A single tripodal ligand stabilizing three different oxidation states (II, III, and IV) of manganese

Article abstract of DOI:10.1039/c1cc12418h

A series of mononuclear Mn^II, Mn^III, and Mn ^IV complexes was prepared using a single tripodal ligand (H ₃L). Addition of a cation (NH₄⁺, K⁺, Na⁺) to [Mn^IIIL] showed a pronounced effect on the redox potentials. Different variants of Jahn-Teller distortion, axial elongation and compression, were observed in the Mn^III complexes.

Full text of DOI:10.1039/c1cc12418h

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COMMUNICATION
A single tripodal ligand stabilizing three different oxidation states
(II, III, and IV) of manganesew
Yukinari Sunatsuki,a Yukana Kishima,a Tamami Kobayashi,a Tomoka Yamaguchi,a
Takayoshi Suzuki,a Masaaki Kojima,*a J. Krzystekb and Markku R. Sundbergc
Received 26th April 2011, Accepted 24th June 2011
DOI: 10.1039/c1cc12418h
A series of mononuclear MnII, MnIII, and MnIV complexes was
prepared using a single tripodal ligand (H3L). Addition of a
cation (NH4+, K+, Na+) to [MnIIIL] showed a pronounced
effect on the redox potentials. Different variants of Jahn–Teller
distortion, axial elongation and compression, were observed in
the MnIII complexes.
We expected that L3ꢀ would stabilize all three major oxidation
states, II, III, and IV, of manganese. These [MnIIL]ꢀ, [MnIIIL],
and [MnIVL]+ complexes would give us a rare opportunity to
compare directly the structures and properties between them.
Manganese is involved in a number of redox enzymes, such
as superoxide dismutase, the oxygen-evolving center of photo-
system II, and catalases.4,5 For these processes to take place
smoothly, the oxidation state of manganese should change
easily. Our complexes will fulfill the requirement. While in
most of the manganese metalloproteins and enzymes the active
sites involve more than one metal ion, the study of mono-
nuclear model complexes is important because evidence for
MnIV reactive intermediates has been obtained in catalytic
reactions of mononuclear MnIII complexes.6 Here, we report
the preparation, structures, and magnetic and electrochemical
properties of mononuclear MnII, MnIII, and MnIV complexes
of a single H3L ligand, as well as the effect of a cation on the
oxidation state of the complexes.
Mikuriya et al.2c reported that the reaction of
MnII(CH3CO2)2ꢁ4H2O with a tridentate ONO Schiff-base
ligand such as o-(salicylideneaminomethyl)phenol (1 : 2) in
methanol in air gave the MnIVO4N2-type complex. We
expected that a similar reaction would afford [MnIIIL] and
[MnIVL]+, and by adding a suitable counteranion (Xꢀ),
[MnIVL]X would be isolated. [MnIIL]ꢀ should form under
anaerobic conditions. The red NH4[MnIIL]ꢁ2CH3OH complex
(1) was prepared by the reaction of MnII(CH3CO2)2ꢁ4H2O,
H3L, NH4CH3CO2, and N(C2H5)3 in a 1 : 2 : 2 : 4 mole ratio in
methanol under a N2 atmosphere. The dark green [MnIIIL]
complex (2) was prepared from MnII(CH3CO2)2ꢁ4H2O and
H3L (1 : 1) in dichloromethane–methanol in air. To isolate the
MnIV complex, [MnIVL]X, the presence of an appropriate
counteranion (Xꢀ) is required, and when we added 1 equiv.
of NaClO4 to the reaction mixture for the preparation of
[MnIIIL] (2), the dark green [MnIVL]ClO4 complex (3) was
obtained. The addition of NaPF6 instead of NaClO4 also
gave an MnIV complex, [MnIVL]PF6 (30). Thus, the oxidizing
agent in these reactions should be O2. Interestingly, upon
addition of KPF6 instead of NaClO4 or NaPF6, dark green
[MnIIILK(PF6)] (4) was isolated. Similarly, the addition of
KNO3 gave [MnIIILK(NO3)] (5). The oxidation states of
manganese were assigned on the basis of the magnetic suscepti-
bility measurements. The meff values at 300 K were 5.98 (1),
Manganese can have a wide variety of oxidation states. Mixed-
valence manganese clusters play an important role in creating
single-molecule magnets (SMMs).1 For example, Langley
et al. reported a {Mn32} complex involving 18MnII, 10MnIII
,
and 4MnIV displaying probable SMM features.1b In such a
polynuclear complex, the Mn ions with different oxidation
states are in different coordination environments. It is not easy
to isolate Mn complexes in the same ligand system with several
different oxidation states. In addition, relatively few mono-
nuclear MnIV complexes have been isolated and structurally
characterized.2 Recently, we have shown that the tripodal H3L
ligand (=1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane,
Fig. 1) containing three imine groups, three phenol groups,
and three methoxy groups is a versatile ligand and can
accommodate a metal ion with differing nature and size.3
Fig. 1 The H3L ligand and complexes 1–4.
a Department of Chemistry, Faculty of Science, Okayama University,
3-1-1 Tsushima-naka, Okayama 700-8530, Japan.
E-mail: mkojima@ksn.biglobe.ne.jp; Fax: (+81)86-255-3712
b National High Magnetic Field Laboratory, Florida State University,
Tallahassee, Florida 32310, USA
c Department of Chemistry, Laboratory of Inorganic Chemistry,
University of Helsinki, POB 55, FIN-0014, Finland
w Electronic supplementary information (ESI) available: Synthesis
and characterisation of the complexes, BVS calculations, X-ray crystal
data of all complexes and molecular structures of 3 and 30, and
preliminary HFEPR results for 1 and 2. CCDC 812372–812377. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1cc12418h
c
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Chem. Commun., 2011, 47, 9149–9151 9149

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Fig. 2 Molecular structures of (a) NH4[MnIIL]ꢁ2CH3OH (1), (b) [MnIIIL] (2), (c) [MnIIILK(PF6)] (4), and (d) [MnIIILK(NO3)] (5) with atom
+
numbering schemes showing the 50% probability ellipsoids. Methanol molecules of 1 and the hydrogen atoms except for NH4 (greenish blue)
have been omitted for clarity. Color code: orange, MnII; green, MnIII; black, C; blue, N; red, O; light blue, F; pink, K; and reddish orange, P.
5.00 (2), 3.90 (3), 3.92 (30), 4.95 (4), and 4.91 mB (5). These
values are in good agreement with the spin-only meff values for
HS MnII (3d5: 5.92 mB, 1), HS MnIII (3d4: 4.89 mB, 2, 4, and 5),
and MnIV (3d3: 3.87 mB, 3 and 30).
Magnetic properties of complexes 1–5 were investigated by
bulk magnetometry, and by high-frequency and -field electron
paramagnetic resonance (HFEPR). In this Communication we
will present magnetic properties of the structurally most inter-
esting complex 4. Preliminary HFEPR results for complexes 1
and 2 are given in Fig. S3 and S4 (ESIw), respectively.
The room-temperature magnetic moment of 4 (4.95 mB)
agrees with the S = 2 ground state of the complex. The steep
drop at low temperatures is a signature of zero-field splitting
(zfs) effects. The full temperature dependence has been fitted to
the spin Hamiltonian of the form
The structures of the complexes were determined by X-ray
diffraction (crystal data, ESIw). In each complex unit the point
group is C1, although the highest point group could be C3. If
only the chromophore is considered, then the site symmetry
around a central Mn cation would be C3v. The coordination
polyhedron in every compound is a trigonally distorted octa-
hedron. In the case of the Mn(III), the Jahn–Teller effect is
operational. There are two deformational E modes resulting in
either elongation or compression in one of the O–Mn–N
directions. Which one of the two deformations will be stabilized
in the solid state, depends on a suitable inter- or intramolecular
interaction. Each Mn ion is coordinated by a fully deprotonated
L3ꢀ ligand via three phenolate oxygen atoms and three imine
H = bBgS + D(S2z ꢀ S(S + 1)/3).
(1)
where D stands for the axial zfs parameter and the remaining
symbols adopt their usual meaning. The resulting values are
|D| = 3.5 ꢂ 0.5 cmꢀ1, and giso = 2.02 ꢂ 0.01. The sign of D
could not be determined. The experimental and simulated
plots of magnetic moment vs. temperature are shown in the
inset to Fig. 3.
+
nitrogen atoms (Fig. 2). In 1, the four H atoms of NH4 are
hydrogen bonded to the six O atoms of the L3ꢀ ligand. In
accordance with the Jahn–Teller theorem, the pseudo-octahedron
about [MnIIIL] (2) is distorted and elongation is observed along
the O1–Mn1–N3 direction. In the crystal of 4, the K atom is
nine-coordinated by three methoxy and three phenolate O
ꢀ
atoms, and three F atoms of PF6 (Fig. 2(c)), and 4 is best
formulated as [MnIIILK(PF6)]. Jahn–Teller distortion is also
observed in 4 and the distortion is seen along the O3–Mn1–N1
direction. However, compression is observed in this complex.
There are many examples of HS MnIII complexes exhibiting a
tetragonal elongation or a rhombic distortion; however, those
that display axially compressed octahedral environments are
uncommon.7 It is to be noted that a closely related MnIII
complex, [MnIIILK(NO3)] (5), shows an elongation along thꢀe
O3–Mn1–N2 direction (Fig. 2(d)). In [MnIVL]ClO4 (3), a ClO4
ion exists as a counteranion (Fig. S1, ESIw), and the coordinate
bonds are shorter than those of the MnIII complexes. The MnII
complex (1) has the longest coordinate bonds among the
five complexes. The differences in coordinate bond lengths
(Tables S1–S3, ESIw) correspond well with the differences in
the effective ionic radii of six-coordinate Mn (HS Mn2+, 0.83 A;
HS Mn3+, 0.645 A; Mn4+, 0.53 A),8 confirming that the
oxidation states of 1, 2, 3, 4, and 5 are II, III, IV, III, and III,
respectively. The bond valence sum (BVS) calculations supported
these assignments (Table S4, ESIw).
Fig. 3 Inset: Experimental plot of effective magnetic moment vs.
temperature for 4 (squares) together with a simulated dependence
(represented by the curve) using spin Hamiltonian parameters S = 2,
|D| = 3.5 cmꢀ1, giso = 2.02. Main part: experimental (black trace) and
simulated (color traces) HFEPR spectra of 4 at 10 K and 331.2 GHz.
Simulation parameters: S = 2, |D| = 4.15, |E| = 0.75 cmꢀ1; giso
=
2.00, isotropic single-crystal linewidth 80 mT. Resonances marked
with an asterisk originate from molecular dioxygen, the resonance
with a plus sign from a g = 2 impurity (presumably Mn(II)).
c
9150 Chem. Commun., 2011, 47, 9149–9151
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The effect decreases in the order: NH4 4 K+ 4 Na+.
+
+
Hydrogen bonding of NH4 or coordination of an alkali
metal ion to [MnIIIL] via three methoxy and three phenolate
oxygen atoms results in a decrease in electron density on Mn,
thus the metal becomes difficult to oxidize to MnIV and easier
to reduce to MnII.
It is known that mononuclear hexacyanidomanganate
complexes can have a wide variety of oxidation states from
MnI ([MnI(CN)6]5ꢀ) to MnIV ([MnIV(CN)6]2ꢀ).10 However,
cyanide is an unusual ligand with both strong s-donating and
p-accepting abilities. To the best of our knowledge, the present
work is the first example of the isolation of Mn complexes of a
single ligand containing oxygen and nitrogen donor atoms
with three different oxidation states whose structures have
been characterized by X-ray analysis.
Fig. 4 CVs of [MnIIIL] (2, black), [MnIIILK(PF6)] (4, red), and
NH4[MnIIL] (1, blue) in acetonitrile containing 0.1 M (n-Bu)4NBF4
at a glassy carbon electrode at a sweep rate of 100 mV sꢀ1
.
All complexes investigated as solids showed a strong
response in HFEPR experiments. The quality of EPR data
for 4 was adversely affected by very strong field-induced
torquing effects that could be only partly prevented by pressing
a pellet. Fig. 3 (main part) shows a low-temperature EPR
spectrum of 4 together with two simulations using the same
magnitude of |D| = 4.15 cmꢀ1, but differing in sign.z Although
the agreement between the simulations and experiment is not as
good as in many other MnIII complexes studied so far (or other
MnIII complexes in the discussed series), it is clearly much better
for the case of positive D than for negative D. Thus, as expected
by a simple ligand-field theory, and indeed previously observed
by EPR,7e the Jahn–Teller compression results in positive D.
We note at this point that certain MnIII complexes have been
shown to exhibit polymorphism, with different structures
showing either a Jahn–Teller elongation or a compression.7f,9
Such a possibility cannot be excluded for 4.
As described above, 2 ([MnIIIL]) was obtained by the
reaction of MnII(CH3CO2)2ꢁ4H2O and H3L in air, while
addition of NaClO4 and NaPF6 to this reaction mixture
yielded [MnIVL]ClO4 (3) and [MnIVL]PF6 (30), respectively.
We were intrigued by the fact that the addition of KPF6
gave the MnIII complex, [MnIIILK(PF6)] (4) rather than
[MnIVL]PF6 (30). The results suggest that K+ prevents the
oxidation of MnII from proceeding to MnIV. To study the
effect of K+ on the redox potential, we compared the cyclic
voltammograms (CVs) of the two MnIII complexes, [MnIIIL] (2)
and [MnIIILK(PF6)] (4), in acetonitrile (Fig. 4). Both complexes
Notes and references
z The spin Hamiltonian used in the simulations was the same as in
eqn (1) except for an additional zfs rhombic term, E.
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ꢀ0.156 V; 4: E11/2
=
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(2: E21/2 = ꢀ0.727 V; 4: E12/2 = ꢀ0.505 V). The MnIV/MnIII
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potential than that of the other complexes, 2–4 (Fig. 4). When
NH4+ (1 equiv.) was added to a solution of 2, the same CV as
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that of 1 was obtained. Thus, countercations show
a
pronounced effect on the redox potentials of [MnIIIL].
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9149–9151 9151