Efficient Synthesis of Salts of the Tetrakis(perfluoroorgano)borate anion, [B(CF=CF2)4]-

Article abstract of DOI:10.1021/om030610i

The synthesis and the NMR spectra of the tetrakis(trifluorovinyl)borate salts M[B(CF=CF₂)₄] (M = Li, K, Cs, [NR¹R ₃²) as well as the X-ray crystal structure of the tetramethylammonium salt are reporte

Full text of DOI:10.1021/om030610i

172
Organometallics 2004, 23, 172-174
Efficien t Syn th esis of Sa lts of th e
Tetr a k is(p er flu or oor ga n o)bor a te An ion , [B(CF dCF 2)4]-
Nicolay Yu. Adonin and Vadim V. Bardin
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, SB RAS,
Acad. Lavrentjev Avenue, 9, 630090 Novosibirsk, Russia
Ulrich Flo¨rke
Department of Chemistry, University Paderborn, Warburger Strasse 100,
D-33098 Paderborn, Germany
Hermann-J osef Frohn*
Institute of Chemistry, Inorganic Chemistry, University Duisburg-Essen,
Lotharstrasse 1, D-47048 Duisburg, Germany
Received September 24, 2003
Summary: The synthesis and the NMR spectra of the
(R ) F, Cl, C2F5, C6F13, C4F9, C4H9, C6H5), and the
Cs[CF2dCFB(CF3)2F] salt6 were prepared and charac-
terized. To our knowledge, borates that contain more
than one trifluorovinyl substituent on boron have not
been described to date.8
We report here the synthesis and the NMR spectra
of tetrakis(trifluorovinyl)borate salts, M[B(CFdCF2)4]
(1), as well as the X-ray crystal structure of the tet-
ramethylammonium salt. These salts were prepared by
the reaction of boron trichloride with bis(trifluorovinyl)-
di-n-butyltin (2) in the presence of a tri- or tetraalkyl-
ammonium or alkali-metal chloride in dichloromethane
(eq 1).
tetrakis(trifluorovinyl)borate salts M[B(CFdCF2)4] (M )
Li, K, Cs, [NR1R2 ]) as well as the X-ray crystal structure
3
of the tetramethylammonium salt are reported. These
salts were prepared by the reaction of boron trichloride
with bis(trifluorovinyl)di-n-butyltin in the presence of tri-
and tetraalkylammonium or alkali-metal chlorides in
dichloromethane.
Weakly coordinating anions have been studied inten-
sively in the past decade because to their importance
in pure and applied chemistry.1 The tetrakis(perfluo-
roorgano)borates M[B(RF)4] contain such weakly coor-
dinating anions.1a Until 2000 they have been repre-
sented only by the tetrakis(pentafluorophenyl)borate
salts M[B(C6F5)4] (M ) Li, Na, K, (C6H5)3C, HNRR′2,
etc.) and related perfluoroaryl compounds, which were
prepared by the reaction of the respective aryllithium
reagent with boron trihalide.2
2(CF2dCF)2SnBu-n2
+ MCl +
2
M[B(CFdCF ) ]
BCl3 f
+ n-Bu2SnCl2 (1)
2 4
1
In 2001 the synthesis of the first tetrakis(perfluoro-
alkyl)borate salts, M[B(CF3)4], was reported.3 In con-
trast to M[B(C6F5)4], the M[B(CF3)4] salts could not be
prepared by stepwise introduction of nucleophilic CF3
groups. Salts of the [B(CF3)4]- anion were obtained by
the fluorination of the cyano groups in M[B(CN)4] using
ClF3 in anhydrous HF. The M[B(CFdCF2)4] salts and
related (perfluoroalkenyl)borates have remained un-
known, although the first (trifluorovinyl)boranes,
(CF2dCF)nBX3-n (n ) 1-3; X ) F, Cl, Br), were
synthesized more than 40 years ago.4 Other (trifluo-
rovinyl)boranes, (CF2dCF)nBX3-n (n ) 1, 2; X ) OAlk,
Me), were prepared later.5 Recently, the donor-acceptor
complexes CF2dCFB(CF3)2‚NRMe2 (R ) H, Me, Bz),6
the series of (alken-1-yl)trifluoroborates M[RCFdCFBF3]7
M ) NR1R2 (R1 ) H, Bz, Me, n-Bu;
3
R2 ) Me, Et, n-Bu), Li, K, Cs
NMR-scale reactions for elucidating the reaction
parameters were carried out under either homogeneous
(M ) NR1R23 (R1 ) H, Bz, Me, n-Bu; R2 ) Me, Et, n-Bu))
or heterogeneous (M ) Li, K, Cs) conditions at 25 °C.
The addition of a solution of BCl3 into a mixture of
(CF2dCF)2SnBu-n2 and [C6H5CH2N(CH3)3]Cl in dichlo-
romethane gave salt 1 in 98% yield (Table 1, entry 1).
Both trifluorovinyl groups of (CF2dCF)2SnBu-n2 can be
transferred to boron (Table 1, entries 1-3).
The order of mixing of the reagents is important.
When BCl3 was added to a slight excess of [NR1R2 ]Cl
3
before the tin compound was added, the formation of 1
(1) (a) Strauss, S. H. Chem. Rev. 1993, 93, 927-942. (b) Reed, C. A.
Acc. Chem. Res. 1998, 31, 133-139. (c) Piers, W. E. Chem. Eur. J . 1998,
4, 13-18.
(5) (a) Sterlin, R. N.; Isaev, V. L.; Zakharov, G. M.; Martin, B.;
Knunyanz, I. L. Zh. Vses. Khim. Obshch. 1967, 12, 475-477; Chem.
Abstr. 1968, 68, 13025r. (b) Stampf, E. J .; Odom, J . D. J . Organomet.
Chem. 1976, 108, 1-11.
(2) For recent reviews, see: (a) Chen, E. Y.-X.; Marks, T. J . Chem.
Rev. 2000, 100, 1391-1434. (b) Piers, W. E.; Chivers, T. Chem. Soc.
Rev. 1997, 26, 345-354.
(3) Bernhardt, E.; Henkel, G.; Willner, H.; Pawelke, G.; Bu¨rger, H.
Chem. Eur. J . 2001, 7, 4696-4705.
(4) (a) Kaesz, H. D.; Stafford, S. L.; Stone, F. G. A. J . Am. Chem.
Soc. 1959, 81, 6336. (b) Stafford, S. L.; Stone, F. G. A. J . Am. Chem.
Soc. 1960, 82, 6238-6240. (c) Coyle, T. D.; Stafford, S. L.; Stone, F. G.
A. Spectrochim. Acta 1961, 17, 968-976.
(6) Brauer, D. J .; Pawelke, G. J . Organomet. Chem. 2000, 604, 43-
51.
(7) Frohn, H.-J .; Bardin, V. V. Z. Anorg. Allg. Chem. 2001, 627,
2499-2505.
(8) Independently, G. Pawelke et al. obtained [B(CFdCF2)4]- and
related anions by a different route (private communication).
10.1021/om030610i CCC: $27.50 © 2004 American Chemical Society
Publication on Web 12/12/2003

Communications
Organometallics, Vol. 23, No. 2, 2004 173
Ta ble 1. Rea ction of (CF 2dCF )2Sn Bu -n 2 (2) a n d
BCl3 in CH2Cl2 in th e P r esen ce of MCl
amt of
yield, %
reagent,
mmol
M[B(CFd B(CFd
CF2)4]a
CF2)3
b
entry
M
2
BCl3 MCl
1
2
3
4
[C6H5CH2NMe3] 0.40 0.11 0.10
[C6H5CH2NMe3] 0.20 0.11 0.10
[C6H5CH2NMe3] 0.22 0.11 0.10
[C6H5CH2NMe3] 0.20 0.10 0.11
>98
80
95
traces
18
9
c
5
6
7
8
9
10
[Bu4N]
[Me4N]
Li
K
Cs
-
0.22 0.11 0.10
0.22 0.11 0.10
0.22 0.10 0.28
1.42 0.50 0.51
0.22 0.10 0.86
0.22 0.1
85
10
92d
30b,d
26b,d
25b,d
traces
>98
a
b
The yield was calculated based on MCl. The yield was
calculated based on BCl3. c First the BCl3-CH2Cl2 solution was
added to MCl in CH2Cl2, followed by (CF2dCF)2SnBu-n2. Prepar-
d
ative yield.
was suppressed owing to the formation of the [BCl4]-
anion, which is stable in dichloromethane (shown by 11
B
NMR)9 (Table 1, entry 4).
Tetramethyl- and tetra-n-butylammonium chlorides
formed the corresponding salts 1 in high yield (Table 1,
entries 5 and 6). In dichloromethane the insoluble
chlorides MCl also promote the formation of M[B-
(CFdCF2)4], but their yields were not greater than 30%
(Table 1, entries 7-9).
In the absence of MCl the 2:1 molar reaction of 2 and
BCl3 gave, even when the correct stoichiometry was
used, only tris(trifluorovinyl)borane (3) in 98% yield
(Table 1, entry 10). It should be noted that the reaction
of BCl3 with an excess of the methyl derivative
(CF2dCF)2SnMe2 in the absence of a source of chloride
afforded a mixture of (perfluorovinyl)chloroboranes that
was difficult to separate.4b
For a preparative-scale synthesis we used a modified
procedure for mixing the reagents and the tetramethyl-
ammonium chloride. The product [NMe4][B(CFdCF2)4]
could be isolated easily from the reaction mixture as a
white solid by addition of pentane, whereas in the case
of the benzyltrimethyl- and tetra-n-butylammonium
salts only oils were formed after the addition of pentane.
[NMe4][B(CFdCF2)4] is a white crystalline salt, which
is soluble in CH2Cl2, acetone, acetonitrile, methanol, and
water but insoluble in hydrocarbons.
The 19F, 11B, and 13C NMR spectra of M[B(CFdCF2)4]
were measured and compared with those of the struc-
turally related salts K[CF2dCFBF3] and Cs[CF2dCFB-
(CF3)2F] and the borane (CF2dCF)3B (3) (Table 2, see
Supporting Information). The 19F NMR spectra of
M[B(CFdCF2)4] consist of three signals with equal
integrals at ca. -100, -124, and -183 ppm, and their
positions are practically independent of the nature of
the cation M ) K, Cs, Me4N or the solvent (Table 2).
Replacement of BF3 in [CF2dCFBF3]- by B(CF3)2F or
by B(CFdCF2)3 had practically no influence on the 19F
NMR chemical shift values of the fluorine atoms F2 on
carbon C-2 of the trifluorovinyl group. However, the
fluorine atom F1 at carbon C-1 in [B(CFdCF2)4]- is
significantly deshielded relative to those in the
[CF2dCFBX2F]- anions (X ) F, CF3). In contrast to the
F igu r e 1. 11B NMR spectrum of K[B(CFdCF2)4] in
CD3CN: an overlapping quintet of quintets of quintets
3
3
(2J B,F ) 21.5, J B,F ) 3.2, J B,F ) 2.2 Hz).
strongly deviating position of F1 in 1 (∆δ approximately
13 ppm), the 13C NMR chemical shift δ(C-1) of all three
borate anions differs only by approximately 4 ppm,
whereas δ(C-2) differs by approximately only 1 ppm
(Table 2).
The 11B NMR signal of the tetrakis(trifluorovinyl)-
borate ion (Figure 1) is located at approximately -21
ppm as a quintet of overlapping quintets of quintets
caused by the 11B-19F spin-spin coupling of four
trifluorovinyl groups each with three magnetically in-
equivalent fluorine atoms (Figure 1). This δ(11B) value
is close to the chemical shift of the tetramethylborate
anion (-20.2 ppm),10,11 the tetrakis(trifluoromethyl)-
borate anion (-18.9 ppm),3 and the tetravinylborate
anion (-16.1 ppm)11 and is strongly shifted to a low
frequency relative to the 11B signals of the anion
[CF2dCFBF3]- (0.51 ppm) or of borane 3 (45.74 ppm)
(see Table 2).
The structure of the [B(CFdCF2)4]- anion in its
tetramethylammonium salt and important bond lengths
and bond angles are presented in Figure 2 (for crystal
data and details of the structure determination see the
Supporting Information).12 [NMe4][B(CFdCF2)4] (1) crys-
tallizes in the tetragonal crystal system (space group
4h) with Z ) 2. The centers of the cation and anion are
both located on special positions with point symmetry
4h. At 153(2) K only the [NMe4]+ cation is disordered over
(9) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spec-
troscopy of Boron Compounds; Springer-Verlag: Berlin, Heidelberg,
New York, 1978; p 388.
(10) No¨th, H.; Vahrenkamp, H. J . Organomet. Chem. 1968, 12, 23-
36.
(11) Thompson, R. J .; Davis, J . C. Inorg. Chem. 1965, 4, 1464-1467.

174 Organometallics, Vol. 23, No. 2, 2004
Communications
atom bonded to C(1), which reduces the polar component
of the C(1)-F(1) bond.
Exp er im en ta l Section . Gen er a l P r oced u r es. All
reactions were carried out under an atmosphere of dry
argon. A 2.5 M solution of n-BuLi in hexanes (Aldrich),
di-n-butyltin dichloride (Fluka), 1.0 M BCl3 in hexanes
(Aldrich), 1.0 M BCl3 in CH2Cl2 (Aldrich), and anhy-
drous ether (Baker) were used as supplied. NMR spectra
were recorded on a Bruker AVANCE 300 (FT 300.13
MHz, 1H; 96.29 MHz,11B; 75.47 MHz,13C; 282.40
MHz,19F) spectrometer. The chemical shifts are refer-
enced to TMS (1H, 13C), BF3‚OEt3/CDCl3 15% v/v (11B),
and CCl3F (19F, with C6F6 as secondary reference
(-162.9 ppm)), respectively.
Rea ction of BCl3 w ith (CF 2dCF )2Sn Bu -n 2 in th e
P r esen ce of MCl. MCl, (CF2dCF)2SnBu-n2,13 C6H5CF3
(internal standard) (5 µL, 0.041 mmol), and CH2Cl2 (0.5
mL) were placed in sequence into a NMR tube under
an atmosphere of dry argon. After addition of BCl3 (1.0
M solution in hexanes), the reaction mixture was kept
at 20 °C for 20 min and analyzed by 19F (yields, see
Table 1) and 11B NMR spectroscopy.
P r ep a r a tion of [Me4N][B(CF dCF 2)4]. To a stirred
solution of (CF2dCF)2SnBu-n2 (400 µL, 561.6 mg, 1.42
mmol) in CH2Cl2 (2 mL) was added a BCl3 solution (1.0
M in CH2Cl2, 0.5 mL, 0.5 mmol) in one portion. The
mixture was stirred for 10 min at 20 °C and formed a
light brown solution. Anhydrous [Me4N]Cl (61 mg, 0.55
mmol) was added in 5 portions within 50 min. The
resulting solution was stirred at 20 °C for an additional
2 h and was then diluted with pentane (7 mL). The
white precipitate was filtered, washed with pentane (3
× 10 mL), and dried in a vacuum desiccator overnight.
The yield of [Me4N][B(CFdCF2)4] was 184 mg (90%).
Mp: 156-157 °C (recrystallized from CH2Cl2, sealed
capillary). Tonset(endothermic) ) 158.9 °C (DSC). Anal.
Calcd for C12H12BF12N (409.02): C, 35.24; H, 2.96; N,
3.42; F, 55.74. Found: C, 35.09; H, 2.90; N, 3.42; F,
55.65%.
F igu r e 2. Anion in the [NMe4][B(CFdCF2)4] salt. Dis-
placement ellipsoids are drawn at the 40% probability level.
Selected bond lengths (Å) and angles (deg): B-C(1) )
1.618(3), C(1)-C(2) ) 1.287(4), C(1)-F(1) ) 1.381(4), C(2)-
F(2) ) 1.330(4), C(2)-F(3) ) 1.321(4); C(2)-C(1)-B )
132.1(3), C(2)-C(1)-F(1) ) 112.1(3), F(1)-C(1)-B ) 115.7-
(2), C(1)-C(2)-F(2) ) 126.3(4), C(1)-C(2)-F(3) ) 125.0-
(3), F(2)-C(2)-F(3) ) 108.7(3), C(1)-B-C(1A) ) 108.01-
(11), C(1)-B-C(1B) ) 112.4(2).
two positions, with occupation factors of 0.77(1) and
0.23(1).
The perfluorovinyl group of the anion has a strong
structural similarity to that in the neutral adduct
CF2dCFB(CF3)2‚NHMe2.6 The main deviations involve
the C-B and the F(2)-C(2) distance, which are trans
to each other and longer in the anion than in the adduct.
Additionally, the angle B-C(1)-C(2) is smaller, whereas
F(1)-C(1)-B is larger in the anion than in the adduct.
The long distance C(1)-F(1) in the anion of 1 may be
caused by the opposite inductive effects of the F and B
Cr ysta ls for X-r a y Str u ctu r a l An a lysis. [Me4N]-
[B(CFdCF2)4] (30 mg) was dissolved in boiling CH2Cl2
and filtered into a glass tube, which was placed in a
closed flask over pentane. Within 3 weeks crystals of
quality sufficient for X-ray analysis were obtained.
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support by the Deutsche Forschungsgemein-
schaft, the Russian Foundation for Basic Research, and
the Fonds der Chemischen Industrie.
(12) Crystal structure analysis of [NMe4][B(CFdCF2)4]: C12H12
-
BF12N, SMART APEX (Bruker), Mo KR radiation (λ ) 0.710 73 Å,
graphite monochromator), T ) 153(2) K, colorless crystal (0.35 × 0.20
× 0.15 mm3), tetragonal, space group 4h, a ) 9.9305(9) Å, c ) 8.2531-
(10) Å, V ) 813.88(14) Å3, Z ) 2, Fcalcd ) 1.669 Mg m-3, µ(Mo KR) )
0.194 mm-1, F(000) ) 408, 4751 measured reflections (2.90 < θ <
28.22°), absorption correction (SADABS14), minimum/maximum trans-
mission 0.524/0.589, 981 independent intensities, structure solution
by direct and conventional Fourier methods, refinement based on F2
and 981 intensities, 70 parameters, all but hydrogen atoms refined
anisotropically, H atoms located from difference maps and refined at
idealized positions with riding model, R1(I > 2σ(I)) ) 0.049, wR2(all
data) ) 0.130, maximum ∆/σ < 0.001, minimum/maximum height in
∆F map ) -0.16/0.26 e/Å3. In the absence of significant anomalous
scattering effects, the Flack parameter and thus the determination of
absolute structure is essentially meaningless. Accordingly, Friedel
pairs were merged. Programs for solution and refinement were from
the SHELXTL suite.14 Crystallographic data, excluding structure
factors, have been deposited as Supporting Information with the
Cambridge Crystallographic Data Centre, with deposition number
CCDC-212214. Data can be obtained free of charge on application to
the CCDC, 12 Union Road, Cambridge CB2 1 EZ, U.K. (e-mail:
deposit@ccdc.cam.ac.uk).
Su p p or tin g In for m a tion Ava ila ble: Table 2 containing
NMR spectral data of M[B(CFdCF2)4] and related compounds,
and details of the X-ray crystal structure study, including
tables of crystal data and structure refinement details, atomic
coordinates, bond lengths and angles, and anisotropic dis-
placement parameters for [NMe4][B(CFdCF2)4]. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM030610I
(13) The stannane (CF2dCF)2SnBu-n2 was prepared by trifluorovi-
nylation of Cl2SnBu-n2 with CF2dCFLi (generated from CF3CFH2 and
n-BuLi15) in ether in 95% preparative yield.
(14) SADABS (Version 2.03) and SHELXTL (Version 6.10); Bruker
AXS Inc., Madison, WI, 2002.
(15) (a) Burdon, J .; Coe, P. L.; Haslock, I. B.; Powell, R. L. J . Chem.
Soc., Chem. Commun. 1996, 49-50. (b) Banger, K. K.; Brisdon, A. K.;
Gupta, A. J . Chem. Soc., Chem. Commun. 1997, 139-140. (c) Burdon,
J .; Coe, P. L.; Haslock, I. B.; Powell, R. L. J . Fluorine Chem. 1997, 85,
151-153.