Covalent Postassembly Modification and Water Adsorption of Pd3 Self-Assembled Trinuclear Barrels

Three new ditopic imidazole ligands (2-4) were synthesized in high yields and characterized by various spectroscopic techniques. These ligands resulted in the formation of [3 + 6] self-assembled trinuclear barrels (5-7) in quantitative yields by stoichiometric combination of individual ligands and Pd(NO3)2 in DMSO. All the three assemblies (5-7) were characterized by (1)H NMR and ESI-MS analysis, and subsequently, structures of the complexes 5 and 6 were confirmed by single-crystal X-ray diffraction studies. Structure analysis reveals the presence of NO3(-) counteranions in the intermolecular channels/pockets, which could potentially act as H-bonding sites between adsorbed water molecules within the channels. In fact, both the assemblies (5 and 6) showed water uptake (136.58, and 123.78 cm(3) g(-1), respectively) at ambient temperature under maximum allowable humidity. In addition, free aldehyde group present in the bridging ligand in complex 7 provides reactive site for postassembly modification. Herein, Knoevenagel condensation with Meldrum's acid was utilized under mild conditions by targeting aldehyde group appended in prefabricated complex 7 and transformed into a different complex (8) with altered functional group. Such postassembly functionalization enables incorporation of a new functional group without disrupting the integrity of the trifacial structure.