Tetra-, hexa- and octanuclear copper hydride complexes supported by tridentate phosphine ligands

Abstract

Multinuclear copper hydride complexes were synthesized by using a triphosphine, bis(diphenylphosphinomethyl)phenylphosphine (dpmp). The reaction of [Cu(MeCN)₄]PF₆ with dpmp in 2 : 1 ratio in the presence of Me₄NBH₄ or NaBH₄ yielded a hexanuclear complex, [Cu₆(μ₃-H)₅(μ-dpmp)₃]PF₆ (1), together with a minor product [Cu₈(μ-H)₂(μ₄-H)₄(μ-dpmp)₄](PF₆)₂ (2) in a very low yield. Complex 1 was also prepared from [CuH(PPh₃)]₆, [Cu(MeCN)₄]PF₆, and dpmp in 87%, but the yield of 2 could not be improved presumably due to its instability in solution. A tetranuclear complex, [Cu₄(μ-H)(μ₃-H)₂(μ-dpmp)₃]PF₆ (4), was obtained from the reaction of [Cu(MeCN)₄]PF₆ with dpmp in 4 : 3 ratio, or that of [Cu₃(μ-dpmp)₂(MeCN)₄](PF₆)₃ (3) and dpmp, in the presence of Me₄NBH₄ for both cases. The structures of 1, 2, and 4 were determined by X-ray crystallography, and the positions of hydride ligands were elucidated by DFT optimization. Complex 1 consists of a distorted trigonal anti-prismatic Cu₆ core bridged by three L-shaped dpmp ligands and five μ₃-hydrides, and 2 is composed of three edge-shared tetrahedral Cu₄ units supported by four L-shaped dpmp ligands and two μ₂-hydrides in the two outer Cu₄ units and four μ₄-hydride ligands in the central Cu₄ units. The structures of two outer Cu₄ units in 2 resemble those of 4, where a Cu₄ tetrahedron is supported by two L-shaped dpmp ligands and one dpmp-κ²P,P′ bridge, in addition to one μ₂- and two μ₃-hydrides. Natural bond orbital analyses suggest that the hydrides act as glue between the multinuclear copper centres through electron-deficient delocalized bonding interactions, which result in partial electron migration from hydrides to Cu^I centres.