New Ru(II) Complexes with Anionic and Neutral N-Donor Ligands as Epoxidation Catalysts: An Evaluation of Geometrical and Electronic Effects

The synthesis of a family of new Ru complexes containing the meridional trpy ligand with general formula [Ru-II(T)-(D)(X)(n+) (T = 2,2':6',2"-terpyridine (trpy); D = 3,5-dimethyl-2-(2-pyridyl)pyrrolate (pyrpy), and 2-(1-Methyl-3-pyrazolyl)pyridine (pypz-Me); X = Cl, H2O) have been described. All complexes have been spectroscopically characterized in solution through H-1 NMR and UV-vis techniques, and the chloro complexes have also been characterized in the solid state through monocrystal X-ray diffraction analysis. The pyrpy ligand undergoes an oxidation at the 3-position of the pyrrolate ring during the formation of the corresponding aqua complex thus generating the analogous compound containing the oxidized ligand pyrpy-O. The redox properties of all complexes have also been studied by means of CV and DPV techniques, where both geometrical (trans vs cis) and electronic (neutral vs anionic) effects can be unveiled and rationalized. Finally, the reactivity of the whole set of Ru-OH2 complexes has been tested with regard to the epoxidation of different alkenes with Phl(OAc)(2). In all cases good selectivities and conversions were obtained. Furthermore, total retention of the initial cis configuration was achieved when using these catalysts to epoxidize cis-beta-methylstyrene.