A tetranuclear Sn(IV) 4-thiazolecarboxylate complex: synthesis, structure and catalytic behavior in the bulk ring-opening polymerization of glycolide

2.1 Synthesis and general characterization

Among the large variety of metal complexes investigated as initiators, tin complexes have shown excellent catalytic activity for ROP of cyclic esters to produce high molecular weight polymers with narrow molecular weight distribution [17–20]. For instance, the industrially most commonly used catalyst for the preparation of PGA is stannous octanoate [19]. In this study, we selected the chelating-type ligand [4-tzc]⁻ to react with various tin salts including tin(II) dichloride, tin(II) oxalate, tin(II) trifluoromethanesulfonate, triphenyltin(IV) chloride and dimethyltin(IV) dichloride. Single crystals of 1 were obtained from the reaction of 4-Htzc and dimethyltin(IV) dichloride with NaOH in MeOH-H₂O mixed solvent under ambient conditions, while only white precipitates or microcrystalline products were afforded in the other cases. Complex 1 is stable to air and moisture and insoluble in common solvents such as water, dimethyl sulphoxide, alcohol, acetonitrile, chloroform and acetone. The insolubility prevented the characterization of complex 1 by solution NMR. The elemental analysis of 1 matches well with the molecular formula as evaluated by X-ray diffraction analysis (below). In the IR spectrum of 1, the antisymmetric and symmetric carboxylate stretching vibrations are found in the range of 1590–1665 cm⁻¹ and 1400–1440 cm⁻¹, respectively. The absence of the characteristic band ~1680 cm⁻¹ for the free 4-Htzc molecule indicates the complete deprotonation of the carboxyl group, which is also consistent with the crystal structure as described below.

The solid-state ¹³C NMR spectrum of complex 1 (Fig. 1) shows two characteristic peaks of the carboxylate carbon atoms at 161.7 and 166.1 ppm. Signals of the methyl carbon atoms are observed in the range of 1.9–19.1 ppm, while the resonances of the carbon atoms of the triazole ring appear at 124.6–156.1 ppm.

Fig. 1:

The solid-state ¹³C NMR spectrum of complex 1.

2.2 Description of the crystal and molecular structure

Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the monoclinic space group P2₁/c with Z = 2. It shows a tetranuclear structure with crystallographically imposed centrosymmetry. A perspective view of the molecular structure of 1 is shown in Fig. 2a. There are two crystallographically independent Sn(IV) ions (Sn1 and Sn2). The Sn1 ion exhibits a distorted pentagonal bipyramidal geometry, which is completed by seven atoms including four oxygen atoms (O2, O3, O5 and O5#1) from two tzc anions and two bridging O^2– ligands as well as one nitrogen atom (N2) from one tzc anion in the equatorial plane, and two methyl groups in apical positions. The Sn–O bond distances vary from 2.103(3) to 2.479(4) Å, and the Sn–C bond distances are 2.102(6) and 2.106(6) Å, while the Sn–N bond distance of 2.769 (6) Å is similar to those reported for other tetranuclear Sn(IV) complexes [21]. However, the Sn2 ion is five-coordinated by three oxygen atoms (O1, O3#1 and O5) from two tzc anions and one O^2– ligand with the Sn–O bond distances in the range of 1.981(3)–2.207(4) Å and two carbon atoms (C11 and C12) with the Sn–C bond distances of 2.094(8) and 2.098(7) Å, respectively. The Addison parameter τ of 0.66 suggests that the coordination sphere around the Sn2 atom is properly considered as a distorted trigonal bipyramid.

Fig. 2:

Views of (a) the molecular structure of 1 (symmetry code: #1, – x + 1, – y, – z + 2) and (b) the 2D supramolecular architecture (C4–H4···O2 hydrogen bonds are shown as dashed lines; other hydrogen atoms are omitted for clarity).

The tzc anions display two types of coordination modes, where one adopts a chelating/bridging coordination mode via one nitrogen atom and one carboxylate oxygen atom to connect the Sn1 and Sn2#1 atoms, while the other bridges the Sn1 and Sn2 atoms by the carboxylate group in a μ₂-η¹:η¹-syn-syn-bridging fashion. The atoms O²⁻ atom O5 are located in the midpoint of the three Sn(IV) atoms (Sn1, Sn1#1 and Sn2) and adopt a μ₃-bridging mode with Sn–O–Sn angles of 104.6(1)°, 117.7(1)° and 137.4(2)°. The sum of these angles is 359.7°, indicating almost perfect planarity of these oxygen atoms. Two pairs of symmetry-related Sn(IV) atoms are linked by four tzc ligands, forming the tetranuclear molecule with the shortest Sn···Sn distance being 3.383(1) Å.

As is shown in Fig. 2b, adjacent tetranuclear units are connected by weak C4–H4···O2ⁱ hydrogen bond interactions (H···O/C···O distances: 2.616/3.492 Å, angle: 157°, i = x, – y + 1/2, z – 1/2) between thiazole rings and oxygen atoms of the μ₂-η¹:η¹-syn-syn-bridging carboxylate group to generate a 2D supramolecular layer along the bc plane. Furthermore, the uncoordinated carboxylate oxygen atoms are H-bonded to neighboring layers via C3–H3···O4ⁱⁱ hydrogen bonds g (H···O/C···O distances: 2.38/3.016 Å, angle: 125°, i = x – 1, y, z – 1), resulting in a 3D supramolecular architecture.

2.3 Powder X-ray diffraction and thermal stability analyses

The crystalline phase purity of the bulk samples of 1 was confirmed by powder X-ray diffraction (PXRD) (see Fig. 3). The experimental PXRD pattern matches with the simulated one obtained from single-crystal diffraction data, indicative of pure products. To estimate the thermal stability of 1, a thermogravimetric analysis (TGA) of a crystalline sample was performed from room temperature to 800°C under nitrogen atmosphere. As depicted in Fig. 4, complex 1 is thermally stable to ca. 260°C where the weight loss starts to end at ca. 450°C. Furthermore, to verify the stability of complex 1 at the reaction temperature of ROP, PXRD experiments for the as-synthesized sample of 1 were carried out at different temperatures (150°C, 200°C and 230°C). The results indicate that even the PXRD pattern at 230°C is still in agreement with the experimental pattern at room temperature.

Fig. 3:

XRPD patterns for 1 as simulated (black) from the single-crystal data and as-synthesized 1 at 25°C (red), 150°C (blue), 200°C (magenta) and 230°C (green).

Fig. 4:

TGA curve of complex 1.

2.4 Ring-opening polymerization of glycolide

The solid-state bulk polymerization is the key for the fabrication of high molecular weight PGA without any organic solvents, where the melt/solid polycondensation of glycolide generally occurs in the temperature of 180–230°C [22]. Such a reaction process is very fast, and PGA samples can be analyzed by isothermal differential scanning calorimeter (DSC). In this work, the bulk solvent-free melt ROP of glycolide initiated by the as-synthesized complex 1 was studied via the DSC method. The data such as the reaction rate constant, activation energy and reaction order were determined in a similar way as described in our previous work [23]. The representative polymerization data are summarized in Table 1. It was found that complex 1 could initiate the ROP of glycolide with kinetic reaction orders ranging from 0.32 to 0.48 in the range 200–220°C. The dependence of the reaction rate constant on the reaction temperature revealed that the bulk polymerization is relatively faster at higher temperature, which is also consistent with the enthalpy changes. Furthermore, by investigating the relationship between conversion and reaction time at 210°C (Fig. 5), the conversion of glycolide could be completed after 3.5 min. Because the bulk polymerization can afford high molecular weight polyesters, the polymerization of glycolide initiated by complex 1 was carried out at 210°C. It was found that the molecular weight (M_n) of the resultant PGA is 55.5 kDa with the molecular weight distribution (M_w/M_n) of 1.65. On the basis of the above experimental results and previous literature [24–29], one possible reaction mechanism for the present ROP of glycolide may be the coordination-insertion mechanism. The monomer glycolide is coordinated to the Sn(IV) center in the first stage, forming an alkoxyl tin(IV) species [6]. Then, the chain propagation process occurs through a coordination-insertion process.

Fig. 5:

Plot of conversion of glycolide versus reaction time with 1 as an initiator at 210°C.

In summary, a new tetranuclear Sn(IV) 4-thiazolecarboxylate complex was synthesized and structurally characterized. The complex can promote the bulk ROP of glycolide to afford the PGA with high molecular weight. The further work on the design and synthesis of more highly active, low cost and less toxic complexes as initiators toward ROP of glycolide is proceeding in our laboratory.

3.1 Synthesis of (Me₂Sn)₄(μ₃-O)₂(tzc)₄ (1)

A mixture of dimethyltin(IV) dichloride (43.9 mg, 0.2 mmol), Htzc (25.8 mg, 0.2 mmol) and NaOH (8.0 mg, 0.2 mmol) was dissolved in MeOH-H₂O (v/v = 4:1, 6 mL) mixed solvent with stirring for 30 min. The resulting solution was filtrated and then left to stand at room temperature. Colorless needle-like crystals of 1 were isolated in ca. 42% yield (23.9 mg, based on Htzc). – Anal. for C₂₄H₃₂N₄O₁₀S₄Sn₄ (%): calcd. C 29.29, H 2.83, N 4.92; found, C 30.08, H 2.81, N 4.83. – IR (cm⁻¹, KBr pellet): v = 3138 (m), 3119 (w), 3076 (w), 3042 (m), 2917 (w), 1665 (s), 1593 (vs), 1505 (m), 1492 (m), 1436 (s), 1419 (s), 1379 (m), 1331 (s), 1308 (vs), 1220 (m), 1196 (w), 1095 (w), 946 (m), 883 (m), 1331 (s), 1308 (vs), 1220 (m), 1196 (w), 1095 (w), 946 (m), 883 (m), 856 (m), 843 (m), 780 (s), 755 (m), 663 (s), 623 (m), 583 (m), 566 (m), 520 (m), 487 (s), 431 (m). – ¹³C NMR (100.6 MHz, 9.4 T, solid state): δ = 1.9, 3.4, 18.5, 19.1 (all methyl carbon atoms), 124.6, 131.1, 151.5, 151.5, 156.1, 156.1 (all carbon atoms on the thiazole rings), 161.7, 166.1 (carboxylate carbon atoms).

3.2 Glycolide polymerization procedure

The isothermal experiments of glycolide polymerization were performed on a Perkin-Elmer DSC under a nitrogen atmosphere (50 mL min⁻¹), and the DSC data were analyzed by Pyris Kinetic Analysis software. Indium (156.6°C) was employed as a standard sample for the calibration of temperature and heat. A mixture of glycolide (10 g) and complex 1 (24.6 mg) was grinded for 30 min at room temperature, and a sample (5 mg) was taken for DSC measurements. The experiments were run in aluminum pans. The samples were heated at a heating rate of 600°C min⁻¹ to the temperature in the range of 200–220°C, and then kept for 15 min.

Molecular weights (M_n and M_w) and molecular weight dispersities (M_w/M_n) were measured by gel permeation chromatography. The measurements were performed at 40°C on a Waters 1525 binary system equipped with a Waters 2414 Refractive Index detector and a Waters 2487 dual λ absorption (UV, λ_abs = 220 nm) detector. In the case of the analyses performed using a solution of sodium trifluoroacetate (0.68 g, 5 mmol) in 1,1,1,3,3,3-hexafluoro-2-propanol (1000 mL) as eluent at a flow rate of 0.6 mL min⁻¹, a system of four Styragel HR columns (7.8 × 300 mm; range 103–106 Å) was employed. The molecular weights were calculated with respect to poly(methyl methacrylate) standards (M_n ranging from 7000 to 200000).

3.3 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 [31], and the program Saint was used for integration of the diffraction profiles [32]. The structure was solved by Direct Methods using Shelxs of the Shelxtl program package and refined anisotropically for all non-H atoms by full-matrix least squares on F² with Shelxl [33–36]. In general, hydrogen atoms were located geometrically and allowed to ride during the subsequent refinement. Crystallographic data and numbers pertinent to data collection and structure refinement are summarized in Table 2. Selected bond lengths and angles are listed in Table 3.

CCDC 1441860 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.