Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology) of Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China.
Dalton Trans. 2018 Oct 7;47(37):13142-13150. doi: 10.1039/c8dt02148a. Epub 2018 Aug 31.
The conversion of carbon dioxide (CO) to fuels or value-added chemicals by a photocatalytic system has recently been of growing research interest. One of the challenges is the development of new catalysts with high activity and low cost. Cobalt complexes have long been used as catalysts for the reduction of CO in either electrochemical or photochemical systems. Recently, a series of cis-Co complexes of tetradentate pyridine-amine ligands (N-ligands) exhibited high activity in the reduction of CO in homogeneous photocatalytic systems. However, only CO was obtained as the reduction product. In this regard, herein, we report a novel cis-Co complex C1 supported by an N ligand derivatized with TPA (TPA = tris(2-pyridylmethyl)amine). In contrast to the aforementioned Co catalysts, which contain two halogen atoms at cis-positions, C1 contains one oxygen atom at one cis-coordination site. The structure of C1 was fully characterized by MS, elemental analysis, and single-crystal X-ray diffraction. Experiments on the photocatalytic reduction of CO revealed that C1 is able to convert CO to not only CO but also formate in a homogeneous system containing C1 as a catalyst, Ir(ppy) as a photosensitizer, and triethylamine as an electron donor under visible-light irradiation. The catalytic activity and distribution of reduction products of this system are highly affected by the solvent environment. The presence of water in this system enhances the efficiency of 2H-to-H and CO-to-formate conversions. Electrochemical and steady-state emission quenching experiments indicate that photoinduced electron transfer from excited Ir(ppy) to C1 is thermodynamically feasible. A photogenerated Co species is suggested to be the active species involved in the reduction of CO and protons. DFT calculations were performed to elucidate the catalytic pathways of the formation of CO, formate, and H in this system; four pathways, namely, one for the formation of CO, one for the formation of hydrogen, and two for the formation of formate, were suggested. The results revealed that the oxygen atom at the cis-coordination site in C1 plays an important role in stabilizing the transition state during the transformation of CO at the cobalt center.
二氧化碳（CO）转化为燃料或增值化学品的光催化系统最近引起了越来越多的研究兴趣。其中一个挑战是开发具有高活性和低成本的新型催化剂。钴配合物长期以来一直被用作电化学或光化学系统中 CO 还原的催化剂。最近，一系列具有四齿吡啶-胺配体（N-配体）的顺式-Co 配合物在均相光催化系统中 CO 还原中表现出高活性。然而，仅获得 CO 作为还原产物。在这方面，本文报道了一种新型的顺式-Co 配合物 C1，它由 N 配体衍生的 TPA（TPA=三（2-吡啶甲基）胺）支持。与上述含有两个卤素原子在顺式位置的 Co 催化剂不同，C1 在一个顺式配位位置含有一个氧原子。C1 的结构通过 MS、元素分析和单晶 X 射线衍射进行了全面表征。CO 的光催化还原实验表明，C1 能够将 CO 不仅转化为 CO，而且在含有 C1 作为催化剂、Ir(ppy)作为光敏剂和三乙胺作为电子供体的均相体系中，在可见光照射下也可以转化为甲酸盐。该体系的还原产物的催化活性和分布受溶剂环境的影响很大。该体系中存在水可提高 2H 到 H 和 CO 到甲酸盐的转化效率。电化学和稳态发射猝灭实验表明，从激发态 Ir(ppy)到 C1 的光致电子转移在热力学上是可行的。建议光生 Co 物种是参与 CO 和质子还原的活性物质。进行了密度泛函理论计算以阐明该体系中 CO、甲酸盐和 H 的形成的催化途径；提出了四种途径，即 CO 的形成途径、氢的形成途径和甲酸盐的形成途径。结果表明，C1 中顺式配位位置的氧原子在钴中心 CO 转化过程中对过渡态的稳定起着重要作用。
