Kinetics and mechanism of substitution of bis(tptz)iron(II) by 2,2′-bipyridine and 1,10-phenanthroline

The kinetics of substitution of Fe(tptz)(2)(2+) by 2,2'-bipyridine and 1,10-phenanthroline have been investigated in acetate buffers in the 3.6-5.6 pH range employing stopped-flow spectrophotometry. These reactions are very fast and complete within 5 s. The rate of substitution is linearly dependent on [phen] and [bpy](2), and increases with the increase in pH. Suitable mechanisms have been proposed involving the unprotonated form of the entering ligand, viz. bipyridine/phenanthroline as the reactive species. The pK(a) values of bipyridine and phenanthroline, determined from the kinetic data, are in agreement with the literature values. It is concluded that the substitutions of iron(II)-diimine complexes also occur by an associative mechanism.