Reactions of co-ordinated ligands. Part 35. Evidence for carbon–carbon double-bond cleavage of cyclopropenes in their reaction with dinuclear cobalt, rhodium, and iridium complexes; crystal structure and protolysis of [Rh2(µ-CO)(µ-COCHCMe2CH)(µ-C5Me5)2]
Abstract
Reaction of 3,3-dimethylcyclopropene with [M1M2(µ-CO)2(η-C5Me5)2](M1= M2= Rh or Co; M1= Rh, M2= Co; M1= Ir, M2= Co; M1= Rh, M2= Ir) affords 1 : 1 adducts, in which the carbon–carbon double bond has been broken. An X-ray crystallographic study on the dirhodium complex showed that the molecule consists of two Rh(η-C5Me5) fragments bridged by a carbonyl ligand and a complex ligand derived from CO and the cyclopropene. This ligand binds to the Rh2 fragment via a µ-carbene functionality and a µ-ketene. The corresponding reaction of 1,3,3-trimethylcyclopropene with [Co-Rh(µ-CO)2(η-C5Me5)2] and [Rh2(µ-CO)2(η-C5Me5)2] afforded one and two isomeric complexes respectively with related structures, thus providing insight into the reaction mechanism. Additional insight was provided by the observation that 3,3-dimethylcyclopropene reacts with [Co2(µ-CO)2–(η-C5Me5)2], [CoRh(µ-CO)2(η-C5Me5)2], [CoIr(µ-CO)2(η-C5Me5)2], and [RhIr(µ-CO)2(η-C5Me5)2] to afford high yields of single isomeric adducts where ‘carbonyl insertion’ occurs into a carbon to cobalt or iridium bond. The mechanism of formation is discussed in terms of the interaction of cyclo-propene with the two metal centres. The protolysis of the dirhodium species is also examined and leads to regioselective cleavage of the longest Rh–C σ bond via an SE2 reaction pathway.
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