Sc3+ (or HClO4) Activation of a Nonheme FeIII-OOH Intermediate for the Rapid Hydroxylation of Cyclohexane and Benzene

Abstract

[Fe(β-BPMCN)(CH₃CN)₂]²⁺ (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 H₂O₂ and cannot perform benzene hydroxylation. However, addition of Sc³⁺ activates the 1/H₂O₂ reaction mixture to be able to hydroxylate cyclohexane and benzene within seconds at -40 °C. A metastable S = 1/2 Fe^III-(η¹-OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon addition of Sc³⁺ at rates independent of [substrate] but linearly dependent on [Sc³⁺]. HClO₄ elicits comparable reactivity as Sc³⁺ 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^V═O oxidant that is comparably reactive to the fastest nonheme high-valent iron-oxo oxidants found to date.

Figure 1

Yields in the hydroxylation of C₆H₆ (100 equiv relative to 1) or c-C₆H₁₂ (1000 equiv) by 1 (0.7 mM) and 10 equiv of 90% H₂O₂ in CH₃CN at 20 °C under air as a function of [Sc³⁺]. Xⁿ⁺ reflects the averaged results from adding 2 equiv of Al³⁺, Y³⁺, Yb³⁺, or Zn²⁺ (for individual results see Figure S1). 90% H₂O₂ was used as oxidant to minimize Lewis-acid deactivation by water present in the H₂O₂ solution.

Figure 2

Competitive hydroxylations of cyclohexane (c) and benzene (b, left) or nitrobenzene (n, right) with 0.7 mM 1, 10 equiv of H₂O₂, 2 equiv of Sc³⁺, and 600 equiv of total substrate. The numbers on the x-axis represent the substrate ratio in the competition experiments.

Figure 3

Yields in the hydroxylation of C₆H₆ (100 equiv relative to 1) or c-C₆H₁₂ (1000 equiv) by 1 (0.7 mM) and 10 equiv of 90% H₂O₂ in CH₃CN at 20 °C under air as a function of [HClO₄].

Figure 4

(a) UV–vis spectrum of 2 formed at −40 °C in CH₃CN from 1 mM 1 and 20 equiv of H₂O₂. (b) X-band EPR spectrum of 2 obtained at 40 dB at 2 K. (c) Resonance Raman spectrum of 2 formed with 2.5 mM 1 and 20 equiv of H₂O₂ at −30 °C (λ_exc = 561 nm).

Figure 5

(a) Spectral changes in the visible region upon reaction of 0.5 mM 1 (dashed black line) in CH3CN at −40 °C with 20 equiv of H₂O₂ to form 2 (dotted purple lines). Formation of 3 is observed upon subsequent addition of 1 equiv of Sc³⁺ to 2 (solid blue lines). (b) Time traces monitoring nearly instantaneous changes in absorbance at 545 and 800 nm after addition of 1 equiv of Sc³⁺: (black squares) 545 nm; (red circles) 800 nm in the presence of C₆H₆; (green triangles) 545 nm in the presence of C₆H₁₂.

Figure 6

(a) Time traces monitoring decay of 2 at 545 nm in the presence of 185 equiv of cyclohexane (■) and formation of 3 at 800 nm with 100 equiv of benzene (○) showing the effect of increasing [Sc³⁺] (green: 1 equiv; red: 2 equiv; black: 8 equiv of Sc³⁺). (b) [Sc³⁺] dependence of k_obs for 2 decay with cyclohexane as substrate (black squares) or 3 formation (red circles) with benzene as substrate. Data for the effect of HClO₄ addition are shown in Figure S5.

Scheme 1

Proposed Mechanism for the Effect of Sc³⁺ or HCIO4 in Cyclohexane and Benzene Hydroxylation by 1/H₂O₂