Helical System

Simple mixed donor ligands are commonly anticipated to form mononuclear complexes. However, many examples of polynuclear complexes that form readily through prior ligand design or via spontaneous self-assembly processes have appeared. A range of shapes, including dendrimers and helical systems, have been identified. One of the... 

A Helical System Leading to a Multi-intertwined [2]-Catenane... 

Figure IR-9.1. Two skew lines AA and BB which are not orthogonal define a helical system. In the Figure, AA is taken as the axis of a cylinder whose radius is determined by the common normal NN of the two skew-lines. The line BB is a tangent to the above cylinder at its crossing point with NN and defines a helix upon this cylinder. Cases (a) and (b) illustrate a right- and left-handed helix, respectively.

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

For dominantly a-helical systems /c is a constant that can be evaluated at 201 nm because at this wavelength [0]corr=O, i.e.,... 

Inorganic double or triple helices are formed by two or three ligand strands wrapped around linearly disposed metal ions [13], Among cyclic transition metal complexes, circular helicates [nJ cH [ ] "cH is a general notation characterizing circular helicates (cH) with n = number of metal ions and m = helicity (m = 2 for a double helix) have specific features and may be considered as toroidal helices [34]. There are two different kinds of circular helical systems. Some structures self-assemble from the metal ions and the ligands only in the presence of an anion, which could act as a template [34,35,64-67], whereas, in other cases, the circular helicates self-assemble from the metal ions and the ligands alone [68-70]. 

In contrast to most other circular helical systems, 44 assembles from the metal ions and the ligands alone without the support of an anionic template. 

We prepared three bifunctional redox protein maquettes based on 12 16-, and 20-mer three-helix bundles. In each case, the helix was capped with a Co(III) tris-bipyridyl electron acceptor and also functionalized with a C-terminal viologen (l-ethyl-V-ethyl-4,4 -bipyridinium) donor. Electron transfer (ET) was initiated by pulse radiolysis and flash photolysis and followed spectrometrically to determine average, concentration-independent, first-order rates for the 16-mer and 20-mer maquettes. For the 16-mer bundle, the a-helical content was adjusted by the addition of urea or trifluoroethanol to solutions containing the metal-loprotein. This conformational flexibility under different solvent conditions was exploited to probe the effects of helical secondary structure on ET rates. In addition to describing experimental results from these helical systems, this chapter discusses several additional metalloprotein models from the recent literature. 

AA, BB non normal skew-lines define a right helical system... 

We have prepared and characterized a range of complexes of the new redox active Fctpy ligand, including [Fe(Fctpy)2][PF6]2 (blue), [Cu(Fctpy)2][PF6]2 (red) and [Ru(Fctpy)2][PF6]2- Each of these complexes shows two oxidation processes one is associated with the ferrocene substituent (+0.20, +0.30 and +0.22V respectively) and the other with the other metal centre (Fe, +0.8 IV, Co, -0.18V and Ru, +0.90V). These data have allowed us to define a Hammet redox active terminators into oligomers and into multiple helical systems. 

The C2SHel helical system is very different and the carbon backbone is also very different from that of the linear ladder, C2SLad. The carbon skeleton left over after removing the sulfurs for the purpose of the perturbation analysis is not a highly delocalized n-electron system. The HOCO and LUCO of C2SHel are illustrated in Figure 7.15. 

Keywords chiral helical systems Chiroptical properties Computational studies Helical phosphorous ligands Resolution of thiahelicenes Synthesis of heterocycles Thiahelicenes... 

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