Open-Framework Structures of Anhydrous Sr(CF3COO)2 and Ba(CF3COO)2

Article abstract of DOI:10.1021/acs.inorgchem.5b02081

Anhydrous Sr(CF₃COO)₂ and Ba(CF₃COO)₂ open-framework structures featuring three-dimensional connectivity of metal-oxygen polyhedra were crystallized from a mixture of water and CF₃COOH. Crystallization was induced via evaporation of the solvent mixture under a dry nitrogen flow. This approach differs from that routinely employed for crystallization of metal trifluoroacetates, which achieves solvent evaporation by heating under air and yields hydrated salts. Thermogravimetric and differential thermal analysis as well as single-crystal and synchrotron powder X-ray diffraction were employed to characterize the alkaline-earth trifluoroacetate products. Neither thermal analysis nor single-crystal X-ray diffraction detected the presence of crystallization water molecules, demonstrating these trifluoroacetates can be obtained in anhydrous form. Single-crystal X-ray diffraction studies showed that Sr(CF₃COO)₂ and Ba(CF₃COO)₂ are isostructural and crystallize in the rhombohedral R3ˉ space group. Both compounds belong to the class of organic-inorganic extended hybrids and exhibit an open-framework structural motif with three-dimensional connectivity of the metal-oxygen polyhedra and one-dimensional channels along the c axis. The channels are decorated with the trifluoromethyl groups of the trifluoroacetate ligands, and their average (minimum) diameters are ～3.75 (2.60) and 3.45 (2.25) ? for Sr(CF₃COO)₂ and Ba(CF₃COO)₂, respectively. This size range is comparable to the kinetic diameter of small molecules such as hydrogen (2.3 ?). Chemical substitution of barium for strontium affects not only the diameter of the channels but also the spatial arrangement of the trifluoromethyl groups within the channels and the coordination environment of the metal atoms. The different coordination requirements of the strontium and barium atoms are accommodated through the displacement of one of the two chemically distinct trifluoroacetate ligands relative to the metal center.

Full text of DOI:10.1021/acs.inorgchem.5b02081

Article
pubs.acs.org/IC
Open-Framework Structures of Anhydrous Sr(CF3COO)2 and
Ba(CF3COO)2
K. Tauni Dissanayake,† Laura M. Mendoza,† Philip D. Martin,† Leopoldo Suescun,‡
and Federico A. Rabuffetti*,†
†Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States
‡Cryssmat-Lab/Cat
́
edra de Física/DETEMA, Facultad de Química, Universidad de la Republica, Montevideo 11800, Uruguay
́
S
* Supporting Information
ABSTRACT: Anhydrous Sr(CF3COO)2 and Ba(CF3COO)2 open-framework
structures featuring three-dimensional connectivity of metal−oxygen polyhedra
were crystallized from a mixture of water and CF3COOH. Crystallization was
induced via evaporation of the solvent mixture under a dry nitrogen flow. This
approach differs from that routinely employed for crystallization of metal
trifluoroacetates, which achieves solvent evaporation by heating under air and
yields hydrated salts. Thermogravimetric and differential thermal analysis as
well as single-crystal and synchrotron powder X-ray diffraction were employed
to characterize the alkaline-earth trifluoroacetate products. Neither thermal
analysis nor single-crystal X-ray diffraction detected the presence of
crystallization water molecules, demonstrating these trifluoroacetates can be
obtained in anhydrous form. Single-crystal X-ray diffraction studies showed
that Sr(CF COO) and Ba(CF COO) are isostructural and crystallize in the rhombohedral R3 space group. Both compounds
̅
3
2
3
2
belong to the class of organic−inorganic extended hybrids and exhibit an open-framework structural motif with three-
dimensional connectivity of the metal−oxygen polyhedra and one-dimensional channels along the c axis. The channels are
decorated with the trifluoromethyl groups of the trifluoroacetate ligands, and their average (minimum) diameters are ∼3.75
(2.60) and 3.45 (2.25) Å for Sr(CF3COO)2 and Ba(CF3COO)2, respectively. This size range is comparable to the kinetic
diameter of small molecules such as hydrogen (2.3 Å). Chemical substitution of barium for strontium affects not only the
diameter of the channels but also the spatial arrangement of the trifluoromethyl groups within the channels and the coordination
environment of the metal atoms. The different coordination requirements of the strontium and barium atoms are accommodated
through the displacement of one of the two chemically distinct trifluoroacetate ligands relative to the metal center.
trifluoroacetates are the two main sources of contamination.
The former can be avoided by storing and handling the coating
solution in a moisture-free atmosphere; however, the
crystallization of anhydrous trifluoroacetate precursors has
proven to be elusive. As a result, synthetic methods targeting
anhydrous coating solutions have focused on removing the
crystallization water released upon dissolution of the
trifluoroacetates,1,9−11 rather than exploring new routes for
crystallizing anhydrous trifluoroacetate precursors.
In addition, previous investigations of metal trifluoroacetates
in the solid state have focused on understanding the
physicochemical aspects of their thermal decomposition;12−17
investigation of their crystallochemical aspects has compara-
tively languished. Specifically in the case of alkaline-earth
trifluoroacetates, only the crystal structure of Ca(CF3COO)2·
H2O has been reported;15 the crystal structures of Sr-
(CF3COO)2 and Ba(CF3COO)2 remain unknown. This
fundamental knowledge gap persists despite the fact that the
relevance of the precursors’ structure in their solution chemistry
INTRODUCTION
■
Alkaline-earth metal trifluoroacetates constitute an important
class of precursors employed in solution-based routes to a wide
range of functional materials, including high-temperature
superconducting oxide thin films (e.g., YBa2Cu3O7),1,2
upconversion nanocrystals (e.g., Er:Yb:SrF2),3 and nano-
structured fluoride ceramics (e.g., MgF2 and CaF2).4,5 Ba-
(CF3COO)2·xH2O is the most extensively studied member of
this family of materials because of its use in the preparation of
YBa2Cu3O7 thin films via chemical solution deposition of metal
trifluoroacetates. This approach employs a stoichiometric
mixture of the corresponding metal trifluoroacetates to prepare
a coating solution that is subsequently deposited on a substrate
and subjected to thermal treatment to induce decomposition of
the metal−organic precursors. It has been extensively acknowl-
edged that the purity of the coating solution, specifically its
water content, directly affects the functionality of the
superconducting oxide film.1,6−8 Thus, significant efforts have
been devoted to the preparation of anhydrous coating solutions
of metal trifluoroacetates.1,7,8 Chemisorption of atmospheric
moisture during storage and handling and release of
crystallization water during the dissolution of the metal
Received: September 9, 2015
© XXXX American Chemical Society
A
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Figure 1. Thermogravimetric and differential thermal analysis of (a) Sr(CF3COO)2 and (b) Ba(CF3COO)2.
has been acknowledged.16 From this perspective, elucidation of
the crystal structures of Sr(CF3COO)2 and Ba(CF3COO)2 is
needed to establish structure−solution chemistry relationships
that could guide synthetic approaches employed in a variety of
areas; these include the preparation of coating solutions for the
deposition of high-temperature superconducting oxide thin
films, the synthesis of upconversion nanocrystals via thermal
decomposition of trifluoroacetates in high-boiling point organic
solvents, and the preparation of nanostructured fluoride
ceramics via sol−gel routes.
the metal carbonate occurred within 1−3 min and, as expected, was
accompanied by vigorous gas evolution; a colorless, optically
transparent solution was obtained. Crystallization of the desired
metal trifluoroacetate was accomplished by solvent evaporation, which
was induced by flowing dry nitrogen gas over the precursor solution.
The reaction flask containing the precursor solution was connected to
a nitrogen gas line through one of its necks; a glass adapter was placed
on the other neck to allow venting of the nitrogen gas. Then, it was
immersed in a sand bath at 65 °C, and the nitrogen gas flow was
started. A needle-valve rotameter was employed to regulate the
nitrogen flow rate. A constant flow rate of 200 mL min−1 was
employed for both Sr(CF3COO)2 and Ba(CF3COO)2. The system
was left undisturbed for 48 h at 65 °C under flowing nitrogen. After 48
h, white Sr(CF3COO)2 and Ba(CF3COO)2 powders were obtained at
the bottom of the reaction flasks. Because of their deliquescent nature,
these were stored in a glovebox (oxygen and water levels of <1 ppm)
at room temperature for thermal and X-ray diffraction studies. Both
trifluoroacetates were observed to deliquesce within minutes if
exposed to ambient moisture.
Thermal Analysis. Thermogravimetric analysis (TGA) of the
powders was conducted using a Pyris 1 TGA analyzer (Perkin-Elmer).
Samples (∼5−10 mg) were heated at 35 °C for 30 min under flowing
nitrogen (20 mL min−1) and then heated to 600 °C at a rate of 10 °C
min−1 under an identical atmosphere. Differential thermal analysis
(DTA) was conducted using an SDT2960 TGA-DTA analyzer (TA
Instruments) under identical conditions. Samples were exposed to the
atmosphere for <30 s prior to thermal analysis.
Single-Crystal X-ray Diffraction. Colorless single crystals of
Sr(CF3COO)2 and Ba(CF3COO)2 were recovered from the
polycrystalline powders and used for structural determination. Both
crystals were columnar with approximate dimensions of 0.2 mm × 0.2
mm × 0.3 mm. These were mounted in Paratone N oil on a Bruker X8
Apex single-crystal diffractometer, and X-ray intensities were measured
at −173 °C using Mo Kα radiation (λ = 0.71073 Å). Frames were
integrated using Bruker SAINT software. Experimental data were
corrected for Lorentz, polarization, and absorption effects; for the
latter, the multiscan method was employed. Structures were
determined using direct methods and Fourier synthesis as embedded
in the SHELXL-2014/7 software.19 Resulting crystal structures were
visualized using the VESTA software.20 Crystal data of Sr(CF3COO)2
and Ba(CF3COO)2 were deposited in the Cambridge Crystallographic
Data Centre as entries 1421987 and 1421988, respectively. Structural
models derived from single-crystal X-ray diffraction studies were
employed to estimate the average and minimum diameters of the one-
dimensional channels, as will be shown below.
In the course of our developments of anhydrous metal−
organic precursors suitable for the synthesis of upconversion
nanocrystals, we crystallized Sr(CF3COO)2 and Ba(CF3COO)2
powders from a mixture of water and CF3COOH by inducing
evaporation of the solvent mixture with a dry nitrogen flow.
Our approach to crystallization of metal trifluoroacetates was
drastically different from that first proposed by Roberts for the
crystallization of rare-earth trifluoroacetates,18 which achieves
solvent evaporation by heating under air. Interestingly, neither
thermal analysis nor single-crystal X-ray diffraction (XRD)
detected the presence of crystallization water molecules,
indicating that Sr(CF3COO)2 and Ba(CF3COO)2 can be
obtained in anhydrous form using the solvent evaporation
method presented herein. Single-crystal XRD studies showed
that Sr(CF3COO)2 and Ba(CF3COO)2 are isostructural and
revealed an unexpected open-framework structural motif
consisting of a network of metal−oxygen polyhedra with
three-dimensional connectivity and one-dimensional channels
along the c axis. The channels are decorated with trifluor-
omethyl groups of the trifluoroacetate ligands. Their average
(minimum) diameters are ∼3.75 (2.60) and 3.45 (2.25) Å for
Sr(CF3COO)2 and Ba(CF3COO)2, respectively. This size
range is comparable to the kinetic diameter of small molecules
such as hydrogen (2.3 Å). Chemical substitution of the alkaline-
earth metal affects not only the diameter of the channels but
also the spatial arrangement of the trifluoromethyl groups
within the channels. The significance of these findings is
discussed from the perspective of employing Sr(CF3COO)2
and Ba(CF3COO)2 as precursors in solution-based approaches
to functional solid-state materials and as functional materials
themselves.
Synchrotron Powder X-ray Diffraction. Synchrotron XRD
patterns of the polycrystalline Sr(CF3COO)2 and Ba(CF3COO)2
powders were collected in the 2θ range of 1−40° at the 11-BM line
of the Advanced Photon Source of Argonne National Laboratory
(Argonne, IL). Samples were packed in double-nesting Kapton tubes,
and both ends of the tubes were sealed with clay. Packing was
conducted inside the glovebox to minimize degradation of the samples.
Diffraction patterns were collected in transmission mode. An incident
photon energy of 29.936 keV (λ = 0.4141650 Å) was employed. The
EXPERIMENTAL SECTION
■
Crystallization of Sr(CF3COO)2 and Ba(CF3COO)2. SrCO3
(99.9%, Sigma-Aldrich), BaCO3 (99.99%, Sigma-Aldrich), and
anhydrous CF3COOH (99%, Sigma-Aldrich) were used as received.
Double-distilled water was employed as the solvent. A mixture of 5 mL
of double-distilled water and 1 mL of CF3COOH was prepared in a 50
mL two-neck round-bottom flask. Then, 2 mmol of the corresponding
metal carbonate was added to this solution. Quantitative dissolution of
B
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Figure 2. Crystal structures of (a−d) Sr(CF3COO)2 and (e−h) Ba(CF3COO)2. Atoms are colored green (Sr and Ba), brown (C), red (O), and light
blue (F). For each structure, the following are shown: (a and e) ball-and-stick representation, (b and f) polyhedral representation showing one-
dimensional channels parallel to the c axis, along with the diameter of the circular projection of the centroids of the fluorine atoms on the (001)
plane (C−C, C−F, and C−O bonds have been omitted for the sake of clarity), (c and g) coordination environment of the alkaline-earth metal,
including the two distinct trifluoroacetate ligands, and (d and h) spatial arrangement of the two distinct trifluoromethyl groups in the one-
dimensional channels. The F(4) atom has been included in the coordination sphere of barium (g), but not in that of strontium (c); see the text for
details.
step size and scan speed were 0.001° and 0.01° s−1, respectively.
tigations of the thermal decomposition of metal trifluoroace-
tates.12,13,16,17 This is further confirmed by inspecting the DTA
Diffraction patterns were recorded at 25 °C.
curves, in which two peaks are observed for Sr(CF3COO)2
RESULTS AND DISCUSSION
(305 and 379 °C) and Ba(CF3COO)2 (366 and 377 °C). The
presence of exothermic peaks [379 °C for Sr(CF3COO)2 and
366 and 377 °C for Ba(CF3COO)2] confirms that crystal-
lization of the corresponding fluoride occurs during this weight
loss. It is worth noting the presence of an endothermic peak at
305 °C in the DTA curve of Sr(CF3COO)2. The process giving
rise to this peak precedes the crystallization of the SrF2 phase
and is not observed in the DTA curve of the barium
counterpart, suggesting the decomposition of Sr(CF3COO)2
and Ba(CF3COO)2 occurs through different pathways.
Although an investigation of these pathways is beyond the
scope of this work, it is important to note that the temperature
at which the endothermic process occurs prior to the
crystallization of SrF2 is too high to be attributed to the
desorption and vaporization of solvent molecules (i.e., H2O and
CF3COOH) coordinated to the metal atoms or bonded to the
■
Thermograms of Sr(CF3COO)2 and Ba(CF3COO)2 are shown
in panels a and b of Figure 1, respectively. Sr(CF3COO)2 and
Ba(CF3COO)2 exhibit total weight losses of 62.2 and 51.3 wt
%, respectively, upon being heated to 600 °C under dry
nitrogen. Both experimental results are in excellent agreement
with the theoretical weight losses expected upon decomposition
of the trifluoroacetates to the corresponding SrF2 (60.0 wt %)
and BaF2 (51.8 wt %) phases. Powder XRD patterns of the
decomposition products confirmed both trifluoroacetates
decompose to the corresponding fluorite phase (Figure S1).
A single weight loss in the 295−385 and 300−390 °C ranges is
observed for Sr(CF3COO)2 and Ba(CF3COO)2, respectively.
Inspection of the first derivative of the thermograms (Figure
S2), however, indicates that multiple processes occur within
this temperature range, in agreement with previous inves-
C
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Table 1. Crystal and Structural Determination Data of Sr(CF3COO)2 and Ba(CF3COO)2
Sr(CF3COO)2
Ba(CF3COO)2
formula weight (g)
temperature (°C)
wavelength (Å)
crystal system
313.66
364.71
−173(2)
0.71073
trigonal
trigonal
space group
R3
̅
R3
̅
lattice parameters
a = b = 21.9891(7) Å
c = 9.4696(9) Å
α = β = 90°
γ = 120°
a = b = 21.2541(18) Å
c = 10.4984(10) Å
α = β = 90°
γ = 120°
4107.1(8)
18
volume (Å3)
3965.3(5)
Z
18
calculated density (g cm−3
)
2.364
2.645
μ (mm−1
)
6.212
4.442
F(000)
2664
2988
θ range (deg)
1.85−45.35 (0.50 Å resolution)
−43 ≤ h ≤ 42
−44 ≤ k ≤ 41
−18 ≤ l ≤ 18
79321
1.92−26.36 (0.80 Å resolution)
−26 ≤ h ≤ 26
−25 ≤ k ≤ 21
−13 ≤ l ≤ 13
21122
index ranges
no. of reflections collected
no. of independent reflections
7364 (Rint = 5.05%)
1871 (Rint = 4.44%)
100.0
independent reflection coverage (%) 99.5
refinement method
full-matrix least squares on F2
∑w(Fo − Fc2)2
2
function minimized
weighting scheme
w = 1/[σ2(Fo ) + (0.0270P)2 + 0.3321P], where
w = 1/[σ2(Fo ) + (0.0157P)2 + 35.938P], where
2
2
2
2
P = (Fo + 2Fc2)/3
P = (Fo + 2Fc2)/3
data, restraints, parameters
Δ/σmax
7364, 0, 136
0.003
1871, 0, 136
0.001
largest difference peak and hole
0.833 and −0.702
1.484 and −0.680
(e Å−3
)
root-mean-square deviation (e Å−3
)
0.132
0.143
R
2.86% [5648 data with I > 2σ(I)]
5.31% (all data)
2.31% [1545 data with I > 2σ(I)]
3.85% (all data)
Rw
5.13% [5648 data with I > 2σ(I)]
5.54% (all data)
4.45% [1545 data with I > 2σ(I)]
5.10% (all data)
goodness of fit on F2
1.020
1.097
trifluoromethyl groups via hydrogen bonding. Indeed, extensive
investigation of the thermal decomposition of alkaline-earth
and rare-earth metal trifluoroacetates has shown that
desorption of crystallization water molecules takes place
below 160 °C.5,12,13,15 Further, no simultaneous weight losses
and sharp endothermic peaks that could indicate the presence
of crystallization solvent molecules are observed in the low-
temperature range (100−160 °C) of the TGA and DTA curves
of Sr(CF3COO)2 and Ba(CF3COO)2. This is in agreement
with results of single-crystal XRD studies, which demonstrate
the absence of crystallization solvent molecules in the structures
of both Sr(CF3COO)2 and Ba(CF3COO)2 (vide infra). We
attribute the anhydrous character of these compounds to the
crystallization approach employed in this work, that is, to the
use of a dry nitrogen flow to induce evaporation of the solvent.
Evaporating the solvent under atmospheric conditions, instead,
yields the hydrated products.13,14 The relevance of this finding
lies on the demonstration that Sr(CF3COO)2 and Ba-
(CF3COO)2 can be crystallized in anhydrous form provided
that an adequate crystallization method is employed.
given in Tables S1−S4 and Figure S3. Sr(CF3COO)2 and
Ba(CF3COO)2 are isostructural and crystallize in the
rhombohedral R3 space group, with the unit cell containing
̅
18 formula units. The unit cell of Sr(CF3COO)2 is slightly
larger along the a and b axes than that of its barium counterpart
(∼21.99 vs 21.25 Å), whereas that of Ba(CF3COO)2 is larger
along the c axis (∼10.45 vs 9.47 Å). Overall, the unit cell
volume expands by ∼3.5% upon substitution of barium for
strontium. The most remarkable feature of these hybrid
structures is their open-framework nature due to presence of
infinite channels that run parallel to the c axis (Figure 2a,e).
Using a polyhedral representation, these channels can be
visualized as the assembly of a corona of metal−oxygen
polyhedra and the trifluoromethyl groups of the trifluoroacetate
ligands, which are directed inward (Figure 2d,f). It is likely that
the electrostatic repulsion between the negatively charged
trifluoromethyl groups plays an important role in the formation
of these channels. It should be mentioned that coordination
polyhedra are used here just to facilitate visualization of the
crystal structure; the highly distorted coordination environ-
ments of the metal atoms prevents them from being described
in terms of distortions of regular polyhedra (vide infra). The
average diameter of the channels can be estimated using the
diameter of a cylinder with a volume equal to that of the void
space in the channel; void spaces of 105(12) and 98(9) Å3 were
The crystal structures of Sr(CF3COO)2 and Ba(CF3COO)2
determined by single-crystal XRD are shown in Figure 2.
Crystal and structural determination data are listed in Table 1.
Fractional atomic coordinates, anisotropic atomic thermal
displacement parameters, and thermal ellipsoid plots are
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computed for Sr(CF3COO)2 and Ba(CF3COO)2, respectively,
using the PLATON software.21 This translates into average
diameters of ∼3.75 Å for Sr(CF3COO)2 and ∼3.45 Å for
Ba(CF3COO)2; both values are comparable to the kinetic
diameter of small molecules such as carbon dioxide (3.4 Å) and
methane (3.8 Å).22 The projection of the centroids of the
fluorine atoms of the trifluoromethyl groups on the (001) plane
yields circles of diameter ∼5.54 and 5.19 Å for Sr(CF3COO)2
and Ba(CF3COO)2, respectively (Figure 2b,f). If we account
for the van der Waals radius of the fluorine atom (1.47 Å),
these values translate into minimum diameters of ∼2.60 and
2.25 Å for Sr(CF3COO)2 and Ba(CF3COO)2, respectively.
Both values are comparable to the kinetic diameter of the
hydrogen molecule (2.3 Å).22 Both structures belong to the
organic−inorganic hybrid framework class of materials
reviewed by Rao et al.23 in 2004 and Cheetham et al.24 in
2006. Specifically, they are reminiscent of the nickel succinate
reported by Forster and Cheetham in 200225 and of the
cadmium malonate reported by Vaidhyanathan et al. in 2003.26
Despite featuring dicarboxylates as ligands, those hybrid
frameworks feature three remarkable similarities with the
Sr(CF3COO)2 and Ba(CF3COO)2 structures reported herein:
(1) a honeycomb arrangement of the metal−oxygen polyhedra
and one-dimensional channels when viewed down the c axis,
(2) three-dimensional connectivity of the metal−oxygen
polyhedra, and (3) the substituents of the organic ligand (i.e.,
methylene groups) lining the channels. Some of these features
are also observed in the structure of the manganese glutarate
framework reported by Vaidhyanathan et al.26 and in that of the
nickel glutarate framework reported by Guillou et al.27 In terms
of the size of the channels, Forster and Cheetham reported an
8.0 (4.0) Å atom-to-atom distance for the widest (narrowest)
cavities present in nickel succinate.25 This size range is
comparable to that observed for the average and minimum
diameters of the channels present in Sr(CF3COO)2 and
Ba(CF3COO)2. We decided to investigate whether further
chemical control of the size (and eventually shape) of the
channels could be achieved by substituting calcium for
strontium, but no open-framework structure was observed for
Ca(CF3COO)2 crystallized under similar conditions; a layered
structure similar to that reported by Khristov et al. was
observed instead (see the Supporting Information).15
Table 2. Selected Metal−Oxygen and Metal−Fluoride
Distances
distance (Å)
distance (Å)
Sr−O(1)
Sr−O(1)′
Sr−O(2)
Sr−O(2)′
Sr−O(3)
Sr−O(3)′
Sr−O(4)
2.4979(6)
2.7095(7)
2.5110(7)
2.7119(7)
2.5785(7)
2.5866(7)
2.4623(7)
Ba−O(1)
Ba−O(1)′
Ba−O(2)
Ba−O(2)′
Ba−O(3)
Ba−O(3)′
Ba−O(4)
Ba−O(4)′
Ba−F(4)
2.654(3)
2.813(3)
2.707(3)
2.968(3)
2.686(3)
2.863(3)
2.676(3)
3.050(3)
3.056(2)
Sr−F(4)
2.8619(6)
alkaline-earth metal atoms. The coordination environment of
strontium features strontium−oxygen distances in the 2.46−
2.71 Å range, and the metal atom is depicted as
heptacoordinated. A Sr···F(4) short distance of 2.86 Å is also
observed; as a result, non-negligible bonding interactions
between the two atoms may be present.28 Barium−oxygen
distances in the 2.65−3.05 Å range and a barium−fluoride
distance of 3.06 Å are observed in the coordination
environment of barium; in this case, the metal atom is depicted
as nonacoordinated. Short Ba···F contacts within the 2.85−3.30
Å range have been reported previously in organometallic
compounds containing large alkaline-earth metal atoms and
fluorinated ligands.29,30 Altogether, these results show that
chemical substitution of the barium for strontium induces
changes in the coordination environment of the metal atom,
and that these changes are accommodated through the
displacement of one of the two chemically distinct trifluor-
oacetate ligands relative to the metal center.
The phase purity of the polycrystalline Sr(CF3COO)2 and
Ba(CF3COO)2 powders was probed using synchrotron powder
XRD. The corresponding experimental diffraction patterns are
shown in Figure 3. Sr(CF3COO)2 powder exhibits just a trace
amount of secondary phases. Indeed, with the exception of two
low-intensity diffraction maxima appearing at 2θ ∼ 2.51 and
6.28°, all the peaks in the pattern of Sr(CF3COO)2 can be
indexed to the alkaline-earth trifluoroacetate. In the case of
The coordination environment of the alkaline-earth metal is
heavily distorted in both Sr(CF3COO)2 and Ba(CF3COO)2
(Figure 2c,g). In the case of Sr(CF3COO)2, the trifluoroacetate
ligand is coordinated to the metal atoms via the oxygen atoms
of the carboxylate group. Two coordination modes can be
distinguished. In the first mode, the carboxylate acts a bidentate
bridging ligand: three strontium atoms coordinate to one
carboxylate group, with oxygen atoms O(1) and O(2) shared
by two metal atoms. In the second mode, only the O(3) atom
of the carboxylate group bridges two strontium atoms, whereas
O(4) coordinates to a single metal atom. In the case of
Ba(CF3COO)2, two chemically distinct trifluoroacetate ligands
are present, as well, but both of their carboxylate groups
coordinate to the metal atom in a bidentate bridging mode. The
presence of two chemically distinct trifluoroacetate ligands in
both Sr(CF3COO)2 and Ba(CF3COO)2 affects not only the
coordination environment of the metal atom but also the spatial
arrangement of the trifluoromethyl groups exposed in the one-
dimensional channels (Figure 2g,h). Selected metal−oxygen
and metal−fluoride distances are listed in Table 2 to further
illustrate the difference in the coordination environments of the
Figure 3. Experimental powder XRD patterns of Sr(CF3COO)2 and
Ba(CF3COO)2. Tick marks depict the position of the diffraction
maxima calculated using the structural models derived from single-
crystal studies. Diffraction maxima resulting from secondary phases are
shown in the insets using asterisks. Rietveld analysis was possible only
for Sr(CF3COO)2 (see the text and Figure S4 for details).
E
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Ba(CF3COO)2, the presence of secondary phases is more
noticeable; the alkaline-earth trifluoroacetate remains, however,
the major phase. Attempts to quantitatively analyze the powder
patterns of Sr(CF3COO)2 and Ba(CF3COO)2 using Rietveld
refinements were only partially successful because of the
damage suffered by both samples upon exposure to the X-ray
beam (i.e., peak broadening and shift). In the case of
Sr(CF3COO)2, an adequate fit to the experimental pattern
could be obtained using the structural model derived from
single-crystal studies (Figure S4). In contrast, adequate
modeling of the shape of the diffraction maxima of
Ba(CF3COO)2 was not possible. From a qualitative standpoint,
however, results from powder XRD studies demonstrate that
polycrystalline Sr(CF3COO)2 and Ba(CF3COO)2 with open-
framework structures can be obtained with adequate phase
purity using the crystallization approach presented in this work.
scope of application to areas such as the adsorption of small
gases.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02081.
Powder XRD patterns of the thermal decomposition
products of Sr(CF3COO)2 and Ba(CF3COO)2, differ-
ential thermograms (first-derivative) of Sr(CF3COO)2
and Ba(CF3COO)2, additional crystallographic informa-
tion about Sr(CF3COO)2 and Ba(CF3COO)2, Rietveld
analysis of Sr(CF3COO)2, and thermal analysis and
single-crystal XRD study of Ca(CF3COO)2 (PDF)
Crystallographic information file (CIF)
Crystallographic information file (CIF)
Crystallographic information file (CIF)
CONCLUSIONS
■
In summary, crystallization of the anhydrous Sr(CF3COO)2
and Ba(CF3COO)2 open-framework structures was accom-
plished by inducing evaporation of the H2O/CF3COOH
solvent mixture with a dry nitrogen flow. This approach to
crystallization of metal trifluoroacetates departs from that first
proposed by Roberts for the crystallization of rare-earth
trifluoroacetates18 and was routinely employed to crystallize a
variety of metal trifluoroacetate salts. Powder XRD demon-
strated both trifluoroacetate powders could be obtained with
adequate phase purity; only trace and minor amounts of
secondary phases were observed in Sr(CF3COO)2 and
Ba(CF3COO)2, respectively. The anhydrous nature of the
trifluoroacetates was confirmed by thermal analysis and single-
crystal XRD, which did not detect the presence of
crystallization solvent molecules. Sr(CF3COO)2 and Ba-
(CF3COO)2 are isostructural and belong to the organic−
inorganic extended hybrids class of materials. They exhibit an
open-framework structural motif with three-dimensional
connectivity of the metal−oxygen polyhedra and one-dimen-
sional channels along the c axis. The channels are decorated
with the trifluoromethyl groups of the trifluoroacetate ligands,
and their average (minimum) diameters were estimated to be
∼3.75 (2.60) and 3.45 (2.25) Å for Sr(CF3COO)2 and
Ba(CF3COO)2, respectively. This size range is comparable to
the kinetic diameter of molecules such as hydrogen (2.3 Å).
From this perspective, we believe that an investigation of the
ability of these structures to accommodate guest molecules
would be worthwhile. Chemical substitution of barium for
strontium affected not only the diameter of the channels but
also the spatial arrangement of the trifluoromethyl groups
within the channels and the coordination environment of the
metal atom. The different coordination requirements of the
strontium and barium atoms were found to be accommodated
through the displacement of one of the two chemically distinct
trifluoroacetate ligands relative to the metal center.
AUTHOR INFORMATION
Corresponding Author
*E-mail: far@chem.wayne.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the financial support of the
Department of Chemistry at Wayne State University. They
also thank the Lumigen Instrument Center at Wayne State
University for the use of the Bruker X8 Apex single-crystal X-
ray diffractometer and Drs. Stephanie L. Brock and Charles H.
Winter for the use of the TGA and DTA analyzers. Use of the
Advanced Photon Source at Argonne National Laboratory was
supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under Contract DE-AC02-
06CH11357.
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