Tuning the pitch in helical polymers

A strategy for tuning the pitch of metal-organic helical polymers by choice of metal ion or counter anion has been demonstrated.          

Helical architectures are common in nature. Understanding the processes of self-organisation reveals techniques for controlling structures synthesised in the laboratory, and it is known that flexible, strand-like ligands can be used to build metal-organic helical polymers.          

Xu-Dong Chen and Thomas Mak of the Chinese University of Hong Kong have explored the potential of the ligand 2-pyridinyl-3-pyridinylmethanone as a building block for metal-organic helices. They synthesised a series of nine single-stranded helical complexes by reacting the ligand with different metal salts.          

Six, made with various silver salts, resulted in spring-like helices with the two pyridyl nitrogen atoms on opposite sides of the helix, known as the anti mode. The helical pitch was dependent both on the size of the counter ion and to what extent it is embedded in the helical grooves. They also found that complexes with cobalt and zinc formed in the anti mode, with the pitch length this time dependent on the metal ion.         

A complex made with a copper salt had the pyridyl nitrogen atoms on the same side of the helix (the syn mode).          

The authors believe this work is a valuable additional technique for designing synthetic helical structures. Jane Crawshaw