Synthesis and characterization of a sterically encumbered unsymmetrical 9-borafluorene, its pyridine adduct, and its dilithium salt

Article abstract of DOI:10.1021/om000868y

The reaction of 2,6-(4-t-BuC₆H₄)₂C₆H₃Li with BH₂Cl·SMe₂ in hexane or Et₂O solution affords the 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. Addition of pyridine gives the colorless crystalline adduct 1·py. Compound 1 is readily reduced to the deep red heteroaromatic dianionic (μ₂-η⁵,η⁵-1-(4-tert-butylphenyl)-7-t ert-butyl- 9-(bis-2,6-(4-tert-butylphenyl)phenyl)-9-borafluorenyl)bis(diethyl ether)dilithium (2) with excess lithium powder in Et₂O solution. Reactions of the dianionic 2 with various metal salts leads to reduction of these salts, and bright yellow 1 is recovered inessentially quantitative yields. Compounds 1, 1·py, and 2 were characterized by¹H, ¹³C, and ¹¹B NMR spectroscopy and compounds 1·py and 2 also by single-crystal X-ray diffraction.

Full text of DOI:10.1021/om000868y

844
Organometallics 2001, 20, 844-849
Syn th esis a n d Ch a r a cter iza tion of a Ster ica lly
En cu m ber ed Un sym m etr ica l 9-Bor a flu or en e, Its P yr id in e
Ad d u ct, a n d Its Dilith iu m Sa lt
Rudolf J . Wehmschulte,*,† Masood A. Khan,† Brendan Twamley,‡ and
Berthold Schiemenz†
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval,
Room 208, Norman, Oklahoma 73019, and University of Idaho, Morill Hall 109,
PO Box 443010, Moscow, Idaho 83844-3010
Received October 11, 2000
The reaction of 2,6-(4-t-BuC6H4)2C6H3Li with BH2Cl‚SMe2 in hexane or Et2O solution
affords the 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.
Addition of pyridine gives the colorless crystalline adduct 1‚p y. Compound 1 is readily
reduced to the deep red heteroaromatic dianionic (µ2-η5,η5-1-(4-tert-butylphenyl)-7-tert-butyl-
9-(bis-2,6-(4-tert-butylphenyl)phenyl)-9-borafluorenyl)bis(diethyl ether)dilithium (2) with
excess lithium powder in Et2O solution. Reactions of the dianionic 2 with various metal
salts leads to reduction of these salts, and bright yellow 1 is recovered in essentially
1
quantitative yields. Compounds 1, 1‚p y, and 2 were characterized by H, 13C, and 11B NMR
spectroscopy and compounds 1‚p y and 2 also by single-crystal X-ray diffraction.
In tr od u ction
be successfully transferred to the boratabenzene and
borole systems. However, the syntheses of 1- and
2-boranaphthalenes, 9-borafluorenes, and related com-
pounds are still very demanding and not very practical
for routine use. In the early 1960s, Ko¨ster and co-
workers reported that 9-borafluorenes were formed
upon thermolysis of 2-biphenyldialkylboranes,11 and in
1973, Bickelhaupt and van Veen showed that the
pyridine adduct of 2-biphenylborane could also be
converted into the corresponding pyridine adduct of
9-borafluorene.12 Both methods required temperatures
of about 200 °C.
Assuming that a three-coordinate boron hydride spe-
cies is the active intermediate in these ring-closure
reactions, we set out to synthesize a series of base-free
m-terphenylboranes, [ArBH2]2, and study their potential
as precursors for unsymmetrical 9-borafluorenes via
low-temperature C-H activation (eq 1). Unsymmetrical
9-borafluorenes such as the one shown in eq 1 could be
of interest as ligands in catalytic systems.
The synthesis of the first stable boratafluorene com-
plex [CpCo(C5H5BPh)]+ by Herberich et al.1 in 1970 has
sparked interest in boratabenzenes, boroles, and deriva-
tives as potential replacements for the ubiquitous cy-
clopentadienyl ligand in organometallic compounds.2
More recently, Ashe and Bazan3-6 have successfully
introduced these compounds as valuable ligands for
“single-site” olefin polymerization catalysts. The most
important feature of these systems is the ease of tuning
the electronic properties by simply varying the exocyclic
substituent on the boron atom. Steric properties can be
affected by introducing different substituents around
the ring system. Benzannulation (e.g., in the cases of
indenyl or fluorenyl) is among the more common ways
of increasing the size of the cyclopentadienyl ligand.
Initial results from the groups of Ashe,7 Bazan,8 Her-
berich,9 and Paetzhold10 indicate that this concept can
* Corresponding author. Fax: +14053256111. E-mail: rjwehmschulte@
ou.edu.
Here, we report about the unexpected formation of
the unsymmetrical 9-borafluorene 1 even below room
temperature, the crystalline pyridine adduct 1‚p y, and
the reduction of 1 to the dianionic 2.
† University of Oklahoma.
‡ University of Idaho.
(1) Herberich, G. E.; Greiss, G.; Heil, H. F. Angew. Chem., Int. Ed.
Engl. 1970, 9, 805-806.
(2) Herberich, G. E.; Ohst, H. Adv. Organomet. Chem. 1986, 25,
199-236.
(3) Bazan, G. C.; Donnelly, S. J .; Rodriguez, G. J . Am. Chem. Soc.
1995, 117, 2671-2672.
Exp er im en ta l Section
(4) Bazan, G. C.; Rodriguez, G.; Ashe, A. J ., III; Al-Ahmad, S.;
Mu¨ller, C. J . Am. Chem. Soc. 1996, 118, 2291-2292.
(5) Bazan, G. C.; Rodriguez, G.; Ashe, A. J ., III; Al-Ahmad, S.;
Kampf, J . W. Organometallics 1997, 16, 2492-2494.
(6) Ashe, A. J ., III; Al-Ahmad, S.; Fang, X. J . Organomet. Chem.
1999, 581, 92-97.
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 Na/K alloy and
(7) Ashe, A. J ., III; Fang, X.; Kampf, J . W. Organometallics 1999,
18, 466-473.
(10) Boese, R.; Finke, N.; Henkelmann, J .; Maier, G.; Paetzold, P.;
Reisenauer, H. P.; Schmid, G. Chem. Ber. 1985, 118, 1644-1654.
(11) Ko¨ster, R.; Benedikt, G. Angew. Chem. 1963, 75, 419.
(12) Veen, R. v.; Bickelhaupt, F. J . Organomet. Chem. 1973, 47, 33-
38.
(8) Lee, R. A.; Lachicotte, R. J .; Bazan, G. C. J . Am. Chem. Soc.
1998, 120, 6037-6046.
(9) Herberich, G. E.; Eigendorf, U.; Englert, U. Chem. Ber. 1993,
126, 1397-1402.
10.1021/om000868y CCC: $20.00 © 2001 American Chemical Society
Publication on Web 01/31/2001

Unsymmetrical 9-Borafluorene
Organometallics, Vol. 20, No. 5, 2001 845
Ta ble 1. Deter m in a tion of K for th e Dissocia tion
of 1‚p y
minitial
mg
/
K ) [1][py]/[1‚p y]/
[1‚p y]/mol L-1
[py]/mol L-1
mol L-1
6(1)
10(1)
16(1)
5.33 × 10-3
10.12 × 10-3
17.32 × 10-3
3.50 × 10-3
4.60 × 10-3
6.23 × 10-3
2.30 × 10-3
2.09 × 10-3
2.24 × 10-3
Cl‚SMe2 (0.67 mL, 6.4 mmol, 0.71 g) was added with a syringe.
The reaction mixture was kept at -78 °C for 20 min, slowly
warmed to room temperature, and stirred overnight (ca. 12
h). A color change to yellow commenced around 0 °C. Filtration
and removal of the volatile materials afforded a bright yellow
solid which contained ca. 75% 1 according to 1H NMR
spectroscopy. A portion of this solid (0.60 g) was dissolved in
hexanes (15 mL) and treated with pyridine (0.2 mL, 0.19 g,
2.5 mmol), whereupon the color immediately faded and a small
amount of a colorless precipitate formed. Filtration followed
by concentration of the pale yellow filtrate to ca. 5-7 mL and
storage at room temperature afforded large (2-3 mm) colorless
crystals of 1‚p y (0.23 g). Concentration of the mother liquor
and cooling to -28 °C gave another 0.1 g of product. Yield:
54% (based on 2,6-(4-t-BuC6H4)2C6H3Br). Anal. Calcd for
C
57.75H63.75BN: C, 88.62; H, 8.21; N, 1.79. Found: C, 88.36; H,
8.69; N, 1.98. Mp softens at 70 °C, changes color to yellow at
85-90 °C, melts with gas evolution at 125-140 °C. H NMR
degassed twice prior to use. BH2Cl‚SMe2, 4-tert-butylbro-
mobenzene, 2-bromobiphenyl, m-dichlorobenzene, n-butyl-
lithium (1.6 M in hexanes), lithium powder, FeCl2, FeBr2,
ZrCl4, CpZrCl3, and SnCl2 were obtained from commercial
suppliers. Cu(MeCN)PF6 was synthesized according to the
literature.13 1H and 13C NMR spectra were recorded on a
Varian Unity Plus 400 MHz spectrometer and 11B NMR
spectra on a Varian VXR 500S spectrometer. 1H NMR chemical
shift values were determined relative to the residual protons
in C6D6 or CDCl3 as internal reference (δ 7.15 or 7.26 ppm).
13C NMR spectra were referenced to the solvent signals (δ
128.0 or 77.0 ppm). 11B NMR spectra were referenced to a
CDCl3 solution of BF3‚OEt2 as external standard (δ 0 ppm).
UV-vis spectra were recorded on a Shimadzu UV-2401PC
spectrophotometer. FAB mass spectra were measured with a
VG ZAB-E mass spectrometer using 3-nitrobenzyl alcohol as
the matrix material. Melting points were determined in Pyrex
capillary tubes (sealed under nitrogen where appropriate) with
a Mel-Temp apparatus and are uncorrected.
1
(400 MHz, C6D6): 8.52 (s, br, 1H), 8.08 (s, br, 1H), 8.03 (s, br,
1H), 7.37 (t, 3H), J ) 7.6 Hz, 7.19 (s, br, 4H), 6.81 (s, br, 2H),
6.69 (s, br, 2H), 6.53 (s, br, 1H), 6.13 (s, br, 1H), 5.61 (s, br,
1H), 5.52 (s, br, 1H), 1.41 (s, br, 9H), 1.34 (s, br, 9H), 1.30 (s,
9H), 0.98 (s, br, 9H). 11B NMR (160.38 MHz, C6D6): 4.2 (s, br,
w1/2 ≈ 1470 Hz).
Deter m in a tion of th e Dissocia tion Con sta n t of 1‚p y.
1H NMR spectra of three samples containing different amounts
of 1‚p y (6, 10, 16 mg) in C6D6 (0.88 mL) were collected at 20
°C, and the relative concentrations of free pyridine, [py], and
of 1‚p y, [1‚p y], were determined by integration of the signals
at 8.52 (o-H, free pyridine) and 6.13 and 5.56 ppm (p- and m-H,
coordinated pyridine in 1‚p y). With [py] ) [1] and cinitial) [1‚
p y] + [py] the equilibrium constant K ) 0.0022(5) mol L-1
was obtained (Table 1).
(µ2-η5,η5-1-(4-ter t-Bu t ylp h en yl)-7-ter t-b u t yl-9-(b is-2,6-
(4-ter t-b u t ylp h en yl)p h en yl)-9-b or a flu or en yl)b is(d iet h -
yl eth er )d ilith iu m (2). Lithium powder (high sodium, Ald-
rich, 0.31 g) was added to a -15 °C Et2O solution (60 mL) of
crude 1, synthesized as above from 2,6-(4-t-BuC6H4)2C6H3Br
(2.79 g, 6.6 mmol). The reaction mixture, which turned dark
green after a few minutes, was held at -15 °C for 30 min,
2,6-(4-t-Bu C6H4)2C6H3Br was synthesized according to a
modification of the literature procedure14 allowing for large-
scale syntheses as described by Power et al.15 after quenching
with Br2 at -15 °C instead of I2 as a colorless crystalline solid
1
(101 g, 53% yield). Mp: 145-9 °C. H NMR (400 MHz, C6D6):
slowly warmed to room temperature, filtered through
a
7.43 (d, o- or m-H (4-t-BuC6H4), 4H), 3J HH ) 8.0 Hz, 7.31 (d, o-
medium-porosity glass frit, and concentrated to ca. 10 mL.
Small crystals began to form after a few minutes. Cooling to
-28 °C for a day afforded a second crop of deep red, almost
black crystals (1.20 g, 42% yield based on 2,6-(4-t-BuC6H4)2C6H3-
Br). Anal. Calcd for C60H77BLi2O2: C, 84.29; H, 9.08. Found:
C, 84.10; H, 8.81. Mp: 180-180 °C (dec with gas evolution
3
or m-H (4-t-BuC6H4), 4H), 7.17 (d, m-H, 2H), J HH ) 7.4 Hz,
7.02 (t, p-H, 1H), 1.22 (s, Me, 18H). 13C{1H} NMR (100 MHz,
CDCl3): 150.3, 143.7, 139.1, 130.1 (m-C), 129.2 (o- or m-C, 4-t-
BuC6H4), 126.8 (p-C), 124.8 (o- or m-C, 4-t-BuC6H4), 123.2 (i-
C), 34.6 (C(CH3)3), 31.4 (CH3).
1-(4-ter t-Bu tylp h en yl)-7-ter t-bu tyl-9-(bis-2,6-(4-ter t-bu -
tylp h en yl)p h en yl)-9-bor a flu or en e‚p yr id in e (1‚p y). A so-
lution of 2,6-(4-t-BuC6H4)2C6H3Br (2.41 g, 5.7 mmol) in hexanes
(50 mL) was treated with n-butyllithium in hexanes (4.0 mL,
1.6 M, 6.4 mmol, 10% excess) with cooling in an ice bath. After
ca. 10 min the ice bath was removed and the pale yellow
reaction mixture was slowly warmed to room temperature and
stirred for 1 h. The fine colorless precipitate was dissolved by
heating to reflux for 2-3 min. After cooling to room temper-
ature solvents were removed from the cloudy reaction mixture
under reduced pressure. The resulting off-white solid was
suspended in hexanes (50 mL) and cooled to -78 °C, and BH2-
1
and color change to yellow). H NMR (400 MHz, C6D6): 8.15
(d, 1H, J ) 0.8 Hz), 8.02 (d, 1H, J ) 9.2 Hz), 7.97 (d, 1H, J )
8.0 Hz), 7.28 (s, br, 4H), 7.52 (d, 2H, J ) 8.2 Hz), 7.31 (d, 2H,
J ) 8.2 Hz), 7.21, 7.20 (A2B system, 3H, J ≈ 8 Hz), 7.02 (d,
4H, J ) 8.4 Hz), 6.90 (dd, 1H, J ) 9.2 Hz, 0.8 Hz), 6.84 (d, 1H,
J ) 6.4 Hz), 6.69 (dd, 1H, J ) 8.0 Hz, 6.4 Hz), 2.50 (q, 8H,
OCH2, J ) 7.2 Hz), 1.52 (s, 9H, t-Bu), 1.46 (s, 9H, t-Bu), 1.04
(s, 18H, t-Bu), 0.16 (t, 12 H, OCH2CH3, J ) 7.2 Hz). 13C{1H}
NMR (100.57 MHz, C6D6): 148.35, 147.46, 147.22, 147.19,
144.54, 143.15, 138.27, 128.97, 128.75, 128.65, 125.42, 125.09,
125.00, 123.41, 120.70, 120.02, 117.70, 113.39, 112.27, 111.93,
110.41, 66.37 (OCH2), 34.89, 34.40, 34.15 (C(CH3)3), 32.27,
31.86, 31.31 (C(CH3)3), 13.50 (OCH2CH3). 11B NMR (160.38
MHz, C6D6): 13.6 (s, br, w1/2 ≈ 430 Hz). 7Li NMR (155.44 MHz,
C6D6): -10.3 (s, w1/2 ≈ 20 Hz). UV-vis (hexanes): 380 nm (sh,
ꢀ ) 5420), 435 nm (ꢀ ) 8450), 595 nm (ꢀ ) 1160).
(13) Kubas, G. J . Inorg. Synth. 1990, 28, 68-70.
(14) Saednya, A.; Hart, H. Synthesis 1996, 1455-1458.
(15) Simons, R. S.; Haubrich, S. T.; Mork, B. V.; Niemeyer, M.;
Power, P. P. Main Group Chem. 1998, 2, 275-283.

846 Organometallics, Vol. 20, No. 5, 2001
Wehmschulte et al.
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r a l Refin em en t
1-(4-ter t-Bu tylp h en yl)-7-ter t-bu tyl-9-(bis-2,6-(4-ter t-bu -
tylp h en yl)p h en yl)-9-bor a flu or en e (1). SnCl2 (0.16 g, 0.82
mmol) was added to a -78 °C solution of 2 (0.70 g, 0.82 mmol)
in Et2O (50 mL). The mixture was slowly warmed to room
temperature. At a bath temperature of ca. -20 °C the deep
red color of the solution began to fade and a fine dark solid
began to precipitate. After stirring at room temperature for
an additional 2 h the solvent was removed under reduced
pressure and the remaining dark solid was extracted with
hexanes (50 mL). After filtration through Celite and removal
of the solvent under reduced pressure 1 was obtained in
essentially quantitative yield as a bright yellow solid. Anal.
Calcd for C52H57B: C, 90.15; H, 8.29. Found: C, 89.23; H, 8.69.
Mp: 162-4 °C 1H NMR (400 MHz, C6D6): 7.40 (d, 2H), J )
7.6 Hz, 7.35 (d, 4H), J ) 8.4 Hz, 7.26 (t, 1H) J ) 7.6 Hz, 7.26
(d, 1H), J ) 7.6 Hz, 7.20 (dd, 1H), J ) 7.2 Hz, 0.8 Hz, 7.15 (d,
2H), J ) 8.0 Hz, 7.10 (dd, 1H), J ) 7.6 Hz, 2.0 Hz, 7.05 (d,
1H), J ) 2.0 Hz, 7.00 (d, 2H), J ) 8.0 Hz, 6.95 (d, 4H), J ) 8.4
Hz, 1.30 (s, 9H), 1.09 (s, 18H), 1.03 (s, 9H). 13C{1H} NMR
(100.57 MHz, C6D6): 154.38, 150.70, 150.29, 149.67, 149.50,
149.33, 145.46, 140.93, 138.20, 134.64, 131.70, 129.87, 129.64,
129.43, 128.79, 128.74, 126.94, 125.18, 123.91, 119.14, 118.02,
34.46, 34.38, 34.33 (C(CH3)3), 31.49, 31.33, 31.30 (C(CH3)3).
UV-vis (hexanes): 347 nm (ꢀ ) 6180), 428 nm (ꢀ ) 480). MS
(FAB): m/z 692.7 (M+).
Deta ils for 1‚p y‚1/8C6H14 a n d 2
1‚p y‚1/8C6H14
2
empirical formula
fw
C
57.75H63.75BN
C60H77BLi2O2
854.91
782.66
temp, K
wavelength, Å
cryst syst
space group
a, Å
173(2)
203(2)
0.71073
triclinic
P1h
0.71073
monoclinic
P2(1)/c
13.4883(3)
35.4783(5)
12.2052(2)
90
114.3670(10)
90
5320.42(11)
4
15.9622(19)
17.283(2)
17.758(2)
88.829(10)
78.656(9)
87.525(10)
4798.3(10)
4
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
D
calcd, Mg/m3
1.083
0.061
1689
1.067
0.061
1856
µ(Mo KR), mm-1
F(000)
cryst size, mm3
cryst color and habit
2θmax, deg
0.64 × 0.56 × 0.52 0.33 × 0.20 × 0.10
colorless block
25.01
red needle
22.50
6878
no. of observns
no. of variables
R1a [I>2σ(I)]
16 502
1114
0.0743
0.1899
1.009
592
0.0882
0.2012
1.069
wR2b [I>2σ(I)]
goodness-of-fit on F2
Rea ction s of 2 w ith th e Meta l Sa lts F eCl2, F eBr 2, Cu -
(MeCN)4P F 6, Zr Cl4, a n d Cp Zr Cl3. A solution of 2 (ca. 0.5
mmol) in Et2O or THF (10-20 mL) was added slowly to a -78
°C slurry of an equimolar amount of the metal salts in Et2O
or THF (FeBr2). In the reaction of Cu(MeCN)4PF6 with 2, 2
equiv of the copper salt was used. The mixture was kept at
-78 °C for 1 h, slowly warmed to room temperature, and
stirred for several hours. In the reactions involving the iron
and copper salts a dark fine solid precipitated (magnetic in
the iron salt reactions), and the supernatant solution was
transparent brownish yellow. Removal of the solvent under
reduced pressure followed by extraction with hexanes (20-30
mL), filtration, and removal of the solvent under reduced
pressure afforded 1 in essentially quantitative yields. Actual
isolated yields were lower due to mechanical losses. In the
reactions involving the zirconium compounds deep red solu-
tions were obtained whose NMR spectra where those of
uncomplexed 1. From these reaction mixtures 1 could be
separated by addition of pyridine (0.1-0.2 mL) as 1‚p y after
change of solvent to hexanes and filtration.
largest diff peak, e Å-3 0.737
0.593
a
b
2
2
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) (∑w||Fo| - |Fc|| /∑w|Fo| )1/2
.
by full-matrix least-squares procedures on F2 using all reflec-
tions. The crystallographic programs employed were those of
the SHELXTL program suite.17 In the final refinement cycles
all the non-hydrogen atoms were refined anisotropically, and
all the hydrogen atoms were included in the refinement with
idealized parameters except the hydrogen atoms belonging to
the disordered hexane solvent molecule, which were not
included in the refinement.
Crystals of 2 were treated similarly to those of 1‚p y with
the exception that they were removed from the flask inside a
drybox. The data for 2 were collected at 203(2) K on a Siemens
SMART 1000 diffractometer using Mo KR (λ ) 0.71073 A)
radiation. The data were corrected for Lorentz and polarization
effects, and an empirical absorption correction was applied.18
The structure was solved by direct methods and refined by
full-matrix least-squares procedures on F2 using all reflections.
The crystallographic programs employed were those of the
SHELXTL program suite.17 Several t-Bu groups showed
disorder and were modeled with refined occupancies. Six atoms
were kept isotropic. One Et2O molecule was disordered.
Attempts to model the whole Et2O molecule as disordered were
unsatisfactory. The terminal CH3 was modeled with two
positions and the remaining atoms were left with large thermal
parameters. The data were limited to 45° due to poor diffrac-
tion. In the final refinement cycles all the non-hydrogen atoms
were refined anisotropically except six carbon atoms belonging
to the disordered t-Bu groups, and all the hydrogen atoms were
included in the refinement with idealized parameters. Some
details of the data collections and refinements are given in
Table 2, and selected bond distances and angles are listed in
Tables 2 and 3. Further details are given in the Supporting
Information.
X-r a y Cr ysta llogr a p h y. Crystals of sufficient quality for
single-crystal X-ray diffraction were obtained by crystallization
from hexanes (1‚p y) or diethyl ether (2) at room temperature.
Compound 1‚p y was found to crystallize in two different
modifications, 1‚p y‚1/8C6H14 and 1‚p y‚1/2C6H14. The data set
of the latter modification suffered from disorder problems and
low data intensity. Due to these problems only the higher
quality data of the modification 1‚p y‚1/8C6H14 are the subject
of the discussion in this paper, and information about the other
data set is provided in the Supporting Information. Neverthe-
less, the metric parameters for 1‚p y are essentially identical
in both datasets.
Crystals of 1‚p y‚1/8C6H14 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.16 The data for 1‚p y‚1/8C6H14
were collected at 173(2) K on a Siemens P4 diffractometer
using Mo KR (λ ) 0.71073 A) radiation. The data were
corrected for Lorentz and polarization effects. An absorption
correction was not applied since it was judged to be insignifi-
cant. The structure was solved by direct methods and refined
Resu lts a n d Discu ssion
Monosubstituted boranes [RBH2]2 can be obtained by
the reaction of an organolithium compound RLi with
(17) SHELXTL Version 5.1; Bruker AXS, 1997.
(18) SADABS: an empirical absorption program by G. M. Sheldrick
using the method described by Blessing, R. H. Acta Crystallogr. 1995,
A51, 33-38.
(16) Hope, H. In Experimental Organometallic Chemistry; Wayda,
A. L., Darensbourg, M. Y., Eds.; American Chemical Society: Wash-
ington, DC, 1987; ACS Symposium Series 357, Chapter 10.

Unsymmetrical 9-Borafluorene
Organometallics, Vol. 20, No. 5, 2001 847
Ta ble 3. Selected Bon d Len gth s (Å) for
1‚p y‚1/8C6H14 a n d 2
1‚p y‚1/8C6H14
2
B(1)-N(1)
1.638(3)
1.638(4)
1.640(4)
1.645(4)
1.407(4)
1.468(4)
1.402(4)
1.641(3)
1.632(4)
1.645(4)
1.647(4)
1.409(3)
1.468(4)
1.414(4)
B(1)-C(1)
1.607(6)
1.544(6)
1.536(6)
2.317(9)
2.242(9)
2.247(9)
2.302(9)
2.328(9)
2.302(9)
2.250(9)
2.252(9)
2.253(8)
2.263(9)
1.915(8)
1.946(8)
1.479(6)
1.423(6)
1.472(5)
B(1)-C(7)
B(1)-C(27)
B(1)-C(38)
B(1)‚‚‚Li(1)
Li(1)‚‚‚C(38)
Li(1)‚‚‚C(33)
Li(1)‚‚‚C(32)
Li(1)‚‚‚C(27)
B(1)‚‚‚Li(2)
Li(2)‚‚‚C(32)
Li(2)‚‚‚C(33)
Li(2)‚‚‚C(38)
Li(2)‚‚‚C(27)
Li(1)-O(1)
Li(2)-O(2)
C(27)-C(32)
C(32)-C(33)
C(33)-C(38)
B(1)-C(16)
B(1)-C(42)
C(7)-C(8)
C(8)-C(11)
C(11)-C(16)
B(2)-N(2)
B(2)-C(64)
B(2)-C(73)
B(2)-C(99)
C(64)-C(65)
C(65)-C(68)
C(68)-C(73)
F igu r e 1. Thermal ellipsoid plot (30% probability el-
lipsoids) showing the molecular structure of one of the two
independent molecules of 1‚p y. Hydrogen atoms are omit-
ted for clarity.
dimethyl sulfide-stabilized H2ClB‚SMe2.19 Indeed, the
terphenyl-substituted species [2,6-(o-tol)2C6H3BH2]2 and
[2,6-Mes2C6H3BH2]2 (Mes ) 2,4,6-Me3C6H2) have been
synthesized according to this route.20 However, when
we now reacted 2,6-(4-t-BuC6H4)2C6H3Li with H2ClB‚
SMe2 in hexanes or diethyl ether at -78 °C followed by
stirring at room temperature for ca. 12 h, no [2,6-(4-t-
BuC6H4)2C6H3BH2]2 was formed, but instead a bright
yellow, fluorescing solution was obtained. The 1H NMR
spectrum of the crude product after removal of the
solvent under reduced pressure indicated the presence
of several compounds, with 1 (eq 2) being the main
product (50-75%) as gauged by integration of the tert-
butyl region of the spectrum.
Ta ble 4. Selected Bon d An gles (d eg) for
1‚p y‚1/8C6H14 a n d 2
1‚p y‚1/8C6H14
2
C(7)-B(1)-N(1)
C(7)-B(1)-C(16)
N(1)-B(1)-C(16)
C(7)-B(1)-C(42)
N(1)-B(1)-C(42)
101.76(19) C(27)-B(1)-C(1)
126.6(4)
130.3(4)
103.0(3)
108.6(3)
109.2(3)
98.9(2)
107.1(2)
116.4(2)
114.2(2)
C(38)-B(1)-C(1)
C(38)-B(1)-C(27)
C(32)-C(27)-B(1)
C(33)-C(38)-B(1)
C(16)-B(1)-C(42) 116.5(2)
C(32)-C(33)-C(38) 109.5(3)
C(64)-B(2)-N(2)
C(64)-B(2)-C(73)
N(2)-B(2)-C(73)
101.13(19) C(33)-C(32)-C(27) 109.6(3)
99.09(19)
107.87(19)
C(64)-B(2)-C(99) 115.2(2)
N(2)-B(2)-C(99)
113.63(19)
C(73)-B(2)-C(99) 117.8(2)
yields of 1 are 70-75% in hexanes but only 50-53% in
Et2O solution. Attempts to perform the reaction in THF
afforded only the protonated terphenyl substituent, i.e.,
the arene 2,6-(4-t-BuC6H4)2C6H4. In hexane and Et2O
solution the borafluorene formation (C-H activation)
is facile and apparently commences even below room
temperature (ca. 0-10 °C), as indicated by the appear-
ance of the intense yellow color. This reaction provides
a facile route to unsymmetrical 9-borafluorenes. We are
currently studying the scope of this reaction.
Crystals of 1‚p y contain two independent molecules
and a disordered hexane molecule (with an occupancy
of 0.25) per asymmetric unit. Some selected bond
lengths and angles of both independent molecules are
given in Tables 3 and 4. There are no large structural
differences in the two independent molecules, and for
brevity, only the parameters of one of the molecules are
discussed here. The structure of 1‚p y features a planar,
unsymmetrically substituted 9-borafluorene unit. The
boron atom is further coordinated to the m-terphenyl
group 2,6-(4-t-BuC6H4)2C6H3- and one pyridine in a
distorted tetrahedral fashion. The B-N (1.638(4) Å)
distance is somewhat elongated compared to those of
the pyridine adduct of 9-chloro-9-borafluorene (d(B-N)
) 1.612(3) Å)21 and other pyridine adducts of boranes
(see, e.g., (C6F5)3B‚(NC5H4(4-Me2N), d(B-N) ) 1.604
Å;22 tetraethylbis(µ2-2-(5-methyl)pyridyl)diborane, d(B-
Although the high solubility of 1 in hexanes, toluene,
diethyl ether, or THF prevented the purification by
crystallization, addition of a slight excess of pyridine
caused a color change to almost colorless, and large
colorless crystals of the adduct 1‚p y formed after several
days at room temperature. The structure of 1‚p y was
determined by X-ray crystallography (Figure 1) and will
be discussed below.
The mechanism of the formation of 1 is not yet fully
understood. The presence of donor solvents appears to
suppress the formation of 1. According to 1H NMR
spectroscopic investigations of the crude products, the
(21) Narula, C. K.; No¨th, H. Inorg. Chem. 1985, 24, 2532-2539.
(22) Lesley, M. J . G.; Woodward, A.; Taylor, N. J .; Marder, T. B.;
Cazenobe, I.; Ledoux, I.; Zyss, J .; Thornton, A.; Bruce, D. W.; Kakkar,
A. K. Chem. Mater. 1998, 10, 1355-1365.
(19) Wehmschulte, R.; Ruhlandt-Senge, K.; Olmstead, M. M.; Petrie,
M. A.; Power, P. P. J . Chem. Soc., Dalton Trans. 1994, 2113-2117.
(20) Wehmschulte, R. J .; Khan, M. A. Unpublished results, 1999.

848 Organometallics, Vol. 20, No. 5, 2001
Wehmschulte et al.
N) ) 1.610 Å23). The B-C(aryl) distances (1.638(4)-
1.645(4) Å) are within the range observed for four-
coordinate organoboron compounds (d(B-C) ) 1.59-
1.65 Å).24 The smallest angle at the boron atom is the
internal C(7)-B(1)-C(16) angle with a value of 98.9-
(2)°, and the widest angles with values of 114.2(2)-
116.5(2)° all involve the terphenyl substituent (i.e.,
C(42)). Further evidence for steric crowding is found in
the short N(1)‚‚‚C and C‚‚‚C contacts between the
pyridine and the C(31)-C(36) aromatic ring (e.g., N(1)‚
‚‚C(34) ) 3.020 Å, N(1)‚‚‚C(35) ) 2.971 Å, C(57)‚‚‚C(34)
) 2.869 Å) as well as short C/B‚‚‚C contacts between
the borole unit and the C(43)-C(48) aromatic ring (e.g.,
B(1)‚‚‚C(43) ) 2.989 Å, C(7)‚‚‚C(44) ) 3.034 Å, C(16)‚‚
‚C(48) ) 3.197 Å).
Consistent with the elongated B(1)-N(1) bond is the
observation that dissolution of colorless 1‚p y in hexanes
or benzene affords bright yellow solutions, suggesting
partial dissociation of the pyridine ligand according to
eq 3.
Similar equilibria have been observed for other steri-
cally crowded boranes.25 Determination of the relative
concentration of 1‚p y and pyridine in three samples of
different concentrations using 1H NMR spectroscopy
gave the equilibrium constant K ) 2.2(0.5) × 10-3 mol
L-1. However, attempts to remove the pyridine by
distillation from a toluene solution of 1‚p y or by vacuum
distillation from melted 1‚p y at 150-170 °C resulted
in decomposition.
Boroles are readily reduced to the respective dianions
owing in part to the empty pz-orbital on the boron atom.
Stirring of an Et2O solution of crude 1 with excess
lithium powder at -10 °C gave a dark green solution,
which was concentrated to give deep red, almost black
plates of 2 in moderate yields (eq 4).
F igu r e 2. Thermal ellipsoid plot (30% probability el-
lipsoids) showing the molecular structure of 2. Hydrogen
atoms are omitted for clarity.
359.9°), with the narrow angle of 103.0(3)° being the
internal C(27)-B(1)-C(38) angle of the boratafluorene.
The two external angles are wider, with C(1)-B(1)-
C(27) ) 126.6° and C(1)-B(1)-C(38) ) 130.3°. The two
diethyl ether-solvated lithium cations are positioned
almost symmetrically above and below the plane of the
central C4B ring of the boratafluorene unit. The Li‚‚‚C
distances range from 2.242(9) (Li(1)‚‚‚C(38)) to 2.328-
(9) Å (Li(1)‚‚‚C(27)) and average 2.280(36) and 2.254(5)
Å for Li(1) and Li(2), respectively. The Li‚‚‚B distances
are slightly longer, with Li(1)‚‚‚B(1) ) 2.317(9) and Li‚
‚‚B(2) ) 2.302(9) Å. Similar values have been reported
for the related lithium salts of a 9-borafluorene,26
a
2-benzoborole,9 and two boroles.27,28 The central ring of
the terphenyl substituent is rotated 80.4° with respect
to the boratafluorene plane so that the m-(4-t-BuC6H4)
rings create pockets for the Li+ cations. The interactions
of the Li+ cations with these rings are weak, with the
shortest Li‚‚‚C distances ranging from 2.82 (Li(2)‚‚‚
C(18)) to 3.04 Å (Li(1)‚‚‚C(12)). However, the close
proximity of these rings pushes the Li-coordinated Et2O
molecules away from the Li(1)‚‚‚Li(2) axis by 35.9° (O(1))
and 39.3° (O(2)). The Li-O bond lengths with Li(1)-
O(1) ) 1.915(8) and Li(2)-O(2) ) 1.946(8) Å are only
slightly elongated by this interaction and are well within
the range observed for organolithium compounds with
Li+-π-interactions.26,29-32 The internal B-C distances
The structure of 2 (Figure 2) features a planar
9-boratafluorene unit. The coordination at the boron
atom is distorted trigonal planar (∑(angles at boron) )
(25) Brown, H. C.; Soderquist, J . A. J . Org. Chem. 1980, 45, 846-
849.
(26) Grigsby, W. J .; Power, P. P. J . Am. Chem. Soc. 1996, 118, 7981-
7988.
(23) Murafuji, T.; Mouri, R.; Sugihara, Y.; Takakura, K.; Mikata,
Y.; Yano, S. Tetrahedron 1996, 52, 13933-13938.
(24) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987, S1-S19.
(27) Herberich, G. E.; Hostalek, M.; Laven, R.; Boese, R. Angew.
Chem., Int. Ed. Engl. 1990, 29, 317-318.
(28) Herberich, G. E.; Wagner, T.; Marx, H.-W. J . Organomet. Chem.
1995, 502, 67-74.

Unsymmetrical 9-Borafluorene
Organometallics, Vol. 20, No. 5, 2001 849
F igu r e 3. Schematic drawing of the 9-borafluorene core
of 2 showing the bond lengths (Å).
B(1)-C(27) and B(1)-C(38) of 1.544(6) and 1.536(6) Å,
respectively, are about 2-4% shorter than a typical
B-C single bond (1.57-1.59 Å33-35), indicating some
multiple bonding at the boron. The annulated aromatic
rings are also slightly perturbed toward alternating
elongated and shortened bonds, with the longest bonds
belonging to the central C4B ring (Figure 3). A similar
distortion is found in related boratafluorene com-
pounds26 as well as in fluorenyl anions, albeit less
pronounced in the latter species.31,32,36-39
Unfortunately, preliminary investigations showed
that the reactions of 2 with the metal halide salts ZrCl4,
CpZrCl3, FeCl2, FeBr2, and Cu(MeCN)4PF6 at low tem-
peratures in diethyl ether or THF solutions only led to
reduction of the metal halide salts, and the neutral
species 1 could be recovered essentially quantitatively
from these solutions. An analogous tendency to reduce
metal salts rather than to form metal complexes was
observed for the dianion of a tetra-tert-butyl-s-in-
dacene.40 However, given that it has not been possible
so far to isolate pure 1 either by crystallization of the
crude product of the reaction of 2,6-(4-t-BuC6H4)2C6H3-
Li with H2ClB‚SMe2 (vide supra) or by removal of the
coordinated pyridine in 1‚p y, the controlled oxidation
of 2 by metal salts represents a viable route to uncom-
plexed 1. We have found that using SnCl2 in Et2O at
-78 °C as the oxidizing reagent affords 1 in essentially
quantitative yields after a simple workup procedure (see
also ref 41). Compound 1 is a bright yellow fluorescent
solid which is very soluble in standard organic solvents.
Solid 1 or solutions of 1 are stable toward short-time
exposure to atmospheric oxygen and moisture. Interest-
ingly, 1 decomposes when heated above ca. 100 °C. One
explanation for the low thermal stability of 1 may be
that the neutral 9-borafluorene is isoelectronic with the
formally antiaromatic fluorenyl cation.42-44 The reactiv-
ity of 1 and 2 toward a variety of main group and
transition metal complexes is currently under investiga-
tion.
The highfield shift of the CH2 and CH3 hydrogens of
1
the coordinated Et2O molecules in the H NMR spec-
trum (δH ) 2.50 and 0.16 ppm) and that of the Li+
cations in the 7Li NMR spectrum (δLi ) -10.3 ppm)
strongly suggest that the structure of 2 is maintained
in C6D6 solution. The UV/vis spectrum displays two
intensive bands in the visible region (435 nm (ꢀ ) 8450),
595 nm (ꢀ ) 1160)), which disappear upon oxidation to
the neutral borafluorene 1.
One of the exciting aspects of the dianionic complex
2 is the possibility to form transition metal complexes
by metathesis reactions with transition metal halides
(eq 5).
(29) Culp, R. D.; Cowley, A. H. Organometallics 1996, 15, 5380-
5384.
(30) Harder, S.; Lutz, M.; Straub, A. W. G. Organometallics 1997,
16, 107-113.
(31) Ko¨nemann, M.; Erker, G.; Fro¨hlich, R.; Wu¨rthwein, E.-U. J .
Am. Chem. Soc. 1997, 119, 11155-11164.
(32) Hakansson, M.; Ottosson, C.-H.; Boman, A.; J ohnels, D. Orga-
nometallics 1998, 17, 1208-1214.
(33) Bartell, L. S.; Carroll, B. L. J . Chem. Phys. 1965, 42, 3076-
3078.
(34) Zettler, F.; Hausen, H. D.; Hess, H. J . Organomet. Chem. 1974,
72, 157-162.
(35) Olmstead, M. M.; Power, P. P. J . Am. Chem. Soc. 1986, 108,
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96, 5441-5448.
Ack n ow led gm en t. Financial support for this work
from the University of Oklahoma is gratefully acknowl-
edged. We also thank Dr. Larry Russon for recording
the MS spectra and Professor Robert P. Houser for the
synthesis of Cu(MeCN)4PF6.
(37) Corbelin, S.; Kopf, J .; Weiss, E. Chem. Ber. 1991, 124, 2417-
2422.
(38) U¨ ffing, C.; Ko¨ppe, R.; Schno¨ckel, H. Organometallics 1998, 17,
3512-3515.
(39) Neander, S.; Tio, F. E.; Buschmann, R.; Behrens, U.; Olbrich,
F. J . Organomet. Chem. 1999, 582, 58-65.
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O’Hare, D. Organometallics 2000, 19, 1071-1076.
(41) Herberich, G. E.; Ohst, H. Chem. Ber. 1985, 118, 4303-4313.
(42) Olah, G. A.; Prakash, G. K. S.; Liang, G.; Westerman, P. W.;
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1980, 102, 4485-4492.
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‚p y‚1/8C6H14, 1‚p y‚1/2C6H14, and 2. Data are also given as
CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
(43) Amyes, T. L.; Richard, J . P.; Novak, M. J . Am. Chem. Soc. 1992,
114, 8032-8041.
(44) J iao, H.; Schleyer, P. v. R.; Mo, Y.; McAllister, M. A.; Tidwell,
T. T. J . Am. Chem. Soc. 1997, 119, 7075-7083.
OM000868Y