Exploiting the Electronic Tuneability of Carboranes as Supports for Frustrated Lewis Pairs

The first example of a carborane with a catecholborolyl substituent, [1-Bcat-2-Ph-closo-1,2-C₂B₁₀H₁₀] (1), has been prepared and characterized and shown to act as the Lewis acid component of an intermolecular frustrated Lewis pair in catalyzing a Michael addition. In combination with B(C₆F₅)₃ the C-carboranylphosphine [1-PPh₂-closo-1,2-C₂B₁₀H₁₁] (IVa) is found to be comparable with PPh₂(C₆F₅) in its ability to catalyze hydrosilylation, whilst the more strongly basic B-carboranylphosphine [9-PPh₂-closo-1,7-C₂B₁₀H₁₁] (V) is less effective and the very weakly basic species [μ-2,2'-PPh-{1-(1'-1',2'-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀}] (IX) is completely ineffective. Base strengths are rank-ordered via measurement of the ¹J ³¹P-⁷⁷Se coupling constants of the phosphineselenides [1-SePPh₂-closo-1,2-C₂B₁₀H₁₁] (2), [9-SePPh₂-closo-1,7-C₂B₁₀H₁₁] (3), and [SePPh₂(C₆F₅)] (4).