A synthetic molecular pentafoil knot

Abstract

Knots are being discovered with increasing frequency in both biological and synthetic macromolecules and have been fundamental topological targets for chemical synthesis for the past two decades. Here, we report on the synthesis of the most complex non-DNA molecular knot prepared to date: the self-assembly of five bis-aldehyde and five bis-amine building blocks about five metal cations and one chloride anion to form a 160-atom-loop molecular pentafoil knot (five crossing points). The structure and topology of the knot is established by NMR spectroscopy, mass spectrometry and X-ray crystallography, revealing a symmetrical closed-loop double helicate with the chloride anion held at the centre of the pentafoil knot by ten CH ... Cl -hydrogen bonds. The one-pot self-assembly reaction features an exceptional number of different design elements-some well precedented and others less well known within the context of directing the formation of (supra)molecular species. We anticipate that the strategies and tactics used here can be applied to the rational synthesis of other higher-order interlocked molecular architectures.K nots are important structural features in DNA 1 , are found in some proteins [2][3][4][5] and are thought to play a significant role in the physical properties of both natural and synthetic polymers 6,7 . Although billions of prime knots are known to mathematics 8 , to date the only ones to have succumbed to chemical synthesis using building blocks other than DNA are the topologically trivial unknot (that is, a simple closed loop without any crossing points) and the next simplest knot (featuring three crossing points), the trefoil knot 9,10 . A pentafoil knot-also known as a cinquefoil knot or Solomon's seal knot (the 5 1 knot in Alexander-Briggs notation 11 )-is a torus knot 12 with five crossing points, is inherently chiral, and is the fourth prime knot (following the unknot, trefoil knot and figure-of-eight knot) in terms of number of crossing points and complexity 8,11,12 .Sauvage reported the first molecular knot synthesis 13 , using a linear metal helicate 14 to generate the three crossing points required for a trefoil knot. Although other syntheses of trefoil knots have been reported [15][16][17][18][19][20][21][22] (as have composites of trefoil knots 23 and other molecular topologies such as catenanes [24][25][26][27][28] and Borromean links 29 ), higher-order molecular knots remain elusive. Here, we report on the synthesis of a molecular pentafoil knot that combines the use of metal helicates to create crossover points 30 , anion template assembly to form a cyclic array of the correct size [31][32][33] , and the joining of the metal complexes by reversible imine bond formation 34-37 aided by the gauche effect 38 to make the continuous 160-atom-long covalent backbone of the most complex non-DNA molecular knot prepared to date.So far, attempts to make molecular knots with more than three crossing points by extending the linear helicate strategy of Sauvage to ligands with more coordination sites...

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

“…For instance by mixing templates which favour the formation of trefoils and 8 19 knots, respectively, we find that pentafoil knots, practically absent in the monodisperse topological phase diagram, can now be observed. Another interesting avenue in which our work can be further pursued in the future could be to allow to for some flexibility of the templates, as would be relevant for molecular or polymeric building blocks 26 .…”

“…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.…”

Anion receptor chemistry: highlights from 2008 and 2009

“…Leigh and co-workers have reported the self-assembly of five bis-aldehydes, five bis-amines, five metal cations and one chloride anion to form a 160-atom covalent pentafoil knot. 71 The authors first described that the reaction of bis-aldehyde 123 and a series of amines (124a-j) in the presence of FeCl 2 in DMSO at 60 1C forms a symmetric and cyclic pentafoil knot, as confirmed by 1 H NMR and ESI-MS spectroscopy. It was found that anilines (124j) and amines with two substituents on the amine-bearing carbon atom (124h,i) could not form pentafoil knots with 123 and FeCl 2 , but all other amines tested did.…”

Jamming of Knots along a Tensioned Chain

“…9 Recently, researchers have also determined facile methods to create knotted DNA via an electric field, 10,11 and this work has led to microfluidic experiments examining how the coil−stretch transition is affected by the presence of knots.…”