Expanded Aza-Helicenes Featuring Inner Rim Metal Coordination

Abstract

Extended fused 1,10-phenanthroline derivatives bearing heteroatoms at the inner rim, especially those extended into helical structures, are highly intriguing molecules for metal–ligand coordination and host–guest chemistry. However, such fully aromatic ligands have rarely been reported, possibly due to the combined synthetic challenges arising from ring strain, steric hindrance, and the coordination ability of nitrogen atoms on aromatic rings. Herein, we report two such derivatives: a coplanar ligand (4N7) and an expanded azahelicene (5N9) synthesized via thermodynamically driven multifold ring-closing olefin metathesis. Their structures were unambiguously confirmed by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. Both compounds exhibited strong binding affinities toward transition metals, with the Ag(I) complexes (Ag@4N7, Ag@5N9) showing the highest stability─substantially exceeding that of a nonfused, flexible analog (5N-OP). Thermodynamic analysis revealed that structural preorganization in the fused ligands imparts a significant entropic contribution to metal complexation. This work establishes thermodynamically driven ring-closing metathesis as a powerful approach for accessing synthetically challenging azahelicene ligands and highlights an entropy-based design principle for achieving highly efficient metal binding.