Monoalkyl-di-µ-methylene-bis[(η-pentamethylcyclopentadienyl)-rhodium(IV)] complexes and the intramolecular migration of alkyl groups between two metal atoms

Abstract

Reaction of the trans dimethyl complex [{(C₅Me₅)Rh}₂(µ-CH₂)₂Me₂](1) with one equivalent of acid in the presence of acetonitrile gave the methyl–acetonitrile complex [{(C₅Me₅)Rh}₂(µ-CH₂)₂(Me)(MeCN)]PF₆(2a); the acetonitrile could be replaced by other ligands to give [{(C₅Me₅)Rh}₂-(µ-CH₂)₂(Me)(L)]PF₆[L = Bu^tCN (2b), PhCN (2c), pyridine (2d), 2-methylpyridine (2e), or CO (2f)]. Reaction of (2a) with halide gave [{(C₅Me₅)Rh}₂(µ-CH₂)₂(Me)X][X = Cl (3) or I (4)]. The other monoalkyl complexes [{(C₅Me₅)Rh}₂(µ-CH₂)₂(R)(MeCN)]PF₆[R = Et (9), Prⁿ(10), or Buⁿ(11)] were obtained analogously from reaction of the appropriate dialkyl complexes [{(C₅Me₅)Rh}₂(µ-CH₂)₂R₂] which were in turn synthesised from the trans dichloro-complex [{(C₅Me₅)Rh}₂(µ-CH₂)₂Cl₂]. The n.m.r. spectra of (2a) showed the presence of cis and trans isomers (ratio ca. 1 : 2) at –80 °C and of dynamic behaviour at higher temperatures. The dynamic behaviour arises from loss of the MeCN, movement of the methyl into a bridging position in a transition state, followed by readdition of the MeCN. Overall this corresponds to an intramolecular migration of the methyl from one rhodium to the other. The other complexes (2) behave similarly but (2d) and (2f) show the ‘frozen-out’ spectra even at +22 °C. Under identical conditions the complexes (9)–(11) exhibited similar behaviour to (2a), but the rates of alkyl migration were ca. 10 times faster. Complex (2a) also disproportionated to give (1) on reaction with base; this involves an intermolecular methyl migration. The other alkyl complexes did not undergo this reaction. The halide complexes (3) and (4) were rigid and of cis configuration in benzene but showed more complex behaviour in dichloromethane.