A robust heterodimeric bis-Rh(iii)-porphyrin macrocycle for the self-assembly of a kinetically stable [2]-rotaxane

We report the self-assembly of a robust di-nuclear tetralactam macrocyle based on two symmetric components: a Rh(iii) bis-porphyrin and a bis-pyridyl ligand. We probe the binding properties of the tetralactam macrocycle with adipamide derivatives using H-1 NMR spectroscopy. On the one hand, we show that the binding of the adipamide having linear alkyl chains that can thread through the intact macrocycle's cavity produces a weakly bound 1 : 1 complex stabilized by four intermolecular hydrogen bonds and featuring a preferred binding geometry of [2]pseudorotaxane topology. On the other hand, we detect the formation of two different complexes in the binding of an analogous adipamide possessing bulky stoppers (dumb-bell axle). The initial addition of the dumb-bell guest induces the formation of a 1 : 1 complex featuring fast exchange kinetics on the H-1 chemical shift timescale and exo-cyclic (non-threaded) binding geometry. Notably, in the presence of a large excess of the dumb-bell guest and at suitable concentrations of the macrocycle (>5 mM) we observe the emergence of a second species displaying slow exchange kinetics. This observation allows the undisputed assignment of a [2]rotaxane topology to the second complex. The significant increase in kinetic stability featured by the di-nuclear Rh(iii) [2]rotaxane complex contrasts with its reduction in thermodynamic stability (more than one order of magnitude) compared to the previously described di-nuclear Zn(ii) counterpart.

Automated summary

We report the self-assembly of a di-nuclear tetralactam macrocyle based on two symmetric components and investigate its binding properties with adipamide derivatives. We observe the formation of a weakly bound 1:1 complex with linear alkyl chains and a preferred binding geometry of [2]pseudorotaxane topology. In the presence of bulky stoppers, two different complexes are formed, with the second complex displaying slow exchange kinetics and a [2]rotaxane topology.