Iron chemistry, from molecular clusters to nanoparticles: A nice example of chemical versatility

Iron is an important element in the environment, due to its presence in soils and natural waters in the form of molecular complexes or colloids. Dissolution of ferric oxides in natural conditions (including acid-base and redox phenomena, microbial mediation and photo-chemistry) is of major importance in the cycling of iron. Iron oxides are also present in living organisms (eg. plants, bacteria, molluscs, birds and humans). Fe(II) complexes are the active center of haemoglobin and ferredoxins. Various biomineralization processes involve Fe(II) and Fe (III) species for the regulation of iron concentration in organism (ferritin) or to produce different oxides, such as goethite, magnetite, lepidocrocite. The structural chemistry of iron oxy(hydr)oxides is very rich and diversified. Because of the great diversity of physicochemical conditions in the environment (e.g. acidity, redox conditions, bacterial activity, temperature, salinity, presence of organic or inorganic ligands), practically all the iron oxide phases can be found in the natural environment. All of the structural types can also be formed from solutions by « chimie douce » giving rise to a puzzling chemistry. We present the main aspects of iron chemistry in aqueous medium, especially the condensation phenomena which lead to the formation of clusters or nanoparticles. Their formation is interpreted with illustrative mechanisms building a bridge between solution chemistry and solid state chemistry. The main factors orienting the crystallization process of solids and controlling the particle size are examinated, including the synthesis of magnetic phases and nanomaterials. The properties of finely divided magnetic materials closely depend on the size of the particles and their state of dispersion and aggregation. It is therefore very important to carefully understand and control the synthesis of particles and their surface state.