Oligomeric pentafluorophenylboron azides

Article abstract of DOI:10.1039/b007626k

The dichloroborane C6F5BC12 reacted with two equivalents trimethylsilyl azide to form C6F5B(N3)2 which trimerizes in the solid state to give [CtFjBj) 1, the first example of an azido substituted N,N,-trisCdiazoJlriazatriboratacyclohexane. Its monomer was

Full text of DOI:10.1039/b007626k

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Oligomeric pentafluorophenylboron azides
Wolfgang Fraenk, Thomas M. Klapötke,* Burkhard Krumm, Heinrich Nöth, Max Suter and
Marcus Warchhold
Department of Chemistry, Ludwig-Maximilians University of Munich, Butenandtstr. 5-13 (D),
D-81377 Munich, Germany. Fax: (ϩ49) 89-2180-7492; E-mail: tmk@cup.uni-muenchen.de
Received 20th September 2000, Accepted 19th October 2000
First published as an Advance Article on the web 23rd November 2000
The dichloroborane C6F5BCl2 reacted with two equivalents trimethylsilyl azide to form C6F5B(N3)2 which trimerizes
in the solid state to give [C6F5B(N3)2]3 1, the first example of an azido substituted N,NЈ,NЉ-tris(diazo)triazatriborata-
cyclohexane. Its monomer was trapped by pyridine yielding C6F5B(N3)2ؒpy 2. This is in contrast to (BF2N3)3 3 which
is trimeric in solution and in the solid state, as shown by 11B NMR spectroscopy.
Introduction
The chemistry of boron azides commenced in 1954 with the
synthesis of boron triazide and lithium tetraazidoborate.1 Over
the next decades mainly Paetzold and co-workers extensively
studied their chemistry.2a–d,3 The preparation and thermally
promoted decomposition of boron azides leads, depending
on the nature of the substituents, to iminoboranes, diazadi-
boretidines or borazines.2c,3 Cryoscopic mass determination
as well as X-ray diffraction studies on boron azides showed
that these species are monomeric except for the trimeric boron
dihalide azides (BX2N3)3 (X = F, Cl or Br).2c,4a–d
Recently we reported on the irreversible dimerization of
bis(pentafluorophenyl)boron azide, which was first indicated in
1983, being the first example of a N,NЈ-diazodiazadiborata-
cyclobutane.5,6 In this report we present results of further
exploration of this chemistry, the reversible trimerization of
the new pentafluorophenylboron diazide [C6F5B(N3)2]3 1, the
first example of an azido N,NЈ,NЉ-tris(diazo)triazatriborata-
cyclohexane, and trapping of its monomer by pyridine yielding
the adduct C6F5B(N3)2ؒpy 2. Here, the unique case of structural
characterization of both monomeric and trimeric C6F5B(N3)2
is accomplished. Furthermore, not reported NMR data of
the known (BF2N3)3 3 are given. The influence of electron-
withdrawing substituents on the electron deficient boron atom
and the difference between halogeno and multiply substituted
pentafluorophenyl boron azides is discussed.
Results and discussion
Fig. 1 Top: Molecular structure of 1 with thermal ellipsoids drawn at
the 25% probability level. Bottom: View of the B(N3)2 units, showing
the B3N3 heterocycle of 1 with thermal ellipsoids drawn at the 25%
probability level. Selected bond lengths (Å) and angles (Њ): B(1)–N(1)
1.501(3), N(1)–N(2) 1.229(3), N(2)–N(3) 1.129(2), B(1)–N(4) 1.602(3),
B(1)–N(7) 1.602(3), N(4)–N(5) 1.271(2), N(5)–N(6) 1.108(2), C(1)–B(1)
1.619(3), N(1)–N(2)–N(3) 174.6(2), N(4)–N(5)–N(6) 178.8(2), B(1)–
N(1)–N(2) 121.4(2), B(1)–N(4)–N(5) 115.8(2), N(7)–B(1)–N(4) 98.5(2),
B(1)–N(4)–B(2) 125.8(2).
Pentafluorophenylboron dichloride7 was treated with two
equivalents trimethylsilyl azide in a dichloromethane solution
(eqn. 1). The diazide 1 was obtained as a colorless, highly
NMR spectrum of 1 three resonances for the two azide groups
(see Table 1) and in the 19F NMR spectrum three resonances for
the fluorine atoms of the pentafluorophenyl groups are found.
These findings suggest the formation of monomeric C6F5B(N3)2
but differ from the solid state of 1, shown by IR, Raman and
X-ray diffraction, proving solid 1 as a trimeric boron diazide
(Fig. 1). (Note: a low-temperature NMR spectrum is not
possible due to the low solubility of trimeric 1.)
moisture sensitive, explosive solid with a melting point of
36–39 ЊC. It is soluble in benzene and dichloromethane but
insoluble in hexane and decomposes in chloroform. The 11B
NMR spectrum shows a resonance at δ 34.6, which is in the
region for three-coordinated boron8 and is in contrast to the
resonance of δ 0.5 observed for trimeric (BF2N3)3 3. In the 14
N
DOI: 10.1039/b007626k
J. Chem. Soc., Dalton Trans., 2000, 4635–4638
This journal is © The Royal Society of Chemistry 2000
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Table 1 Selected spectroscopic data of oligomeric boron azides a
Compound
δ
11B
δ
14N (Nβ,Nγ,Nα)
νasym(N3)/cmϪ1 IR/Raman
mp/ЊC
Ref.
(BF2N3)3
(BCl2N3)3
(BBr2N3)3
0.5
n.r.
n.r.
62/4.9
43.9
34.6
Ϫ154, Ϫ175, Ϫ300
n. r.
n. r.
n. r.
Ϫ144, Ϫ160, Ϫ322
Ϫ149, Ϫ168, Ϫ277
2208, 2154/2236
50
67
9, This work
10
10
2(b), 2(c)
6
2219, 2210/2219, 2210
2215, 2200/2215, 2200
2128
2202/2209
2200, 2180, 2142/2206, 2169, 2135
94.5
54c
b
Me2BN3
d
[(C6F5)2BN3]2 d
76–80 (decomp)
36–39
[C6F5B(N3)2]3
This work
a An oligomeric structure is also predicted for solid B(N3)3.1,2b b Shows a temperature dependent oligomerization.2c c 720 Torr (bp). d In solution
monomer.
Table 2 Selected bond lengths [Å] and angles [Њ] of oligomeric boron azides (average values)
Compound
B–N
Nα–Nβ
Nβ–Nγ
N–N–N
B–N–N
Ref.
(BCl2N3)3
[(C6F5)2BN3]2
[C6F5B(N3)2]3
1.58
1.60
1.50
1.60
1.25
1.24
1.23
1.27
1.09
1.11
1.13
1.11
178
178
175
179
116
130
121
116
10
6
a
This work
a Top column: terminal azide groups. Bottom column: bridging azide groups.
The triazatriborata heterocycle shows a slightly twisted
boat conformation (Fig. 1) comparable with that found for
(BCl2N3)3.4d The X-ray study nicely displays the difference
between bridging and terminal azide groups. The B–Nring dis-
tances of 1.60 Å (Table 2) are longer than the B–N distances
of 1.50 Å found for the covalently bound azide groups, which
correspond to typical B–N single bonds. A difference is
also observed for the N–N–N angles of the azide groups and
the Nβ–Nγ distances. The bridging azide groups are close to
linearity (N–N–N 178–179Њ), the other azide groups are slightly
bent with an N–N–N angle of 175Њ which is in accord with the
structures of other boron azides previously determined.4
᎐
The N –N distance of 1.11 Å (N᎐N 1.098 Å) found for the
᎐
β
γ
bridging azide groups is shorter than for the corresponding
distance found for the terminal azide groups Nβ–Nγ 1.13 Å.
The vibrational spectra also reveal the presence of two dif-
ferent azide species. The Raman spectrum of solid 1 shows
three absorptions in the region of the antisymmetric stretching
vibration of the azide group (νasymN3) at 2206, 2169 and 2135
cmϪ1. The absorption shifted to a higher wavenumber at
2206 cmϪ1 corresponds to the bridging azide groups (νasymN3:
(BF2N3)3 2236,9 [(C6F5)2BN3]2 2209 cmϪ1 6), the other to
terminal azide groups. In the IR spectrum νasymN3 vibrations
were found at 2200, 2180 and 2142 cmϪ1. The low melting
point of 1, which suggests the dissociation of the trimer,
encouraged us to observe 1 in the liquid phase at 40 ЊC by
Raman spectroscopy (Fig. 2). The characteristic absorption
for νasymN3 at 2206 cmϪ1 for the bridging azide groups dis-
appears and the νasymN3 bands were now found at 2170 and
2156 cmϪ1 which confirms the dissociation of 1 into monomeric
C6F5B(N3)2. Bis(pentafluorophenyl)boron azide decomposes at
its melting point and therefore monomeric (C6F5)2BN3 can only
be detected in solution. Characteristic data of oligomeric boron
azides, that are known to our knowledge, are listed in Tables 1
and 2.
The reaction of C6F5BCl2 with trimethylsilyl azide in the
presence of pyridine yielded a stable adduct C6F5B(N3)2ؒpy 2, a
non-explosive solid being soluble in toluene, dichloromethane
and chloroform but insoluble in hexane. The crystal structure
of 2 (Fig. 3) revealed the monomeric nature of pentafluoro-
phenylboron diazide–pyridine. The B–Nα distance of 1.54 Å
is comparable with the corresponding distance found for 1,
the B–Npy distance of 1.61 Å is similar to the B–Nring distance in
1. The Nα–Nβ (1.21 Å) and the Nβ–Nγ (1.14 Å) bond distances
are between a N–N double (1.24 Å) and a N–N triple bond
(1.098 Å). The Raman and the IR spectrum show both the
Fig. 2 Raman spectra of solid 1 (bottom), liquid 1 (top).
Fig. 3 Molecular structure of 2 with thermal ellipsoids drawn at the
25% probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (Њ): B(1)–N(2) 1.537(3), N(2)–N(3)
1.211(3), N(3)–N(4) 1.130(3), B(1)–N(1) 1.612(3), B(1)–C(1) 1.633(3),
N(2)–N(3)–N(4) 174.3(2), B(1)–N(2)–N(3) 121.0(2), N(1)–B(1)–N(2)
101.6(2), N(2)–B(1)–N(5) 109.5(2).
characteristic νasymN3 vibrations at 2146 and 2130 cmϪ1 (IR
νasymN3: C6H5B(N3)2ؒpy 2120/2090 cmϪ1 2d). The 11B NMR
resonance is found at δ 1.4, a region typical for four-
coordinated boron; the 14N NMR spectrum shows four
resonances for the azide and the pyridine nitrogen atoms.
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J. Chem. Soc., Dalton Trans., 2000, 4635–4638

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Table 3 Crystal data and structural refinements for compounds 1 and 2
1
2
Empirical formula
Formula weight
C18B3F15N18
785.79
C11H5BF5N7
341.03
T/K
Crystal system
Space group
193
Monoclinic
C2/c
193
Orthorhombic
P212121
a/Å
b/Å
c/Å
14.918(2)
13.042(1)
28.840(2)
97.463(1)
555.2(8)
7.5637(6)
13.040(1)
13.4308(9)
β/Њ
V/Å3
1324.7(2)
Z
8
4
µ/mmϪ1
0.195
0.160
Reflections collected
Independent reflections
Observed reflections
R1, wR2
(all data)
Flack parameter
15689
5471 (Rint = 0.026)
4072
0.0395, 0.0957 [I > 4σ(I)]
0.0596, 0.1051
7697
2687 (Rint = 0.020)
2131
0.0380, 0.0885 [I > 4σ(I)]
0.0546, 0.0945
0.1(7)
solution was monitored by 19F NMR spectroscopy showing
complete conversion into monomeric compound 1. The
solution was concentrated and cooled to Ϫ25 ЊC. Colorless
crystals formed, isolated and dried in vacuo. Yield 0.13 g
(50%), mp 36–39 ЊC. IR (Nujol): 3405vw, 2200/2180/2142 (vs,
νasymN3), 1650vs, 1523vs, 1487vs,1477vs, 1381s, 1299s, 1260w,
1197s, 1162s, 1146s, 1105s, 1081s, 980 (sh), 821s, 765w, 738m,
724m, 674m, 644s, 579w, 543w, 488vw and 447vw cmϪ1. Raman
(50 mW): 2206/2169/2135 (1–5, νasymN3) 1650 (3), 1391 (2), 1331
(3), 1301 (1), 1240 (2), 1143 (1), 1114 (1), 962 (1), 826 (1), 726
(1), 582 (5), 488 (8), 467 (4), 449 (5), 426 (4), 392 (4), 371 (3), 339
(3), 315 (3), 272 (3), 246 (4), 226 (5), 217 (6), 186 (5), 118 (10)
Conclusion
The behaviour of pentafluorophenylboron dichloride towards
azide was investigated and found to be contrary to that of
boron trihalides. The boron halides BX3 (X = F, Cl or Br) react
with only one equivalent of Me3SiN3 to give BX2N3 which
immediately stabilize by irreversible formation of trimers.
However, C6F5BCl2 reacts with two equivalents of Me3SiN3 to
form the diazide 1. The electron-withdrawing effect of the
pentafluorophenyl group is, compared to the halogen atoms,
obviously weaker and therefore interactions between the
C6F5B(N3)2 molecules are only observed in the solid state. The
non-fluorinated analogue, C6H5B(N3)2, has been reported and
characterized by NMR data and mass spectrum,2a but no
information regarding its structure was given. The relatively low
melting point of 1 indicates a low dissociation energy of this
trimeric boron azide. As an adduct with pyridine, monomeric
C6F5B(N3)2 can be stabilized.
and 86 (6) cmϪ1 13C-{19F} NMR (100.6 MHz, C6D6): δ 146.6
.
(o-C), 142.2 (p-C), 136.4 (m-C) and 103 (br, CB). 11B NMR
(128.3 MHz, C6D6): δ 34.6s. 14N NMR [28.9 MHz, C6D6,
∆ν1/2/Hz]: δ Ϫ149 (60, Nβ), Ϫ168 (150, Nγ) and Ϫ277 (900, Nα).
19F NMR (376.1 MHz, C6D6): δ Ϫ131.7 (m, o-F, 2F), Ϫ147.7
(m, p-F, 1F) and Ϫ160.6 (m, m-F, 2F). Calc. for C6BF5N6:
C, 27.5; N, 32.1. Found: C, 28.3; N, 30.9%.
Experimental
C6F5B(N3)2ؒC5H5N 2. 2 mmol trimethylsilyl azide (0.230 g)
was added to a solution of 1 mmol (0.248 g) C6F5BCl2 and
1 mmol (0.080 g) pyridine in 10 mL CH2Cl2 at Ϫ78 ЊC. After
stirring for 12 h at ambient temperature, the solvent was
removed in vacuo and the remaining oil crystallized from
CH2Cl2 giving compound 2 as colorless crystals. Yield 0.27 g
(80%), mp 88–90 ЊC. IR (Nujol): 3110w, 3105w, 2146/2130
(vs, νasymN3) 1648s, 1628m, 1523s, 1458vs, 1383vw, 1350m,
1332s, 1320m, 1291m, 1214m, 1107vs, 1027vw, 972s, 963s, 950s,
924vs, 858w, 837s, 765vs, 727vw, 690vs, 660vw, 630vw, 610vw,
580vw and 491vw cmϪ1. Raman (150 mW): 3159 (1), 3104 (5),
3093 (4), 2146/2130 (1, νasymN3) 1648 (1), 1629 (1), 1579 (2),
1379 (1), 1351 (2), 1332 (2), 1216 (2), 1164 (1), 1098 (1), 1028
(10), 950 (1), 651 (2), 581 (2), 492 (4), 479 (2), 447 (3), 429 (3),
General
All manipulations of air and moisture sensitive materials were
performed under an inert atmosphere of dry nitrogen using
standard Schlenk techniques. Solvents were dried and degassed
by standard methods. Raman spectra were recorded on a Perkin-
Elmer 2000 NIR FT-Raman spectrometer, infrared spectra
on a Nicolet 520 FT-IR spectrometer as neat solids between
KBr plates. The elemental analyses were performed with a
C, H, N-Analysator Elementar Vario EL instrument. NMR
spectra were recorded on a JEOL EX400 instrument. Chemical
shifts are recorded with respect to (CH3)4Si (1H, 13C), BF3ؒOEt2
(11B), CH3NO2 (14N) and CFCl3 (19F). For the determination
of the melting points, samples were heated in capillaries in
a Büchi B540 instrument. C6F5H, Me2SnCl2, BF3, BCl3 (1.0 M
in hexane), Me3SiN3, n-BuLi (2.5 M in hexane) and pyridine
were used as received (Aldrich, Fluka, FluoroChem). (C6F5)2-
SnMe2 and C6F5BCl2 were prepared as described.7,11 Elemental
analyses were performed for all compounds, but only useful
results are reported. Mass spectra were recorded, but were
not expressive regarding the structures. CAUTION: compounds
1 and 3 are explosive; appropriate safety precautions must
be taken.
395 (3), 281 (1), 260 (1), 215 (2), 178 (2) and 120 (7) cmϪ1
.
1H NMR (400 MHz, CDCl3): δ 8.78 (m, 2H), 8.26 (m, 1H)
and 7.79 (m, 2H). 13C-{1H} NMR (100.6 MHz, CDCl3): δ 147.9
(o-C, dm, 1JCF 247.9), 144.3 (o-C, s), 143.4 (p-C, s), 140.9 (p-C,
1
1
dm, JCF 253.3), 137.3 (m-C, dm, JCF 249.8 Hz), 126.3 (m-C,
s) and 112 (br, CB). 11B NMR (128.3 MHz, CDCl3): δ 1.4s.
14N NMR [28.9 MHz, CDCl3, ∆ν1/2/Hz]: δ Ϫ143 (80, Nβ), Ϫ145
(350, N-py), Ϫ201 (250, Nγ) and Ϫ316 (650, Nα). 19F NMR
(376.1 MHz, CDCl3): δ Ϫ135.4 (m, o-F, 2F), Ϫ154.3 (m,
p-F, 1F) and Ϫ162.3 (m, m-F, 2F). Calc. for C11H5BF5N7:
C, 38.7; H, 1.5; N, 28.8. Found: C, 38.7; H, 1.4; N, 28.4%.
Preparations
[C6F5B(N3)2]3 1. A solution of 1 mmol (0.248 g) C6F5BCl2 in
CH2Cl2 (10 mL) was treated with 2 mmol trimethylsilyl azide
at Ϫ78 ЊC. After stirring for 12 h at ambient temperature the
(BF2N3)3 3. (see also Ref. 10). BF3 (0.068 g, 1 mmol) was con-
densed onto a frozen solution of trimethylsilyl azide (0.12 g,
J. Chem. Soc., Dalton Trans., 2000, 4635–4638
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1 mmol) in CH2Cl2 (10 mL) at Ϫ196 ЊC. The solution was
allowed to warm up slowly and stirred for 6 h at room tem-
perature. The solvent and all volatile products were removed
by vacuum evaporation leaving a colorless, explosive, highly
moisture sensitive solid. The solid was purified by sublimation
(1013 mbar/ 45 ЊC). Yield 0.07 g (75%), mp 50 ЊC. IR (Nujol):
3410m, 3300s 2208/2154 (vs/s, νasymN3), 1340w, 1297vs, 1257s,
1149s, 1053m, 933m, 910m, 845m, 664w, 631vs and 509vw
cmϪ1. Raman (100 mW): 2236 (7, νasymN3), 1255 (2, νs N3),
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4638
J. Chem. Soc., Dalton Trans., 2000, 4635–4638