Iron is an important metal in biology, and mixed species containing both Fe(II) and Fe(III) centres are thought to be vital to the biochemical function of many enzymes. 

A team of German chemists have synthesised and characterised a complex that mimics just such an unsymmetrical diiron species that displays, according to its creators, a 'previously unobserved' Fe(II)/Fe(III) dimer ground state of S = 3/2. 

Lead by Karl Wieghardt at the Max-Planck-Institut für Bioanorganische Chemie, Mülheim, the team prepared the complex by reducing the 1,2-diketone ligand bis(2,6-diisopropylphenyl)glyoxal with 1.5 equivalents of sodium metal in the presence of iron(II) bromide and 1,2-dimethoxyethane. A bulky diketone ligand was chosen as it provided the necessary steric protection to stabilise the activation of the ligand, while allowing further chemistry to take place at the nearby metal centres. 

According to Wieghardt, the complex's highly unsymmetrical crystal structure, comprising of a trigonal bipyramidal Fe(II) and a square pyramidal Fe(III) centre, is responsible for this ground state because of its unusual electronic structure, adding that for synthetic reasons, in most model complexes the ligand environment is symmetrical - resulting in a normal ground state of S = 1/2. 

Having isolated this single complex of his 1,2-diketone ligand, Wieghardt is now investigating the wider scope of its reactions: '[It] should [also] provide a rich coordination chemistry with [other] transition metals,' he said. 

David J. Parker