Structural chemistry of borazines

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)₃has 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)₃with Me₃SiNMe₂allows 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 F₅C₆rings stands

almost perpendicular to the borazine plane while the other

two are twisted by 70°. Aminolysis of (F₅C₆N=BBr)₃yields

(F₅C₆N=BNH₂)₃with NH₂groups that are coplanar with the

borazine ring. The phenyl groups of (HN=BPh)₃are 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]₃are obtained. The latter two react

with CH₂Cl₂by 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)₃and (MeN=BiPr)₃are 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.

tion with borazine planes 4.2 Å apart from each other. Char- Germany, 2008)

four-membered B₂N₂rings 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 B₃N₃rings 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 D_6hsymmetry, borazine exhibits D_3hsymmetry in the

gas phase^[3]but C₂symmetry 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)₃depends on the halogen atom X

and increases in the order of X = I Ͻ Br Ͻ Cl Ͻ F. The

compound (HN=BI)₃is unstable at ambient tempera-

ture,^[27](HN=BBr)₃decomposes slowly at ambient tem-

perature but rapidly at 90 °C^[28]whereas (HN=BCl)₃can 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)₃can 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

3186

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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)₃determine its structure we repeated its preparation from an

with SbF₃by ¹¹B NMR spectroscopy and observed a quan- excess of BCl₃and 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)₃with Me₃SiNMe₂were

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 BCl₃with 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 BCl₃in 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)₂with BCl₃in

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)₃occurs 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 BCl₂group 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 MeBCl₂as shown by the base peak at m/e = 145 groups in CH₂Cl₂solution 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 BCl₂substituents leads to better shi-

trichloroborazine, (Cl₂BN=BCl)₃5.^[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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[a] B. Anand, H. Nöth, H. Schwenk-Kircher, A. Troll

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)₂and BNMe₂groups 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)₃and (HN=Bmes)₃by the silazane cleavage

route.^[44]

Synthesis of Bromoborazines (RN=BBr)₃

In contrast to the B-trihaloborazines (HN=BX)₃no

The aminosilane iPrN(SiMe₃)₂had to be prepared in a

three step reaction from iPrNH₂and Me₃SiCl to give

iPrNHSiMe₃followed by metallation with LiBu to produce

iPrN(Li)SiMe₃which on reaction with Me₃SiCl gave

iPrN(SiMe₃)₂. The compound tBuN(SiMe₃)₂was prepared

analogously.^[44]However its reaction with MeBBr₂in 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)₃has so far been reported although these types

of borazines are well documented.^[10]We prepared

(MeN=BBr)₃, 11, by treating BBr₃with (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 BBr₃and

F₅C₆NH₂in the presence of NMe₃as 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)SiMe₃because 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 NH₂groups 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^–1due to B₃N₃ring vibrations. This band

is 35 cm^–1higher than that found in 11 (1364 cm^–1). The

band at 1342 cm^–1can be assigned to the exocyclic B–N

vibrations in 13.^[42,43]BN deformation modes can be found

at 734 cm^–1for 12 and at 736 cm^–1for 13. Two strong bands

at 3526 and 3448 cm^–1can 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)₃crystallises in the trigonal space

¯

group R3 and shows D_3hpoint symmetry. The C₃axis

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)₃seem 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)₃and (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)₃in 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 CF₃CH₂group 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

B₃N₃ring 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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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 BCl₂plane 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)₃crystallises 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 BCl₂groups 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 BCl₂groups 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 BCl₂groups 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)₃implies that the boron longer uniform. The B1–N2 and B1A–N2 bonds have more

atoms of the BCl₂groups 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 BCl₂groups. This has been experi- B–N bond lengths and bond angles become different and

mentally verified by the reaction of (Cl₂BN=BCl)₃with 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)₂groups 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)₂groups 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 C₂axis

Me₂N groups form with the BN₃plane of B[(NMe₂)₂]₃are which gives the molecule C₂point 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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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 F₅C₆group bonded to N2 and 82.2(1)° for those

bonded to N1 and N1A.

A similar distortion results for the F₅C₆ring. 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 R₁value 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 F₅C₆groups 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

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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 B₃N₃ring 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)₃was 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)₃and [(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, ¹J_H,C= 18 Hz; ²J_H,Si= 39 Hz). ¹³C NMR (100 MHz):

uent effects on the structure of borazines will almost cer-

tainly provide interesting results.

Experimental Section

1

δ = 37.6, 3.83 ppm (s, J_C,H= 18 Hz, J_C,Si= 29 Hz). ¹¹B NMR

(64 MHz): δ = 37.6 ppm (s, h_1/2= 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 N₂using special storage flasks. If not stated otherwise,

all reagents were of commercial grade. The compounds 2,4,6-tri-

1

(d, J_Si,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)₃was 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, ¹J_H,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-K_α-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, J_H,C= 17 Hz).

¹¹B NMR (64 MHz): δ = 35.5 ppm (s, h_1/2= 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)₃at 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, J_H,Si= 5 Hz, 18 H),

Eur. J. Inorg. Chem. 2008, 3186–3199

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 (h_1/2= 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]₃are 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

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

M_r[gmol^Ϫ1

Cryst. size [mm]

Cryst. system

Space group

a [Å]

]

0.23ϫ0.3ϫ0.5

tetragonal

P4(1)2(1)2

9.8256(1)

16.4708(2)

90

¯

R3c

P1

11.388(3)

6.835(2)

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

γ [°]

V [Å³]

1334.5(8)

4

1.205

1590.13(3)

4

1.779

2328.3(2)

4

1.133

Z

ρ(calcd.) [Mgm^Ϫ3

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 [eÅ^Ϫ3

]

[a] w^–1= σ²F_o+ (xP)²+ yP; P = (F_o+ F_c)/3.

2

3196

www.eurjic.org

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, h_1/2= 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 (h_1/2

=

400 Hz, B1, B1A, B2), 30.8 (h_1/2= 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] 1}H 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)

7.203(2)

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

M_r[gmol^Ϫ1

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.) [Mgm^Ϫ3

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 [eÅ^Ϫ3

]

[a] w^–1= σ²F_o+ (xP)²+ yP; P = (F_o+ F_c)/3.

2

Eur. J. Inorg. Chem. 2008, 3186–3199

www.eurjic.org

3197

[a] B. Anand, H. Nöth, H. Schwenk-Kircher, A. Troll

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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), BCH₃not 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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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,

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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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Structural Chemistry of Borazines

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Received: February 14, 2008

Published Online: June 3, 2008

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3199

Article Doi

Structural chemistry of borazines

DOI: 10.1002/ejic.200800169

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Article abstract of DOI:10.1002/ejic.200800169

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