A novel single-site manganese(ii) complex of a pyridine derivative as a catalase mimetic for disproportionation of H2O2 in water

Article abstract of DOI:10.1039/c3dt50288k

A novel single site Mn(ii) complex was successfully synthesized and tested in the aqueous disproportionation of hydrogen peroxide. The complex was found to be stable (HR-XAS) and exhibits catalase-like activity in neutral pH. Theoretical calculations suggested a reaction mechanism involving two complexes, changes in metal oxidation state and proton shuttling. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt50288k

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A novel single-site manganese(II) complex of a pyridine
derivative as a catalase mimetic for disproportionation
of H O in water†
Cite this: Dalton Trans., 2013, 42, 7761
2
2
a
b,c
d
Małgorzata Zienkiewicz, Jakub Szlachetko,* Christian Lothschütz,
e
a
b
Maciej Hodorowicz, Agnieszka Jabłońska-Wawrzycka, Jacinto Sá* and
a
Barbara Barszcz*
Received 28th January 2013,
Accepted 18th March 2013
A novel single site Mn(II) complex was successfully synthesized and tested in the aqueous disproportiona-
tion of hydrogen peroxide. The complex was found to be stable (HR-XAS) and exhibits catalase-like
activity in neutral pH. Theoretical calculations suggested a reaction mechanism involving two complexes,
changes in metal oxidation state and proton shuttling.
DOI: 10.1039/c3dt50288k
www.rsc.org/dalton
as vitamin E derivatives, performed poorly in clinical trials due to
the low reaction rates. Catalytic antioxidants have much higher
1
. Introduction
9
1
Oxidative stress is an in vivo phenomenon related to the kinetic rates than stoichiometric antioxidants, making them
10
uneven production and elimination rates of reactive oxygen attractive for novel therapeutics. Most catalases contain four
species by endogenous antioxidants, generally enzymes. Super- porphyrinheme (iron) groups; however, some bacteria possess
−
11
oxide (O ˙ ) and hydrogen peroxide (H O ) are two of the best- di-Mn-containing catalases. It is not surprising that synthetic
2
2 2
12
13
characterized oxidative species known to cause oxidative mimics contain either Mn or Fe, nevertheless only synthetic
stress. The superoxide is produced in vivo by leakage from the oxidants contain single metal sites in their composition.
respiration pathway and the oxidative burst of neutrophils,
Low molecular weight Mn(III) salen complexes, such as
whereas the hydrogen peroxide is the product of superoxide EUK-134, exhibit potent and selective dual function, namely
2
3
14
dismutase. Both can be produced by sunlight UVA and UVB.
superoxide dismutase and catalase. The EUK class of Mn(III)
Excessive amounts of oxidants damage the proteins and salen complexes was found to be active for animal models of
1
5
16
17
lipids leading to cell death. Oxidative stress has been linked to a stroke, cardiac ischemia, Parkinson’s disease, reverse age-
18
19
plethora of pathophysiological conditions, such as Alzheimer’s related learning deficits in mice, autoimmune disease, kidney
4
5
6
7
20
21
and Parkinson’s diseases, aging, cancer, genetic damage,
reperfusion injury, and seizure-induced neuronal death.
Nitrogen-containing ligands, such as imidazoles, are
In nature, enzymes called catalases carry the catalytic dis- another class of single site Mn catalase-like complexes. Imid-
proportionation of hydrogen peroxide according to eqn (1): azoles and their derivatives with strong π-donating ability were
found to accelerate H disproportionation. The complexes
8
and tissue damage in cardiac ischemic/reperfusion injury.
2
2
2 2
O
2
3
2H2O2 ! 2H2O þ O2
ð1Þ show high activity in organic solvents but extremely low reac-
tivity or even none in aqueous solutions with a pH < 8 (physio-
Synthetic antioxidants can be divided into two classes, logical pH values). The activity improved when a base was
stoichiometric and catalytic. Stoichiometric antioxidants, such added as a promoter. In this paper we report a novel manga-
nese(II) complex with a pyridine substitute as a ligand. The
complex was stable and showed catalase-like activity in neutral
aqueous solution.
a
Institute of Chemistry, Jan Kochanowski University in Kielce, 15G Świętokrzyska
Str., 25-406 Kielce, Poland. E-mail: basiab@ujk.kielce.pl
b
Paul Scherrer Institute (PSI), 5232 Villigen, Switzerland.
E-mail: jakub.szlachetko@psi.ch, jacinto.sa@psi.ch; Tel: +41563102910
c
Institute of Physics, Jan Kochanowski Universityin Kielce, 25-406 Kielce, Poland
2. Experimental
d
Syngenta Crop Protection, Münchwilen AG, Breitenloh-5, CH-4333 Muenchwilen,
Switzerland
Materials
MnSO ·H
-hydroxymethylpyridine (2-CH
e
Faculty of Chemistry, Jagiellonian University, 30-060 Kraków, Poland
4
2
O purchased from the Merck Chemical Company,
OHpy) (Aldrich 99.7%) and
PrOH (P.O.Ch. Gliwice) were used as received.
†
Electronic supplementary information (ESI) available: Further refinements of
2
2
the crystal structure (Fig. S1 and S2, Table S1). CCDC 908641. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt50288k
i
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Synthesis of the Mn(II) complex
mirror. The collimated X-rays were monochromatized by
means of a double Si(111) crystal monochromator and focused
by a toroidally bent Rh mirror. On the sample the photon flux
A water solution (20 mL) of MnSO
4
·H
2
O (0.4199 g, 2.5 mmol)
OHpy (0.5421 g,
mmol) in 20 mL of pure PrOH. The molar ratio M : L was
: 2. The solution was stirred for 5 h at room temperature and
was added dropwise to a solution of 2-CH
5
1
2
1
1
−1
i
was 7–8 × 10 photons s with an energy resolution of ΔE/E
4
2
≈
1.4 × 10− and a spot size of 100 μm . For calibration a 4 μm
thick Mn foil was used. X-ray detection was performed using a
wavelength-dispersive spectrometer. To avoid any scanning
components during the acquisition, the spectrometer was
left to crystallize. After three months, fair olive crystals were
obtained. The crystals were filtered, washed with 20 mL of
EtOH and dried under vacuum. The product was collected
with 69% yield.
2
7
operated in the von Hamos geometry. The X-rays emitted
from the sample were diffracted by a Ge(660) crystal bent cylin-
drically to a radius of curvature of 25.4 cm. For the detection
of the diffracted photons, a 1D-array, single photon counting
MythenII was employed. The spectrometer was operated in the
vertical scattering geometry.
Methods
Chemical analysis. Elemental analysis was run on a Model
Elemental Vario EL III CHN Analyzer.
Thermal analysis. The thermal gravimetric analysis (TG/
DTG) was carried out in a TG/SDTA 851 Mettler-Toledo
thermo-balance. The experiment was performed in a nitrogen
e
Computational details
−
1
The calculations were performed using the Gaussian 09
program package, using the B3LYP hybrid functional with cor-
relation consistent basis set cc-PVDZ for the light atoms and a
pseudopotential contracting 10 electrons at the manganese
atmosphere at a heating rate of 5 K min using an Al O cru-
2
3
cible. 3.8651 mg of the complex and a temperature range of
5–800 °C were used. The thermo-analytical curves were
2
e
obtained using STAR System Mettler-Toledo software. DSC
differential scanning calorimetry) analysis was carried out
2
8
center.
(
using a DSC Mettler-Toledo Instrument in the standard closed
sample, static air atmosphere, and the heating rate of 5 °C
Catalytic test
−
1
min
.
The complex was tested in the disproportionation of hydrogen
peroxide at 25 °C. The reaction was performed in a sealed
semi-batch reactor, thermostated, and sonicated in an ultra-
Crystal structure determination
Diffraction data of the single crystal were collected at room sonic bath. Fig. 1 illustrates the reactor setup. 0.25 mmol of
3
temperature on an Xcalibur, Sapphire 2 diffractometer with the complex was charged into the reactor containing 80 cm of
3
graphite-monochromated MoK radiation (λ = 0.71073 Å). Cor- Milli-Q quality water, and continuously flushed with 25 cm
α
−
1
rections for Lorentz, polarization and absorption effects were min of argon. The reaction was started, after the stabiliz-
applied. The structures were solved by direct methods using ation of the mass spectrometer, by adding 20 mmol of H O
2 2
the SHELXS program package and refined using a full-matrix (30% w/w from Merck) through a spectrum with a syringe. The
least squares procedure on F using SHELXL-97. Anisotropic evolution of molecular oxygen was monitored by a quadrupole
2
4
2
5
2
22
displacement parameters for all non-hydrogen atoms and iso- mass-spectrometer (QMS) from Hiden Analytical. To deter-
tropic temperature factors for hydrogen atoms were intro- mine the amount of O
duced. The figures were made using DIAMOND software.
2
evolved, the QMS sensitivity factor was
determined using known concentrations of O and tracer
Ar m/z = 36, 0.3% abundance in pure Ar), a common
2
6
2
(
Infrared (IR) and Raman analysis
procedure in gas phase reactions monitored by a mass
spectrometer. The presented O evolution data represent exclu-
sively the performance of the catalyst because the un-catalysed
Infrared spectra were recorded on a Nicolet 380 FT-IR type
2
−
1
spectrophotometer in the 4000–500 cm spectral range using
an ATR-diffusive reflection method. Fourier transform Raman
scattering measurements (FT-RS) were performed at room
temperature with
spectrometer, resolution 4 cm
a PerkinElemer RamanStation™ 400F
−
1
.
Magnetic measurement
Magnetic measurements were carried out on a magnetic sus-
ceptibility balance (Sherwood Scientific) at room temperature
by Gouy’s method using Hg[Co(NCS) ] as a standard.
4
Resonant inelastic X-ray spectroscopy (RIXS)
The RIXS maps were performed at the SuperXAS beamline
at the Swiss Light Source, Paul Scherrer Institute, Switzerland.
The X-ray beam delivered by the 2.9 Tesla super-cooled
Fig. 1 Schematic representation of the reactor used to evaluate the catalytic
performance of the Mn complex. MFC: mass flow controller, QMS: quadrupole
bending magnet was collimated by a spherically bent Rh mass spectrometer.
7
762 | Dalton Trans., 2013, 42, 7761–7767
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O
2
evolution was subtracted from the measured data. The un- Table 2 Crystal data and structure refinement for [Mn(2-CH
2 4
OHpy)(SO )-
2 n
(H O)]
catalysed data were estimated using blank experiments, in
which we followed the same procedure as described, except
the presence of the catalyst in the reaction mixture.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system, space group
Unit cell dimensions
a (Å)
6 9 6
C H MnNO S
278.14
293(2)
0.71073
1
Monoclinic, P2 /c
3
. Results and discussion
9.4275(8)
7.9796(5) Å
13.0018(9)
90
91.422(7)
90
977.79(12)
4, 1.889
1.571
b (Å)
c (Å)
Elemental analysis was performed to confirm the stoichio-
metry of the complex. The CHN composition for C NO
in %) obtained experimentally was C 26.08, H 3.17, and
6
H
9
6
SMn α (°)
β (°)
γ (°)
(
N 4.97, consistent with calculated values (C 25.91, H 3.26,
N 5.04).
The results of the thermal decomposition experiment (it is
after all just one experiment) are compiled in Table 1. The
Mn(II) complex decomposes in three stages in a N
phere. The first step begins at 100 °C and is attributed to the
3
Cell volume (Å )
Z, Calculated density (g cm−
3
)
)
Absorption coefficient (mm−
F(000)
1
564
Crystal size (mm)
0.30 × 0.25 × 0.20
3.13 to 27.72
2
atmos- θ Range for data collection (°)
Limiting indices
−8 ≤ h ≤ 12, −10 ≤ k ≤ 4,
−
16 ≤ l ≤ 15
release of H O molecules and part of the organic ligand
2
Reflections collected/unique/observed
3726/2124 [Rint = 0.0195]
(observed 10.16%, calculated 11.16%). The dehydration is an [I > 2σ(I)]
Completeness to 2θ (%)
Absorption correction
2θ = 25.00° (99.9)
Semi-empirical from
equivalents
endothermic process at maximum 225 °C. The second mass
loss of ca. 33.90% (calculated 34.56%) corresponds to the loss
of pyridine ring fragments. The last stage occurring at 345 °C Maximum and minimum transmission
is ascribed to the release of SO2 from the sulphate ion
0.7440 and 0.6500
Full-matrix least-squares
on F
Refinement method
2
2
9
(observed 22.98%, calculated 23.00%). The final residue of
Data/restraints/parameters
Goodness-of-fit on F
Final R indices [I > 2σ(I)]
R indices (all data)
2124/2/148
1.042
2
the decomposition is MnO
The single crystal data and structure refinements are
reported in Table 2. The Mn(II) complex crystallizes with
2
.
R
R
1
= 0.0334, wR
= 0.0440, wR
2
= 0.0801
= 0.0860
0.390 and −0.483
1
2
Largest differences in peak
3
the orthorhombic P2
1
/c space group. A perspective view of the and hole (e Å )
complex is shown in Fig. 2. The asymmetric unit of the
complex consists of two bivalent manganese metal centers,
two water molecules, two 2-hydroxymethylpyridines and two
sulphate(VI) anions. The Mn(1) cation is six-coordinated by one
water molecule (O(11)), one 2-CH OHpy (acting as a bidentate
2
ligand), through N(2) of the pyridine ring, and an oxygen atom
O(8) of the hydroxymethyl group. The chelating five-membered
(N(2)–Mn(1)–O(8)–C(7)–C(1)) does not exhibit planarity due to
values of torsion angles of −29.60(2)°. Mn(1) also coordinates
to three oxygen atoms O(10), O(9), O(12) of three mono-tri-
dentate bridging sulphate(VI) ions (ESI Fig. S1,† μ ). A similar
3
2
−
coordination mode of SO
(
4
ions is also reported in the Ln(III)
Ln = La, Pr, Nd, Sm), Mn(II) and Cd(II) complexes. High
3
0
charge and denticity/bridging capability of sulphate(VI) groups
gives a great degree of coordination flexibility, which provides
intriguing and unique architectures. According to the Harris
Table 1 Thermo-analytical results of a manganese(II) complex
Fig. 2 DIAMOND diagram showing the coordination environment of Mn(II) in
Mn(2-CH OHpy)(SO )(H O)]
Mass loss/%
[
2
4
2
n
.
TG
Step range/°C Measured Calculated Assignment
classification, a sulphate anion can exhibit from the μ to the
μ10 potential types of coordination modes. The Mn–N bond
length is 2.247(2) Å whereas the Mn–O distances are in the
I
100–230 10.16
230–345 33.90
11.16
34.56
H
2
O, 0.12 L (fragments of
2
3
1
aromatic ring)
0.88 L (fragment of ligand
donor atom containing)
II
range 2.1022(2)–2.2325(2) Å, these distances being compar-
III
Σ
345–800 22.98
23.00
68.72
SO
2
2
7
67.04
able to those reported elsewhere.
Coordination sphere
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(a chromophore of {MnO N} type) is distorted octahedral with the parallel-displaced aromatic rings of 2-CH OHpy, giving a
5 2
a basal plane formed by N(2), O(8), O(11), and O(12) atoms. 3D network (Fig. 3). The distance between centroids of
Apical positions are occupied by O(9) and O(10) atoms. The 3.680(2) Å (offset 0.948 Å) corresponds well with the relatively
distortion in a Mn(II) complex is apparent not only in bond- found parameter in a large number of organometallic com-
lengths but also in bond-angles (O(12)–Mn(1)–N(2), 98.28°, plexes. It is also interesting to notice that the coordinated
N(2)–Mn(1)–O(8), 72.37°) (Table 3).
water molecules form intermolecular hydrogen bonds with the
4
–Mn–SO –
Detailed analysis of the single crystal data can be found in sulfate ligand serving to cross-link the –Mn–SO
4
Table S1, Fig. 3 and S2.† Fig. S2† shows the 2D fragment of the chains (Table 3). The Mn⋯Mn distances are in the range
network with the marked mimic system of the catalase viewed 5.1117(3)–6.679(3) Å.
along the [1 0 0] direction. For clarity, the ring ligands are
2
The most important spectral bands of the [Mn(2-CH OHpy)-
omitted. The complexes are held together with donor bifur- (SO )(H O)] complex and the free ligand, and their assign-
4
2
n
cated H-bonds between the water molecule and the oxygen ments are listed in Table 4. The IR spectrum is given in
atoms of the sulfate ion, or between the hydroxymethyl group Fig. S3.†
2
−
of the pyridine ring and the O atoms of the SO4 ions. The
layers are connected together by non-covalent intermolecular aqueous disproportionation of hydrogen peroxide at neutral
interactions of π-aromatic clouds, which are observed between pH is depicted in Fig. 4, and the cumulative O liberation is
presented in ESI Fig. S4.† It is easily observable that the O2
evolution started as soon as H was added to the reaction
mixture. The O evolution increased steadily for 10 min before
reaching a stable value. The reaction rate calculated at steady
The catalytic activity of [Mn(2-CH OHpy)(SO )(H O)] in the
2 4 2 n
2
2 2
O
Table 3 Selected bond lengths (Å) and valence angles (°) for a Mn(II) complex
2
Bond
lengths (Å)
Valance
angles (°)
−
1
evolution was found to be 0.19 μmol min , which equates to a
turnover frequency (TOF) of 7.44 × 10− min−1. The activity was
found to be stable for the duration of the reaction. Based on
the measured TOF, in order for each manganese center to
4
Mn(1)–N(2)
Mn(1)–O(8)
Mn(1)–O(9)
Mn(1)–O(10)
Mn(1)–O(11)
Mn(1)–O(12)
2.247(2)
2.2325(2)
2.2020(2)
2.1770(2)
2.166(2)
2.1022(2)
O(12)–Mn(1)–O(11)
O(12)–Mn(1)–O(10)
O(11)–Mn(1)–O(10)
O(12)–Mn(1)–O(9)
O(11)–Mn(1)–O(9)
O(10)–Mn(1)–O(9)
O(12)–Mn(1)–O(8)
O(11)–Mn(1)–O(8)
O(10)–Mn(1)–O(18)
O(9)–Mn(1)–O(8)
O(12)–Mn(1)–N(2)
O(11)–Mn(1)–N(2)
O(10)–Mn(1)–N(2)
O(9)–Mn(1)–N(2)
99.29(8)
84.26(7)
95.64(7)
95.42(7) react at least once, the reaction must be carried out for at least
85.68(7)
178.68(7)
170.02(8)
2
0 h. Therefore, the reaction mixture was sealed and left to
react for five extra days. After five days the reaction rate
90.41(7) dropped by ca. 20%, which is due to the decrease of H
2 2
O
92.52(7)
87.59(7)
98.28(8)
161.28(8)
92.48(7)
86.29(7)
Table 4 The characteristic IR (Raman) bands for the Mn(II) complex with
2
2-CH )
OHpy and a free ligand (in cm−
1
Assignment
2
2-CH OHpy
2 4 2 n
[Mn(2-CH OHpy)(SO )(H O)]
ν
ν
δ
ν
OH
H
2
O
—
3213b
1054
3342
3222
OH ligand
C–O
1097 (1065), 1044 (1023) Δ = 43
1606 (1607), 1594, 1572 (1573),
1491 (1096), 1434 (1453) Δ = 12
1165 (1160), 1157 (1137)
CvC
,
CvN
1595, 1572,
479, 1435
1
Breath. ring
vibrations
νSO
1149
2−
4
—
1059 (1050)
Fig. 3 View of a 3D supramolecular network constructed by interlayer π⋯π
interactions. View along the [0 1 0] direction. Cg(1) denotes the gravity six-mem-
bered [N(2), C(1), C(6), C(5), C(4), C(3)] ring centre.
Fig. 4 Oxygen evolution rate from the disproportionation of H
ence of [Mn(2-CH OHpy)(SO )(H O)] in water at 25 °C.
2 2
O in the pres-
2
4
2
n
7
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concentration in solution. Nevertheless, the catalyst remained complex changed the oxidation state from Mn(II) to Mn(III), i.e.,
active, which confirms that the reaction is catalytic, not there is no evidence of complexes in oxidation of Mn(II).
stoichiometric.
2 2
The addition of H O resulted in a shift in the pre-edge
In situ RIXS maps were collected to determine the electronic feature to lower energy and a drop in white-line intensity,
structure of the metal center. The HR-XAS spectrum of a crys- indicative of the Mn being reduced to Mn(II). From signal
talline complex (Fig. 5a) has a pre-edge feature at 6536.5 eV deconvolution, we estimated that 20% of the Mn centers got
3
2
and an inflection point at 6543.9 eV ascribed to Mn(II). The reduced from Mn(III) to Mn(II) due to the presence of H
room temperature magnetic moment of the powdered complex The changes in the metal oxidation state upon addition of
is 5.60 B.M. The calculated value of μeff is close to that suggest the involvement of a redox mechanism.
expected for uncoupled five independent spins (S = 5/2; μeff Theoretical calculations were performed to establish a poss-
.92 B.M.), indicative of a high-spin complex with a valence ible reaction pathway (Fig. 6). According to calculations, the
2 2
O .
2 2
H O
=
5
3
3
state of Mn(II), consistent with the XAS experiments.
initial step is the activation of the complex, i.e., oxidation of
Upon dissolution in water, the pre-edge and the inflection Mn(II) to Mn(III), as detected by RIXS experiments. This is
point shifted by about 0.5 eV to higher energy, and the white- achieved due to the release of the proton bonded to the bridge
line intensity increased, suggestive of change in the oxidation oxygen. This was confirmed by the drop in reaction pH, from
state, i.e., from Mn(II) to Mn(III). The changes are evident in the 7.2 to 5.5. After activation, H
full RIXS maps presented in Fig. 5 (I) and (II), and in the res- center (step 2), resulting in the loss of coordinated water mole-
onant Kβ XES (Fig. 5c) analysis. However, with non-resonance cules and abstraction of a proton from H . Since we did not
2 2
O is coordinated to the metal
2 2
O
Kβ XES only a slight shift in energy was detected. It is worth use a base to do so, one of the ligands acts as a proton accep-
mentioning that within our experimental sensitivity all the tor. From the quantum chemical calculations we determined
the internal energy (ΔU) for the two possible proton acceptors
in the structure, namely sulphate and deprotonated bridge
oxygen. We calculated a ΔU of −69.89 kJ, when the proton is
abstracted by the sulphate group, which is much lower than
the alternative site.
As aforementioned, most of the single site Mn catalase-like
complexes containing nitrogen ligands, such as imidazoles, do
not work particularly well in water, and not at all under acidic
conditions, which contrasts with what we report. Therefore, we
performed calculations to estimate the effect of water on the
reaction. The calculations assigned a pivotal role to water,
Fig. 5 RIXS measurements of the complex (I) solid; (II) complex in water and
(III) complex during the reaction. The side panels (a) to (c) represent cuts across
different regions in the RIXS maps for the three measured samples. (a) High-
resolution X-ray absorption spectroscopy (HR-XAS); (b) non-resonant Kβ X-ray
emission spectroscopy (XES) and (c) resonant Kβ XES. Black: solid; red: complex
in water; blue: complex during the reaction.
Fig. 6 Proposed reaction mechanism for H
CH OHpy)(SO )(H O)] in water.
2 2
O disproportionation over [Mn(2-
2
4
2
n
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since its presence decreases dramatically the reaction complex concentration. A tenfold increase in complex’s con-
enthalpy:
centration leads to a dramatic increase of oxygen liberation,
however, under these conditions the RIXS maps could not be
measured due to experimental setup limitations. Nevertheless,
the presented results confirm the given complex as a catalyst,
and corroborate both the theoretical calculations and the
measured RIXS.
ðaÞ MnðpyolÞSO4ðaqÞ þ H2O2 ! MnðpyolÞSO4ðH2O2Þ þ H2O
ΔE ¼ ꢀ69:89 kJ
ðbÞ MnðpyolÞSO4ðaqÞ þ H2O2 ! MnðpyolÞSO4ðH2O2ÞðaqÞ
ΔE ¼ ꢀ161:97 kJ pyol ¼ 2-CH2OHpy
4
. Conclusions
Reaction (b) is energetically favoured because water sits in
between the sulphate and peroxo ligand, shuttling proton
from the substrate to the acceptor (Fig. 7). We cannot make
any statements concerning the entropy of this process, but
there is no doubt about the fact that reaction (a) should be
entropically favoured.
In conclusion, we propose a novel single site manganese(II)
complex active in aqueous disproportionation of H O at
2
2
physiological pH. The proposed reaction mechanism suggests
an initial step in which the manganese complex is oxidized
due to the loss of a bridge oxygen proton. This is followed by
peroxide coordination and proton abstraction by the sulphate
group mediated by water. For the liberation of molecular
oxygen and catalyst regeneration, a second complex molecule
is required, which affects the overall catalytic performance
because the reaction rates are diffusion limited. The proposed
mechanism is the first attempt to try to understand single site
Mn complexes reactivity, however, this is a preliminary work
and much more must be done to fully understand the reaction
pathway of biomimetic small-molecule models of Mn catalase.
The reaction proceeds by abstraction of the second proton
from the coordinated peroxide group, which requires a second
complex with Mn(III) oxidation. The process in essence is
similar to the first abstraction, namely water shuttles the
proton, which gets coordinated to the sulphate group, leading
to a fast release of molecular oxygen. The catalyst rehydrates
and the sulphate group remains protonated. This part of the
3
4
mechanism has some parallels to the Naruta et al. model, in
which the dioxygen evolution in aqueous media was believed
to involve the formation of a hydroxide-bridge between metal
centers. However, in their case the initial Mn oxidation state
was (III), which was oxidized to Mn(IV), and both metal centers
were bonded to a porphyrin.
Acknowledgements
In order for the catalytic cycle to be closed, the sulphate We thank the SuperXAS beam line at the Swiss Light Source
group must be regenerated to the un-protonated form. Calcu- for providing the beam time.
lations suggest that a second H O molecule coordinates
2
2
between two complex molecules with protonated sulphate
groups, which induces a reduction of the metal center from Notes and references
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1
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K. Hensley, K. A. Robinson, S. P. Gabbita, S. Salsman and
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peroxide, once more mediated by water, which results in the
dissociation of the peroxide molecule into two water molecules
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Mn(III).
The theoretical calculations reveal the reason for the low
TOF measured. Specifically two complexes need to come
together at different parts of the catalytic cycle with the active
centre having specific characteristics. The complex’s low con-
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