Article
pubs.acs.org/IC
Halide Influence on Molecular and Supramolecular Arrangements of
Iron Complexes with a 3,5-Bis(2-Pyridyl)-1,2,4,6-Thiatriazine Ligand
Katie L. M. Harriman,† Irina A. Kühne,† Alicea A. Leitch,† Ilia Korobkov,† Rodolphe Clérac,*,‡,§
Muralee Murugesu,*,† and Jaclyn L. Brusso*,†
†
Department of Chemistry and Biomolecular Science, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada
CNRS, CRPP, UPR 8641, F-33600 Pessac, France
§
Univ. Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France
‡
S Supporting Information
*
ABSTRACT: A series of iron centered complexes, namely,
[Fe(Py2TTA)Cl2] (1), [Fe(Py2TTA)Br2] (2), and [Fe(μF)(Py2TTAO)F]∞ (3), were isolated via complexation of 3,5bis(2-pyridyl)-1,2,4,6-thiatriazine (Py2TTAH) with various
ferric halides (e.g., FeF3, FeCl3, and FeBr3). Comparison of
the optical and electrochemical spectroscopy, structural
analysis, and magnetic studies reveal numerous similarities
between the chlorido (1) and bromido (2) derivatives, which
crystallize as discrete five-coordinate iron centered complexes
with coordination geometries that are intermediate between
trigonal bipyramidal and square base pyramid. Conversely, the
fluorido derivative (3) results in a completely different structure due to oxidation of the ligand and the formation of a onedimensional coordination polymer held together through a bridging fluoride ion. Consequently, the spectroscopic and magnetic
behavior of 3 differs significantly compared with 1 and 2. Complexes 1 and 2 exhibit paramagnetic properties typical for a
mononuclear S = 5/2 system with weak intermolecular antiferromagnetic interactions at low temperatures, whereas complex 3
demonstrates significant exchange couplings within the chain and weak antiferromagnetic interchain interactions, which stabilize
a canted antiferromagnetic state below 4.2 K.
■
INTRODUCTION
In recent years, the design and development of sulfur and
nitrogen containing heterocycles have received a great deal of
interest due in large part to their suitability in a variety of
applications.1−6 In that regard, heterocycles containing the
thiazyl moiety are particularly attractive as the S−N unit not
only affords an unpaired electron, but the incorporation of
heavy heteroatoms into planar frameworks also provides the
potential for structural control through S---N′ and N---HC′
interactions and better opportunities for enhanced magnetic
and electronic interactions by virtue of greater orbital
overlap.7−9 Thus, targeting such radicals could lead to unique
physical properties (e.g., inherent conductivity) along with the
potential for strong magnetic interactions between the spin
carriers. Recent developments on thiazyl-based radicals provide
a glimpse of the aforementioned properties such as magnetization (M) bistability (i.e., M vs H hysteresis associated with a
magnet behavior), which can be harnessed through long-range
ordering via strong communication between the radical
centers.10−13 With that said, relatively little is known about
the coordination chemistry of these systems. For example, by
coordinating highly delocalized π-conjugated radicals, strong
communication between the spin carriers may be induced. In
order to achieve such behavior, it is necessary to introduce
© 2016 American Chemical Society
highly coordinating pockets into the thiazyl-based radical
framework. With this in mind, we have focused our attention
on the synthesis and coordination chemistry of 3,5-bis(2pyridyl)-1,2,4,6-thiatriazine (Py2TTAH; Scheme 1),14 which
provides an ideal terpyridine-like stable tridentate coordination
pocket while retaining the “active” N−S−N moieties.
Both symmetrically and asymmetrically substituted thiatriazinyl (TTA) radicals have been reported, most of which are
partially functionalized by a phenyl group.15−18 With respect to
metal coordination, the reactivity of symmetric (Ph2TTA) and
asymmetric (3-phenyl-5-trifluoromethyl-TTA) derivatives with
[Cr(Cp)(CO3)2] has been explored.19,20 As expected, the TTA
substituents played a role in the coordination leading to an η1adduct bonded through the sulfur atom in Ph2TTA, whereas
the asymmetric ligand resulted in an η2 SN linkage. In both
cases, coordination through the central nitrogen of the TTA
ring did not occur. By employing heteroaromatic substituents,
as in Py2TTA, coordination through the chelating effect is
promoted rendering the N−S−N portion of the heterocyclic
ring devoid of substituents. This therefore permits these
heteroatoms to engage in supramolecular interactions, which
Received: February 15, 2016
Published: May 6, 2016
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a molecular complex or participate in the coordination
environment. Halide ions have also been known to act as
monatomic bridges between metal centers in multimetallic
complexes and coordination polymers.26 Notably, due to their
relative sizes and charge, halides often dictate the packing
arrangement of molecules in the crystal lattice. For example,
smaller highly electronegative F− ions can be efficient electron
withdrawing groups, whereas the larger and more dispersed
negative charge of Cl− and Br− can enhance stability of the
resulting complexes. In addition to size and electronegativity,
the acidity of different halides can also significantly influence
the formation of metal complexes and the overall structural
arrangement.27
In order to further investigate the role of the ligand and the
halide anion in coordination complex formation, we have
focused our attention on synthesizing Fe-Py2TTA complexes
with F−, Cl−, Br−, and I− anions. These halide anions were
explored to probe their role in the supramolecular packing
arrangement and influence on the resulting physical properties.
To that end, we herein report the synthesis, optoelectronic, and
solid-state properties of a family of Fe complexes with the
Py2TTA ligand, which are coordinated to fluoride, chloride, and
bromide.
Scheme 1. Versatility and Reactivity of 3,5-Bis(2-pyridyl)1,2,4,6-thiatriazine (Py2TTAH): (ii) X = H; (iii) X = M
has been shown to direct crystal packing in the solid-state and
facilitate conductive or magnetic exchange pathways in thiazyl
based radical complexes.21 What is particularly interesting about
this ligand system is its ambiguity in regard to oxidation states
as both the anionic and neutral states are isolatable,22 while
maintaining the topology of terpy. More specifically, the neutral
radical Py2TTA may be prepared from treatment of Py2TTAH
with N-chlorosuccinimide in the presence of base (e.g., 4dimethylaminopyridine: DMAP; Scheme 1). While Py2TTAH
is surprisingly robust with a high tolerance toward aqueous,
basic, and thermal conditions, once in the radical form (i.e.,
Py2TTA), it is highly susceptible to oxidation affording the 4hydro-1-oxo-1,2,4,6-thiatriazine (1-oxo-Py2TTAH, see Scheme
1).22
As a first step toward exploring the potentially rich
coordination chemistry of Py2TTA, we recently reported the
synthesis and detailed study of a mononuclear iron complex
([Fe(Py2TTA)Cl2]; 1). This unique system can be prepared via
the reaction between FeIII and Py2TTAH or through the
treatment of FeII with the neutral radical Py2TTA.23 Once
isolated, [Fe(Py2TTA)Cl2] is quite stable in the solid-state;
however, if rigorously degassed solvents were not used, metalassisted aerial oxidation (Py2TTA to 1-oxo-Py2TTA, see
Scheme 1) was observed upon dissolution. While initially this
was rather intriguing, such behavior has been reported by Clark
et al. where they demonstrated that Lewis-acidic CuII ions
catalyze the aerial oxidation of benzothiadiazines to benzothiadiazine S-oxides.24 On the basis of that study, it is reasonable to
anticipate that other Lewis acids may also catalyze such
oxidation. Although FeCl3 salts are also Lewis-acidic, in the case
of complex 1 aerial oxidation of the TTA ring occurs following
isolation of the coordination complex, not during the synthesis.
As anticipated, in complex 1 coordination to the ligand
occurs through the tridentate terpy-like pocket leaving the
N−S−N portion of the TTA ligand devoid of substituents
permitting these heteroatoms to participate in supramolecular
interactions. As a result, within the packing motif of
[Fe(Py2TTA)Cl2], there are two dominant structural features,
ligand−ligand and halogen−halogen. The latter are nonnegligible as the intermolecular Cl---Cl interactions are in
good agreement with the sum of the van der Waals radii. In
addition to directing packing in the solid state, halide ions are
known to play various important roles in coordination
chemistry.25 They can act as noncoordinating counterions for
■
RESULTS AND DISCUSSION
Synthesis. Synthesis of the bromido derivative [Fe(Py2TTA)Br2] (2) followed a similar route as the preparation
of [Fe(Py2TTA)Cl2] (1; Scheme 2).23 Namely, an acetonitrile
Scheme 2. Synthesis of Complexes 1, 2, and 3a
a
Reagents and conditions: (a) FeCl3, DMF, MeOH, RT; (b) FeBr3,
MeCN, CHCl3, RT; (c) FeF3, DMF, MeOH, diethyl ether, RT.
(MeCN) solution of FeBr3 was layered over a chloroform
(CHCl3) solution of Py2TTAH affording green-brown blocklike crystals of 2 suitable for single crystal X-ray analysis (for
experimental details see Supporting Information). In order to
isolate the fluorido derivative, a heterogeneous reaction was
required due to the limited solubility of FeF3. As such, after a
slurry of FeF3 in a DMF/MeOH solution of Py2TTAH was
stirred overnight, the mother liquor was separated and placed in
a diethyl ether bath at room temperature. After several days,
green needle-shaped crystals of [Fe(μ-F)(1-oxo-Py2TTA)F]∞
(3) suitable for single crystal X-ray analysis were obtained.
Although the F, Cl, and Br complexes were isolated following
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the aforementioned procedures, the iodo-derivative remains
elusive, despite our best efforts.
Interestingly, the reactions involving FeCl3 and FeBr3
resulted in mononuclear complexes with very similar structures
(vide inf ra), whereas replacing the halide with fluorine (i.e.,
FeF3) afforded complex 3 in which oxidation of the ligand
(Py2TTA to 1-oxo-Py2TTA, see Scheme 1) occurs, and a
coordination polymer is formed as a result of a bridging fluoride
ion. Initially, oxidation of the ligand was surprising as
Py2TTAH is remarkably robust with a high tolerance toward
aqueous, basic, and thermal conditions; it is only instability that
occurs under acidic conditions, eliciting opening of the
thiatriazine ring leading to N-bridgehead-1,2,5-thiadiazoium
salts.14 However, as demonstrated here, the thiatriazine ring is
prone to oxidation when coordinated to iron. Such reactivity of
the TTA ring has only previously been observed when the
brominated derivative (3,5-bis(2-pyridyl)-1-bromo-1,2,4,6-thiatriazine) or radical (3,5-bis(2-pyridyl)-1,2,4,6-thiatriazinyl) is
exposed to oxygen or moisture affording 3,5-bis(2-pyridyl)-4hydro-1-oxo-1,2,4,6-thiatriazine.22 Intriguingly, in the case of
complexes 1 and 2, aerial oxidation of the TTA ring occurs
following isolation of the coordination complex, whereas
oxidation occurs during the reaction for 3. The difference
between these complexes may be attributed to the relatively
high electronegativity of the fluoride ions in comparison to the
later halides, enhancing aerial oxidation. Furthermore, the
relatively small size of the fluoride promotes octahedral
coordination. Nonetheless, the susceptibility toward oxidation
when coordinated to iron is quite interesting and opens an
avenue for metal-non-innocent ligand based catalysis.28−31
Electrochemical Studies. The redox behavior of complexes 1 and 2 were probed by cyclic voltammetry in
dichloromethane (DCM). In the case of complex 3, electrochemical studies could not be carried out due to its insoluble
nature. As was previously reported, 1 shows two quasireversible redox processes at 0.923 V and −0.037 V (vs
SCE), which are attributed to the Py2TTA 0/+1 and FeII/FeIII
process, respectively.23 Similar behavior is observed for
[Fe(Py2TTA)Br2], with two quasi-reversible redox processes
visible within a similar range (0.915 and 0.056 V vs SCE, Figure
1). On the basis of this study, it appears halide exchange leads
to an anodic shift in the FeII/FeIII redox potential (by 93 mV),
while the Py2TTA 0/+1 process remains relatively constant
(i.e., 8 mV cathodic shift). Furthermore, both 1 and 2 are
susceptible to metal-assisted ligand oxidation in solution, as
observed by the additional redox process occurring at 0.272 V23
and 0.366 V, respectively. While this could be avoided for 1 by
rigorously degassing the solvent (i.e., freeze−pump−thaw)
prior to collecting the cyclic voltammegram,23 for complex 2
oxidation of the TTA ligand in solution is unavoidable, even
under those conditions.
Optical Spectroscopy. To probe the effect of halide
exchange on the electronic structure, solution absorption and
solid state diffuse reflectance spectroscopic studies were carried
out on complexes 1, 2, and 3 in the range of 200−1200 nm.
Because of the insolubility of complex 3, only solid-state diffuse
reflectance was feasible. For comparative purposes, Py2TTAH
and 1-oxo-Py2TTAH were investigated, the results of which are
presented in Figure 2. As may be expected, the absorption
profile of [Fe(Py2TTA)Br2] is quite similar to that of
[Fe(Py2TTA)Cl2] with ligand based transitions in the high
energy region (273 and 272 nm; cf. 235 and 261 nm for
Py2TTAH) as well as a series of less intense bands in the visible
Figure 1. CV scan of [Fe(Py2TTA)Cl2] (1; top blue line),
[Fe(Py2TTA)Br2] (2; bottom red line) in DCM solution (0.1 M
[Bu4N][PF6], 100 mV/s). Dashed lines are provided to emphasize the
difference in redox processes between 1 and 2.
region (e.g., 300−400 nm) characteristic of metal centered d−d
transitions. Compounds 1 and 2 also exhibit broad and
relatively intense low energy transitions observed at 500−1100
nm, which may be indicative of some radical character on the
ligand since near-IR bands are often observed in coordination
complexes of paramagnetic ligands.32 The diffuse reflectance
measurements are in good agreement with the solution based
absorption profiles. While solution measurements could not be
performed on [Fe(μ-F)(1-oxo-Py2TTA)F]∞, the solid state
diffuse reflectance spectrum reveals that 3 does not absorb in
the NIR region, which is consistent with the lack of potential to
form a radical due to oxidation of the ligand. [Fe(μ-F)(1-oxoPy2TTA)F]∞ does, however, absorb in the high energy (226
nm) and visible (658 nm) region, which is consistent with
ligand based transitions (cf. 224 and 514 nm for 1-oxoPy2TTAH) and metal centered d−d transitions, respectively.
Structural Studies. While [Fe(Py2TTA)Cl2] (1) crystallizes in the orthorhombic space group Pnma,23 the bromido
derivative [Fe(Py2TTA)Br2] (2) crystallizes in the triclinic
space group P1̅. The variation in the space groups adopted by 1
and 2 has consequences leading to subtle differences in the
packing of the molecules (vide inf ra). However, it should be
noted that although 1 and 2 crystallize into different space
groups, their solid-state structures are remarkably similar as is
highlighted in Figures 3, 4, and 5.
In both cases, the metal ion adopts a five-coordinate
coordination arrangement with the iron center residing within
the plane of the tridentate Py2TTA ligand, coordinated through
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Figure 4. Crystal packing of [Fe(Py2TTA)Cl2] (1) viewed along the baxis (top) and [Fe(Py2TTA)Br2] (2) viewed along the a-axis
(bottom).
Figure 2. Solution absorption (top) and solid-state diffuse reflectance
(bottom) spectra at room temperature for [Fe(Py2TTA)Cl2] (1; blue
line), [Fe(Py2TTA)Br2] (2; red line), [Fe(μ-F)(1-oxo-Py2TTA)F]∞
(3; green line), Py2TTAH (black line), and 1-oxo-Py2TTAH (gray
line).
Figure 3. Molecular structure of [Fe(Py2TTA)Cl2] (1; left) and
[Fe(Py2TTA)Br2] (2; right). H atoms are omitted for clarity.
the N1, N4, and N5 atoms. The remaining metal coordination
environment is filled by two halide anions above and below the
molecular framework. On the basis of the Addison and Reedijk
structural parameter (τ),33 the coordination geometry of 1 is
best described as intermediate between trigonal bipyramidal
and square base pyramid (0.495), whereas 2 more closely
resembles square pyramidal (0.283). Considering the Fe−N
bond lengths (Table 1), the two corresponding to the pyridyl
rings (N4 and N5) are essentially equivalent and are consistent
with what is commonly observed for Fe−N bonds in terpybased systems.34−37 The other, which represents the interaction
between iron and the nitrogen on the TTA ring (i.e., N1), is
shorter than average indicative of π-bonding interactions.23,38,39
Although the bond lengths between the central iron center and
Figure 5. Stacking of four molecules of 1 (top) and 2 (bottom) with
intermolecular interactions highlighted by dashed lines (Cl---Cl in
bright green; N---N in blue; S---S in yellow; Fe---Fe in dark green;
Br---Br in olive green). Intermolecular distances are provided.
the Py2TTA ligand are very similar for both complexes, the
FeX distances are slightly elongated for the bromido
derivative, as expected due to the larger ionic size of the
bromide ion in comparison to the chloride. Furthermore, in
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Table 1. Selected Bond Lengths of Complexes
[Fe(Py2TTA)Cl2] (1), [Fe(Py2TTA)Br2] (2), and [Fe(μF)(1-oxo-Py2TTA)F]∞ (3)a
a
distance (Å)
1
2
3
Fe1−N1
Fe1−N4
Fe1−N5
Fe1−X1
Fe1−X2
1.97
2.12
2.14
2.21
2.21
1.97
2.12
2.10
2.32
2.36
2.05
2.11
2.13
1.80
1.95
X being Cl, Br, and F in 1, 2, and 3, respectively.
complex 2 the ligand displays lower planarity than in 1;
however, that is likely due to the different crystallographic space
groups and the consequence of 1 being situated on a mirror
plane.
In the solid-state, complexes 1 and 2 are both highly ordered
forming layered structures that are dominated by two structural
features, ligand−ligand and halogen−halogen interactions.
While these layers are evenly separated in complex 1, this is
no longer the case in 2 (Figure 5 and Supporting Information,
Figure S1), which may be attributed to the larger size of the
bromide ions. Nonetheless, in both complexes the TTA
fragment of one molecule overlaps with another one on the
next layer (Figure 4), resulting in a number of close
intermolecular contacts between neighboring TTA rings
(Figure 5).
As well, the pyridyl rings between the ligand of neighboring
molecules overlap in a zig-zag fashion for complex 1, whereas in
2 these layers are aligned parallel to the b-axis (Figure 4).
Although numerous similarities exist between the solid-state
structures of 1 and 2, the subtle discrepancies are likely a result
of the difference in anion size and electronegativity. This is
perhaps exemplified by considering the halide−halide interactions between molecules of alternating layers. In 1, the
chloride−chloride distance of 3.52 Å, resulting from the
chloride ion of one molecule interacting with another two
layers away, is in good agreement with the sum of the van der
Waals radii.40 Conversely, in complex 2 the bromide ions are
no longer superposed, resulting in a distance between the
bromide ions of 3.81 Å, a value slightly elongated with respect
to complex 1 and is nominally larger than the sum of the van
der Waals radii.40
While numerous similarities exist between the chlorido and
bromido derivatives, replacing the halogen with fluoride ions
results in a completely different structure. This is attributed to
the reaction between Py2TTAH and FeF3 leading to oxidation
of the ligand and the formation of a coordination polymer.
Crystals of 3 belong to the monoclinic space group P21/c and
consists of Fe chains held together through a single bridging
fluoride (Figure 6). The metal ion adopts a distorted octahedral
coordination arrangement with the iron center coordinated to
the tridentate oxidized Py2TTA ligand (1-oxo-Py2TTA) in the
equatorial plane through its three nitrogen atoms (N1, N4, and
N5). The remaining equatorial position is occupied by a
fluoride ion (F1). The two additional fluoride ions (F2 and
F2a), which occupy the axial positions, are μ-bridging to the
next iron center in the 1D coordination polymer. In
[Fe(μ-F)(1-oxo-Py2TTA)F]∞, the ligand is no longer planar
due to oxidation of the sulfur atom (S1) of the TTA ring.22
Considering the Fe−N bond lengths (see Table 1), the two N
atoms corresponding to the pyridyl rings (N4 and N5) are in a
range similar to those for complexes 1 and 2. The remaining
Figure 6. Molecular structure of [Fe(μ-F)(1-oxo-Py2TTA)F]∞ with H
atoms omitted for clarity.
Fe−N bond (Fe1N1), which represents the interaction
between iron and the nitrogen on the TTA ring, is slightly
larger in comparison to those in complexes 1 and 2, while still
remaining in good agreement to other previously reported iron
complexes with terpy-like ligands.41−43 The Fe−F bond lengths
fall in the range of 1.80−1.91 Å. This, coupled with the
coordination environment and oxidation of the TTA ring
leading to an anionic ligand, suggests the presence of FeIII metal
ions.44,45
Unlike complexes 1 and 2 in which a number of close
ligand−ligand interactions are observed, only one close π−π
interaction exists within the polymeric chain of 3 (C12---C2 =
3.37 Å), which is likely due to the bridging fluoride ion.
Furthermore, the fluoride bridged Fe chains in 3, which are
aligned along the c-axis, are held together through O---H and
N---H interactions between the 1-oxo-Py2TTA ligands on
neighboring polymers. This leads to interchain Fe---Fe
distances of 8.24, 13.83, and 11.49 Å, as highlighted in Figure 7.
Magnetic Properties. The dc magnetic susceptibility, χ, of
compounds 1−3 was investigated under 1000 Oe field in the
temperature range 1.85−300 K on polycrystalline samples. The
corresponding χT vs T plots are presented in Figure 8. At room
temperature, the χT values of complexes 1, 2, and 3 are 4.34,
4.36, and 2.73 cm3 K mol−1, respectively. For 1 and 2, the
observed χT values are in good agreement with the expected
spin only value of 4.375 cm3 K mol−1 for a Fe(III) S = 5/2
center. In the case of 3, the observed low value can be
attributed to non-negligible antiferromagnetic interactions
between the Fe centers observable even at room temperature
(vide inf ra). The χT curves of 1 and 2 exhibit similar
temperature dependent profiles over the entire temperature
range, which may be expected due to the similarity between
their molecular structures. The decrease of the χT product is
gradual with minimal values of 0.18 cm3 K mol−1 for 1 and 2 at
1.9 K. Such temperature decreases can be attributed to
intermolecular antiferromagnetic interactions between the
spin carriers estimated at zJ/kB = −2.9 K (z and J being the
number of next-neighbors and the average exchange interaction
between carriers respectively) from the Curie−Weiss fits of the
experimental data shown in Figure 8 (note that g values of
2.05(5) and 2.06(5) were obtained from the Curie−Weiss fits
for the Fe(III) site in 1 and 2 respectively). For 3, the variable
temperature χT curve differs significantly from the other two
compounds; upon decrease of the temperature the χT product
decreases rapidly to reach a minimum value of 0.19 cm3 K
mol−1 at 9.5 K indicating dominant antiferromagnetic
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developed by Fisher46 in the 1960s, was used to fit the
experimental data, considering the following spin Hamiltonian:
Ĥ = −2J ∑ Si⃗ ·Si⃗ + 1 + gμB H⃗ ·∑ Si⃗
i
i
As shown in Figure 8, an excellent fit is obtained down to 10
K with g = 2.02(5) and J/k B = −12.5(1) K. The
antiferromagnetic interaction in 3 is comparable to fluoridebased systems47 and especially the one, −8.7 K, determined in
the (imidH)2[Fe2oxF6] compound (imidH: imidazolium cation,
ox: oxalate)48 that also displays a single fluorido-bridge between
Fe(III) centers. Close inspection of the χT vs T data below 9 K
(Figure 8 inset) reveals a χT product increase with a maximum
of 0.39 cm3 K mol−1 at 3 K and a final decrease to reach 0.28
cm3 K mol−1 at 1.85 K.
This peculiar thermal behavior is associated with a magnetic
susceptibility (Figure 9a) that becomes strongly dependent on
the applied dc field below 5 K, suggesting the stabilization of a
magnetically ordered ground state. Accordingly, the ac
susceptibility exhibits a small but significant out-of-phase signal
below 5 K (Figure 9b), consistent with a 3D magnetically
ordered phase, which possesses a small spontaneous magnetization as expected for a canted antiferromagnetic state. It is
worth noting that ferromagnetic or ferrimagnetic states can be
easily ruled out in 3 as the observed χ″ signal and the 1.85 K
magnetization (that reaches only 0.22 μB at 7 T; Figure 10a,b)
are both small. Below 5 K, a hysteresis effect on the M vs H
curve is also observed reproducibly in the magnetically ordered
phase with a coercive field, Hcoer, and remnant magnetization,
Mrem, which reach 360 Oe and 0.0237 μB, respectively, at 1.85 K
(Figures 9c and 10c). From the field dependence of the
magnetization shown in Figure 10b,c, the small remnant
magnetization (<0.025 μB) resulting from the noncompensation of the two sublattices in the canted antiferromagnetic
phase has been deduced and plotted as a function of the
temperature in Figure 11. Remarkably below 4 K, the
experimental Mrem vs T data compare almost perfectly with
the temperature variation of the Ising order parameter (m) in
the mean field approximation (that is the solution of the selfconsistent equation m = tanh(mTC/T)).49,50 This excellent
theory/experiment agreement corroborates the direct correlation between the temperature dependence of remnant
magnetization and the magnetic phase transition with TC =
4.2 K and Mrem (0 K) = 0.0243 μB. Between 4.2 and 8 K above
the canted antiferromagnetic phase, fluctuation effects are
clearly observed on the Mrem vs T data (Figure 11). The
presence of short-range order between TC and about 7 K is also
detected by ac susceptibility measurements, which show a
broad χ″ peak and a nonzero χ″ value below 7 K (Figure 9b).
As detailed in the previous paragraphs, combined dc and ac
magnetic measurements clearly establish the canted antiferromagnetic ground state of 3 below 4.2 K; therefore it is
interesting to schematize the possible spin order in relation to
the crystal structure. In 3, the presence of two Fe(III)
orientations along the chains is seen in Figures 7 and 12 with
a Fe−F−Fe angle of 146.9° or α = 33.1°. This structural angle
is indeed remarkably larger than the magnetic angle, αmagnetic, of
about 1.1° between the two uncompensated sublattices of the
canted antiferromagnetic phase (Figure 12; note that αmagnetic is
estimated from Mrem (0 K) = 0.0243 μB and the well-established
geometrical relation: sin(αmagnetic/2) = 2Mrem/Msat, with Mrem
(0 K) = 0.0243 μB and Msat = gSμB = 5.05 μB being the
magnetization expected at high field after saturation).
Figure 7. Packing arrangement of the fluoride bridged Fe chains in
[Fe(μ-F)(1-oxo-Py2TTA)F]∞ (3) as viewed from the side (along the
a*-axis; top). Bird’s-eye view of the 1D coordination polymers of 3
along the c-axis illustrating the separation between chains (bottom).
Intermolecular Fe---Fe distances are highlighted by green dashed lines.
Figure 8. Temperature dependence of the χT product at 1000 Oe (χ is
defined as magnetic susceptibility equal to M/H per mole of Fe
center) for 1 (black), 2 (green), and 3 (blue; Inset: zoom of the data
below 20 K). The solid lines indicate the best fit using the magnetic
models described in the text.
interactions between the FeIII centers through the single
fluoride-bridge. On the basis of the 1D structure of 3 (vide
supra), the Heisenberg chain model of classical S = 5/2 spins (J
being the magnetic interaction between the Fe(III) spins),
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Figure 10. For 3: field dependences of the magnetization below 10 K,
(a) between 0 and 7 T after a zero-field cooled at the indicated
temperature, (b) between 0 and 0.5 T after a zero-field cooled at the
indicated temperature, and (c) between −0.12 and +0.12 T cycling the
dc field from −7 to 7 T. Solid lines are visual guides.
Figure 9. For 3: temperature dependences of (a) the dc magnetic
susceptibility at different dc fields below 10 000 Oe (χ is defined as
molar magnetic susceptibility equal to M/H per mole), (b) the ac
susceptibility at different ac frequency in zero-dc field and (c) the
coercive field extracted from Figure 10c. Solid lines are visual guides.
complexation of Py2TTAH with various ferric halides (e.g.,
FeF3, FeCl3, and FeBr3) in order to investigate the role of the
ligand and halide anion in coordination complex formation.
Furthermore, these Fe-Py2TTA complexes with F−, Cl−, and
Br− anions were explored to probe the effect of halide
substitution (i.e., size, electronegativity, etc.) on the supramolecular packing arrangement and to examine their influence
on the subsequent physical properties. Comparison of the
electrochemical and optical spectroscopy, structural analysis,
and magnetic studies reveal numerous similarities between the
chlorido (1) and bromido (2) derivatives. More specifically,
[Fe(Py2TTA)Cl2] and [Fe(Py2TTA)Br2] crystallize as discrete
five-coordinate iron centered complexes with intermediate
coordination geometries between trigonal bipyramidal and
square base pyramid. As may be expected due to their structural
similarities, electrochemical, optical, and magnetic studies reveal
the physical properties of 1 and 2 are quite comparable.
Conversely, replacing the halogen with fluoride anions results
in a completely different structure, which is likely due to
This result demonstrates that the magnetic exchange along
the chain (J/kB = −12.5 K) is dominating the spin orientations
without overcoming completely the Fe(III) magnetic anisotropy that induces an αmagnetic angle different from zero. It is
also interesting to deduce from Figure 12 that the canted
antiferromagnetic phase in 3 is necessarily the result of
interchain antiferromagnetic interactions dominated by those
involving different sublattices, which is the only possible
situation that induces a noncompensation of the chain
magnetization. This spin arrangement is certainly favored by
the staggered arrangement of the chains along the c axis (Figure
12).
■
CONCLUSIONS
In summary, a series of iron complexes, namely, [Fe(Py2TTA)Cl 2 ] (1), [Fe(Py 2 TTA)Br2 ] (2), and [Fe(μ-F)(1-oxoPy2TTA)F]∞ (3), have been prepared and isolated via
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Inorg. Chem. 2016, 55, 5375−5383
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Inorganic Chemistry
intermolecular antiferromagnetic interactions, whereas complex
3 demonstrates significant exchange couplings within the chain
and weak interchain interactions, which stabilize a canted
antiferromagnetic ordered state below 4.2 K.
On the basis of the studies described here, it is clear the
counterion has a significant impact on the coordination
environment and packing arrangement in the solid-state of
the resulting coordination complex or polymer. As well, the
reactivity of the ligand with respect to metal-assisted aerial
oxidation appears to be influenced by the halide. Investigations
to further probe and understand the aforementioned metalassisted aerial oxidation are currently underway.
■
ASSOCIATED CONTENT
S Supporting Information
*
Figure 11. Temperature dependences of the remnant magnetization
for 3 estimated from the data shown in Figure 10b,c. The solid black
line is a visual guide, while the red line is shown the best fit of the data
by the order parameter of the mean-field incompressible Ising
model.49,50
. The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00357.
■
Synthetic procedures, electrochemistry, crystallography,
and magnetic measurements (PDF)
Crystallographic information files (CIF1, CIF2)
AUTHOR INFORMATION
Corresponding Authors
*(R.C.) E-mail: [email protected].
*(M.M.) E-mail: [email protected].
*(J.L.B.) E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors thank the University of Ottawa, the Canadian
Foundation for Innovation (CFI), NSERC (Discovery and RTI
grants), the CNRS, the Aquitaine region, the ANR and the
University of Bordeaux for their financial support.
■
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