Assembly of cobalt-p-sulfonatothiacalix[4]arene frameworks with phosphate, phosphite and phenylphosphonate ligands

2.1 Materials and instruments

The sodium salt of p-sulfonatothiacalix[4]arene (Na₄H₄TC4AS) was synthesized by the literature method [16], and the other reagents were obtained commercially and used as received. FT-IR spectra (KBr pellets) were recorded on a Bruker Vertex 70 spectrometer (Figure S1; Supporting Information available online). Elemental analysis of C, H, N and S was carried out on a VarioEL instrument. Thermogravimetric analyses (TGA, Figure S2) were monitored by a NETZSCH STA 449F3 apparatus from 20 to 800 °C at a rate of about 10 °C min⁻¹ in air. Magnetic susceptibility measurements were performed with a Quantum Design MPMS XL-5 SQUID system in the temperature range of 2–300 K.

2.2 Syntheses of compounds CIAC-253–CIAC-255

Purple single-crystal blocks of CIAC-253 were obtained by the solvothermal reaction of a mixture of Na₄H₄TC4AS (0.05 g, 0.05 mmol), Co(ClO₄)₂·6H₂O (0.05 g, 0.1 mmol), H₃PO₃ (0.02 g, 0.2 mmol), N-methyl-2-pyrrolidinone (NMP) (5.0 mL), N,N-dimethylformamide (DMF) (5.0 mL) and triethylamine (0.1 mL) in a 20 mL Teflon-lined autoclave which was heated to 130 °C for three days. Subsequently, the autoclave was slowly cooled to 20 °C at a rate of 4  °C·h⁻¹. The crystals were collected by filtration. Yield: 36% based on p-sulfonatothiacalix[4]arene. – Elemental analysis: calculated (%) for [(C₂H₅)₃NH]₄[Co₄Cl(H₄TC4AS)]₄(HPO₃)₈·3H₂O·7DMF·16NMP (i.e. C₂₂₁H₃₁₉N₂₇Co₁₆O₁₁₄P₈S₃₂Cl₄), C 35.22, H 4.27, N 5.02, S 13.61; found C 35.24, H 4.31, N 4.98, S 13.62. – FT-IR (cm⁻¹): 3450(m), 3055(w), 2978(w), 2936(m), 2875(w), 2800(w), 2740(m), 2484(m), 2355(s), 1664(s), 1456(s), 1308(w), 1260(w), 1193(m), 1132(s), 1076(m), 1040(s), 967(m), 752(m), 621(s), 550(w).

Purple single-crystal blocks of CIAC-254 were obtained from a mixture of Na₄H₄TC4AS (0.05 g, 0.05 mmol), Co(ClO₄)₂·6H₂O (0.05 g, 0.1 mmol), H₃PO₄ (10 µL), CH₃OH (5.0 mL) and DMF (5.0 mL) under similar solvothermal conditions employed as for CIAC-253. Yield: 42% based on p-sulfonatothiacalix[4]arene. – Elemental analysis: calculated (%) for H₁₂[Co₄Cl(H₄TC4AS)]₄(PO₄)₈·25H₂O·16DMF·4CH₃OH (i.e. C₁₄₈H₂₀₆N₁₆Co₁₆O₁₄₁P₈S₃₂Cl₄), C 26.05, H 3.04, N 3.28, S 15.04; found C 26.09, H 3.33, N 3.34, S 15.53. – FT-IR (cm⁻¹): 3457(w), 3043(w), 2941(w), 2815(w), 1670(s), 1573(m), 1466(s), 1312(w), 1270(m), 1192(w), 1140(m), 1088(m), 1042(s), 884(s), 758(m), 628(s), 575(s), 521(w).

Red single-crystal blocks of CIAC-255 were obtained from a mixture of Na₄H₄TC4AS (0.05 g, 0.05 mmol), CoCl₂·6H₂O (0.05 g, 0.2 mmol), phenylphosphonic acid (Ph-PO₃H₂) (0.02 g, 0.1 mmol), NMP (5.0 mL), DMF (5.0 mL) and triethylamine (0.1 mL) under similar solvothermal conditions as applied for CIAC-253. Yield: 38% based on p-sulfonatothiacalix[4]arene. – Elemental analysis: calculated (%) for [(C₂H₅)₃NH]₃[Co₄Cl(H₄TC4AS)]₃(C₆H₅PO₃)₆·31H₂O·4NMP·DMF (i.e. C₁₄₉H₁₉₅N₈Co₁₂O₁₀₂P₆S₂₄Cl₃), C 32.54, H 3.57, N 2.04, S 13.99; found C 32.54, H 3.39, N 2.04, S 13.43. – FT-IR (cm⁻¹): 3443(w), 3057(w), 2921(w), 2870(w), 2512(w), 1656(s), 1573(m), 1401(s), 1307(m), 1267(m), 1191(m), 1126(s), 1038(s), 935(m), 884(m), 749(m), 698(m), 624(s), 576(w), 554(w).

2.3 Single crystal X-ray diffraction

The intensity data was recorded on a Bruker APEX-II CCD system with CuKα radiation (λ = 1.54178 Å). The crystal structures were solved by Direct Methods and refined by employing full-matrix least-squares on F ² (Shelxtl-2018) [32]. It was not possible to model the disordered solvent molecules appropriately even with low-temperature data obtained at about 190 K. The diffraction data was treated by the SQUEEZE routine [33], a part of the Platon package, which dramatically improved the agreement indices. Non-hydrogen atoms except some disordered ones were refined anisotropically. The hydrogen atoms of the organic ligands were generated theoretically onto the specific atoms and refined isotropically with fixed displacement parameters. Crystal data for CIAC-253–CIAC-255 are tabulated in Table 1.

CCDC 2101292–2101294 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.

3.1 Crystal structures of compounds CIAC-253–CIAC-255

All three compounds feature isolated coordination entities based on the shuttlecock-like Co₄-TC4AS SBUs, quadrilateral ones for CIAC-253–CIAC-254 and a triangular one for CIAC-255 (Figure 1). Compounds CIAC-253 and CIAC-255 crystallize in the monoclinic system, but with different space groups, i.e. C2/m for CIAC-253 and C2/c for CIAC-255. Compound CIAC-254 crystallizes in the triclinic system with the space group P 1 ‾ . In all three compounds, four adjacent cobalt atoms in a Co₄ square arrangement are bonded by a conic TC4AS molecule with four bridging sulfur atoms and four phenolic oxygen atoms to from a shuttlecock-like Co₄-TC4AS SBU. A μ ₄-Cl anion is bonded at the bottom of each shuttlecock. The HPO₃ ²⁻, PO₄ ³⁻ and Ph-PO₃ ²⁻ anions act as the linkers. All the cobalt atoms are six-coordinated by two μ ₂-phenoxy oxygen atoms, one sulfur bridge, one μ ₄-Cl anion and two oxygen atoms from two different HPO₃ ²⁻ (CIAC-253), PO₄ ³⁻ (CIAC-254) or Ph-PO₃ ²⁻ anions (CIAC-255). All the Co–O bond lengths are in the normal range (1.97–2.14 Å for Co–O bonds). Bond valence sum (BVS) calculations indicated that all the cobalt ions in these compounds are divalent [34]. All R-PO₃ ⁿ⁻ (R = H, O and Ph) anions have a [4.211] coordination mode according to the Harris notation (Figure 2) [27, 35]. In CIAC-253, four Co₄-TC4AS SBUs are bridged by four pairs of HPO₃ ²⁻ anions into a quadrilateral framework (Figure 1b and c) with a 16-membered ring composed of four cobalt, four phosphorus, and eight oxygen atoms (Figure 1d). The repeating unit for the 16-membered ring is -[Co−O−P−O]- unit. The longest distance between the diametrically opposed atoms in the ring is about 8.71 Å (Figure 1d). The hexadecagonal pocket of CIAC-253 has a depth of ca. 6.4 Å. The outer dimension of the entity is approximately 21.7 × 21.7 Å². Because an oxygen atom of the PO₄ ³⁻ anion does not bond the metal atom, the PO₄ ³⁻ anion has a similar coordination mode as HPO₃ ²⁻, and CIAC-254 has a similar structure as that of CIAC-253 (Figure 1e and f). The longest distance between diametrically opposed atoms in the ring is about 8.7 Å (Figure 1g) and the hexadecagonal pocket has a depth of ca. 6.4 Å. The outer dimension of the entity is approximately 21.4 × 21.4 Å. Differently, in CIAC-255, three Co₄-TC4AS SBUs are interconnected by three pairs of Ph-PO₃ ²⁻ anions into a triangular framework (Figure 1h and i) which has a 12-membered ring composing of three cobalt, three phosphorus and six oxygen atoms (Figure 1j). The repeating unit for the 12-membered ring is also the -[Co−O−P−O]- unit. The longest distance between the diametrically opposed atoms in the ring is about 6.4 Å (Figure 1j).

Figure 1:

A shuttlecock-like Co₄Cl-TC4AS SBU (a), windows frame-like coordination squares in CIAC-253 (b, c) and CIAC-254 (e, f), and triangular framework (h, i) in CIAC-255. Solid yellow lines highlight the ring (d, g and j) in CIAC-253–CIAC-255. The cyan cones and yellow spheres represent the calixarene molecules and the inner cavity, respectively.

Figure 2:

Coordination modes of the HPO₃ ²⁻, PO₄ ³⁻ and Ph-PO₃ ²⁻ ligands.

Three-dimensional (3D) extended structures are formed through supramolecular interactions between the isolated frameworks and some counterparts (Figure 3). All three compounds thus exhibit porous extended structures.

Figure 3:

Supramolecular extended structures of compounds CIAC-253–CIAC-255.

Comparing the structures of the three compounds and their bridging ligands, one can find that the bridge having a phenyl group interconnects the shuttlecock-like SBUs into a triangular framework while those without a phenyl group bridge the SBUs into the quadrilateral ones. It thus appears that the phenyl group of the bridging ligand induces the formation of a different framework. As shown in Figure 4, the benzene ring of the Ph-PO₃ ²⁻ ligand forms π···π and H···π interactions with the benzene ring of the TC4AS molecules, which stabilizes the triangular framework. A similar coordination triangle was reported for the product of the reaction of p-phenylthiacalix[4]arene (H₄PTC4A), Ph-PO₃H₂ and NaOH with CoCl₂·6H₂O [28]. Differently, in the triangle reported here, the benzene ring of the Ph-PO₃ ²⁻ anion interacts with PTC4A by π···π interactions (Figure S3). On the other hand, the stacking effect between the quadrilateral frameworks in CIAC-253 and CIAC-254 prevent the self-assembly of the structures as an open helmet-shaped Co₂₀ or an octahedral Co₂₄ nanocage [29].

Figure 4:

π···π and H···π interactions between the phenyl group of Ph-PO₃ ²⁻ ligands and the TC4AS molecules.

3.2 Magnetic properties

The magnetic properties of many metal-calixarene compounds were investigated because calixarenes are ideal macrocycle molecules to bond and separate metal centers [20, 21]. The temperature-dependent magnetic susceptibility of compounds CIAC-253–CIAC-255 was measured on freshly prepared samples in an applied field of 1 kOe (1 kOe = 7.96 × 10⁴ A m⁻¹) at T = 2–300 K. As shown in Figure 5, the χ _M T values for compounds CIAC-253–CIAC-255 at 300 K are 39.5, 35.2, and 24.5 cm³ K mol⁻¹, respectively, which are slightly larger than the spin-only value of 16 isolated high-spin d ⁷ ions (30.0 cm³ mol⁻¹ K, for CIAC-253 and CIAC-254) or 12 isolated high-spin d ⁷ ions (22.5 cm³ mol⁻¹ K, for CIAC-255) due to the strong orbital contributions of the distorted octahedral Co^II ions. As temperature decreases, the χ _M T values decrease gradually to the minimums of 0.78, 0.78, and 1.54 cm³ mol⁻¹ K at 2 K, respectively. For all three compounds, the reciprocal molar susceptibilities (1/χ _M) in the range 50–300 K follow the Curie-Weiss Law of 1/χ _M = (T − θ)/C with the Curie constants (C) being 46.08, 40.10, and 33.68 cm³ mol⁻¹ K and the Weiss constant (θ) being −45.69, −35.91, and −33.07 K, respectively. The negative Weiss constant (θ) suggests an antiferromagnetic interaction between the metal centers and/or the spin–orbit coupling effect of Co^II [36], [37], [38].

Figure 5:

Plots of χ _M T versus T and 1/χ _M versus T for CIAC-253 (a), CIAC-254 (b) and CIAC-255 (c) in 1 kOe field.