Engineered π⋯π interactions favour supramolecular dimers X@[FeL3]2 (X = Cl, Br, I): solid state and solution structure

Abstract

Ditopic bis-pyrazolylpyridine ligands usually react with divalent metal ions (M²⁺) to produce dinuclear triple-stranded helicates [M₂L₃]⁴⁺ or, via π⋯π interactions, dimers of monoatomic complexes ([ML₃]₂)⁴⁺. The introduction of an additional benzene ring at each end of ligand L increases the number of aromatic contacts within the supramolecular aggregate by 40%, driving the self-recognition process in an irreversible manner. Consequently, the mixing of new bis-pyrazolylquinoline L2 with FeX₂ salts leads to crystallization of the tripartite high-spin assemblies (X@[Fe(L2)₃]₂)³⁺ (X = Cl, Br or I). The aggregates exhibit exceptional stability, as confirmed by a combination of paramagnetic ¹H NMR techniques, demonstrating their persistence in solution. Our investigations further reveal that the guests Br⁻ and I⁻ are retained inside the associate in solution but Cl⁻ is immediately released, resulting in the formation of the empty supramolecular dimer ([Fe(L2)₃]₂)⁴⁺.