Unifying Evaluation of the Technical Performances of Iron-Tetra-amido Macrocyclic Ligand Oxidation Catalysts

The main features of iron-tetra-amido macrocyclic ligand complex (a sub-branch of TAML) catalysis of peroxide oxidations are rationalized by a two-step mechanism: Fe^III + H₂O₂ → Active catalyst (Ac) (k_I), and Ac + Substrate (S) → Fe^III + Product (k_II). TAML activators also undergo inactivation under catalytic conditions: Ac → Inactive catalyst (k_i). The recently developed relationship, ln­(S₀/S_∞) = (k_II/k_i)­[Fe^III]_tot, where S₀ and S_∞ are [S] at time t = 0 and ∞, respectively, gives access to k_i under any conditions. Analysis of the rate constants k_I, k_II, and k_i at the environmentally significant pH of 7 for a broad series of TAML activators has revealed a 6 orders of magnitude reactivity differential in both k_II and k_i and 3 orders differential in k_I. Linear free energy relationships linking k_II with k_i and k_I reveal that the reactivity toward substrates is related to the instability of the active TAML intermediates and suggest that the reactivity in all three processes derives from a common electronic origin. The reactivities of TAML activators and the horseradish peroxidase enzyme are critically compared.