Mechanochemical synthesis and characterization of hydrated and dehydrated crystalline strontium terephtalates

Article abstract of DOI:10.1002/zaac.201200550

A successful mechanochemical synthesis of strontium terephthalate trihydrate is described for the first time. The dehydration of Sr(C ₈H₄O₄)-3H₂O occurs at about 100 °C and results in a well-defined strontium terephtalate, Sr(C ₈H₄O₄), thermally stable up to 550 °C. Both compounds are not described so far in the literature. Their structures were solved by ab initio structure determination and subsequent Rietveld refinement of the powder diffraction data. Further methods like DTA-TG, MAS NMR and FT-IR spectroscopy, and BET measurements were used to characterize these compounds. Copyright

Full text of DOI:10.1002/zaac.201200550

ARTICLE
DOI: 10.1002/zaac.201200550
Mechanochemical Synthesis and Characterization of Hydrated and
Dehydrated Crystalline Strontium Terephtalates
Gudrun Scholz,*[a] Franziska Emmerling,[b] Maik Dreger,[a] and Erhard Kemnitz*[a]
Keywords: Mechanochemical synthesis; Ab initio structure determination; MAS NMR spectroscopy; Strontium
Abstract. A successful mechanochemical synthesis of strontium Their structures were solved by ab initio structure determination and
terephthalate trihydrate is described for the first time. The dehydration
subsequent Rietveld refinement of the powder diffraction data. Further
of Sr(C8H4O4)·3H2O occurs at about 100 °C and results in a well- methods like DTA-TG, MAS NMR and FT-IR spectroscopy, and BET
defined strontium terephtalate, Sr(C8H4O4), thermally stable up to
550 °C. Both compounds are not described so far in the literature.
measurements were used to characterize these compounds.
Structures of coordination polymers of calcium and barium
as alkaline earth metals with terephthalic acid (benzene-1,4-
dicarboxylic acid) were described in the literature. The crystal
structure of a calcium terephthalate trihydrate, Ca(C8H4O4)·
3H2O, was determined in 1972.[8] In that case, crystals were
simply grown from an aqueous solution of calcium acetate,
potassium hydroxide, and terephthalic acid. Barium
terephthalate, Ba(C8H4O4), was crystallized from aqueous
Introduction
While transition metals like copper, silver, or manganese are
frequently used as central metals in metal-organic frameworks,
alkaline earth metals became only recently attractive re-
sources.[1] For a long time this was a largely undeveloped area
of coordination chemistry. The use of the alkaline earth metals
in this context promises interesting new topologies of the re-
sulting hybrid materials, different from their transition metal
counterparts. Additional importance of such frameworks is
gained by the organic linker properties that bind to them.
Meanwhile, predominantly hydro – and solvothermal condi-
tions are used to synthesize alkaline earth organic-inorganic
hybrid frameworks.[1] In such reactions, the alkaline earth met-
als are introduced as hydrated compounds of chlorides,[2,3] ni-
trates,[4] and acetates,[4,5] next to halogenates[6] and alkoxids.[6]
On the other hand, multivalent acids were used as ligands such
as, for example, pyridine-2,6-dicarboxylic acid N-oxide
(H2PDCO),[2] pyridine-2,4,6-tricarboxylic acid (ptCH3),[7] or
2,5-dihydroxyterephthalic acid (H4dhtp).[4] This points to the
crucial role of carboxylic acid groups as linker to alkaline earth
metal ions. Appropriate hydro- or solvothermal reactions are
usually performed in autoclaves for several days (mostly from
one to seven days) at temperatures above 100 °C.
solution as well and its structure was described in reference[9]
.
Very recently, in 2011, crystals of a strontium terephtalate
monohydrate, Sr(C8H4O4)·H2O, were prepared after reaction
in an autoclave for 7 d and the structure was published in
the literature.[10] Also, single crystals of Sr(H2dhtp)(H2O) were
prepared by hydrothermal reactions.[4] So far, a strontium
terephthalate trihydrate or a strontium terephthalate compound
without was not described in the literature.
Microwave and solid state syntheses are in general very rare
in this field. To date, only very few papers demonstrated that
an alkaline earth metal organic framework can be synthesized
through mechanochemical reactions. The latter was shown for
the reaction of MgO with naproxen under the special influence
of liquid-assisted grinding (LAG).[11] Recent studies showed
that mechanochemical conversions of metal oxides, carbon-
ates, or acetates with carboxylic acid ligands into coordination
polymers are successful especially exploring the possibilities
of LAG.[12–15]
Based on these findings and the fact that alkaline earth metal
fluorides MF2 (M = Ca, Sr, Ba) are very easy accessible simply
by shaking corresponding hydroxides with NH4F, and even
more by milling,[16] Sr(OH)2·8H2O was chosen as suitable
starting material for mechanochemical reactions.
The presented study is showing that Sr(C8H4O4)·3 H2O can
be prepared by an easy mechanochemical reaction of
Sr(OH)2·8H2O with terephthalic acid. Subsequent thermal
treatment gives access to stable Sr(C8H4O4). Structure and
properties of these new compounds are determined applying
* PD Dr. G. Scholz
Fax: +49-30-2093-7277
E-Mail: Gudrun.Scholz@rz.hu-berlin.de
* Prof. Dr. E. Kemnitz
E-Mail: Erhard.Kemnitz@chemie.hu-berlin.de
[a] Department of Chemistry
Humboldt-Universität zu Berlin
Brook-Taylor-Str. 2
12489 Berlin, Germany
[b] BAM Federal Institute for Materials Research and Testing
Richard-Willstaetter- Str. 11
12489 Berlin, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201200550 or from the au-
thor.
Z. Anorg. Allg. Chem. 2013, 639, (5), 689–693
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
689

G. Scholz, F. Emmerling, M. Dreger, E. Kemnitz
ARTICLE
1
1
different methods like XRD, H and H-13C CP MAS NMR,
FT-IR, thermal analysis, and BET surface measurements.
Results and Discussion
An easy to perform mechanochemical reaction of the two
components Sr(OH)2·8H2O and terephthalic acid resulted in a
white powder, which did not need further processing. Both
chosen molar ratios of Sr(OH)2·8H2O:1,4-(COOH)2-C6H4 as
(i) 1:1or (ii) 1:2 resulted in powders with almost the same
XRD pattern. In the latter case reflections, indicating residues
of terephtalic acid, superimpose the XRD powder pattern of
the new compound.[17] For the first case (i.e. molar ratio 1:1)
the powder pattern is given in Figure 1a together with the sim-
ulation obtained after Rietveld refinement. Amplitudes and
line widths show the good crystallinity of the sample. Reflec-
tions of the starting materials Sr(OH)2·8H2O (PDF-card: 27–
1438) and terephthalic acid (PDF-card: 31–1916[18]) disap-
peared during the mechanochemical reaction. Results of the
elemental analysis support the formation of a strontium
terephthalate trihydrate (Table 1), that can be described by the
following reaction equation:
Sr(OH)2·8H2O + C6H4(COOH)2 Ǟ Sr(C6H4(COO)2)·3H2O + 7H2O
(1)
The structure of Sr(C8H4O4)·3H2O consists of alternating
layers of terephthalic anions (cf. Figure 1b). Strontium cations
have the coordination number eight, which is in agreement
with the coordination numbers of calcium, strontium, and bar-
ium in related terephthalate compounds.[8–10]
The eightfold coordination of strontium comprises four
terephthalate oxygen atoms: Two different anions act as mono-
dentate ligands, whereas a further terephthatalic anion acts as
bidentate ligand. The coordination environment is completed
by four oxygen atoms stemming from water molecules. Two
of the latter four oxygen atoms coordinate to two strontium
atoms. For the monoclinic crystal system the space group
P21/a was determined (cf. Table 2).
The structure of Sr(C8H4O4)·3H2O is isotypic to the crystal
structure of Ca(C8H4O4)·3H2O determined by Matsuzaki et
al.[8] In both compounds a network of hydrogen bonds between
water molecules and carboxyl groups stabilizes the structure.
Similar to strontium terephthalate monohydrate[10] inorganic
chains of SrO8 polyhedra are formed sharing three almost co-
planar oxygen atoms (Figure 1b).
Figure 1. Rietveld refinement of Sr(C8H4O4)·3H2O (a) Observed in-
tensities are indicated by dots, the best-fit profile is indicated by a
solid line, the difference is shown by a grey line. The vertical bars
correspond to the positions of the Bragg peaks. Crystal structure of
Sr(C8H4O4)·3H2O and coordination around the strontium atoms (b).
The thermoanalytical curves show the expected mass loss
around 100 °C due to the water release (Figure 2). Surpris-
ingly, after releasing the main water content the mass of the
compound is stable up to about 550 °C followed by a complete
decomposition at higher temperatures (Figure 2). Therefore,
the mechanochemically synthesized Sr(C8H4O4)·3H2O was
separately annealed in a furnace under comparable conditions
and quenched at 300 or 400 °C, respectively. Subsequent
analysis with X-ray diffraction resulted in a completely new
powder pattern as depicted in Figure 3a. Reflections of
Sr(C8H4O4)·3H2O completely disappeared. The structure of
the new compound was solved within the same monoclinic
Table 1. Results of elemental analysis and BET surface areas (SBET
of Sr(C8H4O4)·3H2O and Sr(C8H4O4).
)
Sample
C /%
H /%
SBET /m2·g–1
Sr(C8H4O4)·3H2O
Calcd.
Found
9,5
31.4
28.4
3.3
3.0
Sr(C8H4O4)
Calcd.
Found
6.1
38.2
37.0
1.6
1.5
690
www.zaac.wiley-vch.de
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 689–693

Hydrated and Dehydrated Crystalline Strontium Terephtalates
Table 2. Crystal data of Sr(C8H4O4)·3H2O and Sr(C8H4O4).
Sr(C8H4O4)·3H2O
Sr(C8H4O4)
Crystal system
Space group
Cell parameter
monoclinic
monoclinic
P21/a (no. 14)
a = 6.8116 Å
b = 21.8194 Å
c = 7.2598 Å
β = 92.40°
P21/c (no. 14)
a = 7.2761 Å
b = 5.4289 Å
c = 20.3548 Å
β = 108.71°
761.57 Å3
Cell volume
Density
Calculated
Measured
Molar mass
1078.03 Å3
1.884 g·cm–3
1.289 g·cm–3
305.78 g·mol–1
2.196 g·cm–3
2.025 g·cm–3
251.74 g·mol–1
crystal system and respective crystal data are summarized in
Table 2. Along with results of elemental analysis (Table 1), the
new sum formula was calculated as Sr(C8H4O4), i.e., a water
free strontium terephthalate was obtained.
Figure 2. Thermoanalytical curves of Sr(C8H4O4)·3H2O.
This finding is supported by the 1H MAS NMR spectra,
taken both before and after thermal annealing (cf. Figure 4a,
c). The shoulder at 4.6 ppm (Figure 4a), which can be unam-
biguously assigned to the crystal water in Sr(C8H4O4)·3H2O,
completely disappeared after annealing (Figure 4c) and the sig-
nals at about 7 ppm indicate strongly bridged hydrogen atoms.
The water release is confirmed by FT-IR measurements as
well. Broad vibration bands above 3000 cm–1 disappear and
Figure 3. Rietveld refinement and crystal structure of Sr(C8H4O4) (a):
Observed intensities are indicated by dots, the best-fit profile is indi-
cated by a solid line, the difference is shown by a grey line. The verti-
cal bars correspond to the positions of the Bragg peaks; (b) given with
different views and enlargements.
bands of valence vibrations of carboxyl groups are better re- Whereas the strontium atom is coordinated by six oxygen
solved in the water free sample (Figure S1, Supporting Infor- atoms, the larger barium atom is coordinated by eight oxygen
mation).
In the new, more compact compound Sr(C8H4O4), the coor-
atoms.
1
The H-13C CP MAS NMR spectrum of Sr(C8H4O4)·3H2O
dination number of strontium is reduced to six ({SrO6 units}). (Figure 4b) again indicates, from the small line widths, the
Every coordinated oxygen atom stems from a monodentately good crystallinity of the sample. From eight different carbon
bridged terephthalic anion. The strontium atoms are coordi- positions addressed by the structure solution, six different car-
nated octahedrally by six oxygen atoms. Four of the six oxy- bon positions are resolved by solid state NMR: four carbon
gen atoms coordinate to two strontium atoms, whereas the two positions belonging to benzene rings (below 140 ppm) and two
remaining oxygen atoms coordinate to one strontium atom. As carbon positions belonging to carboxyl groups (close to
1
a consequence each SrO6 unit shares two edges with adjacent 180 ppm). As expected, the H-13C CP spectrum of the dehy-
SrO6 octahedra resulting in a chain structure parallel to the drated compound Sr(C8H4O4) depicts 13C signals in compar-
crystallographic a axis.
able regions (Figure 4d), which are, however, not identical to
The structures of Sr(C8H4O4) and Ba(C8H4O4)[9] are inde- those of hydrated sample (Figure 4b). The exclusive mono-
pendent from each other. The differences mainly stems from dentate bridging of terephthalic anions basically contribute to
the different coordination environment of the metal atoms. a simplification of the 13C spectrum of Sr(C8H4O4).
Z. Anorg. Allg. Chem. 2013, 689–693
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
691

G. Scholz, F. Emmerling, M. Dreger, E. Kemnitz
ARTICLE
Figure 5. Adsorption (crosses) and desorption (empty circles) iso-
therms for nitrogen at 77 K of (a) Sr(C8H4O4)·3H2O, and (b)
Sr(C8H4O4).
Figure 4. 1H MAS (νrot = 25 kHz) and 1H-13C CP MAS (νrot
=
10 kHz) NMR spectra of Sr(C8H4O4)·3H2O (a,b) and Sr(C8H4O4)
(c,d). H-13C CP spectra are shown for a contact time of 3.5 ms.
1
Bearing in mind that mechanical milling is usually con-
nected with a decrease of particle sizes, it is surprising that the
milling product here is of good crystallinity. The easy access
by milling and subsequent thermal treatment is encouraging
for possible syntheses of further coordination polymers with
alkaline earth metals.
Both new strontium terephthalate compounds have very
small BET surface areas (cf. Table 1), which can already as-
sumed from narrow reflections (cf. Figure 1a, Figure 3a) and
thereby good crystallinity. Similar adsorption/desorption iso-
therms (Figure 5) and comparable pore size distributions (Fig-
ure S2, Supporting Information) complete the characterization
for these new compounds.
Experimental Section
Preparation: Powders of Sr(OH)2·8H2O (Aldrich, chemical purity
95%) and terephthalic acid [1,4-(COOH)2-C6H4] (Aldrich, chemical
purity 98%) were used as commercially delivered.
Conclusions
Phase pure Sr(C8H4O4)·3H2O was prepared by milling pow-
ders of strontium hydroxide octahydrate Sr(OH)2·8H2O with
terephthalic acid in a stoichiometric ratio of 1:1 for 1 h in a
planetary mill. Dehydration at 100 °C results in a new, water-
free strontium terephthalate, Sr(C8H4O4). Structures of both
compounds were solved ab initio from powder diffraction data
and refined by Rietveld refinement.
Milling experiments were performed with a commercial planetary mill
“Pulverisette 7 premium line” (Fritsch, Germany) under access of air
with two molar ratios of Sr(OH)2·8H2O:1,4-(COOH)2-C6H4 as (i) 1:1
and (ii) 1:2. The total powder mass in each silicon nitride grinding
bowl, assembled with five silicon nitride balls, was always 1 g. Thus,
a ball to powder mass ratio of 14.5 was adjusted. Samples were milled
with a rotational speed of 600 rpm for 1 h.
692
www.zaac.wiley-vch.de
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 689–693

Hydrated and Dehydrated Crystalline Strontium Terephtalates
Thermal Annealing: A high temperature furnace (Carbolite chamber BET: Gas adsorption experiments were carried out on a ASAP 2010
furnace, model RHF 16/3) along with open platinum crucibles was
used for annealing experiments of milling products. Using a heating
rate of 10 K·min–1 each sample was annealed for 2 h isothermally at
300 °C and 400 °C, respectively, and cooled down in the furnace ac-
cording to Newtons rule.
(Micromeritics) using a 1 Torr pressure transducer with a resolution of
5ϫ10–5 Torr (0.007 Pa). The adsorptive used was nitrogen at a tem-
perature of 77 K. Prior to the measurements, the samples were de-
gassed at 120 °C for 4 h in vacuo.
X-ray Diffraction: X-ray diffractograms were measured with a XRD-
3003-TT diffractometer (Seiffert & Co., Freiberg) with Cu-Kα radia-
tion (λ = 1.542 Å; 2 θ range: 5° Յ 2 θ Յ 64°; step scan: 0.05°, step
time: 5 s). Reflections were compared with diffractograms of the
JCPDS-PDF data base.[18]
FT IR: Infrared spectra were recorded with a Equinox 55 IR micro-
scope (Bruker).
Supporting Information (see footnote on the first page of this article):
FT-IR spectrum of Sr(C8H4O4)·3H2O and Sr(C8H4O4) (Figure S1).
BJH pore size distributions determined for Sr(C8H4O4)·3H2O (a) and
Sr(C8H4O4) (b) (Figure S2).
Powder diffraction measurements for structure solution were per-
formed with a D8 Discover diffractometer (Bruker AXS, Karlsruhe,
Germany) operated in transmission geometry (Cu-Kα1 radiation, λ =
1.54056 Å), equipped with a Lynxeye detector. Samples were mea-
sured in a 2θ range of 5–90° with a step size of 0.009° and 5 s per
step. The structure determinations were carried out based on the pow-
der XRD patterns. Indexing and structure solution were performed
using the open source program FOX.[19] The unit cell and space group
was confirmed using CHEKCELL.[20] FOX uses global-optimization
algorithms to solve the structure performing trials in direct space. This
search algorithm uses random sampling coupled with simulated tem-
perature annealing to locate the global minimum of the Figure-of-merit
factor. To reduce the total degrees of freedom parts of the molecules
were added to a rigid group. The terephtalic acid was set rigid with
the exception of the carboxyl oxygen and hydrogen atoms. The crystal
structures of both compounds were solved by the simulated annealing
procedure on a standard personal computer within 6 h (Sr(C8H4O4)·
3H2O) and 4 h (Sr(C8H4O4)), finding the deepest minimum of the cost
function several times during the procedure. To complete the structure
determination, the structural solution obtained from MC/SA was sub-
sequently subject to a Rietveld refinement employing the TOPAS soft-
ware.[21] Figure 1a and Figure 3a show the result of the Rietveld re-
finements illustrating the agreement of the simulated powder pattern
with the measured one. The refinement converged at an Rwp = 8.89%
(Rp = 6.73%) for the structure of Sr(C8H4O4)·3H2O and Rwp = 8.50%
(Rp = 6.59%) for Sr(C8H4O4).
Acknowledgements
We thank Dr. M. Feist (HU Berlin) for DTA measurements, A. Zimath-
ies and C. Prinz (BAM Berlin) for gas adsorption and density measure-
ments, and A. Kohl (BAM Berlin) for IR spectroscopic measurements.
References
[1] K. M. Fromm, Coord. Chem. Rev. 2008, 252, 856.
[2] L. L. Wen, F. M. Wang, X. K. Leng, M. M. Wang, Q. J. Meng,
H. Z. Zhu, J. Inorg. Organomet. Polym. 2010, 20, 313.
[3] S. Chen, Q. Shuai, S. Gao, Z. Anorg. Allg. Chem. 2008, 634,
1591.
[4] P. D. C. Dietzel, R. Blom, H. Fjellvag, Z. Anorg. Allg. Chem.
2009, 635, 1953.
[5] E. H. L. Falcao, R. K. Feller, G. Wu, F. Wudi, A. K. Cheetham,
Inorg. Chem. 2008, 47, 8336.
[6] K. M. Fromm, Dalton Trans. 2006, 5103.
[7] M. C. Das, S. K. Ghosh, E. C. Sanudo, P. K. Bharadwaj, Dalton
Trans. 2009, 1644.
[8] T. Matsuzaki, Y. Iitaka, Acta Crystallogr., Sect. B 1972, 28, 1977.
[9] S. M. F. Lo, S. S. Y. Chui, I. D. Williams, Acta Crystallogr., Sect.
C 1998, 54, 1846.
[10] L. Yang, D. Zhao, G. Li, Acta Crystallogr. Sect. E 2011, 67, m282.
[11] T. Friscic Halasz, F. C. Strobridge, R. E. Dinnebier, R. S. Stein,
L. Fabian, C. Curfs, CrystEngComm 2011, 13, 3125.
[12] T. Friscic, L. Fabian, CrystEngComm 2009, 11, 743.
[13] F. C. Strobridge, N. Judas, T. Friscic, CrystEngComm 2010, 12,
2409.
Crystallographic data (excluding structure factors) for the structures 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
numbers CCDC-922248 and CCDC-922249. (Fax: +44-1223-336-033;
E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)
Elemental Analysis: An EURO EA equipment (HEKAtech GmbH)
was used for carbon, nitrogen, and hydrogen content determinations.
[14] K. Fujii, A. L. Garay, J. R. Hill, E. Sbircea, Z. Pan, M. C. Xu,
D. C. Apperley, S. L. James, K. D. M. Harris, Chem. Commun.
2010, 46, 7572.
DTA-TG: The thermal behavior was studied with a NETZSCH
thermoanalyzer STA 409 C Skimmer®, being additionally equipped
with a conventional high-temperature SiC oven. A DTA-TG sample
carrier system with platinum crucibles (baker, 0.8 mL) and Pt/PtRh10
thermocouples were used. Measurements were performed under nitro-
gen atmosphere applying a heating rate of 10 K·min–1.
[15] P. Wang, G. Li, Y. Chen, S. Chen, S. L. James, W. Yuan, Crys-
tEngComm 2012, 14, 1994.
[16] M. Dreger, G. Scholz, E. Kemnitz, Solid State Sci. 2012, 14, 528.
[17] M. Dreger, Diplomarbeit, Humboldt-Universität zu Berlin 2011.
[18] JCPDS-ICDD, International Centre for Diffraction Data: PDF-2
Database (Sets 1–51 plus 70–89). PA 19073–3273 USA, Release
2001. – PCPDFWIN Version 2.2.
[19] V. Favre-Nicolin, R. Cerny, J. Appl. Crystallogr. 2002, 35, 734.
[20] J. Laugier, B. Bochu, Chekcell 2001.
[21] Topas, Version 2.0, General Profile and Structure Analysis Soft-
ware for Powder Diffraction Data (User Manual), Bruker AXS,
Karlsruhe, Germany, 2000.
MAS NMR: 1H and 1H-13C CP MAS NMR spectra were recorded
with a Bruker AVANCE 400 spectrometer (Larmor frequencies: ν1H
=
400.1 MHz, ν13C = 100.6 MHz) using both a 2.5 mm and a 4 mm MAS
1
probe (Bruker Biospin). H MAS studies were made with a π/2 pulse
length of 4 μs and a recycle delay of 5 s. Values of the isotropic
chemical shifts of 1H and 13C are given with respect to TMS. Different
contact times of the 1H-13C CP MAS NMR experiments between
0.2 ms and 8.5 ms were tested.
Received: December 17, 2012
Published Online: March 5, 2013
Z. Anorg. Allg. Chem. 2013, 689–693
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
693