Cu(I)/Zn2+ exchange has no geometrical effect in a cyclic [4]rotaxane whereas it induces rearrangement in a simpler [3]rotaxane

The dynamic behaviour of a [4]rotaxane consisting of two rigid bis-macrocycles and two linear fragments threading the rings, was compared to that of a simple model, namely a [3]rotaxane, consisting of roughly one half of the full [4]rotaxane molecule. Although the coordination sites are strictly identical in both compounds, it was observed that demetalation or metal exchange (CuI/Zn2+) has very limited effect on the [4]rotaxane geometry whereas the same reactions have a profound effect on the geometrical arrangement of the [3]rotaxane. This observation is a clear demonstration that the number of mechanical bonds is determining in terms of flexibility and rigidity, the more constrained [4]rotaxane being more difficult to distort than its smaller congener.