C O M M U N I C A T I O N S
Although water stable, cation [3]+ does not react with fluoride in
pure water. Nevertheless, when dissolved in CHCl3, [3]+ captures
fluoride from water to form 3-F. For example, shaking a biphasic
mixture consisting of TBAF in D2O (2.7 × 10-1 M, 0.5 mL) and
[3]OTf in CDCl3 (6.9 × 10-2 M, 0.5 mL) results in a 82%
conversion of [3]OTf into 3-F after a few minutes. To our
knowledge, such a biphasic fluoride capture is unprecedented for
any boron-based fluoride receptors. To provide additional compara-
tive data, we have carried out the same experiments using I-III
and found that these chelating boranes fail to capture fluoride under
these biphasic conditions.
Figure 1. Crystal structure of one of the independent molecules of [3]+ in
[3][OTf] (left) and 3-F (right) (50% ellipsoid). Pertinent parameters are
provided in the text.
The results that we present in this Communication indicate that
cationic boranes such as [3]+ may be well suited as molecular
recognition units for fluoride in water under biphasic conditions.
We propose that the high fluoride affinity of [3]+ results from
favorable Coulombic forces which stabilize the B-F bond against
heterolysis. We are currently investigating the use of cationic
boranes for the fluorescent sensing of fluoride.
Acknowledgment. We thank the NSF (CAREER Grant CHE-
0094264), the Welch Foundation (Grant A-1423), and the PRF
(Grant 44832-AC4) for supporting this research.
Figure 2. (left) Portion of the NMR showing the resonance of the hydrogen
atom hydrogen-bonded to the fluorine atom. The peaks marked by /
correspond to mesityl CH resonances; (right) contour plot of the electron
density of [3-F] in the B-F‚‚‚H plane along with bond paths and bond
critical points.
Note Added in Proof. After submission of this Communication,
the complexation of fluoride by a phosphonium borane19 and by
triarylborane-functionalized polystyrenes20 has been reported.
the formation of a hydrogen bond with a neighboring hydrogen-
bond donor group. This situation is reminiscent to that encountered
in the ammonium fluoroborate [(η5-C5H5)Fe{η5-C5H3(BF3)(CH2-
NMe2H)}] which features a N-H‚‚‚F-B hydrogen bond.13
Colorless crystals of 3-F were obtained by evaporation of a Et2O
solution of 3-F. As indicated by the X-ray crystal structure (Figure
1), the fluorine atom is trapped between the boron atom and the
methylene group. The B-F bond length of 1.486(4) Å is not
significantly longer than those found in triarylfluoroborate anions
(1.47 Å), thus indicating the presence of an usual polar covalent
B-F linkage. Accordingly, the sum of the coordination angles at
boron (Σ(C-B-C) ) 340.71°) indicates substantial pyramidalization.
The distance of 2.826(4) Å separating the fluorine atom and the
methylene carbon atom confirms the presence of a C-H‚‚‚F-B
hydrogen bond. Hydrogen bonds involving C(sp3)-H groups are
rare14 especially for fluoroborate species.15 In an effort to better
understand this unusual C-H‚‚‚F-B linkage, the structure of 3-F
was optimized using DFT methods (B3LYP, 6-31+g(d′) for the
boron and fluorine, 6-31g for all other atoms). The optimized
structure is close to that determined experimentally. In particular,
the calculated B-F (1.528 Å) and F-C(01) (2.818 Å) distances
are within a few hundreds of an angstro¨m from that observed in
the crystal (1.486 and 2.826 Å, respectively). Analysis of the
topology of the electron density using the AIM2000 program16
shows the presence of a bond path between the hydrogen and
fluorine atom of the C-H‚‚‚F-B bridge (Figure 2). Moreover, the
value of the electron density (F(r) ) 2.5 × 10-2 e bohr-3) and the
Supporting Information Available: Experimental details and X-ray
crystallographic data for [3][OTf] and 3-F in CIF format. This material
References
(1) Martinez-Manez, R.; Sancenon, F. Chem. ReV. 2003, 103, 4419-4476.
Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. Engl. 2001, 40, 486-516.
Gale, P. A. Coord. Chem. ReV. 2003, 240, 191-221. Schmidtchen, F. P.;
Berger, M. Chem. ReV. 1997, 97, 1609-1646.
(2) Boiocchi, M.; Del Boca, L.; Gomez, D. E.; Fabbrizzi, L.; Licchelli, M.;
Monzani, E. J. Am. Chem. Soc. 2004, 126, 16507-16514.
(3) Lin, Z.-H.; Ou, S.-J.; Duan, C.-Y.; Zhang, B.-G.; Bai, Z.-P. Chem.
Commun. 2006, 624-626.
(4) Wang, S.; Liu, X. Y.; Bai, D. R. Angew. Chem., Int. Ed. 2006, Early
View. Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am. Chem.
Soc. 2002, 124, 8816-8817. Yamaguchi, S.; Akiyama, S.; Tamao, K. J.
Am. Chem. Soc. 2001, 123, 11372-11375. Yamaguchi, S.; Akiyama, S.;
Tamao, K. J. Am. Chem. Soc. 2001, 123, 11372-11375. Liu, Z.-Q.; Fang,
Q.; Cao, D.-X.; Wang, D.; Xu, G.-B. Org. Lett. 2004, 6, 2933-2936.
(5) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000, 122,
6335-6336.
(6) Katz, H. E. J. Org. Chem. 1985, 50, 5027-5032. Chase, P. A.; Henderson,
L. D.; Piers, W. E.; Parvez, M.; Clegg, W.; Elsegood, M. R. J.
Organometallics 2006, 25, 349-357. Mela¨ımi, M.; Gabba¨ı, F. P. AdV.
Organomet. Chem. 2005, 53, 61-99.
(7) 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.
(8) Sole´, S.; Gabba¨ı, F. P. Chem. Commun. 2004, 1284-1285.
(9) Mela¨ımi, M.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2005, 127, 9680-9681.
(10) Hudnall, T. W.; Mela¨ımi, M.; Gabba¨ı, F. P. Org. Lett. 2006, 8, 2747-
2749.
(11) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252-6254.
(12) Hoefelmeyer, J. D.; Gabba¨ı, F. P. Organometallics 2002, 21, 982-985.
(13) Bresner, C.; Aldridge, S.; Fallis, I. A.; Jones, C.; Ooi, L.-L. Angew. Chem.,
Int. Ed. 2005, 44, 3606-3609.
(14) Chan, M. C. W.; Kui, S. C. F.; Cole, J. M.; McIntyre, G. J.; Matsui, S.;
Zhu, N.; Tam, K.-H. Chem.sEur. J. 2006, 12, 2607-2619. Mountford,
A. J.; Hughes, D. L.; Lancaster, S. J. Chem. Commun. 2003, 2148-2149.
(15) Collman, J. P.; Christian, P. A.; Current, S.; Denisevich, P.; Halbert, T.
R.; Schmittou, E. R.; Hodgson, K. O. Inorg. Chem. 1976, 15, 223-227.
(16) Konig, F. B.; Schonbohm, J.; Bayles, D. J. Comput. Chem. 2001, 22,
545-559.
(17) Kolandaivel, P.; Nirmala, V. J. Mol. Struct. 2004, 694, 33-38.
(18) Bryantsev, V. S.; Hay, B. P. J. Am. Chem. Soc. 2005, 127, 8282-8283.
(19) Agou, T.; Kobayashi, J.; Kawashima, T. Inorg. Chem., published online
2
Laplacian value (-1/4∇ F(r) ) -2.4 × 10-2 e bohr-5) at the
H‚‚‚F bond critical point fall within the expected range for a
moderately strong hydrogen bond.17 Although moderately strong,
this interaction may serve to increase the stability of the fluoroborate
motif.18
The fluoride binding constant of [3]+ in 75/25 (vol) THF/MeOH
is equal to 5.0 ((0.5) × 106 M-1 as indicated by a UV titration
experiment carried out by monitoring the absorption of [3]+ at λmax
) 352 nm (ꢀ ) 11850). Under these conditions, Mes3B does not
form any detectable quantities of [Mes3BF]-. Encouraged by these
results, we attempted to test the water compatibility of [3]+.
(20) Parab, K.; Venkatasubbaiah, K.; Jaekle, F. J. Am. Chem. Soc. 2006, 127,
13748-13749.
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