Sc³⁺ (or HClO₄) Activation of a Nonheme Fe^III–OOH Intermediate for the Rapid Hydroxylation of Cyclohexane and Benzene

Abstract

[Fe(β-BPMCN)(CHCN)] (1, BPMCN = N,N' -bis(pyridyl-2-methyl)- N,N' -dimethyl- trans-1,2-diaminocyclo-hexane) is a relatively poor catalyst for cyclohexane oxidation by HO and cannot perform benzene hydroxylation. However, addition of Sc activates the 1/HO reaction mixture to be able to hydroxylate cyclohexane and benzene within seconds at -40 °C. A metastable S = 1/2 Fe-(η-OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon addition of Sc at rates independent of [substrate] but linearly dependent on [Sc]. HClO elicits comparable reactivity as Sc at the same concentration. We thus postulate that these additives both facilitate O-O bond heterolysis of 2 to form a common highly electrophilic Fe═O oxidant that is comparably reactive to the fastest nonheme high-valent iron-oxo oxidants found to date.

Activation of a Non‐Heme FeIII‐OOH by a Second FeIII to Hydroxylate Strong C−H Bonds: Possible Implications for Soluble Methane Monooxygenase

“…Control experiments show that neither Fe III -(OTf) 3 nor Fe II (OTf) 2 alone leads to the observed reactivity under similar reaction conditions (Table S1). When the results of Fe III (OTf) 3 activation of the 1/H 2 O 2 system are compared to those previously reported for Sc 3+ and HClO 4 activation, [14] it is clear that Fe 3+ is more effective than either Sc 3+ or HClO 4 in substrate oxidation (Table 1and Figure 1). Forbenzene hydroxylation, aturnover number of 5.4 is obtained with 2equiv Fe 3+ ,w hich is about 30 % higher than found for Sc III (OTf) 3 or HClO 4 .F or cyclohexane oxidation with 2equiv Fe III (OTf) 3 , aT ON of 7.5 is observed for cyclohexanol, which is almost two-fold higher than that reported for Sc III -(OTf) 3 or HClO 4 .F urthermore,t he A/K ratio for Fe III (OTf) 3 increases five-fold relative to that observed for Sc III (OTf) 3 due to the lower yield of cyclohexanone.…”

“…[9][10][11] Lewis acids can also assist in cleaving OÀOb onds in peroxo(hydro) intermediates to form high-valent metal-oxo species.F or [(TMC)Fe III -h 2 -O 2 ] + , Lewis acids such as Sc 3+ and Y 3+ induce cleavage of the OÀO bond to form the [(TMC)Fe IV (O)] 2+ . [14] Fe 3+ and Fe 2+ are generally known for their redox properties but can potentially act as Lewis acids as well. [14] Fe 3+ and Fe 2+ are generally known for their redox properties but can potentially act as Lewis acids as well.…”

“…[15] In 2018, we have shown that redox-inactive Lewis acid Sc 3+ activates this system to form apowerful oxidant that can efficiently catalyze cyclohexane oxidation within seconds at 25 8 8C. [14] Addition-Scheme 1. Proposed active oxidants for a) diiron and b) monoiron non-heme enzymes.…”

“…[12,13] Recently,w eh ave demonstrated that Lewis acidic Sc 3+ activates the [Fe III (b-BPMCN)(OOH)] 2+ intermediate to form ar eactive Fe V = O oxidant that cleaves strong C À Hb onds within seconds at À40 8 8C. [14] Fe 3+ and Fe 2+ are generally known for their redox properties but can potentially act as Lewis acids as well. However,neither Fe 3+ nor Fe 2+ has to date been reported to activate Fe III -peroxo or hydroperoxo species.Here we report the activation of the [Fe III (b-BPMCN)(OOH)] 2+ intermediate by as econd iron(III) ion to form as trong oxidant that hydroxylates both cyclohexane and benzene efficiently.…”

Density functional approximations for consistent spin and oxidation states of oxoiron complexes

“…This shed new light on the scandium‐capped complex, with theoretical predictions confirmed experimentally in 2015 by Prakash et al, who were able to carry out characterization by X‐ray, Mössbauer and EPR spectroscopy and unambiguously confirmed the high spin iron(III) state. However, irrespective of the reassignment of the oxidation state, the seminal work by Fukuzumi and Nam opened up a complete new approach for the stabilization of volatile species by complexation with a Lewis acid, and has brought forward new insights into the electronic structure of these short‐lived species.…”