Synthesis, characterization and properties of a glycol-coordinated ?-Keggin-type Al13 chloride

Article abstract of DOI:10.1039/c8cc01363b

Herein we present the first example of a glycol-coordinated ?-Keggin Al₁₃ chloride (gl-?-Al₁₃), which is the first chelated version since discovery of Al₁₃ in 1960. The molecular structure consists of [AlO₄Al₁₂(OH)₁₂(OC₂H₄OH)₁₂]Cl₇·H₂O units with chelating mono-anionic ethylene glycol units replacing one bridging and one terminal oxygen site.

Full text of DOI:10.1039/c8cc01363b

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Chem. Commun., 2018, DOI: 10.1039/C8CC01363B.
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Reactivity differences of Sc
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Synthesis, characterization and properties of a glycol-coordinated
ε-Keggin-type Al chloride
1
3
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
b
c
c
Bin Gu, Chenglin Sun,* James C. Fettinger, William H. Casey, Alla Dikhtiarenko, Jorge Gascon,
d
e
f
f
Kamila Koichumanova, Karthick Babu Sai Sankar Gupta, Hero Jan Heeres and Songbo He*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein we present the first example of a glycol-coordinated ε- salts is unknown since single crystals have not been isolated so
Keggin Al13 chloride (gl-ε-Al13), which is the first chelated version far.
since discovery of Al13 in 1960. The molecular structure consists of
[
AlO
4 2 4 7 2
Al12(OH)12(OC H OH)12]Cl ·H O units with chelating mono- The conventional hydrolysis method for Al13 synthesis brings
+
anionic ethylene glycol units replacing one bridging and one high concentrations of countercations, usually Na , into the
terminal oxygen site.
system, which suppresses Al13 chloride crystallization and
1
, 2, 5, 12
purification.
In this work, we report a simple
The hydrolysis of aluminum salts in water produces an array of
polyoxoaluminum clusters which are widely used in catalysis,
water treatment, antiperspirant additives and pillaring agents
solvothermal method that allows the salts of the ε-Al13 isomer
to crystallize in a new glycol-chelated form (gl-ε-Al13) with the
7
+
4 2 2
ion stoichiometry: [AlO Al12(OH)12(O-CH -CH -OH)12] . The
1
, 2
for clays.
The Baker-Figgis-Keggin Al clusters, having the
13
overall charge is compensated by chloride ions and the
ethylene glycol eliminates the bound water molecules,
possibly suppressing hydrolysis. Ethylene glycol is present as a
mono-anion coordinated in a bidentate fashion. The oxo
replaces a μ -OH in each Al cap and a hydroxo replaces a
7
stoichiometry: [AlO Al (OH) (OH ) ] , are the most common
+
4
12
24
2 12
multinuclear ions. The five isomers have an Al(O) tetrahedral
4
center surrounded by 4 trimeric aluminum-hydroxide caps
(
Al ) with aluminum coordinated in an octahedral fashion. The
3
2
3
3
rotation of these Al caps results in α, β, γ, δ and ε isomers.
3
terminal water (Figure 2). Single crystals can be crystallized
within a few hours rather than days to weeks. To the best of
our knowledge, it is the first example of an ε-Al13 isomer
directly crystallized as a chloride salt or in a chelated form.
The ε-Al isomer, which is the most thermodynamically
1
3
4
favored, was first isolated by Johansson as sulfate and
5
selenate salts. The δ-Al isomer is less stable and was first
1
3
6
,
7
isolated by Nazar.
The γ-Al13 isomer was recently
1
crystallized by Smart and other derivatives have been
The catalytic properties of the gl-ε-Al13 cluster are
demonstrated by employing it as a heterogeneous catalyst for
the synthesis of secondary amines via direct N-
alkylation/amination of primary amines (i.e., aniline) and
alcohols (i.e., benzyl alcohol). Such a catalyst is attractive as it
8
reported. In most cases, the Keggin-type clusters are
crystallized as sulfate, sulfonate, or selenate salts because the
2
interaction of SO4 or SeO4 with aluminum polycations
-
2-
3
facilitates the crystallization process. Although there have
, 9
4
, 10-12
13, 14
been many efforts,
the precise structure of ε-Al13 chloride
involves green chemistry methods
and could conceivably
replace C-N bond formation using expensive homogeneous
1
5-17
catalysts, such as Pd, Ru and Ir complexes.
The
heterogeneous catalyst is also attractive because separation
1
does not require toxic additives, i.e., ligands and base , as
8
may be the case with homogeneous catalysts. Alumina,
1
9-22
aluminosilicates and zeolites,
e.g., Cu, Ni, Pd, Ag, Au, Ru and Pt) catalysts
recently reported. As we show, the gl-ε-Al exhibits excellent
and the supported metal
2
3-29
(
have been
1
3
primary amine conversion and secondary amine selectivity.
Single crystals (yield ca. 90%) of the gl-ε-Al13 were isolated as
chloride salts through solvothermal synthesis carried out in a
o
Teflon-lined stainless steel autoclave (100 mL) at 150 C for 6
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3
1
Correspondingly, bond-valence analysis, performed using
3
VaList software, indicates that the cationic Al13 moiety can be
2
7
+
4 2 2 2
described by the formula [AlO Al12(μ -OH)12(OCH CH OH)12] .
As expected for the Keggin isomers, the gl-ε-Al13 contains one
crystallographically unique, tetrahedrally coordinated Al(III)
cation with Al-O bond length of 1.825(4) Å. It also contains
twelve Al(III) cations that are each octahedrally coordinated.
The inner-coordination sphere of each Al(O)
oxo, two µ -OH, two µ -O-glycols and a single µ
The glycols are part of a bidentate chelate that terminates at
O, 2.4 g) was the other end in a µ -OH-glycol replacing what is normally a
6
includes a µ
4
-O
Figure 1. SEM and particle-size distribution of single-crystal gl-ε-
2
3
2
-OH-glycol.
Al13
.
h. Aluminium chloride hexahydrate (AlCl
3
·6H
2
2
dissolved in ethylene glycol (EG, 10 mL) under magnetic water molecule bound to the Al13
stirring, followed by adding ethanol (40 mL) to maintain a
.
-
transparent solution. After being cooled down to room Two crystallographically nonequivalent Cl anions, acting as
temperature, the mixture was centrifuged (5000 rpm) to counter anions, provide overall charge balance for the
-
collect the white solids, which were further washed 3 times by structure. Four Cl anions (Figure 2-C and D, Cl1) are located in
o
ethanol and dried at 60 C.
the cavity along the axis of the AlO facets (Figure 2-C), in a
4
7+
10
cation, which
ring of μ -OH, and hydrogen-bond to the Al
2
1
SEM images of gl-ε-Al13 chloride (Figure 1) show an octahedral is also indicated by the previous modeling. Another three Cl
13
1
-
morphology, with particles in the order of 3 to 30 μm and very anions sharing positions with the crystallization water
smooth surfaces.
molecules (Figure 2, Cl2) are disordered. In the crystal
structure of the gl-ε-Keggin Al , the distorted hexagonal
1
3
The gl-ε-Al13 structure crystallizes in the cubic space group F- channels run between the molecules along [111] direction
3m and has the formula of [Al C H O ][Cl] [H O]. The (Figure S2) possess a significant free volume where additional
4
1
3
24 72 40
7
2
structure is shown in Figure 2-A and B. Structural parameters water, ethanol or ethylene glycol, may be located and have
3
3
are listed in Table S1. The essential structure of the ε-Al1
3
Keggin cluster, including the +7 overall charge, is preserved.
been removed using the program Platon-Squeeze (see SI).
3
0,
A comparison between the powder XRD and the simulated
XRD patterns from single crystal analysis is shown in Figure S3,
which indicates high crystallinity and purity of the synthesized
compound.
FT-IR and Raman spectra of gl-ε-Al13 shown in Figure 3 have a
few similar vibrational bands belonging to species that are
both Raman and IR active. Two broad bands at 3500 and 3100
-1
cm are assigned to the OH stretching vibrations of the gl-ε-
34
Al13 and hydrogen-bonded OH groups of coordinated water,
respectively. Presence of water can also be confirmed by the
-1
clear water-bending mode in FT-IR at 1636 cm . In the 3000-
-
1
2
800 cm region which is typical for methyl and methylene C-
H stretching modes, the peaks are less clear due to underlying
Figure 2. (A) [AlO
The central Al(O)
4
Al(OH)12(OC
is drawn in green, the twelve Al(O)
2
H
4
OH)12]Cl
7
·H
2
O (gl-ε-Al13) structure.
are drawn in
4
6
light purple, oxygens are drawn in red, carbons are drawn in grey
and hydrogens are drawn in white. (B) Ethylene glycols chelate to
Al(O)
water, forming a μ-(OH-CH
bridging oxo between two Al(O)
6
by having one hydroxyl functional group replace a terminal
), and the other end coordinating to a
, forming a μ -(O-CH ). (C, D) The
located Cl counterions are located near a ring of μ -OH and
between two μ -OH. The Cl(2) site is completely occupied; 5/6 of
the time as Cl and 1/6 as a water.
2
6
3
2
-
2
2
-
Figure 3. FT-IR and Raman spectra of single-crystal gl-ε-Al13
.
2
| J. Name., 2012, 00, 1-3
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O-H broad band. However, 2964, 2892 and 2820 cm bands
COMMUNICATION
-1
Table 1. N-alkylation of aniline with benzyl alcohol using
single crystal gl-ε-Al13 chloride salt as the catalyst.
3
5
can be assigned to CH
2
- bands of ethoxy-groups of gl-ε-Al13
- bending modes at 1469,
419, 1412, 1369 and 1350 cm and their overtones at 2800-
.
These groups also give rise to CH
2
-1,
1
2
-1
-1
600 cm . Sharp bands at 1096, 996 and 891 cm are assigned
to Al-O-C bonds. However, presence of solvent C-OH and CH
x
, 36
1
Catalyst/Reactant
Conversion/Selectivity
(%)
bands cannot be excluded (ethanol and ethylene glycol).
(
mmol)
Reaction
time (h)
-
1
The strong band at 702 cm can be attributed to Al-O-Al
Entry
structural unit of gl-ε-Al13 chloride and is identical to the ε-
3
ε-Al13
1
2
1
3
4
5
7, 38
Al13
.
0
-1
0
1
1
1
1
1
1
1
1
1
1
3
1
1
1
1
2
3
1
12
12
12
12
12
12
12
12
4
0
0
0
0
0
1
The H MAS SSNMR spectrum of gl-ε-Al13 chloride (Figure 4-A)
0-2
0
0
0
0
shows two resonances at 1.3 ppm and 3.8 ppm, which are
1
1
1
1
2
2
-1
-2
-3
-4
-1
-2
0.06
0.19
0.43
0.6
0.6
0.6
0.6
22.3
43.3
87.3
90.4
100
100
35.5
5.4
3.8
4.9
4.4
0.2
0.2
0.6
94.6
96.2
95.1
95.6
85.8
89.7
99.5
0
assigned to bridging -OH and -OCH . The broad resonance peak
2
3
9, 40
0
at 4-6.5 ppm is ascribed to physically adsorbed water.
In
1
3
the C NMR spectrum, two adjacent resonances are observed
at 60.2 ppm and 63.3 ppm (Figure 4-B), which clearly indicate
the coexistence of two different chemical environments for
the methylene groups. One of them is slightly more deshielded
0
0
3.4
2.7
0
2
7
than the other, probably due to hydrogen bonding. The Al
NMR spectrum (Figure 4-C) displays a sharp peak at 61.6 ppm,
which could be assigned to AlO species, and a broad peak
4
3-1
4
1-43
centered at 4.4 ppm corresponding to AlO6.
The broadness
displays the presence of large quadrupole couplings and a
possibility of presence of multiple Al environments, as
4
4
auto-generated pressure, followed by cooling in the cold water
and centrifuging. The liquid products were analyzed on an
Agilent 7890A GC equipped with FID and mass spectrometer. It
can be seen from Table 1 that when catalyst loading reaches
expected.
The Al/Cl atom ratio of gl-ε-Al13 chloride derived from XPS
analysis (Figure S4) is 1.95, which is in accordance with the
chemical formula (1.86), elemental analysis (1.85) and the
SEM-EDS analysis (1.84). Spectra fitting of Cl(2p) (Figure S4)
reveals two intensive peaks centered at 197.7 eV and 199.3
eV, which correspond to Cl1 and Cl2. The Cl2/Cl1 ratio is 0.79,
which consists well with the above theoretical value 0.75.
0
.6 mmol/1 mmol aniline, aniline
Ent. 1-4). 100% aniline conversion can be obtained by
increasing alcohol/aniline ratio (Ent. 2-1 and 2-2) with the
compensation of more di-alkylation to form tertiary amine
Instead of hydrogen autotransfer (HA) or borrowing hydrogen
1 conversion surpasses 90%
(
5.
1
3, 14
(BH) strategy,
direct N-alkylation of primary amine with
alcohol over the single crystal gl-ε-Al13 chloride seems to
This gl-ε-Al13 chloride was used as catalyst for the N-alkylation
of aniline with benzyl alcohol (Table 1). Aniline, benzyl alcohol
and gl-ε-Al13 were successively added to 5.0 mL o-xylene in a
follow well what is expected from intermolecular dehydration,
2
1
which were also reported by Ready et al. (0% yield of
secondary amine over SiO and 98% yield of over nanosized
zeolite beta), Sreenivasulu et al. (100% selectivity of imine
4
2
4
15 mL quartz tube followed by purging with nitrogen to
2
0
3
4
remove oxygen. The reactor tube was sealed and transferred
1
9
over nano-aluminosilicate) and Srinivasu et al. (60% yield of
into a preheated oil bath to perform N-Alkylation at 150 °C and
over 2-D hexagonal aluminosilicate). Extending the reaction
time from 4 hours (Ent. 3-1) to 12 hours (Ent. 2-2) increases
aniline conversion while decreases the selectivity of
9.5% to 89.7%.
4 from
9
These results indicate promise for the gl-ε-Al13 chloride to be
useful in green catalysis. However, the significance of
successful synthesis of gl-ε-Al13 chloride even transcends the
catalytic properties. The Keggin ions are among the few
cationic clusters of nanometer sizes and attempts to isolate
clusters made of metals besides aluminum have met with little
success. There are still few other clusters of trivalent metals,
4
5-48
although they have been speculated about for decades
.
Attempts at aqueous synthesis of new metal cation clusters
often fail because of uncontrolled hydrolysis - the bound water
molecules deprotonated leading to uncontrolled gel
Figure 4. MAS solid state NMR spectra of single-crystal gl-ε-Al13
.
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formation. This point was emphasized recently by Sadeghi et 20. P. Sreenivasulu, N. Viswanadham and S. K. Saxena, J. Mater.
4
9
Chem. A, 2014, 2, 7354.
1. M. M. Reddy, M. A. Kumar, P. Swamy, M. Naresh, K. Srujana, L.
Satyanarayana, A. Venugopal and N. Narender, Green Chem.,
al. who obtained a version of the Fe13 as a Keggin cluster only
by replacing the terminal water molecules with organic
ligands. In this paper we suggest a simple means of
accomplishing this replacement, which indicates a new
strategy for expanding the library of hydrolytic clusters. The
replacement of bound water molecules with bridging glycols
may allow dissolution of the ε-Al13 into a wider range of
organic solvents (Table S8 and Figure S5, however the Al13 was
not found to be stable in those solvents). If so, it suggests a
2
2013, 15, 3474.
2
2
2
2. F. Valot, F. Fache, R. Jacquot, M. Spagnol and M. Lemaire,
Tetrahedron Lett., 1999, 40, 3689.
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uselessly in aqueous solutions, including perhaps clusters of
2
2
6. M. Yin, S. He, Z. Yu, K. Wu, L. Wang and C. Sun, Chin. J. Catal.,
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S. He thanks financial support from National Natural Science
Foundation of China (No. 21206161). WHC acknowledges
financial support by the NSF Phase-2 CCI, Center for
Sustainable Materials Chemistry (NSF CHE-1102637). WHC and
JCF thank the National Science Foundation (Grant CHE-
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Synthesis, characterization and properties of a glycol-coordinated ε-Keggin-
type Al chloride
13
Bin Gu, Chenglin Sun,* James C. Fettinger, William H. Casey, Alla Dikhtiarenko, Jorge Gascon, Kamila
Koichumanova, Karthick Babu Sai Sankar Gupta, Hero Jan Heeres and Songbo He*
Crystals of glycol-coordinated ε-Al13 ions were isolated for the first time as chloride salts.