Azide ion recognition in water-CHCl3 using a chelating phosphonium borane as a receptor

Article abstract of DOI:10.1039/b905232a

In H₂O-CHCl₃, the phosphonium borane [ortho-(Mes ₂B)C₆H₄(PMePh₂)]⁺ selectively complexes azide anions to afford ortho-(Mes₂(N ₃)B)C₆H₄

Full text of DOI:10.1039/b905232a

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Azide ion recognition in water–CHCl3 using a chelating phosphonium
borane as a receptorw
Youngmin Kim,a Todd W. Hudnall,a Ghenwa Bouhadir,b Didier Bourissou*b
a
and Franc¸ ois P. Gabbaı*
¨
Received (in Berkeley, CA, USA) 16th March 2009, Accepted 6th May 2009
First published as an Advance Article on the web 27th May 2009
DOI: 10.1039/b905232a
In
H2O–CHCl3, the
phosphonium
borane
[ortho-
phosphazide 2 (Chart 1) thus suggesting that the boron and
phosphorus atoms of such compounds are well positioned to
interact with the terminal nitrogen atom of an organic azide.8 In
this communication, we now show that [1]+ can be used for the
complexation of N3ꢀ. We also report that this complexation is
selective in biphasic organic solvent–water mixtures.
(Mes2B)C6H4(PMePh2)]+ selectively complexes azide anions
to afford ortho-(Mes2(N3)B)C6H4(PMePh2) in which the
boron-bound azide anion is stabilized by an interaction with
the adjacent phosphorus atom.
Like cyanide, azide (N3ꢀ) is a toxic anion which binds to and
deactivates cytochrome oxidase enzymes. This anion is also
found in explosives such as NaN3 or Pb(N3)2 used in airbags
and detonators, respectively. Because of its toxicity, as well as
its use in explosive devices, the development of simple methods
In the initial phase of this work, we decided to carefully
investigate the behavior of [1]+ in aqueous solution. Having
previously noted that [1]+ is not stable at neutral pH,7 its
behavior at acidic pH was investigated by UV-vis spectro-
scopy. At pH 2.3, the characteristic absorption band of [1]+ at
330 nm can be readily observed indicating that the boron
center retains sp2 hybridization.9 Upon elevation of the pH to
3.5, the intensity of this band decreases before precipitation of
a new product occurs. Acidification of the solution leads to
the rapid disappearance of this new product and triggers a
resurrection of the original absorption spectrum of [1]+. These
observations suggest that the new product is the hydroxide
adduct ortho-(Mes2(HO)B)C6H4(PMePh2) (1–OH). Its forma-
tion at such a low pH suggests that the pKR+ of [1]+ (eqn (1))
which cannot be accurately determined because of the precipi-
tation of 1–OH, is in the 3–4 range. This value is much lower
than that measured for [para-(Mes2B)C6H4(PMePh2)]+
(pKR+ = 7.3)6 thus confirming that [1]+ is a stronger Lewis
acid which cannot be used in neutral water.
for the molecular recognition of N3 is of interest.1 Earlier
ꢀ
contributions have shown that this anion can be selectively
captured by organic2 or bimetallicꢀ3 hosts to form complexes in
which each extremities of the N3 anion forms hydrogen or
coordination bonds with the binding sites of the host. More
recently, the chelation of this anion by group 13 bidentate
Lewis acids has been reported4 leading us to question whether
water compatible polyfunctional boranes could be used for the
molecular recognition of this anion in aqueous environments.
As part of our interest in the chemistry of cationic
boranes,5–7 we have recently reported the synthesis of [1]+
(as an Iꢀ salt) and investigated its affinity for halide anions.7 In
the context of these studies, we found that [1]+ complexes Fꢀ
in MeOH to form the zwitterionic fluoroborate 1–F in which
the Fꢀ anion is asymmetrically chelated by the B/P+ bidentate
Lewis acid (Chart 1). In parallel, we have also shown that
PhN3 reacts with ortho-(Mes2B)C6H4(P(i-Pr)2) to afford the
KRþ
Ð
½1ꢁþ þ2H2O
1ꢀOH þ H3Oþ
ð1Þ
In an effort to overcome this limitation while still being able
to use [1]+ for the capture of aqueous anions, we decided to
consider the use of a biphasic solvent mixture. To this end, we
investigated the behavior of [1]+ (0.0125 M) in D2O–CDCl3
(1 : 3 vol.) by monitoring the 31P and/or 1H NMR spectrum of
the organic phase. After 2.5 h, analysis of the organic phase
indicated the presence of [1]+ as the major species (91%,
d(31P) 23.9 ppm) and 1–OH as a minor species (9%, d(31P)
20.8 ppm) (Scheme 1). In agreement with this view,
Chart 1
a 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
b University of Toulouse, UPS, LHFA, 118 route de Narbonne,
F-31062 Toulouse, France. CNRS, LHFA UMR 5069, F-31062
Toulouse, France
w Electronic supplementary information (ESI) available: Spectroscopic
and computational details. CCDC 724078. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b905232a
Scheme 1
ꢂc
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acidification of the aqueous phase (by addition of HBr, 0.07 M)
leads to complete disappearance of 1–OH leaving [1]+ as the
only remaining species. The 11B NMR resonance of 1–OH at
0.2 ppm is close to the value of 0.3 ppm observed for para-
(Mes2(HO)B)C6H4(PMePh2).6 Formation of 1–OH is quanti-
tative when 1.2 eq. of NaOH is added to the aqueous phase.z
Encouraged by these observations, we set out to investigate
the ability of [1]+ to capture anions in D2O–CDCl3.z When
2 eq. of the sodium salts of Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, H2PO4ꢀ were
used, no new species (except for 1–OH) could be detected in
the organic phase indicating the lack of affinity of [1]+ for
these anions. In the case of Fꢀ (2 eq.), analysis of the organic
phase after 20 mins indicated the formation of 1–F (d(31P)
28.3 ppm, JP–F = 24.0 Hz ) in 7% yield (as well as 1–OH in
10% yield). The low yield observed for 1–F suggests that its
formation is disfavored by the efficient solvation of Fꢀ in the
aqueous phase. By contrast, a rapid reaction was observed
with N3ꢀ (2 eq.) leading to the formation of 1–N3 in 66% yield
(as well as 1–OH in 8% yield) after 20 min as indicated by the
detection of a new 31P NMR resonance at 25.5 ppm
(Scheme 1). The rapid formation of 1–N3 not only corroborates
Fig. 2 NBO contour plot showing the lp(N) - s*(P–C) interaction of
1–N3.
cates that the N(1) atom occupies an axial coordination site
directly opposite to a phenyl ring. In agreement with this view,
we note: a slight elongation of the P(1)–C(14) bond (1.819(3) A)
when compared to the P(1)–C(8) bond (1.793(5) A) and a
noticeable increase in the sum of the C(1)–P(1)–C(7),
C(1)–P(1)–C(8), and C(7)–P(1)–C(8) bond angles a on going
from [1]+ to 1–N3 (a = 335.21 in [1]+ vs. 341.71 in 1–N3).7
Altogether, these structural results support the presence of a
B–N-P interaction reminiscent of the B–F-P bridging motif
observed in 1–F.7 These results show that the phosphonium is
acting as a Lewis acid,13 a property that has only rarely been
exploited in the context of anion recognition.7
ꢀ 10
the lipophilic behavior of N3
,
but also suggests a strong
interaction of the anion with the Lewis acidic borane. In order
to shed light on this point, 1–N3 was isolated by reaction
of [1]I with NaN3 in methanol and fully characterized.z The
diagnostic antisymmetric stretching vibration of the azide
group11 was observed at 2109 cmꢀ1 in the IR spectrum.
Examination of the 13C NMR spectrum indicates that the
phosphorus bound phenyl groups are either equivalent or in
rapid exchange, the latter being supported by the results of the
X-ray analysis.
The DFT optimized structure of 1–N3 is in excellent
agreement with that experimentally determined. In particular,
the P(1)–N(1) separation of 2.799 A is almost identical to
that observed in the crystal (2.790(2) A). A natural bond
orbital (NBO) analysis performed at the optimized geometry
indicates that the P(1)–N(1) bond is best described as
donor–acceptor interactions involving a nitrogen lone-pair
as a donor and the phosphorus–carbon s*-orbital as the
acceptor (Fig. 2). Moreover, zeroing the Kohn–Sham matrix
elements corresponding to the lp(N) - s*(P–C) interaction
leads to an increase of the total energy of the molecule
by a deletion energy of 5.8 kcal molꢀ1. This deletion
energy, which provides a measure of the strength of the
lp(N) - s*(P–C) interaction, is comparable to that of a strong
hydrogen-bond,14 and very similar to that computed for the
lp(F) - s*(P–C) interaction present in 1–F (5.0 kcal molꢀ1).7
It is important to point out that 1–N3 is further stabilized by
favourable electrostatic effects.
Examination of the crystal structure of 1–N3 confirmed
ꢀ
the presence of an N3 anion bound to the boron center
via a B(1)–N(1) bond of 1.623(4) A (Fig. 1) which is slightly
longer than that measured in the [(C6H5)3B(N3)]ꢀ complex
(1.601(2) A).11y The resulting B(1)–N(1)–N(2) (129.6(2)1)
angle is close to the ideal value of 1201 while the azide remains
essentially linear (N(1)–N(2)–N(3) = 175.1(3)1). Interestingly,
ꢀ
the N(1) atom of the N3 anion is also engaged in a weak
interaction with the P(1) atom. The N(1)–P(1) distance
(2.790(2) A) is well within the sum of the van der Waals radii
of the two elements (ca. 3.35 A).12 Other conspicuous features
include N(2)–N(1)–P(1) angle of 129.6(2)1 which is close to
1201; (ii) the N(1)–P(1)–C(14) angle of 177.9(1)1 which indi-
In summary, the results reported here show thatꢀcationic
boranes can be used for the capture of aqueous N3 anions
under biphasic conditions. The ability of [1]+ to transport
this anion in organic phases originates from favourable
Coulombic effects which stabilize the B–N3 linkage against
dissociation. Last but not least, the high selectivity observed in
these phase-transfer reactions most likely results from the
lipophilic character of the azide anion as well as from its
ability to interact with both the boron and phosphorus Lewis
acidic sites of the receptor.
This work was supported by the National Science Founda-
tion (CHE-0646916), the Welch Foundation (A-1423), the
Petroleum Research Funds (Grant 44832-AC4) and the US
Army Medical Research Institute of Chemical Defense as well
as by the Centre National de la Recherche Scientifique, the
´
Universite Paul Sabatier, and the Agence Nationale de la
Fig. 1 Structure of 1–N3 (50% ellipsoid, H-atoms omitted for
clarity). Pertinent metrical parameters are provided in the text.
Recherche (program BILI).
ꢂc
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Notes and references
z Anion complexation under biphasic conditions: These experiments
were carried out by shaking a biphasic mixture consisting of [1]I in
CDCl3 (0.0125 M, 0.6 mL) and the sodium salt of the anion in D2O
(0.15 M, 0.1 mL) in a 5 mm NMR tube. The reactions were monitored
by 31P and 1H NMR spectroscopy. 1–N3: [1]I (0.030 g, 0.046 mmol)
was dissolved in MeOH (10 mL) and treated with NaN3 (6 mg,
0.092 mmol). After 30 min, the resulting precipitate was isolated by
filtration and washed with MeOH (15.1 mg, 53% yield). 1H NMR
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(499.88 MHz, CDCl3): 1.76 (bs, 12H, Mes-CH3), 2.11 (d, JH–P
=
13 Hz, 3H, P-CH3), 2.19 (s, 6H, Mes-CH3), 6.57–6.79 (bm, 4H,
Mes-CH), 6.92 (m, 1H, Ar-CH), 7.05 (bm, 1H, Ar-CH), 7.27
(bm, 5H, Ar-CH), 7.45 (bm, 1H, Ar-CH), 7.59 (bm, 3H, Ar-CH),
7.59–7.62 (bm, 2H, Ar-CH), 7.69 (bt, 1H, Ar-CH). 13C NMR
(100.5 MHz, CDCl3): d 14.60 (d, JC–P = 65.3 Hz, P-CH3), 20.78
(Mes-CH3), 24.41 (br, Mes-CH3), 120.02 (d, JC–P = 91.9 Hz,
P-Cphipso), 124.21 (d, JC–P = 14.4 Hz), 129.20 (br, Mes-CH), 129.46
(d, JC–P = 12.2 Hz, P-Cphmeta), 131.165 (br, P-Cphortho), 131.70
(d, JC–P = 3.8 Hz), 132.51 (d, JC–P = 3.1 Hz, P-Cphpara), 133.60
(Mes-CMe), 136.34 (d, JC–P = 18.2 Hz), 140.06 (d, JC–P = 16.7 Hz),
141.841 (br), 172.497. 11B NMR (128.2 MHz, CDCl3): d-14.8.
31P NMR (161.75 MHz, CDCl3): 25.5. Anal. calcd for C37H39BN3P:
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1
under vacuum (50 mg, 61% yield). H NMR (399.572 MHz, CDCl3):
d 1.03 (s, 1H, B–OH), 1.85 (bs, 12H, Mes-CH3), 2.21 (s, 6H, Mes-CH3),
2.28 (d, 3H, JH–P = 14 Hz, P–CH3), 6.61 (s, 4H, Mes-CH), 6.93–7.02
(m, 2H), 7.26–7.48 (m, 9H), 7.56–7.58 (m, 2H), 7.63–7.67 (m, 1H).
13C NMR (100.5 MHz, CDCl3): d 16.95 (d, JC–P = 73.7 Hz, P–CH3),
20.76 (Mes-CH3), 24.94 (br, Mes-CH3), 121.59 (d, JC–P = 101 Hz,
P–Cphipso), 123.48 (d, JC–P = 16 Hz), 128.87 (br, Mes-CH), 128.93
(d, JC–P = 11.3 Hz, P–Cphmeta), 131.14 (br, P–Cphortho), 131.52
(d, JC–P = 3 Hz), 132.50 (Mes–CMe), 135.76 (d, JC–P = 19 Hz),
137.65 (d, JC–P = 18.2 Hz), 140.90, 155.56, 178.04. 11B NMR
(128.2 MHz, CDCl3): d 0.2. 31P NMR (161.75 MHz, CDCl3): d 20.8.
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P21/n,
a = 17.702(2), b = 8.9863(12), c = 19.979(3) A,
b = 95.172(9)1, V = 3165.3(7) A3, Z = 4, T = 213(2) K, m =
0.117 mmꢀ1, 27,389 reflections collected, 4956 unique (Rint = 0.0582),
R1 = 0.0525 [I 4 2s(I)], wR2 = 0.1496 (all data).
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ꢂc
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Chem. Commun., 2009, 3729–3731 | 3731