Thermodynamics, Kinetics, and Mechanism of the Stepwise Dissociation and Formation of Tris(l-lysinehydroxamato)iron(III) in Aqueous Acid

pK a values for the hydroxamic acid, α-NH3 +, and ε-NH3 + groups of l-lysinehydroxamic acid (LyHA, H3L2+) were found to be 6.87, 8.89, and 10.76, respectively, in aqueous solution (I = 0.1 M, NaClO4) at 25 °C. O,O coordination to Fe(III) by LyHA is supported by H+ stoichiometry, UV−vis spectral shifts, and a shift in νCO from 1648 to 1592 cm-1 upon formation of mono(l-lysinehydroxamato)tetra(aquo)iron(III) (Fe(H2L)(H2O)4 4+). The stepwise formation of tris(l-lysinehydroxamato)iron(III) from Fe(H2O)6 3+ and H3L2+ was characterized by spectrophotometric titration, and the values for log β1, log β2, and log β3 are 6.80(9), 12.4(2), and 16.1(2), respectively, at 25 °C and I = 2.0 M (NaClO4). Stopped-flow spectrophotometry was used to study the proton-driven stepwise ligand dissociation kinetics of tris(l-lysinehydroxamato)iron(III) at 25 °C and I = 2.0 M (HClO4/NaClO4). Defining k n and k - n as the stepwise ligand dissociation and association rate constants and n as the number of bound LyHA ligands, k 3, k - 3, k 2, k - 2, k 1, and k - 1 are 3.0 × 104, 2.4 × 101, 3.9 × 102, 1.9 × 101, 1.4 × 10-1, and 1.2 × 10-1 M-1 s-1, respectively. These rate and equilibrium constants are compared with corresponding constants for Fe(III) complexes of acetohydroxamic acid (AHA) and N-methylacetohydroxamic acid (NMAHA) in the form of a linear free energy relationship. The role of electrostatics in these complexation reactions to form the highly charged Fe(LyHA)3 6+ species is discussed, and an interchange mechanism mediated by charge repulsion is presented. The reduction potential for tris(l-lysinehydroxamato)iron(III) is −214 mV (vs NHE), and a comparison to other hydroxamic acid complexes of Fe(III) is made through a correlation between E 1/2 and pFe.