An Overview of Various Applications of Cadmium Carboxylate Coordination Polymers

This review highlights the most recent applications of Cd(II)-carboxylate-based coordination polymers (Cd(II)-CBCPs), such as sensors, catalysts, and storage materials, in comparison with those of Zn(II) counterparts. A wide range of species with luminescence properties were designed by using proper organic fluorophores, especially a carboxylate bridging ligand combined with an ancillary N-donor species, both with a rigid structure. These characteristics, combined with the arrangement in Cd(II)-CBCPs’ structure and the intermolecular interaction, enable the sensing behavior of a plethora of various inorganic and organic pollutants. In addition, the Lewis acid behavior of Cd(II) was investigated either in developing valuable heterogeneous catalysts in acetalization, cyanosilylation, Henry or Strecker reactions, Knoevenagel condensation, or dyes or drug elimination from wastewater through photocatalysis. Furthermore, the pores structure of such derivatives induced the ability of some species to store gases or toxic dyes. Applications such as in herbicides, antibacterials, and electronic devices are also described together with their ability to generate nano-CdO species.


Introduction
Cd(II) complexes similar to Zn(II) might seem unattractive to researchers due to their diamagnetism.
Similarly, a comparison between these ions involving their relationship with living organisms indicates that zinc is an essential element while cadmium is recognized for its toxicity.Cd(II) toxicity is well documented: WHO includes it in the list of the most dangerous chemicals and the International Agency for Research on Cancer lists it as a group 1b carcinogen [1,2].Data indicate its accumulation in the human body from the food chain [3] and water sources [4], breathing, smoking and exposure to particles resulting from ore exploitation and processing [5,6].Some human activities such as combustion of fossil fuels, waste, and metal ores are also sources of the spread the cadmium into environment [4] from where it can be also retained by humans over time.Fortunately, cadmium toxicity and its adverse effects on the human body together with its elimination through chelation therapy are well documented [4][5][6][7][8][9].
On the other hand, similar to Zn(II), Cd(II) exhibits a good Lewis acid activity that allows its involvement in several catalytic processes.But, despite the continuous interest in the structural aspects of Cd(II) species, research contributions describing such applications are currently limited due to its toxicity [10].Moreover, its versatile coordination behavior generates valuable applications as sensors based on luminescent properties for species with properly selected ligands [11,12].Furthermore, its ability to adopt various stereochemistry allows for a diverse structural arrangement in the network, which generates pores and thus Molecules 2024, 29, 3874 2 of 32 induces the ability of some materials to store gaseous or solid species similar to the Zn(II) counterpart [13].
Some of these properties are characteristic of a polymeric structure, which can be achieved by a suitable molar ratio and by mixing a carboxylate derivative with another appropriate bridging ligand.Usually, polycarboxylate derivatives possess an enhanced ability to generate two-or three-atom connections between metallic ions in coordination polymer (CP) structures (Figure 1) [11,14].
As can be seen from Figure 1, besides the most common two-or three-atom bridges, other bridges of up to seven atoms connected in the network structure have also been reported.It is worth mentioning that in some complex structures, two or three different coordination modes of the bridging carboxylate group may even appear.Hence, generally, several aromatic and heterocyclic two-, three-or tetracarboxylate derivatives have usually been used for the synthesis of Cd(II)-carboxylate-based coordination polymers (Cd(II)-CBCPs) with diverse structures and properties.
achieved by a suitable molar ratio and by mixing a carboxylate derivative with another appropriate bridging ligand.Usually, polycarboxylate derivatives possess an enhanced ability to generate two-or three-atom connections between metallic ions in coordination polymer (CP) structures (Figure 1) [11,14].

Dicarboxylate backbone
Tricarboxylate backbone Tetracarboxylate backbone As can be seen from Figure 1, besides the most common two-or three-atom bridges, other bridges of up to seven atoms connected in the network structure have also been reported.It is worth mentioning that in some complex structures, two or three different coordination modes of the bridging carboxylate group may even appear.Hence, generally, several aromatic and heterocyclic two-, three-or tetracarboxylate derivatives have usually been used for the synthesis of Cd(II)-carboxylate-based coordination polymers (Cd(II)-CBCPs) with diverse structures and properties.
A burgeoning class of adsorbent materials is represented by metal-organic frameworks (MOFs), species with desirable attributes such as a tunable crystal structure and morphology, ultrahigh specific surface area and porosity, adaptable surface chemistry, high thermal stability, open metal sites, and high degree of crystallinity and robustness [15,16].They also exhibit high compatibility with other materials leading to composites with valuable physical and chemical properties [17].Current efforts in the field are centered on enhancing the specific surface area and the gas-binding affinity, both favored by Cd(II) cations due to their large volume.
Conversely, by comparison with other CBCPs, it is rather difficult to control both morphology and structure [16] as well as create a mixture either of proper organic linkers (carboxylate and a N-donor ligand) or to combine Cd(II) with another cation in the structure of this kind of species.
On the other hand, Cd(II), similar to Zn(II), shows advantages in developing CPs.These advantages are supported by its d 10 configuration that allows for a flexible stereochemistry and consequently for its geometries to accommodate ligand parameters easily.It varies from tetrahedral stereochemistry through trigonal, bipyramidal, and square pyramidal to octahedral stereochemistry, adopting different distortion degrees.The stereochemistry control is achieved by a proper selection of either ligands or molar ratio.
A literature survey evidenced that some carboxylate derivatives, di-and three-carboxylate benzene derivatives with a rigid backbone, exhibit the ability to develop polymeric structures by coordination with Cd(II).Their packing features can then be controlled through polydentate species acting as bridges, either a carboxylate alone or in combination with another ligand.These supplementary ligands are usually heterocycle Nbased derivatives, such as pyridine, thiazole, imidazole, bipyridine, phenanthroline, or a combination of such derivatives.Besides modulation of both topology and porosity, these species can also provide an additional fluorophore source into the complex.
As a result of the rich structural chemistry of Cd(II)-CBCPs, these compounds exhibit potential applications as sensors for an inorganic or organic compound [12,15], materials A burgeoning class of adsorbent materials is represented by metal-organic frameworks (MOFs), species with desirable attributes such as a tunable crystal structure and morphology, ultrahigh specific surface area and porosity, adaptable surface chemistry, high thermal stability, open metal sites, and high degree of crystallinity and robustness [15,16].They also exhibit high compatibility with other materials leading to composites with valuable physical and chemical properties [17].Current efforts in the field are centered on enhancing the specific surface area and the gas-binding affinity, both favored by Cd(II) cations due to their large volume.
Conversely, by comparison with other CBCPs, it is rather difficult to control both morphology and structure [16] as well as create a mixture either of proper organic linkers (carboxylate and a N-donor ligand) or to combine Cd(II) with another cation in the structure of this kind of species.
On the other hand, Cd(II), similar to Zn(II), shows advantages in developing CPs.These advantages are supported by its d 10 configuration that allows for a flexible stereochemistry and consequently for its geometries to accommodate ligand parameters easily.It varies from tetrahedral stereochemistry through trigonal, bipyramidal, and square pyramidal to octahedral stereochemistry, adopting different distortion degrees.The stereochemistry control is achieved by a proper selection of either ligands or molar ratio.
A literature survey evidenced that some carboxylate derivatives, di-and three-carboxylate benzene derivatives with a rigid backbone, exhibit the ability to develop polymeric structures by coordination with Cd(II).Their packing features can then be controlled through polydentate species acting as bridges, either a carboxylate alone or in combination with another ligand.These supplementary ligands are usually heterocycle N-based derivatives, such as pyridine, thiazole, imidazole, bipyridine, phenanthroline, or a combination of such derivatives.Besides modulation of both topology and porosity, these species can also provide an additional fluorophore source into the complex.
The most recent data on the above-mentioned applications are summarized by categories in the following sections.
The most recent data on the above-mentioned applications are summarized by categories in the following sections.Once the main objectives of this review paper were established, a literature review was conducted to identify the most relevant papers that address our objectives.For this purpose, databases were used, such as Clarivate Analytics' Web of Science (WoS), Elsevier's Scopus (Scopus), and Science Direct, for searching papers.The following keywords and/or combinations of keywords have been used: "cadmium coordination polymer", "cadmium" + "carboxylate ligand", "cadmium" + "carboxylate complexes" + "luminescence"/"sensor"/"storage"/"catalyst"/"adsorbent"/"properties".This procedure helped identify several categories of papers (reviews, reports, communications), from which the suitable ones have been selected for this review, and classified using properties and applications as criteria.Furthermore, the reference lists of these papers were scanned to identify further related papers.As a result, the database obtained was enriched with additional papers discovered with the snowballing technique applied to initially classified papers.All the classified papers were used for extracting data that was further included in the review paper, emphasizing the applicability of cadmium carboxylate coordination polymers.
As today's area of coordination polymers is vast and developing fast, it was considered suitable that this review includes papers published in the last decade.

Basic Applications of Cadmium (II) Carboxylate Coordination Polymers
The Cd(II) species described below either exhibit fluorescence or possess stable and robust framework structures with a proper porosity that allows retention, sometime selective, of small guest molecules like gases, dyes, or drugs.Based on photocatalytic properties, some Cd(II)-CBCPs were also studied as potential agents able to remove dyes or drugs from wastewater, while additional applications include their use as herbicides, antibacterial species, as drugs in uterine fibrosis treatment, and in electronic devices like gate dielectrics, proton conductive materials, or cathodes in Li-Se batteries.Special attention is also paid to the use of such CPs for the nano-CdO generation, another material with valuable uses is several fields.Once the main objectives of this review paper were established, a literature review was conducted to identify the most relevant papers that address our objectives.For this purpose, databases were used, such as Clarivate Analytics' Web of Science (WoS), Elsevier's Scopus (Scopus), and Science Direct, for searching papers.The following keywords and/or combinations of keywords have been used: "cadmium coordination polymer", "cadmium" + "carboxylate ligand", "cadmium" + "carboxylate complexes" + "luminescence"/"sensor"/"storage"/"catalyst"/"adsorbent"/"properties".This procedure helped identify several categories of papers (reviews, reports, communications), from which the suitable ones have been selected for this review, and classified using properties and applications as criteria.Furthermore, the reference lists of these papers were scanned to identify further related papers.As a result, the database obtained was enriched with additional papers discovered with the snowballing technique applied to initially classified papers.All the classified papers were used for extracting data that was further included in the review paper, emphasizing the applicability of cadmium carboxylate coordination polymers.
As today's area of coordination polymers is vast and developing fast, it was considered suitable that this review includes papers published in the last decade.

Basic Applications of Cadmium (II) Carboxylate Coordination Polymers
The Cd(II) species described below either exhibit fluorescence or possess stable and robust framework structures with a proper porosity that allows retention, sometime selective, of small guest molecules like gases, dyes, or drugs.Based on photocatalytic properties, some Cd(II)-CBCPs were also studied as potential agents able to remove dyes or drugs from wastewater, while additional applications include their use as herbicides, antibacterial species, as drugs in uterine fibrosis treatment, and in electronic devices like gate dielectrics, proton conductive materials, or cathodes in Li-Se batteries.Special attention is also paid to the use of such CPs for the nano-CdO generation, another material with valuable uses is several fields.
Table 1 presents examples of Cd(II)-carboxylate-based coordination polymers with diverse applications as well as the ligands' nature, synthesis method, and characteristics.

Sensors Based on Cadmium (II) Carboxylate Coordination Polymers
Some water pollutants are harmful for both humans and the environment.These species are the result of industrial, hospital, and domestic activities; they are antibiotics, dyes, and nitro-derivatives, as well as cationic and anionic inorganic harmful compounds [22].Hence, their monitoring requires both sensitive and selective species such as fluorescent Zn(II) and Cd(II) complexes [12,15].Similar to Zn(II) species, the luminescence of Cd(II)-CBCPs can be related to π-electron-rich fluorescent ligands.The luminescence in these cases can originate from intra-ligand (IL), ligand-to-ligand (LLCT), or metal-to-ligand (MLCT) spin-allowed charge-transfer processes [11,12].The role of a fluorescent ligand can be played by the carboxylate linker, the supplementary N-donor ligand, or both.Since d 10 ions such as Zn(II) and Cd(II) have the ability to regulate the emission wavelength of organic materials, several species based on benzene polycarboxylate rigid tectons were developed as luminescent materials , some of them with the ability to selectively detect certain organic or inorganic species.
The sensing of nitroaromatic compounds in aqueous media is an important aspect for the protection of both the environment and human health.For this purpose, Cd(II) CP [Cd 3 (bpy) 3 (cia) 2 ] n (2) was characterized as a one-dimensional ladder-like chain based on 5-((4-carboxybenzyl) oxy) isophthalic acid (5-H 3 cia) and 2,2 ′ -bipyridine (2,2 ′ -bpy) (Figure 2b).The compound detects nitrobenzene (NBZ) in an aqueous solution with high sensitivity and selectivity, with a limit of detection (LOD) of 3.03 × 10 −9 M. The fluorescence-quenching mechanism consists of NB absorption in the pore, thus blocking thus the LLCT [46].4-phenylenebis (1H-imidazole-2,4,5-triyl))tetrabenzoic acid; DMF = N,N ′ -dimethylformamide) was successfully constructed under solvothermal conditions.The structural analysis revealed a 3D framework and exhibited good stability in aqueous solutions within the pH range between 4 and 12.Its intense luminescence emission could be used to quickly and sensitively detect Fe(III), MnO 4 − , and 2,4,6-trinitrophenol (TNP) in aqueous solutions with a high quenching constant and a low detection limit, even in the presence of other competitor ions [47].
The detection of Cu(II) and Ni(II) cations in an aqueous medium was investigated for [Cd 2 (btc) 2 (phen) 2 (H 2 O) 2 ] n (16) (H 3 btc = 1,3,5-benzentricarboxylic acid; phen = 1,10phenantroline) based on their fluorescence properties.For both cations, the detection is achieved with a fast response, a wide linear range, and a very low detection limit.These features allow the use of CP for real water samples without any matrix interference [58].
naphthalene, ACN = acetonitrile) adopts a 4-fold interpenetrated network.This species behaves as a bis-color excited fluorescent sensor with a high sensitivity to vitamin B2.Interestingly, at an excitation wavelength of 230 nm, this was significantly quenched by a pesticide called nitenpyram, while at an excitation wavelength of 290 nm, the highest fluorescence quenching was achieved for imidacloprid [67].
), exhibits fascinating one-dimensional in-plane channels functionalized with active pyridine-N sites, and an excellent water and chemical stability.Furthermore, a species with a dual-emissive ratiometric fluorescent sensor for 2-(2-methoxyethoxy) acetic acid (MEAA) was obtained by Eu(III) functionalization.This aspect is important because MEAA is a metabolite of 2-(2-methoxyethoxy)ethanol, which affects the DNA and, as a result, exhibits teratogenic and toxic effects [68].
To conclude, in order to obtain Cd(II)-CBCPs with a broad range of emission energy, the selection of multifunctional organic ligands is of utmost importance.Data indicated that the polycarboxylate ligands, especially those derived from rigid aromatic derivatives with both a large π-conjugated backbone and the ability to act as bridge, are excellent candidates to design such fluorescent-based materials.The properties can be further modulated by the appropriate selection of a second N-donor ligand (i.e., pyridine, imidazole, thiadiazole derivative) exhibiting the same characteristics as carboxylate ligands.
The fluorescence quenching by inorganic or organic species with a volume that matches the pore dimensions enables their specific recognition and thus the possibility to develop sensors that may be used in the monitoring process of pollutants.
The Cd-CBCPs presented have proven their efficacy for the detection of different drugs and bioactive species (STZ, NFZ, SDZ, NFT, TEC, NOR, AA), cations (Fe(III), Cu(II), Ni(II)), anionic species (CrO  To conclude, in order to obtain Cd(II)-CBCPs with a broad range of emission energy, the selection of multifunctional organic ligands is of utmost importance.Data indicated that the polycarboxylate ligands, especially those derived from rigid aromatic derivatives with both a large π-conjugated backbone and the ability to act as bridge, are excellent candidates to design such fluorescent-based materials.The properties can be further modulated by the appropriate selection of a second N-donor ligand (i.e., pyridine, imidazole, thiadiazole derivative) exhibiting the same characteristics as carboxylate ligands.
The fluorescence quenching by inorganic or organic species with a volume that matches the pore dimensions enables their specific recognition and thus the possibility to develop sensors that may be used in the monitoring process of pollutants.
All compounds described above as sensors are neutral species that exhibit a robust network, being both water and pH stable.Also, these share a common characteristic of the excitation wavelength in the UV domain, considering the fact that this comes from π→π* or n→π* transitions of the ligands, usually the N-based ancillary one.
An analysis of species described in this section indicates that the sensing ability of Cd-CBCPs comes from a correlation between fluorescence quenching and architecture All compounds described above as sensors are neutral species that exhibit a robust network, being both water and pH stable.Also, these share a common characteristic of the excitation wavelength in the UV domain, considering the fact that this comes from π→π* or n→π* transitions of the ligands, usually the N-based ancillary one.
An analysis of species described in this section indicates that the sensing ability of Cd-CBCPs comes from a correlation between fluorescence quenching and architecture collapse in interaction with a given analyte.For the majority of these species, the former process can be assigned either to the photoinduced electron transfer (PET) or to the inner filter effect (IFE).The PET effect is involved in quenching when the Cd-CBCP material has a LUMO with a higher energy than the analyte, and as a result, the electron transfer from the fluorescent material to the analyte occurs.When the excitation radiation of the fluorescent species is absorbed by the analyte, then the IFE mechanism occurs.Obviously, a combination of these two mechanisms is also possible.

Catalysts Based on Cadmium (II) Carboxylate Coordination Polymers
It has been proven that cadmium (II) centers present good catalytic performances based on their Lewis acid ability.Also, the identification of heterogeneous catalysts derived from CPs is a topic of great interest.These catalysts are more desirable than the homogeneous ones, for example, due to their easy recovery.From this perspective, the efficacy of CPs is due to their superiority over other materials, since CPs present a high surface area, large pores, and a high number of Lewis acid centers [70].
Consequently, CP formulated as [Cd(ipc)(Cl)(H 2 O)] n (32) (Hipc = 5-imidazol-1-yl)-2-pyridine carboxylic acid) (Figure 3a) was evaluated for its catalytic activity in the acetalization reaction and the results indicated that it is an efficient Lewis acid catalyst.As the lifetime and reusability of catalysts are important indicators for their efficient use, the complex (32) was tested from these perspectives and it has given a 78, 81, and 82% yield for the second, third, and fourth runs, respectively.This catalyst can be readily recovered after being processed and reused [71].
Good Lewis catalytic activity of complex { bromide) in a cyanosilylation reaction has been demonstrated by a high yield of the product obtained (93%), easy recovery, and the potential for reuse.Although the authors in [72] stated that it is possible to determine the catalytic activity by the cadmium ions with coordinated water molecules or a pyridinium unit, the catalytic centers of the cyanosilylation reaction are subjected to further studies.
Another CP formulated, [Cd 2 (1,4-ndc) 2 (DMF) 2 ] n (34), was reported [73] due to catalytic activity for a cyanosilylation reaction of aromatic aldehydes with nitro substituents in different positions.This was the result of the presence of micropores inside the structure after partial dissociation of DMF molecules. The acid) were tested as heterogeneous catalysts for the Henry reaction [74].According to the literature [75], the Henry reaction, also known as the nitroaldol reaction, is a valuable tool used for C-C bond formation and the resultant product (nitroaldol) may lead to different oxygen-and nitrogen-containing derivatives.Also, the Henry reaction requires catalysts and many studies are investigating what the appropriate catalyst or catalytic system is.As for CPs (35) and (36), the catalytic activity was explored in the transformations of various aldehyde substrates and nitroethane to obtain corresponding β-nitro alcohol products.The results revealed that complex (35) is more efficient than (36).The yield obtained was 57 and 25%, respectively.
Another reaction catalyzed by CPs is Knoevenagel condensation, which consists of the formation of C-C bonds during condensation of an aldehyde or ketone with active methylene groups under acidic or alkaline conditions [76].For example, complex [Cd(1,2-bdc-OH)(DMF) 2 •DMF] n (37) was tested as heterogeneous catalyst in Knoevenagel condensation to obtain benzylidene malononitrile.The results indicated that small amounts of complex produce, within three minutes, a rapid conversion to 82%, which after 30 min, reaches 92%, and after 60 min, is up to 94%.These findings are more so an indicator of the excellent catalytic activity of (37) as it is used in small amounts and it presents tolerable reusability after four cycles [77].(38) (2,7-H 2 cdc = 9H-carbazole-2,7dicarboxylic acid) composed from negatively charged 3D frameworks with 2-fold interpenetration was reported [78] to present catalytic activity in a Knoevenagel reaction, more specific for reactions between benzaldehyde and different methylene substrates in a DMF medium.The results were satisfactory since for malononitrile, the conversion after 2 h reached 100% at room temperature.
In addition, complex {[Cd(hipamifba 3c) has proven its efficacy as a heterogeneous catalyst for the Strecker reaction involving aromatic and cyclic ketones and aromatic aldehydes to obtain high yields of α-aminonitriles that are part of various clinical drugs [81].
The hydrogen evolution reaction (HER) is the easiest electrochemical manner of producing high-purity hydrogen using a catalyst [82].The importance of this reaction is easily understood, even more so as hydrogen is an important fuel for the future.Also, finding proper catalysts is an important matter.Therefore, CP {[Cd 2 (ddb)(Hbimb)]•3H 2 O} n (21) (Hddb = 3,5-di(2 ′ ,4 ′ -dicarboxylphenyl)benzoic acid; Hbimb = ortho-bis(imidazole-1ylmethyl)benzene) has been tested for this and the results supported its potential as an electrocatalyst for HER [63].
On top of the catalytic effect of CPs over different types of chemical processes (Henry reaction, Knoevenagel condensation, cyanosilylation, etc.), there is also their catalytic performances on the degradation of different organic dyes.The challenge with organic dyes is to find the suitable catalyst for their decomposition since they are broadly used in various domains due to accessible costs and the variety of colors, but are hazardous for the environment (they degrade slowly and are nontoxic).
The exploration of the photocatalytic effects of CPs was extended to methyl violet (MV) degradation by Cai and coworkers [53], who investigated the degradation of MV in polluted waters under effect of complex (11).Also, complexes (25) and (26) were reported due to their high catalytic activity for MV dye degradation [65].
The photocatalytic performances of { (47) (1,5-bis(2methylimidazolil-1-yl)pentane) for the degradation of MV was also explored also by other authors [86].Studies evidenced that after 40 min of exposure to UV light in the presence of (47), 47% of MV was decomposed, while in control experiments, photodecomposition of MV was limited to 11% in the same circumstances.
The photocatalytic decomposition of MV and rhodamine B (RhB) under the influence of complex [Cd(1,2-bdc)(bip)(H 2 O)] n (48) (bip = 1,3-bis(2-methyl-imidazol-1-yl)propane) was tested [87].The results evidenced that the rate of MV photodegradation is slower than the RhB rate.This behavior was allegedly related to the nature of dyes.In addition, the degradation of MV and RhB was not observed, not even after 40 min under natural illumination/dark conditions or UV light without a catalyst.The photocatalytic behavior of the compound after three cycles was similar to that observed for the fresh catalyst.CP {[Zn 2 (pa) 2 (bip) 2 ]•6H 2 O} n presents a lower photocatalytic effect for MV and RhB compared to the behavior of ( 48 50) (H 2 suc = succinic acid, bbmb = 4,4 ′ -bis(benzimidazol-1-ylmethyl)biphenyl) (Figure 3d) [88].The catalytic effect consists of the formation of hydroxyl radicals, which present potential for organic pollutant degradation.The degradation efficiency of MO under the influence of (49) and ( 50) increased along with the reaction time to 92.8% and 85.4%, respectively.In the absence of a catalyst, self-degradation of MO reached 16.6%.
The exploration of the photocatalytic effects of CPs was extended to methyl violet (MV) degradation by Cai and coworkers [53], who investigated the degradation of MV in polluted waters under effect of complex (11).Also, complexes ( 25) and (26) were reported due to their high catalytic activity for MV dye degradation [65].
The photocatalytic decomposition of MV and rhodamine B (RhB) under the influence of complex [Cd(1,2-bdc)(bip)(H2O)]n (48) (bip = 1,3-bis(2-methyl-imidazol-1-yl)propane) was tested [87].The results evidenced that the rate of MV photodegradation is slower than the RhB rate.This behavior was allegedly related to the nature of dyes.In addition, the degradation of MV and RhB was not observed, not even after 40 min under natural illumination/dark conditions or UV light without a catalyst.The photocatalytic behavior of the compound after three cycles was similar to that observed for the fresh catalyst.CP {[Zn2(pa)2(bip)2]•6H2O}n presents a lower photocatalytic effect for MV and RhB compared to the behavior of ( 48), evidenced by lower rate constants.
The Cd(II) CP {[Cd(Hipa)(Hiz)(H2O)2]⋅3H2O}n (52) (H3ipa = 5-hydroxy-isophthalic acid; Hiz = imidazole) was recently obtained under solvothermal conditions.This complex and its ZnO-doped composite material exhibit dual behavior as adsorbents and photocatalysts of methyl blue (MB), methyl orange (MO), and crystal violet (CV) dye degradation.It is worth mentioning that the composite exhibits a slightly improved surface area with a reduced photogenerated electron-hole ratio, hence displaying enhanced photodegradation activity Valuable heterogeneous catalysts for acetalization, cyanosilylation, Henry or Strecker reactions, and Knoevenagel condensation processes were developed based on good Lewis acidity of Cd(II) centers from CBCPs.Furthermore, such species show the ability to remove a wide range of pollutants dyes (AB-92, MB, MV, RhB, and MO) from wastewater through photocatalysis assisted or not by oxidants such hydrogen peroxide or persulfate by Fenton-like processes.The Cd-CBCPs discussed significantly increased the degradation efficiency of organic dyes to over 90% in many cases.Valuable heterogeneous catalysts for acetalization, cyanosilylation, Henry or Strecker reactions, and Knoevenagel condensation processes were developed based on good Lewis acidity of Cd(II) centers from CBCPs.Furthermore, such species show the ability to remove a wide range of pollutants dyes (AB-92, MB, MV, RhB, and MO) from wastewater through photocatalysis assisted or not by oxidants such hydrogen peroxide or persulfate by Fentonlike processes.The Cd-CBCPs discussed significantly increased the degradation efficiency of organic dyes to over 90% in many cases.
The species with catalytic activity are neutral complexes that exhibit good stability in water within a wide pH range to provide several recycling runs.Moreover, these are CP or MOF microporous materials with structural features that ensure a good accessibility of precursors to Cd(II) centers.The differences come from the shape and the dimension of the pore, which as a result, accommodate well to the shape and size of a certain substrate.
The catalytic mechanism is based on the coordination of one or both species involved in the process at Cd(II) Lewis acid centers and their further activation.Instead of photocatalysis, the mechanism consists of electron excitation from HOMO to LUMO of Cd-CBCPs, under UV irradiation.In the next step, the generated electrons are transferred to the surface of the material where they produce •O 2 − from O 2 .Finally, the superoxide in interaction with water molecules produces the •OH active species that degrade the organic pollutants.

Adsorbent Materials Based on Porous Cadmium (II) Carboxylate Coordination Polymers
The design of CPs with versatile structural features, in particular with high surface areas and cavities of certain dimensions, has drawn a lot of attention lately, mainly due to their applications.Therefore, such CPs are able to either selectively adsorb gases or to trap organic molecules (dyes, solvents, and drugs) [91].
An interesting and useful application similar to CP is the capturing of iodine for use as medicine.Consequently, Naskar and his team [99] 4c) is able to reversibly uptake iodine from an organic medium (more than 98%).The structural analysis of CP (62) showed polycatenation with microporous channels, while the size of the pores was 17.2x8.31Å 2  and filled with lattice H 2 O and DMF molecules.After removal of the solvent molecules by heating at 120 • C, the resultant species was able to uptake both iodine and N 2 .
A MOF with a rod-packing framework {(Me 2 NH 2 ) 3 [Cd 5 (atnc) 6 ]•18DMA•2H 2 O} n (H 3 atnc = 1-amino-2,4,6-tris(5-naphtalenecarboxylic) acid; DMA = N,N-dimethylacetamide) (63), built up from a rigid tricarboxylate ligand, represents a promising adsorbent material applied for the separation and purification of C 2 hydrocarbons.The abundant pore surface, the uncoordinated amine groups, and the partitioned pore space of a suitable size act synergistically to provide both selective recognition ability and the confinement effect towards C 2 hydrocarbons for this compound.As a result, it displays promising potential for adsorptive separation of C 2 -CH 4 and C 2 -CO 2 mixtures.In addition, the MOF skeleton remains intact in aqueous solutions within a wide pH range of 3 to 11 [100].
Besides the adsorption properties over different gaseous molecules, the literature presents an extended series of CPs able to adsorb organic dyes.This behavior gained interest more so as organic dyes are water pollutants and pose an environmental risk.In addition, the use of magnetic adsorbents presents a lot of advantages in this matter.Hence, a CP that contains 1,2,3,4-benzenetetracarboxylate formulated as [Cd(btca)(ppz)] n (66) (H 4 btca = 1,2,3,4-benzene tetracarboxylic acid, ppz = piperazine) was magnetized with iron oxide nanoparticles and it was subjected to investigation of adsorption behavior of MB and Chicago Sky Blue (CSB) [102].After assaying dosage, pH, shaking time, equilibrium, and thermodynamic and kinetic studies, it has been evidenced that (66) presented more selective removal of CSB in comparison with MB, while the adsorption capacity was of 64 and 3.6 mg g −1 , respectively.
A CP that resulted from H 2 pda and 4,4 ′ -azobis(pyridine) (abpy), [Cd(1,2-pda)(abpy) 0.5 (H 2 O)] n (67) was reported due to its ability to adsorb MB from an aqueous solution [103].The adsorption capacity of complex (67) within 240 min was 315.2 mg/g, which was almost double that found for a similar Zn-containing CP.In addition, after 240 min, the MB adsorbed quantity decreased, and at the end (after 1440 min), it was found as 5.2 mg g −1 , suggesting that MB was desorbed.
An acetate-based CP, [Cd 4 (CH 3 COO)(µ-OH) 4 (C 2 H 5 OH)] n (68), has been employed in adsorption tests of aromatic dyes [104].It has been found that CP (68) presents a maximum adsorption capacity at a basic pH.Thus, adsorption efficacy is 62.7% at pH 11 and 18.5% at pH 3. In addition, the maximum adsorption property was observed at room temperature.The authors stated that hydrogen bonds, electrostatic interactions, ion exchange, and π-π interactions are involved in physical adsorption.
Another interesting behavior of Cd(II) CPs is their ability to remove different drugs by adsorption.For example, complex [Cd 2 (btc) 2 (phen) 2 (H 2 O) 2 ] n (16) has been used to remove diclofenac from drinking and tap water [58].Studies have demonstrated its efficiency on this, and the maximum adsorption capacity was 1822 mg g −1 .
Therefore, Cd(II) CP (22) has been tested as an adsorbent for TEC [64].Based on the large specific surface area (171.72 m 2 g −1 ) and pore volume (0.31 cm 3 g −1 ), this compound adsorbed 64.493 mg g −1 of TEC at pH 7 and 298 K.In addition, its pore size favor TEC entering and binding to the adsorption site; the authors indicated a π-π interaction between these species.Luo and coworkers [107] reported the adsorbent properties of CP [Cd 3 (dpa)(HCO 2 )(bpp) 3 ] n (74) (H 2 dpa = diphenic acid; bpp = 1,3-bis(4-pyridyl)propane) for dichromate anions.This property was useful for removing Cr(VI) oxyanion from water.The adsorption of dichromate reached an equilibrium after 15 h, and the maximum quantity adsorbed was 2.451 mg g −1 .
Some Cd(II)-CBCPs with proper channels and pores are able to perform the selective adsorption of gases (CO 2 , N 2 , CH 4 , and H 2 ) or retain oxoanions (Cr 2 O 7 2− ), organic toxic species like solvents (methanol), dyes (MB, MO, CSB, RhB/6G, and CR), and drugs (diclofenac, tetracycline) from different media.This ability of Cd(II) CPs can be exploited to reduce the greenhouse effect by capturing CO 2 and CH 4 , to separate a certain gas from a mixture or to depollute wastewater from industry or hospitals.Some MOF species have exhibited their potential to act as adsorbents of C 2 -CH 4 and C 2 -CO 2 mixtures.
Most compounds described above as adsorbent materials are neutral species that exhibit a microporous and robust network, and are stable in water and in a wide pH range.Their high porosity coupled with a good surface area make these materials proficient adsorbent for gases or a liquid-phase adsorbate molecule.Their differences come from the shape, pore dimensions, and core charge that, as result, lead to selective uptake related to the size and charge of the retained species.Moreover, the nature of electrostatic or noncovalent interactions (hydrogen bonds and π-π interactions) with uncoordinated groups as well as the ion exchange involved in adsorption are different from one compound to another.It is worth mentioning that for such species, it is also important to preserve their crystalline structure after an adsorption−desorption cycle.
The mechanism of adsorption consists of electrostatic and/or non-covalent interactions between Cd(II)-CBCPs and guest molecules, except for charged dyes where an ion exchange process can be involved.remove diclofenac from drinking and tap water [58].Studies have demonstrated its efficiency on this, and the maximum adsorption capacity was 1822 mg g −1 .Therefore, Cd(II) CP (22) has been tested as an adsorbent for TEC [64].Based on the large specific surface area (171.72 m 2 g −1 ) and pore volume (0.31 cm 3 g −1 ), this compound adsorbed 64.493 mg g −1 of TEC at pH 7 and 298 K.In addition, its pore size favor TEC entering and binding to the adsorption site; the authors indicated a π-π interaction between these species.Luo and coworkers [107] reported the adsorbent properties of CP [Cd3(dpa)(HCO2)(bpp)3]n (74) (H2dpa = diphenic acid; bpp = 1,3-bis(4-pyridyl)propane) for dichromate anions.This property was useful for removing Cr(VI) oxyanion from water.The adsorption of dichromate reached an equilibrium after 15 h, and the maximum quantity adsorbed was 2.451 mg g −1 .
Some Cd(II)-CBCPs with proper channels and pores are able to perform the selective adsorption of gases (CO2, N2, CH4, and H2) or retain oxoanions (Cr2O7 2-), organic toxic species like solvents (methanol), dyes (MB, MO, CSB, RhB/6G, and CR), and drugs (diclofenac, tetracycline) from different media.This ability of Cd(II) CPs can be exploited to reduce the greenhouse effect by capturing CO2 and CH4, to separate a certain gas from a mixture or to depollute wastewater from industry or hospitals.Some MOF species have exhibited their potential to act as adsorbents of C2-CH4 and C2-CO2 mixtures.

Cadmium (II) Carboxylate Coordination Polymers with Miscellaneous Applications
The Cd(II) CPs present comprehensive application domains, as depicted in the sections presented above.However, there are several applications that cannot be presented in a structured manner, and they are therefore gathered and described here.
For example, compound (51) has been tested for its herbicidal activity [89].To accomplish this objective, the inhibitory rate was evaluated in comparison with the ligand over Brassica napus L. and Echinochloa crusgalli L. at different concentrations.The rates of inhibition of Brassica napus L. root growth was 29.4-39.3,22.8-30.9,and 10.1-14.2% at 100, 50, and 10 ppm, respectively, suggesting low herbicidal activity against this plant.Contrariwise, the inhibition rates of Echinochloa crusgalli L. root growth were high at 100  A CP resulting from a self-assembly reaction between Cd(II) and ligands 5-methoxyphtalate (5-meo-ip) and 1,3-bis(2-methyl-imidazol-1-yl)propane (bip), formulated as [Cd(5-meo-ip)(bip)] n (76), has been reported due to its treatment of uterine fibroids combined with ultrasound therapy [109].In addition, a similar CP with Zn(II) presented much weaker biological activity than (71).
The bactericidal potential of CP [Cd(phac) 2 (Dabco)(H 2 O)] n (77) (Hphac = phenylacetic acid; dabco = 1,4-diazabicyclo[2.2.2]octane) (Figure 5a) has been evidenced against Staphylococcus aureus and Escherichia coli.Investigation of inhibitory effect of CP components revealed significant inhibitory effect of cadmium salts in comparison with Phac and pillars which exhibit lower effects and absence of activity, respectively [110].The antibacterial activity was evaluated as diameter of the inhibition zone and evidenced a slightly better activity on S. aureus strains.78) (b) (adapted from ref. [110,111]).
Besides the applications of Cd(II)-CBCPs structured in previous subchapters Sections 2.1-2.3 and presented extensively in, a several miscellaneous ones have been reported: the herbicidal activity over Echinochloa crusgalli L., the phyto-growth inhibitory effects on B. Campestris L. and E. utilis Ohwi et Yabuno, the bactericidal activities against S. aureus and E. coli.Also, the potential use for treatment of uterine fibroids associated with ultrasound therapy has been demonstrated for one species.
Furthermore, finding new energy sources is connected with Cd(II)-CBCPs which may be incorporated into energy storage or electronic devices, or may be converted into CdO nanoparticles suitable for solar cells, or even gas sensors.Studies evidenced that a proper selection of both carboxylate and ancillary ligands, as well as the reaction condition provide Cd CPs with structural features (dielectric behavior, proton conductivity) useful for electronic device design.
Taking into account all the information related to Cd(II)-CBCPs and presented in this paper, it may be concluded that although Cd(II) ions are not suitable for biological applications due to their intrinsic toxicity, there are many other domains where they have demonstrated their complete usefulness.

Conclusions
The aim of this paper was to gather and organize the most relevant data regarding Cd(II)-CBCPs, emphasizing the application areas of these species.
Recent progress in both coordination chemistry and crystal engineering has allowed the design and synthesis of a wide range of Cd(II)-CBCPs with desired structures and properties by choosing a suitable mixture of organic ligands.During recent years, the  78) (b) (adapted from ref. [110,111]).
Lately, to respond to the ever-increasing need for new energy sources, researches focused on proton conductive materials, in particular to CPs with such properties.Consequently, two CPs (81) were reported due to their super high proton conductivities [112].The key elements for such materials are continuous hydrogen bonded network and ligands with hydrophilic units and from this perspective (80) and ( 81) contain HCBIA ligand that fulfill the requirements to generate protonconductive CPs.At 100 • C and 98% relative humidity (RH), proton conductivities of ( 80) and (81) are 5.09 × 10 −3 and 3.41 × 10 −3 S•cm −1 , respectively.
Besides the catalytic degradation or adsorption by different materials, organic dyes could be removed by flocculation, an efficient route to treat wastewaters.The exploration of new flocculants has led to CP { 10]phenantroline) which present high specific and efficient flocculation effect on Congo Red (CR) [115].
Besides the applications of Cd(II)-CBCPs structured in previous subchapters Sections 2.1-2.3 and presented extensively in, a several miscellaneous ones have been reported: the herbicidal activity over Echinochloa crusgalli L., the phyto-growth inhibitory effects on B. Campestris L. and E. utilis Ohwi et Yabuno, the bactericidal activities against S. aureus and E. coli.Also, the potential use for treatment of uterine fibroids associated with ultrasound therapy has been demonstrated for one species.
Furthermore, finding new energy sources is connected with Cd(II)-CBCPs which may be incorporated into energy storage or electronic devices, or may be converted into CdO nanoparticles suitable for solar cells, or even gas sensors.Studies evidenced that a proper selection of both carboxylate and ancillary ligands, as well as the reaction condition provide Cd CPs with structural features (dielectric behavior, proton conductivity) useful for electronic device design.
Taking into account all the information related to Cd(II)-CBCPs and presented in this paper, it may be concluded that although Cd(II) ions are not suitable for biological applications due to their intrinsic toxicity, there are many other domains where they have demonstrated their complete usefulness.

Conclusions
The aim of this paper was to gather and organize the most relevant data regarding Cd(II)-CBCPs, emphasizing the application areas of these species.
Recent progress in both coordination chemistry and crystal engineering has allowed the design and synthesis of a wide range of Cd(II)-CBCPs with desired structures and properties by choosing a suitable mixture of organic ligands.During recent years, the combination of rigid-backbone polycarboxylate and heterocycle N-donor ligands has been confirmed as one of the most useful building strategies to obtain such derivatives.The polycarboxylate ligands usually act as multiple-Cd(II) center connectors depending on the number of carboxylate groups, the molar ratio, both substituents' nature, and ancillary ligands with a similar aspect mentioned for Zn-CBCPs [11].Similar to other similar CPs, both structure and morphology are difficult to control since several factors, such as the molar ratio, synthesis method, reaction conditions (solvent, temperature, and pH), stereochemical versatility of Cd(II), and coordinative ability of the carboxylate and ancillary ligand, are involved.
Due to their Lewis acid character, Cd(II) ions present excellent catalytic properties and consequently, many reported Cd(II)-CBCPs were subjected to tests of heterogeneous catalytic behavior.Their capability to catalyze under certain conditions has been proven, with satisfying and even high yields from acetalization cyanosilylation and Henry, Strecker, and Knoevenagel reactions.In addition, many Cd(II)-CBCPs with suitable pores and channels into their structures have adsorptive properties for gases, organic molecules, or medicines.Besides these, some Cd(II)-CBCPs have proven herbicidal and bactericidal activities and were further used as raw materials for CdO nanoparticle synthesis.
Finally, this paper provides an overview and gives a summary of current knowledge regarding applications for Cd(II)-CBCPs, which are quite comprehensive and comparable with those of Zn-CBCPs, with the exception of the biological field [11].In addition, it is worth mentioning that in most cases, the Cd(II)-CBCP properties are a consequence of their structures (dimensions of the pores, hydrogen bonds, presence of certain ligands into composition, etc.).

Further Perspectives
In recent years, the CP domain has been a very active research area that has advanced considerably and is based on either finding new materials with predefined properties or on improving existing ones.With regard to Cd(II)-CBCPs, design of new species is possible using proper ligands, mainly polycarboxylates with a rigid backbone functionalized with substituents with coordinative sites or combining carboxylic derivatives with other polydentate ligands with donors other than nitrogen atoms.The domain of fluorescence can be extended by including in the Cd(II)-CBCP network some known luminophores such as the lanthanide ions, for which several studies on species similar to Zn(II) have proven their capacity to modulate the optical properties of such derivatives so far.
Considering the properties already revealed and the applications of Cd(II)-CBCPs, future perspectives are related to the extension of catalytic properties over other reactions besides those already mentioned, or to the improvement of the yield of studied species.
In addition, the design of species able to adsorb many gases, particularly those with a greenhouse effect, could be an enormous achievement, considering their negative effect on the environment.
Even if the literature data regarding Cd(II)-CBCPs with biological properties are very scarce, a barrier that cannot be overcome is gaining new candidates with biological properties considering the known toxicity of Cd(II) ions.
Also, involvement of Cd(II)-CBCPs in technologies such as networks of electronic devices or finding new energy sources could be further investigated and extended, since the results reported so far are promising.
Finally, we must conclude that despite all data cited in this paper being from 2014 to the present date, there is still plenty of room for improvement and development in the Cd(II)-CBCP domain.The information in our literature review may support future exploration studies.

Table 1 .
Examples of Cd(II)-carboxylate-based coordination polymers with diverse applications.