One‐ versus Two‐Electron Oxidation of Complexed Guanidino‐Functionalized Aromatic Compounds

Abstract

AbstractHerein, we report a rational synthetic access to dinuclear CuII complexes with radical monocationic guanidine ligands. The starting point was the first directed synthesis of a dinuclear CuII complex of the redox‐active, neutral guanidino‐functionalized aromatic (GFA) compound 1,2,4,5‐tetrakis(tetramethylguanidino)benzene (1) as ligand by treatment of 1 with Cu(OAc)2. The neutral complex [1{Cu(OAc)2}2] was then oxidized with several oxidizing reagents. With I2 two‐electron oxidation occurred, thus leading to green salts of the dication [1{Cu(OAc)2}2]2+. In contrast, with AgPF6 or AgSbF6 one‐electron oxidation was observed to yield red salts of the monocation [1{Cu(OAc)2}2]+, which is a three‐spin system with one unpaired electron at each copper atom and at the ligand unit. Superconducting quantum interference device (SQUID) magnetometric measurements confirm a quartet electronic ground state that arises from strong ferromagnetic copper–ligand coupling. Intense charge‐transfer transitions in the visible region (576 and 536 nm) were observed in the electronic absorption spectra and assigned to strong ligand–ligand (1·+←OAc) charge‐transfer (LLCT) excitations. These LLCT bands open up the possibility of photoinduced redox reactions with [1{Cu(OAc)2}2]+ driven by restoration of the aromatic system at the GFA ligand.