Competing H_2 versus Intramolecular C-H Activation at a Dinuclear Nickel Complex via Metal-Metal Cooperative Oxidative Addition

摘要

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_2 upon substrate binding. Here we present a pyrazolate-based bis(β-diketiminato) ligand [L~(Ph)]~(3-) with bulky m-terphenyl substituents that can host two Ni-H units in close proximity. Complexes [L~(ph)(Ni~Ⅱ-H)_2]~- (3) are prone to intramolecular reductive H_2 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(sp~2)-H oxidative addition to the dinickel(Ⅰ) intermediate [L~(Ph)Ni~Ⅰ_2]~- (4). While Ni~Ⅰ species have been implicated in catalytic C-H functionalization, discrete activation of C-H bonds at Ni~Ⅰ complexes has rarely been described. The reversible H_2 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 [L~(Ph) Ni~Ⅰ_2]~- intermediate 4 either via elimination of H_2 from dihydride 3 or via reductive C-H elimination from monohydride 2. The latter does not release any H_2 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.