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 2JPH coupling constants are approximately equal but of opposite sign, and fast exchange between the methylene protons in solution leads to an observed 2JPH 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 1JPSe 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.