Cyanide ion complexation by a cationic borane

Article abstract of DOI:10.1039/b715534d

While we have previously reported that [1-(Mes₂B)-8-(Me ₃NCH₂)-C₁₀H₆]⁺ ([2] ⁺) complexes fluoride ions to form [1-(Mes₂FB)-8-(Me ₃NCH₂)-C₁₀H₆] (2-F), we now show that this cationic borane also complexes cyanide to form [1-(Mes ₂(NC)B)-8-(Me₃NCH₂)-C₁₀H ₆] (2-CN). This reaction also occurs under biphasic conditions (H₂O-CHCl₃) and may serve to transport cyanide in organic phases. The zwitterionic cyanoborate 2-CN has been fully characterized and its crystal structure determined. UV-vis titration experiments carried out in THF indicate that [2]⁺ has a higher affinity for fluoride (K > 10 ⁸ M^-1) than cyanide (K = 8.0 (±0.5) × 10 ⁵ M^-1). Steric effects which impede cyanide binding to the sterically congested boron center of [2]⁺ are most likely at the origin of this selectivity. Finally, electrochemical studies indicate that [2]⁺ is significantly more electrophilic than its neutral precursor 1-(Mes₂B)-8-(Me₂NCH₂)-(C₁₀H ₆) (1). These studies also show that reduction of [2]⁺ is irreversible, possibly because of elimination of the NMe₃ moiety under reductive conditions. In fact, [2]OTf reacts with NaBH₄ to afford 1-(Mes₂B)-8-(CH₃)-(C₁₀H₆) (4) which has also been fully characterized. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715534d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Cyanide ion complexation by a cationic borane†‡
Ching-Wen Chiu and Franc¸ois P. Gabba¨ı*
Received 9th October 2007, Accepted 8th November 2007
First published as an Advance Article on the web 4th December 2007
DOI: 10.1039/b715534d
While we have previously reported that [1-(Mes2B)-8-(Me3NCH2)-C10H6]+ ([2]+) complexes fluoride ions
to form [1-(Mes2FB)-8-(Me3NCH2)-C10H6] (2-F), we now show that this cationic borane also complexes
cyanide to form [1-(Mes2(NC)B)-8-(Me3NCH2)-C10H6] (2-CN). This reaction also occurs under
biphasic conditions (H2O–CHCl3) and may serve to transport cyanide in organic phases. The
zwitterionic cyanoborate 2-CN has been fully characterized and its crystal structure determined. UV-vis
titration experiments carried out in THF indicate that [2]+ has a higher affinity for fluoride (K >
108 M−1) than cyanide (K = 8.0 ( 0.5) × 105 M−1). Steric effects which impede cyanide binding to the
sterically congested boron center of [2]+ are most likely at the origin of this selectivity. Finally,
electrochemical studies indicate that [2]+ is significantly more electrophilic than its neutral precursor
1-(Mes2B)-8-(Me2NCH2)-(C10H6) (1). These studies also show that reduction of [2]+ is irreversible,
possibly because of elimination of the NMe3 moiety under reductive conditions. In fact, [2]OTf reacts
with NaBH4 to afford 1-(Mes2B)-8-(CH3)-(C10H6) (4) which has also been fully characterized.
Introduction
Results and discussion
In the past few years, a great deal of attention has been devoted
to the use of cationic boranes1 for the molecular recognition of
fluoride anions.2–10 Because of favorable Coulombic effects, such
boranes are sometimes able to overcome the hydration energy
of fluoride to form the corresponding zwitterionic fluoroborate
derivative in aqueous solutions.3,4 These reactions are quite useful
because they lay the basis for the development of new fluoride
ion probes that can operate in aqueous solutions. In a recent
development, our group has demonstrated that a similar approach
could be used for the sensing of cyanide in water at neutral pH.3
For example, the ammonium borane [p-(Mes2B)C6H4(NMe3)]+
captures cyanide ions in aqueous solutions to form a very stable
zwitterionic ammonium cyanoborate complex.3
Although both 1 and [2]OTf have been previously characterized,
the crystal structure of 1 had not been determined. This compound
crystallizes in the monoclinic space group P21/c with four
molecules in the unit cell (Fig. 1, Table 1). The carbon–boron
bond lengths of 1 are unremarkable and the boron adopts a
ꢀ
trigonal planar geometry ( angles = 359.0◦). The methylene carbon
atom C(01) of the CH2NMe2 moiety is separated from the boron
˚
atom B(1) by only 3.121 A indicating significant steric congestion.
As a result, the B(1)–C(1)–C(9) (128.7(2)◦) and C(01)–C(8)–C(9)
(123.7(2)◦) angles substantially deviate from the ideal value of
120◦. These distortions are comparable to those observed in the
structure of [2]OTf.2
In a previous communication, we reported the synthesis of
the cationic borane [1-(Mes2B)-8-(Me3NCH2)-C10H6]+ ([2]+) by
reaction of 1-(Mes2B)-8-(Me2NCH2)-(C10H6) (1) with MeOTf.
We also reported that this cationic borane has a high affinity
for fluoride whose complexation results in the formation of [1-
(Mes2(F)B)-8-(Me3NCH2)-C10H6] (2-F).2 In this paper, we now
report on the cyanide binding affinity of [2]+. We also report on
the unusual electrochemical properties of [2]+ which can be used
to monitor cyanide binding. Ultimately, these studies may result
in the development of new methods for sensing or complexing the
toxic cyanide anion whose use for harmful purposes is a growing
concern.
Fig. 1 ORTEP plot of the molecular structure of 1 with thermal
ellipsoids set at 50% probability level. Hydrogen ato◦ms are omitted for
˚
clarity. Selective bond distances [A] and bond angles [ ]: B–C(1) 1.570(3),
Department of Chemistry, Texas A&M University, College Station, Texas,
77843, USA. E-mail: francois@tamu.edu; Fax: +1 979 845 4719; Tel: +1
979 862 2070
† This paper is dedicated to Professor Ken Wade on the occasion of his
75th birthday.
B–C(11) 1.576(3), B–C(21) 1.587(3), C(1)–C(2) 1.385(3), C(1)–C(9)
1.450(3), C(7)–C(8) 1.379(3), C(8)–C(9) 1.428(3), C(8)–C(01) 1.516(3);
C(1)–B–C(11) 122.0(2), C(1)–B–C(21) 116.5(2), C(11)–B–C(21) 120.5(2),
C(2)–C(1)–C(9) 117.3(2), C(2)–C(1)–B 111.9(2), C(9)–C(1)–B 128.7(2),
C(7)–C(8)–C(9) 119.4(2), C(7)–C(8)–C(01) 116.9(2), C(9)–C(8)–C(01)
123.7(2).
‡ CCDC reference numbers 663259 and 663260. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b715534d
814 | Dalton Trans., 2008, 814–817
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Table 1 Crystal data and structure refinement for 1 and 2-CN
with the reduction peak potential of −2.48 V recorded for 1
indicate that [2]+ is substantially more electrophilic than 1. This
observation can be easily reconciled by considering the fact that
[2]+ is a cation which is therefore expected to be more electrophilic.
The irreversibility of the reduction of [2]+ suggested that the
resulting radical anion decomposes possibly through loss of NMe3.
Interestingly, reduction of [2]OTf with NaBH4 in methanol
produces 1-(Mes2B)-8-(CH3)–(C10H6) (3) (Scheme 1). This reac-
tion most likely proceeds by nucleophilic displacement of the
NMe3 group by a hydride. The spectroscopic features of 3 are
Crystal data
1
2-CN
Formula
Mr
C31H36BN
433.42
C33H39BN2
474.47
Crystal size/mm
Crystal system
Space group
0.42 × 0.27 × 0.16
Monoclinic
P2(1)/c
15.8202(13)
10.2595(8)
17.3207(14)
90.00
116.620(1)
90.00
2513.5(3)
4
0.21 × 0.06 × 0.03
Monoclinic
P2(1)/c
16.009(3)
10.329(2)
17.540(4)
90.00
111.12(3)
90.00
2705.5(9)
4
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
V/A
Z
1
similar to those of 1. The H NMR spectrum of 3 exhibits six
distinct resonances that correspond to the aromatic CH groups
of the unsymmetrically substituted naphthalene backbone. In
addition, the methyl groups of the mesityl substituents give rise
to broadened multiple resonances indicating that 3 has a highly
congested structure. The resonance of the methyl group bound to
the 8-position appears at 2.29 ppm as a broad singlet. Finally, the
3
˚
qcalc/g cm−3
l(Mo-Ka)/mm−1
F(000)
1.145
0.064
936
1.165
0.066
1024
Data collection
T/K
Scan mode
hkl range
presence of a trigonal planar boron center is confirmed by the 11
NMR signal detected at 70 ppm.
B
110
x
110
x
−19 → 15, −12 → 11, −19 → 19, −11 → 12,
−20 → 20
12734
−14 → 20
13047
Measured refl.
Unique refl., [Rint
]
4544, [0.0660]
4757, [0.0763]
4757
Refl. used for refinement 4544
Refined parameters
295
326
R1,a wR2 [all data]
0.0815, 0.1399
0.417/−0.358
0.1393, 0.1961
0.444/−0.594
b
−3
˚
qfin (max/min)/e A
ꢀ
ꢀ
ꢀ
ꢀ
Scheme 1
a R1 = ꢀFo| − |Fcꢀ/ |F|. b wR2 = {[ w(Fo − Fc2)2]/[ w(Fo2)2]}
;
2
1/2
w = 1/[r2(Fo2) + (ap)2 + bp]; p = (Fo + 2Fc2)/3; a = 0.0 (1), 0.49 (2-CN);
b = 3.8 (1), 12.73 (2-CN).
2
Encouraged by the high affinity that [2]+ displays for fluoride,2
we decided to investigate its reaction with cyanide ions. This
cationic borane is swiftly converted into zwitterionic 2-CN when
treated with NaCN in MeOH (Scheme 1). The 11B NMR signal of
2-CN appears at −12.2 ppm as expected for a tetrahedral boron
atom. Formation of this cyanide complex noticeably affects the 1H
NMR resonances of the diastereotopic methylene hydrogen atom.
The two resonances are shifted downfield and appear at 4.66 and
5.46 ppm (vs. 3.69 and 4.81 ppm in [2]+). In the IR spectrum, the
cyanide stretching band appears at 2163 cm−1. Formation of 2-
CN can also be monitored using differential pulsed voltammetry.
Indeed, addition of cyanide ions to a solution of [2]OTf in THF
with nBu4NPF6 as a supporting electrolyte leads to a progressive
decrease of the peak corresponding to the reduction of the boron
center (Fig. 3). This decrease is caused by binding of the cyanide
In an attempt to better understand the chemical properties of
1 and [2]OTf, we have studied their electrochemistry (Fig. 2). The
cyclic voltammogram of 1 in THF shows a reversible reduction
wave at E1/2 −2.53 V (vs. Fc/Fc+) which is followed by undefined
irreversible processes. The redox behavior of this derivative is
similar to that of other triarylboranes which typically display a
single reversible reduction wave corresponding to the formation
of a radical anion.11–20 The reduction potential of 1 is slightly
more negative than that reported for dimesityl-1-naphthylborane
(−2.41 V, vs. Fc/Fc+) in agreement with the electron releasing
properties of the CH2NMe2 substituent present at the 8-position.21
Interestingly, the cyclic voltammogram of [2]+ in THF only shows
an irreversible wave at Epeak −2.18 V (vs. Fc/Fc+). A comparison
Fig. 2 Cyclic voltammograms of 1 (top) and [2]+ (bottom) in THF with
a glassy-carbon working electrode (0.1 M nBu4NPF6). Scan rates: m = 300
mV s−1 for 1 and 100 mVs−1 for [2]+.
Fig. 3 Changes in the differential pulsed voltammogram of [2]OTf
(0.001 M) observed upon the addition of nBu4NCN (0.087 M in CH2Cl2)
to a THF solution.
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ion to the boron center which can no longer be reduced because of
its coordinative saturation. Similar observations have been made
upon addition of fluoride ions to bidentate boranes.22
The crystal structure of 2-CN has been determined (Table 1,
Fig. 4). This compound crystallizes in the monoclinic space
1. UV-vis titrations carried out in THF indicate that the cyanide
binding constant of 1 is at least three orders of magnitude lower
than that of [2]+ thus suggesting that favorable Coulombic effects
are essential to the cyanide binding ability of [2]+. Although cation
[2]+ does not react with cyanide in water, it is able to transport
it into organic phases. For example, shaking a biphasic mixture
consisting of NaCN in D2O (0.5 ml, 7.6 × 10−2 M) and [2]OTf
in CDCl3 (0.5 ml, 7.21 × 10−2M) results in a 32% conversion of
[2]OTf into 2-CN after a few minutes.
group P21/c with four molecules in the unit cell. The sum of
ꢀ
the Caryl–B–Caryl angles (
= 343.5◦) indicates substantial
(C–B–C)
pyramidalization of the boron atom which is more acute than
ꢀ
that observed in 2-F (
= 340.7◦).2 The B(1)–C(30) bond
(C–B–C)
connecting the carbon atom of the cyanide anion to the boron
center (1.527(6) A) is significantly shorter than those typically
found in triarylcyanoborate anions such as [Ph3BCN] (1.65 A).
It is also important to note that the B(1)–C(1) bond length of
˚
−
23
˚
˚
1.791(7) A is abnormally long, especially when compared to the
˚
˚
B(1)–C(1) bond lengths of 1.678(5) A measured in 2-F and 1.573 A
measured in [2]OTf. The B(1)–C(1)–C(9) angle of 132.8(4)◦ in
2-CN is also larger than that present in 2-F (130.6(3)◦). These
unusual metrical parameters and, in particular, the length of the
B(1)–C(1) bond indicates that the steric congestions present in 2-
CN are more important than in 2-F. The larger size of the cyanide
anion is certainly responsible for this effect.
Fig. 5 Left: Changes in the UV-vis absorption spectra of a solution of
[2]+ (5.16 × 10−5M in THF) upon the addition of a NaCN solution (9.14
× 10−3M in methanol); Right: resulting fluoride binding isotherm.
Conclusions
The results presented in this paper provide additional evidence for
the suitability of cationic boranes for the complexation of cyanide
ions. The relatively high cyanide binding affinity of [2]+ can be
assigned to favorable Coulombic effects which increase the anion
affinity of the receptor. This conclusion is also in agreement with
the redox properties of [2]+ which is more easily reduced than its
neutral precursor 1. Last but not least, these results allow us to
establish that [2]+ has a higher affinity for fluoride than cyanide.
Keeping in mind that cyanide is much more basic than fluoride,
the observed selectivity arises, at least in part, from steric effects
which impede cyanide binding to the boron center of [2]+.
Fig.
4
ORTEP plot of the molecular structure of 2-CN with
thermal ellipsoids set at 50% probability level. Hydrogen atoms are
omitted for clarity. Selective bond distances [A] and bond angles
˚
[◦]: B(1)–C(1) 1.791(7), B(1)–C(11) 1.633(6), B(1)–C(21) 1.726(7),
B(1)–C(30) 1.527(6), N(2)–C(30) 1.074(5), C(1)–C(2) 1.354(6), C(1)–C(9)
1.457(6), C(7)–C(8) 1.328(6), C(8)–C(9) 1.405(6), C(8)–C(01) 1.527(6);
B(1)–C(30)–N(2) 117.0(5), C(1)–B(1)–C(11) 113.1(4), C(1)–B(1)–C(21)
112.9(4), C(11)–B(1)–C(21) 117.5(4), C(2)–C(1)–C(9) 110.2(4),
C(2)–C(1)–B(1) 116.3(4), C(9)–C(1)–B(1) 132.8(4), C(7)–C(8)–C(9)
117.0(4), C(7)–C(8)–C(01) 117.8(4), C(9)–C(8)–C(01) 125.0(4).
Experimental
Materials and methods
[2]OTf was synthesized according to the published procedure. UV-
vis spectra were recorded on HP8453. IR spectra were obtained
using a Bruker Tensor 37 infrared spectrophotometer. NMR
spectra were recorded on Varian Unity Inova 400 FT NMR
A UV titration experiment carried out by monitoring the
absorption of [2]+ at kmax 352 nm (e = 11435 dm3 mol−1 cm−1)
shows that the cyanide binding constant of [2]+ in THF is equal
to 8.0 ( 0.5) × 105 M−1 (Fig. 5). Under the same conditions, the
fluoride binding constant of [2]+ cannot be accurately measured
and exceeds 108 M−1. These results indicate that [2]+ shows a
greater selectivity for fluoride than cyanide which is surprising
1
(399.63 MHz for H, 128.2 MHz for 11B, and 100.50 MHz for
13C) spectrometer. Chemical shifts d are given in ppm and are
referenced against external Me4Si (1H, 13C), and BF3·OEt2 (11B).
Crystallography
since cyanide (pKa = 9.3) is more basic than fluoride (pKa
=
3.18). Presumably, the larger size of the cyanide anion impedes
binding to the sterically crowded boron center of [2]+. Similar
effects have been observed in the case of [o-Mes2B-C6H4-NMe3]+
which binds fluoride but not cyanide in aqueous media.3 In an
effort to assess the influence of the cationic nature of [2]+ on its
cyanide binding ability, we have also studied the cyanide affinity of
Single crystals of 1 were obtained from evaporation of a CH2Cl2–
hexane solution. Colorless crystals of 2-CN were obtained from
slow evaporation of the methanol solution. The crystallographic
measurement of 1 and 2-CN were performed using a Siemens
SMART-CCD area detector diffractometer with a graphite-
˚
monochromated Mo-Ka radiation (k = 0.71073 A). The crystal
816 | Dalton Trans., 2008, 814–817
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was mounted onto a nylon loop with Apiezon grease. The structure
was solved by direct methods, which successfully located most
of the non-hydrogen atoms. Subsequent refinement on F2 with
the SHELXTL/PC package (version 5.1) allowed location of the
remaining non-hydrogen atoms.
isotherm which indicated that the fluoride binding constant of [2]+
is greater than 1 × 108 M−1.
UV-vis titration in THF with cyanide
A THF solution of [2]OTf (5.16 × 10−5M, 3 ml) was titrated with
incremental amounts of cyanide by addition of NaCN in methanol
(9.14 × 10−3M). The absorbance was monitored at kmax = 352 nm
(e = 11435 dm3 mol−1 cm−1 for [2]+, e = 394 dm3 mol−1 cm−1 for 2-
CN). The experimental data obtained were fitted to a 1 : 1 binding
isotherm which indicated that the cyanide binding constant of
[2]+is 8.0 ( 0.5) × 105 M−1.
Synthesis of 3
Excess NaBH4 was added into a methanol solution of the [2]OTf
salt (100 mg, 0.17 mmol) at room temperature and stirred for
10 min. After reaction, the solvent was removed under reduced
pressure and the white solid residue was extracted with 10 ml of
hexane three times. By removing the hexane, compound 3 was
1
Acknowledgements
obtained as a light yellow solid (24 mg, yield = 36%). H NMR
(CDCl3, 400 MHz, 213 K): d 0.92 (s, 3H), 1.89 (s, 3H), 2.03 (s,
This work was supported by NSF (CHE-0646916), the Welch
Foundation (A-1423), the Petroleum Research Funds (Grant
44832-AC4) and the US Army Medical Research Institute of
Chemical Defense.
3H), 2.17 (s, 3H), 2.31 (bs, 9H), 6.47 (s, 1H), 6.75 (s, 1H), 6.82 (s,
1H), 6.90 (s, 1H), 7.29 (d, 1H, 3JH–H = 7.2 Hz), 7.35 (t, 1H, 3JH–H
=
8 Hz), 7.40 (t, 1H, 3JH–H = 7.6 Hz), 7.45 (d, 1H, 3JH–H = 7.2 Hz),
7.74 (d, 1H, 3JH–H = 7.6 Hz), 7.92 (d, 1H, 3JH–H = 8 Hz). 13C NMR
(CDCl3, 100.5 MHz, 213K) d 21.2, 21.3, 22.7, 23.3, 23.6, 24.3,
25.0, 124.9, 125.3, 127.0, 127.8, 128.4, 128.7, 128.8, 128.9, 132.7,
133.0, 133.6, 135.8, 135.9, 138.6, 139.0, 140.7, 141.0. 11B NMR
(CDCl3, 128.2 MHz, 213K): d 69.6.
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[2]OTf (100 mg, 0.17 mmol) was mixed with eight equivalents
of NaCN (66 mg, 1.35 mmol) in methanol at room temperature.
After stirring for 1 h, the solvent was removed under reduced
pressure and the residual white solid was extracted with diethyl
ether. The combined ether solution was dried to give 2-CN as a
white powder (58 mg, yield 72%). Single crystals of 2-CN were
obtained from evaporation of a methanol solution. 1H NMR
(CDCl3, 399.59 MHz, 263K): d 1.21 (s, 3H, Mes–CH3), 1.79 (s,
3H, Mes–CH3), 2.15 (s, 3H, Mes–CH3), 2.19 (s, 3H, Mes–CH3),
2.20 (s, 3H, Mes–CH3), 2.24 (s, 9H, NMe3), 2.46 (s, 3H, Mes–
2
CH3), 4.66 (d, 1H, JH–H = 12.8 Hz, nap-CH2–NMe3+), 5.46 (d,
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6.61 (s, 1H, Mes–CH), 6.90 (s, 1H, Mes–CH), 7.16–7.23 (m, 2H,
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A THF solution of [2]OTf (4.95 × 10−5M, 3 ml) was titrated with
incremental amounts of fluoride by addition of TBAF in THF
(4.46 × 10−3M). The absorbance was monitored at kmax = 352 nm
(e = 11435 dm3 mol−1 cm−1 for [2]+, e = 135 dm3 mol−1 cm−1 for
2-F). The experimental data obtained were fitted to a 1 : 1 binding
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