Copper Carboxylate-phosphonates: Syntheses,Crystal Structures and Magnetic Properties①

XU Yan

(Department of Materials Science and Engineering, Suqian University, Suqian223800, China)

ABSTRACT Two novel copper carboxyly-phosphonates, namely, Cu2.5(5-pnc)(SO4)0.5(OH)(H2O)0.5 (1) and Cu0.5(5-pncH2)(H2O)1.5 (2) (5-pncH3 = 5-phosphono-1-naphthalenecarboxylic acid), have been synthesized and characterized by X-ray diffraction, infrared spectroscopy, elemental analysis, and thermogravimetric analysis. In compound 1, each {PO3C} tetrahedron is corner-shared with two {Cu(1)O4}, two {Cu(2)O5} and one {Cu(3)O5},thus forming a one-dimensional inorganic chain along the c axis containing 8-membered rings of [Cu3O4S] and 19-membered cages of [Cu5O10P4]. The inorganic chains are further connected by a 5-pnc3- ligand to generate a three-dimensional framework. Compound 2 exhibits a one-dimensional structure, in which the inorganic chains of[Cu-O-Cu]n are connected by the organic ligands through hydrogen bonding interactions, forming an infinite two-dimensional layer. Magnetic measurements of 1 indicate that dominant antiferromagnetic interactions are mediated between the Cu2+ centers.

Keywords: copper, carboxylate-phosphonate, crystal structure, magnetic properties;

1 INTRODUCTION

Metal phosphonate materials have been receiving an increasing amount of attention due to their significance in the fields of ion-exchange, catalysis, and gas adsorption, as well as having optical, magnetic and proton conducting properties[1-8]. Great efforts have been devoted to the syntheses of metal phosphonates with versatile architectures and interesting physical or chemical functions during the past two decades[9-11]. Among them, metal phosphonates containing carboxylate groups are of particular interest as they provide additional coordination sites for metal ions[12,13]. A series of metal phosphonate compounds have been reported based on 2/3/4-carboxyphenylphosphonic acid (2/3/4-cppH3),featuring 1D, 2D and 3D structures, together with interesting magnetic, dielectric and chiroptical properties[14-18]. The(4-carboxynaphthalen-1-yl)phosphonic acid (4-cnappH3) is analogues to 4-cppH3except with an expanded aromatic moiety. Some metal phosphonates based on this ligand have been described, including a cobalt compound with a 3D framework structure showing enantioenrichment[19]and three copper compounds with layered structures[20], as well as four manganese phosphonates with 3D framework or 2D layered structures[21]. However, the 5-phosphono-1-naphthalenecarboxylic acid (5-pncH3) is a positional isomeric ligand of 4-cnappH3. As far as we are aware, only one example of metal phosphonates based on this ligand has been reported[22].

To explore new aromatic carboxylate-phosphonate materials, herein we report two new copper phosphonates based on 5-pncH3, formulated as Cu2.5(5-pnc)(SO4)0.5(OH)(H2O)0.5(1) and Cu0.5(5-pncH2)(H2O)1.5(2). Compound 1 shows a 3D framework structure, whereas 2 exhibits a 1D chain structure.The magnetic properties of compounds 1 and 2 are also investigated.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All reagents and solvents employed in this work were commercially available and used without further purification.(5-Carboxynaphthalen-1-yl)phosphonic acid (5-pncH3) was synthesized following a previous procedure[19]. Elemental analyses (C and H) were performed on a Perkin-Elmer 240C elemental analyzer. IR spectra were recorded on a Bruker Tensor 27 spectrometer in the range of 400～4000 cm-1using KBr pellets. Thermal analyses were performed using a Mettler Toledo TGA/DSC thermoanalyzer in a temperature range of 50～800 °C in N2flow (20 mL/min) at a heating rate of 10 °C/min. Powder X-ray diffraction (XRD) data were collected on a Bruker D8 ADVANCE X-ray powder diffractometer with CuKαradiation (λ= 1.54056 Å) in a range of 5.00～50.00° at room temperature. The magnetization data were recorded on a Quantum Design MPMSXL7 SQUID magnetometer and a vibrating sample magnetometer (VSM) of the Quantum Design MPMS SQUID VSM system. The diamagnetic contribution of the sample itself was estimated from Pascal’s constant[23].

2. 2 Synthesis of Cu2.5(5-pnc)(SO4)0.5(OH)(H2O)0.5 (1)

A mixture of Cu(SO4)2·5H2O (0.2 mmol, 0.0512 g),5-pncH3(0.1mmol, 0.0256 g) and 10 mL of a mixed solution of H2O/methanol (1:1 in volume) was kept in a Teflon-lined autoclave at 140 °C for 2 days. After cooling the autoclave to room temperature, light green block-like crystals were obtained as a monophasic material, judged by powder X-ray diffraction pattern. Yield: 43.9% based on 5-pncH3.Elemental analysis (%) calcd. for C11H9O9PS0.5Cu2.5: C,26.90; H, 1.85. Found: C, 26.53; H, 1.76. IR (KBr, cm-1):3554m, 3125br, 1596w, 1541s, 1402m, 1381m, 1327w,1205w, 1089s, 1041s, 956s, 801m, 771m, 591m, 543w, 498w,466w.

2. 3 Synthesis of Cu0.5(5-pncH2)(H2O)1.5 (2)

The Cu(ClO4)2·6H2O (0.2 mmol, 0.0742 g), 5-pncH3(0.1mmol, 0.0254 g) and 10 mL water were stirred for 2 h.Then two drops of 0.5 M NaOH were added into the solution.The mixture was kept in a Teflon-lined autoclave at 140 °C for 2 days. After cooling the autoclave to room temperature,light green rod-like crystals were obtained as a monophasic material, judged by powder X-ray diffraction pattern. Yield:10.3% based on 5-pncH3. Elemental analysis (%) calcd. for C11H11O6.5PCu0.5: C, 42.62; H, 3.55. Found: C, 42.31; H,3.51. IR (KBr, cm-1): 2985br, 2645m, 1681s, 1511m, 1427m,1334w, 1295m, 1132s, 1065s, 1028s, 917m, 802s, 594m,534w, 502w, 464w.

2. 4 X-ray data collection and structure determination

Single crystals of 1 (0.12mm × 0.12mm × 0.11mm) and 2(0.20mm × 0.20mm × 0.20mm) were used for indexing and intensity data collection on a Bruker APEX II CCD diffractometer using graphite-monochromated MoKαradiation (λ= 0.71073 Å) at 293 K. A hemisphere of data was collected in theθranges of 4.2730～70.9370° for 1 and 2.1070～73.5890° for 2. The data were integrated using the Siemens SAINT program[24], with the intensities corrected for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Empirical absorption and extinction corrections were applied. The structures were solved by direct methods and refined onF2by full-matrix least-squares using SHELXL[25]. All the non-hydrogen atoms were located from Fourier maps, and refined anisotropically. All H atoms were put in calculated positions using the riding mode, and refined isotropically with the isotropic vibration parameters related to the non-H atom to which they are bonded.Crystallographic data and selected bond lengths and bond angles for compounds 1 and 2 are listed in Tables 1 and 2,respectively.

Table 1. Crystallographic Data for Compounds 1 and 2

aR = Σ||Fo| – |Fc||/Σ|Fo|, wR = [Σw(Fo2 – Fc2)2/Σw(Fo2)2]1/2

Table 2. Selected Bond Lengths (Å) and Bond Angles (°)

3 RESULTS AND DISCUSSION

3. 1 Crystal structures of 1 and 2

Compounds 1 and 2 were synthesized by reactions of ligand 5-pncH3and different copper salts (Cu(SO4)2·5H2O for 1 and Cu(ClO4)2·6H2O for 2) at 140 °C under solvothermal for 1 and hydrothermal for 2 reaction conditions (Fig.1a). Although the carboxyly-phosphonate acid ligand and metal are the same, their structures are completely different. It is found that the anions of metal original play critical roles in self-assembly.

Fig.1. (a) Schematic synthesis of 1 and 2. Coordination modes of 5-pncH3 in compounds 1 (b) and 2 (c)

Single crystal structural determination reveals that 1 crystalizes in monoclinic system, space groupC2/c. Each asymmetric unit of compound 1 consists of 2.5 Cu atoms, 1 5-pnc3-ligand, 0.5 coordination SO42-, 1 coordination OH-,and 0.5 coordination H2O. The Cu(1) displays a distorted planar quadrilateral geometry. Four coordination sites are occupied by four phosphonate oxygen atoms (O(1), O(1A),O(3B), O(3C)) from four equivalent 5-pnc3-ligands(Symmetry codes: A: 2–x,y, 1.5–z; B: 2–x, 2–y, 2–z; C:x,2–y,–0.5+z). Cu(2) and Cu(3) are each five-coordinated with a distorted trigonal-bipyramidal geometry, surrounded by two phosphonate oxygen, one carboxylate oxygen, one hydroxyl oxygen, and one sulfate oxygen atoms for Cu(2)and one phosphonate oxygen, one carboxylate oxygen, two hydroxyl oxygen, and water molecule oxygen atoms for Cu(3) (Fig.2a). The Cu–O bond lengths (1.900(8)～2.197(10) Å) are comparable to those in the other copper phosphonate compounds[20].

The 5-pnc3-ligand is fully deprotonated. It serves as a hepta-dentate ligand connecting seven Cu atoms using its three phosphonate oxygen and two carboxylate oxygen donors (Fig.1b). Each SO42-behaves as a bidentate metal linker bridging two equivalent Cu atoms through two oxygen atoms. Each OH-actsμ3-O(H) bridging three Cu atoms.Each {PO3C} tetrahedron is corner-shared with two{Cu(1)O4}, two {Cu(2)O5} and one {Cu(3)O5}, thus forming a one-dimensional inorganic chain along thecaxis containing 8-membered rings of [Cu3O4S] and 19-membered cages of [Cu5O10P4] (Fig.2b). The inorganic chains are further connected by a 5-pnc3-ligand, generating a three-dimensional framework (Fig.2c).

Fig.2. (a) Building unit with labelling except carbon atoms in compound 1. (b) The inorganic chain of structure.(c) Packing diagram of structure 1 viewed along the caxis. The hydrogen atoms attached to carbon are omitted for clarity. Symmetry codes: A: –x+2, –y+2, –z+2; B: x, –y+2, z–1/2; C: –x+2, y, –z+3/2; D: –x+2, y, –z+5/2;E: x+1/2, y–1/2, z+1; F: x–1/2, y+1/2, z–1; G –x+3/2, y+1/2, z+3/2

Compound 2 crystallizes in orthorhombic space groupPbcm. The asymmetric unit contains 0.5 Cu2+ion, 1 5-pncH2-and 1.5 coordination water molecules (Fig.3b).The Cu2+ion has a distorted square-pyramidal coordination environment. The basal positions are occupied by two phosphonate oxygens from two equivalent 5-pncH2-ligands,and two water molecule oxygen atoms (Cu–O: 1.9802(16)～2.0012(17) Å). The apical position is filled with coordination water molecule oxygen atoms, presenting an elongated Cu–O(5A) bond length (2.245(3) Å), which is similar to those reported in other Cu2+phosphonates[20]. In addition,O(5B) from another coordination water molecule shows a weak contact with the Cu2+ion (Cu(1)–O(5B): 2.675(4) Å),thus forming an inorganic chain running along the b axis containing [Cu–O–Cu]n.

The 5-pncH2-ligand is singly protonated at O3 and O7. It serves as a monodentate ligand, coordinating one Cu2+ion through its one phosphonate oxygen atom (Fig.1c). The naphthalenecarboxyly groups of 5-pncH2-ligand are pendant on the two sides of the inorganic chain (Fig.3a). C–H···πstacking interaction (3.412 Å) is observed among the adjacent naphthalene rings of the 5-pncH2-ligand. The inorganic chains are further connected by organic ligands through hydrogen bond interactions (O(6)…O(7) = 2.7031 Å)from neighbouring carboxyly groups. Consequently, an infinite 2-D layer in thebcplane is constructed (Fig.3b).

Fig.3. (a) Building unit with labelling except carbon atoms in compound 2. (b) Hydrogen bond interactions and C–H···π stacking interaction for structure 2. All H atoms except those attached to the COO- are omitted for clarity. Symmetry codes: A: x, y,–z+1/2; B: –x+1, y–1/2, –z+1/2; C: –x+1, y+1/2, –z+1/2; D: x+1, –y+1/2, –z+1; E: –x+2, –y+1, –z+1; F: x+1, y, z

3. 3 FTIR and powder X-ray diffraction

The FTIR spectra of ligand 5-pncH3, compounds 1 and 2 are shown in Fig.4a. The bands appearing at 1695, 1266 cm-1and 1687, 1289 cm-1(for 5-pncH3and compound 2)correspond to the stretching vibration of uncoordinated COO,while the peaks of coordinated COO appear at 1542, 1412 cm-1. The enhanced intensity at 1136 and 620 cm-1is attributed to the ClO4-anion in compound 1. The vibration bands of phosphonate groups appear at 1098, 1058, 1020 and 948 cm-1for ligand 5-pncH3, 1094, 1044 and 948 cm-1for compound 1 and 1126, 1067, 1027 and 916 cm-1for compound 2. The results indicate that the coordination modes of carboxylate and phosphonate groups are different in compounds 1 and 2. The experimental and simulated powder X-ray diffraction (PXRD) patterns of 1 and 2 are shown in Fig.4b and 4c. The experimental PXRD patterns at room temperature are in good agreement with the simulated ones based on single-crystal X-ray solution, demonstrating the phase purity of the bulk products. The differences in reflection intensities are probably due to the preferred orientation effects.

Fig.4. FTIR spectra (a) and PXRD patterns for compounds 1 (b) and 2 (c)

3. 4 Thermal stability

To compare their thermal stabilities, thermogravimetric analyses were performed on fresh samples under a nitrogen gas flow. Compound 1 shows a three-step decomposition process, indicating its stability below 290 °C. The organic components decompose and the structure collapses above 290 °C. Compound 2 also experiences a three-step decomposition in the temperature range of 50～800 °C, as shown in Fig.5. The first step is observed below 140 °C with weight loss of 4.5%, and the second step in the 260～350 °C region to 4.3%. The total weight loss is 8.8%, which corresponds to the removal of 1.5 water molecules (calcd.8.72%). The third steps observed above 350 °C are attributed to the burning of organic components and the collapse of the framework structure.

Fig.5. Thermogravimetric analyses for compounds 1 and 2

3. 5 Magnetic properties

The magnetic properties of the two compounds were investigated using polycrystalline samples. Fig.6 shows χMT vs. T plots of compounds 1 and 2 measured under a dc field of 1000 Oe. The room temperature χMT value per Cu is 0.41 and 0.43 cm3·K·mol-1for compounds 1 and 2, respectively,close to that expected for non-interacting Cu2+ions with S =1/2 and g = 2.2 (0.45 cm3·K·mol-1). The χMT value decreases continuously upon cooling, suggesting that antiferromagnetic (AF) interaction is dominant between the magnetic centers. The presence of AF interaction is also confirmed by a larger negative Weiss constant (–148.5,–59.46 K for compounds 1 and 2, respectively), determined by the susceptibility data above 100 K.

Fig.6. Temperature dependence of χMT (□) and 1/χM (○) for compound 1

4 CONCLUSION

In conclusion, the syntheses, structures and magnetic properties of two new copper carboxylate phosphonates based on 5-pncH3ligand, namely, Cu2.5(5-pnc)(SO4)0.5(OH)(H2O)0.5(1) and Cu0.5(5-pncH2)(H2O)1.5(2), have been reported.Compound 1 features a three-dimensional framework structure with inorganic chains connected by the 5-pncHligand, while compound 2 displays a one-dimensional structure in which the inorganic chains are connected by organic ligands through hydrogen bond interactions, forming an infinite 2-D layer. Magnetic measurements reveal that dominant antiferromagnetic interactions are mediated between the Cu2+centers in compounds 1 and 2.