REDOX KINETICS OF METAL-COMPLEXES IN NONAQUEOUS SOLUTIONS - OXIDATION OF A SERIES OF TRIS(1,10-PHENANTHROLINE)IRON(II) IONS BY HEXAKIS(TRIMETHYLPHOSPHATE)IRON(III) IN ACETONITRILE - A REACTIVITY-SELECTIVITY RELATIONSHIP

The kinetics of outer-sphere oxidation of Fe(Xphen)3(2+) ions (X = H or several methyl substituents) in acetonitrile (MeCN) solution by iron(III), introduced as Fe(tmp)6(ClO4)3 (tmp = trimethylphosphate), have been investigated at 25-degrees-C. The reactions are very complex because of solvation equilibria between tmp and MeCN coordinated at Fe3+, with a reduction potential difference of 0.20V for the replacement of one tmp by MeCN. This makes the various solvate species highly different in driving force. The second essential feature is the high charge-type of + 2/ + 3. This brings about strong acceleration by salt because of ion association reducing the work necessary to overcome the Coulombic repulsion in forming the precursor complex. The task was to deconvolute two kinds of speciation: the ionic and the solvate speciations. The analysis suggests the concurrent existence of five Fe(tmp)n3+ (n = 2-6) species among which four species (n=2-5) are reacting, with an additional mono and bis perchlorate ion pair for each n. Extra complications arise as some of the solvation equilibria are not always fast compared to the redox reactions, leading to non-first order rate constants. Although the rate constants could not be defined with the desired precision, at least two results are worth noting: (i) The relative effects of driving force and charge are highlighted in controlling overall reactivity. (ii) The stronger the reducing power of the Fe(Xphen)3(2+) moiety, the less it can distinguish between the various solvate species (reactivity-selectivity relationship).