Self‐Assembly of a Circular Double Helicate

Abstract

Dedicated to Professor Guy Ourisson on the occasion of his 70th hirtlidaj.Helicates, metal complexes that form double" -41 or triple in solution and/or in the solid state, are generated by self-assembly of two or three ligand strands around suitable metal ions. Whereas the double helix of DNA results from the assembly of two complementary chains of nucleotides through hydrogen bonding, helicates form spontaneously from linear ligands composed of a series of discrete binding subunits through metal ion coordination. The process may be considered as resulting from the operation of a programmed system, in which the steric and interactional information stored in the ligands is read out by the metal ions following the algorithm detined by their coordination geometry.[31 Thus homostranded d~u b l e [ ' .~,~I and t r i~l e [~.~] helicates have been obtained from chains of 2,T-bipyridine (bpy) or 2,2 : 6',2"-terpyridine (terpy) groups connected by short covalent bridges, and metal ions having tetrahedral or octahedral coordination geometry. Recently, a heteroduplex helicate based on a tris-bpy strand, a tris-terpy strand, and pentacoordinated Cu" ions has been described.['] The DNA double helix is usually acyclic but can also be closed into a cyclic structure, a circular DNA, as found in some viruses. By analogy, it would be of great interest to devise means of generating closed, circular helicates. We describe herein the self-assembly of such a supramolecular architecture. The trisbpy ligand L and iron(i1) chloride have been found to form spontaneously and quantitatively a circular double helicate, cH (Fig. I), in other words, a double helicate closed into a ring, as indicated by 'H N M R spectroscopy and mass spectrometry as well as by X-ray crystal structure determination.

Supramolecular self-assembly codes for functional structures

“…A large number of low-molecular-weight molecules are obviously needed to create assemblies on the scale of macromolecules, approximately 10 4 -10 7 Da. Current synthetic strategies to create very large controlled assemblies are inspired by previous examples of smaller systems, such as melamine-cyanurate spheres (Whitesides et al 1991(Whitesides et al , 1995 (diameter approximately 2.5 nm) and helicates (Koert et al 1990;Hasenknopf et al 1996Hasenknopf et al , 1997. While these structures did not display any particular function, they established an important principle that small molecules can self-assemble into well-defined structures.…”

A synthetic molecular pentafoil 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 have proved unsuccessful 30 , probably because joining the ends of each strand with the required connectivity becomes increasingly difficult as helicate length increases. An alternative way to use helicates (which generate the necessary crossing points) to make higher-order topologies could be to use cyclic structures or grids, where there is no requirement for there to be long distances between the ends of the strands that are to be connected.…”

“…An alternative way to use helicates (which generate the necessary crossing points) to make higher-order topologies could be to use cyclic structures or grids, where there is no requirement for there to be long distances between the ends of the strands that are to be connected. To generate the five crossing points required for a pentafoil knot, we modified a motif for generating cyclic metal helicates discovered by Lehn in the mid-1990s [31][32][33] . Lehn found that double-stranded circular helicates of various sizes could be obtained using Fe(II) or Ni(II) ions with tris(bipyridine) ligands with very short inter-bipyridine spacers that destabilized the normally preferred three-metal-ion linear triple helicate.…”

“…The reaction conditions (ethylene glycol, 170 8C) [31][32][33] used to form cyclic helicates with tris(bipyridine) ligands are not compatible with imine bond formation. Because the product distribution between cyclic double helicate, linear triple helicate and polymer is a delicate thermodynamic balance that could change significantly depending on the structure of the building blocks, reaction conditions, solvent and the nature and stoichiometry of the metal and anions, we first investigated whether it was possible to translate the self-assembly chemistry of the tris(bipyridine) ligands to an imine system.…”

Bis-amidinium calixarenes: Templates for self-assembled receptors

“…Less attention has been paid to producing self-assembled 'supermolecules' capable of performing a function such as binding a 'third-party' guest species. Recently, very elegant examples of self-assembled receptors capable of binding anions have been reported by Lehn and coworkers [2] and Stoddart et al [3].…”