Pentanuclear helicates

The linear helicate forms first as a kinetic product, upon heating for a few minutes at 170°C in ethylene glycol. Longer reaction times resulted in disassembly of the initial structure and the formation of the thermodynamic product consisting of a circular pentanuclear helicate. This structure is apparently templated by the chloride anion, which is bound strongly inside the cavity of the helical macrocycle. 

Fig. 3 Expression of two different circular helicates via anion recognition in a DCL self-assembled by metal coordination. The smaller CL anion amplifies the formation of a pentanuclear helicate, whereas the larger S042, SiF62, and BF4 amplify the formation of a hexanuclear helicate...

Figure 10.79 X-ray molecular structures of cyclic helicates (a) pentanuclear species formed from 10.124 and templated by Cl 87 (b) hexanuclear species formed from 10.126 and templated by C104 (reproduced by permission of The Royal Society of Chemistry).

In the presence of the smaller chloride ion the self-assembly generates the pentanuclear circular helicate [i] cH 33 (Figure 21). With the larger anions S04 , BF4 , and SiF ", the hexanuclear architecture is formed while the Br anion of intermediate size yields a mixture of [5] cH and [d] cH. The charge of the anion has apparently little influence on the structure formed as [<5] cH is obtained with mono- and divalent anions. The structure depends rather on the size of the anion to be included in the circular helicates. These differences in the products formed, might be considered as resulting from a templating effect of the anion... 

FIGURE 5 Programmed assemblies of bi- to pentanuclear double-stranded helicates (redrawn after Lehn and Rigault, 1988). 

Figure 5 Depiction of the rationai design of the tower-like pentanuclear assembly of [Eu5(12)(15)3](C104)i5 showing how the symmetrical ligands (12) and (14) (a) and (b) can be combined to give the unsymmetrical ligand (15) (c). The helicate and cage compounds are crystal structures, whereas the pentanuclear tower is a calculated model. (Adapted with permission from Ref 32. Copyright (2011) American Chemical Society.)...

Their strategy was successful and they were able to isolate the pentafoil knot in 44% yield. In addition to the metal-assisted templation of the strands to form a circular helicate precursor, the chloride anion was shown to assist the formation of the pentanuclear adduct. The use of 3,6-dioxo-l,8-diaminooctane allowed the knots to be closed while simple aliphatic diamines failed to produce a clean closing procedure. The synergy of metal and anion coordinations was employed simultaneously to template the formation of a pentafoil knot structure. 

Aroussi B. E., S. Zebret, C. Besnard, P. Perrottet, J. Hamacek, Rational Design of a Ternary Supramolecular System Self-Assembly of Pentanuclear Lanthanide Helicates. J. Am. Chem. Soc. 133, 10764-10767 (2011). 

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