Developing a Highly Reducing Heterogeneous Nickel Photocatalyst with Photoexcited Hantzsch Esters

While intricate designs have been implemented to transform noble-metal complexes and sophisticated organic molecules into reactive photocatalysts for the single-electron reduction of unactivated alkyl halides, the development of highly reducing photocatalytic systems based on earth-abundant metals is still in its early stages. Herein, we show that a simple terpyridine-ligated nickel(II) complex, integrated into an imine-linked covalent organic framework, can be readily reduced by photoexcited Hantzsch esters to produce a heterogeneous nickel(I) photocatalyst with a fairly low excited-state oxidation potential of approximately -3.5 V (referenced to the saturated calomel electrode). The efficient electron transfer processes within the framework support and the confinement effects that stabilize the nickel(I) species contribute to the improved performance of this innovative heterogeneous nickel photocatalysis in comparison to its homogeneous counterparts. Notably, the current system allows for substantial reductions in catalyst loadings and exhibits high recyclability in promoting reductive couplings between challenging organohalide electrophiles and allyl sulfones. Our research indicates that employing framework-based heterogenization strategies can facilitate the development of new classes of robust photoredox catalysts with earth-abundant metals for sustainable and practical organic synthesis.