Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States.
Inorg Chem. 2020 May 4;59(9):5854-5864. doi: 10.1021/acs.inorgchem.9b03341. Epub 2020 Apr 23.
Previously, we reported an iron(III) complex with 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(2,4-ditertbutyl-phenol) as a ligand (Fe(dhbpy)Cl, ) as catalytically competent for the electrochemical reduction of CO to formate (Faradaic efficiency FE = 68 ± 4%). In mechanistic experiments, an essential component was found to be a pre-equilibrium reaction involving the association of the proton donor with the catalyst, which preceded proton transfer to the Fe-bound O atoms upon reduction of the Fe center. Here, we report the synthesis, structural characterization, and reactivity of two iron(III) compounds with 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(2-methoxy-4-methylphenol) (crebpy[H], Fe(crebpy)Cl, ) and 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(4-(-butyl)benzene-1,2-diol) (catbpy[H], Fe(catbpy), ) as ligands, where pendent -OMe and -OH groups are poised to modify the protonation reaction involving the Fe-bound O atoms. Differences in selectivity and activity for the electrocatalytic reduction of carbon dioxide (CO) to formate (HCO) between complexes - were assessed via cyclic voltammetry and controlled potential electrolysis (CPE) experiments in ,-dimethylformamide. Mechanistic studies suggest that the O atoms in the secondary coordination sphere are important for relaying the exogenous proton donor to the active site through a preconcentration effect, which leads to the (partial catalytic current density for formate) increasing by 3.3-fold for and 1.2-fold for in comparison to the of . These results also suggest that there is a difference in the strength of the interaction between the pendent functional groups and the sacrificial proton donor between and , resulting in quantifiable differences in catalytic activity and efficiency. CPE experiments demonstrate an increased FE = 85 ± 2% for , whereas had a lower FE = 71 ± 3%, with CO and H generated as co-products in each case to reach mass balance. These results indicate that using secondary sphere moieties to modulate metal-ligand interactions and multisite electron and proton transfer reactivity in the primary coordination sphere through reactant preconcentration can be a powerful strategy for enhancing electrocatalytic activity and selectivity.
先前，我们报道了一种以 6,6'-([2,2'-联吡啶]-6,6'-二基)双(2,4-二叔丁基-苯酚)为配体的铁(III)配合物（Fe(dhbpy)Cl， ）作为电化学还原 CO 为甲酸盐（法拉第效率 FE = 68 ± 4%）的催化能力。在机理实验中，发现一个重要的组成部分是一个预平衡反应，涉及质子供体与催化剂的缔合，这在铁中心还原时先于质子转移到与 Fe 结合的 O 原子。在这里，我们报告了两种铁(III)化合物的合成、结构表征和反应性，它们的配体分别为 6,6'-([2,2'-联吡啶]-6,6'-二基)双(2-甲氧基-4-甲基苯酚)（crebpy[H]，Fe(crebpy)Cl， ）和 6,6'-([2,2'-联吡啶]-6,6'-二基)双(4-(-丁基)苯-1,2-二醇)（catbpy[H]，Fe(catbpy)， ），其中悬垂的-OMe 和-OH 基团能够修饰涉及与 Fe 结合的 O 原子的质子化反应。通过在,-二甲基甲酰胺中的循环伏安法和恒电位电解（CPE）实验评估了这些配合物对二氧化碳（CO）电催化还原为甲酸盐（HCO）的选择性和活性差异。机理研究表明，次级配位球中的 O 原子对于通过预浓缩效应将外源性质子供体传递到活性位点很重要，这导致 （甲酸盐的部分催化电流密度）对于 和 分别增加了 3.3 倍和 1.2 倍，而 为 。这些结果还表明，在与牺牲质子供体之间，悬垂官能团与 的相互作用强度存在差异，导致催化活性和效率存在可量化的差异。CPE 实验表明， 具有更高的 FE = 85 ± 2%，而 则具有较低的 FE = 71 ± 3%，在每种情况下，CO 和 H 都作为副产物生成，以达到质量平衡。这些结果表明，通过反应物预浓缩来调节金属-配体相互作用和主配位球中的多站点电子和质子转移反应性，使用次级球基团可以成为增强电催化活性和选择性的有力策略。
