CC BY
12Corroies Корролы
Шкрогетэроц!/! vs\ ы
http://macroheterocycles.isuct.ru
Paper
Статья
DOI: 10.6060/mhc200183l
pH-Dependent Electrochemically Catalyzed Oxygen Reduction Behaviors of o-Substituted Co(III) Corroles
Wei Tang,a Yuanyuan Qiu,b Xiaonan Li,a@1 Rodah C. Soy,c John Mack,c@2 Tebello Nyokong,c and Xu Liangb@3
Department of Children Health Care, Nanjing Children's Hospital, Nanjing Medical University, 210008 Nanjing, PR China
bSchool of Chemistry and Chemical Engineering, Jiangsu University, 212013 Zhenjiang, PR China
'Institute for Nanotechnology Innovation, Department of Chemistry, Rhodes University, 6140 Makhanda, South Africa
Corresponding author E-mail: xiaonan6189@163.com @2Corresponding author E-mail: j.mack@ru.ac.za @3Corresponding author E-mail: Liangxu@ujs.edu.cn
Herein, a series of three o-substituted Co(III) corroles with electron-donating/withdrawing moieties have been prepared andfully characterized. The different functional substituents at o-position of meso-phenyl rings results in tunable local environment, and unusual pH-dependent electrochemically catalyzed oxygen reduction behaviors were clearly observed.
Keywords: Co(III) corroles, local environment, electrochemistry, TD-DFT calculations, oxygen reductions.
pH- Зависимое электрохимически катализируемое окислительно -восстановительное поведение о-замещенных Co(III) корролов
В. Танг,а Ю. Киу,ь К. Ли,а@ Р. Сой,с Дж. Мак,с@2 Т. Ниоконг,с К. Лиангь@3
аОтдел детского здравоохранения, Нанкинская детская больница, Нанкинский медицинский университет, 210008 Нанкин, Китай
ъШкола химии и химической инженерии, Университет Цзянсу 212013 Чжэньцзян, Китай
Институт инноваций в области нанотехнологии, кафедра химии, Университет Родса, 6140 Маканда, ЮАР
@lE-mail: xiaonan6189@163.com
@2E-mail: j.mack@ru.ac.za n
@3E-mail: Liangxu@ujs.edu.c
В работе были получены и полностью охарактеризованы три о-замещенных Со(Ш)-коррола с электронодо-норными/электроноакцепторными фрагментами. Различные функциональные группы в о-положении мезо-фенильных колец приводят к локальным изменениям, отчетливо наблюдалось необычное рН-зависимое электрохимически катализируемое окислительно-восстановительное поведение.
Ключевые слова: Co(Ш)-юрролы, локальное окружение, электрохимия, TD-DFT расчеты, восстановление кислорода.
Introduction
Earth-abundant first row transition metal corrole complexes have played an important role in fundamental research due to their unique molecular structures and attractive properties.11-41 In comparison to porphyrins, corroles have three inner N-H protons and are ring-contracted with a smaller macrocyclic cavity.[5-6] First row transition metal corroles have been widely used as effective electrochemical catalysts for small molecule activations, such as hydrogen evolution, oxygen reduction/evolution and CO2 reduction reactions (HERs, ORRs/OERs and CO2RRs) through homogenous and/or heterogenous prodecures.[7-10] Several strategies have been used to modulate the catalytic efficiency of synthetic metallocorroles. For example, the introduction of functional substituents at the meso- and/or ^-positions of metallocorroles rings can be used to enhance the catalytic efficiency, through perturbation to the electronic structure of the complexes.[11-14] Research has also focused on modulating the electron transfer properties between the catalytic center of metal corroles and carbon supports.[15-18] Metal-locorrole based covalent organic frameworks have been also prepared through chemical and/or electrochemical polymerization to obtain highly efficient catalysts.[19-20] Interestingly, the super-structure of metallocorroles have also been reported to enhance their electrochemical catalysts. For example, the introduction of a "hangman" ligand enables the precise control of the placement of a proton-donating or -accepting moiety over the face of a macrocyclic redox site.[21-24] Thus, it is reasonable to assume that changes to the local environment of metallcorroles could be an effective strategy for enhancing the catalytic efficiency. According to the crystal structures of Co(III)PPh3-triarylcorrole, o- and m-functional units of meso-phenyl rings generally lie at the distal side of the corrole rings due to steric hindrance related to the triphenylphosphine ligands. Herein, we describe how o-substituted Co(III)-triarylcorroles modulate electrochemically catalyzed oxygen reductions, by combining electrochemical studies with an analysis of the trends in the optical spectra and TD-DFT calculations will also be described.
Experimental
Synthesis of Co(III)-5,10,15-triphenylcorroles (3a). The general experimental procedures are described in the Supporting
information. The Co(III)PPh3-corroles were synthesized through a metal-insertion reaction of the corresponding free base corroles (0.05 mmol) by using excess cobalt acetate (0.25 mmol) and triphenylphosphine dissolved in 25 mL of CH2Cl2/CH3OH (v/v = 1:4) solution at 75 °C under N2 for 1 h. After removal of the solvent on a rotary evaporator, the crude product was purified by silica gel column chromatography and recrystallized. Free base corrole 2a was prepared according to literature procedure.[25] The target o-substituted Co(III)-corrole 3a was obtained as deep-red solid state compound in a 55.6 % yield (0.047 g). MALDI-TOF: m/z = 843.29 (Calcd. [M-PPh3]+ = 844.84). IR (KBr) vmax cm-1: 3424 s, 3053 w, 2972 w, 2924 w, 2364 w, 1597 m, 157l"w, 1556 w, 1535 w, 1504 m, 1433 s, 1344 m, 1318 m, 1223 w, 1175 w, 1088 w, 1070 w, 1052 vs, 1015 s, 984 m, 881 w, 843 w, 787 m, 747 s, 715 s, 699 s, 669 w, 521 vs. 'H NMR (400 MHz, CDCl3) SH ppm: 8.58 (2H, d, J = 4.2 Hz), 8.32 (2H, d, J = 4.5 Hz), 8.15-8.03 (4H, d, J = 4.0 Hz), 7.99 (3H, s), 7.72-7.48 (11H, m), 7.41-7.31 (1H, d, J = 7.0 Hz), 7.05 (3H, t, J = 7.2 Hz), 6.76-6.64 (6H, t, J = 6.9 Hz), 4.72 (6H, dd, J1 = 9.8 Hz, J2 = 8.4 Hz).
Synthesis of Co(III)-5,10,15-tri(o-nitrophenyl)corrole (3b). The general synthetic procedure is same as that for 3a, with the exception that o-nitrobenzaldehyde was used. The target compound was successfully obtained in a 64.5 % yield (0.0320 g). MALDI-TOF: m/z = 979.64 (Calcd. [M-PPh3]+ = 979.83). IR (KBr) v cm-1: 3444 s, 3056 w, 2358 w, 1733 w, 1652 w, 1606 w, 1570 w,
max
1558 w, 1526 vs, 1456 w, 1436 m, 1353 vs, 1320 w, 1260 w, 1224 w, 1161 w, 1052 m, 1016 m, 984 m, 874 w, 847 m, 787 m, 741 s, 716 s, 693 s, 666 w, 521 s. « NMR (400 MHz, CDCl3) SH ppm: 8.72-8.61 (1H, dd, J = 37.4 Hz, J2 = 4.3 Hz), 8.46-8.37 (1H, m), 8.33-8.15 (5H, m), 8.11 (1H, d, J = 4.8 Hz), 8.08-7.98 (2H, dd, J = 12.8 Hz, J2 = 6.2 Hz), 7.98-7.88 (2H, m), 7.87-7.66 (6H, m), 7.62 (1H, t, J= 7.1 Hz), 7.03 (3H, t, J = 7.2 Hz), 6.73 (6H, m), 4.91-4.66 (6H, m).
Synthesis of Co(III)-5,10,15-tri(o-methoxylphenyl)corrole (3c). The general synthetic procedure is same as that for 3a, with the exception that o-methoxybenzaldehyde was used. The target compound was successfully obtained in a 62.3 % yield (0.0289 g). MALDI-TOF: m/z = 933.42 (Calcd. [M-PPh3]+ = 934.92). IR (KBr) v cm-1: 3442 s, 3057 w, 2927 w, 2829 w, 2359 w, 2324 w, 1643 m,
max
1575 w, 1536 w, 1488 m, 1457 m, 1432 s, 1344 w, 1317 m, 1286 w, 1248 s, 1158 w, 1117 m, 1047 s, 1017 s, 983 m, 855 w, 787 m, 750 s, 711 m, 660 w, 618 w, 521 s. B NMR (400 MHz, CDCl3) SH ppm: 8.37 (1H, m), 8.17 (1H, d, J = 30.4 Hz), 8.10-7.81 (5H, m), 7.80-7.41 (8H, m), 7.23-7.10 (4H, m), 7.08-6.95 (4H, m), 6.76-6.60 (6H, m), 4.99-4.72 (6H, m), 3.64-3.43 (7H, m), 3.27 (2H, d, J = 6.0 Hz).
Results and Discussion
Free base corroles (2a-c) were synthesized according to literature procedures from a reaction of dipyrromethane 1 and an arylaldehyde (Scheme 1).[25] Co(III)-Triarylcor-roles (3a-c) were then synthesized by a metal-insertion
Scheme 1. Synthetic procedure of o-substituted Co(III)-corroles 3. Макрогетер0циmbl/Macroheterocycles 2020 13(2) 156-162
Figure 1. The UV-Visible absorption spectra of 3a-c in CH2Cl2.
reaction of 2a-c by using excess cobalt acetate and tri-phenylphosphine, and were purified by silica gel column chromatography and recrystallized. MALDI-TOF MS for 3a revealed an intense parent peak at m/z = 843.29 (Calcd. [M-PPh3]+ = 844.84), providing direct evidence that the Co(III)-triphenylcorrole 3a target compound was successfully prepared. Similar parent peaks were observed for 3b and 3c. In the 'H NMR spectra of 3a-c, the proton signals for both the meso-substituents and pyrrole rings lie beyond 7.40 ppm and the other three signals at 7.10, 6.70 and 4.50 ppm can be assigned to the triphenylphosphine axial ligand (Figure S1, see ESI). The peaks from 3056 and 2970 cm-1, and at 1608 and 1490 cm-1 in the FT-IR spectra, can be assigned to C-H stretching and benzene skeleton vibrations, respectively. The peaks at 1526 and 1353 cm-1, and at 1248 and 1117 cm-1 can be assigned, respectively, to the -NO2 and -OMe groups of Co(III)PPh3 corroles 3b and 3c (Figure S2, see ESI).
The electronic structures of the n-systems of por-phyrinoids and their optical spectra (Figures 1 and 2) can be readily understood by using Gouterman's 4-orbital model[26] and Michl's perimeter model[27] as conceptual frameworks through a consideration of how different structural perturbations alter the energies of the frontier n-molecular orbitals (n-MOs) of a parent hydrocarbon perimeter.[28] The n-MOs of the C15H153- parent perimeter of corroles are arranged in an ML = 0, ±1, ±2, ±3, ±4, ±5, ±6, ±7 sequence in ascending energy terms due to their angular nodal properties. The highest occupied and lowest occupied molecular orbitals (HOMO and LUMO) have ML values of ±4 and ±5, respectively. Michl[27] introduced an a, s, -a, -s nomenclature (Figure 3) for the MOs that are derived from the HOMO (a, s) and LUMO (-a, -s) of the parent perimeter depending on whether a nodal plane is aligned with the z-axis (a, -a) or there are large MO coefficients (s, -s). When the UV-Visible absorption spectra of 3a-c (Figure 1) are compared to analogous bands in the spectra of Co(III)-triarylcorroles,[10] the introduction of electron-withdrawing o-nitrophenyl and donating o-methoxylphenyl substituents at meso-positions results in only minor-shifts of the Q-bands (3a: 561 and 584 nm, 3b: 565 and 590 nm, and 3c: 559 and 584 nm). The only significant spectral differences that are observed are related to a splitting of the 5-band of Co(III)-corrole 3b. TD-DFT calculations demonstrate that this can be assigned
to the presence of weak transitions into MOs localized on the meso-aryl rings (Figures 2 and 3 and Table S1, see ESI). The MO energies structures of 3a-c, only differ based on the weak inductive effect of the meso-aryl groups. The introduction of electron-withdrawing and -donating groups at the o-positions of 3b and 3c results in a uniform stabilization or destabilization of the n-MO energies and the HOMO-LUMO gaps remain almost unchanged (Figures 2 and 3).
To gain further insight into the electronic structures and redox properties of 3a-c, cyclic and differential pulse voltammetry (CV and DPV) measurements were carried
Figure 2. The TD-DFT calculations of 3a-c at the CAM-B3LYP/6-31G(d) level of theory. The Q- and 5-bands that are associated with the main spin-allowed transitions between the a, s, -a and -s MOs of Michl's perimeter model are highlighted with red diamonds and blue diamonds are used to highlight transitions associated with the 3d orbitals of the central Co(III) ion. Purple and black diamonds are used to highlight transitions into three MOs localized on the meso-aryl groups due to the presence of the nitro groups and other n ^ n* transitions, respectively.
Figure 3. The MO energies of 3a-c at the CAM-B3LYP/6-31G(d) level of theory (bottom). The a, s, -a and -s MOs of Michl's perimeter model are highlighted with thicker black lines and the 3d orbitals of the central Co(III) ion are highlighted and blue with stars. MOs associated with the meso-aryl groups of 3b that lie between the -a and -s MOs are highlighted in purple. Red diamonds are used to highlight the HOMO-LUMO gaps which are plotted against a secondary axis. The angular nodal patterns of the a, s, -a, -s and 3dz2 MOs of 3a are shown at an isosuface of 0.02 a.u. (top).
out in o-dichlorobenzene (o-DCB), so that redox potential (Ey) values could be clearly derived (Figure 4 and Table 1). The effect of reduction is more complex, however, due to the presence of a 3d orbital associated with the central Co(III) ion and two MOs associated with the meso-nitro-phenyl rings in addition to the -a and -s MOs. Co(III)-Cor-roles have been reported to undergo a facile one-electron reduction to generate a [Co(II)corrole]- species together with the removal of axial PPh3 ligand during the first reduction step, followed by a one-electron reduction on the rings and at the metal center as the second and third steps (sometimes the reduction of the meso-substituents is also observed, for example with the nitrophenyl-unit of 3a), respectively. While there is a positive shift of the oxidation curves and a negative shift of the reduction curves when the redox data of electron-deficient 3b are compared to those for Co(III)-triphenylcorrole 3a, and the potential values of electron-rich Co(III)-corrole 3c were negatively shifted (Figure 4, Table 1). The poorly defined cathodic reduction peak and the negative shifts of the Ey value for the Co(III)/Co(II) processes are mainly caused by the loss
of the PPh3 axial ligand and an associated reaction due to electron transfer. The trend observed in the gaps between the first reduction and oxidation steps is consistent with the slight blue shift of the main 5-band that is observed spectroscopically for the compound with more strongly electron-withdrawing meso-aryl rings.
The Co(III)-triarylcorrole/rGO (Co(III)PPh3-triaryl-corrole/rGO-graphene) composites were examined as electrocatalysts for use in ORRs, and the stability of the Co(III)-corroles in both strongly acidic and strong base media was confirmed by using previously reported procedures.[10] The next step was to confirm the p^ effect on the ORRs catalysed by Co(III)-triarylcorrole/rGO composites. Catalyst-loaded GCEs were immersed in 0.5 M H2SO4 solution and in 0.1 M NaOH for the ORR measurements (Figure 5). In strongly acidic media (p^ = 1), the onset potentials of Co(III)-corroles 3a-c under an O2 atmosphere were arranged at 0.493 V for 3c, 0.498 V for 3b, and 0.508 V for 3a (Table 2). When the p^ value increased to 4.0, the order of onset potentials changed to 3a (0.474 V) > 3b (0.448 V) > 3c (0.419 V). When Co(III)-corroles 3a-c were
Table 1. Potential values (E%, V) of Co(III)-corroles 3a-c.
E'A Ox II E'A Ox I E'A Red I E'A Red II E'A Red III
3a 1.10 0.60 -0.58 -1.55 -
3b 1.18 0.73 -0.50 -1.24 -1.37
3c 1.12 0.48 -0.67 -1.66 -
Figure 4. CV (left) and DPV (right) measurements of 3a (top), 3b (middle) and 3c (bottom) in o-DCB containing 0.1 M TBAP.
Table 2. Active potentials of electrochemically catalyzed oxygen reductions of 3a-c at different pH values.
pH value 1st 2nd 3rd
1.0 3a (0.508 V) 3c (0.498 V) 3b (0.493 V)
4.0 3a (0.474 V) 3b (0.448 V) 3c (0.419 V)
7.0 3b (0.475 V) 3a (0.432 V) 3c (0.427 V)
10.0 3b (0.601 V) 3a (0.579 V) 3c (0.565 V)
13.0 3b (0.795 V) 3c (0.784 V) 3a (0.769 V)
tested in the neutralized and basic media, Co(III)-corroles 3b, with o-nitrophenyl substituents, revealed better performance on the electrochemically catalyzed oxygen reductions. The active potentials vs рЯ relationship is shown in Figure 6. Considering 3b has an electron-withdrawing o-nitrophenyl unit, the interactions between the O2 molecule
and the catalytic Co(III) center in negative OH- charge abundant media (pH > 7) must be stronger than for the other complexes. Thus, the electrochemically catalysed oxygen reduction could be enhanced. Meanwhile, 3c with the electron-donating o-methoxylphenyl group, has a better performance in H+ positive charge abundant media (pH = 1). It is clear from the investigation of the electronic structures of the Co(III)-triarylcorroles that the introduction of o-functionalized Co(III)PPh3-corroles has a minor effect, thus the tuneable electrocatalytic properties can be assigned to the change of local environment of the catalytic center of Co(III)PPh3-corroles 3a-c.
Conclusions
A series of three o-substituted Co(III)-corroles with electron-donating/withdrawing moieties has been prepared
Figure 5. CV measurements of 3a-c at various pH values under an O2 atmosphere.
Figure 6. The active potentials vs pH values derived from CV measurements of 3a-c.
and fully characterized. When different electron-donating and -withdrawing units were introduced at the o-positions of Co(III)-corroles, the influence on the tunable catalytic behavior can be assigned primarily to changes in the local environment since the inductive effects of the o-substituents are relatively weak. Considering super-structured metallocorroles have a wide range of application, the current study will provide useful information for future molecular design related to enhanced electrochemically catalyzed energy-related small molecule activations.
Acknowledgements. This work was supported by the National Natural Science Foundation of China (21701058, 21703086), the Natural Science Foundation of Jiangsu Province (BK20160499), the State Key Laboratory of
Coordination Chemistry (SKLCC1817), the Key Laboratory of Functional Inorganic Material Chemistry (Hei-longjiang University) of Ministry of Education, the China post-doc foundation (2018M642183), the Lanzhou High Talent Innovation and Entrepreneurship Project (2018-RC-105) and the Jiangsu University (17JDG035). The theoretical calculations were carried out at the Centre for High Performance Computing in Cape Town. Dr. Wei Tang also acknowledges the financial support from Jiangsu Province (F201648) and the Zhenjiang Key Research Plan (SH2019055).
References
1. Ganguly S., Ghosh A. Acc. Chem. Res. 2019, 52, 2003-2014.
2. Lim H., Thomas K.E., Hedman B., Hodgson K.O., Ghosh A., Solomon E.I. Inorg. Chem. 2019, 58, 6722-6730.
3. Ghosh A. Chem. Rev. 2017, 117, 3798-3881.
4. Buckley H.L., Rubin L.K., Chrominski M., McNicholas B.J., Tsen K.H.Y., Gryko D.T., Arnold J. Inorg. Chem. 2014, 53, 7941-7950.
5. Guo Y.X., Gu T.T., Li P.F., Fu B., Sun L., Zhu W.H., Xu H.J., Liang X. Inorg. Chem. Acta 2019, 496, 119067.
6. Petrova D.V., Semeikin A.S., Berezina N.M., Berezin M.B., Bazanov M.I. Macroheterocycles 2019, 12, 119-128.
7. Kosa M., Levy N., Elbaz L., Major D.T. J. Phys. Chem. C 2018, 122, 17686-17694.
8. Levy N., Mahammed A., Kosa M., Major D.T., Gross Z., Elbaz L. Angew. Chem. Int. Ed. 2015, 54, 14080-14084.
9. Liang X., Niu Y.J., Zhang Q.C., Mack J., Yi X.Y., Hlatsh-wayo Z., Nyokong T., Li M.Z., Zhu W.H. Dalton Trans. 2017, 46, 6912-6920.
10. Niu Y. J., Li M.Z., Zhang Q.C., Zhu W.H., Mack J., Fomo G., Nyokong T., Liang X. Dyes Pigm. 2017, 142, 416-428.
11. Barata J.F.B., Neves M., Faustino M., Tome C., Cavaleiro J. Chem. Rev. 2017, 117, 3192-3253.
12. Zhang X.F., Wang Y., Zhu W.H., Mack J., Soy R.C., Nyokong T., Liang X. Dyes Pigm. 2020, 175, 108124.
13. Berezina N.M., Bazanov M.I., Maksimova A.A., Semeikin A.S. Russ. J. Phys. Chem. A 2017, 91, 2377-2382.
14. Zhu W.H., Huang T.T., Qin M.F., Li M.Z., Mack J., Liang X. J. Electroanal. Chem. 2016, 774, 58-65.
15. Berezina N.M., Thao V.T., Berezin D.B., Bazanov M.I. Russ. J. Inorg. Chem. 2017, 62, 1617-1621.
16. Lei H.T., Li X.L., Meng J., Zheng H.Q., Zhang W., Cao R. ACS Catalysis 2019, 9, 4320-4344.
17. Li H., Li X.L., Lei H.T., Zhou G.J., Zhang W., Cao R. Chem-SusChem 2019, 12, 801-806.
18. Stefanelli M., Pomarico G., Tortora L., Nardis S., Fronczek F.R., McCandless G.T., Smith K.M., Manowong M., Fang Y.Y., Chen P., Kadish K.M., Rosa A., Ricciard G., Paolesse R. Inorg. Chem. 2012, 51, 6928-6942.
19. Friedman A., Saltsman I., Gross Z., Elbaz L. Electrochim. Acta 2019, 310, 13-19.
20. Friedman A., Landau L., Gonen S., Gross Z., Elbaz L. ACS Catalysis 2018, 8, 5024-5031.
21. Sinha W., Mahammed A., Fridman N., Diskin-Posner Y., Shimon L., Gross Z. Chem. Commun. 2019, 55, 11912-11915.
22. Lai W.Z., Cao R., Dong G., Shaik S., Yao J.N., Chen H. J. Phys. Chem. Lett. 2012, 3, 2315-2319.
23. Dogutan D.K., Stoian S.A., McGuire R., Schwalbe M., Teets T., Nocera F.G. J. Am. Chem. Soc. 2011, 133, 131-140.
24. Dogutan D.K., McGuire R.J., Nocera D.G. J. Am. Chem. Soc. 2011, 133, 9178-9180.
25. Li M.Z., Zhu W.H., Mack J., Mkhize S., Nyokong T., Liang X. Chin. J. Struct. Chem. 2017, 36, 367-380.
26. Gouterman M. In: The Porphyrins. Vol. III, Part A (Dolphin D., Ed.) New York: Academic Press, 1978. pp. 1-165.
27. Michl J. Tetrahedron 1984, 40, 3845-3934.
28. Mack J. Chem. Rev. 2017, 117, 3444-3478.
Received 26.01.2020 Accepted 06.05.2020
