CpCo-Mediated Reactions of Cyclopropenones: A DFT Study

Quantum chemical calculations using density functional theory at the B3LYP/6-311G(d)/Wachters+f level have been carried out to model possible pathways for the reaction of CpCo(CO)(2) (15) with cyclopropenone to yield CpCo-capped p-benzoquinone complexes. For a number of intermediates, including all relevant 16-electron species, energies and geometries of the triplet electronic state were also obtained, which permitted us to locate four crossings of the singlet and triplet potential surfaces. It was found that first a metallacyclobutenone complex (21) is formed. Our calculations indicate a dissociative mechanism; the previously observed associative mechanism in ligand substitution reactions of 15 is shown to be a consequence of lower reaction temperatures. The addition of a second cyclopropenone unit to 21 reveals a higher energy path (path I) than does the addition of CO to 21, followed by acetylene addition (path II). The latter path is favored, which has already been shown by labeling and trapping experiments. Despite favorable energetics, the paths involving the triplet surface have not been observed experimentally; we tentatively attribute this to the very short lifetime of the initially formed triplet.