Sustainable Hexa-Nuclear Copper(II) Metallamacrocycle for Heterogeneous Dehydrocyclization-Based Synthesis of Quinolines and 1,8-Naphthyridines.

We demonstrate a convenient approach to synthesize a sustainable and novel hexa-nuclear copper(II) metallamacrocycle, [Cu6{N3C6H3(COO)2}6(phen)6], using inexpensive and readily available precursors viz., Cu(NO3)2·6H2O, 5-azidoisopthalic acid and o-phenanthroline in a stoichiometric ratio. Structural elucidation by single-crystal X-ray diffraction (SC-XRD) confirms a covalently connected hexagonal chairlike topology in which six Cu(II) centers are interconnected by azidoisophthalate ligands, affording hydrophobic cavities of around 6-7 Å. Significant π-π interactions between ancillary phenanthroline coligands promote clustering among hexa-nuclear metallamacrocycles, thereby enhancing the accessibility of the catalytic site. Leveraging these characteristics, the complex was employed as a heterogeneous catalyst for solvent-free dehydrogenation of secondary alcohols and single pot stepwise process (SPSS) cascade synthesis of quinoline and 1,8-naphthyridine derivatives presenting moderate-to-good yields, satisfactory turnover numbers (TONs) and turnover frequencies (TOFs). The robust catalyst exhibits very good recyclability, and mechanistic insights are revealed through control experiments. This catalytic protocol integrates oxidant-free and solvent-free conditions, minimizes waste generation, ensures high atom economy, and catalyst recyclability enables efficient catalysis under moderate temperatures.