Encapsulation controlled single molecule magnetism in tetrathiafulvalene- capped cyanide-bridged cubes

Article abstract of DOI:10.1039/c2dt30908d

New TTF-based (TTF = tetrathiafulvalene) ligands, L1 and L2 (L1 = α-(4′-methyl-4,5-ethylenedithiotetrathiafulvalene-5′-thio) -α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene and L2 = α-(4′-methyl-4,5-dimethylthiotetrathiafulvalene-5′-thio) -α′-[tris-2,2,2-(1

Full text of DOI:10.1039/c2dt30908d

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An international journal of inorganic chemistry
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Volume 41 | Number 44 | 28 November 2012 | Pages 13543–13766
ISSN 1477-9226
COVER ARTICLE
Oshio et al.
Encapsulation controlled single molecule magnetism in
tetrathiafulvalene-capped cyanide-bridged cubes

Dynamic ArtVicielweALritnicklesOnline
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 13601
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PAPER
Encapsulation controlled single molecule magnetism in tetrathiafulvalene-
capped cyanide-bridged cubes†
Kiyotaka Mitsumoto,a Hiroyuki Nishikawa,a,b Graham N. Newtona and Hiroki Oshio*a
Received 26th April 2012, Accepted 11th July 2012
DOI: 10.1039/c2dt30908d
New TTF-based (TTF = tetrathiafulvalene) ligands, L1 and L2 (L1 = α-(4′-methyl-4,5-
ethylenedithiotetrathiafulvalene-5′-thio)-α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene and L2 =
α-(4′-methyl-4,5-dimethylthiotetrathiafulvalene-5′-thio)-α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene),
possessing tris-pyrazolyl coordination sites, were synthesized. The reactions of Ni(BF4)2·6H2O with the
TTF-ligands (L1 and L2), n-Bu4N[Fe(CN)3(tp or pztp)] (tp = hydrotris(pyrazol-1-yl)borate and pztp =
tetrakis(pyrazol-1-yl)borate) in the presence of additional counter ions afforded two cyanide-bridged
octanuclear complexes: [FeIII4NiII4(CN)12(pztp)4(L1)4](BF4)4 (1) and [FeIII4NiII4(CN)12(pztp)4(L2)4]-
(PF6)4 (2). Using a similar procedure to that employed in the synthesis of complex 2, with the addition of
sodium tetraphenylborate, yielded a two-electron-reduced compound, Na[FeIII2FeII NiII (CN)12(tp)4(L2)4]-
2
4
(BF4)3 (3), in which a sodium ion was encapsulated by the cube. The host–guest complex 3 showed
enhanced redox behaviour and while magnetic susceptibility measurements revealed ferromagnetic
interactions to be operative in all three complexes, the cation encapsulation behaviour of 3 led it to exhibit
single molecule magnet-type properties.
multi-redox behaviour and mixed valency;8 therefore, their deri-
Introduction
vatization with conducting moieties may afford materials dis-
playing synergetic properties, combining conductivity with
additional functionality.9 Numerous cyanide-bridged polynuclear
clusters have been reported in recent years, and one family, the
[M4M′4(μ-CN)12]n+ octanuclear cubic clusters constructed with
[Fe(CN)3Rtp]− (R = H, hydrotris(pyrazol-1-yl)borate, R = pyra-
zole, tetrakis(pyrazol-1-yl)borate) building blocks, have shown
particularly rich physical properties including reversible multi-
redox behaviour, electron transfer coupled spin transition
(ETCST) and single molecule magnetism (SMM).10 We have
introduced TTF moieties into the capping ligand, tris(pyrazol-1-
yl)methane, and investigated the host–guest chemistry of the
system, and herein report the structures and physical properties
of three TTF-based complexes: [FeIII4NiII4(CN)12(pztp)4(L1)4]-
(BF4)4 (1), [FeIII4NiII4(CN)12(pztp)4(L2)4](PF6)4 (2), and
Na⊂[FeIII2FeII2NiII4(CN)12(tp)4(L2)4](BF4)3 (3) where L1 =
α-(4′-methyl-4,5-ethylenedithiotetrathiafulvalene-5′-thio)-α′-
[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene, L2 = α-(4′-methyl-
4,5-dimethylthiotetrathiafulvalene-5′-thio)-α′-[tris-2,2,2-(1-
pyrazolyl)ethoxy]-p-xylene), pztp = tetrakis(pyrazol-1-yl)borate)
and tp = hydrotris(pyrazol-1-yl)borate.
Electrically conductive materials based on tetrathiafulvalene
(TTF) derivatives have been shown to display unique physical
properties, such as d–π magnetic interactions and magnetic field
induced superconductivity.1,2 To date, such molecular magnetic
conductors have primarily been prepared by the combination of
TTF derivatives with metal halides.2 The TTF-based metal com-
plexes reported have shown weak d–π interactions between the
localized metal ion spins and the conductive electrons of the
ligand π-systems, rendering them candidates for the construction
of magnetic conductors if the selected metal ion has a suitably
large spin that can be delocalized to the conducting moieties. In
the past ten years, Ouahab and co-workers have reported a
family of pyridyl functionalized TTF ligands and their metal
complexes, [M(hfac)2(TTF-py)2] (M = Mn, Cu; hfac = hexa-
fluoroacetylacetonate; TTF-py
=
4-(2-tetrathiafulvalenyl-
ethenyl)pyridine),3 and there have also been a few reports of
multinuclear TTF-based clusters.3–6 Multinuclear complexes can
display physical characteristics such as superparamagnetism,7
aGraduate School of Pure and Applied Sciences, University of Tsukuba,
Tennodai 1-1-1, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: oshio@chem.tsukuba.ac.jp
Results and discussion
bCollege of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki
310-8512, Japan
Syntheses
†Electronic supplementary information (ESI) available: MS spectra of 2
and 3, IR spectra of 1–3, cyclic voltammograms of L1 and L2, Figures
of coulometry of 2 and 3, electronic absorption spectra of 2 and 3, and
magnetization of 1–3. CCDC 858647. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2dt30908d
TTF-based ligands L1 and L2, were synthesized following the
method depicted in Scheme 1. Tris-2,2,2-(1-pyrazolyl)ethanol,
prepared by the reported procedure,11 was converted to α-bromo-
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Scheme 1 Syntheses of TTF-based ligands L1 and L2.
Fig. 1 Crystal structure of 14+ with Fe in purple; Ni, green; C, grey; N,
blue; B, pink; O, red; S, yellow. Hydrogen atoms, BF4 anions and
Scheme 2 Structures of [Fe(CN)3(pztp)]− and [Fe(CN)3(tp)]−.
−
solvent molecules are omitted for clarity.
α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene (4) by the reaction
with sodium hydride and α,α′-dibromo-p-xylene, in 91% yield.
The cyanoethyl-protected TTF derivatives of 4-(2-cyano-
magnetic data (vide infra) suggested that 1 is composed of four
low-spin (LS) FeIII and four NiII ions. The six coordination sites
of the iron ions are occupied by the three nitrogen atoms of a tri-
dentate pztp− ligand and the remaining facial positions are co-
ordinated by three cyanide carbon atoms. The octahedral
coordination environments of the NiII ions consist of N6 donor
sets from three cyanide nitrogen atoms and the tridentate tris-
pyrazolyl ligand (L1). Four TTF groups are attached to the cubic
core via xylene spacers. The average coordination bond lengths
around the nickel and iron ions at 100 K are in the range of
2.01(1)–2.12(1) Å and 1.90(2)–2.00(1) Å, respectively, charac-
teristic of NiII and LS FeIII ions. The vertex angles of the cube
are close to right angles (87.0(6)–93.6(4)°), and the Fe–C–N and
Ni–N–C interactions are approximately linear, with bond angles
of 171.3(9)–179.0(12) and 173.8(10)–178.3(10)°, respectively.
The bond lengths and angles within the TTF skeleton reflect the
valence states of the TTF moiety. The average central CvC
bond length of the TTF moieties is 1.36(2) Å, and the dihedral
angles between the dithiolylidene planes are 13.6(1), 21.3(1),
26.0(1) and 41.5(1)°, values characteristic of neutral TTF
derivatives.15
ethylthio)-5-methyl-4′,5′-ethylenedithiotetrathiafulvalene
and
4-(2-cyanoethylthio)-5-methyl-4′,5′-dimethylthiotetrathiafulvalene12
were converted to L1 and L2 by deprotection with caesium
hydroxide, followed by treatment with 4, in 89 and 75% yield,
respectively. Complexes 1 and 2 were prepared by the reactions
of (n-Bu4N)[Fe(CN)3(pztp)]13 and Ni(BF4)2·6H2O with L1
and L2 respectively (Scheme 2). The reaction of (n-Bu4N)-
[Fe(CN)3(tp)]14 with Ni(BF4)2·6H2O, L2 and NaBPh4 yielded
the mixed valence encapsulation complex 3. All complexes were
isolated as red crystals.
Structures
Single crystals of 1, suitable for X-ray diffraction studies, were
grown by the slow evaporation of a MeOH/acetone solution.‡
Single crystals of 2 and 3 were also obtained and their diffraction
data collected; however, the crystals were highly unstable due to
the evaporation of lattice solvent molecules, and the data could
not be processed to a level suitable for publication. XRD
measurements revealed that 2 and 3 became amorphous upon
desolvation. The crystallographic parameters of 1 are shown
below.
The complex units of 1 are arranged into a 3D network super-
structure through intermolecular interactions between three of
the TTF moieties: TTF1, TTF2 and TTF3 (where TTF1 is
appended to Ni1, TTF2 to Ni2 and so on), while TTF4 is non-
interacting. TTF1 forms a face-to-face π–π interaction with its
symmetry generated equivalent TTF1′ (interplanar distance =
3.69(1) Å), and side-on interactions with TTF2 and TTF3. In
order to estimate the magnitude of the intermolecular interaction
between the units, overlap integrals (S) between the highest
occupied molecular orbitals of the TTF moieties were calculated
for complex 1 at the extended Hückel level.16 Of the three inde-
pendent interactions, the face-to-face π–π interaction resulted in
the largest overlap (S1–1 = 11.42 × 10−3 eV) while the integrals
of the side-by-side interactions were one order of magnitude
smaller (S1–2 = 1.28 × 10−3, S1–3 = −1.40 × 10−3 eV) (Fig. S1†).
The ESI mass spectra of 2 and 3 were measured (Fig. S2
and S3†) to confirm their compositions. In 2 a one-electron
1 crystallized (with the formula 1·4H2O·24MeOH·23C2H6O)
ˉ
in the triclinic space group P1 and despite weak diffraction data
the structure was elucidated, albeit with relatively large thermal
parameters, and is depicted in Fig. 1. 14+ has a cubic core com-
posed of four iron and four nickel ions alternately bridged by
twelve cyanide ions. Charge consideration, Mössbauer and
‡Crystallographic
data
for
1:
[C172H152N68B4Fe4Ni4O4S28]-
(BF4)4·4H2O·24MeOH·23C2H6O, Mr = 7158.88, triclinic, space group
ˉ
P1, a = 17.318(5) Å, b = 29.061(8) Å, c = 29.958(8) Å, α = 70.275(4)°,
β = 89.925(4)°, γ = 74.594(4)°, V = 13 618 (6) Å3, Z = 2, T = 100 K. A
total of 65 760 reflections were collected (2.46° < 2θ < 50.6°) of which
48 300 unique reflections (Rint = 0.0883) were measured. R1 = 0.1501,
wR2 = 0.3913 (I > 2σ(I)). Disordered solvent molecules in the crystal
voids have been removed from calculations by the SQUEEZE
program.25 Details of restraints and SQUEEZE results are provided in
the cif file. CCDC 858647.†
reduction peak corresponding to [M–4(PF6)]3+ (M
=
[Fe4Ni4(CN)12(pztp)4(L2)4](PF6)4), was observed at 1547.6 m/z,
13602 | Dalton Trans., 2012, 41, 13601–13608
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Fig. 3 Cyclic voltammograms of 2 (top) and 3 (bottom) at a carbon
electrode in a concentration of 1.0 mM in MeCN, using n-Bu4NPF6 as
an electrolyte, at a scan rate of 0.025 V s−1
.
Fig. 2 Mössbauer spectra of 1 (a), 2 (b), and 3 (c) at 20 K. The solid
Electrochemistry
lines are calculated Lorentzian curves using the parameters in the text.
Cyclic voltammograms (CV) were measured for L1, L2, 2 and 3
in acetonitrile, and the results are depicted in Fig. 3 and
Fig. S5.† CV measurements could not be performed for 1 due to
its low solubility in organic solvents. L1 and L2 showed quasi-
reversible redox waves at E0′ = 0.45 and 0.77 V (versus SCE)
and E0′ = 0.45 and 0.75 V, respectively, which were attributed to
the two-step one-electron redox behaviour of the TTF moiety.
The CV of 2 showed the quasi-reversible two-step oxidation for
the TTF groups at E0′ = 0.46 and 0.76 V, and irreversible
reduction waves below 0.35 V that can be ascribed to the
reductions of the four FeIII ions. Note that the natural potential
of 2 was 0.38 V. Bulk coulometry measurements of 2 with an
applied potential of 1.2 V (Fig. S6†) suggested that the number
of electrons involved in the oxidation processes was 7.9 e−,
attributed to two four electron oxidation processes corresponding
to the oxidation of all four TTF groups to their dicationic state.
The CV for 3 showed two reversible oxidation waves at E0′ =
0.46 and 0.74 V for the TTF moieties and two reversible
reduction waves at E0′ = 0.23 and 0.03 V for the iron ions. Con-
trolled potential coulometry for 3 was performed at 1.2 V and
−0.4 V (Fig. S7 and S8†), and suggested that the numbers
of electrons involved in the oxidation and reduction processes
were 10.1 e− and 1.7 e− per molecule, respectively. It was there-
fore intimated that the oxidation waves accounted for the
oxidation of two FeII ions in addition to the four TTF groups
(total 10 e−), while the reduction peaks represented the single
electron reduction of two FeIII ions (total 2 e−). The oxidation
numbers of the component ions in 3 were thus assigned as
{Na+⊂[FeIII2FeII2NiII4(TTF0)4]}3+. The differential pulse voltam-
mogram (DPV) of 3, measured in 1.0 mM acetonitrile solution
(Fig. 4), supports this conclusion. In the DPV measurement, two
oxidation and two reduction peaks were observed at 0.44 and
0.73 V, and 0.25 and 0.05 V, respectively. The current peaks in
the DPV were analysed using theoretical predictions,18 suppos-
ing that the observed oxidation waves were the sum of the two
electron oxidation events on each TTF moiety and the single
electron oxidation of each FeII ion. Least squares calculations
determined that the oxidation processes of TTF1+/TTF2+, TTF0/
TTF+, [FeIII3FeII1Ni4]3+/[FeIII4Ni4]4+, and [FeIII2FeII2Ni4]2+/
(calcd: 1547.5 m/z) in addition to a peak corresponding to the
native state [M–4(PF6)]4+ at 1160.6 (1160.7) m/z. In contrast,
The data suggested that complex 3 contained an encapsulated
sodium ion within its cubic core, similar to some previously
reported examples.10e,f All peaks observed represented the
cluster with the addition of one sodium ion ([M–3(BF4)]n+; M =
Na[Fe4Ni4(CN)12(tp)4(L2)4](BF4)3, n = 2, 3, 4), with the di-, tri-
and tetracation ([M–3(BF4)+H+]4+) appearing at 2200.6
(2200.5), 1467.1 (1467.1) and 1100.6 (1100.5) m/z respectively.
The fact that the sodium ion is associated with the cationic
cluster in all observed species strongly suggests that the stability
of the cation–cation assembly is conferred through encapsula-
tion, allowing the sodium ion to exist in the π-electron-rich
cluster core (Fig. S3†).
57Fe Mössbauer spectra
The 57Fe Mössbauer spectra were recorded for 1–3 (Fig. 2).
The Mössbauer spectra of 1 and 2 at 20 K showed a single
quadrupole doublet with the Mössbauer parameters (relative to
metallic iron) of δ = 0.045 and ΔEQ = 1.07 mm s−1 for 1, and δ
= 0.059 and ΔEQ = 1.08 mm s−1 for 2, characteristic of LS FeIII
ions. On the other hand, the spectrum of 3 at 20 K was com-
posed of two quadrupole doublets (with δ = 0.046 and ΔEQ
=
1.07 mm s−1, and δ = 0.21 and ΔEQ = 0.44 mm s−1), which
were assigned to LS FeIII and LS FeII species, respectively.
The peak area ratio of FeIII/FeII was 0.43/0.57, which is in
good agreement with that (FeIII/FeII = 1) expected from the
number of counterions indicated by elemental analysis. The IR
spectra of 1–3 are shown in Fig. S4.† Note that the IR spectrum
of 3 exhibited three peaks in the measured frequency region
(at 2168, 2110 and 2086 cm−1), corresponding to νCN stretching
modes characteristic of MIII–CN–MII and MII–CN–MII bridges,
in contrast to the spectra of 1 and 2 which both showed single
peaks at 2170 and 2173 cm−1 respectively (Fig. S3†).10d,17
It is reasonable to conclude that 3 has the formula Na[FeIII2FeII
-
2
NiII4(CN)12(tp)4L24](BF4)3.
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Fig. 5 Electronic absorption spectra of 3 upon electrochemical oxi-
dation from 0.36 to 1.00 V (potential vs. SCE, 1.0 mM 3 and 0.1 M
n-Bu4NPF6 in acetonitrile, Pt mesh working electrode, and Pt wire
counter electrode). Arrows represent absorbance changes upon electro-
chemical oxidation.
Fig. 4 Differential pulse voltammogram of 3. De-convoluted peaks are
overlaid and summed to yield the simulated data (dotted line).
[FeIII3FeII1Ni4]3+ occurred at 0.73, 0.43, 0.70, and 0.50 V,
respectively. The potential separation between waves is a
measure of the stability of a mixed valence species. Thus the
stabilities of [FeIII1FeII3NiII4]1+, [FeIII2FeII2NiII4]2+, and
tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) exhibits LMCT
bands around 420 nm.10a,d,19 3 exhibited an LMCT band at
440 nm (ε = 7.90 × 103 M−1 cm−1), where ε was lower than that
of 2 due to the [FeIII2FeII2NiII4] state. Controlled potential
absorption spectra of 3 were recorded by applying potentials of
0.36 to −0.40 V and 0.36–1.00 V (versus SCE) (Fig. S10† and
5), respectively. In the negative potential scans from 0.36 to
−0.40V (Fig. S10†), the shoulder peak at 440 nm, corresponding
to the LMCT band of the [FeIII(CN)3(tp)]− group decreased as
the potential was lowered, due to the reduction of
{Na⊂[FeIII2FeII2NiII4(TTF0)4]}3+ to {Na⊂[FeII4NiII4(TTF0)4]}1+.
In the positive potential scans from 0.36 to 1. 00 V (Fig. 5), on
the other hand, new absorption bands initially appeared at
448 nm and 750 nm, corresponding to TTF˙+ monomers and
radical dimers respectively, in agreement with previously
reported spectral data.20 As the potential was increased beyond
0.75 V the absorption bands characteristic of the TTF˙+ cation
radicals decreased in intensity and disappeared as the potential
approached 1.00 V, while a new band, representing the TTF2+
dication appeared at 646 nm.20b,d The LMCT band correspond-
ing to the FeIII units was observed at 440 nm. Note that no IVCT
(intervalence charge transfer) band between the TTF moieties
and iron centres was observed in this experiment. One isosbestic
point (at 366 nm) was observed in the scan range of 0.36–0.75 V,
[FeIII3FeII1NiII 3+ could be represented by their comproportiona-
]
4
tion constants (Kc) as defined by the following equations:
Kc1
Ð
2þ
1þ
4
½FeI4INiI4Iꢀ þ ½Fe2IIIFe2IINi4IIꢀ
2½FeIIIFeIINiIIꢀ
1
3
Kc2
Ð
1þ
3þ
2þ
4
½FeI1IIFeI3INiI4Iꢀ þ ½FeI3IIFe1IINiI4Iꢀ
2½FeIIIFeIINiIIꢀ
2
2
Kc3
Ð
2þ
4þ
3þ
4
½FeI2IIFe2IINi4IIꢀ þ ½Fe4IIINiI4Iꢀ
2½FeIIIFeIINiIIꢀ
3
1
where Kci = exp(FΔE/RT) (i = 1–3), and F and R are the Faraday
and gas constants, respectively. The potential differences (ΔE) of
0.20, 0.25, and 0.20 V yielded the comproportionation constants
of Kc1 = 2.7 × 103; Kc2 = 1.9 × 104; and Kc3 = 2.7 × 103 for
[FeIII1FeII3NiII4], [FeIII2FeII2NiII4] and, [FeIII3FeII1NiII4] respec-
tively, indicating intermediate thermodynamic stability of the
mixed valence states.10d
The CV results illustrate the effect of the sodium ion encapsu-
lation within the cluster core. Cation encapsulation has been
shown to result in positive shifting of reduction potentials in
cyanide-bridged cubes,10f by withdrawing electron density from
the cyanide bridging ligands and changing the ligand field
strength around the metal centres. Through sodium ion encapsu-
lation, complex 3 shows greatly shifted FeIII reduction potentials
compared to 2, leading to its isolation as a mixed valence
complex, and its reversible solution state redox behaviour.
corresponding to the oxidation of {Na⊂[FeIII2FeII2NiII
-
4
(TTF0)4]}3+ to {Na⊂[FeIII3FeII1NiII4(TTF˙+)4]}8+. In contrast,
three isosbestic points (at 361, 514 and 727 nm) were seen in the
scan range of 0.75–1.00V, representing the oxidation of
{Na⊂[FeIII3FeII NiII (TTF˙+)4]}8+ to {Na⊂[FeIII4NiII (TTF2+)4]}13+
.
1
4
4
Electronic spectra
UV-vis-NIR absorption spectra of 2 and 3 were recorded in
0.400 mM acetonitrile solutions (Fig. S9†). 2 showed an
absorption band with a shoulder peak at 440 nm (ε = 147 ×
104 M−1 cm−1), which was assigned to the ligand-to-metal
charge transfer (LMCT) band of the [FeIII(CN)3(X)]− chromo-
phores. Note that in general, [FeIII(CN)3(X)]− (X = tp, pztp, and
Magnetic properties
Magnetic susceptibility data of 1–3 were collected in the temp-
erature range of 1.8–300 K and their magnetization data were
measured at 1.8 K. The χmT versus T plots of 1–3 are shown
in Fig. 6, and the magnetization data are depicted in Fig. S11.†
13604 | Dalton Trans., 2012, 41, 13601–13608
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Fig. 7 In- and out-of-phase ac susceptibility signals versus temperature
for 3.
Fig. 6 χmT versus T plots of 1, 2 and 3, in blue, red and green respect-
ively, measured in an applied magnetic field of 500 Oe. The solid lines
were calculated by the parameters given in the text. Note that the
exchange coupling constants and g values for 1 and 2 are very similar.
(S = 1/2 with g = 2.7). 3 showed gradually increasing χmT
values as the temperature was lowered, reaching a maximum
value of 13.34 emu mol−1 K at 5 K, followed by a decrease
down to 12.81 emu mol−1 K at 1.8 K. The magnetic suscepti-
bility data in the temperature range of 1.8–300 K was analysed
using MAGPACK, where the following spin Hamiltonian was
used: H = −2Jiso[(S1 + S2)(S5 + S6) + S3 × S6 + S4 × S5] with the
spin operators of Si (i = 1–4 for NiII and i = 5 and 6 for FeIII).
The obtained exchange coupling constant was Jiso = +5.4 K with
g = 2.33. The magnetization value at 1.8 K was 9.06 Nβ at
50 kOe, lower than those for 1 and 2, due to the two diamagnetic
LS FeII ions in 3. The ac magnetic susceptibility for 3 was
measured (Fig. 7), and both frequency dependent in- and out-of-
phase signals were observed, indicative of SMM behaviour.
SMMs require uniaxial magnetic anisotropy,23 and DFT calcu-
lations performed on [Fe(CN)3(tp*)]− showed that the principal
axis of anisotropy in the complex anion lies along the Fe⋯Btp*
The χmT values at 300 K of 1 and 2 were 8.02 and
8.25 emu mol−1 K, respectively, which correspond to the value
(8.02 emu mol−1 K) expected for four uncorrelated NiII ions (S =
1, g = 2.321) and four FeIII ions (S = 1/2, g = 2.7). Note that
Holmes reported that the g value for a FeIII ion in {[(pztp)-
FeIII(CN)3]4[CoL]4[ClO4]4}·13DMF·4H2O (L = 2,2,2-(1-pyrazolyl)-
ethanol) was 2.7.10c The χmT values of 1 and 2 gradually
increased as the temperature was lowered, reaching the
maximum values of 19.82 and 20.53 emu mol−1 K at 5 K, fol-
lowed by decreases to 19.21 and 19.76 emu mol−1 K at 1.8 K,
respectively. Magnetic susceptibility data in the intermediate
temperature region suggested the occurrence of ferromagnetic
interactions between LS FeIII and NiII ions. The magnetic data
of 1 and 2 were analysed by MAGPACK22 with the spin
Hamiltonian of H = −2Jiso[S1(S6 + S7 + S8) + S2(S5 + S7 + S8) +
S3(S5 + S6 + S8) + S4(S5 + S6 + S7)],10a,b where Jiso is the isotro-
pic exchange coupling constant between the LS FeIII and NiII
ions and Si (i = 1–8) represents the spin operators (i = 1–4 for
NiII and i = 5–8 for FeIII). Analyses using the data above 10 K
vector.24 The symmetry of the core structures of 1 and 2 ([FeIII
-
4
NiII4]) can be recognized as quasi Td, with the principal axes of
the [FeIII(CN)3(pztp)]− units directed towards the centre of the
cube, resulting in the cancellation of the magnetic anisotropy. In
3, however, the two-electron reduced species, [FeIII2FeII2NiII
]
4
has quasi C2 symmetry and the uniaxial magnetic anisotropy of
the two FeIII corner units cannot be cancelled by the diamagnetic
FeII units, resulting in a residual easy axis of magnetization.
yielded g = 2.30 and Jiso = +6.2 K for 1 and g = 2.31 and Jiso
=
+6.1 K for 2, corresponding to the g and Jiso values previously
reported for Fe–Ni cubes which have been in the ranges of
2.2–2.3 and 8.5–9.5 K, respectively.10a,b Magnetization exper-
iments conducted at 1.8 K (Fig. S11†) gave unsaturated values
of 11.5 Nβ for 1 and 11.2 Nβ for 2 at 50 kOe, which are also
comparable to the values previously reported for [Fe4Ni4]
cubes.10a,b
Ac magnetic susceptibility measurements for 1 and 2 were
performed in the range of 1.8–4.0 K under a 3 G ac field oscilla-
ting at 1000 Hz. No out-of-phase signals were observed, indica-
ting that the compounds were not SMMs or that their energy
barriers for magnetization reversal were prohibitively small.
The χmT value of 3 was 6.62 emu mol−1 K at 300 K, which
corresponds to the value (6.66 emu mol−1 K) expected for four
uncorrelated NiII ions (S = 1 with g = 2.321) and two FeIII ions
Experimental
X-ray crystallography
The crystals were mounted on glass capillaries, and data were
collected at −173 °C (Bruker SMART APEXII diffractometer
coupled with a CCD area detector with graphite monochromated
Mo Kα (λ = 0.71073 Å) radiation). The structure was solved
using direct methods and expanded using Fourier techniques
within the SHELXTL program. Empirical absorption corrections
were carried out using SADABS. In the structure analyses, non-
hydrogen atoms were refined with anisotropic thermal para-
meters. Hydrogen atoms were included in calculated positions
and refined with isotropic thermal parameters riding on those of
the parent atoms.
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Dalton Trans., 2012, 41, 13601–13608 | 13605

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Physical measurements
was warmed to room temperature and stirred for 24 h. After
evaporation of THF under vacuum, the residue was extracted
twice with diethyl ether (100 mL). The organic layer was then
washed with water (100 mL) and dried over anhydrous Na2SO4,
and the solvent removed under vacuum. The resulting organic
residue was purified using silica-gel column chromatography
(CH2Cl2 as the eluent) to yield 4 as a pale yellow oil (3.18 g,
7.45 mmol, 91%). Anal. calcd. for C19H19N6BrO·0.5H2O:
C 52.30, H 4.62, N 19.26. Found: C 52.24, H 4.79, N 19.03%.
1H NMR (270 MHz, acetone-d6): δ 7.59 (3H, d, pz), 7.50 (3H,
d, pz), 7.40 (2H, d), 7.21 (2H, d), 6.34 (3H, t, pz), 5.13 (2H, s),
4.61 (2H, s), 4.56 (2H, s). 13C NMR (600 MHz, CDCl3):
141.37, 137.49, 130.91, 128.63, 106.60, 89.84, 73.59, 33.18.
Magnetic susceptibility data were collected using a Quantum
Design MPMS-5S SQUID magnetometer. The temperature scan
rate was fixed to 1 K min−1, and each measurement was per-
formed 60 s after the temperature had stabilized. Ac magnetic
susceptibility measurements were carried out using a Quantum
Design PPMS with ACMS probe attachment. Magnetic data
were corrected for the diamagnetic contribution of the sample
holder, and the diamagnetism of the samples was accounted for
using Pascal’s constants. Mössbauer experiments were carried
out using a 57Co/Rh source in a constant-acceleration trans-
mission spectrometer (Topologic Systems) equipped with an
Iwatani HE05/CW404 Cryostat. The spectra were recorded at
20 K. The spectrometer was calibrated using standard α-Fe foil.
UV-vis-NIR absorption spectra were recorded on a SHIMADZU
UV-3150 spectrometer. Infrared absorption spectra were
measured on KBr pellet samples using a SHIMADZU FT-IR
8400 spectrometer. Electrochemical measurements were carried
out using a BAS 620A electrochemical analyser. Cyclic voltam-
metry and differential pulse voltammetry measurements were
carried out in a standard one-compartment cell under N2 at
20 °C equipped with a platinum-wire counter electrode, a satu-
rated calomel electrode (SCE) as the reference electrode, and a
glassy carbon (GC) working electrode. Bulk electrolysis with
Coulometry was measured with a porous carbon working elec-
trode. Controlled potential absorption spectra were recorded on a
SHIMADZU UV-3150 spectrometer equipped with BAS 620A,
an optical electrochemical cell, Pt mesh working electrode and
Pt wire counter electrode. The measurements were performed in
acetonitrile with 0.1 M tetra-n-butylammonium hexafluorophos-
phate (n-Bu4NPF6) as the supporting electrolyte. Mass spectra
(electrospray ionization (ESI), positive mode) were recorded for
MeCN/MeOH solutions using an Applied Biosystems QStar
Pulsar i spectrometer.
Synthesis of α-(4′-methyl-4,5-ethylenedithiotetrathiafulvalene-5′-
thio)-α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene (L1)
To a solution of 4-(2-cyanoethylthio)-5-methyl-4′,5′-ethylene-
dithiotetrathiafulvalene (500 mg, 1.27 mmol) in dry THF
(70 mL), CsOH·H2O (233 mg, 1.39 mmol) was added. The
resulting mixture was stirred for 8 h before 4 (542 mg,
1.27 mmol) was added, and the mixture was stirred for a further
4 h. After evaporation of the THF under vacuum, the residue
was extracted with CH2Cl2 (100 mL). The organic layer was
washed with water (100 mL) and dried over anhydrous Na2SO4,
and the solvent was removed under vacuum. The resulting
organic residue was purified using silica-gel column chromato-
graphy with CH2Cl2 as the eluent. A red oil was obtained and
dissolved in acetonitrile, which was concentrated to yield L1 as
a red solid (654 mg, 0.953 mmol, 75%). Anal. calcd for
C28H26N6OS7·CH3CN: C 49.49, H 4.01 N 13.47. Found:
C 49.52, H 3.87 N 13.37%. 1H NMR (270 MHz, CDCl3): δ 7.65
(3H, d, pz), 7.40 (3H, d, pz), 7.14 (4H, dd), 6.33 (3H, t, pz),
5.09 (2H, s), 4.49 (2H, s), 3.82 (2H, s), 3.29(4H, s), 1.59 (3H,
s). 13C NMR (600 MHz, CDCl3): 141.23, 137.92, 137.05,
136.29, 130.82, 129.01, 127.87, 117.84, 113.83, 106.42, 89.78,
73.91, 73.21, 40.35, 30.40, 14.95.
Materials
All solvents and chemicals were reagent-grade, purchased com-
mercially, and used without further purification unless otherwise
noted. THF was dried with sodium, using benzophenone as an
indicator. MeOH was distilled from dried with iodine and mag-
nesium. Tetra-n-butylammonium hexafluorophosphate, used as
the electrolyte in the electrochemical measurements, was recrys-
tallized from ethanol solution before use. Tris-2,2,2-(1-pyrazo-
lyl)ethanol,11 4-(2-cyanoethylthio)-5-methyl-4′,5′-ethylenedithio-
tetrathiafulvalene, 4-(2-cyanoethylthio)-5-methyl-4′,5′-dimethyl-
thiotetrathiafulvalene,12 n-Bu4N[Fe(CN)3pztp],13 and n-Bu4N-
[Fe(CN)3tp]14 were prepared according to the reported methods.
Synthesis of α-(4′-methyl-4,5-dimethylthiotetrathiafulvalene-5′-
thio)-α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene (L2)
To a solution of 4-(2-cyanoethylthio)-5-methyl-4′,5′-dimethyl-
thiotetrathiafulvalene (300 mg, 0.759 mmol) in dry MeOH
(100 mL), CsOH·H2O (164 mg, 0.987 mmol) was added. The
resulting mixture was stirred for 8 h before 4 (324 mg,
0.759 mmol) was added, and the mixture was stirred for a
further 4 h. After evaporation of the MeOH under vacuum, the
residue was extracted with CH2Cl2 (100 mL). The organic layer
was washed with water (100 mL) and dried over anhydrous
Na2SO4, and the solvent was removed under vacuum. The result-
ing organic residue was purified using silica-gel column
chromatography (CH2Cl2 as the eluent) to yield L2 (464 mg,
0.676 mmol, 89%). Anal. calcd for C28H28N6OS7: C 48.81,
Synthesis of α-bromo-α′-[tris-2,2,2-(1-pyrazolyl)ethoxy]-p-xylene
(4)
To a stirred suspension of NaH (224 mg, 9.33 mmol) in dry
THF (30 mL), a solution of tris-2,2,2-(1-pyrazolyl)ethanol
(2.00 g, 8.19 mmol) in dry THF (30 mL) was added at room
temperature. The mixture was stirred for 30 min before being
1
H 4.09 N 12.19. Found: C 48.87, H 4.15 N 12.24%. H NMR
(270 MHz, CDCl3): δ 7.65 (3H, d, pz), 7.40 (3H, d, pz), 7.14
(4H, dd), 6.33 (3H, t, pz), 5.09 (2H, s), 4.49 (2H, s), 3.82 (2H,
s), 2.43 (3H, s), 2.42 (3H, s), 1.59 (3H, s). 13C NMR (600 MHz,
CDCl3): 141.33, 137.98, 137.15, 136.37, 130.88, 129.64,
added to
a solution of α,α′-dibromo-p-xylene (12.6 g,
47.7 mmol) in dry THF (30 mL) at 0 °C. The resulting mixture
13606 | Dalton Trans., 2012, 41, 13601–13608
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View Article Online
128.57, 127.49, 117.98, 106.53, 89.84, 73.86, 73.35, 40.03,
19.20, 14.85.
susceptibility, indicative of SMM behaviour. The observation of
cation encapsulation controlled redox and magnetic properties is
an important development in the field of cyanide-bridged
molecular species, and reminiscent of the way in which the pro-
perties of Prussian Blue and its analogues can be influenced
through cation doping. The presented research offers a further
bridge between cyanide cluster chemistry and material science,
and extensions of this research are underway to exploit the host–
guest behaviour of such complexes. No electronic interactions
were observed between the octanuclear cube and the TTF groups
in these complexes, but they are potential building blocks for the
synthesis of molecular-based magnetic conductors. Studies on
their development as conductive materials are in progress.
Synthesis of [FeIII4NiII4(CN)12(pztp)4(L1)4](BF4)4 (1)
Ni(BF4)2·6H2O (5.4 mg, 0.016 mmol) and ligand L1 (11 mg,
0.016 mmol) were combined in methanol (2 mL). The solution
was stirred for 5 min. The resulting orange solution was com-
bined with a solution of n-Bu4NBF4 (26 mg, 0.08 mmol) and
n-Bu4N[Fe(CN)3(pztp)] (11 mg, 0.016 mmol) in acetone
(2 mL). Slow evaporation yielded 1 as red crystals (6.5 mg,
0.0013 mmol, 8.1%). Anal. calcd for C172H152N68B8F16Fe4-
Ni4O4S28·2(CH3)2CO: C 41.94, H 3.24, N 18.68. Found:
C 41.90, H 3.34, N 18.78%.
Acknowledgements
Synthesis of [FeIII4NiII4(CN)12(pztp)4(L2)4](PF6)4 (2)
This work was supported by a Grant-in-Aid for Scientific
Research for Priority Area “Coordination Programming” (area
2107) from MEXT, Japan. K. M. thanks the JSPS Young Scien-
tist Fellowship.
Ni(BF4)2·6H2O (5.4 mg, 0.016 mmol) and L2 (11 mg,
0.016 mmol) were combined in ethanol (2 mL). The solution
was stirred for 5 min. The resulting orange solution was com-
bined with a solution of n-Bu4NPF6 (31 mg, 0.08 mmol) and
n-Bu4N[Fe(CN)3(pztp)] (11 mg, 0.016 mmol) in acetone
(2 mL). Slow evaporation yielded 2 as red crystals (10 mg,
0.0019 mmol, 12%). Anal. calcd for C172H160N68B4F24Fe4-
Ni4O4P4S28·2EtOH·2H2O: C 39.52, H 3.28, N 17.80. Found:
C 39.72, H 3.48, N 17.50%. MS (ESI MS, m/z) calcd for
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13608 | Dalton Trans., 2012, 41, 13601–13608
This journal is © The Royal Society of Chemistry 2012