Single-Molecule-Magnet FeII4FeIII2 and Antiferromagnetic FeIII4 Coordination Clusters

Article abstract of DOI:10.1021/acs.inorgchem.9b00828

Supported by endogenous (part of the ligand, in-built) phenoxo bridges provided by the ligand 2,6-bis[{{(5-bromo-2-hydroxybenzyl)}{(2-(pyridylethyl)}amino}methyl]-4-methylphenol) (H₃L), in its deprotonated form, exogenous (not part of the ligan

Full text of DOI:10.1021/acs.inorgchem.9b00828

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
II
III
III
Single-Molecule-Magnet Fe Fe and Antiferromagnetic Fe
4
2
4
Coordination Clusters
†
‡
‡
,†
Suman K. Barman, Joan Cano, Francesc Lloret, and Rabindranath Mukherjee*
†
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
‡
Departament de Química, Inorgan
̀
ica/Instituto de Ciencia Molecular (ICMOL), Universitat de Valen
̀
cia, Polígono de la Coma, s/n,
4
̀
6980 Paterna (Valencia), Spain
*
S Supporting Information
ABSTRACT: Supported by endogenous (part of the ligand,
in-built) phenoxo bridges provided by the ligand 2,6-bis[{{(5-
bromo-2-hydroxybenzyl)}{(2-(pyridylethyl)}amino}methyl]-
4
-methylphenol) (H L), in its deprotonated form, exogenous
3
(
not part of the ligand, externally added or generated) oxo-/
h y d r o x o -
a n d
2
a c e t a t o - b r i d g e d
I I
I I I
[
[
Fe Fe
(O) (O CMe) (L) ]·4Et O (1) and
4
2
2
4
2
2
III
Fe (OH) (O CMe) (L) ](ClO )·3MeCN·2H O (2) coor-
4
2
2
3
2
4
2
dination clusters have been synthesized and structurally
characterized. Complexes 1 and 2 have μ -O and μ -OH
4
3
bridges, respectively. Magnetic studies on 1 reveal slow
magnetic relaxation below 2 K. Both in-phase (χ′ ) and out-
M
of-phase (χ″ ) magnetic susceptibility were found to be frequency dependent. This is typical of a single-molecule magnet
M
−
7
−1
−1
(
SMM) with τ = 1.9(2) × 10
s
and E = 5.1(3) cm . Assuming that E corresponds to the energy splitting of the ground
a a
0
−
1
spin state (S = 2) by the zero-filed-splitting (zfs), E = 4|D| (D is the axial zfs parameter), D ≈ − 1.3 cm could be estimated.
a
−
1
For 2, three types of magnetic interactions are observed: J = −56.5(3), J = −71.6(4), and J = +4.5(2) cm . Considering the
A
B
C
observed structural parameters, the magnetic behavior for both of the coordination clusters 1 and 2 has been rationalized.
INTRODUCTION
complexes is of significant importance because of their
■
“
existence in the active sites of several nonheme diiron-
Synthesis of high-nuclearity transition-metal clusters through
coordination-driven self-assembly” continues to be of special
interest, owing to their aesthetically pleasing structures,
interesting magnetic properties,
8
containing proteins and enzymes.
From the viewpoint of magnetic property driven applica-
3
−5,9−11
1
,2
tions, single-molecule magnets (SMMs)
are very
and the possibility of
useful, as these discrete molecules are capable of retaining a
net magnetization even after removal of the applied magnetic
field. In other words, SMMs refer to slow relaxation of
observing slow magnetic relaxation effects at the molecular
3
level. Application potentials of such coordination clusters
include high-density information storage, quantum computing,
and spintronic technology.
3
4
,5
magnetization of purely molecular origin. Two fundamental
In order to utilize these
criteria for obtaining efficient 3d-SMM behavior are (i) a large
ground-state spin (S) and (ii) a high negative magnetic
anisotropy, quantified by the axial zero-field splitting parameter
properties, it is important to examine the topology of the
synthesized clusters for a better understanding of the origin of
the observed properties, which are associated with structural
features of the polynuclear complexes. The increasing interest
in this field is justified by the intellectual challenge in
controlling and manipulating the self-assembly process. The
choice of the bridging ligand(s) is crucial for the formation of
these clusters.
(
D).
In order to maximize molecular ground-state spin, synthesis
of polynuclear coordination clusters has been a good
9e
strategy. Here an ensemble of paramagnetic ions couple
their spins via superexchange pathways to achieve large-spin
9
d,10a−c,11b,g
ground states.
maximum high-spin regime is observed only at low temper-
However, in most cases the
To successfully synthesize such coordination clusters,
carboxylates act as ideal linkers between metal ions for
propagation of metal−ligand coordination units due to their
11g
atures (T ≲ 30 K). Stronger electronic alignment to observe
large ground-state spin is constrained by superexchange
interaction. The extent of superexchange is dependent on
structural features: e.g., arrangement of metal ions, nature of
6
divergent bridging capabilities. In fact, we have investigated
magnetostructural properties of acetato-bridged coordination
7
a,b
polymers
and discrete coordination clusters of various
It is appropriate to mention here that under-
7
c−f
nuclearity.
standing the electronic and magnetic properties of μ-oxo-, μ-
Received: March 22, 2019
hydroxo-, and μ-carboxylato-bridged high-nuclearity iron
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00828
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
bridging groups, bridge angle, dihedral angle between the
planes containing metal ions, etc. Thus, systematic inves-
tigation on magnetostructural studies is very important and
relevant to obtain coordination clusters that can exhibit
thermally well isolated ground states, even at room temper-
1
2a
ature and beyond.
The second requirement for SMM behavior, large magnetic
anisotropy, is very difficult to achieve in a controllable manner,
and that is why clusters with large spin values do not always
show SMM behavior. Magnetic anisotropy (D) is related to
Jahn−Teller distortion. The ligand-field approach is handy in
this regard to judiciously choose the metal ion, with potentially
12b
II
high magnetic anisotropy.
In this context, Fe ion is a
suitable candidate, as it can provide a high-spin ground state
II
and, by manipulating the coordination geometry of the Fe
ion, magnetic anisotropy can be controlled. Thus, the synthesis
13
14
of high-nuclearity iron(II) and iron(III) coordination
clusters has been of continued interest. It is appropriate to
mention here that recently there has been a totally new
synthetic procedure established for the guaranteed preparation
of high-spin coordination clusters and polymers with exciting
15
magnetic properties, including SMM behavior. Moreover,
synthesis of mixed-valent coordination clusters has attracted
much attention owing to their electrochemical properties,
small-molecule activation, catalysis, and magnetic applica-
Figure 1.
organic layers were dried over anhydrous Na SO . Solvent
1
4d,f,16
tions.
In mixed-valent clusters, the existence of several
redox sites with different spins makes it challenging to
understand their ground-state electronic properties due to
the presence of delocalization. Thus, the development of
mixed-valent coordination clusters is quite relevant from the
viewpoint of understanding their magnetostructural as well as
electronic structural properties. From this perspective, herein
we describe the synthesis and magnetic properties of
phenolato-, oxo-, and acetato-bridged hexanuclear mixed-
2
4
1
evaporation led to isolation of the amine. Yield: 1.6 g, 76%. H
NMR (CDCl , 500 MHz): δ 3.02 (2H, t, −CH −), 3.108 (2H, t,
3
2
−CH −), 3.96 (2H, s, −CH −), 6.69 (2H, s, aromatic H), 7.12 (1H,
2
2
t, pyridine 5-H), 7.20 (1H, d, pyridine 3-H), 7.60 (1H, t, pyridine 4-
H), 8.52 (1H, d, pyridine 6-H).
Step 2. To a solution of 2,6-bis(chloromethyl)-4-methylphenol
0.414 g, 0.002 mol) dissolved in dry THF (10 mL) was added
(
II
III
dropwise a solution in dry THF (7 mL) of the amine (1.228 g, 0.004
valent [Fe Fe (O) (O CMe) (L) ]·4Et O (1) and tetranu-
4
2
2
2
4
2
2
mol), obtained in step 1, followed by addition of Et N (0.408 g, 0.004
III
4
3
clear [Fe (OH) (O CMe) (L) ](ClO )·3MeCN·2H O (2)
2
2
3
2
4
2
mol) at 273 K. The mixture was stirred for 24 h at 298 K and filtered
to remove a white solid. The filtrate was evaporated to dryness, and
CH Cl (20 mL) was added. The solution was washed with brine
coordination clusters. The coordination clusters are endoge-
nously supported by two phenol-based 2,6-bis[{{(5-bromo-2-
hydroxybenzyl)}{(2-(pyridylethyl)}amino}methyl]-4-methyl-
2
2
solution and water. The organic layer was dried over anhydrous
Na SO . Solvent was evaporated to obtain the desired ligand H L, as a
yellowish white solid. Yield: 1.1 g, 73%. H NMR (CDCl , 500 MHz):
δ 2.16 (3H, s, −CH ), 2.82 (4H, t, −CH −), 3.06 (4H, t, −CH −),
3
−
phenol) (H3L) (Figure 1), in their deprotonated form (L ).
2
4
3
1
To the best of our knowledge, 1 is the first example of a mixed-
3
II
4
III
2
valent Fe Fe coordination cluster to exhibit SMM behavior.
3
2
2
3
.68 (4H, s, −CH −), 3.72 (4H, s, −CH −), 6.60 (2H, d, aromatic
2
2
A temperature-dependent magnetic study reveals that for 2
three types of magnetic interactions are observed. On the basis
of structural parameters, the observed magnetic behavior of
coordination clusters 1 and 2 has been rationalized. For a
better understanding and estimation of the magnetic-exchange
parameters DFT calculations have been performed on 1.
H), 6.74 (2H, s, aromatic H), 7.00 (2H, d, aromatic H), 7.09 (2H, s,
aromatic H),7.15 (2H, t, pyridine 5-H), 7.18 (2H, d, pyridine 3-H),
7
.54 (2H, t, pyridine 4-H), 8.50 (2H, d, pyridine 6-H).
II
III
Synthesis of Complexes. [Fe Fe (O) (O CMe) (L) ]·4Et O (1).
4
2
2
2
4
2
2
Method A. To a mixture of H L (0.100 g, 0.13 mmol) and
3
NaO CMe·3H O (0.088 g, 0.65 mmol) in MeOH (4 mL) was added
2
2
II
solid [Fe (H O) ](CF SO ) (0.120 g, 0.26 mmol), under anaerobic
2
6
3
3 2
conditions (mBraun glovebox). After 5 min of stirring, solid
EXPERIMENTAL SECTION
III
■
Fe (ClO ) ·6H O (0.050 g, 0.13 mmol) was added portionwise to
4
3
2
General Considerations. All reagents were obtained from
it and the mixture was stirred. After 6 h, the reaction mixture was
commercial sources and used as received. Solvents were dried/
filtered through a G-4 frit and the filtrate was kept for Et O diffusion
2
7
purified as reported previously. 2,6-Bis(chloromethyl)-4-methylphe-
under anaerobic conditions, affording single crystals suitable for X-ray
diffraction studies. Yield: 0.090 g, 58%. Anal. Calcd for
C H Br Fe N O (fw 2386.78): C, 49.31; H, 5.12; N, 4.70.
1
7
18
nol and [Fe(H O) ](CF SO ) were prepared by following the
2
6
3
3 2
reported procedures.
9
8
122
4
6
8
20
Synthesis of H L. The ligand H L was synthesized in two steps.
Step 1. To a solution of 2-pyridylethylamine (0.850 g, 0.007 mol)
in MeOH (20 mL) was added solid 5-bromo-2-hydroxybenzaldehyde
Found: C, 49.10; H, 4.85; N, 4.82. UV−vis (MeCN): λmax, nm (ε,
3
3
−1 −1
M
cm ) 415 (1900).
Method B. To a stirred mixture of H L (0.100 g, 0.13 mmol) and
3
(
1.40 g, 0.007 mol) portionwise. This mixture was refluxed for 3 h and
NaO CMe·3H O (0.088 g, 0.65 mmol) in MeOH (5 mL) was added
2 2
then cooled to 298 K. Solid NaBH (0.304 g, 0.008 mol) was then
solid [Fe(H O) ](CF SO ) (0.120 g, 0.26 mmol), under anaerobic
2 6 3 3 2
4
III
added portionwise with continuous stirring. After 12 h of stirring, the
volatiles of the mixture were removed under reduced pressure, and
then water (20 mL) was added. The mixture was neutralized with
MeCO H and extracted with CH Cl (3 × 15 mL). The combined
conditions (mBraun glovebox). After 10 min, solid Fe (CF SO )
3 3 3
(0.065 g, 0.13 mmol) was added portionwise and the mixture was
stirred for 5 h. The reaction mixture was then filtered through a G-4
frit, and the solution was kept for Et O diffusion, under anaerobic
2
2
2
2
B
DOI: 10.1021/acs.inorgchem.9b00828
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
conditions. Single crystals suitable for X-ray diffraction studies were
obtained. Yield: 0.100 g, 64%.
It should be noted that both methods provided identical products, as
were 0.0654 (0.01513) and the Flack parameter (x) value was
0.041(2). After twin refinement, the values became 0.0618 (0.01513)
with x = 0.027(6), suggesting that the absolute configuration is best
described by the latter coordinate assignment. The figures were made
verified by X-ray crystallography.
III
20b
[
3
Fe (OH) (O CMe) (L) ](ClO )·3MeCN·2H O (2). To a solution of
with the Mercury program.
4
2
2
3
2
4
2
H L (0.051 g, 0.068 mmol) in MeOH (3 mL), first solid NaO CMe·
Pertinent crystallographic parameters are summarized in Tables S1
and S2. CCDC 1849015 (1) and 1849001 (2) contain supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
2
III
3
(
H O (0.051 g, 0.375 mmol) and then solid Fe (ClO ) ·6H O
2
4
3
2
0.048 g, 0.136 mmol) were added. The reddish brown solution thus
obtained was stirred for 6 h and filtered. The filtrate was kept for Et O
2
vapor diffusion. A dark brown crystalline solid was obtained. Slow
evaporation of a MeCN solution of this solid led to single crystals,
suitable for X-ray diffraction studies. Yield: 0.045 g, ∼65%. Anal.
Calcd for C H Br ClFe N O (fw 2178.9): C, 47.36; H, 4.31; N,
Computational Details. DFT calculations were carried out using
21
the Gaussian 09 package to estimate the magnitude of the magnetic
coupling between the magnetic centers in 1. These calculations were
performed with the B3LYP hybrid functional; the quadratic
convergence approach and a guess function was generated with the
86
94
4
4
11 20
7
.07. Found: C, 47.51; H, 4.21; N, 6.95. UV−vis (MeCN): λmax, nm
−1 −1
(ε, M cm ): 465 (9180), 345 (sh) (11 100).
22−24
Caution! Perchlorate salts of compounds containing organic ligands are
fragment tool using the same program.
Two ways to build the
potentially explosive!
atomic orbitals were used: (i) triple-ζ and double-ζ all-electron basis
sets proposed by Ahlrichs et al. were employed for the metal ion and
the rest of the atoms, respectively; (ii) the same triple-ζ basis set was
Physical Measurements. Elemental analyses were obtained using
a Thermo Quest EA1110 CHNS-O instrument (Italy). UV−vis
25,26
spectra were recorded on an Agilent 8453 diode-array spectropho-
employed for all atoms.
The magnetic coupling states were
1
tometer. H NMR spectra (CDCl ) were obtained on a JEOL LA 500
obtained from the relative energies of the broken-symmetry (BS)
singlet spin state from the high-spin state with parallel local spin
moments. More details about the use of the broken-symmetry
approach to evaluate magnetic coupling constants can be found in the
3
(
500 MHz) spectrometer. Chemical shifts are reported in ppm
referenced to TMS. Variable-temperature magnetic susceptibility
measurements in the solid state were performed with a Quantum
2
7−29
Design (Valen
̀
cia, Spain) SQUID magnetometer. Variable-temper-
literature.
A polarizable continuum model (PCM) was
ature (2−300 K) direct current (dc) magnetic susceptibility
measurements on crushed crystals of 1 and 2 under applied fields
of 0.25 (T < 30 K) and 5.0 kG (T ≥ 30 K) and variable-field (0−5 T)
magnetization measurements in the temperature range 2.0−10.0 K
were carried out with a Quantum Design SQUID magnetometer.
Variable-temperature (2.0−10.0 K) alternating current (ac) magnetic
susceptibility measurements at frequencies in the range 0.6−10.0 kHz
under ±4.0 G oscillating field and different applied static dc fields in
the range 0−1.0 kG were performed for 1 with a Quantum Design
Physical Property Measurement System (PPMS). Data were corrected
for the diamagnetism of the constituent atoms and the sample holder.
The crushed samples of 1 and 2 were restrained with n-eicosane to
prevent any displacement due to their significant magnetic anisotropy.
Crystal Structure Determination. Single crystals of suitable
dimension were used for data collection. Diffraction intensities were
collected on a Bruker SMART APEX CCD diffractometer, with
graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 100(2)
K. For data reduction the “Bruker Saint Plus” program was used. Data
were corrected for Lorentz and polarization effects; empirical
absorption correction (SADABS) was applied. For 1, the structure
introduced in the calculations with the parameters corresponding to
30
acetonitrile. High-spin iron(II) complexes show an incomplete t2g
shell, and therefore searching for the most stable electronic
configuration is a difficult task, even more so when a large number
II
of Fe ions are present. This situation can be made simpler by
II
reducing the number of Fe ions. Only two metal ions are involved in
each magnetic coupling in our molecular models (see below).
II
III
Therefore, they were kept and the remaining Fe and Fe ions were
II
III
replaced by diamagnetic Zn and Ga ions, preserving the overall
charge of the complex and avoiding a cutting of the molecule that
could introduce unwanted electronic effects.
RESULTS AND DISCUSSION
■
Synthesis of the Ligand H L and the Coordination
3
Complexes Thereof. The ligand H L was synthesized in two
3
steps. The first step involved Schiff base condensation between
2
-pyridylethylamine and 5-bromo-2-hydroxybenzaldehyde, fol-
lowed by reduction with NaBH to produce the secondary
4
1
9a,b
amine. In the second step, the amine was treated with 2,6-
was solved by SHELXT and refined with SHELXL-2016,
incorporated into the Olex2 1.2-alpha crystallographic package.
For 2, the structure was solved by SIR-97 and refined by full-matrix
least-squares methods, based on F using SHELXL-97, expanded by
20a
bis(chloromethyl)-4-methylphenol and Et N in THF. This
3
afforded H L (Figure 1) as a yellowish white solid, which was
3
2
1
characterized by its H NMR spectrum (Figure S1).
1
9a
difference Fourier syntheses, and refined with SHELXL-2014, as
The choice of H L in this work derives impetus from our
3
7e
incorporated in the WinGX 2014.1 crystallographic collective
recent results on the hydrolysis of carboxy esters under
ambient conditions by dinickel(II) complexes; recently we
19c
package. The positions of the hydrogen atoms were calculated by
assuming ideal geometries but not refined. All non-hydrogen atoms
were refined with anisotropic thermal parameters by full-matrix least-
II
reported the series of dinuclear complexes [Ni (L*)(H O) -
2
2
2
II
(
(
2
MeOH)](ClO )·MeOH, [Ni (HL*)(μ-O CMe)(H O) ]-
2
4
2
2
2
2
squares procedures on F .
The structural analysis of 1 revealed distorted Et O molecules. The
atoms O11, O112, and O13 of the Et O molecules are situated on a
II
ClO )·4H O, [Ni (H L*)(μ-O CMe) ](ClO )·CH Cl ·
4
2
2
2
2
2
4
2
2
2
II
H O, [Ni (HL*)(μ-O CEt)(H O) ](ClO ), and particu-
2
2
2
2
2
4
2
II
larly the pentanuclear complex [Ni (H L*) (μ -OH) (μ-
crystallographically imposed 2-fold axis. C44, C45, C46, C47, C52,
and C53 are generated by the symmetry operator −3/2 − x, 3/2 − y,
z. Fe1, Fe2, and Fe3 of 1 are situated on a crystallographically
imposed glide plane. The other half of the molecules is generated by
the symmetry operator −1/2 − x, 3/2 − y, z. The anisotropic
5
2
2
3
2
O CMe) ](ClO ) ·MeCO Et, with use of H L* (=2,6-bis-
2 4 4 2 2 3
[{{(5-bromo-2-hydroxybenzyl)(N′,N′′-(dimethylamino)-
ethyl)}amino}methyl]-4-methylphenol). The difference be-
tween the two ligands is only replacing the N′,N′′-dimethyl
arms of H L* by 2-pyridyl arms, present in H L. Notably, in
displacement parameters of the highly distorted Et O molecules were
2
3
3
constrained, using distance restraints. To make the ADPs of the atoms
reasonable, the rigid-bond restraints DELU, SADI, SIMU, and ISOR
were used on the solvent molecules. For 1 the convergence of the
Flack parameter was taken into account, during the final cycles of
least-squares refinement. At this stage, all the x, y, z coordinates for 1
were inverted (using both TWIN and BASF entry) to test the
absolute configuration. In fact, before refinement the R1 (wR2) values
II
none of these Ni complexes did bromophenolate units act as
bridging ligands. Barring only one case, in all other complexes
these units coordinated as phenol(s), not in the common
phenolate form. Moreover, using asymmetrical ligands
providing both a bridging and a terminal phenolate to a pair
II III
of iron ions, mixed-valent binuclear Fe Fe complexes have
C
DOI: 10.1021/acs.inorgchem.9b00828
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
1
been reported. Anticipating that the bromophenolate units of
H L may act as bridging ligands, we attempted the synthesis of
3
II
III
Fe /Fe coordination cluters with H L and the present work
3
is the outcome of such a synthetic exploration.
II
The reaction of H L, NaO CMe·3H O, [Fe (H O) ]-
3 3 2 4 3 2
respectively) in MeOH, under anaerobic conditions, followed
3
2
2
2
6
III
(
CF SO ) , and Fe (ClO ) ·6H O (1:5:2:1 mol ratio,
II
III
by vapor diffusion of Et O, afforded [Fe Fe 2(O)2-
2
4
(
O CMe) (L) ]·4Et O (1). The identical complex in better
2 4 2 2
III
yield was obtained when Fe (CF SO ) was used instead of
3
3 3
III
Fe (ClO ) ·6H O. The reaction of H L, NaO CMe·3H O,
4
3
2
3
2
2
and Fe(ClO ) ·6H O, (1:5.5:2 mol ratio, respectively) in
4
3
2
MeOH, followed by vapor diffusion of Et O, afforded a
2
microcrystalline solid. The slow evaporation of a MeCN
III
4
solution of this solid yielded [Fe (OH) (O CMe) (L) ]-
2
2
3
2
(
ClO )·3MeCN·2H O (2). We should admit that, after initial
4 2
serendipity in the synthesis, we were successful in the designed
synthesis of 1 and 2. The formation of the coordination
clusters 1 and 2 is summarized in eqs 1 and 2, respectively.
II
2
H L + 10NaO CMe·3H O + 4[Fe (H O) ](CFSO )
3
2
2
2
6
3
3 2
III
+
2Fe (CFSO ) (MeOH, Et O)
3 3 3 2
II
III
→
[Fe Fe (O) (O CMe) (L) ]·4Et O + 6MeCO H
4
2
2
2
4
2
2
2
+
10NaO SCF + 4CFSO H + 52H O
(1)
3
3
3
3
2
III
2
H L + 11NaO CMe·3H O + 4Fe (ClO ) ·
3
2
2
4 3
6
H O(MeOH, MeCN, Et O)
2 2
III
→
[Fe (OH) (O CMe) (L) ](ClO )·3MeCN·
4
2
2
3
2
4
2
H O + 8MeCO H + 11NaClO + 53H O
(2)
2
2
4
2
Complexes 1 and 2 display UV−vis spectra assignable to
II
13a
metal-to-ligand (Fe → pyridine)
(
and ligand-to-metal
PhO /OH to Fe ) charge-transfer (MLCT/LMCT)
transitions at 415 nm for 1 and at 465 and 345 nm (sh) for
. X-ray structure determinations of 1 and 2 (see below)
authenticated the composition of the coordination clusters. It
−
−
III
2
Figure 2. (a) Perspective view of metal coordination environment
II III
4 2
13
and (b) schematic representation and (c) butterfly-like Fe Fe core
should be mentioned here that a sizable number of iron(II)
and a large number of iron(III) coordination clusters are
known. To the best of our knowledge, only a handful of mixed-
II
III
of [Fe Fe (O) (O CMe) (L) ]·4Et O (1).
14
4
2
2
2
4
2
2
II III
14d,f,32
valent Fe Fe (≥Fe3) coordination clusters are known,
oxo (μ -bridging mode) and acetate bridges connect Fe2 and
4
II
2
with oxo, alkoxo, and acetato bridges. Hence, the successful
synthesis of coordination cluster 1, supported by phenoxo, oxo,
and acetato bridges, deserves special attention. From a
Fe2* with other symmetry-related Fe units. A schematic
II
4
III
2
presentation of the Fe Fe core is displayed in Figure 2. The
II
4
III
Fe Fe core assumes a butterfly-like topology. It should be
2
II
magnetostructural perspective, therefore, synthesizing Fe
mentioned here that syntheses of mixed-valent coordination
II III
14d,f,16,32
and mixed-valent Fe Fe (≥Fe3) coordination clusters and
understanding their structural as well as topological features
are of paramount importance.
clusters
not only are challenging from synthetic
aspects but are also of significant interest because of their
application in magnetic material, redox catalysis, etc.
D e s c r i p t i o n o f t h e S t r u c t u r e o f
The Fe1 (and symmetry-related Fe1*) centers are held by
the endogenous phenoxo O1 with Fe1−O1−Fe1* = 103.3(5)°
and exogenous oxo O3 with Fe1−O3−Fe1* = 101.1(5)°.
Other four-coordinations to Fe1 (and Fe1*) centers consist of
II
III
[
Fe Fe (O) (O CMe) (L) ]·4Et O (1). An X-ray structural
4 2 2 2 4 2 2
analysis of 1 reveals a hexameric core (Figure 2), bridged by
two endogenous phenoxides and two exogenous oxo and four
exogenous acetates. The molecule sits on a crystallographically
imposed mirror plane. Selected metric parameters are given in
Table 1 and Table S3. From the bond distances (Table 1) and
3
−
pyridyl N1, tertiary amine N2, a phenolate O2 from one L ,
and O4 from an acetate. Thus, Fe1/Fe1* centers have a
II
distorted Fe N O coordination environment. The Fe1 (and
2
4
33
bond valence sum (BVS) analysis (Table 2), it is evident that
Fe1*) centers are 3.208 Å apart. The Fe1 and Fe2 centers are
held by the oxo bridge O3 with Fe1−O3−Fe2 = 134.21(7)°, at
a distance of 3.75 Å. Fe1 and Fe2* centers are bridged by
phenolate O2 with Fe1−O2−Fe2* = 94.7(3)° and oxo O3
with Fe1−O3−Fe2* = 98.86(6)°. In addition, an acetate
II
III
the Fe1 and Fe3 centers are Fe and the Fe2 center is Fe
Table 2). Thus, if there is any ambiguity, it is due to
(
II
delocalization. Two dimeric Fe units are connected by two
2
III
Fe ions: Fe2 and symmetry-related Fe2*. In addition to
terminal phenolato bridges from L connecting a Fe unit,
3−
II
2
bridges Fe1 and Fe2* in a μ -1,3 syn-syn bridging mode (O4
2
D
DOI: 10.1021/acs.inorgchem.9b00828
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Inorganic Chemistry
Table 1. Selected Bond Lengths (Å) in [Fe Fe (O) (O CMe) (L) ]·4Et O (1)
Article
II
III
4
2
2
2
4
2
2
Fe1−O1
Fe1−O2
Fe1−O3
Fe1−O4
Fe1−N1
Fe1−N2
2.045(7)
2.104(8)
2.077(7)
2.045(8)
2.246(10)
2.189(10)
Fe2−O2
Fe2−O3
Fe2−O5
Fe2−O6
Fe2−O8
Fe2−O9
2.107(8)
1.999(8)
2.070(8)
2.051(8)
2.024(8)
1.936(7)
Fe3−O7
Fe3−O8
Fe3−O9
Fe3−O10
Fe3−N3
Fe3−N4
2.097(8)
2.204(8)
2.146(7)
2.071(7)
2.231(10)
2.243(11)
Table 2. Bond Valence Sums (BVS) for the Fe Centers in 1
and 2
FeII
a
FeIII
b
Fe center in 2
FeII
a
FeIII
b
Fe center in 1
Fe1
Fe2
Fe3
2.22
2.72
1.95
2.42
2.91
2.12
Fe1
Fe2
Fe3
2.73
2.89
2.89
2.75
2.95
3.09
3.09
2.98
Fe4
a
II
b
III
Considering all as Fe . Considering all as Fe .
6
to Fe1 and O5 to Fe2*). Fe2 (and Fe2*) centers are further
coordinated by O6 of an acetate, an oxo O9, and a phenolate
3−
O8, from another L . The distance between Fe1 and Fe2* is
3
.09 Å. O3 and O9 oxo groups bridge Fe2 and Fe2*, at a
distance of 2.96 Å, with Fe2−O3−Fe2* = 95.9(5)° and Fe2−
O9−Fe2* = 100.1(5)°. Thus, Fe2/Fe2* centers assume a
III
distorted Fe O coordination sphere. It should be noted here
6
that oxo groups O3 and O9 exist in μ -bridging mode: O3
4
bridges Fe1, Fe1*, Fe2, and Fe2*, and O9 bridges Fe2, Fe2*,
Fe3, and Fe3*. The Fe2 and Fe3 centers are held at a distance
of 3.783 Å by the oxo bridge O9 with Fe2−O9−Fe3 =
1
35.88(6)°. Fe2 and Fe3* (and Fe2* and Fe3) centers are
held at 3.04 Å by three bridging moieties: an oxo O9 (Fe2−
O9−Fe3* = 96.18(7)°), a phenolate O8 (Fe2−O8−Fe3* =
9
1.9(3)°), and a μ -1,3 syn-syn bridging acetate O6 and O7
2
providing coordination to Fe2 and Fe3*, respectively. Fe3
(
and Fe3*) are bridged by a phenolate O10 with Fe3−O10−
Fe3* = 105.0(5)° and oxo O9 with Fe3−O9−Fe3* =
1
00.0(5)°. The resultant distance between Fe3 and Fe3* is
.29 Å. Fe3 (and Fe3*) centers are further coordinated by
3
3
−
pyridyl N3, tertiary amine N4, and phenolate O8 from L and
O7 from an acetate. Thus, Fe3/Fe3* centers attain a distorted
II
Fe N O coordination sphere.
2
4
An analysis of the packing diagram of 1 reveals the presence
of an intermolecular C−H···Br hydrogen-bonding interac-
Figure 3. (a) Perspective view of metal coordination environment,
3
4
tion. The Br5 on the phenolate ring is involved in H-bonding
with H10 of the pyridine ring of neighboring molecule with
C10−H10···Br5 = 2.766 Å. The H17 of the phenolate ring is
III
III
(
(
b) schematic representation, and (c) Fe core in [Fe 4(OH)2-
4
O CMe) (L) ](ClO )·3MeCN·2H O (2).
2
3
2
4
2
involved in H-bonding with O13 of Et O (C17−H17···O13 =
2
3
5,36
confirm the presence of only FeIII ions. Four iron(III) centers
2
.465 Å).
These two H-bonding interactions (C10−H10···
Br5 and C17−H17···O13) gives rise to a 1D chainlike
structure (Figure S2). On the other hand, Br1 of other
phenolate participates in H-bonding interaction with H5B of
are bridged by phenoxo, acetato, and hydroxo groups. A
III
4
schematic presentation of the Fe core is displayed in Figure
3. Unlike the case for 1, here the hydroxo group coordinates in
the −CH − group, which is connected to the phenolate of
a μ -bridging mode. Selected metric parameters are given in
2
3
another hexamer unit with C5−H5B···Br1 = 2.996 Å (Figure
S3). This intermolecular H-bonding connects the aforemen-
tioned 1D chains in such a way that a helical network results
Table 3. Two dimeric units involving Fe1, Fe2 and Fe3, Fe4
are connected through hydroxo and acetato bridges. Each
iron(III) center attains a distorted-octahedral geometry. Fe1
and Fe2 centers are bridged by phenolato O4 with Fe1−O4−
Fe2 = 93.7(5)° and hydroxo O13 with Fe1−O13−Fe2 =
103.8(2)°. Fe1 and Fe2 centers are further bridged by another
(
Figure S4). All H-bonding parameters for 1 are given in Table
S4.
D e s c r i p t i o n o f t h e S t r u c t u r e o f
III
[
Fe (OH) (O CMe) (L) ](ClO )·3MeCN·2H O (2). A crys-
exogenous acetate through a μ -1,3 syn-syn bridging mode. As
4
2
2
3
2
4
2
2
tallographic study reveals that the asymmetric unit of 2 consists
of an Fe core (Figure 3) with two L ligands. Metrical
a result, the Fe1 and Fe2 centers are held at a distance of 3.036
Å. The Fe1 center is further coordinated by pyridyl N2, tertiary
III
4
3−
3−
parameters (Table 3 and Table S5) and BVS values (Table 2)
amine N5, phenolato O5 from L , and an acetate O9. Thus,
E
DOI: 10.1021/acs.inorgchem.9b00828
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Inorganic Chemistry
Article
Table 3. Selected Bond Lengths (Å) in [FeIII (OH) (O CMe) (L) ](ClO )·3MeCN·2H O (2)
4
2
2
3
2
4
2
Fe1−O4
Fe1−O5
Fe1−O9
Fe1−O13
Fe1−N2
Fe1−N5
2.060(5)
1.945(6)
2.071(5)
1.836(5)
2.210(6)
2.223(7)
Fe2−O2
Fe2−O7
Fe2−O10
Fe2−O4
Fe2−O13
Fe2−O14
1.933(5)
2.049(5)
2.074(5)
2.099(5)
2.018(5)
1.893(5)
Fe3−O3
2.117(5)
1.920(5)
2.055(5)
1.901(5)
2.076(6)
2.000(5)
Fe4−O1
Fe4−O3
Fe4−O12
Fe4−O14
Fe4−N1
Fe4−N4
1.937(5)
Fe3−O6
Fe3−O11
Fe3−O13
Fe3−O8
Fe3−O14
2.056(5)
2.059(5)
1.837(5)
2.227(7)
2.224(6)
Fe1 has an N O coordination environment. It is worth
methylene hydrogen (H03A) attached to the bromophenolate
of a neighboring tetramer with C036−H03A···Br2 = 2.753 Å
and another methylene hydrogen (H03C) of ethylpyridine arm
of another tetramer with C038−H03C···Br2 = 2.962 Å (Figure
S7). On the other hand, Br3 is H-bonded with the methylene
hydrogen (H08A) attached to the bromophenolate of a
neighboring tetramer with C083−H08A···Br3 = 2.711 Å
(Figure S7). All H-bonding parameters for 2 are collected in
Table S6.
2
4
mentioning here that the central phenolate O5 does not act as
a bridging moiety; it is terminally bound to Fe1. Thus, tertiary
amine N6 and pyridyl N8 on the other arm of the lignad
remain uncoordinated and the bromophenolate O6 binds to
iron center Fe3. Another bromophenolate O4 acts as a bridge
between Fe1 and Fe2. Fe2 is further coordinated by hydroxo
O13, two acetates O7 and O10, another bromophenolate O2
from another ligand, and hydroxo O14. Thus, Fe2 has an O6
coordination environment.
The Fe2 and Fe3 centers are bridged by two hydroxo
groups: O13 with Fe2−O13−Fe3 = 95.3(2)° and O14 with
Fe2−O14−Fe3 = 96.1(2)°. This results in an Fe2···Fe3
distance of 2.898 Å. Fe3 is bridged with Fe1 by hydroxo O13
with Fe1−O13−Fe3 = 159.7(3)°. The distance between Fe1
and Fe3 is 3.679 Å. In addition to hydroxo O13 and O14, Fe3
is coordinated by two phenolates O3 and O6 and two acetates
O8 and O11. The Fe3 and Fe4 centers are connected by
bromophenolate O3 with Fe3−O3−Fe4 = 93.0(2)° and
hydroxo O14 with Fe3−O14−Fe4 = 103.9(2)°. These two
iron centers (O11 coordinates to Fe3 and O12 coordinates to
Fe4) are bridged by an acetate group. The resultant distance
between Fe3 and Fe4 is 3.025 Å. Fe4 is connected to Fe2 by an
O14 hydroxo bridge with Fe2−O14−Fe4 = 158.6(3)°. Fe2
and Fe4 are 3.665 Å apart. In addition to phenolato O3 and
hydroxo O14, the coordination sites of Fe4 are satisfied by
another phenolate O1, acetate O12, pyridyl N1, and tertiary
Magnetism. For high-nuclearity coordination clusters of
transition-metal ions, possibility of the occurrence of various
exchange-interactions exists between the magnetic centers.
Obviously, the research target involves satisfactory fitting of
magnetic data with appropriate theoretical equations and
providing a rationale for the observed behavior. In order to
explore the magnetic properties of coordination clusters
I I
I I I
[
[
Fe Fe
(O) (O CMe) (L) ]·4Et O (1) and
4
2 2 2 4 2 2
III
4
Fe (OH) (O CMe) (L) ](ClO )·3MeCN·2H O (2), vari-
2
2
3
2
4
2
able-temperature magnetic susceptibility data were collected
on powdered crystalline solids.
II
III
[
Fe Fe (O) (O CMe) (L) ]·4Et O (1). A plot of χ T (χ
4 2 2 2 4 2 2 M M
being the magnetic susceptibility per six metal ions) vs T for 1
is displayed in Figure 4. At 300 K, the χ T value is ca. 14.8 cm
3
M
amine N4. Thus, Fe4 assumes an N O4 coordination
2
environment. As stated earlier, like O5, the central phenolate
O1 of other ligand also does not act as a bridge; rather, it binds
only to Fe4. As a result the tertiary amine N3 and pyridyl N9
on the other arm of L3 remain uncoordinated and the
bromophenolate O2 binds to another iron center Fe2. Thus, in
this structure two corners are empty (Figure S5). Such an
environment could be utilized for further binding.
−
An analysis of the packing diagram of 2 reveals that
perchlorate oxygens hold three tetramers and a MeCN
molecule (solvent of crystallization) through extensive C−
3
5,36
H···O interactions
(Figure S6). Perchlorate O15 is
involved in an H-bonding interaction with methylene hydro-
gen, attached to the phenolate ring of a neighboring tetramer
with C036−H03B···O15 = 2.609 Å. The O16 of perchlorate
interacts with C−H (H049) of the phenolate ring of a
neighboring tetramer with C049−H049···O16 = 2.555 Å. The
O16 is also H-bonded to the methyl hydrogen of MeCN with
C4W−H4W1···O16 = 2.622 Å. The O17 has an H-bonding
interaction with the methyl hydrogen (H10C) of the acetate of
the third neighboring tetramer with C103−H10C···O17 =
Figure 4. Thermal dependence of the χ T product for 1: ( )
○
M
experimental data; (−) best-fit curve through eq 3a (see text). The
simulation was performed with the following parameters: g(Fe ) =
III
II
−1
−1
2
.0, g(Fe ) = 2.12, J = −36.5 cm , D = −10 cm .
b
mol− K. This value is smaller than the spin-only value
expected for (ca. 20.8 cm mol K, with g = 2.0) four
magnetically noninteracting high-spin Fe (S = 2) ions and
two high-spin Fe (S = 5/2) ions. This fact, together with the
1
3
−1
II
III
2
.489 Å. On the other hand, O18 is involved in an H-bonding
interaction with the methylene hydrogen (H04B) of the
central phenolate of the first tetramer (which has an H-
bonding interaction with O15) with C043−H04B···O18 =
continuous decrease of χ T upon lowering the temperature for
1, indicates the existence of an important antiferromagnetic
M
interaction among the paramagnetic centers, a feature which is
II
2
2
.662 Å. There are extensive C−H···Br interactions in 2. Br2
commonly found for phenoxo- and/or hydroxo-bridged Fe ,
III
II III
31,37
interacts simultaneously with two hydrogens: one from the
Fe , and Fe Fe centers.
However, in spite of having an
2
F
DOI: 10.1021/acs.inorgchem.9b00828
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Inorganic Chemistry
Article
a
even number of paramagnetic centers, the χ T value does not
Scheme 2. Simplified Magnetic-Exchange Pathways in 1
M
become zero, even at 2.0 K. In fact, χ T exhibits an incipient
M
plateau in the temperature range 35−15 K. Below 15 K, χ T
M
3
−1
decreases again to ca. 3.3 cm mol K at 2.0 K. The value of
χ T at the plateau (ca. 4.5 cm mol K) is greater than that
expected for a low-lying S = 2 spin state (ca. 3.0 cm mol K)
but smaller than that expected for an S = 3 spin state (6.0 cm
mol K), suggesting a possible thermal population of these
spin states, which would be closer in energy.
3
−1
M
3
−1
3
−
1
a
An inspection of the molecular structure of 1 indicates the
presence of two different paramagnetic ions, Fe(II) (Fe1 and
Fe3) and Fe(III) (Fe2), with several exchange pathways
See legend to Scheme 1 for numbering.
magnetic properties of 1 can be described through the spin
Hamiltonian given by eq 3a:
(Scheme 1): (a) the paramagnetic Fe1a/Fe2a, Fe2a/Fe3a,
a
Scheme 1. Magnetic-Exchange Pattern in 1
̂
̂ ̂ ̂ ̂ ̂ ̂ ̂ ̂
= −J (S S + S S + S S + S S )
a
H
exchange
1a 2a
2a 3a
1b 2b
2b 3b
̂ ̂ ̂ ̂ ̂ ̂ ̂ ̂
J (S S + S S + S S + S S ) −
b
−
−
1a 2b
2a 1b
2a 3b
3a 2b
̂
̂
̂
̂
̂
̂
J (S S + S S ) − J (S S )
1a 1b
3a 3b
d
2a 2b
(
3a)
c
The presence of high-spin iron(II) ions requires incorporation
into the general Hamiltonian of terms that consider zero-field
splitting (zfs) of these ions (eq 3b) and Zeeman factor (eq 3c)
for each type of metal ion (eq 3d).
2
2
2
2
a
H
̂
= D(S + S + S + S − 8)
Dashed lines represent a bridging syn-syn carboxylate; see Table 4.
zfs
z1a
z1b
z3a
z3b
(3b)
The values of αna,mb correspond to Fe−O−Fe angles involved in the
magnetic-exchange pathways (see Table 4). Fe1a and Fe1b stand for
Fe1 and Fe1*, respectively, Fe2a and Fe2b stand for Fe2 and Fe2*,
respectively, and Fe3a and Fe3b stand for Fe3 and Fe3*, respectively,
as per X-ray structure labeling.
̂
̂
̂
̂
̂
̂
̂
2b
H
= βH[g (S + S + S + S ) + g (S + S )
Zeeman
1a
3a
1b
3b
2a
A
B
(
3c)
̂
̂
̂
̂
Zeeman
H = H
+ H + H
exchange
zfs
(3d)
The coexistence of several exchange pathways with a large
number of parameters governing them makes the analysis of
the experimental magnetic behavior difficult and ambiguous.
The use of additional tools is essential in such cases.
Fe1b/Fe2b, and Fe2b/Fe3b pairs are connected through
phenoxo, oxo, and syn-syn acetate bridges, (b) oxo and
phenoxo bridges occur for the Fe3a/Fe3b and Fe1a/Fe1b
pairs, (c) the Fe2a/Fe2b pair is connected through two oxo
groups, and (d) the Fe2a/Fe3b, Fe3a/Fe2b, Fe1a/Fe2b and
Fe2a/Fe1b pairs are linked only by an oxo group. A summary
of the more relevant structural parameters is given in Table 4.
One can see therein the existence of more than three different
magnetic couplings, as shown in Scheme 2. In light of this, the
DFT calculations have proven to be very useful in the past to
identify the nature and quantitatively determine the magnitude
of the magnetic couplings in polynuclear complexes. The
II
presence of several high-spin Fe ions with low-lying excited
states makes it very difficult to find the most stable electronic
configuration for each one of the proposed spin configurations
through the process of evaluating the magnetic coupling
Table 4. Selection of More Relevant Geometrical Parameters of 1 Involved in the Magnetic Coupling Together with Calculated
J Values
a
b
,
bc
coupling
metal ions
pathway
μ-O
μ-O
d(M···M) (Å)
α(MOM) (deg)
J
J
n n J
A B
III
II
J1a,2b/J1b,2a
J2a,3b/J2b,3a
J2a,2b
Fe /Fe
3.756
3.783
2.968
134.2
135.9
95.8
−49.8
−38.8
+20.9
−48.4
−38.1
+17.1
−968.0
−762.0
+427.5
III
II
Fe /Fe
III
III
Fe /Fe
μ-O
μ-O
μ-O
μ-OPh
100.1
98.9
94.7
III
II
J1a,2a/J1b,2b
Fe /Fe
3.097
3.040
+9.5
+6.7
+9.1
+8.0
+182.0
+160.0
μ-O CMe
2
III
II
J2a,3a/J2b,3b
Fe /Fe
μ-O
μ-OPh
96.2
91.9
μ- O CMe
μ-O
2
II
II
J1a,1b
J3a,3b
Fe /Fe
3.208
3.287
101.1
103.3
100.0
104.5
−4.6
−2.2
−3.3
−3.3
−52.8
−52.8
μ-OPh
μ-O
μ-OPh
II
II
Fe /Fe
a
b
c
Short atomic basis set (see Experimental Section). Extended atomic basis set (see Experimental Section). n and n are the number of unpaired
A
B
electrons at the paramagnetic sites A and B involved in each magnetic coupling
G
DOI: 10.1021/acs.inorgchem.9b00828
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Inorganic Chemistry
Article
constant. The task becomes easier by delimiting the number of
paramagnetic centers to only those involved in each magnetic
coupling. However, a reduction of the molecular entity can
induce electronic effects, particularly in 1 where there are some
monatomic pathways connecting the metal ions. As demon-
strated in the past, a satisfactory settlement is replacing those
metal ions which are not involved in the studied magnetic
coupling by similar but diamagnetic metal ions. In the case of
Even though the molecular symmetry imposes seven
different magnetic couplings, they are grouped in four sets
according to the type and number of bridging ligands involved
in them, as specified above. Because the values of J within each
set are similar, the pattern of the magnetic interactions in 1 can
be roughly simplified, reducing significantly the degree of
complexity of the system and making possible the analysis of
its magnetic data. In this sense, the applied conditions are J
=
=
=
a
II
III
II
1
, Zn and Ga ions are good alternatives to replace the Fe
J
1a,2a = J1b,2b = J2a,3a = J2b,3b, J
1a,1b = J3a,3b, J = J2a,2b, g
b
= J1a,2b = J1b,2a = J2a,3b = J2b,3a, J
c
III
and Fe ions, respectively, keeping the value of the overall
charge of the polynuclear complex invariant. The computed
values of the magnetic coupling constants in 1 through this
approach are given in Table 4.
J
d
A
= g1a = g3a = g1b = g3b, and g
B
= g2a
g . The predominant magnetic coupling (J ) would lead to
2b
b
two S = 3/2 units that will interact through the other exchange
pathways at low temperatures. This is an idealized picture
because Jb is not much stronger than other magnetic
One problem to rationalize the theoretical results resides in
II
III
interactions, but it is useful for a qualitative discussion. J is
the coexistence of Fe and Fe ions in 1, leading to three
d
ferromagnetic and stronger than J and J , and it would stabilize
possible combinations of them in pairs related to the magnetic
a
c
2
2
2
a low-lying S = 3 spin state. However, the ferromagnetic J and
couplings. It is understandable that d(z )- and d(x −y )-type
a
II
III
antiferromagnetic J interactions play an antagonistic role
orbitals for six-coordinate Fe and Fe ions are often the main
orbitals involved in magnetic interaction. A comparison
between different values of J in 1 can roughly be done by
using the n n J product, where n and n are the number of
c
versus J promoting a low-lying singlet spin state. The subtle
d
interplay among these three weaker magnetic couplings
establishes the ground spin state. As an example, the weakest
J coupling can be considered negligible and only J and J
a
A
B
A
B
unpaired electrons on each paramagnetic site involved in a
c
d
1
would tune the ground spin state. Independent of the size of Jb,
magnetic coupling. Let us consider two observations. First, for
and as a function of the J /J ratio, there are values for this
polynuclear complexes with some 3d metal ions, it is well-
known that the magnetic couplings through μ-oxo, μ-hydroxo,
and μ-alkoxo/μ-phenoxo (μ-OR) bridges follow similar
pathways; their nature and the magnitude depend on the
angle at the bridgehead atom (α). Second, ferromagnetic
couplings are usual when this angle is small (close to 90°).
However, the nature of the coupling becomes antiferromag-
netic when it exceeds a value known as the magic angle. This
angle is different depending on the metal ions involved. In
both cases, the magnetic coupling is strengthened as the value
of α departs from the magic angle. These features that were
mainly observed in systems with pathways mediated by two μ-
OR groups should also occur in systems with only one μ-OR
magnetic pathway, although in the latter case, no examples are
known with small α values. In this sense, just what one could
expect for the J1a,2b, J1b,2a, J2a,3b, and J2b,3a magnetic couplings
through a μ-oxo pathway and very large α values is strong
antiferromagnetic interactions (Table 4). Two monatomic
oxygen bridges in J2a,2b, J1a,1b, and J3a,3b contribute to the
exchange pathway, making possible less obtuse values of the α
angle. This feature leads to a weak antiferromagnetic
interaction for J1a,1b and J3a,3b and an even smaller value of α
would turn it to ferromagnetic. This last case would be
enhanced in J1a,2a, J1b,2b, J2a,3a, and J2b,3b because they exhibit the
smallest values for α. Nevertheless, the presence of a third
bridge (syn-syn carboxylato) would change the situation. This
type of carboxylate coordination is known to mediate
antiferromagnetic coupling; however, as its effect is weaker
than those of the μ-oxo and μ-phenoxo bridges, a weak
ferromagnetic coupling results. Finally, the theoretical results
have a physical meaning and provide us with an adequate set of
values of J to be used as starting parameters to analyze the
experimental magnetic behavior. The energy diagram for 1,
obtained through the magnetic coupling constants found with
a more extended basis set (seventh column in Table 4) points
to a S = 0 ground state with very close low-lying excited states
a
d
quotient where a singlet or a septuplet state is more stable, but
it exists in a narrow range of values of this quotient, where both
states are intercrossed (Figure 5 and Figure S8).
Figure 5. Detail of the low-lying spin levels of the energy spectrum of
1
(see Figure S8). Data were obtained assuming values for J , J , and J
b c d
−
1
of −30, 0, and +10 cm , respectively.
In addition to the large number of variable parameters (g
II
III
factors for Fe and Fe ions and four magnetic coupling
constants), our attempts to simulate the experimental data lead
to a low-lying singlet spin state in order to reproduce the drop
of χ T at low temperature. Evidently, the iron(II) ion has an
M
axial zfs (D) that can account for the behavior below 40 K.
Given the fact that χ T tends to a nonzero value at 2.0 K and
M
II
keeping in mind that the high-spin Fe ion shows an integer S
value, only a negative D value should be envisaged, and so, a
situation with S = 0, 1, 2, and 3 quasi-degenerate ground spin
states would be expected. Even when a negative D parameter is
included in the model, the fitting process leads to situations
−
1
(S = 1, 2, and 3 at 11.7, 36.0, and 74.6 cm , respectively),
making it possible to exchange ground and excited states by
moderate or slight modifications of the values of J values.
H
DOI: 10.1021/acs.inorgchem.9b00828
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
with low-lying singlet spin states. This fact is a consequence of
the difficulty in catching those cases with other ground states
or a nearly degenerate situation, being so limited in the range
of the J /J ratio. Additionally, there is also a partial correlation
susceptibility measurements, in the presence of a dc magnetic
external field (Figures S11 and S12). Despite the fact that the
expected maxima could not be observed down to 1.9 K (the
lowest temperature of our susceptometer), this behavior is
d
a
38
between J and J . The inclusion of the J coupling in this
typical of a SMM. Thus, under the assumption that the SMM
relaxation has just one characteristic time corresponding to a
Debye relaxation process driven by one activation energy (Ea),
the relaxation time (τ) may be written in terms of the
b
d
c
discussion makes it very complicated to obtain a clear-cut
answer. Whatever it would be, we can conclude that the
presence of a paramagnetic ground state or a degenerate
II
39
scenario and a local axial zfs for six-coordinate Fe ion are
Arrhenius law τ = τ exp(E /kT). Taking into account that
0
a
ensured. More conclusive assertions are precluded, as it is
possible to simulate the experimental data with values for the
parameters qualitatively similar to those obtained by the
χ′′ /χ′ = 2πντ, where ν is the experimental ac field exciting
M
M
frequency, and considering that the adiabatic susceptibility is
zero, one obtains eq 4, which allows a rough evaluation of the
values of E and τ .
−
1
−1
theoretical calculations (J = 0.0 cm , J = −27.9 cm , J =
a
b
c
a
0
−
1
−1
+
2.7 cm , and J = +11.4 cm ) or include in the model only
d
ln(χ″ /χ′ ) = ln(2πντ ) + E /kT
(4)
M
M
o
a
the stronger intratrinuclear J magnetic coupling together with
b
−
1
II
a local D parameter of around −10 cm for the Fe ion
This method has already been applied to the evaluation of
40
(Figure 4).
these factors in several SMMs in the literature. Figure 7
Although the bridging ligands in 1 and 2 (see below) are
basically the same, their magnetic exchange parameters (J) are
not strictly comparable, because they represent different metal
ions. However, the values obtained for 2 (see below) follow
the same trend observed and discussed for 1.
The magnetization data of 1 increase with the magnetic field
and attain ca. 4.1 μ at 5 T and 2.0 K. This value is very close
B
to that expected for a spin quintuplet (4.24 μ , with g = 2.12).
B
Nevertheless, the experimental curve deviates significantly
from that calculated through the Hamiltonian of eq 2 (solid
line in Figure S9). This behavior is characteristic of the
existence of zfs and close excited states with different S values
or at least one of them. Both situations occur in 1. Thus, we
were unable to determine unambiguously the magnitude of
this zfs due to the overparameterization and the large
correlation with the values of J.
The existence of this zfs and the consequent anisotropy of
the ground spin state, possibly S = 2 or S = 3, must be
responsible for the apparent slow magnetic relaxation observed
Figure 7. Logarithm of the χ′′ /χ′ ratio vs 1/T plot for 1 at several
M
M
frequencies (from Figure 6 and Figure S10). The solid lines are the
in 1 below 2 K. Figure 6 shows the out-of-phase (χ′′ )
best-fit curves (see text).
M
alternating-current (ac) magnetic susceptibility measurements
for different frequencies in the absence of any external
magnetic field. The corresponding in-phase (χ′ ) magnetic
M
shows the fit of the experimental data by using eq 4. The values
−
7
−1
−1
susceptibility is displayed in Figure S10. As one can see, both
obtained were τ = 1.9(2) × 10
s
and E = 5.1(3) cm .
a
40,41
0
χ′ and χ′′ are frequency-dependent below 4.0 K. It should
M
M
They lie within the range of those expected for a SMM.
be noted that similar behavior was observed for ac magnetic
Assuming that the E corresponds to the energy splitting of the
a
ground spin state, S = 2, by zfs, E = 4|D|, D being the axial zfs
a
−
1
parameter, D ≈ − 1.3 cm may be estimated.
Two relevant examples showing SMM behavior are Fe (S
II
9
=
14) coordination cluster, bridged by hydroxo and acetate
13d
11i
groups, and metal−metal-bonded cluster Fe (S = 11/2).
To our knowledge, 1 provides first example of a mixed-valent
Fe Fe coordination cluster exhibiting SMM behavior.
3
II
III
4
2
III
[Fe (OH) (O CMe) (L) ](ClO )·3MeCN·2H O (2). Figure 8
4 2 2 3 2 4 2
displays an χ T vs T plot (χ being the magnetic susceptibility
per four metal ions) for 2. χ T at 300 K is ca. 3.3 cm mol K,
M
M
3
−1
M
a value which is well below the spin-only value expected for
III
four magnetically noninteracting high-spin Fe centers (17.5
3
−1
cm mol K with S = 5/2). Upon cooling, χ T decreases and
M
approaches zero below 20 K. These features suggest the
presence of a prominent antiferromagnetic interaction.
Although the structures and magnetic properties of several
III
Fe clusters have been the subject of previous reports, a clear
rationalization of a possible magnetostructural correlation is
still lacking. In this context, further studies on Fe multi-
Figure 6. Frequency dependence of the out-of-phase (χ′′ )
M
III
component of the ac susceptibility for 1 in zero static field under a
±
4 G oscillating field.
nuclear complexes are relevant to understand the structural
I
DOI: 10.1021/acs.inorgchem.9b00828
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
These results point out that the interaction between Fe2 and
Fe3 is ferromagnetic and the remaining interactions are
antiferromagnetic. The crystal structure shows that Fe2 and
Fe3 are bridged by two hydroxo moieties with values of 95.3
and 96.1° for Fe2−O13−Fe3 and Fe2−O14−Fe3, respec-
tively. Since these angles are close to 90°, the orthogonality
between the magnetic orbitals of Fe2 and Fe3 would be
1
achieved. Whereas this orthogonality tends toward ferromag-
43
netism, the syn-syn acetate bridge favors antiferromagnet-
44,45
ism.
+4.5 cm ) results. Fe1 and Fe2 are bridged by syn-syn
acetate and hydroxo bridges (O13) with Fe1−O13−Fe2 =
03.8° (which is well above 90°) and a phenoxo (O4) with
As a consequence, a moderate ferromagnetic coupling
−
1
(
1
Fe1−O4−Fe2 = 93.7(2)°, which is also close to 90°. Now, the
combined effect of the syn-syn acetate and hydroxo bridges
overcomes the contribution through the phenoxo bridge and a
−
1
Figure 8. Temperature dependence of magnetic susceptibility data for
significant antiferromagnetic coupling (−56.5 cm ) occurs. A
similar situation is expected for Fe3 and Fe4 centers. Finally,
the values of the bridgehead O13 (between Fe1 and Fe3) and
O14 (between Fe2 and Fe4) atoms are 159.7 and 158.6°,
respectively. These angles are well above 90°, thus leading to
III
[
Fe (L) (OH) (O CMe) (L) ](ClO )·3MeCN·2H O (2).
4
2
2
2
3
2
4
2
features which govern the magnetic interactions between the
III
Fe centers.
Annalysis of the molecular structure of 2 suggests the
presence of three types of exchange pathways (Scheme 3): (a)
−1
an antiferromagnetic coupling (−71.6 cm ). Given that the
values of the Fe1−O13−Fe3 and Fe2−O14−Fe4 angles are
quite close to 180°, in comparison to the bridging angles
between Fe1 and Fe2 or Fe3 and Fe4, JB would be
Scheme 3. Schematic Presentation of FeIII Core in 2
4
antiferromagnetic in nature and stronger than that of J , as
A
found through the fit of the magnetic data.
SUMMARY AND CONCLUDING REMARKS
■
With a phenol-based binucleating ligand in its deprotonated
3
−
form L , the successful syntheses of mixed-valent
II
III
III
[
Fe Fe (O) (O CMe) (L) ]·4Et O (1) and [Fe 4(OH)2-
4 2 2 2 4 2 2
(
O CMe) (L) ](ClO )·3MeCN·2H O (2) coordination clus-
2
3
2
4
2
ters have been achieved. The coordination cluster 1 with a
butterfly-like topology has μ -O and acetate bridges, in
4
addition to phenoxo bridges (both bridging and terminal)
3
−
from L . To our knowledge, 1 provides the first example of a
Fe1 and Fe2 as well as Fe3 and Fe4 are connected through
phenoxo, hydroxo, and syn-syn acetate bridges; (b) two
hydroxo and a syn-syn acetato group act as bridges between
Fe2 and Fe3; (c) Fe1 and Fe3 as well as Fe2 and Fe4 are
connected through a single hydroxo bridge. An inspection of
the Fe −O−Fe angles (α ) allows us to assume that α ≈ α
II III
32
mixed-valent Fe Fe (≥Fe ) coordination cluster, supported
3
by phenoxo, oxo, and carboxylato bridges. The coordination
cluster 2 has μ -OH, acetato, and terminal phenoxo bridges
3
3−
from L . In 2, two diiron(III) units involving Fe1, Fe2 and
Fe3, Fe4 are connected through hydroxo and acetato bridges,
i
j
ij
12
34
III
4
giving rise to a Fe core. Magnetic studies reveal that the
and α ≈ α , and so, the pattern of magnetic interactions in 2
1
3
24
II
4
III
2
Fe Fe coordination cluster exhibits SMM behavior with an
can be described by the spin Hamiltonian of eq 5
S = 2 spin ground state. The existence of zero-field splitting
and the consequent anisotropy of the ground spin state (S = 2)
must be responsible for the apparent slow magnetic relaxation
observed for 1 below 2 K. To our knowledge, 1 is the first
̂
̂
̂
̂
̂
̂
̂
̂
̂ ̂
2 3
H = −J (S S + S S ) − J (S S + S S ) − J (S S )
A
1 2
4
3 4
B
1 3
2 4
C
+
gβH ∑ S
i
II
III
i=1
(5)
mixed-valent Fe
4 2
Fe coordination cluster to exhibit SMM
behavior. A magnetostructural analysis demonstrates that the
where J = J = J , J = J = J , J = J and g = g = g = g =
1
2
34
A
13
24
B
23
C
1
2
3
4
III
III
nature and mode of bridging and Fe −O(H)−Fe angle(s)
g. However, despite this great simplification three exchange
parameters (J , J , and J ) are too many to unambiguously
III
connecting Fe centers are key factors affecting the effective
A
B
C
magnetic properties. An analysis of structural parameters of 2
reveals the following: (i) J values approach ferromagnetic,
when the bridging angle(s) are close to 90°, (ii) an increase in
describe the χ T curve, whose dependence on the temperature
M
is almost linear. The set of physically reasonable best-fit
parameters obtained through this Hamiltonian and using full-
matrix diagonalization, as implemented in the VPMAG
III
III
Fe ···Fe separation leads to an increased antiferromagnetic
interaction, and (iii) when iron centers are singly hydroxo
bridged, significant orbital overlap occurs, which in turn leads
to effective antiferromagnetic interaction, in comparison to
double/triple hydroxo bridges.
4
2
−1
program package, are J = −56.5(3) cm , J = −71.6(4)
A
B
−
1
−1
cm , J = +4.5(2) cm , and g = 1.99(1). These values have to
C
be considered with great caution because of the aforemen-
tioned reasons.
J
DOI: 10.1021/acs.inorgchem.9b00828
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
4) (a) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00828.
■
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frequency dependence of the in-phase components of ac
susceptibility for 1 under an external field of 1000 G and
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Accession Codes
CCDC 1849001 and 1849015 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
molecularmagnets. Nature 2001, 410, 789−793. (b) Urdampilleta,
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AUTHOR INFORMATION
Corresponding Author
R.M.: e-mail, rnm@iitk.ac.in; tel, +91-512-2597437; fax, +91-
12-2597436.
Pyridyl)alkylamine-Appended Carboxylates: Magnetic Properties.
■
New J. Chem. 2010, 34, 2357−2365 and references therein. .
II
(
7) (a) Arora, H.; Lloret, F.; Mukherjee, R. One-Dimensional Co
*
II
II
and Cu Coordination Polymers and a Discrete Cu Complex of
4
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Carboxylate-Appended (2-Pyridyl)alkylamine Ligands: Spin-Canting
and Anti-/Ferromagnetic Coupling. Inorg. Chem. 2009, 48, 1158−
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ORCID
Rabindranath Mukherjee: 0000-0003-0739-5896
Notes
The authors declare no competing financial interest.
II
II
II
Coordination Polymers of Mn , Cu , and Zn Supported by
Carboxylate-Appended (2-Pyridyl)-alkylamine Ligands − Structure
and Magnetism. Eur. J. Inorg. Chem. 2009, 2009, 3317−3325.
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c) Arora, H.; Lloret, F.; Mukherjee, R. Molecular Squares of Ni
ACKNOWLEDGMENTS
This work was supported by a J.C. Bose fellowship grant by the
Department of Science & Technology (DST), Government of
■
II
and Cu : Ferromagnetic Exchange Interaction Mediated by Syn−Anti
Carboxylate-Bridging. Dalton Trans. 2009, 9759−9769. (d) Arora, H.;
Cano, J.; Lloret, F.; Mukherjee, R. Unprecedented Heptacopper(II)
Cluster with Body-Centred Anti-Prismatic Topology. Structure,
Magnetism and Density Functional Study. Dalton Trans. 2011, 40,
10055−10062. (e) Barman, S. K.; Lloret, F.; Mukherjee, R. A
Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced Hydrol-
ysis of Carboxyesters under Ambient Conditions. Inorg. Chem. 2016,
India and Ministerio Espan
Projects CTQ2016-75068P and CTQ2016-75671P), Unidad
de Excelencia Marıa de Maetzu (MDM-2015-0538). R.M.
̃
ol de Ciencia e Innovacion
́
(
́
sincerely thanks the DST for a J.C. Bose fellowship. S.K.B.
acknowledges the award of an SRF by the CSIR. We sincerely
thank Dr. Arunava Sengupta for his help in the synthesis of 1
and its X-ray structure determination. Comments of the
reviewers were very helpful at the revision stage.
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Carboxylate-Bridged Ni Cluster with a Distorted Cubane Topology:
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DEDICATION
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Carboxylate-Bridged Diiron Metalloproteins: Strategies and Tactics.
Chem. Rev. 2004, 104, 987−1012. (b) Sazinsky, M. H.; Lippard, S. J.
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Dedicated to Prof. V. Chandrasekhar on the occasion of his
0th birthday.
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