A quantum chemical study of the mechanisms of olefin addition to group 9 transition metal dioxo compounds

The mechanistic aspects of ethylene addition to MO(2)(CH(2))(CH(3)) (M=Co, Rh, Ir) have been investigated with a Hartree–Fock/DFT hybrid functional at the MO6/LACVP* and B3LYP/LACVP* levels of theory to elucidate the reaction pathways on the singlet, doublet and triplet potential energy surfaces (PES). In the reaction of the IrO(2)CH(2)CH(3) complex with ethylene, [3 + 2](C,O) addition is the most plausible pathway on the singlet PES, the [3 + 2](O,O) addition is the most favoured pathway on the doublet surface whiles the stepwise [1 + 1] addition involving the oxygen atom of the complex in the first step and the carbon atom of the complex in the second step is the most plausible pathway on the triplet PES. For the reaction of the RhO(2)(CH(2))(CH(3)) complex, the [2 + 2](Rh,O) addition pathway is the most favoured on the singlet surface, the [2 + 2](Rh,C) is the most plausible pathway on the triplet PES and [3 + 2](C,O) is the most plausible on the doublet surface. For the reactions of the CoO(2)(CH(2))(CH(3)) complex, the [1 + 2](O) addition is the most plausible on the singlet PES, [3 + 2](C=Co=O) cycloaddition to form the five–membered intermediate is the most preferred pathway on the doublet PES, whiles on the triplet PES the preferred pathway is the [3 + 2] addition across the O=Co=O bond of the metal complex. The reactions of olefins with the Co dioxo complex have lower activation barriers for the preferred [3 + 2] and [2 + 2] addition pathways as well as fewer side reactions than those of the rhodium and iridium systems. This could imply that the cobalt dioxo complexes can efficiently and selectively catalyze specific reactions in oxidation of olefins than Rh and Ir oxo complexes will do and therefore Co oxo complexes may be better catalysts for specific oxidation reactions of olefins than Rh and Ir complexes are. The activation barriers for the formation of the four—or five-membered metallacycle intermediates through [2 + 2] or [3 + 2] cyclo-addition are lower on the triplet PES than on the singlet PES for the formation of similar analogues. There are fewer competitive reaction pathways on the triplet surface than on the singlet PES. Also, cycloadditions that seem impossible on the singlet PES seem possible on the doublet and or triplet PESs, this is the case typically for the Rh and Co complexes, illustrating the importance of multiple spin states in organometallic reactions. [Figure: see text]