Large, weakly basic bis(carboranyl)phosphines: an experimental and computational study

The bis(carboranyl)phosphines [μ-2,2′-PPh-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (I) and [μ-2,2′-PEt-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (1) have been prepared and spectroscopically and structurally characterised. Crystallographic and DFT computational studies of 1 suggest that the orientation of the ethyl group, relative to the bis(carborane), is the result of intramolecular dihydrogen bonding. This orientation is such that the magnitudes of the ²J_PH coupling constants are approximately equal but of opposite sign, and fast exchange between the methylene protons in solution leads to an observed ²J_PH close to zero. The steric properties of I, 1 and their derivatives [μ-2,2′-P(Ph)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (2) and [μ-2,2′-P(Et)AuCl-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (3) have been assessed by Tolman cone angle and percent buried volume calculations, from which it is concluded that the bis(carboranyl)phosphines I and 1 are comparable to PCy₃ in their steric demands. The selenides [μ-2,2′-P(Ph)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (4) and [μ-2,2′-P(Et)Se-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (5) have also been prepared and characterised. The ¹J_PSe coupling constants for 4 and 5 are the largest reported so far for carboranylphosphine selenides and indicate that I and 1 are very weakly basic.