Kinetics and mechanism of the reaction of trans-(diaqua)(N,N '-ethylene bis-(salicylidineiminato)cobalt(III) with ascorbic acid

The kinetics of the reactions of trans-[Co(Salen)(OH2)(2)](+) (Salen = N.N' -ethylene bis (salicylidineiminate) with ascorbic acid (H(2)Asc) have been studied under varying conditions of pH, [ascorbic acid](T) and temperature at 0.5 mol dm(-3) ionic strength. The initial fast reactions observed in th e stopped flow time scale are due to the complex formation between the reac tants. This occurs in two phases i.e., the formation of the trans[(aqua)(as corbato)Co-III(Salen)] and its transformation to the corresponding ascorbat e chelate. The rate constants and the activation parameters for the formati on of the monobonded and chelate ascorbate complexes are reported. The low values of Delta H-double dagger and negative values of Delta S-double dagge r for the complexation reaction favour associative interchange mechanism(I- 2). The hydroxide in trans-[Co(Salen)(OH)(OH2)] marginally accelerates subs titution of the aqua ligand by HAsc(-) and the trans-[Co(Salen)(OH)(AscH)]( -) is considered to undergo fast internal proton transfer to generate trans -[Co(Salen)(OH2)(Asc)](-) which undergoes chelation of the Co-III centre by the bound ascorbate moiety; the latter reaction is, however, 15 times slow er than the corresponding reaction of trans[Co(Salen)(OH,)(AscH)]. The fast er complexation reactions are followed by the slow redox reactions. The rat e constant for the internal reduction of Co-III by the coordinated ascorbat e in the chelate [Co(Salen)(AscH)] is 5 - 10 times (25 degrees C - 45 degre es C) faster than the same for [Co(Salen)(Asc)](-). This trend in reactivit y is due to the low value of Delta H-double dagger for the former af though the high negative value of Delta S-double dagger compensates at least part ly the overriding effect of the activation enthalpy. The internal redox occ urs via innersphere mechanism. We also have observed a redox path involving trans- [Co(Salen)(OH2)(AscH(2))](+) and H(2)Asc for which electron transfe r most likely involves outersphere mechanism.