Water-stable manganese(IV) complex of a N2O4-donor non-schiff-base ligand: Synthesis, structure, and multifrequency high-field electron paramagnetic resonance studies

Article abstract of DOI:10.1021/ic4030958

A novel water-stable (t_1/2 ～ 6.8 days) mononuclear manganese(IV) complex of a hexacoordinating non-Schiff-base ligand (H ₄L) with N₂O₄-donor atoms has been synthesized and characterized crystallographically. High-frequency electron paramagnetic resonance experiments performed on a single crystal reveal a manganese(IV) ion with an S = ³/₂ ground spin state that displays a large single-ion anisotropy, setting the record of mononuclear manganese(IV) complexes reported so far. In addition, spin-echo experiments reveal a spin-spin relaxation time T₂ ～ 500 ns.

Full text of DOI:10.1021/ic4030958

Article
pubs.acs.org/IC
Water-Stable Manganese(IV) Complex of a N2O4‑Donor Non-Schiff-
Base Ligand: Synthesis, Structure, and Multifrequency High-Field
Electron Paramagnetic Resonance Studies
Malay Dolai,†,‡ Asma Amjad,‡,§ Mainak Debnath,† Johan van Tol,⊥,∥ Enrique del Barco,*,§
and Mahammad Ali*,†
†Department of Chemistry, Jadavpur University, Kolkata 700 032, India
§Department of Physics, University of Central Florida, Orlando, Florida 32816, United States
⊥National High Magnetic Field Laboratory (NHMFL), Florida State University, Tallahassee, Florida 32310, United States
∥Department of Physics, Florida State University, Tallahassee, Florida 32310, United States
S
* Supporting Information
ABSTRACT: A novel water-stable (t1/2 ∼ 6.8 days)
mononuclear manganese(IV) complex of a hexacoordinating
non-Schiff-base ligand (H4L) with N2O4-donor atoms has
been synthesized and characterized crystallographically. High-
frequency electron paramagnetic resonance experiments
performed on a single crystal reveal a manganese(IV) ion
3
with an S = /2 ground spin state that displays a large single-
ion anisotropy, setting the record of mononuclear manganese-
(IV) complexes reported so far. In addition, spin−echo
experiments reveal a spin−spin relaxation time T2 ∼ 500 ns.
N2-donor environment, [MnIV(bigH)3]4+, was found to be
INTRODUCTION
■
water-stable.5
Manganese is an essential component of various biological
However, there were only a few manganese(IV) Schiff base
redox processes, viz., in catalases1 and in oxygen-evolving
complexes that were either isolated from or reacted in aqueous
photosystem II (PS-II),2,3 where the active intermediate
media.11
generated is believed to have manganese in the +4 oxidation
The magnetostructural properties of mononuclear
state. Hence, the studies associated with manganese(IV) are
manganese(IV) complexes (S = 3/2), such as zero-field splitting
important in bioinorganic chemistry. Manganese(IV) is also
(zfs), are of great interest. However, despite numerous X-band
relevant in molecular magnetism because it is a constituent of
(9.4 GHz) EPR studies of such complexes, a precise
some crucial single-molecule magnets (SMMs).4 Although a
determination of these parameters has been scarce.12−15 The
number of oxo-bridged multinuclear manganese(IV) complexes
zfs values of Manganese(IV) impurities in magnesium and
are known to be stable enough for the successful study of their
aluminum oxide are exceptions, while the record zfs value
found for Manganese(IV) remains D = 1.6 cm−1.15 In this
solution chemistry, mononuclear nonoxomanganese(IV) com-
plexes that are stable in an aqueous medium have been scarcely
paper, we disclose an aqueous stable manganese(IV) complex,
reported.5 This situation has considerably limited our knowl-
which happens to have a record magnetic anisotropy, as
edge of the aqueous chemistry of MnIV, a key species in PS-II.
delineated by single-crystal high-frequency EPR studies.
The first structurally characterized mononuclear manganese-
(IV) complexes were reported independently by the Pierpont,6
EXPERIMENTAL METHODS
Christou,7 and Pecoraro8,9 groups in the 1980s. These
■
Chemicals and Reagents. The starting materials for synthesis of
compounds have been critical to the understanding of MnIV
the H4L ligand, viz., 2,4-di-tert-butylphenol (Lancaster), formaldehyde
in biology because they provide detailed electron paramagnetic
resonance (EPR) spectral evaluation of Manganese(IV). A
(Merck India), and N,N-bis(2-hydroxyethyl)ethylenediamine (Aldrich
Chemicals), were of reagent-grade and were used as received. MnCl2·
CSD search (CSD version 5.35, Nov 2013) on MnN2O4
coordination resulted in 1917 hits, out of which only 34 were
found to be based on mononuclear manganese(IV) complexes.
Again, out of these 34 hits only 8 complexes were found to be
of non-Schiff-base ligands,10 but none of these were found to be
water-stable. Only one complex of the biguanide ligand in the
4H2O, triethylamine (TEA), and solvents like ethanol, petroleum
ether, and methanol (MeOH; Merck India) were of reagent-grade and
were dried by standard methods before use.
Received: September 13, 2013
© XXXX American Chemical Society
A
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Synthesis of the H4L Ligand. The ligand (H4L) was prepared by
using simple Mannich condensation.16 In a typical procedure, N,N-
bis(2-hydroxyethyl)ethylenediamine (2.22 g, 15 mmol) in 50 mL of
MeOH along with 2,4-di-tert-butylphenol (6.19 g, 30 mmol) and
formaldehyde (3 mL, 41%, 35 mmol) was stirred for 2 days. The
solution was kept in air to partially evaporate the solvent (about 20
mL). A white solid appeared within 2 days. After filtration, the solid
was washed with cold MeOH and dried in air.
Synthesis of Complex [Mn(L)]. MnCl2 (0.039 g, 0.20 mmol) was
dissolved in 20 mL of MeOH, and then hexadentate H4L (0.117 g,
0.20 mmol), deprotonated separately by treatment with TEA (0.081 g,
0.80 mmol) in MeOH, was added to this solution with stirring. The
dark-pink color of the solution appeared almost immediately. Stirring
was continued for 1 h and then filtered off. The filtrate was kept aside
undisturbed for slow evaporation. After the mother liquor was allowed
to stand for 2 days, dark-pink rod-shaped crystals were obtained and
collected by filtration. The isolated suitable single crystals were
subjected to X-ray studies. Elem anal. Calcd: C, 68.00; H, 8.87; N,
4.40. Found: C, 67.8. H, 8.79; N, 4.50.
EPR Spectroscopy Measurements. The magnetic properties of
the complex were probed via multifrequency single-crystal high-field
EPR (HF-EPR) studies. The experiments were conducted at the
Electron Magnetic Resonance (EMR) Facility at the NHMFL
(Tallahassee, FL). A heterodyne quasi-optical spectrometer equipped
with a 12.5 T superconducting magnet was used for single-crystal
measurements at high frequencies (>230 GHz).18,19
RESULTS AND DISCUSSION
■
The complex was prepared by the reaction of MnCl2 in 20 mL
of MeOH with H4L (obtained from our previous work; Scheme
1)16 in 20 mL of MeOH in the presence of TEA. The colorless
Scheme 1. Hexadentate H4L Ligand
Crystallography. Single-crystal X-ray data of the complex was
collected on a Bruker SMART APEX-II CCD diffractometer using
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data
collection, reduction, structure solution, and refinement were
performed using the Bruker APEX-II (v2.0-2) program suite. All
available reflections to 2θmax were harvested and corrected for Lorentz
and polarization factors with Bruker SAINT plus. Reflections were then
corrected for absorption, interframe scaling, and other systematic
errors with SADABS. The structures were solved by direct methods
and refined by means of a full matrix least-squares technique based on
F2 with the SHELX-97 software package.17 All of the non-hydrogen
atoms were refined with anisotropic thermal parameters. All of the
hydrogen atoms belonging to carbon atoms were placed in their
geometrically idealized positions. Drawings of the molecules were
generated with the PLATON-1.16 and Mercury 2.4 programs. The
crystallographic data are given in Table 1.
solution immediately transformed to dark purple, yielding dark-
purple rod-shaped crystals suitable for single-crystal X-ray
studies after 2 days. Single-crystal X-ray diffraction analysis
reveals that the compound crystallizes in a monoclinic P21/c
space group. The compound is a monomeric neutral species,
[MnIV(L)], in which the manganese(IV) ion has a highly
distorted octahedral geometry with four oxygen and two
nitrogen atoms of the L4− ligands coordinated to a manganese
atom, which creates a C2 symmetry axis along the N−Mn−N
axis (selected Mn−N1 and Mn−N2 bond lengths and angles
are given in Table 2). The six coordination positions are
Table 1. Crystal Data and Structure Refinement of the
Complex 1
Table 2. Important Bond Lengths (Å) and Angles (deg) of
Compound 1
empirical formula
fw
C36 H56 Mn N2 O4(ccdc no.-947606)
Bond Distances
635.77
temperature (K)
wavelength (Å)
cryst syst
273
Mn1−O1
Mn1−O2
Mn1−O4
1.8898(16)
1.8492(16)
1.8641(17)
Mn1−O3
Mn1−N1
Mn1−N2
1.8647(15)
2.0763(18)
2.0600(18)
0.71073
monoclinic
P21/c (No. 14)
space group
unit cell dimensions
a (Å)
Bond Angles
O1−Mn1−O2
O1−Mn1−O3
O1−Mn1−O4
O2−Mn1−O4
O2−Mn1−O3
O3−Mn1−O4
O3−Mn1−N2
O4−Mn1−N2
95.80(7)
91.70(7)
168.87(7)
91.28(7)
93.36(7)
96.45(7)
89.91(7)
82.13(7)
O1−Mn1−N1
O1−Mn1−N2
O2−Mn1−N1
O2−Mn1−N2
O3−Mn1−N1
O4−Mn1−N1
N1−Mn1−N2
81.82(7)
90.35(7)
91.32(7)
172.94(7)
172.37(7)
89.46(7)
86.11(7)
14.0488(4)
b (Å)
9.9929(3)
c (Å)
24.7991(7)
α (deg)
90.00
β (deg)
volume (Å3)
93.193(2)
3476.10(17)
Z
4
density (calcd) (g cm−3
)
1.215
abs coeff μ(Mo Kα) (mm−1
)
0.419
F(000)
1372
cryst size (mm3)
0.10 × 0.14 × 0.18
−18:18, −13:13, −32:32
1.6−27.6°
occupied by atoms O1 and O4 (alkoxo oxygens), O2 and O3
(phenoxo oxygens), and N1 and N2 (amine nitrogens) of the
ligand L4−. The phenolic oxygen atoms are trans to the nitrogen
atoms, while alkoxides are cis. The axial oxygen atoms (O1 and
O4) have distances of 1.890 and 1.864 Å from the
manganese(IV) center, while in the basal plane, the MnIV−Oi
(i = 2 and 3) distances are 1.849 and 1.865 Å and MnIV−Ni (i =
1 and 2) are 2.076 and 2.060 Å, respectively. The four opposite
bond angles around the manganese center at the basal plane,
namely, O3−Mn−N2 (89.91°) versus N1−Mn−O2 (91.32°)
index ranges
θ range for data collection
total, unique data, R(int)
obsd data [I > 2.0 σ(I)]
55622, 8062, 0.085
5626
completeness (%) to θ = 27.6°
abs corrn
100
empirical
no. of ref, no. of param
GOF on F2
8062, 400
1.04
final R indices [I > 2σ(I)]
R1 = 0.0493, wR2 = 0.1306
B
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and O3−Mn−O2 (93.36°) versus N1−Mn−N2 (86.1°), are
nearly identical.
The molecular view of the complex in the crystallographic ac
plane (see Figure 1) shows two different orientations (A and B)
Figure 1. Molecular views of the manganese(IV) complex in the
crystallographic ac plane showing the two different molecular
orientations (A and B), related by mirror reflection about the ac
plane. Hydrogen atoms are omitted for clarity. Symmetry operators: a,
x, y, z; b, −x, 0.5 + y, 0.5 − z; c, −x, −y, −z; d, x, 0.5 − y, 0.5 + z.
Figure 2. (a) Representation of the axial magnetoanisotropy energy of
the molecular spin, which shows an easy magnetic xy plane. (b) Single-
crystal spectrum observed at 239 GHz and 10 K, with the external
magnetic field applied within the easy-anisotropy plane. (c) Zeeman
energy diagram of the spin levels of MnIV with the field applied along
both the hard-anisotropy z axis (dashed green lines) and the easy-
anisotropy xy plane (continuous black lines) of the molecule. The
expected transitions induced by microwaves of frequency 239 GHz are
indicated by arrows (purple arrows indicate the absorptions shown in
panel b).
of the molecules within the crystal, related to each other by
reflection symmetry about the mirror plane ac. Strong
hydrogen-bonding interactions are presented in Figure S1 in
the SI, which leads to a 1D chain (Figure S2 in the SI). The
bond valence sum (BVS) of the manganese ion was calculated
to be 3.97, which is consistent with the +4 oxidation state of the
central manganese atom (Table S2 in the SI). The stability of
the complex was determined spectrophotometrically and found
to be kd = 1.17 × 10−6 s−1 in 50% (v/v) MeOH−H2O (Figure
S3 in the SI), which indicates fair stability of the complex in an
organic aqueous medium with t1/2 ∼ 6.8 days.
3/2, D = 1.65 cm−1, and isotropic g = 2. For the data presented
in this work, which is taken at high frequencies (i.e., EPR
transitions occur at fields far from anticrossings between
opposite spin levels; see Figure 2c), the effect of a reasonable
2
2
rhombic anisotropy term, E(Sx − Sy ), would be completely
negligible; thus, it is not included in eq 1. Note that the value of
the zfs parameter (D = 1.65 cm−1) is larger than those
previously reported in the literature.12−15 Figure 2 shows the
Zeeman energy levels of the MnIV ion as a function of the field
applied along both the hard-anisotropy axis (z axis) and the
easy-anisotropy plane (perpendicular to the z axis) calculated
by direct diagonalization of the Hamiltonian in eq 1 using the
parameters given above. The arrows indicate the three main
expected transitions for 239 GHz, which are observed in the
experiment (see Figure 2b for the H ⊥ z case), corresponding
Single-Crystal EPR Spectroscopy. HF-EPR experiments
have been performed in a single crystal of the mentioned
compound. The study reveals spectra containing sharp
3
resonances that can be directly associated with the spin /2
system, hence allowing precise comparisons with theoretical
simulations. A detailed study of the angular dependence of the
EPR spectra for this sample has been conducted at two
frequencies, 239 and 336 GHz. As will be shown below, the
angle-dependent EPR study of the ground-state transition
within the S = 3/2 multiplet reveals an axial magnetic symmetry
with an easy-anisotropy plane perpendicular to the z axis (hard
axis). Figure 2a shows a sketch representing the magneto-
anisotropic energy of the molecular spin. The 239 GHz EPR
spectrum obtained at 10 K with the field along the easy-
anisotropy plane (perpendicular to the z axis) is shown in
Figure 2b. Three strong absorption peaks are observed at ∼6.9,
∼8.5, and ∼10 T, with the lowest-field peak displaying the
highest intensity, as expected from a positive axial anisotropy
parameter (D > 0) for this orientation of the magnetic field.
The multiplicity of the peaks results from the two molecular
orientations within the sample, as discussed below. No more
peaks appear at fields below the range given in the figure.
The EPR spectra obtained in our study can be simulated by a
complete diagonalization of the spin Hamiltonian:
3
to transitions between consecutive ms levels within the S = /2
multiplet. In particular, the three transitions observed with the
field applied within the easy-anisotropy plane correspond to ms
= (−3/2 → −1/2)⊥ (∼6.9 T), ms = (−1/2 → 1/2)⊥ (∼8.5 T), and
3
ms = (1/2 → /2)⊥ (∼10 T). The fact that the strongest EPR
absorption peak corresponds to the lowest-field transition
indicates that the axial anisotropy parameter D is positive,
resulting in a hard-anisotropy axis along the x axis (i.e., an easy-
anisotropy xy plane). Note that when the field is applied along
the z axis, ms = (−3/2 → −1/2)∥ becomes the highest-field
3
transition, as seen in Figure 2c, while ms = (1/2 → /2)∥
becomes the lowest-field one. As shown below, the angular
modulation of the main EPR transitions can be employed to
determine the anisotropy parameters of the molecules with a
high degree of accuracy.
In order to establish the direction of the anisotropy axes
within the crystal, EPR spectra have been recorded at different
orientations of the magnetic field with respect to the crystal
faces. Figure 3 shows the angular dependence of the ground-
2
H = DSz + gμBB·S
(1)
where D is the axial zfs parameter. The corresponding values of
the parameters used to fit all of the data in this article are S =
C
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is so and each molecular orientation has the anisotropy axis at a
different direction, their EPR responses should be different for a
given orientation of the applied magnetic field, unless the field
is applied along the ac plane, which is the molecular mirror
plane. Because all of the EPR peaks in Figure 3 follow
approximately the same angular dependence, we conclude that
the zx rotation plane coincides with the crystallographic ac
plane (i.e., parallel to one of the crystal faces, as depicted in the
sketch of Figure 3). In this situation, the field is never applied
exactly along the anisotropy z axis of any of the molecular
orientations because it is likely located in the N1−Mn−N2
molecular planes, which are tilted away from the ac plane by
15.85°.
To account for the possible misalignment between the
molecular anisotropy axes and the crystal faces, the orientation
of the main axes of the zfs tensor is left to be determined by the
Euler angles α, β, and γ (=0°). The best fits of the data
presented in Figure 3 were indeed achieved for α = 15.85° (and
β = 55 and 145° for orientations A and B, respectively). One
must keep in mind that the small deviations between the
experimental and calculated results may be a consequence of
the uncertainty in the alignment of the crystal with respect to
the main axes of the magnet used to obtain the spectra. In
addition, the presence of the two molecular orientations within
the crystal complicates the analysis because the two resulting
modulations superimpose for several angles.
Figure 3. Angle dependence of the main EPR peaks observed at 10 K
for a frequency of 239.2 (circles) and 336 GHz (stars). The field is
rotated in a plane perpendicular to the long crystal axis (see the
sketch). The lines represent the calculated response obtained from
diagonalization of the Hamiltonian in eq 1
state EPR transition at 239.2 GHz (circles) and 336 GHz
(stars), with the magnetic field rotated in a plane that is related
to the crystal faces, as depicted in the sketch of Figure 3. The
crystal is oriented such that the y axis lies along the longest
crystal axis and the x axis is perpendicular to the flattest crystal
face. This measurement corresponds to the external magnetic
field being rotated at an angle θ from the z axis within the zx
plane (ϕ = 0; red plane in Figure 3). The three main direct
To show this, Figure 4 includes the 336 GHz modulation of
the ms = (−3/2 → −1/2) (low-field data) and ms = (−1/2 → 1/2)
3
transitions within the S = /2 multiplet can be followed in the
response recorded at the lowest frequency (239 GHz). Only
two transitions were observed within the field range of the
experiment (<12 T) for the highest frequency (336 GHz). The
lines in Figure 3 represent the calculated behavior of the
respective transitions obtained by diagonalization of the
Hamiltonian in eq 1 with the parameters given above (see
below for more details). Specifically, ms = (−3/2 → −1/2) is
indicated with black lines, ms = (−1/2 → 1/2)⊥ with green lines,
3
and ms = (1/2 → /2) with orange lines, using continuous and
dashed lines for 239 and 336 GHz, respectively.
As mentioned before, one could clearly observe a 2-fold
modulation of the main ms = (−3/2 → −1/2) spin transition,
with minima at θ = 90° and a maximum in the peak field
position when the field is applied along the z axis (θ = 0°),
which corresponds to the hard-anisotropy axis (see below for
details). The strong anisotropic nature of this molecule is
evident from the strong modulation of the main EPR
absorption, which shifts by almost 5 T between the easy-
anisotropy plane (6.8 T for 239 GHz) and the hard-anisotropy
axis (11.5 T for 239 GHz), giving this compound the record zfs
value within other mononuclear manganese(IV) systems. The
octahedral coordination of this molecule is strongly distorted
by the disposition of the two large Mn−N bonds, significantly
lowering the molecular symmetry. In fact, it is likely that the
axial anisotropy axis of the molecule originates from the
deformation produced by these bonds and may lie within the
plane formed by N1−Mn−N2. This plane is tilted away from
the crystallographic ac plane by 15.85° for one of the molecular
orientations, while for the other orientation, the N1−Mn−N2
plane results from reflection on the opposite site of the ac
mirror plane; i.e., the two distinct N1−Mn−N2 planes are
titled by 31.70° with respect to each other (see Figure 1). If this
Figure 4. EPR absorption peaks obtained by rotating the magnetic
field within the xy plane, which contains the crystal’s physical longest
axis. The dual modulation observed in the data corresponds to the
response of the two molecular orientations in the crystal. The blue
circles indicate the correspondence with the data obtained in the zx-
plane rotation shown in Figure 3.
(high-field data) spin transitions resulting from a second
rotation, orthogonal to the one shown in Figure 3, which was
performed by rotating the field by an angle ϕ within the xy
plane (θ = −90°; blue plane in the sketch of Figure 4). The
data along the vertical dashed line in Figure 4 (black stars
enclosed by blue circles) correspond to the data shown in
Figure 3 (blue-circled stars at ϕ = 90°). The weak but dual 2-
fold modulation (ΔH ∼0.5 T) of the data corresponds to EPR
D
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Inorganic Chemistry
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absorptions coming from the two manganese molecular
orientations within the crystal. The ∼90° shift between the
two modulation patterns establishes that the anisotropy z axes
of the molecules are located in the N1−Mn−N2 planes such
that their projections into the xy plane are shifted by 90°. This
modulation is indicative of the magnetic field being rotated
slightly away from the easy-anisotropy plane of the molecules,
for which the anisotropy axes of the molecules need to be
rotated by the Euler angles given above (with the
corresponding 90° shift in the respective β values). Note that
the value of α agrees with the orientation of the N1−Mn−N2
planes with respect to the mirror ac plane and leads to a fit of
the θ-rotation data in Figure 3 up to a high degree of accuracy
but can only account for a 0.2 T modulation in the ϕ-rotation
data of Figure 4 (not shown). It is not possible to conclude
about the exact orientation of the axial symmetry axis of the
molecule from the EPR measurements because we do not have
face-indexed X-ray diffraction data, so that it lies within the
N1−Mn−N2 plane is just a fair guess. In addition, the
complexity of the EPR spectra resulting from the presence of
two molecular orientations does not permit a better
determination of their relative disposition with respect to the
crystal from the experimental data (e.g., the dual modulation is
difficult to distinguish at 239 GHz), other than the fact that the
projections of their main anisotropy axes into the xy plane are
shifted by ∼90°, in agreement with the spatial disposition
obtained from X-ray diffraction data. Nevertheless, the data and
analysis presented in this work are sufficiently clear to
demonstrate the strong anisotropic nature of this mononuclear
manganese compound, which is the main purpose of this work.
Pulse EPR spectroscopy was also employed to determine the
characteristic spin−spin relaxation time of this system. Spin−
echo measurements and the consequent echo decay were
studied via HF-EPR as a function of the temperature and
magnetic field orientations at 240 GHz. The experiments were
performed using a two-pulse echo sequence (p1−τ−p2−τ−
echo), where the delay τ between pulses was varied. Both
pulses were 140 ns in length to optimize the echo amplitude
and time resolution, for a maximum incident power on the
sample of 20 mW. The initial τ = 210 ns. Echo signals were
only observed at the lowest achievable temperature (1.5 K) for
the ms = −3/2 to −1/2 transition when a large part of the
decoherence due to electron-spin fluctuations is quenched,20
with an exponential decay for this sample characterized with T2
= 500 ns (see Figure 5). Note that this is a high-concentration
compound where T2 relaxation due to dipolar electron−
electron spin dominates. This relaxation pathway can be
quenched at high electron-spin polarizations.20 This effect has
been observed in several single-crystalline iron-based molecular
magnets, but no spin−spin relaxation has so far been measured
directly in manganese-based molecular magnets. This work has
relevance to that. Spin−echo spectroscopy was also performed
by monitoring the echo intensity as a function of the magnetic
field at different orientations within the zx plane of the crystal,
similar to the continuous-wave (CW) measurements shown in
Figure 3. The inset to Figure 5 shows the corresponding
magnetic field values of the maximum echo intensity and its fit
using the Hamiltonian of eq 1 with the same parameters as
those used above.
Figure 5. Exponential decay of the spin−echo amplitude measured at
239.2 GHz and 1.5 K. The inset shows the angular variation of the
field position of the echo, following the same anisotropic behavior
observed in the CW EPR measurements.
previously reported complexes are either water-insoluble or
highly unstable in aqueous solution. Detailed single-crystal HF-
EPR spectroscopy measurements show that the manganese(IV)
3
ion presents an S = /2 ground spin state with a strong axial
anisotropy of the easy-anisotropy plane type. Mononuclear
manganese(IV) complexes are known to exhibit moderate
magnetic anisotropies (a few gigahertz). The current sample
presents large single-ion anisotropy, setting the record of
mononuclear manganese(IV) complexes reported so far. In
addition, spin−echo experiments reveal a spin−spin relaxation
time T2 ∼ 500 ns. Although this molecule is not a SMM, its
strong anisotropic character and its stability in water may
enable future technological applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in CIF format (CCDC 947606),
hydrogen-bonding interactions, BVS calculations, and kinetic
trace for the decomposition of [Mn(L)]. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: delbarco@physics.ucf.edu.
*E-mail: mali@chemistry.jdvu.ac.in.
■
Author Contributions
‡These authors contributed equally to this work. M.D. with the
chemical synthesis and A.A. with the physical characterization.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.A. acknowledges financial support from CSIR [Grant
02(2490)/11/EMR-II], UGC [Grant 39-735/2010(SR)], and
DST (Grant SR/S1/IC-20/2012), New Delhi, India. M.D.
thanks UGC for a SRF fellowship. A.A. and E.d.B. acknowledge
support from the National Science Foundation (Grant ECCS-
1001755). We are grateful to Stephen Hill for his support while
these experiments were being performed at the NHMFL. Work
CONCLUSIONS
■
We have reported the synthesis of a water-stable manganese-
(IV) mononuclear compound with t1/2 ∼ 6.8 days. Most of the
E
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at the NHMFL is supported by the National Science
Foundation (Grant DMR1157490), the State of Florida, and
the U.S. Department of Energy.
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