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Article

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

by
Xiuqi Kang
,
Chao Ren
,
Zhenzhong Mei
,
Xiaoxiang Fan
,
Jijun Xue
,
Yongliang Shao
* and
Jinzhong Gu
*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(22), 7474; https://doi.org/10.3390/molecules28227474
Submission received: 7 October 2023 / Revised: 25 October 2023 / Accepted: 31 October 2023 / Published: 8 November 2023
(This article belongs to the Section Inorganic Chemistry)
Browse Figures
Versions Notes

Abstract

:
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.

Graphical Abstract

1. Introduction

Nowadays, a growing number of transition metal coordination polymers (CPs) serve as crystalline materials because their various topologies and structures [1,2,3]. These functional compounds have prospective applications in gas separation and adsorption [4,5,6], the development of sensors [7,8] and heterogeneous catalysts [9,10,11], and many other fields [12,13,14]. Solvothermal and hydrothermal synthetic methods are widely used in the assembly of coordination polymers [15,16,17,18]. In the process of self-assembly, many reaction parameters (pH, temperature, concentration, solvent, etc.) as well as the intrinsic characteristics of the organic ligands and metal centers [19,20,21,22,23,24,25,26] affect the structures of the coordination polymers.
As the main kind of organic ligands for synthesizing coordination polymers, aromatic multi-carboxylic acids are especially attractive given their adjustable deprotonation and charges, tunable coordination modes, and good thermal stability [27,28,29,30]. Since 2019, our group has been working on the synthesis of coordination polymers with various carboxylic acids and exploring their catalytic performance in organic reactions [31,32,33]. Recently, our group used a pyridine-dicarboxylic acid, 4,4′-(pyridine-3,5-diyl)dibenzoic acid (H2pdba), to synthesize a series of transition metal coordination polymers. The catalytic performance of these polymers was evaluated [34]. As a continuation of that research, 5-(3′,5′-dicarboxylphenyl)nicotinic acid (H3dpna, Scheme 1) has been selected in the present study as a little studied pyridine-tricarboxylate building block.
As a linker, H3dpna 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) H3dpna bears some sites for coordination (six carboxyl O sites and one pyridine N site). (c) The presence of a free Npyridine 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 H3dpna-based frameworks were investigated. Thus, the present work can aid in developing this field.
Hence, we report in this work the synthesis and analysis of seven new coordination polymers obtained from metal(II) salts, H3dpna, and different auxiliary ligands. The structures of these polymers vary from a 2D sheet (1–4, 6 and 7) to a 3D metal–organic framework (MOF, 5), namely [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).
The catalytic properties of the obtained coordination polymers have also been evaluated in Knoevenagel reactions between benzaldehyde and malononitrile.

2. Results and Discussion

2.1. Hydrothermal Synthesis of Polymers 1–7

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 (Hdpna2−and dpna3−, Scheme 2).

2.2. Crystal Structure of 1

As depicted in Figure 1a, there are one Co(II) center, one μ3-Hdpna2− 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-Hdpna2 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 Hdpna2 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-Hdpna2− linkers connect the adjacent Co(II) centers to give a dimeric Co2 subunit with a Co∙∙∙Co distance of 4.021(3) Å (Figure 1b). These Co2 subunits are further connected by the µ3-Hdpna2− linkers and μ-dpey ligands into a 2D network (Figure 1c). This 2D structure is composed of the 5-linked Co1 nodes, 3-linked µ3-Hdpna2− nodes, and 2-connected µ-dpey linkers (Figure 1d). This is a topology and a point symbol of (42.67.8)(42.6).

2.3. Crystal Structure of 2

The asymmetric unit of 2 consists of five distinct Zn(II) centers (Zn1 with 1/2 occupancy; Zn2–Zn5 with full occupancy), three μ6-dpna3− blocks, and three phen supporting ligands. The Zn1 and Zn2 atoms are six-coordinate and feature distorted octahedral {ZnO6} environments (Figure 2a). The Zn3, Zn4, and Zn5 centers are five-coordinate and display distorted trigonal bipyramidal {ZnN2O3} geometries. The Zn–O [2.003(2)–2.187(2) Å] and Zn–N [2.097(2)–2.214(2)Å] distances are comparable to those in Zn(II) derivatives [5,6,12]. In 2, the dpna3− blocks behave as μ6-spacers (mode II, Scheme 2), in which the COO− groups exhibit bridging bidentate modes, while the N donor of the pyridine ring remains uncoordinated. The three adjacent Zn(II) atoms (Zn1, Zn5, and Zn5i) are clustered by means of four carboxylate groups and two carboxyl oxygen atoms of six different μ6-dpna3− blocks, generating a trizinc subunit (Figure 2b). Meanwhile, another Zn3 (Zn2, Zn3, and Zn4i) subunit is formed (Figure 2c). These Zn3 subunits are further assembled by the remaining carboxylate groups of the µ6-dpna3− blocks to give a 2D network (Figure 2d). The 2D network in this structure is built from the 3- and 6-linked Zn1/Zn2 and 6-linked µ6-dpna3− nodes (Figure 2e). It can be defined as a trinodal net with a topology and point symbol of (412.63)(43)2(46.69)2.

2.4. Crystal Structures of 3 and 4

Since coordination polymers 3 and 4 are isostructural, only polymer 4 is described herein (Figure 3). The asymmetric unit of Zn(II) polymer 4 bears two Zn(II) atoms (Zn1 with full occupancy, Zn2 with 1/2 occupancy), one µ6-dpna3−block, and one 2,2′-bipy auxiliary ligand (Figure 3a). The five-coordinate Zn1 atom possesses a distorted trigonal bipyramidal {ZnN2O3} environment, which is formed by three carboxyl oxygen atoms from three distinct µ6-dpna3− blocks and a pair of N2,2′-bipy donors. The Zn2 center is six-coordinate and assumes as a distorted octahedral {ZnO6} environment, based on six carboxyl oxygen atoms coming from six different µ6-dpna3− blocks. The Zn–O [2.018(2)–2.134(2) Å] and Zn–N [2.100(3)–2.164(3) Å] bonds are within standard values [13,17,26]. The dpna3− block acts as a µ6-linker (mode II, Scheme 2). The Zn3 subunit is found (Figure 3b), which is similar to that in polymer 2. These Zn3 subunits are further assembled by the other carboxylate groups of the µ6-dpna3− blocks to produce a 2D network (Figure 3c). This 2D structure is composed of the 3- and 6-linked Zn1/Zn2 nodes, and 6-linked µ6-dpna3− nodes (Figure 3d). It is a topology and a point symbol of (412.63)(43)2(46.69)2.

2.5. Crystal Structure of 5

Compound 5 is a 3D coordination polymer (Figure 4) and its asymmetric unit reveals two Co(II) centers (Co1 with 1/2 occupancy, Co2 with full occupancy), one µ3-dpna3− block, one µ-4,4′-bipy moiety, four H2O ligands, and one free water molecule (Figure 4a). As shown in Figure 4a, the Co1 center is six-coordinate and forms a distorted octahedral {CoN2O4} geometry. It is completed by two carboxyl oxygen atoms from two μ3-dpna3−, two O donors from two H2O ligands, and two N4,4′-bipy from two µ-4,4′-bipy moieties. The six-coordinate Co2 atom also displays a distorted octahedral {CoN2O4} geometry filled by one carboxyl oxygen atom from one μ3-dpna3−, two N donors from two distinct µ-4,4′-bipy moieties, and three O atoms of three H2O ligands. The Co–O [2.078(2)–2.131(2) Å] and Co–N [2.157(2)–2.160(2) Å] bonds are within standard values [17,21,22]. The dpna3− moiety adopts a μ3-coordination fashion (mode III, Scheme 2), in which COO− groups exhibit monodentate or uncoordinated fashions; the N donor of the pyridine ring is coordinated to the Co(II) center. The μ3-dpna3− blocks and µ-4,4′-bipy moieties form multiple connections to the Co centers to form a 3D framework (Figure 4b). The 3D network is assembled from the 3- and 4-linked Co1/Co2 centers, the 3-connected µ3-dpna3− nodes, and 2-connected µ-4,4′-bipy linkers (Figure 4c). The resulting network possesses a topology with a net point symbol of (6.102)4(62.124).

2.6. 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 (Figure 5a). In the cation, the Co2 center adopts a distorted octahedral {CoN2O4} environment filled by four oxygen atoms from the four H2O 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 {CoNO5} environment, which is constituted by three carboxyl oxygen and nitrogen atoms from four different μ3-dpna3− 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) derivatives [23,24,25]. The dpna3− block acts as a µ3-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 µ3-dpna3− 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 µ3-dpna3− nodes (Figure 5c). It is a fes topology with a point symbol of (4.82).

2.7. Crystal Structure of 7

This compound has a 2D metal–organic network structure. Its asymmetric unit contains two distinct Mn(II) centers (Mn1 with full occupancy; Mn2 with 1/2 occupancy and located on a 2-fold rotation axis), a µ6-dpna3− block, and a bridging dpea ligand. The Mn1 atom is five-coordinated and forms a distorted trigonal bipyramidal {MnN2O3} geometry, which is completed by three carboxyl oxygen donors coming from three distinct µ6-dpna3− moieties and two N donors from two dpea ligand (Figure 6a). The Mn2 center is six-coordinate and positioned on a 2-fold rotation axis; Mn2 is surrounded by six O donors from six distinct µ6-dpna3− blocks, thus forming a distorted octahedral {MnO6} environment. The Mn–O [2.112(2)–2.178(2) Å] and Mn–N [2.207(2)–2.313(2)Å] lengths are comparable to those in related Mn(II) derivatives [23,26,32]. In 7, the dpna3− block behaves as a μ6-spacer (Scheme 2, mode II), in which the COO− groups exhibit μ-bridging bidentate modes. Two Mn1 and one Mn2 centers are bridged by four carboxylate groups and two carboxyl O atoms from six different µ6-dpna3− blocks, thus forming a centrosymmetric trimanganese(II) cluster (Figure 6b). These Mn3 clusters are additionally interlinked by the µ6-dpna3− blocks to give an intricate 2D metal–organic layer (Figure 6c). This 2D structure is composed of the 5- and 6-linked Mn1/Mn2 nodes, 6-linked µ6-dpna3− nodes, and 2-connected μ-dpea linkers (Figure 6d). It is a topology and a point symbol of (3.46.56.62)2(32.43.54.6)2(412.63).

2.8. 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.

2.9. 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 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 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 18–20). 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, CH3OH, 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., –NO2 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., –CH3 or –OCH3) 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).

3. Experiments

3.1. Materials and Measurements

Except where otherwise noted, all chemicals were commercially available and were used as received. H3dpna 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 N2 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. CDCl3 was used as the solvent for 1H NMR measurements on a JNM ECS 400 M spectrometer (Bruker BioSpin AG, Fällanden, Switzerland).

3.2. Synthesis of [Co(μ3-Hdpna)(μ-dpey)]n·nH2O (1)

H3dpna (57.4 mg, 0.2 mmol), CoCl2·6H2O (47.6 mg, 0.2 mmol), dpey (36.4 mg, 0.2 mmol), NaOH (16.0 mg, 0.4 mmol), and H2O (10 mL) were sealed into a 20 mL Teflon cup. After the resulting mixture was kept at 160 °C for three days, it was cooled to room temperature, and pink block-shaped crystals were picked out and washed with water. (yield: 45% based on H3dpna). Calculated for C26H19CoN3O7: C 57.36, N 7.72, H 3.52%. Found: C 57.66, N 7.63, H 3.50%. FTIR (KBr, cm−1): 3463 w, 3043 w, 1708 m, 1608 s, 1589 s, 1437 s, 1405 s, 1325 w, 1288 w, 1265 w, 1205 w, 1176 w, 1136 w, 1073 w, 1017 w, 989 w, 901 w, 838 m, 793 w, 765 w, 701w, 625 w, 562 w.

3.3. Synthesis of [Zn4.5(μ6-dpna)3(phen)3]n (2)

H3dpna (57.4 mg, 0.2 mmol), ZnCl2 (40.9 mg, 0.3 mmol), phen (59.4 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H2O (10 mL) were sealed into a 20 mL Teflon cup. After the resulting mixture was kept at 160 °C for three days, it was cooled to room temperature, and colorless block-shaped crystals were picked out and washed with water (yield: 47% based on H3dpna). Calculated for C78H42Zn4.5N9O18: C 55.51, N 7.47, H 2.51%. Found: C 55.23, N 7.44, H 2.53%. FTIR (KBr, cm−1): 1628 m, 1608 m, 1577 w, 1516 w, 1425 s, 1345 s, 1305 w, 1277 w, 1176 w, 1145 w, 1102 w, 1026 w, 933 w, 905 w, 845 w, 777 m, 726 m, 686 w, 641 w, 565 w, 538 w.

3.4. Synthesis of [Co1.5(μ6-dpna)(2,2′-bipy)]n (3)

H3dpna (57.4 mg, 0.2 mmol), CoCl2·6H2O (71.4 mg, 0.3 mmol), 2,2′-bipy (46.8 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H2O (10 mL) were sealed into a 20 mL Teflon cup. After the resulting mixture was kept at 160 °C for three days, it was cooled to room temperature, and pink block-shaped crystals were picked out and washed with water (yield: 44% based on H3dpna). Calculated for C24H14Co1.5N3O6: C 54.51, N 7.95, H 2.67%. Found: C 54.73, N 7.74, H 2.68%. FTIR (KBr, cm−1): 1625 s, 1601 s, 1473 w, 1425 s, 1361 s, 1305 w, 1276 w, 1185 w, 1154 w, 1101 w, 1054 w, 1026 w, 973 w, 933 w, 897 w, 833 w, 762 m, 721 m, 686 w, 650 w, 629 w, 565 w, 534 w.

3.5. Synthesis of [Zn1.5(μ6-dpna)(2,2′-bipy)]n (4)

Compound 4 was synthesized following a procedure similar to 3 except using ZnCl2 (40.9 mg, 0.3 mmol) instead of CoCl2·6H2O. Some colorless block-shaped crystals of compound 4 were collected (yield: 53% based on H3dpna). Calculated for C48H28Zn3N6O12: C 53.53, N 7.80, H 2.62%. Found: C 53.27, N 7.77, H 2.64%. FTIR (KBr, cm−1): 1632 s, 1608 s, 1476 w, 1425 s, 1376 w, 1340 w, 1276 w, 1181 w, 1149 w, 1102 w, 1053 w, 1021 w, 941 w, 902 w, 838 w, 790 m, 769 w, 729 m, 686 w, 650 w, 626 w, 565 w, 530 w.

3.6. Synthesis of [Co3(μ3-dpna)2(4,4′-bipy)2(H2O)8]n·2nH2O (5)

H3dpna (57.4 mg, 0.2 mmol), CoCl2·6H2O (71.4 mg, 0.3 mmol), 4,4′-bipy (46.8 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H2O (10 mL) were sealed into a 20 mL Teflon cup. After the resulting mixture was kept at 160 °C for three days, it was cooled to room temperature, and pink needle-shaped crystals were picked out and washed with water (yield: 42% based on H3dpna). Calculated for C24H24Co1.5N3O11: C 46.58, N 6.79, H 3.91%. Found: C 46.83, N 6.75, H 3.93%. FTIR (KBr, cm−1): 3283 m, 1613 s, 1565 s, 1401 s, 1365 s, 1281 w, 1229 w, 1185 w, 1137 w, 1069 w, 1029 w, 1009 w, 897 w, 821 m, 789 w, 701 m, 634 w, 526 w.

3.7. Synthesis of [Co(bpb)2(H2O)4]n[Co2(μ3-dpna)2(H2O)4]n·3nH2O (6)

H3dpna (57.4 mg, 0.2 mmol), CoCl2·6H2O (71.4 mg, 0.3 mmol), bpb (69.7 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H2O (10 mL) were sealed into a 20 mL Teflon cup. After the resulting mixture was kept at 160 °C for three days, it was cooled to room temperature, and red block-shaped crystals were picked out and washed with water (yield: 45% based on H3dpna). Calculated for C60H58Co3N6O23: C 51.18, N 5.97, H 4.15%. Found: C 51.35, N 6.01, H 4.13%. FTIR (KBr, cm−1): 3404 m, 3227 m, 3027 w, 1665 w, 1601 s, 1557 m, 1469 m, 1413 s, 1369 s, 1297 w, 1221 w, 1197 w, 1154 w, 1117 w, 1065 w, 1014 w, 909 w, 802 m, 781 m, 717 m, 658 w, 574 w.

3.8. Synthesis of [Mn1.5(μ6-dpna)(μ-dpea)]n (7)

H3dpna (57.4mg, 0.2 mmol), MnCl2·4H2O (59.4 mg, 0.3 mmol), dpea (55.2 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H2O (10 mL) were sealed into a 20 mL Teflon cup. After the resulting mixture was kept at 160 °C for three days, it was cooled to room temperature, and yellow block-shaped crystals were picked out and washed with water (yield: 47% based on H3dpna). Calculated for C52H36Mn3N6O12: C 56.69, N 7.63, H 3.29%. Found: C 56.94, N 7.47, H 3.31%. FTIR (KBr, cm−1): 1608 s, 1569 s, 1501 w, 1421 s, 1353 s, 1310 w, 1281 w, 1229 w, 1181 w, 1149 w, 1105 w, 1069 w, 1021 w, 902 w, 833 w, 786 m, 722 m, 681 w, 586 w, 553 w.

3.9. 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/CuKα radiation; λ = 0.71073/1.54184 Å). The structures were solved by direct methods and refined by full-matrix least-squares on F2 with SHELXS-97 and SHELXL-97 [44]. The C, O, and N atoms were refined anisotropically using full-matrix least-squares methods on F2. 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.
Topological analysis of the obtained coordination polymers was performed with ToposPro software (Version 5.5.2.1) by producing abbreviated (underlying) nets, therein bridging ligands were simplified to the centroids [45,46].

3.10. 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 CH3OH) 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 CDCl3 and analyzed by 1H 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.

4. Conclusions

The present work revealed the application of H3dpna 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 benzaldehydes and malononitrile. Polymers 2–4 and 7 revealed excellent catalytic performance in the Knoevenagel reaction of aldehydes with malononitrile.

Supplementary Materials

The following data are available online at https://www.mdpi.com/article/10.3390/molecules28227474/s1. Figure S1: FT-IR spectra; Figure S2: TGA curves; Figure S3: PXRD patterns and additional catalysis data; Figure S4: Accumulation of 2-benzylidenemalononitrile vs. time in the Knoevenagel condensation of benzaldehyde with malononitrile catalyzed by 4; Figure S5: Catalyst recycling experiments in the Knoevenagel condensation of benzaldehyde with malononitrile catalyzed by 4; Figure S6: PXRD patterns for 4: simulated (red), before (black), and after (blue) catalysis; Figure S7: Example of the integration in the 1H NMR spectrum of the reaction mixture for the determination of Knoevenagel condensation product (conditions of Table 2, entry 6); Tables S1 and S2: Selected bonding and H-bonding parameters for 1, 5, and 6; Table S3: Substrate scope of catalytic reaction.

Author Contributions

Writing—original draft, X.K.; Investigation, C.R.; Data curation, Z.M.; Software, X.F.; Methodology, J.X.; Resources. Y.S.; Writing—review and editing, J.X. and J.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the 111 Project of MOE (111-2-17).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article or supplementary material.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 28 07474 sch001
Scheme 1. Structures of H3dpna and auxiliary ligands.
Scheme 1. Structures of H3dpna and auxiliary ligands.
Molecules 28 07474 sch001
Molecules 28 07474 sch002
Scheme 2. The coordination modes of Hdpna2−/dpna3−.
Scheme 2. The coordination modes of Hdpna2−/dpna3−.
Molecules 28 07474 sch002
Molecules 28 07474 g001aMolecules 28 07474 g001b
Figure 1. Structure of 1. (a) Coordination environment at Co(II) atom; hydrogen atoms are ignored except for COOH group. (b) The Co2 subunit. (c) The 2D sheet (view down the c axis). (d) Topological representation of a 3-connected network with topology (view down the c axis); 5-connected Co1 nodes (pink), centroids of 3-connected µ3-Hdpna2− nodes (gray), centroids of 2-connected µ-dpey linkers (blue).
Figure 1. Structure of 1. (a) Coordination environment at Co(II) atom; hydrogen atoms are ignored except for COOH group. (b) The Co2 subunit. (c) The 2D sheet (view down the c axis). (d) Topological representation of a 3-connected network with topology (view down the c axis); 5-connected Co1 nodes (pink), centroids of 3-connected µ3-Hdpna2− nodes (gray), centroids of 2-connected µ-dpey linkers (blue).
Molecules 28 07474 g001aMolecules 28 07474 g001b
Molecules 28 07474 g002
Figure 2. Structure of 2. (a) Coordination environment at Zn(II) centers; hydrogen atoms are ignored. (b,c) Zn3 subunits; the phen moieties are ignored. (d) The 2D sheet (view down the b axis). (e) Topological representation of a 2-nodal network with topology (view down the b axis); Zn series nodes (cyan), centroids of 6-linked µ6-dpna3− nodes (gray).
Figure 2. Structure of 2. (a) Coordination environment at Zn(II) centers; hydrogen atoms are ignored. (b,c) Zn3 subunits; the phen moieties are ignored. (d) The 2D sheet (view down the b axis). (e) Topological representation of a 2-nodal network with topology (view down the b axis); Zn series nodes (cyan), centroids of 6-linked µ6-dpna3− nodes (gray).
Molecules 28 07474 g002
Molecules 28 07474 g003
Figure 3. Structure of 4. (a) Coordination environment of Zn(II) atoms; hydrogen atoms are ignored. (b) The Zn3 subunit. (c) Two-dimensional metal–organic network (view down the c axis). (d) Topological representation of a 3,6-connected network with topology (view along the c axis); 3- and 6-linked Zn1/Zn2 nodes (cyan), centroids of 6-connected µ6-dpna3− nodes (gray).
Figure 3. Structure of 4. (a) Coordination environment of Zn(II) atoms; hydrogen atoms are ignored. (b) The Zn3 subunit. (c) Two-dimensional metal–organic network (view down the c axis). (d) Topological representation of a 3,6-connected network with topology (view along the c axis); 3- and 6-linked Zn1/Zn2 nodes (cyan), centroids of 6-connected µ6-dpna3− nodes (gray).
Molecules 28 07474 g003
Molecules 28 07474 g004
Figure 4. Structure of 5. (a) Coordination environments at Co(II) atoms; hydrogen atoms are ignored. (b) Three-dimensional metal–organic framework (view down the c axis). (c) Topological representation of a 2,3,4-connected framework with topology (view down the c axis); 3- and 4-linked Co1/Co2 nodes (pink), centroids of 3-connected µ3-dpna3− nodes (gray), centroids of 2-connected µ-4,4′-bipy linkers (blue).
Figure 4. Structure of 5. (a) Coordination environments at Co(II) atoms; hydrogen atoms are ignored. (b) Three-dimensional metal–organic framework (view down the c axis). (c) Topological representation of a 2,3,4-connected framework with topology (view down the c axis); 3- and 4-linked Co1/Co2 nodes (pink), centroids of 3-connected µ3-dpna3− nodes (gray), centroids of 2-connected µ-4,4′-bipy linkers (blue).
Molecules 28 07474 g004
Molecules 28 07474 g005
Figure 5. Structure of 6. (a) Coordination environment at Co(II) atoms; hydrogen atoms are ignored. (b) Two-dimensional cation metal–organic network (view down the a axis). (c) Topological representation of a 3-connected network with a fes topology (view down the a axis); 3-connected Co1 nodes (pink), centroids of 3-connected µ3-dpna3− nodes (gray).
Figure 5. Structure of 6. (a) Coordination environment at Co(II) atoms; hydrogen atoms are ignored. (b) Two-dimensional cation metal–organic network (view down the a axis). (c) Topological representation of a 3-connected network with a fes topology (view down the a axis); 3-connected Co1 nodes (pink), centroids of 3-connected µ3-dpna3− nodes (gray).
Molecules 28 07474 g005
Molecules 28 07474 g006
Figure 6. Structure of 7. (a) Coordination environment at Mn(II) atoms; hydrogen atoms are ignored. (b) Mn3 unit. (c) Two-dimensional metal–organic network (view down the c axis). (d) Topological representation of a 2,5,6-connected network with topology (view down the c axis); Mn series nodes (light green), centroids of 6-connected µ6-dpna3− nodes (gray), centroids of 2-connected μ-dpea linkers (blue).
Figure 6. Structure of 7. (a) Coordination environment at Mn(II) atoms; hydrogen atoms are ignored. (b) Mn3 unit. (c) Two-dimensional metal–organic network (view down the c axis). (d) Topological representation of a 2,5,6-connected network with topology (view down the c axis); Mn series nodes (light green), centroids of 6-connected µ6-dpna3− nodes (gray), centroids of 2-connected μ-dpea linkers (blue).
Molecules 28 07474 g006
Molecules 28 07474 sch003
Scheme 3. Knoevenagel condensation of benzaldehyde with malononitrile.
Scheme 3. Knoevenagel condensation of benzaldehyde with malononitrile.
Molecules 28 07474 sch003
Table 1. Summary of crystal data for compounds 1–7.
Table 1. Summary of crystal data for compounds 1–7.
Compound1234
Chemical formulaC26H19CoN3O7C78H42Zn4.5N9O18C24H14Co1.5N3O6C48H28Zn3N6O12
Molecular weight544.371687.37528.781076.87
Crystal systemTriclinicTriclinicTriclinicTriclinic
Space groupP − 1P − 1P − 1P − 1
a/Å9.3127(5)10.2995(2)9.52279(17)9.5341(4)
b/Å12.2573(5)14.4165(3)10.17273(19)10.2025(3)
c/Å12.3293(6)23.8214(5)12.5987(2)12.6077(5)
α/(°)119.515(5)88.9997(15)76.6446(16)76.362(3)
β/(°)103.370(4)81.6471(16)71.2712(17)70.953(4)
γ/(°)97.284(4)84.2605(16)78.5857(16)78.498(3)
V/Å31142.36(11)3481.98(12)1114.40(4)1116.49(8)
Z2221
F(000)5581704535544
Crystal size/mm0.09 × 0.03 × 0.030.06 × 0.04 × 0.030.09 × 0.05 × 0.040.10 × 0.07 × 0.05
θ range for data collection1.991–25.501.871–75.713.766–67.9822.073–25.999
Limiting indices−11 ≤ h ≤ 11, −14 ≤ k ≤ 14, −14 ≤ l ≤ 14−12 ≤ h ≤ 12, −18 ≤ k ≤ 17, −28 ≤ l ≤ 30−11 ≤ h ≤ 7, −12 ≤ k ≤ 11, −15 ≤ l ≤ 14−11 ≤ h ≤ 11, −12 ≤ k ≤ 11, −15 ≤ l ≤ 15
Reflections collected/unique (Rint)4240/3434 (0.0413)14,040/11,155 (0.0271)4016/3810 (0.0245)4401/3497 (0.0426)
Dc/(Mg·cm−3)1.5831.6091.5761.602
μ/mm−10.8072.4349.2621.671
Data/restraints/parameters4240/7/33514,040/168/10934016/0/3134401/0/313
Goodness-of-fit on F21.0651.1151.0271.008
Final R [(I ≥ 2σ(I))] R1, wR20.0465, 0.12030.0407, 0.11760.0324, 0.08750.0355, 0.0880
R (all data) R1, wR20.0590, 0.12760.0504, 0.12420.0341, 0.08850.0485, 0.0942
Largest diff. peak & hole/(e·Å−3)0.905 & −0.4500.513 & −0.5160.864 & −0.4350.561 & −0.393
Compound567
Chemical formulaC24H24Co1.5N3O11C60H58Co3N6O23C52H36Mn3N6O12
Molecular weight618.861407.911101.69
Crystal systemMonoclinicMonoclinicTriclinic
Space groupP21/nC2/cP − 1
a/Å6.82955(7)27.5511(3)9.6797(6)
b/Å18.1486(2)13.9096(2)10.2210(6)
c/Å19.02692(18)15.26792(19)12.8368(7)
α/(°)9090100.562(5)
β/(°)94.8812(10)90.8226(11)103.381(5)
γ/(°)9090102.206(5)
V/Å32349.77(4)5850.44(14)1171.04(12)
Z441
F(000)12702900561
Crystal size/mm0.09 × 0.04 × 0.030.05 × 0.03 × 0.020.08 × 0.04 × 0.03
θ range for data collection3.371–67.9973.203–77.1062.102–25.496
Limiting indices−8 ≤ h ≤ 6, −17 ≤ k ≤ 21, −22 ≤ l ≤ 22−34 ≤ h ≤ 34, −17 ≤ k ≤ 17, −11 ≤ l ≤ 19−11 ≤ h ≤ 11, −12 ≤ k ≤ 12, −15 ≤ l ≤ 15
Reflections collected/unique (Rint)4273/3958 (0.0357)5732/4870 (0.0347)4357/3733 (0.0281)
Dc/(Mg·cm−3)1.7491.5981.562
μ/mm−19.0457.3610.870
Data/restraints/parameters4273/1/3645732/0/4284357/0/331
Goodness-of-fit on F21.0441.0471.022
Final R [(I ≥ 2σ(I))] R1, wR20.0321, 0.07900.0381, 0.10180.0303, 0.0740
R (all data) R1, wR20.0354. 0.08040.0472, 0.10730.0374, 0.0767
Largest diff. peak & hole/(e·Å−3)0.226 & −0.4640.333 & −0.4330.306 & −0.236
Table 2. Knoevenagel condensation of benzaldehyde with malononitrile.
Table 2. Knoevenagel condensation of benzaldehyde with malononitrile.
EntryCatalystT(°C)Time (min)Catalyst Loading (mol%)SolventYield a (%)
1425102.0CH3OH45
2425202.0CH3OH62
3425302.0CH3OH73
4425402.0CH3OH84
5425502.0CH3OH95
6425602.0CH3OH100
7425601.0CH3OH94
8425602.0H2O98
9425602.0C2H5OH95
10425602.0CH3CN88
11425602.0CHCl365
12125602.0CH3OH89
13225602.0CH3OH100
14325602.0CH3OH100
15525602.0CH3OH86
16625602.0CH3OH82
17725602.0CH3OH100
18Blank2560−CH3OH20
19ZnCl225602.0CH3OH32
20H3dpna25602.0CH3OH26
a Calculated by 1H NMR spectroscopy: mol(product)/mol(aldehyde + product) × 100.
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Kang, X.; Ren, C.; Mei, Z.; Fan, X.; Xue, J.; Shao, Y.; Gu, J. Hydrothermal Assembly, Structural Multiplicity, and Catalytic Knoevenagel Condensation Reaction of a Series of Coordination Polymers Based on a Pyridine-Tricarboxylic Acid. Molecules 2023, 28, 7474. https://doi.org/10.3390/molecules28227474

AMA Style

Kang X, Ren C, Mei Z, Fan X, Xue J, Shao Y, Gu J. Hydrothermal Assembly, Structural Multiplicity, and Catalytic Knoevenagel Condensation Reaction of a Series of Coordination Polymers Based on a Pyridine-Tricarboxylic Acid. Molecules. 2023; 28(22):7474. https://doi.org/10.3390/molecules28227474

Chicago/Turabian Style

Kang, Xiuqi, Chao Ren, Zhenzhong Mei, Xiaoxiang Fan, Jijun Xue, Yongliang Shao, and Jinzhong Gu. 2023. "Hydrothermal Assembly, Structural Multiplicity, and Catalytic Knoevenagel Condensation Reaction of a Series of Coordination Polymers Based on a Pyridine-Tricarboxylic Acid" Molecules 28, no. 22: 7474. https://doi.org/10.3390/molecules28227474

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