Solvent-Induced Reversible Spin-Crossover in a 3D Hofmann-Type Coordination Polymer and Unusual Enhancement of the Lattice Cooperativity at the Desolvated State

Article abstract of DOI:10.1021/acs.inorgchem.0c02240

The new 3D Hofmann-type coordination polymer [Fe(dpyu){Pt(CN)4}]·9H2O [dpyu = 1,3-di(pyridin-4-yl)urea] exhibits reversible interchange between two- and one-step spin-crossover behavior, associated with desorption/resorption of lattice water molecules. Solvent water removal also induces an increase of the spin-transition temperature, indicating strong lattice cooperativity, observed for the first time in a 3D Hofmann-type coordination polymer.

Full text of DOI:10.1021/acs.inorgchem.0c02240

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Solvent-Induced Reversible Spin-Crossover in a 3D Hofmann-Type
Coordination Polymer and Unusual Enhancement of the Lattice
Cooperativity at the Desolvated State
Dibya Jyoti Mondal, Subhadip Roy, Jyoti Yadav, Matthias Zeller, and Sanjit Konar*
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ABSTRACT: The new 3D Hofmann-type coordination polymer [Fe(dpyu){Pt(CN)4}]·9H2O [dpyu = 1,3-di(pyridin-4-yl)urea]
exhibits reversible interchange between two- and one-step spin-crossover behavior, associated with desorption/resorption of lattice
water molecules. Solvent water removal also induces an increase of the spin-transition temperature, indicating strong lattice
cooperativity, observed for the first time in a 3D Hofmann-type coordination polymer.
istable materials have generated significant interest in the
sites.8,9 The 3D Hofmann-type metal−organic framework
reported by Kepert and co-workers, namely,
dpsmePt·2/3dpsme·xEtOH·yH2O8 [where dpsme = 4,4′-di-
(pyridylthio)methane], represents an exemplary case where a
unique hysteretic three-step spin transition has been reported.
The SCO properties in Hofmann-type coordination polymers
are also found to be affected by the guest shape, size, and
location, cavity within the host lattice, functional groups
present in the guest and host, interaction type and strength,
and guest adsorption/desorption processes.4a
1
Bfield of materials science.
Spin-crossover (SCO)
complexes, one of the archetypical bistable materials, generally
contain d4−d7 metal ions capable of switching between the
high-spin (HS) and low-spin (LS) state under an external
stimulus (for example, temperature, pressure, light, magnetic
field, guest molecules, and so on).2 Simultaneously, the SCO
phenomenon can also induce remarkable changes in the
various material properties ranging from mechanical to optical
to magnetic to polarizability, etc., thus paving the way for
numerous potential applications such as switching devices,
multiswitches, and high-order data storage.3
In the present study, to construct a Hofmann-type
coordination polymer, a bent-flexible pillar ligand, namely,
[1,3-di(pyridin-4-yl)urea] (dpyu; Scheme 1), was selected that
Many SCO complexes have been synthesized over the past
decades by varying the nature of ligands, noncoordinating
counterions, and solvent molecules, and a wide range of SCO
behaviors, including hysteretic, complete, incomplete, abrupt,
and gradual one-step and multistep transitions, have been
reported.4 The results have shown that attaining elastic
cooperativity between metal centers through the maximization
of intermolecular interactions, such as hydrogen bonding, π···π
stacking, van der Waals, etc., is key to strategically developing
functional SCO materials for potential applications.2b,5
Another successful strategy involves the design of SCO ion-
bearing coordination polymeric networks, where interactions
between the spin-transition centers are facilitated by strong
coordination bonding.5,6 Indeed, a porous SCO coordination
polymer featuring fine-tuned host−guest interaction can
combine the benefits of the above two strategies to render
multipath propagation of elastic interactions throughout the
lattice.5 In this context, a fascinating platform to obtain
differing degrees of cooperativity is offered by the Hofmann-
type SCO coordination polymers, where four CN groups from
tetracyanometalate bridging ligands occupy the basal positions
of [FeN6] sites and monodentate or bis-monodentate ligands
link the axial positions into 2D or 3D networks.2c,4a,5,7
Multistep transitions have been targeted successfully in this
type of system via clever modification of the bimetallic
interlayer spacing and the generation of inequivalent SCO
Scheme 1. Representation of the dpyu Ligand
is functionalized with a hydrogen-bond-capable urea backbone
to promote both internetwork urea···urea synthons as well as
urea−solvent (guest) interactions.10 Thus, a new 3D
coordination polymer, [Fe(dpyu){Pt(CN)4}]·9H2O [1(Pt)·
9H2O], has been synthesized and characterized. The
compound exhibits cooperative single-step SCO behavior
with a 9 K thermal hysteresis loop (T1/2↑ = 190 K and
T1/2↓ = 181 K). Remarkedly, a two-step SCO behavior has
been observed and the spin-transition temperature increases
upon desorption of guest solvent water molecules at the
Received: July 28, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c02240
© XXXX American Chemical Society
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expense of hysteresis, signifying an improvement of the lattice
cooperativity. This phenomenon is unprecedented in 3D
Hofmann-type coordination polymers. The resorption of water
molecules reestablishes the one-step SCO behavior.
Light-yellow hexagonal-shaped single crystals of 1(Pt)·9H2O
were obtained using slow reactant diffusion by layering an
aqueous solution of Fe(ClO4)2·6H2O onto an ethanolic
solution of the dpyu ligand and K2[Pt(CN)4] (see the
Supporting Information for details).
The 100 K single-crystal data of 1(Pt)·9H2O (see the
Supporting Information for details) reveal that the compound
is a typical 3D Hofmann-type coordination polymer.4a It
crystallizes in the orthorhombic crystal system with the space
group Pmmm.
The asymmetric unit of 1(Pt)·9H2O features one iron(II)
center, one independent platinum atom with a half-disordered
ligand, a CN fragment, and a lattice water molecule (Figure
1a). The geometry of the iron(II) atom is slightly distorted
from a regular octahedron. The four equatorial positions are
coordinated to the symmetry-equivalent cyanide nitrogen
atoms of [Pt(CN)4]2− fragments, and the two axial sites are
taken up by the nitrogen atoms of bridging dipyridyl urea
ligands (Figure 1b). The dpyu ligand and resolved solvate
water molecules exhibit systematic symmetry-imposed disorder
(see Figure S1 for details) that is partially static by a whole
rotation of the entire ligand and partially dynamic by slight
offsets of the ligands from a crystallographic mirror plane,
induced by hydrogen bonding to the symmetry-disordered
water molecule [Figure 1c; established by thermogravimetric
analysis (TGA) measurements]. The lattice void space that
accounts for about half the lattice volume (at least 46%; see
Figure 1d,e and the Supporting Information) contains an
additional eight unresolved water solvate molecules. Further
structural details can be found in the Supporting Information.
Upon an increase of the temperature, the symmetry-induced
disorder, large void space, and pronounced liberation of the
ligands lead to a substantial drop in data quality, and no useful
numerical data related to, e.g., spin states at the iron center,
can be extracted anymore. The general structure is, however,
maintained [also evidenced by powder X-ray diffraction
(PXRD)]. It is worth pointing out that the water molecule
hydrogen bonded to the urea fragment at 100 K is not resolved
at 220 K anymore, indicating that the O−HOurea hydrogen
bonds are broken at this temperature or become highly
fluxional.
Figure 1. (a) Asymmetric unit of the compound with an atom
numbering scheme. (b) View of the framework along the ac plane
(only one of four symmetry-equivalent disordered components is
shown). (c) Propagation of the network architecture: one ladder
down and another ladder up on the other side (alternative disordered
components are marked gray). Only the one crystallographically
derived water molecule is shown. (d) Perspective view the of the
network along the a axis without solvent. A total of 46% of the cell is
solvent-accessible through the large channel visible here. (e) Space-
filling perspective view of the network along the a axis without
solvent.
With a further decrease in the temperature, the χMT value
gradually drops in a very gradual fashion to a certain level until
it reaches 1.95 cm3 K mol−1 at ca. 125 K. It then decreases
again more rapidly because of zero-field splitting for the rest of
the HS iron(II) species.11,12 Differential scanning calorimetry
(DSC) measurement was performed in the cooling and heating
modes (Figure S4). The heat flow (mW) versus T plot shows
two well-distinguished peaks in the cooling and heating modes
and reflects the changes in the slope of χMT versus T of 1(Pt)·
9H2O. The observed critical temperature values (T1/2↓ = 185
K and T1/2↑ = 198 K) are in agreement with the T1/2 values
from the magnetic data. The average enthalpy and entropy
changes in the cooling (exothermic) and heating (endother-
mic) modes are ΔH = 7.11 kJ mol−1 and ΔS = 36.32 J mol−1
K−1, consistent with the values typically displayed by the 3D
Hofmann SCO compounds.13
To investigate the SCO properties of the framework
compound, variable-temperature magnetic susceptibility data
of 1(Pt)·9H2O (powdered sample prepared by grinding the
single crystals) were recorded in the temperature range of 2−
300 K in both warming and cooling modes with a rate of 3 K
min−1 (Figure 2). The phase purity of the bulk sample was
confirmed by the agreement of the patterns from PXRD with
the single-crystal structure data (Figure S8). At 300 K, the
value of χMT (3.40 cm3 K mol−1) suggests a HS iron(II) state.
Upon cooling, this value remains almost constant up to 204
K, but upon further cooling, it abruptly drops until it reaches,
at 150 K, a value of 2.09 cm3 K mol−1, which coincides with
3
the /5 fraction of the HS state to the LS site (HS0.6LS0.4).
This confirms the one-step incomplete SCO process with
critical temperatures T1/2↑ = 190 K and T1/2↓ = 181 K in the
heating and cooling modes, respectively, with a pronounced
hysteresis loop around 9 K due to the strong cooperativity.
To explore the effect of guest molecules on the SCO
behavior from the viewpoint of host−guest chemistry, we
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To capture the overall prospects of structural changes,
variable-temperature synchrotron PXRD was employed,
revealing the single-phase nature across the spin-state
transition range. The shifts in the Bragg reflections correlate
well with the temperature dependence of the molar magnetic
susceptibility data of the solvated complex and its solvent-free
form (Figure 3).
Figure 2. (a) Variable-temperature χMT of 1(Pt)·9H2O: blue, freshly
prepared crystalline sample; green, desolvated sample; red, resolvated
sample. (b) Hysteresis loop in solvated 1(Pt)·9H2O. (c) Derivative
graph of the desolvated sample, where the red dashed line indicates
the room temperature spin-state transition.
investigated desorption of the solvent molecules upon heating
of the hydrated polycrystalline sample 1(Pt)·9H2O at 540 K
according to TGA (Figure S5). Using the desolvated material,
we measured the temperature dependence of χMT in the range
2
of 380−2 K, revealing a two-step transition in the form of /3
1
and /3 HS states (Figure. 2a). At 380 K, the χMT value of
1(Pt) is equal to 3.06 cm3 K mol−1, indicating a HS iron(II)
center, which is sustained down to 316 K upon cooling. At 248
K, the χMT value drops in a first step to 2.48 cm3 K mol−1 and
in a second step to 1.25 cm3 K mol−1 at 143 K. Below this
temperature, χMT decreases further, presumably because of
zero-field splitting of the HS iron(II) fraction. As per the first-
order derivative plot of χMT versus T (Figure 2c), the first and
second step transition temperatures T1/2 are 292 and 195 K,
respectively. The heating loop follows the same path, without
any hysteresis, and represents an incomplete spin-conversion
state of the iron(II) sites to LS. The first spin-state switching is
at room temperature, which is an important property of any
SCO material for practical application purposes (Figure S6). In
the absence of a single-crystal X-ray structure because of the
partial degradation of crystallinity with dehydration, it was not
possible to establish a magnetostructural correlation. However,
the phenomenon of two-step SCO behavior with an increase of
the spin-transition temperature may be tentatively attributed to
the removal of elastic lattice frustration from the guests and
host−guest interactions, which compete with host−host
interactions.14 Moreover, upon desorption, the H2O···urea
hydrogen-bonding interactions, which are unfavorable for the
HS state because of increased electron density to the iron(II)
sites, are eliminated. Consequently, elimination of the solvate
water molecules might favor the HS state over the LS state.15
Figure 3. Variable-temperature synchrotron PXRD peak evolution
over the range of 11.0−12.0° and schematic graphical diagrams of the
iron(II) sites drawn as circles (orange = LS and green = HS): solvated
complex 1(Pt)·9H2O (a); desolvated complex 1(Pt) (b).
Furthermore, variable-temperature Raman spectroscopy14
was performed at three different temperatures to gain some
insight into the vibrational details of the HS and LS states of
the solvated and desolvated complexes as well as to check
retention of the structural framework after solvent water
removal (Figure S7). Most importantly, in the Raman
spectrum, for the CN vibrational mode, the wavenumber
decreases with an increase in the temperature due to a decrease
in the CN bond length (strength) from the LS to HS state.
The solvated complex exhibits well-resolved doublet peaks at
2187 and 2205 cm−1 at 80 K (LS) corresponding to the
symmetric and asymmetric stretching modes of the cyanide
vibration, respectively, and a prominent band at 2201 cm−1
representing the symmetric cyanide stretch at both 250 and
400 K (HS). Similarly, for the desolvated complex, doublet
C
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peaks are observed at 2193 and 2206 cm−1 for the 2/3 LS state
Bhopal 462066, India; Department of Chemistry, The ICFAI
University Tripura, Mohanpur, Agartala, Tripura 799210,
India; orcid.org/0000-0002-0097-6113
Jyoti Yadav − Department of Chemistry, Indian Institute of
Science Education and Research (IISER) Bhopal, Bhauri,
Bhopal 462066, India
Matthias Zeller − Department of Chemistry, Purdue University,
West Lafayette, Indiana 47907, United States; orcid.org/
0000-0002-3305-852X
1
(80 K) and at 2190 and 2203 cm−1 for the /3 LS state (250
K). A shoulder band appears at 2199 cm−1 in the HS state
(400 K).14 Interestingly, the SCO compound also features
reversible guest exchange multifunctional properties because
the single-step SCO behavior can be recovered when the
desorbed sample is resorbed with water molecules. The parent
framework is robust toward repeated solvation and resolvation,
as evidenced by the PXRD pattern (Figure S8).
In summary, a new 3D Hofmann-type coordination polymer
of the formula [Fe(dpyu){Pt(CN)4}]·9H2O [where dpyu =
1,3-di(pyridin-4-yl)urea] with single-step hysteric SCO behav-
ior has been reported. Although the hysteresis loop is lost, a
two-step SCO behavior has been observed and the spin-
transition temperature is increased upon desorption of guest
solvent water molecules. This is in stark contrast to what is
typically observed in 3D Hofmann-type coordination polymers
containing guest solvent molecules, where the cooperativity is
less pronounced in the desolvated phase because of removal of
the host−guest hydrogen-bonding interactions. The rare
phenomenon that guest removal positively impacts the lattice
cooperativity in this 3D Hoffman-type coordination polymer
can be roughly ascribed to the elimination of elastic frustration
and host−guest interactions resulting in the proliferation of
host−host interactions. The present investigation also revealed
reversible guest exchange properties by recovering the one-step
spin transition of the complex through resolvation of the
desolvated analogue. This endows the host system with a
sensory function. Overall, such a compound holds the potential
to be an excellent candidate for a futuristic switchable
molecular material.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02240
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Science and Engineering Research
Board (Project SERB/CRG/2018/000072), Government of
India. D.J.M. thanks CSIR, India, for a fellowship, IISER
Bhopal for instrumentation facilities, RRCAT-Indore, DAE,
Government of India, for the synchrotron radiation source at
Beamline-12, UGC−DAE Consortium for Scientific Research,
Indore for DSC analysis, and also Dr. Surajit Saha, Department
of Physics, IISER Bhopal, for Raman spectroscopy.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02240.
■
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Experimental information, syntheses, PXRD, Fourier
transform infrared, magnetic and calorimetric measure-
ment data, Raman spectra, and crystal data and
refinement details (PDF)
Accession Codes
CCDC 2012230 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Sanjit Konar − Department of Chemistry, Indian Institute of
Science Education and Research (IISER) Bhopal, Bhauri,
Bhopal 462066, India; orcid.org/0000-0002-1584-6258;
Phone: +91-755-2691313; Email: skonar@iiserb.ac.in;
Fax: +91-755-2692392
Authors
Dibya Jyoti Mondal − Department of Chemistry, Indian
Institute of Science Education and Research (IISER) Bhopal,
Bhauri, Bhopal 462066, India
Subhadip Roy − Department of Chemistry, Indian Institute of
Science Education and Research (IISER) Bhopal, Bhauri,
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