Reactivity and O2 Formation by Mn(IV)- and Mn(V)-Hydroxo Species Stabilized within a Polyfluoroxometalate Framework

Article abstract of DOI:10.1021/jacs.5b03456

Manganese(IV,V)-hydroxo and oxo complexes are often implicated in both catalytic oxygenation and water oxidation reactions. Much of the research in this area is designed to structurally and/or functionally mimic enzymes. On the other hand, the tendency of such mimics to decompose under strong oxidizing conditions makes the use of molecular inorganic oxide clusters an enticing alternative for practical applications. In this context it is important to understand the reactivity of conceivable reactive intermediates in such an oxide-based chemical environment. Herein, a polyfluoroxometalate (PFOM) monosubstituted with manganese, [NaH₂(Mn-L)W₁₇F₆O₅₅]^q-, has allowed the isolation of a series of compounds, Mn(II, III, IV and V), within the PFOM framework. Magnetic susceptibility measurements show that all the compounds are high spin. XPS and XANES measurements confirmed the assigned oxidation states. EXAFS measurements indicate that Mn(II)PFOM and Mn(III)PFOM have terminal aqua ligands and Mn(V)PFOM has a terminal hydroxo ligand. The data are more ambiguous for Mn(IV)PFOM where both terminal aqua and hydroxo ligands can be rationalized, but the reactivity observed more likely supports a formulation of Mn(IV)PFOM as having a terminal hydroxo ligand. Reactivity studies in water showed unexpectedly that both Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM are very poor oxygen-atom donors; however, both are highly reactive in electron transfer oxidations such as the oxidation of 3-mercaptopropionic acid to the corresponding disulfide. The Mn(IV)-OH-PFOM compound reacted in water to form O₂, while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K⁺, Rb⁺, and Cs⁺) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of Li⁺ and Na⁺ did not lead to aggregates. Aggregation leads to a lowering of the entropic barrier of the reaction without changing the free energy barrier. The observation that O₂ formation is fastest in the presence of Cs⁺ and ～fourth order in Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O₂ formation in an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermediate appears to be slower for O₂ formation.

Full text of DOI:10.1021/jacs.5b03456

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Article
Reactivity and O2 Formation by Mn(IV)- and Mn(V)-Hydroxo
Species Stabilized within a Polyfluoroxometalate Framework
Roy E. Schreiber, Hagai Cohen, Gregory Leitus, Sharon
G. Wolf, Ang Zhou, Lawrence Que, and Ronny Neumann
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b03456 • Publication Date (Web): 12 Jun 2015
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Journal of the American Chemical Society
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Reactivity and O Formation by Mn(IV)- and Mn(V)-Hydroxo Species
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Stabilized within a Polyfluoroxometalate Framework
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Roy E. Schreiber, Hagai Cohen, Gregory Leitus, Sharon G. Wolf, Ang Zhou, Lawrence Que,
¶
†
Jr., and Ronny Neumann*
†
‡
¶
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100
Department for Chemical Research Support, Weizmann Institute of Science, Rehovot, Israel 76100
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, MN 55455,
United States
Ronny. Neumann@weizmann.ac.il
ABSTRACT: Manganese(IV,V) hydroxo and oxo complexes are often implicated in both catalytic oxygenation and water
oxidation reactions. Much of the research in this area is designed to structurally and/or functionally mimic enzymes. On
the other hand, the tendency of such mimics to decompose under strong oxidizing conditions makes the use of molecular
inorganic oxide clusters an enticing alternative for practical applications. In this context it is important to understand the
reactivity of conceivable reactive intermediates in such an oxide-based chemical environment. Herein, a polyfluoroxo-
q–
metalate (PFOM) mono-substituted with manganese, [NaH (Mn-L)W F O ] , has allowed the isolation of a series of
2
17
6
55
compounds, Mn(II, III, IV and V), within the PFOM framework. Magnetic susceptibility measurements show that all the
compounds are high spin. XPS and XANES measurements confirmed the assigned oxidation states. EXAFS measurements
indicate that Mn(II)PFOM and Mn(III)PFOM have terminal aqua ligands and Mn(V)PFOM has a terminal hydroxo lig-
and. The data is more ambiguous for Mn(IV)PFOM where both terminal aqua and hydroxo ligands can be rationalized,
but the reactivity observed more likely supports a formulation of Mn(IV)PFOM as having a terminal hydroxo ligand. Re-
activity studies in water showed unexpectedly that both Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM are very poor oxygen-
atom donors, however, both are highly reactive in electron transfer oxidations such as the oxidation of 3-
mercaptopropionic acid to the corresponding disulfide. The Mn(IV)-OH-PFOM compound reacted in water to form O
2
+
+
while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K , Rb , and
+
Cs ) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of
+
+
Li and Na did not lead to aggregates. Aggregation leads to a lowering of the entropic barrier of the reaction without
+
th
changing the free energy barrier. The observation that O formation is fastest in the presence of Cs and ~4 order in
2
Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O formation in
an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermedi-
2
ate appears to be slower for O formation.
2
oxometalates, typically anionic oxide clusters of molyb-
denum and tungsten, are a class of compounds that are
indeed stable to even very strongly oxidizing environ-
ments such as ozone that also can be soluble in water.
Importantly, insertion of a transition metal into the poly-
oxometalate to form so-called transition metal-
substituted polyoxometalates can lead to a formulation
where a transition metal is ligated by a polyoxometalate.
First row transition metal (Co, Mn) containing polyoxo-
metalates have been reported as water oxidation cata-
Introduction
Manganese-based catalysts are of interest in various re-
actions involving low valent species, for example as mim-
ics of superoxide dismustase and hydrogen peroxide cat-
•
–
1,2
alases that decompose O2 and H O . On the other hand
2
2
higher valent species are important in the context of wa-
3
4
ter oxidation and oxygenation reactions. A key issue in
this later field has been the isolation and identification of
high valent manganese species, such as Mn(V)-oxo,
Mn(IV)-oxo and Mn(IV)-hydroxo species, designed to
lead to understanding of manganese-based oxygenations
9
lysts, and Mn containing polyoxometalates have been
1
0
used in various oxygenation reactions. Such polyoxo-
metalates acting as ligands have also been shown to have
a good potential to stabilize high oxidation state species.
In this area a Mn(V)-oxo species was identified as an ef-
and water oxidation, that is, O evolving reactions. Such
investigations have been carried out using various coor-
2
5
6
dination platforms such as porphyrins, corroles, cor-
7
8
1
1
rolozines, and others. For the development of practical
catalysts it is important to develop and study compounds
that will be stable in highly oxidizing environments. Poly-
fective oxygen donor in an organic solvent, and a dimeric
Co(III)-oxyl species was efficient for O formation and C-
2
1
2
H bond activation in water. Transition metal substituted
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polyfluoroxometalates of the quasi Wells-Dawson struc- isomer can be assured by carefully controlling the tem-
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ture, Figure 1, are a sub-class of the analogous polyoxo-
metalate compounds, where an electron withdrawing but
π-donating fluorine atom is an axial ligand to the substi-
tuted transition metal and trans to the purported reaction
site. These polyfluoroxometalates have been only sparsely
studied, for example, as epoxidation catalysts with H O
perature of the preparation to 80-85 °C; lower tempera-
tures lead to some formation of the α isomer. This indi-
2
cates that the α isomer is thermodynamically favored.
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19
The F NMR spectral analysis and peak assignment with
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19
H/ F 2D HETCOR NMR are presented in the supporting
2
2
information, Figures S1-S3. It was assumed that after the
1
3
(
TM = Ni(II)) and as catalysts for aerobic hydroxylation
metathetical exchange of Zn(II) with Mn(II) the α isomer
1
and oxidative dehydrogenation of alkylated arenes(TM =
is retained. Interestingly, based on research on metathet-
ical exchange of Zn(II) with other metals it was discov-
ered by NMR experiments that for a Co(III) substituted
compound there can exist both a dimeric and monomeric
1
4
V(V)). Their investigation in the context of stabilization
of high valent species in water not been reported nor has
there been a study of their reactivity.
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form. The paramagnetism of Mn(II)PFOM precludes
In the research presented below we present the synthe-
sis and characterization of a series of manganese substi-
such a NMR analysis in this case, but we were able to
crystallize some of the dimer. The structure obtained by
X-ray diffraction, Figure S4, clearly shows that the substi-
tuted
polyfluoroxometalate
anions,
[NaH (Mn-
2
q–
L)W F O ] where Mn-L is Mn(II)-H O, Mn(III)-H O,
1
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2
2
tution of Mn(II) is in the α “belt” position. Additional
1
Mn(IV)-OH and Mn(V)-OH. Furthermore, we present the
quite surprising reactivity profiles in water of the Mn(IV)-
OH and Mn(V)-OH containing compounds. Both are very
effective in electron transfer oxidation but are surprising-
ly very poor oxygen-atom donors. The Mn(V)-OH com-
pound is stable in water, while the Mn(IV)-OH com-
oxidation states of manganese, III, IV and V, are accessi-
ble as presented in Scheme 1 by oxidation with peroxodi-
2–
sulfate, S O8 . As will be shown below, only monomers of
2
these compounds were obtained.
pound reacts to yield O . In the latter reaction we have
2
Scheme 1. Preparation of manganese substituted
polyfluoroxometalates.
observed a strong catalytic effect of alkali cations that is
explained by formation of aggregates in solution and
shows the possible viability of a tetra Mn(IV)-OH inter-
mediate as an oxygen forming species.
The efficient formation of Mn(V)PFOM requires elevat-
ed temperatures, and the synthesis of Mn(IV)PFOM is
–
more practical using ozone or monopersulfate, HSO ,
5
delivered as the triple salt, Oxone. The manifestation of
the different colors can be observed in the UV-vis spectra,
Figure 2.
Figure 1. Ball and stick (left) and polyhedral (right) represen-
q–
tation of a [NaH (TM-H O)W F O ] polyfluoroxometalate
2
2
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55
anion where the transition metal (TM) is in the α “belt” posi-
1
tion (see below). Na-green, H-gray, TM-blue, W-black, F-
yellow, O-red.
Results and Discussion
Synthesis and characterization. The manganese(II)
substituted polyfluoroxometalate with an aqua ligand at
the terminal position, Figure 1, where TM = Mn(II),
K [NaH Mn(II)(H O)W F O ], Mn(II)PFOM was known
from the literature. The procedure for its synthesis in-
volves first the preparation of the analogous zinc substi-
Figure 2. UV-vis spectra of the various manganese substitut-
ed polyfluoroxometalates, 3.4 mM in water. Peaks:
Mn(III)PFOM – 493, 521, 746 nm; Mn(IV)PFOM – 540 (sh),
9
2
2
17
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1
3
6
15 (sh) nm; Mn(V)PFOM – 576, 718 nm
tuted
compound,
K [NaH Zn(II)(H O)W F O ],
9 2 2 17 6 55
In order to confirm that the monomeric quasi Wells-
(Zn(II)PFOM) followed by metathatical exchange of
Zn(II) with Mn(II). In principle, the Zn/Mn atom may
occupy an α “belt” or an α “capped” position. In a novel
Dawson structure of the polyfluoroxometalates was re-
tained during the oxidation reactions, single crystal X-ray
diffraction measurements were made. The crystal struc-
tures, Figure S5, see also the cif files in the supporting
information, clearly show that indeed the original basic
1
2
observation reported here, we found that in the initial
preparation of Zn(II)PFOM, the formation of only the α1
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structure was unchanged as shown in Figure 1 although
there is a disorder of the location of manganese in the α1
belt” position preventing clear identification of the ter-
minal ligand by this method. This also prevents the de-
termination of the manganese oxidation state, and of
course spin state, by X-ray diffraction through bond va-
lence sum (BVS) analysis. However this information can
be obtained from XPS and XAS (XANES and EXAFS) and
magnetic susceptibility measurements.
These features correspond to crystal field transitions from
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the core 1s levels to the half-occupied or empty 3d levels,
which can gain some 4p character depending on the ex-
“
1
7
tent of deviation from centrosymmetry. Thus, complexes
with more distorted geometries exhibit larger pre-edge
areas than the complexes with less distorted geometries.
A 50:50 pseudo Voigt function was utilized to model both
the rising edge and the peak so as to get the pre-edge area
value, Table 1 and Figure S8. The data in Table 1 show that
all the MnPFOM species share a pre-edge peak at 6540.6
The XPS data are summarized in Figure 3 where the dif-
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±
0.5 eV, which increases in area with the Mn oxidation
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ference in binding energies between the Mn 2p and W
18
state. The 10-fold increase in the pre-edge peak area ob-
served on going from Mn(II) to Mn(V) likely reflects a
growing distortion from octahedral symmetry as the Mn
center gets higher in oxidation state.
7/2
f
3/2
7/2
4
orbitals, Mn 2p – W 4f , is presented; more details
can be found in Figure S6. This data presentation was
carried out to rectify charging effects implicit to the XPS
technique, which gives reliable corrected binding energies
under the realistic assumption that the binding energy of
7/2
the W 4f electrons do not change as a function of the
oxidation state of manganese. As one may observe, an
excellent linear correlation between the corrected binding
energy and the proposed oxidation state of manganese; a
difference of ~0.31 eV in binding energy between each
oxidation state was found.
X-ray absorption spectroscopic measurements were
made as well in order to verify the assignment of the
manganese oxidation state in the MnPFOM series and
gain some structural insight. The X-ray absorption near-
edge structure (XANES) spectra of the four MnPFOM
complexes are shown in Figure 4 and the corresponding
properties are summarized in Table 1. Visual examination
of the spectra reveals the edge energy increasing from
Mn(II)PFOM to Mn(V)PFOM; a more quantitative analy-
sis of the data indicates a 2- to 3-eV step for each incre-
ment in the Mn oxidation state. As in the case of the XPS
measurements, there is a very good linear correlation be-
tween the oxidation state of manganese and the K-edge
energy, Figure S7. Such a trend has been previously re-
ported for a series of manganese oxides, MnO, Mn O
Figure 4. Mn K-edge XANES of MnPFOM species measured
at 30 K. Inset: Pre-edge areas of the MnPFOM species.
Table 1. Mn K-edge XANES and EXAFS data for the
MnPFOM series (E = 6539 eV for Mn metal).
0
E
0, eV
E
,
Pre-edge Best EXAFS
pre-edge
eV
area
fits
3
4
1
6
Mn O , and MnO .
2
3
2
Mn(II)
6547.0
6540.1
6540.2
1.6
6O @ 2.13 Å
a
Mn(III) 6550.3
3.5
3O @ 1.93 Å
2
O @ 2.20 Å
4O @ 1.92 Å
O @ 2.23 Å
Mn(IV) 6552.4
6540.6
6541.1
7.7
2
Mn(V)
6554.5
15.4
1O @ 1.78 Å
4O @ 1.92 Å
1O @ 2.30 Å
Mn K-edge energies for MnO (6546.2 eV), Mn O (6550.1
2
3
19
eV) and MnO (6553.5 eV) have been reported. (a) The best
2
fit uses only five scatterers. this is well within the standard
error of 25% for determining coordination numbers and is
fully consistent with a 6-coordinate Mn center for the whole
series.
3
/2
EXAFS data were collected and analyzed for all four
complexes in the series. The best fits are listed in Table 2;
more detailed analysis and the spectra may be found in
the supporting information, Figures S9-S12. Since the
atomic mass and size between O and F are quite similar it
is not possible to differentiate the scatters between them
Figure 3. Corrected binding energies of Mn 2p as a function
of the oxidation state of manganese.
In contrast to the K-edge energies, the pre-edge fea-
tures before the main K-edge crest provide different in-
sights into the nature of each Mn center in the series.
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from the EXAFS data. Thus, the spectrum of Mn(II)PFOM mind that Mn(IV)-OH-PFOM reacts very fast (<1 s) with
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is best fit with one shell of 6 O scatterers at 2.13 Å (Figure
S10). These bond lengths are consistent with bond lengths
observed in first row transition metal substituted polyox-
ometalates where the transition metal is in the same
Mn(II)-H O-PFOM; Mn(V)OH-PFOM does not react with
2
Mn(III)-H O-PFOM and Mn(V)OH-PFOM reacts rather
2
slowly, t = 6.5 min, with Mn(II)-H O-PFOM (equations
1
/2
2
1- 3).
nearest
neighbor
environment,
such
as
1
2– 19
[ZnW(MnH O) (ZnW O ) ] . This result, together with
2
2
9
34 2
the observation that this complex has the smallest pre-
edge area of the series, is consistent with a highly sym-
5
metric octahedral center, as expected for a high-spin d
+
-
-
O-atom transfer to the water soluble phosphine (Na /
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Mn(II) center. On the other hand, Mn(III)PFOM should
exhibit a Jahn-Teller distortion and is best fit with two
subshells of O atoms at 1.93 and 2.20 Å (Figure S11).
Mn(IV)PFOM is best fit with 4 O scatterers at 1.92 Å and 2
O scatterers at 2.23 Å (Figure S12). That an O scatterer at
1.7 Å is not required to obtain a good fit excludes the pos-
+
O SC H ) P to the corresponding phosphine oxide (Na /
3
6
4 3
O SC H ) P=O with the two manganese hydroxo species
showed significant differences. A reaction of a 3.3 mM
solution of Mn(IV)-OH-PFOM with one equivalent of
3
6
4 3
+
-
(
Na / O SC H ) P at room temperature was complete
3 6 4 3
+
-
within a t1/2 = 0.5 s with a ~50 % yield of (Na /
O SC H ) P=O, Figure S14. On the other hand, a reaction
2
0
sibility of a Mn=O unit being present in this complex.
3
6
4 3
The pre-edge area of Mn(IV)PFOM sample is 7.7, which
also suggests that the Mn ligand environment is not very
distorted from centrosymmetry, unlike what might be
expected for a Mn=O complex. Lastly, Mn(V)PFOM is
best fit with an O-atom at 1.78 Å, 4 O-atoms at 1.92 Å, and
an O-atom at 2.30 Å (Figure S13). These results also ex-
clude the possibility of a Mn(V)=O unit, which is ex-
of a 3.3 mM solution of Mn(V)-OH-PFOM with one
+
–
equivalent of (Na PhSO ) P at room temperature was
3
3
+
-
very slow (t1/2 = ~1 h) and yielded ~100 % (Na /
O SC H ) P=O. One can conclude that these reactions
3
6
4 3
occur according to equations 4 and 5, respectively. De-
spite the fast oxygenation of the phosphine with Mn(IV)-
OH-PFOM, this compound was surprisingly inert in the
oxidation of water soluble substrates such as dimethyl-
sulfoxide to dimethylsulfone or allyl alcohol to the corre-
sponding epoxide. More obviously Mn(V)-OH-PFOM was
also not reactive to these substrates. Clearly, both hy-
droxo compounds can be considered inferior O-atom do-
nors to organic substrates. The very low reactivity of the
Mn(V) species supports the assignment of the compound
as having a terminal OH ligand, Mn(V)-OH-PFOM. In
contrast, a Mn(V)-oxo species in another polyoxometalate
21
pected to have a bond length of about 1.6 Å; instead, the
1.78-Å O scatterer of the best fit would appear to point to
1
8
the presence of a Mn(V)–OH unit. An authentic Mn(V)-
hydroxo compound has apparently not yet been prepared,
but in our experience it would not be surprising for the
oxygen-rich PFOM ligand environment to support such a
high-valent unit.
The magnetic susceptibilities of the various MnPFOM
compounds as solids were measured at room temperature
using a magnetic balance. Magnetic susceptibility was
also measured during cooling and heating of the samples
at 2 ≤ T ≤ 300 K at H=5000 Oe using a SQUID magnetom-
eter, Figure S13. The data was fitted well using the Curie
equation at 100-300 K. Most importantly, the data clearly
11
framework and in organic solvent have been shown to be
quite reactive.
22
show that the compounds are high spin, Table 2.
Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM were also
tested as electron transfer oxidants. In the outer sphere
oxidation of potassium ferrocyanide, the data shows (see
supporting information, Figure S15) that the reaction of
3.3 mM Mn(V)-OH-PFOM with one equivalent ferrocya-
nide was about 40 times faster than the analogous reac-
tion with Mn(IV)OH-PFOM (equations 6 and 7).
Table 2. Results of magnetic measurements of
MnPFOM
a
S
g
g√(S(S+1)), BM
ꢀeff, BM
5.7
b
Mn(II)
5/2
2
2.0
6.21
4.58
4.01
3.64
Mn(III)
1.87
2.06
2.58
4.6
Mn(IV) 3/2
Mn(V)
3.9
1
3.2
Other S values gave calculated g-factors that were much
farther from g = 2 indicating that the given S values best fit
the data, see Table S1. (a) Room temperature measurement
with magnetic balance. (b) Set at g = 2 after measurement of
the EPR spectrum indicating so.
A different outcome however was observed in the oxi-
dation of 3-mercaptopropionic acid, RSH (R
=
CH CH COOH) to the corresponding disulfide. The for-
2
2
1
mation of disulfide was verified and quantified by H
NMR, while the reaction kinetics was measured by UV-vis
spectroscopy. The reaction of Mn(V)-OH-PFOM (3.3.
mM) with 2 equivalents of RSH had a t1/2 = 8.8 sec and
Reactivity. The water-soluble Mn(IV)-OH-PFOM and
Mn(V)-OH-PFOM compounds were investigated with
respect to their reactivity in O-atom transfer, electron
afforded RSSR and Mn(III)-H O-PFOM in ~100 % yield. In
2
transfer oxidation, and formation of O as related to water
oxidation. Within this context, it is important to keep in
contrast, the oxidation of RSH by Mn(IV)-OH-PFOM un-
der similar conditions was an order of magnitude faster
2
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with t1/2 = 0.8 sec to yield RSSR in a ~50% yield and
Mn(III)-H O-PFOM. The latter reacted very slowly over
PFOM, which are ~2 in the presence of KCl, and ~4 in the
presence of CsCl.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
4
8 h wherein the remaining RSH was also oxidized to
RSSR to and formed Mn(II)-H O-PFOM. The stoichiome-
2
try and reactivity is summarized in equations 7 and 8.
The difference in the reactivity of Mn(IV)-OH-PFOM
and Mn(V)-OH-PFOM in these two electron transfer re-
actions can be explained as follows: The oxidation of fer-
rocyanide is a classic example of an outer sphere reaction
and therefore the compound with the higher potential,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
23
Mn(V)-OH-PFOM, reacts more quickly. On the other
hand, the thiol oxidation is more likely to be an inner
sphere reaction requiring thiolate coordination at the
metal center. Thus ligand exchange between the thiol and
bound hydroxide should precede the electron transfer
step. As the terminal Mn-OH bond of Mn(IV)-OH-PFOM
is longer and weaker than that of Mn(V)-OH-PFOM, the
reaction is more facile for the former.
During the preparation of the Mn(IV)-OH-PFOM and
Mn(V)-OH-PFOM compounds it was noticed that
Mn(IV)-OH-PFOM tended to be slowly reduced in water
to yield Mn(III)-H O-PFOM, while Mn(V)-OH-PFOM was
Figure 5. Bottom: Profiles of the reaction of 4.5 mM Mn(IV)-
OH-PFOM in the presence of 70 mM of MCl (M=Li, Na, K,
Rb or Cs) at 80°C. KF yielded essentially the same result as
2
indefinitely stable. This suggested that Mn(IV)-OH-
PFOM might have reacted in water to yield an oxidized
product. Thus, 0.4 mL of 46 mM Mn(IV)-OH-PFOM in
double distilled water were inserted into a 2 mL vial and
sealed with a septum. The vial was then purged with CO2
KCl and KBr was oxidized to Br . Top: ln/ln plots of the reac-
2
tion rates versus concentration generated from the reaction
profiles. Experimental data (colored) and linear fit (black).
Slope: KCl = 2.2; CsCl = 4.1. Note that the noise in the UV-vis
measurement is amplified in the data manipulation.
for 10 min in order to remove all air. 120 μL of N O were
2
inserted to the vial as an internal standard. The vial was
left overnight at 70°C to react. Finally, the amount of O2
in the gas phase of the vial was quantified by gas chroma-
It is well-known that the solubility of polyoxometalates
tends to decrease as one goes from Li to Cs counter cati-
24
tography by integrating the peak area of O versus that of
ons. This fact led us to investigate the possibility that
perhaps colloid clusters of polyoxometalates are being
formed in solution. Note that the solutions are completely
transparent to the naked eye and remain so at the reac-
tion temperature of 80 °C, meaning that there are no par-
ticles >500 nm in size. Formation of clusters could facili-
2
N O. Indeed for every mol of Mn(IV)-OH-PFOM the for-
2
mation of ~0.25 mol O was observed and a balanced
2
equation would indicate the following reaction stoichi-
ometry, equation 10.
tate the O forming reactions since having the reaction
2
centers in close proximity would obviously be important
in a multi-centered, multi-electron reaction. Such for-
mation of clusters could change the free energy, enthalpy
and entropy of activation of the reaction and also the re-
action mechanism. Since the reaction orders are different
in the presence of the different cations, it is difficult to
make highly accurate comparisons of the activation pa-
rameters. Despite this Eyring plots of reactions, Figure
S17, in the presence of KCl and RbCl gave the following
activation parameters assuming second order reactions:
The formation of O from Mn(IV)-OH-PFOM stands in
2
sharp contrast to its low reactivity as a oxygen atom do-
nor. Further investigation showed that the addition of
salts, originally KCl, had a strong effect on the rate of
formation of O2 and formation of Mn(III)-H O-PFOM
2
from Mn(IV)-OH-PFOM. Reaction profiles measured by
UV-vis spectrometry at 80 °C in the presence of various
alkali metal cations by the disappearance of Mn(IV)-OH-
PFOM are presented in Figure 5, bottom. Although the
reaction in its entirety is not very fast, the catalytic effect
+
+
+
+
+
Cs > Rb > K > Na ~ none ~ Li is striking. Another no-
table feature is the change in the reaction order in
Mn(IV)-OH-PFOM. Plotting, Figure 5, top, ln(rate) = ln(–
d[Mn(IV)-OH-PFOM]/dt) versus ln([Mn(IV)-OH-PFOM]
gives the slopes as the reaction order in Mn(IV)-OH-
‡
‡
‡
2
KCl: ꢁH = 65±2 kJ/mol; ꢁS = -104 ± 6 J/mol K; ꢁG
=
98
96±7 kJ/mol.
‡
‡
‡
RbCl: ꢁH = 41±2 kJ/mol; ꢁS = -168 ± 6 J/mol K; ꢁG 298
= 92±5 kJ/mol.
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Journal of the American Chemical Society
Page 6 of 13
‡
Clearly for RbCl the ꢁS is considerably lower than the
large so that a relatively accurate EDX measurement can
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
‡
‡
ꢁ
S observed for KCl, although the ꢁG
tions are the same within the experimental error of the
measurement. One may conclude that the transition state
is more ordered in the series Cs > Rb > K > Na ≥ Li .
of both reac-
be made using cryo-STEM. The results, Figure S18, show
that these colloid particles are indeed composed of poly-
oxometalate anions with the surrounding Cs cations and
water.
298
+
+
+
+
+
+
The activation parameters for the Cs case could not be
measured because of precipitate formation at lower tem-
peratures and a large deviation from second order.
In order to evaluate if clusters are indeed being formed
in the reaction solution and taking to advantage the high
contrast the presence of tungsten atoms provide, cryo
transmission and cryo scanning-transmission electron
microscopy (TEM) and (STEM) measurements were made
to directly observe the polyoxometalatate in solution. The
solutions were heated to 60 °C, and flash-frozen directly,
thus preserving the experimental conditions. Figures 6
and 7 present STEM and TEM images, respectively, that
clearly show differences in the arrangement of polyoxo-
metalates in frozen vitreous glass solution. The TEM
shows the large particles while STEM was used to survey
the space between the larger particles. The solutions pre-
pared in the presence of LiCl are completely homogene-
ous with no evidence for the formation of any clusters.
Solutions containing NaCl are quite similar with perhaps
formation of some very small assemblies. On the other
hand, Mn(IV)-OH-PFOM in the presence of KCl leads to
the observation of small colloidal particles, that is 2-5 nm
and larger particles of ~50 nm. In the presence of CsCl
two different types of structures are observable; dendrite
type structures that are more easily viewed by STEM and
large spherical colloids that can be seen by TEM.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 7. Cryo-TEM pictures from solutions of 4.5 mM
Mn(IV)-OH-PFOM and 70 mM of MCl. M = K (left), and Cs
(right) at 60°C. Scale bar is 200 nm, dark is high contrast, ie
W.
Figure 8. Size distributions obtained from DOSY NMR exper-
iments. The solutions were 4.5 mM Zn(II)PFOM and 70 mM
of MCl (M=Li, Na, K, Rb or Cs) at 80 °C in D O.
2
In order to support the observation of large polyoxo-
metalate assemblies in solution by an entirely different
technique, diffusion NMR, DOSY, was used to approxi-
25
mate particle size. In this measurement we focused on
the hydrogen atom inside the PFOM structure, Figure 1.
Since Mn(IV)-OH-PFOM is paramagnetic and shows no
NMR signal, the isostructural diamagnetic Zn(II)PFOM
was used instead. The results shown in Figure 8 reveal
that in the presence of LiCl or NaCl, particles in the one-
nanometer region are present. Since the Zn(II)PFOM pol-
yoxometalate itself has such nanometric dimensions, the
DOSY experiment supports the electron microscope ob-
servation showing the absence of aggregates and the ab-
sence of a catalytic effect for O formation in the presence
2
of LiCl or NaCl, Figure 6. On the other hand when KCl,
RbCl or CsCl are added to Zn(II)PFOM in water there is a
bimodal distribution that varies according to the alkali
cation added. In addition to the single Zn(II)PFOM mole-
+
cules present in all cases for K , one can see particles
+
ranging from ~10 nm to 100 nm. For Cs , the larger aggre-
Figure 6. Cryo-STEM pictures from solutions of 4.5 mM
Mn(IV)-OH-PFOM and 70 mM of MCl. M = Li (top left), Na
gates are clearly present, although it appears that only few
+
aggregates in the 10-50 nm region are formed. The Rb
(
top right), K (bottom left), and Cs (bottom right) at 60°C.
cation shows intermediate behavior. It should be noted
that the analysis of DOSY data is based on the assumption
that spherical particles are formed; the EM data clearly
show that also fractal type aggregates are present. There-
Scale bar is 50 nm, white is high contrast, ie W.
Some of the colloidal particles formed from 4.5 mM
Mn(IV)-OH-PFOM with 70 mM of CsCl are sufficiently
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Journal of the American Chemical Society
fore, although the DOSY experiments support the obser-
vation of aggregates or colloids in the presence of KCl,
RbCl or CsCl, it is likely a poor indicator of the actual size
distribution of the larger particles.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
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2
2
2
2
2
3
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3
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3
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3
4
4
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4
4
4
4
4
4
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5
5
5
5
5
5
5
5
6
There are several points worth emphasizing concerning
the reactivity profiles of Mn(V)-OH-PFOM and Mn(IV)-
OH-PFOM in general and more specifically the O for-
2
mation reaction that was observed from Mn(IV)-OH-
PFOM.
The reactivity of Mn(IV)-OH-PFOM in the presence
heavier alkali metal salts, such as CsCl is especially note-
worthy taking into account the considerable rate en-
hancement observed and the fourth order reaction in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Mn(V)-OH-PFOM: This higher oxidation state and high
spin compound (S =1), which is very stable in the solid
state and in water, is a very poor O-atom donor even to
highly reactive phosphines. On the other hand, it has a
sufficiently high oxidation potential for the very fast out-
er-sphere electron transfer oxidation of ferricyanide and
the somewhat slower inner sphere electron transfer oxi-
dation of a water-soluble thiol to yield the corresponding
26
27
Mn(IV)-OH-PFOM. The well accepted Kok cycle pro-
poses the formation of O from the yet to be isolated or
directly observed highly reactive S state of the tetraman-
ganese cluster, that is the oxygen evolving complex (OEC)
2
4
3
,28
of PSII. There appears to still be debate concerning the
oxidation states of manganese in S although a tetra-
Mn(IV) complex appears most likely for S
disulfide. It showed no formation of O and was very sta-
4
2
28
.
ble in water. Since this seems to be the first such Mn(V)-
OH specie that has been reported it is presently difficult
to put this low reactivity in a broader perspective.
3
From this
research one could hypothesize that a four centered
Mn(IV)-OH-PFOM arrangement could be possible
through colloid formation of Mn(IV)-OH-PFOM in the
presence of heavier alkali metals. This would allow for-
mation of O from an S like OEC involving four mole-
Mn(IV)-OH-PFOM: The high spin Mn(IV)-OH-PFOM
compound is a somewhat better O-atom donor, although
its reactivity appears to be limited to phosphines. It is a
weaker outer sphere electron transfer oxidant, but in the
electron transfer oxidation of 3-mercaptopropionic acid to
the corresponding disulfide it is more reactive than
Mn(V)-OH-PFOM presumably because is an inner sphere
oxidation, requiring ligand exchange. The formation of O2
in water from Mn(IV)-OH-PFOM is especially interesting.
2
4
cules of Mn(IV)-OH-PFOM, Scheme 2, pathway C. In
order to see if such a hypothesis is reasonable in an in-
termolecular assembly, in Figure 9 we present a TEM im-
age of a colloid where on the edge of the sphere one can
observe a high degree of ordering of the PFOM molecules,
which have a ~ 1 nm cross-section, within the colloidal
structure. Furthermore in Figure 10 we present the Mn-
Mn distances as can be deduced from the crystal structure
of Mn(IV)-OH-PFOM. Clearly the Mn-Mn distances are
significantly longer in such an intermolecular assembly
than the distances, <3 Å, known for the OEC unit. For an
As Mn(III)-H O-PFOM is the reaction product four
2
equivalents of Mn(IV)-OH-PFOM are needed to form 1
equivalent of O as shown in equation 10. In a bimolecular
2
mechanism two simple scenarios can be envisioned,
Scheme 2. Assuming Mn(IV)-OH-PFOM is a one electron
oxidant one can propose that H O , which was not de-
O-O bond to be formed some H O and cations would
2
2
2
need to be “squeezed” out of the interstitial space be-
tween the Mn atoms. However, and perhaps paradoxical-
tectable, is formed in the rate determining step and is
then decomposed quickly, as was separately observed,
ly, these longer Mn-Mn distances slow down the O for-
2
with additional Mn(IV)-OH-PFOM to yield O , pathway
2
mation reaction considerably allowing its observation
from a complex, Mn(IV)-OH-PFOM, that has been well
characterized. In this way we can conclude that a high
spin tetra-Mn(IV)-OH OEC can be considered as a viable,
but of course not proven, possibility for the OEC, and
also would be an interesting target for synthetic oxide
based analogs of a water oxidation catalyst.
A. Alternatively, Mn(IV)-OH-PFOM could be a two-
electron oxidant, yielding O in one step and Mn(II)-H O-
2
2
PFOM, pathway B. The latter would react very fast with
Mn(IV)-OH-PFOM by comproportionation, equation 1, to
yield Mn(III)-H O-PFOM. When Mn-Mn distances are on
2
the average large this would be the expected reaction
mechanism. The reactivity of Mn(IV)-OH-PFOM does not
support a recent suggestion that initial O-O bond for-
mation with the oxygen evolving complex does not in-
IV
8j
volve an Mn -OH unit.
Scheme 2. Conceivable pathways for formation of O2.
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Figure 9. A zoom in on a cryo-TEM image of a colloid parti-
cle of Mn(IV)-OH-PFOM. Scale bar – 50 nm.
electron microscope results. Most interestingly, in the
+
th
1
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7
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9
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1
1
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1
1
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1
1
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5
5
6
presence of Cs the reaction is both the fastest and ~ 4
order in Mn(IV)-OH-PFOM. This supports a mechanistic
hypothesis that a 4 Mn(IV)-OH centered complex is via-
ble and a possible reactive state for O formation. Notably,
2
this observation was made possible because in such an
intermolecular assembly the Mn-Mn distances are much
longer than known for the oxygen evolving complex of
PSII. The slower reaction enabled the study of the reactiv-
ity of an isolated species. It would appear that a bimolecu-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
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3
4
5
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9
0
1
2
3
4
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8
9
0
1
2
3
4
5
6
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8
9
0
lar mechanism for O formation is also feasible but signif-
2
icantly slower.
Experimental Part
Synthesis.
K [NaH Mn(II)(H O)W F O ]•19H O,
8 2 2 17 6 55 2
Mn(II)PFOM. The Mn(II) substituted polyfluoroxomet-
alate, Mn(II)PFOM, was synthesized by a previously re-
1
3
ported method. First the Zn(II) substituted polyfluorox-
ometalate is prepared by dissolving 44 g of Na WO •2H O
2
Figure 10. Possible intermolecular Mn-Mn distances in the
crystal of K [NaH Mn(IV)(OH)W F O ]•15H O considering
optimal organization of 4 PFOM molecules.
8
2
17
6
55
2
2
4
in 100 mL of de-ionized water and heating to 80-85 °C. To
this solution are added about 14 mL of 40% HF bringing it
to pH = 4.5. The solution is then stirred for 1 hour and
filtered. The filtrate is then reheated to 80 °C and a solu-
Conclusions
A manganese(II)-substituted polyfluoroxometalate was
used as a synthetic platform for the isolation of four dif-
tion of 7 g Zn(CH CO ) •2H O in 10 mL de-ionized water
3 2 2 2
ferent high spin species, Mn(II)-H O-PFOM, Mn(III)-
is added dropwise. The solution is let to stir for an addi-
tional 1 h and then filtered. Note that carefully controlling
the temperature insures that the zinc atom is substituted
at the belt position only. This filtrate is precipitated by
addition of 4.5 g KCl and the precipitate is recrystallized
in 5.5 mL de-ionized water. Yield – 3 g. See the supporting
information for further analysis of Zn(II)PFOM. Then 3 g
Zn(II)PFOM dissolved in 27 mL acetate buffer, pH=5, is
2
H O-PFOM, Mn(IV)-OH-PFOM, and Mn(V)-OH-PFOM
2
as determined from magnetic susceptibility, XPS and XAS
(XANES and EXAFS) measurements. The coordination
environment around manganese for the Mn(IV)-OH-
PFOM and Mn(V)-OH-PFOM has four oxygen atoms in
the equatorial plane, an accessible axial hydroxo terminal
ligand and an inaccessible fluorine atom trans to the hy-
droxo ligand. It would appear that the fluorine atom as
an electron withdrawing, but π donating ligand plays a
key role in the stabilization of Mn(IV)-OH-PFOM and
Mn(V)-OH-PFOM. Both Mn(IV)-OH-PFOM and Mn(V)-
OH-PFOM in water were found to be poor oxygen atom
heated to 50 °C and 324 mg MnSO •H O in 2 mL acetate
4 2
buffer, pH=5, are added dropwise. The solution turns
brown, is stirred for 30 min and then cooled and precipi-
tated with a saturated solution of KCl. Yield - 2.4 g (80%).
Elemental analysis Calcd (Exp): Na – 0.47 (0.35); K – 7.16
+
-
(6.46); Mn – 1.12 (0.76); W – 63.63 (62.67); H O (TGA) –
donors, although the Mn(IV)-OH-PFOM oxidized (Na /
2
+
-
–1
O SC H ) P to (Na / O SC H ) P=O at room temperature
5.64 (7.34). IR - 724, 776, 813, 882, 949 cm . Crystalliza-
3
6
4
3
3
6
4 3
tion yielded mostly a dimer, which is in equilibrium with
the monomer as explained above. See Table 3 below for
details on the dimer.
within a few seconds (t1/2 = 0.5 s). The analogous reaction
with Mn(V)-OH-PFOM was much slower, t1/2 = 1 h. Both
Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM were reactive
as one-electron oxidants. The results show that Mn(V)-
OH-PFOM is the better outer sphere oxidant with ferro-
cyanide as the substrate as would be expected for a higher
valent species. On the other hand Mn(IV)-OH-PFOM was
more reactive in what appears to be an inner sphere oxi-
dation of 3-mercaptopropionic acid that requires nucleo-
philic substitution of the terminal hydroxo ligand that is
less strongly bonded in Mn(IV)-OH-PFOM.
K [NaH Mn(III)(H O)W F O ]•13H O, Mn(III)PFOM.
8
2
2
17
6
55
2
Mn(II)PFOM, 200 mg, were dissolved in 6 mL de-ionized
water and 114 mg K S O were added. The solution was
2
2
8
heated until the purple color started turning brown.
Mn(III)PFOM was crystallized in the presence of ethanol
vapor that serves both as a reducing agent and an anti-
(III)
solvent. After a few days purple crystals of Mn PFOM
were formed. Yield – 180 mg (90%). Elemental analysis
Calcd (Exp): Na – 0.47 (0.37); K – 6.42 (6.18); Mn – 1.13
The formation of O from Mn(IV)-OH-PFOM is the
2
most notable aspect related to reactivity in this research.
(0.87); W – 64.14 (64.73); H
737, 786, 888, 954 cm .
2
O (TGA) – 6.77 (4.94). IR -
–1
The O formation reaction is catalyzed by the presence of
2
+
+
+
alkali metal cations, K , Rb and Cs . The catalytic effect is
due to formation of aggregates that were of a fractal and
colloidal nature as visualized by cryo STEM and cryo
K [NaH Mn(IV)(OH)W F O ]•15H O, Mn(IV)PFOM.
8
2
17
6
55
2
Method 1 – 2 g of Mn(II)PFOM were dissolved in 10 mL of
double distilled water and treated with a flow of 6.2 mg
ozone per min for 30 min. The color of the solution
changed to dark brown and Mn(IV)PFOM is collected
+
+
TEM. Addition of Li and Na did not lead to the for-
mation of aggregates. A more indirect measurement of
aggregation by diffusion NMR (DOSY) supported the
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from a frozen solution by lyophilization. Yield – 1.7 g
85%). Method 2 – 2 g Mn(II)PFOM were dissolved in 35
After 30 min the pressure tube was cooled and a small
1
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5
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9
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1
1
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1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
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5
5
5
5
5
5
5
6
(
amount of fine brown precipitate that was formed is re-
moved by centrifugation. The filtrate was then reduced to
30 mL by evaporation and Mn(V)PFOM was precipitated
with a saturated KCl solution. Finally the product is re-
crystallized in 1 mL de-ionized water at 80 °C. Yield – 0.9
g (57%). Elemental analysis Calcd (Exp): Na – 0.47 (0.35);
mL de-ionized water; 2 g Oxone was added and the solu-
tion was heated to 80°C. The solution changed color from
light brown to purple and finally to dark brown, where
upon it is cooled in ice water and Mn(IV)PFOM is collect-
ed by precipitation with saturated KCl. X-ray quality crys-
tals were grown at 4 °C. Yield – 1.2 g (60%). Elemental
analysis Calcd (Exp): Na – 0.47 (0.35); K – 6.42 (6.40); Mn
K – 5.66 (5.46); Mn – 1.13 (0.84); W – 64.67 (63.99); H O
2
–1
(TGA) – 5.23 (7.43). IR -739, 778, 886, 955 cm .
–
1.13 (0.72); W – 64.16 (65.71); H O (TGA) – 6.45 (5.63).
2
–
1
X-ray Crystallography. Data were collected and pro-
cessed either by a Nonius KappaCCD diffractometer or a
Bruker Appex2 KappaCCD diffractometer; MoKα
(λ=0.71073 Å), graphite monochromator. The data were
processed with Denzo-scalepack and the structures were
solved by direct methods with SHELXS. The data is sum-
marized in Table 3.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
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3
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7
8
9
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8
9
0
IR - 731, 784, 836, 890, 954 cm .
K [NaH Mn(V)(OH)W F O ]•20H O, Mn(V)PFOM. 2 g
7
2
17
6
55
2
of Mn(II)PFOM were dissolved in 65 mL double distilled
water and placed into a pressure tube. 1.6 g of K S O were
2
2
8
added. The pressure tube was sealed and heated in an oil-
bath at 160 °C. A color change from yellow light brown to
purple to dark brown and finally to green was observed.
Table 3. Crystallographic Data for Compounds Mn(II)-PFOM, Mn(III)-PFOM, Mn(IV)-PFOM, and Mn(V)-PFOM,
Mn(II)PFOM
Mn(III)-PFOM
Mn(IV)-PFOM
Mn(V)-PFOM
F Mn O W Na 40O
F MnO W Na 20O
8K
12
2
110
34
2
6
56
17
formula
F MnO W Na 13O 9K F MnO W Na 15O 9K
6
56
17
6
56
17
18K
−
1
formula weight (g mol ) 9738.56
4760.39
triclinic
P-1
4805.28
monoclinic
C2/c
4846.18
triclinic
P-1
crystal system,
space group
T (K)
monoclinic
C2/c
100(2)
43.248(9)
12.198(2)
29.370(6)
90.00
100(2)
120(2)
22.604(5)
12.289(3)
29.349(6)
90.00
100(2)
a (Å)
12.2907(7)
12.8548(7)
29.2936(16)
87.980(2)
90.005(2)
61.462(2)
4062.6(4)
2
12.2185(13)
12.8189(13)
13.5340(15)
104.662(5)
90.032(5)
118.350(5)
1786.6(3)
1
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
101.13(3)
90.00
92.40(3)
90.00
3
V (Å )
15202(5)
4
8145(3)
4
Z
−
3
Dcalc (g cm )
4.255
3.891
3.918
4.504
−
1
μ (mm )
26.394
85127
24.630
33331
24.623
8278
28.016
collected reflections
reflections[I > 2σ(I)]
Rint
13985
10224
18455
7539
13537
0.0655
1001/108
0.0572
0.1555
0.0514
0.0509
522/43
0.0506
0.1341
0.0449
1035/166
0.0359
0.0884
1.047
Parameters/restraints
934/116
0.0577
0.1576
a
R(F) [I>2σ(I)]
2
a
wR(F ) (all data)
GOF
1.093
1.031
1.084
a
2
2
o
2
2
2
o
2
1/2
(
F) = Σ∥F − F ∥/Σ|F |. wR(F ) = {Σ[w(F − F ) ]/Σ[w(F ) ]} .
o
c
o
c
samples at 2 ≤ T ≤ 300 K and H = 5000 Oe by SQUID
magnetometer (MPMS3). The diamagnetic correction was
made by fitting Mn(II)PFOM assuming no TIP and apply-
ing the same correction to all other oxidation states.
Magnetic Susceptibility. Magnetic susceptibility was
measured at RT using a Guoy magnetic balance. The dia-
magnetic correction was made by measuring the magnet-
ic susceptibility of the structurally similar Zn(II)PFOM.
The number of uncoupled electrons was calculated as-
suming no spin-orbit coupling. Magnetic susceptibility
was also measured during cooling and heating of the
X-ray Photoelectron Spectroscopy (XPS). XPS meas-
urements were conducted on a Kratos Axis-Ultra DLD
with a monochromatized Al source operated at 15 W. De-
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 13
tection at pass energies ranging between 20-80 eV was 70mM MCl (M=Li, Na, K, Rb or Cs). The measurements
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used, whereas most of our line-shape analysis was based
on data recorded at pass energy = 40 eV. Charge compen-
sation was attempted by means of a flood gun, used at 1.8
A and 0.5 V filament bias, while the charge balance was
set to 0.5 V. The X-ray source was with a pass energy of 40
were performed on a Bruker 400 MHz NMR using the
dstebpgp3s pulse sequence provided by Bruker at 353.2K.
The parameters used were: diffusion period Δ=0.2s and
the gradient pulse length δ=7.5 ms. 32 different exponen-
tially distributed gradient strengths were measured. The
results were analyzed by the CONTIN method provided
in the TopSpin program.
7
/2
5/2
3/2
eV. Data collected for W 4f , W 4f and W 5p was
baseline corrected with a Shirley-type background and
fitted with a function consisting of a series of 3 Gaussian
equations adapted in advance for the specific line shape
Quantification of O Evolution. 0.4 mL of 46 mM
2
3
/2
1/2
Mn(IV)-OH-PFOM in double distilled water were insert-
ed into a 2 mL vial and sealed with a septum. The vial was
0
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0
of these peaks. Mn 2p and Mn 2p peaks were treated
in a similar way. See the supporting information for the
7
/2
then purged with CO
air. 120 μL of N O were inserted to the vial as an internal
standard. The vial was left overnight at 70°C to react. Fi-
nally, the amount of O in the gas phase of the vial was
2
for 10 min in order to remove all
equations used. Finally, the binding energies of W 4f
3
/2
2
were subtracted from those of Mn 2p in order to correct
for the differential charging effects, assuming that the W
4f binding energy is negligibly modified only for the
different oxidation states of manganese.
7/2
2
quantified by gas chromatography by integrating the peak
area of O versus that of N O. The gas chromatograph
2
2
X-ray Absorption Spectroscopy (XAS). Samples for
the XAS experiments were prepared by diluting the
MnPFOM solid with solid boron nitride in a mass ratio of
instrument that was used was a HP-6890 equipped with a
micro-thermal conductivity detector and a ShinCarbon
ST 80/100 Micropacked Column 2mX0.53mm ID and a
250 µL injection port valve. Helium was used as carrier
gas. Retention times O -1.1 min; CO -7.9 min; N O-9.4
1
:6. The mixture was thoroughly ground for 30 min and a
uniform thin layer of the sample was brushed onto a piece
of tape, which was then folded to ensure a homogeneous
powder distribution. XAS data were collected at beamline
X3B at the National Synchrotron Light Source (NSLS) of
Brookhaven National Laboratory. Mn K-edge XAS data
were collected for frozen solutions maintained at ∼20 K
over the energy range 6.3-7.4 keV. A Mn foil spectrum
was measured simultaneously for internal energy calibra-
tion using the first inflection point of the K-edge energy
2
2
2
min.
ASSOCIATED CONTENT
Supporting Information. Additional NMR, XPS, XAS, ki-
netic, X-ray diffraction and magnetic susceptibility data. This
material is available free of charge via the Internet at
http://pubs.acs.org.”
AUTHOR INFORMATION
(
6539.0 eV). Data were obtained as fluorescence excita-
tion spectra using a solid-state germanium detector
Canberra). Data reduction, averaging, and normalization
Corresponding Author
(
29
* Email: Ronny.Neumann@weizmann.ac.il
were performed using the program EXAFSPAK. The
coordination number of a given shell was a fixed parame-
ter and was varied iteratively in integer steps while the
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
2
bond lengths (R) and mean-square deviation (σ ) were
allowed to freely float. The amplitude reduction factor
was fixed at 0.9 while the edge-shift parameter E was
0
allowed to float as a single value for all shells. The pre-
ACKNOWLEDGMENT
3
0
edge features were fit using the Fityk program with
pseudo-Voigt functions composed of 50:50 Gaussi-
an/Lorentzian functions (Figure S6).
This research at the Weizmann Institute was supported by
the Israel Science Foundation grant # 763/14, the I-CORE
Program of the Planning and Budgeting Committee and The
Israel Science Foundation (grant 152/11), the Helen and Mar-
tin Kimmel Center for Molecular Design. Electron microsco-
py was performed at the Irving and Cherna Moscowitz Cen-
ter for Nano and Bio-Nano Imaging of the Weizmann Insti-
tute of Science. R.N. is the Rebecca and Israel Sieff Professor
of Chemistry. The work carried out at the University of Min-
nesota was supported by the U.S. National Science Founda-
tion (CHE-1361773 to L.Q.). XAS data were collected on
beamline X3B at the National Synchrotron Radiation
Lightsource (NSLS), which is supported by the U.S. Depart-
ment of Energy under Contract No. DE-AC02-98CH10886.
Use of beamline X3B was made possible by the Center for
Synchrotron Biosciences grant, P30-EB-00998, from the Na-
tional Institute of Biomedical Imaging and Bioengineering.
Electron Microscopy (STEM and TEM). Samples for
electron microscopy were prepared from solutions of 4.5
mM Mn(IV)-OH-PFOM and 70 mM of MCl (M=Li, Na, K,
Rb or Cs) at 60 °C. A drop of solution was put on a Quan-
tifoil grid kept at high humidity and maintained at 60 °C
in the chamber of the automated plunging apparatus
(Leica EM-GP). The grid was then blotted with filter pa-
per and plunged into liquid ethane. Samples were trans-
ferred under liquid nitrogen to a Gatan 626 cryoholder,
and visualized by TEM and STEM under low-dose cryo-
conditions with a Tecnai G2 TWIN-F20 microscope. TEM
images were recorded on a Gatan US4000 CCD camera,
and STEM images were recorded with a Fischione
HAADF detector.
REFERENCES
Diffusion Ordered Spectroscopy (DOSY). Samples
were prepared from solutions of 4.5 mM Zn(II)PFOM and
10
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Journal of the American Chemical Society
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23. Note that the actual oxidation potentials could not be
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1
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1
1
1
1
1
1
1
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measured by CV because they are higher than the oxidation
potentials of water, see Figure S19.
2
4. Spitsyn, V. I.; Babaev, N. B. Russ. J. Inorg. Chem. 1960, å,
80-585.
5. In solution the polyoxometalate-based colloids are in dy-
5
2
namic equilibrium and not static. The time scale of a dynamic
light scattering (DLS) measurement is such that unreliable re-
sults were obtained.
2
6. It is our supposition that the lower rates observed upon
+
+
addition of K and Rb are related to the degree of aggregation
and/or a combination of a four- and two- manganese centered
reaction pathways.
0
1
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0
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0
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9
0
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9
0
2
7. Kok, B.; Forbush, B.; McGloin, M. Photochem. Photobiol.
970, 11, 467–475.
8. (a) Krewald, V.; Retegan, M.; Cox, N.; Messinger, J.; Lubitz,
W.; DeBeer, S.; Neese, F.; Pantazis, D. A. Chem. Sci. 2015, 6, 1676-
695. (b) Cox, N.; Retegan, M. Neese, F.; Pantazis, D. A. Boussac,
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A.; Lubitz, W. Science, 2014, 345, 804-808.
29. George, G. N. EXAFSPAK; Stanford Synchrotron Radiation
Laboratory: Stanford, CA, 1990.
30. Fityk: a general-purpose peak fitting program, Wojdyr, M.
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