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

TAML complex is oxidized by H₂O₂ or ^tBuOOH in water at pH < 10 into the corresponding iron(IV) μ-oxo-bridged dimer 2, which oxidizes readily ring-substituted thioanisoles p-XC₆H₄SMe (X=H, MeO, Me, Cl, CN) into the corresponding sulfoxides with regeneration of 1. The oxidation studied under pseudo-first-order conditions using 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-XC₆H₄SMe]/(1 + b[p-XC₆H₄SMe]) 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-NO₂C₆H₄SMe by H₂O₂ 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-NO₂C₆H₄SMe with rate constants k_m and k_d which were found to be (13 ± 1) × 10⁴ and (0.32 ± 0.01) × 10⁴ M⁻¹ s⁻¹, respectively. The difference in the rate constants is the key for resolving the dilemma of faster catalysis versus slower single-turnover reactivity of TAML activators in water.

Graphical abstract

Keywords:

• Iron complex
• TAML
• Kinetics
• Mechanism
• Thioanisoles
• Stopped-flow

Acknowledgement

ADR thanks Genoa R. Warner for critical reading of the manuscript and helpful comments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

Dedicated to Prof. Rudi van Eldik on the occasion of his 70th birthday.

Additional information

Funding

Operating support is acknowledged from the Heinz Endowments (T.J.C.); the Institute for Green Science (T.J.C.); the Environmental Protection Agency [grant number RD 83 to T.J.C.].