Anion-Directed Formation and Degradation of an Interlocked Metallohelicate

Although there are many examples of catenanes, those of more complex mechanically interlocked molecular architectures are rare. Additionally, little attention has been paid to the degradation of such interlocked systems into their starting complexes, although formation and degradation are complementary phenomena and are equally important. Interlocked metallohelicate, [(Pd₂L₄)₂]⁸⁺ (<b>2</b>⁸⁺), is a quadruply interlocked molecular architecture consisting of two mechanically interlocked monomers, [Pd₂L₄]⁴⁺ (<b>1</b>⁴⁺). <b>2</b>⁸⁺ has three internal cavities, each of which encapsulates one NO₃^– ion (1:3 host–guest complex, <b>2</b>⊃(NO₃|NO₃|NO₃)⁵⁺) and is characterized by unusual thermodynamic stability. However, both the driving force for the dimerization and the origin of the thermodynamic stability remain unclear. To clarify these issues, BF₄^–, PF₆^–, and OTf^– have been used to demonstrate that the dimerization is driven by the anion template effect. Interestingly, the stability of <b>2</b>⁸⁺ strongly depends on the encapsulated anions (<b>2</b>⊃(NO₃|NO₃|NO₃)⁵⁺ ≫ <b>2</b>⊃(BF₄|BF₄|BF₄)⁵⁺). The origins of this differing thermodynamic stability have been shown through detailed investigations to be due to the differences in the stabilization of the interlocked structure by the host–guest interaction and the size of the anion. We have found that 2-naphthalenesulfonate (ONs^–) induces the monomerization of <b>2</b>⊃(NO₃|NO₃|NO₃)⁵⁺ via intermediate <b>2</b>⊃(ONs|NO₃|ONs)⁵⁺, which is formed by anion exchange. On the basis of this finding, and using <i>p</i>-toluenesulfonate (OTs^–), the physical separation of <b>2</b>⊃(NO₃|NO₃|NO₃)⁵⁺ and <b>1</b>⁴⁺ as OTs^– salt was accomplished