期刊
ACS CATALYSIS
卷 11, 期 6, 页码 3257-3267出版社
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.0c04518
关键词
electrocatalysis; CO2 reduction; iron complexes; NHC ligands; macrocyclic ligands; heme analogues; spectro-electrochemistry
资金
- University of Gottingen
- Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) [217133147/SFB 1073]
The study investigates the electrochemical CO2 reduction using a macrocyclic tetracarbene iron complex with N-donor supporting ligands. The iron complex exhibits high selectivity and catalytic efficiency in converting CO2 to CO, showing promising potential for further exploration in reductive chemistry.
Molecular first-row transition metal complexes for electrocatalytic CO2 reduction mostly feature N-donor supporting ligands, iron porphyrins being among the most prominent catalysts. Introducing N-heterocyclic carbene (NHC) ligation has previously shown promising effects for some systems, yet the application of NHC iron complexes for electrochemical CO2 reduction has so far remained unreported. Herein, we show that the macrocyclic tetracarbene iron complex [LFe(MeCN)(2)](OTf)(2)(1(OTf)(2)), which can be described as an organometallic heme analogue, mediates selective electrocatalytic CO2 -to-CO conversion with a faradaic efficiency of over 90% and a very high initial observed catalytic rate constant (k(obs)) of 3300 s(-1). Replacement of an axial MeCN ligand by CO increases the catalyst stability and turnover number, while the rate of catalysis decreases only slightly (k(obs) = 3100 s(-1)). Ferrous complexes with one or two axial CO ligands, [LFe(MeCN(CO)](OTf)(2)(2(OTf)(2)) and [LFe(CO)(2)](OTf)(2)(3(OTf)(2)), have been isolated and fully characterized. Based on linear sweep voltammogram spectroelectro-IR studies for 1(2+) and 2(2+), both under the N-2 and CO2 atmosphere, a mechanistic scenario in anhydrous acetonitrile is proposed. It involves two molecules of CO2 and results in CO and CO32- formation, whereby the first CO2 binds to the doubly reduced, pentacoordinated [LFe0(CO)] species. This work commences the exploration of the reductive chemistry by the widely tunable macrocyclic tetracarbene iron motif, which is topologically similar to hemes but electronically distinct as the strongly sigma-donating and redox inactive NHC scaffold leads to metal-centered reduction and population of the exposed d(z)(2) orbital, in contrast to ligand-based orbitals in the analogous porphyrin systems.
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