Decoding Proton-Coupled Electron Transfer with Potential-p Ka Diagrams: Applications to Catalysis.
Decoding Proton-Coupled Electron Transfer with Potential-p Ka Diagrams: Applications to Catalysis.
复制标题
用势 p Ka 图解码质子耦合电子转移：催化应用。
DOI：
10.1021/acs.inorgchem.8b03368
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发表时间：
2019
影响因子：
4.6
通讯作者：
J. Dempsey
中科院分区：
文献类型：
--
作者：
Eric S. Rountree;B. McCarthy;J. Dempsey
据报道，[NiII（p2phn20亿）2]2+ （p2phn20亿= 1,5-二苯基-3,7-二苯基-1,5-二氮杂-3,7-二磷酸环辛烷）催化制氢的应用电位随质子源pka在乙腈中的变化而变化。相比之下，大多数分子催化剂表现出与pka无关的催化电位。利用实验确定的与还原和质子化相关的热化学参数，构建了[NiII(p2phnbn)2]2+的耦合Pourbaix图。一层描述涉及配体质子化的质子耦合电子转移反应，第二层描述基于金属的质子化。通过实验确定的15个pka单位的E cat/2值，以及检测反应中间体的互补的停止流动快速混合实验，叠加在该图上，支持了一种机制，其中质子耦合电子转移过程支持依赖于pka的催化过程，涉及配体的质子化，而不是金属中心。对于pka值在6-10.6范围内的质子源，形成的初始物质是双还原、双质子化的物质[Ni0(P2PhN2BnH)2]2+，尽管与形成金属质子化的同分异体相比，这种质子耦合电子转移反应的过电位更高。在这个复合物中，每个配体在外显位被质子化，每个配体上的两个胺基团结合一个质子并使其远离金属中心。该物质经过非常缓慢的异构化形成内质子化的氢化物[HNiII（p2phn20亿）（P2PhN2BnH）]2+，可以释放氢来结束催化循环。重要的是，这种缓慢的异构化不会扰乱最初建立的质子耦合电子转移平衡，将催化置于热力学控制之下。耦合Pourbaix图和互补的停流研究揭示了反应机理的新细节，从而预测了这种依赖于pka的活性如何在其他多电子、多质子转化的分子催化剂中产生。
The applied potential at which [NiII(P2PhN2Bn)2]2+ (P2PhN2Bn = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) catalyzes hydrogen production is reported to vary as a function of proton source p Ka in acetonitrile. By contrast, most molecular catalysts exhibit catalytic onsets at p Ka-independent potentials. Using experimentally determined thermochemical parameters associated with reduction and protonation, a coupled Pourbaix diagram is constructed for [NiII(P2PhN2Bn)2]2+. One layer describes proton-coupled electron transfer reactivity involving ligand-based protonation, and the second describes metal-based protonation. An overlay of this diagram with experimentally determined E cat/2 values spanning 15 p Ka units, along with complementary stopped-flow rapid mixing experiments to detect reaction intermediates, supports a mechanism in which the proton-coupled electron transfer processes underpinning the p Ka-dependent catalytic processes involve protonation of the ligand, not the metal center. For proton sources with p Ka values in the range 6-10.6, the initial species formed is the doubly reduced, doubly protonated species [Ni0(P2PhN2BnH)2]2+, despite a higher overpotential for this proton-coupled electron transfer reaction in comparison to forming the metal-protonated isomer. In this complex, each ligand is protonated in the exo position with the two amine moieties on each ligand binding a single proton and positioning it away from the metal center. This species undergoes very slow isomerization to form an endo-protonated hydride species [HNiII(P2PhN2Bn)(P2PhN2BnH)]2+ that can release hydrogen to close the catalytic cycle. Importantly, this slow isomerization does not perturb the initially established proton-coupled electron transfer equilibrium, placing catalysis under thermodynamic control. New details revealed about the reaction mechanism from the coupled Pourbaix diagram and the complementary stopped-flow studies lead to predictions as to how this p Ka-dependent activity might be engendered in other molecular catalysts for multi-electron, multi-proton transformations.