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

Masanari Hirahara; Sho Nagai; Kosuke Takahashi; Shunsuke Watabe; Taisei Sato; Kenji Saito; Tatsuto Yui; Yasushi Umemura; Masayuki Yagi

doi:10.1021/acs.inorgchem.7b00978

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

Inorg Chem. 2017 Sep 5;56(17):10235-10246. doi: 10.1021/acs.inorgchem.7b00978. Epub 2017 Aug 24.

Authors

Masanari Hirahara¹, Sho Nagai², Kosuke Takahashi², Shunsuke Watabe², Taisei Sato², Kenji Saito², Tatsuto Yui², Yasushi Umemura¹, Masayuki Yagi²

Affiliations

¹ Department of Applied Chemistry, National Defense Academy of Japan , Hashirimizu 1-10-20, Yokosuka, Kanagawa 239-8686, Japan.
² Department of Materials Science and Technology, Faculty of Engineering, Niigata University , 8050 Ikarashi-2, Niigata 950-2181, Japan.

PMID: 28836776
DOI: 10.1021/acs.inorgchem.7b00978

Abstract

proximal,proximal-(p,p)-[Ru^II₂(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 Ru^II(μ-OH)Ru^II 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 Ru^II(μ-OH)Ru^II 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.