Supramolecular associates of para-aminobenzoic acid with N- and N,O-heterocyclic molecules

Abstract

A series of novel supramolecular multicomponent complexes of para-aminobenzoic acid (PABA) with N-containing cyclic molecules and aza-crown ethers, (H₂PPz)·(PABA)₂·2H₂O (1), (H₂teta)·(PABA)₂·H₂O (2), (H₂cyclen)·(PABA)₂·2H₂O (3), (H₂-diazonia-12C4)·(PABA)₂·2H₂O (4), (H-azonia-18C6)·(PABA)·3H₂O (5) (PPz = piperazine, teta = meso-5,7,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, cyclen = 1,4,7,10-tetraazacyclododecane, diaza-12-crown-4 = 1,7-dioxa-4,10-diazacyclododecane and aza-18-crown-6 = 1,4,7,10,13-pentaoxa-16-azacyclooctadecane) has been synthesized. Crystallographic studies reveal that all complexes are organic salts formed due to the proton transfer from the carboxylic group of PABA to the amino group of the macrocycle, and thus are comprised of PABA anion, macrocyclic mono- or dication and a variable number of water molecules. In all complexes the cations and anions are associated via (H₂N⁺)NH⋯O(COO⁻) charge-assisted hydrogen bonds. The number of PABA involved in the complex equals to the number of N-protonated binding sites in the cyclic molecule. Water molecules play a significant role in the formation of the supramolecular architecture.