Syntheses, structures, and properties of six new coordination polymers constructed from N-heterocyclic multicarboxylic acids

Abstract

Six new coordination polymers, {[Zn₂(Hpdtc)₂(Hbpe)₂]·5H₂O} (1), [Zn₂(pdtc)(bpe)_1.5]_n (2), [Zn(Hpztc)(Hbpe)(H₂O)] (3), {[Zn₂(pztc)(bpe)₂(H₂O]·9H₂O}_n (4), {[Cu₂(pdtc)(bpy)(H₂O)₂]·3H₂O}_n (5) and {[Cu₂(pdtc)(bpy)(H₂O)₂]·2H₂O}_n (6) [H₄pdtc = pyridine-2,3,5,6-tetracarboxylic acid, H₄pztc = pyrazine-2,3,5,6-tetracarboxylic acid, bpy = 4,4′-dipyridine, and bpe = 1,2-bis(4-pyridyl)ethene], have been synthesized under control by modifying the reaction conditions. Their structural diversities range from zero-dimensional (0D) (1, 3), two-dimensional (2D) (2, 5), to porous three-dimensional (3D) networks (4, 6). The pH values of the reagents of 1 and 3 are around 3, which results in the ligands of 1 and 3 being partly protonated. Both 1 and 3 show 0D structures. So we tried to modify the pH value to improve the structures. 2 and 4 were synthesized at higher pH values (5.8, 5.1). 2 and 4 have 2D layer structure and 3D pillar-layered framework, respectively. 5 and 6 were synthesized from the same reagents, 5 was synthesized at room temperature, while 6 was obtained under hydrothermal conditions. The coordination environment of Cu(II) centers in 6 is similar to 5 except that one oxygen atom from a water molecule for Cu1 in 5 is replaced by the oxygen atom from the pdtc⁴⁻ ligand in 6, which results in different structures: 5 shows 2D network structure, 6 has a 3D porous framework. The PXRD, thermal stabilities and luminescent properties of these complexes were investigated in detail.

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Introduction

The design and synthesis of metal–organic frameworks (MOFs) has become a subject of extensive investigation due to their unmatched structural versatilities and potential applications such as gas storage, separation, luminescence, catalysis, sensors and magnetism.^1–6 In recent years, the porous metal–organic frameworks (PMOFs) especially pillar-layered ones have attracted significant attention due to their structural diversities and chemical functionalities through simple modification of the pillar module and layer-mediated bridges.^7–9 The rational method constructing these porous frameworks is to connect well-defined two-dimensional (2D) layers with appropriate pillars, which is the so-called pillaring strategy.¹⁰ Over the past decades, great effort has been invested in the purposeful design and controllable synthesis of the functional complexes,¹¹ which is still a hard task. As we all know, except metal ions and bridging ligands, many other factors can affect the structures of MOFs, for instance pH value, hydrothermal/solvothermal conditions, temperature, counter-ions, auxiliary ligands, etc. Therefore, appropriate control over influences from various factors is necessary for directed synthesis of MOF materials.

To achieve the desired networks, an important family of N-heterocyclic ligands is often applied. Here, we chose pyridine-2,3,5,6-tetracarboxylic acid (H₄pdtc) and pyrazine-2,3,5,6-tetracarboxylic acid (H₄pztc) as bridging ligands. Although some compounds of the two ligands have been synthesized,^12–19 there still have been an active field to prepare various coordination polymers with predictable structures and functional properties. For instance, the compound constructed by Cu(II), H₄pdtc and 4,4′-bpy has been synthesized,²⁰ but in our work two new coordination polymers (5, 6) with different structures are obtained under different conditions. Both the H₄pdtc and H₄pztc contain rich coordination sites, which can adopt a variety of coordination modes in the structure. Furthermore, the pillar-like bridging ligands: 1,2-bis(4-pyridyl)ethene (bpe) and 4,4′-dipyridine (4,4′-bpy) can modify the structure and properties of the resulting materials through cooperative coordination together with carboxyl group to meet the requirement of coordination geometries of metal ions in the assembly process.^21–25 From the report, some complexes constructed by transitions metal nodes, pyridine dicarboxylic acid and bpe ligand have been synthesized and show different structures. Considering that H₄pdtc and H₄pztc ligands contain more coordination sites, furthermore, the complexes composed of H₄pdtc/H₄pztc and bpe are not reported.^26–32 So we chose H₄pdtc/H₄pztc to construct the metal–organic frameworks. Herein, we successfully synthesized six new coordination polymers with the formulas {[Zn₂(Hpdtc)₂(Hbpe)₂]·5H₂O} (1), [Zn₂(pdtc)(bpe)_1.5]_n (2), [Zn(Hpztc)(Hbpe)(H₂O)] (3), {[Zn₂(pztc)(bpe)₂(H₂O)]·9H₂O}_n (4), {[Cu₂(pdtc)(bpy)(H₂O)₂]·3H₂O}_n (5) and {[Cu₂(pdtc)(bpy)(H₂O)₂]·2H₂O}_n (6). The structures, thermal stabilities and luminescent properties of these complexes have been investigated in detail.

Experimental

Materials and methods

K₄pdtc was synthesized according to the literature method reported by our group³³ and H₄pztc was synthesized by Wolff's synthesis.³⁴ Other chemicals for synthesis were purchased from commercial sources and used without purification. Infrared spectra were obtained in KBr pellets on a Bruker TENOR 27 spectrophotometer in the range of 400–4000 cm⁻¹. Elemental analyses for C, H and N were measured on a Perkin-Elmer 240 CHN elemental analyzer. Powder X-ray diffraction (PXRD) data were performed on a Rigaka D/max 2500v/pc X-ray powder diffractometer with Cu-Kα radiation (λ = 1.540598). Thermogravimetric analyses (TGA) were carried out on a NETZSCH TG 209 instrument with a heating rate of 10 °C min⁻¹. Fluorescence spectra were investigated with an F-4500 fluorescence spectrophotometer at room temperature.

Syntheses of the complexes

{[Zn₂(Hpdtc)₂(Hbpe)₂]·5H₂O} (1). A mixture of K₄pdtc (0.0204 g, 0.05 mmol), Zn(NO₃)₂·6H₂O (0.0447 g, 0.15 mmol), bpe (0.0182 g, 0.10 mmol), HNO₃ (5%), H₂O (10 mL) and ethanol (2 mL) was stirred for 1 h in a 25 mL Teflon high-temperature kettle and kept at 140 °C for 2 days, and then cooled to room temperature at a rate of 1.5 °C h⁻¹. Pale yellow crystals of 1 were obtained by filtration. Yield: 48% based on Zn. Elemental analysis (%) calcd. for C₄₂H₃₆Zn₂N₆O₂₁ (fw = 1091.51): C, 46.22; H, 3.32; N, 7.70; found: C, 46.20; H, 3.35; N, 7.67. IR (KBr pellet, cm⁻¹): 3399m, 1660vs, 1617vs, 1452m, 1325m, 1323m, 1031w, 827m, 553m.

[Zn₂(pdtc)(bpe)_1.5]_n (2). Compound 2 was synthesized through the same synthetic procedure as that for complex 1 except that HNO₃ (5%) was not added. Yellow crystals of 2 were obtained by filtration. Yield: 43% based on Zn. Elemental analysis (%) calcd. for C₂₇H₁₆Zn₂N₄O₈ (fw = 655.18): C, 49.50; H, 2.46; N, 8.55; found: C, 49.52; H, 2.45; N, 8.51. IR (KBr pellet, cm⁻¹): 3404w, 1672vs, 1617vs, 1592vs, 1425m, 1352m, 1324m, 1161w, 824m, 551m.

Zn(Hpztc)(Hbpe)(H₂O)] (3). A mixture of H₄pztc (0.0032 g, 0.0125 mmol), Zn(NO₃)₂·6H₂O (0.0074 g, 0.025 mmol), bpe (0.0046 g, 0.025 mmol), H₂O (10 mL) and ethanol (2 mL) was stirred for 2 h in a 25 mL Teflon high-temperature kettle and kept at 140 °C for 2 days, and then cooled to room temperature at a rate of 1.5 °C h⁻¹. Colorless crystals of 3 were obtained by filtration. Yield: 51% based on Zn. Elemental analysis (%) calcd. For C₂₀H₁₄ZnN₄O₉ (fw = 519.72): C, 46.22; H, 2.72; N, 10.78; found: C, 49.19; H, 2.73; N, 10.76. IR (KBr pellet, cm⁻¹): 3417vs, 1713w, 1657vs, 1613vs, 1398m, 1303m, 1143m, 986w, 883w.

{[Zn₂(pztc)(bpe)₂(H₂O)]·9H₂O}_n (4). Compound 4 was synthesized through the same synthetic procedure as that for complex 3 except that KOH (5%) was added. Yellow crystals of 4 were obtained by filtration. Yield: 45% based on Zn. Elemental analysis (%) calcd. For C₁₆H₂₁ZnN₃O_8.50 (fw = 456.73): C, 42.08; H, 4.63; N, 9.20; found: C, 42.09; H, 4.61; N, 9.19. IR (KBr pellet, cm⁻¹): 3386s, 1601vs, 1425m, 1306m, 1161w, 973w, 826m, 548m.

{[Cu₂(pdtc)(bpy)(H₂O)₂]·3H₂O}_n (5). A mixture of Cu(CH₃COO)₂·H₂O (0.0332 g, 0.2 mmol) dissolved in 5 mL H₂O and 4,4′-bpy (0.0288 g, 0.15 mmol) dissolved in 2 mL ethanol was stirred for 4 h at room temperature. Then the aqueous solution of K₄pdtc (0.0407 g, 0.1 mmol) (5 mL) was added. The resulting mixture was continuously stirred for 1 h and filtered. Blue crystals were obtained from the filtrate after slow evaporation of the solution at the temperature for 20 days. Yield: 48% based on Cu. Elemental analysis (%) calcd. For C₁₉H₁₉Cu₂N₃O₁₃ (fw = 624.45): C, 36.55; H, 3.07; N, 6.73; found: C, 36.56; H, 3.05; N, 6.72. IR (KBr pellet, cm⁻¹): 3440vs, 1636vs, 1541m, 1508m, 1420w, 1388w, 1320w, 1155m, 821w, 620m, 583m.

{[Cu₂(pdtc)(bpy)(H₂O)₂]·2H₂O}_n (6). A mixture of K₄pdtc (0.0407 g, 0.1 mmol), Cu(CH₃COO)₂·H₂O (0.0332 g, 0.2 mmol), 4,4′-bpy (0.0288 g, 0.15 mmol), H₂O (10 mL) and ethanol (2 ml) was stirred for 1 h in a 25 mL Teflon high-temperature kettle and kept at 150 °C for 1 day, and then cooled to room temperature naturally. Dark blue crystals of 6 were obtained. Yield: 53% based on Cu. Elemental analysis (%) calcd. For C₁₉H₁₇Cu₂N₃O₁₂ (fw = 606.44): C, 37.63; H, 2.83; N, 6.93; found: C, 37.65; H, 2.82; N, 6.91. IR (KBr pellet, cm⁻¹): 3390s, 1611vs, 1413m, 1381s, 1317s, 1219w, 1152m, 1065m, 819m, 648m.

X-ray single crystal structure determination

Single-crystal X-ray diffraction analyses of the complexes were carried out on a BRUKER SMART-1000 CCD diffractometer equipped with a graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) by using ω–ϕ scan. The structures were solved by direct method using the program SHELXS-97, and refined anisotropically by a full-matrix least-squares technique based on F² using SHELXL-97 for all non-hydrogen atoms. Hydrogen atoms were located and included at their calculated positions. Crystal data collection and refinement details for coordination polymers 1–6 are summarized in Table 1. Selected bond lengths and angles for 1–6 are listed in ESI.†

Table 1 Crystal data and structure refinements for coordination polymers 1–6

Complexes	1	2	3	4	5	6
Formula	C42H36Zn2N6O21	C27H16Zn2N4O8	C20H14ZnN4O9	C16H21ZnN3O8.50	C19H19Cu2N3O13	C19H17Cu2N3O12
Formula weight	1091.51	655.18	519.72	456.73	624.45	606.44
Temperature (K)	113(2)	113(2)	113(2)	113(2)	113(2)	113(2)
Wavelength (Å)	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073
Crystal system	Monoclinic	Monoclinic	Triclinic	Triclinic	Monoclinic	Monoclinic
Space group	P21/c	P21/n	P1̄	P1̄	C2/c	P21/n
a (Å)	24.516(4)	8.5377(17)	7.562(3)	7.4083(15)	16.349(3)	7.791(1)
b (Å)	7.3474(13)	16.953(3)	11.277(3)	10.890(2)	16.979(3)	17.630(4)
c (Å)	23.623(4)	17.256(4)	12.205(3)	13.632(3)	17.583(4)	15.874(3)
α (deg)	90	90	71.997(14)	66.67(3)	90	90
β (deg)	99.650(2)	102.99(3)	87.887(18)	78.17(3)	112.86(3)	96.99(3)
γ (deg)	90	90	74.766(14)	71.78(3)	90	90
Volume (Å^3)	4195.0(12)	2433.7(8)	949.4(5)	955.1(3)	4497.4(16)	2164.3(8)
Z	4	4	2	2	8	4
Calculated density (Mg m^−3)	1.728	1.788	1.818	1.588	1.844	1.861
Absorption coefficient (mm^−1)	1.242	2.035	1.363	1.339	1.969	2.040
F (000)	2232	1320	528	472	2528	1224
Crystal size (mm)	0.20 × 0.18 × 0.12	0.20 × 0.18 × 0.12	0.20 × 0.18 × 0.12	0.20 × 0.18 × 0.12	0.10 × 0.04 × 0.04	0.08 × 0.04 × 0.04
θ Range for data collection (deg)	1.69 to 27.85	2.40 to 27.89	1.76 to 27.93	2.11 to 27.86	2.64 to 25.02	2.31 to 27.86
Limiting indices	−32 ≤ h ≤ 32	−11 ≤ h ≤ 11	−9 ≤ h ≤ 9	−9 ≤ h ≤ 9	−19 ≤ h ≤ 19	−10 ≤ h ≤ 10
	−9 ≤ k ≤ 9	−22 ≤ k ≤ 15	−14 ≤ k ≤ 14	−13 ≤ k ≤ 14	−15 ≤ k ≤ 20	−23 ≤ k ≤ 23
	−31 ≤ l ≤ 31	−22 ≤ l ≤ 22	−15 ≤ l ≤ 16	−14 ≤ l ≤ 17	−20 ≤ l ≤ 20	−16 ≤ l ≤ 20
Reflections collected	42141	21703	12267	9423	14906	19348
Independent reflection	9948 [Rint = 0.0621]	5784 [Rint = 0.0724]	4508 [Rint = 0.0521]	4441 [Rint = 0.0410]	3913 [Rint = 0.1512]	5129 [Rint = 0.0353]
Completeness	99.5%	99.5%	99.2%	97.9%	98.7%	99.5%
Max. and min. transmission	0.8653 and 0.7893	0.7923 and 0.6864	0.8535 and 0.7722	0.8559 and 0.7756	0.9254 and 0.8274	0.9228 and 0.8538
Data/restraints/parameters	9948/16/680	5784/0/370	4508/4/320	4441/27/298	3913/12/344	5129/11/357
Goodness-of-fit on F^2	1.099	0.997	0.999	1.043	1.061	1.070
Final R indices [I > 2σ(I)]	R1 = 0.0538	R1 = 0.0450	R1 = 0.0368	R1 = 0.030	R1 = 0.0751	R1 = 0.0341
	wR2 = 0.1134	wR2 = 0.0877	wR2 = 0.0805	wR2 = 0.1120	wR2 = 0.1687	wR2 = 0.0776
R indices (all data)	R1 = 0.0718	R1 = 0.0640	R1 = 0.0504	R1 = 0.0481	R1 = 0.0981	R1 = 0.0429
	wR2 = 0.1214	wR2 = 0.0949	wR2 = 0.0864	wR2 = 0.1159	wR2 = 0.1834	wR2 = 0.0817
Largest diff. peak and hole (e Å^−3)	1.325 and −0.475	0.619 and −0.755	0.690 and −0.504	0.559 and −0.663	1.082 and −1.206	1.021 and −0.581

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Result and discussion

Description of crystal structures

Crystal structure of {[Zn₂(Hpdtc)₂(Hbpe)₂]·5H₂O} (1). Complex 1 is a dinuclear zinc complex and crystallized in a monoclinic system, P2₁/c space group. The dinuclear unit contains two Zn(II) cations, two Hpdtc³⁻ ligands, two Hbpe⁺ ligands and five lattice water molecules, as shown in Fig. 1a. The Zn1 and Zn2 atoms show similar coordinated environments, which are five coordinated in distorted trigonal bipyramidal geometry bonding three oxygen atoms from Hpdtc³⁻ anions and two nitrogen atoms from Hpdtc³⁻ and Hbpe⁺ ligands. The Hpdtc³⁻ ligand exhibits monodentate/chelating coordination mode briging two Zn(II) atoms to build the binuclear structure. The Zn–O bond lengths range from 1.959(2)–2.144(2) Å, and the Zn–N bond lengths are in the range of 2.024(2)–2.048(2) Å.

Fig. 1 (a) The molecular structure of complex 1 with 50% probability ellipsoids, lattice water molecules have been omitted for clarity. (b) 3D packing diagrams of 1 connected by hydrogen bonds.

In complex 1, the ligands were partly protonated: one carboxyl group of the Hpdtc³⁻ ligand and one nitrogen atom of Hbpe⁺ ligand were protonated respectively, which prevented the compound from assembling high-dimensional structure. But the interesting thing is that the hydrogen-bonding interaction exists in 1 (Table 2), which holds the complex together to form 3D network structure (Fig. 1b). Bond lengths and angles involving these hydrogen bonds are given in ESI.†

Table 2 Hydrogen-bonding contacts (Å, °) in complex {[Zn₂(Hpdtc)₂(bpe)₂]·5H₂O} (1)^a

Donor–H⋯acceptor	d(D–H)	d(H⋯A)	d(D⋯A)	∠D–H⋯A
a Symmetry code: #2: x − 1, y + 1, z; #3: x, −y + 1/2, z − 1/2; #5: x, −y + 1/2, z − 1/2; #6: −x + 1, −y + 2, −z + 1.
N(4)–H(4)⋯O(17)#2	0.919(10)	2.22(3)	2.916(4)	132(3)
N(6)–H(6)⋯O(17)#3	0.910(10)	1.867(16)	2.741(4)	160(3)
O(17)–H(17A)⋯O(4)#5	0.90(2)	1.83(2)	2.707(3)	165(3)
O(17)–H(17B)⋯O(8)#6	0.845(10)	2.155(11)	2.992(3)	171(3)

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Crystal structure of [Zn₂(pdtc)(bpe)_1.5]_n (2). Compound 2 represents a 2D framework and crystallizes in the monoclinic space group P2₁/n. As shown in Fig. 2a, the asymmetric unit of 2 consists of two Zn(II) ions, one pdtc⁴⁻ anion and one and a half bpe ligands. The Zn1 adopts a distorted trigonal bipyramidal geometry and is directly coordinated by two nitrogen atoms from pdtc⁴⁻ and bpe ligands, and three oxygen atoms from two pdtc⁴⁻ ligands. While the Zn2 center is four coordinated, and shows a distorted tetrahedron geometry bonding three oxygen atoms from three pdtc⁴⁻ anions and one nitrogen atom from bpe ligand.

Fig. 2 (a) The molecular structure of compound 2 with 50% probability ellipsoids. (b) 1D chain connected by pdtc⁴⁻ and Zn(II). (c) 1D chain connected by bpe. (d) The 2D layer structure of compound 2.

Two antiparallel pdtc⁴⁻ anions four Zn(II) atoms (two Zn1, two Zn2) are interconnected to assemble into a bilayer building block [Zn₄(pdtc)₂]. Each building block links two equivalent blocks into a 1D chain (Fig. 2b). This 1D chain is joined into a 2D layer structure by pillar-like bpe ligands. As Fig. 2c shown, each [Zn₄(pdtc)₂] building block links four bpe ligands through Zn–N coordination. However, only two bpe ligands play a role in connecting two [Zn₄(pdtc)₂] building blocks. One nitrogen atom of another two bpe ligands coordinates to Zn(II) atom. The 2D layer structure of 2 is presented in Fig. 2d.

Crystal structure of [Zn(Hpztc)(Hbpe)(H₂O)] (3). Complex 3 crystallizes in the space group and the asymmetric unit contains one Zn(II) center, one Hpztc³⁻ anion, one Hbpe⁺ cation, one coordinated water molecule (Fig. 3a). Each Zn(II) center adopts a distorted octahedral arrangement and is coordinated with two oxygen atoms (O1 and O8) from Hpztc³⁻ anion and two nitrogen atoms (N1 and N3) from Hpztc³⁻ and Hbpe⁺ ligands in the equatorial plane, the axial positions are occupied by two oxygen atoms (O4A and O9) from another Hpztc³⁻ and water molecule, respectively. As shown in Fig. 3b, the parallel Zn(II) ions assemble into a binuclear building block [Zn₂(Hpztc)₂(Hbpe)₂(H₂O)₂] through the interconnection of O4 and O4A.

Fig. 3 (a) The molecular structure of complex 3 with 50% probability ellipsoids. (b) The binuclear building block [Zn₂(Hpztc)₂(Hbpe)₂(H₂O)₂] of 3. The symmetry code: (A): 1 − x, −y, −z. (c) 2D layer structure of 3 connected by hydrogen bonds. (d) 3D packing diagrams of 3 connected by hydrogen bonds.

The coordination mode of the ligands were analogous to complex 1: Hpztc³⁻ and Hbpe⁺ ligands were protonated and hydrogen-bonding interactions existed in 3 (Table 3). As the Fig. 3c shown, the binuclear building blocks assembled the infinite 2D layer structure through interaction of N4–H4⋯O5 and O9–H9B⋯O3. Furthermore, the 2D layers were further assembled into a 3D supramolecular framework (Fig. 3d) through the hydrogen-bonding interaction (O9–H9A⋯O8).

Table 3 Hydrogen-bonding contacts (Å, °) in complex Zn(Hpztc)(Hbpe)(H₂O)] (3)^a

Donor–H⋯acceptor	d(D–H)	d(H⋯A)	d(D⋯A)	∠D–H⋯A
a Symmetry code: #2: x − 1, y + 1, z + 1; #3: −x + 1, −y + 1, −z; #4: −x + 2, −y, −z.
N(4)–H(4)⋯O(5)#2	0.900(10)	1.92(2)	2.670(3)	140(3)
N(4)–H(4)⋯N(2)#2	0.900(10)	2.396(19)	3.175(3)	145(3)
O(9)–H(9A)⋯O(8)#3	0.845(9)	1.984(10)	2.824(2)	172(2)
O(9)–H(9B)⋯O(3)#4	0.851(9)	1.826(10)	2.658(2)	166(2)

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Crystal structure of {[Zn₂(pztc)(bpe)₂(H₂O)]·9H₂O}_n (4). Compound 4 displays a 3D architecture with the triclinic space group As shown in Fig. 4a, the asymmetric unit contains two Zn(II) atoms, half pdtc⁴⁻ anion and two semimolecule bpe ligands, two coordinated water molecules and several lattice water. Zn1 atom shows a distorted octahedral geometry bonding four nitrogen atoms from two pdtc⁴⁻ anion and two bpe ligands respectively in the equatorial plane and two oxygen atoms from the two pdtc⁴⁻ anions in the axial positions. Zn2 atom adopts a distorted octahedral geometry too. But it is coordinated by four oxygen atoms from two pdtc⁴⁻ anions and two water molecules in the equatorial plane, the axial positions are occupied by two nitrogen atoms from two bpe ligands. The pdtc⁴⁻ anion links four Zn(II) atoms (two Zn1, two Zn2) through monodentate and bidentate chelating coordination modes.

Fig. 4 (a) The crystal structure of 4, the symmetry code: (A) −x + 2, −y + 1, −z. (b and c) 2D network of 4 connected by pdtc⁴⁻ and Zn(II) with sticks mode. (d) The 3D pillar-layered structure of 4 with sticks mode. (e) Space-filling mode of the channel structures along the a axis.

The pdtc⁴⁻ ligands are joined to form 1D chain by Zn1 atoms. Then the 1D chains are connected by Zn2 atoms coordinating O4 and O4b to assemble the 2D layer structure (Fig. 4b and c). As the Fig. 4d shown, the adjacent layers are further extended by bpe linkers to form a 3D pillar-layered framework. The bpe ligands like pillars holding the pdtc⁴⁻ layers together. Furthermore, compound 4 exists 1D channels with the size of 5.839 Å × 8.857 Å along the a axis, showing in Fig. 4e.

Crystal structure of {[Cu₂(pdtc)(bpy)(H₂O)₂]·3H₂O}_n (5). Single crystal X-ray diffraction analysis reveals that compound 5 crystallizes in the space group C2/c and the asymmetric unit contains two Cu(II) centers, one pdtc⁴⁻ anion, one 4,4′-bpy ligand, three coordinated water molecules (Fig. 5a). Both of the two copper ions are five coordinated in distorted square pyramidal geometry. Cu1 is coordinated by two oxygen atoms (O2, O3), one nitrogen atom (N1) form pdtc⁴⁻ anion and another nitrogen atom (N2) from 4,4′-bpy ligand in the bottom plane, and the apical position is occupied by one oxygen atom (O9) from one water molecule. Cu2 is bonding three oxygen atoms (O6A, O7, O10) from two pdtc⁴⁻ anions and one water molecule and one nitrogen atom (N3B) from 4,4′-bpy ligand in the bottom plane, and the oxygen atom (O11E) from another water molecule in the apical position, but the distance of Cu2–O11E (2.461 Å) is longer than the other bond lengths of Cu–O, so the bond of Cu2–O11E is not given out in the figure.

Fig. 5 (a) The crystal structure of compound 5 with 50% probability ellipsoids. (b) 1D zigzag chain connected by pdtc⁴⁻ and Cu(II). (c) The 2D network structure of compound 5.

In compound 5, the pdtc⁴⁻ ligand bonds three copper atoms: one Cu1 through tridentate chelating coordination mode; two Cu2 through monodentate coordination mode. The Cu2 atoms and pdtc⁴⁻ ligands connect up and assemble into a zigzag 1D chain (Fig. 5b) through bonding O6A and O7 atoms of the adjacent pdtc⁴⁻ ligands. The two nitrogen atoms of 4,4′-bpy ligand coordinate with Cu1 and Cu2 respectively, which links the zigzag chains into 2D network structure (Fig. 5c).

Crystal structure of {[Cu₂(pdtc)(bpy)(H₂O)₂]·2H₂O}_n (6). Single crystal X-ray diffraction analysis reveals that compound 6 has a 3D open framework with many 1D channels and has a monoclinic system, P2₁/n space group. As Fig. 6a shown, the asymmetric unit of 6 contains two Cu(II) centers, one pdtc⁴⁻ anion, one 4,4′-bpy ligand, two coordinated water molecules and two lattice water molecules. The coordination environment of Cu(II) centers (Cu1, Cu2) is similar to 5 except that one oxygen atom (O9) from water molecule for Cu1 in 5 is replaced by the oxygen atom (O3B) from the pdtc⁴⁻ ligand in 6. In the structure of 6, two antiparallel pdtc⁴⁻ ligands and two copper atoms are interconnected through two oxygen atoms (O3, O3B) to assemble into a binuclear building block [Cu₂(pdtc⁴⁻)₂]. The building block links four Cu2 atoms, then the building blocks and Cu2 atoms connect alternately to form a 2D network structure (Fig. 6b). In the Fig. 6c, the 4,4′-bpy ligands connect the 2D layers into a 3D pillar-layered framework along c axis. It is noteworthy that 1D channels with dimensions of 8.766 Å × 10.153 Å exist in the structure (Fig. 6d).

Fig. 6 (a) The crystal structure of 6 with 50% probability ellipsoids, lattice water molecules have been omitted for clarity. (b) 2D network of 6 connected by pdtc⁴⁻ and Zn(II). (c) The 3D porous structure of 6 with sticks mode. (d) Space-filling mode of the channel structures along the c axis.

Synthesis and structural features

To the best of our knowledge, when the pH value of the solution is about 3, it is easier to get the crystals of the complexes constructed by H₄pdtc or H₄pztc. So we adjusted the pH value to 3.0 using HNO₃ (5%) for complex 1. The pH value of the reagent for complex 3 is 2.9. The crystal structures of 1 and 3 show that both H₄pdtc/H₄pztc and bpe ligands are partly protonated, which prevented the compound from assembling high-dimensional structure. Then we tried to modify the pH value to improve the structure. The synthetic procedures of 1 and 2 are similar except that the HNO₃ (5%) was not added. The pH value of 2 is 5.8. We adjusted the pH value to 5.1 by adding KOH (5%), then we got the crystals of 4. Single crystal X-ray diffraction analysis reveals that 2 and 4 have 2D layer structure and 3D pillar-layered framework, respectively. Compound 5 (2D network structure) was synthesized at room temperature, and we got the crystals of 6 (3D porous structure) through hydrothermal method at 423 K. The pH value has a very important influence on the formation and structure of the complexes. Comparing 5 and 6, the temperature is the main influence factor in formation of the structures.

PXRD patterns and thermal gravimetric analysis

Powder X-ray diffraction (PXRD) experiments on the synthesized samples of the coordination polymers were carried out to confirm the phase purity of the bulk materials. Fig. S1† showed that all major peaks matched well with the simulated PXRD patterns. Some missing or extra minor peaks could be attributed to the impurities present in these samples. To further test the stability of the porous frameworks of 4 and 6, we investigated the powder samples of 4 and 6 by X-ray diffraction analysis at 180 °C. The PXRD experiments of 4 and 6 in different reagents (acetone and chloroform) further indicated their stability (Fig. S2a and b†). Compounds 4 and 6 could be the best candidates of adsorption materials.

To study the thermal stabilities of these complexes, thermal gravimetric analysis (TGA) of complexes 1, 2, 3 and 6 was carried out by heating the samples from 30 to 800 °C in air atmosphere, and the TG curves were depicted in Fig. 7. For complex 1, the first weight loss of 7.03% from 50 to 140 °C is attributed to the lattice water molecules (calcd 8.24%). The second weight loss of 78.01% (calcd 76.92%), occurring at the range of 250–655 °C, corresponds to the loss of Hpdtc³⁻ and Hbpe⁺ ligands. Finally, the residue of 14.96% (calcd 14.84%) shows ZnO component. The TG curve of 2 shows one weight loss of 73.12% (calcd 75.15%) from 285 to 640 °C, corresponding to the structural collapse due to the removal of pdtc⁴⁻ and bpe ligands. For complex 3, the weight loss of 3.83% (calcd 3.46%) at the range of 120–202 °C is assigned to the loss of coordinated water molecule. Subsequently, the second weight loss step from 269 to 485 °C is attributed to the decompositions of Hpztc³⁻ and Hbpe⁺ ligands. The TGA diagram of 6 shows two main weight loss in the curve. The first loss of 9.81% (calcd 11.87%) from 97 to 260 °C is associated with the loss of lattice water molecules and coordinated water molecules, successively. The second step is attributed to the loss of pdtc⁴⁻ and 4,4′-bpy ligands, and then the framework decomposes. Finally, the residue of 27.62% (calcd 26.38%) shows CuO component.

Fig. 7 TG curves of 1, 2, 3, 6.

Luminescence properties

Recent years, luminescent MOFs have been widely examined for their potential applications in fluorescent sensors, nonlinear optics, photocatalysis, electroluminescent devices, etc. From the report, we know that luminescence can arise from direct organic ligands excitation, metal-centered emission, charge-transfer such as ligand-to-metal charge transfer (LMCT) and metal-to-ligand charge transfer (MLCT) and guest molecules.³⁵ Considering this, we investigated the luminescent properties of 1–4 and the free ligands in the solid state at room temperature. The main emission peaks of K₄pdtc, H₄pztc and bpe are at 489 nm (λ_ex = 300 nm), 480 nm (λ_ex = 300 nm), 392 nm (λ_ex = 280 nm), respectively. These emission can be attributed to the π* → n or π* → π transitions.^36–38 1–4 exhibit broad fluorescence emission bonds at 437 nm, 470 nm, 451 nm, 499 nm upon excitation at 320 nm, respectively (Fig. 8a and b), which are blue/red shifted relative to the free K₄pdtc, H₄pztc and bpe ligands. The emissions of 1–4 can be tentatively assigned to the charge transfer of ligands to Zn(II) centers (LMCT),^39,40 because, from the report, the highest occupied molecular orbital (HOMO) is presumably the π-bonding orbital from the pyridyl rings, and the lowest unoccupied molecular orbital (LUMO) is localized mostly on the metal centers.^41,42 We also investigated the luminescent properties of 4 after being heated 1 h at 180 °C and being soaked in acetone for 1 day, respectively. As the Fig. 9 shown, the emission shoulder peaks (507 nm, 510 nm) of 4 after being heated and soaked in acetone had slight red shifts comparing 4 (499 nm). These shifts can be attributed to the influence of the guest molecules. These investigations are very useful to study the potential photoactive materials.

Fig. 8 (a) The solid-state photoluminescent spectrums of 1, 2 at room temperature. (b) The solid-state photoluminescent spectrums of 3, 4 at room temperature.

Fig. 9 The solid-state photoluminescent spectrums of 4 and after being heated at 180 °C for 1 h and soaked in acetone for 1 day.

Conclusion

In summary, six new complexes were synthesized by modifying the reaction conditions. 1 and 2 were obtained by using the same reagent of different pH values, which resulted in two different structures (complex 1 shows 0D, 2 is 2D layer structure). And the analogous condition happened to complex 3 (0D) and 4 (3D porous pillar-layered structure). Based on the above information, the pH value is important in the formation of 1, 2, 3 and 4. We got the crystals of 5 at room temperature, while 6 was synthesized under hydrothermal conditions. It's worth mentioning that both 4 and 6 present 3D porous pillar-layered frameworks. Subsequent effort will be focused on the preparation and adsorption property of novel polymers by using the H₄pdtc/H₄pztc, bpe and more metal ions under different conditions.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (no. 21001078 and 21271137).

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Footnote

† Electronic supplementary information (ESI) available. CCDC (972278-1, 972279-2, 972280-3, 972281-4, 972282-5, 972283-6). For crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra00180j

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