Synthesis and structural characterization of pentaarylboroles and their dianions

Article abstract of DOI:10.1021/om8002812

A series of 1-aryl-2,3,4,5-tetraphenylboroles [Ph₄C ₄BAr] [Ar = p-MeC₆H₄ (2), p-Me ₃SiC₆H₄ (3), p-FC₆H₄ (4)] were synthesized. The X-ray crystallography of 2-4 revealed that they have short distances between the borole boron atom and the phenyl rings of the neighboring molecules, suggesting the existence of certain intermolecular interaction. Despite tins interaction, the borole rings in 2-4 remain planar and contain substantial bond alternation in the butadiene skeleton, which is consistent with the theoretically optimized structures of the singlet boroles. These results demonstrated that the boroles still have antiaromatic character in the crystalline state. Compound 3 and pentaphenylborole [Ph₄C ₄BPh] (5) were further reacted with potassium or potassium graphite to form potassium borole dianion salts [K₂[Ph₄C ₄B(p-Me₃SiC₆H₄)]] (6) and [K ₂(Ph₄C₄BPh)] (7), respectively. In their X-ray crystal structures, compounds 6 and 7 are polymeric and have two K+ ions lying on both sides of the borole plane with η⁵-coordination.

Full text of DOI:10.1021/om8002812

3496
Organometallics 2008, 27, 3496–3501
Synthesis and Structural Characterization of Pentaarylboroles and
Their Dianions
Cheuk-Wai So, Daisuke Watanabe, Atsushi Wakamiya, and Shigehiro Yamaguchi*
Department of Chemistry, Graduate School of Science, Nagoya UniVersity, and SORST, Japan Science and
Technology Agency, Furo, Chikusa, Nagoya 464-8602, Japan
ReceiVed March 31, 2008
A series of l-aryl-2,3,4,5-tetraphenylboroles [Ph4C4BAr] [Ar ) p-MeC6H4 (2), p-Me3SiC6H4 (3), p-FC6H4
(4)] were synthesized. The X-ray crystallography of 2-4 revealed that they have short distances between
the borole boron atom and the phenyl rings of the neighboring molecules, suggesting the existence of
certain intermolecular interaction. Despite this interaction, the borole rings in 2-4 remain planar and
contain substantial bond alternation in the butadiene skeleton, which is consistent with the theoretically
optimized structures of the singlet boroles. These results demonstrated that the boroles still have
antiaromatic character in the crystalline state. Compound 3 and pentaphenylborole [Ph4C4BPh] (5) were
further reacted with potassium or potassium graphite to form potassium borole dianion salts [K2{Ph4C4B(p-
Me3SiC6H4)}] (6) and [K2(Ph4C4BPh)] (7), respectively. In their X-ray crystal structures, compounds 6
and 7 are polymeric and have two K+ ions lying on both sides of the borole plane with η5-coordination.
field transitions cannot be detected.8a However, the borole is a
Introduction
strong Lewis acid and very prone to oxidation and Diels-Alder
The cyclopentadienyl cation (C5H5+), one of the fundamental
4π electron systems, is of great importance in the understanding
of antiaromaticity.1 Its triplet ground state predicted from simple
Hu¨ckel theory has been confirmed by the electron spin resonance
(ESR) spectra,2 photoelectron spectroscopy,3 and recent ab initio
calculations.4 The nucleus-independent chemical shift (NICS)
calculations showed that the triplet C5H5+ as well as other triplet
4nπ electron systems are aromatic, while their singlet derivates
are antiaromatic.5 Until now, the solid-state structure of a simple
cyclopentadienyl cation is still unknown.6 On the other hand,
in the case of the pentaphenylcyclopentadienyl cation (Ph5C5+),
the ESR spectroscopic detection of half-field transitions showed
that it has a thermally populated and low-lying excited triplet
state.7 In this regard, pentaphenylborole [Ph4C4BPh],8 isoelec-
tronic with Ph5C5+, would be a suitable molecule to elucidate
the antiaromatic character of the singlet 4π electron system.
Unlike Ph5C5+, it does not consist of a low-lying excited triplet
state, according to its ESR measurement, in which the half-
reactions.8,9 The high reactivity has retarded the progress of its
chemistry. While pentaphenylborole was first synthesized by
Eisch and co-workers in 1969,8a its X-ray crystal structure has
been determined by Braunschweig et al. recently.10 Interestingly,
they revealed that a certain intermolecular interaction between
the borole boron atom and the substituents of the neighboring
borole compounds in the crystalline state affects the bond
alternation in the borole ring.
Herein, we report the synthesis and structural characterization
of a series of pentaaryl-substituted boroles having various
p-substituted phenyl groups on the boron atom. The X-ray
structural data of the pentaarylboroles provide a new perspective
to understand the nature of the borole antiaromatic system. Thus,
in our compounds, while an intermolecular interaction is
observed similar to that in the case of [Ph4C4BPh], the boroles
still have substantial bond alternation in the borole ring. We
will discuss the effect of the intermolecular interaction in the
crystalline state toward the antiaromaticity.
Besides antiaromaticity, the borole is also of interest in view
of reduction, since it has a vacant 2pz orbital on the boron atom
to accept electrons from the alkali metal. This would lead to
the formation of a dianion with 6π aromatic character,11a which
should be a useful building block for synthesizing novel
organometallic complexes.11–13 By reacting with potassium or
potassium graphite, we have succeeded in transforming some
of the pentaarylboroles into the corresponding dianions. The
features of their crystal structures will be described.
* Corresponding author. E-mail: yamaguchi@chem.nagoya-u.ac.jp.
(1) Allen, A. D.; Tidwell, T. T. Chem. ReV. 2001, 101, 1333.
(2) Saunders, M.; Berger, R.; Jaffe, A.; McBride, J. M.; O’Neill, J.;
Breslow, R.; Hoffman, J. M., Jr.; Perchonock, C.; Wasserman, E.; Hutton,
R. S.; Kuck, V. J. J. Am. Chem. Soc. 1973, 95, 3017.
(3) Wo¨rner, H. J.; Merkt, F. Angew. Chem., Int. Ed. 2006, 45, 293.
(4) (a) Jiao, H.; Schleyer, P.v. R.; Mo, Y.; McAllister, M. A.; Tidwell,
T. T. J. Am. Chem. Soc. 1997, 119, 7075. (b) Reindl, B.; Schleyer, P. v. R.
J. Comput. Chem. 1998, 19, 1402. (c) Shiota, Y.; Kondo, M.; Yoshizawa,
K. J. Chem. Phys. 2001, 115, 9243.
(5) (a) Gogonea, V.; Schleyer, P. v. R.; Schreiner, P. R. Angew. Chem.,
Int. Ed. 1998, 37, 1945. See also: (b) Baird, N. C. J. Am. Chem. Soc. 1972,
94, 4941. (c) Dougherty, R. C. J. Am. Chem. Soc. 1971, 93, 7187. (d) Aihara,
J. Bull. Chem. Soc. Jpn. 1978, 51, 1788. (e) Fratev, F.; Monev, V.;
Janoschek, R. Tetrahedron 1982, 38, 2929. (f) Jug, K.; Malar, E. J. P. J.
Mol. Struct. (THEOCHEM) 1987, 153, 221. (g) Wan, P.; Shukla, D. Chem.
ReV. 1993, 93, 571.
(6) (a) Otto, M.; Scheschkewitz, D.; Kato, T.; Midland, M. M.; Lambert,
J. B.; Bertrand, G. Angew. Chem., Int. Ed. 2002, 41, 2275. (b) Mu¨ller, T.
Angew. Chem., Int. Ed. 2002, 41, 2276. (c) Lambert, J. B. Angew. Chem.,
Int. Ed. 2002, 41, 2278. (d) Lambert, J. B.; Lin, L.; Rassolov, V. Angew.
Chem., Int. Ed. 2002, 41, 1429.
(7) (a) Breslow, R.; Chang, H. W.; Hill, R.; Wasserman, E. J. Am. Chem.
Soc. 1967, 89, 1112. (b) Breslow, R.; Chang, H. W.; Yager, W. A. J. Am.
Chem. Soc. 1963, 85, 2033.
(8) (a) Eisch, J. J.; Hota, N. K.; Kozima, S. J. Am. Chem. Soc. 1969,
91, 4575. (b) Eisch, J. J.; Galle, J. E.; Kozima, S. J. Am. Chem. Soc. 1986,
108, 379. (c) Eisch, J. J.; Galle, J. E.; Shafii, B.; Rheingold, A. L.
Organometallics 1990, 9, 2342.
(9) Fagan, P. J.; Nugent, W. A.; Calabrese, J. C. J. Am. Chem. Soc.
1994, 116, 1880.
(10) Braunschweig, H.; Ferna´ndez, I.; Frenking, G.; Kupfer, T. Angew.
Chem., Int. Ed. 2008, 47, 1951.
10.1021/om8002812 CCC: $40.75
2008 American Chemical Society
Publication on Web 06/10/2008

Synthesis of Pentaarylboroles and Their Dianions
Organometallics, Vol. 27, No. 14, 2008 3497
Scheme 1. Synthesis of Pentaarylboroles 2-4
ring has less significant bond alternation in the butadiene moiety
[C(1)-C(2) 1.428(2) Å, C(2)-C(3) 1.470(2) Å, B-C(1)
1.525(2) Å], which is totally different from the theoretically
optimized structure for the singlet state. They explain this
discrepancy is due to the intermolecular phenylfboron interac-
tion, which leads to some extent of delocalization of the 4π
electrons in the borole ring. We found that compounds 2-4
also have comparable or even shorter B---C(phenyl) intermo-
lecular distances, suggesting certain intermolecular interaction
between the phenyl C-H bond and the boron atom. In contrast,
the butadiene moiety in the borole ring has a large bond
alternation, which is similar to that of the theoretically optimized
structure for the singlet borole.14
Thus, in the packing of [Ph4C4B(p-CH3C6H4)] (2), the boron
center is sandwiched between two phenyl substituents of the
adjacent molecules. The shortest intermolecular distance be-
tween the boron atom and the C(phenyl) atom of the adjacent
borole in 2 is 3.610 Å. However, unlike [Ph4C4BPh] (5), the
bonding within the four-carbon butadiene backbone is highly
alternating. The short C(1)-C(2) bond (1.362(4) Å) and long
C(2)-C(3) bond (1.529(6) Å) correspond to double and single
bonds, respectively. Similarly, [Ph4C4B(p-Me3SiC6H4)] (3) also
consists of dimeric subunits with the phenyl substituent at the
C(1) or C(4) position of the borole ring lying above or below
the borole ring of the neighboring molecule. The shortest
intermolecular B---C(phenyl) distance (3.553Å) is shorter than
that of [Ph4C4BPh] (5) (3.653Å), but the bonding within the
four-carbon butadiene backbone has pronounced bond alterna-
tion [C(1)-C(2) 1.358(3) Å, C(2)-C(3) 1.524(3) Å, B-C(1)
1.605(3) Å]. In the crystal packing of [Ph4C4B(p-FC6H4)] (4),
similar to pentaphenylborole 5, dimeric subunits are present with
the aryl substituent at the boron center lying above or below
the borole ring of the contiguous molecule. The shortest
intermolecular B---C(p-FC6H4) distance (3.776 Å) in 4 is slightly
longer than that of [Ph4C4BPh] (5) (3.653 Å). Although the
p-FC6H4 group is introduced into the boron center, there is no
dramatic change in the bond distance for 4 [C(1)-C(2) 1.362(2)
Å, C(2)-C(3) 1.518(2) Å, B-C(1) 1.577(3) Å] compared with
those of 2 and 3.
These results demonstrate that the short interaction between
the borole boron atom and the phenyl group of the neighboring
molecule in the crystal packing is a common feature. The high
Lewis acidity of the boron atom enhanced by the antiaromatic
character may need stabilization by certain intermolecular B---
phenyl(C-H bond) interactions. This interaction needs not to
affect the planarity of the borole ring, as seen in compound 5.
However, compounds 2-4 maintain pronounced bond alterna-
tion in the borole butadiene moiety, which is different from 5.
Similar bond alternation is also found in the molecular structure
of [Ph4C4BFc],10 in which it has a short Fe---B distance (2.825
Å).
The observed crystal structures of 2-4 are very comparable
to their calculated structures without consideration of the
intermolecular interaction. These compounds were fully opti-
mized with the DFT-variant UB3LYP as implemented in the
Gaussian G03 program suite employing a basis set termed
6-31G*.15 The calculated energy differences between the singlet
ground state and the triplet state of 2-4 are 15.9, 15.4, and
15.7 kcal mol-1, respectively. These results show that the triplet
Results and Discussion
Synthesis of Pentaarylboroles. The displacement reaction
of 1,1-dimethyl-2,3,4,5-tetraphenylstannole [Ph4C4SnMe2] (1)
with a stoichiometric amount of aryldichloroboranes ArBCl2
in toluene or CH2Cl2 afforded unsolvated pentaarylboroles
[Ph4C4BAr] [Ar ) p-MeC6H4 (2), p- Me3SiC6H4 (3), p-FC6H4
(4)] (Scheme 1). Pentaphenylborole [Ph4C4BPh] (5)8,10 and
ferrocenylborole [Ph4C4BFc] (Fc ) ferrocenyl)10 were previ-
ously synthesized in a similar manner. Compounds 2 and 3 are
soluble only in CH2Cl2 at room temperature, and they were fully
verified by NMR spectroscopy and elemental analysis. The 11
B
NMR signals of 2 and 3 are δ 66.4 and 66.0 ppm, respectively.
On the contrary, 4 shows extremely low solubility in nonco-
ordinating solvents (e.g., benzene-d6 and CD2Cl2); therefore the
13C and 11B NMR data for compound 4 could not be obtained
due to its poor solubility.
Structural Characterization of Pentaarylboroles. Crystals
of 2-4, which were suitable for X-ray crystallography, were
obtained by slow cooling of the corresponding hot saturated
toluene or CH2Cl2 solution. Their crystallographic data and
selected structural parameters are summarized in Tables 1 and
2, respectively, and their ORTEP drawings are shown in Figures
1 and 2. In all cases, the borole rings are planar and the aryl
substituents are tilted toward this plane. The dihedral angles
between the aryl substituent at the boron center and the borole
ring [17.0° (2), 46.4° (3), 30.1° (4)] are smaller than those
between the phenyl substituents and the borole ring [average:
57.5° (2), 47.5° (3), 52.4° (4)].
We compare the structures of compounds 2-4 with that of
pentaphenylborole 5, which was reported by Braunschweig et
al.10 They reported that 5 consists of dimeric subunits with the
phenyl substituent at the boron center lying above or below the
borole ring of the neighboring molecule. The shortest intermo-
lecular B---C(phenyl) distance is 3.653 Å. Notably, the borole
(11) (a) Herberich, G. E.; Hostalek, M.; Laven, R.; Boese, R. Angew.
Chem., Int. Ed. Engl. 1990, 29, 317. (b) Herberich, G. E.; Buller, B.;
Hessner, B.; Oschmann, W. J. Organomet. Chem. 1980, 195, 253. (c) Hong,
F.-E.; Eigenbrot, C. W.; Fehlner, T. P. J. Am. Chem. Soc. 1989, 111, 949.
(d) Herberich, G. E.; Hengesbach, J.; Ko¨lle, U.; Huttner, G.; Frank, A.
Angew. Chem., Int. Ed. Engl. 1976, 15, 433.
(12) (a) Herberich, G. E.; Hessner, B.; Boveleth, W.; Lu¨the, H.; Saive,
R.; Zelenka, L. Angew. Chem., Int. Ed. Engl. 1983, 22, 996. (b) Gleiter,
R.; Hyla-Kryspin, I.; Herberich, G. E. J. Organomet. Chem. 1994, 478, 95.
(c) Hyla-Kryspin, I.; Gleiter, R.; Herberich, G. E.; Be´nard, M. Organome-
tallics 1994, 13, 1795. (d) Braunstein, P.; Englert, U.; Herberich, G. E.;
Neuschu¨tz, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1010. (e) Braunstein,
P.; Englert, U.; Herberich, G. E.; Neuschu¨tz, M.; Schmidt, M. U. J. Chem.
Soc., Dalton Trans. 1999, 2807.
(13) (a) Quan, R. W.; Bazan, G. C.; Kiely, A. F.; Schaefer, W. P.;
Bercaw, J. E. J. Am. Chem. Soc. 1994, 116, 4489. (b) Sperry, C. K.; Cotter,
W. D.; Lee, R. A.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1998,
120, 7791. (c) Pindado, G. J.; Lancaster, S. J.; Thornton-Pett, M.; Bochmann,
M. J. Am. Chem. Soc. 1998, 120, 6816. (d) Woodman, T. J.; Thornton-
Pett, M.; Bochmann, M. Chem. Commun. 2001, 329. (e) Woodman, T. J.;
Thornton-Pett, M.; Hughes, D. L.; Bochmann, M. Organometallics 2001,
20, 4080. (f) Loginov, D. A.; Muratov, D. V.; Petrovskii, P. V.; Starikova,
Z. A.; Corsini, M.; Laschi, F.; Fabrizi, F. de B.; Zanello, P.; Kudinov, A. R.
Eur. J. Inorg. Chem. 2005, 1737. (g) Muratov, D. V.; Petrovskii, P. V.;
Starikova, Z. A.; Herberich, G. E.; Kudinov, A. R. J. Organomet. Chem.
2006, 691, 3251.
(14) Schleyer, P. v. R.; Freeman, P. K.; Jiao, H.; Goldfuss, B. Angew.
Chem., Int. Ed. Engl. 1995, 34, 337.
(15) Frisch, M. J. ; et al. Gaussian 03, reVision C.02; Gaussian, Inc.:
Wallingford, CT, 2004. For the complete reference, see the Supporting
Information.

3498 Organometallics, Vol. 27, No. 14, 2008
So et al.
Table 1. Crystallographic Data for Compounds 2-4
2
3
4
formula
fw
color
cryst syst
space group
a (Å)
C35H27B
458.38
purple
tetragonal
P41212 (#92)
10.099(8)
10.099(8)
25.157(2)
90.000
90.000
90.000
2566.0(3)
4
1.187
C37H33BSi
516.53
blue
monoclinic
C2/c (#15)
18.900(5)
15.875(4)
19.674(5)
90.000
C34H24BF
462.34
greenish-blue
monoclinic
P21/n (#14)
10.975(5)
9.582(4)
24.482(11)
90.000
97.028(7)
90.000
2555(2)
4
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
94.149(4)
90.000
5888(3)
8
dcalcd (g cm-3
F(000)
)
1.159
2168
0.103
1.202
968
0.073
968
0.066
µ (mm-1
)
cryst size (mm)
2θ range (deg)
index range
0.10 × 0.10 × 0.10
2.17-25.49
-12 e h e 12,
-12 e k e 12,
-30 e l e 27
16 891
0.20 × 0.05 × 0.02
2.77-25.00
-22 e h e 22,
-13 e k e 18,
-21 e l e 22
19 415
0.20 × 0.20 × 0.20
2.83-25.50
-12 e h e 13,
-6 e k e 11,
-28 e l e 29
8419
no. of rflns collected
no. of indep rflns
2396
5134
4581
R1, wR2 (I < 2σ(I))
0.0811, 0.2055
0.0863, 0.2112
1.163
2396/0/166
0.354 to -0.249
0.0633, 0.1568
0.0806, 0.1692
1.107
5134/56/475
0.513 to -0.343
0.0499, 0.1359
0.0659, 0.1498
1.065
4581/0/325
0.240 to -0.240
R1, wR2 (all data)
goodness of fit, F2
no. of data/restraints/params
largest diff (e Å-3
)
Table 2. Selected Bond Distances (Å) and Angles (deg) for
isoelectronic to the cyclopentadienyl anion, were also prepared
by the reaction of corresponding pentaarylboroles with excess
potassium or potassium graphite in THF at room temperature
(Scheme 2). The 11B NMR signals of 6 (δ 29.6 ppm) and 7 (δ
28.7 ppm) show upfield shifts compared to those of neutral
precursors 3 (δ 66.0 ppm) and 5 (δ 62.8 ppm). They are stable
at room temperature under inert atmosphere. Compound 6 was
isolated as red solids, which formed a crystalline derivative with
tetramethylethylenediamine (tmeda) (Figure 3).
Compounds 2-4
2
3
4
B(1)-C(1)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-B(1)
B(1)-C(5)
1.578(5)
1.362(4)
1.529(6)
1.362(4)
1.578(5)
1.537(7)
1.605(3)
1.358(3)
1.524(3)
1.357(3)
1.606(3)
1.513(8)
1.577(3)
1.362(2)
1.518(2)
1.365(2)
1.576(3)
1.536(3)
B(1)-C(1)-C(2)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(4)-B(1)-C(1)
C(4)-B(1)-C(5)
C(1)-B(1)-C(5)
107.4(3)
107.2(2)
111.30(19)
111.35(19)
102.9(2)
128.9(3)
127.6(4)
107.16(14)
110.46(15)
111.36(15)
104.39(14)
127.85(16)
127.71(16)
Herberich and his co-workers have already reported the
synthesis of [K2(Ph4C4BPh)] (7), but it has not been structurally
characterized.11b We have determined the structures of 6 and 7
by X-ray crystallography. The crystallographic data of 6 and 7
are summarized in Table 3, and selected bond distances and
angles are shown in Table 4. In these compounds, two potassium
ions locate above and below the borole plane with η5-
coordination. The borole ring C4B is planar and the aryl
substituents are tilted toward this plane. The bond distances of
C(1)-C(2) [1.441(4) Å (6), 1.469(2) Å (7)] and C(3)-C(4)
[1.452(4) Å (6), 1.448(2) Å (7)] are longer, while the C(2)-C(3)
[1.431(4) Å (6), 1.409(2) Å (7)], B(1)-C(1) [1.531(4) Å (6),
1.543(2) Å (7)], and B(1)-C(4) bonds [1.538(4) Å (6), 1.522(2)
Å (7)] are shorter, compared with those of 3 and 5, respectively.
Thus, the bond alternation in the borole butadiene moiety is
significantly smaller compared to the neutral precursors 3 and
5. These bond patterns demonstrate the aromatic character (6π
electron system) in the C4B rings of 6 and 7.
In the structure of 6 · tmeda, dimeric subunits, in which two
borole dianions are bridged by coordinating the p-Me3SiC6H4
benzene ring with K(2), form a polymeric structure by bonding
tmeda to the potassium center K(1), as shown in Figure 3.
Among the potassium atoms K(1) and K(2), the K(1) atom has
a stronger coordination with the borole ring than K(2) (for
example, B(1)-K(1) 2.982(3) Å, B(1)-K(2) 3.158(3) Å).
Similarly, the dianion 7 has a polymeric structure through the
coordination of the potassium atoms K(1) and K(2) with the
phenyl substituents at C(1) and C(4) of the adjacent borole
dianion molecules (Figure 4). These compounds would have
110.54(17)
110.54(17)
104.0(4)
128.02(19)
128.02(19)
states of 2-4 are not significantly low-lying and the aryl
substituents on the boron atom do not affect the energy
differences between the singlet and triplet state. The calculated
structural parameters of the singlet boroles 2-4 [2: B(1)C(1)
1.589 Å, C(1)-C(2) 1.363 Å, C(2)-C(3) 1.532 Å; 3: B(1)-C(1)
1.588 Å, C(1)-C(2) 1.363 Å, C(2)-C(3) 1.533 Å; 4: B(1)-C(1)
1.588 Å, C(1)-C(2) 1.363 Å, C(2)-C(3) 1.533 Å] are in good
agreement with the crystallographic data. Therefore, we can
conclude that compounds 2-4 still hold the antiaromatic
character in the crystalline state even with the B---phenyl
intermolecular interaction. Their antiaromaticity can be evaluated
by the nucleus-independent chemical shift calculations at the
HF/6-31+G**//B3LYP/6-31G* level [NICS(0) value: +12.65
ppm (2), +12.78 ppm (3), +12.94 ppm (4)],16 while the NICS(0)
value of the isoelectronic pentaphenylcyclopentadienyl cation
(C5Ph5+) is +21.51 ppm.
Synthesis
and
Structural
Characterization
of
Pentaarylborole Dianions. Pentaaryl-substituted borole dian-
ions [K2(Ph4C4BAr)] [Ar ) p-Me3SiC6H4 (6), C6H5 (7)],
(16) Schleyer, P.v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van Eikema
Hommes, N. J. R. J. Am. Chem. Soc. 1996, 118, 6317.

Synthesis of Pentaarylboroles and Their Dianions
Organometallics, Vol. 27, No. 14, 2008 3499
Figure 2. Dimeric subunits in the crystal lattices of (a) [Ph4C4B(p-
CH3C6H4)] (2), (b) [Ph4C4B(p-Me3SiC6H4)] (3), and (c) [Ph4C4B(p-
FC6H4)] (4) (50% probability for thermal ellipsoids).
Figure 1. ORTEP drawings of (a) 2, (b) 3, and (c) 4 (50%
probability for thermal ellipsoids). Hydrogen atoms are omitted for
clarity.
Scheme 2. Synthesis of Borole Dianions 6 and 7
great potential as precursors for novel multidecker-type orga-
nometallic polymers.12,13
Experimental Section
OptiMelt MPA100 instrument and are uncorrected. Dry solvents
(hexane, CH2Cl2, ether, toluene, and THF) of dehydrated grade were
purchased from Kanto Chemical Co. Ltd. [Ph4C4SnMe2] (1),8
[Ph4C4BPh] (5),8 [p-CH3C6H4BCl2],17 [p- Me3SiC6H4BCl2],18 and
[p-FC6H4BCl2]19 were prepared as described in the literature.
[Ph4C4B(p-CH3C6H4)] (2). A solution of p-CH3C6H4BCl2 (0.18
g, 1.01 mmol) in toluene (15 mL) was added dropwise to
[Ph4C4SnMe2] (1) (0.51 g, 1.01 mmol) in toluene (15 mL) at room
temperature. The reaction mixture was stirred for 18 h, and a dark
General Procedures. All manipulations were carried out under
an inert atmosphere of argon gas by standard Schlenk techniques.
The 1H, 13C, and 11B NMR spectra were recorded on a JEOL AL-
400 spectrometer. The NMR spectra were recorded in CD2Cl2 or
1
THF-d8, and the chemical shifts are relative to SiMe4 for H and
13C and BF3 · Et2O for 11B, respectively. Elemental analyses were
measured on a Yanaco MT-6 instrument by the Chemical Instru-
mentation Facility, Nagoya University. Melting points were mea-
sured in a sealed glass tube on a Stanford Research Systems

3500 Organometallics, Vol. 27, No. 14, 2008
So et al.
Table 3. Crystallographic Data for Compounds 6 and 7
6
7
formula
fw
color
C61H81BK2N2O4.5Si C42H41BK2O2
1031.38
red
666.76
red
cryst syst
space group
a (Å)
b (Å)
c (Å)
triclinic
P1 (#2)
triclinic
P1 (#2)
j
j
14.755 (1)
14.739(2)
16.506(2)
80.90(7)
65.208(6)
63.435(5)
2912.7
2
12.980(3)
13.155(3)
13.210(3)
106.0948(16)
112.6165(17)
108.9784(16)
1747.5(7)
2
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
dcalcd (g cm-3
F(000)
)
1.176
1108
0.230
1.267
704
0.307
µ (mm-1
)
cryst size (mm)
2θ range (deg)
index range
0.10 × 0.10 × 0.10 0.20 × 0.20 × 0.20
2.54-25.00
-17 e h e 17,
-17 e k e 17,
-18 e l e 19
19 749
2.68-25.50
-14 e h e 15,
-15 e k e 15,
-15 e l e 14
12 218
no. of rflns collected
no. of indep rflns
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
goodness of fit, F2
10 97
6363
0.0571, 0.1401
0.0760, 0.1531
1.063
0.0309, 0.0789
0.0376, 0.0822
1.068
no. of data/restraints/params 10 097/ 0/685
6363/0/424
0.436 to -0.294
largest diff peaks (e Å-3
)
1.087 to -0.406
Table 4. Selected Bond Distances (Å) and Angles (deg) for
Compounds 6 and 7
6
7
B(1)-C(1)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
B(1)-C(4)
B(1)-C(5)
B(1)-K(1)
B(1)-K(2)
1.531(4)
1.441(4)
1.431(4)
1.452(4)
1.538(4)
1.583(4)
2.982(3)
3.158(3)
1.543(2)
1.469(2)
1.409(2)
1.448(2)
1.522(2)
1.572(2)
3.1773(17)
3.0361(17)
B(1)-C(1)-C(2)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-B(1)
C(1)-B(1)-C(4)
C(1)-B(1)-C(5)
C(4)-B(1)-C(5)
107.8(2)
110.3(2)
110.1(2)
107.2(2)
104.6(2)
125.5(2)
129.1(2)
106.44(12)
110.09(12)
111.02(12)
107.41(12)
105.04(12)
129.19(13)
125.48(13)
δ 2.32 (s, 3H, CH3), 6.76-7.16 (m, 24H, Ar). 13C{1H} NMR
(CD2Cl2): δ 21.9 (CH3), 125.5, 127.1, 127.2, 127.4, 127.6, 128.0,
129.4, 129.7, 136.6, 137.8, 140.0, 144.1, 162.5 (Ar) (two signals
for the carbon atoms bonding to the boron atom are not observed
due to the quadrupolar relaxation). 11B{1H} NMR (CD2Cl2): δ 66.4.
Anal. Calcd for C35H27B: C, 91.71; H, 5.94. Found: C, 91.61; H,
5.94.
[Ph4C4B(p-Me3SiC6H4)] (3). An analogous experimental pro-
cedure to that for 2 was used. The reaction of p-Me3SiC6H4BCl2
(0.24 g, 1.03 mmol) and 1 (0.52 g, 1.03 mmol) in toluene (30 mL)
afforded 0.49 g (0.95 mmol) of 3 as blue crystals in 92% yield.
Mp: 148 °C. 1H NMR (CD2Cl2): δ 0.22 (s, 9H SiMe3), 6.77-6.80
(m, 2H, Ar), 6.91-7.20 (m, 20H, Ar), 7.33-7.35 (m, 2H, Ar).
13C{1H} NMR (CD2Cl2): δ -1.49 (SiMe3), 125.6, 127.2, 127.37,
127.4, 127.6, 129.4, 129.7, 131.9, 136.5, 139.8, 146.9, 162.8 (Ar)
(two signals for the carbon atoms bonding to the boron atoms are
not observed due to the quadrupolar relaxation). 11B{1H} NMR
(CD2Cl2): δ 66.0. Anal. Calcd for C37H33BSi: C, 86.03; H, 6.44.
Found: C, 86.02; H, 6.43.
Figure
3.
ORTEP
drawings
of
[K2{C4Ph4B(p-
Me3SiC6H4)}] · (tmeda)0.5 · (THF)3 (50% probability thermal el-
lipsoids): (a) perspective view of the molecule and (b) its polymeric
structure. Hydrogen atoms and solvent molecules (TMEDA and
THF) are omitted for clarity.
blue solution was formed. Volatiles of the mixture were removed
under reduced pressure, and Me2SnCl2 was then evacuated from
the residue by sublimation in Vacuo at 40 °C. The residue was
extracted with hot toluene (8 mL) and filtered. The solution was
cooled slowly to room temperature to yield 0.39 g (0.85 mmol) of
2 as purple crystals in 85% yield. Mp: 162 °C. 1H NMR (CD2Cl2):
(17) Muetterties, E. L. J. Am. Chem. Soc. 1959, 81, 2597.
(18) Kaufmann, D. Chem. Ber. 1987, 120, 901.
(19) Fraenk, W.; Klapo¨tke, T. M.; Krumm, B.; Mayer, P.; No¨th, H.;
Piotrowski, H.; Suter, M. J. Fluorine Chem. 2001, 112, 73.
[Ph4C4B(p-FC6H4)] (4). An analogous experimental procedure
to that for 2 was used. The reaction of p-FC6H4BCl2 (0.18 g, 1.01

Synthesis of Pentaarylboroles and Their Dianions
Organometallics, Vol. 27, No. 14, 2008 3501
Figure 4. ORTEP drawings of [K2(Ph4C4BPh)] · (THF)2 (50% probability for thermal ellipsoids): (a) perspective view of the monomeric
unit and (b) its polymeric structure. Hydrogen atoms are omitted for clarity.
mmol) and 1 (0.51 g, 1.01 mmol) in CH2Cl2 (30 mL) afforded 3.9
mg (0.0084 mmol) of 4 as greenish-blue crystals in 0.8% yield.
Mp: 153 °C. 1H NMR (CD2Cl2): δ 6.54-6.58 (m, 2H, Ar),
6.83-7.05 (m, 20H, Ar), 7.37-7.39 (m, 2H, Ar). Anal. Calcd for
C34H24BF: C, 88.32; H, 5.23. Found: C, 88.05; H, 5.17.
on their respective parent atoms. A summary of crystal data for
2-4, 6, and 7 is given in Tables 1 and 3, respectively.
For compound 3, the trimethylsilylphenyl moiety (C5-C10, Si1,
and C11-C13) and one phenyl ring (C26-C31) are disordered and
solved using appropriate disordered models. Thus, two sets of
trimethylsilylphenyl moieties, (C5A-C10A, Si1A, C11A, C12, C13)
and (C5B-C10B, Si1B, C11B, C12, C13), were placed and their
occupancies were refined to be 0.50. Similarly, two sets of phenyl
rings, (C26A-C31A) and (C26B-C31B), were placed and their
occupancies were refined to be 0.46 and 0.54, respectively. All non-
hydrogen atoms, except for C5B, were refined anisotropically, and
all hydrogen atoms, except for those on C11A and C11B, were
placed using AFIX instructions.
For compound 6, besides THF and tmeda coordinating to the
potassium, THF and tmeda molecules are included as solvent
molecules in a lattice. One disordered THF solvent molecule, (O5,
C62-C65), sits on the symmetry axis, which is placed with its
occupancy as 0.5.
Computational Method. All calculations were conducted using
the Gaussian 03 series of electronic structure programs.15 The
geometries were optimized with the unrestricted Becke hybrid
(UB3LYP) method at the 6-31G* level for singlet and triplet borole
derivatives. Their minimum energies were confirmed by the
frequency calculations. The energy differences between the singlet
state and the triplet state were corrected using zero-point energy.
The NICS calculations were carried out at the HF/6-31+G** level
for the optimized geometries at the B3LYP/6-31G* level.
[K2{Ph4C4B(p-Me3SiC6H4)}] (6). THF (30 mL) was added to a
mixture of 3 (0.52 g, 1.01 mmol) and excess potassium chunks
(0.39 g, 0.01 mol) at ambient temperature. The resulting red solution
was stirred for 17 h. The insoluble solid was filtered. Volatiles of
the mixture were removed under reduced pressure. The residue was
washed with 10 mL of toluene three times to give 0.52 g of 6 as
a red powder. Mp: 72 °C (dec). 1H NMR (THF-d8): δ 0.16 (s, 9H
SiMe3), 1.77-1.78 (m, 2H, THF), 3.59-3.62 (m, 2H, THF),
6.33-6.36 (m, 2H, Ar), 6.54-6.78 (m, 18H, Ar), 6.92 (m, 2H,
Ar).7.29 (m, 2H, Ar). 13C{1H} NMR (THF-d8): δ 0.61 (SiMe3),
118.3, 120.9, 124.9 127.6, 127.7, 129.9, 130.0, 130.7, 132.0, 132.3,
133.4, 136.5, 147.2, 152.3 (Ar). 11B{1H} NMR (THF-d8): δ 28.7.
Attempts to obtain acceptable elemental analysis data for compound
6 failed due to its extreme air sensitivity. Red crystals of 6, which
were suitable for X-ray crystallography, were obtained from its
concentrated TMEDA/THF solution.
[K2(Ph4C4BPh)] (7). THF (3.5 mL) was added to a mixture of
5 (0.23 g, 0.53 mmol) and an excess amount of potassium graphite
(0.18 g, 4.6 mmol) at ambient temperature. The resulting dark red
solution was stirred for 34 h. The insoluble solid was filtered off,
and hexane was diffused into the red filtrate to afford 0.34 g (0.52
mmol) of 7 · (THF)2 as a red powder in 98% yield. The spectro-
scopic data are the same as those reported in the literature.11b Red
crystals of 7 · (THF)2, which were suitable for X-ray crystallography,
were obtained from its concentrated THF solution.
Acknowledgment. C.-W.S. thanks Japan Society for the
Promotion of Science for a Research Fellowship. This work
was partially supported by Grants-in-Aid (Nos. 19685004
and 19675001) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan, and SORST, Japan
Science and Technology Agency.
X-ray Structure Analysis. Intensity data for compounds 2-4,
6, and 7 were collected using a Rigaku single crystal CCD, by
means of graphite-monochromatized Mo KR radiation (λ ) 0.71073
Å). The crystals of 2-4, 6, and 7 were measured at 100(2) K. The
structures were solved by direct phase determination (SHELXTL-
97)20 and refined by full-matrix least-squares methods on F2. All
non-hydrogen atoms were subjected to anisotropic refinement.
Hydrogen atoms were generated geometrically and allowed to ride
Supporting Information Available: Complete ref 15, calculated
molecular coordinates for 2-5, and X-ray data (CIF) of 2-4, 6,
and 7. This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Sheldrick, G. M. SHELX-97, Program for the Refinement of Crystal
Structures; University of Gottingen: Gottingen, Germany, 1997.
OM8002812