Structure Effects of Metal Corroles on Energy-Related Small Molecule

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Review

Structure Effects of Metal Corroles on EnergyRelated Small Molecule Activation Reactions Haitao Lei, Xialiang Li, Jia Meng, Haoquan Zheng, Wei Zhang, and Rui Cao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00310 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Catalysis

Structure Effects of Metal Corroles on Energy-Related Small Molecule Activation Reactions

Haitao Lei, Xialiang Li, Jia Meng, Haoquan Zheng, Wei Zhang, and Rui Cao*

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China.

*Correspondence E-mail: [email protected]

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Abstract: The increasing global energy and environmental crises make it urgent to find and use renewable, clean and environmentally benign new energy resources. New energy conversion schemes based on small molecule activation reactions, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR), have been proposed. For example, catalytic water splitting provides a promising way to convert solar energy to chemical energy, which is stored as the hydrogen fuel. Hydrogen oxidation in fuel cells is an energy-releasing process to generate electric energy and water as the only product. Although such a hydrogen-based new energy scheme is appealing, its viability is dependent on highly efficient and robust catalysts for these small molecule activation reactions. Recently, a variety of metal corroles have been demonstrated to be efficient in catalyzing HER, OER, and ORR. The redox-active trianionic corrole ligands can afford a rigid four-coordinated square-planar molecular structure and are very effective in stabilizing high-valent metal centers. These features make metal corroles very attractive to serve as catalysts for these processes. More importantly, the structure of metal corroles can be systematically modified, which provides an ideal system to investigate the structure-reactivity relationship with aims to obtain fundamental knowledge on catalyst design and also catalytic mechanism. In this review, we summarize recently reported metal corrole catalysts for HER, OER and ORR, and pay particular attention to the structure effects on these reactions.


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Keywords: small molecule activation, hydrogen evolution, oxygen evolution, oxygen reduction, metal corrole

1. Introduction 1.1. Energy conversion schemes based on HER, OER and ORR Small molecule activation reactions are significant in many biological processes and have been extensively investigated by scientists.1-6 Among these reactions, energy-related small molecule activation has recently attracted increasing interests because of the global energy demands and environmental concerns related to the use of fossil fuels.7-11 Nature presents many fascinating examples for highly efficient energy conversion schemes based on small molecule activation reactions. For example, photosynthesis by green plants and other organisms converts solar energy to chemical energy in a scale of sustaining the whole life circle on the earth.11-13 This process is initiated at photosystem II (PSII), where a chlorophyll called P680 with a magnesium porphyrin core absorbs solar light to drive photo-induced electron transfer. The resulted P680•+ can then retrieve an electron from the Mn4CaOx cluster of the oxygen-evolving complex (OEC).1,2,14 When four oxidizing equivalents are stored at the OEC through four such steps, two water molecules can be oxidized to release one O2 molecule. The four electrons and four protons generated during this water oxidation process are used for converting carbon dioxide to carbohydrates.8,9,11 These processes together realize the conversion of solar energy to chemical energy with the latter stored in chemical bonds. This natural photosynthesis presents a promising 3 ACS Paragon Plus Environment

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scenario for human beings to design and develop new energy conversion schemes to convert, store and utilize solar energy in an economical and efficient way. One appealing energy conversion scheme is based on water splitting to make H2 and O2. Hydrogen is an ideal energy carrier because it is carbon-free and its combustion product is only water. Hydrogen fuel cells can then convert this chemical energy to electrical energy. Unlike the natural photosynthesis process, the electrons and protons released from water oxidation are combined directly to produce H2. This direct coupling is beneficial from a kinetic point of view to avoid significant structural rearrangements, which happen in other energy carrier molecules to accommodate multiple bond cleavage and formation steps. In this energy conversion scheme, four fundamental small molecule activation reactions are involved, including the hydrogen evolution reaction (HER),15-32 the oxygen evolution reaction (OER),33-45 the oxygen reduction reaction (ORR),46-63 and the hydrogen oxidation reaction.64-66 During these reactions, the precise coupling between electron transfer and chemical bond (i.e., H−H and O−O bonds) cleavage and formation is required.67-71 As a consequence, the viability of this energy conversion scheme is dependent on the high efficiency of these reactions. For HER, metal hydride species are generally proposed to be involved as key intermediates in catalytic cycles.24,25 Subsequent H−H bond formation can happen through either homolytic bimolecular coupling between two such hydride species or heterolytic protonolysis with a proton to release H2 molecule.26-30,72,73 For OER, high-valent metal oxo/oxyl species are usually considered as key intermediates.33,34 4 ACS Paragon Plus Environment

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The O−O bond can be formed by either water nucleophilic attack (WNA)74-77 to these metal-oxo/oxyl units or by coupling between two such metal-oxo/oxyl units.78,79 Further oxidation of the resulted peroxo species will lead to the evolution of O2 molecules. For ORR, O2 molecules can be reduced by either two electrons to give H2O2 or four electrons to produce H2O.80 Although the 4e reduction of O2 is more favored from an energy point of view, the selective production of H2O2 is also significant because H2O2 is an important oxidant in industry and it is an alternative fuel with the benefits of easy storage and transportation.81,82 The selectivity is determined by metal-oxygen bond strength and also by proton and electron transfer rates.83-88 The efficiency of these reactions is dependent on the precise coupling between electron transfer and atom transfer. In other words, the cleavage and formation of chemical bonds, including the H−H and O−O bonds, should be coupled with electron transfer to avoid the formation of intermediates that are high in energy.67-71 Catalysts will play crucial roles in controlling and regulating the atom transfer and electron transfer in a concerted manner in these processes and thus can significantly improve the efficiency of these reactions, particularly the sluggish OER and ORR reactions.89 Importantly, in addition to the energy conversion scheme presented above, these three reactions are also valuable in many other energy-related new techniques. For example, electrocatalytic HER and OER can be coupled with other small molecule activation reactions, such as the coupling of halide oxidation with HER90-92 and the coupling of CO2 reduction with OER.7,10,93 Through these electrocatalytic 5 ACS Paragon Plus Environment

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reactions, the electrical energy generated from renewable energy sources can be converted to chemical energy, which is easier to be stored and transported. The ORR is significant in many types of fuel cells and metal-air batteries.46-48 In general, the reduction of O2 on the cathode electrode and the oxidation of fuels (i.e., H2, methane, methanol, ethanol) on the anode electrode are combined to generate electrical energy. For metal-air batteries, its discharging process also couples the O2 reduction on the cathode electrode and the metal oxidation on the anode electrode. During these processes of chemical to electrical energy conversion, the efficiency and selectivity of ORR are significant determinants for the overall performance of these new energy techniques.

1.2. Brief summary of porphyrin- and corrole-based catalyst systems Porphyrins and the derivatives are used by nature to perform a wide variety of small molecule activation reactions that are vitally significant in biology, including O2 binding and activation and peroxide degradation.5,6,94-96 These heterocyclic macrocycles consist of four pyrroles interconnected by four methine linkers, giving a conjugated ring with 18 π electrons delocalized on an aromatic system (Figure 1). This structural feature makes porphyrins to have very strong absorption in the visible light region. By losing two N−H protons, the resulted dianionic porphyrin macrocycle ligands can afford a four-coordinated square-planar coordination environment with an appropriate space to accommodate most transition metal ions and main group elements. In the last decades, investigating the catalytic features of metal porphyrins 6 ACS Paragon Plus Environment

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for biomimetic small molecule activation reactions have received increasing interests because of the following reasons.97 First, the molecular structure of metal porphyrins can be unambiguously determined, and it has sufficient stability in both solid and solution states. This is crucial for mechanistic studies. Second, their physical and chemical properties can be systematically tuned and studied by changing substituents at the meso- and β-positions. Third, the introduction of additional functional groups at the second coordination sphere is accessible. Fourth, the rigid square-planar coordination geometry of porphyrin ligands provides an ideal system to study the substrate binding and the trans ligand effect in catalysis. Fifth, metal porphyrins have strong and characteristic electronic absorption features in the visible-light range, which is beneficial for mechanistic studies in homogeneous solutions. Sixth, the redox-active porphyrin ligands can participate in electron transfer, making metal porphyrins valuable in multi-electron catalytic processes.

Figure 1. Porphyrin and corrole skeletal structures.

From these studies, one can see that the structure of metal porphyrins plays crucial roles in determining their catalytic performance for various small molecule 7 ACS Paragon Plus Environment

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activation reactions. For example, cytochrome c oxidase (CcO), which belongs to a superfamily of heme/Cu oxidases, catalyzes the biological O2 reduction to H2O.5,6,94,95 Based on biomimetic studies from Collman,85,98,99 Dey,87,100,101 Karlin,102-104 Naruta,105-107 Boitrel108,109 and others, it is suggested that, in CcO, the proximate CuII ion, the Tyr244 residue, the distal pocket environment, and the trans imidazole ligand of Fe together guarantee the complete 4e reduction of O2 to H2O. This 4e ORR is vital because the release of partially reduced oxygen species is extremely harmful to organisms. In addition, with the use of Fe porphyrins, electrocatalytic studies from Collman,84,110 Mayer,111,112 Dey86,88 and others demonstrated the effects of second coordination sphere structures, hydrogen bonding interactions, and electron transfer rates on catalyzing the selective 4e ORR. These studies shed light on the reaction mechanism of ORR and also provide significant information to the design of efficient molecular catalysts for ORR. As the derivatives of metal porphyrins, metal corroles have also recently attracted much attention as catalysts for small molecule activation reactions, including HER, OER and ORR.50,113-117 Corroles are a class of tetrapyrrolic macrocycle complexes that have a skeleton structure similar to porphyrin ring (Figure 1). Unlike porphyrins having 20 carbon atoms, corrole macrocycles lack one meso-carbon atom and have two pyrrole units directly linked together. Although corrole ligands can also provide the N4 square-planar coordination geometry, this structure change makes the corrole ring has a smaller coordination cavity and a reduced molecular symmetry. More importantly, the lack of one meso-carbon atom also makes the deprotonated 8 ACS Paragon Plus Environment

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corrole macrocycles to become trianionic ligands, which are more effective in stabilizing metal ions in their high oxidation states. This feature is significant in OER and ORR catalysis, in which high-valent metal centers are generally considered to be involved as key intermediates in the process of the O−O bond formation and breaking. In addition, due to the stabilization effect of corrole macrocycles on high-valent metal centers, the low-valent metal ions coordinated by corrole ligands usually have very strong reducing potentials, which are beneficial for HER and ORR catalysis. Despite this structural difference and its related effects in coordination, both metal porphyrins and metal corroles do share similarities in many physical and chemical properties. For example, corroles also have strong electronic absorption features known as Soret and Q bands. Corrole macrocycles are aromatic and are redox-active. The substituents at the meso- and β-positions can introduce similar electronic effects for metal corroles. These features of metal corroles are parallel to those of metal porphyrins. As a consequence, the knowledge obtained in the porphyrin chemistry is valuable in predicting and understanding the chemistry of corroles. Corroles have been known for more than 50 years since the first report by Johnson and Kay,118 but the meso-unsubstituted corrole rings are not stable and are easily oxidized through an oxidative coupling mechanism at the meso-positions. In 1999, Gross and co-workers reported a one-pot synthesis to make stable meso-substituted corroles with good efficiency and yield.119 Since then, the corrole chemistry, especially its uses as ligands for metal ions, started to grow rapidly. Recently, metal corroles have been explored in many areas, including catalysis, 9 ACS Paragon Plus Environment

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sensing,

photoactive

arrays,

dye-sensitized

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solar

cells,

and

medicinal

applications.50,120-127 Several comprehensive reviews have been published to highlight the potential uses of corroles in these areas.120,127 Particularly, in the very recent special issue “Expanded, Contracted, and Isomeric Porphyrins” published in Chemical Reviews in 2017,128 the synthesis and chemistry of corroles have been summarized. For example, Gryko and co-workers described the synthetic methods to make various corroles.129 Cavaleiro and co-workers covered the strategies for corrole functionalization at both the meso- and β-positions of the macrocycle.130 Termini, Gross, Gray and co-workers summarized the recent progress of anticancer research with corroles.131 Kadish, Ou and co-workers described the electrochemistry of corroles in non-aqueous media.132 Cao and co-workers summarized the catalytic features of corroles in energy-related small molecule activation reactions.97 For catalytic small molecule activation, such as HER, OER and ORR, metal corroles have been found to be highly active and robust in catalyzing these reactions. It is also found that using various metal ions and introducing different substituents at the meso- and β-positions can significantly change the catalytic properties of corroles. This not only provides a valuable strategy to investigate the structural effects of these macrocycles on these catalytic reactions but also highlights the promising applications of metal corroles in these catalyses.

1.3. Scope of this review Because of the importance of small molecule activation reactions in 10 ACS Paragon Plus Environment

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energy-related areas and also the rapidly grown chemistry of metal corroles in these catalytic reactions, this review article will summarize recent progress made on catalytic HER, OER and ORR with metal corroles. In the first part, we will emphasize the structural effects of metal corroles on their catalytic properties, including the substituent effects at the meso- and β-positions, the trans axial ligand effects, and the effects of the second coordination sphere structures. In the second part, we will discuss the catalytic features of dinuclear metal systems, including biscorroles and porphyrin-corrole dyads. In the third part, we will describe the molecular engineering of corrole-based materials with potential practical uses. Better understanding the structure-function relationship is significant for chemists to design and develop new catalysts with high efficiency and stability. This information is also valuable for other molecular catalytic systems. In addition, investigating the structure effects will shed light on the mechanism of catalytic reactions. It is necessary to note that previous review articles from Kadish132 and from us97 have summarized the electrochemical properties of metal corroles and their uses in catalytic HER, OER and ORR. This review has new information in the following three aspects. First, it covers new progress made in the last two and half years, during which time more significant achievements of corrole-based catalyses have been reported in the literature. Second, instead of simply describing catalytic performance, we will focus on the structural effects of metal corroles on catalyses. Consequently, more efforts will be devoted to discussing the improvements in catalysis with appropriate structural modifications. Third, we will pay special attention to the immobilization of well-designed 11 ACS Paragon Plus Environment

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corrole-based catalysts on electrode materials. In order to realize the practical use of molecular catalysts, one can image that grafting these catalysts onto appropriate electrode materials is required. Therefore, this review is aimed to provide valuable information on the catalyst design with improved catalytic performance and with promising future use in energy-related new technologies.

2. Structure Effects on HER, OER and ORR 2.1. Substituent effects at the meso-position It is established that the physical and chemical properties of metal corroles can be controlled by changing the substituents at the three meso-positions of the macrocycle. This substituent effect plays crucial roles in determining the stability and the redox property of metal corroles, which are both significant factors considered in the catalysis of small molecule activation reactions. Many articles have been published regarding the influence of meso-substituent on the electrochemical and spectral properties of metal corroles.133-138 Notably, electron-withdrawing groups can decrease the electron density on the aromatic corrole macrocycle, and thus increase their tolerance of oxidative degradation. The easier reduction is observed for corroles with strong electron-withdrawing groups. This effect will lead to the formation of active low-valent metal centers at relatively small negative potentials, which can usually decrease the overpotentials of catalytic reduction reactions. Kadish and co-workers synthesized a series of CoIII corroles with different meso-phenyl substituents (complexes 1-5, Figure 2),139 and 12 ACS Paragon Plus Environment

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examined them as ORR electrocatalysts on edge-plane pyrolytic graphite (EPG) electrodes

in

acidic

solutions.

Cyclic voltammograms

(CVs)

of

1-5

in

dichloromethane containing 0.1 M tetrabutylammonium perchlorate, (Bu4N)ClO4, showed that these five complexes all displayed a reversible oxidation wave in the range of 0.10 to 0.19 V versus ferrocene (all potentials recorded in nonaqueous solutions are referenced to ferrocene unless otherwise noted). Importantly, corroles with electron-withdrawing meso-substituent groups have relatively large oxidation potentials. In other words, they can be easier to be reduced at the cathodic electrode for electrocatalytic reduction reactions.

Figure 2. Co corrole complexes 1-8 studied as ORR electrocatalysts.

The electrocatalytic ORR studies were carried out in an air-saturated 1.0 M HClO4 solution, and the results were summarized in Table 1. As shown in this table, the ORR catalytic waves of 1-5 have peak potentials varying from Ep = 0.34 to 0.39 V versus RHE (reversible hydrogen electrode, all potentials recorded in aqueous solutions are referenced to RHE unless otherwise noted). This result indicates that different meso-substituents on corroles 1-5 do not have strong effect on the peak 13 ACS Paragon Plus Environment

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potentials of ORR. However, the H2O2 production is 87.7% for 1, 75.6% for 2, 72.7% for 3, 69.2% for 4, and 64.9% for 5, which corresponds to the number of electrons transferred per O2 molecule n = 2.2 for 1, 2.4 for 2, 2.6 for 3, 2.7 for 4, and 2.8 for 5. Therefore, although the peak potentials are not strongly influenced by different meso-substituents of 1-5, the 4e reduction of O2 to give H2O is improved with corroles containing strong electron-withdrawing substituents. This difference in the ORR selectivity may be due to the stronger binding affinity of negatively-charged reduced O2 species (i.e., peroxide species) on the Co corroles with stronger electron-withdrawing substituents. As a consequence, the release of reduced O2 species becomes difficult, and its further reduction to give H2O is more likely to happen under catalytic conditions.


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Table 1. Electrocatalytic ORR by 1-5 Adsorbed on EPG Electrodes in Air-Saturated 1 M HClO4 Aqueous Solutionsa

a

catalyst

Ep

E1/2

%H2O2

n

1

0.39

0.46

87.7

2.2

2

0.34

0.46

75.6

2.4

3

0.36

0.47

72.7

2.6

4

0.39

0.46

69.2

2.7

5

0.39

0.46

64.9

2.8

The potentials are versus RHE. E1/2 is the half wave potential of the limiting current.

To further confirm the effect of electron-withdrawing meso-substituents on the ORR selectivity, Kadish and co-workers studied the ORR using Co corroles 3 and 6-8 (Figure 2),140 which contain 0-3 nitro units at the para-position of the three meso-phenyl groups. The CVs of 3 and 6-8 in dichloromethane showed the same trend as observed for 1-5: the addition of strong electron-withdrawing nitro units leads to the positive shift in potential for the redox couples. The electrocatalytic ORR studies were carried out in air-saturated 1.0 M HClO4 aqueous solutions, and the results are summarized in Table 2. Similar to 1-5, there is no obvious difference for the reduction peak potentials for 3 and 6-8, while the n value of ORR is 2.6 for 3, 2.7 for 6, 2.9 for 7, and 3.0 for 8. This result indicates that the nitro-phenyl 15 ACS Paragon Plus Environment

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meso-substituents on the corrole macrocycle can improve the 4e ORR selectivity to H2O in acidic media. This improvement is likely due to the role of electro-withdrawing meso-substituents to assist Co corroles in binding reduced O2 species. However, the catalytic ORR peak potentials are similar for these Co corroles, which indicates that the potential is not strongly influenced by differences in the number of NO2 substituents on the corrole.

Table 2. Electrocatalytic ORR by 6-8 Adsorbed on EPG Electrodes in Air-Saturated 1.0 M HClO4 Aqueous Solutionsa

a

catalyst

Ep

E1/2

%H2O2

n

6

0.39

0.52

65

2.7

7

0.38

0.53

55

2.9

8

0.41

0.52

50

3.0


Cao and co-workers designed and synthesized four Cu corroles 9-12 (Figure 3),114 and investigated their catalytic HER features in acetonitrile. Because of relatively small reducing potentials of low-valent Cu species, molecular Cu complexes have been rarely reported as active catalysts for HER. However, by using corrole ligands, CuIII oxidation states can be stabilized. This leads to much enhanced 16 ACS Paragon Plus Environment

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reducing power of low-valent Cu species. As a consequence, reduced Cu corroles become active for proton reduction to evolve H2.

Figure 3. Cu corroles 9-12 studied as HER electrocatalysts.

The CVs of 9-12 were first measured in dry acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate, (Bu4N)PF6. The CV of 9 had two reversible 1e reduction waves at −0.12 and −1.42 V, which could be assigned to the CuIII/II and CuII/I couples, respectively. Under the same condition, the CV of 12 had two reversible 1e reduction waves at −0.29 and −2.14 V. Comparison of CVs of 9 and 12 showed that the replacement of the strong electron-withdrawing p-nitro-phenyl group at the 10-meso-position of 9 by an electron-donating p-methoxyphenyl group of 17 ACS Paragon Plus Environment

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12 caused the cathodic shift of the reduction potentials. Remarkably, the second reduction wave has a very large cathodic shift of 720 mV. Because mechanism studies showed that the 2e-reduced species of these Cu corroles were the catalytically active species for HER, this large cathodic shift suggested that Cu corrole 9 was much more efficient than 12 to catalyze HER. Complex 10 also showed two reversible 1e reduction waves at −0.15 and −1.28 V. Although the second reduction wave of 10 has a 140-mV anodic shift as compared to that of 9, complex 10 will undergo an oxidative dimerization through two β-carbon atoms in acidic solution, and thus it is not a competent HER catalyst. This difference in stability is noteworthy. Complexes 9, 10 and 12 have very similar Cu corrole structures except their different substituents at the 10-meso-position. Although complex 10 is not stable in acidic solution, Cu corroles 9 and 12 are very stable under the same acidic condition. Thus, these results suggest that different meso-substituents of corrole macrocycles not only affects their redox properties but also have significant effects on their stabilities. In order to overcome this oxidative dimerization, the β-brominated analogue 11 was synthesized. However, despite that 11 showed two reversible 1e reduction waves at 0.26 and −1.41 V, it is also not a competent HER catalyst due to its quite low solubility in acetonitrile. Subsequent electrocatalytic studies showed that 9 was highly active to catalyze HER in acetonitrile in the presence of trifluoroacetic acid (TFA) as the proton source. The pronounced catalytic wave has an onset at −0.95 V, which corresponds to an onset overpotential of only 50 mV. The potential measured at the onset of the catalytic wave, i.e., 0.1 mA cm−2, is used to determine the onset overpotential, which is 18 ACS Paragon Plus Environment

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generally reported and used for comparing the performance of molecular catalysts. Under optimized conditions, a remarkable ratio of icat/ip = 303 can be obtained, in which icat is the catalytic plateau current, and ip is the peak current of the second reduction wave of 9 in the absence of acid. Notably, the molecular nature of the catalysis with 9 was confirmed. Mechanism studies suggest that the 2e-reduced form of 9 is the catalytically active species for HER. Its reaction with a proton generates a Cu−H intermediate, which can be identified under stopped-flow measurements. This hydride intermediate can evolve H2 through three possible pathways (Figure 4). According to experimental and theoretical calculations, pathway C is suggested to be involved in the electrocatalytic HER with 9: the Cu−H intermediate will be further reduced by one electron and then undergo protonolysis with a second proton to produce H2.


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Figure 4. Proposed electrocatalytic cycle for H2 evolution with 9. Redrawn from ref 114. Copyright 2015 American Chemical Society.

Recently, Liang and co-workers examined a series of Cu corroles as HER catalysts in non-aqueous solutions. These Cu corroles (13-16, Figure 5) contains zero, one, two and five fluorine atoms on the 10-meso-phenyl group.141 The effects of these different 10-meso-phenyl substituents on the optical and redox properties of these Cu corroles were then studied by using electronic absorption spectroscopy and electrochemistry. Interestingly, as the number of fluorine atoms increases, the Q band exhibits a blue shift from 624 to 593 nm. The CVs and DPVs (differential pulse voltammograms) of 13-16 were measured in dry o-dichlorobenzene containing 0.1 M (Bu4N)ClO4. With the continuing increase of the number of fluorine atoms on the 20 ACS Paragon Plus Environment

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10-meso-phenyl group, the potentials of the CuIII/II redox couple have an anodic shift from −0.49 to −0.38 V. These results suggested that the introduction of electron-withdrawing substituents at the 10-meso-position of CuIII corroles caused the positive shift of the reduction potentials. Furthermore, the electrocatalytic HER was carried out in benzonitrile in the presence of TFA as the proton source. Upon the addition of TFA to the solution of these Cu corroles, pronounced catalytic waves were observed with the onset appearing at around −1.27 V. As expected, there is a clear increase in catalytic HER performance as the number of fluorine atoms on the 10-meso-phenyl groups increases. Thus, complex 16 exhibits the most active HER activity. These authors also synthesized a series of CoIII corroles (17-23, Figure 5) containing different electron-withdrawing substituents at the 10-meso-position.142 As expected, complex 23 showed much higher catalytic activity than other structural analogs. These results demonstrate that the electron-withdrawing substituents at the meso-positions of metal corroles have a substantially large influence on the catalytic HER properties.


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Figure 5. Metal corroles 13-23 studied as HER electrocatalysts.

2.2. Substituent effects at the β-position In addition to the meso-positions, substituents at the β-positions of corroles can also be changed, and the same β-substituents will usually have more substantial effects than the meso-substituents on the electrochemical properties of metal corroles. For example, by adding nitro substituents to the β-pyrrole positions, the resulted metal corroles will have larger positive shifts in redox potentials than those metal corroles with nitro substituents introduced at the meso-positions. More examples are corroles with Ag, Fe, Cu, and Ir metal ions,143-147 which exhibit striking shifts in redox potentials up to 800 mV by introducing β-NO2 substituents. Other commonly used 22 ACS Paragon Plus Environment

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electron-withdrawing β-substituents include halide atoms (i.e., F, Cl and Br). On the other hand, introducing β-substituents can increase the stability of some kinds of metal corroles under certain conditions by avoiding the oxidative dimerization through the β-carbon atoms.147 In order to investigate the influences of β-substituents on ORR, Elbaz and co-workers synthesized four Co complexes of 5,10,15-tris(pentafluorophenyl)corrole containing H atoms (24, Figure 6) or different halide atoms at the eight β-positions (F, 25; Cl, 26; Br, 27, Figure 6).148 When loaded directly on glassy carbon (GC) electrodes for ORR tests in acidic (0.5 M H2SO4) and basic (0.1 M KOH) aqueous solutions, their catalytic ORR activities have the order 27 > 26 ≈ 24 > 25, although F is considered to have the strongest electron-withdrawing ability in this series. Interestingly, when these Co corroles were absorbed on a high surface area carbon electrode (BP2000), their difference for electrocatalytic ORR is small, but the reaction mechanism is different. The n values determined per O2 reduction were 3.6, 2.8, 3.4 and 3.4 for 24, 25, 26, and 27, respectively. Gross and co-workers also investigated the electrocatalytic ORR feature of Co corroles 27 and 24 in aqueous solutions.149 The introduction of eight bromide groups at the β-positions caused the anodic shifts of the redox potentials. As these authors expected, 27 exhibited much lower ORR overpotential than its non-brominated analogue 24 did in aqueous solutions. The CV of 27 immobilized on carbon materials showed significant catalytic current with an onset potential at 0.76 V in an O2-saturated pH 0 buffer solution, but the onset potential of the catalytic ORR wave 23 ACS Paragon Plus Environment

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with 24 is at 0.64 V under the same conditions. Moreover, the n value determined per O2 reduction was 3.8 and 3.9 at pH 3-4 for 27, which is larger than the n value of 2.7 with its analogue 24. These results highlighted the effects of bromide groups introduced at the β-pyrrole positions of Co corroles. They not only lead to positive shifts in the redox potentials but also change the ORR from a 2e pathway to a 4e pathway, which is more favored from an energy point of view. The results from pH-dependence kinetic currents and Tafel studies indicated that the absorption of O2 on the CoIII site is the rate-determining step. In addition, Gross, Elbaz, and co-workers have compared the ORR performance of Co corrole 27 with its analogue containing different central mental ions (i.e., Mn, 28; Fe, 29; Ni, 30, Figure 6; and Cu, 11, Figure 3).150,151 These metal corroles were absorbed on BP2000 for electrocatalytic ORR in 0.5 M H2SO4 solutions. Their catalytic ORR activities have the order 27 > 29 > 30 > 28 > 11. It is suggested that the first 1e reduction of these metal corroles will occur on the metal center for the early transition metal ions (Mn, Fe and Co), whereas it will occur on the corrole macrocycle for the late transition metal ions (Ni and Cu). A volcano-like plot was obtained by plotting the onset potentials determined from experiments and the O2 absorption free energies obtained from density functional theory (DFT) calculations. This result indicated that 27 has the most negative binding free energy to outperform its analogues for electrocatalytic ORR.152


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Figure 6. Metal corroles 24-30 studied as electrocatalysts.

More importantly, Dey, Gross, and co-workers have isolated the O2-adduct of 1e-reduced Co corroles 24 and 26, and studied them by using electron paramagnetic resonance (EPR), resonance Raman, and DFT calculations.153 On the basis of their results, the electronic structure of these Co−O2 adducts has a low spin CoIII bound with an S = 1/2 superoxide. The binding affinity between O2 and CoII is only slightly reduced with the electron-withdrawing β-chloride groups. The Raman and DFT results showed that the Co−O bond in the O2-adduct of 24 was only slightly stronger than that in the O2-adduct of 26. Consequently, the O−O bond in the O2-adduct of 26 was stronger than that in the O2-adduct of 24 by showing only 8-10 cm−1 increase in the O−O stretch. This work suggests that with the replacement of H atoms in 24 by eight chloride atoms in 26, the binding affinity with O2 does not decrease significantly, but the CoIII/II reduction potential shifts to the positive direction by more than 210 mV. This change is therefore beneficial for electrocatalytic ORR. The β-substitution effect is also investigated for HER. Cao and co-workers 25 ACS Paragon Plus Environment

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reported HER catalyzed by Co corrole 24 in 0.5 M H2SO4 under N2.113 When coated on a GC electrode, the CV of 24 showed an obvious catalytic wave with the onset at −0.70 V. During electrolysis under anaerobic conditions, H2 gas was detected and quantified by using gas chromatography, giving the Faradaic efficiency of H2 evolution greater than 96%. The turnover frequency (TOF) for the HER was 1010 and 6150 s−1 at −0.80 and −0.90 V, respectively. Gross and co-workers reported the HER feature of Co corrole 25 under the same conditions.154 When 25 was loaded directly on an EPG electrode, a large electrocatalytic wave was observed with the onset at −0.30 V. This difference in HER performance between 24 and 25 is due to their different β-substituents. With the strong electro-withdrawing fluoride substituents at the β-pyrrole positions, the onset potential of the HER with 25 shifts to the positive direction by about 0.40 V as compared to that with its non-fluorinated analogue 24 under the same conditions. In order to further study the effect of β-substituents on catalytic HER, Gross, Dey, and co-workers used Co corroles 24-27.116 Electrochemical measurements showed an irreversible CoIII/II redox couple and a reversible CoII/I redox couple for all four complexes. The second reduction potentials were −1.88, −1.46, −1.29, and −1.27 V for 24, 25, 26, and 27, respectively, which implied that all halogenated complexes have a positive reduction potential shift as compared to complex 24. However, for 25-27, the reduction potential does not shift to the positive direction, as expected, with the increase of the electro-withdrawing effect of the β-substituents. DFT calculation showed that although F has a stronger σ-electron withdrawing effect than Cl and Br, it 26 ACS Paragon Plus Environment

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is also a stronger π-donating substituent to the corrole π system. As a result, the π-donating effect of F counteracts its σ-electron withdrawing effect. The CV of 1.0 mM 27, 26, and 25 in degassed acetonitrile solutions containing 0.1 M (Bu4N)ClO4 in the presence of TFA showed catalytic waves with onset potentials at −0.89, −1.04, and −1.09 V, respectively. The catalytic HER onset overpotentials thus have the order 25 > 26 > 27, which is similar to the trend obtained from the second reduction potentials. These authors have also proposed a catalytic cycle for HER with Co corroles, in which a CoI species was considered to be the catalytically active species (Figure 7).

Figure 7. Proposed reaction mechanism for H2 evolution catalyzed by Co corroles. Redrawn with permission from ref 116. Copyright 2014 Royal Society of Chemistry.

2.3. Trans axial ligand effects In coordination chemistry, a ligand will affect the lability and the reactivity of its trans-position ligand. This so-called trans effect plays significant roles in 27 ACS Paragon Plus Environment

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determining the biological activities of various heme proteins with different trans axial ligands.5,96,155,156 For example, cytochrome P450 contains a heme unit with an axial cysteine thiolate ligand and can catalyze the reaction of O2 molecules with diverse substrates, while cytochrome c oxidase contains a heme unit with an axial histidine imidazole group and can catalyze the reduction of O2 to H2O. These trans axial ligands are believed to regulate the activities of various heme proteins through a so-called “push effect”. As inspired by nature, Dey and co-workers investigated in detail the trans axial ligand effects of a variety of biomimetic Fe porphyrins on the activity and selectivity of electrocatalytic ORR.83,86,87 For example, Fe porphyrins with different thiolate, phenolate and imidazole ligands have been designed and synthesized, and it is shown that the distinct “push effect” from these axial ligands causes different ORR kinetics and selectivity. Unlike metal porphyrins, only few studies reporting the trans axial ligand effects of metal corroles on catalytic small molecule activation are known in the literature.157-159 In 2014, Cao and co-workers first reported 24-py (py = pyridine) as a bifunctional catalyst for both OER and HER with high activity and stability.113 Results from both experimental and theoretical studies lead to proposed reaction mechanisms for OER (Figure 8) and for HER (Figure 9). In the case of OER, there is an axial ligand trans to the Co-bound water molecule, which is activated on the Co center through two proton-coupled electron transfer (PCET) steps (Figure 8). Moreover, water nucleophilic attack (WNA) to the proposed key CoV=O unit is responsible for the O−O bond formation. It is thus suggested that this axial ligand trans to the 28 ACS Paragon Plus Environment

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terminal oxo unit will have substantial effect on its activity during the O−O bond formation step. On the other hand, for HER, the axial ligand on the Co ion will affect the protonation of the 2e-reduced Co corrole species to form the Co-hydride intermediate and also affect the subsequent heterolytic protonolysis of Co-hydride to give H2 (Figure 9).

Figure 8. Proposed catalytic cycle for water oxidation with 24-L. Reprinted with permission from ref 160. Copyright 2017 Royal Society of Chemistry.


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Figure 9. Proposed catalytic cycle for hydrogen evolution with 24-L. Reprinted with permission from ref 162. Copyright 2017 Wiley-VCH.

In order to study these trans axial ligand effects, Cao and co-workers synthesized six Co corroles bearing different axial ligands (Figure 10), and compared their electrocatalytic features for OER.160 The electron-donating ability of these axial ligands are in the following order: thi > im-Me > py-OMe > py-NMe2 > py > py-CN. The reaction of the axial-ligand-free Co corrole 24 with these axial ligands could be followed by using UV-vis spectroscopy. Upon axial coordination, the UV-vis spectrum of 24 showed a red shift for the Soret band and an increase of the absorption for the Q band. The resulted Co corroles with axial ligands were fully characterized 30 ACS Paragon Plus Environment

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by NMR and elemental analysis. Importantly, Co corroles 24-py and 24-py-OMe were structurally characterized by single crystal X-ray diffraction methods. The X-ray structures of 24-py and 24-py-OMe are almost identical except the coordination at the two axial positions. For 24-py, it contains two py molecules, but for 24-py-OMe, it has two py-OMe molecules. The CV of these Co corroles were first measured in acetonitrile containing 0.1 M (Bu4N)PF6. They all showed two well-defined reversible 1e oxidation waves, which could be assigned to the formal CoIV/III and CoV/IV redox couples, respectively (Table 3). As expected, the electron-donating axial ligands increase the electron densities on Co ions and thus decrease the oxidation potentials of Co corroles. This is considered to be beneficial to decrease the OER overpotentials according to the recent work by Maseras, Llobet and co-workers.161

Figure 10. Co corrole 24 with six different axial ligands for OER catalysis.


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Table 3. Electrochemical Oxidation Potentials of Co Corrole 24-L in Acetonitrilea

E1/2

a

catalyst

CoIV/III

CoV/IV

24-py-CN

0.36

0.89

24-py

0.30

0.88

24-py-NMe2

0.28

0.85

24-py-OMe

0.21

0.78

24-im-Me

0.19

0.87

24-thi

0.19

0.56

The potentials are versus ferrocene. E1/2 is the middle potential of redox couples.

When loaded on electrodes, the electrocatalytic OER performance of these Co corroles was measured in 0.1 M pH 7.0 phosphate buffer, showing pronounced catalytic waves for OER. Importantly, with the increase of the axial ligand electron-donating abilities, the onset overpotentials of catalytic currents with these Co corroles decrease from 580 to 510 mV, and the catalytic current at 2.09 V increase from 0.78 to 1.95 mA cm−2. These trends are consistent with the oxidation potentials presented in acetonitrile solutions. It is suggested that the CoV=O bond will become weaker with a strong electron-donating trans ligand. This may benefit the catalytic 32 ACS Paragon Plus Environment

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OER by decreasing the activation energy barrier of the O−O bond formation step. Cao and co-workers also synthesized 31-PPh3 (PPh3 = triphenylphosphine), 31-py (Figure 11), and 24-py (Figure 10), and investigated the trans effect of the axial ligand on catalytic HER.162 The CVs of 31-PPh3, 31-py, and 24-py were first measured in acetonitrile under N2 atmosphere (Figure 12). For 31-PPh3, there are two reversible 1e reduction waves at E1/2 = −0.76 and −1.89 V, which can be ascribed to the formal CoIII/II and CoII/I couples, respectively. For 31-py, the two reduction waves are E1/2 = −0.60 and −1.89 V, and for 24-py, these values are E1/2 = −0.59 and −1.83 V. As compared with 31-py and 24-py, the first reduction of 31-PPh3 has a large cathodic shift of 160 mV, which is consistent with the stronger electron-donating ability of the triphenylphosphine ligand.

Figure 11. Co corrole 31 with two different axial ligands for HER.

The electrocatalytic HER with 31-PPh3, 31-py, and 24-py was carried out in acetonitrile with addition of benzoic acid as the proton source. The first reduction 33 ACS Paragon Plus Environment

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waves of these Co corroles maintained almost unchanged in the presence of benzoic acid, but their second reduction waves became large catalytic waves. The onset potentials of the catalytic waves with these Co corroles are similar, appearing at −1.65 V. This result implies that the 2e-reduced Co corroles with the formal CoI oxidation state is the catalytically active species for HER. The similarity of onset potentials is consistent with the very close reduction potentials of the second reduction waves of 31-PPh3, 31-py, and 24-py. Although the onset overpotentials of these Co corroles for HER are close, their catalytic activities are different. By adding the same concentration of benzoic acid, 31-PPh3 displays the highest catalytic current under the same conditions, which is due to the strong electron-donating ability of the trans axial triphenylphosphine ligand. For example, the catalytic current measured at −2.0 V for 31-PPh3 with addition of 24 equivalents of benzoic acid is 119 μA. These values are 62 and 97 μA for 31-py and 24-py, respectively. This result is significant to demonstrate that by simply changing pyridine to triphenylphosphine axial ligand, the catalytic current with 31-PPh3 is almost twice of that with 31-py, highlighting the trans axial ligand effect on HER. It is suggested that with strong electron-donating axial ligand, the electron density (basicity) of the Co center is increased, and thus the electrophilic attack of a proton to the Co center becomes more favored.


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Figure 12. CVs of 24-py, 31-py, and 31-PPh3 in acetonitrile. Conditions: 0.1 M Bu4NPF6, GC electrode, 50 mV s−1 scan rate. Reprinted from ref 162. Copyright 2017 Wiley-VCH.

2.4. The effects of second coordination sphere structure The effects of the second coordination sphere structures on small molecule activation reactions have been investigated in both biological and biomimetic systems. The functional groups at the second coordination sphere are expected to have little or no direct interaction with the metal ions but can interact with substrates that are bound to and/or activated at the active sites. Synthetic complexes, whose molecular structures particularly the second coordination sphere structures can be systematically modified, are beneficial to study these effects. For example, DuBois and co-workers have done pioneering works on developing Ni-diphosphine molecules bearing proton relays in the second coordination sphere,163 and have investigated their catalytic 35 ACS Paragon Plus Environment

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features for hydrogen evolution and oxidation reactions. More recently, many other molecular metal complexes containing functional groups at the second coordination spheres are used as catalysts for HER,28,29,163-167 OER,165,168-170 and ORR.83,88,171 Particularly, by using metal porphyrins, the effects of the second coordination sphere structures on catalytic ORR have been well demonstrated by Collman, Nocera, Dey, and others.68,84,85,88,112,172 These examples include the incorporation of functional groups as proton accepters or proton relays, functional groups to facilitate substrate binding and to improve intermediate stability through electrostatic and hydrogen bonding interactions.88 In order to study the second coordination sphere effects, Nocera and co-workers have developed a series of hangman porphyrins and corroles that contain acid/base groups appending at the ligand backbone.28,29,164,168,171,173,174 In 2011, they reported Fe corrole 32 containing a pendant carboxyl group (Figure 13), and investigated its feature to catalyze the disproportionation of hydrogen peroxide.173 It is expected that this carboxyl group will enhance the PCET process and thus promote the disproportionation of hydrogen peroxide. However, it was found that if the Fe IV state of complex 32 was used as the starting catalyst, the oxidative degradation of Fe corrole was observed, which was likely due to the formation of high-valent Fe-oxo intermediates during the catalytic H2O2 disproportionation process. Consequently, they used the FeIII state of 32 as the starting catalyst in a subsequent work,174 and showed that the FeIII state of 32 is efficient by functioning as a H2O2 dismutation catalyst. Although the catalyst will still decompose, the initial rate of the FeIII state of 36 ACS Paragon Plus Environment

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32 outperforms that of its structural analogue 33, which bears no such carboxyl group (Figure 13). On the basis of these experimental results, a catalytic cycle for the H2O2 dismutation with 32 was proposed by these authors. As shown in Figure 14, H2O2 first binds to the FeIII center to form a FeIII–OOH adduct. Subsequent protonation and loss of water give the FeIV=O porphyrin cation radical, which then reacts with another H2O2 to release O2 and H2O. The proton transfer rate from the pendant carboxyl group is very fast (> 106 s−1). Such a facile proton transfer can facilitate the protonation of the peroxy species and formation of the oxo, which is thought to be the origin of the increased activity.

Figure 13. Hangman Fe and Co corroles 32-37 studied as electrocatalysts.

Nocera and co-workers also synthesized several CoIII corroles 34-37 37 ACS Paragon Plus Environment

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containing pendant carboxyl groups (Figure 13),171 and examined them as ORR electrocatalysts in 0.5 M H2SO4 aqueous solutions. By loading on multi-walled carbon nanotubes (CNTs), these hybrids can catalyze ORR with the measured n values and half-wave potentials (E1/2) summarized in Table 4. These results show that the n values are quite similar (i.e., in the range of 2.5-2.9) and the E1/2 values are all between 0.52-0.56 V. This result was similar to the report from Elbaz and co-workers,148 who showed that the immobilization of corroles on high surface-area carbon materials would weaken the substituent effects on ORR. This is likely caused by the intrinsic activity of these carbon materials for electrocatalytic 2e ORR.175,176

Figure 14. Proposed reaction mechanism of H2O2 activation catalyzed by hangman Fe corroles. Redrawn from ref 174. Copyright 2012 Royal Society of Chemistry.


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Table 4. Electrocatalytic ORR by 24 and 34-37 Adsorbed on CNTs in 0.5 M H2SO4 Aqueous Solutionsa

a

catalyst

E1/2

%H2O

n

24

0.56

40

2.7

34

0.54

55

2.9

35

0.58

30

2.6

36

0.52

25

2.5

37

0.56

41

2.8

The potential are versus RHE. E1/2 is the half wave potential of the limiting current.

On the other hand, Nocera and co-workers examined Co corrole 34 (Figure 13) and its β-octafluoro analogue 34-F8 as OER electrocatalysts in 0.1 M pH 7 phosphate buffer.168 It was found that both Co corroles 34 and 34-F8 were much more efficient in catalyzing water oxidation to evolve O2 than the simple Co corrole 24, showing the valuable effect of the appending carboxyl group for OER. Moreover, 34-F8 outperforms 34, further highlighting the positive effect of strong electron-withdrawing β-fluoride substituents. The onset of the catalytic wave with 34-F8 occurred at 1.86 V, corresponding to an onset overpotential of ~600 mV. Electrolysis with 34-F8 in 0.1 M pH 7 phosphate buffer at 2.01 V produced O2 with an almost 100% Faradaic efficiency and a TOF of 0.81 s−1. It is proposed by the authors that the hangman 39 ACS Paragon Plus Environment

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carboxyl group could promote the O−O bond formation by pre-organizing water molecules within the hangman cleft. In order to have a deeper understanding of the role of hangman carboxyl group in OER, Lai, Cao, and co-workers used DFT calculations to study the O−O bond formation step with 34-F8 and its analogues with other transition metal ions.170 It is suggested that the appending carboxyl group can function as an intramolecular proton acceptor to facilitate the water nucleophilic attack to the terminal metal-oxo/oxyl units for the O−O bond formation. The calculated activation energy barriers are in the order CoV 42 > 41 > 40, which was consistent with the basicity trend of these pendants. Significantly, a slope of −55 mV per pKa was obtained by plotting the overpotential on the pKa of the appended group. This result implies that there is a rate-determining PCET step in the OER cycle and the appended base group can enhance the OER activity by acting as an intramolecular base to facilitate the nucleophilic attack of a water molecule to a proposed Co-oxo unit. For HER studies, these Co corroles were deposited onto GC electrodes, and their CVs were measured in 0.1 M pH 7.0 phosphate buffer solutions. Interestingly, the catalytic HER activity shows the same order 43 > 42 > 41 > 40, 42 ACS Paragon Plus Environment

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which is observed for OER studies. It is suggested that the appending –CH2PO(OH)2 group has the highest protonation level to provide large local proton concentrations. Therefore, this –CH2PO(OH)2 group functions as a proton pond to facilitate the HER. In addition to appending acid/base substituents, the steric effect of the second coordination sphere is also important in catalysis. Goldberg and coworkers synthesized a series of metal complexes of tris(2,4,6-triphenylphenyl) corrole and investigated them as catalysts for hydrogen atom transfer (HAT) reactions.180-182 In these complexes, the metal center is sterically protected by three bulky meso-tris(2,4,6-triphenylphenyl) substituents. Significantly, these bulky metal corroles show much higher HAT activities than simple metal corroles, which is due to the steric effect to stabilize high-valent metal complexes. Similar steric effect is also reported for oxygen atom transfer (OAT) reactions.124,183 Although such steric effect of metal corroles on HER, OER and ORR has not been well documented in the literature, it is known for metal porphyrins. For example, Sakai and co-workers synthesized a Co porphyrin bearing four fluorinated phenylsulfonate groups at meso positions.184 As compared to its non-fluorinated analogue, the steric hindrance of the fluorinated phenylsulfonate group can increase the stability of the catalyst during photocatalytic OER and also change the O−O bond formation from a bimolecular oxyl-oxyl coupling mechanism to a water or hydroxide nucleophilic attack mechanism.

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In nature, dinuclear [NiFe] and [FeFe] hydrogenases are used to catalyze the reversible oxidation of H2 and the proton reduction to H2,3,4 respectively. The OEC contains a Mn4CaOx cluster for the water oxidation reaction to evolve O2.1,2 The dinuclear site of CcOs, which consists of a Cu ion and a Fe porphyrin unit, is responsible for the selective 4e O2 reduction to H2O.87,94 Because multiple electrons are transferred during these HER, OER and ORR processes, multinuclear metal complexes have been long considered to be necessary to catalyze these reactions. Although recent works have demonstrated that these reactions can be mediated by mononuclear metal complexes or single metal sites, scientists still believe that the cooperation between two metal ions will benefits these multi-electron processes by avoiding the involvement of intermediates that are high in energy. On the basis of these considerations, a variety of biscorroles and porphyrin-corrole dyads have been designed and examined as catalysts. In 2005, Kadish, Guilard and co-workers studied the ORR performance of CoIII biscorroles, CoII porphyrin-CoIII corrole dyads, and CoIII corrole monomer (44-53, Figure 16).185 The electrocatalytic studies were carried out in 0.1 M HClO4 by using catalyst-loaded graphite electrodes as the work electrode. The peak potential (Ep) and the half-wave potential (E1/2) of the O2 reduction wave, and the n value were summarized in Table 5. Three conclusions could be drawn from these data. First, Co III corroles are the catalytically active sites for ORR. The catalytic activities of Co III biscorroles and CoIII corrole monomer are similar, although CoIII biscorroles exhibit a slightly higher selectivity for the 4e ORR. Second, dinuclear catalysts 45 and 49 44 ACS Paragon Plus Environment

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ACS Catalysis

having an anthracene spacer show the highest efficiency for ORR. This is likely due to the rigid anthracene bridge, which can minimize the lateral slippage of these macrocycles and thus maintain a face-to-face arrangement. Third, CoII porphyrin-CoIII corrole dyads outperform CoIII biscorroles in the 4e ORR. On one hand, the second CoIII corrole unit may reduce the basicity of the O2 adduct, which is not favored for the 4e ORR process. On the other hand, the alkyl-substituted electron-rich porphyrins can increase the binding of O2 on the Co sites. As a result, CoII porphyrin-CoIII corrole dyad 45 exhibits the best ORR property.

Figure 16. Co corroles 44-53 studied as ORR catalysts.


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Table 5. Electrocatalytic ORR by 44-53 Loaded on EPG Electrodes in Air-Saturated 1.0 M HClO4 Aqueous Solutionsa

a

catalyst

Ep

E1/2

n

44

0.60

0.62

2.9

45

0.64

0.71

3.9

46

0.62

0.70

3.7

47

0.62

0.69

3.7

48

0.58

0.65

3.5

49

0.60

0.63

3.4

50

0.59

0.61

2.4

51

0.58

0.61

2.9

52

0.57

0.59

3.4

53

0.57

0.59

3.1

The potentials are versus RHE. Ep is the peak potential of the catalytic ORR wave.

E1/2 is the half wave potential of the limiting current.

Kadish, Guilard and co-workers also synthesized a series of free-base porphyrin-CoIII corrole dyads 54-56 (Figure 17) and studied their electrocatalytic ORR features.186 Compared with CoII porphyrin-CoIII corrole dyads, these free-base porphyrin-CoIII corrole dyads, lacking a second Co center, will greatly simplify the 46 ACS Paragon Plus Environment

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redox behavior and lead to a clearer understanding of how the electrons transfer during reduction processes. The CoIII/CoII redox couple was found at −0.58 V for 54, −0.77 V for 55, and −0.76 V for 56. This result indicates that the use of anthracene spacer in 54 may provide a more appropriate geometry, and thus effectively shift the reduction potential to the anodic direction by more than 180 mV. Consequently, the anthracene spacer is more appealing to be used in such a face-to-face structure for ORR, which is consistent with the earlier result from these authors.185 Interestingly, different electrocatalytic ORR activities were observed by carrying out the measurement in air-saturated 1.0 M HClO4 and 1.0 M HCl aqueous solutions. Compared with in HClO4, the half-wave potential of catalytic ORR in HCl shifts negatively by 60-70 mV. This difference is likely due to the coordination of a Cl− ion on the oxidized Co center. However, because the Cl− will rapidly dissociate upon the reduction of CoIV to CoIII, the ORR selectivity is almost the same in HClO4 and HCl aqueous solutions. In addition, the electrochemical behavior of free-base porphyrin-CoIII corrole dyads is different from that of CoIII corrole monomer, the former has a higher selectivity for the 4e ORR.


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Figure 17. Free-base porphyrin-Co corrole dyads 54-56 studied as ORR catalysts.

In order to further demonstrate the role of porphyrin units in ORR processes, Kadish, Guilard and co-workers examined the catalytic ORR behavior of seven heterobimetallic porphyrin-corrole dyads 57-63 (Figure 18), each containing a CoIV corrole unit and a FeIII or MnIII porphyrin unit.187 The ORR performance was recorded in air-saturated 1.0 M HClO4 aqueous solutions with catalyst-loaded graphite electrodes. Interestingly, these catalysts all exhibited two well-defined catalytic ORR waves with their corresponding half-wave potentials and n values summarized in Table 6. The first reduction wave with a range of E1/2 = 0.54 to 0.63 V is assigned to the Co-corrole-catalyzed ORR. This assignment is based on previous ORR measurements with the free-base porphyrin-CoIII corrole dyads.186 The second reduction wave with a range of E1/2 = 0.08 to 0.22 V is assigned to the porphyrin-catalyzed reduction of H2O2 or O2 to H2O. The results show that these heterobimetallic porphyrin-corrole dyads do not show improved ORR efficiency and selectivity as compared to free-base porphyrin-CoIII corrole dyads. Notably, the best performance in terms of E1/2 and n values was obtained with the CoII porphyrin-CoIII 48 ACS Paragon Plus Environment

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corrole dyads. The superior activity of the CoII porphyrin-CoIII corrole dyads over other heterobimetallic Fe/Mn porphyrin-Co corrole dyads, free-base porphyrin-Co corrole dyads, Co biscorroles, and Co corrole monomers is likely due to the fact that the active form of the catalyst contains the CoII porphyrin moiety.

Figure 18. Cofacial heterobimetallic Fe/Mn porphyrin-Co corrole dyads 57-63 studied as ORR catalysts.


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Table 6. Electrocatalytic ORR by 57-63 Adsorbed on EPG Electrodes in Air-Saturated 1 M HClO4 Aqueous Solutionsa

the first wave

a

the second wave

catalyst

E1/2

n

E1/2

n

57

0.63

2.8

0.20

3.0

58

0.57

2.6

0.16

3.2

59

0.59

2.8

0.21

3.2

60

0.54

2.6

0.18

3.1

61

0.57

2.5

0.08

2.5

62

0.60

2.6

0.22

2.7

63

0.56

2.8

0.10

2.8


In 2009, Kadish, Fukuzumi, Guilard and co-workers investigated the reaction mechanism of O2 reduction with a series of CoII porphyrin-CoIII corrole dyads 47, 48 (Figure 16) and 64-67 (Figure 19).188 By comparing their electrochemical and spectroscopic features with those of mononuclear CoIII corroles and CoII porphyrins, the first and third reductions of these dyads can be assigned to the corrole-centered processes, while the second reduction is assigned to the porphyrin-centered process. The sites of electron transfer are illustrated in Figure 20. 50 ACS Paragon Plus Environment

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ACS Catalysis

Figure 19. Cofacial binuclear Co porphyrin-corrole dyads 64-67 studied as ORR catalysts.

Figure 20. Proposed reduction sites of cofacial binuclear Co porphyrin-corrole dyads. Redrawn from ref 188. Copyright 2009 American Chemical Society.

The electrocatalytic ORR performance of these Co porphyrin-corrole dyads was recorded in air-saturated 1.0 M HClO4 with the E1/2 and n values summarized in 51 ACS Paragon Plus Environment

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Table 7. Interestingly, the dyads 65-67 with bulky mesityl substituents at the 5,15-meso positions of the corrole macrocycle show a negative shift of E1/2 value and a preference of the 2e ORR. One possible explanation for the low catalytic 4e ORR selectivity of these Co porphyrin-corrole dyads is that the steric hindrance of the bulky mesityl substituents causes the lateral slippage between the corrole and porphyrin macrocycles. This may thus weaken the cooperation between two Co ions for binding and activating O2 molecules.

Table 7. Electrocatalytic ORR by 64-67 Adsorbed on EPG Electrodes in Air-Saturated 1.0 M HClO4 Aqueous Solutionsa

a

catalyst

Ep

E1/2

n

64

0.59

0.64

3.1

65

0.49

0.56

2.5

66

0.49

0.56

2.4

67

0.51

0.57

2.5


In contrast, when the catalytic ORR was carried out in benzonitrile using 1,1’-dimethylferrocene (Me2Fc) as the reductant and HClO4 as the proton source, the n value of 47 and 65 is higher than that of 48, 64, 66 and 67. The different geometry


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of these Co porphyrin-corrole dyads adsorbed on the graphite electrode and dissolved in benzonitrile may lead to the difference of their catalytic selectivities. It is suggested that the effects of the corrole meso-substituents are more important under heterogeneous conditions, while the effects of the spacers become more dominant under homogenous conditions. The reaction mechanism of ORR is proposed by these authors (Figure 21).

Figure 21. Proposed reaction mechanism of O2 reduction catalyzed by cofacial binuclear Co porphyrin-corrole dyads. Redrawn from ref 188. Copyright 2009 American Chemical Society.

Unlike ORR, very few studies are know in the literature using dinuclear metal corroles for OER studies. Sun and co-workers reported the electrocatalytic OER features of mononuclear and dinuclear Mn and Cu corroles 68-71 (Figure 22).189 It is 53 ACS Paragon Plus Environment

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found that Mn corroles 68 and 70 are active for the electrocatalytic OER, but their Cu analogues are almost inactive. It is worth noting that dinuclear Mn corrole 70 is almost three times more efficient than mononuclear Mn corrole 68. In a subsequent computational work,190 Privalov and co-workers suggested that for both mononuclear and dinuclear Mn corroles, the O−O bond is formed by the nucleophilic attack of a hydroxyl anion or a water molecule to a formal MnV=O unit through either the concerted or the two-step pathways. In the concerted pathway, hydroxyl ion directly attacks the Mn-oxo. In the two-step pathway, hydroxyl ion first coordinates to the Mn ion and then reacts with the Mn-oxo unit. It is suggested from calculations that the energy differences between the two pathways are moderate, and thus the reaction may in fact proceed via both pathways under catalytic conditions. For the dinuclear complex 70, the coupling of the two Mn-oxo groups to form the O−O bond is not suggested according to the calculated results. Therefore, the higher activity of 70 is more likely due to the pre-organization and pre-bonding of hydroxyl ions and/or water molecules in the dinuclear Mn corrole cleft, which is favored for subsequent water oxidation.


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Figure 22. Mononuclear and binuclear Mn/Cu corroles 68-71 studied as OER catalysts.

4. Immobilization of Molecular Catalyst Immobilization of molecular catalysts on electrode materials is essential for their practical uses. Materials with grafted metal porphyrins and metal corroles have been explored as electrocatalysts for HER,20,162,191 OER,115,192 and ORR.185-188 In 2015, Zhang, Cao and co-workers reported the ORR electrocatalysis of Co corrole 24 (Figure 6) adsorbed on CNTs.193 When loading the resulted hybrid 24-CNT onto GC electrodes, this material displays an obvious catalytic wave in O2-saturated 0.5 M H2SO4 aqueous solutions with the onset and peak potentials at 0.80 and 0.62 V, respectively. As a control, the GC electrode with directly loaded complex 24 displays the catalytic ORR wave with the onset and peak potentials at 0.62 and 0.40 V, 55 ACS Paragon Plus Environment

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respectively. Moreover, the n value of ORR for 24-CNT (3.6) is much larger than that of 24 (2.7) and CNTs (2.5), and 24-CNT shows no H2O2 reduction behavior, excluding a 2e+2e pathway for O2 reduction. Thus, the synergistic interactions between Co corrole molecules and CNTs increase the activity and selectivity for the 4e ORR process. In order to increase the interaction between Co corrole molecules and CNTs, Cao and co-workers designed and synthesized a pyrene-modified Co corrole 31-py (Figure 11).115 For the resulted hybrid 31-CNT, the presence of strong π-π interaction between the pyrene group and CNTs is first demonstrated by electronic absorption spectroscopy and fluorescence quenching experiments. In UV-vis, the absorption bands at 346 and 381 nm in 31 blue shifts to 334 and 372 nm in 31-CNT. No such shifts were observed for its analogue 24 and 24-CNT. In addition, the strong emission of 31 centered at 437 nm (with an excitation at 277 nm), which is attributed to the pyrene moiety, is almost completely quenched upon the addition of CNTs. These results together indicate the presence of strong interactions between pyrene moieties of 31 and CNTs. When loaded on GC electrodes, hybrid 31-CNT outperforms 24-CNT for catalytic ORR in O2-saturated 0.5 M H2SO4 aqueous solutions. The n value of 3.8 was calculated for 31-CNT, which is much larger than those of 31 (2.5) and CNTs (2.5) and is also larger than that of 24-CNT (3.6). In addition, after rinsing these hybrids with dichloromethane, the rinsed 31-CNT retained the initial ORR performance, while the rinsed 24-CNT lost almost completely its catalytic activity. These results show 56 ACS Paragon Plus Environment

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that the strong π-π interaction between the pyrene group and CNTs can not only enhance the catalytic ORR activity by increasing electron transfer efficiency but also can increase the stability of the molecular catalysts on CNTs. In addition to ORR, Cao and co-workers demonstrate that 31-CNT is also more efficient than 24-CNT to catalyze HER.162 Therefore, these studies show that noncovalent immobilization of metal corroles on carbon supports through strong π-π interactions is a simple and direct method to produce hybrid catalyst-carbon materials with efficient electrocatalytic capabilities. Very recently, Cao and co-workers developed the covalent immobilization of Co corroles on CNTs and investigated the effects of different immobilization methods on HER and OER.194 The azido-containing Co corrole 72 was grafted to CNTs via covalent bonds to make hybrids H1 and H2 (Figure 23). As shown in this figure, Co corrole 72 can be covalently attached to CNTs with short conjugated linkers in H1 or with long alkane chains in H2. The successful attachment of intact 72 molecules in both H1 and H2 is confirmed by using electrochemical measurements, and infrared, UV-vis and Raman spectroscopic methods. Remarkably, GC electrodes loaded with these hybrids display high efficiency and stability for both HER and OER in pH 0-14 aqueous solutions. This is the first example in the literature to report a bifunctional catalyst for both H2 and O2 evolution reactions in pH 0-14 aqueous solutions. Comparing the performance of the same Co corrole active sites on CNTs via different covalent and noncovalent immobilization methods is possible. The results show that H1 is the most efficient electrocatalyst for both OER and HER in all pHs, which is 57 ACS Paragon Plus Environment

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due to the high electron transport ability through the short conjugated linkers between the catalyst molecules and CNTs.

Figure 23. Schematic representation showing the immobilization of Co corrole 72 on CNTs through covalent attachments. Reprinted from ref 194. Copyright 2018 Wiley-VCH.

In a subsequent work, Cao and co-workers compared two different amidation methods to attach Co corroles onto oxidized CNTs bearing abundant surface carboxyl groups.195 As shown in Figure 24, Co corrole 73 can be attached to CNTs by using the general amidation method via the activation of the carboxyl group with sulfinyl dichloride (route A) or using the direct amidation coupling method (route B). The resulted hybrids, 73-CNT-A and 73-CNT-B, were fully characterized, confirming the successful attachment of intact molecules of 73 on CNTs in both cases. The catalytic OER and HER activities of 73-CNT-A and 73-CNT-B were recorded in aqueous solutions using GC electrodes. The same activity trend 73-CNT-B > 73-CNT-A for 58 ACS Paragon Plus Environment

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ACS Catalysis

both OER and HER is observed in pH 0, 7 and 14 solutions. This difference is caused by the different amidation conditions during covalent immobilization. Raman studies show that after treating with sulfinyl dichloride at 110 °C, the structural defects of CNTs increase. Morphology analysis shows that the nanotubes of 73-CNT-A become coalescent as compared to CNTs and 73-CNT-B, indicating the partial destruction of the multi-walled CNT structures in 73-CNT-A during treatment with strong acidic and oxidative sulfinyl dichloride at 110 °C. It is known that the graphite structure is crucial for the conductivity of CNTs and the conductivity is important in electrocatalysis. Therefore, it is rational that 73-CNT-B gives better performance for electrocatalytic HER and OER, since it is prepared under mild conditions and its CNT structures can be better preserved.


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Figure 24. Schematic representation showing the immobilization of Co corrole 73 on CNTs through two different routes. Reprinted from ref 195. Copyright 2019 Wiley-VCH.

In 2017, Kadish and co-workers reported the immobilization of Co corroles on graphene oxide (GO) and examined the resulted material as electrocatalysts for ORR.196 Co complex of 5,15-bis(4-methylphenyl)-10-(4-aminophenyl)corrole 74 was grafted to GO via covalent amide bonds to make 74-GO (Figure 25). The electrocatalytic ORR activity of 74-GO was recorded in air-saturated 0.1 M KOH and 0.5 M H2SO4 solutions. In 0.1 M KOH solution, 74-GO was able to catalyze 4e ORR to H2O, which contains both the direct 4e pathway and the 2e+2e pathway. In 0.5 M H2SO4 solution, a 2e ORR process was observed, and H2O2 was the only product formed during O2 reduction in the acidic media.


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ACS Catalysis

Figure 25. Schematic representation showing the immobilization of Co corrole 74 on graphene through covalent attachments. Reprinted from ref 196. Copyright 2017 American Chemical Society.

In 2016, Schöfberger, Dey, and co-workers reported Mn corroles as bifunctional electrocatalysts for OER and ORR from neutral to alkaline conditions.197 In acetonitrile solution, complex 75 (Figure 26) showed an obvious catalytic current starting at 0.89 V upon the addition of an NaOH aqueous solution. The pseudo-first-order rate constant (kcat) was calculated to be 11.4 s−1 with 25 mM NaOH. In aqueous solution, 75 was attached onto an EPG electrode to investigate its electrocatalytic performance. At pH 11, a large catalytic OER current was observed at potentials > 0.81 V. In addition to OER, 75 can also catalyze ORR with the n = 2.3 and the reaction rate = 3.81 × 103 M−1 s−1 at 0.21 V. 61 ACS Paragon Plus Environment

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Figure 26. Mn corroles 75 and 76 studied as OER and ORR catalysts.

Importantly, the adsorption behaviors and electronic properties of Mn corroles on different solid surfaces were studied. The octadecyl chains were introduced to make Mn corrole 76 (Figure 26), which will have proper orientation and adsorption at the solid-liquid interface during the scanning tunneling microscopy (STM) measurements. The octadecyl chains allow 76 to exist on the basal plane of highly ordered pyrolytic graphite (HOPG) in the form of complete individual molecules. Based on electrochemical studies, the surface immobilized catalyst exists in the MnIII state, and its catalytic performance is similar to that of the catalyst dissolved in solution. Elbaz, Gross, and co-workers reported an electropolymerized-Co corrole material, which has higher catalytic ORR activity than its monomeric Co corrole analogue 77 due to the 3D polymeric structures (Figure 27).198 First, an indium tin oxide (ITO) electrode was dipped into an acetonitrile solution containing 0.1 M (Bu4N)BF4 and 2 mM 77. Subsequent CV potential scanning between 0.45 and 0.90 V was carried out to make electropolymerized 77. After several CV cycles, 77 was 62 ACS Paragon Plus Environment

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polymerized to form CoTAC on the ITO electrode. Electrochemical quartz crystal microbalance and FTIR-ATR spectroscopy were used to study the polymerization mechanism of polyCoTAC.

Figure 27. Schematic representation showing the electrochemical polymerization and the growth of the 3D polymeric structures of 77 on GC electrodes.

Their electrocatalytic ORR activities were measured in 0.5 M H2SO4 and 0.1 M KOH aqueous solutions. The ORR onset potential (Eonset), the half-wave potential (E1/2), and n are summarized in Table 8. These results indicate that polyCoTAC shows higher electrocatalytic activity and selectivity than its monomer in both acidic and alkaline solutions. The improved ORR activity and selectivity are likely due to the close distance of metal centers in polyCoTAC. Additionally, when the GC electrode was replaced by BP2000, the electrocatalytic ORR performance could be further improved. Notably, similar electropolymerization method was reported by Aguirre 63 ACS Paragon Plus Environment

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and co-workers in 2013,199 in which Cu corrole was electropolymerized into polymers for ORR studies.

Table 8. Electrocatalytic ORR by 77 and polyCoTAC in O2-Saturated 0.5 M H2SO4 and 0.1 M KOH Aqueous Solutionsa

GC condition

catalyst

Eonset

E1/2

n

Eonset

E1/2

n

CoTAC

0.62

0.40

1.77

0.81

0.62

2.18

0.73

0.58

2.41

0.84

0.64

3.13

CoTAC

0.79

0.58

2.50

0.84

0.72

3.10

polyCoTAC

0.83

0.70

3.10

0.93

0.83

3.50

0.5 M H2SO4 polyCoTAC

0.1 M KOH

a

BP2000

The potential are versus RHE. Eonset is the onset potential. E1/2 is the half wave

potential of the limiting current.

5. Pyrolyzed Corrole for Material Catalyst Since Jasinski investigated Co phthalocyanines as ORR catalysts for fuel cells in 1964,200 the M-N4 unit is thought to be a catalytically active site for ORR. As structural analogues of phthalocyanines, porphyrins and corroles have recently been extensively studied for ORR. In addition, porphyrin, corrin, and corrole ligands can all establish a rigid and stable M-N4 coordination environment for most transition


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metal ions, and after pyrolyzation, they can provide ideal carbon-supported materials with the M-N4 unit, which is considered as the ORR active stie.201-203 In 2012, Wang, Chen and co-workers first reported the pyrolyzed Co corrole as a non-precious metal catalyst for ORR.204 The Co triphenylphosphine complex of 5,10,15-triphenylcorrole was mixed with carbon black and was pyrolyzed at various temperature of 300, 500, 700, and 900 °C, forming carbon-black-supported pyrolyzed Co

corrole

ORR

catalysts

py-Co-corrole/C-300,

py-Co-corrole/C-500,

py-Co-corrole/C-700, and py-Cocorrole/C-900, respectively. Their ORR activities were recorded in O2-saturated 0.1 HClO4 solutions. Their results show that py-Co-corrole/C-700 exhibits less overpotential and the highest activity for the 4e ORR. A possible explanation is that the pyrolysis changes the coordination structure and oxidation state of Co corrole, causing the increase of ORR activity. In 2014, in order to study the effect of structure on ORR activity, Wang and co-workers

prepared

carbon-black-supported

pyrolyzed

Co

corrin,

Co

triphenylcorrole, and Co tetramethoxyphenylporphyrin.205 After pyrolyzing at 700 °C, these catalysts were coated on electrodes to examine the ORR properties in O2-saturated 0.1 M HClO4 solutions. Their electrocatalytic ORR features are summarized in Table 9. Based on these results, py-Co-corrin/C shows higher activity and selectivity than the other two catalysts. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were used to analyze the structure of the Co-N4 unit before and after pyrolyzation. It is suggested that the Co-N4 unit is largely preserved during pyrolysis for py-Co-corrin, 65 ACS Paragon Plus Environment

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whereas this structure is partially destroyed in py-Co-corrole and py-Co-porphyrin. Therefore, the structure and the coordination of Co-based macrocyclic complexes strongly affect their ORR activities.

Table 9. Electrocatalytic ORR by py-Co-corrin/C, py-Co-corrole/C, and py-Co-porphyrin/C in 0.1 M HClO4 Aqueous Solutions at 0.3 V versus RHE.

catalyst

j (mA cm−2)

%H2O

n

py-Co-Corrin/C

5.7

5

3.90

py-Co-Corrole/C

4.5

7

3.86

py-Co-Porphyrin/C

2.7

29

3.42

In

2013,

Wang,

Chen

and

co-workers

also

reported

that

carbon-black-supported pyrolyzed (nitrosyl) FeIII triphenylcorrole was efficient for the direct 4e reduction of O2 in O2-saturated 0.1 HClO4 solutions.206 The in situ XANES and EXAFS were used to understand the relationship among the adsorption site, the catalyst surface structure and the electrochemical potential in ORR processes. As a result, the excellent performance of py-Fe-corrole/C can be attributed to the network structure of polyaromatic hydrocarbons, the quaternary (graphitic)-type N atoms, and the coordination structure of the Fe-N4 unit. Recently, Chen and co-workers reported that carbon-black-supported pyrolyzed

CoIII

10-ferrocenyl-5,15-diphenylcorrole 66 ACS Paragon Plus Environment

was

a

highly

efficient

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electrocatalyst for the 4e reduction of O2.203 Ferrocene was introduced to provide a source of iron. After pyrolyzed at 500 °C, the presence of Co and Fe is confirmed by XRD and XPS. Interestingly, the bimetallic electrocatalyst exhibits a high electron transfer number of more than 3.95, which is superior to the parent monometallic py-Co-corrole.

6. Conclusions and Outlooks In this review, we summarized recent achievements made in using metal corroles to catalyze HER, OER, and ORR, and focused on discussing the corrole structure effects on these reactions. As significant components in the new energy conversion schemes, these energy-related small molecule activation reactions have been extensively studied. Although noble metals and their complexes are active for these reactions, the very low natural abundance and the high price of these precious metals limit their widespread uses and large-scale applications. Therefore, developing cheap and active catalysts based on the first-row transition metal elements for HER, OER, and ORR has attracted increasing interests in the last decade. Notably, a variety of molecular and material catalysts of Mn, Fe, Co, Ni, and Cu have been reported to be able to catalyze these reactions with reasonably high efficiency and stability,25,27,33-35,48,55 and more importantly, mechanism studies using homogeneous molecular catalysts can provide new and valuable insights into better understanding these catalytic processes. The fundamental knowledge obtained from these studies is crucial for the rational design of more efficient and robust catalysts that are promising 67 ACS Paragon Plus Environment

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to be practically used. Among these catalysts, metal porphyrins and metal corroles and their derived materials are of particular interests because of the following reasons. First, heme proteins, which contain Fe porphyrin active sites, are widely found in nature to regulate the O2 chemistry, including the binding, transport, storage, and activation of O2. In these processes, reversible O−O bond formation and cleavage are catalyzed at the Fe porphyrin centers. Many other metalloproteins containing metal porphyrin structures are used in nature to catalyze the degradation of peroxides (i.e., peroxidase and catalase) and to mediate the biological electron transfer processes (i.e., cytochrome b and c, P680). Second, deprotonated porphyrins and corroles are negatively charged ligands. They can in general coordinate most transition metal and main group elements to afford rigid and stable four-coordinated square-planar structures with very high binding affinities. This feature is crucial for homogeneous mechanism studies as the molecular structure is very clear and can be largely maintained during catalytic processes. Third, negatively charged porphyrin and corrole ligands can stabilize metal ions of high oxidation states and offer low-valent metal ions large reducing powers. Because high-valent and low-valent metal ions are involved in OER/ORR and HER, respectively, metal porphyrins and metal corroles are appealing to catalyze these reactions. Fourth, the redox-active feature of porphyrin and corrole macrocycles is important in these multi-electron transfer reactions. The electrons and electron holes can be stored at macrocycles to avoid the involvement of metal ions having too high or too low oxidation states. Fifth, the electronic structure 68 ACS Paragon Plus Environment

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of porphyrins and corroles can be modified by introducing substituents at meso- and β-positions. The second coordination sphere structure can also be modified by appending intramolecular acid/base groups, hydrogen-bonding groups, and charged groups, which will have potential interactions with the metal-bound substrates. Sixth, metal porphyrins and metal corroles have rich electronic absorption features, which are beneficial for mechanism studies. Seventh, metal porphyrins and metal corroles can be used as the starting material to construct carbon-supported materials with the ideal M-N4 structure. This structure is believed to be the catalytically active site for O2 binding and activation in ORR catalysis. Compared with porphyrins, deprotonated corroles are trianionic ligands and are more capable of stabilizing metal ions in their high oxidation states. However, unlike metal porphyrins, metal corroles have been relatively much less studied, which is largely due to the challenging in molecular design and synthesis. Recent achievements have been made in developing new and efficient synthetic methodologies for corroles. Because this topic is beyond the scope of this review and it has been well summarized in other recent reviews,120,126,129,130 we do not cover this topic in the current review. This review focuses on the use of metal corroles as catalysts for HER, OER, and ORR. According to the results presented to date, the smallest overpotential for the HER, OER, and ORR reactions with metal corrole catalysts summarized in this review is 241,154 370194 and 400198 mV, respectively. Particular interests are placed to the structure effects on these reactions. As summarized here, the electronic structure of the corrole macrocycles can be 69 ACS Paragon Plus Environment

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systemically tuned by changing the substituents at the meso- and β-positions. This will alter the redox properties of metal corroles, which are key determinants for these small molecule activation reactions. In general, electron-withdrawing substituents can decrease the electron density of metal corroles and thus (1) lead to the anodic shift in redox potentials, which will benefit HER and ORR catalyses and (2) increase their stability under oxidative conditions, which will benefit OER catalysis. On the other hand, the electronic structure of the central metal ions can also be modified through the use of different axial ligands. In addition, the rigid four-coordinated square-planar structure of metal corroles is ideal to study the axial trans effect. The so-called “push effect” of the axial trans ligand can increase the electron density of the metal center and thus can weaken the coordination bond and increase the pKa of its trans-position ligand. As a consequence, this effect will affect the lability and the reactivity of its trans-position ligand. Although appending groups at the second coordination spheres will have little or no influence on the electronic structures of corroles, they can have hydrogen bonding and/or electrostatic interactions with metal-bound substrates, and thus decrease the activation energy barrier through stabilizing key intermediates. Different roles of acid/base groups appended at the second coordination spheres are suggested in HER, OER, and ORR. For HER, the acid form functions as an intramolecular proton relay. For OER, the base form can function as a proton acceptor to facilitate the water nucleophilic attack to a metal-oxo/oxyl unit for the O–O bond formation. For ORR, the acid group can provide protons and also can stabilize the partially reduced O2 adduct through hydrogen bonding interactions. 70 ACS Paragon Plus Environment

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Although the substituent effects at the meso-position and β-position, the trans axial ligand effects, and the effects of second coordination sphere structure on these small molecule activation reactions, have been demonstrated in previous studies, it is still at the very early stage for us to comprehensively understand these structural effects largely because of the following two reasons. First, the description of these structural effects on catalytic activity and selectivity in a quantitative manner is lacking. Second, the research on the collaborative and synergic effects of these structural modifications is very rare in the literature. In order to obtain fundamental knowledge to guide the design of sophisticated catalytic systems, it is necessary to (1) further develop the methodology of porphyrin and corrole syntheses to install more functional groups at the ligand framework, (2) build up a strategy to quantitatively correlate the structure modifications with the corresponding catalytic activity and selectivity, and (3) build up a unified criterion for the evaluation of catalytic performance under different experimental conditions. Recently, Mayer and Savéant developed reasonable criteria for analyzing overpotentials, reaction rates, turnover frequencies, and other key parameters related to the evaluation of catalytic performance.207,208 Nevertheless, these criteria are also challenging to be used to directly compare different catalysts from various catalytic systems and conditions. In addition, it should be noted that the molecular nature and the stability of these molecular catalysts have to be clearly confirmed prior to the evaluation of their catalytic performances. It is known that molecular metal complexes may undergo decomposition under catalytic conditions to form heterogeneous materials, which act 71 ACS Paragon Plus Environment

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as the real catalysts for these small molecule activation reactions.209,210 Therefore, it is necessary to approve the molecular nature of the catalysis with metal coordination complexes. In addition to mononuclear metal corroles, dinuclear metal corroles and porphyrin-corrole dyads have also been studied as catalysts mainly for ORR. In general, improved efficiency and selectivity are observed for dinuclear catalysts. Possible explanations include: (1) the two metal ions are both involved in the synergistic binding of one O2 molecule to form O2-bridged species;188 (2) one metal ion binds the O2 molecule, while the other provides an extra reducing equivalent and/or plays as a Lewis acid to stabilize the resulting O2-adduct;185 (3) the two metal ions will function independently in separate catalytic processes.187 In order to realize the practical use of metal corroles in electrocatalysis, it is required to immobilize them on various electrode materials. Different methods have been reported, including direct loading via simple physical adsorption, immobilization through noncovalent π-π interactions and through covalent linkers. Covalent immobilization via short conjugated linkers is preferred, which can provide fast electron transfer rates between the catalyst molecules and the supporting electrode materials. This will significantly improve the efficiency and stability of the molecular catalysts. In addition, these molecular complexes can be pyrolyzed to make material catalysts. The macrocycle ligands will be converted to carbon-based supporting materials, and the metal corrole coordination sties will form the M-N4 units, which are proposed to be the active sites for the ORR electrocatalysis. 72 ACS Paragon Plus Environment

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Although a variety of corrole-based catalysts have been reported for HER, OER, and ORR, more efforts are still required to further improve the efficiency, selectivity and stability. The knowledge obtained from previous studies is valuable, showing that the structural modifications play significant roles in regulating the performance of metal corrole catalysts. Moreover, metal corroles have been demonstrated to be active for the reduction of carbon dioxide.211 The exploration of metal corroles on this catalysis and other energy-related small molecule activation reactions will become very active.

Notes The authors declare no competing financial interest.

Acknowledgments We are grateful for the support from the “Thousand Talents Program” of China, the Fok Ying-Tong Education Foundation for Outstanding Young Teachers in University, the National Natural Science Foundation of China under Grants 21101170, 21573139, and 21773146, the China Postdoctoral Science Foundation (2018M631120), the Fundamental Research Funds for the Central Universities, and the Research Funds of Shaanxi Normal University.


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