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

Abstract

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) 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– t BuO2H 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.

The effects of metals and ligands on the oxidation of n-octane using iridium and rhodium “PNP” aminodiphosphine complexes

“…Outer-sphere (A) and inner-sphere (B) and free-radical (C) mechanisms of C-H bond activation in alkanes. 85,86 involvement of mechanism A during oxidation using these catalysts, which is in agreement with the findings of Chepaikin et al 85 In contrast, Ir-based catalysts very dominantly follow mechanisms B and C.…”

“…Also, the free-radical mechanism (C) is known to result in deep oxidation and forms products such as ketones and acids as discussed for mechanism (B). 86 It is important to note that in the majority of cases, the latter two mechanisms dominate. However, generally, and considering the selectivity for different products observed over the Rh-based catalysts (except for 2c), the data support the slight Scheme 1.…”

“…These tests showed that the mechanism was not highly dependent on the structures of the catalysts, suggesting the involvement of mechanism B. Ketone formation, due to deeper oxidation, is common when TBHP (t-BuOOH) is used as an oxidant. 20,87 It can be postulated that the reaction mainly proceeds through the formation of hydroxyl or free radicals (mechanisms B and C), especially in the case of the Ir catalysts. 84,88…”

Pyrazolyl-pyrimidones inhibit the function of human solute carrier protein SLC11A2 (hDMT1) by metal chelation

“…The formation of such 5-membered ring chelate has been recently established by X-ray crystallography of a ruthenium complex containing both a pyrazolyl-pyridine and a pyrazolyl-pyridone ligand and investigated as C-H oxidation catalyst. 22 Unfortunately, UVvis titration of 13 could not be used to provide any evidence for metal complexation since the UV-vis spectrum did not significantly change upon titration with Fe 2+ and Cd 2+ . Attempts to crystallize the iron complex of 13 as well as investigation by 1 H-NMR and mass spectrometry similarly did not yield any indication of complex formation.…”

Hemilabile N‐heterocyclic carbene and nitrogen ligands on Fe (II) catalyst for utilization of CO2 into cyclic carbonate

“…[ 19 ] The difference of the coordination ability between the two ligands results in the weaker ligand preferring to dissociate from the metal center, which will provide one or more coordination sites for the activation of substrate molecule at metal center. Our group has successfully synthesized a variety of transition metal complexes bearing hybrid ligands such as Fe, [ 18,20 ] Pd, [ 21 ] Ru, [ 22 ] and Ni catalysts, [ 23 ] which have been given rise to unique and high reactivity in cycloaddition of CO 2 , coupling reaction, and oxidative reaction. In this work, we now report a simple, practical, and convenient synthetic procedure for Fe (II) complexes based on the trans effect between N ‐heterocyclic carbene (NHC) and nitrogen ligands.…”