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 electrochemistry, 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 reduction 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 reduction, 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. Foot to the wave analysis (FOWA) obtained from cyclic voltammetry experiments allowed us to calculate the catalytic rate constant (k) for mediated CO2 reduction. DFT calculations for compounds and CO2 were carried out to rationalize the effect of electronic structure on the catalytic rate constant (k) for CO2 reduction.

2. Results 2.1. Electrochemical Response of Cobalt Coordination Compounds Cyclic voltammetry in its normalized current representation i/v1/2 of Co(II) complexes at different scan rates are presented in the Supporting Information. For the compound II [Co (bipy)3](BF4)2, a reversible electron transfer I and a quasi-reversible electron transfer II 2+ I + II 2+ III, associated with the redox couples [Co (bipy)3] /[Co (bipy)3] and [Co (bipy)3] / III 3+ [Co (bipy)3] , were observed (see Figure S1) [24]. Furthermore, a bi-electronic quasi- reversible reduction II with a coupled chemical reaction was detected [24]. According to the literature, the dissociation of a bipy ligand and the formation of a product with I − − a square geometry [Co (bipy )2] have been proposed, which is stabilized by the electronic conﬁguration d8 in the electrogenerated Co(I) species [25]. This dissociation has been reported in the literature for Co, Fe, Ni, and Ru complexes with polypyridine II ligands [26–31]. For [Co (phen)3](BF4)2, a similar electrochemical response has been observed (see Figure S2). For processes, I, II, and III, the redox potentials were calculated ◦ with the equation E = (Epa+ Epc)/2 (see Table1). According to the evidence presented, the following mechanism is conﬁrmed:

II 2+ III 3+ − ◦ + [Co (L)3] [Co (L)3] + 1e E (III) = −0.059 V vs. Fc/Fc (8)

II 2+ − I + ◦ + [Co (L)3] +1e [Co (L)3] E (I) = −1.351 V vs. Fc/Fc (9) I + − I − − ◦ + [Co (L)3] +2e [Co (L)(L )2] E (II) = −1.962 V vs. Fc/Fc (10) I − − I − − [Co (L)(L )2] → [Co (L )2] + L L = phen or bipy (11) Catalysts 2021, 11, 948 4 of 19

◦ II 2+ Table 1. Redox potential values E of the processes I, II, and III of the compounds [Co (L)3] and II 2+ [Co (L´)2] and their catalytic constant values for mediated CO2 reduction.

k Compound E◦(I) E◦(II) E◦(III) M−1 s−1 2+ 1 [Co(bipy)3] −1.351 −1.962 −0.059 9.24 × 10 2+ 2 [Co(phen)3] −1.580 −2.286 −0.245 1.60 × 10 2+ 3 [Co(3,4,7,8-tm-phen)3] −1.514 −2.197 −0.204 2.40 × 10 2+ 1 [Co(5,6-dm-phen)3] −1.364 −2.074 −0.115 5.19 × 10 2+ 2 [Co(4,7-dph-phen)3] −1.252 −1.915 −0.090 1.64 × 10 2+ 1 [Co(terpy)3] −1.404 −2.283 −0.341 5.78 × 10 2+ a [Co(4-Cl-terpy)3] −1.067 −1.964 −0.002 n.o. a Epc to 100 mV/s. (bipy = 2,2’-bipyridine), (3,4,7,8-tm-phen = 3,4,7,8-tetramethyl-1,10-phenanthroline), (5,6- dm-phen = 5,6-dimethyl-1,10-phenanthroline), (4,7-dph-phen = 4,7-diphenyl-1,10-phenanthroline), (terpy = terpyridine), and (4-Cl-terpy = −chloro-terpyridine).

II II The coordination compounds [Co (3,4,7,8-tetramethyl-phen)3](BF4)2 and [Co (4,7- diphenyl-phen)3](BF4)2 exhibited an electrochemically reversible behavior in all their processes, exchanging one electron in I and III and two electrons in II. For the compound II [Co (5,6-dimethyl-3,4,7,8-tetramethyl-phen)3](BF4)2, mono electronic electron transfer processes were observed in I and III, while for process II, a bi-electronic quasi-reversible electron transfer was registered (see Figures S3–S5). The redox potential values are pre- II sented in Table1. The cyclic voltammetry response of [Co (terpy)2](BF4)2 presented three reversible mono-electronic transfers. According to the above and considering the π acceptor character of the polypyridine ligands, the following mechanism is presented (see Figure S6) [25]:

II 2+ III 3+ − ◦ + [Co (terpy)2] [Co (terpy)2] + 1e E (III) = −0.341 V vs. Fc/Fc (12)

II 2+ − I + ◦ + [Co (terpy)2] + 1e [Co (terpy)2] E (I) = −1.404 V vs. Fc/Fc (13) I + − I − ◦ + [Co (terpy)2] + 1e [Co (terpy)(terpy )] E (II) = −2.286 V vs. Fc/Fc (14) II For the compound [Co (4-Cl-terpy)2](BF4)2, three reduction signals Ic, IIc, and IIIc and two oxidation signals Ia and IIIa can be observed (see Figure S6). Processes I and III correspond to the redox couples Co(II)/Co(I) and Co(II)/Co(III), respectively. The fact that the normalized current i/v1/2 for process IIc decreases with scan rate and the oxidation signal IIa is absent indicate a ECE mechanism [24]. The change in the mechanism associated with the presence of the chlorine atom in the terpyridine complexes can be related to the weakening of the Co-N bond, which promotes the ligand dissociation. This fact has been reported in the literature for Co, Fe, Ni, and Ru complexes with polypyridine ligands, already mentioned previously [26–31]. The following mechanism is proposed:

II 2+ III 3+ − ◦ + [Co (4-Cl-terpy)2] [Co (4-Cl-terpy)2] +1e E (III) = −0.002 V vs. Fc/Fc (15)

II 2+ − I + ◦ + [Co (4-Cl-terpy)2] +1e [Co (4-Cl-terpy)2] E (I) = −1.067 V vs. Fc/Fc (16)

I + − I − [Co (4-Cl-terpy)2] +1e → [Co (4-Cl-terpy)(Cl-terpy )] E (17) [CoI(4-Cl-terpy)(4-Cl-terpy−)] → [CoI(4Cl-terpy)]+ + (4-Cl-terpy)− C (18) [CoI(4Cl-terpy)]+ + 1e− → [CoI(4Cl-terpy−)] E (19)

2.2. Molecular Catalysis of CO2 Once the electrochemical behavior of the cobalt (II) coordination compounds was presented, the molecular catalysis for the reduction of CO2 was studied. Figure1 presents Catalysts 2021, 11, x FOR PEER REVIEW 5 of 19

Once the electrochemical behavior of the cobalt (II) coordination compounds was presented, the molecular catalysis for the reduction of CO2 was studied. Figure 1 presents a comparative study of the electrochemical response of the [CoII(bipy)3](BF4)2 in MeCN Catalysts 2021, 11, 948 5 of 19 solution in the presence and absence of CO2. As can be observed, signal Ic is not altered; however, the IIc signal (associated with the bi-electronic reduction) presents an increase in current due to molecular catalysis processes. The disappearance of Ia and IIa is also a comparative study of the electrochemical response of the [CoII(bipy) ](BF ) in MeCN associated with the catalytic process that implies the consumption3 of4 the2 cobalt redox me- solution in the presence and absence of CO2. As can be observed, signal Ic is not altered; + diatorhowever,. The appearance the IIc signal (associatedof a new signal with the I’ bi-electronicat −0.772 V reduction) vs. Fc/Fc presents is related an increase to the oxidation of a newin current electrogenerated due to molecular species. catalysis It processes.should be The noted disappearance that the reduction of Ia and IIa of isCO also2 is activated at −1.7associated V vs. Fc/Fc with+, the where catalytic an process increase that in implies the current the consumption of process of II the is cobaltobserved. redox mediator. The appearance of a new signal I’ at −0.772 V vs. Fc/Fc+ is related to the oxidation of a new electrogenerated species. It should be noted that the reduction of CO2 is activated at −1.7 V vs. Fc/Fc+, where an increase in the current of process II is observed.

100 IIIa Ia 50 I´ IIa 0

-50 IIIc ) Ic -1/2 -100

A V

m -150

( N 1/2 2 CO i/v -200 2

-250 IIc

-300

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 E (V vs Fc/Fc+)

II Figure 1. Cyclic voltammetry of a 1 mM [Co (bipy)3](BF4)2 solution in MeCN with 0.1 M of TBAPF6, bubbled with N (black line) and CO (red line). Scan rate of 100 mVs−1 obtained from open-circuit Figure 1. Cyclic voltammetry2 of a 1 2mM [CoII(bipy)3](BF4)2 solution in MeCN with 0.1 M of TBAPF6, potential to cathodic direction. Glassy carbon disk as a working electrode. bubbled with N2 (black line) and CO2 (red line). Scan rate of 100 mVs−1 obtained from open-circuit potential Subsequently,to cathodic direction. cyclic voltammetry Glassy carbon experiments disk as were a working carried outelectrode. at different scan rates. Figure2 shows its normalized current representation (i/v 1/2), in which all the mentioned signalsSubsequently, converge to cyclic the same voltammetry potentials. experiments were carried out at different scan II rates. FigureFrom 2 cyclic shows voltammetry its normalized experiments current of the representation complex [Co (bipy) (i/v31/2](BF), i4n)2 whichin the ab- all the men- sence and presence of CO2, at different scan rates, it is possible to calculate the catalytic tionedconstant signals using converge FOWA in to process the same IIc, according potentials. to the following expression [32,33]: q 2.24 RT 2kC0 ik Fv A = (20) i0 ◦ P + exp F E − E 1 RT II 2+ III 3+ [Co L3] /[Co L3]

Catalysts 2021, 11, x FOR PEER REVIEW 6 of 19

100 Catalysts 2021, 11, x FOR PEER REVIEW I' IIIa 6 of 19 50 Ia IIa* 0 Catalysts 2021, 11, 948 6 of 19 -50 IIIc Ic

) -100

-1/2 100 -150

A V I' IIIa m -20050 Ia

( IIa* 50 mV/s

1/2 -250 0 100 mV/s

i/v 250 mV/s -300-50 500 mV/s 750 mV/s IIIc -350 100 mV/s Ic 1500 mV/s ) -100 IIc

-1/2 -400 -150 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5

A V + m -200 E (V vs Fc/Fc )

( 50 mV/s 1/2 -250 100 mV/s i/v II Figure 2. Normalized current cyclic voltammetry of 1 mM [Co (bipy)3](BF 2504) mV/s2 solution in MeCN with 0.1 M of TBAPF-3006 as supporting electrolyte, bubbled with CO2 from open -500circuit mV/s potential to cathodic direction. Glassy carbon disk as a working electrode. 750 mV/s -350 100 mV/s IIc 1500 mV/s From cyclic voltammetry experiments of the complex [CoII(bipy)3](BF4)2 in the absence and -400presence of CO2, at different scan rates, it is possible to calculate the catalytic constant using FOWA-2.5 in process-2.0 IIc, -1.5according-1.0 to the following-0.5 expression0.0 0.5 [32,33]: E (V vs Fc/Fc+) 푅푇 2.24√ 2푘퐶0 푖푘 퐹푣 퐴 II Figure 2. Normalized current cyclic voltammetry of 1 mM [Co (bipy)3](BF4)2 solution in MeCN 0 = (20) 푖푃 퐹 ° II Figurewith 2. 0.1Normalized M of TBAPF current6 as supporting1 cyclic+푒푥푝 voltammetry[ electrolyte,(퐸 − 퐸 bubbled of 12 + withmM CO[Co2 3from+(bipy))] open-circuit3](BF4)2 solution potential in to MeCN with 푅푇 [퐶표퐼퐼퐿 ] /[퐶표퐼퐼퐼퐿 ] cathodic direction. Glassy carbon disk as a working electrode.3 3 0.1 M of TBAPF6 as supporting electrolyte, bubbled with CO2 from open-circuit potential to cathodic 퐹 ° directionThe .value Glassy of carbonthe slope disk of asa grapha working ik/ip° vselectrode.. 1/(1 +푒푥푝 [ (퐸 − 퐸 ◦ 2+ 3+ )]) ◦ 푅푇F 퐼퐼 퐼퐼퐼 The value of the slope of a graph ik/ip vs. 1/(1 + exp E − E [퐶표 퐿3] 2+/[퐶표 퐿3]3+ ) RT [CoII L ] /[CoIII L ] 3 −1 3−1 allows the calculation of k, which, in this case, corresponds to a value of 9.24 M− s− (see allows the calculation of k, which, in this case, corresponds to a value of 9.24II M 1 s 1 (see FigureFrom 3). cyclic voltammetry experiments of the complex [Co (bipy)3](BF4)2 in the ab- Figure3). sence and presence of CO2, at different scan rates, it is possible to calculate the catalytic

constant3.0 using FOWA in process IIc, according4.0 to the following expression [32,33]: 50 mV/s 100 mV/s 3.5 2.5 250 mV/s 500 mV/s 3.0푅푇 0 750 mV/s 2.24√ 2푘퐶 2.5 퐴 2.0 푖푘 1000 mV/s ) 퐹푣

-1

= 1500 mV/s s 2.0 (20)

-1 p 0 ° 푖 퐹 ° 1.5 푃 1.5

i / i / i 1 +푒푥푝 [ (퐸 − 퐸 2+ 3+ )] 퐼퐼 퐼퐼퐼 1 -1 -1 푅푇 [퐶표 퐿3] /[k퐶표 = 9.24 ×퐿 103] (M s )

log k (M k log 1.0 1.0 0.5 퐹 ° 0.5 0.0 The value of the slope of a graph ik/ip° vs. 1/(1 +푒푥푝 [ (퐸 − 퐸 퐼퐼 2+ 퐼퐼퐼 3+ )]) -0.5 푅푇 [퐶표 퐿3] /[퐶표 퐿3] 0.0 0.0 0.1 0.2 0.3 0.4 0.5 -1.0 −1 −1 allows the calculation of k, which, in this case, corresponds-1.4 -1.2 -1.0 -0.8 -0.6to -0.4a value-0.2 0.0 of0.2 9.24 M s (see 1 1 + exp[ (F/RT) (E - E )] Figure 3). MLred / MLox log n (V/s)

(a) (b)

3.0 II 4.0 Figure 3. (a) FOWA of a 1 mM [Co (bipy) 50 mV/s 3](BF4)2 complex in MeCN with 0.1 M of TBAPF6 as 100 mV/s 3.5 II Figuresupporting 3. 2.5(a) FOWA electrolyte. of a 1 mM (b) Logarithmic[Co (bipy) 3250](BF rate mV/s4 constant)2 complex obtained in MeCN from with FOWA. 0.1 M of TBAPF6 as sup- 500 mV/s 3.0 porting electrolyte. (b) Logarithmic rate constant 750 mV/s obtained from FOWA. 2.5 2.0Similar to what was previously 1000 discussed, mV/s Figure4 presents) a comparative study of

-1

1500 mV/s II s 2.0

-1

the° electrochemical response of the complex [Co (phen)3](BF4)2 in a MeCN solution in the 1.5 1.5 presence i / i and absence of carbon dioxide. As can be observed, process III andk = 9.24 the × 10 Ic1 (M signal-1 s-1)

log k (M k log 1.0 are not1.0 altered; however, the IIc signal presents an increase in current related to molecular catalysis. The disappearance of the IIa and Ia signals and the0.5 appearance of a new broad 0.5 + 0.0 signal (I’a) in −0.650 V vs. Fc/Fc are also associated with the mediated CO2 reduction. -0.5 0.0 0.0 0.1 0.2 0.3 0.4 0.5 -1.0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 1

1 + exp[ (F/RT) (E - EMLred / MLox)] log n (V/s)

(a) (b)

Figure 3. (a) FOWA of a 1 mM [CoII(bipy)3](BF4)2 complex in MeCN with 0.1 M of TBAPF6 as supporting electrolyte. (b) Logarithmic rate constant obtained from FOWA.

Catalysts 2021, 11, x FOR PEER REVIEW 7 of 19

Similar to what was previously discussed, Figure 4 presents a comparative study of the electrochemical response of the complex [CoII(phen)3](BF4)2 in a MeCN solution in the presence and absence of carbon dioxide. As can be observed, process III and the Ic signal are not altered; however, the IIc signal presents an increase in current related to molecular catalysis. The disappearance of the IIa and Ia signals and the appearance of a new broad signal (I’a) in −0.650 V vs. Fc/Fc+ are also associated with the mediated CO2 reduction. Catalysts 2021, 11, 948 7 of 19

100 Ia IIIa IIa I'a 0 IIIc -100 Ic IIc

) -200

-1/2

A V

m -300

( N 1/2 2 CO i/v -400 2

-500

-600 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 E (V vs Fc/Fc+)

II Figure 4. Cyclic voltammetry of a 1 mM [Co (phen)3](BF4)2 solution with 0.1 M of TBAPF6 in MeCN, II −1 Figurebubbled 4. Cyclic with Nvoltammetry2 (black line) andof a CO 1 mM2 (red [Co line).(phen) Scan rate3](BF of4 100)2 solution mVs from with open-circuit 0.1 M of TBAPF potential6 in MeCN, bubbledto cathodic with direction.N2 (black Glassy line) and carbon CO disk2 (red as a line). working Scan electrode. rate of 100 mVs−1 from open-circuit potential Catalysts 2021, 11, x FOR PEER REVIEW 8 of 19 to cathodic direction. Glassy carbon disk as a working electrode. Cyclic voltammetry experiments were carried out at different scan rates, and their normalized current representations (i/v1/2) are shown in Figure5. The catalytic constant Cyclic voltammetry experiments were carried out at differentII scan rates, and their was calculated using FOWA, and the respective plots for the [Co (phen)3](BF4)2 complex 1/2 normalizedare shown current in Figure representations6, where a value of (i/v k = 160) are M −shown1 s−1 was in obtained.Figure 5. The catalytic constant was calculated using FOWA, and the respective plots for the [CoII(phen)3](BF4)2 complex are shown in Figure 6, where a value of k = 160 M−1 s−1 was obtained. 100 I'a IIIa Ia 0 IIIc -100 Ic

-200

)

-1/2 -300

A V

( -400 IIc 50 mV/s

1/2 100 mV/s i/v -500 250 mV/s 500 mV/s -600 750 mV/s 1000 mV/s -700 1500 mV/s

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 E (V vs Fc/Fc+)

II Figure 5. Normalized current cyclic voltammetry of 1 mM [Co (phen)3](BF4)2 solution in MeCN II Figurewith 5.0.1 Normalized M of TBAPF current6 as supporting cyclic voltammetry electrolyte, bubbled of 1 mM with [Co CO(phen)2 from open-circuit3](BF4)2 solution potential in MeCN to with 0.1 Mcathodic of TBAPF direction.6 as support Glassy carboning electrolyte, disk as a working bubbled electrode. with CO2 from open-circuit potential to cathodic direction. Glassy carbon disk as a working electrode.

5.0 4.0 4.5 50 mV 100 mV 3.5 4.0 250 mV 3.0 750 mV 3.5 1000 mV 2.5

)

1500 mV -1

3.0 s 2.0

-1 ° 2.5 1.5 i / i / i 2 -1 -1 2.0 k = 1.60 × 10 (M s )

log k (M k log 1.0 1.5 0.5 1.0 0.0 0.5 -0.5 0.0 -1.0 0.0 0.1 0.2 0.3 0.4 0.5 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 1 log n (V/s) 1 + exp[ (F/RT) (E - EMLred / MLox)]

(a) (b)

Figure 6. (a) FOWA of a 1 mM [CoII(phen)3](BF4)2 complex in MeCN with 0.1 M of TBAPF6 as supporting electrolyte. (b) Logarithmic rate constant obtained from FOWA.

On the other hand, in the electrochemical response of the compound [CoII(terpy)2](BF4)2 in the presence of CO2, changes in current for signals IIc and IIa were observed, as well as the absence of some other species associated with an intermediate (see Figure 7). This fact suggests that this compound presents an efficient catalyst regeneration compared to the previously presented compounds. Cyclic voltammetry experiments were performed at different scan rates, and, from these experiments, the catalytic constant was calculated using FOWA (see Figures 8 and 9), obtaining a value of 57.8 M−1 s−1. For the complex [CoII(4-Cl-terpy)2](BF4)2, no molecular catalysis was observed in the presence of CO2 associated with the fact that the rate constant of the coupled reaction in IIc deactivates the catalyst (see Figures 10 and 11). The same analysis was performed for the remaining compounds, and the catalytic constant k values are reported in Table 1 and Figures S8–S13.

Catalysts 2021, 11, x FOR PEER REVIEW 8 of 19

100 I'a IIIa Ia 0 IIIc -100 Ic

-200

)

-1/2 -300

A V

( -400 IIc 50 mV/s

1/2 100 mV/s i/v -500 250 mV/s 500 mV/s -600 750 mV/s 1000 mV/s -700 1500 mV/s

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 E (V vs Fc/Fc+)

Figure 5. Normalized current cyclic voltammetry of 1 mM [CoII(phen)3](BF4)2 solution in MeCN with Catalysts 2021, 11, 948 0.1 M of TBAPF6 as supporting electrolyte, bubbled with CO2 from open-circuit potential to8 of cathodic 19 direction. Glassy carbon disk as a working electrode.

5.0 4.0 4.5 50 mV 100 mV 3.5 4.0 250 mV 3.0 750 mV 3.5 1000 mV 2.5

)

1500 mV -1

3.0 s 2.0

-1 ° 2.5 1.5 i / i / i 2 -1 -1 2.0 k = 1.60 × 10 (M s )

log k (M k log 1.0 1.5 0.5 1.0 0.0 0.5 -0.5 0.0 -1.0 0.0 0.1 0.2 0.3 0.4 0.5 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 1 log n (V/s) 1 + exp[ (F/RT) (E - EMLred / MLox)]

(a) (b)

II II FigureFigure 6. ( 6.a) FOWA(a) FOWA of ofa 1 a mM 1 mM [Co [Co(phen)(phen)3](BF3](BF4)42 )complex2 complex in in MeCN MeCN with with 0.1 0.1 M of TBAPF66 asas sup- portsupportinging electrolyte. electrolyte. (b) Logarithmic (b) Logarithmic rate rateconstant constant obtained obtained from from FOWA FOWA..

II OnOn the the other other hand, hand, in the electrochemical in the electrochemical response of the compoundresponse [Co of (terpy) the 2](BF compound4)2 in the presence of CO2, changes in current for signals IIc and IIa were observed, as well as the [CoII(terpy)2](BF4)2 in the presence of CO2, changes in current for signals IIc and IIa were absence of some other species associated with an intermediate (see Figure7). This fact suggests observed,that this as compound well as the presents absence an efficient of some catalyst other regeneration species associated compared with to the an previously intermediate (seepresented Figure 7 compounds.). This fact suggests Cyclic voltammetry that this compound experiments presents were performed an efficient at different catalyst scan regen- erationrates, compared and, from theseto the experiments, previously the presented catalytic constant compounds. was calculated Cyclic voltammetry using FOWA (see experi- −1 −1 II Catalysts 2021, 11, x FOR PEER REVIEW mentsFigures were8 and performed9), obtaining at adifferent value of 57.8 scan M rates,s and. For, thefrom complex these [Coexperiments(4-Cl-terpy), the2](BF9 ofcatalytic4 )192,

constantno molecular was calculated catalysis was using observed FOWA in the(see presence Figures of 8 CO and2 associated 9), obtaining with thea value fact that of 57.8 the M−1 s−1. rateFor constantthe complex of the [Co coupledII(4-Cl reaction-terpy) in2](BF IIc deactivates4)2, no molecular the catalyst catalysis (see Figures was observed 10 and 11 ).in the The same analysis was performed for the remaining compounds, and the catalytic constant k presence of CO2 associated with the fact that the rate constant of the coupled reaction in values are reported in Table1 and Figures S8–S13. IIc deactivates the catalyst (see Figures 10 and 11). The same analysis was performed for the remaining100 compounds, and the catalytic constant k values are reported in Table 1 and Figures S8–S13. IIIa

50 Ia IIa 0

)

-1/2 -50 IIIc

A V

m Ic

( N 1/2 -100 IIc 2 CO i/v 2

-150

-200 -3 -2 -1 0 E (V vs Fc/Fc+)

II Figure 7. Normalized current cyclic voltammetry of 1 mM [Co (terpy)2](BF4)2 solution in MeCN II Figure 7. Normalizedwith 0.1 M ofcurrent TBAPF cyclic6 as supporting voltammetry electrolyte, of 1 bubbledmM [Co with(terpy) N2 (black2](BF4 line))2 solution and CO 2in(red MeCN line) with 0.1 M ofobtained TBAPF from6 as open-circuit supporting potential electrolyte, to cathodic bubbled direction. with Glassy N2 (black carbon line) disk as and a working CO2 (red electrode. line) obtained from open-circuit potential to cathodic direction. Glassy carbon disk as a working electrode.

100 IIIa Ia 50 IIa Ia* 0

) -50 -1/2 IIIc

A V -100

( Ic 50 mV

1/2 100 mV i/v -150 250 mV 500 mV -200 750 mV 1000 mV IIc 1500 mV -250 -3 -2 -1 0 E (V vs Fc/Fc+)

Figure 8. Normalized current cyclic voltammetry of 1 mM [CoII(terpy)2](BF4)2 solution in MeCN with 0.1 M of TBAPF6 as supporting electrolyte, bubbled with CO2 from open-circuit potential to cathodic direction. Glassy carbon disk as a working electrode.

Catalysts 2021, 11, x FOR PEER REVIEW 9 of 19

100 IIIa

50 Ia IIa 0

)

-1/2 -50 IIIc

A V

m Ic

( N 1/2 -100 IIc 2 CO i/v 2

-150

-200 -3 -2 -1 0 E (V vs Fc/Fc+)

Figure 7. Normalized current cyclic voltammetry of 1 mM [CoII(terpy)2](BF4)2 solution in MeCN with 0.1 M of TBAPF6 as supporting electrolyte, bubbled with N2 (black line) and CO2 (red line) obtained from open-circuit potential to cathodic direction. Glassy carbon disk as a working elec- Catalysts 2021, 11,trode. 948 9 of 19

100 IIIa Ia 50 IIa Ia* 0

) -50 -1/2 IIIc

A V -100

( Ic 50 mV

1/2 100 mV i/v -150 250 mV 500 mV -200 750 mV 1000 mV IIc 1500 mV -250 Catalysts 2021, 11, x FOR PEER REVIEW -3 -2 -1 0 10 of 19

E (V vs Fc/Fc+)

II Catalysts 2021, 11, x FOR PEER REVIEW Figure 8. Normalized current cyclic voltammetry of 1 mM [Co (terpy)2](BF4)2 solution10 in of MeCN 19 Figure 8. Normalized current cyclic voltammetry of 1 mM [CoII(terpy)2](BF4)2 solution in MeCN with 0.1 M of TBAPF6 as supporting electrolyte, bubbled with CO2 from open-circuit potential to 3.0with 0.1 M of TBAPF6 as support 50 mV ing electrolyte,4.0 bubbled with CO2 from open-circuit potential to cathodic direction. 100 mV Glassy carbon disk as a working electrode. cathodic direction. Glassy c 250arbon mV disk as a working3.5 electrode. 2.5 500 mV 3.0 750 mV 3.0 50 mV 4.0 1000 mV 100 mV 2.5 1 -1 -1 2.0 ) k = 5.78 × 10 (M s ) 1500 mV 250-1 mV 3.5 2.5 s 2.0

-1 p 500 mV

° 3.0 750 mV 1.5 1.5

i / i / i 1000 mV 2.5 1 -1 -1 2.0 ) k = 5.78 × 10 (M s ) 1500 mV -1

log k (M k log 1.0 s 2.0

-1 1.0 p

° 1.5 0.5 1.5

i / i / i 0.0 0.5 (M k log 1.0 1.0 -0.5 0.5 0.0 0.5 -1.0 0.0 0.0 0.1 0.2 0.3 0.4 0.5 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 1 -0.5 0.0 log n (V/s) 1 + exp[ (F/RT) (E - EMLred / MLox)] -1.0 0.0 0.1 0.2 0.3 0.4 0.5 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 1 1 + exp[ (F/RT) (E - E )] log n (V/s) (a) MLred / MLox (b)

II (a) (b) Figure 9. (a) FOWA of a 1 mM [Co (terpy)3](BF4)2 complex in MeCN with 0.1 M of TBAPF6 as supporting electrolyte. (b) Logarithmic rate constant obtainedII II from FOWA. FigureFigure 9. (a 9.) FOWA(a) FOWA of a 1 of mM a 1 [Co mM(terpy) [Co (terpy)3](BF4)32](BF complex4)2 complex in MeCN in MeCNwith 0.1 with M of 0.1 TBAPF M of6 TBAPFas sup-6 as portsupportinging electrolyte. electrolyte. (b) Logarithmic (b) Logarithmic rate constant rate constant obtained obtained from FOWA. from FOWA.

100 100 IIIa Ia IIIa Ia

0 IIa 0 IIa

) IIIc ) IIIc

-1/2

-1/2 Ic Ic

A V

-100 A V m -100

( ( N 1/2 N 1/2 2 2 CO i/v CO i/v 2 2

-200 -200 IIc IIc

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 + + E (V vs Fc/Fc ) E (V vs Fc/Fc ) II FigureFigure 10. Cyclic 10. Cyclic voltammetry voltammetry of a of 1 amM 1 mM [Co [CoII(4-Cl(4-Cl-terpy)-terpy)2](BF2](BF4)2 solution4)2 solution in MeCN in MeCN with with 0.1 M 0.1 of M of II Figure 10. Cyclic voltammetry of a 1 mM [Co (4-Cl-terpy)2](BF4)2 solution in MeCN with−1 0.1 M− of1 TBAPFTBAPF6, bubbled6, bubbled with withN2 (black N2 (black line) and line) CO and2 (red CO line).2 (red Scan line). rate Scan of 100 rate mVs of 100 obtained mVs fromobtained open from- −1 TBAPF6, bubbled withcircuit open-circuitN2 potential(black line) potentialto cathodic and CO to direction cathodic2 (red line)..direction. Glassy Scan carbon rate Glassy of disk 100 carbon as mVs a working disk obtained as a electrode. working from electrode. open- circuit potential to cathodic direction. Glassy carbon disk as a working electrode.

80 IIIa 60 Ia 80 40 IIIa 60 Ia 20 IIa 40 0 20 IIa -20 0 -40 ) IIIc -60 -20 -1/2 -40 -80 Ic

A V ) IIIc

m -100

-60 ( -1/2 -120 -80 1/2 50 mV Ic 100 mV

i/v

A V -140

m -100 250 mV

( -160 500 mV -120 1/2 -180 50 mV 750 mV 100 mV i/v -140 -200 IIc 1000 mV 250 mV 1500 mV -160 -220 500 mV -180 -2.0 -1.5 -1.0 750 mV -0.5 0.0 0.5 -200 IIc 1000 mV E (V 1500vs Fc/Fc mV +) -220

-2.0 -1.5 -1.0 -0.5 0.0 0.5 II Figure 11. Normalized currentE (V cyclicvs Fc/Fc + voltammetry) of 1 mM [Co (4-Cl-terpy)2](BF4)2 solution in MeCN with 0.1 M of TBAPF6 as supporting electrolyte, bubbled with CO2 obtained from open-circuit potential to cathodic direction. Glassy carbon disk as a working electrode. Figure 11. Normalized current cyclic voltammetry of 1 mM [CoII(4-Cl-terpy)2](BF4)2 solution in MeCN with 0.1 M of TBAPF6 as supporting electrolyte, bubbled with CO2 obtained from open-circuit potential to cathodic direction. Glassy carbon disk as a working electrode.

Catalysts 2021, 11, x FOR PEER REVIEW 10 of 19

3.0 50 mV 4.0 100 mV 250 mV 3.5 2.5 500 mV 3.0 750 mV 1000 mV 2.5 1 -1 -1 2.0 ) k = 5.78 × 10 (M s ) 1500 mV -1 s 2.0

-1

° 1.5 1.5

i / i / i

log k (M k log 1.0 1.0 0.5

0.5 0.0

-0.5 0.0 -1.0 0.0 0.1 0.2 0.3 0.4 0.5 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 1 log n (V/s) 1 + exp[ (F/RT) (E - EMLred / MLox)]

(a) (b)

Figure 9. (a) FOWA of a 1 mM [CoII(terpy)3](BF4)2 complex in MeCN with 0.1 M of TBAPF6 as supporting electrolyte. (b) Logarithmic rate constant obtained from FOWA.

100 IIIa Ia

0 IIa

) IIIc

-1/2 Ic

A V -100

( N 1/2 2 CO i/v 2

-200 IIc

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 E (V vs Fc/Fc+)

Figure 10. Cyclic voltammetry of a 1 mM [CoII(4-Cl-terpy)2](BF4)2 solution in MeCN with 0.1 M of TBAPF6, bubbled with N2 (black line) and CO2 (red line). Scan rate of 100 mVs−1 obtained from open- Catalystscircuit2021, 11 potential, 948 to cathodic direction. Glassy carbon disk as a working electrode. 10 of 19

80 IIIa 60 Ia 40 20 IIa 0 -20 -40 ) IIIc -60

-1/2 -80 Ic

A V

m -100

( -120

1/2 50 mV 100 mV i/v -140 250 mV -160 500 mV -180 750 mV -200 IIc 1000 mV 1500 mV -220 Catalysts 2021, 11, x FOR PEER REVIEW 11 of 19 -2.0 -1.5 -1.0 -0.5 0.0 0.5 E (V vs Fc/Fc+)

II Figure 11. Normalized current cyclic voltammetry of 1 mM [Co (4-Cl-terpy)2](BF4)2 solution in An inspection of the obtained values of the catalyticII constant for CO2 reduction (see Figure 11. NormalizedMeCN current with 0.1 cyclic M of TBAPF voltammetry6 as supporting of 1 electrolyte, mM [Co bubbled(4-Cl with-terpy) CO22obtained](BF4)2 fromsolution open-circuit in Table 1and Figure 12) indicates that the more negative the redox potential for process II MeCN with 0.1 M of TBAPFpotential6 toas cathodicsupport direction.ing electrolyte, Glassy carbon bubbled disk as awith working CO electrode.2 obtained from open-cir- with the absence of a coupled reaction, the higher the value of k. In this sense, the com- cuit potential to cathodic direction. Glassy carbon disk as a working electrode. pound [Co(3,4,7,8An inspection-tm- ofphen) the obtained3]2+ present valuesed the of the highest catalytic value constant of k, forcontrary CO2 reduction to that observed (see in theTable compound1and Figure [Co(4 12) indicates-Cl-terpy) that3]2+ the that more present negativeed a the coupled redox potential reaction for with process a rate II with constant the absence of a coupled reaction, the higher the value of k. In this sense, the compound higher than that of the time2+ window used in the voltammetry experiments (see Figure 11 [Co(3,4,7,8-tm-phen)3] presented the highest value of k,2+ contrary to that observed in S715). In the case of the compound2+ [Co(4,7-dphen-phen)3] , the presence of the aromatic the compound [Co(4-Cl-terpy)3] that presented a coupled reaction with a rate constant ringshigher stabilize thand that the of charge the time on window them, making used in the it a voltammetry better molecular experiments catalyst (see despite Figure 11having). a less negative redox potential value than its analog2+ with the 5,6-dmethyl-1,10-phena- In the case of the compound [Co(4,7-dphen-phen)3] , the presence of the aromatic rings nathrolinestabilized. In the the charge case of on the them, compounds making it a[Co(bipy) better molecular3]2+, [Co(phen) catalyst3 despite]2+, and having[Co(terpy) a less3]2+, it can negativebe inferred redox that potential the electronic value than delocalization its analog with effect the 5,6-dmethyl-1,10-phenanathroline. of three conjugated aromatic rings 2+ 2+ 2+ of theIn theelectrogenerated case of the compounds species in [Co(bipy) process3] II ,increase [Co(phen)d the3] ,catalytic and [Co(terpy) constant.3] , it can be inferred that the electronic delocalization effect of three conjugated aromatic rings of the electrogenerated species in process II increased the catalytic constant.

4.0 3.5 3.0 2.5

) 2.0

-1

-1 1.5

1.0 2+ [Co(3,4,7,8-tm-phen)3] 2+ 0.5 [Co(4,7-dph-phen)3]

log k (M log 2+ 0.0 [Co(phen)3] 2+ [Co(bipy)3] -0.5 2+ [Co(terpy)2] -1.0 2+ [Co(5,6-dm-phen)3] -1.5 -2.0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 log n (V/s)

Figure 12. Rate constant as a function of log v for the Co(II) compounds studied in this work. Figure 12. Rate constant as a function of log v for the Co(II) compounds studied in this work.

2.3. Molecular Orbitals Description To relate the molecular and electronic structure with the molecular catalysis activity of CO2 reduction, DFT calculations of the species involved in the electron transfer processes were carried out, using the appropriate multiplicity according to the experimental evidence and the convergence criteria with the level of calculation used. For the [CoII(L)3]2+ and [CoII(L´)2]2+ cations, calculations were performed considering a total spin quantum number (S) S = 3/2, according to the reported magnetic moments values [34]. For the species involved in the first electron transfer I, the electrogeneration of the species [CoI(L)3]+ and [CoI(L´)2]+, a value of S = 1 was considered. Figure 13a,b present the semi-occupied molecular orbitals (SOMOs) of the species [CoII(bipy)3]2+ and [CoI(bipy)3]+, in which the electronic density is observed to be over the metallic center and the aromatic rings of the ligand. Figure 13c shows the SOMO for the species proposed in process II [CoI(bipy)(bipy−)2]− (S = 1), where the electron density is appreciated exclusively on the aromatic rings, as has been proposed in the literature for the electrochemical reduction of coordinated diiminic ligands [26–31]. For [CoII(phen)3]2+, DFT calculations indicate that SOMO has a higher contribution of the ligands with respect to its analog with bipy (see Figure 14). In the case of the rest of the compound with phenanthroline ligands [CoII(L)3]2+ (see Figures S14–S16), the same behavior is observed. For the reduced species with substituted phenanthroline ligands [CoI(L)3]+ (S = 1) and [CoI(L)(L−)2]− (S = 2), it is observed that the electron density is on the metal center for the first case and exclusively on the ligand

Catalysts 2021, 11, 948 11 of 19

2.3. Molecular Orbitals Description To relate the molecular and electronic structure with the molecular catalysis activity of CO2 reduction, DFT calculations of the species involved in the electron transfer processes were carried out, using the appropriate multiplicity according to the experimental evidence and the convergence criteria with the level of calculation used. For the II 2+ II 2+ [Co (L)3] and [Co (L´)2] cations, calculations were performed considering a total spin quantum number (S) S = 3/2, according to the reported magnetic moments values [34]. For the species involved in the ﬁrst electron transfer I, the electrogeneration of the species I + I + [Co (L)3] and [Co (L´)2] , a value of S = 1 was considered. Figure 13a,b present the II 2+ I + semi-occupied molecular orbitals (SOMOs) of the species [Co (bipy)3] and [Co (bipy)3] , in which the electronic density is observed to be over the metallic center and the aromatic rings of the ligand. Figure 13c shows the SOMO for the species proposed in process II I − − [Co (bipy)(bipy )2] (S = 1), where the electron density is appreciated exclusively on the aromatic rings, as has been proposed in the literature for the electrochemical reduction II 2+ of coordinated diiminic ligands [26–31]. For [Co (phen)3] , DFT calculations indicate that SOMO has a higher contribution of the ligands with respect to its analog with bipy (see Figure 14). In the case of the rest of the compound with phenanthroline ligands II 2+ [Co (L)3] (see Figures S14–S16), the same behavior is observed. For the reduced species I + I − − with substituted phenanthroline ligands [Co (L)3] (S = 1) and [Co (L)(L )2] (S = 2), it is observed that the electron density is on the metal center for the ﬁrst case and exclusively on the ligand in the second case (see Figures S14–S16). This fact can be attributed to the conjugation effect of the three fused aromatic rings in the phenanthroline ligands. For the II 2+ II 2+ [Co (terpy)3] and [Co (4-Cl-terpy)3] , the electron density of the SOMOs is over the metallic center and the aromatic rings of the ligand (see Figure 15 and Figure S17). For the I + I + species [Co (terpy)3] and [Co (4-Cl-terpy)3] , the electronic density is located in the metal- − Catalysts 2021, 11, x FOR PEER REVIEWlic centers. On the other hand, for the species proposed in process II [CoI(terpy)(terpy13 of 19)]

(S = 5/2) and [CoI(4-Cl-terpy)(4-Cl-terpy−)] (S = 3/2), it is observed that the electronic density is located on the ligands.

(a) (b) (c)

Figure 13. SOMO diagrams of the species (a) [CoII II(bipy)2+3]2+, (b) [CoI I(bipy)3+]+, and (c) [CoII(bipy)(bipy−)2]−.− Figure 13. SOMO diagrams of the species (a) [Co (bipy)3] ,(b) [Co (bipy)3] , and (c) [Co (bipy)(bipy )2] .

(a) (b) (c)

Figure 14. SOMO diagrams of the species (a) [CoII(phen)3]2+, (b) [CoI(phen)3]+, and (c) [CoI(phen)(phen−)2]−.

(a) (b) (c)

I Figure 15. SOMO diagrams of the species (a) [CoII(terpy)3]2+, (b) [CoI(terpy)2]+, and (c) [Co (terpy)(terpy−)].

CatalystsCatalysts 2021 2021, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 1313 of of 19 19

(a) (b) (c) Catalysts 2021, 11, 948(a) (b) (c) 12 of 19 II 2+ I + I − − FigureFigure 1 133. .SOMO SOMO diagrams diagrams of of the the species species ( a(a)) [Co [CoII(bipy)(bipy)3]3]2+, ,( b(b)) [Co [CoI(bipy)(bipy)3]3]+, ,and and ( (cc) )[Co [CoI(bipy)(bipy(bipy)(bipy−))2]2]−. .

(a)(a) (b)(b) (c)(c)

II II 2+2+ I I ++ II −− − − FigureFigureFigure 14. 1 144SOMO. .SOMO SOMO diagrams diagrams diagrams of of of the the the species species species (a ( )a(a [Co)) [Co [Co(phen)II(phen)(phen)3]3]3]2+,(, ,( b(b)) [Co [CoI(phen)(phen)3]3]]+, ,,and and ( (c (c) )[Co [Co [CoI(phen)(phen(phen)(phen(phen)(phen−))2]2)]−2. .] .

((aa)) ((bb)) ((cc))

II II 2+2+ I I ++ III −− FigureFigure 15. 1 155.SOMO .SOMO SOMO diagrams diagrams diagrams of of of the the the species species species ( a( )a(a) [Co) [Co [Co(terpy)II(terpy)(terpy)33]]3]2+,,( ,( b(b)) [Co [CoI(terpy)(terpy)(terpy)22]2]]+, ,,and and and ( (c (cc) ))[Co [Co [Co(terpy)(terpy(terpy)(terpy(terpy)(terpy−)])].)]. .

In Table2, the SOMO energy values, along with the catalytic constant values for the CO2-mediated electrochemical reduction k, are presented. It can be seen that with increase in the number of electrons in the species in question, its energy becomes more positive. The compound with most positive energy and the highest accumulation of electrons would be expected to have the highest capacity to electrochemically catalyze CO2, measured through its catalytic constant k. Considering this idea, the complex with the ligands bipy, phen, 3,4,7,8-tm-phen, and 5,6-dm-phen should present very similar catalytic constants k. However, the presence of the coupled reaction decreases the k values. For the case of 2+ the compound [Co(4,7-dphen-phen)3] , the fact that its species with charge accumulation I − − [Co (4,7-dphen-phen)(4,7-dphen-phen )2] presents a highly extended electron density distribution over all its aromatic rings, inferred from SOMO, increases the local reactivity I − sites with CO2. The coupled reaction for the [Co (4-Cl-terpy)(4-Cl-terpy )] species does not allow its SOMO energy value to be considered to establish any tendency. Catalysts 2021, 11, 948 13 of 19

II 2+ Table 2. SOMO energy values for the proposed species for the reduction of [Co (L)3] and II 2+ [Co (L´)2] cations and their catalytic constant values for mediated CO2 reduction.

SOMO Energy k CO2 Qcomplex Chemical Species −1 −1 (kcal/mol) M s Qcomplex II 2+ [Co (bipy)3] −250.2 I + 1 [Co (bipy)3] −117.6 9.24 × 10 2.29 I − − [Co (bipy)(bipy )2] 64.1 II 2+ [Co (phen)3] −244.0 I + 2 [Co (phen)3] −116.1 1.60 × 10 3.50 II − − [Co (phen)(phen )2] 65.7 [CoII(3,4,7,8-tm-phen) ]2+ 3 −227.1 [CoI(3,4,7,8-tm-phen) ]+ 3 −103.4 2.40 × 103 4.56 [CoI(3,4,7,8-tm-phen)(3,4,7,8-tm- − − 65.5 phen )2] II 2+ [Co (5,6-dm-phen)3] −230.0 I + 1 [Co (5,6-dm-phen)3] −110.7 5.19 × 10 3.3 I − − [Co (5,6-dm-phen)(5,6-dm-phen )2] 65.9 [CoII(4,7-dphen-phen) ]2+ 3 −205.1 [CoI(4,7-dphen-phen) ]+ 3 −106.2 1.64 × 102 4.4 [CoI(4,7-dphen-phen)(4,7-dphen- − − 46.4 phen )2] II 2+ [Co (terpy)3] −247.7 I + 1 [Co (terpy)3] −117.7 5.78 × 10 1.1 [CoI(terpy)(terpy−)] 15.8 II 2+ [Co (4-Cl-terpy)3] −259.2 I + [Co (4-Cl-terpy)3] −131.7 n.o. n.o. [CoI(4-Cl-terpy)(4-Cl-terpy−)] −19.4 (bipy = 2,2’-bipyridine), (3,4,7,8-tm-phen = 3,4,7,8-tetramethyl-1,10-phenanthroline), (5,6-dm-phen = 5,6-dimethyl- 1,10-phenanthroline), (4,7-dph-phen = 4,7-diphenyl-1,10-phenanthroline), (terpy = terpyridine), and (4-Cl-terpy = 4-chloro-terpyridine).

Another factor that should be considered to understand the relationship between SOMO energy and catalytic processes is the faradaic efﬁciency, which is calculated by means of bulk electrolysis experiments. Nevertheless, the difference in time scale between cyclic voltammetry and bulk electrolysis experiments could give different mechanisms and products. Besides, sophisticated analytical techniques to identify and quantify the product are needed. Hence, a simple strategy with similar information can be proposed. Considering that CO2 is not reduced directly to the electrode but in a catalytic pathway by the electrolyzed complex, we propose the use of the charges for the reduction of CO2 complexes in the absence (Qcomplex ) and presence of CO2 (Qcomplex ), obtained from cyclic voltammetry experiments. The following ratio can be used as a measurement of the efﬁciency of the CO2 transformation:

CO2 CO2 Qcomplex mol complex = (21) Qcomplex mol complex

This ratio can be used as an approach for the ability to regenerate the reduced complex by its reaction with CO2 in a cyclic voltammetry time scale, which includes several processes such as the deactivation of the catalyst. Table2 shows the calculated ratios for Co(II) complexes. The low values for the complex with the ligands bipy, phen, and 5,6-dm-phen in comparison with the complex containing the ligand 3,4,7,8-tm−phenanthroline is related with the diminution of the catalytic constant k for similar SOMO values. With the same level of theory, the boundary orbital energies of CO2 and related species in its reduction were calculated (see Table3). As an approximation, a charge transfer of the Catalysts 2021, 11, 948 14 of 19

species with charge accumulation in the cobalt complexes through their SOMOs with the •− 2− LUMOs of the CO2, CO2, CO2 , and CO2 can be considered for the molecular catalysis I − − of CO2. The SOMO values for the [Co (L)(L )2] species were about 65 kcal/mol, with these being very similar to the LUMO energy values of CO2, conﬁrming that the ﬁrst step •− 2− involves the reduction of CO2 directly. The LUMO energy values for the CO2 and CO2 species suggest that their reduction requires their coordination to lower their activation energy. This interaction can be through either their oxygen or carbon atoms, as shown in Figures S18 and S19, depending on the symmetry and energy of the available orbitals of the metallic center.

Table 3. HOMO, SOMO, and LUMO energy values for species involved in CO2 reduction.

HOMO Energy SUMO Energy LUMO Energy Chemical Species (kcal/mol) (kcal/mol) (kcal/mol)

CO2 −145.22 − 66.52 •− CO2 − 197.64 320.88 2− CO2 459.64 − 571.33

2.4. Identiﬁcation of Products

To identify the products from the molecular catalysis of CO2 reduction with cobalt complexes, macro-electrolysis experiments were carried out in a two-compartment cell, applying a potential value of -2.2 V vs. Fc/Fc+. A color change was observed in the solution, from pale yellow to intense red. We explored the presence of methanol, ethanol, and carbon monoxide by means of UPLC (ultra-performance liquid chromatography) and gas-chromatography (see experimental section for conditions). However, none of these products were detected. From the solution obtained in macro-electrolysis experiments, a white precipitate was obtained and analyzed with Fourier Transform infrared (FT-IR) spectroscopy in Attenuated Total reﬂection (ATR) mode. A broad signal around 1630 cm−1 was detected. The anhydrous conditions and the FT-IR response, similar to Na2C2O4 spectra, suggest that oxalate is the main product of CO2 reduction with Co(II) complexes

3. Methodology 3.1. Synthesis and Characterization of Coordination Compounds Coordination compounds were synthesized and characterized according to the literature [34]. The characterization was carried out by means, UV-vis, Thermo Evolution Array spectrophotometer (Massachusetts-USA), magnetic susceptibility, Johnson-Matthey (London-UK) DG8 5HJ balance, Raman spectroscopy, Thermo Scientiﬁc DXR Raman microscope (Massachusetts-USA), and infrared spectroscopy, Shimadzu IRAfﬁnity-1S spectrophotometer (Kyoto-Japan).

3.2. Electrochemical Characterization The electrochemical studies were carried out using a potentiostat/galvanostat Biologic SP-300 (Lyon-France), using 1 mM solutions of each coordination compound in ultra-dry acetonitrile (MeCN), with 0.1 M of tetrabutylammonium hexaﬂuorophosphate (TBAPF6) as a supporting electrolyte. Before preparing the solutions, all complexes were dried at 100 ◦C in a vacuum oven for 24 h. A conventional cell with a three-electrode arrangement was used. Glassy carbon (Φ = 3 mm) was used as the working electrode (WE). Platinum wire was used as a counter electrode (CE) and silver wire as a pseudo-reference electrode (RE). The reference electrode was prepared with a silver wire immersed in 0.1 M of silver nitrate solution (AgNO3) in MeCN within a separate compartment, connected through a Vycor membrane. All potentials are referenced to the Fc/Fc+ couple, as recommended by the IUPAC [35]. Catalysts 2021, 11, 948 15 of 19

Prior to each measurement, the solutions were bubbled with nitrogen (N2) or carbon dioxide (CO2) for 5 min. The cleaning of the working electrode was carried out by means of manual polishing with 0.1 µm diamond powder, and then rinsed with water and placed in an ultrasonic bath for one minute, to be rinsed again with water and dried with acetone. Voltamperograms were obtained at different scan rates from the open-circuit potential (Eoc) to the cathodic direction. Ohmic drop compensation (iR) was performed by the positive feedback method. The calculation of the solution resistance (Ru) was determined by the impedance measurement (ZIR). The determination of the catalytic constant of the system was evaluated using the foot of the wave analysis (FOWA) [36].

3.3. DFT Calculations For redox mediators, density functional theory [37–39] calculations were implemented using Gaussian 09 [40]. Full geometry optimization without symmetry constraints was performed using the B3LYP [41–43] density functional, and Lanl2mb [44,45] basis sets were used. Optimized geometries of local minima were veriﬁed by the number of imaginary frequencies (which should be zero). Previous studies have indicated that DFT results are accurate to describe stabilities and equilibrium geometries for B3LYP density functional and Lanl2mb basis set combinations [46].

3.4. Product Identiﬁcation

Here, 5 mL of CO2-saturated acetonitrile solution containing 1 mM of complex and 0.1 M of TBAPF6 in a two-compartment cell with Toray carbon paper electrodes with an area of 2 cm2 was used for macro-electrolysis experiments. The potentiostat/galvanostat Biologic SP-300 was used, applying a potential at −2.2 V vs. Fc/Fc+ V for 10 min. A color change was observed in the solution, from pale yellow to intense orange. Methanol and ethanol analyses were carried out in an ultra-performance liquid chromatography system (UPLC) model Acquity H-Class (Waters®, Mildford, MA, USA). The UPLC system had a cooling autosampler, a quaternary solvent manager, a 2414 Model Refractive Index (RI) detector, and an oven for the analytical column. The chromatographic determination was made in a Biorad Aminex HPX-87H cation-exchange column (300 mm × 7.8 mm), using a mobile phase of 0.008 N of sulfuric acid at a flow rate (isocratic mode) of 0.4 mL/min, an oven temperature of 30 ◦C, a temperature cell RI of 35 ◦C, and an injection volume of 10 mL. Peak heights were measured using Empower3 chromatography ® software (Waters , Mildford, MA, USA). Ethanol or methanol:H2O solutions containing 0.7:0.7, 1.5:1.5, 3.1:3.1, 6.2:6.2, 12.5:12.5, 25:25, and 50:50% were used as reference solutions for the calibration curve (three replicates). Ethanol or methanol were identified by comparison of the retention times. All reference solutions and samples were filtered using a 0.2 mm nylon membrane. For carbon monoxide identification, a CG Thermo Scientific Trace 1300 model (Thermo Scientific, Madison, USA) was used. For separating the target gas component, a Traceplot TG-BOND Msieve 5A, 0.32 mm × 30 cm−1 × 30 m, was employed. The detection was performed by using a thermal conductivity detector (TCD) and the output signal was monitored using Chromeleon Software (Version 7.0). For measurements, a 50 µL gas sample was injected using a gas-tight syringe Hamilton (Reno-USA) via the injection port. The injector and detector temperatures were 120 ◦C and 100 ◦C, respectively. For GC separation, the sample was injected via the sample loop into the separation column, which was heated at 50 ◦C, with a split ratio of 30. Ultrapure He at a flow rate of 5.0 mL min−1 was used as a carrier gas. The data were estimated by automated integration of the area under the resolved chromatographic profile using Chromeleon Software (Version 7.0). FT-IR measurements were carried out in a Shimadzu IRAffinity-1S spectrophotometer (Kyoto-Japan) with an ATR module and a range from 4500 to 650 cm−1. Catalysts 2021, 11, 948 16 of 19

4. Conclusions II 2+ The redox behavior was explored for tris and bis chelate Co(II) complexes [Co (L)3] II 2+ + 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). DFT calculations gave a reliable description II 2+ III 3+ for the electronic distribution of the chemical species generated [Co (L)3] , [Co (L)3] I + I − − II 2+ III 3+ I + I − [Co (L)3] , [Co (L)(L )2] , [Co (L’)2] , [Co (L’)2] , [Co (L’)2] , and [Co (L’)(L’ )]. It was demonstrated that for cobalt complexes, negative redox potential values for ligand reduction, and the absence of coupled reaction, increased the value of k. The 2+ compound [Co(3,4,7,8-tm-1,10-phen)3] presented the highest k value of all the complexes 2+ studied in this work; moreover, the complex [Co(4-Cl-terpy)3] did not show catalytic activity. Electrogenerated cobalt species with a high accumulation of electrons and more positive SOMO energy showed a high catalytic constant k. For complexes with the same SOMO energies, the presence of coupled chemical reactions diminished their values of catalytic constant k. QCO2 The ratio complex was proposed as the measurement of the efﬁciency of the CO trans- Qcomplex 2 formation. Low values of this ratio are related with a diminution of catalytic constant k. The anhydrous conditions in electrochemical experiments and the evidence obtained from analytical techniques allowed us to suggest that oxalate is the main product of CO2 reduction with Co(II) complexes.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10.3 390/catal11080948/s1. Figure S1: Normalized current cyclic voltammetry of 1 mM [CoII(bipy)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2 obtained from open circuit potential to cathodic direction. Glassy carbon disk as a working electrode. Figure S2. Normalized current cyclic voltammetry of 1 mM [CoII(phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2 obtained from open circuit potential to cathodic direction Glassy carbon disk as working electrode. Figure S3. Normalized current cyclic voltammetry of 1 mM [CoII(3,4,7,8-tm-phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2. Glassy carbon disk as a working electrode. Figure S4. Normalized current cyclic voltammetry of 1 mM [CoII(4,7-dphen-phen)3](BF4)2solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2. Glassy carbon disk as a working electrode. Figure S5. Normalized current cyclic voltammetry of 1 mM [CoII(5,6-dm-phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2. Glassy carbon disk as a working electrode. Figure S6. Normalized current cyclic voltammetry of 1 mM [CoII(terpy)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2 obtained from open circuit potential to cathodic direction Glassy carbon disk as a working electrode. Figure S7. Normalized current cyclic voltammetry of 1 mM [CoII(4-Cl-terpy)2](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2 obtained from open circuit potential to cathodic direction. Glassy carbon disk as a working electrode. Figure S8. Cyclic voltammetry of a 1 mM [CoII(3,4,7,8-tm-phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2 (black line) and CO2 (red line). Scan rate 100 mVs−1 from open circuit potential in cathodic direction. Glassy carbon disk as a working electrode. Figure S9. Normalized current cyclic voltammetry of 1 mM [CoII(3,4,7,8-tm-phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with CO2 from open circuit potential in cathodic direction. Glassy carbon disk as a working electrode. Figure S10. Cyclic voltammetry of a 1 mM [Co(4,7-diphen-phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, −1 bubbled with N2 (black line) and CO2 (red line). Scan rate 100 mVs from open circuit potential in cathodic direction. Glassy carbon disk as a working electrode. Figure S11. Normalized current cyclic voltammetry of 1 mM [Co(4,7-diphen-phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with CO2 from open circuit potential in cathodic sense. Glassy carbon disk as a working electrode. Figure S12. Cyclic voltammetry of a 1 mM [Co(5,6-dm-phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with N2 (black line) and −1 CO2 (red line). Scan rate 100 mVs from open circuit potential in cathodic sense. Glassy carbon disk as a working electrode. Figure S13. Normalized current cyclic voltammetry of 1 mM [Co(5,6-dm- Catalysts 2021, 11, 948 17 of 19

phen)3](BF4)2 solution in MeCN with 0.1 M TBAPF6 as supporting electrolyte, bubbled with CO2 from open circuit potential in cathodic sense. Glassy carbon disk as a working electrode. Figure S14. SOMO diagrams of the species (a) [CoII(3,4,7,8-tm-phen)3]2+, (b) [CoI(3,4,7,8-tm-phen)3]+ and (c) [CoI(3,4,7,8-tm-phen)2(3,4,7,8-tm-phen−)2]−. Figure S15. SOMO diagrams of the species (a) [CoII(5,6- dm-phen)3]2+, (b) [CoI(5,6-dm-phen)3]+ and (c) [CoI(5,6-dm-phen)2(5,6-dm-phen−)2]−. Figure S16. SOMO diagrams of the species (a) [CoII(4,7-dphen-phen)3]2+, (b) [CoI(4,7-dphen-phen)3]+ and (c) [CoI(4,7-dphen-phen)2(4,7-dphen-phen−)2]−. Figure S17. SOMO diagrams of the species (a) [CoII(4-Cl- terpy)2]2+, (b) [CoI(4-Cl-terpy)2]+ and [CoI(4-Cl-terpy)(4-Cl-terpy−)]. Figure S18. Diagrams (a) HOMO •− 2− •− of CO2, (b) SOMO CO2 and (c) HOMO of CO2 . Figure S19. LUMO diagrams (a) CO2, (b) CO2 2− and (c) CO2 . Author Contributions: J.P.F.R.-C., cobalt (II) DFT calculations and FOWA analyses; G.T.T., prepara- tion of cobalt complexes; V.R.-D., electrochemical studies, writing—reviewing and editing; Y.R.-V., analytical techniques for product identification; M.L.J.-G., bulk electrolysis experiments; M.C.-R., electrochemical experiments; A.M., reviewing and editing inorganic chemistry aspects; L.O.-F., head of the research, conceptualization, writing—reviewing and editing. All authors have read and agreed to the published version of the manuscript. Funding: Apoyo a la Infraestructura 2016 (269102), CONACyT-SEP- Ciencia Básica (288069), Labora- torio Nacional de Supercómputo del Sureste de México (LNS), Project No. 2018030022c. Acknowledgments: The authors thank CONACyT “Apoyo a la Infraestructura” (269102) and “CONACyT-SEP- Ciencia Básica” (288069). JPFRC thanks CONACYT for scholarship. The authors thankfully acknowledge computer resources, technical advice, and support provided by Laboratorio Nacional de Supercómputo del Sureste de México (LNS), a member of the CONACYT national laboratories Project No. 2018030022c. Conflicts of Interest: The authors declare no conflict of interest.

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