Synthesis and Structure of Prussian Blue Analogues

The Electrochemical Society Interface • Fall 2002 olecular magnetism is an evolving discipline relying on quantum chemistry, orbitals and Hamiltonians, organic and coordination chemistries, physics, and devices. Among the exciting molecule-based magnetic systems, Prussian blue analogues occupy a peculiar position. Prussian blue itself was described for the first time in 1704. Since this time, the history of Prussian blue analogues has been marked by electron transfer processes. It appears appropriate to present some of them in this article. After a brief introduction to their chemistry and structures, we comment on the identity between Prussian and Turnbull blue, the low temperature ferromagnetism of Prussian blue, the sensitivity of physical properties (magnetic, optical, electrochromism) to metal oxidation states, the recent discovery of photomagnetism of iron-cobalt analogues, and the design of thermodynamical machines. Molecular magnetism is a new field in science dealing with the conception, design, study, and the use of molecular magnetic materials with new but predictable properties.1 In this way, it follows and goes beyond magnetochemistry, which simply dealt with the magnetic properties of chemical systems. It is multidisciplinary in nature, relying on quantum mechanical principles to explain the properties, synthetic molecular chemistry, physical measurements, devices, biology (looking at biological magnetic models), and the like. Molecular magnetism covers various topics (low spin/high spin crossover systems, exchange coupled oligonuclear compounds, organic magnets, transition metal magnets, mixed organic-inorganic magnets, etc.). We focus on some examples taken from our own work, where electron transfer and oxidation states, familiar to readers of this magazine, play a peculiar role. Prussian blue analogues have a long history in chemistry since the 18th century; they renew unexpectedly the chemistry of cyanides in the last ten years. The magnetic part of this story is of interest in that it can throw light on the steps followed in molecular magnetism in a rational approach to secure a wanted physical property. In other words, we attempt to answer to the question: Is it possible to assemble molecules in mild conditions and in a rational way to get magnetic materials whose properties can be used to ensure some useful functions in a molecule-based device?