# Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Jing Zhang, Ping She, Qiang Xu, Fengkun Tian, Heng Rao, Jun-sheng Qin,

Julien Bonin, Marc Robert

► To cite this version:

Jing Zhang, Ping She, Qiang Xu, Fengkun Tian, Heng Rao, et al. Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst. ChemSusChem, 2024, 17 (12), pp.e202301892. 10.1002/cssc.202301892. hal-04703878

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# Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Jing Zhang<sup>+</sup>,<sup>[a]</sup> Ping She<sup>+</sup>,<sup>[a]</sup> Qiang Xu,<sup>[a]</sup> Fengkun Tian,<sup>[a]</sup> Heng Rao,<sup>*[a]</sup> Jun-Sheng Qin,<sup>*[a]</sup> Julien Bonin,<sup>[b]</sup> and Marc Robert<sup>*[b, c]</sup>

Inspired by natural enzymes, this study presents a nickel-based molecular catalyst, $[Ni^{l}(N_{2}S_{2})]Cl_{2}$ ($NiN_{2}S_{2}$, $N_{2}S_{2}$ = 2,11-dithia[3,3](2,6)pyridinophane), for the photochemical catalytic reduction of $CO_{2}$ under visible light. The catalyst was synthesized and characterized using various techniques, including liquid chromatography-high resolution mass spectrometry (LC-HRMS), UV-Visible spectroscopy, and X-ray crystallography. The crystallographic analysis revealed a slightly distorted octahedral coordination geometry with a mononuclear $Ni^{2+}$ cation, two nitrogen atoms and two sulfur atoms. Photocatalytic $CO_{2}$ reduction experiments were performed in homogeneous conditions using the catalyst in combination with $[Ru(bpy)_{3}]Cl_{2}$.

($bpy$=2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. The catalyst achieved a high selectivity of 89% towards CO and a remarkable turnover number (TON) of 7991 during 8 h of visible light irradiation under $CO_{2}$ in the presence of phenol as a co-substrate. The turnover frequency (TOF) in the initial 6 h was 1079 $h^{-1}$, with an apparent quantum yield (AQY) of 1.08%. Controlled experiments confirmed the dependency on the catalyst, light, and sacrificial electron donor for the $CO_{2}$ reduction process. These findings demonstrate this bioinspired nickel molecular catalyst could be effective for fast and efficient photochemical catalytic reduction of $CO_{2}$ to CO.

## Introduction

$CO_{2}$ resource exploitation has emerged as a significant way to contribute to "carbon neutrality" and to develop clean energy technology. In Nature, green plants employ chlorophyll to convert solar energy into chemical energy through photosynthesis while fixing $CO_{2}$ from the atmosphere to generate carbohydrates. They produce more than ca. 100 billion tons of biomass every year, which is equivalent to ca. 100 terawatts of stored energy. Therefore, converting solar energy into chemical energy and fixing $CO_{2}$ in the environment through artificial photosynthesis has emerged as a promising way to address energy issues.

$CO_{2}$ reduction is an energy demanding transformation for which catalysts are required.<sup>[1]</sup> Molecular catalysts mainly consist of metal-organic complexes which offer high atomic utilization efficiency, various structural modifications, high catalytic activity and selectivity. Additionally, molecular catalysts allow advanced spectroscopy analysis for multi-step reactions involving several electron transfers and intermediates during the reaction process. Homogeneous photocatalytic $CO_{2}$ reduction systems employing molecular catalysts have made significant progress in recent years. However, they often consist of noble metal centers, such as Re,<sup>[2]</sup> Ru,<sup>[3]</sup> and Ir,<sup>[4]</sup> which are rare and costly, preventing possible scale-up. Therefore, non-precious metal-based catalysts have attracted intense attention in recent years. They are widely used in catalysis research because they can both act as nucleophiles or electrophiles in chemical reactions, forming intermediates and lowering the activation energy. Recently, impressive results have been achieved with molecular catalysts based on Fe,<sup>[5]</sup> Co,<sup>[6]</sup> Ni<sup>[1d,7]</sup> or Cu<sup>[8]</sup> for $CO_{2}$ reduction. Among many examples, iron porphyrins with trifluoromethanesulfonic acid substituents exhibited good reduction activity and could selectively catalyze the reduction of $CO_{2}$ to CO with a turnover number (TON) of 5500.<sup>[5d]</sup> A water soluble cobalt porphyrin was reported to achieve a high $CO_{2}$ reduction activity with TON > 8000 in pure water system.<sup>[6d]</sup> A copper quaterpyridine catalyst showed even better photocatalytic activity for CO production, with a TON of 12400.<sup>[8a]</sup> In addition to CO, formic acid was also obtained as a 2-electron reduction product. For example, a Ni-based catalyst was recently reported with a TON of 14000 for formic acid.<sup>[1d]</sup> Most of the efficient non-noble metal molecular catalysts reported so far have improved stability due to their nitrogen atoms able to coordinate and stabilize metals, such as porphyrins<sup>[5e-n,6b-f-9]</sup> and phthalocyanines.<sup>[10]</sup> Other molecular catalysts have also used

<footnote>[a] J. Zhang,<sup>+</sup> Dr. P. She,<sup>+</sup> Q. Xu, F. Tian, Prof. Dr. H. Rao, Prof. Dr. J.-S. Qin State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, International Center of Future Science, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China
E-mail: rao@jlu.edu.cn
qin@jlu.edu.cn</footnote>

<footnote>[b] Prof. Dr. J. Bonin, Prof. Dr. M. Robert Université Paris Cité, CNRS, Laboratoire d'Electrochimie Moléculaire (LEM), F-75013 Paris, France
E-mail: robert@u-paris.fr</footnote>

<footnote>[c] Prof. Dr. M. Robert Institut Universitaire de France (IUF), F-75005 Paris, France
E-mail: robert@u-paris.fr</footnote>

<footnote>[^1]: These authors contributed equally to this work.</footnote>

<footnote>[^2]: Supporting information for this article is available on the WWW under https://doi.org/10.1002/cssc.202301892</footnote>

<footnote>[^3]: © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_3.png" /></figure>

phosphorus[3a,c,4c] or sulfur[1d,7b,e,f,11] heteroatoms to coordinate with transition metal centers for stabilizing the complexes.

Nickel is a versatile transition metal which possesses various oxidation states (+4, +3, +2, +1, and 0) and is widely employed in photocatalytic reactions.[12] In Nature, several classes of Ni-enzymes[13] have been identified, including hydrogenases, ureases, carbon monoxide dehydrogenase (CODH), methyl-coenzyme M reductase and superoxide dismutase. The ligands of these enzymes contain nitrogen or sulfur atoms, which are Lewis bases[14] capable of donating electrons to the metal through σ or π bonds. These electron-donating properties increase electron density of the reducing metal center while maintaining strong metal-ligand covalence. Inspired by these enzymes, various biomimetic nickel catalysts have recently been developed for the photocatalytic CO₂ production.[12a,15] However, catalysts containing thiol ligands for CO₂ reduction are scarce in the literature.[1d,7b,e,f,11] Recently, inspired by the CODH enzyme, two Ni(II) complexes with N₂S₂-type[7f] tetradentate ligand and N₃S₂-type[7e] pentadentate ligand, respectively, were reported to catalyze the reduction of CO₂ to CO with high selectivity albeit with low TONs as compared to the state-of-the-art catalysts reported so far.[7d,16]

Herein, we report a Ni-based molecular catalyst [Ni(N₂S₂)]Cl₂ (NiN₂S₂, N₂S₂ = 2,11-dithia[3,3](2,6)pyridinophane)[17] containing thiol and pyridine ligands for the photochemical catalytic CO₂ reduction in an acetonitrile (MeCN)/H₂O mixed solvent, using [Ru(bpy)₃]Cl₂ (bpy = 2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. This homogeneous system exhibits high activity and selectivity for CO₂ reduction under visible light irradiation, offering a new approach for designing metal complex photocatalysts.

## Results and Discussion

The N₂S₂ ligand was synthesized with minor modifications according to the literature[18] and obtained with a high yield of 92% (Figures S1–S4). NiN₂S₂ was synthesized by reacting N₂S₂ with one equivalent of NiCl₂ in CH₂Cl₂, with 63% yield. The characterization of NiN₂S₂ was performed by liquid chromatography-high resolution mass spectrometry (LC-HRMS, Figure S6), UV-Visible spectroscopy (Figure S7) and X-ray crystallography (Table S1). Details of the synthesis and characterization are given in the Supporting Material.

As shown in Figure 1, NiN₂S₂ presents a slightly distorted octahedral coordination geometry with a mononuclear Ni²⁺ cation, two sulfur atoms and two nitrogen atoms, as shown by the crystalline structure. The two sulfur atoms occupy the *trans* biaxial position and the two pyridine nitrogen atoms occupy the *cis* position on the equatorial plane. Two chloride ions equilibrate charges. The sulfur and nitrogen atoms in the N₂S₂ ligand coordinate with the nickel center, increasing its charge density and thus potentially promoting its CO₂ reduction reactivity. The Ni–N and Ni–S bond distances in NiN₂S₂ (Table S2) are comparable to those of other similar complexes.[7e,f, 19] However, the Ni–S bond distances are longer

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Crystallographic parameters of NiN₂S₂ from single-crystal X-ray diffraction. Element code: nickel (purple), carbon (grey), nitrogen (blue), sulphur (yellow) and chlorine (green). Hydrogen atoms are omitted for clarity.</figcaption></figure>

than the Ni–N bond due to two factors. One is the larger size of the sulfur atom compared to the nitrogen atom, which leads to a more diffuse electron density and an extended bond length. The other is the weaker coordination bond between the sulfur lone pair and the metal *d* orbital as compared to the nitrogen lone pair and the metal *d* orbital, which lowers the bond strength. Moreover, the sulfur doping and the steric effects created by ligand distortion make the Ni–N bond distances in NiN₂S₂ about 0.12 Å longer than in Ni(cyclam)(ClO₄)₂[20] and Ni-porphyrin.[21]

Photochemical CO₂ reduction was typically performed in a homemade quartz reactor containing a mixture of MeCN/H₂O (9.0 mL, v/v = 2:1), NiN₂S₂ (1.0 µM), [Ru(bpy)₃]Cl₂ (0.7 mM) and BIH (0.02 M) saturated with CO₂. Figure 2 shows that during 72 h of visible light irradiation, 83 ± 3 µmol of CO was generated corresponding to a large TON of 9228 ± 321, with a turnover frequency (TOF) of 487 h⁻¹ in the first 2 h and an apparent quantum yield (AQY) of 0.25%. Additionally, CO

<figure><img src="image_5.png" /><figcaption>**Figure 2.** TON and yield (CO in red and H₂ in black) during visible light (420 nm) irradiation (72 h) of a MeCN/H₂O solution (9.0 mL, v/v = 2:1) containing NiN₂S₂ (1.0 µM), [Ru(bpy)₃]Cl₂ (0.7 mM) and BIH (0.02 M) under CO₂ atmosphere. The error bands represent the standard error of the mean, which was determined from three runs.</figcaption></figure>
selectivity was 85%. The main by-product was H₂ (15% selectivity) and trace amounts of formic acid were also detected in the liquid phase by ion chromatography (IC) analysis. Compared with other nickel-based homogeneous catalysts containing only Ni–N bonds (Table S3), this result illustrates that the introduction of Ni–S bonds significantly improves the performance of photocatalytic CO₂ reduction.

As illustrated in Figures 3 and S8, a series of control experiments were carried out to investigate how the various components affect the photocatalytic CO₂ reduction process. In the dark, no product was detected, confirming that the reaction is a light-driven process. A negligible amount of CO was observed without NiN₂S₂ (probably due to [Ru(bpy)₃]Cl₂ low reactivity), indicating that the CO₂ reduction system requires NiN₂S₂ to perform catalysis. Only H₂ was formed under Ar atmosphere, suggesting that CO₂ should be the sole source of CO. To further verify that CO₂ was the initial carbon source, we also performed experiments with ¹³CO₂ as the labelled reactant (Figure S9), and only ¹³CO was generated in this experiment, further confirming that CO originated from CO₂. As illustrated in Figure 3, almost no CO was generated without [Ru(bpy)₃]Cl₂ or BIH, proving that both [Ru(bpy)₃]Cl₂ as a photosensitizer and BIH as a sacrificial electron donor are essential for the reaction. As Ru is a noble element, we tried a non-noble metal Cu-

sensitizer [Cu(bathocuproine)(xantphos)](PF₆) (bathocuproine = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene)[22] and an pure organic sensitizer 1,3-dicyano-2,4,5,6-tetrakis(N,N-diphenylamino)-benzene (4DPAIPN)[23] in the same system (Table 1). We also performed experiments with various sacrificial electron donors (triethylamine, TEA, and triethanolamine, TEOA, as shown in Table 1) and varying the concentration of BIH (Figure S10).

According to our results, the optimal combination was [Ru(bpy)₃]Cl₂ and 0.02 M BIH. We further studied the effect of the concentration of NiN₂S₂ (Figure S11) and observed that the yield of CO increases proportionally with increasing concentration of NiN₂S₂, following a first-order dependence at low (< 1.0 µM) concentration. At higher catalyst concentration, the yield of CO increased slowly, thus showing the beginning of a saturation regime. This observation suggests that the reaction is, at higher concentration of catalyst, under the control of both CO₂ binding and protonation steps. To rule out the possible formation of metal nanoparticles (NPs) from the molecular catalyst degradation, we performed a mercury poison test by adding a drop of mercury (Figure S12). It exhibited no significant difference on the yield of CO, excluding NPs formation. Dynamic light scattering (DLS) experiments (Figure S13) further proved that the catalytic process is homogeneous.

Finally, we tested the addition of an acid as a co-substrate, so as to accelerate the reaction, since it is well established that acids can not only stabilize the adduct between the CO₂ and the active catalyst but also significantly boost the carbon-oxygen bond cleavage.[24] We used trifluoroethanol (TFE) and phenol (PhOH) (see entries 6–10, Table 1), which are stronger acids than water, and in both cases the production rate for CO was significantly enhanced without compromising the selectivity. As illustrated in Figure 4, phenol, the strongest acid used in the study, led to the production of 71.92 µmol of CO during 8 h irradiation, corresponding to a TON of 7991, a selectivity for CO of 89% and an apparent quantum yield of 1.08%. A remarkable turnover frequency of 1079 h⁻¹ (0.30 s⁻¹) was obtained in the first 6 hours.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Yield obtained during control experiments (6 h irradiation) with either no light, no catalyst, no photosensitizer, no electron donor or under Ar atmosphere compared to the complete system.</figcaption></figure>

<table><caption>**Table 1.** CO yield and selectivity during the photochemical reduction of CO₂ under various conditions.ª</caption><thead><tr><th>Entry</th><th>Photosensitizer</th><th>Electron donor</th><th>Acid</th><th>Products</th><th>n<sub>CO₂</sub>, µmol</th><th>CO Selectivity, %</th></tr></thead><tbody><tr><td>1</td><td>[Ru(bpy)₃]Cl₂</td><td>BIH</td><td>None</td><td>CO, H₂</td><td>14.72</td><td>95</td></tr><tr><td>2<sup>b)</sup></td><td>[Ru(bpy)₃]Cl₂</td><td>TEA</td><td>None</td><td>CO, H₂</td><td>0.19</td><td>19</td></tr><tr><td>3<sup>c)</sup></td><td>[Ru(bpy)₃]Cl₂</td><td>TEOA</td><td>None</td><td>CO, H₂</td><td>2.10</td><td>90</td></tr><tr><td>4</td><td>[Cu(bathocuproine) (xantphos)](PF₆)</td><td>BIH</td><td>None</td><td>CO, H₂</td><td>0.57</td><td>84</td></tr><tr><td>5</td><td>4DPAIPN</td><td>BIH</td><td>None</td><td>CO, H₂</td><td>0.06</td><td>7</td></tr><tr><td>6</td><td>[Ru(bpy)₃]Cl₂</td><td>BIH</td><td>0.1 M TFE</td><td>CO, H₂</td><td>25.61</td><td>79</td></tr><tr><td>7</td><td>[Ru(bpy)₃]Cl₂</td><td>BIH</td><td>0.2 M TFE</td><td>CO, H₂</td><td>35.62</td><td>86</td></tr><tr><td>8</td><td>[Ru(bpy)₃]Cl₂</td><td>BIH</td><td>0.1 M PhOH</td><td>CO, H₂</td><td>39.54</td><td>92</td></tr><tr><td>9</td><td>[Ru(bpy)₃]Cl₂</td><td>BIH</td><td>0.2 M PhOH</td><td>CO, H₂</td><td>58.28</td><td>89</td></tr></tbody></table>

a) Standard conditions: photosensitizer (0.7 mM), NiN₂S₂ (1.0 µM), and BIH (0.02 M) in 9.0 mL CO₂-saturated solution under 420 nm LED irradiation for 6 h.

b) [TEA] = 0.02 M. c) [TEOA] = 0.02 M.
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Irradiation with visible light (420 nm) of a MeCN/H₂O solution (9.0 mL, v/v = 2:1) containing NiN₂S₂ (1.0 µM), [Ru(bpy)₃]Cl₂ (0.7 mM), BiH (0.02 M) and weak acid under CO₂ atmosphere. Top: Yield obtained (CO in red and H₂ in black) during a 6 h experiment. Bottom: TON and yield (CO in red and H₂ in black) in the presence of 0.2 M PhOH as a function of time. The error bands represent the standard error of the mean, which was determined from three runs.</figcaption></figure>

Our data shows that NiN₂S₂ exhibits high photocatalytic performance. However, the production of CO stops after 37 h of reaction. A series of experiments were thus conducted to analyze the possible limiting factors. After 37 h of irradiation, either initial concentration of NiN₂S₂, [Ru(bpy)₃]Cl₂ or BiH was re-added, respectively (Figure S14). The activity of the photocatalytic system was partially recovered after the addition of [Ru(bpy)₃]Cl₂ or BiH, and it reached a plateau in 7 h. Since one BiH molecule provides two electrons to form BI⁺,[25] enough for the formation of CO or H₂, the initial BiH is slightly in excess. Similar results could also be observed when we re-added both BiH and NiN₂S₂. However, the catalytic activity was significantly recovered after re-adding both BiH and [Ru(bpy)₃]Cl₂, showing that the photosensitizer [Ru(bpy)₃]Cl₂ and sacrificial electron donor BiH present a limited stability whereas catalyst NiN₂S₂ seems to be highly stable, even under prolonged irradiation.

In order to investigate the mechanism of CO₂ reduction by NiN₂S₂, we conducted cyclic voltammetry (CV) measurements with a glassy carbon (GC) working electrode, a Pt wire counter electrode and a saturated calomel electrode (SCE) reference electrode. Two reduction waves at -1.28 V and -1.58 V vs. SCE were observed under Ar atmosphere (Figures S15–16), which were assigned to the Niill/1 and Niil/0 one-electron reduction processes, respectively. These waves very likely coincide with the concomitant loss of the chloride anions. Under CO₂ atmosphere, the catalytic activity starts at the first reduction

with a marked increase in current. The photocatalytic reaction mechanism was further explored by UV-Visible spectroscopy under Xe lamp irradiation. Individual UV-visible spectra of the NiN₂S₂, [Ru(bpy)₃]Cl₂ and BiH are shown in Figures S7 and S17, respectively. In Figure 5a–b, under Ar or CO₂ atmosphere in the absence of [Ru(bpy)₃]Cl₂, we observed no significant change in the absorption spectrum. In Figure 5c, with the full system (catalyst, photosensitizer and sacrificial electron donor) under Ar atmosphere, no change in the absorption spectrum was observed as well. On the contrary, under CO₂ atmosphere, the absorbance of [Ru(bpy)₃]Cl₂ and BiH gradually decreased (Figure 5d), suggesting that electron transfer occurred in the three-component system under CO₂ atmosphere.

To elucidate the electron transfer pathway in the photochemical process, we performed emission quenching experiments and luminescence lifetime experiments. The steady-state photoluminescence (PL) intensity for [Ru(bpy)₃]Cl₂ was efficiently quenched by BiH but not quenched by NiN₂S₂, indicating a reductive quenching pathway in the photochemical catalytic process (Figure S18). According to the Stern-Volmer analysis, the reductive quenching rate constant of *3[RuII(bpy)₃]²⁺ by BiH is 2.9×10⁹ M⁻¹s⁻¹. The luminescence lifetime (Figure S19) of *3[RuII(bpy)₃]²⁺ (219.6±0.4 ns) was not modified with the addition of NiN₂S₂ (222.5±0.4 ns). However, it decreased to 112.2±0.3 ns with the addition of BiH, further demonstrating a reductive quenching pathway is followed in the photochemical catalytic process.

The nanosecond transient absorption spectra of the photocatalytic system were measured to further elucidate the electron transfer dynamics (Figure 6, Figures S20–22). In Figure 6a–d, after laser excitation at 355 nm, a positive absorption band of the excited state *3[RuII(bpy)₃]²⁺ appeared at 365 nm and a negative bleaching band of the ground state [RuII(bpy)₃]²⁺ appeared at 447 nm,[26] respectively. The addition of NiN₂S₂ did not affect the absorption signal (Figure 6b). However, a new absorption band emerged at 510 nm[27] with the addition of BiH (Figure 6c), indicating the *3[RuII(bpy)₃]²⁺ was reductively quenched by BiH to form the [RuI(bpy)₃]⁺. In Figure 6d–f, the addition of NiN₂S₂ reduced the signal growth of [RuI(bpy)₃]⁺ from 96 ns to 65 ns and reduced the signal decay of [RuI(bpy)₃]⁺ from 199 ns to 144 ns. This finding shows that NiN₂S₂ plays a key role in the formation and conversion of [RuI(bpy)₃]⁺, which is capable of efficiently transfer electrons to NiN₂S₂ for the subsequent catalytic reaction.

Based on all the above data, we propose the following reaction mechanism (Scheme 1). Upon light excitation, [Ru(bpy)₃]²⁺ photosensitizer is promoted to its triplet metal-to-ligand charge transfer (MLCT) excited state *3[RuII(bpy)₃]²⁺. The latter is reductively quenched by BiH to generate [RuI(bpy)₃]⁺ and the one electron oxidized species BI⁺ may quickly deprotonate to give a very strong electron donor BI⁺ (E<sub>ox</sub> = -2.06 V vs. Fc⁺/Fc).[28] [RuI(bpy)₃]⁺ transfers an electron to [NiIIN₂S₂]²⁺ to form the reactive [NiI'N₂S₂]⁺ species, which will combine CO₂ to form [NiI'N₂S₂-CO₂]⁺ intermediate. [NiI'N₂S₂-CO₂]⁺ further obtains an electron from BI⁺ to form [NiI⁰N₂S₂-CO₂]⁺ species. Finally, [NiI⁰N₂S₂-CO₂]⁺ led to CO formation from protonation of the adduct and O–H bond cleavage, and returns
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 5.** UV-Visible spectral changes upon irradiation of a MeCN/H₂O (3.0 mL, v/v = 2:1) solution containing NiN₂S₂ (30 µM) and BIH (50 µM) a) under Ar atmosphere and b) under CO₂ atmosphere. c) UV-Visible spectral changes upon irradiation of a CO₂-saturated MeCN/H₂O (3.0 mL, v/v = 2:1) solution containing NiN₂S₂ (0.1 µM), [Ru(bpy)₃]Cl₂ (70 µM) and BIH (2.0 mM). d) Without NiN₂S₂. T = 298 K, using Xe lamp irradiation (with a λ = 420 nm cut-off filter, 179 mW cm⁻²).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Scheme 1.** Proposed mechanism for the CO₂ photochemical catalytic conversion to CO by catalyst NiN₂S₂ combined with [Ru(bpy)₃]Cl₂ photosensitizer, and BIH sacrificial electron donor upon visible light irradiation.</figcaption></figure>

to the [Ni¹N₂S₂]²⁺ state (possibly after a dissociation step to release a bound CO), accomplishing a catalytic cycle. Our results suggest that the rate-determining step (RDS) for CO₂ reduction by NiN₂S₂ may be the CO₂ coordination to the reduced [Ni¹N₂S₂]⁺ species, although further *operando* characterization is required to investigate the intermediate derived from NiN₂S₂.

## Conclusions

In summary, we have developed an efficient photocatalytic CO₂ reduction system using homogeneous catalyst NiN₂S₂, bearing a sulfur-containing macrocyclic ligand inspired by the CODH enzyme to produce CO selectively in the presence of [Ru-(bpy)₃]²⁺ as photosensitizer and BIH as electron donor. High production rate, up to 10³ h⁻¹, was achieved with phenol as a co-substrate along with 89% selectivity for CO and a high turnover number (close to 8000) during 8 h of irradiation. These performances make the catalyst one of the most active in the photochemical catalytic CO₂ reduction to date. We are currently optimizing the coordination environments of the ligands to further investigate their effects on catalyst performance.

## Supporting Information

Additional experimental details are given in the Supporting Information, including chemicals and catalyst preparation, instruments, ¹H and ¹³C NMR spectra, LC-HRMS and GC-MS data, UV-Visible spectra, control and optimization experiments, DLS spectra, addition experiments, electrochemistry data, Stern-Volmer plot, luminescence lifetime and transient absorption
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 6.** Transient absorption spectra following 355 nm laser excitation (pulse duration 1 ns, pulse energy 7 µJ, repetition rate 1 kHz) in MeCN/H₂O (v/v = 9:1) solution of a) 0.15 mM [Ru(bpy)₃]Cl₂ (0–300 ns after laser pulse excitation); b) 0.15 mM [Ru(bpy)₃]Cl₂ and 0.1 mM NiN₂S₂ (0–300 ns after laser pulse excitation); c) 0.15 mM [Ru(bpy)₃]Cl₂ and 1.0 mM BIH (0–300 ns after laser pulse excitation). d) 0.15 mM [Ru(bpy)₃]Cl₂, 1.0 mM BIH and 0.1 mM NiN₂S₂ (0–300 ns after laser pulse excitation). e) Kinetic analysis of the [Ru'(bpy)₃]⁺ signal growth probed at 510 nm without NiN₂S₂ (black trace) and with NiN₂S₂ (red trace). f) Kinetic analysis of the [Ru'(bpy)₃]⁺ signal decay probed at 510 nm without NiN₂S₂ (black trace) and with NiN₂S₂ (red trace).</figcaption></figure>

spectra measurements, X-ray crystallographic data and comparison of catalytic activity with previous data from literature.

Additional references cited within the Supporting Information.[29–41]

## Acknowledgements

This work was financially supported in part by the Natural Science Foundation of Jilin Province (No. SKL202302017), the National Natural Science Foundation of China (Grant Nos. 21901084, 21905106 and 22279041), the National Key Research and Development Program of China (No. 2022YFC2105800), the 111 Project (B17020), and the Specific Research Fund of the Innovation Platform for Academicians of Hainan Province, China (YSPTZX202321). Partial financial support to M.R. from the Institut Universitaire de France (IUF) is warmly thanked.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** molecular catalyst · CO₂ reduction · visible light · Nickel complex · homogeneous catalysis

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Manuscript received: December 16, 2023

Revised manuscript received: January 17, 2024

Accepted manuscript online: February 7, 2024

Version of record online: February 27, 2024