A highly reactive triboracyclobutane and its dianion: Two-electron homoaromatics with nonclassical and classical σ skeletons

Article abstract of DOI:10.1021/om021046z

The first triboracyclobutane, 2c, a two-electron homoaromatic having a nonclassical σ skeleton, was generated at -80°C and stabilized as its dianion 2a^2-, which was characterized by the crystal structure determination of its dilithium salt and which possesses a classical σ skeleton.

Full text of DOI:10.1021/om021046z

1594
Organometallics 2003, 22, 1594-1596
A High ly Rea ctive Tr ibor a cyclobu ta n e a n d Its Dia n ion :
Tw o-Electr on Hom oa r om a tics w ith Non cla ssica l a n d
Cla ssica l σ Sk eleton s†
Peter Amseis,‡ Wahid Mesbah,‡ Carsten Pra¨sang,‡ Matthias Hofmann,§
Gertraud Geiseler,‡ Werner Massa,‡ and Armin Berndt*,‡
Fachbereich Chemie der Philipps-Universita¨t Marburg, Hans-Meerwein-Strasse,
35032 Marburg, Germany, and Anorganisch-Chemisches Institut,
Universita¨t Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Received December 27, 2002
Sch em e 1. Com p u ted En er gy Differ en ces for
Non a r om a tic (1A, 2A) a n d Ar om a tic (1B, 2B, 1C,
2C) Str u ctu r es of Tr ibor a cyclop r op a n e a n d
Tr ibor a cyclobu ta n ea
Summary: The first triboracyclobutane, 2c, a two-
electron homoaromatic having a nonclassical σ skeleton,
was generated at -80 °C and stabilized as its dianion
2a 2-, which was characterized by the crystal structure
determination of its dilithium salt and which possesses
a classical σ skeleton.
The classical D3h structure of triboracyclopropane
(triborirane; 1A) (Scheme 1), with three 2c-2e B-B
bonds, is much less stable (61.6 kcal mol-1 at the MP2/
6-31G* level) than its alternative aromatic state 1B,
which has lower (C2v) symmetry and two cyclically
delocalized π electrons (symbolized by a circle).1 One
pair of BB electrons in 1A is transformed into the
3c-2e π electrons of 1B and a second pair into a 3c-2e
σ bond (drawn as a dashed triangle).2 Thus, 1B has a
nonclassical σ skeleton like the global minimum 1C,
which is further stabilized by 6.9 kcal mol-1 due to the
formation of an additional BHB bridge. A classical σ
skeleton as in 1A is to be expected only for the dianion
1A2-.1 Our own DFT computations3 revealed tribora-
cyclobutane (triboretane) to be the homoform of tribo-
racyclopropane: the two-electron homoaromatic 2B
containing a 3c-2e σ bond and a 3c-2e π bond between
the three boron centers is considerably lower in energy
than planar 2A with two classical 2c-2e B-B σ bonds.
The global minimum 2C is slightly more stable due to
an additional BHB bridge. The homoaromaticity of the
dianion 2A2- with two classical 2c-2e B-B bonds was
a
A dashed triangle symbolizes a cyclic three-center-two-
electron (3c-2e) σ bond and a circle a cyclic 3c-2e π bond.
discussed already on the basis of computations.4 Here
we present the generation and NMR characterization
of the triboracyclobutane 2c (Scheme 2), which pos-
sesses, aside from substituents, an aryl bridge instead
of the hydrogen bridge in 2C.
* To whom correspondence should be addressed. E-mail: berndt@
chemie.uni-marburg.de. Fax: 0049/6421/2828917.
† Dedicated to Professor Paul von Rague´ Schleyer.
‡ Philipps-Universita¨t Marburg.
§ Universita¨t Heidelberg.
(1) Korkin, A. A.; Schleyer, P. v. R.; McKee, M. L. Inorg. Chem. 1995,
34, 961. McKee, M. L. Inorg. Chem. 1999, 38, 321.
(2) Berndt, A.; Steiner, D.; Schweikart, D.; Balzereit, C.; Menzel,
M.; Winkler, H.-J .; Mehle, S.; Unverzagt, M.; Happel, T.; Schleyer, P.
v. R.; Subramanian, G.; Hofmann, M. Adv. Boron Chem. 1997, 61.
(3) (a) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA,
1998. (b) Becke, A. D. J . Chem. Phys. 1993, 98, 1372, 5648. (c) Lee, C.;
Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
The highly reactive triboracyclobutane 2c is obtained
when the borenate 35 is protonated with HCl in diethyl
ether at -80 °C. When the temperature is raised to -60
°C, the deep red solution turns yellow within about 40
min. This indicates the formation of 4, which can be
(4) Steiner, D.; Winkler, H.-J .; Balzereit, C.; Happel, T.; Hofmann,
M.; Subramanian, G.; Schleyer, P. v. R.; Massa, W.; Berndt, A. Angew.
Chem. 1996, 108, 2123; Angew. Chem., Int. Ed. Engl. 1996, 35, 1990.
(5) (a) Scheschkewitz, D.; Ghaffari, A.; Amseis, P.; Unverzagt, M.;
Subramanian, G.; Hofmann, M.; Schleyer, P. v. R.; Schaefer, H. F.,
III; Geiseler, G.; Massa, W.; Berndt, A., Angew. Chem. 2000, 112, 1329;
Angew. Chem., Int. Ed. 2000, 39, 1272. (b) Unverzagt, M.; Subrama-
nian, G.; Hofmann, M.; Schleyer, P. v. R.; Berger, S.; Harms, K.; Massa,
W.; Berndt, A. Angew. Chem. 1997, 109, 1567; Angew. Chem., Int. Ed.
1997, 36, 1469.
10.1021/om021046z CCC: $25.00 © 2003 American Chemical Society
Publication on Web 03/15/2003

Communications
Organometallics, Vol. 22, No. 8, 2003 1595
Sch em e 2. Gen er a tion of th e High ly Rea ctive
Tr ibor a cyclobu ta n e 2c, Sta biliza tion of 2c a s Its
Dia n ion 2a 2-, a n d Reoxid a tion of 2a 2- to 2ca
a
12
During the formation of 2c from 3, the C-B bond of 3
F igu r e 1. Structure of (2a )Li2‚3THF‚0.5C5H12
.
The
marked with an arrow is cleaved by protonation. The rear-
rangement of 2c into 4 requires activation (cleavage) of the
C-H and C-B bonds of 2c, marked with arrows. During all
these transformations, the two-electron aromaticity is retained.
Legend: R ) SiMe3, Dur ) 2,3,5,6-tetramethylphenyl; for 5a ,
R1 ) 1-neopentyl-3,3-bis(trimethylsilyl)allenyl, R2 ) neopentyl,
solvent molecule and most of the hydrogen atoms are
omitted for clarity. Selected bond distances (pm) and angles
(deg): B1-B2 ) 162.5(4), B2-B3 ) 163.1(3), B1-B3 )
182.8(4), C1-B1 ) 162.3(3), C1-B3 ) 161.5(3), B1-C10
) 161.5(3), B2-C20 ) 160.2(3), B3-C2 ) 161.6(4),
C1-Si1 ) 182.3(2), Li1-B2 ) 232.3(5), Li1-B3
)
R3 ) Ar ) Dur; for 5b, R1 ) R2 ) t-Bu, R3 ) Ar
)
252.9(5), Li2-B1 ) 230.9(5), Li2-B2 ) 222.0(5), Li1-O1
) 199.8(5), Li1-O2 ) 198.2(5), Li2-O3 ) 190.8(5),
Li1-C20 ) 247.2(5), Li2-C20 ) 225.4(5), Li2-C10 )
230.7(5), Li2-C21 ) 273.1(5); B1-B2-B3 ) 68.3(2),
B2-B1-C1 ) 104.0(2), B2-B3-C1 ) 104.1(2), B1-C1-
B3 ) 68.8(2); C1-B1-B3-B2 ) -145.0(2).
2,4,6-trimethylphenyl.
isolated in 70% yield after warming and crystallization.6
At -100 to -80 °C, 2c can be stabilized as its dianion
2a 2- by reaction with 2 equiv of Li naphthalenide in
THF.7
The dianion 2a 2- forms a triple contact ion with two
lithium cations which coordinate one (Li2) or two (Li1)
additional THF molecules. Neither of the two Li cations
is located above or below the dianion, as found for the
dilithium salt of the dianion of tetrakis(trimethylsilyl)-
cyclobutadiene by Sekiguchi,10 but both are coordinated
side-on to the B-B bonds almost within the plane of
the boron atoms (Li1-B2-B3-B1 ) 170.2°, Li2-B1-
B2-B3 ) -168.7°).11 In solution, NMR signals for both
stereoisomers of (2a )Li2 are observed, one having the
After the bulk (about 90%) of naphthalene is removed
by sublimation under high vacuum, the red dilithium
salt of 2a 2- was obtained by crystallization from pen-
tane. The crystal structure of (2a )Li2‚3THF‚0.5C5H12
is shown in Figure 1.
8
(6) A solution of HCl in Et2O (3.15 mL, 6.3 mmol) was added to a
solution of 3.5 g (6.3 mmol) of 3 in 80 mL of Et2O at -80 °C. After 30
min of stirring at -80 °C followed by warming to room temperature,
all volatile components were removed. The residue was digested with
50 mL of pentane, and LiCl was separated by a G4 reversed frit and
washed with 10 mL of pentane. After the volume was reduced to 30
mL and the solution cooled to -30 °C, 2.2 g (70%) of 4 was obtained
as colorless crystals, mp 109-110°C. 1H NMR (500 MHz, CDCl3, 25
°C): δ -0.54 (s, 9H, SiMe3), 0.04 (s, 9H, SiMe3), 0.42 (d, 2H, 3J (H,H)
) 6 Hz, CH2Si), 2.21 (s, 24H, o-, m-CH3), 6.2 (br, 1H, BH), 6.94 (s, 2H,
p-H). 13C NMR (125 MHz, CDCl3, 25 °C): δ -0.3 (q, SiMe3), 0.3 (q,
SiMe3), 13.9 (t, 3J (C,H) ) 105 Hz, CH2Si), 19.7, 20.1, 21.1 (each q, o-,
m-CH3), 129.7 (s, i-C), 131.4 (s, CB2), 132.6 (d, p-C), 133.3, 134.2, 138.0,
138.8 (each s, o-, m-C). 11B NMR (160 MHz, CDCl3, 25 °C): δ 37 (2B),
62.
(7) A solution of HCl in Et2O (3.38 mL, 6.75 mmol) was added to a
solution of 3.75 g (6.75 mmol) of 3 in 80 mL of Et2O at -100 °C. After
30 min of stirring at -100 °C, a freshly prepared and precooled solution
of lithium naphthalenide in THF (15 mL, 13.5 mmol) was added within
5 min using a syringe. After 1 h the solution was warmed to room
temperature and all volatile components were removed. The residue
was digested with pentane, and LiCl was separated by a reversed frit.
After the solvent was removed, the bulk of naphthalene was separated
by sublimation under high vacuum. Cooling a pentane solution of the
mixture obtained to -30 °C led to red crystals of (2a )Li2‚3THF‚
0.5C5H12, mp >165 °C. 1H NMR (500 MHz, [D8]THF, 25 °C): δ -0.03
(s, 9H, SiMe3), 0.05 (s, 9H, SiMe3), 0.27 (br, 1H, SiCH), 0.34, 0.40 (each
br, each 1H, diastereotopic CH2Si), 2.08, 2.09, 2.37, 2.41 (br) (each s,
together 24H, o-, m-CH3), 6.36, 6.47 (each s, each 1H, p-H). 13C NMR
(125 MHz, [D8]THF, 25 °C): δ 2.8, 3.3 (each br, SiMe3) 14.9 (br,
CH2Si), 20.4, 21.0, 21.6 (o-, m-CH3), 32.6 (br, CHSi), 125.8, 127.3 (each
d, p-C), 132.0, 132.3, 134.2, 136.2 (each br, o-, m-C), 156, 158 (each br
s, i-C). 11B NMR (160 MHz, [D8]toluene): -80 °C, δ 10, 13, 16, 21, 40,
51; 25 °C, δ 14, 22, 42, 54; 65 °C, δ 16, 24, 48.
(8) Crystal structure determination for (2a )Li2‚3THF‚0.5C5H12: red
crystal from pentane, 0.25 × 0.20 × 0.15 mm:
C42.5H77B3Li2O3Si2,
triclinic, space group P1h, Z ) 2, a ) 1146.5(1) pm, b ) 1302.6(1) pm,
c ) 1792.0(2) pm, R ) 76.65(1)°, â ) 81.43(1)°, γ ) 64.29(1)°, V )
2342.4(4) × 10-30 m3, F ) 1.047 Mg m-3. Data collection was carried
out on a Stoe IPDS area detector system, with T ) -80 °C. A total of
37 137 reflections were measured up to θ ) 24.9°; there were 8199
unique reflections (Rint ) 0.0832) and 5137 with I > 2σ(I). The structure
was solved using direct and Fourier methods9 and refined by full matrix
2
least-squares methods (based on Fo
,
SHELXL-979); anisotropic
displacement parameters were used for all non-H atoms in the final
cycles, and H atoms were treated by a mixture of independent (H1,
H2a, H2b) and constrained refinement. R1 ) 0.0523 (I > 2σ(I)), and
wR2 ) 0.1489 (all data). Crystallographic data (excluding structure
factors) for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Center as supplementary
publication no. CCDC-175173. Copies of the data can be obtained free
of charge on application to the CCDC, 12 Union Road, Cambridge CB2
1EZ, U.K. (fax, int. code + 44(1223)336-033; e-mail, deposit@ccdc.
cam.ac.uk).
(9) Sheldrick, G. M. SHELX-97, Programs for the Solution and
Refinement of Crystal Structures; University of Go¨ttingen, Go¨ttingen,
Germany, 1997.
(10) Sekiguchi, A.; Matsuo, T.; Watanabe, H., J . Am. Chem. Soc.
2000, 122, 5652; Sekiguchi, A.; Tanaka, M.; Matsuo, T.; Watanabe,
H., Angew. Chem. 2001, 113, 1721, Angew. Chem., Int. Ed. Engl. 2001,
40, 1675.

1596 Organometallics, Vol. 22, No. 8, 2003
Communications
Sch em e 3. Ch em ica l Sh ifts of Sk eleton Atom s As
Com p u ted 3 for Mod el Molecu les 2A′, 2B′, a n d 2C′
a n d Exp er im en ta lly Deter m in ed for 2ca
skeleton SiMe3 substituent in an endo position as found
in the solid state and the other in an exo position.
Coalescence of the room-temperature 11B signals (δ 42
and 54) at 65 °C indicates a barrier of 14 kcal mol-1 for
the exchange of the stereoisomers of (2a )Li2 by ring
inversion. Computations for the unsubstituted prototype
2A2- (without gegenions) led to barriers of 15.5 (MP2/
6-31G*) and 11.2 kcal mol-1 (B3LYP/6-311+G**).4
The unsymmetrical substitution pattern of 2c is re-
tained in the dianion 2a 2-: the ring carbon atom has
boron neighbors with different substituents, is tetraco-
ordinate, and is still connected to a hydrogen atom.
Compound 4, which was also characterized by a crystal
structure determination, however, shows a symmetrical
pattern of substituents: the ring carbon is tricoordinate,
and the hydrogen atom bound to the ring carbon in 2c
is found at the CH2SiMe3-substituted boron atom in 4.
During the conversion of 2c into 4, a C-H and a C-B
bond have to be cleaved under very mild conditions. This
can be understood as the consequence of strong electron
deficiency in 2c due to three adjacent boron centers
without strong donor substituents.
Compound 2c was definitely characterized by analy-
a
1
ses of its H, 13C, and 11B NMR spectra,13 which had to
Legend: R ) SiMe3, Dur ) 2,3,5,6-tetramethylphenyl.
be measured below -80 °C because of the easy rear-
rangement of 2c into 4. Suitable solutions of 2c in [D10]-
diethyl ether or [D8]toluene were obtained by oxidation
of (2a )Li2‚3THF with 1,2-dibromoethane at -90 °C
directly in NMR tubes.13 The presence of a bridging aryl
substituent is indicated by 13C NMR signals for two
deshielded and one shielded carbon atom at δ(13C) 156,
155, and 118 ppm, respectively. Deshielding of ortho and
shielding of ipso carbon atoms is characteristic for aryl
substituents bridging the B-B bond of two-electron
aromatics: for 5a and 5b similar chemical shifts (δ(13C)
151, 124 and δ(13C) 160, 113 ppm, respectively) were
observed.14 Convincing evidence for the aromatic nature
of 2c comes from its 11B NMR chemical shifts in
comparison to those computed for model molecules 2A′,
2B′, and 2C′ (Scheme 3).
For classical 2A′, which is folded to benefit from
hyperconjugation, strongly deshielded boron atoms are
predictedsas is to be expected for tricoordinate boron
centers connected to further boron atoms. 1,2,4-Tribo-
racyclopentanes, for example, are characterized by
δ(11B) 97-101 for the B-B moiety.5 In the aromatic
species 2B′ without an aryl bridge, all boron centers are
shielded as compared to those in 2A′. Only the two
tetracoordinate boron atoms of 2B′ show chemical shifts
comparable to two centers of 2c. In contrast, all the
experimental 11B chemical shifts for 2c agree well with
those computed for the two-electron aromatic model 2C′
with a phenyl bridge (see Scheme 3).
Thus, triboracyclobutane 2c is a two-electron ho-
moaromatic containing a nonclassical σ skeleton: i.e., a
homotriboracyclopropane. In contrast, the homoform of
benzene, cycloheptatriene, is not a homoaromatic spe-
cies: a homo bridge interrupts the cyclic delocalization
of six electrons but not that of two electrons. This is due
to the considerably larger aromatic stabilization energy
of two-electron aromatics as compared to that of six-
electron aromatics. Schleyer has repeatedly pointed out
this fact.15
(11) “Side-on” coordination two lithium cations to B-B σ bonds was
computed to be preferred for the triple contact ion of 1A2- over π
coordination.1
(12) Brandenburg, K. DIAMOND 2.1c; Crystal Impact GbR, Bonn,
Germany, 1999.
(13) A 2-fold excess of 1,2-dibromoethane was added to an NMR
sample of (2a )Li2‚3THF (about 100 mg, 0.6 mL of [D10]Et2O or [D8]-
toluene) at -110 °C. The tube was transferred to the spectrometer as
fast as possible, warmed to -90 °C inside the magnet, and cooled again.
2c: 1H NMR (500 MHz, [D10]Et2O, -100 °C) δ 0.05 (s, 9H, SiMe3),
0.11 (s, 9H, SiMe3), 0.91 (s, 1H, SiCH), 1.1 (hidden by solvent, localized
Ack n ow led gm en t. We acknowledge the DFG and
the FCI for financial support.
by H,H correlation), 1.52 (each d, each 1H, 2J (H,H)
) 10 Hz,
diastereotopic CH2Si), 1.45, 1.94, 2.03, 2.16, 2.28, 2.45, 2.49 (each s,
together 24H, o-, m-CH3), 6.79, 7.16 (each s, each 1H, p-H); 13C NMR
(125 MHz, [D10]Et2O, -100 °C) δ 0.9, 1.3 (SiMe3), 11.8 (br, CH2Si),
20.4, 20.5, 20.6, 20.9, 21.0, 21.6, 22.3 (o-, m-CH3), 118.4, 135.8 (each
br s, i-C), 132.7, 138.3 (each d, p-C), 132.3, 132.8, 134.0, 134.6, 139.4,
139.5, 155.0, 155.6 (each s, o-, m-C), the signal of BCHSi is hidden by
the solvent and in [D8]toluene it can be unambiguously localized at δ
15.8; 11B NMR (160 MHz, [D8]toluene, -105 °C) δ 14, 25, 29.
(14) Menzel, M.; Winkler, H.-J .; Ablelom, T.; Steiner, D.; Fau, S.;
Frenking, G.; Massa, W.; Berndt, A. Angew. Chem. 1995, 107, 1476;
Angew. Chem., Int. Ed. Engl. 1995, 34, 1340. Pilz, M.; Allwohn, J .;
Massa, W.; Berndt, A. Angew. Chem. 1990, 102, 436; Angew. Chem.,
Int. Ed. Engl. 1990, 29, 399.
Su p p or tin g In for m a tion Ava ila ble: Figures and tables
giving details of the crystal structure determination and the
DFT computations. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM021046Z
(15) Budzelaar, P. H. M.; Schleyer, P. v. R. J . Am. Chem. Soc. 1986,
108, 3967, and literature cited therein.