A Potentiometric Method to Measure Thermodynamic Hydricity of Organohydrides and Transition Metal Hydrides

Thermodynamic hydricity ( Δ G ° H − ) is the free energy for loss of hydride from a molecule such as a transition metal hydride or an organohydride. Knowledge of hydricity values for hydride donor and hydride acceptor molecules can be used to predict whether hydride transfer reactions are thermodynamically favorable. In this report, a fast and operationally simple approach to the measurement of hydricity values is demonstrated, using open-circuit potential measurements. The method employs only materials and reagents that are commonly available, such as Pt wire and H 2 gas. Low concentration of samples (0.5 to 2 mM) can be used, and a measurement can be completed in about 2 h. Results are presented for transition metal and organohydride molecules with hydricity values spanning a range of ∼20 kcal mol –1, with Δ G ° H − from 59.3 ± 0.3 up to 74.8 ± 0.1 kcal mol –1 . An average error of 0.01 V based on replicate data collected was obtained, and this corresponds to an average uncertainty in hydricity of about 0.4 kcal mol –1 .