Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy
Rachel F. Wallick, Sagnik Chakrabarti, John H. Burke, Richard Gnewkow, Ju Byeong Chae, Thomas Rossi, Ioanna Mantouvalou, Birgit Kanngießer, Mattis Fondell, Sebastian Eckert, Conner Dykstra, Laura Smith, Josh Vura‐Weis, Liviu M. Mirica, Renske M. van der Veen
Abstract
High Resolution Image Download MS PowerPoint Slide Photoassisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances the reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein, we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni( o -tol)Cl (dtb = 4,4′-di- tert -butyl, bpy = bipyridine, o -tol = ortho -tolyl), in solution. We unambiguously confirm via direct experimental evidence that the long-lived (∼5 ns) excited state is a tetrahedral metal-centered triplet state. These results demonstrate the power of ultrafast X-ray spectroscopies to unambiguously elucidate the nature of excited states in important transition-metal-based photocatalytic systems.