A variety of metal–organic and supramolecular networks constructed from a new flexible multifunctional building block bearing picolinate and terephthalate functionalities: hydrothermal self-assembly, structural features, magnetic and luminescent properties

Abstract

A novel flexible multifunctional building block (H₃L) bearing picolinate and terephthalate functionalities was designed and applied for the hydrothermal self-assembly generation of a series of coordination compounds [Co(H₂L)₂(H₂O)₂] (1), [M(HL)(H₂O)]_n {M = Cd (2) and Mn (3)}, {[Mn_1.5(L)(phen)(H₂O)₂]·H₂O}_n (4), [Zn₃(L)₂(H₂O)₆]_n (5), and {[Zn₃(L)₂(py) (H₂O)₄]·4H₂O}_n (6) {wherein H₃L = 2-(4-carboxypyridin-3-yl)-terephthalic acid, phen = 1,10-phenanthroline, py = pyridine}. All the obtained products 1–6 were fully characterized by IR spectroscopy, elemental, thermogravimetric, powder and single-crystal X-ray diffraction analyses. By adjusting the molar ratio of NaOH and H₃L ligand, the latter becomes partially deprotonated to form the H₂L⁻ blocks in 1 and HL²⁻ moieties in 2 and 3, or completely deprotonated to create the L³⁻ units in 4–6. The structures of the obtained compounds range from a discrete 0D monomer 1 and 1D coordination polymers 4 and 5 to 2D coordination polymers 2 and 3, and an intricate 3D metal–organic framework 6; their detailed topological classification was also performed. The structures of 1–5 are further extended [0D → 3D (1), 1D → 3D (4, 5), and 2D → 3D (2, 3)] into supramolecular networks by means of multiple hydrogen bonds. The results reveal that the nature of metal(II) ion, molar ratio between NaOH and H₃L ligand, and the presence (optional) of auxiliary ligand play a significant role in determining dimensionality, topology and other structural features of the obtained products. Magnetic susceptibility measurements indicate that compounds 3 and 4 have dominating antiferromagnetic couplings between metal centers. Furthermore, luminescent properties of 2, 5, and 6 were also investigated.

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Introduction

In recent years, the design of new metal–organic and supramolecular networks has become of high interest owing to the vast variety of their structural architectures, topological characteristics, and different applications as functional materials.^1–10 However, it is still a great challenge to generate compounds with desirable structural features and properties, since a multitude of factors can influence the result, such as coordination geometries of metal nodes, connectivity of main organic ligands due to the presence of distinct functional groups, introduction of auxiliary ligands, variation in reaction temperatures, ratios of reagents, types of solvents, pH values, crystallization conditions, etc.^11–15 In this regard, diverse multicarboxylic or heterocyclic carboxylic acids are commonly applied as multifunctional building blocks in generating metal–organic networks, not only because of their ability to lead to different coordination modes and exhibit high thermal stability, but also due to their possibility to act as good H-bond donors and acceptors, thus allowing an additional supramolecular level stabilization of resulting compounds.^16–25 Apart from multifunctional carboxylate ligands, 1,10-phenanthroline (phen) and pyridine (py) are frequently applied as simple secondary N-donor building blocks to tune the coordination environment and stabilize structures, because of their efficient π⋯π stacking and H-bonding interactions.^16,17,26,27

Being interested in the exploration of new multifunctional polycarboxylate building blocks for the synthesis of metal–organic and supramolecular networks, we have designed a novel flexible multifunctional building block containing both chelating picolinate and linking terephthalate functionalities, namely 2-(4-carboxypyridin-3-yl)terephthalic acid (H₃L). Then, we have probed the hydrothermal self-assembly reactions of different metal(II) chlorides (M = Co, Cd, Mn, and Zn) with H₃L as a principal building block, along with optional 1,10-phenanthroline (phen) or pyridine (py) auxiliary ligands. The choice of these reactants was governed by the following considerations. (1) The coordination chemistry of H₃L is virtually unexplored, as confirmed by a search of Cambridge Crystallographic Database (CSD) that disclosed no structurally characterized compounds derived from this organic building block. (2) H₃L comprises three carboxylic groups that can become partially or completely deprotonated, depending on the molar ratio of NaOH and H₃L ligand. (3) In addition, H₃L contains a pyridyl and a phenyl ring with structural flexibility and conformation freedom; rotation of the C–C single bond between the aromatic rings can lead to various coordination geometries of metal ions. Thus, the potential coordination sites, one nitrogen atom from pyridyl ring and six oxygen atoms of three carboxylate groups, may supply anions with three different compensating charges and various acidity-dependent coordination modes. (4) The addition into the reaction system of an N-donor auxiliary ligand may improve the crystallization of metal–organic networks and reinforce their structures.

Thus, in the present study, we have generated by a hydrothermal self-assembly protocol a series of new coordination compounds 1–6, namely a discrete 0D complex [Co(H₂L)₂(H₂O)₂] (1), 1D coordination polymers {[Mn_1.5(L)(phen)(H₂O)₂]·H₂O}_n (4) and [Zn₃(L)₂(H₂O)₆]_n (5), 2D coordination polymers [M(HL)(H₂O)]_n (M = Cd (2), and Mn (3)), and a 3D metal–organic framework {[Zn₃(L)₂(py)(H₂O)₄]·4H₂O}_n (6). To our knowledge, these products represent the first coordination compounds obtained from H₃L. Their structural diversity suggests that the molar ratio of NaOH and H₃L ligand, type of metal node and presence of auxiliary ligand play an important role in the generation, structural and topological characteristics of 1–6. The study thus reports the characterization of 1–6 by IR spectroscopy, elemental, thermogravimetric, powder and single-crystal X-ray diffraction, and topological analyses. In addition, magnetic (for 3 and 4) and luminescent (for 2, 5 and 6) properties were investigated.

Experimental

Materials and methods

All chemicals and solvents were of A.R. grade and obtained from commercial sources and used without further purification. 2-(4-Carboxypyridin-3-yl)terephthalic acid (H₃L) was synthesized by adopting a related procedure.^{28,29 1}H and ¹³C NMR spectra were performed on a Bruker Advance III 400 spectrometer. Carbon, hydrogen and nitrogen content in 1–6 was determined using an Elementar Vario EL elemental analyzer. IR spectra were recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TGA) was 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 (λ = 1.54060 Å), in which the X-ray tube was operated at 40 kV and 40 mV. 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 for the solid samples on an Edinburgh FLS920 fluorescence spectrometer at room temperature.

2-(4-Carboxypyridin-3-yl)terephthalic acid (H₃L)

A mixture of ethyl 5-bromonicotinate (115.0 mg, 0.50 mmol), 2,5-bis(ethoxycarbonyl)phenylboronic acid (143.0 mg, 0.55 mmol), 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 (v/v 36 : 1) to give diethyl 2-(4-(ethoxycarbonyl)pyridin-3-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 41% yield based on ethyl 5-bromonicotinate. Calcd for C₁₄H₉NO₆: C 58.54, H 3.16, N 4.88%. Found: C 58.21, H 3.18, N 4.83%. IR (KBr, cm⁻¹): 1721 s, 1379 w, 1302 w, 1224 m, 1168 w, 1138 w, 1094 w, 1031 w, 918 w, 879 w, 804 w, 787 w, 764 w, 754 w, 730 w, 654 w. ¹H NMR (400 MHz, DMSO-d₆): δ 13.38 (s, 3H), δ 8.68 (s, 1H), δ 8.11 (t, 2H), δ 8.03–7.99 (m, 2H), δ 7.96–7.91 (m, 1H). ¹³C NMR (100 MHz, DMSO-d₆): δ 167.9 (COOH), 166.3 (COOH), 166.0 (COOH), 148.6, 147.1, 139.2, 137.6, 137.0, 135.3, 133.4, 131.4, 130.4, 129.3, 124.2 (aromatic C).

Synthesis of [Co(H₂L)₂(H₂O)₂] (1)

A mixture of CoCl₂·6H₂O (71.0 mg, 0.3 mmol), H₃L (73.0 mg, 0.3 mmol), NaOH (12 mg, 0.3 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. Pink needle-shaped crystals were isolated manually, washed with distilled water and dried to give 1. Yield: 35% (based on H₃L). Calcd for C₂₈H₂₀CoN₂O₁₄: C 50.39, H 3.02, N 4.20%. Found: C 50.78, H 2.83, N 4.41%. IR (KBr, cm⁻¹): 3420 w, 3046 m, 3067 w, 1716 s, 1699 s, 1613 m, 1598 s, 1563 m, 1398 m, 1314 w, 1267 w, 1256 w, 1176 w, 1134 m, 1035 w, 926 m, 866 m, 812 w, 801 w, 768 w, 755 m, 702 m, 646 w, 608 w, 542 w, 468 w.

Synthesis of [M(HL)(H₂O)]_n (M = Cd (2) and Mn (3))

A mixture of MCl₂·xH₂O (x = 1 for 2 and x = 4 for 3, 0.3 mmol), H₃L (73.0 mg, 0.3 mmol), NaOH (24 mg, 0.6 mmol) and H₂O (10 mL) was stirred at room temperature for 15 min, and 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. Crystals of 2 and 3 were isolated manually and washed with distilled water. Yield: 70% for 2, 60% for 3 (based on H₃L). Calcd for C₁₄H₉CdNO₇: C 40.46, H 2.18, N 3.37%. Found: C 40.11, H 2.43, N 3.67%. IR (KBr, cm⁻¹): 3407 w, 1712 m, 1618 s, 1585 s, 1557 m, 1527 w, 1482 m, 1378 s, 1296 w, 1273 w, 1242 w, 1149 s, 1033 w, 978 w, 906 w, 890 m, 868 w, 834 w, 809 w, 770 m, 704 w, 651 w, 562 m, 479 m. Calcd for C₁₄H₉MnNO₇ (3): C 46.95, H 2.53, N 3.91%. Found: C 47.36, H 2.47, N 3.85%. IR (KBr, cm⁻¹): 3406 w, 1713 s, 1621 s, 1586 s, 1557 s, 1482 w, 1390 s, 1299 w, 1242 s, 1147 w, 1121 m, 1044 w, 1032 m, 978 w, 948 w, 860 m, 814 m, 757 m, 748 w, 703 m, 662 w, 648 w, 567 m, 491 w, 476 w.

Synthesis of {[Mn_1.5(L)(phen)(H₂O)₂]·H₂O}_n (4)

A mixture of MnCl₂·6H₂O (59.4 mg, 0.3 mmol), H₃L (72.0 mg, 0.3 mmol), phen (60.0 mg, 0.3 mmol), NaOH (36 mg, 0.9 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. Yellow block-shaped crystals were isolated manually, washed with distilled water and dried to give 4. Yield: 50% (based on H₃L). Calcd for C₂₆H₂₀Mn_1.5N₃O₉: C 51.97, H 3.36, N 6.99%. Found: C 51.48, H 3.29, N 7.37%. IR (KBr, cm⁻¹): 3465 w, 1619 s, 1577 s, 1516 m, 1478 w, 1427 m, 1370 s, 1266 m, 1170 w, 1146 w, 1128 w, 1098 w, 1048 w, 1034 m, 985 w, 948 w, 928 w, 895 w, 847 s, 816 m, 777 s, 726 m, 708 m, 664 w, 637 w, 576 w, 554 w, 490 w, 464 w.

Synthesis of [Zn₃(L)₂(H₂O)₆]_n (5)

A mixture of ZnCl₂ (41.0 mg, 0.3 mmol), H₃L (72.0 mg, 0.3 mmol), NaOH (36 mg, 0.9 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 were isolated manually, washed with distilled water and dried to give 5. Yield: 50% (based on H₃L). Calcd for C₂₈H₂₄Zn₃N₂O₁₈: C 38.53, H 2.77, N 3.21%. Found: C 38.84, H 2.49, N 3.38%. IR (KBr, cm⁻¹): 3416 m, 1617 s, 1566 s, 1484 m, 1412 s, 1378 s, 1300 m, 1268 w, 1250 w, 1142 w, 1051 w, 1040 w, 986 w, 937 w, 878 w, 838 m, 773 s, 701 w, 652 w, 578 w, 507 w.

Synthesis of {[Zn₃(L)₂(py)(H₂O)₄]·4H₂O}_n (6)

The preparation of 6 was similar to that of 5 except using py (0.5 mL, 6.3 mmol) instead of NaOH. After being cooled to room temperature, colorless block-shaped crystals of 6 were isolated manually, and washed with distilled water. Yield: 50% (based on H₃L). Calcd for C₃₃H₃₃Zn₃N₃O₂₀: C 40.12, H 3.37, N 4.25%. Found: C 39.85, H 3.61, N 4.57%. IR (KBr, cm⁻¹): 3366 m, 1635 s, 1575 s, 1551 m, 1489 m, 1453 m, 1421 m, 1359 s, 1271 m, 1135 w, 1076 w, 1055 w, 1037 w, 927 w, 879 m, 845 m, 814 w, 786 w, 772 m, 701 m, 654 w, 582 w, 484 w.

X-ray crystal data collection and structure determination

The single-crystal X-ray data collection for 1–6 was performed on a Bruker Smart CCD 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.³⁰ All the non-hydrogen atoms were refined anisotropically by full-matrix least-squares methods on F². All the hydrogen atoms (except those bound to water molecules) were placed in calculated positions with fixed isotropic thermal parameters and included in structure factor calculations at 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 crystal data for 1–6 are summarized in Table 1 and selected bond lengths are listed in Table S1.† Hydrogen bonds in the compounds 1–6 are given in Table S2.†

Table 1 The crystallographic data for 1–6

Compound	1	2	3	4	5	6
Chemical formula	C28H20CoN2O14	C14H9CdNO7	C14H9MnNO7	C26H20Mn1.5N3O9	C28H24Zn3N2O18	C33H33Zn3N3O20
Formula weight	667.39	415.62	358.16	600.86	872.60	987.73
Crystal system	Monoclinic	Orthorhombic	Orthorhombic	Triclinic	Monoclinic	Orthorhombic
Space group	P21/c	Pnna	Pnna	P1̄	C2/c	Pbcn
a/Å	16.9118(11)	14.481(4)	14.5661(10)	7.226(8)	29.702(3)	15.5051(7)
b/Å	6.8877(6)	25.238(7)	25.0223(17)	10.388(12)	6.1660(3)	12.2437(5)
c/Å	11.8438(10)	7.5208(19)	7.4491(6)	17.46(2)	16.6016(8)	18.0992(5)
α/°	90	90	90	104.832(14)	90	90
β/°	105.872(9)	90	90	93.230(12)	94.853(6)	90
γ/°	90	90	90	104.588(13)	90	90
V/Å^3	1327.00(18)	2748.5(12)	2715.0(3)	1216(2)	3029.5(3)	3436.0(2)
T/K	293(2)	296(2)	293(2)	296(2)	291(2)	293(2)
Z	2	8	8	2	4	4
Dc/g cm^−3	1.670	2.009	1.752	1.641	1.913	1.909
μ/mm^−1	0.729	1.628	1.012	0.854	2.448	2.176
F(000)	682	1632	1448	613	1760	2008
Refl. measured	4550	18 100	6744	8606	3310	9140
Unique refl. (Rint)	2343 (0.0495)	2427 (0.0210)	2401 (0.0389)	4153 (0.0372)	3310 (0.0806)	3029 (0.0486)
GOF on F^2	1.064	1.044	1.026	1.048	0.95	1.058
R1 [I > 2σ(I)]	0.0510	0.0234	0.0412	0.0443	0.0395	0.0463
wR2 [I > 2σ(I)]	0.0902	0.0633	0.0975	0.1110	0.0784	0.1185

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Results and discussion

Crystal structures

Crystal structure of 1. The discrete 0D monomer 1 crystallizes in the monoclinic space group P21/c. Its asymmetric unit contains one Co(II) atom (half occupancy, located at an inversion center), one H₂L⁻ ligand, and one coordinated water molecule. As shown in Fig. 1a, the six-coordinate Co1 center is bound by two carboxylate O atoms and two N atoms in equatorial sites from two symmetry equivalent H₂L⁻ ligands (these are mutually trans) and two axial O atoms from two equivalent H₂O moieties, thus forming an ideal octahedral {CoN₂O₄} geometry. The Co–N bonds are 2.071(2) Å, while the Co–O distances vary from 2.033(3) to 2.189(2) Å; all these distances are comparable to those found in the reported Co(II) compounds.^16,17 In 1, the H₂L⁻ acts as a terminal ligand (Scheme 1, mode I), in which the deprotonated carboxylate group is in the η¹:η⁰ monodentate mode. The dihedral angle between the pyridyl and phenyl rings in the H₂L⁻ block is 59.95°. The discrete monomeric units of 1 are interlinked by the strong O–H⋯O hydrogen bonds to form a 3D supramolecular framework (Fig. 1b and Table S2, ESI†). For the sake of topological analysis, a concept of the simplified underlying net was followed.²⁷ Hence, the molecular [Co(H₂L)₂(H₂O)₂] units were reduced to the respective centroids and considered as the 8-connected nodes. The obtained underlying framework (Fig. 1c) is a uninodal 8-connected 3D net with a rare 8T9 topology, which is defined by the point symbol of (3⁶.4¹⁰.5¹¹.6).

Fig. 1 Structural fragments of 1. (a) Coordination environment of the Co(II) atom. Symmetry code: i = −x + 2, −y + 1, −z + 1. (b) Perspective of the 3D H-bonded framework along the ac plane (blue dashed lines represent the H-bonds). (c) Topological representation of the underlying 3D H-bonded framework showing a uninodal 8-connected net with the 8T9 topology; view along the a axis; color codes: centroids of 8-connected [Co(H₂L)₂(H₂O)₂] molecular nodes (magenta).

Scheme 1 Structural formulae of ligands used in this work.

Crystal structures of 2 and 3. Single-crystal X-ray analyses reveal that compounds 2 and 3 are isostructural; the structure of 2 is described in detail as a representative example (Fig. 2). The asymmetric unit of 2 contains two crystallographically unique Cd(II) atoms (Cd1 and Cd2 with half occupancy, they are located on a 2-fold rotation axis), one HL²⁻ ligand, and one H₂O moiety. As depicted in Fig. 2a, the Cd1 center is coordinated by four O and two N atoms from the four different HL²⁻ blocks and possesses a distorted octahedral {CdN₂O₄} coordination environment with the equatorial plane defined by the pairs of O1 and O1iii (mutually cis), and N1i and N1ii atoms (mutually cis); the axial positions are taken by the O5i and O5ii atoms. The six-coordinate Cd2 center reveals a distorted octahedral {CdO₆} geometry, filled by the pairs of carboxylate O2/O2iv (mutually trans) and O5ii/O5v (mutually cis) atoms, and a pair of water ligands O7/O7iv (mutually cis). The Cd–O [2.218(2)–2.423(2) Å] and Cd–N [2.324(2) Å] bond lengths are in good agreement with those observed in some other related Cd(II) compounds.^16,17 In 2, the HL²⁻ spacer exhibits a μ₄-coordination mode (Scheme 1, mode II), in which the two deprotonated carboxylate groups show the μ₂-η¹:η¹ and μ₂-η²:η⁰ bidentate modes. The dihedral angle between the pyridyl and phenyl rings in HL²⁻ is 55.37°. The HL²⁻ moieties alternately link the adjacent Cd(II) centers to form a 1D chain motif with the Cd⋯Cd separation of 4.003(2) Å (Fig. 2b). These 1D motifs are arranged into a 2D sheet structure by further coordination interactions of the HL²⁻ ligands to Cd(II) ions (Fig. 2b). For topological analysis,³¹ the 2D metal–organic network in 2 was simplified (terminal H₂O ligands omitted, μ₄-HL²⁻ moieties reduced to centroids) to give an underlying binodal 4,4-connected layer (Fig. 2d). It is built from the 4-connected topologically equivalent Cd1/Cd2 and μ₄-HL²⁻ nodes and features an undocumented topology^31–33 defined by the point symbol of (4³.6².8). The novelty of this topology was confirmed by a search of different databases.^31–33 Furthermore, the neighboring metal–organic sheets in 2 are assembled, through the O–H⋯O hydrogen bonds, into a complex 3D supramolecular framework (Fig. 2c).

Fig. 2 Structural fragments of 2. (a) Coordination environment of the Cd(II) atoms. Symmetry codes: i = x + 1/2, y, −z + 2; ii = x + 1/2, −y + 1/2, z + 1/2; iii = x, −y + 1/2, −z + 5/2; iv = x, −y + 1/2, −z + 3/2; v = x + 1/2, y, −z + 1. (b) Perspective of the 2D metal–organic sheet along the ac plane. (c) Perspective of the 3D supramolecular framework along the ab plane (blue dashed lines represent the H-bonds). Symmetry code: i = −x + 3/2, −y + 1, z + 1. (d) Topological representation of the underlying 2D metal–organic network showing a binodal 4,4-connected layer with the unique topology defined by the point symbol of (4³.6².8); view along the b axis; color codes: 4-connected Cd1 and Cd2 nodes (turquoise), centroids of 4-connected μ₄-HL nodes (gray).

Crystal structure of 4. This compound crystallizes in the triclinic space group P1̄ and features a linear 1D metal–organic chain structure (Fig. 3). The asymmetric unit bears two crystallographically independent Mn(II) atoms (Mn1 with full occupancy; Mn2 with half-occupancy, located at an inversion center), one L³⁻ and one phen ligand, two H₂O ligands and two water molecules of crystallization. As depicted in Fig. 3a, the six-coordinate Mn1 atom adopts a distorted octahedral {MnN₃O₃} geometry in which the equatorial plane is provided by the N2 and N3 atoms from phen ligand and the N1 and one O3i atoms (mutually cis) of two different L³⁻ blocks, while the O1 atom from the L³⁻ moiety and the O7 water ligand occupy the axial positions. The six-coordinate Mn2 atom possesses a slightly distorted octahedral {MnO₆} coordination environment, filled by the pairs of carboxylate O4/O4iv and O5ii/O5iii atoms, and a pair of O8/O8iv water ligands; within each pair, the symmetry equivalent atoms are in mutually trans positions. The Mn–O [2.103(3)–2.210(3) Å] and Mn–N [2.243(3)–2.336(3) Å] bond lengths are in good agreement with those observed in some other Mn(II) compounds.^16,17 In 4, the L³⁻ ligands show a μ₄-coordination mode (Scheme 1, mode III), in which the carboxylate groups are either η¹:η⁰ monodentate or η¹:η¹ bidentate. The dihedral angle between the pyridyl and phenyl rings in the L³⁻ ligand is 52.09°. The three neighboring Mn(II) ions are bridged by means of two carboxylate groups from the two different L³⁻ moieties, giving rise to a centro-symmetric trinuclear Mn(II) subunit (Fig. 3b). In this trimanganese(II) unit, the Mn⋯Mn distance of 4.954(3) Å is significantly longer than those reported for other carboxylate-bridged trinuclear Mn(II) complexes.^17,34 The adjacent Mn₃ subunits are further linked by L³⁻ blocks into a 1D chain (Fig. 3b), having the shortest distance of 10.388(3) Å between the neighboring Mn₃ units. For the sake of topological analysis, an underlying 1D metal–organic network of 4 was obtained upon reducing the μ₄-L ligands to their centroids and eliminating terminal H₂O and phen moieties, thus giving rise to a binodal 4,4-connected chain (Fig. 3d). Interestingly, this chain possesses an unreported topology^29–31 defined by the point symbol of (3².4.5².6)₂(3².4².5²), wherein the (3².4.5².6) and (3².4².5²) indices correspond to the μ₄-L and Mn2 nodes, respectively. Besides, the 1D coordination chains are held into a 2D network by H-bonding interactions and also involving crystallization H₂O molecules (Table S2†). It is further extended into a 3D supramolecular framework by π–π stacking interactions (Fig. 3c).

Fig. 3 Structural fragments of 4. (a) Coordination environment of the Mn(II) atoms. Symmetry codes: i = x, y − 1, z; ii = −x + 1, −y + 2, −z + 1; iii = x, y + 1, z; iv = −x + 1, −y + 3, −z + 1. (b) Perspective of a 1D metal–organic chain along the bc plane. (c) Perspective of a 3D supramolecular framework along the bc plane. (d) Topological representation of the underlying 1D metal–organic network showing a binodal 4,4-connected chain with the unique topology defined by the point symbol of (3².4.5².6)₂(3².4².5²); view along the a axis; color codes: 2-connected Mn1 linkers and 4-connected Mn2 nodes (green), centroids of 4-connected μ₄-L nodes (gray).

Crystal structure of 5. The compound 5 crystallizes in the monoclinic space group C2/c and also features a linear 1D metal–organic chain structure (Fig. 4). In the asymmetric unit, there are two crystallographically independent Zn(II) atoms (Zn1 with full occupancy; Zn2 with half occupancy, located on a 2-fold rotation axis), one L³⁻ and three coordinated water molecules. As depicted in Fig. 4a, the four-coordinate Zn1 atom adopts a distorted {ZnO₄} tetrahedral geometry, filled by two O atoms from two different L³⁻ ligands and two O atoms from two coordinated H₂O molecules. The six-coordinate Zn2 atom adopts a distorted {ZnN₂O₄} octahedral geometry in which the equatorial sites are taken by a pair of O6/O6ii atoms (mutually cis) from two different L³⁻ ligands and a pair of O9/O9ii atoms (mutually cis) from two coordinated water molecules, while the N1 and N1ii atoms of two individual L³⁻ blocks occupy the axial positions. The Zn–O and Zn–N bond lengths are in the range of 1.935(2)–2.118(3) and 2.127(3) Å, respectively. In 5, the L³⁻ ligands show a μ₃-coordination mode (Scheme 1, mode IV), in which all carboxylate groups are η¹:η⁰ monodentate. The dihedral angle between the pyridyl and phenyl rings in the L³⁻ ligand is 66.62°. The carboxylate groups of the L³⁻ blocks alternately bridge neighboring Zn(II) atoms to form a ladder-like coordination chain (Fig. 4b). It was analyzed from a topological viewpoint following the concept of the simplified underlying net.^31,35 After eliminating terminal H₂O ligands and contracting the μ₃-L spacers to their centroids, a uninodal underlying chain was generated (Fig. 4d). It is composed of the 3-connected μ₃-L nodes and 2-connected Zn1/Zn2 linkers, disclosing the SP 1-periodic net (4,4)(0,2) topology with the point symbol of (4².6).³¹ The adjacent 1D coordination chains are further extended (1D → 3D) by multiple hydrogen bonds (Fig. 4c, Table S2†) into a supramolecular framework.

Fig. 4 Structural fragments of 5. (a) Coordination environment of the Zn(II) atoms. Symmetry codes: i = x, −y + 1, z − 1/2; ii = −x + 1, y, −z + 3/2. (b) Perspective of a 1D ladder-like metal–organic chain along the ac plane. (c) Perspective of a 3D supramolecular framework along the ac plane. (d) Topological representation of the underlying 1D metal–organic network showing a uninodal ladder-like 3-connected chain with the SP 1-periodic net (4,4) (0,2) topology; view along the b axis; color codes: 2-connected Zn1 and Zn2 linkers (pale green), centroids of 3-connected μ₃-L nodes (gray).

Crystal structure of 6. The asymmetric unit of 6 consists of two symmetry non-equivalent Zn(II) atoms (Zn1 with full occupancy; Zn2 with half occupancy, located on a 2-fold rotation axis), one μ₄-L³⁻ ligand, a half of py moiety, two H₂O ligands and two water molecules of crystallization. As depicted in Fig. 5a, both Zn1 and Zn2 atoms are five-coordinate and possess a {ZnNO₄} trigonal-bipyramid environment. The Zn1 atom is coordinated by three O and one N atom from the three different L³⁻ blocks, and one O atom from one coordinated H₂O molecule. The Zn2 atom is bound by two O atoms from two different L³⁻ ligands, one N atom from one py moiety, and two O atoms from two coordinated water molecules. The Zn–O and Zn–N bond lengths are in the range of 1.957(3)–2.147(4) and 2.053(6)–2.065(4) Å, respectively. All the Zn–O and Zn–N distances in 5 and 6 are comparable to those in other reported Zn(II) compounds.^16–18 In 6, the L³⁻ ligands show a μ₄-coordination mode (Scheme 1, mode V), in which two carboxylate groups are η¹:η⁰ monodentate and the third group is μ₂-η¹:η¹ bidentate. The dihedral angle between the pyridyl and phenyl rings in the L³⁻ ligand is 51.68°. The carboxylate groups of the L³⁻ ligands multiply bridge the adjacent Zn(II) ions to form a complex 3D metal–organic framework (Fig. 5b). It features channels [13.75 × 4.923 Å measured by atom-to-atom distances], which are filled with water molecules of crystallization. If seen down the c axis (Fig. 5c), the framework of 6 also displays channels with a size of ca. 9.574 × 5.192 Å measured by atom-to-atom distances. Upon removal of water molecules of crystallization, we computed by PLATON an effective free volume that is 10.2% of the crystal volume.³⁶ However, after eliminating both coordinated and guest water molecules, the effective free volume attains 12.5% of the crystal volume of 6. In contrast to 5, the 3D metal–organic structure of 6 is more complex due to the presence of μ₄-L blocks. To perform the topological classification, the metal–organic framework of 6 was simplified by omitting terminal H₂O and pyridine ligands and by reducing the μ₄-L blocks to their centroids. The obtained underlying 3D net is composed of the 2-connected Zn2 linkers, 3-connected Zn1 nodes, and 4-connected μ₄-L nodes (Fig. 5d). Its topological analysis³¹ revealed a binodal 3,4-connected framework with the fsc-3,4-Pbcn-2 topology and the point symbol of (6³)(6⁵.8), wherein the (6³) and (6⁵.8) notations are those of Zn1 and μ₄-L nodes, respectively. The present topological type is rather rare, as confirmed by a search of different databases.^31–33

Fig. 5 Structural fragments of 6. (a) Coordination environment of the Zn(II) atoms. Symmetry codes: i = x, −y + 1, z + 1/2; ii = −x + 1/2, y + 1/2, z; iii = −x + 1, y, −z + 1/2. (b) Perspective of the 3D porous framework along the ac plane. (c) Perspective of the 3D porous framework along the ab plane. (d) Topological representation of the underlying 3D metal–organic framework showing a binodal 3,4-connected net with the fsc-3,4-Pbcn-2 topology; view along the c axis; color codes: 2-connected Zn2 linkers and 3-connected Zn1 nodes (pale green), centroids of 4-connected μ₄-L nodes (gray).

Coordination modes of 2-(4-carboxypyridin-3-yl)terephthalate blocks and structural comparison

As depicted in Scheme 2, the ligands derived from H₃L exhibit versatile coordination modes in compounds 1–6. The H₃L block exists in partially and fully deprotonated forms, namely H₂L⁻, HL²⁻ and L³⁻, depending on the molar ratio of NaOH and H₃L ligand. For example, when the molar ratios of NaOH and H₃L are 1 : 1 and 2 : 1, the H₃L ligands become partially deprotonated to give H₂L⁻ and HL²⁻ forms in 1–3. When the molar ratio of NaOH and H₃L is adjusted to 3 : 1, the ligand is completely deprotonated to afford the L³⁻ form in 4 and 5. Five different coordination modes of ligands derived from H₃L were observed in compounds 1–6, in which the carboxylate groups adopt three typical modes, that is, η¹:η⁰ monodentate and μ₂-η²:η⁰ and μ₂-η¹:η¹ bidentate modes. The pyridyl N atom of the principal ligand always acts as a donor to metal ions in 1–6, reinforcing the binding of the adjacent carboxylate group. To facilitate the generation of a certain coordination environment around metal ions in the assembly process, the C–C single bond between the pyridyl and phenyl rings of the H₂L⁻, HL²⁻ or L³⁻ ligand can relatively freely rotate, with the corresponding dihedral angles being in the range of 51.68–66.62°. The obtained results indicate that the ligands derived from H₃L can adopt various coordination modes to meet the central metal ions in the compounds, and behave as either terminal (in 1) or different μ₃- (in 5) and μ₄-building blocks (in 2, 3, 4, and 6).

Scheme 2 Various coordination modes of H₂L⁻/HL²⁻/L³⁻ in compounds 1–6.

Apart from different dimensionality of metal–organic networks that ranges from 0D (in 1) and 1D (in 4 and 5) to 2D (in 2 and 3) and 3D (in 6), all the obtained products also reveal distinct topologies, including their rare or even unprecedented types. In fact, the topological analysis of the coordination networks disclosed: (i) a uninodal 3-connected 1D chain with the SP 1-periodic net (4,4)(0,2) topology in 5, (ii) a binodal 3,4-connected 3D framework with a rare fsc-3,4-Pbcn-2 topology in 6, and (iii) a binodal 4,4-connected 1D chain in 4 or 2D layer in 2 and 3 with the unique topologies. A rare 8T9 topology was also identified in a 3D supramolecular network in 1.

It should also be mentioned that while attempting the hydrothermal self-assembly syntheses of compounds 1–6, the reactions of different metal(II) chlorides with H₃L as a single building block and also in the presence of an ancillary ligand (phen or py) were tested. Only in the case of 4 and 6, the incorporation of phen and py moieties occurred. In other cases, we were not able to obtain crystalline samples of sufficient quality to be characterized by single crystal X-ray diffraction, or the reaction attempts were not successful. We predict that further optimization of reaction conditions might allow the introduction of various N-donor ancillary blocks (e.g., by substituting H₂O ligands in 1–3 and 5), thus resulting in the generation of mixed-ligand coordination compounds. The exploration of H₃L as a versatile building block for the synthesis of mixed-ligand derivatives will be pursued.

Thermogravimetric analysis and PXRD results

The thermal stability of compounds 1–6 was studied under nitrogen atmosphere by thermogravimetric analysis (TGA) and the obtained plots are given in Fig. S1.† The compound 1 releases its two water ligands (exptl, 5.7%; calcd, 5.4%) in the 88–169 °C range, followed by the decomposition at 238 °C. The TGA curves of 2 and 3 indicate that there is one distinct thermal effect in the 131–194 and 138–194 °C range, respectively, which corresponds to the removal of one coordinated H₂O molecule (exptl, 4.7% (2), 5.4% (3); calcd, 4.3% (2), 5.0% (3)). Further heating up to 369 and 377 °C, respectively, leads to the decomposition of dehydrated samples. For 4, the weight loss associated with the removal of two H₂O ligands and one water molecule of crystallization is observed in the 141–268 °C interval (exptl, 8.7%; calcd, 9.0%), and the decomposition of the remaining solid begins at 308 °C. For 5, there is one distinct thermal effect in the 164–228 °C range that corresponds to the removal of six coordinated H₂O molecules (exptl, 12.2%; calcd, 12.4%). Further heating up to 336 °C leads to the decomposition of a dehydrated sample. Compound 6 loses its four lattice and four coordinated water molecules (exptl, 14.9%; calcd, 14.6%) in the 164–228 °C range, followed by decomposition at 388 °C.

Powder X-ray diffraction (PXRD) experiments for compounds 1–6 have been carried out at room temperature to identify whether the crystal structures can represent the bulk samples. As shown in Fig. S2,† the peak positions of the PXRD patterns closely match the simulated ones, thus indicating that the as-synthesized bulk materials are pure products.

Luminescent properties

The emission spectra of H₃L and its cadmium(II) and zinc(II) compounds 2, 5 and 6 were recorded at room temperature in the solid state (Fig. 6). The uncoordinated H₃L exhibits a weak photoluminescence with an emission maximum at 454 nm if excited at 365 nm. However, the metal–organic networks 2, 5 and 6 reveal the substantially more intense emission bands with the maxima at 439 nm (λ_ex = 352 nm), 404 nm (λ_ex = 345 nm), and 430 nm (λ_ex = 341 nm), respectively. These bands are presumably associated with the intraligand (π* → n or π* → π) emission.³⁷ The observed enhancement of luminescence in 2, 5, and 6 can depend on the type of ligands present in these compounds and their binding modes.³⁸ A stronger emission intensity in 6 over 2 and 5 can potentially be explained by structural differences in these compounds, namely because of higher dimensionality of 6 and the presence of auxiliary pyridine ligand.

Fig. 6 Solid state emission spectra of H₃L, 2, 5, and 6 (λ_ex of 365, 352, 345, and 341 nm, respectively).

Magnetic properties

Variable-temperature magnetic susceptibility studies were carried out on powder samples of manganese(II) derivatives 3 and 4 in the 2–300 K temperature range. For 3, as shown in Fig. 7, the χ_MT value at 300 K is 4.41 cm³ mol⁻¹ K, which is close to the value of 4.38 cm³ mol⁻¹ K expected for one magnetically isolated high-spin Mn(II) center (S_Mn = 5/2, g = 2.0). The χ_MT values steadily decrease on lowering the temperature and reach the minimum of 0.176 cm³ mol⁻¹ K at 2.00 K. Between 50 and 300 K, the magnetic susceptibility can be fitted to the Curie–Weiss law with C_M = 5.04 cm³ mol⁻¹ K and θ = −42.9 K. These results indicate an antiferromagnetic interaction between the adjacent Mn(II) ions. According to the crystal structure of 3, compound 3 can be considered as 1D chain from the viewpoint of magnetism. We tried to fit the magnetic data of 3 using the following expression³⁹ for a 1D Mn(II) chain:

χ_chain = (Ng²β²/kT)[A + Bx²][1 + Cx + Dx³]⁻¹

with A = 2.9167, B = 208.04, C = 15.543, D = 2707.2, and x = ∣J∣/kT.

Fig. 7 Temperature dependence of χ_MT (○) and 1/χ_M (□) vs. T for compound 3. The red line represents the best fit to the equations in the text. The blue line shows the Curie–Weiss fitting.

Using this rough model, the susceptibilities for 3 were simulated, leading to J = −2.75 cm⁻¹, g = 2.06, and the agreement factor R = 5.83 × 10⁻⁶ (R = ∑(χ_obsT − χ_calcT)²/∑(χ_obsT)²). The J parameter confirms that an antiferromagnetic exchange coupling exists between the adjacent Mn(II) centers, which is agreement with a negative θ value.

For 4, the χ_MT value at 300 K is 6.60 cm³ mol⁻¹ K, which is close to the value of 6.57 cm³ mol⁻¹ K expected for one and a half of magnetically isolated high-spin Mn(II) centers (S_Mn = 5/2, g = 2.0). Upon cooling, the χ_MT value drops down very slowly from 6.60 cm³ mol⁻¹ K at 300 K to 6.42 cm³ mol⁻¹ K at 104 K and then decreases steeply to 2.22 cm³ mol⁻¹ K at 2 K (Fig. 8). The χ_M⁻¹ vs. T plot for 4 in the 2–300 K interval obeys the Curie–Weiss law with a Weiss constant θ of −4.45 K and a Curie constant C of 6.71 cm³ mol⁻¹ K. The negative value of θ and the decrease of the χ_MT should be attributed to the overall antiferromagnetic coupling between the Mn(II) centers within the Mn₃ unit. According to the structure of compound 4, there is one set of magnetic exchange pathway within the trinuclear cluster via carboxylate bridge (Fig. 3d). We tried to fit the magnetic data of 4 using the following expression⁴⁰ for the linear trinuclear Mn(II) motif:

Ĥ = −2J₁₂S⃑₁S⃑₂ − 2J₂₃S⃑₂S⃑₃ − 2J₁₃S⃑₁S⃑₃

where J₁₂ = J₂₃ = J₁, J₁₃ = J₂ (J₁₂ and J₂₃ are the exchange interactions between the “central” Mn(II) and two “outer” Mn(II) atoms; J₂ is the exchange interaction between the “outer” Mn(II) ions within a Mn₃ unit), zJ′ refers to the intercluster coupling constant in the 1D chain. This model gives satisfactory results with the superexchange parameters: J₁/k_B = −1.35 K, J₂/k_B = −0.45 K, zJ′/k_B = −0.18 K, and g = 2.01. The agreement factor defined by R = ∑(χ_mT_exp − χ_mT_calc)²/∑(χ_mT_exp)² is 9.86 × 10⁻⁴. These values confirm the presence of antiferromagnetic interaction between the Mn(II) ions within a trinuclear subunit. The intercluster magnetic interaction (zJ′) is rather small, indicating that the exchange interactions between two magnetic clusters are very weak, which is probably due to a long separation [10.388(3) Å] of the adjacent magnetic subunits.

Fig. 8 Temperature dependence of χ_MT (○) and 1/χ_M (□) vs. T for compound 4. The red line represents the best fit to the equations in the text. The blue line shows the Curie–Weiss fitting.

Conclusions

In this work, by selecting different metal(II) nodes (M(II) = Co, Cd, Mn, Zn) and adjusting the molar ratio of NaOH and H₃L ligand and/or the type of the aromatic N-donor auxiliary ligand (optional), a new series of transition metal coordination compounds driven by the 2-(4-carboxypyridin-3-yl)-terephthalic acid blocks were synthesized by the hydrothermal self-assembly method. The structures of the obtained products vary from the 0D monomer (1) and 1D coordination chains (4 and 5) to 2D metal–organic layers (2 and 3) and 3D framework (6) with distinct architectures and topologies, including their unprecedented types. The structural diversity of 1–6 demonstrates that the molar ratio of NaOH and H₃L ligand and the nature of the metal ion and auxiliary ligand play a crucial role in the assembly of these distinct products which, to our knowledge,³³ represent the first coordination compounds derived from H₃L. Thus, the study also opens up the application of 2-(4-carboxypyridin-3-yl)terephthalic acid as a virtually unexplored but rather versatile multifunctional building block for the generation of diverse coordination compounds.

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 bonding (Table S1) and H-bonding (Table S2) distances, TGA plots (Fig. S1) and PXRD patterns (Fig. S2). CCDC 1057602–1057606 and 1058046. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra16580f

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