Homogeneous Catalysis of the Photoreduction of Water by Visible Light. 3. Mediation by Polypyridine Complexes of Ruthenium(II) and Cobalt(II)

Abstract

Irradiation of polypyridineruthenium(I1) complexes (RuL~') in aqueous solutions containing ascorbate (HAs-) and L' (2,2'-bipyridine or other polypyridines) with visible light yields reduced polypyridine L'H, and H,. The quantum yield of H, is increased greatly when cobalt(I1)is also present and is optimal, tP =0.13,with L =bpy, L' =4,4'-(CH,hbpy (2 x \0-' M), [Co(I1») =0.01M, [HAs-) =0.7 M at pH 4.5-5.5. The quantum yield of L'H, is~0.01 when L' =bpy at~\0-' M, [HAs-) =0.7 M at pH -5 but diminishes greatly at pH~6 or when cobalt(I1) is present. The photochemical experiments and pulse-radiolysis studies indicate that the quantum yields reflect the relative rates of some twenty elementary reactions. The rate constant for the first, quenching of°Ruq' by HAs-to give RuL; and ascorbate radical, spans the range 0.2 x 10 7

Splitting Water with Cobalt

“…Following these pioneering observations, a wide range of cobalt complexes, ranging from [Co(bipy) n ] 2+ species (n = 1-3; Figure 5) [57,115,[126][127][128] to the cobalt-polypyridinium complexes 39 and 40 ( Figure 18) [129] to some macrocyclic Co II complexes, including 1, [130] that are based on the ligands shown in Figure 19 [130,131] or the cobaloxime 11 [131] have been used. [132] Figure 16.…”

“…In the presence of TEOA however, oxidative quenching ( Figure 16) yielding [Co(bpy) 3 ] + is faster than conversion of PS* into PS À . [57,60,128] With cobaloxime as a catalyst, a reductive quenching process dominates in the presence of TEOA. In the case of Hbridged cobaloximes, this observation is consistent with the quite negative Co II /Co I redox potential.…”

Wasserspaltung mit Cobalt

“…Nach dieser bahnbrechenden Entdeckung wurde ein breites Spektrum an Cobaltkomplexen getestet: von [Co(bipy) n ] 2+ (n = 1-3, Abbildung 5) [57,115,[126][127][128] über die CobaltPolypyridinium-Komplexe 39 und 40 (Abbildung 18) [129] bis hin zu Co II -Komplexen (u. a. 1 [130] ) mit makrocyclischen Liganden (Abbildung 19) [130,131] oder dem Cobaloxim 11.…”

“…[57,60,128] Mit Cobaloxim-Katalysatoren und in Gegenwart von TEOA ist der reduktive Löschprozess vorherrschend. Im Fall der H-verbrückten Cobaloxime ist dies im Einklang mit dem hohen negativen Co II /Co I -Redoxpotential.…”

Kinetics and Mechanism of Two-Electron Reduction of Surfacial Ferric Ions of Colloidal Ferrofluid byl-Ascorbic Acid in Aqueous Acidic Medium

“…In an excellent résumè of the complex redox behavior of H 2 A, it is pointed out that the oxidation and reduction reactions of H 2 A and its redox companions are complicated by the intervention of simultaneous proton transfer reactions. Ascorbic acid (vitamin C) being a donor of two protons and two electrons is widely used as a reducing agent in chemical (1,2), biochemical (3)(4)(5)(6)(7)(8), biological (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14), photochemical (15,16), and biomedical reactions (17)(18)(19)(20). Recent observations proved that the ascorbic acid is a potential two-electron reductant for cleavage of N-N bonds at a transition metal center converting hydrazines and azide to ammonia and/or amine in acidic solution (21).…”