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.

Abstract

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.