A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

Abstract

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes Mn^V(O)(ttppc) (4) and Mn^III(H₂O)(ttppc) (1·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O-H bond of 2,4-di- tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k₂ = 2.73(12) × 10⁴ M^-1 s^-1, which is ∼3 orders of magnitude larger than 4 ( k₂ = 17.4(1) M^-1 s^-1). Reaction of 3 with a series of para-substituted 2,6-di- tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k₂ = 510(10)-36(1.4) M^-1 s^-1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes.

Figure 1.

Displacement ellipsoid plots (35% probability level) for Mn^IV(Cl)(ttppc) (2) at 110(2) K. Hydrogen atoms omitted for clarity.

Figure 2.

UV-vis spectral titration of 2 with NBu₄OH (0, 0.33, 0.66, 1.0 equiv) in toluene/CH₃CN (100:1 v/v) at 23 °C. Inset: changes in the X-band EPR spectra of 2 (blue line) upon titration with Bu₄NOH (0 – 1.0 equiv) in toluene/CH₃CN (4:1 v/v) at 12 K (final spectrum, orange line). (* = radical impurity (S = ½)).

Figure 3.

Displacement ellipsoid plot (35% probability level) for Mn^IV(OH)(ttppc) (3) at 110(2) K. Hydrogen atoms (except H1) omitted for clarity.

Figure 4.

Displacement ellipsoid plot (35% probability level) for Mn^V(O)(ttppc) (4) at 110(2) K. H atoms omitted for clarity.

Figure 5.

Displacement ellipsoid plots (50% probability level) for the Mn–O unit in 4 at 110(2) K, from crystals grown over different time periods. The structures were obtained from crystals of 4 grown for 1 week (4a), 2 weeks (4b), and 1 month (4c). All bond distances are given for the major component of the Mn–O group which is disordered over two positions (~7:3 occupancy) above and below the plane of the corrole.

Figure 6.

Displacement ellipsoid plot (35% probability level) for Mn^III(H₂O)(ttppc) (1•H₂O) at 110(2) K. H atoms (except for those attached to O) omitted for clarity.

Figure 7.

a) Reaction of Mn^V(O)(ttppc) (4) and 2,4-DTBP. b) Time-resolved UV−vis spectral changes for the reaction between (4) (15 μM) and 2,4-DTBP (0.5 mM) in benzene at 23 °C. Inset: change in absorbance vs time for the growth of Mn^III(ttppc) (660 nm) (green circles) with the best fit line (black). c) Plot of pseudo-first order rate constants (k_obs) versus [2,4-DTBP-OH] (black circles) and [2,4-DTBP-OD] (red squares). d) Reaction of Mn^IV(OH)(ttppc) (3) and 2,4-DTBP. e) Time-resolved UV−vis spectral changes for the reaction between (3) (15 μM) and 2,4-DTBP (0.42 mM) in benzene at 23 °C. Inset: change in absorbance vs time for the growth of Mn^III(ttppc) (660 nm) (green circles) with the best fit line (black). f) Plot of pseudo-first order rate constants (k_obs) versus [2,4-DTBP-OH] (black circles) and [2,4-DTBP-OD] (red squares).

Figure 8.

Hammett (a) and Marcus (b) plots for the reaction of Mn^IV(OH)(ttppc) (3) and 4-X-2,6-DTBP (X = -OMe, -Me, -tBu, -H) in benzene at 23 °C.

Scheme 1.

Net H-atom Transfers in the Active Sites of CYP and OEC

Scheme 2.

Possible Mechanisms for H-atom Abstraction by an Mn^V(O) Complex

Chart 1.

Examples of Mn^IV(OH)_n (n = 1, 2) Complexes