Mechanistic Studies on the Oxidation of Hydroquinone by an Oxo-bridged Diiron(III,III) Complex in Weakly Acidic Aqueous Media

Abstract

The kinetics of oxidation of hydroquinone (H 2 Q) by a l-oxo-bridged diiron(III,III) complex, ½Fe 2 ðl-OÞ-ðphenÞ 4 ðH 2 OÞ 2 4þ (1) has been investigated in aqueous media at 25.0°C in presence of an excess of 1,10-phenanthroline (phen). The overall redox rate increases with increase in [H + ]. The title complex (1) and its conjugate bases, [Fe 2 ðl-OÞðphenÞ 4 ðH 2 OÞ(OH) 3þ (2) and [Fe 2 ðl-OÞðphenÞ 4 ðOHÞ 2 2þ (3), participate in the reaction with H 2 Q as the only kinetically reactive reducing species. Rate constants (in dm 3 mol )1 s )1 ) for the parallel reactions ð1Þ þ H 2 Q À! Products; ð2Þ þ H 2 Q À! Products and that for ð3Þ þ H 2 Q À! Products are, respectively, 500±40, 100±6 and 30±2. Substantial rate retardation in D 2 O media in comparison to that in H 2 O media suggests that electron transfer is coupled with proton movements in the rate-determining step.

Mechanistic Insights into Iron-Catalyzed C–H Bond Activation and C–C Coupling

“…μ-Oxodiiron complexes are known to play a role in oxygen atom transfer reactions, but their role in C–C coupling reactions has not been established. − The complexes [FeL1­(Cl 2 )] + , [FeL4­(Cl 2 )] + , and [FeL5­(Cl) 2 ] + were explored due to the well-documented facile formation of μ-oxodiiron complexes in solutions with a pH above 1. − [FeL4­(Cl 2 )] + and [FeL5­(Cl) 2 ] + (Figure ) were added to the series to explore the impact of steric bulk on the N-atoms because it was hypothesized that the steric bulk in ligands L4 and L5 would impact the formation or stability of the μ-oxodiiron complexes. In the presence of Et 3 N and H 2 O, [FeL1­(Cl 2 )] + and [FeL4­(Cl 2 )] + readily formed μ-oxodiiron complexes 1 and 2 (Scheme ), respectively.…”

Proton-Coupled Electron Transfer

“…Reaction rates were greatly diminished in D 2 O pointing to EPT in the rate-determining step. 127 In a subsequent paper it was demonstrated that the Fe dimer quantitatively oxidizes pyruvic acid to acetic acid and CO 2 . 128 The pentadentate pyridyl iron complex, Fe(PY5)(H 2 O) 2+ (PY5 is depicted in figure 31 Ru Complexes: The Ru(IV)-oxo complex, [Ru IV (tpa)(H 2 O)(O)](PF 6 ) 2 (depicted in figure 33), is an efficient catalyst for the oxidation of hydrocarbons with water as the solvent and oxygen source using Ce IV as a sacrificial oxidant.…”

Proton-Coupled Electron Transfer

“…Reaction rates were greatly diminished in D 2 O pointing to EPT in the rate-determining step. 127 In a subsequent paper it was demonstrated that the Fe dimer quantitatively oxidizes pyruvic acid to acetic acid and CO 2 . 128 The pentadentate pyridyl iron complex, Fe(PY5)(H 2 O) 2+ (PY5 is depicted in figure 31), was investigated as a mimic of lipoxygenase and suggested to react through an Fe(IV)-oxo intermediate.…”