Synthesis of halogen bonding and crown ether-functionalised Ni(ii)-salen receptors for cation, anion and ion-pair recognition

Abstract

With the objective of utilising supramolecular host–guest chemistry as a potential method to stabilise Lewis basic intermediates in transition metal salen catalytic reactions, the synthesis of two families of Ni(II) salen complexes functionalised with appended supramolecular recognition motifs for the binding of charged guest species is reported. Detailed ¹H NMR anion binding studies in competitive DMSO-d₆/CDCl₃ mixed solvent media reveal the appendage of neutral halogen bonding donor groups facilitates the binding of Cl⁻, Br⁻ and I⁻ halide anions in close proximity to the Ni(II) salen transition metal centre, with halide binding constants >10³ M⁻¹. Furthermore, the incorporation of benzo-15-crown-5 (B15C5) alkali metal cation binding motifs enables the selective, near-quantitative, formation of an intramolecular bis-B15C5 sandwich potassium cation complex, which results in a binding cavity for a co-bound iodide anion, enabling binding of an ion-pair proximal to the Ni(II) salen complex centre.