Self-assembly properties of zinc(ii) complexes with azo ligands grafted with dodecyl chains: towards supramolecular materials driven by coordination and hydrophobic effect

Abstract

Two zinc(II) complexes with azopyridine or azopyrimidine featuring dodecyl chains have been synthesized, crystallographically characterized and analyzed in the framework of quantum chemistry. In the mononuclear complex 1, the metal centre has a distorted octahedral geometry with two molecules of 2-(4-dodecyloxyphenylazo)pyrimidine connected in a bidentate fashion, while the remaining coordination sites are occupied by two monodentate nitrate anions. Considering the complex 2, a linear arrangement of three zinc atoms linked by acetate ions was observed. The central zinc atom, situated on the inversion center, is in a nearly perfect octahedral environment, while the outer symmetry-related zinc atoms have a distorted octahedral geometry and they coordinate to three acetate groups and to one molecule of 2-(4-dodecyloxyphenylazo)pyridine in a bidentate manner. In 1, enantiomers locally deracemize so that the coordinated units form homochiral ribbons, while the dodecyl chains from the neighbouring ribbons interdigitate to form layers of molecules. Compound 2 shows a comparable layered packing arrangement. Theoretical investigations of the supramolecular energetic landscape were conducted using density-functional theory (DFT) formalism, quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) computational tools. Quantifying the strength of polar and hydrophobic interactions revealed that H⋯H interactions, hydrophobic in nature, dominate the crystal arrangement of these molecules. The obtained results pave a pathway towards understanding self-organized molecular systems that reach the nano- and micrometer scales.