Ni-Ir Interaction Enables a Dark Reactive State in Nickel Catalysis

Nickel-iridium metallophotoredox catalysis has emerged as an effective strategy for promoting C-C and C-heteroatom bond-forming reactions, overcoming the often sluggish reductive elimination from Ni(II) intermediates, thereby expanding the synthetic utility of nickel catalysis. Despite extensive study, however, its precise mechanism has remained largely unresolved because of the experimental difficulty of disentangling the two catalytic centers and the complex electronic structure of nickel. Here, we directly probe the ultrafast electronic dynamics of both metal centers using element-specific femtosecond time-resolved X-ray absorption spectroscopy at an X-ray free-electron laser (XFEL). We identify a transient higher-energy triplet metal-to-ligand charge-transfer (3MLCT) state of nickel that appears exclusively in the presence of the iridium photocatalyst, providing evidence for the energy transfer within a preassociated Ni-Ir complex. Comparison with quantum calculations further shows that this state promotes access to a repulsive Ni-C σ-σ* state, thereby suggesting a possible excited-state pathway relevant to reductive elimination. These findings reveal how the iridium photocatalyst activates the nickel catalytic cycle and highlight selective population of optically dark yet reactive excited states as a general strategy for enabling otherwise inaccessible reaction pathways.