@misc{oai:ir.soken.ac.jp:00000213,
 author = {和田, 亨 and ワダ, トオル and WADA, Tohru},
 month = {2016-02-17},
 note = {In recent years, there has been a significant interest in the reactivity of polynuclear metal-oxo complexes from the viewpoints of development of oxidation catalysts which work under mild conditions and simulate enzymatic oxidations of chemically inert molecules such as methane monooxigenase.  Metal aqua complexes would be converted to metal-oxo ones through metal-hydroxo complexes either by treatment with strong base under aprotic conditions or alkalization under aqueous conditions.  Deprotonation of metal-aqua complexes, however, usually results in preferential formation of p-oxo dimers or p-oxo polymer rather than metal-oxo complexes due to the reaction of strong basic metal-oxo complexes with metal-aqua ones remained in solutions.  Depression of an increase in the basicity of metal-oxo complexes, therefore, would enable smooth conversion from an aqua ligand to an oxo one on metals.
  In order to relize smooth conversion of Ru(OH) complex to Ru(O) complex, the author introduced quinone ligands into dinuclear ruthenium-hydroxo complexes.  Quinine ligands bonded to Ru serve as two electrone reservoirs because the bonding modes of the ligands to metals are classified to quinone, semiquinone and catechol.  Dinuclear ruthenium-hydroxo complex with quinone ligands is ,therfore, expected to easily eliminate protons without the formation of μ-oxo dimers.  The purpose of this study is that i) syntheses of dinuclear ruthenium-hydroxo complexes with quinone ligands and elucidation of their acid-base equilibrium and redox properties, ii) comparison of the catalytic activity for some oxidation reactions with that of mononuclear and bipyridine analogs, iii) evaluation of the role of the quinone ligands and the dinuclear framework of the complexes in those oxidation reactions.
  A novel bridging ligand 1,8-bis (2, 2': 6', 2"-terpyridyl) anthracene (btpyan) was synthesized by three reactions from 1,8-diformylanthracene and this structure was determined by X-ray crystallography.  Face-to-face dimeric complexes was prepared by the connection with two [Ru(L)(OH)]+ units (L = 3,6-di-tert-butyl-1,2-benzoquinone (Q) and 2,2'-bipyridine (bpy)) by btpyan.  An addition of tBuOK (2.0 equiv.) to a methanolic solution of [RuII2(OH)2(Q)2(btpyan)](SbF6)2 ([1](SbF6)2) resulted in the generation of [RuII2(O)2(SQ)2(btpyan)]0 (3,6-tBu2SQ = 3,6-di(fert-butyl)- 1,2-semiquinone) due to the reduction of quinone coupled with the dissociation of the hydroxo protons.  The resultant [RuII2(O)2(SQ)2(btpyan)]0 underwent the ligand-localized oxidation at E1/2 = +0.40 V (vs Ag/AgCl) to give [RuII2(O)2(Q)2(btpyan)]2+ in Me0H.  Furthermore, metal-localized oxidation of [RuII2(O)2(Q)2(btpyan)]2+ at Ep = +1.2 V in CF3CH20H/ether or water gave [RuIII2(O)2(Q)2(btpyan)]4+.
  Cyclic voltammetry showed that [1]2+ oxidized water to dioxygene at potentials more positive than +1.2 V in CF3CH2OH and H2O (pH>3.0).  The controlled-potential electrolysis of [1]2+ at +1.70 V in the presence of H2O in CF3CH2OH evolved dioxygen with a current efficiency of 91% (21 turnovers).  The turnover numbers of O2 evolution increased up to 33500 when the electrolysis was conducted in water (pH 4.0) by using a [1](SbF6)2-modified indium-tin-oxide (ITO) electrode.  Active species of this water oxidation would be [RuIII2(O)2(Q)2(btpyan)]4+ from the electrochemical and spectroscopic studies.  On the other hand, the analogous [RuII2(OH)2(bpy)2(btpyan)](SbF6)2 showed neither dissociation of the hydroxo protons even in the presence of large excess of tBuOK nor activity for the oxidation of H2O under similar conditions.  These results indicate that the quinone ligands of [1]2+ play a key role in the water-oxidation.
  Deprotonation and two-electron oxidation of [1]2+ gave [RuII2(O)2(Q)2(btpyan)]2+ which has an ability for the oxidations of 1,3-cychrohexadiene and 1,2- dihydronaphtalene to corresponding aromatic compounds via hydrogen atom abstraction in the presence of AgClO4 and tBu0K.  The yield of naphthalene was 710% based on the amount of [1]2+ used in the catalytic conditions.  On the other hand, mononuclear complex [RuII(OH2)(Q)(Ph-terpy)]2+ (Ph-terpy = 4'-phenyl-2, 2': 6',2"-terpyridine) was converted to [RuII(OH)(Q)(Ph-terpy)]2+ under the similar conditions, but displayed low activity for the oxidation compared with the dinuclear complex [1]2+.
  The oxidized forms of [1]2+ showed the catalytic activity toward the water oxidation to dioxygen and the oxidation of hydrocarbons.  It is worthy to note that two Ru-O units and quinone ligands play the key roles in the O-O bond formation in the water oxidation and also in the simultaneous abstraction of H atoms from vicinal two C-H bonds in the oxidation of organic substrates.  Thus, both ruthenium and quinone work as redox centers in the oxidation reactions., application/pdf, 総研大甲第514号},
 title = {Studies on Bis(hydroxoruthenium) Complexes with Quinones as Noninnocent Ligands and Their Catalytic Ability for Oxidation Reactions},
 year = {}
}