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Highly Active and Robust Ruthenium Complexes Based on Hemilability of Hybrid Ligands for C–H Oxidation
journal contribution
posted on 2020-02-24, 15:58 authored by Chun-Bo Bo, Qingqing Bu, Xue Li, Ge Ma, Donghui Wei, Cheng Guo, Bin Dai, Ning LiuEvaluation
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) of 114 h–1, which is the highest TOF value 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 stabilize the Ru center; however, nitrogen ligands with
a relatively weaker donor ability release from the Ru center, so that
they induce a reaction. UV–vis, high-resolution electrospray
ionization mass spectrometry (ESI-MS), electron paramagnetic resonance
(EPR) spectrometry, the trapping of radicals, 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.