A new pentadentate ligand forms both a di- and a mononuclear Mn II complex: Electrochemical, spectroscopic and superoxide dismutase activity studies

Article abstract of DOI:10.1002/ejic.200601236

The X-ray crystal structure of the dinuclear complex [1(PF ₆)₂] derived from a new ligand bearing both imidazole and phenolato moieties, namely N-(2-hydroxybenzyl)-N,N′-bis[2-(N- methylimidazolyl)methyl]ethane-1,2-diamine (LH), is de

Full text of DOI:10.1002/ejic.200601236

FULL PAPER
DOI: 10.1002/ejic.200601236
A New Pentadentate Ligand Forms Both a Di- and a Mononuclear MnII
Complex: Electrochemical, Spectroscopic and Superoxide Dismutase Activity
Studies
Federico Cisnetti,[a,b] Anne-Sophie Lefèvre,[a,b] Régis Guillot,[b,c] François Lambert,[a,b]
Guillaume Blain,[b,c] Elodie Anxolabéhère-Mallart,[b,d] and Clotilde Policar*[a,b]
Keywords: Manganese(II) / Enzyme models / SOD mimic / N,O ligands / Bioinorganic chemistry
The X-ray crystal structure of the dinuclear complex
[1(PF6)2] derived from a new ligand bearing both imidazole
and phenolato moieties, namely N-(2-hydroxybenzyl)-N,NЈ-
bis[2-(N-methylimidazolyl)methyl]ethane-1,2-diamine (LH),
is described and its properties in organic solvent (CH3CN)
investigated (EPR, electrochemistry). [1(PF6)2] is shown to be
a mononuclear MnII species in aqueous solution and displays
an efficient SOD-like activity, as measured by the McCord–
Fridovich assay performed both in conventional phosphate
buffer and in a noncoordinating buffer (PIPES).
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
biliverdin manganese complexes[48] have been reported to
react with superoxide either as a true SOD mimetic or as a
scavenger.
Introduction
Superoxide is known to be a toxic reactive oxygen species
(ROS) involved in oxidative stress[1–4] and is considered to
be responsible for several pathologic situations. Under nor-
mal conditions, its concentration is kept low (8–40 p)[5,6]
by metalloenzymes, known as superoxide dismutases
(SODs), that catalyze its dismutation. As the therapeutic
use of purified enzymes has major drawbacks,[3] different
low molecular weight complexes have been synthesized for
this purpose using either copper, iron, manganese, or cobalt
ions. As Mn derivatives do not lead to Fenton chemistry,
special interest has been devoted to Mn complexes (oxi-
dation states II, III, or IV) as antisuperoxide agents for pos-
sible therapeutic use.[2–4] Free MnII [7–9] or Mn coordinated
to salen derivatives,[8,10–12] cyclic polyamines,[3,13–19] tri-
or dipodal ligands,[20–28] 1,2-ethanediamine-based li-
gands,[29–32] desferrioxamine derivatives[13,33,34] polyamino-
carboxylato[35–37] or polycarboxylato ligands,[38] pep-
tides,[39–41] as well as MnIII porphyrins,[42–47] or dinuclear
One of the goals of our research is the synthesis of bioac-
tive complexes of therapeutic interest in the case of oxida-
tive stress.[2–4] In the past few years we have synthesized a
series of Mn complexes containing tripodal amines as tetra-
dentate ligands that show an SOD-like activity, according
to the results of the indirect McCord–Fridovich as-
say[24,27,28] and pulsed radiolysis experiments.[26,27] These
complexes were designed as bio-inspired functional SOD
mimics bearing in mind that the active site of Mn-based
SODs has a trigonal-bipyramidal N3O2 core with three his-
tidines, a monodentate aspartate, and an aqua or hydroxido
ligand.[49]
Pentadentate ligands based on a 1,2-ethanediamine moi-
ety are interesting alternatives to the previous tetradentate
N-tripod series because they offer a larger number of pos-
sible variations.[50–52] We report here the synthesis of a com-
plex with a new pentadentate ligand L [LH = N-(2-hydroxy-
benzyl)-N,NЈ-bis[2-(N-methylimidazolyl)methyl]ethane-1,2-
diamine], which contains a 1,2-ethanediamine backbone
bearing one phenol (mimicking the monodentate carboxyl-
ate present at the Mn-SOD active site) and two 1-methyl-
imidazole groups (mimicking the histidine side-chains) as
coordinating groups (Scheme 1). We describe the structure
and solution characterization (EPR and UV/Vis spec-
troscopy, electrochemistry) of the dinuclear complex [(L)-
MnMn(L)](PF6)2 [1(PF6)2] in organic solution and show
that in aqueous solution this complex is in the monometal-
lic form [(L)Mn]+ (EPR, cyclic voltammetry). The SOD-
like activity of the [(L)Mn]+ species has been measured in
aqueous solution using the conventional McCord–
[a] Equipe de Chimie Biorganique et Bioinorganique, Institut de
Chimie Moléculaire et des Matériaux d’Orsay, Univ. Paris-Sud,
UMR 8182,
91405 Orsay, France
E-mail: cpolicar@icmo.u-psud.fr
[b] Institut de Chimie Moléculaire et des Matériaux d’Orsay,
CNRS, UMR 8182,
91405 Orsay, France
[c] Institut de Chimie Moléculaire et des Matériaux d’Orsay, Univ.
Paris-Sud, UMR 8182,
91405 Orsay, France
[d] Equipe de Chimie Inorganique, Institut de Chimie Moléculaire
et des Matériaux d’Orsay, Univ. Paris-Sud, UMR 8182,
91405 Orsay, France
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
4472
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Di- and Mononuclear MnII Complexes
Fridovich test under noncoordinating conditions. The good
activity found is consistent with the redox potential of the
[(L)Mn]+ species measured by CV.
Scheme 1. Synthetic route to LH.
Figure 1. Ellipsoid plot of the centrosymmetric cationic moiety 1,
produced with Mercury 1.4.2.[56] Ellipsoids are drawn at 50% prob-
–
ability level. PF6 and H atoms have been omitted for clarity.
Results and Discussion
The synthesis of LH is straightforward. N,NЈ-Bis[2-(N-
methylimidazolyl)methyl]ethane-1,2-diamine (2) was ob-
tained by a conventional two-step reductive amination pro-
cedure starting from 1-methyl-2-imidazolecarboxal-
dehyde[53] and 1,2-ethanediamine, as published else-
where.[54] The dissymmetric pentadentate ligand containing
a phenolic group was then obtained by treating 2 with sali-
cylaldehyde to obtain the aminal 3 (Scheme 1), which was
reductively opened with NaBH3CN in the presence of an
acid.[55] The whole synthesis proceeded with high yields.
The reaction of crude LH with MnBr2 in warm methanolic
solution led to the formation of a MnII complex. Crystals
suitable for X-ray diffraction were grown by cooling this
solution to room temperature under argon in the presence
of hexafluorophosphate anion.
Table 1. Selected bond distances [Å] and angles [°] in the cationic
complex 1. The uncertainties correspond to 2σ (σ is given in the
cif file).
N4–Mn 2.205(8) O–Mn–O
79.2(2) O–Mn–N2 128.5(2)
N2–Mn 2.420(8) O–Mn–N4 94.2(3) N1–Mn–N2 76.8(3)
N1–Mn 2.354(8) N4–Mn–N2 71.0(3) N4–Mn–N1 113.7(3)
O–Mn
2.111(6) N1–Mn–O 83.1(1) N3–Mn–O 109.9(3)
OЈ–Mn 2.172(6) N1–Mn–N3 72.8(2) N3–Mn–N2 86.0(3)
N3–Mn 2.226(8) N3–Mn–O 89.0(2) N4–Mn–O 97.1(3)
ethane-1,2-diamine].[51] Such a dinuclear structure in which
manganese atoms show a distorted octahedral geometry is
in line with the fact that MnII complexes, due to their half-
filled d5 shell, have no electronic coordination preference.[57]
EPR Spectroscopy
Description of the Structure
The X-ray crystal structure of this complex revealed it to
The X-band EPR spectrum of a 1.1 m acetonitrile solu-
be dinuclear with the formula [(L)MnMn(L)](PF6)2 tion of 1(PF6)2 in the presence of 0.1  tetrabutylammo-
[1(PF6)2]. A view of cation 1 is presented in Figure 1. This nium perchlorate was recorded at 7 K in both the perpen-
cation is dinuclear and centrosymmetric with a central di- dicular and parallel modes. The spectrum recorded in the
µ-phenolato Mn2O2 diamond core. Selected bond lengths perpendicular mode exhibits features extending from 0 to
and angles are reported in Table 1.
800 mT (Figure 2), which arise from the superimposition of
Each MnII ion is hexacoordinate, with four nitrogen the signal of the five paramagnetic spin states S= 1 to 5
atoms of the anionic form L– of the ligand and two oxygen and is characteristic of dinuclear MnII complexes.[51,52,58–63]
atoms from the phenolato groups, the first one from the The band at around 260 mT (B) shows a nice hyperfine
same ligand and the other one from the ligand chelating structure with a field separation between two lines of about
the second MnII ion. The Mn environment departs from 41 G (see insert Figure 2), which is typical of dinuclear
that of a regular octahedron (see Table 1). The longest coor- MnIIMnII complexes. Increasing the temperature induced
dination bond is that to the secondary amine N2, and the only slight modifications in the spectral profile [the band at
N2–Mn–O angle is much larger than the ideal value for an around 204 mT (A) and the intensity of the signal at
octahedron. These distances and deformations are similar 330 mT (C)]. Unlike previous observations with a dinuclear
to those reported for similar MnII dinuclear com- bis-µ-phenolato MnIIMnII complex with a J-value of
pounds,[50,51] in particular to the dinuclear phenolate- –1.5 cm–1,[50] the intensities do not reach a maximum but
bridged dication [(mLЈ)MnIIMnII(mLЈ)]2+ [mLЈH = N-(2- decrease on increasing the temperature from 7 to 30 K (see
hydroxybenzyl)-NЈ-methyl-N,NЈ-bis(2-pyridylmethyl)- Figure 2). This thermal behavior is consistent with the very
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C. Policar et al.
FULL PAPER
weak J-value estimated from the measurement of the mag-
netic susceptibility as a function of temperature (see Experi-
mental Section).
Figure 3. Cyclic voltammograms of 1(PF6)2 (1.1 m in CH3CN
with 0.1  tetrabutylammonium perchlorate) under argon (v =
0.1 Vs–1; working electrode: glassy carbon; counter electrode: Pt
wire; reference electrode: Ag/AgClO4) at 20 (a) –25 (b), and –25 °C
after electrolysis at E = +0.8 V vs. SCE (c). PE = potential of the
preparative electrolysis.
Figure 2. X-band EPR spectrum of 1(PF6)2 (1.1 m in CH3CN
containing 0.1  tetrabutylammonium perchlorate) at 7 K. Micro-
wave frequency: 9.63 GHz; microwave power: 2 mW; modulation
amplitude: 0.5 mT; modulation frequency: 100 kHz; gain: 40 dB.
Electrochemical Access to MnIII
recorded at –25 °C before (Figure 3, b) and after (Figure 3,
The cyclic voltammogram of a 1.1 m acetonitrile solu-
c) complete electrolysis at +0.8 V vs. SCE. After electrolysis
tion of 1(PF6)2 is shown in Figure 3. The CV trace shows a
a
one anodic process was detected at Ep = +1.35 V vs. SCE
a
well-defined anodic wave (wave 1 in Figure 3a) at Ep
=
0.58 V vs. SCE and a well-defined cathodic wave on the
c
reverse scan at Ep = 0.39 V vs. SCE (wave 1Ј). The differ-
c
ence between these two values (∆Ep = Epa – Ep ) of 190 mV
is indicative of a slow electron transfer. A similar behavior
has previously been reported for related dinuclear
MnIIMnII complexes with pyridine instead of imidazole
moieties.[50] Controlled potential coulometry performed at
+0.8 V vs. SCE indicated a one electron per Mn process for
the first oxidation. Thus, the first anodic wave (1/1Ј; E1/2
= 0.485 V vs. SCE)[64] can be attributed to a two-electron
MnIIMnII Ǟ MnIIIMnIII process. This oxidation occurs at
lower potential than that reported for the pyridine-contain-
ing ligand mLЈ[51] as the imidazole group has a higher elec-
tron-donating effect than pyridine, which leads to greater
stabilization of the MnIII oxidation state.
Two other anodic peaks (labeled 2 and 3 in Figure 3, a)
a
are observed at Ep = 1.4 and 1.7 V vs. SCE respectively.
These second and third oxidation processes are tentatively
attributed to MnIII to MnIV and phenolato oxidation pro-
cesses.[50,65]
In an effort to determine the exact nature of the resulting
MnIII species, a controlled potential electrolysis was per-
formed at +0.8 V vs. SCE at –25 °C. Aliquots of the electro-
lyzed solution were collected during the course of the elec-
trolysis and their UV/Vis absorption (Figure 4) and EPR
Figure 4. Evolution of the UV/Vis spectrum of 1(PF6)2 (1.1 m
acetonitrile solution) during bulk electrolysis at E = 0.8 V vs. SCE
before electrolysis (a), after 0.5 electron/Mn (b), and after 1 elec-
spectra recorded. Figure 3 shows the cyclic voltammograms tron/Mn (c) at room temperature. Path length = 0.1 cm.
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Di- and Mononuclear MnII Complexes
c
and one irreversible cathodic process at Ep = +0.1 V vs.
The EPR spectra of a 1 m solution in PIPES (Figure 5)
SCE when scanning towards low potential (labeled 4 in Fig- displays a large six-line signal at g = 2 (∆Bav = 96 G) char-
ure 3, c).
acteristic of a mononuclear MnII center. The low-intensity
As expected for a MnII oxidation state, the UV/Vis spec- signal at g ≈ 3.94 is typical for distorted MnII environ-
trum of 1 (Figure 4) only shows features in the UV region
(λ1 = 287 nm, ε1 = 4909 –1 cm–1) associated with π–π*
transitions of the ligand. The colorless solution turns pink
upon oxidation at 0.8 V and two new absorption bands ap-
pear at λ2 = 403 and λ3 = 672 nm. Based on a complete
conversion of the initial MnII into MnIII, the extinction co-
efficients calculated per Mn ion are ε2 = 2181 and ε3
=
432 –1 cm–1. Similar bands have been reported in the litera-
ture for mononuclear MnIII complexes[66] or dinuclear
MnIIIMnIII complexes with phenolato/pyridine ligands.[65]
The band at 403 nm can therefore be attributed to a phe-
nolato to MnIII LMCT transition. Due to the lower value
of its extinction coefficient, and previous reports in the lit-
erature, the band at 672 nm can be attributed to a d-d tran-
sition.[67,68]
The intensity of the X-band perpendicular mode EPR
spectrum of the initial MnIIMnII complex decreased during
the course of the electrolysis. After completion of the elec-
trolysis (1 electron/1 Mn), the pink solution was EPR-silent
in the perpendicular mode (data not shown), as expected
for a MnIII complex. Unfortunately, no signal was detected
when a parallel mode EPR spectrum was recorded for the
pink solution.
Figure 5. X-band EPR spectra of 1(PF6)2 (10–3  in 50 m aqueous
PIPES buffer, pH 7.5). Microwave frequency: 9.38 GHz; microwave
power: 2.0 mW; modulation amplitude: 0.5 mT; modulation fre-
quency: 100 kHz; gain: 20 dB.
The above-mentioned experimental data confirm the
MnIII oxidation state of the oxidized species, although the
exact nature of the MnIII species remains uncertain as a
dinuclear bis-µ-phenolato MnIIIMnIII complex is likely to
be unstable in solution. As reported previously in the litera-
ture, the bis-µ-phenolato bridge can easily break and gener-
ate two mononuclear [(L)MnIII(S)]2+ complexes, where S
can either be a water molecule or a solvent molecule.[51] It
should be noted that wave 2 disappeared upon addition of
2,6-lutidine[69] and a new anodic wave appeared at 1.02 V
vs. SCE. Based on these results (see Figure S1 in the elec-
tronic supporting information), wave 2 can be assigned to the
[(L)MnIII(OH2)]2+/[(L)MnIV(OH2)]3+ oxidation process and
the wave at 1.02 V to the [(L)MnIII(OH)]+/[(L)MnIV-
(OH)]2+ oxidation process as deprotonation of the aqua
complex lowers the MnIV/MnIII redox potential. Wave 4 can
tentatively be assigned to reduction of the mononuclear
[(L)MnIII(OH2)]2+ species into [(L)MnII(OH2)]+.
Nature of the Complex in Water
Cyclic voltammetry and the EPR spectrum recorded in
CH3CN are consistent with a dinuclear [(L)MnMn(L)]2+
species (1; see above). In order to investigate the behavior
of the complex in aqueous solution, electrochemistry and
EPR spectra were recorded in a noncoordinating buffer,
namely piperazine-N,NЈ-bis(2-ethanesulfonic acid) (PIPES;
50 m, pH 7.5).[38,70]
Figure 6. Cyclic voltammograms in aqueous solution: (A) 1(PF6)2
(10–3 ) in aqueous PIPES buffer (pH 7.5, 50 m) and (B) in aque-
ous phosphate buffer (pH 7.8, 50 m); (C) MnCl2 (10–3 ) in aque-
ous PIPES buffer (pH 7.5, 50 m) and (D) in aqueous phosphate
buffer (pH 7.8, 50 m); (E) 1(PF6)2 (10–5 ) in aqueous PIPES
buffer (pH 7.5, 50 m; the signal recorded for pure PIPES buffer
has been subtracted). Working electrode: glassy carbon; counter
electrode: Pt wire; reference electrode: SCE.
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C. Policar et al.
FULL PAPER
ments.[71–74] This species will be labeled [(L)Mn]+ in the fol-
lowing discussion, although there is probably a water mole-
cule coordinated to the metal center.
Figure 6 shows the cyclic voltammogram of [1(PF6)2] dis-
solved in PIPES (1 m; trace A). Trace A displays a well-
a
defined anodic wave at Ep = 0.282 V vs. SCE and a well-
c
defined cathodic wave on the reverse scan at Ep = 0.116 V
vs. SCE (E1/2 = 0.199 V vs. SCE and ∆Ep = 165 mV). Nei-
ther MnCl2 (trace C in Figure 6) nor Mn(ClO4)2 (not
shown) dissolved in PIPES show any wave at these poten-
tials. A similar voltammogram, although less reversible, was
recorded for 1(PF6)2 dissolved in pH 7.8 phosphate buffer
(see Figure 6 trace B, E1/2 = 0.184 V vs. SCE; Epa = 0.334 V,
c
Ep = 0.037 V vs. SCE; ∆Ep = 331 mV).
Taken together, the EPR spectra and cyclic voltammog-
rams unambiguously indicate that in aqueous solution the
metal-containing species is therefore mononuclear MnII
(EPR) coordinated to L (the CV signature is different from
that of uncoordinated Mn2+).
Figure 7. Determination of the IC50 value for [(L)Mn]+ in PIPES
buffer (50 m, pH 7.5; exp. points: filled circles; linear curve fit:
solid line) and phosphate buffer (50 m, pH 7.8; exp. points: open
circles; linear curve fit: dot-dashed line). Inset: comparison with
MnCl2 in 50 m PIPES (exp. points: triangles; linear fit: dashed
line). The assay was performed by recording the reduction of ferri-
cytochrome c at 550 nm as a function of time. The slope of the line
A550nm = f(t) before addition of the SOD mimic is s1 and s2 is the
slope after addition of the SOD mimic. (s1 – s2)/s2 was plotted
against the complex concentration to provide a line, and the IC50
was then obtained for (s1 – s2)/s2 = 1.[79]
The nature of this species was also investigated at 10–5

both by CV and EPR spectroscopy. The electrochemical
signature of the mononuclear [(L)Mn]+ complex was still
observed, with an intensity two orders of magnitude
smaller, as expected for a concentration that is two orders of
magnitude smaller. This indicates that no de-coordination
occurs at this low concentration and is also consistent with
the electrospray mass spectrometry analysis of a 10–5

Table 2. IC50 values.
aqueous solution of 1(PF6)2, which shows a peak assigned
to the mononuclear [(L)Mn]+ cation (with spacings in the
M multiplet equal to 1) and no peaks assignable to either 1
(at Mdinuclear/2, with expected spacings of 1/2) or to mono-
cationic adducts.
[a,b]
IC50 buffer
kMcCF [–1 s–1 [c]
]
Mn2+ (MnCl2) IC50 PIPES = (7.3Ϯ0.5)ϫ10–6


–
–
[d]
aq
[(L)Mn]+
IC50 PIPES = (7.2Ϯ0.4)ϫ10–7
IC50 Phosphate = (8.1Ϯ0.3)ϫ10–7

(7.0Ϯ0.3)ϫ106
[a] 50% Inhibition concentration. [b] Uncertainties were obtained
from the fitting linearization procedure. [c] kMcCF values were recal-
culated from the IC50 value. [d] kMcCF was not recalculated as free
MnII is known to be catalytically inactive.[2] See ref.[4] for a com-
parison with similar N4O complexes.
SOD-Like Activity in Aqueous Buffer
IC50 values are dependent on the concentration of the
The mononuclear [(L)Mn]+ cation was tested for its su-
peroxide dismutase like activity using the McCord–Frido-
vich assay,[34,75] as published previously,[24,27] with ferricyto-
chrome c as the detector. Two different buffers, namely
phosphate buffer (pH 7.8, 50 m) and the noncoordinating
buffer PIPES (pH 7.5, 50 m), were used, as in a previous
study by Fridovich et al.[38] To the best of our knowledge,
the PIPES buffer has only been used in the original publica-
tion by Fridovich et al.[38] even though these conditions ap-
pear to be a good alternative to phosphate buffer condi-
tions.
detector, therefore in order to determine the activity we re-
[27]
calculated the apparent kinetic constant kMcCF
.
At the
IC50 value, the kinetics of the reduction of ferricytochrome
c by superoxide is reduced by 50% as 50% of the superoxide
is consumed by the ferricytochrome c and 50% by the com-
plex (auto-dismutation being negligible in comparison).
The apparent kinetic constant can thus be defined as kMcCF
[27,76]
= kdetector ϫ[detector]/IC50
and can be derived in the
case of the phosphate buffer, for which the kinetic constant
kcytC has been measured [kcytC (pH 7.8, 50 m phosphate,
21 °C) = 2.6ϫ105 –1 s–1].[77,78] The complex [(L)Mn]+
The complex [(L)Mn]+ displays very similar SOD-like ac- therefore displays an apparent catalytic rate kMcCF of
tivity under both sets of conditions, as shown by the inhibi- (7.0Ϯ0.3)ϫ106 –1 s–1.
tion of the reduction of ferricytochrome c. As shown in
The activity reported here is one of the best reported for
Figure 7, the activity of the complex is different from that the series of MnII complexes with a similar coordination
of Mn2+aq (see insert in Figure 7 and Table 2), which is con- sphere (N/O coordination sphere with N from imidazole or
sistent with the fact that the complex remains coordinated pyridine and O from carboxylato or phenolato) reported by
in dilute aqueous solution, as evidenced by the CV and ES- us and others,[23–25,27,28,80] which makes sense in light of the
MS experiments at 10–5  (see above).
redox potential for the mononuclear [(L)Mn]+ species mea-
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Di- and Mononuclear MnII Complexes
ethanol bath, and then washed carefully with ethanol and dried.
The counter electrode for bulk electrolysis was a piece of Pt sepa-
rated from the rest of the solution with a fritted bridge. The work-
ing electrode was a Pt gauze (about 20 cm2). The solvent used was
(a) distilled acetonitrile with tetrabutylammonium perchlorate
added to obtain a 0.1  supporting electrolyte or (b) doubly dis-
tilled water with either phosphate or PIPES buffer (50 m). The
reference electrode was (a) an Ag/AgClO4 electrode (0.3 V vs. SCE)
for experiments in CH3CN and (b) an SCE electrode for experi-
ments in water. In both cases, the reference electrode was isolated
from the rest of the solution with a fritted bridge. Temperature
regulation (20 °C) was ensured by a Julabo circulation cryostat.
sured in aqueous buffer. As the catalysis of the dismutation
of superoxide is a redox process involving both MnII and
MnIII oxidation states, the potential of the MnII/MnIII cou-
ple is a key parameter.[27] It should lie between the potential
–
of the two couples O2/O2– and O2 /H2O2 with the optimum
being the midpoint, that is 0.12 V vs. SCE (at pH 7), which
is the case for all SODs.[27,81,82] The mononuclear complex
[(L)Mn]+ (E1/2 = 0.199 V vs. SCE) fulfills this requirement.
Conclusions
The dinuclear bis-µ-phenolato complex [(L)MnIIMnII-
(L)] (PF6)2 [1(PF6)2], which contains a new amino/imid-
azole pentadentate N4O ligand, has been synthesized and
its structure resolved by X-ray diffraction. An X-band EPR
spectroscopy investigation has confirmed the dinuclear na-
ture of the compound in acetonitrile solution. The cyclic
voltammogram of 1 in acetonitrile solution displays a
quasi-reversible anodic process at E1/2 = 0.485 V vs. SCE
assigned to the two-electron oxidation of the MnIIMnII spe-
cies into the MnIIIMnIII species. The electrochemical forma-
tion of a MnIII species has been followed by UV/Vis and
EPR spectroscopy. In aqueous solution, the dinuclear com-
plex has been shown to be converted into a cationic mono-
nuclear complex [(L)Mn]+ with a redox potential of +0.199
V vs. SCE. Its superoxide dismutase-like (SOD-like) activity
has been evaluated by the means of the McCord–Fridovich
assay. The complex has a potent SOD-like activity, with an
apparent catalytic rate, kMcCF, of (7.0Ϯ0.3)ϫ106 –1 s–1.
The development of manganese complexes based on 1,2-
ethanediamine is of interest for the synthesis of efficient
SOD mimics.
2-(2-Hydroxybenzyl)-N,NЈ-bis[2-(N-methylimidazolyl)methyl]imid-
azolidine (3): A solution of compound 2[54] (0.500 g, 2.01 mmol,
1 equiv.) in absolute ethanol (10 mL) was mixed with salicylalde-
hyde (0.246 g, 215 µL, 2.01 mmol, 1 equiv.). After stirring the reac-
tion mixture for 24 h at ambient temperature, anhydrous sodium
sulfate (0.5 g) was added to eliminate water. After stirring for a
further 30 min the reaction mixture was filtered and the solvent
evaporated to give the crude product as a yellow oil (0.700 g, quan-
1
titative yield). H NMR (360 MHz, CDCl3, 23 °C): δ = = 7.22 (t,
J = 7.8 Hz, 1 H, CHPhOH), 7.06 (d, J = 7.3 Hz, 1 H, CHPhOH), 6.8
(m, 4 H, HAr), 6.70 (s, 2 H, CHIm), 3.91 (s, 1 H, CHaminal), 3.78 (d,
J = 13.3 Hz, 2 H, CH2-Im), 3.51 (d, J = 13.3 Hz, 2 H, CH2-Im),
3.35 (s, 6 H, CH3), 3.05 (m, 2 H, NCH2CH2N), 2.79 (m, 2 H,
NCH2CH2N) ppm. 13C NMR (90.6 MHz, CDCl3, 23 °C): δ =
157.8 (CAr-OH), 144.0 (CquatIm), 131.2, 130.4, 119.0, and 116.6
(CPhOHH), 127.1 and 121.7 (CImH), 120.6 (CquatPhOH), 88.1
(CHaminal), 49.6 and 48.0 (CH2CIm, N-CH2-CH2-N), 32.4 (Me)
ppm. ES-MS: m/z (%) 353.4 (100) [M + H]+.
N-(2-Hydroxybenzyl)-N,NЈ-bis[2-(N-methylimidazolyl)methyl]-
ethane-1,2-diamine (LH): NaBH3CN (0.107 g, 1.70 mmol, 1 equiv.)
and then CF3COOH (0.390 g, 340 mL, 3.40 mmol, 2 equiv.) were
added to a solution of 3 (0.600 g, 1.70 mmol, 1 equiv.) in absolute
ethanol (30 mL). After stirring for 2 h, 1  NaOH (10 mL) was
added and the ethanol was evaporated. CH2Cl2 (20 mL) was then
added and the pH adjusted to 9 by addition of 1  HCl. The or-
ganic phase was decanted and the aqueous phase extracted with
3ϫ20 mL of CH2Cl2. The combined organic fractions were con-
centrated to obtain the product as a yellow oil (0.600 g, quantita-
tive yield). 1H NMR (300 MHz,CDCl3, 23 °C): δ = 7.18 (t, J =
7.5 Hz, 1 H, HAr), 6.92 (d, J = 7.5 Hz, 1 H, HAr), 6.8 (m, 6 H,
HAr), 3.74 (s, 2 H, N-CH2-CAr), 3.71, (s, 2 H, N-CH2-CAr), 3.70 (s,
2 H, N-CH2-CAr), 3.58 (s, 3 H, N-CH3), 3.40 (s, 3 H, N-CH3), 2.78
(m, 4 H, CH2-CH2) ppm. 13C NMR (75.5 MHz, CDCl3, 23 °C): δ
= 157.0 (CAr-OH), 145.1 and 144.7 (CquatIm), 130.1, 128.6, 126.3,
122.7, 121.0, 120.7, 118.4, and 116.1 (CAr-H), 123.1 (CquatPhOH),
56.1, 51.6, 49.3, 45.4, and 43.4 (CH2), 32.8 and 32.3 (N-CH3) ppm.
Experimental Section
All reagents were bought from Acros Organics and used without
further purification. 1-Methyl-imidazole carboxaldehyde was syn-
thesized according to Iberson et al.[53] N,NЈ-Bis[2-(N-methylimida-
zolyl)methyl]ethane-1,2-diamine was synthesized according to La-
ronde et al.[54] NMR spectra were recorded with a Bruker AV360
or DRX300 spectrometer. IR spectra (KBr pellets) were recorded
with a Bruker IFS 66 FT-IR spectrometer. Microanalysis was per-
formed by the Service de Microanalyse de l’ICSN (Gif-sur-Yvette,
France) for C,H,N, and by the Service Central d’Analyse du CNRS
(Vernaison, France) for other elements.
ES-MS: m/z (%) 355.3 [M + H]+ (100). IR (KBr pellet): νmax = 757
˜
Electrospray-ionization mass spectra were recorded with a Finni-
gan Mat 95S in the BE configuration at low resolution. X-band
EPR spectra were recorded with a Bruker ELEXSYS 500 spectrom-
eter fitted with an Oxford Instrument continuous-flow liquid he-
lium or nitrogen cryostat and a temperature control system. UV/
Vis spectra were recorded with a Cary 300 Bio Spectrophotometer
with a 0.1-cm optical path quartz cuvette. The McCord–Fridovich
assay was performed on the same apparatus at 25 °C and with a
1-cm optical path quartz cuvette. All electrochemical experiments
were performed under argon. Cyclic voltammetry and coulometry
were performed with a M273 EGG PAR potentiostat. The counter
electrode for cyclic voltammetry was a Pt wire and the working
electrode a glassy carbon disk, which was carefully polished before
each voltammogram with a 1-µm diamond paste, sonicated in an
(δimidazol), 1282 (νC–O), 1454 and 1487 cm–1 (νC=N).
[Mn2(L)2](PF6)2: NEt3 (16 mg, 21 µL, 0.155 mmol, 0.5 equiv.) was
added to a solution of LH (0.110 g, 0.310 mmol, 1 equiv.) in MeOH
(40 mL) and argon was bubbled into the solution to remove O2. A
deoxygenated solution of MnBr2 (67 mg, 0.310 mmol, 1 equiv.) in
MeOH (10 mL) was added to this mixture and the solution heated
at 50 °C for 30 min. A deoxygenated solution of NH4PF6 (101 mg,
0.621 mmol, 2 equiv.) in methanol (1 mL) was then added and the
resulting solution allowed to cool overnight. The complex started
to crystallize out of solution, and 92 mg (40% yield) of nearly col-
orless crystals were isolated by filtration. ES-MS [dilute aqueous
solution (5ϫ10–5  and 5ϫ10–6 )]: m/z (%) 408.6 [(L)Mn]+ (100),
353.3 [LH + H]+ (23). The peak separation within the 408.6 mul-
Eur. J. Inorg. Chem. 2007, 4472–4480
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4477

C. Policar et al.
FULL PAPER
tiplet is +1, which indicates a +1 charge and is consistent with the
Table 3. Crystallographic data for 1(PF6)2.
monometallic species [(L)Mn]+. IR (KBr pellet): ν
= 760
˜
max
Formula
Fw
C38H50Mn2N12O2·2(PF6)
1106.72
(δimidazol), 837 (PF6), 1290 (νC–O), 1447 and 1484 cm–1 (νC=N).
C38H50F12Mn2N12O2P2 (1106.7): calcd. C 41.24, H 4.55, Mn 9.93,
N 15.19, P 5.60; found C 40.99, H 4.61, Mn 9.36, N 14.98, P 5.48.
Crystal system
Space group
triclinic
P1
¯
a [Å]
b [Å]
c [Å]
α [°]
9.2677(12)
11.0617(14)
13.2606(16)
107.965(2)
107.114(3)
92.810(3)
1221.2(3)
1
566
0.71073
273(2)
1.505
Magnetic Measurements and Fitting Procedures: Magnetization
data were collected for ground crystals with a Quantum Design
MPMS5 magnetometer in the 2–300 K temperature region (applied
field: 10 kG). The data were corrected for the diamagnetism contri-
β [°]
butions of the sample and of the cell. At 300 K, χT
=
γ [°]
8.01 cm3 Kmol–1. This value remained constant down to about
50 K upon lowering the temperature and then decreased abruptly
to a value of 0.65 cm3 Kmol–1 at 2 K. This behavior is indicative
of an antiferromagnetic coupling between the two MnII ions. The
experimental data were fitted to the expression for a dinuclear MnII
(S1 = 5/2)–MnII (S2 = 5/2) complex using the Hamiltonian H =
–JS1S2 (Marquardt algorithm from KaleidaGraph). A g-value of
1.94 was obtained from the fitting of χT = f(T) at high tempera-
tures. A J-value of –0.9 cm–1 was obtained by fitting χ = f(T) using
g = 1.94. As shown in Figure 8, a good agreement was obtained
for 1(PF6)2 in the whole temperature range for both χT and χ.
V [Å3]
Z
F(000)
λ [Å]
T [K]
ρcalcd. [Mgm–3]
µ(Mo-Kα) [mm–1]
Crystal size [mm3]
θ range [°]
0.675
0.20ϫ0.14ϫ0.06
3.00–26.19
6141
3291
0.0252
Number of data collected
Number of unique data
R(int)
Absorption correction
Number of variable parameters
Number of observed reflections[a]
R[b] obsd., all
SADABS
307
2639
0.0565, 0.0702
0.1671, 0.1795
1.075
[c]
Rw obsd., all
S
Largest diff. peak and hole [eÅ–3]
0.805, –0.450
[a] Data with Fo Ͼ 4σ(Fo). [b] R = Σ||Fo|–|Fc||/Σ|Fo|. [c] Rw
=
[Σw(|Fo2|–|Fc |)2/Σw|Fo2|2]1/2
.
2
monitored first in the absence of any putative SOD mimic and then
after addition of the mimic. The slope of the line A550nm = f(t)
before addition of the SOD mimic is s1 and s2 is the slope after
addition of the SOD mimic. (s1 – s2)/s2 was plotted against the
complex concentration to provide a line and the IC50 was then ob-
tained for (s1 – s2)/s2 = 1. Experiments were performed in duplicate
for each buffer.
Figure 8. Thermal variation of χT (triangles) and χ (squares) vs.
temperature for 1(PF6)2. For clarity, only 30% of the recorded data
are shown. The solid lines correspond to the data calculated with
the parameters g = 1.94 and J = –0.9 cm–1 with H = –JS1S2.
Supporting Information (see footnote on the first page of this arti-
cle): Figure S1, Evolution of the cyclic voltammogram of [1(PF6)2]
upon addition of base (2,6-lutidine).
X-ray Crystallography: X-ray diffraction data for 1(PF6)2 were col-
lected with a Kappa X8 APPEX II Bruker diffractometer with
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) and
corrected for Lorentz, polarization, and absorption effects. The
structures were solved by direct methods using SHELXS-97[83] and
refined against F2 by full-matrix least-squares techniques using
SHELXL-97[84] with anisotropic displacement parameters for all
non-hydrogen atoms. Hydrogen atoms were located on a difference
Fourier map and introduced into the calculations as a riding model
with isotropic thermal parameters. All calculations were performed
with the crystallographic software package WINGX.[85] The crystal
data collection and refinement parameters are given in Table 3.
Acknowledgments
We thank Eric Rivière for recording the magnetic measurements
and Geneviève Blondin for useful discussions on the EPR spec-
troscopy. This work was supported by the French government
(grant no. ACI-2004 JC2044 to C. P.).
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Received: December 22, 2006
[78] It should be noted that the kinetic constant for the reaction
between ferricytochrome c in PIPES at pH 7.5 has not been
Published Online: August 9, 2007
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