Interconversion of Formic Acid and Carbon Dioxide by Proton-Responsive, Half-Sandwich CpIr^III Complexes: A Computational Mechanistic Investigation

Dihydrogen (H₂) has many desirable features as a fuel, but utilization of H₂ is limited due to storage and transportation problems. A promising solution to these issues is reversible storage of hydrogen in the form of liquid-phase chemicals such as formic acid (FA), which could be accomplished by the development of efficient and robust catalysts. Recently, proton-responsive, half-sandwich CpIr^III (where Cp∗ = pentamethylcyclopentadienyl anion) complexes capable of reversible hydrogen storage via interconversion between H₂/CO₂ and formic acid/formate in water have been reported. This interconversion is performed via CO₂ hydrogenation and FA dehydrogenation reactions and modulated by the pH of the medium. We report the results of a computational investigation of the mechanistic aspects of reversible hydrogen storage via two of these catalysts: namely, [CpIr(4DHBP)]²⁺ (4DHBP = 4,4′-dihydroxy-2,2′-bipyridine) and [CpIr(6DHBP)]²⁺ (6DHBP = 6,6′-dihydroxy-2,2′-bipyridine). Distinct features of the catalytic cycles of [CpIr(4DHBP)]²⁺ and [CpIr(6DHBP)]²⁺ for CO₂ hydrogenation and FA dehydrogenation reactions are demonstrated using density functional theory (DFT) calculations employing a speciation approach and probing deuterium kinetic isotope effects (KIE). In addition to the mechanistic insights and principles for the design of improved next-generation catalysts, the validation of computational methods for the investigation of the hydrogenation and dehydrogenation reactions is addressed.