Synthesis and characterization of the Co(ii), Ni(ii), and Cu(ii) complexes with a 1,2,4-triazine derivative ligand

3.2.1 Compositions

The manufactured ligand was employed to capture Co(ii), Ni(ii), and Cu(ii) ions from aqueous solutions. The ligand–metal complexes are referred to as Co–L, Ni–L, and Cu–L, respectively. After preparing and purifying the ligand–metal complexes, their elemental compositions, including the presence and number of carbon, hydrogen, nitrogen, sulfur, and metal atoms, were analyzed. The metal and water contents were determined using a gravimetric technique, while the contents of carbon, hydrogen, nitrogen, and sulfur were determined using a CHN elemental analyzer. The compositions of the synthesized complexes, including the quantities of carbon, hydrogen, nitrogen, sulfur, metal, and water, are described in detail below:

1. Complex Co–L: the complex has the chemical formula C₁₇H₃₂BrN₄O₁₂SClCo, with a molecular weight of 690.66 g/mol. The microanalytical data show the following percentages (%): found (calculated): C, 29.66 (29.54); H, 4.80 (4.63); N, 7.96 (8.11); S, 4.85 (4.63); Cl, 5.40 (5.13); Co, 8.36 (8.53); and H₂O, 26.91 (26.06).

2. Complex Ni–L: the complex has the chemical formula C₁₇H₂₈BrN₄O₁₀SClNi, with a molecular weight of 654.42 g/mol. The microanalytical data show the following percentages (%): found (calculated): C, 31.31 (31.17); H, 4.52 (4.28); N, 8.45 (8.56); S, 4.73 (4.89); Cl, 5.65 (5.42); Ni, 8.74 (8.97); and H₂O, 22.19 (22.0).

3. Complex Cu–L: the complex has the chemical formula C₁₇H₁₈BrN₄O₅SClCu, with a molecular weight of 569.28 g/mol. The microanalytical data shows the following percentages (%): found (calculated): C, 35.59 (35.83); H, 3.45 (3.16); N, 10.0 (9.84); S, 5.49 (5.62); Cl, 6.08 (6.23); Cu, 11.30 (11.16); and H₂O, 9.72 (9.49).

The measured values for carbon, hydrogen, nitrogen, sulfur, chlorine, metals, and water match the calculated values, confirming the accuracy of the chemical analysis. The consistency between the measured and calculated data provides strong evidence supporting the reliability and precision of the analytical methods used. The compositions of the ligand–metal complexes suggest a 1:1 reaction stoichiometry (ligand to metal), which aligns with the proposed general compositions of [CoL(H₂O)₄]‧Cl·6H₂O for the ligand complex with Co(ii), [NiL(H₂O)₄]·Cl·4H₂O for the ligand complex with Ni(ii), and [CuL(H₂O)Cl]·2H₂O for the ligand complex with Cu(ii), respectively. The suggested chemical formulas C₁₇H₃₂BrN₄O₁₂SClCo, C₁₇H₂₈BrN₄O₁₀SClNi, and C₁₇H₁₈BrN₄O₅SClCu represent the corresponding Co–L, Ni–L, and Cu–L complexes.

3.2.2 UV-Visible spectroscopy and conductivity measurements

DMSO solutions containing the uncoordinated ligand, the Co–L complex, the Ni–L complex, and the Cu–L complex were analyzed using a UV-Vis spectrometer, and the resulting spectra are presented in Figure 2. The analysis of these solutions offered valuable insights into the spectral characteristics of the different ligand–metal complexes, which can be further explored to enhance our understanding of their structural and electronic properties. Table 1 lists the characteristic UV-Vis band wavelengths and their assigned transitions for the free ligand and its complexes. The UV-Vis spectrum of the uncoordinated ligand exhibits a broad range of absorption. It absorbs light wavelengths from 200 to 600 nm. Within this range, it displays a prominent absorption band at 520 nm, as well as two medium-intensity twin bands at 268 and 324 nm. The strong 520 nm band is approximately three times more intense than the twin bands. The width of the intense 520 nm band is around 125 nm. The 520 nm band is likely attributed to n → π* electronic transitions, which involve the promotion of an electron from a non-bonding orbital (n) to an anti-bonding π orbital. In contrast, the twin bands at 268 and 324 nm can be attributed to π → π* electronic transitions, where an electron is promoted from a bonding π orbital to an anti-bonding π* orbital. The ligand does not absorb any light beyond 600 nm. The UV-Vis spectrum of the uncoordinated ligand exhibits an enhanced intensity and broadening of the 520 nm band upon coordination with Co(ii), Ni(ii), and Cu(ii) ions. Additionally, the band’s maximum shifts from 520 nm in the uncoordinated ligand to 535 nm in the coordinated ligand. This observed shift in the band maximum and the broadening of the 520 nm band suggest a change in the electronic structure of the ligand upon coordination with the metal ions. The enhanced intensity of the 520 nm band may indicate an increase in the molar absorptivity of the ligand, potentially due to the formation of a more stable coordination complex. After coordination with the metal ions, the uncoordinated ligand’s UV-Vis spectrum, which originally featured twin absorption bands at 268 and 324 nm, is now characterized by a single absorption band at 303 nm in the ligand complexes with Co(ii), Ni(ii), and Cu(ii) ions. This bathochromic shift and simplification of the absorption spectrum suggest a change in the electronic structure of the ligand upon coordination with the metal centers.

Figure 2

UV-Vis absorption profiles of the uncoordinated ligand and the ligand–metal complexes.

Conductivity measurements offer valuable insights into the charge dynamics and ionization states of organic–metal compounds. In this study, we used a digital conductivity meter to measure the conductivity of the uncoordinated ligand and its metal complexes at room temperature. This approach allows us to elucidate the charge transfer and electron delocalization within these systems, which is crucial for understanding their structural, electronic, and functional characteristics. We found that the molar conductance of the uncoordinated ligand in DMSO was 8.13 Λm, confirming its non-conducting nature. In stark contrast, the molar conductance values of the Co–L, Ni–L, and Cu–L complexes in DMSO were 52.10, 61.05, and 30.28 Λm, respectively. The significantly higher values for Co–L and Ni–L strongly indicate their electrolytic properties, which can be attributed to the presence of one chloride ion outside the coordination sphere of these complexes, facilitating ion mobility and thus higher conductivity. The molar conductance of the Cu–L complex, while lower than Co–L and Ni–L, still indicates a slightly electrolytic nature. This suggests that, although a chloride ion is within its coordination sphere, it can still dissociate and exist as a free ion in solution. This dissociation increases the solution’s conductivity, explaining the relatively higher conductivity of the Cu–L complex compared to a neutral complex in DMSO.

3.2.3 FT-IR spectroscopy

Infrared spectroscopy is a powerful analytical technique that offers valuable insights into the coordination environment of ligands. This method reveals changes in the spectra of metal complexes, providing information about their structure and bonding. The synthesized ligand–metal complexes were analyzed using FT-IR spectroscopy, and the recorded spectra are included in the Supplementary Data file. The resulting IR spectral data (in cm⁻¹) obtained from their FT-IR analyses are as follows:

1. Complex Co–L: 3,300 and 3,077 ν(N–H) of the –NH₂ group, 3,198 ν(N–H) of the –NH group, 1,732 ν(C═O), 1,644 δ_sciss(N–H), 1,582 ν(C═N), 1,486 ν(C═C), 1,450 δ(C–O–H), 1,350 ν(C═S), 1,294 ν(C–O), 1,223 ν_asym(C–N), 1,171 ν(C–Br), 1,065 ν_sym(C–N), 1,018 ν(C‒C), 868 ν(C═S), 690 ν(O–H) out-of-plane bending, 545 ν(M‒O), 497 ν(M‒N).

2. Complex Ni–L: 3,310 and 3,080 ν(N–H) of the –NH₂ group, 3,200 ν(N–H) of the –NH group, 1,731 ν(C═O), 1,644 δ_sciss(N–H), 1,585 ν(C═N), 1,487 ν(C═C), 1,450 δ(C–O–H), 1,349 ν(C═S), 1,294 ν(C–O), 1,222 ν_asym(C–N), 1,172 ν(C–Br), 1,065 ν_sym(C–N), 1,018 ν(C‒C), 860 ν(C═S), 690 ν(O–H) out-of-plane bending, 548 ν(M‒O), 495 ν(M‒N).

3. Complex Cu–L: 3,308 and 3,074 ν(N–H) of –NH₂ group, 3,199 ν(N–H) of –NH group, 1,726 ν(C═O), 1,644 δ_sciss(N–H), 1,580 ν(C═N), 1,487 ν(C═C), 1,451 δ(C–O–H), 1,356 ν(C═S), 1,294 ν(C–O), 1,224 ν_asym(C–N), 1,172 ν(C–Br), 1,065 ν_sym(C–N), 1,018 ν(C‒C), 858 ν(C═S), 691 ν(O–H) out-of-plane bending, 553 ν(M‒O), 498 ν(M‒N).

FT-IR spectral analysis of the uncoordinated ligand and its corresponding ligand–metal complexes shows significant changes in the frequency and intensity of various characteristic bands. These observed shifts can be directly attributed to the anticipated modifications in the symmetry and electronic structure of the ligand upon complexation, in comparison to the uncoordinated state. The uncoordinated ligand consists of six functional groups: –OH, –NH₂, –NH, C═O, C═N, and C═S. The –OH group vibration produced a distinct and intense IR band at 3,414 cm⁻¹. The –NH₂ group vibration generated a distinct and intense IR band at 3,306 cm⁻¹ and a weaker band at 3,075 cm⁻¹. The –NH group exhibited a distinct and intense absorption band at 3,198 cm⁻¹. After the ligand coordinated with Co(ii), Ni(ii), and Cu(ii) ions, the characteristic band of the ν(O‒H) vibrational mode was no longer observed in the FT-IR spectra of the resulting ligand–metal complexes, indicating deprotonation. The formation of complexes between the ligand and the studied metal ions leads to the deprotonation of the ligand. This results in the disappearance of the characteristic IR band corresponding to the ν(O‒H) vibrational mode of the hydroxyl functional group. The deprotonated hydroxyl group of the ligand participates in complexation with the Co(ii), Ni(ii), and Cu(ii) ions, leading to the emergence of new and medium-intensity IR bands at 545, 548, and 553 cm⁻¹ for the Co–L, Ni–L, and Cu–L complexes, respectively, which originate from the ν(M‒O) vibrations [21]. The interaction between the ligand and the studied metal ions had a negligible effect on the vibrational frequencies of the –NH₂, –NH, C═O, and C═S functional groups. Their frequencies were only slightly altered or remained largely unaffected in the ligand–metal complexes compared to the uncoordinated ligand. The band corresponding to the ν(C═N) vibrations exhibited significant shifts when the ligand formed complexes with the studied metal ions. This band was observed at 1,527 cm⁻¹ in the uncoordinated ligand, but it shifted to 1,582 cm⁻¹ in the Co–L complex, 1,585 cm⁻¹ in the Ni–L complex, and 1,580 cm⁻¹ in the Cu–L complex. The participation of the ligand’s C═N bond in complexation with the Co(ii), Ni(ii), and Cu(ii) ions led to the emergence of new and medium-intensity IR bands at 497, 495, and 498 cm⁻¹ for the Co–L, Ni–L, and Cu–L complexes, respectively, which could be attributed to the ν(M‒N) vibrations, indicating that the ligand coordinated with the studied metal ions through the nitrogen lone pair electrons of the C═N group [22]. The acquired spectral, elemental, and conductivity data were employed to suggest appropriate chemical structures for the synthesized Co–L, Ni–L, and Cu–L complexes, as shown in Figure 3.

Figure 3

Chemical structures proposed for synthesized ligand–metal complexes.

3.2.4 XRD reflections and TEM imaging

XRD and TEM analysis provided detailed information about the structural properties, phase purity, morphology, and particle characteristics of the synthesized ligand–metal complexes. The XRD reflections for the uncoordinated ligand and ligand–metal complexes were acquired at an accelerating voltage of 40 kV, a current of 30 mA, over an angular range of 5–70°, and a temperature of 25°C; the recorded XRD spectra are included in the Supplementary Data file. The XRD instrument utilized a Cu K_α1 source and a Ge monochromator. The XRD spectroscopic data presented in Table 2 highlight the most significant peaks for the uncoordinated ligand and the ligand–metal complexes, providing valuable insights into their crystalline structure and phase composition. The XRD analysis of the uncoordinated ligand revealed four distinct peaks: a very intense peak, a moderately strong peak, and two peaks of medium intensity. The most prominent and strongest peak was observed at a Bragg angle 2θ of 21.4222°. Additionally, a moderately strong peak was found at a Bragg angle 2θ of 25.310°. Furthermore, two medium-intensity diffraction peaks were detected at 2θ values of 28.105 and 42.130°. The Co–L complex demonstrated a single, extremely intense diffraction peak at a Bragg angle 2θ of 21.450°. Additionally, the X-ray diffraction pattern of this complex exhibited ten supplementary low-intensity peaks. This X-ray diffraction profile of the Co–L complex indicates a highly crystalline structure with a regular and well-ordered arrangement of atoms or molecules within its frameworks, suggesting a highly organized and structured material. The XRD patterns of the Ni–L and Cu–L complexes showed two and three XRD reflections, respectively. The Ni–L complex exhibited a very strong diffraction peak at a Bragg angle 2θ of 21.759°, and a medium-intensity peak at a 2θ angle of 33.077°. Conversely, the Cu–L complex presented a most prominent and intense peak at a Bragg angle 2θ of 21.895°, along with two medium-intensity peaks at 2θ angles of 20.172° and 25.642°.

Micrographs obtained using transmission electron microscopy (TEM) offer comprehensive insights into the structural features, distributions, shapes, and particle dimensions of the materials, equipping researchers with a thorough understanding of their properties and potential applications. In this work, we employed a high-resolution microscope to visualize and explore the microstructural variations among the synthesized ligand–metal complexes, providing valuable information about their structural characteristics. The resulting images are shown in Figures 4–7, allowing for a detailed examination and analysis of the sample’s morphology and composition. The high-quality and well-focused TEM images show that most of the uncoordinated ligand particles had an elongated, rod-like shape. The ligand rods varied significantly in size and thickness, with an average length of approximately 300 nm and an average width of around 50 nm. Specifically, the rods ranged in length from around 200 nm to 400 nm, and their widths spanned a range of 40–60 nm. The coordination of the ligand with the Co, Ni, and Cu ions resulted in a transition from a rod-like to a more irregular, stone-like morphology of the particles. The resulting stone-like particles of the Co–L, Ni–L, and Cu–L complexes exhibited a range of shapes, sizes, and thicknesses, and these structures tended to group together, forming cluster-like arrangements. The differences in shapes, sizes, and thicknesses, as well as the cluster-like arrangement of these stone-like particles, could have resulted in a significantly increased surface area of the complexes, potentially enhancing their catalytic performance to a greater degree. The increased surface area and unique morphology of the stone-like particles may have also contributed to other beneficial properties, such as improved adsorption capacity or enhanced mass transfer kinetics, which could further optimize the catalytic activity of the Co–L, Ni–L, and Cu–L complexes.

Figure 4

TEM images of the uncoordinated ligand.

Figure 5

TEM images of the Co–L complex.

Figure 6

TEM images of the Ni–L complex.

Figure 7

TEM images of the Cu–L complex.

3.2.5 Examination of thermal patterns

Spectral, elemental, and conductivity data obtained were used to propose suitable chemical structures for the synthesized Co–L, Ni–L, and Cu–L complexes. These data suggest general compositions of [CoL(H₂O)₄]‧Cl·6H₂O for the ligand complex with Co(ii), [NiL(H₂O)₄]·Cl·4H₂O for the ligand complex with Ni(ii), and [CuL(H₂O)Cl]·2H₂O. To further confirm the proposed compositions and structures of the created ligand–metal complexes, the next step is to characterize them through thermogravimetric measurements. The thermal degradation patterns can provide additional support for the proposed compositions and structures of the synthesized ligand–metal complexes. This analysis can also offer valuable insights into the thermal stability and decomposition characteristics of these coordination compounds. Table 3 lists the expected thermal degradation patterns for the uncoordinated ligand and the ligand–metal complexes, derived from their thermograms shown in Figure 8. An examination of the products’ thermograms revealed the following observations:

1. A well-aligned trend emerged between the calculated and experimental results. This finding supports the stoichiometry identified through empirical inquiry. The robust consistency between the calculated and experimental data offers compelling evidence that the stoichiometry accurately reflects the composition of the synthesized ligand–metal complexes.

2. The free ligand demonstrated high thermal stability, withstanding temperatures up to 230°C without degradation. However, combining the ligand with Co(ii), Ni(ii), and Cu(ii) ions significantly diminished its thermal stability, lowering it to below 200°C, likely due to the relatively weak ligand–metal bonds. Forming the ligand–metal bonds reduces the thermal endurance and mechanical durability of the free ligand. This implies that the ligand’s structural strength and resistance to physical stress are weakened, rendering it more susceptible to thermal degradation and damage at lower temperatures compared to the uncombined ligand.

3. The thermal stability of the uncoordinated ligand and its metal complexes showed a decreasing trend in the following order: free ligand > Co–L > Ni–L > Cu–L. This pattern is influenced by various factors, including the strength of the metal–ligand bond, the size and charge of the metal ion, as well as the specific coordination environment surrounding the metal center. These aspects all contribute to the observed differences in thermal stability.

4. The complexes formed by the ligand and Ni(ii) or Cu(ii) ions underwent a three-step degradation process. Conversely, the Co–L complex exhibited a two-step degradation process.

5. The Co(ii), Ni(ii), and Cu(ii) ion complexes contained six, four, and two hydrated water molecules, respectively. Their thermal degradation began at approximately 200, 175, and 150°C, respectively. This indicates that increased hydration correlates with greater thermal stability: the Co(ii) complex, with six hydrated water molecules, was the most stable, while the Cu(ii) complex, with two, was the least stable.

6. The thermal degradation of the synthesized metal–ligand complexes resulted in metal oxide compounds as the final products. The metal oxide compounds of CoO and CuO were contaminated with some residual carbon from the decomposition process, whereas the NiO contained no residual carbon from the decomposition process.

Figure 8

Thermograms of the uncoordinated ligand and the ligand–metal complexes.