Structural and electrochemical investigations of the high fluoride affinity of sterically hindered 1,8-bis(boryl)naphthalenes

Article abstract of DOI:10.1021/ic060709s

The reaction of 10-bromo-9-oxa-10-boraanthracene with the tetrakis(tetrahydrofuran)lithium salt of dimesityl-1,8-naphthalenediylborate in diethyl ether affords 1-(dimesitylboryl)-8-(10′-bora-9′-oxaanthryl) naphthalene (2). This diborane reacts with [Me₃SiF ₂][S(NMe₂)₃)] to afford the anionic complex [2-μ₂-F]^-, which has been isolated as a [S(NMe ₂)₃]⁺ salt. The cyclic voltammograms of diborane 2 as well as 1-(dimesitylboryl)-8-(10′-bora-9′-thiaanthryl) naphthalene (1) exhibit two reversible reductions at E_1/2 = -2.200 and -2.566 V (vs FcH/FcH⁺) for 1 and E_1/2 = -2.248 and -2.620 V (vs FcH/FcH⁺) for 2 corresponding to the sequential reduction of the two boron centers. These two waves simultaneously disappear upon fluoride addition, thus indicating the formation of fluoride chelate complexes [1-μ₂-F]^- and [2-μ₂-F] ^-. To identify the origin of the high fluoride affinity displayed by these diboranes, the structures of 2 and [2-μ₂-F]^- have been studied experimentally and computationally. The crystallographic studies show that the structure of 2 is distorted, thus indicating the presence of important steric repulsions between the neighboring boryl moieties. By contrast, the structure of the anionic complex [2-μ₂-F]^- is much more sterically relaxed than that of 2, as indicated by a reduction of the B-B distance from 3.279(4) A in 2 to 2.922(7) A in [2-μ₂- F]^-. The structural results suggest that the high fluoride affinity displayed by 2 results, at least in part, from the relief of steric repulsions induced by fluoride binding. Finally, the nature of the bonding as well as the strength of the interactions involved in the B-F-B bridge of [2-μ₂-F]^- has been studied using density functional theory calculations and Atoms-ln-Molecules analyses. These calculations indicate that the enthalpic gain associated with the formation of two B-F bonds in [2-μ₂-F]^- only amounts to a fraction of the energy of a terminal B-F bond. These calculations also suggest that the relief of steric repulsions induced by fluoride binding in 2 may contribute to the high fluoride affinity of these types of molecules.

Full text of DOI:10.1021/ic060709s

Inorg. Chem. 2006, 45, 8136−8143
Structural and Electrochemical Investigations of the High Fluoride
Affinity of Sterically Hindered 1,8-Bis(boryl)naphthalenes
Mohand Mela1ımi, Ste´phane Sole´, Ching-Wen Chiu, Huadong Wang, and Franc¸ois P. Gabba1ı*
Chemistry TAMU 3255, Texas A&M UniVersity, College Station, Texas 77843-3255
Received April 27, 2006
The reaction of 10-bromo-9-oxa-10-boraanthracene with the tetrakis(tetrahydrofuran)lithium salt of dimesityl-1,8-
naphthalenediylborate in diethyl ether affords 1-(dimesitylboryl)-8-(10 -bora-9 -oxaanthryl)naphthalene (2). This diborane
reacts with [Me3SiF2][S(NMe2)3)] to afford the anionic complex [2-
2-F]-, which has been isolated as a [S(NMe2)3]+
salt. The cyclic voltammograms of diborane 2 as well as 1-(dimesitylboryl)-8-(10 -bora-9 -thiaanthryl)naphthalene
(1) exhibit two reversible reductions at E1 ) −2.200 and
2.566 V (vs FcH/FcH+) for 1 and E1 ) −2.248 and
′
′
µ
′
′
−
/
2
/2
−
2.620 V (vs FcH/FcH+) for 2 corresponding to the sequential reduction of the two boron centers. These two
waves simultaneously disappear upon fluoride addition, thus indicating the formation of fluoride chelate complexes
[1-
2-F]- and [2- 2-F]-. To identify the origin of the high fluoride affinity displayed by these diboranes, the structures
of 2 and [2-
2-F]- have been studied experimentally and computationally. The crystallographic studies show that
the structure of 2 is distorted, thus indicating the presence of important steric repulsions between the neighboring
boryl moieties. By contrast, the structure of the anionic complex [2-
2-F]- is much more sterically relaxed than that
of 2, as indicated by a reduction of the B B distance from 3.279(4) Å in 2 to 2.922(7) Å in [2-
2-F]-. The structural
µ
µ
µ
µ
−
µ
results suggest that the high fluoride affinity displayed by 2 results, at least in part, from the relief of steric repulsions
induced by fluoride binding. Finally, the nature of the bonding as well as the strength of the interactions involved
in the B
Molecules analyses. These calculations indicate that the enthalpic gain associated with the formation of two B
bonds in [2- F bond. These calculations also
2-F]- only amounts to a fraction of the energy of a terminal B
−F−B bridge of [2-µ
2-F]- has been studied using density functional theory calculations and Atoms-In-
−F
µ
−
suggest that the relief of steric repulsions induced by fluoride binding in 2 may contribute to the high fluoride
affinity of these types of molecules.
Introduction
bridges the two boron centers. It is generally assumed that
the chelate structure of these complexes is largely responsible
for their unusual stability. For example, in the case of
fluorinated 1,2-diborylbenzene derivatives, some of the
resulting anionic complexes are sufficiently stable to coexist
with cationic olefin polymerization catalysts.10,11 Similarly,
the hydride complex of 1,8-bis(dimethylboryl)naphthalene
fails to reduce benzaldehyde.12-15
Bidentate boranes including 1,2-diborylbenzenes and 1,8-
diborylnaphthalenes have attracted a great deal of interest
as receptors for electron-rich substrates.1-9 It has been clearly
demonstrated that such diboranes interact with anionic
substrates to form chelate complexes in which the anion
* To whom correspondence should be addressed. E-mail: gabbai@
mail.chem.tamu.edu.
(1) Piers, W. E.; Irvine, G. J.; Williams, V. C. Eur. J. Inorg. Chem. 2000,
2131-2142.
(8) (a) Hergel, A.; Pritzkow, H.; Siebert, W. Angew. Chem., Int. Ed. Engl.
1994, 33, 1247-1248. (b) Weinman, W.; Hergel, A.; Deforth, T.;
Kra¨mer, A.; Pritzkow. H.; Siebert, W. Z. Naturforsch., B 1996, 51b,
1104-1110.
(9) Reilly, M.; Oh, T. Tetrahedron Lett. 1995, 36, 221-223.
(10) 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. Henderson, L. D.; Piers, W. E.; Irvine, G. J.; McDonald,
R. Organometallics 2002, 21, 340-345.
(11) Lewis, S. P.; Taylor, N. J.; Piers, W. E.; Collins, S. J. Am. Chem.
Soc. 2003, 125, 14686-14687.
(12) Katz, H. E. J. Org. Chem. 1985, 50, 5027-5032.
(2) Gabba¨ı, F. P. Angew. Chem., Int. Ed. 2003, 42, 2218-2221. Hoe-
felmeyer, J. D.; Schulte, M.; Tschinkl, M.; Gabbai, F. P. Coord. Chem.
ReV. 2002, 235, 93-103.
(3) Melaimi, M.; Gabbai, F. P. AdV. Inorg. Chem. 2005, 53, 61-99.
(4) Chen, E. Y.-C.; Marks, T. J. Chem. ReV. 2000, 100, 1391-1434.
(5) Eisch, J. J.; Kotowicz, B. W. Eur. J. Inorg. Chem. 1998, 761-769.
(6) 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.
(7) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P. N.; Marks, T.
J. Angew. Chem., Int. Ed. 2000, 39, 1312-1316.
8136 Inorganic Chemistry, Vol. 45, No. 20, 2006
10.1021/ic060709s CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/29/2006

Sterically Hindered 1,8-Bis(boryl)naphthalenes
Scheme 1
Table 1. Crystal Data, Data Collection, and Structure Refinement for 2
and [[2-µ2-F][S(NMe2)3]
2
[2-µ2-F][S(NMe2)3]
Crystal Data
C40H36B2O
554.31
0.56 × 0.47 × 0.40
monoclinic
P21/n
16.058(14)
10.799(10)
17.637(16)
100.280(16)
3009(5)
formula
Mr
C46H54B2FN3OS
737.60
0.61 × 0.43 × 0.41
orthorhombic
Pbca
13.579(14)
12.12(2)
28.62(3)
cryst size (mm3)
cryst syst
space group
a (Å)
Recent results obtained in our laboratory further substanti-
ate the stabilities of such anionic chelates. Indeed, we recently
reported that 1-(dimesitylboryl)-8-(10′-bora-9′-thiaanthryl)-
naphthalene (1) behaves as a bidentate Lewis acid and readily
complexes fluoride to afford the anionic chelate complex
[1-µ2-F]- (Scheme 1).16 Titration experiments carried out in
tetrahydrofuran (THF) indicate that the fluoride binding
constant of 1 exceeds 5 × 109 M-1. This binding constant is
at least 3 orders of magnitude greater than that measured
for the monofunctional boranes including Mes3B.16-18 This
large difference suggests that the cooperative binding of the
anion by the two boron centers may be responsible for the
high fluoride affinity of 1. Because the crystal structure of
1 remains unknown, a careful analysis of the structural
reorganization accompanying fluoride binding has not been
possible. Since the structural changes induced by fluoride
binding may provide additional insight into the high fluoride
affinity of such diboranes, we set out to prepare a crystalline
diborane, which, along with its fluoride complex, could be
structurally studied.
In this paper, we report the synthesis of 1-(dimesitylboryl)-
8-(10′-bora-9′-oxaanthryl)naphthalene (2), a novel diborane
that is a close analogue of 1. Using both structural and
computational techniques, we show that the high fluoride
affinity displayed by this novel diborane results, at least in
part, from the relief of steric repulsions induced by fluoride
binding. Additionally, we demonstrate that the unusual
electrochemical properties of 1 and 2 can serve to signal
fluoride binding.
b (Å)
c (Å)
â (deg)
V (Å3)
8207(15)
8
1.194
0.122
3152
Z
F
4
calc (g cm-3
)
1.223
0.070
1176
µ (mm-1
F(000)
)
Data Collection
110(2)
ω
-19 f +17,
-12 f +11,
-20 f +20
15 328
5280 [0.0798]
5280
T (K)
scan mode
hkl range
110(2)
ω
-13 f +15,
-23 f +22,
-25 f +31
34 900
5822 [0.1066]
5822
measd reflns
unique reflns [Rint
reflns used for
refinement
]
Refinement
388
0.0585, 0.1746
0.569, -0.550
refined param
554
R1,a wR2b [I > 2σ(I)]
0.0753, 0.1528
0.951, -0.549
F
fin (max/min) (e Å-3
)
handled under a N2 atmosphere using standard Schlenk and
glovebox techniques. Elemental analyses were performed by
Atlantic Microlab (Norcross, GA). All melting points were mea-
sured on samples in sealed capillaries and are uncorrected. NMR
spectra were recorded on Inova-400 FT NMR (399.63 MHz for
1H, 376.03 MHz for 19F, 128.22 MHz for 11B, and 100.50 MHz
for 13C) by using an internal deuterium lock. Chemical shifts, δ,
are given in ppm. Spectra are internally referenced to Me4Si (1H
and 13C, δ ) 0 ppm) and externally referenced to BF3‚OEt2 (11B,
δ ) 0 ppm), CFCl3 (19F, δ ) 0 ppm), and HgCl2 in DMSO (199Hg,
δ ) -1501.6 ppm).
Crystallography. The crystallographic measurements were
performed using a Siemens SMART-CCD area detector diffracto-
meter, with graphite-monochromated Mo KR radiation (λ )
0.710 69 Å). Specimens of suitable size and quality were selected
and mounted onto a glass fiber with Apiezon grease. The structures
were solved by direct methods, which successfully located most
of the non-hydrogen atoms. Subsequent refinement on F 2 using
the SHELXTL/PC package (version 5.1) allowed location of the
remaining non-hydrogen atoms. Further crystallographic details can
be found in Table 1.
Electrochemistry. Electrochemical experiments were performed
with an electrochemical 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 into a small glass tube fitted with a porous Vycor frit
at the tip and filled with a CH3CN solution containing nBu4NPF6
(0.3 M) and Ag(NO3) (0.005 M). All three electrodes were
immersed in a deoxygenated THF solution (5 mL) containing nBu4-
NPF6 (0.3 M) as a support electrolyte and the diborane (1 or 2)
(0.003 M). The electrolyte was used as purchased. In all cases,
ferrocene was used as an internal standard, and all reduction
potentials are reported with respect to E1/2 of the FcH/FcH+
redox couple.
Experimental Section
General Procedures. Diborane 1,16 tetrakis(tetrahydrofuran)-
lithium dimesityl-1,8-naphthalenediylborate,19 and bis[2-(trimeth-
ylsilanyl)phenyl] ether20 were prepared according to the reported
procedures. TASF ([Me3SiF2][S(NMe2)3)]) was purchased from
Aldrich and used as provided. Chloroform was distilled over CaH2
and THF over a Na/K amalgam. Air-sensitive compounds were
(13) Katz, H. E. J. Am. Chem. Soc. 1985, 107, 1420-1421.
(14) Katz, H. E. Organometallics 1987, 6, 1134-1136.
(15) Katz, H. E. Inclusion Compd. 1991, 4, 391-405.
(16) Sole´, S.; Gabba¨ı, F. P. Chem. Commun. 2004, 1284-1285.
(17) (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
122, 6335-6336. (b) Yamaguchi, S.; Shirasaka, T.; Tamao, K. Org.
Lett. 2000, 2, 4129-4132. (c) Yamaguchi, S.; Akiyama, S.; Tamao,
K. J. Am. Chem. Soc. 2001, 123, 11372-11375. (d) Yamaguchi, S.;
Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2002, 124,
8816-8817. (e) Kubo, Y.; Yamamoto, M.; Ikeda, M.; Takeuchi, M.;
Shinkai, S.; Yamaguchi, S.; Tamao, K. Angew. Chem., Int. Ed. 2003,
42, 2036-2040.
(18) Liu, Z.-Q.; Fang, Q.; Cao, D.-X.; Wang, D.; Xu, G.-B. Org. Lett. 2004,
6, 2933-2936.
(19) Hoefelmeyer, J. D.; Gabba¨ı, F. P. Organometallics 2002, 21, 982-
985.
(20) Gilman, H.; Miles, D. H. J. Org. Chem. 1958, 23, 1363-1365.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8137

Mela1ımi et al.
Theoretical Calculations. Density functional theory (DFT)
calculations (full geometry optimization) were carried out with
Gaussian0321 using the gradient-corrected Becke exchange func-
tional (B3LYP)22 and the Lee-Yang-Parr correlation functional.23
A 6-31+g(d′) basis set was used for the boron, oxygen, and
coordinating fluorine atoms.24 A 6-31g basis set was used for all
carbon and hydrogen atoms.25 Frequency calculations, which were
carried out on the optimized structure of each compound, confirmed
the absence of any imaginary frequencies. Frontier orbitals were
obtained from the optimized geometry. The electron density of the
DFT-optimized structure of the fluoroborates was subjected to an
Atoms-In-Molecules analysis26 using AIM2000.27 To calculate the
fluoride ion affinity (FIA) of the boranes, the optimized geometries
of the boranes and fluoroborates were subjected to a single-point
energy calculation using the gradient-corrected Becke exchange
functional (B3LYP) and the Lee-Yang-Parr correlation functional
and the 6-311+g(2d,p) basis set for all atoms.28 The fluoride ion
affinities were calculated as per eqs 1 and 2. The reaction enthalpies
∆H were derived from the energy of each molecule (from the single-
point calculation) corrected to enthalpy by the “thermal correction
to enthalpy term” obtained in the frequency calculations.
solid was extracted with diethyl ether (30 mL). This solution was
filtered and concentrated. The compound, namely, 10-bromo-9-
oxa-10-boraanthracene, crystallized upon cooling to -50 °C. The
solvent was removed by filtration, and the crystals were washed
with cold diethyl ether (2 × 10 mL) and dried under vacuum. The
compound was not purified any further and was used as such for
the synthesis of diborane 2. Yield: 2.84 g (70%). Mp: 104 °C. 1H
NMR (CDCl3): δ 7.37 (ddd, 2H, 3JH-H ) 7.2 Hz, 3JH-H ) 7.6 Hz,
3
4JH-H ) 1.2 Hz, BC-CH-CH), 7.54 (dd, 2H, JH-H ) 8.4 Hz,
4JH-H ) 1.2 Hz, OC-CH), 7.78 (ddd, 2H, 3JH-H ) 7.2 Hz, 3JH-H
) 8.4 Hz, 4JH-H ) 1.6 Hz, OC-CH-CH), 8.32 (dd, 2H, 3JH-H
)
4
7.6 Hz, JH-H ) 1.6 Hz, BC-CH). 13C NMR (CDCl3): δ 117.7,
123.0, 135.4, 135.5 (8C, CH), 159.4 (2C, O-C). B-C was not
detected. 11B NMR (CDCl3): δ +52.3.
1-(Dimesitylboryl)-8-(10′-bora-9′-oxaanthryl)naphthalene (2).
A solution of 10-bromo-9-oxa-10-boraanthracene (390 mg, 1.50
mmol) in diethyl ether (10 mL) was added to a suspension of
tetrakis(tetrahydrofuran)lithium dimesityl-1,8-naphthalenediylborate
(670 mg, 1.00 mmol) in diethyl ether (40 mL) at 25 °C. The mixture
was stirred 2 h at room temperature, and the solution was filtered.
The solvent was removed under vacuum, and the resulting solid
was extracted with dichloromethane (20 mL). Following filtration
and evaporation of the solvent, the colorless solid was washed with
ethanol (2 × 50 mL) and dried under vacuum. This solid was
recrystallized from a dichloromethane/hexane (1:1, v/v) mixture,
affording colorless crystals of 2. Yield: 205 mg (37%). Large
monocrystals could be obtained by slow evaporation of a diethyl
borane + F- ∆H8 fluoroborate
FIA ) -∆H
(1)
(2)
10-Bromo-9-oxa-10-boraanthracene. This compound has been
previously reported.29 The following synthetic procedure differs
from that found in the literature. A solution of boron tribromide
(5.5 mL, 45.3 mmol) in dichloromethane (10 mL) was added
dropwise under nitrogen to a solution of bis[2-(trimethylsilanyl)-
phenyl] ether (5 g, 19.3 mmol) in dichloromethane (20 mL) at -78
°C. This reaction mixture was allowed to warm to room temperature
and stirred for 1 h. The solvent was evaporated, and the resulting
1
ether solution. Mp: 239 °C. H NMR (CDCl3): δ 0.93 (s, 3H,
Mes-CH3), 1.23 (s, 3H, Mes-CH3), 1.42 (s, 3H, Mes-CH3), 1.71
(s, 3H, Mes-CH3), 1.90 (s, 3H, Mes-CH3), 2.19 (s, 3H, Mes-
CH3), 6.02 (s, 1H, Mes-CH), 6.55 (s, 1H, Mes-CH), 6.66 (s, 1H,
Mes-CH), 6.91 (m, 1H, CH), 6.95 (m, 1H, CH), 7.20 (dd, 1H,
4
3
3JH-H ) 7.2 Hz, JH-H ) 1.6 Hz, CH), 7.26 (d, 1H, JH-H ) 8.0
3
Hz, CH), 7.39 (d, 1H, JH-H ) 8.4 Hz, CH), 7.47-7.55 (m, 6H,
3
4
CH), 7.85 (dd, 1H, JH-H ) 7.2 Hz, JH-H ) 1.6 Hz, CH), 7.99
(dd, 1H, 3JH-H ) 7.2 Hz, 4JH-H ) 2.4 Hz, CH), 8.13 (dd, 1H, 3JH-H
) 8.0 Hz, 4JH-H ) 1.6 Hz, CH). 13C NMR (CDCl3): δ 20.99, 21.05,
22.24, 22.70, 22.97, 25.39 (6C, Mes-CH3), 116.3, 117.1, 120.9,
121.9, 124.6, 125.6, 127.7, 127.9, 129.0, 129.3, 130.1, 131.7, 133.4,
133.8, 135.2, 135.9, 138.4, 140.9 (18C, CH), 123.2, 123.7, 138.9,
151.9, 144.6, 147.3 (6C, B-C), 134.0, 137.6, 139.9 (2C), 140.9 (2C),
141.5, 143.7 (8C, C-C), 159.7, 160.2 (2C, O-C). 11B NMR
(CDCl3): δ +51.6, +72.4. Anal. Calcd for C40H36B2O: C, 86.67;
H, 6.55. Found: C, 86.45; H, 6.52.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(22) Becke, A. J. Chem. Phys. 1993, 98, 5648-5652.
Synthesis of [2-µ2-F][S(NMe2)3]. A solution of TASF (45 mg,
0.16 mmol) in THF (5 mL) was added to a solution of 2 (100 mg,
0.18 mmol) in THF (5 mL) at 25 °C. After 15 min, the solvent
was evaporated and the residue was washed with two portions of
diethyl ether (20 mL). The remaining white solid was dried under
vacuum. Large colorless monocrystals could be obtained by slow
evaporation of an acetonitrile solution. Yield: 100 mg (85%). Mp:
(23) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200-206.
1
238 °C. H NMR (acetone-d6): 1.04 (s, 3H, Mes-CH3), 1.50 (d,
(24) (a) Foresman, J. B.; Frisch, Æ. Exploring Chemistry with Electronic
Structure Methods, 2nd ed.; Gaussian Inc.: Pittsburgh, PA, 1996; p
100. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28,
213-222.
(25) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257-2261.
(26) Bader, R. F. W. Atoms In Molecules: A Quantum Theory; Oxford
University Press: Oxford, U.K., 1990.
(27) AIM 2000: Biegler-Ko¨nig, F. W.; Scho¨nbohm, J.; Bayles, D. J.
Comput. Chem. 2001, 22, 545-549. http://www.aim2000.de.
(28) (a) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem.
Phys. 1980, 72, 650-654. (b) Spintzagel, G. W.; Clark, T.; Schleyer,
P. v. R.; Hehre, W. J. J. Comput. Chem. 1987, 8, 1109-1116.
(29) Roth, H. J.; Miller, B. Arch. Pharm. (Weinheim, Ger.) 1964, 297,
524-528.
3H, JH-F ) 4.4 Hz, Mes-CH3), 1.66 (s, 3H, Mes-CH3), 1.79 (d,
3H, JH-F ) 6.0 Hz, Mes-CH3), 2.00 (s, 3H, Mes-CH3), 2.06 (s,
3H, Mes-CH3), 2.97 (s, 18H, NCH3), 5.90 (s, 1H, Mes-CH), 6.17
3
(m, 2H, CH), 6.35 (d, 1H, JH-H ) 7.2 Hz, CH), 6.60 (m, 2H,
3
CH), 6.83-6.89 (m, 3H, CH), 6.97 (d, 1H, JH-H ) 7.6 Hz, CH),
7.04 (m, 1H, CH), 7.08 (dd, 1H, 3JH-H ) 6.8 Hz, 4JH-H ) 1.2 Hz,
CH), 7.12-7.18 (m, 2H, CH), 7.58 (dd, 1H, 3JH-H ) 7.6 Hz, 4JH-H
3
) 1.2 Hz, CH), 7.64 (d, 1H, JH-H ) 8.0 Hz, CH). 13C NMR
(acetone-d6): δ 20.77 (d, 2C, JC-F ) 3.4 Hz), 24.20 (d, 1C, JC-F
)
5.3 Hz), 24.71 (s, 1C), 25.11 (s, 1C), 26.11 (d, 1C, JC-F ) 7.2 Hz,
Mes-CH3), 38.40 (s, 6C, NCH3), 114.2 (s), 115.4 (d, JC-F ) 2.2
8138 Inorganic Chemistry, Vol. 45, No. 20, 2006

Sterically Hindered 1,8-Bis(boryl)naphthalenes
a
Scheme 2
a TASF ) [Me3SiF2][S(NMe2)3]
Hz), 120.7 (s), 120.8 (s), 124.3 (d, JC-F ) 1.9 Hz), 124.9 (s), 125.2
(s), 126.3 (d, JC-F ) 2.0 Hz), 126.4 (s), 127.2 (d, JC-F ) 3.0 Hz),
128.4 (s), 128.7 (s), 129.3 (s), 129.4 (d, JC-F ) 2.0 Hz), 129.7 (d,
JC-F ) 7.2 Hz), 130.0 (d, JC-F ) 8.8 Hz), 137.0 (s), 137.9 (s)
(18C, CH), 138.8 (s), 133.1 (s), 133.3 (d, JC-F ) 2.7 Hz), 140.8
(s), 140.8 (d, JC-F ) 2.6 Hz), 140.9 (s), 141.8 (d, JC-F ) 6.9 Hz),
142.7 (s), 144.4 (s), 158.9 (s) (10C, C-C), 131.4, 134.2, 148.2,
150.4, 151.3, 155.0 (br, 6C, B-C). 11B NMR (acetone-d6): δ
+11.7, +17.6. 19F NMR (acetone-d6): δ -178.6. Anal. Calcd for
C46H54N3B2OSF: C, 74.90; H, 7.38; N, 5.70. Found: C, 73.75; H,
7.34; N, 5.63.
Results and Discussion
Synthesis, Structure, and Characterization of Diborane
2. Diborane 2 was prepared by the reaction of 10-bromo-
9-oxa-10-boraanthracene with the Li(THF)4 salt of dimesityl-
1,8-naphthalenediylborate19 and was isolated as a colorless
solid in 37% yield (Scheme 2). The properties of this new
diborane are close to those of 1. It is soluble in chloroform,
Figure 1. ORTEP view of 2 (50% ellipsoid; hydrogen atoms omitted for
clarity). Selected distances [Å] and angles [deg]: B1-C1 1.581(3), B1-
C11 1.598(3), B1-C21 1.573(3), B1-B2 3.279(4), B2-C8 1.577(3), B2-
C31 1.535(4), B2-C41 1.535(4), B2-C21 2.965(3); C1-B1-C11 117.77-
(19), C1-B1-C21 122.55(19), C11-B1-C21 118.52(19), C8-B2-C31
121.6(2), C8-B1-C41 122.1(2), C31-B2-C41 114.2(2), C9-C1-B1
128.43(19), C9-C8-B2 129.91(19), C8-C9-C1 124.23(19).
1
THF, and pyridine. As for 1, the H NMR spectrum of 2
exhibits six distinct resonances that correspond to the
aromatic CH groups of the unsymmetrically substituted
naphthalene backbone. In addition, four aryl protons and six
distinct methyl groups are observed for the mesityl substit-
uents, indicating the existence of a congested structure. The
11B NMR spectrum of 2 shows two resonances at 51.6 and
72.4 ppm, confirming the presence of two different boron
centers. This derivative crystallizes in the P21/n space group
with four molecules per unit cell. The structure of 2 is similar
to those of previous diboranes such as 1-(dimesitylboryl)-
8-(diphenylboryl)naphthalene (Figure 1 and Table 1).19 As
for other sterically hindered peri-substituted naphthalene
derivatives, the B1-C1-C9 [128.43(19)°], B2-C8-C9
[129.91(19)°], and C1-C9-C8 [124.23(19)°] angles sub-
stantially deviate from the ideal value of 120°. As a result
of this steric crowding, the two boron centers are separated
by 3.279(4) Å. This separation is much larger than the B-B
distance of 3.002(2) Å observed in 1,8-bis(diphenylboryl)-
naphthalene, which possesses less sterically demanding aryl
substituents.30 Diborane 2 features a short distance of 2.965-
(3) Å between the ipso carbon (C21) atom of one of the
mesityl groups and the boron of the 9-oxa-10-boraanthryl
unit (B2). This short distance, which is 0.23 Å longer than
that observed in the structure of the more Lewis acidic
1-(dimesitylboryl)-8-(borafluorenyl)naphthalene,31 may in-
dicate the presence of a weak bonding interaction. The slight
Figure 2. Left: DFT orbital picture showing the LUMO of 2 (isovalue
) 0.35). Hydrogen atoms omitted for clarity. Right: overlay representation
of theoretical (black) and X-ray (gray) structures for compound 2.
pyramidalization of the boron center B2 (ΣC-B2-C ) 357.9°)
as well as the nonlinear B1-C21-C24 angle (170.7°) is in
agreement with this view.
Calculated Structure and Electrochemistry of the
Diboranes. The structure of 2 was computationally optimized
using DFT methods [B3LYP and 6-31+g(d′) for the boron
and oxygen atoms and 6-31g for all other atoms]. The
optimized geometry of diborane 2 is close to that determined
experimentally (Figure 2), although the calculated distance
of 3.413 Å separating the boron centers is slightly larger
than that observed in the crystal structure [3.279(4) Å].
Examination of the orbitals reveals that the empty boron p
orbitals largely contribute to the lowest unoccupied molecular
orbital (LUMO; Figure 2) and are oriented toward one
another in a transannular fashion.
(30) Hoefelmeyer, J. D.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2000, 122,
9054-9055.
The cyclic voltammograms of diboranes 1 and 2 are very
similar to one another and feature two reversible reduction
(31) Hoefelmeyer, J. D.; Sole´, S.; Gabba¨ı, F. P. Dalton Trans. 2004,
1254-1258.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8139

Mela1ımi et al.
Figure 3. Cyclic voltammograms of 1 (E1/2 ) -2.200 and 2.566 V) and
2 (E1/2 ) -2.248 and -2.620 V) in THF with a glassy-carbon working
Figure 4. Changes in the differential pulsed voltammogram of 2 observed
upon the addition of nBu4F to a THF solution (0.02 M). Similar observations
are made for 1.
electrode (0.3 M nBu4PF6). Scan rates: ν ) 50 mV s-1
.
waves (Figure 3). The first reduction at E1/2 ) -2.200 V
for 1 and E1/2 ) -2.248 V for 2 most likely corresponds to
the formation of a radical anion in which the unpaired
electron occupies a molecular orbital bearing large contri-
butions from the boron p orbitals.30,31 The potential for the
first reduction of 1 and 2 is markedly more positive than
that of other triarylboranes32-40 such as dimesityl-1-naphth-
ylborane, which is reduced at E1/2 ) -2.41 V (vs FcH/
FcH+).31 The direct σ delocalization of the unpaired electron
over the two boron centers can be used to rationalize the
observation that 1 and 2 are more easily reduced than
dimesityl-1-naphthylborane.31 The redox behavior of 1 and
2 is reminiscent of that reported for 9,10-dihydro-9,10-
dimethyl-9,10-diboraanthracene, whose reduction is facili-
tated by its antiaromatic character.41 By analogy with some
of our previous investigations,31 the second reduction wave
at E1/2 ) -2.566 V for 1 and E1/2 ) -2.620 V for 2
corresponds to the formation of a dianion, which exists either
as a boron-boron σ-bonded derivative or as a diradical. A
comparison of the reduction potentials of these two diboranes
indicates that 1 is more easily reduced than 2. Presumably,
this phenomenon indicates that the 9-oxa-10-boraanthryl
moiety of 2 is more aromatic and, therefore, less electrophilic
than the 9-thia-10-boraanthryl moiety of 1. This effect
possibly originates from the lesser amount of mixing of the
sulfur pπ orbital with the π system of the boraanthryl moiety
in 1.
boron centers. These properties, and more specifically the
reduction that these diboranes undergo, should be greatly
affected by the binding of nucleophiles to the boron centers.
For these reasons, we decided to determine whether cyclic
voltammetry and differential pulsed voltammetry could serve
to probe anion complexation by these diboranes. To this end,
we monitored the cyclic and differential pulsed voltammo-
gram of 1 and 2 upon incremental addition of fluoride,
chloride, bromide, iodide, acetate, nitrate, and nitrite. The
addition of chloride, bromide, iodide, acetate, nitrate, and
nitrite to a THF solution of 1 or 2 containing nBu4PF6 as a
supporting electrolyte does not result in any changes in the
cyclic voltammogram of 1 and 2, indicating that these anions
do not bind to the diboranes. In contrast, incremental addition
of fluoride results in the progressive disappearance of the
two reduction waves, which are no longer detected after the
addition of 1 equiv of fluoride (Figure 4). These two waves
disappear concomitantly and not one after another. The
synchronous disappearance of these two waves indicates that
fluoride simultaneously binds to the two boron centers of 1
and 2. This observation further supports the formation of
chelate complexes in solution. For all anions, and especially
nitrite (pKa ) 3.29) and acetate (pKa ) 4.76), which are more
basic than fluoride (pKa ) 3.17), the narrow size of the
binding pocket of 1 and 2 is most likely responsible for the
observed selectivity. These investigations show that 1 and 2
constitute novel boron-based electrochemical fluoride sen-
sors.42
Anion Binding. The electrochemical properties of these
diboranes directly reflect the coordinative unsaturation of the
To collect additional evidences for the binding of fluoride
to the diboranes, we have also studied the binding of anions
by UV-vis spectroscopy in THF. As previously reported
for 1, the addition of fluoride to a THF solution of 2 results
in quantitative quenching of the absorption of the diborane
at 360 nm (ꢀ ) 16 000 mol-1 cm-1), indicating the formation
(32) Krause, E.; Polack, H. Ber. 1926, 59, 777-785.
(33) Chu, T. L.; Weissman, T. J. J. Am. Chem. Soc. 1956, 78, 23-26.
(34) Leffler, J. E.; Watts, G. B.; Tanigaki, T.; Dolan, E.; Miller, D. S. J.
Am. Chem. Soc. 1970, 92, 6825-6830.
(35) Eisch, J. J.; Dluzniewski, T.; Behrooz, M. Heteroatom Chem. 1993,
4, 235-241.
(36) Elschenbroich, C.; Kuhlkamp, P.; Behrendt, A.; Harms, K. Chem. Ber.
1996, 129, 859-869.
1
of [2-µ2-F]-. The same conclusion is reached by H NMR
(37) Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1986, 108, 4235-
measurements, which show the complete disappearance of
the resonance of the free borane and the appearance of broad
resonances that can be assigned to the anionic chelate
complex. Because of the magnitude of the binding, we have
not been able to determine the absolute fluoride binding
constant of 2. Attempts to accurately determine the relative
4236.
(38) Brown, H. C.; Dodson, V. H. J. Am. Chem. Soc. 1957, 79, 2302-
2306.
(39) Weissman, S. I.; van Willigen, H. J. Am. Chem. Soc. 1965, 87, 2285-
2286.
(40) For related studies dealing with the reduction of boranes, see: Kwaan,
R. J.; Harlan, C. J.; Norton, J. R. Organometallics 2001, 20, 3818-
3820. Venkatasubbaiah, K.; Zakharov, L. N.; Kassel, W. S.; Rheingold,
A. L.; Ja¨kle, F. Angew. Chem., Int. Ed. 2005, 44, 5428-5433.
(41) Mueller, P.; Huck, S.; Ko¨ppel, H.; Pritzkow, H.; Siebert, W. Z.
Naturforsch., B 1995, 50b, 1476-1484.
(42) Bresner, C.; Aldridge, S.; Fallis, I. A.; Jones, C.; Ooi, L.-L. Angew.
Chem., Int. Ed. 2005, 44, 3606-3609.
8140 Inorganic Chemistry, Vol. 45, No. 20, 2006

Sterically Hindered 1,8-Bis(boryl)naphthalenes
fragment in 2. This conclusion is in agreement with the
observation that the reduction potential of 2 is more negative
than that of 1. As mentioned earlier, this effect most likely
originates from the increased aromaticity of the 9-chalcogena-
10-boraanthryl moiety in 2 when compared to 1. As expected
from the bridging location of the fluorine atom, B1-F and
B2-F bond lengths are significantly longer than that
observed in the tris(anthryl)fluoroborate anion (1.47 Å).17c
The B1-F-B2 angle of 126.6(3)° deviates from the ideal
value of 109°, which would be predicted for a sp3-hybridized
fluorine atom. While this distortion may be imposed by the
rigid structure, it is important to note that a fluoride anion
bridging two triarylboron centers may actually prefer to adopt
a linear geometry. Such geometries have been observed in
heavier group 13 derivatives such as [(C6F5)3GaFGa(C6F5)3]-.44
As observed for [1-µ2-F]-, the cooperative binding of the
fluoride anion leads to pyramidalization of both boron centers
Figure 5. ORTEP view of [2-µ2-F][S(NMe2)3] (30% ellipsoid; hydrogen
atoms and the S(NMe2)3 countercation are omitted for clarity). Selected
distances [Å] and angles [deg]: B1-F1 1.634(5), B2-F1 1.636(6), B1-
C1 1.624(7), B1-C11 1.656 (6), B1-C21 1.646(6), B1-B2 2.922(7), B2-
C8 1.617(8), B2-C31 1.597(7), B2-C41 1.615(7); B1-F1-B2 126.6(3),
C1-B1-F1 102.0(3), C8-B2-F1 105.9(4), C1-B1-C11 115.8(4), C1-
B1-C21 113.7(4), C11-B1-C21 115.9(3), C8-B2-C31 119.1(4), C8-
B1-C41 113.0(4), C31-B2-C41 109.2(4), C9-C1-B1 122.5(4), C9-
C8-B2 124.7(4), C1-C9-C8 120.9(4).
as indicated by the sum of the C-B-C angles (∑C-B1-C
)
345.4°; ∑C-B2-C ) 341.3°). Moreover, the C9-C1-B1
[122.5(4)°], C9-C8-B2 [124.7(4)°], and C1-C9-C8 [120.9-
(4)°] angles in [2-µ2-F]- are closer to the ideal value of 120°
when compared to those in 2 [C9-C1-B1 ) 128.43(19)°,
C9-C8-B2 ) 129.91(19)°, and C1-C9-C8 ) 124.23-
(19)°]. Accordingly, the distance separating the two boron
centers B1 and B2 is reduced from 3.279(4) Å in 2 to 2.922-
(7) Å in [2-µ2-F]-. These structural data suggest that the
important steric congestion present in 2 largely disappears
in [2-µ2-F]-, where the arylboron substituents adopt a more
divergent orientation.
Theoretical Investigation of the Fluoride Complex of
2. The DFT-optimized structure of [2-µ2-F]- [B3LYP and
6-31+g(d′) for the boron, fluorine, and oxygen atoms and
6-31 g for all other atoms] is close to that determined
experimentally (Figure 6). The calculated B1-F (1.64 Å)
and B2-F (1.67 Å) bonds are within a few hundreds of an
angstro¨m from those observed in the crystal [1.634(5) and
1.633(5) Å, respectively]. A very good match is also
observed in the B1-F-B2 angle [exptl 126.6(3)°; calcd
126.8°] as well as in the sum of the C-B-C angles
(∑C-B1-C: exptl 345.4°; calcd 345.4°. ∑C-B2-C: exptl 341.3°;
calcd 341.8°). To compare the optimized structure of [2-µ2-
F]- to that of related systems, we optimized the geometry
of the simple fluoroborates [I-F]- and [II-F]-, whose
structures are depicted in Chart 1. For [I-F]- and [II-F]-,
the B-F bond distances of 1.49 and 1.48 Å are close to that
observed experimentally in the tris(anthryl)fluoroborate anion
(1.47 Å).17c They are also substantially shorter than those
computed for [2-µ2-F]- (average 1.65 Å). Qualitatively, these
differences in bond lengths show that the bridging fluorine
atom in [2-µ2-F]- forms weaker B-F bonds than the terminal
fluorine atom in [I-F]- and [II-F]-. Similar conclusions can
be derived from an AIM analysis,26 which indicates that the
values of the densities at the B-F bond critical points in
[2-µ2-F]- [F(r) ) 0.080 e bohr-3 for B1-F and F(r) ) 0.074
e bohr-3 for B2-F] are significantly smaller than those found
at the B-F bond critical point of [I-F]- [F(r) ) 0.126 e
fluoride binding constants of 1 and 2 have been complicated
by the slow kinetics of fluoride exchange between the two
receptors.
Isolation and Structural Characterization of the Fluo-
ride Complex of 2. Reaction of 2 with TASF leads to the
formation of [2-µ2-F][S(NMe2)3], which has been isolated
1
as a colorless crystalline material. The H NMR spectrum
of [2-µ2-F]- differs from that of 2 but is still characteristic
of an unsymmetrically substituted naphthalene species. The
11B NMR resonances of [2-µ2-F]- at 11.7 and 17.6 ppm are
in the range expected for tetrahedral boron centers, which is
consistent with the coordination of the fluoride anion. The
19F NMR resonance of the bridging fluorine atom appears
at -178.6 ppm, which is comparable to the chemical shift
observed in other fluoride-bridged species6 including [1-µ2-
F]-.16
The [2-µ2-F][S(NMe2)3] salt has been studied by single-
crystal X-ray diffraction. Examination of the structure of the
anion [2-µ2-F]- (Figure 5) confirms the presence of a fluorine
atom, which bridges the boron centers via B-F bonds of
comparable lengths (average 1.635 Å). These bonds are
distinctly longer that those formed by the bridging fluorine
in [C6F4-1,2-[(BF2)2(µ2-F)]]- [average 1.487(4) Å].43 The
accessibility and high Lewis acidity of the difluoroboryl
moieties in [C6F4-1,2-[(BF2)2]]- are probably at the origin
of this difference. Nonetheless, a close match is observed
between the structure [2-µ2-F]- and that of [1-µ2-F]-.16 The
B1-F bond of 1.634(5) Å involving the boron center of the
dimesityl boryl in [2-µ2-F]- is almost identical with that
measured in [1-µ2-F]- [1.633(5) Å]. The B2-F bond, which
involves the boron atom of the 9-chalcogena-10-boraanthryl
fragment, is noticeably longer in [2-µ2-F]- [1.636(7) Å] than
in [1-µ2-F]- [1.585(5) Å]. This difference indicates that the
boron atom of the 9-thia-10-boraanthryl fragment in 1 is a
greater Lewis acid than that of the 9-oxa-10-boraanthryl
(43) Chase, P. A.; Henderson, L. D.; Piers, W. E.; Parvez, M.; Clegg, W.;
(44) Chen, M.-C.; Roberts, J. A. S.; Marks, T. J. Organometallics 2004,
23, 932-935.
Elsegood, M. R. J. Organometallics 2006, 25, 349-357.
Inorganic Chemistry, Vol. 45, No. 20, 2006 8141

Mela1ımi et al.
methods for such calculations have been previously pub-
lished,45,46 we chose to calculate the FIA of the boranes
directly from the energy of the free Lewis acid, F-, and the
fluoroborate. To validate our computational approach, we
first calculated the FIA of CF2O. The structures of CF3O-
and CF2O were first optimized using the B3LYP functional
and 6-31g(d′)/6-31g mixed basis set. The optimized mol-
ecules as well as F- were subsequently subjected to a single-
point energy calculation using the B3LYP functional and a
6-311+g(2d,p) basis set for all atoms. The relative enthalpies
derived from these single-point energy calculations afforded
48.8 kcal mol-1 for the FIA of CF2O. This value is very
close to that experimentally determined (49.9 kcal mol-1),45
thus pointing to the adequacy of our computational methods.
Application of this method to the boranes afforded the
following FIAs: 60.74 kcal mol-1 for I, 64.18 kcal mol-1
for II, 66.07 kcal mol-1 for III, and 73.28 kcal mol-1 for 2.
These values indicate that the enthalpic gain associated with
the formation of two B-F bonds in [III-F]- and [2-µ2-F]-
is relatively small and only amounts to a fraction of the
energy of the terminal B-F bond. In other words, the fluoride
ion in [III-F]- and [2-µ2-F]- forms two relatively weak B-F
bonds whose cumulated energies only slightly exceed that
of the terminal B-F bonds of [I-F]- and [II-F]-. Although
not as marked as one may have expected, the difference
observed in the FIAs of mono- and bifunctional boranes may
be responsible for the high fluoride binding displayed by
bifunctional boranes such as 2 in solution. In an attempt to
understand why 2 has a higher calculated FIA than III, we
compared the structural changes accompanying fluoride
complexation by III and 2 (Figure 6). To this end, we
examined the C-C-B angles R and â as defined in Figure
6. For III, fluoride binding induces changes of the R (124.9°
f 115.1°) and â (126.8° f 116.3°) angles around their ideal
values of 120°. In the case of 2, however, the geometry of
the free borane is more sterically distorted, as indicated by
the elevated values of the R (129.7°) and â (129.3°) angles
and the large B-B separation of 3.41 Å. Fluoride binding
leads to a decrease of these angles (R ) 123.8°; â ) 125.5°),
which become closer to their ideal values of 120°, indicating
a relief of the steric repulsion. Thus, we propose that the
important steric congestion present in 2 may, in fact,
destabilize the free form of the borane, thus favoring the
formation of the less sterically encumbered fluoride adduct.
In other words, the high steric compression present between
the boryl units of 2 serves to “clamp” the fluoride anion more
effectively. This last view is certainly in agreement with the
observation of shorter B-F bonds in [2-µ2-F]- than in [III-
F]-. Related arguments have been used to explain the unusual
basicity of naphthalene-based proton sponges, where the lone
pair-lone pair repulsion occurring between the nitrogen
atoms serves to destabilize the free base and facilitates its
protonation.47
Figure 6. Calculated structures of 2, III, [2-µ2-F]-, and [III-F]-.
Chart 1. Borane and Fluoroborate Complexes Studied Theoretically
bohr-3] and [II-F]- [F(r) ) 0.130 e bohr-3]. We also
investigated computationally the possible existence of [I-F-
II]- as a stable gas-phase species. Independently of the
starting structures, full geometry optimization always led to
complete dissociation of one of the boranes. These results
indicate that the stabilization associated with the formation
of a B-F-B bridge, if any, is not sufficient to overcome
the steric repulsions resulting from the proximity of I and
II. To compare the structure of [2-µ2-F]- to another model
featuring a B-F-B bridge, we computed the structure of
[III-F]-, in which dimesitylboryl and 9-oxa-10-boraanthryl
moieties are linked by an o-phenylene backbone (Figure 6).
The optimized geometry corresponds to a complex featuring
a bridging fluorine atom with B-F bonds of 1.63 and 1.76
Å. The B-F bond of 1.76 Å is noticeably longer than those
computed for [2-µ2-F]-. The AIM analysis of [III-F]-
indicates that the densities at the B-F bond critical points
[F(r) ) 0.084 e bohr-3 for B1-F and F(r) ) 0.063 e bohr-3
for B2-F] are similar to those calculated in [2-µ2-F]-.
(45) Christe, K. O.; Dixon, D. A.; McLemore, D.; Wilson, W. W.; Sheehy,
J.; Bootz, J. A. J. Fluorine Chem. 2000, 101, 151-153.
(46) Krossing, I.; Raabe, I. Chem.sEur. J. 2004, 10, 5017-5030.
(47) Peraekylae, M. J. Org. Chem. 1996, 61, 7420-7425.
To better assess the strength of the interactions involving
the bridging fluoride in [2-µ2-F]- and [III-F]-, we have
computed the gas-phase FIAs of I-III and 2. While different
8142 Inorganic Chemistry, Vol. 45, No. 20, 2006

Sterically Hindered 1,8-Bis(boryl)naphthalenes
Concluding Remarks
rationalize the high fluoride affinity of the diboranes
described in these studies, we have performed a series of
computations on 2 and several model compounds. These
calculations indicate that the enthalpic gain associated with
the formation of two B-F bonds in fluoride adducts of
bidentate Lewis acids is moderate. These calculations also
suggest that the relief of steric repulsions induced by fluoride
binding in 2 may contribute to the high fluoride affinity of
these types of molecules. Ongoing studies are focused on
the experimental determination of the enthalpy and entropy
changes accompanying fluoride binding in solution.
The synthetic results presented in this paper further
demonstrate the suitability of the Li(THF)4 salt of dimesityl-
1,8-naphthalenediylborate as a starting material for the
synthesis of asymmetrically substituted 1,8-diborylnaphtha-
lenes. A key aspect of this synthetic method is that it allows
for the synthesis of highly sterically hindered derivatives such
as 1 and 2. The steric hindrance present in these diboranes
is directly reflected by their NMR spectra as well as by a
X-ray single-crystal structure that shows, in the case of 2,
noticeable distortions of the diborane backbone. These
diboranes are remarkably selective fluoride anion receptors
and fail to interact with bulkier and sometimes more basic
anions. The origin of this selectivity possibly results from
the narrow size of the binding pocket, which can only
accommodate small anions. The formation of the chelate
fluoride complexes [1-µ2-F]- and [2-µ2-F]- can be readily
observed by NMR and UV spectroscopy as well as by cyclic
voltammetry and differential pulsed voltammetry. The latter
techniques, which show the synchronous disappearance of
the reduction waves of the two electroactive boryl moieties
upon fluoride addition, confirm the simultaneous coordina-
tion of the fluoride anion to both boron centers. Finally, to
Acknowledgment. This work was supported by the
Welch Foundation (Grant A-1423) and the National Science
Foundation (Grant CHE 00-94264, Career Award to F.P.G.,
and Grant CHE 00-77917, for the purchase of NMR
instrumentation). We thank Lisa Perez for her help with the
calculations.
Supporting Information Available: Crystallographic data in
CIF format for 2 and [2-µ2-F][S(NMe2)3] and computational details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC060709S
Inorganic Chemistry, Vol. 45, No. 20, 2006 8143