Radical-Doped Metal-Organic Framework: Route to Nanoscale Defects and Magnetostructural Functionalities

Article abstract of DOI:10.1021/acs.inorgchem.9b00696

Nanosized structural defects in metal-organic frameworks (MOFs) attract growing attention and often remarkably enhance functional properties of these materials for various applications. In this work, a series of MOFs [Cu₂(TPTA)_1-x(BDPBTR)_x] (H₄TPTA, [1,1′:3′,1″-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid; H₄BDPBTR, 1,3-bis(3,5-dicarboxyphenyl)-1,2,4-benzotriazin-4-yl radical)) with a new stable radical linker doped into the structure has been synthesized and investigated using Electron Paramagnetic Resonance (EPR). Mixed linkers H₄TPTA and H₄BDPBTR were used to bridge copper(II) paddle-wheel units into a porous framework, where H₄BDPBTR is the close structural analogue of H₄TPTA. MOFs with various x = 0-0.4 were investigated. EPR studies indicated that the radical linker binds to the copper(II) units differently compared to diamagnetic linker, resulting in the formation of nanosized structural defects. Moreover, remarkable kinetic phenomena were observed upon cooling of this MOF, where slow structural rearrangements and concomitant changes of magnetic interactions were induced. Thus, our findings demonstrate that doping of structurally mimicking radical linkers into MOFs represents an efficient approach for designing target nanosized defects and introducing new magnetostructural functionalities for a variety of applications.

Full text of DOI:10.1021/acs.inorgchem.9b00696

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Radical-Doped Metal−Organic Framework: Route to Nanoscale
Defects and Magnetostructural Functionalities
‡
‡
,‡
Artem S. Poryvaev,†,§ Daniil M. Polyukhov,†,∥ Eva Gjuzi, Frank Hoffmann, Michael Froba,*
̈
and Matvey V. Fedin*,†,§
†International Tomography Center SB RAS, Institutskaya str. 3a, Novosibirsk, 630090, Russia
‡Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany
§Novosibirsk State University, Pirogova str. 2, Novosibirsk, 630090, Russia
∥N.N. Vorozhtsov Institute of Organic Chemistry SB RAS, Novosibirsk, 630090, Russia
S
* Supporting Information
ABSTRACT: Nanosized structural defects in metal−organic
frameworks (MOFs) attract growing attention and often
remarkably enhance functional properties of these materials
for various applications. In this work, a series of MOFs
[Cu2(TPTA)1−x(BDPBTR)x] (H4TPTA, [1,1′:3′,1″-terphen-
yl]-3,3′′,5,5′′-tetracarboxylic acid; H4BDPBTR, 1,3-bis(3,5-
dicarboxyphenyl)-1,2,4-benzotriazin-4-yl radical)) with a new
stable radical linker doped into the structure has been
synthesized and investigated using Electron Paramagnetic
Resonance (EPR). Mixed linkers H4TPTA and H4BDPBTR
were used to bridge copper(II) paddle-wheel units into a porous
framework, where H4BDPBTR is the close structural analogue
of H4TPTA. MOFs with various x = 0−0.4 were investigated.
EPR studies indicated that the radical linker binds to the
copper(II) units differently compared to diamagnetic linker, resulting in the formation of nanosized structural defects.
Moreover, remarkable kinetic phenomena were observed upon cooling of this MOF, where slow structural rearrangements and
concomitant changes of magnetic interactions were induced. Thus, our findings demonstrate that doping of structurally
mimicking radical linkers into MOFs represents an efficient approach for designing target nanosized defects and introducing
new magnetostructural functionalities for a variety of applications.
Magnetic properties in MOFs usually are introduced by
open-shell metal ions or lanthanide ions.16−18 The famous
MOF Cu3(btc)2, also known as HKUST-1, shows relatively
strong antiferromagnetic coupling as this MOF is built up by
dimeric copper(II) paddle-wheel (PW) units. However, there
is only a very weak exchange between neighboring PWs.25 The
reason is that the size of even short diamagnetic linker bridges
prevents spin coupling throughout the framework, meaning
that the dimeric units remain magnetically isolated. A potential
way of designing MOFs with long-range magnetic order,
therefore, is to introduce spin density between the metal
centers. The growing family of stable organo radicals is a
suitable choice in this case.26
I. INTRODUCTION
Metal−organic frameworks (MOFs) are a fascinating class of
self-assembling, porous crystalline materials, which are built up
by the coordination of metal ions or clusters with organic
bridging ligands.1−3 Since their discovery over 20 years ago,
the field of MOFs has been marked by astonishing develop-
ment and enormous interest of the scientific community and
industry. Such high appeal is due to the MOFs high structural
and functional adjustability that makes them amenable for an
ever-expanding potential for application scope.4−7 In addition
to applications such as gas storage and separation,8−11
catalysis,12,13 chemical sensor technology,14 and drug release,15
which have been investigated for some time, MOFs with
special properties like magnetism,16 luminescence,17 ferroelec-
tricity,18 redox activity,19 switchable behavior,20 and proton
conductivity21 are attracting increasing interest today.
Remarkably, in a number of contexts it was found that
nanosized structural defects occurring in MOFs might strongly
enhance their functionalities in gas uptake, catalysis, and many
other various applications.22−24
Radicals can be incorporated into MOFs in different ways:
by way of noncovalently bound guests within the voids, as
pendant radicals on metals or ligands,27,28 and in the form of
structurally integrated organic radicals.29,30 However, the only
way to obtain long-range magnetic ordering is to use
Received: March 11, 2019
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of Radical Linker H4BDPBTR 1
a
Reagents and conditions: (I) NaNO2, HCl (37%), EtOH, H2O, 2 h, −1 °C; (II) SnCl2·2H2O, HCl (37%), 2 h, rt; (III) dimethyl 5-
(chlorocarbonyl)benzene-1,3-dioate, THF, NEt3, 4.5 h, 0 °C − rt; (IV) o-iodoaniline, CuI, K2CO3, DMSO, Ar, 15 h, 90 °C; (V) AcOH, 10 min,
140 °C; (VI) NaOH (aq., 2 M), DCM, 18 h, rt; (VII) KOH, H2O, THF, 3 h, reflux, HCl (1 M).
MOFs were activated in vacuum at room temperature after the
following solvent exchange protocol was performed under an argon
atmosphere. The synthesis solvent was exchanged against tetrahy-
drofuran three times, followed by an exchange against dichloro-
methane once. The solvent was replaced with fresh solvent after each
24 h.
Powder X-ray diffraction (PXRD) measurements were carried out
at room temperature using a Panalytical MPD X’Pert Pro X-ray
diffractometer with Cu/Kα radiation. The standard angle range was
2θ = 5−50°, with a step width of 0.1° and a counting time of 20 s.
The evaluation was done with the software X’Pert HighScore Plus
2.2.5 from PANalytical.
Nitrogen physisorption measurements were recorded with a
Quantachrome QUADRASORB-SI-MP at 77 K. The specific surface
area was determined by using the Brunauer−Emmett−Teller (BET)
micropore assistant provided in the Quantachrome ASiQwin software.
EPR studies were performed using commercial Bruker Elexsys
E580 X/Q-band EPR spectrometer equipped with Oxford Instru-
ments temperature control system (T = 4−300 K). In all cases,
powdered samples were placed into the quartz sample tubes (OD =
2.8 mm) and sealed off at vacuum. CW EPR spectra were recorded at
conditions avoiding unwanted modulation broadening. Two-pulse
Hahn echo sequence π/2−τ−π−τ−echo was used to record echo-
detected EPR spectra, and a standard HYSCORE (Hyperfine Sublevel
Correlation Spectroscopy) sequence was used for nuclear modulation
experiments (pulse lengths 12/24 ns for π/2 and π flip angles).58 For
all simulations EasySpin toolbox for Matlab was used.59
structurally integrated radical linkers as rigid spin mediators
between the inorganic building units.31 A well-suited radical
linker candidate for realizing such MOFs is based on the
benzotriazin-4-yl motif, also known as Blatter’s radical. Due to
the extended conjugated π-system this radical has a high
thermal and chemical stability, even in air.32,33 The additional
benefits of this radical are the structural adaptability and the
already proven application for metal coordination.34−37
In this work we report the synthesis and investigation of the
new MOF series Cu2(TPTA)1−x(BDPBTR)x (H4TPTA,
[1,1′:3′,1″-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid 2;
H4BDPBTR, 1,3-bis(3,5-dicarboxyphenyl)-1,2,4-benzotriazin-
4-yl radical) 1) with a novel stable radical linker integrated
in different concentrations into a framework resembling the
PCN-306 host structure.38 Since both radicals and copper(II)
ions are paramagnetic, we employ methods of Electron
Paramagnetic Resonance (EPR) for characterization of
structural and magnetic properties of these MOFs. EPR
methods proved to be very useful in studies of structure and
functions of MOFs recently.39−56 Pulse and continuous wave
(CW) EPR techniques were also actively used to study MOFs
containing copper(II) PW units,48−56 but hitherto not for
MOFs containing additionally radical organic linkers. Below
we describe our variable-temperature CW and pulse EPR
studies of Cu2(TPTA)1−x(BDPBTR)x, aiming at deriving key
structural information on the coordination modes of the
radicals and generated structural defects, the spin-coupling
behavior between radicals and the PW units, and the flexibility
of the frameworks of this type.
II.2. Synthesis. Synthesis of the Diamagnetic Linker H4TPTA.
Synthesis of [1,1′:3′,1″-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid (2,
H4TPTA) was carried out according to Liu et al.38
Synthesis of the Radical Linker H4BDPBTR 1. Following Scheme 1,
dimethyl 2-aminobenzene-1,3-dicarboxylate 3 was converted to the
hydrazine 4 via the diazonium salt. Hydrazide 5 was obtained by
addition of dimethyl 5-(chlorocarbonyl)benzene-1,3-dioate under
hydrogen chloride cleavage. Based on a synthesis of the group of
Koutentis, a one-pot-like synthesis without purification of the
intermediates was performed to provide the radical linker 1.60 The
synthesis can be classified by four steps: the copper-catalyzed Ma
cross-coupling between hydrazide 5 and o-iodoaniline, a cyclization
step in boiling glacial acetic acid, the oxidation of the triazine 1a in the
II. EXPERIMENTAL SECTION
II.1. General Methods and Materials. All solvents were used in
HPLC grade quality. All reagents and solvents were purchased from
commercial sources and used as received without further purification.
Dimethyl 5-(chlorocarbonyl)benzene-1,3-dioate was prepared accord-
ing to literature procedures.57
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presence of base, and completion of methyl ester cleavage, followed
by precipitation of the linker 1 under mild acidic conditions (see
Section I of Supporting Information (SI) for characterization).61
Synthesis of the Mixed-Linker Radical-Containing MOFs. In a
typical synthesis of Cu2(TPTA)1−x(BDPBTR)x, a solution of the
respective linkers and copper(II) nitrate trihydrate in N,N-
dimethylformamide (DMF) was acidified with nitric acid and the
reaction mixture was treated solvothermally for 7 d at 60 °C under an
argon atmosphere in a crimped glass. This procedure furnished blue
to black (depending on x, i.e., the ratio of the radical to nonradical
linker composition) nanocrystalline powder.
20%, the obtained surface area decreases significantly. Since
single-crystal study was not available for this series of MOFs,
we implemented EPR for obtaining the complementary
structural information and elucidation of magnetic interactions.
III.2. Paramagnetic Sites and Magnetic Motif. In
principle, there can be two types of paramagnetic centers in
such radical-containing MOFs: dimeric copper(II) sites in the
PW units and radical centers. In addition, previous studies of
the PW containing MOFs have indicated that often signals of
single copper(II) can be present as well.48 These can be either
copper(II) sites in defect PWs where, for some reason, the
second copper ion is in diamagnetic copper(I) oxidation state.
Alternatively, there can be some extra-framework monomeric
copper(II) species located inside the pores of MOFs and
giving characteristic EPR signals.48
III. RESULTS AND DISCUSSION
III.1. Structural Characterization. A series of compounds
[Cu2(TPTA)1−x(BDPBTR)x] with different amount of radical
linker x = 0−0.4 (for brevity, below we use designations “x = ...
MOF”) were synthesized and characterized using PXRD and
nitrogen physisorption. The integrity of the reisolated linkers
was investigated by 1H NMR spectroscopy (Figure S13).
Unfortunately, the radical linker was found to be not stable
enough in the presence of copper salts, meaning that the actual
content of radical linkers that is incorporated into the MOF is
lower compared to the content in the initial synthesis mixture
(see SI and the next section). More important, the crystallinity
of the MOF decreases progressively upon addition of radical
linker (Figure 1b). When more than 40% of radical linker is
introduced (x > 0.4), only an amorphous phase can be
detected. Results from nitrogen physisorption correlate with
this finding (Figure 1c). For initial linker amounts higher than
Figure 2 shows the variable-temperature CW EPR data for x
= 0.1 MOF (see Figures S19 and S20 in SI for x = 0 and 0.2
MOFs).
Figure 2. X-band CW EPR spectra of x = 0.1 MOF vs temperature
(indicated on the right). Signal of single copper(II) is marked by
“Cu”, of free radical by “R”, of dimeric copper(II) paddle-wheel units
by “D”, of exchange-narrowed line as “Ex”, of internal reference in the
resonator by asterisk. (a) High mw power I1. The spectra at 100−298
K are normalized to the internal reference. Blue trace shows the
simulation of spectrum at T = 100 K. (b) Low mw power I2 = I1/
1000. The spectra are normalized to the internal reference, the
spectrum at T = 10 K is scaled by a factor of 0.5.
Different species in this sample have much different electron
relaxation times. In order to observe fast-relaxing copper(II)
dimers we need to apply high microwave (mw) power, which
leads to a suppression of signal of slow-relaxing radical due to a
mw saturation. Therefore, Figure 2a,b shows the EPR spectra
recorded at two values of mw power differing by a factor of
1000 (30 db).
Figure 1. Chemical structure of the building blocks of studied MOFs
(a), PXRD data for x = 0−0.4 (b), and nitrogen physisorption
isotherms of activated MOFs measured at 77 K, x = 0−0.4 (c).
At both values of mw power, depending on temperature, we
observe signals of copper(II) dimers in the PWs (D), signals of
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radicals (R), signals of single copper(II) (Cu), and broad
exchange-narrowed line (Ex). It is well-known for the PWs that
the exchange coupling between two copper(II) ions is strong
and antiferromagnetic (singlet−triplet splitting ∼220 cm−1);
therefore, at temperatures below ∼80 K all dimeric copper(II)
units become EPR-silent.48,62 As the temperature increases, the
excited triplet state becomes populated, giving rise to the
dimeric signals, most clearly visible at 100 and 200 K.
Simulation yields zero-field splitting (ZFS) parameters typical
for the copper(II) PWs: D = 328 mT (0.306 cm−1) and E ≈
0.48,62 Upon further population of the triplet state, the
probability of finding neighboring copper(II) PWs in para-
magnetic state becomes high enough and the spectrum begins
to coalesce (the phenomenon known as exchange narrow-
ing63). The spectrum at 298 K is clearly contributed by a broad
exchange-narrowed line on top of the smoothed dimeric
signals, meaning that the exchange coupling between
neighboring PWs becomes significant. Note that the spectral
changes with temperature shown in Figure 2 and Figure S19
(SI) are very similar to those reported previously for variable-
temperature CW EPR on copper(II) PW containing MOF
Cu3(btc)2(H2O)3 (without any radical linkers).48 Therefore,
based on the analysis of variable temperature CW EPR, we
propose that exchange interaction pathways propagate between
PWs via the connecting phenyl rings (Figure 3).
Using the obtained concentrations of single copper(II), we
can also obtain the concentration of radicals on the basis of
EPR spectra at 10 K (Figure 2b). The calculated concentration
of radicals in x = 0.1 MOF does not exceed ∼2% relative to the
overall amount of linkers (instead of expected 10%). This
unambiguously means that the radicals partly decompose or
are coupled with other paramagnetic sites in the MOF. Since
an increase in the amount of loaded radical linkers correlates
with a larger amount of single copper(II) sites, we plausibly
conclude that the radicals either perturb the formation of
copper(II) PW units, or they couple with one of the copper(II)
sites in the formed PWs resulting in EPR-silent copper-radical
dyads. In the latter case, the second (uncoupled) copper(II)
ion in the PW becomes EPR-visible. The absolute numbers for
the concentration of single copper(II) and the amount of
loaded radical linker agree only qualitatively (10% loading
yields ∼3% of single copper(II), 20% loading yields ∼20% of
single copper(II)). The reason for poor quantitative agreement
is, perhaps, the uncontrolled amount of extra-framework
copper(II) species, which can also form diamagnetic
complexes with radical linker either during the synthesis or
postsynthetically.
In order to study the local coordination environment of
single copper(II) centers, we applied pulse EPR method
HYSCORE (Hyperfine Sublevel Correlation Spectroscopy),64
which allows revealing the weak hyperfine interactions (HFIs)
between electron spin of copper(II) and neighboring nuclear
spins. Figure 4 shows the HYSCORE spectra obtained on the
single copper(II) signals for MOFs with x = 0 and 0.1 at two
magnetic field positions B = 285 mT (g|| orientation) and B =
338 mT (g⊥ region). First, the signals due to the HFIs with
surrounding protons are visible near their Larmor frequency
ν(1H) ∼ 14 MHz. These proton spectra are drastically
different in the two orientations: the couplings observed at g||
orientation (Figure 4a, wide ridges across diagonal) are much
larger compared to those at g⊥ orientation (Figure 4b). This
implies the presence of close proton(s) in axial coordination of
copper(II), whereas in equatorial plane mainly remote protons
are visible. Simulation of HYSCORE spectra at g|| orientation
(Figure S25) shows that the dipolar HFI is T ≈ 1.7 MHz,
which translates into electron−nucleus distance of ∼3.6 Å.
Therefore, the obtained data can be most plausibly rationalized
by assuming that single copper(II) centers are localized in the
PW units with an additional axial ligand being the DMF
molecule, whose aldehyde proton is 3.6−4 Å away from the
copper(II). The abundance of DMF molecules unremovable
from MOF by our activation protocol was also supported by
CHN analysis (Table S2, SI), whereas the octahedral geometry
of the copper(II) environment is confirmed by the analysis of
its echo-detected EPR spectrum (Figure S26).
Figure 3. Exchange interaction pathways in x = 0 and 0.1 MOFs
(anticipated structure) at high temperature limit. The pathways
inherent for x = 0 MOF are shaded in yellow, the additional pathway
via the radical linker in x = 0.1 MOF is shaded in red.
At T = 10 K all copper(II) PWs are in the ground S = 0
state, and we observe only signals of single copper(II) and
organic radical (Figure 2b). However, as the temperature
increases, the signal of organic radical (g ∼ 2.0) decreases and
disappears completely at T = 200 K and above (Figure 2b).
This unambiguously means that the radical becomes involved
in the exchange-coupled network of the PWs at high
temperatures and conducts electron density between parallel
magnetic chains (Figure 3).
The signals of the single copper(II) (Cu) are observed at 10
K for any value of x (Figures 2 and S21). The analysis of
temperature-dependent CW EPR data allows us to estimate
the ratio of single to dimeric copper(II) sites (Figures 2, S19,
and S20), which gives approximately 1, 3, and 20% of single
copper(II) for x = 0, 0.1, and 0.2, respectively. The increase of
the single copper(II) contribution at higher x can also be
clearly seen in Figure S21 (SI).
More importantly, in the case of x = 0.1 MOF we observe
strong enhancement of signals with ν ∼ 1 MHz compared to x
= 0 MOF. These signals are clearly enhanced in both field
positions, and can only be assigned to the nitrogen (14N)
nuclei emerged around the single copper(II) spins (Figure
S25). Given that actual amounts of radical and single
copper(II) in x = 0.1 MOF are ∼2% and ∼3%, respectively,
it is statistically improbable that the single copper(II)
spontaneously locates near the radical linker. Therefore,
HYSCORE data prove that the formation of the single
copper(II) is driven by the radical linker, resulting in their
close mutual location. In other words, the radical linker
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Figure 4. X-band HYSCORE spectra of MOFs with x = 0 (left) and 0.1 (right) detected at two field positions 285 (a) and 338 mT (b). The
1
regions of H and 14N signals (for x = 0.1) are marked.
perturbs the local MOF structure and causes single copper(II)
defect sites.
occurring in such MOFs. We noticed that when a sample is
placed at 10 K, its echo-detected (ED) EPR spectrum evolves
on the time scale of several hours (Figure 5). Thermalization
of the sample in the cryostat of our spectrometer normally
takes a few minutes, and after a maximum of 15−20 min the
sample reaches perfect equilibrium with the cryostat. This was
specially verified once again under the same conditions using
several reference samples, and in all cases complete thermal-
ization was achieved within half an hour. It could also be
Finally, variable-temperature CW EPR data on x = 0.2
compound shed further light into formation of single
copper(II) defects and the role of radical linker (Figure
S20). Remarkably, no characteristic signals of dimers are visible
at any temperatures for this sample, nor the broad line of
extended exchange-coupled network of the PW units. This
likely means that already for x = 0.2 there is a strongly
decreased amount of PW units where both copper(II) ions are
paramagnetic. As is evident from Figure 1 above, the material
becomes more amorphous upon increase of the radical content
x, and at the same time the number of paramagnetic
copper(II)−copper(II) dimers decreases.
In summary, EPR studies indicate that the radical linkers
bind to the copper(II) units differently compared to
diamagnetic linkers and introduce structural defects in their
vicinity. This is in perfect agreement with the trend of
decreasing crystallinity upon increasing x, as revealed by
PXRD. The doping of radical linkers leads to a formation of
single copper(II) defects in the PWs, meaning that the second
copper is reduced to the diamagnetic copper(I) state. Below
we expand on this idea and propose that the radical linker is
the “dangling linker” type of defect in MOF, which is attached
by two carboxylate groups to the PW on the one side, but
freely dangles on the other. We also propose possible scenario
why such mismatch leads to a formation of mixed-valence
defect PWs.
III.3. Temperature-Induced Slow Structural Rear-
rangements. Pulse EPR experiments discussed in the
previous section were recorded at liquid helium temperatures
about 10 K. At such temperatures CW EPR studies on radical
and single copper(II) sites are problematic due to severe
microwave saturation, but pulse EPR becomes straightforward
to implement. Surprisingly, in addition to the results discussed
above, these studies revealed some peculiar kinetic phenomena
Figure 5. Pulse (echo-detected) EPR spectra of x = 0.1 MOF, taken
from room temperature, then inserted into 10 K and measured as a
function of time. Time delay after insertion is shown on the left,
magnification factor is shown on the right. Signal assignment is
sketched on top, where “Cu” refers to the single copper(II) defects
and “R” to the radical.
E
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hypothesized that thermal contact/conductivity of highly
porous material sealed off at vacuum in the quartz tube is
much lower compared to common samples. To verify this, we
used another reference MOF sample45 containing copper(II)
PWs: Cu-doped [Zn2(1,4-bdc)2(dabco)] prepared for EPR
measurement in the same way, whose physical properties must
not be noticeably different. Figure 6 shows that there is no
further signal growth beyond ∼30 min after insertion of the
sample into the cryostat.
is known as Temperature-Induced Excited Spin State Trapping
(TIESST).70,71 We therefore suppose that the origin of the
observed phenomena in our MOFs can be generally similar.
For most of the materials, the thermal capacity drastically
drops down with temperature. This means that, given the same
temperature gradient, the cooling rate accelerates at low T.
Having this in mind, we compared the kinetic effects for x =
0.1 MOF in two situations: (i) the sample was kept for ∼4 h at
room temperature and then introduced into cryostat at 10 K
(the experiments discussed above), and (ii) the sample was
thermalized in liquid nitrogen (T = 77 K) for ∼4 h and then
immediately introduced into cryostat at 10 K. The kinetic
behavior in both cases is illustrated in Figure S27 (SI). No
remarkable differences were found between (i) and (ii), except
for some minor changes in the shapes of kinetic curves at short
times. This means that the structural trapping occurs
essentially during the temperature change below T < 77 K.
This also once again confirms that the observed kinetic
phenomena do not owe to the slow thermalization of the
sample, because in this case, one would reasonably assume that
the temperature jump from 293 to 10 K would require more
time than the jump from 77 to 10 K.
Finally, we investigated the electron relaxation rate of single
copper(II) and radical centers vs time after the sample was
placed to 10 K (Figure S24). If relaxation rates of copper(II)
and radical would change with time differently, this could
impact their relative intensities in ED EPR spectra and be one
more explanation of the observed kinetic phenomena.
However, no noticeable changes in longitudinal relaxation
(T1) were observed versus time, proving that the evolution of
the spectral shapes in Figure 5 owes purely to the changing
amounts of copper(II) and radical centers.
Figure 6. Intensities of pulse EPR signals measured as a function of
time upon insertion of sample into 10 K. Intensities of single copper
defects (Cu) and free radical (R) signals in x = 0.1 MOF are plotted,
the intensity of Cu in x = 0 MOF, and, finally, the intensity of
copper(II) signal in reference MOF Cu(II)-doped [Zn2(1,4-
bdc)2(dabco)], bare (ref 1) and loaded with CO2 (ref 2). Y-axis
shows the normalized echo intensity (I0 is the intensity immediately
after insertion of sample into cryostat).
The data in Figures 5 and 6 clearly indicate that the amount
of spins visible by pulse EPR grows with time, and this is also
confirmed qualitatively by CW EPR (Figure S22). Remarkably,
the overall growth of the signal of single copper(II) defects is
two times larger than that of the radical, for both x = 0.1 and
0.05 samples. This means, that kinetic process leads to the
“formation” of two EPR-visible single copper(II) ions per one
EPR-visible radical. This might happen if the fragile high-
temperature (HT) structure incorporates exchange-coupled
(not covalently bound) spin triads, where signals of closely
located copper(II) defects and/or radical centers are strongly
broadened and therefore EPR-invisible. The low-temperature
(LT) structure should have these paramagnetic centers better
separated from each other, resulting in the onset of their
individual EPR signals.
At the moment we cannot provide the exact structural model
for such rearrangement; however, we can propose a plausible
scenario for that. The radical linker has slightly different
geometry compared to the diamagnetic analogue. Therefore, it
likely binds to the PW via carboxylate groups on the one end
with high efficiency, but the binding on the other end can be
hindered, leading to a “dangling linker” defect in the MOF
structure (Figure 7). In this case, the two neighboring PWs
should have vacant copper(II) sites becoming accessible for
competing reactions. In our preparation conditions (thermal
treatment) DMF easily decomposes to formaldehyde, which is
able to reduce copper(II) to copper(I) and produce formate
that readily occupies vacant positions in the PWs. Very
recently, Fu et al. carried out Density Functional Theory
(DFT) calculations on defective Zr clusters that are present in
UiO-66 and found out that “missing linker” Zr defective sites
In the case of x = 0.1 MOF shown in Figure 5, we observed
the growth of both signals of single copper(II) and radical
versus time after insertion at 10 K. The overall increase of
intensity is dramatic, being about 30-fold during 4 h (Figure
6). Very similar behavior was observed for x = 0.05 MOF
(Figure S23). In the case of x = 0, we detected much weaker
growth (∼6-fold) completed within shorter time (∼75 min).
In the case of x = 0.2, no kinetic effects were observed, perhaps
due to the high amount of defect sites and relatively low
crystallinity of the structure. Thus, we conclude that kinetic
effects are moderately present in x = 0 radical-free MOF, they
are noticeably enhanced for x = 0.05−0.1, and they disappear
for x = 0.2 and higher. This is all coherent with the previous
conclusions that doping of radical linkers introduces structural
defects in this MOF, still keeping its crystallinity bearable at
small x, but ultimately losing it at x > 0.4.
Slow kinetic effects must be connected to some structural
rearrangements (see below), and, generally, many flexible
MOFs exhibiting guest- or temperature-induced rearrange-
ments are known.65−69 We speculate that incorporation of
small amounts of defects for x = 0.05−0.1 MOFs makes the
local structure more fragile and prone to rearrangements. It is
overall unlikely that the whole structure of MOF undergoes
rearrangement; however, local structure of the defect sites
might be more flexible and have more freedom for conforma-
tional change without perturbing much the regular lattice.
Then why are these rearrangements so slow? In principle, it
is known that some bistable compounds can be kinetically
trapped in a high-temperature structural state upon shock-
freezing. For example, in spin-crossover research such an effect
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Surprisingly, for x = 0.05−0.1, the defected structure reveals
noticeable slow structural rearrangements upon shock-freezing
at liquid helium temperatures. The local high-temperature
structure of the defect sites appears to be kinetically trapped
and relaxes to a low-temperature structure on a time scale of a
few hours. Upon such structural rearrangement, which can be
reversed by heating, the magnetic interactions and properties
of the MOF are modulated.
A number of works addressed the role of the nanosized
structural defects in MOFs recently. Such defects, either
inherent or deliberately engineered, might remarkably enhance
functional properties of these materials for applications as gas
separation and storage, catalysis and, potentially, many
more.22−46 Our present work clearly underlines that
introduction of structurally mimicking radical linkers into
MOFs is an interesting and promising approach for generation
of site defects and bringing new magnetostructural function-
alities into MOFs. This strategy is to be explored in more detail
and applied in MOF research in the future.
Figure 7. Sketch of the two potential wells corresponding to the two
hypothetical structures, which refer to the high-temperature (HT)
and low-temperature (LT) conformations of the dangling BDPBTR
linker. Low temperature structure corresponds to spatially separated
copper(II) and radical centers, whereas high-temperature (kinetically
trapped) structure has closely located copper(II) and radical ions and
stronger exchange coupling between them (shaded in yellow).
can be considerably stabilized by bridging bidentate binding
acetate species or also by a mixture of acetate and water, each
in monodentate binding mode.72 In such case, we arrive at the
dangling radical linker and two mixed-valence copper(II)−
copper(I) units next to it (Figure 7). In order to explain the
kinetic phenomena, we need to assume that the dangling linker
has two preferred orientations with the outer phenyl ring being
closer to or farther away from the mixed-valence PW (high-
and low-temperature structures, respectively). The two
orientations (two potential minima in Figure 7) can possibly
originate from two conformations of the linker (folded or
twisted). Alternatively, they can be the consequence of a subtle
thermal shrinking/expansion of the MOF itself. Note that
similar explanation can also be applied to the less pronounced
(but still present) kinetic phenomena in x = 0 MOF (Figure
6), with the only difference that radical center is not involved.
Last but not least, we have found that the ability of kinetic
behavior depends on the sample aging. Freshly prepared
samples do show kinetic effects, but after keeping the samples
in an argon atmosphere for 3−6 months the above effects get
noticeably smaller and fully disappear in approximately 1 year.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00696.
■
S
Supplementary characterization data of the synthesized
linkers (procedure, IR, NMR, TG/MS, elemental
analysis) and supplementary EPR data (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: froeba@chemie.uni-hamburg.de.
*E-mail: mfedin@tomo.nsc.ru.
■
ORCID
Matvey V. Fedin: 0000-0002-0537-5755
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by RFBR (18-29-04013). We are
very thankful to P. A. Demakov and Prof. Dr. D. N. Dybtsev
(NIIC SB RAS) for providing us with the reference sample
Cu(II)-doped [Zn2(1,4-bdc)2(dabco)]. D.M.P. thanks the
Ministry of Science and Higher Education (MSHE) of the
Russian Federation (Grant 14.W03.31.0034). We thank
MSHE for access to the EPR equipment.
CONCLUSIONS
■
In this work, the free radical bearing mixed-linker MOF series
[Cu2(TPTA)1−x(BDPBTR)x] was synthesized and character-
ized with PXRD, nitrogen physisorption and extensively
studied by EPR. The progressive decrease of the crystallinity
upon increase of x was evidenced by PXRD, and above x ∼ 0.4,
the material appears to be fully amorphous. Physisorption
measurements correlate with this result and are presenting a
loss of porosity already above x ∼ 0.2. In agreement with this,
CW and pulse EPR indicate that the radical linker H4BDPBTR
reacts with the copper precursor complex differently compared
to diamagnetic linker H4TPTA. In particular, the amount of
single copper(II) centers increases upon addition of radical
linker in a series of x = 0, 0.1, and 0.2, implying the increasing
amount of nanosized defects in the structure. At the same time,
HYSCORE indicates the presence of 14N nuclei coming from
H4BDPBTR linker surrounding single (defect) copper(II) sites
for x = 0.1. Thus, doping radical linker of generally close
structure into Cu2(TPTA) MOF results in generation of point
defects in the framework, and for a small concentrations of
radical linker (e.g., 5−10%) the initial structure, crystallinity,
and porosity of the host MOF remain unperturbed.
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DOI: 10.1021/acs.inorgchem.9b00696
Inorg. Chem. XXXX, XXX, XXX−XXX