The state of Al(III) in alcohol solutions of aluminum alkoxide as probed by 27Al and 13C NMR and small-angle X-ray scattering

Article abstract of DOI:10.1134/S0036023608100161

Solutions of aluminum alkoxides obtained by interaction of aluminum metal with methyl, ethyl, and isopropyl alcohols were studied by ²⁷Al and ¹³C NMR and small-angle X-ray scattering. Alkoxides with a tetrahedral environment of aluminum prevail in methanol solutions, and those with an octahedral environment of aluminum predominate in ethanol solutions. In isopropyl alcohol at 293 K, polynuclear alkoxides with tetrahedral, octahedral, and pentacoordinated aluminum environments coexist. The structure of polynuclear complexes was refined by comparison of their calculated dimensions with small-angle X-ray scattering data.

Full text of DOI:10.1134/S0036023608100161

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 10, pp. 1621–1627. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © M.A. Fedotov, V.V. Molchanov, R.N. Zotov, F.V. Tuzikov, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 10, pp. 1735–1741.
PHYSICAL METHODS
OF INVESTIGATION
The State of Al(III) in Alcohol Solutions of Aluminum Alkoxide
as Probed by 27Al and 13C NMR
and Small-Angle X-Ray Scattering
M. A. Fedotov, V. V. Molchanov, R. N. Zotov, and F. V. Tuzikov
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 5, Novosibirsk, 630090 Russia
Received April 24, 2007
Abstract—Solutions of aluminum alkoxides obtained by interaction of aluminum metal with methyl, ethyl,
and isopropyl alcohols were studied by 27Al and 13C NMR and small-angle X-ray scattering. Alkoxides with a
tetrahedral environment of aluminum prevail in methanol solutions, and those with an octahedral environment
of aluminum predominate in ethanol solutions. In isopropyl alcohol at 293 K, polynuclear alkoxides with tet-
rahedral, octahedral, and pentacoordinated aluminum environments coexist. The structure of polynuclear com-
plexes was refined by comparison of their calculated dimensions with small-angle X-ray scattering data.
DOI: 10.1134/S0036023608100161
A survey of patents shows that the most efficient cat- detection of broad lines against the background of nar-
alysts of reforming, hydrofining of oil fractions, the row signals. The spectra obtained in stronger fields can
Fischer–Tropsch process, and selective oxidation of be expected to provide more information due to the
carbon monoxide in hydrogen are obtained when spe- “slowing down” of exchange between different species
cial purity alumina is used as a catalyst support. Alu- on the NMR time scale and the better resolution of
mina is obtained by hydrolysis of aluminum alkoxides. broad lines.
Of special interest are aluminum oxides modified with
other elements, as a rule, in the oxidation state +4. The
EXPERIMENTAL
method of their synthesis [1, 2] involves combined
hydrolysis of aluminum alkoxides and å4+ compounds.
Study of aluminum species and their evolution in the
course of hydrolysis makes it possible to control the
properties of resulting solid precipitates [3]. The first
step should be a study of the state of aluminum alkox-
ides in alcohol solutions, which is dealt with in this
work.
Samples of aluminum alkoxides were prepared by
dissolving aluminum metal in the corresponding dry
alcohol under heating and vigorous stirring. The pro-
cess was activated by introducing iodine into a solution
up to a concentration of ~1 mmol/L. The following
samples were prepared: a 0.05 M solution of tri-
methoxyaluminum in methyl alcohol (I), a 1 M solu-
tion of triethoxyaluminum in ethyl alcohol (II), and a
0.3 M solution of triisopropoxyaluminum in isopropyl
alcohol.
The 27Al and 13ë NMR spectra were recorded on a
Bruker AVANCE-400 spectrometer at 104.26 and
100.13 MHz, respectively. Spectral decomposition was
performed with the WINNMR program.
In the pioneering study of a benzene solution of alu-
27
minum triisopropoxide by Al NMR, only one signal
at δ = 0 was observed [4]. For solutions of aluminum
27
chloride in alcohols (AlCl3 + ROH/CH3CN), two Al
NMR signals at δ = 10 and 5.5 ppm were observed,
1
which were assigned on the basis of H NMR data to
[Al(MeOH)6]3+ and [Al(MeOH)6 – xClx]3 – x [5]. Solutions
Small-angle X-ray scattering (SAXS) patterns were
measured on a SAXS diffractometer (Siemens, Anton
Paar, and Hecus-Braun) with a Kratky small-angle
camera at a temperature of 20 0.1°ë. An X-ray tube
with a copper anode was used (λCuKα = 1.54 Å). Sam-
ples of triisopropoxyaluminum solutions and the sol-
vent were in turn placed in a sealed quartz capillary cell
with a wall thickness of 0.01 mm (the sample layer
thickness was 1.0 mm). Measurements were carried out
in isopropyl alcohol were not studied in [5].
1H [6] and 13C [7] NMR studies showed that the
resulting complexes are labile at room temperature.
Solutions of (Bu4N)[Al(OR)4] in nonpolar organic sol-
vents were studied by 27Al NMR in a field of 4.7 T [8].
Polynuclear species have been isolated from alkoxide
solutions [9–11], which suggests that a fraction of alu-
minum exists in solution as polynuclear complexes.
All early studies were carried out on spectrometers in the range h = 0.013–0.609 Å–1(h = 4π(sinθ)/λ, 2θ is
with magnetic fields of 1–2.3 T, which prevented the the scattering angle). To apply corrections for back-
1621

1622
FEDOTOV et al.
*
140
100
60
20
–20 δ, ppm
27
Fig. 1. Al NMR spectrum of sample I at 283 K.
ground scattering, the sample absorption coefficients the second component. A further decrease in tempera-
were also measured [18]. The SAXS data were cor- ture leads to line broadening, which obscures the pres-
rected for absorption, collimation, and solvent scatter- ence of components.
ing (isopropyl alcohol).
According to the 27Al NMR chemical shift scale,
line A in the spectrum of sample I (58 ppm) is due to
aluminum atoms in a tetrahedral oxygen environment
and line B (29 ppm), to pentacoordinated aluminum
atoms. The presence of a narrow, weak line at 72 ppm,
which can be assigned to the mononuclear methoxide
[Al(OMe)3(HOMe)], indicates that broad lines are due
RESULTS AND DISCUSSION
27
The Al NMR spectra of sample I in the tempera-
ture range 296–253 K (Fig. 1, table) are represented by
a single line with a width of ~5 kHz. At 283 K, the line
becomes asymmetric, which points to the existence of to polynuclear species.
27Al NMR parameters of aluminum alkoxides
Sample, T, K
I, 296
A, δ(W), %a
B, δ(W), %
C, δ(W), %
D, δ(W), %
58 2.5(5000)
58 2(4400), 77%
61 3(5300)
283
273
29 2(4500), 23%
253
60 3(5400)
Average
II, 297
283
59 1.5
–0.3 0.1(55), 38%
–0.4 0.2(110), 35%
–0.3 0.3(200), 89%
0.2 0.5(230), 62%
0.7 0.5(260), 65%
1.6 0.7(260), 11%
273
253
–0.4 0.3 (350), 94% 2.9 1(300), 6%
–0.35 0.1 1.4
Average
III, 296
283
1
62 2(4200), 54% 35.8 1.5(3500), 27%
66 3(6300), 43% 36.7 2(4500), 24%
60 4(7200), 41% 32.9 2.5(5200), 30%
4.6(730), 11%
5.5(2400), 17%
5.5(1400), 16%
5.1 0.5 ppm
2.2(150), 8%
2.1(310), 17%
2.3(380), 13%
2.2 0.3 ppm
273
Average
62.7 1.7 ppm
35.1 1.4 ppm
a
Chemical shift, ppm (line width, Hz), % is the line intensity (the entire spectrum is 100%).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 10 2008

THE STATE OF Al(III) IN ALCOHOL SOLUTIONS OF ALUMINUM ALKOXIDE
1623
60
50
40
30
20
10
δ, ppm
13
Fig. 2. C NMR spectrum of sample II at 303 K.
8
6
4
2
0
–2
–4
–6
–8
δ, ppm
27
Fig. 3. Al NMR spectrum of sample II at 297 K.
The line width W of quadrupole nuclei, including of pentacoordinated aluminum was studied ab initio in
27Al nuclei, is proportional to the squared electric field gra-
[14], and the structures of pentacoordinated complexes
of ethylenediamine with aluminum isopropoxide were
studied in [15].
dient (EFG) and the rotational correlation time τÒ [13]:
W ~ (EFG)2τÒ.
(1)
Figure 2 shows the 13C NMR spectrum of an ethoxy-
aluminum sample. The signals at 60.1 and 20.7 ppm are
due to the methylene and methyl carbon atoms of the
ethanol bound to aluminum, respectively, and the other
signals arise from the atoms of free molecules. As is
seen, the ligand exchange is slow on the carbon NMR
time scale. This does not rule out fast intracomplex
exchange of ligands or proton at 303 K.
The large width of line A can be due to ligand
exchange, lowering of the symmetry of the aluminum
environment, or an increase in τÒ caused by an increase
in the molecular mass of the complex.A decrease in tem-
perature slows down the exchange, and the dependence
of the line width on temperature shows that the line width
is dominated by quadrupole relaxation (Eq. (1)).
To comply with the electroneutrality, polynuclear
complexes composed of tetrahedra or square pyramids
should be cyclic. Tetramers of aluminum alkoxides
were studied by X-ray crystallography [14].
The aluminum spectra indicate the presence of two
octahedral complexes, their intensity ratio changing
with decreasing temperature (Fig. 3, table), and slow
(on the NMR scale) exchange takes place. At the lowest
temperature, the line widths for these complexes are
nearly the same, which is evidence of almost the same
The formation of a pentacoordinated aluminum
environment can be predetermined by steric hindrances
generated upon the formation of a ring of octahedra.
The possibility of the formation of organic compounds nuclearity of these species. Both components should be
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 10 2008

1624
FEDOTOV et al.
160
120
80
40
0
–40
δ, ppm
27
Fig. 4. Al NMR spectrum of sample III at 296 K.
uncharged since their concentration ratio changes with clear species. Comparison of the line widths for the
temperature.
dimer (5 ppm) and the monomer (2 ppm) shows the
degree of line broadening caused by the distortion of
the octahedron. Larger widths of signals A and B as
compared with that of the signal due to the dimer point
to a higher nuclearity of the corresponding species. It
might be suggested that a solution contains anionic and
cationic aluminum species that form ion pairs. In this
case, the ratio between the cationic (octahedral) and
anionic (tetrahedral) complexes should be constant to
comply with electroneutrality. However, with cooling,
the ratio between the species corresponding to the
NMR lines changes (as in the case of ethoxyalumi-
num). This is evidence that isopropoxyaluminum spe-
cies have zero charge. Although spectral decomposition
into components is somewhat arbitrary, the change in
the shape of the envelope confirms a change in the ratio
between species upon cooling of the solution. The
structures of complexes proposed on the basis of NMR
data are shown in Fig. 5.
To confirm these structures, a solution of triisopro-
poxyaluminum was studied by the SAXS method [18].
The character of the SAXS pattern for the aluminum
triisopropoxide (Fig. 6) shows that, to a first approxi-
mation, form factors close to homogeneous equiaxed
nanoparticles (the axis ratio of no more than 1 : 3) can
be used. Structural models of such particles can be
homogeneous spheres [19].
27
The Al NMR spectra of sample III (Fig. 4) show
four lines (their parameters are in the table). It turns out
that, in a strong field at room temperature, the alumi-
num species in sample III are involved in slow
27
13
exchange on both the Al and C NMR scale. At the
same time, the exchange between the bound and free
states of the ligands at 303 K is fast, which follows from
the 13C NMR spectrum. Such a situation is not surpris-
ing since the exchange does not lead to a change in the
aluminum environment. It can also be assumed that
exchange follows the associative mechanism, in which
the intermediate state has minimal effect on 27Al NMR
parameters.
According to the 27Al NMR chemical shift scale
[12], signal A in the spectrum of sample III is due to
aluminum in a tetrahedral environment, signal B corre-
sponds to pentacoordinated aluminum, and signals C
and D arise from an octahedral oxygen environment of
aluminum. Inasmuch as no integer ratio was found
between the intensities of any two of the signals A, B,
C, and D, they should be assigned to individual species.
Taking into account the position and small width of
signal D, it can be attributed to a mononuclear octahe-
dral complex, presumably, fac-Al(OPri)3(PriOH)3 (for
ML13L23 , the fac isomer will have the smallest EFG). By
analogy with aqueous solutions of aluminum hydroxo
complexes [16, 17], signal C can be assigned to the
dimer (PriOH)2(PriO)2Al(µ–OPri)2Al(OHPri)2(OPri)2.
After applying hardware corrections, the SAXS data
(Fig. 6) on a sample in different states were used to cal-
culate (as a solution to inverse scattering problems) the
size distribution functions for nanoparticles in samples
(Dv(R)) as a histogram (volume distribution as a per-
Signals A and B in the spectra of isopropoxide solu-
tions, like in the methoxide system, are due to polynu- centage).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 10 2008

THE STATE OF Al(III) IN ALCOHOL SOLUTIONS OF ALUMINUM ALKOXIDE
1625
B
A
Pr
Pr
O
H
O
H
H
O
Pr
O
O
Pr
O
Pr
H
Pr
H
O
H
Al
O
O
O
Al
Pr
Pr
H
Al
O
H
O
O
Pr
O
H
O
O
Pr
H
Pr
Pr
Pr
H
Pr
D
C
Fig. 5. Proposed structures of the complexes in solution III: A is tetrahedral aluminum, B is pentacoordinated aluminum, C is the
dimer of octahedra, and D is an octahedron.
width. This means that the 27Al NMR spectrum showed
all basic aluminum species existing in solutions.
As follows from the histogram in Fig. 6, a major
fraction of complexes in an aluminum triisopropoxide
solution corresponds to the size range from 0.2 to 0.7 nm
(in radius). As shown by NMR, aluminum forms four
different species in solution; therefore, we consider
four major peaks in this range. The presence of other
peaks on the histogram (Fig. 6) can be due to the lim-
ited information provided by SAXS and the formation
of bulkier aggregates of aluminum complexes in the
solution.
The suggested cyclic structures make it possible to
construct uncharged polynuclear complexes with the
[OPri]1– ligand and explain the appearance of pentaco-
ordinated aluminum. Such structures quite closely
match the size and the ratio of aluminum species. An
increase in the number of aluminum atoms in such rings
to six leads to an exceeding of the limiting size (as
probed b y SAXS), whereas a decrease in the number
of aluminum atoms leads to a contradiction with data
obtained by different methods.
For comparison with NMR data (Fig. 4), we divided
the volume ratios of the complexes shown in Fig. 6 by
the volume of the proposed species and thereby
obtained the numbers of aluminum atoms in the corre-
sponding band, using the spherical approximation for
all complexes (Fig. 7). Figure 7 shows that the molar ratios
of complexes obtained by the SAXS and NMR methods
are consistent with each other and that their sizes do not
contradict the structures suggested in Fig. 5.
Thus, in alcohol solutions of aluminum alkoxides
(R = OMe, OEt, and OPri), there exists a set of alumi-
num complexes with a tetrahedral and octahedral envi-
ronment, as well as pentacoordinated aluminum com-
plexes. The ratio between the species changes with a
change in temperature.
Tetrahedral and pentacoordinated aluminum species
predominate in solutions of trimethoxy- and triisopro-
poxyaluminum, and octahedral species prevail in solu-
tions of triethoxyaluminum. A larger part of aluminum
in a triisopropoxyaluminum solution is bound in
uncharged polynuclear complexes, presumably as four-
Thus, NMR findings allowed us to draw the conclu-
sion about a polynuclear structure of tetra- and penta-
coordinated aluminum. The SAXS data point to the
absence of noticeable amounts of particles with a radius
more than 0.6 nm that could defy detection in the 27Al
NMR spectrum because of having too large a line membered rings. Pentacoordinated aluminum com-
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 10 2008

1626
FEDOTOV et al.
h3+J(h), arb. units
35
(‡)
30
25
20
15
10
5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
h, Å–1
DV(R), arb. units
120
(b)
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
R, Å
Fig. 6. (a) SAXS pattern of solution III and (b) the volume distribution of particles in this solution. h = 4π(sinθ)/λ, where 2θ is the
scattering angle and λ is the wavelength.
Content of complexes in solution, %
70
A
SAXS
NMR
60
50
40
30
20
10
0
B
D
3
C
4
5
6
R, Å
Fig. 7. Content of aluminum in the complexes in solution III as probed by SAXS and NMR (see caption to Fig. 5 for designations).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 10 2008

THE STATE OF Al(III) IN ALCOHOL SOLUTIONS OF ALUMINUM ALKOXIDE
1627
plexes were identified (with a 27Al NMR chemical shift
in the range 29–37 ppm).
9. T. Gelbrich, U. Dumichen, and P. Jorchel, Acta Crystal-
logr., Sect. C: Cryst. Struct. Commun. 55 (6), 856
(1999).
Our findings open the possibility of controlling the
properties of fine aluminum oxide.
10. W. E. Rhine, D. P. Eyman, and S. J. Schaner, Polyhedron
18 (6), 905 (1999).
11. J. A. Meese-Markscheffel, R. Weinann, H. Schumann,
and J. W. Gilje, Inorg. Chem. 32 (25), 5894 (1993).
REFERENCES
12. Multinuclear NMR, Ed. by J. Mason (Plenum, New
York, 1987).
1. A. Montoya, P. del Angela, and T. Viveros, J. Mater.
Chem. 11, 944 (2001).
13. A. Abragam, Principles of Nuclear Magnetism (Oxford
Univ. Press, Oxford, 1961; Inostrannaya Literatura,
Moscow, 1963).
2. S. Inamura, H. Miyamoto,Y. Imaida, et al., J. Mater. Sci.
281, 4913 (1994).
14. D. X. Atwood, J. A. Jegier, S. Liu, et al., Organometallics
3. O. P. Krivoruchko, R. A. Buyanov, M. A. Fedotov, and
18 (6), 976 (1999).
L. M. Plyasova, Zh. Neorg. Khim. 23 (7), 1798 (1978).
15. J.-P. Laussac, E. Renee, J. Galy, and J.-P. Laurent, J.
4. H. Haraguchi and S. Fujiwara, J. Phys. Chem. 73 (10),
Coord. Chem. 12 (3), 133 (1983).
3467 (1969).
16. M. A. Fedotov, R. A. Buyanov, and O. P. Krivoruchko,
Izv. Akad. Nauk SSSR, Ser. Khim., No. 10, 2183 (1977).
17. J. W. Akitt, N. N. Greenwood, B. L. Khandelwall, and
G. D. Lester, J. Chem. Soc., Dalton Trans., No. 5, 604
(1972).
18. F. V. Tuzikov, N. A. Tuzikova, and R. V. Gamov, et al,
Poverkhnost’. Rentgen., Sinkhrotron. Neitron. Issled.,
No. 10, 84 (2004).
19. D. I. Svergun and L. A. Feigin, Small-Angle X-Ray and
Neutron Scattering (Nauka, Moscow, 1986) [in Rus-
sian].
5. Yu. A. Buslaev, V. P. Tarasov, S. P. Petrosyants, and
G. A. Kirakosyan, Dokl. Akad. Nauk SSSR 241 (4), 848
(1978).
6. V. V. Kon’shin and B. N. Chernyshev, Zh. Strukt. Khim.
24 (1), 137 (1983).
7. G. W. Stockton and J. S. Martin, Can. J. Chem. 52 (5),
744 (1974).
ˇ
ˇ
z
8. S. Hermanek, O. ri , J. Fusek, et al., J. Chem. Soc.,
K
Perkin Trans., No. 8, 987 (1989).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 10 2008