An iron(II) spin-transition compound with thiol anchoring groups

Article abstract of DOI:10.1002/ejic.200800212

The synthesis and characterisation of the complex [FeII(L)2]-(ClO4)2 [L = S-(4-{[2,6-(Dipyrazol-1-yl)pyrid-4-yl]ethynyl}-phenyl) ethanethioate] carrying both a spin-transition core and acetyl-protected terminal thiol groups is described. The compound exhibits a reversible, thermally driven solid-state spin transition at 277 K with a hysteresis loop of ca. 8 K. The unique combination of the bistable spin-transition property with thiol anchoring groups sets the basis for the investigation of the spin-transition switching phenomenon at a single-molecule level. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800212

SHORT COMMUNICATION
DOI: 10.1002/ejic.200800212
An Iron(II) Spin-Transition Compound with Thiol Anchoring Groups
Rajadurai Chandrasekar,[a][‡] Frank Schramm,[a] Olaf Fuhr,[a] and Mario Ruben*[a]
Keywords: Iron / Spin crossover effect / Magnetic properties / S ligands / N ligands
The synthesis and characterisation of the complex [FeII(L)2]-
(ClO4)2 [L = S-(4-{[2,6-(Dipyrazol-1-yl)pyrid-4-yl]ethynyl}-
phenyl) ethanethioate] carrying both a spin-transition core
and acetyl-protected terminal thiol groups is described. The
compound exhibits a reversible, thermally driven solid-state
spin transition at 277 K with a hysteresis loop of ca. 8 K. The
unique combination of the bistable spin-transition property
with thiol anchoring groups sets the basis for the investiga-
tion of the spin-transition switching phenomenon at a single-
molecule level.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
publications demonstrate the principle of the spin-state
transition process in different environments or dimensions.
For example, (i) in polymeric matrices,[1a] (ii) in surface-
grown iron(II) coordination polymer-based multilayer thin
films,[6] (iii) in Langmuir–Blodgett (LB) multilayers of mo-
nomeric and metallo-supramolecular polymeric com-
plexes[7] and (iv) in nanoparticles of FeII complexes.[8]
Furthermore, an important ingredient of the nanodevice
“tool box” is the molecular wire concept, where the current/
voltage (I/U) characteristics of single molecules are investi-
gated between Au electrodes.[9] Such molecular wires are
designed, synthesized and studied at a single-molecule level
to scrutinize the effect of external stimuli such as electric
field, laser light, molecular conformational changes, molec-
ular symmetry and temperature on the single-molecule elec-
tric transport.[10]
In this vein, ST centres are of interest because they are
some of the best-known forms of inorganic electronic
switches. However, exploitation of the real potential of the
ST complexes to construct a switching magnetic nanocom-
ponent is still in its infancy due to the design and more
notably to synthetic challenges. To the best of our knowl-
edge no report is available on the synthesis of an ST com-
plex bearing thiol anchoring groups based on a molecular
wire concept.
In order to manipulate iron(II) ST units under surface
conditions, we have rationally designed and synthesized the
complex [FeII(L)2](ClO4)2 (1) [L = S-(4-{[2,6-(dipyrazol-1-
yl)pyrid-4-yl]ethynyl}phenyl ethanethioate]. Compound 1
consists of three main components: (i) an octahedral
iron(II) ST unit,[5] (ii) two highly π-conjugated phenylethy-
nyl linker moieties and (iii) two acetyl-protected thiol an-
choring groups to facilitate contact with noble and coinage
metal (gold, silver, mercury, copper, palladium and plati-
num) electrodes or surfaces upon deprotection of the acetyl
groups (Scheme 1).
Introduction
Molecular spin-transition (ST) units made of iron(II)
metal ions in an octahedral coordination environment may
be considered as potential electronic components[1] in the
construction of switching molecular devices in different di-
mensional scales (thin films, SAMs, nanoparticles, -wires,
etc.) since their physical properties change strongly in de-
pendence on their intrinsic spin states. The electronic
ground state of an ST unit is represented by the low-spin
(LS) state, while excitation leads to the metastable high-spin
(HS) state. The spin switching is basically a result of the
intraorbital electron-density reorganisation within the
iron(II) metal ion. The observed molecular mechanical
movement (elongation ↔ contraction) during spin transi-
tion is caused by the population of antibonding eg* orbitals
in the HS case, which leads to Fe–N bond length variations
of approximately 0.2 Å. The spin-state switching is entropy-
driven and can be reversibly triggered by external param-
eters like temperature, pressure and electromagnetic radia-
tion (visible light, X-ray, etc.).
The shaping of ST units for possible use in molecular
electronic nanodevices (e.g. switching arrays, wires, etc.) is
receiving growing interest. This rising interest is strongly
driven by the spin-state-dependent electronic, magnetic,
photophysical, and mechanical properties of ST com-
pounds. The vast majority of the studies dealing with
iron(II) spin-transition compounds has been carried out un-
der solid-state bulk conditions.[1–5] Only a small number of
[a] Institute of Nanotechnology, Research Centre, Karlsruhe,
P. O. Box 3640, 76021 Karlsruhe, Germany
Fax: +49-724-782-6781
E-mail: Mario.Ruben@int.fzk.de
[‡] Present address: School of Chemistry, University of Hyderabad,
Central University
P. O. Box, Gachi Bowli, Hyderabad 500046, A.P. India
Eur. J. Inorg. Chem. 2008, 2649–2653
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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R. Chandrasekar, F. Schramm, O. Fuhr, M. Ruben
SHORT COMMUNICATION
Scheme 1. Schematic cartoon representation of the spin transition
molecule [FeII(L)2](ClO4)2 (1) with thiol anchoring groups.
Scheme 2. Synthesis of ligand L and its iron(II) molecular complex
1. Reagents and conditions: (a) Pd(PPh3)4/CuI/Et3N/thf/N2, (b)
K2CO3/MeOH/CH2Cl2, (c) Pd(PPh3)2Cl2/CuI/iPr2NEt/thf/N2, (d)
FeClO4 hydrate.
Results and Discussion
We have achieved the synthesis of our target ligand L in
eight steps starting from citrazinic acid. The synthesis of the L yielding a dark-red single-crystalline material in 58%
4-iodo-2,6-di(pyrazol-1-yl)pyridine precursor 2 (Scheme 2) yield (Scheme 2).[5] Recrystallization was carried out in
was recently reported by us.[2]
methanol, and the obtained red crystals were used for X-
Starting from this iodo precursor, two subsequent Sono- ray diffraction and magnetic studies.
gashira coupling reactions were employed to introduce the Single-crystal X-ray diffraction analysis revealed the tri-
¯
conjugated phenylethynyl linkers: First, the connection of a clinic space group P1 of 1 (Figure 1). In the crystal lattice,
π-conjugated ethynyl moiety at the 4Ј-position of the 2,6- each molecular complex 1 is sourrounded by three dichloro-
di(pyrazol-1-yl)pyridine backbone was established, followed methane lattice solvent molecules and two disordered per-
by the coupling of a (thioacetyl)benzene group to the de- chlorate counter anions. At 180 K, the coordination envi-
protected ethynyl linker. The precursor of the anchoring ronment of the iron(II) metal ion can be described as a
group 4-iodo-1-(thioacetyl)benzene was prepared as re- distorted octahedron. The average Fe–N bond lengths vary-
ported in the literature.[11] Due to deprotection of the acetyl ing from 1.889 Å to 1.980 Å indicate the LS state of the
group leading to the formation of dimeric dithiol coupling iron(II) ion at 180 K. The oxygen atoms of the thioacetyl
side products, the final yield of L was only 20%. The purifi- units exhibit the following intermolecular hydrogen-bond
cation of ligand L was carried out on a silica gel column. interactions with pyrazole and pyridine hydrogen atoms:
The metal coordination reaction was achieved in a mixture d(O1···H1Apyrazole
)
=
2.392 Å, d(O2···H7Apyridine
)
=
of dichloromethane and methanol at room temperature 2.407 Å, d(O2···H9Apyrazole) = 2.446 Å. The sulfur atoms of
from 1 equiv. of FeII(ClO4)2 hydrate and 2 equiv. of ligand the thioacetyl units also show short contacts with the pyr-
Figure 1. ORTEP (50% probability) plot of molecular complex 1 with atomic labels. Hydrogen atoms, perchlorate counteranions and
dichloromethane lattice solvent molecules are omitted for clarity. Selected bond lengths [Å]: Fe–N(1) 1.980(4), Fe–N(3) 1.892(4), Fe–N(5)
1.971(4), Fe–N(6) 1.969(4), Fe–N(8) 1.889(4), Fe–N(10) 1.959(4). Selected bond angles [°]: N(8)–Fe–N(3) 177.71(18), N(3)–Fe–N(6)
100.46(16), N(10)–Fe–N(6) 159.76(15), N(8)–Fe–N(5) 102.12(17), N(5)–Fe–N(1) 160.03(16), N(8)–Fe–N(10) 80.10(16), N(3)–Fe–N(10)
99.79(16), N(8)–Fe–N(6) 79.68(16), N(3)–Fe–N(5) 80.17(16), N(10)–Fe–N(5) 91.47(17), N(6)–Fe–N(5) 91.86(17), N(8)–Fe–N(1) 97.76(17),
N(3)–Fe–N(1) 79.95(17), N(6)–Fe–N(1) 93.59(17).
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2649–2653

An Iron(II) Spin-Transition Compound with Thiol Anchoring Groups
spectrometer with solvent-proton signal as internal standard. Infra-
red spectra were recorded using KBr-pressed pellets with a Perkin–
Elmer Spectrum GX FT-IR spectrometer. MALDI-TOF MS data
were acquired with a Voyager-DE PRO Bio spectrometry workst-
ation. Elemental analyses were carried out with a Vario Micro
cube. X-ray data collection was performed with a STOE IPDS II
diffractometer with graphite-monochromated Mo-Kα radiation at
180 K. The structure was solved by direct methods (SHELX-97).
Refinement was done with anisotropic temperature factors for all
non-hydrogen atoms. Temperature-dependent static susceptibilities
azole hydrogen atom [d(S1···H9Apyrazole) = 2.899 Å] and
pyridine carbon atom [d(S2···C28) = 3.368 Å]. The two ace-
tyl-protected thiol end groups are situated at a distance
d(S1···S2) of 2.6 nm.
The temperature-dependent bulk magnetic measurement
of molecular wire 1 in the range of 4.5–380 K is shown in
Figure 2. The measurements were performed both at heat-
ing (Ȇ) and cooling (ȇ) cycles, which revealed a reversible
ST behaviour with a hysteresis loop. At 380 K the value of
χT is 3.25 emuKmol–1, which is close to the expected value of 1 were recorded with an MPMS-5S (Quantum Design) SQUID
magnetometer over a temperature range of 4.5–380 K in a homo-
geneous 0.1 T external magnetic field. Gelatin capsules were used
as sample containers. The diamagnetic corrections of the molar
magnetic susceptibilities were applied using well-known Pascal’s
constants. Caution: Perchlorate salts of metal complexes with or-
ganic ligands are potentially explosive; only small quantities of
such compounds should be prepared and handled with much care!
for an HS iron(II) ion in the S = 2 state. The ST tempera-
tures for the heating (T1/2Ȇ = 273 K) and cooling (T1/2ȇ
= 281 K) modes revealed the occurrence of a ca. 8 K wide
thermal hysteresis loop. Below ca. 200 K, the χT value is
almost constant and reached
a minimum value of
0.07 emuKmol–1 at 4.5 K, indicating the LS state of com-
plex 1. This is also in good agreement with the low-spin
state observed in the single-crystal data at 180 K. The spin
transition is not lattice-solvent-assisted since the air-dried
powder sample exhibits a very similar ST behaviour.
Crystal Data for 1: C45H36Cl8FeN10O10S2; triclinic; space group
¯
P1; a = 10.831(2) Å; b = 13.480(3) Å; c = 19.774(4) Å; α =
76.50(3)°; β = 82.62(3)°; γ = 70.39(3)°; V = 2640.3(9) Å3; crystal
size = 0.22ϫ0.11ϫ0.06 mm; index ranges: –12Յ h Յ 12, –15Յ
k Յ 15, –23Յ l Յ 22; θ range for data collection: 1.64–24.72°;
completeness to θ: 97.1%; Z = 2; Dobsd. = 1.611 g/m3; F(000) =
1300; reflections with IϾσ(I) = 5876; µ = 0.836 mm–1; R(Fo) =
0.0644; Rw(F) = 0.1623; GOF on F2 = 1.026; T = 180(2) K. CCDC-
668334 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
2,6-Di(pyrazol-1-yl)-4-(trimethylsilylethynyl)pyridine (3): Freshly
distilled dry diisopropylamine (150 mL) was placed into a 250 mL
Schlenk vessel, and nitrogen was bubbled through it for 30 min. 4-
Iodo-2,6-di(pyrazol-1-yl)pyridine
(4 g,11.9 mmol),
Pd(PPh3)4
Figure 2. χT vs. T plot for complex 1 from 4.5 to 380 K (ȇ cooling
mode and Ȇ heating mode) measured with a 0.1 T applied DC
magnetic field. The sample was cooled from 300 to 4.5 K, and the
χT was first measured upon warming from 4.5 to 380 K, followed
by the cooling mode.
(1.38 g, 1.2 mmol) and CuI (228 mg, 1.2 mmol) were added, and
nitrogen was bubbled through the mixture for another 1 h. (Tri-
methylsilyl)acetylene (7 mL, 52 mmol) was added dropwise to this
mixture with a syringe through a septum while stirring was contin-
ued at room temperature under nitrogen for 3 d. An additional
portion of (trimethylsilyl)acetylene (4 mL, 30 mmol) was added
dropwise, and the mixture was stirred at room temperature for an-
other 3 d. The workup procedure has to be taken carefully to avoid
both the presence of strong bases and temperatures above 40 °C.
The solvent was removed in vacuo by rotary evaporation, and the
obtained black crude solid was purified by column chromatography
on neutral alumina using a mixture of ethyl acetate/hexane (1:50)
as eluent. The colourless, blue-fluorescent fraction (Rf = 0.43) was
concentrated to afford 3.4 g (11.1 mmol, 93% yield) of white crys-
talline 3. 1H NMR (300 MHz, CDCl3, 25 °C): δ = 0.29 [s, 9 H,
Conclusions
We have designed, synthesized and characterized for the
first time a novel iron(II) ST complex bearing acetyl-pro-
tected thiol anchor groups. The X-ray structure of complex
1 in the LS state revealed a distance of separation of 2.6 nm
between the two thiol end groups. The molecular complex
undergoes a thermally reversible ST at 277 K with a hyster-
esis loop width of ca. 8 K. Investigations of the tunable elec-
tronic transport through the molecular ST unit at the sin-
gle-molecule level is in progress.[12]
3
4
(CH3)3Si], 6.51 (td, J = 2.4, J = 0.9 Hz, 2 H, pyrazole), 7.78 (d,
3J = 1.5 Hz, 2 H, pyrazole), 7.91 (s, 2 H, pyridine), 8.55 (dd, J =
3
2.4, 4J = 0.6 Hz, 2 H, pyrazole) ppm. 13C NMR (75 MHz, CDCl3,
25 °C): δ = –0.26 [(CH3)3Si], 101.4 (CϵC), 101.6 (CϵC), 108.3,
111.9, 127.2, 136.6, 142.7, 150.3 ppm. FT-IR (KBr): ν = 3156,
˜
3115, 2955, 2898, 2171, 1722, 1615, 1554, 1464, 1398, 1252, 1206,
1109, 1044, 894, 847, 754 cm–1. MALDI-TOF: experiment: m/z
(relative intensity of isotopic distribution) = 496.63 (100%), 497.63
(35%), 498.63 (15%); simulation: m/z (relative intensity of isotopic
distribution) = 496.19 (100%), 497.19 (35%), 498.19 (5%).
Experimental Section
Materials: All reagents and solvents used in this study are commer-
cially available and were used without further purification. 4-Iodo-
1-(thioacetyl)benzene and 4-iodo-2,6-di(pyrazol-1-yl)pyridine were
synthesized according to reported procedures.[11,2] 1H and 13C
NMR spectroscopic data were recorded with a Bruker DPX 300
4-Ethynyl-2,6-di(pyrazol-1-yl)pyridine (4): Compound
3
(2 g,
6.5 mmol) was dissolved in dichloromethane (80 mL) followed by
the addition of methanol (80 mL). Potassium carbonate (7 g,
Eur. J. Inorg. Chem. 2008, 2649–2653
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2651

R. Chandrasekar, F. Schramm, O. Fuhr, M. Ruben
SHORT COMMUNICATION
50 mmol) was added to this mixture, and it was stirred at room
temperature overnight (18 h). The solvents were evaporated from
the yellowish solution in vacuo by rotary evaporation, and the
crude solid was purified by column chromatography on neutral alu-
mina using ethyl acetate/hexane (1:40) as eluent to afford 1.3 g
Acknowledgments
The authors thank the Deutsche Forschungsgemeinschaft (DFG)
for financial support within the frame of the project “Kondo Mole-
cules”. We deeply thank Prof. Dr. Horst Hahn for providing the
instrument facilities and Sven Stahl for carrying out the elemental
analysis.
(85% yield) of
4
as white crystalline solid. 1H NMR
3
(300 MHz,CDCl3, 25 °C): δ = 3.38 (s, 1 H, CϵH), 6.50 (td, J =
4
3
4
2.7, J = 0.6 Hz, 2 H, pyrazole), 7.76 (dd, J = 1.5, J = 0.6 Hz, 2
H, pyrazole), 7.92 (s, 2 H, pyridine), 8.53 (dd, 3J = 2.7, 4J = 0.6 Hz,
2 H, pyrazole) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 80.84
(CϵC), 83.11 (CϵC), 108.58, 112.33, 127.43, 135.86, 142.99,
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150.55 ppm. FT-IR (KBr): ν = 3217, 3157, 3106, 2727, 2109, 1758,
˜
1612, 1557, 1526, 1473, 1401, 1266, 1209, 1108, 1047, 955, 867,
749 cm–1. MALDI-TOF: experiment: m/z (relative intensity of iso-
topic distribution) = 496.63 (100%), 497.63 (35%), 498.63 (15%);
simulation: m/z (relative intensity of isotopic distribution) = 496.19
(100%), 497.19 (35%), 498.19 (5%).
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S-(4-{[2,6-(Dipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl) Ethanethioate
(L): 4 (1.08 g, 4.6 mmol), 4-iodo-1-(thioacetyl)benzene (1.02 g,
3.7 mmol), Pd(PPh3)2Cl2 (145 mg, 180 µmol) and CuI (70 mg,
370 µmol) were placed into a 100 mL Schlenk tube, and the mixture
was purged with nitrogen several times. To this mixture were added
20 mL each of freshly distilled thf and diisopropylamine, and the
reaction mixture was stirred at room temperature for 3 d. Acetic
anhydride (8 mL) was added to the reaction mixture, and all sol-
vents were removed in vacuo by rotary evaporation. The dark and
resinous crude reaction product was purified by column
chromatography on silica gel using a mixture of ethyl acetate/hex-
ane (1:4) as eluent to afford a mustard-coloured solid, which
needed a second chromatographic purification on silica with a mix-
ture of ethyl acetate/hexane (1:9) as eluent. The yellow-brownish
product with Rf ≈ 0.37 (ethyl acetate/hexane, 1:4) was recrystallized
from hot diethyl ether to afford 0.28 g (20% yield with respect to
the iodo compound) of a yellowish crystalline material which was
suitable for single-crystal X-ray diffraction. 1H NMR (300 MHz,
CDCl3, 25 °C): δ = 2.44 (s, 3 H, CH3COS), 6.50 (m, 2 H, pyrazole),
7.50 (dd, 3J = 43.5, 4J = 8.1 Hz, 4 H, phenyl), 7.77 (d, 3J = 1.5 Hz,
3
2 H, pyrazole), 7.96 (s, 2 H, pyridine), 8.55 (d, J = 2.4 Hz, 2 H,
pyrazole) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 30.76
(CH3), 88.32 (CϵC), 94.42 (CϵC), 108.6, 111.8, 123.4, 127.5,
130.0, 133.0, 134.7, 136.7, 143.0, 150.6, 193.50 (C=O) ppm. FT-IR
(KBr): ν = 3109, 2924, 2854, 2220, 1696, 1608, 1553, 1468, 1398,
˜
1261, 1208, 1123, 1042, 955, 863, 762 cm–1. MALDI-TOF: experi-
ment: m/z (relative intensity of isotopic distribution) = 496.63
(100%), 497.63 (35%), 498.63 (15%); simulation: m/z (relative in-
tensity of isotopic distribution) = 496.19 (100%), 497.19 (35%),
498.19 (5%).
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Bis[S-(4-{[2,6-(dipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl)ethanethio-
ato]iron(II) Diperchlorate (1): In a 100 mL round-bottomed flask
ligand L (67 mg, 0.17 mmol) was dissolved in dichloromethane
(15 mL), and methanolic Fe(ClO4)2 hydrate (32 mg, 0.085 mmol)
was added to the ligand solution. The mixture was stirred at room
temperature for 15 min, and then a portion of dichloromethane
(10 mL) was added, after which formation of a slight precipitate
was observed. The precipitate was filtered off and discarded. Meth-
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obtain dark red crystals (56 mg, yield 58%). FT-IR (KBr): ν =
˜
3122, 2220, 1696, 1620, 1558, 1527, 1457, 1406, 1268, 1217, 1093,
971, 833, 767, 624 cm–1. C42H30Cl2FeN10O10S2·1/3CH2Cl2·
1/3CH3OH (1064.62): calcd. C 48.13, H 3.03, N 13.16, S 6.02;
found C 48.12, H 3.09, N 13.13, S 6.22.
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An Iron(II) Spin-Transition Compound with Thiol Anchoring Groups
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Received: February 28, 2008
Published Online: April 18, 2008
Eur. J. Inorg. Chem. 2008, 2649–2653
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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