Conformational mobility of 7,16-bis(4-methoxybenzyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane in molecular and proton-transfer complexes: X-ray and DFT studies

Abstract

For the first time the bibracchial-crown ether 7,16-bis(4-methoxybenzyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (1) and its molecular and proton-transfer complexes were isolated and characterized by X-ray single-crystal diffraction. Hydrogen bonding between the neutral molecules is present in the binary complex 1·(H₂NCS)₂ (3) giving rise to a tape structure. The proton migration from an inorganic acid to a macrocyclic molecule results in doubly protonated cations (1-H₂)²⁺ and (2-H₂)²⁺ (where 2 is the parent 7,16-dibenzyl-1,4,10,13-tetraoxo-7,16-diazacyclooctadecane) giving rise to the ionic complexes (1-H₂)·[ClO₄]₂·2H₂O (4), (1-H₂)·[NbF₆]₂·2H₂O (5), (1-H₂)·[TaF₆]₂·2H₂O (6), (1-H₂)·[BF₄]₂ (7) and (2-H₂)·[ClO₄]₂·2H₂O (8) sustained by a system of charge-assisted hydrogen bonding. The macrocyclic entities in 1–8 differ by the conformation of the crown ring and the arrangement of the pendant arms. To rationalize the different conformations of 1 in comparison with the relative compounds based on 2, the theoretical quantum chemical calculations on the DFT (B3LYP) level were performed. The contribution of the methoxy group that provides the C–H⋯O(OCH₃) hydrogen bonding to the overall system of intermolecular interactions has been estimated by comparison with the complexes based on 2.