Bistable Hofmann-Type FeIISpin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces

Article abstract of DOI:10.1021/acs.inorgchem.1c01010

Aiming at investigating the suitability of Hofmann-type two-dimensional (2D) coordination polymers {FeII(Lax)2[MII(CN)4]} to be processed as single monolayers and probed as spin crossover (SCO) junctions in spintronic devices, the synthesis and characterization of the MII derivatives (MII = Pd and Pt) with sulfur-rich axial ligands (Lax = 4-methyl- and 4-ethyl-disulfanylpyridine) have been conducted. The thermal dependence of the magnetic and calorimetric properties confirmed the occurrence of strong cooperative SCO behavior in the temperature interval of 100-225 K, featuring hysteresis loops 44 and 32.5 K/21 K wide for PtII-methyl and PtII/PdII-ethyl derivatives, while the PdII-methyl derivative undergoes a much less cooperative multistep SCO. Excluding PtII-methyl, the remaining compounds display light-induced excited spin-state trapping at 10 K with TLIESST temperatures in the range of 50-70 K. Single-crystal studies performed in the temperature interval 100-250 K confirmed the layered structure and the occurrence of complete transformation between the high- and low-spin states of the FeII center for the four compounds. Strong positional disorder seems to be the source of elastic frustration driving the multistep SCO observed for the PdII-methyl derivative. It is expected that the peripheral disulfanyl groups will favor anchoring and growing of the monolayer on gold substrates and optimal electron transport in the device.

Full text of DOI:10.1021/acs.inorgchem.1c01010

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Article
Bistable Hofmann-Type FeII Spin-Crossover Two-Dimensional
Polymers of 4‑Alkyldisulfanylpyridine for Prospective Grafting of
Monolayers on Metallic Surfaces
Rubén Turo-Cortés, Francisco Javier Valverde-Munoz,* Manuel Meneses-Sánchez, M. Carmen Munoz,
̃ ̃
Carlos Bartual-Murgui, and José Antonio Real*
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ABSTRACT: Aiming at investigating the suitability of Hofmann-
type two-dimensional (2D) coordination polymers
{FeII(Lax)2[MII(CN)4]} to be processed as single monolayers
and probed as spin crossover (SCO) junctions in spintronic
devices, the synthesis and characterization of the MII derivatives
(MII = Pd and Pt) with sulfur-rich axial ligands (Lax = 4-methyl-
and 4-ethyl-disulfanylpyridine) have been conducted. The thermal
dependence of the magnetic and calorimetric properties confirmed
the occurrence of strong cooperative SCO behavior in the
temperature interval of 100−225 K, featuring hysteresis loops 44
and 32.5 K/21 K wide for PtII-methyl and PtII/PdII-ethyl
derivatives, while the PdII-methyl derivative undergoes a much
less cooperative multistep SCO. Excluding PtII-methyl, the
remaining compounds display light-induced excited spin-state trapping at 10 K with TLIESST temperatures in the range of 50−70
K. Single-crystal studies performed in the temperature interval 100−250 K confirmed the layered structure and the occurrence of
complete transformation between the high- and low-spin states of the FeII center for the four compounds. Strong positional disorder
seems to be the source of elastic frustration driving the multistep SCO observed for the PdII-methyl derivative. It is expected that the
peripheral disulfanyl groups will favor anchoring and growing of the monolayer on gold substrates and optimal electron transport in
the device.
materials where the SCO synchronically interplays with other
relevant physicochemical propertiese.g., porosity (host−
guest chemistry), liquid crystalline properties, crystal-to-crystal
phase transitions, luminescence or chiralityin a synergetic
fashion in the same crystal is one of the fundamental goals in
the field. This requires a rational design of the synthesis at
macroscopic scale and precise control of essential elusive SCO
parameters, such as critical temperature (T1/2), abruptness,
hysteresis width, and completeness. Relevant achievements of
this strategy include the combination of SCO and nonlinear
optical properties,4 electronic conduction,5 electrolumines-
cence,6 fluorescence,7 liquid-crystalline properties,8 porosi-
ty,2d,g molecular recognition,9 photoswitchable magnets,10
chirality,11 room-temperature photoisomers and reactions,12
INTRODUCTION
■
Bistable molecular materials with switchable properties are
appealing candidates for developing technological applications,
e.g., sensors for information storage. Iron(II) spin crossover
(SCO) complexes afford excellent examples of molecular
bistability, because they reversibly switch between the high-
spin (HS, t2g4eg2) and low-spin (LS, t2g6eg0) electronic states in
response to a variety of external stimuli such as temperature,
pressure, light, adsorption of analytes or extrinsic phase
transitions. This is particularly true when the spin changing
centers are strongly coupled to each other, since the spin state
change manifests cooperatively conferring hysteretic behavior
(memory effect) to the magnetic, optical, structural, mechan-
ical, and electric properties associated with the material.1
The SCO research is a very active and multidisciplinary field
that spreads in many complementary directions. The synthesis
and characterization of interesting mononuclear, polynuclear,
and one-dimensional to three-dimensional (1D−3D) poly-
meric SCO systems has increased exponentially during the last
two decades, affording new SCO behaviors2 which, in turn,
have inspired new sophisticated physical techniques and
theoretical models.1e,3 To engineer new multifunctional
Received: April 1, 2021
Published: May 28, 2021
https://doi.org/10.1021/acs.inorgchem.1c01010
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Figure 1. Magnetic and photomagnetic properties of MpyS2Me (M = Pt (a), Pd (b)) and MpyS2Et (M = Pt (c), Pd (d)). Cooling, heating and
photoswitching processes are represented in blue, red, and green, respectively.
etc. The ultimate goal is the construction of sensing materials
capable of acting as switchers in response to changes of
ambient conditions (temperature, humidity, chemical con-
taminants, etc.). Furthermore, the potential implementation of
SCO materials into electronic and spintronic devices is a new
concept of paramount importance that has fuelled sophisti-
cated studies aiming at controlling the electron transport
(charge and spin) processing SCO materials as ultrathin films
on surfaces.13
characterization, and spin crossover properties of four
complexes generically formulated {FeII(pyS2R)2[MII(CN)4]}n
(MpyS2R, where MII = Pd, Pt and R = Me, Et), where the axial
organic ligand pyS2R is 4-methyl/ethyldisulfanylpyridine (R =
Me, Et). In contrast to the mentioned above multilayer studies
based of the LPE technique, the axially coordinated pyridine
ligand functionalized in 4-position with a reactive alkyldisul-
fanyl group opens the possibility to process the resulting 2D
coordination polymers as robust single monolayer arrays of
elastically coupled SCO centers deposited on suitable surfaces
to be probed as SCO junctions. This approach was inspired by
a relevant pioneer work by Mallouk et al. about the growth of
thin films of the porous 3D Hofmann clathrate {Ni(4,4′-
bipyridine)[Pt(CN)4]} anchored through a monolayer of 4-
pyridyl ethyldisulfide on gold substrates.20 A similar strategy
has recently led to the production of molecular monolayers
prepared by simple immersion of the substrate in highly
diluted solutions of mononuclear FeII SCO complexes on gold
substrates and successfully tested as spintronic devices.26
Two-dimensional (2D) Hofmann-type FeII coordination
polymers with general formula {FeII(Lax)2[MII(CN)4]} repre-
sent an important source of SCO compounds, where MII = PtII,
PdII, or NiII and Lax is a terminal monotopic axial ligand based
on pyridine/pyridine-like2d,14 and triazole rings.15 The FeII
ions are equatorially connected through square-planar
[MII(CN)4]2− anionic metalloligands affording robust infinite
[FeII[MII(CN)4]∞ layers that are the origin of the cooperativity
typically exhibited by these compounds. The layers stack on
top each other interdigitating the axial ligands Lax whose nature
(length, donor−acceptor substituents, etc.) plays an important
role in the modulation of the cooperativity through changes in
the interlayer spacing and flexibility of the layers (corrugation),
factors that may influence the inclusion of guest molecules.
It has recently been shown that 2D Hofmann-type
coordination polymers can be processed as ultrathin films
under mild conditions (RT) by applying the layer-by-layer
liquid phase epitaxy (LPE) methodology,16−18 at variance of
the homologous 3D derivatives, which require very low
temperatures.19−25 Processing of these materials as ultrathin
films is a requirement to keep small electrode separation in
vertical transport devices to ensure a functional current flow
but, obviously, it can seriously compromise the SCO
properties. For example, synchrotron XAS studies showed
that, for film thicknesses above ca. 12 nm, the 2D coordination
polymer {FeII(pyridine)2[PtII(CN)4]} presents a cooperative
SCO behavior similar to that observed for the microcrystalline
sample.17 However, below this threshold value, the coopera-
tivity and completeness of the spin transition are exponentially
attenuated since the films lose cohesion conferring to its
structure a high degree of mosaicity constituted of practically
unconnected nanoislands. The nature of the axial ligand and its
dramatic influence on the coalescence of the thin film
deposited on Au substrates has also been investigated for
two new 2D Hofmann compounds {FeII(pyrimidine)2
[PtII(CN)4]} and {FeII(isoquinoline)2[PtII(CN)4]}, together
with their transport properties.18
RESULTS
■
Synthesis. All the samples MpyS2R (where M = Pt, Pd and
R = Me, Et) were prepared as single crystals from slow
diffusion techniques in water−methanol solutions (see the
Experimental Section). According to chemical and thermog-
ravimetric analyses (see Figure S1 in the Supporting
Information), the single crystals resulted to be unsolvated
and decompose above 420 K.
Spin Crossover Properties. Figure 1 shows the magnetic
and photomagnetic properties of the title compounds in the
form of the product χMT vs T, where χM is the molar magnetic
susceptibility and T is the temperature. At 300 K, the χMT
value is ca. 3.70 cm3 K/mol for the four derivatives consistently
with a fully populated HS state with a strong orbital
contribution. Upon cooling at 1 K/min, χMT remains constant
down to 183 K for PtpyS2Me and decreases abruptly to 0.4
cm3 K/mol in the interval 182−170 K, then decreases
gradually to attain a value of 0.2 cm3 K/mol at 100 K, where
the LS state is practically fully populated. The profile of the
χMT vs T curve in the heating mode is similar to that of the
cooling mode but shifted to high temperatures, defining a
hysteresis loop ΔT = 44 K wide with the equilibrium
temperatures, T1/2, at which the populations of the HS and
LS centers are equal at 0.5, equal to 180 K and 224 K for the
cooling and heating branches, respectively. This strong
cooperative SCO behavior contrasts drastically with that
shown by the isostructural (vide infra) homologous PdpyS2Me
derivative. χMT = 3.70 cm3 K/mol remains constant down to
In the search for new FeII Hofmann-type 2D coordination
polymers, here, we report on the preparation, structural
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Figure 2. Thermal dependence of ΔCp vs T for PtpyS2Me (left) and PdpyS2Me (right). Note that for the latter the step below 150 K could not be
recorded (see text). Cooling and heating modes are represented in blue and red, respectively.
Figure 3. (Left) Molecular fragment of PtpyS2Me showing the atom numbering of the asymmetric unit. (Right) Packing of two consecutive layers
(only one of the two possible orientations of the −S−S−Me moiety is shown.
232 K; however, below this temperature, it decreases gradually
in a succession of slightly marked steps, reaching a value of
0.15 cm3 K/mol at 100 K. Except for the lower step, which
shows a small hysteresis between 130 K and 138 K, the
cooling−heating profiles are practically superposed. The
corresponding characteristic T1/2 temperature is 170 K. The
SCO profile for the MpyS2Et derivatives is similar to that of
PtpyS2Me, featuring strong cooperative hysteretic behaviors
with T1/2 temperatures 121.5 and 154.0 K (ΔT = 32.5 K) for
M = Pt and 111.0 and 132.0 K (ΔT = 21.0 K) for M = Pd, in
the cooling and heating modes, respectively.
Photogeneration of the fully populated metastable HS*
state, the so-called “light-induced excited spin state trapping
(LIESST) experiment”,27 was performed by irradiating micro-
crystalline samples of the title compounds with green light (λ =
532 nm) at 10 K. Under these conditions, all the samples but
PtpyS2Me display the LIESST effect and saturate at χMT
values of 2.08 cm3 K/mol for PdpyS2Me and 2.80 cm3 K/mol
for MpyS2Et (M = Pt, Pd). Subsequently, the light was
switched off and the temperature increased at a rate of 0.3 K/
min inducing a gradual increase of χMT to a value of 2.36 cm3
K/mol at 26 K for PdpyS2Me and 3.54 cm3 K/mol at ca. 48 K
for MpyS2Et (M = Pt, Pd), which corresponds to ca. 64% and
96% of the maximum value observed at 300 K, respectively.
This increase in χMT reflects the thermal population of
different microstates originated from the zero-field splitting of
the HS* spin state. At higher temperatures, χMT decreases
rapidly until joining the thermal SCO curve at ca. 65 K
(PdpyS2Me), 69 K (PtpyS2Et), and 76 K(PdpyS2Et),
indicating that the metastable HS* state has relaxed back to
the stable LS state. The corresponding TLIESST temperatures,
evaluated as ∂(χMT)/∂T,28 are 50.0 K (PdpyS2Me) and 68−70
K (MpyS2Et, M = Pt, Pd). These temperatures are consistent
with the inverse-energy-gap law, i.e., the metastability of the
photogenerated HS* species decreases as the stability of the LS
increases, namely as T1/2 increases.29
The SCO behavior was also investigated through the
thermal dependence of the heat capacity at constant pressure,
ΔCp, for MpyS2Me (M = Pt, Pd) (Figure 2). The low SCO
temperatures observed for both ethyl derivatives prevented us
to evaluate their thermodynamic parameters. The average
enthalpy ΔH and entropy variations ΔS (= ΔH/T1/2) are,
respectively, 16.12 kJ/mol and 79.84 J/K mol for PtpyS2Me
and 7.68 kJ/mol and 45.18 J/K mol for PdpyS2Me. The ΔH
and ΔS values found for PtpyS2Me are comparable to those
reported for similar Hofmann-type coordination polymers with
comparable cooperative SCO.1b,2d However, for PdpyS2Me
these values are considerably smaller due to the fact that ca.
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Table 1. Selected Bond Lengths and Angles for MpyS2Me (M = Pd, Pt)
PdpyS2Me, 120 K
PdpyS2Me, 250 K
PtpyS2Me, 120 K
PtpyS2Me, 250 K
Selected Bond Lengths [Å]
2.219(5)
Fe−N(1)
Fe−N(2)
Fe−N(3)
Pd−C(1)
Pd−C(2)
Pt−C(1)
Pt−C(2)
C(1)−N(2)
C(2)−N(3)
1.987(3)
1.942(3)
1.943(3)
1.991(3)
1.992(3)
1.992(14)
1.943(14)
1.945(14)
2.23(3)
2.13(2)
2.11(2)
2.132(4)
2.137(4)
1.991(4)
1.988(5)
1.975(17)
1.978(17)
1.15(3)
1.97(2)
1.93(2)
1.16(3)
1.21(3)
1.149(4)
1.153(5)
1.131(7)
1.130(7)
1.15(3)
Selected Bond Angles (°)
90.1(2)
N(1)−Fe−N(2)
N(1)−Fe−N(3)
N(2)−Fe−N(3)
ΣFe
90.06(12)
90.13(12)
90.01(10)
0.8
90.7(6)
90.5(6)
91.0(6)
8.8
91.6(11)
91.0(11)
90.2(6)
11.2
90.0(2)
90.04(14)
0.56
C(1)−N(2)−Fe
C(2)−N(3)−Fe
179.8(2)
179.8(3)
179.7(5)
179.9(5)
178(2)
177(2)
169(2)
178(2)
27% of the SCO occurs out of the temperature window of our
calorimeter, an extrapolation to 100% gives ΔH = 10.5 kJ/mol
and ΔS = 62 J/Kmol (see also Figure S2 in the Supporting
Information). These extrapolated values are still smaller than
those observed for PtpyS2Me but consistent with the much
less cooperative gradual SCO and lower T1/2 temperature of
the homologous Pd derivative. The T1/2 values obtained from
the calorimetric measures are virtually the same than those
obtained from magnetism (see Figure S2). As it can be seen
from Figure 2, the surprisingly distinct nature of both SCO
behaviors, hysteretic versus multistepped, are clearly reflected
in the ΔCp vs T plots.
consecutive layers interdigitate in such a way that the pyS2Me
axial ligands of one layer point toward the center of the square
windows of the adjacent layers, with the distance between the
{FeII [MII(CN)4]2}n layers being equal to 10.36 Å (M = Pt)
2
and 10.75 Å (M = Pd). The S−S−CH3 tails display positional
disorder in two equivalent positions for the Pt derivative while
the disorder is considerably more severe for the Pd derivative
also involving the pyridine groups (see Figure S3 in the
Supporting Information). At 250 K, the structures are
essentially the same, being the most significant differences,
with respect to those at 120 K, the increase of the ⟨Fe−N⟩
bond length by 0.2 Å and the change of color of the crystals to
yellow. Both facts are perfectly consistent with the full
population of the FeII HS state in agreement with the
magnetic data. In addition, the change to the HS state in the Pt
derivative is accompanied by a small degree of corrugation.
The angle defined between the equatorial FeIIN4 and the
[PtII(CN)4]2− square planes is 7.64°. Consistently, the Fe−
N2−C1 angle decreases 9° from 178(2)° in the LS state until
169(2)° in the HS state. In addition, the separation of two
consecutive [Fe2M2]n layers increases by 0.38 Å until 10.74 Å.
In contrast, the layers remain perfectly flat for the Pd derivative
while the separation between consecutive layers increases by
0.2 Å until 10.95 Å (see Figure S2).
Structure of MpyS2Et. The crystal structures of MpyS2Et,
M = Pt and Pd, were investigated at 100 and 250 K turning out
to be isostructural. At 100 K, the red crystals of both
derivatives display a monoclinic I2/m unit cell that changes to
monoclinic C2/m at 250 K, where the crystals are yellow (see
Table S2). Table 2 contains a selection of significant bond
lengths and angles including the angular distortion parameter
ΣFe. The asymmetric unit contains one slightly distorted
[FeIIN6] octahedral site defined by two distinct pyS2Et axial
ligands coordinated, respectively, via N1 and N2 and two
distinct [M(CN)4]2− groups coordinated, respectively, via N3
and N4 (Figure 4). The two pyridine rings of pyS2Et and the
FeII center lie in a reflection plane which bisects the equatorial
N3−Fe−N3′ and N4−Fe−N4′ angles. At 100 K, the ⟨Fe−N⟩
is 1.961(5) and 1.968(4) Å for the Pt and Pd derivatives,
respectively, are consistent with the FeII centers in a fully
populated LS state, whereas, at 250 K, these average bond
lengths increase by 0.20−0.21 Å attaining typical values for the
Single-Crystal Structure Analysis. Structure of
MpyS2Me. The crystal structure of MpyS2Me (M = Pt and
Pd) was investigated at 120 and 250 K; it turned out to be
̅
isostructural and crystallized in the triclinic P1 space group. A
selection of relevant crystallographic data for MpyS2Me (M =
Pt, Pd) is given in Table S1 in the Supporting Information. At
120 K, the structure is characterized by a crystallographically
unique FeII site lying in an inversion center defining a slightly
elongated [FeIIN6] octahedron. A representative fragment of
the structure including the atom numbering is shown in Figure
3 (left). Table 1 contains a selection of significant bond lengths
and angles, together with the corresponding average angular
distortion parameter ΣFe, which is defined as the sum of
deviations from the ideal octahedron/tetrahedron of the 12
“cis” bond angles, ∑i1=21 |θi − 90°|. The equatorial positions are
occupied by the N2 and N3 atoms of the CN groups belonging
to the [PtII(CN)4]2− bridging ligands, while the axial positions
are occupied by the N1 atom of the pyridine group of the
pyS2Me ligand. The average ⟨Fe−N⟩ bond length, 1.960(14) Å
(M = Pt) and 1.957(3) Å (M = Pd), are typical of the FeII site
in the LS state and consistent with the magnetic data and the
characteristic deep red color of the crystals at same
temperature. The ΣFe parameter, almost 0 for the Pd derivative
and relatively much larger for the Pt derivative, denote that the
angular distortion in both compounds is very small and
practically independent of the spin state.
Each FeII site is bridged to four equivalent FeII sites through
four equivalent square-planar [PtII(CN)4]2− bridges defining
2D layers in which the equatorial planes of the [FeIIN6] and
[PtIIC4] centers are strictly coplanar (Figure 3, right). Two
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(Pd) and 9° (Pt) when moving from the LS to the HS state.
The separation between two consecutive the layers, measured
from the average plane defined by M1 and M2, is very similar
for the two derivatives and practically does not change with
temperature (11.15−11.52 Å).
Table 2. Selected Bond Lengths and Angles for MpyS2Et (M
= Pd, Pt)
PdpyS2Et,
100 K
PdpyS2Et,
250 K
PtpyS2Et,
100 K
PtpyS2Et,
250 K
Selected Bond Lengths [Å]
Fe−N(1)
Fe−N(2
Fe−N(3)
Fe−N(4)
Pd(1)−C(1)
Pd(2)−C(2)
Pt(1)−C(1)
Pt(2)−C(2)
C(1)−N(3)
C(2)−N(4)
2.001(5)
2.218(8)
2.237(9)
2.170(5)
2.140(5)
1.987(7)
2.005(6)
1.997(7)
2.205(7)
2.202(9)
2.162(5)
2.133(5)
DISCUSSION
2.017(5)
1.946(4)
1.948(4)
1.992(4)
2.003(5)
2.004(6)
1.940(5)
1.944(5)
■
Since the first SCO Hofmann type 2D coordination polymer
{FeII(pyridine)2[MII(CN)4]}, MII = Ni,30 and its isostructural
PdII and PtII counterparts31 were reported, this family of
compounds has been steadily growing until recently (see refs
2d, g, 14, 15, 18). Despite their high insolubility, their synthesis
based on formal replacement of the axial pyridines with
homologous N-donor ligands can be addressed in a
straightforward manner to obtain samples constituted ex-
clusively of single crystals by employing liquid−liquid slow
diffusion techniques, which is the safest way to get pure
samples with reliable SCO properties for this type of
compounds. The strong cooperative SCO behavior featuring
well-shaped symmetric hysteresis loops expressed by many of
these compounds is likely the most appealing aspect, which
explains the interest and growth of this family of compounds.
This cooperativity seems to be originated mainly from the
robust nature of the {FeII[MII(CN)4]}n layers where all SCO
centers are strongly coupled. Indeed, most of the
[FeII(L)2[MII(CN)4] compounds show hysteresis widths
ranging in the interval of 10−30 K, but it has also been
reported hysteresis close to 40 K32a,18b or even larger (50−65
K).15g It is reasonable to consider that the nature of the axial
ligands and included guest molecules play an important role in
the modulation of the observed cooperativity, however, this is a
fact that generally has not obvious rationalization. In addition,
it is also worth noting that the hysteresis width seems not to be
correlated with the length of the axial ligand, namely, with the
separation between the {FeII[MII(CN)4]}n layers. For example,
interdigitation of the relatively long ligands L = 4-
styrylpyridine (n = 0.5) and 4-(2-phenylethyl)pyridine (n =
0) in [FeII(L)2[PtII(CN)4]·nMeOH separates the
1.981(5)
1.989(5)
1.160(7)
1.159(7)
1.987(5)
1.993(5)
1.142(6)
1.141(7)
1.146(5)
1.146(5)
1.145(8)
1.126(8)
Selected Bond Angles [°]
N(1)−Fe−N(3)
N(1)−Fe−N(4)
N(2)−Fe−N(3)
N(2)−Fe−N(4)
N(3)−Fe−N(4)
N(3)−Fe−N(3)
N(4)−Fe−N(4)
Σ
91.29(14)
90.72(14)
86.83(14)
91.18(14)
88.43(14)
91.9(2)
91.2(2)
19.96
169.7(4)
178.0(3)
90.7(2)
91.3(2)
86.7(2)
91.2(2)
88.3(2)
90.4(3)
93.0(3)
19.8
91.4(2)
90.6(2)
86.6(2)
91.5(2)
88.3(2)
92.0(3)
91.3(3)
20.5
90.5(2)
91.2(2)
86.6(2)
91.7(2)
88.5(2)
90.2(3)
92.7(3)
19.5
C(1)−N(3)−Fe
C(2)−N(4)−Fe
158.8(6)
177.7(6)
169.3(4)
178.1(4)
160.3(5)
177.1(5)
FeII in the HS state [2.166(9) and 2.179(9) Å, respectively].
The ΣFe parameter is small (∼20°) and remains almost
constant upon SCO. There are two crystallographically distinct
[MII(CN)4]2− groups and two pairs of them connect each FeII
center to four equivalent atoms defining an irregularly
corrugated layer. Indeed, at 100 K, the angle defined between
the [M1II(CN)4]2−/[M2II(CN)4]2− squares and the equatorial
plane of the FeII center is, respectively, 4.04°/4.24° and
20.47°/21.40° and increase by ca. 36% up to 6.22°−6.56° and
32.26°−35.82° for M = Pt/Pd at 250 K. The change of this
angle occurs through the Fe−N3−C1, which decrease 10.9°
Figure 4. (Left) Molecular fragment of PtpyS2Et showing the atom numbering of the asymmetric unit. (Right) Packing of three consecutive layers
(only one of the two possible orientations of the −S−S−Et moiety is shown.
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{FeII[PtII(CN)4]}n layers by ca. 13.85 Å and although both
compounds display sharp SCO transitions they lack of
hysteretic behavior.32b In contrast, the closely related axial
ligands L = 3-phenylazo-pyridine and 4-phenylazopyridine in
[FeII(L)2[PdII(CN)4] with similar interdigitation induce
abrupt hysteretic spin transitions with ΔT1/2 = 12 and 17 K,
respectively.32c An additional difficulty when dealing with this
very narrow plateaus, while the Pt derivative displays a sharp
cooperative spin transition with large hysteresis. The most
significant structural difference between them is the occurrence
of positional disorder of the pyridine and S−S−CH3 groups
over four orientations in the Pd derivative, which remains in
the HS and LS states. This behavior is reminiscent of that
found, among others, for the 2D coordination polymer
{FeII[HgII(SCN)3]2(4,4′-bipy)2}n where a sequence of differ-
ent phases characterized by distinct HS/LS fractions and
symmetry breaking results from competition between SCO
and structural 4,4′-bipy ligand ordering. For this system, it was
possible to identify a correlation between the internal dihedral
angle adopted by the 4,4′-bipy ligand and each particular step
(spin state phase) as being responsible for the observed
multistability.33 From a phenomenological point of view,
thermally induced multistep SCO behavior is associated with
elastic frustration,3b,c namely, the occurrence of subtle balances
between opposed intermolecular interactions that drive the
HS ↔ LS transformation in fractional steps consistent with
different concentrations of HS and LS centers (with or without
ordering). For PdpyS2Me, the more conspicuous positional
disorder may be the source of subtle balances between
interlayer interactions and/or distortions of the [FeN6]
centers. However, to precisely identify the structural
constraints favoring the steps, is for most of the known
multistep SCO examples a major difficulty in particular when
the steps are poorly defined.
type of compounds is that the spin crossover nature (T1/2
,
ΔT1/2, completeness, abruptness, etc.) may be strongly affected
by the degree of crystallinity. A relevant example has been
recently observed for {FeII(pyridine)2[PtII(CN)4]} (separation
between layers {FeII[PtII(CN)4]}n ca. 7.6 Å). In its precipitated
microcrystalline form, it displays a SCO centered at 212 K with
a hysteresis 8 K wide, which is characterized by a remarkable
residual fraction (15%−19%) of inactive HS centers. In
contrast, the same compound exclusively constituted of single
crystals shows a complete well-shaped SCO centered at 234 K
and a hysteresis 42 K wide (see Figure S4 in the Supporting
Information).17 Rapid precipitation of these highly insoluble
compounds usually produces microcrystalline samples consist-
ing of submicrometric/nanometric crystallites, dramatically
influencing the SCO via the increase of crystal defects, and
hence consisting of the residual HS molar fraction in the LS
phase, which, in turn, is reflected on a decrease of the T1/2, of
cooperativity (ΔT1/2) and completeness of the SCO.
In the present study, the SCO behavior has been
investigated for samples exclusively constituted of single
crystals. Except for PdpyS2Me, the SCO behavior of the title
compounds MpyS2R (R = Me, Et; M = Pd, Pt) retain the
general features described for other Hofmann-type 2D
coordination polymers. Compound PtpyS2Me undergoes a
particularly strong cooperative transition with a hysteresis ΔT
= 44 K wide, which, despite an interlayer distance increase of
∼2−3 Å, because of the presence of the flexible −S−S−CH3
moieties, it is virtually the same than the SCO observed for
single crystals of {FeII(pyridine)2[PtII(CN)4]}. The only
CONCLUSIONS
■
Here, we have described the synthesis, structure, magnetic,
photomagnetic, and calorimetric properties of four new
Hofmann-type 2D SCO coordination polymers. Three of
them show strong cooperative SCO properties, featuring wide
thermal hysteresis, in particular compound PtpyS2Me, while its
isostructural Pd counterpart surprisingly displays a multi-
stepped transition without hysteresis, most likely due to the
occurrence of additional disorder in the structure. The
MpyS2Et derivatives, which have the lowest T1/2 of the series,
show complete LIESST effect. In contrast, the LIESST effect is
incomplete for PdpyS2Me and vanishes completely for
PtpyS2Me because of their higher T1/2 values.
av
noticeable difference is observed for the average T1/2 value,
which is 32 K less than that observed for the pyridine
derivative. This result also supports the idea mentioned above
that separation between the layers does not substantially affect
the cooperativity.
Replacement of the methyl group by the ethyl group in
MpyS2R does not change significantly the separation between
The results here reported correspond to the first step in a
more challenging work whose ultimate objective is to graft
these Hofmann-type 2D SCO coordination polymers as
monolayers on metallic surfaces (e.g., Au) to be probed as
junctions for spintronic devices in which the switchable SCO
centers can be used to modulate the junction conductance (see
Scheme I). The choice of 4-alkyldisulfanylpyridines as axial
ligands was based on the well-known fact that S atoms ensure
appropriate interaction between the molecular wires and the
electrodes. Preliminary work on this second objective confirms
its feasibility and definitive conclusions will be reported in due
time.
av
the layers but involves a considerable decrease in T1/2 from
202 K to 138 K (64 K) for the Pt derivative. This fact could
tentatively be correlated with a higher corrugation of the layers
in the ethyl derivatives. This fact is clearly reflected in the
decrease from 180° of one of the two Fe−N−C−Pt moieties.
For PtpyS2Me, the angle Fe−N2−C1(Pt) is 168.5°, while the
equivalent angle for PtpyS2Et, Fe−N3−C1(Pt), is 158.8°, both
in the HS state, and they change to 178.0° and 169.6° in the
LS state, respectively. Obviously, the larger misalignment of
the N−C−Pt moiety, with respect to the 3d orbitals of FeII in
the ethyl derivative, must necessarily decrease the σ and π
overlaps, thereby decreasing the ligand field felt by the FeII
centers. Another important difference pointing to the same
direction is that the angular distortion ΣFe (see Tables 1 and 2)
is significantly larger for PtpyS2Et than for its methyl
counterpart.
EXPERIMENTAL SECTION
■
Materials and Reagents. Iron(II) tetrafluoroborate hexahydrate,
potassium tetracyanoplatinate(II) trihydrate, potassium
tetracyanopalladate(II) hydrate, 4-mercaptopyridine, and methyl
methanethiosulfonate were obtained from commercial sources and
used as received without further purification. Ethyl methanethiosul-
fonate was synthesized following a literature procedure.34
Surprisingly, even though both MpyS2Me (M = Pd, Pt)
compounds are isostructural, their SCO properties are
drastically different to each other. The Pd derivative shows a
relatively gradual multistep behavior (ca. 6 steps) separated by
Synthesis of Methyl/Ethyl(4-pyridyl)disulfide. The synthesis of
methyl(4-pyridyl)disulfide was performed using a method previously
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Single Crystal X-ray Diffraction. Single-crystal X-ray data were
collected on an Oxford Diffraction Supernova diffractometer using
graphite monochromated Mo Kα radiation (λ = 0.71073 Å). A
multiscan absorption correction was performed. The structures were
solved by direct methods using SHELXS-2014 and refined by full
matrix least-squares on F2 using SHELXL-2014.36 Non-hydrogen
atoms were refined anisotropically and hydrogen atoms were placed in
calculated positions refined using idealized geometries (riding model)
and assigned fixed isotropic displacement parameters. CCDC
2072898 (100 K) and 2072899 (250 K) (PdpyS2Et); 2072901
(250 K) and 2072902 (129 K) (PdpyS2Me); 2072900 (100 K) and
2072905 (250 K) (PtpyS2Et); and 2072903 (120 K) and 2072904
(250 K) (PtpyS2Me) contain the supplementary crystallographic data
for this article. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Scheme I. Model of Device Constituted of a Monolayer of
MpyS2Me Deposited on an Au Substrate
described.35 Stoichiometric amounts of NaOH (5 mmol), 4-
mercaptopyridine (5 mmol), and methylmethanethiosulfonate (5
mmol) were dissolved in water (10 mL). A white turbidness appears
immediately which slowly transforms to a yellow oil. The reaction
mixture was stirred at room temperature for 30 min and then
extracted with CH2Cl2. The organic phase was dried with MgSO4 and
subsequently evaporated to obtain a yellow oil, which was purified by
SiO2 column chromatography using a toluene-ethyl acetate mixture
(80:20) as eluent. 300 MHz, CDCl3, δ/ppm: 8.49 (2H), 7.44 (2H),
2.46 (3H). The same route was followed for the synthesis of ethyl(4-
pyridyl)disulfide using the noncommercial precursor ethyl meth-
anethiosulfonate. 300 MHz 1H-RMN, CDCl3, δ/ppm: 8.47 (2H),
7.45 (2H), 2.77 (2H), 1.32 (3H).
Synthesis of Complexes MpyS2R (M = Pd, Pt; R = Me, Et). All the
samples were constituted of single crystals exclusively obtained
through slow liquid-to-liquid diffusion methods using a 10-mL-total-
volume H-shaped tube. One arm of the tube was filled with 1 mL of
H2O:MeOH (1:1) solution containing a mixture of 33.7 mg of
Fe(BF4)2·6H2O (0.1 mmol) and 40/44 mg (0.25 mmol) of methyl/
ethyl(4-pyridyl)disulfide, whereas the other one was filled with an
aqueous solution (1 mL) of 44/35 (0.1 mmol) of K2[M(CN)4] (M =
PtII/PdII). The rest of the tube was carefully filled with a
methanol:water (1:1) solution, closed with parafilm and left to
stand at room temperature. Light yellow cubic single crystals of
MpyS2R were obtained after 2 weeks. Elemental analysis: Calculated
for PtpyS2Me [C16H14N6S4FePt (669.5) (%)]: C 28.70; H 2.11; N
12.55. Found (%): C 29.11; H 2.08; N 12.78. Calculated for
PdpyS2Me [C16H14N6S4FePd (580.8) (%)]: C 33.09; H 2.43; N
14.47. Found (%): C 33.57; H 2.15; N 14.65. Calculated for PtpyS2Et
[C18H18N6S4FePt (697.6) (%)]: C 30.99; H 2.60; N 12.05. Found
(%): C 30.52; H 2.52; N 12.35. Calculated for PdpyS2Et
[C18H18N6S4FePd (608.9) (%)]: C 35.51; H 2.98; N 13.80. Found
(%): C 35.17; H 2.90; N 14.01.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01010.
Relevant crystallographic data (Tables S1 and S2);
thermogram for MpyS2Me and MpyS2Et (M = Pd, Pt)
(Figure S1); comparison of the thermal dependence of
ΔCp and ∂(χMT)/∂T curves for PdpyS2Me (Figure S2);
positional disorder of the pyS2Me ligands of PdpyS2Me
at 250 K (Figure S3); comparison of the χMT vs T plots
for {Fe(pyridine)2[Pt(CN)4]} obtained by direct
precipitation and by slow diffusion as single crystals
(Figure S4) (PDF)
Accession Codes
CCDC 2072898−2072905 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Francisco Javier Valverde-Munoz − Instituto de Ciencia
̃
Molecular/Departamento de Química Inorgánica,
Universidad de Valencia, 46980 Paterna, Valencia, Spain;
orcid.org/0000-0003-3578-5445;
Email: francisco.valverde@uv.es
Physical Measurements. Magnetic Measurements. Variable
temperature magnetic susceptibility data were recorded with a
Quantum Design MPMS2 SQUID magnetometer equipped with a
7 T magnet, operating at 1 T and at temperatures of 1.8−400 K.
Experimental susceptibilities were corrected for diamagnetism of the
constituent atoms by the use of Pascal’s constants.
José Antonio Real − Instituto de Ciencia Molecular/
Departamento de Química Inorgánica, Universidad de
Valencia, 46980 Paterna, Valencia, Spain; orcid.org/
0000-0002-2302-561X; Email: jose.a.real@uv.es
Authors
Calorimetric Measurements. DSC measurements were performed
using a differential scanning calorimeter (Mettler Toledo, Model DSC
821e). Low temperatures were obtained with an aluminum block
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 drybox under a flow of dry nitrogen gas to avoid water
condensation. The measurements were performed using ∼15 mg of
microcrystalline samples of MpyS2Me (M = Pt, Pd) sealed in
aluminum pans with a mechanical crimp. Temperature and heat flow
calibrations were made with standard samples of indium by using its
melting transition (429.6 K, 28.45 J g−1). An overall accuracy of 0.2
K in temperature and 2% in the heat capacity is estimated. The
uncertainty increases for the determination of the anomalous enthalpy
and entropy due to the subtraction of an unknown baseline.
Rubén Turo-Cortés − Instituto de Ciencia Molecular/
Departamento de Química Inorgánica, Universidad de
Valencia, 46980 Paterna, Valencia, Spain
Manuel Meneses-Sánchez − Instituto de Ciencia Molecular/
Departamento de Química Inorgánica, Universidad de
Valencia, 46980 Paterna, Valencia, Spain
M. Carmen Munoz − Departamento de Física Aplicada,
̃
̀
̀
Universitat Politecnica de Valencia, Valencia, Spain
Carlos Bartual-Murgui − Instituto de Ciencia Molecular/
Departamento de Química Inorgánica, Universidad de
Valencia, 46980 Paterna, Valencia, Spain; orcid.org/
0000-0003-1547-8018
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01010
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Spanish Ministerio de Ciencia
e Innovación (MICINN) and FEDER founds (No. PID2019-
106147GB-I00), Unidad de Excelencia María de Maeztu (Nos.
MDM2015-0538-18-2, CEX2019-000919-M) and EU Frame-
work FET-OPEN project COSMICS (Grant Agreement No.
766726). R.T.C. and M.M.S. thank the MICINN for a
predoctoral grant.
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