
==== Front
RSC Adv
RSC Adv
RA
RSCACL
RSC Advances
2046-2069
The Royal Society of Chemistry

d4ra05352d
10.1039/d4ra05352d
Chemistry
Hydrothermal synthesis, structures, and catalytic performance of five coordination compounds driven by 5-aminoisophthalic acid†
† Electronic supplementary information (ESI) available: optical images (Fig. S1), FTIR spectra (Fig. S2), PXRD patterns (Fig. S3), additional catalysis data (Fig. S4–S7) and structural parameters (Tables S1 and S2). CCDC 2367606–2367610. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ra05352d

Mei Zhen-Zhong a
Wang Hong-Yu a
Ren Chao a
https://orcid.org/0000-0002-6441-4064
N-4207-2014
Yang Ying a
https://orcid.org/0000-0001-6704-7370
Gu Jin-Zhong a
a College of Chemistry and Chemical Engineering, Lanzhou University Lanzhou 730000 People's Republic of China gujzh@lzu.edu.cn

3 9 2024
29 8 2024
3 9 2024
14 38 2816028167
24 7 2024
21 8 2024
This journal is © The Royal Society of Chemistry
2024
The Royal Society of Chemistry
https://creativecommons.org/licenses/by-nc/3.0/ This article is licensed under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given and it is not used for commercial purposes.
An amino-functionalized-dicarboxylic acid, 5-aminoisophthalic acid (H2aipa), was used as a versatile building block to synthesize a series of five novel coordination compounds under hydrothermal conditions and formulated as [Co(μ3-aipa)(2,2′-H2biim)]n (1), [Ni2(μ-aipa)2(2,2′-H2biim)2(H2O)4]·4H2O (2), {[Cd(μ3-aipa)(2,2′-H2biim)]·H2O}n (3), {[Ni(μ-aipa)(μ-bpb)]·0.5bpb·H2O}n (4), and {[Ni2(μ-aipa)(μ3-aipa)(μ-dpea)2(H2O)][Ni(μ-aipa)(μ-dpea)(H2O)]·8H2O}n (5). Three supporting ligands (2,2′-biimidazole (H2biim),1,4-bis(pyrid-4-yl)benzene (bpb), and 1,2-di(4-pyridyl)ethane (dpea)) were used in the synthesis. The structures of the studied products 1–5 vary significantly, ranging from a 0D dimer (2), 2D sheets (1, 3 and 4) to 3D + 2D interpenetrated frameworks (5). Furthermore, these compounds were evaluated as heterogeneous catalysts for the Knoevenagel reaction, achieving high product yields under optimized conditions. In addition, we also investigated various reaction parameters, substrate scope, and assessed the feasibility of catalyst recycling. This thorough investigation highlights the versatility of H2aipa as a dicarboxylate building block in the formation of functional coordination polymers.

The structural and catalytic properties of five new compounds were investigated.

pubstatusPaginated Article
==== Body
pmcIntroduction

Coordination polymers (CPs) are an important class of crystal materials that are constructed using self-assembly of metal ions with organic bridging ligands.1–3 Their versatile topologies and chemical properties4,5 make them ideal candidates for diverse applications such as gas adsorption and separation,6–9 heterogeneous catalysis,10–12 sensing,8,13,14 and biomaterials.15 To study the process of mineral formation under supercritical conditions in the laboratory, researchers initially employed hydrothermal and solvothermal synthesis methods. These days, hydrothermal synthesis is extensively utilized in the construction of CPs, especially for the preparation of CPs crystals.16–19 The self-assembly in these reactions is highly sensitive to several variables including the types of metal ion, the concentration of reactants as well as the pH value of the solution.20–23 Polycarboxylate ligands are commonly used as versatile organic linkers in the synthesis of metal clusters and polymers.24,25 This preference stems from their partially or fully deprotonated sites, which enable the development of different structural topologies and exhibit a variety of coordination modalities.26 These ligands, in particular, can function as terminal unidentate, chelating, bridging bidentate and bridging tridentate ligands.27

During the last few years, our research has focused on designing new functional coordination polymers using polycarboxylic ligands.10,18,28 In this study, we chose 5-aminoisophthalic acid (H2aipa, Scheme 1) as a amino-functionalized dicarboxylate building block.

Scheme 1 Structures of H2aipa block and auxiliary ligands.

H2aipa exhibits several intriguing properties that make it an attractive linker: (a) H2aipa has five coordination sites, including one amino nitrogen and four carboxyl oxygen atoms. (b) The free amino nitrogen atom can act as a base site for catalysis, facilitating reactions such as the Henry reaction, cyanosilylation and Knoevenagel condensation.28–30

Here, we report the synthesis and characterization of five novel coordination compounds derived from metal(ii) salts, H2aipa, and three auxiliary ligands. These compounds exhibit diverse structures, ranging from a 0D dimer (2), 2D sheets (1, 3 and 4) to 3D + 2D interpenetrated frameworks (5), specifically [Co(μ3-aipa)(2,2′-H2biim)]n (1), [Ni2(μ-aipa)2(2,2′-H2biim)2(H2O)4]·4H2O (2), {[Cd(μ3-aipa)(2,2′-H2biim)]·H2O}n (3), {[Ni(μ-aipa)(μ-bpb)]·0.5bpb·H2O}n (4), and {[Ni2(μ-aipa)(μ3-aipa)(μ-dpea)2(H2O)][Ni(μ-aipa)(μ-dpea)(H2O)]·8H2O}n (5).

In addition, we have also evaluated the catalytic activities of these compounds in the Knoevenagel reaction between pyridine-3-aldehyde with propanedinitrile.

Experimental section

General methods

All chemicals were purchased commercially utilized exactly as supplied. To record the FTIR spectra (KBr discs), a Bruker EQUINOX 55 spectrometer was utilized. Elemental analyses (EA) for carbon, hydrogen, and nitrogen in compounds 1–5 were conducted using an ElementarVario EL elemental analyzer. Thermogravimetric analyses (TGA) were conducted using a LINSEIS STA PT1600 thermal analyzer under a nitrogen flow and heating at a rate of 10 °C min−1. The powder X-ray diffraction patterns (PXRD) of the compounds were collected using a Rigaku-Dmax 2400 diffractometer (CuKα radiation; λ = 1.54060 Å, Rigaku Corporation, Tokyo, Japan). For 1H NMR experiments, a JNM ECS 400 M spectrometer was used with CDCl3 as the solvent.

Synthesis of [Co(μ3-aipa)(2,2′-H2biim)]n (1)

A 20 mL Teflon cup was sealed with the following contents: H2aipa (36.2 mg, 0.2 mmol), H2biim (26.8 mg, 0.2 mmol), CoCl2·6H2O (47.6 mg, 0.2 mmol), NaOH (16.0 mg, 0.4 mmol), and H2O (10 mL). After three days of heating the mixture at 160 °C, it was allowed to cool to ambient temperature. Purple block-shaped crystals were extracted and rinsed with water. Yield: 43% based on H2aipa. Computed for C14H11CoN5O4: C 45.18, N 18.82, H 2.98%. Found: C 45.43, N 18.49, H 3.00%. FTIR (KBr, cm−1): 3578 w, 3309 w, 1620 w, 1595 m, 1517 m, 1412 s, 1327 s, 1244 w, 1100 w, 1058 w, 995 w, 961 w, 920 w, 845 w, 778 m, 730 w, 666 w, 621 w 580 w, 547 w.

Synthesis of [Ni2(μ-aipa)2(2,2′-H2biim)2(H2O)4]·4H2O (2)

Compound 2 was synthesized using a method analogous to CP 1 by substituting NiCl2·6H2O (47.6 mg, 0.2 mmol) for CoCl2·6H2O. The synthesis yielded green block-shaped crystals of compound 2, with a 42% yield based on H2aipa. Computed for C28H38Ni2N10O16: C 37.87, N 15.77, H 4.31%. Found: C 38.03, N 15.85, H 4.28%. FTIR (KBr, cm−1): 3273 m, 3168 w, 3120 w, 1617 w, 1554 s, 1487 w, 1371 s, 1270 w, 1192 w, 1128 w, 1054 w, 991 w, 964 w, 924 w, 864 w, 818 w, 785 m, 748 w, 715 m, 610 w, 570 w.

Synthesis of {[Cd(μ3-aipa)(2,2′-H2biim)]·H2O}n (3)

CP 3 was synthesized using a method analogous to CP 1 by substituting CdCl2·H2O (40.2 mg, 0.2 mmol) for CoCl2·6H2O. The synthesis yielded yellow block-shaped crystals of CP 3, with a 45% yield based on H2aipa. Computed for C14H13CdN5O5: C 37.90, N 15.78, H 2.95%. Found: C 38.15, N 16.01, H 2.96%. FTIR (KBr, cm−1): 3304 m, 3266 m, 3180 w, 1613 w, 1580 m, 1546 s, 1397 s, 1349 s, 1323 s, 1252 w, 1114 m, 1021 w, 964 w, 920 w, 853 w, 782 m, 734 w, 673 w, 595 w, 540 w.

Synthesis of {[Ni(μ-aipa)(μ-bpb)]·0.5bpb·H2O}n (4)

A 20 mL Teflon cup was sealed with the following contents: H2aipa (36.2 mg, 0.2 mmol), bpb (46.4 mg, 0.2 mmol), NiCl2·6H2O (47.6 mg, 0.2 mmol), NaOH (16.0 mg, 0.4 mmol), and H2O (10 mL). After three days of heating the mixture at 160 °C, it was allowed to cool to ambient temperature. Green block-shaped crystals were extracted and rinsed with water. Yield: 46% based on H2aipa. Computed for C32H25N4NiO5: C 63.61, N 9.27, H 4.17%. Found: C 63.90, N 9.23, H 4.20%. FTIR (KBr, cm−1): 3359 m, 3309 m, 3083 w, 1614 s, 1546 s, 1480 w, 1386 s, 1226 w, 1110 w, 1073 w, 1013 w, 905 w, 804 m, 726 w, 678 w, 618 w, 573 w.

Synthesis of {[Ni2(μ-aipa)(μ3-aipa)(μ-dpea)2(H2O)][Ni(μ-aipa)(μ-dpea)(H2O)]·8H2O}n (5)

A 20 mL Teflon cup was sealed with the following contents: H2aipa (36.2 mg, 0.2 mmol), dpea (36.8 mg, 0.2 mmol), NiCl2·6H2O (47.6 mg, 0.2 mmol), NaOH (16.0 mg, 0.4 mmol), and H2O (10 mL). After three days of heating the mixture at 160 °C, it was allowed to cool to ambient temperature. Green block-shaped crystals were extracted and rinsed with water. Yield: 43% based on H2aipa. Computed for C60H71N9Ni3O22: C 49.83, N 8.72, H 4.95%. Found: C 50.17, N 8.75, H 4.91%. FTIR (KBr, cm−1): 3445 w, 3363 m, 3236 w, 1621 m, 1546 s, 1468 w, 1431 m, 1382 s, 1222 w, 1099 w, 1069 w, 1024 w, 957 w, 894 w, 831 w, 789 m, 726 m, 673 w, 606 w, 547 w.

Single crystal X-ray diffraction & topological analysis

A Bruker APEX-II CCD diffractometer was used to collect the crystal data of compounds 1–5 using graphite-monochromated Mo/CuKα radiation; λ = 0.71073/1.54178 Å). SHELXS-97 and SHELXL-97 software were used for the determination of the structures.31 Detailed crystal parameters and structural refinements can be found in Table 1, with selected bond parameters are listed in Tables S1 and S2 (ESI).† The supplementary crystallographic data for compounds 1–5 are available in CCDC 2367606–2367610.

Crystal data for 1–5

Compound	1	2	3	4	5	
Chemical formula	C14H11CoN5O	C28H38Ni2N10O16	C14H13CdN5O5	C32H25N4NiO5	C60H71N9Ni3O22	
Formula weight	372.21	888.10	443.68	604.27	1446.24	
Crystal system	Triclinic	Monoclinic	Triclinic	Monoclinic	Triclinic	
Space group	P1̄	P21/c	P1̄	P21/n	P1	
a/Å	7.6951(3)	11.9238(3)	7.89650(10)	11.0968(4)	10.1262(3)	
b/Å	8.0250(2)	8.0959(2)	8.16800(10)	22.3594(7)	13.2389(3)	
c/Å	11.2161(6)	18.3412(5)	11.4250(2)	11.6156(4)	13.3575(3)	
α/°	83.335(3)	90	83.7430(10)	90	88.8838(17)	
β/°	76.981(4)	95.998(2)	77.408(2)	107.065(4)	75.6606(19)	
γ/°	81.112(3)	90	81.2660(10)	90	74.270(2)	
V Å−3	664.34(5)	1760.85(8)	708.630(19)	2755.14(18)	1667.75(7)	
T/K	303(2)	303(2)	285(2)	293(2)	303(2)	
Z	2	2	2	4	1	
D c/g cm−3	1.861	1.675	1.995	1.457	1.297	
μ/mm−1	1.326	1.159	12.657	0.754	1.530	
F(000)	378	920	420	1252	674	
Refl. measured	2470	3277	2577	5127	8599	
Unique refl. (Rint)	2264 (0.0319)	2914 (0.0332)	2463 (0.0291)	4389 (0.0261)	8204 (0.0380)	
GOF on F2	1.136	1.031	1.037	1.051	1.066	
R 1[I > 2σ(I)]a	0.0369	0.0324	0.0226	0.0300	0.0420	
wR2[I > 2σ(I)]b	0.0759	0.0864	0.0579	0.0788	0.1184	
a R 1 = Σ||Fo| − |Fc||/|Fo|.

b wR2 = {Σ[w(Fo2 − Fc2)]2/Σ[w(Fo2)]2}1/2.

Topological analysis of the obtained coordination polymers was conducted using ToposPro software. This involved generating a simplified underlying net, wherein bridging ligands were reduced to the centroids.32,33

Catalytic activity in Knoevenagel reaction

The following ingredients were mixed in a suspension and stirred at 25 °C for a required reaction time: catalyst (2.0 mol%), aromatic aldehyde (0.50 mmol), propanedinitrile (1.0 mmol), and solvent (1.0 mL, usually CH3OH). Centrifugation was subsequently used to extract the catalyst. A crude solid product was obtained by evaporating the filtrate using a rotary evaporator. The amount of this solid product was ascertained by 1H NMR spectroscopy (JNM ECS 400M spectrometer) after it was dissolved in CDCl3 (Fig. S7, ESI†). The catalyst was recovered by centrifugation, washed with methanol, allowed to dry at room temperature, and subsequently reused in further reactions, following the same procedure for recycling experiments.

Results and discussion

Hydrothermal synthesis of compounds 1–5

A series of reactions were conducted under hydrothermal conditions to assess the potential of 5-aminoisophthalic acid (H2aipa) as a building block for synthesizing coordination polymers. These reactions involved a mixture of metal(ii) salts, H2aipa, sodium hydroxide, and an auxiliary ligand selected from the following H2biim, bpb, and dpea. The hydrothermal synthesis was carried out at 160 °C for three days, followed by a gradual reduction in temperature to promote the crystallization of CPs. The resulting products were collected in substantial yields and analyzed using standard techniques. The synthesis repeatability of the five complexes is good and the yields are stable. The experimental PXRD patterns of products 1–5 agree with the computed diffractograms (Fig. S3†), suggesting their homogeneous phase purity. Structural discrepancies among compounds 1–5 could be attributed to the varying auxiliary ligands and the coordination properties of the metal(ii) centers.

Description of structures

[Co(μ3-aipa)(2,2′-H2biim)]n (1)

The asymmetric unit of CP 1 contains a μ3-aipa2− block, a Co(ii) center, and a H2biim auxiliary ligand (Fig. 1a). The Co1 center is six-coordinate with the Co1 center being coordinated by three carboxyl oxygen and one N atoms from three μ3-aipa2− linkers and two N donors from the H2biim moiety and forms a distorted octahedral {CoN3O3} geometry. The Co–O and Co–N bond lengths, which measure 2.044(2)–2.399(2) and 2.087(2)–2.230(3) Å, are within the typical ranges for Co(ii) compounds.18,28 In CP 1, the aipa2− ligand exhibits the coordination mode I (Scheme 2) with two monodentate or bidentate COO− groups. Furthermore, the nitrogen atom of the NH2 is involved in coordination. The μ3-aipa2− blocks connect the Co(ii) centers, forming a 2D sheet (Fig. 1b) with hcb topology (Fig. 1c).

Fig. 1 Structure of CP 1. (a) Coordination environment at cobalt(ii) atom. (b) The two-dimensional layer viewed along the c axis. (c) The sheet exhibiting a hcb topology, observed along the c axis.

Scheme 2 The coordination modes of aipa2−.

[Ni2(μ-aipa)2(2,2′-H2biim)2(H2O)4]·4H2O (2)

One Ni(ii) atom, one μ-aipa2− block, one H2biim, two H2O ligands and two lattice water molecules form the asymmetric unit of compound 2 (Fig. 2a). The six-coordinate Ni1 atom shows a distorted octahedral {NiN3O3} environment. It comprises one carboxyl oxygen and one N atoms from two μ-aipa2− blocks, two O donors from two H2O ligands, and a pair of N atoms of a H2biim moiety. The Ni–O [2.021(2)–2.120(2) Å] and Ni–N [2.072(2)–2.164(2) Å] bond lengths fall within the expected ranges.18,28 The cpic2− block functions as a μ-linker (mode II, Scheme 2). Fig. 2b shows that a Ni2 molecule is formed when two μ-aipa2− blocks link two Ni1 atoms. These Ni2 molecules are further assembled into a 3D H-bonded supramolecule framework (Fig. 2c).

Fig. 2 Structure of dimer 2. (a) Coordination environment at nickel(ii) atom. (b) The Ni2 molecule. (c) A 3D H-bonded supramolecule framework; view along the b axis.

{[Cd(μ3-aipa)(2,2′-H2biim)]·H2O}n (3)

CP 3 is an asymmetric unit with one μ3-aipa2− block, one cadmium(ii) center, one H2biim supporting ligand, and one lattice water molecule. The five-coordinate Cd1 atom has a distorted trigonal bipyramidal {CdO2N3} geometry (Fig. 3a), made up of one nitrogen and two carboxyl oxygen atoms from three distinct μ3-aipa2− blocks and two Ndonor from the H2biim ligand. The Cd–O [2.248(2)–2.285(2) Å] and Cd–N [2.257(2)–2.447(3) Å] distances are comparable to those in Cd(ii) derivatives.28,29 In 3, the aipa2− block behaves as a μ3-spacer (mode III, Scheme 2). The adjacent cadmium(ii) atoms are linked by μ3-aipa2− blocks, generating a 2D sheet (Fig. 3b) with a hcb topology (Fig. 3c).

Fig. 3 Structure of CP 3. (a) Coordination environment at cadmium(ii) center. (b) The two-dimensional layer viewed along the c axis. (c) Sheet exhibiting a hcb topology, observed along the c axis.

{[Ni(μ-aipa)(μ-bpb)]·0.5bpb·H2O}n (4)

CP 4 is an asymmetric unit with one Ni(ii) center, one μ-aipa2− block, a half of μ-bpb, moiety and one free water molecule. The compound forms a two-dimensional coordination polymer (Fig. 4a). As shown in Fig. 4a, the nickel center is six-coordinate and form a distorted octahedral {NiO4N2} geometry. The geometry is completed by two Nbpb atoms from two μ-bpb moieties and four carboxyl oxygen atoms from two μ-aipa2−. The Ni–O [2.052(2)–2.192(2) Å] and Ni–N [2.050(2)–2.062(2) Å] bond lengths fall within expected ranges.18,28 The aipa2− moiety adopts μ-coordination fashion (mode IV, Scheme 2). The aipa2− and bpb ligands linked neighbouring Ni(ii) centers, resulting a 2D layer (Fig. 4b) with a new topology and point symbol of (84.122)(8)2 (Fig. 4c).

Fig. 4 Structure of 4. (a) Coordination environment at the nickel(ii) atom. (b) The two-dimensional layer viewed along the a axis. (c) Sheet exhibiting a new topology, viewed along the a axis.

{[Ni2(μ-aipa)(μ3-aipa)(μ-dpea)2(H2O)][Ni(μ-aipa)(μ-dpea)(H2O)]·8H2O}n (5)

The structure consists of a [Ni(μ-aipa)(μ-dpea)(H2O)]n 2D sheet and a [Ni2(μ-aipa)(μ3-aipa)(μ-dpea)2(H2O)]n 3D framework. CP 5 is an asymmetric unit with three distinct nickel(ii) centers, three aipa2− blocks, three dpea, two coordinated water molecules, and eight lattice water molecules. The Ni1 and Ni3 atoms exhibit distorted octahedral {NiO4N2} coordination geometries, which include three carboxyl oxygen atoms from two distinct aipa2− blocks, one O atom from the H2O ligand, and two N donors from two dpea ligands (Fig. 5a). The six-coordinate nickel 2 center forms a distorted octahedral {NiO3N3} configuration. It is surrounded by one nitrogen and three oxygen atoms from three distinct aipa2− blocks, as well as two N atoms of two dpea ligands. The Ni–O [2.002(3)–2.158(4) Å] and Ni–N [2.070(4)–2.145(4) Å] bond lengths are similar to those observed in analogous Ni(ii) derivatives.18,28 The aipa2− blocks act as μ- or μ3-linkers (modes IV and V, Scheme 2). The dpea shows a bridging coordination fashion. The μ-aipa, μ3-aipa and μ-dpea blocks linked Ni1 and Ni2 centers to form a 3D framework (Fig. 5b). Meanwhile, the Ni3 atoms were connected by the μ3-aipa and μ-dpea ligands to generate a 2D layer (Fig. 5c). Finally, 3D + 2D interpenetrated frameworks were formed (Fig. 5d) with a new topology and point symbol of (84·122)(8)2 (Fig. 5e and f).

Fig. 5 Structure of 5. (a) Coordination environment at nickel(ii) atoms. (b) Three-dimensional framework along the c axis. (c) Two-dimensional metal–organic network along the b axis. (d) 3D + 2D interpenetrated frameworks along the c axis. (e) Topological representation of the framework along the c axis. (f) Two 3D + 2D interpenetrated frameworks shown by different colours (green and gray).

TGA & PXRD data

The assessment of thermal stability for compounds 1–5 was carried out using thermogravimetric analysis (TGA) under a N2 atmosphere within a temperature range of 23–800 °C (Fig. 6). The CP 1 does not contain coordinated or crystallization water, so its network is stable up to 395 °C. At temperatures 72 to 292 °C, dimer 2 lost four lattice and four coordinated water molecules (exptl 15.9%; calculated 16.2%), whereas the dehydrated sample held steady until 316 °C. For CP 3, a weight loss of 4.3% (calcd 4.1%) occurred at temperatures from 28 and 193 °C, associated with the release of one lattice water molecule. In CP 4, one lattice water and a half of bpb were lost at 28–193 °C (exptl, 22.0%; calcd, 22.2%), with the dehydrated sample is stable up to 343 °C. For CP 5, a mass loss between 29–190 °C indicated the release of eight lattice water molecules and two coordinated water molecules (exptl 12.1%, calculated 12.4%), whereas the sample held steady until 335 °C.

Fig. 6 TGA plots of 1–5.

Power X-ray diffractograms were acquired at 25 °C for compounds 1–5 (Fig. S3†). Phase purity of all samples was established by analyzing the experimental patterns and comparing them with the simulated ones (based on CIF data).

Catalytic Knoevenagel reaction

We investigated compounds 1–5 for their potential as heterogeneous catalysts in the Knoevenagel reaction, which involves a combination of propanedinitrile and a variety of aldehydes, considering the possibility of different coordination complexes acting as catalysts in this reaction.28–30,34,35 Using pyridine-3-aldehyde as a model substrate, we reacted it with malononitrile at 25 °C in methanol to synthesize the related product. As a model substrate, pyridine-3-aldehyde was reacted with malononitrile at 25 °C in methanol (Scheme 3 and Table 2). Furthermore, we conducted a thorough investigation of various reaction parameters, which included the duration of the reaction, the type of solvent used, catalyst loading, potential for catalyst reuse, and the scope of substrates involved. CP 3 exhibited the highest activity, achieving a >99 conversion rate of pyridine-3-aldehyde (Table 2). It was then used to study the effects of different reaction parameters. The yield increased from 55 to >99% when the reaction time was extended from 10 to 60 min (Table 2, entries 1–6). The impact of catalyst amount was also examined, and the results showed that increasing the loading of catalyst from 1 to 2 mol% increased product yield from 95 to >99% (entries 6 and 11). It is worth mentioning that various solvents, including methanol, were tested in these reactions. Chloroform, acetonitrile, water, and ethanol all have lower product yields (66–98% product yields). Compounds 1, 2, 4 and 5 exhibit reduced activity when compared to CP 3, achieving product yields in the 82–90% range (entries 12–15, Table 2). It is worth mentioning that the Knoevenagel reaction of pyridine-3-aldehyde is much less efficient without a catalyst (only 20% product yield) or when H2aipa or CdCl2 are used as catalysts (28% yield or 24% yield), respectively under comparable reaction conditions (entries 16–18, Table 2). The improved performance of CP 3 is likely due to the presence of unsaturated coordination sites in the Cd(ii) centers, which makes it easier to access the substrates.28,30,32

Scheme 3 Knoevenagel reaction of pyridine-3-aldehyde with propanedinitrile.

Knoevenagel reaction of pyridine-3-aldehyde with propanedinitrilea

Entry	Catalyst	Reaction time (min)	Catalyst loading (mol%)	Solvent	Product yieldb (%)	
1	3	10	2.0	CH3OH	55	
2	3	20	2.0	CH3OH	75	
3	3	30	2.0	CH3OH	84	
4	3	40	2.0	CH3OH	91	
5	3	50	2.0	CH3OH	96	
6	3	60	2.0	CH3OH	>99	
7	3	60	2.0	H2O	98	
8	3	60	2.0	C2H5OH	96	
9	3	60	2.0	CH3CN	86	
10	3	60	2.0	CHCl3	66	
11	3	60	1.0	CH3OH	95	
12	1	60	2.0	CH3OH	82	
13	2	60	2.0	CH3OH	87	
14	4	60	2.0	CH3OH	85	
15	5	60	2.0	CH3OH	90	
16	Blank	60	—	CH3OH	20	
17	CdCl2·2H2O	60	2.0	CH3OH	32	
18	H2aipa	60	2.0	CH3OH	26	
a Conditions: pyridine-3-aldehyde (0.5 mmol), propanedinitrile (1.0 mmol), catalyst (1–2 mol%), solvent (1.0 mL), 25 °C.

b Yield based on 1H NMR analysis: [moles of product per mol of aldehyde substrate] × 100%.

A range of substituted benzaldehyde substrates was evaluated to determine the substrate scope in the Knoevenagel reaction with propanedinitrile. These reactions were carried out under optimized conditions (2.0 mol% 3, methanol, 25 °C, 1 h). Table 3 shows that the yields of the respective products ranged from 33 to >99%. Benzaldehydes with strong electron-withdrawing groups, such as nitro, chloro, bromo and fluoro substituents, demonstrated the highest efficiency (entries 2–7, Table 3). This increased efficiency is likely due to the higher electrophilicity of these substrates. However, benzaldehydes with electron-donating groups, such as methyl or methoxy groups, resulted in lower product yields (entries 9 and 10, Table 3).

Substrate scope for Cd-catalyzed Knoevenagel reaction of substituted benzaldehydes with propanedinitrilea

Entry	Substituted benzaldehyde	Product yieldb (%)	
1	Benzaldehyde	>99	
2	2-Nitrobenzaldehyde	>99	
3	3-Nitrobenzaldehyde	>99	
4	4-Nitrobenzaldehyde	>99	
5	4-Chlorobenzaldehyde	>99	
6	4-Bromobenzaldehyde	>99	
7	4-Fluorobenzaldehyde	>99	
8	Pyridine-4-aldehyde	>99	
9	4-Methylbenzaldehyde	97	
10	4-Methoxybenzaldehyde	66	
11	4-Hydroxybenzaldehyde	33	
a Conditions: aldehyde (0.5 mmol), propanedinitrile (1.0 mmol), catalyst 3 (2.0 mol%), CH3OH (1.0 mL), 25 °C.

b Yield on the basis of 1H NMR analysis: [moles of product per mol of aldehyde substrate] × 100%.

The recyclability of catalyst 3 was evaluated. After every reaction cycle, the catalyst was separated by centrifugation, rinsed inmethanol, air-dried at approximately 25 °C, and reused in the subsequent cycle. The data show that CP 3 retains its activity for at least five more reaction cycles (Fig. S5†). Additionally, PXRD patterns show that the structure of CP 3 remains intact (Fig. S6†), despite the occurrence of multiple new signals or widened peaks. These changes are likely due to the existence of certain or a decrease in crystallinity after multiple catalytic cycles.

Referring to previous studies in this area,36–40 we propose a possible reaction mechanism for the Knoevenagel condensation catalyzed by 3 (Scheme S1, ESI†). The unsaturated Cd(ii) metal centers of the catalyst (5-coordinate cadmium centers) eventually act as the Lewis acid sites interacting with the H–C <svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata> Created by potrace 1.16, written by Peter Selinger 2001-2019 </metadata><g transform="translate(1.000000,15.000000) scale(0.017500,-0.017500)" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg> O functionality of pyridine-3-aldehyde, leading to its polarization and an enhanced electrophilicity of the corresponding carbon atom. Such a polarization can facilitate a nucleophilic attack of this site by propanedinitrile acting as a nucleophile precursor. On the other hand, an interaction between the Lewis acid site and the –CN group of malononitrile augments an acidic character of the methylene functionality and enhances its deprotonation. The basic sites present in 3 (O-carboxylate sites) can easily abstract H+ from the –CH2– group to give rise to a nucleophile that would attack the H–CO moiety of pyridine-3-aldehyde and result in the C–C bond formation, followed by the dehydration to give the 2-(pyridin-3-ylmethylene)malononitrile product.

Conclusions

This study showed the use of H2aipa as a amino-functionalized dicarboxylate precursor to synthesize five novel coordination compounds using a hydrothermal method. These compounds 1–5 exhibit diverse structural features, ranging from 0D dimer (compound 2), 2D sheets (CPs 1, 3 and 4) to 3D + 2D interpenetrated frameworks (CP 5). The catalytic potential of compounds 1–5 was evaluated in Knoevenagel reaction, which involved pyridine-3-aldehyde and propanedinitrile. CP 3 showed significant catalytic efficiency in this reaction.

Data availability

The data supporting this article have been included as part of the ESI.† CCDC-2367606–2367610 (compounds 1–5) contain the supplementary crystallographic data for this paper.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-014-D4RA05352D-s001

RA-014-D4RA05352D-s002

This work was supported by the Science and Technology Projects by State Nickel Cobalt New Material Engineering Technology Research Center, Lanzhou, China. The grant number is GCZX2023JSKF0005.
==== Refs
Notes and references

Chakraborty G. Park I. H. Medishetty R. Vittal J. J. Chem. Rev. 2021 121 3751 3891 33630582
Maurin G. Serre C. Cooper A. Ferey G. Chem. Soc. Rev. 2017 46 3104 3107 28561090
Chen Z. J. Kirlikovali K. O. Li P. Farha O. K. Acc. Chem. Res. 2022 55 579 591 35112832
Macreadie L. K. Babarao R. Setter C. J. Lee S. J. Qazvini O. T. Seeber A. J. Tsanaktsidis J. Telfer S. G. Batten S. R. Hill M. R. Angew. Chem., Int. Ed. 2020 59 6090 6098
Dong X. Y. Si Y. Yang J. S. Zhang C. Han Z. Luo P. Wang Z. Y. Zang S. Q. Mak T. C. W. Nat. Commun. 2020 11 3678 32699338
Gu S. F. Xiong X. H. Gong L. L. Zhang H. P. Xu Y. Feng X. F. Luo F. Inorg. Chem. 2021 60 8211 8217 34018393
Fan L. Liu Z. Zhang Y. Zhao D. Yang J. Zhang X. Inorg. Chem. Commun. 2019 107 107463
Wu D. Liu J. Jin J. Cheng J. Wang M. Yang G. Wang Y. Y. Cryst. Growth Des. 2019 19 6774 6783
Zhao Y. Wang L. Fan N. N. Han M. L. Yang G. P. Ma L. F. Cryst. Growth Des. 2018 18 7114 7121
Kang X. Q. Ren C. Mei Z. Z. Fan X. X. Xue J. J. Shao Y. L. Gu J. Z. Molecules 2023 28 7474 38005197
Zheng Y. Shen Q. Li Z. Jing X. Duan C. Inorg. Chem. 2022 61 11156 11164 35799381
Markad D. Mandal S. K. Dalton Trans. 2018 47 5928 5932 29675542
Mörtel M. Oschwald J. Scheurer A. Drewello T. Khusniyarov M. M. Inorg. Chem. 2021 60 14230 14237 34403241
Rashid A. Mondal S. Ghosh P. Molecules 2023 28 1231 36770897
Hung P. Q. Lin P. Y. Wang X. H. Ho J. A. J. Chin. Chem. Soc. 2023 70 1284 1296
Qin Y. She P. Huang X. Huang W. Zhao Q. Coord. Chem. Rev. 2020 416 213331
Hong L. C. Lin L. Y. Wei L. Fei K. Y. Chin. J. Struct. Chem. 2021 40 363 368
Kang X. Q. Wang J. H. Gu J. Z. Chin. J. Inorg. Chem. 2023 39 2385 2392
Singh R. S. Paitandi R. P. Gupta R. K. Pandey D. S. Coord. Chem. Rev. 2020 414 213269
Islam S. Tripathi S. Hossain A. Seth S. K. Mukhopadhyay S. J. Mol. Struct. 2022 1265 133373
Fonseca D. Pérez-Torres A. F. Cobo J. Zapata-Rivera J. Hurtado J. J. Macías M. A. CrystEngComm 2022 24 2982 2991
Xu M. Liang G. Wang S. Ma X. Liang G. Ni Q. J. Mol. Struct. 2020 1217 128411
Kong J. J. Shao D. Zhang J. C. Jiang Y. X. Ji C. L. Huang X. C. CrystEngComm 2019 21 749 757
Li Q. P. Qian J. J. RSC Adv. 2014 4 32391 32397
Qian J. J. Jiang F. L. Su K. Z. Pan J. Zhang L. J. Li X. J. Yuan D. Q. Hong M. C. J. Mater. Chem. A 2013 1 10631 10634
Huang R.-W. Li B. Zhang Y.-Q. Zhao Y. Zang S.-Q. Xu H. Inorg. Chem. Commun. 2014 39 106 109
Chai X. Zhang H. Zhang S. Cao Y. Chen Y. J. Solid State Chem. 2009 182 1889 1898
Fan X. X. Wang H. Y. Gu J. Z. Lv D. Y. Zhang B. Xue J. J. Kirillova M. V. Kirillov A. M. Inorg. Chem. 2023 62 17612 17624 37847556
Zhai Z. W. Yang S. H. Lv Y. R. Du C. X. Li L. K. Zang S. Q. Dalton Trans. 2019 48 4007 4014 30843564
Cai Y. Q. Peng Y. Q. Song G. H. Catal. Lett. 2006 109 61 64
Sheldrick G. M. , SHELXS-97; Program for X-Ray Crystal Structure Determination, University of Gottingen, Göttingen, Germany, 1997
Blatov V. A. , IUCrCompComm Newsletter, 2006, vol. 7 , pp. 4–38
Blatov V. A. Shevchenko A. P. Proserpio D. M. Cryst. Growth Des. 2014 14 3576 3586
Chen H. Fan L. Hu T. Zhang X. Inorg. Chem. 2021 60 3384 3392 33595310
Almasi M. Zelenak V. Opanasenko M. Cejka J. Dalton Trans. 2014 43 3730 3738 24435475
Seal N. Karmakar A. Mondal P. P. Kundu S. Neogi S. ACS Appl. Mater. Interfaces 2024 16 41721 41733 39087733
Parmar B. Patel P. Murali V. Rachuri Y. Kureshy R. I. Khan N.-u. H. Suresh E. Inorg. Chem. Front. 2018 5 2630 2640
Pandey R. Singh D. Thakur N. Raj K. K. ACS Omega 2021 6 13240 13259 34056473
Saghian M. Dehghanpour S. bayatani Z. Sci. Rep. 2023 13 15563 37731034
Chand R. Karmakar A. Kundu S. Neogi S. Small 2024 2404085
