Heterometallic coordination polymers as heterogeneous electrocatalysts

Abstract

Heterometallic coordination polymers are emerging as a class of crystalline materials for sustainable energy production via electrocatalysis, thanks to the synergistic and cooperative effects of different kinds of metal centres present in these polymers. The development of this class of materials mainly relies on screening experiments using a one-pot protocol of mixing metal ions; notably, the incorporation of different metal ions into desired positions in a single polymeric structure is quite difficult. This review article briefly summarizes the synthesis methods for heterometallic coordination polymers that show heterogeneous electrocatalytic activity. In addition, the relationships between the molecular structure and hydrogen evolution (water/proton reduction), oxygen evolution (water oxidation), CO₂ reduction, oxygen reduction, and some important organic reactions are reviewed to offer new ideas for designing efficient energy conversion materials and developing new classes of heterometallic coordination polymers.

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Naoto Kuwamura

Naoto Kuwamura received his B.S., M.S. and Ph.D. degrees from Osaka City University in 2006, 2008 and 2011, respectively. He then successively moved to the Max-Planck Institute of Bioinorganic Chemistry (currently, Max-Planck Institute for Chemical Energy Conversion) as a postdoctoral research fellow. In 2012, he moved to Osaka University where he is currently an assistant professor. His research interest is directed toward the creation and voltammetric analysis of heterometallic multinuclear complexes based on thiolate ligands that show unique redox properties in both solution and solid states.

Takumi Konno

Takumi Konno received his Ph.D. degree from the University of Tsukuba in 1985. He continued his work there before moving to the University of Cincinnati in 1986. He became an assistant professor at the University of Tsukuba in 1987 and was promoted to a lecturer in 1994. In 1997, he moved to Gunma University as an associate professor and was promoted to a full professor in 1998. He was appointed as a professor at Osaka University in 2000. His research interests are currently focused on creating a new class of functional multinuclear and metallosupramolecular coordination compounds based on metal complexes with simple thiolate ligands.

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1. Introduction

Coordination polymers (CPs) are crystalline solids with infinite structures that are sustained by coordination bonds between metal ions or metal clusters and organic linker molecules, including metal–organic frameworks (MOFs) as a CP subclass.¹ Since the definition of CPs in 1964,² this class of compounds has continuously been a hot research topic due to not only their diverse structural aspects but also their wide range of applications, such as sensors,^3,4 gas storage materials,^5,6 magnets,^7,8 and catalysts.^9,10 One of the important characteristics of CPs is their crystallinity, which allows X-ray crystallography to be used to determine their structures at an atomic level. To date, a large variety of compositions and structures of CPs have been designed and synthesized using various organic ligands, metal ions, synthesis conditions, protocols, and in some cases, post-synthesis treatments. In addition, studies on the relationships between the structures of CPs and their functionalities have been conducted to develop desirable functions,¹¹ which have been summarized in recent review articles.¹²

CPs can be classified into homometallic (one kind of metal ion) and heterometallic (more than two kinds of metal ions) species. Recently, growing interest has been directed towards heterometallic CPs because the presence of different kinds of metal ions in one structure often provides synergistic and cooperative effects on their functionalities; notably, these effects are not realized in homometallic CPs.^13–17 For example, heterometallic CPs have been increasingly studied as a new class of heterogeneous catalysts for photochemical reactions¹⁸ and organic reactions¹⁹ due to their uniform arrangement of redox/catalytically active sites in their network structures. The availability of CPs as heterogeneous electrocatalysts is also a recent hot topic²⁰ because electrocatalysis is an energy-related technology that can contribute to the future development of sustainable energy-related reactions, such as hydrogen evolution, oxygen evolution, oxygen reduction, CO₂ reduction, and organic transformation. However, in most cases, CPs have been used as precursors to obtain inorganic composite materials by annealing at high temperature, which results in the complete decomposition of their original frameworks.^21–24 Thus, studies on the use of CPs themselves as electrocatalysts are needed to realize cost-effective and energy-efficient electrocatalytic reactions.

In this review article, we summarize heterometallic CPs that have been applied as heterogeneous electrocatalysts. First, the synthesis approaches for incorporating two or more kinds of metal ions into a single polymeric structure are summarized. The incorporation of several kinds of metal ions in a single CP structure has commonly been achieved by one-pot reactions in which different kinds of metal ions with different coordination properties are mixed with organic ligands.²⁵ Another method to achieve this, so-called metalloligand approach, is the use of isolated metal complexes that possess donor site(s) for binding metal ions to form heterometallic multinuclear structures.²⁶ Following the first section, the relevance of the structural aspects of CPs, such as their dimensionalities, combinations of metal ions, coordination environments, and porosities, to their electrocatalytic activities is described, together with representative examples. Closely related review articles that summarize MOF-based electrocatalysts have appeared previously.^27–29 Very recently, heterotrimetallic MOFs that show electrocatalytic oxygen evolution have also been reviewed.^30,31 This review article focuses on summarizing the synthesis methodologies of heterometallic CPs and their utility for their various ‘heterogeneous’ electrocatalytic activities from the viewpoint of coordination chemistry.

2. Synthesis methods for heterometallic coordination polymers

Conventionally, CPs have been produced by simple mixing of organic ligands and metal ions in a one-pot manner.³² On the other hand, the synthesis situation of heterometallic CPs is different, and three synthesis techniques have been applied to incorporate two or more kinds of metal ions into polymer structures (Fig. 1): (i) a self-assembly method (or one-pot synthesis), (ii) a metalloligand approach,³³ and (iii) post-synthesis modification.³⁴

Fig. 1 General synthesis methods for heterometallic coordination polymers: (a) self-assembly method, (b) metalloligand approach, and (c) post-synthesis modification.

The simplest and most popular synthesis approach is the conventional (i) self-assembly method, where several kinds of metal ions are mixed with organic and/or inorganic ligands under specific conditions, and a large number of heterometallic CPs have been produced by this method so far.³⁵ While this approach does not involve the pre-treatment of starting materials, different kinds of metal ions with similar characteristics, such as ionic radius, charge, and Lewis acidity, normally need to be selected for this synthesis approach. In addition, CPs synthesized by this approach are often produced as non-crystalline solids,³⁶ which prevents the determination of their structures at an atomic level by X-ray crystallography. Therefore, the limitations of employed metal ions and the difficulty in structural determination are serious problems in producing CPs by the self-assembly method.

An alternative synthesis method that has recently attracted the attention of researchers is (ii) the metalloligand approach.³⁷ In this approach, metal complexes bearing donor site(s) are used as a ‘metalloligand’, which reacts with target metal ions to form heterometallic CPs. This method allows for the control of the structures and dimensionalities of CPs via the proper choice of metalloligands and target metal ions, which act as the nodes and linkers in the produced polymeric structures, respectively. For the synthesis of heterometallic CPs that show electrocatalytic activities, redox-active metalloligands bearing open metal site(s) are used,³⁸ and the design and preparation of metalloligands are important for this synthesis approach.

To synthesize heterometallic CPs that show electrocatalytic activities, (iii) a post-synthesis modification, in which catalytically active guest species are incorporated into the framework of a pre-synthesized CP, has also been employed.³⁹ In this approach, guest species are designed to interact with the framework of CPs and to migrate into the pores of CPs. The exchange of metal ions in CPs by catalytically active metal ions is also useful to produce electrocatalytic CPs.⁴⁰ In general, this approach is applicable for CPs having inherently labile metal ions such as Ni^II, Zn^II, and Cd^II, which can be replaced by external metal ions.

For heterometallic CPs that show electrocatalytic activities, the important structural factors are a (i) redox centre (metal ion) with one or more vacant coordination site(s), (ii) mass transport space and/or channel, (iii) conductive linkage, and (iv) stable framework structure.⁴¹ A short distance between the metal centres is also important for electronic communication between the catalytic centres in heterometallic CPs.^42–44 In some cases, additives such as metal clusters (redox centre), Nafion (ion-conductive material or binder), and activated carbon (electrically conductive material) are incorporated to endow heterometallic CPs with electrocatalytic activity. Currently, optimizing the electrode fabrication method makes it possible to investigate the redox behaviour of heterometallic CPs that are insoluble in any solvents without decomposition.^45–47

3. H₂ evolution electrocatalysis

Hydrogen gas (H₂) is a clean and renewable energy source that is an alternative to fossil fuels because H₂ produces only water when reacted with O₂. To generate this renewable energy source, water (or proton) reduction catalysts are in high demand.⁴⁸ In the past few decades, coordination chemistry has widely contributed to the research field of electrocatalysts for water reduction based on the synthesis of new coordination compounds.⁴⁹ Early works on the hydrogen evolution system started from Prussian white which was homometallic Fe^II CP.^50,51 Note that it is easy to incorporate two kinds of metal ions into Prussian blue analogues by mixing hexacyanido complexes and metal ions in a self-assembly method because many types of hexacyanido complexes with different metal centres have been discovered.⁵² Abe et al. demonstrated that the integration of Fe^III and Ru^II in a Prussian blue analogue leads to enhanced hydrogen evolution activity with a turnover frequency (TOF) of 2.5 × 10² s⁻¹ at an onset potential of −0.6 V (vs. Ag/AgCl).^53,54 Other combinations using Ni^II, Co^II, Pb^II, Cu^II, Mn^II, and Zn^II as the metal components of Prussian white analogues also exhibit appreciable electrocatalytic activities for hydrogen evolution.⁵⁵

Polyoxometalates (POMs) are inorganic cluster species that generally comprise early transition metals such as W, Mo, and V.⁵⁶ This class of compounds is the most popular heterometallic electrocatalyst for H₂ evolution because it is easy to integrate different kinds of metal ions into a POM framework.⁵⁷ While homometallic POMs are generally unstable in non-acidic solution, several heterometallic POMs can be handled in neutral water, which allows them to be used as building blocks for the preparation of POM-based CPs.⁵⁸ It has been reported that a Zn^II–Mo^V-POM-based CP exhibits hydrogen evolution activity at pH 1, showing a Tafel slope of 96 mV deg⁻¹ with an overpotential of 180 mV.⁵⁹ Ma et al. reported a Cu^II-MOF that contains W^VI-POM molecules.⁶⁰ The incorporation of W^VI-POM molecules into the MOF provided high chemical stability, allowing the material to exhibit electrocatalytic activity for H₂ evolution at a low overpotential of 131 mV with a Tafel slope of 51 mV deg⁻¹. The exchange of countercations in anionic POMs by metal complexes is also an effective way to obtain heterometallic CPs consisting of POM molecules.⁶¹ Gomez-Mingot et al. studied the inclusion effect of complex cations on framework structures and electrocatalytic activities using anionic Zn^II–Mo^V–Mo^VI-POMs as a host compound.⁶² The inclusion of cationic species strongly affected the catalytic hydrogen evolution activity, resulting in a TOF of 9.3 × 10⁻² s⁻¹ with an overpotential of 419 mV.⁶²

As part of our study on the creation of heterometallic CPs based on S-donating metalloligands with D-penicillaminate (D-pen),^63–65 we recently discovered (in 2017) the first example of a heterotrimetallic coordination polymer that showed heterogeneous catalytic activity for hydrogen evolution under electrochemical conditions.⁶⁶ In this study, we employed a Pt^II metalloligand with D-pen,⁶⁷ which reacted stepwise with Pd^II and Ni^II ions to produce a heterotrimetallic CP containing all three group 10 metal ions. In this compound, Pt^II₂Pd^II₂ tetranuclear units were linked by Ni^II ions through D-pen carboxylate groups in a 1D chain structure.⁶⁶ The combination of the Pt^II metalloligand with Pd^II and Ni^II in the (Pt^II₂Pd^II₂Ni^II)_n coordination polymer significantly enhanced heterogeneous catalytic activity for electrochemical hydrogen evolution, showing a TOF of 0.011 s⁻¹ at an onset potential of −0.98 V (vs. Ag/AgCl). More recently, we prepared 2D and 3D (Pt^II₂Pd^II₂Mn^II₂)_n heterotrimetallic CPs by using Mn^II instead of Ni^II as linker ions, the dimensionality of which was controlled by counteranions (Cl vs. Br).⁶⁸ The 2D and 3D (Pt^II₂Pd^II₂Mn^II₂)_n CPs showed activities much higher than the 1D (Pt^II₂Pd^II₂Ni^II)_n CP, which was explained by the stronger Lewis acidity of the active centres of Pd^II in the Pt^II₂Pd^II₂ units and an increase in framework robustness due to their higher dimensionalities. Budnikova's group developed ferrocene-based metalloligands that coordinated to 3d metal ions to form 1D heterobimetallic CPs.⁶⁹ Due to the presence of ferrocenyl units, the heterobimetallic CPs were redox active. This coordination contributed to a higher electrocatalytic activity for H₂ evolution, showing Tafel slopes of 120 mV deg⁻¹ and 110 mV deg⁻¹ at overpotentials of 450 mV and 340 mV for the heterobimetallic CPs with Co^II and Zn^II, respectively. Additionally, these 1D heterobimetallic CPs have been extended to higher-dimensional CPs by the addition of 4,4′-bipydine ligands (Fig. 2).⁷⁰ The CPs with Ni^II and Co^II showed Tafel slopes of 60 mV deg⁻¹ and 65 mV deg⁻¹ at overpotentials of 350 mV and 400 mV, respectively, which is the best performance for 4,4′-bipydine-incorporated CPs. A combination of high electrical conductivity and the presence of exposed catalytically active centres is important in MOFs to act as excellent heterogeneous electrocatalysts. Indeed, Zhang and Chen et al. demonstrated that Ni^II–Co^II and Ni^II–Cu^II 2D MOFs containing a π-conjugated organic ligand (hexaiminohexaazatrinaphthalene) showed high electrocatalytic activity;⁷¹ the electrocatalytic performance for H₂ evolution was evaluated by Tafel slopes of 98.2 mV deg⁻¹ and 101.5 mV deg⁻¹ with overpotentials of 207 mV and 162 mV for Ni^II–Co^II and Ni^II–Cu^II MOFs, respectively. Farha and Hupp et al. reported a Zr^IV-based MOF that was functionalized with MoS_x catalytic units,⁷² which exhibited high electrocatalytic activity for H₂ evolution. This compound was prepared via the post-synthesis method, with a Zr^IV-based MOF being reacted with Mo^VI species in the presence of H₂S gas. The presence of large pores in the crystal lattice of the precursor MOF, together with the high stability of its framework towards chemical stimuli, allowed the application of the post-synthesis method. Other examples of heterometallic CPs that act as heterogeneous electrocatalysts for H₂ evolution are summarized in Table 1.

Fig. 2 Co^II/Ni^II–Fe^II bimetallic coordination polymers as a H₂ evolution electrocatalyst: (a) synthesis scheme, (b) CV curves in MeCN containing 0.1 M ^tBu₄NBF₄ with a sample-modified glassy carbon electrode, (c) LSV curves in 0.5 M H₂SO₄ at a scan rate of 0.05 V s⁻¹, and (d) their corresponding Tafel plots. Adapted from ref. 70 with permission from The Royal Society of Chemistry.

Table 1 Structural and electrocatalytic parameters for pristine H₂ evolution electrocatalysts based on their heterometallic coordination polymers

Compound	Dimension	Metal	Organic ligand/metalloligand	Onset potential	η	TOF	Tafel slope	Electrolyte	Ref.
a The value was estimated by us based on the experimental data in the literature.
Iron(III) ruthenocyanide	3D	Fe^III and Ru^II	CN^−	−0.6 V (vs. Ag/AgCl)	—	2.5 × 10^2 s^−1 ^a	—	HCl–KCl (pH 1.5)	54
[Zn(fcdHp)]n	1D	Zn^II and Fe^II	1,1′-Ferrocenylenebis(H-phosphinate) (fcdHp)	−0.220 V (vs. RHE)	340 mV (@ 10 mA cm^−2)	4.5 × 10^−3 s^−1	110 mV dec^−1	0.5 M H2SO4	69
[Co(fcdHp)]n	1D	Co^II and Fe^II	1,1′-Ferrocenylenebis(H-phosphinate) (fcdHp)	−0.300 V (vs. RHE)	450 mV (@ 10 mA cm^−2)	1.5 × 10^−3 s^−1	120 mV dec^−1	0.5 M H2SO4	69
[Ni(fcdHp)(4,4′-bpy)]n	3D	Ni^II and Fe^II	1,1′-Ferrocenylenebis(H-phosphinate) (fcdHp) and 4,4′-bipyridine (4,4′-bpy)	−0.270 V (vs. RHE)	350 mV (@ 10 mA cm^−2)	2.1 × 10^−3 s^−1	60 mV dec^−1	0.5 M H2SO4	70
[Co(fcdHp)(4,4′-bpy)]n	3D	Co^II and Fe^II	1,1′-Ferrocenylenebis(H-phosphinate) (fcdHp) and 4,4′-bipyridine (4,5′-bpy)	−0.320 V (vs. RHE)	400 mV (@ 10 mA cm^−2)	2.8 × 10^−4 s^−1	65 mV dec^−1	0.5 M H2SO4	70
[Ni(H2O)4Pd2Pt2(NH3)4(D-pen)4]Cl2	1D	Pd^II, Pt^II and Ni^II	D-Penicillaminate (D-pen)	−0.98 V (vs. Ag/AgCl)	—	1.1 × 10^−2 s^−1	—	0.1 K LiClO4	64
[Mn2Cl2(H2O)6Pd2Pt2(NH3)4(D-pen)4]Cl2	2D	Pd^II, Pt^II and Mn^II	D-Penicillaminate (D-pen)	−1.0 V (vs. Ag/AgCl)	—	1.7 × 10^−2 s^−1	—	0.1 K LiClO4	68
[Mn2(H2O)6Pd2Pt2(NH3)4(D-pen)4]Br4	3D	Pd^II, Pt^II and Mn^II	D-Penicillaminate (D-pen)	−1.0 V (vs. Ag/AgCl)	—	4.1 × 10^−2 s^−1	—	0.1 K LiClO4	68
MOF
[Ni3{Co3(HAHATN)2}]	2D	Ni^II and Co^II	Hexaiminohexaazatrinaphthalene (HAHATN)	—	207 mV (@ 10 mA cm^−2)	—	98.2 mV dec^−1	0.1 M KOH	71
[Ni3{Cu3(HAHATN)2}]	2D	Ni^II and Cu^II	Hexaiminohexaazatrinaphthalene (HAHATN)	—	162 mV (@ 10 mA cm^−2)	—	101.5 mV dec^−1	0.1 M KOH	71
POM-based compounds
(Bu4N)3[PMo^V8Mo^VI4O36(OH)4Zn4](BTB)4/3	3D	Zn^II and Mo^V/VI	1,3,5-Benzenetribenzoate (BTB)	−0.180 V (vs. RHE)	237 mV (@ 10 mA cm^−2)	—	96 mV dec^−1	0.5 M H2SO4	59
(Bu4N)3[PMo^V8Mo^VI4O37(OH)3Zn4](BPT)	3D	Zn^II and Mo^V/VI	[1,1′-Bipheynyl]-3,4′,5-tricarboxylate (BPT)	−0.304 V (vs. RHE)	392 mV (@ 10 mA cm^−2)	—	137 mV dec^−1	0.5 M H2SO4	59
[Co(bpy)3][PMo^V8Mo^VI4O37(OH)3Zn4](trim)Co(bpy)2(H2O)	1D	Co^II, Zn^II and Mo^V/VI	2,2′-Bipyridine (bpy) and 1,3,5-benzenetricarboxylate (trim)	−0.41 V (vs. Ag/AgCl)	452 mV (@ 10 mA cm^−2)	4.5 × 10^−2 s^−1 ^a	—	0.1 M H2SO4	62
(Bu4N)4[PMo^V8Mo^VI4O37(OH)3Zn4](BTB)4/3·1.5(H3BTB)	3D	Zn^II and Mo^V/VI	1,3,5-Benzenetribenzoate (BTB)	−0.45 V (vs. Ag/AgCl)	576 mV (@ 10 mA cm^−2)	9.3 × 10^−2 s^−1 ^a	—	0.1 M H2SO4	62
(Bu4N)2[Co(bpy)3][PMo^V8Mo^VI4O37(OH)3Zn4](BTB)4/3·1.5(H3BTB)	3D	Co^II, Zn^II and Mo^V/VI	2,2′-Bipyridine (bpy) and 1,3,5-benzenetribenzoate (BTB)	−0.31 V (vs. Ag/AgCl)	419 mV (@ 10 mA cm^−2)	9.3 × 10^−2 s^−1 ^a	—	0.1 M H2SO4	62
[Ru(bpy)3]4[PMo^V8Mo^VI4O38(OH)2Zn4]2(trim)2	3D	Ru^II, Zn^II and Mo^V/VI	2,2′-Bipyridine (bpy) and 1,3,5-benzenetricarboxylate (trim)	−0.45 V (vs. Ag/AgCl)	617 mV (@ 10 mA cm^−2)	6.8 × 10^−2 s^−1 ^a	—	0.1 M H2SO4	62
[PPh4]6[PMo^V8Mo^VI4O37(OH)3Zn4]2(trim)2	3D	Zn^II and Mo^V/VI	1,3,5-Benzenetricarboxylate (trim)	−0.27 V (vs. Ag/AgCl)	335 mV (@ 10 mA cm^−2)	3.9 × 10^−1 s^−1 ^a	—	0.1 M H2SO4	62
[Ru(bpy)3]3[PMo^V8Mo^VI4O37(OH)3Zn4Cl]2(biphen)2	2D	Ru^II, Zn^II and Mo^V/VI	2,2′-Bipyridine (bpy) and 4,4′-biphenyldicarboxylate (biphen)	−0.26 V (vs. Ag/AgCl)	337 mV (@ 10 mA cm^−2)	3.0 × 10^−1 s^−1 ^a	—	0.1 M H2SO4	62
[Cu^II6(pzta)6(bpy)3(P2W18O62)]	1D	Cu^II and W^VI	2,2′-Bipyridine (bpy) and 5-(2-pyrazinyl)tetrazole (pzta)	−0.076 V (vs. RHE)	131 mV (@ 10 mA cm^−2)	—	51 mV dec^−1	—	60
[Cu^II6(pzta)6(bpy)3(As2W18O62)]	1D	Cu^II and W^VI	2,2′-Bipyridine (bpy) and 5-(2-pyrazinyl)tetrazole (pzta)	−0.101 V (vs. RHE)	192 mV (@ 10 mA cm^−2)	—	79 mV dec^−1	—	60

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4. O₂ evolution electrocatalysis

While oxygen (O₂) evolution is an important anodic half reaction of water splitting and is paired with cathodic hydrogen (H₂) evolution, its reaction suffers from sluggish kinetics because of the requirement of four electrons and four protons for O–O bond formation.⁷³ Thus, efficient electrocatalysts must be developed to overcome the kinetic barrier of O₂ formation.⁷⁴ To date, considerable efforts have been devoted to investigating molecular electrocatalysts for O₂ evolution by using discrete metal complexes and homometallic CPs.⁷⁵ An early work on O₂-evolving electrocatalysts based on heterometallic CPs used Prussian blue analogues.⁷⁶ A film of a Co^II–Fe^II Prussian blue analogue was attached on an electrode by electrochemical deposition and water oxidation was performed to produce O₂ gas with a TOF of 0.5 s⁻¹ at an overpotential of 550 mV. The electrocatalytic activity of this compound is due to its open channel structure for mass transport and electron conductivity and the presence of catalytically active Co^II centres. The Karadas group employed Prussian blue analogues with other combinations of metal ions, such as Co^II–Co^III, Co^II–Cr^III, Co^II–Fe^II, and Co^II–Fe^III, to investigate the effect of metal ions co-existing with Co^II ions in the Prussian blue framework on their electrocatalytic activities for water oxidation (Fig. 3).⁷⁷ They showed that the electron density of the catalytic centre of Co^II was affected by the co-existing metal ions, which was strongly related to their electrocatalytic activities. The catalytic activities of Prussian blue analogues are not high because their catalytically active sites (related to the so-called surface concentration) are not well exposed; all the metal centres have an octahedral geometry and are coordinated by six cyanide ligands, except for the terminal metal ions of the network structure. To increase catalytic activity, the Karadas group incorporated a pyridyl-based polymer, poly(4-vinylpyridine), into Prussian blue analogues,⁷⁸ which resulted in the production of amorphous solids with high surface concentrations and low overpotentials. A Ni^II–Fe^III bimetallic CP with phthalocyanine ligands also acts as a heterogeneous electrocatalyst for water oxidation.⁷⁹ In this case, both the Ni^II and Fe^III ions are assumed to be catalytic centres in the 2D sheet structure. The catalytic activity of the Ni^II–Fe^III CP is higher than the activities of the corresponding Ni^II and Fe^III homometallic CPs, indicating the positive synergistic effect of the two kinds of metal centres on catalysing the water oxidation reaction.

Fig. 3 Co^II–M^III bimetallic Prussian blue analogues as O₂ evolution electrocatalysts: (a) CV curves in 50 mM KP_i electrolyte at pH 7 with a sample-modified, fluorine-doped tin oxide (FTO) electrode at a scan rate of 0.05 V s⁻¹, (b) their corresponding Tafel plots, and (c) DFT calculation results. Adapted from ref. 77 with permission from John Wiley and Sons.

Recently, our group reported the catalytic behaviour of Pt^II–Cu^II–Zn^II heterotrimetallic CPs with perchlorate or chloride counteranions for water oxidation; these compounds were synthesized via the metalloligand approach using a Pt^II metalloligand with D-pen (Fig. 4).⁸⁰ In these compounds, Pt^II₂Cu^II₂ tetranuclear units were linked by Zn^II ions through D-pen carboxylate groups in a polymeric structure. Notably, the CPs had different dimensional structures (2D vs. 3D) and different coordination geometries around Cu^II centres (square plane vs. octahedron) that were dependent on the counteranion (ClO₄⁻vs. Cl⁻). These differences largely influenced the heterogeneous electrocatalytic activity for water oxidation; the perchlorate salt with square-planar Cu^II centres showed a catalytic activity much higher than that of the chloride salt with octahedral Cu^II centres. While the electronic effect due to the coexistence of heterometallic ions remains unclear, this study revealed that the introduction of Cu^II ions into heterometallic CPs leads to heterogeneous electrocatalytic activity for water oxidation that is tuned by the linkage mode of Zn^II ions affecting the geometry of the Cu^II centres.

Fig. 4 Pt^II–Cu^II–Zn^II trimetallic coordination polymers as O₂ evolution electrocatalysts: (a) synthesis schemes, (b) packing structures of (left) a Cl salt and (right) a ClO₄ salt, and (c) LSV curves in CH₃CN–H₂O containing 0.1 M KPF₆ with a sample-modified glassy carbon electrode at a scan rate of 0.01 V s⁻¹. Adapted from ref. 80 with permission from The Royal Society of Chemistry.

In heterometallic MOF systems, dinuclear or trinuclear heterometallic units bridged by carboxylate groups are normally designed as a cluster node.⁸¹ This class of compounds has been synthesized by the self-assembly method or post-synthesis transformation. Li and Lan explored Fe^II–Fe^III, Co^II–Fe^III, Ni^II–Fe^III, and Zn^II–Fe^III MOFs having a tricarboxylate ligand, all of which showed electrocatalytic activity for oxygen evolution.⁸² These MOFs were synthesized by the reaction of the trinuclear MFe^III₂ cluster, [MFe₂(μ₃-O)(CH₃COO)₆(H₂O)₃] (M = Fe^II, Co^II, Ni^II, Zn^II), with biphyenyl-3,4′,5-tricarboxylate under solvothermal conditions. In the MOFs, the trinuclear cluster units were linked by the tricarboxylate ligands in a 3D network structure. The electrocatalytic behaviour was studied by using a carbon cloth electrode attached to a mixture of MOF and acetylene black via the drop-casting method. The heterometallic MOFs exhibited a more negative potential and a higher current density in the linear sweep voltammograms (LSVs) compared with the homometallic Fe^II–Fe^III MOF, indicating that the heterometallic MOFs performed electrocatalysis faster. In particular, the Ni^II–Fe^III MOF displayed an overpotential of 365 mV, which was lower than those of other heterometallic MOFs (Co^II–Fe^III: 376 mV and Zn^II–Fe^III: 522 mV), a homometallic Fe^II–Fe^III MOF (555 mV), and commercial IrO₂ (390 mV). Another Ni^II–Fe^III MOF with a 2D sheet structure was reported by Chen and Zhao.⁸³ This MOF was grown on nickel foam via one-step chemical bath deposition, where nickel foam was mixed with 2,6-naphthalenedicarboxylate, nickel acetate, and iron(III) nitrate in water. This material showed an overpotential of 240 mV at 10 mA cm⁻² and a TOF of 3.8 s⁻¹, which are greater than those of the corresponding homometallic MOF and IrO₂ catalysts. Other examples of heterometallic CPs that act as heterogeneous electrocatalysts for O₂ evolution are summarized in Table 2.

Table 2 Structural and electrocatalytic parameters for pristine O₂ evolution electrocatalysts based on their heterometallic coordination polymers

Compound	Dimension	Metal	Organic ligand/metalloligand	Onset potential	η	TOF	Tafel slope	Electrolyte	Ref.
K2xCo(2−x)[Fe(CN)6] (0.85 < x < 0.95)	3D	Co^II and Fe^III	CN^−	+1.10 V (vs. NHE)	400 mV (@ 1 mA cm^−2)	2 × 10^−3 s^−1 (η = 300 mV) and 0.5 s^−1 (η = 550 mV)	85–95 mV dec^−1	pH 7 KPi (50 mM) + 1 M KNO3	76
KaCob[Fe(CN)6]·xH2O	3D	Co^II and Fe^II	CN^−	—	692 mV (@ 1 mA cm^−2)	3.0 × 10^−3 s^−1 (η = 400 mV)	121 mV dec^−1	pH 7 KPi (50 mM) + 1 M KNO3	77
KaCob[Fe(CN)6]·xH2O	3D	Co^II and Fe^III	CN^−	—	661 mV (@ 1 mA cm^−2)	4.4 × 10^−3 s^−1 (η = 400 mV)	127 mV dec^−1	pH 7 KPi (50 mM) + 1 M KNO3	77
KaCob[Cr(CN)6]·xH2O	3D	Co^II and Cr^III	CN^−	—	578 mV (@ 1 mA cm^−2)	5.0 × 10^−3 s^−1 (η = 400 mV)	96 mV dec^−1	pH 7 KPi (50 mM) + 1 M KNO3	77
[CoFe(CN)6@FTO]	3D	Co^II and Fe^II	CN^−	—	600 mV (@ 1 mA cm^−2)	2.6 × 10^−3 s^−1 (η = 262 mV)	111 mV dec^−1	pH 7 KPi (50 mM) + 1 M KNO3	78
[CoFe(CN)6-PVP@FTO]	3D	Co^II and Fe^II	CN^−	—	510 mV (@ 1 mA cm^−2)	2.6 × 10^−3 s^−1 (η = 284 mV)	121 mV dec^−1	pH 7 KPi (50 mM) + 1 M KNO3	78
Fe0.5Ni0.5Pc-CP	2D	Fe^II and Ni^II	(2(3),9(10),16(17),23(24)-Tetraiodophthalocyanine and 2(3),9(10),16(17),23(24)-tetra(ethynyl)phthalocyanine)	—	317 mV (@ 10 mA cm^−2)	43.4 s^−1 (η = 317 mV)	116 mV dec^−1	1.0 M KOH	79
[ZnCl(H2O)Cu2Pt2(NH3)4(D-pen)4]Cl	1D	Cu^II, Pt^II and Zn^II	D-Penicillaminate (D-pen)	+0.75 V (vs. Ag/AgCl)	133 mV	18 h^−1	—	0.1 K KPF6	80
[ZnCu2Pt2(NH3)4(D-pen)4](ClO4)2	3D	Cu^II, Pt^II and Zn^II	D-Penicillaminate (D-pen)	+0.75 V (vs. Ag/AgCl)	133 mV	27 h^−1	—	0.1 K KPF6	80
MOF
Fe/Nix-MIL-53 (x = 1.6, 2.0, 2.4)	2D	Fe^III and Ni^II	1,4-Bezenedicarboxylate (1,4-BDC)	—	258, 258 and 244 mV (@ 10 mA cm^−2)	—	37.8, 45.5 and 48.7 mV dec^−1	1.0 M KOH	82
Fe/Ni2.4/Mx-MIL-53 (M = Co, Mn; x = 0.2, 0.4)	2D	Fe^III, Ni^II and M^II	1,4-Bezenedicarboxylate (1,5-BDC)	—	219 mV (@ 10 mA cm^−2)	—	53.5 mV dec^−1	1.0 M KOH	82
Ni0.8Fe0.2(C12H6O4)(H2O)4	3D	Fe^III and Ni^II	2,6-Naphthalenedicarboxylic acid	—	240 mV (@ 10 mA cm^−2)	3.8 s^−1 (η = 400 mV)	34 mV dec^−1	0.1 M KOH	83

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5. CO₂ reduction electrocatalysis

To realize a future carbon-neutral society, the electrochemical conversion of CO₂ into useful chemical species has been intensively investigated in the past decade.⁸⁴ A homometallic Cu^II MOF was first applied for electrocatalytic CO₂ reduction in 2012,⁸⁵ and then, an increasing number of MOFs have been studied for electrochemical CO₂ reduction. An early work on electrocatalytic CO₂ reduction using a heterometallic MOF was performed by Yaghi and Yang in 2015. They applied a Co^II–Al^III MOF containing 4,4′,4′′,4′′′-(porphyrin-5,10,15,20-tetrayl)tetrabenzoate as a linker ligand for electrocatalytic CO₂ reduction.⁸⁶ In this case, a homometallic Al-MOF was directly grown on the electrode by the self-assembly method under solvothermal conditions, and then, the Co^II–Al^III MOF electrocatalyst was prepared by the post-binding of Co^II to the empty sites of the porphyrin units; this catalyst exhibited >76% selective CO production with a turnover number (TON) of 1400 and a stability of 7 hours. Kubiak et al. reported a Zr^IV MOF with Fe^III porphyrin units as a CO₂ reduction electrocatalyst, where the Fe^III porphyrin unit acted as a catalytic site in the Zr^IV MOF that was grown on the conductive electrode surface.⁸⁷ This compound showed a CO₂/CO overpotential of ∼650 mV, Faraday efficiency of ∼100%, TOF of 0.13 s⁻¹, and TON of 1520, and all of these values are superior to those of the corresponding homometallic catalysts.

While the aforementioned two reports did not show a cooperative effect due to the presence of multi- and heterometal ions, a report by Beobide et al. demonstrated that Zn^II–Cu^II, Ru^III–Cu^II, and Pd^II–Cu^II MOFs that were prepared by post-synthesis transmetallation of HKUST-1(Cu) containing benzene-1,3,5-tricarboxylate (BTC) exhibit a heterometallic effect.⁸⁸ The sample solid was dispersed in a Nafion–isopropanol solution and drop-cast on an electrode to be applied for electrocatalytic CO₂ reduction. Although the amount of transmetallated metal ions was less than 19%, the CO₂ reduction catalytic activity was strikingly increased by this heterometal doping. In particular, the selective formation of EtOH reached 100%.

In 2020, Dong and Feng et al. reported 2D sheet Cu^II–Zn^II MOFs with phthalocyanine ligands, which showed the synergistic electroreduction of carbon dioxide (Fig. 5).⁸⁹ The solid sample was prepared by the self-assembly method under solvothermal conditions and loaded on an electrode together with a mixture of carbon nanotubes and Nafion. These conjugated 2D sheet MOFs revealed excellent catalytic performance with a CO product selectivity of 88%, TOF of 0.39 s⁻¹, and durability of >10 hours. Notably, this material showed that the molar H₂/CO ratio was reduced from 4 : 1 to 1 : 7 when the metal centres and applied potentials were varied. Cu^II ions coordinated by phthalocyanine-N4 in a square-planar geometry and Zn^II ions coordinated by two catecholato units in a square-planar geometry were assigned as the active centres for hydrogen evolution and CO production, respectively. The Cu^II and Zn^II centres synergistically inhibited H₂ production and promoted CO production via electrons that passed through π-conjugated ligands. Examples of MOFs that show electrocatalytic CO₂ reduction have been summarized in a recent review article.⁹⁰

Fig. 5 Cu^II–Zn^II bimetallic coordination polymers as a CO₂ reduction electrocatalyst: (a) packing structure, (b) a proposed synergistic catalytic scheme based on DFT calculations, (c) partial current and (d) Faraday efficiency of CO production at different potentials. Adapted from ref. 89.

6. O₂ reduction electrocatalysis

Oxygen reduction is an important chemical reaction in the design of fuel cell devices and metal–air batteries.⁹¹ The best catalyst for oxygen reduction is platinum metal containing carbon (Pt/C), but platinum is expensive and scarce.⁹² Therefore, non-precious alternative catalysts for oxygen reduction are in high demand, especially those that meet the requirements of large-scale commercial applications. One of the major studies related to alternative catalysts is the use of composite materials derived from CPs or MOFs.⁹³ However, their preparation requires energy-consuming pyrolysis/calcination under harsh synthesis conditions, which results in the destruction of their original ordered structures and a loss of active sites.⁹⁴ The use of CPs for oxygen reduction reactions is still at an early research phase, having started only in 2012,⁹⁵ and studies using heterometallic CPs have rarely been reported.^96–100 The Dehghanpour group reported that a Zr-MOF with Zr^IV₆ oxo-clusters linked by Fe^III porphyrins acted as an oxygen reduction catalyst.⁹⁶ The Zr^IV–Fe^III MOF was further mixed with pyridine-functionalized graphene and then drop-cast on a glassy carbon electrode. The modified electrode revealed enhanced catalytic activity for oxygen reduction in cyclic voltammograms with a maximum current that was 61.6 times larger than that of the bare electrode. In addition, this MOF system showed improved stability and repeatability. They suggested that the 3D MOF structure prevented the aggregation of the active units of the Fe^III porphyrin, contrasting the case of conventional discrete molecular systems. The Morris group also studied the catalytic behaviour of the same Zr^IV–Fe^III MOF.⁹⁷ They directly grew a MOF film on a FTO electrode by a solvothermal method without using graphene. The thin film of the heterometallic Zr^IV–Fe^III MOF had enough conductivity to perform catalytic oxygen reduction without any conductive additives. Electrochemical characterization revealed high selectivity for H₂O (4e⁻ reduction product) over H₂O₂ (2e⁻ reduction product) with this material.

Jiang and Yao et al. reported an ethyl-linked Fe^III/Co^II polymer conjugated with phthalocyanine that acted as a heterogeneous oxygen reduction catalyst.⁹⁸ The conjugated polymers were synthesized by a Sonogashira coupling reaction between Fe^III and Co^II phthalocyanine monomers in Fe : Co ratios of 100 : 0, 50 : 50, and 0 : 100. The homo- and heterometallic polymers were loaded on glassy carbon electrodes with 50% carbon black (Vulcan XC-72), and their catalytic activities for oxygen reduction were evaluated by CV, LSV, and rotating ring-disk electrode (RRDE) measurements. The heterometallic conjugated polymer revealed a catalytic activity superior to that of the corresponding homometallic polymers, indicating a synergistic effect between the Fe^III and Co^II centres that were located in close proximity to each other in the heterometallic polymer.

Zhang et al. employed a Mn^II₁₃ coordination polymer containing Fe^III ions as an oxygen reduction catalyst (Fig. 6).⁹⁹ They expected that the Fe^III ions dispersed into the coordination polymer would show catalytic activity, although Fe^III ions tend to form an aggregate that is catalytically inactive. The Fe^III ions were introduced by immersing freshly prepared crystals of the porous Mn^II₁₃ coordination polymer with tert-butylphosphonate and 4,4′-trimethylenedipyridine in an aqueous solution of FeCl₃. The obtained microcrystals were characterized by RRDE and LSV measurements using a glassy carbon electrode loaded with a sample-Nafion mixture. While the Mn^II₁₃ coordination polymer itself exhibited a catalytic current during oxygen reduction, as observed by LSV, the doped Fe^III ions led to an improved onset potential and a smaller Tafel slope (130 mV dec⁻¹), indicative of a synergistic effect due to the Fe^III and Mn^II centres. Furthermore, this heterometallic coordination polymer was highly tolerant towards methanol, which is a typical fuel crossover.

Fig. 6 Fe^III–Mn^II bimetallic coordination polymers as an O₂ reduction electrocatalyst: (a) synthesis scheme and structures, (b) CV curves in 1.0 M CH₃OH containing a N₂- and O₂-saturated 0.1 M solution of KOH at a scan rate of 0.1 V s⁻¹, (c) LSV curves at a scan rate of 0.01 V s⁻¹, (d) oxygen reduction polarization curves at different rotation rates, and (e) oxygen reduction polarization curves before and after 5000 potential cycles. Adapted from ref. 99 with permission from The Royal Society of Chemistry.

In general, catalytic oxygen reduction reactions require a low overpotential, high kinetics (Tafel slope, TOF and TON), high selectivity (H₂O vs. H₂O₂vs. H₂) and high stability. Thus, heterometallic CPs are promising materials for these catalytic reactions due to their robust framework and the synergistic and cooperative effects of their heterometal ions.¹⁰⁰ While their low conductivities may be a weak point, this issue has now been overcome by the use of redox-active frameworks or the inclusion of conductive species in the framework. Further development of heterometallic CPs as oxygen reduction electrocatalysts will have a significant impact on this research field.

7. Organic reaction electrocatalysis

Heterometallic CPs have also been studied for the catalytic activation of small organic molecules such as alcohols, organic acids, and organic halides.¹⁰¹ The Kolotilov group reported the electrocatalytic dehalogenation of organic halides using a Fe^III–Co^II–Ni^II heterotrimetallic CP (Fig. 7).¹⁰² They prepared a mononuclear nickel(II) complex with a Schiff base ligand that was derived from the hydrazine of 4-pyridinecarboxylic acid and 2-pyridinecarbaldehyde to act as a N-donating metalloligand. By mixing this nickel(II) metalloligand with the trigonal trinuclear cluster, [Fe₂CoO(piv)₆], which consists of Co^II and Fe^III ions bridged by pivalate (piv) ligands, the Fe^III–Co^II–Ni^II heterotrimetallic CP was isolated as a crystalline solid. The nickel(II) metalloligand is a ligand-based redox-active species that shows two reduction processes in homogeneous cyclic voltammograms. The first reduction current increased with the addition of CHCl₃, indicating that the nickel(II) metalloligand itself had electrocatalytic activity for the dehalogenation of organic halides. Thus, this group showed that the Fe^III–Co^II–Ni^II heterotrimetallic CP functioned as a heterogeneous electrocatalyst for the dehalogenation of organic halides due to the presence of the nickel(II) metalloligand.

Fig. 7 A Fe^III–Ni^II bimetallic coordination polymer as a dehalogenation electrocatalyst: (a) synthesis scheme and structures, (b) CV curves of the Ni^II metalloligand in DMF with the addition of CHCl₃, (c) CV curves of the suspension containing the Fe^III–Ni^II bimetallic coordination polymer in CH₃CN with the addition of CHCl₃, and (d) catalytic activities vs. redox potentials of the catalysts and the halides. Adapted with permission from ref. 102, Copyright 2014 American Chemical Society.

Since electrocatalytic current is commonly much higher than the current of a standard redox process, it is often used as the signal for molecular sensors working in solution.¹⁰³ Furthermore, the electrocatalytic response is specific for substrates, which is also an advantage for the design of selective and sensitive molecular sensors. Ma and Pang et al. reported a 3d–4f heterometallic MOF compound that consisted of Mn^II–W^VI Dawson-type POM units linked by Ce^III species with N-oxide bipyridine ligands.¹⁰⁴ The Mn^II–W^VI–Ce^III MOF was prepared by a reaction of Mn^II–W^VI POM with Ce^III species. Cyclic voltammetry using a sample-modified carbon paste electrode showed that the oxidation peak currents for the Mn^II and Ce^III centres substantially increased with the addition of either ascorbic acid or dopamine. This result indicated that the oxidations of ascorbic acid and dopamine were catalysed by the Mn^II–W^VI–Ce^III MOF. On the other hand, this current increase was not observed for the Mn^II–W^VI POM. The Ma group established another detection system using a heterometallic CP.¹⁰⁵ They prepared a film of a cyano-bridged heterometallic CP containing poly(L-citrulline)/lanthanide on a glassy carbon electrode, and it was applied for the electrochemical detection of 3-nitropropionic acid. A linear relationship between the peak currents due to reduction of 3-nitropropionic acid and the 3-nitropropionic acid concentration was obtained for the range of 40.0–4.00 × 10³ μM. Wang et al. conducted the electrocatalytic oxidation of ethanol using a Nd–Fe–Mo cyano-bridged CP.¹⁰⁶ The fabrication of this coordination polymer on a platinum electrode was performed by electrochemical deposition. While platinum atoms on the electrode surface were assigned to catalytic reaction centres, the heterometallic CP provided a cooperative effect reconstructing the environment around the platinum surface, improving the ethanol oxidation kinetics.

In the past decade, an increasing number of studies have been devoted to heterometallic CPs as promising heterogeneous catalysts for a variety of organic reactions, such as the cyanosilylation of aldehydes,¹⁰⁷ oxidation of cyclohexene,¹⁰⁸ olefin epoxidation,¹⁰⁹ the Knoevenagel condensation,¹¹⁰ cyanation of aldehydes,¹¹¹ sulfoxidation,¹¹² imine formation from alcohols and amines,¹¹³ regioselective halogenation of phenol,¹¹⁴ hydrogenation,¹¹⁵ desulfurization,¹¹⁶ the Henry reaction,¹¹⁷ C–S cross-coupling reactions,¹¹⁸ organophosphate hydrolysis,¹¹⁹ oxidative hydroxylation of phenylboronic acid,¹²⁰ nitroaldol reactions,¹²¹ acetalization of benzaldehyde,¹²² and reduction of nitrophenol.¹²³ The above catalytic reactions often involve redox reactions using chemical reagents (i.e., H₂O₂, O₂, and NaBH₄), and their corresponding electrocatalytic reactions have not been reported to date. This observation is most likely due to the difficulty in the fabrication/loading of CPs on electrodes, which requires tight and close contact between the electrode and sample. Furthermore, the complexity of analysing electrochemical responses due to organic species using voltammograms prevents the development and expansion of this research field. Further trials are needed to elucidate the mechanisms of heterogeneous, electrocatalytic organic transformations, in which synergistic effects are expected due to the different kinds of metal centres existing in heterometallic CPs.

8. Summary and perspectives

While CPs have been applied to obtain inorganic composite materials by their pyrolysis in materials science, recently, an increasing number of studies have been conducted on their direct employment as heterogeneous electrocatalysts because they are (i) crystalline compounds whose molecular and packing structures can be determined at an atomic-level resolution by X-ray crystallography, (ii) solid materials that can offer high stability in the solid state, and (iii) hybrid compounds that contain metal ions available for catalytically active sites. The tunability of the active sites in the structure, such as the oxidation states and the coordination geometries of metal ions and the distances between metal centres, is also the reason for the recent interest from researchers. In particular, heterogeneous electrocatalytic reactions using heterometallic CPs containing more than two kinds of metal ions, which can be prepared by the self-assembly of metal ions and ligands in a one-pot reaction, the metalloligand approach using metal complexes with several donor sites, or the post-synthesis modification of CPs by metal exchange reactions, are a recent hot topic in this research area, because their metal centres cause synergistic and cooperative effects in electrocatalytic reactions. Therefore, these CPs can lead to enhanced electrocatalytic performances, which are highly expected for this class of compounds. To date, hydrogen evolution (water/proton reduction), oxygen evolution (water oxidation), oxygen reduction, carbon dioxide reduction, and organic transformation have been studied as heterogeneous electrocatalytic reactions using CPs, all of which depend on the differences in the geometries and oxidation states of metal ions incorporated in the CPs.

Porous coordination polymers (PCPs) with MOFs are a major class of heterometallic CPs, and they have been a target of study for their effects on heterogeneous electrocatalytic activity. This is because porous structures allow the insertion and accommodation of catalytically active and electrically conductive species, electrolytes, substrates, and solvents, which can overcome the weakness of CPs in various applications. While the best performance of heterogeneous electrocatalytic activity has been achieved by conductive MOFs, some non-porous CPs have also shown electrocatalytic activities due to the addition of external species, such as Nafion and carbon materials, which serve as binders and electrical conductors, respectively. As part of studies on heterogeneous electrocatalysis, methods for sample loading on electrodes, such as crystal growth, electrodeposition, and transcription, have also been developed by applied chemists.⁴⁷

The electronic communication between the catalytically active centres of metal ions can enhance not only the electrical conductivity but also the synergistic effects, leading to a higher performance during electrocatalytic reactions. In coordination chemistry, it is quite important to determine the best combinations of metal ions that lead to the enhancement in electrocatalytic activity and to elucidate enhancement mechanisms. As described in this review article, the favourable combinations of metal ions in CPs appear to be dependent on the type of electrocatalytic reaction, and the design of metal combinations that cause high activities relies on the large number of screening experiments based on reported heterometallic systems. The insufficient number of reports on the heterogeneous electrocatalytic activities of heterometallic CPs and the lack of theoretical studies make it difficult to predict their synergistic effects on electrocatalytic activities. Thus, fundamental studies focused on investigating the relationships between structures and catalytic activities and the synthesis of new heterometallic CPs with unique molecular and crystal structures are highly desired, although arguments exist regarding whether the original polymeric structures are fully retained in the course of electrocatalytic reactions.

Most reports on heterogeneous electrocatalysis using heterometallic CPs have appeared only in recent decades. In this research field, the discovery of new heterometallic CPs that show excellent electrocatalytic activities should be promising via cooperative studies on coordination chemistry, catalytic chemistry, and electrochemistry.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI (grant numbers 18H05344 and 19K05667).

Notes and references

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