A Mn(II) coordination polymer from a polycarboxylate-containing ligand: synthesis, structural characterization, and properties

2.1 Preparation

MnCl₂·4H₂O reacts with the neutral, four-fold protonated pyridine-3,5-dicarbox(3,5-dicarboxylatoanilide) (H₄L) in the presence of KOH under the hydrothermal condition at 120°C to produce the complex [Mn₂(L)(H₂O)₂]·H₂O (1) which is stable in air.

2.2 Structural description of [Mn₂(L)(H₂O)₂]·H₂O (1)

Complex 1 was obtained, which is a 3D framework based on Mn(II) and the four-fold deprotonated ligand L⁴⁻. It crystallizes in the monoclinic space group C2/c (Table 1). The asymmetric unit of 1 contains one Mn(II), half of L⁴⁻, one coordinated water, and 1/2 molecule of lattice water. Mn(II) is pentacoordinated (Fig. 1a) with distorted trigonal bipyramid coordination geometry, by four carboxylate O atoms and one water O atom. The bond distances around Mn(II) are from 2.0641(15) to 2.2106(19) Å; the coordination bond angles are in the range of 81.91(7) to 165.01(7)° (Table 2). In complex 1, the H₄L was wholly deprotonated as a centrosymmetric L⁴⁻ anion (Scheme 1). Two different carboxylate groups all exhibit μ₂-η¹:η¹-bridging coordination mode, but the pyridyl N atom is free of coordination. So each L⁴⁻ ligand links eight metal ions as a μ₈-bridge, and each Mn(II) is coordinated by four L⁴⁻. This kind of interconnection extends infinitely to form a 3D framework (Fig. 1c). If only half of centrosymmetric L⁴⁻ is considered, a 2D network can be viewed (Fig. 1b). Topological analysis shows that the architecture of 1 can be simplified as a binodal (4,8)-connected 3D framework with flu (4¹².6¹².8⁴)(4⁶)₂ topology (Fig. 1d) [7].

Scheme 1:

Coordination modes of H₄L appearing in complex 1.

Fig. 1:

(a) The coordination environment of Mn(II) in complex 1 with the ellipsoids drawn at the 30% probability level. Hydrogen atoms and lattice water molecules are omitted for clarity. (b) View of the 2D network in 1. (c) View of the 3D architecture of 1. (d) Topological view of the 3D framework of 1.

2.3 Thermal stability of complex 1

Thermogravimetric analysis (TGA) was carried out for complex 1, and the result is shown in Fig. 2. There is a weight loss of 3.0% in the temperature range of 90–120°C which corresponds to the liberation of lattice water (calcd 2.8%). A weight loss of 5.6% from 150 to 220°C is assigned to the release of coordinated water (calcd 5.5%), and the decomposition of the residue can be observed at 420°C.

Fig. 2:

TGA curve of complex 1.

2.4 Magnetic properties of complex 1

The Mn(II) cations in 1 are bridged by carboxylate, which may mediate magnetic interactions [8]. Thus, the temperature dependence of the magnetic susceptibility of 1 was investigated from 300 to 1.8 K with an applied magnetic field of 2000 Oe. The χ_M, χ_M⁻¹, and χ_MT vs. T curves 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 7.66 emu K mol⁻¹ which is larger than the value expected for a magnetically isolated Mn(II) cation (4.38 emu K mol⁻¹, g=2.0) due to spin–orbital coupling, indicating a significant orbital contribution [8]. The temperature dependence of χ_M⁻¹ above 50 K obeys the Curie–Weiss equation of χ_M⁻¹=(T–θ)/C with the constants C=8.27 cm³ mol⁻¹ K and θ=−18.4 K.

The negative value of θ and the shape of the χ_MT vs. T curve suggest that there may exist antiferromagnetic interactions between the neighboring Mn(II) centers [9]. In order to estimate the strength of the magnetic interactions in 1, the following equation was used [10]:

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=8.34 cm³ mol⁻¹ K and E₁/k=10.6 K agree with those given in a previous report [10]. The value of −E₂/k=−1.68 K corresponding to J=−3.76 K further proves that antiferromagnetic interactions exist between neighboring Mn(II) ions in 1 [11].

3.1 Preparation of [Mn₂(L)(H₂O)₂]·H₂O (1)

The reaction mixture of H₄L (0.05 mmol, 24.7 mg), MnCl₂·4H₂O (0.10 mmol, 14.3 mg), 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, colorless block crystals of 1 were collected by filtration and washed with water and ethanol several times (yield 50% based on H₄L). – C₂₃H₁₇N₃O₁₃Mn₂ (653.28): calcd. C 42.29, H 2.62, N 6.43; found C 42.50, H 2.42, N 6.26%. – IR (KBr pellet, cm⁻¹): ν=3459 (m), 1621 (s), 1570 (s), 1468 (s), 1440 (s), 1417 (s), 1359 (s), 1238 (m), 1139 (m), 1061 (s), 879 (m), 762 (s), 733 (m), 647 (m), 586 (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 T=293(2) K. The diffraction data were integrated by using the program Saint [12], which was also used for the intensity corrections for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the program Sadabs [13]. 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 Shelxs/l-97 crystallographic software package [14], [15], [16], [17], [18]. In 1, all hydrogen atoms at C atoms were generated geometrically, while the ones of O1, O10, and N1 were found at reasonable positions in the difference Fourier maps and located there. The bond distance of N(1)–H(1) was restricted to 0.87±0.01 Å for a more reasonable length. The details of crystal parameters, data collection, and the graphite refinement are summarized in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 1501754 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.