The Impact of a Proton Relay in Binuclear α-Diimine-Mn(CO)3 Complexes on the CO2 Reduction Catalysis
The Impact of a Proton Relay in Binuclear α-Diimine-Mn(CO)3 Complexes on the CO2 Reduction Catalysis
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DOI：
10.1021/acs.inorgchem.9b00992
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发表时间：
2019-08-19
影响因子：
4.6
通讯作者：
Siewert, Inke
中科院分区：
文献类型：
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作者：
Fokin, Igor;Denisiuk, Alisa;Siewert, Inke
在本文中，我们描述了在金属中心附近具有内部质子源的双和单核α-二亚胺-Mn（CO）（3）络合物的氧化还原化学及其在电化学驱动的CO2还原反应中的催化活性。为了解决的两个金属网站和质子源的影响，我们研究了一个双核配合物与苯酚部分，1，双核锰配合物与甲氧基苯酚单元，而不是，2，和单核类似物与苯酚单元，3。光谱电化学研究表明，1和3在DMF中分别还原生成[Mn-2（H-1 L1）（CO）（6）Br]和[Mn（H-1 L3）（CO）（3）] H-2，这对配合物来说是氧化还原中性的，相当于苯酚单元的去质子化.该反应可能通过内部质子从苯酚部分转移到还原的金属中心而进行，形成Mn-H物质。2在还原过程中发生二聚反应，生成[Mn-2（L-2）（CO）（6）]（2），而1和3则不发生二聚反应。1、2和3的还原伴随着溴化物损失，最终的物种分别表示[Mn-2（H-1 L1）（CO）（6）]（3-）、[Mn-2（L-2）（CO）（6）]（2-）和[Mn（H-1 L3）（CO）（3）]（2-）。1和2是电化学CO2还原反应中的活性催化剂，而3在施加的电势下快速分解。因此，第二氧化还原活性单元对于增强的稳定性至关重要。1中的质子传递改变了dmf/水混合物中CO2的2 H（+）/2 e（-）还原产物的动力学。对于2，CO是唯一的产物，而甲酸盐和CO以相似的量形成，在1的存在下分别为40%和50%。因此，从苯酚部分到金属中心的内部质子转移形成推定的Mn-H物质和随后的CO2插入的反应速率以及还原的金属中心与CO2形成CO的反应速率是相似的。在扫描速率为0.1 V s（-1）时，相对于CO2到CO的标准氧化还原电位的过电位和所观察到的催化总速率常数，1比2高，也就是说，OH基团由于内部质子转移而有利于催化。
Herein, we describe the redox chemistry of bi- and mononuclear alpha-diimine-Mn(CO)(3) complexes with an internal proton source in close proximity to the metal centers and their catalytic activity in the electrochemically driven CO2 reduction reactions. In order to address the impact of the two metal sites and of the proton source, we investigate a binuclear complex with phenol moiety, 1, a binuclear Mn complex with methoxyphenol unit instead, 2, and the mononuclear analogue with a phenol unit, 3. Spectroelectrochemical investigation of the complexes in dmf under a nitrogen atmosphere indicates that 1 and 3 undergo a reductive H-2 formation forming [Mn-2(H-1L1)(CO)(6)Br] and [Mn(H-1L3)(CO)(3)], respectively, which is redox neutral for the complex and equivalent to a deprotonation of the phenol unit. The reaction likely proceeds via internal proton transfer from the phenol moiety to the reduced metal center forming a Mn-H species. 2 dimerizes during reduction, forming [Mn-2(L-2)(CO)(6)](2), but 1 and 3 do not. Reduction of 1, 2, and 3 is accompanied by bromide loss, and the final species represent [Mn-2(H-1L1)(CO)(6)](3-), [Mn-2(L-2)(CO)(6)](2-), and [Mn(H-1L3)(CO)(3)](2-), respectively. 1 and 2 are active catalysts in the electrochemical CO2 reduction reaction, whereas 3 decomposes quickly under an applied potential. Thus, the second redox active unit is crucial for enhanced stability. The proton relay in 1 alters the kinetics for the 2H(+)/2e(-) reduced products of CO2 in dmf/water mixtures. For 2, CO is the only product, whereas formate and CO are formed in similar amounts, 40% and 50%, respectively, in the presence of 1. Thus, the reaction rate for the internal proton transfer from the phenol moiety to the metal center forming the putative Mn-H species and subsequent CO2 insertion as well as the reaction rate of the reduced metal center with CO2 forming CO are similar. The overpotential with regard to the standard redox potential of CO2 to CO and the observed overall rate constant for catalysis at scan rates of 0.1 V s(-1) are higher with 1 than with 2, that is, the OH group is beneficial for catalysis due to the internal proton transfer.