Spin Coupling and Magnetic Anisotropy in 1D Complexes with Manganese(III) Units and Carboxylate Bridges – Synthesis, Analysis, Calculations, and Models

Article abstract of DOI:10.1002/ejic.201800003

We report the synthesis, structure and magnetic properties of a series of Mn^III complexes with N,N′-ethylenebis(5-methoxysalicylideneiminate) (5MeO-salen) and acetate ligands, namely, [Mn₂(5MeO-salen)₂(CH₃COO)]X

Full text of DOI:10.1002/ejic.201800003

A Journal of
Accepted Article
Title: Spin Coupling and Magnetic Anisotropy in 1D Complexes with
Manganese(III) Units and Carboxylate Bridges. Synthesis and
Analysis, Calculations and Models.
Authors: Fanica Cimpoesu, Tanta Spataru, Cristina Maria Buta, Horst
Borrmann, Olivia Georgeta Moga, and Marilena Ferbinteanu
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800003
Link to VoR: http://dx.doi.org/10.1002/ejic.201800003

10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
Spin Coupling and Magnetic Anisotropy in 1D Complexes with
Manganese(III) Units and Carboxylate Bridges. Synthesis and
Analysis, Calculations and Models.
Fanica Cimpoesu,[a] Tanta Spataru,[a] Cristina Maria Buta, [a, c] Horst Borrmann,[b] Olivia Georgeta
Moga,[c] and Marilena Ferbinteanu*[c]
Abstract: We report the synthesis, structure and magnetic
denotes the N,N'-Bis(salicylidene) ethylenediamine molecule,
represent one of the most accessible class of Mn(III) building
properties of a series of Mn(III) complexes based on 5MeO-salen,
N,N'-ethylenebis(5-methoxysalicylideneiminate), and acetate ligands, units, the axial positions, perpendicularly to the mean plane of
-
-
namely [Mn2(5MeO-salen)2(CH3COO)]X, X=ClO4 (1), PF6 (2),
the ligand, being at the same time easy magnetization axes and
assembling points, binding the bridging agents.
CF3SO3 (3). The [Mn(5MeO-salen)]+ form oxo-bridged dimers whose
-
further assembling by acetate groups, leads to zigzag chains. There
are two alternating types of dimeric {Mn2O2} units, each of them
intrinsically ferromagnetic and anisotropic, but mutually coupled
antiferromagneticaly along the chain. Magneto-structural analyses
were carried out with the help of ab initio calculations, understanding
the subtle factors of local magnetic anisotropy combined with the
exchange effects. A qualitative mechanism of the exchange coupling
was advanced, in the continuation of ab initio calculations, explaining
the ferromagnetism inside the phenoxo-bridged dimer units and the
antiferro behaviour of the acetate bridge.
The use of Mn(III) salen-type dimeric complexes as
anisotropic building blocks in SCM-type systems was pioneered
by Miyasaka.[7] A particular achievement is the building of 1D
coordination polymers,[8,9] behaving as Single Chain Magnets
(SCM).
A
particular
SCM
design[10]
binuclear
bridged
units
the
(where
[Mn2(naphtmen)2(H2O)2](ClO4)2
H2naphtmen is N,N’-(1, 1, 2, 2- tetra-methyl-ethylene) bis
naphthylidene-imine) through a ligand having a nucleobase
pattern: 6-amino-9-β-carboxyethylpurine. Aside assembling the
1D complexes, this ligand links the chains, by hydrogen bond.
The coupling is ferromagnetic inside the Mn(III) dimeric units
and antiferromagnetic along the nucleobase-alike ligand. The
spin canting determined by the anisotropy of Mn(III) ions leads
to a non-cancelled magnetic moment, whose dynamics is
interpreted as a SCM feature.
Introduction
The molecular magnetism,[1] as distinct branch of coordination
chemistry, contributes with academic insight to the interests in
developing new functional materials. In this respect, the design
of supramolecular coordination networks on the basis of
molecular building blocks concept, afforded a large number of
compounds with different structural patterns, enabling case
studies for various structure-property correlations.[2] One may
say that the main focus of the nowadays magneto-chemistry
stays in producing and understanding the so-called Single
Molecule Magnets (SMMs)[3] and related systems. Extended 1D
analogues of the SMMs are the Single Chain Magnets (SCM)[4].
The manganese(III) ion is one of the best suited d-type
ions to acquire SMM systems, being a good carrier of magnetic
anisotropy, its complexes showing a large structural variety.[5. 6]
The tetradentate Schiff-base salen type ligands, where salen
Although not possessing special SCM or SMM properties,
system interesting for the molecular structure and
a
supramolecular assembling is obtained with a bicompartmental
Schiff base H23MeO-salen: N,N’-ethylene bis(3-methoxy
salicylaldiimine), where the methoxy groups provide a potential
second coordination site.[11] The dimeric [Mn2(3MeO-
salen)2(H2O)2]2+ units are assembled by hydrogen bonds. E.g. in
the [Mn2(3MeO-salen)2(H2O)2] (NO3)2 material, the aqua ligands,
placed in the outer apical positions of the Mn(III) coordination
spheres are linked to the nitrate counter-ions, forming chains of
dimers. In a crystal isomeric form, the chains are realized with
hydrogen bonds from the aqua ligand of one monomeric Mn(III)
complex approaching the MeO groups of the other unit. Other
supramolecular architectures of this complex unit are found,
including a case where the open coordination compartment is
occupied by potassium ions.
Complicate architectures are realized combining Mn(III)
salen-type complexes with large polyoxometalate clusters.[12] An
interesting result is the successful attempt to produce multi-
property systems, like conducting magnets, in hybrid materials
using dimers of the manganese(III) complex with the
5MeOsaltmen ligand (where 5MeOsaltmen is N,N'-(1,1,2,2-
tetramethylethylene)bis(5-methoxysalicylideneiminate)) for the
magnetic ingredient, while the conducting function is given by
stacks of [Ni(dmit)2]0,-1 complexes in mixed valence regime, with
neutral and negative charges (where dmit is 2-thioxo-1,3-
dithiole-4,5-dithiolate).[13]
[a]
[b]
[c]
Dr. F. Cimpoesu, Dr. T. Spataru, C.M. Buta
Institute of Physical Chemistry
Splaiul Independentei 202, Bucharest 060041, Romania
Prof. H.Bormann,
Max-Planck-Institute for Chemical Physics of Solids
Nöthnitzer Straße, 4001187 Dresden, Germany
O.G. Moga, C.M. Buta, Assoc. Prof. M. Ferbinteanu
Inorganic Chemistry Department
Faculty of Chemistry, University of Bucharest
Dumbrava Rosie 23, Bucharest 020462, Romania
E-mail: marilena.cimpoesu@g.unibuc.ro
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
In a large number of examples the Mn(III) dimers are
identified with individual SMM behaviour,[14] a property from
where the SCM features are emerging. One of us contributed to
one of the first SCM systems, cooperating with Miyasaka’s
group, synthesizing and analysing a coordination chain based
on the [Mn(5MeO-salen)]+ unit,[15] where 5MeO-salen is N,N'-
ethylenebis(5-methoxysalicylideneiminate). This was assembled
with Fe(III) hexa-cyanide complex anions, generating a Mn-Mn-
Fe-Mn-Mn-Fe- chain alternation. In the subsequent analyses on
this system, we realized that the SMM qualities of the
manganese dimers are the important actors. This issue, debated
collaboratively, was reflected in a work produced in the same
group and in the same interval with our above cited work. Thus,
in the discussion of the [Mn2(saltmen)2(ReO4)2] compound,
realized with the H2saltmen Schiff base, N,N’-(1,1,2,2-tetra-
methyl-ethylene) bis (salicylidene-imine), the intrinsic SMM
features of the on Mn(III) dimeric units were firstly
acknowledged.[16]
this discussion remained at elementary level.[29] The spin-canting,
either interpreted as determined by the so-called Dzyaloshinskii-
Moriya coupling,[30] or as anisotropy manifestation,[31] is due to
the imbrication of exchange and spin-orbit coupling, being one of
the effects that help to achieve long-range magnetic ordering.[32]
However, such aspects are rarely discussed in concrete details,
being often limited at the qualitative invocation. Our attention
devoted to the magnetic anisotropy will attempt bringing new
contributions in this respect also.
Results and Discussion
Structure Description.
Here we report chain coordination polymers with a (Mn-
Mn-RCOO)n skeleton, made of dimeric sequences bridged in
syn-anti fashion by acetate moieties, bringing relevant on the
magneto-structural considerations. The three new single-crystal
structures are [Mn2(5MeO-salen)2(CH3COO)]X, where 5MeO-
salen is N,N'-ethylenebis(5-methoxysalicylideneiminate) and
X=ClO4- (1), PF6- (2), CF3SO3- (3).
The cationic binuclear units are formed by the link between
one manganese centre and the phenoxo oxygen atom from the
neighbouring Schiff base ligand. The cationic chains are
surrounded by perchlorate anions in 1, hexafluoro-phosphate
and one methanol molecule in 2, and triflate anions in 3. The
system 1 is illustrated in the Figure 1, the other two being
presented in the Supporting Information. In each compound, the
manganese centres are crystallographically different, labelled
Mn(1) and Mn(2). Each manganese centre is hexa-coordinated
by two nitrogen and four oxygen atoms, having an elongated
octahedron structure. Four equatorial positions are occupied by
the two nitrogen and two oxygen atoms from the deprotonated
tetradentate 5MeO-salen ligand.
A chemical proof that the Mn(III) dimeric units are the key
carriers is the fact that polymeric chains incorporating single
Mn(salen)-type nodes do not show SMM or SCM features, as is
the case of a system[17] assembling [Mn(5MeO-salen)]+ and
[Fe(tp)CN3]- (where Tp− is
= hydro-tris-pyrazolyl- borate),
yielding alternating -Mn-Fe-Mn-Fe- heterometallic complexes. A
very interesting result is the incorporation of a Mn(III) dimeric
SMM in
a
3D hetero-bimetallic oxalate lattice with
a
[Mn(II)Cr(III)(ox)3] skeleton.[18]
Reportedly, the Mn(III)···Mn(III) magnetic interaction within
the manganese dimeric units can be both ferromagnetic and
antiferromagnetic, depending on the Mn…Mn distance and Mn-
O(phenoxo)-Mn angles, parameters controlled by the apical
ligands, but also by subtle packing effects.[19] Experimental
structural studies were combined with Density Functional Theory
(DFT)[20] calculations, revealing -in part- the parameters tuning
the nature of the exchange coupling.[21]
Using paramagnetic complexes as bridges, their magnetic
behaviour is interplayed with those of the manganese units.
Aiming to focus on the anisotropy due to manganese units, we
oriented ourselves toward simpler ligands as bridges,
considering, in the work presented herein, the acetate ligand.
The carboxylate group is one of the most widely used bridging
unit, its versatility determining multiple coordination modes.[22, 23]
Besides, the carboxylates are convenient for a preliminary
control of coordination pattern, existing clear correlations
between ν(COO) frequencies and the carboxylate bringing
geometries.[24] Previous articles of 1D structures based on
acetate-bridged manganese(III) dimers remained at the level of
bare structure reports, without focusing in details of magnetic
properties, magnetic anisotropy or assembling features.[25] One-
dimensional systems assembled with bridging deprotonated
adipic and E,E-1,3- butadiene- 1,4-dicarboxylic acids where
characterized with slow relaxation of magnetization and SCM
behaviour.[26]
Syntheses and magnetic properties of [Mn(III)salen]+ chain
complexes with other carboxylate groups such as formate,[27]
Figure 1. The structure of the compound 1. The hydrogen atoms are omitted
chloro-acetate[28] or
a
series[29] consisting in cinnamate,
for clarity.
phenylacetate and benzoate, have been reported. The chains
were characterized as having a spin-canted pattern. However,
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10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
The bond lengths Mn-N and Mn-O vary within the ranges
1.968(2)- 1.998(2)Å and 1.881(2)-1.940(1) Å, respectively.
General crystallographic data are given in Table 1, selected
bonds and angles being summarized in Table S1 from
Supporting Information.
Table 1. Summary of the crystallographic data for 1, 2 and 3
1
2
3
formula
C38H39ClMn2N4O14
C39H42F6Mn2N4O11
C39H39F3Mn2N4O13S
P
formula
weight
crystal
921.07
triclinic
997.61
970.68
triclinic
Triclinic
In all three compounds, can be observed longer Mn-N and
Mn-O equatorial bond lengths longer for Mn(2) centres,
compared with Mn(1) sites. The longer bond lengths are
correlated with the strain and the ligand conformation. For the
Mn(2) centre, all the Schiff base present a more relaxed planar
conformation, while for the Mn(1), the ligands have a rather
strained conformation, with stepped pattern, for compounds 1-2
and umbrella-like for 3 (Figure 2). Each manganese centre is
deviated out of the ligand plane, towards the axial oxygen atom
belonging to the bridging carboxylate group. The acetate-type
axial positions of the Mn(1) and Mn(2) sites are established with
the O9 and O10 oxygen atoms, respectively, having the
corresponding bond lengths: 2.119 Å and 2.113 Å for 1, 2.149 Å
and 2.114 Å for 2 and 2.149 Å and 2.114 Å for 3.
system
space group
P
1
(No. 2)
P
1
(No. 2)
P
1
(No. 2)
a (Å)
b (Å)
c (Å)
 (o)
 (o)
 (o )
V (Å3)
Z
11.0684(5)
11.7949(9)
16.0906(3
81.973(13)
67.281(9)
85.00(1)
1917.4(2)
2
11.771(6)
11.824(5)
15.966(7)
72.038(10)
78.804(11)
80.694(12)
2061.0(16)
2
11.3151(6)
11.8514(7)
16.1846(9)
79.249(2)
70.659(2)
86.081(2)
2011.87(19)
2

calcd, g/cm3
F000
1.595
948
17435
1.609
1024
10526
1.602
996
14573
no. of
reflections
no. of
observations
no. of
variables
 (Mo K),
cm-1
6573
528
7074
610
4684
564
8.04
7.44
7.65
T, K
93
0.040
93
0.040
100
0.0376
R1a
(I>2.00(I))
R1a (all data)
wR2b (all
data)
0.045
0.112
0.044
0.110
0.0311
0.0802
GOF
1.064
1.045
1.062
1
2
2
2
[a]
[b]
F0
R 



F0  F0

/
w F

R 
F0  F0
/

0


The packing diagrams of the 1-3 crystal structures show,
for each compound, the zig-zag chain arrangement of the two
different out-of-plane manganese dimers alternating units (A and
B) (see Figure 3). The supramolecular interactions are mainly
hydrogen-bonds established between the anions and the Schiff
base ligand. The slipped stacking π-π interactions appear inside
each dimer unit, but are not important for the inter-chain
architecture.
Figure 2. Different salen-type Schiff base ligand conformation in compounds
1-3 and the details for the both out-of-plane Mn dimer units type (A and B).
In line with Jahn-Teller alike distortions expected for the
Mn(III) complexes, the axial Mn-O bonds are significantly longer
than the equatorial bonds. The syn-anti coordination mode for
the bridging carboxylate group imposes a zig-zag feature for the
chains and a distance of 5.233 Å (for 1), 5.226 (for 2) and 5.433
Å (for 3) respectively, between the Mn(1) and Mn(2) centres.
Each second axial position is occupied by a phenoxo oxygen
atom of the neighbouring ligand from the next Mn centre with the
same symmetry. In this way, two types of centrosymmetric
cationic binuclear units are formed, named in the following: A
unit, including the Mn(1) centres and B unit, including the Mn(2)
ones.
Figure 3. Packing diagram of the crystal structure of compound 1, showing for
each compound the zig-zag chain pattern of the two different out-of-plane
Mn(III) dimeric units. The hydrogen atoms are omitted for clarity.
Voltammetry measurements
The Mn-O-phenoxo-Mn angles are, in the A and B units:
103.26o (A unit in 1), 98.55o (B unit in 1); 101.94o(A unit in 2),
87.79o (B unit in 2), 98.43o (A unit in 3) and 102.16o (B unit in 3).
The Mn…Mn distances within each unit A and B are: 3.391 Å (A
unit in 1), 3.436 Å (B unit in 1); 3.394 Å (A unit in 2), 3.729 Å (B
unit in 2) and 3.303 Å (A unit in 3), 3.443 Å (B unit in 3).
The obtained oxidation peaks, at almost the same positive
potential for all three compounds, indicated that the processes
take place on the metal centre of the complex. As is shown in
the Figure 4, the voltammetry measurements upon anodic
scanning suggest the progressive oxidation of the complexes, all
the systems showing peaks at approximately the same abscissa
points. The relatively small variation in the intensity profiles is
due to kinetic specifics induced by the different anions. The
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10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
observed curves are most probably the superposed processes
of the multiple species resulted from the dissociation equilibria of
the complexes in the solution. Since the ligand itself shows an
oxidation peak at about 0.8 eV, must presume that the same
peak, visible for all the complexes, is due to the dissociated free
ligand. The peaks at 1.2 V can be tentatively assigned to the
Mn(III)Mn(IV) oxidation of the monomeric [Mn(5MeO-Salen)]+
units, whose axial coordination is probably completed with
solvent, as ligand. We can speculate that the shoulders visible
around 1.1 V are due to the oxidation of the free manganese
ions (or better said, solvated), appeared altogether with the free
ligand, by the dissolution of the complexes. The ligand itself is
showing no peak in the 1.0-1.4 V range. The processes are not
reversible because, once dissociated, the complexes cannot
return to previous stages. Varying the parameters of the process
the situation remains approximately the same. For instance,
starting the scan from -0.8 V negative domain, the pattern and
positions of the peaks in the positive side of potential are
conserved (See Supporting Information), varying only the
Magnetic properties
We recorded the magnetic susceptibility () as function of
temperature (T) as well as the magnetization (M) as function of
field (B) and temperature, for all the three compounds, but for
sake of conciseness we will present only the data of the system
2 (see Figure 5). The other ones, relatively similar, are shown in
the Supporting Information.
The T vs. T curves show antiferromagnetic-like pattern,
but the picture of the active interactions is, in fact, more complex.
There are multiple parameters, implied by the non-equivalence
of the manganese units, aside the interplay between exchange
coupling and the anisotropy effects. The topology of species and
the related exchange coupling contacts are schematized in the
Figure 6, including also the representation of the relative
orientations of magnetization axes, perpendicularly to the salen
ligand averaged planes.
Having two different dimeric building blocks, we must
assign, in principle, three coupling parameters, JA and JB inside
each binuclear unit and JC for their bridging trough acetate. The
description of magnetic anisotropy with the general Zero Field
Splitting (ZFS) Hamiltonian demands, in general, two
parameters per manganese mononuclear species, namely DA
and EA for the constituents of one dimeric unit, while DB and EB
for the other one. The interpretation of magnetic properties is
complicated also by the extended nature of the system. The
infinite chain (A1A2-B1B2)n can be conceived as a closed ring
with a sufficiently large number of elements, n, enforcing a
connection between the first A1 node and the n-th B2 one, in
order to achieve the complete equivalence of each i-th
tetrameric sequence along the extended structure.
current intensity, because of
underlying kinetics.
a
different conditioning of
60
1
2
3
3
1
50
40
30
20
10
0
2
We must consider formally a tetranuclear repeating unit,
since, otherwise, we cannot express, in topological sense, the
inversion operation determined by the crystallographic symmetry
(inversion inside the A1A2 and B1B2 dimers).
0.0
0.4
0.8
1.2
1.6
Potential(V)
Figure 4. The voltammograms for 1-3 recorded at a concentration of 5x 10-4 M,
within the potential range 0 to 1.5V.
8
6
6
5
5T
4T
4
3
2
1
0

3T
4
2T
1T
2
T (K)
B/T (T/K)
0
0
100
200
300
0
1
2
3
Figure 5. Magnetic data for compound 2. Left side: T vs, T curve with an inset showing the qualitative pattern of the magnetization swept at up and down field.
Right side: Magnetization (M, in Bohr magneton units) represented as function of field and temperature dependence (B/T).
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10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
The non-diagonal elements in the above HZFS(α) one-site
Hamiltonian lead to cross-elements in the matrix of the extended
system. In such circumstances, the n=1 case is the sole
tractable object. In spite of inherent limitations, even this
smallest system can retain the basic elements of the interaction,
being taken in the following as basis for the semi-quantitative
interpretation of the magnetic data in this class of systems. With
the presumed J=JA=JB equivalence of the intra-dimer couplings,
re-labelling by J'=JC the coupling over the acetate bridge, and
enforcing a formal contact between A1 and B2 ends, in order to
simulate the continuity, the Hamiltonian of the minimal
tetranuclear is ascribed as follows:
JA
JC
JB JC
JA
JC
JB
A1(1) A2(1)
B1(1)B2(1)
A1(2)A2(2)
B1(2)B2(2)
Figure 6. The topology of magnetic chains, showing the different units and
exhange couplings and local orientations of easy magnetization axes.
The general expression for a closed chain is:
n
ˆ
ˆ
ˆ
ˆ
ˆ
B2 (i)

HHDVV  2
J
SA (i) S
 JB
S
S
B1(i)
A
A2 (i)
(1)
(2)
1
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
2
.
(4)
H  2J

SA  SA  SB  SB

 2J'

SA  SB  SA  SB

i1
1
2
1
2
2
1
1
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
B1(n)
 JC
S
S
 JC
S
S

 2JC
S
S
A2 (1)
A2 (i)
B1(i)
A2 (i)
B1(i1)
The succession of sites with the {+ α,+ α,- α,- α} pattern in
the tilting of anisotropy axes, with respect of the mean line of the
chain, is formally ascribed by the following notation:
(5)
{A ,A2 ,B1,B2 ,}
n
1
2
1
3



z
ˆ
ˆ
HZFS  2
D

S


S(S 1)

 
K 

K (i)
A
A2
B
1
B2
i1
K
1
ˆ
ˆ
ˆ
ˆ
ˆ
HZFS  HZFS



 HZFS



 HZFS



 HZFS



Each Mn(III) unit carries a magnetic anisotropy with easy
magnetization axis perpendicular to the average plane of the
5MeO-salen ligand. Because the symmetry is quasi-tetragonal,
the E parameter is negligible, in comparison to the magnitude of
D, as proven also by the following ab initio calculations. For a
given centre, the ZFS Hamiltonian can be transformed to a
diagonal form, with respect of the D parameter, if the local z axis
is aligned to the easy magnetization axis. The local ZFS
Hamiltonian for a species having a tilt by the α angle, with
respect of the z axis, and D -only ZFS (i.e. E=0), is described by
the following matrix:
Considering that the system under a B magnetic field, the
Hamiltonian is completed with the Zeeman Hamiltonian, each
site contributing with a gBSB term, assuming a common g
factor for all the sites. In the given circumstances, namely
anisotropic system with different local axes, the Sz ceases to be
an overall good quantum number and the popular van Vleck
formula cannot be used. In turn, must use the general
expressions based on the sum of state function Z (see the final
section dedicated to methodological details). Besides,
accounting the anisotropy, dependence is made explicit with
respect of the directions from which the probe field is applied.
We proceeded to a rather advanced algorithm, fitting
simultaneously the magnetization and susceptibility data. From
experimental source, we have multiple experimental curves for
magnetization, dependence on field and temperature (M vs. B
and T), aside the standard susceptibility data (T vs. T). Then,
we handle a rather rich information, in a non-trivial treatment,
that enables the several parameters of the model.
The results of the fit are outlined in the Table 2. Using the
coupled fit procedures, we avoided the problem of parametric
uncertainty that arises when monotonous T vs. T patterns are
to be considered in over-parameterization circumstances. The fit
is good for T vs. T curves and moderate for the M vs. B/T
series, this limitation being the price paid to the limited
tetranuclear sequence that stayed at the basis of the modelling.
However, the dimension is a bottleneck issue and cannot be
encompassed, the difficulty of the problem increasing
exponentially, being intractable at higher levels.




1
2
3
2
3
2
2
D

1 3cos

2

D sin

2

D sin



0
0
0




















3
1
1
2
3
3
2
2

D sin
2


D

1 3cos
2

D sin

2
2
2

D sin
2
3
4
2
1
3
2
1
1
2
1
3
3
2
3
2
2
H
ZFS () 
D sin



D sin
2


D

1 3cos

D sin

2

D sin
(3)
2
2
2
2
3
1
3
2
3
2
0
0
D sin



D sin



D

1 3cos

2
D sin

2

2
2
4
2
3
2
1
2
2




0
D sin



D sin

2

D

1 3cos

2
2


This matrix is written in the basis of Sz = {-2,-1,0,1,2} spin
projections. For an isolated single-site this matrix has the same
eigenvalues as the =0 alignment, but the trigonometric factors
become non-trivial when inserted in the whole Hamiltonian of an
extended system, with different orientations of the local
anisotropy axes. If choose the z axis along the translation vector
of the chain, then the normal to the {MnN2} planes of A- and B-
type units are inclined with +38.1 and -34.4 degrees with respect
of this line, if take compound 1 as example, the other cases
being similar. The magnetic anisotropy can be simplified if
choose an averaged absolute value of the tilt, α=36.25 degrees,
with +α for the units A and -α for the units B. Another idealization
is the assumption of equal coupling parameters inside the dimer
units. For a system with n sites of spin S, the Hamiltonian matrix
has the dimension (2S+1)n. Aiming to consider a (A1A2-B1B2)n
oligomer as surrogate for the infinite chain, the Hamiltonian
scales as 625n, being prohibitive, with customary procedures,
already at the n=2 stage (when matrix dimension reaches almost
400000). The discussed alternation of anisotropy axes precludes
the block factorization that occurs in the case of ZFS with
coaxial D-only local Hamiltonians.
Table 2. The results of simultaneous fit to the T vs. T and M vs. B/T curves
for compounds 1-3.
Parameters
Compound
1
2
3
Landé factor
ZFS (cm-1)
g
2.020
-5.13
1.998
-2.73
+0.50
-0.65
2.010
-5.87
+0.33
-1.16
D =DA=DB
J =JA=JB
J' =JC
+0.26
-1.08
Exchange (cm-1)
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10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
In order to describe the situation at one of A or B types of
centres, we considered oxo-bridged dimers where one site is
replaced by Co(III), working CASSCF-SO calculations on {Mn-
Co}A and {Mn-Co}B systems. The lowest five states from such
calculations can be fitted well to a ZFS model, obtaining DA= -
3.706 cm-1, EA= 0.067 cm-1 for the A-type site and DB= -3.246
cm-1, EB= 0.121 cm-1 for the B-type one. Note that the sign of E-
parameter is not important, the ZFS spectrum being the same
for both ±|E| values. At the same time, on may conclude that as
compared with D, the E magnitude is small and can be
neglected, in order to simplify the parameter list (as we actually
did in the fit model).
The Figure 7 shows a comprehensive account of the
various results of calculations characterizing one complex unit
belonging the species A from system 2. The ZFS spectrum
resulted from the CASSCF-SO calculation on the ground state is
illustrated in the left panel (a). The energy levels depicted in the
right side of panel (b) correspond to the computed CASSCF
states, labelled according to the tetragonal idealization. The
reversed CASSCF energy diagram, shifted with the barycentre
in zero, can be assimilated to the ligand field orbital scheme.
.
All the systems were evidenced with ferromagnetic, J > 0,
coupling inside the oxo-bridged dimeric units and
antiferromagnetic, J' < 0, over the carboxylate linkage. The ZFS
parameter is negative, as expected for the axially elongated d4
ions. In spite of the introduced simplifications, the advance is
sensibly higher in comparison to existing interpretations on
similar systems, considering the works cited in introduction.
The ab initio approach of the magnetic properties.
Ab initio multi-configuration calculations using Complete
Active Space Self Consistent Field (CASSCF)[33] procedures
were performed on the system 2. The computed values are
JA=+0.165 cm-1, JB=+0.154 cm-1 and JC=-0.035 cm-1, supporting
qualitatively the fit results, i.e. yielding ferromagnetic coupling
inside the oxo-bridged units and antiferromagnetism for the
carboxylate bridge sequence, in spite of the fact that the
absolute values from the fit to the experiment do not match very
well. The ZFS parameters were extracted from spin-orbit (SO)
calculations, subsequent to the CASSCF orbital optimization.
Figure 7. Synopsis of orbital scheme determining the (a) ZFS groundstate scheme, (b) spectral terms and ligand field effects and (c) the magnetic orbitals
involved in the exchange coupling from the corresponding dimer unit. The calculations are illustrated on a unit belonging to the species A from compound 2.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
The groundstate is similar to
a
configuration
Thus, a pz-type atomic orbital (AO) from a bridging oxygen
atom is involved in -type bonding with the central metal ion of
its unit and in -type overlapping with the metal ion of the other
unit. The interacting couples figured in panels (a) z2-pz-z2 and (b)
yz-pz-yz from Figure 8, contribute to ferromagnetic channels,
containing orthogonality relationships, since the bridging pz
component cannot interact simultaneously with both d partners.
One may imagine other couples leading to delocalized
overlapping, such as z2-pz-yz, but one may assume the
dominance of the frontier orbital couple, z2-pz-z2, determining the
overall ferromagnetism over the oxo-bridges. Such a regularity is
supported by the almost rectangular shape of the Mn2O2 frame,
holding the relative purity of  and  interaction labels. A more
advanced distortion leads to the - mixing and to the mutual
|(xz)(yz)(xy)(z2)| which, in quasi-tetragonal effective symmetry,
5
can be assigned to a B1g term. This configuration can be
interpreted as a hole in the x2-y2 function, which correspondingly
takes the higher position in the effective orbital scheme.
Conversely, the energies of the highest spectral terms are
related with the lowest effective d orbital, assignable to the
quasi-degenerate doublet eg ≡ (xz, yz). The effective tetragonal
symmetry can be approximated, if neglect the spacing between
xz and yz components (about 715 cm-1), small in comparison
with the total gap of the scheme, about 23500 cm-1.
Using the standard ligand field parameters for a tetragonal
case, Dq, Ds and Dt, (see formulas written aside the levels from
Figure 7b), the quantities computed for the A-type Mn(III) site of
compound
2
are: 10DqA=19732.2 cm-1, DsA=2676.3 cm-1,
overlapping that favours the antiferromagnetism. Such a
DtA=918.5 cm-1. For the species B, the scheme is qualitatively
similar, showing a slightly higher ligand field strength, with the
following parameters: 10DqB=22014.5 cm-1, DsB=2849.8 cm-1,
DtB=1296.8 cm-1
The orbitals depicted in section (c) are the functions
obtained by the symmetry localization of the canonical orbitals
resulted from the CASSCF calculation of the {Mn2}A oxo-bridged
dimer. Considering that the canonical molecular orbitals (MOs)
were implied in the determination of exchange coupling
parameter, the localized functions have also the meaning of
magnetic orbitals. Observing details of the depicted orbitals and
underlining the key features in Figure 8, one may advance a
qualitative explanation for the ferromagnetic coupling along the
oxo-bridges and antiferromagnetic over the carboxylate ligand.
situation occurs for the interaction along the carboxylate bridge.
The panels (c) and (d) from Figure 8 suggest nonbonding and
weakly bonding MOs on the acetate that interact simultaneously
with both metal ions, leading to effective overlap and
antiferromagnetism. Note that, in (a) and (b), the lobes located
on the bridge show overlap only towards one Mn centre, while
the overlap against the other is forbidden. Conversely, in (c) and
(d) cases there are elements of bridge orbitals that are
simultaneously overlapping with both metal ions.
Conclusions
We developed an extended and systematic analysis
dedicated to experimental, computational and conceptual
aspects of the molecular magnetism based on dimeric building
blocks of Mn(III)-salen type complexes, which turn to be a rather
large class of systems, with promising further chemistry.
Particularly, we used the [Mn(III)(5MeO-salen)]+ unit, which in
(a)
(b)
previous instances[15] led to
a SCM system with overall
ferromagnetic coupling. The SCM features are induced by the
intrinsic SMM characteristics of the [Mn2(5MeO-salen)2]2+
binuclear units, formed in the sequence of extended assemblies.
We present a new series of coordination chains, resulted from
the bridging of {Mn2} with carboxylate groups. Because of the
antiferromagnetic coupling over the Mn-OCO-Mn bridge, the
systems cannot be claimed as SCM, but their magnetic
behaviour is naturally interesting, because of the embodied
dimeric SMMs.
(c)
(d)
The interpretation of magnetic properties of an infinite
system is non-trivial, having a complex interaction scheme (two
or three types of exchange coupling constants, J, anisotropy
described by local D parameters, plus a spin canting due to the
tilted local easy axes along the chain). We proceeded with care,
performing a salient modelling of these features on finite
systems, obtaining reasonable effective parameters.
The data were complemented with state of the art ab initio
calculations, CASSCF followed by spin-orbit (SO) treatments,
accounting the exchange coupling and ZFS parameters in semi-
quantitative way. The ferromagnetism inside the dimeric
Figure 8. Different interaction patterns explaining the nature of exchange
coupling in the dimeric sequences of the discussed chains: (a) the σ-type and
(b) the π-type interaction channels in oxo-bridged {Mn2} dimeric units
suggesting ferromagnetism determined by the orthogonality of magnetic
orbitals; (c) the σ-type and (d) the π-type interaction in carboxylate bridged
moiety showing mutual overlap of magnetic orbitals and antiferromagnetism.
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10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
using Fourier techniques.[41] All calculations were performed using the
CrystalStructure[42] crystallographic software package, except for
refinement, which was performed using SHELXL-97.[43] The diffraction
data for 3 were collected on SMART-1000/CCDD (Bruker AXS) area
detector using the standard procedure (Mo-K radiation). The data
integration and reduction were undertaken with SAINT and XPREP.[44]
The structure was solved by direct methods and refined using least-
squares methods on F2 with SHELXL-97.[43] The non-hydrogen atoms
were modelled with anisotropic displacement parameters, and hydrogen
atoms were placed by difference Fourier syntheses and refined
isotropically. CCDC- 848820 (for 1), 848821 (for 2), 848822 (for 3)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif.
sequences is explained by orbital interactions.[34-37] Namely, the
bridge functions that establish an overlap with one of the metal
ions is orthogonal towards the other one. In turn, the acetate
contact, having a lower local symmetry, realizes overlapping
with both metal ions, leading to antiferromagnetism. The
ferromagnetism inside the Mn(III) binuclears, combined with the
anisotropy given by the negative D parameter of the ZFS
Hamiltonian, makes these moieties intrinsic SMMs.[38-39]
However, because of the inter-dimer antifferomagnetic coupling,
the systems are not SCMs, having yet interesting magneto-
structural features.
Cyclic Voltammetry. The voltammetry measurements were carried out
using a conventional three electrode glass cell, coupled to an PAR 273 A
scanning potentiostat, while the system was controlled by a PC unit. The
working electrode was a small platinum disk with about 0.07 cm2 area;
the counter electrode was a platinum coiled wire and the reference
electrode an Ag/AgCl electrode immersed in a lithium perchlorate (0.1 M)
in acetonitrile. All solutions were prepared using acetonitrile. The
voltammograms were recorded at a concentration of the investigated
compounds of 5x 10-4 M, within the potential range 0 to 1.5 V, or -0.8V to
1.5 V. As the supporting electrolyte, lithium perchlorate (Aldrich), were
used, at a concentration of 0.1 M.
Experimental Section
Materials. All reagents were of analytical grade and were used without
further purification.
Caution! Perchlorate salts are potentially explosive and should only be
handled in small quantities.
Preparation of Complexes. The Schiff base ligand, N, N”-bis(5-
Methoxy-salicylidene)-1,2-diaminoethane (H25MeOsalen) was prepared
by the stoichiometric condensation reaction of 5- methoxy-
salicylaldehyde and 1,2-ethanediamine, in methanol.
Magnetic measurements. Magnetic measurements for 1-3 were carried
out on
a Quantum Design SQUID MPMS 5S magnetometer on
polycrystalline samples. The dc measurements were made using an
external magnetic field of 1000 Oe over a temperature range of 1.8-300K
and ac measurements using a 3Oe magnetic field oscillating at 100-1000
Hz. Diamagnetic corrections were estimated from Pascal constants.
[Mn2(5Meosalen)2(μ-CH3COO)]n(ClO4)n (1): A methanol solution (20
mL) of Mn(CH3COO)3•2H2O (0.268 g, 1 mmol) was added to a methanol
solution (40 mL) of the Schiff base ligand H25MeOSalen (0.324g, 1
mmol). After stirring for 30 min, NaClO4 (0.07g, 0.5 mmol) was added.
After 30 min heating at 50oC, the brown solution was filtered hot. The
filtrate was left to stand for one week at room temperature to form brown-
black crystals. Yield: 74%. Crystals were used for X-ray and magnetic
measurements. Analysis: Calc. for C38H39ClMn2N4O14 (M=921.07) C，
49.55; H，4.27; N，6.08 %．Found C，49.63; H，4.17; N，6.12 %. IR
Simulation of the magnetic properties. Expressing the magnitude of
external field by B and its orientation by the of the  and φ polar angles,
we worked a general Z(B,,φ) for anisotropic sum of the states function.
The explicit field dependence is introduced by the Zeeman components.
Finally, because the experimental data are statistic averages, due to
random orientation of the microcrystals in the sample, we shall consider
the space integration, when simulate the magnetization,
-
(KBr) (σ /cm-1): 1652(C=N), 1626(C=N), (C=0), 1107, 625 (ClO4 ).
[Mn2(5Meosalen)2(μ-CH3COO)]n(PF6)n•nCH3OH (2): This compound
was prepared following an analogous procedure, mentioned above for 1
using NH4PF6 (0.081g, 0.5 mmol) instead of the perchlorate anion. Yield:
64.3%. Crystals were used for X-ray and magnetic measurements.
Analysis: Calc. for C39H42F6Mn2N4O11P (M=997.61) C，46.95; H，4.24;
N，5.62 %．Found C，46.88; H，4.21; N，5.57 %. IR (KBr) (σ /cm-1):
 2
d
1




M  NAkBT
ln(Z(B,,)
sin()dd
 
0 0
dB
4


or magnetic susceptibility,
-
1639(C=N), 853 (PF6 ).
d2
1
 2






  NAkBT
2 ln(Z(B,,)
sin()dd
 
0 0
dB
4
[Mn2(5Meosalen)2(μ-CH3COO)]n(CF3SO3)n (3): An analogous method as
described for 1 was used, by replacing NaClO4 with NaCF3SO3 (0.086g,
0.5 mmol). Yield: 54.2%. Crystals were used for X-ray and magnetic
measurements. Analysis: Calc. for C39H39F3Mn2N4O13S (M=970.68) C，
48.26; H，4.05; N，5.77% Found C，48.22; H，3.99; N，5.83 %. IR


Here NA and kB are the Avogadro and Boltzmann universal constants.
The basic part of the procedure consists in solving the eigenvalues εi(B, ,
φ), (i=1 to 625) of a Hamiltonian matrix constructed in the above
discussed manner, with the J, J', D and g parameters. The sum of the
state is expressed then:
-
(KBr) (σ /cm-1): 1636(C=N), 1158, 1030, 845, 638 (CF3SO3 ).
Physical Measurements. Infrared spectra were measured on a KBr disk
with a Jasco FT-IR 4200 spectrometer.


i (B,,)



Z(B,,)  exp 


kBT
i


X-ray Crystal Structure Analysis. Single crystals of 1 - 3 were prepared
according to the described method in the synthesis section. Data
collections for 1-2 were made on a Rigaku Saturn CCD diffractometer
with graphite monochromated Mo-K radiation (λ=0.71069 Å). The
structure was solved by heavy-atom Patterson methods[40] and expanded
The Hamiltonian and consequently the Z function are including an outer
magnetic field B, applied from a direction expressed with polar angled 
and φ, i.e. having Cartesian components Bx=Bsin()cos(φ),
By=Bsin()sin(φ), Bz =Bcos(). The derivatives from the Z function are
solved numerically, taking finite differences after solving the Hamiltonian
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800003
European Journal of Inorganic Chemistry
FULL PAPER
at the B, B+dB, B-dB, where dB=0.001 T. The dB small perturbation has
the same polar orientation as the external field. If the outer field is null,
then the  and φ dependence concerns the orientation of the applied dB
perturbation. The integration contained in the orientation averaging is
[4]
[5]
a) A. Caneschi, D. Gatteschi, C. S. Lalioti, R. Sessoli, G. Venturi, A.
Vindigni, A. Rettori, M. G. Pini, Novak, M. A. Angew. Chem. Int. Ed.
2001, 40, 1760-1763; b) R. Lescouëzec, J. Vaissermann, C. Ruiz-
Pérez, F. Lloret, R. Carrasco, M. Julve, M. Verdaguer, Y. Dromzée, D.
Gatteschi, W. Wernsdorfer, Angew. Chem. Int. Ed. 2003, 42, 1483; c) J.
S. Miller, Adv. Mater. 2002, 14, 1105.
also considered numerically, taking
a summation (corresponding
weighted with the spherical sin() factors) over a grid of 12 points for the
 coordinate and 24 points in the φ scan. As a whole, with the repeated
solving of the large Hamiltonian matrix, as required by the numerical
differentiation and integration procedures, the computation is rather
demanding.
C. E. Hulme, M. Watkinson, M. Haynes, R. G. Pritchard, C. A. McAuliffe,
N. Jaiboon, B. Beagley, A. Sousa, J. Chem. Soc., Dalton Trans. 1997,
1805–1814.
[6]
[7]
R. Bagai, G. Christou, Chem. Soc. Rev. 2009, 38, 1011–1026.
H. Miyasaka, A. Saitoh, S. Abe, Coord. Chem. Rev. 2007, 251, 2622–
2664.
Ab initio calculations. Complete Active Space Self Consistent
(CASSCF) calculations were performed with the GAMESS program[45]
using the 6-311G* basis set for the Mn, N, or O atoms, and 6-31G for the
C or H atoms. The molecular models are cut from the experimental
structure of system 2. For the building blocks of A or B type we
[8]
a) R. Clerac, H. Miyasaka, M. Yamashita, C. Coulon, J. Am. Chem. Soc.
2002, 124, 12837; b) H. Miyasaka, R. Clerac, K. Mizushima, K. Sugiura,
M. Yamashita, W. Wernsdorfer, C. Coulon, Inorg. Chem. 2003, 42,
8203; c) H. Miyasaka, T. Madanbashi, K. Sugimoto, Y. Nakazawa, W.
Wernsdorfer, K. Sugiura, M. Yamashita, C. Coulon, R. Clerac, Chem.
Eur. J. 2006, 12, 7028; d) A. Saitoh, H. Miyasaka, M. Yamashita, R.
Clerac, J. Mater. Chem. 2007, 17, 2002; e) H. Miyasaka, A. Saitoh, M.
Yamashita, R. Clerac, Dalton Trans. 2008, 10, 2422.
considered the {Mn2}A
≡
[Mn2(5-MeOSalen)2(CH3COOLi)2]2+
or
B
molecular models, where Li atoms were imposed as prosthetic elements
mimicking the next Mn(III) ions in the chain. In this way, the carboxylate
moiety from a terminating Mn-carboxylate-Li sequence contains a part of
the electrostatic polarization encountered in the chain, having a better
description of corresponding contribution of this group to the axial ligand
field. In order to estimate the exchange coupling over the carboxylate,
[9]
T.-T. Wang, M. Ren, S.-S. Bao, Z.-S. Cai, B. Liu, Z.-H. Zheng, Z.-L.
Xua, L.-M. Zheng, Dalton Trans. 2015, 44, 4271–4279.
[10] W.-X. Zhang, T. Shiga, H. Miyasaka, M. Yamashita, J. Am. Chem. Soc.
2012, 134, 6908-6911.
bridge
an
asymmetric
molecular
unit
{MnAMnB}
≡
[(LiOH)Mn(MeOSalen)(CH3COO)(MeOSalen)Mn(LiOH)]2+ was computed,
with artificial terminating groups, namely Li+, instead of the following MnIII
ion, and HO- instead of phenoxo group from next salen-type ligand that
completes the axial coordination of the studied centre. The active spaces
of the CASSCF(n,m) calculations (n electrons in m orbitals) were
selected in accordance the considered problem. Thus, for estimating
exchange coupling on selected dimer sequences, {Mn2}A, {Mn2}B, and
{MnA,MnB}, we performed CASSCF(8,10) calculations corresponding to
the d4-d4 case. Focusing on local ligand field on a given d4 site, the
[11] A. Gutierrez, M. F. Perpinan, A. E. Sanchez, M. C. Torralba, V.
Gonzalez, Inorg. Chim. Acta 2016, 453, 169–178.
[12] a) Y. Sawada, W. Kosaka, Y. Hayashi, H. Miyasaka, Inorg. Chem.
2012, 51, 4824-4832; b) Q. Wu, Y.-G. Li, Y.-H. Wang, R. Cleìrac, Y. Lu,
E.-B. Wang, Chem. Commun. 2009, 5743.
[13] K. Kubo, T. Shiga, T. Yamamoto, A. Tajima, T. Moriwaki, Y. Ikemoto, M.
Yamashita, E. Sessini, M. L. Mercuri, P. Deplano, Y. Nakazawa, R.
Kato, Inorg. Chem. 2011, 50 (19), 9337–9344.
[14] a) C. Kachi-Terajima, H. Miyasaka, K. Sugiura, R. Clérac, H. Nojiri,
Inorg. Chem. 2006, 45, 4381; b) C. Kachi-Terajima, H. Miyasaka, A.
Saitoh, N. Shirakawa, M. Yamashita, R. Clérac, Inorg. Chem. 2007, 46,
5861; c) H. Hiraga, H. Miyasaka, S. Takaishi, T. Kajiwara, M.
Yamashita, Inorg. Chim. Acta 2008, 361, 3863; d) H. Hiraga, H.
Miyasaka, R. Clérac, M. Fourmigue., M. Yamashita, Inorg. Chem. 2009,
48, 2887; e) H. Miyasaka, T. Nezu, K. Sugimoto, K. Sugiura, M.
Yamashita, R. Clérac, Chem. Eur. J. 2005, 11, 1592; f) H. J. Choi, J. J.
Sokol, J. R. Long, Inorg. Chem. 2004, 43, 1606.
CASSCF(4,5) calculations were performed on {CoMn}A
[(CH3COOLi)CoIII(5-MeOSalen)MnIII(5-MeOSalen)(CH3COOLi)]2+
≡
or
B
idealized models, where a site was replaced with diamagnetic Co(III) and
excluded from the active space. Applying ab initio spin-orbit (SO)
procedures, subsequently to the CASSCF(4,5) calculations, we
accounted for the magnetic anisotropy resulted from the Zero Field
Splitting (ZFS) effects.
[15] M. Ferbinteanu, H. Miyasaka, W. Wernsdorfer, K. Nakata, K. Sugiura,
M. Yamashita, C. Coulon, R. Clerac, J. Am. Chem. Soc. 2005, 127,
3090-3099.
Acknowledgements
[16] H. Miyasaka, R. Clerac, W. Wernsdorfer, L. Lecren, C. Bonhomme, K.-I.
Sugiura, M. Yamashita, Angew. Chem. Int. Ed. 2004, 43, 2801–2805.
[17] S. Wang, M. Ferbinteanu, M. Yamashita, Inorg. Chem. 2007, 46, 610-
612.
The authors acknowledge financial support from ECOSTBIO
COST action 1305 and the Romanian Ministry of Education and
Research through the UEFISCDI research grant PN3-Idei PCE-
108/2017.
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Keywords: Manganese complexes • Magnetic anisotropy • Spin
Hamiltonian• Ab Initio calculations• Exchange mechanisms
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Entry for the Table of Contents (Please choose one layout)
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A
new series of compounds
incorporating Mn(III) dimeric
building block behaving as
Fanica Cimpoesu, Tanta Spataru,
Cristina Maria Buta, Horst Borrmann,
Olivia Georgeta Moga, and
a
molecular magnet, due to intrinsic
magnetic anisotropy, is reported.
Advanced analyses are performed,
complemented with transparent
qualitative mechanisms.
Marilena Ferbinteanu*
Page No. – Page No.
Spin Coupling and Magnetic
Anisotropy in 1D Complexes with
Manganese(III) Units and
Carboxylate Bridges. Synthesis and
Analysis, Calculations and Models.
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