A convenient route to carbon-substituted derivatives of nido-5,6-C2B8H12

Article abstract of DOI:10.1016/S0022-328X(97)00535-4

An alternative route to the parent nido-5,6-C₂B₈H₁₂ dicarbaborane is reported together with a convenient synthesis of its carbon-substituted derivatives. The method is based on reactions between 4-(Me₂S)-arachno-B₉H₁₃ and alkynes R¹R²C₂ (where R¹R² = H,H; Me, Me; Ph,H, and Ph,Ph) in toluene at reflux. The characteristic reaction mode is a dicarbon insertion into the 9-vertex arachno cluster to produce a series of the 5,6-R¹R²-mdo-5,6-C₂B₈H₁₀ species combined with concomitant elimination of one {BH} vertex. The products were characterised by high-field ¹H and ¹¹B NMR spectroscopy and mass spectrometry associated with [¹¹B-¹¹B]-COSY and ¹H{¹¹B(selective)} measurements that permitted complete assignments of all resonances to individual cluster {BH} units.

Full text of DOI:10.1016/S0022-328X(97)00535-4

Ž
.
Journal of Organometallic Chemistry 550 1998 125–130
A convenient route to carbon-substituted derivatives of
1
nido-5,6-C2 B8H12
a
b
Bohumil Stıbr a,b,), Francesc Teixidor a,2, Clara Vinas , Jirı Fusek
ˇ
´
˜
ˇ´
a
Institute of Material Sciences, Campus de la UAB, Bellaterra 08913, Spain
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 250 68 Rez, Czech Republic
b
ˇ
ˇ
Received 27 January 1997
Abstract
An alternative route to the parent nido-5,6-C2B8H12 dicarbaborane is reported together with a convenient synthesis of its
carbon-substituted derivatives. The method is based on reactions between 4- Me2S -arachno-B9H13 and alkynes R1R2C2 where
Ž
.
Ž
1
2
.
R R sH,H; Me, Me; Ph,H, and Ph,Ph in toluene at reflux. The characteristic reaction mode is a dicarbon insertion into the 9-vertex
arachno cluster to produce a series of the 5,6-R1R2-nido-5,6-C2B8H10 species combined with concomitant elimination of one BH
Ä
4
vertex. The products were characterised by high-field 1H and 11B NMR spectroscopy and mass spectrometry associated with
1
11B–11B -COSY and H B selective measurements that permitted complete assignments of all resonances to individual cluster BH
w
x
Ä11
Ž
.4
Ä
4
units. q 1998 Elsevier Science S.A.
Keywords: Carbon-substituted derivatives; Dicarbaborane; Aqueous solution; Synthesis
1. Introduction
The first report on the synthesis of nido-5,6-C2 B8 H12
and its 5,6-dimethyl derivative is that by Schaeffer’s
3
w
x
w x
The neutral, 10-vertex nido
2–4 dicarbaborane
group 18 . These compounds were isolated from reac-
5,6-C2 B8 H12 has become one of the most essential
synthons of dicarbaborane chemistry
stance, it has been employed as a starting material for
the preparation of other basic dicarbaboranes, such as
tions between the octaborane nido-B8 H12 and alkynes
4
w
x
Ž .
5,6 . For in-
as in Eq. 1 :
nidoyB8 H12 qR2C2
closo-1,2-C2 B H10, closo-1,6-C2 B8 H10, closo-1,10-
™5,6yR2 ynidoy5,6yC2 B8 H10 qH2
where RsH and Me.
1
Ž .
w8
x
2y
C 2 B8 H
,
nido-6,9-C 2 B8 H10
,
w
arachno-4,6-
x w
10w x
C2 B7 H13 7 , arachno-6,9-C2 B8 H14 8–10 , arachno-
This method is rather inconvenient because of gen-
eral inaccessibility and instability of the starting B8 H12
xy
w
x
4,5-C2 B6 H
C4 B H
8 w 14x
11 , the derivatives of arachno-
12,13 , and 14-vertex methyltricarbaazabo-
ranes 14 . As we have recently reported, 5,6-C2 B8 H12
has become a source of tricarbollides, the first represen-
w11
x
w
x
borane 19 . Plesek and Hermanek have developed a
ˇ
ˇ ´
w
x
more convenient method 20,21 starting from the read-
w
x
w
x
ily available nido-7,8-C2 B9 H12 anion 22,23 . This
anion can be oxidised by FeCl3 P6 H2O in an acidified
aqueous solution to give a 45% yield of nido-5,6-
w
x
tatives of the 11-vertex series of tricarbaboranes 15–17 .
Ž .
C2 B8 H12 , as in stoichiometric Eq. 2 :
)
y
Corresponding author.
C2 B9 H12 qFeq3 qH3Oqq2 H2O
1 Dedicated to Professor K. Wade on the occasion of his 65th
birthday for his essential contributions to the area of cluster boron
chemistry. All cluster chemists know Wade’s electron-counting rules
and use them for the benefit of chemistry.
™C B H qB OH 3 qFeq2 q2 H2
2
Ž .
Ž
.
2
8
12
The disadvantage of this method is that the yields of
this reaction decrease significantly when substituted
2 Also corresponding author.
3 for classification of cluster compounds according to the number
w
xy
derivatives of the nido-7,8-C2 B9 H12 anion are em-
ployed, which complicates a reasonable synthesis of the
w
x
of skeletal electrons see, for example, Refs. 1–4 .
4
w
x
See, for example, Refs. 5,6 .
0022-328Xr98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.

ˇ
(
)
B. Stıbr et al.rJournal of Organometallic Chemistry 550 1998 125–130
126
´
literature 31 . Residual solvent 11 H resonances were
used as internal secondary standards. Coupling con-
w
x
substituted derivatives of the nido-5,6-C2 B8 H12 series
ˇ
Ž
B. Stıbr et al., Collect. Czech. Chem. Commun.,
´
1
Ž11
.
1
.
manuscript in preparation . Preliminary results from our
stants J B– H are taken from resolution-enhanced
11 B spectra with digital resolution 8 Hz and are given in
Hz.
and Shore’s groups indicated that also the 9-vertex
w
x
w x
arachno clusters, 4-CB8 H14 24 and B9 H13 POEt2 25 ,
can be employed as a convenient source for the prepara-
tion of the C-substituted derivatives of nido-5,6-
C2 B8 H12. Here we report a convenient synthesis of
these derivatives from the stable and readily available
(
)
( )
1
2.3. Preparation of 4- Me2 S -arachno-B9 H13
A modification of the method reported for the Et2 S
Ž
.
w
x
ligand-derivative 4- Me2 S -arachno-B9 H13. Simplified
structures and numbering of compounds discussed in
this work are in general structures 1–3 below, with the
vertices of individual polyhedra denoting cluster BH
units.
analogue 32 was employed. A solution of B10 H14
Ž
.
1.22 g; 10 mmol was dissolved in 10 ml of dimethyl
sulphide and the solution was left to stand at ambient
Ž
.
temperature for 48 h hydrogen evolution . The excess
dimethyl sulphide was then distilled off under vacuum
Ž
.
into a y788C trap ca. 8 ml recovered . The residual
solid was heated with 15 ml of methanol at reflux for 2
Ž
.
2. Experimental
h hydrogen evolution , and the solution obtained was
left to crystallise for 12 h. The white crystalline material
was isolated by filtration, washed with 20 ml of hexane,
and vacuum-dried to give the first crop of 1 946 mg .
To remove possible contaminants, the hexane washings
rotaevaporated and subjected to preparative TLC in
2.1. General
Ž
.
The Aldrich toluene was dried over Narbenzo-
phenone, hexane and CH2Cl2 were dried over CaH2 ,
and all were freshly distilled before use. Technical
acetylene was passed through a y788C trap and the
Ž
. Ž
50% CH2Cl2rhexane vrv column chromatography
.
may be used as well to collect the main band of RF
Ž
.
substituted alkynes Aldrich were of reagent grade and
used as purchased. All reactions were carried out under
0.5, which was extracted by CH2Cl2. Evaporation of
the extract gave the second crop 363 mg; total yield
1309 mg, 76% of 1, which was identified by NMR
spectroscopy.
Ž
w
x
.
dry dinitrogen 26 , although some manipulations, such
as preparative TLC and column chromatography, were
carried out in air. Preparative TLC was carried out
Ž
(
)
using silica gel G with a fluorescent indicator Aldrich,
2.4. Preparation of nido-5,6-C2 B8 H12 2a
.
type UV 254 as the stationary phase on plates of
dimensions 200=200=1 mm, made on glass formers
from aqueous slurries followed by drying in air at 808C.
Column chromatography was performed using silica gel
Acetylene was bubbled through a solution of com-
Ž
.
pound 1 861 mg, 5 mmol in 50 ml of toluene, while
the temperature of the reaction mixture was raised to
reflux and the bubbling continued intensively for addi-
Ž
.
Silpearl, Kavalier and the purity of individual chro-
Ž
.
matographic fractions was checked by analytical TLC
tional 15 min dihydrogen evolution . The mixture was
then cooled to room temperature and the toluene dis-
tilled off using a standard rotary evaporator. The resid-
ual oily material was dried in vacuo at ambient tempera-
ture, and the solid material obtained was sublimed at
Ž
on Silufol Kavalier, silica gel on aluminium foil; detec-
tion by UV 254 or diiodine vapour, followed by 2%
.
aqueous AgNO3 spray .
Ž
.
2.2. Physical measurements
508C bath onto a finger cooled to 08C. The sublimate
Ž
the average of 330–368 mg, 53–60%, from several
.
Low-resolution mass spectra were obtained using a
Finnigan Mat Magnum ion trap quadrupole mass spec-
trometer equipped with a heated inlet option, as devel-
experiments was then removed and identified as 2a by
5
NMR spectroscopy.
Ž
(
)
oped by Spectronex AG, Basle, Switzerland 70 eV, EI
ionisation . Proton H and boron
2.5. Preparation of 5,6-Me2-nido-5,6-C2 B8 H10 2b
.
Ž1
.
Ž11
.
B NMR spec-
troscopy was performed at 11.75 T on a Varian X
Under cooling to 08C, to a solution of compound 1
210 mg, 1.22 mmol in 20 ml of toluene was added a
11
11
w
x
Ž
.
L-500 instrument. The procedure for B– B -COSY
1
w
x
Ä11
Ž
.4 w x
Ž
27,28 and H- B selective 29 NMR experiments
were essentially as described in other recent papers
solution of Me2C2 20 ml of a 0.65 M solution in
.
toluene; 12.9 mmol of Me2C2 and the reaction mixture
was heated at reflux for 15 min slow dihydrogen
w
x
Ž
from our laboratories 30 . NMR chemical shifts d are
Ž
.
given in ppm to high-frequency low field of Js
Ž
.
.
32.083971 MHz nominally F3BPOEt2 in CDCl3 for
11
5 A similar BH vertex removal via hydroboration was recently
Ž
Ž
.
Ž
B quoted "0.5 ppm and Js100 MHz SiMe4 for
1
.
H quoted "0.05 ppm , J being defined as in the
w x
observed in the nido-4,5-C2 B6 H10 series 33 .

ˇ
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)
B. Stıbr et al.rJournal of Organometallic Chemistry 550 1998 125–130
127
´
.
.
Ž
.
evolution . The mixture was then cooled to room tem-
mg, 2.8 mmol for 15 min dihydrogen evolution . The
orange mixture was then cooled to room temperature
and the toluene distilled off. The residual oily material
was dried in vacuo at room temperature, and the oily
material obtained was subjected to preparative TLC,
using hexane as the mobile phase. The main band of RF
perature and the toluene distilled off as in the preceding
experiment. The residual oily material was dried in
vacuo at room temperature and the oily material ob-
tained was subjected to preparative TLC using a 10%
CHCl3 solution in hexane as the mobile phase. The
main band of RF prep. 0.45 was removed by CH2Cl2 ,
the extract was evaporated, and the residual liquid dried
in vacuo at room temperature. This procedure was
followed by distillation into a 08C trap of the vacuum
Ž
.
Ž
.
prep. 0.35 was removed by CH2Cl2 , the extract was
evaporated, and the residual liquid dried in vacuo at
room temperature to obtain 144 mg 52% of compound
Ž
.
Ž
2c, which was identified by NMR spectroscopy see
Ž
.
. w
Table 1 and mass spectrometry mass calcd. for
line to obtain 130 mg 85% of a liquid compound 2b.
12
C181B18 H16 200, found 200 23% . Compound 2c can
Ž
.
Ž
.x
This was identified by NMR spectroscopy see Table 1
12
and mass spectrometry mass calcd. for C141B18 H16
be isolated as a low-melting solid by careful distillation
w
Ž
.
x
Ž
.
152; found 152 15% . For larger-scale preparations, it
is convenient to carry out the reaction in benzene, using
a sealed ampoule or an autoclave to prevent the volatili-
sation of the alkyne from the reaction mixture.
at ca. 808C bath , collecting the distillate on the walls
of a sublimation tube at 08C or in the 08C trap of a
vacuum line.
(
)
2.7. Preparation of 5,6-Ph2-nido-5,6-C2 B8 H10 2d and
1,2-Ph2-closo-1,2-C2 B8 H8 3a
(
)
(
)
2.6. Preparation of 6-Ph-nido-5,6-C2 B8 H11 2c
Ž
.
Ž
.
A solution of compound 1 250 mg, 1.4 mmol in 20
A solution of compound 1 250 mg, 1.4 mmol in 20
Ž
Ž
ml of toluene was heated at reflux with PhC2 H 286
ml of toluene was heated at reflux with Ph2C2 498 mg,
Table 1
NMR data
1
Ž
Ž
..
Compound
Nucleus
11Ba
d assignment, JBH Hz
b
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
4- Me2 S -arachno-B9 H13, 1
17.0 B7, 150 , 3.3 B1, 137 ,y17.5 B5,9, 137 ,y22.6 B6,8,128 , y24.8 B4, 135 ,
Ž
.
y40.4 B2,3, 146
11 B–11Bc B7–B6,8; B7–B2,3; B1–B4,5; B1–B4; B1–B2,3; B5,9–B4; B5,9–B2,3; B6,8–B2,3
1Hd
5,6-Me2-nido-5,6-C2 B8 H10, 2b 11 Ba
4.08 H7 , 3.05 H1 , 2.55 Me2 S, 6 H , 1.98 exo-H6,8 , 1.83 H5,9 , 0.47 H2,3 ,
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
0.04 endo-H4 , y0.01 endo H6,8 , y3.52 mH5,6rmH8,9, 2 H
e
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
4.1 B1, 156 , 2.1 B7, 147 , y0.1 B8, 153 , y4.6 B3, 146 , y6.5 B9,10, ;156 ,
Ž
.
Ž
.
y21.1 B2, 174 , y41.0 B4, 153
11 B–11Bc crosspeaks: B1–B3; B1–B10; B1–B2; B1–B4; B7–B8; B7–B3; B7–B2, B8–B3; B8–B9;
B8–B4; B3–B2; B3–B4; B9–B4, B10–B4;
1Hd
3.46 H1 , 3.28 H7 , 3.01 H9 or H10 , 2.88 H8 , 2.72 H3 , 2.57 H10 or H9 ,
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
2.15 and 2.13 5- and 6-Me, 3PH each , 1.02 H2 , 0.85 H4 , y2.32 mH9,10 , y2.64 mH8,9
e
Ž
.
Ž
Ž
.
.
Ž
.
Ž
.
Ž
.
6-Ph-nido-5,6-C2 B8 H11 2c
11 Ba
2.8 B1, 7 ; 143 , 2.0 B8, 147 , y4.0 B3, 149 , y5.6 B9, 165 , y9.8 B10, 156r28 ,
Ž
.
y21.6 B2, 181 , y39.5 B4, 152
11 B–11Bc crosspeaks: B1,7–B8; B1,7–B3; B1,7–B10; B1,7–B2; B1,7–B4; B8–B3; B8–B4; B3–B2;
B3–B4; B9–B4, B10–B4;
1Hd
7.62 Ph, m, 2 H , 7.43 Ph, m, 3 H , 4.96 H5 , 3.70 H1 , 3.49 H8 , 3.19 H7 , 3.16 H9 ,
3.03 H3 , 2.77 H10 , 1.57 H2 , 0.82 H4 , y2.11 mH9,10 , y2.64 mH8,9
Ž
.
Ž
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
.
Ž
.
Ž
.
.
Ž
.
Ž
.
Ž
.
Ž
e
e
e
e
9
e
e
5,6-Ph2-nido-5,6-C2 B8 H10, 2d 11 Ba,e
4.3 B1,y , 3.1 B7,y , 2.3 B8,y , y3.4 B3,y , y4,7 B ,y y5.3 B10,y
Ž
.
Ž
.
Ž
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
.
y22.6 B2, 174 , y39.2 B4, 156
11 B–11Bc crosspeaks: B1–B3; B1–B2; B1–B4; B7–B8f, B7–B3; B8–B3; B8–B4; B3–B2; B3–B4;
B9–B4, B10–B4
1Hd
7.24-6.76 5,6-Ph2 , m, 10 H , 3.75 H1 , 3.50 H7 , 3.22 H8 , 3.07 H3 , 3.27 H9 , 2.92 H10 ,
Ž
.
Ž
.
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
Ž
.
1.90 H2 , 0.91 H4 , y1.58 mH9,10 , y2.05 mH8,9
1,2-Ph2-closo-1,2-C2 B8 H8, 3a 11 Ba,e
32.5 B10, 172 , y5.1 B4, 169 , y16.7 B3,5, 178 , y20.6 B6,9, 166 , y23.6 B7,8, 154
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
11B–11Bc B10–B6,9; B10–B7,8; B4–B3,5; B4–B7,8; B3,5–B6,9; B3,5–B7,8
1Hd
7.63-6.94 1,2-Ph2 , m, 10 H , 5.98 H10 , 3.17 H3,5 , 2.78 H4 , 1.51 H6,9 , 1.41 H7,8
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
a
11
Ž11
.
Ä1
Ž
.4
w
x
d
B values in CDCl3 determined from
B
H broadband measurements with assignments by 11 B–11B -COSY NMR spectroscopy.
b Broad doublet.
c
Ä1
.4
Ž
.4
Ž
Measured under the conditions of H broadband decoupling.
d
1
1
Ä11
Ž
.4
Ä11
.4
Ž
.
Assignments by H- B broadband and H- B selective NMR spectroscopy; unless stated otherwise msmultiplet , all signals are singlets
1
Ä11
Ž
in the H- B broadband NMR spectrum.
e Values uncertain due to peak overlap.
f Uncertain crosspeaks due to close proximity of resonances.

ˇ
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)
B. Stıbr et al.rJournal of Organometallic Chemistry 550 1998 125–130
128
´
1
2
.
Ž
.
Ž
.
2.8 mmol for 15 min dihydrogen evolution . The
orange mixture was then cooled to room temperature
and the toluene distilled off. The residual oily material
was dried in vacuo at room temperature and the oily
residue obtained was subjected to preparative TLC us-
general formula 7,8-R R - Me2 S -nido-7,8-C2 B9 H9
and of the starting compound 1. This inseparable mix-
ture can be, however, readily removed from compounds
of types 2 and 3 by chromatography.
Ž
ing hexane as the mobile phase three subsequent devel-
opments on the same TLC plate to develop two main
bands of RF prep. ca. 0.45 and 0.35. These were
removed by CH2Cl2 , the extracts were evaporated, and
the residual materials dried in vacuo at room tempera-
.
Ž
.
Ž
.
ture to obtain 131 mg 36% of the closo compound 3a
12
mass calcd. for C14 B18 H18 274, found 274 13% and
11
w
Ž
.x
Ž
.
w
190 mg 52% of the nido species 2d mass calcd. for
12
C14 B18 H20 276, found 276 24% , respectively, from
11
Ž
.x
individual bands. Both compounds thus isolated were
Ž
.
further identified by NMR spectroscopy see Table 1 .
Analytical samples can be purified and isolated as solid
materials by careful volatilisation at ca. 80–1008C
Ž
.
bath , collecting the condensate on the walls of a
sublimation tube at 08C.
3. Results and Discussion
Ž
.
Ž .
Treatment of 4- Me2 S -arachno-B9 H13 1 with ap-
proximately a twofold excess of alkynes R1R2 C2 in
Ž
.
toluene reflux, 15 min resulted in the formation of a
series of the 5,6-R1R2-nido-5,6-C2 B8 H10 species of
1
2
w
Ž
.
general structure 2 where R , R sH, H 2a ; Me, Me
Ž
.
Ž
.
Ž
.x
2b ; Ph, H 2c , and Ph, Ph 2d in yields 53–60, 85,
52, and 52%, respectively. The compounds were iso-
lated by the evaporation of the toluene, followed by
preparative TLC separation of the main products in
Ž
.
hexane or its mixtures with CH2Cl2 or CHCl3 and
further purification by distillation or sublimation. Col-
umn chromatography may be employed larger-scale
syntheses of individual compounds. As suggested by
NMR spectroscopy, the reactions are accompanied, to a
variable extent, by the formation of a complex mixture
that apparently consists of diverse positional isomers of
Scheme 1.

ˇ
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)
B. Stıbr et al.rJournal of Organometallic Chemistry 550 1998 125–130
129
´
w
x
The formation of the nido 10-vertex dicarbaboranes
of type 2 seems to be in agreement with the stoichiom-
etry as in Eq. 3 :
the parent compound 2a 20,21 . Another advantage is a
Ž
.
simple availability of the borane adduct 4- Me2 S -
arachno-B9 H13. This compound is considerably stable
for longer storage and can be prepared directly from the
commercially available decaborane. Another positive
Ž .
2 R1R2 C2 qMe2 SPB9 H13
Ž
™R1R2 C2 B8 H10 qMe2 SPBH2C2 H R1R2
3
Ž .
aspect is the short reaction time of the synthesis 15
min and facile isolation of the reaction products. The
derivatives of 2a can be used as indispensable starting
materials for the development of substitution chemistry
in the currently developing areas of dicarbaboranes
.
Ž
As shown in a simplified Scheme 1 exo and endo
.
hydrogens omitted for clarity , the reaction is consistent
with the insertion of both alkyne carbons into the
w
x
w
x
w
9-vertex arachno cage of compound 1. The reaction
proceeds under simultaneous elimination of the B 4
5,6 , heterocarboranes 14 , and tricarbaboranes 15–
Ž .
x
17 .
vertex, which may be removed as Me SPBH2C2 H
w
x 2
R1R2 by hydroboration at that site 33 . This borane
adduct is apparently decomposed by exposure to air
during the chromatographical experiment; nevertheless,
the fate of the allylborane decomposition products was
not followed. It should be noted that the reaction with
phenyl acetylene proceeds in a highly regiospecific
manner, only with the formation of 6-Ph-nido-5,6-
Acknowledgements
Ž
We thank Banco Bilbao de Vizcaya BBV Founda-
ˇ
.
Ž
.
tion, Grant to B.S. , the CIRIT project QNF92-43B ,
Grant Agency of the Czech Republic Grant No.
Ž
.
203r97r0060 , and Dr. Z. Plzak for the mass spectra.
´
´
Ž
.
Ž
C2 B8 H11 2c as a single product of type 2 without
The involvement of Drs. T. Jelınek and J. Holub in the
.
formation of the isomeric 5-Ph-nido-5,6-C2 B8 H11 . This
revision of some experiments is kindly appreciated.
regiospecificity may be consistent with steric require-
ments of the bulky Ph substituent during the insertion
process. In the case of diphenyl acetylene, the reaction
is accompanied by the dehydrogenation of 2d and for-
References
Ž
.
mation of 1,2-Ph2-closo-1,2-C2 B8 H8 3a , a closo
counterpart of 2d; the dehydrogenation being apparently
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Ä
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Ä
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1
11
11
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´
ˇ
ˇ ´
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B. Stıbr, J. Plesek, S. Hermanek, Collect. Czech. Chem. Com-
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´
´
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