Sterically hindered Re- and Mn-CO2 reduction catalysts for solar energy conversion.
Sterically hindered Re- and Mn-CO2 reduction catalysts for solar energy conversion.
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DOI：
10.1039/d0dt00252f
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发表时间：
2020-02
影响因子：
4
通讯作者：
James D. Shipp;Heather Carson;Steven J. P. Spall;S. Parker;D. Chekulaev;Natalie A Jones;M. Y. Mel’nikov;C. Robertson;A. Meijer;J. Weinstein
中科院分区：
文献类型：
--
作者：
James D. Shipp;Heather Carson;Steven J. P. Spall;S. Parker;D. Chekulaev;Natalie A Jones;M. Y. Mel’nikov;C. Robertson;A. Meijer;J. Weinstein
合成了6，6‘-位带有空间位阻取代基的联吡啶配体的新型分子Re和Mn三甲酰配合物[M(HPEAB)(CO)3(X)](M/X=Re/Cl，Mn/Br；HPEAB=6，6’-{N-(4-hexylphenyl)-N(ethyl)-amido}-2，2‘-bipyridine))，并用单晶X-射线衍射法对其进行了表征，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，催化剂对CO_2的电化学还原是有效的，生成CO(稀土络合物在MeCN中，Mn-络合物在95：5(v/v)MeCN：H2O混合液中)。通过对溶液中CO2的补充，证明了催化剂的可回收性。新型催化剂的还原潜力与以前报道的类似结构的络合物相似，波脚分析的结果也显示出类似的最大转化率。在红外光谱电化学数据的基础上，结合密度泛函理论计算，提出了前催化剂的活化机理。结果表明，锰催化剂的典型二聚反应是通过引入空间位阻基团来防止的，而再催化剂在氯离子损失后经历了通常的机理。在牺牲施主(三乙胺)的存在下，没有观察到Re络合物的光化学还原，这是由于Re的三重态激发态寿命短(3.6 ns)，不足以进行扩散控制的电子转移。重要的是，在MeCN：H2O(95：5)中，[Mn(HPEAB)(CO)3Br]在红光照射(λ>；600 nm)下被锌卟啉光敏化时，可以作为二氧化碳还原催化剂；以这种方式光活化锰催化剂的报道只有一例。因此，这项工作证明了空间保护的稀土和锰二亚胺羰基催化剂的广泛应用，其中CO的生成速度和产率可以根据金属中心和催化条件进行调整，其优点是通过空间保护空位来抑制不必要的副反应。
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.