Synthesis, structure, and cyclic voltammetry of 4,6-bis(dimesitylboryl) dibenzofuran: Isolation of 4,6-dilithiobenzofuran and 4,5-dilithio-9,9- dimethylxanthene as tmeda adducts

Article abstract of DOI:10.1021/om050165m

The reaction of dibenzofuran and 9,9-dimethylxanthene with sec-BuLi and n-BuLi, respectively, in the presence of N,N,N′,N′- tetramethylethylenediamine (tmeda) in hexane followed by storage at -20°C results in the precipitation of 4,6-dilithiodibenzofuran-

Full text of DOI:10.1021/om050165m

2898
Organometallics 2005, 24, 2898-2902
Synthesis, Structure, and Cyclic Voltammetry of
4,6-Bis(dimesitylboryl)dibenzofuran: Isolation of
4,6-Dilithiobenzofuran and
4,5-Dilithio-9,9-dimethylxanthene as tmeda Adducts
Huadong Wang and Franc¸ois P. Gabba¨ı*
Chemistry TAMU 3255, Texas A&M University, College Station, Texas 77843-3255
Received March 7, 2005
The reaction of dibenzofuran and 9,9-dimethylxanthene with sec-BuLi and n-BuLi,
respectively, in the presence of N,N,N′,N′-tetramethylethylenediamine (tmeda) in hexane
followed by storage at -20 °C results in the precipitation of 4,6-dilithiodibenzofuran-1.5-
(tmeda) (1) and 4,5-dilithio-9,9-dimethylxanthene-1.5(tmeda) (2). Both of these adducts form
tetranuclear dimers with the lithium atoms bridging the deprotonated aromatic ligands.
The reaction of 1 with dimesitylboronfluoride affords 4,6-bis(dimesitylboryl)dibenzofuran
(3), which has been fully characterized. Cyclic voltammetry of compound 3 shows two
reduction waves at E1/2 -2.45 and -2.81 V (vs Fc/Fc+), which substantiates significant
electronic coupling of the two boron centers. Computational studies show that the p-orbital
of the two boron centers participates equally to the LUMO, thus providing grounds for the
observed electronic coupling.
Introduction
ions.3,4,7,14 The reduction chemistry of some of these
derivatives has also been studied. In particular, we
showed that the reduction of 1,8-bis(diphenylboryl)-
naphthalene yields a radical anion that features a
boron-boron one-electron σ-bond whose formation is
facilitated by the short boron-boron separation enforced
by the naphthalene backbone.17 It occurred to us that a
different set of both Lewis acidic and redox properties
might be observed in diboranes featuring an increased
separation between the Lewis acidic centers. In this
contribution, we report the synthesis and structures of
4,6-dilithiodibenzofuran-1.5(tmeda), 4,5-dilithio-9,9-
dimethylxanthene-1.5(tmeda), and 4,6-bis(dimesityl-
boryl)dibenzofuran,18 as well as the redox properties of
the latter.
Polyfunctional boranes have been widely investigated
as polydentate Lewis acids. Following the pioneering
work of Shriver and Biallas, who showed that methoxide
anions are effectively chelated by 1,2-bis(difluoroboryle-
thane),1 a number of bidentate boranes in which the
boron centers are linked by a rigid spacer have been
prepared.2 In this research, much effort has focused on
the chemistry of 1,8-diborylnaphthalenes2,3-8 and 1,2-
diborylbenzenes.2,9-16 Owing to the preorganization and
proximity of the Lewis acidic centers, such derivatives
are powerful complexing agents for small anions and
form chelate complexes with fluoride and hydroxide
* To whom correspondence should be addressed. E-mail: francois@
tamu.edu.
(1) Schriver, D. F.; Biallas, M. J. J. Am. Chem. Soc. 1967, 89, 1078-
1081.
Results and Discussion
(2) Gabba¨ı, F. P. Angew. Chem., Int. Ed. 2003, 42, 2218-2221.
(3) Katz, H. E. J. Org. Chem. 1985, 50, 5027-5032.
(4) Katz, H. E. J. Am. Chem. Soc. 1985, 107, 1420-1421.
(5) Katz, H. E. Organometallics 1987, 6, 1134-1136.
(6) Katz, H. E. Inclusion Compd. 1991, 4, 391-405.
(7) Sol, S.; Gabba, F. P. Chem. Commun. 2004, 1284-1285.
(8) Hoefelmeyer, J. D.; Schulte, M.; Tschinkl, M.; Gabba¨ı, F. P.
Coord. Chem. Rev. 2002, 235, 93-103.
The dilithiation of dibenzofuran by reaction with sec-
BuLi has been previously reported, but the resulting
dilithium salt has not been isolated.19-21 We found that
the reaction of dibenzofuran with 3 equiv of sec-BuLi
and N,N,N′,N′-tetramethylethylenediamine (tmeda) in
hexane followed by storage at -20 °C for 24 h resulted
in the precipitation of 4,6-dilithiodibenzofuran-1.5-
(tmeda) (1) in the form of dark brown crystals (Scheme
(9) Eisch, J. J.; Kotowicz, B. W. Eur. J. Inorg. Chem. 1998, 761-
769.
(10) Lewis, S. P.; Taylor, N. J.; Piers, W. E.; Collins, S. J. Am. Chem.
Soc. 2003, 125, 14686-14687.
(11) Henderson, L. D.; Piers, W. E.; Irvine, G. J.; McDonald, R.
Organometallics 2002, 21, 340-345.
(17) Hoefelmeyer, J. D.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2000, 122,
9054-9055.
(12) Williams, V. C.; Irvine, G. J.; Piers, W. E.; Li, Z.; Collins, S.;
Clegg, W.; Elsegood, M. R. J.; Marder, T. B. Organometallics 2000,
19, 1619-1621.
(18) For recent examples of bimetallic complexes featuring a diben-
zofuran-4,6-diyl backbone, see: Deng, Y. Q.; Chang, C. J.; Nocera, D.
G. J. Am. Chem. Soc. 2000, 122, 410-411. Cottone, A.; Scott, M. J.
Organometallics 2002, 21, 3610-3627. Clare, B.; Sarker, N.; Shoe-
maker, R.; Hagadorn, J. R. Inorg. Chem. 2004, 43, 1159-1166.
(19) Haenel, M. W.; Jakubik, D.; Rothenberger, E.; Schroth, G.
Chem. Ber. 1991, 124, 1705-1710.
(13) Piers, W. E.; Irvine, G. J.; Williams, V. C. Eur. J. Inorg. Chem.
2000, 2131-2142.
(14) Williams, V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.;
Collins, S.; Marder, T. B. J. Am. Chem. Soc. 1999, 121, 3244-3245.
(15) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Marks, T. J.; Nickias,
P. N. Organometallics 2002, 21, 4159-4168.
(20) Tsang, K. Y.; Diaz, H.; Graciani, N.; Kelly, J. W. J. Am. Chem.
Soc. 1994, 116, 3988-4005.
(21) Gilman, H.; Young, R. V. J. Am. Chem. Soc. 1935, 57, 1121.
(16) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P. N.; Marks,
T. J. Angew. Chem., Int. Ed. 2000, 39, 1312-1316.
10.1021/om050165m CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/26/2005

4,6-Bis(dimesitylboryl)dibenzofuran
Organometallics, Vol. 24, No. 12, 2005 2899
Scheme 1
1). Encouraged by these results, we decided to attempt
the isolation of 4,5-dilithio-9,9-dimethylxanthene.22 This
objective was successfully achieved by treatment of 9,9-
dimethylxanthene with n-BuLi and tmeda in hexane,
which, upon cooling, resulted in the precipitation of 4,5-
dilithio-9,9-dimethylxanthene-1.5(tmeda) (2) as a pink
crystalline solid (Scheme 1). Compounds 1 and 2 are
extremely air sensitive and ignite when exposed to air.
The 1H NMR spectra of 1 (in pyridine-d5) and 2 (in
benzene-d6) are in agreement with the dimetalation of
the backbone.
Compound 1 crystallizes in the P21 space group and
forms a coordination polymer (Figure 1).23 This coordi-
nation polymer consists of dimeric units of 4,6-dilithio-
dibenzofuran that are linked by a bridging tmeda
molecule. In each dimeric unit, the dibenzofuran-4,6-
diyl moieties are bridged by four lithium atoms that are
coordinated to the deprotonated 4- and 6-positions of
the organic backbone. The resulting C-Li bond dis-
tances observed in 1 range from 2.16 to 2.33 Å and are
similar to those found in the structure of other aryl-
lithium derivatives such as phenyllithium-tmeda (av
C-Li ) 2.21 Å).24,25 Similarly to the situation encoun-
tered in the structure of phenyllithium-tmeda, the
lithium atoms Li(1) and Li(2) are chelated by tmeda
molecules and are tetracoordinated. Owing to positional
disorder affecting the tmeda ligands, the Li-N bond
distances are somewhat unreliable. The lithium atoms
Li(3) and Li(4) are coordinated to a single nitrogen atom
provided by the bridging tmeda ligand. As a result, these
Figure 1. Stick and ball representation of 1. H atoms
omitted for clarity. Only one conformation of the disordered
tmeda ligand is shown. Selected bond lengths (Å) and
angles (deg): Li(1)-N(5) 2.076(18), Li(1)-N(6) 2.154(17),
Li(1)-C(4) 2.229(16), Li(1)-C(24) 2.280(17),), Li(2)-N(3)
2.23(3), Li(2)-N(4) 2.21(2), Li(2)-C(4) 2.226(17), Li(2)-
C(24) 2.235(17), Li(3)-N(2) 2.167(15), Li(3)-C(6) 2.333(14),
Li(3)-C(26) 2.361(15), Li(4)-N(1) 2.022(16), Li(4)-C(6)
2.157(15), Li(4)-C(26) 2.231(14); N(5)-Li(1)-N(6) 91.4(7),
C(4)-Li(1)-C(26) 108.9(7), N(4)-Li(2)-N(3) 84.7(8), C(4)-
Li(2)-C(24) 110.7(7), N(2)-Li(3)-C(6) 112.0(6), N(2)-
Li(3)-C(24) 106.8(6), C(6)-Li(3)-C(26) 107.2(6), N(1)-
Li(4)-C(6) 124.7(7), N(1)-Li(4)-C(26) 115.2(6), C(6)-
Li(4)-C(26) 118.8(7).
two lithium atoms are tricoordinated, as sometimes
observed in the structure of bulky aryllithium deriva-
tives.26,27 While the Li(4) atom adopts a trigonal planar
arrangement (∑angles ) 358.7°), the coordination geom-
etry of the Li(3) atom is distinctly pyramidal (∑angles
)
326.0°), which is unusual but not unprecedented in
sterically crowded systems.28,29 Compound 2 crystallizes
in the Pbcn space group and forms a C2 symmetrical
dimer in which the two doubly deprotonated 9,9-
dimethylxanthenes are linked by four lithium atoms
(Figure 2).23 The Li(1) atom, which bridges the C(4) and
C(5A) carbon atoms (Li(1)-C(4) ) 2.209(3) Å and
Li(1)-C(5A) ) 2.402(3) Å), is also chelated by a tmeda
molecule (Li(1)-N(1) ) 2.120(3) Å, Li(1)-N(2) )
2.254(3) Å). As a result, its coordination geometry
resembles that encountered in phenyllithium-tmeda. As
indicated by the relatively short Li(2)-O (2.045(3) Å),
Li(2)-C(10) (2.394(3) Å), and Li(2)-C(4) (2.274(3) Å)
distances, one of the 9,9-dimethylxanthene-4,5-diyl
groups acts as a η3-ligand for the Li(2) atom. The
coordination sphere of this lithium atom is completed
(22) For recent examples of bimetallic complexes featuring a 9,9-
dimethylxanthene-4,5-diyl backbone, see: Malaise, G.; Barloy, L.;
Osborn, J. A. Tetrahedron Lett. 2001, 42, 7417-7419. Bronger, R. P.
J.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M. Organometallics 2003,
22, 5338-5369. Raebiger, J. W.; Miedaner, A.; Curtis, C. J.; Miller, S.
M.; Anderson, O. P.; DuBois, D. L. J. Am. Chem. Soc. 2004, 126, 5502-
5514.
(23) Crystal data: 1: C42H60Li4N6O2, M ) 708.72, monoclinic space
group P21, a ) 12.644(2) Å, b ) 14.450(3) Å, c ) 13.141(3) Å, â )
118.007(3)°, V ) 2119.6(7) Å3, Z ) 2, Fcalcd ) 1.110 g cm-3, F(000) )
by
a deprotonated carbon atom (Li(2)-C(5A) )
764, T ) 110(2) K, 9469 measured reflections, 5749 unique (Rint
)
0.0389), µ ) 0.067 mm-1, R1 (I > σ) ) 0.0814, wR2 (I > 2σ) ) 0.1611
for 482 parameters. 2: C42H60Li4N6O2, M ) 792.88, orthorhombic space
group Pbcn, a ) 13.030(2) Å, b ) 17.639(3) Å, c ) 20.807(4) Å, V )
4782.2(15) Å3, Z ) 4, Fcalcd ) 1.101 g cm-3, F(000) ) 1720, T ) 110(2)
K, 29 265 measured reflections, 5892 unique (Rint ) 0.0645), µ ) 0.066
mm-1, R1 (I > 2σ) ) 0.0493, wR2 (I > 2σ) ) 0.1329 for 271 parameters.
3-toluene: C55H58B2O, M ) 776.84, monoclinic space group P21/c, a )
15.976(3) Å, b ) 18.738(4) Å, c ) 16.169(3) Å, â ) 114.29(3)°, V )
4411.7(15) Å3, Z ) 4, Fcalcd ) 1.139 g cm-3, F(000) ) 1624, T ) 293(2)
K, 20 177 measured reflections, 6854 unique (Rint ) 0.0351), µ ) 0.065
mm-1, SADABS absorption correction, Tmin/Tmax 0.688081, R1 (I > 2σ)
) 0.0871, wR2 (I > 2σ) ) 0.1720 for 572 parameters.
2.142(3) Å) provided by another 9,9-dimethylxanthene-
4,5-diyl ligand and a nitrogen atom of a bridging tmeda
molecule (Li(2)-N(3) ) 2.184(3) Å). All Li-C and Li-N
distances observed in 2 fall within the expected range
for such linkages.24,25
(26) Girolami, G. S.; Riehl, M. E.; Suslick, K. S.; Wilson, S. R.
Organometallics 1992, 11, 3907-3910.
(27) Rabe, G. W.; Sommer, R. D.; Rheingold, A. L. Organometallics
2000, 19, 5537-5540.
(24) Schubert, U.; Neugebauer, W.; Schleyer, P. V. Chem Commun.
1982, 1184-1185.
(28) Goldfuss, B.; Eisentrager, F. Aust. J. Chem. 2000, 53, 209-
212.
(25) Dinnebier, R. E.; Behrens, U.; Olbrich, F. J. Am. Chem. Soc.
1998, 120, 1430-1433.
(29) Smith, G. D.; Fanwick, P. E.; Rothwell, I. P. Inorg. Chem. 1989,
28, 618-620.

2900 Organometallics, Vol. 24, No. 12, 2005
Wang and Gabbai
Figure 3. ORTEP view of 3. Ellipsoids are drawn at the
50% probability level. H atoms omitted for clarity. Selected
bond lengths (Å) and angles (deg): B(1)-C(31) 1.572(6),
B(1)-C(21) 1.576(5), B(1)-C(4) 1.592(6), B(2)-C(41)
1.577(6), B(2)-C(6) 1.580(6), B(2)-C(51) 1.578(6), C(31)-
B(1)-C(21) 122.2(4), C(31)-B(1)-C(4) 121.5(3), C(21)-
B(1)-C(4) 116.2(4), C(41)-B(2)-C(6) 118.6(3), C(41)-
B(2)-C(51) 122.7(4), C(6)-B(2)-C(51) 118.7(4), C(4A)-
C(4)-B(1) 125.9(4), C(6A)-C(6)-B(2) 125.7(4).
Figure 2. ORTEP view of 2. Ellipsoids are drawn at the
50% probability level. H atoms omitted for clarity. Selected
bond lengths (Å) and angles (deg): Li(1)-N(1) 2.130(3),
Li(1)-C(4) 2.209(3), Li(1)-N(2) 2.254(3), Li(1)-C(5A) 2.402-
(3), Li(2)-O 2.045(3), Li(2)-C(5A) 2.142(3), Li(2)-N(3)
2.184(3), Li(2)-C(4) 2.247(3), Li(2)-C(10) 2.394(3), N(1)-
Li(1)-C(4) 117.67(13), N(1)-Li(1)-N(2) 83.08(10), C(4)-
Li(1)-N(2) 117.49(13), N(1)-Li(1)-C(5A) 110.17(12), C(4)-
Li(1)-C(5A) 105.31(11), N(2)-Li(1)-C(5A) 122.23(12),
O-Li(2)-C(5A) 129.71(13), O-Li(2)-N(3) 114.18(12),
C(5A)-Li(2)-N(3) 112.85(12), O-Li(2)-C(4) 66.53(9),
C(5A)-Li(2)-C(4) 113.33(12), N(3)-Li(2)-C(4) 108.47(12),
O-Li(2)-C(10) 36.41(6), C(5A)-Li(2)-C(10) 140.82(13),
N(3)-Li(2)-C(10) 102.16(11), C(4)-Li(2)-C(10) 34.85(6).
respectively, forms a moderate dihedral angle of 35.7°
and 34.0° with the plane containing the dibenzofuran-
4,6-diyl backbone. The moderate values of these angles
suggest the operative conjugation of the boron empty
p-orbital with the π-system of the dibenzofuran-4,6-diyl
backbone. In fact, the dihedral angles that each of the
trigonal coordination planes of the B(1) and B(2) centers
forms with the plane containing the aromatic core of
the mesityl ligands (59.4°, 39.0°, 48.4°, 60.1°) are
distinctly larger. Finally, the boron centers are sepa-
rated by 5.79 Å.
Scheme 2
The structure of this derivative was computationally
optimized using density functional theory (DFT) (B3LYP,
6-31+G* for the boron centers, 6-31G for all other
atoms).33 The fully optimized geometry is close to that
observed in the solid state, and all bond distances
correspond accurately to those experimentally observed
(av B-C: obs 1.579 Å, calc 1.578 Å) (Figure 4). It is also
important to note that the dihedral angles formed by
the trigonal coordination planes of each boron center
and the plane of the aromatic ligands are also in
reasonable agreement (dihedral angles formed between
the boron trigonal planes and (i) the plane of the
dibenzofuran backbone for B(1): obs 35.7°, calc 38.7°;
for B(2) obs 34.0°, calc 38.8°; (ii) the plane of the mesityl
subsiutents: av values for B(1) and B(2): obs 51.7°, calc
51.8°). Examination of the B3LYP orbitals indicates that
the LUMO of 3 bears a large contribution from the
p-orbitals of the two boron centers and the π-system of
the dibenzofuran-4,6-diyl backbone (Figure 4).
Adduct 1 reacts with 2 equiv of dimesitylboronfluoride
to afford 4,6-bis(dimesitylboryl)dibenzofuran (3), which
was isolated in a pure form by recrystallization from
toluene (Scheme 2). Despite repeated efforts, attempts
to synthesize and isolate 4,5-bis(dimesitylboryl)-9,9-
dimethylxanthene by reaction of 2 with dimesityl-
1
boronfluoride have not been successful.30 The H and
13C NMR spectra of 3 feature the expected resonances
for a 4,6-disubstituted dibenzofuran derivative and also
confirm the presence of the mesityl ligands. The 11B
NMR spectrum of 3 features a resonance at 74 ppm,
comparable to that observed in other dimesitylarylboron
derivatives.31
Compound 3 crystallizes in the P21/c space group with
one interstitial molecule of toluene (Figure 3).23 As
indicated by the C(4A)-C(4)-B(1) (125.9(4)°) and C(6A)-
C(6)-B(2) (125.7(4)°) angles, which deviate from the
ideal value of 120° (Figure 3), the structure of 3 is
sterically crowded. Each boron center adopts a trigonal
planar arrangement (∑angles ) 359.9° at B(1) and B(2)).
All carbon-boron distances in 3 are comparable to those
found in the structure of other triarylboranes such as
triphenylboron (av 1.58 Å).32 Interestingly, each of the
trigonal coordination planes of the B(1) and B(2) centers,
(32) Zettler, F.; Hausen, H. D.; Hess, H. J. Organomet. Chem. 1974,
72, 157-162.
(33) DFT calculations (full geometry optimization) were carried out
by using the gradient-corrected Becke exchange functional (B3LYP)
(Becke, A. J. Chem. Phys. 1993, 98, 5648-5652) and the Lee-Yang-
Parr correlation functional (Lee, C.; Yang, W.; Parr, R. G. Phys. Rev.
B 1988, 37, 785-789 and Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H.
Chem. Phys. Lett. 1989, 157, 200-206). A 6-31+G* basis set was used
for the boron atoms. (Clark, T.; Chandrasekhar, J.; Schleyer, P. v. R.
J. Comput. Chem. 1983, 4, 294-301; Krishnam, R.; Binkley, J. S.;
Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650-654; Gill, P. M.
W.; Johnson, B. G.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1992,
197, 499-505). A 6-31G basis set was used for all other carbon and
hydrogen atoms (Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem.
Phys. 1972, 56, 2257-2261). Following full optimization of the
geometry of 3, a frequency calculation was carried out and indicated
that the optimized structure was a true minimum.
(30) Evidence for the formation of this diborane has been obtained,
but its isolation in a pure form has failed.
(31) Hoefelmeyer, J. D.; Sole´, S.; Gabba¨ı, F. P. Dalton Trans. 2004,
1254-1258.

4,6-Bis(dimesitylboryl)dibenzofuran
Organometallics, Vol. 24, No. 12, 2005 2901
that observed for 4,4′-bis(dimesitylboryl)biphenyl (∆E1/2
) 0.25 V),44 thus indicating substantial electronic
coupling of the two boron centers.45 As indicated by the
aforementioned DFT calculation, both boron centers
participate equally to the nondegenerate LUMO, thus
providing grounds for this electronic coupling. Combina-
tion of diborane 3 with 18-C-6 and Na/Hg produced a
dark green paramagnetic solution, which presumably
contains the radical anion 3•-. The EPR spectrum of this
solution consists of a signal at g ) 2.00; however, the
11B hyperfine coupling could not be resolved.
Conclusion
Figure 4. DFT orbital picture showing the LUMO of 3
(isovalue ) 0.025). H atoms omitted for clarity.
The present investigations indicate that 4,6-dilithio-
dibenzofuran (1) and 4,5-dilithio-9,9-dimethylxanthene
(2) can be isolated in the form of tmeda adducts.
Isolation of these solid adducts, which can be kept under
an inert atmosphere for extended periods of time, is a
significant achievement that should provide new syn-
thetic possibilities for the incorporation of the dibenzo-
furan-4,6-diyl and 9,9-dimethylxanthene-4,5-diyl back-
bone in organic and organometallic compounds. The
preparation of diborane 3 by reaction of 1 with dimesi-
tylboronfluoride illustrates this idea. Structural, com-
putational, and electrochemical studies of diborane 3
indicate that the dibenzofuran-4,6-diyl backbone should,
in principle, promote antiferromagnetic coupling of spin
carriers directly attached to the 4- and 6-positions.
Experimental Section
Figure 5. Cyclic voltammogram of 3 in THF with a glassy
carbon working electrode (0.2 M n-Bu4PF6). Scan rates:
ν ) 500 mV/s (A); 200 mV/s (B); 50 mV/s (C).
General Procedures. Dibenzofuran was purchased from
Aldrich and purified by recrystallization from ethanol. tmeda
was purchased from Aldrich and dried over Na/K. 9,9-Di-
methylxanthene, dimesitylboronfluoride, n-butyllithium (1.6
M in hexane), and sec-butyllithium (1.4 M in cyclohexane) were
purchased from Aldrich and used without purification. Tolu-
ene, pentane, and hexane were dried by reflux under N2 over
CaH2 and freshly distilled prior to use. Air-sensitive com-
pounds were handled under N2 atmosphere using standard
Schlenk and glovebox techniques. Elemental analyses were
performed at Atlantic Microlab (Norcross, GA). Melting points
were measured on samples in sealed capillaries and are
uncorrected. 1H and 13C NMR spectra were recorded on Inova
300 and 500 spectrometers. Chemical shifts for 1H and 13C
NMR spectra are reported with respect to Me4Si standard (δ
) 0 ppm). 11B NMR spectra were recorded on an Inova 400
spectrometer. Chemical shifts for 11B NMR spectra are re-
ported with respect to Et2O‚BF3 standard (δ ) 0 ppm). EPR
spectra were recorded on a Bruker X-band EPR spectrometer
(model ESP 300E).
The cyclic voltammogram of 3 in THF shows a
reversible wave at E1/2(1) ) -2.45 V (vs Fc/Fc+) (Figure
5). The potential of this reduction is comparable to that
observed for other triarylboranes34-43 and corresponds
to the formation of a radical anion. While the second
reduction of triarylboranes is always irreversible, the
cyclic voltammogram of 3 features a second reduction
wave at E1/2(2) ) -2.81 V (vs Fc/Fc+), which appears
reversible at elevated scan rate (ν ) 500 mV/s) and
irreversible at slower scan rates (ν < 400 mV/s). The
scan rate dependence of this second reduction wave
suggests that the dianion generated at this potential
undergoes rapid chemical reactions. The potential dif-
ference ∆E1/2 of 0.36 V (ν ) 500 mV/s) observed between
the two reduction waves in the CV of 3 is larger than
Electrochemical experiments were performed with an elec-
trochemical analyzer from CH Instruments (Model 610A) with
a glassy-carbon working electrode and a platinum auxiliary
electrode. The reference electrode was built from a silver wire
inserted in a small glass tube fitted with a porous Vycor frit
at the tip and filled with a CH3CN solution containing
(nBu)4NPF6 (0.2 M) and AgNO3 (0.005 M). All three electrodes
(34) Krause, E.; Polack, H. Ber. 1926, 59, 777-785.
(35) Chu, T. L.; Weissman, T. J. J. Am. Chem. Soc. 1956, 78, 23-
26.
(36) Leffler, J. E.; Watts, G. B.; Tanigaki, T.; Dolan, E.; Miller, D.
S. J. Am. Chem. Soc. 1970, 92, 6825-6830.
(37) Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1986, 108,
4235-4236.
(38) Eisch, J. J.; Dluzniewski, T.; Behrooz, M. Heteroat. Chem. 1993,
4, 235-241.
were immersed in
a THF solution (8 mL) containing
(39) Harlan, C. J.; Hascall, T.; Fujita, E.; Norton, J. R. J. Am. Chem.
Soc. 1999, 121, 7274-7275.
(nBu)4NPF6 (0.2 M) as a support electrolyte and the diborane
3 (0.003 M). The electrolyte was dried under vacuum prior to
use. In all cases, ferrocene was used as an internal standard,
(40) Kwaan, R. J.; Harlan, C. J.; Norton, J. R. Organometallics 2001,
20, 3818-3820.
(41) Elschenbroich, C.; Kuhlkamp, P.; Behrendt, A.; Harms, K.
Chem. Ber. 1996, 129, 859-869.
(42) Brown, H. C.; Dodson, V. H. J. Am. Chem. Soc. 1957, 79, 2302-
2306.
(44) Fiedler, J.; Zalisˇ, S.; Klein, A.; Hornung, F. M.; Kaim, W. Inorg.
Chem. 1996, 35, 3039-3043.
(45) Kaim, W.; Schulz, A. Angew. Chem., Int. Ed. Engl. 1984, 23,
615-616.
(43) Weissman, S. I.; van Willigen, H. J. Am. Chem. Soc. 1965, 87,
2285-2286.

2902 Organometallics, Vol. 24, No. 12, 2005
Wang and Gabbai
and all reduction potentials are reported with respect to the
E1/2 of the Fc+/Fc redox couple.
at room temperature. The resulting mixture was stirred for 5
h and then filtered. The solvent was removed under vacuum,
and the resulting solid was further purified by crystallization
from toluene. Yield: 46% (198 mg). Melting point: 217-220
4,6-Dilithiodibenzofuran-1.5(tmeda) (1). sec-Butyllith-
ium (1.4 M) in cyclohexane (6.4 mL, 9.0 mmol) was slowly
added to a suspension of dibenzofuran (500 mg, 3.0 mmol) and
tmeda (1.35 mL, 9.0 mmol) in hexane (2 mL) at room
temperature. After standing at room temperature for 24 h, the
solution was cooled to -20 °C. Compound 1 precipitated as a
dark brown solid and was washed with hexane (10 mL).
Yield: 71% (750 mg). 1H NMR (pyridine-d5, 500 MHz): δ 2.15
(s, 18 H, tmeda-CH3), 2.38 (s, 6 H, tmeda -CH2), 7.35 (pseudo
t, Japparent ) 7.0 Hz, 2 H, dibenzo-CH), 7.46 (d, J ) 7.3 Hz, 2
H, dibenzo-CH), 8.03 (d, J ) 7.6 Hz, 2 H, dibenzo-CH).
4,5-Dilithio-9,9-dimethylxanthene-1.5(tmeda) (2). n-
Butyllithium (1.6 M) in hexane (4.6 mL, 7.4 mmol) was slowly
added to a solution of 9,9-dimethylxanthene (524 mg, 2.5
mmol) and tmeda (0.9 mL, 6.0 mmol) in hexane (2 mL) at room
temperature. After standing at room temperature for 24 h, the
solution was cooled to -20 °C. Compound 2 precipitated as a
pink solid and was washed with pentane (10 mL). Yield: 71%
(701 mg). 1H NMR (benzene-d6, 300 MHz): δ 1.74 (s, 6 H,
tmeda-CH2), 1.80 (s, 24 H, tmeda-CH3 and xanthene-CH3), 7.28
(pseudo t, Japparent ) 7.0 Hz, 2 H, xanthene-CH), 7.35 (d, J )
7.5 Hz, 2 H, xanthene -CH), 7.97 (d, J ) 6 Hz, 2 H, xanthene
-CH).
1
°C. H NMR (benzene-d6, 500 MHz): δ 2.16 (s, 24 H, Mes-o-
CH3), 2.17 (s, 12 H, Mes-p-CH3), 6.70 (s, 8 H, Mes-CH), 7.10
(pseudo t, J ) 7.4 Hz, 2 H, dibenzo-CH), 7.41 (d, J ) 7.3 Hz,
2 H, dibenzo-CH), 7.75 (d, J ) 7.5 Hz, 2 H, dibenzo-CH). 13C
NMR (benzene-d6, 125.9 MHz): δ 21.4 (Mes-p-CH3), 23.2 (Mes-
o-CH3), 122.9 (Mes-o-C), 123.1 (Mes-p-C), 123.7 (dibenzo-C1a/
9a), 128.7 (Mes-m-CH), 132.4 (Mes-CB), 133.2 (dibenzo-C3/7
or 2/8 or 1/9), 138.8 (dibenzo-C3/7 or 2/8 or 1/9), 140.9 (dibenzo-
C3/7 or 2/8 or 1/9), 142.3 (dibenzo-CB), 159.4 (dibenzo-C4a/
6a). 11B NMR (chloroform-d, 160.6 MHz): δ 74. Anal. Calcd
for C48H50OB2: C 86.76; H 7.58. Found: C 86.20; H 7.60.
Acknowledgment. Support from the Robert A.
Welch Foundation (Grant A-1423) and the National
Science Foundation (CAREER grant CHE-0094264 and
NMR instrumentation grant CHE 00-77917) is grate-
fully acknowledged. We thank Lisa Pe´rez for her help
with the calculations.
Supporting Information Available: Crystallographic
information in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
4,6-Bis(dimesitylboryl)dibezofuran (3). A solution of
dimesitylboronfluoride (350 mg, 1.31 mmol) in toluene (5 mL)
was added into a suspension of freshly isolated 4,6-dilithio-
dibenzofuran-1.5(tmeda) (264 mg, 0.75 mmol) in toluene (5 mL)
OM050165M