CH3)3)2BXCH3 (1(X = O,S)) have been determined by single-crystal X-ray crystallography. Derivative 1(X 0) crystallizes in the triclinic space group P1BAR with Z = 2, a = 8.155(3) angstrom, b = 10.230(6) angstrom, c = 11.328(5) angstrom, alpha = 65.62(4)-degrees, 72.70(3)-degrees, gamma = 82.12(4)-degrees, R = 0.073, and R(w) = 0.083 at -90-degrees-C. Derivative 1(X = S) crystallizes in the monoclinic space group P2(1)/c with Z = 4, a = 13.509(8) angstrom, b = 8.132(5) angstrom, c = 16.079(6) angstrom, beta = 99.66(4)-degrees, R = 0.067, and R(w) = 0.089 at 25-degrees-C. The boron atoms adopt approximate trigonal planar geometries, and the XC moieties lie in the C2BX planes, an orientation about the B-X bond that maximizes Bppi-Xppi bonding. The mesitylene rings are rotated approximately 60-degrees with respect to the C2BX plane, which prohibits significant Bppi-aryl interaction. Thus, the crystal structures of 1(X = O,S) offer benchmarks for comparing discrete Bppi-Xppi bonds: B-0 = 1.351(5) angstrom, B-O-C = 123.6(3)-degrees, C-B-O-C = 173.8(3)-degrees, C'-B-O-C = -4.0(5)-degrees; B-S = 1.792(6) angstrom, B-S-C = 109.4(3)-degrees, C-B-S-C = 175.9(4)-degrees, C'-B-S-C = -4.3(6)-degrees. A comparison of the B and X effective radii (calculated by assuming the B-C and X-C lengths represent single bonds) indicates that the B-0 bond is stronger than the B-S bond. Ab initio molecular orbital calculations have been carried out on the model compounds H2BXH (2(X = O,S)). The geometries of 2 have been optimized at the SCF level for various rotational orientations about the B-X bonds. The ground-state geometries of 2 are analogous to those observed experimentally, with the X-H bonds lying in the trigonal planes of the boron atoms. Mirroring the dynamic behavior observed experimentally, the energy barrier found for rotation about the B-X bond of 2(X = S) is larger than that for 2(X = O). Mulliken population analysis suggests, with respect to the BH2pi-acceptor moiety, that the OH and SH groups are comparable pi donors in the ground-state geometry (H-B-X-H = 0, 1800), but the OH group is a much better pi donor than the SH group in the transition-state geometry (H-B-X-H = 90-degrees). Thus the trend in the barriers to rotation is attributed to a greater stabilization of the transition state by oxygen and not a stronger Bppi-Sppi bond in the ground state. Accordingly, rotational barriers about the B-X bonds of R2BOR' and R2BSR' complexes are not measures of their relative B-X pi-bond strengths.