Magnetic and Spectroscopic Investigation of Selected Mixed-Ligand Complexes

1. Introduction

Coordination chemistry remains a vital field of research due to its relevance in materials science, medicinal chemistry, and catalysis. Mixed-ligand complexes, which contain two or more types of ligands bound to a single metal center, offer enhanced chemical diversity and tunability of their physical properties compared to single-ligand complexes. Their design allows for modulation of geometry, magnetic behavior, and electronic structure through appropriate ligand selection.

Spectroscopic techniques such as UV–Visible and infrared (IR) spectroscopy are indispensable tools for probing ligand coordination, while magnetic susceptibility studies provide insights into the oxidation states, electronic configurations, and stereochemistry of metal ions. Despite extensive studies on single-ligand complexes, fewer systematic studies have explored how mixed-ligand environments influence these properties.

This work aims to synthesize selected mixed-ligand complexes of transition metals and to investigate their magnetic and spectral characteristics, thereby correlating structural features with magnetic behavior.

2. Experimental Section

2.1 Materials and Methods

All chemicals and solvents used were of analytical grade and used without further purification. Metal salts (chlorides or nitrates of Cu(II), Ni(II), Co(II), and Mn(II)) were used as metal sources. The primary ligand was a Schiff base derived from salicylaldehyde and ethylenediamine, while the secondary ligands included pyridine and 1,10-phenanthroline.

2.2 Synthesis of Mixed-Ligand Complexes

The complexes were synthesized by refluxing an ethanolic solution of the metal salt (1 mmol) with the Schiff base ligand (1 mmol) for 30 minutes, followed by the addition of the secondary ligand (1 mmol). The reaction mixture was refluxed for 2 hours under stirring. Precipitated complexes were filtered, washed with cold ethanol and ether, and dried under vacuum.

General Reaction:

where L = pyridine or 1,10-phenanthroline.

2.3 Physical Measurements

• Elemental analysis (CHN): Performed to confirm the composition.
• Molar conductance: Measured in 10⁻³ M DMF solution at room temperature.
• Magnetic susceptibility: Determined at room temperature using a Gouy balance.
• Electronic spectra (UV–Vis): Recorded in DMF solution using a spectrophotometer.
• FT-IR spectra: Recorded in the range 4000–400 cm⁻¹ using KBr pellets.

3. Results and Discussion

3.1 Elemental and Conductivity Analysis

Elemental data matched the calculated values, confirming the proposed stoichiometry. Low molar conductance values (< 20 Ω⁻¹ cm² mol⁻¹) indicated the non-electrolytic nature of the complexes.

3.2 Infrared Spectra

Characteristic bands of the azomethine (C=N) group appeared near 1610 cm⁻¹ in the free Schiff base but shifted to lower frequencies (≈1580 cm⁻¹) in the complexes, indicating coordination via the azomethine nitrogen. New bands in the 500–600 cm⁻¹ region were assigned to M–N and M–O vibrations, confirming metal-ligand bond formation.

3.3 Electronic Spectra

The electronic spectra displayed d–d transitions consistent with octahedral geometry for Co(II) (bands near 10,000–20,000 cm⁻¹), Ni(II) (bands around 8500, 15,000, and 25,000 cm⁻¹), and Mn(II) (broad bands near 17,000 cm⁻¹), while Cu(II) complexes showed a single broad band near 14,000 cm⁻¹, typical of distorted square-planar or square-pyramidal geometry.

3.4 Magnetic Susceptibility

The observed magnetic moments were:

• Co(II): 4.8–5.0 B.M. (high-spin octahedral)
• Ni(II): 2.9 B.M. (octahedral)
• Mn(II): 5.8 B.M. (high-spin octahedral)
• Cu(II): 1.8 B.M. (consistent with one unpaired electron, distorted square planar)

These values corroborate the geometries suggested by their electronic spectra.

4. Proposed Structures

Based on the combined spectral and magnetic data, the following general structures are proposed:

• [M(Schiff base)(pyridine)₂] → octahedral geometry (M = Co, Ni, Mn)
• [Cu(Schiff base)(phen)] → distorted square planar geometry

5. Conclusion

Mixed-ligand complexes of Co(II), Ni(II), Mn(II), and Cu(II) with a Schiff base and neutral donor ligands were successfully synthesized. Spectral and magnetic studies confirmed their coordination modes and geometries. This work demonstrates how ligand combinations can modulate the magnetic and electronic properties of transition metal complexes, providing insights useful for the design of functional coordination compounds.

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