Kinetic Analysis of the Conversion of Nonheme (Alkylperoxo)iron(III) Species to Iron(IV) Complexes

Abstract

Low-spin mononuclear alkylperoxoiron(III) complexes decompose by peroxide O-O bond homolysis to form iron(IV) species. We examined the kinetics of previously reported homolysis reactions for alkylperoxoiron(III) intermediates supported by TPA (tris-(2-pyridylmethyl)amine) in CH 3 CN solution and promoted by pyridine-N-oxide, by BPMCN (N,N-bis(2-pyridylmethyl)-N,Ndimethyl-trans-1,2-diaminocyclohexane) in its cis-β configuration in CH 3 CN and CH 2 Cl 2 , as well as for the previously unreported chemistry of TPA and 5-Me 3 TPA intermediates in acetone. Each of these reactions forms an oxoiron(IV) complex, except for the β-BPMCN reaction in CH 2 Cl 2 that yields a novel (hydroxo)alkylperoxoiron(IV) product. Temperature-dependent rate measurements suggest a common reaction trajectory for each of these reactions, and verify previous theoretical estimates of a ca. 60 kJ/mole enthalpic barrier to homolysis. However, both the tetradentate supporting ligand and exogenous ligands in the sixth octahedral coordination site significantly perturb the homolyses, such that observed rates can vary over two orders of magnitude at a given temperature. This is manifested as a compensation effect in which increasing activation enthalpy is offset by increasingly favorable activation entropy. Moreover, the applied kinetic model is consistent with geometric isomerism in the low-spin alkylperoxoiron(III) intermediates, wherein the alkylperoxo ligand is coordinated in either of the inequivalent cis sites afforded by the non-heme ligands.

X-ray Absorption Spectroscopy and Reactivity of Thiolate-Ligated FeIII−OOR Complexes

“…These studies also suggest that thiolate-ligated 1a – 3a do not decompose via O-O bond cleavage to give high-valent iron-oxo species. These results contrast previous work on non-thiolate-ligated, low-spin, alkylperoxo-iron(III) complexes, which decompose via homolytic O-O bond cleavage to give Fe IV (O) complexes 40,48,49. Furthermore, an increase in the electron-releasing nature of the thiolate ligands in 1a – 3a correlates with faster rates of decay for these species, in agreement with a weakening of the Fe-O bond as seen by RR and now EXAFS studies, and supported by DFT.…”

Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)–Alkylperoxo Complexes

“…One would expect that a rate-determining reaction step involving O–O bond cleavage would display a positive activation entropy, which is contrary to our experimental results. Negative activation entropies are also seen in O–O bond-cleaving reactions involving high-spin ( S = 5/2) Fe(III)–OO t Bu complexes, 86 and peroxo-bridged (TMP)Fe-(III)–O–O–Fe(III)(TMP). 87 One possible explanation for the negative activation entropies seen with the peroxo compounds described in this study is that the transition state leading to successful cleavage of the O–O bond has a shorter, more ordered interaction between the Mn ion and N(3,4).…”

Manganese and Iron Catalysts in Alkyd Paints and Coatings

“…Such adducts are wellknown for cobalt, manganese and iron complexes (see [8][9][10][11] for some representative examples of characterized metal-alkylperoxo species). These adducts may decompose to form peroxyl or alkoxy radicals (Reactions (8) and (9), respectively) [4]:…”

“…Reactions showing the formation of peroxyl radicals (8) and alkoxy radicals (9) from a metal-alkylperoxyl intermediate obtained by reaction of the metal complex with an alkylhydroperoxide (7) [4].…”