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. 2024 Apr 9;29(8):1694.
doi: 10.3390/molecules29081694.

Electrocatalytic Reduction of CO2 to CO by Molecular Cobalt-Polypyridine Diamine Complexes

Yong Yang  1 , Fang Xie  1 , Jiahui Chen  1 , Si Qiu  1 , Na Qiang  1 , Ming Lu  1 , Zhongli Peng  1 , Jing Yang  2 , Guocong Liu  1
Affiliations

Electrocatalytic Reduction of CO2 to CO by Molecular Cobalt-Polypyridine Diamine Complexes

Yong Yang et al. Molecules. .

Abstract

Cobalt complexes have previously been reported to exhibit high faradaic efficiency in reducing CO2 to CO. Herein, we synthesized capsule-like cobalt-polypyridine diamine complexes [Co(L1)](BF4)2 (1) and [Co(L2) (CH3CN)](BF4)2 (2) as catalysts for the electrocatalytic reduction of CO2. Under catalytic conditions, complexes 1 and 2 demonstrated the electrocatalytic reduction of CO2 to CO in the presence or absence of CH3OH as a proton source. Experimental and computational studies revealed that complexes 1 and 2 undergo two consecutive reversible one-electron reductions on the cobalt core, followed by the addition of CO2 to form a metallocarboxylate intermediate [CoII(L)-CO22-]0. This crucial reaction intermediate, which governs the catalytic cycle, was successfully detected using high resolution mass spectrometry (HRMS). In situ Fourier-transform infrared spectrometer (FTIR) analysis showed that methanol can enhance the rate of carbon-oxygen bond cleavage of the metallocarboxylate intermediate. DFT studies on [CoII(L)-CO22-]0 have suggested that the doubly reduced species attacks CO2 on the C atom through the dz2 orbital, while the interaction with CO2 is further stabilized by the π interaction between the metal dxz or dxz orbital with p orbitals on the O atoms. Further reductions generate a metal carbonyl intermediate [CoI(L)-CO]+, which ultimately releases CO.

Keywords: CO2 reduction; cobalt; electrocatalysis; molecular catalyst.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Scheme 1
Scheme 1
The structures of the target cobalt complexes 1 and 2.
Figure 1
Figure 1
Molecular structures of complexes 1 and 2. Counter ions and hydrogen atoms are omitted for clarity.
Figure 2
Figure 2
Cyclic voltammograms of complex 1 in CH3CN with 0.1 M nBu4NPF6 as the supporting electrolyte (I) under Ar; under CO2 (II) in the absence of and (III) in the presence of 4 M methanol. The arrow in the CV diagram represents the initial scan direction.
Figure 3
Figure 3
IR spectrum of freshly prepared [Co0(L1)]0 (black line), which is bubbled with dry CO2 (above) and 13CO2 (down) (red line), then 2 M methanol is added (blue line). These lines were recorded by in situ FTIR during the reaction process. The tests are performed in a CH3CN/THF (1:1) mixture under Ar or CO2 with 10.0 mM [Co0(L1)]0.
Figure 4
Figure 4
The observed and calculated (inset) HRMS of the protonated product of [CoII(L1)−CO2−]0 generated from CO2 coordinating to [Co0(L1)]0.
Figure 5
Figure 5
Differential IR-SEC spectrum of complex 1 (10.0 mM) recorded after electrolysis at −1.85 V (vs. Fc+/0) for 10 s in CO2 saturated CH3CN solution.
Scheme 2
Scheme 2
A proposed mechanism for electroreduction of CO2 to CO catalyzed by complex 1.
See this image and copyright information in PMC

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