Inclusion of a dithiadiazolyl radical in a seemingly non-porous solid

Article abstract of DOI:10.1039/c7cc06678c

Inclusion of the dithiadiazolyl radical PhCNSSN into the dynamically porous metallocycle [Cu₂(L1)₂Cl₄], where L1 is the bidentate ligand 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene, has been achieved by gas phase diffusion. Single crystal X-ray diffraction, powder X-ray diffraction, UV-visible spectroscopy, EPR and SQUID magnetometry studies confirm inclusion of the radical into this seemingly non-porous material, and illustrate the antiferromagnetic coupling between the paramagnetic host and guest species. The radical guest is readily released by heating or by the addition of solvent (CH₂Cl₂).

Full text of DOI:10.1039/c7cc06678c

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Inclusion of a dithiadiazolyl radical in a seemingly
non-porous solid†
Cite this:DOI: 10.1039/c7cc06678c
a
a
b
a
Varvara I. Nikolayenko,
Javier Campo,
Leonard J. Barbour,
Ana Arauzo,
b
c
Received 25th August 2017,
Accepted 21st September 2017
Jeremy M. Rawson
and Delia A. Haynes
*
DOI: 10.1039/c7cc06678c
rsc.li/chemcomm
Inclusion of the dithiadiazolyl radical PhCNSSNꢀ into the dynamically
The majority of known HOF-radical inclusion compounds
porous metallocycle [Cu2(L1)2Cl4], where L1 is the bidentate ligand were obtained by co-crystallisation of the guest with an organic
1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene, has been achieved host from solution or the gas phase.12–17 A recent review by
by gas phase diffusion. Single crystal X-ray diffraction, powder X-ray D’Alessandro et al. illustrates the increase in reports of MOFs
diffraction, UV-visible spectroscopy, EPR and SQUID magnetometry containing radical moieties, either incorporated into the host
studies confirm inclusion of the radical into this seemingly non-porous backbone or as encapsulated guests.18 However, the number of
material, and illustrate the antiferromagnetic coupling between the reported inclusion compounds containing paramagnetic guests
paramagnetic host and guest species. The radical guest is readily still remains low. In fact, there are only two reports of the
released by heating or by the addition of solvent (CH2Cl2).
inclusion of thiazyl radicals into metal-containing frameworks,
both arising from our groups, and both involving diamagnetic
The evolution and exploitation of host–guest interactions for use host frameworks. One study reports the inclusion of benzo-
in gas sequestration and storage,1,2 chemical sensing,3–5 and dithiazolyl and methylbenzodithiazolyl into MIL-53(Al),19 while
heterogeneous catalysis6–8 have garnered considerable interest the other reports the inclusion of PhCNSSNꢀ into Faujasite.20
in recent years owing to the rapid expansion of the number of Notably, in the former complex the walls of the host cavity rotate
known dynamically porous solid-state materials.9 As this field to accommodate the guest, indicating a responsive relationship
of chemistry becomes more established, researchers are focusing between the host and the guest.
on property development, including magnetic behaviour,
Previous studies in the Barbour group21 have shown the
light modulation, spin-state bistability, electron, hole and ion copper metallocycle [Cu2(L1)2Cl4] (Scheme 1, hereafter 1a) to be
mobility, ferroelectricity and luminescence. Each of the afore- a dynamic material, capable of undergoing multiple single
mentioned properties is governed by host–guest interactions: crystal-to-single crystal (SC–SC) guest-inclusion reactions.
sensors require one entity to impose some measurable change A variety of guests have been included in the metallocyclic
on the other, for example,10 while orientation-specific binding host, yielding host–guest systems 1–11, where the host adapts
can lead to regio- and/or enantioselective transformations in the size of the cavity to best suit a particular guest via rotation
catalysis.11 The inclusion of radical guest molecules into porous of the imidazole rings.21 Since 1a is a highly stable, dynamic
materials such as hydrogen-bonded frameworks (HOFs), metal– host that is inherently paramagnetic, we decided to investigate
organic frameworks (MOFs) or discrete cavities (such as those inclusion of a thiazyl radical (4-phenyl-1,2,3,5-dithiadiazolyl,
found in metallocycles) offers the potential for strong host–guest PhCNSSNꢀ, Scheme 1) into this material, with the aim of inducing
and guest–guest electronic interactions, potentially resulting in
interesting magnetic or transport properties.
a Department of Chemistry and Polymer Science, Stellenbosch University,
P. Bag X1 Matieland, 7602, Stellenbosch, South Africa. E-mail: dhaynes@sun.ac.za
b
´
Aragon Materials Science Institute (CSIC-University of Zaragoza) and Condensed
Matter Physics Dept. Facultad de Ciencias, 50009 Zaragoza, Spain
c Department of Chemistry and Biochemistry, University of Windsor,
401 Sunset Avenue, Windsor, ON, N9B 3P4, Canada
Scheme 1 Schematic representation of the synthesis of 12 from 1a
([Cu2(L1)2Cl4] where L = 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene)
and phenyl-1,2,3,5-dithiadiazolyl.
† Electronic supplementary information (ESI) available: Full experimental details,
synthesis, TGA, DSC, EPR SQUID and DFT. CCDC 1566860–1566863. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc06678c
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communication between the spin on the host and the spin (Table S1, ESI†). The structure shows that the space group of
on the guest. the host (P21/c) is maintained in 12, but there is an increase in
Since guest encapsulation is accompanied by cooperative the length of the b axis, with a corresponding increase in the
motion of the host, inducing a highly specific change in the unit cell volume. In addition to the metal, ligand and chloride
solvent-accessible space and in turn in the chemical environ- counterions present in the asymmetric unit of 1a, the asym-
ment of the metal, single-crystal (SC) X-ray data in conjunction metric unit of 12 also contains one dithiadiazolyl radical, which
with variable temperature solid-state EPR were employed to is disordered across an inversion centre with a site occupancy
obtain a clearer understanding of the host–guest interaction.
of ca. 30%, resulting in a guest occupancy of ca. 60%.
The host was prepared as its methanol solvate 1, following
Closer examination of the two crystal structures determined
the previously reported synthesis,21 and was activated by heating at 100 K (Fig. 1) shows notable differences in the Cu–Cu
under dynamic vacuum at 100 1C for 24 hours to eliminate the distances, as well in the phenyl(centroid)–phenyl(centroid) and
included methanol, yielding the apohost 1a. Bulk phase purity imidazole(centroid)–imidazole(centroid) distances. In order to
was established using powder X-ray diffraction (Fig. S2, ESI†). accommodate the radical guest, the copper metallocycle expands
Previous work conducted by Barbour, Haynes and co-workers along the shorter dimension (Cu–Cu), while contracting along
has shown that materials that are solid but sufficiently volatile at the longer dimension (phenyl–phenyl). In 12, one of the two
room temperature can be included into framework hosts by imidazole moieties is disordered over two positions with the
sorption of the guest directly from the gas phase.19,21,22 This is minor fragment having a site occupancy of 30%. This disorder
also possible for metallocycles such as 1a.21 The volatility of is attributed to weak hydrogen bonding between the nitrogen
PhCNSSNꢀ permitted its inclusion into the apohost 1a when the atom of the radical guest and the hydrogen atom of the host
two species were heated together in vacuo at 80 1C (Fig. S1, ESI†). imidazole. Extension of the ASU shows the radicals encapsulated
After 24 hours of exposure to vapours of the radical, crystals of 1a within stacks of metallocycles (Fig. 2). Guest inclusion results in
had undergone a distinct colour change from green to deep red two cavities within the host merging, analogous to the behaviour
(almost black), 12 (Fig. 1). The UV absorption spectra of the two of the host on inclusion of naphthalene.21 Merged cavities are
samples (Fig. S9, ESI†) confirm the observed colour change, with separated from one another because the radical guest is too large
1a absorbing the majority of the visible spectrum except in the to be included between every metallocycle pair. However, the
green region, while 12 does not transmit any light.
CuÁ Á ÁCu distances between metallocycles remain constant along
The powder pattern of bulk 12 is similar to that of 1a the metallocycle stack (Fig. 2b). The distortion of the host lattice
(Fig. S3, ESI†), albeit with some small differences in peak positions/ is manifested in a subtle change in the coordination geometry at
intensities, indicating that the radical has been included in the the four-coordinate copper(II) centre. For the host complex, the
host without significant structural changes to the crystal structure. t4 parameter (defined as [360 À (a + b)]/141 where a and b are the
Notably there is no evidence for neat PhCNSSNꢀ in the PXRD, two largest angles subtended at Cu23) is 0.37, intermediate
confirming that the observed colour change is not as a result of between square-planar (t4 = 0) and tetrahedral (t4 = 1) geometries.
crystallisation of the radical on the surface of the host crystals.
In 12 the geometry at Cu is slightly more compressed towards
The structure of 12 was elucidated by means of single-crystal square planar (t4 = 0.32).
X-ray diffraction, as inclusion of the radical is a SC–SC process
No degradation of 12 was noted using PXRD or EPR, even
when samples had been exposed to air in ambient conditions for
extended periods of time (Fig. S4, ESI†). This enhanced stability
of the radical within the metallocycle is in stark contrast to the
pure radical, which degrades when not kept under an inert
atmosphere. In addition, TGA data show no significant mass
Fig. 1 Comparison of the structures of 1a (left, 100 K) and 12 (right, 100 K).
Vertical green arrows show CuÁ ÁÁCu distances. Horizontal green arrows show
distances between the centroids of the phenyl rings on the ligands in the
metallocycle. Vertical blue and red lines show the distances between the
centroids on the imidazole rings, where red and blue relate to the two
components of the disorder. The insets show photographs of solid 1a and 12.
Fig. 2 (a) A perspective view of a column of 1a metallocycles showing the
discrete guest-accessible voids as a grey transparent surface (V = 97 Å3 per
ASU). (b) A perspective view of a column of 12 showing merged guest-
filled voids (V = 145 Å3 per host). Metallocycles stack with a 401 slant
relative to the a axis.
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Fig. 4 Temperature dependence of wT for 1a (blue circles) and 12 (purple
circles). The inset is an expansion of the data between 0 and 50 K.
Fig. 3 Solid-state EPR spectra of (a) 1a and (b) 12 at room temperature
(solid lines = experimental data, dotted line = simulation); (c) solution EPR
spectrum of dithiadiazolyl radical recovered from 12 by immersion in
CH2Cl2.
spectroscopy (2.12). M vs. H studies at 1.8 and 10 K are nicely
replicated with two S = 1/2 ions using g = 2.12 and a mean field
correction term to take into account weak anti-ferromagnetic
interactions (y = À0.95 K, see ESI†). Radical inclusion leads to a
marked change in the paramagnetism of the sample (Fig. 4). Down
to 50 K the magnetism once again follows Curie–Weiss behaviour
but the Curie constant is a little larger (C = 0.954 emu K molÀ1),
consistent with an increase in the total number of spin centres
in the sample. The Curie constant equates to 24% S = 1/2 (based
on TGA data on the same sample recorded after the SQUID
measurements) with g = 2.0 and two S = 1/2 Cu2+ centres with
g = 2.15. The Weiss constant (y = À5.7 K), reflects weak
antiferromagnetic interactions which could comprise both
host-host and host–guest interactions. A subtle change in
loss for 12 up to 100 1C (Fig. S5, ESI†). These findings are
consistent with previous studies, where thiazyl radicals included
in porous materials were shown to be stable for up to a month.19
Solid-state EPR spectroscopy was used to further probe
the structures of both 1a and the new radical-containing
paramagnetic hybrid material 12. Both 1a and 12 yield essen-
tially axial spectra with a small rhombic distortion (Fig. 3
2À
and ESI†). Previous EPR studies on distorted CuCl4 anions
(0.72 4 t4 4 0.61) produce rhombic spectra with g8 4 g> with
increasing distortions towards square planar, leading to a
reduction in the magnitude of the g-tensor.24 In this context
it is notable that both 1a and 12 also exhibit g8 4 g>, with
smaller g-values for 12, which is closer to square planar. The
average g-values for 1a and 12 (2.12 and 2.11, respectively) are
smaller than those for CuCl42À, consistent with geometries
significantly closer to square planar (t4 = 0.37 and 0.32).
Notably the EPR spectrum of 12 does not exhibit an observable
component associated with the PhCNSSN radical (g = 2.01).
The EPR linewidth of the radical inclusion complex is sub-
stantially greater than that of the host framework, which can be
attributed to dipolar broadening between radical and host, or
through some exchange interaction, such that the radical no
longer behaves as an isolated S = 1/2 spin system. The presence
of the radical in the host framework was confirmed by displace-
ment of the guest by addition of CH2Cl2 (Fig. 3c).
gradient is observed below 20 K (C = 0.772 emu K molÀ1
,
y = À0.63 K), consistent with antiferromagnetic coupling
between host Cu2+ ions and the guest PhCNSSN radicals. This
interpretation is supported by DFT calculations (see ESI†), which
reveal exchange couplings between the radical and Cu2+ of À10
and À11 cmÀ1 for the two copper atoms in the metallocycle.
In conclusion, the incorporation of a dithiadiazolyl radical
into the dynamic paramagnetic metallocycle 1a results in an
unusual paramagnetic hybrid material, 12, in which SQUID and
DFT studies support the presence of exchange coupling between
host and guest. The stability of the radical is significantly enhanced
by inclusion in the metallocyclic host.
We thank the National Research Foundation of South Africa
and Stellenbosch University for funding. J. M. R. acknowledges
the support of the Canada Research Chair Program. J. C.
acknowledges the Grant No. MAT2015-68200-C2-2-P from the
Spanish Ministry of Economy and Competitiveness.
To examine the nature of the host–guest interaction, vari-
able temperature dc SQUID magnetometry measurements were
made on 1a and 12. The parent host framework was found to
exhibit Curie–Weiss paramagnetism across the range 1.8–300 K
with C = 0.842 emu K molÀ1 and y = À2.0 K, consistent with very
weak antiferromagnetic interactions between two S = 1/2 ions
Conflicts of interest
with g = 2.12, in agreement with the value determined by EPR There are no conflicts of interest to declare.
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