Deprotonation-induced changes in π-electron systems of iridium(III) and rhodium(III) complexes with azophenine

Abstract

N,N′,N′′,N′′′-Tetraphenyl-2,5-diamino-1,4-benzoquinonediimines (RAp·H2) are used as unique bridging ligands containing localized π-electron systems at each of the two NCCCNH moieties, which are susceptible to changes on coordination of the N atoms to Lewis acid(s). In this contribution, we disclose deprotonation-induced changes in the π-electron systems, spectroscopic properties, and electronic state of cationic cyclometalated iridium(III) and rhodium(III) complexes with 4-EtAp·H2, i.e., [M(ppy)2(4-EtAp·H2)]Cl (ppy: 2-phenylpyridinate; M: Ir(III) (1·H2+) and Rh(III) (2·H2+)). The cationic complexes possessed localized π-electron systems, but the NCCCNH moieties were isomerized from those in free 4-EtAp·H2. Although 4-EtAp·H2 showed no responsiveness to tetrabutylammonium hydroxide (TBAOH) as a base, the reaction of 1·H2+ and 2·H2+ with TBAOH afforded monodeprotonated neutral complexes [M(ppy)2(4-EtAp·H)] (M: Ir(III) (1·H) and Rh(III) (2·H)). UV–vis absorption spectral titrations of 1·H2+ and 2·H2+ with TBAOH revealed the occurrence of monodeprotonation even in the presence of 4 equivalents of TBAOH. The cationic complexes were regenerated from 1·H and 2·H upon addition of p-toluenesulfonic acid. X-ray crystallography of 2·H showed that the complex has localized and delocalized π-electron systems at the NCCCNH and NCCCN moieties, respectively. The UV–vis absorption spectra of 1·H2+ and 2·H2+ in CH2Cl2 present intense bands around 500 nm, and those of 1·H and 2·H exhibit intense bands around 450 nm and broad bands around 700 nm. Density functional theory (DFT), time-dependent DFT, and natural transition orbital calculations suggest that the intense bands of all complexes and the broad bands of 1·H and 2·H stem from π–π* transitions in the localized and delocalized π-electron systems, respectively, with MLCT transition.