Protonation induced self-complementarity of rod-like Cu(NTA)(bpeH) units and their layered supramolecular self-assembly entrapping heptamer like water clusters

Abstract

Molecular self-assembly is the most important methodology for synthesizing novel materials including nano-materials. The ultimate challenge for scientists is to predict the self-assembled pattern of a set of molecular components. The labile nature of self-assembly arises from weak intermolecular forces which makes the outcome diverse, as small changes in physicochemical condition changes the course of self-assembly strongly. A one-pot reaction of 1,2-bis(4-pyridyl)ethylene (bpe), potassium salt of nitrilo-tri-acetic acid (K3NTA) and basic CuCO3 in an acidic medium (around pH = 4) give rise to a metal–organic complex with molecular formula C18H17CuN3O6.6(H2O) (henceforth it will be referred to as Cu-NTA-bpeH, where bpeH is singly protonated 1,2-bis(4-pyridyl)ethylene) whose self-assembly in the solid state shows that even a simple protonation event can drastically influence the course of the self-assembly. Further complication may arise from the participation of solvent water molecules in an unpredictable manner like the appearance of a new water-carboxylate heptamer cluster in the present crystal structure. The study of this simple system shows that pH-controlled protonation can lead to self-complementary metal organic units with interesting supramolecular architecture. The self-assembled system has been characterized by X-ray structural studies, Hirshfeld fingerprint and surface analysis, DFT computational studies as well as TG-DTA studies. The present crystal, which is analogous to clathrate hydrate, can possibly act as gas absorption and storage medium for future use.

Keywords

• Water heptamer cluster
• Heptamer mimic
• NTA
• Bpe
• Hydrogen bonding interaction
• Hydrophobic cavity