Influence of the substitution pattern of four naphthalenedicarboxylic acids on the structures and properties of group 13 metal-organic frameworks and coordination polymers

Article abstract of DOI:10.1039/d0dt00387e

Metal-organic frameworks containing Ga³⁺ ions and four differently substituted naphthalenedicarboxylates (ndc^2-) have been synthesized and characterized. The Ga³⁺ ions are six-fold coordinated by oxygen atoms in all title compounds, but two different inorganic building units, i.e. trans corner-sharing and cis,trans edge-sharing octahedra are observed. Crystal structures were validated by Rietveld refinements against powder X-ray diffraction data. [Ga(OH)(1,4-ndc)]·2H₂O crystallizes in a non-breathing MIL-53 type structure with two different pore sizes (5.5 × 5.5 ? and 9 × 9 ?). It is non-porous with respect to nitrogen but has a water adsorption capacity of about 155 mg g^-1 and a thermal stability of up to 240 °C. The dense compound [Ga(OH)(1,8-ndc)] crystallizes in a new layered structure motif, which is related to the crystal structure of MIL-122 ([Al(OH)((O₂C)₄C₆H₂)]). The third and fourth compounds [Ga₂(OH)₄(2,3-ndc)]·H₂O and [Ga(OH)(2,6-ndc)]·H₂O are isoreticular to CAU-15 ([Al₂(OH)₄(2,3-bdc)]·H₂O) and MIL-69 ([Al(OH)(2,6-ndc)]·H₂O), respectively. The last two compounds are non-porous toward nitrogen but reversible dehydration was demonstrated. For comparison, the two new compounds [Al(OH)(1,8-ndc)] and [Al₂(OH)₄(2,3-ndc)]·H₂O, which are isostructural to the newly described gallium compounds, were also synthesized and fully characterized. The Al-containing coordination polymers exhibit higher temperature stabilities compared to their isostructural Ga compounds.

Full text of DOI:10.1039/d0dt00387e

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DOI: 10.1039/D0DT00387E
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ARTICLE
Influence of the Substitution Pattern of four Naphthalene-
dicarboxylic Acids on the Structures and Properties of Group 13
Metal-organic Frameworks and Coordination Polymers
Received 00th January 20xx,
Accepted 00th January 20xx
Timo Rabe,a Harm Pewe,a Helge Reinsch,a Tom Willhammar,b Erik Svensson Grapeb and Norbert
Stock*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Metal-organic frameworks containing Ga3+ ions and four differently substituted naphthalenedicarboxylates (ndc2-) have
been synthesized and characterized. The Ga3+ ions are six-fold coordinated by oxygen atoms in all title compounds, but two
different inorganic building units, i.e. trans corner-sharing and cis,trans edge-sharing octahedra are observed. Crystal
structures were validated by Rietveld refinements to powder X-ray diffraction data. [Ga(OH)(1,4-ndc)] ּ 2 H2O crystallizes in
a non-breathing MIL-53 type structure with two different pore sizes (5.5 x 5.5 Å and 9 x 9 Å). It is non-porous with respect
to nitrogen but has a water adsorption capacity of about 155 mg/g and a thermal stability of up to 240 °C. The dense
compound [Ga(OH)(1,8-ndc)] crystallizes in a new layered structure motif, which is related to the crystal structure of MIL-
122 ([Al(OH)((O2C)4C6H2)]). The third and fourth compounds [Ga2(OH)4(2,3-ndc)] ּ H2O and [Ga(OH)(2,6-ndc)] ּ H2O are
isoreticular to CAU-15 ([Al2(OH)4(2,3-bdc)] ּ H2O) and MIL-69 ([Al(OH)(2,6-ndc)] ּ H2O), respectively. The last two
compounds are non-porous toward nitrogen but reversible dehydration was demonstrated. For comparison, the two new
compounds [Al(OH)(1,8-ndc)] and [Al2(OH)4(2,3-ndc)] ּ H2O, which are isostructural to the newly described gallium
compounds, were also synthesized and fully characterized. The Al-containing coordination polymers exhibit higher
temperature stabilities compared to their isostructural Ga compounds.
MOFs and CPs has been less often reported.4 This is probably
due to the high price resulting from the fact that its abundance
in the earth crust is rather low when compared to aluminium
Introduction
In the past decades metal-organic frameworks (MOFs) have
become one of the most studied topics in inorganic chemistry.1
These mostly crystalline solids, which are built from inorganic
(metal ions or metal-oxygen clusters) and organic building units
(ligands or linkers), are especially interesting due to various
structural motifs that can be created. In case the framework
contains potential voids the classification of MOF is applicable,
otherwise, the term coordination polymer (CP) is used. Often,
an intentional choice of components will lead to materials with
a defined pore network and specific properties. This makes
MOFs a very interesting class of porous materials for
applications in fields such as heat transformation, drug delivery,
gas-storage and -separation or catalysis.2 MOFs containing Al3+
ions in the inorganic building unit (IBU) have been intensively
studied during the last decade.3,4 The compounds are attractive
candidates for industrial applications due to their high chemical
and thermal stability.5 Additionally, Al salts as starting materials
are relatively cheap, because of the widespread use of the
element in industry and its high abundance in the earth crust.6
The use of the heavier homologue gallium for the synthesis of
(Al:Ga = 4390 : 1)6 and its costly refining process.7 Although
most of the reported Ga- and Al-MOFs are isostructural,5 they
can exhibit different properties, for example the structural
flexibility of MIL-53 type compounds, [M(OH)(1,4-bdc)] (M =
Al3+, Ga3+; 1,4-H2bdc = benzene-1,4-dicarboxylic acid).8–10 On
the other hand Ga-MOFs have been reported that are not
known for aluminium. An example is MIL-124, [Ga2(OH)4(Hbtc)]
(H3btc = benzene-1,2,4-tricarboxylic acid), which when
functionalized with Eu3+ ions can be used as a highly selective
and accurate sensor for Fe3+/Fe2+ ions.11 We are interested in
the synthesis and characterisation of MOFs and CPs containing
tri- and tetravalent metal ions.5,12 Based on our previous work
on Al-MOFs,5 we have now started a systematic investigation on
Ga-MOFs. Here we report our results on the use of four
differently substituted naphthalenedicarboxylic acids (Fig. 1) as
starting materials employing solvothermal syntheses.
a. Christian-Albrechts-Universität zu Kiel, Max-Eyth-Straße 2, 24118 Kiel.
b. University of Stockholm, 10691, Stockholm, Frescativägen 8.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Figure 1. The four different naphthalenedicarboxylic acids employed in our systematic
investigations (1) naphthalene-1,4-dicarboxylic acid (1,4-H2ndc), (2) naphthalene-1,8-
dicarboxylic dianhydride, (3) napthalene-2,3-dicarboxylic acid (2,3-H2ndc) and (4)
napthalene-2,6-dicarboxylic acid (2,6-H2ndc).
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the PXRD data did not lead to satisfactory results. Hence small
single crystals of [Ga(OH)(1,4-ndc)] (Fig. 2) were studied by
electron diffraction on a JEOL JEM-2100 transmission electron
microscope.
Experimental
Materials and Methods
View Article Online
DOI: 10.1039/D0DT00387E
Gallium nitrate hydrate (Ga(NO3)3 · 7 H2O (Fig. S1 and Tab. S1),
ABCR, 99.99 % puratrem), aluminum sulphate octadeca-
hydrate (Al2(SO4)3 · 18 H2O, Grüssing, reinst), aluminum nitrate
nona-hydrate
naphthalene-1,4-dicarboxylic acid (1,4-H2ndc, TCI, > 95 %),
naphthalene-1,8-dicarboxylic anhydride (TCI, > 98%),
(Al(NO3)3 · 9
H2O,
Grüssing,
reinst)
naphthalene-2,3-dicarboxylic acid (2,3-H2ndc, TCI, > 98%),
naphthalene-2,6-dicarboxylic acid (2,6-H2ndc, TCI, > 98 %) were
commercially purchased and used without further purification.
Powder X-ray diffraction data (PXRD) was collected on a Stadi
MP equipped with a MYTHEN 1K detector (CuKα1-radiation). IR-
spectra were measured on a Bruker ALPHA-FT-IR A220/D-01
using an ATR-unit. The CHNS-measurements were performed
with
a
HEKAtech Euro EA Elemental Analyser.
Figure 2. A rod shaped single crystal of [Ga(OH)(1,4-ndc)] can be seen with a square cross
section of ~300-500 nm and an estimated length of 3 µm (left). Right: electron diffraction
pattern of one frame of a rotation series of a randomly orientated crystal.
Thermogravimetric data was collected on a NETZSCH STA 409
CD analyser (airflow = 75 mL/min, heating rate = 4 K/min) and
on a Lindseis STA PT 1000 (nitrogen flow = 8 dm3/h, heating rate
= 4K/min). Sorption measurements were carried out using a BEL
Japan Inc. BELSORP-max (nitrogen (77 K) and water vapour
(298 K)). Depending on the compound, the samples were
treated for 16 h at a temperature between 160 and 250 °C
under reduced pressure prior to the measurement. The
syntheses of the gallium compounds were carried out in 20 ml
The crystal structure was solved using SHELX15 and refined
against the three-dimensional electron diffraction data
converging with a R1 of 0.248 without any geometric restraints.
Details regarding the structure determination can be found in
the SI (Tab. S2 and Fig. S3). The structures of (M = Al3+, Ga3+)
[M(OH)(1,8-ndc)], [M2(OH)4(2,3-ndc)] and [Ga(OH)(2,6-ndc)]
were refined from PXRD data (Rietveld plots for Ga3+ in Fig. 3).
In all cases the initial structure models were created ab initio
using Material Studios,16 details can be found in the SI (Fig. S5,
S7, S9, S11 and S13). For [Ga(OH)(2,6-ndc)] the structural
information of the isoreticular aluminium compound MIL-6917
was used as a starting model. All structure models were refined
from PXRD data with the Rietveld method18 in TOPAS academics
(Tab. 2).19 CCDC entries 1981186-1981191 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
glass reactors using
a Biotage® Initiator and Initiator+
microwave system. The syntheses of the aluminium compounds
were carried out in regular ventilation ovens using custom
made steel autoclaves with Teflon® inserts (total volume of 2 or
20 ml).13 Temperature-dependent PXRD data (TD-PXRD) was
collected with a Stoe capillary furnace in 0.5 mm quartz
capillaries.
Syntheses
A summary of reaction parameters for the syntheses of the title
compounds is given in Table 1. It includes solvent, molar metal
to linker ratio (M:L), reaction temperature (T) and time (t) as
well as the heating method (MW = microwave-assisted
reaction, conv. = conventional heating). The full and detailed
syntheses procedures, results of the CH-analyses, and yields can
be found in the supporting information (SI sect. 2).
Results and Discussions
Within this investigation, six new CPs/ MOFs were obtained by
hydro- and solvothermal syntheses. The following section
describes the most important results which were observed
during the synthesis optimisation. While the Ga-MOFs were
synthesized under microwave-assisted heating within 4 hours,
the Al-MOFs were only obtained in our study using longer
reaction times and conventional heating. Water as a solvent is
crucial for the synthesis of crystalline products. The employed
metal salts were nitrates except for the synthesis of
[Al2(OH)4(2,3-ndc)] for which the use of aluminium sulphate
octadeca-hydrate resulted in the most crystalline reaction
product. In contrast to the aluminium compounds, the
syntheses conditions, i.e. reaction-time and temperature, of all
gallium compounds are very similar. Yields of the gallium
reactions are higher compared to the aluminium reactions (SI,
sect. 2). Sodium hydroxide had to be added to the reaction
mixture in order to achieve a higher solubility of the organic
Table 1. Overview of the parameters for the synthesis of [Ga(OH)(1,4-ndc)], [M(OH)(1,8-
ndc)], [M2(OH)4(2,3-ndc)] M = Al3+ or Ga3+ and [Ga(OH)(2,6-ndc)] (conv. = conventional
heating, MW = microwave-assisted heating, DMF = N,N’-dimethylformamide).
Product
solvent
H2O
H2O
H2O
MeOH/H2O
H2O
M : L
1 : 0.8
1 : 1.4
1 : 1.4
1 : 1.2
1 : 1
T, t /°C, h
140, 4
140, 4
140, 4
150, 4
heating
MW
MW
MW
MW
[Ga(OH)(1,4-ndc)]
[Ga(OH)(1,8-ndc)]
[Ga2(OH)4(2,3-ndc)]
[Ga(OH)(2,6-ndc)]
[Al(OH)(1,8-ndc)]
[Al2(OH)4(2,3-ndc)]
160, 42
120, 14
conv.
conv.
H2O/DMF
1 : 2.1
Structure solution and refinement
[Ga(OH)(1,4-ndc)]
corresponding Al-compound,14 but a structure refinement of
crystallizes
isostructural
to
the
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DOI: 10.1039/D0DT00387E
Figure 3. Rietveld refinement plots for [Ga(OH)(1,8-ndc)], [Ga2(OH)4(2,3-ndc)] and [Ga(OH)(2,6-ndc)]. The observed intensities are shown in black, the calculated intensities in red. The
blue line is the difference plot, vertical bars mark the Bragg reflection positions.
Table 2. Crystal data of the four Ga compounds and their aluminium analogues. All crystal structures were refined from PXRD data using the Rietveld method.
compound
crystal system
space group
metal ion
a / Å
[M(OH)(1,4-ndc)]
tetragonal
P42/nmc (No. 137) P4/nmm (No. 129)
[M(OH)(1,8-ndc)]
monoclinic
P21/c (No. 14)
[M2(OH)4(2,3-ndc)]
monoclinic
P2/c (No. 13)
[M(OH)(2,6-ndc)]
monoclinic
C2/c (No. 15)
Ga3+ this work
21.3392(3)
21.3392(3)
6.7142(2)
90
Al3+20
21.1012(7)
21.1012(7)
6.6095(4)
Ga3+ this work
Al3+ this work
16.973(2)
6.633(6)
10.041(5)
90
Ga3+ this work
Al3+ this work
7.593(2)
12.860(2)
9.715(1)
90
Ga3+ this work
Al3+17
24.598(2)
7.5305(6)
6.5472(5)
90
17.0649(2)
6.7215(3)
10.0314(3)
90
6.9193(1)
12.7504(3)
9.7223(2)
90
24.5518(1)
7.1261(5)
6.6983(4)
90
b / Å
c / Å
α / °
90
β / °
γ / °
V / Å3
GoF
Rwp / %
RBragg / %
90
90
3057.4(1)
2.74
90
90
2942.9(2)
116.222(5)
90
1032.2(1)
2.54
116.505(9)
90
1011.6(2)
2.86
127.503(2)
90
680.5(1)
2.26
135.335(8)
90
666.8(2)
2.31
104.225(6)
90
1135.9(1)
3.84
106.863(8)
90
1160.6(2)
-
-
6.5
-
7.4
5.7
4.6
1.9
5.8
2.0
4.8
2.6
5.4
2.3
4.8
1.8
1.4
6.2
reactant and in the case of napthalene-1,8-dicarboxylic Interestingly, the gallium compound crystallizes in a different
anhydride to initiate the hydrolysis of the anhydride and ring space group (P42/nmc vs. P4/nmm for [Al(OH)(1,4-ndc)]). The
opening reaction.21 Dependent on the substitution pattern of IBU is composed of trans corner-sharing GaO4(OH)2-polyhedra
the naphthalenedicarboxylic acid, MOFs or CPs with four which are arranged along the c-axis (Fig. 4 b). The
different structures crystallize, which are described in the interconnection of the IBU is accomplished through the
following section.
naphthalene-1,4-dicarboxylate ions (Ga-O distances range from
1.87(9) to 1.96(1) Å), forming smaller and larger square-like
one-dimensional channels with a maximum diameter of 5.5 x
5.5 Å and 9 x 9 Å, respectively. Each channel is enclosed in four
directions by naphthyl groups. The channels are slightly larger
in comparison to the ones observed for the aluminum
compound (3.0 x 3.0 Å and 7.7 x 7.7 Å) due to the larger ionic
radius of Ga3+ compared to Al3+ (0.620 Å and 0.535 Å).20,22 In
[Al(OH)(1,4-ndc)] two ring orientations pointing into the smaller
channels are observed whereas for the Ga analogue only one is
found. Water molecules are located in the larger channels.
Structure description of [Ga(OH)(1,4-ndc)] · 2 H2O
The hydrothermal reaction of naphthalene-1,4-dicarboxylic acid
with Ga3+ salts leads to the formation of a MIL-53 type structure
(Fig. 4), which has been previously reported for the isostructural
Al compound.20
Structure description of [M(OH)(1,8-ndc)]
The use of the anhydride of 1,8-H2ndc as the starting material
in the synthesis of aluminium and gallium carboxylates led to
two new isostructural coordination polymers of composition
[M(OH)(1,8-ndc)], with M = Al3+ and Ga3+. In the crystal
structure the well-known IBU of trans corner-sharing M(OH)2O4
-polyhedra is observed. In contrast to the structure of MIL-53
these chains are only interconnected to two other IBUs by 1,8-
naphthalenedicarboxylate ions (Al-O and Ga-O distances range
Figure 4. (a) Crystal structure of [Ga(OH)(1,4-ndc)] along the c-axis in with smaller 5.5 x
5.5 Å and larger channels 9 x 9 Å (marked by a smaller and a bigger yellow spheres), (b)
arrangement of the trans corner-sharing Ga(OH)2O4-polyhedra with one GaO6 unit
shown in red.
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from 1.88(3)-1.97(4) and 1.82(4)-2.05(4) Å, respectively) and a The position of the crystal water in [M2(OH)4(2,3-VniedwcA)]rti·clHe O2Onlinies
layered structure is formed (Fig. 5). ABAB stacking of the layers shown in Figure 7. Based on O···O diDstOaIn: 1c0e.s103b9e/Dtw0DeTe0n03t8h7eE
along the a-axis is observed with van der Waals interactions assigned bridging hydroxyl group and the water molecules (Al
between the naphthyl rings. The crystal structure is related to dOw···O1 = 2.60(5) Å and Ga dOw···O1 = 2.91(5) Å) H-bonding is
the one of MIL-122 [M2(OH)2(C14O8H4)] (M = Al3+, Ga3+, In3+), anticipated.25
which contains the same IBU but being interconnected by
1,4,5,8-naphthalenetetracarboxylate ions to
dimensional framework.23
a
three-
Figure 7. Position of the water molecule in the structure of [M2(OH)4(2,3-ndc)] · H2O, M3+
= Al3+ or Ga3+
.
The crystal structure of [M2(OH)4(2,3-ndc)] · H2O is similar to
the ones of MIL-12426 [Ga2(OH)4(Htrim)] (H3trim = trimelletic
acid) and CAU-1527 [Al4(OH)8(1,2-bdc)2] · H2O. While the IBUs
and their interconnection are the same the interlayer distance
varies with the size of the organic moiety (phenyl- vs. naphthyl
groups). Furthermore [M2(OH)4(2,3-ndc)] · H2O exhibits no
structural changes upon water ad- and desorption in contrast to
CAU-15.27
Figure 5. (a) Crystal structure of [M(OH)(1,8-ndc)] with M = Al3+ or Ga3+. (a) Two stacked
layers viewed along the b-axis. (b) Perspective view and representation of a section of a
layer with IBUs of trans corner-sharing M(OH)2O4-polyhedra bridged by
naphthalenedicarboxylate ions. (c) View of one layer along the c-axis.
Structure description of [M2(OH)4(2,3-ndc)] · H2O
The hydrothermal reaction of Ga3+ and Al3+ ions with
naphthalene-2,3-dicarboxylic acid results in an isostructural
compound with one-dimensional IBUs composed of alternating
cis, trans edge-sharing MO2(OH)4-polyhedra (Fig. 6 c). These
chains are interconnected by the naphthalene-2,3-
dicarboxylate ions. Al-O and Ga-O distances range from 1.92(3)
to 1.93(2) and 1.98(2) to 2.00(6) Å, respectively. ABAB stacking
of the layers is observed (Fig. 6 a) and the distance of about
3.5 Å between slightly parallel displaced phenylrings hints at the
presence of weak π-π-interactions.24
Structure description of [Ga(OH)(2,6-ndc)] · H2O
Under hydrothermal conditions the reaction of Ga3+ ions with
naphthalene-2,6-dicarboxylic acid leads to a MOF with a three-
dimensional network, which is isostructural to MIL-69, the
corresponding Al-MOF first described by T. Loiseau et al.
(Fig. 8 a).17
Figure 8. Crystal structure of [Ga(OH)(2,6-ndc)]. (a) View of the trans corner-sharing
chains of Ga(OH)2O4-polyhedra connected by naphthalene-2,6-dicarboxylate ions along
the c-axis and (b) the space filling model (H2O omitted for clarity).
Figure 6. Crystal structure of [M2(OH)4(2,3-ndc)], with M3+ = Al3+ or Ga3+. (a) Perspective
view of two layers. (b) View along the b-axis showing the chain of alternatingly cis,trans
edge-sharing M(OH)2O4-polyhedra interconnected by naphthalene-2,3-dicarboxylate
ions. (c) IBU composed of alternatingly cis,trans edge-sharing M(OH)2O4-polyhedra (H2O
omitted for clarity).
The IBU is composed of chains of trans corner-sharing
GaO4(OH)2-polyhedra which are connected through the
carboxylate groups of the linker forming a three-dimensional
framework (Ga-O distances range from 1.92(8) to 1.99(3) Å).
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Taking the van der Waals radii into account, the dimensions of are already known with aluminium23,30 or indium23,31,32
.
View Article Online
29
23
the one-dimensional channel along the c-axis can be estimated Example are MIL-53,8,10,33 MIL-68,31 MIL-96 and MIL-122. .
DOI: 10.1039/D0DT00387E
as ca. 3 Å x 3 Å, being theoretically accessible for small guest
molecules like H2O (Fig. 8). Based on the O···O distances IR-spectroscopy
between the bridging hydroxyl group and the
The IR-spectra of all six title compounds are presented in Figure
crystallographically located water molecules in the pores
10. The strong vibrational bands in the range of 1515-1557 cm-1
(dOw···O1 = 2.54(5) Å) H-bonding is anticipated. While in previous
and 1410-1360 cm-1 can be assigned to the antisymmetric and
studies an open pore form of Al-MIL-69 (aBET = 1308 m2/g,
symmetric stretching vibrations of the carboxylate groups,
denoted as DUT-4), was reported when the synthesis was
respectively. Aromatic -C=C- stretching vibrations of the
carried out in DMF instead of water,28 analogous reactions in
naphthalene rings correspond to the signals at 1460-1491 cm-1.
DMF with Ga3+ salts resulted exclusively in the formation of non-
The spectra of [Ga(OH)(1,4-ndc)] · 2 H2O (black), [Ga(OH)(1,8-
crystalline products.
ndc)]·H2O (red) and [Ga(OH)(2,6-ndc)] · H2O (blue) show clear
bands at around 3035 cm-1, which can be linked to the aromatic
Comparison of the structures
-C-H stretching vibrations.34
In the crystal structures of the six title compounds, the M3+ ions
are all octahedrally coordinated with Al-O and Ga-O distances
ranging from 1.88(3) Å to 1.97(4) and 1.82(1) Å to 2.05(4) Å,
respectively. These values are in good agreement with values
reported in the literature.26,27,29 The crystal structures depend
strongly on the substitution pattern of the linker molecules. The
dominant structural motif for the IBU consist of trans corner-
sharing chains of MO4(OH)2-polyhedra, which is observed in
four of the six structures. This IBU is obtained when 1,4-, 1,8-
and 2,6-H2ndc are used in the synthesis. The vicinal
arrangement of the two –COOH groups in 2,3-H2ndc leads to
chains of alternatingly cis,trans edge-sharing M(OH)2O4-
polyhedra. This clearly hints at a structure-directing role of the
vicinal substitution pattern of the aromatic ring. The
arrangement of the –COOH groups in 1,4- and 2,6-H2ndc results
in the formation of three-dimensional framework structures,
while closer proximity of the –COOH groups in 2,3- and 1,8-
H2ndc leads to layered structures due to sterical effects.
Schematic drawings of the crystal structures demonstrating the
influence of the substitution patterns of the linker molecules
are shown in Figure 9.
Figure 10. IR-spectra of [Ga(OH)(1,4-ndc)] · 2 H2O (black), [Ga(OH)(1,8-ndc)] (red),
[Al(OH)(1,8-ndc)] (dark yellow), [Ga2(OH)4(2,3-ndc)] · H2O (green), [Al2(OH)4(2,3-ndc)]
· H2O (purple) and [Ga(OH)(2,6-ndc)] · H2O (blue).
For [Ga2(OH)4(2,3-ndc)] (green) the observed vibrational bands
in the range of 3250-3500 cm-1 are caused by trapped lattice
water molecules and thus support the results of the structure
determination (Fig. 7). The vibration bands in the region of 3460
to 3670 cm-1, which can be attributed to either the bridging
µ-OH group or the -OH groups of water molecules, is prominent
in the spectra of [Ga(OH)(1,8-ndc)] (red) and [Ga2(OH)4(2,3-
ndc)], but of smaller intensity in the spectrum of [Ga(OH)(2,6-
ndc)] (blue) and [Ga(OH)(1,4-ndc)] (black). Comparing the
similar spectra of the isoreticular compounds [M(OH)(1,8-ndc)]
(Fig. 10, red and dark yellow line) and [M2(OH)4(2,3-ndc)] (Fig.
10, green and purple line), a significant shift of the bands
corresponding to the vibrations of the µ-OH group and water
molecules towards higher wavenumbers is observed. This is
related to the higher charge-to-radius ratio caused by the
smaller ionic radius of Al3+ and thus stronger bonding of
aluminium with the µ-OH groups as predicted by the HSAB
principle.35 Another shift of bands between the µ-OH groups of
[M(OH)(1,8-ndc)] and [M2(OH)4(2,3-ndc)] is observed, most
likely resulting from the different M-O-M bond angles in the
IBUs which is well-known from zeolites.36 For [Ga(OH)(1,4-ndc)]
and [Ga(OH)(2,6-ndc)] no vibrational bands of water are
Figure 9. Schematic representation of the crystal structures observed in this study. The
black spheres represent chains of trans corner-sharing M(OH)2O4-polyhedra, green
spheres chains of cis,trans edge-sharing M(OH)2O4-polyhedra and the blue hexagons the
benzene rings.
Our study on the use of naphthalenedicarboxylic acid confirms
previous reports. Based on the hitherto available literature, it is
very common for MOFs with gallium to yield structures which
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Table 3. Results of the thermogravimetric analysis for all six title compounds performed at air.
Product/step
1. (solvent) / wt%
2. (ligand) / wt%
residue mass / wt%
obs
11
0
calcd
10.7 (2 H2O)
0
 T / °C
RT-240
-
obs
63.1
70.3
79.5
54.2
64.0
60.9
calcd
 T / °C
240-850
RT-940
observed
25.9
29.7
20.5
42.3
calculated
27.8
31.2
19.7
42.6
[Ga(OH)(1,4-ndc)] · 2 H2O
[Ga(OH)(1,8-ndc)]
61.5
68.8
80.3
53.4
66.1
65.0
[Al(OH)(1,8-ndc)]
0
0
-
RT-700
[Ga2(OH)4(2,3-ndc)] · H2O
[Al2(OH)4(2,3-ndc)] · H2O
[Ga(OH)(2,6-ndc)] · H2O
3.5
3.5
3.5
4.1 (1 H2O)
5.1 (1 H2O)
5.4 (1 H2O)
RT-205
RT-270
RT-220
205-780
270-700
220-1000
32.5
35.6
28.8
29.4
observed after the samples have been washed with ethanol and Sorption measurements
dried at 80 °C. The IR-spectra and detailed assignment of
selected vibrational bands for all compounds can be found in
the SI (Fig. S15-20 and Tab. S9-14).
Since [Al(OH)(1,4-ndc)] is known to be porous to H2O, N2 and
H2O sorption measurements were carried out for [Ga(OH)(1,4-
ndc)] at 77 and 298 K respectively (Fig. S37 and Fig. 12).
Thermal analysis
The presence of water molecules in the crystal structures of the
title compounds is confirmed by thermogravimetric
measurements (Figure 11, Table 3). Loss of the solvent
molecules is detected up to 240 °C and for [Ga(OH)(1,4-
ndc)] · 2 H2O desorption is observed below 100 °C, which is
probably due to weak host-guest interactions.37 It is anticipated
that the samples adsorbed water, while stored at ambient
conditions before the measurement.
Figure 12. Water adsorption isotherm of [Ga(OH)(1,4-ndc)] · 2 H2O.
While no adsorption of N2 was observed the water vapour
isotherm exhibits a sigmoidal shape with a steep rise at
p/p0 = 0.27 and a maximum uptake of 155 mg/g.39 The uptake is
comparable to the one reported for the aluminium analogue
[Al(OH)(1,4-ndc)] · 2 H2O (maximum uptake 162 mg/g with a
steep rise at 0.2 p/p0). The theoretical water uptake for
[Ga(OH)(1,4-ndc)] is much higher, calculations with PLATON40
predict a maximum uptake of 302 mg/g. Water sorption
measurements of [M2(OH)4(2,3-ndc)], M = Al3+, Ga3+ and
[Ga(OH)(2,6-ndc)] show uptake against water vapour with a
maximum capacity of 63, 68 and 89 mg/g, respectively (Fig. S34-
Figure 11. TG curves of [Ga(OH)(1,4-ndc)] · 2 H2O (black), [Ga(OH)(1,8-ndc)] (red),
[Ga2(OH)4(2,3-ndc)] · H2O (green), [Ga(OH)(2,6-ndc)] · H2O (blue), [Al(OH)(1,8-ndc)] 36). All four compounds remain stable as proven by PXRD
(dark yellow) and [Al2(OH)4(2,3-ndc)] · H2O (purple).
measurements (Fig. S43-S48). No uptake is observed when N2 is
used as the adsorptive at 77 K (Fig. S37-S42).
Decomposition of the compounds under formation of β-Ga2O3
(Fig. S21) or X-ray amorphous Al2O3 is observed above ca.
250 °C. In general it is hard to predict, whether Al- or Ga-
Conclusions
compounds have a higher thermal stability.8,38 For the
We report the synthesis, structure determination and full
isostructural Al- and Ga-compounds in this study the former
characterisation of two MOFs and four CPs containing Ga3+ or
decompose at higher temperatures. These observations are
Al3+ ions and differently substituted naphthalenedicarboxylate
also confirmed by temperature dependent PXRD
ions. In one case the crystal structure was solved using three-
measurements (Fig. S28-S33).
dimensional electron diffraction, while for most of the others
structure molecular modelling had to be employed to get a
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7
D. R. Lide, CRC Handbook of Chemistry and Physics, Taylor &
starting models for the Rietveld refinement. All crystal
structures were refined from PXRD data and contain either an
IBU composed of trans corner-sharing M(OH)2O4-polyhedra or
an IBU of cis,trans edge-sharing M(OH)2O4-polyhedra. Both IBUs
are well-known for group 13 MOFs and CPs. The substitution
patterns of naphthalenedicarboxylic acids, have a crucial
influence on the crystal structure. While 2D-motifs were
favoured from naphthalenedicarboxylic acids with close
arrangements of the -COOH groups such as 1,8-H2ndc or 2,3-
H2ndc, 3D-motifs were obtained using naphthalenedicarboxylic
acids with distant arrangements as in 1,4-H2ndc or 2,6-H2ndc.
The IR-spectra show the expected bands of characteristic
functional groups, along with a prominent shift for the µ-OH
groups of the isostructural title compounds. Comparison of the
isostructural Al and Ga compounds demonstrate a higher
thermal stability of the Al compounds. Furthermore water
vapour adsorption measurements revealed reversible de-
/rehydration for [Ga2(OH)4(2,3-ndc)] · H2O, [Al2(OH)4(2,3-
ndc)] · H2O and [Ga(OH)(2,6-ndc)] · H2O as well as a total water
uptake of 155 mg · g-1 for [Ga(OH)(1,4-ndc)] · 2 H2O with a
sigmoidal isotherm.
View Article Online
Francis, Cleveland, Ohio, Ed. 84, 2004.DOI: 10.1039/D0DT00387E
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Synopsis
In a systematic investigation on Al- and Ga-MOFs and CPs containing differently substituted
naphthalenedicarboxylate ions six compound were obatined and characterized by Rietveld
refinement, thermogravimetric measurements, elemental analysis, IR-spectroscopy and sorption
measurements.