Coordination ability of amino acid hydrazide ligands and their influence on magnetic properties in copper(ii) coordination polymers

Abstract

Newly synthesized amino acid hydrazide ligands (L1 = N,N′-di-L-leucine hydrazide; L2 = N,N′-di-L-phenylalanine hydrazide) have shown excellent reactivity and stability, as well as extraordinary coordination ability. Reactions of L1/L2 with copper(II) ions and 4,4′-bipyridine (4,4′-bpy) as a secondary spacer yielded new coordination polymers {Cu(L1)_0.5(4,4′-bpy)(H₂O)(NO₃)}_n·1.5nH₂O (1) and {Cu₃(L2)₂(4,4′-bpy)(H₂O)₂}_n(NO₃)_2n·3nH₂O (2). UV/vis spectroscopy together with structural analysis showed that the deprotonated ligands L1²⁻/L2²⁻ with six available donor atoms per ligand have the ability for strong chelation to copper(II) ions, acting as bis-bidentate (L1²⁻) and bridging bi + tridentate (L2²⁻) ligands. Compound 1 contains copper(II) ions simultaneously bridged with short hydrazide bridges (4.725 Å) and long 4,4′-bpy spacers (11.144 Å) forming a two-dimensional coordination network. In compound 2, a pair of L2²⁻ molecules bridges three copper(II) ions, which are further interconnected with 4,4′-bpy into an overall one-dimensional chain-like structure. Although bulkier, the phenylalanine ligand (L2) was found to be more flexible compared to the leucine ligand (L1), exhibiting additional intramolecular contact: cation–π interaction. In both compounds, the hydrazide (–N–N–) bridge mediates the antiferromagnetic interaction between copper(II) ions. Hydrogen bonds have a strong influence on the crystal packing and physical (magnetic) properties. The reported compounds are first examples of hydrazide-bridged metal centres featuring amino acid-based ligands.