Copper complexes helical

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.

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... 

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). 

Cho and Lim compared the effect of a variety of lateral substituents on the thermal behavior of peripherally octasubstituted, metal-free phthalocyanines and their copper complexes. Despite being an interesting study, the characterization of the mesophases was tentatively made by optical microscopy and thus some doubts concerning mesophase identification still persist. The results of this study are gathered in Table 8 along with some metal-free and copper complexes discussed above to allow comparison. The free base with a chiral chain exhibited a texture resembling that of the cholesteric phase, whereas that of the copper complex was not identified. Compounds substituted with chiral chains were room-temperature liquid crystals, whatever the length and number of asymmetric carbon atoms, and the columns described a helical twist. ... 

A new asymmetric urea-copper complex catalyst that is capable of helical chirality inversion has been developed. The enantioselectivity of the asymmetric conjugate addition of diethyl malonate to trans- -nitrostyrene has been found to depend on the helicity of the catalyst. Either enantiomer of the product can be predetermined by selection of the oxidation state of the copper ion. Facile interconversion between the... 

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

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. 

We realized that improvement in the yields of trefoil knots would be determined by (i) the proportion of double helix precursor formed compared with face-to-face open chain complex (Figure 18) (ii) the spatial arrangement of the four reacting ends of the helical dinuclear complex. This latter factor will reflect the extent of winding of the two molecular strings interlaced around the copper atoms. The various complexes synthesized and studied are depicted in Figure 21 [96]. 

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. 

Our template synthesis of knots implies that the target molecules are obtained as cationic dicopper(I) complexes. Therefore we considered the possibility of interconverting both enantiomers into a pair of diastereomeric salts [137, 138] by combining them with an optically active anion. Binaphthyl phosphate (BNP") [139] drew our attention because its chirality arises from the binaphthyl core, which is twisted. This helical structure is of the same type as that of die copper double helix, precursor of the knot. Besides, both compounds are aromatic and, thus, we could expect some potentially helpful stacking interactions [87],... 

Complex IV. Cytochrome c oxidase (ubiquinol-cytochrome c oxidoreductase). Complex IV from mammalian mitochondria contains 13 subunits. All of them have been sequenced, and the three-dimensional structure of the complete complex is known (Fig. 18-10).125-127 The simpler cytochrome c oxidase from Paracoccus denitrificans is similar but consists of only three subunits. These are homologous in sequence to those of the large subunits I, II, and III of the mitochondrial complex. The three-dimensional structure of the Paracoccus complex is also known. Its basic structure is nearly identical to that of the catalytic core of subunits I, II, and III of the mitochondrial complex (Fig. 18-10,A).128 All three subunits have transmembrane helices. Subunit III seems to be structural in function, while subunits I and II contain the oxidoreductase centers two hemes a (a and a3) and two different copper centers, CuA (which contains two Cu2+) and a third Cu2+ (CuB) which exists in an EPR-silent exchange coupled pair with a3. Bound Mg2+ and Zn2+ are also present in the locations indicated in Fig. 18-10. 

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