Interwoven, knotted, and helical structures abound in nature – from the familiar double helix of DNA to the complex folds of proteins. Understanding the ins and outs of such twists and turns is important not only for understanding the form of many biopolymers, but can provide insights into their function and how they are generated in the first place. Synthetic, higher-order non-DNA molecular knots remained elusive until the recently described synthesis of a molecular pentafoil knot, a closed-loop pentameric cyclic Fe(II) double helicate.

David A. Leigh, University of Edinburgh, UK, and colleagues have studied in detail the specific reaction parameters that allow each structure in this pentameric cyclic Fe(II) double helicate to form. They have characterized using spectroscopy and crystallography, 11 pentameric circular Fe(II) double helicates formed by imine condensation of alkyl monoamines with a common bis(formylpyridine)bipyridyl-derived building block and Fe(II) and chloride ions. 

Reactant stoichiometry, concentration, solvent, nature and amount of anion are all factors that affect the assembly process. The role of chloride in the assembly process was not to be limited to that of a simple template. By using certain chiral amines, pentameric cyclic helices of single handedness could be isolated.

This information is useful for the rational synthesis of higher-order topologically complex molecular architectures.

• Pentameric Circular Iron(II) Double Helicates and a Molecular Pentafoil Knot,
  Jean-François Ayme, Jonathon E. Beves, David A. Leigh, Roy T. McBurney, Kari Rissanen, David Schultz,
  J. Am. Chem. Soc. 2012.
  DOI: 10.1021/ja303355v