Synthesis and properties of a cationic bidentate lewis acid

Article abstract of DOI:10.1021/ic700360a

As part of our efforts to increase the fluoride affinity of bidentate Lewis acids, we have set out to determine if the F^- anion chelation occurring in such systems can be complemented by favorable Coulombic attractions. To explore this idea, th

Full text of DOI:10.1021/ic700360a

Inorg. Chem. 2007, 46, 8132−8138
Synthesis and Properties of a Cationic Bidentate Lewis Acid
Min Hyung Lee and Franc¸ois P. Gabba1ı *
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
Received February 26, 2007
As part of our efforts to increase the fluoride affinity of bidentate Lewis acids, we have set out to determine if the
F- anion chelation occurring in such systems can be complemented by favorable Coulombic attractions. To explore
this idea, the neutral B/Hg bidentate Lewis acid 1-
analogue [1- Mes2B -8- (2,6-Me2-4-Me3NC6H2)Hg
3 as well as the triflate salt of [4]+ react with [S(NMe2)3][Me3SiF2] to afford the corresponding fluoride complexes
[3-
2-F]- and [4- 2-F]. Spectroscopic and structural studies confirm that the F- anion bridges the two Lewis acidic
centers in both [3-
{
}
Mes2B
}-8-{(2,6-Me2-4-Me2NC6H2)Hg}C10H6 (3) and its cationic
{
}
{
C10H6]+ ([4]+) have been synthesized and studied. Compound
µ
µ
µ
2-F]- and [4-
µ2-F]. UV−vis titration experiments carried out in tetrahydrofuran/water (9/1, v/v)
mixtures indicate that the fluoride binding constants of 3 and [4]+ are clearly differentiated and are equal to 1.3
0.1)
1
(
±
×
102 M- and 6.2 (
±
0.2)
×
104 M-1, respectively. The enhanced fluoride binding constant of [4]+, when
compared to 3, confirms that the chelate effect occurring in these types of fluoride receptors can be combined with
favorable Coulombic attractions to strengthen the host
−
guest interaction. Cation [4]+ remains highly selective for
-
F- over other environmentally abundant anions including Cl-, Br-, NO3-, H2PO4-, and HSO4 and shows only a
weak response to OAc-. Finally, the addition of an aqueous solution of Al3 to a solution containing [4-
to complete regeneration of [4]+, showing that F- binding is reversible.
µ2-F] leads
+
Introduction
neutral boranes are receiving increasing attention.5 Neutral
triarylboranes, for example, complex F- anions to form the
corresponding triarylfluoroborate anions.6 While this property
has long been overlooked in the context of anion sensing,
Yamaguchi et al. showed that such boranes can serve as
fluoride sensors with binding constants ranging from 105 to
106 M-1 in organic solvents.7,8 Since these seminal contribu-
tions, a number of other groups have reported related
The ion recognition of fluoride in aqueous media is
attracting a great deal of interest because of the importance
of this anion in the areas of health and defense. A plethora
of organic hosts that interact with the anionic guests via
hydrogen bonds have been reported.1 Unfortunately, these
receptors are typically incompetent in aqueous environments
where water interferes with the host-guest interactions.2
Because of these complications, fluorophilic Lewis acidic
derivatives3,4 including boronic acids, boronic esters, and
(5) Bresner, C.; Day, J. K.; Coombs, N. D.; Fallis, I. A.; Aldridge, S.;
Coles, S. J.; Hursthouse, M. B. Dalton Trans. 2006, 3660. Bresner,
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Commun. 2005, 945. Shiratori, H.; Ohno, T.; Nozaki, K.; Osuka, A.
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* To whom correspondence should be addressed. E-mail:
francois@tamu.edu. Tel: +1-979-862-2070. Fax: +1-979-845-4719.
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K.; Hamilton, A. D. Coord. Chem. ReV. 2003, 240, 101. Bondy, C.
R.; Loeb, S. J. Coord. Chem. ReV. 2003, 240, 77. Gale, P. A. Coord.
Chem. ReV. 2003, 240, 191. Sessler, J. L.; Camiolo, S.; Gale, P. A.
Coord. Chem. ReV. 2003, 240, 17. Schmidtchen, F. P.; Berger, M.
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8132 Inorganic Chemistry, Vol. 46, No. 20, 2007
10.1021/ic700360a CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/02/2007

Cationic Bidentate Lewis Acid
results.9,10 However, despite the elevated binding constants
and advantageous photophysical properties, triarylboranes are
unable to overcome the large hydration enthalpy of the F-
anion and cannot be used in water.8
be exercised when synthesizing and handling compounds. Tetrakis-
(tetrahydrofuran)lithium dimesityl-1,8-naphthalenediylborate (1)
was synthesized by the published procedures.16,17 Methyl triflate
(MeOTf) and [S(NMe2)3][Me3SiF2] (TASF) were purchased from
Aldrich, Hg(OAc)2 was purchased from Fisher Scientific, 3,5,N,N-
tetramethylaniline and n-Bu4NF‚3H2O (TBAF) from Alfa Aesar
were purchased, and all were used as received. Solvents were dried
by passing them through an alumina column (n-hexane and
CH2Cl2) or by reflux under N2 over Na/K (Et2O and THF). Air-
sensitive compounds were handled under a N2 atmosphere using
standard Schlenk and glovebox techniques. UV-vis spectra were
recorded on a HP8453 spectrophotometer. Elemental analyses were
performed at Atlantic Microlab (Norcross, GA). NMR spectra were
recorded on Varian Inova 400 FT NMR spectrometer (399.59 MHz
for 1H, 375.99 MHz for 19F, 128.02 MHz for 11B, 100.48 MHz for
13C, and 71.57 MHz for 199Hg) at ambient temperature. Chemical
shifts are given in ppm and are referenced against external Me4Si
(1H, 13C), BF3‚Et2O (11B), CFCl3 (19F), and HgMe2 (199Hg).
Crystallography. The crystallographic measurement was per-
formed using a Bruker Apex II-CCD area detector diffractometer,
with a graphite-monochromated Mo KR radiation (λ ) 0.710
69 Å). A specimen of suitable size and quality was selected and
mounted onto a glass fiber with apiezon grease. The structure was
solved by direct methods, which successfully located most of the
non-H atoms. Subsequent refinement on F 2 using the SHELXTL/
PC package (version 5.1) allowed the location of the remaining
non-H atoms. In the case of [4-µ2-F]MeCN, the largest residual
peak of 2.487 e Å-3 is located at 0.935 Å from the Hg2 atom and
most likely results from absorption.
In order to increase the binding constants of borane-based
receptors, several groups, including ours, have explored the
use of bidentate boranes that chelate F-.9,11-13 Our efforts
in this area have focused on the design of naphthalene-based
derivatives such as I.9 Although the fluoride binding
constants of such receptors exceed those of monofunctional
analogues by 3 or 4 orders of magnitude, these compounds
cannot be used in water because they slowly hydrolyze.9 Such
limitations do not seem to affect the heteronuclear B/Hg
bidentate Lewis acid II, which captures F- in tetrahydrofuran
(THF)/water mixtures without decomposition.13 In addition
to investigating derivatives whose fluoride affinity is in-
creased by the chelating properties of the receptor, several
groups, including ours, are studying the use of cationic
boranes.14,15 The initial results obtained in this area indicate
that the fluoride affinity of such receptors is greatly increased
by Coulombic effects. For example, the phosphonium borane
[III]+, unlike any of its neutral analogues, captures F- in
aqueous solution.15 In principle, a further increase in the
fluoride affinity of B-based receptors could be obtained if
the chelate effect occurring in a bidentate Lewis acid was
complemented by favorable Coulombic attractions. In an
effort to validate this hypothesis, we have synthesized a
cationic B/Hg bidentate Lewis acid and compared its fluoride
affinity to that of its neutral precursor.
Synthesis of (2,6-Me2-4-Me2NC6H2)HgCl (2). 3,5,N,N-Tetra-
methylaniline (10 mmol, 1.49 g) was added to a slurry of Hg(OAc)2
(10 mmol, 3.19 g) in EtOH (15 mL) at room temperature. After it
was stirred for 6 h, the mixture was filtered and washed with EtOH.
The resulting white solid was dissolved in warm acetone (20 mL),
and EtOH (10 mL) was added to the solution. Next, the solution
was treated with a saturated aqueous solution of LiCl (20 mL),
which resulted in the formation of a white precipitate. After 30
min of stirring, the precipitate was isolated by filtration and
successively washed with water and n-hexane. Drying in vacuo at
1
50 °C overnight afforded 2 as a white powder (3.11 g, 81%). H
Experimental Section
NMR (CDCl3): δ 2.41 (s, 6H, Ar-CH3), 2.94 (s, 6H, NCH3), 6.45
(s, 2H, Ar-H). 13C NMR (CDCl3): δ 26.71 (Ar-CH3), 40.26
(NCH3), 110.89 (Ar-C3), 140.45 (Ar-C1), 142.63 (Ar-C2),
151.31 (Ar-C4). 199Hg NMR (CDCl3): δ -984.3. Anal. Calcd
for C10H14ClHgN: C, 31.26; H, 3.67. Found: C, 30.98; H, 3.55.
General Considerations. Caution! Because of the toxicity of
the mercury compounds discussed in these studies, great care should
(7) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
122, 6335. Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J.
Am. Chem. Soc. 2002, 124, 8816. Kubo, Y.; Yamamoto, M.; Ikeda,
M.; Takeuchi, M.; Shinkai, S.; Yamaguchi, S.; Tamao, K. Angew.
Chem., Int. Ed. 2003, 42, 2036.
(8) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001,
123, 11372.
(9) Sole´, S.; Gabba¨ı, F. P. Chem. Commun. 2004, 1284.
(10) Parab, K.; Venkatasubbaiah, K.; Ja¨kle, F. J. Am. Chem. Soc. 2006,
128, 12879. Miyata, M.; Chujo, Y. Polym. J. (Tokyo, Jpn.) 2002, 34,
967. Liu, Z.-Q.; Shi, M.; Li, F.-Y.; Fang, Q.; Chen, Z.-H.; Yi, T.;
Huang, C.-H. Org. Lett. 2005, 7, 5481. Liu, X. Y.; Bai, D. R.; Wang,
S. Angew. Chem., Int. Ed. 2006, 45, 5475. Agou, T.; Kobayashi, J.;
Kawashima, T. Org. Lett. 2005, 7, 4373. Sakuda, E.; Funahashi, A.;
Kitamura, N. Inorg. Chem. 2006, 45, 10670.
(11) Katz, H. E. J. Org. Chem. 1985, 50, 5027. Chase, P. A.; Henderson,
L. D.; Piers, W. E.; Parvez, M.; Clegg, W.; Elsegood, M. R. J.
Organometallics 2006, 25, 349. Mela¨ımi, M.; Gabba¨ı, F. P. AdV.
Organomet. Chem. 2005, 53, 61. Katz, H. E. J. Am. Chem. Soc. 1985,
107, 1420. Katz, H. E. J. Am. Chem. Soc. 1986, 108, 7640. Katz, H.
E. Inclusion Compd. 1991, 4, 391. Gabba¨ı, F. P. Angew. Chem., Int.
Ed. 2003, 42, 2218. Piers, W. E.; Irvine, G. J.; Williams, V. C. Eur.
J. Inorg. Chem. 2000, 2131.
Synthesis of 1-{Mes2B}-8-{(2,6-Me2-4-Me2NC6H2)Hg}C10H6
(3). To the solid mixture of 1 (500 mg, 0.745 mmol) and 2 (286
mg, 0.745 mmol) was added cold (-20 °C) THF (10 mL). After it
was stirred overnight at room temperature, the solvent was removed
under reduced pressure and the residue was extracted with n-hexane
(30 mL). Following filtration and evaporation of the solvent, the
light-yellow solid was washed with acetonitrile (3 × 4 mL) and
1
dried under vacuum to yield an ivory powder (378 mg, 70%). H
NMR (CDCl3): δ 1.29 (s, 3H, Mes-CH3), 1.70 (s, 3H, Mes-
CH3), 1.74 (s, 3H, Mes-CH3), 2.10 (s, 6H, Ar-CH3), 2.19 (s, 3H,
Mes-CH3), 2.29 (s, 3H, Mes-CH3), 2.37 (s, 3H, Mes-CH3), 2.91
(13) Mela¨ımi, M.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2005, 127, 9680.
(14) Chiu, C.-W.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2006, 128, 14248. Agou,
T.; Kobayashi, J.; Kawashima, T. Inorg. Chem. 2006, 45, 9137.
(15) Lee, M. H.; Agou, T.; Kobayashi, J.; Kawashima, T.; Gabba¨ı, F. P.
Chem. Commun. 2007, 1133.
(12) Williams, V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.; Collins,
S.; Marder, T. B. J. Am. Chem. Soc. 1999, 121, 3244.
(16) Hoefelmeyer, J. D.; Gabba¨ı, F. P. Organometallics 2002, 21, 982.
(17) Schulte, M.; Gabba¨ı, F. P. Chem.sEur. J. 2002, 8, 3802.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8133

Lee and Gabba1ı
(s, 6H, NCH3), 6.42 (br s, 1H, Mes-CH), 6.46 (s, 2H, Ar-H),
6.61 (br s, 1H, Mes-CH), 6.74 (br s, 1H, Mes-CH), 6.81 (br s,
1H, Mes-CH), 7.35-7.38 (m, 2H, Naph-CH), 7.43 (dd, 1H, 3JH-H
) 7.2 Hz, 4JH-H ) 1.6 Hz, Naph-CH), 7.55 (dd, 1H, 3JH-H ) 8.0
125.16, 125.65, 127.63, 128.19, 129.47, 129.75, 129.86, 133.80,
133.83, 134.37, 137.57, 137.93, 138.61, 139.04, 139.91, 141.51,
142.03, 143.08, 144.31, 145.45, 146.34, 146.68, 152.13, 171.96,
173.46. 199Hg NMR (DMSO-d6): δ -693.9. UV-vis (THF):
4
3
Hz, JH-H ) 6.8 Hz, Naph-CH), 7.78 (dd, 1H, JH-H ) 8.2 Hz,
4JH-H ) 1.6 Hz, Naph-CH), 7.95 (dd, 1H, 3JH-H ) 8.0 Hz, 4JH-H
) 1.6 Hz, Naph-CH). 13C NMR (CDCl3): δ 21.25 (Ar-CH3),
22.36, 23.51, 24.68, 25.26, 25.55 (Mes-CH3), 57.62 (NCH3),
111.67, 125.21, 125.35, 127.61, 128.14, 129.18, 129.56, 130.07,
133.52, 134.28, 134.72, 136.47, 138.21, 139.55, 139.97, 140.51,
141.30, 142.40, 143.76, 144.66, 145.43, 146.68, 150.33, 152.91,
157.35, 176.51. 11B NMR (CDCl3): δ +72.1. 199Hg NMR
(CDCl3): δ -584.6. UV-vis (THF): λmax/nm (log ꢀ) 363 (4.04).
Anal. Calcd for C38H42BHgN: C, 63.03; H, 5.85. Found: C, 62.91;
H, 5.97.
λmax/nm (log ꢀ) 362 (4.01). Anal. Calcd for C40H45BF3HgNO3S:
C, 54.09; H, 5.11. Found: C, 54.27; H, 5.16.
Synthesis of 1-{Mes2B}-(µ2-F)-8-{(2,6-Me2-4-Me3NC6H2)Hg}-
C10H6 ([4-µ2-F]). TASF (21 mg, 0.076 mmol) was added to a slurry
of [4][OTf] (60 mg, 0.068 mmol) in CH2Cl2 (2 mL) at room
temperature. Shortly after the addition was complete, the reaction
mixture turned into a homogeneous solution. Further stirring for
2 h led to the formation of a precipitate, which was isolated by
filtration and washed with cold CH2Cl2 (2 × 1 mL). Drying in
vacuo afforded [4-µ2-F] as a white powder (32 mg, 62%). Single
crystals of [4-µ2-F] were obtained by slow evaporation of an
acetonitrile solution. 1H NMR (acetone-d6): δ 1.63 (br s, 9H, Mes-
CH3), 2.06 (br s, 9H, Mes-CH3), 2.28 (s, 6H, Ar-CH3), 3.79 (s,
9H, NCH3), 5.89 (br s, 1H, Mes-CH), 6.36 (br s, 2H, Mes-CH),
Synthesis of [S(NMe2)3][1-{Mes2B}-(µ2-F)-8-{(2,6-Me2-4-
Me2NC6H2)Hg}C10H6] ([S(NMe2)3][3-µ2-F]). To a solid mixture
of 3 (20 mg, 0.028 mmol) and TASF (7.7 mg, 0.028 mmol) was
added CH2Cl2 (2 mL) at room temperature. The reaction mixture
was stirred for 30 min, layered with n-hexane, and stored at -20
°C to induce the crystallization of [S(NMe2)3][3-µ2-F]. After
filtration and washing with ether (2 × 1 mL), the product was
3
6.54 (br s, 1H, Mes-CH), 7.02 (t, 1H, JH-H ) 7.2 Hz, Naph-
3
CH), 7.23-7.26 (m, 2H, Naph-CH), 7.31 (d, 1H, JH-H
)
)
)
3
6.8 Hz, Naph-CH), 7.45 (s, 2H, Ar-H), 7.50 (d, 1H, JH-H
3
4
8.0 Hz, Naph-CH), 7.60 (dd, 1H, JH-H ) 6.8 Hz, JH-H
1
obtained as colorless crystals (18 mg, 71%). H NMR (acetone-
2.4 Hz, Naph-CH). 13C NMR (acetone-d6): δ 21.11 (Ar-CH3),
24.61, 25.11, 25.59, 25.62 (Mes-CH3), 57.62 (NCH3), 116.30,
123.23, 125.33, 127.11, 128.42 (br), 129.00 (br), 129.50 (br),
131.62, 133.19, 133.25, 136.19, 136.22, 136.44, 141.25, 143.30,
146.51, 147.43, 147.47, 147.72, 152.15 (br), 157.17 (br), 161.95
(br), 174.26, 174.36, 175.73. 19F NMR (acetone-d6): δ -164.7.
d6): δ 1.68 (br s, 9H, Mes-CH3), 2.06 (br s, 9H, Mes-CH3), 2.12
(s, 6H, Ar-CH3), 2.82 (s, 6H, NCH3), 3.00 (s, 18H, S(NMe2)),
6.32 (br s, 4H, Mes-CH), 6.36 (s, 2H, Ar-H), 6.99 (t, 1H, 3JH-H
3
) 7.2 Hz, Naph-CH), 7.21 (d, 2H, JH-H ) 5.6 Hz, Naph-CH),
3
4
7.30 (dd, 1H, JH-H ) 6.8 Hz, JH-H ) 1.2 Hz, Naph-CH), 7.46
3
4
(dd, 1H, JH-H ) 8.0 Hz, JH-H ) 1.6 Hz, Naph-CH), 7.54 (t,
1H, 3JH-H ) 4.8 Hz, Naph-CH). 13C NMR (acetone-d6): δ 21.12
(Ar-CH3), 25.21 (br s, Mes-CH3), 25.79, 25.81 (Mes-CH3), 38.65
(S(NMe2)), 41.08 (NCH3), 111.98, 123.17, 125.12, 126.96, 128.93,
129.04 (br), 132.75, 132.81, 136.08, 136.12, 136.45, 145.32, 147.94,
147.98, 150.46, 158.60, 178.92, 179.03. 19F NMR (acetone-d6): δ
-165.1. 11B NMR (acetone-d6): δ +8.7. 199Hg NMR (acetone-
11B NMR (acetone-d6): δ +8.9. 199Hg NMR (acetone-d6):
-759.8 (d, JHg-F ) 122.0 Hz). Anal. Calcd for C39H45BFHgN:
δ
1
C, 61.78; H, 5.98. Found: C, 61.78; H, 6.01.
UV-Vis Titrations in THF. A THF solution of 3 or [4][OTf]
(5 × 10-5 M, 3.0 mL) was titrated with incremental amounts of
F- anions by the addition of a solution of TBAF in THF (3.69 ×
10-3 M). The absorption was monitored at λmax ) 363 nm (ꢀ )
10 900) for 3 and at λmax ) 362 nm (ꢀ ) 10 100) for [4][OTf].
The experimental data obtained were fitted to a 1:1 binding
isotherm.
1
d6): δ -708.6 (d, JHg-F ) 109.8 Hz). Anal. Calcd for C44H60-
BFHgN4S: C, 58.24; H, 6.66. Found: C, 57.98; H, 6.76.
Synthesis of [1-{Mes2B}-8-{(2,6-Me2-4-Me3NC6H2)Hg}C10H6]-
[OTf] (4[OTf]). An excess of MeOTf (0.1 mL) was slowly added
to a CH2Cl2 solution (5 mL) of 3 (200 mg, 0.276 mmol) at room
temperature. Within a few minutes, a white solid began to
precipitate. After stirring for 2 h, the solid was isolated by filtration
and washed twice with Et2O (2 × 10 mL). Drying under vacuum
afforded [4][OTf] as a white powder (231 mg, 94%). Colorless
single crystals could be obtained by slow evaporation of a methanol
UV-Vis Titrations in THF/H2O. A THF/H2O (9/1, v/v)
solution of 3 or [4][OTf] (4.5 × 10-5 M, 3.0 mL) was titrated with
incremental amounts of F- anions by the addition of a solution of
TBAF in THF (1.07 × 10-1 M for 3 and 6.66 × 10-3 M for [4]-
[OTf]). The absorption was monitored at λmax ) 361 nm (ꢀ )
10 300) for 3 and at λmax ) 363 nm (ꢀ ) 9870) for [4][OTf]. The
experimental data obtained were fitted to a 1:1 binding isotherm.
In order to confirm that fluoride complexation by [4]+ is reversible,
the following experiment was carried out. To a solution of [4][OTf]
[3.0 mL, 4.5 × 10-5 M; THF/H2O (9/1, v/v)] was added 5 µL of
TBAF in THF (6.75 × 10-2 M). After the system had reached
equilibrium, aliquots of an aqueous solution of AlCl3 (0.01 M,
2 f 10 µL) were added incrementally and the absorption was
recorded after each addition.
1
solution of [4][OTf]. H NMR (DMSO-d6): δ 1.23 (s, 3H, Mes-
CH3), 1.50 (s, 3H, Mes-CH3), 1.65 (s, 3H, Mes-CH3), 2.18 (s,
6H, Ar-CH3), 2.21 (br s, 6H, Mes-CH3), 2.31 (s, 3H, Mes-CH3),
3.57 (s, 9H, NCH3), 6.31 (br s, 1H, Mes-CH), 6.74 (br s, 2H,
3
Mes-CH), 6.83 (br s, 1H, Mes-CH), 7.31 (d, 1H, JH-H
)
6.8 Hz, Naph-CH), 7.45 (t, 1H, 3JH-H ) 7.6 Hz, Naph-CH), 7.57
(s, 2H, Ar-H), 7.60 (t, 1H, 3JH-H ) 7.2 Hz, Naph-CH), 7.65 (d,
3
3
1H, JH-H ) 6.0 Hz, Naph-CH), 7.91 (d, 1H, JH-H ) 7.6 Hz,
3
1
Naph-CH), 8.08 (d, 1H, JH-H ) 8.4 Hz, Naph-CH). H NMR
(acetone-d6): δ 1.39 (s, 3H, Mes-CH3), 1.59 (s, 3H, Mes-CH3),
1.73 (s, 3H, Mes-CH3), 2.22 (br s, 6H, Mes-CH3), 2.28 (s, 6H,
Ar-CH3), 2.36 (s, 3H, Mes-CH3), 3.86 (s, 9H, NCH3), 6.34 (br s,
1H, Mes-CH), 6.77 (br s, 2H, Mes-CH), 6.85 (br s, 1H, Mes-
CH), 7.42-7.49 (m, 2H, Naph-CH), 7.59-7.64 (m, 2H, Naph-
Theoretical Calculations. Density functional theory (DFT)
calculations (full geometry optimization) were carried out with
Gaussian 0318 using the BP86 functional with the following basis
sets: 6-31g for all C and H atoms,19 6-31+g(d′) for the B and N
atoms,20 and Stuttgart RSC 1997 ECP for the Hg centers.21
Frequency calculations, which were carried out on the optimized
structure of each compound, confirmed the absence of imaginary
frequencies. Frontier orbitals were obtained from the optimized
geometry.
CH), 7.74 (s, 2H, Ar-H), 7.93 (dd, 1H, 3JH-H ) 7.2 Hz, 4JH-H
)
)
3
4
2.8 Hz, Naph-CH), 8.11 (dd, 1H, JH-H ) 7.6 Hz, JH-H
1.6 Hz, Naph-CH). 13C NMR (DMSO-d6): δ 20.88 (Ar-CH3),
21.90, 23.05, 24.22, 24.89 (Mes-CH3), 56.30 (NCH3), 117.34,
8134 Inorganic Chemistry, Vol. 46, No. 20, 2007

Cationic Bidentate Lewis Acid
Scheme 1a
NMR signal detected at 72.1 ppm for 3 is comparable to
that of II and confirms the presence of a base-free trigonal-
planar B center.13 In the case of [4][OTf], the 11B NMR
resonance could not be detected despite extended acquisition
times. The 199Hg NMR signals of 3 and [4][OTf] are
observed at -584.6 and -693.9 ppm, respectively, and are
199
both in the expected range for a diarylmercurial.23 The
-
Hg NMR chemical shift of [4][OTf] is close to that of II
(-741.9 ppm). In THF, the UV spectra of 3 and [4][OTf]
feature a broad absorption band at 363 nm (ꢀ363 ) 10 100)
for 3 and 362 nm (ꢀ362 ) 10 100) for [4][OTf]. These bands
are clearly red-shifted when compared to the absorption of
naphthalene (λmax ) 286 nm) and confirm the presence of a
three-coordinate B center, which mediates conjugation of the
naphthalenediyl and mesityl substituents.13
a (i) THF, 25 °C, 70%. (ii) MeOTf, CH2Cl2, 25 °C, 94%.
Single crystals of [4][OTf] have been analyzed by X-ray
diffraction. These crystals belong to the monoclinic space
group P21/c (Figure 1 and Table 1). The B1 atom adopts a
trigonal-planar geometry (ΣC-B-C ) 359.2°) and is separated
from the Hg1 atom by only 3.328(4) Å, which is very similar
to the B-Hg distance observed in II.13 The B1-C1-C9
[128.6(3)°] and Hg1-C8-C9 [130.4(3)°] angles substan-
tially deviate from the ideal value of 120° and indicate the
existence of a crowded structure. The short C20-Hg1
distance of 3.022(4) Å indicates the presence of a secondary
Hg-π interaction involving the ipso-C atom of one of the
mesityl groups. This Hg-π interaction leads to a noticeable
deviation of the C8-Hg1-C29 angle [167.7(2)°] from
linearity. Such an interaction has been observed in II13 and
is reminiscent of the B-π interactions sometimes observed
in 1,8-diborylnaphthalenes.24
Results and Discussion
Synthesis and Characterization. Reaction of 116 with 2,
which was obtained by mercuration of N,N,3,5-tetramethy-
laniline,22 afforded the neutral heteronuclear Lewis acid 3
as an ivory solid in 70% yield (Scheme 1). This new
derivative reacts with MeOTf to afford the ammonium triflate
salt [4][OTf]. Both 3 and [4][OTf] are air- and water-stable.
While 3 dissolves in common organic solvents, solubilization
of [4][OTf] requires polar solvents such as acetone or
dimethyl sulfoxide (DMSO). The identities of 3 and [4][OTf]
have been confirmed by multinuclear NMR spectroscopy and
1
elemental analysis. In both cases, the H NMR spectrum
exhibits six distinct resonances that correspond to the
aromatic CH groups of the unsymmetrically substituted
naphthalene backbone. Four aryl and six methyl proton
resonances are observed for the two mesityl groups, thus
indicating the existence of a congested structure. The proton
resonances of the dimethylamino group in 3 and the
trimethylammonium group in [4][OTf] appear at 2.91 ppm
(CDCl3) and 3.57 ppm (DMSO-d6), respectively. The 11B
Compounds 3 and [4][OTf] can be converted into their
respective fluoride complexes [S(NMe2)3][3-µ2-F] and [4-µ2-
F] by reaction with TASF in CH2Cl2 (Scheme 2). The
chelation of the F- anion in both complexes is supported by
their NMR spectroscopic features, which are very similar to
those of [II-µ2-F]- (Table 2). Some of the salient features
include the following: the 11B NMR signal, which clearly
indicates the presence of a four-coordinate B center; the 19
F
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. 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.
(19) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257.
NMR signal, which is close to that observed in other fluoride-
bridged species;9,12,13 the detection of a doublet in the 199Hg
NMR spectrum, confirming the presence of a Hg-F linkage.
1
The difference observed in JHg-F for [3-µ2-F]- and
[4-µ2-F] suggests that the Hg center of [4]+ is more
fluorophilic than that of 3.
Crystals of [4-µ2-F] can be obtained from CH3CN. These
crystals belong to the monoclinic space group P21/c (Table
1). The asymmetric unit contains two independent molecules,
denoted as A and B, which have very similar structures
(Figure 1). The B-F bond lengths of 1.474(8) Å (A) and
1.488(8) Å (B) are comparable to those found in triarylfluo-
roborates.8,15 The Hg-F bonds of 2.618(3) Å (A) and 2.646-
(4) Å (B) correspond to strong secondary interactions that
are well within the sum of the van der Waals radii of Hg
(1.75 Å)25 and F (1.30-1.38 Å).26 These Hg-F distances
(20) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J.
Comput. Chem. 1983, 4, 294. Krishnan, R.; Binkley, J. S.; Seeger,
R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. Gill, P. M. W.; Johnson,
B. G.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1992, 197, 499.
(21) Dunning, T. H. J.; Hay, P. J. Modern Theoretical Chemistry; Schaefer,
H. F. I., Ed.; Plenum: New York, 1976.
(23) Rowland, K. E.; Thomas, R. D. Magn. Reson. Chem. 1985, 23, 916.
(24) Hoefelmeyer, J. D.; Sole´, S.; Gabba¨ı, F. P. Dalton Trans. 2004, 1254.
(25) Pyykko, P.; Straka, M. Phys. Chem. Chem. Phys. 2000, 2, 2489.
(22) Nicholson, B. K.; Whitley, S. K. J. Organomet. Chem. 2004, 689,
515.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8135

Lee and Gabba1ı
Figure 1. Crystal structures of [4]+ in [4][OTf] and [4-µ2-F] (50% ellipsoid). For [4-µ2-F], only molecule A is shown. H atoms were omitted for clarity.
Selected bond lengths (Å) and angles (deg) are as follows. [4]+: B1-C1 1.576(6), B1-C11 1.582(6), B1-C20 1.570(6), Hg1-C8 2.084(4), Hg1-C29
2.092(4); C1-B1-C11 115.3(3), C1-B1-C20 125.1(3), C11-B1-C20 118.8(3), B1-C1-C9 128.6(3), Hg1-C8-C9 130.4(3), C8-Hg1-C29 167.7(2).
[4-µ2-F] [the metrical parameters of the second independent molecule (B) are provided in brackets]: B1-C1 1.654(8) [1.667(10)], B1-C11 1.656(8) [1.663-
(10)], B1-C20 1.679(9) [1.669(10)], B1-F1 1.474(8) [1.488(8)], Hg1-F1 2.618(3) [2.646(4)], Hg1-C8 2.092(6) [2.079(6)], Hg1-C29 2.093(6) [2.086-
(6)]; C1-B1-C11 110.3(5) [109.7(5)], C1-B1-C20 119.2(5) [119.0(6)], C11-B1-C20 109.0(5) [112.2(5)], B1-C1-C9 125.1(5) [126.1(6)], Hg1-C8-
C9 129.0(4) [127.5(5)], C8-Hg1-C29 162.2(2) [164.1(2)], B1-F1-Hg1 98.2(3) [96.8(3)].
Table 1. Crystal Data, Data Collection, and Structure Refinement for [4][OTf] and [4-µ2-F]
[4][OTf]
[4-µ2-F]‚MeCN
C41H48BFHgN2
Crystal Data
formula
Mr
C40H45BF3HgNO3S
888.23
799.21
cryst size (mm3)
cryst syst
space group
a (Å)
0.24 × 0.12 × 0.05
monoclinic
P21/c
25.412(5)
13.069(3)
11.038(2)
96.08(3)
3645.3(13)
4
0.40 × 0.15 × 0.05
monoclinic
P21/c
23.700(5)
11.660(2)
32.091(11)
125.81(2)
7192(3)
8
b (Å)
c (Å)
â (deg)
V (Å3)
Z
F
calc (g cm-3
)
1.618
4.334
1776
1.476
4.316
3216
µ (mm-1
F(000)
)
Data Collection
T (K)
140(2)
140(2)
scan mode
hkl range
measd reflns
ω
ω
-34 f +33, -17 f +17, -14 f +13
-28 f +28, -13 f +13, -38 f +38
31 409
66 176
unique reflns [Rint
reflns used for refinement
]
8755 [0.0985]
8755
12 657 [0.0470]
12657
Refinement
refined param
GOF on F 2
451
1.003
0.0437, 0.0791
1.258, -0.550
817
1.004
0.0492, 0.1241
2.487, -1.624
R1,a wR2b all data
F
fin (max/min) (e Å-3
)
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {[∑w(Fo - Fc )2]/[∑w(Fo )2]}1/2
.
are slightly longer than those found in [II-µ2-F]- [2.589(2)
Å], which possesses a less sterically hindered and possibly
more Lewis acidic Hg center. Nevertheless, they remain
within the range observed in other complexes such as [(CF3)2-
Hg(µ2-F)2Hg(CF3)2]2- (2.39 and 2.42 Å)27 or the fluoride
adduct of a tetranuclear mercuracarborand (2.56 and 2.65
Å).4 In comparison to [II-µ2-F]-, it is also interesting to note
that the longer Hg-F distances observed in [4-µ2-F] cor-
1
relate, as expected, with a smaller JHg-F. Coordination of
the fluoride results in substantial pyramidalization of the B
centers [ΣC-B1-C ) 338.5° (A) and 340.9° (B)] along with
an increased deviation of the C-Hg-C angles [162.2(2)°
(A) and 164.1(2)° (B)] from linearity. This deviation is
comparable to that observed in the anionic complex [(CF3)2-
Hg(µ2-F)2Hg(CF3)2]2- [162.1(5)°]27 and provides further
(26) Nyburg, S. C.; Faerman, C. H. Acta Crystallogr., Sect. B: Struct. Sci.
1985, B41, 274.
(27) Viets, D.; Lork, E.; Watson, P. G.; Mews, R. Angew. Chem., Int. Ed.
1997, 36, 623.
8136 Inorganic Chemistry, Vol. 46, No. 20, 2007

Cationic Bidentate Lewis Acid
Scheme 2a
a (i) [Me3SiF2][S(NMe2)3], CH2Cl2, 25 °C, 71%. (ii) [Me3SiF2][S(NMe2)3], CH2Cl2, 25 °C, 62%.
Table 2. NMR Spectroscopy Data for [II-µ2-F]-,13 [3-µ2-F]-, and
[4-µ2-F]
[II-µ2-F]-
[3-µ2-F]-
[4-µ2-F]
δ(11B) in ppm
8.5
-164.3
-811.8
135.2
8.7
-165.1
-708.6
109.8
8.9
-164.7
-759.8
122.0
δ(19F) in ppm
δ(199Hg) in ppm
1JHg-F in Hz
amination of the binding isotherm obtained for both 3 and
[4]+ shows a linear decrease of the absorbance, which reaches
the baseline after the addition of exactly 1 equiv. These
results indicate the formation of the 1:1 complexes [3-µ2-
F]- and [4-µ2-F], whose stability constants exceed the value
of 107 M-1 measurable by a UV-vis titration. Bearing in
mind that the stability constant of [Mes3BF]- is equal to 3.3
((0.4) × 105 M-1 in THF,9 these results suggest that the
fluoride ion affinity of 3 and [4]+ is increased mainly by a
chelate effect. Both 3 and [4]+ also complex fluoride in THF/
water (9/1, v/v) mixtures. In this more competitive medium,
the fluoride binding constants of 3 and [4]+ are clearly
differentiated and are respectively equal to 1.3 ((0.1) × 102
and 6.2 ((0.2) × 104 M-1 (Figure 3, top). The enhanced
fluoride binding constant of [4]+ when compared to 3
confirms that the chelate effect occurring in these types of
fluoride receptors can be combined with favorable Coulombic
attractions to strengthen the host-guest interaction. We also
note that, despite its cationic character, [4]+ remains highly
selective for fluoride over other environmentally abundant
anions. For example, the addition of 10 equiv of Cl-, Br-,
Figure 2. Optimized geometries and LUMOs (isodensity value ) 0.03)
of 3 (left) and [4]+ (right).
evidence for the presence of a significant interaction between
the Hg and F atoms in [4-µ2-F].
Calculations. The structures of 3 and [4]+ have been
optimized using DFT methods. The two structures are very
similar and show separations of 3.38 Å (3) and 3.32 Å ([4]+)
between the B and Hg atoms. This distance is comparable
to the separation of 3.328(4) Å observed in the crystal
structure of [4][OTf]. The secondary Hg-π interaction
involving the ipso-C atom of one of the mesityl groups and
the Hg atom is also present in the optimized structure of
[4]+ (3.04 Å) and can be compared to the distance of 3.022(4)
Å observed in the crystal structure of [4]+. As shown in
Figure 2, the lowest unoccupied molecular orbital (LUMO)
of 3 is dominated by the B atom. By contrast, the LUMO of
[4]+ is distributed between the two Lewis acidic elements
and features a lobe that spans the B and Hg centers. This
orbital also shows a substantial contribution from an anti-
bonding E orbital of the benzene core of the aniline moiety.
The difference observed in the makeup of the LUMO of these
two molecules indicates that conversion of the amino group
into an ammonium group on going from 3 to [4]+ results in
a lowering of the Hg vacant orbitals, allowing them to mix
more efficiently with the B orbital. In turn, it can be
anticipated that the Hg center of [4]+ will show an increased
participation in the binding of anionic guests.
NO3 , H2PO4 , and HSO4- does not significantly affect the
UV-vis spectrum of a 4.5 × 10-5 M solution of [4]+ in
THF/H2O (9/1, v/v), while the same experiment carried out
with F- leads to a 93% decrease of the absorbance at
363 nm (Figure 4). A weak response is detected with OAc-
(9% absorbance decrease upon the addition of 10 equiv),
which is usual for B-based Lewis acids.8 Because of the
-
-
-
spontaneous formation of I3 under these conditions, UV-
vis spectroscopy could not be used to study iodide binding.
However, NMR studies in DMSO-d6 show that iodide does
not interact with [4]+. Finally, the addition of an aqueous
solution of Al3+ to a solution containing [4-µ2-F] leads to
complete regeneration of [4]+, showing that fluoride binding
is reversible (Figure 3, bottom).15
F- Anion Binding Studies. In order to determine how
the anion binding properties of such receptors are influenced
by the presence of a cationic ammonium group, the fluoride
binding properties of 3 and [4]+ have been studied and
compared. The addition of TBAF to a THF solution of 3
and [4]+ results in a decrease of the absorbance at 363 and
362 nm for 3 and [4]+, respectively. This phenomenon results
from population of the empty B p orbital, which can no
longer promote conjugation throughout the molecule. Ex-
Conclusion
The results reported in this paper show that the fluoride
binding constant of heteronuclear B/Hg bidentate Lewis acids
can be substantially increased by introduction of a cationic
Inorganic Chemistry, Vol. 46, No. 20, 2007 8137

Lee and Gabba1ı
Figure 3. Top: Changes in the UV-vis absorption spectra of a solution of 3 and [4][OTf] [3 mL, 4.5 × 10-5 M in THF/H2O (9/1, v/v)] upon the addition
of a TBAF solution in THF. Bottom: Changes in the UV-vis absorption of a solution of [4][OTf] (3 mL, 4.5 × 10-5 M) in THF/H2O (9/1, v/v) after the
addition of a TBAF solution (left, 6.75 × 10-2 M in THF), followed by successive additions of an aqueous AlCl3 solution (right, 0.01 M).
ultimately complement the chelate effect occurring in these
types of bidentate receptors. This study constitutes a useful
milestone for the design of water-compatible fluoride sensors.
Future improvement may require that the cationic moiety
be directly attached to the triarylboron part of the receptor.
Acknowledgment. We gratefully acknowledge the fol-
lowing agencies for funding: the Korea Research Foundation
(Grant KRF-2005-214-C00211), the U.S. Army Medical
Research Institute of Chemical Defense, the Welch Founda-
tion (Grant A-1423), the NSF (CHE-0646916), and the PRF
(Grant 44832-AC4).
Figure 4. Percent decrease of the absorbance of a solution of [4][OTf]
Supporting Information Available: Optimized geometries of
(4.5 × 10-5 M) in THF/H2O (9/1, v/v) at 363 nm in the presence of 10
3 and [4]+, UV-vis titration data in THF, and X-ray crystal-
equiv of various anions.
lographic data for [4][OTf] and [4-µ2-F] in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
moiety. This increase results from favorable Coulombic
attractions that strengthen the host-guest interaction and
IC700360A
8138 Inorganic Chemistry, Vol. 46, No. 20, 2007