Electrochemical and spectroelectrochemical studies of C-benzodiazaborolyl-ortho-carboranes

Fifteen C-diazaborolyl-ortho-carboranes, 1-R'-2-R ''-1,2-C2B10H10, where R' represents the groups 2-(1,3-Et-2-1,3,2-N2BC6H4)-, 2-(1,3-Ph-2-1,3,2-N2BC6H4)-, 2-(1,3-Ph-2-5,6-Me-2-1,3,2-N2BC6H2)-, 2-(1,3-Pr-i(2)-1,3,2-N2B-C6H4)-, and 2-(1,3,2-N2BC6H6)-and where R '' is H, Me, Ph, Bu-t or SiMe3, were synthesized. Cyclic voltammetry studies of the compounds showed irreversible oxidation waves which are caused by the oxidation of the heterocycle. Those C-diazaborolyl-ortho-carboranes with Ph, tBu and SiMe3 substituents at the adjacent C-atom of the cage displayed two one-electron reduction waves reflecting the formation of stable radical monoanions with unusual (2n + 3) skeletal electron counts. The geometries of these anions were determined by combinations of infrared, UV-visible spectroelectrochemical and computational studies. Additionally the structures of seven new C-diazaborolyl-ortho-carboranes and one new 2-bromo-1,3,2-benzodiazaborole were determined by X-ray crystallography and compared with previously obtained structures.