Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO)3 Complexes with Proton-Responsive Ligands in CO2 Reduction Catalysis
Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO)3 Complexes with Proton-Responsive Ligands in CO2 Reduction Catalysis
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DOI：
10.1002/chem.201806398
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发表时间：
2019-04-11
影响因子：
4.3
通讯作者：
Siewert, Inke
中科院分区：
文献类型：
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作者：
Du, Jia-Pei;Wilting, Alexander;Siewert, Inke
本文介绍了单核和双核α-二亚胺-Re（CO）（3）质子响应配体配合物的还原化学及其在电化学驱动的CO2还原催化中的应用。这项工作的目的是研究1）两个金属离子的紧密接近和2）内部质子源对催化的影响。因此，三种不同的Re配合物，一种是具有中心苯酚单元的双核配合物，3，两种是单核配合物，一种是具有中心苯酚单元的单核配合物，1，一种是具有甲氧基单元的单核配合物，2。所有配合物在CO2到CO的转化中都是活性的，并且CO总是主要产物。三种配合物在DMF/水混合溶剂中的催化速率常数K-cat均比在纯DMF（DMF=N，N-二甲基甲酰胺）中高得多，过电位也比纯DMF低。循环伏安法（CV）的研究表明，在基板的情况下，这是由于加速氯离子损失后，在DMF/水的混合物中的初始还原相比，纯DMF。氯离子损失对于随后的CO2结合是必需的，并且该步骤在水的存在下大约快十倍[2：k（Cl）（DMF）约为1.7 s（-1）; k（Cl）（DMF/H2O）约为20 s（-1）]。具有质子响应性苯酚单元的双核配合物3比单核配合物具有更高的活性。在水的存在下，观察到的速率常数k（obs），为3是4倍高于2，在没有水的情况下，甚至10倍。因此，两个金属中心对催化是有益的。最后，研究表明，苯酚单元对催化速率没有影响，它甚至减缓了CO2到CO的转化。这是由于非生产性的竞争性副反应：在初始还原后，1和3失去Cl-或经历还原性OH去质子化形成酚盐单元。酚盐可以结合到金属中心，阻断第六配位位点用于CO2活化。在DMF中，O-H键断裂和Cl-离子损失的速率常数相近[1：k（Cl）（DMF）约为2 s（-1），k（OH）约为1.5 s（-1）]，在水/DMF中Cl-损失快得多。因此，对催化速率的影响在DMF中更显著。然而，酸性质子使催化的过电位降低约150 mV。
Here, the reduction chemistry of mono- and binuclear alpha-Diimine-Re(CO)(3) complexes with proton responsive ligands and their application in the electrochemically-driven CO2 reduction catalysis are presented. The work was aimed to investigate the impact of 1) two metal ions in close proximity and 2) an internal proton source on catalysis. Therefore, three different Re complexes, a binuclear one with a central phenol unit, 3, and two mononuclear, one having a central phenol unit, 1, and one with a methoxy unit, 2, were utilised. All complexes are active in the CO2-to-CO conversion and CO is always the major product. The catalytic rate constant K-cat for all three complexes is much higher and the overpotential is lower in DMF/water mixtures than in pure DMF (DMF=N,N-dimethylformamide). Cyclic voltammetry (CV) studies in the absence of substrate revealed that this is due to an accelerated chloride ion loss after initial reduction in DMF/water mixtures in comparison to pure DMF. Chloride ion loss is necessary for subsequent CO2 binding and this step is around ten times faster in the presence of water [2: k(Cl)(DMF) approximate to 1.7 s(-1); k(Cl)(DMF/H2O) approximate to 20 s(-1)]. The binuclear complex 3 with a proton responsive phenol unit is more active than the mononuclear complexes. In the presence of water, the observed rate constant k(obs), for 3 is four times higher than of 2, in the absence of water even ten times. Thus, the two metal centres are beneficial for catalysis. Lastly, the investigation showed that the phenol unit has no impact on the rate of the catalysis, it even slows down the CO2-to-CO conversion. This is due to an unproductive, competitive side reaction: After initial reduction, 1 and 3 loose either Cl- or undergo a reductive OH deprotonation forming a phenolate unit. The phenolate could bind to the metal centre blocking the sixth coordination site for CO2 activation. In DMF, O-H bond breaking and Cl- ion loss have similar rate constants [1: k(Cl)(DMF) approximate to 2 s(-1), k(OH) approximate to 1.5 s(-1)], in water/DMF Cl- loss is much faster. Thus, the effect on the catalytic rate is more pronounced in DMF. However, the acidic protons lower the overpotential of the catalysis by about 150 mV.