Zheng Tao, Zhu Mo, Waqas Muhammad, Umair Ahmad, Zaheer Muhammad, Yang Jinxian, Duan Xiaozheng, Li Lianwei
Department of Chemical Physics, University of Science and Technology of China Hefei Anhui 230026 China
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences Changchun Jilin 130022 China
RSC Adv. 2018 Nov 19;8(68):38818-38830. doi: 10.1039/c8ra08012g. eCollection 2018 Nov 16.
In this work, we analyze the catalytic mechanism of P4VP-Ru(bda) polyelectrolyte-metal complex (PMC) as a water oxidation catalyst and elucidate how the unique slow diffusion and multi-charge properties of the polyelectrolyte ligand dominate the catalytic process. Four poly(4-vinyl pyridine)-Ru(bda) (P4VP-Ru) PMCs with different chain lengths and controlled Ru loading amounts were prepared and used as catalysts for catalytic water oxidation. These catalysts present excellent catalytic performance with turnover numbers (TON) from ∼1200 to ∼1700 because of the good hydration properties. Surprisingly, the combined catalysis kinetics and kinetic isotope effect (KIE) studies for P4VP-Ru PMCs confirm the single-site water nucleophilic attack (WNA) mechanism in catalysis, rather than the interaction between two metal oxide units (I2M). A combination of dynamic light scattering characterization, zeta-potential measurement and molecular dynamics simulation reveals that the slow diffusion and multi-charge properties of the polyelectrolyte ligand are responsible for the observed mechanism difference between the P4VP-Ru PMC system and small-molecule multi-nuclear system, though the two systems actually own a similar structural feature (flexible linkages between Ru centers). Our experimental and simulation results highlight the fact that though the existence of flexible linkages between Ru centers could provide large conformation entropy for the occurrence of Ru-dimerization in small-molecule and neutral polymer systems, the entropy elasticity could not overcome the electrostatic interaction energy in the PMC system. Clearly, this work unambiguously clarified why both intra-chain and inter-chain Ru-dimerization (I2M) are prohibited for the PMC system from a perspective of macromolecular chemistry and physics.
在本工作中，我们分析了聚（4-乙烯基吡啶）-钌（联苯二甲酸）（P4VP-Ru）聚电解质-金属配合物（PMC）作为水氧化催化剂的催化机制，并阐明了聚电解质配体独特的缓慢扩散和多电荷性质如何主导催化过程。制备了四种具有不同链长和可控钌负载量的聚（4-乙烯基吡啶）-钌（P4VP-Ru）PMC，并将其用作催化水氧化的催化剂。由于良好的水合性质，这些催化剂表现出优异的催化性能，周转数（TON）约为1200至1700。令人惊讶的是，对P4VP-Ru PMC的催化动力学和动力学同位素效应（KIE）联合研究证实了催化过程中的单位点水亲核攻击（WNA）机制，而非两个金属氧化物单元之间的相互作用（I2M）。动态光散射表征、zeta电位测量和分子动力学模拟相结合表明，聚电解质配体的缓慢扩散和多电荷性质是P4VP-Ru PMC系统与小分子多核系统之间观察到的机制差异的原因，尽管这两个系统实际上具有相似的结构特征（Ru中心之间的柔性连接）。我们的实验和模拟结果突出了这样一个事实，即尽管Ru中心之间存在柔性连接可以为小分子和中性聚合物系统中Ru二聚化的发生提供大的构象熵，但熵弹性无法克服PMC系统中的静电相互作用能。显然，这项工作从高分子化学和物理的角度明确阐明了为什么PMC系统中链内和链间Ru二聚化（I2M）均被禁止。
