Ruthenium Phosphinimine Complex as a Fast-Initiating Olefin Metathesis Catalyst with Competing Catalytic Cycles

A ruthenium-based olefin metathesis (OM) catalyst bearing a monodentate triphenylphosphinimine ligand, Ru1, was synthesized, characterized, and its activity for the homocoupling of terminal alkenes was investigated. Utilizing 1-hexene as a model substrate, the empirical rate law for Ru1 was found to be first-order in alkene and complex (indicating that both species were involved in the rate-limiting step), with a rate constant of 0.697 ± 0.050 M–1 s–1. Moreover, the experimentally determined activation parameters ΔS⧧ and ΔH⧧ (−48.7 ± 5.1 eu and 3.19 ± 0.15 kcal/mol, respectively) were consistent with an associative or associative interchange ligand substitution reaction. When considering the ΔG⧧ (298 K) value of 17.7 kcal/mol, Ru1 ranked among the fastest initiating ruthenium-based OM catalysts reported in the literature. Density functional theory (DFT) calculations were also performed to explore potential catalytic mechanisms. Two pathways were considered: a traditional mechanism where the phosphinimine ligand de-coordinated and an alternative mechanism where the phosphine donor de-coordinated. Although the energy differences between the two pathways were typically fairly small (1.4–3.5 kcal/mol), the alternative pathway with phosphine de-coordination was energetically more favorable. It is anticipated, however, that both cycles are working in tandem during the catalytic reaction. In addition to kinetic studies, the stability of Ru1 was explored using 1-hexene as a model substrate. The phosphinimine catalyst was found to be mildly oxygen-sensitive and moisture-tolerant. Furthermore, Ru1 was determined to be prone to bimolecular decomposition, through the crystallographic characterization of a key degradation product. There was also strong evidence for NH exchange between the tricyclohexylphosphine and triphenylphosphinimine moieties. Lastly, the substrate scope of Ru1 in regard to α-olefins was explored. Catalytic efficiency dropped with more electron-deficient alkenes, as well as with increasing steric bulk on the substrate, which was consistent with the proposed catalytic mechanism.