Unsymmetrical 9-borafluorenes via low-temperature C-H activation of m-terphenylboranes

Article abstract of DOI:10.1021/om020611v

The reaction of 2,6-(4-t-BuC₆H₄)₂C₆H₃Li and with H₂ClB·SMe₂ or HCl₂B·SMe₂ in hexane solution afforded the m-terphenyl-substituted unsymmetrical 9-borafluorene 1-(4-tert-butylphenyl)-7-tert-butyl-9-(bis-2,6-(4-tert-butylphenyl) phenyl)-9-borafluorene, 1, in good to moderate yields. The related reaction of 2,6-(3,5-Me₂C₆H₃)₂C₆ H₃Li with BH₂Cl·SMe₂ or BHCl₂·SMe₂ in toluene solution gave 1-(3,5-dimethylphenyl)-6,8-dimethyl-9-(bis-2,6-(3,5-dimethylphenyl)phenyl)-9-bor afluorene, 3. Compounds 1 and 3 are air-stable fluorescent solids. The reactions of 2,6-(2-MeC₆H₄)₂C₆H₃Li or 2,6-Mes₂C₆H₃Li (which possess either two or no o- and o″-hydrogens) with H₂ClB·SMe₂ gave the primary boranes [2,6-(2-MeC₆H₄)₂C₆H₃ BH₂]₂, 4, and [2,6-Mes₂C₆H₃BH₂]₂, 5, respectively. Quenching of the reaction of 2,6-(4-t-BuC₆H₄)₂C₆H₃Li with H₂ClB·SMe₂ after 1.5 h with pyridine resulted in the isolation of the primary borane [2,6-(4-t-BuC₆H₄)₂C₆H₃ BH₂]₂, 2, as the pyridine adduct 2·py, which after thermolysis at 190°C gave 1-(4-tert-butylphenyl)-7-tert-butyl-9-borafluorene·pyridine, 7·py. Heating a C₆D₆ solution of 4 to 60-70°C led to C-H activation and formation of a 1:1 adduct of monomeric 4 and 1-(2-methylphenyl)-5-methyl-9-borafluorene, 12. Reaction of 2 equiv of 2,6-(4-t-BuC₆H₄)₂C₆H₃Li with H₂ClB·SMe₂ in hexane solution followed by addition of THF gave the very crowded diterphenyl borate [2,6-(4-t-BuC₆H₄)₂C₆ H₃]₂B (μ-H)₂ Li(THF)₂]₂, 11, which can be converted to 1 by simple addition of water. Prolonged exposure of 1 to concentrated aqueous HCl led to B-C bond cleavage and formation of the sterically very crowded diterphenylborinic acid [2,6-(4-t-BuC₆H₄)₂ C₆H₃]₂BOH, 15. All compounds have been characterized by ¹H, ¹³C, and ¹¹B NMR spectroscopy and mass spectrometry, and compounds 1, 4, and 11 have also been characterized by single-crystal X-ray crystallography.

Full text of DOI:10.1021/om020611v

Organometallics 2003, 22, 83-92
83
Un sym m etr ica l 9-Bor a flu or en es via Low -Tem p er a tu r e
C-H Activa tion of m -Ter p h en ylbor a n es
Rudolf J . Wehmschulte,* Armando A. Diaz, and Masood A. Khan
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval,
Room 208, Norman, Oklahoma 73019
Received J uly 29, 2002
The reaction of 2,6-(4-t-BuC6H4)2C6H3Li and with H2ClB‚SMe2 or HCl2B‚SMe2 in hexane
solution afforded the m-terphenyl-substituted unsymmetrical 9-borafluorene 1-(4-tert-
butylphenyl)-7-tert-butyl-9-(bis-2,6-(4-tert-butylphenyl)phenyl)-9-borafluorene, 1, in good to
moderate yields. The related reaction of 2,6-(3,5-Me2C6H3)2C6H3Li with BH2Cl‚SMe2 or BHCl2‚
SMe2 in toluene solution gave 1-(3,5-dimethylphenyl)-6,8-dimethyl-9-(bis-2,6-(3,5-dimeth-
ylphenyl)phenyl)-9-borafluorene, 3. Compounds 1 and 3 are air-stable fluorescent solids.
The reactions of 2,6-(2-MeC6H4)2C6H3Li or 2,6-Mes2C6H3Li (which possess either two or no
o- and o′′-hydrogens) with H2ClB‚SMe2 gave the primary boranes [2,6-(2-MeC6H4)2C6H3-
BH2]2, 4, and [2,6-Mes2C6H3BH2]2, 5, respectively. Quenching of the reaction of 2,6-(4-t-
BuC6H4)2C6H3Li with H2ClB‚SMe2 after 1.5 h with pyridine resulted in the isolation of the
primary borane [2,6-(4-t-BuC6H4)2C6H3BH2]2, 2, as the pyridine adduct 2‚py, which after
thermolysis at 190 °C gave 1-(4-tert-butylphenyl)-7-tert-butyl-9-borafluorene‚pyridine, 7‚
py. Heating a C6D6 solution of 4 to 60-70 °C led to C-H activation and formation of a 1:1
adduct of monomeric 4 and 1-(2-methylphenyl)-5-methyl-9-borafluorene, 12. Reaction of 2
equiv of 2,6-(4-t-BuC6H4)2C6H3Li with H2ClB‚SMe2 in hexane solution followed by addition
of THF gave the very crowded diterphenyl borate [2,6-(4-t-BuC6H4)2C6H3]2B(µ-H)2Li(THF)2]2,
11, which can be converted to 1 by simple addition of water. Prolonged exposure of 1 to
concentrated aqueous HCl led to B-C bond cleavage and formation of the sterically very
crowded diterphenylborinic acid [2,6-(4-t-BuC6H4)2C6H3]2BOH, 15. All compounds have been
characterized by 1H, 13C, and 11B NMR spectroscopy and mass spectrometry, and compounds
1, 4, and 11 have also been characterized by single-crystal X-ray crystallography.
In tr od u ction
In an effort to explore the coordination properties of
unsymmetrical 9-borafluorenes as potential alternatives
to the ubiquitous cyclopentadienide-based ligand sys-
tems,1 we discovered 9-borafluorene formation via a
remarkably facile intramolecular C-H activation even
below room temperature.2 The reaction of 2,6-(4-t-
BuC6H4)2C6H3Li with H2ClB‚SMe2 afforded the steri-
cally encumbered terphenyl-substituted 9-borafluorene
1 in yields of up to 75% (eq 1).
Typically, the synthesis of 9-borafluorenes requires
either o,o′-biphenylmercury compounds,3 which are
highly toxic, or temperatures of at least 150 °C in
thermolysis reactions of 2-biphenyldialkylboranes.4 Thus,
9-borafluorene formation via a low-temperature C-H
activation involving an intermediate primary borane
[2,6-(4-t-BuC6H4)2C6H3BH(µ-H)]2, 2, constitutes a sig-
nificant improvement with respect to the previous
methods. Furthermore, the steric protection brought on
by the bulky m-terphenyl substituent in 9-borafluorene
1 leads to enhanced stability. For example, 1 is even
* Correspondingauthor.Fax: +14053256111.E-mail: rjwehmschulte@
ou.edu.
(1) Togni, A., Halterman, R. L., Eds.; Metallocenes: Synthesis,
Reactivity, Applications; Wiley-VCH: Weinheim, New York, 1998.
(2) Wehmschulte, R. J .; Khan, M. A.; Twamley, B.; Schiemenz, B.
Organometallics 2001, 20, 844-849.
(3) Narula, C. K.; No¨th, H. J . Organomet. Chem. 1985, 281, 131-
134.
(4) Ko¨ster, R.; Benedikt, G. Angew. Chem. 1963, 75, 419.
10.1021/om020611v CCC: $25.00 © 2003 American Chemical Society
Publication on Web 11/27/2002

84 Organometallics, Vol. 22, No. 1, 2003
Wehmschulte et al.
Sch em e 1
F igu r e 1. Thermal ellipsoid plot (30% probability el-
lipsoids) showing the molecular structure of 1. Hydrogen
atoms are omitted for clarity.
stable toward brief exposure to concentrated aqueous
HCl. In the doubly reduced form of 1 the lithium cations
are positioned above and below the five-membered
borole ring in an almost symmetrical fashion. They are
further protected by weak interactions with the flanking
4-t-BuC6H4 groups of the m-terphenyl substituent.2
Transition metals are expected to be coordinated in a
similar manner. In fact, preliminary spectroscopic data
of a Cr(CO)3-containing complex of 1 are in agreement
with this coordination mode.5 Other potential applica-
tions of air-stable, bright yellow-green fluorescent 9-bo-
rafluorenes could be as fluoride ion sensors6 or electronic
materials such as chromophores in organic LEDs.7
Therefore, we have further investigated the scope of the
9-borafluorene formation from readily available m-
terphenyl precursors. We have prepared three ad-
ditional m-terphenyl compounds which differ in the
substitution pattern of the o- and o′′-aromatic rings
(Scheme 1). Here, we report on the reactions of the
m-terphenyllithium compounds derived from the pre-
cursors A to D with H2ClB‚SMe2 or HCl2B‚SMe2 to
afford either 9-borafluorenes or primary boranes
[TerphBH(µ-H)]2 (Terph ) m-terphenyl). Furthermore,
wehaveinvestigated thereaction of2,6-(4-t-BuC6H4)2C6H3-
Li with H2ClB‚SMe2 in more detail. The results of
thermolysis reactions of the new primary boranes [2,6-
(2-MeC6H4)2C6H3BH(µ-H)]2, 4, and [2,6-Mes2C6H3BH-
(µ-H)]2, 5, are also presented.
the terphenyl-substituted 9-borafluorene 3, albeit in
lower yields (eq 2).
Whereas the high solubility of 1 in standard organic
solvents prevents the isolation of 1 by crystallization
from the reaction mixture (vide infra), 3 is less soluble
in hexanes or Et2O and can be isolated by this method.
Both compounds are bright yellow-green fluorescent,
air- and moisture-stable solids. 9-Borafluorenes are
isoelectronic with the formally antiaromatic fluorenyl
+ 8-10
cation C13H9
.
To assess the extent of π-electron
delocalization and antiaromaticity on the metric pa-
rameters of the borafluorene ring system, a crystal
structure analysis of 1 was undertaken. The structure
of 1 (Figure 1) features an unsymmetrical, essentially
planar 9-borafluorene core with the boron being slightly
pushed out of the borafluorene plane due to close C-C
contacts involving the C(37)-C(42) and the C(17)-C(22)
aromatic rings: C(17)‚‚‚C(37) ) 3.502 Å, C(17)‚‚‚C(42)
) 3.424 Å, C(22)‚‚‚C(42) ) 3.230 Å. The geometry at
Resu lts a n d Discu ssion
We have previously reported that the reaction of 2,6-
(4-t-BuC6H4)2C6H3Li with H2ClB‚SMe2 in hexanes af-
forded the sterically encumbered borafluorene 1 in up
to 50-75% yield.2 Similarly, the corresponding reaction
of 2,6-(3,5-Me2C6H3)2C6H3Li with H2ClB‚SMe2 afforded
(8) Olah, G. A.; Prakash, G. K. S.; Liang, G.; Westerman, P. W.;
Kunde, K.; Chandrasekhar, J .; Schleyer, P. v. R. J . Am. Chem. Soc.
1980, 102, 4485-4492.
(5) Wehmschulte, R. J . Unpublished results, 2002.
(6) Yamaguchi, S.; Akiyama, S.; Tamao, K. J . Am. Chem. Soc. 2001,
123, 11372-11375.
(7) Klaus, M.; Wegner, G. Electronic Materials: The Oligomer
Approach; Wiley-VCH: Weinheim, New York, 1998.
(9) Amyes, T. L.; Richard, J . P.; Novak, M. J . Am. Chem. Soc. 1992,
114, 8032-8041.
(10) J iao, H.; Schleyer, P. v. R.; Mo, Y.; McAllister, M. A.; Tidwell,
T. T. J . Am. Chem. Soc. 1997, 119, 7075-7083.

Unsymmetrical 9-Borafluorenes
Organometallics, Vol. 22, No. 1, 2003 85
9-N,N-(di-tert-butylboryl)methyl-9-borafluorene14 and
1.556(4) and 1.549(3) Å in 9-pentafluorophenyloctafluoro-
9-borafluorene15). Due to the 9-borafluorene structure
constraints, the internal C(9)-B(1)-C(16) angle with
a value of 103.5(4)° is significantly smaller than the
other two C-B-C angles with values of 128.3(4)° and
127.9(4)°. Very similar values were observed for the two
aforementioned 9-borafluorenes14,15 and, interestingly,
also for the doubly reduced dilithium salt of 1, 1‚
2LiOEt2.2 The terphenyl substituent is rotated out of
the 9-borafluorene plane by 65° and the central C(37)-
C(42) ring is almost parallel to the C(17)-C(22) aro-
matic ring. With the exception of the C(11)-C(16) bond
distance of 1.434(6) Å, which may be caused by minor
disorder problems, the metric parameters indicate only
little distortion and thus little antiaromaticity, and thus
the 9-borafluorene group may be best described as a
substituted biphenyl.
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r a l Refin em en t
Deta ils for 1, 4, a n d 11
1
4
11
emp formula
fw
T, K
C
52H57
B
C40H38B2
540.32
173(2)
C
60H76BLiO2
692.79
173(2)
846.96
103(2)
wavelength, Å
cryst syst
space group
a, Å
0.71073
monoclinic
P2(1)/c
14.400(3)
20.794(5)
15.672(5)
90
115.900(13)
90
4221.5(19)
4
1.090
0.71073
triclinic
P1h
0.71073
orthorhombic
P2(1)2(1)2(1)
12.3136(8)
15.1506(10)
27.4491(18)
90
11.2756(14)
11.298(2)
12.691(2)
77.854(12)
88.050(9)
86.787(12)
1577.7(4)
2
b, Å
c, Å
R, deg
â, deg
90
γ, deg
90
V, Å3
5120.9(6)
4
Z
D
calcd, Mg/m3
1.137
1.099
µ(Mo KR), mm-1 0.061
0.063
0.063
F(000)
1496
576
1840
cryst size, mm3
0.64 × 0.44 × 0.36 × 0.32 × 0.32 × 0.24 ×
0.34
0.24
0.16
cryst color and
habit
yellow plate
colorless prism colorless plate
Contrary to the results above, the reaction of 2,6-(2-
MeC6H4)2C6H3Li and 2,6-Mes2C6H3Li16 (Mes ) 2,4,6-
Me3C6H2) with H2ClB‚SMe2 afforded the expected pri-
mary boranes [2,6-(2-MeC6H4)2C6H3BH(µ-H)]2, 4, and
[2,6-Mes2C6H3BH(µ-H)]2, 5, in moderate to good yields
(eq 3).
2θmax, deg
46.00
5836
547
0.0830
0.1677
1.058
48.48
5034
399
0.0580
0.1236
1.056
53.00
10595
596
0.0389
0.0903
1.029
no. of observns
no. of variables
a
R1 [I>2σ(I)]
b
wR2 [I>2σ(I)]
goodness-of-fit
on F2
largest diff peak, 0.279
0.202
0.229
e Å-3
a
b
2
2
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) (∑w||Fo| - |Fc|| /∑w|Fo| )1/2
.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 1
B(1)-C(9)
B(1)-C(16)
B(1)-C(42)
C(8)-C(9)
C(8)-C(11)
C(11)-C(16)
1.573(6)
1.556(7)
1.575(7)
1.391(6)
1.472(6)
1.434(6)
C(16)-B(1)-C(9)
C(16)-B(1)-C(42)
C(9)-B(1)-C(42)
C(8)-C(9)-B(1)
C(9)-C(8)-C(11)
C(16)-C(11)-C(8)
C(11)-C(16)-B(1)
103.5(4)
128.3(4)
127.9(4)
107.7(4)
111.4(4)
109.4(4)
107.4(4)
Both compounds are colorless solids that are stable
toward brief exposure to air. The structure of 4 was
confirmed by X-ray diffraction as a dimer with bridging
hydrogens (Figure 2). The B-C, B-Hterm, and B-Hbridg.
bond distances with average values of 1.574(4), 1.09-
(3), and 1.24(3) Å, respectively, are slightly shorter than
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 4
17
those observed for [MeBH(µ-H)]2 or [(Me3Si)3CBH(µ-
H)]2,18 and the B‚‚‚B separation in 4 with a value of
1.787(5) Å lies between those reported for [MeBH(µ-H)]2
or [(Me3Si)3CBH(µ-H)]2. The terphenyl substituents are
rotated 71.9° with respect to each other, which may be
a result of steric repulsion considering that the related
dimeric aluminum hydrides [2,6-Mes2C6H3AlH(µ-H)]2
and [2,6-Trip2C6H3AlH(µ-H)]2 (Trip ) 2,4,6-i-Pr3C6H3)19
feature an almost parallel alignment of the terphenyl
substituents. Contrary to the situation in solution only
one isomer is found in the solid state of 4. Here, all
methyl groups are pointed outward. The internal H-
B-H angles are relatively narrow, with values of 87-
B(1)-C(13)
B(1)-H(1)
B(1)-H(2)
B(1)-H(3)
B(1)‚‚‚B(2)
B(2)-C(33)
B(2)-H(2)
B(2)-H(3)
B(2)-H(4)
1.574(4)
1.11(3)
1.22(3)
1.27(3)
1.787(5)
1.575(4)
1.22(3)
1.24(3)
1.07(3)
C(13)-B(1)-H(1)
C(13)-B(1)-H(2)
C(13)-B(1)-H(3)
H(1)-B(1)-H(2)
H(1)-B(1)-H(3)
H(2)-B(1)-H(3)
C(33)-B(2)-H(2)
C(33)-B(2)-H(3)
C(33)-B(2)-H(4)
H(2)-B(2)-H(3)
H(2)-B(2)-H(4)
H(3)-B(2)-H(4)
123(2)
108(2)
115.5(12)
111(2)
107(2)
87(2)
114(2)
115.8(12)
120(2)
88(2)
105(2)
109(2)
Ta ble 4. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 11
B(1)-C(16)
B(1)-C(42)
B(1)-H(1)
B(1)-H(2)
Li(1)-H(1)
Li(1)-H(2)
Li(1)-(O1)
Li(1)-O(2)
1.643(2)
1.645(2)
1.175(14)
1.180(14)
1.851(14)
1.852(14)
1.906(3)
1.903(3)
C(16)-B(1)-C(42)
C(16)-B(1)-H(1)
C(16)-B(1)-H(2)
C(42)-B(1)-H(1)
C(42)-B(1)-H(2)
H(1)-B(1)-H(2)
H(1)-Li(1)-H(2)
O(1)-Li(1)-O(2)
111.44(12)
113.1(8)
108.3(8)
108.6(8)
109.9(8)
105.3(11)
60.8(7)
(11) Zettler, F.; Hausen, H. D.; Hess, H. J . Organomet. Chem. 1974,
72, 157-162.
(12) Kahr, B.; J ackson, J . E.; Ward, D. L.; J ang, S. H.; Blount, J . F.
Acta Crystallogr., Sect. B: Struct. Sci. 1992, B48, 324-329.
(13) Olmstead, M. M.; Power, P. P. J . Am. Chem. Soc. 1986, 108,
4235-4236.
(14) Ma¨nnig, D.; No¨th, H.; Prigge, H.; Rotsch, A. R.; Gopinathan,
S.; Wilson, J . W. J . Organomet. Chem. 1986, 310, 1-20.
(15) Chase, P. A.; Piers, W. E.; Patrick, B. O. J . Am. Chem. Soc.
2000, 122, 12911-12912.
(16) Ruhlandt-Senge, K.; Ellison, J . J .; Wehmschulte, R. J .; Pauer,
F.; Power, P. P. J . Am. Chem. Soc. 1993, 115, 11353-11357.
(17) Hedberg, L.; Hedberg, K.; Kohler, D. A.; Ritter, D. M.; Scho-
maker, V. J . Am. Chem. Soc. 1980, 102, 3430-3434.
(18) Al-J uaid, S. S.; Eaborn, C.; Hitchcock, P. B.; Kundu, K. K.;
Molla, M. E.; Smith, J . D. J . Organomet. Chem. 1990, 385, 13-21.
(19) Wehmschulte, R. J .; Grigsby, W. J .; Schiemenz, B.; Bartlett,
R. A.; Power, P. P. Inorg. Chem. 1996, 35, 6694-6702.
104.61(15)
the boron atom is distorted trigonal planar. The B-C
bond lengths (Table 2) with values of 1.556(7), 1.573-
(6), and 1.575(7) Å are typical for B-Caryl bonds found
in other three-coordinate aryl boranes (e.g., 1.589(5) and
1.571(3) Å in Ph3B,11 1.574(4) and 1.565(7) Å in [2,6-
(MeO)2C6H3]3B,12 and 1.579(2) and 1.580(3) Å in Mes3B13)
and 9-borafluorenes (e.g., 1.592(6) and 1.588(6) Å in

86 Organometallics, Vol. 22, No. 1, 2003
Wehmschulte et al.
Sch em e 2
F igu r e 2. Thermal ellipsoid plot (30% probability el-
lipsoids) showing the molecular structure of 4. Hydrogen
atoms except of those bound to boron are omitted for clarity.
After stirring a hexane solution of crude 2 (synthe-
sized as above) at room temperature for 3 days es-
sentially all of 2 was consumed, and the main products
were the 9-borafluorenes 1 and 7 (eq 4). These could be
separated and isolated as pyridine adducts by fractional
crystallization in the presence of excess pyridine.
(2)° and 88(2)°, whereas the external C-B-H angles
with values of 123(2)° and 120(2)° are significantly wid-
er. The dimeric structures of 4 and 5 are maintained in
solution, which is evidenced by the occurrence of sepa-
rate 1H NMR signals for the terminal and bridging
1
hydrides. Furthermore, the H and 13C{1H} NMR spec-
tra of 4 are complicated due to the presence of several
isomers which originate in the different orientations the
o-tolyl groups can assume with respect to each other.
The formation of the terphenyl-9-borafluorenes 1 and
3 instead of the corresponding primary boranes [2,6-(4-
t-BuC6H4)2C6H3BH(µ-H)]2, 2, and [2,6-(3,5-Me2C6H3)2-
C6H3BH(µ-H)]2, 6, in the reaction of 2,6-(4-t-BuC6H4)2-
C6H3Li or 2,6-(3,5-Me2C6H3)2C6H3Li with H2ClB‚SMe2
is remarkable due to two factors: (i) the facile activation
of an aromatic C-H bond by a primary borane ArBH2
and (ii) the ease of the aryl migration despite the large
size of the aryl substituent. To better understand the
factors that govern the borafluorene formation, the
reaction of 2,6-(4-t-BuC6H4)2C6H3Li with H2ClB‚SMe2
was investigated in greater detail.
If the reaction is stopped after only 1.5 h by removal
of the volatile materials under reduced pressure, the
main product observed in the residue is the primary
borane [2,6-(4-t-BuC6H4)2C6H3BH(µ-H)]2, 2. Compounds
1‚py, H3B‚SMe2, and LiBH4 have been identified among
the other products by 1H and 11B NMR spectroscopy
after addition of excess pyridine. Redissolution of a
portion of the crude product in hexanes followed by
addition of pyridine afforded, after workup, the adduct
2‚py as a pale yellow crystalline solid. Contrary to
uncomplexed 2, the adduct 2‚py is stable at room
temperature with respect to C-H activation. Indeed,
heating to the melting point (165 °C) is required before
a gas evolution and formation of the 9-borafluorene 7‚
py is observed. This corresponds well with Bickelhaupt’s
report of the synthesis of the parent 9-borafluorene‚py
by thermolysis of the pyridine adduct 2-biphenylborane‚
py at 260 °C.20
The formation of 7 may be explained by C-H activa-
tion of one of the o- or o′′-aromatic hydrogens in 2.
Unfortunately, these results do not explain the forma-
tion of 1, for which there are several possible pathways,
two of which are discussed below (Scheme 2): 7 reacts
with 2 to give 1 and B2H6 or 2 rearranges to form B2H6
and the very crowded diterphenyl borane [2,6-(4-t-
BuC6H4)2C6H3]2BH (8), which readily undergoes C-H
activation to afford 1. The fact that 8 has not yet been
isolated or detected by spectroscopic techniques suggests
either that 8 is very short-lived or that it is not an
intermediate in the formation of 1. The isolation of the
borafluorene 9, an analogue of 7, as the pyridine adduct
9‚py from the reaction of 2,6-(3,5-Me2C6H3)2C6H3Li with
H2ClB‚SMe2 indicates that both compounds 1 and 3 may
be formed by the same mechanism.
The terphenyl-9-borafluorenes 1 and 3 can also
be synthesized by the reaction of 2 equiv of 2,6-
(4-t-BuC6-H4)2C6H3Li or 2,6-(3,5-Me2C6H3)2C6H3Li with
(20) Veen, R. v.; Bickelhaupt, F. J . Organomet. Chem. 1973, 47, 33-
38.

Unsymmetrical 9-Borafluorenes
Organometallics, Vol. 22, No. 1, 2003 87
HCl2B‚SMe2 (eq 5).
F igu r e 3. Thermal ellipsoid plot (30% probability el-
lipsoids) showing the molecular structure of 11. Hydrogen
atoms except of those bound to boron are omitted for clarity.
Although the reaction mechanism is not known at this
time, it seems likely that the very crowded diterphenyl
boranes [2,6-(4-t-BuC6H4)2C6H3]2BH, 8, and [2,6-(3,5-
Me2C6H3)2C6H3]2BH, 10, may be intermediates.
In an effort to evaluate the potential role of a diter-
phenylborate Terph2BH2Li as a possible intermediate
in the formation of 1 and 3 as well as an alternative ac-
cess to the very crowded 8, 2 equiv of 2,6-(4-t-BuC6H4)2-
C6H3Li was reacted with H2ClB‚SMe2 according to eq
6.
1.643(2) Å for B(1)-C(16) and 1.645(2) Å for B(1)-C(42)
are typical for four-coordinate B-Caryl distances.21 The
lithium cation is solvated by two THF molecules and
the two bridging hydrides in a distorted tetrahedral
fashion in which the long Li-H distances (compared to
the B-H distances) lead to a narrow H(1)-Li(1)-H(2)
angle of 60.8(7)°. With the exception of the close
aryl‚‚‚aryl interactions the structure of 11 corresponds
closely to that of Mes2B(µ-H)2Li(DME).22 It should also
be noted that diterphenyl compounds are quite rare and
have so far been limited to a few gallium and indium
species.23-25 The steric crowding in these species is
expressed in the wide Cipso-M-Cipso angles with values
ranging from 134.4° to 153.3°.
Compound 11 is stable in solution under anhydrous
and anaerobic conditions and decomposes only at its
melting point of 190-198 °C with gas evolution and
color change to yellow. Thus, its role as an intermediate
in the formation of 1 may be ruled out, but its existence
supports our proposal that the diterphenylboranes 8 and
10 may be intermediates in the formation of 1 and 3
according to eq 5. To generate 8, borate 11 was reacted
with Me3SiCl or H2O in C6D6 solution in analogy with
The 11B NMR spectrum of the crude product indicated
the formation of the [2,6-(4-t-BuC6H4)2C6H3]2BH2- anion
with a triplet at -20.0 ppm and a B-H coupling con-
stant of 73 Hz. Attempts to isolate this species by crys-
tallization from hexanes were unsuccessful, but the
THF-solvated compound [2,6-(4-t-BuC6H4)2C6H3]2B(µ-
H)2Li(THF)2, 11, was eventually obtained in the form
of small pale yellow needles after addition of THF to
the hexanes solution of the crude product. The crystal
structure (Figure 3) shows that there is remarkably
little distortion despite the large size of the terphenyl
substituents. The terphenyl groups are rotated with
respect to each other so that they adopt an approximate
staggered arrangement of the flanking aryl groups and
also generate a cleft large enough to accommodate the
lithium cation and the two THF molecules. This results
in an almost parallel alignment of the C(43)-C(48) with
the C(11)-C(16) aromatic ring and the C(37)-C(42)
with the C(15)-C(10) aromatic ring with C‚‚‚C distances
not below 3.456 Å, with the exception of C(8)‚‚‚C(42) and
C(16)‚‚‚C(43), with values of 3.066 and 3.083 Å, respec-
tively. The geometry at the boron center is essentially
tetrahedral with only minor deviations from the ideal
109.5° angles with C(16)-B(1)-C(42) ) 111.44(12)°,
H(1)-B(1)-H(2) ) 105.3(11)°, and C(16)-B(1)-H(1) )
113.1(8)°. Similarly the B-C distances with values of
the synthesis of Trip2BH26 or [Mes*AlH2]2 from the
27
corresponding borate or alanate. Whereas no reaction
was observed between 11 and Me3SiCl even after 24 h
at room temperature, the reaction of 11 with water led
to a gentle gas evolution, which was accompanied by a
gradual color change from pale yellow to bright yellow
fluorescent. The formation of 1 was verified by 1H NMR
spectroscopy, but no signals attributable to 8 were
detected. Thus, we propose that the reaction of 11 with
H2O leads to the formation of 8, which immediately
(21) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987, S1-S19.
(22) Hooz, J .; Akiyama, S.; Cedar, F. J .; Bennett, M. J .; Tuggle, R.
M. J . Am. Chem. Soc. 1974, 96, 274-276.
(23) Li, X.-W.; Pennington, W. T.; Robinson, G. H. Organometallics
1995, 14, 2109-2111.
(24) Crittendon, R. C.; Li, X.-W.; Su, J .; Robinson, G. H. Organo-
metallics 1997, 16, 2443-2447.
(25) Crittendon, R. C.; Beck, B. C.; Su, J .; Li, X.-W.; Robinson, G.
H. Organometallics 1999, 18, 156-160.
(26) Pelter, A.; Smith, K.; Buss, D.; Norbury, A. Tetrahedron Lett.
1991, 32, 6239-6242.
(27) Wehmschulte, R. J .; Power, P. P. Inorg. Chem. 1994, 33, 5611-
5612.

88 Organometallics, Vol. 22, No. 1, 2003
rearranges to 1 (eq 7).
Wehmschulte et al.
Further heating of the C6D6 solution of 13 at 74-78
°C for 4 days led to the gradual disappearance of the
1H NMR signals at 1.70 and 1.81 associated with 13.
In addition, formation of hydrogen (δH ) 4.47 ppm)28
and a color change to yellow fluorescent, which appears
to be characteristic for 9-borafluorenes,2,4 were observed.
The 11B NMR spectrum displayed two new signals of
approximately equal intensity at 28.6 and 0.0 ppm.
Addition of 4-(dimethylamino)pyridine (DMAP, a stron-
ger base than pyridine) resulted in the disappearance
of these peaks and the appearance of two new signals
at -2.5 and -12.5 ppm, respectively. In a separate
experiment in which a more dilute sample was ther-
molyzed and pyridine was added instead of DMAP, a
broad singlet at 7.3 and a quartet at -10.7 (J B-H ) 99
Hz) were observed in the 11B NMR spectrum. The latter
signal is probably due to H3B‚py,29 whereas the chemical
shift of the former signal corresponds with the one found
for 1‚py. In both cases the 1H NMR spectra showed only
severely broadened signals, in accordance with the
equilibrium given in eq 9.
Th er m olysis Rea ction s. To gain further insight into
the formation of 1 and 3 and also to explore the scope
of the 9-borafluorene formation via C-H activation, the
thermolysis of the primary boranes 4 and 5 was inves-
tigated. Heating of both compounds to their respective
melting points led to gas evolution and the formation
of mixtures of compounds. Thermolysis of a C6D6 solu-
tion of 4 at 68-71 °C for ca. 2.75 h resulted in the dis-
appearance of the 1H NMR signals of 4 and a color
change of the solution to yellow. The 11B NMR spectrum
shows two signals of equal intensity at 24.1 and 12.1
ppm. Addition of excess pyridine significantly simplified
the methyl region of the 1H NMR spectrum, and the
signals were identified as belonging to a 1:1 mixture of
4‚py and the 9-borafluorene 12‚py by comparison with
original samples (see Experimental Section for the syn-
thesis of 4‚py and 12‚py). The same reaction also occurs
at room temperature, albeit much more slowly. It takes
approximately five months for the disappearance of the
1H NMR signals of 4 from a sample of 4 kept in C6D6
solution at room temperature in a sealed NMR tube.
No further reaction is observed under these conditions.
On the basis of these findings we propose the initial
product of the thermolysis to be the dimer 13, consisting
of one monomer each of 4 and 12 (eq 8). Addition of pyr-
idine cleaves the B-H-B bridges and afforded the ob-
served 1:1 mixture of 4‚py and the 9-borafluorene 12‚py.
Attempts to isolate 14 or its pyridine or DMAP
adducts have not yet been successful, in part due to the
difficulty of determining the completion of the ther-
molysis and the high solubility of the products in
aliphatic or aromatic solvents.
Although the bond strengths of benzylic C-H bonds
are lower than those for aromatic C-H bonds (85 kcal/
mol for C6H5CH2-H vs 110 kcal/mol for C6H5-H30), no
reaction took place when 5 was heated to ca. 130 °C in
C7D8 solution in a sealed NMR tube. Gas evolution,
however, was observed when neat 5 was heated to its
melting point, but only a complicated mixture was
1
detected via H NMR spectroscopy. Whereas the mech-
anism for the C-H activation in the m-terphenyl borane
compounds discussed earlier is not known, the fact that
5 does not undergo C-H activation under mild condi-
tions suggests that an electrophilic attack of the Lewis
acidic boron center on the aromatic ring system may
be an important step in this reaction.
(28) Millar, J . M.; Kastrup, R. V.; Melchior, M. T.; Horvath, I. T.;
Hoff, C. D.; Crabtree, R. H. J . Am. Chem. Soc. 1990, 112, 9643-9645.
(29) No¨th, H.; Wrackmeyer, B. Chem. Ber. 1974, 107, 3070-3088.
(30) CRC Handbook of Chemistry and Physics; Weast, R. C., Ed.;
CRC Press: Boca Raton, FL, 1984; p F-195.
(31) Wehmschulte, R. J .; Khan, M. A.; Hossain, S. I. Inorg. Chem.
2001, 40, 2756-2762.

Unsymmetrical 9-Borafluorenes
Organometallics, Vol. 22, No. 1, 2003 89
Sta bility a n d P u r ifica tion of 1 a n d Syn th esis of
15. Due to its very high solubility in common organic
solvents, 1 could not be isolated by crystallization from
the reaction mixture. We have previously shown that
the doubly reduced dianionic 1‚2LiOEt2 is readily
oxidized by various metal salts, and pure 1 can be
obtained from these reactions.2 However, this method
requires the prior synthesis and isolation of the highly
air- and moisture-sensitive 1‚2LiOEt2. Previous work
has also shown that the air-stable pyridine adduct 1‚
py can be isolated from the reaction mixture containing
1 and pyridine by crystallization and that the pyridine
was not bound very tightly.2 As 1‚py is sensitive toward
elevated temperatures, removal of the coordinated py-
ridine by distillation was not feasible. We then inves-
tigated the trapping of pyridine by a strong acid and
found ca. 6 M aqueous HCl suitable. Addition of 2 to 3
equiv of HCl to an Et2O solution of 1‚py at room
temperature led to an immediate brightening of the
yellow color of the Et2O phase and a colorless solid
formed in the aqueous phase. Separation of the Et2O
phase, followed by exchange of the solvent to petroleum
ether and subsequent drying over MgSO4, afforded pure
1 after removal of the solvent under reduced pressure.
Activated Al2O3 may also be used to remove the pyri-
dine, but up to 50% loss of 1 occurred due to its apparent
strong adsorption on alumina. Prolonged exposure (1
week) of 1‚py to concentrated aqueous HCl in the
presence of air led to cleavage of one of the B-C bonds
of the 9-borafluorene unit and the formation of the
diterphenyl borinic acid [2,6-(4-t-BuC6H4)2C6H3]2BOH
(15) (eq 10).
hydrogens are available, as in the case of substituent
C, the primary borane [2,6-(2-MeC6H4)2C6H3BH(µ-H)]2
(4) can be isolated, and C-H activation to afford the
9-borafluorene requires elevated temperatures (ca. 60-
70 °C). Investigations of the reaction of 2,6-(4-t-
BuC6H4)2C6H3Li with H2ClB‚SMe2 and the thermolysis
of 4 indicate that the primary boranes 2 and 4 react to
give the 9-borafluorenes 7 and 12 and, in a second step,
undergo ligand migration to give 1 and 14. 9-Borafluo-
renes 1 and 3 can also be prepared by the reaction of 2
equiv of TerphLi with HCl2B‚SMe2, in which case the
very crowded diterphenyl boranes 8 and 10 are the most
likely intermediates. The (short-lived) existence of 8 is
further supported by the isolation of the diterphenylbo-
rate 11 and its conversion into 1 upon addition of H2O.
Compounds 1 and 3 are air-stable and fluorescent. A
study to exploit these properties for application as, for
example, fluoride sensor is under way.
Exp er im en ta l Section
Gen er a l P r oced u r es. All work was performed under
anaerobic and anhydrous conditions by using either modified
Schlenk techniques or an Innovative Technologies drybox.
Solvents were freshly distilled under N2 from sodium, potas-
sium, or sodium/potassium alloy and degassed twice prior to
use. H2ClB‚SMe2, HCl2B‚SMe2, and n-butyllithium (1.6 M in
hexanes) were obtained from commercial suppliers. Com-
pounds 1,2 1‚py,2 2,6-(4-t-BuC6H4)2C6H3Br,2 2,6-(2-MeC6H4)2-
C6H3I,31 2,6-(3,5-Me2C6H3)2C6H3I,32,33 and 2,6-Mes2C6H3I16 were
synthesized according to literature methods.34,35 NMR spectra
were recorded on a Varian Mercury 300 MHz, a Varian Unity
Plus 400 MHz, or a Varian VXR-500S spectrometer, and 1H
NMR chemical shift values were determined relative to the
residual protons in C6D6 or C7D8 as internal reference (δ )
7.15 or 2.09 ppm). 13C NMR spectra were referenced to the
solvent signal (δ ) 128.0 or 20.4 ppm) and 11B NMR spectra
to an external solution of F3B‚OEt2 in CDCl3. Infrared spectra
were recorded in the range 4000-200 cm-1 as a Nujol mull
between CsI plates using a Nicolet Nexus 470 FTIR spectrom-
eter. UV-vis spectra were recorded on a Shimadzu UV-
2401PC spectrophotometer and fluorescence spectra on a
Shimadzu RF5301 spectrofluorimenter. FAB mass spectra
were measured with a VG ZAB-E mass spectrometer using
3-nitrobenzyl alcohol as the matrix material, electron impact
mass spectra with a Finnigan MAT Polaris spectrometer, and
electrospray mass spectra with a Micromass Q-ToF spectrom-
eter. Melting points were determined in Pyrex capillary tubes
(sealed under nitrogen where appropriate) with a Mel-Temp
apparatus and are uncorrected. Elemental analyses were
performed at Complete Analysis Laboratories Inc. in Parsip-
pany, NJ .
1-(4-ter t-Bu tylp h en yl)-7-ter t-bu tyl-9-(bis-2,6-(4-ter t-bu -
tylph en yl)ph en yl)-9-bor aflu or en e‚pyr idin e (1‚py). Method
A: see ref 2. Method B: A solution of 2,6-(4-t-BuC6H4)2C6H3-
Br (3.90 g, 9.25 mmol) in hexanes (50 mL) was treated with a
hexane solution of n-BuLi (6.3 mL, 10.1 mmol, 1.6 M) at 0 °C.
The ice bath was removed after 10 min, and the mixture was
allowed to warm to room temperature. After 1 h the off-white
suspension was briefly heated to reflux and then cooled to room
temperature. The volatile material was removed under re-
duced pressure (0.05 Torr), and the resulting solid was
suspended in hexanes (50 mL). The resulting suspension was
cooled to -78 °C, and H2ClB‚SMe2 (0.53 mL, 4.6 mmol) was
added via syringe. No reaction was observed. The reaction
mixture was slowly warmed to room temperature and stirred
Unfortunately, the crystals of 15 were not suitable
for X-ray crystallography, but a crowded structure
similar to 11 or [2,6-Ph2C6H3]2GaI25 is expected.
Su m m a r y
Terphenyl-substituted unsymmetrical 9-borafluorenes
are readily available at or below room temperature by
the reaction of TerphLi with either H2ClB‚SMe2 or
HCl2B‚SMe2 provided the terphenyl substituent pos-
sesses four o- and o′′-hydrogens. If only two of these
(32) Lu¨ning, U.; Wangnick, C.; Peters, K.; Von Schnering, H. G.
Chem. Ber. 1991, 124, 397-402.
(33) Chen, C.-T.; Gantzel, P.; Siegel, J . S.; Baldridge, K. K.; English,
R. B.; Ho, D. M. Angew. Chem., Int. Ed. Engl. 1996, 34, 2657-2660.
(34) Saednya, A.; Hart, H. Synthesis 1996, 1455-1458.
(35) Simons, R. S.; Haubrich, S. T.; Mork, B. V.; Niemeyer, M.;
Power, P. P. Main Group Chem. 1998, 2, 275-283.

90 Organometallics, Vol. 22, No. 1, 2003
Wehmschulte et al.
overnight to afford a cloudy yellow fluorescent solution. After
filtration through a medium-porosity frit the volatile materials
were removed under reduced pressure (0.05 Torr) to give a
bright yellow solid. After redissolution in hexanes (70 mL)
pyridine (1.5 mL, 18.5 mmol) was added, whereupon the color
immediately faded and the solution became slightly cloudy.
Warming to 40-50 °C for a few minutes resulted in a clear,
pale brown solution. Cooling to -30 °C for two weeks afforded
colorless crystals that were covered with a small amount of
an oily substance. Recrystallization from hexanes at room
temperature for 1 day afforded well-shaped crystals of 1‚py
(0.87 g). An additional batch (0.87 g) was obtained from the
concentrated mother liquor of the first crop. Yield: 49%.
P u r ifica tion of 1. A solution of 1‚py (0.81 g, 1.05 mmol) in
Et2O (40 mL) was treated with ca. 1 mL of HClconc. The yellow
solution became cloudy immediately and clarified after a few
minutes. The bright yellow Et2O phase was decanted from the
aqueous phase and dried with MgSO4 (30 min). After filtration
and removal of the solvent under reduced pressure the
resulting yellow glassy solid was redissolved in petroleum
ether (40 mL) and stirred again with MgSO4 (1 h). Filtration
followed by removal of the solvent under reduced pressure
afforded pure 1. Yield: 86%. Crystals suitable for X-ray
diffraction were obtained by slow evaporation of the solvent
from a concentrated solution of 1 in Et2O (ca. 200 mg in 2 mL).
Fluorescence spectrum (hexanes): emission: 495, 515 nm;
excitation: 316, 390, 436, 451 nm.
1-(3,5-Dim eth ylp h en yl)-6,8-d im eth yl-9-(bis-2,6-(3,5-d i-
m eth ylp h en yl)p h en yl)-9-bor a flu or en e (3). Method A: H2-
ClB‚SMe2 (0.64 mL, 6.1 mmol) was added slowly via syringe
to a suspension of 2,6-(3,5-Me3C6H3)2C6H3Li (1.79 g, 6.1 mmol)
in hexanes (50 mL) at -78 °C. After 5 min the dry ice bath
was removed and the reaction mixture was warmed to room
temperature and stirred overnight to give a cloudy, bright
yellow solution. After filtration the volatile material was
distilled off under reduced pressure. The remaining yellow oil
was extracted with hexanes (40 mL) to afford a yellow solution
and 3 as a yellow-green solid that did not dissolve. Yield (based
on 2,6-(3,5-Me3C6H3)2C6H3Li): 21%. Anal. Calcd for C44H41B:
C, 91.02; H, 7.12. Found: C, 90.87; H, 6.98. Mp: 148-152 °C.
1H NMR (C6D6, 300 MHz): 7.42 (d, J ) 7.2 Hz, m-H, 2H), 7.29
(dd, J ) 7.2 Hz, 1.2 Hz, 1H), 7.24 (t, J ) 7.2 Hz, p-H, 1H),
7.19 (s, o-H(3,5-Me2C6H3), 4H), 7.08 (s, 1H), 7.03 (t, J ) 7.2
Hz, 1H), 6.92 (dd, J ) 7.2 Hz, 1.2 Hz, 1H), 6.84 (s, 2H), 6.69
(s, 1H), 6.53 (s, 2H), 6.36 (s, 1H), 2.08 (s, CH3, 6H), 1.99 (s,
CH3, 3H), 1.98 (s, CH3, 12H), 1.92 (s, CH3, 3H). 13C{1H} NMR
(75.45 MHz, C6D6): 153.13, 152.68, 149.25, 144.88, 144.23,
143.62, 143.10, 141.60, 137.69, 136.14, 132.88, 131.43, 130.79,
129.40, 128.52, 126.71, 126.11, 117.98, 117.94, 21.94, 21.71,
21.36, 20.55. UV-vis (CH2Cl2): 353 nm (ꢀ ) 6140), 410 nm (ꢀ
) 430). Fluorescence spectrum (THF): emission: 505 nm;
excitation: 314, 380(sh), 433 nm. MS (EI, 70 eV): m/z 580.1
(M+, 100).
Method B: HCl2B‚SMe2 (0.23 mL, 1.9 mmol) dissolved in
hexanes (7-8 mL) was added dropwise via cannula to a
suspension of 2,6-(3,5-Me3C6H3)2C6H3Li (1.12 g, 3.8 mmol) in
toluene (30 mL) at -78 °C. The reaction mixture was slowly
warmed to room temperature and stirred overnight (ca. 12 h).
After filtration the yellow fluorescent solution was pumped to
dryness and the remaining yellow solid was extracted with
hexanes (45 mL) to leave the essentially hexane-insoluble 3
behind (0.31 g). After concentration of the supernatant liquid
to 15-20 mL and subsequent cooling to -30 °C for 1 day an
additional crop of 3 was obtained (0.02 g). Yield: 29%.
Rea ction of 2,6-(4-t-Bu C6H4)2C6H3Li w ith H2ClB‚SMe2.
Freshly prepared 2,6-(4-t-BuC6H4)2C6H3Li (from 2,6-(4-t-
BuC6H4)2C6H3Br (4.15 g, 9.85 mmol), as above) was suspended
in Et2O (50 mL), and H2ClB‚SMe2 (1.12 mL, 10.8 mmol) was
added via syringe at 0 °C. The reaction mixture was kept at 0
°C for ca. 45 min and then slowly warmed to 5-10 °C,
whereupon the color changed to yellow. The volatile material
was removed under reduced pressure, and the resulting yellow,
glassy solid was stored at -30 °C.
2,6-(4-t-Bu C6H4)2C6H3BH2‚C5H5N (2‚p y). A sample (2 g)
of the crude product from the reaction of 2,6-(4-t-BuC6H4)2C6H3-
Br with H2ClB‚SMe2 (vide supra) was dissolved in hexanes
(40 mL), and the resulting cloudy (LiCl) yellow solution was
treated with pyridine (1 mL). The color faded immediately,
and after 1.5 h some fine, colorless needles formed. The
supernatant liquid was decanted and cooled to -28 °C for 2
days to give colorless needles of 2‚py (0.25 g). A second batch
(0.15 g) was obtained by recrystallization of the needles that
had formed just after the pyridine addition. Mp: 165-166 °C
2,6-(2-MeC6H4)2C6H3Li. A suspension of 2,6-(2-MeC6H4)2-
C6H3I (3.88 g, 10.1 mmol) in hexanes (50 mL) was treated with
n-butyllithium (6.9 mL, 11.0 mmol, 1.6 M in hexanes) at 0 °C.
After initial partial clarification of the solution a fine, colorless
solid began to precipitate. The reaction mixture was warmed
to room temperature and stirred overnight (ca. 12 h). The fine,
colorless solid was collected on a glass frit and dried under
reduced pressure. Yield: 87%. 1H NMR(C6D6, 400 MHz): 7.12
(m, ArH, 5H), 6.92 (m, ArH, 6H), 2.06 (s, Me, 6H). 13C{1H}
NMR (100 MHz, C6D6): 176.5 (br, i-C), 151.4, 146.3, 136.8
(quaternary carbons), 131.8, 127.6, 127.4, 127.3, 124.4, 20.9
(Me).
1
(with gas evolution). H NMR (300 MHz, C6D6): 7.55 (d, J )
8.4 Hz, o- or m-H(4-t-BuC6H4), 4H), 7.50 (m, o-H(py), 2H), 7.46
(d, J ) 7.4 Hz, m-H, 2H), 7.33 (t, J ) 7.4 Hz, p-H, 1H), 7.19
(d, J ) 8.4 Hz, o- or m-H(4-t-BuC6H4), 4H), 6.36 (m, p-H(py),
1H), 5.90 (m, m-H(py), 2H), 4.08 (s, br, w1/2 ) 180 Hz, BH,
2H), 1.24 (s, CH3, 18 Hz). 13C{1H} NMR (100.57 MHz, C6D6):
149.19, 147.98, 147.01, 144.42, 137.00, 129.98, 128.77, 125.92,
124.34, 123.63, 34.34 (C(CH3)3), 31.61 (CH3). 11B NMR (160.38
MHz, C6D6): -5.4 (s, br), w1/2 ≈ 420 Hz. IR: 2417(m), 2361(sh),
2338(m), 2296(sh) cm-1, ν (BH). MS (EI, 70 eV, 210 °C): m/z
431.2 (M+ - H2, 16), 416.2 (M+ - H2 - Me, 23), m/z 352.2 (M+
- H2 - py, 76), m/z 337.3 (M+ - H2 - Me - py, 100).
2,6-(3,5-Me3C6H 3)2C6H 3Li. A solution of 2,6-(3,5-Me3C6-
H3)2C6H3I (4.40 g, 10.7 mmol) in hexanes (45 mL) was treated
with n-butyllithium (6.9 mL, 11.0 mmol, 1.6 M in hexanes) at
0 °C. The ice bath was removed, and the pale yellow reaction
mixture was stirred overnight. A small amount of colorless
solid was separated, and the clear, pale yellow solution was
pumped to dryness to afford an off-white solid. Yield: 95%.
1H NMR (C6D6, 300 MHz): 7.47 (d, J ) 7.2 Hz, m-H, 2H), 7.31
(t, J ) 7.2 Hz, p-H, 1H), 7.20 (s, o-H(3,5-Me3C6H3), 4H), 6.58
(s, 20 (s, p-H(3,5-Me3C6H3), 2H), 2.00 (s, CH3, 12H). 13C{1H}
NMR (75.45 MHz, C6D6): 152.22, 147.78, 140.08, 128.55,
126.32, 124.66, 124.14, 21.25 (CH3). 7Li NMR (C6D6, 155.44
MHz): 1.95 (s).
[2,6-(2-MeC6H4)2C6H3BH(µ-H)]2 (4). H2ClB‚SMe2 (0.92
mL, 8.8 mmol, 0.98 g) was added via syringe to a suspension
of 2,6-(2-MeC6H4)2C6H3Li (2.34 g, 8.8 mmol) in hexanes (50
mL) at -78 °C. The mixture was slowly warmed to room
temperature and stirred for 4 h, whereupon the color changed
to pale yellow, and a fine colorless precipitate formed. After
filtration and removal of the volatile materials under reduced
pressure the pale yellow solid was redissolved in hexanes (5
mL). Cooling to -28 °C for 3 days afforded small colorless
crystals (1.36 g). A second batch of larger crystals was obtained
from the concentrated mother liquor (ca. 1-2 mL) after 1 day
at room temperature. Yield: 1.47 g (61%). Anal. Calcd for
C
40H38B2: C, 88.91; H, 7.09. Found: C, 88.45; H, 6.99. Mp:
softens at 105 °C, melts with gas evolution at 120-125 °C,
and turns yellow. 1H NMR (400 MHz, C6D6): 7.03-6.83 (m,
aromatic H, 22H), 3.75 (s, broad, w1/2 ≈ 260 Hz, B-H, 2H), 1.89,
1.88, 1.83, 1.82, 1.754, 1.749 (o-Me, 12H, rel intensity: 0.07:
0.13:0.23:0.34:0.09:0.14), 1.52 (s, br, w1/2 ≈ 80 Hz, B-H, 2H).
11B NMR (160.38 MHz, C6D6): 19.8 (s, br), w1/2 ≈ 460 Hz. MS
(FAB, 3NBA matrix): m/z 269.1 (M+/2 - H). IR: 2513 cm-1
(m), ν(BH).

Unsymmetrical 9-Borafluorenes
Organometallics, Vol. 22, No. 1, 2003 91
2,6-(2-MeC6H4)2C6H3BH2‚p y (4‚p y). Pyridine (0.18 mL, 2.2
mmol) was added to a solution of 8 (0.30 g, 0.55 mmol) in
hexanes (30 mL) at room temperature to give a colorless cloudy
solution. After 2 h stirring 4‚py was isolated by removal of
the volatile materials under reduced pressure as a colorless
powder. Mp: 128 °C. 1H NMR (300 MHz, C6D6): 7.48 (m,
o-H(py), 2H), 7.23 (m, 4H), 7.11 (m, 3H), 6.96 (m, 4H), 6.29
(m, p-H(py), 1H), 5.94 (m, m-H(py), 2H, 3.5 (s, br, w1/2 ) 250
Hz, B-H, 2H), 2.44 and 2.07 (s, Me, 6H, syn and anti). 13C-
{1H} NMR (75.45 MHz, C6D6): 148.54. 148.25, 147.08, 147.03,
146.89, 146.77, 137.60, 137.35, 136.63, 130.67, 129.72, 129.40,
128.96, 126.32, 126.24, 126.29, 125.71, 125.15, 124.91, 123.90,
123.81, 21.18, 20.99. 11B NMR (160.38 MHz, C6D6): -6.1 (s,
br), w1/2 ≈ 320 Hz. IR: 2338(m), 2345(m). MS (EI, 70 eV): m/z
349.2 (M+, 44), 347.2 (M+ - H2, 38), 268.1 (M+ - H2 - py,
100).
MHz, C6D6): 7.6 (s, br), w1/2 ≈ 350 Hz. IR: 2312(w). MS (EI,
70 eV, 225 °C): m/z 431.2 (M+, 48), 416.2 (M+ Me, 100), m/z
352.2 (M+ - py, 61), m/z 337.3 (M+ - Me - py, 96).
1-(3,5-Dim et h ylp h en yl)-6,8-d im et h yl-9-b or a flu or en e‚
p yr id in e (9‚p y). Pyridine (1.0 mL, 12.2 mmol) was added to
the yellow mother liquor (50 mL) obtained in the synthesis of
3 according to method A. Concentration to 10-15 mL followed
by cooling to -30 °C for 3 days afforded a small amount of a
colorless solid (70 mg) and a yellow oil. The colorless solid was
identified as pure 9‚py, whereas the oil was found to be impure
1
9‚py. Mp: 162-167 °C. H NMR (C6D6, 300 MHz): 7.92 (dd,
J ) 7.5 Hz, 0.9 Hz, 1H), 7.70, (s, 1H), 7.69 (m, o-H(py), 2H),
7.42 (t, J ) 7.5 Hz, 1H), 7.27 (dd, J ) 7.5 Hz, 0.9 Hz), 7.08 (s,
o-H(3,5-Me2C6H3), 2H), 6.93 (s, 1H), 6.67 (s, 1H), 6.39 (m,
p-H(py), 1H), 5.93 (m, m-H(py), 2H), 4.59 (s, br, w1/2 ) 190
Hz, B-H, 1H), 2.43 (s, CH3, 3H), 2.13 (s, CH3, 3H), 2.07 (s, 3,5-
(CH3)2C6H3, 3H). 13C{1H} NMR (75.45 MHz, C6D6): 150.20,
150.02, 146.62, 145.33, 145.10, 139.05, 138.43, 136.62, 136.44,
128.56, 127.60, 127.21, 127.03, 126.53, 124.01, 118.60, 118.33,
21.92, 21.32 (3,5-(CH3)2C6H3), 21.27. 11B NMR (160.38 MHz,
C6D6): -1.0 (s, br), w1/2 ≈ 350 Hz. IR: 2341(m). MS (EI, 70
eV): m/z 375.3 (M+, 100), 296.1 (M+ - py, 99).
[2,6-Mes2C6H3BH(µ-H)]2 (5). A solution of [2,6-Mes2C6H3-
Li]2 (0.90 g, 1.4 mmol) in Et2O (30 mL) was treated with H2-
ClB‚SMe2 (0.29 mL, 2.8 mmol, 0.31 g) at -78 °C. The clear,
colorless solution was held at -78 °C for an hour and then
slowly warmed to room temperature, whereupon
a fine,
colorless precipitate formed. After stirring for an additional 2
h at room temperature the mixture was filtered through a
medium-porosity glass frit. Volatile materials were removed
from the filtrate under reduced pressure, and the remaining
pale yellow viscous oil was extracted with ca. 70 mL of
hexanes. Filtration followed by concentration to ca. 50 mL and
cooling to -28 °C overnight afforded large (2-3 mm) colorless
crystals. Yield: 0.49 g (54%). Anal. Calcd for C48H54B2: C,
88.35; H, 8.34. Found: C, 88.16; H, 8.46. Mp: turns opaque
[2,6-(4-t-Bu C6H4)2C6H3]2B(µ-H)2Li[O(CH2CH2)2]2 (11). H2-
ClB‚SMe2 (0.21 mL, 0.22 g, 2.0 mmol) was added dropwise via
syringe to a suspension of 2,6-(4-t-BuC6H4)2C6H3Li (1.40 g, 4.0
mmol) in hexanes at -78 °C. The mixture was kept at this
temperature for 1 h, warmed to room temperature, and stirred
for 18 h to give a pale yellow cloudy solution. After filtration
the clear yellow filtrate was concentrated to ca. 2 mL and
cooled to - 40 °C for 2 days. As no crystals were obtained, the
remaining solid was distilled off under reduced pressure, and
the resulting yellow glass was redissolved in hexanes (30 mL).
THF (2 mL) was added, and a fluffy colorless precipitate
formed immediately. Partial dissolution of the precipitate by
gentle warming with a heatgun (ca. 40-50 °C) for 2-3 min
followed by slow cooling to room temperature afforded 11 as
fine, colorless to pale yellow needles. Yield: 22%. Mp: 190-
198 °C with gas evolution and color change to yellow fluores-
cent. 1H NMR (300 MHz, C6D6): 7.32 (s, o- and m-H, 16H),
7.02 (m, p- and m- H, 6H), 3.18 (s, br, OCH2, 8H), 2.17 (q, 1:1:
1:1 rel int, J H-B) 73 Hz, B-H, 2H), 1.39 (s, CH3, 36H), 1.23 (s,
br, CH2, 8H). 13C{1H} NMR (75.45 MHz, C6D6): 148.27, 147.16,
146.26, 129.50, 128.47, 124.02, 123.25, 68.49 (OCH2), 34.60
(C(CH3)3), 32.06 (CH3), 25.60 (CH2). 11B NMR (160.38 MHz,
C6D6): -20.0 (t, J ) 73 Hz, w1/2 ) 27 Hz). FTIR (CsI plates,
1
at ca. 150 °C and melts with gas evolution at 235-240 °C. H
3
NMR (400 MHz, C6D6): 7.13 (t, p-H, 2H), J HH) 8.0 Hz, 6.84
(d, m-H, 4H), 6.82 (s, m-H(Mes), 8H), 3.36 (s, br, w1/2 ≈ 270
Hz, B-H, 2H), 2.23 (s, p-Me, 12H), 1.80 (s, o-Me, 24H), 0.95 (s,
br, w1/2 ≈ 95 Hz, B-H, 2H), 13C{1H} NMR (100 MHz, C6D6):
146.3, 140.4, 136.1, 135.6, 129.4 (p-C), 128.5 (m-C(Mes)), 127.4
(m-C), 21.2 (p-Me), 20.6 (o-Me). 11B NMR (160.38 MHz, C6D6):
17.6 (s, br), w1/2 ≈ 290 Hz. IR: 2546 cm-1 (m), ν (BH). MS (EI,
70 eV): m/z 652.0 (M+, 17), 326.1 (M+/2, 70), 324.1 (M+/2 -
H2, 100).
1-(4-t er t -Bu t ylp h e n yl)-7-t er t -b u t yl-9-b or a flu or e n e ‚
p yr id in e (7‚p y). Method A: The remainder (ca. 2 g) of the
crude product from the reaction of 2,6-(4-t-BuC6H4)2C6H3Br
with H2ClB‚SMe2 (vide supra) was dissolved in hexanes (40
mL), and the resulting cloudy (LiCl) yellow solution was stirred
at room temperature for 3 days, filtered, and treated with
pyridine (1 mL). The fine precipitate was redissolved by gentle
warming (40 °C). After standing at room temperature for 1
day a yellow-orange oil separated, which was shown to be a
mixture of mainly 7‚py and some 2‚py. Cooling of the super-
natant liquid to -28 °C afforded pure 7‚py in the form of small
crystalline spheres (0.1 g). Removal of the solvent from the
mother liquor resulted in a pale yellow glassy solid, which was
impure 1‚py.
mineral oil): 2234(m), 2210(m), 2168(m) cm-1
.
1 -( 2 -M e t h y l p h e n y l ) -5 -m e t h y l -9 -b o r a f l u o r e n e ‚
p yr id in e (12‚p y). A Schlenk flask was charged with 4‚py
(0.065 g) and placed into a 200 °C oil bath. After 2 min the
powder began to melt, and a minute later a smooth gas
evolution commenced from the yellow liquid which stopped
after about 2 min. After cooling to room temperature the melt
solidified into a yellow glass. Anal. Calcd for C25H22BN: C,
1
86.47; H, 6.39. Found: C, 86.23; H, 6.50. Mp: 74-78 °C. H
NMR (300 MHz, C6D6): 8.09 (d, J ) 7.8 Hz, 1H), 7.75 (s, br,
0.5H), 7.42 (m, 4H), 7.10 (m, 4.5H), 6.96 (t, J ) 7.8 Hz, 1H),
6.68 (s, br, 0.5H), 6.45 (s, br, 2H), 6.27 (s, br, 0.5H), 5.96 (s,
br, 2H), 4.25 (s, br, w1/2 ≈ 230 Hz, B-H, 1H), 2.80 (s, CH3, 3H),
2.45 and 1.45 (s, br, Me, 3H in 0.55:0.45 ratio). 13C{1H} NMR
(75.45 MHz, C6D6): 146.55, 138.56, 132.62, 131.37, 130.44,
129.90, 128.96, 126.52, 126.14, 125.77, 125.67, 124.29, 122.96,
23.24, 20.81, 20.14. 11B NMR (160.38 MHz, C6D6): -0.6 (s, br),
w1/2 ≈ 300 Hz. IR: 2351(m). MS (EI, 70 eV): m/z 347.1 (M+,
70), 268.2 (M+ - py, 75).
[2,6-(4-t-Bu C6H4)2C6H3]2BOH (15). A solution of 1‚py (0.41
g, 0.53 mmol) in Et2O (30 mL) was treated with concentrated
HCl (1 mL) to give a cloudy solution. No significant color
change was observed after 6 days at room temperature. As a
sample taken at this time contained still mainly unreacted 1,
additional HCl (1 mL) was added and stirring was continued
for another week, whereupon the color faded. The Et2O phase
Method B: A Schlenk flask charged with 2‚py (0.075 g, 0.17
mmol) was placed into a 190 °C preheated oil bath. The
crystals began to melt after approximately 1 min with gentle
gas evolution. The gas evolution stopped after 2 min, leaving
a pale yellow liquid behind, which solidified into a pale yellow
glass after cooling to room temperature. Anal. Calcd for C31H34
-
BN: C, 86.30; H, 7.94. Found: C, 86.35; H, 8.03. Mp: 113-
115 °C. 1H NMR(C6D6, 400 MHz): 7.97 (d, J ) 8.0 Hz, 1H),
7.93 (d, J ) 7.6 Hz, 1H), 7.77 (s, 1H), 7.74 (m, o-H(py), 2H),
7.53 (d, J ) 8.0 Hz, o- or m-H(4-t-BuC6H4), 2H), 7.47 (m, 2H),
7.33 (d, J ) 7.6 Hz, 1H), 7.02 (d, J ) 8.0 Hz, o- or m-H(4-t-
BuC6H4), 2H), 6.35 (m, p-H(py), 1H), 5.93 (m, m-H(py), 2H),
4.75 (s, br, w1/2 ) 195 Hz, 1H), 1.36 (s, CH3, 9H), 1.18 (s, CH3,
9H). 13C{1H} NMR (100.57 MHz, C6D6): 150.37, 148.91,
148.34, 147.50, 146.74, 145.05, 142.38, 138.66, 128.85, 128.09.
126.44, 124.55, 124.42, 124.18, 119.62, 118.50, 34.82 (C(CH3)3),
34.29 (C(CH3)3), 31.96 (CH3), 31.56 (CH3). 11B NMR (160.38

92 Organometallics, Vol. 22, No. 1, 2003
Wehmschulte et al.
was decanted from the aqueous phase, and the solvent was
distilled off under reduced pressure (ca. 400 Torr). The
remaining pale yellow oil was dissolved in petroleum ether
(40 mL) and dried over MgSO4. Removal of the solvent at
reduced pressure (ca. 400 Torr) afforded 15 as a colorless solid,
which was recrystallized from petroleum ether (30 mL) at -30
°C for one week to give small colorless needles. Yield: 17%.
Anal. Calcd for C52H59BO: C, 87.86; H, 8.37. Found: C, 87.93;
for 1 and 4 but for 11 by a multiscan method from equivalent
reflections. The structures were solved by direct methods using
the SHELXTL system37 and refined by full-matrix least-
squares on F2 including all reflections. All the non-hydrogen
atom were refined anisotropically, and all the hydrogen atoms
were included in the refinement with idealized parameters
with the exception of the hydrogen atoms attached to the B
atoms in the structures of 4 and 11, which were located in the
difference map and refined isotropically. The tert-butyl group
C(24)-C(26) in 1 displayed rotational disorder and was refined
using split occupancies of 0.70 for C(24)-C(26) and 0.30 for
C(24A)-C(26A). Some details of the crystal data and refine-
ment are given in Table 1, and selected bond distances and
angles are listed in Tables 2, 3, and 4. Further details are given
in the Supporting Information.
1
H, 8.52. Mp: 193-197 °C. H NMR (300 MHz, CDCl3): 7.24
(d, J ) 8.7 Hz, o-or m-H(4-t-BuC6H4), 8H), 7.12 (d, J ) 8.7
Hz, o-or m-H(4-t-BuC6H4), 8H), 7.06 (t, J ) 7.2 Hz, p-H, 2H),
6.88 (d, J ) 7.2 Hz, m-H, 4H), 5.94 (s, B-OH, 1H), 1.33 (s, Me,
36H). 13C{1H} NMR (75.45 MHz, CDCl3): 148.84, 146.86,
140.69, 129.18, 128.79, 128.08, 124.35, 34.47 (C(CH3)3), 31.35
(CH3). 11B NMR (160.38 MHz, CDCl3): 47 (s, br, w1/2 ) 1500
Hz). IR: 3537 (m), ν(O-H). MS (EI, 70 eV, 270 °C): m/z 485.2
(M+ - 4 CH2C(CH3)2 - H, 75).
Ack n ow led gm en t. Financial support for this work
from the University of Oklahoma and the donors of the
Petroleum Research Fund, administered by the ACS,
is gratefully acknowledged. We are also grateful to the
NSF for purchase of the CCD diffractometer.
X-r a y Cr ysta llogr a p h y. Crystals of 1 were obtained by
slow evaporation of a concentrated Et2O solution of 1 at room
temperature. Crystals were removed from the Schlenk tube
under a stream of N2-gas and immediately covered with a layer
of hydrocarbon oil. A suitable crystal was selected, attached
to a glass fiber, and immediately placed in the low-temperature
nitrogen stream.36 The data for 1 and 4 were collected at 173(2)
K on a Siemens P4 diffractometer, and those for 11 at 103(2)
K on a Bruker Apex diffractometer using Mo KR (λ ) 0.71073
A) radiation. The data were corrected for Lorentz and polar-
ization effects. Absorption corrections were not applied
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
tables, numbering schemes, and unit cell plots for compounds
1, 4, and 11. Data are also given as CIF files. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM020611V
(36) Hope, H. In Experimental Organometallic Chemistry; Wayda,
A. L., Darensbourg, M. Y., Eds.; ACS Symposium Series 357; American
Chemical Society: Washington, DC, 1987; Chapter 10.
(37) SHELXTL Versions 5.1 and 6.12; Bruker AXS: Madison, WI,
1997.