Syntheses and structures of [(Me3N)3SiNHLi] 4, (C4H8O)Al[NHSi(NMe2) 3]3, ((Me2N)3SiNH)3Al and Li(THF)2+[((Me2N)3SiNH) 4Al]-

Article abstract of DOI:10.1039/b209605f

The preparation of lithium tris(dimethylamino)silylamide, [(Me ₃N)₃SiNHLi]₄, 2, and its reaction with aluminium trichloride to give tris(dimethylamino)silylamino)(tetrahydrofuran) alane, (C₄H₈O)Al[NHSi(NMe₂)₃] ₃, 3, and bis(tetrahydrofuran)lithium tetrakis(tris(dimethylamino) silylamino)alanate, Li(THF)₂⁺[((Me₂N) ₃SiNH)₄Al]^-, 6, is reported, together with the crystal structures of 2, 3 and 6. Tris(dimethylamino)alane, ((Me ₂N)₃SiNH)₃Al, was obtained by reaction of tris(dimethylamino)silylamine with aluminium triethyl. The ammonolytic gelation of 3 to an imidoaluminosilicate gel is also described. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b209605f

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Syntheses and structures of [(Me3N)3SiNHLi]4,
(C4H8O)Al[NHSi(NMe2)3]3, ((Me2N)3SiNH)3Al and
Li(THF)2؉[((Me2N)3SiNH)4Al]؊
John S. Bradley,a Fei Cheng,*a Stephen J. Archibald,a Ralf Supplit,a Riccardo Rovai,b
Christian W. Lehmann,b Carl Krügerb and Frédéric Lefebvrec
a Department of Chemistry, University of Hull, Cottingham Road, Hull, UK HU6 7RX.
E-mail: f.cheng@hull.co.uk
b Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1,
D-45470 Mülheim an der Ruhr, Germany
c Laboratoire de Chimie, Organométallique de Surface, 44Bd Dull November 1918, 69616,
Villeurbanne Cedex, France
Received 1st October 2002, Accepted 4th December 2002
First published as an Advance Article on the web 28th March 2003
The preparation of lithium tris(dimethylamino)silylamide, [(Me3N)3SiNHLi]4, 2, and its reaction with aluminium
trichloride to give tris(dimethylamino)silylamino)(tetrahydrofuran)alane, (C4H8O)Al[NHSi(NMe2)3]3, 3, and
bis(tetrahydrofuran)lithium tetrakis(tris(dimethylamino)silylamino)alanate, Li(THF)2 [((Me2N)3SiNH)4Al]Ϫ, 6,
ϩ
is reported, together with the crystal structures of 2, 3 and 6. Tris(dimethylamino)alane, ((Me2N)3SiNH)3Al, was
obtained by reaction of tris(dimethylamino)silylamine with aluminium triethyl. The ammonolytic gelation of 3 to an
imidoaluminosilicate gel is also described.
The co-ammonolysis of mixtures of metalalkylamides has
recently been reported as a route to multinary silicon-based
Introduction
The chemistry of compounds in which a main-group element is
bound to a maximum number of nitrogen atoms is under scru-
tiny as a potential source of precursor molecules for the chem-
ical synthesis of nitride-based inorganic materials. We recently
reported a method for the preparation, by a non-aqueous,
ammonolytic sol–gel technique, of a high surface area porous
silicon diimide gel by the acid catalyzed ammonolysis of tris-
(dimethylamino)silylamine, (Me2N)3SiNH2, TDSA, 1, thus
establishing an imide analog to the hydrolytic sol–gel chemistry
of orthosilicate esters to form silica gels.1 This chemistry has
two immediate areas of impact. First, since silicon diimide
is a precursor for high purity silicon nitride,2 this sol–gel
method leading to imidosilicate gels provides a potentially
important method for the preparation and processing of
ceramics based on silicon nitride, and similarly for nitridosilicate
solids.3 Secondly there is current interest in the preparation of
micro- and mesoporous non-oxide solids analogous to the
wide range of porous oxide materials. Materials in this form are
now recognised as offering technological opportunities beyond
their traditional uses as adsorbents and catalysts and catalyst
supports,4 and as the ammonolytic sol–gel method produces
high surface area porous imidosilicate solids, we anticipate its
extensive use in this application.
This chemistry has a further related application to the pre-
parative chemistry of crystalline multinary nitridosilicates,5,6
recently reviewed by Schnick and Huppertz3 These important
materials, which are the nitride analogs to the oxosilicates
which dominate the inorganic chemistry of silicon, are pre-
pared by solid state synthesis techniques at high temperature,
for example by the reaction between metal powders and silicon
diimide under r.f. heating.7 The high temperatures used in these
preparations are necessary to overcome the slow diffusion rates
of atoms in the solid particles of reactants and products. As a
general synthetic strategy these diffusion rates are increased by
use of high reaction temperatures (1000–1500 ЊC), and thus, as
a consequence of the high temperatures involved, only the most
thermodynamically stable products are obtained. The use of
bimetallic imidosilicate gels as starting materials in which the
metal atoms are already dispersed at atomic level should allow
a wider range of crystalline multinary nitridosilicates to be
prepared, at lower temperatures.
nitride ceramics.8 We are following an analogous sol–gel route
to these important materials but using as starting materials
presynthesised multinary silylamides. The chemistry of 1 is
particularly ammenable to the preparation of a wide range of
such compounds. The synthetic strategy we have adopted is
to assemble precursor molecules containing, in addition to
peripheral Si(NMe2)3 groups, an M(NHSiN3)n core. In such
compounds each of the ceramogenic metal centres is in an
exclusively MN4 environment. By these means we expect to be
able to use the proven ammonolytic susceptibility1 of the
Si(NMe2)3 periphery of the molecule to develop ternary imido-
silicate gels in which M and Si atoms are interdispersed on the
atomic level, thus extending the ammonolytic sol–gel method to
the preparation of multinary nitride ceramics.
The compounds whose syntheses and structures we report
here were designed as a means to allow us to explore further the
use of single source precursors for the preparation via
ammonolytic sol–gel chemistry of imidometalosilicates (nitro-
gen analogues to oxidic metalosilicates), and, from them, of
metal nitridosilicates. Given the extensive chemistry of alumino-
silicates in oxide chemistry, we chose imidoaluminosilicates
as our first target system. As has been recognized by Roesky
and coworkers9 such nitrogen analogues to aluminosilicates
were unknown until 1997.
We report here the use of 1 via its lithium salt, lithium
tris(dimethylamino)silylamide, [(Me2N)3SiNHLi]4, 2, as
a
reagent for the preparation of two new imidoaluminosilicates,
tris(tris(dimethylamino)silylamino)alane as its THF adduct,
(THF)Al(NH(Si(NMe2)3)3, 3. THF-free tris(tris(dimethyl-
amino)silylamino)alane, Al(NH(Si(NMe2)3)3, 4, was also pre-
pared by the protolysis reaction of 1 with triethylaluminium.
Dilithio(tetrakis(tris(dimethylamino)silylamino))alanate, as its
bis-THF adduct, [(THF)2Li]ϩ[((Me2N)3SiNH)4Al]Ϫ, 6, was
prepared by reaction of 2 with anhydrous aluminium trichlor-
ide in the appropriate molar ratios. The X-ray crystal structures
of 2, for which we anticipate extensive application in the syn-
thesis of silylamide compounds, of 3 and of 6 are reported.
In the context of their potential as precursors for carbon-free
imidosilicate gels none of these compounds contain Si–C or
Al–C bonds, and each of the ceramogenic elements Si and Al
are in exclusively MNn environment. We show also that 3 and 4
D a l t o n T r a n s . , 2 0 0 3 , 1 8 4 6 – 1 8 5 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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are useful precursors for the preparation of imidoalumino-
silicate gels.
ring molecules to be isolated. Examples of this are the TMEDA
and PMDETA complexes of pyrrrolidinyl lithium12 in which
three Li2N2 rings form a condensed four-rung ladder as is
the case for 2. In 2 the necessary complexing agent, which
coordinates to the terminal lithium atoms, is the additional
functionality provided by the dimethylamino group of one of
the tris(dimethylamino)silylamido groups, and thus 2 is a
self-terminating ladder structure.
Results and discussion
Lithium tris(dimethylamino)silylamide, [(Me2N)3SiNHLi]4, 2
Compound 1, prepared from dimethylamine, silicon tetra-
chloride and ammonia,1 reacted with one equivalent of
n-butyllithium in mixed pentane–hexane solution at 0 ЊC with
evolution of one equivalent of butane. The product, 2, was
isolated in quantitative yield by crystallization at Ϫ80 ЊC from
the reaction mixture as a white, air-sensitive solid, and was
characterized by 1H, 13C, 15N and 29Si NMR and FTIR spectro-
scopy. The IR spectrum of the crystalline solid showed clearly
the disappearance of the two characteristic absorptions at 3420
and 3390 cmϪ1 due to the symmetric and asymmetric NH
stretching modes of the SiNH2 group in 1. A new band at 3354
cmϪ1 was assigned to the ν(NH) stretching mode of a SiNHLi
group, consistent with lithiation of the amino group of 1. The
1H NMR confirms the occurrence of the deprotonation reac-
tion and comprises two signals, the first at 2.50 ppm, assigned
to the Si(NMe2)3 (18H), and a second at Ϫ1.70 ppm assigned to
the Si–NHLi proton (1H) in the appropriate ratio for 2 (for 1,
only one resonance is seen for the methyl protons at 2.46 ppm,
while the Si–NH protons are found at 0.31 ppm). The 13C NMR
shows only one resonance at 39.28 ppm. The 15N spectrum
shows two resonances, the first at Ϫ373 ppm assigned to the
NMe2 nitrogen ( Ϫ374 ppm in 1), and the second at Ϫ364 ppm,
assigned to the Si–NHLi nitrogen.
The 1H and 13C NMR spectra of 2 require comment since as
can be seen from Fig. 1, several environments are distinguish-
able for the dimethylamino methyl groups. At the lowest level of
differentiation the dimethyamino groups are either associated
with a lithium cation or are free, and the observation of only a
single resonance in the 13C and 1H spectra implies an exchange
of these environments in solution which is rapid on the NMR
time scale.
Tris(dimethylaminosilylamino)(tetrahydrofuran)alane, 3
The reaction of 2 with aluminium trichloride in THF under
mild conditions results, after addition of pentane, in the pre-
cipitation of a quantitative amount of lithium chloride and the
formation of 3 in high yield. IR and NMR spectroscopy were
consistent with the formulation of 3 as tris(dimethylamino-
silylamino)(tetrahydrofuran)alane, (C4H8O)Al[NHSi(NMe2)3]3.
The IR spectrum exhibited bands at 3335 cmϪ1 (ν(NH)) and
1555 cmϪ1 (δ(NH)) in addition to absorptions attributable to
1
(ν(CH)) at 2980, 2835 and 2769 cmϪ1. The H NMR spectrum
exhibited a singlet at 2.27 ppm and a low intensity broad reson-
ance at 0.00 ppm ascribable to SiNMe and SiNHAl groups
respectively. In addition a triplet at 3.90 and a quintet at 1.33
ppm due to THF were observed. 13C NMR showed three reson-
ances which are assigned to the dimethylamino groups (38.58
ppm) and to THF (69.12 and 25.10 ppm), respectively. The 29Si
NMR spectrum contained only one singlet at Ϫ31.65 ppm. All
attempts to obtain acceptable elemental analyses for 3 were
unsuccessful. Mass spectrometry was also unsuccessful, result-
ing in the facile displacement of THF from 3 and subsequent
decomposition.
Crystals of 2 suitable for X-ray diffraction were grown
from pentane solution at Ϫ80 ЊC. The structure of 2 is shown in
Fig. 1.
Crystals of 3 suitable for X-ray diffraction were grown from
pentane solution at Ϫ80 ЊC. The appearance of the crystals
suggested some decomposition had occurred prior to attempt-
ing data collection. This may be due to loss of THF from the
crystal sample and resulted in a lower quality data set, however
a satisfactory refinement was achieved
The molecular structure of 3 is shown in Fig. 2. The central
aluminium atom in 3 is bonded to three tris(dimethylamino)-
silylamino groups. The molecule contains an approximately
tetrahedral AlON3 core but with considerable variation among
the three Al–N bonds (1.722(4), 1.774(4) and 1.817(4) Å. The
angles subtended at the central Al atom by each of the three
nitrogen atoms and the THF oxygen atom varied from
97.00(15) to 119.00(19)Њ. The geometry of the AlON3 core of
3 reflects the larger steric bulk of the tris(dimethylamino)-
silylamino groups in comparison to THF.
Fig. 1 ORTEP10 plot of the molecular structure of compound 2.
Hydrogen atoms in the dimethylamino groups are omitted for clarity.
Selected bond lengths (Å) and angles (Њ): Si(1)–N(1) 1.674(2), Si(1)–
N(2) 1.704(3), Si(1)–N(3) 1.763(2), Si(1)–N(4) 1.722(3), Li(1)–N(5)
2.007(5), Li(1)–N(1) 2.058(5), Li(1)–N(4) 2.619(5), Li(1)–N(5)*
1.980(5); N(1)–Si(1)–N(4) 106.3, N(2)–Si(1)–N(1) 115.7, N(3)–Si(1)–
N(1) 107.5, N(3)–Si(1)–N(2) 108.3, N(1)*–Li(1)–N(5) 104.9, N(1)*–
Li(1)–N(1) 105.2, N(5)–Li(1)–N(4) 122.6, N(5)–Li(1)–N(1) 135.2,
N(1)*–Li(1)–N(4) 116.2. Equivalent positions are generated by the
symmetry operator: Ϫx, Ϫyϩz, Ϫz.
Tris(tris(dimethylamino)silylamino)alane, Al(NH(Si(NMe2)3)3, 4
Attempts to prepare a THF-free form of 3 by reaction of 2 with
AlCl3 in pentane were unsuccessful, resulting only in low yields
of Liϩ[((Me2N)3SiNH)4Al]Ϫ, (see 5 and 6 below). An alternative
approach, taking advantage of the weak acidity of the NH2
protons in 1 was more fruitful. When three equivalents of 1 in
toluene were allowed to react with one equivalent of aluminium
triethyl, added dropwise as toluene solution, at 50 ЊC, the pro-
tolysis reaction shown in eqn. (1) occurs smoothly over a period
of 24 h and affords a quantitative yield of ethane (measured
volumetrically) and, on evaporation of the toluene, the new
silylamidoalane Al[NHSi(NMe2)3]3, 4, as a viscous oil.
The molecule possesses a crystallographic inversion centre,
and comprises a central Li2N2 ring which shares two opposite
Li–N edges with two further Li2N2 rings forming a four-rung
ladder. All three Li2N2 rings are planar, and the dihedral angles
between the central ring and the outer rings are 135.5 0.5Њ.
Condensed ladder structures such as this are often found in
lithium amide chemistry when the lithium cation is complexed
by a Lewis base.11 Uncomplexed lithium amides commonly
comprise (LiN)x rings condensed in extensive amorphous struc-
tures, but complexation of the cation allows small condensed
All attempts to crystallise 4 were unsuccessful, and the
compound was characterised solely by IR, NMR, cryoscopic
D a l t o n T r a n s . , 2 0 0 3 , 1 8 4 6 – 1 8 5 1
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(eqn. (2)). Filtration and evaporation of the solvent gave an
colorless oily product identified by elemental analysis, FTIR
and NMR spectroscopy as Liϩ[((Me2N)3SiNH)4Al]Ϫ, 5. Addi-
tion of two equivalents of THF to a pentane solution of this
product and cooling to Ϫ80 ЊC yielded the white crystalline
THF adduct Li(THF)2ϩ[((Me2N)3SiNH)4Al]Ϫ, 6. Crystals of 6
suitable for X-ray diffraction were grown from pentane–THF
solution at Ϫ80 ЊC. The structure of 6 is shown in Fig. 3. The
molecule possesses crystallographic C2 symmetry, with the
central aluminium atom bound to four tris(dimethylamino)-
silylamino nitrogen atoms in a distorted tetrahedral fashion.
The two Al–N(1) distances involving imido nitrogen atoms
which are also coordinated to the lithium cation are slightly
longer (1.893 Å) than the remaining two Al–N(2) distances
(1.842 Å). The N(1)–Al–N(1)* angle in the four-membered
AlN2Li ring is more acute (102.8(1)Њ) than the N(2)–Al–N(2)*
angles (119.1(1)Њ).
Fig. 2 ORTEP10 plot of the molecular structure of compound 3.
Hydrogen atoms in the dimethylamino groups and THF are omitted for
clarity. Selected bond lengths (Å) and angles (Њ): Al(1)–N(1) 1.722(4),
Al(1)–N(2) 1.817(4), Al(1)–N(3) 1.774(4), Al(1)–O(1) 1.877(3), Si(1)–
N(1) 1.583(4), Si(2)–N(2) 1.689(4), Si(3)–N(3) 1.689(4); N(1)–Al(1)–
N(3) 119.00(19), N(1)–Al(1)–N(2) 114.11(18), N(3)–Al(1)–N(2)
113.84(17), N(1)–Al(1)–O(1) 97.00(15), N(3)–Al(1)–O(1) 104.86(16),
N(2)–Al(1)–O(1) 104.82(16), Si(1)–N(1)–Al(1) 129.1(2), Si(2)–N(2)–
Al(1) 133.3(2), Si(3)–N(3)–Al(1) 137.7(4).
(1)
molecular weight determination and elemental analysis. The
comparison between the IR spectra of the starting silylamine 1
and the product 4 shows the disappearance of the two ν(NH2)
bands at 3450 and 3400 cmϪ1, characteristic of 1, and the
appearance of a new absorbance at 3330 cmϪ1, assigned to
the Al–NH–Si linkage. Mass spectroscopic analysis was not
possible for 4, owing to decomposition during the measure-
ment liberating 1 and dimethylamine, probably through self-
condensation reactions. The molecular weight of 4 was
measured cryoscopically in benzene, and showed it to be
monomeric in solution (measured 548, calculated for 4 552).
Fig. 3 ORTEP10 plot of the molecular structure of compound 6.
Hydrogen atoms in the dimethylamino groups and THF are omitted for
clarity. Selected bond lengths (Å) and angles (Њ): Al(1)–N(1) 1.893(2),
Al(1)–N(2) 1.842(2), Si(1)–N(1) 1.718(1), Si(2)–N(2) 1.694(1), Li(1)–
N(1) 2.139(3), Li(1)–O(1Ј) 2.003(2); N(1)*–Al(1)–N(1) 1.028, N(2)*–
Al(1)–N(2) 119.1, Al(1)–N(1)–Li(1) 84.8, N(1)*–Li(1)–N(1) 87.5,
O(1Ј)*–Li(1)–O(1Ј) 95.0. Equivalent positions are generated by the
symmetry operator: Ϫxϩ2, y, Ϫzϩ1/2.
1
The H NMR and 13C NMR spectra contain only singlets, at
2.8 ppm and 41 ppm, respectively, evidence for the presence of a
single type of dimethylaminosilylamide group, while reson-
ances corresponding to the NH protons were not observed.
These data allow the formulation of 4 as monomeric
tris(tris(dimethylamino)silylamino)alane, Al(NH(Si(NMe2)3)3.
This is one of very few examples of non-spirocyclic triamido-
alanes. These include the sterically hindered dialkylamino-
alanes, such as Al(NMe2)3,13 Al(NEt2)3,14 Al(NPri2)3,15 and
Al[N(SiMe3)2]3.16 Moreover there is only one example reported
of a trisubstituted primary amidoalane Al((NHPh)3.17 An
intriguing cage imidoaluminosilicate was reported by Roesky
and coworkers from the reaction of the sterically hindered
triaminosilane RSi(NH2)3 (R = 2,6-iPr2C6H3NSiMe3) with tri-
methylaluminium, in a reaction analogous to that which we
have used here.18
(2)
Compound 6 shows the same structural features as the previ-
ously reported aluminium amide (Et2O)2Liϩ[Al(NMe2)4]Ϫ,19,20
in which the aluminium is in a tetrahedral coordination
environment, as is the lithium cation which is coordinated to
two diethylether molecules as external ligands and to two di-
methylamine nitrogen atoms. Compound 6 is nevertheless the
first structurally characterized molecular compound containing
an aluminium bound to four primary amide groups, suggesting
that the silylamide anion [(Me2N)3SiNH]Ϫ, due to its steric
dimensions, is particularly effective in stabilising otherwise
labile classes of metal imide molecules.
Bis(tetrahydrofuranato)dilithio(tetrakistris(dimethylamino)-
silylamino)alanate, [(THF)2Li؉]2[((Me2N)3SiNH)4Al]؊, 6
Ammonolysis and gelation of 3 and 4
The isolation of low yield of an anionic tetrakis(silylamido)-
alanate in the attempted synthesis of 4 from AlCl3 and 2 in
pentane in a 3 : 1 molar ratio prompted us to prepare this
compound in high yield by reaction of 2 with AlCl3 in a 4 : 1
ratio. The reaction proceeded smoothly in pentane at 20 ЊC
with the precipitation of 3.0 equivalents of lithium chloride
Having demonstrated previously the facile ammonolysis of
TDSA to give silicon diimide gels,1,21 we anticipated the reac-
tion of ammonia with these new ternary imidosilicates would
yield ternary imide gels.
On treating a solution of 3 in dry terahydrofuran with
a cooled (Ϫ80 ЊC) solution of ammonia in dry THF in the
D a l t o n T r a n s . , 2 0 0 3 , 1 8 4 6 – 1 8 5 1
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presence of a catalytic quantity of trifluoromethanesulfonic
acid at room temperature for 18 h followed by 72 h at 50 ЊC, a
translucent gel formed filling about half of the original volume
of the reaction mixture. Decantation of the solution above the
gel and evaporation of the remaining solvent and the di-
methylamine produced in the gelation process by a stream of
nitrogen followed by drying under reduced pressure at 50 ЊC for
6 h yielded a translucent white solid. The IR spectrum of this
gel is shown in Fig. 4. In addition to the broad ν(N–H) band at
its bis-THF adduct, [(THF)2Li]ϩ[((Me2N)3SiNH)4Al]Ϫ, 6 were
obtained when 2 was reacted with aluminium chloride in 4 : 1
molar ratio. It has been shown that 1 is a good starting material
for the synthesis of NH bridged imidosilicates based on the
acidity of its SiNH2 protons.
High surface area aluminoimidosilicate gels were successfully
prepared from the catalytic ammonolysis of 3 and 4 in dry
THF, thus extending the chemistry of 1 to the preparation of
multinary silicon based imide gels. The synthesis of a variety of
bimetallic silylamides derived from 1, and their uses as sources
of imidosilicate gels for the preparation of ternary nitride
ceramics are under active investigation.
3350–3450 cmϪ1, ν(N–C) at 1176 cmϪ1 and ν(Si–N) at 920 cmϪ1
,
there is a broad band at 712 cmϪ1 which can be ascribed to
ν(Al–N).22 The low intensity of ν(CH) bands from 2790 to 2973
cmϪ1 indicates that most of the dimethylamino groups in com-
pound 3 (Fig. 4(a)) have been removed. Both 29Si CP-MAS
NMR and 27Al CP-MAS NMR showed only single resonances
at Ϫ40 ppm and 120 ppm, respectively, corresponding to the
presence of Si[N4] and AlN4 species.23,24 13C CP-MAS NMR
showed only one broad resonance at 38 ppm ascribed to
residual dimethylamino groups, and no resonances due to THF
were observed. These results suggest that ammonolysis of 3
produced a silicon aluminium imide gel containing residual di-
methylamino groups, and that the THF in compound 3 has
been removed during the gelation. The gel exhibits a meso-
porous structure (type IV physisorption isotherm), with an
average pore diameter of 3.7 nm and a BET surface area of
Experimental
General procedures
All the procedures were performeded under protective nitrogen
atmosphere using standard Schlenk techniques or a nitrogen-
filled glove box. Silicon tetrachloride, dimethylamine and
ammonia were purchased from Fluka and used as received.
Aluminium trichloride was purchased from Aldrich and puri-
fied by sublimation. Trifluoromethansulfonic acid, n-butyl-
lithium (1.6 M in hexane) and aluminium triethyl were
purchased from Aldrich and used as received. Solvents were
dried and distilled under dry nitrogen by the usual methods.
High-resolution NMR spectra were obtained on a JEOL
JNM-LA400FT NMR spectrometer using C6D6 as solvent. IR
spectra were recorded on a Nicolet Magna-500 FTIR spectro-
meter. MS spectra were obtained by fractional distillation of
the sample on Finnigan MAT 8200 and 8400 spectrometers.
Elemental analyses were performed by Kolbe Microanalitiches
Labor (Mülheim an der ruhr, Germany). 29Si and 13C CP-MAS
NMR were recorded on a Bruker DSX-300 spectrometer oper-
ating at frequencies of 59.6 and 75.5 MHz, respectively. 27Al
was measured at frequency of 78.1 MHz with a single pulse
sequence. Nitrogen adsorption isotherms were performed at 77
K using a Micrometritics ASAP 2010 instrument, and surface
area was determined from BET analysis.
528 m2 gϪ1
.
Syntheses
(Me2N)3SiNH2, 1. 1 was prepared from silicon tetrachloride,
dimethylamine and ammonia by a modification of the previ-
ously reported method.1 Anhydrous dimethylamine (127 mL,
85.1 g, 1.90 mol) was condensed at Ϫ50 ЊC into a graduated
glass trap, and then passed under dry nitrogen into a solution
of silcon tetrachloride (47.2 g, 0.28 mol) in diethyl ether (1.5 L)
at Ϫ40 ЊC. The resulting suspension was stirred for 15 h
during which time the mixture had reached room temperature)
(ca. 20 ЊC). The resulting dimethylammonium chloride was
removed by filtration and washed with diethyl ether (2 × 200
mL). The combined ether solutions of (Me2N)3SiCl were
cooled to Ϫ50 ЊC and excess anhydrous liquid ammonia
(64 g, 4.3 mol) was added in two aliquots. After stirring for 15 h
the precipitated ammonium chloride was removed by filtration
at room temperature, and the filtrate concentrated under
reduced pressure leaving a colourless liquid residue of tris-
(dimethylamino)silylamine (Me2N)3SiNH2 (99.2% purity by
GC/MS). Yield 35.1 g (0,20 mol; 74% based on SiCl4) bp
186 ЊC (760 Torr). The product was distilled at 60 ЊC/0.1
Torr and stored under dry nitrogen. 1H NMR (400 MHz,
C6D6): δ 2.5 (s, 18H, Si(N(CH3)2)3, 0.31 (w, 2H, NH2Si); 13C
NMR (400 MHz, C6D6): δ 37.88 (SiN(CH3)2); 29Si NMR:
δ Ϫ33.78 ppm; IR (cmϪ1): 3474 (w), 3405 (w) (ν(NH2)); 1553
(m) (δ(NH)).
Fig. 4 IR spectra of (a) (THF)Al[NHSi(NMe2)3]3, 3 and (b) the gel
from 3; (a) and (b) were recorded by neat and KBr plate technique,
rspectively
A similar procedure using 4 as the gel precursor resulted in a
transparent gel after only 24 h, which after separation, washing
with THF and drying was shown to be homogeneous by EDAX
analysis (5 nm probe size). FTIR showed the presence of resid-
ual dimethylamino groups at a similar level to the gel derived
from 3, which could be removed by further treatment with
ammonia, yielding an aluminoimidosilicate gel with a surface
area of 310 m2 gϪ1 and a broad pore size distribution in the
mesopore range.
Summary
Two novel tris(tris(dimethylamino)silylamino)alanes: (THF)Al-
(NH(Si(NMe2)3)3, 3 and Al(NH(Si(NMe2)3)3, 4 were prepared
by reaction of tris(dimethylamino)silylamidolithium, 2 with
aluminium chloride and tris(dimethylamino)silylamine, 1 with
aluminium triethyl, respectively. Dilithio(tetrakis(tris(dimethyl-
amino)silylamino))alanate, [Li]ϩ[((Me2N)3SiNH)4Al]Ϫ, 5, and
[(Me2N)3SiNHLi]4, 2. To a rapidly stirred solution of 1 (3.8 g,
21.6 mmol) in pentane (150 mL) at 0 ЊC was added n-butyl-
lithium (13.4 mL, 1.6 M, 1.0 equivalents) under argon. Gas
evolution was observed, and after stirring at 0 ЊC for 6 h the
D a l t o n T r a n s . , 2 0 0 3 , 1 8 4 6 – 1 8 5 1
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solution was cooled to Ϫ80 ЊC for 18 h. The resulting white
crystalline solid, 3, was isolated by decantation and washing
with cold (–80 ЊC) pentane. Yield 2.9 g (16.4 mmol, 76%).
Evaporation of the combined decantate and washings followed
by crystallisation from pentane at Ϫ80 ЊC yielded a further 0.9 g
of 3 (5.2 mmol, 24%). Calc. for C24H76N16Si4Li4: C, 39.5; H,
10.5; N, 30.7; Li, 3.81; Si, 15.4. Found: C, 39.5; H, 10.4; N, 30.7;
Li, 3.74; Si, 15.3%. 1H NMR (600 MHz, C5D12): δ 2.50 (s, 18H,
NCH3), Ϫ1.70 (s, 1H, NH); 13C NMR (75 MHz, C5D12): δ 39.28
(NCH3); 29Si NMR (79.5 MHz, C5D12): δ Ϫ21.5; 15N NMR
(60.8 MHz, C5D12, vs. external CH3NO2): δ Ϫ364 (NH), Ϫ373
(N(CH3)2); IR (KBr, cmϪ1): 3449 (m) (ν(NH)); 2949 (s),
2910 (s), 2846 (s), 2813 (s) (ν(CH3)); 1549 (w); 1483 (m), 1465
(m) (δ(NH)); 995 (s) (ν(SiN)).
into a toluene solution of AlEt3 (1.05 g, 9.2 mmol; Si : Al ratio
1 : 3) at 50 ЊC. The reaction mixture was then stirred for 24 h at
50 ЊC. Overall 581 mL of gas, identified by MS as ethane, were
evolved as measure by a gas burette (calculated 618 mL, 94% of
the theoretical value). The solvent was then removed under
reduced pressure, and a pale yellow oil was obtained. Calc. for
C18H57N12Si9Al3: C, 39.1; H, 10.3; N, 30.4; Si, 14.7; Al, 4.88.
Found: C, 38.6; H, 9.8; N, 30.9; Si, 15.2; Al, 5.00%; Si/Al = 2.92;
MW (cryoscopy in benzene) 548 (calculated for Al[NHSi-
1
(NMe2)3]3 553). H NMR (300 MHz, C6D6): δ 2.84 (s, NCH2);
13C NMR (300 MHz, C6D6): δ 41 (s, NCH2); IR (KBr, cmϪ1):
3330 (w) (ν(NH)); 2972 (s), 2831 (s), 2783 (s) (ν(CH)); 1541 (w)
(δ(NH)); 984(s) (ν(SiN)).
A crystal of [(Me2N)3SiNHLi]4 suitable for X-ray diffraction
experiment was transferred under argon into a Pypex capillary,
which was then sealed in vacuo and collected on Enraf-Nonius
CAD4 Vierkreis. The structure was solved by direct methods
(SHELXS-86)25 and refined against F 2 (SHELXL-97).26
Li[Al(NHSi(NMe2)3]4, 5. To a rapidly stirred suspension of
AlCl3 (250 mg, 1.9 mmol) in pentane (10 mL) was added drop-
wise at room temperature a solution of (Me2N)3SiNHLi, 2,
(1.36 g, 7.5 mmol) in pentane (15 mL). The reaction mixture
was stirred for 18 h. The resulting precipitate of lithium chlor-
ide was removed by filtration, and the filtrate was evaporated
under reduced pressure to give a colourless oil, characterised as
Li[Al(NHSi(NMe2)3]4, 5, (1.29 g, 94% based on AlCl3). Calc.
for C24H76N16LiSi4Al4: C, 39.0; H, 10.4; N, 30.3; Si, 15.7.
(C4H8O)Al[NHSi(NMe2)3]3, 3. A solution of 5.89 g (32.4
mmol) LiTDSA in 50 mL THF was added to a solution of
1.45 g (10.9 mmol) AlCl3 in 50 mL THF at 0 ЊC. After 30 min,
the reaction mixture was returned to room temperature and
stirred for about 10 h. After THF was exchanged by same
volume of pentane, the resulting precipitate of lithium
chloride was removed by filtration, and the filtrate was evapor-
ated under reduced pressure to give a slightly yellow oil. The
oil was dissolved in 25 mL pentane and cooled to Ϫ80 ЊC
for three days giving white crystals. After separating from
pentane by decantation and washing twice with cooled (ca.
Ϫ80 ЊC) pentane, the crystals were dried under reduced
1
Found: C, 39.1; H, 10.2; N, 30.4; Si, 15.5%. H NMR (600
MHz, C7D8): δ 2.61 (s, 72H, Si(N(CH3)2)3), 0.05 (br s, 4H,
SiNH); 13C NMR (75 MHz, C7D8): δ 38.8 (s, SiN((CH3)2)3); 29Si
NMR (279.5 MHz, C6D6): δ Ϫ30.2 Si(N(CH3)2)3); 15N NMR
(60.8 MHz, C7D8, vs. external CH3NO2): δ Ϫ358.4 (NH),
Ϫ373.0 (N(CH3)2); IR (liquid film, cmϪ1): 3327 (m) (ν(NH));
2974 (s), 2839 (s), 2783 (s) (ν(CH3)); 1551 (m) (δ(NH)); 1182 (s)
(ν(SiN)); 986 (s).
The bis(tetrahydrofuran) adduct of 5, 6, was crystallised
from solutions of 5 by addition of 2.0 equivalents of THF and
cooling to Ϫ80 ЊC. A crystal of 6 suitable for X-ray diffraction
was transferred under argon into a Pypex capillary, which was
then sealed in vacuo and data collected on Siemens SMART
CCD diffractometer. The structure was solved by direct
methods (SHELXS-86)25 and refined against F 2 (SHELXL-
97).26
1
precursor. Yield (5.49 g, 8.77 mmol, 80 %). H NMR: δ 2.72
(s, 54 H, NCH3), 3.90 (t, 4 H, CH2O), 1.33 (qnt, 4 H,
CH2CH2CH2); 13C NMR: δ 38.58 (NCH3), 69.12 (CH2O), 25.10
(CH2CH2CH2). 29Si NMR: δ Ϫ31.65. IR (neat, cmϪ1): 3335 (m)
(ν(NH)); 2980 (s), 2835 (s), 2769 (s) (ν(CH)); 1555 (m) (δ(NH));
1170 (s); 992 (s).
A crystals of (C4H8O)Al[NHSi(NMe2)3]3, suitable for single
crystal X-ray crystallography, was coated with an Perfluoro-
polyether PFO-XR75 oil under a constant nitrogen flow. The
crystal with oil coating was rapidly transferred to the gonio-
meter head at 150 K and data collected on a STOE IPDS II
diffractometer. The structure was solved by direct methods
(SHELXS-86)25 and refined against F 2 (SHELXL-97).26 Data
was collected to θ = 35Њ, however the poor crystal quality
due to partial decomposition prior to data collection gave a
high number of weak reflections and a high value of Rint. The
structure was refined against truncated data sets with θ = 22.5,
25 or 30Њ for comparison and the solution presented with
θ = 30Њ.
Crystallographic data
Crystal structure determination of compound 2 [(Me2N)3-
SiNHLi]4: C24H76Li4N16Si4; M = 729.13; monoclinic, space
group P21/n; a = 11.5670(5), b = 8.9071(8), c = 22.3754(16) Å;
3
α = 90, β = 96.861(5), γ = 90Њ; V = 2288.8(3) Å ; T = 293 K;
Z = 2; Dc = 1.058 Mg mϪ3; µ(Mo-Kα) = 0.165 mm Ϫ1; F(000) =
800; 5345 reflections collected, 5217 independent (Rint = 0.0159)
used in refinement. R1 = 0.0487 on F > 4σ(F), wR2 = 0.181 on
all data.
Crystal structure determination of compound 3 (C4H8O)-
Al[NHSi(NMe2)3]3: C22H65AlN12OSi3; M = 625.11; monoclinic,
space group P21/c; a = 9.7875(16), b = 17.413(2), c = 21.810(4)
Å; α = 90, β = 112.536(14), γ = 90Њ V = 3433.2(9) Å 3; T = 150 K;
Z = 4; Dc = 1.209 Mg mϪ3; µ(Mo-Kα) = 0.200 mmϪ1; F(000) =
Preparation of gel from 3. To a solution of 3 (4.1 g, 6.5 mmol)
in dry terahydrofuran (205 mL) was added trifluoromethane-
sulfonic acid (40 uL, 0.45 mmol). After stirring for 30 min, a
cooled (ca. Ϫ80 ЊC) solution of ammonia (100 mmol) in dry
THF (9 mL) was added. After standing quiescent at room tem-
perature for 18 h, the mixture was warmed to 50 ЊC for 72 h, to
give a translucent gel filling about half of the original volume
of the reaction mixture. Decantation of the solution above the
gel and evaporation of remaining solvent and dimethylamine in
the gel by a stream of nitrogen followed by drying under
reduced pressure at 50 ЊC for 6 h yielded a translucent white
solid. 29Si CP-MRS NMR: δ Ϫ40 ppm; 27Al CP-MRS NMR:
δ 120 ppm; 13C CP-MRS NMR: δ 38 ppm; IR (KBr, cmϪ1): 3355
(m) ((ν(NH)); 2973 (w), 2861 (w), 2776 (w) (ν(CH)); 1564 (w)
(δ(NH)); 1176 (s) (δ(N–C)); 920 (s) (ν(Si–N)); 715 (w) (ν(Al–N)).
1376, 29973 reflections collected, 10569 independent (Rint
=
0.2043) used in refinement. R1 = 0.0716 on F > 4σ(F), wR2 =
0.1620 on all data.
ϩ
Crystal structure determination of compound 6 Li(THF)2
-
[((Me2N)3SiNH)4Al]Ϫ: C32H92AlLiN16O2Si4; M = 879.50; mono-
clinic, space group C2/c (no. 15); a = 12.2318(4), b = 21.7844(9),
c = 20.2211(6) Å; α = 90, β = 105.3130(10), γ = 90Њ; V = 5196.9(3)
Å3; T = 100 K; Z = 4; Dc = 1.124 Mg mϪ3; µ(Mo-Kα) = 0.350
mmϪ1
; F(000) = 1936, 27683 reflections collected, 9089
independent (Rint = 0.0294) used in refinement. R1 = 0.0407 on
F > 4σ(F), wR2 = 0.118 on all data.
CCDC reference numbers 172029, 194606 and 194607.
See http://www.rsc.org/suppdata/dt/b2/b209605f/ for crystal-
lographic data in CIF or other electronic format.
Al(NHSi(NMe2)3)3, 4. (Me2N)3SiNH2 (5.1 g, 29 mmol)
diluted in 20 mL of toluene was added dropwise into 50 mL
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15 P. J. Brothers, R. J. Wehmschulte, M. Olmstead, K. Ruhlandt-Senge,
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