Switching of Spin States Triggered by a Phase Transition: Spin-Crossover Properties of Self-Assembled Iron(II) Complexes with Alkyl-Tethered Triazole Ligands

Article abstract of DOI:10.1021/ja038088e

Iron(II) complexes of triazole derivatives having two C12 and C16 long alkyl chains, (C₁₂trz)Fe^II and (C₁₆trz)Fe^II, serve as novel spin-crossover materials, which display a spin-state transition in response to a phase transition. In contrast, a triazole complex with two C8 alkyl chains ((C₈trz)Fe^II) exhibits only a poor response. EXAFS and XRD analyses of (C₁₆trz)Fe^II indicate an interdigitating self-assembled structure of polynuclear iron(II) species. According to DSC, VT-IR, and VT-XRD profiles, the spin-state transition is triggered by melting of the interdigitating alkyl chains, which is likely responsible for the lock-and-release feature of the spin state. By virtue of the thermoreversibility of the phase transition, the spin crossover could be repeated without deterioration. Copyright

Full text of DOI:10.1021/ja038088e

Published on Web 11/08/2003
Switching of Spin States Triggered by a Phase Transition: Spin-Crossover
Properties of Self-Assembled Iron(II) Complexes with Alkyl-Tethered Triazole
Ligands
Tsuyohiko Fujigaya,† Dong-Lin Jiang,*,‡ and Takuzo Aida*,†,‡
Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan, and Aida Nanospace Project, Exploratory Research for AdVanced Technology
(ERATO), Japan Science and Technology Agency (JST), 2-41 Aomi, Koto-ku, Tokyo 135-0064, Japan
Received August 23, 2003; E-mail: jiang@nanospace.miraikan.jst.go.jp; aida@macro.t.u-tokyo.ac.jp
Metal complexes with d4-d7 electron configurations can adopt
Chart 1
two different magnetic states, that is, high-spin (HS) and low-spin
(LS) states, which can cross over thermally or by electronic
excitation.1 Spin-crossover phenomena have attracted attention in
relation to their potential applications in molecular electronics.2 An
important challenge is to fabricate spin-crossover materials that can
respond to external stimuli such as light and electric/magnetic fields.
Gu¨tlich and co-workers have reported liquid-crystalline (LC) ligands
for the preparation of iron(II) complexes,3 where one may expect
the LC-phase transitions could trigger the spin-state transition.
However, the phase transitions in that example occur at much higher
temperatures than for the spin crossover, so that these two transition
events are not synchronous.
Here, we report the first successful example of “phase transition”-
triggered spin crossover by use of self-assembled iron(II) complexes
having triazole ligands with two long alkyl chains (Cntrz)FeII (Chart
1). The newly designed triazole derivatives (Cntrz) serve as bidentate
ligands that covalently bridge the iron(II) centers to form rigid
coordination polymers. In the self-assembled state, the polynuclear
species thus formed are fastened by the interdigitation of the long
alkyl chains, resulting in a cooperative magnetic response among
the iron(II) sites. Cntrz ligands (n [number of carbon atoms in each
alkyl chain] ) 8, 12, and 16) were synthesized by coupling of the
corresponding 3,5-dialkoxybenzoic acids with 4-amino-1,2,4-tria-
zole and unambiguously characterized by 1H/13C NMR and FT-IR
spectroscopies as well as MALDI-TOF-MS spectrometry. Iron(II)
complexes (Cntrz)FeII were prepared by refluxing Cntrz with Fe-
(ClO4)2‚2H2O in THF.4
spin-state transition (Figure 1A). Thus, the spin-crossover profiles
of (Cntrz)FeII are strongly affected by the length of the alkyl chains.
EXAFS analysis4 of solid (C16trz)FeII at 296 K displayed peaks
at 1.7, 3.5, and 7 Å, assignable to Fe-N, neighboring Fe-Fe, and
linear Fe-Fe-Fe scatterings, respectively, indicating the presence
of iron(II) polynuclear chains.7 The X-ray diffraction (XRD) pattern4
of (C16trz)FeII at 296 K showed a sharp peak in the small angle
region with a d spacing of 36.5 Å. This value most likely suggests
an interdigitation of the long alkyl groups to give parallel-aligned
polynuclear iron(II) chains, whose center-to-center separation is
estimated to be 36 Å (Chart 1). A similar XRD pattern with a
smaller d spacing (30.2 Å) was observed for (C12trz)FeII. Therefore,
the iron(II) centers are fastened with one another not only by ligation
with the triazole ligands but also through interdigitation with the
long alkyl chains.
Infrared spectroscopy of (C16trz)FeII at 296 K displayed CH2
stretching vibrations at 2850 (νsym) and 2920 cm-1 (νanti), which
were blue-shifted to 2853 and 2923 cm-1, respectively, at 313 K
on heating. Thus, the alkyl chains of low-spin (C16trz)FeII are
crystallized and adopt a stretched conformation, while those of the
high-spin complex adopt a shrunken conformation. Furthermore,
these spectral changes were thermally reversible. We also inves-
tigated the variable-temperature XRD pattern of (C16trz)FeII, where
the d spacing of 36.5 Å, observed at 296 K, was decreased to 35.0
Å at 313 K on heating, and then reverted to the original value when
cooled to 296 K.4 In relation to these observations, differential
At 296 K, solid (C16trz)FeII was colored pink, characteristic of
d-d electronic transitions of low-spin iron(II) species. Discoloration
occurred upon heating to give a virtually white solid at 313 K. On
cooling below the phase transition, the white solid returned to its
original color.4 This thermochromism reflects the spin-state transi-
tion between LS and HS states. The magnetic susceptibility profile
of (C16trz)FeII (Figure 1C) showed that the øT value, on heating
from 70 K,5 gradually increased from 0.8 cm3 K mol-1 and then
displayed a jump from 2.0 to 2.9 cm3 K mol-1 at 298-328 K
(inflection temperature [Tc] ) 310 K), characteristic of the LS-to-
HS transition.6 On the other hand, when the resulting sample in
the HS state was cooled from 350 to 70 K, the øT value decreased
from 2.9 cm3 K mol-1 through a drop at 318-298 K with a
hysteresis loop width of 5 K. (C12trz)FeII with shorter alkyl chains
(Figure 1B) exhibited a spin-crossover profile similar to that of
(C16trz)FeII, but spin transition occurred at a lower temperature Tc
) 276 K. On the other hand, (C8trz)FeII hardly showed a clear
† The University of Tokyo.
‡ ERATO Aida Nanospace Project, JST.
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J. AM. CHEM. SOC. 2003, 125, 14690-14691
10.1021/ja038088e CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. DSC profiles of (Cntrz)FeII (n ) 8, 12, and 16) on (A) second
heating and (B) second cooling at a rate of 5 K min-1
.
spin state. The successful synchronization of spin crossover with
phase transition would provide an opportunity to switch the spin
state by external stimuli. Elaboration of self-assembling triazole
ligands and development of, for example, photoresponsive dopants
are the subjects worthy of further investigation.
Acknowledgment. We thank JASCO for VT-IR spectroscopy,
and Dr. T. Sasagawa and Prof. H. Takagi of the University of Tokyo
for magnetic susceptibility measurements.
Figure 1. Magnetic susceptibility profiles of (A) (Cntrz)FeII, (B) (C12trz)-
FeII, and (C) (C16trz)FeII.
Supporting Information Available: Details of the synthesis and
spectral data for Cntrz and (Cntrz)FeII (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
scanning calorimetry (DSC) of both (C16trz)FeII and (C12trz)FeII
displayed single endothermic and exothermic peaks, respectively
(Figure 2A and B).4,8 However, (C16trz)FeII showed higher transition
temperatures and larger |∆H| values than those of (C12trz)FeII. More
importantly, the transition temperatures in DSC on heating were
in good agreement with their spin-transition temperatures, Tc. In
sharp contrast, (C8trz)FeII exhibited very broad DSC peaks with
small |∆H| values. All of the above results clearly demonstrate that
the spin crossover of (Cntrz)FeII (n ) 12 and 16) is triggered by
the phase transition. At low temperatures, the crystalline alkyl chains
of (Cntrz)FeII, upon interdigitation, most likely lock the Fe-N bond
distance of the low-spin iron(II) complex. On heating, the above
lock can be released by melting of the alkyl chains, thereby
permitting elongation of the Fe-N bond, necessary for transition
to the HS state.2b By virtue of the thermoreversibility of the phase
transition, the spin crossover can be repeated without deterioration.
In summary, we have succeeded in the first demonstration of
synchronous spin crossover and phase transition, by use of self-
assembling triazole ligands with long alkyl chains for the poly-
nucleation of iron(II) species. This approach allowed mesoscopic
cooperativity among the magnetic species through interdigitation
of the alkyl chains and enabled a “lock-and-release” feature of the
References
(1) Beattie, J. K. AdV. Inorg. Chem. 1988, 32, 1-49.
(2) (a) Kahn, O.; Launay, J. P. Chemtronics 1988, 3, 140-151. (b) Kahn,
O.; Kro¨ber, J.; Jay, C. AdV. Mater. 1992, 4, 718-728. (c) Kahn, O.; Jay,
C. Science 1998, 279, 44-48. (d) Real, J. A.; Gaspar, A. B.; Niel, V.;
Munoz, M. C. Coord. Chem. ReV. 2002, 236, 121-141.
(3) Galyametdinov, Y.; Ksenofontov, V.; Prosvirin, A.; Ovchinnikov, I.;
Ivanova, G.; Gu¨tlich P.; Haase, W. Angew. Chem., Int. Ed. 2001, 40,
4269-4271.
(4) See Supporting Information.
(5) A high-spin character remained even at very low temperatures. A similar
trend has been reported for an analogous ClO4--carrying Fe(II) complex
with mono-octadecyltriazole and has been claimed to be due to the steric
bulks of the large counteranion and long alkyl chain, see: Roubeau, O.;
Alcazar G. J.; Balskus, E.; Kolnaar, J.; Haasnoot, J.; Reedijk, J. New J.
Chem. 2001, 25, 144-150.
(6) The theoretical magnetic susceptibilities of HS and LS states have been
estimated to be 3.0 and 0 cm3 K mol-1, respectively, see: Goodwin, H.
A. Coord. Chem. ReV. 1976, 18, 293-325.
(7) (a) Michalowicz, A.; Moscovic, J.; Garcia Y.; Kahn, O. J. Synchrotron
Radiat. 1999, 6, 231-232. (b) Kojima, N.; Murakami, Y.; Komatsu, T.;
Yokoyama, T. Synth. Met. 1999, 103, 2154. (c) Garcia, Y.; Kahn, O.;
Rabardel, L.; Chansou, B.; Salmon, L.; Tuchagues, J. P. Inorg. Chem.
1999, 38, 4663-4670.
(8) Badia, A.; Singh, S.; Deners, L.; Cuccia, L.; Brown, G. R.; Lennox, R.
B. Chem.-Eur. J. 1996, 2, 359-363.
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