Supramolecular architecture based on high-lacunary sandwich-type building blocks: synthesis, characterization, and properties

2.1 Synthesis of 1

(NH₄)₁₂[Fe₂(AsMo₇O₂₇)₂]·12H₂O (0.29 g, 0.1 mmol) was dissolved in 20 mL distilled water and stirred for 10 min. Then, 0.013 mL (0.2 mmol) ethylenediamine dissolved in 10 mL 0.02 m hydrochloric acid was added dropwise within 10 min. The mixture was kept stirring for 1 h at room temperature. The resulting reaction mixture was further refluxed for 3 h, cooled to room temperature, and filtered. Sealed with a parafilm with a few of tiny pores, a flask was kept for slow evaporation at room temperature. After 1 week, yellow block crystals were obtained (yield 35% based on Mo). – IR spectrum: see Fig. S3 (Supplementary Information available online). – Anal. for 1: Calcd. C 1.48, H 1.67, Mo 41.41, Fe 3.44; found C 1.43, H 1.52, Mo 41.12, Fe 3.19%. TG analysis indicated that there are about six interstitial water molecules in crystals of 1 (Fig. S4; Supplementary Information).

2.2 X-ray structure determination

The intensity data of 1 were collected at 296(2) K on a Bruker D8 SMART APEX II CCD diffractometer with kappa geometry and MoK_α radiation (λ = 0.71073 Å). Suitable crystals were mounted in a thin glass capillary and transferred to the goniometer. A multi-scan absorption correction was applied. Data integration was performed using Saint [23]. Routine Lorentz and polarization corrections were applied. Multi-scan absorption corrections were performed using Sadabs [24]. Molybdenum, iron, and arsenic atoms were located by Direct Methods, and successive Fourier syntheses revealed the remaining atoms. Refinements were achieved by full-matrix least squares on F² using the Shelxtl crystallographic software package [25–27]. In the final refinement, all the non-H atoms were anisotropically refined. H atoms on the C and N atoms of ethylenediammonium cations were fixed on the calculated positions. The H atoms on the water molecules could not be located from the difference Fourier maps and were directly included in the final molecular formula. The NH₄⁺ cations could not be precisely found from the difference Fourier maps. Therefore, the ammonium ions were also modeled as oxygen atoms. The routine SQUEEZE in Platon [28] was used to further estimate the number of solvate water molecules and remove their electron densities from the crystal structure. The elemental analysis and the charge balance confirmed their presence. The highest residual peak and the deepest hole are 2.61 and –1.51 e Å^–3, respectively. The detailed crystal data and structure refinement are given in Table 1. Bond lengths and angles around the As, Mo, and Fe centers are listed in the Supplementary Information (Tables S1 and S2).

CCDC 1057001 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.

3.1 Description of the crystal structure

Single crystal X-ray diffraction analysis has shown that the basic structural unit of 1 contains a polyoxoanion [Fe₂(AsMo₇O₂₇)₂]^12–, two ethylenediammonium cations, eight ammonium cations, and six water molecules (Fig. 1). The structural feature of [Fe₂(AsMo₇O₂₇)₂]^12– is similar to that of a previously report one [20]. Two AsMo₇O₂₇^9– (abbreviated {AsMo₇}) units are fused together by two center Fe^III ions. The {AsMo₇} unit can be deduced from hexalacunary derivatives of mono-{MoO₆} capped α-Keggin anions AsMo₁₂O₄₀^3– that lost six edge-sharing {MoO₆} octahedra exposing five nucleophilic oxygen atoms. The {AsMo₇} unit can be viewed as a multi-dentate ligand which can provide five coordination sites to chelate metal ions. The As and Mo atoms in the {AsMo₇} unit exhibit a tri- and octahedral coordination geometry, respectively. The bond lengths As–O and the bond angles O–As–O are in the ranges of 1.761(6)–1.825(7) Å and 99.2(3)–101.4(3)°, respectively. The bond lengths Mo–O and bond angles O–Mo–O vary in the ranges 1.692(8)–2.435(7) Å and 70.1(2)–171.8(4)°, respectively (see Table S1). Both Fe^III ions sited in the central belt exhibit a hexa-coordinated environment and are bridged by two μ₃-O atoms derived from {AsO₃} units. The bond lengths Fe–O and the bond angles O–Fe–O are in the ranges of 1.966(7)–2.039(7) Å and 79.5(3)–179.5(3)°, respectively. All of these bond lengths and bond angles are within the normal ranges observed in other POM complexes [29, 30].

Fig. 1:

Combined polyhedral and ball and stick representation of the polyoxoanion of [Fe₂(AsMo₇O₂₇)₂](H₂en)₂(NH₄)₈·6H₂O (1). Mo blue octahedra, Fe green, As purple, O in polyanion: white; C blue. Lattice water molecules and ammonium cations are omitted for clarity.

As an interesting feature of 1, ethylenediammonium cations (H₂en²⁺) stabilize the [Fe₂(AsMo₇O₂₇)₂]^12– anions, as is shown in Fig. 2. The counterions H₂en²⁺ reside in the interspace of adjacent anionic units affording the first example of a hexalacunary sandwich-type supramolecular hybrid. The H₂en²⁺ cations act as donors for the neighboring anions [Fe₂(AsMo₇O₂₇)₂]^12– in N–H···O hydrogen bonds (see Table 2) and form a multiple circular hydrogen bonding environment (see Fig. 2). The [Fe₂(AsMo₇O₂₇)₂]^12– units can be abstracted as an eight-connected node that links with eight H₂en²⁺ cations. The H₂en²⁺ linker can be viewed as a two-connected node. Thus, the open framework of 1 can be abstracted into a 2D (8,2)-connected net supramolecular framework (see Figs. S1 and S2; Supplementary Information).

Fig. 2:

Packing arrangement of compound 1 on the [100] plane, exhibiting the multiple circular hydrogen bond environment between [Fe₂(AsMo₇O₂₇)₂]^12– polyoxoanions and H₂en²⁺ cations. Mo blue octahedra, Fe green, As purple, O in polyanion: blue stick; O in ethylenediammonium red, C blue. Lattice water molecules and ammonium cations are omitted for clarity.

Fig. 3:

Temperature dependence of the χ_mT product for 1. The solid line is the fitting line.

3.2 Magnetic properties

The DC measurements of 1 were performed on a powdered sample. Since all the As and Mo atoms in 1 are diamagnetic, the magnetic behavior originates from the Fe^III ions. As Fig. 3 shows, the χ_mT value at room temperature is 8.91 cm³ mol^–1 K, which is slightly higher than the expected value of 8.75 cm³ mol^–1 K for two spin-only uncoupled Fe(III) (S = 5/2, g = 2 for each Fe(III) ion). As the temperature was lowered, the χ_mT value remained constant to below 50 K, and then diminished monotonically reaching to 0.33 cm³ mol^–1 K at 2.0 K. The behavior indicates the presence of significant antiferromagnetic interaction between two Fe^III ions.

Considering the long interdimer distance of 1, the direct magnetic coupling interactions can be neglected. The susceptibility data were simulated by the isotropic exchange Hamiltonian with H = –JS₁S₂. The analytical expression of the susceptibility derived from this model was reported by Xue et al. [31]. The best fitting results between the calculated and experimental data were found with J = –2.2 cm^–1 and g = 1.99. The negative J value indicates strong antiferromagnetic Fe–Fe interaction in 1, the result being similar to that of a previous work [31].

3.3 Antitumor activity

The MTT method was introduced to evaluate the antitumor effect of the title compound and its precursor (NH₄)₁₂ [Fe₂(AsMo₇O₂₇)₂]·12H₂O for HepG-2 cells in vitro. The inhibitory effects and effective cell 50% lethal concentration (IC₅₀) are given in Table 3. Both compound 1 and its precursor exhibit antitumor activities for HepG-2 cells in a dose dependent manner with the concentrations of 1–100 μm. The inhibitory effect of 1 is higher than that of the precursor. The inhibition rates of 1 on HepG-2 cells are 25%, 35%, 45%, 61%, and 74% (1), and those of the precursor are 22%, 32%, 40%, 59%, and 55%. The 50% lethal concentration (IC₅₀) value against HepG-2 cell lines for 1 is 221.18 μm mL^–1, while the value of (NH₄)₁₂[Fe₂(AsMo₇O₂₇)₂]·12H₂O is 313.06 μm mL^–1, which indicates that 1 possesses higher antitumor activities.