Photochemistry and Photophysics of Nickel Complexes

Abstract

Over the past century, transition metal catalysis has fundamentally changed chemistry, delivering novel methods for the synthesis of organic molecules and sustainable approaches to challenges facing society. Accordingly, the continued discovery and development of novel transition metal catalyzed reactions has the potential to impact most individuals and industries either directly, as in the production of life saving pharmaceuticals and fine chemicals, or indirectly in downstream applications. The work presented herein highlights the development and study of nickel-catalyzed photochemical transformations. I report the development of a C(sp3)–H cross-coupling platform enabled by the catalytic generation of chlorine radicals via nickel and photoredox catalysis. Preliminary mechanistic studies suggest that photolysis of a Ni(III) aryl chloride intermediate, generated by photoredox-mediated single-electron oxidation, leads to elimination of a chlorine radical in what amounts to the sequential capture of two photons. This novel reaction platform is then leveraged towards the redox‐neutral formylation of aryl chlorides. The scalable benchtop approach provides a distinct advantage over traditional reductive carbonylation in that no carbon monoxide, pressurized gas, or stoichiometric reductant is employed. The mild conditions give unprecedented scope from abundant and complex aryl chloride starting materials. Finally, these synthetic studies motivated a spectroscopic investigation of the photophysics and photochemistry of Ni(II) aryl halide complexes common to cross-coupling and Ni/photoredox reactions. Computational and ultrafast spectroscopic studies suggest that these complexes feature long-lived and reactive excited states, implicating Ni as an underexplored alternative to precious metal photocatalysts.