Mechanistic Evaluation of a Nickel Proton Reduction Catalyst Using Time-Resolved X‑ray Absorption Spectroscopy

We report the light-induced electronic and geometric changes taking place in “real time” of a multimolecular [Ru­(bpy)3]2+/[Ni­(PPh 2NPh 2)2(CH3CN)]2+/ascorbic acid photocatalytic system by time-resolved X-ray absorption spectroscopy (tr-XAS) in the nano- to microsecond time regime. Using tr-XAS allows us to observe the diffusion-governed electron transfer between the excited photosensitizer and the nickel­(II) proton reduction catalyst on the nanosecond time scale followed by formation of a transient distorted tetrahedral Ni­(I) intermediate. A 50-fold increase in the decay lifetime of the Ni­(I) species, in the presence of the electron donor, shows that the favored catalytic pathway occurs through reductive quenching of the excited photosensitizer followed by electron transfer to the catalyst. Lack of protonation of the Ni­(I) amine groups within our experimental tr-XAS time window suggests that proton binding is the rate limiting step for H2 photocatalysis by this system. This study is supported by molecular orbital density functional theory (DFT-MO) calculations providing relevant information for ongoing synthetic efforts on “DuBois-type” nickel complexes as well as mechanistic time-resolved understanding of the photoevolution processes in the catalytic cycle for the rational design of molecular hydrogen-evolving photocatalysts.