Mixed valence bimetallic Cu complexes with a urea-backboned tetradentate ligand

Abstract

Valence-delocalized bimetallic copper cores are found at the biological CuA sites, which are well-known for their electron transfer reactivity accompanying the transition between fully delocalized Class III and partially delocalized Class II sites. Understanding the electron transfer reactivity of CuA sites when both delocalizations are available within the same ligand platform would be helpful, but few complexes with the same ligand scaffold have been reported. Here, we report the syntheses and characterization of two different classes of bimetallic copper complexes Na(THF)x{NMe2[NNNN]Cu}2 (1) and NMe2[NNNN]Cu2(OAc) (2) using the same tetradentate ligand NMe2[NNNN]. X-ray crystallographic studies revealed that the coordination environments around bimetallic cores in 1 are distorted, whereas those for 2 are symmetric. The electron paramagnetic resonance (EPR) spectrum of 1 was deconvoluted using the different contributions from each copper, which was supported by the characteristic intervalence charge transfer (IVCT) bands of Robin-Day Class II partial delocalization observed in the UV-vis spectrum. The symmetric coordination environment of 2 matches the fully delocalized Class III copper ions, as confirmed by the characteristic septet hyperfine splitting in the EPR spectrum and IVCT band in the UV-vis spectrum. The crystallographic data and electronic spectra were successfully reproduced using the DFT and TD-DFT calculations. Frontier molecular orbital (FMO) analysis revealed that the SOMO of 1 is 3dz²-based, while that of 2 is 3dx²-y²-based. Finally, the observed structural and electronic variations of bimetallic mixed valence cores gave rise to the reactivity differences as demonstrated with tests of O2 and DPPH radical scavenging assays.