Competing H2 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 H2 upon substrate binding. Here we present a pyrazolate-based bis­(β-diketiminato) ligand [LPh]3– with bulky m-terphenyl substituents that can host two Ni–H units in close proximity. Complexes [LPh(NiII–H)2]− (3) are prone to intramolecular reductive H2 elimination, and an equilibrium between 3 and orthometalated dinickel­(II) monohydride complexes 2 is evidenced. 2 is shown to form via intramolecular metal–metal cooperative phenyl group C­(sp2)-H oxidative addition to the dinickel­(I) intermediate [LPhNiI 2]− (4). While NiI species have been implicated in catalytic C–H functionalization, discrete activation of C–H bonds at NiI complexes has rarely been described. The reversible H2 and C–H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron reductant 4 from either 2 or 3, which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of interconversion of 2 and 3 and also for subsequent thermally driven formation of a twice orthometalated dinickel­(II) complex 6. X-ray crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with 2 and 3. The present system allows for the unmasking of a highly reactive [LPhNiI 2]− intermediate 4 either via elimination of H2 from dihydride 3 or via reductive C–H elimination from monohydride 2. The latter does not release any H2 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.