A Mn(II) complex with an amide-containing ligand: synthesis, structural characterization, and magnetic properties

2.1 Structural description of [Mn₂(L)(H₂O)₃]·(H₂O)_4.8 (1)

The single-crystal X-ray diffraction analysis has shown that complex 1 consists of a 3D framework in the triclinic crystal system with space group P1̅ and Z=2 (Table 1). The asymmetric unit of 1 contains two Mn(II) cations, one anionic L⁴⁻ ligand, three coordinated and 4.8 interstitial water molecules (Fig. 1a). Each manganese cation is six-coordinated and exhibits octahedral coordination geometry [MnO₆]. The bond distances around the manganese cations are from 2.209(3) to 2.234(2) Å for Mn(1) and from 2.106(3) to 2.333(2) Å for Mn(2); the bond angles are in the range of 58.68(9)–172.70(10)° for Mn(1) and in the range of 57.18(8)–168.24(9)° for Mn(2) (Table 2). The L⁴⁻ ligand contains four carboxylate groups, which adopt μ₁-η¹:η⁰-monodentate, μ₁-η¹:η¹-chelating, μ₂-η¹:η¹-bridging, and μ₂-η²:η¹-chelating/bridging coordination modes, respectively. Interestingly, one O atom of an amide moiety also takes part in coordination (Scheme 1). Two carboxylate groups link two Mn(II) centers to form a secondary building unit (SBU) [Mn₂(COO)₂] with the Mn···Mn distance of 3.60 Å, which is shorter than the sum of two van der Waals radii (3.94 Å) [7]. The interconnections of the L⁴⁻ and the SBUs are repeated infinitely to yield chain structures (Fig. 1b). Adjacent chains are further linked to fabricate the 3D framework architecture through the coordination of the μ₁-η¹:η⁰-monodentate carboxylate and the amide O atoms (Fig. 1c). The topological analysis was carried out to get insight into the structure of 1. Each SBU [Mn₂(COO)₂] is neighbored by five L⁴⁻ ligand and thus can be treated as a five-connector, while each L⁴⁻ ligand connects five SBUs and can be viewed as a five-connector node. Therefore, the resulting structure of 1 can be simplified as a uninodal five-connected 3D bnn framework with (4⁶.6⁴) topology (Fig. 1d) [8].

Fig. 1:

(a) The coordination environment of the Mn(II) ions in 1 with ellipsoids drawn at the 30% probability level. The hydrogen atoms are omitted for clarity; (b) view of the chain structure in 1; (c) view of the 3D architecture of 1; (d) view of the uninodal five-connected 3D bnn framework of 1 with (4⁶.6⁴) topology.

2.2 PXRD and thermogravimetric analysis measurements of complex 1

The phase purity of 1 could be proven by powder X-ray diffraction (PXRD) analysis. As shown in Fig. 2, the pattern of the bulk sample was in agreement with the pattern simulated from the single crystal data.

Fig. 2:

The powder X-ray diffraction pattern of complex 1.

Thermogravimetric analysis (TGA) was carried out for complex 1, and the result is shown in Fig. 3. A continuous weight loss (17.4%) in the temperature range of 95°C–188°C, corresponding to the release of lattice and coordinated water (calc. 17.24%), and the decomposition of the residue can be observed from 311°C.

Fig. 3:

The thermogravimetric analysis curve of complex 1.

2.3 Magnetic properties

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 (1 kOe=7.96×10⁴ A m⁻¹). The χ_M, χ_M⁻¹, and χ_MT vs. T curves for 1 are shown in Fig. 4. The value of χ_MT at 300 K is 5.56 emu K mol⁻¹ which is larger than the value expected for magnetically isolated Mn(II) atom (4.38 emu K mol⁻¹, g=2.0) due to spin-orbit coupling, indicating a significant orbital contribution [9]. The temperature dependence of χ_M⁻¹ above T=50 K obeys the Curie-Weiss equation of χ_M⁻¹=(T–θ)/C with the Curie-Weiss constants C=5.74 cm³ mol⁻¹ K, θ=–9.02 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 [10].

Fig. 4:

Temperature dependences of magnetic susceptibility: (a) of χ_M and χ_MT for 1; (b) of χ_M⁻¹ for 1. The red solid line represents the fitted curve.

In order to estimate the strength of the magnetic interactions in 1, the following equation was used [11]: χ_MT=Aexp(–E₁/kT)+Bexp(–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=5.84 cm³ mol⁻¹ K and E₁/k=11.81 K agree with those given in a previous report [12]. The value of –E₂/k=–0.529 K, corresponding to J=–1.06 K, further proved that antiferromagnetic interactions exist between neighboring Mn(II) ions in 1 [13].

3.1 Preparation of [Mn₂(L)(H₂O)₃]·(H₂O)_4.8 (1)

The mixture of MnCl₂·4H₂O 0.20 mmol (39.6 mg), H₄L (56.8 mg, 0.1 mmol), and 0.5 mL aqueous tetrabutylammonium hydroxide solution (10%, w/w) in 10 mL H₂O was sealed in a 16 mL Teflon-lined stainless steel container and heated at 120°C for 3 days. Then the oven was cooled down at a rate of 10°C h⁻¹. Colorless needle crystals of 1 were obtained with an approximate yield of 40% based on H₄L. –C₃₀H_31.6N₂O_17.8Mn₂ (814.48): calcd. C 44.24, H 3.91, N 3.44; found C 44.02, H4.19, N 3.65%.–IR (KBr pellet, cm⁻¹): ν=3462 (m), 1622 (s), 1548 (s), 1533 (m), 1478 (s), 1455 (s), 1416 (s), 1356 (s), 1243 (s), 1192 (s), 1101 (s), 943 (s), 877 (m), 734 (m), 657 (m), 570 (m).

3.2 X-ray structure determination

The crystallographic data collection was carried out using a Rigaku Rapid II imaging plate area detectorusing graphite-monochromatized MoKα radiation (λ=0.71073 Å) at T=200 K. The diffraction data were integrated by using the program Saint [14],which was also used for the intensity corrections for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the program Sadabs [15]. The structure was solved by direct methods, and all nonhydrogen atoms were refined anisotropically on F² by the full-matrix least-squares techniques using the Shelxl-97crystallographic software package [11], [16], [17]. All hydrogen atoms at C atoms were generated geometrically. The hydrogen atoms at N1, O11, O13, and O14 could be found at reasonable positions in the difference Fourier maps and located there, while the other hydrogen atoms of water could not be located and thus were not included in the refinement. The details of crystal parameters, data collection, and refinements are summarized in Table 1; selected bond lengths and angles are listed in Table 2.

CCDC 1874703 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.