Construction and properties of cobalt(II)/copper(II) coordination polymers based on N-donor ligands and polycarboxylates mixed ligands

Abstract

Metal–organic coordination polymers (MOCPs) are well known organic–inorganic hybrids with infinite structures consisting of metal ions/clusters and organic ligands linked through coordination interactions. MOCPs can be constructed from one or more than one organic bridging ligands (mixed-ligands) and different metal ions. The previous reports prove the fact that the nature of organic ligands and metal ions dominates the final structures as well as properties of the MOCPs in a certain way. Therefore, we focus on discussing the cobalt(II)/copper(II) coordination polymers constructed from the mixed-ligands of polycarboxylates and N-donor ligands, which may possess potential applications in the fields of electrochemistry, electrocatalysis, magnetism and photocatalysis. In this review, we summarize some typical Co(II)/Cu(II) MOCPs based on the mixed bridging organic ligands, aiming to discuss their versatile synthesis methods, topologies and structural influence factors, as well as their tunable properties. All of these aspects are highlighted in this review, which seeks to guide further investigations of cobalt(II)/copper(II) coordination polymers.

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1 Introduction

Metal–organic coordination polymers (MOCPs) are an intriguing class of hybrid materials, which exist as infinite crystalline lattices extended from inorganic metal ions (or clusters) and organic ligands supported by coordination interactions.^1–5 In the past few decades, a variety of typical examples for such crystalline materials with attractive architectures and potential applications in luminescence, heterogeneous catalysis, gas storage/separation, as well as magnetism have attracted significant interest.^6–9 The MOCPs containing cobalt(II)/copper(II) ions are particularly interesting because of their excellent physical and chemical properties, especially for their potential applications as electrochemical, electrocatalysis, magnetism and photocatalytic materials.^10–17 It is well known that many factors can affect the ultimate structures of the target cobalt(II)/copper(II) MOCPs, such as organic ligands, reactants ratios, systematic pH values and reaction temperatures etc., however, the final structures and properties of cobalt(II)/copper(II) MOCPs are determined in large measure by the nature of organic ligands.¹⁰ So the selection of proper organic ligands is an important factor in the self-assembling process of cobalt(II)/copper(II) MOCPs, even small structural changes of the organic ligands such as the symmetry, length, and flexibility or functional groups can produce obvious differences in the final coordination frameworks and topological networks.

In recent years, the mixed-ligand system containing two or more organic bridging ligands has been widely adopted to generate new cobalt(II)/copper(II) MOCPs with diversified topologies and interesting properties.^11–16 Firstly, the polycarboxylate ligands as a kind of O-donor anionic bridging ligands have been extensively used to construct novel cobalt(II)/copper(II) MOCPs owing to their versatile coordination modes and high structural stability.^13–17 Secondly, the neutral N-donor organic ligands can be properly selected as the co-ligands and introduced into the cobalt(II)/copper(II)-polycarboxylate systems, which may connect the cobalt(II)/copper(II) ions/clusters to modify the final architectures and physical properties of target cobalt(II)/copper(II) MOCPs. Thus, this is one of the most efficient synthetic strategies to construct multifunctional MOCPs.

To date, in the reported cobalt(II)/copper(II) MOCPs composed of mixed organic ligands, the neutral multidentate N-donor organic bridging ligands with two or more N and/or O-involving functional groups have been widely applied as the effective tectons, which show different binding abilities and can construct diverse coordination networks.^11,12 However, the extensive researches towards the rational design and construction of mixed-ligand cobalt(II)/copper(II) coordination polymers have shown that bis(imidazole)/bis(triazole)/bis(pyridyl) derivatives and polycarboxylate ligands represent the most reliable and typical building blocks which can be jointly applied to synthesize a wide range of desired coordination networks.^13–16 A choice of such connectors in coordination assembly can be rationalized based on the following considerations: (i) the neutral N-donor ligands normally bind to the cobalt(II)/copper(II) ions as the rod-like bidentate tectons; (ii) the polycarboxylate ligands can not only provide various coordination modes upon binding to metals, but also play the role of counterions.^13–16 As a result, by combining the advantages of such two types of ligands, the so-called mixed-ligand synthetic strategy can be rationally proposed.^18,19 Recently, this strategy has also been applied to design and prepare a series of cobalt(II)/copper(II)-based MOCPs with diverse bis(imidazole)/bis(triazole)/bis(pyridyl) and polycarboxylate ligands, which show emerging applications in electrochemistry, magnetism and catalysis.^{13–16,20–28}

This paper would focus on the significant advances in cobalt(II)/copper(II) MOCPs based on N-donor ligands and polycarboxylates mixed-ligands with potential applications in electrochemistry, electrocatalysis, magnetism, and photocatalysis. Part of the ligands discussed in this paper are shown in Schemes 1 and 2.

Scheme 1 Part of N-donor ligands used in this review.

Scheme 2 Part of polycarboxylate ligands used in this review.

2 Synthesis method and structural influence factors of the cobalt(II)/copper(II) MOCPs based on the mixed ligands

The acid–base system is the most important and flourishing branch for preparing the cobalt(II)/copper(II) MOCPs based on the mixed ligands.⁴ Naturally, polycarboxylate anions and cobalt(II)/copper(II) ions are perfect partners that can compensate charge balance, coordination deficiency, and weak interaction all at once.⁴ In this context, it is valuable to propose the rational synthetic strategy to regulate the networks of cobalt(II)/copper(II) mixed-ligand MOCPs by designing or selecting the suitable neutral N-donor ligands and polycarboxylates, because the intrinsic characteristic of organic ligands (such as spacer, positional isomer and substituent groups) have different effects on the assembly of MOCPs.^2,4,29 From another aspect, there are many synthetic methods for obtaining MOCPs in the literatures, such as hydro(solvo)thermal methods,^30,31 saturation methods,³² diffusion methods,³³ microwave,³⁴ and ultrasonic methods.³⁵ Thereinto, the hydro(solvo)thermal method is originally and commonly used for the synthesis of cobalt(II)/copper(II) MOCPs constructed from the mixed ligands.^36–39 Thus, in this paper, cobalt(II)/copper(II) MOCPs containing the mixed ligands synthesized by hydro(solvo)thermal and diffusion method would be focused on.

2.1 Cobalt(II)/copper(II) MOCPs based on the mixed ligands by hydrothermal synthesis

Generally, hydrothermal synthesis method can be viewed as the most popular method for preparing cobalt(II)/copper(II) MOCPs based on the mixed ligands.^{36–64,66–89} Under these conditions, the reduced viscosity of water enhances the diffusion process and thus extraction of solids and crystal growth from solution are favoured.^{36–64,66–89} During the hydrothermal assembly process, many factors may affect the crystallization and structural construction of the prospective cobalt(II)/copper(II) MOCPs, such as organic ligands (including N-donor ligands and the polycarboxylate ligands), reactants ratio, and pH value, etc.

2.1.1 Cobalt(II) MOCPs based on the mixed ligands. It is well known that the polycarboxylate ligands play an important role in determining the final structures of target complexes. A straightforward example is provided by the cobalt(II)–bib (bib = 1,4-bis(imidazolyl)benzene, Scheme 1) matrix via incorporation of three structurally related benzoate acids (Scheme 2).⁴⁴ Owing to the different position of the carboxylic group, the benzoates adopt different bridging modes to connect metal centers into various 1D chains. These chains are further connected by bib rods to afford a series of interpenetrating networks (Fig. 1). Therefore, the topological frameworks are built up using the benzoate-bridging chains, and the benzoates play a dominant role in producing mixed-ligands-based networks.⁴⁴ To investigate the effect of organic anions on the coordination frameworks, shortly after this work, three novel cobalt(II) MOCPs have been successfully fabricated under hydrothermal condition.⁴⁵ The results reveal that the carboxylic building blocks with different conformations play a significant role in promoting the diversity of the observed structural motifs (Fig. 2).⁴⁵ By introducing the methyl (–CH₃) into the imidazole groups, three new Co(II) coordination polymers with different interpenetrating and topological motifs have been isolated through varying the assistant dicarboxylates.⁴⁶ These results herein indicate that the conformations of carboxylates have a great influence on the structures of resulting cobalt(II) MOCPs (Fig. 3). It is believed that more cobalt(II) MOCPs containing flexible bis(imidazole) and polycarboxylate anions with interesting structures as well as physical properties will be documented.⁴⁶

Fig. 1 Influence of the polycarboxylic anions on the structures of the MOCPs. Reprinted with permission from ref. 44.

Fig. 2 Influence of the polycarboxylic anions on the structures of the MOCPs. Reprinted with permission from ref. 45.

Fig. 3 Influence of the polycarboxylic anions on the structures of the MOCPs. Reprinted with permission from ref. 46.

Moreover, the flexible bis(benzimidazole) derivatives as a kind of excellent N-donor ligands show various conformations when coordinating with transition metal ions.^54–56 In our previous work, we synthesized a series of coordination polymers based on flexible bis(benzimidazole) and carboxylates mixed ligands (Fig. 4).^54–56 In contrast to the flexible bis(benzimidazole) ligands, the semi-rigid bis(benzimidazole) ligands have rigid backbones. In order to explore the effect of the auxiliary ligands on the structures of MOCPs based on the semi-rigid bis(benzimidazole) ligand, different types of carboxylic acids were selected to construct new MOCPs (Fig. 4). As a result, the organic carboxylates have an important effect on the coordination geometries of the Co(II) ions and the final dimensionalities of the complexes.^57,58

Fig. 4 Influence of the polycarboxylic anions on the structures of the MOCPs. Reprinted with permission from ref. 57 and 58.

As a part of continuous efforts towards, two pairs of novel complexes have been successfully prepared from the appropriate combination of cobalt(II) ions with the rigid 5-amino-isophthalic acid (aip) (Scheme 2) and flexible bis(imidazole) ligands in different ratios, which further enriches the coordination chemistry of these ligands.⁴⁸ All of the four complexes possess 2D structures and pack into 3D supramolecular frameworks through intermolecular N–H⋯O hydrogen bonds (Fig. 5). All the frameworks have been analyzed by the topological approach. By comparison of the two pairs of cobalt(II) complexes, when the metal–ligand ratio and flexible linkers are changed, the aip ligand presents various coordinated modes, thus resulting in four unique structures.⁴⁸

Fig. 5 Influence of the reaction radio on the structures of the MOCPs. Reprinted with permission from ref. 48.

Furthermore, altering bidentate N-donor ligand is an effective strategy for construction of MOCPs with interesting structures and properties.^49,50 When using bis-imidazole/bis-pyridyl ligands, four novel cobalt(II) MOCPs based on a flexible 3,3′,4,4′-oxydiphthalic acid (oa) (Scheme 2) have been synthesized through hydrothermal reactions (Fig. 6).⁴⁹ Moreover, several factors influence the self-assembly process of MOCPs based on flexible organic ligands; among those, the angle between the positions of coordinating groups, i.e. the coordination angle of the organic ligands, is an important factor that alters the dimensionality and topology of the coordination networks.⁵⁰ The self-assembly of a (flexible, flexible) mixed-ligand system reveals the amendment of dimensionality according to the length of organic ligands, which is dependent on the coordination angle of the organic ligands.⁵⁰ This concept has been discussed elaborately in some articles by using flexible ligands to prepare a series of complexes.⁵⁰

Fig. 6 Influence of the N-donor ligands on the structures of the MOCPs. Reprinted with permission from ref. 49.

It is well known that the pH of reaction system plays a crucial role in the construction of metal carboxylate frameworks.⁵¹ For 5-nitro-1,2,3-benzenetricarboxylic acid (3-nbtc) (Scheme 2), the pH value of the reaction solution can lead to different degree of deprotonation, which may affect not only the ligand coordination ability but also its charge and even coordination modes.⁵¹ In this context, two new complexes assembled by Co(II) salt, 3-nbta and 1,6-bis(triazole)hexane (bth) (Scheme 1) have been synthesized and characterized. The results indicate that pH value of reaction system of metal–H₃nbta has vital influence on structures of the result products (Fig. 7).⁵¹ Moreover, the steric hindrance created by the secondary ligand at the metal coordination sphere plays an important role in driving the self-assembly process.^52,53 By using flexible pyridyl ligands as secondary ligands along with carboxylic acids, the conformation, and position of the ligating atom in the secondary ligand have a substantial role in the formation of diverse architectures.^52,53

Fig. 7 Influence of the pH on the structures of the MOCPs. Reprinted with permission from ref. 51.

2.1.2 Copper(II) MOCPs based on the mixed ligands. By using flexible bidentate 1,1′-(1,4-butanediyl)bis(imidazole) ligand (bimb) and m-phthalate anion ligand (1,2-bdc) (Schemes 1 and 2), the first molecular architecture {[Cu(bimb)(1,2-bdc)(H₂O)]·5H₂O}_n with the unprecedented topology of a four-connected 8⁶ net has been obtained with larger circuit (Fig. 8).⁵⁹

Fig. 8 (a) Schematic topology for 8⁶ nets; (b) channels in the 3D framework. Reprinted with permission from ref. 59.

As a continuation of such work, six novel copper(II) MOCPs with different topologies have been further synthesized by Ma's group due to the effect of the organic anions.^60,61

Recently, Ma's group selected six structurally different organic acids as the anion ligands and two flexible bis(imidazole) ligands as the secondary ligands and synthetized nine new copper(II) MOCPs with different framework structures, and they have investigated the effect of the organic acids on the structures.⁶² The structural features of the organic acids such as shape, flexibility and length of the spacers are the main reason for the structural differences of the Cu(II) complexes.⁶² To study the influence of the spacers between the triazole groups on the structures, a copper(II) MOCP was synthesized by using 1,3-bis(1,2,4-triazol-1-ylmethyl)benzene (mbtz) and 1,2,4,5-benzenetetracarboxylate (btec) (Schemes 1 and 2).⁶³ Each btec ligand connects four Cu(II) atoms through its four –COO groups, resulting in a planar 2D [Cu(btec)_0.5]_n network. The Cu(II) atoms are further coordinated with mbtz ligands to fulfil their coordination geometry. However the mbtz ligands do not increase the dimension of the 2D network.⁶³ In addition, Lysenko and co-workers have employed tetrafunctional adamantanecarboxylic acid (H₄adtc) in combination with bistriazole ligands (tr₂pr = 1,3-bis(1,2,4-triazol-4-yl)propane and tr₂ad = 1,3-bis(1,2,4-triazol-4-yl)adamantane) for the construction of novel Cu(II) MOCPs based on discrete coordination clusters (Fig. 9).⁶⁴

Fig. 9 Influence of the N-donor ligands on the structures of the Cu(II) MOCPs. Reprinted with permission from ref. 64.

The triazole and amide groups may provide more potential coordination sites,^64–69 and the amide group is an excellent hydrogen acceptor and donor,^66–69 which conduces to the formation of hydrogen bonding interactions and may further influences the final structures of target MOCPs. These features may allow bis-triazole-bis-amide ligands, combining the triazole and amide groups, to adopt versatile coordination modes and play an important role in constructing MOCPs.^66–69 In order to investigate the effect of polycarboxylates on the networks based on bis-triazole-bis-amide ligands, we designed two flexible bis-triazole-bis-amide ligands with different spacer length to assemble with different aromatic dicarboxylic/tricarboxylic acids, and obtained a series of Cu(II) complexes (Fig. 10).^67,69 As a continuation of this kind of ligand, we also designed a semi-rigid bis-triazole-bis-amide N,N′-di(4H-1,2,4-triazole)cyclohexane-1,4-dicarboxamide to construct new MOCPs.^66,68

Fig. 10 Influence of the polycarboxylic anions, N-donor ligands and pH on the structures of the MOCPs. Reprinted with permission from ref. 66–69.

Cui's group has devoted to exploratory research toward developing new examples of 3D high-connected MOCPs with the mixed nodes based on 1,2,4,5-benzenetetracarboxylic acid (H₄btec) and btx (1,4-bis(1,2,4-triazol-1-ylmethyl)-benzene) ligands,^70,71 they have successfully synthesized and structurally characterized two new complexes {[Co₅(btec)₂(btx)(μ₃-OH)₂(H₂O)₂]·2H₂O}_n and [Cu₂(btec)(btx)_1.5]_n, which represent two unusual 3D binodal (4,10)- and (4,7)-connected MOCPs (Fig. 11).

Fig. 11 Influence of the metal ions on the structures of the MOCPs. Reprinted with permission from ref. 70 and 71.

As the contribution of hydrothermal synthesis, our group has been working on the syntheses of Co(II)/Cu(II)-based complexes containing rigid/semi-rigid/flexible bis-pyridyl-bis-amide ligands, and a series of MOCPs with various structures have been constructed.^72–89 The structural diversities indicate that the bis-pyridyl-bis-amide ligands not only are good candidates for the construction of coordination polymers but also play an important role in tuning the structures. Moreover, the polycarboxylates also greatly contribute to the structural diversities of the final frameworks.^72–89

2.2 Cobalt(II)/copper(II) MOCPs based on the mixed ligands by solvothermal synthesis

The difference of solubility between organic and inorganic components in the same solvent is often a barrier in the formation of single crystals, solvothermal experiments can be a good alternative as solubilities of starting materials can be increased.^90–95 This crystallization technique is a non-equilibrium synthesis and may lead to metastable products. This can be influenced mainly by the cooling speed at the end of the reaction.^90–95

2.2.1 Cobalt(II) MOCPs based on the mixed ligands. In this regard, some investigation on a series of cobalt(II) MOCPs under solvothermal conditions have been obtained.⁹⁰ Four complexes based on bbi and 5-iipa mixed ligands by using different solvent systems display 3D supramolecular structures, in which the open channels are occupied by different guest molecules (Fig. 12). In addition, the role of the halogen-related interaction in the assembly of MOCPs and the conformations of flexible ligand can be influenced by different solvents. The combination of the rigid and flexible ancillary ligands can be used to obtain diverse networks in different solvents and can be considered as candidate to design more intriguing architectures.⁹⁰

Fig. 12 Influence of the solvents on the structures of the MOCPs. Reprinted with permission from ref. 90.

By using solvothermal synthesis method, three entanglement systems based on the flexible and the rigid bis(imidazole) ligands, including two polyrotaxane-like and one interpenetrating frameworks have been obtained.⁹¹ The flexible nature of spacer for the bis(imidazole) allows the ligands to bend and rotate when it coordinates to metal centers. In particular, they favor forming a 2-membered metallacycle when adopting cis-conformation. While the rigid bis(imidazole) ligand tends to construct the coordination polymers exhibiting high porosity, the potential void allows a number of individual nets participating in interpenetration with each other (Fig. 13).⁹¹

Fig. 13 Influence of the N-donor ligands on the structures of the MOCPs. Reprinted with permission from ref. 91.

To investigate the role of three isomeric ligands 1,2-, 1,3-, 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene (1,2-mbtz, 1,3-mbtz, 1,4-mbtz) and rigid 5-nitroisophthalate (nbpdc) (Schemes 1 and 2) in the construction of cobalt(II) MOCPs, three Co(II) MOCPs were synthesized. The structural versatility of the three complexes show that the structures can be tuned by the position isomers. The successful synthesis of these complexes not only provides an intriguing example of a polythreaded system but also provides new perspectives to devise novel extended entanglements (Fig. 14).⁹²

Fig. 14 Influence of the position of N-donor ligands on the structures of the MOCPs. Reprinted with permission from ref. 92.

2.2.2 Copper(II) MOCPs based on the mixed ligands. On the basis of combination of 5-iodo-isophthalic acid (5-iip) (Scheme 2) ligand and various N-donor ligands, seven divalent copper(II) MOCPs have been synthesized and characterized (Fig. 15).⁹³ X-ray analysis revealed that these complexes exhibit different intriguing supramolecular architectures, in which various halogen bonding interaction are found. These results have demonstrated that the presence of iodine atom and carboxylate groups in the same system can lead to crossing-pairing such as O⋯I halogen bonding, I⋯I, or I⋯π interactions, which are effective and reliable tools in supramolecular coordination assemblies. 5-H₂iipa is an effective ligand, which is useful to better understand the synthon selectivity in multifunctional crystal structures. In addition, the introduction of N-containing auxiliary ligands into a metal–polycarboxylate system often leads to structural changes and affords new frameworks.⁹³ Recently, five new copper(II) MOCPs have been synthesized by using 4-carboxybenzaldehyde (4-fba) and 1,n-bis(imidazol-l-ylmethyl)benzene (1,n-bix; n = 2, 3, 4) (Schemes 1 and 2) as ligands. The position isomers of the N-donor ligands are the main reason for the structural differences of the Cu(II) complexes.⁹⁴

Fig. 15 Influence of the N-donor ligands on the structures of the MOCPs. Reprinted with permission from ref. 93.

It has been demonstrated that the same Cu(II)/oxalate/twisted N,N′-ligands system can give rise to different complexes with different structures and properties.⁹⁵ The different complexes are obtained by controlling the synthetic procedures. In this way, four new complexes ranging from a zwitterionic complex to 1D, 2D or 3D MOCPs have been synthesized and characterized. This approach represents an example of the versatility of coordination polymer chemistry (Fig. 16). These complexes have proven to be suitable systems to trap lattice water molecules with different morphologies. In this respect, hydrogen bonds play different, but always crucial, roles in all structures as they can stabilize networks, networks with solvent/guest and solvent/guest with themselves.⁹⁵

Fig. 16 Influence of the pH on the structures of the MOCPs. Reprinted with permission from ref. 95.

2.3 Cobalt(II)/copper(II) MOCPs based on the mixed ligands by diffusion method

Diffusion method is preferred to get single crystals suitable for X-ray diffraction analysis instead of non- or poly-crystalline products, especially if the products are poorly soluble.^96,97

Of particular interest, the self-assembly of various kinds of coordination frameworks is one of the means to obtain MOCPs with new modes of entanglements in the interpenetrating architectures. Hou et al. have studied the self-assembly of trans, trans-muconic acid (H₂muco), a ligand with longer spacer, 4,4′-bis(pyridyl)ethylene (bpe) and cobalt(II) ions by slow diffusion method, then two 3D interpenetrating structures formed in one-pot reaction have been obtained (Fig. 17).⁹⁶

Fig. 17 Two solvent accessible CPs formed in a one-pot crystallization. Reprinted with permission from ref. 96.

Interestingly, it is easy to get metallogelators from the solvent diffusion method. Dastidar et al. have reported a series of Cu(II)/Co(II) CPs derived from two bis-pyridyl-bis-amide ligands namely N,N′-bis-(3-pyridyl)-isophthalamide (1,3-bpta) and N,N′-bis-(3-pyridyl)terephthalamide (1,4-bpta), and various dicarboxylates (Scheme 3).⁹⁷

Scheme 3 The outline of the target complexes. Reprinted with permission from ref. 97.

3 The properties and applications of cobalt(II)/copper(II) MOCPs based on the mixed ligands

Cobalt(II)/copper(II) MOCPs have been of great interest in recent years, not only stem from their fascinating structures but also from their potential applications as new materials.^98–104 In this part of the review, we will present examples of how the cobalt(II)/copper(II) MOCPs based on the mixed-ligand are promising as materials for applications in the fields of electrochemistry, electrocatalysis, magnetism and photocatalysis.

3.1 Electrochemical behaviors and electrocatalytic properties of cobalt(II)/copper(II) CPs based on the mixed ligands

Recently, various efforts have been made to study electrochemistry and electrocatalytic activities of cobalt(II)/copper(II) MOCPs.^98–100 Up to now, there are several kinds of method to fabricate chemically modified electrodes (CMEs) according to the way in which materials are attached into the substrates, such as Langmuir–Blodgett (LB),¹⁰⁵ chemical vapor deposition (CVD),¹⁰⁶ self-assembling (SA),¹⁰⁷ drop-casting,¹⁰⁸ bulk modification,¹⁰⁹ etc. Among of all, chemically bulk-modified carbon paste electrodes (CPE) have been widely used in cobalt(II)/copper(II)-based CPs because of their low background current, easy fabrication and good performance.^101,110 Renewing the surface of chemically modified CPE avoids the memory effects and the contamination or deactivation of the surface between consecutive measurements.^101,110

Recently, our group has been working on the syntheses of Co(II)/Cu(II)-based complexes containing rigid/semi-rigid/flexible bis-triazole/bis-pyridyl ligands, and a series of MOCPs with various electrochemical behaviors have been constructed.^{54,55,58,67,72–79,83–89}

3.1.1 Electrochemical properties of cobalt(II) MOCPs based on the mixed ligands. Our group reported the electrochemical behavior of a Co(II) complex with a bis-pyridyl-bis-amide ligand (3-bpcb = N,N′-bis(3-pyridinecarboxamide)-1,4-benzene) and 5-aminoisophthalic acid (5-H₂AIP), [Co(3-bpcb)_0.5(5-H₂AIP)]·2H₂O], in which a bulk-modified carbon paste electrode (Co-CPE) was used to measure the cyclic voltammograms at different scan rates in 0.1 M H₂SO₄ aqueous solution.⁷⁰ Fig. 18 shows the cyclic voltammetric behavior for Co-CPE in 0.1 M H₂SO₄ aqueous solution at different scan rates. Redox peaks I–I′ correspond to one-electron process. When the scan rates were varied from 40 to 500 mV s⁻¹, the peak currents were proportional to the scan rates, which indicated the redox process of Co-CPE is surface-controlled. Although the electrochemical behaviors of some MOCPs based on Co(II) ions have been reported, their electrochemical properties are different.^{54,55,58,75,76,83,88,89} The mean peak potentials E_1/2 = (E_pa + E_pc)/2 ranges from −34 to 660 mV. The differences of the mean peak potential among Co(II) complexes may be ascribed to the different electrolyte solutions, the different coordination environments of Co(II) ions and their final diverse structures.

Fig. 18 Cyclic voltammograms of the Co-CPE in 0.1 M H₂SO₄ aqueous solution at different scan rates (from inner to outer: 40, 60, 80, 100, 120, 140, 160, 180, 200, 250, 300, 350, 400, 450 and 500 mV s⁻¹). The inset shows the plots of the anodic and cathodic peak currents against scan rates. Reprinted with permission from ref. 76.

It is well known that the electroreduction of nitrite requires a large overpotential at most electrode surfaces and no obvious response is observed at the bare CPE at the presence of the nitrite. However, for Co-CPE, with the addition of nitrite, the reduction peak currents increase gradually while the corresponding oxidation peak currents decrease dramatically in the range of +300 to −250 mV (Fig. 19). The result indicates that Co-CPE has good electrocatalytic activity toward the reduction of nitrite.

Fig. 19 Cyclic voltammograms of the bare CPE in 0.1 M H₂SO₄ solution containing 2.0 mmol L⁻¹ KNO₂ (a), and Co-CPE in 0.1 M H₂SO₄ solution containing (b–e): 0.0, 2.0, 4.0 and 6.0 mmol L⁻¹ KNO₂. Scan rate: 100 mV s⁻¹. Reprinted with permission from ref. 76.

3.1.2 Electrochemical properties of copper(II) MOCPs based on the mixed ligands. Among those important properties of copper(II) complexes, the ability to undergo reversible mono-electron redox process endows them to be very attractive as the potential electrochemical and electrocatalytic materials. As shown in Fig. 20, the voltammetric behavior of the copper(II) MOCP bulk-modified carbon paste electrode (Cu-CPE) in the 0.1 M phosphates buffer aqueous solution (pH = 2) at different scan rates was recorded,¹⁰¹ exhibiting quasi-reversible peaks in the potential range of +0.6 to −0.3 V, attributable to the redox of Cu(II)/Cu(I). Among the Cu-CPEs, their mean peak potentials E_1/2 = (E_pa + E_pc)/2 range from −133 to 597 mV.^{55,67,72–75,77–79,84–88} The differences of the mean peak potentials among Cu(II) complexes could be ascribed to their different electrolyte solutions, the different coordination environments of Cu(II) ions and their final frameworks.

Fig. 20 Cyclic voltammograms of Cu-CPE in 0.1 M phosphates buffer solution (pH = 2) at different scan rates (from inner to outer) 50, 70, 90, 120, 150, 200, 250, 300, 350 and 400 mV s⁻¹. Inset: plots of peak currents vs. scan rate. Potential vs. SCE. Reprinted with permission from ref. 101.

Bromate is a disinfectant by-product contaminant found in drinking water, and is formed during the ozonation of source water containing bromide.¹⁰² The overpotential for BrO₃⁻ reduction is high and therefore an efficient electrocatalyst would be beneficial.¹⁰² Various types of systems have been reported for bromate detection in the recent years.¹⁰³ Our work was to fabricate a Cu(II) complex bulk-modified CPE capable of the electrocatalytic reduction of bromate.¹⁰¹ The peak currents for cathodic reduction of bromate are proportional to the bromate concentrations (Fig. 21a).¹⁰¹ These results indicate that at sufficiently negative potential the reaction is controlled by surface-confined of the bromate species, which is the ideal case for quantitative applications.

Fig. 21 Cyclic voltammograms of a bare CPE containing 2 mM KBrO₃ (a) KNO₂ (b) H₂O₂ (c) and a Cu-CPE in pH = 2 phosphate buffer solution containing different BrO₃⁻ (a) NO₂⁻ (b) H₂O₂ (c) concentrations. Scan rate: 50 mV s⁻¹. Inset: the variation of cathodic peak currents vs. bromate (a) nitrite (b) hydrogen peroxide (c) concentrations. Reprinted with permission from ref. 101.

Some reports on electrocatalytic reduction of nitrite at the surface of MOCPs bulk-modified CPEs have been studied.^55,76,85,101 Our work indicates that the Cu(II) complex bulk-modified CPE also shows good electrocatalytic activity toward the reduction of nitrite.¹⁰¹ With addition of nitrite, the reduction peak currents increase markedly while the corresponding oxidation peak currents decrease markedly (Fig. 21b). The electrochemical catalytic pathway is probably the reduction of NO₂⁻ to NO and then further reduction to N₂O in the aqueous solution.^55,76,85,101

H₂O₂ is usually employed in pollution control, bleaching of textile and paper products and in the processing of petrochemicals, minerals, food and various products.¹⁰⁴ Moreover, H₂O₂ is the product of various oxidases in countless biological reactions, which has been used in the development of first generation biosensors.¹⁰⁴ As is known, the electroreduction of H₂O₂ requires a large overpotential. Fig. 21c proves that the cathodic currents increase while the corresponding anodic currents decrease at the Cu-CPE. The peak currents for cathodic reduction of hydrogen peroxide are proportional to the concentration of H₂O₂. The process of electroreduction of hydrogen peroxide at the surface of Cu-CPE is similar to that of the bromate and the nitrite.¹⁰¹

3.2 Magnetism properties of cobalt(II)/copper(II) MOCPs based on the mixed ligands

Cobalt(II)/copper(II)-based MOCPs strategy is furthermore employed for the design of molecular-based magnets.^{2,9,27,111–113} Indeed antiferromagnetism, ferrimagnetism and ferromagnetism are cooperative phenomena of the magnetic spins within a solid. They require an interaction or coupling between the spins of the paramagnetic centres.^2,9 The building of metal–organic frameworks allows the choice of the coupling parameters. The magnetic coordination polymers have to own a residual permanent magnetization at zero-field for an as high as possible Tc (critical temperature).^2,9

3.2.1 Magnetism properties of cobalt(II) MOCPs based on the mixed ligands. Recently, magnetic behavior is found for cobalt(II)/copper(II) MOCPs built up with the mixed ligands containing polycarboxylate and N-donor ligands. With the use of polycarboxylates, the polynuclear cobalt(II)/copper(II) clusters will be easily constructed, because the carboxyl groups may induce core aggregation or link these discrete clusters into extended networks, which may be ascribed to their various coordination modes that provide superexchange pathways for magnetic coupling among paramagnetic cobalt(II)/copper(II) ions.^{9,27,111–114} For example, two new cobalt(II) MOCPs, namely {[Co₅(μ₃-OH)₂(m-pda)₃(bix)₄]·2ClO₄}_n and {[Co₂(p-pda)₂(bix)₂(H₂O)]·H₂O}_n, were prepared by hydrothermal reactions of cobalt(II) salt with two isomeric dicarboxyl tectons 1,3-phenylenediacetic acid (m-pda) and 1,4-phenylenediacetic acid (p-pda, Scheme 1), along with 1,3-bis(imidazol-L-ylmethyl)benzene (bix, Scheme 2).¹¹¹ Magnetic susceptibility measurements indicate that both of the two complexes show weak antiferromagnetic interactions between the adjacent Co(II) ions (as shown in Fig. 22).¹¹¹

Fig. 22 The weak antiferromagnetic interactions between the adjacent Co(II) ions. Reprinted with permission from ref. 111.

As part of efforts in developing new magnets, Bu and co-workers have obtained three new Co(II) MOCPs with a semi-dicarboxylate-like ligands as the subligands.¹¹⁴ The investigation flexible di(1H-imidazol-1-yl)methane ligand and different of the magnetic properties of these complexes demonstrates that they all display FM coupling between Co(II) ions but AFM interaction between Co₃ or Co₂ units (Fig. 23).

Fig. 23 χ_mT vs. T plots of the complexes at 1 kOe and 1/χ_m vs. T plots of the complexes, respectively. The red solid lines are the best fits to the Curie–Weiss laws, respectively.

The magnetic properties of cobalt(II) MOCPs have generally been well explored and they illustrate the potential of producing magnets of many different ferrimagnet,^{21,24,48,49,114} antiferromagnet^{21,22,25,38,43,45–57,89,111} and only a few unusual single molecular magnet.²⁶

3.2.2 Magnetism properties of copper(II) MOCPs based on the mixed ligands. Li and co-workers have illustrated the magnetic behavior of a interesting copper(II) MOCP {[Cu₉(OH)₆(bte)₂(sip)₄(H₂O)₃]·6H₂O}_n, which is prepared by the hydrothermal reaction of Cu(NO₃)₂, 1,2-bis(1,2,4-triazol-1-yl)ethane (bte) and 5-sulfoisophthalate (sip) and displays a novel 3D metal–organic coordination network (Fig. 24).¹¹² This copper(II) MOCP shows dominant antiferromagnetic coupling interactions and good photocatalytic activity for the degradation of methyl orange.¹¹² In addition, LaDuca and co-workers have generated a serious of copper(II) MOCPs with diverse structures by hydrothermal assembly of the divalent copper salt, aromatic polycarboxylates, and some isomeric dipyridylamide ligands.^27,113 Adjustment of nitrogen donor geometric position at the carbonyl-bearing terminus of the dipyridylamide ligand resulted in very significant differences in topology and magnetic properties (Fig. 25).^27,113 Our group also reported a series of Cu(II) MOCPs based on polycarboxylate and bis-triazole/bis-pyridyl mixed ligands with various structures and magnetic properties.^{67–69,84,89}

Fig. 24 (a) The [Cu₉(μ₃-OH)₆] enneanuclear copper(II) cluster; (b) plot of μ_eff vs. T; (c) Plot of 1/χ_m vs. T. Reprinted with permission from ref. 112.

Fig. 25 χ_mT(T) plots. The best fit to eqn is shown as a thin dark line, respectively. Reprinted with permission from ref. 113.

It is clear that copper(II) MOCPs will continue to provide new properties and improve existing knowledge of their structures as well as their magnetic properties. Most of the magnetic properties of copper(II) MOCPs show antiferromagnet^{67–69,84,9,112} and less of ferrimagnet.²⁷

3.3 Photocatalytic properties of cobalt(II)/copper(II) MOCPs based on the mixed-ligands

Photocatalysis is a “green” technology for the treatment of all kinds of contaminants, which has many advantages over other treatment methods. For instance, the use of the environmentally friendly oxidant O₂, which can realize experiments under the ambient temperature reaction condition, and complete the oxidation of the MOCPs, even at low concentrations.^115,116 To date, TiO₂ has undoubtedly proven to be the most excellent photocatalyst for the oxidative decomposition of many organic pollutants under UV irradiation.^117,118 Furthermore, the development of photocatalysis as applied, in particular, to organic pollutants concentration abatement or removal from wastewater under ultraviolet light has received great research attention in catalysis field.^115–118 A new emerging application of cobalt(II)/copper(II) MOCPs is photocatalysis, and some MOCPs have been demonstrated to be efficient photocatalysts on the green degradation of organic pollutants.^119–121

3.3.1 Photocatalytic properties of cobalt(II) MOCPs based on the mixed ligands. A 4-connected cobalt(II) MOCP, [Co₂(L1)(L2)]·4.25H₂O, based on tetrakis[4-(carboxyphenyl)-oxamethyl]methane acid (L1) and tetrakis(imidazol-1-ylmethyl)methane (L2) has been synthesized under hydrothermal conditions. The photocatalytic activities for dye (MB, RhB and X₃B) degradation under either UV or visible-light irradiation have been reported (Fig. 26).¹²¹ Our group has selected a series of flexible/semi-rigid/rigid bis-pyridyl ligands as the main ligands combining with polycarboxylates to construct cobalt(II) MOCPs, investigating the photocatalytic properties of target cobalt(II) MOCPs under UV/vis irradiation.^81,82,88,89 The difference in the catalytic activity under UV/vis irradiation among them may trace back to the discrepancy in the different structures of the cobalt(II) MOCPs.

Fig. 26 The UV-vis absorption spectra of the MB (a)/RhB (c)/X₃B (e) solution during the decomposition reaction. Photocatalytic degradation of MB (a) RhB (c) X₃B (e) solution under UV with the use of the complex; the black curve is the control experiment without any catalyst. Reprinted with permission from ref. 121.

3.3.2 Photocatalytic properties of copper(II) MOCPs based on the mixed ligands. The relatively wide band gap limits further application of the photocatalytic material in the visible-light region (λ > 400 nm).^117,118 In view of the efficient utilization of visible light, the largest proportion of the solar spectrum and artificial light sources, the development of a photocatalyst with high activity under a wide range of visible-light irradiation is indispensable.¹²² Currently, five new copper(II) MOCPs with the conjugated 1,2,4,5-benzenetetracarboxylic acid (btec) and 4,4′-bis(1-Imidazolyl)biphenyl (bimb) (Schemes 1 and 2) have been obtained and found to show different structures and topologies.¹²² Remarkably, the presence of visible regions transitions motivated scientist to explore applications of these complexes in heterogeneous photocatalysis. These complexes exhibit photocatalytic activities higher than that of commercial TiO₂ (Degussa P-25) under visible irradiation for the degradation of X₃B as a model pollutant, which is recognized as being difficult to decompose. Meanwhile, the synergistic effect of H₂O₂ and MOCP on the photodegradation of X₃B has been studied, which could obviously enhance the degradation rate of X₃B under visible light. The successful synthesis of these complexes provides access to a promising path in the search for stable new visible light-driven photocatalysts (Fig. 27).¹²² On the other hand, our group have studied the photocatalytic activities of lots of copper(II) MOCPs toward the degradation of MB/MO/RhB in UV light to detect the photocatalytic efficiencies in the wastewater treatment, which implying that such copper(II) MOCPs might be potential photocatalysts.^{79–82,86,88}

Fig. 27 Concentration changes of X₃B as a function of irradiation time for the complex. Conditions: (I) H₂O₂/complex/dark, (II) complex/visible light, (III) H₂O₂/visible light, (IV) H₂O₂/complex/visible light. Reprinted with permission from ref. 122.

4 Conclusions and outlook

The aim of this review is to present a brief summary to introduce the assemblies and structures of cobalt(II)/copper(II)-based mixed-ligand MOCPs with potential applications in electrocatalysis, magnetism, and photocatalytic properties. In this direction, the most effective approach for construction of cobalt(II)/copper(II) MOCPs is to combine both bis-imidazole/bis-triazole/bis-pyridyl derivatives and polycarboxylates by the hydro(solvo)thermal and diffusion methods. Further, the design of organic ligands is mostly practiced by modifying the ligand backbones containing bis-imidazole/bis-triazole/bis-pyridyl functional groups. The mixed-ligand synthetic strategy will be highly affected by the organic ligands (such as the spacers, positional isomer, substituent groups and the types of organic ligands) and some other factors including solvent, pH, metal ion, reactants ratio and synthetic route. These can establish an essential methodology for the rational design and construction of the desired MOCPs with promising properties and potential applications.

Although reports on cobalt(II)/copper(II)-based mixed-ligand MOCPs are ever increasing, it is important to recognize that controlling the outcome in terms of the resultant prospective structures and functions is indeed a daunting task. This is because formation of MOCPs is a result of crystallization, which is influenced by numerous and often not controllable factors such as solvent, pH, metal ion, reactants ratio, synthetic route, etc. Generally speaking, the results obtained are unpredictable. Therefore, efforts should be made to understand various factors that influence the formation of the resultant MOCPs. On the other hand, although much of the work during the last decade was motivated by promising properties and potential applications, such as electrocatalysis, magnetism and photocatalysis, several important discoveries in the last few years are opening new and significant opportunities for cobalt(II)/copper(II)-based mixed-ligand MOCPs beyond the gas storage and separations. We foresee that there may be a burgeoning field of porous cobalt(II)/copper(II)-based mixed-ligand MOCPs, which should have a brilliant future and are undergoing explosive growth.

Acknowledgements

The supports of the National Natural Science Foundation of China (no. 21171025, 21471021), New Century Excellent Talents in University (NCET-09-0853) and Program of Innovative Research Team in University of Liaoning Province (LT2012020) are gratefully acknowledged.

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