Decomposition reactions of dimethyl-di-µ-methylene-bis(η-pentamethylcyclopentadienyl)dirhodium and their relation to the mechanism of the Fischer-Tropsch reactions; the formation of propylene from three C1ligands

Abstract

Thermal decomposition of [{C5(CH3)5Rh}2(µ-CH2)2(CH3)2](1) at temperatures from 275 to 375 °C yielded methane, propylene, ethylene, and some ethane. Using (1) selectively labelled with 13C in the Rh–methylene and/or the Rh–methyl ligands showed that (a) the gases are formed in intramolecular decompositions not involving the C5(CH3)5 rings, (b) the methane arises from both the rhodium–methyls and the rhodium–methylenes, and (c) the C2 and C3 gases arise predominantly from the coupling of a Rh–methyl and one or two Rh–methylenes; direct coupling of two methylenes or of two methyls is not a favourable process here. Very similar gas mixtures are formed (but at 20–50 °C) on reaction of complex (1) with excess IrCl62–(or other one-electron oxidisers and electrophiles). Carbon-13 and deuterium labelling studies show that these reactions are again intramolecular and do not involve the C5(CH3)5 rings, or the coupling of two methyl or two methylene ligands. Methane arises mainly by combination of a Rh–CH3 and a methylene hydrogen, probably after a two-electron oxidation, leaving a transient species (A) formulated as [{C5(CH3)5Rh}2(µ-CH2)(µ-CH)(CH3)]2+. The C2 products must be formed from (A) by the coupling of the Rh–CH3 with the methylene, to give an Rh–ethyl intermediate which β-eliminates to give ethylene (or acquires a hydrogen to give ethane). The labelling shows propylene to arise from the coupling of one methyl and two methylenes. It can be formed via migration of the methyl onto the µ-methyne in (A), giving a µ-methylene-µ-ethylidene species which couples to give propylene directly. Implicit in the route is the need for two metal centres to allow three C1 fragments to couple together to form propylene. Labelling studies rule out appreciable ethylene formation from a Rh–ethylidene. Direct coupling of the methylenes does occur in the thermal decompositions of [{C5(CH3)5Rh}2(µ-CH2)2X2](X = halide, SCN, or N3), to give ethylene; the main product is again methane. The data are contrasted with results from the decomposition of the iridium analogue of (1) and of [C5(CH3)5Ir(CH3)4]. The relationships of the mechanisms proposed to current models for the mechanism of the Fischer-Tropsch reaction on metal surfaces are discussed.