Scientists from Hong Kong have demonstrated a strategy for tuning the pitch of metal-organic helical polymers by choice of metal ion or counter anion. 

Helical architectures are common in nature. In imitation of this, scientists have sought to recreate them. Understanding the processes of self-organisation reveals techniques for controlling structures synthesised in the laboratory. For example 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 the reaction of L 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. They found that the helical pitch was dependent both on the size of the counter ion and to what extent it is embedded in the helical grooves. The researchers also found that complexes with cobalt and zinc formed in the anti mode, with the pitch length this time dependent on the metal ion. 

When a different complex was made, this time with a copper salt, the pyridyl nitrogen atoms arose on the same side of the helix (the syn mode). 

The authors believe that this tuning of helical pitch by choice of counter anion or metal ion could be a highly valuable addition to the techniques available for the design of synthetic helical structures. .

Jane Crawshaw