Making up a foursome: Self‐assembly of 4,4′‐(3‐pyridinemethoxy)benzophenone (L) and Pd(NO3)2 resulted in the quantitative formation of a quadruply stranded metallohelicate [Pd2(L)4], which undergoes spontaneous dimerization to an unprecedented chiral interlocked metallohelicate [Pd2(L)4]2 (see X‐ray structure; C black, H white, N blue, O red).
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, [(Pd(2)L(4))(2)](8+) (2(8+)), is a quadruply interlocked molecular architecture consisting of two mechanically interlocked monomers, [Pd(2)L(4)](4+) (1(4+)). 2(8+) has three internal cavities, each of which encapsulates one NO(3)(-) ion (1:3 host-guest complex, 2⊃(NO(3)|NO(3)|NO(3))(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, BF(4)(-), PF(6)(-), and OTf(-) have been used to demonstrate that the dimerization is driven by the anion template effect. Interestingly, the stability of 2(8+) strongly depends on the encapsulated anions (2⊃(NO(3)|NO(3)|NO(3))(5+) ≫ 2⊃(BF(4)|BF(4)|BF(4))(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 2⊃(NO(3)|NO(3)|NO(3))(5+) via intermediate 2⊃(ONs|NO(3)|ONs)(5+), which is formed by anion exchange. On the basis of this finding, and using p-toluenesulfonate (OTs(-)), the physical separation of 2⊃(NO(3)|NO(3)|NO(3))(5+) and 1(4+) as OTs(-) salt was accomplished.
Über die Stränge: 4,4′‐(3‐Pyridinmethoxy)benzophenon (L) und Pd(NO3)2 bilden durch Selbstorganisation quantitativ das viersträngige Metallohelicat [Pd2(L)4], das spontan zu einem neuartigen chiralen verzahnten Metallohelicat [Pd2(L)4]2 dimerisiert (siehe Struktur; C grau, H weiß, N blau, O rot).
Interlocked dimer 2, which is composed of two physically interlocked monomers 1, has three cavities (cavity A × 2 and cavity B × 1) and can encapsulate three anions, such as NO and BF, one anion per cavity. There are six possible encapsulation patterns, A-F; two (A and F) contain only one kind of anion and the others (B-E) contain both NO and BF at the same time with different ratios and with different positions. Anion competition experiments showed that in addition to F, which encapsulates three NO ions, C, in which NO and BF ions are captured in cavities A and cavity B, respectively, was selectively formed. Detailed investigations have revealed that B-E were formed by dimerization, but three of the four were subjected to anion exchange and converged into C or F. This selective formation can be explained by the fact that NO is a better anion template than BF, as well as the molecular structure of the interlocked dimer; cavities A are surrounded by four bridging ligands and can be accessed by free anions, whereas no space available for anion exchange is present around cavity B because this cavity is surrounded by eight bridging ligands. Therefore, the BF ions in cavities A are expelled by free NO, but the BF ion in cavity B is not, resulting in the selection of C and F. We have found that the volume of the cavity influenced anion recognition. New interlocked dimer 3, which has smaller cavities than those of 2, captured three NO ions to form F, whereas only a small amount of an interlocked dimer that contains both NO and BF was formed.
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