Hydrothermal Assembly, Structural Multiplicity, and Catalytic Knoevenagel Condensation Reaction of a Series of Coordination Polymers Based on a Pyridine-Tricarboxylic Acid

A pyridine-tricarboxylic acid, 5-(3′,5′-dicarboxylphenyl)nicotinic acid (H3dpna), was employed as a adjustable block to assemble a series of coordination polymers under hydrothermal conditions. The seven new coordination polymers were formulated as [Co(μ3-Hdpna)(μ-dpey)]n·nH2O (1), [Zn4.5(μ6-dpna)3(phen)3]n (2), [Co1.5(μ6-dpna)(2,2′-bipy)]n (3), [Zn1.5(μ6-dpna)(2,2′-bipy)]n (4), [Co3(μ3-dpna)2(4,4′-bipy)2(H2O)8]n·2nH2O (5),[Co(bpb)2(H2O)4]n[Co2(μ3-dpna)2(H2O)4]n·3nH2O (6), and [Mn1.5(μ6-dpna)(μ-dpea)]n (7), wherein 1,2-di(4-pyridyl)ethylene (dpey), 1,10-phenanthroline (phen), 2,2′-bipyridine(2,2′-bipy),4,4′-bipyridine(4,4′-bipy),1,4-bis(pyrid-4-yl)benzene (bpb), and 1,2-di(4-pyridyl)ethane (dpea) were employed as auxiliary ligands. The structural variation of polymers 1–7 spans the range from a 2D sheet (1–4, 6, and 7) to a 3D metal–organic framework (MOF, 5). Polymers 1–7 were investigated as heterogeneous catalysts in the Knoevenagel condensation reaction, leading to high condensation product yields (up to 100%) under optimized conditions. Various reaction conditions, substrate scope, and catalyst recycling were also researched. This work broadens the application of H3dpna as a versatile tricarboxylate block for the fabrication of functional coordination polymers.

The catalytic properties of the obtained coordination polymers have also been evaluated in Knoevenagel reactions between benzaldehyde and malononitrile.

Hydrothermal Synthesis of Polymers1-7
To evaluate 5-(3′,5′-dicarboxylphenyl)nicotinic acid (H3dpna) as a promising block for the fabrication of coordination polymers, some reactions were performed under hydrothermal conditions, using a mixture containing a metal(II) chloride, H3dpna (main ligand), sodium hydroxide, and an auxiliary ligand selected from dpey, phen, 2,2′-bipy, 4,4′-bipy, bpb, and dpea.These hydrothermal synthetic reactions were carried out at 160 °C for three days, and then continued through a slow cooling of the mixtures and the crystallization of coordination polymers, as described in Table 1.The polymers were collected in 42-53% yields and characterized using standard methods.The obtained PXRD patterns of 1-7 are consistent with the simulated ones (Figure S3), confirming the formation of the homogeneous phase of the obtained polymers.The structural differences in 1-7 might be attributed to the type of auxiliary ligand selected as well as the coordination fashion of the metal(II) centers.In polymers 1-7, the blocks generated upon the deprotonation of H3dpna showed four different coordination fashions and are partially or totally deprotonated (Hdpna 2− and dpna 3− , Scheme 2).

Scheme 1. Structures of H 3 dpna and auxiliary ligands.
As a linker, H 3 dpna has a lot of interesting characteristics that favor its exploration.(a) There is some rotation between the phenyl and pyridine rings, leading to improved adaptability to coordination preferences of metal(II) centers.(b) H 3 dpna bears some sites for coordination (six carboxyl O sites and one pyridine N site).(c) The presence of a free N pyridine atom can provide a base site for catalysis, including Knoevenagel condensation [33,34].(d) In 2017-2021, several Mn(II)-, Ni(II)-, Co(II)-, Zn(II)-, and Cd(II)dpna coordination polymers were synthesized [35][36][37][38].Although their magnetism, luminescence, selective adsorption, and proton conductivities were studied, the catalytic properties of only two H 3 dpna-based frameworks were investigated.Thus, the present work can aid in developing this field.
The catalytic properties of the obtained coordination polymers have also been evaluated in Knoevenagel reactions between benzaldehyde and malononitrile.

Hydrothermal Synthesis of Polymers 1-7
To evaluate 5-(3 ,5 -dicarboxylphenyl)nicotinic acid (H 3 dpna) as a promising block for the fabrication of coordination polymers, some reactions were performed under hydrothermal conditions, using a mixture containing a metal(II) chloride, H 3 dpna (main ligand), sodium hydroxide, and an auxiliary ligand selected from dpey, phen, 2,2 -bipy, 4,4 -bipy, bpb, and dpea.These hydrothermal synthetic reactions were carried out at 160 • C for three days, and then continued through a slow cooling of the mixtures and the crystallization of coordination polymers, as described in Table 1.The polymers were collected in 42-53% yields and characterized using standard methods.The obtained PXRD patterns of 1-7 are consistent with the simulated ones (Figure S3), confirming the formation of the homogeneous phase of the obtained polymers.The structural differences in 1-7 might be attributed to the type of auxiliary ligand selected as well as the coordination fashion of the metal(II) centers.In polymers 1-7, the blocks generated upon the deprotonation of H 3 dpna showed four different coordination fashions and are partially or totally deprotonated (Hdpna 2− and dpna 3− , Scheme 2).

Crystal Structure of 6
The crystal structure of compound 6 is composed of one cation [Co(bpb)2(H2O)4] 2+ one anion 2D sheet [Co2(μ3-dpna)2(H2O)4] 2− n, and three lattice water molecules (Figur 5a).In the cation, the Co2 center adopts a distorted octahedral {CoN2O4} environmen filled by four oxygen atoms from the four H2O ligands and a pair of N donors of two in dividual bpb auxiliary ligands.In the cation 2D sheet, the Co1 center is also six-coordinate and displays a distorted octahedral {CoNO5} environment, which is con stituted by three carboxyl oxygen and nitrogen atoms from four different μ3-dpna 3 blocks and two oxygen donors from two H2O ligands.The Co-O [2.048(3)-2194(2) Å and Co-N [2.102(2)-2.141(2)Å] distances are comparable to those in related Co(II) de rivatives [23][24][25].The dpna 3− block acts as a μ3-linker (mode IV, Scheme 2) with thre COO − groups being monodentate, bidentate, or uncoordinated; the N donor of the pyri dine ring is coordinated to the Co(II) center.The bpb auxiliary ligand shows a termina coordination fashion.The μ3-dpna 3− blocks link to the Co1 centers to form a cation 2D metal-organic network (Figure 5b).This 2D structure is composed of the 3-linked Co1

Crystal Structure of 6
The crystal structure of compound 6 is composed of one cation [Co(bpb) 2 (H 2 O) 4 ] 2+ , one anion 2D sheet [Co 2 (µ 3 -dpna) 2 (H 2 O) 4 ] 2− n , and three lattice water molecules (Figure 5a).In the cation, the Co2 center adopts a distorted octahedral {CoN 2 O 4 } environment filled by four oxygen atoms from the four H 2 O ligands and a pair of N donors of two individual bpb auxiliary ligands.In the cation 2D sheet, the Co1 center is also six-coordinate and displays a distorted octahedral {CoNO 5 } environment, which is constituted by three carboxyl oxygen and nitrogen atoms from four different µ 3 -dpna 3− blocks and two oxygen donors from two H 2 O ligands.The Co-O [2.048(3)-2194( 2) Å] and Co-N [2.102(2)-2.141(2)Å] distances are comparable to those in related Co(II) derivatives [23][24][25].The dpna 3− block acts as a µ 3 -linker (mode IV, Scheme 2) with three COO − groups being monodentate, bidentate, or uncoordinated; the N donor of the pyridine ring is coordinated to the Co(II) center.The bpb auxiliary ligand shows a terminal coordination fashion.The µ 3 -dpna 3− blocks link to the Co1 centers to form a cation 2D metal-organic network (Figure 5b).This 2D structure is composed of the 3-linked Co1 nodes and 3-linked µ 3 -dpna 3− nodes (Figure 5c).It is a fes topology with a point symbol of (4.8 2 ).

TGA and PXRD Data
In an atmosphere of nitrogen, the thermal stability of polymers 1-7 were investigated by TGA (Figure S2) in the range of 21-800 °C.In compound 1, one lattice water molecule was lost at 157-192 °C (exptl 3.2%, calcd 3.3%), and the dehydrated sample was stable up to 324 °C.Polymers 2-4 and 7 do not contain free solvent or coordinated H2O, and were stable until 382, 347, 383, and 373 °C, respectively.In 5, two lattice water molecule and eight water ligands were lost at 140-240 °C (exptl, 14.2%; calcd, 14.5%), and the dehydrated framework was stable up to 265 °C.For polymer 6, a loss of mass at 51-152 °C was attributed to the loss of three lattice and eight coordinated water molecules (exptl, 14.3%; calcd, 14.1%), while the sample maintained stability until 335 °C.
For all the obtained products 1-7, power X-ray diffractograms were collected at 25 °C (Figure S3).The comparison between the experimental patterns and the simulated ones (coming from CIF data) showed that all samples are pure.

Catalytic Knoevenagel Condensation
Given the ability of some coordination polymers to catalyze the Knoevenagel condensation reaction of aldehydes and active methylene compounds [39][40][41], the reaction was performed to evaluate the catalytic activities of polymers 1-7.Firstly, benzaldehyde and malononitrile were employed as reagents and catalytic experiments were carried out in CH3OH at 25 °C, leading to the generation of 2-benzylidenemalononitrile (Scheme 3, Table 2).The influence of important parameters on the reaction yield was also studied, including the time, solvent, temperature, and amount of catalyst.
Preliminary screening of all the obtained polymers revealed that compounds 2-4 and 7 were the most promising.Because polymer 4 had a higher yield when it was synthesized, this polymer was used to further optimize the reaction parameters.The reaction time had a great influence on the yield; the product yield increased from 45 to 100% (Table 2, entries 1-6; Figure S4) when the reaction was extended from 10 min to 60 min.The influence of catalyst amount was also investigated, resulting in an increase in prod-

TGA and PXRD Data
In an atmosphere of nitrogen, the thermal stability of polymers 1-7 were investigated by TGA (Figure S2) in the range of 21-800 • C. In compound 1, one lattice water molecule was lost at 157-192 • C (exptl 3.2%, calcd 3.3%), and the dehydrated sample was stable up to 324 • C. Polymers 2-4 and 7 do not contain free solvent or coordinated H 2 O, and were stable until 382, 347, 383, and 373 • C, respectively.In 5, two lattice water molecule and eight water ligands were lost at 140-240 • C (exptl, 14.2%; calcd, 14.5%), and the dehydrated framework was stable up to 265 • C. For polymer 6, a loss of mass at 51-152 • C was attributed to the loss of three lattice and eight coordinated water molecules (exptl, 14.3%; calcd, 14.1%), while the sample maintained stability until 335 • C.
For all the obtained products 1-7, power X-ray diffractograms were collected at 25 • C (Figure S3).The comparison between the experimental patterns and the simulated ones (coming from CIF data) showed that all samples are pure.

Catalytic Knoevenagel Condensation
Given the ability of some coordination polymers to catalyze the Knoevenagel condensation reaction of aldehydes and active methylene compounds [39][40][41], the reaction was performed to evaluate the catalytic activities of polymers 1-7.Firstly, benzaldehyde and malononitrile were employed as reagents and catalytic experiments were carried out in CH 3 OH at 25 • C, leading to the generation of 2-benzylidenemalononitrile (Scheme 3, Table 2).The influence of important parameters on the reaction yield was also studied, including the time, solvent, temperature, and amount of catalyst.
Molecules 2023, 28, x FOR PEER REVIEW 10 of 15 uct yield from 94 to 100% when the catalyst amount was increased from 1 to 2 mol% (Table 2, entries 6 and 7).Although the highest reaction yield was achieved in CH3OH, water, ethanol, acetonitrile, and chloroform were also used as alternative solvents, leading to lower product yields (65-98%).In the absence of the above catalysts, the catalytic reaction was not efficient (Table 2, entries [18][19][20].Although no correlation between catalytic performance and the structure of coordination polymer catalyst could be concluded, the superior performance of polymers 2-4 and 7 can be attributed to the existence of the non-saturated coordination site in the metal centers [42,43].Preliminary screening of all the obtained polymers revealed that compounds 2-4 and 7 were the most promising.Because polymer 4 had a higher yield when it was synthesized, this polymer was used to further optimize the reaction parameters.The reaction time had a great influence on the yield; the product yield increased from 45 to 100% (Table 2, entries 1-6; Figure S4) when the reaction was extended from 10 min to 60 min.The influence of catalyst amount was also investigated, resulting in an increase in product yield from 94 to 100% when the catalyst amount was increased from 1 to 2 mol% (Table 2, entries 6 and 7).Although the highest reaction yield was achieved in CH 3 OH, water, ethanol, acetonitrile, and chloroform were also used as alternative solvents, leading to lower product yields (65-98%).In the absence of the above catalysts, the catalytic reaction was not efficient (Table 2, entries [18][19][20].Although no correlation between catalytic performance and the structure of coordination polymer catalyst could be concluded, the superior performance of polymers 2-4 and 7 can be attributed to the existence of the non-saturated coordination site in the metal centers [42,43]. Next, some benzaldehyde substrates bearing functional groups were reacted under the best conditions (2.0 mol% 4, CH 3 OH, 60 min).The respective products of the reaction were obtained in yields ranging from 33 to 100% (Table S3).Benzaldehyde and tis derivatives bearing an electron-attracting group (e.g., -NO 2 or -Cl) showed a higher reactivity (Table S3, entries 1-5).This could be attributed to an increased electrophilic characteristic of the above benzaldehydes.Benzaldehyde substrates bearing electron-donor groups (e.g., -CH 3 or -OCH 3 ) had lower yields of products (Table S3, entries 6 and 7).
Catalyst recycling tests (Figure S5) confirm that polymer 4 maintains its catalytic activity during five cycles, which was borne out by the similarity of product yields.Moreover, the PXRD data also support the maintenance of the structure of catalyst 4 (Figure S6).

Materials and Measurements
Except where otherwise noted, all chemicals were commercially available and were used as received.H 3 dpna was acquired from Yanshen Tec.Co., Ltd.(Changchun, China).A Bruker EQUINOX 55 spectrometer (Bruker Corporation, Billerica, MA, USA) was used to record the FTIR spectra (KBr discs).The C, H, and N elemental analyses (EA) for 1-7 were conducted using an Elementar Vario EL elemental analyzer (Elementar, Langenselbold, Germany).An LINSEIS STA PT1600 thermal analyzer (Linseis Messgeräte GmbH, Selb, Germany) was used for the thermogravimetric analyses (TGAs) under a N 2 flow with a heating rate of 10 • C/min.A Rigaku-Dmax 2400 diffractometer (Cu-Kα radiation; λ= 1.54060 Å, Rigaku Corporation, Tokyo, Japan) was used to collect the powder X-ray diffraction (PXRD) patterns of the obtained compounds.CDCl 3 was used as the solvent for 1 H NMR measurements on a JNM ECS 400 M spectrometer (Bruker BioSpin AG, Fällanden, Switzerland).

Single Crystal X-ray Diffraction and Topological Analysis
The crystal data of polymers 1-7 were collected by a Bruker APEX-II CCD diffractometer (graphite-monochromated Mo/CuK α radiation; λ = 0.71073/1.54184Å).The structures were solved by direct methods and refined by full-matrix least-squares on F 2 with SHELXS-97 and SHELXL-97 [44].The C, O, and N atoms were refined anisotropically using full-matrix least-squares methods on F 2 .The H atoms were placed in calculated positions through riding models.In 2, the phen ligands (N4, C43-C54, N5) were split over two sites and refined with 0.70 and 0.30 occupancies.The crystal parameters and structural refinements are summarized in Table 1.The selected bond parameters are collected in Tables S1 and S2 (Supplementary Materials).CCDC 2299559-2299565 contains the supplementary crystallographic data of 1-7.

Catalytic Activity in Knoevenagel Condensation Reaction
At 25 • C, a suspension containing catalyst (typically 2 mol%), aromatic aldehyde (0.50 mmol, benzaldehyde as a model substrate), malononitrile (1.0 mmol), and solvent (1.0 mL, typically CH 3 OH) was stirred for the desired reaction time.Then, the catalyst was removed by centrifugation.The filtrate was evaporated using a rotary evaporator to obtain a crude solid product.This was dissolved in CDCl 3 and analyzed by 1 H NMR spectroscopy (JNM ECS 400 M spectrometer) for the quantification of the product (Figure S7).In order to run recycling tests, the catalyst was recollected by centrifugation, washed with methanol, dried at room temperature, and reused in subsequent steps as depicted above.

Conclusions
The present work revealed the application of H 3 dpna as a pyridine-tricarboxylate block for assembling novel coordination polymers.As a result, seven new coordination polymers were synthesized using the hydrothermal method.The structures of polymers 1-7 ranged from a 2D network (1-4, 6 and 7) to metal-organic framework (5).The catalytic application of polymers 1-7 was evaluated in Knoevenagel reactions involving benzalde-
2− block, one µ-dpey auxiliary ligand, and one free water molecule in the asymmetric unit of polymer 1.The six-coordinate Co1 center possesses a distorted octahedral {CoN 2 O 4 } environment populated by four carboxyl oxygen atoms from three µ 3 -Hdpna 2 linkers and two N donors from two different µ-dpey moieties.The bond lengths of Co-O and Co-N are 2.017(2)-2.246(2)and 2.154(3)-2.156(2)Å, respectively; these are within the normal ranges observed in related Co(II) compounds [7,13].In 1, the Hdpna 2 ligand takes the coordination model (Scheme 2) with two COO − groups being bidentate or bridging bidentate, while the nitrogen site of the pyridine ring remains uncoordinated.Two carboxylate groups of two µ 3 -Hdpna 2− linkers connect the adjacent Co(II) centers to give a dimeric Co 2 subunit with a Co•••Co distance of 4.021(3) Å (Figure , there are one Co(II) center, one μ3-Hdpna 2− block, one μ-dpey auxiliary ligand, and one free water molecule in the asymmetric unit of polymer 1.The six-coordinate Co1 center possesses a distorted octahedral {CoN2O4} environment populated by four carboxyl oxygen atoms from three μ3-Hdpna 2 linkers and two N donors from two different μ-dpey moieties.The bond lengths of Co-O and Co-N are 2.017(2)-2.246(2)and 2.154(3)-2.156(2)Å, respectively; these are within the normal ranges observed in related Co(II) compounds [7,13].In 1, the Hdpna 2 ligand takes the coordination model (Scheme 2) with two COO − groups being bidentate or bridging bidentate, while the nitrogen site of the pyridine ring remains uncoordinated.Two carboxylate groups of two μ3-Hdpna 2− linkers connect the adjacent Co(II) centers to give a dimeric Co2 subunit with a Co•••Co distance of 4.021(3) Å (Figure