Highly Reduced Alkali-Metal Nickelates: Synthesis, Structure, Catalytic Applications, and Alkali-Metal Effects

Low-valent nickel complexes have recently emerged as powerful reagents in challenging cross-coupling reactions and other bond forming and breaking processes. These transformations typically rely on nickel's ability to adopt a range of different oxidation states and engage in reversible one and two electron reactivity. While the synthesis, structure, and reactivity of low-valent Ni(I) and Ni(0) complexes are well established, examples of Ni complexes featuring formally negative oxidation states remain scarce, and in the few reported complexes, the actual oxidation state of nickel and the nature of bonding are poorly understood. Herein, we report a novel family of highly reduced alkali-metal nickelate complexes, [{(THF)_<i>x</i>AM}₂Ni(^4-Mestb)_<i>y</i>] (where AM = alkali-metal; ^4-Mestb = (<i>E</i>)-4,4-dimethylstilbene; <i>x</i> = 1,2; <i>y</i> = 2,3), which is highly electron rich. The complete alkali-metal set ranging from lithium to cesium were accessed and characterized in the solid state (except the Cs congener) and in solution. The lithium nickelate complex was studied by experimental charge density to comprehensively elucidate the structure, electronics, and bonding of this unique system. The stoichiometric and catalytic reactivity toward C-F activation, reductive norbornene coupling, hydrosilylation, and alkene isomerization were investigated and benchmarked against homometallic Ni(0) systems.