Mechanistic Analysis of Metallaphotoredox C–N Coupling: Photocatalysis Initiates and Perpetuates Ni(I)/Ni(III) Coupling Activity

The combined use of reaction kinetic
analysis, ultrafast spectroscopy,
and stoichiometric organometallic studies has enabled the elucidation
of the mechanistic underpinnings to a photocatalytic C–N cross-coupling
reaction. Steady-state and ultrafast spectroscopic techniques were
used to track the excited-state evolution of the employed iridium
photocatalyst, determine the resting states of both iridium and nickel
catalysts, and uncover the photochemical mechanism for reductive activation
of the nickel cocatalyst. Stoichiometric organometallic studies along
with a comprehensive kinetic study of the reaction, including rate–driving
force analysis, unveiled the crucial role of photocatalysis in both
initiating and sustaining a Ni­(I)/Ni­(III) cross-coupling mechanism.
The insights gleaned from this study further enabled the discovery
of a new photocatalyst providing a >30-fold rate increase.