Activation of a High-Valent Manganese–Oxo Complex by a Nonmetallic Lewis Acid

Abstract

The reaction of a manganese(V)–oxo porphyrinoid complex with the Lewis acid B(C₆F₅)₃ leads to reversible stabilization of the valence tautomer Mn^IV(O)(π-radical cation). The latter complex, in combination with B(C₆F₅)₃, reacts with ArO–H substrates via formal hydrogen-atom transfer and exhibits dramatically increased reaction rates over the Mn^V(O) starting material.

Short abstract

The addition of the nonmetal Lewis acid B(C₆F₅)₃ to a low-spin manganese(V)−oxo corrolazine (Cz) stabilizes the valence tautomer Mn^IV(O)(Cz^•+). Cryospray mass spectrometry, together with other data, supports formation of the 1:1 complex Mn^IV(O)(Cz^•+):B(C₆F₅)₃. The rates of hydrogen-atom abstraction from O−H bonds are significantly enhanced for the latter species. This work provides insight regarding the influence of both the electronic structure and Lewis acids on the reactivity of high-valent metal−oxo complexes.

High-valent metal–oxo species are invoked as key players in both biological and synthetic oxidations. Examples include an Fe^IV(O) porphyrin π-radical cation, the key intermediate in cytochrome P450,¹ and postulated Mn^IV/Mn^V(O) intermediates in photosynthetic water oxidation.² In heme systems such as P450, there are two possible valence tautomers, Fe^V(O)(p) or Fe^IV(O)(p^•+), and the redox-active nature of the porphyrin ligand plays a critical role in stabilizing the latter. The synthesis of analogous transition-metal complexes with redox-active ligands that enhance reactivity has been targeted for multielectron catalysis.³ The design of complexes that can undergo controlled valence tautomerism is also important for molecular device applications.⁴

Previously, we introduced a new method for controlling valence tautomerism in a Mn^V(O) porphyrinoid complex.⁵ The addition of the Lewis acidic Zn^II ion was shown to convert Mn^V(O)(TBP₈Cz) to the valence tautomer Mn^IV(O)(TBP₈Cz^•+). The influence of Lewis acids on the reactivity of biologically relevant high-valent metal–oxo species is of intense current interest,⁶ especially because of the Ca²⁺ ion involved in manganese-mediated water oxidation.² Herein, we show for the first time that a nonmetal ion Lewis acid, B(C₆F₅)₃, can induce valence tautomerism in a high-valent metal–oxo complex. We find that the addition of B(C₆F₅)₃ also causes dramatically enhanced hydrogen-atom abstraction reactivity toward phenol substrates.

The reaction of Mn^V(O)(TBP₈Cz) with 1 equiv of B(C₆F₅)₃ in CH₂Cl₂ at room temperature (Scheme 1) was monitored by UV–vis spectroscopy. An immediate color change from the green solution for Mn^V(O)(TBP₈Cz) to a red-brown solution was observed together with the disappearance of the spectrum for Mn^V(O)(TBP₈Cz) (λ_max = 420 and 634 nm) and the appearance of new peaks at λ_max = 420 and 789 nm (Figure 1). The broadening and decrease in the intensity of the Soret band at 420 nm together with the formation of a relatively weak band in the near-IR region at 789 nm is characteristic of the formation of a porphyrinoid π-radical cation.⁵

Scheme 1. Formation of Mn^IV(O)(TBP₈Cz^•+).

Figure 1.

(a) Spectral titration of Mn^V(O)(TBP₈Cz) (5 μM) + B(C₆F₅)₃ (0.1–1.0 equiv) in CH₂Cl₂. (b) CSIMS(+) of [Mn(O)(TBP₈Cz):B(C₆F₅)₃ + H]⁺ at −50 °C.

The new spectrum matches that observed previously upon the addition of the Lewis acid Zn^II(OTf)₂ to the Mn^V(O) complex, which was shown to convert the low-spin (S = 0) Mn^V complex into a high-spin triplet (S = 1) (or possibly quintet S = 2) state with an electronic configuration best described as a manganese(IV) corrolazine π-radical cation.⁵ UV–vis spectral titrations were performed with B(C₆F₅)₃, and tight isosbestic behavior was seen throughout the titration (Figure 1a). A plot of the absorbance at 789 nm versus [B(C₆F₅)₃] (Figure S1 in the Supporting Information, SI) reaches a plateau at ∼1 equiv of B(C₆F₅)₃, and no further spectral changes are seen with the addition of more B(C₆F₅)₃. Assuming that triarylborane reversibly binds to the Mn^V(O) complex under equilibrium conditions, a good fit of the data can be obtained with a model for a one-to-one binding isotherm. This fit gives an association constant (K_a) of 2.0 × 10⁷ M^–1, close to that measured for Zn²⁺ [K_a(Zn²⁺) = 4.0 × 10⁶ M^–1].⁵

The ¹H NMR spectrum of Mn^V(O)(TBP₈Cz) + B(C₆F₅)₃ (1:1 molar ratio) in CDCl₃ reveals paramagnetically shifted and broadened peaks, as opposed to the sharp, diamagnetic spectrum seen for low-spin Mn^V(O)(TBP₈Cz). The paramagnetic NMR spectrum is consistent with the formation of a Mn^IV(O)(TBP₈Cz^•+) complex, in which a high-spin (S = ³/₂) Mn^IV ion is either ferromagnetically (S = 2) or antiferromagnetically (S = 1) coupled to the corrolazine π- radical cation (S = ¹/₂). An Evans method measurement in CDCl₃ gives μ_eff = 4.19 μ_B, which falls between the predicted spin-only values of 2.83 and 4.90 μ_B for the triplet and quintet spin states, respectively. The X-band electron paramagnetic resonance (EPR) spectrum of Mn^IV(O)(TBP₈Cz^•+) in CH₂Cl₂ at 12 K showed only relatively weak EPR-active impurities, consistent with the main product having an integer spin (S = 1 or 2). The UV–vis, NMR, and EPR data all support the conclusion that the nonmetallic Lewis acid B(C₆F₅)₃ stabilizes the open-shell valence tautomer Mn^IV(O)(TBP₈Cz^•+).

Attempts to isolate the 1:1 complex Mn^IV(O)(TBP₈Cz^•+):B(C₆F₅)₃ in the solid state led to significant reduction, giving a Mn^III(TBP₈Cz) product. However, it was possible to characterize the B(C₆F₅)₃ complex in solution by cryospray ionization mass spectrometry (CSIMS). The reaction of Mn^V(O)(TBP₈Cz) + B(C₆F₅)₃ (1 equiv) in CH₂Cl₂ at 20 °C was monitored by UV–vis to ensure good conversion to Mn^IV(O)(TBP₈Cz^•+) and then analyzed directly by CSIMS (Figure 1b). A cluster centered at m/z 1939.7619 is observed, and the high-resolution, isotopic distribution pattern matches well for the complex of formula [Mn(O)(TBP₈Cz):B(C₆F₅)₃ + H]⁺. The cluster observed at m/z 1426.7712 corresponds to [Mn(O)(TBP₈Cz)]⁺, which likely results from fragmentation of the B(C₆F₅)₃ complex (Figure S4 in the SI). These data provide strong evidence for the formation of a 1:1 complex between the manganese–oxo corrolazine and the Lewis acidic triarylborane.

Athough we have yet to obtain direct structural information for the Zn^II- or borane-derived complexes, B(C₆F₅)₃ is anticipated to bind to the oxo ligand of Mn^IV(O)(TBP₈Cz^•+). Precedent for this conclusion can be seen in the X-ray structure of an isoelectronic Re^V(O)(B(C₆F₅)₃) complex, in which the boron atom is bound directly to the terminal oxo ligand.⁷ In addition, it was proposed that Sc³⁺ coordinates directly to the oxo group of nonheme Mn^IV(O) complexes based on X-ray absorption spectroscopy and other supporting data.^6b,6c

The reversibility of the formation of Mn^IV(O)(TBP₈Cz^•+) was examined with the addition of fluoride anion. Triarylboranes are known to readily form fluoroborate products ([FB(Ar)₃]⁻) upon the addition of F^–.^8a The reagent [((CH₃)₂N)₃S]⁺[F₂Si(CH₃)₃]⁻ (TASF) was employed as a soluble, anhydrous fluoride source. The addition of TASF to a freshly prepared solution of Mn^IV(O)(TBP₈Cz^•+):B(C₆F₅)₃ led to rapid recovery of the starting low-spin Mn^V(O) complex, as seen by UV–vis (λ_max = 634 nm; Figure S2 in the SI), together with a small amount of the reduced Mn^III complex [Mn^III(TBP₈Cz)(F)]⁻.^9a The addition of excess chloride anion (Bu₄NCl), in contrast, produced no observable change by UV–vis. The difference in the reactivity of F^– versus Cl^– parallels their independent reactivity toward triarylboranes, in which Cl^– binds only weakly to B(Ar)₃.^8b

The reactivity of Mn^IV(O)(TBP₈Cz^•+):B(C₆F₅)₃ in hydrogen-atom-transfer (HAT) reactions was examined with phenol substrates. The reaction with 2,4-di-tert-butylphenol (2,4-DTBP) was monitored by UV–vis (Figure 2). Isosbestic conversion to a final spectrum with λ_max = 443 and 727 nm was observed and is similar to that seen for [Mn^IV(TBP₈Cz)]⁺.^9b EPR spectroscopy gives a well-resolved spectrum with g ∼ 4 and 2 and a hyperfine splitting consistent with ⁵⁵Mn (I = ⁵/₂; Figure 2b). The spectrum was satisfactorily simulated with a fictitious spin of S′ = ¹/₂, and g = [4.61, 4.20, 1.92]; A_iso(⁵⁵Mn) = [82, 86, 50] G. These parameters are within the range of previously reported high-spin manganese(IV) corrolazines (S = ³/₂) and other hs manganese(IV) porphyrinoid complexes.^9b These data indicate that the reaction with 2,4-DTBP results in a one-electron reduction of Mn^IV(O)(TBP₈Cz^•+), consistent with hydrogen-atom abstraction from the O–H bond.

Figure 2.

(a) UV–vis spectral changes (0–60 s) for Mn^IV(O)(TBP₈Cz^•+):B(C₆F₅)₃ + 2,4-DTBP (300 equiv) at 25 °C. (b) EPR spectrum (12 K) after reaction with 2,4-DTBP: exptl, black line; simulation, red line.

The product of oxidation of 2,4-DTBP was identified by gas chromatography with flame ionization detection as the expected bisphenol dimer (eq 1). A maximal yield of 0.5 equiv of dimer per manganese complex is predicted if the manganese complex serves as a one-electron oxidant, and a yield of 0.39 equiv based on manganese was obtained. This result further supports the Mn^IV(O)(TBP₈Cz^•+) complex functioning as a one-electron oxidant, in stark contrast to the reactivity seen for the low-spin Mn^V(O) complex, which acts as a two-electron oxidant toward phenol and C–H substrates.⁹

1

The kinetics for phenol substrates were measured to gain insight into the reactivity of Mn^IV(O)(TBP₈Cz^•+):B(C₆F₅)₃. Reactions with excess 2,4-DTBP were monitored by UV–vis and found to be first-order over 5 half-lives (Figure S10 in the SI). A plot of [2,4-DTBP] versus k_obs was linear, consistent with the rate law rate = k₂[Mn(O)][ArOH]. These methods yielded second-order rate constants, k₂, for both B(C₆F₅)₃ and Zn^II, as well as the more sterically hindered 2,4,6-tri-tert-butylphenol (2,4,6-TTBP), and are compared in Table 1. The generation of Mn^IV(O)(TBP₈Cz^•+) by either Zn^II or B(C₆F₅)₃ leads to a significant increase (up to 130-fold) in reactivity compared to the starting low-spin Mn^V(O) complex for both phenol substrates. The identity of the Lewis acid further influences the reaction rates, with triarylborane being more reactive in both cases. In addition, the much slower rate constants for the more sterically hindered 2,4,6-TTBP indicate a mechanism involving HAT. Support for this mechanism was obtained by measuring a kinetic isotope effect of 3.2 ± 0.3 for 2,4,6-TTBP-OD (Figure S11 in the SI).¹⁰ It was reported that the addition of Sc³⁺ to a nonheme Mn^IV(O) complex caused a decrease in HAT rates, attributed to steric hindrance from the Sc³⁺ ion.^6b Our results appear to contrast these findings, with Lewis acids strongly increasing the HAT reactivity of a Mn^IV(O)(Cz^•+) complex. However, this comparison is complicated by the fact that the inherent HAT reactivity of the valence tautomer with the electronic structure Mn^IV(O)(Cz^•+) is not known. The influence of the electronic structure on the reactivity of high-valent metal–oxo complexes remains an area of intense debate.¹¹

Table 1. Rate Constants for Oxidation of Phenol Substrates.

substrate	Lewis acid	k2 (M–1 s–1)	kacid/knone
2,4-DTPB	Zn2+	17 ± 1	5.9
	B(C6F5)3	107 ± 8	37
	none	2.9 ± 0.1
2,4,6-TTBP	Zn2+	0.157 ± 0.008	2.1
	B(C6F5)3	9.5 ± 0.7	130
	none	0.074 ± 0.007

In summary, we have demonstrated for the first time that a nonmetal ion Lewis acid can induce reversible valence tautomerism in a metalloporphyrinoid compound. We have also shown that the HAT reactivity of a Mn^IV(O)(porphyrinoid^•+) complex in the presence of Lewis acids is strongly enhanced compared to its closed-shell Mn^V(O) valence tautomer. This work provides new insight regarding how to control valence tautomerism in porphyrinoid compounds, as well as on how Lewis acids influence the reactivity of high-valent metal–oxo species.

Supporting Information Available

Experimental procedures, kinetic studies, and EPR and MS data. This material is available free of charge via the Internet at http://pubs.acs.org.

The authors declare no competing financial interest.

Funding Statement

National Institutes of Health, United States