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Polypyridine Complexes on the Molecular Catalysis of CO2 Reduction

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Polypyridine Complexes on the Molecular Catalysis of CO2 Reduction catalysts Article The Role of Redox Potential and Molecular Structure of Co(II)-Polypyridine Complexes on the Molecular Catalysis of CO2 Reduction Juan Pablo F. Rebolledo-Chávez 1 , Gionnany Teodoro Toral 1, Vanesa Ramírez-Delgado 1, Yolanda Reyes-Vidal 1 , Martha L. Jiménez-González 1, Marisela Cruz-Ramírez 2, Angel Mendoza 3 and Luis Ortiz-Frade 1,* 1 Centro de Investigación y Desarrollo Tecnológico en Electroquímica S.C. Parque Tecnológico Querétaro, Departamento de Electroquímica, 76703 Santiago de Querétaro, Mexico; [email protected] (J.P.F.R.-C.); [email protected] (G.T.T.); [email protected] (V.R.-D.); [email protected] (Y.R.-V.); [email protected] (M.L.J.-G.) 2 División de Química y Energías Renovables, Universidad Tecnológica de San Juan del Río, 76800 San Juan del Río, Mexico; [email protected] 3 Centro de Química, Instituto de Ciencias, Benemérita Universidad Autónoma de Puebla, Ciudad Universitaria, Col. San Manuel, 72570 Puebla, Mexico; [email protected] * Correspondence: [email protected] or [email protected] Abstract: In this work, we report the electrochemical response of a family of Co(II) complexes, II 2+ II 2+ [Co (L)3] and [Co (L’)2] (L = 2,2’-bipyridine, 1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10- Citation: Rebolledo-Chávez, J.P.F.; phenanthroline, 5,6-dimethyl-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline; L’ = ter- Toral, G.T.; Ramírez-Delgado, V.; pyridine and 4-chloro-terpyridine), in the presence and absence of CO2 in order to understand the Reyes-Vidal, Y.; Jiménez-González, role of the redox potential and molecular structure on the molecular catalysis of CO2 reduction. The M.L.; Cruz-Ramírez, M.; Mendoza, A.; II 2+ III 3+ − tris chelate complexes exhibited three electron transfer processes [Co (L)3] [Co (L)3] + 1e , Ortiz-Frade, L. The Role of Redox [CoII(L) ]2++1e− [CoI(L) ]+, and [CoI(L) ]+ + 2e- [CoI(L)(L−) ]−. In the case of complexes with Potential and Molecular Structure of 3 3 3 2 1,10-phen and 2,2-bipy, the third redox process showed a coupled chemical reaction [CoI(L)(L−) ]− Co(II)-Polypyridine Complexes on 2 ! [CoI(L−) ]− + L. For bis chelate complexes, three electron transfer processes associated with the the Molecular Catalysis of CO2 2 II III 3+ II 2+ I + I + I − Reduction. Catalysts 2021, 11, 948. redox couples [Co (L)2]/[Co (L)2] , [Co (L)2] /[Co (L)2] , and [Co (L)2] /[Co (L)(L )] were https://doi.org/10.3390/catal11080948 registered, including a coupled chemical reaction only for the complex containing the ligand 4- chloro-terpyridine. Foot to the wave analysis (FOWA) obtained from cyclic voltammetry experiments Academic Editors: Charles allowed us to calculate the catalytic rate constant (k) for the molecular catalysis of CO2 reduction. E. Creissen and Souvik Roy 2+ The complex [Co(3,4,7,8-tm-1,10-phen)3] presented a high k value; moreover, the complex [Co(4- 2+ Cl-terpy)3] did not show catalytic activity, indicating that the more negative redox potential and Received: 30 June 2021 the absence of the coupled chemical reaction increased the molecular catalysis. Density functional Accepted: 4 August 2021 theory (DFT) calculations for compounds and CO2 were obtained to rationalize the effect of electronic Published: 8 August 2021 structure on the catalytic rate constant (k) of CO2 reduction. Publisher’s Note: MDPI stays neutral Keywords: molecular catalysis; CO2 reduction; Co(II)-polypyridine complexes; DFT calculations with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction The transformation of carbon dioxide into value-added products has stimulated the creativity and curiosity of many generations of scientists [1]. This molecule represents an Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. alternative carbon raw material that has the advantages of being renewable, cheap, safe, This article is an open access article and nontoxic. Carbon dioxide can be transformed by biological, chemical, photochemical, distributed under the terms and reforming, inorganic, and electrochemical methods, with a wide variety of products [2]. conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Catalysts 2021, 11, 948. https://doi.org/10.3390/catal11080948 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 948 2 of 19 From an electrochemical point of view, it is possible to carry out CO2 reduction. In the absence of proton donors, two consecutive reduction processes have been proposed, with typical thermodynamic redox potentials: ◦− ◦ CO2 $ CO2 E = −1.90 V vs. NHE (1) ◦− 2− ◦ CO2 $ CO2 E = −1.2 V vs. NHE (2) It has been established that in the first reduction, a change in the geometry of the CO2 ◦− takes place, from linear to bent-type CO2 [3], making slow electrode kinetics with a high overpotential [4,5]. Hence, experimentally, the reduction of CO2 on a Pt electrode requires much more negative potentials for the formation of the radical anion CO2·- than −1.90 V vs. NHE (normal hydrogen electrode) in water and −1.97 V vs. NHE in DMF [6,7]. In the presence of proton donors, the electrochemical reduction of carbon dioxide can give different reaction products, including carbon monoxide, formic acid, methanol, and methane, as shown below [6]: E◦(V) vs NHE + − CO2 + 2H +2e ! HCO2H −0.61 (3) + − CO2 + 2H + 2e ! CO + H2O −0.52 (4) + − CO2 + 4H + 4e ! HCHO + H2O −0.48 (5) + − CO2 + 6H + 6e ! CH3OH + H2O 0.38 (6) + − CO2+2H + 8e ! CH4 + H2O −0.24 (7) Indeed, the processes are carried out at a very negative potential with a slow electron transfer, which is expected for small molecules such as CO2 with high solvation and reorganization energies. On the other hand, CO2 can react with its radical anion and by the addition of one electron to produce carbon monoxide and carbonate in a (ECE- DISP) mechanism. Another possible reaction is the dimerization between two radical anions, which leads to the formation of the divalent oxalate anion. As in the case of CO2, there are many electrochemical reactions that require a significant overpotential to be appreciated. Hence, different electrocatalytic materials can be used for this purpose [8–10]. However, the control of crystallinity and the number of defects make it difficult to control the electrocatalytic processes. Another approach to catalyzing electrochemical reactions is the use of molecules as catalysts, which can be viewed as a mediated process or molecular catalysis. In electro- chemistry, molecular catalysis can be defined as the use of molecules, whether in solution or immobilized on the electrode, susceptible to being reduced to transfer electrons to a target molecule with slow electrode kinetics such as CO2, in a concerted or sequential manner [8,10,11]. Coordination compounds with polypyridine ligands represent one of the most studied classes of molecular catalysts for CO2 reduction. As they stabilize various oxidation states for metals and can store electrons in π* orbitals as a function of their substituents, these ligands present a great variety in the type of denticity, which includes bipyridines, terpyridines, tetrapyridines, phenanthroline, and their derivatives [12]. First, studies found in the literature correspond to the use of the compound [Re(bipy) (CO)3Cl] as a catalyst in the electrochemical reduction of carbon dioxide, where the only product obtained was carbon monoxide, with a faradaic efficiency of 98% [13]. 2+ Ruthenium compounds with polypyridine ligands [Ru(bipy)2(CO)2] and [Ru(bipy)2 (CO)Cl]+ have been studied in DMF-water mixtures [14], applying a potential of −2.14 V vs. Fc+-Fc, giving different products depending on the pH of the solution [12]. On the other hand, the use of metal complexes with earth-abundant elements and polypyridine ligands for the electrochemical reduction of CO2 has attracted the attention Catalysts 2021, 11, 948 3 of 19 again in recent years [12]. The use of several Mn-bipy complexes has been reported as electrocatalysts in the reduction of CO2 to CO [15–18]. A series of Ni(II) compounds with N-heterocyclic ligands with carbenes and pyridine have the ability to carry out the electrochemical conversion of CO2 to CO [19,20]. The 2+ Ni complex [Ni(bipy)3] presents a bi-electronic reduction at a potential of −1.58 V vs. + 0 Fc /Fc. The dissociation of a bipyridine has been proposed by generating [Ni(bipy)2] , which, in the presence of CO2, increases the catalytic current associated with its reduction, − giving CO and CO3 as products [21]. In Fe(II) coordination compounds with polypyridine ligands, vacant coordination sites are not necessary for the molecular catalysis of CO2. The mechanism involves the outer sphere homogeneous electron transfer between carbon dioxide and one of the reduced forms of the complexes [22,23]. The use of cobalt complexes with pyridylimine ligands for the electrochemical reduc- tion of CO2 has been informed from the literature [19], reporting the production of carbon I/0 monoxide at a potential very close to the redox potential of the Co couple (E1/2 = −1.88 V vs. Fc+/Fc) [19]. Cobalt compounds with phenanthrolines have also been studied [20], for 2+ example, [Co(phen)3] , which presents an increase in the catalytic current in the presence of CO2, associated with the reduction of the latter to formate [19]. Despite the wide variety of earth-abundant metal complexes reported for CO2 re- duction, there are no systematic studies that indicate the effect of redox potential and molecular structure (i.e., the metal center and the nature of the ligands) in the molecular catalysis of carbon dioxide. Therefore, in this work, we studied the electrochemical response II 2+ II 2+ of a family of Co(II) complexes, [Co (L)3] and [Co (L´)2] (L = 2,2’-bipyridine, 1,10- phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline, 5,6-dimethyl-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline; L’=terpyridine and 4-chloro-terpyridine), in the presence and absence of CO2 in order to understand the role of redox potential and the electronic structure of metal complexes on the molecular catalysis of CO2.
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