A Mixed N-Heterocyclic Carbene/2,2′-Bipyridine-Supported Robust Ruthenium(II) Oxidation Precatalyst for Benzylic C−H Oxidation

Impact of Bidentate Pyridyl-Mesoionic Carbene Ligands: Structural, (Spectro)Electrochemical, Photophysical, and Theoretical Investigations on Ruthenium(II) Complexes

We present here new synthetic strategies for the isolation of a series of Ru(II) complexes with pyridyl-mesoionic carbene ligands (MIC) of the 1,2,3-triazole-5-ylidene type, in which the bpy ligands (bpy = 2,2'-bipyridine) of the archetypical [Ru(bpy)3]2+ have been successively replaced by one, two, or three pyridyl-MIC ligands. Three new complexes have been isolated and investigated via NMR spectroscopy and single-crystal X-ray diffraction analysis. The incorporation of one MIC unit shifts the potential of the metal-centered oxidation about 160 mV to more cathodic potential in cyclic voltammetry, demonstrating the extraordinary σ-donor ability of the pyridyl-MIC ligand, while the π-acceptor capacities are dominated by the bpy ligand, as indicated by electron paramagnetic resonance spectroelectrochemistry (EPR-SEC). The replacement of all bpy ligands by the pyridyl-MIC ligand results in an anoidic shift of the ligand-centered reduction by 390 mV compared to the well-established [Ru(bpy)3]2+ complex. In addition, UV/vis/NIR-SEC in combination with theoretical calculations provided detailed insights into the electronic structures of the respective redox states, taking into account the total number of pyridyl-MIC ligands incorporated in the Ru(II) complexes. The luminescence quantum yield and lifetimes were determined by time-resolved absorption and emission spectroscopy. An estimation of the excited state redox potentials conclusively showed that the pyridyl-MIC ligand can tune the photoredox activity of the isolated complexes to stronger photoreductants. These observations can provide new strategies for the design of photocatalysts and photosensitizers based on MICs.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

Synthesis of Xanthones, Thioxanthones and Acridones by a Metal-Free Photocatalytic Oxidation Using Visible Light and Molecular Oxygen

9H-Xanthenes, 9H-thioxanthenes and 9,10-dihydroacridines can be easily oxidized to the corresponding xanthones, thioxanthones and acridones, respectively, by a simple photo-oxidation procedure carried out using molecular oxygen as oxidant under the irradiation of visible blue light and in the presence of riboflavin tetraacetate as a metal-free photocatalyst. The obtained yields are high or quantitative.

Site-selective unidirectional benzylic sp3 C–H oxidation of dodecahydrotriphenylene with RuCl3–NaIO4: formation of benzylic ketones

Dodecahydrotriphenylene, a higher homologue of trindane chemoselectively undergoes unidirectional benzylic sp³ C–H oxidation and the central benzene ring remains intact unlike that in trindane under similar reaction conditions. RuO₄ which generally attacks sp² C–H to form oxidative products is found to give benzylic ketones via sp³ C–H oxidation. Density functional theory (DFT) calculations have also been performed to analyse the potential energy, energy barrier and HOMO–LUMO energy gap of the products.

Dodecahydrotriphenylene, a higher homologue of trindane chemoselectively undergoes unidirectional benzylic sp³ C–H oxidation and the central benzene ring remains intact unlike that in trindane under similar reaction conditions.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

Coordination Booster-Catalyst Assembly: Remote Osmium Outperforming Ruthenium in Boosting Catalytic Activity

Presented herein is a set of bimetallic and trimetallic "coordination booster-catalyst" assemblies in which the coordination complexes [Ru^II (terpy)₂ ] and [Os^II (terpy)₂ ] acted as boosters for enhancement of the catalytic activity of [Ru^II (NHC)(para-cymene)]-based catalytic site. The boosters accelerated the oxidative loss of para-cymene from the catalytic site to generate the active catalyst during the oxidation of alkenes and alkynes into corresponding aldehydes, ketones and diketones. It was found that the boosting efficiency of the [Os^II (terpy)₂ ] units was considerably higher than its congener [Ru^II (terpy)₂ ] unit in these assemblies. Mechanistic studies were conducted to understand this unique improvement.

Aerobic oxidation of fluorene to fluorenone over Co–Cu bimetal oxides

Aerobic oxidation of fluorene to fluorenone was achieved over Co–Cu bimetal oxides using O₂ as an oxidant in the absence of a radical initiator. Co–Cu bimetal oxides showed better catalytic performance than CuO and Co₃O₄.