Multifunctional Aromatic Carboxylic Acids as Versatile Building Blocks for Hydrothermal Design of Coordination Polymers

Selected recent examples of coordination polymers (CPs) or metal-organic frameworks (MOFs) constructed from different multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores have been discussed. Despite being still little explored in crystal engineering research, such types of semi-rigid, thermally stable, multifunctional and versatile carboxylic acid building blocks have become very promising toward the hydrothermal synthesis of metal-organic architectures possessing distinct structural features, topologies, and functional properties. Thus, the main aim of this mini-review has been to motivate further research toward the synthesis and application of coordination polymers assembled from polycarboxylic acids with phenyl-pyridine or biphenyl cores. The importance of different reaction parameters and hydrothermal conditions on the generation and structural types of CPs or MOFs has also been highlighted. The influence of the type of main dior tricarboxylate ligand, nature of metal node, stoichiometry and molar ratio of reagents, temperature, and presence of auxiliary ligands or templates has been showcased. Selected examples of highly porous or luminescent CPs, compounds with unusual magnetic properties, and frameworks for selective sensing applications have been described.


Introduction and Scope
In recent years, various crystalline metal-organic architectures (MOAs) including coordination polymers (CPs) or metal-organic frameworks (MOFs) have been an object of very intense research that spans from the fields of crystal design and engineering to chemistry of functional materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15].In particular, a very interesting research direction concerns the search for new and versatile organic building blocks that can be applied for the design of unusual metal-organic architectures with desirable structural features and notable functional properties [16][17][18][19].Despite considerable progress achieved in this field, the assembly of coordination polymers or metal-organic frameworks in a predictable way is often a difficult task.This is mainly because the assembly of such compounds can depend on various factors, such as the nature and coordination properties of metal nodes [20,21], connectivity and type of organic building blocks [22][23][24], reaction conditions and stoichiometry [25,26], and effects of templates [27][28][29] or supporting ligands [30,31].
A high diversity of aromatic polycarboxylic acids has been extensively applied as multifunctional building blocks in designing novel metal-organic networks [32,33].Among such building blocks, flexible ligands containing biphenyl and phenyl-pyridine cores with a varying number and position of carboxylic groups as well as distinct locations of N-pyridyl functionality have attracted a special interest [34,35].It can be justified by a possibility of two adjacent phenyl and/or pyridine rings to rotate around the C-C single bond and thus conform to a coordination environment of metal nodes.Besides, the presence of several carboxylic groups with a varying degree of deprotonation in addition to an optional N-pyridyl functionality can provide multiple and distinct coordination sites, thus leading to different coordination fashions and resulting in the assembly of structurally distinct coordination polymers [36][37][38].Furthermore, depending on a deprotonation degree and crystal packing arrangement, these aromatic polycarboxylate ligands can behave as good H-bond acceptors and donors, thus furnishing an extra stabilization of metal-organic structures and facilitating their crystallization.
Hence, the main objective of the present work consists in highlighting selected recent examples of coordination polymers that were hydrothermally assembled from a series of multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores (Scheme 1).These carboxylic acids are still very poorly explored toward the design of CPs or MOFs, but can constitute an interesting type of semi-rigid, thermally stable, multifunctional, and versatile building blocks in crystal engineering research.Thus, the present study briefly discusses the general aspects of hydrothermal synthesis of selected coordination polymers derived from the aromatic carboxylic acids shown in Scheme 1.Some of them represent isomeric biphenyl tricarboxylate blocks (H 3 bptc and H 3 btc), while other are isomeric phenyl-pyridine tricarboxylate blocks (H 3 cptc, H 3 dcppa, and H 3 cpta).The study also highlights the influence of various parameters (main ligand type, metal node, molar ratio and stoichiometry, temperature, presence of auxiliary ligand or template) on structural diversity of the obtained products.
For selected examples of CPs, functional properties and applications are also highlighted.
A high diversity of aromatic polycarboxylic acids has been extensively applied as multifunctional building blocks in designing novel metal-organic networks [32,33].Among such building blocks, flexible ligands containing biphenyl and phenyl-pyridine cores with a varying number and position of carboxylic groups as well as distinct locations of N-pyridyl functionality have attracted a special interest [34,35].It can be justified by a possibility of two adjacent phenyl and/or pyridine rings to rotate around the C-C single bond and thus conform to a coordination environment of metal nodes.Besides, the presence of several carboxylic groups with a varying degree of deprotonation in addition to an optional N-pyridyl functionality can provide multiple and distinct coordination sites, thus leading to different coordination fashions and resulting in the assembly of structurally distinct coordination polymers [36][37][38].Furthermore, depending on a deprotonation degree and crystal packing arrangement, these aromatic polycarboxylate ligands can behave as good H-bond acceptors and donors, thus furnishing an extra stabilization of metal-organic structures and facilitating their crystallization.
Hence, the main objective of the present work consists in highlighting selected recent examples of coordination polymers that were hydrothermally assembled from a series of multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores (Scheme 1).These carboxylic acids are still very poorly explored toward the design of CPs or MOFs, but can constitute an interesting type of semi-rigid, thermally stable, multifunctional, and versatile building blocks in crystal engineering research.Thus, the present study briefly discusses the general aspects of hydrothermal synthesis of selected coordination polymers derived from the aromatic carboxylic acids shown in Scheme 1.Some of them represent isomeric biphenyl tricarboxylate blocks (H3bptc and H3btc), while other are isomeric phenyl-pyridine tricarboxylate blocks (H3cptc, H3dcppa, and H3cpta).The study also highlights the influence of various parameters (main ligand type, metal node, molar ratio and stoichiometry, temperature, presence of auxiliary ligand or template) on structural diversity of the obtained products.For selected examples of CPs, functional properties and applications are also highlighted.
Scheme 1.Ten selected multifunctional carboxylic acids used as building blocks for the design of CPs or MOFs.Scheme 1.Ten selected multifunctional carboxylic acids used as building blocks for the design of CPs or MOFs.

Advantages of Hydrothermal Synthesis
Hydrothermal synthesis is commonly applied toward the design of metal-organic networks [39][40][41] and refers to the synthesis and crystallization of coordination compounds that occur under hydrothermal conditions, typically in a hermetically sealed aqueous solution at elevated temperatures and pressures.The hydrothermal synthesis features a number of important advantages over other common methods for preparing CPs, namely: (i) high reactivity of reactants and unique synthetic conditions in terms of a combination of pressures and temperatures; (ii) growth of good quality single crystals (Figure 1) or microcrystalline phases with no need for additional work-up and purification; (iii) possible control of solution or interface reactions, formation of metastable and unique structures that cannot be generated by other methods; (iv) use of water as a green organic-solvent-free reaction medium that can also aid crystallization by supplying labile H 2 O ligands to complete coordination environment of metal nodes; and (v) relative simplicity of the equipment.

Advantages of Hydrothermal Synthesis
Hydrothermal synthesis is commonly applied toward the design of metal-organic networks [39][40][41] and refers to the synthesis and crystallization of coordination compounds that occur under hydrothermal conditions, typically in a hermetically sealed aqueous solution at elevated temperatures and pressures.The hydrothermal synthesis features a number of important advantages over other common methods for preparing CPs, namely: (i) high reactivity of reactants and unique synthetic conditions in terms of a combination of pressures and temperatures; (ii) growth of good quality single crystals (Figure 1) or microcrystalline phases with no need for additional work-up and purification; (iii) possible control of solution or interface reactions, formation of metastable and unique structures that cannot be generated by other methods; (iv) use of water as a green organicsolvent-free reaction medium that can also aid crystallization by supplying labile H2O ligands to complete coordination environment of metal nodes; and (v) relative simplicity of the equipment.For coordination polymers driven by multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores (Scheme 1), typical synthetic procedure begins with mixing, in water at ambient temperature and under constant stirring, a metal nitrate or chloride salt, a main carboxylic acid building block, and an auxiliary ligand (optional) [42][43][44].The obtained mixture is then treated with sodium hydroxide as a typical base to adjust the solution pH value in the range of 5-7.Then, the reaction mixture is sealed in a Teflon-lined stainless steel autoclave and subjected to the hydrothermal treatment at 80-210 °C for 2 or 3 days in an oven, followed by gradual cooling to ambient temperature at a rate of 10 °C/h (Figure 2).The autoclaves are opened after being kept at ambient temperature for 24 h.The obtained crystalline solids are filtered off and washed (optional) or isolated manually to furnish a coordination polymer product (Figure 1).For coordination polymers driven by multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores (Scheme 1), typical synthetic procedure begins with mixing, in water at ambient temperature and under constant stirring, a metal nitrate or chloride salt, a main carboxylic acid building block, and an auxiliary ligand (optional) [42][43][44].The obtained mixture is then treated with sodium hydroxide as a typical base to adjust the solution pH value in the range of 5-7.Then, the reaction mixture is sealed in a Teflon-lined stainless steel autoclave and subjected to the hydrothermal treatment at 80-210 • C for 2 or 3 days in an oven, followed by gradual cooling to ambient temperature at a rate of 10 • C/h (Figure 2).The autoclaves are opened after being kept at ambient temperature for 24 h.The obtained crystalline solids are filtered off and washed (optional) or isolated manually to furnish a coordination polymer product (Figure 1).

Advantages of Hydrothermal Synthesis
Hydrothermal synthesis is commonly applied toward the design of metal-organic networks [39][40][41] and refers to the synthesis and crystallization of coordination compounds that occur under hydrothermal conditions, typically in a hermetically sealed aqueous solution at elevated temperatures and pressures.The hydrothermal synthesis features a number of important advantages over other common methods for preparing CPs, namely: (i) high reactivity of reactants and unique synthetic conditions in terms of a combination of pressures and temperatures; (ii) growth of good quality single crystals (Figure 1) or microcrystalline phases with no need for additional work-up and purification; (iii) possible control of solution or interface reactions, formation of metastable and unique structures that cannot be generated by other methods; (iv) use of water as a green organicsolvent-free reaction medium that can also aid crystallization by supplying labile H2O ligands to complete coordination environment of metal nodes; and (v) relative simplicity of the equipment.For coordination polymers driven by multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores (Scheme 1), typical synthetic procedure begins with mixing, in water at ambient temperature and under constant stirring, a metal nitrate or chloride salt, a main carboxylic acid building block, and an auxiliary ligand (optional) [42][43][44].The obtained mixture is then treated with sodium hydroxide as a typical base to adjust the solution pH value in the range of 5-7.Then, the reaction mixture is sealed in a Teflon-lined stainless steel autoclave and subjected to the hydrothermal treatment at 80-210 °C for 2 or 3 days in an oven, followed by gradual cooling to ambient temperature at a rate of 10 °C/h (Figure 2).The autoclaves are opened after being kept at ambient temperature for 24 h.The obtained crystalline solids are filtered off and washed (optional) or isolated manually to furnish a coordination polymer product (Figure 1).

Effect of Building Block Type
The type of the main carboxylic acid ligand (Scheme 1) is one of the structure-defining factors during the hydrothermal synthesis of CPs.Selected examples of different metal-organic networks that were obtained under similar reaction conditions are summarized in Table 1.For example, the use of different dicarboxylic acids (H 2 cpna, H 2 cppa, or H 2 bpydc) as main building blocks and 1,10-phenanthroline as an auxiliary ligand led to the generation of distinct manganese(II) derivatives 1−3 (Figure 3), the structures of which range from a 1D ladder [Mn(µ 3 -cpna)(phen)(H 2 O)] n (1) and 1D zigzag chain [Mn(µ-cppa)(phen)(H 2 O)] n (2) to a 3D MOF [Mn(µ 4 -bpydc)(phen)] n (3).The use of a cobalt(II) metal source in combination with the isomeric H 2 cpna or H 2 cppa ligands and 2,2 -bipyridyl resulted in the assembly of a 2D metal-organic layer [ (5).Similar structure-defining influence of tricarboxylic acid building blocks can be observed in other zinc(II) (6, 7) and manganese(II) (8, 9) coordination compounds (Table 1).

Effect of Metal Source
The type of metal node also plays an important structure-defining role in the hydrothermal generation of coordination polymers.This is primarily associated with different coordination behavior and ligand affinity of distinct metal centers, their charges and ionic radii.Selected examples of CPs assembled under identical reaction conditions but using different metal sources are collected in Table 2.In particular, an interesting series of compounds 14−16 can be built from H3btc and phen Adapted from [42,44,45].

Effect of Metal Source
The type of metal node also plays an important structure-defining role in the hydrothermal generation of coordination polymers.This is primarily associated with different coordination behavior and ligand affinity of distinct metal centers, their charges and ionic radii.Selected examples of CPs assembled under identical reaction conditions but using different metal sources are collected in Table 2.In particular, an interesting series of compounds 14−16 can be built from H 3 btc and phen ligands by using different metal(II) chlorides, namely a 1D chain{[Cd(µ 3 -Hbtc)(phen (16).Notably, despite the diversity of these structures, they all feature a monoprotonated tricarboxylic acid block, Hbtc 2− .
Apart from the nature of metal, the type of anion in a starting metal salt can also influence the resulting structure.

Effect of Reagents Molar Ratio
In the synthesis of CPs, a proportion between metal node and main carboxylate ligand can be easily modified, what can cause a change of the coordination number of metal ions and affect the resulting structure.In addition, change of the molar ratio between main building block and alkali metal hydroxide used as a pH-regulator can result in a partial or full deprotonation of polycarboxylic acid ligand.As shown in Table 3, both 3D MOFs {[Co3(µ4-btc)2(µ-H2O)2(py)4(H2O)2]•(py)2}n (19) and {[Co3.5(µ6-btc)2(µ3-OH)(py)2(H2O)3]•H2O}n(20) were obtained under exactly the same conditions, except using a slightly different molar ratio between CoCl2•6H2O and H3btc (1.5:1 for 19 and 1.77:1 for 20).However, these products feature very different structures and topologies (Figure 5).The structures of product pairs 21/22 and 23/24 (Table 3) also differ significantly on varying the NaOH:H2cppa and NaOH:H3bptc molar ratios, respectively.In these cases, an excess of sodium hydroxide leads to a complete deprotonation of H2cppa in 22 or a generation of additional µ3-OH

Effect of Reagents Molar Ratio
In the synthesis of CPs, a proportion between metal node and main carboxylate ligand can be easily modified, what can cause a change of the coordination number of metal ions and affect the resulting structure.In addition, change of the molar ratio between main building block and alkali metal hydroxide used as a pH-regulator can result in a partial or full deprotonation of polycarboxylic acid ligand.As shown in Table 3 (20) were obtained under exactly the same conditions, except using a slightly different molar ratio between CoCl 2 •6H 2 O and H 3 btc (1.5:1 for 19 and 1.77:1 for 20).However, these products feature very different structures and topologies (Figure 5).The structures of product pairs 21/22 and 23/24 (Table 3) also differ significantly on varying the NaOH:H 2 cppa and NaOH:H 3 bptc molar ratios, respectively.In these cases, an excess of sodium hydroxide leads to a complete deprotonation of H 2 cppa in 22 or a generation of additional µ 3 -OH linkers in 24, thus making these structures more complicated in comparison with their counterparts assembled using a lower amount of NaOH.linkers in 24, thus making these structures more complicated in comparison with their counterparts assembled using a lower amount of NaOH.
Table 3. Selected examples of CPs showing an effect of reagents molar ratio on product structure.

Effect of Auxiliary Ligand
The presence of an additional auxiliary ligand also plays an important role in the hydrothermal synthesis of CPs, especially by facilitating product crystallization.Introduction of a common

Effect of Reaction Temperature
The reaction temperature during the synthesis of metal-organic networks also has a significant impact on the final product structure.
Table 3. Selected examples of CPs showing an effect of reagents molar ratio on product structure.

Effect of Auxiliary Ligand
The presence of an additional auxiliary ligand also plays an important role in the hydrothermal synthesis of CPs, especially by facilitating product crystallization.Introduction of a common

Effect of Auxiliary Ligand
The presence of an additional auxiliary ligand also plays an important role in the hydrothermal synthesis of CPs, especially by facilitating product crystallization.Introduction of a common auxiliary N,N-donor ligand such as 2,2 -bipyridine of 1,10-phenanthroline usually changes the coordination environment of metal centers, thus resulting in the generation of different structures (Table 5).For example, the reaction of a cobalt(II) salt with H 2 cppa with no auxiliary ligand leads to a 2D coordination polymer [Co(µ 3 -cppa)(H 2 O) 2 ] n (27) 7) were prepared under the same synthetic conditions except the introduction of phen in 35.As can be seen from various examples collected in Table 5, the use of the N,N-donor auxiliary ligands tends to facilitate the formation of CPs with a lower dimensionality if compared to the systems without an auxiliary ligand.However, rather complex 3D MOF {[Cd 3 (µ 5 -btc) 2 (phen) 2 (H 2 O)]•H 2 O} n (31) can also be generated in the presence of the auxiliary ligand (Table 5).auxiliary N,N-donor ligand such as 2,2′-bipyridine of 1,10-phenanthroline usually changes the coordination environment of metal centers, thus resulting in the generation of different structures (Table 5).For example, the reaction of a cobalt(II) salt with H2cppa with no auxiliary ligand leads to a 2D coordination polymer [Co(µ3-cppa)(H2O)2]n ( 27), whereas simpler 1D zigzag chain products {[Co(µ-cppa)(2,2′-bpy)(H2O)]•H2O}n (28) and [Co(µ-cppa)(phen)(H2O)]n (29) are generated in the presence of 2,2′-bpy or phen, respectively.Similarly, structurally distinct CPs {[Nd(µ-Hcpna)2(µcpna)2(H2O)2]•3H2O}n (34) and {[Nd(µ-Hcpna)2(µ4-cpna)2(phen)]•2H2O}n (35) (Figure 7) were prepared under the same synthetic conditions except the introduction of phen in 35.As can be seen from various examples collected in Table 5, the use of the N,N-donor auxiliary ligands tends to facilitate the formation of CPs with a lower dimensionality if compared to the systems without an auxiliary ligand.However, rather complex 3D MOF {[Cd3(µ5-btc)2(phen)2(H2O)]•H2O}n (31) can also be generated in the presence of the auxiliary ligand (Table 5).

Effect of Template
Template-assisted synthesis of CPs has attracted a special attention as a promising approach toward tunable architectures or structures that might be difficult to access by routine synthetic methods [47,53,54].Various inorganic ions or organic molecules can be used as templating agents in the hydrothermal synthesis of coordination polymers.In particular, 4,4′-bipyridine acts not only as a common linker in CPs but is frequently applied as a template.Selected pairs of structurally distinct coordination polymers obtained with or without template are summarized in Table 6.For example, although compounds {[Ni3(µ4-dcppa)2(H2O)6]•2H2O}n (42) and {[Ni3(µ5-dcppa)2(H2O)6]•2H2O}n (43) were prepared under similar reaction conditions except using 4,4′-bipy as a templating agent in 43, they feature structures of different dimensionality and topology (Figure 8).

Effect of Template
Template-assisted synthesis of CPs has attracted a special attention as a promising approach toward tunable architectures or structures that might be difficult to access by routine synthetic methods [47,53,54].Various inorganic ions or organic molecules can be used as templating agents in the hydrothermal synthesis of coordination polymers.In particular, 4,4 -bipyridine acts not only as a common linker in CPs but is frequently applied as a template.Selected pairs of structurally distinct coordination polymers obtained with or without template are summarized in Table 6 (43) were prepared under similar reaction conditions except using 4,4 -bipy as a templating agent in 43, they feature structures of different dimensionality and topology (Figure 8).

Effect of Two Main Ligands
Although a substantial number of coordination polymers incorporating various kinds of carboxylate ligands has been reported [56], the examples of heteroleptic networks constructed from a combination of two kinds of biphenyl or phenyl-pyridine carboxylate building blocks (Scheme 1) are barely known.It is primarily caused by different solubility of such ligands, distinct coordination modes and charges, as well as ligand competition for metal node during the hydrothermal synthesis and crystallization.The latter factor may often lead to the formation of a mixture of simpler products containing only one main building block rather than more complex products comprising both carboxylate ligands.The competition between two main carboxylate building blocks for metal nodes can be even more pronounced when the reaction mixture also contains an additional auxiliary ligand along with water as a solvent and frequent terminal ligand source.The effect of two different types of biphenyl carboxylate moieties on the structure of the resulting metal-organic network remains  46), respectively (Table 7, Figure 9).

Effect of Two Main Ligands
Although a substantial number of coordination polymers incorporating various kinds of carboxylate ligands has been reported [56], the examples of heteroleptic networks constructed from a combination of two kinds of biphenyl or phenyl-pyridine carboxylate building blocks (Scheme 1) are barely known.It is primarily caused by different solubility of such ligands, distinct coordination modes and charges, as well as ligand competition for metal node during the hydrothermal synthesis and crystallization.The latter factor may often lead to the formation of a mixture of simpler products containing only one main building block rather than more complex products comprising both carboxylate ligands.The competition between two main carboxylate building blocks for metal nodes can be even more pronounced when the reaction mixture also contains an additional auxiliary ligand along with water as a solvent and frequent terminal ligand source.The effect of two different types of biphenyl carboxylate moieties on the structure of the resulting metal-organic network remains poorly studied.Notable examples of CPs combining two kinds of biphenyl carboxylate blocks include a 2D network [Cd 2 (µ 5 -cpic) 2 (µ-bpdc) 0.5 (phen) 2 ] n (45) and a 3D MOF [Co 2 (µ 7 -btc) 2 (µ-bpydc) 0.5 (py) 3 ] n (47) that feature distinct structures and topologies in comparison with their counterparts {[Cd 2 (µ 4 -cpic (46), respectively (Table 7, Figure 9).

Highly Porous MOFs
Some coordination polymers based on multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores possess the highly porous structures and excellent stability (Table 8).These properties make these materials rather promising for exploring CO 2 capture and gas storage applications.As illustrated in Table 8 and Figure 8).

Highly Porous MOFs
Some coordination polymers based on multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores possess the highly porous structures and excellent stability (Table 8).These properties make these materials rather promising for exploring CO2 capture and gas storage applications.As illustrated in Table 8 and Figure 8).

Highly Luminescent Materials
MOFs based on the europium(III) and terbium(III) nodes are highly luminescent compounds.As illustrated in Table 9 and Figure

Compounds with Unusual Magnetic Properties
Some coordination polymers derived from multifunctional carboxylic acids with phenylpyridine or biphenyl cores can exhibit unusual magnetic properties.Selected examples are highlighted in Table 10.In particular, Du and co-workers assembled a 3D MOF, {[Dy2(µ4pyip)3(H2O)4]•2DMF•3H2O}n (54), using H2pyip as a building block.This compound possesses the pcu topology and exhibits a slow magnetization relaxation behavior (Figure 12

Highly Luminescent Materials
MOFs based on the europium(III) and terbium(III) nodes are highly luminescent compounds.As illustrated in Table 9 and Figure

Highly Luminescent Materials
MOFs based on the europium(III) and terbium(III) nodes are highly luminescent compounds.As illustrated in Table 9 and Figure

Compounds with Unusual Magnetic Properties
Some coordination polymers derived from multifunctional carboxylic acids with phenylpyridine or biphenyl cores can exhibit unusual magnetic properties.Selected examples are highlighted in Table 10.In particular, Du and co-workers assembled a 3D MOF, {[Dy2(µ4pyip)3(H2O)4]•2DMF•3H2O}n (54), using H2pyip as a building block.This compound possesses the pcu topology and exhibits a slow magnetization relaxation behavior (Figure 12

Selective Sensing Materials
It is known that some fluorescent MOF materials are sensitive to the presence or absence of guest solvent molecules.As illustrated in Table 11 and Figure 13, Wen and co-workers reported a 3D MOF based on the H2pyip ligand, [Zn(µ3-pyip)(bimb)•(H2O)]n (58).This MOF exhibits the first report of a MOF material as a promising luminescent probe for detecting pesticides.This compound is also unique by allowing a detection of both pesticides and solvent molecules simultaneously.Other examples of sensing MOFs are shown in Table 11.

Selective Sensing Materials
It is known that some fluorescent MOF materials are sensitive to the presence or absence of guest solvent molecules.As illustrated in Table 11 and Figure 13, Wen and co-workers reported a 3D MOF based on the H 2 pyip ligand, [Zn(µ 3 -pyip)(bimb)•(H 2 O)] n (58).This MOF exhibits the first report of a MOF material as a promising luminescent probe for detecting pesticides.This compound is also unique by allowing a detection of both pesticides and solvent molecules simultaneously.Other examples of sensing MOFs are shown in Table 11.

Selective Sensing Materials
It is known that some fluorescent MOF materials are sensitive to the presence or absence of guest solvent molecules.As illustrated in Table 11 and Figure 13, Wen and co-workers reported a 3D MOF based on the H2pyip ligand, [Zn(µ3-pyip)(bimb)•(H2O)]n (58).This MOF exhibits the first report of a MOF material as a promising luminescent probe for detecting pesticides.This compound is also unique by allowing a detection of both pesticides and solvent molecules simultaneously.Other examples of sensing MOFs are shown in Table 11.

Conclusions and Outlook
In this mini-review, we featured selected recent examples of coordination polymers (CPs) or metal-organic frameworks (MOFs) that were constructed from various multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores (Scheme 1).Despite being still little explored, these types of semi-rigid, thermally stable, and versatile building blocks appear to be very promising for the hydrothermal synthesis of metal-organic networks with different structural characteristics, topologies, and functional properties.The present work also highlighted an importance of different reaction parameters and conditions on the assembly and structural diversity of coordination polymers.The effects of the type of main carboxylate ligand, kind of metal node, stoichiometry and molar ratio of reagents, temperature, presence or absence of auxiliary ligands or templates were showcased.In addition, some examples of highly porous MOFs, notable luminescent materials, compounds with unusual magnetic properties, and frameworks for selective sensing applications were described.
We believe the application of multifunctional carboxylic acids containing phenyl-pyridine or biphenyl cores toward the design of coordination polymers will be continued, leading to new series of coordination compounds and derived materials with fascinating structural features and notable functional properties.Future research might focus on: (A) widening the family of multicarboxylate building blocks to new members with additional functional groups; (B) diversifying the types of metal nodes; (C) assembling heterometallic metal-organic architectures; (D) optimizing the conditions of the hydrothermal synthesis and crystallization; (E) predicting the structural and topological characteristics; and (F) broadening the types of possible applications of the obtained coordination polymers.

Figure 1 .
Figure 1.Images showing examples of single crystals of Ni (a), Cd (b), and Cu (c) coordination polymers generated hydrothermally.

Figure 2 .
Figure 2. Images of the Teflon-lined stainless steel autoclaves (a) and an oven with temperature control (b) typically applied for the hydrothermal generation of CPs.

Figure 1 .
Figure 1.Images showing examples of single crystals of Ni (a), Cd (b), and Cu (c) coordination polymers generated hydrothermally.

Figure 1 .
Figure 1.Images showing examples of single crystals of Ni (a), Cd (b), and Cu (c) coordination polymers generated hydrothermally.

Figure 2 .
Figure 2. Images of the Teflon-lined stainless steel autoclaves (a) and an oven with temperature control (b) typically applied for the hydrothermal generation of CPs.

Figure 2 .
Figure 2. Images of the Teflon-lined stainless steel autoclaves (a) and an oven with temperature control (b) typically applied for the hydrothermal generation of CPs.

Table 1 .
Selected examples of coordination polymers (CPs) showing an effect of main carboxylate ligand on product structure.

Table 1 .
Selected examples of coordination polymers (CPs) showing an effect of main carboxylate ligand on product structure.

Table 2 .
Selected examples of CPs showing an effect of metal source on product structure.

Table 2 .
Selected examples of CPs showing an effect of metal source on product structure.

Table 4 .
Selected examples of CPs showing an effect of reaction temperature on product structure.

Table 5 .
Selected examples of CPs showing an effect of auxiliary ligand on product structure.

Table 5 .
Selected examples of CPs showing an effect of auxiliary ligand on product structure.

Table 6 .
Selected examples of CPs showing an effect of template on product structure.

Table 6 .
Selected examples of CPs showing an effect of template on product structure.

Table 7 .
Selected examples of CPs showing an effect of two main carboxylate ligands on product structure.

Table 7 .
Selected examples of CPs showing an effect of two main carboxylate ligands on product structure.

Table 9 .
Selected examples of highly luminescent MOFs.

Table 9 .
Selected examples of highly luminescent MOFs.

Table 9 .
Selected examples of highly luminescent MOFs.

Table 10 .
Selected examples of CPs with unusual magnetic properties.

Table 10 .
Selected examples of CPs with unusual magnetic properties.

Table 11 .
Selected examples of MOFs with selective sensing behavior.

Table 10 .
Selected examples of CPs with unusual magnetic properties.

Table 11 .
Selected examples of MOFs with selective sensing behavior.

Table 11 .
Selected examples of MOFs with selective sensing behavior.