Pairwise H/D Exchange and H Substitution at a Bimetallic Dinickel(II) Complex Featuring Two Terminal Hydrides.

A compartmental ligand scaffold HL with two β-diketiminato binding sites spanned by a pyrazolate bridge gave a series of dinuclear nickel(II) dihydride complexes M[LNi(H)], M = Na (Na·2) and K (K·2), which were isolated after reacting the precursor complex [LNi(μ-Br)] (1) with MHBEt (M = Na and K). Crystallographic characterization showed the two hydride ligands to be directed into the bimetallic pocket, closely interacting with the alkali metal cation. Treatment of K·2 with dibenzo(18-crown-6) led to the separated ion pair [LNi(H)][K(DB18C6)] (2[K(DB18C6)]). Reaction of Na·2 or K·2 with D was investigated by a suite of H and H NMR experiments, revealing an unusual pairwise H/D exchange process that synchronously involves both Ni-H moieties without H/D scrambling. A mechanistic picture was provided by DFT calculations which suggested facile recombination of the two terminal hydrides within the bimetallic cleft, with a moderate enthalpic barrier of ∼62 kJ/mol, to give H and an antiferromagnetically coupled [LNi] species. This was confirmed by SQUID monitoring during H release from solid 2[K(DB18C6)]. Interaction with the Lewis acid cation (Na or K) significantly stabilizes the dihydride core. Kinetic data for the M[L(Ni-H)] → H transition derived from 2D H EXSY spectra confirmed first-order dependence of H release on M·2 concentration and a strong effect of the alkali metal cation M. Treating [LNi(D)] with phenylacetylene led to D and dinickel(II) complex 3 with a twice reduced styrene-1,2-diyl bridging unit in the bimetallic pocket. Complexes [LNi(H)] having two adjacent terminal hydrides thus represent a masked version of a highly reactive dinickel(I) core. Storing two reducing equivalents in adjacent metal hydrides that evolve H upon substrate binding is reminiscent of the proposed N binding step at the FeMo cofactor of nitrogenase, suggesting the use of the present bimetallic scaffold for reductive bioinspired activation of a range of inert small molecules.