Spin-crossover behavior and electrical conduction property in iron(ii) complexes with tetrathiafulvalene moieties

Article abstract of DOI:10.1039/c0dt01092h

Iron(ii) complexes with neutral and oxidized tetrathiafulvalene (TTF) moieties were prepared and X-ray crystallographic analyses, magnetic and electrical resistivity measurements suggested an interaction of spin transition and electrical conductivity.

Full text of DOI:10.1039/c0dt01092h

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Spin-crossover behavior and electrical conduction property in iron(II)
complexes with tetrathiafulvalene moieties†
Masayuki Nihei,a Nobukazu Takahashi,a Hiroyuki Nishikawab and Hiroki Oshio*a
Received 24th August 2010, Accepted 29th October 2010
DOI: 10.1039/c0dt01092h
Iron(II) complexes with neutral and oxidized tetrathiafulva-
lene (TTF) moieties were prepared and X-ray crystallographic
analyses, magnetic and electrical resistivity measurements
suggested an interaction of spin transition and electrical
conductivity.
magnetic and electrical resistivity measurements suggested that 2
exhibits an interaction of SCO and electric conductivity.
The novel tridentate ligand with a TTF moiety, dppTTF, was
synthesized by the Wittig reaction between formyltetrathiafulva-
lene and the phosphonium salt of a dpp derivative (see ESI†). The
reaction of dppTTF with Fe(BF4)2·6H2O, Na(BPh4) in MeNO2,
followed by recrystallization from MeNO2/diethyl ether, yielded
dark purple crystals of 1.
X-ray structural analysis of 1 was performed at 120 K, and an
ORTEP diagram of the complex cation is depicted in Fig. 1a.
1 crystallized in the monoclinic space group P21/c and the
asymmetric unit contains a complex cation and two BPh4- anions.
In the complex cation of 1, two tridentate coordination sites in
dppTTFs coordinate to an iron(II) ion and two peripheral TTF
moieties are linked to the central core via ethylene bridges. The
iron(II) ion has a distorted octahedral coordination structure
with six nitrogen atoms from the two tridentate dppTTF ligands.
A multi-functional system, which exhibits coexistence or synergy
of two different physical properties, is one of the most attractive
research targets in molecule-based materials, because the interplay
of different physical properties may lead to novel behaviors.1
Hybrid materials, composed of magnetic and conducting moieties,
have been reported to exhibit interesting multi-functionalities such
as the coexistence of ferromagnetism and metallic conductivity,
and field-induced superconducting transitions.2 However, the
switching of electrical conductivity in hybrid materials by external
stimuli is still challenging. Spin-crossover (SCO),3 a spin transition
between high-spin (HS) and low-spin (LS) states associated with
relatively large structural changes, can function as a switch
for the electrical conductivity in hybrid systems, as electrical
conductivity in molecular conductors is very sensitive to external
and chemical pressures.4 A major approach employed to obtain
multi-functional materials exhibiting both electrical conductivity
and SCO phenomena is to combine conducting anions and SCO
cations in the same crystal lattice.5 This approach has recently
led to an interaction of SCO and electrical conductivity being
reported.5a An alternative approach is by the preparation of
metal complexes with ligands possessing potentially conducting
moieties such as tetrathiafulvalene (TTF) and its derivatives.
Although some metal complexes with neutral or oxidized TTF
moieties have been reported,6 to the best of our knowledge,
there have been no reports on the observations of both SCO
and electrical conductivity in such complexes. On the other
hand, [FeII(dpp)2]X2 (dpp = 2,6-di(pyrazol-1-yl)pyridine) is one
of the most studied SCO complexes, and chemical modifi-
cation of the ligand, dpp, leads to different spin transition
behavior.7 We report here two novel iron(II) complexes with
TTF moieties, [Fe(dppTTF)2](BPh4)2·MeNO2·0.5Et2O (1) and
[Fe(dppTTF)2][Ni(mnt)2]2(BF4)·PhCN (2) (dppTTF = 1-{2-(1,3-
dithiol-2ylinene)-1,3-dithiolyl}-2-{2,6-bis(1-pyrazolyl)pyridyl}-
ethylene, mnt = maleonitriledithiolate).‡ X-ray structural analyses,
˚
The Fe–N bond lengths are in the range of 1.914(6)–2.006(7) A,
characteristic of LS iron(II) ions. Distortion of the coordination
geometry from the ideal octahedron was quantified by using R
parameters (= R | 90-q | (◦), where the q value is the bite angle of
the two coordinated ligands).8 The R value of 89.7◦ for the central
iron(II) ion in 1 affirms that the iron(II) ion is in the LS state.
The complex cation has two crystallographically independent TTF
moieties. The central C C bond lengths in TTF derivatives are
sensitive to the oxidation state, and oxidized species have longer
C
C bonds than that in neutral species.6 In 1, the central C
C
˚
bond lenghts are 1.34(1) and 1.33(1) A for C16–C17 and C35–C36,
respectively, which are typical values for neutral TTF derivatives.
aGraduate School of Pure and Applied Sciences, University of Tsukuba, Tenn-
odai 1-1-1, Tsukuba, Ibaraki, Japan. E-mail: oshio@chem.tsukuba.ac.jp;
Fax: +81-29-853-4238; Tel: +81-29-853-4238
bCollege of Science, Ibaraki University, Bunkyo 2-1-1, Mito, Ibaraki, Japan.
E-mail: nisikawa@mx.ibaraki.ac.jp; Fax: +81-29-228-8371; Tel: +81-29-
228-8371
† Electronic supplementary information (ESI) available: Additional figures
and experimental procedure. CCDC reference numbers 796347–796349.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0dt01092h
Fig. 1 ORTEP diagrams of (a) complex cation in 1, and (b) 2 with 30%
probability of thermal ellipsoids.
2154 | Dalton Trans., 2011, 40, 2154–2156
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The Royal Society of Chemistry 2011
©

In 1, the shortest intermolecular S ◊ ◊ ◊ S contact between TTF
˚
moieties was 3.722(4) A, suggesting no significant intermolecular
interactions between TTFs. X-ray structural analysis at higher
temperature could not be done as the crystals were unstable due
to solvent loss.
Magnetic susceptibility measurements for 1 were performed in
the temperature range of 5–300 K (Fig. S1†). The cmT value
was 2.79 emu mol-1 K at 300 K, which is close to the Curie
constant (3.00 emu mol-1 K with g = 2) for HS iron(II) ions. As the
temperature was lowered, the cmT values showed gradual decrease
centered at ca. 200 K, and reached the minimum value of 0.15 emu
mol-1 K at 5 K, suggesting the occurrence of gradual SCO to the
diamagnetic LS state in the iron(II) ions.
Cyclic voltammograms of dppTTF and 1 in MeCN are
displayed in Fig. 2. The dppTTF showed two reversible ox-
idation waves at 0.40 and 0.76 V vs. SCE (Fig. 3a), which
is close to the redox potentials of TTF (0.38 and 0.77 V vs.
SCE).9 The two oxidation waves are, therefore, assignable to
the two-stepped one-electron oxidation process in TTF moiety
Fig. 3 Temperature dependence of cmT values and resistivity in 2.
and one PhCN molecule (Fig. 1b and Fig. S3†). In the complex
cation, the iron(II) ion has an average ◦coordination bond length
∑
(dppTTF/dppTTF+ /dppTTF2+). In 1, three quasi-reversible ox-
˚
idation waves were observed at 0.46, 0.81 and 1.26 V (Fig. 3b
and Fig. S2†). The first two oxidation waves correspond to two-
electron oxidation of the TTF moieties, and the third wave is
assignable to the Fe(II)/Fe(III) couple. The positive shift of the
oxidation potentials of TTF moieties in 1 compared with that in
dppTTF is due to the electron withdrawing effect of the iron(II)
center as observed in other metal complexes with TTF moieties.6
The quasi-reversible oxidation behavior of TTF moieties and the
higher oxidation potential of the central iron(II) ion in 1 suggested
of 2.106(6) A and the R value of 133.9 at 300 K. The structural
feature clearly revealed that the electronic state of the iron ion is HS
iron(II). [Ni1]- and [Ni2]- have square planar structures with the
˚
average coordination bond lengths of Ni1–S = 2.166(3) A and Ni2–
-
˚
S = 2.141(3) A, respectively, which are typical values for [Ni(mnt)2]
anions.10 Charge considerations and structural features of each
constituent unit suggest that the complex cation has +3 charge. The
complex cation in 2 has two TTF moieties, TTF1 and TTF2, with
S1 – S4 and S5 – S8 atoms, respectively. The central C C bond
∑
that the oxidized species composed of TTF+ radicals and the
lengths are 1.38(1) A both in TTF1 and TTF2, and the apparently
˚
iron(II) ion are relatively stable at room temperature.
longer C C bond than that in 1 suggested that the oxidation states
of TTF1 and TTF2 are either a mixed-valent state composed of
∑
TTF+ and TTF0, or an averaged oxidation state of TTF0.5+. In
2, the TTF moieties and [Ni(mnt)2]- anions form 2-D layers on a
ab plane and the layers are linked through the [Fe(dppTTF)2]3+,
where TTF and dpp are bridged by ethylene groups (Fig. S3 and
S4†). The [Ni1]- moieties are strongly dimerized with an interplane
-
˚
distance of 3.460 A. The [Ni1] dimers and weakly dimerized TTF1
moieties form 1-D columnar structures with a [TTF1]-[TTF1]-
[Ni1]-[Ni1] repeating unit. On the other hand, weakly dimerized
TTF2 and [Ni2]- moieties are aligned perpendicular to the [TTF1]-
[Ni1] columns with short S ◊ ◊ ◊ S contacts, forming herringbone-
like 2-D layers. In the layers each constituent unit interacts with
˚
S ◊ ◊ ◊ S contacts in the range of 3.402–3.671 A (Table S1†). As
the temperature was lowered, significant structural changes were
observed at 200 K while the space group remained unchanged.
The average c◦oordination bond length and the R value are 2.05(1)
˚
A and 115.9 , respectively, suggesting the occurrence of SCO
from the HS to the LS state. It should be noted that the S ◊ ◊ ◊ S
distances between TTF and [Ni(mnt)2]- anions in the 2-D layers
were significantly changed and the dimerized constituent units
became closer with the structural changes in the iron(II) centers
(Table S1†).
In the magnetic susceptibility measurements for 2, the cmT
values showed gradual change associated with the SCO of the
iron(II) ions (Fig. 3). The cmT value (3.44 emu mol-1 K) at 330 K
corresponds to the theoretical value (3.75 emu mol-1 K) for the
one isolated HS iron(II) ion and two isolated S = 1/2 radicals,
suggesting that the TTF and [Ni2]- moieties are paramagnetic
Fig. 2 Cyclic voltammograms for (a) dppTTF and (b) 1.
Galvanostatic (I = 5 mA) oxidation of [Fe(dppTTF)2]2+ with
(Bu4N)[Ni(mnt)2] and (Bu4N)BF4 in PhCN under nitrogen atom-
osphere gave dark brown crystals of 2 (Fig. 1b). X-ray structural
analyses of 2 were performed at 200 and 300 K. 2 crystallized in
¯
the triclinic space group P1, and the asymmetric unit is composed
of one complex cation, two [Ni(mnt)2]- anions ([Ni1]- and [Ni2]-),
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2154–2156 | 2155
©

and the strongly dimerized [Ni1]- moieties are in the singlet
state, even at 330 K. The cmT value is 0.80 emu mol-1 K at
35 K, corresponding to the expected value (0.75 emu mol-1 K)
for one LS iron(II) ions and two magnetically isolated radicals.
The sudden decrease of cmT values below 25 K is attributed
to antiferromagnetic interactions between radicals (Fig. S5†). It
should be noted that the spin transition gradually occurred in the
temperature range of 170–300 K.
The temperature dependence of electrical resistivity was mea-
sured on a single crystal of 2 by the four-probe dc method
(Fig. 3). The resistivities of 2 along crystallographic c axis showed
semiconducting behavior above 280 K and the conductivity at
room temperature was estimated to be 2.6 ¥ 10-3 S cm-1. Although
the conducting pathway in 2 is not clear, the herringbone-
like 2-D structure along bc plane might be responsible for the
semiconducting behavior. As the temperature was lowered, an
anomaly was observed in the temperature range of 160–280 K,
which corresponds well to the temperature range for the spin
transition of iron centers. The activation energy below and above
the anomaly were 129 meV (126–160 K) and 119 meV (280–
360 K), respectively. 2 shows SCO with relatively large structural
modification depending on the temperatures, and the modulation
of the electric conductivity might be, therefore, ascribable to the
chemical pressure induced by SCO in the iron(II) core.
In summary, the novel iron(II) complexes with TTF moieties
linked by ethylene bonds were presented. 1 showed reversible
redox behavior and thermal SCO. The galvanostatic oxidation
of [Fe(dppTTF)2]2+ with [Ni(mnt)2]- anions yielded the oxidized
complex 2. Magnetic and resistivity measurements suggested
that 2 showed an interaction of spin transition and conducting
behavior. Detailed variable temperature structural analyses and
photomagnetic experiments are currently underway to clarify
the mechanism of conductivity and to explore the switching of
conductivity by light irradiation.
Crystal data for 1 at 120 K: C89H74B2Fe1N11O2.5S8, monoclinic P21/c,
◦
˚
˚
˚
a = 21.330(2) A, b = 22.146(2) A, c = 17.307(2) A, b = 95.123(2) , V =
3
-3
˚
8143(1) A , Z = 4, dc. = 1.364 Mg m , 45908 reflections measured, 15968
unique reflections (Rint = 0.1533). Final R1 = 0.1026 and wR2 = 0.2129 (I
¯
> 2sI). Crystal data for 2 at 200 K: C61H31B1F4Fe1N19Ni2S16, triclinic P1,
◦
˚
˚
˚
a = 13.688(2) A, b = 14.900(2) A, c = 18.364(2) A, a = 89.572(3) , b =
◦
◦
3
-3
˚
83.176(3) , g = 74.454(3) , V = 3581.9(7) A , Z = 2, dc. = 1.672 Mg m ,
16246 reflections measured, 10251 unique reflections (Rint = 0.0629). Final
¯
R1 = 0.0874 and wR2 = 0.2084 (I > 2sI). at 300 K: triclinic P1, a = 13.828(2)
◦
◦
˚
˚
˚
A, b = 15◦.156(2) A, c = 18.255(3) A, a = 89.722(2) , b = 83.344(2) , g =
3
-3
˚
74.195(2) , V = 3654.8(9) A , Z = 2, dc. = 1.638 Mg m , 14934 reflections
measured, 10226 unique reflections (Rint = 0.0198). Final R1 = 0.0779 and
wR2 = 0.2198 (I > 2sI).
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research
on Innovative Areas (“Coordination Programming” Area 2107,
No. 21108006) from MEXT, Japan, and the Kurata Memorial
Hitachi Science and Technology Foundation.
Notes and references
‡ Synthesis of 1: The reaction of Fe(BF4)2·6H2O (8 mg, 0.022 mmol) with
dppTTF (20 mg, 0.045 mmol) in MeNO2 (5 mL) gave a dark purple
solution. Na(BPh4) (80 mg, 0.23 mmol) was added to the reaction mixture,
and the resulting solution was filtered. Dark purple crystals of 1 were
obtained by slow diffusion of dietyl ether into the filtrate. Anal. Calcd.
for dried 1 (C87H69B2Fe1N11O2S8) : C, 62.55; H, 4.40; N, 9.22%. Found: C,
62.47; H, 4.28; N, 9.14%.
Synthesis of 2: The reaction of Fe(BF4)2·6H2O (8 mg, 0.022 mmol) with
dppTTF (20 mg, 0.045 mmol) in PhCN (14 mL) gave a dark purple
solution. (Bu4N)[Ni(mnt)2] (40 mg, 0.07 mmol) and (Bu4N)BF4 (147
mg, 0.45 mmol) were added to the reaction mixture, and galvanostatic
oxidation (I = 5 mA) of the resulting solution gave dark brown crystals of
2. Anal. Calcd. for C61H35B1Fe1N19Ni2O2S16: C, 39.84; H, 1.92; N, 14.47%.
Found: C, 40.10; H, 1.95; N, 14.18%.
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2156 | Dalton Trans., 2011, 40, 2154–2156
This journal is
The Royal Society of Chemistry 2011
©