Synthesis, crystal structure, and magnetic properties of an azido-bridged Mn(II) complex [C₃H₅NH₃][Mn(N₃)₃]

2.1 Preparation and structure of the title complex

Under ambient conditions, the reaction of cyclopropylamine hydrochloride, MnCl₂·4H₂O and NaN₃ in an aqueous solution yielded the product [C₃H₅NH₃][Mn(N₃)₃]. Slow evaporation of the solvent led to the crystallization of the title compound. The structure of 1 was characterized by 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 2057 cm⁻¹ can be assigned to ν(N–N) stretching vibration of the N₃⁻ ligands.

Fig. 1:

Infrared spectrum of compound 1 recorded in a KBr pellet 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 Pbca with Z=8 (Table 1). Its asymmetric unit consists of one Mn^II ion, three N₃⁻ anions and one cyclopropylammonium (CPA) cation.

As shown in Fig. 2, the Mn^II cations are connected by six N₃⁻ anions to form a three-dimensional [Mn(N₃)₃]_nⁿ⁻ network, and CPA cations reside in the cavities of the Mn²⁺–N₃⁻ network for charge balance, giving rise to an ABO₃-type perovskite-like compound of distorted MnN₆ octahedra, in which the cations reside within the well-matched cage-like host framework to form a host-guest compound. The Mn−N distances in the ab plane are between 2.177 and 2.276 Å, and the angles N–Mn–N are between 83.22° and 95.37°. Two nitrogen atoms contributed by two N₃⁻ anions in the c direction have a Mn−N distance of 2.265 or 2.221 Å and the angle of N(4)–Mn–N(7) is 176.7° (Fig. 3, Table 2). All Mn²⁺ ions are connected by the EE azide anions to form the 3D architecture (Fig. 2). The coordination geometry of the Mn atoms in the crystal structure is a distorted octahedron (Fig. 3). The ammonium groups of the cations form N−H···N hydrogen bonds to the [Mn(N₃)₃]_nⁿ⁻ network with donor–acceptor distances, as shown in Fig. 2.

Fig. 2:

The perovskite-related structure of 1 including hydrogen bonding interactions. Parts of the hydrogen atoms are omitted for clarity.

Fig. 3:

Coordination environments of 1 including the atom numbering. Symmetry codes: ^ax+1/2, y, −z+1/2; ^b−x+1/2, y+1/2, z; ^cx−1/2, −y+3/2, −z+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. 4, the χ_MT value at room temperature is 3.613 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. 4, the χ_MT value rises slowly with the temperature rising from 1.8 to 300 K. At 300 K, the χ_MT value reaches the maximum 3.613 cm³ K mol⁻¹. At 2.0 K, the field dependence of the magnetization of complex 1 was measured, as shown in Fig. 5. With the increase of the magnetic field, the value of M is also increasing and is far from saturation. At the highest magnetic field of 70 kOe, the value of M reaches 0.30 μ_B, much less than the common value of 2–3 μ_B for a Mn²⁺ center (at 2 K, the effective spin is S=5/2) with a common value of g (1 kOe=7.96×10⁴ A m⁻¹).

Fig. 4:

Temperature-dependent magnetic susceptibility of 1 measured at 1000 Oe. (The red line represents its linear fit curve from 150 to 300 K).

Fig. 5:

Magnetization isotherm of 1 at 2 K.

Between 150 and 300 K, the magnetic susceptibilities can be fitted to the Curie-Weiss law [χ_M=C/(T–θ)] with θ=−120 K. The pattern of the χ_MT curves in the high-temperature section and the negative Weiss constant θ are indicative of an antiferromagnetic coupling. As a versatile bridging ligand, the azide anion can bind to transition metal atoms with different coordination modes, such as μ-1,1[(end-on)(EO)] and μ-1,3[(end-end)(EE)], which can allow for the assembly of complexes with a wide range of magnetic behavior. Generally, azide bridges can transmit ferromagnetic couplings in the EO coordination, and antiferromagnetic couplings in the EE coordination [21], [22]. For compound 1, antiferromagnetic couplings are transmitted by the azide anions in the EE coordination.

To further clarify the magnetic properties of 1, the zero-field-cooled (ZFC) and field-cooled (FC) profiles were measured under a field of 10 Oe (Fig. 6). The deviation of the ZFC profile from the FC profile at about 70 K manifests the existence of magnetic phase transformation, revealing the long distance ordering and antiferromagnetism for 1.

Fig. 6:

The temperature dependence of the FC and ZFC magnetization of 1 under a field of 10 Oe.

4.1 Materials and measurements

All starting reagents and solvents 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. X-ray data for the title complex were collected on a Bruker Smart APEX II diffractometer. The IR spectrum was recorded in the range of 4000–400 cm⁻¹ on a Tensor 27 OPUS (Bruker) FT-IR spectrometer from KBr pellets. Magnetic measurement of the powder sample of the title complex was performed on a Quantum Design SQUID VSM magnetometer.

4.2 Synthesis of the title complex

MnCl₂·4H₂O (1 mmol, 0.189 g) in 3 mL methanol was slowly added to a methanol solution (12 mL) of sodium azide (5 mmol, 0.325 g) and cyclopropylamine hydrochloride (5 mmol, 0.467 g). The resultant clear solution was allowed to stand at room temperature. Three weeks later, pale-green block-shaped crystals of complex 1 were deposited from the solution in about 5% yield based on Mn. – Analysis for MnC₃H₈N₁₀: calcd. C 15.07, H 3.37, N 54.93; found C 14.57, H 3.07, N 53.53. – FT-IR (KBr, cm⁻¹): 3373(m), 2057(s), 1627(m), 1016(m), 614(m).

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 (0.18×0.12×0.12 mm³) were collected on a Bruker Smart APEX II diffractometer with graphite-monochromatized MoK_α radiation (λ=0.71073 Å) by using a φ-ω scan mode at 296(2) K. An absorption correction was applied using the program SADABS [23]. The structure was solved by direct methods [24] with SHELXTL [25], [26] and refined by full matrix least-squares techniques on F² with the SHELXTL program. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated geometrically.

CCDC 1520560 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.