Spin crossover and paramagnetic behaviour in two-dimensional iron(II) coordination polymers with stilbazole push-pull ligands

Article abstract of DOI:10.1071/CH09100

The suitability of the stilbazole pushpull ligands, 4′- dimethylaminostilbazole (DMAS) and 4′-diethylaminostilbazole (DEAS), for the construction of bimetallic Fe^IIAg^I/Au^I cyanide-based coordination polymers that exhibit s

Full text of DOI:10.1071/CH09100

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2009, 62, 1155–1165
Spin Crossover and Paramagnetic Behaviour in
Two-Dimensional Iron(II) Coordination Polymers
with Stilbazole Push–Pull Ligands∗
Gloria Agustí,A Ana Belén Gaspar,A,D M. Carmen Muñoz,B Pascal G. Lacroix,C
and José Antonio RealA,D
AInstitut de Ciència Molecular (ICMOL)/Departament de Química Inorgànica,
Universitat de València, Edifici d’Instituts de Paterna, Apartat de Correus 22085,
46071 València, Spain.
BDepartament de Física Aplicada, Universitat Politécnica de València, Camí de Vera s/n,
46022 València, Spain.
CLaboratoire de Chimie de Coordination, CNRS, 205 route de Narbonne, Toulouse, France.
DCorresponding authors. Email: ana.b.gaspar@uv.es; jose.a.real@uv.es
The suitability of the stilbazole push–pull ligands, 4ꢀ-dimethylaminostilbazole (DMAS) and 4ꢀ-diethylaminostilbazole
(DEAS), for the construction of bimetallic FeII–AgI/AuI cyanide-based coordination polymers that exhibit spin crossover
properties is investigated. The structural and physical characterization of four novel two-dimensional FeII polymers for-
mulated as {Fe(DMAS)2[Ag(DMAS)(CN)2]2} (1) and {Fe(L)2[M(CN)2]2} (L = DMAS, M =Au (2); DEAS, Ag (3);
DEAS, Au (4)) is reported. Polymers 1 and 4 are paramagnetic over the whole range of temperatures studied (5–300 K),
whereas 2 and 3 exhibit spin crossover properties.
Manuscript received: 19 February 2009.
Manuscript accepted: 7 April 2009.
Introduction
hyperpolarizability upon spin conversion is directly related to
the structural changes, which accompany the process. This pos-
sibility has also been envisioned by Létard et al. at the level of
the cubic NLO response for the SCO compound [FeL2(NCS)2]
(L = N-(2-pyridylmethylene)aminobiphenyl).[10] The influence
of the SCO process on the quadratic NLO response in molecular
materials still waits for experimental confirmation though.
As a first step in the development of FeII SCO coordina-
tion polymers that exhibit a quadratic NLO response, we aim to
investigate the suitability of stilbazole push–pull ligands and
the metal–cyanide counter-ions [MI(CN)2]− (MI =Ag, Au),
[MII(CN)4]2−, and (MII = Ni, Pd, Pt, Cd) for the construction
of FeII coordination polymers with SCO properties.
Functional coordination polymers with switch properties and
memory transduction are of considerable interest in view of their
potential technological applications. The construction of coordi-
nation compounds that display physical and chemical properties
ranging from magnetism, conductivity, or optical properties
to functions as sorption–desorption, exchange, separation, and
catalysis is an important topic in current chemistry.[1,2]
The incorporation of mechanically, electronically, or opti-
cally active building blocks as essential structural components in
the construction of such functional materials has been scarcely
explored.[1c,3] In this regard, the use of FeII spin crossover
(SCO) building blocks for the construction of functional coor-
dination polymers has been shown as a suitable strategy as
they change reversibly their magnetic, structural, dielectric, and
optical properties in response to stimuli such as a variation of
temperature,[4] pressure,[4a,5] guest sorption,[6c] structural phase
transition (crystal–liquid crystal),[7] and light irradiation.[4a,8]
Recently, Lacroix and coworkers have reported on a com-
putational approach aimed at investigating possible quadratic
non-linear optical (NLO) properties modulation achieved upon
spin transition in an iron(ii) Schiff base complex [Fe(5-
NO2-sal-N(1,4,7,10))].[9] The investigation has revealed that
the S = 2 ↔ S = 0 SCO process should result in an increase
of the quadratic NLO response by ∼25% of the initial
value. It is suggested that the enhancement of the molecular
Cyanide-bridged FeII coordination polymers have been
shown to exhibit interesting thermal-, pressure-, and light-
induced SCO properties[11–17] but they can also combine their
cooperative spin transition properties (magnetic, chromatic,
and structural) with different chemical properties such as
crystalline-state ligand-substitution reactions with remarkable
structural changes[13d] or metallophilicity.[14a,17] Stilbazolium-
based units with extremely large NLO responses have been
widely investigated since the 1980s in Langmuir–Blodgett
films,[18] self-assembled multilayers,[19] intercalated NLO
materials,[20] or single crystals,[21] with second harmonic gen-
eration (SHG) efficiencies up to 1000 times that of urea in some
cases.[21a]
∗Dedicated to Professor Keith S. Murray on the occasion of his 65th birthday.
© CSIRO 2009
10.1071/CH09100
0004-9425/09/091155

RESEARCH FRONT
1156
G. Agustí et al.
Table 1. Crystal data for 1, 3, and 4
Parameter
1
3
4
293 K
293 K
293 K
100 K
Empirical formula
Mr
Crystal system
Space group
a [Å]
C64H64N12Ag2Fe
1272.86
Triclinic
P-1
9.94500(10)
10.3290(2)
32.8450(9)
88.7530(10)
87.4150(10)
61.2120(10)
2953.90(10)
2
C38H40N8Ag2Fe
880.37
Monoclinic
P21/c
9.8050(3)
14.8280(4)
15.3100(5)
C57H60N12Ag3Fe1.5
1320.56
Monoclinic
P21/c
12.9300(2)
14.5500(2)
30.5520(5)
C38H40N8Au2Fe
1058.56
Monoclinic
P21/c
9.8610(4)
14.6920(6)
15.1840(5)
b [Å]
c [Å]
α [◦]
β [◦]
121.8050(10)
107.7360(10)
121.596(3)
γ [◦]
V [Å3]
Z
1891.67(10)
2
1.546
888
1.441
5474.60(14)
4
1.602
2664
1873.73(12)
2
1.876
1016
8.231
Dc [mg cm−3
F(000)
]
1.431
1304
0.949
µ (MoKα) [mm−1
]
1.494
Crystal size [mm3]
No. of unique reflections
No. of reflections [I > 2σ(I)]
R1 [I > 2σ(I)]A
0.02 × 0.02 × 0.04
0.02 × 0.04 × 0.04
0.03 × 0.04 × 0.06
8825
4254
12252
7748
0.0519
0.1289
0.914
4246
3065
0.0792
0.1469
0.940
2971
3208
0.0477
0.1386
0.855
0.0345
0.0872
1.051
wR [I > 2σ(I)]B
S
AR1 = ꢀ||Fo| − |Fc||/ꢀ|Fo|.
BwR = ꢀ[[w(Fo2 − Fc2)2]/ꢀ[w(F2o)2]]1/2, where w = 1/[σ2(Fo2) + (mP)2 + nP] [where P = (Fo2 + 2F2c)/3, m = 0.0889 (1), 0.1334 (3), 0.0934 (3 (120 K)), and
0.0570 (4); n = 0.0000 (1), 0.0000 (3), 2.9187 (3 (120 K)), and 4.1306 (4)].
Here we report the synthesis, and structural and physical
characterization of four novel two-dimensional (2D) FeII poly-
mers formulated as {Fe(DMAS)2[Ag(DMAS)(CN)2]2} (1) and
{Fe(L)2[M(CN)2]2} (L = DMAS, M =Au (2); DEAS, Ag (3);
DEAS, Au (4)), where DMAS = 4ꢀ-dimethylaminostilbazole
and DEAS = 4ꢀ-diethylaminostilbazole. Polymers 1 and 4 are
paramagnetic over the whole range of temperature studied
(5–300 K), whereas 2 and 3 exhibit SCO properties.
C(32)–Ag(1)–N(9) = 95.6(4)◦, C(33)–Ag(1)–N(11) = 105.7(5)◦,
and C(34)–Ag(1)–N(11) = 96.8(5)◦].
The three building blocks are interconnected to define
edge-sharing rhombuses {[Fe2[Ag(1)(DMAS)(CN)2][Ag(2)
(DMAS)(CN)2]}2 where the distances [Fe–Ag(1)–Fei] and
[Fe–Ag(2)–Feii] are 10.327(3) and 10.327(3) Å, respectively
[(i = −x, y, −z − 1/2), (ii = −x, y, −z + 1/2)] (Fig. 2a). The iron
atom lies at the inversion centre of an elongated octahedron. The
equatorial bond lengths are defined by the nitrogen atoms of four
[AgI(DMAS)(CN)2]− groups. They are shorter than those of the
axial positions occupied by the nitrogen atoms of the DMAS lig-
ands [Fe–N(3)eq = 2.164(10) Å, Fe–N(4)eq = 2.159(10) Å, Fe–
Results and Discussion
Crystal Structures of Compounds 1, 3, and 4
N(5)eq = 2.181(12) Å, Fe–N(6)eq = 2.154(13) Å, Fe–N(1)ax
=
The crystal structures of 1 and 4 have been solved at 293 K
while the structure of compound 3 has been solved at 293 and
100 K. Relevant crystallographic data as well as bond lengths
and angles are gathered in Tables 1 and 2, respectively.
2.216(10) Å, and Fe–N(2)ax = 2.224(11) Å]. The Fe–N average
bond length of 2.183(13) Å is consistent with a high spin (HS)
state for the FeII ions and matches with the reported magnetic
data at this temperature (see below).
The edges of the rhombuses strongly deviate from 180◦
since the angles defined by the connectors are C(31)–
Ag(1)–C(32) = 158.3(4)◦ and C(33)–Ag(2)–C(34) = 157.4(5)◦
(Fig. 2a). The complementary angles of this pseudo-rhombus
are 91.3(4)◦, 87.2(4)◦, 91.3(4)◦, and 90.6(4)◦. Furthermore, the
Ag(1)andAg(2)atomsare, respectively, 0.706(2)and0.702(2) Å
out of the plane containing the four Fe atoms but in opposite
directions.
Consequently the [FeN6] octahedrons are markedly tilted to
generate strongly corrugated 2D layers, which spread on the xy
plane and stack along the [001] direction (Fig. 2b). Two consec-
utive layers are organized in such a way that the iron atoms
of one layer are projected at a distance of 17.563(4) Å near
to the barycentre of the [Fe4] rhombuses of contiguous layers
(Fig. 2c).The space between two consecutive layers is filled with
the DMAS ligands of the Ag(1), Ag(2), and Fe sites that inter-
digitate along the [100] direction. There is no noticeable C· · ·C
Structure of 1
Compound 1 crystallizes in the triclinic P-1 space group.
Fig. 1 displays an ORTEP view at 293 K around the FeII
centre with the corresponding atom numbering scheme. The
structure is constituted of four geometrically distinct build-
ing blocks: an FeII ion, a DMAS ligand, and two pseudo-
trigonal [AgC2N3] sites. The pseudotrigonal [Ag(1)C2N3]
and [Ag(2)C2N3] sites are defined by the in-situ generated
[Ag(1)(DMAS)(CN)2]− and [Ag(2)(DMAS)(CN)2]− bridging
moieties. The four atoms of the pseudotrigonal site lie prac-
tically in the same plane with Ag–N distances considerably
longer than the Ag–C ones, Ag(1)–N(9) = 2.535(10) Å, Ag(2)–
N(11) = 2.543(11) Å
versus Ag(1)–C(31) = 2.059(13) Å,
Ag(1)–C(32) = 2.051(14) Å, Ag(2)–C(33) = 2.071(15) Å, and
Ag(2)–C(34) = 2.079(17) Å. The corresponding bond angles
strongly deviate from 120◦ [C(31)–Ag(1)–N(9) = 106.9(4)◦,

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Spin Crossover and Paramagnetic Behaviour of Fe(ii) Polymers Based on Stilbazole Push–Pull Ligands
1157
Table 2. Selected bond lengths [Å] and angles [◦] for 1, 3, and 4
3
4
1
3
4
1
293 K
293 K
293 K
293 K
293 K
100 K
293 K
100 K
Bond length
Fe–N(1)
Fe–N(2)
Fe–N(3)
Fe–N(4)
Fe–N(5)
Fe–N(6)
Ag–C(18)
Ag–C(19)
N(2)–Fe–N(3)
N(2)–Fe–N(4)
N(2)–Fe–N(5)
N(2)–Fe–N(6)
N(3)–Fe–N(4)
N(3)–Fe–N(5)
N(3)–Fe–N(6)
N(4)–Fe–N(5)
88.91(14)
91.4(2)
86.5(4)
88.5(4)
92.0(4)
92.6(4)
174.7(4)
87.2(4)
91.3(4)
91.3(4)
90.6(4)
175.1(4)
2.214(3)
2.170(3)
2.168(3)
2.209(6)
2.190(6)
2.193(6)
2.216(10)
2.224(11)
2.164(10)
2.159(10)
2.181(12)
2.154(13)
2.066(4)
2.062(4)
N(4)–Fe–N(6)
N(5)–Fe–N(6)
Au–C(18)
Au–C(19)
1.992(7)
1.990(6)
C(18)–Ag–C(19)
C(18)–Au–C(19)
N(9)–Ag(1)–C(31)
N(9)–Ag(1)–C(32)
C(31)–Ag(1)–C(32)
N(11)–Ag(2)–C(33)
N(11)–Ag(2)–C(34)
C(33)–Ag(2)–C(34)
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–N(3)
N(2)–Fe(1)–N(3)
N(5)–Fe(2)–N(6)
N(5)–Fe(2)–N(7)
N(5)–Fe(2)–N(8)
N(5)–Fe(2)–N(9)
N(5)–Fe(2)–N(12)
N(6)–Fe(2)–N(7)
N(6)–Fe(2)–N(8)
N(6)–Fe(2)–N(9)
N(6)–Fe(2)–N(12)
N(7)–Fe(2)–N(8)
N(7)–Fe(2)–N(9)
N(7)–Fe(2)–N(12)
N(8)–Fe(2)–N(9)
N(8)–Fe(2)–N(12)
N(9)–Fe(2)–N(12)
C(18)–Ag(1)–C(54)
C(19)–Ag(2)–C(55)
C(56)–Ag(3)–C(57)
174.9(2)
Ag(1)–N(9)
Ag(1)–C(31)
Ag(1)–C(32)
Ag(2)–N(11)
Ag(2)–C(33)
Ag(2)–C(34)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(2)–N(5)
Fe(2)–N(6)
Fe(2)–N(7)
Fe(2)–N(8)
Fe(2)–N(9)
Fe(2)–N(12)
Ag(1)–C(18)
Ag(1)–C(54)
Ag(2)–C(19)
Ag(2)–C(55)
Ag(3)–C(56)
Ag(3)–C(57)
2.535(10)
2.059(13)
2.051(14)
2.543(11)
2.071(15)
2.079(17)
177.9(3)
106.1(4)
95.6(4)
158.3(4)
105.7(5)
96.8(5)
2.009(4)
1.951(4)
1.947(4)
2.209(4)
2.203(4)
2.155(4)
2.163(4)
2.165(4)
2.158(4)
2.062(5)
2.059(5)
2.058(5)
2.048(5)
2.064(5)
2.057(5)
157.4(5)
89.9(2)
89.7(2)
90.2(2)
179.01(13)
89.28(14)
90.09(14)
90.30(14)
89.6(2)
91.66(14)
90.24(14)
89.37(14)
89.5(2)
88.83(14)
91.6(2)
178.8(2)
179.44(13)
91.2(2)
Bond angle
N(1)–Fe–N(2)
N(1)–Fe–N(3)
N(1)–Fe–N(4)
N(1)–Fe–N(5)
N(1)–Fe–N(6)
90.06(13)
89.77(14)
90.3(2)
90.2(2)
178.8(4)
93.0(4)
92.0(4)
86.9(4)
88.5(4)
88.4(2)
175.4(2)
174.5(2)
173.7(2)
contact between the DMAS ligands since the shortest distance
is ∼3.80 Å.
whose edge and angles are 10.6570(3) Å (3) [10.5640(4) Å (4)]
and 88.91(14) and 91.09(14)◦ (3) [88.6(2) and 91.4(2)◦ (4)],
respectively.TheoctahedralsurroundingoftheFeII ionisdefined
by the DEAS ligands that occupy the axial positions, whereas
the equatorial ones are occupied by the [MI(CN)2]− groups
(Fig. 5a). The average bond distances Fe–Nax = 2.214(3) Å (3)
[2.209(6) Å (4)] and Fe–Neq = 2.169(3) Å (3) [2.192(6) Å (4)]
indicate a HS state for the FeII ions. The edges of the squares
deviate slightly from 180◦ because the angles defined by the
connectors are C(18)–Ag–C(19) = 174.9(2)◦ and C(18)–Au–
C(19) = 177.9(3)◦. The AgI (3) and AuI (4) atoms are, respec-
tively, 0.390(2) and 0.250(2) Å out of the plane that contains the
four Fe atoms but in opposite directions.
Structures of 3 and 4
Compounds 3 and 4 are isostructural and crystallize in the P21/c
space group. Compound 3 undergoes a 33% spin transition as
the magnetic data indicates (see below); consequently, its crystal
structure has been investigated at temperatures above (293 K)
and below (100 K) the critical temperature Tc ≈ 140 K. Com-
pound 4 is paramagnetic between 5 and 300 K, and its crystal
structure has been studied at 293 K. Given that 3 and 4 are
isostructural at 293 K, their crystal structure will be described
together.
The {[Fe2[MI(CN)2]}2 motif decorated with the DEAS lig-
ands spread in the yz plane leads to a 2D polymeric structure.
The slightly corrugated layers stack in the [100] direction
with a distance between consecutive layers of 9.8050(3) Å (3)
[9.8610(4) Å (4)] (Fig. 5b). The space between layers is filled
with the DEAS ligands.
Figs 3 and 4 gather the ORTEP view around the Fe centre with
the corresponding atom numbering scheme for 3 and 4, respec-
tively. In difference to compound 1, compounds 3 and 4 are made
up of three building blocks: an FeII ion, a DEAS ligand, and
one [MIC2N2]− site (MI =Ag (3), Au (4)). The building blocks
interconnect to define edge-sharing squares {[Fe2[MI(CN)2]}2

RESEARCH FRONT
1158
G. Agustí et al.
C(63)
N(12)
C(60)
C(59)
C(58)
C(57)
C(56)
C(55)
C(52)
C(51)
C(64)
C(15)
(a)
C(14)
N(7)
C(61)
C(62)
C(11)
C(12)
C(13)
C(7)
C(10)
C(9)
C(8)
C(53)
C(6)
C(3)
C(50)
N(11)
C(2)
C(1)
C(54)
C(4)
C(5)
N(1)
N(3)
N(4Ј)
C(34)
N(6)
Fe
C(31)
C(33)
Ag(1)
Ag(2)
N(5Ј)
y
x
C(32)
N(5)
C(20)
C(35)
C(19)
C(36)
C(37)
C(40)
N(4)
N(2)
z
N(9)
C(39)
C(38)
C(16)
C(17)
(b)
C(18)
C(21)
C(22)
C(41)
C(23)
C(42)
C(28)
C(47)
C(46)
C(45)
C(24)
C(43)
C(25)
C(44)
N(10)
C(27)
C(26)
N(8)
C(30)
C(29)
C(49)
C(48)
Fig. 1. ORTEP diagram of (1) at 293 K with the corresponding atom
numbering scheme. Displacement ellipsoids are shown at 50% probability
levels.
z
xy
(c)
Compound 3 undergoes a crystallographic phase transition
concomitantly with the change of the spin state of ∼33% of
the FeII ions at Tc ≈ 140 K. There is no change of space group
but the cell parameters are considerable different at 120 K
(Table 1). The parameters a and b expand and contract, respec-
tively, by 3.1250(2) Å and 0.2780(3) Å, whereas c practically
doubles in value (30.5520(5) Å). In addition, β decreases down
to 107.7360(10)◦. Consequently, the volume cell expands from
1891.67(10) to 5474.60(14) Å3. The crystal structure changes
involve not only the FeII environment but the whole crystal struc-
ture. Fig. 6 displays an ORTEP view around the FeII centres at
100 K with the corresponding atom numbering scheme.
x
y
Fig. 2. Crystal structure of 1. (a) View of the edge-sharing rhombuses
{[Fe2[Ag(1)(DMAS)(CN)2][Ag(2)(DMAS)(CN)2]}2. (b) View of the 2D
layers in the xy plane, which stacks in the z direction. (c) View of the crystal
packing of the 2D layers in the z direction emphasizing the displacement
between consecutive 2D layers. Atom colour code: Fe: red; Ag(1): magenta;
Ag(2): yellow; N: green; C: blue.
There are two crystallographically independent FeII ions at
this temperature, Fe(1) and Fe(2), and three crystallographically
independent [AgI(CN)2]− groups. The average Fe–N bond
length for Fe(2) (2.176(4) Å) and Fe(1) (1.969(4) Å) agrees well
with the Fe–N bond distances typically found for an FeII ion in
the HS and low-spin (LS) states, respectively. There are six FeII
atoms per unit cell, two Fe(1) and four Fe(2). This singular fact
perfectly correlates with the reported magnetic data at this tem-
perature denoting that ∼33% of the FeII ions change spin state.
Each Fe(1) site is surrounded by four Fe(2) sites,
which are connected through the groups [Ag(1)(CN)2]− and
[Ag(2)(CN)2]−, whereas each Fe(2) site is connected to two
Fe(1) through the [Ag(1)(CN)2]− and [Ag(2)(CN)2]− groups
and to two Fe(2) sites through the [Ag(3)(CN)2]− group as
it is depicted in Fig. 7a. The [AgI(CN)2]− groups are almost
linear because C(18)–Ag(1)–C(54) = 175.4(2)◦, C(19)–Ag(2)–
C(55) = 174.5(2)◦, and C(56)–Ag(3)–C(57) = 173.7(2)◦. The
resulting edge-sharing rhombuses {[Fe(1)–Fe(2)–Fe(2)]}4
spread in the yz plane, which leads to a 2D polymeric structure.
The slightly corrugated layers stack in the [100] direction but
there are no remarkable intermolecular contacts between con-
secutive layers (Fig. 7b). The space between consecutive layers

RESEARCH FRONT
Spin Crossover and Paramagnetic Behaviour of Fe(ii) Polymers Based on Stilbazole Push–Pull Ligands
1159
C(14)
C(14)
C(16)
C(15)
C(17)
N(4)
C(15)
C(17)
C(16)
C(11)
N(4)
C(11)
C(10)
C(10)
C(9)
C(12)
C(12)
C(13)
C(9)
C(13)
C(8)
C(8)
C(7)
C(7)
C(3)
C(6)
C(6)
C(2)
C(1)
C(4)
C(5)
C(3)
C(4)
C(2)
C(1)
N(1)
C(19)
N(3)
C(5)
N(3)
N(1)
Fe
C(19)
Fe
N(2)
Au
C(18)
Ag
C(18)
N(2)
Fig. 3. ORTEP diagram of 3 at 293 K with the corresponding atom
numbering scheme. Displacement ellipsoids are shown at 50% probability
levels.
is filled by the DEAS ligands being of the distance 9.710(2) Å
(given as the Fe(2)· · ·Fe(2) distance).
Fig. 4. ORTEP diagram of 4 at 293 K with the corresponding atom
numbering scheme. Displacement ellipsoids are shown at 50% probability
levels.
Powder X-Ray Diffraction Study of Compound 2
The powder X-ray diffraction pattern (PXRD) of 2 was recorded
at 293 K. It is compared with those calculated for 1 and 4 from
the X-ray single-crystal data in SFig. 5 (Accessory Publication).
The diffraction patterns of compounds 1 and 2 present some
similarities, whereas they are very different to that found for 4.
Compounds 1 and 2 present an intense peak in the region of low
angles, at 2θ = 5.30◦ and 5.40◦ for 2 and 1, respectively. In the
case of 1, this peak corresponds to the reflection plane with a
Miller index (001) (d ≈ 16.391 Å) defined by {Fe[Ag(CN)2]}n
layers separated by the DMAS ligands. From these PXRD pro-
files one can conclude that although compounds 1 and 2 are
not isostructural their structures must be closely related. Taking
into account the analytical data (magnetism, energy-dispersive
X-ray analysis (EDXA), and CHN analysis) of compound 2 it
is reasonable to propose a similar 2D structure as reported for 1
but with simple [Au(CN)2]− building blocks. It is worth men-
tioning that, unlike AgI and CuI, expansion of the coordination
sphere of AuI to give a [Au(CN)2DMAS]− species during the
crystallization process is highly improbable because of the inert
nature of the [Au(CN)2]− anion.[17]
Magnetic Properties of Compounds 1–4
The magnetic data expressed in the form of χMT versus T, χM
being the molar magnetic susceptibility and T the temperature,
is shown in Figs 8 and 9 for compounds 1–4. The tempera-
ture dependence of the magnetic susceptibility was recorded on
microcrystalline samples over the 300 to 5 K range in a field of
1T at a rate of 2 K min−1. At 300 K, the χMT values 3.15 (1),
3.66 (2), 3.30 (3), and 3.69 (4) cm3 K mol−1 are in the range of
values expected for an FeII ion in the HS state.
For 1 and 4, this value is practically temperature independent
between 300 and 100 K. Compound 4 exhibits a slight decrease
of χMT between 100 and 70 K, which could be associated to the
spin transition of a small fraction of iron(ii) ions (less than 5%).
Below 70 K, χMT diminishes for both compounds most probably

RESEARCH FRONT
1160
G. Agustí et al.
second spin transition, which is much more continuous than the
first one. The continuous two-step spin transition observed in 2
can be considered practically complete because the χMT value at
5 K is 0.34 cm3 K mol−1. In the warming mode the temperature
dependence of χMT matches that of the cooling mode.
(a)
Like 2, compound 3 undergoes a spin transition but only
∼33% of the FeII centres change their spin state. The spin tran-
sition is more cooperative than that observed for 2 being centred
atT↓c = 138 K andT↑c = 144 K in the cooling and warming mode,
respectively (Tc is the critical temperature of the spin transition
for a first-order phase transition). The temperature dependence
of the magnetic susceptibility recorded in the warming mode
denotes a very narrow hysteresis of 6 K wide for 3. The χMT
value belowT↓c found for 3 indicates that around 66% of the FeII
remain in the HS state (2.3 cm3 K mol−1); the decrease in χMT
value below 100 K is ascribed to zero field splitting of the S = 2
spin state of FeII ions and/or intermolecular antiferromagnetic
interactions.
Compounds 1 and 4 show a paramagnetic behaviour over the
whole temperature range under study, whereas SCO behaviour
is observed in the case of compounds 2 and 3.
x
y
z
Differential Scanning Calorimetry of Compound 3
Differential scanning calorimetry (DSC) measurements for 3
were carried out in the 120–300 K temperature range at a rate
of 5 K min−1 in the cooling and warming mode. The thermal
dependence of the anomalous heat capacity, ꢁCp, obtained from
DSC measurements is shown for 3 in Fig. 10. For 3, anomalies
in the heat capacity appear in the cooling and warming mode,
(b)
respectively, at T↓c = 137 K and at T↑c = 146 K. In the warming
mode the anomaly in the ꢁCp appear at different temperatures,
denoting that the spin transition is accompanied by a few Kelvin
hysteresis width. These values agree reasonably well with those
observed from the χMT versus T plot. The small differences in
critical temperatures between DSC and magnetic data have their
origin in the different rate at which the experiments were done.
The derived thermodynamic parameters ꢁHSCO and ꢁSSCO for
compound 3 are 3.5 kJ mol−1 and 25.4 J K−1 mol−1
.
z
x
General Discussion
In this contribution we have reported the synthesis of stilbazole
ligands DEAS and DMAS and demonstrated their suitability in
combination with [Ag(CN2)]2− and [Au(CN2)]2− coordinating
counter-ions for the synthesis of 2D FeII coordination polymers
that exhibit SCO properties.
Although the synthesis of DMAS and DEAS has previously
been reported in the literature,[22,23] we have used a differ-
ent synthetic route, the so called ‘anil synthesis’,[24] which
provides higher yields. This synthetic method has been suc-
cessfully applied in the synthesis of numerous o, m, and
p-styrylpyridines.[25]
y
Fig. 5. Crystal structure of 3 and 4 at 293 K. (a) View of the edge-sharing
squares {[Fe2[MI(CN)2]}2 (MI =Ag (3), Au(4)). (b) View of the crystal
packing of the 2D layers in the x direction. Atom colour code: Fe: red; MI:
yellow; N: green; C: blue.
because of zero field splitting of the S = 2 spin state of FeII ions
and/or intermolecular antiferromagnetic interactions.
The χMT value remains practically constant for compound 2
down to 265 K. It then decreases abruptly reaching a plateau
centred at around 2.12 cm3 K mol−1 in the temperature range of
200–188 K over which χMT varies smoothly. The χMT value in
this flat region corresponds to ∼50% of the complete spin tran-
sition taking place in two differentiated steps. The first step has
a characteristic temperature T11/2 = 226 K (T1/2 is the character-
istic temperature at which the number of SCO molecules in both
spin states is equal;T1/2 is used in the case of second-order phase
transitions). BelowT = 188 K a second drop in the χMT versusT
curve centred at T21/2 = 132 K is observed; this corresponds to a
Compounds 1–4 have been synthesized in the form of sin-
gle crystals (twins in the case of 2) using a slow diffusion
technique in a H-shaped vessel in analogy with precedent
examples of cyanide-bridge FeII SCO polymers.[11–18] How-
ever, microcrystalline powders can also be obtained by direct
synthesis.[13a,17b]
The crystal structures of 1–4 are made up of layers of
[FeMI(CN)2]n decorated with DMAS or DEAS ligands coor-
dinated to the FeII ions and to the AgI ions in the case
of 1. The layers formed by sharing [FeMI(CN)2]n square-
like (3, 4) or rhombus-like (1) units stack in the crystal

RESEARCH FRONT
Spin Crossover and Paramagnetic Behaviour of Fe(ii) Polymers Based on Stilbazole Push–Pull Ligands
1161
C(52)
C(53)
C(51)
C(50)
C(47)
N(11)
C(48)
C(49)
C(44)
C(46)
C(45)
C(43)
C(42)
C(39)
C(38)
C(40)
C(41)
C(37)
N(6)
N(8)
Fe(2)
Fe(1)
N(2)
C(18)
Ag(1)
C(57)
N(3)
Ag(2) C(19)
Ag(3Ј)
N(12)
C(55)
C(54)
N(1)
C(5)
C(56)
N(9)
Ag(3)
N(7)
C(1)
N(5) Ag(1Ј)
C(2)
C(3)
C(6)
C(20)
C(21)
C(4)
C(24)
C(23)
C(26)
C(22)
C(25)
C(7)
C(8)
C(9)
C(27)
C(28)
C(29)
C(30)
C(32)
C(31)
C(13)
C(12)
C(10)
C(11)
N(10)
C(35)
N(4)
C(16)
C(17)
C(33)
C(34)
C(36)
C(14)
C(15)
Fig. 6. ORTEP diagram of 3 at 100 K with the corresponding atom numbering scheme. Displacement ellipsoids are
shown at 50% probability levels.
without noticeable contacts between adjacent layers. This is
an unprecedented result taking into account the compounds
{Fe(3-F-pyridine)2[Ag(CN)2]2} where [FeAg(CN)2]4 square-
like units interact by pairs that form double layers held together
by argentophilic interactions.[17c] However, it is clear that the
structure adopted by compounds 1–4 prevents any argento-
philic or aurophilic interactions because the distance between
adjacent layers imposed by the DMAS or DEAS ligands is
too long [Fe· · ·Fe = 17.563(4) Å (1), Fe· · ·Fe = 9.8050(3) Å (3),
Fe· · ·Fe = 9.8610(4) Å (4)].
that the spin transition takes place for 2 at higher tempera-
tures in comparison with {Fe(3-F-pyridine)2[Ag(CN)2]2}[17c]
and {Fe(py)2[Ag(CN)2]2}[11] where T11/2 and T21/2 are centred
around 160 and 90 K, respectively.
A much steeper conversion is observed for 3, which is
also centred at relatively low temperatures (T↓c = 138 K and
T↑c = 144 K). Approximately 33% of the FeII centres undergo
a spin-state transition that takes place concomitantly with a
crystallographic phase transition. Nevertheless, this magnetic
behaviour it is not surprising because there are some well
documented iron(ii) SCO compounds for which incomplete
spin conversion at low temperatures has been observed. This
behaviour has been interpreted in terms of: (i) the existence of
different Fe crystallographic sites,[17c] (ii) the existence of dif-
ferent interpenetrated networks where only the Fe centres of one
network undergo SCO,[14d] (iii) the kinetic effects at low tem-
perature that block the SCO process,[4] and (iv) the occurrence
of a crystallographic phase transition.[17b,c]
As far as the SCO properties are concerned, 2 and 3
undergo a spin state transition whereas 1 and 4 are para-
magnetic over the whole range of temperatures studied
(5–300 K). Compound 2 displays a two-step spin transition
possibly as a result of the occurrence of two crystallo-
graphically distinct iron(ii) sites as it has been evidenced
for the related compounds {Fe(3-F-pyridine)2[Ag(CN)2]2}[17c]
and {Fe(py)2[Ag(CN)2]2}.[11] Similarly to the latter com-
pounds, the two-step transition in 2 is poorly cooperative.
The high-temperature step takes place in an interval of 50 K
(T11/2 = 226 K), while the low-temperature step occurs in an
interval of around 80 K (T21/2 = 132 K). It is interesting to note
The overall enthalpy and entropy variations associated with
the spin transition in 3 were found to be ꢁH = 3.5 kJ mol−1
and ꢁS = 25.4 J K−1 mol−1.These values correspond to the spin
transition of 33% of the total Fe sites. Extrapolating the enthalpy

RESEARCH FRONT
1162
G. Agustí et al.
(a)
(a)
3.5
3
2.5
2
1.5
1
0.5
0
1
0
50
100
150
200
250
300
x
T [K]
(b)
y
z
4
3.5
3
(b)
2.5
2
1.5
1
0.5
0
2
y
x
z
0
50
100
150
200
250
300
T [K]
Fig. 7. Crystal structure of 3 120 K. (a) View of the edge-sharing romb-
huses {[Fe(1)–Fe(2)–Fe(2)]}4. (b) View of the crystal packing of the 2D
layers in the x direction. Atom colour code: Fe: red; Ag(1): orange; Ag(2):
yellow; Ag(3): pink; N: green; C: blue.
Fig. 8. Temperature dependent magnetic susceptibility measurements for
1 and 2 at atmospheric pressure (rate of measurements: 2 K min−1).
and entropy variations to 100% of spin conversion give us the
real ꢁH and ꢁS variations associated exclusively with the Fe(1)
site, namely 10.5 kJ mol−1 and 76.2 J K−1 mol−1. As expected,
the ꢁS values are, in general, significantly larger than the
entropy variation that results from the change in spin-only val-
ues (ꢁS = R ln [(2SHS + 1)/(2SLS + 1)] = 13.4 J K−1 mol−1 for
SHS = 2 and SLS = 0). The excess of entropy mainly corre-
sponds to the structural reorganization that takes place concomi-
tantly with the spin-state change. The structural reorganization
accounts mainly for the Fe–N bond distances, which are on
average 0.2 Å shorter in the LS state. The thermodynamic para-
meters found for 3 are comparable to those observed in the FeII
cyanide-bridge polymers.[12–17]
The paramagnetic behaviour of 4 can be understood in
terms of a decrease of the crystal field strength at the iron
centres. It is caused by the more electronegative character of
the Au atoms of the [Au(CN)2]− ligands in comparison with
the Ag atoms. Indeed, the Au–C bond length is observed to be
notably shorter than the Ag–C one (see for instance Table 2).
This fact is finally reflected as weaker donor and acceptor
capabilities of the N atom of the [Au(CN)2]− ligand.[12–17]
Nevertheless, for 1 it is difficult to understand the para-
magnetic behaviour because the Fe–N average bond length
(2.183(13) Å) is comparable to that observed for 3 and other
related SCO 2D polymers {Fe(3-X-pyridine)2[Ag(CN)2][Ag(3-
X-pyridine)(CN)2]}·3-X-py (X = Br, I).[17c]
A cooperative first-order spin transition is observed when
efficient propagation of the molecular changes that stem from
the SCO metal ion through the crystal lattice takes place. In
coordination polymers, an effective pathway of communication
is the covalent bond linking of the SCO centres, however, inter-
molecular contacts (π–π and/or hydrogen bonds), intermetallic
interactions (Cu· · ·Cu, Ag· · ·Ag, or Au· · ·Au), and guest or sol-
vent inclusion also play an important role in the cooperative
mechanism. Therefore, the weak cooperativity associated with
the SCO process in 2 and 3 is not surprising because the only
path of communication between FeII centres is the cyanide
linkage.[12–17]

RESEARCH FRONT
Spin Crossover and Paramagnetic Behaviour of Fe(ii) Polymers Based on Stilbazole Push–Pull Ligands
1163
(a)
3.5
3
1200
800
400
2.5
2
0
Ϫ400
Ϫ800
Ϫ1200
1.5
1
3
3
132
136
140
T [K]
144
148
0
50
100
150
200
250
300
Fig. 10. DSC measurements performed at the rate of 5 K min−1 for 3.
T [K]
(b)
attached to the sample holder, refrigerated with a flow of liquid
nitrogen, and stabilized at a temperature of 110 K. The sample
holder was kept in a dry-box under a flow of dry nitrogen gas
to avoid water condensation. The measurements were carried
out using around 20 mg of a powdered sample sealed in alu-
minium pans with a mechanical crimp. Temperature and heat
flow calibrations were made with standard samples of indium
by using its melting (429.6 K, 28.45 J g−1) transition. An overall
accuracy of 0.2 K in the temperature and 2% in the heat flow is
estimated. The thermodynamic parameters were analyzed with
Netzsch Proteus software (NETZSCH-Geraetebau GMBH). 1H
NMR spectroscopic measurements were done with an Avance
DRX Bruker 300 MHz spectrometer. Microanalysis was done
by using PV 9760 EDAX Microanalysis with a PHILIPS XL 30
ESEM scanning electron microscope.
3.5
3
2.5
2
1.5
1
0.5
0
4
Single-Crystal X-Ray Diffraction
Diffraction data for 1, 3, and 4 were collected with a Nonius
Kappa-CCD single crystal diffractometer using MoKα radia-
tion (λ 0.71073 Å). A multi-scan absorption correction was
found to have no significant effect on the refinement results.
The structures were solved by direct methods using SHELXS-97
and refined by full-matrix least-squares on F2 using SHELXL-
97.[26] All non-hydrogen atoms were refined anisotropically.
CCDC 718239–718242 contain the supplementary crystallo-
graphic data for compounds 1, 3, and 4. This data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
0
50
100
150
200
250
300
T [K]
Fig. 9. Temperature dependent magnetic susceptibility measurements for
3 and 4 at atmospheric pressure (rate of measurements: 2 K min−1).
In summary, we have demonstrated the possibility to obtain
FeII SCO polymers that incorporate stilbazole push–pull lig-
ands. A further step in this research is to investigate the possible
quadratic NLO response of these compounds.
Experimental
PXRD
Physical Measurements
The equipment used for the PXRD characterization of compound
2 was a Seifert XRD 3003 TT diffractometer, with Bragg–
Brentano geometry and a Cu tube working at 40 kV with Ni filter
(0.3 mm primary slit, 0.3 mm secondary slit, 0.2 mm detector
slit, and scintillation detector).
Variable-temperature magnetic susceptibility measurements of
samples 1–4 (20–30 mg) were recorded with a Quantum Design
MPMS2 SQUID susceptometer equipped with a 5.5T magnet,
operating at 1T and at temperatures from 1.8 to 375 K. The
susceptometer was calibrated with (NH4)2Mn(SO4)2·12H2O.
Experimental susceptibilities were corrected for diamagnetism
of the constituent atoms by the use of Pascal’s constants. Calori-
metric measurements have been performed using a differential
scanning calorimeter (Mettler Toledo DSC 821e). Low tem-
peratures were obtained with an aluminium block, which was
Materials
K[Ag(CN)2], K[Au(CN)2], Fe(BF4)2·6H2O, and all organic
reagents and solvents were purchased from commercial sources
and used as received.

RESEARCH FRONT
1164
G. Agustí et al.
7Ј
7Ј
8Ј
N
N
6Ј
6Ј
5Ј
4Ј
8
5Ј
4Ј
5
6
5
6
7
7
4
2
1
2
4
1
N
N
N
N
3Ј
N
3Ј
N
3
3
2Ј
1Ј
1Ј 2Ј
Scheme 1.
Scheme 2.
Synthesis of DMAS and DEAS
7.27 (H3, 1H, d, J 11.09), 7.24 (H2ꢀ, 2H, d, J 5.99), 7.11 (H5ꢀ,
2H, d, J 8.83), 6.77 (H4, 1H, d, J 16.17), 6.67 (H6, 2H, d, J 8.88),
6.63 (H3ꢀ, 1H, d, J 11.73), 6.52 (H6ꢀ, 2H, d, J 8.92), 6.24 (H4ꢀ,
1H, d, J 12.14), 3.40 (H7, 2H, q, J 7.03), 3.35 (H7ꢀ, 2H, q, J
7.06), 1.20 (H8, 3H, t, J 7.04), 1.15 (H8ꢀ, 3H, t, J 7.07).
Schemes of the ligands’ synthesis are given in SFigs 1 and
2 (see Accessory Publication). The syntheses were performed
under argon atmosphere and all glassware used was necessar-
ily dried previously in the oven. 4-Methylpyridine (1.78 mL,
20 mmol) and sodium hydride (0.5 g, 21 mmol) were mixed
with 10 mL of distilled N,Nꢀ-dimethylformamide (DMF). The
mixture was heated at 60◦C for 2 h. After this time the
correspondent aldehyde (4-(dimethylamino)benzaldehyde or 4-
(diethylamino)benzaldehyde) was added and the mixture was
kept at this temperature for 7 h.The resulting solution was cooled
to room temperature and poured onto 150 mL of methanol. Yel-
low bright precipitates appeared immediately.They were filtered
off and washed with a mixture of methanol/cold water (1/1) and
dried under vacuum. Yield: 55% (DMAS) and 62% (DEAS).
1H NMR spectra of the ligands were recorded at 300 MHz
(SFigs 3 and 4, see Accessory Publication). It is worth noting
that these stilbazole ligands undergo isomerization in solution
at room temperature (see Schemes 1 and 2). Therefore, signals
that belong to both of the isomers can be seen in the 1H NMR
spectra. For that reason, the spectra were recorded using two
different deuterated solvents, deuterated chloroform (CDCl3),
and deuterated dimethyl sulfoxide ((D6)DMSO). The ratio of
both cis and trans isomers is not constant in solution and it
depends on the polarity of the solvent. For both of the ligands the
ratio of the cis isomer is higher in (D6)DMSO, while the majority
is trans in CDCl3. The assignment of the signals was as follows:
Synthesis of Compounds 1–4
Synthesis of compounds 1–4 was performed using slow dif-
fusion H-shaped vessels under an argon atmosphere. One side
of the H-shaped vessel contained a mixture of Fe(BF4)2·6H2O
(0.148 mmol, 50 mg) and DMAS or DEAS ligands (0.29 mmol,
66 and 75 mg, respectively) in methanol/dichloromethane (1/1,
2 mL). The other side contained a water solution (2 mL) of
KM(CN)2 (0, 59 mg (Ag), and 85 mg (Au)). The H-shaped ves-
sel was filled with methanol. Crystals of 1, 3, and 4 suitable for
single crystal X-ray analysis were obtained after 3 weeks while
compound 2 was obtained as twins.
{Fe(DMAS)2[Ag(DMAS)(CN)2]2} (1)
Prismaticorangecrystals.Yield:84%. νmax (KBr)/cm−1 2146
ν(C≡N), 1592 ν(C=N). (Found: C 56.01, H 4.73, N 13.21. Calc.
for C64H64Ag2Fe N12: C 56.13, H 4.61, N 13.36%.) EDXA
found: 33% Fe, 67% Ag.
{Fe(DMAS)2[Au(CN)2]2} (2)
Dark red twins.Yield: 62%. νmax (KBr)/cm−1 2171 ν(C≡N),
1597 ν(C=N). (Found: C 28.93, H 2.16, N 10.99. Calc. for
C32H36Au2FeN8: C 29.33, H 2.07, N 10.80%.) EDXA found:
35% Fe, 65% Au.
DMAS
δH (300 MHz, (D6)DMSO) 8.46 (H1, 2H, d, J 5.98), 8.44
(H1ꢀ, 2H, d, J 6.04), 7.48 (H2, 2H, d, J 8.58), 7.46 (H5, 2H, d, J
5.94), 7.42 (H3, 1H, d, J 16.42), 7.22 (H2ꢀ, 2H, d, J 6.00), 7.07
(H5ꢀ, 2H, d, J 8.83), 6.94 (H4, 1H, d, J 16.39), 6.73 (H6, 2H, d, J
8.83), 6.66 (H3ꢀ, 1H, d, J 12.24), 6.60 (H6ꢀ, 2H, d, J 8.87), 6.32
(H4ꢀ, 1H, d, J 12.21), 2.95 (H7, 3H, s), 2.89 (H7ꢀ, 3H, s).
δH (300 MHz, CDCl3) 8.49 (H1, 2H, d, J 4.98), 8.45 (H1ꢀ,
2H, d, J 5.20), 7.44 (H2, 2H, d, J 8.84), 7.38 (H5, 2H, d, J 6.17),
7.30 (H3, 1H, d, J 16.12), 7.24 (H2ꢀ, 2H, d, J 5.88), 7.12 (H5ꢀ,
2H, d, J 8.78), 6.80 (H4, 1H, d, J 16.19), 6.71 (H6, 2H, d, J 8.84),
6.68 (H3ꢀ, 1H, d, J 11.32), 6.57 (H6ꢀ, 2H, d, J 8.88), 6.28 (H4ꢀ,
1H, d, J 12.13), 3.02 (H7, 3H, s), 2.96 (H7ꢀ, 3H, s).
{Fe(DEAS)2[Ag(CN)2]2} (3)
Prismatic red crystals. Yield: 62%. νmax (KBr)/cm−1 2152
ν(C≡N), 1593 ν(C=N). (Found: C 39.93, H 3.36, N 13.09. Calc.
for C38H40Ag2FeN8: C 40.16, H 3.21, N 13.38%.) EDXA found:
34% Fe, 66% Ag.
{Fe(DEAS)2[Au(CN)2]2} (4)
Prismatic red crystals. Yield: 57%. νmax (KBr)/cm−1 2146
ν(C≡N), 1574 ν(C=N). (Found: C 30.99, H 2.24, N 11.01. Calc.
for C38H40Au2FeN8: C 31.29, H 2.50, N 10.42%.) EDXA found:
37% Fe, 63% Au.
DEAS
Accessory Publication
δH (300 MHz, (D6)DMSO) 8.45 (H1, 2H, d, J 6.05), 8.44
(H1ꢀ, 2H, d, J 5.59), 7.45 (H2, 2H, d, J 6.25), 7.44 (H5, 2H, d, J
8.78), 7.39 (H3, 1H, d, J 16.49), 7.24 (H2ꢀ, 2H, d, J 5.99), 7.05
(H5ꢀ, 2H, d, J 8.84), 6.89 (H4, 1H, d, J 16.34), 6.67 (H6, 2H, d, J
8.85), 6.62 (H3ꢀ, 1H, d, J 12.22), 6.53 (H6ꢀ, 2H, d, J 8.91), 6.27
(H4ꢀ, 1H, d, J 12.25), 3.37 (H7, 2H, q, J 7.049), 3.29 (H7ꢀ, 2H,
q, J 7.033), 1.11 (H8, 3H, t, J 6.97), 1.05 (H8ꢀ, 3H, t, J 7.03).
δH (300 MHz, CDCl3) 8.48 (H1, 2H, d, J 6.13), 8.46 (H1ꢀ,
2H, d, J 5.92), 7.42 (H2, 2H, d, J 8.89), 7.35 (H5, 2H, d, J 6.18),
Schemes of the synthesis of the ligands DMAS and DEAS, 1H
NMR spectra of the ligands DMAS and DEAS, PXRD at 293 K
of compounds 1, 3, and 4, and CIFS of compounds 1, 3, and 4
are available from the Journal’s website.
Acknowledgements
Financial support is acknowledged from the Spanish Ministerio de Edu-
caciónyCiencia(MEC)(CTQ2007-64727), fromtheGeneralitatValenciana

RESEARCH FRONT
Spin Crossover and Paramagnetic Behaviour of Fe(ii) Polymers Based on Stilbazole Push–Pull Ligands
1165
(GVPRE/2008/150 and ACOMPO9/236), and from the European network
of excellence MAGMANET (Contract: NMP3-CT-2005-515767-2). A.B.G.
thanks the Spanish MEC for a research contract (Programa Ramón y Cajal).
(c) T. Kitazawa, M. Takahashi, M. Enomoto, A. Miyazaki,
T. Enoki, M. Takeda, J. Radioanal. Nucl. Chem. 1999, 239, 285.
doi:10.1007/BF02349498
(d) T. Kitazawa, M. Eguchi, M. Takeda, Mol. Cryst. Liq. Cryst. 2000,
341, 527. doi:10.1080/10587250008026193
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