Ammonium boranes for the selective complexation of cyanide or fluoride ions in water

Article abstract of DOI:10.1021/ja073793z

With the recognition of aqueous fluoride and cyanide ions as an objective, we have investigated the anion binding properties of two isomeric ammonium boranes, namely [p-(Mes₂B)C₆H₄(NMe ₃)]⁺ ([1]⁺) and [o-(Mes₂B)C ₆H₄(NMe₃)]⁺ ([2]⁺). These cationic boranes, which could be obtained by reaction of the known 4- and 2-dimesitylboryl-N,N-dimethylaniline with MeOTf, have been investigated both experimentally and computationally. They both react with fluoride and cyanide ions in organic solvents to afford the corresponding fluoroborate/ or cyanoborate/ammonium zwitterions 1F, 1CN, 2F, and 2CN. In aqueous solution, however, these cationic boranes behave as remarkably selective receptors. Indeed, [1]⁺ only complexes cyanide ions while [2]⁺ only complexes fluoride ions. In H₂O/DMSO 60:40 vol (HEPES 6 mM, pH 7), the cyanide binding constant of [1]⁺ and the fluoride binding constant of [2]⁺ are respectively equal to 3.9 (±0.1) × 10⁸ and 910 (±50) M^-1. Structural and computational studies indicate that both steric and electronic effects contribute to the unusual selectivity displayed by these cationic boranes. Owing to favorable Coulombic effects, the para-derivative [1]⁺ has a very high affinity for cyanide; yet these effects are not sufficiently intense to allow complexation of the more efficiently hydrated and less basic fluoride anion. In the case of the orthro-derivative [2]⁺, the proximity of the ammonium moiety leads to an increase in the Lewis acidity of the boron center thus making fluoride binding possible. However, steric effects prevent cyanide coordination to the boron center of [2]⁺. Finally, cation [1] ⁺ and [2]⁺ bind their dedicated anions reversibly and show a negligible response in the presence of other common anions including Cl ^-, Br^-, I^-, NO₃^-, OAc^-, H₂PO₄^-, and HSO ₄^-.

Full text of DOI:10.1021/ja073793z

Published on Web 09/11/2007
Ammonium Boranes for the Selective Complexation of
Cyanide or Fluoride Ions in Water
Todd W. Hudnall and Franc¸ois P. Gabba¨ı*
Contribution from the Department of Chemistry, Texas A&M UniVersity,
College Station, Texas 77843
Received June 8, 2007; E-mail: francois@tamu.edu
Abstract: With the recognition of aqueous fluoride and cyanide ions as an objective, we have investigated
the anion binding properties of two isomeric ammonium boranes, namely [p-(Mes2B)C6H4(NMe3)]+ ([1]+)
and [o-(Mes2B)C6H4(NMe3)]+ ([2]+). These cationic boranes, which could be obtained by reaction of the
known 4- and 2-dimesitylboryl-N,N-dimethylaniline with MeOTf, have been investigated both experimentally
and computationally. They both react with fluoride and cyanide ions in organic solvents to afford the
corresponding fluoroborate/ or cyanoborate/ammonium zwitterions 1F, 1CN, 2F, and 2CN. In aqueous
solution, however, these cationic boranes behave as remarkably selective receptors. Indeed, [1]+ only
complexes cyanide ions while [2]+ only complexes fluoride ions. In H2O/DMSO 60:40 vol (HEPES 6 mM,
pH 7), the cyanide binding constant of [1]+ and the fluoride binding constant of [2]+ are respectively equal
to 3.9 ((0.1) × 108 and 910 ((50) M-1. Structural and computational studies indicate that both steric and
electronic effects contribute to the unusual selectivity displayed by these cationic boranes. Owing to favorable
Coulombic effects, the para-derivative [1]+ has a very high affinity for cyanide; yet these effects are not
sufficiently intense to allow complexation of the more efficiently hydrated and less basic fluoride anion. In
the case of the ortho-derivative [2]+, the proximity of the ammonium moiety leads to an increase in the
Lewis acidity of the boron center thus making fluoride binding possible. However, steric effects prevent
cyanide coordination to the boron center of [2]+. Finally, cation [1]+ and [2]+ bind their dedicated anions
reversibly and show a negligible response in the presence of other common anions including Cl-, Br-, I-,
NO3-, OAc-, H2PO4-, and HSO4
.
-
Introduction
with the host-guest interactions. An added complication exists
for the cyanide ion whose competitive protonation (pKa (HCN)
) 9.3) complicates its capture in neutral water. Despite these
challenges, recent efforts have afforded a number of receptors17-23
which sometimes function in water.18,19,22,24 Most of the cyanide
receptors are electrophilic organic reagents which undergo C-C
bond forming reactions with cyanide. Some of these reactions
necessitate basic pH and their reversibility is not always well
documented. Other cyanide receptors or probes with interesting
Cyanide is a toxic anion which binds to and deactivates the
cytochrome c oxidase enzyme with sometimes fatal conse-
quences.1 Because cyanide is widely available in both research
and industrial settings, its use for harmful purposes or its release
in the environment are sources of concern.2 For these reasons,
the development of methods that can sense this anion in water
has become a topical objective. Another important nucleophilc
anion is the fluoride anion. This anion is often added to drinking
water and toothpaste because of its beneficial effects in dental
health. It is also administered in the treatment of osteoporosis.3
As widely documented, however, excessive fluoride intake can
be problematic and can lead to the development of dental or
skeletal fluorosis.4 Thus, as for cyanide, the detection of fluoride
levels in water is an important task.
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11978
J. AM. CHEM. SOC. 2007, 129, 11978-11986
10.1021/ja073793z CCC: $37.00 © 2007 American Chemical Society

Complexation of Cyanide or Fluoride Ions in Water
A R T I C L E S
properties include boronic acids,25-29 boron-subphthalocya-
nines,24,30 zinc-porphyrins,31,32 iron-hemes,33 and transition metal
complexes.34 For fluoride ions, the high hydration enthalpy of
-504 kJ/mol is certainly one of the factors making recognition
of this anion in aqueous solution especially difficult.6,7 To
circumvent these difficulties, several groups are currently
studying receptors in which the fluoride binding site is a Lewis
acidic element such as boron,25,35-55 aluminum,56 or tin.57 Recent
advances in this area suggest that cationic boranes58-61 such as
[I]+, [II]+, and [III]+ may be competent for fluoride capture in
water. Indeed, [I]+ and [II]+ promote the transfer of fluoride
ions from water into organic or solid phases59,62 while [III]+
binds fluoride ions in H2O/MeOH 90:10 vol with a binding
cyanide in water. While this possibility has long gone unnoticed,
the group of Ja¨kle showed recently that polymers containing
pendant triarylboranes can serve to probe cyanide in organic
solvents.46 Hoping to achieve the recognition of cyanide in
water, we have now turned our attention to cationic boron-based
receptors. On the basis of the results that we have obtained on
the recognition of fluoride ions,63 we hypothesized that cationic
boranes may be particularly well-adapted for cyanide complex-
ation because of favorable Coulombic receptor-anion attractions.
In this paper, we report the synthesis of the cationic borane
[p-(Mes2B)C6H4(NMe3)]+ ([1]+) which serves as a selective
receptor for cyanide ions in aqueous solutions. We also report
the synthesis of its structural isomer [o-(Mes2B)C6H4(NMe3)]+
([2]+) which only complexes fluoride ions in aqueous solutions.
constant of 1000 ((100) M-1 63
.
It has long been established that triarylboranes interact with
cyanide to form the corresponding cyanoborate complexes. For
example, the [Ph3BCN]- anion can be used for the precipitation
of cesium ions.64 This simple observation suggests that water-
stable triarylboranes could be used for the complexation of
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Discussion and Results
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Synthesis, Structure, and Properties. The ammonium
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dimethylaniline65-67 with MeOTf in diethyl ether (Scheme 1).
These salts dissolve in polar solvents such as acetone, aceto-
nitrile and DMSO but are insoluble in hydrocarbon solvents
and diethyl ether. [1]OTf and [2]OTf are hygroscopic colorless
solids which have been characterized by NMR spectroscopy
and elemental analysis. The 1H NMR spectra of [1]OTf (CDCl3)
and [2]OTf (acetone-d6) feature all expected resonances for the
aromatic CH groups of the phenylene core. The aryl and methyl
proton resonances of the two mesityl groups of [2]OTf are split
into broad multiple signals thus indicating the existence of a
congested structure. The proton resonance of the trimethylam-
monium group in [1]OTf and [2]OTf appears at 3.74 ppm
(CDCl3) and 3.75 ppm (acetone-d6), respectively. The broad
11B NMR signals at 74 ppm for [1]+ and 66 ppm for [2]+
(CDCl3), respectively, are characteristic of triaryl boranes.
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Scheme 1 a
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The crystal structures of [1][OTf] and [2][OTf] have been
determined (Table 1, Figure 1). Salt [1][OTf] crystallizes in the
triclinic space group P1h as a toluene solvate with two molecules
in the asymmetric unit (Figure 1). Both molecules have very
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9
J. AM. CHEM. SOC. VOL. 129, NO. 39, 2007 11979

A R T I C L E S
Hudnall and Gabba¨ı
Table 1. Crystal Data, Data Collection, and Structure Refinement for [1][OTf]-0.5(Toluene), [2][OTf], 1CN-Acetone and 2F
[1][OTf]-0.5(toluene)
[2][OTf]
1CN-acetone
2F
Cystal Data
C28H35BF3NO3S
533.44
0.22 × 0.16 × 0.13
monoclinic
P21/c
14.061(4)
12.976(3)
16.329(4)
formula
Mr
C
31.50H39BF3NO3S
C31H41BN2O
468.47
0.28 × 0.10 × 0.08
monoclinic
P21/n
9.3438(17)
12.777(2)
23.658(4)
C27H35BFN
403.37
579.51
crystal size (mm3)
crystal system
space group
a (Å)
0.23 × 0.20 × 0.20
0.29 × 0.21 × 0.20
monoclinic
P21
8.4441(11)
15.338(2)
8.8250(11)
triclinic
P1h
8.9107(8)
16.1875(15)
23.213(2)
109.8110(10)
95.6030(10)
90.4530(10)
3132.1(5)
4
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
113.423(5)
99.547(2)
95.774(2)
2733.8(12)
4
1.296
0.169
1128
2785.4(9)
4
1.117
0.066
1016
1137.2(3)
2
1.178
0.072
436
Z
F
calcd (g cm-3
)
1.229
0.153
1228
µ (mm-1
F(000)
)
Data Collection
110(2)
T (K)
110(2)
110(2)
110(2)
scan mode
hkl range
ω
ω
ω
ω
-11 f +11
-21 f +21
-30 f +30
29051
-16 f +15
-14 f +14
-12 f +18
11848
-8 f +8
-11 f +11
-20 f +20
12791
-10 f +9
-20 f +19
-11 f +11
8020
measd reflns
unique reflns [Rint
reflns used for refinement
]
14466 [0.0394]
14466
4286 [0.0819]
4286
2017 [0.0502]
2017
4989 [0.0710]
4989
Refinement
335
1.024
0.1105, 0.1604
0.520, -0.461
refined parameters
GOF on F2
730
1.024
0.1094, 0.1172
0.399, -0.363
316
1.002
0.0622, 0.1364
0.276, -0.236
271
1.004
0.0668, 0.1485
0.321, -0.282
R1,a wR2,b all data
F
fin (max/min) (e Å-3
)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {[∑w(Fo - Fc )2]/[∑w(Fo )2]}1/2
.
2
2
2
Figure 1. Crystal structure of [1]+ in [1]OTf-0.5(toluene) (left) and [2]+ in [2]OTf (right) (50% ellipsoid, H-atoms omitted); selected bond lengths (Å) and
angles (deg). [1]+ (the metrical parameters of the second independent molecule are provided in brackets): B(1)-C(11) 1.569(3) [1.577(3)], B(1)-C(21)
1.572(3) [1.578(3)], B(1)-C(1) 1.567(3) [1.559(3)]; C(11)-B(1)-C(21) 125.8(2) [125.6(2)], C(11)-B(1)-C(1) 117.5(2) [116.9(2)], C(21)-B(1)-C(1)
117.0(2) [117.4(2)]. [2]+: B(1)-C(21) 1.574(7), B(1)-C(11) 1.578(7), B(1)-C(1) 1.582(7), C(1)-C(2) 1.426(6); C(21)-B(1)-C(11) 121.5(4), C(21)-
B(1)-C(1) 116.1(4), C(11)-B(1)-C(1) 122.1(4), C(2)-C(1)-B(1) 135.8(4), C(1)-C(2)-N(1) 121.8(4).
similar structures and do not form unusually short contacts with
the triflate anions. Both molecules feature a trigonal planar boron
center as indicated by the sum of the Caryl-B-Caryl angles
(∑(C-B-C) ) 360°). All carbon-boron bond distances fall in
the 1.557(4)-1.580(4) Å range and are comparable to those
found in the starting material.66,67 Salt [2][OTf] crystallizes in
the P21/c space group with one molecule in the asymmetric unit
(Figure 1). As for [1]+, the boron center is trigonal planar
(∑(C-B-C) ) 359.7°) and does not interact with the triflate anion.
The boron center of the ortho-derivative [2]+ is separated from
the methyl carbon atom C(7) by only 3.175(7) Å. This short
separation indicates that the unsaturated boron center is sterically
encumbered. This conclusion is in agreement with (i) the large
B(1)-C(1)-C(2) angle (135.8(4)°) which substantially deviates
from the value of 126.95° observed in the starting borane
2-dimesitylboryl-N,N-dimethylaniline;65 (ii) the elongated C(1)-
C(2) bond of 1.426(6) Å. Despite these distortions, the boron-
carbon bonds which are in the 1.574(7)-1.582(7) Å range
remain within the norm.65
The structures of cations [1]+ and [2]+ have been optimized
using DFT methods (B3LYP, 6-31g(d) for all aromatic carbon,
boron, and nitrogen atoms, 6-31g for all other atoms) and
9
11980 J. AM. CHEM. SOC. VOL. 129, NO. 39, 2007

Complexation of Cyanide or Fluoride Ions in Water
A R T I C L E S
Figure 2. HOMO and LUMO of [1]+ and [2]+ (isodensity value ) 0.05, H-atoms omitted for clarity).
Scheme 2 a
a Conditions: (a) X ) F: TBAF in CHCl3,, 94% for 1-F and 88% for
2-F; X ) CN: NaCN, MeOH, 95% for 1-CN and 68% for 2-CN.
of the LUMOs in [1]+ and [2]+ are also substantially lower
than those calculated for the LUMO of Mes2BPh (-1.58 ev) at
the same level of theory and with the same solvation model.
This last comparison indicates that the positive charge of the
cationic boranes substantially increases their Lewis acid-
ity.58,63,70,71
In H2O/DMSO 60:40 vol, the UV spectra of [1]+ and [2]+
feature a broad absorption band at 320 nm (ꢀ320 ) 9104) for
[1]+ and 321 nm (ꢀ321 ) 9200) for [2]+ (Figure 3). Time-
dependent density functional theory calculations carried out
using the PCM/water solvation model suggest that the low-
energy edge of the absorption spectrum is in fact dominated by
the HOMO-LUMO transition. Because of the localization of
the frontier orbitals, the HOMO-LUMO transition can be
regarded as an intramolecular “ligand-to-element” charge-
transfer transition. In agreement with this view, we note that
both [1]+ and [2]+ give rise to a bright fluorescence whose
maximum wavelength depends on the polarity of the solvent
(Figure 3). Like other boron based charge-transfer chromo-
phores,49,72-74 both [1]+ and [2]+ show positive fluorosolvato-
chromism which suggests that the dipole of these molecules is
larger in the excited state than in the ground state.72,75,76
Figure 3. Absorption (in H2O/DMSO 60:40 vol) and emission spectra of
[1]+ (above) and [2]+ (below).
subjected to single point energy calculations using the polariz-
able continuum model68,69 (PCM) with water as a solvent.
Inspection of the frontier orbitals shows that the lowest
unoccupied molecular orbital (LUMO) bears an important
contribution from the boron-empty p-orbital while the highest
occupied molecular orbital (HOMO) is localized on the mesityl
rings (Figure 2). The energy of the LUMO in the ortho-isomer
[2]+ (-2.12 ev) is lower than in the para-isomer [1]+ (-2.02
eV) which suggests that [2]+ is a stronger Lewis acid. Since
the ammonium moiety is closer to the boron center in [2]+ than
in [1]+, the higher Lewis acidity of [2]+ can also be anticipated
on the basis of a simple inductive effect argument. The energies
(70) Welch, G. C.; Cabrera, L.; Hollink, E.; Stephan, D. W. Abstracts of Papers;
232nd ACS National Meeting, San Francisco, CA, Sept. 10-14, 2006;
American Chemical Society: Washington, DC, 2006.
(71) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006,
314, 1124.
(72) Stahl, R.; Lambert, C.; Kaiser, C.; Wortmann, R.; Jakober, R. Chem. Eur.
J. 2006, 12, 2358.
(73) Yamaguchi, S.; Shirasaka, T.; Tamao, K. Org. Lett. 2000, 2, 4129.
(68) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett. 1996,
255, 327.
(69) Tomasi, J.; Cammi, R.; Mennucci, B. Int. J. Quantum Chem. 1999, 75,
783.
9
J. AM. CHEM. SOC. VOL. 129, NO. 39, 2007 11981

A R T I C L E S
Hudnall and Gabba¨ı
Figure 4. Crystal structure of 1CN (left) and 2F (right) (50% ellipsoid, H-atoms omitted); selected bond lengths (Å) and angles (deg). 1CN: B(1)-C(10)
1.618(8), B(1)-C(1) 1.644(7), B(1)-C(21) 1.661(7), B(1)-C(11) 1.662(7), C(10)-B(1)-C(1) 105.0(3), C(10)-B(1)-C(21) 101.7(3); C(1)-B(1)-C(21)
116.8(4), C(10)-B(1)-C(11) 111.0(4), C(1)-B(1)-C(11) 106.6(3), C(21)-B(1)-C(11) 115.1(4). 2F: B(1)-F(1) 1.465(3), B(1)-C(21) 1.668(4), B(1)-
C(11) 1.671(3), B(1)-C(1) 1.684(4), F(1)-C(7) 2.964(3), C(1)-C(2) 1.411(4); F(1)-B(1)-C(21) 105.35(19), F(1)-B(1)-C(11) 105.18(18), C(21)-B(1)-
C(11) 116.3(2), F(1)-B(1)-C(1) 108.23(19), C(21)-B(1)-C(1) 108.10(18), C(11)-B(1)-C(1) 113.1(2), C(2)-C(1)-B(1) 132.4(2).
Figure 5. Changes in the UV-vis absorption spectra of a solution of (top) [1]OTf (3 mL, 5 × 10-5 M in H2O/DMSO 60:40 vol; HEPES 6 mM, pH 7) upon
addition of a NaCN solution (3 × 10-3 M in H2O); (bottom) [2]OTf (3 mL, 5 × 10-5 M in H2O/DMSO 60:40 vol; HEPES 6 mM, pH 7) upon addition of
a NaF solution (0.3 M in H2O).
Scheme 3
Cyanide and Fluoride Ion Complexation in Organic
Solvents. Salts [1]OTf and [2]OTf are converted into their
corresponding cyanide complexes 1CN and 2CN upon reaction
with NaCN in MeOH (Scheme 2). Analogously, the fluoride
complexes 1F and 2F form quantitatively when [1]OTf and [2]-
OTf are allowed to react with TBAF in CHCl3 (Scheme 2).
These cyanide and fluoride complexes are zwitterions related
to compounds such as [o-((C6F5)2(HO)B)C6H4(NHPh2)]77 and
[p-((n-Bu)3B)C6H4(NMe3)]78 which have been previously re-
ized by NMR spectroscopy and elemental analysis. In all cases,
ported. Zwitterions 1CN, 2CN, 1F, and 2F have been character-
the 11B NMR signal is in the expected range for a four-
coordinate boron center (-12.9 ppm for 1CN, -14.2 ppm for
2CN, 5.1 ppm for 1F, 7.3 ppm for 2F). For the fluoride complex
1F and 2F, the 19F NMR signal at -170 and -158 ppm,
respectively, is close to that observed in compounds featuring
triarylfluoroborate moieties.42,45,55,63 The 1H NMR spectrum of
1F and 1CN features four distinct resonances corresponding to
the hydrogen nuclei of the p-phenylene ring. The inequivalence
(74) Zhao, C. H.; Wakamiya, A.; Inukai, Y.; Yamaguchi, S. J. Am. Chem. Soc.
2006, 128, 15934.
(75) Cavalli, V.; Silva, D. C.; Machado, C.; Machado, V. G.; Soldi, V. J.
Fluoresc. 2006, 16, 77.
(76) Baraldi, I.; Brancolini, G.; Momicchioli, F.; Ponterini, G.; Vanossi, D. Chem.
Phys. Lett. 2003, 288, 309.
(77) Roesler, R.; Piers, W. E.; Parvez, M. J. Organomet. Chem. 2003, 680,
218.
(78) Lambert, C.; Stadler, S.; Bourhill, G.; Braeuchle, C. Angew. Chem., Int.
Ed. 1996, 35, 644.
11982 J. AM. CHEM. SOC. VOL. 129, NO. 39, 2007
9

Complexation of Cyanide or Fluoride Ions in Water
A R T I C L E S
Figure 6. Fluorescence of a 5 µM solution of [1]+ in H2O/MeOH 90:10
vol before (left) and after (right) the addition of 1 equiv of cyanide. The
cells are illuminated with a hand-held UV lamp.
of these signals indicates that rotation about the B-C bond
connecting the boron atom to the p-phenylene moiety is
restricted because of steric effects. The IR spectra of 1CN and
2CN also feature an intense cyanide stretching band at 2162
and 2167 cm-1, respectively.
The crystal structures of 1CN and 2F have been determined
(Table 1, Figure 4). These compounds crystallize in the
monoclinic space group P21/n for 1CN and P21 for 2F. In both
cases, the sum of the Caryl-B-Caryl angles (∑(C-B-C) ) 323.5°
for 1CN, 337.5° for 2F) indicates substantial pyramidalization
of the boron atom. In 1CN, the B(1)-C(10) bond connecting
the carbon atom of the cyanide anion to the boron center (1.618-
(8) Å) is comparable to those typically found in triarylcyanobo-
rate anions such as [Ph3BCN]- (1.65 Å).79 The same type of
comment can be made about the B(1)-F(1) bond length of 2F
(1.465(3) (4) Å) which is comparable to those found in
triarylfluoroborate anions (1.47 Å).42,63 The most important
feature in the structure of 2F concerns the B(1)-C(1)-C(2)
angle of 132.4(2)° which remains much larger than the ideal
value of 120°. This large angle is comparable to that present in
[2]+ (135.8(4)°) thus indicating that the pyramidalization of the
boron center does not allow for a great deal of steric relief.
The short distance of 2.964(3) Å separating the F(1) fluorine
atom from the C(7) methyl carbon atom provides additional
evidence for the steric constraints present in this derivative. It
can be anticipated that these steric constraints will be even more
acute in 2-CN, whose crystal structure has not been determined.
Figure 7. Top: Percent decrease of the absorbance of a solution of [1]-
[OTf] (5.2 × 10-5 M) in H2O/DMSO (60:40 vol) at 320 nm in the presence
of 80 equiv of various anions. Bottom: Percent decrease of the absorbance
of a solution of [2][OTf] (4.9 × 10-5 M) in H2O/DMSO (60:40 vol) at 321
nm in the presence of 80 equiv of various anions. For F-, only 20 equiv of
the anion was added to avoid precipitation.
data80 indicates that the cyanide binding constant of [1]+ is equal
to 3.9 ((0.1) × 108 M-1 at pH 7. These experiments show that
[1]+ has a very high affinity for cyanide which is bound
selectively over fluoride in aqueous solution. Since Mes3B does
not complex cyanide under these conditions, the elevated
cyanide binding constant of [1]+ probably results from favorable
Coulombic effects. These effects increase the Lewis acidity of
[1]+ by contributing electrostatically to the receptor-anion
interaction. Interestingly, however, the observed selectivity
indicates that these Coulombic effects are not sufficiently intense
to permit fluoride binding in aqueous solution. A factor
contributing to this selectivity is the high hydration enthalpy
and associated low basicity of fluoride (∆H°hyd ) -504 kJ/
mol, pKa(HF) ) 3.18), which this ammonium borane receptor is
unable to overcome. Because of its high cyanide binding
constant, [1]+ can be used for the naked-eye detection of cyanide
in the µM range. For example, the fluorescence of a 5 µM
solution of [1]+ in H2O/MeOH 90:10 vol is quenched in the
presence of 1 equiv of cyanide as shown in Figure 6.
Interestingly, the behavior of [2]+ appears to be the direct
opposite of that of [1]+. Indeed, the UV-vis absorption
spectrum of [2]+ in H2O/DMSO 95:5 vol is not affected in the
presence of cyanide while the addition of fluoride ions induces
a rapid quenching of the band at 321 nm followed by
precipitation of 2F (Scheme 3). When the same experiment was
repeated in buffered H2O/DMSO 60:40 vol (HEPES 6 mM, pH
7) precipitation does not occur until after the addition of 20
equiv of fluoride anions. Fitting of the resulting data affords a
fluoride binding constant of 910 ((50) M-1 (Figure 5). This
result indicates that [2]+ is a selective receptor for fluoride ions
Anion Complexation in Aqueous Solution. The anion
binding properties of [1]+ and [2]+ have been investigated in
aqueous solution using UV-vis spectroscopy. This technique
is particularly well adapted because binding of a nucleophile
to the boron center disrupts the frontier orbitals of the borane
leading to drastic changes of their photophysical properties.42
Remarkably, the UV-vis absorption spectrum of [1]+ in H2O/
DMSO 95:5 vol is not affected in the presence of fluoride ions
while addition of cyanide ions induces a rapid quenching of
the band at 320 nm followed by precipitation of 1CN (Scheme
3). In buffered H2O/DMSO 60:40 vol (HEPES 6 mM, pH 7)
precipitation does not occur (Figure 5). Because of the low
acidity of HCN, the cyanide binding constant of [1]+ can only
be determined if one considers the competing protonation of
the anion. To this end, we measured the pKa of HCN in H2O/
DMSO 60:40 vol by a potentiometric titration and found it equal
to 9.3 ((0.01). On the basis of this value, analysis of the titration
(79) Kuz’mina, L. G.; Struchkov, Y. T.; Lemenovsky, D. A.; Urazowsky, I. F.
(80) A full derivation of the equation used to fit the data is provided in the
Supporting Information.
J. Organomet. Chem. 1984, 277, 147.
9
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A R T I C L E S
Hudnall and Gabba¨ı
Figure 8. Top: Changes in the UV-vis absorption spectrum of a solution of [1][OTf] (3 mL, 5.25 × 10-5 M) in H2O/DMSO (60:40 vol) after addition
of a NaCN solution (left, 3.0 × 10-2 M in H2O) followed by the acidification with HCl to pH 0.5 (right). Bottom: Changes in the UV-vis absorption
spectrum of a solution of [2][OTf] (3 mL, 3.32 × 10-5 M) in H2O/DMSO (60:40 vol) after addition of a NaF solution (left, 3.0 × 10-1 M in H2O) followed
by additions of an aqueous AlCl3 solution (right, 0.01 M).
in aqueous solution whose fluoride binding constant is close to
that of [III]+.63 In this case, the selectivity most probably arises
from a combination of effects: the steric crowding of the boron
binding pocket probably hampers binding of the cyanide ion
which is larger than the fluoride ion; since cyanide binding to
[2]+ occurs in organic solvents, the observed lack of affinity of
[2]+ for cyanide in aqueous solution must also be the result of
the increased solvation and protonation of the anion which
makes its complexation thermodynamically unfavorable.
These experiments also allow us to draw some conclusions
on the respective anion affinity of [1]+ and [2]+. Since only
[2]+ binds fluoride, it can be concluded that the presence of
the trimethylammonium group ortho to the boron center leads
to an increase in the Lewis acidity of the derivative. This
conclusion is in agreement with the computational results which
suggest that the LUMO in [2]+ has a lower energy than in [1]+.
Thus, for fluoride, the anion affinity seems to be governed by
the Lewis acidity of the boron center. A different set of rules
seems to apply to the larger cyanide anion. For this anion, steric
effects appear to supersede the apparent higher Lewis acidity
of [2]+ and prevent coordination of the cyanide anion to the
sterically hindered boron center. Accordingly, the reduced steric
congestion of the boron center in [1]+ makes coordination of
the cyanide anion possible. It is also important to note that,
unlike [1]+ or [2]+, neutral boranes such as Mes3B do not
complex fluoride or cyanide ions in H2O/DMSO 95:5 vol or
H2O/DMSO 60:40 vol. This difference in reactivity indicates
that the charge of [1]+ or [2]+ plays a determining role in
increasing the Lewis acidity of these derivatives.
To test the anion binding selectivity of [1]+ and [2]+, their
absorption spectrum has been monitored upon the addition of
-
various anions including Cl-, Br-, I-, OAc-, NO3-, and HSO4
.
Despite the addition of an 80-fold excess of anions, the
absorbance of the band at 320 nm for [1]+ and 321 nm for [2]+
shows no or negligible quenching in the presence of these anions
thus confirming the selectivity of these receptors (Figure 7).
The absorption spectra of [1]+ and [2]+ are also not altered
when a neutral phosphate buffer is used indicating that binding
of H2PO4 does not occur. Finally, cyanide binding to [1]+ is
-
reversible and can be modulated by adjusting the pH. Thus,
when the pH of a solution containing 1CN is lowered from 7
to 0.5, the absorption at 320 nm increases, indicating liberation
of the cyanide anion and regeneration of free [1]+ (Figure 8).
Finally, addition of an aqueous solution of Al3+ to a solution
containing 2-F leads to complete regeneration of [2]+ showing
that fluoride binding is reversible (Figure 8).63
Conclusion
This work demonstrates that cationic boranes such as [1]+
serve as selective receptors for cyanide in water at neutral pH.
While many cyanide receptors are known, few are effective in
aqueous solution at neutral pH,33 conditions under which the
cyanide anion exists mostly in a protonated form. The unusual
cyanide binding properties of [1]+ can be assigned to favorable
Coulombic effects which increase the Lewis acidity of the boron
atom and strengthen the receptor-cyanide interaction. Another
important aspect of this work concerns the anion binding
selectivity of these cationic boranes which can be tuned using
9
11984 J. AM. CHEM. SOC. VOL. 129, NO. 39, 2007

Complexation of Cyanide or Fluoride Ions in Water
A R T I C L E S
0.948 mmol) in Et2O (15 mL) at room temperature. The mixture was
stirred overnight which resulted in the formation of a white solid. The
solid was collected by filtration, washed with Et2O (3 × 10 mL), and
dried in under vacuum to afford [2]OTf as a white powder (0.198 g,
39% yield): mp 219 °C. H NMR (399.9 MHz, CDCl3): δ 1.37 (bs,
3H, Mes-CH3), 1.84 (bs, 3H, Mes-CH3), 1.95 (bs, 3H, Mes-CH3), 2.20
both steric and electronic effects. Indeed, when the trimethy-
lammonium functionality is positioned ortho to the boron center
as in [2]+, the Lewis acidity of the ammonium borane is
increased, making fluoride binding possible. However, in this
case, the increased steric crowding of the boron center prevents
coordination of the larger cyanide anion.
1
(bs, 3H, Mes-CH3), 2.26 (bs, 6H, Mes-CH3), 3.55 (s, 9H, N-CH3),
3
6.66-6.88 (bm, 4H, Mes-CH), 7.43 (d, 2H, JH-H ) 4.4 Hz, phenyl-
Experimental Section
CH), 7.72 (m, 1H, 3JH-H ) 4.0 Hz, phenyl-CH), 8.08 (d, 1H, 3JH-H
)
General Considerations. 4-(Dimesitylboryl)-N,N-dimethylaniline
and 2-(dimesitylboryl)-N,N-dimethylaniline were synthesized by pub-
lished procedures.65-67 4-Bromo-N,N-dimethylaniline was purchased
from Oakwood Products Inc., methyl triflate, dimesitylboron fluoride,
and N,N-dimethylaniline were purchased from Aldrich, and n-Bu4NF‚
3H2O (TBAF) was purchased from Fluka. Et2O and THF were dried
by reflux over Na/K. Hexane was dried by passing through a column
charged with activated alumina. Air-sensitive compounds were handled
under a N2 atmosphere using standard Schlenk and glovebox techniques.
UV-vis and fluorescence spectra were recorded on a HP8453 and an
Aminco-Bowman 2 luminescence spectrophotometer, respectively. IR
spectra were obtained using a Bruker Tensor 37 infrared spectropho-
tometer. Elemental analyses were performed at Atlantic Microlab
(Norcross, GA). NMR spectra were recorded on a Varian Unity Inova
400 FT NMR (399.59 MHz for 1H, 375.99 MHz for 19F, 128.19 MHz
for 11B, 100.45 MHz for 13C) spectrometer at ambient temperature.
Chemical shifts δ are given in ppm and are referenced against external
Me4Si (1H, 13C), BF3‚Et2O (11B), and CFCl3 (19F). Potentiometric
titrations were carried out using a SympHony gel-filled combination
electrode from VWR International with a PHM290 pH stat controller
from Radiometer Analytical. Melting points were measured on samples
in sealed capillaries and are uncorrected.
4.4 Hz, phenyl-CH). 13C NMR (100.5 MHz, CDCl3): δ 21.18 (2C,
Mes-p-CH3), 22.55 (1C, Mes-o-CH3), 23.19 (2C, Mes-o-CH3), 24.74
(1C, Mes-o-CH3), 58.25 (N-CH3), 120.50 (q, 1JC-F ) 319.69 Hz, CF3),
122.63 (phenyl-CH), 129.42 (bs, Mes-CMe), 129.66 (bs, Mes-CMe),
129.91 (bs, Mes-CMe), 130.19 (bs, Mes-CMe), 130.49 (phenyl-CH),
133.53 (phenyl-CH), 135.52 (phenyl-CH), 140.01 (bs, B-CMes), 140.65
(bs, N-Cphenyl), 141.38 (bs, Mes-CMe), 141.71 (Mes-CMe), 141.96
(phenyl-CB), 143.13 (bs, B-CMes), 150.66 (4C, Mes-CH). 11B NMR
(128.2 MHz, CDCl3): δ +67.6. 19F NMR (375.9 MHz, CDCl3): δ
-74.4. UV-vis (CHCl3): λmax/nm (log ꢀ) 328 (4.08). Anal. Calcd for
C28H35BNSO3F3: C, 63.04; H, 6.61. Found: C, 62.82; H, 6.75.
Synthesis of 1F. Addition of a solution of TBAF (60 mg,
0.187 mmol) in chloroform (10 mL) to a solution of [1]OTf (100 mg,
0.187 mmol) in chloroform (10 mL) resulted in the formation of a
precipitate. After stirring for 30 min, the mixture was filtered. This
solid was washed with chloroform and dried under vacuum to afford
1F-0.5(CHCl3) as a white powder: 71 mg, 94% yield, mp 260 °C. 1H
NMR (400 MHz, acetonitrile-d3): δ 1.85 (s, 12H, Mes-CH3), 2.12 (s,
6H, Mes-CH3), 3.45 (s, 9H, N-CH3), 6.45 (s, 4H, Mes-CH), 7.02-
7.48 (bm, 4H, phenyl-CH). 13C NMR (100.5 MHz, acetonitrile-d3): δ
20.85 (Mes-CH3), 25.30 (Mes-CH3), 57.79 (N-CH3), 117.00 (Mes-o-
CMe), 123.41 (phenyl-CB), 129.11 (Mes-CH), 132.69 (phenyl-CH),
134.21(B-CMes), 135.66 (Mes-p-CMe), 142.10 (phenyl-CH), 144.07
(phenyl-CN). 11B NMR (128.2 MHz, acetonitrile-d3): δ 5.1. 19F NMR
Crystallography. Colorless single crystals of [1]OTf-0.5(toluene)
could be obtained by slow evaporation of a 1:1 toluene;CH2Cl2 solution.
Colorless single crystals could be obtained by vapor diffusion of hexane
into a concentrated dichloromethane solution [2]OTf. Single crystals
of 1CN-acetone were obtained by crystallization from a concentrated
acetone solution. Single crystals of 2F were obtained by vapor diffusion
of hexane into a concentrated acetone solution of 2F. The crystal-
lographic measurement of [1]OTf-0.5(toluene), [2]OTf, 1CN-acetone,
and 2F was performed using a Siemens SMART-CCD area detector
diffractometer, with a graphite-monochromated Mo KR radiation (λ )
0.71069 Å). A specimen of suitable size and quality was selected and
mounted onto glass fiber with apiezon grease. The structure was solved
by direct methods, which successfully located most of the non-hydrogen
atoms. Subsequent refinement on F2 using the SHELXTL/PC package
(version 5.1) allowed location of the remaining non-hydrogen atoms.
Synthesis of [1]OTf. Methyl triflate (0.53 g, 3.25 mmol) was added
to a solution of 4-(dimesitylboryl)-N,N-dimethylaniline (1.20 g,
3.25 mmol) in Et2O (30 mL) at room temperature. The mixture was
stirred overnight which resulted in the formation of a white solid. The
solid was collected by filtration, washed with Et2O (3 × 10 mL), and
dried in under vacuum to afford [1]OTf as a white powder (1.35 g,
78% yield). [1]OTf is hygroscopic and captures water when exposed
to air. Analysis of the crystal structure revealed inclusion of one
(375.9 MHz, acetonitrile-d3): δ -169.9 (bs). Anal. Calcd for C27.43H35.43
-
BNFCl1.29 (1F-0.43(CHCl3): C, 72.45; H, 7.85. Found: C, 72.44; H,
7.94.
Synthesis of 2F. 2F was prepared in a 88% yield by following the
procedure used for 1F and was recrystallized from acetonitrile to afford
2F-MeCN: mp 273 °C. 1H NMR (400 MHz, acetone-d6): δ 1.82 (bs,
12H, Mes-CH3), 2.11 (s, 6H, Mes-CH3), 3.81 (d, 9H, JF-H ) 2 Hz,
3
N-CH3), 6.49 (s, 4H, Mes-CH), 7.02 (t, 1H, JH-H ) 7.2 Hz phenyl-
3
CH), 7.12 (t, 1H, JH-H ) 7.6 Hz phenyl-CH), 7.62 (d, 1H,3JH-H
)
7.6 Hz phenyl-CH), 7.67 (d, 1H, 3JH-H ) 8.4 Hz phenyl-CH). 13C NMR
(100.5 MHz, acetone-d6): δ 20.88 (Mes-CH3), 25.56 (Mes-CH3), 57.89
(d, JF-C ) 12.6 Hz, N-CH3), 118.85 (phenyl-CH), 126.69 (phenyl-
CH), 128.39 (phenyl-CH), 129.63 (bs, 8C, Mes-o-CMe, Mes-CH),
132.96 (phenyl-CH), 142.02 (d, JF-C ) 6.9 Hz, B-CMes), 153.15
(phenyl-CNMe3). Phenyl-CB carbon peak was not observed. 11B NMR
(128.2 MHz, acetone-d6): δ 7.3.19F NMR (375.9 MHz, acetone-d6):
δ -157.9 (bs). Anal. Calcd for C29H38BN2F (2F-MeCN): C, 78.37;
H, 8.62. Found: C, 78.72; H, 8.52.
Synthesis of 1CN. Addition of a solution of NaCN (9.2 mg,
187 mmol) in methanol (10 mL) to a solution of [1]OTf (100 mg,
0.187 mmol) in methanol (10 mL) resulted in the formation of a
precipitate. After stirring for 30 min, the mixture was filtered. This
solid was washed with methanol (3 × 10 mL) and dried under vacuum
to afford 1CN-MeOH as a white powder (99 mg, 95% yield): mp
405 °C (dec). 1H NMR (399.9 MHz, acetone-d6): δ 1.90 (s, 12H, Mes-
CH3), 2.11 (s, 6H, Mes-CH3), 3.29 (s, 3H, MeOH), 3.71 (s, 9H,
N-CH3), 6.49 (s, 4H, Mes-CH), 7.18 (bs, 1H, phenyl-CH), 7.37 (bs,
1H, phenyl-CH), 7.69 (bs, 1, phenyl-CH), 8.29 (bs, 1H, phenyl-CH).
13C NMR (100.5 MHz, DMSO-d6): δ 20.44 (Mes-p-CH3), 25.30 (Mes-
o-CH3, 56.26 (N-CH3), 79.89 (B-CN), 116.98 (Mes-CH), 128.62
(phenyl-CH), 131.37 (Mes-o-CMe), 135.63 (bs, B-CMes), 136.53 (bs,
B-Cphenyl), 140.86 (phenyl-CH), 141.02 (N-Cphenyl), 143.69 (Mes-
1
molecule of toluene in the asymmetric unit: mp 210 °C. H NMR
(399.9 MHz, CDCl3): δ 1.93 (s, 12H, Mes-CH3), 2.29 (s, 6H, Mes-
CH3), 3.75 (s, 9H, N-CH3), 6.81 (s, 4H, Mes-CH), 7.64-7.72 (m,
4H, phenyl-CH). 13C NMR (100.5 MHz, CDCl3): δ 21.22, 23.45 (Mes-
CH3), 57.32(N-CH3), 118.64 (Mes-o-C), 128.53 (Mes-CH), 138.05
(phenyl-CH), 139.75 (Mes-p-C), 140.71 (phenyl-CH), 148.89 (B-CMes)
B-Cphenyl, N-Cphenyl, and CF3 carbon peaks were not observed. 11B NMR
(128.2 MHz, CDCl3): δ +74. 19F NMR (375.9 MHz, CDCl3): δ -78.3.
UV-vis (CHCl3): λmax/nm (log ꢀ) 335 (3.93). Anal. Calcd for C28H38-
BNSO4.5F3 ([1]OTf-1.5H2O): C, 61.71; H, 7.16. Found: C, 61.61; H,
6.72.
Synthesis of [2]OTf. Methyl triflate (0.15 mL, 0.948 mmol) was
added to a solution of 2-(dimesitylboryl)-N,N-dimethylaniline (0.35 g,
p-CMe). 11B NMR (128.2 MHz, CDCl3): δ -12.9. IR νCN
)
9
J. AM. CHEM. SOC. VOL. 129, NO. 39, 2007 11985

A R T I C L E S
Hudnall and Gabba¨ı
2162 cm-1. Anal. Calcd for C29H39BON2 (1CN-MeOH): C, 78.72;
H, 8.88. Found: C, 78.83; H, 8.47.
o-CMe). 11B NMR (128.2 MHz, CDCl3): δ -14.2. IR νCN
)
2167 cm-1. Anal. Calcd for C30H38BN3 (2CN-MeCN): C, 79.81; H,
Synthesis of 2CN. 2CN was prepared by following the procedure
used for 1CN. 2CN-MeCN was isolated in a 68% by recrystallization
from hot acetonitrile: mp 329 °C (dec). 1H NMR (399.9 MHz, DMSO-
d6): δ 1.77 (bs, 12H, Mes-CH3), 2.11 (s, 6H, Mes-CH3), 3.72 (s, 9H,
8.48. Found: C, 80.42; H, 8.49.
Acknowledgment. This work was supported by NSF (Grant
CHE-0646916), the Welch Foundation (Grant A-1423), the
Petroleum Research Funds (Grant 44832-AC4) and the U.S.
Army Medical Research Institute of Chemical Defense.
3
N-CH3), 6.53 (s, 4H, Mes-CH), 6.99 (t, 1H, JH-H ) 7.2 Hz phenyl-
3
3
CH), 7.22 (t, 1H, JH-H ) 8.4 Hz phenyl-CH), 7.46 (d, 1, JH-H
)
7.6 Hz phenyl-CH), 7.73 (d, 1H, 3JH-H ) 8.8 Hz phenyl-CH). 13C NMR
(100.5 MHz, DMSO-d6): δ 20.34 (Mes-p-CMe), 25.45 (Mes-o-CMe),
57.65 (N-CMe), 120.18 (phenyl-CH), 120.40 (B-CN), 126.66 (phenyl-
CH), 127.44 (phenyl-CH), 127.78 (phenyl-CH), 129.33 (Mes-CH),
130.06 (phenyl-CB), 132.40 (Mes-p-CMe), 137.25 (B-CMes), 140.16
(B-CMes), 141.96 (phenyl-CN), 142.08 (Mes-o-CMe), 152.05 (Mes-
Supporting Information Available: Experimental details and
X-ray crystallographic data for [1]OTf, [2]OTf, 1CN, and 2F
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA073793Z
9
11986 J. AM. CHEM. SOC. VOL. 129, NO. 39, 2007