Organic chemistry of dinuclear metal centres. Part 5. Generation of hydrocarbons from methylene chains linking two metal centres. Evidence for a dimetallacycle intermediate

Abstract

The hydrocarbon products of the thermal and photochemical decompositions of the complexes [(η-C₅H₅)(CO)₂M¹{µ-(CH₂)_n}M²(CO)₂(η-C₅H₅)](M¹= M²= Fe, n= 3–5; M¹= M²= Ru, n= 3 or 4; M¹= Fe, M²= Ru, n= 3) have been determined and the results interpreted in terms of a transient dimetallacycle which undergoes decomposition viaβ-elimination and reductive elimination processes. Decomposition of the compounds containing three-carbon bridges yields cyclopropane and propene in a ratio strongly dependent upon the identity of the metal atoms and the conditions, factors which are rationalised by the proposed mechanism. But-1-ene, and cis- and trans-but-2-ene are obtained from the decomposition of complexes containing four-carbon chains, but only but-1-ene and trans-but-2-ene are produced from the thermolysis of [(η-C₅H₅)(CO)₂Fe{µ-CH(Me)CH₂CH₂}Fe(CO)₂(η-C₅H₅)]. This is attributed to a methyl-substituted dimetallacyclopentane intermediate adopting for steric reasons a conformation which does not allow the formation of cis-but-2-ene. The low proportion of but-1-ene can also be traced to crowding in the dimetallacyle. Pentane is preferentially evolved from [(η-C₅H₅)(CO)₂Fe{µ-(CH₂)₅}Fe(CO)₂(η-C₅H₅)], with smaller amounts of pent-1-ene, and cis- and trans-pent-2-ene, interpreted as due to a predominantly radical mechanism for the decomposition of this compound. The organometallic products of the decompositions are the dimers [M₂(CO)₄(η-C₅H₅)₂](M = Fe or Ru) and, where appropriate, those of the subsequent photolysis of [Ru₂(CO)₄(η-C₅H₅)₂]. Carbon–carbon bond fission occurs on heating [(η-C₅H₅)(CO)₂Ru{µ-(CH₂)₃}Ru(CO)₂(η-C₅H₅)] with Me₃NO in tetrahydrofuran, giving a low yield of [Ru₂(CO)₂(µ-CO)(µ-CH₂)(η-C₅H₅)₂].