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 Cu(II), Co(II), and Mn(II), 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 Co(II)-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 Cu(II) and Zn(II) 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 Zn(II) or Cd(II) 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 (1)H 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 Zn(II) cation coordinated to the oxophilic interior walls of the ensemble. Interestingly, these Zn(II)-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 d(10) metal ions favor SK formation-no evidence of Cu(II)-, Co(II)-, or Mn(II)-templated SKs has been found, yet a 1:0.9 ratio of BR:SK has been identified by (1)H NMR spectroscopy when Cd(II) is used as the template.

DNA Origami Catenanes Templated by Gold Nanoparticles

“…i) Assembly of an origami ring by connecting four monomers (see also Figure 2b; Figures S8 and S9, Supporting Information). Subsequently, a DNA- [2]-catenane can be achieved through a double cyclization by connecting two other half-rings, which contain junctions IIb and III. iv) Threading the half-ring through the origami ring by connecting to the AuNP at junctions I, giving rise to a 1.5-ring complex.…”

“…As a result, creation of the DNA-[2]-catenanes by further cyclization of the 1.5-ring complex with another half-ring becomes challenging. The enlarged TEM images of the interlocked half-ring complex and the DNA- [2]-catenane in Figure 3c clearly shows the presence of the interconnecting AuNP between the two origami components in the individual structure. When taking all the assembly steps into account, the overall yields of strategies A and B are 4.8% and 23%, respectively.…”

“…Over the last decades, a variety of interlocked molecules have been created, ranging from Borromean rings, [1][2][3] knots, [4,5] catenanes, [6] rotaxanes, [7] pretzelanes [8,9] to Solomon-links [10] and Daisy-chains. Over the last decades, a variety of interlocked molecules have been created, ranging from Borromean rings, [1][2][3] knots, [4,5] catenanes, [6] rotaxanes, [7] pretzelanes [8,9] to Solomon-links [10] and Daisy-chains.…”

An Introduction to the Mechanical Bond

“…Whereas a dynamic combinatorial library (DCL) of 2,6-diformylpyridine (DFP) and 2,2´-bipyridine derived diamine (DAB) ligands leads to the self-assembly 237 328 More recently, a subtle structural modification to the DAB ligand involving the addition of methylene spacers in between the amino and bipyridine moieties was found to lead to the simultaneous self-assembly 352 of a [2]catenane, trefoil knot, and Solomon knot in one pot, providing further evidence for the complexity of the equilibrium governing the formation of these molecular topologies.…”

Mechanostereochemistry and the mechanical bond

“…A prime example is the preparation of molecular Borromean rings (figure 14b), three interlocked but non-catenated rings, in an 18-component reversible self-assembly process in nearly quantitative yields (Chichak et al 2004) When six equivalents each of the dialdehyde 65, the diamine 66 and the Zn(II) template are heated in methanol, the molecular Borromeate 67 is obtained in 95 per cent isolated yield, clearly demonstrating the efficacy of thermodynamic, all-in-one preparative approaches. Interestingly, subtle changes to reaction conditions result in the isolation of a molecular Solomon Link from the same ligand system (Meyer et al 2010). In dynamic systems such as this one, serendipity often plays a role in the synthesis of topologically complex MIMs.…”