Substituents effects on the electrocatalytic CO₂ reduction by cobalt corroles in solution

Abstract

Electrochemical CO₂ reduction catalysis with cobalt corrole complexes in solution is reported. Corroles have attracted attention as contracted and trianionic tetrapyrrolic macrocycles that can be compared to leading porphyrin catalysts for CO₂ reduction, but most studies focus on heterogenized systems with poorly defined electrochemical responses. Electrochemical studies of cobalt corroles bearing axial triphenylphosphine ligands to ensure solubility are reported. The voltammetry provides mechanistic insights supporting CO₂ activation after the formal Co^II/Co^I reduction. The series of cobalt complexes, including a newly designed corrole with mixed perfluorophenyl/ortho-dimethoxyphenyl substituent pattern, provide evidence for electron-rich catalysts having stronger interactions with CO₂. The primary product of CO₂ reduction is CO, formed at a rate of ca. 90 s⁻¹.

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The electrocatalytic reduction of CO₂ to energy-dense compounds is currently being pursued as a sustainable approach for manufacturing fuel (sometimes called eFuel).^1,2 Presently, a multitude of catalytic materials are being studied to achieve a shift in the global energy paradigm;³ and molecular catalysts based on 3d transition metals have attracted interest (in part inspired by analogies to biological energy storage enzymes).^4,5 Metal complexes of porphyrins and phthalocyanines are among the most effective molecular catalysts for CO₂ reduction,^6–8 shown to be capable of catalyzing its transformation into a range of reduced forms, like CO,⁴ HCOO⁻,⁹ CH₃OH,¹⁰ and CH₄¹¹ with high turnover frequencies at moderate overpotentials. There is also increasing activity in employing corroles as metal-chelating ligands for the development of CO₂ reduction catalysts.¹² Corroles act as tri-anionic ligands and supply a coordination core that is about 10% contracted relative to related dianionic tetrapyrrole macrocycles, two factors that can stabilize complexes in high oxidation states while generating complexes that are more reducing or nucleophilic in low oxidation states.^13–15 The first report regarding utilization of cobalt and iron corroles for CO₂ reduction, performed under homogenous conditions (organic solvent, non-modified electrodes) with a comparison to analogous porphyrin complexes,¹⁶ disclosed that electrocatalysis by the former relies on the M⁺¹ rather than the M⁰ oxidation state.¹⁷ This was followed much later by an investigation deducing small benefits of remote OCH₃ groups present in superstructured derivatives.¹⁷ Most recent reports focused on heterogeneous catalysis (aqueous solutions, catalyst-modified electrodes), performed by depositing cobalt corroles substituted with S-PEG-(7)-OMe anchoring groups on a carbon paper electrode.^12,18 Highly reduced products, such as methanol, ethanol, and acetic acid were observed at relatively low overpotentials at pH 6.¹² High selectivity towards CO production was observed for cobalt corrole catalysts bound to carbon nanotubes and deposited on carbon electrodes.^19,20

The earlier studies are promising but also raise questions. In solution, only CO has been observed as a carbon-containing product of CO₂ reduction, while on surfaces either CO or highly reduced C₁ and C₂ products are reported. Mechanistic insight has so far been limited, particularly on surfaces where voltammetry is not typically well-defined. And relatively few structural variations have been made, limiting insight into how corrole substituents impact the electrocatalytic performance and alter the reaction mechanism. Studies of well-defined corroles with good solubility can provide important performance comparisons between homogeneous and heterogeneous systems, as well as providing mechanistic insight that is possible with homogeneous molecular catalysis.²¹

The goals of the present study were to deduce how structural and electronic effects of cobalt(III) corrole complexes affect their performance as CO₂ reduction catalysts. Chart 1 shows the catalysts examined. Two known complexes were included: 1a, with three electron-withdrawing C₆F₅ groups, and the much smaller and more electron-rich 1d, free of any substituent.^22,23 Two complexes were newly designed with either one (1b) or three (1c) 2,6-dimethoxyphenyl groups. The remote OCH₃ moieties might facilitate protons transfer to the active site or stabilize intermediates, while also providing some steric shielding and increasing electron density. Complex 1c is most electron-rich while 1b has only one 2,6-dimethoxyphenyl substituent but two C₆F₅ groups whose role is to make the complex easier to reduce.

Chart 1 The (triphenylphosphine)cobalt(III) corrole complexes investigated in this work, for deducing the effects of meso-substituents on electrocatalytic CO₂ reduction.

The new 5-coordinate low-spin d⁶ (PPh₃)Co^III complexes (1b, 1c) were prepared via metallation of their respective free-base corroles,^24–26 and characterized by high resolution mass spectrometry, UV-vis, ¹H-NMR, and elemental analysis (Fig. S1–S13, ESI†). Their electronic spectra (Fig. 1a) display characteristic split Soret and Q-bands that are distinct from those of 6-coordinate bis-pyridine cobalt(III) corroles.²⁷ Axial PPh₃ coordination increases the stability of the pre-catalysts and has been shown to influence the reactivity of cobalt corrole complexes in the catalytic O₂ reduction reactions.^28,29

Fig. 1 The (a) electronic spectra (in ethyl acetate) of the (triphenylphosphine)cobalt(III) corroles 1a (black), 1b (green), 1c (blue), and 1d (red) and ORTEP presentation (pink-cobalt, blue-nitrogen, gray-carbon, red-oxygen, orange-phosphorus) of the X-ray crystal structures obtained for (b) the metal-free corrole of complex 1b and (c) complex 1c.

X-ray quality crystals were obtained for free-base 1b (Fig. 1b). Notably, the corrole macrocycle exhibits significant distortion from planarity to reduce steric hindrance between the inner protons. In the resulting structure, the NH protons of rings B and D deviate from the mean plane of the four nitrogen atoms by +0.72 Å and −0.47 Å, respectively, while the NH proton of ring C remains nearly in-plane, with a deviation of only +0.009 Å, while in free base 1a,³⁰ these values +0.89 Å and −0.46 Å, and 0.1 Å for ring B. These distortions position the hydrogens at minimal van der Waals distances of 2.07–2.33 Å. Single crystals of complex 1c were also obtained (by slow evaporation from a DCM/heptane/methanol solvent mixture), enabling a solid-state structure determination (Fig. 1c). Examination of the corresponding data reveals quite large angles between the corrole and the 2,6-dimethoxyphenyl groups (70.3°–80.2°) which brings the remote OCH₃ moieties quite close to the center of the N4 coordination core (Fig. 1c). The cobalt ion is in a square pyramidal geometry placed 0.287 Å above the N₄ plane and 0.381 Å from 23 atoms plane (Table 1). The average Co–N bond length is 1.87 Å, and the Co–P distance is 2.207 Å. The through-space distances of the aryl's ortho-methoxy O atoms are 5.143 Å to Co, 2.737 Å to the meso-C atom, and 4.757 Å to the C atom attached to phosphorus which would be most representative of the oxygen atom in the putative Co–CO₂ adduct. The complex shows high deviation of the cobalt ion from the mean plane defined by the 4-core nitrogen atoms and the 23-core atoms as compared to other cobalt corrole (Table 1).

Table 1 Comparison of structural metrics for three of the investigated cobalt corroles

Complex	ΔM4^a (Å)	ΔM23^b (Å)	Co–P (Å)	Co–N^c (Å)
a Deviation of the cobalt ion from the mean plane defined by the 4-core nitrogen atoms.b Deviation of the cobalt ion from the mean plane defined by the 23-ring atoms.c Average bond length of the cobalt(III) ion with the four equatorial N atoms.d Data from ref. 17.e Data obtained in this work.f Data from ref. 18.
1a^d	0.262	0.324	2.210	1.878
1c^e	0.287	0.381	2.207	1.873
1d^f	0.278	0.374	2.209	1.872

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The four cobalt corroles were studied by cyclic voltammetry (CV) in acetonitrile solution with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) as supporting electrolyte. Each complex displays two reductions, which previous work has suggested are both metal-centered.²² At a scan rate of 100 mV s⁻¹, all corrole complexes show quasi-reversible or fully irreversible Co^III/II features with peak potentials (E_pc) of −0.80, −0.93, −1.25 and −0.99 V vs. Fc^+/0 for 1a, 1b, 1c and 1d, respectively (Fig. 2). The correlation between increased irreversibility and the electron-donating capability of the ligand is attributed to faster and/or more favorable dissociation of the axial PPh₃ ligand upon formation of a d⁷ Co^II center. Its reoxidation (E_pa) hence takes places either on the 5-coordinate PPh₃-bound complex (small peak-to-peak separation) or on the 4-coordinate PPh₃-free Co^II corrole (large peak-to-peak separation).²⁸ Both processes apparently occur for 1b and 1d, while for the most electron-poor and hence most Lewis acid 1a the PPh₃ remains mostly bound and for 1c the PPh₃ is completely dissociated upon reduction. Varying the scan rate of the CV of 1c reveals an anodic shift of the Co^III/II irreversible wave at slower scan rates, consistent with ligand dissociation after reduction (Fig. S14 (ESI†). More reducing potentials lead to reversible features at −1.82, −1.96, −2.20 and −2.12 V vs. Fc^+/0 which have been assigned to the formal Co^II/I redox couples of 1a, 1b, 1c and 1d, respectively. On top of affecting the Lewis acidity, the substituents on the meso-carbon induce a monotonic electrochemical response within the examined series.³¹ Both the Co^III/II and Co^II/I processes appear more negative as the electron-donating character of the ligand is increased. Focusing on the reversible Co^II/I process, this translates into a 360 mV more positive E_1/2 in the case of 1a compared to 1c (three C₆F₅ vs. 2,6-dimethoxyphenyl groups) and the same process for 1c is 60 mV more negative relative to 1d. The dianionic 4-coordinate d⁸ cobalt(I) corroles with two electrons in the high energy d_z² orbital may be anticipated to have a high affinity towards electrophiles in general and carbon dioxide in particular.¹²

Fig. 2 Cyclic voltammograms (100 mV s⁻¹) of the Co corrole complexes (1 mM) 1a (black), 1b (green), 1c (blue), and 1d (red) in anhydrous MeCN/TBAPF₆, 0.1 M electrolyte solution under N₂ atmosphere.

Examination of the CVs of the complexes in MeCN containing 1% H₂O under a nitrogen atmosphere uncovered that both the first and second reduction waves were unaffected (Fig. 3, red traces). This observation suggests that the electrochemically generated Co^II and Co^I species are not sufficiently reactive toward proton reduction, i.e. that they not catalyze the hydrogen evolution reaction (HER) under these conditions. In contrast, when the atmosphere was switched from N₂ to CO₂ in the presence of 1% H₂O the second reduction wave-corresponding to the Co^II/I redox couple-became irreversible with a notable increase in current (Fig. 3, blue traces). This implies a strong interaction between the electrochemically generated Co^I species and CO₂, likely resulting in the formation of [Co–CO₂]²⁻ or [Co–CO₂H]⁻ adducts. The complexes bearing electron-withdrawing C₆F₅ substituents (1a and 1b) did not exhibit an anodic shift in the Co^II/I couple when moving from N₂ to CO₂ atmosphere, suggesting quite a weak interaction with CO₂. In contrast, the much more electron-rich complexes (1c and 1d) displayed pronounced anodic shifts (Fig. 3c and d), consistent with enhanced rate constants for CO₂ binding due to high nucleophilicity of metal centre.

Fig. 3 Cyclic voltammograms recorded for (a) 1a, (b) 1b, (c) 1c and (d) 1d dissolved in anhydrous CH₃CN under N₂, 1% H₂O under N₂, and 1% H₂O under CO₂ (the black, red and blue traces, respectively).

Following the indications regarding CO₂ binding to the cobalt(I) corroles, an increase in current consistent with electrocatalysis was observed at potentials slightly more negative than the Co^II/I redox couple. These were also much better defined and most pronounced for 1c and 1d, with the latter reaching current densities of up to 2.05 mA cm⁻² (Fig. 3, blue traces). Full analysis of catalysis by 1d is difficult since the curve crossing observed in its CV suggests a side reaction. But catalyst 1c has a well-defined voltammogram that includes a plateau shape, which enables a rate analysis to determine the observed rate constant for the turnover-limiting catalytic step (k_obs), which is equivalent to the turnover frequency under these conditions (TOF_max). Based on the ratio of the catalytic current (i_c) to the current of the second reduction (i_p) at scan rates where i_c/i_p becomes nearly constant, the rate constant for the catalytic process was estimated as k_obs = 90 s⁻¹ (Fig. S16, S17 and Table S2, ESI†). It is likely that 1d has similar or even larger activity, but its voltammogram did not meet the criteria for quantitative analysis. This set of observations is in line with previous reports where Co^I was identified as the active species for CO₂ binding, leading to electrocatalytic CO₂ reduction.

These investigations were followed by identifying and characterizing the gaseous products obtained by performing controlled potential electrolysis (CPE, Fig. S18, ESI†) over 20 minutes with all the catalysts in the series in CH₃CN solution containing 1 mM catalyst and 1% H₂O under CO₂ atmosphere. The results summarized in Table 2 show that almost no products were obtained via catalysis by 1a, while for the other three catalysts CO gas was the major product, accompanied by a very small amount of H₂. Similar performance was observed for catalysts 1b and 1c. The latter aspect is fully consistent with the earlier described indications (the red traces in Fig. 3), which is important since CO₂ reduction catalysis is very often hampered by HER activity.³² Increasing the H₂O content to 10% or adding phenol (50 mM) as a stronger acid did not improve the CO₂ reduction selectivity to CO for 1c (Table S4, ESI†). That 1a is the most sluggish catalyst is also in accord with the rather small catalytic waves seen in Fig. 3a, which suggests that while having electron-withdrawing meso-C substituents is beneficial for accessing the cobalt(I) state at less negative potentials that also decreases its reactivity regarding CO₂ activation. Comparison of the three much more electron-rich complexes suggests that the hanging OCH₃ moieties present in catalysts 1b and 1c are not advantageous, at least under the conditions of limited amounts of a weak proton donor like water. In fact, complex 1d with no meso-C substituents provided somewhat better results in terms of both faradaic efficacy (FE) and diminished HER activity. A rinse test was performed with the best-behaved catalyst 1c, with the electrode surface rinsed well with solvent after an electrolysis before a second CPE was conducted in electrolyte containing no dissolved catalyst. Almost no current passed in the second electrolysis (Fig. S19, ESI†), suggesting that no active heterogeneous catalyst was deposited onto the surface during CPE.

Table 2 Results after 20 min CPE under CO₂ with 1% H₂O in MeCN TBAPF₆ electrolyte. E_CPE is the applied potential in V vs. Fc^+/0

Cat	E1/2 vs. Fc^+/0	ECPE (V)	FECO^a (%)	FEH2^a (%)
a The faradaic efficiency values for CO and H2 (FECO and FEH2) are based on headspace analysis.b Performing CPE for 150 min provides 11% CO and 2% H2.
1a^b	−1.86	−2.2	<1	<1
1b	−1.96	−2.2	31	2
1c	−2.20	−2.4	33	1
1d	−2.12	−2.4	37	<1

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In summary, we have investigated electrochemical CO₂ reduction by Co^III corrole complexes that differ significantly in terms of the meso-C substituents: electron-withdrawing C₆F₅ substituents only, combinations of those and electron-donating 2,6-dimethoxyphenyl groups, and no substituents. None of the complexes efficiently catalyzed hydrogen evolution under homogenous conditions with 1% H₂O as proton source, but all the three electron-rich complexes facilitated electrocatalytic CO₂ reduction to CO with moderate FE. The studies here uncover the importance of delicate tuning of redox potentials for achieving good catalytic activity and introduce one new catalyst that proved to be the best-defined for rate analysis and provide insight into the mechanism of CO₂ reduction with soluble corroles.

It is noteworthy that no highly reduced products were detected, whereas products including methanol, ethanol, and acetic acid have been observed in heterogenized corroles in low to moderate faradaic efficiency.^12,18–20 The conditions are slightly different between those reports and the present study, with fully aqueous conditions possible with heterogeneous catalysis while mixed acetonitrile/water was used here to ensure homogeneous conditions. Specific interactions with the support materials may also give rise to changes in electronic structure, as proposed for Co phthalocyanine catalysts.³³ While further studies are needed to fully elucidate differences in homogeneous vs. heterogeneous CO₂ reduction with corroles, this work again shows that the product distribution can differ significantly between the systems. Whereas experimental analysis of individual reaction steps is difficult in the heterogeneous catalysts, the homogeneous system described here enabled insight into the ligand dissociation and CO₂ binding steps in the mechanism of this emerging class of CO₂ reduction electrocatalysts.

This research was supported by the grants from the Israel Ministry of Innovation Science and Technology (MOST) and from the Israel Council for Higher Education-VATAT (grant #1024460). Electrochemical studies were supported by the Center for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0021173. Some NMR spectroscopy work was conducted in the University of North Carolina's Department of Chemistry NMR Core Laboratory, supported by the U.S. National Science Foundation under grant no. CHE-1828183. S. F. acknowledges the Ramón Areces Foundation for a postdoctoral research fellowship.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

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Footnote

† Electronic supplementary information (ESI) available: Characterization like UV-Vis, ¹H NMR, mass and CV Data. CCDC 2421462 and 2421463. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5cc02717a

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