Agust´ı et al.
{Fe(pyridine)2[Ni(CN)4]}.4 Later on, Real and co-workers
reported the isostructural [Pd(CN)4]2- and [Pt(CN)4]2- SCO
derivatives and the 3D system {Fe(pyrazine)[MII(CN)4]} (MII
) Ni, Pd, and Pt),5 which display hysteresis loops ca. 25 K
wide containing room temperature when conveniently
treated.6 It has been demonstrated recently that the spin
change in the Pt derivative can be triggered within the
bistable region using a pulsed laser around room temperature7
and that it can be grown as thin films on gold surfaces
without losing essentially its properties.8 More recently,
cooperative spin transitions have also been reported for the
2D polymers {Fe(5-bromopyrimidine)2[MII(CN)4]} (MII )
Ni, Pd, and Pt) with critical temperatures in the temperature
range 170-225 K.9 All of these compounds display a marked
change of color upon spin conversion. On the basis of the
results above, we started a systematic study that also included
the cyanometalate bridging ligands [MI(CN)2]- (MI ) Cu,
Ag, and Au). The [MI(CN)2]- building blocks have afforded
a number of singular 1-3D structures with a rich variety of
topologies and interesting properties. For instance, the
systems {Fe(L)x[MI(CN)2]2} · nG [MI ) Ag; L ) pyrazine
(x ) 1, nG ) 1 pyrazine), 4,4′-bipyridine (x ) 2), and
bis(pyridylethylene) (x ) 2)] are doubly interpenetrated 3D
polymers.10 The former is LS at 300 K, while the other two
complexes display strong cooperative spin transitions. Triply
interpenetrated 3D SCO networks with NbO topology have
also been reported for {Fe(3-cyanopyridine)2[MI(CN)2]2} ·
0.1 Å, which results in an abnormally large variation of the
unit cell volume. When the organic ligand L is pyrimidine,
a singular family of coordination SCO polymers was
obtained. The {Fe(pmd)(H2O)[MI(CN)2]2} · H2O (MI ) Ag
and Au) coordination polymers, constituted of triply inter-
penetrated 3D frameworks, undergo cooperative spin transi-
tions with thermal hysteresis (ca. 8 K) as well as a singular
reversible ligand-exchange reaction in the solid state involv-
ing coordinated water and pyrimidine ligands, giving the 3D
polymers {Fe(pmd)[MI(CN)2]2} (MI ) Ag and Au), which
display different magnetic behavior.12 Pressure-tuneable
thermal hysteresis and a piezohysteresis loop at room
temperature on the silver derivative have been demonstrated
from its magnetic and optical properties. Pressure allows one
to tune the hysteresis width and to place the hysteresis loop
at will in a large range of temperatures, including room
temperature, without losing its well-defined square shape.13
Three additional silver(I) polymers were isolated, two of
which are architectural isomers of {Fe(pmd)2[Ag(CN)2]2}14
and {Fe(pmd)[Ag(CN)2][Ag2(CN)3]}.15 The latter shows a
rather complicated self-interpenetrated 3D structure with
strong Ag · · · Ag contacts [3.286(2)-2.934(3) Å] dependent
on the spin state and a singular thermo- and photoinduced
two-step spin transition. More recently, a new series of
complexes {Fe(3-Xpy)2[M(CN)2]2} (MI ) Ag16 and Au17)
based on 3-halogenpyridine ligands (3-Xpy; X ) F, Cl, Br,
and I) was undertaken. Silver and gold derivatives display
similar structures made up of the stacking of pairs of slightly
corrugated 2D polymeric networks. The pairs of layers are
held together by strong metallophilic interactions [3.2727(11)-
2.9635(11) and 3.580(8)-3.0137(8) Å for silver and gold
derivatives, respectively]. These crystalline materials are fully
HS at 300 K. However, in the [Ag(CN)2]- polymers, the
3-Fpy and 3-Clpy derivatives undergo thermally induced two-
step and half-spin transitions, respectively, while only the
3-Fpy polymer displays a half-spin transition in the gold
derivatives. As usual, the spin transition in these compounds
is coupled with a marked color change from pale yellow in
the HS state to deep red in the LS state. Interestingly, the
silver derivatives afford a second type of red-colored
crystalline materials in the presence of an excess of 3-Brpy
or 3-Ipy where the LS state is strongly stabilized at 300 K.
In these clathrate materials, two additional 3-Xpy molecules
are included in the framework, one is coordinated to a
[Ag(CN)2]- anion and the other one remains as an uncoor-
dinated guest molecule, thereby inducing dissociation of the
double layers and, consequently, breaking the argentophilic
interactions. Inclusion of an uncoordinated 3-Ipy guest
molecule in the FeII-3-Ipy-[Au(CN)2]- system also afforded
11
2
nH2O (MI ) Ag or Au and n ) /3). Significant metallo-
philic interactions [3.256(2)-3.1593(6) and 3.4212(13)-
3.3952(17) Å for silver and gold derivatives, respectively]
occur between consecutive networks for both derivatives.
Interestingly, the Ag · · · Ag interactions strongly depend on
the spin state of the FeII atoms displaying a variation of ca.
(2) (a) Real, J. A.; Gaspar, A. B.; Mun˜oz, M. C. Dalton Trans. 2005,
2062. (b) Gaspar, A. B.; Ksenofontov, V.; Seredyuk, M.; Gu¨tlich, P.
Coord. Chem. ReV. 2005, 249, 2661. (c) Real, J. A.; Gaspar, A. B.;
Niel, V.; Mun˜oz, M. C. Coord. Chem. ReV. 2003, 236, 121. (d)
Bousseksou, A.; Molna´r, G.; Matouzenko, G. Eur. J. Inorg. Chem.
2004, 4353.
(3) (a) Vreugdenhil, W.; van Diemen, J. H.; De Graaff, R. A. G.; Haasnoot,
J. G.; Reedijk, J.; Kahn, O.; Zarembowitch, J. Polyhedron 1990, 9,
2971. (b) Real, J. A.; Andre´s, E.; Mun˜oz, M. C.; Julve, M.; Granier,
T.; Bousseksou, A.; Varret, F. Science 1995, 268, 265. (c) Kahn, O.;
Martinez, J. C. Science 1998, 279, 44. (d) Garcia, Y.; Kahn, O.;
Rabardel, L.; Chansou, B.; Salmon, L.; Tuchagues, J. P. Inorg. Chem.
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K. S.; Cashion, J. D. Science 2002, 298, 1762. (f) Garcia, Y.; Niel,
V.; Real, J. A. Top. Curr. Chem. 2004, 233, 229.
(4) Kitazawa, T.; Gomi, Y.; Takahashi, M.; Takeda, M.; Enemoto, A.;
Miyazaki, T.; Enoki, T. J. Mater. Chem. 1996, 6, 119.
(5) Niel, V.; Martinez-Agudo, J. M.; Mun˜oz, M. C.; Gaspar, A. B.; Real,
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(6) Tayagaki, T.; Galet, A.; Molna´r, G.; Mun˜oz, M. C.; Zwick, A.; Tanaka,
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(7) Bonhommeau, S.; Molna´r, G.; Galet, A.; Zwick, A.; Real, J. A.;
McGarvey, J. J.; Bousseksou, A. Angew. Chem., Int. Ed. 2005, 44,
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(8) (a) Cobo, S.; Molna´r, G.; Real, J. A.; Bousseksou, A. Angew. Chem.,
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3372 Inorganic Chemistry, Vol. 48, No. 8, 2009