# Self-Photosensitizing Cobalt Complexes for Photocatalytic CO2 Reduction Coupled with CH3OH Oxidation

Ji-hong Zhang, Zhao-ming Ge, Di-chang Zhong, Jing-lin Zuo, Marc Robert, Tong-bu Lu

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To cite this version:

Ji-hong Zhang, Zhao-ming Ge, Di-chang Zhong, Jing-lin Zuo, Marc Robert, et al. Self-Photosensitizing Cobalt Complexes for Photocatalytic CO2 Reduction Coupled with CH3OH Oxidation. Angewandte Chemie International Edition, In press, 10.1002/anie.202506060. [hal-05056526]

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## Homogeneous Photosynthesis

How to cite: Angew. Chem. Int. Ed. **2025**, e202506060
doi.org/10.1002/anie.202506060

# Self-Photosensitizing Cobalt Complexes for Photocatalytic CO₂ Reduction Coupled with CH₃OH Oxidation

Ji-Hong Zhang⁺, Zhao-Ming Ge⁺, Di-Chang Zhong,* Jing-Lin Zuo, Marc Robert,* and Tong-Bu Lu*

**Abstract:** The use of metal complexes as homogeneous molecular catalysts has attracted considerable attention regarding photocatalytic CO₂ reduction. Enhancing these complexes with photosensitivity and photooxidation capabilities, aiming to create multifunctional molecular devices, presents significant challenges. In response to these challenges, we successfully designed and synthesized three innovative metal complexes. The complexes demonstrate a remarkable ability to perform CO₂ photoreduction in tandem with methanol photooxidation, allowing for the simultaneous production of formic acid without requiring additional photosensitizers and electron sacrificial reductants. An optimal turnover number (TON) value of 855 was obtained under simulated sunlight. Even under natural sunlight, the TON can reach 207, much higher than the value of the physical mixture of the photocatalytic reductive and oxidative moieties. Spectroscopic studies and density functional theory (DFT) calculations revealed that integrating reduction and oxidation sites in one molecular catalyst can promote charge transfer kinetics and enhance activity for CO₂ reduction and methanol oxidation. This is the first report that non-noble metal homogeneous catalysts can simultaneously possess photosensitivity, photoreduction, and photo-oxidation functions, offering new insights into designing homogeneous catalysts for artificial photosynthesis.

## Introduction

The global focus on achieving carbon neutrality has intensified in response to the energy crisis and environmental

problems stemming from the significant surge in CO₂ emissions because of the excessive combustion of fossil fuels.[1-4] Reduction of CO₂ to value-added chemicals by photocatalysis is recognized as one of the most promising technologies for sustainable carbon recycling.[5-12] Conventional homogeneous photocatalytic systems for CO₂ reduction usually involve a photosensitizer (PS), a catalyst (CAT), and a sacrificial reductant (SR; Scheme 1a).[13-28] Driven by light, the photosensitizer plays an important role in transferring reducing equivalents from the sacrificial reductant to the catalytic centre in the catalyst. The latter coordinates CO₂ and reduces it with the help of protons as cocatalysts. In these systems, electron transfers occur upon diffusional collisions between these components, leading to low efficiency and limitations by the diffusional process itself and not by the intrinsic activity of the catalyst.

To accelerate intermolecular electron transfer from photosensitizer to catalyst, covalently linking the photosensitive and catalytic moiety has been conceived and evidenced as an effective approach (Scheme 1b).[29-40] For instance, Nakajima group developed a binuclear Ru(II)-Re(I) complex by connecting a Ru photosensitive unit with a Re catalytic unit, which can serve as a photocatalyst to reduce CO₂ to CO, with a turnover number (TON) value of over 1000.[32] Kamada et al. reported an efficient tetradentate PNNP-type Ir photocatalyst, which achieved a TON of 2560 for CO₂ reduction to HCOOH and CO in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial reductant.[41] Very recently, Yuan and co-workers synthesized a multimetallic Fe₂Na₃ purpurin complex, which can serve as a self-photosensitizing catalyst for CO₂ reduction, achieving a TON value of 2625 and a selectivity of 91%

[*] J.-H. Zhang⁺, Z.-M. Ge⁺, D.-C. Zhong, T.-B. Lu
Institute for New Energy Materials and Low Carbon Technologies,
School of Materials Science and Engineering, Tianjin University of
Technology, Tianjin 300384, China
E-mail: dczhong@email.tjut.edu.cn
lutongbu@tjut.edu.cn

J.-H. Zhang⁺
Key Laboratory of Jiangxi Province for Persistent Pollutants Control
and Resources Recycle, Nanchang Hangkong University, Nanchang
330063, China

J.-L. Zuo
School of Chemistry & Chemical Engineering, State Key Lab
Coordination Chemistry, Nanjing University, Nanjing 210023, China

M. Robert
Institut Parisien de Chimie Moléculaire, Sorbonne Université, CNRS,
IPCM, Paris F-75005, France
E-mail: marc.robert@sorbonne-universite.fr

M. Robert
Institut Universitaire de France (IUF), Paris F-75005, France

[+] Both authors contributed equally to this work.

<figure><img src="image_3.png" /></figure>

Additional supporting information can be found online in the
Supporting Information section

<figure><img src="image_4.png" /></figure>

© 2025 The Author(s). Angewandte Chemie International Edition
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.
<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Schematic representations of homogeneous photocatalytic CO₂-reduction systems: a) Conventional system; b) covalently bonded PS (photosensitizer) and CAT (catalyst) system; c) covalently bonded PS and CAT system without added external SR (sacrificial reductant) as reported in this work.</figcaption></figure>

to CO in 120 h, with BIH as a sacrificial reductant.[42]
Remarkably, significant progress has been made in acceler-
ating electron transfer between photosensitizer and catalyst
in homogeneous photocatalytic CO₂ reduction. Neverthe-
less, noble-metal catalysts/photosensitizers and/or sacrificial
reductants are necessary for these photocatalytic systems. In
particular, costly sacrificial reductants, often amines such as
BIH, triethanolamine (TEOA), triethylamine (TEA), and 1-
benzyl-1,4-dihydronicotinamide (BNAH) should be avoided.
Alternative organic compounds that can be oxidized to high
value-added products may be used instead and should get
more attention (Scheme 1c). To the best of our knowledge,
homogeneous photocatalytic systems for CO₂ reduction
based on non-noble-metal catalysts/photosensitizers in the
absence of sacrificial reductants have not been reported to
date. Thus, it would be meaningful to develop homoge-
neous multifunctional photocatalysts integrating photosensi-
tive unit, catalytic reductive and catalytic oxidative moieties,
so as to achieve the coupling of CO₂ reduction with a useful
organic oxidation reaction.

Electron-rich tetrathiafulvalene (TTF) is a powerful elec-
tron donor with good photon-absorption properties.[43–45]
After donating electrons, the resulting cation radical TTF⁺
possesses a strong oxidation capacity. A covalently bonded
TTF with a non-noble metal complex may thus act as a multi-
functional catalyst achieving the coupling of CO₂ photoreduc-
tion with an organic oxidation reaction. Herein, on the basis
of chemical modification of a mononuclear [CoL¹]²⁺ complex
(L¹ = 4-amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole) with TTF

groups, we obtained an example of non-noble metal complex of [CoL²]²⁺ (L² = (E)-1-[(2,2'-bi(1,3-dithiolylidene)]-4-yl)-N-(3,5-di(pyridine-2-yl)-4H-1,2,4-triazol-4-yl) methenamine), which contain a mononuclear Co(II) catalytic reduction moiety, and two TTF moieties playing the role of photosensitizer (Figure 1a). Photocatalytic experiments show that [CoL²]²⁺ complex possesses activity for CO₂ reduction and CH₃OH oxidation, simultaneously synthesizing formic acid (HCOOH), and achieving a high turnover number (TON) value of 855 for HCOOH in the absence of additional photosensitizers or sacrificial reductants under laboratory light source. It surpasses the performance of most reported non-noble metal self-photosensitizing molecular catalysts and even outperforms noble-metal self-photosensitizing molecular catalysts (Table S1). Moreover, it was found that natural sunlight could also drive this catalytic system to produce HCOOH, with a TON value up to 207. Spectroscopic experiments and density functional theory (DFT) calculations revealed that integrating oxidation and reduction moieties in one complex promotes charge transfer kinetics and enhances the activity for CO₂ reduction and CH₃OH oxidation. Using the same protocol, the resulting [NiL²]²⁺ and [CuL²]⁺ also show activity for CO₂ photoreduction and CH₃OH photo-oxidation, illustrating the generality of this approach.

**Results and Discussion**

The ligands L¹ and L² were synthesized by modified lit-
erature methods (see details in Supporting Information,
Figures S1–S6).[46–48] Subsequently, [CoL¹]²⁺ and [CoL²]²⁺
were obtained by the reaction of Co(H₃₃C₁₆SO₃)₂(H₂O)₂
and Co(NO₃)₂·6H₂O with corresponding ligands at room
temperature, respectively. Their structures were characterized
by IR spectroscopy, elemental analysis, UV–vis absorption
spectroscopies, liquid chromatography-mass spectrometry
(LC-MS), X-ray photoelectron spectroscopic (XPS), and
X-ray absorption spectroscopy (XAS) (Figures S7–S18). UV–
vis absorption spectra reveal a sharp peak at 288 nm
(εmax = 42 024 M⁻¹ cm⁻¹) for [CoL¹]²⁺ (Figure S9). The
UV–vis spectrum of TTF displays three absorption peaks at
305, 362, and 448 nm (ε = 9712, 1631, and 261 M⁻¹ cm⁻¹,
respectively, Figure S4). In the case of [CoL²]²⁺, a maximum
peak at 297 nm (εmax = 51 819 M⁻¹ cm⁻¹) and a broad
band spanning from 378 to 634 nm are observed (Figure
S10), suggesting a redshift in the absorption peak of [CoL²]²⁺
following the incorporation of TTF. The results of LC-MS
measurements for [CoL¹]²⁺ and [CoL²]²⁺ in CH₃OH show
ion peaks at m/z = 840.36 and 1024.92, corresponding to the
species of [[CoL¹]²⁺ + H₃₃C₁₆SO₃⁻]⁺ and [[CoL²]²⁺ + NO₃⁻]⁺,
respectively (Figures S11–S14). The XPS measurements
showed that the binding energy peaks for Co 2p₁/₂ and Co
2p₃/₂ appear at 796.62 and 780.78 eV, respectively, indicating
the chemical valence of cobalt in [CoL²]²⁺ is +2 (Figure
S15).[34] Similarly, the oxidation state of cobalt in [CoL¹]²⁺
was also determined to be +2 (Figure S16). Furthermore, X-
ray absorption near-edge structure (XANES) analysis reveals
that the absorption edge of [CoL²]²⁺ closely resembles that
of CoO (Figure S17), further corroborating the +2 valence
<figure><img src="image_6.png" /><figcaption>**Figure 1.** a) Chemical structures of homogeneous catalysts of [CoL¹]²⁺ and [CoL²]²⁺. b) Time-dependent HCOOH generation from the photocatalytic CO₂ reduction coupled with CH₃OH oxidation using [CoL¹]²⁺ or [CoL²]²⁺ (The error bars are standard deviations calculated from the results of three parallel experiments). c) Control experiments for the photocatalytic CO₂ reduction coupled with CH₃OH oxidation using [CoL²]²⁺. d) Photocatalytic performance of [CoL²]²⁺ for the photocatalytic CO₂ reduction coupled with CH₃OH oxidation driven by sunlight, the error bars represent the temperature range of photocatalytic set-up during catalysis under the sunlight irradiation from 9:00 am to 4:00 pm (Date: 9–15/8/2023; Position: TJUT, Xiqing district, Tianjin). Catalytic conditions: Catalysts (1 µM), 5 mL CO₂-saturated CH₃OH/H₂O (ν:ν = 4:1) solution, and 300 W Xe lamp (320 < λ < 780 nm; light intensity: 200 mW cm⁻²), irradiation time 14 h, and temperature T = 25 °C.</figcaption></figure>

state of cobalt in [CoL¹]²⁺, consistent with the XPS and LC-MS findings. Moreover, fitting results indicate that the simulated Co Fourier transform-extended X-ray absorption fine structure (FT-EXAFS) spectra based on our architectural model are in good agreement with the experimental results (Figure S18). Taken together, all these observations suggest the successful syntheses of [CoL¹]²⁺ and [CoL²]²⁺, which are stable in CH₃OH solution.

The experiments of photocatalytic CO₂ reduction with CH₃OH oxidation were performed with a 300 W Xe lamp (320 < λ < 780 nm), using [CoL¹]²⁺ or [CoL²]²⁺ as a photocatalyst under CO₂ atmosphere in 5 mL CH₃OH/H₂O (ν:ν = 4:1) solution, without additional photosensitizers and sacrificial reductants (see the Supporting Information). The possibly generated gaseous and liquid products were detected and quantified by gas chromatography (Figure S19) and ion chromatography (Figure S20), respectively. The results show that for [CoL¹]²⁺, negligible gaseous and liquid products were detected during the photocatalytic process, while for [CoL²]²⁺, a large amount of HCOOH liquid product was observed. Specifically, as shown in Figures 1b, S21, after illumination for 14 h, no HCOOH was detected in the photocatalytic reaction over 1 µM of [CoL¹]²⁺. Under similar conditions, a significant amount of HCOOH can be generated and detected over 1 µM of [CoL²]²⁺, corresponding to a TON value of 855 (two-thirds of the formic acid was

produced comes from CO₂ reduction and one-third from CH₃OH oxidation) (Figures 1b, S21). The apparent quantum yield (AQY) for HCOOH production was determined to be 0.24%, based on the photocatalytic result using an LED light during a 10 h (λ = 365 nm) experiment (Figure S22). Besides HCOOH, trace amount of HCHO was also detected (Figures S23, S24), which is the intermediate product during the process of CH₃OH oxidation to HCOOH.[49] Despite the quantum yield for HCOOH production is higher than most reported self-photosensitive homogeneous catalysts,[29–40] the value also indicates that large number of photons have not been utilized. Dynamic light scattering (DLS) measurements showed that there were no particles in the catalytic system using [CoL²]²⁺ after 14 h irradiation (Figure S25). The result of photocatalytic CO₂ reduction coupled with CH₃OH oxidation in the presence of mercury shows that the amount of HCOOH generated is almost equal to that without mercury, suggesting that the catalytic reaction is homogeneous (Figure S26).[15] However, the rate of HCOOH generation after 14 h decreased. The results of UV–vis spectrum of the reaction solution after catalysis for 14 h indicate that the decline in catalytic activity is primarily due to the photodegradation of [CoL²]²⁺ (Figures S27, S28).

It is interesting to observe that [CoL¹]²⁺ and [CoL²]²⁺ show completely different photocatalytic activity in coupling CO₂ reduction with CH₃OH oxidation. Considering that the
only structural difference between [CoL¹]²⁺ and [CoL²]²⁺ is that there are two TTF groups covalently bonded in [CoL²]²⁺, we speculate that TTF plays a significant role in this com-proportionation reaction. To confirm this speculation, further photocatalytic reaction over the physical mixture of [CoL¹]²⁺ and TTF ([CoL¹]²⁺&TTF) was performed. It was found that 0.90 µmol of HCOOH was detected (Figure S21). Although the amount of HCOOH generated by [CoL¹]²⁺&TTF with molar ratio of 1:2 is much lower than that of [CoL²]²⁺ in which TTF is covalently bonded to [CoL¹]²⁺, this result still demonstrates the important role of TTF in this coupling reduction and oxidation reaction. Further increase of the TTF amount, the HCOOH yield of [CoL¹]²⁺ and TTF catalytic systems increased (Figure S28). When the molar ratio of [CoL¹]²⁺ to TTF was 1:50, a maximum amount of HCOOH was generated. However, upon further increasing the TTF amount, a decline in the yield of HCOOH was observed, which may be due to the excessive TTF hindering the light absorption and utilization.[50] Besides, trace amounts of HCOOH were detected with L² as catalyst (Figure S21), which also confirms the role of TTF in the photocatalytic process. The covalent bonding of TTF with [CoL¹]²⁺ through a Schiff base reaction results in the creation of [CoL²]²⁺, a compound with enhanced capabilities. This new complex exhibits photosensitive properties and can perform both photoreduction and photo-oxidation functions. By integrating these multiple functions and satisfying the requirements for photocatalytic reactions, [CoL²]²⁺ demonstrates exceptional photocatalytic performance in coupling CO₂ reduction with CH₃OH oxidation, simultaneously synthesizing HCOOH. The different activity of [CoL²]²⁺ and [CoL¹]²⁺&TTF also highlights that the covalent bonding is beneficial for accelerating the electron transfer between TTF and the Co(II) catalytic center, which has been observed in other heterogeneously photocatalytic systems with molecular catalysts.[44,51–54]

To further explore this strategy based on ligand L², we synthesized the Ni(II) complex ([NiL²]²⁺) and the Cu(I) complex ([CuL²]⁺) by a similar synthetic procedure (Figure S29). The IR spectroscopy, elemental analysis, UV–vis absorption spectroscopies, LC-MS, XPS, and XAS demonstrated the successful syntheses of [NiL²]²⁺ and [CuL²]⁺ (Figures S30–S41). The UV–vis spectrum of [NiL²]²⁺ and [CuL²]⁺ also displays a maximum peak at 296.5 nm (εmax = 51 158 and 58 056 M⁻¹ cm⁻¹, respectively) and a broad band spanning 378 to 634 nm (Figures S32, S33), which is similar to [CoL²]²⁺. The Ni 2p XPS spectra in [NiL²]²⁺ show two main characteristic peaks at 873.32 and 855.75 eV, which are assigned to the Ni 2p₁/₂ and Ni 2p₃/₂, respectively, indicating the chemical valence of nickel in [NiL²]²⁺ is +2 (Figure S38).[34] The Auger electron spectroscopy (AES) of the Cu LMM only showed a peak at 570.30 eV assignable to Cu(I) species (Figure S39), suggesting the oxidation valence of copper in [CuL²]⁺ is +1.[55] Photocatalytic experiments showed that as expected, both [NiL²]²⁺ and [CuL²]⁺ are photosensitive, and can achieve CO₂ photoreduction coupled with CH₃OH photooxidation, to simultaneously synthesize HCOOH, corresponding to TON values of 405 and 62, respectively (Figure S42). A trace amount of intermediate product HCHO was also detected (Figure S43). The apparent

quantum yields of [NiL²]²⁺ and [CuL²]⁺ to HCOOH were determined to be 0.13% and 0.0051%, respectively (Figure S44). The above results suggest that the introduction of TTF into active metal complexes is really an effective approach to developing non-noble catalysts for artificial photosynthesis.

A series of control experiments with [CoL²]²⁺ as a catalyst were carried out to thoroughly investigate this photocatalytic reaction of coupling CO₂ reduction with CH₃OH oxidation to HCOOH. First, the photocatalytic reactions were performed in the absence of [CoL²]²⁺, CH₃OH, or illumination, no HCOOH was detected (Figure 1c), suggesting that [CoL²]²⁺, CH₃OH, and light are all indispensable for the photocatalytic reaction to occur. Second, the photocatalytic reaction was carried out using Ar instead of CO₂. The result shows that no HCOOH was generated either (Figure 1c), indicating that CO₂ is the possible carbon source for HCOOH production. This conclusion was directly evidenced by the isotopic labeling experiment with ¹³CO₂ instead of CO₂ (see discussion below). No H₂ was detected under Ar atmosphere. It may be attributed to the affinity between the cobalt site and the hydrogen atom, which significantly diminishes the likelihood of hydrogen–hydrogen binding, thereby inhibiting H₂ evolution.[56] Third, the ratio of CH₃OH/H₂O (v:v) was explored. It was found that the photocatalytic activity showed a volcano-type with the ratio of CH₃OH/H₂O (Figure S45). When the ratio of CH₃OH/H₂O is 4:1, a maximum amount of HCOOH was obtained. Fourth, it was found that the yield of HCOOH increased with the concentration of the catalyst. At a catalyst concentration of 5 µM, the TON remained as high as 310 (Figure S46). Upon scaling up the catalytic system (from 5 to 30 mL total volume), the yield of HCOOH reached 20.1 µmol, consistent with a TON of 671 (Figure S47).

The excellent photocatalytic activity of [CoL²]²⁺ for coupling CO₂ reduction with CH₃OH oxidation was further evaluated under outdoor natural sunlight illumination at ambient conditions. The results show that the sunlight can also drive the coupling reactions of CO₂ reduction and CH₃OH oxidation over [CoL²]²⁺, with a maximal TON value of 207 to HCOOH upon illumination for 8 h by natural sunlight on 15th August, 2023 (Figures 1d, S48). Although the TON value for HCOOH (using [CoL²]²⁺) under sunlight is lower than that under the laboratory light source, it illustrates the excellent activity of [CoL²]²⁺ in photocatalytic CO₂ reduction coupled with CH₃OH oxidation.

To further demonstrate the coupling of CO₂ photoreduction and CH₃OH photooxidation, photocatalytic reaction with [CoL²]²⁺ under Ar was performed in the presence of 10 mM AgNO₃. The result shows that 2.29 µmol HCOOH was generated (Figure S49). It should be noted that as mentioned above, no formic acid was formed in the control photocatalytic experiment under argon (Figure 1c). Thus, the generation of HCOOH in the presence of AgNO₃ should be attributed to the CH₃OH oxidation, in which the Ag⁺ serves as electron scavenger, benefiting to the CH₃OH oxidation by the positive TTF moiety. This result shows that the half-reaction of CH₃OH photooxidation could be achieved by [CoL²]²⁺. On the other hand, the photocatalytic CO₂ reduction reaction by [CoL²]²⁺ was further carried out using TEOA instead of CH₃OH as the electron sacrificial reductant.
<figure><img src="image_7.png" /><figcaption>**Figure 2.** a) ¹³C NMR spectra of products generated in the photocatalytic system containing CO₂ and CH₃OH by [CoL²]²⁺; b) ¹³C NMR and c) ¹H NMR spectra of products generated in the photocatalytic system containing ¹³CO₂ and CH₃OH by [CoL²]²⁺; d) ¹H NMR spectra of products generated in the photocatalytic system containing CO₂ and ¹³CH₃OH by [CoL²]²⁺. Operando FTIR spectra of [CoL²]²⁺ during the photocatalytic CO₂ reduction coupled with CH₃OH oxidation, showing the characteristic absorption peaks of reaction intermediates with the increase of illumination time, e) 2700–3100 cm⁻¹ and f) 1150–1900 cm⁻¹.</figcaption></figure>

A large amount of HCOOH was also generated and observed, indicating that the half-reaction of CO₂ photoreduction could also be realized by [CoL²]²⁺ (Figure S49). Isotopic labeling experiments were further conducted to confirm the origin of the generated HCOOH. As shown in Figure 2, in the presence of ¹³CO₂ and CH₃OH system, the ¹³C NMR spectrum shows the H¹³COOH signal at 170.8 ppm,[⁵⁷,⁵⁸] confirming that HCOOH originates from CO₂ reduction (Figure 2a,b). In addition, the ¹H NMR spectrum displays two split peaks at δ = 8.01 and 8.50 ppm respectively (Figure 2c), which can be assigned to the H¹³COOH generated from ¹³CO₂ reduction.[⁵⁸] Meanwhile, a typical peak at δ = 8.25 ppm appears, which can be identified as HCOOH originating from CH₃OH oxidation. Moreover, the integrated area ratio of peaks belonging to H¹³COOH and HCOOH is about 2:1, which is consistent with the stoichiometric ratio (CO₂ + 2e⁻ + 2H⁺ → HCOOH; CH₃OH + H₂O + 4H⁺ → 4H⁺ + HCOOH). To further verify the product of CH₃OH oxidation, the photocatalytic experiment by [CoL²]²⁺ with CO₂ and ¹³CH₃OH as feedstocks was performed. As shown in Figure 2d, both the doublet associated with H¹³COOH and singlet assigned to HCOOH can also be observed in the ¹H NMR spectrum, and the integrated area ratio of peaks originated from CO₂ reduction and CH₃OH oxidation can be calculated to be about 2:1. The results of isotopic labeling experiments clearly show that the generated HCOOH is from both CO₂ reduction and CH₃OH oxidation, which also con-

firms that CO₂ photoreduction and CH₃OH photooxidation are simultaneously achieved by [CoL²]²⁺.

The reaction intermediates during CO₂ reduction coupled with CH₃OH oxidation using [CoL²]²⁺ were monitored by Fourier transform infrared (FTIR) spectroscopy. As shown in Figure 2e,f, new peaks appeared under photoirradiation, and their intensities gradually increased with the illumination time from 0 to 20 min. The peaks at 2981, 2974, 2967, 2950, 2923, 2865, 2844 and 2825 cm⁻¹ may be assigned to the stretching and bending vibration of C–H in *OCH₃, deciphering a multistep dehydrogenation process of the CH₃OH oxidation over [CoL²]²⁺.[⁵⁹] In addition, the reaction intermediates in the process of CO₂ reduction over [CoL²]²⁺ were also monitored. The peak located at 1641 cm⁻¹ can be attributed to a *COOH intermediate (C atom binding to the Co site).*[⁶⁰] which indicates the CO₂ undergoes first proton-coupled electron transfer (PCET) process to form the *COOH intermediate. In addition, another gradually enhanced peak at 1384 cm⁻¹ can be assigned to the *OCOH,[⁶¹] implying the *COOH transforms to *OCOH (O atom binding to the Co site) through isomerization, the latter being a key intermediate in CO₂-to-HCOOH conversion. Of note, no peaks related to either *OCH₃, *COOH, or *OCOH intermediate were detected in the absence of CH₃OH or CO₂ (Figures S50, S51). These FTIR results add support to the conclusion that photocatalytic coupling reaction of CO₂ reduction with CH₃OH oxidation occurs with [CoL²]²⁺ as catalyst.
<figure><img src="image_8.png" /><figcaption>**Figure 3.** EPR signals of a) [Col<sup>2</sup>]<sup>2+</sup> and b) [Col<sup>1</sup>]<sup>2+</sup> under the dark and light illumination in the presence of DMPO as the spin-trapping reagents; high-resolution XPS spectra of c) Co 2p and d) S 2p in [Col<sup>2</sup>]<sup>2+</sup> in dark and upon illumination by a Xe lamp; e) PL spectra of TTF (100 µM) and [Col<sup>2</sup>]<sup>2+</sup> (100 µM) in CH<sub>3</sub>OH solution; f) the time-resolved photoluminescence (TRPL) spectra of TTF and [Col<sup>2</sup>]<sup>2+</sup>.</figcaption></figure>

In order to get insights in the reaction mechanism, the electrochemical behaviors of [Col<sup>1</sup>]<sup>2+</sup> and [Col<sup>2</sup>]<sup>2+</sup> were investigated by cyclic voltammetry (CV) in CH<sub>3</sub>OH/H<sub>2</sub>O (v:v = 4:1) under the Ar and CO<sub>2</sub> atmosphere. As shown in Figure S52, irreversible reduction peaks at -1.0 V versus (vs.) normal hydrogen electrode (NHE) for [Col<sup>1</sup>]<sup>2+</sup>, and -0.71 versus NHE for [Col<sup>2</sup>]<sup>2+</sup> were observed, which corresponds to the reduction of Co<sup>II</sup> to Co<sup>I</sup>. These values are more negative than the theoretical CO<sub>2</sub> reduction potential (CO<sub>2</sub>/HCOOH, -0.61 V vs. NHE). In addition, the catalytic current for [Col<sup>1</sup>]<sup>2+</sup> and [Col<sup>2</sup>]<sup>2+</sup> in the presence of CO<sub>2</sub> is obviously enhanced compared to those in Ar, manifesting their potential catalytic activity for CO<sub>2</sub> reduction. Furthermore, [Col<sup>2</sup>]<sup>2+</sup> exhibits two redox waves assignable to the redox processes of TTF/TTF<sup>+</sup> and TTF<sup>+</sup>/TTF<sup>2+</sup> at +0.92 and +1.23 V, respectively (Figure S53), which is more positive than the theoretical oxidation potential of CH<sub>3</sub>OH to HCOOH (0.10 V vs. NHE).[62] These redox properties imply that it is thermodynamically feasible for [Col<sup>2</sup>]<sup>2+</sup> to produce HCOOH via CO<sub>2</sub> reduction and CH<sub>3</sub>OH oxidation.

Electron paramagnetic resonance (EPR) of [Col<sup>2</sup>]<sup>2+</sup> was performed to better uncover the electron-transfer process. The EPR spectrum of [Col<sup>2</sup>]<sup>2+</sup> shows a strong signal at g = 2.13 in the dark, which is ascribed to the signal of paramagnetic Co(II) (Figure S54).[63] Upon light illumination, the signal of Co(II) displays evident decrease, indicating that part of Co(II) accepts an electron from TTF moiety to form diamagnetic Co(I) (Table S2).[64,65] In addition, the EPR measurement was further performed in CH<sub>3</sub>OH to detect radical species during photocatalytic experiments using 5,5-dimethyl-

1-pyrroline N-oxide (DMPO) used as a spin-trap agent. As shown in Figure 3a, a typical six characteristic signals for DMPO-CH<sub>3</sub>O· originating from CH<sub>3</sub>OH oxidation were markedly observed with [Col<sup>2</sup>]<sup>2+</sup> upon illumination,[66] while no signal of DMPO-CH<sub>3</sub>O· can be detected with [Col<sup>1</sup>]<sup>2+</sup> (Figure 3b), indicating that the methanol oxidation process occurs due to the introduced group of TTF moiety. The electron transfer pathway in [Col<sup>2</sup>]<sup>2+</sup> was further investigated by in-situ light-illuminated XPS measurements. As shown in Figure 3c,d, the [Col<sup>2</sup>]<sup>2+</sup> exhibited two peaks at 780.78 and 796.62 eV in dark conditions, which can be assigned to the Co 2p<sub>3/2</sub> and Co 2p<sub>1/2</sub>, respectively.[67] Upon light illumination, the Co 2p binding energy displayed a slightly negative shift (0.36–0.37 eV; Figures 3c, S55), suggesting an increase in the electron density at the Co, which is consistent with the EPR result. Meanwhile, the binding energy of S 2p<sub>3/2</sub> and S 2p<sub>1/2</sub> at 163.76 and 164.98 eV respectively with [Col<sup>2</sup>]<sup>2+</sup> in dark conditions underwent markedly positive shift (0.23–0.25 eV) upon light illumination (Figures 3d, S55),[52] which indicates a decrease in the electron density on the TTF groups. The results of the XPS and EPR measurements provide solid evidence that the electron migrates from the TTF groups to Co(II) in [Col<sup>2</sup>]<sup>2+</sup>, and the Co(II) and TTF serve as reduction and oxidation sites, respectively.[68] To reveal more details about the catalytic mechanism of [Col<sup>2</sup>]<sup>2+</sup> in photochemical CO<sub>2</sub> reduction, luminescence quenching experiments of the excited state TTF* were performed with the addition of [Col<sup>1</sup>]<sup>2+</sup> or CH<sub>3</sub>OH. As shown in Figure S56, TTF exhibits strong fluorescence at 412 nm upon excitation at 300 nm. The fluorescent intensity of TTF dramatically
decreased with the gradual addition of [CoL¹]²⁺. By contrast, the addition of CH₃OH did not induce significant quenching to the excited TTF*. These observations illustrate that the quenched mode of TTF* can be assigned to an oxidative quenching mechanism.

The photoluminescence (PL) and time-resolved photo-
luminescence (TRPL) spectra were further performed with
[CoL²]²⁺. As shown in Figure 3e, the PL spectrum of TTF
exhibits a strong characteristic peak at 406 nm, while the
luminescence intensity of [CoL²]²⁺ is significantly smaller
than that of TTF. This indicates that the [CoL²]²⁺ can
competitively promote charge transfer from its excited state.
Additionally, the TRPL spectra revealed a reduced average
exciton lifetime of 2.5 ns for TTF to 0.9 ns for [CoL²]²⁺,
also illustrating that [CoL²]²⁺ can accelerate the charge
transfer (Figure 3f). The UV-vis absorption spectra of TTF
show an absorption peak at 310 nm, and after irradiation
for 14 h with an Xe lamp, photodegradation of TTF was
observed (Figure S57). However, the [CoL²]²⁺ shows better
robustness, as the main absorption peak of [CoL²]²⁺ can
still be observed after 14 h illumination (Figure S27). The
enhanced durability of TTF when inserted in [CoL²]²⁺ can
be attributed to the rapid electron transfer from TTF to
Co(II) center, and further reduction by CH₃OH, which
avoids overoxidation of TTF itself. Furthermore, the HOMO-
LUMO gaps of [CoL¹]²⁺ and [CoL²]²⁺ were calculated to be
3.17 and 0.84 eV, respectively (Figures S58, S59), indicating
that the introduction of the TTF group reduces the HOMO-
LUMO gap, thereby accelerating electron transfer during
the photocatalytic production of HCOOH.<sup>[69,70]</sup> A similar
trend was observed in [NiL²]²⁺ and [CuL²]⁺ (Figures S60,
S61). Moreover, the CV of [NiL²]²⁺ and [CuL²]⁺ shows a
primary reduction peak at -0.77 and -0.94 V versus NHE
(Figure S62), respectively. These values are more negative
than [CoL²]²⁺ (-0.71 V vs. NHE), illustrating that [CoL²]²⁺
possesses higher photocatalytic activity over [NiL²]²⁺ and
[CuL²]⁺,<sup>[71]</sup> which well supports the experimental results.

Based on the above results, a reaction scheme for the photocatalytic CO₂ reduction coupled with CH₃OH oxidation using [CoL²]²⁺ as catalyst can be proposed (Figure S63). The photoactive TTF moiety in [CoL²]²⁺ is first activated under light illumination to an excited state TTF*, which is oxidatively quenched by electron transfer to the Co(II) center. The resulting Co(I) further binds the CO₂ molecule. The CO₂ molecule is thus activated to *CO₂, which undergoes a PCET process to form *COOH intermediate. The *COOH isomerizes and finally generates HCOOH. At the same time, the positive TTF moiety activates the O–H bond of CH₃OH to generate *OCH₃ intermediate, which undergoes multistep dehydrogenation and oxygen-coupled processes to generate the oxidation product of HCOOH.

The catalytic mechanism of [CoL²]²⁺ for photocatalytic
CO₂ reduction to HCOOH has been further investigated at
a molecular level by DFT calculations (see the Supporting
Information, Figures S64, S65). After detailed calculations, a
reasonable reaction pathway has been presented in Figure 4a.
First, [CoL²]²⁺ obtains one electron to generate [CoL²]⁺-a,
during which the CoII in [CoL²]²⁺ is reduced to CoI. Second,
the CoI in [CoL²]⁺-a captures a CO₂, and then the CO₂

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Proposed catalytic mechanism for the reduction of CO₂ to HCOOH with [CoL²]²⁺.</figcaption></figure>

accepts 2e⁻ from Co¹ to be reduced to CO₂²⁻ through the
transition state of TS1 to generate [CoL²]⁺-b. The energy
barrier for this transition state is 10.61 kcal mol⁻¹ (Figures
S64, S65). Third, [CoL²]⁺-b goes through a PCET process
to generate [CoL²]⁺-c. Fourth, the -COOH in [CoL²]⁺-c
transforms to —OCHO through the transition state of TS2
to generate [CoL²]⁺-d. The energy barrier calculated for TS2
is 10.61 kcal mol⁻¹. Finally, [CoL²]⁺-d is protonated to yield
[CoL²]²⁺-e via transition state of TS3. The energy barrier
calculated for TS3 is 8.76 kcal mol⁻¹. After the release of
HCOOH in [CoL²]²⁺-e, [CoL²]²⁺ is regenerated and the
photocatalytic cycle restarts. This mechanism is similar to that
reported by Chen et al.[⁷²]

**Conclusion**

In summary, we designed innovative self-photosensitizing
non-noble single metal complexes, which achieve the pho-
tocatalytic CO₂ reduction coupled with CH₃OH oxidation
to simultaneously synthesize formic acid, without the addi-
tion of any photosensitizers and sacrificial reductants. The
spectroscopic studies and DFT calculations revealed that
integrating reduction and oxidation moieties in one complex
promotes charge transfer kinetics, thus enhancing the activity
of CO₂ reduction coupled with CH₃OH oxidation. This work
opens new perspectives toward the development of molecular
catalysts for artificial photosynthesis.

**Acknowledgements**

This work was supported by the National Natural Science
Foundation of China (22271218), the National Key R&D
Program of China (2022YFA1502902), and the Natural
Science Foundation of Tianjin City (24JCZDJC00220). Partial
financial support from the Institut Universitaire de France
(IUF) is gratefully acknowledged.

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: CO₂ reduction • Homogeneous catalyst • Methanol oxidation • Non-noble metal complex • Self-photosensitizing

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Manuscript received: March 16, 2025

Revised manuscript received: April 15, 2025

Accepted manuscript online: April 15, 2025

Version of record online: ■■■
Research Article

Homogeneous Photosynthesis

J.-H. Zhang, Z.-M. Ge, D.-
C. Zhong*, J.-L. Zuo, M. Robert*,
T.-B. Lu* e202506060

Self-Photosensitizing Cobalt Complexes
for Photocatalytic CO₂ Reduction Coupled
with CH₃OH Oxidation

Self-photosensitizing non-noble metal
complexes achieve CO₂ photoreduction
coupled with CH₃OH photooxidation,
simultaneously synthesizing formic
acid in the absence of any additional
photosensitizers and electron sacrificial
reductants.

<figure><img src="image_10.png" /></figure>