Synthesis and Structures of Dimesityldiboranes(4)

An efficient route to B,B′-dihalo-B,B′-dimesityldiboranes(4) is reported, as well as the conversion of these and of B,B′-dimesityl-B,B′-dimethoxydiborane(4) into other B,B′-dimesityldiboranes B2mes2X2 (X = OR, SR, NR2, NHR). The substituents X influence the shielding of the 11B nucleus in the expected manner. Hindered rotation about the B–N bond is observed for X = pyrrolyl, while in solution there is free rotation about the B–C bond. However, X-ray structural results reveal that the mesityl group is almost orthogonal to the B(B)CY plane in the solid state. The shortest B–B bond is observed in mes2B2I2, indicating that the iodide is the strongest Lewis acid in the reported series of mes2B2Y2 compounds. This is also reflected in short B–C bonds. Moreover, the two mes(Y)B units also approach orthogonality in these diborane(4) compounds, and thus enforce this as a typical structural feature of diborane(4) derivatives. For this reason, the planar C(N)B–B(N)C unit of B,B′-di(benzylamino)-B,B′-dimesityldiborane(4) is regarded as exceptional for aminodiboranes(4).