OrthometalatedN-(Benzophenoxazine)-o-aminophenol: Phenolato versus Phenoxyl States

The molecular and electronic structures of the orthometalated ruthenium(III) and osmium(III) complexes of N -(benzophenoxazine)- o -aminophenol ( OX LH 2 ) that exhibits versatile redox activities are reported. The redox chemistry of OX LH 2 is remarkably different from that of N -(aryl)- o -aminophenol ( AP LH 2 ). The study established that OX LH 2 is redox noninnocent and is a precursor of a phenoxyl radical. One of the C-H bonds of OX LH 2 is activated by ions, and OX LH 2 reveals three different redox states as dianionic phenolato ( OX L 2- ), monoanionic phenoxyl radical ( OX L •- ), and zwitterionic phenoxium cation ( OX L ± ) states. The reaction of OX LH 2 with [Ru II (PPh 3 ) 3 Cl 2 ] in boiling toluene in air affords an orthometalated OX L 2- complex of ruthenium(III), trans -[( OX L 2- )Ru III (PPh 3 ) 2 (Cl)] ( 1 ), whereas the similar reaction with [Os II (PPh 3 ) 3 Br 2 ] yields an orthometalated OX L •- complex, cis -[( OX L •- )Os III (PPh 3 )Br 2 ] ( 2 ). 1 and 2 exhibit ligand-based reversible redox waves due to OX L •- / OX L 2- , OX L ± / OX L •- , and M III /M II couples. The 1 + ion is a OX L •- complex of ruthenium(III). 2 - exhibits temperature-dependent valence tautomerism due to [Os II ( OX L •- ) ↔ Os III ( OX L 2- )] equilibrium. 2 2- is a OX L 2- complex of osmium(II), while 1 2+ and 2 + are OX L ± complexes of metal(III). 2 is an oxidant and effective catalyst for oxidation of 3,5-di- tert -butylcatechol to the corresponding quinone, and the turnover number is 119.7 h -1 . The UV-vis-NIR absorption spectrum of 1 displays an NIR band at 800 nm due to an intra-ligand-charge-transfer transition, which is absent in 2 incorporating a OX L •- radical. The molecular and electronic structures of 1 and 2 and their oxidized and reduced analogues were confirmed by single-crystal X-ray crystallography, variable-temperature electron paramagnetic resonance spectroscopy, spectroelectrochemical measurements, and density functional theory calculations.