A 2D Brickwall-like Copper(II) Coordination Polymer Based on Phenyliminodiacetate and 4,4?-Bipyridine:Synthesis, Crystal Structure and Magnetic Property①

WANG Xiao-Bing LU Zheng-An LU Wen-Guan

(Department of Chemistry, Shaoguan University, Shaoguan 512005, China)

1 INTRODUCTION

Very recently, because of the structural diversities and potential applications in many fields, such as gas adsorption, catalysis, luminescence, magnetic,etc., metal-organic coordination polymers (CPs)have attracted extensive attention to chemical researchers[1-10]. One of the most effective strategies to construct functional CPs with unique structures is to employ multinuclear metal cluster as the secondary building unit (SBU), then connecting these SUBs by bridging ligands as linkers, often leading to the formation of novel CPs with extended higher dimensional framework structures and desired function properties[11-13]. Usually, as an auxiliary ligand, rigid rod-like neutral N,N?-donor bridging ligands, such as 4,4?-bipyridine (4,4?-bipy), pyrazine etc., can also be used as bridged ligands to connect these SBUs to form higher dimensional structures[11,12].

As a flexible organic ligand, phenyliminodiacetic acid (C6H5N(CH2COOH)2, H2L) or its derivatives have two kinds of coordination sites to participate in the coordination process, namely, not only containing one amino nitrogen atom but also exten-ding two flexible acetic arms from its aminodiacetic group. Two flexible acetates, herein, have good coordination capacities and diverse coordination modes, such as monodentate, bidentate, chelate,and so on. Furthermore, H2L can also be used as a tridentate ligand to bind the metal ions to form two five-membered rings of MOCCN by one amino nitrogen atom and two acetates. So, H2L or its derivatives have been widely used in the architecture of CPs[14-19]. Taking these properties into account,we and others have also selected flexible H2L as the main ligand and rigid neutral N,N?-donor ligands,such as 4,4?-bipy, pyrazine, 2,2?-bipyridine (2,2?-bipy), 1,10-phenanthroline (1,10?-phen), etc. as the auxiliary ligand to react with transition metal ions to construct and obtain a series of new CPs based on the H2L ligand[15-17]. Here we report the preparation and crystal structure of another novel copper(II)coordination polymer of {[Cu3L2(4,4?-bipy)4]-(ClO4)2·20H2O}n(1) based on the mixed ligands of H2L and 4,4?-bipy. It contains rare tri- nuclear copper(II) clusters of [Cu3L2]2+, and repre- sents a 2D cation brickwall-like network of [Cu3L2-(4,4?-bipy)4]n2n+connected by 4,4?-bipy ligands. Furthermore, its thermal stability and magnetic property have also been investigated.

2 EXPERIMENTAL

2.1 Materials and physical measurements

Phenyliminodiacetic acid (H2L) was prepared according to the literature procedure[14]. The other chemicals were commercially available and used without further purification. An elemental analysis(EA) was carried out using a Vario EL elemental analyzer. Infrared spectroscopy (IR) was acquired in a Nicolet Avatar-370 FT-IR spectrometer with KBr pellets. Powder X-ray diffraction (PXRD) measurement was performed on a Bruker D8-ADVANCE X-ray diffractometer with a CuKα radiation (λ =0.15418 nm). Thermogravimetric analysis (TGA)data were carried out on a Netzsch TG-209 thermogravimetry analyzer under N2atmosphere in the temperature range of 20～800 ℃ at a heating rate of 10 ℃/min. Magnetic susceptibility data were collected at 2～300 K in a field of 1000 Oe with a Quantum Design MPMS-XL7 SQUID.

2.2 Synthesis of {[Cu3L2(4,4?-bipy)4](ClO4)2·20H2O}n (1)

A solution of CuSO4·5H2O (0.125 g, 0.5 mmol)in methanol (20 mL) was added dropwise to an equimolar aqueous solution of H2L (0.105 g, 0.5 mmol). The reaction mixture was stirred for 1 h.Then a solution of 4,4?-bipy (0.156 g, 1.0 mmol) in methanol (10 mL) was added to it. After stirring for 0.5 h, NaClO4·H2O (0.140 g, 1.0 mmoL) was added and the mixture was stirred for 0.5 h, and then filtered. The mother liquor was allowed to evaporate slowly at room temperature. Blue block-shaped crystals of 1 were collected by filtration after one week. Yield: 75% (based on Cu). Anal. Calcd. for C60H90Cl2Cu3N10O36(%): C, 40.28; H, 5.07; N, 7.83.Found (%): C, 41.63; H, 4.86; N, 7.95. IR (KBr pellet, cm-1): 3415 (vs), 1637 (s), 1617 (s), 1501 (w),1443 (m), 1122 (s), 871 (w), 623 (m), 467 (w).

2.3 Crystallographic data collection and structure determination

A blue block-shaped crystal with dimensions of 0.30mm × 0.30mm × 0.28mm was selected for X-ray diffraction. The diffraction data were obtained at 293(2) K using a Bruker Smart 1000 CCD diffractometer equipped with a graphitemonochromatic MoKα radiation (λ = 0.071073 nm). 15824 reflections were collected in the range of 1.47≤θ≤25.00o by using the ω/2θ scan mode. Among them,6929 were independent (Rint= 0.0347) and 4910 were observed (I ≥ 2σ(I)). The structure was solved by direct methods and refined by full-matrix least-squares techniques on F2using the SHELXS-97 and SHELXL-97 programs[20,21]. All hydrogen atoms of L2-and 4,4?-bipy ligands were placed in the calculated positions with fixed isotropic thermal parameters and included in the structure factor calculations in the final stage of full-matrix leastsquares refinement. The positions of oxygen atoms in ClO4-and one lattice water molecule (O(5)) are dually disordered, and its occupies were fixed according to their FVAR values. The hydrogen atoms in lattice water molecules were not included.The largest peak and deepest hole on the final difference Fourier map are 735 and –478 e·nm-3,respecttively. The final refinement converged at R =0.0487 and wR = 0.1204 (w = 1/[σ2(Fo2) +(0.0657P)2+ 23.6255P], where P = (Fo2+ 2Fc2)/3),(Δ/σ)max= 0.001 and S = 1.036. The selected bond lengths and bond angles are given in Table 1.

Table 1. Selected Bond Lengths (nm) and Bond Angles (o) of 1

3 RESULTS AND DISCUSSION

Complex 1 was obtained with medium yield by the reaction of H2L, CuSO4·5H2O, NaClO4·H2O and 4,4?-bipy in water/methanol followed by slow evaporating the solution at room temperature. Additionally, it is worth noting that using Cu(NO3)2·3H2O or Cu(Ac)2·H2O instead of CuSO4·5H2O to react with H2L, NaClO4·H2O and 4,4?-bipy under the same conditions can also produce 1 in various yields.This indicates that the influence of metal precursor to the formation of 1 is negligible. IR spectra of 1 show broad O–H stretching bands about 3415 cm-1resulting from the existence of lattice water molecules in the structure. The bands at 1617 and 1443 cm-1can be assigned to the antisymmetric vas(COO-)and symmetric stretching frequency vs(COO-), and the absorption bands for ClO4-could be found at 1122 and 623 cm-1, respectively. The phase homogeneity of the bulk sample 1 was identified by powder X-ray diffraction (PXRD). As shown in Fig.1, all the peaks of experimental result at room temperature closely match to the simulated one generated from the single-crystal diffraction data,which show the bulky sample is pure.

Fig. 1. Experimental and simulated PXRD patterns of 1

Fig. 2. Coordination environment of Cu(II) ions, bridging mode of μ2-L2- ligand and [Cu3L2]2+ cluster (a), the coordination polyhedron of Cu(II) ions (b and c) in 1 (Thermal ellipsoids are drawn at the 30% level, and all the hydrogen atoms were omitted for clarity. Symmetry codes: A: x, –y + 1, z + 1/2; B: –x + 2, –y + 1, –z + 2;C: –x + 2, –y, –z + 2; D: x, –y, z + 1/2; E: –x + 2, y, –z + 5/2)

Fig. 3. In 1, view of the 2D cation brickwall-like layer structure of [Cu3L2(4,4?-bipy)4]2+ along the 101 plane (a), b axis (c)and c axis (d), respectively (All benzene rings of L2- and the hydrogen atoms were omitted for clarity).Topological view of the 2D cation brickwall-like layer in 1 showing (4,4) grid structure by the linkages of 4,4?-bipy ligands and [Cu3L2]2+ clusters as nodes (b)

Fig. 4. In 1, view of the 3D supramolecular structure assembled via the interlayer stacking interactions along the 101 plane (a), b axis (b) and c axis (c), respectively (The disordered ClO4- counter anions are represented in space-filling mode. All lattice water molecules were omitted for clarity in b and c)

The result of single-crystal X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic system with space group C2/c, and its structure features a 2D cation brickwall-like network layer of[Cu3L2(4,4?-bipy)4]n2n+with an unprecedented trinuclear unit of [Cu3L2]2+core as the second building unit (SBU) (Fig. 2a). The extended structure may be described in terms of the interconnected [Cu3L2]2+subunits by 4,4?-bipy ligands. Three arranged Cu(II)ions are clustered by virtue of two L2-ligands. The L2-ligand, herein, can be viewed as a μ2-connecter.The two carboxylate groups exhibit two different coordination modes. One is monodentate, and the other is syn-anti bridging bidentate, which is very similar to the complex we had reported before[17]. In 1, the asymmetrical unit contains two crystallographically independent Cu(II) ions (Cu(1) and Cu(2)). The Cu(2) is located on an inversion center,and shows different coordination environments and geometry from Cu(1). The Cu(1) and Cu(2) ions are bridged by one carboxylate of μ2-L2-with syn-anti bridging bidentate coordination mode, and the distance between Cu(1) and Cu(2) is 0.5859 nm,which is longer than the syn-syn bridging bidentate coordination mode in several compounds having been reported[22,23]. Two Cu(II) ions (Cu(1) and Cu(1E)) and two L2-ligands are related to each other by this inversion center (Cu(2)) in the SBU, in which the Cu(1)···Cu(2)···Cu(1E) angle is 170.450(8)o, indicating Cu(1), Cu(2) and Cu(1E) in SBU are not completely in a straight line. The Cu(1)shows a slightly distorted tetragonal pyramidal coordination geometry (Fig. 2b) and is five-coordinated with two oxygen atoms (O(1) and O(3)) and one nitrogen atom (N(1)) from one μ2-L2-, and two nitrogen atoms (N(2) and N(3)) from two individual 4,4?-bipy ligands. O(1), O(3), N(1) and N(2) locate on the square quasi-plane, and the vertical position is occupied by N(3). The lengths of Cu(1)–O bands are 0.1939(3) and 0.1954(3) nm, and those of Cu(1)–N range from 0.2008(3) to 0.2227(3) nm. The bond angles around Cu(1) are in the range of 83.34(12)～163.52(12)o. The Cu(2) is six-coordinated with four nitrogen atoms (N(4A), N(4B), N(5C) and N(5D))from four individual 4,4?-bipy ligands in the equatorial plane, and two oxygen atoms (O(4) and O(4E)) from two individual μ2-L2-ligands in the axial positions, forming a slightly distorted octahedral geometry (Fig. 2c). The axial Cu(2)–O bond lengths (0.2380(3) nm) are slightly longer than the equatorial Cu(2)–N bond lengths (0.2028(3) and 0.2043(3) nm) because of the Jahn-Teller effect.Therefore, based on the crystal- field theory, the electronic configuration of Cu(2) may be(t2g)6(dx2–y2)1(dz2)2. The axial bond angle of O(4)–Cu(2)–O(4E) is 169.34(13)o, and the bond angles around Cu(2) in the equatorial plane fall in the 88.11(18)～91.78(12)o range. Both Cu(II)–N and Cu(II)–O bond lengths around the Cu(II) ions (Cu(1)and Cu(2)) in 1 are well-matched to those observed in similar compounds[14,17]. In 1, each subunit of[Cu3L2]2+is further connected to four others via rigid rod-like neutral N,N?-donor bridging 4,4?-bipy ligands to form a 2D cation brickwall-like network layer of [Cu3L2(4,4?-bipy)4]n2n+with a tetragonal window of about 1.1240nm × 1.1105nm (measured by the Cu···Cu distances) (Fig 3a). If the [Cu3L2]2+subunits are taken as nodes and twin bridging 4,4?-bipy as linkers of a topological network, a (4,4)grid topological architecture with grid length of 1.3380nm × 1.3053nm (measured between the[Cu3L2]2+node distances) is generated (Fig. 3b). As shown in Fig. 4, the neighboring brickwall-like layers are further stacked in a staggered manner via the interlayer stacking interactions to generate a 3D supramolecular structure with 1D open square channels. These open square channels are filled with disordered ClO4-counter anions and lattice water molecules (Fig. 4a).

Fig. 5. Thermogrvimetric analysis curve of 1

Fig. 6. Temperature dependence of 1/χM and χMT versus T plots for 1

Thermogravimetric analysis (TGA, Fig. 5)indicates that the lattice water molecules in 1 could be removed form the channel under 220 ℃, and the weight loss of 16.09% is less than that calculated(20.14%), indicating part of lattice water molecules may have been lost before heating. After the loss of lattice water molecules, the 2D framework began to decompose upon further heating.

Magnetic susceptibility measurements on temperature dependence were investigated on Quantum Design MPMS-XL7 SQUID at 2～300 K in a field of 1000 Oe. As shown in Fig. 6, the χMT value of 1 is 1.733 cm3·mol-1·K at room temperature, which is close to the theoretical value (1.732 cm3·mol-1·K) for three uncoupled spin Cu(II) ions (SCu= 1/2, g = 2.0).By decreasing the temperature, the χMT value decreases gradually approaching to 1.418 cm3·mol-1·K at 20 K, then drops rapidly. At 2.0 K, the χMT value is equal to 0.956 cm3·mol-1·K. The result probably indicates a typical antiferromagnetic behavior between the Cu(II) ions within [Cu3L2]2+trinuclear cluster units because of the coordinate bridging action through the carboxylic ions (Fig. 2a). Due to the long distance of bridging 4,4?-bipy ligands, there is no obvious magnetic exchange interaction between the adjacent trinuclear [Cu3L2]2+cluster units(Fig. 3a). The magnetic susceptibilities over the temperature range of 2～300 K can be fitted based on the Curie-Weiss Law χM= C/(T – θ) with C =1.696 cm3·mol-1and θ = –5.986 K, further suggesting the antiferromagnetic coupling between the Cu(II) ions within [Cu3L2]2+trinuclear cluster units.The obvious decrease of χMT value below 20 K may be attributed to the antiferromagnetic coupling interactions within [Cu3L2]2+trinuclear cluster units or zero-field splitting of the ground state[24].

4 CONCLUSION

In summary, a new Cu(II) coordination polymer of {[Cu3L2(4,4?-bipy)4](ClO4)2·20H2O}n(1) was obtained based on mixed ligand of flexible phenyliminodiacetate (L2-) and rigid 4,4?-bipyridine(4,4?-bipy). The single-crystal X-ray diffraction analysis that 1 exhibits a 2D cation brickwall-like layer structure of [Cu3L2(4,4?-bipy)4]n2n+built from the trinuclear [Cu3L2]2+secondary building units(SBUs) and 4,4?-bipy linkers. These adjacent 2D cation brickwall-like layers are further stacked in a staggered fashion by the interlayer stacking interactions to generate a 3D supramolecular structure with 1D open square channels, in which the counter anions ClO4-and lattice water molecules are filled in these channels. Furthermore, magnetic susceptibility measurement of 1 indicates the presence of antiferromagnetic interactions between the neighboring Cu(II) ions.

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