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

Benzimidazoles as Metal-Free and Recyclable Hydrides for CO2 Reduction to Formate

We report a novel metal-free chemical reduction of CO2 by a recyclable benzimidazole-based organo-hydride, whose choice was guided by quantum chemical calculations. Notably, benzimidazole-based hydride donors rival the hydride-donating abilities of noble metal-based hydrides such as [Ru(tpy)(bpy)H]+ and [Pt(depe)2H]+. Chemical CO2 reduction to the formate anion (HCOO) was carried out in the absence of biological enzymes, a sacrificial Lewis acid, or a base to activate the substrate or reductant. 13CO2 experiments confirmed the formation of H13COO by CO2 reduction with the formate product characterized by 1H-NMR and 13C-NMR spectroscopies, and ESI-MS. The highest formate yield of 66% was obtained in the presence of potassium tetrafluoroborate under mild conditions. The likely role of exogenous salt additives in this reaction is to stabilize and shift the equilibrium towards the ionic products. After CO2 reduction, the benzimidazole-based hydride donor was quantitatively oxidized to its aromatic benzimidazolium cation, establishing its recyclability. In addition, we electrochemically reduced the benzimidazolium cation to its organo-hydride form in quantitative yield, demonstrating its potential for electrocatalytic CO2 reduction. These results serve as a proof of concept for the electrocatalytic reduction of CO2 by sustainable, recyclable and metal-free organo-hydrides