A DFT computational study of phosphine ligand dissociation versus hemilability in a Grubbs-type precatalyst containing a bidentate ligand during alkene metathesis

Abstract

Using density functional theory, the metathesis reaction of 1-octene in the presence of a Grubbs-type ruthenium alkylidene complex bearing a chelating pyridinyl alcoholate ligand, [RuCl(L)(O∧ N)( = CHPh)] (L = H2IMes or PCy3, O∧ N = 1-(2′-pyridinyl)cyclohexan-1-olate)), was investigated. The complete geometry optimisation and activation energy of various activation steps in the dissociative mechanism were performed at the GGA-PW91/DNP level of theory using Accelrys Materials Studio® 4.0. Two possible precatalyst initiations were explored, i.e. the dissociation of the labile N-atom of the O∧ N-ligand as well as the dissociation of ligand L, due to the belief that ruthenium-catalysed metathesis reactions proceed through 14-electron intermediates. The formation of the catalytically active heptylidene species is kinetically favoured for both the first- and second-generation chelating complexes. The computational results are in agreement with the experimental results obtained with NMR for the second-generation system. The computational results suggest that both phosphine ligand dissociation and hemilability may play a role in the metathesis reaction with the first-generation system.