Spin crossover intermediate plateau stabilization in a flexible 2-D Hofmann-type coordination polymer

Article abstract of DOI:10.1039/c4cc01079e

The abrupt and hysteretic two-step spin crossover in a new triazole-based 2-D Hofmann-type complex shows a record breaking 120 K intermediate plateau (IP) region stabilized by negative cooperative interactions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01079e

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Spin crossover intermediate plateau stabilization
in a flexible 2-D Hofmann-type coordination
polymer†
Cite this: Chem. Commun., 2014,
50, 3838
Received 11th February 2014,
Accepted 22nd February 2014
Y. Maximilian Klein,ab Natasha F. Sciortino,a Florence Ragon,a
Catherine E. Housecroft,b Cameron J. Keperta and Suzanne M. Neville*a
DOI: 10.1039/c4cc01079e
www.rsc.org/chemcomm
The abrupt and hysteretic two-step spin crossover in a new magnetically inequivalent SCO sites. Such structural deviations may
II
triazole-based 2-D Hofmann-type complex shows a record break- be subtle, such as in the dinuclear material [Fe2 (ddpp)2(NCS)4]ꢀ
ing 120 K intermediate plateau (IP) region stabilized by negative 4CH2Cl2 (ddpp = 2,5-di(20,200-dipyridylamino)pyridine), for which the
cooperative interactions.
coordinative linkage of SCO sites generates slight iron(II) site inequi-
valence,22 or dramatic, such as in the hybrid material [FeII(dpp)2]-
Spin crossover (SCO) complexes are of current interest because of [Ni(mnt)2]ꢀMeNO2 (dpp
= 2,6-bis(pyrazol-1-yl)pyridine), which
their potential use in advanced technologies, such as memory storage undergoes multiple spin-state conversions of both components.13
devices and molecular sensing.1–5 Many of these applications require An alternative, or additional path to multi-stability is through an
hysteretic and abrupt complete SCO transitions centred around or accompanying symmetry-breaking event triggered by or coincident
near room temperature.1,3–5 To prepare materials with optimal with SCO. There are several recent examples of this type, such as
properties and then transform them into functional and purposeful [FeIII(nsal2trien)]SCN and [FeIII(L2)]ClO4, which show long-range
materials we must fully understand the link between structure and order at the IP owing to a symmetry conversion.18,23
function.6 This is why many SCO studies reported include detailed
We are interested in designing a rational platform for routinely
structural analysis coupled with magnetic studies with an eye towards achieving multi-stability and then exploiting this to systematically
fine tuning solid state structural features such as intra- and inter- tune and optimize multi-stable SCO properties. The multi-stable
molecular interactions, which are of key consequence in attaining magnetic characters to optimize include: abrupt and hysteretic
lattice cooperativity. Using such information, recent studies have transitions; high transition temperatures; and large regions of ther-
taken steps towards the incorporation of SCO materials into useful mal stability for each step.3 Towards attaining abrupt and hysteretic
devices through probing the effect of miniaturisation on long-range transitions with high transition temperatures, we focus here on the
communication by preparing, for example, nanoparticles, nano- Hofmann-type framework system, comprised of [FeIIMII(CN)4] 2-D
crystals and thin layers.7–10
grids (MII = Pt, Pd, Ni) linked or separated by aromatic N-donor
There are varied SCO behaviour characters which are possible, ligands.16,19,24–32 These systems show high SCO cooperativity, facili-
such as complete, incomplete, abrupt, gradual one-step and multi- tated by the bimetallic layered structure and furthermore, provide a
step transitions, and each are well documented in the literature.2,11 flexible platform for fine-tuning inter- and intra-molecular interac-
Multi-step transitions, i.e. those that display three- or more regions of tions via synthetic manipulation of the interlayer spacing (host or
thermally stable, distinct magnetic states (e.g. HS–HS, HS–LS, LS–LS, guest related). Importantly, multi-step transitions are a conceptually
HS = high spin, LS = low spin), are a recent feature in the literature proven target feature in Hofmann-type materials due to the distor-
owing to the enhanced possibilities these materials provide for table nature of the bimetallic layer largely driven by solid state
additional information storage capacity (ref. 12–21 and references interactions.16,17,19,29,32,33 The generation of inequivalent SCO sites
therein). The most simplistic structural feature that enables a multi- can be conceivably targeted in Hofmann-type materials through
step transition is the presence of two- (or more) structurally and exploiting the connectivity in the interlayer spacing, i.e. the incorpora-
tion of ligands with unsymmetric binding mode and/or interaction
a School of Chemistry, The University of Sydney, Sydney, NSW, 2006 Australia.
sites. Whilst to date the majority of Hofmann-type materials have
focused on pyridyl N-donor ligands33 we utilize a 1,2,4-triazole group
which through its monodentate binding mode provides a less
symmetric ligand coordination environment. This binding further-
more generates multiple (unsymmetric) interaction site possibilities
via the unbound atoms. Finally, in choosing this type of ligand
E-mail: suzanne.neville@sydney.edu.au
b Department of Chemistry, University of Basel, Spitalstrasse 51, CH-4056,
Switzerland
† Electronic supplementary information (ESI) available: Experimental, single
crystal and powder X-ray diffraction. CCDC 963504–963506. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c4cc01079e
3838 | Chem. Commun., 2014, 50, 3838--3840
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Table 1 Selected single crystal structural parameters
Parameters (Fe1, Fe2)
100 K
180 K
260 K
FeII spin state
LS, LS
9.2, 4.4
1.95, 1.94
125.1, 121.8
16.73, 9.31
HS, LS
21.2, 1.2
2.15, 1.95
130.7, 122.2
22.09, 8.36
HS, HS
11.3, 5.4
2.15, 2.16
131.3, 121.3
20.44, 9.65
Sa (1)
b
hdFe–N
i
(Å)
Fe–N–N(trz)c (1) (A, B)
Fe–N(C)4 tiltd (1)
a
Octahedral distortion parameter calculated by sum of |90 ꢁ y| for
the twelve cis-N–Fe–N angles in the octahedron.5 b Average Fe–N
distance. c Fe–N–N(trz) angle (A and B, as defined in Fig. 1(b)). d Equatorial
out-of-plane octahedral tilt angle.
The single crystal structural data collected at 260 K reveal that
both Fe1 and Fe2 are in the HS state (d
, Table 1) and that the
hFe–Ni
coordination environment of Fe1 is considerably more distorted
than that of Fe2, which displays an almost perfect octahedron
(octahedral distortion parameter, SFe-1: 14.41, SFe-2: 3.21, Table 1).
This large distortive difference indicates that a multi-step (or half-
step) transition is possible as with cooling the HS to LS transition
will be considerably less favoured for Fe2 than Fe1.6 Further
exemplifying the greater distortion to Fe1 than Fe2 is the more
pronounced octahedral tilt out of the [FePd(CN)4] plane evident
both visually (Fig. 1(c)) and quantitatively (Table 1). Subsequent
magnetic susceptibility measurements indeed reveal a striking multi-
stable character detailing an abrupt and hysteretic two-step SCO with a
record-breaking IP region of 120 K (Fig. 2(a)).14 Detailing the magnetic
curve, the wMT values remain constant at ca. 3.4 cm3 molꢁ1
K
with cooling from room temperature, indicating that iron(II)
exists in the HS state, until 230 K, where there is a rapid
Fig. 1 (a) N-Thiophenylidene-4H-1,2,4-triazol-4-amine (thtrz), (b) the
asymmetric unit of 1 (excluding solvent molecules), (c) a 2-D layer with
ligand interactions and larger guest-filled spaces. (d and e) Adjacent layers
inter-digitate with ligand p-stacking interactions (---).
platform we can tailor the interactions (host–host or host–guest) in
the interlayer space with a specific interest towards stabilizing mixed
HS–LS species to attain large thermally stable IP regions.
The material [Fe(thtrz)2Pd(CN)4]ꢀ(EtOH)(H2O) (thtrz = N-thiophenyl-
idene-4H-1,2,4-triazol-4-amine, Fig. 1(a)), 1, is readily formed as single
crystals or a pure bulk polycrystalline powder. Full single-crystal
structural analysis at 260 K revealed FeIIN6 octahedral environments
coordinated equatorially by N-donor [Pd(CN)4]2ꢁ metallo-ligands and
axially to terminal N1-donor thtrz ligands (Fig. 1(b)). The overall
structure motif adopted, of a 2-D inter-digitated pillared-layered
Hofmann-type topology, consists of undulating 2-D [FePd(CN)4] grids
separated by thtrz ligands (Fig. 1(c)). The layers stack such that the thtrz
ligands align in an eclipsed face-to-face fashion generating an array of
p–p interactions and defining a pseudo 3-D character (Fig. 1(d and e)).
The structurally distinct iron(II) sites (Fe1 and Fe2) and associated
ligands (LFe1 and LFe2, Fig. 1(b)) are generated by self-complementary
hydrogen-bonding interactions between pairs of ligands which direct
the ligand geometry by ‘locking-in’ the Fe–N–N(trz) angle (Fig. 1(b) A
and B, Table 1). Notably, this results in vastly different angles of ligand
approach into the interlayer spacing and generates alternating regions
of ligand hydrogen-bonding and guest-filled 1-D pores (Fig. 1(c)). The
guests are additionally involved in intermolecular interactions with the
host framework (Fig. S5, ESI†).
Fig. 2 (a) wMT versus temperature (1 K minꢁ1) for 1 (cooling (blue) and heating
(red) curves). Inset: schematic of HS(yellow)–HS to HS–LS(purple) to LS–LS
transition of Fe1 and Fe2 (interactions —). (b) Variable temperature synchrotron
powder X-ray diffraction data single peak evolution over the range 7.8–8.41.
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drop in wMT to 1.6 cm3 molꢁ1 K. This value remains constant
until 130 K where there is a further rapid decrease in wMT to
0.05 cm3 molꢁ1 K representing a complete spin state conversion
of iron(II) sites to LS. The heating curve shows the same two-step SCO
behaviour with hysteresis loops of 19 K (T1/2k: 123 K T1/2m: 142 K)
and 16 K (T1/2k: 228 K T1/2m: 244 K). The IP corresponds to a 50 : 50
ratio of HS and LS iron(II) sites distributed in the crystalline lattice.
To gain an overall perspective of the structural changes accom-
panying the two-step switching in variable temperature synchrotron-
based powder X-ray diffraction was used and most notably revealed a
single-phase behaviour over the entire SCO temperature range
(Fig. 2(b)). The powder diffraction data additionally revealed pro-
nounced, abrupt shifts in Bragg reflections that mirror the magnetic
data (Fig. S11, ESI†). Most importantly, parallel single crystal data
collected at the IP (180 K) and LS state (100 K) confirm the iron(II) spin
state progression (HS–HS to HS–LS to LS–LS, respectively, for Fe1–Fe2)
and reveal definitively the underlying fundamental reason for the
prolonged thermal existence of the IP region. Surprisingly, with
cooling from 260 to 180 K, concomitant with the HS to LS transition
at Fe2, the octahedral distortion of Fe1 increases dramatically (11.3 -
21.21, Fig. 2(a) inset). Additionally, the sequence of octahedral distor-
tion of Fe2 with heating from 100 to 180 K (4.4 - 1.21) indicates that
the LS to HS transition at Fe1 stabilises the LS state at Fe2. In
particular, with each respective HS 2 LS transition at either Fe1 or
Fe2, as the Fe–N bond lengths and overall octahedral volume vary this
causes a mechano-elastic stress resulting in stabilization through
distortive forces and thus significantly enhancing the temperature
stability range of the IP region. We attribute these findings to a
negative cooperative impact imparted by the hydrogen-bonding net-
work interconnecting the ligands associated with Fe1 and Fe2 which
inhibits the entire lattice converting from HS to LS in one-step.
Negative cooperativity has been noted previously, for example in
dinuclear materials in which the lower temperature HS to LS transi-
tion is completely inhibited, resulting in a half one-step SCO.34,35
In summary, we have shown that the combination of 2-D
Hofmann-type materials and aromatic ligands with inherent bind-
ing asymmetry can promote the formation of structurally and
magnetically distinct SCO sites towards predictably attaining
multi-step SCO behaviours. Furthermore, we highlight that inter-
connecting spin switching sites can facilitate negative cooperativity
and stabilize IP regions, where here a remarkable 120 K IP has been
achieved by this method. We are currently looking into the host–
guest properties within these flexible 2-D porous materials which
promise adaptable and versatile structure-function response.
This work was supported by a Fellowships and Discovery Project
funding from the Australian Research Council. Use of the Advanced
Photon Source was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Contract
No. DE-AC02-06CH11357; we thank Dr Gregory J. Halder. We thank
the International Synchrotron Access Program (ISAP) for funding.
Use of the Australian Synchrotron was undertaken at MX-1.
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3840 | Chem. Commun., 2014, 50, 3838--3840
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