Synthesis and reactivity of nickel hydride complexes of an α-diimine ligand

Reaction of L⁰NiBr₂ with 2 equiv of NaH yielded the Ni^II hydride complex [(L^•-)Ni(μ-H) ₂Ni(L^•-)] (1) (L = [(2,6-iPr₂C ₆H₃)NC(Me)]₂; L⁰ represents the neutral ligand, L^•- is its radical-anionic form, and L ^2- denotes the dianion) in good yield. Stepwise reduction of complex 1 led to a series of nickel hydrides. Reduction of 1 with 1 equiv of sodium metal afforded a singly reduced species [Na(DME)₃][(L ^•-)Ni(μ-H)₂Ni(L^•-)] (2a) (DME = 1,2-dimethoxyethane), which contains a mixed-valent core [Ni(μ-H) ₂Ni]⁺. With 2 equiv of Na a doubly reduced species [Na(DME)]₂[L^2-Ni(μ-H)₂NiL^2-] (3a) was obtained, in which each monoanion (L^•-) in the precursor 1 has been reduced to L^2-. By using potassium as the reducing agent, two analogous species [K(DME)₄][(L^•-)Ni(μ-H) ₂Ni(L^•-)] (2b) and [K(DME)]₂[L ^2-Ni(μ-H)₂NiL^2-] (3b) were obtained. Further treatment of 3b with 2 equiv of K led to a trinuclear complex [K(DME)(THF)]₂K₂[L^2-Ni(μ-H) ₂Ni(μ-H)₂NiL^2-] (4), which contains one Ni^II and two Ni^I centers with a triplet ground state. When 1 and 3a were warmed in toluene or benzene, respectively, three reverse-sandwich dinickel complexes, [(L^•-)Ni(μ- η³:η³-C₇H₈)Ni(L ^•-)] (5) and [Na(DME)]₂[L^2-Ni(μ- η³:η³-C₆H₅R)NiL ^2-] (6: R = CH₃; 7: R = H), were isolated. Reaction of 1 with Me₃SiN₃ gave the N₃-bridged complex [(L^•-)Ni(μ-η¹-N₃) ₂Ni(L^•-)] (8). The crystal structures of complexes 1-8 have been determined by X-ray diffraction, and their electronic structures have been fully studied by EPR/NMR spectroscopy.