Organic chemistry of dinuclear metal centres. Part 14. Synthesis, X-Ray structure, and reactivity of the ruthenium–ruthenium double-bonded complex [Ru2(μ-CO)(μ-C2Ph2)(η-C5H5)2]

Abstract

Ultraviolet irradiation of the metallacycle [Ru₂(CO)(μ-CO){μ-C(O)C₂Ph₂}(η-C₅H₅)₂] (1) in tetrahydrofuran (thf) gives the complex [Ru₂(μ-CO)(μ-C₂Ph₂)(η-C₅H₅)₂] (2), shown by X-ray diffraction to have a ruthenium–ruthenium double bond [RuRu 2.505(1) Å] bridged transversely by a diphenylacetylene ligand. The loss of two molecules of CO in forming (2) is reversible; under 100 atm of CO at 50 °C complex (2) is converted into (1) in 60% yield. Treatment of unsaturated complex (2) with diazoalkanes RCHN₂ (R = H, Me, or CO₂Et) results in the corresponding uptake of two alkylidene units to form [Ru₂(CO)(μ-CHR){η-C(Ph)C(Ph)CHR}(η-C₅H₅)₂], existing as isomers for R = Me or CO₂Et due to differing orientations of the μ-CHR substituent. The structure of [Ru₂(CO)(μ-CH₂){μ-C(Ph)C(Ph)CH₂}(η-C₅H₅)₂] (3) has been established by X-ray diffraction, revealing that one methylene co-ordinates to the dinuclear metal centre while the other links with the alkyne. There are non-bonding C–C distances of 3.07 Å between the two μ-carbons of the complex, but only 2.78 Å separating the μ-CH₂ carbon and the CH₂ carbon of the C(Ph)C(Ph)CH₂ ligand. On thermolysis the latter two carbons link, accompanied by other processes, to afford [Ru₂(CO)(μ-CO){μ-C(Ph)C(Ph)CHMe}(η-C₅H₅)₂] (5). A co-product of the reaction of diazoethane with (2) is the di-μ-vinyl complex [Ru₂(CO) (μ-CHCH₂){μ-C(Ph)CHPh}(η-C₅H₅)₂] (8). X-ray diffraction reveals that the two β-carbons of the vinyl groups are 2.99 Å apart and it is these rather than the two μ(α) carbons (3.06 Å apart) which link on thermolysis, affording complex (5) once more. Thermolysis of [Ru₂(CO)(μ-CHCO₂Et){μ-C(Ph)C(Ph)CH(CO₂Et)}(η-C₅H₅)₂] does not effect carbon–carbon bond formation. Instead, CO is ejected and its site occupied by an oxygen of a carboethoxy group in the complex [Ru₂(μ-CHCO₂Et){μ-C(Ph)C(Ph)CHC(O)OEt}(η-C₅H₅)₂]. Treatment of complex (1) with BH₃·thf or LiMe–HBF₄–NaBH₄ converts the metallacyclic ketone group into CH₂ or CHMe respectively, yielding [Ru₂(CO)(μ-CO){μ-C(Ph)C(Ph)CHR}(η-C₅H₅)₂] (R = H or Me). The nature of the processes observed on thermolysis of complexes (3) and (8) suggests the importance of least-motion effects in determining the course of carbon–carbon bond formation at a dinuclear metal centre.