Bonding and Catalytic Application of Ruthenium N-Heterocyclic Carbene Complexes Featuring Triazole, Triazolylidene, and Imidazolylidene Ligands

The first mixed normal/abnormal NHC ruthenium(II) complexes bearing triazole/imidazole-based ligands are presented. The ruthenium complexes show differing coordination geometries depending on the ligand sphere and the coordination of the triazole. Two Ru(II) monocarbene complexes have been obtained, with a 2-imidazolylidene carbene and the 1,2,3-triazoles acting as neutral nitrogen donors. In addition, a tridentate Ru(II) complex featuring a normal 2-imidazolylidene as well as two abnormal 1,2,3-triazolylidene carbene moieties is described. Both structural motifs are evaluated in transfer hydrogenation of acetophenone, with the electron-rich Ru(II) tri-NHC exhibiting the highest conversion.