Predicting Hydride Donor Strength via Quantum Chemical Calculations of Hydride Transfer Activation Free Energy

We propose a method to approximate the kinetic properties of hydride donor species by relating the nucleophilicity (<i>N</i>) of a hydride to the activation free energy <i>Δ<i>G</i></i>^⧧ of its corresponding hydride transfer reaction. <i>N</i> is a kinetic parameter related to the hydride transfer rate constant that quantifies a nucleophilic hydridic species’ tendency to donate. Our method estimates <i>N</i> using quantum chemical calculations to compute <i>Δ<i>G</i></i>^⧧ for hydride transfers from hydride donors to CO₂ in solution. A linear correlation for each class of hydrides is then established between experimentally determined <i>N</i> values and the computationally predicted <i>Δ<i>G</i></i>^⧧; this relationship can then be used to predict nucleophilicity for different hydride donors within each class. This approach is employed to determine <i>N</i> for four different classes of hydride donors: two organic (carbon-based and benzimidazole-based) and two inorganic (boron and silicon) hydride classes. We argue that silicon and boron hydrides are driven by the formation of the more stable Si–O or B–O bond. In contrast, the carbon-based hydrides considered herein are driven by the stability acquired upon rearomatization, a feature making these species of particular interest, because they both exhibit catalytic behavior and can be recycled