Distinct stepwise reduction of a nickel-nickel-bonded compound containing an α-diimine ligand: From perpendicular to coaxial structures

A nickel-nickel-bonded complex, [{Ni(μ-L^.-)}₂] (1; L=[(2,6-iPr₂C₆H₃)NC(Me)]₂), was synthesized from reduction of the LNiBr₂ precursor by sodium metal. Further controllable reduction of 1 with 1.0, 2.0 and 3.0 equiv of Na, respectively, afforded the singly, doubly, and triply reduced compounds [Na(DME)₃]×[{Ni(μ-L^.-)}₂] (2; DME=1,2-dimethoxyethane), [Na(Et₂O)]Na[(L^.-)Ni-NiL ^2-] (3), and [Na(Et₂O)]₂Na[L ^2-Ni-NiL^2-] (4). Here L represents the neutral ligand, L^.- denotes its radical monoanion, and L^2- is the dianion. All of the four compounds feature a short Ni-Ni bond from 2.2957(6) to 2.4649(8) Å. Interestingly, they display two different structures: the perpendicular (1 and 2) and the coaxial (3 and 4) structure, in which the metal-metal bond axis is perpendicular to or collinear with the axes of the α-diimine ligands, respectively. The electronic structures, Ni-Ni bonding nature, and energetic comparisons of the two structure types were investigated by DFT computations. For a nickel and a dimer: Controllable reduction of a nickel-nickel-bonded compound with sodium affords three Ni-Ni-bonded compounds, that display two distinctly different structures: the perpendicular and the coaxial structure (see scheme). The electronic structures, Ni-Ni bonding nature, and energetic comparison of the two structure types were investigated by EPR spectroscopy and DFT computations.