On the track of novel triel-stabilised silylaminoiminoborenes

Article abstract of DOI:10.1002/chem.200802669

Abstract: Reactions between the halogen triels AlClMe₂, AlBr₃, GaCl₃ and the silylaminofluoroboranes (Me ₃Si)₂N-B(F)NRSiMe₃ (R = SiMe₃, CMe₃) afforded the silylaminoim

Full text of DOI:10.1002/chem.200802669

DOI: 10.1002/chem.200802669
On the Track of Novel Triel-Stabilised Silylaminoiminoborenes
Holger Ott, Christoph Matthes, Arne Ringe, Jçrg Magull, Dietmar Stalke, and
Uwe Klingebiel*[a]
Abstract: Reactions between the halo-
gen triels AlClMe2, AlBr3, GaCl3 and
the silylaminofluoroboranes (Me3Si)2N-
nofluoroboranes were synthesised. Be-
cause almost no open-chain diamino-
fluoroboranes had been structurally
characterised previously, corresponding
fluoroboranes containing no silyl
groups were crystallised for purposes
of comparison. In complex reactions
with the arylsilylaminofluoroboranes
A
[(2,6-(iPr)2C6H3)ACHTUGNTERN(NUG Me3Si)NB(F)NR2,
forded the silylaminoiminoborenes,
which were isolated as the triel ad-
R=iPr, iBu], amine adducts of boreni-
um salts such as [(iPr)2NH!
ꢀ
ducts, such as Me3Si
N
B(Bu)NH-2,6-(iPr)2C6H3]+AlCl4 (13)
Keywords: aluminium
· diamino-
N
were obtained.
fluoroboranes · gallium · iminobor-
enes · X-ray diffraction
reaction path to other fluoroboranes,
novel aryl- and silyl-substituted diami-
Introduction
A widely used method to generate dicoordinated boron
atoms in iminoborines is the elimination of halosilanes from
the corresponding silylaminohaloborane [Eq. (1)].[3b,f]
ꢀ
ꢀ
Even though B N and C C units are isoelectronic, compari-
son of their chemical properties shows significant differen-
ꢀ
ces. Amine–borane (H3N BH3) is a solid at room tempera-
ꢀ
ture, for example, whereas ethane (H3C CH3) is a gas. The
ꢀ
ꢀ
differences between the B N and C C bonds can be ascri-
bed to their different polarities.[1] Ethane is non-polar,
whereas amine–borane (H3B–NH3) has
a large dipole
ꢀ
moment. The simplest unsaturated B N compound is ami-
noborane (H2N=BH2), which is isoelectronic with ethene. It
is unstable and readily forms cyclic ring compounds such as
The elimination often requires heating of the starting mate-
rial. Sometimes temperatures up to 5008C are necessary. Be-
cause iminoborines are thermodynamically unstable with re-
spect to their oligomerisation, cyclodiborazanes or higher
cyclisation products are isolated.[2,4] The elimination in nor-
ꢀ
the cyclohexane analogue (H2N BH2)3 and can only be sta-
bilised by shielding the double bond with bulky groups.[2]
ꢁ
The chemistry of iminoborines (-B N-), which are isova-
mally becoming more difficult with increasing boron halo-
[3d]
ꢁ
ꢀ
ꢀ
lence electronic with -C C-, is in many respects similar to
gen bond strength from B I to B F compounds.
that of the alkynes: they oligomerise, polymerise and under-
The synthesis of silylaminoborinium cations is of special
ꢀ
go addition of HX. However, the polarity and weakness of
interest because of the weak Si N bond, which can easily be
[2,3]
ꢀ
the B N bonds results in higher reactivity.
cleaved. Therefore, all members of the series [(Me3Si)4ꢀn
-
ꢀ
(Me3C)nN2B]+BBr4 have been synthesised by use of excess
tribromoborane [general synthetic route Eq. (2)].[3a,c,d]
[a] H. Ott, Dr. C. Matthes, Dr. A. Ringe, Prof. Dr. J. Magull,
Prof. Dr. D. Stalke, Prof. Dr. U. Klingebiel
Institut fꢀr Anorganische Chemie
Georg-August-Universitꢁt Gçttingen
Tammannstraße 4, 37077 Gçttingen (Germany)
Fax : (+49)551-39-3052
E-mail: uklinge@gwdg.de
Recently, we investigated the reaction between
[(Me3Si)2N]2BF and AlCl3 and isolated the silylaminoimino-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802669.
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Chem. Eur. J. 2009, 15, 4602 – 4609

FULL PAPER
borene (1) as the first aluminium adduct of an iminoborene
that could also be investigated by X-ray diffraction
[Eq. (3)].[5]
Analogously to the synthesis of the asymmetrical silylimino-
borene
(AlCl3)SiMe3 (5),[5] the reaction between Me3C
B(F)-N(SiMe3)2 and GaCl3 yielded the [tert-butyl(trimethyl-
trichloroalane
adduct
Me3CACHTNUGTRENNNUG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
silyl)amino]-trimethylsilyliminoborene GaCl3 adduct
[Eq. (5)].
6
This formation of a stabilised aminoiminoborene and not of
a borinium cation prompted the idea to combine the fluoro-
silane elimination of as yet unknown silylaminofluorobor-
anes with the addition of halo-triels to prepare novel and so
far inaccessible aminoiminoborenes.
Again, the imine group is formed at the bis(trimethylsilyl)-
substituted nitrogen atom, as is also observed in the corre-
sponding AlCl3 adduct 5.[5] This emphasises the original
finding that the silyl groups show a stronger inductive
effect[6] than the tBu substituents, leading to a stronger
Results and Discussion
Lewis basic nitrogen atom at the N
ACHTUNGTREN(NGNU SiMe3)2 site rather than
Bis(trimethylsilyl)amino- and [tert-butyl(trimethylsilyl)ami-
no]-(trimethylsilyl)iminoborene triel adducts: In order to es-
tablish the reaction route to triel stabilised aminoiminobor-
at the N(SiMe3)tBu site. This was verified by a natural bond
order (NBO) analysis of the starting material in which the
disilylated nitrogen atom shows a much higher negative
ACHTUNGTRENNUNG
enes as shown in Equation 3, bis
A
N
charge than the asymmetrically substituted NACTHGUNETRNNU(G SiMe3)tBu ni-
trogen atom (ꢀ1.60 e vs ꢀ1.21 e).[5] This trend is not surpris-
ing, because various charge density investigations of com-
pounds with silicon atoms in positions adjacent to first row
elements show high positive integrated charges at the silicon
atoms (above +2 e).[7] This is normally balanced by partial
negative charges on the neighbouring atoms. The most elec-
tronegative atom among those, the nitrogen atom in this
case, carries the highest charge.
GaCl3. This led to the formation of the triel adducts of the
bis(trimethylsilyl)aminotrimethylsilyliminoborenes 2–4, with
fluorotrimethylsilane acting as the leaving group [Eq. (4)].
We can deduce that this synthetic route can be applied more
generally for group 13 halides (except boron) to yield the
silylamino-silyliminoborenes.
Surprisingly, it is well established in amine chemistry that
silyl substitution decreases the donor strength and basicity
of the amine relative to the carbon analogues, which at first
sight contradicts the findings above.[8] The reason for this is
the drastic change in the hybridisation of the nitrogen atom
if a silyl substituent is present.[9] The sp3-hybridised lone
pair of the nitrogen atom becomes less nucleophilic on
changing to an sp2 hybridisation state with silyl substi-
tuents.[3c] This causes a decreased reactivity. In our case,
though, sp2 hybridisation is already present at both nitrogen
atoms in the starting materials so that the selectivity found
for 5 and 6 is based only on the nitrogen atom charges, so
the nucleophilic attack proceeds at the disilylated amine.
Single crystals of 6 were obtained from a diethyl ether so-
lution. The adduct 6 (Figure 1) crystallises in the monoclinic
space group P21/n and is isomorphous with the AlCl3 ana-
logue.[5] It consists of well-separated monomers that display
SiMe3/tBu disorder. The site occupation factor ratio refined
to 0.7:0.3, which prevents a detailed analysis on the amino
substituent part. Nevertheless, clear tendencies are visible.
Abstract in German: Die Reaktion von Halogentrielen
(AlClMe2, AlBr3 und GaCl3) mit Silylaminofluorboranen
(Me3Si)2NB(F)NRSiMe3 (R=SiMe3, CMe3) ergab Silylami-
noiminoborene, die als Trieladdukte isoliert wurden (z.B.
Me3SiACHTUNGTRENNUNG(Cl3Ga)NBNRSiMe3 (6)). Um diese Syntheseroute auf
andere Fluorborane auszuweiten wurden aryl- und silyl-sub-
stituierte Diaminofluorborane synthetisiert. Zu Vergleichs-
ACHTUNGTRENNUNGzwecken wurden ebenso die zugehçrigen nichtsilylierten Ver-
ꢀ
The mean B N bond length to the amino nitrogen atom
bindungen kristallisiert, da bislang kaum offenkettige Diami-
nofluorborane strukturell charakterisiert wurden. Aminad-
dukte von Boreniumsalzen z.B. [(iPr)2NH!B(Bu)NH-2,6-
(N2) is longer (135 pm) than that to the imino nitrogen
atom (N1, 133 pm). These are exactly in the range of related
Lewis acid base adducts of silylaminoiminoborenes[5] and
carboaminoiminoborenes.[10] The N-B-N unit is almost
linear, and the four atoms of the R2NB units on each side of
the molecule lie in the same plane. Both planes are close to
ꢀ
(iPr)2C6H3]+AlCl4 (13) konnten in komplexen Reaktionen
mit
Arylsilylaminofluorboranen
[(2,6-(iPr)2C6H3)-
ACHTUNGTRENNUNG
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U. Klingebiel et al.
tometer with use of graphite monochromated molybdenum
radiation. Crystallographic details are shown in Table 2, and
selected interatomic distances and angles in 7, 9,[14] 10 and
11 are listed in Table 3. The atom labelling is consistent
throughout the molecular structures and can be taken from
Figures 2 and 3.
The central structural motif of all diaminofluoroboranes is
a planar NB(F)N unit with an sp2 hybridised boron atom
(mean deviation from idealised plane: 0.09 pm; sum of the
bond angles: 360.08). Remarkably, two different constitu-
tional arrangements of the aryl substituents can be ob-
served. In 7 and 9, the aryl ring is oriented cis to the fluorine
atom (7, Figure 2), whereas in 10 and 11 it is arranged in the
trans position (11, Figure 3). Thus, once the second substitu-
ent of the arylamine is not a hydrogen atom, the sterically
disfavoured position trans to the fluorine atom (from now
on the reference atom for cis/trans nomenclature will be F)
is occupied by the aryl ring. The bulk of this substituent is
reduced through its adoption of an orientation in which the
ring plane forms almost a 908 angle with the NB(F)N unit
(10: 94.88; 11: 91.28). Accordingly, the steric demand of the
isopropyl groups of the second amino substituent is mini-
mised by a conformation in which the hydrogen atom at C-
Figure 1. Molecular structure of [tert-butyl(trimethylsilyl)amino]-(trime-
thylsilyl)iminoborene trichlorogallium adduct (6). Disordered parts of
the molecule, as well as hydrogen atoms, are omitted for clarity. Aniso-
tropic displacement parameters are depicted at the 50% probability
level.
orthogonal (858), so an allene-type bonding situation can be
ꢀ
assumed. The Ga N bond length (194 pm) is in the range of
the sum of the covalent radii (195 pm[11]) and exactly the
same as that in a gallium carboiminoborene adduct[9a]
(194 pm).
New arylaminofluoroboranes: The formation of 1–6 prompt-
ed the investigation of the analogous triel reactions with
novel silylaminofluoroboranes. For reasons of comparison,
we also synthesised related hydrogen- or carbo-substituted
arylaminofluoroboranes. We prepared the (arylamino)-alkyl-
AHCTUNGTREG(NNNU trans) lie in the NB(F)N plane and face the aryl ring (iso-
propyl “shovels” bent away from the aryl ring). Exactly the
opposite orientation is present in 7 (isopropyl “shovels”
bent away from the fluorine atom) if the aryl substituent
points to the side with the fluorine atom (cis). Consequently,
both bulky groups (Dip and TMS) in 9 are oriented towards
the fluorine atom (cis).
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
boranes with lithium 2,6-diisopropylanilide, isopropylanilide
and trimethylsilylanilide [Eq. (6) and Table 1] by the syn-
thetic route of Kçlle and Nçth.[3c,12]
The steric demand of the substituents can be monitored
by means of the N1-B1-N2 bond angle. In 9 the trans groups
are small (H and Me) and the narrowest bond angle can be
found [124.8(2)8]. This angle widens in 7 [126.9(1)8] and 10
[128.0(2)8], whereas in 11 the N1-B1-N2 angle is as much as
132.9(3)8. The differences between 10 and 11 can be ascri-
bed to the different aryl ring tilts (see above; almost perfect-
ly rectangular with respect to the boron plane in 11) and the
hybridisation of the connected nitrogen atom.
The degrees of sp2 hybridisation (fully or partly sp3) at
both nitrogen atoms, as well as the dihedral angles between
the pz orbitals, determine the extent of p interaction with
the unoccupied pz orbital of the boron atom. These orbitals
are almost ideally aligned (angles between orbitals: N1,
B1=3.48; N2, B1=1.48) and perfect sp2 hybridisation is
present at both nitrogen atoms (sum of the bond angles:
Table 1. Residues for compounds 7–12.
7
8
9
10
11
12
R
R’
iPr
H
iBu
H
Me/SiMe3
H
iPr
iPr
iPr
SiMe3
iBu
SiMe3
ꢀ
The diaminofluoroboranes can be synthesised in excellent
yields and purities because they can easily be distilled and
crystallised. Crystal structural analyses of four model com-
pounds (7, 9, 10, 11) give further insight into the bonding sit-
uation in diaminofluoroboranes, of which only a few exam-
ples had previously been structurally characterised by X-ray
methods.[13] They were chosen in order to investigate the in-
fluence of substituents either with different steric demand
or that were believed to stabilise the borane. Crystals of the
diaminofluoroboranes were obtained from n-hexane solu-
tions and were measured on a Bruker Smart Apex II diffrac-
360.08) in 7. Both B N bond lengths are therefore short
ꢀ
ꢀ
[B1 N1: 141.4(2) pm; B1 N2: 139.6(2) pm] in relation to a
standard B N single bond (149 pm[15]) but far from values
ꢀ
found for aminoiminoborenes (ca. 133 pm[5,9]). Nevertheless,
they are comparable to the formal single bonds in a super-
mesityl-substituted aminoiminoborine[16] [139.2(5) pm] and
in a bis(trimethyl)silylaminodisilylamino-substituted fluoro-
borane[17] [143.4(1) and 144.2(1) pm]. According to Paet-
zold, who suggested a value of 141 pm as a typical R2B=NR2
ꢀ
bond length, we have to classify the B N bonds in 7, 9, 10
and 11 likewise.[2]
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Chem. Eur. J. 2009, 15, 4602 – 4609

Stabilised Silylaminoiminoborenes
FULL PAPER
Table 2. Crystallographic details for 6, 7, 9, 10 and 11 (100(2) K) and for 13 and 16 (133(2) K).
Compound
6
7
9
10
11
13
16
CCDC no.
empirical formula
molecular weight [gmolꢀ1
crystal system
space group
a [pm]
713642
C10H27BCl3GaN2Si2
418.40
713643
C18H32BFN2
306.27
713644
C16H30BFN2Si
308.32
713645
C21H38BFN2
348.34
monoclinic
C2/c
3232.4(1)
1830.6(1)
3016.1(2)
102.514(1)
17.4231(12)
32
713646
713647
713648
C21H40BFN2Si C22H42AlBCl4N2 C8H19AlCl3N
]
378.45
514.17
262.57
P21/n
814.5(1)
1918.5(2)
12.967(1)
91.690(1)
2.0253(3)
4
1.372
1.863
2.64- 26.68
0.999
C2/c
1714.7(1)
1100.3(1)
2170.1(2)
112.593(1)
3.7802(6)
8
1.076
0.069
2.25–25.35
0.999
P21/c
663.2(1)
995.5(1)
2832.9(4)
90.720(2)
1.8701(4)
4
1.095
0.130
2.50–25.33
0.997
P21/c
1698.9(1)
1419.5(1)
1944.0(1)
90.383(1)
4.6879(4)
8
1.072
0.115
1.87–26.04
0.999
P21/n
1504.2(3)
1022.4(2)
2003.0(4)
111.89(3)
2.8584(10)
4
1.195
0.457
1.46–25.91
0.994
P21/c
8224.4(5)
9425.4(5)
1762.7(1)
91.772(5)
1.3658(2)
4
1.277
0.699
2.31–24.68
0.986
b [pm]
c [pm]
b [8]
V [nm3]
Z
1calcd [Mgmꢀ3
]
1.062
0.067
2.21–25.36
0.998
m [mmꢀ1
q range [8]
]
completeness
no. reflections
23740
13521
16421
64214
66018
25115
18792
unique reflections (Rint
data/restraints/parameters
goodness of fit
)
4325 (0.0226)
4325/416/266
1.094
3470 (0.0373) 3392 (0.0568) 15952 (0.0574) 9242 (0.0394)
5523 (0.0774)
5523/0/288
0.999
2290 (0.0542)
2290/0/122
1.050
3470/0/232
1.009
3392/1/201
1.066
15952/0/941
1.010
9242/4/509
1.071
R1 [I > 2s(I)]
0.0215
0.0401
0.0431
0.0517
0.0392
0.0469
0.0216
wR2 (all data)
0.0508
0.275/ꢀ0.197
0.0982
0.169/ꢀ0.187
0.1055
0.237/ꢀ0.243
0.1494
0.469/ꢀ0.189
0.1022
0.277/ꢀ0.272
0.1093
0.382/ꢀ0.413
0.0589
0.252/ꢀ0.244
largest diff. peak/hole [eꢃꢀ3
]
Table 3. Selected interatomic distances [pm] and angles [8] in compounds
7, 9, 10 and 11.
7
9
10[a]
11[a]
ꢀ
B F
ꢀ
B N1
136.8(2)
141.4(2)
139.6(2)
143.4(2)
83.2(19)[b]
148.0(2)
148.0(2)
126.9(1)
114.7(2)
118.4(2)
125.4(2)
135.8(3)
140.3(3)
141.4(3)
143.4(2)
89.1(9)[b]
174.5(2)
148.2(2)
124.8(2)
116.3(2)
118.9(2)
126.8(2)
136.4(3)
142.9(2)
140.9(3)
143.5(3)
149.0(3)
148.5(3)
147.7(3)
128.0(2)
115.5(2)
116.6(2)
123.5(2)
137.1(2)
143.5(2)
140.9(2)
144.6(2)
177.6(1)
148.9(2)
147.7(2)
132.9(2)
111.8(1)
115.4(2)
124.1(1)
ꢀ
B N2
ꢀ
N1 C1
ꢀ
N1 R’
ꢀ
N2 Rcis
ꢀ
N2 Rtrans
N1-B-N2
F-B-N1
F-B-N2
B-N1-C1
[a] Value averaged over four (10) or two (11) molecules in the asymmet-
ric unit. [b] H atom positions were taken from the final difference map
and refined.
Figure 3. Molecular structure of di(isopropyl)amino-[N-trimethylsilyl-2,6-
di(isopropyl)anilino]fluoroborane (11, only one molecule in the asymmet-
ric unit shown). Hydrogen atoms are omitted for clarity. Anisotropic dis-
placement parameters are depicted at the 50% probability level.
leads to an imperfect overlap of the pz orbitals (16.78) and,
together with the increased steric demand of the silyl sub-
ꢀ
stituent, to the longest B N2 bond [141.4(3) pm] in the
series presented here. In the two compounds (7 and 9) the
aryl rings are tilted by 658 and 758 relative to the NB(F)N
planes and do not participate in the p stabilisation of the
boron atoms. The same is true for 10 and 11 (values see
above).
Figure 2. Molecular structure of di(isopropyl)amino-2,6-di(isopropyl)ani-
lino-fluoroborane (7). Disordered parts of the molecule and constrained
hydrogen atoms are omitted for clarity. Anisotropic displacement param-
eters are depicted at the 50% probability level.
As soon as the aryl groups are arranged trans, increases in
ꢀ
the B N1 bond lengths are evident [10: 142.9(2) pm; 11:
143.5(2) pm]. This can be explained by the need for ideal ar-
rangements of the sterically demanding groups. The methyl
groups of the N-bound isopropyl substituent in 10 point to-
wards the fluorine atom. The halide resides at the bisection
of the two methyl groups. The aryl ring is thereby forced
The trimethylsilyl substitution in 9 causes a slight pyra-
midalisation at N2 (sum of the bond angles: 357.48) in which
the silyl group is bent out of the NB(F)NC plane. This also
Chem. Eur. J. 2009, 15, 4602 – 4609
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4605

U. Klingebiel et al.
further out of the NB(F)N plane because of the isopropyl
groups on the aryl substituent, which interfere with the
other amine substituent. This leads to a slight pyramidalisa-
tion at N1 (sum of the bond angles: 359.18) and a tilt be-
tween the lone pair orbital and the boron pz orbital of 27.58.
The trimethylsilyl group in 11 is even more bulky, but be-
ꢀ
cause of the longer Si N bond [177.6(1) pm] relative to the
2
ꢀ
C N bond in 10 [149.0(3) pm] the nitrogen atom stays sp
hybridised (sum of the bond angles: 359.98) with only a
slight p orbital tilt (38).
ꢀ
The bulky substituents even influence the B F bond
lengths. The longest is found in 11 [137.1(2) pm], whereas
ꢀ
the sterically least affected 9 shows the shortest B F bond
[135.8(3) pm]. The halide–boron bond lengths can be com-
pared to the related cases in the tetrasilyl-substituted amino-
fluoroborane[16] [137.1(1) pm] and also to the maximum in
Figure 4. Molecular structure of di(isopropyl)amine-2,6-di(isopropyl)ani-
lino-n-butyl-borenium tetrachloroaluminate (13). Disordered parts of the
molecule and constrained hydrogen atoms are omitted for clarity. Aniso-
tropic displacement parameters are depicted at the 50% probability
level.
ꢀ
the histogram of reported B F single bonds (136 pm) with a
tricoordinate boron atom.[18]
Conversions with AlCl3: With the novel silyl-substituted di-
A
C13-B and C13B(N2)N1 plane: 93.88). The longest carbon–
ꢀ
ꢀ
carbon bond can thus be found between C13 C14 [C13
ꢀ
ꢀ
G
C14: 154.3(3) pm; C14 C15: 149.1(4) pm; C15 C16:
propyl)phenyl]amino-di(isopropyl)aminofluoroborane (11)
was treated with one equivalent of AlCl3 in diethyl ether
and dichloromethane. Elimination of Me3SiF was detected
in the NMR spectra, but it was impossible to isolate the de-
sired product. However, we grew crystals of a by-product or
decomposition product at 08C from the resulting polymer-
151.2(4) pm]. The boron–carbon bond is 156.4(3) pm long
ꢀ
and just slightly shorter than a standard B C single bond
(158 pm[14]).
The boron nitrogen bond lengths differ significantly as a
result of the two bonding modes present in 13. B N1 can be
p-interaction-reinforced single bond
[137.8(3) pm]. The bond length is in the range between
those found for the diaminofluoroboranes and the amino-
ꢀ
classified as
a
ꢀ
ide. The borinium cation 13 with AlCl4 as counter-ion was
characterised as a di(isopropyl)-
A
ACHTUNGTNERiNUNG minoborenes. However, it is
structural analysis.
close to the value of the related
ꢀ
According to Nçthꢄs nomen-
clature, 13 would correctly be
B N bond [138.6(3) pm] report-
ed for 1,3-dimethyl-2-(diphenyl-
called
a
borenium cation.[3c]
ACHTUNGTRNEaNUNG mino)-1,3,2-diazaborolidinium
Nçth reported several possible
routes to related tricoordinated boron cations in his review,
but we cannot yet explain the mechanism of the borenium
cation formation. Interestingly, repeated reactions always
led to the same results.
Compound 13 crystallises in the monoclinic space group
P21/c with one ion pair in the asymmetric unit (Figure 4). It
belongs to the class of the rare examples of borenium cat-
ions characterised by X-ray diffraction. To the best of our
knowledge, it is even the first crystal structure of a boreni-
um cation in which the boron atom is not part of a ring
system.
triflate (14), which is one of the
structurally characterised bore-
nium cations.[19]
ꢀ
The second B N bond is much longer and a coordinative
ꢀ
interaction (B N2: 157.1(3) pm]. It is thus on the same
ꢀ
scale as those of amine-boranes (R3B NR3) selected by
Paetzold as model compounds for a boron nitrogen single
bond (158 pm).[2] The corresponding bond in 14 is shorter
[154.7(3) pm], most probably due to the limited flexibility of
the donating aminoethylene side chain. Moreover, we have
ꢀ
to emphasise that no interactions between the AlCl4 anions
and the borenium cations are present in the solid state. The
aluminium chlorine distances are thus in the normal range
of 211.9(1) to 214.9(1) pm.
Even though we had not been able to isolate the desired
aminoiminoborene when using the silylarylaminofluorobor-
ane 11, we performed the same reaction with the isobutyl-
substituted starting material 12. Once more the fluorosilane
elimination takes place in the initial reaction step, but it is
followed by the addition of Me3SiF to the iminoborine,
which results in a methyl substitution at the boron atom and
The boron atom is sp2 hybridised (sum of the bond
angles: 360.08) with all connected atoms in the same plane
(mean deviation from idealised plane: 0.79 pm). The same is
true for the tricoordinated nitrogen atom N1 (mean devia-
tion from idealised plane: 0.15 pm; sum of the bond angles:
360.08), and an almost perfectly aligned pz orbital orienta-
tion between B and N1 is adopted (tilt angle: 38). In addi-
tion, the butyl chain participates in the p-stabilisation of the
boron atom through hyperconjugation (angle between C14-
4606
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 4602 – 4609

Stabilised Silylaminoiminoborenes
FULL PAPER
a dimethylfluorosilane group at the aryl-substituted nitrogen
atom. In the final conversion the isobutylamino nitrogen
atom was protonated[20] and below 08C it was possible to
Conclusion
We can conclude that the novel direct synthetic route to
triel stabilised aminoiminoborenes can be easily extended to
a variety of halogen triels. Unfortunately, the aryl-substitut-
ed diaminofluoroboranes proved to be inappropriate start-
ing materials in similar reactions. Nevertheless, we were
able to study the bonding situations in a series of diamino-
fluoroboranes. In complex reactions with those starting ma-
terials we could only isolate by-products or decomposition
products. Even so, the isolated compounds were structurally
interesting.
isolate diACHTUNGTRENNUNG(isobutyl)amine adduct of [(N-dimethylfluorosilyl)-
2,6-di(isopropyl)anilino)]-methylborenium tetrachloroalumi-
nate (15) from the resulting residual oil as a crystalline ma-
terial (Scheme 1). Unfortunately, no useful structure model
could be refined because of the poor crystal quality; only
the atom connectivity could be deduced from the diffraction
experiment. The borenium salt 15 is thermally unstable
above room temperature; under these conditions the di(iso-
propyl)amine trichloroalane adduct (16) was isolated
(Scheme 1).
Experimental Section
All experiments were performed in oven-dried glassware under purified
nitrogen or argon with use of standard inert gas and vacuum line tech-
niques. All NMR spectra were measured at room temperature on a
Bruker AVANCE 200 or DPX 500 spectrometer with SiMe4, BF3·Et2O,
AlACHTUNTRGNEUNG(NO3)3 or C6F6 as external standards. Mass spectra were obtained with
a MAT 95 spectrometer. The progress of the reactions was checked by
19F NMR spectroscopy. The purities of the compounds were confirmed
by NMR spectroscopy and gas chromatography.
Compounds 2–4: A solution of AlClMe2 (0.1 mol) in n-hexane (100 mL)
(2), of tribromoalane (0.1 mol) in diethyl ether (50 mL) (3) or of trichlor-
ide gallium (0.1 mol) in n-hexane (50 mL) (4) was added to bis-
Scheme 1. Putative reaction sequence for the formation of di-
ACHTUNGTRENNUNG
AHCTUNGERTG[NNUN bis(trimethylsilyl)amine]fluoroborane (0.1 mol) in n-hexane (50 mL).
AHCTUNGTRENNUNG
The reaction mixture was heated at reflux for 48 h and compounds 2–4
were isolated by distillation and recrystallisation from n-hexane.
ACHTUNGTRENNUNG
Chlorodimethylalano(trimethylsilyl)iminobis-(trimethylsilyl)aminoborene
(2): Yield 91%; m.p. 428C; 1H NMR (CDCl3): d=ꢀ0.32 (s, 6H; AlMe2),
0.10 [s, 18H;
NACTHNGUTERNNUG
(SiMe3)2], 0.42 ppm (s, 9H; AlNSiMe3); 13C NMR
The amine triel adduct 16 crystallises in the monoclinic
space group P21/c with one formula unit in the asymmetric
unit (Figure 5). The nitrogen aluminium donor bond is
(CDCl3): d=1.03 (AlMe2), 1.94 [N
ACHTUGNTREN(NUNG SiMe3)2], 2.48 ppm (AlNSiMe3);
11B NMR (CDCl3): d=33.63 ppm; 27Al NMR (CDCl3): d=63.24 ppm;
29Si NMR (CDCl3): d=11.15 (AlNSiMe3), 12.85 ppm [N
ACTHNUTRGEN(UGN SiMe3)2]; MS
(EI): m/z (%): 350 [M]+ (8), 335 [MꢀMe]+ (45), 315 [MꢀCl]+ (5), 259
[MꢀAlClMe2]+ (20).
Tribromoalano(trimethylsilyl)imino-bis(trimethylsilyl)aminoborene (3):
Yield 95%; m.p. 1228C; 1H NMR (CDCl3): d=0.39 [s, 18H; N
0.51 ppm (s, 9H; AlNSiMe3); 13C NMR (CDCl3): d=1.94 [N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
2.30 ppm (AlNSiMe3); 11B NMR (CDCl3): d=26.19 ppm; 27Al NMR
(CDCl3): d=91.47 ppm; 29Si NMR (CDCl3): d=13.70 [AlNSiMe3],
15.74 ppm [NACTHNUGTRNEUNG(SiMe3)2].
Trichlorogallano(trimethylsilyl)imino-bis(trimethylsilyl)aminoborene (4):
Yield 82%; m.p. 1578C; 1H NMR (CDCl3): d=0.33 (s, 9H; GaNSiMe3),
0.36 ppm [s, 18H; N
1.83 ppm (GaNSiMe3); 11B NMR (CDCl3): d=30.10 ppm; 29Si NMR
(CDCl3): d=13.68 (GaNSiMe3), 15.67 ppm [N(SiMe3)2].
(SiMe3)2]; 13C NMR (CDCl3): d=1.74 [N
ACHTUGTNRENNUG ACHTUNGTRENNUNG(SiMe3)2],
AHCTUNGTRENNUNG
Figure 5. Molecular structure of the diACTHNUGTRNEUNG(isobutyl)amine trichloroalane
Trichlorogallano(trimethylsilyl)imino-[tert-butyl(trimethylsilyl)amino]-
borane (6): A solution of trichlorogallium (0.1 mol) in diethyl ether
(50 mL) was added to tert-butyl(trimethylsilyl)amino-bis(trimethylsilyl)a-
mino-fluoroborane (0.1 mol) in diethyl ether (50 mL). The reaction mix-
ture was heated under reflux for 24 h. The crystalline compound 4 was
obtained after removal of the solvent in vacuo and recrystallisation of
the residue from n-hexane. Yield 98%; m.p. 728C; 1H NMR (CDCl3): d=
0.40 (s, 9H; CNSiMe3), 0.44 (s, 9H; GaNSiMe3), 1.47 ppm (s, 9H;
NCMe3); 13C NMR (CDCl3): d=1.93 (CNSiMe3), 2.30 (GaNSiMe3), 32.64
(NCMe3), 58.54 ppm (NCMe3); 11B NMR (CDCl3): d=31.92 ppm;
29Si NMR (CDCl3): d=11.77 (GaNSiMe3), 14.26 ppm (CNSiMe3).
adduct 16. Constrained hydrogen atoms are omitted for clarity. Aniso-
tropic displacement parameters are depicted at the 50% probability
level.
196.0(1) pm long and slightly shorter than the comparable
bonds in methylamine [193.6(4) pm],[21] trimethylamine
[196(1) pm][22] and diphenylamine [198.3(4) pm][23] adducts.
The aluminium chlorine bond lengths vary between 211.5(1)
and 212.8(1) pm and are in the normal range.
Compounds 7–12: A nBuLi solution (15% in n-hexane, 0.2 mol) was
added at 08C variously to 2,6-di(isopropyl)aniline (0.2 mol; 7–9), to (N-
isopropyl)-2,6-di(isopropyl)aniline (0.2 mol; 10) or to (N-trimethylsilyl)-
2,6-di(isopropyl)aniline (0.2 mol; 11, 12) in n-hexane (100 mL). After the
Chem. Eur. J. 2009, 15, 4602 – 4609
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4607

U. Klingebiel et al.
system had been heated at reflux for 2 h, the lithium anilide was dis-
solved in THF (100 mL) and slowly added variously to di(isopropyl)ami-
no-difluoroborane (7, 10, 11), to diACTHNUTRGNE(NUG isobutyl)amino-difluoroborane (8, 12)
Compounds 13, 15 and 16: A solution of trichloroalane (0.1 mol) in di-
ethyl ether (50 mL) was added at ꢀ208C to a solution either of 11
(0.1 mol) in diethyl ether (100 mL) or of 12 (0.1 mol) in diethyl ether
(50 mL) and dichloromethane (50 mL), and the mixture was stirred for
4 h at 08C. Compounds 13 and 15 were crystallised from diethyl ether at
08C and 16 at room temperature.
or to (trimethylsilyl)methylamino-difluoroborane (9[13]) (0.2 mol) and the
mixture was heated to reflux for 3 h. The crude product was separated
from LiF by condensation of the volatile components into a cooling trap
in vacuo. Compounds 7–12 were purified by distillation and recrystallisa-
tion from n-hexane.
Di(isopropyl)amine-2,6-di(isopropyl)anilino-n-butyl-borenium tetrachlor-
oaluminate (13): Yield 20%; decomposition at 258C.
Di(isopropyl)amino-2,6-di(isopropyl)anilino-fluoroborane
(7):
Yield
DiACTHNUTRGENUG(N isobutyl)amine-[(N-dimethylfluorosilyl)-2,6-di(isopropyl)anilino)]-
97%; m.p. 838C; b.p. 708C (0.01 mbar); 1H NMR (CDCl3): d=1.19 (d,
methylborenium tetrachloroaluminate (15): Yield 18%; decomposition at
3
3JH,H =6.8 Hz, 12H; C CHMe2), 1.19 (d, JH,H =6.8 Hz, 12H; N CHMe2),
388C; MS (EI): m/z (%): 407 [MꢀAlCl4]+ (10), 278 [MꢀNH
ACHTUNGTRENNUNG
(iBu)2]+
ꢀ
ꢀ
3.27 (sept, 3JH,H =6.8 Hz, 2H; C CHMe2), 3.37 (sept, 3JH,H =6.8 Hz, 2H;
ꢀ
(30).
N CHMe2), 3.39 (d, 3JH,F =0.6 Hz, 1H; NH), 7.02–7.11 ppm (m, 3H;
ꢀ
DiACTHNUTRGNEUNG
(isobutyl)amine aluminium (16): M.p. 778C; 1H NMR (CDCl3): d=
13
ꢀ
ꢀ
arom. CH); C NMR (CDCl3): d=22.43 (N CHMe2), 23.40 (C CHMe2),
1.06 (d, 3JH,H =6.7 Hz, 12H; CHMe2), 2.19 (m, 2H; CHMe2), 2.94 ppm
(m, 4H; CH2); 13C NMR (CDCl3): d=20.24 (CHMe2), 25.63 (CHMe2),
56.04 ppm (CH2); 27Al NMR (CDCl3): d=103.41 ppm.
ꢀ
ꢀ
28.51 (C CHMe2), 44.58 (N CHMe2), 122.88 (m-C), 125.10 (p-C), 132.43
(o-C), 145.11 ppm (i-C); 11B NMR (CDCl3): d=21.78 ppm; 19F NMR
(CDCl3): d=28.98 ppm; MS (EI): m/z (%): 306 [M]+ (20), 205 (100)
Crystal structure determination: Single crystals were selected from
Schlenk flasks under argon and covered with perfluorated polyether oil
on a microscope slide, which in the case of 6 was cooled with a nitrogen
gas flow by use of the X-TEMP 2.[24] Suitable crystals were mounted on
the tip of a glass fibre fixed to a goniometer head and shock-cooled with
the crystal cooling device. All data were collected with use of monochro-
mated MoKa radiation (l=71.073 pm) variously with a Bruker TXS rotat-
ing anode (6), a Bruker Smart Apex II with D8 goniometer (7, 9, 10, 11)
or a Stoe IPDS II with image plate detector (13, 16).
[MꢀN
Di(isobutyl)amino-2,6-di(isopropyl)anilino-fluoroborane (8): Yield 95%;
m.p. 818C; b.p. 808C (0.01 mbar); 1H NMR (CDCl3): d=0.91 (d, JH,H
ACHTUNGTRENNUNG
(CHMe2)2]+.
ACHTUNGTRENNUNG
3
=
6.6 Hz, 12H; CH2 CHMe2), 1.19 (d, 3JH,H =6.9 Hz, 12H; C CHMe2),
ꢀ
ꢀ
1.88 (m, 2H; CH2 CHMe2), 2.72 (d, 3JH,H =7.4 Hz, 4H; CH2 CHMe2),
ꢀ
ꢀ
3.29 (sept, 3JH,F =6.9 Hz, 2H; C CHMe2), 3.49 (d, 3JH,F =17.2 Hz, 1H;
ꢀ
NH), 7.13–7.36 ppm (m, 3H; arom. CH); 11B NMR (CDCl3): d=
21.99 ppm; 13C NMR (CDCl3): d=20.13 (CH2 CHMe2), 23.46 (C
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
CHMe2), 26.93 (CH2 CHMe2), 28.43 (C CHMe2), 53.56 (CH2 CHMe2),
122.85 (m-C), 125.17 (p-C), 136.04 (o-C), 145.13 ppm (i-C); 19F NMR
(CDCl3): d=22.75 ppm; MS (EI): m/z (%): 334 [M]+ (18), 291
[MꢀCHMe2]+ (18).
Bruker system: The cell determination was performed with the aid of the
APEX2 program package (Bruker AXS, 2007) followed by integration
with SAINT (Bruker AXS, 2007). SADABS (Bruker AXS, 2006) was
used for the empirical absorption correction.
Di(isopropyl)amino-(N-2,6-tri(isopropyl)anilino)-fluoroborane
(10):
=
IPDS system: The cell determination and integration was done with the
aid of X-AREA (Stoe & Cie, 2002) software.
1
3
Yield 97%; b.p. 1108C (0.01 mbar); H NMR (CDCl3): d=0.92 [d, JH,H
6.7 Hz, 12H; N ACHTUNGTRENNUNG
(CHMe2)2], 1.19 (d, 3JH,H =6.8 Hz, 6H; C CHMe2), 1.22
ꢀ
ꢀ
3
3
3
The data were reduced in XPREP and the structures were solved by
direct methods with SHELXS.[25] The refinement against F2 was done
with SHELXL.[26]
ꢀ
(d, JH,H =6.8 Hz, 6H; C CHMe2), 1.29 (dd, JH,H =6.7, JH,F =1.9 Hz, 6H;
C CHMe2), 3.00 [sept, 3JH,H =6.7 Hz, 2H; N
ACTHNUGTRNEUN(G CHMe2)2], 3.02 (sept,
ꢀ
ꢀ
3JH,H =6.7 Hz, 2H; N CHMe2), 3.30 (sept, 3JH,H =6.8 Hz, 2H;
ꢀ
ꢀ
C
CHMe2), 7.01–7.15 ppm (m, 3H; arom. CH); 13C NMR (CDCl3): d=
CCDC-713642 (6), 713643 (7), 713644 (9), 713645 (10), 713646 (11),
713647 (13) and 713648 (16) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
22.56 (d, 3JC,F =4.3 Hz; N CHMe2), 22.87 [N
ACHTNUGTRENUNG(CHMe2)2], 23.46 (C
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
CHMe2), 24.64 (C CHMe2), 27.78 (C CHMe2), 44.35 [N ACHTUNTRGNEUNG(CHMe2)2],
ꢀ
55.29 (N CHMe2), 124.26 (m-C), 125.54 (p-C), 142.14 (o-C), 146.94 ppm
(i-C); 11B NMR (CDCl3): d=23.39 ppm; 19F NMR (CDCl3): d=
48.86 ppm; MS (EI): m/z (%): 348 (15) [M]+, 333 (100) [MꢀMe]+.
Di(isopropyl)amino-(N-trimethylsilyl-2,6-di(isopropyl)anilino)-fluorobor-
ane (11): Yield 95%; m.p. 838C; b.p. 808C (0.01 mbar); 1H NMR
ACTHNUTRGNEUNG
(CDCl3): d=0.10 [d, 5JH,F =2.2 Hz, 9H; Si(CH3)3], 0.97 [d, 3JH,H =6.5 Hz,
12H; C CH
(CH3)2], 1.18 [d, 3JH,H =6.8 Hz, 6H; N CH
E
ꢀ
ꢀ
Acknowledgements
3JH,H =6.8 Hz, 6H; N CH
(CH3)2], 2.94 (sept, 3JH,H =6.5 Hz, 2H;
ꢀ
ꢀ
C
CHMe2), 3.41 (sept, 3JH,H =6.8 Hz, 2H; N CHMe2), 6.8–7.07 ppm (m,
ꢀ
We gratefully acknowledge financial support from the Deutsche For-
schungsgesellschaft, the Volkswagen Stiftung, the Fonds der Chemischen
Industrie (H.O.), and CHEMETALL GmbH Frankfurt.
3H; arom. CH); 13C NMR (CDCl3): d=1.75 [Si
N
ꢀ
ꢀ
ꢀ
ꢀ
(CH3)2], 24.19 [N CH
E
(CH3)2], 27.77 [C CHMe2],
44.07 [d, 3JC,F =2.4 Hz, N CHMe2], 123.59 (p-C), 124.99 (m-C), 140.81
(o-C), 145.74 ppm (i-C); 11B NMR (CDCl3): d=22.61 ppm; 19F NMR
(CDCl3): d=53.00 ppm; 29Si NMR (CDCl3): d=8.30 ppm (d, 3JSi,F =9.6);
MS (EI): m/z (%): 378 [M]+ (3), 363 [MꢀMe]+ (100), 335 [MꢀCHMe2]+
(90).
ꢀ
[1] For example, a) U. Flierler, D. Leusser, H. Ott, G. Kehr, G. Erker,
S. Grimme, D. Stalke, Chem. Eur. J. 2009, 15, 4595–4601, preceding
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Di
ACHTUNGTRENNUNG(isobutyl)amino-(N-trimethylsilyl-2,6-di(isopropyl)anilino)-fluoro-
borane (12): Yield 88%; b.p. 898C (0.01 mbar); 1H NMR (CDCl3): d=
5
3
ꢀ
0.15 [d, JH,F =2.2 Hz, 9H; Si
ACHTNUGTNERUN(GN CH3)3], 0.71 [d, JH,H =6.7 Hz, 12H; C CH-
(CH3)2], 1.22 [d, 3JH,H =6.9 Hz, 6H; CH2 CH
ACTHUGNNTRE(GUN CH3)2], 1.25 [d, JH,H =
3
ꢀ
6.9 Hz, 6H; CH2 CHACHTUNGTRENNUNG
(CH3)2], 2.43 (d, 3JH,H =6.1 Hz, 4H; CH2CHMe2),
ꢀ
2.92 (sept, 3JH,H =6.7 Hz, 2H; C CHMe2), 3.39 (m, 2H; CH2CHMe2),
ꢀ
7.06–7.13 ppm (m, 3H; arom. CH); 11B NMR (CDCl3): d=21.73 ppm;
[2] P. Paetzold, Adv. Inorg. Chem. 1987, 31, 123–170.
13C NMR (CDCl3): d=1.75 [d, 4JC,F =4.5 Hz, Si
(CH3)3], 20.18 [C CH-
ꢀ
[3] a) P. Kçlle, H. Nçth, Chem. Ber. 1986, 119, 3849–3855; b) M. Haase,
U. Klingebiel, Angew. Chem. 1985, 97, 335–336; Angew. Chem. Int.
Ed. Engl. 1985, 24, 324; c) P. Kçlle, H. Nçth, Chem. Rev. 1985, 85,
399–418; d) H. Nçth, S. Weber, B. Rasthofer, C. Narula, A. Kon-
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Richter, T. Thijssen, S. Wꢀrtenberg, Chem. Ber. 1979, 112, 3811–
3827.
ꢀ
A
E
N
CHMe2), 27.81 (CH2CHMe2), 52.56 (d, 3JC,F =4.7 Hz; CH2CHMe2),
123.16 (p-C), 125.25 (m-C), 140.75 (d, 4JC,F =8.4 Hz; o-C), 145.87 ppm (i-
C); 19F NMR (CDCl3): d=46.06 ppm; 29Si NMR (CDCl3): d=8.00 ppm
(d, 3JSi,F =9.5 Hz); MS (EI): m/z (%): 406 [M]+ (6), 391 [MꢀMe]+ (18),
363 [MꢀCHMe2]+ (100).
4608
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 4602 – 4609

Stabilised Silylaminoiminoborenes
FULL PAPER
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[13] Only one other compound without further inorganic substituents or
ring systems at the nitrogen atoms connected to the boron atom is
reported to the best of our knowledge (see ref. [16]).
Received: December 18, 2008
Published online: March 23, 2009
Chem. Eur. J. 2009, 15, 4602 – 4609
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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