Low-coordinate aluminum amides from silylanilines and alkylalanes

Article abstract of DOI:10.1002/ejic.201000869

The aluminum amides R₂AlN(Ar)SiMe₃ [R = Et, Ar = Dipp, 1 (Dipp = 2,6-iPr₂C₆H₃-); R = iBu, Ar = Dipp, 2; R = Et, Ar = Mes, 3 (Mes = 2,4,6-Me₃C₆H ₂-); R = iBu, Ar = Mes, 4] were prepared by ethane or hydrogen elimination reaction between Et₃Al or iBu₂AlH and ArN(H)SiMe₃ in refluxing hexane solution. The LiCl salt elimination route resulted in the formation of the monomeric aluminum amide Et ₂AlN(Mes)SiPh₃ (5) featuring three-coordinate aluminum and nitrogen centers. In addition, the synthesis and characterization of the dinuclear species Ph₂Si{N(Mes)AlEt₂}₂ (6) and the hydride-bridged eight-membered ring compound {MesN(SiMe₃)Al(iBu) (μ-H)}₂(μ-LiH)(μ-iBu₂AlH) (7) are reported. All compounds have been characterized by ¹H and ¹³C{ ¹H} NMR spectroscopy, and compounds 5, 6, and 7 have also been characterized by single-crystal X-ray crystallography.

Full text of DOI:10.1002/ejic.201000869

FULL PAPER
DOI: 10.1002/ejic.201000869
Low-Coordinate Aluminum Amides from Silylanilines and Alkylalanes
Manish Khandelwal,[a] Douglas R. Powell,[b] and Rudolf J. Wehmschulte*[a]
Keywords: Main group elements / Aluminum / Amides / Low-coordination / Lewis acids
The aluminum amides R2AlN(Ar)SiMe3 [R = Et, Ar = Dipp, 1
(Dipp = 2,6-iPr2C6H3-); R = iBu, Ar = Dipp, 2; R = Et, Ar =
Mes, 3 (Mes = 2,4,6-Me3C6H2-); R = iBu, Ar = Mes, 4] were
prepared by ethane or hydrogen elimination reaction be-
centers. In addition, the synthesis and characterization of the
dinuclear species Ph2Si{N(Mes)AlEt2}2 (6) and the hydride-
bridged eight-membered ring compound {MesN(SiMe3)Al-
(iBu)(μ-H)}2(μ-LiH)(μ-iBu2AlH) (7) are reported. All com-
tween Et3Al or iBu2AlH and ArN(H)SiMe3 in refluxing hex- pounds have been characterized by 1H and 13C{1H} NMR
ane solution. The LiCl salt elimination route resulted in the
formation of the monomeric aluminum amide Et2AlN(Mes)-
SiPh3 (5) featuring three-coordinate aluminum and nitrogen
spectroscopy, and compounds 5, 6, and 7 have also been
characterized by single-crystal X-ray crystallography.
butylaluminum amides R2AlN(Ar)SiRЈ3 with the steric pro-
tection being provided by bulky anilides. The monomeric
methylaluminum compounds with very bulky disilylamides
Me2AlN(SiPhtBu2)(SiMetBu2)[6] and Me(Cl)AlN(SiPh-
tBu2)(SiMetBu2)[7] have been reported. A series of dimeth-
ylaluminum silyl anilides Me2AlN(Ar)SiR2RЈ (Ar = Mes,
Dipp; R = Me, RЈ = Me, iPr, tBu, Mes) has been prepared
by the Roesky group, and {Me2AlN(Dipp)SiMe3}2 was
found to be dimeric with bridging methyl substituents.[8]
Contrary to the majority of previously reported aluminum
amides the aluminum precursors Et3Al, Et2AlCl and iBu2-
AlH are commercially available and inexpensive, and the
silyl anilines can be prepared in simple one-step procedures
from the readily available anilines ArNH2 (Ar = Mes, Dipp)
and R3SiCl (R = Me, Ph). In addition, we report the syn-
thesis and characterization of the dinuclear Ph2Si{N(Mes)-
AlEt2}2 and the hydride-bridged eight-membered-ring com-
pound {MesN(SiMe3)Al(iBu)(μ-H)}2(μ-LiH)(μ-iBu2AlH).
Introduction
Neutral aluminum compounds are widely used as cata-
lysts for Lewis-acid-mediated reactions such as Friedel–
Crafts and Diels–Alder reactions, as reagents in alkylation
reactions, as initiators for cationic polymerizations, and co-
catalysts/activators in transition-metal-catalyzed olefin po-
lymerizations.[1,2] Bulky substituents have been employed to
increase the reactivity by providing a three-coordinate
Lewis acidic aluminum center as well as to increase the
selectivity by blocking alternative positions in the sub-
strate.[3] The Lewis acidity of the aluminum center can be
further enhanced by the use of electronegative substituents
such as amides R2N-, alkoxides RO- or aryloxides ArO-
and by the introduction of a positive charge in combination
with a lowering of the coordination number to two as in
[RAlRЈ]+ (R, RЈ = alkyl, aryl). As part of our ongoing in-
vestigations of low-coordinate cationic aluminum and gal-
lium compounds[4] we have become interested in the synthe-
sis of low-coordinate cationic alkylaluminum amides such
as [EtAlN(Ar)SiR3]+. The bulky anilides –N(Ar)SiR3 (Ar = Results and Discussion
Mes, Dipp; Mes = 2,4,6-C6H2–; Dipp = 2,6-iPr2C6H3–; R =
The aluminum amides R2AlN(Ar)SiMe3 (R = Et, Ar =
Me, Ph) are thought to prevent aggregation of the cationic
species in addition to limit cation···anion contacts. The pre-
cursors for these compounds would be dialkylaluminum
amides such as Et2AlN(Ar)SiR3. While the synthesis of
three-coordinate aluminum amides with bulky substituents
on both the aluminum and nitrogen center is well documen-
ted,[5] we describe here the preparation of diethyl and diiso-
Dipp, 1; R = iBu, Ar = Dipp, 2; R = Et, Ar = Mes, 3; R =
iBu, Ar = Mes, 4) were prepared by the ethane or hydrogen
elimination reaction in refluxing hexane according to Equa-
tions (1) and (2).
(1)
[a] Department of Chemistry, Florida Institute of Technology,
150 West University Blvd., Melbourne, FL 32901, USA
Fax: +1-321-674-8951
(2)
E-mail: rwehmsch@fit.edu
Compounds 1–4 were obtained as pale yellow oils, which
could not be purified further. The diethylaluminum amides
1 and 3 were analytically pure, but the diisobutylaluminum
[b] Department of Chemistry and Biochemistry, University of
Oklahoma,
101 Stephenson Parkway, Norman, OK 73019-5251, USA
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M. Khandelwal, D. R. Powell, R. J. Wehmschulte
FULL PAPER
amides 2 and 4 contained approximately 10% of unreacted range of 49–86° are observed.[5] Only those reported for
silylamines. Based on freezing point depression experiments Trip2AlN(H)Dipp and tBu2AlN(Dipp)SiPh3 with values of
1
on 1 and the similarity of their solution H and 13C{1H} 5.5 and 16.1° come close to the value of 2.1° found in 5.
NMR spectra in C6D6, compounds 1–4 are most likely mo-
nomeric in solution. Typically, aluminum amides are di-
meric species with bridging amides such as {iBu2AlN(H)-
Dipp}2,[9] {Neo2AlN(H)Dipp}2 [Neo = neopentyl, –CH2C-
(CH3)3],[10] or {Me2AlN(H)Dipp}2.[11] Introduction of the
bulky Me3Si group in {Me2AlN(Dipp)SiMe3}2 prevented
amide bridging, but methyl bridging via two-electron-three-
center bonds was observed.[8] The present observation that
compounds 1–4 are most likely monomeric in solution may
be explained by the lower bridge-forming tendency of alkyl
groups other than methyl.[12]
In order to obtain a crystalline species, the larger and
crystalline aniline derivative MesN(H)SiPh3 was prepared,
Figure 1. Structure of 5 (50% ellipsoids). H atoms are omitted for
lithiated and reacted with Et2AlCl to give the solid com-
clarity. Important bond lengths [Å] and angles [°]: Al(1)–N(1)
1.813(4), Al(1)–C(29) 1.950(4), Al(1)–C(31) 1.969(5), N(1)–C(2)
1.451(5), Si(1)–N(1) 1.737(3), N(1)–Al(1)–C(29) 123.48(19), N(1)–
Al(1)–C(31) 116.3(2), C(29)–Al(1)–C(31) 120.2(2), Si(1)–N(1)–
Al(1) 127.6(2), C(2)–N(1)–Al(1) 114.5(3), C(2)–N(1)–Si(1)117.9(3).
pound Et2AlN(Mes)SiPh3 (5), see Equation (3). Attempts
to generate the Dipp analogue were unsuccessful.
The reaction of the in-situ generated lithiated diamide
Ph2Si{N(Mes)Li}2 with 2 equiv. Et2AlCl afforded the new
compound Ph2Si{N(Mes)AlEt2}2, 6, in moderate yields
(3)
The crystal structure of 5 (Figure 1) confirms that it is a (Scheme 1). This type of compounds has been previously
monomer with three-coordinate planar aluminum and ni- obtained in two isomers: (CH2)4Si{N(tBu)AlCl2}2 and
trogen centers [Σ(angles): 360° for both centers]. The planes (CH2)5Si{N(tBu)AlMe2}2 form cage structures in which the
at aluminum and nitrogen are essentially coplanar (angle equivalent aluminum centers form a symmetric bridge be-
between normals: 2.1°) allowing for a short Al–N distance tween the amide nitrogen atoms (isomer A).[15]
of 1.813(4) Å as well as potential overlap of the lone pair Me2Si{N(tBu)AlPh2}2 forms a bicyclic structure consisting
on the nitrogen into the empty p-orbital on aluminum. The of two edge-sharing four-membered rings (isomer B).[16] As
1
large size of the SiPh3 group causes a widening of the N(1)– solution H and 13C{1H} NMR spectra of 6 showed only
Al(1)–C(29) and Si(1)–N(1)–Al(1) angles to 123.48(19)° and one set of signals, which were also slightly broadened for
127.6(2)°, respectively. The Al–N distance is among the the AlCH2 group indicating a fluxional behavior, the crystal
shorter ones having been reported for aluminum mono- structure of 7 was determined (Figure 2). Compound 6
amides, see for example Trip2AlN(H)Dipp (Trip = 2,4,6- crystallizes as isomer B featuring a bridging ethyl group.
[14]
iPr3C6H2-),[13] tBu2AlNMes2 and tBu2AlN(Dipp)SiPh3
The SiN2Al ring is essentially planar, whereas the NAl2C
with values of 1.784(3), 1.823(4), and 1.834(3) Å, respec- ring is puckered. The angle between the ring planes is 118°.
tively. Similarly, the angle between the planes at aluminum Both aluminum centers are four-coordinate with a distorted
and nitrogen falls in the lower range of those observed to tetrahedral environment. Al(1) is connected to two amide
date for this class of compounds. Most often angles in the nitrogen atoms, one terminal ethyl and one bridging ethyl
Scheme 1.
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Low-Coordinate Aluminum Amides
group, where as Al(2) is connected to one amide nitrogen, but they differ significantly from one another. The dialkyl-
two terminal ethyl and one bridging ethyl group. The Al–C substituted Al(1C) features two short Al–H bonds with val-
distances with average values of 1.966 and 2.166 Å fall in ues of 1.46(9) and 1.56(9) Å, whereas each of the amide-
the range observed for terminal and bridging Al–C substituted Al(1A) and Al(1B) show a short and a long Al–
groups[12] and are slightly shorter than the values reported
H bond. The short Al–H bonds [Al(1A)–H(1AA)
for Me2Si{N(tBu)AlPh2}2 with average values of 2.019 and 1.35(9) Å, Al(1B)–H(1BB) 1.44(8) Å] involve the Al–H–Li
2.176 Å.[16] The Al–N distances vary with the coordination bridges, and the long Al–H bonds [Al(1A)–H(1AC)
number of the nitrogen center: The Al(1)–N(1) distance in- 1.87(8) Å, Al(1B)–H(1BC) 1.87(9) Å] involve the Al–H–Al
volving a distorted trigonal planar nitrogen [Σ(angles) = bridges. The Li···C contacts are unsymmetrical and vary
359.5°] is 1.833(3) Å and the Al(1)–N(2) and Al(2)–N(2) from 2.581 to 2.896 Å for the C1A ring and from 2.506 to
distances average 1.987 Å.
2.877 Å for the C1B ring. The nitrogen centers are in a tri-
gonal planar environment [Σ(angles) = 359.5°], which is
typical for amines with two or more electropositive substit-
uents. The Li–H bond lengths in 7 are almost 0.3 Å longer
than those observed for the alanates listed above. This is
probably a due to the Li···arene coordination and the con-
straints resulting from this.
Figure 2. Structure of 6 (50% ellipsoids). H atoms are omitted for
clarity. Important bond lengths [Å] and angles [°]: Al(1)–C(31)
1.955(4), Al(1)–C(33) 2.140(4), Al(1)–N(1) 1.833(3), Al(1)–N(2)
1.994(3), Al(2)–C(33) 2.192(4), Al(2)–C(35) 1.980(5), Al(2)–C(37)
1.964(4), Al(2)–N(2) 1.980(3), Si(1)–N(1) 1.714(3), Si(1)–N(2)
1.818(3),
N(1)–Al(1)–N(2)
83.95(12),
C(31)–Al(1)–C(33)
101.90(15), C(31)–Al(1)–N(2) 139.16(14), N(1)–Al(1)–C(31)
124.00(15), C(37)–Al(2)–C(35) 113.8(3), N(2)–Al(2)–C(33)
95.03(13), Al(2)–N(2)–Al(1) 82.93(11), Si(1)–N(2)–Al(1) 87.26(12),
C(13)–N(1)–Si(1) 138.3(2), C(13)–N(1)–Al(1) 125.4(2), Si(1)–N(1)–
Al(1) 95.82(14), N(1)–Si(1)–N(2) 92.97(13), Al(1)–C(33)–Al(2)
74.81(12).
On one occasion, a colorless crystalline compound was
obtained from the reaction of iBu2AlH with MesN(H)
SiMe3. Its crystalline nature, its NMR spectra and the IR Figure 3. Structure of 7. Due to disorder problems the ball and
absorption at 1754 cm–1 clearly showed that it was not the
stick plot is shown here. H atoms except of those bound to Al and
Li are omitted for clarity. Important bond lengths [Å] and angles
expected compound 4, and its structure was determined by
[°]: Al(1A)–C(10A) 1.958(8), Al(1A)–N(1A) 1.834(6), Al(1A)–
single-crystal X-ray crystallography (Figure 3) as the un-
H(1AA) 1.35(9), Al(1A)–H(1AC) 1.87(8), Al(1B)–C(10B) 1.974(9),
usual cyclic species {MesN(SiMe3)Al(iBu)(μ-H)}2(μ-
LiH)(μ-iBu2AlH), 7. Two MesN(SiMe3)Al(iBu) moieties
are connected via hydrogen bridges through a μ-H–Li–H
and a μ-H–Al(iBu)2–H unit to form an eight-membered Al3-
LiH4 ring. While the formation of Al2Li2H4 rings is com-
Al(1B)–N(1B) 1.859(7), Al(1B)–H(1BB) 1.44(8), Al(1B)–H(1BC)
1.87(9), Al(1C)–C(1C) 1.967(13), Al(1C)–C(5C) 1.924(12), Al(1C)–
H(1AC) 1.46(9), Al(1C)–H(1BC) 1.56(9), Li(1)–H(1AA) 2.11(9),
Li(1)–H(1BB) 2.06(9), Li(1)···C(1A–6A) 2.581–2.896, Li(1)···C(1B–
6B) 2.506–2.877, N(1A)–Al(1A)–C(10A) 120.2(3), H(1AA)–
Al(1A)–H(1AC) 100(5), N(1B)–Al(1B)–C(10B) 118.6(3), H(1BB)–
mon for many substituted lithium alanates including Al(1B)–H(1BC) 103(4), C(5C)–Al(1C)–C(1C) 127.7(6), H(1AC)–
Al(1C)–H(1BC) 90(5), H(1AA)–Li(1)–H(1BB) 100(3).
{TriphAl(H)(μ-H)(μ-LiH)(OEt2)1.5
}
(Triph
=
2,4,6-
2
Ph3C6H2–),[17] {(Me3Si)3CAl(tBu)(μ-H)(μ-LiH)(THF)}2,[18]
and {(Me3Si)2CH(Dipp)NAl(H)(μ-H)(μ-LiH)(THF)2}2,[19]
Due to its approximate C2 symmetry, its 1H and 13C{1H}
the Al3LiH4 ring observed here appears to be the first of its NMR spectra are relatively easy to interpret and also indi-
kind. In addition, compound 7 seems to be the only com- cate that the cyclic structure is maintained in solution. It is
pound of this type in which the lithium cation is not sol- currently not clear how compound 7 formed, but an old
vated by ethers or amines, but by the π-electron density of batch of nBuLi that was used to prepare the precursor
two aromatic rings in an η6-fashion. The Al–C and Al–N MesN(H)SiMe3 may be to blame. It is known that nBuLi
distances average 1.956 and 1.848 Å fall within the normal solutions slowly decompose via β-hydrogen elimination to
range for four-coordinate aluminum centers. The Al–H dis- give LiH and butene.[2] This LiH can remain soluble by in-
tances are shorter than the Li–H distances [2.08 Å (avg.)] corporation into various nBuLi clusters[20] and may have
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M. Khandelwal, D. R. Powell, R. J. Wehmschulte
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1
CH(CH3)2], 3.51 [sept, 2 H, CH(CH3)2], 7.05 (s, 3 H, aromatic).
13C{1H} NMR (C6D6, 100.62 MHz): δ = 1.9 (s, broad, AlCH2), 3.1
(SiMe3), 8.7 (AlCH2CH3), 24.5 [CH(CH3)2], 24.9 [CH(CH3)2], 28.8
[CH(CH3)2], 123.9 (m-C), 124.3 (p-C) 144.6 (i-C), 145.1 (o-C). 29Si
NMR (C6D6, 79.49 MHz): δ = 3.88 ppm. C19H36AlNSi (333.56):
calcd. C 68.41, H 10.88; found C 67.41, H 10.65.
been carried over into the reaction with iBu2AlH. The H
NMR spectrum of the precursor did not show any impuri-
ties, however.
Summary
iBu2AlN(Dipp)SiMe3 (2): A solution of DippN(H)SiMe3 (2.7 g,
10.8 mmol) in hexanes (50 mL) was treated with 6.8 g of 25%/wt
hexanes solution of diisobutylaluminum hydride (DIBAL)
(12 mmol, 1.7 g) to afford a colorless viscous oil; yield 4 g, 95%
based on DippN(H)SiMe3. 1H NMR (C6D6, 400.13 MHz): δ = 0.22
(s, 9 H, SiMe3), 0.34 [d, J = 7.6 Hz, 4 H, CH2CH(CH3)2], 1.00 [d,
12 H, J = 6.6 Hz, CH2CH(CH3)2], 1.22 [d, J = 6.9 Hz, 6 H,
CH(CH3)2], 1.24 [d, J = 6.9 Hz, 6 H, CH(CH3)2], 1.97 [sept, 2 H,
Monomeric aluminum amides featuring three-coordinate
Al and N centers can be obtained in simple procedures
from readily available starting materials. Compounds 1–4
were prepared via ethane or hydrogen elimination reactions,
where as compounds 5 and 6 were obtained using salt elimi-
nation metathesis. The crystalline species 5 is a rare example
in which the planes at Al and N are almost coplanar. The
dinuclear compound 6 adopts a ladder structure with an
Al–Et–Al bridge, and the unusual compound 7, which pos-
J
= 6.7 Hz CH2CH(CH3)2], 3.55 [sept, 2 H, J = 6.9 Hz,
CH(CH3)2], 7.05 (m, 3 H, aromatic). 13C{1H} NMR (C6D6): δ =
3.5 (SiMe3), 23.9 [CH2CH(CH3)2], 24.9 [CH(CH3)2], 25.1
sesses an eight-membered Al3LiH4 ring core, was isolated [CH(CH3)2], 26.3 [CH2CH(CH3)2], 28.7 [CH(CH3)2], 28.8
[CH2CH(CH3)2], 124.0 (m-C), 124.4 (p-C), 144.8 (i-C), 145.1 (o-C).
in one instance probably due to impurities in one of the
starting materials. Conversions of compounds 1–5 into the
corresponding cationic species [R2AlN(Ar)SiRЈ3]+ are cur-
rently underway and will be reported in a future contri-
bution.
29Si NMR (C6D6, 79.49 MHz): δ = 3.91 ppm.
Et2AlN(Mes)SiMe3 (3): A solution of MesN(H)SiMe3 (10 mmol,
2.07 g) in hexanes (50 mL) was treated with AlEt3 (10 mmol,
1
1.14 g) to obtain 3 as a light yellow oil; yield 2.5 g, 86%. H NMR
(C6D6, 400.13 MHz): δ = 0.09 (q, J = 8.2 Hz, 4 H, Al-CH2CH3),
0.13 (s, 9 H, SiMe3), 0.98 (t, J = 8.2 Hz, 6 H, Al-CH2CH3), 2.15
[s, 6 H, o-Me(Mes)], 2.16 [s, 3 H, p-Me(Mes)], 6.83 [s, 2 H, m-
H(Mes)], 13C{1H} NMR (C6D6, 100.62 MHz): δ = 1.81 (Al-
CH2CH3), 3.18 (SiMe3), 8.72 (Al-CH2CH3), 21.21 [o-Me(Mes)],
21.19 [p-Me(Mes)], 129.65 (m-C) 131.9 (p-C), 134.5 (o-C), 145.3 (i-
C). 29Si NMR (C6D6, 79.49 MHz): δ = 3.72 ppm. C16H30AlNSi
(291.48): calcd. C 65.93, H 10.37; found C 64.37, H 10.05.
Experimental Section
General Procedures: All experiments were conducted under a nitro-
gen atmosphere using standard Schlenk techniques or in a Vacuum
Atmospheres dry box unless otherwise noted. Dry, oxygen-free sol-
vents were used unless otherwise indicated. NMR spectra were re-
corded on a Bruker Avance 400 MHz spectrometer. 1H NMR
chemical shift values were determined relative to the residual pro-
tons in C6D6 as internal reference (δ = 7.16 ppm), and 13C NMR
spectra were referenced to the solvent signal (δ = 128.39 ppm). 29Si
NMR spectra were referenced to external Me4Si in C6D6. FTIR
spectra were recorded using a Nicolet Magna 550 FTIR spectrome-
ter equipped with ATR in the range of 4000–530 cm–1. Melting
points were determined in Pyrex capillary tubes sealed under nitro-
gen with a Mel-Temp apparatus and are uncorrected. Elemental
analyses were performed by Columbia Analytical Services in Tuc-
iBu2AlN(Mes)SiMe3 (4):
A
solution of MesN(H)SiMe3
(10.0 mmol, 2.07 g) in hexanes (50 mL) was treated with 5.92 g of
25%/wt hexanes solution of DIBAL (10.4 mmol, 1.48 g) to obtain
4 as colorless viscous oil of ca. 90% purity; yield 2.6 g, 75% based
on amine. 1H NMR (C6D6, 400.13 MHz): δ = 0.18 (s, 9 H, SiMe3),
0.29 [d, J = 7.4 Hz, 4 H, CH2CH(CH3)2], 0.96 [d, 12 H, J = 6.5 Hz,
CH2CH(CH3)2], 1.85 [m, 2 H, CH2CH(CH3)2], 2.15 [s, 3 H, p-
Me(Mes)], 2.21 [s, 6 H, o-Me(Mes)], 6.85 [s, 2 H, m-H(Mes)].
13C{1H} NMR (C6D6, 100.62 MHz): δ = 3.5 (SiMe3), 21.2 [p-
Me(Mes)], 21.3 [o-Me(Mes)], 23.8 [CH2CH(CH3)2], 26.3
[CH2CH(CH3)2], 28.9 [CH2CH(CH3)2], 129.7 (m-C), 132.0 (p-C),
134.5 (o-C), 145.6 (i-C). 29Si NMR (C6D6, 79.49 MHz): δ =
3.87 ppm.
[22]
son, AZ. DippN(H)SiMe3,[21] MesN(H)SiMe3 and Ph2Si{N(H)-
[23]
Mes}2 were prepared using modified literature procedures. The
new aluminum amides 1–4 were synthesized according to a general
method.[8]
MesN(H)SiPh3: A solution of MesNH2 (20 mmol, 2.7 g) in ben-
zene (50 mL) was treated with a 1.6 m solution of nBuLi in hexanes
(22 mmol, 13.75 mL, 10% excess) at 0 °C. The resulting slurry was
stirred for 30 min at room temperature followed by addition of tri-
phenylsilyl chloride (20 mmol, 5.89 g) to the reaction mixture. The
resulting mixture was heated at 80 °C for 15 h and then cooled to
room temperature and filtered through a celite bed to remove the
LiCl salt. The solvent was removed under reduced pressure to af-
ford 7.63 g (97% crude yield) of a pale white solid and used as such
without further purification. 1H NMR (C6D6, 400.13 MHz): δ =
2.09 [s, 6 H, o-Me(Mes)], 2.15 [s, 3 H, p-Me(Mes)], 3.05 (br. s,
w1/2 = 6 Hz, 1 H, NH), 6.72 [s, 2 H, m-H(Mes)], 7.18 (m, 9 H, Ph),
7.71 (m, 6 H, Ph). 13C{1H} NMR (C6D6, 100.62 MHz): δ = 20.6
[o-Me(Mes)], 20.7 [p-Me(Mes)], 128.1 [p-C(Ph)], 129.7 [m-C(Mes)],
129.9 [m-C(Ph)], 131.3 [p-C-(Mes)], 132.0 [o-C(Mes)], 135.9 [o-
C(Ph)], 136.5 [i-C(Ph)], 140.1 [i-C(Mes)].
Melting Point Depression Experiment: 0.17 g of 1 was dissolved in
benzene (5.0 g). The freezing point of the mixture was measured
by slowly cooling the mixture, and the temperature was recorded
once every 15 s. This procedure was repeated three times. Calcu-
lated molality 0.102 mol/Kg (molecular weight 333.57 g/mol); ex-
perimental molality 0.103 mol/Kg (molecular weight 330 g/mol).
Et2AlN(Dipp)SiMe3 (1): The synthesis of compounds 1–4 is illus-
trated by the procedure for 1, which is described here. A solution
of DippN(H)SiMe3 (2.0 g, 8.0 mmol) in hexanes (50 mL) was
treated with Et3Al (0.98 g, 8.6 mmol) in hexanes (20 mL) at 0 °C
under nitrogen. The reaction mixture was stirred for 1 h at room
temperature and refluxed for an hour to ensure the completion of
the reaction. The solvent and volatile side products were removed
under reduced pressure to afford a light yellow oil; yield 2.6 g, 97%.
1H NMR (C6D6, 400.13 MHz): δ = 0.15 (q, 4 H, J = 8.2 Hz,
AlCH2), 0.17 (s, 9 H, SiMe3), 1.03 (t, 6 H, J = 8.2 Hz, AlCH2CH3),
1.14 [d, 6 H, J = 6.9 Hz, CH(CH3)2], 1.23 [d, 6 H, J = 6.9 Hz,
Et2AlN(Mes)SiPh3 (5): A 1.6 m solution of n-butyllithium in hex-
anes (14 mmol, 8.75 mL) was added dropwise to a slurry of
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Low-Coordinate Aluminum Amides
MesN(H)SiPh3 (13.7 mmol, 5.39 g) in hexanes (50 mL) at 0 °C. The freezer to obtain a light yellow crystalline solid; m.p. 162–164 °C;
reaction mixture was stirred for 2 h. The solvent and volatile side
products were removed in vacuo. Hexanes (50 mL) was added to
the dry solid followed by addition of AlEt2Cl (13.7 mmol, 1.65 g)
at 0 °C. The reaction mixture was stirred overnight at 50 °C. The
light yellow clear solution was separated from the LiCl by cannula
and concentrated under reduced pressure to obtain a yellow viscous
oil. The viscous oil was dissolved in hexanes (ca. 10 mL) and al-
lowed to crystallize at room temperature for two days. Colorless
blocks were obtained which were used for X-ray diffraction; yield
1.2 g, 18.3%; m.p. 92–95 °C. 1H NMR (400.13 MHz, C6D6): δ =
0.06 (q, J = 8.2 Hz, 4 H, AlCH2CH3), 0.96 (t, J = 8.2 Hz, 6 H, Al-
CH2CH3), 2.13 (s, 6 H, o-Me), 2.17 (s, 3 H, p-Me), 6.75 [s, 2 H, m-
H(Mes)], 7.17 (m, 9 H, Ph), 7.62 (m, 6 H, Ph). 13C{1H} NMR
(100.62 MHz, C6D6): δ = 2.73 (AlCH2CH3), 8.79 (AlCH2CH3),
21.19 (p-Me), 21.97 (o-Me), 128.60 (p-Ph), 130.15 (m-Mes), 130.27
(m-Ph), 132.78 (p-Mes), 135.88 (o-Mes), 136.63 (o-Ph), 138.19 (i-
Ph), 143.41 (i-Mes). C31H36AlNSi (477.69): calcd. C 77.94, H 7.60;
found C 77.7, H 7.0.
yield 2.0 g, 32%. 1H NMR (400.13 MHz, C6D6): δ = 0.74 (br. s,
w1/2 = 40 Hz, 4 H, Al-CH2CH3), 1.08 (t, J = 7.7 Hz, 6 H, Al-
CH2CH3), 2.08 (s, 12 H, o-Me), 2.30 (s, 6 H, p-Me), 6.69 [s, 4 H,
m-H(Mes)]; 6.9–7.02 (m, 6 H, Ph), 7.35 (br. s, 4 H, Ph). 13C{1H}
NMR (100.62 MHz, C6D6): δ = 5.8 (br., Al-CH2CH3), 9.5 (Al-
CH2CH3), 20.9 [p-Me(Mes)], 22.7 [br., o-Me(Mes)], 127.7 [m-
C(Ph)], 130.2 [p-C(Ph)], 130.8 [o-C(Mes)], 134.3 [o-C(Mes)], 136.3
[o-C(Mes)], 137.5, 141.9 [i-C(Mes)]. 29Si NMR (C6D6, 79.49 MHz):
δ = –24.4. C38H52Al2N2Si (618.88): calcd. C 73.75, H 8.47; found
C 73.63, H 8.18.
{MesN(SiMe3)Al(iBu)(μ-H)}2(μ-LiH)(μ-iBu2AlH) (7): A solution
of MesN(H)SiMe3 (prepared from old nBuLi) (2.07 g, 10 mmol) in
hexanes (30 mL) was treated with 6.5 g of 25wt.-% hexanes solu-
tion of DIBAL (1.62 g, 11.4 mmol) at 0 °C under nitrogen. The
reaction mixture was stirred for 1 h at room temperature and re-
fluxed for an hour to ensure the completion of the reaction. The
solvent and volatile side products were removed under reduced
pressure and the resulting light yellow solution viscous oil was dis-
solved in a minimum amount of benzene (3–5 mL) for crystalli-
zation. A colorless crystalline solid was obtained after 1 d at room
temperature; m.p. 254 °C; yield 600 mg, 8% based on amine. 1H
NMR (400.13 MHz, C6D6): δ = 0.19 (s, 18 H, SiMe3), 0.56–0.60
[m, 8 H, CH2CH(CH3)2], 1.23 [d, J = 6.5 Hz, 12 H, CH2CH-
(CH3)2], 1.28 [d, J = 6.5 Hz, 12 H, CH2CH(CH3)2] 1.99 [s, 6 H, p-
Me(Mes)], 2.2 [br., 12 H, o-Me(Mes)], 2.1–2.2 (br., 2 H, Al-H or
Li-H), 2.1–2.2 [br., 4 H, CH2CH(CH3)2], 2.99–3.6 (br., 2 H, Al-H
or Li-H), 6.59 [br., 4 H, m-H(Mes)]. 13C{1H} NMR (100.62 MHz,
C6D6): δ = 3.1 (SiMe3), 20.9 [o-Me(Mes)], 21.0 [p-Me(Mes)], 21.8
[CH2CH(CH3)2], 23.3 [CH2CH(CH3)2], 27.0 [CH2CH(CH3)2], 27.4
[CH2CH(CH3)2], 28.6 [CH2CH(CH3)2], 28.7 [CH2CH(CH3)2],
Ph2Si{N(Mes)AlEt2}2 (6): A 1.6 m solution of nBuLi in hexanes
(20.0 mmol, 12.5 mL) was added dropwise to
a slurry of
Ph2Si{N(H)Mes}2 (9.9 mmol, 4.50 g) in hexanes (50 mL) at 0 °C.
The reaction mixture was warmed to room temperature and stirred
for 2 h. The solvent and volatile side products were removed in
vacuo. Hexanes (50 mL) was added to the dry solid followed by
addition of AlEt2Cl (2.41 g, 20.0 mmol) at 0 °C. The reaction mix-
ture was allowed to stir overnight at room temperature. The prod-
uct precipitated as a white solid, and at this point all volatile side
products were removed, and the remaining dry solid was dissolved
in dichloromethane (50 mL). Lithium chloride was filtered off, and
the colorless filtrate was concentrated to 20 mL and kept in a
Table 1. Crystal data and refinement details for compounds 5–7.
5
6
7
Empirical formula
Formula weight
T [K]
Wavelength [Å]
Crystal system
Space group
C31H36AlNSi
477.68
100(2)
1.54178
monoclinic
P21/c
C38H52Al2N2Si
618.87
100(2)
C40H80Al3LiN2Si2
733.12
100(2)
1.54178
monoclinic
Cc
1.54178
triclinic
¯
P1
a [Å]
b [Å]
c [Å]
α [°]
14.9904(14)
10.1017(10)
17.878(2)
90
95.538(8)
90
2694.6(5)
4
1.177
11.190(4)
12.262(4)
15.112(5)
69.298(14)
79.722(12)
66.716(10)
1779.9(10)
2
11.6332(8)
33.555(2)
12.8301(12)
90
104.585(9)
90
4846.9(6)
4
1.005
β [°]
γ [°]
V [Å3]
Z
Dcalcd. [Mgm–3]
μ(Cu-Kα) [mm–1]
F(000)
1.155
1.259
668
1.213
1024
1.370
1616
Crystal size [mm]
Crystal color and habit
θ [°]
0.25ϫ0.22ϫ0.15
colorless block
4.97–52.63
13572
3082 [R(int) = 0.0849]
3082/0/312
0.1310
0.0533
1.000
1955
0.48ϫ0.28ϫ0.24
yellow block
4.77–67.12
21264
5956 [R(int) = 0.0792]
5956/3/407
0.1585
0.0541
1.003
4090
0.52ϫ0.32ϫ0.26
colorless block
2.63–69.47
17623
4511 [R(int) = 0.0480]
4511/12/406
0.2451
0.0876
1.026
4012
0.06(7)
Reflections collected
Independent reflections
Data/restraints/parameters
wR2 (F2 all data)[a]
R1 (F obsd. data)[a]
Goodness-of-fit on F2
Observed data [I Ͼ 2σ(I)]
Absolute structure parameter
Largest diff. peak and hole [e/Å3]
0.185 and –0.258
0.271 and –0.435
0.972 and –0.711
[a] wR2 = {S [w(Fo – Fc ) ]/S [w(Fo2)2]}1/2 R1 = S||Fo| – |Fc||/S|Fo|.
2
2 2
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525

M. Khandelwal, D. R. Powell, R. J. Wehmschulte
FULL PAPER
131.3 (Mes), 149.9 (Mes). IR: v(Al-H) 1754 cm–1 (br). C40H80Al3-
LiN2Si2 (733.12): calcd. C 65.53, H 11.00; found C 65.13, H 10.70.
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2009.
X-Ray Crystallography: Intensity data for compounds 5–7 were col-
lected using a diffractometer with a Bruker APEX ccd area detec-
tor and graphite-monochromated Cu-Kα radiation (λ = 1.54178 Å)
at 100(2) K. The data were corrected for absorption by the semi-
empirical method.[24] The structures were solved by direct methods
and refined by full-matrix least-squares methods on F2.[25] Hydro-
gen atom positions were initially determined by geometry and re-
fined by a riding model. Non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atom displacement
parameters were set to 1.2 (1.5 for methyl) times the displacement
parameters of the bonded atoms. Hydrogen atoms bonded to the
aluminum centers in 7 were located on a difference map, and their
positions were allowed to refine independently. A single isotropic
displacement parameter was refined for these hydrogen atoms.
Some details of the data collections and refinements are given in
Table 1, and selected bond lengths and angles are given in the Fig-
ure legends.
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CCDC-788550 (for 5), -788551 (for 6), and -788552 (for 7) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
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Financial support for this work from the National Science Founda-
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the National Science Foundation (CHE 03422510) for the purchase
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Received: August 12, 2010
Published Online: December 15, 2010
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