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Publicly Available Published by De Gruyter July 13, 2019

A new Co(II) coordination polymer with the 2-(4-pyridyl)-terephthalate ligand: synthesis, crystal structure and magnetic properties

  • Danian Tian , Runmei Ding , Bingbing Chen and Peipei Cen EMAIL logo

Abstract

A new Co(II) complex, [Co(pta)(H2O)2]n (1), with the 2-(4-pyridyl)-terephthalate ligand (pta2−) has been synthesized and structurally and magnetically characterized. Single crystal X-ray analysis indicates that the unique Co(II) ion in the asymmetric unit of 1 displays stretched octahedral geometry. Compound 1 presents a bimetallic layer structure which is further expanded to a 3D supramolecular network through hydrogen bonding interactions. Magnetic measurements have revealed the temperature-dependent existence of antiferromagnetic and ferromagnetic interactions in compound 1.

1 Introduction

The rational design and synthesis of coordination polymers (CPs) continue to attract great attention because of the numerous possibilities and plenty of synthetic options for tailoring their structures and properties [1], [2], [3], [4]. Clearly, the judicious selection of both ligands and metal cations is the key factor for a control of the structure and properties of the resulting CPs [5], [6], [7], [8], [9], [10]. Because the systems exhibit substantial orbital contributions to the magnetic moments and therefore possess strong magnetic anisotropy, the exchange coupled magnetic cobalt(II) compounds are most fascinating [11], [12], [13], [14]. The research concerning magnetic Co(II) complexes may be in favor of comprehending the fundamental magnetic phenomena and will also help create a valid platform for further exploitation of newfangled functional magnetic materials [15]. Besides, the selection of organic ligands is also of great importance, since their coordination modes, the configuration, and the flexibility could remarkably affect the final structures of CPs [16], [17], [18]. Recently, much attention has been focused on ligands containing both −COO and N-donor groups, which show strong affinities to transitional metal ions and diverse coordination modes. Therefore, 2-(4-pyridyl)-terephthalic acid (H2pta) was chosen as the ligand source. Multifunctional pyridyl-di- and poly-carboxylate ligands have been extensively employed as rigid and bridging donors to intergrate metal ions into CPs [19], [20], [21], [22]. In contrast to fruitful research in bridging carboxylate or pyridyl spacers, limited work has been focused on pyridyl-carboxylate ligands in CPs [23], [24], [25].

In work described in this article, the new CP with the composition [Co(pta)(H2O)2]n (H2pta=2-(4-pyridyl)-terephthalic acid) (1) has been isolated under hydrothermal reaction condition. Its crystal structure has been determined by X-ray crystallographical analysis. In addition, the magnetic properties of compound 1 have been investigated.

2 Results and discussion

2.1 Description of structure

Single-crystal X-ray structural analysis shows that 1 crystallizes in the monoclinic system in space group P21/c (Table 1), exhibiting a bi-layered structure. The asymmetric unit consists of one Co2+ ion, one pta2− dianion, and two coordinated water molecules (Fig. 1a). The Co(II) center indicates octahedral configuration with a CoNO5 unit, completed by three oxygen atoms from three different pta2− ligands [Co(1)–O(1)=2.059(3) Å, Co(1)–O(3)=2.102(3) Å, Co(1)–O(4)=2.108(3) Å], one oxygen atom from a water molecule [Co(1)–O(2W)=2.085(3) Å] in the equatorial plane, as well as one nitrogen atom from a pta2− ligands [Co(1)–N(1)=2.110(4) Å] and one oxygen atom from another water molecule [Co(1)–O(1W)=2.157(3) Å] at the axial positions (Table 2). Obviously, the axial bond distances are longer than the equatorial bond distances, illustrating that the octahedral Co(II) sphere undergoes an axial extension. The pta2− ligand acts as a μ4 node to coordinate with four Co2+ ions via the carboxylate groups and pyridine N atoms (Fig. 1b). The Co ions linked by carboxylate groups produce a layer structure (Fig. 1c), and two layers are combined by pyridyl units to afford a bilayer skeleton. Finally, adjacent bilayers are further interconnected by hydrogen bonding interactions (O1W–H···O2=2.927 Å), leading to a 3D supramolecular network (Fig. 1d). The shortest Co···Co distance is 5.0848(8) Å.

Table 1:

Selected crystallographic data for 1.

Compound1
Empirical formulaC13H11CoNO6
Formula weight336.16
Temperature/K150
Crystal systemMonoclinic
Space groupP21/c
a12.9579(3)
b11.2330(3)
c8.4106(2)
β/deg98.186(2)
V31211.74(5)
Z4
D/g cm−31.84
μ/mm−111.4
F(000)/e684
Refl. total, unique, Rint3683, 2062, 0.0578
Refl. observed [I>2 σ(I)]1844
Parameter refined190
Final R1, wR2 [I>2 σ(I)]0.0584, 0.1461
Final R1, wR2 (all data)0.0644, 0.1555
Goodness-of-fit on F21.002
Δρfin (max, min)/e Å−30.88, −0.93
Fig. 1: (a) Coordination environment of the Co(II) ion in 1. (b) Coordination mode of the pta2− ligand in 1. (c) The layer of 1. (d) The 3D supramolecular network of 1. H atoms have been deleted for clarity.
Fig. 1:

(a) Coordination environment of the Co(II) ion in 1. (b) Coordination mode of the pta2− ligand in 1. (c) The layer of 1. (d) The 3D supramolecular network of 1. H atoms have been deleted for clarity.

Table 2:

Selected bond lengths (Å) and bond angles (deg) for 1a.

Co(1)–O(1)#22.059(3)O(1)–Co(1)–N(1)#288.54(12)
Co(1)–N(1)#12.110(4)O(1)–Co(1)–O(1W)88.10(11)
Co(1)–O(1W)2.157(3)O(1)–Co(1)–O(2W)88.43(10)
Co(1)–O(2W)2.085(3)O(1)–Co(1)–O(3)#398.23(11)
Co(1)–O(3)#32.102(3)O(2W)–Co(1)–N(1)#295.73(13)
Co(1)–O(4)#42.108(8)O(2W)–Co(1)–O(1W)86.45(12)
  1. a Symmetry operations: #1 −x, 1/2+y, 1/2−z; #2 −x, −1/2+y, 1/2−z; #3 +x, 1/2−y, 1/2+z; #4 +x, −1+y, +z; #5 +x, 1/2−y, −1/2+z; #6 +x, 1/2−y, −1/2+z.

2.2 Magnetic properties

The temperature-dependent direct current magnetic susceptibilities of 1 were measured from T=2 to 300 K in a magnetic field of 1 kOe (1 kOe=7.96×104 A m−1). As is shown in Fig. 2, the χMT value at room temperature for 1 is 3.14 cm3 K mol−1, which is clearly larger than the spin-only value (1.875 cm3 K mol−1) for a Co(II) cation (S=3/2 and g=2), implying a significant orbital contribution to the magnetic moment. Upon cooling, the χMT product first decreases smoothly to a minimum of 2.35 cm3 K mol−1 at 10 K, suggesting a dominant antiferromagnetic exchange coupling between the magnetic centers and/or a significant spin-orbit coupling. However, the χMT value rises abruptly to a high maximum of 3.33 cm3 K mol−1 at 2 K, which is probably due to ferromagnetic behavior or a spin canting response that evolves into long-range ordering. Considering the homo-spin character of the compound, the ferromagnetism should be of topological origin.

Fig. 2: Plot of χMT versus T for 1.
Fig. 2:

Plot of χMT versus T for 1.

As is shown in Fig. 3, the rise of magnetization upon increasing the field to 1 T is much more rapid than expected for isolated Co(II) systems, confirming the presence of ferromagnetic coupling between Co(II) ions. The magnetization increases slowly with a value of 2.32 Nβ at H=5 T and T=2.0 K and does not reach a saturation plateau (Msat=3.3 Nβ). In addition, under the oscillating field of 2 Oe and the frequencies of 1000 Hz, the alternating current (AC) magnetic susceptibility experiments with a 0 Oe static field are determined in the temperature range of 2–15 K, and no out-of phase (χM) signals are observed before reaching the temperature drop to 2 K (Fig. 4).

Fig. 3: Plot of magnetization versus H for 1.
Fig. 3:

Plot of magnetization versus H for 1.

Fig. 4: AC magnetic susceptibility measurements for 1 in a 0 Oe static field.
Fig. 4:

AC magnetic susceptibility measurements for 1 in a 0 Oe static field.

3 Conclusion

In the present work, a new Co(II) CP with the 2-(4-pyridyl)-terephthalate ligand has been assembled and structurally and magnetically characterized. The Co(II) centers, which present distorted octahedral coordination geometry, are bridged by the ligands to construct the bilayered structure. Magnetic analyses have revealed that 1 features dominant antiferromagnetic exchange coupling between the magnetic centers at high temperature, whereas the ferromagnetic component only appears at low temperatures due to a spin reorientation. Unfortunately, the performances of slow magnetic relaxation and long-range magnetic ordering are absent in the system. The impetus for the case presented herein is to exploit the potential of pyridyl-carboxylate ligands in the synthesis of Co(II)-based multifunctional compounds; further research is underway.

4 Experimental section

4.1 Materials and physical measurements

All chemicals and solvents were of reagent grade, obtained from commercial sources without further purification. Elemental analysis (C, H, N) was implemented on Perkin Elmer 2400 CHN elemental analyzer. Magnetic measurements were accomplished on polycrystalline samples (18.49 mg for 1) using a Quantum Design MPMS-XL7 superconducting quantum interference device magnetometer (restrained in eicosane to prevent torquing under high fields). The measured magnetic data were corrected for the diamagnetism of the constituent atoms using Pascal’s constants.

4.2 Synthesis of [Co(pta)(H2O)2]n (1)

A mixture of H2pta (0.15 mmol, 0.0365 g), CoCl2·6H2O (0.15 mmol, 0.0357 g), and 6 mL H2O was heated at 160°C for 3 days in a Teflon-lined stainless steel vessel and then cooled at a descent rate of 5 K h−1 to room temperature. Red block-like crystals of 1 were collected by filtration. Yield 48% (based on Co). – Analysis calcd. for C13H11CoNO6 (M=336.16): C 46.45, H 3.30, N 4.17; found C 46.40, H 3.27, N 4.13%.

4.3 Crystallographic data collection and structure refinement

A suitable single crystal of the compound was selected for indexing, and the intensity data were recorded on a Bruker Smart APEX II CCD diffractometer with graphite-monochromatized CuKα radiation (λ=1.54178 Å). Using Olex2 [26], the structure of 1 was solved with the ShelXT [27] structure solution program by using intrinsic phasing, and refined with the ShelXL [27] refinement package by least-squares minimization. All non-hydrogen atoms were refined anisotropically. All the hydrogen atoms were located in difference maps by the program Olex2. Basic information pertaining to the crystal parameters, data collection, and structure refinement is summarized in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 1914358 (1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Funding source: NSFC

Award Identifier / Grant number: 21863009

Award Identifier / Grant number: NZ15075

Award Identifier / Grant number: 2019107490041

Funding statement: This work was financially supported by the NSFC (21863009), the Natural Science Foundation of Ningxia Province (NZ15075), and the Undergraduate Innovative and Entrepreneurial Training Program of Ningxia Province (2019107490041).

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Received: 2019-05-20
Accepted: 2019-06-24
Published Online: 2019-07-13
Published in Print: 2019-08-27

©2019 Walter de Gruyter GmbH, Berlin/Boston

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