Operando Mechanistic Studies of CO2 Hydrogenation by Ruthenium Complexes Using High-Pressure NMR Spectroscopy

As a result of increased energy demands, the ability to both reduce carbon dioxide with dihydrogen to formate and conduct the reverse reaction with the same catalyst is of interest as a method for potential fuel generation and use. Ruthenium bis-(diphosphine) complexes with and without pendant amines were reacted with mixtures of CO2/H-2 gases in the presence of added base to catalytically yield formate; when the base was triethylamine, the reaction was found to be reversible. The reactions were monitored using high-pressure operando H-1 and P-31-{H-1} NMR spectroscopy at 18 degrees C in THF under 40 atm of a 1:1 mixture of H-2 and CO2. The rate of production of formate was correlated with the observation of specific organometallic species by NMR spectroscopy under catalytic conditions, including a hydrido-dihydrogen complex. From this operando study, a mechanism is proposed with two competing catalytic cycles for which the predominant cycle is dependent on which base and catalyst are used. The role of the base is shown to be vital for both catalytic rate and reversibility of the chemical transformation, indicating base selection should be carefully considered.