Methane Formation Induced via Face-to-Face Orientation of Cyclic Fe Porphyrin Dimer in Photocatalytic CO2 Reduction

Iron porphyrins are known to provide CH4 as an eight-electron reduction product of CO2 in a photochemical reaction. However, there are still some aspects of the reaction mechanism that remain unclear. In this study, we synthesized iron porphyrin dimers and carried out the photochemical CO2 reduction reactions in N,N-dimethylacetamide (DMA) containing a photosensitizer in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. We found that, despite a low catalytic turnover number, CH4 was produced only when these porphyrins were facing each other. The close proximity of the cyclic dimers, distinguishing them from a linear Fe porphyrin dimer and monomers, induced multi-electron CO2 reduction, emphasizing the unique role of their structural arrangement in CH4 formation.


Introduction
The rise in atmospheric CO 2 concentration has led to serious impacts on the environment, emphasizing the urgent requirement for CO 2 reduction.Many attempts have been made to utilize light energy to reduce CO 2 and convert it into energy-rich substances similar to photosynthesis.One significant challenge is the selective reduction of CO 2 while suppressing thermodynamically favorable proton reduction, which generates hydrogen, and this has been addressed using metal complexes.For instance, in artificial Z-scheme systems using semiconductor photocatalysts, the incorporation of metal complex catalysts at reduction sites enables selective CO 2 reduction under visible light irradiation.This process effectively extracts electrons from water, and simultaneously minimizes hydrogen generation, producing CO and formic acid [1,2].Abundant earth elements have been used as the central metal; among them, Fe porphyrin complexes have been extensively studied owing to their high selectivity and activity [3,4].The introduction of peripheral proton-donating groups (Fe-o-OH in Figure 1) considerably improved the catalytic activity of Fe porphyrins, resulting in selective CO production [5].In addition, linking Fe porphyrins through an o-phenylene or a urea group enhanced CO production [6,7].The Fe porphyrin substituted with p-trimethylammoniophenyl groups (Fe-p-TMA in Figure 1) also exhibited high CO production activity in the electrochemical CO 2 reduction [8].Interestingly, under photochemical CO 2 reduction conditions using tris(2-phenylpyridine)iridium (Ir(ppy) 3 ) as a photosensitizer and triethylamine (TEA) as a sacrificial electron donor in acetonitrile, Fe-o-OH, and Fe-p-TMA yielded CH 4 along with CO [9].The formation of CH 4 was observed when using an organic photosensitizer, 3,7-(4-biphenyl)-1-naphthalene-10-phenoxazine (Phen2), instead of Ir(ppy) 3 in N,N-dimethylformamide (DMF) solution containing Fe-p-TMA and sacrificial electron donors [10].The reaction mechanism indicated the involvement of the Fe(II)-CO intermediate.However, although computational studies have proposed this reaction mechanism [11], the actual formation mechanism of CH 4 , an eight-electron reduction product of CO 2 , remains unclear.

Results and Discussion
Fe 2 -CP2 m and Fe 2 -CP2 p were, respectively, prepared by demetallation of Zn 2 -CP2 m and Zn 2 -CP2 p [12] to obtain the corresponding free-base porphyrins, followed by the introduction of Fe ions.Fe 2 -P2 was prepared by the introduction of Fe ions into the corresponding free-base precursor [12].In the UV-vis absorption spectra, the Soret band of Fe 2 -P2 red shifted compared with those of Fe 2 -CP2 m , Fe 2 -CP2 p , and Fe(III) tetraphenylporphyrin chloride (FeTPP(Cl)) (Figure 2, left).The red-shifted band is attributed to the head-to-tail excitonic coupling between the two transition dipoles of X in the anticonformation of Fe 2 -P2, indicating that a linear structure is stable in Fe 2 -P2, as observed for the corresponding Zn porphyrins (Figure 1) [12].The cyclic voltammogram (CV) and differential pulse voltammogram (DPV) of Fe 2 -P2 in Ar-saturated DMF showed three reversible redox waves at −0.74, −1.55, and −2.23 V vs. Fc/Fc + (−0.27, −1.08, and −1.76 V vs. SCE) [14], which corresponded, respectively, to Fe(III/II), Fe(II/I), and Fe(I/0) couples (Figure 2, right and Figure S12).The observation of only three redox waves indicates that each porphyrin is reduced independently and that the electronic interaction between the two porphyrins through the bpy linker is negligible.Meanwhile, although the CVs of Fe 2 -CP2 m and Fe 2 -CP2 p showed redox waves at the similar positions to those of Fe 2 -P2, the second redox wave of Fe 2 -CP2 m split into two, indicating that the porphyrins in close proximity, arranged in a face-to-face configuration, exhibit electrical interaction with each other.Under a CO 2 atmosphere, Fe 2 -P2, Fe 2 -CP2 m , and Fe 2 -CP2 p showed similar catalytic currents at the third waves of Fe(I/0) of the Fe porphyrin dimers (Figure 2, right).Lewis acids, known to enhance the activity of Fe porphyrin catalysts for CO 2 reduction [15,16], were anticipated to exhibit differences in their interactions with the bpy moiety between cyclic and linear structures.However, no significant difference among three porphyrin dimers in the catalytic currents was observed for the electrochemical CO 2 reduction even when using water and/or metal ion additives (Figures S13-S15 and Table S1).
Molecules 2024, 29, x FOR PEER REVIEW 3 of 12 second redox wave of Fe2-CP2m split into two, indicating that the porphyrins in close proximity, arranged in a face-to-face configuration, exhibit electrical interaction with each other.Under a CO2 atmosphere, Fe2-P2, Fe2-CP2m, and Fe2-CP2p showed similar catalytic currents at the third waves of Fe(I/0) of the Fe porphyrin dimers (Figure 2, right).Lewis acids, known to enhance the activity of Fe porphyrin catalysts for CO2 reduction [15,16], were anticipated to exhibit differences in their interactions with the bpy moiety between cyclic and linear structures.However, no significant difference among three porphyrin dimers in the catalytic currents was observed for the electrochemical CO2 reduction even when using water and/or metal ion additives (Figures S13-S15 and Table S1).We first attempted to perform photocatalytic CO2 reduction using Fe2-P2 in the presence of Ir(ppy)3 as a photosensitizer and TEA as an electron donor, but no reduction product was detected.The present Fe porphyrin dimers exhibited more negative reduction potentials (−2.17 to −2.23 V vs. Fc/Fc + and −1.70 to −1.76 V vs. SCE for Fe(I/0)) compared with the reported Fe porphyrins, such as Fe-o-OH (−1.57V vs. SCE for Fe(I/0)) and Fe-p-TMA (−1.47 V vs. SCE for Fe(I/0)) [17].According to the energy diagram (Figure S16), the oxidation quenching process [18] of the excited Ir(ppy)3 by either Fe2-P2 or Fe2-CP2m is thermodynamically less favorable when using TEA.Consequently, we used BIH with a stronger reducing power as an electron donor instead of TEA.The energy diagram and results of a phosphorescence quenching experiment support that electron transfer from BIH to the excited Ir(ppy)3 (i.e., reductive quenching) [18], followed by a reduction We first attempted to perform photocatalytic CO 2 reduction using Fe 2 -P2 in the presence of Ir(ppy) 3 as a photosensitizer and TEA as an electron donor, but no reduction product was detected.The present Fe porphyrin dimers exhibited more negative reduction potentials (−2.17 to −2.23 V vs. Fc/Fc + and −1.70 to −1.76 V vs. SCE for Fe(I/0)) compared with the reported Fe porphyrins, such as Fe-o-OH (−1.57V vs. SCE for Fe(I/0)) and Fe-p-TMA (−1.47 V vs. SCE for Fe(I/0)) [17].According to the energy diagram (Figure S16), the oxidation quenching process [18] of the excited Ir(ppy) 3 by either Fe 2 -P2 or Fe 2 -CP2 m is thermodynamically less favorable when using TEA.Consequently, we used BIH with a stronger reducing power as an electron donor instead of TEA.The energy diagram and results of a phosphorescence quenching experiment support that electron transfer from BIH to the excited Ir(ppy) 3 (i.e., reductive quenching) [18], followed by a reduction in the Fe(I) porphyrins, can occur (Figures S17-S19).Here, BIH was used in much larger quantities (10 mM) than the catalyst (10 µM), and it was expected that the reaction could proceed, although it was slightly unfavorable thermodynamically.Under the reaction conditions for the photocatalytic CO 2 reduction using BIH and Fe-o-OH, we observed the production of CO during the catalytic reaction (Table S2).However, the amount of BIH consumed was significantly greater than the production of CO (Figure S21).The NMR spectra after irradiation in the presence of CO 2 revealed the formation of unidentified BIH decomposition products (Figure S22) rather than the formation of BI + , which is typically observed in reactions with the [Ru(bpy) 3 ] 2+ photosensitizer as a two-electron oxidation product of BIH [13].A highly reactive BI radical, formed via the oxidation and deprotonation of BIH, is likely to react with CO 2 to generate the unidentified products (Figure S23).This is hypothesized because Ir(ppy) 3 would not accept the electron from the BI radical.The investigation of the effects of solvents and additives showed that N,N-dimethylacetamide (DMA) [19] promoted CO production and suppressed BIH consumption more than acetonitrile (entries 4−6 in Table S3 and Figure S24).Therefore, in the subsequent experiments, the photoreactions were performed using DMA.
Photocatalytic CO 2 reductions using Fe 2 -P2 and Fe 2 -CP2 m (10 µM) in DMA containing BIH (10 mM) and Ir(ppy) 3 (0.2 mM) under 450 nm light were performed.The turnover numbers (TONs) of the reduction products against the Fe porphyrin dimers are shown in Figure 3.No detectable amounts of CO, H 2 , or CH 4 were found in the absence of any one of the Fe porphyrin dimers, Ir(ppy) 3 , and light, whereas only H 2 was detected under Ar instead of CO 2 .While Fe 2 -P2 and Fe 2 -CP2 m produced CO, the amount of CO was smaller in Fe 2 -CP2 m .Interestingly, a small amount of CH 4 was formed in Fe 2 -CP2 m (Figure 3b).Under the same conditions, CH 4 was not detected in Fe 2 -P2 (Figure 3a), Fe-o-OH, or FeP-phen, which is a model monomer with a diimine ligand.Figure 3b shows CH 4 production with an induction period and lower CO production than that in Figure 3a, indicating that the CH 4 was formed via the reduction in CO, as reported previously [9,10].
in the Fe(I) porphyrins, can occur (Figures S17-S19).Here, BIH was used in much larger quantities (10 mM) than the catalyst (10 µM), and it was expected that the reaction could proceed, although it was slightly unfavorable thermodynamically.Under the reaction conditions for the photocatalytic CO2 reduction using BIH and Fe-o-OH, we observed the production of CO during the catalytic reaction (Table S2).However, the amount of BIH consumed was significantly greater than the production of CO (Figure S21).The NMR spectra after irradiation in the presence of CO2 revealed the formation of unidentified BIH decomposition products (Figure S22) rather than the formation of BI + , which is typically observed in reactions with the [Ru(bpy)3] 2+ photosensitizer as a two-electron oxidation product of BIH [13].A highly reactive BI radical, formed via the oxidation and deprotonation of BIH, is likely to react with CO2 to generate the unidentified products (Figure S23).This is hypothesized because Ir(ppy)3 would not accept the electron from the BI radical.The investigation of the effects of solvents and additives showed that N,Ndimethylacetamide (DMA) [19] promoted CO production and suppressed BIH consumption more than acetonitrile (entries 4−6 in Table S3 and Figure S24).Therefore, in the subsequent experiments, the photoreactions were performed using DMA.
Photocatalytic CO2 reductions using Fe2-P2 and Fe2-CP2m (10 µM) in DMA containing BIH (10 mM) and Ir(ppy)3 (0.2 mM) under 450 nm light were performed.The turnover numbers (TONs) of the reduction products against the Fe porphyrin dimers are shown in Figure 3.No detectable amounts of CO, H2, or CH4 were found in the absence of any one of the Fe porphyrin dimers, Ir(ppy)3, and light, whereas only H2 was detected under Ar instead of CO2.While Fe2-P2 and Fe2-CP2m produced CO, the amount of CO was smaller in Fe2-CP2m.Interestingly, a small amount of CH4 was formed in Fe2-CP2m (Figure 3b).Under the same conditions, CH4 was not detected in Fe2-P2 (Figure 3a), Fe-o-OH, or FeP-phen, which is a model monomer with a diimine ligand.Figure 3b shows CH4 production with an induction period and lower CO production than that in Figure 3a, indicating that the CH4 was formed via the reduction in CO, as reported previously [9,10].Next, we investigated the photocatalytic CO2 reduction reactions in which the organic dye Phen2, rather than Ir(ppy)3, was used as the photosensitizer (Figure 4) [10].During the irradiation of the solution in the absence of the catalyst (Figure S28), BIH decomposition was still observed without reduction products of CO2.However, reducing the light intensity to 5 mW suppressed BIH decomposition.When Fe2-P2 was used as the catalyst, a linear CO formation was observed for up to 4 h under the 5 mW light intensity (Figure S29). Figure 4 shows the TONs of the reduction products against the Fe porphyrin dimers in DMA containing BIH (10 mM) and Phen2 (1.0 mM) under 420 nm light (5 mW).As observed with Ir(ppy)3, CH4 was formed with induction periods in the cyclic structure Next, we investigated the photocatalytic CO 2 reduction reactions in which the organic dye Phen2, rather than Ir(ppy) 3 , was used as the photosensitizer (Figure 4) [10].During the irradiation of the solution in the absence of the catalyst (Figure S28), BIH decomposition was still observed without reduction products of CO 2 .However, reducing the light intensity to 5 mW suppressed BIH decomposition.When Fe 2 -P2 was used as the catalyst, a linear CO formation was observed for up to 4 h under the 5 mW light intensity (Figure S29). Figure 4 shows the TONs of the reduction products against the Fe porphyrin dimers in DMA containing BIH (10 mM) and Phen2 (1.0 mM) under 420 nm light (5 mW).As observed with Ir(ppy) 3 , CH 4 was formed with induction periods in the cyclic structure (Fe 2 -CP2 m and Fe 2 -CP2 p ), whereas no CH 4 was detected in the linear structure (Fe 2 -P2).In addition, the CO productions in Figure 4b,c were smaller than those in Figure 4a.In the previous systems involving Fe-p-TMA as the catalyst and TEA as the electron donor, the addition of a proton source, such as trifluoroethanol (TFE), enhanced the formation of CO and CH 4 [9,10].However, using the cyclic porphyrin dimer, the CH 4 production decreased with the addition of TFE and was completely suppressed by PhOH, while TFE and PhOH enhanced the CO production.The addition of Mg ions decreased the production of CO and CH 4 (Figure S30).
(Fe2-CP2m and Fe2-CP2p), whereas no CH4 was detected in the linear structure (Fe2-P2).In addition, the CO productions in Figure 4b,c were smaller than those in Figure 4a.In the previous systems involving Fe-p-TMA as the catalyst and TEA as the electron donor, the addition of a proton source, such as trifluoroethanol (TFE), enhanced the formation of CO and CH4 [9,10].However, using the cyclic porphyrin dimer, the CH4 production decreased with the addition of TFE and was completely suppressed by PhOH, while TFE and PhOH enhanced the CO production.The addition of Mg ions decreased the production of CO and CH4 (Figure S30).In conventional catalytic reactions using Phen2 as the photosensitizer, the reaction typically involves an oxidative quenching process, where electrons are transferred from the excited Phen2 to the catalyst [18,20].However, this system would proceed via a reductive quenching process involving electron transfer from the electron donor to the excited Phen2.The fluorescence quenching experiments of Phen2 by BIH demonstrate that the electron transfer from BIH to the excited singlet state of Phen was efficient (Figure S32).However, the quenching efficiency of the excited Phen2 (ηq), which was estimated from the Stern-Volmer plot [18], strongly depended on the concentration of BIH because of the shorter fluorescence lifetime ([BIH] = 10 mM, ηq = 7%; [BIH] = 100 mM, and ηq = 41%).Meanwhile, we observed that the TONs were less dependent on the BIH concentration (10−100 mM in Figure 5), suggesting that the electron transfer from BIH mainly occurred not via the excited singlet state but via the long-lived excited triplet state of Phen2 [21], which has a lifetime of 480 µs [22].In conventional catalytic reactions using Phen2 as the photosensitizer, the reaction typically involves an oxidative quenching process, where electrons are transferred from the excited Phen2 to the catalyst [18,20].However, this system would proceed via a reductive quenching process involving electron transfer from the electron donor to the excited Phen2.The fluorescence quenching experiments of Phen2 by BIH demonstrate that the electron transfer from BIH to the excited singlet state of Phen was efficient (Figure S32).However, the quenching efficiency of the excited Phen2 (η q ), which was estimated from the Stern-Volmer plot [18], strongly depended on the concentration of BIH because of the shorter fluorescence lifetime ([BIH] = 10 mM, η q = 7%; [BIH] = 100 mM, and η q = 41%).Meanwhile, we observed that the TONs were less dependent on the BIH concentration (10−100 mM in Figure 5), suggesting that the electron transfer from BIH mainly occurred not via the excited singlet state but via the long-lived excited triplet state of Phen2 [21], which has a lifetime of 480 µs [22].The photocatalytic CO2 reduction using Fe2-CP2p and 100 mM BIH produced CH4 with TON = 3.5 against the Fe porphyrin dimer after 35 h, exceeding the amount of the Fe porphyrin units (Figure S34).The capillary electrophoresis showed that the TON of formic acid reached 38 after irradiation for 18 h in the presence of Fe2-CP2p and BIH (100 mM, Figure S34).We conducted isotopic experiments under 12 CO2 and 13 CO2 atmospheres.In gas chromatography/mass spectrometry, 13 CO (m/z = 29) and 13 CH4 (m/z = 17) were detected under a 13 CO2 atmosphere (Figures S35 and S36), confirming that the carbon source of CO and CH4 was CO2.The 1 H and 13 C NMR spectra were measured in a DMA-d9 solution during irradiation under 12 CO2 and 13 CO2 atmospheres (Figures S37-S39).The spectral changes during light irradiation showed that BIH was almost completely consumed after 21 h, indicating that the catalytic reaction stopped due to the disappearance of BIH.No reduction products, including methanol or formaldehyde, were observed, except for formic acid, which showed a doublet peak at 8.68 ppm with a coupling constant of J13C-H = 175 Hz and a singlet peak at 8.72 ppm in 1 H NMR spectra under 12 CO2 and 13 CO2 atmospheres, respectively (Figure S38).A peak at 167 ppm was assigned to HC(O)-, which was correlated with the doublet proton peak at 8.68 ppm in the heteronuclear multiple bond connectivity (HMBC), observed in the 13 C NMR spectrum under only a 13 CO2 atmosphere (Figures S39 and S40) [19].Formic acid was not observed in the absence of the Fe porphyrins, indicating that formic acid is produced via the CO2 reduction catalyzed by the Fe porphyrins, and it is not directly formed by the chemical reaction between BIH and CO2 [23,24].Furthermore, a peak appeared at 222 ppm in the 13 C NMR spectrum only under a 13 CO2 atmosphere (indicated by an asterisk in Figure S39).The peak can be attributed to the Fe- 13 CO signal [25], indicating the formation of a carbonyl intermediate during irradiation, as observed in previous reports.In addition, only under a 13 CO2 atmosphere an intense peak was observed at 172 ppm (Figure S39), which was correlated with the proton peaks at 7.4, 3.4, 2.7, 2.4, and 1.8 ppm in HMBC (Figure S40).Although no clear attribution could be established, it likely corresponded to the reaction products of BIH and CO2, an adduct of CO2 with the BI radical caused by the oxidation and deprotonation of BIH.The photocatalytic CO 2 reduction using Fe 2 -CP2 p and 100 mM BIH produced CH 4 with TON = 3.5 against the Fe porphyrin dimer after 35 h, exceeding the amount of the Fe porphyrin units (Figure S34).The capillary electrophoresis showed that the TON of formic acid reached 38 after irradiation for 18 h in the presence of Fe 2 -CP2 p and BIH (100 mM, Figure S34).We conducted isotopic experiments under 12 CO 2 and 13 CO 2 atmospheres.In gas chromatography/mass spectrometry, 13 CO (m/z = 29) and 13 CH 4 (m/z = 17) were detected under a 13 CO 2 atmosphere (Figures S35 and S36), confirming that the carbon source of CO and CH 4 was CO 2 .The 1 H and 13 C NMR spectra were measured in a DMA-d 9 solution during irradiation under 12 CO 2 and 13 CO 2 atmospheres (Figures S37-S39).The spectral changes during light irradiation showed that BIH was almost completely consumed after 21 h, indicating that the catalytic reaction stopped due to the disappearance of BIH.No reduction products, including methanol or formaldehyde, were observed, except for formic acid, which showed a doublet peak at 8.68 ppm with a coupling constant of J 13C-H = 175 Hz and a singlet peak at 8.72 ppm in 1 H NMR spectra under 12 CO 2 and 13 CO 2 atmospheres, respectively (Figure S38).A peak at 167 ppm was assigned to HC(O)-, which was correlated with the doublet proton peak at 8.68 ppm in the heteronuclear multiple bond connectivity (HMBC), observed in the 13 C NMR spectrum under only a 13 CO 2 atmosphere (Figures S39 and S40) [19].Formic acid was not observed in the absence of the Fe porphyrins, indicating that formic acid is produced via the CO 2 reduction catalyzed by the Fe porphyrins, and it is not directly formed by the chemical reaction between BIH and CO 2 [23,24].Furthermore, a peak appeared at 222 ppm in the 13 C NMR spectrum only under a 13 CO 2 atmosphere (indicated by an asterisk in Figure S39).The peak can be attributed to the Fe- 13 CO signal [25], indicating the formation of a carbonyl intermediate during irradiation, as observed in previous reports.In addition, only under a 13 CO 2 atmosphere an intense peak was observed at 172 ppm (Figure S39), which was correlated with the proton peaks at 7.4, 3.4, 2.7, 2.4, and 1.8 ppm in HMBC (Figure S40).Although no clear attribution could be established, it likely corresponded to the reaction products of BIH and CO 2 , an adduct of CO 2 with the BI radical caused by the oxidation and deprotonation of BIH.

Synthesis of Fb 2 -CP2 m
The TFA (0.50 mL) was slowly added to Zn 2 -CP2 m (10 mg, 5.9 × 10 −6 mol) in a 10 mL flask, and the mixture was stirred for 2 h.The solution was slowly poured into a saturated NaHCO 3 aqueous solution in an ice bath.The organic layer was transferred to a PFA-coated funnel, and the aqueous layer was extracted with CHCl 3 (10 mL × 3).The combined organic layer was washed with water and dried over anhydrous Na 2 SO 4 .The solvent was evaporated to dryness, resulting in a purple solid, Fb 2 -CP2 m (8.3 mg, 90%). 1

Synthesis of Fb 2 -CP2 p
The TFA (1.0 mL) was slowly added to Zn 2 -CP2 p (8.7 mg, 5.2 × 10 −6 mol) in a 10 mL flask, and the mixture was stirred for 2 h.The solution was slowly poured into a saturated NaHCO 3 aqueous solution in an ice bath.The organic layer was transferred to a PFAcoated funnel and the aqueous layer was extracted with CHCl 3 (10 mL × 3).The combined organic layer was washed with water and dried over anhydrous Na 2 SO 4 .The solvent was evaporated to dryness, giving a purple solid, Fb 2 -CP2 p (7.1 mg, 89%). 1

Photocatalytic CO 2 Reduction
In glass tubes (8.0 mL, i.d.= 10 mm), 1.0 mL of CO 2 -saturated DMA solutions containing BIH was added by 1.0 mL of Ar-saturated DMA solutions containing the Fe porphyrin dimer and the photosensitizer, and the reaction solutions were bubbled with CO 2 gas (purity ≥ 99.995%) for 15 min.Photo-irradiations were carried out using a merrygo-round irradiation apparatus (Iris-MG, Cell Systems, Yokohama, Japan) equipped with LED lamps at λ = 420 nm (FWHM = 18.4 nm).The gaseous reaction products (CO, H 2 , and CH 4 ) were quantified with a gas chromatography system (GC-2014, Shimadzu Science, Kyoto, Japan) equipped with a Shincarbon column (i.d.3.0 mm × 3.0 m) and a thermal conductivity detector (TCD).The product (formate) in the solutions was analyzed with a capillary electrophoresis system (Otuka Electronics Co. CAPI-3300I, Osaka, Japan).

Computational Methods
The DFT calculations were carried out using the Gaussian 09 package of programs [27].Each structure was fully optimized using the B3LYP functional using the 6-31G(d) basis set for all atoms, except Fe, and the standard double-ζ type LANL2DZ basis set with the effective core potential of Hay−Wadt for Fe.The stationary points were verified using the vibrational analysis.

Conclusions
In this study, we show that the photochemical CO 2 reduction yielded CH 4 , an eightelectron reduction product of CO 2 , when Fe porphyrins were placed in a face-to-face arrangement.Although the catalytic turnover number of CH 4 was low in this study, this can be attributed to the significant degradation of BIH.Thus, it is expected that by addressing this issue, we can enhance the catalytic performance.While bimetallic porphyrin complexes have been reported to enhance CO production, to the best of our knowledge, this is the first report on the induction of CH 4 production using bimetallic porphyrins.We anticipate that this finding can contribute to the understanding CH 4 formation mechanisms and provide potential molecular design guidelines for selective CH 4 generation.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29112453/s1, Figure S1 S2; Figure S22: 1 H NMR spectra of the reaction solutions containing BIH and Ir(ppy) 3 before and after irradiation; Figure S23: Decomposition of BIH during irradiation in the presence of PS and CO 2 ; Table S3: Effects of solvents and additives on photocatalytic reaction using Fe-o-OH; Figure S24: Comparison of the reduction products and BIH consumption in Table S3;

Figure 3 .
Figure 3.Time dependence of the reduction products generated during the irradiation of CO 2saturated DMA solutions (2.0 mL) containing (a) Fe 2 -P2 and (b) Fe 2 -CP2 m (10 µM) in the presence of BIH (10 mM) and Ir(ppy) 3 (0.2 mM) at 450 nm using a merry-go-round apparatus equipped with LED lamps (input power: 10 mW).

Figure 5 .
Figure 5. Relationship between the reduction products and the initial concentration of BIH during irradiation at 420 nm for 4 h in CO2-saturated DMA solutions (2.0 mL) containing Fe2-CP2p (10 µM) and Phen2 (1 mM), using a merry-go-round apparatus equipped with LED lamps (input power: 5 mW).

Figure 5 .
Figure 5. Relationship between the reduction products and the initial concentration of BIH during irradiation at 420 nm for 4 h in CO 2 -saturated DMA solutions (2.0 mL) containing Fe 2 -CP2 p (10 µM) and Phen2 (1 mM), using a merry-go-round apparatus equipped with LED lamps (input power: 5 mW).
: UV-vis absorption and fluorescence spectra of Zn 2 -CP2 m and Fb 2 -CP2 m ; Figure S2: MALDI-TOF mass of Fb 2 -CP2 m ; Figure S3: 1 H NMR spectrum of Fb 2 -CP2 m ; Figure S4: UV-vis absorption and fluorescence spectra of Fb 2 -CP2 m and HCl-treated Fe 2 -CP2 m ; Figures S5-S6: MALDI-TOF mass of Fe 2 -CP2 m and Fb 2 -CP2 p ; Figure S7: 1 H NMR spectrum of Fb 2 -CP2; Figure S8: UV-vis absorption spectra of Fe 2 -CP2 p before and after treatment with HCl aqueous solution; Figure S9: MALDI-TOF mass of Fe 2 -CP2 p ; Figure S10: UV-vis absorption and fluorescence spectra of Fb 2 -P2 and brine-treated Fe 2 -P2; Figure S11: MALDI-TOF mass of Fe 2 -P2; Figure S12: DPVs of Fe 2 -P2, Fe 2 -CP2 m , and Fe 2 -CP2 p ; Figure S13: CVs of Fe 2 -P2, Fe 2 -CP2 m , and Fe 2 -CP2 p collected in CO 2 -saturated DMF in the presence of water; Figure S14: CVs of Fe 2 -P2 and Fe 2 -CP2 m collected in CO 2 -saturated DMF in the presence of M(OTf) n (M = Mg (n =2), La and Gd (n = 3)); Figure S15: CVs of Fe 2 -P2 and Fe 2 -CP2 m in CO 2 -saturated DMF in the presence of M(OTf) n (M = Mg (n =2), La and Gd (n = 3)), and water; Table S1: Effect of each additive on the ratio of the maximum current to i p 0 ; Figure S16: Energy diagram of the photoinduced electron transfer during the formation of Fe(0) species via the oxidative quenching process by either Fe-o-OH or the Fe porphyrin dimer, when Ir(ppy) 3 is used as a photosensitizer and TEA as a sacrificial donor; Figure S17: Energy diagram of the photoinduced electron transfer during the formation of Fe(0) species via the reductive quenching process by BIH, when Ir(ppy) 3 is used as a photosensitizer and BIH as a sacrificial donor; Figure S18: UV-vis absorption and emission spectra of Ir(ppy) 3 in Ar-saturated DMA at 298 K in the presence of various amounts of BIH; Figure S19: Stern-Volmer plot of emission quenching of Ir(ppy) 3 by BIH; Table S2: Photocatalytic reaction using Fe-o-OH in acetonitrile; Figure S20: HPLC charts of the reaction solutions containing BIH and Ir(ppy) 3 before and after irradiation; Figure S21: Comparison of the reduction products and BIH consumption in Table Figure S25: Plots of the amounts of the reduction products and the consumption of BIH after 60 min of irradiation at 450 nm using a merry-go-round apparatus equipped with LED lamps versus the concentration of FeP-phen in CO 2 -saturated DMA in the presence of Ir(ppy) 3 and BIH; Figure S26: Time dependence of the reduction products during the irradiation of CO 2 -saturated DMA solutions containing FeP-phen in the presence of BIH and Ir(ppy) 3 at 450 nm; Figure S27: HPLC charts of the resulting solutions after irradiation in Figure S26, and time dependence of the remaining amount of BIH determined using HPLC; Figure S28: Time dependence of the remaining amount of BIH as per irradiation intensity in blank CO 2 -saturated DMA solutions containing BIH and Phen2 during irradiation at 420 nm; Figure S29: Dependence of CO production on light intensity at 420 nm in CO 2 -saturated DMA solutions containing Fe 2 -P2, BIH, and Phen2; Figure S30: TONs of CO, H 2 , and CH 4 during irradiation at 420 nm for 18 h in CO 2 -saturated DMA solutions containing Fe 2 -CP2 p , BIH, and Phen2 in the presence of acids; Figure S31: UV-vis absorption and fluorescence spectra of Phen2 in Ar-saturated DMA in the presence of various amounts of BIH; Figure S32: Stern-Volmer plot of emission quenching of Phen2 by BIH, Figure S33: Relationship between the reduction products and the initial concentration of BIH during irradiation at 420 nm for 4 h and 18 h in CO 2 -saturated DMA solutions containing Fe 2 -CP2 p and Phen2; Figure S34: Time dependence of the reduction products during the irradiation of CO 2 -saturated DMA solutions containing Fe 2 -CP2 p in the presence of BIH and Phen2 at 420 nm, and gas chromatogram of the gaseous reaction products after the irradiation for 35 h; FigureS35: Gas chromatograms of the resulting gas-phase products after irradiation at 420 nm for 14 h under 12 CO 2 or 13 CO 2 atmosphere in DMA solutions containing Fe 2 -CP2 p , BIH, and Phen2 obtained using mass spectroscopy; FigureS36: Mass spectra of CH 4 generated under12 CO 2 and 13 CO 2 atmosphere, and gas chromatogram of the products obtained from the reaction under13 CO 2 atmosphere, plotted for each m/z; FigureS37: 1 H NMR spectra of the reaction solutions containing Fe 2 -CP2 p , BIH, and Phen2 during irradiation at 420 nm under a 13 CO 2 atmosphere; FigureS38: Comparison of the 1 H NMR spectra of reaction solutions containing Fe 2 -CP2 p , BIH, and Phen2 after irradiation at 420 nm for 21 h under12 CO 2 and 13 CO 2 atmospheres; FigureS39:13 C NMR spectra of the reaction solutions containing Fe 2 -CP2 p , BIH, and Phen2 during irradiation at 420 nm under a 13 CO 2 atmosphere; FigureS40: HMBC of the reaction solutions containing Fe 2 -CP2 p , BIH, and Phen2 during irradiation for 21 h at 420 nm under12 CO 2 and 13 CO 2 atmospheres; FigureS41: HSQC of the reaction solutions containing Fe 2 -CP2 p , BIH, and Phen2 during irradiation for 21 h at 420 nm under a 13 CO 2 atmosphere; FigureS42: 1 H-1 H COSY of the reaction solutions containing Fe 2 -CP2 p , BIH, and Phen2 during irradiation for 21 h at 420 nm under a 13 CO 2 atmosphere.Scheme S1.Synthetic routes of Fe2-CP2m, p. Scheme S2.Synthetic route of Fe2-P2.Scheme S3.Synthetic route of FeP-phen.