Nanoscale Borromean Links - The nontrivial link known as the Borromean rings has long been a source of endless fascination among artists, theologians, mathematicians, and scientists. The molecular construction of the Borromean Ring (BR) topology represents a formidable synthetic challenge as they consist of three mutually interlocked, yet noncatenated rings. The BR topology can be viewed as a three-ring system, as described in knot theory, with the sole requirement that the scission of any one of the rings destroys the unique union of the three.
On the assumption that the construction of BRs from small building blocks can be realized by appealing to constitutional dynamic chemistry protocols, the Stoddart group successfully achieved (Science 2004, 304, 1308-1312) the complete molecular construction of the BR topology from 18 individual components under strict dynamic covalent, coordinative, and thermodynamic control (Box). This supramolecular assembly is fixed on the ability to control the placement of 12 organic fragments around 6 transition metals in near quantitative yields. Stabilized by combinations of 12 Đ– Đ stacking interactions and 30 dative bonds, six tridentate and six bidentate ligands are spatially preorganized around six transition metals, such that they preferentially react and form molecular Borromean Rings in a single step, on a gram scale, in yields of greater than 95%.
The successful construction of this framework opens the door to a new molecular entity with unexplored properties. Recently (Chem. Commun. 2005, 3391-3393), the reversible nature of at least some of the 30 dative bonds and 12 imine bonds stabilizing and constituting the three rings of the Borromean Link were assessed in scrambling experiments.
A true Borromean Link requires that the structure can exist without the presence of the template. These “real” BRs have been obtained (Chem. Commun. 2005, 3394-3396) by reducing the dynamic imine bonds followed by removal of the metal ions. From a design point of view, in a bottom-up sense at least, this molecular BR topology provides a unique symmetrical, nanoscale 3-dimensional scaffold into which unique features (e.g., electroactive, photoactive, and chirooptical) can be imbedded at will. With these thoughts in mind, synthetic strategies have been developed (J. Org. Chem. 2005, In press) to append functional groups at the periphery of the structure to give hexasubstituted BRs.
With the intention of further exploiting the symmetry, size and topology of the BRs, we are continuing to alter strategically each of the components, be it the metal or the ligand, using the nearly quantitative synthetic protocol, to try and uncover functions that can emerge from such a form.