Water as an oxygen source: Synthesis, characterization, and reactivity studies of a mononuclear nonheme manganese(IV) oxo complex

Article abstract of DOI:10.1002/anie.201000819

The source of the problem: Experiments with isotopically labeled water can be used to unambiguously assign the source of oxygen in a nonheme manganese(IV) oxo complex. The complex, which was generated using water as an oxygen source and cerium(IV) as an oxidant (see picture), shows reactivities in the activation of C-H bonds of alkyl-functionalized aromatic molecules and the oxidation of aromatic substrates (sub) and benzyl alcohol.

Full text of DOI:10.1002/anie.201000819

Communications
DOI: 10.1002/anie.201000819
Enzyme Models
Water as an Oxygen Source: Synthesis, Characterization, and
Reactivity Studies of a Mononuclear Nonheme Manganese(IV) Oxo
Complex**
Sarvesh C. Sawant, Xiujuan Wu, Jaeheung Cho, Kyung-Bin Cho, Sun Hee Kim, Mi Sook Seo,
Yong-Min Lee, Minoru Kubo, Takashi Ogura, Sason Shaik, and Wonwoo Nam*
High-valent manganese oxo species have been invoked as key
intermediates in the oxidation of organic substrates by heme
and nonheme manganese catalysts, and in water oxidation by
the oxygen-evolving complex (OEC) in photosystem II
(PS II).[1,2] To elucidate the chemical and physical properties
of high-valent manganese oxo intermediates, a number of
heme and nonheme MnIV and MnV oxo complexes have been
synthesized, and characterized by using various spectroscopic
methods and X-ray crystallography. The reactivities of these
species have also been investigated in oxidation reactions,
mononuclear nonheme MnIV oxo complex using water as an
oxygen source and CeIV as an one-electron oxidant. The
spectroscopic characterization and DFT-optimized structure
of the intermediate are also reported. We also report the
reactivity of the nonheme MnIV oxo complex in oxygenation
reactions.
Addition of cerium(IV) ammonium nitrate (CAN; 8 mm)
to a reaction solution containing [MnII(BQCN)](CF3SO3)2 (1;
2 mm; BQCN = N,N’-dimethyl-N,N’-bis(8-quinolyl)cyclohex-
anediamine; see the crystal structure of 1 in Figure 1 and
ꢀ
such as C H bond activation, olefin epoxidation, halogen-
ation, and hydride- and electron-transfer reactions.[3–9]
In PS II, the oxidation of water by the OEC induces the
generation of high-valent MnIV oxo species by a proton-
coupled electron transfer (PCET) mechanism. The oxygen
atom in the MnV oxo intermediate is derived from water.[1]
Biomimetic studies have established the formation of ruthe-
nium oxo complexes in water oxidation in the presence of a
strong oxidant, such as CeIV or [Ru(bpy)3]3+ (bpy = 2,2ꢀ-
bipyridine).[10–12] Very recently, we have generated mononu-
clear nonheme FeIV oxo complexes using water as an oxygen
source and CeIV as an one-electron oxidant.[13] Since it has
been proposed that high-valent manganese oxo species are
generated by the oxidation of water at the OEC of PS II, we
attempted to generate high-valent manganese oxo species in a
similar fashion.[14] Herein, we report the generation of a
Figure 1. a) X-ray structure of [MnII(BQCN)](CF3SO3)2 (1) showing
thermal ellipsoids at 30% probability. Hydrogen atoms are omitted for
clarity. Selected bond distances [ꢀ]: Mn1–N1 2.2048(16), Mn1–N2
2.3364(16), Mn1–O1 2.1491(16). b) Gas-phase DFT-optimized struc-
ture of 2 calculated at the B3LYP/LACVP level. Mulliken spin density
distribution: 1(Mn)=2.54 (2.65) and 1(O)=0.61 (0.50). Values in
parentheses indicate results from structures optimized with water
cluster (H2O)16 explicitly present.
[*] Dr. S. C. Sawant, X. Wu, Dr. J. Cho, Dr. K.-B. Cho, Dr. S. H. Kim,
Dr. M. S. Seo, Dr. Y.-M. Lee, Prof. Dr. W. Nam
Department of Bioinspired Science
Department of Chemistry and Nano Science and
Centre for Biomimetic Systems
Ewha Womans University, Seoul 120–750 (Korea)
Fax: (+82)2-3277-4441
E-mail: wwnam@ewha.ac.kr
Figures S1,S2 and Tables S1,S2 in the Supporting Informa-
tion for the synthesis, characterization, and structural data of
1) gave a green complex 2 with an absorption band at 630 nm
(e ꢁ 400mꢀ1 cmꢀ1) in CH3CN/H2O (9:1) or acetone/H2O (9:1)
at 08C (t1/2 ꢁ 10 h; Figure 2a). The intermediate 2 was also
synthesized by the reaction of 1 with iodosylbenzene (PhIO)
in CH3CN or acetone at ꢀ408C (t1/2 ꢁ 1 h).
Dr. M. Kubo, Prof. Dr. T. Ogura
Picobiology Institute, Graduate School of Life Science
University of Hyogo (Japan)
Prof. Dr. S. Shaik
Department of Organic Chemistry and The Lise Meitner-Minerva
Center for Computational Quantum Chemistry
The Hebrew University of Jerusalem (Israel)
[**] The research at EWU was supported by NRF/MESTof Korea through
the CRI, WCU (R31-2008-000-10010-0), and GRL (2010-00353)
Programs (to W.N.). The research at UH was supported by Grant-in-
Aid for scientific research (C) (no. 21570171; to T.O.) by MEXT
(Japan). S.S. thanks the Israel Science Foundation (Grant 16/06).
The intermediate 2 was then characterized by using
various spectroscopic methods. EPR spectroscopy was used
to demonstrate that 2 has an S = 3/2 MnIV center. The X-band
continuous wave (CW) EPR spectrum of 2 shows a broad
absorption band at geff ꢁ 4, which originates from the zero-
field splitting of the high-spin d3 (S = 3/2) manganese
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000819.
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Chemie
compound (Figure 2b).[7b,15] It has been shown that low-field
signals are prominent when the axial zero-field splitting (D) is
greater than the energy of the incident microwave photon
(hn = 0.32 cmꢀ1 for 9.68 GHz),[7b,8c,15] which is consistent with
our result. The simulated EPR spectrum was consistent with
the experimental spectrum when the axial zero-field splitting
parameter D = 2 cmꢀ1 and the rhombic zero-field parameter
E/D = 0 were used (Figure 2b).[16]
The electrospray ionization mass spectrum (ESI MS) of 2
shows prominent ion peaks at m/z 242.5, 263.0, and 484.1, the
mass and isotope distribution patterns of which correspond to
[MnIV(BQCN)(O)(H2O)]2+ or [MnIV(BQCN)(OH)2]2+ (cal-
culated m/z 242.6), [MnIV(BQCN)(O)(H2O)(CH3CN)]2+ or
[MnIV(BQCN)(OH)2(CH3CN)]2+ (calculated m/z 263.1), and
[MnIV(BQCN)(O)(OH)]+ (calculated m/z 484.2), respec-
tively. When the reaction was carried out in the presence of
isotopically labeled H218O, peaks corresponding to [MnIV-
(BQCN)(18O)(H218O)]2+ or [MnIV(BQCN)(18OH)2]2+, [MnIV-
(BQCN)(18O)(H218O)(CH3CN)]2+ or [MnIV(BQCN)(18OH)2-
(CH3CN)]2+, and [MnIV(BQCN)(18O)(18OH)]+ appeared at
m/z 244.5, 265.0, and 488.1, respectively (Figure 2c). The
experiment with labeled water thus demonstrates that the
oxygen atom in 2 originates from water.
The resonance Raman spectrum of 2, measured in
acetone/H2O (9:1) at ꢀ408C with 407 nm laser excitation,
showed an isotope-sensitive band at 707 cmꢀ1, which shifted
to 676 cmꢀ1 when [18O]-2 was generated in acetone/H218O
(9:1; Figure 2d). The observed isotopic shift of ꢀ31 cmꢀ1 with
18O substitution is in agreement with the calculated value
ꢀ1
ꢀ
(Dncalc = ꢀ31 cm ) for the Mn O diatomic harmonic oscil-
ꢀ1
ꢀ
lator. The observed Mn O frequency at 707 cm is lower
than those reported in a nonheme MnIV oxo complex [MnIV-
(H3buea)(O)]ꢀ (nMn O = 737 cmꢀ1
;
H3buea = tris[(N’-tert-
=
butylureaylato)-N-ethylene]aminato),[7b] and MnIV oxo por-
phyrin complexes (nMn O ꢁ 750 cmꢀ1),[17] but is higher than
=
that observed in a nonheme MnIV hydroxo complex [MnIV-
(Me2EBC)(OH)2]2+ (nMn OH = 664 cmꢀ1
;
Me2EBC = 4,11-
ꢀ
dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane).[8a]
In
ꢀ1
ꢀ
the latter case, the Mn OH band was shifted from 664 cm
to 660 cmꢀ1 when the intermediate was generated in the
presence of D2O.[8a] However, in the present study, we did not
observe the isotopic shift when 2 was prepared in the presence
of D2O, thus suggesting that 2 contains a terminal oxo ligand
and not hydroxo ligands. Based on the above spectroscopic
data, it follows that the intermediate 2 is a MnIV complex with
a terminal oxo ligand that has a double-bond character
between the manganese ion and oxygen atoms, and that the
oxygen atom in the MnIV oxo species originates from water
[Eq. (1)].
Figure 2. a) UV/Vis spectral changes showing the formation of 2 (red
line) in the reaction of 1 (2 mm; blue line) and [CeIV(NO3)6](NH4)2
(8 mm) in CH3CN/H2O (9:1) at 08C. The inset shows the time course
of the formation of 2 monitored at 630 nm. b) X-band CW-EPR
spectrum (red line) of 2 recorded at 5 K and the simulated spectrum
(black line). The hyperfine splitting (ca. 95 G) observed at gꢁ2 results
from a minor impurity of MnII species. c) ESI MS of [18O]-2 formed in
the reaction of 1 and CAN (4 equiv) in CH3CN/H218O (9:1) at 08C. The
insets show the observed isotope distribution patterns for [16O]-2 (left
panel, prepared with H216O) and [18O]-2 (right panel, prepared with
H218O). d) Resonance Raman spectra (407 nm excitation) of [16O]-2
(red line) generated in the reaction of 1 (4 mm) and 4 equiv of CAN in
acetone/H216O mixture (9:1) and [18O]-2 (blue line) formed in acetone/
H218O mixture (9:1) at ꢀ408C. The black line shows the intensity
difference between the two spectra. * indicates peaks that arise from
the solvent.
ꢀ2 eꢀ, ꢀ2 Hþ
2þ
2þ
½ðBQCNÞMnIIꢀOH2ꢂ
½ðBQCNÞMnIV¼Oꢂ
ð1Þ
ƒƒƒƒƒƒ!
DFT calculations at B3LYP/LACV3P*+//B3LYP/LACVP
level[18] were performed to elucidate the structural details of 2
(Figure 1b) and to compare the relative energy of 2 to its
structural tautomer [MnIV(BQCN)(OH)2]2+ (3; see the Sup-
porting Information for technical details). The calculations
show that the solvating water molecules play a key role in the
formation of 2 from the tautomer 3. For instance, when
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www.angewandte.org
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Communications
comparing the gas-phase energies of 2 and 3, the calculations
clearly predicted 3 to be lower in energy (in the order of
12 kcalmolꢀ1, see the Supporting Information). However,
upon addition of a water cluster with randomly distributed
water molecules around the oxygen ligands, 3 no longer exists
as an energy minimum. Instead, 3 spontaneously loses a
proton to the solution to form [MnIV(BQCN)(O)(HO)]+ and
H3O+. This MnIV species is computed to be about 1 kcalmolꢀ1
higher in free energy than 2 (Table S6 in the Supporting
Information). Furthermore, since the reaction mixture
becomes acidic in the presence of cerium ions, protonation
ꢀ
of the Mn OH group to form 2 is entirely plausible and the
corresponding optimized structure is the one as shown in
Figure 1b (see also Figure S7b in the Supporting Informa-
=
tion). The calculated Mn O bond length in 2 is 1.67/1.69 ꢁ
(gas phase/solvated). This bond length is similar to that found
in [MnIV(H3buea)(O)]ꢀ (1.706 ꢁ; determined by DFT calcu-
lations)[7b] and those found in MnIV or MnV oxo porphyrins
(ca. 1.68 ꢁ; determined by EXAFS analysis),[4b,19] in which
ꢀ
the Mn O bond has a double-bond character.
ꢀ
The oxidative reactivity of 2 was explored in the C H
bond activation of alkyl-subsituted aromatic compounds and
in the oxidation of aromatic compounds and benzyl alcohol.
Upon reaction with substrates such as xanthene, 9,10-
dihydroanthracene (DHA), 1,4-cyclohexadiene (CHD), and
fluorene, which have bond dissociation energies (BDEs) in
the range of 75.5–80 kcalmolꢀ1,[20] 2 was consumed with a
pseudo-first-order decay (Figure 3a; see also the Experimen-
tal Section in the Supporting Information for product
analysis). The pseudo-first-order rate constants increased
proportionally with substrate concentration (Figure S3 in the
Supporting Information), from which second-order rate
constants k2 were determined. The corresponding logk2’
values gave a linear correlation with the BDEs of the
substrates (Figure 3b). Furthermore, a kinetic isotope effect
(KIE) value of 3.4(3) was observed in the oxidation of
xanthene by 2. This value is much smaller than those obtained
in the oxidation of xanthene by a MnIV oxo porphyrin complex
(KIE value of 14)[4d] and nonheme FeIV oxo complexes (KIE
values of 10–20),[21] but is similar to those obtained in the
oxidation of DHA by [MnIV(Me2EBC)(OH)2]2+ (KIE value
of 3.3) and MnIV(Me2EBC)(O)2 (KIE value of 3.78).[8c] Based
on the correlation between reaction rates and BDEs of
substrates and the significant KIE value, we conclude that the
Figure 3. a) Changes in the UV/Vis spectrum of 2 (2 mm) upon
addition of xanthene (10 equiv, 20 mm) in CH3CN/H2O (9:1) at 08C.
Inset shows the time course of the decay of 2 monitored at 630 nm.
ꢀ
b) Plot of logk2’ of 2 against the C H BDE of substrates, such as
xanthene (75.5 kcalmolꢀ1), DHA (77 kcalmolꢀ1), CHD (78 kcalmolꢀ1),
and fluorene (80 kcalmolꢀ1). Second-order rate constants k2 were
determined at 08C and then adjusted for reaction stoichiometry to
ꢀ
yield k2’ based on the number of equivalent target C H bonds of
substrates (e.g., 2 for xanthene and fluorene and 4 for DHA and
CHD). c) Hammett plot of logkrel against sp of anthracene derivatives
in the reactions of 2 (1 mm) with para-substituted anthracene deriva-
tives (10 equiv with respect to 2) at 08C.
ꢀ
C H bond activation by 2 occurs by an H-atom abstraction
mechanism.
The MnIV oxo species 2 was also found to hydroxylate
anthracene and para-substituted anthracene derivatives. With
anthracene, the reaction rates increased proportionally with
the substrate concentration to give a second-order rate
constant of 2.0(2) ꢂ 10ꢀ1mꢀ1 sꢀ1 at 08C (Figure S4 and Exper-
imental Section in the Supporting Information). The elec-
tronic effect of the anthracene substituent is apparent from
Figure 3c, which shows a good linear correlation with the sp
of the substituents (i.e., a negative Hammett 1 value of
ꢀ1.8(2)), thus indicating that the manganese oxo group
attacks the aromatic ring by an electrophilic pathway. We
noted that negative Hammett 1 values of ꢀ3.9 and ꢀ8.0 were
observed in aromatic hydroxylation reactions by nonheme
FeIV oxo and FeIV oxo porphyrin p-cation radical complexes,
respectively.[22] Furthermore, a KIE value of 0.98(2) was
determined in the hydroxylation of undeuterated and deu-
terated anthracene derivatives by 2 (Figure S4 in the Sup-
porting Information), thus indicating the addition of an
electrophilic manganese oxo group to the sp2 center of
aromatic ring to form a s adduct.[22,23]
Finally, the intermediate 2 was shown to oxidize benzyl
alcohol to give benzaldehyde as a major product (ca. 90%
yield based on the intermediate generated), with a second-
order rate constant of 4.4(3) ꢂ 10ꢀ3 mꢀ1 sꢀ1 at 08C (Figure S5 in
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Angewandte
Chemie
11489; e) S. Fukuzumi, N. Fujioka, H. Kotani, K. Ohkubo, Y.-M.
the Supporting Information). The benzyl alcohol oxidation
was also carried out with deuterated benzyl alcohol
(C6D5CD2OH), for which a KIE value of 2.1 was obtained.
This KIE value is similar to that obtained in an intermolecular
competitive oxidation of benzyl alcohol and deuterated
benzyl alcohol (KIE value of 2.2) by a nonheme mangane-
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se(II) catalyst, [MnII(BQEN)]2+ (BQEN N,N’-dimethyl-
=
N,N’-bis(8-quinolyl)ethane-1,2-diamine) and peracetic acid
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value of ca. 50).[26]
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effective oxidant in oxidation reactions.
Received: February 10, 2010
Revised: August 27, 2010
Published online: September 21, 2010
Keywords: bioinorganic chemistry · enzyme models ·
.
manganese · oxygenation · reaction mechanisms
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