Exploring structural and electronic effects in three isomers of tris{bis(trifluoromethyl)phenyl}borane: towards the combined electrochemical-frustrated Lewis pair activation of H2

Abstract

Three structural isomers of tris{bis(trifluoromethyl)phenyl}borane have been studied as the acidic component of frustrated Lewis pairs. While the 3,5-substituted isomer is already known to heterolytically cleave H₂ to generate a bridging-hydride; ortho-substituents in the 2,4- and 2,5-isomers quench such reactivity through electron donation into the vacant boron p_z orbital and steric blocking of the boron centre; as shown by electrochemical, structural and computational studies. Electrochemical studies of the corresponding borohydrides identify that the two-electron oxidation of terminal-hydrides occurs at more positive potentials than observed for [HB(C₆F₅)₃]⁻, while the bridging-hydride oxidizes at a higher potential still, comparable to that of free H₂.