Synthesis and Catalytic Evaluation of Ruthenium–Arene Complexes Bearing Imidazol(in)ium-2-thiocarboxylate Ligands

Five new complexes with the generic formula [RuCl2(p-cymene)­(SOC·NHC)] (2–6) were isolated in high yields by reacting
the [RuCl2(p-cymene)]2 dimer
with a range
of imidazol­(in)­ium-2-thiocarboxylate zwitterions bearing cyclohexyl,
2,4,6-trimethylphenyl (mesityl), or 2,6-diisopropylphenyl groups on
their nitrogen atoms in CH2Cl2 at −20 °C. All the products
were fully characterized by IR and NMR spectroscopy, and the molecular
structures of [RuCl2(p-cymene)­(SOC·IMes)] (3) and [RuCl2(p-cymene)­(SOC·SIMes)] (5)
were determined by X-ray diffraction analysis. Coordination of the
NHC·COS ligands took place via the sulfur atom.
A remarkable shielding of the methine proton on the p-cymene isopropyl group was observed by 1H NMR spectroscopy
for complexes 3–6. It is most likely
caused by the aromatic ring current of a neighboring mesityl or 2,6-diisopropylphenyl
substituent. The catalytic activity of compounds 2–6 was probed in the ring-opening metathesis polymerization
(ROMP) of cyclooctene, in the atom transfer radical polymerization
(ATRP) of methyl methacrylate, and in the synthesis of enol esters
from 1-hexyne and 4-acetoxybenzoic acid. In all these reactions, the
[RuCl2(p-cymene)­(SOC·NHC)] complexes displayed performances slightly inferior to those
exhibited by [RuCl2(p-cymene)­(NHC)] species
that result from the reaction of [RuCl2(p-cymene)]2 with NHC·CO2 inner salts. However, they were significantly better catalyst
precursors than the much more robust chelates of the [RuCl­(p-cymene)­(S2C·NHC)]­PF6 type obtained by coordination of NHC·CS2 betaines to the ruthenium dimer. These results suggest that the
Ru–(SOC·NHC) motif undergoes a dethiocarboxylation
under the experimental conditions adopted for the catalytic tests
and leads to the same elusive Ru–NHC active species as the
preformed [RuCl2(p-cymene)­(NHC)] family
of complexes.