Controlled Self-Assembly of [2]-Catenanes Via Tuning the Length and Conjugation Area of Flexible Building Units

The controlled self-assembly of [2]-catenanes represents a significant frontier in supramolecular chemistry, where the delicate balance between linker length and intermolecular interactions dictates the resulting topology. In this study, half-sandwich rhodium-based building blocks and heteroaryl amide ligands were employed as model systems to systematically investigate how linker length and conjugation area influence self-assembly behavior. We found that pyrazine and thiophene ligands (L1 and L2) bearing methylene (-CH2-) linkers readily assemble into D-type [2]-catenanes; however, extending the linker to two methylenes (-CH2-CH2-, L3) completely suppresses catenane formation, yielding only metallacycles. Crystallographic analysis reveals that the ethylene spacer increases the interannular distance beyond the effective range of π-π stacking interactions, thereby favoring the thermodynamically more stable single macrocycle. Remarkably, substitution of thiophene with 2,2'-bipyridine significantly expands the conjugation area while maintaining the linker length, successfully restoring D-type [2]-catenane formation. This result demonstrates that enhanced π-electron delocalization can compensate for the distance penalty. Collectively, our findings establish a cooperative "linker length-conjugation area" mechanism for regulating product topology, providing theoretical guidance for the rational design of flexible long-chain catenanes.