Helical complexes, double-stranded

The occurrence of the redox-driven reversible assembling-disassembling process involving copper complexes of 16 has been verified through cyclic voltammetry experiments at a platinum electrode in a MeCN solution. Figure 2.17 shows the CV profile obtained with a solution of the double-strand helicate complex [ Cu 21 (16)212 +. 

Figure 2.16 The redox-driven disassembling of a dicopper(I) double-strand helicate complex to give two mononuclear copper(II) complexes, in which each strand behaves as a quadridentate ligand. On subsequent reduction, the two mononuclear complexes reassemble to give the helicate. The illustrated process fits well the behavior of copper complexes of 16 in a MeCN solution.

The bidentate ligand 17 forms bis with Cu1 a double-strand helicate complex, whose structure was elucidated through X-ray diffraction studies and is shown in Fig. 2.19. 

Figure 2.19 The molecular structure of the [Cu2,(17>212 1 double-strand helicate complex cation. Cu1 metal centers are represented as spheres. Hydrogen atoms of the two strands have been omitted for clarity. Structure redrawn from data deposited at the Cambridge Crystallographic Data Centre CCDC 641164.

Figure 2.20 Cyclic voltammogram of a MeCN solution of [Cu2 (17)2 2 1 double-strand helicate complex. Supporting electrolyte [Bu4N]C104 scan rate 0.2 V/s internal reference electrode Fc+/Fc. Diagram adapted from Ref. 21.

During the 1980s several laboratories prepared and investigated double-stranded helical complexes, systems containing either pyirolic ligands [75, 76] and derivatives [77-79] (with Zn2+, Ag+, Cu+) or oligomers of 2,2 -bipyridine [80, 81]. Helicates [80-84] may consist of up to five copper centers and these systems are reminiscent of the DNA double helix. 

An infinite ID complex [Ag( 14)( 15) with a single-stranded helical chain structure was obtained through the reaction of 6-amino-1-naphthalenesulfonic acid (H-14) and 2-methylpyrazine (15) with Ag2CC>3.58 Furthermore, when tetra- and hexaethyleneglycol-spaced ligands, 1,1 l-(3,6,9-trioxaundecane)diyl-bis(nicotinate) (16) and l,17-(3,6,9,12,15-pentaoxaheptadecane)diyl-bis(nicotinate) (17), were reacted with silver salts, double-stranded helical complexes (as exhibited in Fig. 11.24) were formed.59... 

The disubstituted polypyridine derivatives, 4, 4" -bis(methylthio)-2,2 6, 2" 6", 2" 6", 2""-quinquepyridine (45) and 4, 4""-bis(methylthio)- 2,2 6, 2" 6",2" 6", 2"" 6 ", 2 ""-sexipyridine (46), in which the alkylthio groups are present on the 4-positions of the penultimate pyridines (with respect to each end) also form bimetallic double-stranded helical complexes with iron(II), cobalt(II), nickel(II), zinc(II) and palladium(II). With the first of these ligands, the palladium species is of type... 

Fig. 9. The dinuclear double-stranded helical complex [Cu2(L,3)2](BF4)2 is utilized as the precursor of the trefoil knot system. (From Scheme 1 in Dietrich-Buchecher, C. D. Sauvage, J.-P. Cian, A. D. Fischer, J. J. Chem. Soc., Chem. Commun. 1994, 2231.)...

Abbreviations DNA deoxyribonucleic acid RNA ribonucleic acid rRNA ribosomal RNA tRNA transfer RNA. A adenine U uracil G guanine C cytosine, poly A polyadenylic acid poly U polyuridylic acid poly C polycytidylic acid poly I poly-inosinic acid poly G polyguanylic acid. The letters r and d refer to the ribose or deoxy-ribose series. Complexes between the polynucleotides are written as follows polyG poly C double-stranded helical complex of poly G and poly C 2 poly G poly C, triple-stranded complex. BSA bovine serum albumin MBSA methylated bovine serum albumin. N nitrogen. AMV Avian myeloblastosis virus NDV Newcastle disease virus. SLE systemic lupus erythematosus. 

Figure 2.13 The dinucleating bis-bidentate ligand 14 forms with M1 metal ions of electronic configuration d10 (e.g., Cu1, Ag1) dimetallic complexes of formula [M2I(14)2]2 +, in which two molecules of 14 are intertwined to give a double helix. Ligands of the type 14 are named helicands and complexes such as 15 are called helicates. In this particular case, we have a double-strand helicate.

However, it has been recently demonstrated by Pallavicini et al. that the lifetime of the dicopper(II) double-strand helicate [ 2 (16)]4 + can be significantly increased by introducing hindering substituents on the framework of 16. In particular, this was shown to occur with the copper complexes of the bis-bidentate ligand 17.21... 

The most easily identifiable characteristics are those related to the shape of the complexes, with their double-stranded helical cores. In this respect, the electrochemical and photochemical properties of Cu2(K-84)2+ are not much different from those of the open-chain helical precursor or its O-methylated version. The strong electronic coupling between the two copper centers is clearly a consequence of the 1,3-phenylene bridges between the two complex subunits and the topological properties of the ligand have virtually no influence. 

Fig. 3a-d. A diagrammatic representation of double-stranded helicates (23, 34] containing (a) two, (b) three, (c) four, and (d) five 2,2 -bipyridine subunits [23], The Cu(I) ions function as a template around which the helicates can assemble. Furthermore, this assembly process exhibits (i) self-self recognition in the preferential pairing to the same ligand in the presence of others in the reaction mixture, and (ii) positive cooperativity in which the complexation of one metal ion facilitates the binding of the next... 

Terpyridine is the simplest oligopyridine capable of forming a double-stranded helicate. Copper complexes with terpyridine derivatives have been reported. In [Cu2(L17)2](PF6)2 (45) and [Cu2(Li8)2](PF6)2 (46), the ligands have essentially distributed themselves to present bidentate domains to one metal center and a single pyridine donor to the other, to give a [4 + 2] double helicate. The two copper atoms involve different coordination environments, one with a distorted tetrahedral geometry and the second in a approximately linear two-coordinate environment (Fig. 10). 

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