Rearrangement Reactions of the Transient Lewis Acids (CF3) 3B and (CF3)3BCF2: An Experimental and Theoretical Study

Article abstract of DOI:10.1021/ic0350640

Short-lived (CF₃)₃B and (CF₃) ₃BCF₂ are generated as intermediates by thermal dissociation of (CF₃)₃BCO and F-abstraction from the weak coordinating anion [B(CF₃)₄]^-, respectively. Both Lewis acids cannot be detected because of their instability with respect to rearrangement reactions at the B-C-F moiety. A cascade of 1,2-fluorine shifts to boron followed by perfluoroalkyl group migrations and also difluorocarbene transfer reactions occur. In the gas phase, (CF ₃)₃B rearranges to a mixture of linear perfluoroalkyldifluoroboranes C_nF_2n+1BF₂ (n = 2-7), while the respective reactions of (CF₃)₃BCF ₂ result in a mixture of linear (n = 2-4) and branched monoperfluoroalkyldifluoroboranes, e.g., (C₂F₅)(CF ₃)FCBF₂. For comparison, the reactions of [CF ₃BF₃]^- and [C₂F₅BF ₃]^- with AsF₅ are studied, and the products in the case of [CF₃BF₃]^- are BF₃ and C₂F₅BF₂ whereas in the case of [C ₂F₅BF₃]^-, C₂F ₅BF₂ is the sole product. In contrast to reports in the literature, it is found that CF₃BF₂ is too unstable at room temperature to be detected. The decomposition of (CF₃) ₃BCO in anhydrous HF leads to a mixture of the new conjugate Bronsted-Lewis acids [H₂F][(CF₃)₃BF] and [H₂F][C₂F₅BF₃]. All reactions are modeled by density functional calculations. The energy barriers of the transition states are low in agreement with the experimental results that (CF₃)₃B and (CF₃)₃BCF₂ are short-lived intermediates. Since CF₂ complexes are key intermediates in the rearrangement reactions of (CF₃)₃B and (CF₃)₃BCF₂, CF₂ affinities of some perfluoroalkylfluoroboranes are presented. CF₂ affinities are compared to CO and F^- affinities of selected boranes showing a trend in Lewis acidity, and its influence on the stability of the complexes is discussed. Fluoride ion affinities are calculated for a variety of different fluoroboranes, including perfluorocarboranes, and compared to those of the title compounds.

Full text of DOI:10.1021/ic0350640

Inorg. Chem. 2004, 43, 490−505
Rearrangement Reactions of the Transient Lewis Acids (CF3)3B and
(CF3)3BCF2: An Experimental and Theoretical Study
Maik Finze, Eduard Bernhardt, Manfred Za1hres, and Helge Willner*
Fakulta¨t 4, Anorganische Chemie, UniVersita¨t Duisburg-Essen, Lotharstrasse 1,
D-47048 Duisburg, Germany
Received September 9, 2003
Short-lived (CF ) B and (CF ) BCF are generated as intermediates by thermal dissociation of (CF ) BCO and F-
3 3
3 3
2
3 3
abstraction from the weak coordinating anion [B(CF ) ]-, respectively. Both Lewis acids cannot be detected because
3 4
of their instability with respect to rearrangement reactions at the B−C−F moiety. A cascade of 1,2-fluorine shifts
to boron followed by perfluoroalkyl group migrations and also difluorocarbene transfer reactions occur. In the gas
phase, (CF ) B rearranges to a mixture of linear perfluoroalkyldifluoroboranes C F2n+1BF (n ) 2−7), while the
3 3
n
2
respective reactions of (CF ) BCF result in a mixture of linear (n ) 2−4) and branched monoperfluoroalkyldif-
3 3
2
luoroboranes, e.g., (C F )(CF )FCBF . For comparison, the reactions of [CF BF ]- and [C F BF ]- with AsF are
2
5
3
2
3
3
2
5
3
5
studied, and the products in the case of [CF BF ]- are BF and C F BF whereas in the case of [C F BF ]-,
3
3
3
2
5
2
2
5
3
C F BF is the sole product. In contrast to reports in the literature, it is found that CF BF is too unstable at room
2
5
2
3
2
temperature to be detected. The decomposition of (CF3)3BCO in anhydrous HF leads to a mixture of the new
conjugate Brønsted−Lewis acids [H F][(CF ) BF] and [H F][C F BF ]. All reactions are modeled by density functional
2
3 3
2
2
5
3
calculations. The energy barriers of the transition states are low in agreement with the experimental results that
(CF ) B and (CF ) BCF are short-lived intermediates. Since CF complexes are key intermediates in the
3 3
3 3
2
2
rearrangement reactions of (CF ) B and (CF ) BCF , CF affinities of some perfluoroalkylfluoroboranes are presented.
3 3
3 3
2
2
CF affinities are compared to CO and F- affinities of selected boranes showing a trend in Lewis acidity, and its
2
influence on the stability of the complexes is discussed. Fluoride ion affinities are calculated for a variety of different
fluoroboranes, including perfluorocarboranes, and compared to those of the title compounds.
Introduction
pounds are still uncommon as difluorocarbene sources.4 In
the presence of strong π-donor ligands tricoordinated tri-
fluoromethylboranes are stable, e.g., (CF3)2BNMe2.2,3 CF3-
BF2 is claimed to be stable at temperatures below -40 °C,
and it was trapped with trimethylamine.5 The syntheses of
In the course of our recent synthesis of the promising weak
coordinating anion [B(CF3)4]-,1 we became interested in
studying its stability and decomposition pathways. Since the
degradation of the tetrakis(trifluoromethyl)borate anion is
initiated by F- abstraction and the formation of the conjugate
Lewis acid (CF3)3BCF2, we became interested in studying
the properties of the difluorocarbene complex and related
trifluoromethylboron Lewis acids, especially (CF3)3B.
Tricoordinated trifluoromethyl boranes, e.g., (CF3)3B, are
known to be unstable with respect to B-F bond formation
and probably loss of difluorocarbene,1-3 but B-CF3 com-
n
CF3BF2 were also reported by using Bu2BCF3 and BF3 as
starting materials,6,7 but similar reactions did not verify the
observation. Hence its existence seems to be doubtful.2,5 In
contrast, a great number of tetracoordinated boron com-
pounds containing trifluoromethyl ligands are well-known
in the literature.1-3 Whereas tricoordinated trifluorometh-
ylboranes are rare, boranes with one perfluoroalkyl substitu-
(3) Pawelke, G.; Bu¨rger, H. Coord. Chem. ReV. 2001, 215, 243.
(4) Brahms, D. L. S.; Daily, W. P. Chem. ReV. 1996, 96, 1585.
(5) Bu¨rger, H.; Grunwald, M.; Pawelke, G. J. Fluorine Chem. 1985, 28,
183.
(6) Parsons, T. D.; Baker, E. D.; Burg, A. B.; Juvinall, G. L. J. Am. Chem.
Soc. 1961, 83, 250.
* Corresponding author. E-mail: willner@uni-wuppertal.de. Phone:
(+49) 202-439-2517. Fax: (+49) 202-439-3052. Present address: FB
C-Anorganische Chemie, Bergische Universita¨t-GH Wuppertal, Gauâstraâe
20, D-42097 Wuppertal, Germany.
(1) Bernhardt, E.; Henkel, G.; Willner, H.; Pawelke, G.; Bu¨rger, H. Chem.
Eur. J. 2001, 7, 4696.
(7) Parsons, T. D.; Self, J. M.; Schaad, L. H. J. Am. Chem. Soc. 1967,
89, 3446.
(2) Pawelke, G.; Bu¨rger, H. Appl. Organomet. Chem. 1996, 10, 147.
490 Inorganic Chemistry, Vol. 43, No. 2, 2004
10.1021/ic0350640 CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/20/2003

Transient Lewis Acids (CF3)3B and (CF3)3BCF2
ent, e.g., CnF2n+1BF2 (n ) 3, 6), are stable.8-10 To our
knowledge no unstabilized tricoordinate boranes of the types
(CnF2n+1)2BF and (CnF2n+1)3B have been reported so far.
The tetrakis(trifluoromethyl)borate anion, [B(CF3)4]-, is
synthesized by fluorination of the tetracyanoborate anion,
[B(CN)4]-,11,12 in anhydrous HF according to eq 1.1
and the rearrangement as well as degradation reactions of
the intermediate (CF3)3BCF2, (iii) the heterogeneous reactions
of K[CF3BF3] and K[C2F5BF3] with AsF5, (iv) a detailed ab
initio study on the stabilities, fluoride ion affinities, and
reaction pathways of fluoro(perfluoroalkyl)boron compounds
and a comparison to related fluoroboron species, and (v) a
19F NMR spectroscopic study on the reaction products.
K[B(CN)4]
8 K[B(CF3)4]
(1)
CIF3/aHF
Experimental Section
[B(CF3)4]- forms many stable salts with a wide variety
of highly reactive cations, e.g., [Co(CO)5]+ 13 and NO+,14
and is stable under reductive (Na/NH3) and oxidative
conditions (F2/aHF), respectively.1 However, in the presence
of concentrated sulfuric acid, K[B(CF3)4] is converted to the
borane carbonyl (CF3)3BCO according to eq 2.15,16
General Procedures and Reagents. (a) Apparatus. Volatile
materials were manipulated in stainless steel or glass vacuum lines
of known volume equipped with capacitance pressure gauges (type
280E Setra Instruments, Acton MA, or type 221AHS-1000 MKS
Baratron, Burlinton, MA). The stainless steel line was fitted with
bellow valves (type BPV25004 Balzers and type SS4BG Nupro)
as well as with Gyrolok and Cajon fittings, while the glass vacuum
line was fitted with PTFE stem valves (Young, London) and NS14.5
standard tapers. The glass vacuum line was equipped with three
U-traps and connected to an IR gas cell (optical path length 200
mm, Si windows 0.5 mm thick) contained in the sample compart-
ment of the FTIR instrument. Anhydrous HF was stored in a PFA
tube (12 mm o.d., 300 mm long), heat sealed at the bottom, and
connected at the top to a stainless steel needle valve (3762 H46Y
Hoke, Cresskill, NJ). For synthetic reactions in HF, a reactor was
used that consists of a 100 mL PFA bulb with a NS29 socket
standard taper (Bohlender, Lauda, Germany) in connection with a
PFA NS29 cone standard taper and a PFA needle valve (type 204-
30 Galtek, Fluoroware, Chaska, Minnesota). The parts were held
together with a metal compression flange, and the reactor was
leakproof (<10-5 mbar L s-1) without using grease. Alternatively,
a V-shaped reactor consisting of two PFA tubes (12 mm o.d., 100
mm length) and a PFA needle valve (Fluoroware, MN) with an
approximate volume of 25 mL was used. Volatile products were
stored in flame-sealed glass ampules under liquid nitrogen in a
storage Dewar vessel. The ampules were opened and resealed using
an ampule key.19
25 °C
[B(CF3)4]-(solv) + H3O+ conc. H SO 8
2
4
(CF3)3BCO(g) + 3HF(solv) (2)
The transformation of one of the CF3 groups to the CO
ligand involves the difluorocarbene complex (CF3)3BCF2,
which is immediately hydrolyzed in sulfuric acid. The
released borane carbonyl, (CF3)3BCO, is stable up to 0 °C
and at room temperature in suitable solvents, such as SO2
or CH2Cl2, for a prolonged period of time.15,16 In anhydrous
HF, (CF3)3BCO forms a new conjugated Brønsted-Lewis
acid, [H2F][(CF3)3BF], (see eq 3) which has been shown to
be a useful reaction medium for the formation of the so far
unknown [Co(CO)5]+ cation in a salt with the weak
coordinating anion [(CF3)3BF]-.17
(CF3)3BCO(solv) + 2HF(l) f
[H2F][(CF3)3BF](solv) + CO(g) (3)
The tris(trifluoromethyl)fluoroborate anion is formed by
exchange of CO against F-. In contrast, in nonacidic media,
(CF3)3BCO reacts with fluoride ions under addition to the
carbonyl carbon atom, resulting in the formation of
[(CF3)3BC(O)F]-.18
In this study we report (i) the rearrangement and decom-
position reactions of the intermediate (CF3)3B in anhydrous
HF and in the gas phase at room and elevated temperatures,
(ii) the reactions of K[B(CF3)4] with selected Lewis acids
Matrix-isolated samples were prepared by passing a gas stream
of Ar (∼3 mmol h-1) over the sample placed in a small U-trap in
front of the matrix support. The U-trap containing C2F5BF2 was
kept at -150 °C. Details of the matrix apparatus have been
described elsewhere.20
(b) Chemicals. Anhydrous HF (Solvay AG, Hannover, Germany)
as well as all standard chemicals and solvents were obtained from
commercial sources. (CF3)3BCO was prepared through the acidic
hydrolysis of K[B(CF3)4] with H2SO4 as reported.15,16 K[B(CF3)4]
was synthesized as described previously from K[B(CN)4].1 K[CF3-
BF3] was formed as side product in some syntheses of K[B(CF3)4].
A sample of K[C2F5BF3] was obtained from Dr. N. V. Ignat’ev
(Merck KGaA, Darmstadt, Germany).21
(c) Synthetic Reactions. (1) Decomposition of (CF3)3BCO in
aHF in the Presence of KHF2. A 100 mL PFA reactor, fitted with
a PTFE coated magnetic stirring bar, was charged with 1.20 g (15.4
mmol) of KHF2. The PFA vessel was connected to a glass vacuum
apparatus and set under vacuum to remove all volatile compounds.
A 810 mg (3.3 mmol) portion of (CF3)3BCO was condensed in
vacuo into the PFA reactor at -196 °C. The PFA vessel was
(8) Frohn, H.-J.; Bardin, V. V. Z. Anorg. Allg. Chem. 2001, 627, 15.
(9) Bardin, V. V.; Idemskaya, S. G.; Frohn, H.-J. Z. Anorg. Allg. Chem.
2002, 628, 883.
(10) Frohn, H.-J.; Bardin, V. V. Z. Anorg. Allg. Chem. 2002, 628, 1853.
(11) Bernhardt, E.; Henkel, G.; Willner, H. Z. Anorg. Allg. Chem. 2000,
626, 560.
(12) Bernhardt, E.; Finze, M.; Willner, H. Z. Anorg. Allg. Chem. 2003,
629, 1229.
(13) Bernhardt, E.; Finze, M.; Berkei, M.; Lehmann, C. W.; Willner, H.;
Aubke, F. Paper in preperation.
(14) Bernhardt, E.; Finze, M.; Willner, H. Paper in preparation.
(15) Finze, M.; Bernhardt, E.; Terheiden, A.; Berkei, M.; Willner, H.;
Christen, D.; Oberhammer, H.; Aubke, F. J. Am. Chem. Soc. 2002,
124, 15385.
(16) Terheiden, A.; Bernhardt, E.; Willner, H.; Aubke, F. Angew. Chem.,
Int. Ed. 2002, 41, 799.
(17) Bernhardt, E.; Finze, M.; Willner, H.; Lehmann, C. W.; Aubke, F.
Angew. Chem., Int. Ed. 2003, 42, 2077.
(18) Finze, M.; Bernhardt, E.; Willner, H.; Lehmann, C. W. Angew. Chem.,
Int. Ed. 2003, 42, 1052.
(19) Gombler, W.; Willner, H. J. Phys. E: Sci. Instrum. 1987, 20, 1286.
(20) Argu¨ello, G. A.; Grothe, H.; Kronberg, M.; Willner, H.; Mack, H. G.
J. Phys. Chem. 1995, 99, 17525.
(21) Ignat’ev, N. V.; Schmidt, M.; Welz-Biermann, U.; Weiden, M.; Heider,
U.; Miller, A.; Willner, H.; Sartori, P. (Merck KGaA). German Patent
DE 10216998.5y, 2002.
Inorganic Chemistry, Vol. 43, No. 2, 2004 491

Finze et al.
Chart 1. 11B and 19F NMR Data of Linear Perfluoroalkylborates
attached to a stainless steel vacuum line, and 3 mL of anhydrous
HF was added at -196 °C. Upon warming the reaction mixture to
room temperature, the HF became liquid, and at approximately -20
°C, the white KHF2 and (CF3)3BCO dissolved in the HF. The
reaction mixture was stirred for 1 h at room temperature. After
cooling to -100 °C, the gas phase above the clear colorless solution
was investigated by IR spectroscopy. The main components of the
gas phase were identified as CO and HCF3, and small quantities of
BF3 were also found. All volatile compounds were removed under
reduced pressure, and the white residue was suspended in aceto-
nitrile. The suspension was filtered to remove excess KHF2. The
solvent was removed using a rotary evaporator giving a white solid.
The solid was dissolved in CD3CN and investigated by 11B and
19F NMR spectroscopy. It consisted of 77.1% K[(CF3)3BF]1 and
22.9% K[C2F5BF3] (see Chart 1).
(2) Decomposition of (CF3)3BCO in Neat aHF. One sidearm
of a V-shaped 25 mL PFA reactor was fitted with a PTFE coated
magnetic stirring bar and was charged with 160 mg (2.6 mmol) of
NaF. The PFA vessel was connected to a glass vacuum apparatus
and set under vacuum to remove all volatile compounds. A 310
mg (1.3 mmol) portion of (CF3)3BCO was transferred in vacuo into
the second sidearm of the PFA reactor at -196 °C. The PFA vessel
was attached to a stainless steel vacuum line, and 3 mL of anhydrous
HF was added to the borane carbonyl at -196 °C. The reaction
mixture was warmed to room temperature and stirred for 1 h. The
clear colorless reaction mixture was poured at room temperature
onto the NaF and stirred for further 10 min. All volatile compounds
were removed under reduced pressure, and the white residue was
suspended in acetonitrile. The suspension was filtered to remove
excess NaHF2. The solvent was removed using a rotary evaporator
492 Inorganic Chemistry, Vol. 43, No. 2, 2004

Transient Lewis Acids (CF3)3B and (CF3)3BCF2
Chart 2. 11B and 19F NMR Data of [(C2F5)(CF3)FCBF3]-
giving a white solid. The solid was dissolved in CD3CN and
investigated by 11B and 19F NMR spectroscopy. It consisted of
29.5% Na[(CF3)3BF],1 69.8% Na[C2F5BF3], and 0.7% Na[BF4] (see
Chart 1).
(3) Gas-Phase Decomposition of (CF3)3BCO: Procedure A.
A 100 mL PFA reactor was connected to a glass vacuum apparatus
and charged with 320 mg (1.3 mmol) of (CF3)3BCO by vacuum
transfer at -196 °C. The reactor was warmed to room temperature
and kept at this temperature for 20 h.
A 50 mL round-bottom glass flask equipped with a valve with
a PTFE stem (Young, London), fitted with a PTFE-coated magnetic
stirring bar, was charged with 200 mg (4.8 mmol) of NaF and 10
mL of CH3CN. The PFA reactor was cooled to -196 °C and the
CO was removed in vacuo. The content of the PFA reactor was
vacuum transferred into the glass flask at -196 °C and stirred for
10 min at room temperature. The suspension was filtered to remove
excess NaF, and the solvent was removed using a rotary evaporator
yielding a white solid. It consisted of a mixture of linear perfluo-
roalkyl borates as identified by 19F NMR spectroscopy of the
following composition: 1.6% Na[BF4], 26.0% Na[C2F5BF3], 39.4%
Na[C3F7BF3], 22.8% Na[C4F9BF3], 7.9% Na[C5F11BF3], 1.6% Na-
[C6F13BF3], and 0.7% Na[C7F15BF3] (see Chart 1).
bar, was charged with 1.0 g (23.8 mmol) of NaF and 10 mL of
CH3CN. The content of the glass finger was vacuum transferred
into the glass flask at -196 °C and stirred for 10 min at room
temperature. The suspension was filtered to remove excess NaF,
and the solvent was removed using a rotary evaporator yielding a
white solid. It consisted of a mixture of perfluoroalkylborates as
identified by their 19F NMR spectra. The main components of
perfluoroalkylborates were the following: 7.6% Na[BF4], 43.3%
Na[C2F5BF3], 13.1% Na[C3F7BF3], 0.5% Na[C4F9BF3], 32.8% Na-
[(C2F5)(CF3)FCBF3], 1.6% Na[(C2F5)2FCBF3], 1.2% Na[(C3F7)-
(CF3)FCBF3] (see Charts 1 and 2).
(6) Reaction of K[CF3BF3] with Gaseous AsF5. A sample of
K[CF3BF3] (198 mg, 1.13 mmol) was added to a 100 mL PFA
reactor. The PFA vessel was connected to a glass vacuum apparatus
and set under vacuum to remove all volatile compounds. A 102
mg (0.60 mmol) portion of AsF5 was transferred in vacuo into the
PFA reactor at -196 °C. The reaction mixture was warmed to room
temperature and stored for 28 h.
A 50 mL round-bottom flask equipped with a valve that has a
PTFE stem (Young, London), fitted with a PTFE-coated magnetic
stirring bar, was filled with 100 mg (2.4 mmol) of NaF and 10 mL
of CH3CN. The content of the PFA reactor was vacuum transferred
into the glass flask at -196 °C and stirred for 10 min at room
temperature. The suspension was filtered to remove excess NaF,
and the solvent was removed using a rotary evaporator yielding a
white solid. The solid was dissolved in CD3CN and investigated
by 11B and 19F NMR spectroscopy. It consisted of 65% Na[BF4]
and 35% Na[C2F5BF3] (see Chart 1) as well as minor amounts of
unidentified products.
(4) Gas-Phase Decomposition of (CF3)3BCO: Procedure B.
A 40 mg (0.16 mmol) portion of (CF3)3BCO was vacuum
transferred into a 500 mL round-bottom glass flask equipped with
a valve with a PTFE stem (Young, London), at -196 °C. The
borane carbonyl was allowed to warm to room temperature and
stored for 4 h (∼22 °C). The flask was cooled to -196 °C, and
CO formed during the reaction was removed under reduced
pressure. The residue was transferred into a 4 mm o.d. NMR tube,
and 1.5 mL of dry CFCl3 was added. The NMR tube was flame
sealed and warmed to 0 °C. The tube was placed inside a thin walled
5 mm o.d. NMR tube with a CD3CN film between both tubes. The
NMR spectra were recorded at 0 °C. The mixture consisted of BF3,
a mixture of linear perfluoroalkylboranes, and (CF3)3BCO as
identified by their 19F NMR spectra of the following composition:
2.8% BF3, 29.4% C2F5BF2, 28.0% C3F7BF2, 10.6% C4F9BF2, 3.1%
C5F11BF2, 0.7% C6F13BF2, and 25.4% (CF3)3BCO. NMR data of
the boranes, 11B NMR (96.92 MHz, CFCl3, 0 °C, BF3‚OEt2): BF3,
δ 9.9 ppm, 1J(11B,19F) ) 16.5 Hz; (CnF2n+1)BF2 (n ) 2-6), δ 19.5
ppm; (CF3)3BCO, δ -18.2 ppm, 2J(11B,19F) ) 33.1 Hz. 19F NMR
(282.41 MHz, CFCl3, 0 °C, CFCl3): BF3, δ -126.4 ppm, 1J(11B,19F)
(7) Reaction of K[C2F5BF3] with Gaseous AsF5. A 10 mL glass
finger equipped with a valve that has a PTFE stem (Young, London)
was charged with 259 mg (1.15 mmol) of K[C2F5BF3]. The white
salt was dried overnight and then 48 mg (0.28 mmol) of AsF5 were
transferred in vacuo into the PFA reactor at -196 °C. The reaction
mixture was warmed to room temperature, and the further reaction
was monitored by IR spectroscopy. After 2 h the reaction was
completed, and C2F5BF2 was obtained with slight impurities of BF3.
11B NMR (96.92 MHz, CFCl3, 0 °C, BF3‚OEt2): δ 19.5 ppm. 19F
) ∼16 Hz; C2F5BF2, δ(CFa3) -84.4 ppm, δ(CFb ) -134.7 ppm,
2
2
4
δ(BFc2) -75.6 ppm, J(11B,19Fc) ) 20-30 Hz, J(19Fa,19Fc) ) 3.9
Hz; C3F7BF2, δ(CFa3) -81.6 ppm, δ(CFb ) -129.4 ppm, δ(CFc2)
2
-134.3 ppm, δ(BFd ) -75.6 ppm, 2J(11B,19Fd) ) 20-30 Hz,
2
4J(19Fa,19Fc) ) 8.0 Hz; C4F9BF2, δ(CFa3) -81.6 ppm, δ(CFb )
2
-127.3 ppm, δ(CFc2) -126.1 ppm, δ(CFd ) -134.7 ppm, δ(BFe2)
NMR (282.41 MHz, CFCl3, 0 °C, CFCl3): δ(CFa3) -84.4 ppm,
2
-77.0 ppm, J(19Fa,19Fc) ) 9.2 Hz, J(19Fa,19Fd) ) 1.9 Hz; C5F11-
4
5
2
δ(CFb ) -134.7 ppm, δ(BFc2) -75.6 ppm, J(11B,19Fc) ) 20-30
2
BF2, δ(CFa3) -81.6 ppm, δ(CFb ) -127.0 ppm, δ(CFc2) -122.5
Hz, J(19Fa,19Fc) ) 3.9 Hz.
4
2
ppm, δ(CFd ) -126.8 ppm, J(19Fa,19Fc) ) 9.8 Hz, J(19Fa,19Fd) )
4
5
2
(d) Instrumentation. (1) Vibrational Spectroscopy. Gas-phase
infrared spectra were recorded with a resolution of 2 cm-1 in the
range 5000-400 cm-1 on a Bruker IFS 66v FTIR instrument.
Matrix infrared spectra were recorded using another Bruker IFS
66v FTIR spectrometer in the reflectance mode employing a transfer
optic. A DTGS detector, together with a KBr/Ge beam splitter, was
used in the region 5000-400 cm-1. In this region, 64 scans were
CO added for each spectrum using an apodized resolution of 1.2
2.4 Hz; C6F13BF2, δ(CFb ) -126.6 ppm, δ(CFc2) -123.4 ppm,
2
δ(CFd ) -121.7 ppm; (CF3)3BCO, δ -58.7 ppm, J ) ∼33 Hz.
2
2
(5) Reaction of K[B(CF3)4] with Gaseous AsF5. A 10 mL glass
finger equipped with a valve that has a PTFE stem (Young, London)
was charged with 169 mg (0.52 mmol) of K[B(CF3)4]. The flask
was connected to a glass vacuum apparatus and set under vacuum
to remove all volatile compounds. A 51 mg (0.30 mmol) portion
of AsF5 was transferred in vacuo into the glass finger at -196 °C.
The reaction mixture was warmed to room temperature and stored
for 20 h.
cm-1
.
(2) NMR Spectroscopy. 19F and 11B NMR spectra were recorded
at room or at low temperatures on a Bruker Avance DRX-300
spectrometer operating at 282.41 or 96.92 MHz for 19F and 11B
nuclei or on a Bruker Avance DRX-500 spectrometer, operating at
A 50 mL round-bottom flask equipped with a valve with a PTFE
stem (Young, London), fitted with a PTFE-coated magnetic stirring
Inorganic Chemistry, Vol. 43, No. 2, 2004 493

Finze et al.
470.59 or 160.46 MHz for 19F and 11B nuclei, respectively. The
NMR signals were referenced against CFCl3 (19F) as internal
standard and BF3‚OEt2 in CD3CN (11B) as external standard.
Concentrations of the investigated samples were in the range 0.1-1
mol L-1. Samples were prepared in 5 mm NMR tubes, equipped
with special valves with PTFE stems (Young, London).22 Dry CD3-
CN was used as solvent.
the introduction of the (CF3)3B fragment into a variety of
molecules, it is of interest to investigate the decomposition
pathway of (CF3)3BCO to gain a deeper insight into the
chemistry of the transient Lewis acid (CF3)3B. The products
of the gas-phase decomposition are linear perfluoroalkylbo-
ranes which are formed by a cascade of intramolecular
fluoride ion abstractions from CF3 ligands and perfluoroalkyl
migrations to the initially formed difluorocarbene complexes.
The formation of boranes with perfluoroalkyl chains other
than C3F7 is also observed and is due to either loss of CF2
(3) Computational Section. Quantum chemical calculations were
performed to support the experimental results presented in this study
and to understand the reaction pathways for the isomerization
reactions of (CF3)3B and (CF3)3BCF2 mainly leading to C3F7BF2
and (C2F5)(CF3)FCBF2, respectively. DFT calculations23 were
carried out using the B3LYP method24-26 with the basis sets
6-311G(d) and 6-311+G(d) as implemented in the Gaussian 98
program suite.27 Frequency calculations were performed for all
species employing the basis set 6-311G(d), and all structures
represent true minima without imaginary frequencies on the
respective hypersurface. Geometries and energies were recalculated
using the 6-311+G(d) basis set because the accuracy of the energies
of anions is improved with the incorporation of diffuse functions.28
Frequency calculations with the more time demanding basis set
6-311+G(d) were performed only for a few model compounds,
for example (CF3)3B and [B(CF3)4]-, and the wavenumbers are very
close to those calculated with the basis set 6-311G(d). Hence,
correction terms derived from calculations with 6-311G(d) were
used. Transition states exhibit one imaginary frequency, and IRC
calculations were performed to verify that the transition states
connect the products and reactants, respectively.29,30 All energies
presented herein are zero point corrected. For enthalpies and free
energies, the thermal contributions are included for 298 K.
(the formation of C2F5BF2) or trapping of CF2 by CnF2n+1
-
BF2 followed by migration of CnF2n+1. A similar degradation
reaction of (CF3)3BCO was also observed in anhydrous HF,
and hence, we have included these results in this report. To
prove our proposed reaction mechanism, the fluorine and
perfluoroalkyl shifts, we have studied the degradation of
(CF3)3B giving C3F7BF2 by quantum chemical calculations.
All reaction intermediates and transition states were calcu-
lated. The small barriers found for the fluoride and perfluo-
roalkyl migrations help to explain why only monoperfluo-
roalkylboranes are observed as products.
The abstraction of a fluoride ion from [B(CF3)4]- leads
to the transient difluorocarbene complex (CF3)3BCF2 which
should react with HF to give the novel conjugated Brønsted-
Lewis acid, [H2F][B(CF3)4]. Although ab initio calculations
predict an even higher stability for (CF3)3BCF2 against loss
of the neutral ligand than for (CF3)3BCO, similar to the
attempted synthesis of (CF3)3B, the isolation of (CF3)3BCF2
failed. The products of the gas-phase degradation of (CF3)3-
BCF2 are linear and also branched pefluoroalkylboranes. This
result indicates a similar rearrangement process as discussed
above for the decomposition of (CF3)3B. We have studied
the degradation process of (CF3)3BCF2 by density functional
theory to confirm our proposed rearrangement mechanism
as already described for (CF3)3B.
Results and Discussion
(I) Investigation of the Transient Lewis Acids (CF3)3B
and (CF3)3BCF2. All attempts on the synthesis of the free
Lewis acid tris(trifluoromethyl)borane, (CF3)3B, have failed
so far. Recently we reported on the synthesis of (CF3)3BCO
which has been shown to behave in some reactions as a
synthon for (CF3)3B, for example in the formation of (CF3)3-
BNCCH3 with acetonitrile15 or the reaction with anhydrous
HF to [H2F][(CF3)3BF].17 Furthermore, (CF3)3BCO undergoes
ligand exchange with 13CO in the gas phase as proven by
the synthesis of (CF3)3B13CO. Since (CF3)3BCO is a useful
reagent for the generation of weak coordinating anions17 and
(a) Decomposition Reactions of (CF3)3BCO in Anhy-
drous HF and in the Gas Phase. In anhydrous HF, (CF3)3-
BCO forms the new Brønsted-Lewis acid, [H2F][(CF3)3BF]
(see eq 3). In the presence of Co2(CO)8 mainly [Co(CO)5]-
[(CF3)3BF] is formed (eq 4), and only small quantities of
[C2F5BF3]- salts are observed as side products.17
(22) Gombler, W.; Willner, H. Int. Lab. 1984, 84.
(23) Kohn, W.; Sham, L. J. Phys. ReV. A 1965, 140, 1133.
(24) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
Co2(CO)8 + 2(CF3)3BCO + 2HF f
2[Co(CO)5][(CF3)3BF] + H2 (4)
(25) Becke, A. D. Phys. ReV. B 1988, 38, 3098.
(26) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 41, 785.
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian, Inc.:
Pittsburgh, PA, 1998.
If the reaction is carried out in the presence of KHF2
instead of Co2(CO)8, 23% of [C2F5BF3]- and 77% of
[(CF3)3BF]- are formed. The product distribution is reversed
to 70% of [C2F5BF3]- and only 30% of [(CF3)3BF]- if no
KHF2 is added due to the enhanced acidity. The [(CF3)3BF]-
anion is the product of the carbonyl group exchange of
(CF3)3BCO by F-. For the formation of the [(C2F5)BF3]-
anion, an intramolecular rearrangement mechanism is pro-
posed (Scheme 1). The ratio of both anions formed,
[(CF3)3BF]- and [C2F5BF3]-, strongly depends (i) on the
availability of F- in the HF solution (the more F- is available
the more (CF3)3B is trapped as [(CF3)3BF]-) and (ii) on the
acidity of the HF solution (the more acidic the reaction
(28) Rienstra-Kiracofe, J. C.; Tschumper, G. S.; Schaefer, H. F., III; Nandi,
S.; Ellison, G. B. Chem. ReV. 2002, 102, 231.
(29) Gonzales, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154.
(30) Gonzales, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
494 Inorganic Chemistry, Vol. 43, No. 2, 2004

Transient Lewis Acids (CF3)3B and (CF3)3BCF2
Scheme 1. Reaction of (CF3)3BCO in Anhydrous HF (∆E(298 Κ))a
Figure 1. Gas phase and solid state IR (Ar matrix) spectra of C2F5BF2.
a Energy: B3LYP/6-311+G(d). ZPC and thermal corrections: B3LYP/
formation of C2F4 occurs due to the low concentration of
CF2. Addition of BF3 to (CF3)3BCO does not change the
course of the reaction, and a nearly identical product
composition is observed. Hence, BF3 is the weakest Lewis
acid in this reaction mixture. On the other hand, BF3 is
capable of reacting with CF2 to yield living polymers under
thermal and photolytic conditions with difluorodiazirine as
CF2 source in great access.31,32
The reaction of K[CF3BF3] with AsF5 at room temperature
led to a mixture of BF3 and C2F5BF2 which were detected
as their corresponding borate anions after reaction with NaF
in CH3CN. No [CF3BF3]- was found after treatment with
NaF. The instability of CF3BF2 is explained by dissociation
into CF2 and BF3 followed by trapping of CF2 with CF3BF2
as C2F5BF2 which is in agreement with the rearrangement
reactions discussed above.
6-311G(d).
medium is, the more the equilibrium between [(CF3)3BF]-
and (CF3)2BF(CF2) shifts toward the difluorocarbene com-
plex). The rearrangement is either initiated by an intramo-
lecular fluoride ion shift from one CF3 group to the B atom
of (CF3)3B or an intermolecular fluoride ion abstraction from
one CF3 ligand in [(CF3)3BF]-. The next step is an intramo-
lecular migration of one CF3 group in the carbene complex
(CF3)2BF(CF2). This step explains the formation of a
perfluoroethyl group. A further intramolecular F- shift from
the remaining CF3 group followed by loss of difluorocarbene
and the addition of a fluoride ion from HF results in
[C2F5BF3]-. Difluorocarbene is immediately trapped by HF
as HCF3 which was observed by IR spectroscopy in the gas
phase over the HF solution.
The decomposition of (CF3)3BCO in the gas phase has
already been investigated by us,15 but some of the results
must be reinterpreted. To underline the reinterpretation, new
additional experiments have been performed as described in
the Experimental Section. To prevent side reactions with the
glass walls, the reaction was performed in PFA vessels. The
resulting gaseous reaction mixture was reacted with sodium
fluoride in acetonitrile to convert the boranes into the
corresponding borates. This procedure has two advantages:
(i) the borates are easier to handle because they are
nonvolatile salts, and (ii) borate anions usually exhibit sharper
NMR signals as the related boranes.
The formation of the observed linear perfluoroalkylborates
[CnF2n+1BF3]- (n ) 2-7) can be rationalized by initial
formation of the carbene complex C2F5BF2(CF2) as shown
in Scheme 1. This carbene complex can undergo two
reactions: (i) the reversible dissociation into C2F5BF2 and
CF2 or (ii) the irreversible migration of the C2F5 group to
the carbon atom of the difluorocarbene ligand (eq 5).
In contrast to CF3BF2, C2F5BF2 can be synthesized from
K[C2F5BF3] by abstraction of one F- ion with AsF5 according
to eq 6.
K[C2F5BF3] + AsF5 f C2F5BF2 + K[AsF6]
(6)
The product was unambiguously characterized by NMR
spectroscopy (see Experimental Section). The gas-phase IR
spectrum of C2F5BF2 is displayed in Figure 1, and the
calculated wavenumbers are in good agreement with the
experimental values (Table 1). In addition, calculated and
reported6,7 wavenumbers for CF3BF2 are also compared. In
contrast to C2F5BF2, the experimental values reported for
CF3BF2 strongly deviate from the calculated data. Hence it
6,7
is very likely that the IR spectrum of CF3BF2 has never
been observed.
Comparison of the IR spectrum of C2F5BF2 in Ar matrix
at 16 K (Figure 1) with the reported IR matrix spectrum of
the low pressure thermolysis products (180 °C) of (CF3)3-
BCO15 reveals that C2F5BF2 is the main product of the
thermolysis besides CO and CF2. Hence the possible ther-
molysis products of (CF3)3BCO like (CF3)3B, (CF3)2BF, and
CF3BF2 are all very short-lived species, and the rearrange-
ment process to yield C2F5BF2 must be very fast.
(b) Rearrangement Reactions of (CF3)3BCF2. All at-
tempts to isolate the difluorocarbene complex (CF3)3BCF2
The formation of perfluoroalkyl chains with more than
two CF2 groups is due to trapping of CF2 by C3F7BF2
followed by transfer of the perfluoroalkyl chain to the C atom
of the carbene ligand. Repeated CF2 addition followed by
migration of the perfluoroalkyl group leads to chain growth.
IR spectra of the gaseous reaction mixture reveal that no
(31) Ogden, P. H.; Mitsch, R. A. J. Heterocycl. Chem. 1968, 5, 41.
(32) Mitsch, R. A.; Ogden, P. H. (Minnesota Mining and Manufacturing
Company). United States Patent US3,493,629, 1970.
Inorganic Chemistry, Vol. 43, No. 2, 2004 495

Finze et al.
Table 1. Observed and Calculateda Band Positions [cm-1] and IR Intensities [km mol-1] of CF3BF2 and C2F5BF2
CF3BF2
C2F5BF2
expt(s)c
calcda
exptb
calcda
expt(g)d
ν˜
I
ν˜
ν˜
I
ν˜
ν˜
assignment
1502
1448
1374
1311
1218
1193
1133
1124
978
748
703
602
589
545
521
470
356
347
312
267
213
179
112
46
1509
1458
1374
1323
1229
1207
1135
1135
980
m
vs
m
s
1509
1457
1377
1341
1222
1222
1139
1139
990
m
vs
m
s
vs
vs
vs
vs
s
νas(10BF2)
νas(11BF2)
νs(BF2)
νs(CF3)
νas(CF3)
νas(CF3)
νs(CF2)
νas(CF2)
ν(CC)
δs(CF3)
π(BF2)
δs(BF2)
δas(CF3)
δs(CF2)
δas(CF3)
F(BF2)
F(CF3)
F(CF3)/τ(CF2)
ν(BC)
ω(CF2)
τ(CF2)/F(CF3)
F(CF2)
δ(BCC)
τ(CF3)
τ(BF2)
1448
1431
1137
1156
1101
336
13
311
265
249
(1460)
m
355
103
194
151
265
244
131
105
10
16
44
0.8
36
22
s
s
s
(1190)
(1080)
s
vs
vs
vs
s
w
w
m
753
692
595
545
0.3
48
51
754
690
607
n.o.
691
607
(690)
w
w
w
0.1
543
525
m
m
499
378
186
174
329
9
1.8
6
3.4
0.2
0.1
0.3
0.6
0.5
3.5
1.0
3.7
1.0
0.00
0.00
6
0.05
8
a B3LYP/6-311G(d). b Attributed band positions for (CF3)BF2. Further unassigned bands: 1400-1350w, 1255m, 1160w, 1040m, 850m, 845m, 725w.6
c Ar matrix, this work. d Gas-phase spectrum at room temperature, this work.
Table 2. Calculateda F-, CF2, and CO Affinities of Perfluoroalkyl Boranes and Selected Bond Parameters of the Addition Products
F- affinity
CF2 affinity
CO affinity
-∆E
-∆H
-∆G
d(B-F)
-∆E
-∆H
-∆G
d(B-CF2)
-∆E
-∆H
-∆G
d(B-CO)
compd
BF3
kJ mol-1 kJ mol-1 kJ mol-1
Å
kJ mol-1 kJ mol-1 kJ mol-1
Å
kJ mol-1 kJ mol-1 kJ mol-1
Å
329.7
392.6
460.8
533.9
401.2
402.6
406.4
401.9
332.2
395.1
463.3
536.4
403.6
405.1
408.9
404.4
293.4
349.5
422.8
490.9
362.1
368.3
368.7
363.4
1.417
1.412
1.413
1.419
1.410
1.408
1.408
1.404
18.6
42.4
101.3
171.0
42.1
21.1
44.9
103.8
173.4
44.6
-19.0
-14.5
51.3
119.4
-11.0
5.9
1.771
1.719
1.624
1.593
1.742
1.745
0.1
2.1
25.1
102.2
2.5b
4.6
27.6
-24.1
-31.2
-15.2
52.3
2.957b
2.887
1.722
1.589c
CF3BF2
(CF3)2BF
(CF3)3B
C2F5BF2
C3F7BF2
C4F9BF2
(C2F5)(CF3)-
FCBF2
104.7
39.1
41.5
a Energy: B3LYP/6-311+G(d). ZPC and thermal corrections: B3LYP/6-311G(d). b Expt values: ∆H ) -7.6 ( 0.3 kJ mol-1 33
Å.34 c Expt values: d(B-CO) ) 1.617(12) Å (gas phase), 1.69(2) Å (solid state).15
,
d(B-CO) ) 2.886(5)
Table 3. Calculateda F--Affinities of the Carbene Complexes
(CF3)nBF3-nCF2 (n ) 0-3)
failed, although DFT calculations predict a higher stability
against elimination of CF2 than of CO from (CF3)3BCO
(Table 2). Attempted syntheses for this target molecule
include (i) the reaction of [Ph3C][B(CF3)4] with Et3SiH and
iPr3Al, (ii) the reaction of NO[B(CF3)4] with Me3SiSiMe3,
(iii) the thermolysis of [p-ClC6H4N2][B(CF3)4] or [o-EtC6H4N2]-
[B(CF3)4], and (iv) the reaction of K[B(CF3)4] with SbF5.
All reactions resulted in complex product mixtures, and in
some cases, C2F5BF2 could be identified as a product.
Since the reaction of K[B(CF3)4] with SbF5 is very
vigorous, AsF5 was chosen as a weaker and gaseous Lewis
acid. The boranes formed after the solid-gas reaction of
K[B(CF3)4] with AsF5 were vacuum transferred onto NaF
with acetonitrile and reacted to the corresponding borates
which were analyzed by NMR spectroscopy. The fluoride
ion abstraction from [B(CF3)4]- with AsF5 proceeds easily
although the calculated gas-phase F- affinities of AsF5 (∆H
F--affinity
-∆E
-∆H
-∆G
compd
kJ mol-1
kJ mol-1
kJ mol-1
BF3(CF2)
443.3
470.9
484.2
506.0
445.8
473.4
486.7
508.5
400.2
438.2
441.8
458.3
CF3BF2(CF2)
(CF3)2BF(CF2)
(CF3)3BCF2
a Energy: B3LYP/6-311+G(d). ZPC and thermal corrections: B3LYP/
6-311G(d).
) 434.4 kJ mol-1) (Table S1) and (CF3)3BCF2 (∆H ) 508.5
kJ mol-1) (Table 3) indicate that no reaction should occur.
[B(CF3)4]- + AsF5 f (CF3)3BCF2 + [AsF6]-
∆H ) +74.1 kJ mol-1 (7)
496 Inorganic Chemistry, Vol. 43, No. 2, 2004

Transient Lewis Acids (CF3)3B and (CF3)3BCF2
Scheme 2. Rearrangement of (CF3)3BCF2 in the Gas Phase (∆E(298 Κ))a
a Energy: B3LYP/6-311+G(d). ZPC and thermal corrections: B3LYP/6-311G(d).
However, the higher lattice energy35 of K[AsF6] (Upot
)
Scheme 3. Formation Pathways of Branched Perfluoroalkylborates
-579 kJ mol-1; Vmol ) 120 Å3) in comparison to K[B(CF3)4]
(Upot ) -479 kJ mol-1; Vmol ) 244 Å3) makes the reaction
exothermic.
K[B(CF3)4] + AsF5 f (CF3)3BCF2 + K[AsF6]
∆H ) -25.9 kJ mol-1 (8)
The carbene complex (CF3)3BCF2 formed initially is
unstable and undergoes rearrangement reactions leading to
the formation of linear perfluoroalkylboranes. In addition,
branched perfluoroalkylboranes are observed. The formation
of the main products C2F5BF2 (43.3%), C3F7BF2 (13.1%),
and (C2F5)(CF3)FCBF2 (32.8%) can be explained according
to Scheme 2. The initial step of the rearrangement process
is a CF3 group migration to the CF2 ligand followed by an
intramolecular fluoride ion abstraction from one of the two
remaining CF3 groups. The transfer of either CF3 or C2F5 in
(C2F5)(CF3)BF(CF2) leads to the formation of (C2F5)2BF or
(C3F7)(CF3)BF, respectively. A further fluoride ion abstrac-
tion in (C3F7)(CF3)BF to yield (C3F7)BF2(CF2) followed by
migration of C3F7 or loss of CF2 gives the final products
C4F9BF2 and C3F7BF2, respectively. (C3F7)BF2(CF2) can
dissociate into the borane and CF2 due to the decreased Lewis
acidity of boron compared to boranes with more than one
perfluoroalkyl group. Unexpected is the instability of (C2F5)2-
BF under the reaction conditions: it was not found in the
reaction mixture. Since the boron atom in (C2F5)2BF is more
Lewis acidic than boron in monoperfluoroalkylboranes,
intramolecular fluoride ion abstraction occurs. (C2F5)(CF3)-
FCBF2 is formed via C2F5BF2(CFCF3) although the inter-
mediate monofluorocarbene complex is less stabilized than
the difluorocarbene complexes that are involved in the
reaction steps described above. Alternative reaction pathways
for (C2F5)2BF are shown in eq 9.
Both routes, (i) a cascade of two 1,2-fluorine shifts as well
as (ii) a 1,3-fluorine shift, lead to the formation of C2F5BF2
and C2F4.
Since C3F7BF2(CF2) acts as a difluorocarbene source, the
formation of other branched boranes (∼3%) can be under-
stood as presented in Scheme 3.
(c) Computational Results for the Model Systems:
CF3BF2 and C2F5BF2. The simplest trifluoromethylfluo-
roborane, CF3BF2, is unstable with respect to reversible
dissociation into BF3 and CF2 followed by trapping of CF2
by CF3BF2 as shown in the Experimental Section. In contrast
to CF3BF2, no decomposition at room temperature was
observed for C2F5BF2. These results are surprising when
(33) Sluyts, E. J.; van der Veken, B. J. J. Am. Chem. Soc. 1996, 118, 440.
(34) Janda, K. C.; Bernstein, L. S.; Steed, J. M.; Novick, S. E.; Klemperer,
W. J. Am. Chem. Soc. 1978, 100, 8074.
(35) Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg.
Chem. 1999, 38, 3609.
+
compared to the isoelectronic carbocations CF3CF2 and
Inorganic Chemistry, Vol. 43, No. 2, 2004 497

Finze et al.
twice as high as the barrier calculated for the 1,2-fluoride
ion migration in CF3BF2 (Figure 2) in agreement with our
experimental observations that C2F5BF2 does not dissociate
into C2F4 and BF3 although the reaction is exergonic (eq 11).
C2F5BF2 f BF3 + C2F4
∆H ) -10.5 kJ mol-1 (∆G ) -49.9 kJ mol-1) (11)
The 1,2-fluorine shift from the CF2 group in C2F5BF2 to
boron was also calculated, and the value for the transition
state is close to the value for the transition state of the 1,3-
fluorine shift in C2F5BF2 (Figure 3). Since it was not possible
to locate a minimum for CF3CFBF3 and the geometry of the
transition state for the 1,2-fluorine shift is close to that
expected for CF3CFBF3, it is likely that CF3CFBF3 has no
local minimum on the respective hypersurface. IRC calcula-
tions indicate that CF3CFBF3 represents a transition state for
a fluoride ion exchange between carbon and boron in C2F5-
BF2. The dissociation of the transient species CF3CFBF3 into
CFCF3 and BF3 or a further 1,2-fluorine shift from CF3 to
CF is endothermic. Hence, no decomposition of C2F5BF2 is
possible (Figure 3).
Figure 2. Calculated isomerization of CF3BF2 to F3BCF2 and dissociation
of F3BCF2 into CF2 and BF3 (energy and geometry, B3LYP/6-311+G(d);
wavenumbers and ZPC, B3LYP/6-311G(d)).
+
C2F5CF2 , because the barrier for a 1,2-fluorine migration
in CF3CF2+ (72.4 kJ mol-1) is calculated to be significantly
higher than the barrier for a 1,3-fluorine shift in CF3CF2-
+
CF2 (27.2 kJ mol-1).36 Unfortunately, only little is known
about fluoride ion migration reactions in carbocations,37,38
and so, no experimental data are available on the stabilities
+
of CF3CF2+ and C2F5CF2 . The transition states for the 1,2-
fluorine shifts in CF3BF2 and C2F5BF2, the 1,3-fluorine shift
in C2F5BF2, and the thermochemistry for the dissociations
of the carbene complexes into BF3 and the carbene were
calculated (Figures 2 and 9). Since in the rearrangement
reactions of (CF3)3B and (CF3)3BCF2 1,2-fluorine shifts are
proposed to be key steps, it is of interest to know the barrier
for the fluoride migration in CF3BF2 as a reference (Figure
2).
Although the 1,2-fluorine shift in CF3BF2 (Figure 2) and
the decomposition of CF3BF2 into BF3 and CF2 are endot-
hermic (∆H ) +71.8 kJ mol-1; ∆G ) +31.7 kJ mol-1), the
decomposition readily proceeds because it is followed by
the loss of CF2. Since CF2 can be trapped, e.g., by the more
Lewis acidic CF3BF2 compared to BF3 giving CF3BF2(CF2),
the loss of CF2 is irreversible and C2F5BF2 is formed via
CF3 group migration (eq 10).
Similar to our observations for CF3BF2 and C2F5BF2,
phosphoranes with C2F5 groups are known to be more stable
than the corresponding compounds with CF3 ligands with
respect to loss of CF2.39,40
(d) Computational Results for the Isomerizations of
(CF3)3B and (CF3)3BCF2. The isomerization of (CF3)3B to
C3F7BF2 (Figure 4) was modeled as a reaction cascade of
fluoride ion and perfluoroalkyl migrations. All reaction
barriers are small, and the overall isomerization energy is
-303.6 kJ mol-1 (∆H ) -305.0 kJ mol-1). The optimized
geometries of the transition states clearly demonstrate that
the rearrangement reactions of the difluorocarbene complexes
to perfluoroalkyl boranes are transfers of perfluoroalkyl
chains from the B atom to the C atom of the carbene ligand
and not an insertion reaction of the difluorocarbene into the
B-C or C-F bond of the perfluoroalkyl chain (Figure 4).
The activation barrier for the fluorine shift of a CF3 group
becomes lower with increasing Lewis acidity of boron due
to more perfluoroalkyl chains attached to boron (Figures 2
and 10). Only in the case of three perfluoroalkyl groups
attached to boron is the fluoride ion migration exothermic.
The reverse trend is observed for the transfer of perfluoro-
alkyl groups to the carbene ligand. The lower the Lewis
acidity of the B atom is, the easier the perfluoroalkyl chain
can migrate. The driving forces for the isomerizations are
the gains in energy due to the formations of B-F from C-F
bonds and of C-C from B-C bonds, respectively.
2CF3BF2 f BF3 + C2F5BF2 ∆H ) -124.3 kJ mol-1 (10)
The small activation barrier for the fluoride ion migration
in CF3BF2 (80.7 kJ mol-1) is the reason for its instability.
The activation barriers for the transfers of perfluoroalkyl
chains in difluorocarbene borane complexes are even smaller
as will be shown for the isomerization of (CF3)3B giving
C3F7BF2 (Figure 3); hence, the intramolecular transfer of the
CF3 group of CF3BF2(CF2) to give C2F5BF2 will readily take
place (eq 10). These results are in agreement with our
experimental findings. Furthermore, this result is also
consistent with earlier reports that BF3 catalyzes the polym-
erization of CF2.31,32
In Figure 5, the isomerization of (CF3)3BCF2 to (C2F5)-
(CF3)FCBF2 is displayed, and the overall reaction energy is
-363.4 kJ mol-1 (∆H ) -365.3 kJ mol-1). The calculations
reveal the same trends for the C-F activations as well as
the perfluoroalkyl transfers involved in the reactions from
The energy of the transition state for a 1,3-fluorine shift
in C2F5BF2 was computed to be 131.1 kJ mol-1 higher than
the ground state energy of C2F5BF2 (Figure 3). This is nearly
(36) Krespan, C. G.; Dixon, D. A. J. Fluorine Chem. 1996, 77, 117.
(37) Nguyen, V.; Mayer, P. S.; Morton, T. H. J. Org. Chem. 2000, 65,
8032.
(39) Semenii, V. Y.; Stepanov, V. A.; Ignat’ev, N. V.; Furin, G. G.;
Yagupol’skii, L. M. J. Gen. Chem. USSR (Engl. Transl.) 1985, 55,
2415.
(38) Shaler, T. A.; Morton, T. H. J. Am. Chem. Soc. 1994, 116, 9222.
(40) Mahler, W. Inorg. Chem. 1963, 2, 230.
498 Inorganic Chemistry, Vol. 43, No. 2, 2004

Transient Lewis Acids (CF3)3B and (CF3)3BCF2
Figure 3. Calculated decomposition of C2F5BF2 (energy and geometry, B3LYP/6-311+G(d); wavenumbers and ZPC, B3LYP/6-311G(d)).
Figure 4. Calculated reaction profile for the isomerization of (CF3)3B to C3F7BF2 (energy and geometry, B3LYP/6-311+G(d); wavenumbers and ZPC,
B3LYP/6-311G(d)).
(CF3)3BCF2 to (C2F5)2BF compared to the isomerization of
(CF3)3B. A more complex case is the last step, the rear-
rangement of (C2F5)2BF giving (C2F5)(CF3)FCBF2. The
isomerization should proceed under C-F activation via a
transition state to give the carbene complex C2F5BF2(CFCF3)
which rearranges via a second transition state to the final
product (C2F5)(CF3)FCBF2. It was not possible to verify by
IRC calculations that the found transition states connect the
reactants and the products. All calculations performed
indicate that the energies of the transition states are very
similar to the energy of the minimum of C2F5BF2(CFCF3)
and hence the potential is very flat in this area. In Figure 5,
one minimum of the monofluorocarbene complex is depicted.
The energies of the located transition states and the energy
of C2F5BF2(CFCF3) differ by only (3 kJ mol-1, and so, the
value for the activation barrier as shown in Figure 5
represents the overall activation barrier for this process very
well. A comparison with the activation barriers found for
1,2- and 1,3-fluorine shifts in C2F5BF2 which are very close
indicates that for the degradation of (C2F5)2BF both reaction
pathways are possible. The 1,2-fluorine migration followed
by C2F5 group transfer leads to the formation of (C2F5)(CF3)-
FCBF2 as already outlined whereas the 1,3-fluorine shift is
followed by dissociation into C2F5BF2 and C2F4. Since in
the reaction mixture C2F5BF2 is one of the main components,
the assumption of a competitive decomposition is very likely.
Inorganic Chemistry, Vol. 43, No. 2, 2004 499

Finze et al.
Figure 5. Calculated reaction profile for the isomerization of (CF3)3BCF2 to (C2F5)(CF3)FCBF2 (energy and geometry, B3LYP/6-311+G(d); wavenumbers
and ZPC, B3LYP/6-311G(d)).
(e) Discussion and Summary of the Results on the
Stability of (CF3)3B and (CF3)3BCF2. Both strong Lewis
acids (CF3)3B and (CF3)3BCF2 are unstable with respect to
degradation to mono perfluoroalkylfluoroboranes at room
temperature as shown experimentally and verified by density
functional calculations. Furthermore, the computational
results on the stabilities of CF3BF2 and C2F5BF2 are also in
agreement with the experimentally observed behavior which
shows that the calculations display the real behavior very
well.
As mentioned above the intramolecular fluoride migrations
presented in this study are the first examples for reactions
of this type in boron chemistry, whereas for other elements
intramolecular C-F activations have been observed in the
past.41 In main group chemistry only for phosphorus and
arsenic compounds have similar reactions been reported to
our knowledge.42,43 In variance to the well-known feature
of C-F activations at R-C atoms of perfluoroalkyl ligands,
the examples for intramolecular transfer reactions onto the
carbene ligand derived through the activation process are
rare. One example is the synthesis of (Et3P)(BF4)Ni{CF-
(PEt3)(CF2)3} from BF3 and (Et3P)2Ni{(CF2)4}.44 Examples
for similar perfluoroalkyl transfer reactions to CF2 complexes
in the literature are very limited in general,41 e.g., the
synthesis of C6F5(CF2)2Cu from C6F5Cu and CF3Cu in DMF
is a typical exception.45,46
(f) F-, CF2, and CO Affinities: A Measure for the Lewis
Acidity of Perfluoroalkylfluoroboranes. The replacement
of a F atom against a perfluoroalkyl group on boron leads
to an enhanced Lewis acidity due to the reduction of π-back-
donation from fluorine to boron. We have studied the F-,
CF2, and CO affinities of the series of fluoro(trifluoromethyl)-
borates (Figure 6, Tables 2 and 3) to demonstrate the changes
in the Lewis acidity and some of its effects. F- and CF2
affinities were calculated because F- abstraction from
trifluoromethyl substituents leading to CF2 ligands is the
initial step of the decomposition reactions of trifluorometh-
ylboranes studied in this contribution. CO affinities are of
interest because during the degradation of (CF3)3BCO carbon
monoxide is released and hence present in the reaction
mixture.
A general trend for the bond strength of F-, CF2, and CO
to boron in the investigated boranes is that CO is bound more
weakly than CF2 and F- (Table 2). CO complexes of BF3
and CF3BF2 are only stabilized by weak van der Waals
interactions. The values calculated for F3BCO are close to
values derived from experimental studies.33,34
The F- affinities underline the trend for the Lewis acidities
as outlined above, for example (CF3)3B is a stronger Lewis
acid than BF3 (Table 2). The electron affinity of the F atom
(41) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. ReV. 1994,
94, 373.
(42) Eujen, R.; Haiges, R. Z. Naturforsch., B: Chem. Sci. 1998, 53b, 1455.
(43) Eujen, R.; Hoge, B. J. Organomet. Chem. 1995, 503, C51.
(44) Burch, R. R.; Calabrese, J. C.; Ittel, S. D. Organometallics 1988, 7,
1642.
(45) Yang, Z.-Y.; Wiemers, D. M.; Burton, D. J. J. Am. Chem. Soc. 1992,
114, 4402.
(46) Yang, Z.-Y.; Burton, D. J. J. Fluorine Chem. 2000, 102, 89.
500 Inorganic Chemistry, Vol. 43, No. 2, 2004

Transient Lewis Acids (CF3)3B and (CF3)3BCF2
Table 4. Calculateda F--Affinities of Selected Boron and Carbon
Lewis Acids
F- affinity
-∆E
-∆H
-∆G
compdb
kJ mol-1 kJ mol-1 kJ mol-1 symmetry
(CF3)2BFCF2
(CF3)3B
484.2
533.9
617.5
426.1
527.9
602.4
623.8
638.0
699.4
752.8
769.1
784.3
486.7
536.4
620.0
428.6
530.4
604.9
626.3
640.4
701.8
755.3
771.6
786.8
441.8
490.9
584.0
374.4
485.3
556.5
587.7
600.6
651.9
708.6
727.7
739.9
Cs
C3h
C3
Cs
Cs
Cs
C3W
C3W
Cs
Cs
C5W
Cs
(CF3)3B tetrahedralc
d
â-BC9F15
d
R-BC9F15
d
γ-BC9F15
i-BC9F15
d
e
i-BC19F19
i-CB11F11
g
g
m-CB11F11
p-CB11F11
o-CB11F11
g
g
a Energy: B3LYP/6-311+G(d). ZPC and thermal corrections: B3LYP/
c
6-311G(d). b Compounds in bold letters are boron Lewis acids.
(CBC)
) 110.81° (adopted from [(CF3)3BF]-). d Perfluoro-1-boradamantane (per-
fluoro-1-boratricyclo[3.3.1.13,7]decane). e Perfluoroboradodecahedrane; i, R,
â, γ, δ, ꢀ: bonds between B atom and unsaturated center, F- abstraction
in R, â, γ, δ, ꢀ position leads to opening of the cage. f Energy without zero
point correction. g i, o, m, and p position of the unsaturated center.
Figure 6. Comparison of the computed structures of (CF3)nBF3-n (n )
0-3) and related CF2 and CO complexes (B3LYP/6-311+G(d)).
found for the transition state connecting CF3BF2 and BF3-
(CF2) (80.7 kJ mol-1) whereas the lowest barrier is found
between (CF3)3B and (CF3)2BF(CF2) (19.4 kJ mol-1). The
gain in Lewis acidity and hence also in F- affinity from CF3-
BF2 to difluoroboranes with longer perfluoroalkyl chains is
rather small (Table 2).
was calculated (336.3 kJ mol-1) to investigate the perfor-
mance of the method applied for the calculations because
electron affinities and hence also F- affinities are known to
be critical cases for calculations.28,47 The difference of the
computational value for the electron affinity of the F atom
to the experimental value (328.1 kJ mol-1)28 is 8.2 kJ mol-1
(Table S1), so no further correction of the computational
data seems to be necessary. Furthermore, the F- affinity
calculated for BF3 (332.2 kJ mol-1) (Table 2) is also in good
agreement with the experimental value (331 ( 8 kJ
mol-1).48,49 Some of the F- affinities presented in this study
correspond very well to those reported in the literature.47,50
The decrease of the CF2 affinities from (CF3)3B to BF3
helps to explain that CnF2nBF2(CF2) is in equilibrium with
the free borane and CF2 and, hence, why CF3BF2 is unknown.
The occurrence of an equilibrium is the main reason for the
formation of linear monoperfluoroalkyldifluoroboranes with
carbon chains longer than three C atoms in the decomposition
of (CF3)3BCO, where (C2F5)BF2(CF2) formed from (C2F5)-
(CF3)BF acts as CF2 source. The trend in CF2 affinity also
gives a clue for the observation of smaller quantities of
reaction products that are due to CF2 transfer in the
rearrangement reaction of (CF3)3BCF2 because in this case
only a minor fraction decomposes via (C3F7)(CF3)BF which
then forms (C3F7)BF2(CF2) and partially dissociates whereas
the main route is via (C2F5)2BF which cannot serve as a CF2
source (Scheme 2).
(g) Comparison of the Influence of π-Back Donation
and Geometry at Boron on the Lewis Acidity of Selected
Perfluoroalkylfluoroboranes, Perfluoroalkylfluoroborate
Anions, Carboranes, and Carboronate Anions. Since our
initial aim is the use of perfluoroborates as weak coordinating
anions, a comparison of the F- affinities of perfluoroalky-
lfluoroborates with other perfluoroborates is of interest.
Especially the evaluation of the stabilizing and destabilizing
parameters for the design of novel boron based weak
coordinating anions is essential. The comparison of the F-
affinities of some different perfluoroboron species with
perfluoroalkylfluoroborates leads to a qualitative view on the
influence of π-back-donation from fluorine to boron and the
geometry at boron (Table 4).
In the series BF3, CF3BF2, (CF3)2BF, and (CF3)3B, the
Lewis acidity toward F- increases for each step by about
70-80 kJ mol-1. These increments display the loss of
π-back-donation in the free Lewis acids. In Table 3, the F-
affinities of the corresponding difluorocarbene complexes
are listed, and interestingly, in the case of [(CF3)3BF]- the
fluoride ion is bound more strongly to boron than to carbon
(Tables 2 and 3, Scheme 1). In the cases of the remaining
borates, the fluoride anion is easier to remove from boron
than from one of the CF3 groups. The increase in F- affinity
and hence in Lewis acidity from BF3 to (CF3)3B is due to
the loss of stabilization for the planar three coordinate borane
by π-back-donation from the fluorine ligands.
The increases in the F- affinities from BF3 to (CF3)3B
underline the trends found for the activation barriers for 1,2-
fluorine shifts as discussed above. The highest barrier is
(47) Christe, K. O.; Dixon, D. A.; McLemore, D.; Wilson, W. W.; Sheehy,
J. A.; Boatz, J. A. J. Fluorine Chem. 2000, 101, 151.
(48) Veljkovic, M.; Neskovic, O.; Zmbov, K. F.; Borshchevsky, A. Y.;
Vaisberg, V. E.; Sidorov, L. N. Rapid Commun. Mass Spectrom. 1991,
5, 37.
(49) Aleshina, V. E.; Borshchevsky, A. Y.; Korobov, M. V.; Sidorov, L.
N. Russ. J. Phys. Chem. (Engl. Transl.) 1996, 70, 1170.
(50) Dixon, D. A.; Christe, K. O. Book of Abstractss225th ACS National
Meeting; New Orleans, LA; American Chemical Society: Washington,
DC, 2003.
The Lewis acidity of (CF3)3B is reduced by 90 kJ mol-1
due to the planar configuration at boron in comparison to
Inorganic Chemistry, Vol. 43, No. 2, 2004 501

Finze et al.
Table 5. Calculateda and Experimental B-F Bond Length in Boranes
fluorine to boron. Hence π-back-donation stabilizes the
borane in these systems. The averaged B-F bond length in
[CB11F12]- compared to B-F bond lengths in fluoroborate
anions (Table 5) is shorter. This finding is unprecedented
taking into account that the coordination number on boron
in [CB11F12]- is six whereas in [(CF3)xBF4-x]- (x ) 0-3)
only four substituents are bound to boron. The shortening
of the B-F bond length in fluorocarborante anions is
attributed to π-back-bonding from fluorine to boron and
hence leading to stabilization of the anion against fluoride
ion abstraction.
and Borate Anions
d(B-F)b
borate
anion
d(B-F)b d(B-F)b
borane
BF3
calcda
calcd,a Å expt, Å
cation
ref
1.317c
1.312
1.306
[BF4]-
1.417
1.412
1.413
1.419
1.365e
1.406d [NH4]+
54
55
56
CF3BF2
[CF3BF3]-
1.391
1.391
1.407
1.369f
K+
Cs+
(CF3)2BF
(CF3)3B
RCB11F10
[(CF3)2BF2]-
[(CF3)3BF]-
[Co(CO)5]+ 17
[Rh(CO)4]+ 57
-
[RCB11F11
]
a B3LYP/6-311+G(d). c Expt d(B-F): 1.310 Å.58 d Corrected for
thermal motion. e R ) F. f R ) Et.
(CF3)3B with a tetrahedral arrangement (geometry increment).
To model the maximum of the Lewis acidity for perfluoro-
alkylboranes, perfluoro-1-boradamantane and perfluorobo-
radodecahedrane were investigated. The fluoride ion at boron
in perfluoro-1-boradamantane and perfluoroboradodecahe-
drane is bound as strong as in (CF3)3B with a tetrahedral
arrangement. But as for [(CF3)3BF]-, the fluoride ion will
be removed from one of the C atoms and not from the B
atom. The position with the weakest bound fluorine ion in
perfluoro-1-boradamantane is in â-position. The abstraction
of the fluoride ion is followed by destruction of the carbon
cage and the formation of a C-C double bond (Figure S1).
This reaction is also predicted for the â-position in perfluo-
roboradodecahedrane (Figure S2) by density functional
calculations. This type of acid catalyzed â-elimination is an
analogue to the Hofmann elimination of alkenes in tetraalky-
lammonium cations by â-hydrogen elimination catalyzed by
F-.51,52
The F- affinities for the four different fluorine positions
in perfluorocarborane, [CB11F12]-, are listed in Table 4, and
the structures of the Lewis acids as well as the anion are
displayed in Figure S3. The fluoride ion at the C atom is
weakest bound in perfluorocarborane. The reason for this
unexpected behavior is the stabilization of the resulting Lewis
acid through a pseudo-Jahn-Teller effect.53 The Lewis
acidities of perfluorocarborane are higher than the ones in
perfluoroalkylboranes (enhanced by 100-200 kJ mol-1). The
enhancement of the Lewis acidity for perfluorocarborane in
comparison to the B atom in perfluoro-1-boradamantane and
(CF3)3B in the tetrahedral arrangement is due to a gain in
π-back-donation from the fluorine atoms of about 130-170
kJ mol-1. While in the system [BF4]-/BF3 the Lewis acid
BF3 is stabilized by π-back-donation, in [CB11F12]-/CB11F11
the anion [CB11F12]- is stabilized by π-back-donation.
Additional evidence for the contrary influence of π-back-
donation from fluorine to boron is also displayed in the trend
in B-F bond length in fluoroboranes, fluoroborate anions,
and fluorocarboranate anions (Table 5). The shorter B-F
bond length in fluoroboranes compared to the related
fluoroborate anions (Table 5) is due to two effects: (i) the
less steric hindrance in the boranes due to the reduction of
the coordination number from four (borate anion) to three
(borane) and (ii) the occurrence of π-back-bonding from
closo-Carboranes containing CF3 groups instead of fluorine
substituents are less stable, probably due to a loss of
stabilization through π-back-donation. The extreme example
[CB11(CF3)12]- tends to explode,59 whereas other trifluorom-
ethylcarboranes, e.g., (CF)2B10(CF3)10, exhibit higher stabil-
ity.60 The decomposition is probably initiated by a fluoride
ion abstraction from a CF3 group followed by opening of
the cluster at the electron deficient C atom, similar to
reactions described for the N atom in MeNB11H11.61 The
resulting strong Lewis acid can attack one of the remaining
CF3 groups, and the decomposition can proceed. The reaction
should be highly exothermic as the comparison to the
energies for the rearrangement reactions described herein
implies. In contrast, the high stability of [CB11F12]- 62 or
n
[RCB11F11]- (R ) Me, Et, Bu)57,63 is demonstrated by the
formation of salts with highly electrophilic cations, e.g., [i-
Pr3Si][EtCB11F11] and [AlMe2][MeCB11F11].63
(h) Comparison of the Stability of Some Borate Anions
with the Sum-Formula [BC4F12]-. Geometries and energies
of some isomers of borate anions with the sum-formula
[BC4F12]- have been calculated (Figure 7). A comparison
of the relative stabilities is of interest since [B(CF3)4]- is a
weak coordinating anion.1 In the investigated series of fluoro-
(perfluoroalkyl)borates the stability increases with an in-
creased number of F atoms attached to boron and for every
additional F atom on boron the species become more stable
by -70 to -80 kJ mol-1. A further trend is the gain in energy
if the BF3 group is attached to a secondary C atom and a
tertiary C atom. Although [(CF3)3CBF3]- is the most stable
isomer of the series, [B(CF3)4]- has an enhanced stability
against fluoride ion abstraction, and so, it is suitable as a
weak coordinating anion. The reason is the high fluoride ion
(54) Yeh, H. J. C.; Ragle, J. L. J. Phys. Chem. 1968, 72, 3688.
(55) Brauer, D. J.; Bu¨rger, H.; Pawelke, G. Inorg. Chem. 1977, 16, 2305.
(56) Brauer, D. J.; Bu¨rger, H.; Pawelke, G. J. Organomet. Chem. 1980,
192, 305.
(57) Lupinetti, A. J.; Havighurst, M. D.; Miller, S. M.; Anderson, O. P.;
Strauss, S. H. J. Am. Chem. Soc. 1999, 121, 11920.
(58) Beeker, K.; Mrinal, K. D.; Gragg, B. R.; Keller-Rudek, H.; List, H.;
Sawodny, W. In Gmelin Handbuch der Anorganischen Chemies
BorVerbindungen; Niedenzu, K., Buschbeck, K.-C., Merlet, P., Eds.;
Springer-Verlag: Berlin, 1978; Vol. 53, Teil 18.
(59) King, B. T.; Michl, J. J. Am. Chem. Soc. 2000, 122, 10255.
(60) Herzog, A.; Callahan, R. P.; Macdonald, C. L. B.; Lynch, V. M.;
Hawthorne, M. F.; Lagow, R. J. Angew. Chem., Int. Ed. 2001, 40,
2121.
(61) Paetzold, P.; Lomme, P.; Englert, U. Z. Anorg. Allg. Chem. 2002,
628, 632.
(62) Strauss, S. H.; Ivanov, S. V.; Lupinetti, A. J. (Colorado State University
Research Foundation). United States Patent US 6,130,357, 2000.
(63) Strauss, S. H.; Ivanov, S. V. (Colorado State University Research
Foundation). Patent WO 02/36557/A2, 2002.
(51) Christe, K. O.; Wilson, W. W.; Wilson, R. D.; Bau, R.; Feng, J.-A. J.
Am. Chem. Soc. 1990, 112, 7619.
(52) Mahjoub, A. R.; Zhang, X.; Seppelt, K. Chem. Eur. J. 1995, 1, 261.
(53) Bersuker, I. B. Vibronic Interactions in Molecules and Crystals;
Springer-Verlag: Berlin, 1989.
502 Inorganic Chemistry, Vol. 43, No. 2, 2004

Transient Lewis Acids (CF3)3B and (CF3)3BCF2
Figure 7. Comparison of the relative stabilities (∆H) of some [C4BF12]-
anions (energy and geometry, B3LYP/6-311+G(d); ZPC and thermal
corrections, B3LYP/6-311G(d)).
affinity of the corresponding Lewis acid (CF3)3BCF2 com-
pared to monoperfluoroalkylborates (Tables 2 and 3).
(II) NMR Studies of Perfluoroalkylborates: Applica-
tion of the SERF Method in 19F NMR Spectroscopy and
Trends in 19F NMR Chemical Shifts. 19F NMR spectros-
copy offers the best possibility to analyze the complex
reaction mixtures obtained in this study. The advantages are
the characteristic 19F NMR chemical shifts as well as the
high sensitivity and access to the quantitative product
distribution. In most cases, it was possible to elucidate the
connectivity of the atoms by using the relative intensities
and the coupling constants. Since the influence of the
quadrupolar moment of the 11B nucleus leads to a broadening
of the lines, the 19F NMR spectra were also recorded applying
11B broad band decoupling. Furthermore by this procedure
the number of parameters, the couplings with 11B, is reduced.
The largest contribution to couplings of 19F nuclei is often
not due to dipolar interactions; hence, other interactions
dominate, e.g., through-space coupling, and it is often
impossible to predict and to assign these coupling constants.64
For example, the 3J(19F, 19F) values are significantly smaller
than the 4J(19F, 19F) values, and in many cases, they cannot
be observed. In some cases, even long-range couplings, e.g.,
Figure 8. 19F{11B} NMR spectrum of [(CFa CFb′Fb′′)(CFd )FcCBFe ]-
3
3
3
(bottom) and F2 projections, nJ(19F,19F), obtained by SERF experiments.
constants are directly measured, the SERF method offers an
easier and more reliable tool for the determination and
assignment of coupling constants in complex systems than
selective decoupling. In this manner it was possible to assign
all signals and coupling constants for [(C2F5)(CF3)FCBF3]-,
and in Figure 8, the 19F{11B} NMR spectrum of the CF3
group at -70.5 ppm and the spectra of the corresponding
coupling constants (F2 projections) are shown. In the case
of [C4F9BF3]-, a few SERF experiments were performed to
prove the proposed assignment of the signals of the CF2
groups. In addition to the SERF measurements the spectra
were also simulated using the program gNMR.69
In Chart 1, the NMR spectroscopic data of the linear
perfluoroalkyl borates are depicted. To offer a complete
overview of the 11B and 19F NMR spectroscopic data of linear
perfluoroalkyl borates known so far, the values of the
[CF3BF3]- anion are included.1 For [CnF2n+1BF3]- (n ) 3-4,
6), the 11B and 19F chemical shifts and a few coupling
constants have been reported previously,8-10 which are in
good agreement with our results. The 19F and the 19F{11B}
NMR spectra of the [C2F5BF3]- anion are displayed in Figure
9. In the 11B decoupled spectrum, small 19F-19F coupling
constants are resolved that cannot be extracted from the
corresponding 11B coupled spectrum. In Figure 10, the 11B
coupled and decoupled spectra of [C3F7BF3]- are displayed,
5J(19F, 19F) and J(19F, 19F), are resolved. The J(19F, 19F)
coupling constants between the F atoms of one CF2 group
to the F atoms of another CF2 group are different and can
only be equivalent by accident.65,66 To solve this problem
we have used the selective refocusing method (SERF).67,68
To our knowledge this is the first example for the use of
this method in 19F NMR spectroscopy. Because the coupling
6
n
(64) Berger, S.; Braun, S.; Kalinowski, H.-O. NMR-Spektroskopie Von
Nichtmetallens19F NMR-Spektroskopie; Georg Thieme Verlag: Stut-
tgart, 1994; Vol. 4.
(65) Harris, R. K.; Woodman, C. M. J. Mol. Spectrosc. 1968, 26, 432.
(66) Hopkins, R. C. J. Mol. Spectrosc. 1966, 20, 321.
(67) Fa¨cke, T.; Berger, S. J. Magn. Reson. 1995, 113A, 114.
(68) Eberstadt, M.; Gemmecker, G.; Mierke, D. F.; Kessler, H. Angew.
Chem. 1995, 107, 1813.
(69) Budzelaar, P. H. M. gNMR, V4.1.0; Chem Research GmbH, 1995-
1999.
Inorganic Chemistry, Vol. 43, No. 2, 2004 503

Finze et al.
Figure 9. 19F NMR (top) and 19F{11B} NMR spectrum (bottom) of
[C2F5BF3]-.
Figure 12. Trends of the 19F NMR chemical shifts of the CF2 and BF3
groups in linear perfluoroalkylborates.
The NMR spectroscopic data obtained for [(C2F5)(CF3)-
FCBF3]- are summarized in Chart 2. The data for [(C3F7)-
(CF3)FCBF3]- and [(C2F5)2FCBF3]- are limited because these
species were only formed in small quantities; [(C2F5)(CF3)-
FCBF3]- could be investigated in detail by 19F NMR
spectroscopy. The measured coupling constants for the two
diastereotopic F atoms attached to the Cb atom (Chart 2)
were corrected to the values of the real coupling constants.70
Since the signs of the effective coupling constants are
unknown, two different sets of coupling constants are
obtained from the corrections. The simulation with gNMR
shows that both sets give the expected line distribution, but
the intensity distribution obtained with the values presented
in Chart 2 is in closer agreement to the measured spectrum.
In Figure 13, the 19F and 19F{11B} NMR spectra are depicted
and compared to the simulated 19F NMR spectrum. A similar
study was already performed for perfluoroisopropyl groups
attached to aryl systems.71 The spectra reveal similarities,
but especially, the coupling constants differ significantly due
to the different influences of the attached groups.
Figure 10. 19F NMR (top) and 19F{11B} NMR spectrum (bottom) of
[C3F7BF3]-.
In the cases of [(C3F7)(CF3)FCBF3]- and [(C2F5)2FCBF3]-,
very complex spectra are expected in comparison to the
simpler spectra of isoelectronic C3F7CF(CF3)2 and (C2F5)2-
CFCF3.72 The observation of signals with similar chemical
shifts and intensities as found for [(C2F5)(CF3)FCBF3]- and
a related trend as reported for the series C2F5CF(CF3)2, C3F7-
73,74
Figure 11. Simulated 19F{11B} NMR (bottom) and 19F{11B} NMR
CF(CF3)2, and (C2F5)2CFCF3
indicates the formation of
spectrum (top) of [C3F7BF3]-.
the above-mentioned species during the rearrangement and
CF2 transfer reactions of (CF3)3BCF2.
and they illustrate that except for the CF3 group the signals
become very complex and that special methods as mentioned
above are necessary to assign the coupling constants and the
signals. The measured and simulated spectra of [C3F7BF3]-
(Figure 11) are in good agreement. The comparison of the
19F NMR chemical shifts of the linear perfluoroalkyl borates
reveals an interesting trend; the CF2 groups in the middle of
the perfluoroalkyl chain converge against -121.3 ppm
(Figure 12). The closer a CF2 group is located to any of the
two ends of the anion, the lower the 19F NMR chemical shifts
becomes. By accident the 19F NMR chemical shifts of the
BF3 groups fit into this trend. The 19F NMR chemical shifts
of the CF3 groups are in a different region (-75 to -84
ppm).
Summary and Conclusion
The investigations of the rearrangement reactions of the
unstable intermediates (CF3)3B and (CF3)3BCF2 formed from
(70) Emsley, J. W.; Feeney, J.; Sutcliffe, L. H. High-Resolution Nuclear
Magnetic Resonance Spectroscopy, 2nd ed.; Pergamon Press: Oxford,
1967; Vol. 1.
(71) Chambers, R. D.; Sutcliffe, L. H.; Tiddy, G. J. T. Trans. Faraday
Soc. 1970, 66, 1025.
(72) Burdon, J.; Creasey, J. C.; Procter, L. D.; Plevey, R. G.; Yeoman, J.
R. N. J. Chem. Soc., Perkin Trans. 2 1991, 445.
(73) Coe, P. L.; Sellers, S. F.; Tatlow, J. C. J. Fluorine Chem. 1981, 18,
417.
(74) Sartori, P.; Velayutham, D.; Ignat’ev, N. V.; Noel, M. J. Fluorine
Chem. 1998, 87, 31.
504 Inorganic Chemistry, Vol. 43, No. 2, 2004

Transient Lewis Acids (CF3)3B and (CF3)3BCF2
Figure 13. Simulated 19F{11B} NMR (bottom), 19F{11B} NMR (middle), and 19F NMR spectrum (top) of [(C2F5)(CF3)FCBF3]-.
the precursors (CF3)3BCO and the weak coordinating anion
[B(CF3)4]- revealed unprecedented rearrangement reactions.
The results rationalized the degradation chemistry of per-
fluoroalkylfluoroborate anions which are of interest as weak
coordinating anions. In the case of (CF3)3B, the final product
of the decomposition in anhydrous HF is [C2F5BF3]-, and
in the gas phase, a mixture of linear perfluoroalkylboranes
CnF2n+1BF2 is observed. For (CF3)3BCF2, a mixture of linear
and branched perfluoroalkylboranes is obtained with the
interesting borane (C2F5)(CF3)FCBF2 as a main product. The
boranes are formed in a cascade of 1,2-fluorine shifts from
a CF3 or a C2F5 group to boron followed by perfluoroalkyl
migrations. The difluorocarbene complexes formed as in-
termediates can also serve as CF2 sources. The 1,2-fluorine
shifts observed are the first examples for intramolecular C-F
activation in boron chemistry, and the intramolecular transfer
reactions of the perfluoroalkyl groups are unusual reactions
in general.
Quantum chemical calculations were performed to gain a
deeper insight into the reaction cascades described above.
The geometries and energies of the transition states for the
rearrangement reactions were calculated, and the low barriers
are in agreement with the experimental findings that neither
(CF3)3B, (CF3)3BCF2, nor the intermediates could be isolated.
As precedents for the C-F activations, the barrier of the
1,2-fluorine shift in CF3BF2 was compared to the barrier of
the 1,2- and 1,3-fluorine shift in C2F5BF2. Since the activation
energy for the isomerization of C2F5BF2 is nearly twice as
large as for the isomerization of CF3BF2, the borane C2F5-
BF2 is easily obtained at room temperature whereas CF3BF2
remains unknown.
F- affinities of a variety of different fluoroboron species
were investigated by theoretical methods. The F- affinities
are closely related to the calculated barriers for the observed
intramolecular fluoride ion abstractions. Because the fluoride
ion affinities are a good measure for the stability of weak
coordinating anions, their knowledge is of broad interest.
Especially, the influence of π-back-donation and the geom-
etry at boron are discussed. Tricoordinated (perfluoroalkyl)-
fluoroboranes are stabilized by π-back-donation from fluorine
ligands to boron and planar arrangement of the ligands leads
to a further gain in energy as modeled with perfluoro-1-
boradamantane. In contrast to tetracoordinated (perfluoro-
alkyl)fluoroborates, closo-carboranate anions are stabilized
by π-back-donation from fluorine.
Acknowledgment. Financial support by the Deutsche
Forschungsgemeinschaft, DFG, and the Fonds der Chemis-
chen Industrie is acknowledged. Furthermore, we are grateful
to Merck KGaA, Darmstadt, Germany, for providing finan-
cial support and chemicals used in these studies. We thank
Professor J. S. Francisco for helpful discussions and Dr. N.
V. Ignat’ev for a sample of K[C2F5BF3].
Supporting Information Available: Table of energies, enthal-
pies and free energies of all compounds and transition states
investigated by DFT calculations, and figures displaying the
calculated structures of the carboranate, adamantane, and dodeca-
hedrane derivatives. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC0350640
Inorganic Chemistry, Vol. 43, No. 2, 2004 505