Ligand-Based PCET Reduction in a Heteroleptic Ni(bpy)(dithiolene) Electrocatalyst Giving Rise to Higher Metal Basicity Required for Hydrogen Evolution
Ligand-Based PCET Reduction in a Heteroleptic Ni(bpy)(dithiolene) Electrocatalyst Giving Rise to Higher Metal Basicity Required for Hydrogen Evolution
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DOI：
10.1002/celc.201900400
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发表时间：
2019-04-15
期刊：
影响因子：
4
通讯作者：
Sakai, Ken
中科院分区：
文献类型：
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作者：
Koshiba, Keita;Yamauchi, Kosei;Sakai, Ken
质子提取导致形成的氢化物物种所需的进化H-2在很大程度上依赖于碱性的d轨道的金属负责这一行动。本文报道了一种正方形平面Ni-II（bpy）（dcbdt）析氢催化剂，通过连续两次单电子还原（bpy= 2，2 '-bipyridine;dcbdt= 4，5-dicyanobenzene-1，2-dithiolate）形成[Ni-I（bpy*）（dcbdt）]（2）后，由于填充Ni d（z）2轨道上的碱性增加，其质子提取速率显著加快。催化剂可能采用EECC机制，其中第一质子化步骤的速率远高于第二步骤的速率，即使需要另一还原的替代路径（即，ECEC“）仍然未被排除在外。我们的DFT计算表明，第一和第二减少相关的电子注入到金属配体反键和π *（bpy）轨道，分别，后者的轨道显示不可忽略的杂交与镍d轨道。此外，均配型催化剂[Ni-II（dcbdt）（2）]（2-）显示出采用EC'EC机理，其中速率决定步骤是氢化物形成步骤，这与注入的电子在两个dcbdt配体（pi*（dcbdt）轨道）上的大部分离域性质一致。这项工作表明了提高金属d轨道的碱性的重要性，与促进质子耦合电子转移（PCET）有关。
Proton abstraction leading to the formation of a hydride species required to evolve H-2 largely relies on the basicity of d orbital of the metal responsible for this action. Here we report that a square-planar Ni-II(bpy)(dcbdt) hydrogen evolution catalyst shows substantial acceleration in the proton abstraction rate due to the increased basicity at the filled Ni d(z)2 orbital after formation of [Ni-I(bpy*)(dcbdt)](2) via consecutive two one-electron reductions (bpy=2,2'-bipyridine;dcbdt=4,5-dicyanobenzene-1,2-dithiolate). The catalyst is likely to adopt the EECC' mechanism in which the rate of the first protonation step is by far higher than that of the second step, even though an alternative path requiring another reduction (i.e., ECEC') remains unexcluded. Our DFT calculations reveal that the first and second reductions are correlated with the electron injection into the metal-ligand anti-bonding and pi*(bpy) orbitals, respectively, where the latter orbital shows non-negligible hybridization with the nickel d orbital. In addition, a homoleptic catalyst [Ni-II(dcbdt)(2)](2-) is shown to adopt the EC'EC mechanism with the rate-determing step being a hydride forming step, consistent with the largely delocalized nature of the injected electron over the two dcbdt ligands (pi*(dcbdt)orbital). This work demonstrates the importance of raising the basicity of the metal d orbital, relevant to promote the proton-coupled electron transfer (PCET).