A manganese(V)-oxo π-cation radical complex: Influence of one-electron oxidation on oxygen-atom transfer

Article abstract of DOI:10.1021/ja2066237

One-electron oxidation of Mn^V-oxo corrolazine 2 affords 2 ⁺, the first example of a Mn^V(O) π-cation radical porphyrinoid complex, which was characterized by UV-vis, EPR, LDI-MS, and DFT methods. Access to 2 and 2⁺ allowed for a direct comparison of their reactivities in oxygen-atom transfer (OAT) reactions. Both complexes are capable of OAT to PPh₃ and RSR substrates, and 2⁺ was found to be a more potent oxidant than 2. Analysis of rate constants and activation parameters, together with DFT calculations, points to a concerted OAT mechanism for 2⁺ and 2 and indicates that the greater electrophilicity of 2⁺ likely plays a dominant role in enhancing its reactivity. These results are relevant to comparisons between Compound I and Compound II in heme enzymes.

Full text of DOI:10.1021/ja2066237

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A Manganese(V)ꢀOxo π-Cation Radical Complex: Influence of
One-Electron Oxidation on Oxygen-Atom Transfer
Katharine A. Prokop,† Heather M. Neu,† Sam P. de Visser,*,‡ and David P. Goldberg*,†
†Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States
‡Manchester Interdisciplinary Biocentre and School of Chemical Engineering and Analytical Science, University of Manchester,
131 Princess Street, Manchester M1 7DN, U.K.
S Supporting Information
b
Scheme 1. One-Electron Oxidation of (TBP8Cz)MnV(O)
ABSTRACT: One-electron oxidation of MnVꢀoxo corro-
lazine 2 affords 2+, the first example of a MnV(O) π-cation
radical porphyrinoid complex, which was characterized by
UVꢀvis, EPR, LDI-MS, and DFT methods. Access to 2 and
2+ allowed for a direct comparison of their reactivities in
oxygen-atom transfer (OAT) reactions. Both complexes are
capable of OAT to PPh3 and RSR substrates, and 2+ was
found to be a more potent oxidant than 2. Analysis of rate
constants and activation parameters, together with DFT
We previously showed that oxidation of the MnIII corrolazine
(TBP8Cz)MnIII (1) [TBP8Cz = octakis(p-tert-butylphenyl)-
corrolazinato3ꢀ] gave the isolable MnVꢀoxo complex
(TBP8Cz)MnV(O) (2).9 Despite the stability of 2, it is capable
of both HAT and OAT oxidation reactions.9a,c,d
calculations, points to a concerted OAT mechanism for 2+
and 2 and indicates that the greater electrophilicity of 2+
likely plays a dominant role in enhancing its reactivity.
These results are relevant to comparisons between Com-
pound I and Compound II in heme enzymes.
Herein we report the oxidation of the neutral MnV(O)
complex 2 to afford the one-electron-oxidized complex
[(TBP8Cz)Mn(O)]+ (2+) (Scheme 1). Spectroscopic analyses
and density functional theory (DFT) calculations have shown
that this complex is best described as a MnV(O) corrolazine
π-cation radical [(TBP8Cz+•)MnV(O)], the first example of a
MnV(O) π-cation radical species. Access to 2 and 2+ has made
possible a comparison of the reactivities of two high-valent
Mn(O) complexes that differ in the same manner as Cpd-I and
Cpd-II. We have determined the OAT reactivities of 2 and 2+
with thioether substrates, providing a direct measure of the
influence of the change in oxidation level on OAT reactions.
The results of kinetic measurements and DFT calculations are in
excellent agreement with each other and provide a mechanistic
framework that addresses the origins of this influence.
Previously, cyclic voltammetry (CV) measurements on 2 in
CH2Cl2 or PhCN revealed a reversible reduction near 0.0 V vs
SCE that was assigned to the metal-centered MnV/MnIV couple.9b
Complex 2 also showed two well-separated oxidations, the first
appearing as a reversible wave centered at 1.02 (1.09) V in CH2Cl2
(PhCN) and the second as an irreversible oxidation at Epa = 1.57
(1.52) V.9b Time-resolved thin-layer UVꢀvis spectra for the con-
trolled-potential oxidation of 2 at 1.20 V in PhCN revealed a decrease
andblueshiftoftheSoretbandat425nm, adecreaseoftheQbandat
633 nm, and an increase in a broad shoulder at 718 nm. These spectral
changes suggested a corrolazine-ring-centered oxidation, although
metal-centered oxidation to give a MnVI complex could not be ruled
out. To gain further insight into the locus of oxidation and potentially
wide range of heme (e.g., cytochrome P450s, peroxidases)1
A
and non-heme metalloenzymes,2 as well as biomimetic
synthetic catalysts,3 mediate the oxidation of organic substrates
via proposed high-valent ironꢀoxo and manganeseꢀoxo inter-
mediates. Fundamental recurring processes in these oxidation
reactions include hydrogen-atom transfer (HAT) and oxygen-
atom transfer (OAT) to/from the metalꢀoxo unit. High-valent
Mn(O) species have also been implicated in photosynthetic
water splitting, in which the critical OꢀO bond may be formed
by nucleophilic attack of H2O/OHꢀ on an electrophilic Mn(O)
unit,4 a process reminiscent of OAT with H2O as the “organic”
substrate. Thus, there is great interest in delineating the structural
and electronic properties of high-valent Fe(O) and Mn(O)
complexes, and a key challenge is to determine what factors are
crucial for controlling their reactivity.5 However, the direct
observation of these metalꢀoxo species is difficult because of
their inherent instability. For example, manganese(III) porphyr-
ins are powerful catalysts for OAT processes, but the direct
examination of high-valent MnV(O) porphyrins is rare.6
Significant efforts have been made to determine the relative
reactivities of iron(IV)ꢀoxo porphyrin+•, designated as Com-
pound I (Cpd-I), versus iron(IV)ꢀoxo porphyrin (Cpd-II), for
both heme enzymes and model complexes.5c,7 To our knowl-
edge, however, there have been no reports of a direct comparison
of the reactivities of a similar pair of Mn(O)(porphyrin+•)/
Mn(O)(porphyrin) complexes, most likely because of a lack of
access to such species.8 Corrolazines, along with their corrole
analogues, impart special stability to high-valent metal ions
that distinguishes them from their porphyrin congeners.
Received: July 15, 2011
Published: September 02, 2011
r
2011 American Chemical Society
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|

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Figure 2. DFT-calculated optimized geometries of the 2A0, 2A00, and 2Δ
states of 2[Mn(O)(H8Cz)]+ (left) and high-lying occupied MOs (right).
Figure 1. (a) UVꢀvis spectra of the oxidation of 2 (10 μM) with
increasing amounts of [Ar03N+•](SbCl6ꢀ) (0ꢀ1.2 equiv) at 25 °C in
CH2Cl2. Inset: X-band EPR spectrum of 2+ (10 μM) at 15 K in CH2Cl2.
(b) Spectral titration to determine the stoichiometry of the reaction.
geometry optimizations and associated energies were successfully
calculated for the doublet and quartet states of [Mn(O)(H8Cz)]+,
wherein all of the peripheral substituents on the Cz ring were
replaced with H atoms. There are three low-lying doublet spin
access a novel Mnꢀoxo complex at the formal MnVI oxidation level,
we targeted the bulk production of this species via chemical oxidation.
Reaction of 2 (10 μM) with 1 equiv of the one-electron
2
2
states: the lowest two are the A0 [δ2(a0)1(a00)2] and A00
[δ2(a0)2(a00)1] states, both representing a [MnV(O)(H8Cz+•)]
oxidant [Ar03N+•](SbCl6 ) (Ar0 = 4-BrC6H4; Eox = 1.16 V vs
ꢀ
configuration, while the third is the Δ [δ1(a0)2(a00)2] state,
2
SCE)10 in CH2Cl2 at 25 °C (Scheme 1) led to the immediate
isosbestic conversion of bright-green 2 to an orange-brown
solution with new peaks at λmax = 410 and 780 nm
(Figure 1a). The decrease in the Soret and Q bands of 2 along
with the appearance of a band at longer wavelength (780 nm) is
similar to the transformation seen upon controlled-potential
oxidation of 2 in PhCN9b and analogous corrole oxidations.11
Oxidation of 2 could also be accomplished with cerium(IV)
ammonium nitrate (CAN), as shown by UVꢀvis spectra when 1
equiv of CAN dissolved in ∼100 μL of CH3CN was added to 2 in
CH2Cl2 (2 mL) [Figure S3 in the Supporting Information (SI)].
A spectral titration with [Ar03N+•] (Figure 1b) confirmed the 1:1
stoichiometry for the electron-transfer process and indicated the
formation of a one-electron-oxidized product, [(Cz)Mn(O)]+.
Orange-brown 2+ is stable in dilute solution for over 24 h, but
significant decomposition occurs upon concentration to dryness.
Despite this relative instability, laser desorption ionization mass
spectrometry (LDI-MS) on samples of 2+ deposited and evapo-
rated rapidly on the LDI-MS target plate gave rise to a prominent
isotopic cluster centered at m/z 1426.8 in positive-ion mode
(Figure S1). This cluster matches the expected theoretical isotope
distribution for 2+. Exchange of the terminal oxo group in 2 with
H218O gave the 18O isotopomer (2-18O) with high isotopic
enrichment (97% 18O), as previously reported.9a One-electron
oxidation of this material resulted in a shift of the isotopic cluster con-
sistent with partial retention of the 18O label (60% 18O) (Figure S2).
It is likely that some scrambling occurred with residual H2Ooriginating
from the solvent or from the oxidant in this case (CAN). These data
confirmed the formulation of 2+ as [(Cz)Mn(O)]+.
representing a [MnVI(O)(H8Cz)]+ configuration. The A00 state
2
was found to be the ground state and is well-separated from the
2A0 and 2Δ states, which lie 16.2 and 37.4 kcal molꢀ1 higher in
energy, respectively. These results are in good agreement with
the conclusion based on the CV, spectroelectrochemistry, and
EPR data that the locus of oxidation upon conversion of 2 to 2+ is
the π system of the Cz ring and not the Mn ion.
The optimized geometry and high-lying occupied molecular
orbitals (MOs) for the 2A00 ground state are shown in Figure 2.
The structural parameters from the optimized geometry for
2[MnV(O)(H8Cz+•)] reveal MnꢀN and MnꢀO bond lengths
that compare well to the distances obtained for 2 from EXAFS
[(MnꢀNpyrrole ave
)
= 1.88(4) Å, MnꢀO = 1.56(2) Å].9b The a00
orbital in Figure 2 is the singly occupied MO and is analogous to
the a1u orbital in porphyrins. This orbital has almost no spin
density on either the meso- or pyrrole N atoms, which agrees with
the lack of hyperfine splitting in the EPR spectrum. Metallopor-
phyrin π-cation radicals [M(por+•)] typically exhibit a singly
occupied a2u orbital analogous to a0 in Figure 2, but the electron-
withdrawing meso-N atoms of the Cz ring likely stabilize this
orbital and lower it below the a00 orbital.14 DFT calculations give
d(MnꢀO) = 1.542 Å for 2[MnV(O)(H8Cz+•)] which is slightly
shorter than either the DFT-derived (1.549 Å)9d or EXAFS-
derived distance for 2. Similarly, the DFT-derived ν(MnꢀO) of
990 cmꢀ1 for 2[MnV(O)(H8Cz+•)] is higher than ν(MnꢀO) =
1
975 cmꢀ1 for [MnV(O)(H8Cz)]. This predicted increase in
ν(MnꢀO) for the π-cation radical is opposite to that seen for
Cpd-I [ν(FeꢀO) = 801ꢀ835 cmꢀ1] versus Cpd-II [ν(FeꢀO) =
843 cmꢀ1] in synthetic porphyrin analogues.15 However, the
former Cpd-I is an a2u ferryl π-cation radical. A better compar-
ison with 2+/2 is [V(O)(por+•)] with a1u symmetry [, υ(VꢀO)=
1002 cmꢀ1) versus [V(O)(por)] (υ(VꢀO)= 987 cmꢀ1).16
The facile generation and relative stability of 2+ allowed us to
investigate its reactivity toward OAT. Complex 2+ reacted rapidly
with 1 equiv of PPh3 at 25 °C to produce OPPh3 in good yield
(71%) (Scheme 2). When this reaction was monitored by UVꢀvis
spectroscopy, isosbestic conversion of 2+ to a new spectrum with
λmax = 440, 678(sh), and 722 nm was observed (Figure S4), which
matches that for [(TBP8Cz)MnIV]+.17 The UVꢀvis data show that
a smooth two-electron OAT process occurred following addition of
PPh3 to 2+. The oxo source was identified as 2+ by 18O isotopic
enrichment of the terminal oxo group. Reaction of 2+-18O with
PPh3 gave 18OPPh3 (55% incorporation by GCꢀMS).
The electron paramagnetic resonance (EPR) spectrum of 2+ at
15 K in CH2Cl2 shows a sharp singlet at g= 2.001 (line width = 15 G)
with no hyperfine splitting from the Mn ion (Figure 1a inset). The
signal was quantified against an external standard, revealing a 77%
yield of 2+. The data suggest that the ground state retains a low-spin d2
configuration for the MnV ion, as seen in the neutral starting material,
with oxidation resulting in a π-cation radical on the corrolazine ring.
Furthermore, the lack of hyperfine splitting from N atoms suggests
that the spin density is concentrated on the pyrrole carbon atoms. The
same EPR spectrum was observed irrespective of the oxidant
employed (CAN or [Ar03N+•]). The EPR spectrum of 2+ is similar
to that observed for metallocorrole π-cation radicals.11,12
To model the electronic structure of 2+, DFT calculations at
the B3LYP level were performed as described previously.13 Full
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Scheme 2. OAT Reactions
Table 1. Kinetic Parameters for OAT Reactions with DBS
2
2+
ka
ΔHq c
ΔSq e
ΔGq (298 K)c
3.8 ꢁ 10ꢀ4 b
16 ( 1 [12.3]d
ꢀ20 ( 4
(6.2 ( 0.2) ꢁ 10ꢀ3
7.0 ( 0.8 [7.1]d
ꢀ45 ( 3
22 ( 2 [23.8]d
20 ( 2 [17.6]d
a Values in Mꢀ1 sꢀ1 at 298 K. b Extrapolated from the ln(k/T) vs
1/T plot. c Values in kcal molꢀ1
.
d DFT-calculated value for the reaction
with DMS. e Values in cal Kꢀ1 molꢀ1
.
Figure 4. Potential energy profiles for the OAT reactions of 2 and 2+
with DMS. Energies (ΔE + ZPE + Esolv) are given, with free energies in
brackets. The optimized transition-state geometries are shown at the right.
Figure 3. Time-resolved UVꢀvis spectra of the reactions between
(left) 2+ (9.5 μM) and DBS (72 mM) at 25 °C to give [(TBP8Cz)-
MnIV]+ (440, 722 nm) and (right) 2 (3.2 μM) and DBS (0.4 M) at 35 °C
to give 1 (443, 689 nm) in toluene. Insets: second-order plots.
similar bimolecular OAT mechanisms. The decrease in ΔHq
for 2+ is somewhat attenuated by its more negative ΔSq value,18
as reflected in the smaller difference (∼2 kcal molꢀ1) in the ΔGq
values for 2+ and 2 at 298 K.
Complex 2+ was also found to be competent to oxidize thermo-
dynamically more challenging thioether substrates to their correspond-
ing sulfoxides. Addition of excess dimethyl sulfide (DMS) to a solution
of 2+ in CH2Cl2 resulted in the production of dimethyl sulfoxide
(DMSO) (88%byGC) (Scheme 2). UVꢀvis spectroscopy revealed
isosbestic conversion of 2+ to [(TBP8Cz)MnIV]+, as seen for the
reaction with PPh3. Analysis of the DMS reaction mixture by EPR
spectroscopy upon completion showed an intense high-spin MnIV
(S = 3/2) signal with g = 4.68, 3.28, and 1.94 and well-resolved 55Mn
hyperfine splitting (Figure S8). The EPR data confirm that MnIV was
produced following OAT from 2+ to thioether substrates.
DFT calculations were employed to compare the mechanisms
of sulfoxidation for 2+ and 2. DMS was used in place of DBS as
the substrate to facilitate the calculations. The potential energy
profiles (Figure 4) reveal single transition states (TSSO) pro-
ceeding to sulfoxide products (PSO) for both oxidants. These
profiles indicate that 2 and 2+ proceed through the same
concerted bimolecular OAT mechanism. This mechanistic con-
clusion agrees well with the experimentally derived ΔSq values
for 2+ and 2 and the results of previous sulfoxidation studies.19
The DFT-derived ΔHq value (ΔE + ZPE + Esolv) for 2+ is
significantly lower than that for 2, in good agreement with the
trend seen experimentally. This large difference in activation
enthalpies is unlikely to arise from changes in MnꢀO bond
strength for 2+ vs 2, given that our DFT calculations predict
relatively minor increases in bond distance and ν(MnꢀO),
which might be expected to increase ΔHq for OAT with 2+.20
However, both the calculated electron affinities (see the SI) and
the experimentally derived reduction potentials for 2+ relative to
2 point to a much higher electrophilicity for 2+, and we suggest
that the ΔHq for OAT is dominated by the much greater
electrophilicity of the oxidant in this case.21 Few systematic studies
are available that have similarly compared OAT reactivities for
closely related metalꢀoxo species differing by only one oxidation
level. Interestingly, however, Brown and co-workers reached
similar conclusions regarding the impact of electrophilicity on
OAT for two ReV(O) complexes differing by only one unit of
charge.22 Similarly, Mayer also highlighted the importance of
electrophilicity in explaining the reactivity of a Re(dioxo) complex.23
A previous study showed that the neutral MnVꢀoxo complex
2 is capable of OAT to PPh3, generating OPPh3 in good yield
(83%) along with MnIII complex 1. Qualitative UVꢀvis data also
suggested that DMS can reduce 2 to 1.9a Here we confirmed that
DMSO (64%) is produced in this reaction. The kinetics of OAT
for 2+ and 2 with DMS under pseudo-first-order conditions was
monitored by UVꢀvis spectroscopy in CH2Cl2 (Figures S6 and
S7). Both complexes exhibited good pseudo-first-order kinetics,
and a linear dependence of the observed rate constants (kobs) on
[DMS] gave the second-order rate constants k(2+) = 0.25 (
0.05 Mꢀ1 sꢀ1 and k(2) = (2.0 ( 0.2) ꢁ 10ꢀ3 Mꢀ1 sꢀ1
,
indicating a large rate enhancement for 2+ over 2 in OAT.
The temperature dependence of k(2+) and k(2) was measured
in toluene with the less volatile n-Bu2S (DBS) in place of DMS as
the substrate. Good isosbestic behavior and pseudo-first-order
kinetics were also observed for this substrate (Figure 3). Plots of
ln(k/T) versus 1/T (Figure S11) were linear for both complexes
and yielded the activation parameters given in Table 1.
The enthalpy of activation for 2, ΔHq =16( 1 kcal molꢀ1, decreased
significantly upon one-electron oxidation, with ΔΔHq = ꢀ9 kcal
molꢀ1 for 2+ versus 2. Large negative ΔSq values were found for
both complexes, indicating that the two oxidants operate through
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In conclusion, we have generated a MnV(O) π-cation radical
complex and shown that it is a more reactive oxidant for OAT
reactions than its one-electron-reduced analogue. Our experi-
mental data and theoretical calculations are in excellent agree-
ment and point to a concerted OAT mechanism for both cationic
and neutral oxidants. A recent study of MnV(O) corroles and
OAT to sulfides invoked a disproportionation mechanism to give
a more reactive MnVI(O)(corrole) as the primary oxidant,
although no direct evidence for this complex was presented.24,25
While we found no evidence for the MnVI oxidation state, our
results indicate that the isoelectronic species MnV(O)(Cz+•) is
indeed a reactive oxidant. In relating our findings to the relative
reactivity of Cpd-I versus Cpd-II in heme enzymes and models,
we suggest that a simple advantage in electrophilicity for Cpd-I
may play a significant role in determining the greater reactivity
usually associated with this species. Further work is necessary to
determine the relative reactivity of 2 versus 2+ in other oxidative
transformations of biological and catalytic relevance.
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Supporting Information. Experimental procedures, Fig-
b
ures S1ꢀS15, and computational details. This material is avail-
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’ AUTHOR INFORMATION
Corresponding Author
sam.devisser@manchester.ac.uk; dpg@jhu.edu
’ ACKNOWLEDGMENT
This work was supported by the NSF (CHE0909587) to D.P.G.
and the Harry and Cleio Greer Fellowship to K.A.P. S.P.d.V. thanks
the National Service of Computational Chemistry Software for CPU
time and the University of Manchester for a travel grant. We thank R.
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