This contribution presents a theoretical and experimental mechanistic study of the process by which actinide dialkyls, Cp2AnR2 (Cp = η5-(CH3)5C5, An = Th or U), undergo facile double carbonylation and C=C bond formation to yield monomeric or dimeric Cp2An2+ complexes of the cis-enediolato ligand, cis-RC(O)=C(O)R. An electronic structural analysis of the proposed actinide bis(acyl) precursor shows that the most stable structure is that of a 20-electron bis(dihaptoacyl), Cp2An(η2-COR)2. In contrast, the stable structure for the analogous titanium complex is found to be the 18-electron Ti(η2-COR)(η1-COR) configuration. A significant factor in stabilizing the actinide 20-electron structure is a favorable interaction which can only occur between a bis(dihaptoacyl) C=O donor molecular orbital and a metal f orbital. The molecularity of C=C fusion has been studied experimentally via a crossover experiment in which a 1:1 mixture of Cp2Th(CH3)2 and Cp2Th(13CH3)2 is carbonylated. Within experimental error, all butenediolate formation occurs at a single metal center. A theoretical analysis of the intramolecular An(η2-COR)2→ AnOC(CH3)=C(CH3)O reaction process has identified a low-energy pathway beginning from a bis(η2-acyl) in which the coplanar C → O vectors point away from each other. Conrotatory twisting of the two η2-COR ligands occurs in concert with C=C fusion, analogous to the process by which two singlet methylenes undergo coupling to form ethylene. In contrast, N C(acyl) π-electron donation in the analogous carbamoyls, An(η2-CONR2)2, is calculated to substantially increase the barrier to C=C coupling. Just such a situation is observed experimentally.
ASJC Scopus subject areas
- Colloid and Surface Chemistry