Oxidized TAML

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... 

Scheme 7. The mechanism of reactions of 1 with peroxides in water that accounts for the bell-shaped pH profiles with maxima around pH 10. Oxidized TAML reacts rapidly with 8.

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

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. 

If step III in Scheme 6 is ignored, a simplification is reached that has been demonstrated to be reasonable with certain substrates (see below). As a consequence, then the 1 catalysts should oxidize targeted reductants in accordance with Eq. (13) (here [Fe-TAML] is the total concentration of all TAML iron in solution). Eq. (13) implies that the catalysis by 1 mimics the steady-state oxidation by peroxidase enzymes, where Compound I is much more reactive than Compound II (55). 

The rate constants k and ku for la calculated from the data in Fig. 14 of ca. 3.5 x 103 and 1.5 x 104M 1s 1 (pH 11, 25°C), respectively, illustrate a very high activity of Fe-TAML activators particularly in terms of k. For example, Oakes and Gratton reported the value of 0.08M s1 for the oxidation of Orange II by p-sulfonated perbenzoic acid under the same conditions (57). 

The rate law of Eq. (15) holds at all pHs, despite the fact that is strongly pH dependent (see below). Free radical oxidation chemistry (60) appears not to be involved in these Fem-TAML catalyzed oxidations to any detectable degree. The efficient hydroxyl radical scavenger, mannitol (61,62), when added over the concentration range (0.5-2.0) x 10 3 M has no effect on the rate. This peroxide oxidation catalyzed by 1 does not proceed extensively via the hydroxyl free radical serving as the reactive intermediate. 

Equilibrium Constants, Rate Constants (in M 1 s 4) and Activation Parameters for the Fem-TAML-CATALYZED Oxidation of 8... 

The similar rate laws (Eqs. (15) and (18)), pH profiles, and the values of the observed second-order rate constants ( and k s) suggest a common reactive intermediate in reactions of Eqs. (14) and (17) (oxidized Fe-TAML in Scheme 6). Taking all three steps... 

Eqs. (19) and (20) were derived applying the steady-state approximation to the oxidized Fe-TAML species and using the mass balance equation [Fe-TAML] = 1 + [oxidized Fe-TAML] ([Fe-TAML] is the total concentration of all iron species, which is significantly lower than the concentrations of H2O2 and ED). The oxidation of ruthenium dye 8 is a zeroth-order reaction in 8. This implies that n[ED] i+ [H202]( i+ m). Eq. (19) becomes very simple, i.e.,... 

The intercept and slope equal (k + km)lkikm and ku/ i m[H202], respectively. An estimate for the ratio ku/km is derived from the slope using the known value of k (e.g., 4.4 x 103M-1s-1 for lk). The kn/km ratio equals 52 for lk and indicates that the oxidized Fe-TAML is significantly more reactive to the ED, Safranine O, than to H202. This extremely important feature of the Fe-TAML activators eliminates unproductive decomposition of H202 in the oxidation of Safranine... 

The mechanistic aspects of peroxidase-like activity of Fem-TAMLs discussed so far concerned mainly the activation by 1 of primary oxidizing agents, such as in Table VI, characterized by... 

In this paper, author reported the reactivity of newly synthesized Fem-superoxo and MnIV-peroxo complexes with nitric Oxide (NO) to follow nitric oxide dioxygenation reactions. Reactions of nonheme Fem—superoxo and Mnlv—pcroxo complexes bearing a common tetraamido macrocyclic igand (TAML), namely [(TAML)Fem(02)]2" and [(TAML)MnIV(02)]2% with NO afford the Fem-N03 complex [(TAML)Fem(N03)]2- and the Mnv-oxo complex [(TAML)Mnv(0)]" plus N02, respectively. 

The isomer shift is generally an excellent marker for the iron oxidation state. The graph of Figure 2.17 shows the correlation between DFT calculated and experimental isomer shifts of the TAML complexes.16 From this graph, we can see that Ferv- and Fem-TAML complexes are nicely grouped. It is also pleasing to see that theoretical isomer shifts correlate well with experimentally determined 8 values, but this shall not concern us here. 

Collins, T.J., Horwitz, C., and Gordon-Wylie, S.W. Project Title TAML Activators General activation of hydrogen peroxide for green oxidation processes, provided by Mary Kirchhoff, Green Chemistry Institute, American Chemical Society, 1999. 

Carbon Dioxide as an Environmentally Friendly Blowing Agent TAML Oxidant Activators ... 

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