Chemistry Reference
In-Depth Information
4
Structural Aspects of Helicates
utzen
Kekule Institute of Organic Chemistry and Biochemistry, University of Bonn, Germany
Martin Berg and Arne L
4.1 Introduction
Spirals and helices are virtually ubiquitous, they can be found in giant objects such as
spiral galaxies, but also in the shells, horns, teeth, and claws of animals, or in plant struc-
tures, as well as in manmade objects like helical springs, wires, and so on. Besides the
interesting mechanical properties and the mathematical models that can be used to
describe them, the attractiveness of their simply appealing structure has fascinated
humans for a long time and caused use of this motif in numerous examples of architecture
and art. Helical structures, however, are not only found in macroscopic objects but are
also well known structural motifs in molecular sciences, as for example, double-stranded
DNA or the a-helical secondary structures of peptides and proteins. Actually, there is
even one naturally occurring dinuclear metal complex known that has such a structure
[1,2]. In this complex that a red yeast uses as a growth factor, three molecules of rhodo-
torulic acid ( 1 ), a diketopiperazine dihydroxamic acid, act as bridging ligands that wind
themselves around two iron(III) centres in a helical fashion (Figure 4.1).
Interestingly, even before the elucidation of the structure of this natural compound a
first example of an artificial dinuclear helical zinc(II) complex {[Zn 2 ( 2 ) 2 ], Figure 4.2}
was reported by J.-H. Fuhrhop and colleagues in 1976 that could be characterized by X-
ray analysis [3]. Within the next decade a few further ligands ( 3 - 5 , Figure 4.2) that form
oligonuclear helical complexes were reported by G. van Koten [4], K.N. Raymond [5],
and J.-M. Lehn [6] who finally coined the term “ helicate ” for coordination compounds
Search WWH ::




Custom Search