Halide Exchange in Second-Generation cis-Dihalo Ruthenium Benzylidene Complexes

The current article discusses the lability of the chloride ligands in cis-dichloro ruthenium benzylidene complexes featuring the NHC ligand N,N′-bis­[2,4,6-(trimethyl)­phenyl]­imidazolin-2-ylidene] (SIMes) and an ester-chelating benzylidene ligand. A simple halide exchange reaction with iodide salts was used to assess the halide's lability in this type of complex. The starting compound bearing the cis-dichloro stereochemistry (cis-1) underwent rapid and selective exchange of the chloride trans to the SIMes ligand (cis-2). The substitution of the chloride cis to the NHC ligand for an iodide was found to be much slower, and the corresponding cis-diiodo compound could be isolated and characterized (cis-3). The latter compound undergoes fast and selective exchange of the iodide trans to the NHC ligand when an excess of a chloride source is added, and the fourth member of the family, bearing a chloride trans to the NHC ligand and a iodide cis to the NHC ligand (cis-4), was obtained. The four compounds have been fully characterized, and their catalytic activity was accessed using ring-opening metathesis polymerization of a norbornene derivative as the benchmark reaction. The activity of the initiators decreased within the series cis-4 > cis-1 > cis-2 > cis-3. The high activity of cis-4 could be traced back to the formation of the highest amounts of the corresponding trans-dihalo species, which are the actual active initiators.