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. 2023 Aug 22;3(6):384–392. doi: 10.1021/acsorginorgau.3c00027

Redox-Active Ligand Assisted Multielectron Catalysis: A Case of Electrocatalyzed CO2-to-CO Conversion

Wen-Wen Yong †,‡, Hong-Tao Zhang †,*, Yu-Hua Guo †, Fei Xie †, Ming-Tian Zhang †,*
PMCID: PMC10704577  PMID: 38075450

Abstract

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The selective reduction of carbon dioxide remains a significant challenge due to the complex multielectron/proton transfer process, which results in a high kinetic barrier and the production of diverse products. Inspired by the electrostatic and H-bonding interactions observed in the second sphere of the [NiFe]-CODH enzyme, researchers have extensively explored these interactions to regulate proton transfer, stabilize intermediates, and ultimately improve the performance of catalytic CO2 reduction. In this work, a series of cobalt(II) tetraphenylporphyrins with varying numbers of redox-active nitro groups were synthesized and evaluated as CO2 reduction electrocatalysts. Analyses of the redox properties of these complexes revealed a consistent relationship between the number of nitro groups and the corresponding accepted electron number of the ligand at −1.59 V vs. Fc+/0. Among the catalysts tested, TNPPCo with four nitro groups exhibited the most efficient catalytic activity with a turnover frequency of 4.9 × 104 s–1 and a catalytic onset potential 820 mV more positive than that of the parent TPPCo. Furthermore, the turnover frequencies of the catalysts increased with a higher number of nitro groups. These results demonstrate the promising design strategy of incorporating multielectron redox-active ligands into CO2 reduction catalysts to enhance catalytic performance.

Keywords: Carbon Dioxide Reduction, Cobalt Porphyrin, Redox-Active Ligand, Metal−Ligand Cooperation, Electrochemical Catalysis

Introduction

Conversion of carbon dioxide into valuable chemicals, such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), and methane (CH4), represents a promising strategy for mitigating the impacts of global warming and climate change.1−3 While the reduction reaction of CO2 (CO2RR) has attracted substantial research attention, selectively catalyzing CO2 reduction at low overpotential remains a challenging task due to the complexities of multielectron/proton transfer and the high activation barrier of CO2.4−10 In this context, the [NiFe] CO dehydrogenases ([NiFe]-CODHs), exemplified in Figure 1a, have demonstrated the ability to catalyze the reversible conversion between CO2 and CO at near the thermodynamic potential under physiological condition.11−16 This remarkable capability has inspired researchers to investigate the structure and functions of [NiFe]-CODH through synthetic models.17−29 Certainly, the distinctive catalytic performance of [NiFe]-CODH arises from collaborative interactions, including NiFe heterobimetallic center, iron–sulfur cluster, and second-sphere cofactors.16,30−38

Figure 1.

Figure 1

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(a) Functional iron–sulfur cluster and second-sphere protein residues regulate the electrons/protons transfer to the Ni–Fe catalytic center in [NiFe]-CODH. (b) Designed molecular structures of Co porphyrins with different electron capacity in this work.

Within the second-sphere environment of [NiFe]-CODH, two crucial components, namely, imidazolium groups derived from histidine and ammonium pendants from lysine, have been identified to stabilize reaction intermediates and promote proton transfer through electrostatic and H-bonding interactions.13,39 Building upon this knowledge, the design strategies involving the second coordination sphere have been widely employed to enhance the catalyst activity and reduce the catalytic overpotential in CO2RR.40−44 For instance, the incorporation of phenol units as the local proton source has shown a significant enhancement of turnover frequency (TOF) of iron tetraphenylporphyrin in the electroreduction of CO2 to CO.45 Furthermore, amines,46−48 amides,49,50 ureas,51−54 and guanidines55,56 have been integrated into catalyst scaffolds as proton shuttles to boost CO2 reduction. Cationic ammonium57−61 and imidazolium62,63 units have also been proven to be effective in stabilizing catalytic intermediates and promoting the C–O bond cleavage through electrostatic effect, thus reducing the energy barrier in CO2 reduction catalysis. Notably, although extensive efforts have been made in the electrostatic and hydrogen bonding design within the second sphere, studies investigating the mimicry of iron–sulfur clusters to regulate electron transfer in CO2 reduction are still scarce. Although some redox-active ligands have been utilized in the design of CO2 reduction catalysts, most of these ligands possess only one-electron capacity.64−68 To the best of our knowledge, the use of redox-active ligands capable of accommodating more than two electrons in CO2 reduction catalysts has not been reported.

Based on the inspiration from [NiFe]-CODH and our recent works toward multielectron transfer catalysis promoted by redox-active ligands,69,70 we herein designed a series of cobalt(II) tetraphenylporphyrins with varying numbers of nitrophenyl units (Figure 1b, NPPCo with one nitro, BNPPCo with two nitros, TriNPPCo with three nitros, TNPPCo with four nitros) for application in electrocatalytic CO2 reduction. Electrochemical investigations have revealed the relationship between the number of electrons transferred and the number of nitro groups in the complexes, providing an orderly electron tuning platform. Intriguingly, the catalytic overpotentials of nitro-substituted catalysts unexpectedly decrease by 820 mV compared to cobalt tetraphenylporphyrin (TPPCo). Furthermore, the catalysts with higher electron storage capacity attributed to the nitroporphyrin ligands have exhibited a 10-fold increase in TOFs for CO2 reduction to CO. This study demonstrates the efficacy of employing multielectron redox-active ligand as a viable strategy to enhance the performance of CO2 electrocatalysts, providing valuable guidance for the rational design of catalysts for multielectron transfer process.

Results and Discussion

Synthesis and Characterization

Nitro-substituted tetraphenylporphyrin ligands (Scheme 1) were synthesized and isolated by a modified literature procedure (the details are given in the Supporting Information).71−73 Corresponding Co(II) porphyrins were prepared by mixing the ligands with Co(OAc)2 salt under reflux conditions (Scheme 1)73 and showed paramagnetic character by NMR. X-ray crystallography (Figure 2) showed that the Co(II) atom in the solid state is coordinated by four N atoms of the ligand and in a square-planar geometry similar to the reported structure,74,75 and all benzene rings are perpendicular to the porphyrin ring to reduce steric hindrance. Notably, for the isolated BNPP ligand and BNPPCo, two nitro units are situated on opposite sides (Figures 2a and S1). These complexes were also characterized by high-resolution mass spectrometry (HRMS, Figures S19–S22). In addition, these cobalt porphyrins also exhibited characteristic Soret (415–420 nm) and Q (540–600 nm) bands in their UV–vis spectra in DMF (Figure S27, Table S1).76−78 To demonstrate the redox properties of the ligands, analogous Zn(II) porphyrins were also prepared and characterized, the details of which are given in the Supporting Information.79

Scheme 1. Synthesis of Nitro-Substituted Tetraphenylporphyrin Co(II) Complexes in This Work.

Scheme 1

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The complex TPPCo was obtained through commercial purchase.

Figure 2.

Figure 2

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Crystallographic structures of BNPPCo (a) and TNPPCo (b) with thermal ellipsoids set at 50% probability. Hydrogen atoms have been omitted for clarity.

The Redox Behaviors of Co(II) Porphyrins

The electrochemical properties of the Co(II) porphyrins were studied by cyclic voltammetry (CV) in a degassed DMF solution with 0.1 M nBu4NPF6. Using BNPPCo as an example, two reversible waves were observed at −1.21 and −1.59 V (vs Fc+/0, Figure 3a). The potential differences between the cathodic and anodic peaks are 61 and 77 mV, respectively, but the second cathodic peak current (−9.2 μA) is twice higher than that of the first wave (−4.6 μA). This result suggests that the electron transfer numbers are different in these two reduction processes. Compared to BNPPCo, the first redox wave of TPPCo is observed at −1.28 V (Figure 3a), which is shifted negatively by 68 mV due to the absence of the nitro group but maintains the same current. The second redox event of TPPCo occurs at −2.41 V, which is 820 mV more negative than that of BNPPCo, but the peak area is equivalent to that of the first peak. When the Co(II) ion in BNPPCo is replaced by a redox-inactive Zn(II) ion, only one reversible wave at −1.53 V is observed within the same range (Figure 3b), which belongs to ligand-centered reduction. Thus, the two reduction waves of BNPPCo can be identified as CoII/CoI couple of Co sites (at −1.21 V)80 and the ligand-centered reduction (at −1.59 V). Similar to BNPPCo, other nitro-substituted tetraphenylporphyrin Co(II) complexes also exhibit two reversible waves at −1.24 V–−1.15 V (Co centered reduction) and −1.59 V (ligand-centered reduction). As the introduced nitro groups increase, the reduction of the metal site becomes easier, but the ligand-centered reductions all occur at the invariant −1.59 V and are insensitive to the electronic effect of the nitro (Figure S29 and Table 1). Additionally, the peak currents of the ligand reduction increase significantly with an increase in the number of nitro groups (Figure S29).

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Figure 3.

Figure 3

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(a) CVs of 0.3 mM BNPPCo and TPPCo under Ar in the initial negative scan direction. (b) CV comparison between BNPPCo and BNPPZn under Ar in the initial negative scan direction. (c) DPV of 0.3 mM BNPPCo under Ar in the initial negative scan direction. (d) Plot of the ratio of electron transfer number (n2/n1) between the second and the first reduction waves on the nitro number of the complexes. Test conditions: glassy carbon electrode as working electrode, in DMF with 0.1 M nBu4NPF6, scan rate of CVs is 100 mV/s.

Table 1. Half-Wave Potentials (E1/2) of the Derived Co(II) or Zn(II) Porphyrins in Degassed N,N-Dimethylformamide Solution.

  potentials in DMF (V vs Fc+/0)a
complexes CoII/I L2–/(2+n)–
TPPCo –1.28 –2.41 (n = 1)
NPPCo –1.24 –1.59 (n = 1)
BNPPCo –1.21 –1.59 (n = 2)
TriNPPCo –1.19 –1.59 (n = 3)
TNPPCo –1.15 –1.59 (n = 4)
BNPPZn   –1.53 (n = 2)
TriNPPZn   –1.53 (n = 3)
TNPPZn   –1.53 (n = 4)
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a

0.1 M nBu4NPF6 was used as the electrolyte solution; potentials were determined by the DPV method.

Differential pulse voltammetry (DPV) to determine the electron transfer number (n) in ligand reduction waves was carried out (Figures 3c and S31). According to eq 1, the ratio of the electron transfer number (n2/n1) is equal to the ratio of the corresponding peak areas (area2/area1). For BNPPCo, the ratio of the area of the second peak to that of the first peak is 1.98 (Figure 3c), indicating that the wave at −1.59 V is a two-electron reduction process. Similarly, the ratios of n2/n1 in other complexes are 0.92 (NPPCo), 3.20 (TriNPPCo), and 4.08 (TNPPCo), respectively, which are linearly correlated to the number of nitro groups (Figure 3d and Table S2). Significantly, the ligand reduction potentials of four cobalt porphyrins are independent of the number of nitro groups, and all are at −1.59 V, consistent with the one-electron reduction of nitrobenzene at −1.55 V vs Fc+/0.81 The above results support that the site of ligand reduction under this condition is the nitrophenyl units rather than the porphyrin ring. And introduced redox nitrophenyl units can act as electron reservoirs, providing a molecular platform for regulating the electron transfer in catalysis.

Catalytic Performance of Co(II) Porphyrins

When the solution was saturated with CO2, BNPPCo showed a significant catalytic current enhancement at the second reduction wave but the first reduction peak remained unchanged (Figure 4a). Scan rate (v) dependence experiments (Figure 4b) showed that the normalized currents at the second peak increased with a decreasing scan rate, consistent with a typical catalytic behavior. The onset potential for this catalytic process appears at −1.50 V vs. Fc+/0. And constant normalized currents of the first peak indicate a diffusion-controlled reduction event. Compared to the electrochemical behavior under argon, this catalytic process corresponds to the CO2 reduction. The controlled-potential electrolysis (CPE) with BNPPCo was then performed at −1.59 V in a CO2-saturated DMF solution. During the 4 h of electrolysis, the electrolytic current remained constant (Figure S32), and gas chromatographic analysis showed that the CO production was continuously released (Figures S33 and S34). This result indicates that this catalytic process is the two-electron reduction of CO2 to CO.

Figure 4.

Figure 4

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(a) CVs with 0.3 mM BNPPCo or without under Ar or saturated CO2 at 100 mV/s at initial negative scan direction. (b) Normalized CVs of 0.3 mM BNPPCo under saturated CO2 at initial negative scan direction with different scan rates. (c) CV comparison between different Co(II) porphyrins under saturated CO2 at initial negative scan direction. (d) Comparison of turnover frequencies (i.e., kobs) of the Co(II) porphyrins with different number of nitro via both FOWA and icat/id methods.

In addition to BNPPCo, other nitro-substituted Co(II) porphyrins are also active for electrocatalytic CO2-to-CO conversion at the same onset potentials (Figures 4c and S35–S39). However, TPPCo without a nitro unit does not start to catalyze CO2 reduction until −2.32 V, suggesting that the redox active nitrophenyl unit can significantly reduce catalytic overpotential by ∼820 mV and make the catalytic overpotential of CO2-to-CO conversion only 270 mV.45 By comparing the catalytic peak currents and the icat/id values (icat is the catalytic peak current, and id is the diffusion current of the CoII/I reduction peak), we showed a dependence between the catalytic activity and the number of nitro groups in the catalysts, with more nitro groups having higher activity (Figure 4c). The maximum catalytic ability of the Co(II) porphyrins was further quantified by foot-of-the-wave analysis (FOWA),82 and the observed maximum turnover frequency (TOFmax) value of TNPPCo (4.9 × 104 s–1) represents a 27-fold increase over the single nitro-substituted NPPCo (1.8 × 103 s–1), a 6-fold increase over BNPPCo (7.5 × 103 s–1), and 2-fold increase over TriNPPCo (2.4 × 103 s–1) (Figures 4d and S40). This result indicates that the nitrophenyl moiety indeed can enhance the efficiency of the Co porphyrin catalyzing CO2-to-CO conversion. Of course, we also evaluated the TOF of the catalysts at the catalytic peak potential (i.e., kinetic condition) by the classical icat/id method (Figure S41).83 Although the observed catalytic rates under kinetic conditions are greatly underestimated in contrast to FOWA conditions, the trend of the nitrophenyl moiety’s regulation on activity remains consistent (Figure 4d). It is worth noting that BNPPZn also responded to the CO2 molecule after the ligand reduction (Figure S42), similar to the reported zinc CO2 reduction catalysts with porphyrin ligand,84,85 but BNPPZn with more positive onset potential. The above results confirm that the redox-active nitrophenyl moieties as electron reservoir do work for the electron transfer of the multielectron catalytic process.

Kinetic and Mechanistic Analysis

Due to the poor coordination ability of CO2, CO2 binding to a metal catalyst becomes a major concern in CO2 reduction catalysis. Here the CO2 binding was first investigated by the spectroelectrochemical technique. When the BNPPCo solution was electrolyzed at the first reduction peak (−1.21 V) in the absence of CO2, the Soret band of the catalyst was only slightly weakened due to the reduction of the Co site (Figure S43), indicating the preservation of the conjugation of the porphyrin. The UV–vis spectral changes in the presence of CO2 were consistent with those without CO2 (Figure S44), combined with the reversible reductive behavior of the CoII/I couple in the CO2 atmosphere (Figure S45), demonstrating that BNPPCo cannot interact with the CO2 molecule after one-electron reduction. While the CPE was carried out at −1.59 V, UV–vis spectra of BNPPCo showed an obvious decrease in the Soret band (420 nm), accompanied by an increase at 288, 364, and 452 nm (Figure 5a). These observations further supported the ligand-centered reduction event at −1.59 V. Interestingly, in the presence of CO2, these spectral changes are no longer observed (Figure 5b), indicating that the ligand-reduced BNPPCo rapidly transfers the stored electrons to the bound CO2 molecule and initiates the catalytic cycle. Furthermore, the catalyst concentration-dependent experiments were conducted by CV, which showed that the currents of the catalytic peak were first-order dependent on the concentration of BNPPCo (Figures 5c and S46), suggesting that a single catalyst molecule participates in the catalytic cycle. In addition, in the presence of varying concentrations of CO2, the catalytic peak currents were first-order dependent on [CO2]0.5 (Figures 5d and S47), corresponding to a first-order relationship between the apparent catalytic rate and [CO2], kobs = kcat [CO2], implying that only one CO2 molecule is involved in the rate-determining step.

Figure 5.

Figure 5

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UV–vis spectral changes of BNPPCo electrolyzed at −1.59 V vs. Fc+/0 for 10 min under Ar (a) and CO2 (b) atmosphere. (c) Plot of linear regression of catalytic peak currents versus the concentration of BNPPCo. (d) Plot of the linear regression of catalytic peak currents of 0.3 mM BNPPCo versus the square root of [CO2].

Notably, during the reverse sweep, CV showed a distinct oxidation peak at −0.58 V. However, after scanning only the first reduction peak, this oxidation peak became invisible (Figures 4a and S45). Interestingly, as the catalytic peak current at −1.59 V increases, so does this oxidation peak current (Figures S46 and S47). According to previous studies on Co catalyst,67 this electrochemical behavior corresponds to the oxidation of cobalt(I) carbonyl species (CoI–CO), suggesting that the formation of CoI–CO species hinders the catalytic turnover due to the challenges in the CO release step. It is known that irradiation can promote the CO release from metal carbonyl complexes.86,87 When the CV of BNPPCo was performed under additional light irradiation, the catalytic current increased significantly, accompanied by a weakening of the oxidation peak at −0.58 V (Figure S48). With an increased number of electrons stored by the ligand, the interaction between the ligand-reduced Co(I) center and CO becomes stronger due to π-backdonation from the electron-rich Co(I) center to the CO π* orbital. As a consequence, an unexpected oxidation peak at −1.36 V was observed in BNPPCo, TriNPPCo, and TNPPCo during the reverse sweep (Figure 4c), which was attributed to the oxidation of ligand-reduced cobalt(I) carbonyl species ((L2-/3–)CoI–CO couple). Based on the above observations, it is suggested that the Co(II) porphyrin catalysts are susceptible to being poisoned by the CO produced under catalytic condition, resulting in lower Faradaic efficiency of 0.8–3.2% (Figures S34–S36) and obviously decreased TOFs of the catalysts under kinetic conditions compared to FOWA conditions (Figure 4d).

Based on the above experimental results and discussions, the proposed mechanism for the reduction of CO2 to CO by BNPPCo (Figure 6) involves the following steps: First, the Co(II) species, (L2–)CoII, is reduced to (L2–)CoI by metal-centered reduction at −1.21 V. The formed (L2–)CoI is further reduced by two-electron ligand-centered reduction at −1.59 V, leading to the formation of the (L4–)CoI species. In the presence of CO2, the (L4–)CoI species binds a CO2 molecule to form the (L2–)CoI–CO2– species by inner-sphere electron transfer. In the absence of proton source, another CO2 molecule typically acts as Lewis acid to facilitate the cleavage of the C–O bond.36,88−90 Consequently, the (L2–)CoI–CO2– species is converted to an (L2–)CoI–CO intermediate with CO32– production. The (L2–)CoI–CO intermediate must release CO before being converted back to (L2–)CoI, but this is unfavorable and thus causes the accumulation of (L2–)CoI–CO in catalysis. Interestingly, lighting in electrocatalysis can indeed play a role in promoting the release of CO, thereby effectively accelerating the catalytic cycle. This phenomenon is attributed to the activation of specific photoactive centers within the catalyst, which enhance the rate of the CO release process.

Figure 6.

Figure 6

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Proposed mechanism of BNPPCo for electrocatalytic CO2 reduction to CO, where L represents the BNPP ligand.

Conclusion

Inspired by the role of the iron–sulfur cluster as a multielectron relay in [NiFe]-CODH, a series of Co porphyrins with varying numbers of redox-active nitrophenyl unit has been designed for electrocatalytic CO2RR. By incorporating more nitrophenyl group into the porphyrin backbone, the corresponding complexes can accept more electrons at the ligand center at same potential, −1.59 V vs. Fc+/0, and exhibit higher catalytic activity for CO2 reduction to CO. The turnover frequency (TOF) of the Co porphyrin substituted with four nitrophenyl groups (TNPPCo) reaches up to 4.9 × 104 s–1 at only 270 mV overpotential, which is 27 times higher than the single-substituted NPPCo. Spectroelectrochemical and kinetic studies indicate that the reduced ligand triggers CO2 binding at the CoI site, resulting in a significantly decrease of the overpotential (by 820 mV) compared to the parent TPPCo. This work highlights the effectiveness of incorporating a multielectron pool type ligand as a strategy to improve the CO2 reduction performance and lower the kinetic barrier. In addition, this strategy shows promise for enabling multielectron CO2 reduction reactions, such as the conversion of CO2 to CH4, although the presently designed NO2-containing ligand is incompatible with protons.

Experimental Section

General Methods and Materials

All reagents were purchased from commercial suppliers and used as received, unless otherwise noted. Ligands (NPP,91 BNPP,72 TirNPP,92 and TNPP73) and complexes (NPPCo,93 TirNPPCo,94 TNPPCo,95 BNPPZn,96 and TNPPZn97) were synthesized according to the previously reported method with proper modifications.79 Details are given in the Supporting Information. TPPCo was purchased from J&K Co. and used after chromatography (petroleum ether/dichloromethane) and recrystallization. All heating reactions were operated on the heating mantle, unless otherwise noted.

NMR spectra were recorded on a Bruker AVANCE III HD 400 (400 MHz) spectrometer. 1H NMR spectra are reported in parts per million (ppm) and are referenced to residual solvent, e.g., 1H(CDCl3): δ 7.26; 13C(CDCl3): 77.0; coupling constants are reported in Hz. 13C NMR spectra were performed as proton-decoupled experiments and are reported in ppm. X-ray diffraction determination was carried out by an X-ray single crystal diffractometer from Rigaku Oxford Diffract, and data collections were performed using four-circle kappa diffractometers equipped with CCD detectors. High-resolution mass spectrometry (HRMS) experiments were conducted by Bruker Daltonics ESI-Q-TOF LC/MS. UV–vis spectra were measured on an Agilent Cary 8454 UV–vis instrument. Gas generated during electrolysis was analyzed by Shimadzu gas chromatograph (GC-2010 Plus).

Electrochemical Measurements

Electrochemical experiments were performed on a CHI-660E electrochemical workstation. The solution was bubbled with argon or CO2 for 10 min before test. A three-electrode system was used. The reference electrode in the organic phase, AgNO3/Ag in 0.1 M AgNO3 DMF solution, was calibrated by Fc+/0. Glassy carbon electrode (0.07 cm2) was used as the working electrode, and Pt wire was used as the counter electrode. Cyclic voltammograms were measured in dimethylformamide (DMF) solution with 0.1 M nBu4NPF6. Controlled potential electrolysis was performed in a gastight electrochemical cell with three ports under CO2 with stirring. The 10 mL DMF solution in the center port contains catalyst and 0.1 M nBu4NPF6, 3 mL DMF solution in the other ports contains 0.1 M nBu4NPF6 each, and the headspace is approximately 23 mL. A glassy carbon disk (2 cm2) was used as the working electrode. Before electrolysis, the cell was degassed by bubbling argon or CO2 gas for at least 30 min.

Synthesis of [5,15-Bis(4-nitrophenyl)-10,20-diphenylporphinato]cobalt, BNPPCo

BNPP (95.2 mg, 0.135 mmol) was dissolved in 40 mL of DMF. To this solution, 10 equiv of Co(OAc)2·4H2O (336.5 mg, 1.35 mmol) was added and heated to reflux for 4 h by the heating mantle (monitored by TLC). DMF was removed by rotary evaporation. The residue was washed by water and separated by centrifugation for several times, and then recrystallized by chloroform/acetone. Purple crystalline solid was obtained (68.4 mg, yield 66.5%). The NMR signal is paramagnetic. Anal. Calcd for BNPPCo, C44H26CoN6O4: C, 69.39; H, 3.44; N, 11.03; found: C, 68.57; H, 3.54; N, 10.73. HRMS for [M]+: calcd for C44H26CoN6O4, 761.1348; found 761.1341.

Synthesis of [5,10,15-Tris(4-nitrophenyl)-20-phenylporphinato]zinc, TriNPPZn

TriNPP (33.3 mg, 0.044 mmol) was dissolved in 130 mL of dichloromethane/chloroform. The solution of Zn(OAc)2·2H2O (97.5 mg, 0.444 mmol) in 15 mL of methanol was then added. While stirring at room temperature for 2 h, the reaction was monitored by TLC. After solvent was removed, the residue was washed by water and separated by centrifugation several times and then recrystallized by chloroform/acetone. A purple crystalline solid was obtained (24.3 mg, yield 67.9%). 1H NMR (400 MHz, CDCl3) δ 9.03 (d, J = 4.8 Hz, 2H), 8.91 (s, 4H), 8.87 (d, J = 3.9 Hz, 2H), 8.66 (d, J = 7.8 Hz, 6H), 8.40 (d, J = 7.9 Hz, 6H), 8.21 (d, J = 6.0 Hz, 2H), 7.79 (d, J = 10.1 Hz, 3H). Anal. Calcd for TriNPPZn, C44H25N7O6Zn: C, 65.00; H, 3.10; N, 12.06; Found: C, 63.42; H, 3.43; N, 11.71. HRMS for [M+H+]+: calcd for C44H26N7O6Zn, 812.1231; found 812.1223. The 13C NMR spectra were not obtained due to the poor solubility of this compound.

Acknowledgments

We acknowledge the National Natural Science Foundation of China (NSFC 21933007 and 22193011) and China Postdoctoral Science Foundation (2023T160358) for funding support.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsorginorgau.3c00027.

  • Additional experimental details, materials, methods, complexes synthesis (1H NMR and HRMS), X-ray crystallography, UV–vis, electrochemical studies (PDF)

Author Contributions

CRediT: Wen-Wen Yong data curation (equal), investigation (equal), methodology (equal), writing-original draft (equal); Hong-Tao Zhang data curation (equal), methodology (equal), writing-original draft (equal); Yu-Hua Guo data curation (equal), investigation (supporting), methodology (supporting); Fei Xie investigation (supporting), validation (supporting); Ming-Tian Zhang conceptualization (lead), funding acquisition (lead), project administration (lead), resources (lead), supervision (lead), writing-original draft (equal), writing-review & editing (lead).

The authors declare no competing financial interest.

Special Issue

Published as part of the ACS Organic & Inorganic Auvirtual special issue “2023 Rising Stars in Organic and Inorganic Chemistry”.

Supplementary Material

gg3c00027_si_001.pdf (6.1MB, pdf)

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Associated Data

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Supplementary Materials

gg3c00027_si_001.pdf (6.1MB, pdf)

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

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