Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid

Article abstract of DOI:10.1002/anie.201802641

Naphthalene oxidation with metal–oxygen intermediates is a difficult reaction in environmental and biological chemistry. Herein, we report that a Mn^IV bis(hydroxo) complex, which was fully characterized by various physicochemical methods, such as ESI-MS, UV/Vis, and EPR analysis, X-ray diffraction, and XAS, can be employed for the oxidation of naphthalene in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the Mn^IV bis(hydroxo) complex gave a one-electron reduction potential of 1.09 V, which is the most positive potential for all reported nonheme Mn^IV bis(hydroxo) species as well as Mn^IV oxo analogues. Kinetic studies, including kinetic isotope effect analysis, suggest that the naphthalene oxidation occurs through a rate-determining electron transfer process.

Full text of DOI:10.1002/anie.201802641

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Accepted Article
Title: Naphthalene Oxidation of a Manganese(IV)-Bis(Hydroxo)
Complex in the Presence of Acid
Authors: Jaeheung Cho, Donghyun Jeong, James Yan, Hyeonju Noh,
Britt Hedman, Keith Hodgson, and Edward Solomon
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802641
Angew. Chem. 10.1002/ange.201802641
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10.1002/anie.201802641
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
Naphthalene Oxidation
Naphthalene Oxidation of a Manganese(IV)-Bis(Hydroxo) Complex in the
Presence of Acid
Donghyun Jeong, James J. Yan, Hyeonju Noh, Britt Hedman*, Keith O. Hodgson*, Edward I.
Solomon* and Jaeheung Cho*
Abs tract: Naphthalene oxidation by metal-oxygen intermediates
is one of difficult reactions in environmental and biological
chemistry. Herein, we report that a MnIV-bis(hydroxo) complex,
which was fully characterized by various physicochemical methods,
such as UV-vis, ESI-MS, EPR, X-ray and XAS, shows the
naphthalene oxidation in the presence of acid to afford 1,4-
naphthoquinone. Redox titration of the MnIV-bis(hydroxo) complex
exhibits one electron reduction potential of 1.09 V, which is the
most positive potential for the previously reported nonheme MnIV-
bis(hydroxo) species as well as MnIV-oxo analogues. Kinetic studies
including kinetic isotope effect suggest that the naphthalene
oxidation by the MnIV-bis(hydroxo) complex in the acid-promoted
reaction occurs via a rate-determining electron transfer process.
Figure 1. ORTEP diagrams of (a) the starting complex 1 and (b) the
intermediate 2 with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms are omitted for clarity except for hydrogen atoms on
hydroxo groups.
The metabolic conversion of polycyclic aromatic hydrocarbons
(PAHs) has received a lot of interests in environmental and
biological chemistry because many PAHs are genotoxic, mutagenic,
and carcinogenic pollutants.[1,2] Three main degradation pathways
are known for microbial catabolism of PAHs; (i) the insertion of
dioxygen molecule into the aromatic ring to form cis-dihydrodiols
by dioxygenases,[3] (ii) hydroxylation or epoxidation of aromatic
compounds by cytochrome P450 via arene oxides,[4] and (iii)
ligninases oxidize PAHs to quinones followed by ring cleavage.[5]
Importantly, these reactions are proposed to be carried out by
metal-reactive oxygen species such as metal-hydroxo, -peroxo and
-oxo intermediates.
Naphthalene, which is the most stable PAH, is known as a
major component of the coal and tar-based industries and a highly
toxic xenobiotic compound.[6] However, naphthalene oxidation has
been challenging reaction due to the low reactivity of aromatic C-H
bond and notoriously high oxidation potential. High-valent metal-
oxo species have been studied on the oxidation of relatively weak
PAHs such as anthracene.[7] Only one example of the MnIV-oxo
complex exhibited sluggish oxidation of naphthalene.[8] Some Fe
catalysts have also investigated the naphthalene oxidation under
catalytic reaction conditions.[9,10] Recently, high-valent metal-
hydroxo intermediates have been suggested as reactive species in
lipoxygenase and ligninolytic enzymes.[11-16] In biomimetic studies,
a few nonheme high-valent metal-hydroxo complexes have been
prepared and exhibited the C-H bond activation reactivity towards
organic substrates,[16-23] whereas there is no example of their
reactivity in aromatic molecules. Herein, we report the very first
example of the oxidation reaction of naphthalene in the presence of
acid by a mononuclear MnIV complex with terminal hydroxide
ligands, [MnIV(TBDAP)(OH)2]2+ (2) [TBDAP = N,N-di-tert-butyl-
2,11-diaza[3.3](2,6)-pyridinophane],[24] together with kinetic
studies in detail. To the best of our knowledge, it is the first
report on the acid-promoted oxidation by the high-valent metal-
hydroxo species.
The starting manganese(II) complex, [MnII(TBDAP)(OTf)2] (1-
(OTf)2) was prepared and characterized using UV-vis absorption
spectroscopy, electrospray ionization mass spectrometry (ESI-MS),
electron paramagnetic resonance (EPR) spectroscopy, and X-ray
crystallography (Figures 1a and 2a; Supporting Information (SI),
Figure S1). Addition of 1.5 equiv of iodosylbenzene (PhIO) to a
solution of 1 resulted in the generation of a green intermediate 2 in
CF3CH2OH/Acetone (v/v = 1:3) at –30 °C showing a characteristic
absorption band at 834 nm with ε = 380 M–1 cm–1 (Figure 2a). The
stoichiometry of PhIO to 1 has been examined by absorption
spectral titration (Figure S2).[25] Complex 2 was metastable at –
30 °C with a half-life (t1/2) of 30 min, allowing us to use it for
spectroscopic characterization and reactivity studies. The
intermediate 2 was also synthesized by adding 8 equiv of
cerium(IV) ammonium nitrate (CAN) to 1 in CH3CN/H2O (v/v =
9:1) at –30 °C, showing the same absorption spectrum (Figure S3)
and enhanced stability (t1/2 = 10 h). The latter method is inspired by
the similar synthetic strategy of Nam and coworkers reported
previously.[26,27] The ESI-MS of 2 exhibits a prominent peak at m/z
= 220.7 (Figure 2b), whose mass and isotope distribution pattern
correspond to [MnIV(TBDAP)(OH)2]2+ (2-16O) (calculated m/z =
[]
D. Jeong, H. Noh, Prof. Dr. J. Cho
Department of Emerging Materials Science, DGIST
Daegu 42988 (Korea)
E-mail: jaeheung@dgist.ac.kr
J. J. Yan, Prof. Dr. K. O. Hodgson, Prof. Dr. E. I. Solomon
Department of Chemistry, Stanford University
Stanford, CA 94305 (USA)
E-mail: edward.solomon@stanford.edu
Prof. Dr. B. Hedman, Prof. Dr. K. O. Hodgson, Prof. Dr. E. I.
Solomon
Stanford Synchrotron Radiation Laboratory, Stanford Linear
Accelerator Center
Menlo Park, California 94025 (USA)
Supporting information for this article is available on the
WWW under http://www.angewandte.org or from the author.
1
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10.1002/anie.201802641
Angewandte Chemie International Edition
Figure 3. Normalized Mn K-edge XAS spectra of 1 (black), 2 (red) and
[MnIII(Me2EBC)(O2)]+ (blue).[33] Inset shows expanded pre-edge region
from 6536 to 6546 eV.
quite similar to [MnIV(Me2EBC)(OH)2]2+.[34] 2 showed variability
in the rising edge (Figure S5). However, the pre-edge intensity and
edge shift remained consistent between the samples, showing they
are all centrosymmetric MnIV species. On the basis of the structural
and spectroscopic data, 2 is assigned as a manganese(IV)
bis(hydroxo) complex.
The one electron reduction potential of 2 was determined from
redox titration. The electron transfer (ET) reactions of 2 with
[FeII(phen)3]2+ (Eox = 1.08 V vs. SCE, Figure S6) occurred in
CF3CH2OH/acetone (v/v = 1:3) at –30 °C and were followed by
monitoring decay of the absorption band at 834 nm of 2, with the
concomitant formation of a broad absorption band at 620 nm due to
[FeIII(phen)3]3+ (Figure S7). ET reduction of 2 were found to be in
equilibrium with [FeII(phen)3]2+, where the final concentration of
[FeIII(phen)3]3+ produced in the ET of 2 increases with increasing
the initial concentration of [FeII(phen)3]2+. The equilibrium constant
(Ket) was determined to be 1.38 and the one electron reduction
potential (Ered) of 2 was then determined to be 1.09 V vs. SCE
using the Nernst equation (Figures S8 and S9). Importantly, to the
best of our knowledge, the value of 1.09 V for one electron
reduction potential of 2 is the most positive potential for the present
nonheme MnIV-bis(hydroxo) as well as MnIV-oxo intermediates.[35]
With the highly positive reduction potential, the aromatic
oxidation reactivity of 2 was investigated in the oxidation reaction
of naphthalene. Upon addition of the large excess of naphthalene to
2 (1.0 mM) in a solvent mixture of CF3CH2OH/acetone (v/v = 1:3)
at –30 °C, however, 2 remained intact and product analysis of the
reaction solution confirms that no oxidized products of naphthalene
were obtained.
Interestingly, upon addition of HOTf (10 mM) to the reaction
solution, 2 became reactive with naphthalene affording the
remarkable absorption band changes with a first-order decay profile
(Figure 4a). It should be noted that spectroscopic characterization
of 2 in the presence of HOTf reveals that 2 remains intact (Figure
S10). It is well known that the presence of acid induces the
enhancement for electrophilic reactivity of high-valent nonheme
Fe- and Mn-oxo complexes by the positive shift of their one-
electron reduction potential, resulting in the change of reaction
mechanism to proton-coupled electron transfer (PCET).[36-41] Thus,
the redox titration of 2 in the presence of HOTf (10 mM) was
performed by using [RuII(Cl-phen)3]2+ (Eox = 1.4 V vs. SCE; see
Figure S11), which is in equilibrium with 2 under the reaction
conditions (Figures S12 and S13). The equilibrium constant (Ket) of
1.73 in the electron transfer reaction between 2 and [RuII(Cl-
phen)3]2+ was obtained (Figures S13 and S14). The one electron
Figure 2. (a) UV-vis absorption spectra of 1 (black) and 2 (red) in
CF3CH2OH/acetone (v/v = 1:3) at –30 °C. Inset shows X-band EPR spectra
of 1 (4.0 mM, black line) and 2 (4.0 mM, red line), which was generated in
CH3CN/H2O (v/v = 4:1) at 293 K. The spectra were recorded at 113 K. (b)
ESI-MS spectrum of 2 (0.10 mM) in CF3CH2OH/acetone (v/v = 1:3). Inset
shows isotope distribution patterns for 2-16O (red) and 2-18O (blue).
220.6). Upon substitution of PhI16O with PhI18O, a mass peak
corresponding to [MnIV(TBDAP)(18OH)2]2+ (2-18O) is obtained at
m/z = 222.7 (calculated m/z = 222.6) (Figure 2b, inset),
demonstrating 2 contains two oxygen atoms. The X-band EPR
spectrum of 2 reveals a broad signal at 품∥ = 4.32 and a six-line
hyperfine pattern (푨⊥ = 10 mT) centered at 품⊥ = 1.99, indicating S
= 3/2 MnIV ground state (Figure 2a, inset).[23,28,29] In contrast, 1
shows the intense resonance at 품∥ = 6.48 and weak hyperfine
pattern (푨⊥ = 9 mT) at 품⊥ = 2.02, which means 1 possesses S = 5/2
MnII center. These contrasting spectral intensities of 1 and 2 are
originated from the different axial zero-field splitting term D.[28-30]
The X-ray crystal structure of [Mn(TBDAP)(OH)2][Ce(NO3)6]
(2-[Ce(NO3)6]) revealed the mononuclear Mn-bis(hydroxo)
complex in distorted octahedral geometry (Figure 1b). Structural
parameters of the [Ce(NO3)6] moiety is quite similar to those of
[(NH4)2CeIV(NO3)6] complex, which supports the formal oxidation
state of cerium is 4+.[31] The presence of one [CeIV(NO3)6]2- counter
anion per Mn in the crystal lattice indicates that the Mn is of the 4+
oxidation state, which is consistent with the results of EPR (vide
supra) and XAS (vide infra). Notably, the Mn-O bond lengths
(1.804 and 1.807 Å) and successful refinement of the hydroxo
groups clearly establish the Mn(OH)2 moiety (Figure S4). The Mn-
O bond distances in 2 are comparable to those in the previously
reported MnIV-bis(hydroxo) complexes, [MnIV(Me2EBC)(OH)2]2+
(1.811 Å) and [MnIV(H,MePytacn)(OH)2]2+ (1.799 Å).[23,32]
Mn K-edge X-ray absorption spectroscopy was performed to
further analyze Mn centers of 1 and 2 (Figure 3). The Mn rising
edges exhibit a noticeable shift toward higher energies in the series
of MnII (1, 6547.6 eV), MnIII ([MnIII(Me2EBC)(O2)]+, 6550.3
eV)[33] and MnIV(2, 6551.4 eV). The pre-edge of 2 has an intensity
of 6 units, consistent with a centrosymmetric complex, which is
2
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10.1002/anie.201802641
Angewandte Chemie International Edition
higher concentrations (Figure S15).[37] From such a mixture of the
first- and second-order dependence of k2 values on [HOTf], the
reaction rate equation is reproduced as Eq. (1),
k2 = k'1[HOTf] + k'2[HOTf]2
(1)
where k'1 and k'2 are the rate constants for the first- and second-
order dependence on [HOTf], respectively. The linear plot of k2
against to [HOTf]2 indicates the involvement of two protons in the
naphthalene oxidation via PCET reaction (Figure S16).
We confirmed that the final product formed in the oxidation of
naphthalene is 1,4-naphthoquinone as the sole oxidized product
(90% yield based on naphthalene) by taking 1H NMR spectra of the
reaction solution (Figure S17). Further, when naphthalene is
oxidized by 2-18O, the oxygen atoms in the 1,4-naphthoquinone
were found to be derived from the MnIV-(OH)2 species (Figure
S18). After the reaction, 2 decayed to a MnIII species (Scheme 1),
which was proved by ESI-MS and EPR (Figures S19 and S20).
To gain mechanistic insights, the kinetic isotope effect (KIE)
was investigated in the hydroxylation of undeuterated and
deuterated naphthalenes by 2 in the presence of HOTf (Figure 4b).
A k2 value of 7.4(4) × 10–1 M–1 s–1 was obtained in the reaction of 2
and naphthalene-d8, giving the calculated kH/kD value of 1.0(3).
Thus, reaction rates of the oxidation of naphthalene and
naphthalene-d8 by 2 are irrelevant to the C-H bond dissociation
energy of aromatic substrates. This result is consistent with the
electron transfer reaction of metal-oxo species.[37,41] The reactivity
of 2 was also investigated with substituted naphthalenes, X-
naphthalene (X = OH, OMe, Me, H) to evaluate the electronic
effect of substituents on naphthalene oxidation by 2 (Figure S21
and Table S3). A good linear correlation was obtained from the
plotting of the electron transfer rates against oxidation peak
potentials (Epc) of naphthalenes, affording the negative slope of –
3.7 (Figure 4c). These kinetic studies suggest that the acid-
Figure 4. Reactions of 2 with naphthalene in CF3CH2OH/acetone (v/v = 1:3)
at -30 °C in the presence of HOTf (10 mM). (a) UV-Vis spectral changes
observed in the reaction of 2 (1.0 mM) with 50 equiv of naphthalene. Inset
shows the time courses monitored at 380 nm in the absence (black circles)
and presence (red circles) of HOTf (10 mM). (b) Plots of pseudo-first-order
rate constants (kobs) against the concentrations of naphthalene (black circles)
and naphthalene-d8 (red circles) to determine second-order rate constants
(k2). (c) Plot of log k2 against Epc of X-naphthalene. X = OH, OMe, Me and
H.
reduction potential of 2 in CF3CH2OH/acetone (v/v = 1:3) at –30 °C
was then determined to be 1.41 V vs. SCE from Nernst equation in
the presence of HOTf (10 mM). Thus, the Ered value of 2 is
significantly shifted in the positive direction from 1.09 V vs. SCE
in the absence of HOTf to 1.41 V vs. SCE in the presence of HOTf
(10 mM). Therefore, the oxidation of naphthalene (Eox = ~1.7 V vs.
SCE)[42,43] could be made possible by the positive shift in reduction
potential of 2 in the presence of HOTf via proton-coupled electron
transfer (PCET).
The pseudo-first-order rate constants (kobs) increased linearly
with an increase of the concentration of naphthalene (Figure 4b),
giving a second-order rate constant (k2) of 7.3(6) × 10–1 M–1 s–1 at –
30 oC. The observed k2 values show the first-order dependence on
[HOTf] at lower concentrations and the second-order dependence at
Scheme 1. Proposed mechanisms of the naphthalene oxidation reaction by 2
in the presence of acid.
3
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10.1002/anie.201802641
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promoted oxidation of naphthalene by 2 occurs via a rate-
determining electron transfer pathway (Scheme 1, pathway a).
It has been documented that the oxidation of polyaromatics to
quinones requires totally six-electron oxidation.[44,45] The
mechanistic proposal for the oxidation of naphthalene by 2 is
shown in Scheme 1. First, the rate-determining electron transfer
from naphthalene to protonated 2 triggers the six-electron oxidation
of naphthalene affording a naphthalene radical cation with
[MnIII(TBDAP)(OH)(OH2)]2+ (Scheme 1, pathway a), followed by
the further oxidations (Scheme 1, pathways b - f). 1-naphthol which
is two-electron oxidized product of naphthalene is supposed to
undergo faster four-electron oxidation, possessing lower oxidation
potential than naphthalene. Indeed, product analysis revealed that
1-naphthol was fully oxidized to 1,4-naphthoquinone with 95%
yield upon reaction with four equiv of 2 in CF3CH2OH/acetone (v/v
= 1:3) at –30 oC. The k2 value (1.9(2) × 102 M–1 s–1) of the
oxidation of 1-naphthol by 2 is much larger than those of
naphthalene oxidation by 2 (Table S3). This result is consistent with
the hypothesis that the initial oxidation of naphthalene to 1-
naphthol is rate-determining step.
Keywords: naphthalene oxidation · manganese · metal-hydroxo
compounds · reaction mechanism
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Thus, the presence of acid enhances the oxidizing power of
MnIV-bis(hydroxo) species to afford an electron transfer from
naphthalene, as shown similarly in high-valent metal-oxo
chemistry.[36-39] On the basis of our results, we suggest that high-
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In summary, we have demonstrated the generation of a unique
nonheme mononuclear bis(hydroxo)manganese(IV) complex
bearing the macrocyclic TBDAP ligand, [MnIV(TBDAP)(OH)2]2+
(2) with full characterization using various spectroscopic methods
and X-ray crystallography. 2 exhibits a prominent ability to oxidize
naphthalene in the presence of acid to afford naphthoquinone in
CF3CH2OH/acetone (v/v = 1:3) at –30 oC. Based on the kinetic and
isotope labeling experiments, we suggest that 2 reacts with
naphthalene to give a MnIII-bis(hydroxo) species and naphthalene
cation radical via the PCET reaction, and then produces a MnII-
(hydroxo) species and 1-naphthol, which undergoes faster four
electron oxidation. Further detailed studies, including the density
functional theory calculations as well as the electron transfer
property of MnIV-bis(hydroxo) species in light of Marcus theory of
outer-sphere electron transfer, to understand the intrinsic properties
of arene hydroxylation by metal-hydroxo species in the presence of
acid are under investigation.
Acknowledgements
The research was supported by NRF (2017R1A2B4005441 to J.C.),
and the Ministry of Science, ICT and Future Planning (DGIST
R&D Program 18-BD-0403, and CGRC 2016M3D3A01913243 to
J.C.) of Korea. Funding for this work was also provided by the NIH
(GM-40392 to E.I.S.). Use of the Stanford Synchrotron Radiation
Lightsource, SLAC National Accelerator Laboratory, is supported
by the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences under Contract No. DE-AC02-76SF00515.
The SSRL Structural Molecular Biology Program is supported by
the DOE Office of Biological and Environmental Research, and by
the National Institutes of Health, National Institute of General
Medical Sciences (P41GM103393 to K.O.H and B.H.).
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10.1002/anie.201802641
Angewandte Chemie International Edition
COMMUNICATION
A MnIV-bis(hydroxo) complex, which
was fully characterized by various
physicochemical methods, shows
the reactivity in the naphthalene
oxidation in the presence of acid to
afford 1,4-naphthoquinone via an
D. Jeong, J. J. Yan, H. Noh, B.
Hedman*, K. O. Hodgson*, E. I.
Solomon* and J. Cho*
Page No. – Page No.
acid-promoted
electron transfer process.
rate-determining
Naphthalene Oxidation of a
Manganese(IV)-Bis(Hydroxo)
Complex in the Presence of Acid
6
This article is protected by copyright. All rights reserved.