A porous and redox active ferrocenedicarboxylic acid based aluminium MOF with a MIL-53 architecture

Article abstract of DOI:10.1039/c9dt03489g

A metallocene based linker 1,1′-ferrocenedicarboxylic acid (H₂FcDC) was used to synthesise the first permanently porous ferrocenedicarboxylate, exhibiting a MIL-53 architecture. This compound Al-MIL-53-FcDC [Al(OH)(FcDC)] is obtained in glass v

Full text of DOI:10.1039/c9dt03489g

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A porous and redox active ferrocenedicarboxylic
acid based aluminium MOF with a MIL-53
architecture†
Cite this: Dalton Trans., 2019, 48,
16737
Jannik Benecke, Sebastian Mangelsen, Tobias A. Engesser, Thomas Weyrich,
Jannik Junge, Norbert Stock
and Helge Reinsch
*
2
A metallocene based linker 1,1’-ferrocenedicarboxylic acid (H FcDC) was used to synthesise the first per-
manently porous ferrocenedicarboxylate, exhibiting a MIL-53 architecture. This compound Al-MIL-53-
FcDC [Al(OH)(FcDC)] is obtained in glass vials under mild synthesis conditions at ≤100 °C and after a short
reaction time of 90 min. The crystal structure was determined from powder X-ray diffraction data and the
O, exhibiting a BET surface area of 340 m g−1
2
.
compound shows porosity towards N
2
and H
2
Furthermore, the MOF was characterised via EPR and Mössbauer spectroscopy. The Mössbauer spectrum
of Al-MIL-53-FcDC shows a characteristic doublet with an isomeric shift of 0.34 mm s−1 and a quadru-
pole splitting of 2.39 mm s− , proving the persistence of the ferrocene moiety. A negligibly small amount
of impurities of ferrocenium ions could be detected by EPR spectroscopy as a complementary technique.
Cyclic voltammetric experiments demonstrated the accessible redox activity of the linker molecule
FcDC2− in Al-MIL-53-FcDC. A reversible oxidation and reduction signal (0.75 V and 0.64 V, respectively,
vs. Ag) of FcDC2− was observed and maintained during forty CV cycles, while the crystallinity of the MOF
remained unchanged after the experiment.
1
Received 28th August 2019,
Accepted 16th October 2019
DOI: 10.1039/c9dt03489g
rsc.li/dalton
1
9
shows high thermal stability up to 400 °C. Different possibili-
ties to synthesise MOFs showing redox activity in the electro-
Introduction
Metal–organic frameworks (MOFs) are mostly porous coordi- chemical potential window of ferrocene have been described.
1
nation polymers exhibiting a modular structure. The modu-
larity of the MOF structures allows us to obtain new materials
2
with tuneable pore sizes and pore surface properties.
3
Therefore, their potential applications such as gas storage,
4
–6
7
8–11
gas separation,
catalysis and drug delivery
have been
investigated. Currently there is also strong interest in the devel-
1
2–14
opment of electrochemical MOF-based devices.
MOFs are
promising as active materials for electrodes, e.g. for hetero-
geneous catalysis, energy storage and sensors, in particular
1
5
when redox activity comes into play. Particularly, MOFs
with redox active species within the framework have been
tested for this application, mostly by cyclic voltammetric
1
6–18
measurements.
Ferrocene is a very promising redox active
1
9
species in this context. This molecular sandwich complex of
the metallocene family exhibits mild and reversible oxidation
at around 0.4 V (versus the saturated calomel electrode) and
Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel,
Max-Eyth-Straße 2, D-24118 Kiel, Germany. E-mail: hreinsch@ac.uni-kiel.de
Fig. 1 Representation of the ferrocene-based linker molecules 1,1’-fer-
rocenediyl-bis-(H-phosphinic acid) (a) and 1,1’-ferrocenedicarboxylic
acid (b). Bottom: ferrocene based molecules for post synthetic modifi-
cation reactions: ferrocenecarboxylic acid (c) and 1,1’-ferrocenediyldi-
methylsilane (d).
†
Electronic supplementary information (ESI) available. CCDC 1922329. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
C9DT03489G
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A variety of ferrocene-based molecules can be used for their “Mössbauer Drive System MR206A” and the “Mössbauer
implementation in coordination polymers or MOFs (Fig. 1). Velocity Transducer MVT-1000” from the company
To date several dense coordination polymers are known “Wissenschaftliche Elektronik GmbH” in Starnberg were used.
5
7
which contain the linker molecule ferrocenedicarboxylic acid As the source of radiation, Co in a rhodium matrix with a
2
0
21–25
(
Fig. 1b), including 1D-,
2D-
and 3D-coordination starting activity of 25mCi was used. Cyclic voltammograms
2
6,27
polymers.
In 2016 the synthesis of a flexible and hydrolyti- were measured on an EG&G Princeton Applied Research/
cally stable manganese based MOF based on 1,1′-ferrocene- Model 273A using an Ag rod as a pseudo reference electrode
2
8
diyl-bis-(H-phosphinate) (Fig. 1a) was reported. Three years and Pt as a counter electrode. The MOF coated gold electrode
later Budnikova et al. published two different Co- and consists of a glass substrate with a 50 Å titanium adlayer and a
2
9
Zn-based coordination polymers with this linker molecule.
200 nm evaporated gold film on the surface.
However, these coordination polymers all lack permanent
porosity. In order to obtain porous, redox active MOFs, post
synthetic modification has hitherto been used to modify suit-
able frameworks. In a very simple approach, the MOF can be
modified by adsorbing ferrocene into the pores of the frame-
Synthesis of Al-MIL-53-FcDC
The compound Al-MIL-53-FcDC-as was synthesised employing
a molar ratio of 1 : 1 for H FcDC (11.3 mg, 0.0412 mmol) and
2
aluminum sulfate 18-hydrate (27.3 mg, 0.0412 mmol). The
reactants were placed in a Duran®-glass reactor (volume
3
0
work, as for example reported for Al-MIL-53. In addition, the
ferrocene unit can be attached to the framework by stronger
bonds. For example NU-1000 exhibiting free coordination sites
for carboxylate molecules could be modified by coordinating
4
mL). A mixture of water (300 µL) and DMF (300 µL) was
added. In the next step 10 µL (0.175 mmol) of acetic acid and
a magnetic stirring bar were placed in the Duran®-glass
reactor. The reactor was sealed and placed in an aluminium
block (Fig. S1†) and heated up to 95 °C for 90 min under stir-
ring. It is worth mentioning that after longer reaction times,
the title compound could not be obtained as a pure product.
After cooling down, the reaction products were filtered off and
the obtained light brown powder was washed with a mixture of
DMF and water (1 : 1 ratio). The light brown powder exhibits
two different crystal morphologies (see the ESI† for details), of
which only the obtained fine powder is discussed herein.
Some additional data on the virtually identical product with
block-like appearance can be found in the ESI.† After syn-
thesis, dimethylformamide molecules located in the pores
of the as-synthesised compound (Al-MIL-53-FcDC-as) were
removed by stirring the MOF in methanol for 36 h at room
temperature. Eventually, this activated MOF (Al-MIL-53-FcDC-
ac) was separated by filtration and dried at room temperature
in air (yield: 32.5%). Complete compositional characterisation
data can be found in the ESI.†
3
1
ferrocenecarboxylic acid (Fig. 1c) to the inorganic nodes. In a
different approach the OH-groups of the MOF Al-MIL-53 could
be modified by covalently anchoring 1,1′-ferrocenediyldi-
3
2
methylsilane (Fig. 1d) to the inorganic unit. However, no
porous MOF with the linker molecule H FcDC has hitherto
2
been reported. Thus, herein we describe the synthesis and full
characterisation of the first porous aluminium based 1,1′-ferro-
cenedicarboxylate MOF with MIL-53 topology.
Experimental section
Materials and methods
Aluminium sulfate 18-hydrate (Al
2
(SO
4 3 2
) ·18H O, Kraft, pure),
1
,1′-ferrocenedicarboxylic acid (H
2
FcDC, Alfa Aesar, 99%) and
acetic acid (VWR, 99%) were used as obtained and without
further purification. PXRD data for Rietveld refinement were
collected on a STOE Stadi MP diffractometer (Cu Kα1 radi-
ation) equipped with a Mythen detector in transmission
geometry. The MIR spectra were recorded on a Bruker
ALPHA-FT-IR A220/D-01 spectrometer with an ATR-unit. UV/vis
spectra were recorded with an Agilent UV/VIS-NIR-spectro-
meter in reflection geometry. Prior to measurement the
Results and discussion
Crystal structure
4
powders were mixed with BaSO . Sorption experiments were After synthesis, occluded DMF molecules have to be removed
carried out using a BEL JAPAN Inc. Belsorpmax apparatus. by solvent exchange with methanol from the pores of the
The samples were heated at 150 °C under reduced pressure title compound, i.e. the raw product must be “activated”.
−
2
(
10 kPa) for 16 h. The sorption isotherms were measured at Throughout this activation procedure, only minute changes in
7
7 K for N and at 298 K for H O and the resulting specific the peak intensities of the PXRD pattern can be observed
2
2
surface area was determined as described in the literature by (Fig. S2†), indicating the absence of any “breathing” effect of
3
3,34
Rouquerol et al. All EPR spectra were recorded with a Bruker the framework.
Similarly, no breathing is observed in vari-
EMXplus spectrometer equipped with a PremiumX microwave able temperature PXRD experiments, proving the rigidity of Al-
bridge and a Bruker HQ X-Band cavity. The measurement MIL-53-FcDC under these conditions (Fig. S3, ESI†). The struc-
experiments were performed at an X-band microwave radiation ture of Al-MIL-53-FcDC was determined from powder X-ray
of 9.86 GHz. For the measurement, the powder was analyzed diffraction data for the activated sample Al-MIL-53-FcDC-ac. It
in the solid state at room temperature. The Mössbauer spec- crystallises in a triclinic unit cell with the space group P ˉ1 and
trum was recorded on a custom made Mössbauer spectrometer the refinement of the PXRD data by the Rietveld method con-
in linear transmission geometry. As power units, the verged with unit cell parameters a = 6.6261(5) Å, b = 10.620(2)
16738 | Dalton Trans., 2019, 48, 16737–16743
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Å, c = 10.642(2) Å, α = 79.896(13) Å, β = 71.512(13) Å and γ = IR and UV/vis spectroscopy
7
1.851(16) Å (Fig. 2). Crystallographic data have been de-
The success of the activation process can be monitored by IR
spectroscopy (Fig. 4, left), by which the bands of DMF mole-
cules in the as synthesised sample (black line), located at
posited with the Cambridge Crystallographic Data Centre
(CCDC 1922329†).
The structure of Al-MIL-53-FcDC (Fig. 3) is based on chains
−1
−1
−1
3
1674 cm (ν CvO), 1254cm (ν N–CH ) and 1091 cm
of AlO
6
-octahedra as the inorganic building unit (IBU). Four of
(
r CH –N), can be identified (DMF bands are labeled with
3
the six coordinated oxygen atoms are part of the coordinating
asterisks). In contrast, no bands of DMF could be detected in
the IR spectrum of the activated sample (red line).
Furthermore different vibrational bands of the linker molecule
2
−
carboxylate groups of the linker FcDC . In addition the alu-
minium-oxygen octahedra are corner sharing via OH-groups
along the a-axis (Fig. 3, blue polyhedra). Every inorganic chain
is connected to four other IBUs by the linker FcDC (yellow
polyhedra) and thus a windmill type channel structure results.
The channel pores have a diameter of at least 3.1 Å (Fig. 3)
assuming a spherical guest molecule shape.
2−
35–38
FcDC could be assigned (Table S4, ESI†).
In addition,
2
−
UV/vis spectroscopy was used to characterise the ferrocene in
the MOF (Fig. 4, right). In the UV/vis spectra of Al-MIL-53-
FcDC, the absorption bands of the electronic transitions of the
ferrocene complex could be clearly detected at 328 nm and
4
45 nm. For H FcDC these transitions are located at 315 nm
2
and 459 nm. According to the literature, the absorption bands
of the corresponding electronic dd-transitions for pure ferro-
1
1
1
1
cene ( A to a E and A to b E ) are observed at 325 nm
1
g
1g
9,40
1g
2g
3
and 440 nm.
The shift between the data for Al-MIL-53-
FcDC, the linker and ferrocene can be explained by the substi-
Fig. 4 Left: IR-spectra of the as synthesised sample (black line) and the
activated solvent exchanged sample (red line). The vibrational bands of
DMF molecules are labelled with asterisks. Right: UV/vis-spectra of
Fig. 2 Rietveld-plot of Al-MIL-53-FcDC-ac with measured (black), cal-
culated (red) and difference plots (blue). Vertical bars mark the allowed
peak positions.
2
H FcDC (black) and the activated sample (red).
Fig. 3 Schematic representation of the structure of the linker (FcDC −) and the IBU (aluminum oxygen octahedra chains) (left) and the structure of
2
Al-MIL-53-FcDC along the a-axis (middle and right). The blue sphere represents a guest molecule with a diameter of 3.1 Å.
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tution with two carboxylic acid or carboxylate groups,
3
9,40
respectively.
Sorption properties
The porosity of the activated compound Al-MIL-53-FcDC was
investigated by measuring nitrogen and water vapour sorption
isotherms (Fig. 5). The samples were first activated under
−
2
reduced pressure (10 kPa) for 16 h at 150 °C. The following
sorption experiments were performed at 77 K (298 K) in the
case of N (H O) as the adsorbent. The nitrogen sorption iso-
2
2
therm exhibits a type-1 shape which is characteristic of micro-
porous compounds. BET evaluation results in a specific
2
−1
surface area of aBET = 340 m
g
. Based on the adsorbed
= 0.5, a micropore volume
amount at a relative pressure of p/p
of 0.13 cm g could be determined. Theoretical calculations
with Platon result in a micropore volume of V
0
3
−1
3
= 0.13 cm
mic
−
1
41
g
, thus being in excellent agreement.
Furthermore, the uptake of water was investigated by
Fig. 6 The EPR spectra of Al-MIL-53-FcDC-ac (black line) and the
2
linker H FcDC (red line).
measuring the vapour sorption isotherm. The experimental
data indicate a nearly linear uptake of water molecules. A
−
1
maximum uptake of 106 mg g was determined at a p/p
value of 0.902. The isotherm shows a slight hysteresis in the p/
range between 0.15 and 0.90. Both samples obtained after
0
MIL-53-FcDC-ac shows two signals. The broad signal at 1725 G
g-factor = 4.08) can be assigned to the oxidised linker mole-
(
p
0
5
cule ferroceniumdicarboxylate (the d system of the iron(III)
center). The g-factor of the unsubstituted ferrocenium cation
the sorption experiments were further characterised by PXRD
measurements and showed unchanged patterns, indicating no
breathing behaviour and the stability of the samples under
these conditions (Fig. S17, ESI†).
42
in solution is given in the literature with a value of 4.35, thus
being in good agreement. The very small second signal at 3525
G (g-factor = 1.998) is attributed to the presence of an un-
identified, negligibly small contamination of the sample with
an organic radical.
EPR and Mössbauer spectroscopy
2
−
Since the linker molecule FcDC
shows potential redox
In addition the sample was characterised via Mössbauer
spectroscopy. The spectrum of the activated sample Al-MIL-53-
activity, Al-MIL-53-FcDC was further characterised by EPR and
Mössbauer spectroscopy. The EPR spectrum shows the pres-
ence of a paramagnetic species, which can be traced back to
FcDC was recorded at 298.15 K (Fig. 7). The isomeric shift (δ
IS
0.33 mm s ) and quadrupole splitting (ΔEQ = 2.38 mm s−1)
−1
=
the oxidised ferrocenium cation of H
2
FcDC (Fig. 6). While the
pristine linker is not EPR active, the EPR spectrum of Al-
2
Fig. 5 The N -sorption isotherm (black triangles) and the water sorp-
tion isotherm (red circles) of the activated sample. The adsorption (de-
sorption) isotherms are represented as filled (empty) symbols.
Fig. 7 The Mössbauer spectrum of the activated sample (black squares)
and the corresponding fit (red line).
16740 | Dalton Trans., 2019, 48, 16737–16743
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of the obtained data are similar to the values observed for
−
1
unsubstituted ferrocene (δ = 0.34 mm s and ΔEQ = 2.39 mm
IS
−
1 43
s
). Furthermore the spectrum of ferrocene shows a charac-
teristic doublet, which is also observed for Al-MIL-53-FcDC.
4
4
The small difference between the measured parameters and
the parameters of pure ferrocene can be explained by the sub-
stitution of the cyclopentadienes with carboxylate groups. The
slight isomeric shift of substituted ferrocenes in comparison
to unsubstituted ferrocene is well known in the literature.
Hence Mössbauer spectroscopy indicates that the linker
molecules of the framework are nearly exclusively present in
the ferrocene form, while EPR spectroscopy revealed that a
small fraction was oxidised to ferrocenium ions. However, the
amount of ferrocenium ions is too small to be quantified by
Mössbauer spectroscopy and thus we estimate the relative
4
5
amount to be less than 5% of the linker molecules.
Fig. 9 CV curves of Al-MIL-53-FcDC on a gold electrode: the 1st cycle
(
black) and the 40th cycle (red) using an Ag rod as a pseudo reference
Redox activity
electrode. For comparison the curves of the pure gold electrode (blue)
and ferrocene in solution (green) are also shown.
To investigate the redox activity of Al-MIL-53-FcDC, cyclic vol-
tammetric experiments on electrodes with the deposited title
compound were carried out. In the redox process, one electron
is removed from the ferrocene unit and in the reduction cycling stability of Al-MIL-53-FcDC could be verified by per-
process the generated ferrocenium ion is vice versa reduced forming 40 cycles at a scan rate of 50 mV. This could be
(
Fig. 8).
For these measurements the MOF was first deposited on a recorded before and after CV experiments (see Fig. S22, ESI†).
gold electrode (see the ESI† for details). The CV experiment Remarkably the intensity of the oxidation/reduction wave for
was carried out with a 0.1 M NaPF
/acetonitrile solution as the the 40th cycle is larger than the signal of the 1st cycle. We
further substantiated by the unchanged PXRD patterns
6
electrolyte and using a silver rod as pseudo reference electrode attribute this to the improved accessibility and/or availability
and Pt as counter electrode. Pristine ferrocene was used as the of the MOF after prolonged cycling.
benchmark, dissolved in the same electrolyte. The CV curves
of the CV experiment between 0.3 V and 1.1 V using Al-MIL-53- only on the outer surface of the MOF or as well throughout the
However, it is unclear yet if the redox reaction takes place
−
FcDC on the gold electrode (1st and 40th cycles), for the bare framework. For the surface reaction, the adsorption of PF
6
on
gold electrode, and for ferrocene dissolved in the electrolyte the particle’s surface would suffice for charge compensation.
are shown in Fig. 9. For the MOF coated electrode a reversible In contrast, the reaction of the complete framework would
−
redox event could be detected with an oxidation wave at 0.75 V necessitate the unlikely adsorption of the large PF
6
anions
and a reduction wave at 0.64 V. This redox process can be into the channels and therefore charge compensation by
+
2−
assigned to the reversible oxidation Fc/Fc of FcDC . The proton release from the OH groups of the IBU would be more
likely. To clarify such remaining questions, a more detailed
study of the redox activity is the topic of ongoing
investigations.
Conclusion
In this contribution we have successfully demonstrated the
synthesis and characterisation of the first permanently porous
1,1′-ferrocenedicarboxylate-based Al-MOF. The title compound
crystallises in the MIL-53 type structure as confirmed by
2
−1
Rietveld refinement. A specific surface area of 340 m g and
3
−1
a pore volume of 0.13 cm g were determined by nitrogen
sorption measurement. UV/vis, EPR and Mössbauer spec-
troscopy methods were used to confirm the oxidation state of
the iron species and the structural integrity of the metallocene
unit. A trace amount of ferrocenium ions was observed, while
the large majority of the linker molecules persist as ferrocene
Fig. 8 Schematic representation of the oxidation and reduction pro-
cesses of Al-MIL-53-FcDC during the CV experiment.
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units. The redox activity of Al-MIL-53-FcDC could be proven in 15 D. M. D’Alessandro, Chem. Commun., 2016, 52, 8957.
CV experiments. The CV curve of Al-MIL-53-FcDC shows 16 K. M. Choi, H. M. Jeong, J. H. Park, Y.-B. Zhang, J. K. Kang
signals which can be assigned to the reversible oxidation and
reduction of the ferrocene unit and the stability during this 17 R. Díaz, M. G. Orcajo, J. A. Botas, G. Calleja and J. Palma,
process was further confirmed by PXRD. Therefore Al-MIL-53- Mater. Lett., 2012, 68, 126.
FcDC is a promising candidate for electrochemical appli- 18 Q. Liu, X. Liu, C. Shi, Y. Zhang, X. Feng, M.-L. Cheng, S. Su
and O. M. Yaghi, ACS Nano, 2014, 8, 7451.
cations of MOFs which will be further evaluated in the future.
and J. Gu, Dalton Trans., 2015, 44, 19175.
9 D. Astruc, Eur. J. Inorg. Chem., 2017, 2017, 6.
0 V. Chandrasekhar and R. K. Metre, Dalton Trans., 2012, 41,
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1
2
Conflicts of interest
2
2
2
2
2
2
1 G. Dong, Z. Bing-guang, D. Chun-ying, C. Xin and M. Qing-
jin, Dalton Trans., 2003, 282.
2 G. Dong, M. Hong, D. Chun-ying, L. Feng and M. Qing-jin,
J. Chem. Soc., Dalton Trans., 2002, 2593.
3 G. Dong, L. Yu-Ting, D. Chun-ying, M. Hong and M. Qing-
jin, Inorg. Chem., 2003, 42, 2519.
4 X. Meng, G. Li, H. Hou, H. Han, Y. Fan, Y. Zhu and C. Du,
J. Organomet. Chem., 2003, 679, 153.
5 Y.-Y. Yang and W.-T. Wong, Chem. Commun., 2002, 34,
There are no conflicts of interest.
Acknowledgements
This project was supported by the German Science Foundation
(RE 4057/1-1).
2716.
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