Engineered π⋯π interactions favour supramolecular dimers $\mathrm{X@[FeL_{3}]_{2}(X = Cl, Br, I)}$: solid state and solution structure

Abstract

Ditopic bis-pyrazolylpyridine ligands usually react with divalent metal ions ($\mathrm{M^{2+}}$) to produce dinuclear triple- stranded helicates $\mathrm{[M_{2}L_{3}]^{4+}}$ or, via $\mathrm{\pi\cdots\pi}$ interactions, dimers of monoatomic complexes $\mathrm{([ML_{3}]_{2})^{4+}}$. 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 $\mathrm{FeX_{2}}$ salts leads to crystallization of the tripartite high-spin assemblies $\mathrm{(X@[Fe(L2)_{3}]_{2})^{3+}(X = Cl, Br or I)}$. The aggregates exhibit exceptional stability, as confirmed by a combination of paramagnetic $\mathrm{^{1}H}$ NMR techniques, demonstrating their persistence in solution. Our investigations further reveal that the guests$\mathrm{Br^{-}}$ and $\mathrm{I^{-}}$ are retained inside the associate in solution but $\mathrm{Cl^{-}}$ is immediately released, resulting in the formation of the empty supramolecular dimer $\mathrm{([Fe(L2)_{3}]_{2})^{4+}}$