A Manganese(V)–Oxo Tetraamido Macrocyclic Ligand (TAML) Cation Radical Complex: Synthesis, Characterization, and Reactivity Studies

Article abstract of DOI:10.1002/chem.201804898

A mononuclear manganese(V)–oxo complex with tetraamido macrocyclic ligand (TAML), [Mn^V(O)(TAML)]^? (1), is a sluggish oxidant in oxidation reactions. Herein, a mononuclear manganese(V)–oxo TAML cation radical complex, [Mn^V(O)(TAML^+.)] (2), is reported. It was synthesized by reacting [Mn^III(TAML)]^? with 3.0 equivalents of [Ru^III(bpy)₃]³⁺ or upon addition of one-electron oxidant to 1 and then characterized thoroughly with various spectroscopic techniques along with DFT calculations. Although 1 is a sluggish oxidant, 2 is a strong oxidant capable of activating C?H bonds of hydrocarbons (i.e., hydrogen atom transfer reaction) and transferring its oxygen atom to thioanisoles and olefins (i.e., oxygen atom transfer reaction).

Full text of DOI:10.1002/chem.201804898

A Journal of
Accepted Article
Title: A Manganese(V)-Oxo TAML Cation Radical Complex: Synthesis,
Characterization, and Reactivity Studies
Authors: Deepika G. Karmalkar, Xiao-Xi Li, Mi Sook Seo, Muniyandi
Sankaralingam, Takehiro Ohta, Ritimukta Sarangi, Seungwoo
Hong, and Wonwoo Nam
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of the final Version of Record (VoR). This work is currently citable by
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201804898
Link to VoR: http://dx.doi.org/10.1002/chem.201804898
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DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Manganese-Oxo
A Manganese(V)-Oxo TAML Cation Radical Complex: Synthesis,
Characterization, and Reactivity Studies**
Deepika G. Karmalkar, Xiao-Xi Li, Mi Sook Seo, Muniyandi Sankaralingam, Takehiro Ohta,
Ritimukta Sarangi,* Seungwoo Hong,* and Wonwoo Nam*
Abstract: A mononuclear manganese(V)-oxo complex bearing
V
–
tetraamido macrocyclic ligand (TAML), [Mn (O)(TAML)] (1), is a
sluggish oxidant in oxidation reactions. Herein, we report a
mononuclear manganese(V)-oxo TAML cation radical complex,
V
+•
[
[
Mn (O)(TAML )] (2), which is synthesized by reacting
III
–
III
3+
Mn (TAML)] with 3.0 equiv of [Ru (bpy)
3
]
or upon addition of
one-electron oxidant to 1 and then characterized thoroughly with
various spectroscopic techniques along with DFT calculations.
While 1 is a sluggish oxidant, 2 is a strong oxidant capable of
activating C-H bonds of hydrocarbons (i.e., hydrogen atom transfer
reaction) and transferring its oxygen atom to thioanisoles and
olefins (i.e., oxygen atom transfer reaction).
V
+•
Scheme 1. Synthesis and Reactivity of [Mn (O)(TAML )] (2)
High-valent iron-oxo species and their manganese-oxo analogues
have been invoked as reactive intermediates in the catalytic
oxidation of organic substrates by metalloenzymes and synthetic
High-valent manganese(V)-oxo species have attracted much
attention recently as plausible intermediates in the oxidation of
organic substrates and the oxygen evolution reaction in Photosystem
1
,2
metal catalysts.
One notable example is the iron(IV)-oxo
3
,4
II (PSII).
In biomimetic studies, a number of Mn(V)-oxo
porphyrin π-cation radical species, referred to as compound I (Cpd
complexes have been synthesized and characterized structurally
1
I), in Cytochromes P450 (CYP450). There is another high-valent
3
and/or spectroscopically. Interestingly, the synthetic Mn(V)-oxo
iron-oxo intermediate found in heme enzymes; that is one-electron
reduced species of Cpd I, which is iron(IV)-oxo porphyrin species,
complexes of porphyrin, corrole, corrolazine, and TAML ligands
3
c,5
exhibited low reactivities in oxidation reactions. In addition to the
Mn(V)-oxo complexes, Mn(IV)-oxo ligand cation radical species,
which are the valence tautomers of the Mn(V)-oxo intermediates,
have been synthesized upon addition of Lewis or Brönsted acids to
the Mn(V)-oxo complexes of corrole and corrolazine, and
reactivities of the Mn(V)-oxo and Mn(IV)-oxo ligand cation radical
complexes were compared in hydrogen atom transfer (HAT) and
1
referred to as compound II (Cpd II). The structural and chemical
properties of Cpd I and Cpd II have been investigated intensively in
enzymatic and biomimetic reactions over the past several decades; it
is well known that Cpd I, iron(IV)-oxo porphyrin π-cation radical, is
much more reactive than Cpd II, iron(IV)-oxo porphyrin, in
1
oxidation reactions.
6
oxygen atom transfer (OAT) reactions. Goldberg and co-workers
[]
D. G. Karmalkar, Dr. X.-X. Li, Dr. M. S. Seo, Dr. M. Sankaralingam,
Prof. Dr. W. Nam
synthesized a novel Mn(V)-oxo corrolazine cation radical complex
upon one-electron oxidation of its corresponding Mn(V)-oxo
Department of Chemistry and Nano Science, Ewha Womans
University, Seoul 03760, Korea, E-mail: wwnam@ewha.ac.kr
7
complex and discussed briefly their reactivities in OAT reaction.
Very recently, we reported an iron(V)-imido TAML cation
radical complex, [Fe (NTs)(TAML )], which was synthesized by
Dr. T. Ohta
V
+• 8a
Picobiology Institute, Graduate School of Life Science, University of
Hyogo, RSC-UH LP Center, Hyogo 679-5148, Japan
V
8b
oxidizing an iron(V)-imido complex, [Fe (NTs)(TAML)], with
III
3+
one-electron oxidants, such as [Fe (bpy)
3
] , tris(4-bromophenyl)-
N]SbCl }, and
Prof. Dr. S. Hong
ammoniumyl hexachloroantimonate {[(4-BrC
6
H
4
)
3
6
Department of Chemistry, Sookmyung Women’s University, Seoul
III
3+
[
Ru (bpy)
3
] . In the study, we have demonstrated that the
oxidation of [Fe (NTs)(TAML)] occurs on the TAML ligand, not
on the iron ion. With this result and the report of Goldberg and co-
workers, we attempted to oxidize the Mn(V)-oxo TAML complex,
04310, Korea, E-mail: hsw@sm.ac.kr
V
Prof. Dr. Ritimukta Sarangi
8
a
Stanford Synchrotron Radiation Lightsource, SLAC National
Accelerator Laboratory, Stanford University, California 94025, USA
E-mail: ritis@slac.stanford.edu
7
V
—
5e
[
Mn (O)(TAML)] (1), with one-electron oxidants. In this
communication, we report the synthesis of
manganese(V)-oxo TAML cation radical
a
mononuclear
complex,
[
] This work was supported by NRF of Korea through CRI (NRF-
2012R1A3A2048842 to W.N.), GRL (NRF-2010-00353 to W.N.),
V
+•
[
[
Mn (O)(TAML )] (2), upon one-electron oxidation of 1 by
MSIP (NRF-2017R1C1B2002037 to S.H.), and Sookmyung
Women’s University Grants (1-1603-2053 to S.H.). The SSRL SMB
resource was supported by the NIH National Institute of General
Medical Sciences (NIGMS) through a Biomedical Technology
Research Resource P41 grant (P41GM103393) and by the DOE
Office of Biological and Environmental Research.
III
3+
3
Ru (bpy) ] (Scheme 1, reaction pathway a). 2 was also generated
III
–
III
3+
by reacting [Mn (TAML)] with 3 equiv of [Ru (bpy)
3
]
(Scheme
1, reaction pathway b). Then, 2 was characterized with various
spectroscopic techniques, demonstrating that 2 is a manganese(V)-
oxo TAML cation radical complex possessing a Mn-O triple bond
with a short Mn-O bond distance (1.54 Å ) and a Mn-O bond
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
-
1
vibrational frequency at 996 cm . Interestingly, while 1 is a sluggish
1
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Chemistry - A European Journal
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`
Figure 1. UV−vis spectral changes observed in the reaction of 1 (2.5 x
–
2
III
3+
–2
1
4
0
mM, black line) and [Ru (bpy)
3
]
(3.0 x 10 mM) in CH
3
CN at –
II
2+
0 ºC. The peak at 450 nm was due to [Ru (bpy)
3
] . Inset shows the
plot of the absorbance change at 620 nm due to 2 upon addition of
3
+
–2
[
3
Ru(bpy) ] to 1 (2.5 x 10 mM) in the increment of 0.2 equiv.
oxidant, 2 is a highly reactive oxidant that is capable of oxidizing
hydrocarbon C-H bonds, thioanisoles, and olefins (Scheme 1, blue
box). Some mechanistic aspects are also discussed in this study.
III
–
When [Mn (TAML)] was reacted with one-electron oxidant,
III
3+
such as [Ru (bpy)
at –40 C, we observed the formation of an intermediate with broad
3
]
2 3
(>3 equiv), in the presence of H O in CH CN
o
Figure 2. (a) The normalized Mn K-edge XAS data for 1 (black line)
and 2 (red line). Inset shows the expanded pre-edge region. (b) Non-
phase-shift corrected Fourier Transform data for 1 (black line) and 2
absorption bands at 620 and 825 nm (Supporting Information (SI),
Figure S1). This intermediate was metastable at –40 C but decayed
o
(
red line). Inset shows the EXAFS comparison (SI, Figure S4).
fast at a higher temperature. Interestingly, this intermediate, denoted
V
–
as 2, was generated when [Mn (O)(TAML)] (1) was treated with
III
]3+. This lack of
at the Mn center upon oxidation with [Ru (bpy)
3
III
3+
III
3+
[
Ru (bpy)
3
] ; addition of 1.2 equiv of [Ru (bpy)
3
]
3
to a CH CN
spectral change at the pre-edge and rising-edge region indicates that
the Mn center remains in the Mn(V) redox state and the oxidation
occurs on the TAML ligand. This is also reflected in the comparison
of the Mn K-edge EXAFS data for 1 and 2 and their respective
Fourier transforms (Figure 2b and SI, Figure S4). The first shell is
identical in both species, while small changes are observed in the
outer shells dominated by TAML-based single and multiple
scattering contributions. FEFF analysis of 1 and 2 confirmed very
similar structures with 1 Mn-O at 1.54 Å and 4 Mn-N at 1.88 Å (SI,
Table S1). While the second and third shell multiple scattering
contributions are very similar in both fits, a larger contribution of
outer-shell multiple-scattering is required to obtain a good fit for 2,
suggesting a change in the TAML ring upon oxidation.
solution of 1 at –40 °C afforded the formation of 2 with electronic
–
1
–1
absorption bands at 620 nm (ε = 7600 M cm ) and 825 nm (ε =
–
1
–1
8
000 M cm ) (Figure 1, red line; also see SI, Experimental
III
3+
Section for the preparation of 1). In contrast to [Ru (bpy)
3
]
(Eox =
1
.24 V vs SCE), other relatively weak one-electron oxidants, such as
III
3+
[
Fe (bpy)
3
]
(Eox
=
1.06
V
vs SCE) and tris(4-
bromophenyl)ammoniumyl hexachloroantimonate (Eox = 1.08 V vs
9
SCE), did not convert 1 to 2 under the reaction conditions. These
results can be interpreted with the electrochemical property of 1 that
was determined with cyclic voltammetry; a quasi-reversible wave
with the oxidation and reduction peak potentials at 1.21 V and 1.01
o
V (vs SCE), respectively, were determined in CH
3
CN at –40 C (SI,
o
Figure S2). Since 2 was metastable (t1/2 of ~40 min) at –40 C, we
were able to characterize it with various spectroscopic techniques.
To understand the electronic structure of 2, density functional
theory (DFT) calculations were performed at the UBP86/def2-
TZVPP//def2-SVP level, and a doublet ground state was obtained
X-band electron paramagnetic resonance (EPR) spectrum of a
CH
=
3
CN solution of 2 at 77 K exhibited a radical signal centered at g
(SI, Table S2). After analyzing the spin density distribution, we
2.002 (SI, Figure S3a); EPR of 1 was silent.5e The signal was
found that there was one spin located at the TAML ligand (SI, Table
S3), which is in line with the experimental EPR result. In other
quantified against an external standard, revealing a >80% yield of 2.
A cold spray ionization time-of-flight mass spectrum (CSI-MS) of 2
exhibited an ion peak at m/z of 546.4 in positive mode, whose mass
2
*
0
*
0
*
0
words, 2 has an electronic configuration of [δxy , π xz , π yz , σ x2-y2
σ
,
*
0
*
1
z2 , π ligand ], which has been confirmed by the orbital analysis
and
isotope
distribution
patterns
correspond
to
results (SI, Figure S5). In calculations, the Mn-O bond vibrational
{
Na[Mn(O)(TAML)(CH
3
CN)
2
]}+ (calculated m/z of 546.1) (SI,
–
1
–1
frequency was found at 996 cm , which shifted to 957 cm upon
1
8
Figure S3b). The ion peak of 2 shifted two-mass unit upon O-
substitution, confirming that 2 contains one oxygen atom in it (SI,
Figure S3b, inset). Upon 441.6 nm-excitation, complex 2 exhibited
1
8
O substitution. This is in good agreement with the experimental
rRaman data (e.g., the shift of 986 cm to 945 cm upon O-
substitution). In addition, similar to 1, the doublet ground state 2
with a Mn-O distance of 1.56 Å (SI, Table S4) also has triple Mn-O
bond character, which is also consistent with the EXAFS result
–
1
–1
18
5
e
–
1
an isotope-sensitive resonance Raman (rRaman) band at 986 cm ,
–
1
18
which shifted to 945 cm upon O-substitution (SI, Figure S3c);
these values are similar to those reported for 1 (e.g., 977 and 937
(vide supra). Based on the results of the spectroscopic
–
1
16
18
5e
-1
cm for 1- O and 1- O, respectively). The Δ16O-18O = 41 cm is
characterization and DFT calculations, we conclude that 2 is a
Mn(V)-oxo TAML cation radical complex with a Mn-O triple bond.
-
1
consistent with that calculated (44 cm ) from Hooke’s law for a
diatomic Mn–O oscillator.
We then investigated the reactivity of 2 in various oxidation
reactions, such as C-H bond activation and the oxidation of
thioanisole and olefin. While 1 was completely unreactive toward
Mn K-edge XAS and EXAFS measurements were performed on
and 2 to determine the geometric structure and oxidation state of 2.
The Mn K-edge XAS data in Figure 2a show no significant change
1
2
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`
Figure 3. (a) Plot of log k
bond activation reactions by 2 at –40 ºC (SI, Table S5). (b) Plot of log
against the Eox values of para-X-substituted thioanisole (X = Cl, Br,
CN, and NO ) derivatives in the sulfoxidation reactions by 2 at –40 ºC
SI, Table S6).
2
’ against C-H BDEs of substrates in the C-H
Figure 4. (a) UV-vis spectral changes observed in the reactions of 2
–2
–2
(
1.3 x 10 mM) with 4-Cl-styrene (6.3 x 10 mM) in CH
3
CN at –40
] . Inset shows time
trace monitored at 620 nm due to 2. (b) Plot of log k against the Eox
k
2
II
2+
ºC . The peak at 450 nm was due to [Ru (bpy)
3
2
2
(
values of X-substituted styrene (X = α-Me, 3-Me, H, and 4-Cl)
derivatives in the epoxidation reactions by 2 at –40 ºC (SI, Table S7).
-
1
substrates with weak C-H bonds, such as xanthene (75.5 kcal mol ),
-
1
IV
9
(
,10-dihydroanthracene (DHA, 77 kcal mol ), 1,4-cyclohexadiene
CHD, 78 kcal mol ), even at room temperature, addition of CHD
to a solution of 2 in CH CN at –40 °C resulted in a rapid
dissapperance of absorption bands at 620 and 825 nm due to 2 (SI,
as discussed above, [Mn (TAML)] may be the product which
-
1
10
III
–
decays to [Mn (TAML)] fast due to its unstability. When the
1
8
18
3
sulfoxidation was performed with ~80% O-labeled 2- O, >70%
8O was found in the sulfoxide product (SI, Figure S15). The latter
1
Figure S6). Substrates having stronger C-H BDE such as fluorene
result suggests again that the source of the oxygen atom in the
sulfoxide product was the Mn-oxo oxidant.
-
1
-1
(80 kcal mol ), triphenylmethane (81 kcal mol ) didn’t react. The
C-H bond activation reactions by 2 under pseudo-first-order
conditions were monitored by UV-vis spectrophotometer. A linear
dependence of the observed decay rate on CHD concentrations gave
We finally investigated the olefin epoxidation using styrene and
deutrated styrene as well as X-substituted (X = α-Me, 3-Me, H, and
4
-Cl) styrenes. Upon addition of 4-Cl-styrene to the solution of 2 in
CH CN at –40 °C, 2 decayed with the first-order kinetics profile
Figure 4a), and the second-order rate constant of 1.2 × 10 M
was determined in the reactions of styrene and styrene-d at –40 °C
SI, Table S7 and Figure S16). The KIE value of 1.0 suggests that
–
1
–1
a second-order rate constant (k
2
) of 4.0 x 10 M
s
(SI, Table S5
3
and Figure S7). Product analysis of the xanthene oxidation by 2
3
-1 -1
(
s
revealed the quantitative formation of xanthone (48(2)% yield).
8
1
8
When the xanthene oxidation was performed with ~80% O-labeled
(
1
8
2
(i.e., 2- O), >75% 18O was found in the xanthone product (SI,
the reaction of 2 with the styrene occurs via an OAT to the C=C
double bond (SI, Figure S17). Product analysis by GC and GC-MS
revealed that benzaldehyde (60(5)%) was formed as the major
product with the formation of a small amount of styrene oxide
Figure S8). This result indicates that the oxygen atom in the product
derived from the Mn-oxo oxidant. We also found that
III
–
[
Mn (TAML)] was formed as the decay product of 2 (SI, Figure
IV
S9); it may be proposed that [Mn (TAML)] was the product but
(
10(2)%). It has been well-documented that further oxidation of
III
–
decayed to [Mn (TAML)] fast due to its instability. A kinetic
isotope effect (KIE) value of 4.1(5) was determined in the oxidation
of xanthene and deuterated xanthene by 2 (SI, Figure S10). We also
styrene oxide under acidic condition resulted in the formation of
1
2
18
benzaldehyde. When 2- O was used, the benzaldehyde product
1
8
contained 70(5)% of O (SI, Figure S18), suggesting that the
2
observed a linear correlation between the k values and the C-H
oxygen in the benzaldehyde product derived from 2. A large
bond dissociation energies (BDEs) of substrates (Figure 3a; also see
SI, Table S5 and Figure S7). Taken together, 2 is a competent
oxidant in C-H bond activation reactions and a hydrogen atom (H-
atom) abstraction from the C-H bonds of substrates by 2 is proposed
as the rate-determining step.
negative slope of –6.1 was obtained by plotting log k
one-electron oxidation potentials (Eox) of X-substituted styrenes
Figure 4b), suggesting that 2 is capable of conducting olefin
epoxidation reactions via electron transfer, followed by oxygen
2
values against
(
1
2
atom transfer.
While 1 was unreactive with thioanisole even at room
temperature, 2 reacted extremely fast with thioanisole even at –40
C; we could not follow the reaction even with a stopped-flow
In summary, a mononuclear manganese(V)-oxo TAML cation
V
+•
radical complex, [Mn (O)(TAML )] (2), was successfully
synthesized by oxidizing manganese(V)-oxo complex,
Mn (O)(TAML)] (1), with one-electron oxidant. As shown in Cpd
o
a
spectrophotometer. We therefore used less reactive para-X-
V
–
[
substituted thioanisoles (X = Cl, Br, CN, and NO
Figure S11) and determined the second-order rate constants (k
the sulfoxidation of para-X-thioanisoles by 2 at –40 °C using a
stopped-flow spectrophotometer (SI, Figure S12). When the k
2
) as substrates (SI,
I and Cpd II in heme systems, 2 is much more reactive than 1 in
HAT and OAT reactions. In future studies, we will focus on
elucidating detailed reaction mechanisms of 2 in various oxidation
reactions to compare with the chemistry of Cpd I in heme enzymes
and models. Finally, we will pursue to synthesize a Mn(VI)-oxo
complex, which is a valence tautomer of the Mn(V)-oxo TAML
cation radical complex, by varying reaction conditions.
2
) for
2
values were plotted against one-electron oxidation potentials (Eox) of
para-X-substituted thioanisoles, a large negative slope of –13 was
obtained (Figure 3b and SI, Table S6; also see SI, Figure S13),
suggesting that the sulfoxidation of para-X-thioanisoles by 2 occurs
via electron transfer, followed by oxygen atom transfer.11
Furthermore, the product analysis of the reaction solution revealed
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
III
–
the formation of PhS(O)CH
3
(~95(3)% yield) and [Mn (TAML)]
as organic and manganese products, respectively (SI, Figure S14);
3
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Chemistry - A European Journal
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`
Keywords: Manganese-Oxo Cation Radical • Cpd I vs Cpd II • C-H
Bond Activation • Sulfoxidation • Olefin Epoxidation
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Chemistry - A European Journal
10.1002/chem.201804898
Entry for the Table of Contents
Manganese-Oxo
Deepika G. Karmalkar, Xiao-Xi Li, Mi
Sook Seo, Muniyandi Sankaralingam,
Takehiro Ohta, Ritimukta Sarangi,*
Seungwoo Hong,* and Wonwoo Nam*
_
_________________
Page – Page
A Manganese(V)-Oxo TAML Cation
Radical Complex: Synthesis,
Characterization, and Reactivity Studies**
V
+•
A mononuclear manganese(V)-oxo TAML cation radical complex, [Mn (O)(TAML )], is
synthesized and characterized with various spectroscopic techniques along with DFT
calculations. This Mn-oxo intermediate is a strong oxidant capable of activating C-H bonds
of hydrocarbons (i.e., hydrogen atom transfer reaction) and transferring its oxygen atom to
thioanisoles and olefins (i.e., oxygen atom transfer reaction).
1
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