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

Dicationic tetranuclear ruthenium octahydride [(Cp* Ru)(4)H-8](2+) (5) with tetrahedral geometry was obtained by reaction of dinuclear ruthenium tetrahydride (Cp* Ru)(2)(mu-H)(4) (1) with an excess of Bronsted acids, such as HBF4 center dot OEt2, in toluene. Monocationic tetraruthenium heptahydride [(Cp* Ru)(4)H-7](+) (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)(4)H-6 (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 Bronsted acid to 8 gave 5 via the formation of 7 as an intermediate. Tetranuclear complex 8 was directly obtained from 1 by heating it in THF at 70 degrees C. Complex 8' and tetraruthenium tetrahydride ((CpRu)-Ru-Et)(4)H-4 (10'), where 8' and 10' possessed eta(5)-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 Ru-4 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.