Kinetics and mechanism of the complex formation between iron (III) and some (salicylato)pentaaminecobalt(III) ions

Article abstract of DOI:10.1021/ic00136a043

Kinetic studies by the stopped-flow technique were made at 20-30°C and I = 1.0 M (ClO₄^-) of the formation and the dissociation of the binuclear complex of Fe(III) with (salicylato)pentaaminecobalt(III) ions, where the pentaamine residues are 5 NH₃, (en)₂(NH₃), and tetraethylenepentamine. The formation of N₅CoSalFe⁴⁺ involves the reaction of the phenol form of the cobalt(III) substrates with Fe(OH₂)₆³⁺ and Fe(OH₂)₅(OH)²⁺. The rate constants and the activation parameters for both the paths are reported and compared with the available data for the corresponding reaction of the Fe(III) species with various other ligands. The carboxylate-bound (salicylato)pentaaminecobalt(III) substrates behave effectively as phenol with regard to their rate of complexation with the Fe(III) species. The rate data for the formation reactions are virtually independent of the nature of the amine function attached to the cobalt(III) center. It is concluded that the I_d mechanism applies in the reaction with Fe(OH₂)₅(OH)²⁺, while for the corresponding reaction with Fe(OH₂)₆³⁺, the mechanism is most likely of the I_a type. The binuclear species N₅CoSalFe⁴⁺ undergoes dissociation to yield the cobalt(III) substrates and the Fe(III) species via spontaneous and acid-catalyzed paths. The rate parameters for both the paths are sensitive to the nature of the amine function and decrease in the sequence tetren > (en)₂(NH₃) > 5 NH₃. A comparison of the rate data for the spontaneous dissociation of the binuclear complexes with that for the mono(phenolato)iron(III) species (C₆H₅OFe²⁺) reveals that the former are ～10³ times kinetically less labile than the latter. This suggests that Fe(III) is chelated by the salicylate moiety through the Co(III)-bound carboxylate group and the free phenol group after deprotonation. The chelate ring opening appears to be rate determining in both the spontaneous and the acid-catalyzed paths. To make possible the comparisons just mentioned, we have made a separate study of the formation and decomposition reactions of the mono(salicylato)iron(III) complex in perchlorate media. Previous work on this system involved only the formation reaction and was done in nitrate media, which we have found to exhibit moderate catalytic behavior.