The Self‐Sorting Behavior of Circular Helicates and Molecular Knots and Links

Abstract

We report on multicomponent self-sorting to form open circular helicates of different sizes from a primary monoamine, Fe II ions, and dialdehyde ligand strands that differ in length and structure by only two oxygen atoms. The corresponding closed circular helicates that are formed from a diamine-a molecular Solomon link and a pentafoil knotalso self-sort, but up to two of the Solomon-link-forming ligand strands can be accommodated within the pentafoil knot structure and are either incorporated or omitted depending on the stage that the components are mixed.The spontaneous segregation of molecular building blocks into discrete species within a mixture is known as selfsorting, [1] a phenomenon that helps to maintain structural control over complex dynamic systems in nature. [2] The use of orthogonal recognition elements is a convenient way to achieve sorting in artificial systems, [1,3] but other methods, [4] including subtle differences in ligand design, [5][6][7] can also be remarkably effective. A beautiful example is the classic experiment by Lehn and co-workers [5] in which a mixture of ligand strands containing two to five 2,2'-bipyridine groups spontaneously self-sort into linear double helicates, each containing two ligands with equal numbers of binding sites, in the presence of Cu I ions.We recently described the synthesis of a molecular Solomon link [8] (a doubly entwined [2]catenane [9] ) and a molecular pentafoil knot, [10] each formed through a combination of metal-ligand coordination, an anion template, and geometric restrictions. These closely related structures are derived from tetra- [8] and pentameric [10] circular helicate scaffolds, respectively, and are assembled from up to 20 common, or similar, components. Here we investigate the self-sorting behavior of both the closed molecular topologies and the open circular helicate scaffolds on which they are based (Figure 1). The study provides insights into the self-assembly processes of the individual species and reveals a subtle interplay between the driving forces and kinetic traps involved in their assembly.Despite their structural similarities (a difference of just two oxygen atoms in length), dialdehydes 1 and 2 react individually with a suitable monoamine and FeCl 2 to generate different-sized circular helicates: tetrameric [8] with 1 and pentameric [10] with 2. To investigate the self-sorting potential of the ligands, a 1:1 mixture of aldehydes 1 and 2 was allowed to react with FeCl 2 and n-hexylamine (3) in [D 6 ]DMSO at 60 8C for 18 h, followed by anion exchange through the addition of an aqueous solution of potassium hexafluorophosphate (Scheme 1). 1 H NMR spectroscopy (Figure 2 a, i) indicated the formation of both tetramer 4 and pentamer 5, the spectrum of the reaction outcome being a superimposition of the spectra from the reaction of the individual aldehydes under similar experimental conditions (Figure 2 a, ii and iii). Electrospray mass spectrometry (ESIMS) confirmed perfect self-sorting, with no detectable formation of...

Two-stage directed self-assembly of a cyclic [3]catenane

“…However, we did not observe discrete species to form when the primary aliphatic amines benzylamine and 2-methoxyethylamine were used in place of an aniline; we infer this may be due to a combination of unfavourable steric effects and the absence of favourable π-stacking interactions when these amines are sorting occurs, resulting in the formation of two or more discrete homoleptic species. 24,25 In the present case, we were able to avoid self-sorting through the design of ligand L A .…”

Self-assembly of dynamic orthoester cryptates

“…Besides providing high-yielding syntheses, DCC generally gives rise to target structures that are dynamic and responsive to external stimuli under the conditions of their preparation. In the context of the emerging field of systems chemistry 18 19 , dynamic macrocycles and cages have served as a valuable testing ground for the investigation and manipulation of complex molecular networks 20 21 22 23 24 25 . Constitutionally dynamic cryptates 26 27 would represent the smallest and simplest conceivable three-dimensional platform for studying molecular complexity; however, to the best of our knowledge, there are no reports of monometallic cryptates yet, which can be prepared and manipulated based on a dynamic covalent exchange reaction 28 29 30 31 .…”

Self-assembling knots of controlled topology by designing the geometry of patchy templates

“…It may further provide a route to design novel synthetic molecular knots, if templates of small enough size can be fabricated. This would extend the current range of synthetic molecular topologies which, notwithstanding recent major technical advancements [23][24][25][26][27][28] , is still presently limited.…”