Hydrogen peroxide production from oxygen and formic acid by homogeneous Ir-Ni catalyst.

Hydrogen peroxide was directly produced from oxygen and formic acid, catalysed by a hetero-dinuclear Ir-Ni complex with two adjacent sites, at ambient temperature. Synergistic catalysis derived from the hetero-dinuclear Ir and Ni centres was demonstrated by comparing its activity to those of the component mononuclear Ir and Ni complexes. A reaction intermediate of Ir-hydrido was detected by UV-vis, ESI-TOF-MS, and 1H NMR spectroscopies. It was revealed that the Ir moiety serves as an active species of Ir-hydrido, reacting with oxygen to afford an Ir-hydroperoxide species through O2 insertion, which is the rate-determining step for H2O2 production. Meanwhile, the Ni moiety promotes H2O2 formation by activating solvents as proton sources. We also found that H2O2 production is strongly affected by the solvent dielectric constants (DE); the highest H2O2 concentration was obtained in ethylene glycol with a moderate DE. The catalytic mechanism of H2O2 production by the Ir-Ni complex was discussed, based on kinetic analysis, isotope labelling experiments, and theoretical DFT calculations.

Formate-driven catalysis and mechanism of an iridium-copper complex for selective aerobic oxidation of aromatic olefins in water.

A hetero-dinuclear IrIII-CuII complex with two adjacent sites was employed as a catalyst for the aerobic oxidation of aromatic olefins driven by formate in water. An IrIII-H intermediate, generated through formate dehydrogenation, was revealed to activate terminal aromatic olefins to afford an Ir-alkyl species, and the process was promoted by a hydrophobic [IrIII-H]-[substrate aromatic ring] interaction in water. The Ir-alkyl species subsequently reacted with dioxygen to yield corresponding methyl ketones and was promoted by the presence of the CuII moiety under acidic conditions. The IrIII-CuII complex exhibited cooperative catalysis in the selective aerobic oxidation of olefins to corresponding methyl ketones, as evidenced by no reactivities observed from the corresponding mononuclear IrIII and CuII complexes, as the individual components of the IrIII-CuII complex. The reaction mechanism afforded by the IrIII-CuII complex in the aerobic oxidation was disclosed by a combination of spectroscopic detection of reaction intermediates, kinetic analysis, and theoretical calculations.

Cooperative Effects of Heterodinuclear IrIII-MII Complexes on Catalytic H2 Evolution from Formic Acid Dehydrogenation in Water.

Novel heterodinuclear IrIII-MII complexes (M = Co, Ni, or Cu) with two adjacent reaction sites were synthesized by using 3,5-bis(2-pyridyl)-pyrazole (Hbpp) as a structure-directing ligand and employed as catalysts for H2 evolution through formic acid dehydrogenation in water. A cooperative effect of the hetero-metal centers was observed in the H2 evolution in comparison with the corresponding mononuclear IrIII and MII complexes as the components of the IrIII-MII complexes. The H2 evolution rate for the IrIII-MII complexes was at most 350-fold higher than that of the mononuclear IrIII complex. The catalytic activity increased in the following order: IrIII-CuII complex < IrIII-CoII complex < IrIII-NiII complex . The IrIII-H intermediates of the IrIII-MII complexes were successfully detected by ultraviolet-visible, 1H nuclear magnetic resonance, and ESI-TOF-MS spectra. The catalytic enhancement of H2 evolution by the IrIII-MII complexes indicates that the IrIII-H species formed in the IrIII moiety act as reactive species and the MII moieties act as acceleration sites by the electronic effect from the MII center to the IrIII center through the bridging bpp- ligand. The IrIII-MII complexes may also activate H2O at the 3d MII centers as a proton source to facilitate H2 evolution. In addition, the affinity of formate for the IrIII-MII complexes was investigated on the basis of Michaelis-Menten plots; the IrIII-CoII and IrIII-NiII complexes exhibited affinities that were relatively higher than that of the IrIII-CuII complex. The catalytic mechanism of H2 evolution by the IrIII-MII complexes was revealed on the basis of spectroscopic detection of reaction intermediates, kinetic analysis, and isotope labeling experiments.

Mechanistic Insight into Synergistic Catalysis of Olefin Hydrogenation by a Hetero-Dinuclear RuII-CoII Complex with Adjacent Reaction Sites.

We have designed and synthesized a hetero-dinuclear RuII-CoII complex with a dinucleating ligand inspired by hetero-dinuclear active sites of metalloenzymes. A synergistic effect between the adjacent RuII and CoII sites has been confirmed in catalytic olefin hydrogenation by the complex, exhibiting a much higher turnover number than those of mononuclear RuII or CoII complexes as the components. A RuII-hydrido species was detected by 1H NMR and electrospray ionization (ESI)-time-of-flight (TOF)-MS measurements as an intermediate to react with olefins, and CoII-bound methanol was suggested to act as a proton source.