Experimental evaluation of the hydricity of hydrosilanes

Despite their widespread use as reductants, the thermodynamic hydricities of hydrosilanes have not been determined experimentally. We report the hydricities of three triarylsilanes, determined by a combination of hydride-exchange reactions and electrochemical measurements of the corresponding silylium cations. The hydricities for Xyl3SiH (89.0 kcal·mol-1), Mes3SiH (86.6 kcal·mol-1), and PEMP3SiH (80.5 kcal·mol-1) were estimated from the one-electron reduction potentials of the silylium ions in 1,2 difluorobenzene and are higher than previously reported values obtained by calculations. The reaction of [PEMP3Si][B(C6F5)4] with cycloheptatriene (CHT) in 1,2 difluorobenzene/C6D6 (4:1) attained an equilibrium (K ≈ 1.3), indicating similar hydricities for CHT and PEMP3SiH.