Efficient proton-templated synthesis of 18- to 38-membered tetraimino(amino)diphenol macrocyclic ligands: structural features and spectroscopic properties

Abstract

A whole range of Robson-type tetraiminodiphenol macrocyclic ligands have been prepared as their perchlorate salts [H₄L](ClO₄)₂ in high yield (ca. 90%) by a single-step [2 + 2] condensation reaction between 4-methyl(or tert-butyl)-2,6-diformyl(or diacyl)phenols and α,ω-diaminoalkanes (C₂-C₁₂) in the presence of acetic acid and NaClO₄. The reduction of these 18- to 38-membered macrocyclic salts with NaBH₄ have afforded corresponding tetraaminodiphenol macrocycles H₂L'. The X-ray crystal structures of two of the tetraiminodiphenol macrocycles with the C₂ and C₄ lateral chains have been determined, and the optimized configurations for all of the macrocycles have been obtained by molecular mechanics calculations. The macrocycles have been characterized by elemental analysis and by IR, absorption, emission, and NMR spectroscopic study. The protonated tetraiminodiphenol macrocycles exhibit strong fluoroscence in methanol, acetonitrile, and nitromethane and undergo quenching when treated with triethylamine. The neutral macrocycles H₂L, isolated by treating [H₄L](ClO₄)₂ with excess of triethylamine, lack luminescence, as do the reduced tetraaminodiphenol macrocycles H₂L'. The hydrolytic cleavage of [H₄L](ClO₄)₂ has been studied.