Kinetic and mechanistic studies of the reactivity of iron(IV) TAMLs toward organic sulfides in water: resolving a fast catalysis versus slower single-turnover reactivity dilemma

Abstract

2015): Kinetic and mechanistic studies of the reactivity of iron(IV) TAMLs towards organic sulfides in water: Resolving a fast catalysis versus slower single turnover reactivity dilemma, Journal of Coordination Chemistry, Dedicated to Prof. Rudi van Eldik on the occasion of his 70 th birthday. TAML complex (1) is oxidized by H 2 O 2 or t BuOOH in water at pH < 10 into the corresponding iron(IV) μ-oxo-bridged dimer 2, which oxidizes readily ring-substituted thioanisoles p-XC 6 H 4 SMe (X = H, MeO, Me, Cl, CN) into the corresponding sulfoxides with regeneration of 1. The oxidation studied under pseudo-first-order conditions by the stopped-flow technique by monitoring the fading of the 420 nm band of 2 follows hyperbolic kinetics according to the rate law k obs = ab[p-XC 6 H 4 SMe]/(1 + b[p-XC 6 H 4 SMe]) at pH 8 and 25 °C. Parameters a, b and ab all decrease for electron-poorer thioanisoles and the Hammett value ρ ~ 1 has been found for ab, which can be associated with the second order rate constants for oxidation of thioanisoles by 2. The kinetics of oxidation of p-NO 2 C 6 H 4 SMe by H 2 O 2 catalyzed by 1 has been studied under steady-state conditions. Covering the concentration of 1 in a 100-fold range has revealed that though first order kinetics in 1 is observed at low catalyst concentrations (below 10 -6 M), there is a significant negative deviation from linearity at [1] > 10 -6 M. The latter was rationalized by the equilibrium between the monomeric and dimeric Fe IV species 2 M ⇌ M-M (K d ), both being able to oxidize p-NO 2 C 6 H 4 SMe with rate constants k m and k d which were found to be (13±1)×10 4 and (0.32±0.01)×10 4 M -1 s -1 , respectively. The difference in the rate constants is the key for resolving Downloaded by [New York University] at 23:15 06 July 2015 2 the dilemma of faster catalysis versus slower single turnover reactivity of TAML activators in water.

Activation of Dioxygen by a TAML Activator in Reverse Micelles: Characterization of an FeIIIFeIVDimer and Associated Catalytic Chemistry

“…18,19 Monomers Fe IV O are known to be more reactive than dimers Fe IV Fe IV in aqueous solutions. 89 Thus, the dominating speciation of iron here as the least oxidized and oxidizing Fe III Fe IV explains the observed reactivity picture.…”

Kinetic Evidence for Reactive Dimeric TAML Iron Species in the Catalytic Oxidation of NADH and a Dye by O2 in AOT Reverse Micelles

“…The second-order pathway in [ 1 ] is a new feature in catalysis by TAML activators. When TAMLs function in water utilizing H 2 O 2 or organic peroxides as primary oxidants, first-order kinetics in [TAML] is commonly observed, , though a reaction order of 1/2 has also been confirmed …”

Zero‐Order Catalysis in TAML‐Catalyzed Oxidation of Imidacloprid, a Neonicotinoid Pesticide

“…In catalysis by TAML activators, such is the pH-controlled dimerization of active iron(IV) species. [17,22] The latter mechanism has been supported by the experimental observations.T he ground-state TAML aggregation may also contribute to the zero-order catalysis. A piece of evidence comes from numerousexamples of considerable line-broadening in EPR spectra of the spin 3/2 iron(III) TAML species in pure aqueous solutionsa tm m concentrations.…”

“…In fact, concentrations of 2 used throughout were always lower than the critical value of 10 À6 m,a bove which the dimerization starts to play ar ole. [17] Also, the dimerization scenario predicts an approximately linear relationb etween the reaction rate andasquare root of the catalystconcentration, which correspondst ot he observed reaction order of 0.5. Formale stimation of the reaction order in 2 from the data in Figure 5u sing the corresponding double log plot ( Figure S7) affords as lope of 0.164 AE 0.006, which is noticeably lower than 0.5.…”

“…Non-linear rate-dependence on TAML concentrations was previously reportedf or the 5-catalyzed oxidation of p-NO 2 C 6 H 4 SMe by H 2 O 2 into the corresponding sulfone. [17] The coverage of TAML concentration in a1 00-fold range revealed that althoughf irst-order kinetics in 5 was observed at low catalyst concentrations (below 10 À6 m), there was an egative deviation from linearity at concentrations above 10 À6 M. The effect was rationalizedb yt he equilibrium between the monomeric (M) and dimeric (M 2 )i ntermediate active Fe IV species (2 mÐM 2 , K d ), both capable of oxidizing the sulfide with the rate constants k M = 13 10 4 and k M2 = 0.3 10 4 m À1 s À1 ,r espectively.T he difference in the values of k M and k M2 (k M > k M2 )w as the key for both accounting for the non-linear dependencea nd resolving the dilemma of faster catalysis versus slower single-turnover reactivity of TAML activators in water. [17] As tronger deviation from linearity was observed for the 3-catalyzed oxidation of 3methyl-4-nitrophenolb yH 2 O 2 at pH 8a nd 25 8C.…”