A novel trisprotonated β-dialdiminate cryptand

Abstract

The β-dialdiminate cryptand L⁴ can be isolated in the tris protonated state, [H₃L⁴]³⁺, from the condensation of malonaldehyde (generated in situ from its tetraacetal derivative) with tris-2-aminoethylamine(tren) in [3 + 2] mode. The cryptate cannot be deprotonated under aqueous conditions, but the neutral cryptand can be made by treatment with KOH in dry methanol. This cryptand is water-sensitive, but may be treated with copper salts under anhydrous conditions to generate highly insoluble copper(II) derivatives. The reduced form of the cryptand L⁵ can be isolated upon borohydride reduction, and used to make dicopper μ-hydroxo cryptates, where strong interaction between the pair of encapsulated paramagnetic cations leads to effective magnetic silence. Crystal structures of L⁴ in three of its tris protonated salts and of the solvated neutral ligand L⁴ are presented, together with that of a μ-hydroxo dicopper(II) cryptate of the fully reduced cryptand, [Cu₂(OH)L⁵]·3ClO₄·MeCN.