Synthesis and characterisation of tetranuclear ruthenium polyhydrido clusters with pseudo-tetrahedral geometry

Dicationic tetranuclear ruthenium octahydride [(Cp*Ru)₄H₈]²⁺ (5) with tetrahedral geometry was obtained by reaction of dinuclear ruthenium tetrahydride (Cp*Ru)₂(μ-H)₄ (1) with an excess of Brønsted acids, such as HBF₄·OEt₂, in toluene. Monocationic tetraruthenium heptahydride [(Cp*Ru)₄H₇]⁺ (7) was obtained by dropwise addition of a diluted acid to a rigorously stirred solution of 1 at ambient temperature. Dication 5 was converted into monocationic heptahydrido complex 7 in high yield by treatment with sodium methoxide or sodium hydride. The direct conversion of 5 into neutral hexahydrido complex (Cp*Ru)₄H₆ (8) was achieved in a highly efficient manner by treating 5 with LiAlH₄ in tetrahydrofuran (THF). The conversion of 5 into 8 was reversible, and the addition of a Brønsted acid to 8 gave 5via the formation of 7 as an intermediate. Tetranuclear complex 8 was directly obtained from 1 by heating it in THF at 70 °C. Complex 8' and tetraruthenium tetrahydride (Cp^EtRu)₄H₄ (10'), where 8' and 10' possessed η⁵-C₅EtMe₄ ligands instead of Cp* ligands, were mutually related by the elimination/addition of dihydrogen. The structures of 5, 7, 8, and 10' were determined by X-ray diffraction, and the Ru₄ core structure and the coordination mode of hydrido ligands were discussed based on density functional theory (DFT) calculations for model compounds where the methyl groups of Cp* ligands were replaced with hydrogen atoms.