Rapid Access to Encapsulated Molecular Rotors via Coordination-Driven Macrocycle Formation

Abstract

Macrocycle formation that relies upon trans metal coordination of appropriately placed pyridine ligands within an arylene ethynylene construct provides rapid and reliable access to molecular rotators encapsulated within macrocyclic stators. Showing no significant close contacts to the central rotators, X-ray crystallography of Ag^I -coordinated macrocycles provides plausibility for unobstructed rotation or wobbling of rotators within the central cavity. Solid-state ¹³ C NMR of Pd^II -coordinated macrocycles supports the notion of unobstructed movement of simple arenes in the crystal lattice. Solution ¹ H NMR studies indicate complete and immediate macrocycle formation upon the introduction of Pd^II to the pyridyl-based ligand at room temperature. Moreover, the formed macrocycle is stable in solution; a lack of significant changes in the ¹ H NMR spectrum upon cooling to -50 °C is consistent with the absence of dynamic behavior. The synthetic route to these macrocycles is expedient and modular, providing access to rather complex constructs in four simple steps involving Sonogashira coupling and deprotection reactions.

Keywords: arylene ethynylenes; crystal engineering; macrocycles; metal complexes; molecular rotors.