Characterization of a Stable Ruthenium Complex with an Oxyl Radical

The ruthenium oxyl radical complex, [RuII(trpy)(Bu2SQ)O•-] (trpy = 2,2‘:6‘,2‘ ‘-terpyridine, Bu2SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone) was prepared for the first time by the double deprotonation
of the aqua ligand of [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2. [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2 is reversibly converted
to [RuIII(trpy)(Bu2SQ)(OH-)]+ upon dissociation of the aqua proton (pKa 5.5). Deprotonation of the hydroxo
proton gave rise to intramolecular electron transfer from the resultant O2- to Ru−dioxolene. The resultant
[RuII(trpy)(Bu2SQ)O•-] showed antiferromagnetic behavior with a RuII−semiquinone moiety and oxyl radical,
the latter of which was characterized by a spin trapping technique. The most characteristic structural feature
of [RuII(trpy)(Bu2SQ)O•-] is a long Ru−O bond length (2.042(6) Å) as the first terminal metal−O bond with
a single bond length. To elucidate the substituent effect of a quinone ligand, [RuIII(trpy)(4ClSQ)(OH2)](ClO4)2 (4ClSQ = 4-chloro-1,2-benzosemiquinone) was prepared and we compared the deprotonation
behavior of the aqua ligand with that of [RuIII(trpy)(Bu2SQ)(OH2)](ClO4)2. Deprotonation of the aqua
ligand of [RuIII(trpy)(4ClSQ)(OH2)](ClO4)2 induced intramolecular electron transfer from OH- to the [RuIII(4ClSQ)] moiety affording [RuII(trpy)(4ClSQ)(OH•)]+, which then probably changed to [RuII(trpy)(4ClSQ)O•-].
The antiferromagnetic interactions (J values) between RuII-semiquinone and the oxyl radical for
[RuII(trpy)(Bu2SQ)O•-] and for [RuII(trpy)(4ClSQ)O•-] were 2J = −0.67 cm-1 and −1.97 cm-1, respectively.