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

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.