A new alkali metal salt of tetrafluoroterephthalic acid (H 2tF-BDC)

Article abstract of DOI:10.1002/zaac.201400006

The DMF solvate of lithium tetrafluoroterephthalate [Li₂(tF-BDC) (DMF)₂] (H₂tF-BDC = tetrafluoroterephthalic acid) was synthesized and structurally characterized from X-ray single crystal data (Pbcn, Z = 8). Li⁺ (CN = 4) is coordinated tetrahedrally by four oxygen atoms of three different tF-BDC^2- linkers and one DMF molecule. The LiO₄ tetrahedra are connected by common vertices to chains, which are bridged by tF-BDC^2- linkers to form a 2D polymeric unit. The DMF molecules are oriented perpendicular to these polymeric sheets, which are stacked along [100] in an AB fashion and held together by weak van der Waals interactions. Single crystals of [Li₂(tF-BDC)(DMF)₂] were synthesized by precipitation from an ethanol/DMF solvent mixture, but single-phase samples were only obtained by a mechanochemical approach starting from lithium acetate and H₂tF-BDC by adding DMF. The thermal stability of [Li₂(tF-BDC)(DMF)₂] was investigated by DTA/TG measurements. The (endothermic) decomposition starts at 170 °C and is followed by a second exothermic decomposition at approx. 300 °C. The weight losses are in good agreement with a release of DMF and CO₂. LiF is identified in the residue after heating at 500 °C. Copyright

Full text of DOI:10.1002/zaac.201400006

SHORT COMMUNICATION
DOI: 10.1002/zaac.201400006
[Li2(tF-BDC)(DMF)2]: A New Alkali Metal Salt of Tetrafluoroterephthalic
Acid (H2tF-BDC)
Benedikt Dolfus[a] and Uwe Ruschewitz*[a]
Dedicated to Professor Gerd Meyer on the Occasion of His 65th Birthday
Keywords: Coordination polymer; Crystal structure; Lithium; Mechanochemical synthesis; Perfluorinated carboxylates
Abstract. The DMF solvate of lithium tetrafluoroterephthalate
[Li2(tF-BDC)(DMF)2] (H2tF-BDC = tetrafluoroterephthalic acid) was
synthesized and structurally characterized from X-ray single crystal
data (Pbcn, Z = 8). Li+ (CN = 4) is coordinated tetrahedrally by four
[Li2(tF-BDC)(DMF)2] were synthesized by precipitation from an
ethanol/DMF solvent mixture, but single-phase samples were only ob-
tained by
a mechanochemical approach starting from lithium
acetate and H2tF-BDC by adding DMF. The thermal stability of
oxygen atoms of three different tF-BDC2– linkers and one DMF mole- [Li2(tF-BDC)(DMF)2] was investigated by DTA/TG measurements.
cule. The LiO4 tetrahedra are connected by common vertices to chains, The (endothermic) decomposition starts at 170 °C and is followed by
which are bridged by tF-BDC2– linkers to form a 2D polymeric unit.
The DMF molecules are oriented perpendicular to these polymeric
sheets, which are stacked along [100] in an AB fashion and held to-
gether by weak van der Waals interactions. Single crystals of
a second exothermic decomposition at approx. 300 °C. The weight
losses are in good agreement with a release of DMF and CO2. LiF is
identified in the residue after heating at 500 °C.
ported up to now. Obviously, the synthetic procedures for non-
fluorinated MOFs cannot simply be transferred to their fluori-
nated congeners.
Introduction
Coordination polymers (CPs) and porous metal-organic
frameworks (MOFs) with fluorinated or perfluorinated linker
ligands are supposed to show improved properties compared
with their non-fluorinated congeners.[1] Consequently, a high
H2 adsorption capacity was found for porous FMOF-1.[2] This
result was corroborated by a theoretical investigation,[3] but in
another experimental work only a slightly enhanced H2 adsorp-
tion capacity was reported.[4] Banerjee et al. showed for some
isostructural porous coordination polymers with fluorinated
and non-fluorinated ligands that the H2 uptake is “system-spe-
cific”,[5] i.e. the influence of the crystal structure on the ad-
sorption properties cannot be neglected. In this respect, it is
interesting to note that isostructural fluorinated congeners of
the well-known MOFs, MOF-5[6] and MIL-53,[7] are still un-
known. In both terephthalate (1,4-benzenedicarboxylate =
BDC2–) is incorporated as linking ligand. Despite the fact
that its perfluorinated counterpart tetrafluoroterephthalate
(tF-BDC2–) is commercially available and convenient synthe-
ses have been reported in the literature,[8,9] no isostructural or
even porous homoleptic MOF with tF-BDC2– has been re-
In a different approach focusing on the optical properties
it was reported that CPs with perfluorinated ligands show a
significantly enhanced luminescence.[10] In this context, we
observed a bright luminescence in CPs of 4f elements with
tetrafluoroterephthalate (tF-BDC2–) as bridging ligand.[11] Ob-
viously, the replacement of C–H by C–F entities reduces the
quenching of luminescent properties by C–H vibrations signifi-
cantly. Unfortunately, the synthesis of CPs with tF-BDC2– as
linker is sometimes hampered by the low solubility of
H2tF-BDC in some solvents. This problem might be circum-
vented by using salts of H2tF-BDC. Therefore, we have started
to investigate salts of H2tF-BDC in more detail and described
the synthesis and structural characterization of anhydrous
+ [9]
MI2(tF-BDC) with MI = NH4 , K+,[12] and Rb+.[12] In this
contribution we will report another example, namely
[Li2(tF-BDC)(DMF)2].
Results and Discussion
Single crystals of [Li2(tF-BDC)(DMF)2] precipitated from a
solution of LiCl·H2O and H2tF-BDC (tetrafluoroterephthalic
acid) in EtOH/DMF (3:1, v:v), in which triethylamine was
slowly diffused. A single crystal suitable for an X-ray single
crystal structure analysis was isolated from this precipitate, but
elemental analysis as well as X-ray powder diffraction (Figure
S1, Supporting Information) revealed that the sample is not
single-phase. Therefore, a mechanochemical synthesis was de-
* Prof. Dr. U. Ruschewitz
Fax: +49-221-470-3933
E-Mail: Uwe.Ruschewitz@uni-koeln.de
[a] Department für Chemie
Universität zu Köln
Greinstraße 6
50939 Köln, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201400006 or from the au-
thor.
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1235

B. Dolfus, U. Ruschewitz
SHORT COMMUNICATION
veloped, in which Li(CH3COO)·2H2O and H2tF-BDC were
carefully ground while adding DMF. Elemental analysis as
well as X-ray powder diffraction (Figure S2, Supporting Infor-
mation) confirmed that a single-phase sample was obtained.
[Li2(tF-BDC)(DMF)2] crystallizes in the orthorhombic
space group Pbcn with Z = 8. The asymmetric unit consists of
two symmetry independent Li+ cations, one tF-BDC2– anion,
and two symmetry independent DMF molecules (Figure 1).
Each Li+ cation is coordinated by four oxygen atoms stemming
from three different tF-BDC2– anions and one DMF molecule.
The Li–O distances range from 1.916(6) Å (O201) to
1.986(7) Å (O200) for Li50 and from 1.912(6) Å (O203) to
1.992(6) Å (O200) for Li51. No Li···F contacts shorter than
3 Å are found. The O–Li–O angles within the coordination
polyhedra range from 100.7(4)° to 118.2(3)° for Li50 and from
103.8(3)° to 114.5(3)° for Li51. Thus, the values are close to
the ideal value for a tetrahedron. Both DMF molecules show
a strong disorder, which is reflected in somewhat unusual
bonding distances and angles within the molecules as well as
large thermal ellipsoids (Figure 1, see also Experimental Sec-
tion). The distances and angles within the tF-BDC2– anion are
in the expected range. The carboxylate groups are tilted
[71.6(3)° and 89.3(3)°] with respect to the plane of the benzene
ring like in many fluorinated benzenedicarboxylates. The
reasons have already been discussed elsewhere.[13]
Figure 2. View of the 2D polymeric unit in [Li2(tF-BDC)(DMF)2] in
a projection along [100]. LiO4 tetrahedra are emphasized.
The thermal stability of [Li2(tF-BDC)(DMF)2] was investi-
gated by a DTA/TG measurement in the range of 25–500 °C
in an argon atmosphere (Figure S6, Supporting Information).
An endothermic signal starting at approx. 170 °C (mass loss:
ca. 26%) and a strong exothermic signal starting above 300 °C
(mass loss: ca. 28%) were observed. For the release of one
DMF molecule a theoretical value of 18.5% and for one CO2
molecule 11.1% are calculated. So for the release of two DMF
and two CO2 molecules a remaining mass of 40.8% is ex-
pected, which is in very good agreement with the mass ob-
tained above 330 °C. Accordingly, in the residue after heating
to 500 °C LiF was identified (Figure S7, Supporting Infor-
mation) next to unidentified amorphous components. Heating
[Li2(tF-BDC)(DMF)2] to 195 °C and 260 °C resulted in poly-
crystalline materials, which show significant changes of their
XRPD patterns (Figure S8, Supporting Information). But the
modest quality of the patterns did not allow a further elucida-
tion of their crystal structures.
Conclusions
We have synthesized and structurally characterized the first
lithium salt of tetrafluoroterephthalate, which crystallizes as a
DMF solvate. Its crystal structure consists of layered poly-
meric structural units. A single-phase sample was obtained by
grinding Li(CH3COO) and H2tF-BDC under DMF. The
weaker acid (acetic acid) is released. This mechanochemical
approach emerges as an interesting synthetic approach for the
synthesis of new tetrafluoroterephthalates. The crystallinity of
the samples obtained is surprisingly high (Figure S2, Support-
ing Information). However, single crystals could only be
Figure 1. ORTEP plot of [Li2(tF-BDC)(DMF)2] with labelling of the
atoms of the asymmetric unit. Thermal ellipsoids are drawn at the 30%
probability level.
The LiO4 tetrahedra in [Li2(tF-BDC)(DMF)2] are connected
by common vertices involving oxygen atoms O200 and O202
of the carboxylate groups to form chains along [010]. These
chains are linked by tF-BDC2– anions to form a layer-like
polymeric structural unit (Figure 2). The DMF molecules are
oriented almost perpendicular to these layers, which are
stacked along [100] in an AB fashion and held together by
weak van der Waals interactions. In solvent-free MI2(tF-BDC)
(MI =K+, Rb+) 3D networks have been found.[12]
obtained by crystallization from
a EtOH/DMF solvent
mixture. Currently, we are investigating the potential of
[Li2(tF-BDC)(DMF)2] as starting material for the synthesis of
new coordination polymers with tetrafluoroterephthalate as
linker.
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A Li Salt of Tetrafluoroterephthalic Acid
Table 1. Selected crystal and structure refinement data of
[Li2(tF-BDC)(DMF2] (Stoe IPDS I, Mo-Kα radiation, 20 °C).
Experimental Section
H2tF-BDC was synthesized according to the procedure described in
the literature.[9] Its purity was checked by NMR and X-ray powder
diffraction (Figure S3, Supporting Information).
Molecular formula, Mr /g·mol–1
Crystal system, space group, Z
a /Å, b /Å, c /Å
V /Å3
C14H14F4Li2N2O6, 396.15
Orthorhombic, Pbcn, 8
19.149(1), 9.714(4), 18.589(1)
3458(1)
1.522, 0.14
0.3ϫ0.2ϫ0.1
[Li2(tF-BDC)(DMF)2]: (a) LiCl·H2O (12.1 mg, 0.02 mmol) and
H2tF-BDC (23.4 mg, 0.01 mmol) were dissolved in EtOH/DMF
(1.5 mL, 3:1, v:v) and filled into a small vial, which was closed with
a perforated foil. The vial was placed in a snap cap tube, which had
been filled with EtOH/DMF (4 mL, 3:1, v:v) and triethylamine (3:1,
v:v) before (Figure S4, Supporting Information). After 5 d, colorless
needle-like single crystals were obtained, from which a single crystal
suitable for a single crystal structure analysis was isolated. The re-
sulting XRPD pattern and IR spectrum are given in the Supporting
Information (Figures S1 and S5). Both as well as the elemental analy-
sis indicate that no single-phase sample was obtained. Elemental analy-
sis for Li2C14F4O6H14N2 (396.15 g·mol–1): calcd. C 42.45, H 3.56, N
7.07%; found C 37.27, H 4.07, N 2.87%.
D /g·cm–3, μ /mm–1
Crystal size /mm3
Θmax /°
26.8
Number of reflections
measured
41904
independent
3689 [2105 with I Ͼ 2σ(I)]
Number of parameters
R factors
252
Io Ͼ 2σ(Io)
All data
R1 = 0.088, wR2 = 0.248
R1 = 0.129, wR2 = 0.294
0.189; 1.02
Rint; GooF
Δρmin/max /e·Å–3
–0.63 / 1.18
a)
a) 1.74 Å from N310.
(b) Li(CH3COO)·2H2O (306 mg, 3 mmol) and H2tF-BDC (357 mg,
1.5 mmol) were ground in an agate mortar, while DMF (100 μL) was
added. After the smell of acetic acid was no longer noticed, the re-
sulting product was dried in vacuo. The resulting XRPD pattern is
given in the Supporting Information (Figure S2). No impurity reflec-
tions were detected and the elemental analysis also indicates that a
single-phase sample was obtained. This sample was used for the DTA/
TG measurement. Elemental analysis for Li2C14F4O6H14N2
(396.15 g·mol–1): calcd. C 42.45, H 3.56, N 7.07%; found C 41.24, H
3.78, N 6.58%.
alization and calculation of theoretical XRD patterns from single crys-
tal data.
Elemental Analysis: Elemental analyses were carried out with a
CHNS Euro EA 3000 Analyzer (HEKAtech GmbH).
IR Spectroscopy: FT-IR measurements were carried out on solid KBr
pellets with a Bruker ALPHA-T spectrometer.
DTA/TG Measurements: DTA/TG measurements were performed
with a Netzsch STA 409C/CD using an Al2O3 crucible in a constant
argon stream (50 mL·min–1). The heating rate was 10 °C·min–1 with a
sample mass of 20.4 mg.
Single Crystal Diffraction: A single crystal of [Li2(tF-BDC)(DMF)2]
was isolated from the precipitate of synthesis (a) and measured with a
Stoe IPDS I single crystal diffractometer (T ≈ 20 °C). Data collection
and reduction were performed with the Stoe program package.[14] The
crystal structure was solved by direct methods using SIR-92.[15] The
structural models were completed using difference Fourier maps calcu-
lated with SHELXL-97,[16] which was also used for the refinements.
No absorption correction was applied. Hydrogen atoms were placed
on calculated positions and refined with a “riding” model. All non-
hydrogen atoms were refined anisotropically with the exception of
C212. The methyl groups of both DMF molecules show a high degree
of disorder, as obvious from their large thermal ellipsoids (Figure 1).
C212 could not be modelled adequately with an anisotropic refinement
and was therefore refined isotropically. It was not possible to apply a
split model for the refinement of the methyl groups. The quality of the
data set is restricted by the fact that [Li2(tF-BDC)(DMF)2] only con-
tains light elements so that mainly weak high angle reflections are
obtained. Reducing the 2θ range minimizes the R factors significantly,
but a reflection:parameter ratio Ͻ 10 is obtained. Therefore, in the
final refinement all measured reflections were used. Problems may
also arise from the fact that a needle-shaped crystal was measured with
a Stoe IPDS I diffractometer, which might lead to preferred orientation
effects. A high standard deviation of the b axis (Table 1) is a hint for
that. Nonetheless, the main conclusions of the crystal structure analysis
are not influenced by these problems, so that no low temperature data
were recorded. More details of the crystal structure solution and refine-
ment are given in Table 1.
Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
number CCDC-978884 [Li2(tF-BDC)(DMF)2] (Fax: +44-1223-336-
033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Supporting Information (see footnote on the first page of this article):
Additional experimental and simulated X-ray powder diffraction pat-
terns, IR spectrum, DTA/TG plot and snapshot of the synthesis of
[Li2(tF-BDC)(DMF)2].
Acknowledgements
We thank Peter Kliesen and Dr. Ingo Pantenburg for X-ray single
crystal data acquisition, Malgorzata Smolarek for DTA/TG measure-
ments, Silke Kremer for elemental analysis and Heidi Schwartz for
additional syntheses.
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Received: January 3, 2014
Published Online: February 12, 2014
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Z. Anorg. Allg. Chem. 2014, 1235–1238