EPR spectroscopy of transformations of iridium(III) and iridium(IV) hydroxo complexes in alkaline media. Pankratov D.A., Komozin P.N., Kiselev Yu.M. //Russian Journal of Inorganic Chemistry. 2011. V.56. №11. P.1794-1799.
Processes that occur in strong alkaline solutions of iridium(III) and iridium(IV) hydroxo complexes have been studied by EPR and electronic absorption spectroscopy. It has been demonstrated that dissolution of iridium compounds in alkaline solutions should be accompanied by a series of complicated transformations involving oxygen, which lead to the formation of several binuclear iridium(III, III), (III, IV), and (IV, IV) dioxygen complexes.
Most research into the chemistry of platinum metals has focused on their complexes. However, the chemistry of their hydroxo complexes is still one of the least studied fields. There are both objective (experimental complexity of operation in alkaline and strong alkaline media, tendency to polymerization of many hydroxo compounds, and others) and subjective reasons for this situation. In particular, it is believed that the chemistry of platinum metal hydroxo complexes is insufficiently diverse. Nevertheless, we previously showed the possibility of the existence of platinum(IV) hydroxo complexes as mono- and binuclear as mono- (superoxo-) and bi- (hydroxo- and superoxo-) bridging superoxo complexes of different composition forming under oxidative conditions in strong alkaline media.
The chemistry of iridium hydroxo complexes is a blank spot in the chemistry of platinum group hydroxo compounds. It is traditionally believed that there are hexahydroxoiridium(III) and _iridium(IV) complexes that can interconvert into each other and polymerize in solutions and precipitate from them as hydroxides. Interconversions of iridium hydroxo complexes have been mainly studied by electrochemical methods; i.e., on the one hand, redox transformations occurred under external voltage, and on the other hand, similar studies were carried out after forced removal of oxygen from the reaction solution. As is shown below, oxygen is always involved in redox reactions that occur on dissolving iridium compounds in alkaline solutions. This work deals with interconversions of iridium complexes in strong alkaline media.
It follows from this scheme (where the area of each block is proportional to the number of moles of a reagent involved in reaction) of reactions occurring in alkaline solutions that almost all solutions should contain several different iridium complexes and these solutions should not be treated as those containing individual complexes. In addition to the aforementioned complexes, solutions can contain other dioxygen compounds, including paramagnetic superoxo compounds and oxygen adducts, which is supported by the presence of corresponding weak signal of oxygen radicals in some EPR spectra. Let us remind once more that, for simplicity, in the above analysis, we did not consider the possibility of formation of mixed-ligand hydroxo aqua or hydroxo chloro complexes.