Isostructural Heteronuclear (K⁺/M²⁺: M = Ni, Co, and Zn) One-Dimensional Coordination Polymers of Thiacalix[4]arene Tetraacetate

Results and Discussion

The crystalline products 1-3, {[K2@(TCTC)(𝜇2-M)(H2O)4-(CH3OH)2]·3H2O}n {M = Ni (1 , green), Co (2, pink), and Zn (3, colorless)} were synthesized from the respective reactions of K4TCTC in water and corresponding metal perchlorate except nickel(II) nitrate in methanol. The absence of characteristic bands at 1680-1715 cm−1 in the IR spectra for these products indicates that no protonation of the carboxylates of TCTC4− has been occurred in each complex. The crystal structures of 1-3 were obtained by single-crystal X-ray diffraction analysis (Table 1). The X-ray crystal structures of 1-3 with an isostructural form are shown in Figure 1 and their selected geometric parameters presented in Table 2. Commonly, the products 1, 2, and 3 crystallize in a monoclinic system with space group P21/c adopting a 1D zigzag chain as neutral species without the anions in the coordination sphere and the lattice. Here, the structure of 1 is described as a representative example. Because the structure in Figure 1 is generated through an inversion symmetry, the asymmetric unit of 1 in its coordination sphere contains one TCTC4−, two K atoms, one Ni atom, four water molecules, and two methanol molecules.

In 1, the coordination environment of K1 and K2 atoms accommodated inside the 1,3-alternate calix unit shows no significant difference (Figure 2(a)). Each K atom is in a distorted pentagonal planar array of two monodentate carboxylate oxygens and two phenolic oxygens from two different 1,3-alternate pendant groups. The coordination environment of the K atom is completed by one monodentate water molecule. The bite angles around the K1 atom vary considerably, ranging from 62.0(8)° (O4-K1-O5) to 113.5(2)° (O4-K1-O10), presumably due to the steric strains of the metallacycles formed upon complexation. The K1-O(carboxylate) distances [K1-O5 2.696(7), K1-O11 2.641(6) Å] are found to be appreciably shorter than the K1-O(ether) [K1-O4 2.914(6), K1-O10 2.941(6) Å] and the K1-O(water) [K1-O3W 3.045(10) Å], suggesting that the negatively charged oxygens are bonded more tightly to the potassium center. The K1···K2 separation is 5.702 Å and each K atom is also stabilized by 𝜂3-type cation···π interactions with the aromatic rings (dashed lines in Figure 2(a), K1···C 3.19- 3.45 Å, see Table 3).8 Overall, the K2@TCTC unit in 1 shows the synergic contribution of the three types of oxygen donors (ether, carboxylate, and water) and the aromatic rings to stabilize the dipotassium complexation.

Table 1.Crystal data and structural refinement for 1-3

Figure 1.Isostructural zigzag-type 1D polymeric structure of 1-3, {[K2@(TCTC)(𝜇2-M)(H2O)4(CH3OH)2]·3H2O}n, (1: M = Ni, 2: M = Co, and 3: M = Zn).

Notably, the K2@TCTC units are linearly linked by a 𝜇2-Ni atom to form an infinite 1D zigzag chain structure through Ni-O(carboxylate) bonds [Ni1-O3 2.015(5), Ni2-O9 2.014(6) Å] (Figures 1 and 2(b)). The Ni(II) center is sixcoordinate, being bound to two carboxylate oxygens from different K2@TCTA units in a monodentate manner and two water molecules (Figure 2(a)). The distorted octahedral coordination sphere is completed by two additional methanol molecules.

 

Conclusion

Assembly of 1,3-alternate thiacalix[4]arene potassium tetraacetate with three transition metal salts afforded an isostructural metal-mediated heteronuclear coordination polymers. The present results illustrate how the appropriate modification of the 1,3-alternate thiacalix[4]arene derivative works as an approach in preparing desired metallosupramolecular networks. Further exploration of the synthesis and physical properties of novel metallosupramolecular network species based on the modification of the thiacalix[4]arene derivatives is underway.

 

Experimental

General. All chemicals were of reagent grade and used without further purification. FT-IR spectra were measured with a ThermoFisher Scientific Nicolet iS 10 FT-IR spectrometer using KBr pellet in the range 4000-400 cm−1. The ligand K4TCTC was prepared as reported in the literature.3a

Synthesis of [K2@(TCTC)(𝜇2-Ni)(H2O)4(CH3OH)2]·3H2O}n (1). K4TCTC (20.0 mg, 0.023 mmol) dissolved in water was added to a solution of Ni(NO3)2·6H2O (16 mg, 0.055 mmol) in methanol. Green single crystals suitable for X-ray analysis were obtained by slow evaporation of the resulting solution. mp 321-322 ℃ (decomp.). IR (KBr pellet): 3401, 1598, 1436, 1413, 1330, 1220, 1070, 1018, 846, 802, 761 cm−1.

Table 2.aSymmetry operations: (A) − x, − y+2, − z; (B) − x+1, − y+1, − z.

Figure 2.Crystal structure of 1-3: (a) core coordination unit and (b) a-axis packing structure. Hydrogen atoms and uncoordinated solvent molecules are omitted.

Table 3.Distances of K+···π interactions (Å) in 1 -3

Synthesis of [K2@(TCTC)(𝜇2-Co)(H2O)4(CH3OH)2]·3H2O}n (2). Synthetic procedures of 2 as pink single crystals are same as for 1 except the use of Co(ClO4)2·6H2O (16 mg, 0.044 mmol). mp 318-319 ℃ (decomp.). IR (KBr pellet): 3399, 1604, 1436, 1415, 1330, 1220, 1103, 1072, 1016, 846, 802, 761 cm−1.

Synthesis of [K2@(TCTC)(𝜇2-Zn)(H2O)4(CH3OH)2]·3H2O}n (3). Synthetic procedures of 3 as colorless single crystals are same as for 1 except the use of Zn(ClO4)2·6H2O (16 mg, 0.043 mmol). mp 334-335 ℃ (decomp.). IR (KBr pellet): 3419, 1648, 1434, 1415, 1313, 1220, 1072, 1016, 943, 848, 798, 761 cm−1.

X-ray Crystallography. Crystal data were collected on a Bruker SMART APEX II ULTRA diffractometer equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) generated by a rotating anode. The cell parameters for the compounds were obtained from a least-squares refinement of the spot (from 36 collected frames). Data collection, data reduction, and semi-empirical absorption correction were carried out using the software package of APEX2.9 All of the calculations for the structure determination were carried out using the SHELXTL package.10 In all cases, all nonhydrogen atoms were refined anisotropically and all hydrogen atoms except coordinated and lattice water molecules were placed in idealized positions and refined isotropically in a riding manner along with the their respective parent atoms. In the cases of coordinated water molecules, the initial positions of the hydrogen atoms were obtained from difference electron density maps and refined with riding constraints. The H atoms in the lattice water molecules did not involve in structure analysis because they could not be found from difference electron density maps. In all cases, the methyl group (C34) in the methanol molecule coordinated to M2 ion (M = Ni, Co, or Zn) was disordered over two sites (C34, C34') with equal site occupancy factors of 0.5. Also, the O7W and O7W' atoms in one lattice water molecule have two positions with the site occupancy factors of 0.5, respectively. The crystallographic data for 1-3 are summarized in Table 1.