The magnetism. Single-molecule magnets (SMMs) are individual molecules that

          The synthesis and characterization of
new polynuclear complexes of high spin metal ions is a contemporary topic of
research in coordination chemistry. Motivations for these research activities stem from their
relevance to bioinorganic chemistry246-250 and molecular magnetism.251
Manganese cluster chemistry, in particular, is of current interest because Mn
clusters often display interesting magnetic properties, such as single-molecular magnetism. Single-molecule magnets (SMMs) are
individual molecules that behave as single-domain nanoscale magnetic particles
below their blocking temperature, TB.
122,124,174,175 For Mn and most 3d metal SMMs, this behavior comes from
the combination of a large ground-state spin (S) and Ising-type magnetoanisotropy (negative zero-field splitting
parameter, D).125 SMMs
have been known to often display interesting quantum phenomena such as quantum
tunneling of magnetization (QTM), 126-128 quantum phase interference129,176,177
and quantum entanglement.178,179 Consequently, they have been
proposed as candidates for many potential applications in modern technologies,
such as qubits for quantum computation181-184 and components of
molecular spintronics devices.185,186 The Mn12O12(O2CR)16(H2O)4
(Mn12) family of SMMs have been known since the synthesis and
initial magnetic studies of the R = Me  member were reported in 1980 by Lis,123
and is one of the best
and most thoroughly studied SMMs to date.159,252,253

          For the above
reasons, there continues to be a great need to explore synthetic procedures and
reaction systems to new manganese clusters. In this field, small chelating
and/or bridging organic ligands have been used extensively in the quest of high
nuclearity clusters.254-259 We have been interested in this type of
ligands and in particular in methyl(2-pyridyl)ketone oxime (mpkoH). Due to its
ability to act both as a chelating and a bridging
ligand (Figure 4-1), mpko- has led to the formation of
numerous polynuclear compounds.260-265 In addition, mpko-
ligand has a propensity to promote ferromagnetic interaction in manganese
cluster chemistry.146 Some mpko- ligand based Mn3SMMs
have been employed as building blocks for the construction of supramolecular
aggregates of SMMs.168,170,172

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          In the present work,
we have explored the use of mpkoH in the reaction with Mn12 members
in acetonitrile, anticipating
that mix-valence Mn12 might lead to the formation of some potentially interesting Mn clusters.
We herein report the syntheses, crystal structures and magnetochemical
characterization of two new high nuclearity manganese complexes, Mn9(µ3-O)6(mpko)3(O2CCH3)11
(4-1) and Mn14(µ4-O)2(µ3-O)4(mpko)6(O2CPh)12
(4-2).

4.2 Experimental
Section

4.2.1 Syntheses

          All preparation were performed under
aerobic conditions using chemicals and solvents as received, unless otherwise
stated. Mn12O12(O2CMe)16(H2O)4?2MeCO2H?4H2O
(3) was prepared as described elsewhere.123 Mn12O12(O2CPh)16(H2O)4
(4) was synthesized by ligand
substitution of complex 3 using
benzoic acid by employing standard acetic acid/toluene azeotrope removal method
as described elsewhere.159,309 mpkoH was synthesized as reported.310

4.2.1.1

O6(mpko)3(O2CCH3)11
(4-1)

          To a stirred, dark brown solution of 3 (0.21 g, 0.10 mmol) in acetonitrile
(25 mL) was added mpkoH (0.11 g, 0.80 mmol). The solution of stirred for an
hour, filtered, and the filtrate allowed to stand for slow evaporation at
ambient temperature. Dark brown X-ray quality crystals of 1?4MeCN slowly grew over 3 days. They were collected by filtration,
washed with Et2O (2 × 5 mL), and dried under vacuum; the yield was ~
46% based on Mn. Elemental analysis: Calc. (Found) for 1?H2O (C43H56Mn9N6O32):
C 31.05 (31.33); H 3.39 (3.61); N 5.05 (4.84) %. Selected IR data (KBr, cm-1):
1556(s), 1417(s), 1345(w), 1155(m), 1105(m), 1079(m), 1048(w), 777(w), 713(m),
616(s), 418(w).

4.2.1.2

O12(mpko)6(O2CPh)12(H2O)2
(4-2)

4.2.1.2.1 Method A

           To a stirred, dark brown solution of
4 (0.28 g, 0.10 mmol) in
acetonitrile (25 mL) was added mpkoH (0.11 g, 0.80 mmol). The solution was
stirred for an hour, filtered, and the filtrate
allowed to stand for slow evaporation at ambient temperature. Dark brown X-ray
quality crystals of 2?xMeCN?yH2O slowly grew over 5 days. They were collected by
filtration, washed with Et2O (2 × 5
mL), and dried under vacuum; the yield was ~ 60 % based on Mn. Elemental
analysis: Calc. (Found) for 2?H2O
(C126H108Mn14N12O45): C
46.15 (46.22); H 3.32 (3.39); N 5.13 (5.02) %. Selected IR data (KBr, cm-1):
1599(s), 1559(s), 1447(w), 1405(s), 1175(m), 1151(m), 1068(s), 1025(m), 841(w),
777(w), 716(s), 675(s), 607(s), 459(w), 429(w).

4.2.1.2.2 Method B

           Treatment of stirred, dark brown solution of 4 (0.28 g, 0.10 mmol) in acetonitrile
(25 mL) with mpkoH (0.11 g, 0.80 g) gave a solution of essentially same color.
This was stirred for an hour and the solvent was removed under vacuum by using a rotary evaporator.
The residue was then dissolved in CH2Cl2, filtered and
the filtrate allowed to stand undisturbed in a closed vial at ambient
temperature. Dark brown X-ray quality crystals slowly grew over 3 days. They
were collected by filtration, washed with Et2O (2 × 5 mL), and dried
under vacuum; the yield was ~ 45 % based on Mn. The IR data and magnetic data
confirmed the product to be the same as from method A.

4.2.1.2.3 Method C

           Treatment of stirred, dark brown
solution of 4 (0.28 g, 0.10 mmol) in
CH2Cl2 (25 mL) with mpkoH (0.11 g, 0.80 g) gave a
solution of essentially same color. The solution was stirred for an hour, filtered, and the filtrate
allowed to stand undisturbed in a closed vial at ambient temperature. Dark
brown X-ray quality crystals slowly grew over 3 days. They were collected by
filtration, washed with Et2O (2 × 5 mL), and dried under vacuum; the
yield was ~ 40 % based on Mn. The IR data and magnetic data confirmed the
product to be the same as from method A.

 4.2.2
X-ray Crystallography

The
crystallographic data and structure refinement details for complexes 4-1 and 4-2 are summarized in Table 5-1. Single-crystal X-ray diffraction
data for 4-1 and 4-2 were collected at 100 K on a Bruker
DUO diffractometer using MoK? radiation (l = 0.71073 Å) and an
APEXII CCD area detector. Raw data frames were read by program SAINT206
and integrated using 3D profiling algorithms. 
The resulting data were reduced to produce hkl reflections and their
intensities and estimated standard deviations. 
The data were corrected for Lorentz and polarization effects and
numerical absorption corrections were applied based on indexed and measured
faces. The structures were solved
and refined in SHELXTL2014,206 using full-matrix least-squares
refinement.  The non-H atoms were refined
with anisotropic thermal parameters and all of the H atoms were calculated in idealized positions and refined
riding on their parent atoms.

The asymmetric unit of 4-1 consists of a Mn9
cluster and four acetonitrile solvent molecules. The solvent molecules were
refined with 0.75 occupancy level. In the final cycle of
refinement, 15808 reflections
(of which 7809 are observed with I > 2?(I)) were used to refine 928 parameters and the resulting R1,
wR2 and S (goodness of fit) were 4.68%, 8.96% and 0.858, respectively.

           The asymmetric unit of 4-2 consists of Mn14 cluster,
twelve acetonitrile solvent molecules and traces of water.  The solvent molecules were disordered
and could not be modeled properly, thus program SQUEEZE,207 a part
of the PLATON package of crystallographic software,208 were used to
calculate the solvent disorder area and remove its contribution to the overall
intensity data. The intensity contributions of the traces of water (a half and
a quarter molecules) could not be removed due to their closeness to disorder
regions (those waters’ protons could not be located and were not included in
the final refinement model). There are six disorder regions in the cluster. One
is of half a coordinated pyridine (atoms C1-2-3-4 disordered against
C1′-2′-3′-4′).  Three benzoate ligands
are disordered and their phenyl rings were refined in two positions each. The
other disordered regions lie on opposite end of the cluster and they were
refined in three parts each.  In each
case of disorder, site occupation factors were fixed to add up to unity. In the
case of three disordered rings command SUMP was used in the final
refinement.  The Ueq of atoms of each
disordered PART were restrained to be equivalent.  In the final cycle of refinement, 34585 reflections (of which 16659 are
observed with I > 2 ?(I)) were used to refine 1627 parameters and the resulting R1, wR2
and S (goodness of fit) were 5.59%,
14.06% and 0.955, respectively.

 

 

4.2.3 Physical Measurements

          Infrared spectra were recorded in the
solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 400 –
4000 cm-1 range. Elemental analyses were performed by Atlantic
Microlab, Inc. Variable temperature direct current (dc) and alternating current
(ac) magnetic susceptibility data were collected at University of Florida using
a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7 T dc magnet and
operating in the 1.8 – 300 K range. Samples were embedded in solid eicosane to
prevent torquing. Alternating current magnetic susceptibility measurements were
performed in an oscillating ac field of 3.5 G and a zero dc field. Oscillation
frequencies were in the 50 – 1000 Hz range. Pascal’s constants were used to
estimate the diamagnetic correction, which were subtracted from the
experimental susceptibilities to give the molar paramagnetic susceptibility (?M).266
Magnetization (M)
versus field (H) and temperature data
were fitted by a matrix diagonalization method using the program MAGNET209
to a model which assumes that only the ground-state is populated at the
measurement fields and temperature and takes into account the powder average. Elemental
analyses were performed by Atlantic Microlab, Inc.

4.3 Results and
discussion

4.3.1 Synthetic comments and IR spectra

          The Mn/RCO2-/mpkoH
reaction system has been extensively studied over the last ten years or so,
yielding polynuclear Mn clusters with esthetically beautiful motifs, large
ground state spin values, and SMM behavior. Two representative examples are
triangular

(mpko)3(O2CR)3+
(R = CH3, Ph, Et) clusters with S
= 6,146 and

O4(OMe)(mpko)9(mpkoH)4+
with S = 3 ground state;265
the former exhibits slow magnetic relaxation and the latter is the highest
nuclearity Mn cluster based on mpkoH. Recently, we have started exploring mix-valence Mn12
members as starting material,
aiming to synthesize high nuclearity mix-valence complexes and use mpkoH to
synthesize the same because of
its unique chelating and bridging abilities.

          A variety of reactions differing in
the Mn12 : mpkoH ratio and the reaction solvent(s) were explored in
the course of  identifying the following successful
system. The one-pot reaction of Mn12O12(O2CR)16(H2O)4
(R = CH3 (3)) with mpkoH
in a 1:8 ratio in solvent MeCN, gave solution of same color, upon filtration
and slow evaporation of filtrate, afforded dark brown crystals of a new
nonanuclear complex

O6(mpko)3(O2CCH3)11
(4-1)  in roughly 46% yield. Slight changes in the ratios of starting
materials gave the same product in comparable yield.

          Analogous reaction of mpkoH with Mn12O12(O2CR)16(H2O)4
(R = Ph (4)) in MeCN, yielded dark
brown crystals of a new tetradecanuclear complex

O12(mpko)6(O2CPh)12(H2O)2
(4-2) in approximately 60% yield. 4-2 could also be obtained, but in lower yields of ~40-45% from a two-step reaction using MeCN/CH2Cl2
(method B) or one-pot reaction using CH2Cl2 (method C).

          The IR spectra of complexes 4-1 and 4-2 display several bands in the ~1599 – 1284 cm-1
range, assigned to the stretching vibrations of the aromatic ring of mpko-
ligands, which overlap with the stretches of the carboxylate bands. Thus, they
do not symbolize pure vibrations, making
exact assignments and application of the spectroscopic characterization of
Deacon and Phillips difficult.267

 

4.3.2 Description of Structures

          A
partially labelled representation of complex 4-1 is shown in Figure 4-2. Selected interatomic distances and
angles are listed in Table 4-2.

        Complex 4-1 crystallizes in the monoclinic space group P21/c
with well-separated

O6(mpko)3(O2CCH3)11
molecules. The core Mn9(µ3-O)614+
(Figure 4-3, top) can be described as a central

(µ-O)4+ rod-like subunit (Mn4, Mn5, Mn6,
O2, O3, O4 and O5) attached on either side to two non-symmetric triangular Mn3O
subunits (Figure 4-3, bottom) :

(µ3-O)6+ (Mn1,
Mn2, Mn3 and O1) and

(µ3-O)7+ (Mn7,
Mn8, Mn9 and O6). The former triangular subunit is an isosceles triangle while
the latter is a scalene one. The linkage between the central subunit and the
two Mn3O subunits is provided by the oxido groups of the former,
namely O2, O3, O4 and O5, which are thus converted from µ to µ3,
while additional linkage is provided by 
three mpko- groups which are all of the ?1:?1:?1:µ
types (Figure 4-4). Peripheral ligation about the complete core is provided by
one ?1, five ?1:?1:µ, one ?1:?2:µ
and four ?1:?2:µ3 acetate groups (Figure 4-3).
Mn5 atom is five-coordinate with square pyramidal geometry while the rest of
the Mn atoms are six-coordinate with distorted octahedral geometries.

        Charge balance considerations indicate
a mix-valence

 description of 4-1. This was confirmed qualitatively by metric parameters and bond
valence sum (BVS)268,269 calculations (Table 4-3), which identified
Mn1 as MnII ion and others as MnIII. The latter also
display Jahn-Teller (JT)
distortion which takes the form of axial elongation of the trans Mn-O and/or
Mn-N bonds. The protonation level of O2- groups was also confirmed
by BVS calculations (Table 4-3). Complex 4-1
does not show any significant intermolecular interactions of any kind. Further,
the space filling representation of complex 1 reveals its large nanometer-sized structure with average distances
of ~1.6 nm (across the molecule) and ~1.0 nm (perpendicular to the molecule) (Figure 4-5).

        A partially labelled representation of
complex 4-2 is shown in Figure 4-6.
Selected interatomic distances and angles are listed in Table 4-5.

       Complex 4-2 crystallizes in the orthorhombic space group Pca21 with no crystallographic symmetry.
The cluster comprises a Mn14(µ4-O)2(µ3-O)1018+
core (Figure 4-7, top) which can be conveniently separated into three units: a
central

(µ3-O)4(µ-O)44+
subunit (Mn5, Mn6, Mn7, Mn8,
Mn9, Mn10, O3, O4, O5, O6, O7, O8, O9 and O10) attached on either side
to two virtually symmetry-related

(µ3-O)27+
subunits, which are linked together through 
two µ3-O2- atoms (O4 and O7) and four µ-O2- atoms (O3,
O5, O9, O10) of the central subunit; thus the former and latter O2-
atoms finally become µ4- and µ3- bridging, respectively
(Figure 4-7, bottom). The central

(µ3-O)4(µ-O)44+
subunit consists of four face-sharing partial cubanes: two are Mn3O3+4 types (Mn5, Mn8,
Mn9, O4, O6, O9 and their virtually symmetry related partners Mn6, Mn7, Mn10,
O5, O7, O8)  and the other two are Mn3O4+2
types (Mn5, Mn6, Mn9, O3, O4, O6, O8 and their virtually symmetry related
partners Mn6, Mn9, Mn10, O6, O7, O8, O10). The four metal ions of each

(µ3-O)27+
subunit are located at the four vertices of a butterfly-like unit (Mn1, Mn2,
Mn3, Mn4 and their virtually symmetry related partners Mn11, Mn12, Mn13, Mn14)
and they are connected by two µ3-O2- atoms (O1, O2 and
their virtually symmetry related partners O11, O12). Additional linkage is
provided by the six mpko- groups which are all of the ?1:?1:?1:µ
types, emphasizing the bridging flexibility of this ligand, as well as its
ability to bridge metal ions at different oxidation states (MnII/MnIII).
Peripheral ligation about the complete core is provided by twelve ?1:?1:µ
benzoate groups (Figure 4-8) and two ?1 H2O ligands. All
Mn atoms are six-coordinate with distorted octahedral geometries.            

       Charge balance considerations and an
inspection of the metric parameters indicate a 2MnII, 10MnIII
and 2MnIV description for 4-2.
This was confirmed quantitatively by bond valence calculation (BVS)
calculations (Table 4-5), which identified Mn3 and Mn12 as the MnII
ions, Mn5 and Mn10 as the MnIV and the others as MnIII
ions. The MnIII ions also show Jahn-Teller (JT) distortion which
take the form of axial elongation of the trans Mn-O and/or Mn-N bonds. BVS
calculations also confirm the protonation level of O2- groups (Table
4-5). Complex 4-2 does not form any
significant intermolecular interactions. Further, the space-filling
representation of 4-2 reveals its
nanotubular structure with average distances of ~2.5 nm (across the molecule)
and ~1.8 nm (perpendicular to the molecule) (Fig 4-9).

          In Tables 4-6 and 4-7, we have listed
all the previously reported Mn clusters with nuclearities of 9 and 14,
respectively, for a convenient comparison of their formulae, oxidation states,
and pertinent magnetic data such as their ground state spin (S) values and the nature of predominant
magnetic exchange interactions. The collective results of Table 4-6 show that
complexes 4-1 is the only example Mn9
cluster with

 oxidation level. Similar core topology with
that of 1 is found in Mn9O6(O2CMe)4(pao)8(Hpao)2
cluster,279 but different in oxidation level (

).
Further, complex 4-2 is the second
Mn14 cluster at the

 oxidation state, with the Mn14O8(Hpyaox)14(pyaox)2(N3)26+
cluster288 being the first albeit with a completely different core
topology.

 

 

 

4.3.3 Magnetochemistry

4.3.3.1 Direct Current Magnetic Susceptibility Studies

          Variable-temperature dc
and ac magnetic susceptibility data in a 0.1 T field and in the 5.0 – 300 K
temperature range were collected on a powered microcrystalline samples of 4-1 and 4-2 restrained in eicosane to prevent torquing. The obtained data
are plotted as ?MT versus T shown in Figure 4-10 and Figure 4-11 for complexes 4-1 and 4-2, respectively.

           The ?MT value of
complex 4-1 at 300 K is 27.10 cm3
K mol-1. This value is slightly less than that expected for a
cluster comprising one MnII and eight MnIII
non-interacting ions (28.38 cm3 K mol-1 with g = 2.0). The ?MT product
gradually decreases with decrease in temperature to 2.67 cm3 K mol-1
at 5.0 K. This behavior is indicative of dominant antiferromagnetic
interactions between the metal centers within the molecule. The 5.0 K value
suggests a small spin ground state value of S
= 1/2 or 3/2 for 4-1; the spin-only
(g = 2) values for S = 1/2 and 3/2 are 0.38 and 1.88 cm3
K mol-1, respectively. The size of the Mn9 cluster and
the resulting number of inequivalent exchange constants do not allow to
determine the individual pairwise Mn2 exchange interaction
parameters by applying the Kambe method; direct matrix diagonalization methods
are also computationally unfeasible.

          The ?MT product
for 4-2 steadily decreases with
decreasing temperature from 40.89 cm3 K mol-1 at 300 K to
25.18 cm3 K mol-1 at 45.0 K, then rapidly decreases to
4.28 cm3 K mol-1 at 5.0 K. The profile of plot suggests
predominant antiferromagnetic exchange interactions between the metal centers
within the molecule. The 300 K value is less than the spin-only (g = 2.0) value of 42.5 cm3 K
mol-1 for 2MnII, 10MnIII and 2MnIV
non-interacting ions, further indicating the presence of antiferromagnetic
exchange interactions within the molecule. The 5.0 K value suggests that
complex 4-2 possesses a small ground
state spin value of S = 1 or S = 2; the spin-only (g = 2.0) values for S = 1 and S = 2 are 1.0
and 3.0 cm3 K mol-1, respectively.

4.3.3.2 Magnetization versus
DC magnetic field studies

          We
focused instead to characterize the spin ground state, S, and the zero-field splitting parameter, D of complexes 4-1 and 4-2, by performing magnetization (M) versus dc field measurements at the
applied magnetic fields and temperatures in the 1 – 70 kG and 1.8 – 10.0 K
ranges were plotted as M/NµB vs. H/T
in Figures 4-12 and 4-13, respectively. However,
we could not obtain an acceptable fit for both the complexes using data
collected over the entire field range, which is a common problem created by the
presence of low-lying excited states, especially if some have an S value greater than that of the ground
to state. A general solution to this problem is to employ only data collected
at smaller fields and/or lower temperature; however, it was still not possible
to obtain satisfactory fit even with data below 1 T and 5.0 K. This indicates
that low-lying excited states are populated even at these relatively low
temperatures.

4.3.3.3 Alternating current
magnetic susceptibility studies

          Ac
magnetic susceptibility studies at zero dc field and 3.5 G ac field were also
performed. In the ac susceptibility measurement, the ac magnetic field is
oscillating at a particular frequency and a peak in the out-of-phase ?”M versus T plot is observed when the magnetic
moment vector of the molecule is unable to relax fast enough to retain in-phase
with the oscillating field. As we have described before on many occasions, ac
susceptibility measurements are also a powerful complement to dc measurements
for determining the ground state of a system, because they impede any
complications arising from the presence of a dc field. We thus decided to
perform ac studies on complexes 4-1 and 4-2 as an independent probe of its
ground state, S. These were carried
out in the 1.8 – 15.0 K range using a 3.5 G ac field oscillating at frequencies
in the 50 – 1000 Hz
range. The plots of in-phase (?’M, plotted as ?’MT) and out-of-phase ac
susceptibility data are shown in Figures 4-14 and 4-15 for complexes 4-1 and 4-2, respectively.          

           The ?’MT data of
both the complexes decrease sharply with decreasing temperature below 15 K,
indicating depopulation of excited states with spin S larger than that of the ground state and further supporting the
problematic fits of the dc magnetization data. Extrapolation of the ?’MT data of complex 4-1
from ~6.0 – 0K, where only the ground state will be populated, gives a value of
~1.8 cm3 K mol-1, indicative of an S = 3/2 ground state with g is almost equal to 2. While similar
extrapolation of the ?’MT data of complex 4-2 from ~6.0 – 0 K gives a value close to ~1 cm3 K mol-1,
indicative of an S = 1 ground state
with g = 2. In addition, complexes 4-1 and 4-2 do not display any signals in an out-of-phase ac magnetic
susceptibility down to 1.8 K, indicating the complexes are not SMMs.

4.4 Summary and
Conclusions

          The present work focused of the use
of mpkoH ligand in Mn cluster chemistry is a further demonstration of the
ability of this ligand to
yield complexes with interesting structures. Complex 4-1 is a novel addition to the subfamily of nonanuclear Mn clusters
at the

 level, while the complex 4-2 is a rare example of tetradecanuclear

 complex. Magnetic susceptibility studies
reveal the presence of predominant antiferromagnetic exchange interaction for
both the complexes with small ground state values. Both the complexes do not
exhibit any single-molecule magnetic behavior.  

x

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