A 3D supramolecular architecture based on 2,2′-oxybis(benzoic acid) and trans-1,2-bis(4-pyridyl)ethylene as ligands for Co(II)

2.1 Materials and chemical analysis

The H₂oba and the bpe ligand were purchased from Alfa Aesar Company; all other reagents and solvents employed were commercially available and used without further purification. The C, H, and N microanalyses were carried out with a Perkin–Elmer 2400 elemental analyzer. The IR spectra were recorded with a Shimadzu Prestige-21 spectrometer using the KBr pellet technique. Thermogravimetric (TG) analyses were conducted with a Netzsch STA 449C microanalyzer under normal atmosphere at a heating rate of 10°C min⁻¹. Powder X-ray diffraction (PXRD) patterns were recorded on a Shimadzu XRD-7000 instrument. The magnetic susceptibilities were obtained on crystalline samples using a Quantum Design MPMS SQUID magnetometer.

2.2 Synthesis of {[Co(oba)(bpe)(H₂O)]·1/2bpe}_n (1)

A mixture of 2,2′-oxybis(benzoic acid) (0.0258 g, 0.1 mmol), trans-1,2-bis(4-pyridyl)ethylene (0.0182 g, 0.1 mmol), CoCl₂·6H₂O (0.0238 g, 0.1 mmol), CH₃CH₂OH (3 mL), and H₂O (6 mL) was stirred and heated in a 20 mL Teflon-lined autoclave at 433 K for 4 days, followed by slow cooling (5 K h⁻¹) to room temperature. The resulting mixture was washed with H₂O, and red block crystals were collected and dried in air. Yield: 47% (based on Co). – Elemental analysis C₃₂H₂₅O₆N₃Co (%): calcd for C 63.37, H 4.15, N 6.93; found C 63.57, H 4.23, N 6.86. – IR (KBr, cm⁻¹): 3481 (s), 3024 (s), 2347 (vs), 1626 (s), 1441 (w), 1393 (vs), 1133 (m), 789 (s), 724 (w), 678 (vs), 563 (vs).

2.3 X-ray crystallographic studies

Diffraction data for compound 1 were collected at 296(2) K on a Bruker SMART APEX-II CCD diffractometer employing graphite-monochromated MoK_α radiation (λ=0.71073 Å). A semi-empirical absorption correction was applied using the program Sadabs [14]. The structure was solved by Direct Methods and refined by full-matrix least squares on F² using the programs Shelxs-2014 and Shelxl-2014, respectively [15], [16], [17]. Non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in geometrically calculated positions. H atoms bonded to C atoms were placed in calculated positions and treated using a riding-model approximation, with C–H=0.93 Å and U_iso(H)=1.2U_eq(C) for aromatic H atoms. Water H atoms were located in a difference Fourier map and refined with a restraint of O–H=0.85 and with U_iso(H)=1.5U_eq(O). Similarity restraints were applied to the C–C distances involving chemically equivalent but disordered C atoms (e.g. the disordered C32 and C32A were restrained by ISOR and DFIX, and an error/esd point of [0 1 0] diffraction was omit). Crystallographic data for 1 are listed in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 1515801 (1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 Crystal structures of 1

Single-crystal X-ray diffraction analysis suggests that complex 1 consists of one Co^II ion, one oba²⁻ anion, one bpe molecule, one coordinated water, and a half free bpe molecule. Each Co^II center is six-coordinated by two pyridyl nitrogen donors from two different bpe ligands and four oxygen atoms coming from one oba²⁻ ligand and one coordinated water molecule [Co–N/O=2.011(2)–2.2594(19) Å], forming a distorted CoN₂O₄ octahedral geometry (Fig. 1). The Co–O_{(carboxylate)} and Co–N_(bipy) bond lengths are in agreement with those in carboxylate- and bipy-containing cobalt(II) complexes [18]. The O/N–Co–O/N bond angles are in the range of 78.49(7)°–178.22(9)°. Adjacent Co(II) ions are bridged by bpe ligands to form an infinite polymeric chain running along the crystallographic c axis (Fig. 2). The oba²⁻ ligands adopt a μ₃-tridentate coordination mode, the two phenyl rings being severely twisted, with a dihedral angle of 80.26°. All oba²⁻ ligands are terminal ligands and do not contribute to the chain structure. There are three kinds of hydrogen bonding present in this structure: (i) hydrogen bonds between the coordinated water and carboxylate O atoms of the oba²⁻ anion with the O6–H6B···O1 distance of 2.752(3) Å; (ii) hydrogen bonds between the coordinated water O atoms and N atoms of the free bpe ligand with the O6–H6···N3 distance of 2.754(3) Å; (iii) hydrogen bonds between the C atoms of the bpe ligand and the carboxylate O atoms of the oba²⁻ anion [C18–H18···O5 distance: 3.307(4) Å and C30–H30···O5 distance: 3.205(4) Å] (see Fig. 3 and Table 3). Due to the strong interchain O–H···O hydrogen bonds, the adjacent chains are connected to generate a double-chain structure (Fig. 2). These double chains are joined by O–H···N hydrogen bonding to produce a 2D supramolecular architecture (Fig. 4). Furthermore, through intermolecular C–H···O hydrogen bonds, the layers are linked into a 3D supramolecular structure (Fig. 5).

Fig. 1:

Coordination environment of Co(II) in complex 1. Hydrogen atoms are partially omitted for clarity (symmetry code: #1 x+1, y, z+1).

Fig. 2:

The polymeric chain (top) and double-chain structure (bottom) of complex 1.

Fig. 3:

Hydrogen bonding interactions in complex 1; the symmetry code is as in Table 3.

Fig. 4:

The 2D supramolecular framework of complex 1.

Fig. 5:

The 3D supramolecular structure of complex 1.

3.2 PXRD and TG analysis

PXRD was used to confirm the phase purity of the bulk material of 1 at room temperature (Fig. 6). The experimental diffraction feature peaks of the bulk samples are almost fully consistent with the simulated patterns, indicating an almost pure product of 1. To examine the thermal stability of complex 1, TG analyses were carried out between 20°C and 700°C (Fig. 7). The sample was heated up under a static air atmosphere at a heating rate of 10°C min⁻¹. The TG curve indicates that the weight loss of the complex can be divided into two steps. The first weight loss of 18% from 90°C to 180°C is due to loss of free bpe molecules and coordinated water (calcd 18.0%). The second weight loss of 72% in the temperature range 240°C–440°C corresponds to the release of the remainder ligands (calcd. 72.3%). The final residue seems to be CoO, which has also been further confirmed by a PXRD pattern.

Fig. 6:

PXRD pattern of complex 1.

Fig. 7:

TG curve of complex 1.

3.3 Magnetic properties

The magnetic properties of complex 1 were investigated over the temperature range 5–300 K in a field of 10 kOe (1 kOe=7.96×10⁴ A m⁻¹). The magnetic susceptibility χ_MT versus T plot is shown in Fig. 8. The experimental χ_MT value at 300 K is 1.872 cm³ K mol⁻¹, somewhat less than the spin-only value (1.875 cm³ K mol⁻¹) expected for two isolated spin-only Co(II) ions (S=3/2). The χ_MT value of 1 remains almost constant from 300 to 150 K, and then decreases on further cooling, reaching a value of 0.638 cm³ K mol⁻¹ at 5 K. This behavior indicates a weak antiferromagnetic interaction between the Co(II) ions in the structure. The Co···Co distance through the bpe ligand bridge is 13.717 Å. The temperature dependence of the reciprocal susceptibility (1/χ_M) obeys the Curie–Weiss law above 25 K with θ=−1.36 K, C=1.64 and R=2.384×10⁻⁴ (R=∑[(χ_M)_obs–(χ_M)_calcd]²/∑[(χ_M)_obs]²). The values of θ indicate weak antiferromagnetic interactions between adjacent Co(II) (S=3/2) ions.

Fig. 8:

Thermal variation of χ_MT and 1/χ_M for complex 1 (○, χ_MT experimental values; Δ, 1/χ_M experimental values; and solid lines, 1/χ_M theoretical values).