Skeletal Rearrangement of a μ–η2:η4-Hexatrienyl Ligand at a Diruthenium Site via a μ–η5-Ruthenacyclohexadienyl Intermediate

A diruthenium complex containing a μ–η²:η⁴-1,4-dimethylhexatrienyl ligand, [(Cp*Ru)₂(μ–η²:η⁴-CMe═CH–CH═CMe–CH═CH₂)(μ-H)] (3a), is coordinatively saturated and did not react with any two-electron donors. Instead, it underwent isomerization above 85 °C to a μ–η²:η⁴-2,5-dimethylhexatrienyl complex, [(Cp*Ru)₂(μ–η²:η⁴-CH═CMe–CH═CH–CMe═CH₂)(μ-H)] (4a), in which the methyl substituents on the C₆ moiety migrate from the 1,4- to the 2,5-positions. A methyl acrylate substituted complex, [(Cp*Ru)₂{μ-CMe═CH–CH═CMe–CH═C(H)COOMe}(μ-H)] (3b), was also transformed into [(Cp*Ru)₂{μ-CH═CMe–CH═CH–CMe═C(H)COOMe}(μ-H)] (4b). The molecular structure of 4b, as well as the reaction using ¹³C-labeled ethylene, demonstrated that this skeletal rearrangement involves C═C bond cleavage at the terminal vinyl position followed by the C–C bond formation with the α-carbon atom. DFT calculations at a B3LYP level suggested that this skeletal rearrangement proceeds via a μ–η⁵-ruthenacyclohexadienyl intermediate, [(Cp*Ru)₂(μ–η⁵-CMeCHCHCMeCH−)(Me)] (C). A related μ–η⁵-ruthenacyclohexadienyl complex possessing a terminal hydride, [(Cp*Ru)₂(μ–η⁵-CMeCHCHCHCH−)(H)] (6c), was obtained by the thermolysis of an unsubstituted μ–η²:η⁴-hexatrienyl complex, [(Cp*Ru)₂(μ-CH═CH–CH═CH–CH═CH₂)(μ-H)] (3c). The formation of a stable metallacycle skeleton would promote the C–C bond cleavage, and the subsequent C–C bond formation occurs when it contains a substituent with an α-hydrogen atom that can form a stable μ–η²:η⁴-hexatrienyl skeleton via the β-hydrogen elimination after C–C bond formation.