Formation of active species from ruthenium alkylidene catalysts—an insight from computational perspective

Abstract

Ruthenium alkylidene complexes are commonly used as olefin metathesis catalysts. Initiation of the catalytic process requires formation of a 14-electron active ruthenium species via dissociation of a respective ligand. In the present work, this initiation step has been computationally studied for the Grubbs-type catalysts 3 ), using density functional theory (DFT). Additionally, the extended-transition-state combined with the natural orbitals for the chemical valence (ETS-NOCV) and the interacting quantum atoms (IQA) energy decomposition methods were applied. The computationally determined activity order within both families of the catalysts and the activation parameters are in agreement with reported experimental data. The significance of solvent simulation and the basis set superposition error (BSSE) correction is discussed. ETS-NOCV demonstrates that the bond between the dissociating ligand and the Ru-based fragment is largely ionic followed by the charge delocalizations: σ(Ru-P) and π(Ru-P) and the secondary CH … Cl, CH … π, and CH … HC interactions. In the case of transition state structures, the majority of stabilization stems from London dispersion forces exerted by the efficient CH … Cl, CH … π, and CH … HC interactions. Interestingly, the height of the electronic dissociation barriers is, however, directly connected with the prevalent (unfavourable) changes in the electrostatic and orbital interaction contributions despite the favourable relief in Pauli repulsion and geometry reorganization terms during the activation process. According to the IQA results, the isopropoxy group in the Hoveyda-Grubbs-type catalysts is an efficient donor of intra-molecular interactions which are important for the activity of these catalysts.

Predictive Catalysis in Olefin Metathesis with Ru‐based Catalysts with Annulated C60 Fullerenes in the N‐heterocyclic Carbenes

“…[29] In addition, for the first step we omitted the calculation of the energy barrier, if any, since it is a quasidissociative step. [30,31] The turnover-frequency determining transition state (TDTS) is IV-V for all the complexes under study, [32] which corresponds to the opening of the ruthenacyclobutane complex or the [2 + 2] cycloreversion reaction. After the formation of species V, the Gibbs-energy profiles for reaction pathways involving catalysts A, B, and D follow an energetically slight downhill trajectory without high-energy barriers or highly stabilized intermediates, thus resembling previous reports.…”

“…Although the energy cost of the first step is the largest one regarding the initiation step, to determine the kinetic bottleneck of the reaction pathway it is necessary to look for the barrier of the different TSs with respect to the lowest energy intermediate [29] . In addition, for the first step we omitted the calculation of the energy barrier, if any, since it is a quasi‐dissociative step [30,31] . The turnover‐frequency determining transition state (TDTS) is IV – V for all the complexes under study, [32] which corresponds to the opening of the ruthenacyclobutane complex or the [2+2] cycloreversion reaction.…”

Atoms in molecules in real space: a fertile field for chemical bonding

“…188 By tracking the changes in the pairwise ionic and covalent contributions throughout the whole reaction path, it has been possible to ascertain the key role played by Co⋯O contacts as the driving force for the reaction to take place. The IQA analyses also reveal that bulky ligands of HGCs participate in CH⋯Cl contacts and CH⋯Ru agostic interactions, 182 and may contribute to the weakening of the reactive Ru–O bonds. Overall, these and other examples suggest that the IQA and QTAIM approaches might be critical in building a paradigm of chemical reactivity based on noncovalent interactions as suggested by Cornaton and Djukic.…”

“…This is shown, for example, in a mechanistic study of ammonia fixation by Ir-based complexes, 181 showing how the ionic contribution of the Ir-X bonds with different ligands modulates the activation energies. In another recent study, 182 the breaking of a Ru-O chemical bond during the initiation step of Hoveyda-Grubbs Catalysts (HGCs), 183 which are extensively used in olefine methathesis reactions, has been rationalized thanks to the IQA picture of the reactive Ru-O bond as mainly dominated by electrostatics (roughly 90% of E Ru-O int comes from the classical contribution in eqn (57)). In this way, the impact on the initiation kinetics due to substitution at different positions of HGCs is well understood in terms of their electron donor and withdrawing influence.…”

“…In this way, the impact on the initiation kinetics due to substitution at different positions of HGCs is well understood in terms of their electron donor and withdrawing influence. Moreover, the most labile HGC precatalysts have the smallest values of | E Ru–O int |, 182 which may be considered as a reliable reactivity index.…”