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Highly Active and Robust Ruthenium Complexes Based on Hemilability of Hybrid Ligands for C-H Oxidation.

Evaluation of the hemilability of hybrid ligands provides a key to understand the metal-ligand cooperation in transition metal catalysis. Here, we design and synthesize a type of RuII complexes based on the hemilability of N-heterocyclic carbenes (NHCs), pyridine, and pyrazole, to compare their activity with other reported Ru catalysts in benzylic C-H oxidation. The RuII catalysts showed ultrastrong catalytic activity in water at room temperature and achieved a turnover frequency (TOF) = 114 h-1, which are the highest TOF values ever reported for Ru-catalyzed benzylic C-H oxidation. The addition of tridentate hybrid ligands in the Ru central position has two beneficial effects: NHCs with a stronger donor ability stabilizes the Ru center; however, nitrogen ligands with a relatively weaker donor ability release from the Ru center, so that it induces reaction. UV-vis, high-resolution ESI-MS, EPR spectrometry, the trapping of radical and the density functional theory calculations (DFT) suggested that a cation catalyst L-RuII-tBuO2H is formed via the reaction between starting RuII catalysts and tert-butyl hydroperoxide, which further undergoes a cleavage of the O-O bond to generate a radical and a cation L-RuIII-OH active intermediate.