Two-electron homoaromatics with heteroatom bridges

Article abstract of DOI:10.1016/S0022-328X(03)00396-6

Zwitterionic mono- and bis-homoaromatics 2a-f and 5a comprised of positively charged NMe₂, P(C₆H₅) ₂ or As(C₆H₅)₂ bridges and anionic three-center two-electron (3c2e) delocalized boron heterocyclic units, were prepared and characterized by NMR as well as by X-ray structure analyses. The boron chemical shifts of the trishomoaromatic dianion 12a with an oxygen bridge compare well with those computed ab initio for model 12b. Analysis of the electronic structure of the bishomoaromatic 5u and its anionic analog 11u gives insight into the origin of the trend of increasing effectiveness of BC₂, B₂C and B₃ 3c2e bonds: higher electronegativity of carbon vs boron prevents symmetric delocalization in rings with B₂C and especially BC₂ centers.

Full text of DOI:10.1016/S0022-328X(03)00396-6

Journal of Organometallic Chemistry 680 (2003) 244ꢀ256
/
www.elsevier.com/locate/jorganchem
Two-electron homoaromatics with heteroatom bridges
Yuksel Sahin a, Andreas Ziegler b, Thorsten Happel a, Harald Meyer a,
¨
Michael J. Bayer b, Hans Pritzkow b, Werner Massa a, Matthias Hofmann b,
Paul von Rague´ Schleyer c,1, Walter Siebert b,*, Armin Berndt a,*
a Fachbereich Chemie der Universitat Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany
¨
b Anorganisch-Chemisches Institut der Universitat Heidelberg, Im Neuenheimer Feld 276, D-69120 Heidelberg, Germany
¨
c Computational Chemistry Annex, University of Georgia, Athens, GA 30602-2525, USA
Received 1 March 2003; accepted 29 April 2003
Dedicated to Professor Fred Hawthorne on the occasion of his 75th birthday
Abstract
Zwitterionic mono- and bis-homoaromatics 2aꢀ
/f and 5a comprised of positively charged NMe2, P(C6H5)2 or As(C6H5)2 bridges
and anionic three-center two-electron (3c2e) delocalized boron heterocyclic units, were prepared and characterized by NMR as well
as by X-ray structure analyses. The boron chemical shifts of the trishomoaromatic dianion 12a with an oxygen bridge compare well
with those computed ab initio for model 12b. Analysis of the electronic structure of the bishomoaromatic 5u and its anionic analog
11u gives insight into the origin of the trend of increasing effectiveness of BC2, B2C and B3 3c2e bonds: higher electronegativity of
carbon vs boron prevents symmetric delocalization in rings with B2C and especially BC2 centers.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Ab initio computations; Boron heterocycles; Homoaromaticity; Three-center two-electron bonds; Zwitterions
1. Introduction
for trishomoaromatic cations have been studied compu-
tationally [8].
The concept of homoaromaticity [1] involving three-
center two-electron (3c2e) bonding was applied to the
bis-homocyclopropenyl cation I [2], the tris-homocyclo-
propenyl cation II [3], and the (mono-)-homocyclopro-
penyl cation III [4] (Scheme 1) between 1956 and 1962.
Recently, this class of homoaromatics with bridging
CH2 and similar [5] groups bound by the usual two-
center two-electron (2c2e) bonds was extended by
systems where the bridges involve 3c2e bonds [6].
We now present experimental examples of an unusual
class of neutral mono- and bis-homoaromatics with
charge compensated heteroatom homobridges bound
classically by 2c2e bonds. Such zwitterionic species
involving anionic aromatic boron heterocycles arise
when the nitrogen, phosphorus or arsene bridging
groups bear a positive charge. A dianionic trishomoaro-
matic with an oxygen homobridge is also described.
Hydrogen, sp2-carbon, or boron [7] atoms (cf. IVꢀ
VI),
/
i.e. heteroatoms also can serve as nonclassical [6]
homobridges. Heteroatoms as classical homobridges
* Corresponding authors. Fax: ꢁ
/
49-6421-28-28917 (A.B.). Fax: ꢁ
/
49-6221-54-5609 (W.S.).
E-mail addresses: schleyer@chem.uga.edu (P.v.R. Schleyer),
ci5@ix.urz.uni-heidelberg.de (W. Siebert), berndt@chemie.uni-
marburg.de (A. Berndt).
1
Fax: ꢁ
/
1-706-542-7514.
Scheme 1.
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00396-6

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245
2. Results and discussion
2.2. A bishomoaromatic zwitterion with an ammonium
homobridge
The zwitterionic 1-azonium-2,5-diboracyclopentane-
3-ide 5a is obtained when 6 [12] is reacted with
bis(dimethylamino)borane. The structure of 5a is estab-
lished by X-ray structure analysis. The 11B-NMR
2.1. Monohomoaromatic zwitterions with nitrogen,
phosphorus and arsenic homobridges
The zwitterionic 1-azonium-2,4-diboretane-3-ides
2a,b arose unexpectedly during our attempts to replace
the chlorine atoms of 1 [9a] by amino substituents, using
dimethylamine or pyrrolidine as reagents. A possible
mechanism involves intermediate A. The driving force
for opening the four-membered ring, as shown, is the
formation of the 3c2e bond in the observed products
2a,b. Similarly, the zwitterionic 1-phosphonium-2,4-
diboretane-3-ide 2c is obtained from 3 [9b] and
P(SiMe3)3 at 130 8C, while 2d,e and the 1-arsonium-
2,4-diboretane-3-ide 2f are formed at 25 8C when
diboriranides 4a,b [10] are reacted with chlorodiphenyl-
phosphane or chlorodiphenylarsane, respectively. Reac-
tion of 2e with lithium in diethyl ether yields the 1-
phospha-2,4-diboretane-3-ide 2g (Scheme 2).
chemical shift of one of its skeleton atoms (ꢂ7 ppm)
/
clearly indicates the presence of a pentacoordinate
boron atom in solution. The formation of 5a requires
a trimethylsilyl group to migrate from a carbon to a
boron atom and a hydrogen atom to migrate from this
boron atom to a neighboring carbon. By performing the
reaction at low temperature in an NMR tube, we were
able to identify two intermediates during this rearrange-
ment. The nonclassical 1,2-diboracyclobutane [13] 7a is
formed at ꢂ50 8C. Its structure is deduced from the
/
chemical shifts (12.4 and 35.8) of its skeletal atoms as
compared to those (14.8 and 40.9) of 7b, whose structure
is proven by an X-ray analysis. Upon warming to
ꢂ
/
10 8C, 7a rearranges into the zwitterionic 8a, char-
acterized by a 13C-NMR signal at 236 ppm and a boron
chemical shift of ꢂ5 ppm. Ab initio computations at the
GIAO-SCF/6-31G*//MP2(fc)/6-31G* level yield very
similar chemical shifts for 8u (d13Cꢃ
227 and
d11Bꢃ
7 ppm). At ꢁ10 8C, finally, 8a transforms
The structures of 2aꢀ
structure analyses; those of 2eꢀ
/
d were confirmed by X-ray
g follow from the
/
/
similarity of the chemical shifts of their skeleton atoms
(Table 1) with those of 2d. For comparison the
corresponding chemical shifts of the monohomoaro-
matic 2h [11] with a bis(trimethylsilyl)methylene homo-
bridge are included in Table 1.
/
/
ꢂ
/
/
into 5a. This reaction is easily understood as a 1,2-
hydride migration from a formally negatively charged
boron to a neighboring carbon atom whose partial
Scheme 2.

246
Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ256
/
Table 1
11B and selected 13C-NMR spectroscopic data for 2aꢀ
h
/
2a
2b
2c
2d
44
2e
47
2f
2g
38
2h
d
d
11B
13C
37
151
35
153
39
54
156.9
37
162.9
ꢀ/
ꢀ/
179.0
79
171.2
82
140.8
74
J(13CÃ
/
31P)
ꢀ
/
ꢀ
/
ꢀ/
ꢀ
/
positive charge is indicated by its strong deshielding
(d13Cꢃ
236 in 8a) (Scheme 3).
The rearrangement of 7a into 8a can be explained by
NBO analyses of the electronic structures of 5u and
11u reveal why isoelectronic boron heterocycle struc-
tures (containing formally a 2c2e p bond and an electron
deficient center) are strongly dependent on the number
of ring boron atoms: monocyclic derivatives of 10u have
/
isomerization of the nonclassical 7a to the classical B
(the corresponding opening of unsubstituted 7uꢀBu
/
requires only 6.7 kcal molꢂ1 at the MP2/6-31G* level).
This is followed by attack of the nitrogen of the
hydrogen-bearing boron group at the boron atom of
the three-membered ring to yield the bishomoaromatic
zwitterion C. Migration of a trimethylsilyl group in C to
the neighboring ring carbon leads to the bishomoaro-
matic zwitterion D which, by further migration of the
same silyl group, gives 8a.
undistorted rings with a CÄ
/
C bond [14], whereas
derivatives of 9u [15] and 5a (analogs of 11u) possess
strongly distorted rings due to the 3c2e bonds. Scheme 4
shows the distribution of the cyclically delocalized
electrons over the centers of the 3c2e bond as given by
NBO analyses.
In 10u, 84% of the electrons of the 3c2e bond are
localized at the carbon atoms, only 16% are found at the
boron center. In 9u, however, the electrons involved in
the 3c2e bonding are symmetrically delocalized over the
three boron centers. Both 5u and 11u are in between:
more than 50% of the electrons of the 3c2e bonds are
localized at carbon. Evidently, the electronegativities of
C and B (2.5 and 2.0, respectively) are decisive. Cyclic
delocalization is rather small in 10u; hence, the gain in
energy in forming the 3c2e bond is smaller than the
energy required for distortion of the five-membered ring
into the geometry characteristic for bishomoaromatic
systems. As a consequence, all known monocyclic 1-
boracyclopent-3-enes 10 possess structures without 3c2e
The homoaromatic stabilization energy HSE of 5u,
defined as the energy difference between 5u and the
planar form 5u* containing a 2c2e BÄ
was computed at the MP4/6-311ꢁG**//MP2/6-311ꢁ
G** level to be ꢂ
22.5 kcal molꢂ1. This HSE is between
those of 9u (ꢂ33.1) and 10u (ꢁ
4.5 kcal molꢂ1, both at
/C double bond,
/
/
/
/
/
the same level). To elucidate the influence of the
positively charged homobridge of 5u on the HSE, we
computed 11u with an uncharged methylene bridge and
11u*. The HSE of ꢂ
19.9 kcal molꢂ1 shows that the
influence of a positively charged nitrogen homobridge
/
on the degree of homoaromaticity is rather small.
Scheme 3.

Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ
/256
247
2,4,6-triboracyclohexane 13a with excess lithium
naphthalenide in THF at ꢂ60 8C. Compound 13a was
/
obtained by protonation (HCl at 25 8C) of the dianion
14a, which was synthesized by reduction of 15a (pre-
pared from 15b [18] and LiCH2SiMe3) (Scheme 5).
The structures of 14a×
/
Li2×
/
(Et2O)3 and the stereo-
isomer 13b of 13a were proven by X-ray structure
analyses. The structures of 15a and 13a follow from
the similarity of the chemical shifts of their skeletal
atoms with those of 13b and 15b. The structure of 15b
also was established by X-ray structure analysis [18].
The trans-configuration of 13a is evident from an AB-
multiplet for the protons of the methylene group as a
consequence of the chiral centers at C3 and C5.
The 11B-NMR spectrum of dianion 12a, which could
be observed only at temperatures below ꢂ
shown in Fig. 1. Three sharp signals at ꢂ12.1, ꢂ
and ꢂ 29.2 ppm for
23.9 ppm as compared to d11Bꢃ
/
40 8C, is
/
/15.3
/
/
ꢂ
/
12c, the trishomoaromatic structure of which is estab-
lished by an X-ray analysis [17], strongly hint at similar
structures for 12a and 12c. Deshielding of the boron
atoms of 12a bound to oxygen is to be expected as an
electronegativity effect. Additional support for the
trishomoaromatic nature of 12a was obtained by ab
initio computations of the chemical shifts of model 12b.
Scheme 4.
bonds. This is indicated by their 11B NMR chemical
shifts (92ꢀ96 ppm, characteristic of tricoordinate boron
atoms) and is proven by the X-ray analysis of 1-phenyl-
3,4-dimethyl-3-borolene [14b]. Only bicyclic systems,
which suffer from ring strain in classical conformations,
favor bishomoaromatic structures [16]; these have
The results obtained at the GIAO-SCF/6-31ꢁ
B3LYP/6-31ꢁG* and GIAO-B3LYP/6-31ꢁ
B3LYP/6-31ꢁ
/
G*//
/
/
/
G*//
/
G* levels shown in Fig. 2 are in convin-
cing agreement with the experimental data.
chemical shifts (ꢂ
/
7 to ꢂ9 ppm) characteristic of
/
pentacoordinate boron atoms.
3. Structures
2.3. A trishomoaromatic dianion with an oxygen
homobridge
Compounds 2aꢀ
matic ring systems with heteroatoms bridging two boron
centers. All show folded four-membered rings with B1Ã
C1ÃB2ÃN1 or B1ÃC1ÃB2ÃP torsion angles between
26.2(2) and 19.2(3)8 (see Table 2). The C1ÃB1 and C1Ã
/
d (Figs. 3ꢀ
/
5) are the first homoaro-
/
The trishomoaromatic dianion [17] with an oxygen
homobridge, 12a, was generated by reduction of 1-oxa-
/
/
/
/
/
/
/
Scheme 5.

248
Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ256
/
Fig. 1. 11B-NMR spectrum of 12a in THF at ꢂ
60 8C.
/
Fig. 2. Experimental and computed (GIAO-SCF/6-31G*//B3LYP/6-
31ꢁG* in italics and GIAO-B3LYP/6-31ꢁG*//B3LYP/6-31ꢁ
G*) 11
chemical shifts of 12a and model 12b, respectively.
/
/
/
B
Fig. 5. Molecular structure of 2d×
omitted for clarity; selected bond lengths (pm) and angles (8)
completing Table 2. B1ÃC10 158.1(5), B2ÃC20 158.9(5), C2ÃSi B
189.1(4)ꢀ, P1ÃC B181.0(3)ꢀ, C1ÃB2ÃP1 92.3(2), C30ÃP1ÃC36
104.1(2) (average over both molecules, labels of molecule 1).
/Et2O in the crystal, H-atoms Et2O
/
/
/
/
/
/
/
/
/
/
/
/
B2 bond lengths are short due to additional bonding by
the 3c2e bond which also explains the short transan-
nular B1
/
Á Á Á
/
B2 distance in 2a (as in 2h [11]). The longer
B1/ B2 distances in 2c,d are the consequence of
Á Á Á
/
inherently longer bonds to phosphorus. The small angles
at N1 and P1 are characteristic for homobridges
(compare this feature in 2h). The B1 and B2 distances
to N1 and P1 indicate single bonds to the bridging
atoms. NBO analyses of the prototype molecules 2uN
and 2uP (type 2d,e molecules without any substituents)
show the presence of 3c2e bonds between C1, B1 and
B2, i.e. both are homoaromatic systems.
Fig. 3. Molecular structure of 2a in the crystal, H-atoms omitted for
clarity; selected bond lengths (pm) and angles (8) completing Table 2.
B3Ã
/
N2 142.3(2), C1Ã
/
B2Ã
/
N1 98.2(1), B1Ã
/
C1Ã
/
B2ÃB3 25.7(2).
/
The crystal structure of 5a (Fig. 6) shows a strongly
distorted five-membered ring with short transannular B2
Á Á ÁC1 (182.8(2) pm) and B2Á Á ÁB1 (198.2(3) pm) dis-
tances, small angles at the homobridges (B1ÃN1ÃB2:
77.6(1)8, C1ÃC2ÃB2: 71.3(1)8), and an interplanar angle
of 84.6(8)8 between the best plane through N1ÃB1ÃC1Ã
C2 and the plane N1ÃB2ÃC2. MP2/6-31ꢁG* computa-
/
/
/
/
/
/
/
/
/
/
/
/
/
/
tions for 5u gave transannular distances of 174.3 and
187.8 pm, angles at the homobridges of 73.4 and 69.08
and an interplanar angle of 85.18. For planar 5u* the
corresponding data are 242.4 and 250.7 pm and 104.6
and 104.38. The deformations in 5u (and 5a) as
compared to 5u* are characteristic of bishomoaromatic
five-membered rings [19]. Considerably longer transan-
Fig. 4. Molecular structure of 2c in the crystal, H-atoms omitted for
clarity; selected bond lengths (pm) and angles (8) completing Table 2.
B2Ã
Si3/Si4 224.5(1)/224.7(1), C1Ã
84.2(2).
/
P2 192.4(3), B1Ã
/
Cl1 179.3(3), P1Ã
/
Si1/Si2 226.3(1)/226.8(1), P2Ã
/
/
B2ÃP1 96.7(2), C1Ã
/
/
C2/B1ÃC1ÃB2
/
/

Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ
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249
Table 2
Selected bond lengths (pm), and angles (8) for 2a,c,d (experimental) and 2uN, 2uP (computed at the B3LYP/6-311ꢁ
/G** level)
2a
2c
2d
2uP
2uN
B1Ã
B2Ã
B1Ã
B2Ã
C1Ã
B1
Á Á Á
B1ÃC1Ã
B1ÃXÃB2
C1ÃB1Ã
B1ÃC1Ã
C1ÃB1Ã
/
C1
C1
X
148.9(2)
148.7(2)
159.6(2)
159.0(2) a
156.4(2)
189.5(2)
78.8(1)
73.0(1) a
98.3(1) a
26.2(2) a
147.1(2) a
146.0(4)
148.3(3)
192.7(3) b
200.9(3) b
150.3(3)
209.5(4)
90.8(2)
64.3(1) b
101.1(2) b
21.6(2) b
153.8(2) b
149.9(5)
150.4(5)
193.3(4) b
193.3(4) b
153.4(4)
226.4(5)
97.9(3)
71.7(2) b
92.5(2) b
19.2(3) b
154.9(3) b
148.0
148.0
146.2
146.2
/
a
/
195.9 b
195.9 b
158.3 a
158.3 a
/X
/
B3(C2)
B2
ꢀ/
226.6
ꢀ/
193.1
/
/
/
/
B2
99.9
70.7 b
82.6
74.6 a
/
/
/
/
X
/
/
B2Ã
B2Ã
/
X
/
/
/X
154.6 b
151.8 a
a
Xꢃ
/
N1.
P1.
b
Xꢃ
/
Fig. 7. Structure of 7b×
for clarity; selected bond lengths (pm) and angles (8): C1Ã
C1ÃB2 167.4(2), C1ÃB3 158.6(2), C1ÃC2 159.6(2), B1ÃB2 181.6(3),
B1ÃN1 138.6(2), B1ÃH1 131(2), B2ÃH1 136(2), B2ÃC2 160.3(3), B2Ã
C10 159.9(3), C2ÃSi1/Si2 188.9(2)/188.5(2), B3ÃN2 140.4(3); C1ÃB1Ã
B2 59.6(1), B1ÃC1ÃC2 106.1(1), B1ÃC1ÃB2 69.4(1), B1ÃB2ÃC2
92.9(1), B1ÃB2ÃC1 51.0(1), C2ÃC1ÃB2 58.7(1), C2ÃB2ÃC1 58.2(1),
C1ÃC2ÃB2 63.1(1), C1ÃB3ÃN2 123.2(2); B1ÃC1ÃB2ÃC2 125.3(1),
C1ÃB1ÃC2ÃB2 103.9(2), C1ÃB1ÃB2ÃH1 ꢂ153(1), B1ÃC1ÃB3ÃN2
37.0(3).
/
0.5C6H14 in the crystal, most H-atoms omitted
/
B1 150.8(2),
Fig. 6. Structure of 5a in the crystal, most H-atoms omitted for clarity;
/
/
/
/
selected bond lengths (pm) and angles (8): C1Ã
182.8(2), C1ÃC2 155.6(2), C1ÃSi1 186.6(2), B1ÃN1 156.9(2), B1
198.2(3), B1ÃC10 158.6(3), N1ÃB2 159.6(3), B2ÃC2 158.0(3), B2Ã
202.3(2), C2ÃB3 157.5(3), B3ÃN2 140.2(2), B3ÃC20 160.3(3); B1ÃC1Ã
B2 72.8(1), C1ÃB1ÃN1 106.9(2), B1ÃN1ÃB2 77.6(1), N1ÃB2ÃC1
91.1(1), C2ÃC1ÃB2 55.0(1), C1ÃB2ÃC2 53.7(1), C1ÃC2ÃB2 71.3(1);
B2ÃC1ÃB1ÃN1 ꢂ26.3(1), C1ÃB1ÃN1ÃB2 29.8(2), C1ÃB1ÃB2ÃN1
147.4(2), B1ÃC1ÃB2ÃC2 143.1(2), C1ÃB2ÃC2ÃB3 ꢂ133.6(2).
/
B1 147.9(3), C1Ã
/
B2
/
/
/
/
/
/
/
/
/
Á Á Á
/B2
/
/
/
/
/
/
/
/Si2
/
/
/
/
/
/
/
/
/
/
/
/
/
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ꢂ/
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/
/
nular distances in 5a as compared to 5u can be explained
by the steric repulsion of the three large substituents at
the centers of the 3c2e bond in 5a.
B1 is involved in the 3c2e B1, C1, B2 bond (as well as the
B1, H, B2 bond) despite of its partial double bond to
N1. The sum of the angles around N1 is 360.08.
Compound 7b (Fig. 7) is the first nonclassical 1,2-
diboretane [13] with an amino substituent at only one of
the skeleton boron atoms. 1,2-Diboracyclobutanes with
amino substituents at both boron atoms, like 1,2-
diamino-1,2-diboracyclopentanes [20], have classical
The conformation of 13b is distinctly different from
that of 13c [23] with donor substituents at all boron
atoms and methyl instead of trimethylsilyl groups at the
ring carbon atoms (Fig. 8). While 13c is strongly folded
along the B1ꢀB2 axis, thus containing an almost planar
/
structures [21]. The transannular C1
/
Á Á Á
/
B2 distance of
five-membered ring, 13b is a flat chair: the C1, B1, B2,
O1 plane (maximum deviation from the best plane 0.8
pm) has angles of 24.3(2) and 10.5(2)8, respectively, with
7b, shorter (167.4(2) pm) than in 7c [13] (172.3(3) pm)
with two duryl substituents at the ring boron atoms,
indicates a rather strong 3c2e bond. B1Ã
pm in 7b) is slightly longer than in 7c (147.2(3) pm).
Surprisingly, the B1ÃN1 distance (138.6(2) pm, compare
the internal standard 140.4(3) pm for B3ÃN2) is
characteristic of partial BÃN double bonds [22]. Thus
/
C1 (150.8(2)
the B1, C2, B2 and C1, O, B3 planes. All ring CÃ
bonds in 13b are shorter than the corresponding bonds
in 13c. The bond distances of the silicon atoms bound to
/
B
/
/
the ring are distinctly longer than in the exocyclic H2CÃ
Si bond. All these findings can be explained by
/
/

250
Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ256
/
isoelectronic with the pyrylium cation, 14u is not
aromatic as indicated by its NICS value of 0.8 at the
center of the ring. This is reminiscent of the findings for
borazine and boroxine [25]. Differences in electronega-
tivity of the centers of cyclic delocalization are respon-
sible for the deviation from the Huckel rule. Differences
¨
in electronegativity also are the reason for the non-
homoaromatic nature of 10u despite being isoelectronic
with the strongly homoaromatic 5u, 9u and 11u. As is to
be expected: aromaticity and homoaromaticity are
governed by the same principles!
Fig. 8. Structure of 13b in the crystal, duryl residue at B2 and most H-
atoms omitted for clarity; selected bond lengths (pm) and angles (8):
B1Ã
C1Ã
B2Ã
C1Ã
B2Ã
C1
Á Á Á
/
C1 157.5(3), B1Ã
Si1 191.6(2), B3Ã
C2 156.0(2), B2ÃC20 159.3(2), C2Ã
B1ÃC2 118.0(1), B1ÃC1ÃB3 115.9(1), B3Ã
C2 120.2(1), B2ÃC2ÃB1 112.6(1); B3ÃC1ÃB1Ã
O1ÃB3 ꢂ169.0(2), C1ÃB1Á Á ÁB2ÃC2 ꢂ155.2(2).
/
C2 158.2(2), B1Ã
O1 138.7(2), B3ÃC30 1.583(3), O1Ã
Si2 190.9(4), C3ÃSi3 187.7(8);
O1ÃB2 127.1(1), O1Ã
C2 ꢂ21.6(2), B1Ã
/
C3 157.6(2), C1Ã
/
B3 157.3(2),
/
/
/
/
B2 138.6(2),
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
4. Conclusions
/
/
/
/
/
/
/
/
/
The properties of two-electron homoaromatics with
heteroatom bridges are similar to those of the corre-
sponding species with methylene bridges. The realiza-
tion of the first bishomoaromatic with a delocalized
bond between two boron and one carbon center (B2C)
completes the 3c2e boronꢀcarbon series. The other
/
members of this set have C3, BC2, and B3 centers,
although some of the known examples involve bicyclic
systems. Computations on prototype models show that
the homoaromatic stabilization energies decrease with
the number of carbon centers due to the higher
electronegativity of carbon than boron. The high
electronegativity of oxygen precludes effective cyclic
electron delocalization (aromaticity) in 1-oxa-2,4,6-tri-
boracyclohexane-diide, the isoster of the aromatic
pyrylium cation.
Fig. 9. Structure of 14a×
residues omitted for clarity; selected bond lengths (pm) and angles (8):
B1ÃO1 144.1(8), B1ÃC1 150(1), B1ÃC20 159.5(8), C1ÃB3 155.1(8),
C1ÃSi1 185.6(6), B3ÃC2 155.7(9), B3ÃC3 161.0(8), C2ÃB2 151.4(9),
C2ÃSi3 184.6(6), B2ÃO1 146.2(7), B2ÃC30 158(1), Li1ÃO1 196(1),
Li1ÃO2 196(1), Li1ÃO3 195(1), Li2ÃO4 193(1), Li2Á Á ÁC1 222(1), Li2Á Á Á
B3 230(1), Li2 C2 221(1), Li2Á Á ÁB1 265(1), Li2Á Á ÁB2 266(1), Li2Á Á ÁO1
Á Á Á
285(1); O1ÃB1ÃC1 121.4(5), B1ÃC1ÃB3 114.0(5), C1ÃB3Ã
116.6(5), B3ÃC2ÃB2 117.1(5), C2ÃB2ÃO1 117.4(6); B2ÃO1ÃB1Ã
5.0(8), O1ÃB1ÃC1ÃB3 ꢂ15.9(8), B1ÃC1ÃB3ÃC2 34.2(7), B1Ã
C2ÃB3 ꢂ149.1(7), B3ÃC2ÃB2ÃO1 12.5(8).
/
Li2×
/
(Et2O)3 in the crystal, H-atoms and duryl
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
C2
C1
/
/
/
/
/
/
/
ꢂ/
/
/
/
/
/
/
/
/
C1
/
Á Á Á
/
5. Experimental
/
/
/
/
/
hyperconjugation of ring Ã
boring boron atoms.
The salt 14a×Li2×(Et2O)3 is a contact triple ion (Fig.
/
CÃ
/
Si bonds with two neigh-
5.1. General
/
/
Reactions were carried out under dry argon or
nitrogen, using standard Schlenk techniques. Solvents
were dried, distilled, and saturated with nitrogen.
Glassware was dried with a heat-gun in high vacuum.
1H, 13C-NMR: Bruker DRX 200 spectrometer and
Bruker AC 500, 11B-NMR: Bruker DRX 200 spectro-
9). Li1 is coordinated to two ether molecules and to O1,
Li2 to the remaining atoms of the central ring and to
one Et2O. While the Li2Ã
(221(1) pm) and Li2ÃB3 (230(1) pm) distances are
comparable to those in aromatic diboratabenzene dia-
nions [16b,24], Li2ÃB1 and Li2ÃB2 (265(1) and 266(1)
/
C1 (222(1) pm), Li2Ã
/C2
/
/
/
meter, NMR references are (CH3)4Si and BF3×
/Et2O.
pm) are considerably longer. C1, B1, O1, B2, C2 of the
Mass spectra were obtained with a ZAB-2F VH Micro-
mass CTD spectrometer, high resolution mass spectra
with a Joel MS Station JMS-700 spectrometer. Melting
central ring lie almost in one plane, but the folding along
the C1ꢀC2 axis is 30.1(7)8. This reduces steric hindrance
/
between the substituent at B3 and the trimethylsilyl
substituents at C1 and C2.
points (uncorrected) were measured with a Buchi
¨
Computations of unsubstituted 14u without counter
ions lead to a planar six-membered ring. Despite being
apparatus using capillaries, which were filled under
argon or nitrogen and sealed.

Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ
/256
251
5.2. 3-{Dimethylamino-
5.4. 3-(2,2-Dimethylpropyl)-1-
bis(trimethylsilyl)phosphonium-2-chloro-4-
bis(trimethylsilyl)phosphyl-2,4-diboretane-3-ide (2c)
[bis(trimethylsilyl)(dimethylamino-chloro-
boryl)methyl]boryl}-1-bis(trimethylsilyl)azonium-2,4-
bis(t-butyl)-2,4-diboretane-3-ide (2a)
A mixture of 1.32 g (4.02 mmol) of 3 and 4.20 g (16.76
mmol) of P(SiMe3)3 was heated for 2.5 h at 130 8C and
To a solution of 200 mg (0.43 mmol) of 1 in 10 ml of
hexane 0.8 ml of dimethylamine were added at ꢂ
/
30 8C.
then the volatiles were removed in vacuum. The redꢀ
/
The mixture was allowed to warm to room temperature
(r.t.) and was refluxed for 3 h. After filtration of
insoluble salts the solvent was reduced and the solution
brown residue was distilled at 90ꢀ100 8C/0.02 mbar to
/
give an orange oil, containing 2c and other products. A
solution of the orange oil in CH2Cl2 was kept for several
days at 4 8C to give crystals of yellow 2c (70 mg, 4%)
and traces of colourless [(Me3Si)2PBCl2]2 [26]. 1H-NMR
cooled to ꢂ
145ꢀ147 8C (dec.). Yield: 144 mg (68 %) 2a. H-NMR
(200 MHz, [d8]-toluene, 313 K) dꢃ0.41 (s, 18H, SiMe3),
/
80 8C to yield colourless crystals, m.p.:
1
/
/
(200 MHz, [d8]-toluene, 313 K), dꢃ0.26 (s, 9H, SiMe3),
/
1.04 (s, 18H, CMe3, 2.48 (s, 3H, NMe), 2.58 (s, 3H,
NMe), 2.70 (s, 6H, NMe), 2.73 (s, 3H, NMe), 2.90 (s,
0.29 (s, 9H, SiMe3), 0.38 (s, 9H, SiMe3), 0.42 (s, 9H, Si
Me3), 0.84 (s, 9H, CMe3), 2.02 (s, 2H, CH2); 11B-NMR
3H, NMe), 11B-NMR (64 MHz, CDCl3) dꢃ
/
43, 37 (1:3),
13C-NMR (50 MHz, CDCl3) dꢃ
SiMe3), 6.8 (q, SiMe3), 7.3 (q, SiMe3), 8 (br. s, CB3), 22
dꢃ
/
39, EI-MS: (m/z, %), 494 (Mꢁ, 1), 458 (Mꢁ Ã
/
HCl,
/2.0 (q, SiMe3), 4.9 (q,
B
/
1), 437 (Mꢁ Ã
/
C4H9, 3), 421 (Mꢁ Ã
/
SiMe3, 1), 365
SiMe, 1), 73 (SiMe3ꢁ, 100), 57 (C4H9ꢁ, 6).
(Mꢁ Ã
/
C4H8, Ã
/
12
(br. s, CMe3), 30.0 (s, CMe3), 30.6 (s, CMe3), 30.8 (s,
¯
HR-MS(EI): m/z 494.2094 Mꢁ, Calc.
C
18
1H47 11B2
CMe3), 32.2 (s, CMe3), 37 (br. s, CB2), 41.3 (q, NMe),
42.7 (q, NMe), 44.4 (q, Me), 44.6 (q, Me), 45.0 (q,
35Cl 31P28Si4: 494.2104, (Dꢃ
/
1.0 mmu).
NMe2), 58.1 (q, NMe2), CI-MS m/z (%): 495 (23) [Mꢁ],
5.5. 2,4-Bis(2?,3?,5?,6?-tetramethylphenyl)-3-
bis(trimethylsilyl)methyl-1,1-diphenyl-1-phosphonium-
2,4-diboretane-3-ide (2d)
460 (100) [Mꢁ Ã
/
Cl], HR-MS (EI): m/z 438.3225 [Mꢁ Ã
/
CCH3], Calc. 12C18 H45 11B4 35Cl1 14N3 28Si2: 438.3213,
(Dꢃ 1.2mmu).
1
/
A solution of 0.47 g (2.14 mmol) chlorodiphenylpho-
sphane in 10 ml pentane was added to a cooled
5.3. 3-{Pyrrolidino-[bis(trimethylsilyl)(pyrrolidino-
chloro-boryl)methyl]boryl}-1-
bis(trimethylsilyl)azonium-2,4-bis(t-butyl)-2,4-
diboretane-3-ide (2b)
(ꢂ68 8C) suspension of 1.0 g (2.14 mmol) of the
/
diboriranide 4a in 30 ml Et2O. After warming to r.t.
and 2 h stirring, all volatiles were removed, the residue
digested with 40 ml of pentane, the salt separated by a
D4 revised frit and washed two times with 10 ml
pentane. Removal of the solvent in vacuum yielded
light yellow 2d in NMR-spectroscopic purity. After
Tree hundred and three milligram (4.26 mmol) of
pyrrolidine were added to a solution of 250 mg (0.53
mmol) of 1 in 15 ml of hexane at ꢂ
/
50 8C. The mixture
about 2 weeks in Et2O at ꢂ
m.p. 181ꢀ182, were obtained. Yield: 680 mg (53%) 2d.
1H-NMR (500 MHz, CDCl3, 25 8C): dꢃ
0.03 (s, 18H,
Si(CH3)3), 2.10 (s, 1H, CH), 2.17, 2.27 (each s, each
12H, o- and m-CH3), 7.02 (s, 2H, p-Dur), 7.33ꢀ7.36,
7.40ꢀ7.43, 7.51ꢀ7.57 (m, altogether 10H, HÃ
Ph), 13C-
NMR (125 MHz, CDCl3, 25 8C): dꢃ1.4 (q, 6C,
Si(CH3)3), 19.8, 21.4 (each q, each 4C, o- and m-CH3),
30.1 (d, 1C, HCSi), 128.3 (d, 4C, J(13CÃ31P)ꢃ
10 Hz,
m-Ph), 129.6 (d, 2C, J(13CÃ31P)ꢃ
7 Hz, p-Ph), 130.6 (s,
2C, p-C of Dur), 133.0 (d, 2C, J(13CÃ31P)ꢃ
42 Hz, i-C
of Ph), 133.1 (d, 4C, J(13CÃ31P)ꢃ
9 Hz, o-C of Ph),
134.4 (d, 2C, J(13CÃ31P)ꢃ
10 Hz, o-C of Dur), 141.0
(br., 2C, i-C of Dur), 13C {11B(44 ppm)}-NMR (125
MHz, CDCl3, 25 8C): dꢃ 79
179.0 (d, 1C, J(13CÃ31P)ꢃ
Hz, CB2), 11B-NMR (96 MHz, CDCl3, 25 8C): dꢃ
44.
/
35 8C colourless crystals,
was allowed to warm to r.t. and then was refluxed over
night. After filtration of insoluble salts the solvent and
volatile compounds were removed in vacuum and an
/
/
orange solid was crystallized from toluene. Yield: 210
1
mg (58 %), 2b. H-NMR (200 MHz, [d8]-toluene) dꢃ
/
/
/
/
/
0.23 (s, SiMe3), 0.29 (s, SiMe3), 0.37 (s, SiMe3), 0.59 (s,
SiMe3), 1.09 (s, CMe3), 1.10 (s, CMe3), 1.19 (s, CMe3),
/
1.56 (m, NCH2), 2.96ꢀ
(64 MHz, C6D6) dꢃ
44, 35 (3:1), 13C-NMR (50 MHz,
[d8]-toluene) dꢃ3.7 (q, SiMe3), 5.1 (q, SiMe3), 6.5 (q,
/
3.74 (m, NCH2CH2), 11B-NMR
/
/
/
/
/
/
/
/
SiMe3), 6.6 (q, SiMe3), 24.2 (t, NCH2), 24.7 (t, NCH2),
25.2 (t, NCH2), 25.7 (t, NCH2), 25.8 (t, NCH2), 26.2 (t,
NCH2), 26.9 (t, NCH2), 27.1 (t, NCH2), 27.2 (t, NCH2),
28.0 (t, NCH2), 29.7 (q, CMe3), 30.4 (q, CMe3), 31.1 (q,
CMe3), 48.9 (t, NCH2CH2), 49.8 (t, NCH2CH2), 50.1 (t,
NCH2CH2), 50.3 (t, NCH2CH2), 50.4 (t, NCH2CH2),
50.9 (t, NCH2CH2), 51.1 (t, NCH2CH2), 52.4 (t,
NCH2CH2), 68.5 (t, NCH2CH2), CB not observed,-
/
/
/
/
/
/
/
/
5.6. 3-Bis(trimethylsilyl)methyl-2,4-di-t-butyl-1,1-
diphenyl-1-phosphonium-2,4-diboretane-3-ide (2e)
CI-MS m/z (%): 574 (1) [Mꢁ],-HR-MS (EI): m/z
516.3702 [Mꢁ Ã
/
C(CH3)3], Calc.
C
24
2.0mmu).
1H51 11B4 35Cl1
In analogy to the procedure for 2d, from 1.34 g (6.06
mmol) of chlordiphenylphosphane and 2.0 g (6.06
12
14N3 28Si2: 516.3722 (Dꢃ
/

252
Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ256
/
mmol) of 4b, colourless crystals, m.p. 116 8C, of 2e were
obtained from pentane at ꢂ30 8C. Yield: 2.15 g (72%).
2e: 1H-NMR (CDCl3, 25 8C): dꢃ
0.07 (s, 18H,
Si(CH3)3), 1.07, 1.13 (each s, each 9H, C(CH3)3), 2.38
(s, 1H, HCSi2, 1J(1HÃ31P)ꢃ
2.7 Hz), 7.34ꢀ7.66 (m,
10H, HÃ 40 8C):
Ph), 13C-NMR (100 MHz, CDCl3, ꢂ
dꢃ1.0 (q, 6C, Si(CH3)3), 23.4 (br. s, 1C, C(CH3)3,
2J(13CÃ
2J(13CÃ
3J(13CÃ
3J(13CÃ
3J(13CÃ
3J(13CÃ
2C, i-C, 1J(13CÃ
NMR ([d10]-DME, 25 8C): dꢃ
/
38, 31P-NMR ([d10]-
/
DME, 25 8C): dꢃ 137.4.
/
ꢂ
/
/
5.9. 2,5-Bis(trimethylsilyl)-1,1-dimethyl-2-(2?,3?,5?,6?-
tetramethylphenyl)-4-{[(2?,3?,5?,6?-
tetramethylphenyl)dimethylamino]boryl}-(deloc-2,3,5)-
/
/
/
/
/
/
1-azonium-2,5-diborolane-3-ide (5a)
/
31P)ꢃ
31P)ꢃ
31P)ꢃ
31P)ꢃ
31P)ꢃ
31P)ꢃ
/
8.5 Hz), 24.1 (br. s, 1C, C(CH3)3,
12.2 Hz), 28.3 (d, 1C, HCSi2,
46.5 Hz), 30.7 (q, 3C, C(CH3)3,
4.9 Hz), 31.0 (q, 3C, C(CH3)3,
5.2 Hz), 128.3 (d, 4C, m-C,
10.5 Hz), 129.1 (d, 2C, p-C), 131.9 (s,
/
/
1.06 g (10.6 mmol) bis(dimethylamino)borane were
added to a solution of 3.38 g (7.34 mmol) of compound
/
/
/
/
6 in 150 ml pentane at ꢂ
and 16 h stirring, the volume was reduced to 10 ml and
cooled to ꢂ30 8C yielding colourless crystals, m.p.
148 8C. Yield: 2.8 g (70%) 7b: 1H-NMR (500 MHz,
CDCl3, ꢂ40 8C): dꢃ 0.22, ꢂ0.10 (each s, each 9H,
/
30 8C. After warming to r.t.
/
/
/
/
/
/
31P)ꢃ
49.6 Hz), 133.4 (d, 4C, o-C,
10.4 Hz), 171.2 (br., s, 1C, CB2,
/
2J(13CÃ
2J(13CÃ
dꢃ
/
31P)ꢃ
31P)ꢃ
/
/
/
ꢂ
/
/
/
/
82.1 Hz), 11B-NMR (CDCl3, 25 8C):
Si(CH3)3), 2.27, 2.28, 2.30, 2.32, 2.47 (each s, altogether
24H, o- and m-CH3), 2.35, 2.76, 2.81, 3.27 (each s, each
3H, NMe2), 6.96, 7.02 (each s, each 1H, p-H), 13C-NMR
/
47, 31P-NMR (CDCl3, 25 8C): dꢃ
/
ꢂ39.6.
/
5.7. 3-Bis(trimethylsilyl)methyl-2,4-di-t-butyl-1,1-
diphenyl-1-arsonium-2,4-diboretane-3-ide (2f)
(125 MHz, CDCl3, ꢂ
/
40 8C): dꢃ0.2, 0.5 (each q, each
/
3C, Si(CH3)3), 20.0, 20.1, 20.2, 20.4, 20.5, 20.6, 22.9
(each q, altogether 8C, o- and m-CH3), 32.1 (br. d, 1C,
Analogously to the procedure for 2e, employing 1.33
g (5.0 mmol) chlorodiphenylarsane and 1.65g (5.0
mmol) 4b gave light yellow crystals, m.p. 95 8C (dec.)
B2CH, Jꢃ115 Hz), 39.2, 41.9, 44.6, 44.7 (each q, each
/
1C, NMe2), 75.4 (br. s, 1C, CSiMe3), 129.0, 130.3 (each
d, each 1C, p-C), 132.2, 132.5, 132.6, 132.8, 132.9, 133.9,
136.2, 136.5 (each s, each 1C, o- and m-C), 138.4, 147.1
(each br. s, each 1C, i-C), 11B-NMR (96 MHz, CDCl3,
1
of 2f from 10 ml pentane. Yield: 1.77 g (66.1%). 2f: H-
NMR (CDCl3, 25 8C): dꢃ
1.10 (each s, each 9H, C(CH3)3), 2.16 (s, 1H, HCSi2),
7.33ꢀ7.56 (m, 10H, HÃ
Ph),13C-NMR (100 MHz,
CDCl3, ꢂ35 8C): dꢃ0.8 (q, 6C, Si(CH3)3), 24.1, 24.7
(each s, each 1C, C(CH3)3), 26.1 (d, 1C, HCSi2,
1H)ꢃ
102.4 Hz), 30.4 (q, 6C, C(CH3)3, 128.6
/
0.07 (s, 18H, Si(CH3)3), 1.07,
25 8C): dꢃ
/
ꢂ7, 30, 43.
/
/
/
/
/
5.10. 4,4-Bis(trimethylsilyl)-1-(2?,3?,5?,6?-
1J(13CÃ
/
/
tetramethylphenyl)-2-diethylamino-3-{[(2?,3?,5?,6?-
tetramethylphenyl)dimethylamino]boryl}-3-dehydro-1,2-
mH-(deloc-1,2,3)-1,2-diboretane (7b)
(d, 2C, p-C), 128.7 (d, 4C, m-C), 133.0 (d, 4C, o-C),
134.0 (s, 2C, i-C), 156.9 (br., s, 1C, CB2), 11B-NMR
(CDCl3, 25 8C): dꢃ54.
/
1.98 g (12.69 mmol) bis(diethylamino)borane were
added to a solution of 5.82 g (12.69 mmol) of compound
6 in 200 ml pentane at r.t. After 7 h stirring, all volatiles
were removed and the residue crystallized from n-
5.8. 3-Bis(trimethylsilyl)methyl-2,4-di-t-butyl-1-phenyl-
1-phospha-2,4-diboretane-3-ide (2g)
0.80 g (1.63 mmol) of compound 2e dissolved in 8 ml
THF were added to a suspension of 0.12 g (17.3 mmol)
Li-powder in 15 ml of THF. The reaction mixture slowly
turned dark red. After 4 h, excess Li metal was separated
by a reversed frit and the filtrate evaporated in vacuum.
hexane at ꢂ
(dec.). Yield: 4.1g (53%) 5a: 1H-NMR (500 MHz,
CDCl3, ꢂ10 8C): dꢃ 0.47, 0.07 (each s, each 9H,
Si(CH3)3), 0.95, 0.96, 1.19, 1.22 (each t, each 3H,
NCH2CH3) 2.21, 2.23, 2.24, 2.27, 2.29, 2.39, 2.40, 2.54
/
30 8C. Colourless crystals, m.p. 120 8C
/
/
ꢂ
/
1
Orange solid, yield 0.61g ( 87.4%). 2g: H-NMR ([d10]-
(each s, altogether 24H, o- and m-CH3), 2.95ꢀ
3.16ꢀ3.26, 3.82ꢀ3.87, 4.06ꢀ4.10 (each m, altogether 8H,
NCH2Me), 3.25 (s, 1H, BHB), 6.94, 6.97 (each s, each
1H, p-H), 13C-NMR (125 MHz, CDCl3, ꢂ
10 8C): dꢃ
/3.10,
DME, 25 8C): dꢃ
Si(CH3)3), 0.83, 0.90 (each s, each 9H, C(CH3)3), 2.11
(s, 1H, HCSi2, 1J(1HÃ13C)ꢃ
104.2 Hz), 6.72, 7.30 (each
m, altogether 5H, HÃ
Ph), 13C-NMR (([d10]-DME,
25 8C): dꢃ1.2, 2.8 (each q, each 3C, Si(CH3)3), 25.3
(dd, 1C, HCSi2, J(13CÃ
104.1 Hz), 32.9, 33.9 (each dq, each 3C, C(CH3)3,
3J(13CÃ31P)ꢃ
2.7 Hz), 123.1 (dd, 1C, p-C,
1J(13CÃ1H)ꢃ
156.7 Hz), 125.8 (dd, 2C, m-C,
3J(13CÃ31P)ꢃ3.8 Hz, 1J(13CÃ1H)ꢃ
157.4 Hz), 136.3
(dd, 2C, o-C, 2J(13CÃ31P)ꢃ14.9 Hz, 1J(13CÃ1H), 156.2
Hz), 155.4 (d, 1C, i-C, 1J(13CÃ31P)ꢃ64.8 Hz), 11B-
/
ꢂ
/
0.15, 0.10 (each s, each 9H,
/
/
/
/
/
/
/
/
2.4, 3.2 (each q, each 3C, Si(CH3)3), 13.6, 13.7, 14.5, 15.1
(each q, each 1C, NCH2CH3), 14.8 (br. s, 1C, CSi2),
20.0, 20.2, 20.6, 20.7, 20.7, 20.8, 22.8 (each q, altogether
8C, o- and m-CH3), 35.8 (br. s, 1C, B2C), 39.7, 41.1,
42.3, 45.5 (each t, each 1C, NCH2Me), 129.7, 130.5
(each d, each 1C, p-C), 132.2, 132.7, 132.9, 134.2, 135.2,
137.1, 139.4 (each s, each 1C, o- and m-C), 135.9, 147.3
(each br. s, each 1C, i-C), 11B-NMR (96 MHz, CDCl3,
/
3
1
/
31P)ꢃ
/
17.8 Hz, J(13CÃ
/
1H)ꢃ
/
/
/
/
/
/
/
/
/
/
/
/
/
/
25 8C): dꢃ
/
ꢂ6, 38 (2B).
/

Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ
/256
253
5.11. Generation of the trishomoaromatic trans-2,5-
bis(trimethylsilyl)-2,6-bis(2?,3?,5?,6?-tetramethylphenyl)-
4-trimethylsilylmethyl-1-oxa-2,4,6-triboracyclohexane-
(deloc-2,4,6)-dianion (12a)
18.0 (br., 1C, CH2B), 19.4, 20.1, 21.1, 21.7 (each 2C, o-
and m-CH3), 130.3, 132.6, 132.9, 134.2, 136.4 (10C, o-,
m- and p-C), 150.0 (br., 2C, i-C), the signal for the
skeletal carbon atoms could not be observed under these
conditions. 11B-NMR (96 MHz, Et2O, 25 8C): dꢃ
42,
/
6.6 mmol lithiumnaphtalinide (0.44 ml of a 1.57 molar
61.
THF solution) was added to a precooled (ꢂ78 8C)
/
solution of 0.19 g (3.3 mmol) trans-13a in 15 ml Et2O.
The progress of the reaction was monitored by 11B-
5.14. 2,5-Bis(trimethylsilyl)methyl-2,6-bis(2?,3?,5?,6?-
tetramethylphenyl)-4-trimethylsilylmethyl-1-oxa-2,4,6-
triborabicyclo[3.1.0]hexane (15a)
NMR spectroscopy at ꢂ
peaks at 73 and 51 ppm of 13a were replaced by three
sharp peaks at ꢂ12, ꢂ15 and ꢂ24 ppm (intensity ratio
1:1:1) for 12a. These signals disappeared rapidly upon
warming to ꢂ40 8C.
/
60 8C. After 0.5 h the broad
/
/
/
5.3 ml (4.45 mmol) of Me3SiCH2Li (0.84 M Et2O
solution) were added to a solution of 2.32 g (4.45 mmol)
of compound 15b in 50 ml pentane dropwise. After
stirring over night, all volatiles were removed under high
vacuum. The residue was digested with 20 ml pentane,
LiCl separated by a reversed frit, washed two times with
10 ml pentane. The volume of the combined filtrates was
/
5.12. trans-2,5-Bis(trimethylsilyl)methyl-2,6-
bis(2?,3?,5?,6?-tetramethylphenyl)-4-trimethylsilylmethyl-
1-oxa-2,4,6-triboracyclohexane (13a)
reduced to 10 ml and cooled to ꢂ
/
35 8C. Colourless
crystals. Yield: 1.67 g (72%). 15a: H-NMR (300 MHz,
CDCl3, 25 8C): dꢃ0.11 (s, 18H, SiMe3), 0.23 (s, 9H,
SiMe3) 1.48 (s, 2H, CH2), 2.17, 2.19, 2.30 (each s,
altogether 24H, o- and m-CH3), 6.92 (s, 2H, p-H), 13C-
1
3.7 ml (10 mmol) of HCl in Et2O were added dropwise
to a suspension of 3.0 g (5.11 mmol) of 14a×
in 30 ml Et2O precooled to ꢂ50 8C. After 3 h stirring,
the solvent was removed under high vacuum at ꢂ10 8C,
/
Li2×
/
(Et2O)3
/
/
/
the residue digested with ca. 20 ml pentane, LiCl
separated by a D3 reversed frit and washed two times
with 10 ml pentane. The combined filtrates were
concentrated under high vacuum and cooled to
NMR (75 MHz, CDCl3, 25 8C): dꢃ1.8 (3C, SiMe3), 2.1
/
(6C, SiMe3), 13.9 (br., 1C, CH2B), 19.4, 19.5, 20.4, 21.2
(each 2C, o- and m-CH3), 46.7 (br., 2C, C3,5) 131.3,
132.6, 133.8, 134.4 (10C, o-, m- a. p-C), 140.3 (br., 2C, i-
ꢂ
/
35 8C. Colourless crystals, yield 1.90 g (65%). trans-
C), 11B-NMR (96 MHz, CDCl3, 25 8C): dꢃ
/72, 59 (2B).
1
13a: H-NMR (300 MHz, CDCl3, 25 8C): dꢃ0.00 (s,
18H, (SiMe3)2), 0.19 (s, 9H, SiMe3), 0.83, 1.24 (each d,
each 1H, CH2, J(HH)ꢃ
each 12H, o- and m-CH3), 2.42 (s, 2H, B2CH), 6.96 (s,
2H, p-H), 13C-NMR (75 MHz, CDCl3, 25 8C): dꢃ
2.5
/
5.15. X-ray structure determinations of 2a, 2c, 2d×
/Et2O,
1
/12.0 Hz), 2.21, 2.32 (each s,
5a, 7b×0.5C6H14, 13b, 14a×Li2×(Et2O)3
/
/
/
/
Crystal data and details of the structure determina-
tions are listed in Tables 2 and 3. The structures were
solved by direct methods (SHELXS-86 or -97) [27] and
refined by least-squares methods based on F2 against all
(3C, SiMe3), 2.6 (6C, SiMe3) 20.2, 20.6 (each 4C, o- and
m-CH3), 22.8 (1C, CH2), 47.2 (br., 2C, B2CH,
1J(CH)ꢃ
/
103 Hz), 132.3, 133.5, 136.6 (10C, o-, m- a.
p-C), 141.0 (br., 2C, i-C), 11B-NMR (96 MHz, CDCl3,
25 8C): dꢃ73, 51(2B).
measured reflections (SHELXL-97 and SHELXTL NT5.1
)
/
[27]. For 2a and 2c a semiempirical absorption correc-
tion based on equivalent reflections was applied. All
non-hydrogen atoms were refined using anisotropic
5.13. 2,5-Bis(trimethylsilyl)methyl-2,6-bis(2?,3?,5?,6?-
tetramethylphenyl)-4-trimethylsilylmethyl-1-oxa-2,4,6-
triboracyclohexane-3,5-diide (14a)
displacement parameters. The structure of 2d×
/Et2O
shows two geometrically very similar independent
molecules and two ether molecules in the asymmetric
unit, the listed bond lengths and angles are average
0.2 g (28.8 mmol) Li dust were added to a stirred
solution of 4.0 g (6.98 mmol) 15a in 60 ml Et2O. The
progress of the reaction was monitored by 11B-NMR-
spectroscopy. After 2 h the signals of 15a at 72 and 57
ppm disappeared and two new peaks at 62 and 42 ppm
appeared. Excess Li was separated by a D3 reversed frit,
the volume of solvent reduced to 10 ml and the solution
values. In 7b×
/
0.5C6H14 one of the NEt2 groups is
disordered, the n-hexane solvent molecule is placed on
an inversion center and shows disorder or dynamical
behaviour as well. 14a×
/
Li2×/(Et2O)3 crystallized as a
(1 0 0) reflection twin. With a monoclinic angle close
to 908 the structure could be refined as pseudomerohe-
dral twin (refined twin ratio 28.2(1)%). The very large
asymmetric unit contains two independent moieties,
differing mainly in the orientation of the ether molecules
coordinated to the Li atoms. The geometrical para-
meters are listed for the first unit only because the
second one shows disorder of the ether ethyl residues.
cooled to ꢂ
(20%). 14a×Li2×
25 8C): dꢃ0.43 (s, 18H, SiMe3), 0.64 (s, 9H, SiMe3),
/
35 8C. Colourless crystals, yield 0.79 g
1
/
/
(Et2O)3: H-NMR (500 MHz, CDCl3,
/
1.58 (s, 2H, CH2), 2.10, 2.12, 2.33, 2.58 (each s, each 6H,
o- and m-CH3), 6.77 (s, 2H, p-H), 13C-NMR (125 MHz,
CDCl3, 25 8C): dꢃ3.3 (3C, SiMe3), 6.0 (6C, SiMe3),
/

Table 3
Crystallographic and experimental data for 2a, 2c, 2d×
/
Et2O, 5a, 7b×
/
0.5C6H14, 13b and 14a×
/
Li2×(Et2O)3
/
2a
2c
2d×Et2O
/
5a
7b×/0.5C6H14
13b
14a×
/
Li×(Et2O)3
/
Formula
Formula weight (g molꢂ1
Colour, habit
C22H54B4Cl1N3Si2
495.55
C18H47B2Cl1P2Si4 C44H65B2OPSi2
494.93 718.73
C32H57B3N2Si2
558.41
C39H72B3N2Si2
657.60
C32H57B3OSi3
574.48
C44H85B3Li2O4Si3
808.70
)
Colourless, irregu- Colourless, prism Colourless platelet Colourless, block Colourless, octahe- Colourless, irregu- Colourless, irregular, (100)
lar dral lar twin
0.56ꢄ 0.50ꢄ 0.65ꢄ 0.45 0.48ꢄ
Crystal size (mm3)
/
0.48ꢄ/0.40
0.25ꢄ
/
0.25ꢄ
/
0.10 0.55ꢄ
/
0.35ꢄ
/
0.05 0.40ꢄ
0.30
Triclinic
/
0.30ꢄ
/
/0.25ꢄ/0.25
/
0.50ꢄ
/
/
0.33ꢄ
/
0.24
Crystal system
Space group
Triclinic
¯
1 (No. 2)
Orthorhombic
Pbca (No. 61)
16.323(1)
Triclinic
¯
1 (No. 2)
Monoclinic
Triclinic
¯
P/
Monoclinic
¯
1 (No. 2)
P
/
P
/
P/
P21/c (No. 14)
9.639(1)
20.352(2)
1 (No. 2)
Ia (No. 9)
17.415(1)
18.202(1)
33.547(2)
˚
a (A)
10.0555(3)
10.8553(3)
16.2028(4)
106.395(2)
92.056(3)
113.043(2)
1539.72(7)
2
15.342(1)
18.647(2)
18.714(1)
69.26(1)
67.69(l)
68.96(1)
4473.2(6)
4
9.080(1)
11.338(1)
18.745(1)
77.46(1)
82.68(l)
72.99(1)
1797.1(3)
2
11.724(1)
12.194(1)
13.500(1)
94.31(1)
93.82(1)
107.87(1)
1823.5(3)
2
˚
b (A)
12.3956(8)
30.922(2)
˚
c (A)
21.574(2)
a (8)
b (8)
97.49(1)
90.53(1)
g (8)
3
V (A )
˚
6256.7(7)
8
4196.1(5)
4
1.041
10 634(1)
8
1.010
Z
Calculated density (Mg mꢂ3
/
)
1.069
0.217
1.051
0.382
173(1)
0.71073
1.067
0.145
1.032
0.120
190(1)
1.046
0.152
193(1)
Absorption m (mmꢂ1
)
0.112
193(1)
0.123
Temperature (K)
˚
173(1)
0.71073
193(1)
0.71073
193(1)
0.71073
IPDS (Stoe)
24.0
l(Moꢀ/Ka) (A)
0.71073
IPDS (Stoe)
25.9
0.71073
IPDS (Stoe)
26.0
0.71073
IPDS (Stoe)
25.9
Diffractometer
u max (8)
SMART (Bruker)
28.3
SMART (Bruker) IPDS (Stoe)
26.4 24.1
Reflections: total, unique, observed [ꢀ
4s(F)]
No. of parameters
/
20 361, 7454, 5847 38 585, 6408, 4629 36 494, 13 386,
6423
14 965, 6503,
3675
32 795, 8152, 5221
18 093, 6620, 4926 39 454, 16 332, 9463
505
432
938
374
444
385
1019
R (F ꢀ
/
4s(F))
wR2 (all reflections)
0.0387
0.1078
1.029
0.0393
0.1016
1.039
0.0475
0.1111
0.799
0.0362
0.0847
0.821
0.0420
0.1085
0.918
0.0404
0.1076
0.938
0.0550
0.1252
0.880
GOF, S
3
˚
Residual density (e Aꢂ
/
)
0.53, ꢂ
/
0.35
0.35, ꢂ
/0.32
0.62, ꢂ
/0.29
0.17, ꢂ
/0.18
0.23, ꢂ
/0.30
0.32, ꢂ
/
0.19
0.52, ꢂ0.23
/

Y. Sahin et al. / Journal of Organometallic Chemistry 680 (2003) 244ꢀ
/256
255
[9a] A. Ziegler, H. Pritzkow, W. Siebert, Eur. J. Inorg. Chem. (2001)
387.
[9b] M.J. Bayer, H. Pritzkow, W. Siebert, Eur. J. Inorg. Chem. (2002)
Due to the twinning and disorder problems, the preci-
sion of geometrical parameters is reduced for this
structure.
1293.
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6. Supplementary material
Crystallographic data (excluding structure factors) for
the structures reported in this paper have been deposited
with the Cambridge Crystallographic Data Center:
CCDC-204528 (2a), CCDC-204529 (2c), CCDC-
P.v.R. Schleyer, A. Berndt, Angew. Chem. 104 (1992) 1278;
(b) P. Willershausen, C. Kybart, N. Stamatis, W. Massa, M. Buhl,
¨
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204530 (2d×
(7b×0.5C6H14), CCDC-204533 (13b), CCDC-204534
(14a×Li2×(Et2O)3). Copies of the data can be obtained
free of charge and by application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (Fax: ꢁ44-1223/336-
/
Et2O), CCDC-204531 (5a), CCDC-204532
/
/
/
(b) D. Steiner, C. Balzereit, H.-J. Winkler, N. Stamatis, M.
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¨
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