The Nature of the Barrier to Phosphane Dissociation from Grubbs Olefin Metathesis Catalysts

By using cost‐efficient density functional theory accounting for dispersion in combination with an implicit solvent model, for the first time it has been possible to reproduce activation Gibbs free energies for phosphane dissociation from the Grubbs ruthenium olefin metathesis precatalysts in solution with good accuracy (mean unsigned error compared to experiment <2.5 kcal mol –1 ). The barrier is calculated to be in the range 17.8–25.7 kcal mol –1 for a set of nine catalysts, and is found to be located at intermediate Ru–P distances (ca. 4 Å). The agreement with the experimental activation parameters is gratifying and suggests that the calculations may give insight into these reactions and, in particular, offer resolution as to the individual components of the barriers. The forward (dissociation) barriers are much higher than the corresponding reaction free energies and the reverse reaction, phosphane binding, is associated with a significant barrier (13.2–15.6 kcal mol –1 ). The latter barrier mainly arises from loss of entropy and solute–solvent dispersion interactions for the two fragments prior to Ru–P bond formation. Moreover, the fact that the barrier to phosphane binding is so significant means that the reaction free energy of phosphane dissociation cannot be taken to be identical or similar to the forward barrier, as has occasionally been assumed in earlier studies.