Redox-active nitrogen donor ligands have exhibited broad utility in stabilizing transition metal complexes in unusual formal oxidation states and enabling multielectron redox reactions. In this report, we extend these principles to dinuclear complexes using a naphthyridine-diimine (NDI) framework. Treatment of ((i-Pr)NDI) with Ni(COD)2 (2.0 equiv) yields a Ni(I)-Ni(I) complex in which the two metal centers form a single bond and the ((i-Pr)NDI) ligand is doubly reduced. A homologous series of ((i-Pr)NDI)Ni2 complexes in five oxidation states were synthesized and structurally characterized. Across this series, the ligand ranges from a neutral state in the most oxidized member to a dianionic state in the most reduced. The interplay between metal- and ligand-centered redox activity is interrogated using a variety of experimental techniques in combination with density functional theory models.

An electron rich Ni(I)-Ni(I) bond supported by a doubly reduced naphthyridine-diimine (NDI) ligand reacts rapidly and reversibly with Ph2SiH2 and Et2SiH2 to form stable adducts. The solid-state structures of these complexes reveal binding modes in which the silanes symmetrically span the Ni-Ni bond and exhibit highly distorted H-Si-H angles and elongated Si-H bonds. This process is facilitated by the release of electron density stored in the π-system of the NDI ligand. Based on this dinuclear mode of activation, [NDI]Ni2 complexes are shown to catalyze the high-yielding hydrosilylation of alkenes, dienes, alkynes, aldehydes, ketones, enones, and amides. In comparative studies of alkyne hydrosilylations, the [NDI]Ni2 catalyst is found to be significantly more active than its mononuclear counterparts for aryl-substituted substrates.

A photoassisted Ni-catalyzed reductive cross-coupling between tosyl-protected alkyl aziridines and commercially available (hetero)­aryl iodides is reported. This mild and modular method proceeds in the absence of stoichiometric heterogeneous reductants and uses an inexpensive organic photocatalyst to access medicinally valuable β-phenethylamine derivatives. Unprecedented reactivity was achieved with the activation of cyclic aziridines. Mechanistic studies suggest that the regioselectivity and reactivity observed under these conditions are a result of nucleophilic iodide ring opening of the aziridine to generate an iodoamine as the active electrophile. This strategy also enables cross-coupling with Boc-protected aziridines.

A Ni/photoredox-catalyzed enantioselective reductive coupling of styrene oxides and aryl iodides is reported. This reaction affords access to enantioenriched 2,2-diarylalcohols from racemic epoxides via a stereoconvergent mechanism. Multivariate linear regression (MVLR) analysis with 29 bioxazoline (BiOx) and biimidazoline (BiIm) ligands revealed that enantioselectivity correlates with electronic properties of the ligands, with more electron-donating ligands affording higher ee's. Experimental and computational mechanistic studies were conducted, lending support to the hypothesis that reductive elimination is enantiodetermining and the electronic character of the ligands influences the enantioselectivity by altering the position of the transition state structure along the reaction coordinate. This study demonstrates the benefits of utilizing statistical modeling as a platform for mechanistic understanding and provides new insight into an emerging class of chiral ligands for stereoconvergent Ni and Ni/photoredox cross-coupling.

Photocatalysts convert light into potent reactivity. Here, we report a biohybrid catalyst in which a photosynthetic protein performs broad-spectrum light absorption and subsequent energy transfer to a conjugated photocatalyst, leading to improved yields in test reactions. This strategy has the potential to be generalized for applications in industrial and biological catalysis.