Redox-active diaminoazobenzene complexes of rhodium(iii): synthesis, structure and spectroscopic characterization

Abstract

Herein, the reactivity of an aromatic diamine, 2,2′-diaminoazobenzene 1 [^H₂NL^NH₂], with rhodium salts was investigated. Diverse coordinations, specifically single and double NH deprotonated forms, of the title ligand are obtained during metallation, affording mononuclear 2 and dinuclear 3 complexes, respectively. An unprecedented edge-shared bioctahedral geometry with a syn configuration (sterically encumbered) about the Rh^III₂N₂ core is obtained with pincer-type NNN ligation. Both complexes are redox-active and provide well-defined oxidative responses, and the reversibility of the redox-couples is enhanced with the decreasing temperature. The spectroscopic study indicates the formation of open-shell species derived from the precursor amido complexes, and these species are believed to be ligand-centered π radicals. The incorporation of both electron-poor azo and electron-rich amido moieties imparts the possibility of low-energy electronic transitions within the tridentate ligand (the push–pull effect). Furthermore, the occurrence of ILCT has been substantiated theoretically (TDDFT and NTO).