Double helical metal complexes

Section 9.7). Our work involved first the self-assembly of double-helical metal complexes. This particularly interesting case of self-assembly of a coordination compound represented a first step in our studies on the design of inorganic programmed systems. 

This radical change in outlook builds on the richness of constitutional diversity and the benefits of variability. It stresses the virtues of instructed mixtures [3, 31], such as was revealed in the self-selection processes occurring in the side-by-side self-assembly of double helical metal complexes (helicates), whereby only the correctly paired double helicates were produced from a mixture of ligands and metal ions in dynamic coordination equilibrium [31, 37c]. It is this work that first led us in the early 1990s to envisage a dynamic chemistry bringing into play the constitution of chemical species. 

The first double-helical metal complex described was the tetrahedral bis(zinc formylbiliverdinate) (Figure 5.34). The helical dimer forms spontaneously, if the planar monomeric zinc complex, which carries an axial water ligand, is dehydrated by HCl" . 

Related double-helical metal complexes have been used to investigate the role of cooperativity in self-assembly. Ligands (14a) and (14b) were allowed to react with CUBF4 to form the corresponding helicates (15a) and (15b). The formation of helicates (15a) and (15b) was confirmed in solution by spectrophotometric titration. Two sharp isosbestic points are seen in the spectrum, and the excess absorbance diagram is linear even at a very tow... 

Helicate self-organization-positive cooperativity in the self assembly of double-helical metal-complexes, A. Pfeil and J.-M. Lehn, J. Chem. Soc., Chem. Commun., 1992, 838. 

Lehn, J-M. (1990) Perspectives in supramolecular chemistry, Angew. Chem. Int. Ed. Engl. 29, 1304-1319. Koert, U., Harding, M.M., and Lehn, J.-M. (1990) DNH deoxyribonucleo-helicates Self-assembly of oligonucleosidic double-helical metal complexes. Nature 346, 339-342. 

A variety of supramolecular architectures have also been created by the use of metal coordination. Synthesis of catenanes, knots [88], and double-helical metal complexes (linear or circular heli-cates) [4] are few of the prominent examples. Nanostructured supramolecular squares and capsules are also formed by self-assembly (Figure 13) [89]. 

Self-Assembly of Double-Helical and Triple-Helical Metal Complexes The Helicates... 

Helical complexes are formed by related polytopic heterocyclic ligands [9.78]. A double-helical sodium complex has been obtained [9.79a] as well as heterodinuclear Co(n)Ag(i) [9.79b] and Cu(i)Ag(i) [9.79c] double-helicates. Polyassociation of a helical subunit yields an infinite double-helical Cu(l) complex [9.79d]. When the ligand strand contains two [9.74,9.80,9.81] or three [9.81] terpyridine units double-helical complexes with two or three (see 147 [9.81b]) octahedral metal centres, respectively are obtained. 

Triple Helicates. The steric information contained in the oligo-bipy strands based on bipy units connected in the 6,6 positions is designed to yield double helices on complexation of metal ions undergoing tetrahedral coordination. Steric effects due to the 6,6 -disubstitution hinder the binding of metal ions of octahedral coordination geometry, which would be expected to lead to triple helical complexes. 

While sexipyridine 41 forms double-helical, binuclear complexes with several transition metal ions (see above), it forms only 1 1 complexes with the lan-... 

Double-stranded and triple-stranded helicates as well as double helical and triple helical metal complexes, are formed by the spontaneous organisation of two or three linear polybipyridine ligands of suitable structure into a double or a triple helix by binding of specific metal ions displaying respectively tetrahedral (Cu ) and octahedral (Ni ) coordination geometry. These species are illustrated by the trinuclear double helicate 1 [27] and triple helicate 2 [28] (see also [29]). 

Suitably substituted diazines also form chelate complexes, e.g. 2,3,5,6-tetrakis(a-pyridyl)pyrazine yields red tridentate complexes with Fe11. In the fast development of metallosupramolecular chemistry, many other polydentate ligands, based on 2,2 -bipyridyl units, have been obtained and studied (94CI(L)56). Thus, two molecules of oligopyridine (58) interact with various metal ions (Fe2+, Co2+, Cu2+ etc.) to form a double-helical [M2L2]4+ complex in which each metal is bonded to a tridentate region from each ligand. 

These bound water molecules, in turn, can bond to associated protein and to other atoms of the complex loops found in RNA molecules. The 2 -OH groups also act as ligands for divalent metal ions in some tRNAs and in some RNA catalytic sites. Transient hybrid DNA-RNA double helices also exist within cells and they too usually have the overall shape of A-DNA.55/307 309 However, the minor groove is intermediate in width between that expected for the A and B forms. 

A number of helical and double-helical complexes have been obtained with polypyridine ligands, which bind various metal ions yielding helical and double-helical [9.70-9.74] complexes that present interesting redox and metal-metal interaction properties [9.75]. The graphs of the double-helicates represent braids based on two threads and several crossings [9.1,9.76], that may serve as templates for the synthe-... 

Figure 7-28. The use of metal ions to control the assembly of double-helical complexes. The twisting of the molecular threads is initiated by the co-ordination of metal-binding domains within the ligand to the metal ions. The assembly of the mononuclear compound 7.46 requires the incorporation of a single metal-binding domain in each molecular thread, whereas compound 7.47 requires two metal-binding domains per thread.

When 7.48 reacts with copper(i), which usually forms four-co-ordinate tetrahedral complexes, a double-helical species, 7.49, is indeed formed (Fig. 7-30). This is a genuine self-assembly process - simply treating the ligand with the appropriate metal ion leads to the desired structure. A wide variety of other spacer groups have been incorporated between didentate domains. In practice, some consideration needs to be given to the nature of the spacer group that is selected. If it is too long, or too flexible, other co-ordination possibilities can occur. 

Of course, it is quite possible to further extend these assembly processes to give doublehelical complexes with even more bond crossings. For example, a double-helical complex with three bond-crossings should result from the reaction of a molecular thread containing three metal-binding domains with three tetrahedral metal ions (Fig. 7-32). An example of the assembly of such a trinuclear double-helical complex is seen in the formation of 7.52 from the reaction of 7.51 with silver(i) salts (Fig. 7-33). 

Figure 7-31. The reaction of 7.50 with tetrahedral metal ions gives dinuclear double-helical complexes. The interplay of the metal and ligand requirements are emphasised in this process. Once again, the molecular threads have been shaded differently to clarify the structure of the product.

Figure 7-32. The interaction of a molecular thread containing three didentate metal-binding domains with tetrahedral metal ions should give a trinuclear double-helical complex.

Figure 7-34. The interaction of the potentially hexadentate ligand 7.53 with octahedral metal ions results in a partitioning into two tridentate domains and the formation of dinuclear double-helical complexes.

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