Reactivity of the dimesityl-1,8-naphthalenediylborate anion: Isolation of the borataalkene isomer and synthesis of 1,8-diborylnaphthalenes

Article abstract of DOI:10.1039/b316505a

The anionic boron peri-bridged naphthalene derivative, namely dimesityl-1,8-naphthalenediylborate (1), undergoes a hydrolysis reaction to afford dimesityl-1-naphthylborane (2) whose structure has been determined. Upon standing at room temperature in toluene for an extended period of time, 1 undergoes a ring expansion reaction to afford 8,10,11a-trimethyl-7-mesityl-11aH- 7-boratabenzo[de]anthracene (3). As shown by its crystal structure, compound 3 constitutes a rare example of a borataalkene and features a carbon-boron double bond of 1.475(6) A incorporated in a conjugated hexa-1-boratatriene system. The reaction of 1 with 9-chloro-9-borafluorene and 5-bromo-10,11- dihydrodibenzo[b,f]borepin results in the formation of diboranes 4 and 5 which bear two different boryl moieties at the peri-positions of naphthalene. These diboranes have been characterized by multinuclear NMR spectroscopy and X-ray single crystal analysis. The boron center of the borafluorenyl moiety is π-coordinated to the ipso-corbon of a mesityl group with which it forms a contact of 2.730(3) A. The cyclic voltammogram of 2 in THF shows a quasi-reversible reduction wave at E_1/2 -2.41 V (vs. Fc/Fc ^-) corresponding to the formation of the radical anion. In the case of diboranes 4,5 and 1-(dimesitylboryl)-8-(diphenylboryl)naphthalene (6), two distinct waves are observed at E_1/2 -2.14 and -2.56 V for 4, E _1/2 -2.26 and -2.78 V for 5, and E_1/2 -2.41 and -2.84 V for 6. The first reduction wave most likely indicates the formation of a radical anion in which the unpaired electron is σ-delocalized over the two boron centers.

Full text of DOI:10.1039/b316505a

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Reactivity of the dimesityl-1,8-naphthalenediylborate anion:
isolation of the borataalkene isomer and synthesis of
1,8-diborylnaphthalenes
James D. Hoefelmeyer, Stéphane Solé and François P. Gabbaï*
Chemistry TAMU 3255, Texas A&M University, College Station, Texas 77843-3255, USA.
E-mail: francois@tamu.edu; Fax: 979-845-4719
Received 18th December 2003, Accepted 24th February 2004
First published as an Advance Article on the web 12th March 2004
The anionic boron peri-bridged naphthalene derivative, namely dimesityl-1,8-naphthalenediylborate (1), undergoes
a hydrolysis reaction to afford dimesityl-1-naphthylborane (2) whose structure has been determined. Upon standing
at room temperature in toluene for an extended period of time, 1 undergoes a ring expansion reaction to afford
8,10,11a-trimethyl-7-mesityl-11aH-7-boratabenzo[de]anthracene (3). As shown by its crystal structure, compound 3
constitutes a rare example of a borataalkene and features a carbon–boron double bond of 1.475(6) Å incorporated
in a conjugated hexa-1-boratatriene system. The reaction of 1 with 9-chloro-9-borafluorene and 5-bromo-10,11-
dihydrodibenzo[b,f]borepin results in the formation of diboranes 4 and 5 which bear two different boryl moieties at
the peri-positions of naphthalene. These diboranes have been characterized by multinuclear NMR spectroscopy and
X-ray single crystal analysis. The boron center of the borafluorenyl moiety is π-coordinated to the ipso-carbon of
a mesityl group with which it forms a contact of 2.730(3) Å. The cyclic voltammogram of 2 in THF shows a quasi-
reversible reduction wave at E1/2 Ϫ2.41 V (vs. Fc/Fcϩ) corresponding to the formation of the radical anion. In the
case of diboranes 4, 5 and 1-(dimesitylboryl)-8-(diphenylboryl)naphthalene (6), two distinct waves are observed at
E
1/2 Ϫ2.14 and Ϫ2.56 V for 4, E1/2 Ϫ2.26 and Ϫ2.78 V for 5, and E1/2 Ϫ2.41 and Ϫ2.84 V for 6. The first reduction
wave most likely indicates the formation of a radical anion in which the unpaired electron is σ-delocalized over the
two boron centers.
Introduction
Bidentate boranes constitute one of the simplest class of poly-
functional Lewis acids. Such species exhibit a rich chemistry
and are used as selective receptors for neutral1,2 and anionic
substrates3–5 as well as Lewis acid activators for various
organic6,7 and organometallic reactions.4,5 1,8-Diborylnaphth-
alene derivatives constitute one of the best studied classes of
such compounds.8,9 As demonstrated by previous investi-
gations, these bidentate Lewis acids can complex small anions
such fluoride and hydroxide.3 In addition, our group showed
that the one-electron reduction of 1,8-bis(diphenylboryl)-
naphthalene can lead to the generation of σ-delocalized
radicals.10 As part of our general interest in this chemistry, we
have recently described a rare example of a boron peri-bridged
naphthalene derivative,11 namely dimesityl-1,8-naphthalene-
diylborate (1).12 This strained cyclic borate undergoes ring
opening reactions in the presence of various electrophiles13 and
its reaction with diorganylboronhalides afford unsymmetrically
substituted 1,8-diborylnaphthalenes.12 In this contribution, we
report a series of results that further document the unique
reactivity of this anionic boron peri-bridged naphthalene
derivative.
Scheme 1 Reagents and conditions: (i) THF–H2O, 25 ЊC; (ii) toluene,
25 ЊC, days/tmeda, Ϫ30 ЊC; (iii) 9-chloro-9-borafluorene, Et2O,
Ϫ40 ЊC; (iv) 5-bromo-10,11-dihydrodibenzo[b,f]borepin, Et2O, 25 ЊC.
7
ing the aromatic core of each mesityl ligand (51.1 and 64.3Њ) are
significantly larger than that involving the plane containing the
naphthyl group (36.5Њ). Interestingly, while compound 1 is
indefinitely stable as a solid in a dry inert atmosphere, we
observed that suspensions of its lithium salt in toluene turn to a
red–orange solution after seven days. The same experiment
carried out in toluene-d8 indicated the clean conversion of 1
into a new product (3) which featured two 1H NMR singlets at
5.1 and 5.7 ppm characteristic of ethylenic CH groups. Inte-
gration of the NMR spectrum obtained after 7 days indicated
that 3 had formed in a 70–80% yield (non-isolated). The form-
ation of 3 is accompanied by formation of a small amount of
the hydrolysis product which is also readily detected. Addition
of tmeda to such a solution, followed by storing at Ϫ30 ЊC
overnight results in the crystallization of the new compound
(3-Li(tmeda)2ؒPhMe) in the form of deep purple needle like
Results and discussion
The salt 1-Li(THF)4 is soluble in polar solvents such as THF or
pyridine and partially soluble in aromatic solvents. In the pres-
ence of moisture, 1 undergoes a hydrolysis reaction to afford
dimesityl-1-naphthylborane (2), which has been previously
described (Scheme 1).13,14 In the course of our investigations, we
were able to obtain single crystals of 2 and have determined its
crystal structure (Fig. 1, Table 1). As expected, the carbon–
boron bond lengths of 2 are unremarkable and the boron
adopts a strictly trigonal planar geometry (Σangles = 359.9Њ).
Nevertheless, we note that, as a result of the steric crowding, the
dihedral angles formed between the plane containing the three
carbon atoms bound to the boron center and the plane contain-
D a l t o n T r a n s . , 2 0 0 4 , 1 2 5 4 – 1 2 5 8
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
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Table 1 Crystal data, data collection, and structure refinement for 2, 3-Li(tmeda)2, 4 and 5.
2
3-Li(tmeda)2ؒPhMe
4
5
Crystal data
Formula
Mr
Crystal size/mm
Crystal system
Space group
a/Å
C28H29B
376.32
0.21 × 0.18 × 0.13
Orthorhombic
Pna21
21.9952(19)
8.0123(7)
C47H68BLiN4
706.80
0.19 × 0.09 × 0.07
Monoclinic
Cc
18.007(2)
14.3779(19)
16.731(2)
97.703(3)
4292.7(10)
4
C40H36B2
538.31
0.18 × 0.17 × 0.13
Monoclinic
P21/n
15.8947(14)
10.8667(10)
17.9558(16)
101.358(2)
3040.6(5)
4
C42H40B2
566.36
0.49 × 0.35 × 0.30
Monoclinic
P21/c
16.0226(14)
11.1521(9)
18.2407(16)
97.448(2)
3231.9(5)
4
b/Å
c/Å
12.4897(11)
β/Њ
V/Å3
2201.1(3)
4
1.136
0.063
808
Z
Dc/g cmϪ3
µ(Mo-Kα)/mmϪ1
F(000)/e
1.094
0.062
1544
1.176
0.065
1144
1.164
0.065
1208
Data collection
T /K
Scan mode
hkl Range
Measured refl.
293(2)
ω
Ϫ26/24, Ϫ7/9, Ϫ14/14
10972
110(2)
ω
Ϫ21/13, Ϫ17/15, Ϫ16/19
11105
110(2)
ω
Ϫ18/18, Ϫ12/8, Ϫ20/20
14345
293(2)
ω
Ϫ19/20, Ϫ12/15, Ϫ24/13
19987
Unique refl. (Rint
Refl. used for refinement
)
3867 (0.0423)
3867
5499 (0.0540)
5499
4766 (0.0656)
4766
7472 (0.0455)
7472
Refinement
Refined parameters
R1,a wR2b [I > 2σ(I )]
ρfin (max./min.)/e ÅϪ3
263
478
380
397
0.0584, 0.1353
0.214/Ϫ0.180
0.0541, 0.1245
0.213/Ϫ0.201
0.0642, 0.1843
0.282/Ϫ0.180
0.0791, 0.1841
0.304/Ϫ0.212
a R1 = Σ(Fo Ϫ Fc)/ΣFo. b wR2 = {[Σw(Fo2 Ϫ Fc2)2]/[Σw(Fo )2]}1/2; w = 1/[σ2(Fo ) ϩ (ap)2 ϩ bp]; p = (Fo2 ϩ 2Fc2)/3; a = 0.07 (2), 0.067 (3), 0.1434 (4), 0.0800
(5); b = 0 (2), 0 (3), 0.1177 (4), 0 (5).
2
2
Fig. 2 ORTEP view of 3 in the crystal (30% ellipsoids). Selected bond
lengths (Å) and angles (Њ): B–C(21) 1.475(6), B–C(1) 1.570(6), B–C(11)
1.608(6), C(21)–C(22) 1.422(6), C(21)–C(26) 1.552(5), C(22)–C(23)
1.370(6), C(23)–C(24) 1.440(5), C(24)–C(25) 1.310(6), C(25)–C(26)
1.534(5); C(21)–B–C(1) 117.7(4), C(21)–B–C(11) 127.0(4), C(1)–B–
C(11) 115.3(4), C(22)–C(21)–B 126.4(4), C(22)–C(21)–C(26) 113.8(3),
B–C(21)–C(26) 119.8(4).
Fig. 1 ORTEP view of 2 in the crystal (30% ellipsoids). Selected bond
lengths (Å) and angles (Њ): B–C(1) 1.567(5), B–C(21) 1.587(5), B–C(11)
1.593(5); C(1)–B–C(21) 124.1(3), C(1)–B–C(11) 115.2(3), C(21)–B–
C(11) 120.6(3).
crystals (Scheme 1). While the high air-sensitivity of these
crystals has complicated their analysis, a single crystal X-ray
diffraction study (Table 1, Fig. 2) indicated the formation of
8,10,11a-trimethyl-7-mesityl-11aH-7-boratabenzo[de]anthra-
cene (3), a structural isomer of 1. The isomerization of 1
into 3 can be rationalized on the basis of a mechanism involv-
ing the heterolytic cleavage of one of the boron peri-carbon
bonds followed by nucleophilic attack at the ortho-carbon of
one of the mesityl groups (Scheme 2). Examination of the
structure of 3 indicates that the boron atom adopts a trigonal
planar geometry (Σangles = 360.0Њ) (Fig. 2). The resulting co-
ordination plane forms a dihedral angle of 9.9Њ with the naph-
thalene plane. More importantly, inspection of the bond dis-
tances indicates the existence of a conjugated hexa-1-bora-
tatriene fragment with localized double bonds at B–C(21)
(1.475(6) Å), C(22)–C(23) (1.370(6) Å) and C(24)–C(25)
(1.310(6) Å). It is important to point out that the borataalkenes
Scheme 2
are relatively rare.15–18 The length of the B–C(21) double bond
falls within the range previously established for such a linkage
(1.44 Å in [Mes2BCH2]Ϫ and 1.52 Å in [CH2C6H2(3,5-Me2)-
(4-B{2,4,6-Me3C6H2}2)]Ϫ).16,17 It is also shorter than the B᎐C
᎐
distances of 1.51–1.52 Å observed for coordinated [(C6F5)2-
BCH2]Ϫ ligands.19 The H NMR spectrum of 3-Li(tmeda)2 in
1
THF-d8 shows two distinct resonances for the aromatic CH
D a l t o n T r a n s . , 2 0 0 4 , 1 2 5 4 – 1 2 5 8
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groups of the intact mesityl group indicating that the latter is
not freely rotating. The two singlets corresponding to ethylenic
CH groups of the conjugated hexa-1-boratatriene fragment
appear at 4.7 and 5.7 ppm.
While we have previously studied the reaction of 1 with
dialkyl and diaryl boron halide fragments,12 it occurred to us
that the introduction of flat cyclic boryl moieties at the peri-
position of naphthalene could lead to reduced steric hindrance
and closer approach of the boron centers. With this in mind,
diboranes 4 and 5 were prepared by reaction of compound 1
with 9-chloro-9-borafluorene,20 and 5-bromo-10,11-dihydro-
dibenzo[b,f]borepin (Scheme 1). The latter reagent was
synthesized by reaction of 1,2-bis(2-trimethylsilylphenyl)-
ethane21 with BBr3. Diboranes 4 and 5 are soluble in chloro-
form as well as in more polar solvents such as pyridine and
acetonitrile. The 1H NMR spectra of 4 and 5 confirms the
attachment of the 9-borafluorenyl or 10,11-dihydrodibenzo-
[b,f]borepin moieties and exhibit six distinct resonances that
correspond to the aromatic CH groups of the unsymmetrically
substituted naphthalene backbone. In addition, the spectra
indicate the existence of rigid structures since four aryl protons
and six distinct methyl groups are observed for the mesityl sub-
stituent. The 11B NMR spectrum of 4 shows two resonances at
57 and 71 ppm confirming the presence of two different boron
centers. In the case of 5, the 11B resonances of the two boron
centers merge into a broad signal that is observed at 72 ppm.
Single crystals of both derivatives were readily obtained. The
structures of 4 and 5 are similar to those of previous diboranes
such as 1-(dimesitylboryl)-8-(diphenylboryl)naphthalene12 (6)
(Figs. 3 and 4, Table 1). As in other sterically hindered peri-
substituted naphthalene derivatives, the B(1)–C(1)–C(9) (127.8Њ
in 4, 129.4Њ in 5) and B(2)–C(8)–C(9) (130.6Њ in 4, 132.1Њ in 5)
angles substantially deviate from the ideal value of 120Њ. An
interesting feature in the structure of 4 concerns the presence of
an interaction that occurs between one of the boron mesityl
groups and the boron of the borafluorenyl unit. As shown by
the presence of a short B(2)–C(21) distance of 2.730(3) Å, this
interaction involves the η1-coordination of the ipso-carbon
(C21) of the mesityl group. As a result of this interaction, the
boron center, B(2) is slightly pyramidalized (Σangle = 355.7Њ).
Fig. 4 ORTEP view of 5 in the crystal (20% ellipsoids). Selected bond
lengths (Å) and angles (Њ): B(1)–C(1) 1.565(4), B(1)–C(21) 1.571(4),
B(1)–C(11) 1.590(4), B(2)–C(41) 1.554(4), B(2)–C(31) 1.585(4),
B(2)–C(8) 1.588(4), B(2)–C(21) 3.153(4); C(1)–B(1)–C(21) 122.9(3),
C(1)–B(1)–C(11) 117.2(2), C(21)–B(1)–C(11) 118.8(2), C(41)–B(2)–
C(31) 126.1(3), C(41)–B(2)–C(8) 115.2(2), C(31)–B(2)–C(8) 116.3(3),
C(9)–C(1)–B(1) 129.4(2), C(9)–C(8)–B(2) 132.1(3).
observed in the structures of 4 and 5 substantiate the increased
Lewis acidity of the B(2) boron center of 4. This increased
acidity results from the antiaromatic character of the 9-
borafluorenyl moiety, which favors coordination events that
removes the boron p-orbital from conjugation.22,23 The relief of
strain energy in the borole ring upon boron pyramidalization
also contributes to the increased acidity of the B(2) boron
center.23 Finally, we note that the boron–boron separation in 4
and 5 are, respectively, equal to 3.212 and 3.385 Å.
The cyclic voltammogram of 2 in THF is characteristic of a
triarylborane and shows a partially reversible reduction wave at
E
1/2 Ϫ2.41 V (vs. Fc/Fcϩ). It is followed by an irreversible event
at (Ep)red Ϫ3.36 V. As for other triarylboranes,24 the first reduc-
tion corresponds to the formation of the radical anion. In the
case of diboranes 4–6 (Fig. 5), a partially reversible wave is
observed at E1/2 Ϫ2.14 V for 4, E1/2 Ϫ2.26 V for 5 and E1/2 Ϫ2.41
V for 6. By analogy with some of our earlier work on related
diboranes,10 we suggest that this reduction corresponds to the
formation of the radical anion, which experiences some σ-
delocalization over the two boron centers (Scheme 3). Interest-
Interestingly, compound
5 assumes a similar molecular
structure with an apparent intramolecular contact occurring
between the boron center B(2) and the ipso-carbon (C21) of one
of the mesityl groups. In this case, the resulting B(2)–C(21)
distance is much longer (3.153 Å) indicating a weaker inter-
action, which nevertheless, results in a detectable pyramidaliz-
ation of the B(2) boron center (Σangle = 357.6Њ). The differences
Scheme 3
ingly, this first reduction wave is followed by a second partially
reversible wave at E1/2 Ϫ2.56 V for 4, E1/2 Ϫ2.78 V for 5 and E1/2
Ϫ2.84 V for 6 possibly corresponding to the formation of a
dianion, which exists either as a boron–boron σ-bonded
derivative or as a diradical (Fig. 5).25 In comparison to the other
diboranes, the reduction of 4 is relatively facile as it occurs at a
more positive potential than that of 5 and 6. Once again, this
Fig. 3 ORTEP view of 4 in the crystal (30% ellipsoids). Selected bond
lengths (Å) and angles (Њ): B(1)–C(1) 1.569(3), B(1)–C(21) 1.579(3),
B(1)–C(11) 1.595(3), B(2)–C(8) 1.552(3), B(2)–C(31) 1.570(3),
B(2)–C(41) 1.574(4), B(2)–C(21) 2.730(3); C(1)–B(1)–C(21) 123.64(18),
C(1)–B(1)–C(11) 117.40(18), C(21)–B(1)–C(11) 117.96(17), C(8)–B(2)–
C(31) 126.8(2), C(8)–B(2)–C(41) 125.5(2), C(31)–B(2)–C(41) 103.4(2),
C(9)–C(1)–B(1) 127.77(19), C(9)–C(8)–B(2) 130.62(19).
Fig. 5 Cyclic voltammogram of 5 in THF (scan rate 100 mV sϪ1).
D a l t o n T r a n s . , 2 0 0 4 , 1 2 5 4 – 1 2 5 8
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property can be correlated to the antiaromatic character of the
9-borafluorenyl moiety which favors its reduction.26–28 The
chemical reduction of 4 and 5 has also been studied. Com-
bination of the diboranes with 18-C-6 and K (4) or Na/Hg (5)
results in the production of dark brown (4) or dark purple (5)
(s, 1H, C᎐CH ), 5.66 (s, 1H, C᎐CH ), 6.64 (s, 1H, Mes-CH ),
᎐ ᎐
6.66 (s, 1H, Mes-CH ), 6.72 (dd, 1H, 3J(HH) = 7.0 Hz, 4J(HH) =
1.5 Hz, Naphth-CH ), 6.93 (dd, 1H, 3J(HH) = 7.0 Hz, 3J(HH) =
3
7.0 Hz, Naphth-CH ), 7.25 (t, 1H, J(HH) = 8.0 Hz, Naphth-
CH ), 7.28 (dd, 1H, 3J(HH) = 8.0 Hz, 4J(HH) = 1.5 Hz, Naphth-
CH ), 7.33 (dd, 1H, 3J(HH) = 8.0 Hz, 4J(HH) = 1.5 Hz, Naphth-
CH ), 7.55 (dd, 1H, 3J(HH) = 8.0 Hz, 4J(HH) = 1.5 Hz, Naphth-
CH ). 11B NMR (THF-d8): 45.5 (br). 13C NMR (toluene-d8):
21.59, 22.25, 23.27, 25.06, 25.24, 25.53 (6C, CH3), 122.7, 123.9,
125.1, 125.4, 126.6, 126.9, 128.9, 129.2, 132.3, 134.7 (10C,
CH), quaternary C not observed.
paramagnetic solutions, which presumably contain the radical
Ϫ
anion 4 and 5 Ϫ, respectively. The EPR spectra of these
solutions both consist of a broad signal at g = 2.00. While a
seven-line signal resulting from 11B hyperfine coupling was
observed for the radical anion obtained by reduction of
ؒ
ؒ
1,8-bis(diphenylboryl)naphthalene,10 the 11B hyperfine coupling
Ϫ
ؒ
ؒ
for the presumed 4 and 5 radicals could not be resolved,
possibly because of the unsymmetrical nature of these di-
boranes. Attempts to generate the dianions chemically have not
been successful.
Synthesis of diborane 4
To a suspension of freshly isolated 1-Li(THF)4 (0.80 g, 1.19
mmol) in diethyl ether (5 mL) at Ϫ40 ЊC was added a solution
of 9-chloro-9-borafluorene (0.30 g, 1.51 mmol) in diethyl ether
(5 mL) at Ϫ40 ЊC. An immediate reaction was observed. The
solution was stirred for 12 h at 25 ЊC, and the solvent was
removed by evaporation. The residue was extracted with
chloroform. Following filtration and evaporation of the sol-
vent, the product was recrystallized from a pentane–hexane
mixture, which afforded yellow crystals of 4. Yield: 0.25 g, 39%.
1H NMR (chloroform-d): 1.33 (s, 3H, Mes-CH3), 1.33 (s, 3H,
Mes-CH3), 1.55 (s, 3H, Mes-CH3), 1.90 (s, 3H Mes-CH3), 2.32
(s, 3H, Mes-CH3), 2.48 (s, 3H, Mes-CH3), 6.04 (s, 1H, Mes-
CH ), 6.48 (s, 1H Mes-CH ), 6.69 (s, 1H, Mes-CH ), 6.88 (s, 1H,
Experimental
General considerations
Compound 1,12 9-chloro-9-borafluorene20 and 2,2Ј-dilithio-
bibenzyl29 were synthesized by following the experimental
procedures described in the literature. Trimethylchlorosilane
was purchased from Aldrich and boron tribromide from Alfa
Aesar. Solvents were refluxed over the appropriate drying agent
under N2 and were distilled prior to use. THF and ether were
dried over Na/K. Dichloromethane, chloroform and aceto-
nitrile were dried over CaH2. Air-sensitive compounds were
handled in an N2 atmosphere using standard Schlenk and
glovebox techniques. All melting points were measured on
3
Mes-CH ), 6.90–7.01 (m, 3H, CH ), 7.15 (d, 1H, J(HH) = 7.0
3
Hz, CH ), 7.24 (m, 2H, CH ), 7.31 (d, 1H, J(HH) = 7.0 Hz,
CH ), 7.38 (d, 1H, 3J(HH) = 7.5 Hz, CH ), 7.53 (dd, 1H, 3J(HH)
= 7.0 Hz, 3J(HH) = 7.5 Hz, CH ), 7.59 (dd, 1H, 3J(HH) = 7.5 Hz,
3J(HH) = 8.0 Hz, CH ), 7.68 (d, 1H, 3J(HH) = 7.5 Hz, CH ), 8.03
1
samples in sealed capillaries and are uncorrected. The H and
13C NMR spectra were recorded on a Varian VXR-300 spec-
trometer at 300 and 75.4 MHz, respectively. 11B NMR spectra
were recorded on a Varian Inova 400 broadband spectrometer
at 128.3 MHz. Chemical shifts for the 1H and 13C NMR spectra
are reported with respect to Me4Si standard (δ = 0 ppm). Chem-
ical shifts for the 11B NMR spectra are reported with respect to
BF3ؒOEt2 standard (δ = 0 ppm). EPR spectra were recorded on
a Bruker X-band EPR spectrometer (model ESP 300E).
3
3
(d, 1H, J(HH) = 7.5 Hz, CH ), 8.07 (d, 1H, J(HH) = 7.0 Hz,
CH ), 8.17 (d, 1H, J(HH) = 8.0 Hz, CH ). 13C NMR (chloro-
3
form-d): 21.01, 21.30, 22.21, 22.86, 23.20, 25.40 (6C, Mes-
CH3), 118.2, 119.3, 125.0, 125.8, 126.6, 126.7, 127.9, 128.3,
129.2, 129.7, 130.7, 131.3, 131.4, 132.3, 132.5, 135.4, 136.0,
141.0 (18C, CH), 128.8, 130.8, 133.8, 135.9, 137.7, 140.5, 141.3,
141.4, 142.2 (br), 142.6 (br), 143.9 (br), 145.4 (br), 147.5, 152.8,
153.5 (16C, C-C and B-C). 11B NMR (chloroform-d): 57 (B-
fluorenyl), 71 (B-Mes2).
Crystallography
The crystallographic measurements were performed using a
Siemens SMART-CCD area detector diffractometer, with
graphite-monochromated Mo-Kα radiation (λ = 0.71069 Å).
Specimens of suitable size and quality were selected and
mounted onto glass fibers with Apiezon grease. The structures
were solved by direct methods, which successfully located most
of the non-hydrogen atoms. Subsequent refinement on F 2 using
the SHELXTL/PC package (version 5.1) allowed location of
the remaining non-hydrogen atoms. Further crystallographic
details can be found in Table 1.
Synthesis of 1,2-bis(2-trimethylsilanylphenyl)ethane
Trimethylchlorosilane (6.4 mL, 50.0 mmol) was added to a
solution of 2,2Ј-dilithiobibenzyl (20 mmol) in 100 mL of
diethyl ether at Ϫ78 ЊC. This reaction mixture was allowed to
warm to room temperature and was stirred for 1 h. The reaction
mixture was quenched with HClaq (1 M, 100 mL) and extracted
three times with ether (100 mL). The organic phases were
combined and dried over anhydrous magnesium sulfate. After
filtration, the solvent was removed to yield 1,2-bis(2-trimethyl-
silanylphenyl)ethane as a crude yellow oil. Yield: 98%, 6.45 g.
This compound was found to be spectroscopically identical
to that previously reported.21 It was used without further
purification.
CCDC reference numbers 227178–227181.
See http://www.rsc.org/suppdata/dt/b3/b316505a/ for crystal-
lographic data in CIF or other electronic format.
Synthesis of the borataalkene salt 3-Li(tmeda)2
Synthesis of 5-bromo-10,11-dihydrodibenzo[b,f ]borepin
A suspension of solid 1-Li(THF)4 in toluene (5 mL) was left to
stand for 7 days under N2 which resulted in the production of a
red–orange solution. Addition of TMEDA (5 mL) resulted in
the immediate formation of a deep purple solution. Storage at
Ϫ30 ЊC overnight led to the crystallization of the borataalkene
salt 3-Li(tmeda)2 which was isolated as a toluene solvate. The
compound was purified by successive washings with cold
pentane (2 mL). The compound decomposes upon heating
above room temperature. Because of the high sensitivity of the
compound, an elemental analysis could not be obtained. NMR
yield: 70–80%. 1H NMR (THF-d8): 0.97 (s, 3H, Mes-CH3), 1.56
(s, 3H, Mes-CH3), 1.73 (s, 3H, Mes-CH3), 1.75 (s, 3H,
Mes-CH3), 2.04 (s, 3H, Mes-CH3), 2.24 (s, 3H, Mes-CH3), 4.73
A solution of boron tribromide (4.34 mL, 45.9 mmol) in di-
chloromethane (10 mL) was added dropwise under nitrogen to
a solution of 1,2-bis(2-trimethylsilanylphenyl)ethane (5.00 g,
15.3 mmol) in dichloromethane (40 mL) at Ϫ78 ЊC. This
reaction mixture was allowed to warm to room temperature
and was stirred for 1 h. The solvent was evaporated and the
resulting solid was extracted with diethyl ether (20 mL). This
solution was filtered and concentrated. The product, namely
5-bromo-10,11-dihydrodibenz[b,f]borepin, crystallized upon
standing at Ϫ50 ЊC. The solvent was removed by filtration and
the crystals were washed with cold diethyl ether (2 × 20 mL)
1
and dried under vacuum. Yield: 1.70 g, 41%; mp. 85ЊC. H
D a l t o n T r a n s . , 2 0 0 4 , 1 2 5 4 – 1 2 5 8
1257

View Article Online
NMR (chloroform-d): 3.15 (s, 4H, CH2), 7.20 (d, 2H, 3J(H,H) =
7.6 Hz, H2C–C–CH ), 7.34 (td, 2H, 3J(H,H) = 7.6 Hz, 4J(H,H) =
1.2 Hz, H2C–C–CH–CH ), 7.47 (td, 2H, 3J(H,H) = 7.6 Hz,
References
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4J(H,H) = 1.6 Hz, BC–CH–CH ), 8.53 (dd, 2H, J(H,H) = 7.6
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Synthesis of diborane 5
A
solution of 5-bromo-10,11-dihydrodibenzo[b,f]borepin
(0.15 g, 0.54 mmol) in diethyl ether (10 mL) was added to a
suspension of freshly isolated 1-Li(THF)4 (0.30 g, 0.45 mmol)
in diethyl ether (10 mL) at 25 ЊC. The mixture was stirred for 2 h
at room temperature and the solution was filtered. The solvent
was removed under vacuum and the resulting solid was
extracted with dichloromethane (20 mL). Following filtration
and evaporation of the solvent, the beige solid was washed with
ethanol (2 × 20 mL) and dried under vacuum. The product was
recrystallized from a dichloromethane–hexane mixture. Yield:
89 mg, 35%; mp 327 ЊC (decomp.). Large monocrystals could
be obtained by slow evaporation of a diethyl ether solution.
1H NMR (chloroform-d): 1.21 (s, 3H, Mes-CH3), 1.31 (s, 3H,
Mes-CH3), 1.48 (s, 3H, Mes-CH3), 1.57 (s, 3H, Mes-CH3), 1.76
(s, 3H, Mes-CH3), 2.19 (s, 3H, Mes-CH3), 2.88 (m, 1H, CH(H)),
2.95 (m, 2H, CH2), 3.44 (m, 1H, CH(H )), 6.14 (s, 1H,
Mes-CH ), 6.27 (s, 1H, Mes-CH ), 6.46 (d, 1H, 3J(HH) = 6.8 Hz,
3
CH ), 6.59 (s, 2H, Mes-CH ), 6.73 (td, 1H, J(HH) = 7.6 Hz,
3
4J(HH) = 1.2 Hz, CH ), 6.85 (t, 1H, J(HH) = 7.6 Hz, CH ),
3
6.94 (d, 1H, J(HH) = 7.2 Hz, CH ), 7.10–7.24 (m, 5H, CH ),
3
4
7.40 (m, 2H, CH ), 7.75 (dd, 1H, J(HH) = 7.2 Hz, J(HH) =
1.6 Hz, CH ), 7.94 (dd, 1H, 3J(HH) = 8.0 Hz, 4J(HH) = 1.6 Hz,
3
4
CH ), 8.09 (dd, 1H, J(HH) = 8.0 Hz, J(HH) = 1.6 Hz, CH ).
13C NMR (chloroform-d): 20.86, 21.03, 22.32, 23.34, 23.64,
24.61 (6C, Mes-CH3), 36.87, 37.77 (2C, CH2), 123.6, 124.0,
125.4, 125.6, 127.3, 127.7, 127.8, 128.2, 128.8, 130.2,
130.7, 130.8, 132.1, 134.5, 135.3, 138.9, 144.7, 144.9 (18C, CH),
133.8, 137.7, 139.8, 140.8, 141.1, 141.1, 141.5, 142.9, 152.3,
154.4 (10C, C–C), 137.7, 139.1, 149.1, 149.5, (4C, B–C).
Two B–C not observed. 11B NMR (chloroform-d): 72.2 (br,
2B).
Electrochemistry
Electrochemical experiments were performed with an electro-
chemical analyzer from CH Instruments (Model 610A) with a
glassy-carbon working electrode and a platinum auxiliary elec-
trode at a scan rate of 100 mV sϪ1. The reference electrode was
built from a silver wire inserted a small glass tube fitted with a
porous Vycor frit at the tip and filled with a CH3CN solution
containing (nBu)4NPF6 (0.1 M) and AgNO3 (0.005 M). All
three electrodes were immersed in a THF solution (8 mL) con-
taining (nBu)4NPF6 (0.1 M) as a support electrolyte and the
borane (0.003 M). The electrolyte was dried under vacuum
prior to use. In all cases, ferrocene was used as an internal
standard, and all reduction potentials are reported with respect
to the E1/2 of the Fcϩ/Fc redox couple.
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Acknowledgements
Support from the Welch Foundation (Grant A-1423), the
National Science Foundation (CAREER CHE-0094264) and
the Department of Chemistry at Texas A&M University, is
gratefully acknowledged. The purchase of the X-ray diffract-
ometer was made possible by a grant from the National Science
Foundation (CHE-9807975). Finally, we thank the referees for
their constructive comments.
D a l t o n T r a n s . , 2 0 0 4 , 1 2 5 4 – 1 2 5 8
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