Kinetics and Mechanism of Iron(III)-Nitrilotriacetate Complex Reactions with Phosphate and Acetohydroxamic Acid

The kinetics and mechanism of the substitution of coordinated water in nitrilotriacetate complexes of iron(III) (Fe(NTA)(OH2)2 and Fe(NTA)(OH2)(OH)-) by phosphate (H2PO4- and HPO42-) and acetohydroxamic acid (CH3C(O)N(OH)H) were investigated. The phosphate reactions were found to be pH dependent in the range of 4-8. Phosphate substitution rates are independent of the degree of phosphate protonation, and pH dependence is due to the difference in reactivity of Fe(NTA)(OH2)2 (k = 3.6 × 10^5 M^-1 s^-1) and Fe(NTA)(OH2)(OH)- (k = 2.4 × 10^4 M^-1 s^-1). Substitution by acetohydroxamic acid is insensitive to pH in the range of 4-5.2, and Fe(NTA)(OH2)2 and Fe(NTA)(OH2)(OH)- react at equivalent rates (k = 4.2 × 10^4 and 3.8 × 10^4 M^-1 s^-1, respectively). Evidence for acid-dependent and acid-independent back-reactions was obtained for both the phosphate and acetohydroxamate complexes. Reactivity patterns were analyzed in the context of NTA labilization of coordinated water, and outer-sphere electrostatic and H-bonding influences were analyzed in the precursor complex (Kos).