Structural and spectroscopic characterization of metastable thiolate-ligated manganese(III)-alkylperoxo species

Article abstract of DOI:10.1021/ja205520u

Metastable Mn-peroxo species are proposed to form as key intermediates in biological oxidation reactions involving O₂ and C-H bond activation. The majority of these have yet to be spectroscopically characterized, and their inherent instability, in most cases, precludes structural characterization. Cysteinate-ligated metal-peroxos have been shown to form as reactive intermediates in both heme and nonheme iron enzymes. Herein we report the only examples of isolable Mn(III)-alkylperoxo species, and the first two examples of structurally characterized synthetic thiolate-ligated metal-peroxos. Spectroscopic data, including electronic absorption and IR spectra, and ESI mass spectra for ¹⁶O vs ¹⁸O-labeled metastable Mn(III)-OOR (R = ^tBu, Cm) are discussed, as well as preliminary reactivity.

Full text of DOI:10.1021/ja205520u

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Structural and Spectroscopic Characterization of Metastable
Thiolate-Ligated Manganese(III)ÀAlkylperoxo Species
Michael K. Coggins and Julie A. Kovacs*
Department of Chemistry, University of Washington, Seattle, Washington 98195, United States
S Supporting Information
b
ABSTRACT: Metastable MnÀperoxo species are pro-
posed to form as key intermediates in biological oxidation
reactions involving O2 and CÀH bond activation. The
majority of these have yet to be spectroscopically characterized,
and their inherent instability, in most cases, precludes
structural characterization. Cysteinate-ligated metalÀperoxos
have been shown to form as reactive intermediates in both
heme and nonheme iron enzymes. Herein we report the
only examples of isolable Mn(III)Àalkylperoxo species, and
the first two examples of structurally characterized synthetic
thiolate-ligated metalÀperoxos. Spectroscopic data, in-
cluding electronic absorption and IR spectra, and ESI mass
spectra for 16O vs 18O-labeled metastable Mn(III)ÀOOR
t
(R = Bu, Cm) are discussed, as well as preliminary
reactivity.
Figure 1. ORTEP and ChemDraw diagram of [MnII(SMe2N4(Qui-
noEN))]]+ (1) with hydrogen atoms omitted for clarity. Selected
metrical parameters (Å): MnÀS(1), 2.3835(9); MnÀN(1), 2.170(2);
MnÀN(2), 2.274(2); MnÀN(3), 2.225(3); MnÀN(4), 2.200(3).
etastable metalÀperoxo species form as key intermediates
M
in biological and biomimetic oxidation reactions involving
O2 and the activation of CÀH bonds.1À16 ManganeseÀperoxos
have been shown to play a significant role in reactions catalyzed
by the manganese superoxide dismutase (MnSOD),17À21 and
catalase.22 A manganeseÀperoxo dimer is proposed to form in
class Ib ribonucleotide reductase (Ib RNR),23 as well as in the
Single crystals of our highly reactive and O2-sensitive Mn(II)
complex, [MnII(SMe2N4(QuinoEN))](PF6) (1), were obtained
via ambient temperature vapor diffusion of Et2O into a saturated
MeCN solution. As shown in the ORTEP diagram of Figure 1,
the Mn(II) ion of 1 is found in a tetragonally distorted trigonal
bipyramidal geometry (τ = 0.55)45 with an alkyl thiolate sulfur cis
to an open coordination site. Metrical parameters, a multiline
X-band EPR spectrum (A = 91 G; Figure S1), and magnetic
step preceding O2 evolution by photosystem II (PS II).24À27
A
manganeseÀalkylperoxo is proposed to form as a key intermedi-
ate in manganese lipoxygenase (MnLO)-promoted fatty-acid
oxidation.28À31 The majority of these biological Mn-intermedi-
ates have thus far gone unobserved and therefore have yet to be
spectroscopically characterized. Much of our current under-
standing of the properties of these elusive species has come
through spectroscopic and/or computational studies involving
small-molecule analogues.16,32À37 The inherent instability of
these intermediates usually precludes structural characterization.
Research efforts in our laboratory have specifically focused upon
the isolation and characterization of thiolate-ligated metalÀ
peroxo species.38À41 Cysteinate-ligated metalÀperoxos have
been shown to form as reactive intermediates in the activation
of O2, or derivatives thereof, in both heme (cytochrome P450,
chloroperoxidase) and nonheme (superoxide reductase, cysteine
dioxygenase) iron enzymes.6,39,42À44 Herein we report the first
two examples of structurally characterized synthetic thiolate-
ligated metalÀperoxo species, and the only examples of isolable
Mn(III)Àalkylperoxo species. Spectroscopic data, including
electronic absorption and IR spectra, and ESI mass spectra for
16O vs 18O-labeled Mn(III)ÀOOR (R = tBu, Cm) are discussed,
as well as preliminary reactivity.
susceptibility data (μeff = 5.78 μB (solid state; Figure S2), μeff
=
5.83 μB (CH2Cl2 solution)) are consistent with a high-spin (S =
5/2) ground state. The electronic absorption spectrum of 1
(Figure S3) is featureless throughout the visible region in a
variety of solvents (DCM, MeCN, THF, MeOH), also consistent
with high-spin Mn(II)
Thiolate-ligated 1 (Epa= +592 mV, Epc= +312 mV vs SCE;
Figure S4) reacts immediately with ROOH (R = tBu, cumyl) in
the presence of 2.0 equiv of triethylamine at À15 °C in CH2Cl2
to afford bright blue metastable species, 2a and 2b, characterized
by absorption bands at λmax = 590 nm (460 MÀ1 cmÀ1; Figure 2)
and 596 nm (679 MÀ1 cmÀ1), respectively. Between 1.5 and 1.8
equiv of ROOH per equivalent of 1 were found necessary to
maximize the growth of these absorption bands (Figure 2), and
this was shown to be coincident with the complete disappearance
Received: June 21, 2011
Published: July 21, 2011
r
2011 American Chemical Society
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Figure 4. FT-IR spectrum (Nujol) of [MnIII(SMe2N4(QuinoEN))-
(OOtBu](BPh4) (2a) showing the isotopically sensitive (16OÀ OtBu
16
vs 18OÀ OtBu) alkylperoxo stretches.
18
Figure 2. Anaerobic reaction between [MnII(SMe2N4(QuinoEN))](PF6)
(1, 2.4 mM) and 1.8 equiv of tBuOOH (0.1 M stock solution; added in
0.3 equiv aliquots) monitored by electronic absorption spectroscopy in
CH2Cl2 at À15 °C.
This vibrational frequency is within the expected energy range
(750À900 cmÀ1) for a peroxo ν(OÀO) stretch,3,7,34,35,50,51 and
the observed isotopic shift (Δν(exp) = 57 cmÀ1) is very close to
that predicted based upon a harmonic oscillator approximation
(Δν(calc) = 51 cmÀ1). ESI-MS data (Figure S8) is also
consistent with a Mn(III)Àalkylperoxo species with a mass-to-
charge ratio of 585.2 (m/z(calc) = 585.1) that shifts by approxi-
mately 4 mass units to m/z = 589.1 upon 18O-incorporation.
Unambiguous identification of the putative metastable Mn-
(III)Àalkylperoxo species was provided by structural determina-
tion via XÀ-ray crystaÀllography. Exchange of the counterion of 1
from PF6 to BPh4 permitted us to obtain single crystals of
[MnIII(SMe2N4(QuinoEN))(OOtBu](BPh4) (2a) suitable for
X-ray diffraction studies in ∼60% yield from MeCN/Et2O
(1:5) at À30 °C. We also obtained X-ray-quality crystals of 2b
under similar conditions; however, the resulting structure was
solved at lower resolution (R = 16%). Although the metrical
parameters are less reliable, the structure of [MnIII(SMe2N4(Qui-
noEN))(OOCm](BPh4) (2b) does indeed confirm the pro-
posed structure of, and connectivity for, a Mn(III)Àalkylperoxo
species. The ORTEP diagrams of 2a and 2b are provided along
with selected metrical parameters in Figures 5 and 6, respectively.
Contraction of the MnÀS distance from 2.3835(9) Å in 1 to
2.270(3) Å in 2a clearly reflects an increase in metal oxidation
state from +2 to +3. The solution (CH2Cl2) magnetic moments,
(μeff(2a) = 4.85(4) B.M., μeff(2b) = 4.81(9) B.M.), are also
consistent with oxidized Mn(III) in a high (S = 2) spin-state.
Elongation of the MnÀN(1), MnÀN(3), and MnÀN(4) dis-
tances in 2a are most likely a consequence of both the increase in
coordination number and JahnÀTeller distortion which would
be anticipated for a high-spin Mn(III) ion. In fact, elongation of
the MnÀN(4) distance (from 2.200(3) Å in 1 to 2.522(8) Å) in
2a is so significant that, at least in the solid-state, 2a is best
described as a five-coordinate complex with a dangling quinoline
moiety. The flexibility of the ligand framework in 1 appears to
permit the formation of species with more steric congestion around
the metal center as is seen in structures 2a and 2b (Figure S9).
The MnÀO and OÀO bond lengths of 2a, 1.861(5) and
1.457(7) Å, respectively, are within the expected range for a
metalÀη1-alkylperoxo.52À55 The MnÀOÀO angle (109.2(4°)
in 2a is remarkably acute relative to most metalÀalkylperoxo
species,53À55 and similar to that (105.1(2)°) of the only other
structurally characterized MnÀalkylperoxo, TptBu,iPrMnII(O2C-
Me2Ph).55 This suggests that there is little to no π-bonding
interaction between the S = 2 Mn(III) center of 2a and the
Figure 3. First order kinetics for the decay of [MnIII(SMe2N4(Qui-
noEN))(OOtBu]+ (2a) in the presence of base (Et3N) monitored by
electronic absorption spectroscopy (at λ= 590 nm) in CH2Cl2 at À15 °C.
of the X-band EPR signal of 1. Although the formation of these
metastable species does not appear to be influenced by the
presence of base (Figure 3, and Figures S5ÀS7), their lifetimes
(at À15 °C) are increased by nearly an order of magnitude when
base is present [τ1/2(2a, with base) = 5.9(1.7) Â 103 s,46 τ1/2(2a,
without base) = 7.2(2.4) Â 102 s; Figure 3, Figure S5]. These
rates were unaffected by the addition of high excess amounts
(100 equiv) of substrates known to react with metal-based oxi-
dants,47 suggesting that 2a is incapable of directly oxidizing substrates.
Similarly, Nam and co-workers have found [(tpa)FeIII(OOtBu)]2+
to be unreactive with oxidizable substrates.48 The decay of 2a and
2b was also found to be insensitive to O2, consistent with the
complete oxidation of 1. Upon either warming or prolonged
standing in solution over the course of a few hours at À15 °C
(Figures 3, and S5ÀS7), 2a and 2b each convert to unstable EPR-
silent products.
The vibrational spectrum (FT-IR) of solid samples of 2a,
obtained via precipitation from MeCN with cold Et2O(xs) atÀ40 °C,
displays an isotopically sensitive feature at 888 cmÀ1, which shifts
to 831 cmÀ1 upon 18O-substitution49 using tBu18O18OH(Figure4).
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(in 72% yield), as opposed to cumenol. This would suggest that
the decay pathway for 2b, and possibly for 2a as well, involves
OÀO bond homolysis.56 Given the high spinÀstate of 2b (and
2a), this observation contrasts with previous observations re-
garding the effect of spin-state upon the relative strengths of
ferric peroxo MÀO and OÀO bonds, where high-spin states
were seen to favor MÀO, as opposed to OÀO bond cleavage.57,58
Similarly, Goldberg and co-workers have reported59,60 lowÀspin
(S = 1/2) Fe(III)ÀOOR (R = tBu, Cm) complexes which have
low νFeÀO frequencies and force constants (k) that correlate with
the donor ability of the trans-thiolate ligand, suggesting that they
would preferentially undergo FeÀO bond cleavage despite
theoretical predictions.
In conclusion, we have described the first two examples of
structurally characterized thiolate-ligated metalÀperoxo species,
and the only examples of isolable Mn(III)Àalkylperoxo species.
Manganese(III)Àperoxos are proposed to be key reactive inter-
mediates in a number of manganese-containing metalloenzyme-
promoted reactions. In order to gain insight regarding the
mechanism by which 2a and 2b decay, both in the presence
and absence of substrate, we are currently examining the tem-
perature-dependent kinetics of these reactions. Ideally, these studies
will provide insight into the factors that influence OÀOversusMÀO
bond cleavage in small-molecule transition-metal peroxos.
Figure 5. ORTEP of [MnIII(SMe2N4(QuinoEN))(OOtBu]+ (2a) with
hydrogen atoms omitted for clarity. Selected metrical parameters (Å):
MnÀS(1), 2.270(3); MnÀO(1), 1.861(5); O(1)ÀO(2), 1.457(7);
MnÀN(1), 2.034(7); MnÀN(2), 2.173(7); MnÀN(3), 2.349(7);
Mn N(4), 2.522(8).
3 3 3
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details. This ma-
b
terial is available free of charge via the Internet at http://pubs.acs.
org.
’ AUTHOR INFORMATION
Corresponding Author
kovacs@chem.washington.edu
’ ACKNOWLEDGMENT
NIH funding (#RO1GM45881-19) is gratefully acknowledged.
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