Orthometalated N-(Benzophenoxazine)-o-aminophenol: Phenolato versus Phenoxyl States.

Abstract

The molecular and electronic structures of the orthometalated ruthenium(III) and osmium(III) complexes of N-(benzophenoxazine)-o-aminophenol (OXLH2) that exhibits versatile redox activities are reported. The redox chemistry of OXLH2 is remarkably different from that of N-(aryl)-o-aminophenol (APLH2). The study established that OXLH2 is redox noninnocent and is a precursor of a phenoxyl radical. One of the C–H bonds of OXLH2 is activated by ions, and OXLH2 reveals three different redox states as dianionic phenolato (OXL2–), monoanionic phenoxyl radical (OXL•–), and zwitterionic phenoxium cation (OXL±) states. The reaction of OXLH2 with [RuII(PPh3)3Cl2] in boiling toluene in air affords an orthometalated OXL2– complex of ruthenium(III), trans-[(OXL2–)RuIII(PPh3)2(Cl)] (1), whereas the similar reaction with [OsII(PPh3)3Br2] yields an orthometalated OXL•– complex, cis-[(OXL•–)OsIII(PPh3)Br2] (2). 1 and 2 exhibit ligand-based reversible redox waves due to OXL•–/OXL2–, OXL±/OXL•–, and MIII/MII couples. The 1+ ion is a OXL•– complex of ruthenium(III). 2– exhibits temperature-dependent valence tautomerism due to [OsII(OXL•–) ↔ OsIII(OXL2–)] equilibrium. 22– is a OXL2– complex of osmium(II), while 12+ and 2+ are OXL± 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 OXL•– 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.