Nature of the boron-phosphorus bond in monomeric phosphinoboranes and related compounds

Article abstract of DOI:10.1021/ja00022a034

The nature of the bonding between boron and phosphorus in monomeric phosphinoboranes (also called borylphosphanes, i.e., R₂BPR₂, R and R' = hydrocarbyl groups, B and P three-coordinate) and related species has been studied in some detail. This was accomplished by the synthesis and detailed spectroscopic and structural characterization of a number of rare monomeric phosphinoboranes and related derivatives. These compounds are distinguished by the presence of B-P moieties in which there is a lone pair located on phosphorus adjacent to an empty boron p-orbital. The main conclusion of these studies is that there exist fundamental differences between the B-P compounds and their B-N analogues. The major reason for this is the presence of a large inversion barrier at the phosphorus center, rather than any inherent weakness in B-P π-bonds. The inversion barrier and the strength of the B-P π-interaction can be controlled by electronic and steric factors. In essence, increasing the size and the electropositive character of the phosphorus substituents increases the strength of the B-P π-bond. The compounds studied include Mes₂BP(1-Ad)H, 1; Mes₂BPPh₂, 2; Mes₂BP(t-Bu)₂, 3; Mes₂BPMes₂, 4; Mes₂BP(1-Ad)Li(Et₂O)₂, 5; Mes₂BP(Ph)Li(Et₂O), 6; (Mes₂B)₂PPh, 7; Mes₂BP(1-Ad)PPh₂, 8; Mes₂BP(Ph)SiPh₃, 9; and Mes₂BP(1-Ad)SiMe₃,10 (Mes = 2,4,6-Me₃C₆H₂, 1-Ad = 1-adamantanyl). They were examined by variable-temperature ¹H, ¹¹B, and ³¹P NMR spectroscopy. Most compounds were also characterized by X-ray crystallography. The studies demonstrate that whereas the boron center is always planar, the phosphorus coordination can exhibit considerable geometrical variation between planarity and pyramidicity. There is a good correlation between the degree of pyramidicity at phosphorus and the B-P bond length, which can vary from 1.810 (4) A? for planar phosphorus to 1.948 (3) A? in the most pyramidal species. Variable-temperature ¹H NMR data for 1-3 and 10 reveal inversion barriers of <12.4, <10.4, and <9.7 kcal mol^-1 for 1, 2, and 10, whereas in the almost planar tert-butyl derivative 3 no dynamic behavior was obsereved as low as -97°C. Rotational barriers of 12.4 and 16.9 kcal mol^-1 were also found for 1 and 10. The ¹H NMR data for the compounds 5-9, which have essentially planar geometry at phosphorus, afford rotational barriers (kcal mol^-1) for the B-P multiple bond of 22.3 for 5, 22.2 for 6, 21.3 for 7, 21.3 for 8, and 17.1 for 9. In summary, it may be concluded that B-P π-bonds are comparable in strength to B-N π-bonds. Crystal data with Mo Kα radiation (λ = 0.71069 A?) at 130 K: 1, C₂₈H₃₈BP, monoclinic, P2₁/c, a = 9.461 (2) A?, b = 26.235 (10) A?, c = 10.415 (3) A?, β= 111.93 (2)°, R(3097I > 3σ(I) data) = 0.050; 3, C₂₆H₄₀BP, monoclinic P2₁/c, a = 23.530 (9) A?, b = 12.392 (5) A?, c = 18.119 (8) A?, β = 110.88 (3)°, R(3778I > 3σ(I) data) = 0.077; 4, C₃₆H₄₄BP, monoclinic, P2/n, a = 12.342 (4) A?, b = 7.703 (2) A?, c = 17.021 (5) A?, β= 108.94(2)°, R(1973I > 2σ(I) data) = 0.059; 5, C₄₀H₆₇O₃BPLi, monoclinic, P2₁/H, a = 11.570 (2) A?, b = 20.203 (4) A?, c = 16.053 (5) A?, β= 95.84 (2)°, R(2595I > 3σ(I) data) = 0.065; 9, C₄₂H₄₂BPSi, triclinic, P1?, a = 9.112 (4) A?, b = 10.042 (6) A?, c = 19.479 (12) A?, α = 92.88 (5)°, β= 97.58 (5)°, γ = 97.03 (4)°, R(4180I > 3σ(I) data) = 0.081; 10, C₃₁H₄₆BPSi, orthorhombic, P2₁P2₁P2₁, a = 9.625 (5) A?, b = 11.067 (6) A?, c = 26.676 (12) A?, R(1958I > 2σ(I) data) = 0.056.