Shipp James D, Carson Heather, Spall Steven J P, Parker Simon C, Chekulaev Dimitri, Jones Natalie, Mel'nikov Mikhail Ya, Robertson Craig C, Meijer Anthony J H M, Weinstein Julia A
Department of Chemistry, University of Sheffield, S3 7HF, UK.
Department of Chemistry, Moscow Lomonosov State University, Moscow, Russia.
Dalton Trans. 2020 Apr 7;49(14):4230-4243. doi: 10.1039/d0dt00252f.
Novel molecular Re and Mn tricarbonyl complexes bearing a bipyridyl ligand functionalised with sterically hindering substituents in the 6,6'-position, [M(HPEAB)(CO)3(X)] (M/X = Re/Cl, Mn/Br; HPEAB = 6,6'-{N-(4-hexylphenyl)-N(ethyl)-amido}-2,2'-bipyridine) have been synthesised, fully characterised including by single crystal X-ray crystallography, and their propensity to act as catalysts for the electrochemical and photochemical reduction of CO2 has been established. Controlled potential electrolysis showed that the catalysts are effective for electrochemical CO2-reduction, yielding CO as the product (in MeCN for the Re-complex, in 95 : 5 (v/v) MeCN : H2O mixture for the Mn-complex). The recyclability of the catalysts was demonstrated through replenishment of CO2 within solution. The novel catalysts had similar reduction potentials to previously reported complexes of similar structure, and results of the foot-of-the-wave analysis showed comparable maximum turnover rates, too. The tentative mechanisms for activation of the pre-catalysts were proposed on the basis of IR-spectroelectrochemical data aided by DFT calculations. It is shown that the typical dimerisation of the Mn-catalyst was prevented by incorporation of sterically hindering groups, whilst the Re-catalyst undergoes the usual mechanism following chloride ion loss. No photochemical CO2 reduction was observed for the rhenium complex in the presence of a sacrificial donor (triethylamine), which was attributed to the short triplet excited state lifetime (3.6 ns), insufficient for diffusion-controlled electron transfer. Importantly, [Mn(HPEAB)(CO)3Br] can act as a CO2 reduction catalyst when photosensitised by a zinc porphyrin under red light irradiation (λ > 600 nm) in MeCN : H2O (95 : 5); there has been only one reported example of photoactivating Mn-catalysts with porphyrins in this manner. Thus, this work demonstrates the wide utility of sterically protected Re- and Mn-diimine carbonyl catalysts, where the rate and yield of CO-production can be adjusted based on the metal centre and catalytic conditions, with the advantage of suppressing unwanted side-reactions through steric protection of the vacant coordination site.
合成了新型分子铼和锰的三羰基配合物，其带有在6,6'-位被空间位阻取代基官能化的联吡啶配体，即[M(HPEAB)(CO)₃(X)]（M/X = Re/Cl，Mn/Br；HPEAB = 6,6'-{N-(4-己基苯基)-N(乙基)-氨基}-2,2'-联吡啶），对其进行了全面表征，包括通过单晶X射线晶体学表征，并确定了它们作为二氧化碳电化学和光化学还原催化剂的倾向。控制电位电解表明，这些催化剂对二氧化碳的电化学还原有效，产物为一氧化碳（铼配合物在乙腈中，锰配合物在95 : 5（v/v）乙腈 : 水的混合物中）。通过在溶液中补充二氧化碳证明了催化剂的可循环性。这些新型催化剂具有与先前报道的类似结构配合物相似的还原电位，波峰分析结果也显示出相当的最大周转率。基于红外光谱电化学数据并借助密度泛函理论计算，提出了预催化剂活化的初步机制。结果表明，通过引入空间位阻基团可防止锰催化剂发生典型的二聚化，而铼催化剂在氯离子离去后遵循常规机制。在牺牲供体（三乙胺）存在下，未观察到铼配合物的光化学二氧化碳还原，这归因于其三重激发态寿命较短（3.6 ns），不足以进行扩散控制的电子转移。重要的是，[Mn(HPEAB)(CO)₃Br]在乙腈 : 水（95 : 5）中在红光照射（λ > 600 nm）下被锌卟啉光敏化时可作为二氧化碳还原催化剂；此前仅有一个以这种方式用卟啉光活化锰催化剂的报道实例。因此，这项工作证明了空间保护的铼和锰二亚胺羰基催化剂具有广泛的用途，其中一氧化碳生成的速率和产率可根据金属中心和催化条件进行调节，其优点是通过对空配位位点的空间保护抑制不需要的副反应。
