Mapping the Elimination of Water from Hydroxyvinylidene Complexes of Ruthenium(II): Access to Allenylidene and Vinylvinylidene Complexes in a Stepwise Fashion

Abstract

Reaction of hydroxyvinylidene complexes [Ru(κ1-OAc)(κ2-OAc)(═C═CHC{OH}R1R2)(PPh3)2] (R1 = R2 = Ph; R1 = R2 = Me; R1 = Ph, R2 = Me) with [CPh3]BF4 results in the formation of the cationic carbene species [Ru(κ2-OAc)(OC{Me}OCC{H}═CR1R2)(PPh3)2]BF4. In these complexes, the κ1-acetate ligand has changed its binding mode in order to stabilize the resulting cationic species. The carbene complexes may be deprotonated, although the outcome of the reaction depends markedly on the substituent present. In the case in which R1 = R2 = Ph, the hydrogen on the β-carbon of the organic ligand is removed to afford an allenylidene complex [Ru(κ1-OAc)(κ2-OAc)(═C═C═CPh2)(PPh3)2]. An examination of the structural and spectroscopic parameters for the allenylidene complex indicates that the electronic influence of this ligand is very similar to the corresponding vinylidene and isonitrile analogues. In the cases where R1 = R2 = Me and R1 = Me, R2 = Ph deprotonation occurs at a methyl group to afford vinylvinylidene complexes [Ru(κ1-OAc)(κ2-OAc)(═C═C{H}-CR2═CH2)(PPh3)2] (R2 = Me, Ph). No interconversion between vinylvinylidene and allenylidene complexes was observed. The overall process is analogous to a formal E1-type elimination in which the cationic carbene complex may be viewed as a stabilized carbocation intermediate. A DFT study provided insight into selectivity of the deprotonation step indicating that the greatest relative difference in energy between all the possible isomers of the vinylvinylidene and allenylidene complexes was ca. 20 kJ mol–1. Interconversion between the two forms of the complex by a [1,3]-hydrogen shift appears to be unlikely due to the higher energy of the corresponding transition state; hence the selectivity in the formation of the vinylvinylidene complexes may be due the site of deprotonation being kinetically controlled. An alternative mechanism for this interconversion between vinylvinylidene and allenylidene complexes in cationic half sandwich metal complexes is proposed, which proceeds via a deprotonation/reprotonation pathway.