Syntheses, crystal structures, and characterization of two Mn(II) coordination polymers with bis(4-(1H-imidazol-1-yl)phenyl)methanone ligands

2.1 Preparation and characterization of the complexes

Complexes 1 and 2 were prepared as colorless crystalline products via the combination of BIPMO with the metal ion (MnCl₂ for 1; MnCl₂ and NH₄SCN for 2). The complexes are stable in common solvents such as benzene, hexane, ethanol, and dichloromethane.

The structures of 1 and 2 were characterized by infrared (IR) spectroscopy, elemental analyses, single-crystal X-ray diffraction, and thermogravimetry.

The IR spectra of 1 and 2 were consistent with their formulation. Weak IR bands centered at ca. 3100 cm⁻¹ can be assigned to ν(C–H) of the ligands. A large group of bands in the region 1610–1430 cm⁻¹ are characteristic of ν(C=N) and ν(C=C) stretches of aromatic groups and reveal the presence of ligands BIPMO.

2.2 Molecular structures of 1 and 2

The crystal structure data are summarized in Table 1. Selected bond lengths and angles for 1 and 2 are listed in Table 2. The results of the single-crystal X-ray diffraction analysis indicate that the structures of complexes 1 and 2 are similar. Accordingly, the structure of 1 is described representatively here in detail. Complex 1 crystallizes in the monoclinic space group C2/m with Z = 2. The asymmetric unit contains one Mn(II) ion, two BIPMO ligands, two Cl anions, and two uncoordinating water molecules. As shown in Fig. 1, the basal plane around the Mn(II) ion is formed by four nitrogen atoms of four BIPMO ligands with a Mn–N distance of 2.254(2) Å. The apical positions of the octahedral geometry are occupied by two Cl anions with a distance of 2.5943(8) Å. These parameters are similar to those observed in other Mn(II) complexes [14–16]. Each BIPMO ligand binds to two Mn(II) ions through the terminal N atoms of the imidazole rings, resulting in closed rings. The ring structure looks like a rhombus window of approximate dimensions of 11.20 × 16.47 Å, and the distance between the two Mn(II) ions is 16.469(3) Å. These subunits are linked by sharing the Mn(II) atoms with adjacent subunits, together with the coordination of two Cl anions, to form the resultant infinite chain. The chain structure of 1 is further stabilized by C2–H2···Cl1 (C2···Cl1, 3.146(3) Å; C2–H2···Cl1, 123°, symmetry codes: –x, –y, 1 – z) hydrogen bonds. It should be noted that each chain in compound 1 interacts with two neighboring chains via weak C1–H1···O1 contacts (C1···O1, 3.275(3) Å; C1–H1···O1, 113°, symmetry codes: 1/2 – x, 1/2 – y, –z) along the crystallographic b axis to form a 2D layer. The layer is stabilized by C–H···π interactions between C3–H3 and the Cg1 ring (C3···Cg1 = 3.648(3) Å, symmetry code: 1 – x, y, 1 – z) along the c axis to form a three-dimensional network (Cg1 is the ring comprising the atoms C4–C9 for both 1 and 2) (Table 3, Fig. 2).

Fig. 1:

Coordination environments of complex 1. The hydrogen atoms are omitted for clarity. Symmetry codes: ^#1x, –y, z; ^#2 –x, –y, –z + 1; ^#3 –x, y, –z + 1; ^#4 –x + 1, y, –z.

Fig. 2:

A view of the chain in complex 1.

Compound 2 crystallizes in the triclinic space group, P1̅ with Z = 1. As shown in Fig. 3, the BIPMO ligands, such as 1, connect the Mn(II) ions to generate similar rings of approximate dimensions of 9.73 × 16.97 Å. The distance between the two Mn(II) ions across the ring is longer than that of 1, due to the rotation of an imidazole ring (Scheme 2). The N atoms coordinated with each Mn(II) ion come from four BIPMO ligands and two SCN⁻ anions, with Mn(II) in a distorted octahedral geometry (Figs. 3 and 4).

Scheme 2:

Ligand coordination modes in complexes 1 (left) and 2(right).

Fig. 3:

Coordination environments of complex 2. The hydrogen atoms are omitted for clarity. Symmetry codes: ^#1 –x + 2, –y, –z + 1; ^#2x + 1, y – 1, z – 1; ^#3 –x + 1, –y + 1, –z + 2; ^#4x – 1, y + 1, z + 1.

Fig. 4:

A view of the chain in complex 2.

2.3 Thermal stability

Thermogravimetric analyses (TGA) were carried out for complexes 1 and 2 in order to characterize the compounds more fully in terms of thermal stability (Fig. 5). For complex 1, the weight loss corresponding to the release of free water molecules is observed from 35 to 130°C (observed 5.0%, calculated 4.55%). Then no significant weight loss is observed until the decomposition of the frameworks occurs at 297°C. For complex 2, the weight loss from 50 to 131°C is consistent with the loss of two free methanol molecules (observed 7.2%, calculated 7.42%). The decomposition of the frameworks occurs from 315 to 606°C.

Fig. 5:

TGA curves for complexes 1 and 2.

4.1 Materials and measurements

All solvents, MnCl₂·4H₂O, and NH₄SCN were purchased from Aladdin Industrial Corporation (Shanghai, China). Elemental analyses were performed on an Elementar Vario ELIII elemental analyzer. The IR spectra were recorded on a Bruker Vector 22 spectrophotometer with KBr pellets in the 4000–400 cm⁻¹ region. TGA were carried out on a NETZSCH STA 449F3 unit at a heating rate of 10°C min⁻¹ under a nitrogen–oxygen (80:20) atmosphere.

4.2 Synthesis of bis(4-(1H-imidazol-1-yl)phenyl)methanone

A mixture of imidazole (3.40 g, 50 mmol), anhydrous potassium carbonate (6.90 g, 100 mmol), 4,4′-difluorodiphenylmethanone (5.45 g, 25 mmol), hexadecyltrimethylammonium bromide (50 mg), and dimethyl sulphoxide (30.0 mL) was stirred for a period of 24 h at 80°C and, after cooling to room temperature, was poured into crushed ice (100 mL). Pale yellow precipitates obtained were filtered, washed with distilled water, and dried in air. Yield: 95.6%. – Anal. for C₁₉H₁₄N₄O: calcd. C 72.60, H 4.49, N 17.82; found C 72.47, H 4.38, N 17.74%. – ¹H NMR (500 MHz, CDCl₃) δ 8.00 (s, 1H, imidazole-H), 7.99–7.57 (m, 4H, Ar-H), 7.41 (s, 1H, imidazole-H), 7.29 (s, 1H, imidazole-H). – IR (KBr, cm⁻¹): 3124, 1639, 1599, 1573, 1519, 1481, 1421, 1369, 1325, 1301, 1261, 1180, 1148, 1109, 1055, 956, 926, 899, 860, 831, 766, 746, 662, 622, 519, 473.

4.3 Synthesis of {[Mn(BIPMO)₂Cl₂] · 2(H₂O)}_n (1)

Complex 1 was prepared by a layering method. A buffer solution (5 mL) of methanol was carefully layered over a methanol solution (5 mL) of MnCl₂·4H₂O (19.8 mg, 0.1 mmol). Then a solution of BIPMO (62.8 mg, 0.2 mmol) in methanol (10 mL) was layered over the buffer layer. Colorless block crystals were obtained after a week. Yield: 65% (based on BIPMO). – Anal. for C₃₈H₃₂Cl₂MnN₈O₄: calcd. C 57.73, H 4.08, N, 14.17; found: C 57.51, H 3.91, N 14.04%. – IR (cm⁻¹): 3124, 3083, 1659, 1603, 1556, 1520, 1488, 1470, 1421, 1321, 1306, 1265, 1238, 1176, 1119, 1057, 958, 928, 856, 768, 739, 719, 673, 654, 620, 523, 469.

4.4 Synthesis of {[Mn(BIPMO)₂(SCN)₂] · 2(CH₃OH)}_n (2)

A buffer solution (5 mL) of methanol was carefully layered over a methanol solution (5 mL) of MnCl₂·4H₂O (19.8 mg, 0.1 mmol) and NH₄SCN (15.2 mg, 0.2 mmol). Then a solution of BIPMO (62.8 mg, 0.2 mmol) in methanol (10 mL) was layered over the buffer layer. Colorless block crystals were obtained after 1 week. Yield: 68% (based on BIPMO). – Anal. for C₄₂H₃₆MnN₁₀O₄S₂: calcd. C 58.39, H 4.20, N 16.21; found: C 58.16, H 4.08, N 16.04%. – IR (cm⁻¹): 3141, 2073, 1657, 1637, 1606, 1522, 1490, 1402, 1304, 1284, 1256, 1188, 1119, 1061, 959, 928, 853, 819, 766, 737, 652, 516, 475.

4.5 X-ray crystallography

All measurements were made on an Agilent Technology SuperNova Eos Dual system with a (MoK_α, λ = 0.71073 Å) micro focus source and focusing multilayer mirror optics. The data were collected at a temperature of 293 K and processed using CrysAlis Pro [17]. Absorption corrections were applied using the SADABS program [18]. The structures were solved by Direct Methods [19] with the Shelxtl (version 6.10) program [19, 20] and refined by full matrix least-squares techniques on F² with Shelxtl [19, 20]. All non-hydrogen atoms were refined anisotropically. The ligand hydrogen atoms were localized in their calculated positions and refined using a riding model. The water hydrogen atoms were located in difference maps and refined with d(O–H) = 0.85(2) Å and d(H···H) = 1.35(2) Å distances as restraints.

CCDC 1439673 and 1439674 contain 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.