Group 13 metal carboxylates: Using molecular clusters as hybrid building units in a MIL-53 type framework

Article abstract of DOI:10.1021/cg501189n

Systematic investigation of the reactions of the system AlCl₃·6H₂O/pyridine-2,4,6-tricarboxylic acid (H₃PTC)/pyridine in water yielded two new compounds, both containing the dimeric {AlPTC(μ-OH)(H₂O)}₂^2-unit. With long reaction times, the framework compound [Al(μ-OH){AlPTC(μ-OH)(H₂O)}₂]·2H₂O (CAU-16, compound 1) is obtained, the first example of a framework compound with a metal-organic cluster linker, and bearing the MIL-53 network. Although the compound does not breathe, as other MIL-53 compounds do, it has a maximum uptake of CO₂of 1.76(2) mmol g^-1at 196 K. With shorter reaction times, the molecular compound {Al(HPTC)(μ-OH)(H₂O)}₂(2) was prepared, leading to the proposal of a crystallization scheme for the Al³⁺-pyridine-2,4,6,-tricarboxylic acid system. To determine whether further framework compounds bearing hybrid metal cluster linkers could be prepared, systematic high-throughput investigations of pyridine-2,4,6-tricarboxylic acid in water with Ga³⁺and In³⁺were undertaken. These yielded two chain-type compounds, GaPTC(H₂O)₂(3) and InPTC(H₂O)₂(4), with different coordination chemistries. Optimized syntheses for compounds 1, 2, and 4 are reported.

Full text of DOI:10.1021/cg501189n

Article
pubs.acs.org/crystal
Group 13 Metal Carboxylates: Using Molecular Clusters As Hybrid
Building Units in a MIL-53 Type Framework
,
†,‡
†
†
†
,†
Michael T. Wharmby,* Malte Snoyek, Timo Rhauderwiek, Knut Ritter, and Norbert Stock*
†
Institut fu
̈
r Anorganische Chemie, Christian-Albrechts-Universitat
Diamond Light Source Ltd., Diamond House, Harwell Science & Innovation Campus, Didcot, Oxfordshire OX11 0DE, United
Kingdom
̈
zu Kiel, Max-Eyth-Strasse 2, 24118 Kiel, Germany
‡
*
S Supporting Information
ABSTRACT: Systematic investigation of the reactions of the
system AlCl ·6H O/pyridine-2,4,6-tricarboxylic acid
3
2
(
H PTC)/pyridine in water yielded two new compounds,
3
2−
both containing the dimeric {AlPTC(μ-OH)(H O)}2 unit.
2
With long reaction times, the framework compound [Al(μ-
OH){AlPTC(μ-OH)(H O)} ]·2H O (CAU-16, compound
2
2
2
1
) is obtained, the first example of a framework compound
with a metal−organic cluster linker, and bearing the MIL-53
network. Although the compound does not breathe, as other
−1
MIL-53 compounds do, it has a maximum uptake of CO of 1.76(2) mmol g at 196 K. With shorter reaction times, the
2
molecular compound {Al(HPTC)(μ-OH)(H O)} (2) was prepared, leading to the proposal of a crystallization scheme for the
2
2
3
+
Al -pyridine-2,4,6,-tricarboxylic acid system. To determine whether further framework compounds bearing hybrid metal cluster
linkers could be prepared, systematic high-throughput investigations of pyridine-2,4,6-tricarboxylic acid in water with Ga and
In were undertaken. These yielded two chain-type compounds, GaPTC(H O) (3) and InPTC(H O) (4), with different
3
+
3+
2
2
2
2
coordination chemistries. Optimized syntheses for compounds 1, 2, and 4 are reported.
2
0,21
INTRODUCTION
been extensively investigated with lanthanide cations,
and a
■
number of compounds have also been reported with groups 1
In recent years, coordination polymers have been a focus of
22,23
1
,2
or 2 or transition metal cations.
To date there have been no
structures reported using group 13 metals: Al, Ga, In. These
elements are known to form an extraordinary range of
framework compounds with the trimesate anion (Al;
). The presence of the pyridinyl N atom may
lead to the crystallization of isoreticular structures, with
much research. Their structures are built up from inorganic
and organic building units, which together determine the
topology. Variation of the organic linker in some cases leads to
isoreticular structures. Three approaches to variation of the
linker are commonly used: variation of the organic linker, both
in terms of its size and the functional groups decorating the
1
6,24,25
26−28
29−34
Ga;
In
35−37
3
−5
markedly different adsorption or other properties;
due to the possibility of chelation, entirely new structures all
or
molecule, and use of metal complexes capable of engaging in
6
−8
coordinative bonding to form a framework.
example, to altered adsorption properties and allows
This leads, for
4
together.
9
We report here a systematic study of group 13 metals with
pyridine-2,4,6-tricarboxylic acid. This led to a new variant of the
MIL-53 network, with Al3+ as the framework-forming cation,
wherein the organic linker molecule is replaced by a self-
assembled molecular dimeric aluminum oxide cluster. This new
postsynthetic modification and introduction of catalytic
1
0
capability.
For applications, stability is also an important consideration,
and to this end, the use of tri- or tetravalent metals in syntheses
in combination with tri- or tetratopic linkers has led to the
preparation of highly stable, porous compounds. Examples
phase is labeled CAU-16 (Christian-Albrechts-Universitat
̈
11
3+
porous material number 16).
include NOTT-300, an Al -biphenyltetracarboxylate com-
3
+
12−15
pound; soc-MOF, formed from M (M = Sc, Fe, Ga, In)
and azobenzenetetracarboxylic acid, displaying high-stability
and porosity; and MIL-100, constructed from trimeric M
units (M = Al, Sc, Cr, Mn, Fe)
is one of the most stable, high-porosity framework compounds
EXPERIMENTAL SECTION
■
3+
Reactions were performed under solvothermal conditions in custom-
built high-throughput multiclaves, using either 400 or 2000 μL Teflon
inserts. Initial characterization was performed using a Stoe Stadi P
diffractometer in transmission geometry using Cu Kα1 radiation with
data collected by an image plate detector. Synchrotron powder
1
2,16−19
and trimesic acid, which
3
8
known.
Pyridine-2,4,6-tricarboxylic acid (H PTC) has an arrange-
3
ment of carboxylic acid groups topologically identical to that of
trimesic acid, but incorporates a pyridinyl N atom, forming a
Received: August 8, 2014
Published: August 20, 2014
chelating pocket. The coordination chemistry of H PTC has
3
©
2014 American Chemical Society
5310
dx.doi.org/10.1021/cg501189n | Cryst. Growth Des. 2014, 14, 5310−5317

Crystal Growth & Design
Article
diffraction data were collected at beamline P08 at PETRA III (DESY,
Hamburg, Germany) in Debye−Scherrer geometry at a wavelength of
section S3). From the majority of reactions a highly crystalline
powder later identified as CAU-16 ([Al(μ-OH){AlPTC(μ-
OH)(H O)} ], compound 1) was obtained, although in some
0
.827164 Å with data collected using a Mythen detector (for full
2
2
details of the experimental procedure, see Supporting Information
SI]). Single-crystal X-ray diffraction measurements were performed
on a Stoe IPDS diffractometer equipped with a fine focus sealed tube
Mo K λ = 0.71073 Å). Infrared spectra were recorded on a Bruker
cases this was mixed with a small amount of Bo
From one reaction a single crystal of the phase {Al(HPTC)(μ-
OH)(H O)} , referred to as compound 2 was also obtained. A
̈
hmite impurity.
[
(
2
2
α
focused high-throughput investigation of the phase space
producing the most crystalline CAU-16 was performed to
optimize the synthesis conditions. These conditions were then
used for a single reaction in a 24 reactor multiclave, with a
significantly reduced reaction time (12 h instead of 48 h). This
gave nearly phase pure compound 2.
ALPHA-P A220/D-01 FTIR spectrometer fitted with an ATR unit,
−1
over the spectral range 4000−400 cm . Thermogravimetric analysis
was carried out using a NETSCH STA 429 CD analyzer with a heating
−
1
−1
rate of 4 K min and under flowing air (flow rate 75 mL min ) or a
−
1
TA Instruments TGA Q500 with a heating rate of 5 K min and
under flowing air (60 mL min ). Elemental analysis was performed
−1
using a EuroVector EuroEA elemental analyzer. NMR spectroscopy
Reactions of GaCl , Ga(NO ) ·4H O, and InCl ·4H O with
3
3 3
2
3
2
was performed using a Bruker DRX 500 spectrometer. N sorption
2
H PTC and pyridine in water were also performed. Full details
3
experiments were performed using a BELSORP-mini volumetric
and results of these high-throughput syntheses are presented in
the SI (section S3). Both Ga3+ salts were investigated using a
focused high-throughput array using the optimized conditions
porosimeter equipped with a liquid N bath, while CO sorption
2
2
measurements were made using a Hiden-Isochema IGA gravimetric
porosimeter, fitted with a dry ice/ethanol bath for measurements at
3+
1
96 K or a Huber water bath for measurements at 298 K.
determined for Al . This yielded exclusively mixed-phase
products, and it was not possible to prepare any single phase in
pure form. Nevertheless, across most of the reactions single
crystals of GaPTC(H O) (compound 3) were isolated. With
CHEMICALS
■
2
2
All chemicals were obtained from commercial sources and used
without further purification. Pyridine-2,4,6-tricarboxylic acid
H PTC) was synthesized following the reported method of
InCl ·4H O a high-throughput discovery array using a 24
3
2
reactor multiclave was undertaken, varying InCl ·4H O/
3
2
(
3
H PTC/pyridine ratios in water. Across a wide range of
3
Syper et al., by the oxidation of 2,4,6-trimethylpyridine with
39
phase space, the phase InPTC(H O) (compound 4) was
2
2
potassium permanganate. Purity of the synthesized ligand was
obtained with some variation in crystallinity. The most highly
crystalline compound 4 was obtained from the same phase
region yielding the most crystalline CAU-16. Thus, a focused
high-throughput array with stoichiometric ratios identical to
those used in the Al3+ and Ga systems was performed, which
confirmed the optimized synthesis conditions.
confirmed by NMR spectroscopy in d -DMSO (SI, section S2).
6
HIGH-THROUGHPUT SYNTHESES
■
3+
An initial high-throughput discovery reaction was performed
38
using a 48 reactor multiclave varying the ratios of AlCl3·
6
H O/pyridine-2,4,6-tricarboxylic acid (H PTC)/pyridine in
2
3
Optimized Reaction Conditions for [Al(μ-OH){AlPTC(μ-
water, with a total reaction volume of 250 μL. The reactor was
OH)(H O)} ], Compound 1, CAU-16. Solid H PTC (11 mg,
2
2
3
−1
−
5
heated at a ramp rate of 2.5 °C min to 180 °C and held at this
temperature for 48 h, before cooling at 0.125 °C min− to room
temperature. Figure 1 shows the outcome of these reactions
5
.0 × 10 mol) was placed in a 400 μL Teflon insert, and to
1
this was added in sequence water (63 μL), pyridine (6 μL, 7.5
−5
×
×
10 mol), and aqueous AlCl ·6H O solution (181 μL, 4.14
3 2
(
full details of all high-throughput syntheses are given in the SI,
−1
−3
10 mol dm ). The insert was placed in a 48 reactor, which
was sealed and heated to 180 °C with a heating rate of 1.2 °C
−1
min and kept at this temperature for 48 h. The reaction was
cooled to room temperature over 24 h with a cooling rate of 0.1
−
1
°
C min . Solids were separated by vacuum filtration and dried
in air. The yield for this reaction was not determined due to the
small scale of the reaction. Purity was confirmed by elemental
analysis (Calc for [Al(μ-OH){AlPTC(μ-OH)(H O)} ]: C =
2
2
3
0.98% H = 2.44% N = 4.52%; Found: C = 31.20%, H = 2.33%,
N = 4.60%); IR spectra are included in the SI (section S5).
Optimized Reaction Conditions for {Al(HPTC)(μ-OH)-
H O)} , Compound 2. A reaction mixture with identical
2 2
(
stoichiometry to that used in the preparation of CAU-16
(
above) was heated to 180 °C at a rate of 3 °C min− and held
1
at this temperature for 12 h instead of 48 h. The reactions was
then cooled at a rate of 0.5 °C min− to room temperature.
Solids were separated by vacuum filtration and dried in air.
Large crystals of compound 2 were obtained from this reaction.
Yield for this reaction was not determined due to the small
scale of the reaction Composition of the crystals was confirmed
by elemental analysis (Calc for {Al(HPTC)(μ-OH)(H O)} : C
1
Figure 1. Crystallization diagram showing the compositional space
probed by the discovery array used in the initial investigation of the
AlCl /pyridine-2,4,6-tricarboxylic acid (H PTC)/pyridine (py) sys-
2
2
=
35.44% H = 2.23% N = 5.17%; Found: C = 34.5%, H = 2.4%,
N = 5.0%); IR spectra are included in the SI (section S5).
Optimized Reaction Conditions for InPTC(H O) ,
3
3
tem. Gray circles represent reactions yielding amorphous or poorly
crystalline products; green squares are reactions from which highly
crystalline [Al(μ-OH){AlPTC(μ-OH)(H O)} ]·2H O (CAU-16) was
2
2
−
5
Compound 4. A mixture of H PTC (10 mg, 4.8 × 10
3
2
2
2
−5
obtained. Circled region indicates compositions for which crystalline
products were obtained.
mol), water (135 μL), pyridine (3 μL, 4.2 × 10 mol), and
−1
−3
InCl ·4H O solution in water (112 μL, 6.82 × 10 mol dm ),
3
2
5
311
dx.doi.org/10.1021/cg501189n | Cryst. Growth Des. 2014, 14, 5310−5317

Crystal Growth & Design
Article
a
Table 1. Summary of Crystallographic Results for the Four Compounds Reported
cmpd
1
2
3
4
space group (crystal system)
C2/c (monoclinic)
37.1330(13)
11.5278(4)
6.51553(18)
109.266(3)
−
P2 /c (monoclinic)
P2 /c (monoclinic)
Pbcn (orthorhombic)
9.60049(12)
9.03411(12)
11.17851(15)
90
1
1
a/Å
13.6630(8)
11.7211(8)
6.4221(4)
99.700(5)
0.0562/0.1103
−
10.0568(3)
17.3408(5)
12.1952(3)
111.985(2)
0.0437/0.1093
−
b/Å
c/Å
β/deg
R1/wR2
Rwp/RBragg
−
0.0680/0.0300
0.0678/0.0355
a
Data for compounds 2 and 3 were obtained from single-crystal X-ray diffraction data, while compounds 1 and 4 were analyzed by synchrotron
powder X-ray diffraction. Compound 1: [Al(μ-OH){AlPTC(μ-OH)(H O)} ]·2H O or CAU-16; 2: {Al(HPTC)(μ-OH)(H O)} ; 3:
2
2
2
2
2
GaPTC(H O) ; 4: InPTC(H O)
2
2
2
2.
with a total reaction volume of 250 μL, was placed in a 400 μL
Teflon insert and heated under the same conditions as the
other reaction. Again, yield was not determined due to the
small size of the reaction. Purity was confirmed by elemental
analysis (Calc for InPTC(H O) : C = 26.76% H = 1.68% N =
the reaction time; 12 h reaction times produced large, single
crystals of compound 2, whereas 48 h reactions produced
CAU-16 as a microcrystalline powder. This indicates that
compound 2 may be an intermediate in the synthesis of CAU-
16. To confirm this, syntheses were performed using
2
2
3.90%; Found: C = 26.48%, H = 1.82%, N = 4.06%); IR spectra
compound 2 as a reagent, with additional AlCl solution
3
are included in the SI (section S5).
added to give the correct stoichiometric ratio for CAU-16.
Reaction times of 16 h yielded phase-pure CAU-16 (full details
of the reaction are given in the SI, section S3).
The relationship between compound 2 and CAU-16 can be
clearly understood from their crystal structures (Figures 2 and
STRUCTURAL STUDIES
Crystal Structure Determination of {Al(HPTC)(μ-OH)-
H O)} and GaPTC(H O) . The structures of {Al(HPTC)(μ-
■
(
2
2
2
2
OH)(H O)} (2) and GaPTC(H O) (3) were determined
2
2
2
2
from laboratory single-crystal X-ray diffraction data. Structures
were solved using the direct methods routines of the Sir2011
4
0
package and refined using the SHELXL least-squares routines
41
implemented in the SHELX-2013 package. Results of the final
refinement are summarized in Table 1 while a complete set of
results, including R-factors for both structures, are given in the
SI (Table S8.1).
Crystal Structure Determination of [Al(μ-OH){AlPTC(μ-
OH)(H O)} ] and InPTC(H O) . The structures of [Al(μ-
2
2
2
2
OH){AlPTC(μ-OH)(H O)} ] (CAU-16−compound 1) and
Figure 2. Crystal structure of {Al(HPTC)(μ-OH)(H O)} , com-
pound 2. Aluminum shown in gold; carbon, black; nitrogen, blue;
oxygen red; and protons are shown in pink.
2
2
2
2
InPTC(H O) (compound 4) were determined from synchro-
2
2
tron powder X-ray diffraction data collected at beamline P08 at
42
PETRA III (DESY, Hamburg, Germany). The structure of
CAU-16 was solved using an approach combining the parallel
3
). The asymmetric unit of compound 2 consists of one
43
tempering routines of the FOX package and direct space
HPTC unit, an Al cation, an OH− anion, and a water
2−
3+
44
modeling in Materials Studio. The structure of compound 4
3+
molecule. Al cations are octahedrally coordinated by two O
45
was solved using the direct methods routines of Expo2009
and completed using direct space modeling in Materials
atoms from monodentate carboxylic acid groups and one
2−
pyridinyl N atom, all from the same HPTC unit. The
4
4
Studio. Both structures were refined by the Rietveld Method
coordination sphere is completed by a terminal water molecule
46
using TOPAS-Academic v5. Full details of the experimental
procedure, the structure solution, refinement (including
Rietveld plots) and crystallographic information are given in
the SI (section S7 and Table S8.2), while the results of the final
refinements, including cell parameters and R-factors, are
summarized in Table 1.
and two μ-OH groups. Although the coordination chemistry of
3+
47,48
Al is dominated by oxophilic interactions,
coordination
by N in this structure is enforced by the chelating nature of the
rigid O/N/O pocket of the H PTC linker. Similar coordination
3
schemes have also been reported in the literature with amines
49
having two α-carboxylic acid groups. Monomeric {Al-
(
AlO
HPTC)(μ-OH)(H O)} units are linked through μ-OH, with
2
RESULTS AND DISCUSSION
■
N octahedra edge-sharing to form dimeric clusters (Figure
5
3
+
3+
Al /Pyridine-2,4,6-tricarboxylic Acid/Pyridine. High-
throughput investigations of the reaction system AlCl ·6H O/
2). Such a dimeric motif is common in Al solution chemistry,
with the [Al (μ-OH) (H O) ] cluster observed by NMR
spectroscopy as a product of the hydrolysis of the [Al(H O) ]
4+
3
2
2
2
2
8
3
+
pyridine-2,4,6-tricarboxylic acid (H PTC)/pyridine with water
3
2 6
as solvent, using our in-house developed 48 reactor multi-
clave, showed that the phases [Al(μ-OH){AlPTC(μ-OH)-
complex under slightly basic conditions and as a precursor to
3
8
50−52
the formation of Keggin ions.
In compound 2, the dimeric
(
(
H O)} ] (denoted CAU-16 or compound 1) and {Al-
clusters crystallize to form an extensive H-bonding network.
The structure of CAU-16 ([Al(μ-OH){AlPTC(μ-OH)-
(H O)} ], compound 1) was solved from synchrotron powder
2
2
HPTC)(μ-OH)(H O)} (denoted compound 2) could be
2
2
prepared across a wide range of reaction stoichiometries
Figure 1 and SI, section S3). Both phases are formed using the
same starting stoichiometry, but the final product depends on
2
2
(
diffraction data and is shown in Figure 3. CAU-16 is isoreticular
53
with MIL-53(Al), with the metal−organic dimeric cluster
5
312
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Crystal Growth & Design
Article
by strut length but also by the breathing properties of the
6
0
framework. MIL-53(Al) undergoes a reversible phase
transition, but for compounds containing longer linkers
(
MIL-69/DUT-4; DUT-5; MOF-253) no breathing effect is
observed, and some of the compounds show little or no
porosity to gas molecules.
MIL-69 and CAU-16 both have narrow/closed pore
structures and have similar thermal properties. Thermogravi-
metric analysis (TGA) of both compounds show a similar
pattern of weight losses (Figure 4a, TGA of CAU-16: see SI for
Figure 3. Structure of CAU-16, [Al(μ-OH){AlPTC(μ-OH)(H O)} ]·
2
2
2
H O, (a) shows a detail of the linker, with the dimeric Al (μ-
2
2
OH) (H O) cluster at the core and two PTC3− ions extending
2
2
2
outward to coordinate further Al sites (shown as polyhedra); (b) view
along the c-direction, showing the rhombic channels delimited by the
metal−organic cluster linkers. The unit cell is indicated (black dashes),
while the blue-dashed ellipse indicates the portion of the structure
shown in (a). Aluminum shown in gold; carbon in black; nitrogen,
blue; and oxygen, red.
units observed in compound 2 directly replacing the
terephthalic acid of the prototypical MIL-53 structure. Th−e
asymmetric unit of CAU-16 contains two Al sites, one PTC3
unit, two μ-OH units, one terminal water molecule, and two
isolated physisorbed water molecules. Coordination about the
first Al site is identical to that described for compound 2,
2
−
forming {AlPTC(μ-OH)(H O)}
metal−organic dimeric
2
2
clusters. Unlike in compound 2, however, each 4-position
carboxylate group coordinates two symmetry-equivalent Al sites
in a bridging mode. The octahedral coordination environment
about this second Al site consists of four carboxylate O atoms,
from four different metal−organic linkers and is completed by
−
two μ-OH ions, forming trans corner-sharing chains analogous
53
with that found in MIL-53(Al). Bond valence sums indicate
that both the μ-O sites of the dimers and the trans corner-
5
4,55
sharing chains are OH groups (O7:1.25; O9:1.17).
Further
evidence of two distinct hydroxide environments is provided by
the infrared spectra, which show two resonances at 3673 and
3
562 cm− (Supporting Information, section S5). By
1
Figure 4. Thermal analysis of CAU-16, with the principal weight losses
indicated on the TGA (a). Variable temperature powder X-ray
diffraction data (b) indicate no large structural changes on
dehydration, with changes in intensity of the diffraction peaks
consistent with the loss of physisorbed water molecules from the
channels.
5
3,56
comparison with other MIL-53-like structures,
the higher
wavenumber band is assigned to the μ-OH of the chains (O9)
and therefore the lower band corresponds to the μ-OH of the
dimers (O7). The rhombic unidirectional channels delimited by
the metal−organic cluster linkers have a cross-section of ∼8.5 ×
8
.0 Å and are occupied by physisorbed water molecules, which
form an H-bonding network.
full analysis). Both compounds show an initial loss of
physisorbed water molecules, which in MIL-69 is complete
below 100 °C, whereas for CAU-16 it is complete by 110 °C. In
the variable-temperature X-ray diffraction (VT-PXRD) data for
CAU-16 (Figure 4b), it is accompanied by changes in the
intensities of the diffraction peaks (most noticeably a relative
increase of the 200 and 110 reflections). However, unlike MIL-
53(Al) no breathing-type structural transition is observed.
CAU-16 shows an additional weight loss, not observed for any
of the other MIL-53 compounds, over the range 220−250 °C
which may be attributed to the loss of the coordinating water
The MIL-53 motif exhibited by CAU-16 has been reported
in an isoreticular series of compounds in which different linear
dicarboxylate ions form the framework: terephthalic acid (MIL-
5
3
57
5
4
3(Al) ); naphthalenedicarboxylic acid (MIL-69 and DUT-
8 58
5
); biphenyldicarboxylic acid (DUT-5 ); bipyridinedicarbox-
ylic acid (MOF-253 ); and trans-1,4-cyclohexanedicarboxylic
acid (CAU-13 ). Of these compounds, CAU-16 has the
longest strut (distance measured between carboxylate C
atoms), CAU-16:14.36 Å; next longest, DUT-5:10.20 Å).
However, porosity in these compounds is determined not only
59
5
6
5
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dx.doi.org/10.1021/cg501189n | Cryst. Growth Des. 2014, 14, 5310−5317

Crystal Growth & Design
Article
molecules. VT-PXRD shows that crystallinity is retained by the
compound to more than 300 °C. CAU-16 undergoes a final
decomposition step from about 330 °C, which is at significantly
lower temperature than the other long linker MIL-53-like
structures.
Gas adsorption experiments were performed on CAU-16,
activating the material under dynamic vacuum overnight at 150
by another PTC molecule and subsequent chelation and
crystallization steps leads to compound 2. However, compound
2 represents a kinetic product or intermediate phase, since it
forms with a much shorter reaction time and can be used as a
direct starting material for the synthesis of CAU-16 (see SI,
section S3, Figure S3.3). The free 4-position carboxylate group
4
+
is able to react with further [Al (μ-OH) (H O) ] or
2
2
2
8
3
+
°
C. CAU-16 was found to be nonporous to N at 77 K (SI,
[Al(H O) ] species in solution, leading to the final
2
2 6
section S10), which is not unusual for MIL-53 type materials
crystallization of CAU-16 (compound 1). This proposed
12
reported with other metals, or those with longer struts and
closed/narrow pore structures. Adsorption experiments were
process is summarized in Scheme 1.
57
3+
3+
Ga or In /Pyridine-2,4,6-tricarboxylic Acid/Pyri-
dine. To investigate the possibility of further MIL-53
analogues with the same dimeric linker unit as CAU-16, the
also performed using CO as the adsorbate at two temper-
2
atures: 196 K, allowing access to the widest p/p range possible
0
3
+
and thus measurement of the most complete isotherm; and 298
K, to ensure kinetic effects did not inhibit adsorption or flexible
behavior. At both temperatures, the material is porous to CO2
systems M /H PTC/pyridine (M = Ga, In) were studied.
3
3
+
Optimization arrays identical to that employing Al as the
metal source were performed, using Ga(NO ) , GaCl , and
3 3
3
3+
(Figure 5) showing a type I isotherm, consistent with a
InCl ·4H O. The Ga /H PTC/pyridine system yielded a
3 2 3
multitude of phases, and although further syntheses were
performed, it was not possible to produce any one of these
phase-pure and in the bulk. However, single crystals of the
phase GaPTC(H O) (compound 3) were isolated. For the
2
2
InCl /H PTC/pyridine system, only one phase, InPTC(H O)
3
3
2
2
(
compound 4), was obtained across the entire region
investigated.
The optimized conditions for CAU-16 and for compound 4
are relatively similar, the synthesis of CAU-16 requiring more
pyridine to yield the most crystalline product. H PTC is
3
relatively insoluble in water, and thus it is likely that one role of
pyridine in the reaction is in solubilizing the linker.
Furthermore, in the synthesis of CAU-16, the pyridine is likely
to play a second role in the basic hydrolysis of the
3
+
[
Al(H O) ]
complex, stabilizing the [Al (μ-
2
50,51
2
6
4+
OH) (H O) ] complex.
2
2
8
The crystal structures of compounds 3 and 4 were
determined from single-crystal and synchrotron powder
3
+
diffraction data, respectively. In contrast to the Al system,
both exhibit -M-PTC-M-PTC- chains though with slightly
different connectivities.
Figure 5. Isotherms for the adsorption of CO2 by CAU-16
compound 1) measured at 196 K (green circles - adsorption; red
squares - desorption) and 298 K (blue triangles - adsorption; orange
stars - desorption). Inset shows an enlargement of the region p/p 0−
(
The asymmetric unit of GaPTC(H O)2 (compound 3)
2
3
+
consists of two independent residues, each composed of a Ga
0
3−
cation, a PTC unit, and two coordinating water molecules.
0
.016.
Each Ga3+ cation is octahedrally coordinated in a configuration
3
+
similar to that of the Al site in compound 2. The [Ga(H O) ]
2
6
3
+
nonbreathing material. The low temperature isotherm shows a
hysteresis loop, which is attributed to retention of adsorbates
within the framework due to kinetic effects, since no hysteresis
is observed in the higher-temperature measurement. Dubinin−
cation undergoes hydrolysis less readily than [Al(H
2
O)
] (and this only at high pH values) rather
than dimeric units of the sort observed in compounds 1 (CAU-
6
] ,
−
forming [Ga(OH)
4
5
0,62,63
16) and 2.
group from a neighboring PTC anion along the b-direction
coordinates to the metal, with the GaO N coordination
environment completed by two terminal H O molecules.
Instead, a monodentate 4-position carboxylate
6
1
3−
Radushkevich analysis indicated a maximum loading of CO
2
−1
at 196 K of 1.76(2) mmol g . (Full details of the sorption
experiments and analysis, including diffraction patterns of the
sample before and after adsorption, are given in the SI, section
S10.)
5
2
Coordination by the 4-position carboxylate forms sinusoidal
-Ga-PTC-Ga-PTC- chains (Figure 6). Chains are held together
by H-bonding interactions between the terminal water
molecules and uncoordinated carboxylate O atoms. The
structure is dense and therefore not expected to show any
degree of porosity.
The crystallization process occurring during reactions of Al3+
and H PTC can be interpreted by considering the similarities
3
of the crystal structures and relative reaction times producing
compounds 1 (CAU-16) and 2, as well as by considering the
3
+
solution chemistry of Al in water (Scheme 1). Dimeric
The asymmetric unit of InPTC(H O) (compound 4)
2
2
4
+
3+
3−
[
Al (μ-OH) (H O) ] complexes are known to form from the
consists of one In cation, half a PTC anion, and a
coordinating water molecule. The larger cationic radius of In
(80.0 pm) compared to that of Al (53.5 pm) or Ga (62.0
pm) allow it to adopt a seven-fold coordination state. The
coordination environment of the In site is very similar to that
2
2
2
8
3
+
52
3+
hydrolysis of the [Al(H O) ] complex. Following sub-
2
6
6
4
3+
3+
stitution of one of the H O ligands of the dimer by a PTC
2
ligand, the chelation by PTC and substitution of a further two
3+
H O molecules would be expected to follow rapidly.
2
3+
3+
Substitution of another H O ligand on the second Al cation
of the Ga in compound 3, with an additional coordinative
2
5
314
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Crystal Growth & Design
Article
a
Scheme 1. Proposed Crystallization Process for [Al(μ-OH){AlPTC(μ-OH)(H O)} ]·2H O, CAU-16 (compound 1)
2
2
2
aAluminum hexaaqua ions undergo basic hydrolysis in solution to form the dimers. Ligand substitution of three water molecules by a PTC3− anion
forms a hypothetical intermediate, while reaction with a second equivalent of H PTC forms dimeric compound 2, {AlPTC(μ-OH)(H O)} .
3
2
2
Reaction with a further equivalent of Al3 from solution leads to the final product, CAU-16.
+
-
M-PTC-M-PTC- chain structures are common in the
coordination chemistry of H PTC with transition metals.
3
3+
2+
3+
2+
Reactions of Mn , Fe , Fe , or Co with H PTC result in
chain structures very similar to that of of compound 4.
3
2
3,65,66
Chain structures with monodentate carboxylate groups, similar
to that observed in compound 3, have been reported with
2+
67
Cu .
It is instructive to compare the coordination chemistries of
3
+
3+
3+
Al , Ga , and In in the presence of benzene-1,3,5,-
tricarboxylic acid (H BTC) and pyridine-2,4,6-tricarboxylic
3
26
3+
acid (H PTC). Only MIL-96 has been reported with Ga
3
3+
3+
and H BTC; MIL-96 is also known with Al , In , and also
3
3
+
24,29,68
3+
Cr .
1
With Al , the mesoporous compound MIL-
1
6
25
00(Al) and near mesoporous MIL-110 are also reported,
consisting of trimeric μ -oxo clusters linked in three dimensions
3
3−
3+
by BTC ligands. While the coordination chemistry of Al
and Ga3+ in the presence of BTC anions is dominated by
3−
3
+
trimeric μ -oxo metal clusters, the chemistry of In with
3
H BTC is more diverse, featuring a range of other building
3
blocks in addition to the trimeric μ -oxo metal building
3
3
0,31,33,34
blocks.
However, the results of our syntheses seem to
indicate that such wide structural diversity is not available in the
3
+
Figure 6. Chain structures adopted by Ga3+ (above) and In3+ (below)
system In / H PTC/water, since the highly favorable
3
on reaction with H PTC. In GaPTC(H O) , compound 3, chains are
coordination of the metal in the O/N/O pocket of the ligand
limits the options available to form an extended network.
3
2
2
formed by linking of octahedrally coordinated Ga site through
monodentate 4-position carboxylate groups and the O/N/O pocket of
two PTC3 ions; the sinusoidal topology of the chain is indicated
−
CONCLUSION
(
purple dashes). In InPTC(H O)2 the 4-position carboxylate is
■
2
bidentate and so In sites are seven-fold coordinated, while chains have
a linear configuration. Ga shown in green; In in purple; carbon, black;
nitrogen, blue; oxygen red; and protons are shown in pink.
Four group 13 metal pyridine-2,4,6-tricarboxylate (H PTC)
3
coordination compounds were synthesized, using a high-
throughput multiclave setup, with water as the solvent and in
the presence of pyridine. With Al3+ two phases were identified
and their syntheses optimized: porous [Al(μ-OH){AlPTC(μ-
interaction provided by the bidentate 4-position carboxylate
group (cf. monodentate in compound 3), to give an InO N
OH)(H O)} ] (CAU-16 - Christian-Albrechts-Universitat
̈
6
2 2
coordination environment. Linear -In-PTC-In-PTC- chains are
aligned parallel to the b-direction (Figure 6) and connected by
H-bonding. As for compound 3, the structure is dense and
therefore does not show any porosity.
porous materials number 16, compound 1) and {Al(HPTC)-
(μ-OH)(H O)} (compound 2). It was found that both CAU-
2
2
16 and compound 2 could be prepared from reactions with the
same initial composition, with compound 2 obtained after 12 h
5
315
dx.doi.org/10.1021/cg501189n | Cryst. Growth Des. 2014, 14, 5310−5317

Crystal Growth & Design
Article
and CAU-16 after 48 h. Compound 2 could also be used as a
reagent in the synthesis of CAU-16. Thus, compound 2 is an
intermediate in the synthesis of CAU-16 and a reaction process
was proposed. Compound 2 is a molecular compound,
consisting of a dimeric Al cluster as its core with one
uncoordinated carboxylate group on each of the two PTC
ligands. In CAU-16, this metal−organic cluster unit acts as a
rigid linker, coordinating trans corner-sharing Al-OH-Al chains
through the 4-position carboxylate groups, forming a structure
isoreticular with MIL-53(Al). CAU-16 has a narrow pore
(University of Kiel) is thanked for helpful discussions and
assistance with solution of structures from powder data. Sylvia
Williamson (University of St. Andrews) is thanked for
collection of gas adsorption data. Prof. Anthony Cheetham is
thanked for assistance with funding during writing of the paper.
3
+
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ASSOCIATED CONTENT
Supporting Information
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AUTHOR INFORMATION
Corresponding Authors
Tel. +49-431-880-1675. Fax: +49-431-880-1775. E-mail:
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́ ́
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stock@ac.uni-kiel.de.
E-mail: michael.wharmby@diamond.ac.uk.
6
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*
Author Contributions
́
K.R. first synthesized compounds 1 and 2. M.S. and T.R.
prepared the linker and first synthesized compounds 1 and 4;
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phases. N.S. and M.T.W. jointly wrote the text of this article.
Funding
M.T.W. and N.S. thank the Deutsche Forschungsgemeinschaft
(
(
(DFG, SPP 1362 “Porous Metal-Organic Frameworks”) and
the European Union (Seventh Framework Program FP7/
(
́
ey, G.;
2
007−2013, Grant Agreement No. 228862) for their financial
Haouas, M.; Taulelle, F.; Bourrelly, S.; Llewellyn, P. L.; Latroche, M. J.
Am. Chem. Soc. 2006, 128, 10223−10230.
support.
(25) Volkringer, C.; Popov, D.; Loiseau, T.; Guillou, N.; Ferey, G.;
Notes
Haouas, M.; Taulelle, F.; Mellot-Draznieks, C.; Burghammer, M.;
The authors declare no competing financial interest.
Riekel, C. Nat. Mater. 2007, 6, 760−764.
(
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26) Volkringer, C.; Loiseau, T.; Ferey, G.; Morais, C. M.; Taulelle,
ACKNOWLEDGMENTS
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F.; Montouillout, V.; Massiot, D. Microporous Mesoporous Mater. 2007,
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