Hirahara Masanari, Nagai Sho, Takahashi Kosuke, Watabe Shunsuke, Sato Taisei, Saito Kenji, Yui Tatsuto, Umemura Yasushi, Yagi Masayuki
Department of Applied Chemistry, National Defense Academy of Japan , Hashirimizu 1-10-20, Yokosuka, Kanagawa 239-8686, Japan.
Department of Materials Science and Technology, Faculty of Engineering, Niigata University , 8050 Ikarashi-2, Niigata 950-2181, Japan.
Inorg Chem. 2017 Sep 5;56(17):10235-10246. doi: 10.1021/acs.inorgchem.7b00978. Epub 2017 Aug 24.
proximal,proximal-(p,p)-[Ru(tpy)LXY] (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(μ-OH)Ru 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-[Ru(tpy)L(μ-Cl)] (Ru(μ-Cl)), p,p-[Ru(tpy)L(μ-OH)] (Ru(μ-OH)), p,p-[Ru(tpy)L(OH)(OH)] (Ru(OH)(OH)), and p,p-[Ru(tpy)L(OH)] (Ru(OH)) in aqueous solution. Ru(μ-Cl) converted slowly (10 s) to Ru(μ-OH), and further Ru(μ-OH) converted very slowly (10 s) to Ru(OH)(OH) by the insertion of water to reach equilibrium at pH 8.5-12.3. On the basis of density functional theory (DFT) calculations, Ru(OH)(OH) was predicted to be thermodynamically stable by 13.3 kJ mol in water compared to Ru(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 Ru(μ-OH) to Ru(OH) 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 s) in aqueous solution at pH 8.5-12.3, whereas the insertion of OH ions is accelerated (10-10 s) 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(μ-OH)Ru core of Ru(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-[Ru(tpy)L(μ-OH)] and p,p-[Ru(tpy)L(OH)] formed by protonation from Ru(μ-OH) and Ru(OH)(OH) were predicted to be unstable by 71.3 and 112.4 kJ mol compared to Ru(μ-OH) and Ru(OH)(OH), respectively. The reverse reactions of Ru(μ-OH), Ru(OH)(OH), and Ru(OH) to Ru(μ-Cl) below pH 5 could be caused by lowering the core charge by protonation of the μ-OH or OH ligand.
近端的、近端-(p,p)-[Ru(tpy)LXY]（tpy = 2,2';6',2″-三联吡啶，L = 5-苯基-2,8-二-2-吡啶基-1,9,10-蒽啶，且X和Y = 其他配位位点）会产生结构和功能上不同寻常的Ru(μ-OH)Ru核，该核能够通过关键的水插入到核中来催化水氧化（《无机化学》2015年，第54卷，第7627页）。在此，我们研究了在水溶液中p,p-[Ru(tpy)L(μ-Cl)]（Ru(μ-Cl)）、p,p-[Ru(tpy)L(μ-OH)]（Ru(μ-OH)）、p,p-[Ru(tpy)L(OH)(OH)]（Ru(OH)(OH)）和p,p-[Ru(tpy)L(OH)]（Ru(OH)）之间一系列桥连配体的取代反应。Ru(μ-Cl)缓慢（10秒）转化为Ru(μ-OH)，并且通过水的插入，进一步的Ru(μ-OH)非常缓慢（10分钟）地转化为Ru(OH)(OH)，在pH 8.5 - 12.3达到平衡。基于密度泛函理论（DFT）计算，由于涉及水合、羟基和L主链配体的多重氢键相互作用使核结构特别稳定，预测Ru(OH)(OH)在水中相对于Ru(μ-OH)在热力学上稳定13.3 kJ/mol。通过插入一个OH离子从Ru(μ-OH)到Ru(OH)的观测速率随着OH浓度从10 mM增加到100 mM呈线性增加。在pH 8.5 - 12.3的水溶液中，水插入到核中非常缓慢（约10分钟），而在pH 13.4以上，OH离子的插入加速（10 - 10秒）了2个数量级。包括活化参数在内的动力学数据表明，在pH 8.5 - 12.3时水插入到Ru(μ-OH)的Ru(μ-OH)Ru核中的缔合机制在pH 13.4以上会改变OH离子插入到核中的交换机制，因为OH离子的亲核攻击相对更强。预测由Ru(μ-OH)和Ru(OH)(OH)质子化形成的假设的p,p-[Ru(tpy)L(μ-OH)]和p,p-[Ru(tpy)L(OH)]相对于Ru(μ-OH)和Ru(OH)(OH)分别不稳定71.3和112.4 kJ/mol。在pH 5以下，Ru(μ-OH)、Ru(OH)(OH)和Ru(OH)向Ru(μ-Cl)的逆反应可能是由于μ-OH或OH配体质子化降低了核电荷而引起的。
