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

Abstract

A diruthenium complex containing a μ−η 2 :η 4 -1,4dimethylhexatrienyl ligand, [(Cp*Ru) 2 (μ−η 2 :η 4 -CMe�CH− CH�CMe−CH�CH 2 )(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−η 2 :η 4 -2,5dimethylhexatrienyl complex, [(Cp*Ru) 2 (μ−η 2 :η 4 -CH�CMe− CH�CH−CMe�CH 2 )(μ-H)] (4a), in which the methyl substituents on the C 6 moiety migrate from the 1,4-to the 2,5positions. 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 13 C-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−η 5 -ruthenacyclohexadienyl intermediate, [(Cp*Ru) 2 (μ−η 5 -CMeCHCHCMeCH−)(Me)] (C). A related μ−η 5 -ruthenacyclohexadienyl complex possessing a terminal hydride, [(Cp*Ru) 2 (μ−η 5 -CMeCHCHCHCH−)(H)] (6c), was obtained by the thermolysis of an unsubstituted μ−η 2 :η 4 -hexatrienyl complex, [(Cp*Ru) 2 (μ-CH�CH−CH�CH−CH�CH 2 )(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 αhydrogen atom that can form a stable μ−η 2 :η 4 -hexatrienyl skeleton via the β-hydrogen elimination after C−C bond formation.