Resin-assisted solvothermal synthesis of a manganese(II) coordination polymer with tetrachloroterephthalate

2.1 Synthesis and general characterization

The Mn^II-exchanged resin beads were readily prepared from commercially sulfonated styrene-divinylbenzene cation-exchange resin beads (sodium form) with saturated aqueous solutions of Mn^II chloride. In this case, we used solvothermal synthesis in Teflon autoclaves under autogenous pressure to obtain complex 1 by reacting the Mn^II-resin beads with H₂BDC-Cl₄ in a mixed solvent system (MeOH/DMF). Complex 1 was isolated by a flotation method in carbon tetrachloride (d = 1.60 g cm^–3). The denatured resin floats to the top leaving the more dense 1 (d = 1.62 g cm^–3) to settle to the bottom. When we tried to use MnCl₂ and Mn(OAc)₂ instead of the Mn^II-exchanged resin beads, we obtained only precipitates, and no single crystals were formed. The crystalline phase purity of the bulk samples of complex 1 was confirmed by powder X-ray diffraction (PXRD) (see Fig. 1). The observed data match well with the corresponding simulated ones obtained from the single-crystal data. In the IR spectrum of 1, the broad peak appearing in the region of 3000–3500 cm^–1 indicates the O–H stretching frequency. The antisymmetric and symmetric carboxylate stretching vibrations are found in the range of 1600–1660 cm^–1 and 1325–1430 cm^–1, respectively. The strong peak at ~1733 cm^–1 reveals the presence of the free carboxyl group, which is also consistent with the crystal structure as described below.

Fig. 1:

Experimental and simulated PXRD patterns for complex 1.

2.2 Description of the crystal structure

X-Ray structural determination indicates that complex 1 crystallizes in the monoclinic crystal system with space group C2/c and has a chain structure. The asymmetric unit contains half a Mn^II ion, half a BDC-Cl₄ dianion, three DMF ligands, and half a free (uncoordinated) H₂BDC-Cl₄ molecule. As illustrated in Fig. 2a, each Mn^II center adopts a six-coordinated octahedral geometry provided by six oxygen atoms from two BDC-Cl₄ dianions and four DMF molecules with the Mn–O bond lengths in the range of 2.030(6)–2.270(6) Å. The BDC-Cl₄ dianion shows the monodentate coordination mode for each carboxylate group, connecting the adjacent Mn^II ions into a 1D meandering chain along the [001] direction (see Fig. 2b). The lattice H₂BDC-Cl₄ guest functions as a linker connecting two adjacent parallel coordination chains through intermolecular O4–H4···O2ⁱ hydrogen bonds (H···O/O···O distance: 1.690/2.503(4) Å, angle: 171.9(2)°, i = x– 1/2, –y+ 1/2, z+ 1/2) with the uncoordinated carboxylate O2 atoms of H₂BDC-Cl₄. As a consequence, each chain is connected to four neighboring parallel chains through the lattice H₂BDC-Cl₄ molecules into a 3D supramolecular network (see Fig. 2c).

Fig. 2:

Views of (a) the coordination environment of the Mn^II center in 1 (symmetry codes: #1, –x + 1, y, –z+ 1/2; #2, –x+ 1, –y+ 1, –z), (b) the polymeric chain of 1, and (c) the 3D supramolecular architecture (O–H···O hydrogen bonds are shown as dashed lines; irrelevant hydrogen atoms are omitted for clarity).

2.3 Water solubility

Kondo and co-workers have reported a 2D Cu^II complex {[Cu(CO₃)(bitb)]·2H₂O}_n [bitb = 1,4-bis(imidazole-1-ylmethyl)-2,3,5,6-tetramethylbenzene], which is soluble in MeOH and EtOH [19]. Very recently, water-soluble Ag^I [16], Zn^II and Cd^II [15] as well as Ca^II [17] CPs were observed by us. Complex 1 also exhibits aqueous solubility (ca. 130 mg mL^–1) and is insoluble in common organic solvents (such as ethanol, chloroform, acetonitrile, and DMF) at room temperature. To our knowledge, 1 presents the first example of a water-soluble Mn^II CP. Determination of the molecular conductivity (147.3 × 10^–4 S m² mol^–1) indicates that the CP probably dissociates into ionic species in aqueous solution. In any case, it is improbable that the structural integrity of the polymer is maintained upon solvation in water. Similar to our previous work, we also tried to investigate the water-induced structural transformation by recrystallization of 1 from water. Unfortunately, and consequently, our efforts were not successful.

2.4 SEM and thermal stability

Scanning electron microscopy (SEM) was used to investigate the morphological change occurring on the resin surface after solvothermal reactions (see Fig. 3). The SEM image shows that block shape solids about 1 μm in width and 5 μm in length are present in the case of Mn^II-resin residue. The thermal stability of 1 was studied by thermogravimetric analysis (TGA) from room temperature to 800 °C (see Fig. 4). The TGA curve shows the first weight loss of 31.0% (calculated: 30.7%), occurring from 105 °C to 220 °C, corresponding to the release of coordinated DMF molecules. The remaining framework is decomposed through three consecutive weight losses beginning at 235 °C.

Fig. 3:

SEM images for the (a) as-treated Mn^II-resin bead and (b) Mn^II-resin bead after the solvothermal reaction.

Fig. 4:

TGA curve of complex 1.

In summary, we report the resin-assisted solvothermal synthesis of a water-soluble Mn^II CP with tetrachloroterephthalate. The BDC-Cl₄ dianions serve as bis-monodentate spacers to bridge adjacent Mn^II ions into infinite chains which also feature coordinated DMF molecules. Such chains are expanded to a 3D supramolecular architecture through intermolecular O–H···O interactions with interstitial H₂BDC-Cl₄ molecules. The as-synthesized Mn^II complex is soluble in water upon which the structural integrity of the CP is lost. The present work clearly reveals that metal-exchanged resins can be employed as an alternative metal source to produce new inorganic–organic hybrid materials under solvothermal conditions.

3.1 Synthesis of {[Mn(BDC-Cl₄)(DMF)₄](H₂BDC-Cl₄)}_n (1)

A mixture of Mn^II-resin beads (0.5 g) and H₂BDC-Cl₄ (60.2 mg, 0.2 mmol) in 8 mL of MeOH-DMF (v/v = 3:1) mixed solvent was sealed in a 15 mL Teflon-lined stainless autoclave and heated at 120 °C under autogenous pressure for 24 h, and then cooled to room temperature. Yellow block-shaped crystals were isolated in ca. 40 % yield based on H₂BDC-Cl₄. – Analysis for C₂₈H₃₀Cl₈MnN₄O₁₂ (%): calcd. C 35.28, H 3.17, N 5.88; found C 34.85, H 3.16, N 5.91. – IR (cm^–1, KBr pellet): v = 3450 (br), 2943 (m), 1733 (s), 1656 (s), 1561 (m), 1500 (m), 1422 (m), 1379 (s), 1336 (s), 1255 (s), 1109 (s), 837 (m), 789 (s), 712 (w), 679 (s), 615 (s), 594 (s).

3.2 X-Ray structure determination

The single-crystal X-ray diffraction measurement was performed on a Bruker Apex II CCD diffractometer at ambient temperature with MoK_α radiation (λ = 0.71073 Å). A semiempirical absorption correction was applied using sadabs [21], and the program saint was used for integration of the diffraction profiles [22]. The structure was solved by Direct Methods using the shelxs program of shelxtl packages and refined anisotropically for all non-H atoms by full-matrix least squares on F² with shelxl [23–26]. The coordinated DMF molecules in the compound are disordered over two sites with equivalent occupancy factors. In general, hydrogen atoms were located geometrically and allowed to ride during the subsequent refinement. Further crystallographic data and structural refinement parameters are summarized in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 1037709 contains 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.