Wide thermal hysteresis for the mononuclear Spin-Crossover compound cis-Bis(thiocyanato)bis[N-(2'-pyridylmethylene)-4 -(phenylethynyl)anilino]iron(II)

Full text of DOI:10.1021/ja972441x

J. Am. Chem. Soc. 1997, 119, 10861-10862
10861
Wide Thermal Hysteresis for the Mononuclear
Spin-Crossover Compound
cis-Bis(thiocyanato)bis[N-(2′-pyridylmethylene)-
4-(phenylethynyl)anilino]iron(II)
Jean-Franc¸ois Le´tard,† Philippe Guionneau,†
Epiphane Codjovi,† Olivier Lavastre,‡ Georges Bravic,†
Daniel Chasseau,† and Olivier Kahn*,†
Laboratoire des Sciences Mole´culaires, Institut de
Chimie de la Matie`re Condense´e de Bordeaux
UPR CNRS n°9048, 33608 Pessac Cedex, France
Laboratoire de Chimie de Coordination Organique
UniVersite de Rennes I, Campus de Beaulieu
35042 Rennes, France
Figure 1. øMT versus T plots for 1 in both cooling and warming modes.
The temperature was varied at the rate of 1 K min-1 without over-
shooting. The sample consisted of about 20 mg of small single crystals.
ReceiVed July 21, 1997
The most spectacular example of molecular bistability is
probably offered by the spin-crossover phenomenon. Some dn,
with n ) 4 to 7, first-row transition metal ions in octahedral
surroundings may exhibit a crossover between low-spin (LS)
and high-spin (HS) states.1 To a first approximation, this
situation occurs when the quantum mechanical energy of the
LS state in its equilibrium geometry is slightly lower than the
quantum mechanical energy of the HS state, also in its
equilibrium energy. Above a certain temperature, the thermo-
dynamically stable state may be the HS state. This is due to
the fact that the entropy of the system in the HS state is much
larger than in the LS state (∆S > 0), and the gain is T∆S, where
T, the temperature, compensates the energy loss. This LS /
HS crossover can be thermally induced. When the process takes
place in the solid state, it may be cooperative if the intersite
interactions are strong enough. This cooperativity may lead to
very abrupt transitions along with thermal hystereses. The
thermal hysteresis width defines the temperature range of
bistability for the system.2 One of the main challenges is then
to design spin-crossover compounds exhibiting large bistability
range.
purely molecular lattices consisting of spin-crossover mono-
nuclear molecules could also exhibit wide thermal hysteresis
loops. We report here on a compound of that kind, namely,
cis-bis(thiocyanato)bis[N-(2′-pyridylmethylene)-4-(phenylethy-
nyl)anilino]iron(II) (1). The absence of solvent molecule in the
lattice eliminates the possibility of apparent hysteresis, resulting
from the synergy between LS f HS transformation and removal
of noncoordinated solvent molecules,4 as observed for instance
for [Fe(2-pic)3]Cl2‚H2O (2-pic ) 2-picolylamine).5
The ligand N-(2′-pyridylmethylene)-4-(phenylethynyl)aniline,6
noted hereafter as L, was prepared from 2-pyridinecarbaldehyde
and 4-(phenylethynyl)aniline,7 and the compound 18 was
synthesized under nitrogen by preparing first a solution of 3.5
× 10-4 mol of Fe(NCS)2 in 50 mL of methanol (from the
reaction of Fe(SO4)‚7H2O with KNCS) and adding 7 × 10-4
mol of L in 50 mL of methanol.9 Single crystals were obtained
by slow diffusion of the two solutions in a H-shape tube.
The spin-crossover regime for a sample of 1 made of single
crystals was investigated from the temperature dependence of
øMT, where øM is the molar magnetic susceptibility and T is
the temperature. At room temperature, øMT is equal to 3.5 cm3
K mol-1, which corresponds to what is expected for a HS state,
decreases smoothly down to 3.0 cm3 K mol-1 as T is lowered
down to 204 K, and then drops suddenly around T1/2V ) 194 K.
At 80 K, øMT is close to zero. In the warming mode, the abrupt
It has been suggested and experimentally confirmed that the
cooperativity can be magnified by designing polymeric struc-
tures in which the active sites are linked to each other by
chemical bridges. Some compounds of this kind have been
found to display thermal hysteresis widths reaching ca. 40 K.3
These polymeric compounds, however, are not strictly molecular
any more. The question we were faced with was to see whether
(4) Garcia, Y.; van Koningsbruggen P. J.; Codjovi, E.; Lapouyade, R.;
Kahn, O.; Rabardel, L. J. Mater. Chem. 1997, 7, 857.
† Institut de Chimie de la Matie`re Condense´e de Bordeaux.
(5) (a) Sorai, M.; Ensling, J.; Hasselbach, K. M.; Gu¨tlich, P. Chem. Phys.
1977, 20, 197. (b) Gu¨tlich, P.; Ko¨ppen, H.; Steinha¨user, H. G. Chem. Phys.
Lett. 1980, 74, 3.
‡ Universite de Rennes I.
(1) (a) Goodwin, H. A. Coord. Chem. ReV. 1976, 18, 293. (b) Gu¨tlich,
P. Struct. Bonding (Berlin) 1981, 44, 83. (c) Gu¨tlich, P.; Hauser, A. Coord.
Chem. ReV. 1990, 97, 1. (d) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew.
Chem., Int. Ed. Engl. 1994, 33, 2024 and references therein. (e) Ko¨nig, E.
Prog. Inorg. Chem. 1987, 35, 527. (f) Ko¨nig, E. Struct. Bonding (Berlin)
1991, 76, 51.
(6) 1H NMR (CDCl3, 250 MHz) δ: 8.8 (d, 1H), 8.6 (s, 1H), 8.2 (d, 1H),
7.9 (td, 1H), 7.7-7.3 (m, 10H, NH2). SM: m/e ) 283 (MH+). UV/vis
(CH3CN) λmax (log ꢀ): 293 (4.67), 339 (4.55).
(7) Synthesis of 4-(phenylethynyl)aniline: 0.5% mol of PdCl2(PPh3)2
and 0.5% mol of CuI were added to a carefully deoxygenated solution of
phenylacetylene (2.33 g, 23 mmol) and 4-iodoaniline (5 g, 23 mmol) in
diethylamine (50 mL). After 20 h at room temperature, the solvent was
removed in vacuo and the solid was extracted with diethyl ether. Filtration,
evaporation, and recrystallization from Et2O/pentane (1:3) gave pale yellow
crystals (2.9 g, 67% isolated yield). 1H NMR (CDCl3, 300 MHz) δ: 7.5
(m, 2H, Ph), 7.3 (m, 5H, Ph), 6.6 (m, 2H, Ph), 3.8 (br s, 2H, NH2). 13C
NMR (CDCl3, 75 MHz) δ: 146.74 (t, 2JCH ) 8.6 Hz, C-NH2), 133.02 (dd,
(2) (a) Kahn, O.; Launay, J. P. Chemtronics 1988, 3, 140. (b) Zarem-
bowitch, J.; Kahn, O. New J. Chem. 1991, 15, 181. (c) Kahn, O. Molecular
Magnetism; VCH: New York, 1993.
(3) (a) Vreugdenhil, W.; van Diemen, J. H.; de Graaff, R. A. G.;
Haasnoot, J. G.; Reegijk, J.; van der Kraan, A. M.; Kahn, O.; Zarembowitch,
J. Polyhedron 1990, 9, 2971. (b) Kahn, O.; Kro¨ber, J.; Jay, C. AdV. Mater.
1992, 4, 718. (c) Lavrenova, L. G.; Ikorskii, V. N.; Varnek, V. A.;
Oglezneva, I. M., Larionov S. V. Koord. Khim. 1986, 12, 207. (d)
Lavrenova, L. G.; Ikorskii, V. N.; Varnek, V. A.; Oglezneva, L. M.;
Larionov, S. V. Zh. Struk. Khim. 1993, 34, 145. (e) Sugiyarto, K. H.;
Goodwin, H. A. Aust. J. Chem. 1994, 47, 263. (f) Kro¨ber, J.; Audie`re, J.
P.; Claude, R.; Codjovi, E.; Kahn, O.; Haasnoot, J. G.; Grolie`re, F.; Jay C.;
Bousseksou, A.; Linare`s, J.; Varret, F.; Gonthier-Vassal, A. Chem. Mater.
1994, 6, 1404. (g) Lavrenova, L. G.; Ikorskii, V. N.; Varnek, V. A.;
Oglezneva, I. M.; Larionov, S. V. Polyhedron 1995, 14, 1333. (h) Kahn,
O.; Codjovi, E.; Garcia, Y.; van Koningsbruggen, P. J.; Lapouyade, R.;
Sommier, L. In Molecule-Based Magnetic Materials; Turnbull, M. M.,
Sugimoto, T., Thompson, L. K., Eds.; ACS Symposium Series 644;
American Chemical Society: Washington, DC, 1996.
2
1
2
1JCH ) 161 Hz, JCH ) 6.5 Hz), 131.41 (dt, JCH ) 162 Hz, JCH ) 6.3
1
2
1
Hz), 128.35 (dd, JCH ) 161 Hz, JCH ) 7.5 Hz), 127.73 (dt, JCH ) 161
2
2
1
Hz, JCH ) 7.5 Hz), 123.95 (t, JCH ) 7.2 Hz), 114.81 (dm, JCH ) 157
2
3
3
Hz), 112.61 (t, JCH ) 8.4 Hz), 90.22 (t, JCH ) 5.1 Hz), 87.41 (t, JCH
)
5.2 Hz).
(8) Anal. Calcd for FeC42H28N6S2 (1): C, 68.48; H, 3.80; N, 11.41; S,
8.70; Fe, 7.61. Found: C, 68.78; H, 3.77; N, 11.14; S, 8.85; Fe, 7.60. IR
(KBr, cm-1): 3054, 2060, 1592, 1502, 1441, 841, 758, 691. UV/vis (CH3-
CN) λmax (log ꢀ): 283 (4.72), 335 (4.47), 591 (3.05).
(9) Le´tard, J.-F.; Montant, S.; Guionneau, P.; Martin, P.; Le Calvez, A.;
Freysz, E.; Chasseau, D.; Lapouyade, R.; Kahn, O. J. Chem. Soc., Chem.
Commun. 1997, 745.
S0002-7863(97)02441-4 CCC: $14.00 © 1997 American Chemical Society

10862 J. Am. Chem. Soc., Vol. 119, No. 44, 1997
Communications to the Editor
variation of øMT was observed around T1/2v ) 231 K (Figure
1). T1/2V and T1/2v are defined as the inversion temperatures for
which there are 50% of LS and 50 % of HS molecules in the
cooling and warming modes, respectively. Successive thermal
cycles did not modify the thermal hysteresis loop. It was also
checked that the shape of this loop did not depend on the rate
of the temperature variation, provided that this rate is lower
than 1 K min-1
.
The crystal structure of 1 was solved both at room temperature
in the HS state and at 140 K in the LS state.10 Those two
structures are compared in Figure 2. The LS / HS crossover
is accompanied by a crystallographic phase transition, between
orthorhombic, Pccn, in the LS state and monoclinic, P21/c, in
the HS state. Quite unexpectedly, the symmetry of the low-
temperature phase is higher than that of the high-temperature
phase. The iron atoms are located on 2-fold symmetry axes in
the LS state, while they are in general positions in the HS state.
As expected, the Fe-N bond lengths are significantly shorter,
and the FeN6 core is less distorted in the LS than in the HS
state. Probably the key feature of the LS and HS structures
lies in the presence of very short contacts (3.45 Å in both the
LS and HS states) between phenyl rings belonging to adjacent
molecules. Furthermore, the Fe(NCS)2 linkage of each molecule
is closely nested with the FeL2 linkage of an adjacent molecule
along the c-axis in the LS state (a-axis in the HS state). The
conjunction of this interlocking with the short contacts between
phenyl rings confers a two-dimensional character to the structure
in both the LS (ac planes) and HS (ab planes) states.
The LS f HS crossover for 1 may also be induced optically
at 10 K within the SQUID cavity through irradiation of a single
crystal with the 528 nm line of a Kr+ laser coupled to an optical
fiber (LIESST effect). The yield of the transformation, however,
is far from being quantitative, in contrast with what was
observed with the same equipment for [Fe(ptrz)6](BF4)2 (ptrz
) N-propyltetrazole), in full agreement with the original
observations by Decurtins et al.11 The total relaxation of the
system from the HS to the LS state is above 70 K.
Figure 2. Molecular structure in the LS (140 K) and HS (293 K) states.
(Top) Projection of the LS form along the b-axis. The shortest iron-
iron separation is 8.73 Å (along the c-axis). (Bottom) Projection of the
HS form along the c-axis. The shortest iron-iron separation is 8.49 Å
(along the a-axis).
Let us mention that a similar compound, FeL2(NCSe)2,12 in
which NCSe- replaces NCS-, was also synthesized. This
compound shows a wide hysteresis loop with T1/2V ) 256 K
and T1/2v ) 306 K (on a powder sample). Room temperature
falls within the hysteresis loop.
The title compound, to the best of our knowledge, is the
mononuclear spin crossover species exhibiting the widest
thermal hysteresis loop (37 K), reproducible over successive
thermal cycles, reported so far.13 The cooperativity may be
attributed to both intermolecular π interactions between phenyl
rings and interlocking of the molecular units.
(10) Crystal data for C42H26N6S2Fe: M ) 734; at 293 K, monoclinic,
space group P21/c (Z ) 4); a ) 15.637(1) Å, b ) 14.566(8) Å, c ) 16.821-
(13) Å; R ) 90°, â ) 92.95(4)°, γ ) 90°; V ) 3826(4) Å3; Dc ) 1.27;
Fe-N(CS-) distances [Fe-N(2)) 2.056 Å and Fe-N(5)) 2.055 Å]; Fe-N
(organic ligand) distances [Fe-N(31) ) 2.164 Å, Fe-N(131) ) 2.167 Å,
Fe-N(26) ) 2.246 Å, and Fe-N(126) ) 2.270 Å]; at 140 K, orthorhombic,
space group Pccn (Z ) 8); a ) 14.357(7) Å, b ) 14.291(6) Å, c ) 17.448-
(13) Å; R ) 90°, â ) 90°, γ ) 90°; V ) 3580(4) Å3; Dc ) 1.36; [Fe-N(2)
) 1.948(5) Å, Fe-N(31) ) 1.946(5) Å, and Fe-N(26) ) 1.971(5) Å];
measurements were made with a Nonius CAD-4 diffractometer; Mo KR
(0.710 69 Å); crystal shape, black needles; Bragg angle θ < 65°; empirical
absorption correction 0.926 < T < 1.000; scan type, ω/θ; at 293 K (140
K) 5446 (5474) measured reflections, 2318 (1381) observed [I > 3σ(I)];
structural determination with MITHRIL package and structural refinement
with SHELX93; at 293 K (140 K) 460 (230) parameters; R ) 5.4% (4.0);
Rw ) 5.4% (4.1). The temperature-dependent single-crystal X-ray analyses
show that Fe-N(CS-) bond lengths are shortened by about 5% and Fe-
N(organic) are by 10%, which corresponds to 3% of the unit cell volume
(change of 100 Å3).
Supporting Information Available: Tables giving crystallographic
data, positional parameters, and U values (10 pages). See any current
masthead page for ordering and Internet access instructions.
JA972441X
(12) Anal. Calcd for Fe(L)2(NCSe)2, FeC42H28N6Se2: C, 60.72; H, 3.37;
N, 10.12; Se, 19.04; Fe, 6.75. Found: C, 60.64; H, 3.30; N, 9.68; Se, 18.84;
Fe, 6.76.
(13) For instance, see: (a) Ritter, G.; Ko¨nig, E.; Irler, W.; Goodwin, H.
A. Inorg. Chem. 1978, 17 (2), 224. (b) Ko¨nig, E.; Ritter, G.; Kulshreshtha,
S. K. Chem. ReV. 1985, 85, 219. (c) Bradley, G.; McKee, V.; Nelson, S.
M. J. Chem. Soc., Dalton Trans. 1978, 522.
(11) Decurtins, S.; Gu¨tlich, P.; Ko¨hler, C. P.; Spiering, H.; Hauser, A.
Chem. Phys. Lett. 1984, 105, 1.