Copper(II) complex of a Schiff base of dehydroacetic acid: Characterization and aerobic oxidation of benzyl alcohol

• A molecular copper catalyst for aerobic oxidation of alcohols is described.
• The complex is isolated as a dimer, but it is converted into a monomer in coordinating solvents (alcohols, DMSO).
• The complex undergoes a metal centered reduction (CuII/CuI) redox wave, which suggests that the copper/dioxygen reactivity is responsible for its catalytic activity.
• No additive other than base and dioxygen is required for efficient catalysis.

The copper complex of a tetradentate ligand based on dehydroacetic acid 1 has been prepared and its crystal structure redetermined, showing a copper pair. EPR spectroscopy shows that the dimeric structure is preserved in CH2Cl2 and CH3CN (triplet signal with D = 480 ± 50 MHz, E = 50 ± 25 MHz), but complex 1 is converted into a monomer in coordinating solvents (MeOH, DMF and DMSO). By DFT calculations it is seen that the copper ion geometry in the monomer is essentially square planar in MeOH and DMF, but evolves towards square pyramidal in DMSO due to solvent ligation in axial position. The hydroxyl of the methanol molecules are involved in hydrogen bonds with the carbonyl oxygen of the ligand. Complex 1 displays a quasi-reversible reduction wave at E1/2 = − 0.86 V vs. Fc +/Fc, which is assigned to the CuII/CuI redox couple. It catalyzes the aerobic oxidation of benzylalcohol, with a conversion that reaches 70% after 2 h, by using a loading of 1 mol%.

The complex exhibits a metal-centered reduction wave and catalyzes the aerobic oxidation of benzyl alcohol without additive other than a base.Figure optionsDownload as PowerPoint slide