Cyanido-Bridged FeII–MI Dimetallic Hofmann-Like Spin-Crossover Coordination Polymers Based on 2,6-Naphthyridine

Article abstract of DOI:10.1002/ejic.201700920

Two new 3D spin-crossover (SCO) Hofmann-type coordination polymers {Fe(2,6-naphthy)[Ag(CN)₂][Ag₂(CN)₃]} (1; 2,6-naphthy = 2,6-naphthyridine) and {Fe(2,6-naphthy)[Au(CN)₂]₂}·0.5PhNO₂ (2) were synthesized and characterized. Both derivatives are made up of infinite stacks of {Fe[Ag(CN)₂]₂[Ag₂(CN)₃]}_n and {Fe[Au(CN)₂]₂}_n layered grids connected by pillars of 2,6-naphthy ligands coordinated to the axial positions of the Fe^II centers of alternate layers. The in situ generated [Ag₂(CN)₃]^– linkers define wide rectangular windows that favor the interpenetration of three identical 3D networks, strong argentophilic interactions between them, and the generation of a densely packed structure without accessible void spaces. In contrast, the smaller rhombus-shaped window in 2 affords a structure made up of doubly interpenetrated 3D networks with strong aurophilic interactions between them and accessible voids partially occupied by nitrobenzene molecules. Compound 1 displays a relatively abrupt two-step SCO in the temperature interval 150–215 K, whereas 2 features an incomplete one-step SCO behavior (T_1/2 = 166 K) that extends over 150 K.

Full text of DOI:10.1002/ejic.201700920

A Journal of
Accepted Article
Title: Cyanide bridged FeII-MI bimetallic Hofmann-like spin-crossover
coordination polymers based on 2,6-Naphthyridine
Authors: Lucía Piñeiro-López, Francisco Javier Valverde-Muñoz,
Maksym Seredyuk, Carlos Bartual-Murgui, M. Carmen
Munoz, and Jose Antonio Real
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700920
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10.1002/ejic.201700920
European Journal of Inorganic Chemistry
FULL PAPER
Cyanide bridged FeII-MI bimetallic Hofmann-like spin-crossover
coordination polymers based on 2,6-Naphthyridine
Lucía Piñeiro-López,[a] Francisco-Javier Valverde Muñoz,[a] Maksym Seredyuk,[a] Carlos Bartual
Murgui,[a] M. Carmen Muñoz,[b] José Antonio Real*[a]
Abstract: Two new 3D spin crossover (SCO) Hofmann-type
coordination polymers {Fe(2,6-naphthy)[Ag(CN)2][Ag2(CN)3]} (1) and
the SCO materials at micro- and nano-metric scales or even at
single molecule level has aroused in the last years much interest
in the areas of molecular nanoelectronics and spintronics.[4]
The design and synthesis of new SCO materials is a
necessary step to discover interesting SCO behaviors and
possible applications. The impressive cooperative properties
displayed by the first one- (1D) and two-dimensional (2D) 1,2,4-
triazole-based FeII SCO coordination polymers suggested that
cooperativity could be enhanced by replacing the intermolecular
interactions between SCO centers with more reliable
coordination bonds afforded by suitable bridging ligands.[5,3d]
Consequently, the polymeric approach systematically explored
from the 1990s has afforded a number of relevant two-(2D) and
three-dimensional (3D) SCO coordination polymers (SCO-CP)
and porous metal-organic frameworks (SCO-MOFs).[6]
One of the most prolific series of SCO-CPs/SCO-MOFs
are constituted by cyanide-bridged Fe-MI,II bimetallic 2D and 3D
Hofmann-like compounds. In particular the use of
dicyanometallate linkers such as [MI(CN)2]- (MI = AgI and AuI)
has been fostered since the first 3D complexes
{Fe(L)x[AgI(CN)2]2} reported in 2002.[7] In particular, the complex
{Fe(pz)[AgI(CN)2]2} (pz = pyrazine) is prototypal of a series of
SCO complexes generically formulated {Fe(L)[MI(CN)2]2} (MI =
Ag and Au) where L is a bis-monodentate bridging pyridine-like
ligand. The linear [MI(CN)2]- anions occupy the equatorial
coordination positions of the FeII ions defining an infinite stack of
2D [Fe{MI(CN)2}2]∞ layers pillared by the bridging ligand L which
occupy axial positions of the FeII ions. The layers are organized
in such a way that the ligands L thread the meshes of adjacent
sheets generating two identical interpenetrated 3D networks with
the α-Po topology (Scheme I). While {Fe(pz)[AgI(CN)2]2} is LS at
temperatures below 400 K, it has been recently demonstrated
that the homologous AuI derivative displays a cooperative SCO
between 367 and 349 K.[8] Even more cooperative SCO between
261 and 223 K has been observed for the corresponding 2-
fluoropyrazine (Fpz) and MI = Au derivative.[9] Notably, when
using longer bridging ligands such as bipytz,[10] bpmp,[11] bpp,[12]
or dpb[13] (Scheme 1), and MI = Au, interesting inclusion
chemistry and cooperative SCO behaviors have been described
despite interpenetration. In summary, the use of different organic
ligands and cyanometallate anions has resulted in a Hofmann-
type family, which exhibit customized SCO behaviors coupled
with other interesting properties.[6]
{Fe(2,6-naphthy)[Au(CN)2]2}·(0,5)PhNO2 (2) (2,6-naphthy
naphthyridine) have been synthesized and characterized. Both
derivatives are made up of infinite stacks of
= 2,6-
{Fe[Ag(CN)2]2[Ag2(CN)3]}n and {Fe[Au(CN)2]2}n layered grids pillared
by 2,6-Naphthy ligands coordinated to the axial positions of the FeII
centers of alternate layers. The in situ generated [Ag2(CN)3]- linkers
define wide rectangular windows that favor the interpenetration of
three identical 3D networks, strong argentophilic interactions
between them and generation of a densely packed structure without
accessible void spaces. In contrast, the smaller rhombus-shaped
windows’ size in
2 affords a structure made up of doubly
interpenetrated 3D networks with strong aurophilic interactions
between them and accessible voids partially occupied by
nitrobenzene molecules. 1 displays a relatively abrupt two-step SCO
in the temperature interval 150-215 K whereas 2 features an
incomplete one-step SCO behavior (T1/2 = 166 K) that extends over
150 K.
Introduction
Iron(II) spin crossover (SCO) complexes are a well-known class
of switchable molecular materials. The switching between the
high-spin (HS) and low-spin (LS) states occurs in a reversible,
controllable and detectable manner triggered by external stimuli
(i.e. temperature, pressure, light, or guest molecules) involving
changes in the molecular structure, magnetism, color and
electrical polarizability.[1] The different size of the FeII ion in both
electronic states, is at the origin of cooperative elastic
interactions between the SCO centers in the crystal.[2] When
these interactions are strong enough, the SCO material exhibits
bistability, an appealing property with potential application in the
construction of molecular devices for information storage, signal
processing and sensing.[3] In this respect, amenability to process
[a]
L. Piñeiro-López, F. J. Valverde-Muñoz, M. Seredyuk, C. Bartual-
Murgui, J. A. Real
Institut de Ciència Molecular (ICMol), Departament de Química
Inorgànica
Universitat de València
C/ Catedrático José Beltrán Martínez, 2, 46980 Paterna (Valencia),
Spain
Continuing this work, herein we report the synthesis,
crystal structures and physical properties of two novel FeII SCO
systems based on the bis-monodentate organic ligand 2,6-
E-mail: jose.a.real@uv.es; www.uv.es/smolmat
M. C. Muñoz
[b]
Departament de Física Aplicada
Universitat Politècnica de València
Camino de Vera s/n, 46022 Valencia, Spain
naphthyridine
naphthy)[Ag(CN)2][Ag2(CN)3]}
(2,6-naphthy)
formulated
and
{Fe(2,6-
{Fe(2,6-
(1)
naphthy)[Au(CN)2]2}·(0.5)PhNO2 (2) (PhNO2 = nitrobenzene as
guest molecule).
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/ejic.201700920
European Journal of Inorganic Chemistry
FULL PAPER
Scheme 1. Schematic view of two interpenetrated 3D networks with the α-Po
topology and representation of the mentioned ligands.
The temperature dependence of the χMT product for compound
2 measured at 1 K min-1 is displayed in Figure 1-bottom. At 300
K, the χMT value is ca. 3.35 cm3 K mol-1 and remains constant
down to 220 K. Then gradually decreases, upon cooling,
attaining a value of 0.84 cm3 K mol-1 at 50 K. Below this
temperature χMT decreases slightly most probably due to the
zero-field splitting of the FeII centers that remain HS at low
temperatures. Indeed 75% of the FeII centers undergo spin
transition from the HS state to the LS state in 2. In the heating
mode χMT does not match the values observed for cooling mode
showing an unsymmetrical hysteresis in the low temperature
range (ca. 100-150 K), which practically disappears at higher
temperatures. This behavior could be attributed to steric effects
induced by the nitrobenzene molecules hosted in the pores and
small kinetic effects that do not allow the system to reach the
thermodynamic equilibrium even at temperature scan rates of 1
K min-1. Considering a hypothetical complete SCO for 2, the
equilibrium temperatures are ꢀ!↓/! = 145 K and ꢀ!↑/! = 152 K.
N
N
N
N
N
N
bipydz
pz
N
N
N
N
F
N
N
bipytz
N
N
N
N
Fpz
bpp
N
N
N
N
dpb
N
N
N
N
N
2,6-naphthy
bpmp
Results
Synthesis. The 2,6-naphthy ligand was synthetized following a
modified synthetic route (Supporting Information, Section 1.8.1).
Single crystals of 1 and 2 were synthesized by means of slow
liquid-liquid diffusion techniques using a modified (common) H-
vessel tube. Red and yellow rod-like single crystals of 1 and 2,
respectively, were formed with relative high yield (ca. 50%) four
weeks later. In particular, the occurrence of the relatively rare
[Ag2(CN)3]- species in 1, which have been previously identified in
crystals of some related SCO-CPs[14] but, as far as we know, it
has never been observed in solution or isolated in crystals as
discrete anions of single salts. The incorporation of either
[Ag(CN)2]- or a mix of [Ag(CN)2]- and [Ag2(CN)3]- into the final
polymer is most likely the result of competition between the two
moieties and their associated equilibria (eq. [i] and [ii]). This
competition seems to be influenced by the solvent medium,
reagent concentrations and overall stability and solubility of the
final product. In 1, low concentration of [AgI(CN)2]- and extended
reaction time (liquid-liquid diffusion method) seems to favor the
dissociation of [AgI(CN)2]- to give [AgI2(CN)3]-. However, in the
chosen experimental conditions the synthesis of 1 is perfectly
reproducible.
[AgI(CN)2]- ↔ CN- + AgICN
[i]
[ii]
AgICN + [AgI(CN)2]- ↔ [AgI2(CN)3]-
The presence and quantification of nitrobenzene molecules in 2
were confirmed by single-crystal X-ray diffraction studies and
thermogravimetric analysis (Figure S1, Supporting Information).
Magnetic properties. The plot of χMT vs. T for 1, where χMT is
the molar magnetic susceptibility and T is the temperature, is
displayed in Figure 1-top. At 250 K, the χMT value ca. 3.75 cm3
K mol-1 is consistent with all FeII centers in the HS state and
remains constant down to 198 K. Then, upon cooling, χMT
decreases in two marked steep steps to ca. 0.12 cm3 K mol-1 at
170 K indicating the occurrence of a complete HS→LS spin
transition. The heating mode does not match the cooling mode
showing a narrow asymmetric hysteresis loop (ca. 6 K in
average). The equilibrium temperatures T1/2, at which the molar
Figure 1 Magnetic behaviour displayed by 1 (top) and 2 (bottom). The cooling
and heating modes are shown in blue and red colours, respectively.
fractions of the HS and LS species are equal to 50%, are ꢀ↓
=
!/!
189 K and ꢀ↑ = 195 K for the cooling and heating modes,
!/!
respectively, measured at a temperature scan rate of 1 K min-1.
2
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10.1002/ejic.201700920
European Journal of Inorganic Chemistry
FULL PAPER
elongated [FeN6] octahedron with its axial positions occupied by
two nitrogen atoms of two equivalent 2,6-naphthy ligands. The
equatorial positions are occupied by four nitrogen atoms of two
equivalent pairs of [Ag(CN)2]- and [Ag2(CN)3]- groups (Figure 3).
The average bond length <Fe-N> is, respectively, 1.96 Å and
2.18 Å at 150 and 250 K. This corresponds to a variation of ca.
0.22 Å between the LS and HS states, which is consistent with a
complete spin transition. The sum of deviations from the ideal
!"
octahedron of the 12 “cis” N−Fe−N angles (∑ =
ꢀ! − 90 )
!!!
shows that the coordination center is weakly distorted in the HS
state with ∑ = 24.3° and this value decreases slightly in the LS
state down to 22.8°.
(a)
Figure 2. Calorimetric properties for compound 1. The magnetic behavior of 1
has been included as reference (dashed lines). Blue and red lines correspond
to the cooling and heating modes, respectively.
Calorimetric studies. DSC measurements were carried out for
1. The temperature scan rate in the cooling and heating modes
was 10 K min-1. The anomalous variations of the molar specific
heat due to SCO, in the form of ∆Cp vs T plot, for the cooling and
heating modes is displayed in Figure 2. The ∆Cp vs. T plot
shows the presence of two well separated peaks in the cooling
mode whereas in the heating mode the peaks overlap and
reflect the changes in the slope observed in the χMT vs. T curve
of 1. Moreover, each peak is reminiscent of those observed in
the ∂(χMT)/∂T vs. T plot (Figure S2, Supporting Information). The
obtained critical temperatures (ꢀ!↓/! = 188 K and ꢀ!↑/! = 201 K)
reproduces those obtained from the magnetic data. The average
enthalpy and entropy variations associated with the cooling and
heating modes, ∆H = 14.0 kJ mol-1 and ∆S = 71.6 J K-1 mol-1, are
consistent with the values typically displayed by Hofmann-like
clathrates of FeII featuring strong cooperative SCO behavior.[6]
The DSC measurements could not be performed on 2 because
the temperature window of the spin transition for this compound
extends beyond the limits of our calorimeter.
(b)
Structure. X-ray single-crystal studies were carried out for 1 at
150 and 250 K and for 2 at 250 K. A selection of significant bond
lengths [Å] and angles [º] and relevant crystallographic data are
collected in Tables 1-3, respectively.
Compound 1 displays the monoclinic non-centrosymmetric
Pc space group irrespective of temperature. The structural
motifs in this material are essentially the same at both
Figure 3. ORTEP representation of the representative molecular fragment for
compounds 1 (a) and 2 (b) (thermal ellipsoids are given at 40% probability).
temperatures; thus, the crystal structure at 150
K
is
The equatorial [Ag(CN)2]- and [Ag2(CN)3]- units act as bis-
monodentate bridges connecting equivalent [FeN6] thereby
defining an infinite set of parallel {Fe[Ag(CN)2][Ag2(CN)3]}∞ flat
layers which stack along [100] direction. The grids are defined
by {Fe[AgI(CN)2][AgI2(CN)3]}4 rectangular motifs (Figures 4a,b).
representatively analysed and compared with that found at 250
K. All iron atoms are crystallographically equivalent whereas
three independent Ag atoms can be seen in the asymmetric unit
at both temperatures. Each iron atom defines an axially
3
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10.1002/ejic.201700920
European Journal of Inorganic Chemistry
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The almost linear [AgI(CN)2]- anions connect the FeII centers
along the [010] direction with Fe···Fe distances of 10.255(6) Å
and 10.624(3) Å at 150 and 250 K, respectively. In contrast, the
markedly bent conformation of the [Ag2(CN)3]- anions lead to
corrugated -[Fe-[Ag2(CN)3]-Fe]- chains running along [001] with
Fe···Fe distances of 15.0(2) Å (150 K) and 15.25(8) Å (250 K)
(Figure 4a). In turn, the 2,6-naphthy ligands, axially coordinated
to the iron atoms (Fe···Fe distances are 8.991(7) Å and 9.376(4)
Å at 150 and 250 K, respectively (Figure 4c)), pillar the alternate
layers along [100] thus defining an open 3D framework with the
distorted topology of α-Po. In addition, these links "thread" the
meshes of two adjacent networks allowing the triple
interpenetration of three identical 3D networks (Figure 4d).
Moreover, coupled with the SCO phenomenon, strong
argentophilic interactions take place between these 3D networks
(Figure S3, Supporting information).
framework exhibits
described for 1.
a
much more regular geometry than
Table 2. Selected angles [º] for 1 and 2.
1 (250 K) 1 (150 K) 2 (250 K)
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)
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)
N(4)-Fe-N(6)
N(5)-Fe-N(6)
C(9)-Ag1-C(10)
178.3(5)
87.5(5)
90.8(6)
87.7(6)
91.8(5)
91.4(5)
90.3(6)
91.1(6)
89.6(5)
176.1(8)
94.5(7)
92.3(7)
88.9(7)
84.2(6)
173.1(5)
176.4(9)
178.4(6)
88.1(6)
90.4(6)
89.0(6)
91.1(6)
91.2(6)
90.3(6)
89.7(6)
90.4(6)
178.1(6)
94.9(7)
93.2(7)
86.3(7)
85.6(7)
171.9(6)
175.7(9)
170.0(7)
175.6(8)
178(2)
178.3(5)
89.7(5)
90.6(5)
89.8(5)
89.3(5)
88.9(6)
90.8(5)
89.2(5)
91.7(5)
179.5(5)
90.7(5)
89.6(5)
88.9(6)
90.8(6)
179.1(5)
C(12)-Ag(2)-C(13) 171.6(7)
Table 1. Selected bond lengths [Å] for 1 and 2.
C(11)-Ag3-N(7)
N(3)-C(9)-Ag(1)
N(4)-C(10)-Ag(1)
N(6)-C(12)-Ag(2)
N(7)-C(13)-Ag(2)
N(5)-C(11)-Ag(3)
C(9)-Au(1)-C(10)
C(11)-Au(2)-C(12)
N(3)-C(9)-Au(1)
N(4)-C(10)-Au(1)
N(5)-C(11)-Au(2)
N(6)-C(12)-Au(2)
Σ
177.2(8)
178(2)
174(2)
175(2)
173(2)
171(2)
1 (250 K)
2.264(14)
2.228(13)
2.147(14)
2.148(14)
2.14(2)
2.16(2)
2.048(14)
2.06(2)
2.04(2)
2.06(2)
2.02(2)
2.08(2)
1 (150 K)
2.006(14)
2.01(2)
1.91(2)
1.931(14)
1.95(2)
1.95(2)
2.04(2)
2.03(2)
2.05(2)
2.08(2)
2.03(2)
2.09(2)
3.024(3)
3.147(3)
3.114(2)
2 (250 K)
2.242(13)
2.237(13)
2.134(14)
2.14(2)
176(2)
169(2)
173(2)
164(2)
Fe-N(1)
Fe-N(2)
Fe-N(3)
Fe-N(4)
Fe-N(5)
Fe-N(6)
176.9(8)
177.9(7)
179(2)
176(2)
178(2)
178(2)
9.2
2.127(12)
2.158(14)
Ag(1)-C(9)
Ag(1)-C(10)
Ag(2)-C(12)
Ag(2)-C(13)
Ag(3)-C(11)
Ag(2)-N(7)
Ag(1)-Ag(2)
Ag(1)-Ag(3)
Ag(2)-Ag(3)
Au(1)-C(9)
Au(1)-C(10)
Au(2)-C(11)
Au(2)-C(12)
Au(1)-Au(2)
[FeN6]av
24.3
22.8
3.121(2)
3.281(2)
3.236(2)
The large void space generated by the framework allows
interpenetration of two identical 3D networks. The iron atoms of
one {Fe[Au(CN)2]2}∞ layer are situated below/above but
displaced from the center of the {Fe4[Au(CN)2]4} square grids
belonging to the other layer (Figure 10a). Interestingly, strong
aurophilic interactions (Au···Au distance is ca. 3.16(1) Å) are
observed between two perpendicular edges of the
{Fe4[Au(CN)2]4} square grids of two closest layers (Figure S4,
Supporting Information). Despite the interpenetration, this 3D
expanded Hofmann-like topology displays a channel along the c
axis in which disordered nitrobenzene molecules are located
(Figure 5-bottom). Short C···C contacts (smaller than the sum of
the van der Walls radii of ca. 3.7 Å) evidence the occurrence of
host-guest π-π intermolecular interactions (Figure S5 and Table
S1, Supporting Information). After removing the guest molecules
by SQUEEZE the void space is calculated as 348.6 Å3 per unit
cell, which corresponds to 34.3% of the total volume. The
analysis of the crystal structure of 2 near the LS state was not
possible due to the crack of the single crystals at low
temperature.
2.02(2)
1.97(2)
1.96(2)
1.98(2)
3.1606(12)
2.173
2.181
1.960
The structural analysis of 2 performed at 250 K shows that
this compound crystallizes in the triclinic P-1 space group and
contains one equivalent FeII center. The iron atom defines an
elongated [FeN6] octahedron equatorially coordinated by two
pairs of crystallographically independent [Au(CN)2]- groups
whereas the axial positions are occupied by two equivalent
naphthy ligands (Figure 5). The average <Fe-N> bond length of
2.17(2) Å is consistent with FeII centers in the HS state, in
agreement with the magnetic data at the same temperature. The
angular distortion represented by the parameter ∑ = 9.2º
indicates that the octahedron is even more regular than for 1.
The equatorial [Au(CN)2]- groups act as bridges linking the Fe
centers thereby defining a 2D {Fe[Au(CN)2]2}∞ grid-layered
structure laying parallel to the bc plane. The grid structure is
made up of edge-shared {Fe4[Au(CN)2]4} rhombuses defined by
iron atoms and [Au(CN)2]- linkers (Fe···Fe distances are
10.497(3) and 10.475(3) Å). The almost flat layers stack along
[100] and are pillared by the 2,6-naphthy ligands through the
axial positions of the Fe centers which form chains propagating
along the a axis where the Fe···Fe distance mediated by 2,6-
naphthy is 9.455(3) Å (Figure 5). The resulting 3D α-Po type
Discussion
FeII Hofmann-like 3D SCO-CPs derived from [MI(CN)2]- (MI = Ag,
Au) building blocks have been subject of less attention than
those constituted of [MII(CN)4]2- (MII = Ni, Pd, Pt) building blocks.
[MI(CN)2]- (MI = Ag, Au) linkers generate more open frameworks
4
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10.1002/ejic.201700920
European Journal of Inorganic Chemistry
FULL PAPER
than [MII(CN)4]2- a fact that favors interpenetration and reduces
the effective size of the pores. In addition, the [Ag(CN)2]- linker is
not coordinatively inert being able to expand the coordination
number from 2 (linear) to 3 (T-shaped or trigonal)[15] and even 4
(tetrahedral),[16] thereby affording more complex 3D networks.
Furthermore, in rare cases the [Ag(CN)2]- acts as a chemically
unstable anion affording in situ generated [Ag2(CN)3]- species.
The first observation of this building block in a SCO-PC was
described for the compound {Fe(pmd)[Ag(CN)2][Ag2(CN)3]} (pmd
= pyrimidine).[17] Compared to the title compound 1, replacement
of the organic linear linker 2,6-naphthy with an angular linker
such as pmd, with distinct orientation of the N donor atoms,
originates a very different and complicated 3D framework.
(a)
(b)
(c)
Figure 5. Compound 2: (Top) Fragment of the 3D framework with the α-Po
type network. (Bottom) Interpenetration of two identical network showing
intercalated nitrobenzene molecules. (Yellow = Au, blue = N, grey = C, orange
= Fe).
In both cases the organic ligand acts as a bis-monodentate
bridge generating infinite -[Fe-2,6-naphthy-Fe]-∞ or -[Fe-pmd-
Fe]-∞ parallel chains (Scheme 2). However, the ligand pmd
(d)
imposes
a
62º dihedral angle between the equatorial
coordination planes of [FeN6] occupied by [Ag(CN)2]- and
[Ag2(CN)3]- units (Scheme 2), making impossible to form a
simple stack of pillared layers {Fe[Ag(CN)2][Ag2(CN)3]} as
described for 1, and thereby generating an intricate self-
interpenetrated 3D CP. It is interesting to remark that using the
slightly shorter pz ligand instead of 2,6-naphthy does not favor
the formation of [Ag2(CN)3]- giving {Fe(pz)[Ag(CN)2]2}, which
exhibits the prototypal double interpenetrated α-Po networks
shown in Scheme 1. In contrast, [Ag2(CN)3]- is generated from
self-assembling CdII, pz and [Ag(CN)2]- affording the compound
{CdII(pz)[Ag(CN)2][Ag2(CN)3]},[18] which shares with the title
compound 1 the triply interpenetrated expanded version of
{Fe(pz)[Ag(CN)2]2}.[7]
Figure 4. Compund 1: (a) Fragment of a flat layer {Fe[AgI(CN)2][AgI2(CN)3]}∞.
(b) Two layers pillared by the 2,6-naphthy ligands. (d) Three interpenetrated α-
Po type frameworks. (Yellow = Au, blue = N, grey = C, orange = Fe).
5
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10.1002/ejic.201700920
European Journal of Inorganic Chemistry
FULL PAPER
same way as the herein reported compound 2, all these
derivatives share the same structural feature consisting of
twofold interpenetrating α-Po networks already discussed for
{Fe(pz)[Ag(CN)2]2} (Scheme 1). For the shortest members of the
series, pz and Fpz, the resulting more densely packed
framework without accessible voids, show highly cooperative
SCO properties exhibiting, respectively, ca. 20 and 40 K wide
hysteresis loops at relatively high temperatures. However, the
cooperativity decreases markedly for the remaining members of
the series as the length of the organic bridging ligand increases.
Nevertheless, it has been shown that the presence of suitable
guest molecules in the pores modulates and enhances the
cooperative response of the material in the bpp, dpb and bipytz
derivatives. Although the size of the 2,6-naphthy ligand is closer
to the size of pz and Fpz than bipydz, dpb or bpp ligands, the
resulting compound 2 displays the presence of characteristic 1D
channels running along [001] housing 1/2 nitrobenzene molecule
per FeII which displays short π-π interactions with the host 2,6-
naphthy ligand. Furthermore, interpenetration leads to aurophilic
interactions between the nets in 2 (AuI···AuI distances are ca.
3.16 Å), which are significantly shorter than observed for the pz
and Fpz counterparts.
N
N
N
N
N
N
naphpy
N
N
N
N
N
N
N
pz
N
pmd
N
N
N
N
Scheme 2. Propagation of the {Fe(L)}n chains defined by the organic bis-
monodentate bridging organic ligand L (see text).
There are two additional examples of 3D SCO-CP
exhibiting the [AgI2(CN)3]- building block. One is the compound
{Fe(3,5-CH3py)2[Ag2(CN)3][Ag(CN)2]}
(3,5-CH3py
=
3,5-
dimethylpyridine) in which the monodentate nature of the axial
organic ligand transforms the FeII centers into planar nodes with
coordination number 4 affording an expanded version of the
prototypal structure of CdSO4.[14b] Given the open nature of this
framework, the crystal packing of the 3,5-CH3py derivative
consists of three identical interpenetrated subnets. The other
example is {Fe(dpb)[Ag(CN)2][Ag2(CN)3]}·nSolv where dpb is the
long bridging ligand 1,4-di(pyridin-4-yl)benzene (Scheme 1).[14c]
Conclusions
Two novel bimetallic FeII-MI (MI = Ag and Au) cyano-bridged 3D
frameworks based on the ditopic 2,6-Naphthy ligand namely
{Fe(2,6-naphthy)[Ag(CN)2][Ag2(CN)3]2}
(1)
and
{Fe(2,6-
This compound displays
a very complicated porous 3D
naphthy)[Au(CN)2]2}·(0,5)PhNO2 (2) have been synthesized by
liquid-liquid slow diffusion techniques. 1 exhibits the rare in-situ
framework that can be deconstructed into two identical
interpenetrated networks with the topology of CdSO4 in which
the “pseudo-square” planar FeII nodes are defined by the dpb
and [Ag(CN)2]- bridges, while the [Ag2(CN)3]- groups can be seen
as connectors between the two CdSO4 subnets.
A common characteristic of the pmd, 3,5-CH3py and 2,6-
naphthy SCO-CPs is the occurrence of more or less marked
incipient two-step SCO. Moreover, the title compound 1 displays
a moderate cooperativity characterized by an unsymmetrical
hysteresis loop ca. 6 K wide. In contrast, the dpb derivative with
the organic bridging ligand longer favors the presence of voids
occupied by solvents, which influence the completeness of the
generated
[Ag2(CN)3]-
species
incorporated
in
the
{Fe[Ag(CN)2][Ag(CN)3]}n layers pillared by 2,6-naphthy ligands
that "thread" the meshes of adjacent networks and lead to a
triple interpenetrated 3D framework. This densely packed
structure prevents guest inclusion. Moreover, short Ag···Ag
contacts take place evidencing the occurrence of argentophilic
interactions between the 3D interpenetrated networks. On the
other hand, 2 consists of two interpenetrated 3D networks, made
up of 2D {Fe[Au(CN)2]2}∞ layers pillared by 2,6-naphthy ligands
axially coordinated to the FeII ions. Interestingly, 1D channels
within this pillared structure house nitrobenzene guest molecules
stabilized by host-guest π-π intermolecular interactions and
strong aurophilic interactions between the interpenetrated 3D
networks. 1 and 2 with different structural architectures originate
two distinct SCO behaviors. 1 features a more abrupt two-step
spin transition taking place in an interval of temperature 23 K
wide, while 2 shows a much more gradual spin transition
extending over 150 K. Despite this difference both compounds
exhibit unsymmetrical hysteresis ca. 6 K wide.
transition.
In
particular,
for
{Fe(dpb)[Ag(CN)2][Ag2(CN)3]}·2DMF·CH3CN the two-step SCO
extends over a wide interval of temperatures, 200-75 K,
exhibiting hysteresis loops ca. 10 K wide. Apparently, the
presence of [Ag(CN)2]- and [Ag2(CN)3]- anions confers some
degree of flexibility to the framework allowing accommodation of
the [FeN6] structural changes during the SCO, a fact that
explains the lack of strong cooperative SCO and larger
hysteresis in this series.
The inertness of the [Au(CN)2]- building block against
dissociation and/or increase of the coordination number makes
its structural chemistry much more predictable than that
described for [Ag(CN)2]-. Independently of the bridging organic
ligand length, all derivatives so far reported can be generically
Experimental Section
Materials. K[Ag(CN)2], K[Au(CN)2] and Fe(BF4)2·7H2O were purchased
from commercial sources and used as received whereas the synthesis of
formulated {Fe(L)[Au(CN)2]2}·Guest with
L
=
bipytz,[10a]
bipydz,[10a,b] bpp,[12] dpb,[13] pz[8] and Fpz[9] (Scheme 1). In the
6
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10.1002/ejic.201700920
European Journal of Inorganic Chemistry
FULL PAPER
the 2,6-Naphthy ligand was performed as it is described in Section 1.8.1
in the Supporting Information.
reported previously for other members of the Hofmann-type spin
crossover clathrate family.
Crystal growth. Single crystals of 1 were grown by slow liquid-liquid
diffusion technique using a modified H-vessel with a third tube. The
peripheral tubes (total volume ca. 10 mL) contained Fe(BF4)2·7H2O
(0.077 mmol, 26 mg) and K[Ag(CN)2] (0.154 mmol, 30.6 mg) salts,
respectively. The central tube (total volume ca. 10 mL) contained 2,6-
naphthy ligand (0.077 mmol, 10 mg). Each individual tube was filled with
H2O:MeOH (1:1). Afterwards, the tubes were sealed with parafilm. Four
weeks later, red rod-like single crystals were formed in the tube which
originally contained Fe(BF4)2·7H2O salt with relative high yield (ca. 50%).
C13H6Ag3FeN7 (639.69): C 24.38, H 0.94, N 15.32; found C 23.90, H 0.92,
N 15.10.On the other hand, yellow rod-like single crystals of 2 (yield ca.
50%) were grown in a H-vessel tube (total volume ca. 10 mL) containing
Fe(BF4)2·7H2O (0.077 mmol, 26 mg) and 2,6-naphthy ligand (0.077
mmol, 10 mg) whereas the other contained K[Au(CN)2] (0.154 mmol,
44.4 mg). Each individual tube was first filled with MeOH and secondly
with a PhNO2:MeOH (1:10) solution. Finally, the tubes were sealed with
parafilm. The presence and quantification of the guest molecules were
deduced from TGA analysis and confirmed by means of single-crystal X-
ray diffraction. C15H8.5Au2FeN6.5O (745.57): C 24.14, H 1.14, N 12.20;
found C 24.51, H 1.16, N 12.35.
CCDC 1565402-1565404 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
Table 3. Crystal data for compounds 1 and 2.
Compound
Empirical formula
Mr
1 (250 K)
C13H6Ag3FeN7
639.69
1 (150 K)
2 (250 K)
C15H8.5Au2FeN6.5O
745.57
Crystal system
Space group
Temperature
a (Å)
monoclinic
Pc
250 K
9.3760(8)
triclinic
P-1
250 K
9.4548(9)
150 K
8.991(5)
b (Å)
c (Å)
10.6240(8) 10.255(5) 10.4730(9)
11.2753(9) 11.110(4) 10.4975(12)
90
90
83.369(9)
α (°)
126.714(9) 126.22(3) 85.076(9)
β (°)
90
900.3(2)
2
2.360
600
4.012
0.06 x 0.06 x 0.10
3389
3076
90
826.4(7)
80.288(8)
1015.4(2)
2
2.439
672
15.133
0.08 x 0.08 x 0.18
5127
γ (°)
V (Å3)
Z
Dc (mg cm-3)
F(000)
2.571
Physical characterization. Variable temperature magnetic susceptibility
measurements were carried out by using samples (15-20 mg) consisting
of single crystals of the titled compounds, using a Quantum Design
MPMS2 SQUID susceptometer equipped with a 5.5 T magnet, operating
at 1 T and at temperatures from 300-1.8 K.
µ (Mo-Kα)(mm-1)
Crystal size (mm)
No. of total reflections
No. of reflections
[I>2σ(I)]
4.371
3307
3064
3187
0.0684
0.0753
1.176
0.0637
0.0685
0.836
0.1033
0.1402
1.021
R [I>2σ(I)]
R [all data]
S
Calorimetric measurements were performed using a differential
scanning calorimeter Mettler Toledo DSC 821e. Low temperatures were
obtained with an aluminium 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 dry box under a flow of dry
nitrogen gas to avoid water condensation. The measurements were
carried out using around 20 mg of single crystals of 1 sealed in
aluminium 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.
R1 = Σ ||Fo| - |Fc|| / Σ |Fo|; wR = [ Σ [w(Fo2 - Fc2)2] / Σ [w(Fo2)2]]1/2
w = 1/ [σ2(Fo2) + (m P)2 + n P] where P = (Fo2 + 2Fc2) / 3;
m = 0.1090 (1 (250 K)), 0.1029 (1 (150 K)) 0.1886 (2 (250 K))
n = 1.5595 (1 (250 K)), 29.0173 (1 (150 K))
.
Acknowledgements
This work was supported by the Spanish Ministerio de Economia y
Competitividad (MINECO), FEDER (CTQ2013-46275-P and
CTQ2016-78341-P), Unidad de Excelencia María de Maeztu (MDM-
Single-crystal X-ray data were collected on an Oxford Diffraction
Supernova. In all cases MoKα radiation (λ = 0.71073 Å) was used. A
data scaling and empirical or multiscan absorption correction was
performed. The structures were solved by direct methods using
SHELXS-2014 and refined by fullmatrix least-squares on F2 using
SHELXL-2014.[19] 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. Relevant crystallographic data for 1 and 2 are
gathered in Table 3. Compound 1 shows thermal disorder at 250 and 150
K as well as residual electron density (3.68−4.49 eÅ-3) located very close
to the Ag sites at 150 K (alerts A and B). Nitrobenzene guest molecules
show thermal disorder at 250 K (alerts A and B) for 2 and residual
electron density (2.48−10.94 eÅ-3) located very close to the Fe and Au
atoms (alerts A and B). However, the structures of the host frameworks
as well as in general those of the guest molecules are reasonably well
determined. More importantly, they fully convey all the chemical and
structural meaning required to explain correctly the spin crossover
behavior in this series of compounds and compare well with the data
2015-0538),
and
the
Generalitat
Valenciana
through
PROMETEO/2016/147. L.P.L. and F.J.V.M. thank, respectively, the
Universidad de Valencia and a MINECO for a predoctoral (FPI) grant.
Keywords: Coordination polymers • MOFs • Hofmann
Clathrates • iron(II) complexes• spin crossover
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FULL PAPER
Text for Table of Contents
Interlocked Spin-
Crossover MOFs
The synthesis, structure, magnetic and
calorimetric spin crossover (SCO) properties of
doubly- and triply-interlocked iron(II)
frameworks based on the bismonodentate 2,6-
naphthyridine ligand are described and
discussed in the frame of Hofmann-like
clathrate SCO compounds.
Lucía Piñeiro-López,
Francisco-Javier Valverde
Muñoz, Maksym Serediuk,
Carlos Bartual Murgui, M.
Carmen Muñoz, José
Antonio Real*[
Page No. – Page No.
Cyanide bridged FeII-MI
bimetallic Hofmann-like
spin-crossover
coordination polymers
based on 2,6-
Naphthyridine
9
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