Fluoride anion chelation by a bidentate stibonium-borane Lewis acid

Article abstract of DOI:10.1021/om200499y

The stibonium-borane [o-(Ph₂MeSb)(Mes₂B)C ₆H₄]⁺ (4⁺) has been synthesized and isolated as a triflate salt. Competition experiments carried out with [o-(Ph₂MeP)(Mes₂FB)C₆H₄] indicate that 4⁺ exhibits a much higher fluoride affinity than its lighter phosphorus congener. Structural studies show that the higher fluoride affinity of 4⁺ is correlated to the Lewis acidity of the stibonium center, which engages the boron-bound fluoride anion in the formation of a B-F→Sb bridge. These results illustrate that the Lewis acidity of main-group onium ions increases as the group is descended.

Full text of DOI:10.1021/om200499y

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pubs.acs.org/Organometallics
Fluoride Anion Chelation by a Bidentate StiboniumꢀBorane Lewis
Acid
Casey R. Wade and Franc-ois P. Gabbaï*
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77843-3255, United States
S Supporting Information
b
ABSTRACT: The stiboniumꢀborane [o-(Ph2MeSb)(Mes2B)C6H4]+ (4+) has been
synthesized and isolated as a triflate salt. Competition experiments carried out with
[o-(Ph2MeP)(Mes2FB)C6H4] indicate that 4+ exhibits a much higher fluoride affinity
than its lighter phosphorus congener. Structural studies show that the higher fluoride
affinity of 4+ is correlated to the Lewis acidity of the stibonium center, which engages the
boron-bound fluoride anion in the formation of a BꢀFfSb bridge. These results
illustrate that the Lewis acidity of main-group onium ions increases as the group is
descended.
ollowing the seminal contribution of Biallas and Shriver, who
first showed that bifunctional Lewis acids could be used for
F
anion chelation,1 the chemistry of boron-based bidentate Lewis
acids has been growing at a steady rate,2 leading to numerous
applications in anion sensing3 and organic/organometallic che-
mistry.4,5 A key aspect of this chemistry lies in the stability of the
chelate complex formed between the bidentate host and the
anionic guest.6 Through a series of recent studies, it has been
shown that anion binding at boron could be assisted by an
adjacent onium ion via Coulombic effects.5,7ꢀ9 An in-depth
analysis of some of the complexes isolated suggested that these
favorable Coulombic effects can also be complemented by
donorꢀacceptor interactions involving nonbonding lone pairs
of the anion and low-lying vacant orbitals at the central atom of
the onium group. This situation is illustrated by the structure of
1-F, which features a PfF bond of 2.66 Å, estimated to
contribute 5 kcalmolꢀ1 to the stability of the complex.8 Formation
of such a donorꢀacceptor interaction suggested to us that the latent
Lewis acidity of onium ions could be exploited in anion sensing. On
the basis of this discovery, we have now decided to determine if the
anion affinity of bidentate Lewis acids can be enhanced by varying the
nature of the onium main-group element.
with MeOTf in Et2O afforded the triflate salt of 4+ in moderate
yield as a moisture-sensitive white powder (Scheme 1). The salt
4-OTf is soluble in polar organic solvents such as CHCl3, CH2Cl2,
THF, and methanol but not in hydrocarbon solvents such as
hexanes and pentane as well as Et2O. The 1H NMR spectrum of
4-OTf recorded in CDCl3 displays a signal at 1.91 ppm assigned
to the antimony-bound methyl group. The spectrum also shows a
series of broad resonances corresponding to the mesityl aromatic
CH and o-CH3 groups, which suggests steric crowding and
hindered rotation of the boron substituents in solution. A broad
signal at +80.2 ppm in the 11B NMR spectrum of 4-OTf indicates
the existence of a tricoordinate, “base-free” boron atom.8 This is
supported by the UVꢀvis absorption spectrum of 4-OTf in
CHCl3, which displays a broad absorption band centered at
334 nm, characteristic of aryldimesitylborane chromophores.8,12
This salt quickly reacts with adventitious moisture in CDCl3 to
give rise to an intractable colorless precipitate that has not yet
been identified.
In a first exploration of this idea, we have recently compared
the fluoride anion affinity of the sulfonium and telluronium
derivatives 2+ and 3+ and found that the telluronium derivative 3+
has a significantly higher fluoride anion affinity than the sulfo-
nium borane 2+.9 These results, which can be rationalized by
invoking the greater size, electropositivity, and polarizability of
the chalcogen in 3+, are also in agreement with the stability of
halogen-bonded complexes, which increases with the size of the
halogen.10 In an effort to generalize our approach, we are now
reporting on the synthesis and fluoride affinity of the antimony
analogue of 1+.
Next, we decided to compare the fluoride anion affinity of 4+
to that of its phosphonium analogue 1+. To this end, we studied
the reaction of 4+ with an equimolar amount of 1-F in CDCl3 and
observed the quantitative formation of 4-F and 1+ using multi-
nuclear NMR spectroscopy (Scheme 2). The identity of 4-F was
independently confirmed by the reaction of 4-OTf with 1 equiv
of [S(NMe2)3][Me3SiF2] (TASF) in CDCl3. The 11B NMR
spectrum of 4-F displays a single sharp signal at 7.5 ppm,
indicative of a tetrahedral boron center.8 Similarly, the 19F NMR
The reaction of o-(Ph2Sb)BrC6H411 with 2 equiv of t-BuLi in
Et2O at ꢀ78 ꢀC, followed by addition of Mes2BF, afforded
o-(Ph2Sb)(Mes2B)C6H4. Subsequent treatment of this derivative
Received: June 9, 2011
Published: August 10, 2011
r
2011 American Chemical Society
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Organometallics
COMMUNICATION
Scheme 1. Synthesis of Stibonium/Borane 4-OTf
Scheme 2. Competition Reaction between 4+ and 1-F
Figure 2. Plot of the NBO lpF f σ*SbꢀC donorꢀacceptor interactions
calculated for 4-F. Density isovalues are set to 0.03, and hydrogen atoms
have been omitted for clarity.
the formation of this chelate complex and the bridging location of
the fluoride anion,9 we note that the B(1)ꢀF(1) bond (1.521(4) Å)
is elongated in comparison to terminal BꢀF bonds found in
compounds such as p-(Ph2MeP)C6H4(BFMes2) (1.476(4) Å).13
It is also longer than that observed in the structure of its phosphorus
congener 1-F (1.482(3) Å),8 suggesting that the antimony atom in 4-
F exerts a stronger pull than the phosphorus atom in 1-F. Accord-
ingly, the SbꢀFdistancein4-F (2.450(2) Å) is shorter than the PꢀF
distance in 1-F (2.666(2) Å), despite the larger size of the antimony
atom. Owing to the bridging location of the fluorine atom, the SbꢀF
distance in 4-F is elongated in comparison to that in Ph3MeSbF
(2.069(3) Å)14 but comparable to that observed in the SbꢀFꢀSb
bridges of polymeric Me4SbF (2.369(14) and 2.382(12) Å).15 As a
result of the BꢀFꢀSb bridge, the antimony center adopts a distorted-
trigonal-bipyramidal geometry defined by —(C(14)ꢀSb(1)ꢀ
F(1)) = 173.8(1)ꢀ and ∑—(CeqꢀSbꢀCeq) = 348.1ꢀ.
The geometry of 4-F has been optimized using density
functional theory (DFT) methods (B3LYP functional with the
mixed basis set: aug-cc-pvTz-pp for Sb, 6-31+g(d0) for B and F,
6-31g for C and H). The resulting geometry closely matches that
determined experimentally and corresponds to a true minimum,
as indicated by the absence of imaginary frequencies. Atoms in
molecules (AIM) calculations carried out at the optimized
geometry located a bond path connecting the antimony and
fluorine atoms of 4-F (Figure S3 in Supporting Information).
Although bonding orbitals tend to become more diffuse as the
size of the atom increases, the value of the electron density at
Figure 1. Structure of 4-F. Displacement ellipsoids are scaled to the
50% probability level. Hydrogen atoms have been omitted for clarity,
and phenyl and mesityl groups are depicted in wireframe. Selected bond
lengths (Å) and angles (deg): Sb(1)ꢀF(1), 2.450(2); F(1)ꢀB(1),
1.521(4); Sb(1)ꢀC(1), 2.093(4); Sb(1)ꢀC(7), 2.107(4); Sb(1)ꢀC(8),
2.092(4); Sb(1)ꢀC(14), 2.135(4); C(8)ꢀSb(1)ꢀC(1), 115.85(14);
C(8)ꢀSb(1)ꢀC(7), 111.77(16); C(1)ꢀSb(1)ꢀC(7), 120.50(15);
C(14)ꢀSb(1)ꢀF(1), 173.84(11); C(2)ꢀB(1)ꢀC(20), 118.0(3);
C(2)ꢀB(1)ꢀC(29), 108.6(3); C(20)ꢀB(1)ꢀC(29), 116.7(3).
the SbꢀF bond critical point (BCP) (F(r) = 4.26 ꢁ 10ꢀ2
e
bohrꢀ3) is significantly larger than that determined for the
PꢀF bond of 1ꢀF (F(r) = 2.05 ꢁ 10ꢀ2 e bohrꢀ3).8 The greater
BCP electron density observed in 4-F suggests that the PnꢀF
bond (Pn = P, Sb) in 4-F is stronger than in 1-F. The SbꢀF
bond in 4-F has also been investigated using a natural bond
orbital (NBO) analysis. This analysis reveals the presence of a
lpF f σ*SbꢀC donorꢀacceptor interaction which, as indicated
by deletion calculations, contributes Edel = 15.2 kcal molꢀ1 to
the stability of 4-F (Figure 2). This value greatly exceeds that
calculated for the PꢀF bond of 1-F (Edel = 5.0 kcal molꢀ1),
supporting the greater Lewis acidity of the antimony deriva-
tive. Finally, we have calculated the enthalpy change for the
reaction presented in Scheme 2 by carrying out single-point
calculations on 4+, 1+, 4-F, and 1-F (functional B3LYP, basis sets
aug-cc-pvTz-pp for Sb and 6-311+g(2d,p) for all other atoms).
These calculations indicate that the reaction is exothermic (ΔH =
ꢀ4.9 kcal/mol), thus corroborating the experimental observation
as well as the results of the NBO analysis.
spectrum showsa singleresonance atꢀ140.1ppm, inthe expected
range for a triarylfluoroborate species. The 1H NMR resonance
of the antimony-bound methyl group is shifted upfield to 1.34
ppm and appears as a doublet (JHꢀF = 2.38 Hz) caused by
coupling to the fluorine nucleus. The superior fluoride affinity of
4+ was confirmed by the observation that 4-F remained intact
when treated with 1 equiv of 1+ in CDCl3.
To obtain structural insight into the enhanced fluoride affinity
of 4+, the X-ray crystal structure of 4-F has been determined
(Figure 1). Examination of this structure indicates the formation
of an unprecedented BꢀFꢀSb chelate motif, demonstrating that
4+ can be regarded as a bidentate Lewis acid. In agreement with
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Organometallics
COMMUNICATION
In conclusion, the results presented in this communication
demonstrate that a net enhancement in the anion affinity of
cationic, bidentate boranes can be observed upon introduction of
a stibonium ion as a secondary binding site. This enhancement
results from the Lewis acidity of the stibonium center, which
engages the fluoride anion in a strong donorꢀacceptor interac-
tion. The strength of the latter can be correlated to the increased
size, polarizability, and electropositivity of the heavier pnictogen
element.
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’ ASSOCIATED CONTENT
(13) Lee, M. H.; Agou, T.; Kobayashi, J.; Kawashima, T.; Gabbaï,
F. P. Chem. Commun. 2007, 1133–1135.
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S
Supporting Information. Text, tables, and figures giving
b
full experimental and computational details and a CIF file giving
crystallographic data for 4-F. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: francois@tamu.edu. Tel: (+1)979-845-4719. Fax: (+1)
979-845-4719.
’ ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(No. CHE-0952912), the Welch Foundation (No. A-1423), the
Petroleum Research Fund (Grant 44832-AC4), and the U.S.
Army Medical Research Institute of Chemical Defense.
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