pH-Dependent Cu(II) Coordination Polymers with Tetrazole-1-acetic

DOI: 10.1021/cg070022y. Publication Date (Web): February 21, 2008 ... Romain Gautier and Rodolphe Clérac. Crystal Growth & Design 2017 17 (2), 864-86...
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pH-Dependent Cu(II) Coordination Polymers with Tetrazole-1-acetic Acid: Synthesis, Crystal Structures, EPR and Magnetic Properties Qing Yu,† Xiuqing Zhang,†,‡ Hedong Bian,† Hong Liang,*,† Bin Zhao,‡ Shiping Yan,*,‡ and Daizheng Liao‡

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1140–1146

College of Chemistry and Chemical Engineering, Guangxi Normal UniVersity, Guilin, 541004, P. R. China, and College of Chemistry, Nankai UniVersity, Tianjin, 300071, P. R. China ReceiVed January 8, 2007; ReVised Manuscript ReceiVed December 12, 2007

ABSTRACT: Four novel copper tetrazole-1-acetic acid (Htza) coordination polymers, namely, {[Cu(tza)2(Htza)2] · 2H2O}n (1), [Cu(tza)2]n (2), {[Cu4(tza)4(CH3COO)2(µ3-OH)2(H2O)2] · 2H2O}n (3), and {[Cu4(tza)6(µ3-OH)2] · 4H2O}n (4), were synthesized as pH-dependent products and characterized by elemental analysis, Fourier transform infrared (FT-IR) spectra, electron paramagnetic resonance (EPR), magnetic properties, and single-crystal X-ray diffraction studies. In 1, two neighboring copper ions are bridged by two carboxylato groups from two tza ligands in a syn-anti bridging mode, forming a one-dimensional Cu chain. 2 is a twodimensional (2D) wavelike layer based on {Cu(tza)}4 rings from head-to-tail arranged tza ions. 3 and 4 are 2D coordination polymers containing tetranuclear Cu4(µ3-OH)2 units. Introduction Metal-organic coordination polymers have recently attracted intense attention from chemists, due to their intriguing supramolecular compositions and versatile framework topologies as well as their potential applications as functional materials in molecular magnetism, catalysis, gas sorption, electrical conductivity, optics, etc.1–5 However, in many cases it is quite difficult to design and synthesize a desired structure in a truly deliberate manner because there are many factors that impact the final product structures, such as the coordination geometry of the central metal ion and the shape, solvents, metal–ligand ratio, pH value, and symmetry of the ligand.6–9 Over the past decade, a significant number of efforts have been contributed to the construction of metal-carboxylate complexes with one-, two-, or three-dimensional (1D, 2D, or 3D) framework structures.10–12 At present, there is a rational synthetic strategy widely used in this area that links metal ions with polydentate ligands functioned as connectors.2,13 Polydentate ligands can act as either bridging or chelating ligands, yielding desired networks in metal-organic coordination polymers.14,15 The pH-dependence of the coordination geometry of the transition metal complex in solution has been recently studied extensively in biomimetic inorganic chemistry.16–18 Fabbrizzi and co-workers studied the pH-controlled synthesis of the transition coordination polymers.16 There are some examples of studies to rearrange its coordination sphere by controlling different pH values.19 Encouraged by the previous results, we selected tetrazole-1acetic acid (Htza) to construct novel transition coordination polymers with fascinating structures. As is well-known, carboxylate O atoms and tetrazolyl ring N atoms have good coordination capacities.20 So, tetrazole-1-acetic acid (Htza), with both a carboxylate and tetrazolyl ring, is a multifunctional ligand. Htza has many coordination modes, and (I) and (II) in Scheme 1 have been reported.21 Herein, we report four * To whom correspondence should be addressed. E-mail: lianghongby@ yahoo.com.cn (H.L.); [email protected] (S.Y.). † Guangxi Normal University. ‡ Nankai University.

Scheme 1. Coordination Modes of Htza

coordination polymers prepared by pH-controlled conditions. Three new coordination modes of Htza have been found (Scheme 1). Experimental Section Materials and Apparatus. All starting chemicals were commercially available reagents of analytical grade and were used without further purification. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400II elemental analyzer. FT-IR spectra were recorded from KBr pellets in the range of 4000-450 cm-1 on a PE Spectrum One spectrometer. The EPR spectra were measured on a BRUKER EMX-6/1 EPR spectrometer. The variable-temperature magneticsusceptibility data were measured with a Quantum Design MPMS7 SQUID magnetometer. Diamagnetic corrections were made with Pascal’s constants.22 Synthesis of {[Cu(tza)2(Htza)2] · 2H2O}n (1). Htza (0.7680 g, 6 mmol) was dissolved in distilled water (15 mL). Then CuO (0.2388 g, 3 mmol) was added. The pH value was 1.5. The mixture was stirred at 80 °C for 3 h and then cooled and filtered. The filtrate was allowed to slowly concentrate by evaporation at room temperature. Three months later, dark blue block crystals were obtained. Yield: 76% on the basis of Htza. Anal. Calcd. for 1: C, 23.63; H, 2.97; N, 36.74. Found: C, 23.75; H, 3.12; N, 36.58. IR (KBr pellet) data (ν/cm-1): 3426.49 m br, 3155.96 m, 1751.22 m, 1641.48 s, 1500.00 w, 1416.13 w, 1375.56 m, 1254.88 m, 1099.52 m, 797.88 w, 704.82 w, 677.14 w. Synthesis of [Cu(tza)2]n (2). Htza (0.1280 g, 1.0 mmol) was dissolved in distilled water (5 mL), and a solution of CuCl2 · 2H2O (0.0852 g, 0.5 mmol) in H2O (5 mL) was added dropwise. The pH value was adjusted to 2.5 with triethylamine. The mixture was stirred at 80 °C for 2 h and then cooled and filtered. The filtrate was allowed to slowly concentrate by evaporation at room temperature. One month

10.1021/cg070022y CCC: $40.75  2008 American Chemical Society Published on Web 02/21/2008

Coordination Polymers with Tetrazole-1-acetic acid

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Table 1. Crystal and Structure Refinement Data for Complexes 1–4

empirical formula formula wt T (K) cryst syst space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z dcalc, g/cm3 abs coeff (mm-1) F(000) goodness of fit R1a [I > 2σ(I)] ωR2b (all data) a

1

2

3

4

C12H18CuN16O10 609.96 294(2) monoclinic C2/c 16.105(2) 4.8728(7) 29.455(4) 90 101.130(2) 90 2268.0(6) 4 1.786 1.053 1244 1.113 R1 ) 0.0347 ωR2 ) 0.0434

C6H6CuN8O4 317.73 293(2) monoclinic P21/c 4.7999(7) 11.7936(16) 9.4857(13) 90 103.665(2) 90 521.77(13) 2 2.022 2.123 318 1.057 R1 ) 0.0230 ωR2 ) 0.0673

C8H14Cu2NO9 493.35 294(2) triclinic P1j 8.5325(12) 9.4912(14) 11.0470(16) 106.534(2) 93.207(2) 107.294(2) 809.2(2) 2 2.025 2.697 496 1.029 R1 ) 0.0284 ωR2 ) 0.0756

C9H14Cu2N12O9 561.40 294(2) monoclinic P21/n 8.6369(9) 19.3517(19) 12.0519(13) 90 108.2950(10) 90 1912.5(3) 4 1.950 2.301 1128 1.063 R1 ) 0.0286 ωR2 ) 0.0728

R1 ) Σ|Fo| - |Fc|/|Fo|. b ωR2 ) {[ω(Fo- Fc2)2]/Σ[ω(Fo2)2]}1/2.

later, dark blue prism crystals were obtained. Yield: 70% on the basis of Htza. Anal. Calcd. for 2: C, 22.68; H, 1.90; N, 35.27. Found: C, 22.83; H, 1.99; N, 35.13. IR (KBr pellet) data (ν/cm-1): 3444.04 w br, 3080.67 m, 2992.51 w, 1659.33 s, 1508.88 w, 1453.8 w, 1449.77 w, 1385.96 s, 1295.92 s, 1185.06 s, 1098.52 s, 1019.74 m, 949.33 w,801.99 w, 703.31 m, 597.14 w, 529.78 w. Synthesis of {[Cu4(tza)4(CH3COO)2(µ3-OH)2(H2O)2] · 2H2O}n (3). This was synthesized similarly to 2 by using Cu(NO3)2 · 3H2O instead of CuCl2 · 2H2O, and the pH value was adjusted to 4.0 with a solution of CH3COONa · 3H2O (1 mol/L). Blue block crystals suitable for X-ray diffraction were separated by filtration. Yield: 42% on the basis of Htza. Anal. Calcd. for 3: C, 19.48; H, 2.86; N, 22.71. Found: C, 19.55; H, 2.94; N, 22.57. IR (KBr pellet) data (ν/cm-1): 3473.53 s br, 1624.22 s, 1496.25 w, 1424.55 w, 1385.49 m, 1302.01 w, 1102.83 m, 808.26 w, 712.42 w, 698.37 w, 575.40 w. Synthesis of {[Cu4(tza)6(µ3-OH)2] · 4H2O}n (4). This was synthesized similarly to 1 except the pH value was adjusted to 5.0 with NaOH (1 mol/L) solution. The products, light blue block crystals, were separated by filtration. It was interesting that dark blue prism polymers 2 were obtained after one month from the filtrate. Yield: 53% on the basis of Htza. Anal. Calcd. for 4: C, 19.26; H, 2.51; N, 29.94. Found: C, 19.45; H, 2.39; N, 29.48. IR (KBr pellet) data (ν/cm-1): 3434.46 m br, 3134.66 w, 1655.49 s, 1509.27 w, 1422.49 w, 1395.59 m, 1318.46 w, 1190.69 w, 1109.58 w, 1008.94 w, 918.58 w, 807.68 w, 700.60 w. X-ray Structure Analyses. Diffraction intensities for four complexes were collected on a computer-controlled Bruker SMART 1000 CCD diffractometer equipped with graphite-monochromated Mo KR radiation with a radiation wavelength of 0.71071 Å by using the ω-scan technique. Lorentz polarization and absorption corrections were applied. The structures were solved by direct methods and refined with fullmatrix least-squares technique using the SHELXS-97 and SHELXL97 programs.23 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The organic hydrogen atoms were generated geometrically; the hydrogen atoms of the water molecules were located from difference maps and refined with isotropic temperature factors. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. Crystallographic data and experimental details for structure analyses are summarized in Table 1.

Results and Discussion Synthesis. Compound 1 was prepared from a facile reaction between CuO and Htza in H2O solution at pH 1.5. The complex was easily isolated in blue crystalline form upon evaporating the filtrate at room temperature over three months. The Fourier transform infrared (FT-IR) spectrum suggested that part of the ligands are non-deprotonated (Supporting Information, Figure S1). Another type of compound 2 has been isolated from a solution containing a mixture of copper(II) salts and Htza with

pH 2.5 adjusted by triethylamine. The FT-IR spectrum confirmed that all of the ligands have been deprotonated (Supporting Information, Figure S2). Complex 3 was obtained from the reaction of copper(II) salts and Htza. CH3COONa · 3H2O was added in the reaction as a weak base to adjust the pH value to 4.0. When NaOH was added into the reaction mixture of 1 to adjust the pH value to 5.0, complex 4 was obtained. It is noteworthy that the separation of 4 gave a filtrate with a much lower pH value of 2.5, which produced 2 after standing at room temperature over one month. Coordination polymers 2 and 3 can alternatively be obtained from the reactions similar to those mentioned in the context by using CuO or other copper (II) salts instead of CuCl2 · 2H2O and Cu(NO3)2 · 3H2O. It revealed in the study that the low pH value of 1.5 in the reaction gave compound 1 with Htza partly deprotonated. A higher pH value at 2.5 led to the separation of 2 with Htza fully deprotonated. When the pH value of reaction mixture was further elevated to 4.0 and 5.0, it resulted in not only the full deprotonation of Htza but also the coordination of some OH to Cu(II) in 3 and 4. All of these indicate that the compositions and structures of 1-4 in this study were pH-dependent as revealed by single crystal X-ray diffraction analysis. Complex 1. In 1, half of the ligands are deprotonated. X-ray diffraction determination reveals that the ligands in 1 adopt two kinds of coordination modes: a bidentate bridging coordination mode (I) and a monodentate terminal coordination mode via its 4-N atom (III) (Scheme 1). Each Cu(II) ion lies on the inversion center of an octahedral environment and is coordinated by four O atoms and two N atoms. Two O atoms and two N atoms from two bridging tza ligands and two terminal Htza ligands formed the equatorial plane. And the other two O atoms from two bridging tza ions occupy the two axial sites. As expected, the axial Cu-O bond length of 2.424(2) Å is significantly longer than that in the equatorial plane (2.039(2) Å). The Cu-O and Cu-N bond lengths (Table 2) agree well with published results for related complexes.24 Every two adjacent Cu(II) ions are bridged by two carboxylato groups from two tza ligands in a syn-anti bridging mode with a Cu · · · Cu distance of 4.873 Å. This leads to the construction of a 1D chain along the b axis (Figure 1). All the planes of the tetrazolyl rings at the symmetric site are parallel to each other, while the angle between the tetrazolyl rings from Htza ligands and tza ions is 84.082°.

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Table 2. Selected Bond Lengths (Å) and Bond Angles (°) of Complexes 1–4 1a Cu(1)-O(1) Cu(1)-N(8) Cu(1)-O(2B) O(1A)-Cu(1)-O(1) O(1A)-Cu(1)-N(8A) O(1)-Cu(1)-N(8A) O(1A)-Cu(1)-N(8) O(1)-Cu(1)-N(8) N(8A)-Cu(1)-N(8) O(1A)-Cu(1)-O(2B) O(1)-Cu(1)-O(2B)

1.9499(16) 2.0386(19) 2.4236(18) 180.00(8) 94.42(7) 85.58(7) 85.58(7) 94.42(7) 179.999(1) 93.56(7) 86.44(7)

Cu(1)-O(1A) Cu(1)-N(8A) Cu(1)-O(2C) N(8A)-Cu(1)-O(2B) N(8)-Cu(1)-O(2B) O(1A)-Cu(1)-O(2C) O(1)-Cu(1)-O(2C) N(8A)-Cu(1)-O(2C) N(8)-Cu(1)-O(2C) O(2B)-Cu(1)-O(2C)

1.9499(16) 2.0387(19) 2.4236(18) 97.35(7) 82.65(7) 86.43(7) 93.56(7) 82.64(7) 97.36(7) 179.997(1)

Cu(1)-O(1B) Cu(1)-N(1C) O(1A)-Cu(1)-N(1) O(1A)-Cu(1)-N(1C) N(1)-Cu(1)-N(1C)

1.9282(13) 2.0010(16) 92.11(6) 87.89(6) 180.000(1)

Cu(1)-O(7) Cu(1)-O(7A) Cu(1)-Cu(1A) Cu(2)-O(7A) Cu(2)-N(8) Cu(2)-N(3) O(5)-Cu(1)-N(4) O(5)-Cu(1)-O(7A) N(4)-Cu(1)-O(7A) O(7)-Cu(1)-O(1B) O(7A)-Cu(1)-O(1B) O(2C)-Cu(2)-O(7A) O(7A)-Cu(2)-O(3D) O(7A)-Cu(2)-N(8) O(2C)-Cu(2)-O(1W) O(3D)-Cu(2)-O(1W) O(2C)-Cu(2)-N(3) O(3D)-Cu(2)-N(3)

1.9662(19) 1.9954(19) 2.9720(7) 1.9760(19) 2.023(2) 2.459(3) 89.87(9) 174.89(8) 90.81(8) 90.03(8) 95.39(8) 91.27(8) 94.28(8) 174.01(9) 87.66(9) 92.36(9) 92.26(9) 87.23(9)

Cu(1)-O(7) Cu(1)-N(5) Cu(2)-O(7) Cu(2)-O(6B) Cu(2)-O(2) O(1)-Cu(1)-O(7A) O(1)-Cu(1)-N(5) O(7A)-Cu(1)-N(5) O(7)-Cu(1)-O(3B) N(5)-Cu(1)-O(3B) O(7)-Cu(2)-O(6B) O(7)-Cu(2)-N(9) O(6B)-Cu(2)-N(9) O(4B)-Cu(2)-O(2) N(9)-Cu(2)-O(2)

1.9592(16) 1.992(2) 1.9744(16) 1.9786(17) 2.2425(19) 164.54(8) 91.84(8) 89.66(7) 96.99(6) 90.07(7) 94.47(7) 176.74(8) 86.49(8) 104.04(7) 94.03(8)

2b Cu(1)-O(1A) Cu(1)-N(1) O(1A)-Cu(1)-O(1B) O(1B)-Cu(1)-N(1) O(1B)-Cu(1)-N(1C)

1.9282(13) 2.0010(16) 180.0 87.89(6) 92.11(6) 3c

Cu(1)-O(5) Cu(1)-N(4) Cu(1)-O(1B) Cu(2)-O(2C) Cu(2)-O(3D) Cu(2)-O(1W) O(5)-Cu(1)-O(7) O(7)-Cu(1)-N(4) O(7)-Cu(1)-O(7A) O(5)-Cu(1)-O(1B) N(4)-Cu(1)-O(1B) O(2C)-Cu(2)-O(3D) O(2C)-Cu(2)-N(8) O(3D)-Cu(2)-N(8) O(7A)-Cu(2)-O(1W) N(8)-Cu(2)-O(1W) O(7A)-Cu(2)-N(3) N(8)-Cu(2)-N(3) O(1W)-Cu(2)-N(3)

1.957(2) 1.994(2) 2.251(2) 1.9669(19) 2.0097(19) 2.414(3) 95.92(8) 170.82(9) 82.79(8) 89.55(8) 97.15(8) 174.35(8) 90.02(9) 84.35(9) 100.91(9) 84.99(10) 84.05(8) 90.05(9) 175.04(9) 4d

Cu(1)-O(1) Cu(1)-O(7A) Cu(1)-O(3B) Cu(2)-O(4B) Cu(2)-N(9) O(1)-Cu(1)-O(7) O(7)-Cu(1)-O(7A) O(7)-Cu(1)-N(5) O(1)-Cu(1)-O(3B) O(7A)-Cu(1)-O(3B) O(7)-Cu(2)-O(4B) O(4B)-Cu(2)-O(6B) O(4B)-Cu(2)-N(9) O(7)-Cu(2)-O(2) O(6B)-Cu(2)-O(2)

1.9362(17) 1.9811(16) 2.2635(16) 1.9758(17) 2.000(2) 93.61(7) 82.71(7) 169.48(7) 106.45(7) 88.94(6) 88.84(7) 161.43(8) 89.27(8) 89.01(7) 94.30(8)

Symmetry codes: A, -x + 2, -y, -z + 1; B, x, y - 1, z; C, - x + 2, -y + 1, -z + 1. b Symmetry codes: A, -x, y - 1/2, -z + 1/2; B, x + 1, -y + 3/2, z + 1/2; C, -x + 1, -y + 1, -z + 1. c Symmetry codes: A, -x + 1, -y + 1, -z + 1; B, -x + 2, -y + 1, -z + 1; C, x - 1, y, z; D, -x +2, -y+2, -z + 2. d Symmetry codes: A, -x + 1, -y, -z + 2; B, -x + 2, -y, -z + 2. a

Complex 2. 2 is a 2D wavelike layer based on {Cu(tza)}4 rings from head-to-tail arranged tza ions. Each of the ligand is deprotonated and adopts a bidentate bridging coordination mode (IV). The Cu(II) ion lying on an inversion center is coordinated by two 4-N atoms and two caboxylate O atoms from four tza ligands with Cu-O and Cu-N bond lengths of 1.928 (2) and 2.001 (2) Å, respectively. Therefore, the local coordination geometry of copper center is a square-planar with N2O2 donor set. Each Cu(II) center is linked to four adjacent Cu(II) centers by four bidentate ligands, resulting in a 2D grid with a Cu · · · Cu separation of 8.340(7) Å (Figure 2b).

Complex 3. The ligands in 3 have two coordination modes: a bidentate bridging coordination mode (IV) and a tetradentate bridging coordination (V), as shown in Figure 3a. Complex 3 contains a tetranuclear unit with a center of inversion (Figure 3a). The central Cu4(µ3-OH)2 core is composed of two kinds of crystallographically independent Cu(II) ions (Cu1 and Cu2). The four copper atoms define a parallelogram with the sides of 3.458 (5) Å (Cu1 · · · Cu2A) and 3.389(6) Å (Cu1 · · · Cu2), respectively. The two diagonals of the parallelogram are 2.972(7) Å (Cu1 · · · Cu1A) and 6.169(7) Å (Cu2 · · · Cu2A), respectively. These values are comparable with those observed in other

Coordination Polymers with Tetrazole-1-acetic acid

Figure 1. A fragment of the polymeric chain of complex 1. H2O and H atoms have been omitted for clarity.

similar tetranuclear Cu(II) compounds.25 Except the µ3-OH group, the Cu(II) ions located on the diagonals are further bridged by syn-syn carboxlate groups and N atoms from µ4tza anions. The tetranuclear unit can also be considered as two trinuclear units sharing one edge. Each of these trinuclear units has a bridging µ3-OH group at its center with the Cu-O bond lengths of 1.966(2)-1.995(2) Å. The O atom is 0.550 Å above the Cu3-plane. Cu1 and Cu1A have square-pyramidal geometry. The basal sites are occupied by two oxygen atoms from two µ3-OH groups, one carboxylate oxygen atom from CH3COO- and one nitrogen atom from µ4-tza anion with the Cu-O bond lengths ranging from 1.957(2) to 1.995(2) Å. In the apical position, another µ4-tza anion O coordinates to Cu(II) ion with the Cu-O bond length of 2.251(2) Å. Cu2 and Cu2A can be considered to have an octahedral geometry around them with a N2O4 donor set. These metal centers are surrounded by one carboxylate O atom and one N atom from two µ2-tza anions, one µ3-OH oxygen atom, and one carboxylate O atom from a µ4-tza anion giving rise to a CuNO3 equatorial plane. The axial sites are occupied by one O atom from H2O molecule and one N atom from a µ4-tza anion with the Cu-O and Cu-N bond lengths of 2.414(3) and 2.459(3) Å, respectively. Each pair of neighboring Cu4(µ3-OH)2 units are connected by two µ4-tza anions (coordination mode (V)) to form an infinite 1D band along

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the a axis. Alternatively, another kind of 1D band is constructed via the bridging of each pair of adjacent Cu4(µ3OH)2 units by two µ2-tza anions (coordination mode (IV)). Two types of infinite bands are cross-linked to yield a 2D network (Figure 3b). Complex 4. The ligands in 4 have three coordination modes of (I), (II), and (IV). The polymer 4 contains a tetranuclear unit that is similar to 3 (Figure 4a).The longer and shorter edges of the parallelogram defined by the four Cu(II) ions are 3.383(5) Å (Cu1 · · · Cu2) and 3.272(4) Å (Cu1 · · · Cu2A), respectively. The two diagonals of the parallelogram are 2.958(6) Å (Cu1 · · · Cu1A) and 5.963(7) Å (Cu2 · · · Cu2A), respectively. Different from 3, Cu1 and Cu2A are only bridged by one µ3OH group, while Cu1 and Cu2 are bridged by one µ3-OH group and two carboxylate groups by syn-syn mode from a µ2- and a µ3-tza anions. Each O atom of the OH group is 0.660 Å out of the Cu3-plane, and the Cu-O bond lengths vary from 1.959(2) to 1.981(2) Å. The four copper ions in 4 have tetragonal pyramidal geometry. The basal plane for Cu1 and Cu1A centers contains two µ3OH oxygen atoms, one N atom, and one carboxylate O atom from two different tza anions with average Cu-O bond lengths of 1.959 Å, while the axial site is occupied by a carboxylate O atom from a µ2-tza anion with the Cu-O bond length of 2.264(2) Å. The basal sites around the copper centers (Cu2 and Cu2A) are occupied by one µ3-OH oxygen atom, two carboxylate O, and one N atoms from three different tza anions with average Cu-O bond lengths of 1.976 Å, while a carboxylate O atom occupied the axial site with the Cu-O bond length of 2.243(2) Å. The 2D structure of 4 results from the cross-linking between two kinds of 1D band, which is similar to 3. The difference is that each pair of neighboring Cu4(µ3-OH)2 units are held together by two µ3-tza anions (coordination mode (II)) to form an infinite 1D band along a axis. EPR and Magnetic Properties. The EPR spectra of 1-4 were recorded on solution samples at 120 K. The following parameters were obtained: g ) 2.13 for 1 and 4, g ) 2.14 for 2 and 3, respectively. The magnetic susceptibilities of complexes 1-4 were measured under a 5000 G applied magnetic field in the 2–320 K temperature range. Complex 1. The magnetic behavior of complex 1 is illustrated in Figure 5 in the form of µeff vs T, which reveals the occurrence

Figure 2. (a) The structure of complex 2 showing the repeating asymmetric unit. (b) The 2D network structure of 2. H atoms have been omitted for clarity.

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Figure 3. (a) The structure of complex 3 showing the repeating asymmetric unit. H atoms have been omitted for clarity. (b) Schematic representation of the 2D network structure in complex 3.

of weak antiferromagnetic interactions. The value of µeff at 300 K is 1.72 µB, which is as expected for one uncoupled copper(II) ion (1.73 µB). Upon cooling down of the sample, it remains constant and sharply decreases in the low temperature region. No susceptibility maximum is observed in the magnetic curve until the lowest value of temperature that we have explored (2.0 K). Complex 1 is a chain of copper(II) ions that are bridged by carboxylato groups in a syn-anti bridging mode. In syn-anti Cu-O-C(R)-O′-Cu′ complexes, the contributions from the 2p orbitals of the O and O′ atoms belonging to the magnetic orbitals centered on Cu and Cu′ are unfavorably oriented to give a strong overlap.27 The poor overlapping between the metal centered magnetic orbitals through this kind of bridge accounts for the weak antiferromagnetic coupling observed, and in some cases the relative orientation of the magnetic orbitals can lead to the situation of accidental orthogonality and thus to weak ferromagnetic coupling.28 Taking into account the structure of 1 we fitted its magnetic data through the polynomial expression (eq 1) derived by Hall,29 which describes well the results of Bonner and Fisher30 on a uniformly spaced linear chain of local spin S ) 1/2: 0.25 + 0.144995x + 0.30094x2 Ng2β2 × (1) kT 1 + 1.9862x + 0.68854x2 + 6.0626x3 x ) |J|/kT where the parameters have their usual meaning. The results of the fit through eq 1 leads to J ) -0.36 cm-1, g ) 2.00, R ) 3.97 × χM )

Figure 4. (a) The structure of complex 4 showing the repeating asymmetric unit. H2O and H atoms have been omitted for clarity. (b) Schematic representation of the 2D network structure in complex 4.

Figure 5. Plots of χM vs T and µeff vs T of complex 1. The solid line corresponds to the best theoretical fit (see text).

10-5. R is the agreement factor defined as R ) Σ[(χMT)obs (χMT)calc]2/Σ[(χMT)obs]2.28 The value of J is small as expected for this type of complex exhibiting the syn-anti mode.31

Coordination Polymers with Tetrazole-1-acetic acid

Figure 6. Plots of experimental 1/χM vs T and µeff vs T of complex 2. The solid line shows the Curie–Weiss fitting.

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Figure 8. Plots of χM-1 and µeff vs T of complex 4.

employed (JCu1-Cu2, JCu1-Cu1a, JCu1-Cu2a, JCu2-Cu2a). Unfortunately, we were unable to obtain satisfactory fitting of the susceptibility data. Conclusions In summary, we have reported four novel complexes formed by Htza and divalent copper ions. The strong coordinate abilities of rigid carboxylate groups and flexible acetic groups, as well as nitrogens of the tetrazolyl, endow Htza with abundant coordination modes. By changing the pH of the solution, we obtained four novel complexes. Our research results indicate that, as a promising new type of multifunctional ligand, Htza has a great potential in the field of coordination polymers, and further endeavors for exploration of Htza complexes are underway in our workgroup. Figure 7. Plots of χM-1 and µeff vs T of complex 3.

Complex 2. The magnetic behavior of the complex 2 in the form of 1/χM and µeff vs T plots is shown in Figure 6. The value of µeff at 300 K is 1.78 µB, somewhat larger than that expected for a single uncoupled copper ion (1.73 µB). As the temperature is lowered, susceptibility increases slowly. At 2 K, the value of µeff reaches the maximum (1.88 µB). The lack of interactions throughout practically the whole range of temperature is in agreement with what was expected. The large distances (8.3396 Å) between the copper ions and the absence of a possible “communicating path” make the possible interactions weak. The increase of the µeff value is characteristic of a very weak ferromagnetic interaction. And this is confirmed when 1/χM versus T is plotted (see Figure 6). From it, a Weiss constant (θ) of 1.21 K is obtained, proving the existence of a weak ferromagnetic interaction. Complexes 3 and 4. In 3 and 4, [Cu4(µ3-OH)2] units are linked into a 2D network by tza anions. The magnetic behavior of them in the form of 1/χM and µeff vs T plots is shown in Figures 7 and 8. The Weiss constants are -93.85 and -49.16 K, respectively. Their µeff values are 3.41 and 3.45 µB at room temperature, which are slightly less than that of four noninteracting copper(II) ions (3.46 µB, assuming g ) 2.0).32 Their µeff values gradually decreased to 0.27 and 1.20 µB at 2 K, respectively. This behavior indicates the presence of antiferromagnetic interaction between copper(II) ions. To obtain the magnitude of exchange constants, we attempted to fit the data by a tetramer model25,33 where four independent J values are

Acknowledgment. We gratefully acknowledge the Science Foundation of Guangxi, China, the Youth Science Foundation of Guangxi, and the Teaching and Research Award Programme for Outstanding Young Teachers in Higher Education Institutions of MOE, China. Supporting Information Available: IR spectra and X-ray crystallographic information files (CIF) for compound 1-4. This material is available free of charge via the Internet at http://pubs.acs.org.

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