Above room temperature spin transition in a metallo-supramolecular coordination oligomer/polymer

Article abstract of DOI:10.1039/b702468a

By use of a metallo-supramolecular concept, a linear iron(ii) coordination chain [Fe^II(L)]_n(BF₄)_2n (L = 1,4-bis(1,2′:6′,1″-bispyrazolylpyridin-4′-yl)benzene) was rationally designed and synthesized. The molecula

Full text of DOI:10.1039/b702468a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Above room temperature spin transition in a metallo-supramolecular
coordination oligomer/polymer{
Chandrasekar Rajadurai, Olaf Fuhr, Robert Kruk, Mohammed Ghafari, Horst Hahn and Mario Ruben*
Received (in Cambridge, UK) 15th February 2007, Accepted 1st May 2007
First published as an Advance Article on the web 24th May 2007
DOI: 10.1039/b702468a
By use of a metallo-supramolecular concept, a linear iron(II)
coordination chain [FeII(L)]n(BF4)2n (L = 1,4-bis(1,29:69,10-bis-
pyrazolylpyridin-49-yl)benzene) was rationally designed and
synthesized. The molecular chain shows a reversible spin
transition at 323 K with a ca. 10 K wide hysteresis loop.
The search for spin transition (ST) materials with a transition
temperature (T1/2) around room temperature displaying a wide
hysteresis loop has continued since the first demonstration of a
‘‘ST based display device’’ by Kahn.1 Although recent years have
witnessed great research activity in ST complexes, compounds with
T1/2 in the range of room temperature and above are still scarce.1,2
In contrast to isolated or low-dimensional ST coordination
complexes,
metallo-supramolecular
architectures
(linear,
Hofmann-like or interpenetrated framework) can increase the
molecular cooperativity.3 Due to the enhanced rigidity in higher
dimensional ST architectures, T1/2 might be shifted to the room
temperature region, with a simultaneously widened hysteresis loop.
Previously, Holland et al. have reported the zero dimensional
ST complex FeII[2,6-di(pyrazol-1-yl)pyridine]2(BF4)2 exhibiting
T1/2 at 259 K together with a 3 K wide hysteresis loop.4a
Following the metallo-supramolecular approach,5 we have ration-
ally designed an extended iron(II) based linear rigid coordination
chain using a taylor-made back-to-back 1,4-phenylene-coupled
2,6-di(pyrazol-1-yl)pyridine ligand L (Scheme 1). Conceptually, the
introduction of metallo-supramolecular interactions is expected to
increase T1/2 and to widen the hysteresis loop in comparison with
the monomeric entity.5 Herein, we report the first synthesis of a
linear building block ligand 1,4-bis(1,29:69,10-bispyrazolylpyridin-
49-yl)benzene L, which is subsequently converted into a linear 1-D
iron(II) coordination oligomer/polymer {[FeII(L)]n(BF4)2n} (1). The
metallo-supramolecular coordination chain 1 exhibits ST above
room temperature with a wide hysteresis loop, as proven by
magnetic and Mo¨ssbauer studies (Fig. 2 and 3).
Scheme 1 Synthesis of iron(II) coordination chain (1). Reagents and
conditions: (a) 1,4-phenyldiboronicacid, Pd(PPh3)4, dioxane, 2 M Na2CO3,
95 uC; (b) FeII(BF4)2?6H2O, CH2Cl2, MeOH, 3 days, RT, N2 atmosphere.
a torsion angle (h) of ca. 34u. Compound L can be considered as a
structural alternative ligand in coordination chemistry to the
widely used terpyridyl-based back-to-back ligand systems.6–8 The
supramolecular coordination complex 1 was synthesized by stirring
a mixture of one equivalent of a dichloromethane solution of L
and two equivalents of methanolic Fe(BF4)2?6H2O solution at
room temperature for 3 days under N2 atmosphere to afford an
orange precipitate of 1." Repeated attempts to crystallize the
complex using different techniques resulted only in precipitation of
The ligand L was synthesized via Suzuki cross-coupling reaction
of 4-iodo-2,6-di-pyrazol-1-ylpyridine2e with 1,4-phenyldiboronic
acid (Scheme 1).{ The structure of L was determined by single
crystal X-ray diffraction study,§ which shows linearly arranged 2,6-
…
di(pyrazol-1-yl)pyridine units (N3 N8 distance = 1.14 nm)
(Fig. 1). The pyrazolyl rings are orientated in an all-transoid
conformation around the C–N single bonds. The 2,6-di(pyrazol-
1-yl)pyridine units are almost planar to the 1,4-phenyl bridge with
Institute of Nanotechnology, Research Centre, Karlsruhe,
PB-3640, D-76021, Germany. E-mail: Mario.Ruben@int.fzk.de;
Fax: :(+49) 724-782-6781; Tel: (+49) 724-782-6434
{ Electronic supplementary information (ESI) available: experimental and
FT-ICR MS details. See DOI: 10.1039/b702468a
Fig. 1 ORTEP plot (50% probability) of ligand L: (a) front view, (b) side
view, showing the linear arrangements of the two 2,6-di(pyrazol-1-
yl)pyridine units.
2636 | Chem. Commun., 2007, 2636–2638
This journal is ß The Royal Society of Chemistry 2007

View Article Online
The temperature dependent magnetic measurements of 1 in the
range of 4.5–380 K are shown in Fig. 2. The measurements were
performed during both heating (q) and cooling (Q) cycles, which
revealed a reversible ST behaviour with a wide hysteresis loop. At
380 K the value of xT is 3.02 emu K mol21, which is lower than
the expected value for a high-spin (HS) iron(II) ion in the S = 2
state. This shows that the thermally saturated HS state is situated
above 380 K, which was the upper limit of the experimental set up.
The ST temperatures for the heating (T1/2q = 328 K) and cooling
(T1/2Q = 318 K) modes revealed the occurrence of a ca. 10 K wide
thermal hysteresis. Importantly, the observed T1/2 value of 1 is
above room temperature. Below 225 K, the xT value was almost
constant and reached a minimum value of 0.38 emu K mol21 at
4.5 K. The residual magnetic moment most likely arises from the
high spin state of the water end-capped iron(II) or iron(III)
coordination sites at the linear chain ends. Indeed, the presence of
water molecules in 1 is supported by FTIR spectroscopy (nO–H at
3440 cm21, broad) and FT-ICR mass analysis (ESI,{ Fig. S1).
The zero-field 57Fe Mo¨ssbauer spectra and the hyperfine
parameters of complex 1 measured at 300, 233 and 182 K are
shown in Fig. 3 and Table 1, respectively. Least squares analysis of
the spectra revealed the presence of a mixture of LS and HS states.
At all temperatures, the values of the main doublet parameters are
typical for LS state iron(II) and the minor doublet parameters
correspond to the HS state iron(II). Significantly at 300 K the
relative area of distribution of the LS fraction is ca. 86%, which
further confirms the ST property associated with complex 1. Upon
decreasing the temperature from 300 to 182 K the LS fraction has
increased further from 86% to 91% at the expense of the HS
Fig. 2 The xT vs. T plot for complex 1 from 4.5 to 380 K (Q cooling
mode and q heating mode) measured with a 1000 Oe applied DC
magnetic field. The sample was cooled from 300 to 4.5 K and xT was first
measured upon warming from 4.5 to 380 K, followed by the cooling
mode. Inset shows the dxT/dT curve.
fraction. At 182 K, the presence of 91% LS fraction (diso
=
0.390(1) mm s21; DEQ = 0.738(2) mm s21; C = 0.35(1) mm s21 and
AR = 91%) indicates the near completion of the ST event in
complex 1. Overall, the relative areas of distribution of the HS and
LS components are in good agreement with the magnetic
susceptibility data. Detailed high temperature Mo¨ssbauer spectro-
scopic investigations are currently in progress.
In conclusion, a distinctive iron(II) coordination chain was
synthesized by exploiting for the first time a new back-to-back
ligand with 2,6-di(pyrazol-1-yl)pyridine as terminal binding sites.
The complex undergoes a reversible ST at 323 K with a ca. 10 K
wide hysteresis loop. This approach provides the general concept
for achieving technologically appealing high T1/2 systems by
supramolecular interlinking of the ST-iron(II) centers.5 Both the
high temperature shift of T1/2 and the widening of the hysteresis
can be attributed to the increased cooperativity in the rigid
metallo-supramolecular ST system in comparison to the 0-D, non-
interlinked systems.4 Further investigations of the structural and
Fig. 3 Zero-field 57Fe Mo¨ssbauer spectra of complex 1 recorded at
different temperatures; the solid lines represent the best fit curves with the
parameters given in Table 1.
complex 1. Fourier-transform ion cyclotron resonance mass
spectrometry (FT-ICR MS) analysis of a methanol–DMF–
acetonitrile solution of complex 1 provides evidence for the
presence of a pentanuclear oligomeric unit [Fe5L5–8BF4?(OH2)6]2+
at m/z = 1518.5 (calcd m/z = 1518.81) together with fragmented
smaller nuclear entities (see ESI,{ S4). Since the bulk of compound
1 is not soluble in the same solution, complex 1 is supposed to
represent the higher chain homologue with more than five
repeating units. FTIR spectroscopy (1800–900 cm21 region) of
the precipitate of 1 shows a clear major shift of the ligand bands
and new vibrational modes for the metal complex 1 (see ESI,{ SI
1). In addition, it was shown that the reaction of an analogous,
linear ligand 1,4-bis(2,29:69,20-terpyridin-49-yl)benzene ligand with
metal(II) ions leads to linear coordination polymers.6–8 The
formation of cyclic oligomers can be excluded because of the
rod-like shape of L. Hence, it is reasonable to assume that complex
1 possesses a linear polynuclear structure.
Table 1 Mo¨ssbauer data for complex 1 at different temperaturesa
T/K Component
d
iso/mm s21 DEQ/mm s21 C/mm s21 AR (%)
300 LS
HS
233 LS
HS
182 LS
HS
0.364(2)
0.96(3)
0.390(2)
1.10(6)
0.390(1)
1.2(1)
0.702(2)
1.91(4)
0.740(3)
2.2(1)
0.738(2)
2.6(1)
0.36(1)
0.86
0.37(1)
0.86
0.35(1)
0.86
86
14(1)
92
8(2)
91
9(2)
a
diso: isomer shift relative to a-Fe, DEQ: quadrupole splitting, C:
width of the line, AR: area ratio of the components AHS/Atot
Statistical standard deviations are in parentheses.
.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 2636–2638 | 2637

View Article Online
Transition in Transition Metal Compounds, Top. Curr. Chem., 2004, 233,
1; (c) E. Ko¨nig, Prog. Inorg. Chem., 1987, 35, 527; (d) E. Ko¨nig, Struct.
Bonding, 1991, 76, 51; (e) H. A. Goodwin, Y. Garcia and P. Gu¨tlich,
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) J. A. Real,
A. B. Gaspar, V. Niel and M. C. Mun˜oz, Coord. Chem. Rev., 2003, 236,
121; (h) M. A. Halcrow, Coord. Chem. Rev., 2005, 249, 2880.
2 (a) N. Bre´fuel, S. Shova, J. Lipkowski and J.-P. Tuchagues, Chem.
Mater., 2006, 18, 5467–5479; (b) T. Fujigaya, D. Jiang and T. Aida,
J. Am. Chem. Soc., 2003, 125, 14690–14691; (c) V. Niel, J. M. Martinez-
Agudo, M. C. Mun˜oz, A. B. Gaspar and J. A. Real, Inorg. Chem., 2001,
40, 3838; (d) O. Roubeau, J. G. Haasnoot, E. Codjovi, F. Varret and
J. Reedijk, Chem. Mater., 2002, 14, 2559–2566; (e) C. Rajadurai,
F. Schramm, S. Brink, O. Fuhr, R. Kruk, M. Ghafari and M. Ruben,
Inorg. Chem., 2006, 45, 10019–10021; (f) K. S. Min, A. DiPasquale,
A. L. Rheingold and J. S. Miller, Inorg. Chem., 2007, 46,
1048–1050.
3 (a) T. Kitazawa, Y. Gomi, M. Takahashi, M. Takeda, A. Enomoto,
T. Miyazaki and T. Enoki, J. Mater. Chem., 1996, 6, 119; (b) V. Niel,
A. L. Thompson, M. C. Mun˜oz, A. Galet, A. E. Goeta and J. A. Real,
Angew. Chem., Int. Ed., 2003, 42, 3760; (c) J. A. Real, E. Andre´s,
M. C. Mun˜oz, M. Julve, T. Granier, A. Bousseksou and F. Varret,
Science, 1995, 268, 265; (d) N. Moliner, M. C. Mun˜oz, S. Le´tard,
X. Solans, N. Mene´ndez, A. Goujon, F. Varret and J. A. Real, Inorg.
Chem., 2000, 39, 5390; (e) S. Cobo, M. Ga´bor, J. A. Real and
A. Bousseksou, Angew. Chem., Int. Ed., 2006, 45, 5786–5789; (f) V. Niel,
M. C. Mun˜oz, A. B. Gaspar, A. Galet, G. Levchenko and J. A. Real,
Chem.–Eur. J., 2002, 8, 2446; (g) G. J. Halder, C. Kepert, B. Moubaraki,
K. S. Murray and J. D. Cashion, Science, 2002, 298, 1762.
4 (a) J. M. Holland, J. A. McAllister, Z. Lu, C. A. Kilner, M. Thornton-
Pett and M. A. Halcrow, Chem. Commun., 2001, 577–578; (b)
C. Carbonera, J. S. Costa, V. A. Money, J. Elha¨ık, J. A. K. Howard,
M. A. Halcrow and J. F. Le´tard, Dalton Trans., 2006, 3058; (c)
R. Pritchard, C. A. Kilner and M. A. Halcrow, Chem. Commun., 2007,
577–579.
5 (a) M. Ruben, F. J. Rojo, J. Romero-Salguero and J.-M. Lehn, Angew.
Chem., Int. Ed., 2004, 43, 3644–3662; (b) M. Ruben, U. Ziener, J.-M.
Lehn, V. Ksenofontov, P. Gu¨tlich and G. B. M. Vaughan, Chem.–Eur.
J., 2005, 11, 94–100; (c) M. Ruben, E. Breuning, J.-M. Lehn,
V. Ksenofontov, F. Renz, P. Gu¨tlich and G. B. M. Vaughan, Chem.–
Eur. J., 2003, 9, 4422–4429; (d) 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, 2504–2507.
above room temperature ST characteristics of 1 at the single
molecule level are in progress.9
The authors thank the Deutsche Forschungsgemeinschaft
(DFG) for financial support within the frame of the project
‘‘Kondo Molecules’’. We sincerely thank Dr Oliver Hampe for
FT-ICR mass analysis.
Notes and references
{ 49,49--(1,4-Phenylene)bis(1,29:69,10-bispyrazolylpyridine) (L): A solution
of 1,4-dioxane (50 ml) containing 2 M Na2CO3 (5 ml) was taken in a clean
250 mL flask and N2 gas was bubbled into the solution for 10 min. To this
solution 4-iodo-2,6-di-pyrazol-1-ylpyridine (0.674 g, 2 mmol), 1,4-phenyl-
diboronic acid (0.166 g, 1 mmol) and Pd(PPh3)4 (0.200 g, 0.173 mmol) were
added and heated in the dark at 70 uC for 3 days. The solvent was
evaporated and the crude solid was extracted with a water–CH2Cl2 mixture
several times. The collected organic layers were combined and the
concentrated solution was filtered through a silica column using CH2Cl2
at first to remove coloured impurities and then with ethyl acetate to collect
a colourless solution, which upon evaporation yielded analytically pure
white powder of L. Yield ca. 210 mg; 22%. 1H NMR (300 MHz, CDCl3,
3
298 K): d 8.63 (dd, 4H, JH,H = 2.64 Hz, pyrazole), 8.12 (s, 4H), 7.98 (s,
4H), 7.81 (dd, 4H, 3JH,H = 1.7 Hz, pyrazole), 6.54 (m, 4H, 3JH,H = 4.14 Hz;
4JH,H = 1.7 Hz, pyrazole) ppm. 13C NMR (75 MHz, CDCl3, 298 K): d
153.1, 150.7, 142.5, 138.7, 127.9, 127.3, 108.0, 107.2 ppm. FTIR (KBr, n/
cm21) : 916, 939, 956, 999, 1016, 1036, 1070, 1095, 1124, 1159, 1194, 1206,
1252, 1285, 1311, 1395, 1435, 1461, 1520, 1542, 1561, 1578, 1608, 1738.
MALDI-TOF: m/z (relative intensity of isotopic distribution in %)
experiment: 496.63 (100%), 497.63 (35%), 498.63 (15%); simulation:
496.19 (100%), 497.19 (35%), 498.19 (5%).
§ Crystal data for L: C28H20N10; M = 496.54; orthorhombic; space group
˚
Pca21; a = 9.3026(19), b = 9.4892(19), c = 26.467(5) A; a, b, c = 90u; V =
2336.4(8) s3; crystal size = 0.6 6 0.1 6 0.03 mm; index ranges = 29 ¡ h
¡ 11, 211 ¡ k ¡ 10, 232 ¡ l ¡ 21; theta range for data collection =
2.15–25.68u; Z = 4; Dobsd = 1.412 g cm23; F(000) = 1032; m = 0.091 mm21
;
5658 reflections measured, 1308 unique (Rint = 0.0611), R(Fo) = 0.0527;
Rw(F) = 0.1095; GOF on F2 = 0.926; T = 200 K. CCDC 632648. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b702468a
" Coordination complex (1): 60 mg (0.12 mmol) of ligand L was dissolved
in a deaerated dichloromethane (75 ml). To this a deaerated methanolic
solution containing 81 mg (0.24 mmol) of Fe(BF4)?6H2O was added and
stirred for 3 days under N2 atmosphere. The solvents were evaporated from
the turbid solution and the formed orange precipitate was collected and
washed with dichloromethane–methanol and finally air dried. Yield 82 mg;
94%. FTIR (KBr, n/cm21): 913, 939, 954, 969, 1065 (b), 1084, 1224, 1174,
1212, 1261, 1288, 1339, 1408, 1465, 1497, 1526, 1550, 1566, 1623, 3136,
3440 (broad, O–H stretch). FT-ICR MS (in MeOH–CH3CN–DMF) :
[Fe5L5–8BF4?(OH2)6]2+ at m/z = 1518.5 (exptl), 1518.81 (calcd); [Fe5L4–
8BF4?(OH2)6]2+ at m/z = 1270.5 (exptl), 1270.73 (calcd); [Fe4L3–
5BF4?(OH2)3?H2O]2+ at m/z = 1021.75 (exptl), 1021.17 (calcd). Elemental
analysis (%) calcd for C28H20B2F8FeN10?H2O: C 45.16, H 2.96, N, 18.82;
Found C 45.58, H 3.47, N, 18.41.
6 M. Schu¨tte, D. G. Kurth, M. R. Linford, H. Co¨lfen and H. Mo¨hwald,
Angew. Chem., Int. Ed., 1998, 37, 2891–2893.
7 (a) D. G. Kurth, F. Caruso and C. Schu¨ler, Chem. Commun., 1999,
1579–1580; (b) Y. Bodenthin, U. Pietsch, H. Mo¨hwald and D. G. Kurth,
J. Am. Chem. Soc., 2005, 127, 3110–3114.
8 (a) E. C. Constable and A. M. W. Cargill Thompson, J. Chem. Soc.,
Dalton Trans., 1992, 3467–3475; (b) J. P. Collin, S. Guillerez, J. P. Sauvage,
F. Barigelletti, L. De Cola, L. Flamigni and V. Balzani, Inorg. Chem.,
1991, 30, 4230; (c) J. R. Kirchhoff, D. R. Mcmillin, P. A. Marnot and
J. P. Sauvage, J. Am. Chem. Soc., 1985, 107, 1138; J. P. Collin,
S. Guillerez and J. P. Sauvage, Inorg. Chem., 1990, 29, 5009; J. P. Collin,
S. Guillerez and J. P. Sauvage, J. Chem. Soc., Chem. Commun., 1989,
776.
1 (a) For a general overview, see: (a) O. Kahn and C. Jay Martinez,
Science, 1998, 279, 44; (b) P. Gu¨tlich and H. A. Goodwin, Spin
9 M. Ruben, J.-M. Lehn and P. Mu¨ller, Chem. Soc. Rev., 2006, 35,
1056–1067.
2638 | Chem. Commun., 2007, 2636–2638
This journal is ß The Royal Society of Chemistry 2007