Structural chemistry of borazines

Article abstract of DOI:10.1002/ejic.200800169

The solid-state structure of (HN=BF)₃ has been redetermined. It is characterised by a stacking of the molecules, similar to hexagonal boron nitride. In contrast, (F₃CH₂N=BF)₃ shows no intermolecular interactions between the planar borazine rings. In (Cl ₂BN=BCl)₃, the Cl₂B groups are almost perpendicularly oriented to the planar borazine ring and its B-Cl bonds are shorter than the Cl-B bonds to the ring boron atoms. Reactions of (Cl ₂BN=BCl)₃ with Me₃SiNMe₂ allows a successive Cl/Me₂N exchange. Depending on the molar ratio, the compounds [Cl(Me₂N)BN=BCl]₃, [(Me₂N) ₂BN=BCl]₃ and [(Me₂N)₂BN=BNMe ₂]₃ are obtained. The latter two react with CH ₂Cl₂ by B-N bond cleavage of one ring boron bonded Me ₂N group with formation of an N-H bond. These molecules not only possess a strongly distorted BN hexagon with short B-NH bonds but also a nonplanar borazine ring. The nonplanarity of the borazine ring is even more pronounced in [Me₃SiN=BCl]₃. A planar borazine ring is observed for (MeN=BBr)₃. Its molecules are ordered into stacks by translation with borazine planes 4.2 A apart from each other. Characteristic of the structure are close contacts between neighbouring bromine atoms. Due to the steric demand of pentafluorophenyl groups, the borazine ring of (F₅C₆N=BBr)₃ forms no stacks because one of the three F₅C₆ rings stands almost perpendicular to the borazine plane while the other two are twisted by 70°. Aminolysis of (F ₅C₆N=BBr)₃ yields (F₅C ₆N=BNH₂)₃ with NH₂ groups that are coplanar with the borazine ring. The phenyl groups of (HN=BPh)₃ are twisted by only 30-40° relative to the borazine ring. A threefold crystallographic axis stands perpendicular to the ring plane. The sixfold crystallographic axis of the unit cell generates a channel structure with an internal diameter of 4.5 A. The two isomeric borazines, (iPrN=BMe) ₃ and (MeN=BiPr)₃ are structurally rather similar although the first of these has longer B-N bonds due to the shorter N-C bonds. Moreover, the B-N-B and N-B-N bond angles are slightly different. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800169

FULL PAPER
DOI: 10.1002/ejic.200800169
Structural Chemistry of Borazines[‡]
Benadir Anand,[a] Heinrich Nöth,*[a] Holger Schwenk-Kircher,[a] and Alexander Troll[a]
Dedicated to Prof. Dr. K. Wade on the occasion of his 75th birthday
Keywords: Borazines / Ring distortion
The solid-state structure of (HN=BF)3 has been redeter-
mined. It is characterised by a stacking of the molecules, sim-
ilar to hexagonal boron nitride. In contrast, (F3CH2N=BF)3
shows no intermolecular interactions between the planar
borazine rings. In (Cl2BN=BCl)3, the Cl2B groups are almost
perpendicularly oriented to the planar borazine ring and its
B–Cl bonds are shorter than the Cl–B bonds to the ring boron
atoms. Reactions of (Cl2BN=BCl)3 with Me3SiNMe2 allows a
acteristic of the structure are close contacts between neigh-
bouring bromine atoms. Due to the steric demand of penta-
fluorophenyl groups, the borazine ring of (F5C6N=BBr)3
forms no stacks because one of the three F5C6 rings stands
almost perpendicular to the borazine plane while the other
two are twisted by 70°. Aminolysis of (F5C6N=BBr)3 yields
(F5C6N=BNH2)3 with NH2 groups that are coplanar with the
borazine ring. The phenyl groups of (HN=BPh)3 are twisted
successive Cl/Me2N exchange. Depending on the molar ra- by only 30–40° relative to the borazine ring. A threefold crys-
tio, the compounds [Cl(Me2N)BN=BCl]3, [(Me2N)2BN=BCl]3
and [(Me2N)2BN=BNMe2]3 are obtained. The latter two react
with CH2Cl2 by B–N bond cleavage of one ring boron bonded
tallographic axis stands perpendicular to the ring plane. The
sixfold crystallographic axis of the unit cell generates a chan-
nel structure with an internal diameter of 4.5 Å. The two iso-
Me2N group with formation of an N–H bond. These mole- meric borazines, (iPrN=BMe)3 and (MeN=BiPr)3 are structur-
cules not only possess a strongly distorted BN hexagon with
short B–NH bonds but also a nonplanar borazine ring. The
nonplanarity of the borazine ring is even more pronounced
in [Me3SiN=BCl]3. A planar borazine ring is observed for
(MeN=BBr)3. Its molecules are ordered into stacks by transla-
ally rather similar although the first of these has longer B–N
bonds due to the shorter N–C bonds. Moreover, the B–N–B
and N–B–N bond angles are slightly different.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
tion with borazine planes 4.2 Å apart from each other. Char- Germany, 2008)
four-membered B2N2 rings and six-membered B3N3-boraz-
Introduction
ine rings. Other areas consist of N- or B-metallated boraz-
ines.[21–26] Several of these borazine derivatives possess non-
planar borazine rings and also their B–N bond lengths be-
come different, i.e. the B3N3 rings are distorted. We report
here on the syntheses and structures of new borazine deriv-
atives but also on the structure determination of well
known borazines.
Although isoelectronic and isolobal with benzene, boraz-
ine exhibits antiaromatic character.[2] In contrast to benzene
with D6h symmetry, borazine exhibits D3h symmetry in the
gas phase[3] but C2 symmetry in the solid state.[4] The boron
atoms carry a partial positive charge and the nitrogen
atoms a partial negative charge which leads to BN bond
polarity and this polarity determines its chemical behav-
iour.[5–10] Amongst the BN heterocycles, borazine and its
derivatives have been extensively investigated and the chem-
istry of the former has been described in books[5–9] and
handbooks.[10]
2,4,6-Trihaloborazines and Their Structures
Synthesis of Fluoroborazines (RN=BF)3
Renewed interest in borazines can be associated with the
preparation of BN-fibres and nanotubes[11–14] as precursors
in the formation of BNC- and BSiN-type materials[15–17]
as well as the challenge of preparing BN-“fullerenes”.[18–20]
These latter compounds will be built from edge-sharing
It is well known that the thermal stability of 2,4,6-tri-
haloborazines (HN=BX)3 depends on the halogen atom X
and increases in the order of X = I Ͻ Br Ͻ Cl Ͻ F. The
compound (HN=BI)3 is unstable at ambient tempera-
ture,[27] (HN=BBr)3 decomposes slowly at ambient tem-
perature but rapidly at 90 °C[28] whereas (HN=BCl)3 can be
sublimed at 65 °C. Nevertheless, on storing the latter at
100 °C for an extended time, it decomposes with formation
of HCl and a diborazinyl.[29] (HN=BF)3 can be prepared
[‡] Contribution to the Chemistry of Boron, 260. Part 259: Ref.[1]
[a] Department of Chemistry and Biochemistry, University of
Munich
Butenandtstr. 5-13, 81 377 München
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Structural Chemistry of Borazines
from the trichloroborazine by treatment with various metal the Cl2B groups are perpendicular to the borazine ring. To
fluorides.[30–33] We followed the reaction of (HN=BCl)3 determine its structure we repeated its preparation from an
with SbF3 by 11B NMR spectroscopy and observed a quan- excess of BCl3 and N(SnMe3)3. Single-crystals were grown
titative Cl/F exchange [Equation (1)]. Crystallisation from from CHCl3/hexane solutions. This compound was also
hexane yielded (HN=BF)3, 1, in 89% yield which can be used to prepare Me2N substituted borazines, for example
compared with a reported yield of 40%.[30,33] However, it [(Me2N)2BN=BCl]3, 7 [Equations (4) and (5)]. Because the
was difficult to grow suitable single-crystals for the struc- Me2NB group forms B–N bonds with a high π bond char-
ture determination. Crystallisation at –25 °C from hexane acter we expected that (Me2N)2B groups would have an op-
yielded mainly a powder.[34]
posite effect on the borazine ring compared with the Cl2B
groups. Reactions of (Cl2BN=BCl)3 with Me3SiNMe2 were
performed in 3:1, 6:1 and 9:1 ratios. 11B NMR spectroscopy
showed the formation of compounds 6, 7 and 8 when using
toluene solutions. Treatment of 6 and 7 with CH2Cl2
yielded compounds 9 and 10.
A sample of (F3CCH2N=BF)3 (2) was supplied by A.
Meller.[35]
Synthesis of Chloroborazines (RN=BCl)3
2,4,6-Trichloroborazines can be prepared by various
methods. One of the most convenient of these syntheses in-
volves the reaction of BCl3 with a suitable trimethyl-
silylamine.[27] In case of the N-trimethylsilyl-substituted
chloroborazine 4, a two step reaction is required as shown
below [Equations (2) and (3)].
Reaction (3) was carried out with excess BCl3 in hexane
at reflux.[36] Compound 3 can be isolated by distillation at
89 °C/11 Torr. Single-crystals, obtained from a hexane solu-
tion at –78 °C, were found to melt at 5–7 °C. Compound 3
can also be prepared by treating NaN(SiMe3)2 with BCl3 in
a 1:1 molar ratio in hexane.
The conversion of 3 into the borazine 4[37] was achieved
within two days in boiling xylene. It was isolated by subli-
mation at 95 °C/2 Torr and crystallisation from hexane
yielded single-crystals. Both 3 and 4 hydrolyse readily in
moist air.
The boron nuclei of 4 are better shielded than in 3 (δ11B
= 35.5 vs. 37.6 ppm). On the other hand, the 29Si resonance
of 4 can be observed at lower field (δ = 19.34 ppm) com-
This shows that the substitution of the Cl atoms of
pared with the resonance of 3 (δ = 9.70 ppm) and the same (Cl2BN=BCl)3 occurs stepwise and that the Cl atoms of the
holds for the 1H NMR shifts (4, δ = 0.44; 3, δ = 0.26 ppm). Cl2B groups are more reactive than those of the ClB groups,
In contrast, the reverse is true for the 13C resonances of the a result that is not unexpected as the boron atoms of the
methyl groups as shown for 3 with δ13C = 3.60 and for 4 BCl2 group are more Lewis acidic than the borazine boron
with δ13C = 3.83 ppm.
atoms.
1
The mass spectra of 3 and 4 show only weak parent ions
The reactions (4) and (5) were followed by H and 11B
(≈ 3%). The fragmentation of 3 is dominated by the forma- NMR spectroscopy. The 1H NMR signals of the Me2N
tion of MeBCl2 as shown by the base peak at m/e = 145 groups in CH2Cl2 solution are found in the range from 2.56
for NSi2Me3. In contrast, 4 loses successively two Me3SiCl to 2.70 ppm. Signals at 3.44 (9) and 3.41 ppm (10) showed
molecules and a Cl atom, i.e. the borazine ring remains in- the presence of NH groups. These were absent in the prod-
tact until the compound has lost its substituents.
ucts obtained from toluene solutions. The introduction of
We have already reported another N-substituted 2,4,6- Me2N groups at the BCl2 substituents leads to better shi-
trichloroborazine, (Cl2BN=BCl)3 5.[39] Its structure was de- elded 11B nuclei. The signals are rather broad and can be
duced from NMR spectroscopic data which indicated that observed around 30 ppm. The 11B NMR signal of the
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FULL PAPER
B–Cl groups in the borazines 6 and 7 (δ11B = 30–31 ppm)
overlaps with the signal of the (Me2N)2B group while the
11B NMR signal of the borazine bonded Me2N group re-
sults in a high field shift (25–26 ppm). The presence of
(Me2N)BCl and (Me2N)2B groups can be seen in some
cases by the asymmetry of the signals around 30 ppm.
The mass spectra of compound 6, 9 and 10 show the
parent ions. It can be noted that the fragmentation starts in
all cases with loss of a dimethylaminochloroborane mole-
cule, a Cl atom or a Me2N group. In case of 10, the loss of
B(NMe2)3, B(NMe2)2 and BNMe2 groups is typical.
In order to check whether bulky groups on the boron
and/or nitrogen atoms might lead to a ring distortion of the
borazine as observed for 4 we prepared (iPrN=BMe)3, 15,
and (MeN=BiPr)3, 16 as shown in Equations (10) and (11).
However, we were unsuccessful in synthesising
(tBuN=BMe)3 and (HN=Bmes)3 by the silazane cleavage
route.[44]
Synthesis of Bromoborazines (RN=BBr)3
In contrast to the B-trihaloborazines (HN=BX)3 no
The aminosilane iPrN(SiMe3)2 had to be prepared in a
three step reaction from iPrNH2 and Me3SiCl to give
iPrNHSiMe3 followed by metallation with LiBu to produce
iPrN(Li)SiMe3 which on reaction with Me3SiCl gave
iPrN(SiMe3)2. The compound tBuN(SiMe3)2 was prepared
analogously.[44] However its reaction with MeBBr2 in tolu-
ene or xylene at reflux (5 d) gave only a single 11B-NMR
peak with δ11B = 48 ppm which indicates that no borazine
had been formed. The 29Si NMR spectrum showed peaks
corresponding to the formation of Me3SiBr besides two
structure of
a
1,3,5-triorganyl-2,4,6-trihaloborazine
(RN=BX)3 has so far been reported although these types
of borazines are well documented.[10] We prepared
(MeN=BBr)3, 11, by treating BBr3 with (Me3Si)2NMe as
shown in Equation (6)[27] and obtained single crystals from
hexane solutions.
Meller et al.[40] as well as Glemser et al.[41] have described
the synthesis of (F5C6N=BBr)3, 12, from BBr3 and
F5C6NH2 in the presence of NMe3 as shown in Equa-
tion (7). We obtained 12 by this route in 51% yield.
1
more intensive signals at δ29Si = 3.2 and 8.1 ppm. The H
NMR spectrum showed signals at δ = 0.41 and 0.17 ppm.
These signals may stem from an aminoborane MeBBr–
N(tBu)SiMe3 because aminoboranes of type MeBBrNR2
show 11B NMR signals in the order of 50 to 40 ppm.[45]
X-ray Structures
Compound 12, when treated with an excess of ammonia,
yields the corresponding triaminoborazine 13 which was
isolated in 43% yield [Equation (8)]. The replacement of the
Br atoms of 12 (δ11B = 30.9 ppm) by the NH2 groups leads
to a high field shift of the 11B NMR signal to 25.3 ppm.
Characteristic in the IR spectrum of 13 is a strong broad
band at 1399 cm–1 due to B3N3 ring vibrations. This band
is 35 cm–1 higher than that found in 11 (1364 cm–1). The
band at 1342 cm–1 can be assigned to the exocyclic B–N
vibrations in 13.[42,43] BN deformation modes can be found
at 734 cm–1 for 12 and at 736 cm–1 for 13. Two strong bands
at 3526 and 3448 cm–1 can be assigned to the asymmetric
and symmetric NH stretching vibrations of 13, respectively.
They are at higher wavenumbers than in (H2NB=NH)3
(3509, 3401 and 3185 cm–1).[10] The IR spectrum of 12 is
consistent with the data reported by Meller.[40]
Compound (HN=BF)3 crystallises in the trigonal space
¯
group R3 and shows D3h point symmetry. The C3 axis
stands perpendicular to the planar ring system and passes
through its centre. Table 1 lists the relevant bonding param-
eters for the trihaloborazines[46–49] and Figure 1 shows the
molecular structure of (HN=BF)3. The data indicate that
the B–N bond lengths of (HN=BF)3 seem to be slightly
shorter than in the chloro and bromo derivatives[44,49] al-
though the former is very close to the others considering
the standard deviations. However, the B–F bond is longer
than in BF3 [1.311(1) Å].[50] According to calculations, the
partial double-bond character of the B–Hal bond decreases
from Hal = fluoride (1.44) via chloride (1.20) to bromide
(1.05).
The molecules of 1 are arranged in the trigonal unit cell
in sheets parallel to the ab plane (Figure 2). Therefore, each
boron atom has not only two N atoms next to it within the
ring but is oriented to two N atoms of two neighbouring
borazine rings and each nitrogen atom of a ring has two
Synthesis of Borazines (HN=BR)3 and (RЈN=BR)3
Although the thermally very stable triphenylborazines boron atoms as neighbours in addition to two boron atoms
(RN=BPh) (R = H, Me, Et, Ph) are well known[10] their from neighbouring borazine rings. The distance between
structures have not yet been described. We prepared two borazine rings within each stack is 3.42(2) Å. This is
(PhB=NH)3, 14[27] by the silazane cleavage reaction as only 0.19 Å longer than in hexagonal boron nitride
shown in Equation (9).
(3.33 Å).[51]
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Structural Chemistry of Borazines
Table 1. Relevant bond lengths [Å] and bond angles [°] of the trihaloborazines (HN=BX)3.
[44]
[49]
[48]
[47]
Compound
[BrBN=NH]3 (X-ray) [ClB=NH]3 (X-ray) [FB=NH]3
(Electr. diffr.) [FB=NH]3 (X-ray)
[FB=NH]3 (X-ray)
B–N
B–X
N–H
1.41(1)
1.94 (1)
0.85
1.41.(1)
1.76(2)
0.84
1.43(1)
1.36(1)
0.79
1.418(2)
1.338(3)
0.77
1.420(9)
1.350(8)
N–B–N
B–N–B
120(1)
119.5(9)
119(1)
121(1)
119(1)
121(1)
119.0(2)
121.0(2)
119.9(8)
120.1(8)
Figure 3. The molecular structure of [F3CCH2N=BF]3, 2. Selected
bond lengths [Å]: N1–B2 1.421(3), N1–B1A 1.421(3), N2–B1
1.421(3), B2–N2 1.419(3). B1–F1 1.336(3), B2–F2 1.341(5), N1–C1
1.459(5), C1–C2 1.496(7), C3–C4 1.486(5), C2–F3 1.342(5), C2–F4
1.323(4), C4–F5 1.336(3), C4–F6 1.346(4), C4–F7 1.321(4). Se-
lected bond angles [°]: B1–N1–B1A 119.1(4), N1–B1–N2 120.8(2),
B1–N2 –B2 119.0(2), N2–B2–N2A 121.0(3), N1–B2–F1 119.3(2),
N2–B2–F2 119.5 (2), N1–C1–C2 111.5(3), N2–C3–C4 111.6(3),
B2–N2–C3 120.8(2), C3–C4–F5 110.6(3), C3–C4–F6 112.6(3), C3–
C4–F7 113.5(3), B1–N1–C1 120.3(2). N1–C1–C2 111.5 (3). Se-
lected torsion angles [°]: F2–B2–N2–C3 3.8, B2–N2–C3–C4 89.5,
F1–B1–N2–C3 –3.6, F1–B1–N2–B2 174.5.
Figure 1. The molecular structure of (HN=BF)3 in ORTEP repre-
sentation. Bond lengths [Å]: N1–B1 1.418(2), B1–N1B 1.418(2),
B1–F1 1.338(3), N1–H1 0.77(1). Bond angles [°]: B1–N1–B1A
121.0(2), N1–B1–F1 120.5(1), N1–B1–N1B 119.0(2).
Compound 2 forms orthorhombic colourless crystals,
space group Pnma with Z = 4. This implies that the mole-
cules have crystallographically imposed symmetry. The two
C atoms and one F atom of the CF3CH2 group and of the
para BF group occupy positions on a mirror plane. The
Structures of Chloroborazines (RN=BCl)3
Bis(trimethylsilyl)aminodichloroborane 3, the precursor
B3N3 ring is planar and Figure 3 shows one complete mole- of the borazine 4, crystallises in the monoclinic system,
cule. B–N and B–F bond lengths are similar to those of space group C2/c with Z = 4. The B–N bond of the mole-
(HN=BF)3.
cule lies on a twofold axis making the two Cl atoms and
Figure 2. Packing of compound 1 in the unit cell. View along the c axis.
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[a] B. Anand, H. Nöth, H. Schwenk-Kircher, A. Troll
FULL PAPER
the two Me3Si groups equivalent (see Figure 4). Both the will discuss only the data for one of these molecules (see
boron and the nitrogen atom reside in a planar environ- Figure 6). The most significant result of the structure deter-
ment. They are connected by a short B–N bond [1.384(4) Å] mination is that the borazine ring shows a significant dis-
indicating the presence of strong π bonding. This is empha- tortion towards a chair shaped six-membered ring The
sised by long B–Cl bonds [1.779(2) Å] which are longer Si1N1B1B3 plane is twisted with respect to the N3B3N2B2
than the B–Cl bond in BCl3 [1.742(4) Å],[52,53] the borazine plane by 13.5° and the Cl1B1N1N2 plane is twisted with
(HN=BCl)3 [1.76(2) Å][49] or the B–Cl bond (1.71 Å) of the respect to the N1B3N2N2 plane by 22.7° (see Figure 7).
Cl2B group in the borazine (Cl2BN=BCl)3. The most inter- Although all B and N atoms reside in a planar environ-
esting feature of compound 3 is that the BCl2 plane is ment, each of the BCl bonds is twisted against the mean
twisted relative to the Si2N plane by 31.8°. In spite of this, plane of the borazine by 16.4° and the N–Si bond in the
the B–N bond lengths are on the short side for monoamino- opposite direction by 8.9°.
boranes.[54] The reason for the twisting is the bulkiness of
the Me3Si group as shown by the orientation of the Me3Si
groups: the Cl atoms point to the void between two methyl
groups of a Me3SiN unit (see Figure 5). This orientation
has a steric effect on the Cl atoms by pushing them back-
wards from the Me3SiN group which results in an acute Cl–
B–Cl angle of 111.9°. The Si–C and Si–N bonds and N–Si–
C bond angles show no anomalies.
Figure 6. The molecular structure of the borazine [(Me3Si)2-
N=BCl]3, 4. Selected bond lengths [Å]: B1–N1 1.430(3), B1–N2
1.430(3), B2–N2 1.428(3), B2–N3 1.432(3), B3–N1 1.430(3), B3–
N3 1.429(3), B1–Cl1 1.791(3), B2–Cl2 1.796(3), B3–Cl3 1.795(3),
N1–Si1 1.795(2), N2–Si2 1.802(2), N3–Si3 1.795(2), Si1–C1
1.864(4), Si1–C2 1.861(4), Si1–C3 1.846(4), Si2–C4 1.857(3), Si2–
C5 1.862(3), Si2–C6 1.844(4), Si3–C7 1.859(3), Si3–C8 1.854(4),
Si3–C9 1.852(4). Selected bond angles [°]: N1–B1–N2 123.9(2),
N2–B2–N3 124.4(2), N3–B3–N1 123.9(2), B1–N2–B2 114.3(2), B2–
N3–B3 114.0(2), B3–N1–B1 114.1(2), Cl1–B1–N1 117.3(2), Cl1–
B1–N2 118.3(2), Cl2–B2–N2 118.0(2), Cl2–B2–N3 117.2(2), Cl3–
B3–N1 117.4(2), Cl3–B3–N3 118.1(2). Selected torsion angles [°]:
Cl1–B1–N1–Si1 29.1, Cl1–B1–N2–Si2 –24.2, Cl2–B2–N2–Si2 22.0,
Cl2–B2–N3–Si3 –24.3, Cl3–B3–N3–Si3 28.1, Cl3–B3–N1–Si1
–30.6. Interplanar angles [°]: Si1–N1–B1–B3/N3–B3–N2–B2 13.5,
Cl1–B1–N1–N2/N1–B3–N2–B2 22.7.
Figure 4. The molecular structure of (Me3Si)2N–BCl2, 3. View on
the Cl2BN plane. Selected bond lengths [Å]: B1–N1 1.384(4), B1–
Cl1 1.779(2), N1–Si1 1.778(1), Si1–C1 1.850(2), Si1–C2 1.860(2),
Si1–C3 1.868(2). Selected bonds angles: Cl1–B1–Cl1A 111.9(2),
Cl1–B1–N1 124.1, B1–N1–Si1 119.42(7), N1–Si1–C1 110.13(9),
N1–Si1–C2 110.6(1), N1–Si1–C3 209.5(1). Selected torsion angles
[°]: C1–Si1–N1–B1 89.7, C2–Si1–N1–B1 –32.9, C3–Si1–N1–B1
–146.7. Interplanar angle [°] Cl1–B1–Cl1A/Si1–N1–Si1A 31.7.
Figure 7. Side view of a molecule of 4. Only the atoms directly
bonded to the ring atoms are shown.
Figure 5. View along the BN axis of 3.
¯
Borazine 4 crystallises in the triclinic space group P1
Compound (Cl2BN=BCl)3 crystallises in the tetragonal
with Z = 4. There are two independent molecules in the space group P41212 with Z = 4. The twofold axis passes
unit cell. The two molecules show slightly different bonding through the exocyclic B–N bond and the B–Cl bond oppo-
parameters on the border of the 3σ criterion. Therefore, we site to this B–N bond. Figure 8 shows the molecular struc-
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Structural Chemistry of Borazines
ture of the compound. In contrast to 4 the molecule pos-
sesses a planar borazine ring. The BCl2 groups are twisted
against the borazine ring by 84.7°.
Figure 9. Molecular structure of compound 9. Selected bond
lengths [Å]: B1–N1 1.455(3), B1–N2 1.401(2), B1–N6 1.419(3), B2–
N1 1.421(2), B3–N3 1.423(4), B3–N4 1.410(4), B3–N1 1.499(3),
B2–Cl2 1.820(4), N2–H2A 0.86. Selected bond angles [°]: N2–B1–
N5 117.8(2), N2–B1–N1 116.9(2), N1–B1–N 5 125.3(2), N1–B2–
N1A 126.3(3), N1–B2–Cl2 116.9(2), N4–B3–N3 123.2(2), N1–B3–
N3 117.7(2), N2–B3–N3 119.1(2), B1–N1–B2 116.4(2), B1–N1–B3
124.5(2), B1–N2–H2A 116.4, N1–B2–Cl2 116.9(2), C1–N3–C2
111.8(2), C3–N4–C4 111.8(2), C5–N5–C6 111.6(2), Selected in-
terplanar angles [°]: N1B1N2/N5C5C6 9.9, B1N1B2/N4B3N3 77.1,
C1–N2–C2/N1–B3–N3–N4 22.8, N3–B3–N4/N1–B1–B2–B3 76.8.
Figure 8. Molecular structure of (Cl2BN=BCl)3, 5. Selected bond
lengths [Å]: B1–N1 1.423(4), B1–N2 1.433(5), B2–N2 1.427(4), B2–
N2A 1.427(4), B3–N2 1.456(5), B4–N1 1.458(7), B1–Cl1 1.767(4),
B2–Cl4 1.770(6), B3–Cl2 1.734(5), B3–Cl3 1.718(5), B4–Cl5
1.716(3). Selected bond angles [°]: N1–B1–N2 118.5(3), N1–B1–Cl1
119.3(3), N2–B1–Cl1 119.3(3), N2–B2–N2A 121.4(4), N2–B2–Cl4
119.3(2). B2A–B2–Cl4 119.3(2), B1–N1–B4 120.6(2), B1A–N1–B4
120.6(2), Cl2–B3–Cl3 117.5(3), N2–B3–Cl2 121.8(3), N2–B3–Cl3
121.8(3), N1–B4–Cl5 121.0(2), N1–B4–Cl5A 121.0(2), Cl5–B4–
Cl5A 118.0(4).
The B–N bonds in the borazine ring of 9 are unequal:
B1–N1 1.455(3), B1–N2 1.401(2) and B2–N1 1.421(4) Å.
The exocyclic B–N bonds N1–B3 and N1A–B3A are fairly
long at 1.499(3) Å due to the strong twist of the B(NMe2)2
Considering the standard deviations, the ring B–N bonds group against the ring. The short exocyclic B1–N5 bond
are of equal lengths (average: 1.424 Å). However, the B–N [1.419(3) Å] points to a stronger π bond contribution which
bond to the BCl2 groups are significantly longer [1.458(7), manifests itself also in the large B1A–N2–B1 bond angle of
1.465(5) Å]. The difference in the B–N bond lengths of 127.2(2)°. Also, the N1–B2–N1A bond angle is quite open
0.037 Å represents the loss of B–N π bonding due to the at 126.3(3)° while the B1–N1–B2 and N1–B1–N2 bond
strongly twisted Cl2B groups. Its B–Cl bonds are signifi- angles are smaller at 116.4(2) and 116.9(2)°. This shows that
cantly shorter than the B–Cl bonds at the borazine ring. the bulky substituents distort the borazine ring in such a
This indicates that the B–Cl bond of the BCl2 groups have way that the B–N bond lengths of the borazine become
some degree of π bonding because these B–Cl bonds are longer and the respective B–N–B bond angles smaller. In
shorter than in BCl3. There are two types of intermolecular particular, the long B–Cl bond at atom B2 [1.819(3) Å] indi-
Cl···Cl contacts between the molecules (3.399 and 3.466 Å). cates that the electron distribution in the ring system is no
The structure of (Cl2BN=BCl)3 implies that the boron longer uniform. The B1–N2 and B1A–N2 bonds have more
atoms of the BCl2 groups will be preferentially attacked by π bond character than the other ring B–N bonds.
nucleophiles because the BCl groups of the borazine are
Another consequence of the bulky substituents is that the
sterically shielded by the BCl2 groups. This has been experi- B–N bond lengths and bond angles become different and
mentally verified by the reaction of (Cl2BN=BCl)3 with this also results in a twisting of the substituents out of the
Me3SiNMe2.
borazine plane. Thus, atoms B3 and B3A are twisted out of
The borazine 9 crystallises in the monoclinic space group the ring plane by 7.6° in opposite directions and the in-
C2/c with Z = 4. Therefore, the molecule must have crystal- terplanar angle N3B3N4/N1N2N1A is 77.3°.
lographic symmetry as shown by the mirror plane which
On the other hand, the Me2N groups of the two
passes through the B–Cl and BNH units (see Figure 9). B(NMe2)2 groups are twisted only by 22.0° (N3) and 25.1°
This has, of course, the consequence that the borazine ring (N4) against the N3N4B3N1 plane. Their B–N bonds at N3
is planar. The two bulky B(NMe2)2 groups are twisted and N4 are short for a triaminoborane unit at 1.423(3) and
against the borazine ring by 77.3°. In contrast, the two 1.410(3) Å, respectively.
Me2N groups bonded to the boron atoms of the borazine
The distortions noted for compound 9 become even
ring are twisted against the ring only by 9.8° which allows more pronounced in compound 10. This borazine crystal-
for BN π bonding as shown by the B–N bond length of lises, like 9, in the monoclinic space group C2/c with Z =
1.418(2) Å. In contrast, the interplanar angles which the 4. In this case atoms N6, B2, N1 and H lie on a C2 axis
Me2N groups form with the BN3 plane of B[(NMe2)2]3 are which gives the molecule C2 point group symmetry. The
32.8°[55] i.e. the Me2NB groups in 9 are less twisted against three Me2N groups Me2N3, Me2N2A and Me2N6 as well
the borazine ring than in tris(dimethylamino)borane.
the bis(dimethylamino)boryl groups are twisted in the same
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[a] B. Anand, H. Nöth, H. Schwenk-Kircher, A. Troll
FULL PAPER
sense against the borazine ring which is no longer planar as 122.4(6)° and from 117.5(6) to 119.2(6)°, respectively. This
shown by a torsion angle for N1–B1–N2–B2 of 11.0° (see indicates that the N–B–N bond angles are on average wider
Figure 10).
than the B–N–B bond angles. The B–Br bond lengths are
similar to those in (HN=BBr)3.[44] Figure 11 shows the two
molecules. The molecules are stacked above one another
and the planes of the borazine molecules are, on average,
4 Å apart. The shortest distances between the ring planes
are 3.62 Å and the longest are 4.53 Å. The stacking results,
as in hexagonal boron nitride, in weak intermolecular BN
interactions. Also, intermolecular Br···Br contacts (3.85 Å)
can be observed.
Figure 10. ORTEP plot of the borazine 10. Hydrogen atoms omit-
ted (except the NH hydrogen atom) for clarity. Selected bond
lengths [Å]: B1–N1 1.385(5), B1–N2 1.449(6), B2–N2 1.464(5), B1–
N3 1.422(6), B3–N2 1.488(6), B2–N6 1.477(8), N1–H 0.86(4), N1–
C1 1.464(6), N3–C2 1.454(5), N4–C4 1.466(5), N4–C3 1.462(6),
N6–C7 1.437(6). Selected bond angles [°]: B1–N1–B1A 125.8(5),
N1–B1–N2 118.3(4), B1–N2–B2 120.1(4), N2–B2–N2A 120.4(5),
N1–B1–N3 115.1(4), N2–B1–N3 126.6(4), B1–N2–B3 121.4(3), B2–
N2–B3 120.1(4), N2–B2–N6 119.8(3), N4–B3–N5 121.8(4), N2–
B3–N4 118.3(4). Selected torsion angles [°]: N1–B1–N2–B2 –11.0,
B1–N2–B2–N2A 5.5. Selected interplanar angles [°]: C2–N2–C1/
N1–B1–N2 12.9, N4–B3–N5/B1–N2–N2 111.5, N7–N6–N7A/
N2A–B2–N2 40.7.
Figure 11. ORTEP plot of the two independent molecules of 11
viewed on top of the six-membered ring. Selected bond lengths [Å]:
B1–N1 1.42(1), B2–N1 1.21(1), B2–N2 1.41(1), B3–N2 1.44(1), B3–
N3 1.40(1), B1–N3 1.41(1), B4–N4 1.40(1), B5–N4 1.43(1), B5–
N5 1.44(1), B6–N5 1.41(1), B6–N6 1.44(1), B4–N6 1.44(1), B1–Br1
1.954(8), B2–Br2 1.953(8), B3–Br3 1.939(8), B4–Br4 1.953(8), B5–
Br5 1.933(8), B6–Br6 1.941(8), N1–C1 1.468(9), N2–C2 1.489(9),
N3–C3 1.423(9), N4–C4 1.476(9), N5–C5 1.491(9), N6–C6
1.479(9). Selected bond angles [°]: B1–N1–B2 117.6(6), N1–B2–N2
122.0(7), B2–N2–B3 119.1(6), N2–B3–N3 119.8(7), B3–N3–B1
119.2(6), B4–N4–B5 119.2(6), N5–B6–N6 121.1(7), B6–N5–B5
119.4(6), N5–B5–N4 120.4(7), B4–N6–B6 117.6(6), N4–B5–N5
120.4(7), B5–N5–B6 119.4(6), N5–B6–N6 121.1(7), N1–B1–Br12
118.4(6), N1–B2–Br2 119.4(8), N2–B2–Br2 118.6(6), N2–B3–Br3
119.7(6).
The bond lengths B2–N2 and B2–N2A are 1.464(5) Å,
i.e. 0.04 Å longer than in 9. The Me2N6 group is twisted
against the N2B2N2A plane by 40.9°. This results in a com-
paratively long B2–N6 bond [1.477(8) Å]. In contrast, the
Me2N groups on atoms B1 and B1(A) are twisted against
the borazine ring by 122° and the B–N bonds lengths are
1.422(6) Å.
The shortest B–N bonds in the borazine ring of 10 are
the two B–N bonds to N1 at 1.385(5) Å which is even
shorter than in compound 9. This distortion leads to quite
different endocyclic bond angles: B1–N2–B2 120.1(4), N1–
B1–N2 118.3(4) and B1–N1–B1A are 125.8(5) whereas N2–
B2–N2a is 120.4(5)°. The longest B–N bond found involves
the bis(dimethylamino)boryl group B3 with N2–B3
1.488(6) Å. Its two Me2N groups are twisted against the
B1N2B2B3 plane by 20.2 and 23.4° and the respective B–
N bond lengths are 1.424(6) and 1.417(6) Å. Although the
nitrogen atoms N4 and N5 are also planar, the CϪNϪC
bond angles are sharp at 110.9(4) and 113.6(6)°.
Crystals of the borazine (F5C6N=BBr)3, 12, are mono-
clinic (space group C2/c, Z = 4). A twofold axis passes
through atoms Br1, B1, N2 C7, C10 and F8 as shown in
Figure 12 and the molecule therefore has point group sym-
metry C2. The B–N bonds are longer than in (HN=
[44]
BBr)3
although the B–Br bonds are shorter. The endo-
cyclic bond angles N–B–N are smaller than 120° (on
average 119°) and the B–N–B bonds angles are larger (on
average 121°). Typical is the arrangement of the pentafluo-
rophenyl groups. They stand almost perpendicular to the
borazine ring plane with interplanar angles of 86.4(1)° for
the F5C6 group bonded to N2 and 82.2(1)° for those
bonded to N1 and N1A.
A similar distortion results for the F5C6 ring. The C–C–
C bond angles of the ipso C atoms C1 and C7 are only
117.3(3)° and 117.8(5)°, respectively, and the C–C–C bond
Structures of Bromoborazines (RN=BBr)3
The borazine (MeN=BBr)3, 11, crystallises in the triclinic angle for the ortho C atoms C2/C6 are 121.2(3) and
¯
space group P1 with Z = 4, i.e. there are two independent 121.9(3)° for C8.
molecules in the unit cell. The borazine rings are planar
Figure 13 shows the molecular structure of the amino
and B–N bond lengths span a range from 1.398 to 1.440 Å derivative 13 (F5C6N=BNH2)3. This borazine crystallises in
although these differences have to be considered insignifi- the monoclinic space group I2/a with Z = 4 and the mole-
cant in terms of the standard deviations which, in spite of cules shows point group symmetry C2. The borazine ring is
an R1 value of 4.42%, are rather large (σ = 0.010). The N– therefore planar and the N atoms of the amino groups lie
B–N and B–N–B bond angles span a range from 119.9(6) to in the ring plane. The pentafluorphenyl rings are twisted
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Structural Chemistry of Borazines
Structures of 2,4,6-Triorganylborazines (RN=NRЈ)3
One of the most well known and stable borazines is 2,4,6-
triphenylborazine, 14.[10] Its crystal structure was deter-
mined by I. Manners et al. in 2003.[57] They observed an
orthorhombic unit cell in the space group Pna21. We deter-
mined the structure of 14 already in 1997[44] but report our
result now because we found a different space group. It was
difficult to grow suitable single-crystals because most of the
long needles had diameters less than 0.01 mm. The com-
pound crystallised in the trigonal space group P6cc. Fig-
ure 14 shows an ORTEP plot of the molecule with a view
on top of the planar borazine ring down the c axis. A C3
axis passes through the centre of the ring. The B–N bond
lengths are 1.424(4) Å which is on the short side for B–N
bond lengths in borazines. The N–B–N and B–N–B bond
angles of 14 deviate from the ideal 120° of a hexagon by 3–
4°. Particularly narrow is the N–B–N bond angle of
115.8(3)°. In contrast the B–N–B bond angle is 124.1°. The
respective bond angles of 14 in the orthorhombic cell are
closer to 120°, with B–N–B bond angles ranging from
120.6(1) to 121.1(1)° and N–B–N bond angles from
118.7(2) to 119.90(2)°.[57] Clearly, 14 can exists in two poly-
morphic forms.
Figure 12. ORTEP plot of a molecule of (F5C6N=BBr)3, 12. Se-
lected bond lengths [Å]: B1–N1 1.435(4), B2–N1 1.426(6), B2–N2
1.431(5), B1–Br1 1.907(6), B2–Br2 1.919(4). N1–C1 1.437(5), N2–
C7 1.448(7). Selected bond angles [°]: N1-.B1–N1A 119.3(5), Br1–
B1–N1 120.4(3), N1–B2–Br2 120.6(3), N2–B2–Br2 120.6(3), N1–
B2–N2 118.8(3), B1–N1 B2 120.6(3), B1–N1–C1 118.6(3) B2–N1
C1 120.8(3), B2–N2–C7 119.3(2), B2–N2–B2A 121.3(5), C6–C1–
C2 117.3(3), C9–C7–C8A 117.8(5). Selected interplanar angles [°]:
B1–B2–B2A–N1–N2–N2A/C1–C6 81.4, B1–B2–B2A–N1–N2–
N2A/C7–C8–C9–C8A–C9A 95.2.
against the borazine plane by 102.2° (N1) and 93.8° (N2)
while the planar amino groups are twisted by only 7.4° (B1)
and 8.7° (B2). Thus, the amino groups are almost coplanar
with the ring and, as a consequence, the exocyclic B–N
bonds are rather short at 1.392(4) and 1.410(3) Å, i.e. they
show π-bond character. This is certainly the reason why the
endocyclic B–N bond lengths are comparatively long, at
1.448(2) Å for B1–N1, 1.443(3) Å for B2–N1 and
1.453(2) Å for B2–N2. Similar to the F5C6 groups in 13 one
observes a distortion of the phenyl groups. The C–C–C
bond angle at the ipso-C atom is 116.6(2)° and at the ortho-
C atoms it is 122.0(2)°.
Figure 14. ORTEP plot of compound (HN=PPh)3, 14. Selected
bond lengths [Å]: B1–N1 1.424(4), B1–N1A 1.424(4), B1–C1
1.573(5), N1–H1 0.81(4). Selected bond angles [°]: N1A–B1–N1
115.8(3), N1A–B1–C1 123.1(3), N1–B1–C1 121.0(3), B1B–N1–B1
124.1(3). Torsion angle [°]: C2C1B1N1 25.5.
The phenyl rings are arranged in a propeller like fashion,
the interplanar angle between the phenyl groups and the
borazine ring being 27.7°. This is less than in the isoelec-
tronic 1,3,5-triphenylbenzene where the torsion angles are
36.1, 37.2 and 40.9°.[58] The difference is due to the longer
ring B–N bond lengths compared with the ring C–C bond
lengths.
The molecules are stacked above one another along the
threefold axis (see Figure 15). Each molecule is turned
around by 44.2°. The distance between two ring centres is
3.64 Å which is 0.31 Å longer than in hexagonal BN.
The arrangement of the molecules in the hexagonal unit
cell induces a structural peculiarity. Six stacks of the mole-
cules are arranged around the threefold axis in space to
form a channel as shown in Figure 15. Not taking H atoms
into account, the channel diameter is 7.5 Å (or 4.5 Å taking
the van der Waals radius of H into account).
Figure 13. The molecular structure of (F5C5N=BNH2)3, 13. Se-
lected bond lengths [Å]: B1–N1 1.448(2), B1–N3 1.392(4), B2–N1
1.443(3), B2–N2 1.453(2), B2–N4 1.410(3), N1–C1 1.421(2), N2–
C7 1.421(7). Selected bond angles [°]: N1A–B1–N1 117.2(2), B1–
N1–B2 123.0(2), N1–B2–N2 116.6(2), B2–N2–B2A 123.3(2), N1–
B1–N3 121.4(1), N1A–B1–N3 121,4(1), N1–B2–N4 122.1(2), N2–
B2–N4 121.3(2), B1–N1–C1 117.2(2), B2–N1–C1 119.9(2). C2–C1–
C6 116.4(2), C8–C7–C8A 116.6(2). Selected interplanar angles [°]:
H1–H1A–N3/N1–B1–N1A 7.5, H2–H3–N4/N1–B2–N2 3.6, C1–
C2–C6/B1–N1–B2 103.6, C8–C7–C8A/B2–N2–B2A 90.8.
The borazine (iPrN=BMe)3, 15, crystallises in the mono-
clinic space group P21/n with Z = 4. While the borazine
ring is planar (maximum deviation from the mean plane is
Eur. J. Inorg. Chem. 2008, 3186–3199
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FULL PAPER
The unit cell parameters of the isomeric borazine (MeN-
=BiPr)3, 16, suggest that it crystallises in the tetragonal sys-
tem. However, we were not able to find a structural solution
in a tetragonal space group. A solution was, however, pos-
sible for the orthorhombic system in the P21212 space
group to give what we consider a preliminary result.[59] Fig-
ure 17 shows the molecular structure with the disordered
isopropyl groups. The site occupation factor suggests a
value of 0.5 for the disordered C atoms C4, C7 and C10.
For this reason we will discuss shortly only the bonding
parameters of the B3N3 ring and compare the data with
those of the isomer 15. The B–N bonds of 16 seem to be
slightly longer than in 15 and B–N–B bond angles are
slightly larger. Consequently, the N–B–N bond angles are
smaller than in the isomer 16. A steric effect resulting from
the shorter N–C bond to the iPr group may be responsible
for the longer B–N bond in 17. However, the differences are
small and are more significant for the bond angles than for
the bond lengths.
Figure 15. The channel structure created from stacks of molecules
along a sixfold axis.
0.02 Å), the carbon atoms bonded to the B and N atoms
deviate from the ring plane, the methyl groups by 4–5° and
the central C atom of the isopropyl groups by 1–2°. Fig-
ure 16 shows the molecular structure of 15. The N–B–N
bond angles are close to 118° and the B–N–B angles close
to 122°. B–C bond lengths range from 1.590(4) to
1.600(4) Å. Compared with the B–N bond lengths of
(HN=BMe)3 [1.39(4) Å][10] the B–N bonds are longer
[1.442, 1.444(3) Å]. We assume that this lengthening is due
to the steric effect of the isopropyl groups.
Figure 17. Molecular structure of 16 showing the site disordered
position of the CH atoms of the isopropyl groups.
Discussion and Conclusions
The parent borazine (HB=NH)3 was first prepared by
Stock and Poland[60] in 1926 and its structure was deter-
mined by electron diffraction in 1938.[61] It revealed a
planar six-membered ring with equal B–N bond lengths
and slightly different endocyclic bond angles (119, 121°).
This molecular structure was also observed in the solid-
state, however the symmetry of the borazine molecule
turned out to be C2.[4] E. Wiberg[62] coined the name “inor-
ganic benzene” for borazines because benzenes and the iso-
electronic borazines usually have planar six-membered
rings, although they are chemically quite different. Since
then, numerous calculations at various levels of theory have
been reported attempting to prove or disprove the aromatic
character of borazine. Today, there is evidence that boraz-
Figure 16. Molecular structure of (iPrN=BMe)3, 15. Selected bond
lengths [Å]: N1–B11.444(3), N1–B3 1.443(3), N2–B1 1.441(3), N2–
B2 1.442(3), N3–B2 1.440(3), N3–B3 1.445(3), B1–C4 1.590(4), B2–
C8 1.600(4), B3–C12 1.596(4), N1–C1 1.498(3), N2–C5 1.494(3),
N3–C9 1.497(3). Selected bond angles [°]. N2–B1–N1 117.4(2),
N1–B1–C4 119.7(2), N2–B1–C4 122.8(2), N3–B2–N2 118.1(2),
N3–B2–C8 122.3(2), N1–B3–C12 122.7(2), N1–B3–N3 118.2(2),
N1–B3–C(12) 122.7(2), N3–B3–C12 119.1(2), B3–N1–B1 121.9(2),
B1–N1–C12 116.7(2), B3–N1–C1 121.2(2), B1–N2–B2 122.2(2),
B2–N2–C5 116.5(2), B1–N2–C5 121.2(2), B2–N3–B3 121.3(2), B2–
N3–C9 121.5(2), B3–N3–C9 117.2(2). Selected torsion angles [°]:
N3–B3–N1–B1 7.4, B1–N2B2–N3 –7.9.
The molecules are situated along the b axis. They are ines may have an “antiaromatic” character.[2]
stacked on top of each other due to translation. The axis The N atoms of borazines show Lewis basicity as dem-
through the centre of the borazine ring forms an angle of onstrated, for instance, by the ease of formation of 1:1 ad-
35.2° with the b axis.
ducts with halides of aluminium, gallium or tin[63] whereas
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Structural Chemistry of Borazines
= 25.1 ppm. IR (nujol): ν = 3509 (m), 1531 (w), 1514 (vst), 1505
˜
the B atoms show Lewis acidic character as revealed by the
formation pyridine adducts[56,64] or the formation of borazi-
nium cations such as (Me3B3N3H4)+.[63,65,66]
(vst), 1401 (m), 1333 (m), 1125 (w), 935 (m), 735 (w), 727 (w), 685
(w), 660 (m) cm–1.
[Bis(trimethylsilyl)amino]dichloroborane (3):[36] a) A 250 mL three-
necked round-bottomed flask fitted with a dry-ice reflux condenser
was charged with a solution of N(SiMe3)3 (5.95 g, 25.5 mmol) in
hexane (50 mL). BCl3 (43.0 g, 30.0 mL), condensed in a dry-ice co-
oled dropping funnel, was then diluted with hexane (50 mL). At
–60 °C this solution was dropped into the stirred solution of the
aminosilane. After addition the mixture was allowed to attain room
temperature and was then heated to reflux for 4 h. All volatiles
were then removed from the solution at 25 °C/12 Torr. Distillation
of the remaining oil at b.p. 82 °C/11 Torr yielded 3 (4.51 g, 73%)
as a colourless liquid, m.p. 5 °C.
More recently, it has been shown that the BN bonds of
borazines can become quite unequal by binding one N
atom to a Li, Al, Si, P, As or Sb atom.[21–24] Also, borazin-
ium cations, e.g. (Me3B3N3H4)+, show three different BN
bond lengths.[63–66]
The ring planarity can be lost particularly with the intro-
duction of bulky substituents. At the moment, the most
striking examples are compounds 4, Me3B3N3H2Ti-
[24]
(OtBu)3 and [(EtB=NR)3Cr(CO)3].[67] In addition, there
are now some examples where ring planarity is retained but
the substituents are bent up and/or down from the ring
plane as shown for compounds 15 and 16. In conclusion,
the shape of the borazine ring depends on the steric demand
of the substituents and the electronegativity of the groups
attached either to the N atoms as well as on the B atoms.
b) BCl3 (3.44 g, 2.4 mL, 29.3 mmol) was condensed at –78 °C into
a 250 mL round-bottomed flask and diluted with hexane (60 mL).
At –60 °C a solution of NaN(SiMe3)2 (5.38 g, 29.3 mmol) in hexane
(100 mL) was added while stirring. Stirring was continued over-
night at ambient temperature. The insoluble NaCl was then re-
From this point of view, a more systematic study of substit- moved by filtration and the hexane distilled from the filtrate (25 °C,
12 Torr). The residue was then distilled using a Vigreux column. 3
(3.83 g, 54%) was collected at b.p. 75 °C/5 Torr. It was subsequently
dissolved in hexane (2 mL) hexane. Single-crystals separated on
storing the solution at –25 °C. NMR in C6D6: 1H NMR: δ =
0.26 ppm (s, 1JH,C = 18 Hz; 2JH,Si = 39 Hz). 13C NMR (100 MHz):
uent effects on the structure of borazines will almost cer-
tainly provide interesting results.
Experimental Section
1
1
δ = 37.6, 3.83 ppm (s, JC,H = 18 Hz, JC,Si = 29 Hz). 11B NMR
(64 MHz): δ = 37.6 ppm (s, h1/2 = 140 Hz). 29Si NMR: δ = 9.70 ppm
General: All experiments were performed under anhydrous condi-
tions in an atmosphere of either nitrogen or argon using Schlenk
techniques. The solvents were dried by standard methods and
stored under N2 using special storage flasks. If not stated otherwise,
all reagents were of commercial grade. The compounds 2,4,6-tri-
1
(d, JSi,C = 29 Hz). IR (NaCl): ν = 3041 (w), 2971 (m), 2960 (st),
˜
2894 (m), 1409 (m), 1377 (m), 1321 (st), 1289 (vst), 1268 (st), 1257
(vst), 942 (st), 922 (vst), 875 (st), 850 (st), 803 (m), 770 (m), 695
(m), 684 (m), 637 (w), 620 (m), 552 (w), 534 (m), 472 (w), 395 (m),
302 (w) cm–1. C6H18BCl2NSi2 (242.1): calcd. C 29.77, H 7.49, N
5.79; found C 27.41, H 6.88, N 5.65.
bromo-1,3,5-trimethylborazine,[10]
2,4,6-tribromo-1,3,5-tris(pen-
tafluorophenyl)borazine, m.p. 253 °C,[40,41] were prepared accord-
ing to literature procedures. (F3CCH2N=BF)3 was supplied by
Meller.[35]
2,4,6-Trichloro-1,3,5-tris(trimethylsilyl)borazine (4):[70] A solution
of 3 (4.51 g, 18.6 mmol) in xylene (100 mL) was heated at reflux
for 2 d. A single 11B signal at δ = 35.5 ppm was then present. Me3-
SiCl and the solvent were subsequently removed in vacuo. The solid
residue was subjected to sublimation (95 °C/2 Torr). Yield of 4:
1.59 g, 64%. Compound 4 was dissolved in toluene (10 mL). At
–25 °C 4 crystallised from the solution as colourless prisms, m.p.
140 °C. NMR (C6D6): 1H NMR (400 MHz): δ = 0.44 ppm (s, 1JH,C
NMR: Bruker ACP 200 (1H, 11B, 13C), Jeol GSX 270 (1H, 11B,
29Si), Jeol EX 400 (1H, 11B, 13C): standards: SiMe4, C6D6, BF3OEt2
ext. IR: Nicolet 520 FTIR spectrometer, usually hostaflon/nujol
mulls for solids. MS: Atlas CH7 or Finnigan MAT90, usually at
70 eV; Siemens P4 diffractometer equipped with an area detector
and an LT2 device. Mo-Kα-radiation, graphite monochromator.
Data reduction, structure solution, absorption correction and re-
finement: SAINT, SADABS, SHELXTL and SHELX93.[68] Ther-
mal ellipsoids shown in the figures represent a 25% probability. CH
hydrogens were placed in calculated positions and refined as a ri-
ding model. Positions of N bonded H atoms were taken from the
difference Fourier maps and their positions were refined iso-
tropically. All non-hydrogen atoms were refined with anisotropic
thermal parameters. Elemental analyses were performed at the mi-
croanalytical laboratory of the department.
1
= 17 Hz). 13C NMR (100 MHz): δ = 3.60 ppm (s, JH,C = 17 Hz).
11B NMR (64 MHz): δ = 35.5 ppm (s, h1/2 = 370 Hz). 29Si NMR
(53.7 MHz): δ = 19.34 ppm. IR (nujol/hostaflon): ν = 3048 (w),
˜
2997 (w), 2960 (m), 2901 (w), 1363 (st), 1335 (vst), 1255 (st), 1155
(m), 1141 (st), 1095 (w), 1067 (w), 1053 (w), 1042 (w), 1027 (w),
921 (st), 848 (vst), 779 (st), 685 (m), 677 (m), 670 (m), 643 (w), 638
(w), 619 (w), 597 (w), 513 (w), 475 (w), 399 (w), 364 (w), 350 (w),
297 (w), 270 (m) cm–1. C9H27B3Cl3N3Si3 (400.38): calcd. C 27.00,
H 6.80, N 10.49; found C 27.51, H 6.59, N 10.33.
2,4,6-Trifluoroborazine (1): A 100 mL Schlenk bulb was charged
[Bis(trimethylsilyl)amino]chloro(methyl)borane: MeBCl2 (5.3 g,
with freshly sublimed SbF3 (14 g, 78 mmol) and toluene (30 mL).
At –78 °C stirring was started and a solution of (HN=BCl)3
55 mmol) was condensed into a 250 mL three-necked flask
[69]
equipped with an dry-ice reflux condenser and hexane (10 mL) was
added. The stirred solution was then kept at –60 °C and a suspen-
sion of LiN(SiMe3)2 (8.2 g, 49 mmol) in hexane (100 mL) was
added. The mixture was stirred overnight at ambient temperature.
After filtration and washing with hexane (2ϫ10 mL) all volatiles
were removed from the filtrate at 25 °C/10 Torr. Colourless crystals
separated at –25 °C from a concentrated hexane solution (10 mL).
The yield was not determined; m.p. –8 to –5 °C. The crystal quality
was not good enough for an X-ray structure analysis. NMR
(0.68 g, 3.7 mmol), dissolved in toluene (25 mL), was drop wise
added to the suspension. After warming to ambient temperature
the reaction was followed by 11B NMR spectroscopy: the signal of
(HN=BCl)3 at δ = 30.4 ppm disappeared quickly and within an
hour only the signal of 1 at δ = 25.0 ppm could be observed show-
ing that the solution contained only 1. After filtration (G4 frit) and
storing the filtrate for one day at –25 °C a colourless solid had
formed consisting of tiny, needle shaped single crystals of 1. Yield
0.44 g, (3.3 mmol, 89%), m.p. 121 °C. NMR (C6D6): 11B NMR: δ
2
(C6D6): 1H NMR (400 MHz): δ = 0.21 (s, JH,Si = 5 Hz, 18 H),
Eur. J. Inorg. Chem. 2008, 3186–3199
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3195

[a] B. Anand, H. Nöth, H. Schwenk-Kircher, A. Troll
FULL PAPER
0.76 ppm (s, 3 H). 13C NMR (100 MHz): δ = 4.1 (SiMe), 13.9 ppm 8 ppm (h1/2 = 400 Hz). IR (Nujol, hostaflon): ν = 2981 (st), 2777
˜
(br., BMe). 11B NMR (64 MHz): δ = 49.1 ppm. 29Si NMR
(53.7 MHz): δ = 6.48 ppm. IR (CsBr plates): ν = 3735 (w, br.), 3662
(m), 2509 (st), 1649 (m), 1510 (st), 1459 (vst), 1416 (vst), 1387 (st),
1349 (m), 1220 (w), 1196 (m), 1144 (st), 1131 (st), 1077 (w), 1024
˜
(w, br.), 3412 (w), 2984 (st), 2958 (vst), 2900 (w), 2864 (m), 2801 (st), 965 (vst), 910 (w), 901 (m), 842 (w), 831 (m), 817 (w), 790 (m),
(w), 1940 (w), 1877 (w), 1726 (w), 1648 (w), 1431 (m), 1411 (m), 731 (w), 721 (m), 700 (m), 654 (m), 630 (w), 621 (m), 611 (w), 579
1338 (vst), 1323 (vst), 1290 (vst), 1277 (vst), 1265 (vst), 1256 (vst), (m), 541 (w), 431 (w), 389 (w), 352 (w) cm–1. C6H18B6Cl6N6 (451.8):
1196 (w), 1162 (w), 1148 (w), 1051 (st), 1026 (vst), 944 (m), 905 calcd. C 15.95, H 4.02, N 18.60, Cl 47.08; found C 16.15, H 4.46,
(st), 889 (st), 847 (vst), 768 (st), 688 (st), 625 (m), 614 (st), 553 N 17.53, Cl 46.50.
(w), 533 (w), 499 (w), 477 (m), 450 (m), 386 (m), 303 (m) cm–1.
C7H21BClNSi2 (221.69): calcd. C 37.93, H 9.55, N 6.32; found C
37.34, H 9.03, N 6.41.
1,3,5-Bis[bis(dimethylamino)boryl]-2,4,6-trichloroborazine (7) and 6-
Chloro-2,4-dimethylamino-1,3-bis(dimethylaminoboryl)borazine (9):
A solution of (Cl2BN=BCl)3 (640 mg, 1.84 mmol) in hexane
(10 mL) was cooled to –78 °C. While stirring, Me3SiNMe2 (1.34 g,
1.44 mL, 11.5 mmol) was added with a syringe. The precipitate that
formed was found to dissolve on warming to room temperature.
After stirring for 3 h, Me3SiCl and hexane were removed in vacuo
(1 Torr). The oily product, dissolved C6D6, showed only one 11B
NMR signal (most likely 7). However, when it was dissolved in
dichloromethane (2 mL) and the solution cooled to –25 °C, 0.53 g
of 9 (1.3 mmol, 73%) separated as colourless prisms; m.p. 125 °C.
Attempted Decomposition of (Me3Si)2NB(Cl)Me: (Me3Si)2NB(Cl)-
Me (9.6 g, 40 mmol) was dissolved in toluene (100 mL) and heated
1
at reflux for 10 d. No change in the 11B and H spectra could be
detected. The toluene was then removed in vacuo and replaced by
xylene (100 mL). This solution was then heated to reflux for two
weeks. No change in the NMR spectra could be noted. The xylene
was then removed in vacuo and the pure compounds heated to
reflux (≈ 195 °C). No formation of Me3SiCl was observed.
2,4,6-Trichloro-1,3,5-tris[chloro(dimethylamino)boryl]borazine (6): NMR of 9 (in C6D6): 1H NMR (400 MHz): δ = 2.57 (s, 6 H, NMe),
To a solution of (Cl2BN=BCl)3 (0.48 g, 1.12 mmol) in hexane
(10 mL) was added at –78 °C to a solution of Me3SiNMe2 (397 mg,
3.39 mmol) in hexane (5 mL). A solid formed which dissolved dur-
ing warming to ambient temperature. After three hours stirring,
the hexane and Me3SiCl were removed in vacuo. 427 mg (96%) of
2.60 (s, 12 H, NMe), 2.70 (s, 6 H, NMe), 3.44 ppm (s, 1 H, NH).
13C NMR (100 MHz): δ = 38.1 (s), 39.4(s), 39.5 ppm (s). 11B NMR
(64 MHz): δ = 24.9 with a shoulder at 25.3 (B1, B1A), 29.9 ppm
(B2, B4, B4A); (ratio 2:3). IR (nujol, hostaflon): ν = 3525 (w), 3483
˜
(w), 2983 (st), 2788 (st), 1349 (m), 1220 (m), 1196 (w), 1144 (w),
[Me2N(Cl)BN=BCl]3 are left as a yellow oil. NMR (C6D6): 1H 1129 (st), 1066 (m), 1029 (w), 959 (m), 913 (m), 906 (w), 845 (m),
NMR (400 MHz): δ = 2.62 (s, 9 H, NMe), 2.67 (s, 9 H, NMe). 13
C
830 (st), 807 (w), 798 (w), 736 (w), 713 (m), 701 (w), 654 (w), 637
NMR (100 MHz): δ = 39.35, 39.42. 11B NMR (64 MHz): δ = (w), 631 (w), 616 (w), 616 (w), 566 (w), 546 (w), 520 (w), 478 (w),
Table 2. X-ray data of compounds 1–5, 9.
Compound
Formula
1
2
3
4
5
9
H3B3F3N3
134.48
0.10ϫ0.15ϫ0.40
trigonal
C6H6B3F12N3
380.58
0.24ϫ0.30ϫ0,33
orthorhombic
Pnma
9.481(2)
15.336(3)
9.744(2)
90
90
90
1416.9(5)
12
1.784
C6H18BCl2NSi2
242.10
0.2ϫ0.25ϫ0.6
monoclinic
C2/c
14.972(5)
8.663(4)
10.354(2)
90
96.40(1)
90
C9H27B3Cl3N3Si3
400.39
0.15ϫ0.2ϫ0.5
triclinic
B6Cl9N3
425.94
C12H37B5ClN9
397.01
0.3ϫ0.4ϫ0.6
monoclinic
C2/c
11.4420(6)
17.7561(9)
12.6638(6)
90
115.18(1)
90
Mr [gmolϪ1
Cryst. size [mm]
Cryst. system
Space group
a [Å]
]
0.23ϫ0.3ϫ0.5
tetragonal
P4(1)2(1)2
9.8256(1)
9.8256(1)
16.4708(2)
90
¯
¯
R3c
P1
11.388(3)
11.388(3)
6.835(2)
90
90
120
767.7(4)
6
1.745
10.874(3)
12.099(2)
18.188(4)
78.303(9)
89.893(7)
67.258(9)
2153.3(9)
4
b [Å]
c [Å]
α [°]
β [°]
90
90
γ [°]
V [Å3]
1334.5(8)
4
1.205
1590.13(3)
4
1.779
2328.3(2)
4
1.133
Z
ρ(calcd.) [MgmϪ3
µ [mm–1]
]
1.235
0.588
0.184
0.217
0.624
1.560
0.180
F(000)
Index range
396
744
512
840
816
856
–9 Յ h Յ 4,
13 Յ k Յ 0,
–3 Յ l Յ 5
52.38
–1 Յ h Յ 11,
–18 Յ k Յ 1,
–1 Յ l Յ 11
50.00
–18 Յ h Յ 18,
–10 Յ k Յ Յ 10,
–13 Յ l Յ 8
57.24
–13 Յ h Յ 13,
–16 Յ k Յ 13,
–22 Յ l Յ 23
57.34
–12 Յ h Յ 12,
–12 Յ k Յ 12,
–18 Յ l Յ 21
58.40
–14 Յ h Յ 14,
–21 Յ k Յ 21,
–8 Յ l Յ 16
58.40
2θ [°]
T [K]
173
228
135
127
193
1766
1295
836
0.0294
131
0.0457/0.6152
153(5)
3502
1192
908
0.0264
59
0.0442/1.7617
193(3)
12104
6504
5466
0.0198
397
0.0260/1.8470
213(3)
8287
1731
1658
0.0638
84
0.0654/1.4860
183
6585
1970
1321
0.0351
130
0.0567/4.6672
Refl. collected
Refl. unique
Refl. observed (4σ)
R (int.)
No. variables
Weighting scheme[a]
x/y
0.0303
18
0.02790/1.4390
GOOF
1.109
1.021
1.114
1.151
1.092
1.142
Final R (4σ)
Final wR2
0.0339
0.0825
0.168
0.0454
0.1089
0.200
0.0333
0.0876
0.355
0.0385
0.0850
0.286
0.0505
0.1286
0.446
0.0385
0.0988
0.260
Larg. res. peak [eÅϪ3
]
[a] w–1 = σ2Fo + (xP)2 + yP; P = (Fo + Fc )/3.
2
2
2
2
3196
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 3186–3199

Structural Chemistry of Borazines
399 (w), 326 (w) cm–1. C12H37B5ClN9 (396.92): calcd. C 36.30, H (7.51 g, 12 mmol) in CH2Cl2 (30 mL) was added. The mixture was
9.39, N 31.75, Cl 8.93; found C 36.59, H 9.25, N 30.56, Cl 8.70.
allowed to attain room temperature and the reaction followed by
11B NMR spectroscopy. The starting material signal at δ =
30.9 ppm disappeared within 3 h and was replaced by a signal at δ
= 26.1 ppm. At that time only a single for 13 could be observed at
δ = 26.1 ppm. All volatiles were then evaporated in vacuo and the
residue extracted with toluene (20 mL). After filtration and wash-
ing with toluene (10 mL) crystals separated on cooling to –25 °C.
Yield 3.2 g (43%); m.p. 259 °C (dec.). NMR (C6D6): 1H NMR
(400 MHz): 2.56 ppm (s, br.). 11B NMR (64 MHz): δ = 25.3 ppm
(s, h1/2 = 190 Hz). 19F NMR (376 MHz): δ = 2.7 (m, 2 m-F), 7.7
1,3-Bis[bis(dimethylamino)boryl]-4,6-bis(dimethylamino)borazine
(10): To a stirred solution of (Cl2BN=BCl)3 (670 mg, 1.95 mmol)
in hexane (10 mL) was added Me2NSiMe3 (2.06 g, 2.81 mL,
17.5 mmol) with a syringe at –78°. The colourless precipitate that
formed subsequently dissolved on warming the mixture to room
temperature. After stirring for an additional 3 h the solvents and
Me3SiCl were removed in vacuo. The resultant yellow oil was dis-
solved in CH2Cl2 (2 mL) and the solution stored at –25 °C. Colour-
less prisms of 10 separated as single-crystals. Yield 670 mg
(1.66 mmol, 85%); m.p. 133 °C. NMR (C6D6): 1H NMR
(400 MHz): δ = 2.56 (s, 6 H, NMe), 2.60 (s, 12 H, NMe), 2.66 (s,
24 H, NMe), 3.41 ppm (s, 1 H, NH). 13C NMR (100 MHz): δ =
(m, 1 p-F), 16.9 ppm (m, 2 o-F). IR (Nujol, hostaflon): ν = 3687
˜
(w), 3663 (m w), 3547 (m), 3526 (st), 3469 (m), 3448 (st), 1651 (m),
1610 (vst), 1515 (vst), 1509 (vst), 1489 (st), 1425 (st), 1408 (vst),
1399 (vst), 1342 (st), 1308 (st), 1282 (w), 1249 (w), 1200 (w), 1153
(m), 1126 (w), 1062 (m), 1036 (st), 1026 (m), 1000 (vst), 992 (vst),
941 (w), 913 (w), 882 (w), 831 (w), 806 (m), 785 (m), 736 (st), 688
(m), 681 (m), 660 (m), 607 (w), 569 (m), 463 (m), 449 (m), 440 (m),
391 (st) cm–1. C18H6B3F15N6 (623.69): calcd. C 34.66, H 0.97, N
13.47; found C 34.29, H 1.02, N 12.90.
38.0 s, 39.3 s, 39.4 ppm, s. 11B NMR (64 MHz): δ = 26.1 (h1/2
=
400 Hz, B1, B1A, B2), 30.8 (h1/2 = 350 Hz, B3, B3A), ratio 3:2 –
MS [70 eV, m/z (rel. Intensity); 11B, 35Cl]: 362 (30), 317 (40), 307
(100), 263 (79), 252 (15), 168 (2), 124 (35), 99 (20), 44 (60).
C14H43B5N10 (405.62): calcd. C 41.46, H 10.68, N 34.53; found C
40.25, H 9.89, N 34.23.
2,4,6-Triphenylborazine (14): Prepared as described in ref.[27] In or-
der to remove soluble by-products, the raw material was washed 3
times with toluene (20 mL). Crystallisation from hot hexane
(500 mL for 12 g of triphenylborazine) gave a powder containing
thin needles, most of them not suitable for X-ray crystallography
although some had acceptable diameters (0.09 to 0.12 mm) which
were selected for the structure determination. NMR spectroscopic
data: 11B NMR: δ11B (C6D6) = 34.8 ppm.
2,4,6-Tribromo-1,3,5-trimethylborazine (11):[27] 1H NMR (DMSO):
δ = 6.95 (NH), 7.24, 8.17 ppm (Ph). 13C NMR (DMSO): δ = 127.5,
129.6, 133.2, 136.4 ppm.
2,4,6-Triamino-1,3,5-tris(pentafluorphenyl)borazine (13):
A two-
necked Schlenk bulb connected to a dry-ice reflux condenser was
charged at –78 °C with a solution of dry NH3 (7.7 g, 450 mmol)
and CH2Cl2 (25 mL). With stirring a solution of (F5C6N=BBr)3
Table 3. X-ray data of compounds 10–15.
Compound
Formula
10
11
12
13
14
15
C14H43B5N10
405.63
0.15ϫ0.2ϫ0.2
monoclinic
C2/c
12.43(1)
10.17(1)
19.61(1)
90
90.32(3)
90
2477.8(35)
4
1.087
0.067
888
C3H9B3Br3N3
359.29
C18B3Br3F15N3
815.37
0.20ϫ0.20ϫ0.30
monoclinic
C2/c
16.7592(3)
15.6197(4)
11.7494(3)
90.00
128.395(1)
90.00
2410.6(1)
4
2.247
5.154
1536
C18H6B3F15N6
623.72
0.12ϫ0.15ϫ0.18
monoclinic
I2/a
11.6547(3)
14.9327(3)
13.0087(1)
90.00
93.426(1)
90.00
2259.94(8)
4
1.833
0.198
1224
C18H18B3N3
308.78
0.2ϫ0.2ϫ0.4
hexagonal
P6cc
17.195(3)
17.195(3)
7.203(2)
90
90
120
1844.4(7)
4
1.112
C12H10B3N3
248.82
0.4ϫ0.3ϫ0.3
monoclinic
P21/n
15.386(8)
6.026(3)
19.14(1)
90
111.78(4)
90
1648(1)
4
1.003
0.057
552
Mr [gmolϪ1
Cryst. size [mm]
Cryst. system
Space group
a [Å]
]
0.2ϫ0.4ϫ0.4
triclinic
¯
P1
7.960(2)
9.392(2)
15.467(3)
90.36(3)
91.77(3)
114.48(3)
1051.6(4)
4
b [Å]
c [Å]
α [°]
ß [°]
γ [°]
V [Å3]
Z
ρ(calcd.) [MgmϪ3
µ [mm–1]
]
2.269
11.456
672
0.064
648
F(000)
Index range
–16 Յ h Յ 16,
12 Յ k Յ 12,
–21 Յ l Յ 26
57.94
–7 Յ h Յ 10,
12 Յ k Յ 12,
–19 Յ l Յ 19
58.64
–21 Յ h Յ 21,
18 Յ k Յ 19,
–14 Յ l Յ 14
57.86
–14 Յ h Յ 14,
17 Յ k Յ 18,
–16 Յ l Յ 16
57.04
–1 Յ h Յ 16,
18 Յ k Յ 1,
–8 Յ l Յ 7
45.38
0 Յ h Յ 17,
–6 Յ k Յ 0,
–21 Յ l Յ 19
47–08
2θ [°]
T [K]
203(3)
6792
2049
1065
0.0852
140
0.0264/6.9083
193
6175
3253
2184
0.0253
218
0.0720/0
193
6803
2417
2062
0.0293
194
0.0783/4.7293
183(2)
6393
2242
1781
0.0211
205
0.0483/2.1213
210
3266
841
654
0.0764
77
0.0654/0.0
293(2)
2547
2445
1594
0.0255
172
0.0393/0.8257
Refl. collected
Refl. unique
Refl. observed (4σ)
R (int.)
No. variables
Weighting scheme[a]
x/y
GOOF
1.242
0.972
1.083
1.040
1.134
1.067
Final R (4σ)
Final wR2
0.0822
0.1476
0.249
0.0445
0.1134
1.411
0.0473
0.1208
1.228
0.0380
0.0924
0.209
0.0477
0.1082
0.150
0.0521
0.1159
0.144
Larg. res. peak [eÅϪ3
]
[a] w–1 = σ2Fo + (xP)2 + yP; P = (Fo + Fc )/3.
2
2
2
2
Eur. J. Inorg. Chem. 2008, 3186–3199
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3197

[a] B. Anand, H. Nöth, H. Schwenk-Kircher, A. Troll
FULL PAPER
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1972, 91–95.
1,3,5-Triisopropyl-2,4,6-trimethylborazine (15): N-Isopropyl-hexa-
methyldisilazane (15.5 mL, 62.5 mmol) was placed in a three-
necked flask (200 mL) equipped with a dry-ice cooled reflux con-
denser and a dropping funnel. The system was flushed with nitro-
gen gas and a slow stream of nitrogen was maintained. The flask
was then cooled with liquid nitrogen. MeBBr2 (5.75 mL,
62.2 mmol) was slowly added (over 10 min). On warming, the reac-
tion started at about –80 °C and the mixture quickly reached 60 °C.
After the main reaction had subsided, the solution was kept at
reflux at 95 °C for 24 h. Me3SiBr was then removed by distillation.
Isolation of the product was by distillation; b.p. 120 °C/0.05 Torr.
It crystallised as needles. Yield 2.52 g (49%); m.p. 58 °C. NMR: 1H
NMR (CDCl3): δ = 0.62 (s, 9 H, BMe), 1.32 (d, CH3, 18 H),
3.83 ppm (sept, 3 H). 13C NMR (C6D6): δ = 23.6 (C-CH3),
50.5 ppm (CH), BCH3 not observed. 11B NMR (CDCl3): δ =
36.2 ppm. C12H30B3N3 (248.82): calcd. C 57.93, H 12.15, N 16.89;
found C 56.93, H 11.72, N 16.45.
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22, 321–349; 1980, vol. 2, 1st supplement, 110–123; 1991, vol.
3a, 4th supplement, 182–192; 1991, vol. 4, 4th supplement, 66–
68.
2,4,6-Triisopropyl-1,3,5-trimethylborazine (16): Prepared in analogy
to 15 from iPrBBr2 (15.4 g,72 mmol) and MeN(SiMe3)2 (15.9 mL,
[11] K. J. L. Paciorek, W. L. Krone-Schmid, D. Harris, R. H.
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1
72 mmol). Yield 5.9 g, m.p. 40–45 °C. NMR (CDCl3): H NMR: δ
= 0.67 (s, 9 H, Me), 1.35 (d, 18 H, CMe3), 4.01 ppm (sept, 3 H,
CH). 13C-NMR: δ = 3.8 (BC), 24.1 (C-CH3), 46.9 ppm (NC). 11B-
NMR: δ = 36.6 ppm. C12H30B3N3 (248.82): calcd. C 57.93, H
12.15, N 16.89; found C 57.07, H 11.74, N 16.96.
Structure Determinations: Crystals were put in perfluoroether oil
(if necessary cooled to –30 °C) and a suitable specimen was se-
lected, fixed on top of a glass fibre and then on the goniometer
head which was flushed with cold nitrogen gas (usually –80 °C).
The unit cell was determined from reflections on five sets of 15
frames using the program SMART. Data were collected in the
hemisphere mode on 1200 frames and two different sets of settings.
All non-hydrogen atoms were refined anisotropically. CH hydrogen
atoms were placed in calculated positions and refined isotropically,
riding on the respective C atom. NH hydrogen positions were taken
from the difference Fourier map and refined isotropically. The data
of the Br-containing borazines as well as some of the B-chlorobora-
zines were corrected for absorption. Tables 2 and 3 show relevant
crystallographic data and data related to data collection and refine-
ment.
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CCDC-678182 (for 1), -678183 (for 2), -678184 (for 3), -678185 (for
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 3186–3199

Structural Chemistry of Borazines
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Received: February 14, 2008
Published Online: June 3, 2008
Eur. J. Inorg. Chem. 2008, 3186–3199
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3199