Iron(IV) or iron(V)? Heterolytic or free radical? Oxidation pathways of a TAML activator in acetonitrile at −40 °C

Abstract

The oxidation of TAML complex(1) by various oxidants is explored in MeCN with 0.2% water at -40 °C where the iron(V)oxo complex is stable enough for reliable spectral identification. The iron(V)oxo state is achieved by using NaClO even faster than in the case of previously explored m-chloroperoxybenzoic acid. In contrast, H 2 O 2 and organic peroxides (benzoyl peroxide, tert-butylperoxide and tert-butylhydroperoxide)all convert 1 into the corresponding diiron(IV)-μ-oxo dimer (2) under the same conditions. The latter does not form when (NH 4 ) 2 [Ce(NO 3 ) 6 ] is employed and the Fe IV product obtained does not seem to contain an oxo moiety. In contrast to all other oxidants, the conversion of 1 by t BuOOH into 2 is characterized by non-conventional kinetics and therefore this reaction was explored in some detail. The evidence is presented that this light-, O 2 -, and TEMPO-and base-dependent reaction is a free-radical process under the conditions used.

NaClO-Generated Iron(IV)oxo and Iron(V)oxo TAMLs in Pure Water

“…16 The advantages of using NaClO over m CPBA as an oxidant include higher reactivity and unlimited solubility in water. Figure 1 shows the spectra of 3 and the products formed in the presence of NaClO at pH 10.6 and 13.…”

“…We have recently reported that at −40 °C in MeCN iron(III) TAML 1 is oxidized into the iron(V)oxo species by NaClO and mCPBA though H 2 O 2 and organic peroxides convert 1 into the diiron(IV)(μ-oxo) dimer. 16 The advantages of using NaClO over mCPBA as an oxidant include higher reactivity and unlimited solubility in water. Figure 1 shows the spectra of 3 and the products formed in the presence of NaClO at pH 10.6 and 13.…”

“…The oxidations of 1 by NaClO and mCPBA in MeCN at −40 °C and below occur with spectrally detectable diiron(IV)(μ-oxo) species 21 because the Fe III →Fe IV process is much faster than the Fe IV →Fe V conversion. 16,22 It is worth noting that diiron(IV)(μ-oxo) species are not formed either when 3 was exposed to mCPBA in MeCN at −40 °C. Iron(V)oxo generated under such conditions has UV−vis and EPR spectra identical to those obtained in water.…”

Oxoiron(V) Complexes of Relevance in Oxidation Catalysis of Organic Substrates

“…[Fe V (bTAML)(O)] À was quantitatively generated by adding one equivalent of m-CPBA to a solution of the iron(III) precursor in CH 3 CN. [24] Alternative synthetic routes include the use of NaOCl as oxygen atom donor [25] and CH 3 CN : H 2 O (up to 90 %) as solvent mixture. [26] Photochemical [27] or electrochemical synthetic protocols [28] have also been reported.…”

Homogeneous Catalysis Under Ultradilute Conditions: TAML/NaClO Oxidation of Persistent Metaldehyde

“…We have long known that TAML activators catalyze hypochlorite reactions . NaClO activates TAML catalysts to give Fe V O complexes from −40 °C to room temperature, highlighting a similarity with the most reactive intermediates in TAML/H 2 O 2 systems …”