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(S-0/S-infinity = (k(II/)k(i))[FeIII] tot, where S-0 and S-infinity, are [S] at time t = 0 and infinity, 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.