Structural and spectroscopic characterization of a monomeric side-on manganese(IV) peroxo complex

Article abstract of DOI:10.1002/anie.201201735

Gotcha: The binding and activation of oxygen by a manganese complex is reported. A PS₃ coordination sphere built around a manganese(I) center facilitates the isolation of a monomeric manganese(IV) peroxo complex that is stable at ambient temperature (see picture). The activation of molecular oxygen is proposed via a manganese(II) intermediate with an empty site for the binding of the substrate. Copyright

Full text of DOI:10.1002/anie.201201735

Angewandte
Chemie
DOI: 10.1002/anie.201201735
Oxygen Activation
Structural and Spectroscopic Characterization of a Monomeric Side-On
Manganese(IV) Peroxo Complex**
Chien-Ming Lee,* Chi-He Chuo, Ching-Hui Chen, Cho-Chun Hu, Ming-Hsi Chiang,* Yu-
Jan Tseng, Ching-Han Hu, and Gene-Hsiang Lee
Manganese ions are present in the active sites of several
enzymes that are involved in important reactions, such as
activation of molecular oxygen, detoxification of superoxide
and hydrogen peroxide, and water-splitting chemistry.[1] In the
catalytic cycles of these enzymes, Mn-superoxo or Mn-peroxo
complexes have been implicated as reactive intermediates.
For example, a MnIII-superoxo or Mn-peroxo-HPCA radical
adduct is proposed in Mn-dependent homoprotocatechuate
(HPCA) 2,3-dioxygenase, which catalyzes the ring opening of
HPCAwith incorporation of both oxygen atoms from O2.[2] In
synthetic systems, most of the peroxomanganese complexes
like the Co,[3] Ni,[4] Cu,[5] and Fe[6] analogs are generated from
the H2O2 source. Few examples for the MnII-mediated
reduction of O2 yielding the Mn–O2 adduct were reported.[7–10]
Hoffman and co-workers reported that MnIITPP(py) can act
as an oxygen carrier at low temperature to form a Mn-
(TPP)(O2) complex with a MnIV(O2)2ꢀ formalism and the
binding mode of the peroxo ligand was suggested as
symmetric side-on.[7] Another example reported by the
Wieghardt group is the binuclear [L2Mn2(m-O)2(m-O2)]2+
complex, which contains a peroxo bridge between two MnIV
ions.[9] Recently, Borovik and co-workers showed that reac-
tion of [MnIIH2bupa]ꢀ and O2 at ambient temperature
produces monomeric MnIII-peroxo complexes.[10] In this
system, the noncovalent interactions (hydrogen bonding)
within the secondary coordination sphere support the activa-
tion of O2.[11] In this report, we demonstrate that the complex
[MnI(CO)3(P(C6H3-3-SiMe3-2-S)2(C6H3-3-SiMe3-2-SH))]ꢀ,
1b, with the pendant thiol group in the secondary coordina-
tion sphere can activate molecular oxygen, leading to the
formation of the monomeric, O2-side-on-bound MnIV com-
plex, [Mn(O2)(P(C6H3-3-SiMe3-2-S)3)]ꢀ, 3.
Reaction of cis-[Mn(CO)4(SC6H5)2]ꢀ with one equivalent
of P(C6H3-3-R-2-SH)3 (R = H or SiMe3) in tetrahydrofuran
(THF) solution at 508C, respectively, produced the six-
coordinate [Mn(CO)3(P(C6H3-3-R-2-S)2(C6H3-3-R-2-SH))]ꢀ
(R = H, 1a; SiMe3, 1b) isolated as a yellow solid in high
yield (95% for 1a; 92% for 1b). The IR spectra of 1a and 1b
show the same CO stretching bands at 1989(vs), 1908(s), and
1886(s) (Figure 1), consistent with a tricarbonyl derivative
possessing pseudo C3v symmetry.[12] Complex 1a can be
crystallized by vapor diffusion of diethyl ether into a concen-
trated THF/CH3CN (3:1, v/v) solution at ꢀ208C in nitro-
gen.[13] Figure 1 displays a thermal ellipsoid plot of 1a. The
manganese center is coordinated by one phosphorus, two
thiolate sulfur atoms, and three CO groups in a facial position,
in agreement with the infrared data. The structural data also
reveal that 1a contains a pendant thiol group in the secondary
ꢁ
coordination sphere. An intramolecular [C O···H-S] inter-
action (the S(3)···O(3) distance of 3.526 ꢀ found in 1a) is
present, which is responsible for the broadening of the S–H
stretching band. The stretching vibrations nS-H (in the range of
2500–2000 cmꢀ1) of complexes 1a and 1b were not identi-
fied.[14] Similarly, attempts to identify the chemical shifts of
the thiol protons in 1a and 1b by NMR spectroscopy were not
successful.
Addition of an excess of molecular oxygen to a THF
solution of 1a at ambient temperature for 1 h affords an
orange-red complex 2. The electrospray ionization mass (ESI-
MS) spectrum of the oxygenated species shows a maximum
ion peak at 409.8 (a mass-to-charge ratio, Figure S1 in the
[*] Prof. C.-M. Lee, C.-H. Chuo, C.-H. Chen, Prof. C.-C. Hu
Department of Applied Science, National Taitung University
Taitung city 950 (Taiwan)
E-mail: cmlee@nttu.edu.tw
Dr. M.-H. Chiang
Institute of Chemistry, Academia Sinica
Nankang, Taipei 115 (Taiwan)
E-mail: mhchiang@chem.sinica.edu.tw
Y.-J. Tseng, Prof. C.-H. Hu
Department of Chemistry
National Changhua University of Education
Changhua 500 (Taiwan)
Dr. G.-H. Lee
Instrumentation Center, National Taiwan University
Taipei 107 (Taiwan)
Figure 1. a) ORTEP diagram of [Mn(CO)3(P(C6H4-2-S)2(C6H4-2-SH))]ꢀ,
1a, with thermal ellipsoids at 50% probability and hydrogen atoms
omitted for clarity, and b) FTIR spectra of 1a (solid) and 1b (dash)
measured in the absorbance mode. Selected bond distances [ꢀ]: Mn–
S1 2.3919(11), Mn–S2 2.3929(12), Mn–P1 2.2584(11).
[**] We thank the National Science Council (Taiwan) for financial
support (NSC 99-2113-M-143-002-MY2), and Prof. Wen-Feng Liaw
and Dr. Feng-Chun Lo for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201735.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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Supporting Information), which is the signature of [MnII(P-
2ꢀ
(C6H4-2-S)3)]2 (calcd m/z 409.9). The crystallographic anal-
ysis of single crystals of 2 supports this assignment (Figure 2).
Prolonged stirring of the mixture generates insoluble yellow
solid. Attempts to isolate the insoluble yellow solid were
unsuccessful.
Figure 3. FTIR spectra of 3 (solid line) and 18O-labeled 3 (dashed line).
Inset: a difference spectrum between 3 and its 18O-labeled derivative.
662.1 (calcd, 662.0 m/z) when 18O2 was used in the reaction
(Figure S4 in the Supporting Information) supports the
assignment to {Mn(O2)}.
Figure 2. ORTEP diagram of [MnII(P(C6H4-2-S)3)]22ꢀ, 2, with thermal
ellipsoids at 50% probability and hydrogen atoms omitted for clarity.
Selected bond distances [ꢀ]: Mn1–S1 2.412(2), Mn1–S2 2.491(2),
Mn1–S2A 2.517(2), Mn1–S3A 2.453(3), Mn1–P1 2.687(2), and Mn1–
Mn1A 2.969(2).
The molecular structure of 3 is given in Figure 4. Com-
plex 3 contains a side-on 1:1 Mn–O2 moiety with the
tetradentate ligand [P(C6H3-3-SiMe3-2-S)3]3ꢀ. The dioxygen
is almost symmetrically bound to the Mn ion with the mean
Mn–O distance of 1.878(2) ꢀ, which is longer than the
average MnIV–Operoxo distance of 1.83(2) ꢀ in binuclear
[L2Mn2(m-O)2(m-O2)]2+ complexes.[9] The mean Mn–O dis-
tance of 3 falls into the range of 1.841–1.901 ꢀ for the
monomeric side-on peroxomanganese(III) complexes.[17,19,20]
An acute angle of 43.07(11)8 of aOperoxo(1)-Mn-Operoxo(2)
causes a highly distorted octahedral coordination geometry
around the manganese site. The Mn ion shows a large
deviation of 0.4472(6) ꢀ from the least-square plane based on
In contrast, treating 1b with excess molecular oxygen in
THF solution at room temperature for 1 h generates the
monomeric O2-side-on-bound [Mn(O2)(P(C6H3-3-SiMe3-2-
S)3)]ꢀ, 3. When reaction of 1b and excess O2 in THF was
monitored by UV/Vis spectroscopy at ambient temperature
(Figure S2 in the Supporting Information), three features at
490, 550, and 755 nm gradually developed with decrease of
a band at 395 nm, indicating the formation of 3. Two
absorption bands (at 490 and 550 nm) in the visible region
may be attributed to d–d transitions for the d3 ions.[15] The
infrared spectrum (KBr) of 3 measured at room temperature
shows a feature at 903 cmꢀ1, which is consistent with the O–O
stretching frequency for a peroxometallic complex (nO-O
ꢂ 800–930 cmꢀ1).[16] When 18O2 was used instead, the corre-
sponding 18O–18O vibration was not observed because of
overlap with intense nSi-C bands found between 845 and
875 cmꢀ1 (Figure 3). The difference spectrum of 3 and its 18O-
labeled derivative shows that the nO-O band of 18O-labeled 3
shifts to around 861 cmꢀ1 (inset in Figure 3), roughly similar
to the calculated position based on the difference in masses of
the 16O- and 18O-labled compounds. The 16O–16O vibration of
ꢀ
the three phenylthiolate sulfur atoms. The O O bond length
complex 3 is slightly higher than those (around 885–896 cmꢀ1
)
reported for the monomeric h2-peroxo MnIII systems[11,17] and
the thiolate-ligated manganese(III)-alkylperoxo species.[18]
On the other hand this vibration is red-shifted by 80 cmꢀ1
compared to the Mn(TPP)(O2) complex.[8] The results from
reactions carried out in an 18O2 atmosphere also indicate that
O2 is the source for the formation of 3. The ESI-MS spectra of
the oxygenated species of 1b shows a maximum ion peak at
658.1 m/z (Figure S3 in the Supporting Information), consis-
tent with the mass and calculated isotope distribution of the
[Mn(O2)(P(C6H3-3-SiMe3-2-S)3)]ꢀ anion (calcd, 658.0 m/z).
Furthermore, the shift of four mass units to a m/z value of
Figure 4. ORTEP drawing of [Mn(O2)(P(C6H3-3-SiMe3-2-S)3)]ꢀ, 3, with
thermal ellipsoids at 50% probability and hydrogen atoms omitted for
clarity. Selected bond distances [ꢀ] and angles [deg]: O1–O2 1.379(3),
Mn–S1 2.3370(8), Mn–S2 2.3836(8), Mn–S3 2.3857(8), Mn–P1
2.3036(8), Mn–O1 1.883(2), Mn–O2 1.873(2), O1-Mn-O2 43.07(11),
S1-Mn-S2 110.25(3), S1-Mn-S3 130.59(3), S2-Mn-S3 108.34(3), S1-Mn-
P1 80.92(3), S2-Mn-P1 79.89(3), S3-Mn-P1 76.69(3), P1-Mn-O1
149.60(8), P1-Mn-O2 164.46(9).
2
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Table 1: O–O distance (ꢀ; RO-O), 16O–16O and 18O–18O stretching
frequencies (cmꢀ1; n
O, n O), natural population analysis charges of
the O2 moiety and the metal (qO-O and qMn), and the Wiberg bond index
of 1.379(3) ꢀ in 3 is slightly shorter than those bond lengths of
h2-
16O-16
18O-18
the
structurally
characterized
monomeric
peroxomanganese(III) complexes (1.40–1.43 ꢀ).[17,19,20] The
transfer of less charge from the high-valent MnIV ion to the
coordinated peroxo ligand relative to the charge transfer in
monomeric side-on peroxomanganese(III) complexes may
for O–O and Mn–O bonds (O–OWBI and Mn–OWBI).
Doublet
Quartet
Sextet
RO-O
1.398
1.427
1.339
ꢀ
account for the shorter O O bond length and the higher nO-O
n
n
975
921
ꢀ0.7034
1.1432
1.095
959
906
ꢀ0.7795
1.3286
1.071
1130
1064
ꢀ0.5650
1.3425
1.231
16O-16
O
O
of 3.[21]
18O-18
qO-O
qMn
O–OWBI
ꢀ
Analyses of structural data (O O bond length) and
vibrational energy (nO-O) reveal that the dioxygen ligand of
3 could be described as a peroxo type that is further confirmed
by EPR, SQUID as well as DFT computation. The X-band
EPR spectrum of 3 recorded at 4 K deviates from the ideal
axial pattern with two broad signals (Figure 5a). One strong
Mn–OWBI
0.613/0.783
0.604/0.610
0.172/0.340
out of natural population analysis for the O2 moiety and the
metal, and the Wiberg bond index for O–O and Mn–O bonds
are summarized in Table 1. The O–O distance of the quartet is
the longest. This feature is revealed in the smallest stretching
frequency and Wiberg bond index between the oxygen atoms.
In addition, the atomic spin densities of 3 in the quartet state
are 2.769 on Mn (see Table S1 in the Supporting Information),
indicating that the unpaired electrons are primarily located in
the manganese d orbitals.
The reaction sequences given in Scheme 1 probably
account for the formation of 2 and 3. Molecular oxygen
may initially oxidize the pendant thiol groups[14] of 1a/1b to
form the four-coordinate transient species [MnII(P(C6H3-3-R-
2-S)3)]ꢀ (R = H, A; SiMe3, B), accompanied by the release of
CO and the formation of hydrogen peroxide.[24] The second
possible pathway that is gone through direct oxidation of the
metal site by O2 followed by coordination of the dangling
thiol group could not be ruled out albeit the manganese
center is coordinatively saturated in 1a and 1b.[25] The
attempt to isolate A/B is fruitless. In solution, coexistence
of the starting materials and the products except for A/B was
observed by spectroscopy. We tentatively propose that
dimerization proceeds upon formation of A to afford 2. On
the other hand, the steric hindrance resulting from the SiMe3
groups of B inhibits dimerization and constitutes a pocket
open to the O2 binding (Figure S5 in the Supporting
Information). Treatment of MnII complexes with superoxides
or peroxides in the presence of Lewis bases generates
Figure 5. a) X-band EPR spectrum of 3 at 4 K and b) plots of magnetic
susceptibility (open circles) and cMT (open squares) versus the
temperature for 3. The solid line is the best fit to the theoretical
model.
band at g = 4.37 and a weak one at g = 1.91 are observed. No
55Mn hyperfine coupling can be resolved for both. The
observed result is in agreement with the known spectra for
the MnIV species.[7,15,22] From dominance of the low-field
resonance, it is expected that energy between two Kramerꢁs
doublets of the S = 3/2 manifold is larger than 0.31 cmꢀ1. The
value of D is estimated to be around 0.42 cmꢀ1 if g?e is around
3.40 and g? and gk are around 1.91.[23] The magnetic study of 3
suggests that it is a quartet state complex. The cMT value of 3
remains around 2.1 cm3 Kmolꢀ1 in a temperature range of 20
to 300 K, consistent with the presence of the high-spin MnIV
center (Figure 5b). The rapid drop of the cMT value below
20 K is probably attributed to a contribution of zero-field
splitting (ZFS) and/or intermolecular antiferromagnetic
interactions. The best fit (R2 = 0.9999) to the temperature-
dependent cM data reveals g = 2.14 and D = 0.68 cmꢀ1. The
axial ZFS parameter deduced from the magnetic measure-
ment is comparable with that from the EPR study, suggesting
the decrease of cMT is primarily due to depopulation of the
excited state.
The geometries of the doublet, quartet, and sextet states
of 3 were optimized using mPWPW91/6-31G(d). The quartet
state was found to be the lowest energy species, while the
doublet and sextet state lie 12.3 and 14.6 kcalmolꢀ1 higher in
ꢀ
energy (DG at 298 K), respectively. The O O bond distance,
16O–16O and 18O–18O stretching frequencies, atomic charges
Scheme 1. A proposed mechanism for the formation of 2 and 3.
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ing high rate constants on aldehyde deformylation at low
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excitation band at 550 nm reveals unobservable changes at
room temperature within 1 h. The decreased ability for
nucleophilic attack of the substrates could be the result of
an electronic influence: electronic deficiency of the Mn
center, which is rationalized by its high oxidation state (+ 4)
and the negative reduction potential of ꢀ1.51 V (vs. Fc+/Fc;
Fc = ferrocene; Figure S6 in the Supporting Information).
In summary, we succeeded in the syntheses and structural
characterization of a monomeric side-on [MnIV(O2)(P(C6H3-
3-SiMe3-2-S)3)]ꢀ complex, 3, obtained from oxygenation of 1b
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can support other peroxomanganese(IV) complexes, which
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Further
Received: March 3, 2012
Published online: && &&, &&&&
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Keywords: bioinorganic chemistry · coordination modes ·
enzyme models · manganese · oxygen activation
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Basolo, J. Am. Chem. Soc. 1976, 98, 5473 – 5482.
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Oxygen Activation
Gotcha: The binding and activation of
oxygen by a manganese complex is
reported. A PS3 coordination sphere built
around a manganese(I) center facilitates
the isolation of a monomeric mangane-
se(IV) peroxo complex that is stable at
ambient temperature (see picture). The
activation of molecular oxygen is pro-
posed via a manganese(II) intermediate
with an empty site for the binding of the
substrate.
C.-M. Lee,* C.-H. Chuo, C.-H. Chen,
C.-C. Hu, M.-H. Chiang,* Y.-J. Tseng,
C.-H. Hu, G.-H. Lee
&&&& — &&&&
Structural and Spectroscopic
Characterization of a Monomeric Side-On
Manganese(IV) Peroxo Complex
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!