Molecular Borromean rings

The synthesis of molecular Borromean rings was reported in 2004 by the group of J. Fraser Stoddart.

The so-called Borromeate is made up of three interpenetrated macrocycles formed through templated self assembly as complexes of zinc.

[1] The synthesis of the macrocyclic systems involves self-assembles of two organic building blocks: 2,6-diformylpyridine (an aromatic compound with two aldehyde groups positioned ortho to the nitrogen atom of the pyridine ring) and a symmetric diamine containing a meta-substituted 2,2'-bipyridine group.

Trifluoroacetic acid (TFA) is added to catalyse the imine bond-forming reactions.

[1] The preparation of the tri-ring Borromeate involves a total of 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.

Schematic of a molecular Borromean ring.
Crystal structure reported by Stoddart [ 1 ]
Synthesis of one ring in Borromean ring system from 2,6-diformylpyridine and a diamine in presence of zinc acetate and TFA. The ring-system consists of three such interlocked rings
From left to right: Zinc complex with pyridine group and orthogonal bipyridine groups in Borromeate. Reduction to Borromeand with removal of zinc coordination. Bond cleavage of imine to acetal by action of ethanol