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 Cp*Ir(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, [Cp*Ir(4DHBP)]2+ (4DHBP = 4,4'-dihydroxy-2,2'-bipyridine) and [Cp*Ir(6DHBP)]2+ (6DHBP = 6,6'-dihydroxy-2,2'-bipyridine). Distinct features of the catalytic cycles of [Cp*Ir(4DHBP)]2+ and [Cp*Ir(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.

Hydrogen storage system based on the reversible interconversion between H2/CO2 gas and formic acid in water using molecular catalysts

251st National Meeting (2016)

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