Wallick, R.F.; Chakrabati, S.; Burke, J.H.; Gnewkow, R.; Byeong Chae, Ju; Rossi, T.C.; Mantouvalou, I.; Kanngiesser, B.; Fondell, M.; Eckert, S.; Dykstra, C.; Smith, L.E.; Vura-Weis, J.; Mirica, L.; Van der Veen, R.: Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy. The Journal of Physical Chemistry Letters 15 (2024), p. 4976–4982
10.1021/acs.jpclett.4c00226
Open Access Version (externer Anbieter)
Abstract:
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.