Orthometalated N-(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.