Synthesis and structure of tetrakis(isopropylimino)bis-[tris(dimethylamino) silylamino]alane

Article abstract of DOI:10.1016/j.jorganchem.2004.11.025

A novel cage-type compound tetrakis(isopropylimino)bis[tris(dimethylamino) silylamino]alane, H₆Al₆(NⁱPr) ₄[NSi(NMe₂)₃]₂, (1), was prepared by reaction of tris(dimethylamino)silylamine H₂NSi(NMe ₂)₃ with poly(isopropyliminoalane) (HAlN ⁱPr)₆. The molecular structure of 1 was determined and discussed on the basis of FTIR, NMR and X-ray crystallographic analysis.

Full text of DOI:10.1016/j.jorganchem.2004.11.025

Journal of Organometallic Chemistry 690 (2005) 1205–1208
www.elsevier.com/locate/jorganchem
Synthesis and structure of
tetrakis(isopropylimino)bis-[tris(dimethylamino)silylamino]alane
*
Fei Cheng , Stephen Clark, Stephen J. Archibald, Stephen M. Kelly, John S. Bradley
Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, UK
Received 13 October 2004; accepted 15 November 2004
Abstract
i
A novel cage-type compound tetrakis(isopropylimino)bis[tris(dimethylamino)silylamino]alane, H
was prepared by reaction of tris(dimethylamino)silylamine H NSi(NMe ) with poly(isopropyliminoalane) (HAlN Pr) . The molec-
6 6 4 2 3 2
Al (N Pr) [NSi(NMe ) ] , (1),
i
2
2 3
6
ular structure of 1 was determined and discussed on the basis of FTIR, NMR and X-ray crystallographic analysis.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Dimethylamino; Silylamino; Alane; Synthesis; Structure
1
. Introduction
starting material. The synthetic strategy we have
adopted is to use tris(dimethylamino)silylamine or its
lithium salt as reagents to assemble precursor mole-
cules containing, in addition to Si(NMe2)3 groups,
Al(NHSiN3)n backbones. These precursor materials
should be convertable into high-surface-area silicon
aluminum diimide gels by a similar sol–gel process,
and hence to mesoporous Si–Al–N ceramics. A single-
source precursor molecule (C H O)Al[(NHSi(NMe ) ]
2 3 3
Synthesis and characterization of organometallic
compounds containing Al–N–Si backbones are of cur-
rent interest due to their potential applications as sin-
gle-source precursors for the preparation of silicon
aluminum nitride materials with a defined shape, e.g.,
as coating or fibres [1–3]. We recently reported for
the first time the preparation of tris(dimethyl-
4
8
amino)silylamine,
ammonolysis of
(CH ) N] SiCl, an intermediate prepared from the
[(CH ) N] SiNH
TDSA,
by
was successfully prepared by a reaction of lithium
tris(dimethylamino)silylamide [(CH ) N] SiNHLi with
aluminium chloride [6].
3
2
3
2
tris(dimethylamino)chlorosilane
3
2
3
[
3
2
3
i
reaction of silicon tetrachloride with dimethylamine
4]. A mesoporous silicon diimide gel, which is a pre-
cursor for high surface area mesoporous silicon nitride,
5] was prepared from TDSA via a non-aqueous
Poly(isopropyliminoalane) (HAlN Pr) is a well
6
[
known cage-type compound possessing an (AlN)6
skeleton and has been shown to be a potentially useful
soluble precursor for aluminum nitride [7]. Since AlH
groups are capable of hydrogen elimination reactions
with NH groups, a hybrid pre-ceramic precursor for
[
ammonolytic sol–gel method [4]. We are now actively
seeking to prepare novel ternary Si–Al–N precursor
molecules, using tris(dimethylamino)silylamine as a
Al–Si–C–N nanocomposites has been prepared by
i
the reaction of (HAlN Pr) with a ring-type [Me-
6
Si(H)NH] , but no structure of the precursor molecule
n
*
was reported. [3] Although many cage-type com-
pounds containing the (AlN) skeleton such as (HAl-
Corresponding author. Tel.: +44 1482 465419; fax: +44 1482
4
66410.
E-mail address: f.cheng@hull.ac.uk (F. Cheng).
6
NCH R) (R = Ph, p-MeC H and p-CF C H ) and
4
2
6
6
4
3
6
0
022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.11.025

1206
F. Cheng et al. / Journal of Organometallic Chemistry 690 (2005) 1205–1208
0
0
(
[
RAlNR ) (R = C H NMe ) have been prepared
2
The compound 1 was prepared by a reaction between
i
TDSA and (HAlN Pr) according to reaction (2). Based
6
2
4
8,9] no cage-type molecule containing (AlN) skeleton
6
6
coordinated with silylamino groups has been reported
to date.
on a previous result [3] compound 2, a cage-type mole-
a
i
cule [(Me N) SiNH] H Al (N Pr) produced by
2
3
2
4
6
6
In this paper we report the preparation and structure
of a novel cage-type molecule tetrakis(isopropyli-
hydrogen elimination reaction (Eq. (3)), would be ex-
pected. However, a remarkable reaction took place at
two nitrogen sites to replace two isopropyl chains with
the tris(dimethylamino)silane groups (Eq. (2)).
mino)bis[tris(dimethylamino)silylamino]alane
i
H Al -
6 6
(N Pr) [NSi(NMe ) ] , (1), by a reaction between TDSA
4 2 3 2
and poly(isopropyliminoalane) (HAlN Pr) . The molec-
i
When a TDSA–pentane solution was added to a
6
i
(HAlN Pr) –pentane solution, evolution of a small
amount of gas, probably hydrogen, was observed arising
ular structure of 1 was determined on the basis of
its FTIR, NMR, MS and X-ray crystallographic
spectra. The sol–gel processing of 1 will be discussed
separately.
6
i
from a reaction between AlH groups of (HAlN Pr) with
6
NH groups of TDSA in an undesired site. After further
2
reaction for 18 h at room temperature a white paste was
1
obtained. H NMR analysis showed that the paste was a
mixture of materials including compound 1 but not
2
. Results and discussion
Poly(isopropyliminoalane) (HAlN Pr) was prepared
i
including [(Me N) SiNH] H Al (N Pr) . Compound 1
2
3
2
4
6
6
was separated by recrystallisation from pentane at
ꢀ
i
6
80 °C.
The H NMR spectrum of compound 1 showed that
by a modification of a previous report according to reac-
tion (1). [10]
1
i
the molar ratio of –Si(NMe ) groups to – Pr was 1:2,
see Section 4. The broad signal centred at 131 ppm in
2
3
nꢀheptane
i
i
6
LiAlH
4
þ 6Pr NH
2
!
ðHAlNPr Þ þ 6LiH þ 12H
2
27
the Al NMR spectrum was consistent with an HAlN3
6
ð1Þ
environment [11]. The FTIR analysis confirmed the ab-
sence of NH groups as well as the presence of AlH groups
with different environments [11]. The combination of the
i
Pr
N
H
1
13
29
27
Al
N
Al
i
H, C, Si and Al NMR spectra, FTIR spectrum and
the mass spectrum indicate that tetrakis(isopropyli-
mino)bis[tris(dimethylamino)silylamino]alane H Al -
H
Pr
Al
i
Pr
N
H
6
6
i
+
2H NSi(NMe )
i
H
N
Al
Al
N
Al
N
Pr
H
2
2
3
(N Pr) [NSi(NMe ) ] had indeed been obtained.
4 2 3 2
i
The molecular structure of compound 1 was deter-
Pr
i
mined by single-crystal X-ray diffraction analysis (Table
1). A view of the molecule is shown in Fig. 1 and selected
bond lengths and angles are collected in Table 2. Partial
decomposition occurred prior to data collection due to
H
Pr
ð2Þ
i
Pr
H
N
H
Al
Si(NMe2)
3
Al
N
H
i
Pr
N
Al
i
+
2H N Pr
2
i
H
Al
N
Pr
Table 1
Crystal data for compound 1
N
Al
(
Me N) Si
Al
H
N
2
3
H
i
Pr
Formula
C24H70Al N12Si2
6
Formula weight
Crystal system
Space group
744.4
Orthorhombic
Pbca
i
Pr
N
H
Al
i
H
Pr
˚
a (A)
16.982(2)
13.228(2)
18.797(5)
90
Al
N
H
i
b ( A˚ )
˚
c (A)
Pr
N
Al
+
2H NSi(NMe )
2 2 3
i
H
N
Al
N
Pr
a (°)
b (°)
c (°)
Al
90
90
i
Al
H
Pr
N
H
i
˚
3
Pr
V (A )
4222.7(14)
4
150(2)
ð3Þ
i
Z
Pr
N
H
H
Temperature (K)
˚
(
MeN ) SiN
Al
i
Pr
2
3
k (A)
0.71073
13,922
Al
N
i
No. of reflection collected
No. of independent reflections
No. of data/restraints/parameters
R indices [I > 2r(I)]
R indices (all data)
Largest different peak and hole (eA )
Pr
N
Al
H
3705[Rint = 0.2129]
3705/0/199
+
2H2
i
H
N
Al
N
Pr
Al
R
1
= 0.0545, wR
2
= 0.0762
i
Al
H
Pr
N
R = 0.1583, wR = 0.0996
1
NSi(NMe2)3
2
˚
3
i
0.558 and ꢀ0.278
Pr
H

F. Cheng et al. / Journal of Organometallic Chemistry 690 (2005) 1205–1208
1207
˚
bonds [1.961(3) A (average)]. The Al–N–Al bond angles
in the six-membered rings [122.33 (8)° average] are wider
than the corresponding N–Al–N bond angles [117.39
(17)° average], while in the four-membered rings the for-
mer is 88.56(16)° average, which is slightly smaller than
the latter of 91.39 (17)° average. All of these general
structural features are similar to those observed for
i
(HAlN Pr) as well as other hexameric iminoalanes such
as (HAlN Pr) [14] and (ClAlN Pr) [15].
6
n
i
6
6
The mechanism for the formation of 1 in this syn-
thetic procedure has not yet been established. The evolu-
tion of H gas and the decrease of the relative intensity
2
of Al–H band in the IR spectrum of the paste product
compared to that of compound 1 indicated that reaction
i
3) did occur when TDSA was added to (HAlN Pr)
(
solution. However, the presence of a number of doublets
6
Fig. 1. ORTEP plot of the molecular structure of compound 1 [12].
Hydrogen atoms in the dimethylamino groups are omitted for clarity.
Equivalent positions are generated by the symmetry operator:
1
at 1.1–1.7 ppm (due to (CH ) CH) in the H NMR of
3
2
the oil obtained after separation of 1 from the paste sug-
gests that the cage has been broken during the further
reaction. Compound 1 might form with the cleavage
of Al–N bonds and subsequent rearrangement of the
molecular structure [16].
ꢀ
x + 1,ꢀy, ꢀz.
Table 2
Selected bond distances (A) and angles (°) for compound 1
˚
Bond distances
Bond angles
Al(1)–N(1)
Al(1)–N(3)
1.918(4)
1.904(4)
1.970(4)
1.891(4)
1.904(4)
1.946(4)
1.914(4)
1.908(4)
1.966(4)
N(3)–Al(1)–N(1)
N(3)–Al(1)–N(2)
N(1)–Al(1)–N(2)
N(2)–Al(2)–N(3)
N(2)–Al(2)–N(1)
N(3)–Al(2)–N(1)
N(2)–Al(3)–N(1)
N(2)–Al(3)–N(3)
N(1)–Al(3)–N(3)
117.44(18)
91.13(17)
90.62(18)
119.02(18)
92.18(18)
92.10(15)
115.71(18)
91.10(18)
91.20(18)
123.40(2)
88.48(17)
88.51(17)
122.80(2)
88.60(17)
88.68(17)
120.80(2)
88.16(17)
88.92(18)
a
a
3
. Conclusion
a
Al(1)–N(2)
Al(2)–N(2)
Al(2)–N(3)
a
a
A first example of a stable cage-type silylaminoalane
different
a
i
H Al (N Pr) [NSi(NMe ) ]
Al(2)–N(1)
Al(3)–N(1)
Al(3)–N(2)
with
two
6
6
4
2 3 2
substituents was prepared by a reaction between tris(di-
methylamino)silylamine H NSi(NMe ) and poly(iso-
i
propyliminoalane) (HAlN Pr) . The symmetric nature
a
a
a
2
2 3
Al(3)–N(3)
6
Al(3)–N(1)–Al(1)
Al(3)–N(1)–Al(2)
Al(1)–N(1)–Al(2)
Al(2)–N(2)–Al(3)
Al(2)–N(2)–Al(1)
Al(3)–N(2)–Al(1)
Al(2)–N(3)–Al(1)
Al(2)–N(3)–Al(3)
Al(1)–N(3)–Al(3)
a
a
of this substitution should make it possible to use the pro-
ven ammonolytic chemistry of TDSA to develop a novel
silicon aluminum imido gel in which Al and Si atoms are
interdispersed at the atomic level. The conversion of this
novel compound into silicon aluminum imido gel is being
carried out and will be reported separately.
a
a
a
a
a
Equivalent positions are generated by the symmetry operator:
ꢀx + 1, ꢀy, ꢀz.
4. Experimental
4.1. General
its highly air sensitive nature. However a satisfactory
refinement of this sample was achieved. Compound 1
All procedures were performed under a protective
consists of an hexagonal prism of the (AlN) cage with
6
nitrogen atmosphere using standard Schlenk techniques
or in a nitrogen filled glove box. The solvents pentane
and heptane were freshly distilled from sodium/benzo-
phenone prior to use. Tris(dimethylamino)silylamine
(Me N) SiNH TDSA was prepared according to our
top and bottom faces, which are made of two almost
planar six-membered (AlN) rings related by inversion
3
symmetry. These six-membered rings are linked together
by six transverse Al–N bonds forming six rectangular
side faces of the prism. Each aluminum atom is tetrahe-
drally bonded to three nitrogen atoms and one hydrogen
2
3
2
previous report [4]. Lithium aluminum hydride, isopro-
pylamine and aluminium chloride were obtained from
Aldrich. The NMR spectra were obtained on a JEOL
JNM-LA400FT NMR spectrometer using C D as sol-
i
atom. Two trans-(Me N) Si groups and four trans- Pr
2
3
groups are situated on six nitrogen atoms away from
i
the cage. As with the structure of the parent (HAlN Pr)
6
6
vent. IR spectrum was recorded on a Nicolet Magna-
500 FTIR spectrometer. MS spectra were recorded on
a SHIMADZU GC MS – QP 5050 spectrometer. Single
6
[
13] the Al–N bonds in the six-membered rings [1.907(4)
˚
A (average)] are significantly shorter than the transverse

1208
F. Cheng et al. / Journal of Organometallic Chemistry 690 (2005) 1205–1208
crystal X-ray diffraction data were collected on a STOE
IPDS II diffractometer.
5. Supplementary material
Crystallographic data for the structural analysis
have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC reference number
4
(
.2. Preparation of poly(isopropyliminoalane)
i
HAlN Pr)
6
2
44045 for compound 1. Copies of the information
i
Preparation of (HAlN Pr) was modified from a pre-
may be obtained from The Director, CCDC, 12, Union
Road, Cambridge CB2 1EZ, UK (fax: +44-1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
6
vious report [10]. To a rapidly stirred suspension of
LiAlH (2.52 g, 66.4 mmol) in heptane (50 mL) at 0
4
°
5
C was added a solution of (CH ) CHNH (3.44 g,
3 2 2
8.2 mmol) in heptane (15 mL). The mixture was stirred
at 0 °C for 2 h and then at reflux for 20 h. After filtra-
tion, the filtrate was reduced to 20 mL and cooled at
Acknowledgements
ꢀ
40 °C to yield the product as white crystalline solid.
1
H NMR: d = 1.40 (d, 6H, –CH(CH ) ), 3.61 (m, 1H,
3
2
Part of this work was carried out as part of the EU
project NANOSENSOFLEX. We also thank the Engi-
neering and Physical Sciences Research Council
1
3
–
CH(CH ) ); C NMR: d = 27.1 (s, CH(CH ) ), 48.7
3 2 3 2
27
(
s, –CH(CH ) ); Al NMR: d = 128.6 (s, HAlN ); IR
3
2
3
ꢀ
1
(
Nujol film, cm ): 1860 (s, m(AlH)).
(EPSRC) for financial support.
4
(
.3. Preparation of tetrakis(isopropylimino)bis[tris
dimethyl amino)silylamino]alane
References
A solution of TDSA (2.47 g, 14 mmol) in dry pentane
30 mL) was added to a solution of poly(isopropylimi-
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(
i
[
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6
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1
uum. Yield (1.06 g, 21%). H NMR (ppm): d = 2.63 (s,
1
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3
3
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2
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(
s, Si[(N(CH ) )] ), 28.4 and 25.4 (s, –CH(CH ) ), 49.2
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2
9
s, –CH(CH ) ); Si NMR (ppm): d = ꢀ34.4 (s, Si
3
2
2
[
9] J.E. Park, B.J. Bae, Y. Kim, J.T. Park, H. Suh, Organometal 18
(
7
[
(N(CH ) )] ); Al NMR (ppm): d = 131.0 (s, HAlN );
3 2 3 3
1
1999) 1059, and references therein.
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ꢀ
IR (Nujol, cm ): 2955 (vs), 2929 (vs), 2850 (vs), 2792
[
[
[
[
[
[
[
(
vs), 1882/1873 (s) (m(AlH)), 1465 (s), 1386 (m), 1366
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9
6
20 (s), 821 (m), 749 (vs), 735 (vs), 709 (vs), 670 (s),
1
18 (m), 460 (m) cm ; MS (m/z, EI): 744[1]+.
ꢀ
(
1997) 565.
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1
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imaging plate diffractometer equipped with an Oxford
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not applied. The structure was solved by direct methods
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[
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(
SHELXS-97) [17] and refined by full-matrix least-
2
squares on F using SHELXL-97 [18]. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms
were placed in idealized positions.
[