Double-helicate complexes

Figure 2.18 A square scheme illustrating the disassembling of the [Cu2(16)2]2 + double helicate complex, following Cu -to-Cu" oxidation, and the consequent assembling of two [Cun(16)]2+ mononuclear complexes, following the Cu"-to-Cu reduction. The process ultimately derives from the geometrical coordinative preferences of the two oxidation states Cu1 prefers a tetrahedral coordination, which can be achieved with the double helicate arrangement Cu11 prefers a square coordination geometry, which is fulfilled by the coordination of a single molecule of 16.

Especially attractive was the possibility to connect nucleosides, as has been realized, for instance, with the hexathymidine 141 and with the elongated and alternating strands 142 and 143. These compounds represent artificial oligonucleosides, which may interact with natural polynucleotides or nucleic acids. On treatment with Cu(i), 142 and 143 gave the double-helical complexes 144 and 145, respectively, inside-out analogues of double-stranded nucleic acids, which may be termed deoxy-... 

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-... 

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. 

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.

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.

Bell, T. W., Jousselin, H., Self-Assembly of a Double-Helical Complex of Sodium. Nature 1994, 367, 441-444. 

Bell TW, Jousselin H (1994) Self-assembly of a double-helical complex of sodium. Nature 367 441-444... 

There are many examples of compounds, usually short polymers, that spontaneously form double helical structures. A classic example involves a number of bipyridyl ligands joined by short, flexible spacers. In the presence of Cu(I), which has both an affinity for nitrogen-containing ligands and a preference for a tetrahedral disposition of donor atoms, discrete double helical complexes are formed [23], A similar phenomenon gives rise to a triple helical motif when a metal that preferentially adopts an octahedral coordination geometry is used [24],... 

The preparation of double helices from different bis-chelate ligands and transition metals in all likelihood occurred long ago, yet it is only relatively recently that the first such system was recognised and characterised. Apart from their beauty, their scientific relevance was not at all apparent. One of the earliest dinuclear helical complexes was discovered by Fuhrhop and co-workers in 1976 [28]. Since then, several double helical complexes have been created and characterised, and... 

Complexes of pyridines, bipyridyls, terpyridyls, and other polypyridines differ considerably from those of NH3 and aliphatic amines. They have been intensively studied because many of them have unusual properties for photoinduced energy migrations and charge separation, luminescence, photocatalytic reactions, and water activation. They are also involved in the construction of double-helical complexes, complexes that bind to DNA, and molecular rods and wires28 for fast electron transfers.29... 

Reports of double-helical complexes have appeared in the literature since the sixties. Despite the early interest, it is only more recently that emphasis has been given to the use of metal template synthesis for obtaining a wide range of doubly-and triply-stranded systems. In part, this attention has had its origins in an early report by Lehn et al. in which the spontaneous assembly of a dicopper(I)-containing double helix was described. 

The polypyridine derivatives 48 and 49 also yield double-helical complexes of type [Cu2L2l (L = 48 or 49) and [Ag2L2] (L = 48 or 49) as well as the singlehelical species, [Ru2L(terpy)2] (where L = 49 and terpy is 2,2 6, 2"-terpyridine). The electrochemistry of the double-helical cations indicates substantial electronic coupling between the metals in each case. 

In another approach, it was demonstrated by X-ray diffraction that the optically pure, tertiary phosphine ligand (5,5)-(-i-)-Ph2PCH2CH2P(Ph)CH2CH2P(Ph)CH2CH2-PPh2 yields a left-handed double-helical complex of di-silver(I) as well as a corresponding side-by-side conformer. A molecule of each species is found (together with their associated hexafluorophosphate anions) in each unit cell. 

Figure 1 Structure of the double-helical complex formed between (4) and cadmium II). Reproduced by permission from J. Am. Chem. Soc. 112, 1256-1258 (1990)...

Figure 8 Schematic structure of the double-helical complex formed from (20) and copper(I)...

Surprisingly little work has been carried out on the resolution of homochiral helicates into the two enantiomers. Self-resolution upon crystallization has been observed for two homonuclear triple helicates [37,38], but there seem to be only two well-authenticated cases of enantiomeric resolution, both using antimonyl tartrate the complex [ 02(9)3] " , a dinuclear triple helix [39], and a trinuclear double helical complex of iron(ll) [Fe3(19)2] " with a tm-terpyridyl ligand [40]. The circular dichroism spectrum of [ 02(9)3] " is shown in Figure 13. 

Figure 12 The initial synthesis of a molecular knot used the double helical complex (left) as a precursor, but this is only a minor product with the flexible -(CH2)4- bridging unit. Reproduced with permission from reference 35.

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