Calculation of thermodynamic hydricities and the design of hydride donors for CO 2 reduction

Abstract

We have developed a correlation between experimental and density functional theory-derived results of the hydride-donating power, or "hydricity", of various ruthenium, rhenium, and organic hydride donors. This approach utilizes the correlation between experimental hydricity values and their corresponding calculated free-energy differences between the hydride donors and their conjugate acceptors in acetonitrile, and leads to an extrapolated value of the absolute free energy of the hydride ion without the necessity to calculate it directly. We then use this correlation to predict, from density functional theory-calculated data, hydricity values of ruthenium and rhenium complexes that incorporate the pbnHH ligand-pbnHH = 1,5-dihydro-2-(2-pyridyl)-benzo[b]-1,5-naphthyridine-to model the function of NADPH. These visible light-generated, photocatalytic complexes produced by disproportionation of a protonated-photoreduced dimer of a metal-pbn complex may be valuable for use in reducing CO(2) to fuels such as methanol. The excited-state lifetime of photoexcited [Ru(bpy)(2)(pbnHH)](2+) is found to be about 70 ns, and this excited state can be reductively quenched by triethylamine or 1,4-diazabicyclo[2.2.2]octane to produce the one-electron-reduced [Ru(bpy)(2)(pbnHH)](+) species with half-life exceeding 50 μs, thus opening the door to new opportunities for hydride-transfer reactions leading to CO(2) reduction by producing a species with much increased hydricity.