Synthesis, Structure, and Characterisation of a New Phenolato-Bridged Manganese Complex [Mn2(mL)2]2+: Chemical and Electrochemical Access to a New Mono-μ-Oxo Dimanganese Core Unit

Article abstract of DOI:10.1002/chem.200305515

The dinuclear phenolato-bridged complex [(mL)Mn^IIM-n ^II(mL)](ClO₄)₂ (1(ClO₄) ₂) has been obtained with the new [N₄O] pentadentate ligand mL^- (mLH=N,N′ -bis-(2-pyridylm

Full text of DOI:10.1002/chem.200305515

FULL PAPER
Synthesis, Structure, and Characterisation of a New Phenolato-Bridged
2
+
Manganese Complex [Mn (mL) ] : Chemical and Electrochemical Access to
2
2
a New Mono-m-Oxo Dimanganese Core Unit
[
a]
[a]
[a]
Christelle Hureau, Laurent Sabater, Elodie Anxolabÿhõre-Mallart,*
[
b]
[a]
[c]
[a]
Martine Nierlich, Marie-France Charlot, Florence Gonnet, Eric Riviõre, and
[
a]
Geneviõve Blondin*
III
IV
III
IV
IV
IV
Abstract: The dinuclear phenolato-
electrode (SCE)). Upon chemical oxi-
dation with tBuOOH (10 equiv) at
Mn Mn and Mn Mn /Mn Mn ox-
idation processes, respectively. The
one-electron electrochemical oxidation
II
bridged
complex
[(mL)Mn M-
II
n (mL)](ClO ) (1(ClO ) ) has been
208C, 1 is transformed into the mono-
4
2
4 2
III
obtained with the new [N O] pentaden-
m-oxo
species
[(mL)Mn -(m-O)-
of 2 leads to the mono-m-oxo mixed-
4
ꢀ
III
2+
III
tate ligand mL (mLH=N,N’-bis-(2-
Mn (mL)] (2), which eventually par-
tially evolves into the di-m-oxo species
valent
species
[(mL)Mn -(m-O)-
IV
3+
pyridylmethyl)-N-(2-hydroxybenzyl)-
N’-methyl-ethane-1,2-diamine) and has
been characterised by X-ray crystallog-
raphy. X- and Q-band EPR spectra
were recorded and their variation with
temperature was examined. All spectra
exhibit features extending over 0±
Mn (mL)] (2ox). The UV/Vis spec-
trum of 2ox exhibits one large band at
III
IV
n+
[(mL)Mn -(m-O) -Mn (mL)]
(3) in
2
which one of the aromatic rings of the
ligand is decoordinated. The UV/Vis
spectrum of 2 displays a large absorp-
tion band at 507 nm, which is attribut-
643 nm, which is attributed to the phe-
nolate!Mn
IV
charge-transfer transi-
tion. 2ox can also be obtained by the
direct electrochemical oxidation of 1 in
the presence of an external base. The
2ox and 3 species exhibit a 16-line
EPR signal with first peak to last
trough widths of 125 and 111 mT, re-
spectively. Both spectra have been si-
mulated by using colinear rhombic Mn-
hyperfine tensors. Mechanisms for the
chemical formation of 2 and the elec-
trochemical oxidation of 1 into 2ox are
proposed.
III
ed to a phenolate!Mn charge-trans-
8
1
00 mT at the X band and over 100±
fer transition. The cyclovoltammogram
of 2 exhibits two reversible oxidation
waves, at 0.65 and 1.16 V versus the
450 mT at the Q band, features that
II
are usually observed for dinuclear Mn
complexes. Cyclic voltammetry of 1 ex-
hibits two irreversible oxidation waves
III
III
SCE, corresponding to the Mn Mn /
p
1
p
2
at E =0.89 V and E =1.02 V, accom-
Keywords: electrochemistry ¥ EPR
spectroscopy ¥ manganese ¥ N,O
ligands ¥ oxidation
panied on the reverse scan by an ill-de-
p
fined cathodic wave at E1
0
=0.56 V (all
measured versus the saturated calomel
Introduction
[
a] C. Hureau, L. Sabater, E. Anxolabÿhõre-Mallart, M.-F. Charlot,
E. Riviõre, G. Blondin
Laboratoire de Chimie Inorganique, UMR 8613, LRC-CEA n833 V
Institut de Chimie Molÿculaire et des Matÿriaux d’Orsay
Universitÿ Paris-Sud, 91405 Orsay Cedex (France)
Fax : (+33)1-69-15-47-54
The involvement of manganese in several biological systems
[1,2]
and processes is now well-known.
For example, manga-
nese dinuclear sites are present in metalloproteins such as
[
3,4]
[5]
manganese catalase,
arginase and ribonucleotide reduc-
E-mail: eanxolab@icmo.u-psud.fr
[6]
gblondin@icmo.u-psud.fr
tase. Another very important biological system is the tetra-
nuclear Mn±oxo cluster, a key element of the OEC located
in photosystem II of green plants,
[
7]
[
b] M. Nierlich
[8±12]
CEA Saclay, DRECAM/SCM Bat 125
which catalyses the
9
1191 Gif-sur-Yvette (France)
light-induced oxidation of water into dioxygen.
[
c] F. Gonnet
A photo-induced electron abstraction from the OEC
occurs during each of the four transition states of the cata-
Laboratoire Analyse et Environnement, UMR 8587
B‚timent des Sciences, Universitÿ d’Evry Val d’Essonne
Boulevard FranÁois Mitterrand, 91025 Evry Cedex (France)
[13]
lytic cycle (S ±S of the Kok cycle). Crystal structures of
0
4
the Synechococcus elongatus photosystem-II dark-adapted
S1 state at 3.8 ä resolution and of the Thermosynechococ-
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
[14]
1998
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200305515
Chem. Eur. J. 2004, 10, 1998 ± 2010

1
998 ± 2010
[15]
cus vulcanus photosystem II at 3.7 ä resolution have been
reported, but the spatial arrangement of the Mn ions and
the nature of the oxygenated bridges have not been resolved
[
16]
yet. Information on the structure and valence of the Mn
cluster has mainly been obtained from X-ray absorp-
[
9,12,17,18]
[19±22]
tion
and EPR studies.
Comparison with the
spectroscopic signatures of a wide variety of manganese
compounds was of crucial importance. As a consequence of
unresolved structural details, designing biological manga-
nese models is still a challenge for inorganic chemists, as is
the investigation of the spectroscopic characteristics of these
[
1,23,24]
compounds. On the basis of EXAFS data
theoretical investigations,
as well as
[25±29]
the Mn cluster has been pro-
posed to be a cluster consisting of at least two di-m-oxo- and
one mono-m-oxo-bridged Mn moieties. We and other groups
have synthesised a large number of high-valent Mn±oxo
model complexes with various nitrogeneous ligands and
[29±34]
Scheme 1. The mLH ligand used in this work and various other penta-
dentate ligands for comparison.
cores.
The three prevalent core units in this chemistry
[35±37]
are the di-m-oxo [Mn-(m-O) -Mn],
carboxylato [Mn-(m-O) (m-O CR)-Mn]
oxo-di-m-acetato [Mn-(m-O)(m-OAc) -Mn] moieties.
the di-m-oxo-mono-m-
2
[38±44]
and the mono-m-
2
2
[43,45±49]
new ligand. Indeed, a 1:1 mixture of two dinuclear phenola-
2
II
II
2+
II
II
However, there are very few examples of the unsupported
to-bridged cations, [(L)Mn Mn (L)] and [(L )Mn Mn (-
i i a
[
44,50±56]
2+
mono-m-oxo-bridged [Mn-(m-O)-Mn] system,
especial-
L)] , was obtained, due to oxidation of the secondary
amine group of L into an imine, the ligand thus becoming
Li (L H is shown in Scheme 1), with the concomitant reduc-
tion of Mn into Mn . To avoid such behaviour of the
i
III
IV
ꢀ
ly in the Mn Mn oxidation state. To our knowledge, only
two examples are reported in the literature, one with a por-
phyrinic ligand and one with a non-porphyrinic pentaden-
a
ꢀ
i
[57]
III
II [66]
[34]
[7]
tate ligand.
compounds, we have here used a protected ligand, mLH
The EPR-active S state of the OEC, generated from the
(Scheme 1), where the secondary amine is methylated.
We report here the synthesis and the X-ray characterisa-
2
dark-adapted photosystem II by flash illumination at 273 K,
[19]
II
II
shows the well-known multiline EPR signal.
This spec-
tion of a new phenolato-bridged Mn dimer [(mL)Mn -
II
trum contains at least 19 Mn-hyperfine lines (IMn =5/2) cen-
tred at gꢁ2.0, which arise from an S=1/2 ground state.
Based on the data recorded from different spectroscopic
techniques derived from X-ray absorption, a general consen-
Mn (mL)](ClO ) (1(ClO ) ). We will demonstrate that the
4
2
4
2
chemical oxidation of
[(mL)Mn -(m-O)-Mn (mL)] complex (2), which in turn
1
with tBuOOH leads to the
III
III
2+
III
can be electrochemically oxidised into [(mL)Mn -(m-O)-
III
IV
IV
3+
III
IV 3+
sus has been reached for an Mn /Mn composition of the
Mn (mL)]
(2ox). A [Mn -(m-O) -Mn ]
core unit
2
[
22,58]
OEC in the S state. Attempts to simulate this signal
system, 3, can also be generated and we will demonstrate
that the EPR signatures of 2ox and 3 differ significantly. We
will also show that the electrochemical oxidation of 1 in the
presence of an appropriate base leads to the formation of
2
[59]
and theoretical analysis of the possible spin densities have
shown quite unambiguously that all four Mn ions of the oxo
cluster must be electronically coupled to each other. A thor-
ough study of the EPR features of Mn Mn ±oxo dinuclear
systems showed that the nature of the oxo core unit finely
controls the EPR signature. Subsequently, a detailed multi-
III
IV
III
IV
3+
the desired [(mL)Mn -(m-O)-Mn (mL)]
Consequently, three oxidative equivalents can be produced
species (2ox).
II
II
from the initial Mn Mn complex, with the concomitant for-
[36,60±62]
frequency EPR analysis of dinuclear complexes
has
mation of a mono-m-oxo bridge.
been of great interest in the course of the characterisation
of the OEC tetrameric complex. Even at the X band, both
III
IV 3+[1,33,37,63±65]
III
[
Mn -(m-O) -Mn ]
and [Mn -(m-O) (m-OAc)-
Results and Discussion
2
2
IV 2+[42]
Mn ]
systems present 125 mT (peak to trough), 16-line
spectra that differ in their relative line intensities, while the
Attempts to isolate a manganese complex starting from the
III
IV
3+
III
mono-m-oxo complex [(L’)Mn -(m-O)-Mn (L’)]
(L’H is
shown in Scheme 1) by Horner et al. exhibits a 119 mT,
8-line spectrum, with a peculiar distribution of the line in-
tensities.
In order to obtain a new mono-m-oxo complex, we have
free ligand and the tris-acetato-Mn salt were unsuccessful.
[34]
II
However, the use of an Mn salt leads to the formation of
II
1
the di-m-phenolato-di-Mn complex 1.
II
II
Characterisation of [(mL)Mn Mn (mL)](ClO ) (1(ClO ) ):
4
2
4 2
designed a ligand, L H (Scheme 1), that is very similar to
a
II
the one used by Horner et al. Nevertheless, as we reported
X-ray crystal structure: The structure of the [(mL)Mn -
[66]
II
2+
II
in a previous paper, the synthetic route used for the gen-
Mn (mL)] ion (1) is very similar to the one of [(L)Mn -
i
III
IV
3+
II
2+
[66]
eration of [(L’)Mn -(m-O)-Mn (L’)]
(the reaction of
Mn (L)] described previously. A view of the cation is
i
Mn(OAc) ¥2H O with the free ligand) did not lead to the
presented in Figure 1 and selected bond lengths and angles
3
2
formation of the expected mono-m-oxo complex with the
are listed in Table 1. The complex possesses a C axis that
2
Chem. Eur. J. 2004, 10, 1998 ± 2010
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1999

E. Anxolabÿhõre-Mallart, G. Blondin et al.
FULL PAPER
Magnetic susceptibility measurements: The molar magnetic
susceptibility, c , of a powder sample of 1(ClO ) was meas-
M
4 2
ured in the range 300±2 K. The c T value as a function of
M
temperature, T, is reported in Figure S1 in the Supporting
Information (open circles). At 280 K, the c T value is
M
3
ꢀ1
8
5
.04 cm mol
K
and decreases very slowly down to
.3 cm mol K at 20 K. Below 20 K, the c T value decreas-
es more rapidly, down to 1.14 cm mol K at 3 K. This be-
haviour indicates a weak antiferromagnetic exchange inter-
3
ꢀ1
M
3
ꢀ1
II
action between the two high-spin Mn ions (S =S =5/2).
1
2
ꢀ
1
The best fit is obtained with g=1.95 and J=ꢀ1.8 cm (H=
√
JS S (see the Experimental Section) and is also represent-
1
√
ꢀ
2
ed in Figure S1 (solid line) in the Supporting Information.
These values are in agreement with those reported in the lit-
erature for complexes where two Mn ions are bridged by
II
[67±70]
two phenolates.
Furthermore, the J value obtained for 1
is very close to that obtained for the 1:1 mixture of the two
closely related complexes [(L )Mn Mn (L)](BPh ) ¥H O.
i/a i 4 2 2
II
II
2+
II
II
[66]
Figure 1. X-ray crystal structure of the cation [(mL)Mn Mn (mL)]
.
EPR spectroscopy: X-band (9.4 GHz) and Q-band (34 GHz)
EPR spectra of a powder sample of 1(ClO ) were recorded
over the range 4.2±294 K by using the conventional perpen-
dicular detection mode. The 4.5 K spectra are shown in
Figure 2 and the variation upon increasing the temperature
goes through the middles of the two metallic sites on one
hand and of the two phenolic oxygen atoms on the other
hand. Each manganese ion is hexacoordinated by the four
4
2
ꢀ
nitrogen atoms of the mL ligand and by two oxygen atoms
Table 1. Selected bond lengths [ä] and angles [8] for 1.
MnꢀO1
MnꢀO1
MnꢀN1
2.1225(15)
2.1351(15)
2.3516(19)
MnꢀN2
MnꢀN3
MnꢀN4
2.3079(19)
2.240(2)
2.227(2)
O1ꢀMnꢀN1
N2ꢀMnꢀN4
MnꢀO1ꢀMn’
169.10(6)
150.59(7)
100.01(6)
O1ꢀMnꢀN3
155.09(8)
78.88(6)
O1ꢀMn1ꢀO1’
originating from the phenolato groups, the first coming from
the same ligand and the second from the ligand chelating
the other manganese ion. The manganese environment de-
parts from the regular octahedron usually observed for simi-
II
lar hexacoordinated Mn complexes. The X-ray structure re-
ꢀ
veals that the folding of the methylated ligand mL differs
ꢀ
from that of the related ligand L but is similar to that of
a
ꢀ
[66]
L . This thus confirms the greater flexibility of this kind
i
of ligand when no imine function is present.
II
II
2+
The Mn O diamond core of the [(mL)Mn Mn (mL)]
2
2
cation presents features that are characteristic of the di-m-
II
[66]
phenolato-di-Mn core unit.
The core structure is more
II
II
2+
regular in 1 than that observed in [(L)Mn Mn (L)] . The
Figure 2. a) X- and b) Q-band EPR spectra recorded on a powder sample
i
i
4 2
of 1(ClO ) at 4.5 K. X-band EPR recording conditions: 9.39 GHz micro-
wave frequency, 0.2 mW microwave power, 0.5 mT modulation ampli-
tude, 100 kHz modulation frequency. Q-band EPR recording conditions:
two MnꢀO distances differ by less than 0.02 ä, in compari-
son with a difference of ꢁ0.03 ä in the previously described
complex. The average MnꢀO bond length is slighly longer
in 1 (2.129 ä versus 2.110 ä), while the bridging MnꢀOꢀMn
3
4 GHz microwave frequency, 0.053 mW microwave power, 0.5 mT mod-
ulation amplitude, 100 kHz modulation frequency.
angle is very similar (100.0 versus 100.3(2) and 102.5(2)8).
However, the Mn¥¥¥Mn separation is shorter (3.262 versus
3
7
.280 ä) due to a more open OꢀMnꢀO angle (78.9 versus
is presented in Figure S2 in the Supporting Information. All
spectra exhibit features extending over 0±800 mT at the X
band and over 100±1450 mT at the Q band, as usually ob-
7.3 and 77.68). Similar dihedral angles are observed in both
systems (11.3 versus 11.58).
II
[67,71±74]
served for dinuclear Mn complexes.
It has long been
2000
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chem. Eur. J. 2004, 10, 1998 ± 2010

A Phenolato-Bridged Manganese Complex
1998 ± 2010
demonstrated that these features originate from the super-
imposition of the signatures of the five paramagnetic S spin
and Q bands, a result which proves that the dinuclear struc-
ture of complex 1 is maintained in solution. Unfortunately,
no Mn-hyperfine structure was observed. This can be attrib-
uted to the superimposition, within the same magnetic field
range, of several lines originating from the different S spin
states and/or from inter-S-spin transitions that exhibit slight-
ly distinct Mn-hyperfine structures.
[75]
states (S=1±5). Such mixing makes a detailed analysis of
II
the EPR spectra of dinuclear Mn systems complicated. De-
convolution procedures have previously been developed
that have led, together with the S=1 and S=2 spin-state
traces, to the determination of the exchange coupling con-
stant J between the two antiferromagnetically interacting
II
[76±78]
high-spin Mn ions.
More recently, a new methodology
Chemical oxidation by tBuOOH: It was checked that an
acetonitrile solution of 1 was stable at room temperature for
several days. The chemical oxidation of 1 was performed by
the addition of tBuOOH to an acetonitrile solution of 1 at
room temperature (208C). We will show in the following
has been developed by Blanchard et al. that only relies on a
scrupulous examination of the temperature-dependent spec-
[
79,80]
tra.
Up to three individual S-spin-state signatures are
determined and it has been shown that the J values obtained
are fully consistent with the ones deduced from magnetisa-
tion measurements. Whatever the method, the validity of its
application relies on a Heisenberg exchange interaction that
dominates the other effects contributing to the EPR profiles,
such as the Zeeman interaction and the local zero-field split-
tings. Indeed, a dominant exchange coupling allows the total
section that the species thus obtained, denoted 2, is the un-
III
supported
mono-m-oxo
complex
[(mL)Mn -(m-O)-
III
2+
Mn (mL)] . This species spontaneously evolves into the
di-m-oxo species [Mn -(m-O) -Mn ] (3). When the chemi-
III
IV 3+
2
cal oxidation of 1 is performed in an electrochemical cell,
complex 2 can be electrochemically oxidised into the mixed-
[81]
III
IV
3+
S spin value to be a good quantum number
and allows
valent mono-m-oxo complex [(mL)Mn -(m-O)-Mn (mL)]
(2ox).
consideration of the observed EPR spectra as a linear com-
bination of the temperature-independent individual S-spin-
state signatures weighted by temperature-dependent coeffi-
cients related to the Boltzmann population of the S spin
levels. Due to the low value of the exchange coupling con-
stant deduced from the temperature-dependent magnetisa-
Oxidation of 1 into 2: The optimum conditions for oxidizing
1 into 2 are reached when 10 equivalents of tBuOOH are
added to a millimolar acetonitrile solution of 1(ClO ) .
4
2
After a few minutes of stirring under air, the solution
changes from colourless to dark purple. The resulting spe-
cies 2 has been characterised by different techniques. All
the data shown below were recorded ten minutes after the
addition of tBuOOH at 208C, since we have determined
that the maximum amount of species 2 is available at that
time.
ꢀ
1
tion data of 1 (J=ꢀ1.8 cm ), the Zeeman interaction is a
competitive effect, at least at 34 GHz. This may lead to
[81]
inter-S-spin transitions, that are forbidden otherwise.
Indeed, the lines observed below 600 mT at the Q band may
originate from transitions between the S=0 and S=1 spin
states. A further complication arises from the small energy
gaps between the S spin levels at zero field. Even at liquid
helium temperatures, the S=1±3 spin states are significantly
populated and the EPR spectra recorded at 4.2 K are issued
from, at least, those three contributions. Increasing the tem-
perature leads to increasing contributions from the S=4 and
S=5 spin states. Above 10 K, no new transitions are detect-
ed and further increase of the temperature only leads to
changes in the relative line intensities. Therefore, correct
analysis of the EPR spectra needs the full diagonalisation of
the energy matrix built on the two interacting S =S =5/2
The evolution of the X-band EPR spectra recorded upon
addition of tBuOOH revealed the total disappearance of the
signal of 1. Only a very weak signal is detected at g=2
(data not shown); this signal increases with time (see
below). Thus, the resulting species 2 is EPR-silent. This sug-
III
gests the formation of an Mn complex. Electrospray mass
spectra were recorded in positive detection mode (Figure S3
in the Supporting Information). In the starting solution, qua-
simolecular ions were detected at m/z 931 (monocation, rel-
ative intensity 45%) and 416 (dication, relative intensity
1
2
II
II
ions, with the Heisenberg (isotropic) exchange interaction,
the local Zeeman and zero-field splitting effects and the di-
polar coupling all taken into account together with the ani-
sotropic component of the exchange interaction. All these
interactions are mathematically reproduced by a tensor that
is characterised by three principal values and three principal
directions. These tensors are not necessarily colinear and
Eular angles are added to set their orientations relative to
each other. Hence, the number of unknowns is too impor-
tant and extra experimental data such as high-field, high-fre-
quency spectra are needed to achieve the EPR analysis. This
work is currently in progress.
85%); these correspond to the {[(mL)Mn Mn (mL)](-
+
II
II
2+
ClO )} and the [(mL)Mn Mn (mL)] ions, respectively.
4
After addition of tBuOOH, the peak at m/z 931 significantly
decreases while new ones appear at m/z 947 (monocation)
and 424 (dication) with relative intensities of 7 and 70%, re-
spectively. The increases of 16 and 8 units indicate the inser-
tion of one oxygen atom into complex 1 with the concomi-
II
III
tant oxidation of both Mn ions into Mn . Consequently,
III
these two new peaks may be attributed to the {[(mL)Mn -
III
+
III
(m-O)-Mn (mL)](ClO )}
and the [(mL)Mn -(m-O)-
4
III
2+
Mn (mL)] cations, respectively. Based on the EPR and
mass spectroscopy data, we propose that the species formed
upon the addition of tBuOOH to 1 is the unsupported
Compound 1(ClO ) was dissolved in acetonitrile in the
4
2
III
III
presence of 0.1m tetrabutylammonium perchlorate and EPR
spectra were recorded from frozen solutions (see trace a of
Figure 6). The spectra recorded at 4.2 K present similar fea-
tures to those detected for powder samples at both the X
mono-m-oxo-di-Mn
Mn (mL)] (2).
complex
[(mL)Mn -(m-O)-
III
2+
The evolution of the electronic spectrum of 1 upon oxida-
tion with tBuOOH is represented in Figure 3a. Before oxi-
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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E. Anxolabÿhõre-Mallart, G. Blondin et al.
FULL PAPER
Figure 4. Cyclic voltammograms of 1 mm 1 in acetonitrile in the presence
of 0.1m tetrabutylammonium perchlorate (T=208C, scan rate=
ꢀ
1
1
2
00 mVs ): a) with (dashed line) or without (solid line) 1 equivalent of
,6-lutidine per molecule of 1; b) 10 min after addition of 10 equivalents
of tBuOOH; c) after complete bulk electrolysis at E=1.1 V versus the
SCE in the presence of 1 equivalent of 2,6-lutidine per molecule of 1.
measured versus the standard calomel electrode (SCE)).
1
/2
The E values are reported in Table 2, together with those
Figure 3. a) Evolution of the UV/Vis spectrum of 1 upon addition of
1
(
0 equivalents of tBuOOH to 1 mm solution in CH
dotted line), after 10 min at room temperature (solid line) and after 1 h
at room temperature (dashed line). b) UV/Vis spectrum of 1 before
dotted line) and after (dashed line) complete bulk electrolysis at E=
.1 V versus the SCE in CH CN containing 0.1m tetrabutylammonium
3
CN: before addition
of other dinuclear Mn±oxo complexes. The 510 mV separa-
tion between the two anodic waves (DE ) is far smaller
ox
than the ꢁ800 mV separation generally observed between
(
1
3
perchlorate, together with the UV/Vis spectrum of 2 (solid line) after
electrochemical oxidation at E=1.1 V versus the SCE in a thin-layer
spectroelectrochemical cell.
1/2
III
III
Table 2. E potential values of Mn Mn dinuclear complexes [V versus
SCE].
III
III
III
IV
III
IV
IV
IV
Ligand
Mn Mn /Mn Mn
Mn Mn /Mn Mn
Ref.
ꢀ
L’
0.54
0.65
0.18
0.99
1.16
0.98
[34]
this work
[37]
ꢀ
mL
N,Nbispicen
[
7]
dation, the spectrum is featureless in the visible region, as
II
expected for a Mn complex. Only an intense band below
ꢀ
1
ꢀ1
3
00 nm (e=16000m cm ) is detected; this band can be at-
III
III
III
IV
III
IV
IV
IV
tributed to a p!p* transition within the ligand. After addi-
tion of tBuOOH, the electronic spectrum exhibits four
major absorption bands, located at 507 (e=1020), 376 (e=
the Mn Mn /Mn Mn and Mn Mn /Mn Mn oxidation
[
37,83]
processes in di-m-oxo-di-Mn complexes.
Indeed, the
shape of the voltammogram is reminiscent of that of
ꢀ
1
ꢀ1
III
III
2+
[34]
3580), 317 (e=10160), and 283 nm (e=15520m cm ). The
[(L’)Mn -(m-O)-Mn (L’)] , for which DE =450 mV.
ox
III
absorption band at 507 nm can be attributed to a pheno-
This supports the formation of [(mL)Mn -(m-O)-
Mn (mL)] upon addition of tBuOOH to 1. Therefore, the
oxidation waves observed at E4 =0.65 V and E5 =1.16 V
versus the SCE are attributed to the oxidation of
[(mL)Mn -(m-O)-Mn (mL)]
Mn (mL)] (2ox) and of [(mL)Mn -(m-O)-Mn (mL)]
into [(mL)Mn -(m-O)-Mn (mL)] , respectively. The irre-
versible cathodic wave at E6
III
III
2+
late!Mn charge-transfer transition. Indeed, a very similar
ꢀ
1
ꢀ1
1=2
1=2
band (l=506 nm, e=2100m cm ) has been detected by
[82]
III
+
Neves et al. for the [Mn (bbpen)] complex, where the
[7]
III
III
2+
III
ligands H BBPEN and mLH are related by substitution of
(2) into [(mL)Mn -(m-O)-
2
IV
3+
III
IV
3+
the methyl group in mLH with a 2-hydroxybenzyl group. In
addition, all the features listed above are very similar to
IV
IV
3+
III
III
2+
p
those reported for the complex [(L’)Mn -(m-O)-Mn (L’)]
.
0
=ꢀ0.28 V versus the SCE cor-
[
34]
III
2+
III
3+
responds to the reduction of [(mL)Mn -(m-O)-Mn (mL)]
II
III
The oxidation of 1 was also performed in the electro-
(2) into [(mL)Mn -(m-O)-Mn (mL)] , which is not stable
under the experimental conditions. We propose that at this
oxidation state the dimeric structure breaks down to form
monomeric complexes, as suggested by the presence of the
wave at E =0.29 V versus the SCE on the reverse scan.
chemical cell. The recorded cyclic voltammogram (Fig-
1
4
=2
ure 4b) exhibits two quasireversible anodic waves at E
=
1
=2
0
.65 V (DE =135 mV) and E =1.16 V (DE =100 mV)
p
5
p
p
p
6
and one irreversible cathodic wave at E6
0
=ꢀ0.28 V (all
2002
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chem. Eur. J. 2004, 10, 1998 ± 2010

A Phenolato-Bridged Manganese Complex
1998 ± 2010
Evolution of 2 into 3: As indicated by the evolution of the
EPR spectrum, species 2 is not stable at room temperature.
The very weak signal detected at g=2 after the addition of
tBuOOH to 1 (see above) increases with time and reaches a
maximum after approximately 10 h. This spectrum presents
more reliable technique as far as the g anisotropy is con-
ƒ
cerned. The A hyperfine tensor is found to be of rhombic
1
ƒ
symmetry while the A hyperfine tensor is almost isotropic.
2
In addition, the isotropic components of both tensors are in
a 2:1 ratio and are close to the values reported in the litera-
III
IV 3+
[28,36,37,60,61]
1
7
6 lines spread over 124.5 mT with a regular spacing of
.5 mT (Figure 5b). This signal is characteristic of a mixed-
ture for [Mn -(m-O) -Mn ] complexes.
There-
2
fore, the subscripts 1 and 2 used in the description of the
III
IV
simulation are assigned to the Mn and Mn sites, respec-
tively. We, thus, propose that complex 2 spontaneously
III
IV
n+
evolves into [(mL)Mn -(m-O) -Mn (mL)] , denoted 3, in
2
which one of the aromatic groups of the ligand would be un-
coordinated. The tetracoordination of a potentially penta-
III
dentate ligand has already been observed in [(mL )ClFe -
5
III
2+ [7,84]
(
m-O)-Fe Cl(mL )]
.
The quasi-ideal superimposition
5
of the recorded X-band EPR spectrum with that of
III
IV
3+[7]
[
(mL )Mn -(m-O) -Mn (mL )]
suggests that the pend-
4
2
4
ent arm is the phenol ring, which in addition would be pro-
tonated, thereby leading to an overall 3+ charge for com-
plex 3. In fact, the definite identification of the uncoordinat-
ed arm of the ligand would require isolation of 3, since the
conversion of 2ox into 3 is not complete. Indeed, quantifica-
tion measurements made on the EPR spectrum of a solution
of 2 after 10 h indicate that species 3 only accounts for 15%
of the total. Fortunately, the other species are EPR-silent
III
IV
(
Mn or strongly antiferromagnetically coupled di-Mn
species) and allow the detection of 3 by EPR spectroscopy.
Figure 5. Experimental (solid line) and simulated (dashed line) X-band
EPR spectra recorded on acetonitrile solutions containing 0.1m tetrabu-
Electrochemical oxidation of 2 into 2ox: The generation of
the expected mixed-valent, unsupported, mono-m-oxo com-
III
IV
3+
tylammonium perchlorate: a) [(mL)Mn -(m-O)-Mn (mL)]
(2ox);
III
IV
n+
2
b) [(mL)Mn -(m-O) -Mn (mL)]
(3). Recording conditions: 0.5 mT
III
IV
3+
plex [(mL)Mn -(m-O)-Mn (mL)] (2ox) can be predicted
from the one-electron oxidation of species 2. Indeed cyclic
voltametry, EPR, and UV/Vis data recorded on an electro-
lysed solution of 2 at E=1.1 V versus the SCE demonstrate
the formation of 2ox.
modulation amplitude, 100 kHz modulation frequency, T=100 K, 2 mW
microwave power, n= a) 9.420, b) 9.386 GHz microwave frequency. Sim-
ulation parameters: half-width at half-height=1.15 mT (a), 1.25 mT (b);
agreement factor R=0.030 (a), 0.016 (b).
The oxidation of 1 was performed in the electrochemical
cell. Ten minutes after the addition of tBuOOH had been
completed, the temperature was decreased to ꢀ308C in
order to slow down the spontaneous evolution of 2 (see
above). A cyclic voltamogram was recorded on the electro-
lysed solution after the comsumption of one electron per
molecule of 2 (not shown). Starting from E=1.1 V versus
the SCE, the trace displays one reversible wave when scan-
III
IV 3+
valent [Mn -(m-O) -Mn ]
core complex. A simulation
2
has been performed by using the commonly used hypothesis
for such systems (see the Experimental Section). The pa-
rameters used for the simulation are listed in Table 3. The gƒ
tensor presents a very small anisotropy (g ꢀg =0.02). This
x
z
value is fully consistent with those determined for di-m-oxo
III
IV
and di-m-oxo-m-acetato Mn Mn complexes by using high-
field, high-frequency EPR spectroscopy,
[36,60±62]
1/2
which is a
ning towards negative potentials (E =0.65 V versus the
SCE) and one reversible wave when scanning towards posi-
1
/2
tive potentials (E =1.16 V versus the SCE). These data are
Table 3. EPR parameters determined by simulation performed with
rhombic but colinear tensors. Hyperfine coupling constants are given in
III
fully consistent with the reduction of [(mL)Mn -(m-O)-
IV
3+
III
III
2+
ꢀ
1
Mn (mL)] (2ox) into [(mL)Mn -(m-O)-Mn (mL)] (2)
cm
.
III
IV
3+
and the oxidation of [(mL)Mn -(m-O)-Mn (mL)] (2ox)
into [(mL)Mn -(m-O)-Mn (mL)] , respectively.
g
jA
1
j
jA
2
j
Ref.
IV
IV
4+
III
IV
3+
[
(L’)Mn -(m-O)-Mn (L’)]
x
y
z
2.006 0.0160 0.0062 [34]
1.997 0.0130 0.0059
1.982 0.0091 0.0062
EPR spectra were recorded at T=4.2 K on 100-mL ali-
quots that were taken regularly (every 100 mC) during the
bulk electrolysis of 2. The series of spectra exhibit a multi-
line signal centred at g=2 that increases in intensity as the
electrolysis progresses. A scrupulous examination of this
iso 1.995 0.0127 0.0061
2
3
ox
x
y
z
2.005 0.0146 0.0062
2.003 0.0139 0.0060 this
1.985 0.0089 0.0065 work
III
IV
signal reveals the superimposition of two Mn Mn ±oxo sig-
natures. Actually, the central lines increase remarkably in in-
tensity while the two outmost features remain constant
during the course of the electrolysis. Moreover, the lowest
and highest field lines exactly coincide in position with
iso 1.997 0.0124 0.0062
x
y
z
2.001 0.0164 0.0069
1.996 0.0149 0.0076 this
1.984 0.0114 0.0072 work
iso 1.994 0.0142 0.0072
Chem. Eur. J. 2004, 10, 1998 ± 2010
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2003

E. Anxolabÿhõre-Mallart, G. Blondin et al.
FULL PAPER
II
III
those observed for the EPR signature of the di-m-oxo-di-Mn
values. Mn /Mn oxidation potentials have been reported
at 0.6 V versus the SCE with monodentate phenolato li-
gands.
III
IV
complex 3. We thus suggest that a new Mn Mn species
arises in addition to the small amount of 3 resulting from
the spontaneous evolution of 2.
[91,92]
In the present case, the observed higher value
could originate from a bridging coordination mode of the
phenolate moieties that will not stabilise high oxidation
states of the Mn ions. From a structural point of view, the
studied complex can easily evolve upon oxidation into two
similar mononuclear Mn complexes where the phenolato
group is monodentate. Actually, when increasing the scan
rate or decreasing the temperature, the value of the ratio of
Two different procedures were performed in order to de-
convolute the recorded signal. In a first approach, the EPR
signal associated with 3 was substracted from the recorded
spectra after scaling of the two outmost lines. The second
method involved subtracting two successive spectra. Both
strategies led to the same signal that spread 16 lines over
the range 278±389 mT (first peak to last trough). This spec-
tral width of 111 mT is significantly smaller than the width
1
p
2
p
the peak intensities I :I decreases (data not shown). This
effect can be interpreted by considering a competition be-
tween an electrochemical±electrochemical process and an
electrochemical±chemical mechanism. Indeed, after the
[(mL)Mn Mn (mL)] species has been formed, it can be
further oxidised into [(mL)Mn Mn (mL)]
evolve chemically. Good candidates for the products of this
chemical evolution are Mn and Mn monomeric species
such as [(mL)Mn (S)] where S is either a solvent or a
water molecule. When the scan rate is high enough or the
temperature low enough, the chemical evolution is frozen
III
IV 3+
of 125 mT typically observed for [Mn -(m-O) -Mn ] core
2
complexes. Indeed, this is reminiscent of the spectral width
III
IV
3+ [34]
II
III
3+
value (119 mT) reported for [(L’)Mn -(m-O)-Mn (L’)]
This result strongly suggests that electrochemical oxidation
.
III
III
4+
or it can
III
of
2
leads to the formation of [(mL)Mn -(m-O)-
IV
3+
II
III
Mn (mL)] (2ox). No simulation was attempted on this
deconvoluted signal. As will be described in the next sec-
tion, a genuine EPR signal (with no contaminating 3 spe-
cies) was recorded on a solution of pure 2ox generated by
direct electrolysis of 1 in the presence of base.
II/III
n+
II
III
3+
and the intermediate mixed-valence [(mL)Mn Mn (mL)]
The oxidation of 2 into 2ox was also characterised by in
situspectroelectrochemistry. The UV/Vis spectr mu (Fig-
ure 3b) recorded after the initial solution of 2 has been elec-
trolysed (E=1.1 V versus the SCE) presents three well-de-
fined absorption bands at 643 (e=2300), 414 (e=4500) and
compound can be further oxidised. This two-step oxidation
is irreversible, as demonstrated by the lack of the associated
III
reduction wave, which means that the [(mL)Mn -
III
4+
Mn (mL)] compound is not stable and ultimately evolves
III
2+
into [(mL)Mn (S)] . On the reverse scan, the cathodic
ꢀ
1
ꢀ1
3
25 nm (e=11200m cm ). Such characteristics are similar
process (1’) can then be interpreted as the reduction of this
III
II
III
to those previously observed for the [(L’)Mn -(m-O)-
species. Indeed, Mn /Mn redox processes involving doubly
IV
3+ [34]
III
Mn (L’)]
attributed to the phenolate!Mn charge-transfer transi-
tion, as reported in the literature. The two other
bands are consistent with oxo!Mn charge-transfer transi-
tions, as described for the [Mn (m-O) ] core.
.
The broad absorption band at 643 nm can be
charged Mn ions have been reported in the literature at
IV
[91]
around 0.5 V versus the SCE.
[82,85,86]
IV
Bulk electrolytic oxidation of 1 into 2ox in the presence of
base: Figure 4a shows the evolution of the cyclic voltamme-
try of a solution of 1 upon addition of one equivalent per
[37,64,87,88]
2
2
We shall also report that a solution of 2ox could equally
be obtained by addition of one equivalent of protons to a
freshly prepared solution of 2. This has been fully establish-
manganese ion of 2,6-dimethylpyridine (also named 2,6-luti-
p
3
dine). A new peak appears at E =1.24 V versus the SCE. It
III
ed by the same techniques. Dismutation of di-Mn com-
was confirmed that this new peak is not due to the added lu-
tidine. It is attributed to the oxidation of an
III
IV
plexes generating mixed-valent Mn Mn species is known
[65]
III
+
to be favoured in the presence of protons.
[(mL)Mn (OH)] species resulting from the deprotonation
III
2+
of the coordinated water molecule in [(mL)Mn (OH )]
.
2
III
Electrochemical oxidation: In the following section, we will
demonstrate that 2ox can be prepared by direct electrolysis
of 1.
This mononuclear Mn species originates from the breaking
of the dinuclear structure of 1 when reaching the Mn Mn
II
III
III
III
[93]
or the Mn Mn oxidation state.
A solution of 1 containing 1 equivalent of 2,6-lutidine per
manganese ion was oxidised by controlled-potential electrol-
ysis at E=1.1 V versus the SCE. The solution changed from
colourless to dark purple before turning dark blue. After
1.5 oxidizing equivalents per manganese ion, the current in-
tensity faded. Cyclic voltametry of the resulting solution ex-
Cyclovoltammetry of 1(ClO ) : The cyclic voltammogram of
4
2
1
mm 1 in acetonitrile in the presence of 0.1m tetrabutylam-
monium perchlorate is shown in Figure 4a. The cyclic vol-
tammetry trace of 1 presents two successive anodic waves
p
1
p
2
(
labelled 1 and 2) at E =0.89 V and E =1.02 V, accompa-
1
/2
nied on the reverse scan by an ill-defined cathodic wave (la-
hibits one well-defined reversible (DE =100 mV) oxida-
p
1/2
1/
belled 1’) at E1
0
=0.56 V (all measured versus the SCE). The
tion wave at E =1.16 V, one well-defined reversible (DE
2
1/2
anodic process is attributed to the two-step oxidation of
=135 mV) reduction wave at E =0.65 V and one irrever-
II
II
2+
III
III
4+
p
[
(mL)Mn Mn (mL)]
into [(mL)Mn Mn (mL)] . The
sible reduction wave at E =ꢀ0.28 V (all measured versus
the SCE; Figure 4c). Comparison between traces b and c of
Figure 4 strongly suggests that the product of the bulk elec-
trolysis of 1 is identical to compound 2ox. This is also sup-
ported by UV/Vis and EPR spectra recorded during the
course of the bulk electrolysis.
potential values are slightly higher than the ones reported
II
II
for the related 1:1 mixture of [(L )Mn Mn (L)](ClO ) ¥
i/a
i
4 2
[66]
H O. This can be attributed firstly to greater steric hin-
2
[
89,90]
drance due to the methyl substituent.
However, these
potentials are fairly high compared to the reported literature
2004
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 1998 ± 2010

A Phenolato-Bridged Manganese Complex
1998 ± 2010
The absorption spectrum recorded after the complete
electrolytic oxidation (Figure 3b) presents a drastic differ-
ence in the 400±800 nm range in comparison to the feature-
pound 2ox. The similarities of the EPR signatures of the
present electrolysed solution and of a solution of 2ox (re-
sulting from the electrolysis of 2 as described above) strong-
ly suggest that the two compounds are identical. We thus
propose that the electrolysis of 1 at the working potential of
[94]
less initial spectrum. This spectrum presents two absorp-
ꢀ
1
ꢀ1
tion bands at 650 (e=2300) and 412 nm (e=4430 cm m ).
Furthermore, this trace almost superimposes in the visible
region with that of 2ox (Figure 3b) except for a slightly
lower absorption value around 490 nm. This lower value is a
good indication that the present solution is not contaminat-
ed by other compounds as opposed to what was observed
when 2ox was generated by the electrochemical oxidation
of 2.
III
1.1 V versus the SCE leads to the formation of [(mL)Mn -
IV
3+
(m-O)-Mn (mL)] (2ox).
The EPR spectrum was simulated according to the com-
III
IV
monly used theoretical frame for mixed-valent Mn Mn
systems (Figure 5a). The results are reported in Table 3. The
gƒ tensor presents a very weak anisotropy with an isotropic
component slighlty lower than 2. This is fairly consistent
ƒ
During the electrolysis, 100-mL aliquots of the solution
were taken and the EPR spectra were collected. The evolu-
tion of the X-band EPR signal is reported in Figure 6.
with an S=1/2 system. The A -hyperfine tensor is found to
1
be close to axial symmetry while the Å -hyperfine tensor is
2
almost isotropic. In addition, the isotropic components of
both tensors are in a 2:1 ratio. These features are character-
III
IV
istic of an Mn Mn system, with the subscripts 1 and 2 cor-
III
IV
responding to the Mn and Mn sites, respectively. The
Mn-hyperfine isotropic values determined for the 2ox spe-
ꢀ
4
ꢀ4
ꢀ1
cies (124î10 and 62î10 cm ) are significantly smaller
III
IV 3+
than the ones reported for [Mn -(m-O) -Mn ] core units
averaged values are 140î10 and 70î10 cm ) and close
to those determined for [(L’)Mn -(m-O)-Mn (L’)] (127î
and 61î10 cm ). A detailed examination of the
Mn -hyperfine tensor principal values reveals differences
between 2ox and [(L’)Mn -(m-O)-Mn (L’)] . Firstly, the x
2
ꢀ
4
ꢀ4
ꢀ1
(
III
IV
3+
ꢀ
4
ꢀ4
ꢀ1
1
0
III
III
IV
3+
ꢀ
4
component is significantly smaller in 2ox (146î10 versus
ꢀ
4
ꢀ1
1
60î10 cm ). This is the origin of the smaller X-band
spectral width detected experimentally for 2ox relative to
III
IV
3+
that of [(L’)Mn -(m-O)-Mn (L’)] . Secondly, the rhombici-
ties differ: the (jA jꢀjA j)/(jA jꢀjA j) ratios for 2ox
1
x
1y
1y
1z
III
IV
3+
and [(L’)Mn -(m-O)-Mn (L’)] are equal to 0.14 and 0.77,
respectively. This may originate from the differences in the
III
IV
3+
pentacoordinated ligands. In [(L’)Mn -(m-O)-Mn (L’)]
,
the nitrogen imine atom is in the trans position relative to
the bridging oxo atom and the two pyridine rings are trans
to each other, thereby defining the Jahn±Teller elongation
III
axis of the Mn site. On the other hand, if one assumes that
the coordination of the manganese ions is unchanged in 2ox
relative to the coordination in 1, with the MnꢀO1’ and
Mn’ꢀO1 bonds in 1 being replaced by the Mnꢀ(m-O) links,
Figure 6. Evolution of the X-band EPR spectrum of 1 during bulk elec-
trolysis at E=1.1 V versus the SCE in acetonitrile containing 0.1m tetra-
butylammonium perchlorate: a) before electrolysis; b) after 2.3 electrons
per molecule of 1; c) after 3 electrons per molecule of 1. Recording con-
ditions: 0.5 mT modulation amplitude, 100 kHz modulation frequency,
T=10 K, 0.008 mW microwave power, n=a) 9.38, b) 9.383 and
c) 9.38 GHz microwave frequency.
the two pyridine rings would be cis to each other and the ni-
trogen amine atom, N1 or N1’, would be trans to the bridg-
ing oxo atom. Such differences in the coordination sphere
may lead to a different Jahn±Teller distortion around the
III
Mn centre and thus to differences in the electronic effects
III
that are reflected in the principal values of the Mn -hyper-
fine tensor.
Trace a was recorded before the electrolysis started and cor-
responds to the signature of species 1, as described above.
After consumption of 2.3 electrons per molecule of 1 (Fig-
ure 6b), the signal of 1 disappears and a very weak multiline
signal centred at g=2 appears. After consumption of 3 elec-
trons per molecule of 1, this signal has gained in intensity
and exhibits 16 lines that are regularly separated by approxi-
mately 7 mT; this signal is characteristic of a mixed-valent
Formation of the m-oxo bridges: Starting from the di-m-phe-
nolato-di-Mn complex 1, we have shown that the unsup-
II
ported mono-m-oxo-di-manganese core unit can be formed
upon chemical or electrochemical oxidation. Although it
would be surprising if both oxidations proceeded according
to the same mechanism, the present study affords a unique
opportunity to propose intermediates on the route from the
III
IV
II
Mn Mn
dinuclear complex (Figure 6c). The spectral
Mn to Mn±oxo clusters.
II
width of 111 mT (first peak to last trough) is identical to
that of the X-band EPR signal reported above for com-
Electrochemical interconversion of mononuclear Mn
III
IV
into dinuclear Mn Mn complexes with the concomitant
Chem. Eur. J. 2004, 10, 1998 ± 2010
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2005

E. Anxolabÿhõre-Mallart, G. Blondin et al.
FULL PAPER
III
formation of oxo bridges has been previously reported in
the literature. In these cases, one of the exogenous ligands
tion cannot unambigously be attributed to a specific Mn
system, it is likely that it corresponds to some EPR-silent
(
namely acetate) or the ligand itself served as an internal
species. Thirdly, the unsupported mono-m-oxo bridge system
[55,95,96]
III
III
base for the deprotonation of the water molecules.
exists in the Mn Mn oxidation state as described above
and is thus a meaningful precursor of the ultimate observed
species 2ox. In addition, the oxidation potential of the
Since the ligand:Mn ratio is 1 in the starting material 1 as in
the resulting complex 2ox, we have chosen to add an exter-
nal base to assist the formation of the oxo bridge. The
choice of 2,6-lutidine was guided by the will to use a non-
coordinating base with a high oxidation potential.
III
III 4+
[Mn -(m-O)-Mn ] core complex 2 is 0.65 V versus the
SCE, which leads to the conversion of 2 into 2ox at the
working potential of 1.1 V versus the SCE. It is worth notic-
ing that the one-electron reduction wave of 2ox present on
the cyclic voltammogram recorded at the end of the elec-
trolysis of 1 (Figure 4c) is reversible. This means that, in the
A complete mechanism leading to the formation of 2ox is
proposed in Scheme 2 where B stands for 2,6-lutidine. In the
III
presence of the protonated 2,6-lutidine, the [Mn -(m-O)-
III 4+
Mn ] core unit remains unprotonated.
In order to tackle the role of the base, bulk electrolyses
have been performed with different amounts of 2,6-lutidine.
Preliminary results indicate that when no base or only one
equivalent of 2,6-lutidine per molecule of 1 is present, the
electrolysed solution remains EPR-silent. Furthermore, UV/
III
Vis signatures support the presence of Mn species and the
recording of slightly different absorption spectra depending
on the amount of base introduced suggests distinct chemical
compositions for the oxidised solutions. This work is under
progress.
We have shown that the di-m-oxo-di-Mn complex is also
III
IV
accessible in the mixed-valent Mn Mn state (complex 3).
Consequently, the formation of a second oxo bridge from
the electrochemical oxidation of 1 has been considered by
addition of two extra equivalents of 2,6-lutidine per mole-
cule of 1. When the electrolysis is pursued in the presence
of four equivalents of base per molecule of 1, no condensa-
tion of a second water molecule to generate a di-m-oxo-di-
Mn core complex has been observed. For instance, the UV/
Vis spectrum recorded when the electrolysis is completed is
identical to that of 2ox. This suggests that water molecules
or hydroxy moieties are not able to substitute one of the ar-
Scheme 2. Proposed mechanism for the electrochemical formation of
2
ox.
present scheme, the water molecule comes from residual
water in the acetonitrile solvent. The initial step is the oxi-
II
II
dation of the Mn Mn dimer 1 that leads to the breaking of
the phenolato bridges and the generation of monomeric
III
III
+
Mn complexes. A key species is [(mL)Mn (OH)] which
originates from the deprotonation of the aquo complex
[
III
+
(mL)Mn (OH )] . The condensation of two mononuclear
2
+
III
III
ꢀ
Mn species, one of which being [(mL)Mn (OH)] , leads
omatic arms of the ligand mL .
III
to the dinuclear Mn complex 2. This step requires one ad-
According to the proposed mechanism, complex 2 is an
intermediate in the electrochemical oxidation of 1 into 2ox,
but it can hardly be isolated. When the oxidation of 1 is per-
formed with tBuOOH, 2 has been clearly identified. One
can propose that the insertion of the oxo group involves a
ditional equivalent of base per molecule of 1. Thus, a total
of two equivalents of base per molecule of 1 is needed to
form the oxo bridge. At this point, the applied electrolysis
potential is high enough (E=1.1 V versus the SCE) to oxi-
III
III
2+
IV
dise further the EPR-silent [(mL)Mn -(m-O)-Mn (mL)]
dinuclear species into the mixed-valent EPR-active
high oxidation state of the Mn ion, such as an Mn =O spe-
2
cies, as depicted in Scheme 3. The overall reaction is given
in Equation 2.
III
IV
3+
[
(mL)Mn -(m-O)-Mn (mL)]
complex 2ox. The overall
electrochemical process is summarised in Equation 1.
II
II
2þ
½
½
ðmLÞMn Mn ðmLފ þ tBuOOH !
III III
II
II
2þ
ð2Þ
½
½
ðmLÞMn Mn ðmLފ þ H O þ 2 B !
2þ
ꢀ
þ
2
ðmLÞMn -ðm-OÞ-Mn ðmLފ þ tBuO þ H
ð1Þ
III
IV
3þ
þ
ꢀ
ðmLÞMn -ðm-OÞ-Mn ðmLފ þ 2 BH þ 3 e
The first step in the proposed mechanism would be the
II
It is worth noticing that several experimental arguments
addition of one peroxide molecule onto one of the Mn ions
support the proposed mechanism. Firstly, an EPR-silent step
is indeed observed during the course of the electrolysis
before the growth of the 111 mT width EPR signal that is
of the dinuclear complex 1. The heterolytic breaking of the
IV
OꢀO bond would generate a monomeric Mn ±oxo species,
concomitantly with the release of tert-butylhydroxide. The
III
IV
3+
characteristic of the [(mL)Mn -(m-O)-Mn (mL)] species
ox. Secondly, a transient reddish colour is observed as the
condensation of this high-valent Mn±oxo complex with the
II
2
other Mn moiety of the initial complex 1 leads to the for-
electrolysed solution changes from colourless (complex 1) to
dark blue (complex 2ox). Even though this reddish coloura-
mation of the oxo bridge with the concomitant oxidation of
the Mn site by the Mn ion to result in the experimentally
II
IV
2006
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 1998 ± 2010

A Phenolato-Bridged Manganese Complex
1998 ± 2010
Scheme 3. Proposed mechanism for the chemical formation of 2 and its evolution to 3.
III
III
2+
observed [(mL)Mn -(m-O)-Mn (mL)]
complex (2). A
prepare a pure sample of 2ox is the direct electrochemical
oxidation of 1 in presence of an external base. With the
same ligand, we have thus obtained three types of bridges,
with the Mn ions in different oxidation states.
similar mechanism has already been proposed in the litera-
ture for the alkene epoxidation catalysed by Mn±salen com-
[97]
plexes and has recently been proved by mass spectromet-
[
98,99]
ric measurements.
In these reported cases, the initial
The distinction between the mono-m-oxo and the di-m-oxo
complexes was made possible thanks to a thorough analysis
of the EPR signals of the two species. Indeed the simulated
Mn complexes are in the +III oxidation state and thus the
detected intermediate species is a Mn =O complex. Such a
mechanism is also invoked by Larson and Pecoraro for the
formation of di-m-oxo-di-Mn complexes from Mn Mn
systems with tBuOOH as the oxidant.
ent experimental results and in the absence of low-tempera-
ture experiments, the proposed Mn =O species is only pu-
V
III
IV
parameters for the two Mn Mn mixed-valent complexes
showed Mn-hyperfine isotropic values significantly smaller
for the mono-m-oxo system (2ox) than for the di-m-oxo
system (3). This has already been reported for another
IV
III
III
[
100]
Based on the pres-
IV
III
IV
[34]
mono-m-oxo Mn ±O±Mn complex.
tative. Moreover, the bridging oxygen atom could also come
from a water molecule, since a terminal oxo moiety is sub-
ject to water molecule exchange.
No structural data are available so far for complexes 2
and 2ox. Linear Mn±O±Mn units have been reported for
III
III/IV
2+/3+[34]
[(L’)Mn -(m-O)-Mn
(L’)]
but a bent structure has
III
III
2+
Under the present experimental conditions, complex 2 ul-
been observed in [(m L )(Cl)Fe -(m-O)-Fe (Cl)(m L )]
1
4
1
4
III
[7,84]
ꢀ
timately evolves into a mixture of dinuclear [(mL)Mn -(m-
.
If the two pyridine rings of mL in the m-oxo species
IV
n+
O) -Mn (mL)] (3) and other EPR-silent species. We pro-
are in the cis position relative to each other, as they are in
1, it is unlikely that the core units of complexes 2 and 2ox
are linear. Resonance Raman studies would be of great help
in settling this point.
2
pose that the formation of the second oxo bridge results
from the reaction of tBuOOH with the already present
III
III
2+
[
[
(mL)Mn -(m-O)-Mn (mL)]
complex, to generate an
IV
IV 4+
III
Mn -(m-O) -Mn ]
core species. This second insertion
A mechanism involving the condensation of two Mn
2
would be slower than the first one since it requires the dis-
placement of one of the aromatic arms. The possible break-
ing of one of the aromatic arms cannot be totally excluded
moieties is proposed for the direct electrochemical prepara-
tion of 2ox. The oxo bridge is formed by the deprotonation
of a water molecule, thanks to the presence of an external
base in the solution. This is, to our knowledge, the first time
that an extra base has been added to the medium to assist
the formation of oxo bridges. Based on this electrochemical
method, the preparation of a number of other Mn±oxo clus-
ters can be anticipated. Work in this direction is under prog-
ress with polypyridyl ligands.
IV
in the presence of a strong oxidant. The generated Mn -
IV
Mn species would then oxidise either a solvent molecule
III
or the ligand itself to result in the EPR-active [Mn -(m-O) -
Mn ] core complex 3.
2
IV 3+
IV
Conclusion
A putative Mn =O intermediate is proposed in the mech-
anism of chemical formation of the oxo bridge of complex 2.
Even though no experimental data are available so far, it is
likely that in this case the oxo bridge is the result of the in-
sertion of an oxo moiety rather than of a condensation pro-
cess. Low-temperature experiments as well as stopped-flow
spectroscopy measurements would undoubtedly help in the
assessment of the mechanism. In particular, the recording
II
II
A new Mn Mn dinuclear complex has been obtained with
pentadentate ligand offering an [N O] coordination
a
4
sphere. We have shown that chemical oxidation of this phe-
nolato-bridged Mn Mn species (1) by tBuOOH leads to
the formation of the unsupported mono-m-oxo-bridged com-
plex [(mL)Mn -(m-O)-Mn (mL)] (2). In the presence of
an excess of tBuOOH, complex 2 ultimately evolves into the
mixed-valent di-m-oxo [Mn -(m-O) -Mn ] core complex
3). Complex 2 can also be electrochemically oxidised into
the unsupported mono-m-oxo-bridged mixed-valent complex
(mL)Mn -(m-O)-Mn (mL)]
II
II
III
III
2+
IV
and analysis of a pure Mn =O EPR signal remains a chal-
III
IV 3+
lenging task. It is anticipated that working with bulkier li-
gands will prevent the condensation step and thus favour
the detection of mononuclear intermediates. Several Mn
2
(
III
IV
3+
ꢀ
[
(2ox). Another route to
complexes with similar ligands to mL in which the ethane
Chem. Eur. J. 2004, 10, 1998 ± 2010
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2007

E. Anxolabÿhõre-Mallart, G. Blondin et al.
FULL PAPER
bridge has been replaced by a propane bridge have been ob-
tained and their reactivities towards oxidation are under in-
vestigation.
CH
2
) or 1.5 (CH
3
) times that of the parent atom. The perchlorate anion
is found disordered on two positions and refined with a 0.5 occupation
factor. All calculations were performed on an O2 Silicon Graphics Sta-
tion with the SHELXTL package.
[103,104]
Formula: Mn
2 44 2 8 r
C H50Cl N O10; M =1031.70, T=123(2) K, l=0.71073 ä;
crystallographic system: orthorhombic; space group: Fdd2; a=10.954(2),
3
ꢀ3
b=24.052(5), c=38.982(8) ä, V=10270(4) ä , Z=8; 1=1.334 gcm
,
Experimental Section
ꢀ
1
m=0.655 mm
,
q
max =24.738; hkl ranges: ꢀ12ꢂhꢂ12, ꢀ27ꢂkꢂ28,
45ꢂLꢂ45; reflections measured: 16010; independent reflections:
272; reflections observed with I>2s(I): 3703; parameters: 325; R1=
.0636 jꢀjjF jj/ÆjF j); wR2=0.1664 (wR2=
(R1=ÆjjF
ꢀ
General remarks: Reagents and solvents were purchased commercially
and used as received, except the acetonitrile for electrochemical experi-
ments which was distilled under argon over granular CaCl .
2
4
0
{
o
2
c
o
2
o
2
c
2
2
2
o
2
1/2
2
2
o
2
Æ[w(F ꢀF ) ] /Æ[w(F ) ] } with w=1/[s (F ) + (0.1000P) ], where P=
Caution: Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Only small quantities of these compounds should
be prepared and they should be handled behind suitable protective
shields.
2
o
2
c
ꢀ3
ꢀ3
(
F
+ 2F )/3); (D/s)max =0.020, D1max =0.610 eä , D1min =ꢀ0.379 eä
.
CCDC-215363 contains the supplementary crystallographic data (exclud-
ing structures factors) for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retreiving.html (or from Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223±336±033; or e-mail: deposit@ccdc.cam.ca.uk).
Synthesis of N,N’-bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N’-methyl-
ethane-1,2-diamine (mLH): The synthesis of mLH is adapted from the
[
101]
literature
and consists of the following two-step synthesis.
Physical measurements: Elemental analyses were performed at Service
Central d×Analyses, CNRS Vernaison-France. Infrared spectra were re-
corded on KBr pellets over the range 4000±200 cm with a Perkin±
Elmer Spectrum 1000 spectrophotometer. H NMR spectra were record-
Synthesis of N-(2-hydroxybenzyl)-N’-methyl-ethane-1,2-diamine: N-
Methyl-ethane-1,2-diamine (0.88 mL, 10 mmol) and salicylaldehyde
1.06 mL, 10 mmol) were stirred in methanol (20 mL) at room tempera-
ture for one hour. Methanol (80 mL) was added and the resulting imine
was hydrogenated over Pd/C (10%; 60 mg) under pressure (15 bars)
overnight. The resulting mixture was filtered over celite, washed with
ꢀ1
(
1
ed by using
a Bruker AC-200 (200 MHz) or a Bruker AM-250
(
250 MHz) apparatus.
Magnetic susceptibility measurements: Magnetic susceptibility data were
recorded on an MPMS5 magnetometer (Quantum Design). The calibra-
tion was made at 298 K by using a palladium reference sample furnished
by Quantum Design. The data were collected over a temperature range
of 2±300 K at a magnetic field of 10 kOe and were corrected for diamag-
methanol and concentrated under vacuum. A yellow oil (1.54 g, 85%
1
yield) was obtained; H NMR (200 MHz, CDCl
3
): d=7.15 (t, 1H,
ArꢀH), 6.99 (d, 1H, ArꢀH), 6.80 (m, 2H, ArꢀH), 4.00 (s, 2H,
NꢀCH ꢀAr), 2.75 (s, 4H, Nꢀ(CH ꢀN), 2.43 (s, 3H, CH
)
).
2
2
2
3
Synthesis of N,N’-bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N’-methyl-
ethane-1,2-diamine: 2-Picolyl chloride in hydrochloride form (1.85 g,
1 mmol) was dissolved in water (10 mL) and neutralised by 4m sodium
hydroxide. N-(2-hydroxybenzyl)-N’-methyl-ethane-1,2-diamine (1.0 g,
.5 mmol) was then added. The resulting mixture was heated to 658C
and neutralised progressively over one hour by addition of small aliquots
of 4m sodium hydroxide in order to prevent the pH value exceeding 9.
The mixture was cooled, stirred overnight and then extracted with di-
netism. The c
M
T versus T curves were fitted by using the van Vleck for-
mula and assuming the same g factor for each S spin state. The total spin,
1
S, runs from jS
1
ꢀS
2
j to (S
1 2
+ S ) by unit steps. The exchange Hamiltoni-
√ √
S , in which J stands for the exchange coupling con-
1 2
for the spin operators associated with the electronic
=S =5/2).
√
an used was H=ꢀJS
5
√
√
stant and S
1
and S
2
II
spin of the Mn sites (S
1
2
EPR spectroscopy: EPR spectra were recorded on a Bruker 200D (X
band) and Bruker ELEXSYS 500 (X and Q band) spectrometer equipped
with an Oxford Instrument continuous-flow liquid helium cryostat and a
temperature-control system. Solution spectra were recorded with 0.1m
tetrabutylammonium perchlorate in acetonitrile as solvent.
chloromethane (6î50 mL). The combined CH
with water and dried over Na SO . After concentration under vaccum, a
red oil (1.37 g, 69% yield) was obtained and was purified on an alumina
column with CH Cl /MeOH (10:1) as the eluent. The desired compound
was isolated as the first product (red oil, 0.60 g, 30% yield); H NMR
200 MHz, CDCl ): d=8.50 (m, 2H, H-C N), 7.65±6.70 (m, 10H, H-
Ar), 3.77 (s, 2H, NꢀCH ꢀC N), 3.74 (s, 2H, NꢀCH ꢀC
N), 3.60 (s,
H, NꢀCH ꢀC OH), 2.73 (dt, 2H, NꢀCH ꢀCH
), 2.64 (dt, 2H,
NꢀCH ꢀCH
), 2.12 (s, 3H, CH ).
Synthesis of 1(ClO : mLH (300 mg, 0.83 mmol)and 2,6-dimethylpyri-
dine (2,6-lutidine; 0.2 mL, 2.0 mmol) in methanol (3 mL) were added to
2 2
Cl extracts were washed
2
4
2
2
III
IV
1
Quantification at X band of the mixed-valent di-m-oxo Mn Mn system
was performed by using the di-m-oxo complex [(mL
III
)Mn -(m-O)
2
-
(
3
5 3
H
4
IV
3+
Mn (mL
4
)]
as a reference. The X-band EPR solution spectrum of
2
5
H
3
2
5 3
H
III
IV
[36]
0
.1 mm [(mL
4
)Mn -(m-O)
2
-Mn (mL
4
)](BPh
4
)
2
(ClO
4
)
was recorded at
2
2
6
H
4
2
2
both 20 and 100 K. The spectra were superimposable, a fact demonstrat-
ing that the signal only originates from the S=1/2 ground state at 100 K.
Nonsaturating recording conditions were only achieved at 100 K.
2
2
3
4 2
)
II
Simulations of the X-band EPR spectra were performed by using a pro-
gram described in ref. [57]. An S=1/2 system was considered and the
EPR spectrum profile was assumed to be governed by the Zeeman effect
within the S=1/2 state and the two hyperfine interactions between the
S=1/2 electronic spin on one hand and each manganese I=5/2 nuclear
spin on the other. The Hamiltonian [Eq. (3)] was used with assumption
of colinear tensors.
4 2 2
Mn (ClO ) ¥6H O (300 mg, 0.83 mmol) in methanol (3 mL) and the re-
sulting mixture was stirred for 15 min. Distilled water (5 mL) was added
to the resulting dark-green solution and a pale-yellow powder precipitat-
ed out. The precipitate was filtered, washed with water and carefully
dried on a fritted glass to give 1 (330 mg, 38% yield). Crystals suitable
for X-ray crystallography were obtained by slow diffusion of diethyl
ether in an acetonitrile solution of 1; IR (KBr): n˜ =3447 (w), 2862 (m),
1
601 (w), 1570 (m), 1480 (w), 1445 (m), 1267 (w), 1091 (w), 1015 (s), 884
ꢀ
1
(
s), 843 (s), 765 (m), 624 (m), 611 (m), 576 (s), 473 (s) cm ; elemental
analysis: calcd (%) for C44 Mn (1067.76): C 49.49, H 5.10, N
0.49; found: C 49.56, H 5.02, N 10.17.
Crystallographic data collection and refinement of the structure of
(mL)MnMn(mL)](ClO : A colourless crystal with approximate dimen-
H^ ¼ m
^ ^
^ ~ ^
^ ~ ^
þ SA
ð3Þ
B ~g S þ SA
I
2 2
I
B
1
1
H
54
N
8
O
12Cl
2
2
1
Only the electronic and nuclear spin values are introduced to perform
the EPR simulation and no hypothesis was formulated on the oxidation
states of the manganese ions. A Gaussian line shape was used.
[
4 2
)
sions of 0.20î0.10î0.10 mm was selected. Diffraction collection was car-
ried out on a Nonius diffractometer equipped with a CCD detector. The
lattice parameters were determined from ten images recorded with 28 F
scans and later refined on all data. The data were recorded at 123 K. A
Cyclic voltammetry and bulk electrolysis: Cyclic voltammetry and chrono-
coulometry measurements were recorded with an EGG PAR potentiostat
(M273 model). The counterelectrode was an Au wire, the working elec-
trode was a glassy carbon disc carefully polished before measurement of
each voltammogram with a 1 mm diamond paste, sonicated in ethanol
bath and then washed carefully with ethanol. In absence of such treat-
ment of the working electrode surface, the successive cyclic voltammo-
1
808 F range was scanned with 28 steps with the crystal-to-detector dis-
tance fixed at 30 mm. Data were corrected for Lorentz polarisation. The
structure was solved by direct methods and refined by full-matrix least-
2
squares on F with anisotropic thermal parameters for all non-hydrogen
[
102]
atoms.
Hydrogen atoms were introduced at calculated positions as
riding atoms, with an isotropic displacement parameter equal to 1.2 (CH,
grams were not reproducible. The reference electrode was an Ag/AgClO
4
2008
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 1998 ± 2010

A Phenolato-Bridged Manganese Complex
1998 ± 2010
electrode (0.530 V versus the normal hydrogen electrode) isolated in a
fritted bridge. The solvent used was distilled acetonitrile where tetrabuty-
lammonium perchlorate was added to obtain a 0.1m supporting electro-
lyte when the experiments are carried out at room temperature or a 0.2m
supporting electrolyte for experiments carried out at low temperature
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20 psi) and the declustering potential (20 V). No heating source was
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ꢀ
3
analysed in positive ion mode. Diluted samples (ꢁ10 m) in pure aceto-
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[
[
[
1
2
7] Abbreviations: OEC=oxygen-evolving complex; mLH=N,N’-
bis(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N’-methylethane-1,2-di-
amine;
methyl)ethane-1,2-diamine; mL
methylethane-1,2-diamine; mL
dimethylethane-1,2-diamine;
methylethane-1,2-diamine; N,Nbispicen=N,N-bis(2-pyridylmethyl)-
H
2
BBPEN=N,N’-bis(2-hydroxybenzyl)-N,N’-bis(2-pyridyl-
=N,N,N’-tris(2-pyridylmethyl)-N’-
=N,N’-bis(2-pyridylmethyl)-N,N’-
=N,N’-bis(2-pyridylmethyl)-N-
5
4
m
1
L
4
ethane-1,2-diamine;
mine.
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4
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Received: September 4, 2003 [F5515]
2010
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 1998 ± 2010