Base-enhanced catalytic water oxidation by a carboxylate–bipyridine Ru(II) complex

Abstract

In aqueous solution above pH 2.4 with 4% (vol/vol) CH 3 CN, the complex [Ru II (bda)(isoq) 2 ] (bda is 2,2′-bipyridine-6,6′-dicarboxylate; isoq is isoquinoline) exists as the open-arm chelate, [Ru II (CO 2 -bpy-CO 2 − )(isoq) 2 (NCCH 3 )], as shown by 1 H and 13 C-NMR, X-ray crystallography, and pH titrations. Rates of water oxidation with the open-arm chelate are remarkably enhanced by added proton acceptor bases, as measured by cyclic voltammetry (CV). In 1.0 M PO 4 3-, the calculated half-time for water oxidation is ∼7 μs. The key to the rate accelerations with added bases is direct involvement of the buffer base in either atom-proton transfer (APT) or concerted electron-proton transfer (EPT) pathways. [Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy is 2,2′-bipyridine; Fig. 1], both in solution and on surfaces, reveal mechanisms in which stepwise oxidative activation of aqua precursors to Ru V =O is followed by rate-limiting O-O bond formation (10-15). The results of kinetic and mechanistic studies have revealed the importance of concerted atom-proton transfer (APT) in the O-O bond-forming step. In APT, the O-O bond forms in concert with H + transfer to water or to an added base (11,12,(16)(17)(18)(19). APT can promote dramatic rate enhancements. In a recent study on surface-bound [Ru(Mebimpy)(4,4′-((HO) 2 OPCH 2 ) 2 bpy)(OH 2 )] 2+ [4,4′-((HO) 2 OPCH 2 ) 2 bpy is 4,4′-bis-methlylenephosphonato-2,2′-bipyridine] stabilized by atomic layer deposition, a rate enhancement of ∼10 6 was observed with 0.012 M added PO 4 3− at pH 12 compared with oxidation at pH 1 (20).Sun and coworkers (21, 22) have described the Ru single-site water oxidation catalysts, [Ru II (bda)(L) 2 ] (H 2 bda is 2,2′-bipyridine-6,6′-dicarboxylic acid, HCO 2 -bpy-CO 2 H; L is isoquinoline, 4-picoline, or phthalazine). They undergo rapid and sustained water oxidation catalysis with added Ce IV . A mechanism has been proposed in which initial oxidation to seven coordinate Ru IV is followed by further oxidation to Ru V (O) with O-O coupling to give a peroxo-bridged intermediate, Ru IV O-ORu IV , which undergoes further oxidation and release of O 2 (21, 22). We report here the results of a rate and mechanistic study on electrochemical water oxidation by complex [1], [Ru II (CO 2 -bpy-CO 2 )(isoq) 2 ] (isoq is isoquinoline) (Fig. 1). Evidence is presented for water oxidation by a chelate open form in acidic solutions. The chelate open form displays dramatic rate enhancements with added buffer bases, and the results of a detailed mechanistic study are reported here. − /HPO 4 2− phosphate buffer, I = 0.5 M (NaClO 4 )] in 4% (vol/vol) CH 3 CN at a glassy carbon electrode (GC) (0.071 cm 2 ). The Ag/AgCl [3 M NaCl, 0.21 V vs. normal hydrogen electrode (NHE)] reference electrode was isolated with an electrolyte filled bridge to avoid chloride ion diffusion into the anode compartment. The sample was purged with argon to remove O 2 before each scan, with only O 2 freshly produced in oxidative scans detected on reverse scans at -0.3 V vs. NHE, a...

Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands

“…[7] This behavior is also known for other catalysts and originates from the involvement of proton transfer in the rate-limiting step. [8] However, the [Ru(bda)(L) 2 ] family of catalysts follows a bimolecular pathway for OÀO bond formation.…”

Electron Transfer Mediator Effects in the Oxidative Activation of a Ruthenium Dicarboxylate Water Oxidation Catalyst

“…Under these conditions, MeCN is coordinated to the Ru(II) form of the catalyst, Scheme 4, with pK a = 2.4 for the open-arm −COOH chelate. 11 The first quasi-reversible wave at E 1/2 = 0.62 V vs the saturated calomel electrode (SCE; 0.24 V vs NHE) arises from a 2e − /2H + Ru IV/II couple at this pH. 16 Oxidation of the Ru(II) form of the complex is accompanied by exchange of …”

“…It also highlights the role of electron transfer mediation at the oxide. 11 The larger overpotential for water oxidation at ITO−RuP 2+ in Figure 8 is due to the higher potential of the relay couple with E o = 1.07 V for ITO−RuP 3+/2+ compared to E o = 0.78 V for the Ru V/IV (O) +/0 couple.…”

Light-Driven Water Splitting with a Molecular Electroassembly-Based Core/Shell Photoanode

“…44−46 In a recent mechanistic study on water oxidation by an analogous Ru II (bda) catalyst, water oxidation was shown to occur following oxidation to Ru V (O) + and rate-limiting O atom transfer to water. 31 In the surface-bound assembly under steady-state photolysis conditions, assuming the same ratelimiting step, the assembly would exist as FTO|nanoSnO 2 | TiO 2 (3 nm)|-Ru III -Ru V (O) + , eq 1. Under these conditions, chromophore decomposition as −Ru III presumably competes with rate-limiting water oxidation, explaining the loss in O 2 evolution while maintaining decreased but sustained photocurrents.…”