Synthesis and Reactivity of Nickel Hydride Complexes of an α‑Diimine Ligand

Reaction of L0NiBr2 with 2 equiv of NaH yielded the NiII hydride complex [(L•–)­Ni­(μ-H)2Ni­(L•–)] (1) (L = [(2,6-iPr2C6H3)­NC­(Me)]2; L0 represents the neutral ligand, L•– is its radical-anionic form, and L2– 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[L2–Ni­(μ-H)2NiL2–] (3a) was obtained, in which each monoanion (L•–) in the precursor 1 has been reduced to L2–. By using potassium as the reducing agent, two analogous species [K­(DME)4]­[(L•–)­Ni­(μ-H)2Ni­(L•–)] (2b) and [K­(DME)]2[L2–Ni­(μ-H)2NiL2–] (3b) were obtained. Further treatment of 3b with 2 equiv of K led to a trinuclear complex [K­(DME)­(THF)]2K2[L2–Ni­(μ-H)2Ni­(μ-H)2NiL2–] (4), which contains one NiII and two NiI 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[L2–Ni­(μ-η3:η3-C6H5R)­NiL2–] (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.