77657-15-9Relevant academic research and scientific papers
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 π bonds
Ashby, Michael T.,Sheshtawy, Nader A.
, p. 236 - 243 (2008/10/08)
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=O) crystallizes in the triclinic space group P1 with Z = 2, a = 8.155(3) ?, b = 10.230(6) ?, c = 11.328(5) ?, α = 65.62(4)°, β = 72.70(3)°, γ = 82.12(4)°, R = 0.073, and Rw = 0.083 at -90°C. Derivative 1(X=S) crystallizes in the monoclinic space group P21/c with Z = 4, a = 13.509(8) ?, b = 8.132(5) ?, c = 16.079(6) ?, β = 99.66(4)°, R = 0.067, and Rw = 0.089 at 25°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 Bpπ-Xpπ bonding. The mesitylene rings are rotated ～60° with respect to the C2BX plane, which prohibits significant Bpπ-aryl interaction. Thus, the crystal structures of 1(X=O,S) offer benchmarks for comparing discrete Bpπ-Xpπ bonds: B-O = 1.351(5) ?, B-O-C = 123.6(3)°, C-B-O-C = 173.8(3)°, C′-B-O-C = -4.0(5)°; B-S = 1.792(6) ?, B-S-C = 109.4(3)°, C-B-S-C = 175.9(4)°, C′-B-S-C = -4.3(6)°. 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-O 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 BH2 π-acceptor moiety, that the OH and SH groups are comparable π donors in the ground-state geometry (H-B-X-H = 0, 180°), but the OH group is a much better π donor than the SH group in the transition-state geometry (H-B-X-H = 90°). Thus the trend in the barriers to rotation is attributed to a greater stabilization of the transition state by oxygen and not a stronger Bpπ-Spπ 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 π-bond strengths.
DIMESITYLBORYL COMPOUNDS. IV. SULPHUR DERIVATIVES
Davidson, F.,Wilson, J. W.
, p. 147 - 152 (2007/10/02)
A series of hydrocarbylthiodimesitylboranes (Mes2BSR) has been synthesized, characterised and their 1H, 11B and 13C NMR data recorded.The steric hindrance induced by the four ortho methyl groups is insufficient to prevent rapid hydrolysis in the air of the B-S bonds in compounds where R = alkyl.However, with the bulkier R = aryl groups hydrolytic stability is achieved.The existence of appreciable boron-sulphur ? back-bonding in these compounds is shown by the presence of non-equivalent mesityl groups in the ambient temperature 1H and 13C NMR spectra.The mechanism which leads to removal of this non-equivalence has a free energy of activation ΔG77 = 76.9+/-0.4 kJ mol-1 for R = Ph.
