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

The bis(carboranyl)phosphines [μ-2,2'-PPh-{1-(1'-1',2'-closo-C2 B10 H10 )-1,2-closo-C2 B10 H10 }] (I) and [μ-2,2'-PEt-{1-(1'-1',2'-closo-C2 B10 H10 )-1,2-closo-C2 B10 H10 }] (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 the2 JPH coupling constants are approximately equal but of opposite sign, and fast exchange between the methylene protons in solution leads to an observed2 JPH close to zero. The steric properties of I, 1 and their derivatives [μ-2,2'-P(Ph)AuCl-{1-(1'-1',2'-closo-C2 B10 H10 )-1,2-closo-C2 B10 H10 }] (2) and [μ-2,2'-P(Et)AuCl-{1-(1'-1',2'-closo-C2 B10 H10 )-1,2-closo-C2 B10 H10 }] (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-C2 B10 H10 )-1,2-closo-C2 B10 H10 }] (4) and [μ-2,2'-P(Et)Se-{1-(1'-1',2'-closo-C2 B10 H10 )-1,2-closo-C2 B10 H10 }] (5) have also been prepared and characterised. The1 JPSe 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.