Molecular structures of dimeric and trimeric dineopentylaluminum hydride and trimeric diphenylaluminum hydride

Article abstract of DOI:10.1002/zaac.201300484

The synthesis of dineopentylaluminum hydride, H-Al(CH₂CMe ₃)₂, is known in the literature since 1988. We determined the crystal structure of this important starting material and found different ring sizes with dimeric versus trimeric formula units. The molecular shape depends on the polarity of the solvent used for recrystallization (n-pentane or 1,2-difluorobenzene). The synthesis of diphenylaluminum hydride, which is also long known in the literature, was optimized. It has a trimeric structure in the solid state with an Al₃H₃ heterocycle.

Full text of DOI:10.1002/zaac.201300484

ARTICLE
DOI: 10.1002/zaac.201300484
Molecular Structures of Dimeric and Trimeric Dineopentylaluminum
Hydride and Trimeric Diphenylaluminum Hydride
Werner Uhl,*[a] Christian Appelt,[a] Jana Backs,[a] Hans Klöcker,[a] Andrej Vinogradov,[a]
and Hauke Westenberg[a]
Keywords: Aluminum hydrides; Aluminum; Oligomerization; Three-center bonds
Abstract. The synthesis of dineopentylaluminum hydride, H-
recrystallization (n-pentane or 1,2-difluorobenzene). The synthesis of
Al(CH2CMe3)2, is known in the literature since 1988. We determined
diphenylaluminum hydride, which is also long known in the literature,
the crystal structure of this important starting material and found was optimized. It has a trimeric structure in the solid state with an
different ring sizes with dimeric versus trimeric formula units. The Al3H3 heterocycle.
molecular shape depends on the polarity of the solvent used for
diate steric shielding between that of bis(tert-butyl)aluminum
Introduction
hydride and the commercially available diisobutylaluminum
Dialkylaluminum hydrides are important starting com-
hydride which is a standard reagent in organic transformations.
pounds in preparative organic or inorganic chemistry.[1,2] Due
Its molecular structure in the solid state has not yet been re-
to the coordinative unsaturation and the Lewis acidity of the
ported. Another important starting compound for hydroalumi-
aluminum atoms these hydrides usually form oligomeric spe-
nation reactions is diphenylaluminum hydride. Compared to
cies in solution and the solid state via Al–H–Al 3c-2e bonds.
dialkylaluminum compounds the phenyl groups should en-
Molecular mass determinations verified trimeric formula units
hance the Lewis acidity of the aluminum atoms, which may
or equilibria between dimeric and trimeric forms in solution,[3]
be helpful in many secondary applications. Although this hy-
and the partial dissociation of a dimer into monomeric frag-
dride is known in the literature since 1964,[12] it has been ap-
ments was found with the bulky CH(SiMe3)2 substituent.[4]
plied only in a preliminary reaction with phenylethyne, which
Only two dialkylaluminum hydrides have been characterized
after hydrolytic work-up gave a mixture of at least four prod-
by crystal structure determinations, which verified a dimeric
ucts. Its structure is still unknown.
and a trimeric compound depending on steric shielding. Mono-
meric derivatives have only been observed with the very bulky
aryl groups 2,4,6-tri(tert-butyl)phenyl[5] and 2,6-dimesityl-
Results and Discussion
phenyl.[6] Bis(trimethylsilyl)methyl[4] or triisopropylphenyl
groups[7] lead to dimeric species in the solid state. A dimer
has also been observed for Me2Al-H at a low pressure in the
gas phase at 200 °C.[8] Bis(tert-butyl)aluminum hydride crys-
tallizes as a trimer. In that case the preference of the trimeric
form in spite of the relatively high steric shielding was ex-
plained by an approach of the packing of the approximately
ball-shaped tert-butyl groups to a hexagonal closed array.[9] Its
relatively low solubility in hydrocarbons prevented the deter-
mination of its molecular mass in benzene solution.
Dineopentylaluminum Hydride
Dineopentylaluminum hydride 1 is a colorless solid, which
is readily soluble in hydrocarbons such as n-pentane or benz-
ene. In accordance with 1H NMR spectroscopic data from Ref.
[10] freshly recrystallized samples dissolved in C6D6 show the
1H and 13C NMR signals of two species. Their relative inten-
sities depend on concentration and temperature. In an NMR
experiment at 220 K in [D8]toluene the singlet signals of tert-
butyl groups at δ = 1.19 and 1.16 ppm had an integration ratio
of about 4.6:1. The intensity of the first one steadily decreased
upon warming. At about 283 K both signals had the same in-
tensity, and at 330 K the resonance at δ = 1.19 completely
disappeared. The second resonance at δ = 1.16 was almost
unchanged and showed only the expected continuous shift with
increasing temperature (δ = 1.09 at 380 K). Same holds for the
resonances of the CH2 protons at δ = 0.73 and 0.63 ppm
(220 K), from which only the latter was observed at 380 K (δ
= 0.62 ppm), and for the signals of the Al–H hydrogen atoms
The facile synthesis of dineopentylaluminum hydride 1 from
trineopentylaluminum and LiAlH4 has been reported in 1988
by the group of Beachley.[10] It is an important starting com-
pound in hydroalumination reactions[1,11] and has an interme-
* Prof. Dr. W. Uhl
Fax: +49-251-8336660
E-Mail: uhlw@uni-muenster.de
[a] Institut für Anorganische und Analytische Chemie
Universität Münster
Corrensstraße 30
48149 Münster, Germany
106
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Z. Anorg. Allg. Chem. 2014, 640, (1), 106–109

Dimeric and Trimeric Dineopentylaluminum Hydride and Trimeric Diphenylaluminum Hydride
(δ = 3.29 and 3.14; the latter disappeared). Molecular mass
determinations verified an equilibrium between trimeric and
dimeric formula units, which was shifted towards the dimer
with increasing dilution (see also Ref. [10]). In view of these
results we suppose that the NMR spectra at 300 K show the
resonances of the dimer [1a, δ = 3.33, 1.12, 0.62. 13C NMR:
35.0 (CH3), 27.8 (CH2)] and the trimer [1b, δ = 3.14, 1.15,
0.70. 13C NMR: 35.0 (CH3), 29.0 (CH2)] (Scheme 1). The tri-
mer is the preferred species at low temperature and in concen-
trated solution, while the dimer is the only isomer present at
higher temperature. We did not observe coalescence over the
entire range of temperature. Therefore, a fast exchange be-
tween both oligomeric forms in solution can be excluded,
which in addition should result in resonances with average
Figure 1. Molecular structure of 1a. Displacement ellipsoids are drawn
chemical shifts at higher temperatures. We suppose that the
trimer is completely transformed to the dimeric molecules
upon warming.
at the 40% level. Hydrogen atoms with the exception of the bridging
atoms H1 and H1Ј are omitted. Important bond lengths /pm and
angles /°: Al1–H1 168(1), Al1–H1Ј 171(1), Al1–H1–Al1Ј 101.7(7),
H1–Al1–H1Ј 78.3(7), C11–Al1–C21 130.93(5); symmetry equivalent
atoms generated by –x, –y, –z + 1.
Scheme 1. Equilibrium between dimeric and trimeric H-Al(CH2CMe3)2
(R = CH2CMe3).
Crystals of both forms could be isolated from different sol-
vents. Recrystallization from n-pentane (–30 °C) afforded the
dimer 1a (Figure 1), while the polar non-coordinating solvent
1,2-difluorobenzene (–30 °C) yielded crystals of the trimeric
molecule 1b (Figure 2). Two or three aluminum atoms are
bridged by hydrogen atoms to give Al2H2 or Al3H3 heterocy-
cles. The four-membered ring is planar due to its position on
a crystallographic center of symmetry, and the atoms of the
six-membered heterocycle deviate only slightly from the
average plane (maximum deviation of an atom 2 pm). The Al–
H distances do not significantly depend on the ring size and are
170 pm on average. The endocyclic angles and transannular
distances differ considerably. 1a with the four-membered ring
has relatively acute angles Al–H–Al and H–Al–H of 101.7(7)
and 78.3(7)°, while for 1b much larger values of 144 and 96°
(on average) were found. A very narrow transannular Al···Al
contact of 262.9 pm results for the dinuclear structure of 1a
(H···H = 214 pm) which corresponds to the value detected for
dimeric dimethylaluminum hydride in the gas phase
(262.5 pm).[8] A much longer distance of 322.2 pm was ob-
served for 1b (H···H = 253 pm). These observations may re-
flect a different bonding situation with a closed vs. an open
three-center bonding interaction. The structure of the dimer 1a
is isotypic to the previously published structure of dineopen-
tylgallium hydride, which is exclusively dimeric in solution
and does not show a dimer/trimer equilibrium.[13]
Figure 2. Molecular structure of 1b. Displacement ellipsoids are
drawn at the 40% level. Hydrogen atoms with the exception of
the bridging atoms H1, H2, and H3 are omitted. Important bond
lengths /pm and angles /°: Al–H 162(3) to 176(3), Al–H–Al 142(2) to
145(2), H–Al–H 96(1) to 97(1), C–Al–C 129.3(1) to 132.4(1).
Diphenylaluminum Hydride 2
The synthesis of 2 has been reported in the literature.[12] We
changed the reaction conditions to improve its reproducibility.
2 is formed by treatment of triphenylaluminum with the alane-
ether adduct AlH3·0.19Et2O in toluene at 75 °C for 3 h
(Scheme 2). Higher temperatures resulted in partial decompo-
sition by precipitation of elemental aluminum. Colorless crys-
tals of 2 were obtained after filtration, concentration of the
filtrate, and cooling to –20 °C in 72% yield. The hydrogen
1
atom bonded to aluminum gave a broad resonance in the H
NMR spectrum at δ = 4.63, and the IR spectrum showed a
broad absorption at 1760 cm–1 for the stretching vibration of
the Al–H–Al groups. The monomeric fragment H–Al(C6H5)2
Z. Anorg. Allg. Chem. 2014, 106–109
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107

W. Uhl et al.
ARTICLE
was detected in the mass spectrum. Pure diphenylaluminum groups. The stronger electron pair acceptor capability seems to
hydride is insoluble in cyclopentane or n-hexane and only favor spontaneous secondary reactions such as condensation or
sparingly soluble in toluene or benzene; therefore we were not metallation of aromatic rings with the formation of unprece-
able to determine the molar mass in solution by cryoscopy. dented structural motifs.
Ebulliometric measurements in benzene were reported to result
in an association degree of 2.4.[12]
Experimental Section
General: All manipulations were carried out in a dry argon atmo-
sphere, using standard Schlenk techniques. Toluene was dried with Na/
benzophenone, n-pentane and n-hexane over LiAlH4 and 1,2-difluo-
robenzene over molecular sieves. NMR spectra were recorded in C6D6
at ambient probe temperature using the following Bruker instruments:
Avance I (1H, 400.13; 13C, 100.62 MHz) or Avance III (1H, 400.03;
13C, 100.59 MHz) and referenced internally to residual solvent reso-
nances (chemical shift data in δ). The 13C NMR spectrum was
Scheme 2. Synthesis of diphenylaluminum hydride.
proton-decoupled. The IR spectrum was recorded as paraffin mull
between CsI plates on a Shimadzu Prestige 21 spectrometer. Dineo-
pentylaluminum hydride,[10] triphenylaluminum,[15] and the adduct
AlH3·0.19Et2O[16] were obtained following literature procedures.
Crystal structure determination (Figure 3) revealed a tri-
meric formula unit in the solid state with an almost ideally
planar Al3H3 heterocycle. The largest deviation of an atom
from the average plane is 9 pm (H2). The structural parameters
are quite similar to those of the trimeric dineopentyl derivative
1b. The endocyclic angles Al–H–Al and H–Al–H are 145 and
94° on average, and the transannular Al···Al or H···H distances
are 322 and 248 pm. The Al–C bond lengths [194.3(2) to
195.2(2)] are similar to those to the terminal phenyl groups in
dimeric triphenylaluminum.[14]
Synthesis of Diphenylaluminum Hydride 2: A suspension of tri-
phenylaluminum (1.33 g, 5.15 mmol) and AlH3·0.19Et2O (0.113 g,
2.56 mmol) in toluene (100 mL) was stirred at 75 °C for 3 h. After
cooling to room temperature the mixture was filtered, and the solid
material was washed with toluene (20 mL). The filtrate was concen-
trated in vacuo. A colorless solid of 2 was obtained at –45 °C. It was
isolated and washed with n-hexane. All volatiles were removed in
vacuo (10–3 Torr). Yield: 1.01 g (72%). Mp (argon, sealed capillary):
150–156 °C (dec.). IR (paraffin, CsI plates): ν˜ = 1796 s, br., 1699 s,
br. νAlH; 1580 w, 1558 w phenyl; 1462 vs. (paraffin); 1420 s
νCCarom.; 1378 s paraffin; 1300 vw, 1248 w, 1192 w, 1155 vw, 1099
m δCH; 1084 m δCHarom.; 993 w; 885 w, br., 849 w, br. δAlH; 727
paraffin; 700 vs. phenyl; 629 vw, 617 vw, 476 vs, 457 s, 446 m, 424
w νAlC cm–1. 1H NMR (400 MHz, C6D6, 300 K): δ = 7.64 (4 H,
pseudo-d, ortho-H), 7.23 (2 H, pseudo-t, para-H), 7.16 (4 H,
pseudo-t, meta-H), 4.63 (1 H, br., Al-H). 13C{1H} NMR (100 MHz,
C6D6, 300 K): δ = 138.2 (br., ipso-C and ortho-C), 130.2 (para-C),
128.2 (meta-C). MS (EI, 20 eV, 333 K): m/z (%) = 182 (6) [M
(monomer)]+, 164 (6), 154 (7) [Ph-Ph]+, 78 (100) [Ph-H]+.
X-ray Crystallography: Crystals suitable for X-ray crystallography
were obtained by recrystallization from n-pentane (–30 °C; 1a), 1,2-
difluorobenzene (–30 °C; 1b) and toluene (–20 °C; 2). Intensity data
was collected with Bruker APEX II and Bruker Quasar diffractometers
with monochromated Mo-Kα (1a, 2) and Cu-Kα radiation (1b). Data
reduction was carried out using the program SAINT+.[17] The crystal
structures were solved by direct methods using SHELXTL.[18] Non-
hydrogen atoms were first refined isotropically followed by anisotropic
refinement by full-matrix least-squares calculation based on F2 using
SHELXTL. With the exception of the bridging hydride atoms the hy-
drogen atoms were positioned geometrically and allowed to ride on
their respective parent atoms. A tert-butyl group of 1b was rotationally
disordered. Its atoms were refined with site occupancy factors of 0.35,
0.30, and 0.35. Compound 2 crystallized with half a toluene molecule
per formula unit. The solvent molecule was disordered across a center
of symmetry. The methyl group was refined on two positions with site
occupancy factors of 0.5. Further crystallographic data is summarized
in Table 1.
Figure 3. Molecular structure of 2. Displacement ellipsoids are drawn
at the 40% level. Hydrogen atoms with the exception of the bridging
atoms H1, H2 and H3 are omitted. Important bond lengths /pm
and angles /°: Al–H 163(2) to 173(2), Al–H–Al 145(1) to 146(1),
H–Al–H 94.1(8) to 94.5(8), C–Al–C 124.1(1) to 125.9(1).
Preliminary experiments verified the exceptional reactivity
of the diphenyl compound 2 and its broad applicability in hy-
droalumination reactions. Particularly interesting is the varia-
tion and enhancement of the Lewis-acidity of the aluminum
atoms compared to that of dialkylaluminum compounds, which
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
is caused by the relatively strong –I effect of the phenyl of the data can be obtained free of charge on quoting the depository
108 www.zaac.wiley-vch.de
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 106–109

Dimeric and Trimeric Dineopentylaluminum Hydride and Trimeric Diphenylaluminum Hydride
Table 1. Crystal data and structure refinement for compounds 1a, 1b, and 2.
1a
1b
2·0.5toluene
Empirical formula
Crystal system
Space group
C20H46Al2
triclinic
P1
C30H69Al3
monoclinic
P21
C39.5H37Al3
triclinic
P1
c
¯
¯
Z
1
2
2
T /K
153(2)
0.934
153(2)
0.895
153(2)
1.158
Dcalc /g·cm–3
a /pm
b /pm
c /pm
α /°
623.05(11)
1004.8(2)
1014.9(2)
106.281(3)
90.438(3)
96.706(3)
0.6051(2)
0.118
1120.21(4)
1618.00(7)
1173.55(5)
90
117.033(2)
90
1042.1(2)
1226.1(3)
1447.8(3)
68.03(3)
84.40(3)
82.84(3)
1.6995(6)
0.137
β /°
γ /°
V /nm3
1.8947(1)
0.992
μ /mm–1
Λ
Mo-Kα
Cu-Kα
Mo-Kα
θ Range /°
Independent reflections
Parameters
2.09–30.02
3480 [Rint = 0.0337]
110
0.0445 (2953)
0.1179
4.23–69.00
5589 [Rint = 0.0285]
383
0.0436 (5178)
0.1143
1.52–27.93
8043 [Rint = 0.0471]
401
0.0397 (6096)
0.1084
a)
R1 [I Ͼ 2σ(I))
b)
wR2 (all data)
Largest diff. peak and hole /e·nm–3
407, –184
256, –185
505, –249
a) R1 = Σ||Fo| – |Fc||/Σ|Fo|. b) wR2 = {Σw(|Fo|2–|Fc|2)2/Σ|Fo|2}1/2. c) Absolute structure parameter: 0.01(3).
S. E. Collin, L. A. Whitehurst, P. T. Brain, C. A. Morrison, C. R.
Pulham, B. A. Smart, D. W. H. Rankin, A. Keys, A. R. Barron,
Organometallics 2000, 19, 527.
numbers CCDC-962760 (1a), CCDC-962761 (1b), and CCDC-962762
(2) (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
[9] W. Uhl, Z. Anorg. Allg. Chem. 1989, 570, 37; for the discussion
of the crystal packing of such trimeric bis(tert-butyl)element com-
pounds see: Ref. [13].
Acknowledgements
[10] O. T. Beachley Jr., L. Victoriano, Organometallics 1988, 7, 63.
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The authors gratefully acknowledge support by the Deutsche For-
schungsgemeinschaft.
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Received: September 24, 2013
Published Online: November 22, 2013
Z. Anorg. Allg. Chem. 2014, 106–109
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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109