title: Proton-coupled electron transfer at modified electrodes by multiple pathways.
abstract: In single site water or hydrocarbon oxidation catalysis with polypyridyl Ru complexes such as [Ru(II)(Mebimpy)(bpy)(H(2)O)](2+) [where bpy is 2,2'-bipyridine, and Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog [Ru(II)(Mebimpy)(4,4'-bis-methlylenephosphonato-2,2'-bipyridine)(OH(2))](2+) 2-PO(3)H(2), accessing the reactive states, Ru(V) = O(3+)/Ru(IV) = O(2+), at the electrode interface is typically rate limiting. The higher oxidation states are accessible by proton-coupled electron transfer oxidation of aqua precursors, but access at inert electrodes is kinetically inhibited. The inhibition arises from stepwise mechanisms which impose high energy barriers for 1e- intermediates. Oxidation of the Ru(III)-OH(2+) or forms of 2-PO(3)H(2) to Ru(IV) = O(2+) on planar fluoride-doped SnO(2) electrode and in nanostructured films of Sn(IV)-doped In(2)O(3) and TiO(2) has been investigated with a focus on identifying microscopic phenomena. The results provide direct evidence for important roles for the nature of the electrode, temperature, surface coverage, added buffer base, pH, solvent, and solvent H(2)O/D(2)O isotope effects. In the nonaqueous solvent, propylene carbonate, there is evidence for a role for surface-bound phosphonate groups as proton acceptors.
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
authors.forename: Zuofeng
authors.initials: Z
authors.lastname: Chen
authors.forename: Aaron K
authors.initials: AK
authors.lastname: Vannucci
authors.forename: Javier J
authors.initials: JJ
authors.lastname: Concepcion
authors.forename: Jonah W
authors.initials: JW
authors.lastname: Jurss
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Meyer
mesh_terms: D002384:Catalysis; D004563:Electrochemistry / Q000379:methods; D004566:Electrodes; D004583:Electrons; D007700:Kinetics; D008956:Models, Chemical; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D011725:Pyridines / Q000737:chemistry*; D012428:Ruthenium / Q000737:chemistry*; D017971:Tin Compounds; D014025:Titanium; D014867:Water / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D011725:Pyridines; D017971:Tin Compounds; D014867:Water; C009495:titanium dioxide; D012428:Ruthenium; D014025:Titanium; C045358:stannic oxide