Effects of the Ligand Linkers on Stability of Mixed-Valence Cu(I)Cu(II) and Catalytic Aerobic Alcohol Oxidation Activity

We synthesized a class of ligands that feature single ( L1 ) and dual amine-bis(triazole) chelates ( L2 with a 1,3-phenylene linker and L3 with a 1,5-naphthalene linker). Our findings which were derived from UV-Vis titrations, crystal structure analysis of relevant copper complexes, and DFT calculations indicate the formation of both mononuclear CuBr( L1 ) and dinuclear (µ- Ln )(CuBr) 2 ( Ln = L2 and L3 ) complexes. The catalytic activities of CuBr/Ln, in combination with TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) co-catalyst and NMI ( N -methylimidazole) for aerobic alcohol oxidation, reveal the following activity trend: CuBr/ L3 > CuBr/ L2 > CuBr/ L1 . Furthermore, electrochemical data from in-situ generated CuBr complexes suggest that the higher catalytic performance of CuBr/ L3 is attributed to the presence of less stable mixed-valence and more reducible Cu(I)- L3 -Cu(II) species compared to Cu(I)- L2 -Cu(II). This difference is a result of weaker σ interactions between Cu–N amine , larger bridging π systems, and a longer Cu···Cu distance in the presence of L3 . Additionally, the catalyst system, CuBr/ L3 /TEMPO/NMI, efficiently promotes the aerobic oxidation of benzyl alcohol to benzaldehyde at room temperature in CH 3 CN with a high turnover frequency (TOF) of 39 h − 1 at 1 h.