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Ternary nanomaterials Cu-Cu2O/rGO-NH2 (rGO = reduced graphene oxide) exhibited a synergistic effect in the quantitative catalysis of selective aerobic oxidation of benzyl alcohol. The synergistic effect is attributed to the heterojunctions among the three components and in intrinsic nature, the formation of the heterojunctions lowered the conduction band (CB) energy level and raised the valence band (VB) energy level of the main catalyst Cu2O, which eases electron transfer from the catalyst to O2 in its activation and from the substrate to the catalyst in the oxidation, respectively.

In this work, core–shell Fe3O4@Cu2O and Fe3O4@Cu2O–Cu nanomaterials for aerobic oxidation of benzylic alcohols are reported with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and N-methylimidazole (NMI) as the co-catalysts. To anchor Cu2O nanoparticles around the magnetic particles under solvothermal conditions, the magnetic material Fe3O4 was modified by grafting a layer of L-lysine (L-Lys) to introduce –NH2 groups at the surface of the magnetic particles. With amine groups as the anchor, Cu(NO3)2 was used to co-precipitate the desired Cu2O by using ethylene glycol as the reducing agent. Prolonging the reaction time would lead to over-reduced forms of the magnetic materials in the presence of copper, Fe3O4@Cu2O–Cu. The nanomaterials and its precursors were fully characterized by a variety of spectroscopic techniques. In combination with both TEMPO and NMI, these materials showed excellent catalytic activities in aerobic oxidation of benzylic alcohols under ambient conditions. For most of the benzylic alcohols, the conversion into aldehydes was nearly quantitative with aldehydes as the sole product. The materials were recyclable and robust. Up to 7 repeat runs, its activity dropped less than 10%. The over-reduced materials, Fe3O4@Cu2O–Cu, exhibited slightly better performance in durability. The magnetic properties allowed easy separation after reaction by simply applying an external magnet.

Homogeneous Cu(I)/NMI/TEMPO catalyst system (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) has been investigated for its catalysis on the aerobic oxidation of 1-octanol and other alcohols into aldehydes under room temperature. The catalytic species was found to be a Cu(I) centre coordinated by two NMI molecules and other two weakly bound solvent molecules, [Cu(NMI)2(Sol)2]+ (Sol = solvent). When CuI was used, this species could be [Cu(NMI)I(Sol)2]. Not like being speculated previously, NMI in this system acts solely as a ligand and its role coordinated to the copper centre enhanced the electron density on the metal centre which promoted the O2 binding in the catalysis. The labile solvent binding to the Cu(I) centre is essential to ensure both oxygen and substrate binding. The catalyst system is suitable for the oxidation of various alcohols using a simple reaction setup and workup. In particular, the system possesses strong oxidizing capability in quantitative conversion of benzylic alcohols regardless of the substituents on the phenyl ring and allylic alcohols into aldehydes. A plausible mechanism was also proposed for the catalysis. The aerobic oxidation of primary alcohols at room temperature to corresponding aldehydes was achieved by the catalyst composed by Cu(I) and methyl imidazole (NMI) mediated by TEMPO in acetonitrile. The catalytic species is proposed to be such a Cu(I) complex that two of its four coordinating sites are occupied by a strong ligand(s) and the rest two are weakly bound by solvent molecules.

The surface organometallic chemistry or surface coordination is considered as an important tool for modifying the surface properties and electronic structure of metal centres to achieve enhancement in catalysis. In this piece of work, we report how cuprous oxide (Cu2O) was turned into an efficient catalyst alongside with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and N-methylimidazole (NMI) for catalytic aerobic oxidation of benzylic alcohols into aldehydes under ambient conditions through surface coordination. The coordination not only enhanced the capability of O2-activation via the ligation of NMI to the metal sites, but also the surface structure was engineered in such a way that a vacancy was created for O2-binding, one of the key steps as revealed by DFT calculations. Both experimental and theoretical results suggested that the coordination to the metal sites at the surface by NMI activated cuprous oxide via breaking up its oxo bridge at the surface to form hydroxyl group to allow O2-binding, and increased also the electron density on the metal site to enhance its capability of O2 activation. A plausible mechanism was proposed by using a variety of spectroscopic and electrochemical techniques as well as DFT calculations.