Guest-Adaptable Spin Crossover Properties in a Dinuclear Species Underpinned by Supramolecular Interactions

Article abstract of DOI:10.1021/acs.inorgchem.8b02625

Molecular crystals with guest-adaptable crystalline structures and properties are comparatively rare owing to their inherent reduced structural stability and malleability to support molecular variation. To overcome this intrinsic challenge, here we introd

Full text of DOI:10.1021/acs.inorgchem.8b02625

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Guest-Adaptable Spin Crossover Properties in a Dinuclear Species
Underpinned by Supramolecular Interactions
John E. Clements,† Peter R. Airey,† Florence Ragon,† Vivian Shang,† Cameron J. Kepert,†
and Suzanne M. Neville*,‡
†School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia
‡School of Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia
S
* Supporting Information
ABSTRACT: Molecular crystals with guest-adaptable crystalline structures and
properties are comparatively rare owing to their inherent reduced structural stability
and malleability to support molecular variation. To overcome this intrinsic challenge,
here we introduce structural stabilizing supramolecular interactions into a dinuclear
material and henceforth demonstrate a dynamic structural and spin crossover property
interchange between solvated (A·3MeOH) and desolvated (A·Ø) products (A =
[FeII (o-NTrz)5(NCS)4]; 4-(o-nitrobenzyl)imino-1,2,4-triazole). Relatively uncom-
2
mon for molecular species, the guest molecules in A·3MeOH are evolved (A·Ø) via a
single-crystal to single-crystal transformation with affiliated phase transition resulting
in a reversible transformation from one- to two-step spin crossover (SCO) transition
character. We additionally present the water-saturated product (A·3H2O), which
distinctly shows an abrupt one-step SCO character with a 22 K wide thermal hysteresis
loop. Detailed structure−property analysis highlights that the substantial structural
malleability and guest-adaptable SCO properties of this dinuclear species are afforded by the supportive, yet flexible,
supramolecular interaction pathways derived from the ligand functionalization.
examples include [Fe(bpp)(H2L)](ClO4)2·(guest) (bpp = 2,6-
INTRODUCTION
■
bis(pyrazol-3-yl)pyridine, H2L = 2,6-bis(5-(2-methoxyphenyl)-
Porous materials, including metal−organic frameworks
(MOFs), coordination polymers (PCPs) and organic frame-
works, are intensely investigated for their ability to absorb
molecular guests,1 often with concomitant guest-triggered
modulation to physical properties.2 In cases where guest
induced physical or chemical changes occur, such as color or
luminescence variation, molecular sensing opportunities arise.3
Somewhat counterintuitively, nonporous materials such as
molecular materials can also act as molecular sponges;4
however, examples are relatively rare as guest removal and
replacement chiefly results in lattice collapse due to the
absence of the long-range structural supports that are
intrinsically present in MOFs and PCPs.5 Indeed, one
substantial driver for actively developing the field of functional
molecular materials is their inherent structural malleability,
which in turn may lead to enhanced structural and property
modification.6
Of the various examples of guest-triggered structure and
property variation thus far reported, those with spin-state
adaptable metal ions are important candidates.7 It is well-
established that spin crossover (SCO) activity is extremely
sensitive to local structure variation, whereby even subtle
lattice change occurring alongside guest interchange can confer
substantial magnetic, color and structure variation. While this
inherent platform for host−guest sensing has been intensely
investigated in SCO-PCPs,2a,d,8 it has been somewhat less
extensively exploited in molecular crystals.4b,c,e−h,9 Notable
pyrazol-3-yl)pyridine)), and [FeII (L)2(CH3CN)4](BF4)4·
2
2CH3CN (L = 4-(4-methylphenyl)-3-(3-pyridazinyl)-5-pyrid-
yl-4H-1,2,4-triazole), both showing vapochromism and on−off
SCO switching with guest modulation.4e,f Alongside this,
highlighting the substantial adaptability of nonporous complex
to structural transformations, several independent studies show
a dynamic guest-induced conversion from discrete to 1D chain
structures.4f,h,9j
In conjunction with using molecular variation to induce on-
off molecular switching, various studies on SCO-PCPs have
illustrated that guest-manipulation is a powerful tool to tune
SCO character more subtly by for example inducing multistep
switching exchange.8d,g−k,9i Given that such diversity is
apparent in traditional porous materials, herein we show that
such dynamic, guest-induced SCO switching can also be
observed in molecular materials. For this study, we focus on
the dinuclear species [Fe2(R-trz)5(NCX)4]10 (Scheme 1),
which alongside analogous trinuclear species,11 are rapidly
attracting attention as readily attainable structural protypes of
the widely acclaimed, highly cooperative triazole-containing
1D chains of the type [FeII(R-trz)3]A2 (R-trz = R-1,2,4-
triazole; A = anion).12 To date, as for practically all dinuclear
SCO species,13 spin transitions with comparable cooperativity
Received: September 24, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02625
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Article
a
crystals of A·Ø (100% yield; Figure S1c). Elemental Analysis (%): C
41.16, H 2.47, N 28.41, S 8.97. Found: C 41.00, H 2.56, N 28.32, S
8.77.
Scheme 1. Structures
Powder X-ray Diffraction. Bulk crystalline samples of A·3MeOH
and A·3H2O were carefully ground into a polycrystalline powder in a
slurry of methanol and loaded into glass capillaries. The experimental
patterns were compared to patterns calculated from single crystal X-
ray diffraction data to assess phase purity (Figures S2 and S3).
Thermogravimetric Analysis. Samples of A·3MeOH and A·
3H2O were heated from room temperature to 600 °C at a rate of 1 °C
min−1 under a dry N2 gas flow (20 mL min−1) using a TA Instruments
Discovery Thermogravimetric Analyzer (Figures S4 and S5).
Magnetic Susceptibility Measurements. Data were collected
on a Quantum Design Versalab System with a vibrating sample
magnetometer attachment. Samples were loaded into a polyethylene
sample container and sealed with Teflon tape. Measurements were
taken continuously with a ramp rate of 2 K min−1 (no overshoot)
under a field of 0.3 T, in the range 250−50 K for A·3MeOH and A·
3H2O and 320−50 K for A·Ø. Data were also collected at slower scan
rates (1 and 0.5 K min−1; Figures S6 and S7).
Single Crystal X-ray Diffraction. Data were collected on an
Agilent SuperNova Dual Source diffractometer employing a Cu Kα
radiation source (λ = 1.5418 Å). Data integration and reduction were
performed using CrysAlisPro.14 Structural solution for all materials
was completed within SHELXS-97 and refined using SHELXL-9715
within the X-SEED user interface.16,17 All atoms except those in
disordered positions were refined anisotropically, and hydrogen atoms
were fixed using the riding model. The occupancies of the disordered
structural components (all of which show 2-fold disorder) were
refined using free variables. ORTEP representations (50% probability
ellipsoids) for all structures are shown in Figures S8−14. Tables S1−
S3 contain crystallographic data and refinement details for A·3MeOH,
A·Ø, and A·3H2O, respectively. Table S4 reports selected parameters
for all structures, and Table S5 hydrogen bonding interactions. Figure
S15 shows the hydrogen bonding interactions for each complex with
reference to Table S5. Figures S15−S17 show representative packing
diagrams for A·3MeOH, A·Ø, and A·3H2O, respectively. CCDC
1502200−1502204 and CCDC 1856712 contain the supplementary
crystallographic data.
a
(a) Dinuclear [Fe2(R-trz)5(NCS)4] core structure; (b) o-NTrz, 4-(o-
nitrobenzyl)imino-1,2,4-triazole (blue: N−Fe binding groups, red:
functionality).
to polymeric species are yet to be reported in this family. Here,
in the parallel pursuit of improved communication pathways
and increased lattice stability to guest sorption/desorption, we
include specific ligand functionalization (Scheme 1:red) to
induce supportive, flexible and long-range supramolecular
interaction pathways. By this approach, we show substantial
structural malleability to molecular guests, as supported by
these supramolecular interactions, and a diverse assortment of
guest-induced SCO properties embodied in the one dinuclear
moiety; this includes one- and two-step transitions, and strong
spin-state switching cooperativity in the form of an abrupt and
hysteretic one-step transition.
EXPERIMENTAL SECTION
■
General Procedures. All reagents were commercially available
and were used as received. Iron(II) perchlorate was handled carefully
and in small amounts to avoid any potential explosions.
Ligand Synthesis. 4-Amino-4H-1,2,4-triazole (1.5 g, 17.8 mmol)
and 2-nitrobenzaldehyde (3.3 g, 22 mmol) were dissolved in ethanol
(50 mL). Concentrated sulfuric acid (0.3 mL) was added, and the
solution was heated to reflux for 24 h. Upon cooling, the solution was
concentrated, and the product precipitated on ice. The precipitate was
filtered and washed with distilled water (10 mL), then isopropanol
(10 mL), yielding 4-(o-nitrobenzyl)imino-1,2,4-triazole (o-NTrz) as a
pale-yellow powder (2.6 g, 67%). Mp 126 °C. Elemental Analysis
(%): C 49.77, H 3.25, N 32.25; found: C 49.62, H 3.30, N 32.11. 1H
NMR (300 MHz, MeOD): 9.40 (1H, s), 9.10 (1H, s), 8.22 (1H, dd, J
= 7.9, 0.6) 8.17 (1H, dd, J = 7.5, 1.2), 7.88 (1H, td, J = 7.5, 0.6), 7.81
(1H, td, 7.9, 1.2). ESI-MS (m/z) calcd: 217.06. Found: 217.04.
Complex Synthesis. [Fe2(μ-o-NTrz)3(o-NTrz)2(NCS)4]·3MeOH (A·
3MeOH) and [Fe2(μ-o-NTrz)3(o-NTrz)2(NCS)4]·3H2O (A·3H2O). So-
dium thiocyanate (30 mg, 0.37 mmol), iron(II) perchlorate
hexahydrate (31 mg, 0.85 mmol), and ascorbic acid (5−10 mg)
were dissolved in methanol (2 mL). This solution was added
dropwise to a separately prepared solution of o-NTrz (96 mg, 0.44
mmol) dissolved in methanol (5 mL). Red block-shaped crystals of A·
3MeOH formed over several days (ca. 90% yield; Figure S1a). Orange
rodlike crystals of A·3H2O (Figure S1b) cocrystallize alongside A·
3MeOH (major product (∼75%): A·3MeOH; minor product
(∼25%): A·3H2O). The IR spectra of both phases (and A·Ø) are
identical. IR (cm−1): 2084 (s), 1526 (s), 1347 (s), 1176 (w), 1061
(s), 853 (w), 791 (w), 742 (w), 624 (s). Pure samples of A·3MeOH
and A·3H2O were subsequently prepared by seeding freshly prepared
reaction solutions with the respective phase. Phase purity and solvent
content for these two phases was determined by a combination of
thermogravimetric analysis, powder X-ray diffraction and single crystal
X-ray diffraction. Elemental analysis was conducted on these phases,
but the results were not conducive for proving phase purity, as
although crystals of A·3MeOH are relatively stable in solution,
exposure to air over time results in desolvation (A·Ø see below) and
crystals of A·3H2O dissolve in solution over a period of days and are
not stable as dry crystals.
Variable-temperature single crystal unit cell parameters were
collected over the range 250−90 K at 10 K intervals for A·3MeOH
and A·Ø. The unit cell parameter and volume evolution are plotted
versus temperature in Figure S19.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The novel ligand 4-(o-
nitrobenzyl)imino-1,2,4-triazole (o-NTrz, Scheme 1) was
prepared by the condensation reaction of 4-amino-4H-1,2,4-
triazole and 2-nitrobenzaldehyde. Single crystals of [Fe2(μ-o-
NTrz)3(o-NTrz)2(NCS)4]·3MeOH form over several days
from a methanolic solution of iron(II) perchlorate hexahy-
drate, sodium thiocyanate, ascorbic acid, and o-NTrz. Two
guest-variant phases of the dinuclear species, A·3MeOH and A·
3H2O, cocrystallize under these conditions; with A·3MeOH
the major product (ca. 75:25 for A·3MeOH:A·3H2O). The
solvatomorphs can be visually distinguished by color and
crystal morphology; A·3MeOH forms as dark-red blocks and
A·3H2O as orange elongated blocks (Figure S1). Attempts to
form the latter phase in neat water or water:methanol solutions
were unsuccessful. Notably, once formed, the crystals of A·
3H2O redissolve into the methanolic mother liquor over a
period of days, so they must be utilized immediately upon
crystallization. However, once redissolved, pure phases of
either A·3MeOH or A·3H2O can be reformed from the same
solution with crystal seeding. The A·3MeOH crystals are stable
in solution.
[Fe2(μ-o-NTrz)3(o-NTrz)2(NCS)4] (A·Ø). Crystals of A·3MeOH were
dried under dynamic vacuum for 72 h, producing orange-yellow
B
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Figure 1. Variable-temperature magnetic susceptibility measurement for (a) A·3MeOH, (b) A·Ø, and (c) A·3H2O (250−50−250 K; sweep mode;
scan rate: 2 K min−1). Inset: Spin-state per dinuclear unit at the indicated plateau region, as determined by structural analysis (Fe−Fe; HS: gold;
LS: purple).
Thermogravimetry on A·3MeOH and A·3H2O reveal
substantially distinct guest evolution, dinuclear stability, and
decomposition pathways. For A·3MeOH, three methanol
molecules are evolved together below ca. 100 °C. However,
for A·3H2O, two water molecules are desorbed below 100 °C
and removal of the third requires heating to ca. 150 °C
(Figures S4 and S5). Thermogravimetry furthermore reveals
that the A·3MeOH apohost phase (A·Ø) is present over the
temperature range 100−220 °C, whereas decomposition
occurs soon after guest desorption from A·3H2O. Methanol
desorption from A·3MeOH occurs via a single-crystal to single-
crystal transformation and with a color change from dark-red
to orange-yellow (Figure S1). This process is reversible via
exposure of A·Ø to methanol vapor or neat methanol, although
with loss of monocrystallinity. Water molecule removal from
A·3H2O results in irreversible sample degradation.
Dynamic Spin Crossover Properties. Temperature-
dependent magnetic susceptibility measurements were per-
formed on A·3MeOH, A·Ø and A·3H2O revealing a broad
diversity of SCO transition profile characteristics (Figure 1).
For A·3MeOH (Figure 1a) at 250 K, the χMT value of 6.64
cm K mol−1 is consistent with two HS FeII ions per dinuclear
unit. These values remain constant until 200 K, below which
there is a gradual decrease in χMT values to ca. 150 K followed
by a more abrupt decrease to 115 K. Below approximately 100
K, the χMT values remain constant at ca. 0.25 cm K mol−1,
consistent with two LS FeII ions per dinuclear unit. Negligible
thermal hysteresis is observed over the cooling and heating
modes, and the SCO profile is insensitive to collection scan
rate. The gradual, complete one-step SCO transition for A·
3MeOH shows a transition temperature (T1/2) of 135 K.
For A·Ø (Figure 1b), between 250 and 200 K the χMT
values remain constant at 6.80 cm K mol−1, consistent with
two HS FeII ions per dinuclear unit. Over the range of 200−
175 K, the χMT values decrease rapidly to 3.39 cm K mol−1 and
then again to 0.25 cm K mol−1 over the range of 175−125 K.
The χMT values at the subtle intermediate plateau (ca. 175 K)
are consistent with a 1:1 HS/LS ratio of FeII sites. The χMT
values below approximately 100 K remain constant at ca. 0.25
cm K mol−1, consistent with LS FeII ions. Narrow thermal
hysteresis is observed over the heating and cooling profiles
which shows a subtle scan rate dependence (Figure S6). The
complete, two-step SCO transition of A·Ø (2 K min−1) is
characterized by T1/2(1)↓↑: 181, 185 K and T1/2(2)↓↑: 141, 148
K, with narrow hysteresis (ΔT(1): 4 K, ΔT(2): 7 K). This is the
first evidence of a complete two-step SCO in the [Fe2(R-
trz)5(NCX)4] family.10
For A·3H2O (Figure 1c), with cooling the χMT values
remain constant at ca. 6.74 cm K mol−1 between 250 and 160
K, consistent with two HS FeII ions per dinuclear unit. Below
160 K, the χMT values decrease abruptly to 0.78 cm K mol−1,
indicating a complete, one-step conversion to LS FeII ions. The
χMT values remain constant at this value below 130 K. A wide
thermal hysteresis loop is observed over the cooling and
heating cycles which shows subtle thermal scan rate depend-
ence (Figure S7). The complete and hysteretic one-step SCO
transition of A·3H2O (2 K min−1) is characterized by T1/2↓↑
:
150, 172 K, ΔT: 22 K. This is the first evidence of strong
cooperativity in the [Fe2(R-trz)5(NCX)4] family and among
the widest thermal hysteresis reported thus far for a dinuclear
complex.10
Structure of A·3MeOH. Variable-temperature single
crystal X-ray diffraction unit cell data were collected on A·
3MeOH at 10 K intervals over the temperature range 250 to
90 K revealing a primitive monoclinic symmetry (P21/c) over
the entire one-step SCO profile. A plot of the unit cell
evolution versus temperature maps the gradual one-step spin
transition observed by magnetic susceptibility (Figure S19).
Full-sphere single crystal data were collected at 250 and 90
K (Tables S1 and S4), representative of the high and low
temperature plateaus of the gradual one-step spin transition.
Structural analyses reveal an entire dinuclear complex in the
asymmetric unit, thus defining two crystallographically unique
FeII sites, [Fe1−Fe2] (Figure 2a). The Fe−N bond lengths at
250 K (dFe−N: [2.172(6), 2.166(5) Å]) and 90 K (dFe−N
:
[1.972(6), 1.967(6) Å]) highlight a complete [HS−HS] to
[LS−LS] transition of both FeII sites.7d
The crystalline lattice is propagated by a dense supra-
molecular interaction network comprised of both dinuclear···
dinuclear and dinuclear···MeOH interactions (Figure 2a; Table
S4). Notably, the methanol molecules participate in a
collection of interactions with the nitro-groups alongside
other aromatic interactions, as illustrated in Figure 2b. Various
other dinuclear···dinuclear interactions are present, in most
case involving the nitro-groups (for example, see Figure 2b).
Overall, the presence of a one-step spin transition for A·
3MeOH is somewhat counterintuitive given the existence of
two crystallographically distinct FeII sites; notably, however,
this has been similarly observed in [Fe2(bzCl2trz)5(NCS)4]·
H2O (bzCl2trz = 2,5-dichloride-4-phenylimino-1,2,4-trizole)
and [Fe2(napthtrz)5(NCSe)4]·2DMF·2H2O (napthtrz = naph-
thylimino-1,2,4-triazole).10e,f In the majority of cases, SCO ion
distinction results in some form of multistep switching
behavior, including trapped [HS−LS] species such as observed
in the related dinuclear [Fe2(bztrz)5(NCS)2]·xMeOH (bztrz =
C
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Figure 2. (a) Structural representation of an entire dinuclear moiety of A·3MeOH, which has two distinct FeII sites per dinuclear unit, [Fe1−Fe2];
various dinuclear···dinuclear (red) and dinuclear···methanol (blue) supramolecular interactions. (b) Guest-removal (A·Ø) occurs via a single-
crystal to single-crystal transformation with the phase transition introducing a C2 axis (P21/n → C2/c) and dynamic ligand rotation (inset); single
unique FeII site per dinuclear, [Fe1−Fe1′]; relevant dinuclear···dinuclear (red) supramolecular interactions. Hydrogen atoms, third MeOH
molecule, and secondary ligand disorder not shown.
4-phenylimino-1,2,4-triazole).10d This suggests that this system
is best rationalized as one in which the intermolecular
interactions provide insufficient cooperativity between dinu-
clear units to sharpen the transition to the point where
individual transitions of inequivalent Fe sites (or dinuclear
two-step behavior resulting from intramolecular cooperativity
stabilizing a local HS−LS state) become thermally resolved. An
alternative interpretation is that each dinuclear unit undergoes
a cooperative HS−HS to LS−LS intramolecular transition that
overrides the FeII site inequivalence, but that weak
intermolecular cooperativity broadens this transition; however,
we can largely discount this possibility due to the observation
of two-step behavior in A·Ø, as discussed below. Probing the
structure of A·3MeOH reveals several notable distinctions to
the inner and outer coordination sphere of Fe1 and Fe2 which
support a magneto-structural rationalization. First, the HS state
phase shows a mild but clear difference in the octahedral
distortion about Fe1 and Fe2 (Σ = 29.7 and 23.9°,
respectively).7d This distortive difference is readily accounted
for by the prominent variance to supramolecular interactions
about the Fe1 and Fe2 outer coordination sphere. Most
notably, Fe1 is surrounded by more dinuclear···methanol
interactions than is Fe2 (Figure 2a). Furthermore, while the
SCO behavior is overall one-step, an underlying two-step
behavior exists comprised of distinct regions of more gradual
(i.e., ∼200−150 K) and abrupt (i.e., ∼150−125 K) spin-state
transition pathways (Figure 1a). Such behavior is not
uncommon,18 and here likely indicates that a fraction of the
FeII sites initiate a HS to LS transition, perhaps in this case
related to the presence of disorder at some of the nitro oxygen
atom sites and their related interactions,18b,c and once the LS
concentration is sufficiently high, the remainder of the
transition occurs in a more cooperative fashion. The driving
force for this is likely an enhancement of communication
pathways with LS doping due to compression of dinuclear···
dinuclear and dinuclear···MeOH interactions. Alongside this,
the LS state appears to show substantially reduced strain over
the short Fe···N−N···Fe bridge, as evidenced by the reduced
difference in octahedral distortion between the two FeII site in
each dinuclear in the LS state (ΔΣFe1−Fe2 = 5.8° at 250 K;
ΔΣFe1−Fe2 = 0.8° at 90 K) indicating it is the preferred phase.7d
Dynamic Guest Removal (A·Ø). Molecular methanol
evacuation from A·3MeOH occurs via a single-crystal to single-
crystal transformation from primitive monoclinic (P21/n) to C-
centered monoclinic (C2/c) for A·Ø. This increase in
symmetry imposes a C2 axis through the center of the
dinuclear unit, resulting in only half a dinuclear moiety in the
asymmetric unit (Figure 2b). Consequentially, the two unique
FeII sites observed for A·3MeOH ([Fe1−Fe2]) convert to a
single crystallographically distinct FeII site in A·Ø ([Fe1−
Fe1′]; Figure 2b).
Variable-temperature single crystal X-ray diffraction unit cell
data collected on A·Ø at 10 K intervals over the temperature
range of 250−90 K reveal a retention of the monoclinic C
symmetry (C2/c) over the entire two-step spin transition.
Importantly, a plot of the unit cell evolution versus
temperature clearly shows a two-step character consistent
with that observed by magnetic susceptibility (Figure S19).
Full sphere single crystal data were collected at 250, 150, and
90 K (Tables S2 and S4), representative of the high,
intermediate and low temperature plateau of the two-step
spin transition. Structural analyses reveal a systematic decrease
in average apparent Fe1−N bond lengths (dFe−N: 2.167(11),
2.035(9) and 1.966(10) Å, at 250, 150, and 90 K, respectively)
with the 250 and 90 K lengths, confirming a complete HS to
LS transition (Figure 1b).7d The Fe−N bond lengths at 150 K
are intermediate in length, indicating a 1:1 mix of HS/LS and
LS/HS sites (Figure 1b). This result confirms that within each
dinuclear unit local order of the type [HS−LS] and [LS−HS]
exists, but without long-range order conferred throughout the
crystal lattice. We can discount the other possible spin-state
distribution of pairs of [HS−HS] and [LS−LS] dimers19 on
two grounds: This would result in unit cell doubling or the
emergence of a lower crystalline symmetry if 3D long-range
ordered or diffuse scattering if associated with lower
dimensional and/or short-range order;20 this would likely
result in distinctly different local intramolecular Fe···Fe
distances, leading to a pronounced elongation of the Fe site
atomic displacement parameter in the intermediate phase,
which is not seen (see Figure S10).
A significant outcome of the increased symmetry in A·Ø is
that one of the three μ1,2-bridging ligands is centered over the
C2 axis, resulting in 2-fold disorder of the ligand (Figure 2b).
The most substantial consequence of this disorder is the
presence of the peripheral NO2 group in two opposing
positions in equal ratios (Figure 2b). For this to transpire,
concomitant with methanol removal 50% of the disordered μ-
o-NTrz ligands would need to undergo a solid-state rotation as
depicted in Figure 2. Considering this molecular rotation,21 it
is notable for the guest removal process to proceed in a single-
D
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crystal to single-crystal manner. The preserved crystallinity is
undoubtably due to the structural stabilizing platform provided
by the facile replacement of dinuclear···methanol interactions
in A·3MeOH with new dinuclear···dinuclear interactions, as
illustrated in Figure 2. Providing evidence of the intrinsic role
that the inclusion of the nitro-groups plays in providing
structure adaptability to chemical perturbation, all the new
dinuclear···dinuclear interactions are all formed utilizing the
nitro-groups (Figure 2b).
Alongside the various structural distinctions over the A·
3MeOH to A·Ø transformation, guest removal sees a one-step
transition converted into a two-step SCO character. As for A·
3MeOH, this is somewhat counterintuitive given that the two
crystallographically distinct FeII sites merge into all equivalent
sites in A·Ø. Indeed, equivalent symmetry and disorder was
similarly observed for the related dinuclear complex
[Fe2(saltrz)5(NCS)4]·4MeOH (saltrz = N-salicylidene-4-
amino-1,2,4-triazole);10b however, in this case an abrupt one-
step SCO emerged. Here, the presence of a two-step transition
via the stabilization of a short-range ordered [HS−LS] mixed-
spin state phase is unique. The reasoning for a two-step SCO is
not entirely obvious; however, given the most distinct
structural difference between A·3MeOH and A·Ø is the
bridging ligand rotation and associated array of newly formed
local interactions, it is likely that the disposition of the NO2
groups defines the short-range allocation of [HS−LS] or [LS−
HS] spin-state assignment distribution and hence the two-step
character (i.e., it is likely that the relative spin state
configuration correlates with the local orientation of the
disordered bridging o-NTrz ligand, although the disorder is
such that it is not possible to unambiguously determine the
local spin state/ligand configuration). In the case of
[Fe2(saltrz)5(NCS)4]·4MeOH, it is possible that the inter-
action networks formed by the effective hydrogen bonds of the
ligand OH-groups are more efficient, resulting in a cooperative
SCO transition of all FeII sites, bypassing the [HS−LS] state.
The observation of thermal hysteresis in both steps of the two-
step transition is a point of some interest, in that it indicates
the presence of intermolecular lattice cooperativity despite the
absence of any apparent ordering of the local HS−LS and LS−
HS dinuclear sites (noting that there remains some possibility
that domains of local order exist that lie beyond detection by
Bragg diffraction). Such cooperativity may be expected to arise
if both orientations of the HS−LS unit have a similar
ferroelastic influence on their immediate environment, for
example through a general volume reduction effect.
Figure 3. (a) Structural representation of A·3H2O showing the dense
network of dinuclear···water (blue) and dinuclear···dinuclear (red)
interactions. (b) 1D chain of π-stacking interactions (red dashes)
between neighboring (shown in gray or color) dinuclear species in A·
3H2O.
disordered ligands, and hence nitro-groups is defined by
interactions with water molecules which reside in distinct
pockets at either, but not both sides of the dinuclear unit. The
elegant assembly of dinuclear···water hydrogen bonding
interactions between these and other nitro-groups and triazole
groups are depicted in Figure 3a. It is apparent that the
presence of water molecules in these pockets provides an
optimum pathway for neighboring dinuclear species to interact.
This compact arrangement facilitates further interactions
(dinuclear···dinuclear) between nitro- and triazole groups on
neighboring species (Figure 3a). Alongside hydrogen bonding
interactions, the crystalline lattice of A·3H2O is supported by
one-dimensional chains of π-stacking interactions (Figure 3b)
between neighboring molecular species. Notably, such π-
stacking arrays are absent in A·3MeOH and A·Ø and highlight
the dense packing afforded by the mutual dinuclear···dinuclear
and dinuclear···water interactions in A·3H2O.
The highly cooperative nature of the spin transition of A·
3H2O is broadly consistent with the presence of the much
denser network of intermolecular interactions than is seen in
the methanolic and apohost phases. Despite containing locally
(but not crystallographically) inequivalent FeII sites according
to the asymmetric bridging ligand conformation (as described
above for A·Ø), the observed one-step transition for A·3H2O
indicates that the lattice cooperativity is sufficient to effectively
over-ride these local considerations such that stabilization of
the intermediate HS−LS state seen in A·Ø no longer occurs;
notably, a similar bypassing of anticipated spin state plateau for
inequivalent Fe sites has been seen for example, in the highly
cooperative dinuclear phase [Fe2(saltrz)5(NCS)2]·4MeOH10b
and four-step framework material [Fe3(saltrz)6(M(CN)4)3]·
8(H2O).22 Here, we rationalize the two inequivalent
Water inclusion (A·H2O). Structural analysis was con-
ducted at 225 and 100 K (Tables S3 and S4), representative of
the high and low temperature plateaus of the abrupt one-step
spin transition. At both temperatures, a C-centered monoclinic
(C2/c) symmetry exists, akin to A·Ø. Consequently, the
asymmetric unit comprises half a dinuclear complex with one
crystallographically distinct FeII site per dinuclear unit. Bond
length analysis reveals a systematic decrease in Fe−N bond
lengths (dFe−N: 2.145(9) and 1.974(5) Å, at 225 and 100 K,
respectively), consistent with entirely HS and LS dinuclear
species (Figure 1c). Likewise, the octahedral distortion
parameters of the single FeII site become more regular over
the HS to LS transition (Σ = 18.7(5) and 14.0(3)° for 225 and
100 K, respectively).7d
As for A·Ø, the presence of a C2 axis centered over one of
the three μ1,2-bridging ligands dictates 2-fold disorder of an
entire ligand (Figure 3a). The local disposition of these
E
DOI: 10.1021/acs.inorgchem.8b02625
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
transitions as occurring within a combined single step, with the
wide thermal hysteresis strongly suggesting that it is the
increased lattice cooperativity in this phase that is responsible
for the simultaneous site switching rather than a coincidental
equivalence of ligand field at each local Fe site.
[LS−LS]) is observed in the guest-free state. Detailed
structural analyses show that the major difference between
these guest-substituted dinuclear species is in the complexity
and effectiveness of the supramolecular interaction pathways in
each phase. This study therefore very effectively illustrates the
considerable power of subtle variation in supramolecular
interaction pathways to tune and modulate spin-state switching
cooperativity profiles.
CONCLUSIONS
■
In summary, we present the new, molecular material [FeII (o-
2
Collectively, this study demonstrates that there is vast scope
to explore the guest-sensitive aptitude of molecular materials
which are not typically associated with porosity. Importantly,
the strategic introduction of flexible supramolecular inter-
actions provides access to dynamic structural interplay and
enhanced guest sensitive switching properties. Future studies
will focus on guest and switching diversity in other members of
NTrz)5(NCS)4] in which a flexible network of supramolecular
interactions drives a broad array of guest-sensitive dynamic
structure and spin-state switching properties. In the past, such
guest-sensitive outcomes have predominantly emerged in
traditional porous materials such as SCO-PCPs, as the robust
framework scaffolds can sustain molecular variation and the
pore character can be readily tuned; by this approach, subtle
variation to host−host and host−guest interactions have been
observed to lead to diverse molecular switching outcomes. We
show here that similarly diverse guest-induced SCO variation
can manifest in molecular materials when suitable structural
stabilizing motifs are in place. Of particular advantage, we
illustrate that the inherent flexibility of such supramolecular
networks in molecular crystals allows large structural changes
to occur while retaining crystalline properties. Highlighting
this, the methanolate to guest-free transition occurs via a
single-crystal to single-crystal transformation, which involves
substantial molecular rearrangement to optimize interactions,
including ligand rotation. Further comparison between the
structure of these and the water solvated phase illustrates the
genuine guest-adaptability of the lattice. Of particular note, the
water-solvated phase shows a densification which results in a
compact network of hydrogen-bonding and π-stacking
opportunities, in particular a 1D π-stacking array. This dense
supramolecular interaction network provides superior solid-
state communication pathways, ultimately leading to a highly
cooperative spin-state propagation, which is a rare occurrence
for dinuclear materials. While for this example direct solid-state
structural exchange between all three of the phases does not
occur, the substantial SCO and structure variability available
via a molecular nonporous approach is clearly evident.
the [FeII (R-trz)5(NCS)4] family, which typically support
2
dense interaction networks such as seen here, alongside
expansion to other, more complex, guest molecules. From a
broader perspective, the information gained here on structural
design characteristics suitable for accommodating malleable
structures and dynamic properties can readily be applied to
many other molecular crystals toward evolving molecular
sensing structure and properties.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02625.
■
S
Additional characterization (gravimetry), single crystal,
powder X-ray diffraction and magnetic measurements
(PDF)
Accession Codes
CCDC 1502200−1502204 and 1856712−1856713 contain
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 Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
Beyond the benefit of allowing structural mobility, the guest-
dictated, adaptable molecular crystal platform has provided
enhanced molecular switching variation via its flexibility. By
this approach, we find for the first time that one-step, two-step,
and abrupt and hysteretic spin transition behaviors are
accessible in the one molecular material due to the extreme
sensitivity of SCO to physical environment changes. Indeed,
even within the scope of SCO-active PCPs such as broad
diversity of SCO properties in the one material is
uncommon.8g−k
AUTHOR INFORMATION
Corresponding Author
*E-mail: s.neville@unsw.edu.au.
■
ORCID
Suzanne M. Neville: 0000-0003-4237-4046
Notes
The authors declare no competing financial interest.
From a fundamental perspective, among discrete SCO
materials, dinuclear species are significant as they represent the
most basic prototype of a polymeric species and thus offer
insight into the relative effect of directly linking SCO metal
sites through coordinative bridges. While one-step ([HS−HS]
↔ [LS−LS]) and stepwise transitions (i.e., [HS−HS] ↔
[HS−LS] ↔ [LS−LS]) are prevalent in this class of materials,
strongly cooperative, hysteretic transitions are rare; this is
largely owing to the lack of long-range communication capacity
outside of each discrete dinuclear species. Here, the presence
of a [HS−HS] ↔ [LS−LS] transition is observed both
gradually (methanolate) and very abruptly with pronounced
thermal hysteresis (in the hydrate). Furthermore, a thermally
hysteretic two-step transition ([HS−HS] ↔ [HS−LS] ↔
ACKNOWLEDGMENTS
This work was supported by Fellowship and Discovery Project
funding from the Australian Research Council.
■
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