Tuning the spin-transition properties of pyrene-decorated 2,6-bispyrazolylpyridine based Fe(ii) complexes

Article abstract of DOI:10.1039/c1dt10420a

Two 2,6-bispyrazolylpyridine ligands (bpp) were functionalized with pyrene moieties through linkers of different lengths. In the ligand 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine (L1) the pyrene group is directly connected to the bpp moiety via a C-C

Full text of DOI:10.1039/c1dt10420a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 7564
www.rsc.org/dalton
PAPER
Tuning the spin-transition properties of pyrene-decorated
2,6-bispyrazolylpyridine based Fe(II) complexes†
Rodrigo Gonza´lez-Prieto,a Benoit Fleury,a Frank Schramm,a Giorgio Zoppellaro,a,b Rajadurai Chandrasekar,a,c
Olaf Fuhr,a Sergei Lebedkin,a Manfred Kappesa and Mario Ruben*a,d
Received 14th March 2011, Accepted 11th May 2011
DOI: 10.1039/c1dt10420a
Two 2,6-bispyrazolylpyridine ligands (bpp) were functionalized with pyrene moieties through linkers of
different lengths. In the ligand 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine (L1) the pyrene group is
directly connected to the bpp moiety via a C–C single bond, while in the ligand 4-(2,6-di(1H-pyrazol-
1-yl)pyridin-4-yl)benzyl-4-(pyren-1-yl)butanoate (L2) it is separated by a benzyl ester group involving a
flexible butanoic chain. Subsequent complexation of Fe(II) salts revealed dramatic the influence of the
nature of the pyrene substitution on the spin-transition behaviour of the resulting complexes. Thus,
compound [Fe(L1)2](ClO4)2 (1) is blocked in its high spin state due to constraints caused by a strong
intermolecular p–p stacking in its structure. On the other hand, the flexible chain of ligand L2 in
compounds [Fe(L2)2](ClO4)2 (2) and [Fe(L2)2](BF4)2·CH3CN·H2O (3) prevents structural constraints
allowing for reversible spin transitions. Temperature-dependent studies of the photophysical properties
of compound 3 do not reveal any obvious correlation between the fluorescence of the pyrene group and
the spin state of the spin transition core.
(bpp = 2,6-di(1H-pyrazol-1-yl)pyridine) bearing pyrene groups
in the 4¢-position of the pyridine moiety. Previously, several
[FeII(bpp)2]2+ compounds were shown to display ST in the range
of or above room temperature in the solid state.2–6 On the other
side, the processing of ST compounds into switchable nanostruc-
tures was recently achieved by self-assembly and soft-lithography
techniques.7 Furthermore, integration of metal complexes via
pyrene groups onto single-walled carbon nanotubes (CNT) has
been shown to lead to confined magnetic systems of high interest
for supramolecular spintronic applications.8
Introduction
Spin transition (ST) compounds displaying high transition
temperatures (T1/2), wide thermal hysteresis (DT1/2) and ther-
mochromism have increasingly moved into the focus of scientific
interest due to their potential applicability in molecular devices.1
To develop applications, it is necessary to address the function-
alization of these molecular complexes, e.g. towards facilitating
their processing for device fabrication. This aim can be achieved
by tailored chemical modification of the organic ligands prior to
the metal complexation.
The decoration of [FeII(bpp)2]2+ complexes with pyrene groups
is a straightforward approach towards implementation and in-
tegration of ST compounds into functional device architectures
via supramolecular p–p stacking interactions. In particular, the
study of [FeII(bpp)2]2+ complexes in confined environments allows
for better understanding of the ST mechanism at the molecular
level. There, cooperativity is known to play a key role in the shape
(abruptness and hysteresis) and temperature of the spin transition
in the solid state. Consequently, supramolecular approaches
(coordination polymers, hydrogen bonding, p–p stacking, etc.)
have been applied to molecular ST modules, partially with the
objective being to increase the magnitude of cooperativity and ST
temperatures.9–11 Increasingly, the question of ST cooperativity is
also being studied for Fe(II) centres distributed randomly within
a matrix or on a substrate. However, in order to understand e.g.
hybrid ST materials based on pyrene-modified ST complexes and
sp2-carbon materials (such as graphenes and carbon nanotubes)
the persistence of the ST in the pyrene-modified Fe(II) complexes
themselves has to be verified.
Herein we report on the synthesis, structure and magnetic
properties of three iron(II) complexes of the [FeII(bpp)2]2+-type
aKarlsruhe Institute of Technology (KIT), Institute of Nanotechnology,
Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Ger-
many
bDepartment of Molecular Biosciences, University of Oslo, PO Box 1041
Blindern, Oslo, NO-0316, Norway
cSchool of Chemistry, University of Hyderabad, Prof. C. R. Rao Road, Gachi
Bowli, Hyderabad, 500 046, India
dInstitut de Physique et Chimie des Mate´riaux de Strasbourg (IPCMS),
23 rue de Loess, BP34, 67034, Strasbourg cedex 2, France.
E-mail: mario.ruben@kit.edu; Fax: +49 (0) 7247 82 8976; Tel: +49 7247
82 6781
† Electronic supplementary information (ESI) available: NMR, ESI mass
and IR spectra of ligands L1 and L2, and compounds 1, 2 and 3. Emission
spectra of 3 excited at 270 nm and normalized emission intensity of
polycrystalline pyrene butyric acid as a function of the temperature. X-
Ray crystallographic parameters of 1. CCDC reference number 790319.
For ESI and crystallographic data in CIF or other electronic format, see
DOI: 10.1039/c1dt10420a
7564 | Dalton Trans., 2011, 40, 7564–7570
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Guided by this motivation three different [FeII(bpp)2]2+
complexes have been prepared from two pyrene-substituted
ligands, namely 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine
(L1) and 4-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)benzyl-4-(pyren-
1-yl)butanoate (L2) (Scheme 1). In L1, the pyrene moiety is
directly coupled to the bpp ligand core, while in L2, the pyrene
group is connected to the bpp core through a benzoyl ester
group involving a butanoic acid linker. Thus, the pyrene part
of the ligand is then clearly separated from the coordinating
bpp unit by a flexible C4 chain. Both ligands were then reacted
in dichloromethane with methanolic solutions of iron(II) salts
rendering the complexes [Fe(L1)2](ClO4)2 (1), [Fe(L2)2](ClO4)2 (2)
and [Fe(L2)2](BF4)2·CH3CN·H2O (3).
of 4¢-iodo-2¢,6¢-dipyrazolyl-pyridine12 with pyreneboronic acid. A
Suzuki reaction between 4¢-iodo-2¢,6¢-dipyrazolyl-pyridine and 4-
(hydroxymethyl)phenylboronic acid was also used to synthesize
4-(4¢-hydroxymethylphenyl)-2,6-bis(pyrazol-1-yl)pyridine. Esteri-
fication of pyrene butyric acid with this last compound yielded lig-
and L2 (4-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)benzyl-4-(pyren-
1-yl)butanoate).
A methanolic solution of iron(II) perchlorate was layered over
a dichloromethane solution of two equivalents of L1. Slow
diffusion afforded dark yellow crystals of [Fe(L1)2](ClO4)2 (1)
with sufficient quality for single crystal analysis. In the case of
L2 the solid obtained after the reaction and later evaporation
of the solvents is dissolved in acetonitrile, and diethyl ether was
slowly diffused into this solution. The respective polycrystalline
samples obtained yielded compounds [Fe(L2)2](ClO4)2 (2), and
[Fe(L2)2](BF4)2·CH3CN·H2O (3).
Solid state structure of compound 1
Single crystal X-ray diffraction studies show that compound 1
crystallizes in the C2/c space group with four [FeII(L1)2]2+ ions
and eight perchlorate counter-anions per unit cell (Fig. 1). The
[FeII(L1)2]2+ cation shows a twofold symmetry with the Fe1 atom
lying on the crystallographic twofold axis. The crystal packing
˚
diagram reveals strong intermolecular p–p stacking (d = 3.38 A) of
adjacent inversion related pyrene units of neighbouring complexes
(Fig. 1b and c) forming a 1-D chain-like structure. The Fe(II)
ion is coordinated by two bpp ligands with Fe–N distances
Scheme 1 Molecular structures of ligands L1 and L2.
˚
in the range 2.131(4)–2.202(4) A. These bond distances clearly
indicate the high-spin state (HS) of the iron(II) centre at 180 K.
The coordination polyhedron around the iron(II) ion is strongly
deviated from octahedral. This angular structural distortion is
defined by the angle between the least squares planes formed
by the two bpp ligands (q = 68.9◦) and the interligand angle
Results and discussion
Synthesis
Ligand L1 (2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine) was
obtained as a white powder via a Suzuki cross-coupling reaction
1
2
Fig. 1 (a) ORTEP view of compound 1 (30% probability ellipsoids, symmetry transformation: ¢ ∫ -x, y, - z) measured at 180 K. Selected bond
˚
lengths (A): Fe1–N1 = 2.148(4), Fe1–N3 = 2.131(4), Fe1–N5 = 2.202(4). Selected bond angles (deg): N3¢–Fe1–N3 = 149.63(19), N3–Fe1–N1¢ =
130.42(13), N3–Fe1–N1 = 73.02(13), N3–Fe1–N5¢ = 88.03(14), N3–Fe1–N5 = 71.78(13), N1¢–Fe1–N1 = 94.3(2), N1–Fe1–N5 = 141.53(14), N1–Fe1–N5¢ =
96.80(13), N(5¢)–Fe(1)-N(5) = 97.00(18); hydrogen atoms are omitted for clarity. (b) Molecular packing revealing intermolecular p–p stacking. (c)
Metallo-supramolecular 1D p–p stacked chain structure of the complex. In (b) and (c) the hydrogen atoms and counter ions are omitted for clarity.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7564–7570 | 7565
©

View Article Online
N3¢–Fe1–N3 (f = 149.63(19)◦).5 Unfortunately, compounds 2 and
3 did not provide crystals of sufficient quality for X-ray diffraction.
However, they delivered satisfying analytical data in ESI-TOF
MS, elemental analysis and H NMR and FT-IR spectroscopies
allowing the determination of their composition according to the
formulas indicated above.
In conclusion, the short linker compound 1 remains in the HS
state at all temperatures probed while 2 and 3 exhibit reversible
spin transitions between room-temperature and 90 K; regardless
of the nature of counteranions used and the presence of crystal
solvent molecules.
1
The difference in magnetic behaviour between compound 1
and compounds 2 and 3 can be understood by their structural
differences. At first, the large p-system of the pyrene groups
in compound 1 governs the overall supramolecular structure
leading to strong intermolecular p–p stacking in the final lattice
as demonstrated by the X-ray diffraction study. This effect forces
the coordination sphere around Fe(II) to accommodate to this
exterior constraint. In consequence, this molecular response leads
to a highly distorted coordination octahedron as described above,
favouring compound 1 to remain in the HS state over the
whole temperature range. This has been previously observed in
compounds showing similar◦distortions with q < 76◦ and/or f
< 172◦ (ideally, 90 and 180 , respectively),5 as well as in other
spin transition compounds in which the systems are blocked
in their HS state due to supramolecular constraints.9f ,11c It has
been well recognized that a change of the Fe–ligand bond
Temperature-dependent magnetic properties
Variable temperature magnetic measurements have been recorded
for crystalline samples of compound 1 between 2 and 340 K, both
for cooling and heating mode cycles (Fig. 2). The value of the
cMT product is 3.46 cm3 mol-1 K at 340 K which is close to the
expected value for a HS (S = 2) state iron(II) ion. From 340 K down
to ~50 K, the cMT value is nearly constant indicating the stable
paramagnetic behaviour of the compound without any ST. Below
50 K, the cMT value suddenly decreased reaching a minimum
value of 2.14 cm3 mol-1 K at 2 K. This sudden decrease can be
attributed to the zero-field splitting of the energy level of the Fe(II)
HS ion in an octahedral environment. For compounds 2 and 3,
temperature-dependent magnetic measurements were performed
using polycrystalline samples between 5 and 340 K – also for
cooling and heating mode cycles. The temperature dependence of
cMT of compound 2 showed a value of 3.41 cm3 mol-1 K at 340 K
which is very similar to the value shown by 1 and in the range of
the usual values observed for [Fe(II)(bpp)2]-type compounds in the
HS state.6 Interestingly, in contrast to compound 1, upon cooling
below 300 K the cMT product sharply decreased and reached a
value of 0.46 cm3 mol-1 K at 90 K then remained nearly constant
down to ~25 K. Below 25 K the magnetic moment reached a
minimum value of 0.29 cm3 mol-1 K at 5 K. The latter value
is slightly too high to be attributed only to the Fe(II) LS (S =
0) state. Calculation of the number fraction (f ) involved in the
ST event shows that ca. 87% of the molecules undergo quite
sharp ST, whereas the remaining ca. 13% of the molecules do
not participate in the ST event. Moreover, the residual magnetic
moment (0.29 cm3 mol-1 K) at low temperature may include
trace amounts of paramagnetic impurities. The ST temperature
is centered at T1/2 = 216 K. Similarly, compound 3 also shows a ST
centered at 218 K although exhibiting a more abrupt transition
˚
distances by ca. 0.2 A plays a key role in transmitting the
elastic interaction between the ST sites in the solid state, leading
ultimately to cooperative effects.1b Consequently, the peripheral p–
p stacking interactions of the pyrene moieties prevent structural
rearrangements at the coordination site. Thus, the formation of a
more regular coordination environment, which is a requirement
for the Fe(II) LS state, can not take place and the HS state prevails.
Apparently, such a rearrangement would require too much energy
to overcome the rigidity of the p–p stacking, and is therefore not
possible at low temperature. A similar effect was also observed
recently in another Fe(II) complex bearing sterically hindered bpp-
type ligands and showing incomplete spin transition.13 In this
molecule, [PdCl(dpa)]+ fragments on the 4-position of the bpp
ligand form p–p and M ◊ ◊ ◊ M intermolecular interactions leading
to a supramolecular rigid organization of the molecules in the
crystal lattice, trapping the compound kinetically in its high spin
state at low temperatures.
Although the detailed molecular structure is still unknown for
compounds 2 and 3, the situation must be different owing to the
increased flexibility of the pyrene linker in ligand L2. The absence
of thermal hysteresis loops in their magnetic behaviour also
demonstrates the lack of major cooperativity between molecules of
these compounds in the solid state. Moreover, even if p–p stacking
between adjacent pyrene groups can still occur, the flexible C4 alkyl
chain separator probably acts as a spring absorbing the constraint
imposed by the p–p stacking. This effect attenuates the strain onto
the coordination sphere around Fe(II) enabling the formation of a
LS coordination sphere. Thus, ST is mediated by the more flexible
ligand structure.
and being almost complete with a cMT value of 0.09 cm3 mol-1
K
at 5 K. The cMT value at 340 K (3.18 cm3 mol-1 K) is in the range
of the values obtained for HS state iron(II) complexes.
Temperature-dependent photoluminescence properties
In a recent publication, the group of Bousseksou and co-workers
was able to observe spin state dependent emission quenching
in rhodamine 110 doped [FeII(NH2Trz)3](tosyl)2 nanoparticles
(NH2Trz = 4-amino-1,2,4-triazole).14 To investigate the translation
of this magneto-optical concept onto the (single-)molecular level,
the correlation between the fluorescence of the pyrene group and
Fig. 2 The cT vs T plot of compounds 1, 2, and 3 cycled in the
temperature range of 2–340 K with an applied magnetic field of 0.1 T.
7566 | Dalton Trans., 2011, 40, 7564–7570
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
the respective iron(II) spin state was studied. The photolumines-
cence measurements were carried out on a polycrystalline sample
of compound 3, chosen due its relatively abrupt transition. The
compound was found to be emissive (after excitation at 320 nm)
over the full temperature range investigated (between 17 and
295 K). More specifically, the emission spectrum of compound
3 at 17 K comprises a structured band with vibronic peaks at 373
and 393 nm, as well as a broad band at around 460 nm tailing up
to ca. 700 nm (Fig. 3). Both emissions originate from the pyrene
groups and can be attributed to the “monomer” and “excimer”
pyrene fluorescence, respectively.15 This suggests that there are also
close contacts between pyrene moieties in compound 3 (as found
in the structural investigations for 1) consequently allowing for the
possibility of excimer formation.
Fig. 4 Emission intensity of solid compound 3 as a function of the
temperature. Wavelengths are 320 nm for excitation and 373 nm (monomer)
and 455 nm (excimer) for emission. The solid line is drawn as a guide for
eye.
Conclusion
Two spin transition complexes bearing flexible pyrene linkers
were successfully designed and synthesized: [FeII(L2)2](ClO4)2 (2)
and [FeII(L2)2](BF4)2·CH3CN·H2O (3). Both complexes show very
similar reversible spin transitions with transition temperatures of
216 and 218 K, respectively. In strong contrast, the analogous
short-linker pyrene compound [FeII(L1)2](ClO4)2 [1] does not show
any spin transition and remains blocked in the HS state over the
full temperature range investigated. In addition, no influence of the
spin state on the fluorescence intensities was observed; moderate
temperature-dependent fluorescence quenching was shown to be
non-magnetic in nature by comparison to diamagnetic reference
compounds. In conclusion, we have demonstrated the importance
of the flexibility of the ligand around Fe(II) centres for balancing
structural constraints which prevent the ST process from occurring
and cooperativity. Functionalization of planar surfaces and non-
planar CNTs with compounds 2 and 3, as well as magnetic and
electrical studies of the corresponding devices17 and materials18
are under investigation.
Fig. 3 Emission spectra of compound 3 (polycrystalline) excited at
320 nm at different temperatures. The half-bandwidths of the excitation
and emission monochromators corresponded to 4 nm.
Another peculiar feature of the emission spectra of compound
3 is their notable dependence on the excitation wavelength as
illustrated in a comparison of Fig. 3 and S19† (excitation wave-
lengths of 320 and 270 nm, respectively). For instance, the relative
intensity of the “excimer” band is particularly large at an excitation
wavelength of 270 nm and for temperatures below 150 K. However,
it decreased strongly as the temperature increased. The whole
spectrum then changes dramatically at ambient temperature (Fig.
S20†). The origin of these spectral changes is not clear at the
moment. There are also additional small emission peaks at ~326
and 357 nm (at 270 nm excitation).
Experimental
General experimental methods
Both the “monomer” and “excimer” emissions of pyrene groups
in compound 3 do not show any specific correlation with the
actual spin state of the complex core [FeII(L2)]2+. Measurements
of the emission intensities above and below the spin transition
temperature T1/2 = 218 K reveal a rather continuous dependence
on the temperature as illustrated in Fig. 4 for both emission wave-
lengths. The temperature dependence observed, which manifests a
moderate increase of the emission intensity below ~200 K, seems
to be typical for pyrene-based fluorophors. Correspondingly, a
direct relationship with the spin state of 3, can be ruled out by
comparing with studies of analogous nonmagnetic compounds
(e.g., pyrene butyric acid, see Fig. S20†) which show the same
moderate temperature dependency of the emission intensity as
observed for 3.
All reagents and solvents used in this study are commercially
available and were used without any further purification, except
when stated. 4¢-Iodo-2¢,6¢-dipyrazolyl-pyridine was synthesized
according to the reported procedure.12
1H and 13C NMR spectroscopic data were recorded on a Bruker
DPX 300 and a Bruker Ultrashield plus 500 spectrometers with
solvent-proton as internal standard. Mass data were acquired with
a Voyager-DE PRO Bio spectrometry work station for MALDI-
TOF and with a MicrOTOF-Q II Bruker for ESI-TOF. MALDI
spectra were measured with no additional matrix compound
other than the sample itself. Infrared spectra were recorded using
KBr-pressed pellets with a Perkin-Elmer Spectrum GX FT-IR
spectrometer. Elemental analyses were carried out on a Vario
Micro Cube.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7564–7570 | 7567
©

View Article Online
Magnetic susceptibility
126.38, 125.78, 125.52, 125.01, 124.87, 124.83, 124.31, 111.45,
108.21. ESI-TOF MS in CHCl3 + formic acid (Da): m/z (rel.
intensity, assigned structure) = 279.13 (8%, C21H13N, fragment);
412.21 (100%, C27H18N5, L1 + H+, calc. = 412.16); 434.21 (5%,
C27H17N5Na, L1 + Na+); 823.42 (3%, C54H35N10, L12 + H+).
Temperature-dependent static susceptibilities were recorded using
an MPMS-5S (Quantum Design) SQUID magnetometer in a
homogeneous 0.1 T external magnetic field. Gelatine capsules
were used as sample containers. The very small diamagnetic
contribution of the gelatine capsule had a negligible contribution
to the overall magnetization, which was dominated by the sample.
The diamagnetic corrections of the molar magnetic susceptibilities
were applied using well-known Pascal’s constants.
Synthesis of 4-(4¢-hydroxymethylphenyl)-2,6-bis(pyrazol-
1-yl)pyridine
4¢-Iodo-2¢,6¢-dipyrazolyl-pyridine (2 g, 6 mmol), 4-(hydroxy-
methyl)phenylboronic acid (0.911 g, 6 mmol) and Pd(PPh3)4
(0.700 g, 0.6 mmol, 10%) were suspended in a N2 gas bubbled
solution of 1,4-dioxane (130 mL) and 2 M Na2CO3 (20 mL)
and heated to 70 ◦C for 3 days under a nitrogen atmosphere.
The 1,4-dioxane was removed using a rotary evaporator and the
remaining residue was extracted twice with 100 mL of CHCl3 and
washed with water (3 ¥ 200 mL). The separated organic layer was
dried over MgSO4 and the solvent was removed by evaporation.
The obtained light brown solid was chromatographed on a silica
column using ethyl acetate/hexane (7 : 3) as eluent until the first
non-desired yellow fraction is eliminated. The second colourless
fraction was collected eluting with ethyl acetate/ethanol (17 : 3)
and, upon evaporation, the resultant combined solutions yielded
a white solid of 4-(4¢-hydroxymethylphenyl)-2,6-bis(pyrazol-1-
yl)pyridine (1.4 g, yield 73%). 1H NMR (300 MHz, CDCl3, 25 ◦C, d
(ppm)): 8.60 (d, 2H, pyrazole), 8.10 (d, 2H, pyridine), 7.78 (m, 4H),
7.48 (d, 2H, phenyl), 6.51 (m, 2H, pyrazole), 4.78 (s,◦2H, –CH2–),
2.26 (s, 1H, –OH). 13C NMR (75 MHz, CDCl3, 25 C, d (ppm)):
153.83, 150.60, 142.73, 142.41, 136.53, 127.49, 127.40, 127.30,
108.02, 107.18, 64.75. MALDI-TOF (Da): m/z (rel. intensity,
assigned structure) = 318.13 (100%, C18H16N5O, M + H+, calc. =
318.13); 617.07 (26%, C36H29N10O, fragmentation + association);
635.08 (11%, C36H31N10O2, 2M + H+).
Measurements were recorded for crystalline samples of 1 and
for polycrystalline samples of 2 and 3.
Single crystal X-ray diffraction
X-Ray data collection was performed on a STOE IPDS II
diffractometer with graphite monochromated Mo-Ka radiation at
180 K. Crystallographic and refinement data of 1 are summarized
in a table in the ESI† (CCDC reference number 790319). Structure
solution by direct methods and refinement with anisotropic
temperature factors for all non-hydrogen atoms were carried out
by using the SHELX program package.16 H atoms were calculated
on idealized positions.
Photoluminescence study
Photoluminescence (PL) measurements were performed on a Spex
Fluorolog-3 spectrometer equipped with a 450 W xenon light
source, double excitation and emission monochromators and a
thermoelectrically cooled Hamamatsu R9910 photomultiplier as
a detector. Solid samples (polycrystalline powders) were dispersed
in viscous polyfluoroester (ABCR GmbH), layered between two
1 mm thick quartz plates and mounted on the cold finger of an
optical closed-cycle cryostat (Leybold) operating at ~15–350 K.
Emission was collected at an ~30◦ angle relative to the excitation
light beam. All emission spectra are presented corrected for the
wavelength-dependent response of the spectrometer and detector
(in relative photon flux units).
Synthesis of 4-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)benzyl-4-
(pyren-1-yl)butanoate (L2)
4 - (4¢ - Hydroxymethylphenyl ) - 2,6 - bis (pyrazol - 1 - yl )pyridine
(0.4301 g, 1.4 mmol), 1-pyrenebutyric acid (0.4040 g, 1.4 mmol),
N,N¢-dicyclohexylcarbodiimide (0.2835 g, 1.4 mmol) and
dimethylaminopyridine in catalytic quantities were suspended
in 150 mL of dry dichloromethane, and this reaction mixture
was stirred for two days at room temperature under an inert
atmosphere. After evaporation of solvents, the obtained solid was
chromatographed on a silica column using ethyl acetate/hexane
(1 : 1) as eluent. The white solid obtained after rotary evaporation
of the first colourless fraction was washed with diethyl ether
Synthesis of 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine (L1)
4¢-Iodo-2¢,6¢-dipyrazolyl-pyridine (0.4643 g, 1.38 mmol),
pyreneboronic acid (0.3390 g, 1.38 mmol) and Pd(PPh3)4
(0.1581 g, 0.14 mmol, 10%) were suspended in an Ar gas bubbled
solution of 1,4-dioxane (150 mL) and 2 M Na2CO3 (10 mL)
and heated to 70 ◦C for 5 days under an argon atmosphere.
The 1,4-dioxane was removed using a rotary evaporator and the
remaining residue was extracted twice with 100 mL of CHCl3 and
washed with water (2 ¥ 200 mL). The separated organic layer was
dried over MgSO4 and the solvent was removed by evaporation.
The solid orange residue was washed with methanol to remove the
soluble impurities and to afford a yellowish coloured powder. The
solid residue was column chromatographed on neutral alumina
with ethyl acetate/hexane (1 : 4) as eluent. The second colourless
fraction was collected and the resultant combined solution upon
evaporation yielded a white powder of L1 (0.350 g, yield 62%). 1H
NMR (500 MHz, CDCl3, 25 ◦C, d (ppm)): 8.69 (d, 2H), 8.27–8.02
(m, 11H), 7.78 (d, 2H), 6.55 (m, 2H). 13C NMR (125 MHz,
CDCl3, 25 ◦C, d (ppm)): 155.34, 150.31, 142.62, 134.02, 131.84,
131.49, 130.95, 128.68, 128.35, 128.30, 127.45, 127.43, 127.06,
1
yielding L2 (0.4847 g, yield 61%). H NMR (300 MHz, CDCl3,
25 ◦C, d (ppm)): 8.60 (d, 2H), 8.27 (d, 1H), {8.16–7.95 (m, 9H)},
7.84 (d, 1H), 7.79 (d, 4H), 7.46 (d, 2H), 6,52 (m, 2H), 5,18 (s,
2H), 3,40 (t, 2H), 2.55 (t, 2H), 2.25 (m, 2H). 13C NMR (125 MHz,
CDCl3, 25 ◦C, d (ppm)): 173.36, 153.70, 150.75, 142.53, 137.77,
137.40, 135.67, 131.51, 131.00, 130.11, 128.93, 128.87, 127.59,
127.56, 127.48, 127.36, 126.85, 125.96, 125.21, 125.09, 125.04,
124.93, 123.38, 108.14, 107.34, 65.78, 34.00, 32.85, 26.85. ESI-
TOF MS in CHCl3 + formic acid (Da): m/z (rel. intensity, assigned
structure) = 588.32 (100%, C38H30N5O2, L2 + H+, calc. = 588.24);
610.30 (24%, C38H29N5NaO2, L2 + Na+).
7568 | Dalton Trans., 2011, 40, 7564–7570
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Synthesis of [Fe(L1)2](ClO4)2·(H2O) [1(H2O)]
rotary evaporation, the resulting yellow-orange solid was dissolved
in CH3CN and, after diffusion of diethyl ether into this solution,
the polycrystalline [Fe(L2)2](BF4)2·CH3CN·H2O compound was
obtained (64.4 mg, yield 54%). Elemental analysis found (calcd)
for C78H63B2F8FeN11O5 ([Fe(C38H29O2N5)2](BF4)2·CH3CN·H2O,
1463.90 g mol-1): C 63.72 (64.00)%; H 4.28 (4.34)%, N 10.36
(10.52)%. 1H NMR (300 MHz, CD3CN, 25 ◦C, d (ppm)); 67.00 (s,
pyrazole), 58.65 (s, pyrazole), 39.62 (d, pyrazole), 8.27–7.39 (m),
5.70 (s, –CH2–Ph), 3.34 (t, –CH2–), 2.51 (t, –CH2–). ESI-TOF MS
in CH3CN (Da): m/z (rel. intensity, assigned structure) = 321.62
(35%, FeC38H29N5O2Cl, M2+ - L2); 615.26 (100%, FeC76H58N10O4,
M2+, calc. = 615.20); 909.40 (17%, FeC114H87N15O6, M2+ + L2);
22.3 mg (0.054 mmol) of 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-
yl)pyridine (L1) was dissolved in 7 mL of dichloromethane.
9.80 mg (0.027 mmol) of iron(II) perchlorate hydrate was dissolved
in 2 mL of methanol and this solution was subsequently layered
over the solution of L1. Slow diffusion affords complex 1 as dark
yellow single crystals, which were suitable for single crystal X-ray
diffraction (16.5 mg, yield 56%). Elemental analysis found (calcd)
for C54H36Cl2FeN10O9 {[Fe(L1)2](ClO4)2·H2O, 1095.703 g mol-1}:
1
C 59.28 (59.20)%; H 3.29 (3.31)%, N 12.86 (12.78)%. H NMR
(500 MHz, CD3CN, 25 ◦C, d (ppm)); 63.01 (s, pyrazole), 56.59
(s, pyridine), 37.90 (s, pyrazole), 36.32 (s, pyrazole), 8.46–7.88
(m, pyrene). ESI-TOF MS in CH3CN (Da): m/z (rel. intensity,
assigned structure) = 249.56 (6%, FeC28H21N5O, M2+ - L1 +
CH3OH); 254.08 (8%, FeC29H20N6, M2+ - L1 + CH3CN); 263.09
(9%; FeC29H22N6O2, M2+ - L1 + CH3CN + H2O); 439.17 (100%,
FeC54H34N10, M2+, calc. 439.12); 566.10 (12%, FeC27ClH17N5O4,
-1
1203.04 (7%, FeC152H116N20O8, M2+ + 2L2). FT-IR (KBr): n/cm
=
˜
3135, 3038, 2939, 2875, 2250, 1738, 1630, 1583, 1560, 1526, 1497,
1461, 1431, 1410, 1375, 1338, 1257, 1217, 1176, 1160, 1051, 1034,
972, 914, 843, 827, 792, 780, 769, 710, 680, 643, 630, 591, 520, 492.
Caution. Although we have experienced no difficulties in handling
these complexes, metal–organic perchlorates are potentially explo-
sive and should be handled with care and in small quantities.
M2+ - L1 + ClO4 ); 644.76 (6%, FeC81H51N15, M2+ + L1); 977.28
-
(9%, FeC54ClH34N10O4, M2+ + ClO4 ). FT-IR (KBr): n/cm
=
-
-1
˜
3148, 3114, 3043, 1621, 1567, 1523, 1497, 1457, 1407, 1338, 1235,
1216, 1174, 1099, 1057, 1014, 975, 874, 842, 792, 765, 732, 720,
680, 665, 645, 622, 601, 502.
Acknowledgements
This research was supported by the DFG Collaborative Research
Center TR88 (3MET) under subprojects C5 and C7. We also
acknowledge infrastructure support by KIT and the Helmholtz
Association.
Synthesis of [Fe(L2)2](ClO4)2 (2)
A solution of iron(II) perchlorate hydrate (15.25 mg, 0.042 mmol)
in MeOH (3 mL) was added to 51.5 mg (0.088 mmol) of L2
dissolved in CH2Cl2 (12 mL). Immediately, a colour change from
colourless to yellow occurred and the reaction mixture was stirred
at room temperature for two hours under a N2 atmosphere.
Solvent was removed by rotary evaporation, the resulting yellow-
orange solid was dissolved in CH3CN and diethyl ether was
slowly diffused into this solution. The polycrystalline sample
obtained contains acetonitrile and water as can be deduced
from its elemental analysis. These solvents are slowly lost on
exposure to air yielding, after a few weeks, the compound
[Fe(L2)2](ClO4)2 (2) (44.3 mg, yield 74%). Elemental analysis
found (calcd) for C76H58Cl2FeN10O12 {[Fe(C38H29O2N5)2](ClO4)2,
1430.122 g mol-1): C 63.67 (63.83)%; H 4.170 (4.09)%, N 9.80
Notes and references
1 For a general overview on spin-transition compounds, see: (a) O. Kahn
and C. J. Martinez, Science, 1998, 279, 44; (b) P. Gu¨tlich and H. A.
Goodwin, Top. Curr. Chem., 2004, 233, 1; (c) E. Ko¨nig, Prog. Inorg.
Chem., 1987, 35, 527; (d) E. Ko¨nig, Struct. Bonding (Berlin), 1991,
76, 51; (e) P. Gu¨tlich, Y. Garcia and H. A. Goodwin, Chem. Soc. Rev.,
2000, 29, 419; (f) J. A. Real, A. B. Gaspar, V. Niel and M. C. Mun˜oz,
Coord. Chem. Rev., 2003, 236, 121; (g) M. A. Halcrow, Coord. Chem.
Rev., 2005, 249, 2880; (h) O. Kahn and J. P. Launay, Chemtronics, 1988,
3, 140.
2 J. M. Holland, J. A. McAllister, C. A. Kilner, M. Thornton-Pet, A. J.
Bridgeman and M. A. Halcrow, J. Chem. Soc., Dalton Trans., 2002,
548.
3 J. M. Holland, J. A. McAllister, Z. Lu, C. A. Kilner, M. Thornton-Pett
and M. A. Halcrow, Chem. Commun., 2001, 577.
(9.79)%. H NMR (300 MHz, CD3CN, 25 ◦C, d (ppm)): 66.69
1
4 M. A. Halcrow, Polyhedron, 2007, 26, 3523.
(s, pyrazole), 58.42 (s, pyridine), 39.59 (s, pyrazole), 39.09 (s,
pyrazole), 8.26–7.41 (m), 5.70 (s, –CH2–Ph), 3.33 (t, –CH2–), 2.50
(t, –CH2–). ESI-TOF MS in CH3CN (Da): m/z (rel. intensity,
assigned structure) = 615.20 (100%, FeC76H58N10O4, M2+, calc. =
615.20); 678.14 (30%, FeC38H29N5O2Cl, M2+ - L2 + Cl-); 909.32
(16%, FeC114H87N15O6, M2+ + L2); 1202.94 (7%, FeC152H116N20O8,
M2+ + 2L2); 1265.38 (5%, FeC76H58N10O4Cl, M2+ + Cl-); 1329.36
5 M. A. Halcrow, Coord. Chem. Rev., 2009, 253, 2493 and references
therein.
6 (a) C. Rajadurai, O. Fuhr, R. Kruk, M. Ghafari, H. Hahn and M.
Ruben, Chem. Commun., 2007, 2636; (b) N. T. Madhu, I. Salitrosˇ, F.
Schramm, S. Klyatskaya, O. Fuhr and M. Ruben, C. R. Chim., 2008,
ˇ
ˇ
11, 1166 and references therein; (c) I. Salitrosˇ, N. T. Madhu, R. Boca,
J. Pavlik and M. Ruben, Monatsh. Chem., 2009, 140, 695.
7 (a) M. Cavallini, I. Bergenti, S. Milita, G. Ruani, I. Salitros, Z.-R. Qu,
R. Chandrasekar and M. Ruben, Angew. Chem., Int. Ed., 2008, 47,
8596; (b) S. Cobo, G. Molna´r, J. A. Real and A. Bousseksou, Angew.
Chem., Int. Ed., 2006, 45, 5786; (c) G. Molna´r, S. Cobo, J. A. Real, F.
Carcenac, E. Daran, C. Vieu and A. Bousseksou, Adv. Mater., 2007,
19, 2163; (d) C. Thibault, G. Molna´r, L. Salmon, A. Bousseksou and
C. Vieu, Langmuir, 2010, 26, 1557.
-
-1
(3%, FeC76H58N10O8Cl, M2+ + ClO4 ). FT-IR (KBr): n/cm
=
˜
3135, 3038, 2931, 2866, 1727, 1629, 1581, 1558, 1526, 1496, 1460,
1432, 1410, 1373, 1337, 1258, 1213, 1158, 1094, 1056, 1016, 972,
842, 818, 792, 771, 751, 717, 681, 643, 623, 590, 491.
8 S. Klyatskaya, J. R. Gala´n Mascaro´s, L. Bogani, F. Hennrich, M.
Kappes, W. Wernsdorfer and M. Ruben, J. Am. Chem. Soc., 2009,
131, 15143.
9 (a) O. Kahn, J. Kro¨ber and C. Jay, Adv. Mater., 1992, 4, 718; (b) N.
Moliner, M. C. Mun˜oz, S. Letard, X. Solans, N. Mene´ndez, A. Goujon,
F. Varret and J. A. Real, Inorg. Chem., 2000, 39, 5390; (c) V. Niel, J. M.
Martinez-Agudo, M. C. Mun˜oz, A. B Gaspar and J. A. Real, Inorg.
Chem., 2001, 40, 3838; (d) A. Galet, V. Niel, M. C. Mun˜oz and J. A.
Real, J. Am. Chem. Soc., 2003, 125, 14224; (e) J. A. Real, E. Andre´s, M.
Synthesis of [Fe(L2)2](BF4)2·CH3CN·H2O (3)
A solution of Fe(BF4)2·6H2O (27.50 mg, 0.082 mmol) in MeOH
(3 mL) was added to 98 mg (0.167 mmol) of L2 dissolved in CH2Cl2
(15 mL). Immediately, a colour change from colourless to yellow
occurred and the reaction mixture was stirred at room temperature
for two hours under a N2 atmosphere. Solvent was removed by
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7564–7570 | 7569
©

View Article Online
C. Mun˜oz, M. Julve, T. Granier, A. Bousseksou and F. Varret, Science,
1995, 268, 265; (f) D. L. Reger, J. R. Gardinier, M. D. Smith, A. M.
Shahin, G. J. Long, L. Rebbouh and F. Grandjean, Inorg. Chem., 2005,
44, 1852.
Bravic, C. Gieck, D. Chasseau, W. Tremel and P. Gu¨tlich, Inorg. Chem.,
2005, 44, 9723.
12 C. Rajadurai, F. Schramm, S. Brink, O. Fuhr, M. Ghafari, R. Kruk
and M. Ruben, Inorg. Chem., 2006, 45, 10019.
13 C. A. Tovee, C. A. Kilner, S. A. Barrett, J. A. Thomas and M. A.
Halcrow, Eur. J. Inorg. Chem., 2010, 1007.
14 L. Salmon, G. Molna´r, D. Zitouni, C. Quintero, C. Bergaud, J. C.
Micheau and A. Bousseksou, J. Mater. Chem., 2010, 20, 5499.
15 J. R. Lakowicz, in Principles of Fluorescence Spectroscopy, Kluwer Aca-
demic/Plenum Publishers, New York, 1999, Chapter 3, and references
therein.
16 G. M. Sheldrick, Acta Crystallogr., Sect. A, 2008, 64, 112.
17 (a) M. Ruben, A. Landa, E. Lo¨rtscher, H. Riel, M. Mayor, H. Go¨rls,
H. B. Weber, A. Arnold and F. Evers, Small, 2008, 4, 2229; (b) E. A.
Osorio, T. Bjornholm, J. M. Lehn, M. Ruben and H. S. J. Van Der
Zant, J. Phys.: Condens. Matter, 2008, 20, 374121.
18 (a) N. Lin, S. Stepanov, F. Vidal, K. Kern, M. S. Alam, S. Stro¨msdo¨rfer,
V. Dremov, P. Mu¨ller, A. Landa and M. Ruben, Dalton Trans., 2006,
2794; (b) M. Ruben, Angew. Chem., Int. Ed., 2005, 44, 1594.
10 (a) M. Ruben, F. J. Rojo, J. Romero-Salguero and J. M. Lehn, Angew.
Chem., Int. Ed., 2004, 43, 3644; (b) E. Breuning, M. Ruben, J. M. Lehn,
F. Renz, Y. Garcia, V. Ksenofontov, P. Gu¨tlich, E. Wegelius and K.
Rissanen, Angew. Chem., Int. Ed., 2000, 39, 2504; (c) T. Kitazawa, Y.
Gomi, M. Takahashi, M. Takeda, M. Enomoto, A. Miyazaki and T.
Enoki, J. Mater. Chem., 1996, 6, 119; (d) V. Niel, A. L. Thompson,
A. E. Goeta, C. Enachescu, A. Hauser, A. Galet, M. C. Mun˜oz and
J. A. Real, Chem.–Eur. J., 2005, 11, 2047; (e) A. Galet, M. C. Mun˜oz,
V. M a r t´ınez and J. A. Real, Chem. Commun., 2004, 2268; (f) V. Niel,
M. C. Mun˜oz, A. B. Gaspar, A. Galet, G. Levchenko and J. A. Real,
Chem.–Eur. J., 2002, 8, 2446.
11 (a) R. Bocˇa, M. Bocˇa, L. Dlha´nˇ, K. Falk, H. Fuess, W. Haase, R.
Jarosˇcˇiak, B. Papa´nkova´, F. Renz, M. Vrbova´ and R. Werner, Inorg.
Chem., 2001, 40, 3025; (b) S. Hayami, Z. Gu, H. Yoshiki, A. Fujishima
and O. Sato, J. Am. Chem. Soc., 2001, 123, 11644; (c) Y. Garcia, G.
7570 | Dalton Trans., 2011, 40, 7564–7570
This journal is
The Royal Society of Chemistry 2011
©