Multi-potential-step chronocoulospectrometry for electrocatalytic water oxidation by a mononuclear ruthenium aquo complex immobilized on a mesoporous ITO electrode.

A new mononuclear Ru aquo complex [Ru(C8Otpy)(H2dcbpy)(OH2)]2+ with 4,4'-dicarboxy-2,2'-bipyridine (H2dcbpy) and 4'-octyloxy-2,2':6',2''-terpyridine (C8Otpy) ligands was synthesized to investigate electrocatalytic water oxidation by the complex immobilized on a mesoporous indium-doped tin oxide (meso-ITO) electrode using a multi-potential-step chronocoulospectrometric (MPSCCS) technique. UV-visible absorption spectroscopic data indicated that [Ru(C8Otpy)(dcbpy)(OH2)] (RuOH2) is deprotonated to [Ru(C8Otpy)(dcbpy)(OH)]- (RuOH) on the meso-ITO surface even at pH 5.9 of the electrolyte solution. The cyclic voltammogram (CV) of the RuOH/meso-ITO electrode showed a pH-independent redox response at E1/2 = 0.80 V vs. Ag/AgCl in the pH range of 5-12, being assigned to a non-proton-coupled 1e- redox process of RuIIOH/RuIIIOH. The MPSCCS measurement of the RuOH/meso-ITO electrode between 0.2 and 1.5 V vs. Ag/AgCl showed that RuIV species (tentatively RuIVO) exist in a steady state of the electrocatalysis in the initial stage. This suggests that the electrochemical oxidation from RuIVO to RuVO could compete with the water nucleophilic attack for O-O bond formation involved in the rate-determining step under the employed conditions. The possibility that the water nucleophilic attack on RuIVO could also compete with the electrochemical oxidation of RuIVO to RuVO was suggested by the electrocatalytic water oxidation at a low applied potential of 1.4 V prior to the formation potential of RuVO. The MPSCCS measurement at 1.4 V for 1 h showed that RuOH is gradually transformed into an alternative catalyst (most likely RuOx nanoparticles) on the electrode. The MPSCCS technique is promising to reveal the redox reactions and catalytic aspects of molecular catalysts immobilized on an electrode for water oxidation.

Mechanistic Insight into Reversible Core Structural Changes of Dinuclear µ-Hydroxoruthenium(II) Complexes with a 2,8-Di-2-pyridyl-1,9,10-anthyridine Backbone Prior to Water Oxidation Catalysis.

proximal,proximal-(p,p)-[RuII2(tpy)2LXY]n+ (tpy = 2,2';6',2″-terpyridine, L = 5-phenyl-2,8-di-2-pyridyl-1,9,10-anthyridine, and X and Y = other coordination sites) yields the structurally and functionally unusual RuII(µ-OH)RuII core, which is capable of catalyzing water oxidation with key water insertion to the core (Inorg. Chem. 2015, 54, 7627). Herein, we studied a sequence of bridging-ligand substitution among p,p-[Ru2(tpy)2L(µ-Cl)]3+ (Ru2(µ-Cl)), p,p-[Ru2(tpy)2L(µ-OH)]3+ (Ru2(µ-OH)), p,p-[Ru2(tpy)2L(OH)(OH2)]3+ (Ru2(OH)(OH2)), and p,p-[Ru2(tpy)2L(OH)2]2+ (Ru2(OH)2) in aqueous solution. Ru2(µ-Cl) converted slowly (10-4 s-1) to Ru2(µ-OH), and further Ru2(µ-OH) converted very slowly (10-6 s-1) to Ru2(OH)(OH2) by the insertion of water to reach equilibrium at pH 8.5-12.3. On the basis of density functional theory (DFT) calculations, Ru2(OH)(OH2) was predicted to be thermodynamically stable by 13.3 kJ mol-1 in water compared to Ru2(µ-OH) because of the specially stabilized core structure by multiple hydrogen-bonding interactions involving aquo, hydroxo, and L backbone ligands. The observed rate from Ru2(µ-OH) to Ru2(OH)2 by the insertion of an OH- ion increased linearly with an increase in the OH- concentration from 10 to 100 mM. The water insertion to the core is very slow (∼10-6 s-1) in aqueous solution at pH 8.5-12.3, whereas the insertion of OH- ions is accelerated (10-5-10-4 s-1) above pH 13.4 by 2 orders of magnitude. The kinetic data including activation parameters suggest that the associative mechanism for the insertion of water to the RuII(µ-OH)RuII core of Ru2(µ-OH) at pH 8.5-12.3 alters the interchange mechanism for the insertion of an OH- ion to the core above pH 13.4 because of relatively stronger nucleophilic attack of OH- ions. The hypothesized p,p-[Ru2(tpy)2L(µ-OH2)]4+ and p,p-[Ru2(tpy)2L(OH2)2]4+ formed by protonation from Ru2(µ-OH) and Ru2(OH)(OH2) were predicted to be unstable by 71.3 and 112.4 kJ mol-1 compared to Ru2(µ-OH) and Ru2(OH)(OH2), respectively. The reverse reactions of Ru2(µ-OH), Ru2(OH)(OH2), and Ru2(OH)2 to Ru2(µ-Cl) below pH 5 could be caused by lowering the core charge by protonation of the µ-OH- or OH- ligand.

Distribution of manganese species in an oxidative dimerization reaction of a bis-terpyridine mononuclear manganese (II) complex and their heterogeneous water oxidation activities.

Heterogeneous water oxidation catalyses were studied as a synthetic model of oxygen evolving complex (OEC) in photosynthesis using mica adsorbing various manganese species. Distribution of manganese species formed in the oxidative dimerization reaction of [Mn(II)(terpy)2](2+) (terpy=2,2':6',2″-terpyridine) (1') with various oxidants in water was revealed. 1' was stoichiometrically oxidized to form di-µ-oxo dinuclear manganese complex, [(OH2)(terpy)Mn(III)(µ-O)2Mn(IV)(terpy)(OH2)](3+) (1) by KMnO4 as an oxidant. When Oxone and Ce(IV) oxidants were used, the further oxidation of 1 to [(OH2)(terpy)Mn(IV)(µ-O)2Mn(IV)(terpy)(OH2)](4+) (2) was observed after the oxidative dimerization reaction of 1'. The mica adsorbates with various composition of 1', 1 and 2 were prepared by adding mica suspension to the various oxidant-treated solutions followed by filtration. The heterogeneous water oxidation catalysis by the mica adsorbates was examined using a Ce(IV) oxidant. The observed catalytic activity of the mica adsorbates corresponded to a content of 1 (1ads) adsorbed on mica for KMnO4- and Oxone-treated systems, indicating that 1' (1'ads) and 2 (2ads) adsorbed on mica do not work for the catalysis. The kinetic analysis suggested that 1ads works for the catalysis through cooperation with adjacent 1ads or 2ads, meaning that 2ads assists the cooperative catalysis by 1ads though 2ads is not able to work for the catalysis alone. For the Ce(IV)-treated system, O2 evolution was hardly observed although the sufficient amount of 1ads was contained in the mica adsorbates. This was explained by the impeded penetration of Ce(IV) ions (as an oxidant for water oxidation) into mica by Ce(3+) cations (generated in oxidative dimerization of 1') co-adsorbed with 1ads.