Synthesis and Structure of Alkoxy- and (Aryloxy)alanes. Observation of a Ring-Opening Reaction Involving Tetrahydrofuran

Article abstract of DOI:10.1021/ic970171d

Dihydrogen elimination from aluminum hydrides and alcohols was used to produce new alkoxide and aryloxide complexes. Dimethylethylamine-alane (DMEAA) reacted with 2 equiv of 2,6-diisopropylphenol (DIPH) in THF at -78 °C to give (DIP)₂:A1H·2THF (1) in 94% yield. It was characterized by NMR, IR, MS. and X-ray crystallography and found to be trigonal bipyramidal with both axial sites occupied by THF. Crystal data for 1: C₃₂H₅₁AlO₄, monoclinic, C2/c, a = 21.37(1) A?, b = 9.324(3) A?, c = 15.30(1) A?, β = 94.16(5)°, V = 3040(5)A?³, Z = 4. DMEAA reacted with 2 equiv of triphenylcarbinol, Ph₃COH, in THF at -78 °C to give (Ph₃CO)₂AIH· THF (2) in 42% yield. The lower coordination number was attributed to the larger cone angle of the ligand. Crystal data for 2: C₄₂H₃₉AlO₃, triclinic, P1?, a = 8.482(1) A?, b = 9.067(1) A?, c = 23.59(1)°, α = 90.919(1)°, β = 92.624(1)°, γ = 116.093(1)°, V = 1626.2(2) A?³, Z = 2. Three equivalents of DIPH in THF reacted with DMEAA at -78 °C to form the unexpected compound (DIP)₃(THF)(NMe₂Et)Al (3) in 89% yield. Both solution spectroscopic and X-ray crystallographic data established that THF underwent a ring-opening reaction. Crystal data for 3: C₄₄H₇₀AlNO₄, monoclinic, Cc, a = 23.50(1) A?, b = 10.81(1) A?, c= 19.37(1) A?, β= 122.25(3)°, V = 4162(2) A?, Z = 4.

Full text of DOI:10.1021/ic970171d

4094
Inorg. Chem. 1997, 36, 4094-4098
Synthesis and Structure of Alkoxy- and (Aryloxy)alanes. Observation of a Ring-Opening
Reaction Involving Tetrahydrofuran
John P. Campbell and Wayne L. Gladfelter*
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
ReceiVed February 13, 1997X
Dihydrogen elimination from aluminum hydrides and alcohols was used to produce new alkoxide and aryloxide
complexes. Dimethylethylamine-alane (DMEAA) reacted with 2 equiv of 2,6-diisopropylphenol (DIPH) in THF
at -78 °C to give (DIP)2AlH‚2THF (1) in 94% yield. It was characterized by NMR, IR, MS, and X-ray
crystallography and found to be trigonal bipyramidal with both axial sites occupied by THF. Crystal data for 1:
C32H51AlO4, monoclinic, C2/c, a ) 21.37(1) Å, b ) 9.324(3) Å, c ) 15.30(1) Å, â ) 94.16(5)°, V ) 3040(5)Å3,
Z ) 4. DMEAA reacted with 2 equiv of triphenylcarbinol, Ph3COH, in THF at -78 °C to give (Ph3CO)2AlH‚
THF (2) in 42% yield. The lower coordination number was attributed to the larger cone angle of the ligand.
Crystal data for 2: C42H39AlO3, triclinic, P1h, a ) 8.482(1) Å, b ) 9.067(1) Å, c ) 23.59(1) Å, R ) 90.919(1)°,
â ) 92.624(1)°, γ ) 116.093(1)°, V ) 1626.2(2) Å3, Z ) 2. Three equivalents of DIPH in THF reacted with
DMEAA at -78 °C to form the unexpected compound (DIP)3(THF)(NMe2Et)Al (3) in 89% yield. Both solution
spectroscopic and X-ray crystallographic data established that THF underwent a ring-opening reaction. Crystal
data for 3: C44H70AlNO4, monoclinic, Cc, a ) 23.50(1) Å, b ) 10.81(1) Å, c ) 19.37(1) Å, â ) 122.25(3)°, V
) 4162(2) Å3, Z ) 4.
Introduction
strategy involves using ether solvents to saturate the remaining
coordination sites and prevent the formation of rings, and
clusters or polymers.5,6 An impressive example of this involves
the isolation and structural characterization of AlH3‚2THF and
[AlH3‚THF]2, shown below.7
Aluminum hydrides have attracted recent interest as precur-
sors to solid state materials.1 Two of the molecules that are
under serious consideration for use in the chemical vapor
deposition of aluminum, dimethylethylamine-alane (DMEAA)
and dimethylaluminum hydride (DMAH), contain hydride
ligands.1,2 Much effort is underway to determine the formula
and structure of the surface-bound species on silica, silicon,
metals, and semiconductors. On silica or oxidized silicon
surfaces, free and/or hydrogen-bonded hydroxyl groups are
available to react with incoming molecules of DMEAA3 or
DMAH. This paper summarizes our use of alcohols to prepare
structural and spectroscopic models of the species formed upon
reaction of DMEAA with surface O-H groups.
Structurally characterized alkoxy- and (aryloxy)aluminum
hydrides are rare. A significant obstacle to structural charac-
terization of these compounds is the difficulty in obtaining an
isolable product. Alkoxyalanes tend to form polymeric ag-
gregates of uncertain stoichiometry or intractable solid masses.
Bulky ligands, such as 2,6-di-tert-butyl-4-methylphenoxo (BHT),
have been used effectively to mitigate this problem.4 Another
Experimental Section
General Procedures. All reactions were performed under an inert
atmosphere of Ar or nitrogen using standard Schlenk line techniques,
or in a nitrogen-filled glovebox. Infrared spectra were obtained using
a Mattson Polaris Mod 8 FTIR spectrometer, and NMR spectra were
taken on a Bruker A200 200 MHz spectrometer. A Sciex API III mass
spectrometer (Sciex, Thornhill, Ontario, Canada) was used to obtain
electrospray mass spectra. Elemental analyses were performed by
MHW Laboratories, Phoenix, AZ. Melting points (uncorrected) were
performed in capillaries loaded in the glovebox under nitrogen, and
sealed with silicone grease. Solvents were freshly distilled from sodium
benzophenone ketyl under nitrogen immediately before use. 2,6-
Diisopropylphenol, triphenylcarbinol, and dimethylethylamine were
obtained from Aldrich and used as received. Dimethylethylamine-
alane, DMEAA, was synthesized by one of the two literature
procedures.2,8
X Abstract published in AdVance ACS Abstracts, August 1, 1997.
(1) Simmonds, M. G.; Gladfelter, W. L. In The Chemistry of Metal CVD;
Kodas, T. T., Hampden-Smith, M. J., Eds.; VCH Publishers: New
York, 1994; Vol. 45.
(2) Simmonds, M. G.; Phillips, E. C.; Hwang, J.-W.; Gladfelter, W. L.
Chemtronics 1991, 5, 155.
(5) Wiberg, E.; Amberger, E. Hydrides of the Elements of Main Groups
I-IV; Elsevier: Amsterdam, 1971; Vol. 785.
(6) Chemistry of Aluminum, Gallium, Indium, and Thallium; Downs, A.
J., Ed.; Blackie: Glasgow, 1993.
(3) Elms, F. M.; Lamb, R. N.; Pigram, P. J.; Gardiner, M. G.; Wood, B.
J.; Raston, C. L. J. Chem. Soc., Chem. Commun. 1992, 1423.
(4) Healy, M. D.; Mason, M. R.; Gravelle, P. W.; Bott, S. G.; Barron, A.
R. J. Chem. Soc., Dalton Trans. 1993, 441.
(7) Gorrell, I. B.; Hitchcock, P. B.; Smith, J. D. J. Chem. Soc., Chem.
Commun. 1993, 189.
(8) Frigo, D. M.; van Eijden, G. J. M.; Reuvers, P. J.; Smit, C. J. Chem.
Mater. 1994, 6, 190.
S0020-1669(97)00171-7 CCC: $14.00 © 1997 American Chemical Society

Alkoxy- and (Aryloxy)alanes
Inorganic Chemistry, Vol. 36, No. 18, 1997 4095
Table 1. Crystallographic Parameters for 1-3
Bis(2,6-diisopropylphenolato)aluminum Hydride-Bis(tetrahy-
drofuran) ((DIP)2AlH‚2THF (1)). A 50 mL three-neck round bottom
flask was fitted with a rubber septum, a gas adapter, and a 30 mL
pressure-equalizing dropping funnel and was connected to the Schlenk
line. The setup was carefully flame-dried under a purge of Ar and
allowed to cool. The reaction flask was charged with 15 mL of freshly
distilled THF and 0.71 mL (5.36 mmol) of DMEAA. The addition
funnel was filled with 10 mL of freshly distilled THF and 2.0 mL (10.8
mmol) of 2,6-diisopropylphenol. The reaction flask was cooled using
a dry ice/acetone bath, and the phenol was added dropwise, over 30
min. The cold bath was allowed to warm to room temperature. The
reaction mixture was stirred under Ar for 3 days, after which the solvent
was removed under vacuum, yielding 2.65 g (5.03 mmol, 94%) of white
powdered crystalline (DIP)2AlH‚2THF, melting point 194 °C (dec.).
Anal. Calcd for C32H51AlO4: C, 72.97; H, 9.76. Found: C, 72.8; H,
9.63.
1
2
3
formula
formula wt
space group
a (Å)
b (Å)
c (Å)
C32H51AlO4
526.73
C2/c (No. 15) P1h (No. 2)
C42H39AlO3 C44H70AlNO4
618.71
703.99
Cc (No. 9)
23.50(1)
10.81(1)
19.37(1)
21.37(1)
9.324(3)
15.30(1)
8.482(1)
9.067(1)
23.59(1)
90.919(1)
92.624(1)
116.093(1)
1626.2(2)
2
R (deg)
â (deg)
γ (deg)
vol (Å3)
Z
94.16(5)
122.25(3)
3040(5)
4
4162(2)
4
n/a
F
obs (g/cm3)
F
n/a
1.151
0.710 73
(Mo KR)
0.95
n/a
1.264
0.710 73
(Mo KR)
1.03
calc (g/cm3)
1.123
λ (Å)
0.710 73
(Mo KR)
0.89
SMART
-150(2)
0.0645a
0.1801c
IR (THF): υAl-H ) 1803 cm-1
.
1H NMR (δ ppm, C6D6):
µR(Mo KR) (cm-1
diffractometer
T (°C)
R1 (I > 2σ(I))
R2
)
1.313,1.344 (CH3, d, 12H), 3.58 (OCH2, broad triplet, 4H), 3.73
((CH3)2CH, multiplet, 2H), 7.15 (CH aromatic, m, 3H). Ion spray MS
(m/z, negative ion): 388.3 (100, (DIP)2AlH‚2THF + Li), 355.2 (92.4,
(THF)2Al + Li), 210.2 (40.0, (DIP)Al + Li).
CAD-4
-100(2)
0.061a
0.061b
SMART
-100(2)
0.0913a
0.2295c
Bis(triphenylcarbinolato)aluminum Hydride-Tetrahydrofuran
((Ph3CO)2AlH‚THF (2)). A 50 mL three-neck round bottom flask
was fitted with a rubber septum, a gas adapter, and a 30 mL pressure-
equalizing dropping funnel and was connected to the Schlenk line. The
setup was carefully flame-dried under a purge of Ar and allowed to
cool. The reaction flask was charged with 20 mL of freshly distilled
THF and 1.0 mL (7.56 mmol) of DMEAA. The addition funnel was
filled with 10 mL of freshly distilled THF and 1.96 g (7.53 mmol) of
triphenylcarbinol. The reaction flask was cooled in a dry ice/acetone
bath, and the solution was added dropwise, over 30 min. The cold
bath was allowed to warm to room temperature and removed. A white
precipitate formed overnight. The reaction mixture was allowed to
stir under Ar for 3 days, and the solvent was removed under vacuum.
The remaining white residue was washed with three 20 mL portions
of toluene, then redissolved in THF, and filtered on a 10-20 µ frit.
The solvent was removed under vacuum to yield 0.98 g (1.58 mmol,
42%) of a white solid, (Ph3CO)2AlH‚THF, melting point 155 °C. A
satisfactory elemental analysis could not be obtained.
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) (∑w(|Fo| - |Fc|)2/∑w(Fo)2]1/2
.
2
2
c R2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
search routine collected 25 reflections at low angles, which provided
an initial cell. A search out the axes provided higher angle reflections
between 20° and 52° in 2θ, which were carefully centered and refined
to give the final cell constants and orientation matrix. At this time, a
check was performed for a higher symmetry unit cell; none was found.
ω scans were collected in one shell to 24° using a prescan speed of
16.48°/min, and from 24 to 26° at 4.1°/min. Three strong reflections
were chosen as intensity checks and monitored every hour during data
collection. No decay was observed. Six reflections between 19° and
52° in 2θ were chosen as orientation and reorientation checks, and
monitored every 400 reflections during data collection. Crystallographic
parameters are given in Table 1.
The data were processed and the structure solved and refined using
the TEXSAN 5.0 series of programs. Systematic absences and statistics
revealed the space group to be C2/c (No. 11). The aluminum atom
was located by the direct methods program MITHRIL, and the other
non-hydrogen atoms were found by Fourier techniques and the direct
methods program DIRDIF. The hydride was located in the difference
map and refined isotropically. After a full isotropic refinement, the
IR (THF): υAl-H ) 1850 cm-1 1H NMR (δ, C6D6): 0.85 (OCH2,
.
4H, broad triplet), 3.27 (CH2, 4H, broad triplet), 7.15 (m- and p-H,
18H, multiplet), 7.63 (o-H, d, J ) 6 Hz, 12H). No MS could be
obtained.
Tris(2,6-diisopropylphenolato)(4-(dimethylethylammonium)bu-
tanato)aluminum ((DIP)3(O(CH2)4NMe2Et)Al (3)). A 50 mL three-
neck round bottom flask was fitted with a rubber septum, a gas adapter,
and a 30 mL pressure-equalizing dropping funnel and was connected
to the Schlenk line. The setup was carefully flame-dried under a purge
of Ar and allowed to cool. The reaction flask was charged with 20
mL of freshly distilled THF and 0.95 mL (7.18 mmol) of DMEAA.
The addition funnel was filled with 10 mL of freshly distilled THF
and 4 mL (21.6 mmol) of 2,6-diisopropylphenol. The reaction flask
was cooled in a dry ice/acetone bath, and the phenol was added
dropwise over 30 min. The cold bath was allowed to warm to room
temperature and removed, and the reaction mixture was allowed to stir
under Ar for 5 days. The solvent was then removed in vacuum, yielding
4.51 g (6.41 mmol, 89%) of an off-white solid, (DIP)3(O(CH2)4NMe2-
Et)Al, mp 150-160 °C. Anal. Calcd for C44H70AlNO4: C, 75.07; H,
10.02; N, 1.99. Found: C, 76.05; H, 10.38; N, 1.97.
1H NMR (δ, C6D6): 1.015 (CH3, DIP, 36H), 1.20 (NCH2CH3, 6H),
1.546 (CH2CH2CH2, 4H), 2.474 (NCH3, 6H), 2.770 (NCH2CH3, 6H),
3.146 (NCH2CH2, 2H), 3.601 ((CH3)2CH, 6H), 3.998 (OCH2, THF,
2H), 6.589 (p-CH, 3H), 6.802 (m-CH, 6H).
MS (FABMS, 10 V, m-nitrobenzyl alcohol): m/z ) 710.4 ((DIP)3-
(O(CH2)4NMe2Et)Al + Li).
nonhydridic hydrogens were placed in idealized positions with dCH
)
0.95 Å and B values 20% greater than the Beq of the carbon to which
they were bonded. Their positions were updated every cycle of least
squares using a riding model; they were not refined but were included
in the structure factor calculation. After isotropic refinement of this
model, redundant data were averaged. Anisotropic refinement, per-
formed using full-matrix least squares on F of all non-hydrogen atoms,
converged to a final R ) 0.061 (2σ). An empirical absorption correction
was applied after isotropic refinement using the program DIFABS.
Scattering factors were taken from the usual sources, and the effects
of anomalous dispersion were included for the nonhydrogen atoms.9,10
Table 2 lists selected atomic coordinates.
Crystals of (Ph3CO)2AlH‚THF were grown by layering a 0.050 g
portion of pure product in 1.55 mL of THF with 20 mL of pentane.
Colorless prisms formed within 4 days; however, the quality of these
were poor. After preliminary examination of three crystals a data set
was collected for two of these (one using the CAD-4 and one using
the SMART system). Only the data set collected on the SMART
system (graphite-monochromatized Mo KR radiation with λ ) 0.710 73
Å) was suitable for refinement. This crystal (0.50 × 0.45 × 0.37 mm)
had been mounted on the tip of a glass fiber with STP oil. It was
transferred to the goniometer of a Siemens SMART CCD system and
cooled to -100 °C with a Siemens LT-2 low-temperature device.
X-ray Crystallography. Crystals of (DIP)2AlH‚2THF were grown
from THF at -15 °C, giving colorless blocks. A suitable crystal (0.57
× 0.52 × 0.52 mm) was removed from a Schlenk flask and quickly
covered with oil, transferred to the tip of a glass fiber, and mounted
on the goniometer of the Enraf-Nonius CAD-4 (graphite-monochro-
matized Mo KR radiation with λ ) 0.710 73 Å). It was cooled to
-100 °C with an Enraf-Nonius FR558SH low-temperature device. The
(9) Cromer, D. T.; W., J. T. In International Tables for X-Ray Crystal-
lography, The Kynoch Press: Birmingham, England, 1974; Vol. IV;
Table 2.
(10) Cromer, D. T. In International Tables for X-Ray Crystallography; The
Kynoch Press: Birmingham, England, 1974; Vol. IV; Table 2.

4096 Inorganic Chemistry, Vol. 36, No. 18, 1997
Campbell and Gladfelter
Table 2. Selected Atomic Coordinates (×104) for compounds 1,
2, and 3
the DMEAA dissolved in THF is converted into AlH3‚2THF.
Pure DMEAA in the gas phase exhibits an Al-H stretching
vibration at 1790 cm-1 2
. The THF solutions of DMEAA exhibit
(DIP)2AlH‚2THF (1)
a νAl-H at 1734 cm-1, similar to the value of 1727 cm-1 reported
for THF solutions of AlH3‚2THF.7 Reactions of DIPH with
THF solutions of DMEAA (eq 1) result in either 1 or 3,
depending upon the stoichiometry. 1:1 DIPH/DMEAA gives
the compound (DIP)2AlH‚2THF in low yield (40%), whereas a
2:1 ratio improves this to 94%. Compound 1 is air and moisture
Al1
O1
O2
C1A
H1
5000
3113(1)
4089(2)
3171(2)
4378(3)
142(4)
2500
4542(1)
5678(1)
3925(1)
5000
3195(1)
3503(1)
3260(2)
2500
(Ph3CO)2AlH‚THF (2)
Al(1)
O(1)
O(2)
O(3)
C(1)
C(2)
H(1)
6252(2)
3932(2)
3440(4)
2966(4)
2758(5)
3332(6)
2979(6)
5673(50)
2480(1)
1970(1)
3087(1)
2269(2)
1403(2)
3632(2)
2524(17)
7295(5)
6308(5)
3840(5)
7683(6)
7034(7)
6398(53)
Me2EtN‚AlH3 + 2DIPH -78 °C8
THF
(DIP)2AlH‚2THF + Me2EtN + 2H2 (1)
1
sensitive. The 1H NMR spectrum in C6D6 shows a shift of the
methyl doublet downfield to 1.21 ppm from 1.14 ppm in the
free ligand. The methine protons are shifted from 2.91 ppm in
the free ligand to 3.57 ppm in the product. The hydride signal
could not be observed due to the quadrupolar broadening by
the Al nucleus. The R-methylene protons of the coordinated
THF are shifted to 3.58 ppm from the 3.68 ppm observed in
the free molecule. A unique resonance due to the the â-protons
of the coordinated THF is not observed, but the integration
establishes that it appears at the same chemical shift as the
methyl groups of the isopropyl substituents. No evidence is
found for (DIP)AlH2‚THF, the expected inital product in eq 3,
suggesting that the second H2 elimination is faster than the first.
The trend of increasing reactivity of the Al-H groups with
increasing substitution is broken once two hydrides are replaced,
possibly due to the steric bulk of the phenol.
(DIP)3[Me2EtN(CH2)4O]Al (3)
Al(1)
O(1)
O(2)
O(3)
O(4)
-4335(1)
-4705(2)
-3782(2)
-4971(2)
-3884(2)
-1954(3)
-4697(2)
-3116(3)
-5634(2)
-4095(4)
-3838(3)
-3060(4)
-2672(4)
5459(1)
6898(3)
5523(3)
4377(3)
4962(3)
4596(4)
8135(5)
5347(4)
4354(4)
4901(7)
3752(6)
3527(8)
4529(9)
-1041(1)
-1171(2)
-1383(2)
-1608(2)
-31(2)
N(1)
2278(5)
C(1A)
C(1B)
C(1C)
C(1D)
C(2D)
C(3D)
C(4D)
-1290(4)
-1061(3)
-2195(3)
537(4)
1047(4)
1486(6)
2032(6)
Crystal quality and centering were confirmed by taking a 60 s rotation
frame. A search was performed by taking a series of frames in three
orthogonally related regions of reciprocal space. Each region inves-
tigated was composed of 20 10 s frames, separated by 0.3° in ω. These
were harvested to yield a total of about 100 reflections with intensities
greater than 10σ. An initial unit cell was obtained and checked for
centering and higher symmetry. None was found. The initial cell
constants were refined, and an initial orientation matrix was found.
Data were collected by examining a randomly oriented region of
reciprocal space in three segments; the frames collected in a given
segment were 0.3° apart in ω. The highest resolution data collected
was 0.87 Å. Two 30 s frames were collected, and the data were
summed, thus doubling the dynamic range of the detector. The default
gain on the detector signal was 4×, which was automatically dropped
to 1× when the detector range was exceeded. Final cell constants were
determined during integration of the data using 8192 intense, well-
centered reflections. Crystallographic parameters are given in Table
1.
The structure was solved and refined using the SHELXTL-Plus 5.0
series of programs. The space group was determined to be P1h (No. 2)
based on systematic extinctions and intensity statistics. The structure
was solved using the direct methods program xs. The hydride was
located in the difference map and refined isotropically. The remaining
hydrogens were placed in calculated positions and refined with a riding
model with B values 20% larger than those on the attached carbon
atoms. Full anisotropic refinement of all non-hydrogen atoms was
performed using full-matrix least squares on F2 using the program xl
and converged to a final R1 ) 0.0913 (2σ). Both the poor quality of
the crystals and some disorder in the coordinated THF contributed to
the high R-factor. A semiempirical ψ-scan correction was applied to
the data prior to solution and refinement. The effects of anomalous
dispersion were included for the non-hydrogen atoms.
Reaction of 3:1 DIPH/DMEAA yields (eq 2) 3 in 89% yield.
The zwitterionic compound 3 is not especially air sensitive, but
Me2EtN‚AlH3 + 3DIPH -78 °C8
THF
(DIP)3[Me2EtN(CH2)4O]Al + 3H2 (2)
2
does hydrolyze slowly in the presence of moisture, forming an
intractable mass. This product contains a THF molecule that
has undergone ring-opening. The oxygen remains coordinated
to the aluminum, whereas the amine is attached to the δ-carbon.
1
The H NMR (C6D6) of (DIP)3(Me2EtN(CH2)4O)Al shows a
shift of the DIP ligand methyl protons to 1.01 ppm, as opposed
to 1.2 ppm. The methine protons still exhibit the characteristic
downfield shift to 3.60 ppm. Signals attributable to the
(dimethylethylammonium)butanato ligand are clearly visible in
1
the H NMR spectrum.
Ring-opening of THF by a hydride has been discussed by
Wiberg, who postulated a mechanism involving hydride transfer
to reduce the C-O single bond of a coordinated THF.5
Nucleophilic attack involving a second THF on Cl3Al‚THF has
been proposed as a mechanism of polymerization of THF by
AlCl3.11 As stated above, dissolution of DMEAA in THF leads
to free amine and THF-solvated alane with no evidence of ring-
opening. Considering that solutions of 1 also contain free amine,
yet do not exhibit this reaction, we conclude that the ring-
opening occurs by attack of Me2EtN on (DIP)3Al‚THF. Ap-
parently the cumulative effect of three electronegative ligands
on aluminum is necessary to activate THF at room temperature.
Compound 2 was synthesized by the route shown in eq 3 in
42% isolated yield. The compound is air sensitive, especially
in solution. The lower yield is due to the similar solubilities
Crystals of (DIP)3[O(CH2)4NMe2Et]Al were grown by layering a
0.33 g portion of pure product in 2.0 mL of THF with 15 mL of pentane.
Colorless prisms formed within 2 days, and one having dimensions
0.5 × 0.3 × 0.2 mm was chosen for study. The data collection and
structure solution followed the procedure described for (Ph3CO)2AlH‚
THF. Crystallographic parameters are given in Table 1.
Results and Discussion
Synthesis. All of the reactions described in this paper were
conducted in THF. Based on the IR spectral evidence, most of
(11) Johnson, F. In Friedel-Crafts and Related Reactions; Olah, G. A.,
Ed.; Interscience: New York, 1965; Vol. IV; p 1.

Alkoxy- and (Aryloxy)alanes
Inorganic Chemistry, Vol. 36, No. 18, 1997 4097
Figure 2. TELP thermal ellipsoid drawing of (Ph3CO)2AlH‚THF (2).
Hydrogen atoms are omitted for clarity. Ellipsoids shown are 30%.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
(Ph3CO)2AlH‚THF (2)
Figure 1. ORTEP thermal ellipsoid drawing of (DIP)2AlH‚2THF (1).
Hydrogen atoms are omitted for clarity. Only unique atoms are labeled.
Ellipsoids shown are 50%.
Bond Lengths
Al(1)-H(1)
Al(1)-O(1)
Al(1)-O(2)
Al(1)-O(3)
O(1)-C(1)
O(2)-C(2)
O(3)-C(1G)
1.53(4)
O(3)-C(4G)
C(1)-C(1B)
C(1)-C(1C)
C(1)-C(1A)
C(2)-C(1F)
C(2)-C(1E)
C(2)-C(1D)
1.448(7)
1.532(7)
1.546(7)
1.556(6)
1.529(7)
1.548(7)
1.561(7)
1.691(3)
1.699(4)
1.881(4)
1.404(5)
1.399(6)
1.450(7)
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
(DIP)2AlH‚2THF (1)
Bond Lengths
Al(1)-O(1)
Al(1)-O(2)
Al(1)-H(1)
1.752(2)
2.034(2)
1.58(4)
O(1)-C(1A)
O(2)-C(1B)
O(2)-C(4B)
1.354(3)
1.448(3)
1.453(4)
Angles
Angles
O(2)-Al(1)-H(1)
85.84(9) Al(1)-O(1)-C(1A)
92.6(1) Al(1)-O(2)-C(1B)
H(1)-Al(1)-O(1)
H(1)-Al(1)-O(2)
O(1)-Al(1)-O(2)
H(1)-Al(1)-O(3)
O(1)-Al(1)-O(3)
O(2)-Al(1)-O(3)
O(1)-Al(1)-H(1)
120(2)
119(2)
110.4(2)
100(2)
105.3(2)
98.8(2)
120(2)
O(2)-Al(1)-H(1)
O(3)-Al(1)-H(1)
C(1)-O(1)-Al(1)
C(2)-O(2)-Al(1)
C(1G)-O(3)-C(4G)
C(1G)-O(3)-Al(1)
C(4G)-O(3)-Al(1)
119(2)
100(2)
152.6(3)
150.7(3)
106.5(5)
122.9(4)
122.3(3)
O(1)-Al(1)-O(1)′ 117.5(1)
91.53(7)
O(1)-Al(1)-O(2)
O(1)-Al(1)-O(2)′
137.5(2)
125.2(2)
125.0(2)
O(1)-Al(1)-H(1) 121.27(7) Al(1)-O(2)-C(4B)
O(1)′-Al(1)-H(1) 121.27(7) C(1B)-O(2)-C(4B) 108.7(2)
O(2)-Al(1)-O(2)′ 176.9(1)
Me2EtN‚AlH3 + 2Ph3COH -78 °C8
Table 5. Selected Bond Lengths (Å) and Angles (deg) for
(DIP)3[Me2EtN(CH2)4O]Al (3)
THF
(Ph3CO)2AlH‚2THF + Me2EtN + 2H2 (3)
Bond Lengths
3
Al(1)-O(1)
Al(1)-O(4)
Al(1)-O(2)
Al(1)-O(3)
1.733(3)
1.740(4)
1.747(3)
1.748(4)
O(1)-C(1A)
O(2)-C(1B)
O(3)-C(1C)
O(4)-C(1D)
1.359(6)
1.354(6)
1.354(6)
1.430(8)
of the ligand and product, which renders isolation of the product
difficult. Additionally, a 2-fold excess of DMEAA is required
to ensure full consumption of the alcohol. Like the DIP
compounds, the Ph3COH does not stop at the monosubstituted
product. Unlike the DIP ligand, however, Ph3COH does not
continue to react to form the trisubstituted product even when
excess Ph3COH is present. The Ph3CO ligand may be too
sterically bulky to form the trisubstituted product by this
reaction.
Angles
O(1)-Al(1)-O(4)
O(1)-Al(1)-O(2)
O(4)-Al(1)-O(2)
O(1)-Al(1)-O(3)
O(4)-Al(1)-O(3)
113.4(2)
109.1(2)
108.0(2)
108.4(2)
107.6(2)
O(2)-Al(1)-O(3)
C(1A)-O(1)-Al(1)
C(1B)-O(2)-Al(1)
C(1C)-O(3)-Al(1)
C(1D)-O(4)-Al(1)
110.3(2)
149.0(3)
137.0(3)
139.0(3)
128.8(4)
compound can be considered a derivative of AlH3‚2THF, which
has long been known, but only recently structurally character-
ized.5,7
The 1H NMR spectrum (C6D6) of 2 shows the characteristic
downfield shift of the ligand protons: complex multiplets are
observed at 7.15 and 7.63 ppm, as opposed to a complex
multiplet at 7.15 ppm in the starting alcohol. The coordinated
THF is observed as triplets at 0.85 and 3.27 ppm. The IR
spectrum of 1 in THF shows the characteristic Al-H stretch at
1803 cm-1, while the Al-H stretch of 2 in THF is observed at
1850 cm-1. The substitution of a hydride with an alkoxide is
expected to shift the νAl-H of any remaining hydride ligands to
higher energy.5 The drop in the νAl-H value in going from 2 to
1 is due to the increase in coordination number from 4 to 5. A
similar shift is found for AlH3‚NMe3 and AlH3‚2NMe3.
Structures of 1, 2, and 3. A thermal ellipsoid drawing of
(DIP)2AlH‚2THF is shown in Figure 1; selected bond lengths
and angles are given in Table 3. The compound crystallizes in
space group C2/c, with the Al-H bond vector on the 2-fold
axis. The molecule is trigonal pyramidal with the datively
bonded THF molecules in the axial sites, and the equatorial
sites occupied by the phenoxy groups and the hydride. This
A thermal ellipsoid drawing of 2 is shown in Figure 2, and
selected bond lengths and angles are given in Table 4. The
aluminum is tetrahedrally coordinated, and at 1.53(4) Å the
Al-H bond distance is shorter than found in 1. The Al-O1
and Al-O2 distances are 1.691(3) and 1.699(4) Å, respectively,
approximately 0.05 Å shorter than found in 1. The Al-O1-
C1 and Al-O2-C2 angles are 152.6(3)° and 150.7(3)°. The
Al-O distance to the datively bonded THF is 1.881(4) Å, which
is shorter than found in 1, but comparable to those found in
(BHT)2AlH‚OEt2, (1.907(2) Å).4 Diethyl ether is slightly more
bulky than THF, so one would expect it to be able to approach
less closely to the metal center.
A thermal ellipsoid drawing of 3 is shown in Figure 3, and
selected bond lengths and angles are given in Table 5. The
compound crystallizes in the acentric space group Cc, and the
geometry around the Al is approximately tetrahedral. The Al-O

4098 Inorganic Chemistry, Vol. 36, No. 18, 1997
Campbell and Gladfelter
Table 6. Summary of Structural Data
AlH3‚
(BHT)2AlH‚
1
2THFa
2
Et2Ob
3
Al-H (Å)
1.58(4) 1.51c 1.53(4)
1.47(3)
1.907(2)
1.708c
164c
Al-O(ether) (Å) 2.034c
2.067c 1.881(4)
1.695c
Al-OR (Å)
1.752(2)
1.742c
138c
4
Al-O-C (deg) 137.5(2)
152c
coordination no.
5
5
4
4
a Reference 7. b Reference 4. c Average value.
bond. This can be seen by comparing the Al-THF bond in 1
with the corresponding bond in AlH3‚2THF. The bonds differ
by 0.033 Å, a difference of 4σ using σ ) 0.007 Å for the Al-O
bond of AlH3‚2THF. Both of the five-coordinate complexes
display notably longer Al-O(THF) bonds (by 0.15 Å) compared
to the analogous distance in 2.
Consistent with several related four and five-coordinate Al
compounds,4,14 the Al-OR distances observed in 1-3 are
unusually short, and the observed Al-O-C angles are notably
wide for sp3 oxygens (Table 6). Several explanations for this
phenomenon have been advanced including π donation of
electron density by oxygen to Al4,14,15 and strong 1,3 “geminal”
repulsion between the Al and C atoms attached to the oxygen.16
While these factors may contribute to the short bond and the
wide angles, it should be noted that other factors may also play
a role. Formation of an aluminum-THF dative bond will
transfer electron density to the aluminum, lengthening the bond.
This is consistent with a steadily increasing Al-OR bond length
in three-, four-, and five-coordinate complexes, with zero, one,
and two dative bonds, respectively.4,14 Additionally, the Al-
O-C angle is expected to be soft17,18 so that even relatively
weak nonbonded repulsions16 may distort the angle away from
its ideal value.
Figure 3. TELP thermal ellipsoid drawing of (DIP)3(Me2EtN(CH2)4O)-
Al (3). Hydrogen atoms are omitted for clarity. Ellipsoids shown are
30%.
bond lengths range from 1.733(3) to 1.748(4) Å, with an average
length of 1.742 Å. The Al-O bond to the ring-opened THF is
1.740(4) Å. The Al-O-C angles range from 128.8(4)° to
149.0(3)°, with an average of 138.5°. This average value is
almost identical to the angles found in 1, but narrower than
observed in 2. The intramolecular distance between the tethered
Al and N of the zwitterion is 5.93(1) Å. Examination of
intermolecular distances establish that this is only slightly shorter
than the three closest intermolecular Al-N distances of 7.33-
(2), 7.46(2), and 7.55(2) Å. The distribution of the aluminate
anions and the cationic ammonium groups in 3 represents a
distorted zinc-blende lattice. The dipole moments of all of the
zwitterions are aligned and point in the same direction.
The difficulty in accurately locating hydrogens has led to a
large spread in the Al-H bond lengths. The Al-H distances
range from 1.277 to 1.820 Å for terminal hydrides in the
Cambridge Structural Database (CSD);12,13 the average value
is 1.53(9) Å. Both AlH3‚2THF (dAl-H ) 1.53(4) Å) and 1
(dAl-H ) 1.58(4) Å) have hydrides located in equatorial
positions, and one might expect any major chemical differences
in the Al-H bond to show up upon exchanging H- for the
electronegative DIP. The bond lengths, however, are indistin-
guishable to 2σ and fall on the average for all structurally
characterized Al-H bonds.
These considerations suggest that no one interaction may
control the metrics of these compounds, but that a combination
of intermolecular steric interactions, bond polarity, and π-
bonding may be balanced to control the bond lengths and angles.
Acknowledgment. This work was supported by The Center
for Interfacial Engineering at the University of Minnesota. The
authors wish to acknowledge Dr. Victor G. Young, Jr., and Prof.
Doyle Britton for assistance with the crystallography.
Supporting Information Available: Tables listing detailed crystal-
lographic data, positional parameters, thermal parameters, bond dis-
tances, bond angles, and torsion angles (24 pages). Ordering infor-
mation is given on any current masthead page.
IC970171D
The dative bond between THF and Al involves partial transfer
of electron density to the aluminum without changing the formal
oxidation state. Substitution of a hydride with a DIP withdraws
electron density from the Al, which then compensates by more
strongly binding the THF oxygen and shortening the Al-O
(14) Healy, M. D.; Ziller, J. W.; Barron, A. R. J. Am. Chem. Soc. 1990,
112, 2949.
(15) Barron, A. R.; Dobbs, K. H.; Francl, M. M. J. Am. Chem. Soc. 1991,
113, 39.
(16) Haaland, A. In Coordination Chemistry of Aluminum; Robinson, G.
H., Ed.; VCH: New York, 1993; Vol. 1.
(12) Allen, F. H.; D., J. E.; Galloy, J. J.; Johnson, O.; Kennard, O.; Macrae,
C. F.; Mitchell, E. M.; Smith, J. M.; Watson, D. G. J. Chem. Inf.
Comput. Sci. 1991, 31, 187.
(13) Allen, F. H.; Kennard, O.; Watson, D. G. In Structure Correlation;
Burgi, H.-B., Dunitz, J. D., Eds.; VCH: New York, 1994; Vol. 1, p
71.
(17) Gibbs, G. V.; Meagher, E. P.; Newton, M. D.; Swanson, D. K. In
Structure and Bonding in Crystals; O’Keeffe, M., Navrotsky, A., Eds.;
Academic Press: New York, 1981; Vol. 1, p 195.
(18) Burdett, J. K. In Accurate Molecular Structures: Their Determination
and Importance; Domenicano, A., Hargittai, I., Eds.; Oxford: New
York, 1992; p 498.