Anion-dependent thermo-responsive supramolecular superstructures of Cu(ii) macrocycles

Abstract

This investigation highlights the role of counter anion towards the formation of thermo-responsive supramolecular assemblies of pre-defined discrete molecular Cu(II) macrocycles. A new heteroditopic ligand (L) composed of two terminal triazolyl–pyridine units and a central pyridine-2,6-carboxamide unit connected via ethylene spacers predominantly forms dinuclear Cu(II) macrocycles [L₂Cu₂]X₄ (X = ClO₄⁻, NO₃⁻, Cl⁻) in the presence of respective monoanion-based Cu²⁺ salts. In contrast, dianionic SO₄²⁻-based Cu²⁺ salt produces a mixture of both dinuclear [L₂Cu₂(SO₄)₂] and mononuclear [LCu(SO₄)] Cu(II) macrocycles. Importantly, detailed molecular structure determinations revealed the presence of multiple secondary interactions of the different counter anions with the corresponding macrocycles. The field emission scanning electron microscopy (FESEM) studies confirmed the ability of the individual macrocycles to further assemble upon slow evaporation of the solvent. Morphologically pure rod, sphere, and sheet-like superstructures were obtained for ClO₄⁻, NO₃⁻, and Cl⁻ monoanion-based macrocycles, respectively. In contrast, a mixture of linear ribbon and microflower-like assemblies were obtained for SO₄²⁻-based macrocycles. Moreover, it was found that upon heating, the rod-like assembly of [L₂Cu₂](ClO₄)₄ was gradually transformed into nanoparticles through a microparticle formation, which regenerated the rod-like assembly upon standing at room temperature. This was established from the FESEM, atomic force microscopy (AFM), and dynamic light scattering (DLS) studies.