Synthesis, crystal structure and magnetic properties of the complex [C(NH₂)₃]₂[Mn(N₃)₄] with a polynuclear azido-bridged chain anion

2.1 Preparation and structure of the title complex

Under ambient conditions, the reaction of guanidine hydrochloride, MnCl₂·4H₂O and NaN₃ in a methanol solution yielded the product [C(NH₂)₃]₂[Mn(N₃)₄]. Slow evaporation of the resulting solution led to the crystallization of the title compound. The structure of 1 was characterized by infrared (IR) spectroscopy, elemental analysis and single-crystal X-ray diffraction. The infrared spectrum of 1 was consistent with its formulation (Fig. 1). Strong IR bands centered at 2091 and 2039 cm⁻¹ can be assigned to ν(N–N) stretching vibrations of the N₃⁻ ligands.

Fig. 1:

Infrared spectrum of compound 1 in KBr pellets recorded at room temperature.

A summary of the crystallographic data and the structure refinement is given in Table 1, and selected bond lengths and angles are listed in Table 2. The compound crystallizes in the orthorhombic crystal system with space group Pmna with two formula units MnN₁₂(CH₆N₃)₂ in the unit cell (Table 1). The asymmetric unit thus consists of 1/4 of a Mn^II ion, one N₃⁻ anion and 1/2 guanidinium cation.

As shown in Fig. 2, the Mn^II cations are connected by four N₃⁻ anions in the ab plane to form a [Mn(N₃)₄]_n²⁻ chain, and two guanidinium cations are placed for charge balance between the chains, giving rise to Mn-azido chains of slightly distorted MnN₆ octahedra separated by guanidinium cation bilayers to form an alternating inorganic-organic chain and layer structure. The Mn–N distances in the ab plane are 2.215 Å, and the cis angles N–Mn–N are close to 90°. Two nitrogen atoms contributed by two N₃⁻ anions in the c direction have a Mn−N distance of 2.228 Å and the trans angles N–Mn–N are 180° by symmetry. In the c direction, one N atom of the N₃⁻ anion is connected with Mn^II cations, and the other terminal N atom is unconnected. However, in the ab plane, N₃⁻ anions are connected with Mn^II cations in EE-N₃ coordination modes (Fig. 2, Table 2). The coordination geometry of the Mn atoms in the crystal structure is a weakly distorted octahedron (Fig. 3). The Mn-azido chains are separated by guanidinium cation bilayers (Fig. 2). The two nearest neighbor Mn atoms are joined by N₃⁻ anions, giving rise to a Mn-azido chain, with Mn···Mn distances of 5.292 Å (Fig. 3). The amino groups of the cations form N−H···N hydrogen bonds to the [Mn(N₃)₄]_n²⁻ chains with donor-acceptor distances as shown in Fig. 4.

Fig. 2:

Alternating inorganic-organic chain structure of 1 including the atom numbering. Parts of hydrogen atoms are omitted for clarity.

Fig. 3:

Coordination environments of 1 including the atom numbering. Symmetry codes: ^a–x, –y, –z+1; ^b–x, y, z; ^cx, –y, –z+1; ^d–x+1, y, z; ^ex, –y+1, –z+1. ^f–x, 1–y, –z+1.

Fig. 4:

Hydrogen bonding interactions between the organic and inorganic components in 1.

2.2 Magnetic properties

A variable-temperature magnetic susceptibility measurement was performed on a powder sample of complex 1 from 2 to 300 K. As shown in Fig. 5, the χ_MT value at room temperature is 2.378 cm³ K mol⁻¹, which is smaller than the value (4.375 cm³ K mol⁻¹) expected for one magnetically isolated high-spin Mn(II) ion with S=5/2, revealing the existence of antiferromagnetic interactions transmitted through the azide bridges. As shown in Fig. 5, the χ_MT value rises slowly with the temperature 1.8–300 K. At 300 K, the χ_MT value reaches the maximum 2.378 cm³ K mol⁻¹. Between 150 and 300 K, the magnetic susceptibilities can be fitted to the Curie–Weiss law with θ=–12.5 K. The pattern of the χ_MT curves in the high temperature section and the negative Weiss constant θ value are indicative of an antiferromagnetic coupling.

Fig. 5:

Temperature-dependent magnetic susceptibility of 1 measured at 1 kOe (1 kOe=7.96×10⁴ A m⁻¹).

At 2.0 K, the field-dependence of the magnetization of complex 1 was measured, as shown in Fig. 6. At the highest magnetic field of 70 kOe, the value of M reaches 0.14 μ_B (1 kOe=7.96×10⁴ A m⁻¹). With the increase of the magnetic field, the value of M is also increasing and is far from saturation.

Fig. 6:

Magnetization isotherm of 1 at 2 K.

4.1 Materials and measurements

All starting reagents and solvents (Energy Chemical Company, China) employed for synthesis were commercially available and used without further purification. Elemental analyses (C, H, and N) were performed on a Model 240 Perkin–Elmer elemental analyzer (Perkin-Elmer, USA). The IR spectrum was recorded in the range of 4000~400 cm⁻¹ on a Tensor 27 OPUS (Bruker, Germany) FT-IR spectrometer from KBr pellets. Magnetic measurements on the powder sample of the title complex were performed on a Quantum Design SQUID VSM magnetometer (Quantum Design, USA).

4.2 Synthesis of the title complex

MnCl₂·4H₂O (1 mmol, 0.198 g) in 2 mL methanol was slowly added to a methanol solution (5 mL) of sodium azide (5 mmol, 0.325 g) and guanidine hydrochloride (5 mmol, 0.478 g). The resultant clear solution was allowed to stand at room temperature. One week later, pale yellow block-shaped crystals of complex 1 were deposited from the solution in ca. 10% yield based on Mn. – Analysis for C₂H₁₂MnN₁₈: calcd. C 7.00, H 3.52, N 73.47; found C 6.90, H 3.31, N 73.12. – FT-IR (KBr, cm⁻¹): 3575.8(w), 3425(m), 3340.1(m), 3153.5(m), 2091(s), 2039(s), 1630.6(s), 1555.3(m), 1332.19(m), 1006.65(w), 480(s).

General Caution! Although the samples never exploded during the experiments, metal azide complexes are potentially explosive. Only a small amount of material should be prepared, and it should be handled with caution.

4.3 X-ray crystallography

Single crystal data of complex 1 (0.29 mm×0.20 mm× 0.15 mm) was collected on a Bruker Smart APEX II diffractometer with graphite-monochromatized MoK_α radiation (λ=0.71073 Å) by using an φ–ω scan mode at 296(2) K. An absorption correction was applied using the SADABS program [26]. The structure was solved by Direct Methods [27] with the program SHELXTL [26] and refined by full-matrix least-squares techniques on F² with the SHELXTL program [26], [28]. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated geometrically.

CCDC 1503952 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.