Pairwise H2/D2 Exchange and H2 Substitution at a Bimetallic Dinickel(II) Complex Featuring Two Terminal Hydrides

Abstract

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[LNi2(H)2], M = Na (Na·2) and K (K·2), which were isolated after reacting the precursor complex [LNi2(μ-Br)] (1) with MHBEt3 (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 [LNi2(H)2][K(DB18C6)] (2[K(DB18C6)]). Reaction of Na·2 or K·2 with D2 was investigated by a suite of 1H and 2H NMR experiments, revealing an unusual pairwise H2/D2 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 H2 and an antiferromagnetically coupled [LNiI2]- species. This was confirmed by SQUID monitoring during H2 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)2] → H2 transition derived from 2D 1H EXSY spectra confirmed first-order dependence of H2 release on M·2 concentration and a strong effect of the alkali metal cation M+. Treating [LNi2(D)2]- with phenylacetylene led to D2 and dinickel(II) complex 3- with a twice reduced styrene-1,2-diyl bridging unit in the bimetallic pocket. Complexes [LNiII2(H)2]- 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 H2 upon substrate binding is reminiscent of the proposed N2 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.