Synthesis, structure and Hg²⁺-ion-sensing properties of stable calixazasmaragdyrins

Abstract

The first stable calixazasmaragdyrins containing two meso-sp² and one meso-sp³ carbon atoms were synthesized by [3 + 2] condensation of 2,2′-(1-methylethylidene)bis(pyrrole) and 5,10-diaryltripyrromethane under trifluoroacetic acid catalyzed conditions. The macrocycles were confirmed by high-resolution mass spectrometry and the molecular structures were deduced by detailed 1D and 2D NMR spectroscopy. The single-crystal structural analysis showed the highly strained and distorted nature of a calixazasmaragdyrin macrocycle. The presence of the one meso-sp³ carbon center induces sufficient flexibility into the macrocycle, which, in turn, helps with the stability of the calixazasmaragdyrin macrocycle. The calixazasmargdyrins showed one broad absorption band at ∼425 nm and an ill-defined band at ∼685 nm. The electrochemical studies revealed that the calixazasmaragdyrins are not stable under redox conditions. Because the calixazasmargdyrin macrocycle possesses five pyrrole rings with three ionizable inner NH protons, we investigated anion and cation sensing properties of calixazasmargdyrins. Our studies revealed that the calixazasmaragdyrins do not show any sensing behavior toward anions but exhibited specific sensing behavior toward Hg²⁺ ions as verified by spectral and electrochemical studies.