Synthesis, Characterization, and Structural Determination of the Bimetallic Alkoxide ErAl3(OC3H7i)12

Full text of DOI:10.1021/ic9502685

Inorg. Chem. 1996, 35, 1077-1079
1077
diffraction methods9 and also investigated with the other
methods mentioned.10-12
Synthesis, Characterization, and Structural
Determination of the Bimetallic Alkoxide
ErAl3(OC3H7 )12
Systems of interest to us for thin film preparation of optical
materials are MAl3(OPri)12, with M ) Pr, Nd, Eu, Er, and Yb.
Hydrolysis studies are also currently performed on these systems
as well as on the related compound with M ) Cr, to evaluate
i
M. Wijk,† R. Norrestam,*,‡ M. Nygren,† and
G. Westin†
-
if the shielding Al(OPri)4 shell around Er remains intact and
also to evaluate the kinetics and mechanisms of the hydrolysis.
The present study is focused on the chemical and structural
characterization of ErAl3(OPri)12.
Departments of Inorganic and Structural Chemistry,
Arrhenius Laboratory, Stockholm University,
S-106 91 Stockholm, Sweden
Experimental Section
Preparation. All preparations and the mounting of crystals for the
X-ray diffraction data collection were performed in a glovebox
containing dry, oxygen-free argon atmosphere. The isopropyl alcohol
was distilled over CaH2. The toluene was dried over freshly cut, thin
slices of sodium. Commercial ErCl3 (Strem Chemicals) and Al(OPri)3
(Sigma) were used. Typically, 0.500 g (12.8 mmol) of potassium was
dissolved in 15 mL isopropyl alcohol and 2.612 g (12.8 mmol) of Al-
(OPri)3 was added. After 8 h, 1.166 g (4.26 mmol) ErCl3 and ca 35
mL of toluene were added under stirring, whereafter the reaction was
allowed to proceed for 2 days at room temperature, yielding a pink
solution and a white precipitate (KCl). The solution was removed and
evaporated to dryness. In general, a small amount of K-Al-alkoxide
was found together with the ErAl3(OPri)12. The dried alkoxide was
dissolved in toluene leaving a precipitate of the remaining potassium
aluminium isopropoxide. The pink liquid was removed and isopropyl
alcohol was added and the liquid was then evaporated in steps until a
viscous solution was obtained, from which the crystals grew.
The pink crystals were obtained in a yield of 92% and SEM-EDS
analysis revealed no K or Cl. The solubility of these crystals in a 70:
30 (by volumes) mixture of HOPri and toluene was 0.34 M, while in
HOPri it was much lower, 0.047 M. In toluene the solubility was 0.62
M and in Si(OEt)4 it was ca 0.26 M. The melting point of the
compound was 120-2 °C, and the compound is stable up to at least
140 °C and does not decompose for months in solution.
ReceiVed March 8, 1995
Introduction
In recent, years the demand for new materials with specific
properties has increased dramatically, and new preparation routes
have therefor been developed. The use of metal alkoxides as
precursors for ceramics and thin films is a growing field,
especially for heterometallic alkoxides, since they provide a
means of better control of the stoichiometry, simpler equipment
for manufacture and lower cost. Quite a few heterometallic
alkoxides are known, but only a minor part of them are
structurally characterized.1-6 One possible field of application
for the mixed metal alkoxides containing Pr, Nd, Eu, Er, and
Yb, is for preparing high purity optical materials. The formation
of Er-O clusters, which often occur with solid state synthesis
and sol-gel processing of Er3+ salts, leads to a reduction of
the optical activity, and thus the distance between the dopant
ions must be kept sufficiently large. The reduction of the optical
activity is due to energy transfer between neighboring excited
ions, yielding nonradiative relaxation.7
Characterization. The viscous solution and the crystals were
hydrolyzed and dried, and the overall metal composition was determined
in a scanning electron microscope (SEM, Jeol 820) equipped for energy
dispersive analysis of X-ray spectra (LINK AN 10000). FT-IR spectra
were recorded on a Bruker IFS55 spectrometer. The solid samples
were investigated as KBr tablets and the solutions in a 0.1 mm (mid-
IR) or 5 mm (near-IR) quartz cell. UV/vis spectra were recorded in
Such clustering might be avoided by preparing suitable
precursors for a sol-gel process. If an encapsulation of the Er
with optically silent alkoxy derivatives, e.g. Al, Ti, Zr, Ta, and
Nb, can be made and be preserved during the hydrolysis step,
clustering can be avoided and higher doping levels can be
achieved. The optically silent alkoxy deriviatives surrounding
the Er3+ are more easily dispersed in the glass precursor of the
matrix material (SiO2-TiO2). The bimetallic ErAl (1:3) alkox-
ide is already known, as well as the other corresponding LnAl3
alkoxides.1,3-6 Cryoscopy, mass spectrometry, and NMR stud-
ies have been used to outline the structures of the LnAl3(OPri)12
compounds (Ln ) lanthanide atom). In these studies it is
proposed that all the LnAl3(OPri)12 compounds have the same
structure, in which Ln is surrounded by three Al(OPri)4- units,
giving a six-coordinated central atom. A related compound,
AlAl3(OPri)12, has been structurally determined by single crystal
the range 200-900 nm, with a Philips PU8700 spectrophotometer for
i
0.25 M solutions of ErAl
3(OPr )12 in a 70:30 mixture of HOPri and
toluene in 5 mm quartz cells. A Gallenkamp solid block melting point
apparatus was used to determine the melting point.
Structure Determination. A few selected crystals were mounted
into glass capillaries (φ ) 0.7 mm) that were melt sealed in the
glovebox. Preliminary single-crystal X-ray diffractometer investigations
of the finally selected crystal, using Mo KR radiation, indicated an
orthorhombic space group symmetry, P212121. The crystal had a minor
twin component, but as the final results show, the effects from twinning
were negligible. Unit cell parameters were determined and refined from
the θ-values of 19 accurately centered reflections, as a ) 13.150(1), b
) 17.404(2), and c ) 23.158(3) Å. Single-crystal X-ray diffraction
data were collected at room temperature (21 °C) on a Siemens P4/RA
diffractometer. Absorption correction was performed for effects of the
crystal shape, but the effects from the capillary was neglected. Data
were also corrected for background, Lorentz, and polarization effects.
Preliminary erbium atom positions were obtained by conventional
heavy atom techniques. The remaining non-hydrogen atomic positions
† Department of Inorganic Chemistry.
‡ Department of Structural Chemistry.
(1) Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P. Metal Alkoxides;
Academic Press: London, 1986.
(2) Caulton, K. G.; Hubert-Pfalzgraf, L. G. Chem. ReV. 1990, 90, 969-
95.
(3) Shiner, V. J., Jr.; Whittaker, D.; Fernandez, V. P. J. Am. Chem. Soc.
1963, 85, 2318-22.
(9) Turova, N. Ya.; Kuzonov, V. A.; Yanovskii, A. I.; Bokii, N. G.;
Struchkov, Yu. T.; Taenopolskii, B. L. J. Inorg. Nucl. Chem. 1979,
41, 5-11.
(4) Mehrotra, R. C.; Agrawal, M. M.; Mehrotra, A. Synth. Inorg. Met.
Org. Chem. 1973, 3(2), 181-91.
(5) Oliver, J. G.; Worrall, I. J. J. Chem. Soc. 1970, 845-8.
(6) Mehrotra, R. C.; Mehrotra, A. Proc. Indian Natl. Sci. Acad., Part A
1974, 40, 215-25.
(7) Jerskorzynska, B.; Nilsson, J.; Blixt, P. Res. Optics. 1993, 16-7.
(8) Arvidsson, G.; Henriksson, P. Res. Optics. 1993, 24-5.
(10) Kleinschmidt, D. C.; Shiner, V. J., Jr.; Whittaker, D. J. Org. Chem.
1973, 38(19), 3334-7.
(11) Shiner, V. J., Jr.; Whittaker, D. J. Am. Chem. Soc. 1969, 91, 394-8.
(12) Fieggen, W.; Gerding, H.; Nibbering, N. M. M. Recl. TraV. Chim.
Pays-Bas 1968, 87, 377-83.
0020-1669/96/1335-1077$12.00/0 © 1996 American Chemical Society

1078 Inorganic Chemistry, Vol. 35, No. 4, 1996
Notes
Table 1. Crystallographic Data for the Structural Investigation of
a
ErAl3(OPri)12
formula: ErAl3(OPri)12
fw ) 957.26 g/mol
volume ) 5318.2(13) Å3
Z ) 4
Space group: P212121 (No. 19)
calcd density ) 1.196 g/cm3
Unit cell dimensions: a ) 13.150(1) Å µ ) 1.69 mm-1
b ) 17.464(2) Å
c ) 23.158(3) Å
final R ) 0.053
final Rw ) 0.064
a Definitions: R ) ∑[|F(obs) - F(calc)|]/∑[F(obs)] and Rw
)
∑[|(F(obs) - F(calc))|w]/∑[F(obs)w].
Table 2. Selected Intramolecular Distances (Å) and Angles (deg),
with Esd’s, for ErAl3(OPri)12
Distances
Er1-O1
Er1-O2
Er1-O3
Er1-O5
Er1-O6
Er1-O11
Al1-O2
Al1-O5
Al1-O7
Al1-O12
2.24(2)
2.28(2)
2.20(2)
2.23(2)
2.25(2)
2.24(2)
1.80(2)
1.79(2)
1.68(3)
1.66(3)
Al2-O3
Al2-O6
Al2-O9
Al2-O10
Al3-O1
Al3-O4
Al3-O8
Al3-O11
1.79(2)
1.75(2)
1.67(3)
1.66(3)
1.78(2)
1.70(3)
1.65(3)
1.81(2)
Figure 1. IR spectra of ErAl3(OPri)12 in the solid state (upper curve)
and dissolved in isopropyl alcohol:toluene (70:30) solvent (lower curve).
The plot has been removed where the solvent interactions obscure the
spectrum.
Table 3. UV/Vis Bond Maxima for a 0.25 M Solution of
4
ErAl3(OPri)12 and Ascribed Transitions from the Ground State I15/2
of the Er3+ Ion
Angles
obsd band maxima
band maxima18
O1-Er-O2
O1-Er-O3
O1-Er-O5
O1-Er-O6
O1-Er-O11
O2-Er-O3
O2-Er-O5
O2-Er-O6
O2-Er-O11
O3-Er-O5
O3-Er-O6
O3-Er-O11
O5-Er-O6
O5-Er-O11
O6-Er-O11
98.4(6)
96.6(6)
157.8(7)
104.7(6)
67.0(6)
98.2(6)
67.0(6)
153.2(6)
104.5(6)
101.9(6)
66.2(6)
153.6(6)
94.0(6)
99.3(7)
96.9(6)
O2-Al1-O5
O2-Al1-O7
O2-Al1-O12
O5-Al1-O7
O5-Al1-O12
O7-Al1-O12
O3-Al2-O6
O3-Al2-O9
O3-Al2-O10
O6-Al2-O9
O6-Al2-O10
O9-Al2-O10
O1-Al3-O4
O1-Al3-O8
O1-Al3-O11
O4-Al3-O8
O4-Al3-O11
O8-Al3-O11
87.7(8)
transition
(cm-1 × 10-3
)
(cm-1 × 10-3
)
110.2(11)
110.4(13)
113.4(11)
111.3(11)
119.4(12)
86.4(9)
110.1(11)
112.2(11)
111.2(12)
111.8(12)
120.1(13)
112.2(12)
113.4(12)
87.2(8)
2G7/2
27.94
27.62
27.32
26.43
24.40
22.48
22.10
20.63
19.21
18.36
15.36
12.64
10.26
6.57
27.82
27.58
27.32
26.38
24.48
22.45
22.18
20.49
19.01
18.30
15.18
12.35
10.12
6.49
2K15/2
4G9/2
4G11/2
(2G,4F)9/2
4F3/2
4F5/2,
4F7/2
(2H,4G)11/2
4S3/2
4F9/2
4I9/2
4I11/2
117.6(14)
111.8(11)
110.7(12)
4I13/2
Results and Discussion
were found from subsequent calculations of difference electron density
(∆F) maps. The hydrogen atoms were positioned by assuming ideal
geometry of the alkyl groups and the positions were refined by
constraining the C-H distances to 1.0 Å. Due to the large thermal
vibrations experienced and the limited accuracy of the isopropoxy group
geometries, the O-CR, O-Câ, and CR-Câ bonds were softly con-
strained. The very large differences in CR-Câ distances from the ideal
bond length are most likely due to the large thermal vibrations. In the
final refinement all the metal and oxygen atoms were allowed to vibrate
anisotropically, while the carbon and hydrogen atoms were kept
isotropic. To evaluate whether or not the structure model obtained
describes the correct enantiomer, the method of Rogers was applied,13
which refine a parameter that multiplies the imaginary part of the atomic
scattering factor. The parameter value +0.92(11) obtained shows that
the correct enantiomer is described by the structure model.
Least-squares refinements of the structural model yielded an R value
of 0.053 (Rw ) 0.064): see Table 1 for further details. The final atomic
coordinates with thermal parameters are given as Supporting Informa-
tion; bond distances and selected bond angles are given in Table 2.
The atomic scattering factors used were those for neutral atoms given
in ref 14. The SHELXTL program package15 was used for the
crystallographic calculations. For the analysis of molecular pseudo-
symmetries, routines developed with the MathCAD (Mathsoft Inc. 1994)
were used.
IR spectra of the compound were recorded both for the
crystalline phase and for a 0.25 M in HOPri:toluene solution
(70:30 by volumes). Figure 1 shows the two spectra, covering
the region 1170 - 370 cm-1, which contains the M-O and C-O
vibrations. There is a striking resemblance between the two
spectra, suggesting that the structure of the molecule is very
similar in the solid and solution states. The bands in the region
<800 cm--1 are assigned to Al-O-Er and Al-O vibration
modes, while the bands >800 cm-1 can be assigned to different
C-O and C-C modes originating from bridging and terminal
isopropoxy groups and from the organic part of the ligands.1,16,17
UV/vis and NIR spectra were recorded for a 0.25 M solution
in HOPri:toluene (70:30). All bands are multiplets and the band
maxima and the proposed transitions for each band are given
in Table 3. The proposed transitions follow the scheme outlined
in ref 18.
The obtained molecular structure is shown in Figure 2,
together with the atomic labeling used for the metal and oxygen
atoms. The structure determination reveals a molecular formula
of ErAl3(OPri)12, as proposed by Shiner et al.,3 with the Er being
(16) Barraclough, C. G.; Bradley, D. C.; Lewis, J.; Thomas, I. M. J. Chem.
Soc. 1961, 2601-5.
(13) Rogers, D. Acta Crystallogr. 1981, A37, 734-41.
(14) International Tables of X-ray Crystallography; Kynoch: Birmingham,
U.K., 1974; Vol. IV.
(15) SHELXTL PC, release 4.1. Siemens Analytical X-ray Instruments Inc.,
1990
(17) Bell, J. V.; Heisler, J.; Tannenbaum, H.; Goldenson, J. Anal. Chem.
1953, 25(11), 1720-4.
(18) Yatsimirskii, K. B.; Davidenko, N. K. Coord. Chem. ReV. 1979, 27,
223-73.

Notes
Inorganic Chemistry, Vol. 35, No. 4, 1996 1079
O-C) of the bridging isopropoxy groups have an average value
of 128.2°, while the bond angles of the terminal isopropoxy
groups differ considerably, from 134.3° to 150.8° (average
143.7°). As expected, increasing thermal vibrations are found
at the isopropoxy ends, especially so for the terminal groups.
The main features of the structure of ErAl3(OPri)12 have
previously been proposed for most of the Ln-Al-iso-
propoxides.1,3-6 The structure of AlAl3(OPri)12 has been
determined by single-crystal diffraction methods,9 and the
observed molecular geometry of the compound is related to that
of ErAl3(OPri)12 with respect to that both have an approximate
C3 symmetry. However, the molecular confirmations differ;
i.e. in the first case the central Al has an almost ideal octahedral
coordination, while the title compound has a distorted trigonal
prismatic coordination around the Er. Taking into account the
similarities in stereochemical behaviour and in the size of the
ionic radii of the central ion, most of the Ln-Al-isopropoxides
ought to have a coordination figure around the central ion similar
to that of ErAl3(OPri)12, rather than to that of AlAl3(OPri)12.
The ErAl3(OPri)12 molecule, having a single Er encapsulated
by optically silent, oxygen bridged Al atoms, fulfils the
requirements given above for a precursor to be used in
connection with preparation of optical materials. However, the
molecule must also retain its structure upon dissolution and upon
hydrolysis (cf. above). Narula20 has investigated the hydrolysis
pathway of MAl3(OPri)12 with M ) Ce and La at -78 °C in
organic solvents with up to 12 H2O/MAl3(OPri)12 molecules.
On the basis of 1H NMR data, he suggests that the main features
of the alkoxide structure persist hydrolysis for addition of 1-6
molecules of H2O per MAl3(OPri)12 unit by incorporation of
OH bridges instead of isopropoxy bridges. Preliminary FTIR
studies made by us indicate that ErAl3(OPri)12 retains its
structure upon hydrolysis. Studies of the hydrolysis pathway
of ErAl3(OPri)12 and studies concerning the use of this molecule
as a precursor molecule in connection with preparation of Er-
based wave guides are in progress.
Figure 2. Structure of ErAl3(OPri)12, showing 50% probability thermal
ellipsoids, with the atomic numbering scheme used. For clarity only
the metal and oxygen atoms are included.
coordinated by the oxygen atoms of six bridging isopropoxy
groups. Taking the coordination around the oxygen atoms into
account, the molecular formula can be written ErAl3(µ2-OPri)6-
(OPri)6.
The molecule possesses a 3-fold rotation pseudo-symmetry,
i.e. a noncrystallographically imposed C3 point symmetry, with
the rotation axis perpendicular to the plane through the Al atoms.
The deviation from this rotation symmetry is very minor, as
the largest deviation of any metal or oxygen atoms from an
idealized average structure model with exactly 3-fold symmetry
is only 0.13 Å. The Er deviates only 0.03 Å from the centroid
of the molecule. The coordination geometry around the Er can
be anticipated as a trigonal prism, distorted halfway toward an
octahedron. The distortion, described by the angle relating the
mutual orientation of the two parallel triangular faces of the
prism, is 31°. A 60° distortion angle would give an octahedral
coordination. The Al atoms are coordinated by four isopropoxy
groups, giving the coordination of a slightly distorted tetrahe-
dron. The coordination tetrahedra are rotated by 24° from the
direction of the 3-fold axis.
Acknowledgment. This research was supported financially
by the Swedish National Science Research Council.
The bond distances of Er-O are fairly equal, ranging from
2.201 to 2.283 Å, with an average value of 2.244 Å. For the
Al-O tetrahedra the bond lengths can be divided into two
groups. The longer distances are seen for the bridging µ2-O,
with an average length of 1.792 Å. The other group consists
of Al-O bonds of terminal isopropoxy groups, with an average
bond length of 1.674 Å. The bond distance distribution and
calculated bond valence sums19 for the metal atoms indicates
that all the metal atoms are trivalent. The bond angles (M-
Supporting Information Available: Tables of crystallographic
data, positional and anisotropic thermal parameters for the non-hydrogen
atoms, positional and isotropic thermal parameters for the hydrogen
atoms, and calculated bond distances and angles (5 pages). Ordering
information is given on any current masthead page.
IC9502685
(19) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244-7.
(20) Narula, C. K. Mater. Res. Soc. Symp. Proc. 1992, 271, 181-6.