A Co(II) complex based on a mixed N- and O-donor: synthesis, structural characterization, and properties

2.1 Preparation

Co(NO₃)₂·6H₂O reacts with 5,5′-(pyridine-3,5-dicarbonyl)bis(azanediyl)diisophthalic acid (H₄L) in the presence of KOH under hydrothermal conditions at 120°C to produce the complex [Co₃(HL)₂(H₂O)₄] (1), which is stable in air.

2.2 Structural description of [Co₃(HL)₂(H₂O)₄] (1)

Single crystal X-ray structural analysis revealed that 1 crystallizes in the triclinic space group P1̅, exhibiting a 2D network structure. As shown in Fig. 1a, with the atom numbering scheme, the asymmetric unit of 1 consists of two Co²⁺ cations (one of them is centrosymmetric), one anion HL³⁻, and two coordinating water molecules. The trinuclear unit of three carboxylate-bridged Co(II) cations can be regarded as a secondary building unit (SBU) [Co₃(COO)₄] with a Co···Co distance of 3.54 Å (Fig. 1b), which is shorter than the sum of van der Waals radii (3.84 Å) [11]. Co1 and Co2 are six-coordinated with distorted octahedral coordination geometries with [CoNO₅] and [CoO₆] donor sets, respectively. The bond distances range from 2.071(5) to 2.134(5) Å around Co1 and from 1.939(5) to 2.270(4) Å around Co2. The bond angles range from 82.4(2)° to 178.22(19)° around Co1 and from 57.42(16)° to 180° around Co2. The average bond lengths around Co1 and Co2 are 2.137 and 2.143 Å, respectively (Table 2). The H₄L was partially deprotonated to an HL³⁻ anion, and three carboxylate groups adopt μ₁-η¹:η⁰-monodentate, μ₂-η¹:η¹-bridging, and μ₂-η²:η¹-chelating/bridging coordination modes, respectively (Scheme 1). Each HL³⁻ links four of the above-mentioned SBUs [Co₃(COO)₄] and each SBU is bound by eight HL³⁻. This kind of connectivity repeats infinitely to yield a 2D network structure (Fig. 1c and d). Topology can be used to further analyze the structure of 1; each SBU is bound by eight HL³⁻ and thus treated as an 8-connected node; each HL³⁻ ligand links four SBUs as a 4-connected node. So the structure of 1 can be simplified as a binodal (4,8)-connected 2D network (Fig. 1e). The Point (Schläfli) symbol is (4²⁰.6⁸)(4⁶)₂ [12].

Fig. 1:

(a) The coordination environment of Co(II) cations in complex 1 with the ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity. (b) View of the SBU [Co₃(COO)₄] in 1. (c) View of the 2D structure of 1 along the c axis. (d) The network of 1 with polyhedral configuration. (e) Schematic illustration of the topology of 1.

2.3 IR and thermal stability of complex 1

The band at 1692 cm⁻¹ indicates the existence of carboxylic groups [12]. The split asymmetric and symmetric stretching bands of COO⁻ can be observed (1601, 1542 cm⁻¹ and 1429, 1367 cm⁻¹, respectively).

Thermogravimetric analysis (TGA) was carried out for complex 1, and the result is shown in Fig. 2. There is a weight loss of 5.7% within the temperature range of 140–200°C, corresponding to the release of coordinated water (calcd. 5.9%), and the subsequent decomposition of the framework takes places at 330°C.

Fig. 2:

TGA curve of complex 1.

2.4 Magnetic properties of complex 1

The metal atoms of Co(II) in 1 are bridged by a carboxylate group, which may mediate magnetic interaction [13]. Thus, the temperature dependence of the magnetic susceptibility of 1 was investigated from 300 to 1.8 K with an applied magnetic field of 2 kOe (1 kOe=7.96×10⁴ A m⁻¹). The curves of χ_M, χ_M⁻¹, and χ_MT versus T for 1 are shown in Fig. 3.

Fig. 3:

Temperature dependence of magnetic susceptibility χ_M, χ_M⁻¹, and χ_MT for 1. The solid lines represent the fitted curve.

The value of χ_MT at 300 K is 10.58 emu K mol⁻¹, which is larger than the value expected for a magnetically isolated ion due to the spin–orbital coupling (for a Co(II) atom 1.88 emu K mol⁻¹, g=2.0), indicating a significant orbital contribution [13]. As the temperature is lowered, the χ_MT value decreases smoothly to a value of 0.48 emu K mol⁻¹ at 1.8 K. χ_M increases from 0.035 emu K mol⁻¹ at 300 K to reach a peak of 0.24 emu K mol⁻¹ at 14 K, then decreases to 0.21 emu K mol⁻¹ at 6 K, and increases again to 0.27 emu K mol⁻¹ at 1.8 K. These magnetic behaviors may be attributable to the spin–orbit coupling of Co(II) [12].

The temperature dependence of χ_M⁻¹ above 50 K obeys the Curie–Weiss equation of χ_M⁻¹=(T–θ)/C with the Curie constants of C=11.8 cm³ mol⁻¹ K and θ=−40.4 K. The negative value of θ and the shape of the χ_MT versus T curve suggest that there may exist antiferromagnetic interactions between the neighboring Co(II) centers [14]. In order to estimate the strength of the magnetic interaction, the following equation was used [15]:

χMT=A exp(−E1/kT)+B exp(−E2/kT).

Here, A+B equals the Curie constant (C), and E₁, E₂ represent the “activation energies” corresponding to the spin–orbit coupling and the magnetic exchange interaction, respectively. The obtained values of A+B=11.9 cm³ mol⁻¹ K and E₁/k=13.2 K agree with those given in a previous report [15]. The value of −E₂/k=−1.06 K, corresponding to J=−2.12 K, further proves that an antiferromagnetic interaction exists between neighboring Co(II) cations [16].

3.1 Preparation of [Co₃(HL)₂(H₂O)₄] (1)

The reaction mixture of H₄L 0.05 mmol (24.7 mg), Co(NO₃)₂·6H₂O (29.1 mg, 0.1 mmol), and KOH (11.2 mg, 0.2 mmol) in 10 mL H₂O was sealed in a 16 mL Teflon-lined stainless steel container and heated at 120°C for 3 days. After cooling to room temperature, red block crystals of 1 were collected by filtration and washed with water and ethanol several times (yield 45% based on H₄L). – C₄₆H₃₂N₆O₂₄Co₃ (1229.57): calcd. C 44.93, H 2.62, N 6.83; found C 44.70, H 2.46, N 7.08%. – IR (KBr pellet, cm⁻¹): ν=3412 (m), 1692 (s), 1601 (s), 1542 (s), 1429 (s), 1367 (s), 1320 (s), 1281 (m), 1242 (s), 1188 (m), 1092 (m), 1118 (m), 1101 (m), 992 (m), 930 (m), 866 (m), 842 (s), 821 (m), 772 (s), 736 (s), 698 (m), 635 (m), 580 (m).

3.2 X-ray structure determination

The crystallographic data collection for complex 1 was carried out on a Bruker Smart ApexII CCD area-detector diffractometer using graphite-monochromatized MoK_α radiation (λ=0.71073 Å) at 293(2) K. The diffraction data were integrated by using the program Saint [17], which was also used for the intensity corrections for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the program Sadabs [18]. The structure of 1 was solved by Direct Methods, and all non-hydrogen atoms were refined anisotropically on F² by the full-matrix least-squares techniques using the Shelxl-97 crystallographic software package [19], [20], [21]. In 1, all hydrogen atoms at C atoms were generated geometrically, while the ones of O2, O11, O12, N2, and N3 were found at reasonable positions in the difference Fourier maps and located there. The details of crystal parameters, data collection, and the graphite refinement are summarized in Table 1; selected bond lengths and angles are listed in Table 2.

CCDC 1501753 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.