TAMLs

MECHANISTIC CONSIDERATIONS ON THE REACTIVITY OF GREEN Fe "-TAML ACTIVATORS OF PEROXIDES... 

IV. Catalysis-Relevant Oxidized Forms Derived from Fem-TAMLs 487... 

Chart 1. Feni-TAML activators mentioned in this account are shown as pentacoordinated species with an axial aqua ligand as obtained in the solid state. The aqua complexes are synthesized as such or as the corresponding chloro species with Cl- instead of H20 (with two M+ = Li+, Na +, NR4 counter ions). The subscript Cl as in lei is used throughout to indicate the corresponding chloro species. 

Scheme 1. Speciation of Feni-TAML activators in aqueous solution (solid rectangle) and suggested mechanism of the H+-induced deme-talation (dashed rectangle) — — = free base ligand. From Ref. (13).

Equilibrium Constants for Fem-TAML Activators 1 Reacting with Ligands, L, in Aqueous Solution at 25°C, 0.1 M KPF0 (27)... 

ALEXANDER D. RYABOV AND TERRENCE J. COLLINS III. Kinetics and Mechanisms of Demetalation of Fem-TAML Activators A. Specific Acid Catalysis... 

The k pathway is three times faster in D+/D20 than in H+/H20 for la. The reverse kinetic isotope effect suggests that the rate-limiting event for the k pathway could involve protonation of an amido-nitrogen or an N-Fe bond, forming the stronger N-H bond as the weaker N-Fe bond is cleaved. The k 3 pathway is rationalized as involving pre-equilibrium peripheral protonations of the TAML macrocycle (Scheme 1). The dependence of obs on [H + ] is then given by Eq. (4), which corresponds... 

The rate of acid-induced demetalation depends only slightly on the nature of the head substituents X (Table I). In contrast, the tail-R groups dramatically affect k and, for the most part, k3, suggesting that tail amide O-atoms are sites of peripheral protonation. Thus, the acid tolerant Fem-TAML catalysts with tail electron-withdrawing groups should be more acid resistant and replacement of R = Me with R = F results in a remarkable stabilization. The rate constants (Table I) show that under weakly acidic conditions (pH 2-3), when the k pathway dominates over k3, fluorinated lk is 105-fold more H +-tolerant than la. 

Scheme 2. Proposed mechanism for the phosphate-induced demeta-lation of Fein-TAMLs.

Scheme 3. Stoichiometric mechanism of demetalation of robust FeIII-TAMLs such as lm by picolinic acid (L) in the presence of other pyridine bases (P). Axial aqua ligands are omitted for clarity.

Scheme 4. Proposed general mechanism of demetalation of 1 by picolinic acid accounting for first (la, in the box) and second (lm) orders in the buffer acid concentration. The charge of the Fem-TAML complex is shown outside the bracket and localized charges are shown for the deprotonated pyridine carboxylates. From Ref. (27).

Addition of H202 or t-BuOOH to aqueous solutions of iron(III)-TAML complexes 1 produces brownish-green colors. The spectral changes can be measured by UV/Vis spectroscopy (Fig. 10). Less than a stoichiometric amount of peroxide causes a major increase in the absorbance in the range 350-550 nm. It requires several minutes to obtain invariable spectra at pH below 8.5, but the reaction becomes much faster at pH >9. Two isosbestic points... 

Scheme 5. Speciation of oxidized TAML species derived from 1 in aqueous solution. Axial aqua ligands are not shown.

The UV/Vis, Mossbauer, EXAFS, and EPR spectroscopic data suggest a rather complicated picture regarding the speciation of oxidized TAML species derived from 1 and various oxidants in aqueous solution (Scheme 5). Peroxides ROOH have the capacity to function as two-electron oxidants and usually do. In cases where prior coordination occurs, they can oxidize metal ions via one-electron processes where the 0-0 bond is cleaved homo-lytically or two-electron processes where it is cleaved hetero-lytically. The two-electron oxidation of 1 presumably would give the iron-oxo intermediate 6, two electrons oxidized above the iron(III) state (see below). Before 6 was actually isolated, there... 

V. Mechanism of Catalysis by Tetraamide Macrocyclic Fem-TAML Activators of Hydrogen Peroxide, Functional Catalase-Peroxidase... 

Scheme 6. General mechanistic picture of catalysis by Fem-TAML activators of peroxides that underscores the peroxidase-like (peroxida-tic) and catalase-like (catalatic) activities of the catalysts. Here, S(red) is an oxidizable target substrate. All rate constants are conditional (pH dependent).

Oxidized Fe-TAML could be the iron(V)oxo complex 6, which as noted above can be produced from la and m-chloroperox-ybenzoic acid at low temperatures (—60°C) in a nonaqueous solvent (51). Presumably such an iron(V)oxo complex can behave in a substrate-dependent way as both a two-electron or one-electron oxidant. In the former case, it is returned in one step to the iron(III) state. In the latter, it must first pass through an iron(IV) intermediate. At pH>12, the likely iron(IV) species would be the same compound as is formed from la and H202, (48) namely the iron(IV)-oxo complex 7, which has similar features with [(H20)sFeIV = 0]2 +, (54) or its water adduct 7". At other pHs, other iron(IV) compounds are known to be formed (48). Both the iron(V)-oxo and iron(IV)-oxo complexes as well as the other iron(IV) species could be involved in catalysis by 1 (see Section V.B). The possible involvement of complexes that are in a higher oxidation state than 6 cannot be ruled out. 

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