Self-Assembly of Tetra- and Hexanuclear Circular Helicates

Abstract

The self-assembly of the tris-bipyridine ligands B I and B II with iron(II) salts yields polynuclear complexes displaying structures of cyclic double-helix type, termed circular helicates [n]cH (of order n). With B I in which the bipyridine units in the ligand are connected by ethylene bridges, penta- or hexanuclear architectures [5]cH and [6]cH are obtained, depending on the anion present during the self-assembly process. The elongated tris-bipyridine ligand B II with oxypropylene bridges forms a tetranuclear circular helicate [4]cH, whose structure has been confirmed by crystal structure determination. The possible oligomeric combinations of tris-bipy ligands and iron(II) ions may be considered to constitute the potential members of a virtual combinatorial library, generated via dynamic combinatorial chemistry, from which a specific real constituent of the virtual set of circular helicates is expressed in given conditions.

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.…”

“…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][35][36][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.…”

“…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.…”