Synthesis and investigation of tetraphenyltetrabenzoporphyrins for electrocatalytic reduction of carbon dioxide

We report the synthesis and electrochemical properties of freebase tetraphenyltetrabenzoporphyrin and its complexes of Zn(II), Co(II), Ni(II), Cu(II) and Sn(IV) towards electrochemical reduction of carbon dioxide (CO2). Based on cyclic voltammetry, it is shown that central metals significantly affect the electrocatalytic performance in the reduction of CO2 in terms of reduction potential and catalytic current enhancement. At an applied potential of 1.90 V vs. an Ag/AgCl quasi reference electrode for 20 h, the electrocatalytic reduction of CO2 realized by Zn(II)and Cu(II)-tetraphenyltetrabenzoporphyrins to carbon monoxide resulted in faradaic efficiencies of around 48% and 33%, respectively.


Introduction
Rapid global economic and population growth has led to an increase in fuel consumption in industrial, transportation, commercial and residential sectors. As a major source of energy, vast quantities of fossil fuels are burnt, creating environmental problems due to subsequent creation of greenhouse gases and pollutants. A greenhouse relevant product from fuel combustion processes is carbon dioxide (CO 2 ). Therefore, assiduous scientic attention is directed towards identication of optimal methods to reduce the generation of CO 2 and, at the same time, to convert CO 2 into other useful compounds. One way to address this issue might be the cyclic use of carbon in human society, which stabilizes the CO 2 content in the atmosphere.
The electrochemical reduction processes of CO 2 using catalysts have attracted much attention because they require lower overpotential, compared to the direct reduction of CO 2 , and provide higher product selectivity. [1][2][3][4][5] As economically friendly substitutes for precious-metal catalysts, organometallic electrocatalysts have become popular. [6][7][8][9][10][11][12][13] Among such materials, porphyrin derivatives have been investigated continuously as potential electrochemical catalysts for the reduction of CO 2 . 14, 15 The great advantages of porphyrin compounds regarding this application originate from their high stability and tunability of their electrochemical properties by changing metal centers and substituents at the macrocyclic peripheral positions. Such structural modications do not only enable improvement in the electrocatalytic performance of the systems, but also allow one to investigate the possible catalytic mechanisms. [16][17][18][19] Tetrabenzoporphyrins are one of the very intriguing porphyrin derivatives that exhibit unique photophysical and electrochemical behaviors caused by extension of a p-conjugated structure at the b-positions of the porphyrin core and highly distorted macrocycle core due to meso-substitutions. 20-24 Our previous work described systematic investigation of the photophysical and electrochemical properties of meso-substituted porphyrin and benzoporphyrin derivatives as ternary components for highly efficient bulk-heterojunction solar cells. 22 Our observations on the electrochemical properties of the mesosubstituted benzoporphyrin derivatives obtained from the previous study have led to extended investigation towards their catalytic activities for the electrochemical reduction of CO 2 in this work. To the best of our knowledge, until now there are only a few studies on the electrochemical properties and catalysis of benzoporphyrin derivatives for CO 2 reduction. Ramirez et al. described the electrochemical and photoelectrochemical reduction of CO 2 using a Co(II)-tetrabenzoporphyrin-modied electrode. 25,26 To gain insights into the electrochemical properties of benzoporphyrins towards the electrochemical reduction of CO 2 , a series of metal-free and metallated tetraphenyltetrabenzoporphyrins as shown in Chart 1 were synthesized, characterized and investigated for their electrocatalytic activities for the reduction of CO 2 . Complexes of interest for this study had non-precious-metal centers like Zn(II)-, Co(II)-, Ni(II)-, Cu(II)-and Sn(IV)-, which were previously studied in porphyrin analogs for the electrochemical reduction of CO 2 . [27][28][29][30][31] The overview presented in this work on the effect of the central metals on the electrochemical behavior and electrocatalytic activities of these benzoporphyrins for the reduction of CO 2 will become a useful guideline for further development of several precious-metal-free oligopyrrole-based electrocatalytic systems.

Synthesis
Synthesis of the benzoporphyrins started from previously reported demetallation of Cu-TPBP 32 by sulfuric acid at room temperature for 30 min, resulting in TPBP in 81% yield (Scheme 1). 33 In its 1 H-NMR spectrum, a broad singlet signal of two inner protons at d À1.17 ppm conrmed the formation of TPBP. To obtain the desired metallated benzoporphyrins, TPBP was reacted with Zn(OAc) 2 $2H 2 O, 34 Co(OAc) 2 $4H 2 O 35 and SnCl 2 $2H 2 O 36 as described in the previous studies, leading to Zn-TPBP, Co-TPBP and Sn-TPBP, respectively, in 80-97% yield. Complete metallation of TPBP in these reactions was monitored by the absence of the broad singlet signal of the benzoporphyrinic inner protons at d À1.17 ppm in the 1 H-NMR spectra and disappearance of the characteristic emission peak of TPBP at 787 nm. HR-ESI-MS conrmed the formation of both Zn-TPBP and Co-TPBP by showing their molecular ion peaks at m/z 876.2231 and 871.2273, respectively. Due to difficulties in chromatographic purication, Sn-TPBP could not be completely separated from other byproducts and was obtained in >90% purity based on 1 H-NMR spectroscopy. However, its formation could be conrmed by MALDI-TOF MS showing its molecular ion peak at m/z 966.292. Ni-TPBP was obtained from a known procedure described in detail by Finikova et al. 32 Photophysical properties As mentioned above, variation of the metal center of the benzoporhyrin derivatives does not only signicantly affect the electrochemical behavior, but also the photophysical properties of the molecules. Therefore, absorption and emission of TPBP, Zn-TPBP, Co-TPBP, Ni-TPBP, Cu-TPBP and Sn-TPBP were investigated in this study to provide additional information that should be useful for further applications of these materials in optoelectronics and photoelectrocatalysis. By using UV-Vis spectrophotometry, normalized absorption spectra of all compounds in toluene were obtained as shown in Fig. 1. All compounds exhibited characteristic absorption patterns of the metallated benzoporphyrins having intense Soret bands in the range of 446-466 nm with absorption coefficients (3) of 1.6 Â 10 5 -5.3 Â 10 5 M À1 cm À1 , and the Q-bands in the range of 592-698 nm, as summarized in Table 1. Upon excitation at their absorption maxima, TPBP, Zn-TPBP and Sn-TPBP showed  Table 1, while Ni-TPBP, Cu-TPBP and Co-TPBP gave no signicant emission.

Electrochemical characterization and catalytic activity
All benzoporphyrins were studied by means of cyclic voltammetry to evaluate their catalytic activities towards electrochemical CO 2 reduction. The cyclic voltammograms were recorded in the potential range of 0.0 V to À2.0 V from a 0.1 M TBAPF 6 solution in DMF containing 1.0 mM benzoporphyrin with a scan rate of 50 mV s À1 under the N 2 -and CO 2 -saturated conditions at ambient temperature and pressure. Fig. 3a shows that under the N 2 -saturated conditions, the rst and the second reduction steps of TPBP are characterized by reversible reduction peaks at À1.22 V and À1.52 V, respectively. However, when the solution of TPBP was saturated with CO 2 , the rst irreversible reduction was also observed at À1.22 V with a two-fold increase in peak current from 0.04 mA to 0.08 mA, while its second irreversible reduction peak appeared at À1.84 V with a negative shi by 0.32 V and a peak current enhancement from 0.05 mA to 0.13 mA. The great difference in the electrochemical behavior of TPBP observed in this case, compared with that under the N 2 -saturated conditions, may be attributed to possible binding between CO 2 and inner pyrrolenine nitrogen atoms of the freebase benzoporphyrin macrocycle. The binding CO 2 might form an ionic salt that is stabilized by the TBA cation which may present itself as a peak at more negative potentials. Moreover, new anodic signals were observed in the potential range of À0.40 V to À0.80 V, indicating the generation of unknown products from the irreversible reduction process(es) or the release of captured carbon dioxide. As for the electrochemical reduction of Zn-TPBP (Fig. 3b), a fully reversible rst reduction (around À1.6 V) and as suggested by the scan-rate dependency of the peak current, a quasi-reversible second reduction peak (around À1.8 V) can be observed under N 2 -saturated conditions. A slight current increase for the rst reduction peak and a two-fold current increase (from 0.05 mA to 0.10 mA) for the second one were monitored under CO 2 -saturated conditions signaling the reduction of CO 2 . Unknown oxidation signals were found under both N 2 -and CO 2 -saturated conditions in the potential range of À0.30 V to À0.70 V, which may be attributed to the side products formed during the reduction process. This observation of the increase in current and the change in the shape of the second ligand-based peak suggested that the doubly-reduced Zn-TPBP is the responsible molecule for the reduction of CO 2 .
The cyclic voltammogram of Co-TPBP in the N 2 -saturated solution revealed a reversible reduction peak at À0.75 V stemming from the metal-centered reduction of the molecule. This was followed by two quasi-reversible peaks at À1.29 V and À1.55 V which may be attributed to the multi-step ligandcentered reduction of the meso-substituted benzoporphyrin macrocycle (Fig. 3c). Under the CO 2 -saturated conditions, Co-TPBP showed a very similar reduction pattern with a slight current increase. Compared with planar porphyrin and phthalocyanine derivatives reported as potential catalysts. 37 The difference in the catalytic behavior of Co-TPBP observed herein should stem from the unique saddle-shaped macrocycle distortion of the metallated benzoporphyrins previously studied by several groups. [20][21][22][23][24] The cyclic voltammogram of Ni-TPBP under the N 2 -saturated conditions showed the rst reversible reduction peak at À1.10 V which is ligand-centred and the second quasi-reversible one at À1.90 V that originates from metal-centered reduction. 38 (Fig. 3d). Under the CO 2 -saturated conditions, similar electrochemical features were observed with a 3.6-fold current   enhancement of the second reduction from 0.05 mA to 0.18 mA and a positive shi of its onset potential (E onset ) by 0.20 V, suggesting the catalytic activity of Ni-TPBP towards the electrochemical reduction of CO 2 . In a similar manner, the N 2saturated solution of Cu-TPBP exhibited two major reversible reduction peaks at À1.27 V and À1.90 V which are attributed to the formation of a dianion radical and dianion of the macrocycle, respectively as shown in Fig. 3e. 38,39 In the presence of CO 2 , a considerable current increase from 0.05 mA to 0.13 mA was observed at À1.85 V with a positive shi of E onset by 0.18 V, suggesting the catalytic activity of Cu-TPBP for the electrochemical reduction of CO 2 . Sn-TPBP showed rather complex electrochemical behavior with two major reduction peaks at À0.92 V and À1.36 V, followed by at least ve anodic peaks under the N 2 -saturated conditions, suggesting irreversible structural changes of the molecules upon the reduction processes (Fig. 3f). In the presence of CO 2 , its cyclic voltammogram also exhibited two main reduction peaks at a similar peak potential with even more complex anodic signals and a negligible current enhancement. This was attributed to possible interaction of the axial hydroxyl ligands of Sn-TPBP and CO 2 that might lead to unexpected formation of side products upon reduction.
To further investigate the electrocatalytic performance of each benzoporphyrin towards the electrochemical reduction of CO 2 , all compounds were subjected to constant-potential electrolysis at an applied potential of À1.90 V for 20 h. The results from GC headspace analysis showed that the CO 2 -saturated electrolyte solutions containing TPBP, Co-TPBP, Ni-TPBP and Sn-TPBP did not yield any detectable reduction product. In the case of TPBP, it is likely that the above-mentioned binding between CO 2 and the benzoporphyrin core was relatively strong and therefore suppressed the favorable reduction process of CO 2 molecules. The low current enhancement observed for the above-described cyclic voltammograms of Co-TPBP and Sn-TPBP under the CO 2 atmosphere could imply their low catalytic efficiencies for the electrochemical reduction of CO 2 . As for Ni-TPBP, although the current enhancement was found to be quite high in the cyclic voltammetry study, the reduction process did not lead to any detectable amount of CO or any other gaseous products. It is possible that the reduction process proceeded via other mechanisms and further optimization of the reduction conditions, such as addition of proton sources, pH adjustment and use of a co-catalyst, may be required. When Zn-TPBP and Cu-TPBP were used as catalysts, 8.67 mmol and 12 mmol of CO were detected, corresponding to a faradaic efficiency of 33% and 48% with turnover numbers of 1.7 and 6.6, respectively.

Conclusions
Six tetraphenyltetrabenzoporphyrins, including the freebase derivative and the metal-chelated ones having Zn, Co, Ni, Cu and Sn metal centers, were successfully synthesized and characterized. According to UV-visible and uorescence spectrophotometry, and cyclic voltammetry, the introduction and variation of the metal centers obviously affected the photophysical properties, electrochemical behaviors and catalytic activities for the electrochemical reduction of CO 2 to CO. The results from cyclic voltammetry showed that, compared with the N 2 -saturated conditions, TPBP, Ni-TPBP, Zn-TPBP and Cu-TPBP exhibited a signicant current enhancement, while Co-TPBP and Sn-TPBP gave only a slight increase in the peak current when CO 2 was introduced into the solution. The constant-potential electrolysis revealed the formation of CO as the main reduction product with faradaic efficiencies of 33% and 48% and turnover numbers of 1.7 and 6.6 for Zn-TPBP and Cu-TPBP, respectively. This study suggests that, among the metallobenzoporphyrins of interest, Zn-and Cu-chelated analogs exhibited superior electrocatalytic activities towards the electrochemical reduction of CO 2 to CO. Here, we show the catalytic potential of the metallated benzoporphyrins as alternative CO 2 reduction catalysts using cheaper and more abundant metal centers like Zn and Cu, compared to Re and Pd. Optimization of the reduction conditions, together with structural modication of benzoporphyrin ligands will be further studied and described elsewhere.

Materials and methods
All chemicals were of analytical grade, purchased from commercial suppliers and used as received without further purication. 1 H-NMR and 13 C-NMR spectra were obtained in deuterated chloroform (CDCl 3 ) at 400 megahertz (MHz) for 1 H nuclei and 100 MHz for 13 C nuclei. Chemical shis (d) are reported in parts per million (ppm) relative to the residual CHCl 3 peak (7.26 ppm for 1 H-NMR and 77.0 ppm for 13 C-NMR spectroscopy). Mass spectra were obtained using high-resolution electron spray ionization mass spectrometry (HR-ESI-MS) and matrix-assisted laser desorption ionization-time of ight mass spectrometry (MALDI-TOF MS) with dithranol as a matrix. Ultraviolet-visible (UV-vis) and uorescence spectrophotometry were performed in toluene at room temperature. Molar extinction coefficients (3) are expressed in M À1 cm À1 .

Synthesis and characterization of benzoporphyrin derivatives
meso-Tetraphenyltetrabenzoporphyrin (TPBP). Following a previously published procedure, 33 Cu-TPBP 32 (0.052 g, 0.060 mmol) was reacted with concentrated sulfuric acid (10 mL) at room temperature for 30 min. The resulting reaction mixture was poured into a water/ice mixture and then extracted with dichloromethane. The combined organic phase was dried over anhydrous Na 2 SO 4 and concentrated to dryness. The crude mixture was puried by column chromatography (silica gel, CH 2 Cl 2 /hexanes (2 : 1)) to afford TPBP as a green solid (0.039 g, 81%). 1  meso-Tetraphenyltetrabenzoporphyrinatozinc (Zn-TPBP). Following a previously published procedure, 34 a solution of TPBP (0.131 g, 0.161 mmol) in chloroform (117 mL) was reacted with a solution of Zn(OAc) 2 $2H 2 O (0.177 g, 0.805 mmol) in methanol (13 mL) at room temperature for 12 h. Aer that, the resulting mixture was washed with water, and the organic layer was separated and dried over anhydrous Na 2 SO 4 . Aer removal of the solvent, the crude mixture was puried by column chromatography (silica gel, CH 2 Cl 2 /hexanes (2 : 1)) to afford Zn-TPBP as a greenish blue solid (0.136 g, 97%). 1  meso-Tetraphenyltetrabenzoporphyrinatotin (Sn-TPBP). Following a previously published procedure, 36 a solution of TPBP (0.102 g, 0.125 mmol) and SnCl 2 $2H 2 O (0.141 g, 0.625 mmol) in dimethylformamide (DMF, 5 mL) was treated with pyridine (0.05 mL) and reuxed for 4 h. A blue green precipitate was formed and collected by ltration. Aer that, the resulting crude product was puried by column chromatography (silica gel, CH 2 Cl 2 /MeOH (99 : 1)) to afford Sn-TPBP as a deep green solid (0.047 g, 90%). Due to incomplete purication of column chromatography, the achieved product was obtained with >90% purity, based on 1 H-NMR spectroscopy. 1  Electrochemical studies Background current. A background cyclic voltammogram was obtained in an anhydrous solution (10 mL) of 0.1 M tetrabutylammonium hexauorophosphate (TBAPF 6 ) in DMF. A three-electrode one-compartment cell was used throughout the experiments. A glassy carbon electrode served as a working electrode while a Pt plate was used as a counter electrode. A silver wire coated with silver chloride (Ag/AgCl) was used as a quasi-reference electrode (QRE). The Ag/AgCl QRE was prepared using a procedure described elsewhere 40 and externally calibrated with a ferrocene/ferrocenium redox couple using a potential of 0.72 V vs. a normal hydrogen electrode (NHE) as a reference value. 41 The cyclic voltammograms were recorded at potentials ranging from 0.00 V to À2.00 V vs. a Ag/AgCl QRE at a scan rate of 50 mV s À1 . The potential values were therefore reported with respect to the Ag/AgCl QRE. The solution was purged with nitrogen (N 2 ) or CO 2 for 20 min before each measurement with a ow rate of 0.2 L min À1 . As a control experiment, constant-potential electrolysis of the electrolyte solution under CO 2 -saturated conditions in the absence of the catalyst for 20 h did not yield any CO 2 reduction product.

Electrochemical reduction of CO 2
The catalytic activity of the benzoporphyrins towards electrochemical reduction of CO 2 was investigated by means of cyclic voltammetry and the constant-potential electrolysis using the above-mentioned electrochemical setup at potentials ranging from 0.00 to À2.00 V. Each cyclic voltammogram was collected from a 0.1 M TBAPF 6 solution in anhydrous DMF containing 1.0 mM benzoporphyrin (10 mL) at a scan rate of 50 mV s À1 under N 2 -or CO 2 -saturated conditions. The solution was purged with N 2 or CO 2 for 20 min before each measurement with a ow rate of 0.2 L min À1 . The constant-potential electrolysis of each benzoporphyrin was performed at ambient temperature using the above-mentioned electrochemical setup and at an applied potential of À1.90 V. Aer 20 h, a 2 mL gas sample from headspace gas (total volume was 10 mL) was taken from a reaction vial and analyzed by gas chromatography (GC) equipped with a thermal conductivity detector (TCD). Curves for CO obtained from the experiments were integrated to get the peak area, which was then used to calculate the amount of CO in the headspace using a pre-measured calibration curve.

Conflicts of interest
There are not conicts to declare.