OXIDATION OF COORDINATED DIIMINE LIGANDS IN BASIC SOLUTIONS OF TRIS(DIIMINE)IRON(III), TRIS(DIIMINE)RUTHENIUM(III), AND TRIS(DIIMINE)OSMIUM(III)

The stoichiometrically analogous tris(2,2’-bipyridine) and tris(1,10-phenanthroline) complexes of both iron(III) and osmium(III) have been found to undergo thermal redox and substitution reactions, particularly, in aqueous base. A figure is presented that shows ligand structures and ligand numbering schemes. When tris(diimine)iron(III), -ruthenium(III), or -osmium(III) salts are dissolved in water, reduction to the intensely colored tris(diimine) metal(II) complexes soon becomes apparent. In acid solution, this process is slow, but in alkaline solution it appears to be significantly accelerated. It is known that the kinetics of this reduction process is a complicated plethora of parallel and consecutive reactions, and that metal(II) complexes of differently oxidized diimine ligands are formed in addition to the dominant tris(diimine)metal(II) product. Reduction of these tris(diimine)metal(III) complexes in basic solution is easily monitored by conventional spectrophotometry in the visible range. The osmium(III) complexes are very robust and the free ligand has never been observed as a reaction product. The mechanisms for the first step of the diimine ligand oxidation in basic solution include the following: (1) simple electron transfer, (2) dissociation of a ligand proton, (3) adduct formation at a ligand carbon atom, and (4) dissociation of a metal–nitrogen bond. © 1991 Academic Press Inc.