From monomeric iminosilane to a monomeric aminoalane: Synthesis and crystal structures

Article abstract of DOI:10.1039/a605842f

The reaction between the iminosilane R₂Si=NR′ [R = CMe₃, R′= Si(CMe₃)₂Ph] 1 and AlMe₃ in n-hexane at -78°C affords the first monomeric aminodimethylalane Me₂Al=N-(R′)R″ [R″ = Si(CMe₃)₂Me] 2; the crystal structure of 2 is presented.

Full text of DOI:10.1039/a605842f

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From monomeric iminosilane to a monomeric aminoalane: synthesis and
crystal structures
Jo¨rg Niesmann,a Uwe Klingebiel,*a Mathias Noltemeyera and Roland Boeseb
a Institut fu¨r Anorganische Chemie der Universita¨t, Tammannstr. 4, D-37077 Go¨ttingen, Germany
b Institut fu¨r Anorganische Chemie, Universita¨t-GH Essen, Universita¨tsstr. 5-7, D-45117 Essen, Germany
The reaction between the iminosilane R2SiNNRA [R = CMe3,
RA = Si(CMe3)2Ph] 1 and AlMe3 in n-hexane at 278 °C
affords the first monomeric aminodimethylalane Me2AlNN-
(RA)RB [RB = Si(CMe3)2Me] 2; the crystal structure of 2 is
presented.
associated structures. However, three-coordination is achieved
by the use of bulky substituents.7
For the first time a monomeric aminodimethylalane is
obtained by a nucleophilic methanide-ion migration from
aluminium to silicon. The methanide-ion migration can be
explained by the strong Lewis acidity of the three-coordinate
silicon, which must be stronger than the Lewis acidity of the
three-coordinate aluminium.
Compounds of multiple bond systems involving heavier main
group elements were long considered to be unstable and
synthetically inaccessible.1–3 The first silaethenes and disilenes
were synthesised at the beginning of the 1980s, and the first
iminosilanes were reported in the mid-1980s.1–3
The uncoordinated and thermally stable iminosilane
R2Si = NRA [R = CMe3, RA = Si(CMe3)2Ph] 1 is formed in
quantitative yield in the reaction between the lithiated fluoro-
silylamine compound and SiClMe3.1 It is based on a fluorine/
chlorine exchange with subsequent thermal LiCl elimination
[eqn. (1)].1
CMe3
Si
Me3C
Me3C
CMe3
Me3C Si
hexane
+ AlMe3
Si
N
Ph
N
AlMe2
CMe3
Me
~ 1,3-Me–
Me C
CMe3
SiPh
3
+ Me3SiCl
CMe3 CMe3
Si Si
CMe3
Si
Me3C
MeSi
Me3C
– Me3SiF
Si
(1)
(thf)3Li
F
N
Ph
N
Ph
CMe3
CMe3
N
– LiCl
CMe3 CMe3
Me3C
AlMe2
2
1
The crystal structure of 1 shows a trigonal arrangement of the
C2SiNN unit around the silicon atom. In solid state the
iminosilane is monomeric and the SiNN–Si skeleton is nearly
linear (168.3°). The Si–N bond lengths are drastically different.
The Si–N single bond is 169.6 pm, and the SiNN double bond is
157.2 pm. The results are consistent with Wiberg’s experi-
mental4 and von Rague Schleyer’s calculated5 results.
Scheme 1
C(42)
C(4)
C(43)
C(53)
157.2 pm 169.6 pm
C(41)
C(52)
(Me3C)2Si
N
Si(CMe3)2Ph
C(2)
C(5)
168.3°
Si(1)
156.8 pm 169.5 pm
154.9 pm 168.8 pm
C(51)
Al
(Me3C)2Si
N
Si(CMe3)3
H2Si
N
SiH3
C(3)
N
C(1)
177.8°
175.6°
(ref. 4)
(ref. 5)
C(12)
C(13)
Iminosilanes react with Lewis bases, for example, with ethers
or tertiary amines with formation of adducts. The adducts are
often thermally stable and can be distilled without decomposi-
tion.1,6 Reactions of iminosilanes with Lewis acids are not
described so far. We found that di-tert-butyl(di-tert-butyl-
phenylsilyl)iminosilane 1 reacts with trimethylalane, AlMe3, to
give the monomeric aminoalane, Me2AlN[Si(CMe3)2-
Me]Si(CMe3)2Ph 2 (Scheme 1).†,‡
Si(2)
C(71)
C(8)
C(11)
C(7)
C(63)
C(61)
C(6)
C(72)
C(9)
C(10)
C(73)
Unassociated compounds of the form R2M–NRA2 (M = B,
Al, e.g. 2) have the potential for p bonding between the main
group 3 element (M) and nitrogen through overlap of the lone
pair on nitrogen with the empty p orbital on boron or
aluminium. Such p bonding is well established in molecules
containing boron–nitrogen bonds.
C(62)
Fig. 1 Molecular structure of 2 (ORTEP, thermal ellipsoids at the 50%
probability level). Principal distances (pm) and angles (°): Al–N 186.9(2),
Al–C(2) 195.4(2), Al–C(1) 196.4(2), Al–C(8) 256.4(3), Al–Si(2) 284.43(8),
Si(1)–N 175.2(2), Si(2)–N 175.0(2); C(2)–Al–C(1) 108.03(11), C(1)–Al–N
125.66(9), C(2)–Al–N(1) 124.09(10), S < Al(1) = 357.8°, Si(2)–N–Si(1)
The chemistry of the heavier group 3 elements, e.g. Al, is
dominated by the tendency to form dimers or more highly
133.88(9),
Al–N–Si(2)
103.56(7),
Al–N–Si(1)
122.53(8),
S < N(1) = 360.0°. Torsion angle C(1)–Al–N–Si(1) 257.5°.
Chem. Commun., 1997
365

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H); 13C, d 20.13 (s, SiMe), 1.82 (br s, AlMe2), 22.83/23.56 (s, CMe3),
30.97/31.54 (s, CMe3), 129.29 [s, Ph (C-3, C-5)], 132.56 [s, Ph (C-4)],
137.60 [s, Ph (C-2, C-6)], 137.65 [s, Ph (C-1)]; 29Si, d -3.37 (s, SiPh), 7.13
(s, SiMe).
The structure of 2§ which is depicted in Fig. 1 consists of well
separated Me2AlN(RA)R monomers. The Al–N bond length is
186.9 pm. The geometry of the Al atom is nearly planar (sum of
angles = 357.8°). The C–Al–C angle is 108.03°. The geometry
of the nitrogen centre is also planar (sum of angles = 360.0°)
with the widest angle, 133.9°, between the silyl groups. The
torsion angle of the C(2)–Al–N–Si(1) axis is 257.5°. There is a
weak intramolecular interaction of the electron deficient
aluminium atom with the C(8) atom of the phenyl ring facing
the metal, Al–C(8) 256.4 pm. This decreases the Al–N–Si(2)
angle (103.6°) and brings the Al and Si(2) atoms into close
proximity [Al–Si(2) 284.4 pm]. The Al–N bond length is short
in comparison to the simple sum of the aluminium and nitrogen
covalent radii, i.e. 200 pm, which suggests the presence of Al–N
pp–pp bonding. However, the dihedral angle C(1)–Al–N–Si(1)
of 257.5° makes this very limited. Therefore most of the
shortening of the Al–N bond should be attributed to the
difference of the electronegativity. The Al–N bond length is
comparable the Al–N distances of But2AlN compounds.7
This work was supported by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie.
§ Crystal data for 2: a crystal of size 0.4 3 0.6 3 0.8 mm3 was measured
on a STOE/Siemens AED2 four circle diffractometer with Mo-Ka radiation
at 153 K. The space group is monoclinic, P21/n, with unit-cell dimensions
a = 11.1930(10), b = 15.601(2), c = 16.622(3) Å, b = 105.750(10)°,
U = 2793.6(7) Å3, Z = 4, refined from 30 reflections centred at positive
and negative 2q positions; m = 0.170 mm21 and F(000) = 992. 3401
reflections up to 2q = 45° were collected, of which 3396 were independent
(Rint = 0.014). The structure was solved using direct methods (SHELXS)
and refined on F2 (SHELX93) with no disorder indicated: R1[I
2s(I)] 0.0347, wR2 0.0852; for all 3391 data R1 0.0391,
wR2 = 0.092, GOF = 1.071.
>
=
=
=
Atomic coordinates, bond lengths and angles, and thermal parameters
have been deposited at the Cambridge Crystallographic Data Centre
(CCDC). See Information for Authors, Issue No. 1. Any request to the
CCDC for this material should quote the full literature citation and the
reference number 182/311.
References
1 I. Hemme and U. Klingebiel, Adv. Organomet. Chem., 1996, 39, 159.
2 R. West, Angew. Chem., Int. Ed. Engl., 1987, 26, 1201.
3 A. G. Brook and K. M. Baines, Adv. Organomet. Chem., 1986, 25, 1.
4 N. Wiberg, K. Schurz, G. Reber and G. Mu¨ller, J. Chem. Soc., Chem.
Commun., 1986, 591.
5 P. von Rague´-Schleyer and P.O. Stout, J. Chem. Soc., Chem. Commun.,
1986, 1373.
6 S. Walter, U. Klingebiel and D. Schmidt-Ba¨se, J. Organomet. Chem.,
1991, 412, 319.
Footnotes
† Preparative details. Starting materials: 1,8 yellow oil, bp 108 °C (0.01
Torr), trimethylaluminium, 2.0 m solution in toluene. Reaction under dry
argon. To 4.31
g (11.5 mmol) of di-tert-butyl(di-tert-butylphenyl-
silyl)iminosilane 1,8 which was freshly distilled into a Schlenk bulb, was
added 60 ml of degassed dry hexane. Then 5.74 ml (11.5 mmol, 2.0 m
solution in toluene) trimethylaluminium were added at 278 °C. The
solution was stirred at 278 °C for 2 h and at 25 °C for 1 day. After removal
of the solvent in vacuo (1022 Torr) and recrystallisation of the residue from
toluene crystalline compound 2 was obtained in 74% yield; mp 121 °C; MS
(EI): m/z (%) = 432(30) [M 2 CH3]+, 390(100) [M 2 C4H9]+.
7 P. J. Brothers and P. P. Power, Adv. Organomet. Chem., 1996, 39, 1.
8 D. Grosskopf, L. Marcus, U. Klingebiel and M. Noltemeyer, Phosphorus
Sulfur Silicon, 1994, 97, 113.
‡ NMR data: 2 (C6D6–toluene): 1H, d 20.63 (s, 6H, AlMe2), 0.40 (s, 3H,
SiMe), 1.21 (s, 18H, CMe3), 1.23 (s, 18H, CMe3), 6.97– 8.06 (m, 5H, Ar-
Received, 22nd August 1996; Com. 6/05842F
366
Chem. Commun., 1997