Competing H versus Intramolecular C-H Activation at a Dinuclear Nickel Complex via Metal-Metal Cooperative Oxidative Addition.

Nickel(I) metalloradicals bear great potential for the reductive activation of challenging substrates but are often too unstable to be isolated. Similar chemistry may be enabled by nickel(II) hydrides that store the reducing equivalents in hydride bonds and reductively eliminate H upon substrate binding. Here we present a pyrazolate-based bis(β-diketiminato) ligand [L] with bulky -terphenyl substituents that can host two Ni-H units in close proximity. Complexes [L(Ni-H)] () are prone to intramolecular reductive H elimination, and an equilibrium between and orthometalated dinickel(II) monohydride complexes is evidenced. is shown to form via intramolecular metal-metal cooperative phenyl group C(sp)-H oxidative addition to the dinickel(I) intermediate [LNi] (). While Ni species have been implicated in catalytic C-H functionalization, discrete activation of C-H bonds at Ni complexes has rarely been described. The reversible H and C-H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron reductant from either or , which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of interconversion of and and also for subsequent thermally driven formation of a twice orthometalated dinickel(II) complex . X-ray crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with and . The present system allows for the unmasking of a highly reactive [LNi] intermediate either via elimination of H from dihydride or via reductive C-H elimination from monohydride . The latter does not release any H byproduct and adds a distinct platform for metal-metal cooperative two-electron substrate reductions while circumventing the isolation of any unstable superreduced form of the bimetallic scaffold.