Synthesis of Surface-Analogue Square-Planar Tetranuclear Nickel Hydride Clusters and Bonding to μ4‑NR, -O and -BH Ligands

Tetranuclear Ni complexes were synthesized with bonding to BH, NR, and O in atypical surface-like geometries. The previously reported electron-deficient cluster [( i Pr3P)­Ni]5H6 (1) reacts with N-methylmorpholine oxide to give [( i Pr3P)­Ni]4H4(μ4-O) (2), which contains O coordinated in the center of a square-plane arrangement of Ni atoms. Reaction of 1 with benzonitrile gave the square-planar tetranuclear Ni cluster [( i Pr3P)­Ni]4H4(μ4-NCH2Ph) (3), which contains an imido donor in a square-based-pyramidal geometry. This reaction also gives [( i Pr3P)­Ni­(NCPh)]3 (4), with bridging benzonitrile ligands. Trimer 4 was independently synthesized from the reaction of Ni­(COD)2, i Pr3P, and PhCN. The addition of dihydrogen to a 1:1 mixture of [( i Pr3P)2Ni]2(N2) and ( i Pr3P)2NiCl2 yielded [( i Pr3P)­Ni]4(μ3-H)4(μ2-Cl)2 (5), with a tetrahedral Ni core, in contrast to the square-planar geometries of 2 and 3. The solid-state structure of 5 was determined using both X-ray and neutron diffraction. Reaction of 5 with LiBH4 gave [( i Pr3P)­Ni]4H4(μ4-BH)2] (6) via loss of LiCl and H2.