The dynamic chemistry of molecular Borromean rings and Solomon knots

Abstract

The dynamic solution equilibria between molecular Borromean rings (BRs) and Solomon knots (SKs), assembled from transition metal-templated macrocycles, consisting of exo-bidentate bipyridyl and endo-tridentate diiminopyridyl ligands, have been examined with respect to the choice of the metal template and reaction conditions employed in the synthesis of the metalated BRs, otherwise known as Borromeates. Three new Borromeates, their syntheses templated by CuII, CoII, and MnII, have been characterized extensively (two by X-ray crystallography) to the extent that the metal centers in the assemblies have been shown to be distanced sufficiently from each other not to communicate. The solid-state structure of the CoII-Borromeate reveals that six MeOH molecules, arranged in a [O-H⋯O] hydrogen bonded, chair-like conformation, are located within its oxophilic central cavity. When a mixture of CuII and ZnII is used as the source of templation, there exists a dynamic equilibrium, in MeOH at room temperature, between a mixed-metal BR and a SK, from which the latter has been fractionally crystallized. By employing appropriate synthetic protocols with ZnII or CdII as the template, significant amounts of SKs are formed alongside BRs. Modified crystallization conditions resulted in the isolation of both an all-zinc BR and an all-zinc SK, crystals of which can be separated manually, leading to the full characterization of the all-zinc SK by 1H NMR spectroscopy and X-ray crystallography. This doubly interlocked [2]catenate has been identified retrospectively in recorded spectra, where it was attributed previously to a Borromeate with a ZnII cation coordinated to the oxophilic interior walls of the ensemble. Interestingly, these ZnII-templated assemblies do not interconvert in MeOH at room temperature, indicating the significant influence of both the metal template and solvent on the solution equilibria. It would also appear that d10 metal ions favor SK formation'no evidence of CuII-, CoII-, or MnII-templated SKs has been found, yet a 1:0.9 ratio of BR:SK has been identified by 1H NMR spectroscopy when CdII is used as the template. One thing one minute and another the next: The dynamic solution equilibria between molecular Borromean rings and Solomon knots (see figure) are examined with respect to synthetic conditions, choice of metal template, and crystallization protocols in the self-assembly of these topologically complex mechanically interlocked architectures. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.