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

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.