The influence of alkali-metal ions on the molecular and supramolecular structure of manganese(II) complexes with tetrachlorophthalate ligands

2.1 Synthesis and general characterization

In this work, we selected 1,2-H₂BDC-Cl₄ to construct Mn(II) coordination architectures, aiming to explore the influence of alkali metal ions on the formation such complexes. Complex 1 was prepared by the direct reaction of equimolar quantitaties of 1,2-H₂BDC-Cl₄ and Mn(OAc)₂·4H₂O in EtOH-H₂O solution. Upon addition of an equimolar amount of LiNO₃ to the above reaction system, complex 1 could also be obtained by the same procedure (confirmed by X-ray diffraction). When KNO₃ was used, yellow block-shaped crystals of 2 were produced instead. Using other alkali metal salts (e.g., NaNO₃, RbNO₃, CsNO₃) as the potential structure-directing agents, we have not isolated any solid products suitable for X-ray analysis under the same conditions. Both complexes are stable at room temperature and insoluble in water and common organic solvents, which is consistent with their polymeric nature. In the IR spectra of 1 and 2, the broad peak appearing in the region of 3000–3500 cm^–1 corresponds to the O–H stretching frequencies. Both complexes show characteristic bands of carboxylate groups appearing in the region of 1590–1650 cm^–1 for the antisymmetric stretching vibrations and at 1340–1430 cm^–1 for the symmetric stretching vibrations. In addition, the absence of a strong absorption band around 1715 cm^–1 for the free 1,2-H₂BDC-Cl₄ molecule indicates that the 1,2-H₂BDC-Cl₄ ligands in 1 and 2 are completely deprotonated, which is consistent with the consideration of charge balance.

2.2 Structural descriptions of complexes 1 and 2

[Mn₂(1,2-BDC-Cl₄)₂(H₂O)₉]·H₂O (1) X-ray diffraction ana- lysis has revealed that complex 1 crystallizes in the triclinic space group P1̅ with Z = 2 and has a discrete dinuclear structure. The asymmetric unit is composed of two crystallographically independent Mn(II) ions (namely, Mn1 and Mn2), two 1,2-BDC-Cl₄ dianions, nine metal-coordinated water molecules and one lattice water molecule. As shown in Fig. 1a, the Mn1 ion shows an octahedral coordination coming from one oxygen atom of one 1,2-BDC-Cl₄ dianion and five aqua ligands with the Mn–O distances in the range of 2.125(3)–2.188(3) Å. The distorted octahedral geometry of Mn2 is provided by two oxygen donors of two 1,2-BDC-Cl₄ ligands and four aqua ligands with the Mn–O distances in the range of 2.113(2)–2.260(2) Å. The 1,2-BDC-Cl₄ ligands exhibit two types of coordination modes: one (1,2-BDC-Cl₄^I) is in a μ₂-bridging fashion to connect Mn1 and Mn2 ions with the Mn···Mn distance of 6.454(1) Å, while the other one (1,2-BDC-Cl₄^II) adopts a terminal coordination mode at the Mn2 ion.

Fig. 1

Views of (a) the coordination environments of the Mn1 and Mn2 ions in 1, (b) the water tape of 1, and (c) the 3D supramolecular architecture (Cl···Cl and C–H···Cl interactions are shown as bright green and violet dashed lines, respectively; irrelevant hydrogen atoms are omitted for clarity).

It should be pointed out that there are nine crystallographically independent coordinated aqua ligands [O9, O10, O11, O12, O13, O14, O15, O16, and O17] and one lattice water molecule [O18]. Among them, the coordinated water molecules of O10, O11, O15, O16, and the lattice water molecule of O18 as well as those of symmetry-related ones are arranged to afford a 1D metal-water cluster tape along the a axis, as shown in Fig. 1b (left). The O···O separations vary from 2.749(3) to 3.187 (3) Å with an average value of 2.893(3) Å, which is clearly shorter than the sum of the van der Waals radii (r_vdW for O = 1.52 Å) and slightly longer than that found in liquid water (2.854 Å) [29], whereas the average O···O···O angle is 107.2(1)°. Furthermore, these water tapes are connected to each other through hydrogen bonds with the uncoordinated carboxylate atoms (O6), resulting in a 2D network (Fig. 1b, right). The tetrachlorinated benzene rings project on both the sides of such 2D motifs. Further analysis of the packing of the molecules has revealed that such layers with their parallel arrangement are connected by multiple Cl···Cl interactions (Cl2···Cl5ⁱ distance = 3.336 Å, i = –x–1, –y, –z+1; Cl4···Cl4ⁱⁱ distance = 3.629 Å, ii = –x+1, –y, –z+1; Cl7···Cl8ⁱⁱⁱ distance = 3.632 Å, Cl8···Cl8ⁱⁱⁱ distance = 3.512 Å, iii = –x, –y+1, –z+1) and intermolecular C–H···Cl interactions to generate a 3D supramolecular network (Fig. 1c). It should be pointed out that the Cl···Cl distance in 1 is shorter than twice Pauling’s van der Waals radius of the Cl atom (3.76 Å) [30], and is comparable to that stated by Bondi (3.52 Å) [31].

[MnK₂(1,2-BDC-Cl₄)₂(H₂O)₄]_n(2) Complex 2 crystallizes in the orthorhombic system with space group Pbcn with Z = 4 and shows a 2D heterometallic framework. The asymmetric unit contains half of one Mn(II) ion, one K(I) ion, one 1,2-BDC-Cl₄ dianion, and two aqua ligands. Each Mn(II) ion adopts a distorted octahedral coordination geometry (MnO₆) and is coordinated by four oxygen atoms from four different 1,2-BDC-Cl₄ ligands with Mn–O distances of 2.182(3) and 2.185(3) Å and two terminal aqua ligands with Mn–O distances of 2.273(3) Å (Fig. 2a). The coordination environment around K(I) ion is shown in Fig. 2b. Each K(I) center is embraced by six carboxylate oxygen atoms from four 1,2-BDC-Cl₄ ligands with K–O distances in the range of 2.648(3)–3.287(3) Å, two different aqua ligands with K–O distances of 2.747(5) and 2.935(3) Å, and one chlorine atom from a 1,2-BDC-Cl₄ unit with K–Cl distances of 3.747(2) Å. In contrast to 1, each 1,2-BDC-Cl₄ ligand in 2 is linked to two Mn(II) ions and four K(I) ions with one carboxylate group adopting a μ₄-η²:η²-bridging mode and the other one adopting a μ₄-η³:η¹-bridging fashion. To the best of our knowledge, such a coordination mode has never been observed in metal complexes with phthalate or its substituted analogues. The connectivities lead to the formation of a 2D Mn(II)-K(I) heterometallic framework with the tetrachlorinated benzene rings lying on the two sides and containing a rare example of Mn–O–K linkage (Fig. 2c). Further analysis of the packing of the molecule has revealed that, similar to 1, weak Cl···Cl interactions (Cl1···Cl3ⁱ distance = 3.438 Å, Cl2···Cl4ⁱ distance = 3.643 Å, i = –x+1/2, –y+1/2, z–1/2) can be observed between adjacent coordination layers, which lead to the formation of a 3D supramolecular architecture (Fig. 2d).

Fig. 2

Views of (a) the coordination environment of the Mn(II) ion in 2 (symmetry codes: #1: –x, y, –z+1/2; #2: –x, –y, –z+1; #3: x, –y, z–1/2), (b) the coordination environment of the K(I) ion in 2 (symmetry codes: #1: –x, y, –z+1/2; #3: x, –y, z–1/2; #4: x, –y+1, z–1/2), (c) the 2D heterometallic framework containing layered Mn–O–K linkages, and (d) the 3D supramolecular architecture (Cl···Cl interactions are shown as bright green dashed lines; irrelevant hydrogen atoms are omitted for clarity).

2.2.1 Thermal stability

The thermal stability of complexes 1 and 2 was investigated by thermogravimetric analysis (TGA) yielding the curves depicted in Fig. 3. The TGA trace of 1 shows that the first weight loss of 20.3 % occurring from 50 to ca. 165 °C corresponds to the loss of all water molecules (calculated, 20.1 %). With that, almost no weight loss was observed up to ca. 250 °C, where the complex began to decompose. Complex 2 is stable up to 70 °C and shows the first weight loss of 9.2 % in the temperature range of 70–150 °C corresponding to the release of four coordinated water molecules (calculated, 8.9 %). Subsequently, pyrolysis of the residual coordination framework is observed upon heating above 250 °C with a sharp weight loss. Further heating to 800 °C reveals a gradual weight loss of the sample.

Fig. 3

TGA curves of complexes 1 and 2.

2.2.2 Luminescence properties

To explore their potential applications as luminescent crystalline materials, the emission spectra of complexes 1 and 2 as well as of the free ligand 1,2-H₂BDC-Cl₄ were recorded in the solid state at room temperature (Fig. 4). Excitation of the microcrystalline samples at 336 nm leads to the generation of broad fluorescence emissions in the range of 450–540 nm. The free ligand 1,2-H₂BDC-Cl₄ has a fluorescence emission band centered at 474 nm, which can be ascribed to the π → π* and/or n → π* transitions. For 1 and 2, excitation of the microcrystalline samples at 336 nm leads to the generation of intense fluorescence emissions with similar peak maxima observed in the blue region (1, 473 nm; and 2, 476 nm), which may be attributed to the ligand-centered transitions. In addition, the weaker shoulder peaks of the emission spectra around 508 nm can probably be assigned to intraligand transitions because a similar peak at about 510 nm also appears for 1,2-H₂BDC-Cl₄. Furthermore, the emission intensity of both complexes is obviously weaker than that of the free ligand, which is probably related to their complicated structures and the decay effect of high-energy C–H and/or O–H oscillators from the coordinated and lattice water molecules [32].

Fig. 4

(color online). Solid-state fluorescence emission spectra of 1,2-H₂BDC-Cl₄ and of the complexes 1 and 2.

4.1 Synthesis of [Mn₂(1,2-BDC-Cl₄)₂(H₂O)₉]·H₂O (1)

An ethanol solution (2 mL) of 1,2-H₂BDC-Cl₄ (91.2 mg, 0.3 mmol) was added to an aqueous (6 mL) solution of Mn(OAc)₂·4H₂O (73.5 mg, 0.3 mmol). Then, the mixture was stirred for ca. 30 min. The resulting solution was filtered and left to stand at room temperature. Yellow block-shaped crystals suitable for X-ray analysis were obtained after two weeks in 65 % yield (87.1 mg, on the basis of 1,2-H₂BDC-Cl₄). – Anal. for C₁₆H₂₀Cl₈Mn₂O₁₈ (%): calcd. C 21.50, H 2.26; found C 21.26, H 2.27. – IR (KBr pellet): v = 3423 (br), 1646 (s), 1590 (vs), 1530 (m), 1433 (s), 1404 (m), 1344 (s), 1212 (w), 1131 (w), 918 (w), 843 (m), 672 (m), 650 (m), 615 (m) cm^–1.

4.2 Synthesis of [MnK₂(1,2-BDC-Cl₄)₂- (H₂O)₄]_n (2)

The same synthetic procedure as that described for 1 was used except that KNO₃ (60.6 mg, 0.6 mmol) was added, affording yellow block-shaped crystals of 2 upon slow evaporation of the solvent in 40 % yield (48.5 mg, on the basis of 1,2-H₂BDC-Cl₄). – Anal. for C₁₆H₈Cl₈K₂MnO₁₂ (%): calcd. C 23.76, H 1.00; found C 24.03, H 1.02. – IR (KBr pellet): v = 3407 (br), 1592 (vs), 1532 (m), 1426 (s), 1388 (m), 1339 (s), 1207 (w), 1127 (w), 938 (w), 904 (w), 663 (m), 647 (m), 618 (m) cm^–1.

4.3 X-ray structure determination

The single-crystal X-ray diffraction data for complexes 1 and 2 were collected on a Bruker Apex II CCD diffractometer with MoK_α radiation (λ = 0.71073 Å) at room temperature. A semiempirical absorption correction was applied (SADABS) [33], and the program SAINT was used for integration of the diffraction profiles [34]. The structures were solved by Direct Methods using SHELXS and refined by the full-matrix least-squares techniques on F² using SHELXL [35–38]. All nonhydrogen atoms were refined anisotropically. C-bound H atoms were placed in geometrically calculated positions and refined by using a riding model. H atoms of water were located in difference Fourier maps and refined in subsequent refinement cycles. Further crystallographic details are summarized in Table 1, selected bond lengths and angles are listed in Table 2, and hydrogen bond parameters are listed in Table 3.

CCDC 1013362 and 1013363 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.