Structure and magnetic properties of an unusual homoleptic iron(iii) thiocyanate dimer

Article abstract of DOI:10.1039/c5dt00743g

We describe the structural and variable temperature magnetic susceptibility properties of an unusual homoleptic bimetallic iron(iii) thiocyanate tetraanion. This work represents the first structurally characterized bis(μ-1,3-thiocyanato) dimer of iron(iii). A weak antiferromagnetic exchange interaction is observed between the two iron(iii) ions, which is supported by broken symmetry density functional theory (DFT) calculations.

Full text of DOI:10.1039/c5dt00743g

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ARTICLE
Structure and magnetic properties of an unusual
homoleptic iron(III) thiocyanate dimer
Cite this: DOI: 10.1039/x0xx00000x
D. R. Dinsdale,a A. J. Loughb and M. T. Lemaire*a ,
We describe the structural and variable temperature magnetic susceptibility properties of an
unusual homoleptic bimetallic iron(III) thiocyanate tetraanion. This work represents the first
Received 00th January 2012,
Accepted 00th January 2012
structurally characterized bis
(
ꢀ
-1,3-thiocyanato) dimer of iron(III).
A
weak
DOI: 10.1039/x0xx00000x
antiferromagnetic exchange interaction is observed between the two iron(III) ions, which is
supported by broken symmetry density functional theory (dft) calculations.
www.rsc.org/
complex obtained by single crystal X-ray diffraction, as well as
its spectroscopic and variable temperature magnetic
susceptibility properties, which indicate weak antiferromagnetic
interactions between the iron(III) ions supported by broken
symmetry density functional theory (DFT) calculations.
Introduction
The thiocyanate (SCN-) anion has played an important role in
the development of the field of transition metal coordination
chemistry. There exist a wide array of complexes containing
the ambidentate thiocyanate as a terminally bound ligand (both
N- or S- bound), as a bridiging ligand (both
ꢀ-1,3 end-on-end
and
ꢀ
-1,1 end-on bridging modes) and as a counteranion.1 The
thiocyanate ligand has played a significant role in the
development of complexes that exhibit spin-crossover and in
other molecule-based magnetic materials, providing itself as an
intermediate field ligand to observe the thermal reversibility of
spin states and as a magnetic coupling unit when bridging metal
centers together in polynuclear complexes.2-6
Fig 1. Structure of 1 indicating possible N,S bidentate donor atoms
In a previous study we had used in situ prepared “Fe(NCS)2” to
generate 6-coordinate iron(II) spin-crossover complexes
containing two equivalents of a bidentate ligand bearing one or
two thiophene substituents, and in one case we were able to
electropolymerize the complex to produce a conducting
metallopolymer film.7 We were eager to explore other
polymerizable bidentate ligands in this regard to prepare Wolf
Type 2 conducting polymers where the iron ion is coordinated
directly to the polymer backbone, rather than tethered to the
Experimental
Materials and methods
G
ENERAL PROCEDURES
All reagents were commercially available and used as received
unless otherwise stated. Deaerated and anhydrous solvents
were obtained from a Puresolve PS MD-4 solvent purification
system. 1H/13C-NMR spectra were recorded on a Bruker
Avance 400 MHz spectrometer using deuterated solvents. FT-
IR spectra were recorded on
spectrometer as KBr discs.
recorded on a Shimadzu 3600 UV-Vis-NIR spectrophotometer
in CH2Cl2 solution using quartz cuvette cells. Elemental
analyses were carried out by Canadian Microanalytical
Services, LTD., Delta, BC, Canada. LC-MS experiments were
chain.8 Our target ligand in this strategy
1 was previously
reported9 and we envisioned possible coordination of the 2-
pyridyl donor N atom and thiophene S atom to “Fe(NCS)2” to
produce similar complexes as previously reported.7 However,
rather than obtaining the anticipated 6-coordinate iron(II)
complexes, we obtained a most unusual ionic complex
containing a completely homoleptic thiocyanate iron(III) dimer.
To our knowledge this is the first such reported material and the
a
Shimadzu IRAffinity
UV-Vis measurements were
first bimetallic iron(III) complex with bridging
thiocyanate ligands. Herein we report the structure of this
ꢀ
-1,3 recorded
on
an
Agilent
1260
Infinity
liquid
chromatograph/6530 accurate mass Q-TOF in high resolution
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VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5DT00743G
mode with 70:30 acetonitrile/water using positive mode within 24h. Yield, 20 mg. Calculated for C78H48N14S22Fe2
electrospray ionization (HPLC grade solvents). Differential (found): C, 46.9 (46.1); H, 2.4 (2.4); N, 9.8 (9.2%). HR-MS
Pulse (DP) voltammetry experiments were performed with a (ESI+): Calc’d for C17H12NS3C m/z 326.0126 [MH+], found
Bioanalytical Systems Inc. (BASI) Epsilon electrochemical 326.0124. FT-IR (KBr): 3237 (w), 3094 (w), 2110 (w), 2069
workstation. Compound
2 was dissolved in anhydrous solvent (m sh), 2035 (s), 2021 (m sh), 1618 (w), 1599 (w), 1281 (w),
(CH2Cl2), and then deareated by sparging with N2 gas for 10-15 1229 (w), 1045 (w), 847 (w), 827 (w), 770 (w), 700 (m) cm-1.
min in a low-volume cell. Solution concentrations were
UV-Vis (CH2Cl2), λmax (ε
M-1 cm-1): 331 (7.4 × 103), 508 (5.3 ×
approximately 10-3 M in analyte containing approximately 0.5
M supporting electrolyte (Bu4NPF6). A typical three-electrode
set-up was used including a glassy carbon working electrode,
Ag/AgCl reference electrode, and a platinum wire auxiliary
electrode. The scan rate for all DP experiments was 25 mV/s.
Variable temperature magnetic susceptibility measurements for
103), 560 (sh, 4.1 × 103).
Results and discussion
Synthesis and structural properties
Ligand was synthesized in three steps from commercially
available
bromothiophene
2
was recorded on an MPMS SQUID magnetometer at an
1
external magnetic field of 2000 Oe over a temperature range of
2-300K. The sample was weighed into a gel cap and
diamagnetic contributions were calculated using Pascal’s
constants.
3-bromothiophene.
with HBr/Br2
Bromination
produced
of
3-
2,3,5-
tribromothiophene, which was subjected to Suzuki-Miyura
cross-coupling conditions to yield 3’-bromo-2,2’:5’,2’’-
terthiophene. The final step in the preparation of
1 was a
COMPUTATIONAL DETAILS
Negishi cross-coupling of 3’-bromo-2,2’:5’,2’’-terthiophene
with 2-pyridylzinc bromide. There is one other report that
Single point energy DFT calculations were performed on the
tetra-anion of
2 based on the coordinates obtained from the
describes the preparation of
methodology but our method has the advantage of not having to
use toxic organotin reagents. Reacting two equivalents of
1, via Stille cross-coupling
single crystal X-ray diffraction data using the Gaussian09
(Revision D.01) package10 using the B3LYP hybrid
functional11,12 and the def2-TZVP13 basis set on all atoms. Tight
SCF convergence criteria were used for all calculations. The
program Chemissian14 was used for the preparation of the spin
density distribution figure.
1
with “Fe(NCS)2”, prepared in situ by reaction between hydrated
FeCl2 and NaSCN, in methanol produced not the anticipated
(1)2Fe(NCS)2, where 1 is binding to the ferrous ion in a
bidentate manner, through pyridyl N and terminal thiophene
ring S donor atoms. Rather, a deep purple powder was isolated,
which produced violet needle crystals suitable for X-ray
diffraction by layering hexanes on to a CH2Cl2 solution of the
powder. Single crystal X-ray diffraction revealed an unusual
structure, consisting of a homoleptic bimetallic [Fe2(SCN)10]4-
tetra-anion [containing iron(3+)] with charge balance provided
Synthetic procedures
3’-(pyrid-2-yl)-2,2’:5’,2’’-terthiophene (1).
Under a N2
atmosphere a pre-dried Schlenk flask was charged with
Pd(PPh3)4 (0.15 g, 0.13 mmol, 5.6 mol%) in dry toluene (10
mL). 3’-bromo-2,2’:5’,2’’-terthiophene (0.74 g, 2.3 mmol) was
dissolved in dry toluene (15 mL) and was added to the Schlenk
followed by 2-pyridylzinc bromide solution (7.0 mL of 0.50 M
by four equivalents of [1
H]+ protonated at the pyridine N atom.
It appears that the oxidation of the ferrous ions occurs over the
course of the reaction since recrystallization in air or under an
inert atmosphere produces the same ferric containing material.
To our knowledge, this bimetallic iron(3+) thiocyanate complex
represents the first structurally characterized ferric dimer
THF solution, 3.5 mmol) and the mixture was heated to 100°C
for 72h. The cooled reaction contents were stirred with a 0.5 M
EDTA solution (60 mL) containing 1.0 M K2CO3 (60 mL) for
0.5 hr and then extracted with diethyl ether (3 × 50 mL). The
combined organic extracts were dried over MgSO4, filtered and
rotovaped to dryness. The crude product was chromatographed
over silica gel, eluting with 15:1 hexanes/ethyl acetate to
produce a yellow oil. Yield, 0.36 g (48%). Spectroscopic data
featuring
ꢀ-1,3-thiocyanto bridging ligands. In our literature
search, we could only find two other reports of dimeric iron
complexes with ꢀ-1,3-thiocyanto bridging ligands. These other
reports include an iron(2+) dimer with macrocyclic ligands15,
which was characterized with single crystal X-ray diffraction
and a ferric dimer suspected to be of the form K2[Fe2(SCN)8]16,
which was characterized without single crystal data nor variable
temperature magnetic susceptibility data.
was consistent with a previous reported synthesis of 1 9
.
HR-
MS (ESI+): Calc’d for C17H12NS3C m/z 326.0126 [MH+],
found 326.0124. 1H NMR (CDCl3, 400 MHz):
8.74 (1H, d,
= 4.8 Hz), 7.70 (1H, td, J1 = 7.7 Hz and J2 = 1.8 Hz), 7.62 (1H,
δ
J
s), 7.40 (1H, d,
J = 7.9 Hz), 7.34-7.22 (4H, m), 7.09 (1H, d, J =
The molecular structure of
2 is shown in Figures 1-3. The four
3.2 Hz), 7.08-7.00 (m, 2H). 13C NMR (CDCl3): d 171.2, 154.2,
149.6, 139.1, 136.7, 136.2, 136.1, 135.0, 131.9, 127.9, 127.5,
126.6, 126.5, 124.9, 124.0, 123.9, 122.2
cations and tetra-anion are shown separately for clarity.
Relevant metrical parameters are included in the figure
captions. The tetra-anion includes a bimetallic iron(3+) unit
with two bridging
coordinated by one N- and one S-bound
remaining four coordination sites at each metal are occupied by
terminal N-bound thiocyanates, generating an unusual
homoleptic Fe-N5S coordination sphere at each metal ion. The
Fe-NCSterminal coordinate bond distances range from 1.974(6) to
ꢀ
-1,3-thiocyanato ligands. Each ferric ion is
[1H]+ [Fe2(SCN)10]4- (2). 1 (0.11 g, 0.34 mmol) was dissolved
in methanol (10 mL) and the solution was sparged with N2 gas
for 10 min. FeCl2⋅4H2O (0.021 g, 0.17 mmol) and NaSCN
(0.027 g, 0.34 mmol) were added as solids and the mixture was
heated to reflux for 96 h under an atmosphere of N2 and the
solution turned dark purple over this time. The solution was
rotovaped to dryness to produce a crude dark solid. The crude
solid (3-4 mg batches) was recrystallized by dissolution into
CH2Cl2 and layering hexanes on top carefully (in a ratio of 1:2).
Dark violet needle crystals suitable for X-ray diffraction formed
4
ꢀ-1,3-thiocyanto. The
2.048(7) Å.
The Fe-N bond distances to the bridging
thiocyanates are longer than the bonds to the terminal bound
thiocyanate N atoms [2.088(7) and 2.084(6) Å]. The longest
coordinate bonds are between Fe and S [2.680(2) and 2.676(2)
2 | J. Name., 2012, 00, 1-3
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Å]. The distance between the Fe ions within the anion is 5.740 The disorder in all instances is the result of rotation of the ring
Å. Each coordinated thiocyanate is linear, with near ideal NCS by approximately 180 degrees. The bond distances and angles
bond angles ranging from 177.6(8) to 179.1(7) degrees. The for each of the cations are very similar to each other and typical
bond angles that include Fe and the bridging S and N atoms for other similar molecules of this type. It is likely that the
[S(2)-Fe(1)-N(1) and S(1)-Fe(2)-N(2)], which are important for protonation of
1 occurred in the coordination reaction from the
the transmission of any intramolecular magnetic exchange H3O+ produced from the dissolved FeCl2·4H2O (which may or
coupling between the metal ions, are 88.37(18) and 87.99(18) may not have been oxidized), which is a weak aqua acid with a
degrees, respectively. To our knowledge and surprise,
represents the first bimetallic iron(3+) complex with bridging (methanol) was used in the preparation of
-1,3-thiocyanate ligands characterized by X-ray
2
Ka on the order of 10-9. Alternatively, a protic solvent
2
.
ꢀ
crystallography. We found one report containing single crystal
X-ray diffraction data for a bimetallic ferrous complex with the
same bridge structure from Wieghardt and co-workers.15 In this
complex, the Fe-N [2.14(2)] bond distances to the bridging
thiocyanate anions are significantly longer than the same
distances in
2 [2.088(7) and 2.084(6)], which is consistent with
the ferric oxidation state assignment in
2
.
Fig. 2. Molecular structure of the protonated cation of 2 (one of four protonated
cations in the asymmetric unit is shown for clarity). Displacement ellipsoids at
30% probability (su values in brackets) and the dashed lines indicate a minor
component of disorder. Relevant bond distances (Å) and angles (°): S(1A)-C(4A),
1.732(8); S(1A)-C(1A), 1.732(8); N(1A)-C(5A), 1.346(9); N(1A)-C(9A), 1.347(10);
C(1A)-C(2A), 1.367(10); C(1A)-C(14A), 1.454(9); C(2A)-C(3A), 1.420(10); C(3A)-
C(4A), 1.387(10); C(3A)-C(5A), 1.463(9); C(4A)-C(10A), 1.469(9); C(5A)-C(6A),
1.385(10); C(6A)-C(7A), 1.390(10); C(7A)-C(8A), 1.388(12); C(8A)-C(9A),
1.361(12); S(2A)-C(13A), 1.702(6); S(2A)-C(10A), 1.712(5); C(10A)-C(11A),
1.385(6); C(11A)-C(12A), 1.403(7); C(12A)-C(13A), 1.351(6); S(3A)-C(17A),
1.699(7); S(3A)-C(14A), 1.713(5); C(14A)-C(15A), 1.387(7); C(15A)-C(16A),
1.398(8); C(16A)-C(17A), 1.350(7); S(1E)-C(17E), 1.705(8); C(15E)-C(16E),1.401(8);
C(16E)-C(17E), 1.352(7). C(4A)-S(1A)-C(1A), 93.0(4); C(5A)-N(1A)-C(9A), 123.3(7);
C(2A)-C(1A)-C(14A), 128.3(7); C(2A)-C(1A)-S(1A), 110.3(6); C(14A)-C(1A)-S(1A),
121.4(5); C(1A)-C(2A)-C(3A), 113.8(7); C(4A)-C(3A)-C(2A), 113.0(6); C(4A)-C(3A)-
C(5A), 125.0(7); C(2A)-C(3A)-C(5A), 122.0(6); C(3A)-C(4A)-C(10A), 130.1(6); C(3A)-
C(4A)-S(1A), 109.9(6); C(10A)-C(4A)-S(1A), 120.0(5); N(1A)-C(5A)-C(6A), 118.1(6);
N(1A)-C(5A)-C(3A), 119.9(6); C(5A)-C(6A)-C(7A), 119.8(7); C(8A)-C(7A)-C(6A),
119.7(7); C(9A)-C(8A)-C(7A), 119.2(7); N(1A)-C(9A)-C(8A), 119.9(7).
Fig. 1. Molecular structure of the tetra-anion of
probability). Relevant bond distances (Å) and angles(
2
(displacement ellipsoids at 30%
[with standard
°)
uncertainties (su) in brackets]: Fe(1)-N(5), 1.974(6); Fe(1)-N(6), 2.020(7), Fe(1)-
N(4), 2.039(7); Fe(1)-N(3), 2.040(7); Fe(1)-N(1), 2.088(7); Fe(1)-S(2), 2.680(2);
Fe(2)-N(8), 1.989(6); Fe(2)-N(9), 2.033(7); Fe(2)-N(10), 2.046(7); Fe(2)-N(7),
2.048(7); Fe(2)-N(2), 2.084(6), Fe(2)-S(1), 2.676(2); S(1)-C(1), 1.637(8); S(2)-C(2),
1.638(8); S(3)-C(3), 1.629(8); S(4)-C(4), 1.617(8); S(5)-C(5), 1.596(8); S(6)-C(6),
1.607(8); S(7)-C(7), 1.616(8); S(8)-C(8), 1.598(8); S(9)-C(9), 1.606(8); S(10)-C(10),
1.631(8); N(1)-C(1), 1.161(10); N(2)-C(2), 1.160(10); N(3)-C(3), 1.157(10); N(4)-
C(4), 1.172(10); N(5)-C(5), 1.179(10); N(6)-C(6), 1.183(10); N(7)-C(7), 1.172(10);
N(8)-C(8), 1.173(10); N(9)-C(9), 1.170(10); N(10)-C(10), 1.146(10). N(5)-Fe(1)- The molecular packing of
2 in the ab plane is also shown in
N(6), 97.0(3); N(5)-Fe(1)-N(4), 89.3; N(6)-Fe(1)-N(4), 94.6(3); N(5)-Fe(1)-N(3),
Figure 3. No close intermolecular contacts could be found for
the tetra-anion, which contains the paramagnetic ferric ions. As
90.9(3); N(6)-Fe(1)-N(3), 96.8(3); N(4)-Fe(1)-N(3), 168.3(3); N(5)-Fe(1)-N(1),
174.1(3); N(6)-Fe(1)-N(1), 88.6(3); N(4)-Fe(1)-N(1), 88.0(3); N(3)-Fe(1)-N(1),
90.1(3); N(5)-Fe(1)-S(2), 85.95(19); N(6)-Fe(1)-S(2), 176.5(2); N(4)-Fe(1)-S(2),
83.6(2); N(3)-Fe(1)-S(2), 84.88(19); N(1)-Fe(1)-S(2), 88.37(18); N(8)-Fe(2)-N(9),
96.7(3); N(8)-Fe(2)-N(10), 90.5(3); N(9)-Fe(2)-N(10), 96.9(3); N(8)-Fe(2)-N(7),
90.5(3); N(9)-Fe(2)-N(7), 94.8(3); N(10)-Fe(2)-N(7), 168.0(3); N(8)-Fe(2)-N(2),
174.7(3); N(9)-Fe(2)-N(2), 88.5(3); N(10)-Fe(2)-N(2), 89.8(3); N(7)-Fe(2)-N(2),
88.1(3); N(8)-Fe(2)-S(1), 86.8(2); N(9)-Fe(2)-S(1), 176.18(19); N(10)-Fe(2)-S(1),
84.5(2); N(7)-Fe(2)-S(1), 83.6(2); N(2)-Fe(2)-S(1), 87.99(18).
a result, the variable temperature magnetic properties of
should be those of a well-isolated ferric dimer.
2
The molecular structure of
crystallographically independent cations of protonated
asymmetric unit (one of the cations is shown in Figure 2 and
the other three cations of protonated are presented in the
2
also contains four
1
in the
1
Supporting Information as Figures S1-S3). In each cation, the
2-pyridyl substituent is protonated at N and each N-H acts as a
hydrogen bond donor to S atoms of the anion dimer. In two of
the cations, two of the thiophene rings are disordered, and in
the other two cations, only one thiophene ring is disordered.
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ion occurs simultaneously, as is typical when there is only very
weak electronic coupling between the ions). Anodic processes
at +0.8 and +1.1 V (versus ferrocene) can be attributed to
oxidations of coordinated thiocyanate and the terthienyl cation,
respectively.
The UV-visible spectrum of
2 was also measured in CH2Cl2
soltuion (Figure 5). A low energy absorptions at 508 nm with a
slighltly lower energy shoulder (557 nm) can be assigned as
thiocyanate-to-iron(3+) charge transfer (LMCT) on the basis of
the molar absorptivity values (5.3×103 and 4.2×103 M-1 cm-1).
No weak d-d transitions could be detected in concentrated
solutions, which provides further evidence for the high-spin
ferric oxidation assignments. High energy absorptions (320 nm
and lower) are likely π-π* transitions within [1
H]+.
8000
6000
4000
2000
0
Fig. 3. Molecular packing of 2 in the ab plane. 2 is indicated in black and the four
crystallographically independent cations of [1H]+ are colored red, yellow, green,
and blue. Crystal data. C78H48Fe2N14S22, M = 1998.32, monoclinic, a = 11.6864(4),
300
800
1300
Wavelength / nm
b = 29.2257(9), c = 13.0996(4) Å, U = 4233.9(2) Å3, T = 147(2) K, space group P21
,
Fig. 5. UV-visible-NIR spectrum of
2 recorded in CH2Cl2 at 298K.
Z = 2, 84784 reflections measured, 14940 unique (Rint = 0.0652) which were used
in all calculations. The final wR(F2) was 0.1095 (all data).
Variable temperature magnetic susceptibility
Spectroscopic and electronic properties
The variable temperature magnetic susceptibility properties of
were investigated with a superconducting quantum interference
device (SQUID) over a temperature range of 300 to 2 K.
2
The FT-IR spectrum of
2 is dominated by the CN stretching
absorptions over the range 2150-1950 cm-1, as anticipated for
this structure (Figure S4 in the Supporting Information). The
higher energy absorptions in this region are assigned to the CN
stretching frequencies of the bridging thiocyanates, as has been
observed previously. The terminal thiocyanates absorb at
Single crystals of
2
were carefully harvested from
recrystallization experiments and these crystals were ground
and the pulverized material was placed in gel caps. As
previously noted, the lack of any significant close
slightly lower energy. Other absorptions from [
present in the spectrum.
1
H]+ are also
intermolecular contacts between the tetra-anions in
2 from the
single crystal X-ray diffraction data suggest any temperature
dependent magnetic phenomenon observed will be largely the
result of intramolecular magnetic exchange coupling between
the ferric ions through the thiocyanate bridging ligands. The
variable temperature magnetic properties of
Figure 6 as plots of mT or m versus . The room temperature
value of
mT is 8.2 cm3 K mol-1, which is a little less than the
anticipated value (8.75 cm3 K mol-1) for two uncoupled ferric
ions ( = 2) in the absence of any temperature independent
paramagnetism (TIP). With decreasing temperataure, however,
2 are shown in
χ
χ
T
χ
g
there is
intramolecular
a
steady decrease in
antiferromagnetic
χmT as anticipated for
magnetic
coupling
interactions between the metal ions. The value of
χ
mT at 2 K is
Fig. 4. Differential pulse voltammogram of
containing approximately 0.5 M Bu4NPF6 (scan rate 25 mV/sec)
2
in CH2Cl2 (versus ferrocene) 0.5 cm3 K mol-1, suggesting an
S
= 0 ground state for
2. The
plot of χm versus
T
displays a maximum in χm at low
temperature, as is typically observed in antiferromagnetically
coupled dimers. In fact, we can very nicely fit the experimental
susceptibility data to a simple ferric dimer isotropic spin
Hamiltonian model of the form
Apart from the single crystal X-ray diffraction data, evidence
for the ferric oxidation state in
2 is provided by the differential
pulse (DP) voltammogram (Figure 4). A low potential cathodic
wave is observed (the only cathodic process of note), which is
ascribed to the reduction of ferric ions (the reduction of each
ꢁ
ꢀ ꢂ 2ꢃꢄꢅꢆꢄꢅꢆ
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which provides the following best fit parameters:
= 2.04, = -1 K, = 0.03. The profile of the mT data and the
χm maximum at low temperature indicate weak
J g
= -4 cm-1,
θ
ρ
χ
a
antiferromagnetic interaction between the ferric ions in the
complex. A magnetization versus field experiments at 2 K
confirms the
S = 0 ground state in 2 (Figure S5 in the
Supporting Information).
Fig. 7. Spin density distribution in the tetra-anion component of
TZVP/B3LYP). Isovalue 0.004.
2 (def2-
Conclusions
In summary, we have reported the structural and magnetic
properties of the first bimetallic -1,3-bis(thiocyanto)-bridged
2 shown as plots of χmT vs ferric complex. The structural data indicate a very unusual and
ꢀ
Fig. 6. Variable temperature magnetic susceptibility of
T (red squares) or χm vs T (blue circles) at an external field of 2000 Oe. Black
solid lines represent best fits to the experimental data using the spin
Hamiltonian and best-fit parameters described in the text.
completely homoleptic tetra-anion featuring two ferric ions and
include both bridging and monodentate thiocyanate ligands.
The paramagnetic anionic components are well isolated among
layers of organic cations and this is reflected in the variable
temperature magnetic susceptibility data, which indicates weak
intramolecular antiferromagnetic exchange coupling between
the high-spin ferric ions. The magnetic data could be fit well to
Density functional theory (DFT) calculations
Using the coordinates from the X-ray diffraction data, we
calculated the single point energy of the high-spin (
Broken symmetry (BS)17 states of the tetra-anion component of
using the B3LYP functional with the def2-TZVP basis set on
all atoms. The energy of the BS state was found to be very
slightly lower than the high spin state (
= 106 cm-1). The
energy of the closed-shell singlet state was found to be much
S = 5) and
a
S = 5/2 dimer model. DFT calculations on the tetra-anion
component of the complex support the weak antiferromagnetic
exchange coupling with the spin density mainly localized on
the ferric ions with little delocalization onto the bridging
ligands.
2
ꢁ
E
higher ( E = 2.9 ×
104 cm-1). Using the approach of
ꢁ
Yamaguchi18, we could calculate the magnetic exchange
coupling constant from the energies of the high spin (HS) or BS
wavefunctions and their spin expectation values as follows
Acknowledgements
MTL thanks NSERC (Discovery), CFI, Canada Research
Chairs program and Brandon University for funding. Dr. Paul
Dube (Brockhouse Institute for Materials Research) and Prof.
Fereidoon Razavi (Brock University) are thanked for providing
variable temperature magnetic susceptibility data.
ꢈꢉꢊꢋ ꢇ ꢉꢌꢋꢍ
2ꢃ ꢂ ꢇ
ꢎ
ꢎ
〈
〉
〈
〉
ꢊꢋ ꢇ ꢄ
ꢌꢋ
ꢄ
This procedure provides a calculated magnetic exchange value
of about -4 cm-1, which is identical to the fitted value obtained
from the variable temperature magnetic susceptibility data. To
rationalize the weak exchange coupling we computed the spin
Notes and references
a
Department of Chemistry, Brandon University, Brandon, MB, R7A
6A9, CANADA.
b
Department of Chemistry, University of Toronto, Toronto, ON, M15
density distribution (Figure 7) in the tetra-anion of
2 and found
most of the spin density localized on the ferric ions, with very 3H6, CANADA.
little delocalization onto the bridging thiocyanates (this is likely
the main superexchange pathway for any magnetic
communication in this complex)
Electronic Supplementary Information (ESI) available: Displacement
ellipsoid plots of other crystallographically independent [
H]+ cations,
magnetization versus field data for and crystallographic data for in cif
format. See DOI: 10.1039/b000000x/
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