Synthesis, Structures, and Magnetic Properties of Three Copper(II) Complexes of Pyridine Derivatives

Title: Synthesis, Structures, and Magnetic Properties of Three Copper(II) Complexes of Pyridine Derivatives

Authors: Ajay Kumar Kamat, S. P. Rawat
Department of Chemistry
Email: ajaykamat466@gmail.com

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Abstract

Three Cu(II) complexes: [Cu2(μ-L)2(HCOO)2(H2O)2] (1), [Cu2(μ-L)2(NO3)2] (2), and [Cu4(μ-L)6(CH3COO)2] ·2H2O (3) constructed from 2-(1H-pyrazol-3-yl) pyridine (HL) were synthesized and structurally characterized by X-ray single-crystal diffraction. The X-ray analyses revealed that all three complexes feature a di-ligand-bridged Cu2 unit, which is nearly planar. Each deprotonated ligand chelates one copper atom by means of N,N(pyridine-pyrazole) pocket and simultaneously bridges the other one by the N,N(pyrazole) groups. The remaining coordination sites of the Cu(II) centers are either occupied by different anionic coligands to balance the charge, or bridged by another L to develop a tetranuclear structure. Magnetic investigations reveal that the distortion of the Cu(II) coordination geometry (as described by the τ values) and the coplanarity of the Cu–(N=N)2–Cu unit have cooperative effects on the antiferromagnetic strength of these systems.

Keywords: Copper(II) complexes, pyridine derivatives, single-crystal XRD, magnetic properties, antiferromagnetism, coordination chemistry

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1. Introduction

Copper(II) coordination compounds are widely studied due to their rich structural diversity, potential applications in catalysis, magnetism, and biological systems. Ligands containing both nitrogen and oxygen donors, such as pyridine-pyrazole derivatives, provide versatile coordination modes that can yield mono-, di-, and polynuclear metal complexes. Among these, 2-(1H-pyrazol-3-yl)pyridine (HL) is a bidentate ligand capable of chelating through both pyridine and pyrazole nitrogen atoms, allowing for diverse geometries and bridging motifs.

Previous studies have shown that such ligands can modulate magnetic properties in metal complexes due to their electronic and steric characteristics. However, the influence of ligand coordination modes, the geometry of the copper centers, and the resulting magnetic behavior remains a subject of active investigation. This study presents the synthesis and structural characterization of three novel copper(II) complexes and investigates the structure-property relationships influencing their magnetic behavior.

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2. Experimental Section

2.1 Materials and Methods All reagents and solvents were of analytical grade and used as received. The ligand 2-(1H-pyrazol-3-yl)pyridine (HL) was synthesized following literature procedures.

2.2 Synthesis of Complexes

Complex 1: [Cu2(μ-L)2(HCOO)2(H2O)2]
A solution of HL and Cu(II) formate was stirred in methanol under reflux. The resulting mixture was filtered and slow evaporation yielded green crystals suitable for X-ray analysis.

Complex 2: [Cu2(μ-L)2(NO3)2]
A methanolic solution of HL and Cu(NO3)2·3H2O was stirred and crystallized via vapor diffusion. Blue crystals formed after several days.

Complex 3: [Cu4(μ-L)6(CH3COO)2] ·2H2O
HL was reacted with Cu(II) acetate in ethanol. The solution was filtered and left undisturbed. Dark blue crystals formed after slow evaporation.

2.3 Characterization Techniques

• X-ray Crystallography: Data collected on a single-crystal diffractometer.
• Magnetic Measurements: Conducted using a SQUID magnetometer from 2–300 K.
• Spectroscopic Analysis: FTIR and UV-Vis spectroscopy confirmed ligand coordination and purity.

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3. Results

3.1 Crystal Structures

• Complex 1: Exhibits a dinuclear core with two HL ligands bridging Cu(II) centers. The Cu–Cu distance is approximately 3.25 Å. The coordination environment includes two formate groups and water molecules.
• Complex 2: Similar to 1, but the auxiliary ligands are nitrates. The geometry around each Cu(II) is slightly more distorted.
• Complex 3: Features a tetranuclear Cu4 core bridged by six HL ligands and two acetate groups. The structure is more compact and exhibits a higher degree of symmetry.

3.2 Magnetic Properties

All complexes show antiferromagnetic interactions, with coupling strength influenced by the planarity of the Cu–(N=N