A mononuclear cobalt(III) 3-(2-pyridyl)-5-phenyl-1,2,4-triazolato complex: hydrothermal synthesis, crystal structure, thermostability, and DFT calculations

2.1 Computational results of the 1-Hppt molecule

The optimized geometries of two different conformations shown in Fig. 1 have been obtained. The calculated bond lengths N1–C2, N1–N2, C2–C3, and N4–C3 are 1.335, 1.351, 1.477, and 1.404 Å, respectively.

The molecular total energies (E_total), frontier orbital energy levels (E_HOMO and E_LUMO), and the gaps (ΔE_{(LUMO–HOMO)}) of the two different conformations have been calculated, and the results are listed in Table 1. Compared with the trans-conformation I, the cis-conformation II displays lower total energies, representing the preferred geometry. The surfaces of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the two conformations are shown in Fig. 2. According MO theory, the HOMO and LUMO are the most important factors that influence the properties of a compound [23].

Fig. 1:

Two different conformations of 1-Hppt.

Fig. 2:

Isodensity surfaces of HOMO and LUMO for 1-Hppt (left: conformation I, right: conformation II).

The natural charges and electron configuration for the conformation II have been calculated by using natural bond orbital (NBO) analysis. As shown in Table 2, Mulliken charges and natural charges of the N1, N2, N3, and N4 atoms are obtained as negative values. The natural charge of N3 (−0.52) is more negative than that of the N1 (−0.36) and N2 (−0.26) atoms in the triazole ring. The natural charge of N4 in the pyridine ring is −0.43. The results demonstrate that all four N atoms provide donor position to coordinate with metal ions.

2.2 Synthesis of complex 1

The Co(III) complex 1 was synthesized from CoF₃ and 1-Hppt in water by hydrothermal reaction at 140 °C for 3 days in the presence of terephthalic acid. The latter was not found to be incorporated in the final product which proved to be [Co(ppt)₃] (1) as dark red crystals. The phase purity of bulk 1 was proved by a comparison of the observed and simulated powder X-ray diffraction (PXRD) diagrams as described below.

2.3 Structural description of complex 1

Single-crystal X-ray diffraction analysis reveals that the complex crystallizes in the triclinic system, space group P1̅ with Z = 2 (Table 3). The asymmetric unit of the complex consists of one crystallographically independent Co(III) ion and three [ppt]^– anions. The auxiliary ligand (terephthalic acid) did not take part in coordination. As shown in Fig. 3a, the six-coordinated Co(III) center presents a slightly distorted octahedral geometry. The four equatorial positions are occupied by three nitrogen atoms from triazole rings of three different ligands (Co1–N1 1.874(3), Co1–N5 1.900(3), Co1–N9 1.883(3) Å) and one nitrogen atom from a pyridine ring (Co1–N12 1.976(3) Å). Two nitrogen atoms from pyridine rings of two different ligands are located at the axial positions (Co1–N4 1.953(3), Co1–N8 1.954(3) Å) (Table 4). Compared to the previous reports, the Co–N bond lengths of the title complex are similar to those of Co(III) complexes (<2.0 Å [24]), and deviate from the range of Co(II)–N bond lengths (2.037–2.187 Å [25]). For the ligand, the bond lengths of N1–C2, N1–N2, C2–C3, and N4–C3 are 1.332(5), 1.345(4), 1.453(5), and 1.360(5) Å, respectively, in accordance with the calculated values. Adjacent units are connected to each other via two kinds of π−π stacking interactions between benzene and pyridine rings (Fig. 3b). The distance between two benzene rings is 3.909 Å, but 3.720 Å between benzene and pyridine rings, which is close to the π–π interaction distances reported in [26].

Fig. 3:

(a) Coordination environment of the Co(III) ion in complex 1; (b) intermolecular π–π stacking in the crystal structure of complex 1.

It is interesting to note that all [ppt]^– ligands exhibit conformation II in the structure of the complex, confirming that the preferred conformation calculated by DFT is correct. The ligands display uniform coordination modes with pyridine N4 and triazole N1 atoms chelating the Co(III) ions. The coordination ability of N3 with the most negative charge is probably restricted by steric hindrance.

2.4 Thermogravimetric analysis and PXRD of complex 1

The thermal behavior of 1 was studied by thermal gravimetric (TG) measurements under nitrogen atmosphere with a heating rate of 10 °C min⁻¹. In order to confirm the phase purity of the compound, a PXRD experiment was carried out (Fig. 4). The experimental pattern matches with the simulated pattern, indicating the phase purity of the compound within the limits of experimental error. The sample was heated to 800 °C (Fig. 5). The TG curve shows that a continuous weight loss of 92.3 % is observed in the temperature range from 263 to 660 °C, due to the decomposition of the ligands (calcd. 91.84 %). The final residue mass is 7.7 % of the initial mass, which coincides with the calculated value of elemental cobalt (8.2 %), determined by PXRD (PDF 05-0727).

Fig. 4:

Observed and simulated PXRD diagrams of complex 1.

Fig. 5:

TGA curve for complex 1.

4.1 Materials and instruments

Commercially available solvents, such as 2-cyanopyridine, phenyl hydrazide, absolute methanol, sodium metal, terephthalic acid, and CoF₃, were used as received without further purification (Xi’an Haixin Company, China). ¹H and ¹³C nuclear magnetic resonance (NMR; Beijing Oxford Instrument Company, China) spectra were taken with a Varian 400 spectrometer using tetramethylsilane as an internal standard. NMR spectra were obtained from solutions in dimethyl sulfoxide (DMSO). Elemental analyses (C, H, and N) were performed on a Vario EL III analyzer (Nanjing Hua Xin Analysis Instrument Manufacturing Company, China). Infrared (IR) spectra were obtained using KBr pellets on a BEQ VZNDX 550 FTIR (Shimadzu Corporation) instrument within the 400–4000 cm⁻¹ region. The phase purity of the bulk or polycrystalline samples was confirmed by PXRD (Brucker Company, Germany) measurements executed on a Rigaku RU200 diffractometer at 60 kV, 300 mA with CuK_α radiation (λ = 1.5406 Å), with a scan speed of 5° min⁻¹ and a step size of 0.02° in 2θ. TG analysis was performed on a SETARAM Setsys16 (Nanjing Research Institute, China) instrument under N₂ atmosphere with a heating rate of 10 °C min⁻¹.

4.2 Synthesis of the 1-Hppt ligand

The ligand was synthesized according to [27]. Sodium metal (0.1 g) was added (carefully) to 40 mL absolute methanol followed by the addition of 2-cyanopyridine (2.1 g). The resulting reaction mixture was stirred with a magnetic stirrer for 80 min. Thereafter, it was allowed to cool and phenyl hydrazide (2.1 g) was added. The solution was refluxed for a further 3 h until a yellow solid complex precipitated. The precipitate was filtered off, washed with hot absolute ethanol, and dried in vacuum. The white ligand was precipitated, yield 70 % (based on 2-cyanopyridine). M.p.: 191.8–192.6 °C. – Analysis for C₁₃H₁₀N₃: calcd. C 74.97, H 4.85, N, 20.18; found C 74.92, H 4.83, N, 20.12 %. – IR (film): 3404(s), 3213(s), 3057(m), 2361(m), 1670(s), 1612(s), 1589(w), 1446(s), 1313(s), 1180(w), 1053(m), 792(m), 717(m), 696(w), 579(w) cm⁻¹. – ¹H NMR (400 MHz, [D₆]DMSO, 25 °C, ppm): δ = 6.95 (d, 1H, Ph-H⁴), 7.49–7.51 (d, 2H, Ph-H2/H6), 7.55–7.57 (dd, 2H, Ph-H³/H⁵), 8.60–8.61 (dd, 1H, Pz-H³), 7.87–7.93 (d, 1H, Pz-H⁶), 7.53 (t, 1H, Pz-H⁴), 8.17–8.19 (t, 1H, Pz-H⁵), 10.20 (s, 1H, triazole-H). – ¹³C NMR (75 MHz, [D₆]DMSO, 25 °C, ppm): δ = 163.24, 150.75, 148.20, 147.85, 137.04, 134.70, 131.13, 128.31, 127.76, 124.87, 120.81.

4.3 Synthesis of complex 1

A mixture of CoF₃ (5.8 mg, 0.05 mmol), terephthalic acid (8.3 mg, 0.05 mmol), 1-Hppt (11.1 mg, 0.05 mmol), and water (6 mL) was sealed in a 10 mL Teflon-lined stainless steal vessel and heated at 140 °C for 3 days and then cooled to room temperature at a rate of 5 °C h⁻¹. Dark red crystals of complex 1 were collected in a yield of 40 % (based on Co). – Analysis for C₃₉H₂₇CoN₁₂: calcd. C 64.81, H 3.77, N 23.26; found C 64.75, H 3.73, N 23.20 %. – IR (film): 3599(s), 3447(s), 3063(m), 2822(m), 2662(s), 2548(s), 1690(w), 1618(s), 1576(s), 1464(w), 1425(m), 1287(m), 1111(m), 941(w), 785(w), 735(w), 696(w), 529(w).

4.4 X-ray crystallography

All diffraction data were collected on a Bruker Smart Apex CCD diffractometer with graphite-monochromatized CuK_α radiation (λ = 1.54178 Å) at 293(2) K. Absorption corrections were applied using Sadabs [28]. The structure was solved by Direct Methods using the Shelxs program of the Shelxtl package and refined with Shelxl-97 [29]. The hydrogen atoms were located geometrically. All non-hydrogen atoms were refined anisotropically. Crystal data and numbers pertinent to data collection and refinement are given in Table 3. Selected bond lengths and bond angles are listed in Tabel 4.

CCDC 1034723 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.

4.5 Computational details

Geometry optimization was carried out at the all-electron level using the Gaussian 03 quantum chemistry program package [30] at the B3LYP/6-31G(d) [31, 32] level of theory. Mulliken population analysis, total molecular energies, and NBO calculations were also performed on the B3LYP/6-31G(d) level.