Syntheses, structures, and magnetic properties of three supramolecular isomeric Cu(II) square grid networks: solvents effect on the ligand linkages

Gui-lei Liu *a, Jian-Biao Song b, Qi-ming Qiu b and Hui Li *b
aNational Research Center for Geoanalysis, Beijing 100037, P. R. China. E-mail: lliuguilei2008@163.com; Fax: +86 10 68999561
bKey Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: lihui@bit.edu.cn; Fax: +86 10 68912667

Received 9th December 2019 , Accepted 16th January 2020

First published on 20th January 2020


Reactions of deprotonated (E)-3-(quinolin-4-yl) acrylic acid (QCA) with Cu(NO3)2 in different solvents/templates gave three supramolecular isomers: [Cu(QCA)2]2·6H2O·(CH3)2CO (1), [Cu(QCA)2]·5H2O·CH3OH (2) and [Cu(QCA)2]·DMSO (3). Single-crystal X-ray diffraction analyses revealed that 1, 2 and 3 are all 2D square grid networks, but with different metal fragments and ligand conformations, based on the remarkable effect of solvents. Interestingly, a catenarian water pentamer and a water tape composed of cyclic water pentamers were observed in the crystal lattice of 1 and 2 respectively, which played a crucial role in not only the stabilization but also the template direction of the formation of the 3D stacking crystal host. Moreover, because of the different coordination modes and intramolecular interactions, 1, 2 and 3 showed different magnetic properties.


Introduction

Multifunctional coordination polymers (MCPs) refer to the same compound exhibiting two or more related performances, and their synergistic effects may lead to a new physical and chemical nature or purpose.1 Very recently, the design and construction of MCPs have witnessed explosive growth and are expected to bring important breakthroughs in various technological fields because of their potential applications in different areas of solid-state functional materials.2 However, if the synthesized compound does not have multifunctional properties for a given set of components, it is important to design and synthesize other compounds with different structures and properties by changing the reaction conditions.3 Supramolecular isomerism,4 as reviewed by Moulton and Zaworotko,5 is used to describe the situation where the network structures are different but the whole crystals or the coordination networks have the same chemical composition from the same reactants. Supramolecular isomers usually exhibit different properties due to the nuances of their structure.6 However, rationally realizing the structure–property relationships of supramolecular isomerism systems is still tremendously difficult and generally depends on the subtle variation of assembly environments (such as solvent, temperature, pH value and reactant stoichiometry, etc.).7 The effects of solvents have been proven in the regulation of conformers and topology.8 On the other hand, the control of the supramolecular isomers accompanying the changes in the coordination modes of one or more components by the additive agent is still rare.9

Multifunctional ligands bearing both pyridine and carboxyl groups can be used for the rational construction of coordination networks with different structures and properties because of their diverse binding modes. The uses of these kinds of ligands, such as nicotinate/isonicotinate,10 pyridine acrylic acid,11 bipyridine–carboxylate/dicarboxylate12 and quinoline-2(4,6)-carboxylic acid,13 to obtain intriguing network architectures have been successfully documented. In our previous work, we designed and constructed three novel Cd(II) supramolecular systems with the multifunctional (E)-3-(quinolin-4-yl) acrylic acid (QCA) ligand along with the regulation of auxiliary 4,4′-bipyridine (4,4′-bipy) ligand.14 Herein, as an extension of our research, we report the supramolecular isomerism observed in the coordination assembly of QCA with copper nitrate. The reaction of QCA with Cu(NO3)2·3H2O under room temperature conditions, or conditions of slight heating in the presence of different solvents as template agents, yielded three novel supramolecular isomers [Cu(QCA)2]2·6H2O·(CH3)2CO (1), [Cu(QCA)2]·5H2O·CH3OH (2) and [Cu(QCA)2]·DMSO (3) (Scheme 1).


image file: c9ce01940e-s1.tif
Scheme 1 Synthetic procedures for obtaining three supramolecular isomers.

Compound 1 is a 2D layered coordination polymer with a paddle-wheel dinuclear Cu(II) secondary building unit. Compounds 2 and 3 also display 2D square grid networks but with mononuclear Cu(II) as junction points. They are all further connected by π–π stacking interactions or intermolecular hydrogen bonds to form 3D ordered supramolecular architectures. Interestingly, a catenarian water pentamer and a water tape composed of cyclic water pentamers were observed in the crystal lattice of 1 and 2, respectively. The different magnetic properties of the three compounds have also been determined and are discussed in detail.

Experimental

Materials and physical measurements

All the reagents and solvents were purchased from commercial sources and used without further purification. Quinoline-4-carbaldehyde was purchased from Alfa Aesar, China (Tianjin). The ligand QCA ((E)-3-(quinolin-4-yl) acrylic acid) was synthesized according to previously reported acrylic acid synthetic procedures.14,15 Organic solvents with analytical purity were supplied by commercial sources and used as received. Elemental analyses (C, H, and N) were performed on a Flash EA1112 microanalyzer at the Beijing Institute of Technology. FT-IR spectra were recorded in the Nicolet-360 FT-IR spectrometer as KBr pellets in the 4000–400 cm−1 region. Thermogravimetric analyses (TGA) were carried out on a SEIKO TG/DTA 6200 thermal analyzer from room temperature to 1000 °C at a ramp rate of 10 °C min−1 in a 150 mL min−1 flowing nitrogen atmosphere. X-ray powder diffraction (PXRD) of samples was conducted using a Japan Rigaku D/max γ A X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 0.154060 nm). Variable-temperature magnetic susceptibility was determined on a Quantum Design MPMS-XL-7 (SQUID) magnetometer in the range 2–300 K. Diamagnetic corrections were estimated from Pascal's constants for all constituent atoms.

Synthesis of [Cu(QCA)2]2·6H2O·(CH3)2CO (1)

A mixture of QCA (29.7 mg, 0.15 mmol) and triethylamine (15.1 mg, 0.15 mmol) was dissolved in acetone solution (5 mL) to bring the pH of the solution to about 7 with constant stirring for thirty minutes. The dimethylformamide (DMF) solution of Cu(NO3)2·3H2O (18.1 mg, 0.075 mmol, 5 mL) was added to the aforementioned mixture solution while stirring and then a large amount of blue precipitate was formed. Water (1.5 mL) was added to the aforementioned mixture solution with slight heating (50–80 °C) to clarify the suspension. Dark green, cuboid single crystals suitable for X-ray structure analysis were obtained at room temperature after one week by the slow evaporation of the filtrate with the yield of 78% based on Cu(NO3)2·3H2O. Anal. calcd for C51H50Cu2N4O15: C, 56.46; H, 4.64; N, 5.17%. Found: C, 55.83; H, 4.75; N, 5.03%. IR (KBr pellet, cm−1): 3444(s), 1649(vs), 1602(s), 1586(s); 1512(m), 1403(vs), 1256(w), 1098(w), 974(w), 886(w), 862(w), 761(m), 732(w), 706(w), 614(m), 512(w).

Synthesis of [Cu(QCA)2]·5H2O·CH3OH (2)

A mixture of QCA (29.7 mg, 0.15 mmol) and triethylamine (15.1 mg, 0.15 mmol) was dissolved in methanol solution (CH3OH, 5 mL) to bring the pH of the solution to about 7 with constant stirring for thirty minutes. The DMF solution of Cu(NO3)2·3H2O (18.1 mg, 0.075 mmol, 5 mL) was added to the aforementioned mixture solution while stirring and then a large amount of blue precipitate was formed. Water (5 mL) was added to the aforementioned mixture solution with slight heating to dissolve the precipitate. Purple, pillar-shaped single crystals suitable for X-ray structure analysis were obtained at room temperature after five days by the slow evaporation of the filtrate with a yield of 83% based on Cu(NO3)2·3H2O. Anal. calcd for C25H30CuN2O10: C, 51.55; H, 5.19; N, 4.81%. Found: C, 51.46; H, 5.25; N, 4.69%. IR (KBr pellet, cm−1): 3411(m), 1667(vs), 1514(s), 1466(w); 1399(vs), 1376(vs), 1303(w), 1277(m), 1255(m), 1164(w), 1099(w), 972(m), 908(w), 852(w), 771(m), 710(m), 665(w), 623(w), 426(w).

Synthesis of [Cu(QCA)2]·DMSO (3)

Compound 3 was obtained by following a similar procedure to that of 2, while the blue precipitate or compound 2 was filtered and dissolved in dimethyl sulphoxide (DMSO, 5 mL) and DMF (2 mL). The solution was kept under ambient conditions for six days and then brown, prism-shaped crystals of compound 3 suitable for X-ray structure analysis were obtained as a single phase. Yield: 57% based on Cu(NO3)2·3H2O. Anal. calcd for C26H22CuN2O5S: C, 57.99; H, 4.12; N, 5.20%. Found: C, 58.16; H, 4.25; N, 5.07%. IR (KBr pellet, cm−1): 3446(w), 3073(w), 1682(s), 1636(w), 1616(w), 1586(vs), 1560(s), 1513(s), 1395(vs), 1377(vs), 1307(m), 1240(w), 1083(w), 989(m), 910(w), 862(m), 763(m), 729(m), 709(m), 617(m), 576(w), 532(w), 453(w), 419(m).

Crystal data collection and refinement

Suitable single crystals with dimensions of 0.47 × 0.22 × 0.18 mm, 0.65 × 0.16 × 0.14 mm, and 0.18 × 0.12 × 0.09 mm for compounds 1, 2 and 3 were selected for single-crystal X-ray diffraction analysis, respectively. Data were collected on a Bruker APEX-II CCD diffractometer equipped with a graphite-monochromatic Mo Kα radiation (λ = 0.71073 Å) using the ω scan mode for compound 1 at 296(2) K and 2, 3 at 153(2) K. Unit-cell parameters were determined from the automatic centering of 25 reflections and refined by the least-squares method. All non-hydrogen atoms were located by direct methods and subsequent difference Fourier syntheses. The hydrogen atoms bound to carbon were located by geometrical calculations, and their positions and thermal parameters were fixed during structure refinement. All non-hydrogen atoms were refined by full-matrix least-squares techniques with I > 2σ (I). All refinements were performed using SHELXTL 97.16 The restraint command “ISOR” was applied to the disordered C25, O6, O7, O8 and O9 atoms for compound 1, O3, O4, O5, O6, O7 and C13 atoms for compound 2, and C13 atom for compound 3 to obtain reasonable thermal parameters. Hydrogen atoms could not be located and were not calculated for the disordered methanol in compound 2. The relevant crystallographic data of compounds 1, 2 and 3 are presented in Table 1, while the selected bond lengths and angles are given in the ESI Tables S1–S3.
Table 1 Crystallographic data for compounds 1, 2 and 3
Compounds 1 2 3
a R 1 = Σ||F0| − |FC||/Σ|F0|. b wR2 = Σ[w(F02FC2)2]/Σ[w(F02)2]1/2.
Formula C51H50Cu2N4O15 C25H30CuN2O10 C26H22CuN2O5S
M/mol−1 1086.03 578.02 538.06
T/K 296(2) 153(2) 153(2)
Crystal system Orthorhombic Monoclinic Monoclinic
Space group Pccn C2/c C2/c
a 16.1109(5) 15.773(3) 13.224(3)
b 19.4781(6) 15.649(3) 14.236(3)
c 15.7553(5) 12.824(3) 12.886(3)
α 90 90 90
β 90 109.34(3) 103.52(3)
γ 90 90 90
V3 4944.2(3) 2987.0(10) 2358.6(8)
Z 4 4 4
ρ(calculated)/g·cm−3 1.459 1.285 1.515
μ(Mo Kα)/mm−1 0.934 0.783 1.056
F(000) 2248.0 1196.0 1108.0
2θ range/° 2.53–25.21 1.89–29.13 2.45–29.09
Reflns. collected 55[thin space (1/6-em)]293 11[thin space (1/6-em)]730 7685
Data/restraints/parameters 4433/34/332 2633/50/210 2069/8/162
R 1 [I > 2σ(I)] 0.0506 0.0654 0.0642
wR2b [I > 2σ(I)] 0.1461 0.1944 0.1673
R 1 (all data) 0.0738 0.0741 0.0804
wR2 (all data) 0.1732 0.2002 0.1776
GoF on F2 1.071 1.072 1.082
Δρmax,min/e Å−3 1.065 and −0.772 0.774 and −0.682 1.528 and −1.583


Results and discussion

Syntheses and general characterization

The reaction of QCA with Cu(NO3)2·3H2O led to the formation of three supramolecular isomeric compounds 1, 2 and 3 under ambient or slight heating conditions, while the solvents that were used during the synthesis of compounds were different (Scheme 1). Triethylamine was used to neutralize the acid. It is noteworthy that the solvent is very important for the coordination modes of Cu(II) ions, the QCA ligand and the growth of compounds 1, 2 and 3. The mixture solvents of acetone, DMF and H2O were used in the synthesis of compound 1, while CH3OH, DMF and H2O were used to give compound 2. Solvent-induced supramolecular isomerism occurred and 3 was obtained when compound 2 was filtered and dissolved in DMSO and DMF. All the carboxyl groups in 1–3 were found to be deprotonated and coordinated as supported by the IR spectroscopy data (Fig. S10, ESI) and the results of crystallographic analysis (see below). There are four different coordination modes of ligand QCA (Scheme 2), which exert important influences on the crystalline architectures as described in the following sections. IR spectra show that there are quinoline nitrogen atoms bonded to Cu(II) in all compounds (bands at about 1403 and 614 cm−1). Moreover, these IR spectra reveal the slightly different coordinative behavior of the carboxyl group in all the compounds; the red-shift of ν(C[double bond, length as m-dash]O) compared to the free ligand is 51 cm−1 for compound 1, whereas it is 33 and 18 cm−1 for compounds 2 and 3, respectively.17
image file: c9ce01940e-s2.tif
Scheme 2 Coordination modes of the QCA ligand in the compounds 1 (a, b), 2 (c) and 3 (d).

These IR results are coincident with the crystallographic structural analyses.

Crystal structure of [Cu(QCA)2]2·6H2O·(CH3)2CO (1)

The green crystals of compound 1 crystallized in the orthorhombic space group of Pccn with an asymmetric unit that contained one Cu(II) ion, two deprotonated QCA ligands, two dissociative water molecules (O5, O6), two water molecules (O7, O8) and one acetone molecule assigned to half-occupancy. As shown in Fig. 1a, each Cu(II) center is in a NO4 distorted square pyramidal coordination geometry with one nitrogen atom from one bridged QCA ligand in the axis position (Cu–N bond length of Cu(1)–N(1) is 2.227 (3)Å) and four oxygen atoms from the other four QCA ligands in the equatorial positions (Cu–O bond lengths of Cu(1)–O(1), Cu(1)–O(2), Cu(1)–O(3), Cu(1)–O(4) are 1.972(4), 1.963(3), 1.975(3), and 1.964(4) Å respectively). The distortion of the CuNO4 square-pyramid was indicated by the calculated value of the τ5 factor, which was 0.004 for Cu1 (for ideal square-pyramidal geometry, τ5 = 0).18 The carboxylato oxygen in ligand QCA has only one coordination mode: the bis-monodentate (η2-O) bridging conformation in the synsyn fashion (C(1)–O(1)–Cu(1): 124.9°(3); C(1)#1–O(4)–Cu(1): 124.3°(3); C(13)–O(2)–Cu(1): 123.7°(3); C(13)#1–O(3)–Cu(1): 124.0°(3); #1: −x + 2, −y + 1, −z + 2). Four ligands were connected to two Cu atoms to form a paddle-wheel dinuclear secondary building unit (SBU) with the distance between the two Cu(II) ions of 2.6980(9) Å. It is noteworthy that the QCA ligand exhibited two coordination modes to metal ions (Scheme 1a and b); the nitrogen atoms of the opposing two of the four ligands which were connected in the formation of the SBUs were also coordinated to Cu(II) atoms, so the adjacent SBUs were further bridged by the bis-monodentate bridging QCA ligand (Scheme 1b) to result in an infinite 2D layer framework (Fig. 1b and S1, ESI). The distance between the two SBUs was 12.5262(3) Å; therefore, the 2D layer had grid topology with a side measurement of 12.5262 × 12.5262 Å2 based on the SBU distances (Fig. S3, ESI).
image file: c9ce01940e-f1.tif
Fig. 1 Views of 1: (a) ORTEP representation showing the local coordination environment around the Cu(II) center with 30% thermal ellipsoid probability. Hydrogen atoms have been omitted for clarity. (b) The 2D layer polyhedron structure viewed down the a-axis.

The adjacent 2D layers were further assembled into a 3D ordered supramolecular structure in the form of ABAB through two different kinds of π–π stacking interactions (Fig. 2a and S2, ESI). The quinoline rings of the QCA ligands from the alternate layers are parallel to each other with the distance of 3.7155(1) Å within the square grids of the middle layer, and they are also parallel with the quinoline rings of the bridging QCA ligands of the middle layer with the short centroid–centroid contacts of 3.6014(1) Å (Cg⋯Cg < 3.7 Å) and vertical displacements of 1.4381(1) Å (d[a] < 1.7 Å), which indicate strong π–π stacking interactions among the 2D layers.19


image file: c9ce01940e-f2.tif
Fig. 2 Views of 1: (a) the two different kinds of π–π stacking interactions; (b) H-bonding between the 2D layers; (c) picture of the catenarian water pentamer (a: −x + 1/2, −y + 3/2, z).

On the other hand, the 1D channel, resembling a drum in shape, can be viewed along the c axis (Fig. 3b and S2, ESI). However, the 1D channel is filled with dissociative water and acetone molecules. The solvent water molecules form H-bonds with each other and with the quinoline nitrogen atoms of the no-bridging QCA ligands (the detail H-bonding data can be seen in Table. S4, ESI). As a result, a catenarian water pentamer N2⋯O6⋯O5⋯O7⋯O5A⋯ O6A⋯N2A was observed within the 1D channel (Fig. 2b and 3a). Clearly, the formation of such a catenarian water pentamer plays a crucial role not only in stabilization but also template direction of the formation of the 3D stacking crystal host of 1.20


image file: c9ce01940e-f3.tif
Fig. 3 (a) 3D stacking picture of 1 assembled from H-bonding interactions. (b) The space-filling view of 1 showing 1D channels resembling a drum as viewed along the c axis.

Crystal structure of [Cu(QCA)2]·5H2O·CH3OH (2) and [Cu(QCA)2]·DMSO (3)

Single crystal X-ray crystallographic analysis revealed that 2 and 3 are supramolecular isomers with 1, having the same framework [Cu(QCA)2]n but different crystal structures and solvent molecules; the purple crystals of 2 and brown crystals of 3 all crystallized in the monoclinic space group C2/c. The asymmetric units of 2 and 3 all contained half of the Cu(II) ions and one deprotonated QCA ligand, but with different solvent molecules. One dissociative water molecule (O3), one disordered methanol molecule and three water molecules (O4, O5, O6) were assigned half-occupancy in 2, but there was only one DMSO molecule with half occupancy in 3. The two coordination positions of the metal center in 2 and 3 are all satisfied by two quinoline nitrogen atoms (N1, N1#4) and four carboxylate oxygen atoms (O1, O2, O1#4, O2#4) from four equivalent deprotonated QCA ligands (Fig. 4a and 6a). The two coordinated nitrogen atoms were neighboured by N(1)–Cu(1)–N(1)#4 of 91.10°(2) in 2, but in the opposite axis direction by N(1)–Cu(1)–N(1)#4 of 180.000(1)° in 3. The carboxylato oxygen in ligand QCA has only one coordination mode, the bidentate chelate conformation, but it is not completely isomorphous for 2 (Scheme 2c) and 3 (Scheme 2d). It is noteworthy that the nitrogen atoms of the QCA ligands are all coordinated to the Cu(II) atoms in 2 and 3, so the adjacent Cu(II) atoms are linked by the bidentate chelate bridging QCA ligands to form the 2D networks (Fig. S4 and S8, ESI), and the coordination geometry of the metal centers in 2 and 3 are distorted octahedra (Fig. 4b and 6b). The selected bond distances and angles of 2 and 3 can be seen in Table S2 and S3, respectively. Although the distance of Cu(1)–O(2) (2.645(4) Å for 2, 2.515(4) Å for 3) is significantly longer than the average Cu–O bond length, it can be regarded as coordination bonding since it is less than the sum of the van der Waals radii for copper(II) and oxygen (2.92 Å).21 The transoid bond angles are 178.55(14)° for 2 and 180.000(1)° for 3, while the ranges of the cisoid bond angles in 2 and 3 are 54.80(14)–126.22(14)° and 57.59(13)–127.4(3)° respectively, indicating that the bridging moieties are significantly twisted.
image file: c9ce01940e-f4.tif
Fig. 4 Views of 2: (a) ORTEP representation showing the local coordination environment around the Cu(II) center with 30% thermal ellipsoid probability. Hydrogen atoms have been omitted for clarity (#4: −x, y, −z + 3/2). (b) The 2D layer polyhedron structure picture.

In the sheets of 2 and 3, the distance between the adjacent Cu(II) ions are 11.1094(16) and 10.7693(25) Å, respectively, and all Cu(II) ions are 4-connected, with the shortest circuit being a four-membered ring. These 2D sheets can be simplified to a (4,4) net with the window size of 11.1094 × 11.1094 and 10.7693 × 10.7693 Å2 for 2 and 3, respectively. The adjacent 2D layers are further assembled into a 3D ordered supramolecular structure in the form of ABAB through the strong π–π stacking interactions (Fig. 7b, S5, S7 and S9, ESI), and the quinoline rings of the QCA ligands from the adjacent 2D layers are parallel to each other with the distance of 3.6089(11) and 3.6637(7) Å for 2 and 3, respectively (Fig. 5a and 7a). In addition, the solvent water molecules formed H-bonds with each other and are interconnected to form a tape-like structure containing fused cyclic water pentamers because of the disordered hydrogen atoms of O4 in 2 (Fig. 5b). The O⋯O distances (2.624–2.947 Å) in the hybrid pentamer are comparable to the reported pentamer (2.706–2.918 Å).22 The remaining hydrogen atoms of O3 form two acceptor hydrogen bonds with O1 and O2 of different QCA ligands, giving a bond distance of 2.738 and 2.616 Å, respectively (the detailed H-bonding data can be seen in Table. S5, ESI). Obviously, like the catenarian water pentamers in 1, the formation of such a water tape and the presence of interactions between the water O3 with different QCA ligands also play an important templating role in the formation of the 3D stacking crystal host of 2 (Fig. 5c and S6, ESI).


image file: c9ce01940e-f5.tif
Fig. 5 (a) View of the π–π stacking interactions in 2. (b) View of the circular water pentamer H-bonding chains in 2. (c) 3D supramolecular structure of 2 assembled forms of hydrogen bond interactions.

image file: c9ce01940e-f6.tif
Fig. 6 Views of 3: (a) ORTEP representation showing the local coordination environment around the Cu(II) center with 30% thermal ellipsoid probability. Hydrogen atoms have been omitted for clarity (#4: −x + 2, −y, −z + 2). (b) The 2D layer polyhedron structure picture viewed down the a-axis.

image file: c9ce01940e-f7.tif
Fig. 7 (a) View of the π–π stacking interactions in 3, DMSO molecules are shown in the space-filling mode. (b) Schematic representation of the ABAB stacking structure of 3, in which the balls represent the mononuclear Cu(II) centers.

image file: c9ce01940e-f8.tif
Fig. 8 Temperature-dependence of χMT and χM of 1 at H = 1 kOe from 2 to 300 K. The red line represents the best fit to the Bleaney–Bowers model.

image file: c9ce01940e-f9.tif
Fig. 9 Temperature-dependence of χMT, χM and χM−1 (middle) of 3 at H = 1 kOe from 2 to 300 K. The red line represents the best fit to the Curie–Weiss law.

Thermal stability and powder X-ray diffraction

To examine the architectural and thermal stability of compounds 1–3, thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD) studies were conducted on polycrystalline samples of compounds 1, 2 and 3, and the TG curves are shown in Fig. S11, ESI. The TGA curve of 1 displayed a first weight loss of 5.52% (calcd: 5.34%) at 30–76 °C, corresponding to the loss of one free acetone molecule per unit, and a second weight loss of 5.01% (calcd: 4.97%) at 105–140 °C, corresponding to the complete loss of the lattice water molecules. Its framework was stable up to 230 °C, and then the framework began to collapse, accompanied by the decomposition of the coordinated QCA ligands. Compound 2 showed approximately 21.03% (calcd: 20.96%) weight loss from about 30 to 145 °C, which was attributed to the release of one methanol and five water molecules per formula unit. This was followed by a sharp weight loss at 230–430 °C due to the decomposition of the coordinated QCA ligands. Compound 3 showed a first weight loss of 15.88% (calcd: 14.50%) at 30–145 °C, corresponding to the complete loss of the DMSO molecules. The desolvated framework was stable to 230 °C, and then the framework began to collapse, accompanied by the release of the QCA ligands.

The powder X-ray diffraction (PXRD) pattern of crystallized compounds 1–3 are all coincident with the simulated pattern derived from the X-ray single-crystal data, confirming that the phase purities of the bulk samples were the same as the single crystal (Fig. S12, ESI). In addition, it is clear that the diffraction profiles of compounds 1, 2 and 3 are not the same, indicating that their structures are different.

Magnetic properties

Description of magnetic properties of compound 1. The magnetic behaviors of 1 in the form of χMT vs. T, χMvs. T and χM−1vs. T plots (χM being the molar magnetic susceptibility per two Cu(II) ions) are shown in Fig. 8 and S13, ESI. Under a 1 kOe field, the χMT value of 0.48 cm3·K·mol−1 at 300 K was much lower than the theoretical value of two spin-only Cu(II) ions (0.75 cm3·K·mol−1, S = 1/2, g = 2.0). Upon cooling, the χMT values quickly decreased in the temperature range of 300–50 K to a minimum (0.045 cm3·K·mol−1) at 50 K, and then gradually decreased to 0.027 cm3 mol−1 K at 2.0 K. This behavior indicated the occurrence of strong antiferromagnetism coupling in the temperature range of 300–50 K and weak antiferromagnetism coupling below 50 K in 1. The magnetic susceptibility data above 50 K were best fit to the modified Bleaney–Bowers equation23 for S = 1/2 dimers under a −2JS1S2 spin Hamiltonian:
image file: c9ce01940e-t1.tif
where ρ is the percentage of monomeric impurity; other symbols have their usual meanings. The best-fitting parameters obtained from this simulation are g = 2.21, J = −166.34 cm−1, ρ = 5.06% with the agreement factor R = ∑[(χM)obsd − (χM)calcd]2/∑[(χM)obsd]2 = 2.5 × 10−6. The large negative values of J indicate the existence of strong antiferromagnetic couplings between the Cu(II) ions of the dinuclear structural units.

It is well-known that the magnitude of the antiferromagnetic interaction in these dinuclear copper(II)-carboxylate compounds mainly depends on the bending of the Cu–O–C–O–Cu bridge (φbend, the dihedral angle between the Cu–O–O–Cu and the carboxyl moiety), and the larger the φbend, the greater the decrease in the −2J value. In the case of 1, the φbend is 3.1(2)°, which is smaller than those of [Cu(2-Br-C6H4COO)2(H2O)]2(φbend = 7.3(2)°),24a [Cu2L2(DMF)2]·6H2O (H2L = 3,3′-dimethoxy-4,4′-biphenyldicarboxylic acid) (φbend = 8.8(2)°)24b and [Cu(DMB)2(H2O)2] (HDMB = 2,6-dimethoxybenzoic acid) (φbend = 12.0(2)°),24c and larger than those of reported similar dinuclear copper(II)-carboxylate compounds (φbend = 1.66(2)°, 1.56(2)°, 0.86(2)°).24d,e Thus the −2J value (333 cm−1) of 1 is greater than the above-mentioned compounds with low φbend, mainly because the magnetic data fitting was performed over different temperature ranges in 1 (300–50 K). The magnetic susceptibility data of 1 between 50–2 K obey the Curie–Weiss law,24f with a Curie constant C = 0.046 cm3·K·mol−1 and a Weiss constant θ = −2.62 K. The small negative θ suggests that weak antiferromagnetic coupling exists in 1, and this may be attributed to the magnetic exchange interactions between the dinuclear Cu2 structural units.

Description of the magnetic properties of compounds 2 and 3. The variation of the reciprocal of χMT, and χMversus T for compound 2 is presented in Fig. S14, ESI (where χM is the molar magnetic susceptibility per mononuclear Cu(II) ion). In the temperature range of 2.0–300 K, the χMT value of compound 2 remains almost constant between 0.42 and 0.43 cm3·K·mol−1, which is little higher than the theoretical value of 0.375 cm3·K·mol−1 based on the mononuclear Cu(II) unit (S = 1/2 and assuming g = 2.0) without interactions between the Cu(II) ions. Such a result indicates that although the Cu(II) ions are bridged by QCA anions, there is no obvious magnetic interaction between the Cu(II) ions in 2.

The dc magnetic susceptibility measured on a polycrystalline sample of 3 (applied field of 1 kOe) is shown in Fig. 9 as χMT vs. T, χMvs. T and χM−1vs. T plots (where χM is the molar magnetic susceptibility per mononuclear Cu(II) ion). At room temperature (300 K), the χMT value was 0.42 cm3·K·mol−1, which is a little higher than expected for an uncoupled Cu(II) ion (0.375 cm3·K·mol−1, S = 1/2, g = 2.0). The χMT value gradually decreased to 0.41 cm3·K·mol−1 upon lowering the temperature to about 25 K. Below this temperature the χMT value decreased quickly to reach a minimum value of 0.29 cm3·K·mol−1 at 2 K. The χM−1vs. T plots (300–2 K) obey the Curie–Weiss law with a Curie constant C = 0.42 cm3·K·mol−1 and a Weiss constant θ = −0.64 K, which along with the nature of the χMT vs. T plot indicate a dominant antiferromagnetic interaction among the Cu(II) ions.

Conclusions

Three supramolecular isomeric square grid networks, 1, 2, and 3, were prepared by reacting the (E)-3-(quinolin-4-yl) acrylic acid ligand with Cu(NO3)2 in different solvents/templates. Our analysis based on the structural data suggests that the additive agent exerts a profound influence on the coordination environment of Cu(II) and the linking modes of the QCA ligand, resulting in the isomerisation of the square grid networks with different metal fragments. In the crystal structures of 1 and 2, a catenarian water pentamer and a water tape composed of cyclic water pentamers were observed, respectively, which play a crucial role not only in the stabilization but also in the template direction of the formation of the 3D stacking crystal host. More importantly, when compound 2 was filtered and dissolved in DMSO and DMF, the structural transformation from 2 to 3 was successfully achieved, which again demonstrated the solvent effect on the ligand linkages. Moreover, significant differences in magnetic coupling interactions among the three compounds contributed to a deep understanding of the relationships between the structures and properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21601041 and 21271026).

Notes and references

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Footnote

Electronic supplementary information (ESI) available. CCDC 1577113–1577115. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9ce01940e

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