From Bis(N-Alkylimidazole) to Bis(NH-NHC) Complexes: A New Reactivity Pattern in Rhenium Carbonyl Complexes

The current importance of nitrogen heterocyclic carbene (NHC) ligands in several areas of coordination chemistry and catalysis can hardly be overemphasized. The deprotonation of imidazolium salts is the prime route to NHCs; however, the study of the transformations relating imidazole and NHC ligands is still an emerging area of research. The thermodynamic preference for Nor C-coordination in imidazoles has been theoretically investigated by Crabtree and Eisenstein. Several examples of tautomerization in pyridinic ligands have been published; however apart from the work reported by Sundberg et al. on Ru(II) NHC complexes that were obtained in a very low yield, only a similar tautomerization in a non-chelated imidazole ligand has been proposed by Bergman and Ellman. Rutheniumand iridium-mediated tautomerizations from NHC to imidazole ligands were reported by the groups of Whittlesey and Li respectively, whereas imidazole to NHC tautomerizations aided by chelate ring formation mediated by Ir and Ru were reported by the groups of Grotjahn and of Kuwata and Ikariya respectively. Ruiz and Perandones reported the basepromoted tautomerization of imidazole ligands to NHCs at a manganese (I) center. Our group found that the outcome of related reactions at Re dramatically depends on the nature of the ancillary ligands and the substituents at the nitrogen of the Nalkylimidazole (N-RIm) ligand (see below). As previously found in our goup and shown in Scheme 1, imidazol-2-yl complex A, the product of the deprotonation of [Re(CO)3(N-MeIm)3]OTf (N-MeIm= N-methylimidazole), can be methylated to afford the NHC compound B or protonated to yield C, featuring an NH-NHC ligand. In contrast, employment of [Re(CO)3(N-MesIm)3]OTf (N-MesIm= N-mesitylimidazole) as precursor leads to ring opening. Also, deprotonation of complexes with one N-RIm ligand and either 2,2’-bipyridine or 1,10-phenanthroline results in C-C coupling and activation of one of the pyridine rings of the diimine chelate. This wealth of new reactivity patterns prompted us to extend our studies by investigating the deprotonation of [Re(OTf)(CO)3(N-RIm)2] complexes.