SYNTHESIS OF SALICYLAMIDE-BASED TRANSITION METAL COMPLEXES AND DETERMINATION OF THEIR STRUCTURAL INDIVIDUALITY THROUGH QUANTUM-CHEMICAL CALCULATIONS, SEM-EDS, AND IR SPECTROSCOPIC ANALYSES

ABSTRACT

This study presents the synthesis, structural elucidation, and spectroscopic characterization of a series of transition metal (II) complexes derived from salicylamide ligands. Complexes of the general formulas [MCl₂L₃•H₂O] and [MCl₂(L₃)₂•2(H₂O)] (where M = Mn, Fe, Co, Ni, Cu; L₃ = salicylamide) were obtained under controlled pH conditions (7–9) in ethanolic medium, followed by slow crystallization over 17 days. Structural investigations revealed that the 1:1 metal-to-ligand complexes adopt a tetrahedral geometry with four sp³ bonds, whereas the 1:2 complexes exhibit octahedral coordination environments with six sp³d² bonds, stabilized by coordinated water molecules. Elemental analysis confirmed the proposed stoichiometries with yields ranging from 71–87%. Infrared spectroscopy demonstrated significant shifts in ν(OH), ν(NH), and ν(C=O) stretching vibrations relative to free salicylamide, confirming coordination via the carbonyl oxygen and hydroxylgroups.

АННОТАЦИЯ

В данной работе представлены синтез, структурное исследование и спектроскопическая характеристика ряда комплексов переходных металлов, полученных на основе салициламидных лигандов. Комплексы с общими формулами [MCl₂L₃•H₂O] и [MCl₂(L₃)₂•2(H₂O)] (где M = Mn, Fe, Co, Ni, Cu; L₃ = салициламид) были получены при контролируемых значениях pH (7–9) в этанольной среде с последующей медленной кристаллизацией в течение 17 дней. Структурные исследования показали, что комплексы металл-лиганд в соотношении 1:1 имеют тетраэдрическую геометрию с четырьмя sp³-связями, тогда как комплексы в соотношении 1:2 имеют октаэдрическую координационную среду с шестью sp³d²-связями, стабилизированными координированными молекулами воды. Элементный анализ подтвердил предложенную стехиометрию с выходами 71–87%. Инфракрасная спектроскопия выявила значительные сдвиги валентных колебаний ν(OH), ν(NH) и ν(C=O) относительно свободного салициламида, подтверждая координацию через карбонильный кислород и гидроксильные группы.

Keywords: Transition metal (II) complexes, salicylamide, coordination geometry, infrared spectroscopy, electronic structure, SEM-EDS analysis, metall (II) complex, novocaine hydrochloride, infrared spectroscopy, coordination geometry, microstructural analysis, SEM-EDS characterization.

Ключевые слова: Комплексы переходных металлов (II), салициламид, координационная геометрия, инфракрасная спектроскопия, электронная структура, анализ SEM-EDS, комплекс металла, гидрохлорид новокаина, инфракрасная спектроскопия, координационная геометрия, микроструктурный анализ, характеристика SEM-EDS.

Introduction

Salicylamide (L₃) is an aromatic organic compound containing both amide and hydroxyl functional groups in the ortho-position of a benzene ring. Due to this unique structural arrangement, it possesses significant donor properties, enabling it to act as a bidentate ligand in the formation of coordination complexes with transition metal ions. The ability of salicylamide to coordinate through its carbonyl oxygen and hydroxyl groups provides stability to the resulting complexes and strongly influences their structural, electronic, and biological characteristics [1-3]. In this study, a series of coordination complexes of Mn (II), Fe (II), Co (II), Ni (II), and Cu (II) with salicylamide have been synthesized and investigated. The obtained complexes were characterized by elemental analysis, infrared (IR) spectroscopy, UV–Vis spectroscopy, and powder X-ray diffraction methods. In addition, scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) was employed to determine their morphology and elemental composition. Particular attention has been devoted to the infrared spectral analysis of [MCl₂(L₃) ₂·2(H₂O)]-type complexes in comparison with free salicylamide. The IR spectra clearly demonstrate significant shifts in the vibrational frequencies of hydroxyl, amino, and carbonyl groups upon complex formation, confirming the coordination of salicylamide ligands to the central metal ions. Moreover, new absorption bands attributed to M–O and M–Cl vibrations were observed in the low-frequency region, further supporting the successful synthesis of the desired coordination compounds [4-8].

The structural features and coordination behavior of these complexes are of considerable importance, as they provide insight into the electronic density distribution, geometrical changes, and bond interactions within the molecules. Furthermore, the stability and symmetry of the complexes suggest potential applications in catalysis and biological systems, making the study of salicylamide-based transition metal complexes both scientifically and practically significant [9].

Materials and methods 

Figure 1. The complex [MCl₂(L₃)₂·(H₂O)₂] obtained from MCl₂ and salicylamide

Metal (II) chloride (0.001 mol) and salicylamide (0.002 mol) were dissolved in 20 mL of ethanol, and the two solutions were mixed. To maintain the pH in the range of 7–9, NaOH was added dropwise. The solution was kept at room temperature in a fume hood for 17 days until crystals were formed. The obtained crystals were washed with ethanol to remove impurities.

Figure 2. The [MCl₂L₃·H₂O] complex obtained from MCl₂ and salicylamide

Metal (II) chloride (0.001 mol) and salicylamide (0.001 mol) were dissolved in 20 mL of ethanol, and the two solutions were mixed. To maintain the pH within the range of 7–9, NaOH was added dropwise. The mixture was then stirred magnetically at 40 °C for 2 hours. The solution was left at room temperature in a fume hood for 17 days until crystals formed. The resulting crystals were washed with ethanol to remove impurities. Analysis of the crystal composition revealed that this salicylamide complex also contained coordinated water molecules equivalent to those of salicylamide. In this case, the 1:1 metal-to-salicylamide complex [MCl₂L₃·H₂O] exhibits a tetrahedral configuration characterized by four sp³ bonds, whereas the 1:2 complex [MCl₂(L₃) ₂·(H₂O) ₂] displays an octahedral configuration with six sp³d² bonds.

Results and discussion

Elemental analysis of C, H, N and O in the [MnCl₄] (L¹·H)₂ complex was carried out using a Flash Smart (Thermo Fisher Scientific) analyzer. The instrument operates based on a modified Dumas combustion method, where the amounts of gases released during combustion are determined chromatographically. For the analysis of C, H, N 3–3.5 mg of the sample was sealed in aluminum foil capsules along with 4–5 mg of vanadium(V) oxide and combusted at 960 °C. Oxygen analysis was performed using 2.8–3.2 mg of sample in silver foil capsules at 1050 °C. All processes were conducted under oxygen (100 mL/min) and helium (140 mL/min) gas flows. The complex showed good solubility in water and ethanol, poor solubility in chloroform, and limited solubility in acetone.

Results and discussion

Quantum Chemical Calculations

Figure 3. Bond lengths and molecular electrostatic potential (MEP) distribution in [CuCl₂L₃·H₂O] and [CuCl₂(L₃) ₂·(H₂O) ₂.

Figure 4. Molecular orbital contour diagrams of [CuCl₂L₃·H₂O] (a, b) and [CuCl₂(L₃) ₂·(H₂O) ₂] (c, d), showing orbital interactions in ground and excited states

Understanding the electronic and spatial structures of salicylamide-based Cu (II) complexes is crucial for determining their chemical reactivity, coordination ability, and potential biological activity. The molecular structures of [CuCl₂L₃·H₂O] and [CuCl₂(L₃) ₂·(H₂O) ₂] are compared based on electron density distribution, with a detailed analysis of their coordination degree, geometric variations, and intermolecular interactions. The 1:1 complex exhibits higher electron density and coordination capacity, providing more reactive centers, which suggests stronger activity in catalytic or biological systems. In contrast, the 1:2 complex demonstrates more stable electron density, greater geometric symmetry, and higher structural stability. In both complexes, the donor atoms of salicylamide transfer electrons to the copper ion, thereby stabilizing the molecule. In the molecular structures of [CuCl₂L₃·H₂O] and [CuCl₂(L₃) ₂·(H₂O) ₂], the carbonyl oxygen of salicylamide forms a coordination bond with the central atom. A redistribution of electron density also occurs involving the chloride ligands and the oxygen atoms of water molecules. Consequently, in complexes obtained at different stoichiometric ratios, the bond lengths between the carbonyl oxygen, chloride, water oxygen, and copper ions vary.

The structures and atomic charges of the 1:1 and 1:2 Cu (II)-salicylamide complexes were analyzed. The results show that in the 1:2 complex, the two ligand molecules are arranged symmetrically. This arrangement is attributed to the larger molecular size and the need to minimize steric repulsion, causing the ligands to position themselves as far apart as possible.

Elemental analysis is one of the fundamental techniques used to determine the composition of synthesized coordination compounds. It provides precise quantitative data on the percentages of carbon (C), hydrogen (H), nitrogen (N), chlorine (Cl), and the corresponding metal (M) in the complexes. The experimental results are compared with theoretical calculations based on the molecular formulas of the complexes. The close agreement between calculated and found values confirms the successful synthesis and the proposed stoichiometry of the complexes. As highlighted in Chapter II, a series of complexes were synthesized using L₃ (salicylamide) with Mn (II), Fe (II), Co (II), Ni (II), and Cu (II) ions. Their composition and structure were investigated by elemental analysis (Table 1), UV–Vis and IR spectroscopic studies, and powder X-ray diffraction techniques. In addition, the composition of the complexes was also confirmed using the SEM-EDS method.

Figure 5. Microstructure and EDS analysis of [FeCl₂L₃·H₂O]

Figure 6. Microstructure and EDS analysis of [CuCl₂L₃·H₂O]

 IR Spectroscopic Analysis

Comparison of the IR spectroscopic analysis of [MCl₂(L₃) ₂·2(H₂O)] complexes with the IR spectra of salicylamide. Salicylamide is an organic compound with amide and hydroxyl groups located in the ortho-position of the benzene aromatic ring. Accordingly, its IR spectra exhibit O–H stretching vibrations at 3398 cm⁻¹, N–H stretching vibrations at 3190 cm⁻¹, and C=O stretching vibrations in the range of 1674–1590 cm⁻¹ (Figure 6). In the complexes obtained with salicylamide, these characteristic vibrations are shifted due to complex formation. In particular, the IR spectra of [MCl₂(L₃) ₂·2(H₂O)] complexes show noticeable changes both compared to each other and relative to the IR spectra of free salicylamide (Figure 7).

In the range of 3421–3418 cm⁻¹, strong absorption bands corresponding to the stretching vibrations of water molecules and hydroxyl groups within the complexes were observed. Furthermore, direct coordination of the ligand to the metal through coordination bonding caused the C–H stretching vibrations of the aromatic ring to shift slightly to a lower region around 2940 cm⁻¹. In addition, coordination of the carbonyl group led to its stretching vibrations being observed in the 1728–1636 cm⁻¹ range. This resulted in a shift of 54–46 cm⁻¹ between the carbonyl stretching vibrations of the free ligand and those of the complexes. Coordination bonds involving M–O and M–Cl were also identified in the 620–604 cm⁻¹ region.

Figure 8. IR spectra of [MCl₂(L₃) ₂·2(H₂O)] complexes: [MnCl₂(L₃) ₂·2(H₂O)] (blue), [FeCl₂(L₃)₂·2(H₂O)] (red), [CoCl₂(L₃)₂·2(H₂O)] (pink), [NiCl₂(L₃)₂·2(H₂O)] (green)

Conclusion

A series of Mn (II), Fe (II), Co (II), Ni (II), and Cu (II) coordination complexes with salicylamide ligands were successfully synthesized under controlled pH conditions. Elemental analysis confirmed the stoichiometry of both 1:1 and 1:2 metal-to-ligand complexes, with yields ranging from 71–87%. Structural studies revealed that 1:1 complex adopt tetrahedral geometries with sp³ hybridization, while 1:2 complexes display octahedral configurations stabilized by coordinated water molecules and characterized by sp³d² hybridization.

Infrared spectroscopic analysis demonstrated significant shifts in O–H, N–H, and C=O stretching vibrations compared to free salicylamide, confirming coordination through carbonyl oxygen and hydroxyl groups. Additional bands in the low-frequency region further supported the presence of M–O and M–Cl bonds.

Quantum chemical calculations and SEM-EDS characterization complemented the experimental data, providing insights into electron density distribution, molecular orbital interactions, and elemental composition. The results highlight the strong donor capacity of salicylamide, its ability to stabilize transition metal complexes, and the structural differences arising from metal-to-ligand ratios. These findings suggest that salicylamide-based transition metal complexes possess stable geometries, tunable electronic properties, and potential applications in catalysis and biological systems.

References:

1. C.-Y. Hsu, G. Hameed, I. Ahmad, et al. Nanomagnetic nickel complex based  on  salicylamide  and  L-proline  ligands  for  tetrazole  synthesis  //  Nanoscale Adv. – 2025. – Vol. 7. – PP. 2663–2676.
2. Unknown  author.  Salicylamide  derivatives  for  iron  and  aluminium sequestration: synthesis to complexation studies // J. Trace Elem. Med. Biol. –  2018. – Vol. 50. – PP. 580–590.
3. Ji  H.  Kim,  K.  Ali,  Y.  Oh,  Y.  Seo.  Design,  synthesis  and  biological evaluation of histone deacetylase inhibitor with salicylamide zinc binding group // Medicine (Baltimore) – 2022. – Vol. 101. – PP. 1–8.
4. C.-Y. Hsu, et al. Nanomagnetic nickel complex based on salicylamide and L-proline ligands as heterogeneous catalyst for tetrazole synthesis // Nanoscale Adv. – 2025. – Vol. 7. – PP. 2663–2676.
5. Unknown author. Salicylamide derivatives for Fe and Al sequestration // J. Trace Elem. Med. Biol. – 2018. – Vol. 50. – PP. 580–590.
6. Lei Zhang, J. Y. Zhang. Microwave assisted synthesis of salicylamide via BCl₃ mediated coupling // J. Comb. Chem. – 2005. – Vol. 7. – PP. 622–626.
7. H.  Naeimi,  M.  Moradian.  Synthesis  of  nitro  Schiff  bases  from  5 -nitrosalicylaldehyde and diamines, and their Co(II) complexes // J. Coord. Chem.  –2010. – Vol. 63. – PP. 156–162.
8. P. Nagapandiselvi, C.  Baby, R. Gopalakrishnan. A new Schiff base: (E)-4-((4-chlorophenylimino)methyl)-2-methoxyphenol –  crystal structure and thermal behavior // J. Mol. Struct. – 2014. – Vol. 1056–1057. – PP. 110–120.
9. H. Meng, C. Wang, W. Xi, X. Song, L. Wang. A cationic  tetrahedral Zn(II) cluster based on a new salicylamide imine multidentate ligand: synthesis, structure and fluorescence sensing study // Dalton Trans.  –  2019.  –  Vol. 48. –  PP. 12326–12335.