DFT Study of Internal Alkyne-to-Disubstituted Vinylidene Isomerization in [CpRu(PhCCAr)(dppe)]⁺

Internal alkyne-to-vinylidene isomerization in the Ru complexes ([CpRu­(η²-PhCCC₆H₄R-<i>p</i>)­(dppe)]⁺ (Cp = η⁵-C₅H₅; dppe = Ph₂PCH₂CH₂PPh₂; R = OMe, Cl, CO₂Et)) has been investigated using a combination of quantum mechanics and molecular mechanics methods (QM/MM), such as ONIOM­(B3PW91:UFF), and density functional theory (DFT) calculations. Three kinds of model systems (I–III), each having a different QM region for the ONIOM method, revealed that considering both the quantum effect of the substituent of the aryl group in the η²-alkyne ligand and that of the phenyl groups in the dppe ligand is essential for a correct understanding of this reaction. Several plausible mechanisms have been analyzed by using DFT calculations with the B3PW91 functional. It was found that the isomerization of three complexes (R = OMe, CO₂Et, and Cl) proceeds via a direct 1,2-shift in all cases. The most favorable process in energy was path 3, which involves the orientation change of the alkyne ligand in the transition state. The activation energies were calculated to be 13.7, 15.0, and 16.4 kcal/mol, respectively, for the three complexes. Donor–acceptor analysis demonstrated that the aryl 1,2-shift is a nucleophilic reaction. Furthermore, our calculation results indicated that an electron-donating substituent on the aryl group stabilizes the positive charge on the accepting carbon rather than that on the migrating aryl group itself at the transition state. Therefore, unlike the general nucleophilic reaction, the less-electron-donating aryl group has an advantage in the migration