INTERLOCKING Molecular architecture of Borromean rings (left) can be controlled by embedding a templating feature (zinc ions, represented by silver spheres, right) at the six crossing points. © SCIENCE 2004

Molecular Borromean rings Molecular Borromean rings are the molecular pendants of Borromean rings. The study of Borromean links is part of mechanically-interlocked molecular architectures. One Borromean ring system reported in 2004 is made up of macrocycles formed from 2,6-diformylpyridine and diamine compounds complexed with zinc. 1.molecularBorromean ringsmechanically-interlocked molecular architectures2004macrocyclespyridineaminecomplexedzinc1 The technique of subcomponent self-assembly has been applied to the preparation of a set of copper(I) complexes from diamines and aldehydes in aqueous solution. These complexes may be synthesized alongside one another in solution despite the chemical nonorthogonality of their respective starting materials; thermodynamic equilibration eliminates all mixed products. T he reactivity of these complexes has been studied, revealing that in certain cases, the substitution of both ligands and ligand subcomponents could be independently carried out. In one particular case, a complex was shown to be inert to ligand substitution but readily underwent ligand subcomponent substitution, creating the possibility of a previously undocumented kind of cascade reaction: Once ligand subcomponent substitution had occurred, ligand exchange could then happen, allowing both reactions to be triggered by a single chemical event.

PNAS August 9, 2005 vol. 102 no –11195

This compound was synthesized from two building blocks: 2,6-diformylpyridine and a diamine containing a 2,2'-bipyridine group. Zinc acetate is added as the template for the reaction, resulting in one zinc in each of a total of 6 pentacoordinate complexation sites. Trifluoroacetic acid (TFA) is added to catalyse the imine bond- forming reactions.synthesizedpyridineamine 2,2'-bipyridineZinc acetateTrifluoroacetic acidcatalyseimine The preparation of the tri-ring Borromeate involves a total 18 precursor molecules and is only possible because the building blocks self-assemble through 12 aromatic pi-pi interactions and 30 zinc to nitrogen dative bonds. Because of these interactions, the Borromeate is thermodynamically the most stable reaction product out of potentially many others, and because all the reactions that take place are equilibria, the Borromeate is the predominant reaction product.self-assemblepi-pi interactions dative bondsequilibria

ReductionReduction with sodium borohydride in ethanol affords the neutral Borromeand 2. True to a Borromean system, cleavage of just one imine bond (to an amine and an orthoester) in this structure breaks the mechanical bond between the three constituent macrocycles, releasing the other two individual rings.sodium borohydrideethanol 2imineamineorthoestermechanical bond From left to right: Zinc complex with pyridine group and orthogonal bipyridine groups in Borromeate.orthogonal Reduction to Borromeand with removal of zinc coordination. Bond cleavage of imine to orthoether by action of ethanol Borohydride reduction of a Borromean Ring (BR) complex containing six zinc( II ) ions and 12 imine bonds has resulted in its demetallation, producing a neutral BR compound and also its free macrocycle, following cleavage of at least one of the imine bonds in the ethanolic reaction mixture.

References Molecular Borromean Rings Kelly S. Chichak, Stuart J. Cantrill, Anthony R. Pease, Sheng-Hsien Chiu, Gareth W. V. Cave, Jerry L. Atwood, J. Fraser Stoddart Science, Vol 304, Issue 5675, , 28 May 2004J. Fraser StoddartScience Nanoscale Borromean links for real Andrea J. Peters, Kelly S. Chichak, Stuart J. Cantrill and J. Fraser Stoddart Chemical Communications, 2005, (27), J. Fraser StoddartChemical Communications