A series of mixed-ligand 2D and 3D coordination polymers assembled from a novel multifunctional pyridine-tricarboxylate building block: hydrothermal syntheses, structural and topological diversity, and magnetic and luminescent properties

Abstract

A series of novel mixed-ligand coordination polymers, namely, {[Cd₂(μ₄-L)(μ₃-OH)(phen)₂]·2H₂O}_n (1), {[Mn₃(μ₅-L)₂(phen)₂(H₂O)]·H₂O}_n (2), [Cd₂(μ₃-L)(H₂biim)₂(μ₂-Hbiim)]_n (3), {[Pb₂(μ₄-L)(μ₄-bpdc)_0.5(phen)₂]·H₂O}_n (4), [Cd₂(μ₅-L)(μ₂-bpdc)_0.5(phen)₂]_n (5), and [Zn₂(μ₄-L)(μ₂-bpdc)_0.5(py)₂]_n (6), was hydrothermally synthesized using 4-(5-carboxypyridin-2-yl)isophthalic acid (H₃L) as a new and virtually unexplored multifunctional pyridine-tricarboxylate building block, along with various auxiliary ligands {phen = 1,10-phenanthroline, H₂biim = 2,2′-biimidazole, H₂bpdc = 4,4′-biphenyldicarboxylic acid, and py = pyridine}. All the products 1–6 were characterized by IR spectroscopy, elemental, thermogravimetric, and single-crystal X-ray diffraction analyses. Compounds 1, 3, and 5 reveal 2D metal–organic layers with the kgd, fes, and 3,4,5L5 topology, respectively, whereas the metal–organic frameworks (MOFs) 2, 4, and 6 disclose 3D underlying nets with the unique (2, 4) or ins (6) topology. The results indicate that the nature of the metal(II) ion and type of auxiliary ligand play a significant role in determining the dimensionality, topology and other structural features of the obtained products. Luminescent (for 1 and 3–6) and magnetic (for 2) properties were studied and discussed.

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1. Introduction

Research on coordination polymers has recently attracted lots of attention due to their intriguing architectures and variety of applications.¹ However, it is usually difficult to control the structural and functional characteristics of coordination polymers during the self-assembly syntheses, because many factors can affect the result, such as the coordination geometry of metal ions, connectivity of organic building blocks, stoichiometry, reaction conditions (temperatures, solvents, pH values), and presence of auxiliary ligands.² A rather effective and facile approach to achieve better synthetic predictability consists of the appropriate choice of the organic bridging ligands bearing modifiable backbones, along with the selection of metal centers possessing various coordination preferences. A considerable number of aromatic polycarboxylic acids including N-heterocyclic derivatives has been extensively applied in crystal engineering of coordination polymers.³ Among such ligands, flexible biphenyl polycarboxylic acids have attracted a recent interest due to the presence of two phenyl rings that can be rotated around the C–C single bond, thus facilitating the binding to metal cations. Besides, complete or partial deprotonation of such multicarboxylate blocks can induce the variation of coordination modes and allow the generation of structurally distinct metal–organic frameworks.⁴ Moreover, such ligands can act as H-bond acceptors and donors, depending upon the degree of deprotonation, thus providing an additional structural reinforcement.

Although a substantial number of coordination polymers incorporating various kinds of carboxylate ligands has been reported, the examples of compounds constructed from a combination of two types of biphenyl carboxylate building blocks are barely known, mainly due to different solubility of such ligands, distinct charges and coordination modes, as well as competition with each other during the self-assembly syntheses. In fact, the effect of two different types of biphenyl carboxylate moieties on the structure of the resulting metal–organic framework remains poorly studied. A reaction containing multiple ligands can lead to a mixed-ligand phase, a biphasic mixture, or a pure sample wherein only one ligand is incorporated whereas another one remains in solution. The process can become even more challenging when there is an extra auxiliary ligand apart from two biphenyl carboxylate building blocks. Since it is still very difficult to predict the result of the self-assembly syntheses in these multi-ligand reaction mixtures, a prior examination of the compounds containing several ligands from the same broad family will enhance the understanding of such systems.⁵ Apart from multifunctional carboxylate ligands, 1,10-phenanthroline (phen), 2,2′-biimidazole (H₂biim) and pyridine (py) are frequently applied as simple N-donor auxiliary ligands to tune the coordination environment of metal centers and stabilize the structures of coordination polymers, namely, due to π⋯π stacking and H-bonding interactions.^4a,4b,6–11

Taking into consideration the above-mentioned points, the main objectives of the present study were the following: (1) to design a novel multifunctional polycarboxylate building block for the synthesis of coordination polymers or metal–organic frameworks, (2) to prepare unusual mixed-ligand coordination compounds that would incorporate two different types of biphenyl carboxylate building blocks, and (3) to fully characterize the obtained compounds, explore their structural and topological features and investigate functional properties. Thus, we have attempted a series of hydrothermal self-assembly reactions between different metal(II) chlorides (M = Cd, Mn, Pb and Zn) and 4-(5-carboxypyridin-2-yl)isophthalic acid (H₃L) as a new principal building block, along with optional 4,4′-biphenyldicarboxylic acid (H₂bpdc) as a secondary biphenyl carboxylate block, and also in the presence of an auxiliary ligand selected from 1,10-phenanthroline (phen), 2,2′-biimidazole (H₂biim), or pyridine (py) (Scheme 1). The present study thus reports the syntheses, full characterization, structural and topological features, as well as luminescent or magnetic properties of six new coordination polymers, namely, {[Cd₂(μ₄-L)(μ₃-OH)(phen)₂]·2H₂O}_n (1), {[Mn₃(μ₅-L)₂(phen)₂(H₂O)]·H₂O}_n (2), [Cd₂(μ₃-L)(H₂biim)₂(μ₂-Hbiim)]_n (3), {[Pb₂(μ₄-L)(μ₄-bpdc)_0.5(phen)₂]·H₂O}_n (4), [Cd₂(μ₅-L)(μ₂-bpdc)_0.5(phen)₂]_n (5), and [Zn₂(μ₄-L)(μ₂-bpdc)_0.5(py)₂]_n (6). To our knowledge, the obtained products represent the first structurally characterized examples of coordination compounds derived from H₃L, as attested by a search of the Cambridge Structural Database.

Scheme 1 Structural formulae of used building blocks.

2. Experimental

2.1 Materials and methods

All chemicals and solvents were of A.R. grade and used without further purification, except for H₃L that was synthesized by adapting a literature method.^{12 1}H NMR spectra were performed on a Bruker Advance III 400 spectrometer. Carbon, hydrogen and nitrogen contents were determined using an Elementar Vario EL elemental analyzer. IR spectra were recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer. Thermogravimetric analyses (TGA) were performed under N₂ atmosphere with a heating rate of 10 °C min⁻¹ on a LINSEIS STA PT1600 thermal analyzer. Powder X-ray diffraction patterns (PXRD) were determined with a Rigaku-Dmax 2400 diffractometer using Cu-Kα radiation. Magnetic susceptibility data were collected in the 2–300 K temperature range with a Quantum Design SQUID Magnetometer MPMS XL-7 with a field of 0.1 T. A correction was made for the diamagnetic contribution prior to data analysis. Excitation and emission spectra were recorded at room temperature for the solid samples using an Edinburgh FLS920 fluorescence spectrometer.

2.2 Synthesis of 4-(5-carboxypyridin-2-yl)isophthalic acid (H₃L)

A mixture of ethyl 6-bromonicotinate (115.0 mg, 0.50 mmol) (a), 2,4-bis(ethoxycarbonyl)phenylboronic acid (143.0 mg, 0.55 mmol) (b), K₂CO₃ (210.0 mg, 1.5 mmol), Pd(OAc)₂ (56 mg, 0.025 mmol), and C₂H₅OH (50 mL) was stirred and refluxed for 10 h. After cooling to room temperature, the precipitate was filtered off, and the filtrate was concentrated under reduced pressure. The obtained residue was purified by silica gel column chromatography with dichloromethane–ethyl acetate 36 : 1 to give diethyl 4-(5-(ethoxycarbonyl)pyridin-2-yl)isophthalate as a white solid. A suspension of diester in 1 M NaOH was stirred at 80 °C for 2 h. The solution was cooled with ice and acidified with HCl to pH = 2.5. The white precipitate was collected to give H₃L in 43% yield based on ethyl 6-bromonicotinate. Calcd for C₁₄H₉NO₆ (287.22): C 58.54, H 3.16, N 4.88%. Found: C 58.18, H 3.07, N 4.95%. IR (KBr, cm⁻¹): ν(COO) 1684 s. ¹H NMR (400 MHz, DMSO-d₆): δ 13.43 (s, 3H), δ 7.66–7.69 (m, 2H), δ 8.035 (d, J = 8.0 Hz, 1H), δ 8.186 (s, 1H), δ 8.237 (d, J = 8.0 Hz, 1H), δ 9.044 (s, 1H).

2.3 Synthesis of {[Cd₂(μ₄-L)(μ₃-OH)(phen)₂]·2H₂O}_n (1)

A mixture of CdCl₂·H₂O (60.3 mg, 0.3 mmol), H₃L (57.4 mg, 0.2 mmol), phen (60.0 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H₂O (10 mL) was stirred at room temperature for 15 min, then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160 °C for 3 days, followed by cooling to room temperature at a rate of 10 °C h⁻¹. Colorless block-shaped crystals of 1 were isolated manually, and washed with distilled water and dried (yield 45% based on CdCl₂·H₂O). Anal. calcd for C₃₈H₂₇Cd₂N₅O₉: C, 49.48; H, 2.95; N, 7.59. Found: C 49.28, H 3.06, N 7.34%. IR (KBr, cm⁻¹): 3457 w, 3055 w, 1588 s, 1516 m, 1429 m, 1373 s, 1145 m, 1102 w, 1025 w, 910 w, 861 m, 823 w, 785 m, 728 m, 702 w, 562 w.

2.4 Synthesis of {[Mn₃(μ₅-L)₂(phen)₂(H₂O)]·H₂O}_n (2)

Synthesis of 2 was similar to that of 1 except that CdCl₂·H₂O was replaced by MnCl₂·4H₂O (60.0 mg, 0.3 mmol). Yellow block-shaped crystals of 2 were obtained (yield 60% based on MnCl₂·4H₂O). Anal. calcd for C₅₂H₃₂Mn₃N₆O₁₄: C, 55.29; H, 2.86; N, 7.44. Found: C 55.08, H 2.92, N 7.26. IR (KBr, cm⁻¹): 3429 w, 3055 w, 1599 s, 1515 m, 1424 w, 1389 s, 1145m, 1102 w, 1026 w, 913 w, 848 m, 778 m, 730 m, 698 w, 563 w.

2.5 Synthesis of [Cd₂(μ₃-L)(H₂biim)₂(μ₂-Hbiim)]_n (3)

Synthesis of 3 was similar to that of 1 except that phen was replaced by H₂biim (39.2 mg, 0.30 mmol). Colorless block-shaped crystals of 3 were obtained (yield 35% based on CdCl₂·H₂O). Anal. calcd for C₃₂H₂₃Cd₂N₁₃O₆: C, 42.22; H, 2.55; N, 20.00. Found: C 41.98, H 2.65, N 20.27. IR (KBr, cm⁻¹): 1608 m, 1594 s,1533 m, 1404 m, 1373 s, 1325 w, 1175 w, 1138 m, 1116 m, 1046 w, 992 w, 941 w, 852 w, 778 m, 762 m, 728 w, 711 w, 690 m, 556 w.

2.6 Synthesis of {[Pb₂(μ₄-L)(μ₄-bpdc)_0.5(phen)₂]·H₂O}_n (4)

A mixture of PbCl₂ (111.2 mg, 0.4 mmol), H₃L (57.4 mg, 0.2 mmol), H₂bpdc (24.2 mg, 0.10 mmol), phen (80.0 mg, 0.4 mmol), NaOH (32.0 mg, 0.8 mmol), and H₂O (10 mL) was stirred at room temperature for 15 min, then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160 °C for 3 days, followed by cooling to room temperature at a rate of 10 °C h⁻¹. Colorless block-shaped crystals of 4 were isolated manually, and washed with distilled water and dried (yield 65% based on PbCl₂). Anal. calcd for C₄₅H₂₉Pb₂N₅O₉: C, 45.11; H, 2.44; N, 5.84. Found: C 45.57, H 2.56, N 6.06%. IR (KBr, cm⁻¹): 3436 w, 3053 w, 1579 s, 1530 m, 1424 m, 1370 s, 1145 m, 1098 w, 1025 w, 916 w, 859 m, 830 w, 782 m, 730 m, 708 m, 561 w.

2.7 Synthesis of [Cd₂(μ₅-L)(μ₂-bpdc)_0.5(phen)₂]_n (5)

Synthesis of 5 was similar to that of 4 except using CdCl₂·H₂O (80.4 mg, 0.40 mmol) instead of PbCl₂. Colorless block-shaped crystals of 5 were obtained (yield 60% based on CdCl₂·H₂O). Anal. calcd for C₄₅H₂₇Cd₂N₅O₈: C, 54.56; H, 2.75; N, 7.07. Found: C 54.17, H 2.83, N 7.22. IR (KBr, cm⁻¹): 3408 w, 3060 w, 1583 s, 1537 m, 1428 w, 1372 s, 1140 m, 1101 w, 1024 w, 922 w, 848 m, 776 m, 726 m, 709 w, 640 w.

2.8 Synthesis of [Zn₂(μ₄-L)(μ₂-bpdc)_0.5(py)₂]_n (6)

A mixture of ZnCl₂ (54.5 mg, 0.4 mmol), H₃L (57.4 mg, 0.2 mmol), H₂bpdc (24.2 mg, 0.10 mmol), py (0.5 mL, 6.3 mmol), and H₂O (10 mL) was stirred at room temperature for 15 min, then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160 °C for 3 days, followed by cooling to room temperature at a rate of 10 °C h⁻¹. Colorless block-shaped crystals of 6 were isolated manually, and washed with distilled water and dried (yield 62% based on ZnCl₂). Anal. calcd for C₃₁H₂₀Zn₂N₃O₈: C, 53.70; H, 2.91; N, 6.06. Found: C 53.37, H 3.09, N 6.25. IR (KBr, cm⁻¹): 1607 s, 1545 m, 1487 m, 1450 m, 1396 s, 1219 w, 1159 w, 1132 w, 1068 w, 1044 w, 1014 w, 852 m, 777 m, 724 w, 698 m, 636 w.

3. X-ray crystallography

The X-ray single-crystal data collection for compounds 1–6 was performed on an Agilent SuperNova diffractometer, using graphite-monochromated Mo K_α radiation (λ = 0.71073 Å). Semiempirical absorption corrections were applied using the SADABS program. The structures were solved by direct methods and refined by full-matrix least-squares on F² using the SHELXS-97 and SHELXL-97 programs.¹³ All non-hydrogen atoms were refined anisotropically by full-matrix least-squares methods on F². The structures were solved using direct method, which yielded the positions of all non-hydrogen atoms. All the hydrogen atoms (except the ones bound to water molecules) were placed in calculated positions with fixed isotropic thermal parameters and included in structure factor calculations in the final stage of full-matrix least-squares refinement. The hydrogen atoms of water molecules were located by difference maps and constrained to ride on their parent O atoms. The final chemical formulae of 1–6 were calculated from SQUEEZE¹⁴ results combined with the data of TGA and elemental analysis. In compound 5, four C atoms are disordered over two sites, that is, C41, C42, C44, C45 and C41′, C42′, C44′, C45′, with occupancy of 0.5. In addition, one O atom is also disordered over two sites, O2 and O2A, with occupancy of 0.6 and 0.4. Crystallographic data as well as details of data collection and refinement for 1–6 are summarized in Table 1, whereas selected bond lengths and angles are listed in Table S1 (ESI†). Hydrogen bonds in crystal packing for the compounds 1–6 are given in Table S2 (ESI†).

Table 1 Crystal data and structure refinement for the compounds 1–6

Compound	1	2	3	4	5	6
Empirical formula	C38H27Cd2N5O9	C52H32Mn3N6O14	C32H23Cd2N13O6	C45H29Pb2N5O9	C45H27Cd2N5O8	C31H20Zn2N3O8
Formula weight	922.45	1129.66	910.43	1198.11	990.52	693.24
Crystal system	Triclinic	Monoclinic	Monoclinic	Triclinic	Triclinic	Monoclinic
Space group	P1̄	P21/n	I2/a	P1̄	P1̄	P21/c
a (Å)	12.0855(6)	8.6220(4)	29.1344(11)	11.5662(5)	8.666(3)	8.7544(5)
b (Å)	12.1636(6)	39.658(3)	8.6686(3)	14.0642(12)	15.518(5)	8.4785(7)
c (Å)	13.5127(6)	13.6015(6)	31.9584(14)	14.1354(10)	15.924(6)	39.392(4)
α (°)	98.040(4)	90	90	65.656(8)	103.487(4)	90
β (°)	111.284(4)	92.600(5)	112.448(5)	66.949(5)	95.131(4)	92.684(7)
γ (°)	96.639(4)	90	90	82.089(5)	91.792(4)	90
Volume (Å^3)	1802.69(15)	4646.0(5)	7459.6(5)	1926.9(2)	2071.0(12)	2920.6(4)
Z	2	4	8	2	2	4
Dcalcd (g cm^−3)	1.699	1.615	1.621	2.065	1.588	1.577
M (Mo Kα) (mm^−1)	1.243	0.883	1.200	8.794	1.087	1.699
F (000)	916	2292	3600	1140	948	1404
Data/restraints/parameters	6374/6/487	8216/0/680	6595/0/478	6812/42/545	7060/114/562	6046/24/386
Goodness-of-fit on F^2	1.091	1.051	1.036	1.041	1.103	1.076
Final R indices, [I > 2σ(I)]	R1 = 0.0333, wR2 = 0.0771	R1 = 0.0759, wR2 = 0.1934	R1 = 0.0541, wR2 = 0.1538	R1 = 0.0454, wR2 = 0.0932	R1 = 0.0531, wR2 = 0.1593	R1 = 0.0928, wR2 = 0.2468
R indices (all data)	R1 = 0.0432, wR2 = 0.0873	R1 = 0.1146, wR2 = 0.2230	R1 = 0.0715, wR2 = 0.1725	R1 = 0.0617, wR2 = 0.1025	R1 = 0.0648, wR2 = 0.1676	R1 = 0.1350, wR2 = 0.2620
CCDC no.	1402752	1402753	1402749	1402750	1402748	1402751

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4. Results and discussion

4.1 Synthesis and characterization

The H₃L was synthesized by the Suzuki reaction.¹² The reaction route is given in Scheme 2.

Scheme 2 The synthetic route for H₃L ligand.

To explore 4-(5-carboxypyridin-2-yl)isophthalic acid as a building block toward crystal engineering of metal(II) (Cd, Mn, Pb, Zn) coordination polymers, we have attempted a number of hydrothermal reactions by treating different metal(II) chlorides with H₃L in the presence of some common auxiliary ligands or potential linkers, selected from 1,10-phenanthroline, 2,2′-biimidazole, pyridine, or 4,4′-biphenyldicarboxylic acid. The selection of metal(II) (Cd, Mn, Pb, and Zn) nodes has been governed by a good crystallization behavior of derived polycarboxylate coordination compounds, a particular suitability of such metal ions for the hydrothermal self-assembly synthesis and their coordination versatility,^{2b,2d,3b,3c,4a,4b} as well as interesting luminescent and/or magnetic properties of coordination polymers based on these metals.^{2b,2d,2f,3b,3d,3c,4a,4b,4g} Various stoichiometric ratios of reactants and combinations of auxiliary ligands have been tested, resulting in a few successful synthetic protocols that led to the isolation of single crystals suitable for X-ray diffraction. Compounds 1–3 were obtained by hydrothermal reactions using a mixture of the corresponding metal(II) chloride (CdCl₂·H₂O or MnCl₂·4H₂O) in H₂O with H₃L, phen or H₂biim, and NaOH in a 1.5 : 1 : 1.5 : 3 molar ratio, respectively. Coordination polymers 4 and 5 were synthesized in a similar way but using a mixture of PbCl₂ or CdCl₂·H₂O with H₃L, H₂bpdc, phen, and NaOH as a base in a 4 : 2 : 1 : 4 : 8 molar ratio, respectively. Yet compound 6 was generated in an analogous manner by treating, in aqueous medium, ZnCl₂ with H₃L, H₂bpdc, and py taken in a 4 : 2 : 1 : 63 molar ratio, respectively. In the latter synthesis, pyridine served not only as an auxiliary ligand but also as a base. The structures of the obtained products were established by single crystal X-ray diffraction, analyzed from the topological viewpoint,^15–18 and confirmed further by standard analytical methods.

4.2 Crystal structures

4.2.1 {[Cd₂(μ₄-L)(μ₃-OH)(phen)₂]·2H₂O}_n (1). The asymmetric unit of 1 contains two crystallographically unique Cd(II) ions, one μ₄-L³⁻ block, one μ₃-OH⁻ moiety, two terminal phen ligands, and two solvent water molecules. As depicted in Fig. 1a, both Cd1 and Cd2 atoms possess a distorted octahedral coordination environment. The Cd1 center is coordinated by three O atoms from two different μ₄-L³⁻ blocks, one O atom from μ₃-OH⁻ moiety and two N atoms from one phen ligand. The Cd2 center is bound by two O atoms from the two different μ₄-L³⁻ blocks, two O atoms from two μ₃-OH⁻ moieties, and two N atoms from phen ligand. The lengths of Cd–O bonds range from 2.195(2) to 2.458(3) Å, whereas the Cd–N bonds vary from 2.338(3) to 2.433(3) Å (Table S1†); these are within the normal values observed in various Cd(II) coordination compounds.^{2b,2h,3d,4a,4d,5c,11,17} In 1, the L³⁻ spacer exhibits a μ₄-coordination mode (Scheme 3, mode I), in which the three carboxylate groups show different monodentate or bidentate modes. The dihedral angle between the pyridyl and phenyl rings in the L³⁻ is 42.51°. Two carboxylate groups from two different L³⁻ and two μ₃-OH⁻ moieties bridge the four adjacent Cd(II) atoms to form a tetracadmium(II) unit (Fig. 1b), which is further connected by L³⁻ ligands to generate a 2D metal–organic layer (Fig. 1c). The 2D sheets are further extended into a 3D supramolecular framework by π–π stacking interactions between the adjacent pyridyl planes of phen ligands (Cg⋯Cg = 3.611(2) Å, Fig. S1 and S2, ESI†). To perform the topological classification of 1, its 2D metal–organic network has been simplified by adopting the concept of the simplified underlying net,^15,16 namely, by omitting the terminal phen ligands and contracting the μ₄-L³⁻ and μ₃-OH⁻ nodes to their centroids. The resulting binodal 3,4-connected layer (Fig. 1d) is built from the 3-connected topologically similar Cd1 and μ₃-OH⁻ nodes, as well as the 4-connected Cd2 and μ₄-L³⁻ nodes that are also topologically equivalent. This layer reveals a rather rare 3,4L33 topology¹⁴ with the point symbol of (4².6)(4³.6².8), wherein the (4².6) and (4³.6².8) indices correspond to the pairs of Cd1/μ₃-OH⁻ and Cd2/μ₄-L³⁻ nodes, respectively. Further simplification of this underlying net by treating the tetracadmium(II) [Cd₄(μ₃-OH)₂]⁶⁺ blocks as the 6-connected cluster nodes gives rise to a binodal 3,6-connected layer with the kgd [Shubnikov plane net (3.6.3.6)/dual] topology.^15,16 It is defined by the point symbol of (4³)₂(4⁶.6⁶.8³), with the (4³) and (4⁶.6⁶.8³) notations corresponding to the 3-connected μ₄-L and 6-connected [Cd₄(μ₃-OH)₂]⁶⁺ nodes, respectively.

Fig. 1 Structural fragments of 1. (a) The coordination environment of the Cd(II) ions. The hydrogen atoms are omitted for clarity. Symmetry codes: i = −x + 1, −y + 2, −z + 1; ii = −x, −y + 2, −z; iii = x, y + 1, z. (b) Tetracadmium(II) unit. Symmetry code: i = −x, −y + 2, −z. (c) View of the 2D layer along the bc plane. (d) Topological representation of the underlying 2D binodal 3,4-connected layer with the 3,4L33 topology. Color codes: Cd1 and Cd2 atoms (turquoise), centroids of μ₃-OH⁻ nodes (yellow), centroids of μ₄-L³⁻ nodes (gray).

Scheme 3 Various coordination modes for L³⁻, bpdc²⁻, H₂biim, and Hbiim⁻ in compounds 1–6.

4.2.2 {[Mn₃(μ₅-L)₂(phen)₂(H₂O)]·H₂O}_n (2). The asymmetric unit of 2 consists of three crystallographically independent Mn(II) atoms, two μ₅-L³⁻ blocks, two phen ligands, one coordinated and one lattice water molecule. As depicted in Fig. 2a, both Mn1 and Mn3 atoms are six-coordinate and possess a distorted octahedral coordination environment. The Mn1 center is coordinated by three O atoms from three individual μ₅-L³⁻ blocks, one O atom from coordinated water molecule, and two N atoms from phen ligand. The Mn3 center is bound by four O atoms from the three different μ₅-L³⁻ ligands and two N atoms from phen moiety. The five-coordinate Mn2 center adopts a distorted square pyramidal geometry filled by five carboxylate oxygen atoms from four individual μ₅-L³⁻ ligands. The lengths of the Mn–O bonds are in the 2.053(6)–2.464(8) Å range, while the Mn–N bonds are in the 2.259(6)–2.354(5) Å interval (Table S1†), being comparable to those found in some reported Mn(II) compounds.^{2d,2f,4a,4b,4g,5b} In 2, the L³⁻ ligands exhibit a μ₅-coordination mode (Scheme 3, mode II). The dihedral angles of 50.36 and 54.09° are observed between the pyridyl and phenyl rings in the μ₅-L³⁻ blocks. The carboxylate groups of the μ₅-L³⁻ moieties link the Mn(II) centers to form two different dimanganese(II) motifs with the Mn–Mn separations of 3.898(2) and 4.116(2) Å (Fig. 2b). These Mn₂ units are multiply interlinked by other COO⁻ groups of the L³⁻ blocks to form a 3D metal–organic framework (Fig. 2c, S3 and S4, ESI†). From the topological viewpoint, this framework is very complex due to the presence of three different Mn centers and two types of μ₅-L³⁻ blocks. An underlying net of 2 (Fig. 2c) has been generated after omitting the terminal phen and H₂O moieties, and contracting the μ₅-L³⁻ blocks to the respective centroids. The resulting tetranodal 3,4,5,5-connected net is topologically unique and can be defined by the point symbol of (4².6⁴)(4².6)₂(4³.6³.8⁴)(4³.6⁶.8), in which the (4².6⁴), (4².6), (4³.6³.8⁴), and (4³.6⁶.8) indices are those of the 4-connected Mn2, 3-connected Mn1 and Mn3, and two 5-connected μ₅-L³⁻ nodes, respectively. An unprecedented character of this topology has been confirmed by a search of different databases.^15–17

Fig. 2 Structural fragments of 2. (a) The coordination environment of the Mn(II) ions. The hydrogen atoms are omitted for clarity. Symmetry codes: i = x − 1/2, −y + 1/2, z + 1/2; ii = x + 1/2, −y + 1/2, z + 1/2; iii = x + 1/2, −y + 1/2, z − 1/2; iv = −x + 2, −y, −z + 1; v = x + 1, y, z. (b) Linkage of Mn atoms into dinuclear units. Symmetry codes: i = −x + 2, −y, −z + 1. (c) A perspective of the 3D metal–organic framework along the bc plane. (d) Topological representation of the underlying 3D framework showing a topologically unique tetranodal 3,4,5,5-connected net defined by the point symbol of (4².6⁴)(4².6)₂(4³.6³.8⁴)(4³.6⁶.8); view along the a axis. Color codes: 3-connected Mn1 and Mn3 and 4-connected Mn2 nodes (cyan), centroids of 5-connected μ₅-L³⁻ nodes (gray).

4.2.3 [Cd₂(μ₃-L)(H₂biim)₂(μ₂-Hbiim)]_n (3). The asymmetric unit of 3 possesses two crystallographically unique Cd(II) ions, one μ₃-L³⁻ block, two terminal H₂biim ligands, and one μ₂-Hbiim⁻ linker. Both six-coordinate Cd1 and Cd2 atoms show a distorted octahedral environment (Fig. 3a). The Cd1 center is coordinated by one O and N atom from the same μ₃-L³⁻ block, two N atoms from one H₂biim moiety, and two N atoms from one μ₂-Hbiim⁻ ligand. The Cd2 center is bound by three O atoms from two μ₃-L³⁻ blocks, two N atoms from one H₂biim moiety, and one N atom from μ₂-Hbiim⁻ ligand. The lengths of the Cd–O bonds range from 2.212(5) to 2.528(6) Å, whereas the Cd–N bonds vary from 2.291(6) to 2.485(6) Å; these bonding parameters lie within the typical values observed in various related cadmium(II) derivatives.^{2b,3b,4b,5c,11,17} In 3, the L³⁻ block shows a μ₃-coordination mode (Scheme 3, mode III), in which the carboxylate groups act in the monodentate or bidentate modes. The dihedral angle between the pyridyl and phenyl rings in L³⁻ is 65.88°. The H₂biim ligands take a bidentate chelating mode (Scheme 3, mode IX). The dihedral angles of two imidazole groups are 2.07 and 9.71°. The Hbiim⁻ ligand adopts the μ₂-bridging mode (Scheme 3, mode X), and has the dihedral angle between two imidazole rings of 3.57°. The μ₃-L³⁻ blocks and μ₂-Hbiim⁻ ligands alternately link the adjacent Cd(II) centers to form a 2D metal–organic sheet (Fig. 3b). The neighboring sheets are assembled into a 3D supramolecular framework through N–H⋯O hydrogen bonds (Table S2 and Fig. S5, ESI†). For the sake of topological classification, a 2D metal–organic network of 3 was simplified, namely, by reducing the μ₂-Hbiim⁻ and μ₃-L³⁻ nodes to their centroids and eliminating the terminal H₂biim moieties.^15,18 The obtained underlying net (Fig. 3c) is assembled from the 3-connected Cd2 and μ₃-L³⁻ nodes (topologically equivalent), as well as the 2-connected Cd1 atoms and μ₂-Hbiim⁻ linkers. Its topological analysis discloses a uninodal 3-connected 2D double layer with the fes [Shubnikov plane net (4.8²)] topology.^15,16,18

Fig. 3 Structural fragments of 3. (a) The coordination environment of the Cd(II) ions in 3. The hydrogen atoms are omitted for clarity except ones bond to nitrogen atoms of the H₂biim and Hbiim⁻ ligands. Symmetry codes: i = −x, y − 1/2, −z + 1/2; ii = −x + 1/2, −y + 1/2, −z + 1/2. (b) A perspective of 2D layer along the ab plane. The H₂biim ligands are omitted for clarity. (c) Topological representation of the underlying 2D metal–organic network in 3 showing a uninodal 3-connected layer with the fes topology; view along the c axis. Color codes: 2- and 3-connected Cd1 and Cd2 atoms (turquoise), centroids of 3-connected μ₃-L³⁻ nodes (gray), centroids of 2-connected μ₂-Hbiim⁻ linkers (yellow).

4.2.4 {[Pb₂(μ₄-L)(μ₄-bpdc)_0.5(phen)₂]·H₂O}_n (4). The asymmetric unit of 4 contains two crystallographically unique Pb(II) ions, one μ₄-L³⁻ block, a half of μ₄-bpdc²⁻ spacer, two phen moieties, and one lattice water molecule. As shown in Fig. 4a, the Pb1 atom is seven-coordinate and adopts a distorted pentagonal bipyramidal geometry formed by three O atoms from the two different μ₄-L³⁻ blocks, two O atoms from two distinct μ₄-bpdc²⁻ ligands, and two N atoms from the phen moiety. The five-coordinate Pb2 center is surrounded by three O atoms from the two individual μ₄-L³⁻ blocks and two N atoms from the phen ligand, leading to a distorted trigonal bipyramidal geometry. The Pb–O distances range from 2.350(6) to 2.666(6) Å, whereas the Pb–N distances vary from 2.553(7) to 2.800(7) Å (Table S1†). Although some of these distances are somewhat elongated, they are still well below the sum of the van der Waals radii for Pb and O [3.54 Å] or Pb and N [3.57 Å] atoms,¹⁷ and are also comparable with the bond lengths observed in other Pb(II) compounds.^2b,4b,19 In addition, there are some weak Pb–O interactions in the range of 2.853(7)–3.460(7) Å which, if considered, would allow to describe both Pb1 and Pb2 centers as eight-coordinate. In 4, the L³⁻ block shows a μ₄-coordination mode (Scheme 3, mode IV). The dihedral angle between the pyridyl and phenyl rings in the L³⁻ is 53.72°. The bpdc²⁻ ligand also takes a μ₄-coordination mode (Scheme 3, mode V), in which the two carboxylate groups show a μ₂-bridging tridentate mode. The μ₄-L³⁻ blocks alternately link the adjacent Pb(II) centers to form the 1D chain motifs (Fig. 4b). Such neighboring motifs are pillared by the μ₄-bpdc²⁻ ligands to form a 3D framework (Fig. 4c). The presence of two different Pb centers and two structurally distinct and multiply bridging polycarboxylate ligands in 4 explains a very complex architecture of its 3D metal–organic framework. Hence, to better understand the structure of 4, we have generated an underlying net and carried out its topological analysis. A simplified net (Fig. 4d) has been obtained after reducing the μ₄-L³⁻ and μ₄-bpdc²⁻ blocks to their centroids and eliminating the terminal phen moieties. The obtained trinodal 3,4,4-connected 3D framework is composed of the 4-connected Pb1 and μ₄-bpdc²⁻ nodes (topologically distinct), 3-connected μ₄-L³⁻ nodes (these have mutually common edges), and 2-connected Pb2 atoms. This net reveals an unreported topology that is described by the point symbol of (4.6.8⁴)₂(4².8⁴)(6².8)₂, wherein the (4.6.8⁴), (4².8⁴), and (6².8) indices are those of the Pb1, μ₄-bpdc²⁻, and μ₄-L³⁻ nodes, respectively. The novelty of this topological type has been confirmed by a search of different databases.^15–17

Fig. 4 Structural fragments of 4. (a) The coordination environment of the Pb(II) ions. The hydrogen atoms are omitted for clarity. Symmetry codes: i = −x + 2, −y + 1, −z; ii = x + 1, y, z − 1; iii = −x, −y, −z + 2; iv = x − 1, y, z + 1. (b) 1D chain motif along the c axis. The phen ligands are omitted for clarity. (c) A perspective of 3D metal–organic framework along the b axis. The phen ligands are omitted for clarity. (d) Topological representation of the underlying 3D metal–organic framework in 4 showing a topologically unique trinodal 3,4,4-connected net defined by the point symbol of (4.6.8⁴)₂(4².8⁴)(6².8)₂; view along the b axis. Color codes: 4-connected Pb1 and 2-connected Pb2 atoms (green), centroids of 3-connected μ₄-L³⁻ nodes (gray), centroids of 4-connected μ₄-bpdc²⁻ nodes (yellow).

4.2.5 [Cd₂(μ₅-L)(μ₂-bpdc)_0.5(phen)₂]_n (5). As shown in Fig. 5a, the asymmetric unit of 5 consists of two crystallographically unique Cd(II) ions, one μ₅-L³⁻ block, a half of μ₂-bpdc²⁻ ligand, and two phen moieties. Both Cd1 and Cd2 atoms possess a distorted octahedral coordination environment. The Cd1 center is coordinated by two O atoms from two different μ₅-L³⁻ blocks, two O atoms from one μ₂-bpdc²⁻ moiety, and two N atoms from one phen ligand. The Cd2 center is bound by four O atoms from three individual μ₅-L³⁻ blocks and two N atoms from one phen ligand. The lengths of Cd–O bonds range from 2.233(4) to 2.533(3) Å, while the Cd–N bonds vary from 2.319(5) to 2.381(4) Å. All the Cd–O and Cd–N distances in 1, 3, and 5 (Table S1†) are comparable to those in other reported Cd(II) compounds.^{2b,2h,3b,3d,4a,4b,5c,11,17,20} In 5, the L³⁻ block shows a μ₅-coordination mode (Scheme 3, mode VI), in which the three carboxylate groups adopt the terminal bidentate or μ₂-bridging bidentate and tridentate modes. The dihedral angle between the pyridyl and phenyl rings in L³⁻ is 25.38°. The bpdc²⁻ ligand acts as a μ₂-linker (Scheme 3, mode VII). The μ₅-L³⁻ blocks alternately link the adjacent Cd(II) centers to form a 1D chain motif (Fig. 5b). These neighboring motifs are interconnected by the μ₂-bpdc²⁻ linkers to generate a 2D metal–organic network (Fig. 5c). It is further extended into a 3D supramolecular framework via the weak C–H⋯O hydrogen bond interactions (Fig. S6 and S7, ESI†). To get further insight into an intricate 2D metal–organic network of 5, we have performed its topological analysis.^14,17 An underlying net has been generated by omitting the terminal phen ligands and reducing the μ₅-L³⁻ and μ₂-bpdc²⁻ blocks to their centroids. The resulting trinodal 3,3,5-connected 2D layer (Fig. 5d) is composed of the 3-connected topologically distinct Cd1 and Cd2 nodes, 5-connected μ₅-L³⁻ nodes, and 2-connected μ₂-bpdc²⁻ linkers. This layer features a unique topology¹⁴ that can be defined by the point symbol of (4.6²)(4³)(4⁴.6⁴.8²), wherein the (4.6²), (4³), and (4⁴.6⁴.8²) notations are those of the Cd1, Cd2, and μ₅-L³⁻ nodes respectively. Further simplification of this layer by treating the [Cd₂(μ₂-bpdc)]²⁺ blocks (these are based on the Cd1 atoms) as the 4-connected cluster nodes leads to a trinodal 3,4,5-connected net with a rare 3,4,5L5 topology and point symbol of (4³)₂(4⁴.6²)(4⁵.6⁵)₂ (Fig. 5e).¹⁵

Fig. 5 Structural fragments of 5. (a) The coordination environment of the Cd(II) ions. The hydrogen atoms are omitted for clarity. Symmetry codes: i = x + 1, y, z; ii = −x + 2, −y + 2, −z; iii = −x + 2, −y + 1, −z + 1. (b) 1D chain motif along the b axis. The phen ligands are omitted for clarity. (c) A perspective of 2D metal–organic network along the b axis. (d and e) Topological representations of the underlying 2D metal–organic networks in 5 showing: (d) topologically unique trinodal 3,3,5-connected layer with the point symbol of (4.6²)(4³)(4⁴.6⁴.8²), and (e) further simplified trinodal 3,4,5-connected layer with the 3,4,5L5 topology after treating [Cd₂(μ₂-bpdc)]²⁺ blocks (composed of Cd1 atoms) as cluster nodes; (d and e) rotated views along the b axis. Color codes: (d) 3-connected Cd1 and Cd2 nodes (turquoise), centroids of 5-connected μ₅-L³⁻ nodes (gray), centroids of 2-connected μ₂-bpdc²⁻ linkers (yellow); (e) 3-connected Cd2 nodes (turquoise), centroids of 4-connected [Cd₂(μ₂-bpdc)]²⁺ cluster nodes (yellow), centroids of 5-connected μ₅-L nodes (gray).

4.2.6 [Zn₂(μ₄-L)(μ₂-bpdc)_0.5(py)₂]_n (6). In the asymmetric unit of 6, there are two crystallographically unique Zn(II) ions, one μ₄-L³⁻ block, a half of μ₂-bpdc²⁻ ligand, and two pyridine moieties. As shown in Fig. 6a, the five-coordinate Zn1 atom is surrounded by four O and one N atoms from three individual μ₄-L³⁻ blocks, thus constructing a distorted square pyramidal environment. The Zn2 atom is six-coordinated by two O atoms from one μ₄-L³⁻ block, two O atoms from one μ₂-bpdc²⁻ ligand, and two N atoms from py moieties, forming a distorted octahedral geometry. The Zn–O bond lengths range from 1.935(5) to 2.412(7) Å, whereas the Zn–N distances vary from 2.037(6) to 2.065(6) Å (Table S1†), which are in good agreement with those reported for other Zn(II) derivatives.^{3c,4e,4g,11,17} In 6, the L³⁻ block acts as a μ₄-spacer (Scheme 3, mode VIII), in which the carboxylate groups adopt the terminal bidentate and μ₂-bridging tridentate modes, respectively. The dihedral angle between the pyridyl and phenyl rings in the L³⁻ is 47.70°. The bpdc²⁻ ligand takes the same μ₂-coordination mode (Scheme 3, mode VII) as that in 5. The carboxylate O and pyridyl N atoms of the μ₄-L³⁻ ligands bridge the adjacent Zn(II) ions to form a 2D sheet motif (Fig. 6b). These motifs are further assembled into a 3D metal–organic framework by the linkage of μ₂-bpdc²⁻ blocks with Zn(II) ions (Fig. 6c). A 3D metal–organic framework of 6 has been simplified for the sake of topological classification.¹⁵ Thus, the terminal py ligands have been omitted, whereas the bridging μ₂-bpdc²⁻ and μ₄-L³⁻ moieties have been contracted to their centroids, resulting in an underlying 3D net (Fig. 6d). From the topological viewpoint, this net can be classified as a binodal 3,4-connected framework with the ins (InS) topology.^15,16,18 It is defined by the point symbol of (6³)(6⁵.8), with the (6³) and (6⁵.8) notations corresponding to the 3-connected Zn1 and 4-connected μ₄-L³⁻ nodes, respectively. In this net, the Zn2 atoms and μ₂-bpdc²⁻ blocks act as the 2-connected linkers.

Fig. 6 Structural fragments of 6. (a) The coordination environment of the Zn(II) ions. The hydrogen atoms are omitted. Symmetry codes: i = −x + 2, y + 1/2, −z + 1/2; ii = −x + 1, y + 1/2, −z + 1/2; iii = −x − 1, −y + 1, −z. (b) View of the 2D layer motif along the ab plane. (c) A perspective of 3D metal–organic framework along the b axis. (d) Topological representation of the underlying 3D metal–organic framework showing a binodal 3,4-connected net with the ins topology; view along the a axis. Color codes: 2- and 3-connected Zn2 and Zn1 atoms (cyan), centroids of 4-connected μ₄-L³⁻ nodes (gray), centroids of 2-connected μ₂-bpdc²⁻ linkers (yellow).

4.2.7 Diverse coordination modes of L³⁻ and structural comparison. As shown in Scheme 3, the 4-(5-carboxypyridin-2-yl)isophthalate ligand exhibits versatile coordination modes in products 1–6 acting as a μ₃-, μ₄-, or μ₅-L³⁻ building block, in which three carboxylate groups can adopt one of the following four coordination modes: monodentate, terminal and μ₂-bridging bidentate, and μ₂-bridging tridentate. Besides, the N atom of the pyridine functionality of L³⁻ also acts as a N-donor in the Cd(II) and Zn(II) derivatives 3 and 6, respectively. In polymer 3, the H₂biim ligand takes a bidentate chelating mode whereas the Hbiim⁻ moiety adopts the μ₂-bridging mode. In compound 4, the bpdc²⁻ ligand acts as a μ₄-spacer with both carboxylate groups exhibiting a μ₂-bridging tridentate mode. In 5 and 6, the bpdc²⁻ ligand behaves as a μ₂-pillar. A notable feature of L³⁻ consists of the rotation of the C–C single bond between the pyridyl and phenyl rings (the dihedral angles are in the 25.38–65.88° range), thus fulfilling the requirement of coordination geometries of metal ions during the self-assembly process. The obtained results indicate that the L³⁻ blocks can adopt a diversity of coordination modes to bind the metal centers in the compounds 1–6. Products 1 and 2 were synthesized under the same reaction conditions, except for the type of metal chloride used (CdCl₂·H₂O for 1 and MnCl₂·4H₂O for 2). Their structural differences indicate that the assembly process is metal ion-dependent, thus resulting in a 2D layer structure in 1 and a 3D metal–organic framework in 2. The structural differences between the lead and cadmium containing coordination polymers 4 and 5 can be attributed to the same reason. The distinct structural type of the compounds 1 and 3 was caused by the introduction of different auxiliary ligands (phen for 1 and H₂biim for 3), under the same reaction conditions. All these observations indicate that the type of the metal ions and auxiliary ligands have significant effects on the formation and structures of the resulting coordination polymers.

4.3 Thermal analysis and PXRD results

Powder X-ray diffraction experiments were carried out for compounds 1–6. The patterns for the as-synthesized bulk materials closely match the simulated ones from the single crystal X-ray diffraction data (Fig. S8, ESI†), thus indicating the phase purity of the crystalline samples. To determine the thermal stability of 1–6, their thermal behavior was investigated under nitrogen atmosphere by TGA (Fig. S9, ESI†). Compound 1 lost its two lattice water molecules (exptl, 3.6%; calcd, 3.9%) from 55 to 105 °C, and then the decomposition occurs at 315 °C. For 2, there is one distinct weight loss effect in the 56–125 °C range, corresponding to the release of one lattice water molecule and one H₂O ligand (exptl, 3.4%; calcd, 3.2%). The decomposition of the remaining solid begins at 335 °C. The TGA curves of compounds 3, 5, and 6 show that there is no appreciable weight change up to 330, 295, and 285 °C, respectively, while further heating of the samples leads to their decomposition. For compound 4, the weight loss corresponding to the release of one lattice water molecule was observed from 69 to 116 °C (exptl, 1.4%; calcd, 1.5%), and the decomposition of the dehydrated solid begins at 253 °C. We suppose that the final decomposition products of compounds 1–6 can be the corresponding metal oxides (e.g., CdO for 1, 3, and 5), although a more prolonged heating would be required in some cases to achieve a full decomposition of the samples.

4.4 Luminescent properties

The emission spectra of H₃L, H₂bpdc, and compounds 1 and 3–6 were measured in the solid-state at room temperature and are given in Fig. 7. The “free” H₃L and H₂bpdc ligands display weak photoluminescence with an emission maximum at 473 and 407 nm if excited at 380 and 335 nm, which may be attributed to the π* → n or π* → π transitions.^19,21–29 For compounds 1 and 3–6, the emission bands of higher (4, 6) or significantly higher (1, 3, 5) intensity are detected with the maximum at 415 nm (λ_ex = 335 nm) for 1, 525 nm (λ_ex = 380 nm) for 3, 580 nm (λ_ex = 392 nm) for 4, 469 nm (λ_ex = 376 nm) for 5, and 423 nm (λ_ex = 338 nm) for 6. The emission bands of compounds 1, 5, and 6 were blue-shifted by 58, 4, and 50 nm compared with the “free” H₃L. In contrast, the emission bands of compounds 3 and 4 were red-shifted by 52 and 107 nm relatively to H₃L. If compared with the “free” H₂bpdc, the emission maxima of mixed-ligand derivatives 4, 5, and 6 are red-shifted by 173, 62, and 16 nm, respectively. The observed emission bands cannot be ascribed to the LMCT or MLCT processes. Thus, the intraligand and ligand-to-ligand charge transitions are probably responsible for the emission properties of the compounds 1 and 3–6.^21–29 The enhancement of luminescence in the compounds in comparison with H₃L can be attributed to the binding of ligands to the metal centers, which can effectively increase the rigidity of the ligand and reduce the loss of energy by radiationless decay.^22–29

Fig. 7 Solid state emission spectra of H₃L, H₂bpdc, and compounds 1 and 3–6 at room temperature.

4.5 Magnetic properties

Variable-temperature magnetic susceptibility studies were carried out in the 2–300 K temperature range using a powder sample of compound 2. As shown in Fig. 8, the χ_MT value at 300 K is 12.97 cm³ mol⁻¹ K, which is close to that of 13.12 cm³ mol⁻¹ K expected for three magnetically isolated high-spin Mn(II) centers (S_Mn = 5/2, g = 2.0). The χ_MT values steadily decrease on lowering the temperature and reach the minimum of 2.21 cm³ mol⁻¹ K at 2.00 K. Between 2 and 300 K, the magnetic susceptibility can be fitted to the Curie–Weiss law with C = 13.30 cm³ mol⁻¹ K and θ = −7.58 K. These results indicate an antiferromagnetic interaction between the adjacent Mn(II) ions. According to the crystal structure of 2, there are two sets of magnetic exchange pathways: one is through carboxylate bridges within the two different dimers, the other one is inter-dimers through carboxylate bridges or L³⁻ blocks (Fig. S3 and S4, ESI†). Because of the long inter-dimers Mn–Mn distances (intra- and inter-dimers superexchange distances are 3.898(2)–4.116(2) and 4.902(2)–9.176(2) Å, respectively), it is expected that the intra-dimers interactions in compound 2 are much stronger than the inter-dimers coupling and should dominate its magnetic behavior.

Fig. 8 Temperature dependence of χ_MT (○) and 1/χ_M(□) vs. T for compound 2. The blue line shows the Curie–Weiss fitting.

5. Conclusions

In this work, we have opened up the application of 4-(5-carboxypyridin-2-yl)isophthalic acid (H₃L) as a novel versatile tricarboxylate building block with a pyridine functionality for the design of various mixed-ligand 2D coordination polymers or 3D metal–organic frameworks. Hence, a new series of Cd(II), Mn(II), Pb(II), and Zn(II) coordination polymers has been generated by hydrothermal self-assembly reactions, using different metal(II) chlorides as a metal source and H₃L as an unexplored principal building block, along with various auxiliary ligands. Network dimensionality, structural and topological characteristics of the obtained products depend on the nature of the metal center and the combination of auxiliary ligands. All products have been topologically classified, disclosing the kgd (in 1), fes (in 3), 3,4,5L5 (in 5), and ins (in 6) topologies. Besides, topologically unique multinodal nets have been found in the very complex 3D frameworks of 2 and 4, thus contributing to the identification of novel topological types in MOFs. Further research toward exploring H₃L and related pyridine polycarboxylate building blocks for the design of functional coordination polymers will be pursued.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Project 21201091) and the Fundamental Research Funds for the Central Universities (Project lzujbky-2013-59). AMK acknowledges the Foundation for Science and Technology (FCT), Portugal (PTDC/QUI-QUI/121526/2010, PEst-OE/QUI/UI0100/2013).

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Footnote

† Electronic supplementary information (ESI) available: Selected bond distances and angles (Table S1), hydrogen bonding parameters (Table S2), additional structural representations (Fig. S1–S7), PXRD patterns (Fig. S8) and TGA curves (Fig. S9) of the compounds 1–6, as well as crystallographic data in CIF format. CCDC 1402748–1402753. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13582f

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