English

• 0.   Introduction

  The efficient catalysts are very essential to study oxygen reduction reactions (ORRs) ^[1]. Among the explored catalysts for ORRs, cobalt porphyrin complexes are significant researched compounds in past decades^[2]. It was revealed that the catalytic property and selectivity of cobalt porphyrin complexes for ORRs are highly depend upon the molecular structures of investigated cobalt porphyrin^[2].

  More than decades ago, the ORR catalytic properties of face-to-face (FTF) dimeric cobalt porphyrins (Chart 1) have been studied well^[3]. FTF cobalt porphyrins are composed of two porphyrin macrocycles linked by various bridges located at the meso or β positions of the porphyrins^[3]. FTF cobalt porphyrins show two/four-electron electrocatalytic reduction of O₂ to H₂O₂/H₂O, which strongly relies on their molecular structures^[3]. However, to date, the ORR catalytic properties of non-FTF dimeric cobalt porphyrins have been little-studied. Recently, the zinc porphyrin which containing disulfide ligands was synthesized from metalloporphyrin containing four acetylthio groups as precursor with high yield (Chart 1)^[4]. This work makes us realize that disulfide-bridged dimeric cobalt porphyrin could be obtained from cobalt porphyrins containing only one acetylthio group at the meso position. Under this context, non-FTF dimeric cobalt porphyrins are expected as new candidates to investigate the ORR catalytic properties.

  Figure Chart 1

  

  Figure Chart 1.  TPPCo, FTF cobalt porphyrins and zinc porphyrin containing disulfide ligands

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  1.   Experimental

  The materials and instrument can be found in Sup-porting information. The synthetic scheme of 2Co is shown in Scheme 1.

  Scheme 1

  

  Scheme 1.  Synthesis scheme of dimeric cobalt porphyrin 2Co

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  Synthesis of Por: 2, 2'-(phenylmethylene)bis(1H-pyrrole) (4.44 g, 20 mmol), S-(4-formylphenyl) ethane-thioate (1.80 g, 10 mmol) and benzaldehyde (1.06 g, 10 mmol) were added into 500 mL CH₂ Cl₂. Then 0.5 mL trifluoroacetic acid (TFA) was added under N₂ at room temperature. After 3 h, chloranil (4.90 g, 20 mmol) was added to the mixture. The solution was stirred for extra 1 h. The organic solvent was removed. The crude solid was purified by silica gel column chromatography (CH₂Cl₂/ethyl acetate) and recrystallization by CH₂Cl₂/ methanol mixture to get pure Por in 5.4% yield (370 mg, 0.54 mmol). ¹H NMR (300 MHz, in CDCl₃): δ 8.85 (d, 8H, Ph), 8.24 (m, 8H, Ph), 7.78 (m, 1¹H, Ph), 2.61 (s, 3H, Me), -2.79 (s, 2H, NH). UV-Vis (CH₂Cl₂, λ / nm (ε)): 418 (4.38×10⁵ L·mol^-1·cm^-1), 515 (1.8×10⁴ L· mol^-1·cm^-1), 549 (7.6×10³ L·mol^-1·cm^-1), 593 (4.4×10³ L·mol^-1·cm^-1), 648 (3.8×10³ L·mol^-1·cm^-1).

  Synthesis of 1Co: Compound Por (30 mg, 0.044 mmol) and excess cobalt salt were dissolved in 30 mL CH₂Cl₂/methanol (5∶1, V/V) and the mixture was stirred overnight at 25 ℃. After the ending of reaction, the obtained solid was purified by silica gel column chro-matography to collect 1Co with 90% yield (29 mg, 0.039 mmol). ESI-MS: Calcd. for C₄₆H₃₀CoN₄OS745.15 [M]⁺, Found: 745.42, 701.83. UV-Vis (CH₂Cl₂, λ / nm (ε)): 410 (2.97×10⁵ L·mol^-1·cm^-1), 529 (1.8×10⁴ L·mol^-1·cm^-1). IR (KBr, cm^-1): 1 711, 613 for ν_C=O.

  Synthesis of 2Co: The reaction mixture of the water solution (1 mL) of methylamine (mass fraction: 40%) and 15 mL THF solution of 1Co (15 mg, 0.020 mmol) was stirred for overnight. The obtained solid was washed with methanol and n-hexane to give 2Co in 90% yield (13 mg, 0.009 mmol). ESI-MS: Calcd. for C₈₈H₅₄Co₂N₈S₂: 1 404.26 [M]⁺, Found: 1 404.42, 701.83. UV-Vis (CH₂Cl₂, λ / nm (ε)): 430 (1.85×10⁵ L·mol^-1· cm^-1), 545 (1.3×10⁴ L·mol^-1·cm^-1). IR (KBr, cm^-1): 590 for ν_s-s.

  2.   Results and discussion

  The building block Por as precursor was synthesized by acid-catalyzed condensation reaction^[5]. The treatment of dipyrromethane, benzaldehyde and S-(4-formylphenyl) ethanethioate under acidic condensation conditions gave Por in 5.4% yield (Fig. S1, Supporting information). The regular coordination reaction made the cobalt porphyrin 1Co in 90% yield, and then 1Co was reacted with MeNH₂ (40% in water) in tetrahydrofuran (THF) solution at room temperature under N₂ for overnight^[4] to give the target product 2Co in 85% yield.

  The porphyrin Por and cobalt complexes, 1Co and 2Co, were characterized by NMR, electrospray ionization-mass spectroscopy (ESI-MS), IR, electron paramagnetic resonance spectra, UV-Vis spectroscopies and density functional theory (DFT) calculations. The ESI-MS spectra of 1Co and 2Co showed clear m/z signals of the whole molecular structure and their ionized pieces (Fig.S2 and S3). The IR spectra of 1Co and 2Co showed the disappearance of CO stretching vibrations belonging to the acetylthio group at 1 711 cm^-1 through disulfide-bridge formed reaction (Fig.S4). The ¹H NMR spectra were not recorded for 1Co and 2Co complexes due to the paramagnetic property of cobalt ions. To investigate magnetic properties of the cobalt complexes, electron paramagnetic resonance (EPR) spectra of 1Co and 2Co were examined in CH₂Cl₂ at 298 K (Fig. S5). The EPR spectra of 1Co and 2Co show the behaviors of unpaired electrons in Co porphyrins similar with previous porphyrin cobalt complexes^[6]. The optimized molecular structures of 1Co and 2Co were obtained by DFT calculation at B3LYP/6-31G(d)/SDD level with Gaussian 09 (Fig. 1 and S6). Monomer complex 1Co shows a highly planar molecular structure like regular [18]porphyrin and TPPCo (Chart 1) ^{[2, 7]}. The non-FTF dimer 2Co shows the V-shaped molecular structure with two connected cobalt porphyrin units as a result of the properties and torsion angle of the porphyrin-S-S-porphyrin bond, instead of the face-to-face configura-tion of FTF cobalt porphyrins^[3-4].

  Figure 1

  

  Figure 1.  Optimized structure of 2Co calculated at B3LYP/6-31G(d)/SDD level of DFT

  Six aryl groups of meso-positions were replaced with hydrogen at-oms to reduce computational costs; Hydrogen atoms are omitted for clarity

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  The optical properties of cobalt porphyrins were examined by UV-Vis absorption spectrum. The optical absorption spectra of Por, 1Co and 2Co in dichloromethane (DCM) are displayed in Fig. 2. All the spectra displayed very sharp and narrow Soret-like bands with high molar extinction coefficients and Q-like bands in the near-IR region, which are similar with the typical porphyrin-like absorption^[7]. The Soret-like band of complex 1Co was observed at 410 nm, and the low-intensity Q-like band of 1Co was at 529 nm (Fig. 2). Interestingly, the Soret-and Q-like bands of dimeric complex 2Co were observed at 430 and 545 nm, respectively, which were red-shifted compared with those of monomeric complex 1Co.

  Figure 2

  

  Figure 2.  Absorption spectra of Por (black line), 1Co (red line) and 2Co (blue line) in DCM at room temperature

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  The electrochemical properties of 1Co and 2Co were examined by cyclic voltammogram (CV) in CH₂Cl₂ at room temperature. 0.1 mol·L^-1 tetra-n-butylammonium perchlorate (TBAP) was chosen as supporting electrolyte (Fig. 3). The monomeric compound 1Co exhibited three reversible one electron oxidation potentials at 0.66, 0.95 and 1.14 V (vs SCE), and two ir-/reversible one electron reduction potentials at-0.83 and-1.33 V (vs SCE), respectively^[7]. Our previous work revealed that TPPCo exhibits three reversible one electron oxidation potentials at 0.70, 0.97 and 1.15 V (vs SCE), and two irreversible one electron reduction potentials at-0.87 and-1.38 V (vs SCE), respectively[7b]. Besides, the first reduction of TPPCo is irreversible in CH₂Cl₂ because the generated Co from first reduction can coordinate with CH₂Cl₂ solvent to form σ-bond product as Co-CH₂Cl^[7]. However, the first reversible reduction of 1Co indicate a ring-centered electron transfer which is significant different from TPPCo due to the SAc substituent group as electron withdrawing group probably[7a]. The first and second oxidation of dimeric porphyrin complex 2Co are irreversible and reversible waves with two electrons transferred, respectively. The third oxidation of 2Co at 1.24 and 1.34 V (vs SCE) can be assigned to two porphyrin ring^[7]. First step reduction of 2Co were separated and located at-0.86 and-0.92 V (vs SCE) with one electron transfer while the second reduction is reversible at -1.42 V (vs SCE) with two electrons transfer together. In conclusion, the reduction potential of 2Co is similar to 1Co while the oxidation potential of 2Co is positively shifted. These CV results also indicate that the 1Co and 2Co can capture/lost multi-negative and multi-positive charges^[8].

  Figure 3

  

  Figure 3.  CV curves of 1Co and 2Co in DCM

  Supporting electrolyte: 0.1 mol·L^-1 TBAP, scan rate: 0.1 V·s^-1

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  The ORRs performance of the monomeric 1Co and dimeric 2Co were investigated in 1 mol·L^-1 HClO₄ solution under conventional loading of the catalyst being 0.1 mg·cm^{-2[7a, 9]} . The CV of 1Co and 2Co are il-lustrated in Fig. 4. For the CV measurements, the catalysts were adsorbed on an edge-plane pyrolytic graphite (EPPG) disk electrode under argon and O₂ in 1 mol· L^-1 HClO₄ solution. No reductions of 1Co and 2Co were observed under argon in 1 mol·L^-1 HClO₄ solution, but irreversible reductions were observed at E_pc= 0.42 V (vs RHE) for 1Co and at 0.50 V (vs RHE) for 2Co under O₂ atmosphere in the solution. The CV data indicated that catalytic ORR of cobalt complexes 1Co and 2Co have been observed in 1 mol·L^-1 HClO₄ solution.

  Figure 4

  

  Figure 4.  CV curves of 1Co and 2Co absorbed on an EPPG electrode under Ar (dash line) and O₂ (solid line) in 1 mol·L^-1 HClO₄

  Scan rate: 50 mV·s^-1

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  To gain the values of transferred electrons number (n) during the electroreduction reaction of investigated cobalt complexes 1Co and 2Co, the linear sweep voltammetric measurements of them were examined in the oxygen-saturated 1 mol·L^-1 HClO₄ media (Fig. 5a and 5c). The values of n of ORRs for 1Co and 2Co were determined from steady-state limiting currents on the plateau of current-voltage curves. The Koutecky-Levich (Supporting information) plot gave the slope of diagnostic to estimate the values of n under measurement condition (Fig. 5b and 5d)^[7]. The cobalt complexes showed the values of n of ORR being 2.8~3.5 for 1Co and 3.5~3.6 for 2Co over a potential range of 0.2~0.4V (vs RHE) in the oxygen-saturated 1 mol·L^-1 HClO₄ solution. The TPPCo shows the value of n being 2 in air-saturated 1 mol·L^-1 HClO₄ solution in previous work^[7]. In our tests, the complexes 1Co and 2Co both show bigger values of n than TPPCo. The reason of inducing the bigger values of n are probably the SAc as electron-withdrawing group of 1Co and dimerization of 2Co^[7].

  Figure 5

  

  Figure 5.  Current-voltage curves (a, c) and Koutecky-Levich plots (b, d) of 1Co and 2Co in 1 mol·L^-1 HClO₄

  Scan rate: 10 mV·s^-1

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  It is clear that the ORR of cobalt porphyrin com-plexes involves multi-electron reactions. The processes of ORR are divided into two types. One type is two-step process (Eq.1 and 2). The other is a four-electron pro-cess (Eq.3) and water is product of this process in acid solution^{[7, 10]}. Therefore, two cobalt porphyrin complexes 1Co and 2Co produced the mixture of H₂O₂ and H₂O as products in acid media during oxygen electroreduc-tion reaction because the values of n of 1Co and 2Co were 2.8~3.5 and 3.5~3.6, respectively, over 0.2~0.4 V(vs RHE) potential range.

  $ {2{{\rm{e}}^ - }{\rm{process}}:{\rm{ }}{{\rm{H}}_{\rm{2}}}{\rm{O + 2}}{{\rm{e}}^{\rm{ - }}}{\rm{ + }}{{\rm{O}}_2} \to {\rm{H}}{{\rm{O}}_2}^ - {\rm{ + O}}{{\rm{H}}^ - }} $

  (1)

  $ {2{{\rm{e}}^ - } + {\rm{ }}{{\rm{H}}_{\rm{2}}}{\rm{O}} + {\rm{H}}{{\rm{O}}_2}^ - \to {\rm{ }}3{\rm{O}}{{\rm{H}}^ - }} $

  (2)

  $ {4{{\rm{e}}^ - }{\rm{ process}}:{\rm{ }}{{\rm{O}}_{\rm{2}}}{\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O}} + 4{{\rm{e}}^ - } \to 4{\rm{O}}{{\rm{H}}^ - }} $

  (3)

  The rotating ring-disk electrode (RRDE) were also measured under O₂ in HClO₄ solution (Fig. 6). We found that the ring current of 1Co and 2Co both increased in the range of the disk potentials company-ing with the increased disk current. The disk currents of 1Co and 2Co both increased from 0.6 V, and the current plateau was arrived at around 0.3 V (vs RHE) for complexes 1Co and 2Co.

  Figure 6

  

  Figure 6.  RRDE voltammograms of 1Co and 2Co under O₂ in HClO₄ solution

  Potential of disk electrode was from 0.8 to 0 V (vs RHE); Potential of ring electrode was held at 1.2 V (vs RHE); Rotation rate was 1 600 r·min^-1, and the scan rate was 10 mV·s^-1

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  The calculated peroxide yields of compounds 1Co and 2Co were generated based on Eq. 6 (Supporting information) under these experimental acid conditions (Fig. 7). We found that the ORR catalytic properties of 2Co in acid solution is clearly enhanced relative to the monomeric cobalt porphyrin 1Co. The peroxide yields formed upon the electroreduction of dioxygen for 2Co was almost hold 15%, resulting in a calculated average n of 3.6. The peroxide yields of 1Co was in a range of 31%~34%, resulting in a calculated average n of 3.2, under the same experimental conditions. Besides, we note that the average n numbers obtained from the RRDE tests are in good agreement with those derived from the Koutecky-Levich plots, corroborating that in comparison to 1Co, ORR catalyzed by 2Co undergoes a process closer to the 4e-participated pathway (Eq.3).

  Figure 7

  

  Figure 7.  Values of n and H₂O₂ yields of 1Co (a) and 2Co (b) under dioxygen in 1.0 mol·L^-1 HClO₄

  Potential range was from 0.2 to 0.6 V (vs RHE)

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  The ORR long-time stabilities of 1Co and 2Co were examined by the chronoamperometric I-t test at 0.16 V (vs RHE) as a constant potential to estimate the key parameter for potential applications of these two cobalt complexes (Fig. 8). The linear sweep voltammetry (LSV) curves of 1Co and 2Co were also investigated before and after the I-t test for further comparison. After 300 min continuous testing, the ORR current density of 2Co just dropped around 10%, but that of 1Co dropped more than 20%. This result indicates that the stability of 2Co is slightly better than 1Co.

  Figure 8

  

  Figure 8.  ORR stabilities of 1Co and 2Co examined by LSV curves (a, b) which taken before and after 300 min chronoamperometric I-t test (c, d)

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  Although the attempts to obtaining single crystal of 2Co to investigate clearly molecular structure failed, the reason of superior ORR catalytic properties of 2Co to 1Co is provided considering all collected data and DFT calculation. The mainly reason is likely due to its oligomer nature with two porphyrins brought to closer proximity by the disulfide linker which was defined by the optimized structure of 2Co under theoretical calculation (Fig. 1 and S6)^[2-3]. Previous studies have also shown that the formation of oligomer from molecular complexes is beneficial for their ORR activities^[2-3]. In addition, the disulfide linker in 2Co might further facilitate the through-bond charge transportation, which is essential for the electrocatalyst based on metal-organic complexes^[11]. Therefore, despite of the less-than-four electrons transferred and the end product of H₂O₂ and H₂O mixture during ORR, the dimeric complex 2Co still manifests as a good ORR catalyst in the acidic medium.

  3.   Conclusions

  In summary, a disulfide-bridged non-FTF dimeric cobalt porphyrin 2Co has been synthesized from mono-mer cobalt porphyrin 1Co. The optimized structure of 2Co shows a non-FTF molecular configuration obtained by DFT calculation. The dimerization of por-phyrin in 2Co produced a red-shifted UV-Vis absorp-tion compared with the spectrum of 1Co. The CV mea-surements of 1Co and 2Co indicate that they both can stabilize multi-negative and positive charges. The cata-lytic activities for ORRs of 1Co and 2Co were investigated in acid media. The values of n during ORRs of 1Co and 2Co obtained from the RRDE tests and the Koutecky-Levich plots all indicate that the dimerization of cobalt porphyrins have a slightly positive influence on the catalytic properties for ORRs in the acidic medium. Further studies on the linear oligomers of other metal porphyrin complex to examine catalytic properties and to evaluate the relationship between catalytic properties and molecular structures for ORRs are currently underway in our lab.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Figure Chart 1  TPPCo, FTF cobalt porphyrins and zinc porphyrin containing disulfide ligands

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  Scheme 1  Synthesis scheme of dimeric cobalt porphyrin 2Co

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  Figure 1  Optimized structure of 2Co calculated at B3LYP/6-31G(d)/SDD level of DFT

  Six aryl groups of meso-positions were replaced with hydrogen at-oms to reduce computational costs; Hydrogen atoms are omitted for clarity

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  Figure 2  Absorption spectra of Por (black line), 1Co (red line) and 2Co (blue line) in DCM at room temperature

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  Figure 3  CV curves of 1Co and 2Co in DCM

  Supporting electrolyte: 0.1 mol·L^-1 TBAP, scan rate: 0.1 V·s^-1

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  Figure 4  CV curves of 1Co and 2Co absorbed on an EPPG electrode under Ar (dash line) and O₂ (solid line) in 1 mol·L^-1 HClO₄

  Scan rate: 50 mV·s^-1

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  Figure 5  Current-voltage curves (a, c) and Koutecky-Levich plots (b, d) of 1Co and 2Co in 1 mol·L^-1 HClO₄

  Scan rate: 10 mV·s^-1

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  Figure 6  RRDE voltammograms of 1Co and 2Co under O₂ in HClO₄ solution

  Potential of disk electrode was from 0.8 to 0 V (vs RHE); Potential of ring electrode was held at 1.2 V (vs RHE); Rotation rate was 1 600 r·min^-1, and the scan rate was 10 mV·s^-1

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  Figure 7  Values of n and H₂O₂ yields of 1Co (a) and 2Co (b) under dioxygen in 1.0 mol·L^-1 HClO₄

  Potential range was from 0.2 to 0.6 V (vs RHE)

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  Figure 8  ORR stabilities of 1Co and 2Co examined by LSV curves (a, b) which taken before and after 300 min chronoamperometric I-t test (c, d)

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