Water Oxidation by [(tpy)(H₂O)₂Ru^IIIORu^III(H₂O)₂(tpy)]⁴⁺

Abstract

The complex [(tpy)(C(2)O(4))Ru(III)ORu(III)(C(2)O(4))(tpy)].8H(2)O (1.8H(2)O) (tpy is 2,2':6',2"-terpyridine) has been prepared and characterized by X-ray crystallography and FTIR, resonance Raman, and (1)H NMR spectroscopies. From the results of the X-ray analysis, angleRuORu is 148.5 degrees with a torsional angle (O(22)-Ru(2)-O(1)-Ru(1)-O(12)) of 22.7 degrees and there is a short Ru-O bridge distance of 1.843 Å. 1 undergoes a chemically reversible one-electron, pH-independent oxidation at 0.73 V vs NHE (0.49 V vs SCE) from pH = 4-8 and a pH-dependent, two-electron, chemically irreversible reduction at -0.35 V below pH = 4.0. Addition of 1.8H(2)O to strong acid generates [(tpy)(H(2)O)(2)Ru(III)ORu(III)(H(2)O)(2)(tpy)](4+) (2), which has been characterized by UV-visible, resonance Raman, and (1)H NMR measurements. In pH-dependent cyclic voltammograms there is evidence for a series of redox couples interrelating oxidation states from Ru(II)ORu(II) to Ru(V)ORu(V). In contrast to the "blue dimer", cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+), oxidation state Ru(IV)ORu(IV) (Ru(V)ORu(III)?) does appear as a stable oxidation state. Oxidation of Ru(IV)ORu(IV) by Ce(IV) in 0.1 M HClO(4) is followed by rapid O(2) production and appearance of an anated form of Ru(IV)ORu(IV). O(2) formation is in competition with oxidative cleavage of Ru(V)ORu(V) by Ce(IV) to give [Ru(VI)(tpy)(O)(2)(OH(2))](2+). Anation and oxidative cleavage prevent this complex from being a true catalyst for water oxidation.

Functional models for the oxygen-evolving complex of photosystem II

“…Complex 1 has yielded water-oxidation catalysis by using a number of different primary oxidants including Co 3+ [62], Ce IV [52,63,64], and electrochemical oxidation [51,52,57,61,65]. Using these one-electron oxidant systems has enabled the identification of every oxidation state of 1 between (III,III) and (V,V) [61].…”

“…In addition to 1 and its related complexes [63], there is an emerging family of ruthenium dimer complexes bridged by amine-based ligands [72]. The [Ru 2 II (μ-OAc)(bpp)(tpy) 2 ] 2+ (2) dimer [73] (where bpp = 3,5-di(2-pyridyl)pyrazole and tpy = 2,2':6',2"-terpyridine) (Figure 7) is an example of the new class of complexes with aminebased ligands.…”

“…2 catalyzes oxygen evolution at a rate of 1.4 × 10 −2 s −1 with a turnover number of 18.6 when Ce IV is used as an oxidant. While the increase in rate requires more investigation, the increase in turnover number is at least partially credited to the removal of the oxo bridge present in 1 [63,64,73]. The oxo bridge has been proposed to be a weak point in the structure of 1, facilitating decomposition through disproportionation to a high-valent Ru-oxo monomer and a low-valent Ru-bipyridine complex.…”

“…The challenge in switching from a ruthenium system to a manganese system is largely one of ligand lability [43]. Studies on the decomposition of 1 indicate that the instability of the complex is linked to complications involving the bridging oxo unit [63,64,73]. However, the terminal ligands of Ru complexes are very stably bound.…”

Coordination Chemistry and Solar Fuel Production

“…[66][67][68][69]33 O entendimento do mecanismo de oxidação da água no OEC tem permitido avanços consideráveis no desenvolvimento de catalisadores artificiais. Recentemente, Nocera e colaboradores descreveram um catalisador automontado composto por íons cobalto e fosfato com estrutura semelhante ao OEC natural e também capaz de gerar [75][76][77][78] Para o blue dimer, a oxidação dos centros metálicos à Ru(III)/ Ru(IV) leva a um aumento da acidez dos prótons dos ligantes aquos, com o primeiro pKa decrescendo de 6 para aproximadamente 0. 72 Assim, as espécies com alto estado de oxidação, como [(bpy) 2 Os estudos de catalisadores para oxidação da água a partir de agentes oxidantes como o Ce IV têm permitido grandes avanços no entendimento do mecanismo da produção de O 2 e um maior controle sobre reações secundárias que desativam o catalisador.…”