The guanidinium t-diaqua-bis(oxalato)chromate(III) dihydrate complex: synthesis, crystal structure, EPR spectroscopy and magnetic properties

2.1 IR and UV–Vis spectra of salt 1

The IR spectrum of 1 between 400 and 4000 cm⁻¹ is shown in Figure S1 (Supplementary Material, available online). The broad absorption bands around 3434 and 3286 cm⁻¹ are attributed to ν(N–H) and ν(O–H), respectively. The band centered at 1659 cm⁻¹ is due to ν(C=O), and the bands centered at 1393 and 1267 cm⁻¹ are attributed to ν(C–O) [23]. The bands centered at 900 and 814 cm⁻¹ are assigned to νC–C. The sharp bands observed at 480 and 408 cm⁻¹ are due to Cr–O vibrations [24].

The UV–Vis spectrum of 1 is shown in Figure S2 (Supplementary Material). It exhibits two bands in the visible region, one at 415 nm and the other at 561 nm due to d–d transitions, ⁴A_2g → ⁴T_1g(F) and ⁴A_2g → ⁴T_2g, respectively. These results are in accordance with reported values [21], [25], [26], [27].

2.2 Description of the crystal and molecular structure of salt 1

Single-crystal X-ray structural analysis has shown that crystals of compound 1 belong to the monoclinic system with the space group C2/c and Z = 4 (Table 1). As highlighted in Figure 1a, the structure of 1 consists of one trans-diaquabis(oxalato)chromate(III) [Cr(C₂O₄)₂(H₂O)₂]⁻ complex anion, one CH₆N₃⁺ guanidinium cation compensating the charge of the anion, and two uncoordinated water molecules. In the anionic complex, the six-coordinated Cr^III ion lies on a crystallographic inversion center and the coordination sphere involves four O atoms from two chelating oxalate dianions [Cr–O1, 1.9611(6) Å, Cr–O1ⁱ, 1.9611(6) Å, Cr–O4, 1.9624(5) Å, Cr–O4ⁱ, 1.9624(5) Å] and two O atoms of trans-related water molecules [Cr–O5, 1.9854(7) Å, Cr–O5ⁱ, 1.9854(7) Å]. The Cr–O_water distance [1.9854(7) Å] is longer than those of the Cr–O_oxalate bonds (Table 2). These parameters indicate a slightly elongated octahedron around the Cr^III ion. These bond lengths are comparable with the values reported for salts containing the [Cr(C₂O₄)₂(H₂O)₂]⁻ complex anion in trans configuration [14], [15], [16], [21], [22]. It is worth noting that the imidazolium salt (C₃H₅N₂)[t-Cr(C₂O₄)₂(H₂O)₂]·2H₂O [22] and the present guanidinium salt (CH₆N₃)[t-Cr(C₂O₄)₂(H₂O)₂]·2H₂O are isotypic and they exhibit very similar basic structures except for the distinction of cationic entities. In both salts, each complete cation is generated by a crystallographic twofold rotation axis, with a carbon atom lying on the rotation axis. The bond lengths [C3–N1 1.3166(9) Å, C3–N1ⁱⁱ 1.3166(9) Å, C3–N2 1.3263(17) Å] and angles [N2–C3–N1 119.85(6)°, N2–C3–N1ⁱⁱ 119.85(6)°, N1–C3–N1ⁱⁱ 120.30(12)°] (symmetry code: (ii) −x + 1, y, −z + 3/2) in the CH₆N₃⁺ guanidinium cation show no significant difference from those observed in other guanidinium salts [28], [29], [30].

Figure 1:

(a) Constitutive entities of 1 with atom numbering scheme. (b) Packing diagram of 1 highlighting pillars of the [Cr(C₂O₄)₂(H₂O)₂]⁻ complex anions and guanidinium CH₆N₃⁺ cations, with the next-neighbor cations rotated by an angle of 60° relative to each other.

The structural feature of focal interest in 1 is the formation of pillars of [Cr(C₂O₄)₂(H₂O)₂]⁻ complex anions and CH₆N₃⁺ guanidinium cations, with the next-neighbor cations rotated by an angle of 60° relative to each other (Figure 1b). The intermetallic Cr···Cr distances vary in the range of 6.4561(2)–9.8555(4) Å. The 3D supramolecular architecture of 1 (Figure 2, Table 3) is stabilized through a network of intermolecular hydrogen bonding interactions: N–H···O [2.867(10)–3.017(9) Å] hydrogen bonds linking guanidinium cations and [Cr(C₂O₄)₂(H₂O)₂]⁻ complex anions and O–H···O [2.643(10) Å] hydrogen bonds linking coordinated water molecules and crystallization water molecules.

Figure 2:

View of the three-dimensional supramolecular structure of 1. The dashed lines show hydrogen bonding interactions.

2.3 PXRD and thermogravimetric analysis of salt 1

The phase purity of 1 can be proven by powder X-ray diffraction (PXRD) analysis. As shown in Figure S3 (Supplementary Material), the PXRD pattern of the bulk sample is in good agreement with the pattern simulated from the single-crystal data.

Results of the thermal analyses (Thermogravimetric analysis, TGA and Differential scanning calorimetry, DSC) of a powder sample of 1 is depicted in Figure 3. The TGA curve shows three main decomposition steps. In the range of 100–140 °C, a first weight loss of 10.1%, associated with an endothermic effect, corresponds to the release of two water molecules of crystallization (calcd. 10.0%) to give the anhydrous derivative (CH₆N₃)[Cr(C₂O₄)₂(H₂O)₂]. In the range of 200–240 °C, a second weight loss of 11.7%, associated with an endothermic effect, is ascribed to the decomposition of the guanidinium cation (calcd. 11.7%), leading to the release of CH₂N₂ [31] and formation of the intermediate (NH₄)[Cr(C₂O₄)₂(H₂O)₂]. In the range of 340–360 °C, the latter intermediate undergoes a weight loss of 66.2% associated with an exothermic effect, leading to the decomposition of the framework (calcd. 66.1%) and formation of a dark-green residue which has been proven to be Cr₂O₃.

Figure 3:

TGA (blue) and DSC (red) diagrams for 1.

2.4 EPR spectrum and magnetic properties

The experimental EPR spectrum of a powdered sample of 1 with the resulting parameters measured at room temperature (blue curve) and a simulated EPR spectrum (red curve) are displayed in Figure 4. The two spectra are practically superimposable. The anisotropy of the g parameters along the three principal axes are: g_x = 3.70, g_y = 3.01 and g_z = 2.18. The g anisotropy with three different Eigen values is in accordance with a distortion of the octahedral environment around the chromium(III) complex in 1 [32], [33], [34].

Figure 4:

Experimental (blue) and simulated (red) X-band EPR spectra for 1.

Magnetic susceptibilities of 1 were measured from 300 to 2 K in a magnetic field of 0.1 T. The temperature dependences of X_MT and X_M⁻¹ (χ_M is the molar magnetic susceptibility) are depicted in Figure 5.

Figure 5:

Temperature dependences of X_MT and X_M⁻¹ for 1.

At room temperature, the value of X_MT is ca. 1.74 cm³ K mol⁻¹ which is not far from the expected value (1.87 cm³ K mol⁻¹) for magnetically isolated Cr(III) centers. Upon cooling, the product X_MT remains practically constant down to 50 K, then it decreases more and more rapidly reaching 0.80 cm³ K mol⁻¹ at 2 K. The temperature dependence of X_M^–1 above 50 K obeys the Curie–Weiss equation X_M⁻¹ = (T − θ)/C with the Curie–Weiss constants C = 4.56 cm³ K mol⁻¹, θ = −10.07 K. The negative value of θ and the decrease of X_MT at lower temperatures may be attributed to the zero-field splitting (ZFS) effects associated with the axially elongated Cr(III) ion or to weak antiferromagnetic intermolecular interactions between the Cr(III) spin carriers or to both factors simultaneously [21], [35].

4.1 Materials and physical measurements

All chemicals were of analytical reagent grade and were used without further purification. Elemental analyses for C, H and N were carried out a Fisons-EA 1108 CHN analyzer. The infrared (IR) spectrum of compound 1 was recorded on a Perkin-Elmer 2000 FT-IR spectrometer using KBr pellets in the range of 4000–400 cm⁻¹. The UV–Vis spectrum was obtained on a Bruker HACH DR 3900 spectrophotometer, in water, in the range 300–800 nm. X-ray powder measurements were performed at room temperature. Data were collected on a Bruker D8 Advance A25 diffractometer, in Bragg–Brentano geometry, using CuKα radiation. The simulation from the single-crystal data was performed with the Mercury software [36]. TGA was carried out on a LINSEIS STA PT-1000 thermal analyzer. The powdered sample (20 mg) was heated from 25 to 700 °C with a rate of 10 K min⁻¹ in an air flow. The EPR spectrum was recorded using a Bruker ELEXYS E500 spectrometer operating at 9 GHz at room temperature. The spectrum was simulated with the program WinEPR SimFonia by Bruker [37]. Magnetic susceptibility data were performed on a Quantum Design MPMS-5XL SQUID magnetometer in the temperature range 2–300 K at an applied magnetic field of 0.1 T.

4.2 Synthesis of (CH₆N₃)[t-Cr(C₂O₄)₂(H₂O)₂]·2H₂O (1)

A mixture of guanidinium carbonate (CH₆N₃)₂CO₃ (90 mg; 0.5 mmol) and oxalic acid dihydrate H₂C₂O₄·2H₂O (252 mg; 2 mmol) was added in small portions to a stirred green aqueous solution of chromium trichloride hexahydrate CrCl₃·6H₂O (266 mg; 1 mmol) at 70 °C. After 2 h of stirring in air under ambient conditions, the reaction mixture was gradually cooled to room temperature and filtered. The purple filtrate was allowed to evaporate in a hood at room temperature. Three weeks later, red prismatic crystals suitable for X-ray diffraction were isolated with a yield of 85% based on (CH₆N₃)₂CO₃ – Elemental analysis calcd. for C₅H₁₄CrN₃O₁₂ (M_r = 360.19): C 16.67, H 3.92, N 11.67; found C 16.51, H 4.01, N 11.56%.

4.3 Crystallographic data collection and structure refinement

A suitable single crystal of dimensions 0.32 × 0.18 × 0.17 mm³ was selected for indexing, and the intensity data were recorded at 296 K on a Bruker APEX II CCD diffractometer equipped with a Mo Incoatec Microfocus Source IµS (λ = 0.71073 Å). Using Olex2 [38], the structure of 1 was solved with the program Shelxt [39] by using the intrinsic phasing solution method. The model was refined with Shelxl-2018/3 [40] using least-squares minimization. All nonhydrogen atoms were refined anisotropically and all H atoms linked to N and O atoms were located from the electron density difference map and refined freely. The crystallographic data and structure refinement details are summarized in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 2040246 contains supplementary crystallographic data for 1. These data can be obtained free of charge from The Cambridge Crystallographic Data Center viawww.ccdc.cam.ac.uk/data_request/cif.