The Lewis acidity of borylcarboranes

Abstract

The dimesitylborylcarborane [1-BMes2-2-Me-closo-1,2-C2B10H10] (1) has been prepared. The Lewis acidity of 1 has been assessed by the Gutmann-Beckett method and found to be effectively identical to that of the previously reported 2-H and 2-Ph analogues. Similarly, the new catecholylborylcarboranes [1-Bcat-closo-1,2-C2B10H11] (2) and [1-Bcat-2-Me-closo-1,2-C2B10H10] (3), although more strongly Lewis acidic than the dimesitylborylcarboranes, were found to have Acceptor Numbers very close to that of the known 2-Ph analogue. [μ-2,2ʹ-BPh-(1-1ʹ-closo-1ʹ,2ʹ-C2B10H10)-closo-1,2-C2B10H10] (4), a compound with a boryl fragment attached to two carboranyl units [in the form of bis(ortho-carborane)] has been prepared and found to be highly Lewis acidic, reflecting the strongly electron-withdrawing property of C-bound carboranyl groups. The first example of a compound in which both Lewis acidic and Lewis basic substituents are appended to a single carborane cage, [1-Bcat-7-PPh2-closo-1,7-C2B10H10] (5), is reported. In 5 the Acceptor Number of the Bcat substituent is very close to those of 2 and 3 and the basicity of the PPh2 substituent, assessed by measurement of the 1JPSe NMR coupling constant of the related selenide [1-Bcat-7-P(Se)Ph2-closo-1,7-C2B10H10] (5Se), is very close to that of [1-PPh2-closo-1,7-C2B10H11], suggesting that the properties of both substituents in 5 are effectively unaffected by the presence of the other.