Tris(trifluoromethyl)borane carbonyl, (CF3)3BCO - Synthesis, physical, chemical and spectroscopic properties, gas phase, and solid state structure

Article abstract of DOI:10.1021/ja0209924

Tris(trifluoromethyl)borane carbonyl, (CF₃)₃BCO, is obtained in high yield by the solvolysis of K[B(CF₃)₄] in concentrated sulfuric acid. The in situ hydrolysis of a single bonded CF₃ group is found to be a simple, unprecedented route to a new borane carbonyl. The related, thermally unstable borane carbonyl, (C₆F₅)₃BCO, is synthesized for comparison purposes by the isolation of (C₆F₅)3B in a matrix of solid CO at 16 K and subsequent evaporation of excess CO at 40 K. The colorless liquid and vapor of (CF₃)₃BCO decomposes slowly at room temperature. In the gas phase t_1/2 is found to be 45 min. In the presence of a large excess of ¹³CO, the carbonyl substituent at boron undergoes exchange, which follows a first-order rate law. Its temperature dependence yields an activation energy (E_A) of 112 kJ mol^-1. Low-pressure flash thermolysis of (CF₃)₃BCO with subsequent isolation of the products in iow low-temperature matrixes, indicates a lower thermal stability of the (CF₃)₃B fragment, than is found for (CF₃)₃BCO. Toward nucleophiles (CF₃)₃-BCO reacts in two different ways: Depending on the nucleophilicity of the reagent and the stability of the adducts formed, nucleophilic substitution of CO or nucleophilic addition to the C atom of the carbonyl group are observed. A number of examples for both reaction types are presented in an overview. The molecular structure of (CF₃)₃BCO in the gas phase is obtained by a combined microwave-electron diffraction analysis and in the solid state by single-crystal X-ray diffraction. The molecule possesses C₃ symmetry, since the three CF₃ groups are rotated off the two possible positions required for C_3v symmetry. All bond parameters, determined in the gas phase or in the solid state, are within their standard deviations in fair agreement, except for internuclear distances most noticeably the B-CO bond lengths, which is 1.69(2) A in the solid state and 1.617(12) A in the gas phase. A corresponding shift of v(CO) from 2267 cm^-1 in the solid state to 2251 cm^-1 in the gas phase is noted in the vibrational spectra. The structural and vibrational study is supported by DFT calculations, which provide, in addition to the equilibrium structure, confirmation of experimental vibrational wavenumbers, IR-band intensities, atomic charge distribution, the dipole moment, the B-CO bond energy, and energies for the elimination of CF₂ from (CF₃)_xBF_3-x, x = 1-3. In the vibrational analysis 21 of the expected 26 fundamentals are observed experimentally. The ¹¹B-, ¹³C-, and ¹⁹F-NMR data, as well as the structural parameters of (CF₃)₃BCO, are compared with those of related compounds.

Full text of DOI:10.1021/ja0209924

Published on Web 11/23/2002
Tris(trifluoromethyl)borane Carbonyl, (CF3)3BCOsSynthesis,
Physical, Chemical and Spectroscopic Properties, Gas Phase,
and Solid State Structure
Maik Finze,† Eduard Bernhardt,† Annegret Terheiden,† Michael Berkei,†
Helge Willner,*,† Dines Christen,§ Heinz Oberhammer,*,§ and Friedhelm Aubke‡
Contribution from the Fakulta¨t 4, Anorganische Chemie, Gerhard Mercator UniVersita¨t
Duisburg, Lotharstrasse 1, D-47048 Duisburg, Germany, Institut fu¨r Physikalische und
Theoretische Chemie, UniVersita¨t Tu¨bingen, 72076 Tu¨bingen, Germany, Department of
Chemistry, The UniVersity of British Columbia, VancouVer, British Columbia, V6T1Z1, Canada
Received July 19, 2002
Abstract: Tris(trifluoromethyl)borane carbonyl, (CF3)3BCO, is obtained in high yield by the solvolysis of
K[B(CF3)4] in concentrated sulfuric acid. The in situ hydrolysis of a single bonded CF3 group is found to be
a simple, unprecedented route to a new borane carbonyl. The related, thermally unstable borane carbonyl,
(C6F5)3BCO, is synthesized for comparison purposes by the isolation of (C6F5)3B in a matrix of solid CO at
16 K and subsequent evaporation of excess CO at 40 K. The colorless liquid and vapor of (CF3)3BCO
decomposes slowly at room temperature. In the gas phase t1/2 is found to be 45 min. In the presence of a
large excess of 13CO, the carbonyl substituent at boron undergoes exchange, which follows a first-order
rate law. Its temperature dependence yields an activation energy (EA) of 112 kJ mol-1. Low-pressure flash
thermolysis of (CF3)3BCO with subsequent isolation of the products in low-temperature matrixes, indicates
a lower thermal stability of the (CF3)3B fragment, than is found for (CF3)3BCO. Toward nucleophiles (CF3)3-
BCO reacts in two different ways: Depending on the nucleophilicity of the reagent and the stability of the
adducts formed, nucleophilic substitution of CO or nucleophilic addition to the C atom of the carbonyl group
are observed. A number of examples for both reaction types are presented in an overview. The molecular
structure of (CF3)3BCO in the gas phase is obtained by a combined microwave-electron diffraction analysis
and in the solid state by single-crystal X-ray diffraction. The molecule possesses C3 symmetry, since the
three CF3 groups are rotated off the two possible positions required for C3v symmetry. All bond parameters,
determined in the gas phase or in the solid state, are within their standard deviations in fair agreement,
except for internuclear distances most noticeably the B-CO bond lengths, which is 1.69(2) Å in the solid
state and 1.617(12) Å in the gas phase. A corresponding shift of ν(CO) from 2267 cm-1 in the solid state
to 2251 cm-1 in the gas phase is noted in the vibrational spectra. The structural and vibrational study is
supported by DFT calculations, which provide, in addition to the equilibrium structure, confirmation of
experimental vibrational wavenumbers, IR-band intensities, atomic charge distribution, the dipole moment,
the B-CO bond energy, and energies for the elimination of CF2 from (CF3)xBF3-x, x ) 1-3. In the vibrational
analysis 21 of the expected 26 fundamentals are observed experimentally. The 11B-, 13C-, and 19F-NMR
data, as well as the structural parameters of (CF3)3BCO, are compared with those of related compounds.
Introduction
positively charged central atoms, resulting in a strengthening
of the CO bond by electrostatic contribution.3,4 The simplest
example of a borane carbonyl, H3BCO, has been known since
1937.5 The compound has been extensively characterized by
experimental, structural, and theoretical means.6-9 Approxi-
Borane carbonyls can be viewed on account of their high
CO stretching wavenumbers as main group analogues of
σ-bonded transition metal carbonyl cations.1,2 In both classes
of carbonyls, CO is σ-bonded and the CO bond is polarized by
(3) Goldman, A. S.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1996, 118, 12 159-
12 166.
* To whom correspondence should be addressed. E-mail: willner@uni-
duisburg.de. H. Oberhammer: Telephone: 49-7071-295490. Fax: 49-7071-
295490. E-mail: heinz.oberhammer@uni-tuebingen.de. F. Aubke: Tele-
phone: 1-604-822-2847. E-mail: aubke@chem.ubc.ca.
† Gerhard Mercator Universita¨t Duisburg.
(4) Ehlers, A. W.; Ruiz-Morales, Y.; Baerends, E. J.; Ziegler, T. Inorg. Chem.
1997, 36, 5031.
(5) Burg, A. B.; Schlesinger, H. I. J. Am. Chem. Soc. 1937, 59, 780-787.
(6) Gmelins Handbuch der Anorganischen Chemie, BorVerbindungen, Teil 10;
1980; 1st. Suppl. Volume 1., 1983; 2nd. Suppl. Volume 1, 1987; 3rd. Suppl.
Volume 1, 1994; 4th. Suppl. Volume 1a, and 1996, ed.; Volume 1b 1;
Springer-Verlag: Berlin, Heidelberg, New York, 1976; Vol. 37.
(7) Venkatachar, A. C.; Taylor, R. C.; Kuczkowski, R. L. J. Mol. Struct. 1977,
38, 17.
‡ The University of British Columbia.
§ Universita¨t Tu¨bingen.
(1) Willner, H.; Aubke, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 2402-
2425.
(2) Willner, H.; Aubke, F. In Inorg. Chem. Hightlights; Meyer, G., Naumann,
D., Wesemann, L., Eds.; Wiley-VCH: Weinheim Germany, 2002; p 195.
(8) Jones, L. H.; Taylor, R. C.; Paine, R. T. J. Chem. Phys. 1979, 70, 749.
(9) Bauschlicher, C. W.; Ricca, A. Chem. Phys. Lett. 1995, 237, 14.
9
10.1021/ja0209924 CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 15385-15398
15385

A R T I C L E S
Finze et al.
liquid nitrogen in a storage Dewar vessel. The ampules were opened
and resealed using an ampule key.13
mately twenty additional borane carbonyl derivatives have
become known in the meantime. They are synthesized primarily
by addition of CO to suitable boranes and boron subhalides.6
We have recently reported in a preliminary communication
on the synthesis of tris(trifluoromethyl)borane carbonyl, (CF3)3-
BCO,10 by solvolysis of the [B(CF3)4]- anion11 in concentrated
sulfuric acid. This reaction was found accidentaly by testing
the limits of stability of the very weak coordinating [B(CF3)4]-
anion. The reaction proceeds in a series of ligand transforma-
tions, starting from K[B(CN)4]12 according to (1), featuring a
single C-bonded ligand.
The melting point of (CF3)3BCO was determined on samples placed
inside 6 mm o.d. glass tubes, using a slowly heated stirred water bath,
in the temperature range of 5-10 °C. Vapor pressures were measured
in a small trap, using the above-mentioned capacitance manometer
(AHS-100) in the temperature range between -45 to +10 °C. Five
different cold bathes with ethanol as coolant were used to adjust the
temperature as quickly as possible to prevent decomposition. Measure-
ments at different temperatures were repeated several times on purified
samples to obtain averaged vapor pressure data.
For kinetic measurements, a small glass vessel (1 cm o.d., 10 cm
length) equipped with a glass valve with a PTFE stem (Young, London
UK) was used. For each run 0.015 mmol (CF3)3BCO and 0.8 mmol
13CO were transferred into the evacuated vessel by cooling it to -196
°C. The vessel was placed in a thermostated water bath for the desired
time period. The exchange process was quenched by cooling the cell
to -196 °C. The excess of 13CO was recovered by cryopumping the
gas into a vessel filled with molecular sieve (5 Å) kept at -196 °C.
Subsequently, at -78 °C the decomposition products were removed in
dynamic vacuum. By warming the vessel to room temperature, the solid
residue was finally evaporated into an evacuated IR-cell and the ν-
During these transformations, the B-C bonds remain intact,
whereas the substituents at the central boron atom are altered
first by perfluorination11 and then by partial hydrolysis.10 The
in situ generation of a B-CO group observed,10 is unprec-
edented. The new borane carbonyl is initially identified and
characterized by a molecular mass determination, its IR-
spectrum (where ν(CO) is unprecedentedly high with 2252
cm-1), NMR-data and a limited number of characteristic
reactions.
In this publication, we want to report (i) on details of the
synthesis of (CF3)3BCO and its thermal behavior, studied by
gas phase kinetics, isotopic exchange reactions and low pressure
flash thermolysis, with the decomposition products trapped in
an Ar-matrix and studied by IR spectroscopy, (ii) the synthesis
of the previously unknown reference compound (C6F5)3BCO
by trapping the Lewis acid in a CO matrix, (iii) present an
overview of a number of chemical reactions, that involve either
nucleophilic substitution of the CO ligand or nucleophilic
addition to the C atom of the carbonyl group, (iv) the molecular
structure of (CF3)3BCO in the gas phase, obtained by electron
diffraction combined with microwave spectroscopy, (v) the
molecular structure of (CF3)3BCO, obtained by single-crystal
X-ray diffraction, (vi) vibrational analysis of (CF3)3BCO and
DFT calculations, and (vii) a complete characterization of
(CF3)3BCO by heteronuclear NMR methods.
13
(CO) absorbance ratio A(ν CO)/A(νCO) was measured. This procedure
was performed on 20 samples at bath temperatures of 10.9, 21.0, 25.2,
30.0, and 35.1 °C, respectively. For each temperature, four different
reaction times were chosen (between 7 and 300 min).
The potentiometric titration was performed on a 0.1 molar aqueous
solution of (CF3)3BCO and was repeated several times.
Matrix-isolated samples were prepared by passing a gas stream of
Ar, Ne, or N2 (∼3 mmol h-1) over the sample placed in a small U-trap
in front of the matrix support. A U-trap containing (CF3)3BCO or
(C6F5)3B (in this case CO was used as matrix gas) was kept at -82 or
+35 °C, respectively. In a stainless steel vacuum line with a volume
of 1.1 L, a small amount of a 1:1 CO/BF3 mixture (ca. 0.1 mmol) was
mixed with Ar at a 1:500 ratio. About 1 mmol of this mixture was
directly deposited via a stainless steel capillary on the matrix support,
kept at 16 K. Details of the matrix apparatus have been described
elsewhere.14
(b) Chemicals. CO (standard grade, Messer-Griesheim, Krefeld,
Germany), 13CO (> 99% isotopic enrichement, Deutero GmbH,
Kastellaun, Germany) and C18O (> 99% Ventron, Numbai, India), as
well as all standard chemicals and solvents were obtained from
commercial sources. H3BCO was prepared from B2H6 and CO as
reported.6 K[B(CF3)4] was synthesized as described previously from
K[B(CN)4].11
(c) Synthetic Reactions. (1) (CF3)3BCO. The synthesis was
performed in a “V-shaped” reaction vessel, equipped with a valve with
a PTFE stem (Young, London) and fitted with two 100 mL round-
bottom flasks. Solid K[B(CF3)4] (0.5 g/15.3 mmol) was dissolved in
20 mL of Et2O and transferred into one of the round-bottom flasks,
that contained a PTFE-coated magnetic stirring bar. Ether was removed
under reduced pressure and 25 mL of concentrated H2SO4 were placed
into the second round-bottom flask. The reaction vessel was attached
to a glass vacuum line with a series of three U-traps and evacuated.
H2SO4 was added in 3 portions to K[B(CF3)4] at room temperature,
and the resulting mixture was stirred vigorously. Gas evolution occurred
immediately from the suspension. In the 1st U-trap, a liquid, identified
as HSO3F, that had formed during the reaction, was separated at -30
°C. In the 2nd trap (CF3)3BCO was collected at -100 °C. Highly
volatile byproducts such as BF3 were solidified at -196 °C in the 3rd
U-trap. After completing the addition of H2SO4, the reaction mixture
was stirred at room temperature until no further gas evolution was
observed. The temperature was raised to 45 °C, and the contents of
The comprehensive characterization of (CF3)3BCO described
here, allows a meaningful and informed comparison to other
known carbonyl boranes5-9 and also to σ-bonded metal carbonyl
cations.1,2
Experimental Section
General Procedures and Reagents (a) Apparatus. Volatile materi-
als were manipulated in a glass vacuum-line, equipped with two
capacity pressure gauges (221 AHS-1000 and 221 AHS-100, MKS
Baratron, Burlington, MA) and consisting of three U-traps and valves
with PTFE stems (Young, London UK). The vacuum line was
connected to an IR cell (Optical path length 200 mm, Si windows, 0.5
mm thick), contained in the sample compartment of a FTIR instrument.
Because of this arrangement, it was possible to follow the course of
the reactions closely and to monitor the purification processes of the
products. Products were stored in flame-sealed glass ampules under
(10) Terheiden, A.; Bernhardt, E.; Willner, H.; Aubke, F. Angew. Chem., Int.
Ed. Engl. 2002, 41, 799.
(11) Bernhardt, E.; Henkel, G.; Willner, H.; Pawelke, G.; Bu¨rger, H. Chem.-
Eur. J. 2001, 7, 4696.
(12) Bernhardt, E.; Henkel, G.; Willner, H. Z. Anorg. Allg. Chem. 2000, 626,
560-568.
(13) Gombler, W.; Willner, H. J. Phys. E: Sci. Instruments 1987, 20, 1286.
(14) Argu¨ello, G. A.; Grothe, H.; Kronberg, M.; Willner, H.; Mack, H. G. J.
Phys. Chem. 1995, 99, 17525.
9
15386 J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002

(CF ) BCO−Synthesis
A R T I C L E S
Table 1. Crystallographic Data of (CF3)3BCO at 143 K
3 3
the flask were stirred for approximately 7 h, until the reaction mixture
became clear. 3.31 g of pure (CF3)3BCO (13.5 mmol) were obtained
from the crude product after trap-to-trap distillation at -85 °C which
corresponds to a yield of 88%.
For the syntheses of (CF3)3B13CO and (CF3)3BC18O, a 50 mL round-
bottom flask, fitted with a glass valve (Young, London) was charged
with 0.24 mmol (CF3)3BCO and 5 mmol of 13CO or C18O by vacuum
line transfer. The reaction was allowed to proceed at 35 °C 40 min
and after subsequent cooling to -196 °C, the excess of isotopically
enriched CO was recovered by cryopumping onto molecular sieve (5
Å) at -196 °C. The residue (0.12 mmol) was evaporated and trapped
in vacuo at -78 °C. It contained about 50% of (CF3)3 B13CO or (CF3)3-
BC18O, respectively.
compound
(CF3)3BCO
C4BF9O
245.85
empirical formula
formula weight
crystal system, space group
unit cell dimensions: a [Å]
b [Å]
monoclinic, P21/c (No.14)
7.254(1)
9.959(2)
10.793(2)
91.18(3)
779.5(2)
4
c [Å]
â [°]
unit cell volume: V [Å3]
Z value
F
calc [g m-3
]
2.094
R1, (I > 2σ(I))a
R1, (all data)a
wR2, (all data)b
0.1271
0.1651
0.3937
(2) K2[(CF3)3BCO2]. A 50 mL round-bottom flask equipped with a
valve with a PTFE stem (Young, London), fitted with a PTFE-coated
magnetic stirring bar, was charged with 765 mg of (CF3)3BCO (3.1
mmol). About 10 mL of distilled water were condensed in vacuo, into
the flask, kept at -196 °C. The mixture was allowed to warm to room
temperature. Under vigorous stirring, 1.74 g of KOH (31 mmol)
dissolved in 5 mL of distilled water was added to the clear colorless
solution. After removing most of the water under reduced pressure,
the reaction mixture was extracted with CH3CN three times, in portions
of 100, 100, and 50 mL. The collected CH3CN layers were combined,
dried over K2CO3 and filtered through a fine glass frit. Even after
filtration, the organic solution was cloudy. After removing all volatiles
in vacuo, 780 mg (2.30 mmol) 74% yield of white K2[(CF3)3BCO2]
were obtained that showed no impurities in the NMR spectra.
(3) [Pr3NH][(CF3)3BC(O)OH]. 780 mg of K2[(CF3)3BCO2] (2.30
mmol) were dissolved in 20 mL distilled water. Under vigorous stirring,
2 mL (24 mmol) of concentrated HCl and 0.05 mL (5 mmol) of tri-
n-propyl ammine were added to the clear colorless solution. Im-
mediately after addition small quantities of an off white solid formed
and a small amount of the residual ammine was observed on the bottom
of the flask. The mixture was extracted with CH2Cl2 three times (100
mL, 50 mL and 20 mL, respectively). The collected organic phases
were combined, dried over MgSO4 and filtered through a fine glass
frit, and the solvent was removed under reduced pressure to yield 794
mg of [Pr3NH][(CF3)3BC(O)OH] (2.12 mmol) 92%. NMR data of the
[Pr3NH]+ cation: 1H NMR (300.13 MHz, CD3CN, 25 °C, TMS) δ
9.42 ppm (s, 1 H), 2,96 ppm (t, 6 H), 1,81-1,67 ppm (m, 6H), 0,95
ppm (t, 6H); 13C{1H}-NMR (125.758 MHz, CD3CN, 25 °C, TMS) δ
55.12 ppm (s, 1C), δ 17.76 ppm (s, 1C), δ 11.23 ppm (s, 1C).
(4) (CF3)3BNCCD3. 130 mg (CF3)3BCO (0.5 mmol) were transferred
in vacuo into a 5 mm o.d. NMR tube, equipped with a rotational
symmetrical valve with a PTFE stem (Young, London).15 A 1.5-mL
portion of dry CD3CN were condensed to the borane carbonyl and the
reaction mixture was warmed to room temperature. Gas evolution was
observed in the NMR tube. After 30 min at room temperature, NMR
spectra of the mixture were recorded. Nearly pure (CF3)3BNCCD3 was
identified that contained a small amount (∼4%, 19F NMR) of (CF3)3-
BC(OH)2 as the only impurity.
2 2
2
a R1 ) (∑||Fo| - |Fc|)/∑|Fo|. b Rw ) [∑w(Fo2 - Fc ) /∑wFo ]1/2, weight
scheme w ) [σ2(Fo) + (aP)2 + bP]-1, P ) (max(0, Fo ) + 2Fc )/3, a )
0.1823, b ) 8.1774.
2
2
approximately 20 mm length. The glass cylinders, containing the
crystals, were attached with wax onto goniometer heads.
(e) Instrumentation. (I) Single-Crystal X-ray Diffraction. Dif-
fraction data were collected at -143 K on a Nonius diffractometer
with a CCD camera using Mo KR radiation (λ ) 0.710 69 Å) and a
graphite monochromator. No transformations of the crystal during
cooling from -70 to -130 °C were observed optically, but the reflexes
became sharper on cooling. At the end of the data collection, the first
intensity measurement was repeated, which demonstrated the stability
of the crystal during the X-ray diffraction analysis.
The intensity data were subsequently corrected with the SCALEPACK
program.17 The structure was solved in P21/c (No. 14) by direct methods
with SHELXS-9718,19 and refined with anisotropic temperature factors.20
A summary of experimental details and crystal data is collected in Table
1.
(II) Electron Diffraction. Electron diffraction intensities were
recorded on Kodak Electron Image plates (13 × 18 cm) with a KD-
G2 Diffraktograph21 at 25 and 50 cm nozzle-to-plate distances and with
an accelerating voltage of about 60 kV. The (CF3)3BCO sample was
kept at -6 °C (sublimation pressure about 10 mbar) and the inlet nozzle
was at room temperature. The photographic plates were analyzed by
the usual methods22 and averaged molecular intensities in the s-ranges
2-18 and 8-35 Å-1 (s ) (4π/λ) sin θ/2, λ is the electron wavelength
and θ is the scattering angle) are shown in Figure 1.
(III) Microwave Spectroscopy. The microwave spectrum was
recorded in the X- and K-bands with a conventional stark spectrometer
(modulation frequency 50 kHz). The sample must be allowed to flow
continuously through the cell, which was accomplished by cooling the
sample to -50 °C, the cell to -30 °C and adjusting the valves
accordingly. Broad band as well as high resolution spectra were
recorded. Because the broad band spectra were recorded to monitor
and ascertain the sample flow through the cell, only the high-resolution
spectra were chosen for a subsequent discussion.
(IV) Vibrational Spectroscopy. (a) Gas-phase infrared spectra were
recorded with a resolution of 2 cm-1 in the range 4000-50 cm-1 on a
Bruker IFS 66v FTIR instrument. Matrix infrared spectra were recorded
using another Bruker IFS 66v FT spectrometer in the reflectance mode
employing a transfer optic. A DTGS detector, together with a KBr/Ge
beam splitter was used in the region of 5000-400 cm-1. In this region,
64 scans were co-added for each spectrum using an apodized resolution
of 1.2 or 0.3 cm-1. A Ge-coated 6 µm Mylar beam splitter and a far-
(d) Preparation of Single-Crystals of (CF3)3BCO. Approximately
10 mg (CF3)3BCO were transferred in vacuo into a small glass ampule
(6 mm o.d., 20 cm length) kept at -196 °C. Volatile impurities were
removed under reduced pressure at -75 °C and the sealed ampule was
stored at -70 °C for 6 d. At this temperature, the vapor pressure of
(CF3)3BCO was sufficient (10-2 mbar) for a slow crystallization process.
To prevent decomposition of the single-crystals, used in the subsequent
X-ray diffraction analysis, the crystals were placed on a copper trough,16
cooled to -70 °C. During the preparation the trough was flushed with
dry nitrogen. Suitable crystals were selected under a polarizing
microscope and fitted into glass capillaries (o.d. 0.1 to 0.3 mm). The
capillaries were sealed off at both ends to give glass cylinders of
(17) Otwinowsky, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(18) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990,
467.
(19) Sheldrick, G. M. SHELXTL, Release 5.1 Software Reference Manual; Bruker
AXS, Inc.: Madison, Wisconsin, USA, 1997.
(20) Sheldrick, G. M. Universita¨t Go¨ttingen, 1997.
(15) Gombler, W.; Willner, H. International Laboratory 1984, 84.
(16) Veith, M.; Ba¨ringhausen, H. Acta Crystallogr., Sect. B: Struct. Sci. 1974,
30, 1806.
(21) Oberhammer, H. Molecular Structure by Diffraction Methods; The Chemical
Society: London, 1976; Vol. 4.
(22) Oberhammer, H.; Gombler, W.; Willner, H. J. Mol. Struct. 1981, 70, 273.
9
J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002 15387

A R T I C L E S
Finze et al.
25 °C
[B(CF3)4](solv)- + H3O+ conc. H SO 8
2
4
(CF3)3BCO(g) + 3HF(solv) (2)
The new compound is isolated on account of its volatility. The
byproduct HF is converted in H2SO4 into HSO3F, which is
detected among the less volatile products of a trap-to-trap
condensation.
Precedents for the acid hydrolysis of CF3 groups which result
in the formation of coordinated CO are reported for various
complexes of Mn,23 Fe,23 Mo,23 Ru24, and Pt25,26 according to
eq 3
Figure 1. Experimental (dots) and calculated (full line) molecular intensities
for long (top) and short (bottom) nozzle-to-plate distances and residuals of
(CF3)3BCO.
H2O
+
H
IR DTGS detector were used in the region of 650-80 cm-1. In this
region, 128 scans were co-added for each spectrum, using an apodized
resolution of 0.5 cm-1. (b) Raman spectra of solid (CF3)3BCO, deposited
from the gas phase on a metal finger at -196 °C in high vacuum,
were recorded with a resolution of 2 cm-1 on a Bruker RFS 100/S FT
Raman spectrometer using the 1064 nm excitation (500 mw) of a Nd:
YAG laser (DPY 301 II-N-OEM-500, Coherent, Lu¨beck, Germany).
M-CF3 8 [M ) CF2]+ 2HF8 [M-CO]+
HF
M ) Mn|, Fe|, Mo|, Ru|, Pt|
(3)
The reported hydrolysis of CF3-substituted aryl derivatives in
concentrated sulfuric acid under more severe conditions (>100
°C, >6 h) to carboxy derivatives is a more remote precedent.27
However because in concentrated H2SO4, the oxonium ion,28
H3O+, is found to exist, its involvement in the formation of
(CF3)3BCO (see eq 1) is very likely.
(V) NMR Spectroscopy. 1H-, 19F-, and 11B-NMR spectra were
recorded at room temperature on a Bruker Avance DRX-300 spec-
1
trometer operating at 300.13, 282.41 or 96.92 MHz for H-, 19F-, and
11B-nuclei, respectively. 13C-NMR spectroscopic studies were performed
at room temperature or at -10 °C on a Bruker Avance DRX-500
spectrometer, operating at 125.758 MHz. The NMR signals were
referenced against TMS and CFCl3 as internal standards and BF3‚OEt2
as external standard. Concentrations of the investigated samples were
in the range of 0.1-1 mol L-1. (CF3)3BCO samples for NMR
spectroscopic studies were prepared in 5 mm NMR tubes, equipped
with special valves with PTFE stems (Young, London).15 Dry SO2 or
CD2Cl2 were used as solvents. NMR samples in SO2 were investigated
at -10 °C whereas spectra in CD2Cl2 were recorded at room
temperature. Salts were dissolved in CD3CN and transferred into 5 mm
o.d. NMR tubes and investigated at room temperature.
A potential alternate route to (CF3)3BCO, the CO addition
to the Lewis acid B(CF3)3 is not feasible because the latter
compound is so far not known, despite considerable synthetic
efforts.29 In addition, attempts to add CO to the related Lewis
acid B(C6F5)3,30,31 which is known since 1963, have been
unsuccessful,32 for reasons which will be discussed below.
Hence, the synthesis of (CF3)3BCO described here, is both
unique and unprecedented.
The new compound (CF3)3BCO is a clear, colorless liquid at
room temperature, with a melting point of 9 ( 1 °C. At this
temperature, the vapor pressure is 38 mbar and the sublimation
vapor pressure curve is given by the expression ln(p) ) -6162/T
+ 25.5 (p in mbar, T in K). Above its melting point, (CF3)3-
BCO decomposes rapidly, so that a reliable vapor pressure curve
is not obtainable. The thermal decay of (CF3)3BCO in the gas
phase follows a first-order rate law with t1/2 ) 45 min. The
decomposition products CO and BF3 are identified by IR
spectroscopy. Additional IR bands at 1378, 1327, 1244, 1222,
1150, 1130 (sh), 1068, 991, 937, 876, 751, 605 cm-1 and new
signals in the NMR spectra at δ(11B) ) +19.4 and δ(19F) )
-75.6, -124.7, and -124.3 ppm indicate the formation of CF3-
BF2 in addition to further, unidentified CF-compounds. Hence,
the rate determining step appears to be the dissociation of the
B-CO bond. The resulting B(CF3)3-fragment decomposes fast
(VI) UV Spectroscopy. UV spectra of gaseous samples in a glass
cell (optical path length 10 cm) equipped with quartz windows (Suprasil,
Heraeus, Hanau, Germany) were recorded on a Perkin-Elmer Lambda
900 spectrometer in the spectral range of 190-350 nm. Pressures were
measured with a capacitance manometer (122 A-100, MKS Baratron,
Burlington, MA). To eliminate absorption from atmospheric O2, the
monochromator and the housing of the gas cell were flushed with dry
N2.
The uncertainties of IR and UV absorption cross sections (determined
on base e) are estimated to be 5 to 10%.
(VII) DSC Measurements. Thermo-analytical measurements were
made with a Netzsch DSC204 instrument. Temperature and sensitivity
calibrations in the temperature range of 20-500 °C were carried out
with naphthalene, benzoic acid, KNO3, AgNO3, LiNO3, and CsCl.
About 5-10 mg of the solid samples were weighed and contained in
sealed aluminum crucibles. They were studied in the temperature range
of 20-500 °C with a heating rate of 5 K min-1; throughout this process,
the furnace was flushed with dry nitrogen. For the evaluation of the
output, the Netzsch Protens 4.0 software was employed.
(23) Richmond, T. G.; Crespi, A. M.; Shriver, D. F. Organometallics 1984, 3,
314.
(24) Clark, G. R.; Hoskins, S. V.; Roper, W. R. J. Organomet. Chem. 1982,
234, C9.
(25) Michelin, R. A.; Facchin, G. J. Organomet. Chem. 1985, 279, C25.
(26) Appleton, T. G.; Berry, R. D.; Hall, J. R.; Neale, D. W. J. Organomet.
Chem. 1989, 364, 249.
(27) Baasner, B.; Hagemann, H.; Tatlow, J. C.; 4 ed.; Houben-Weyl, 2000; Vol.
E10b/Part 2, p 418.
(28) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon
Press: Oxford, UK, 1984.
(29) Ansorge, A.; Brauer, D. J.; Bu¨rger, H.; Krumm, B.; Pawelke, G. J.
Organomet. Chem. 1993, 446, 25-35.
(30) Massey, A. G.; Park, A. J.; Stone, F. G. A. Proc. Chem. Soc. 1963, 212.
(31) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
(32) Jacobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.;
Meyer, O. Organometallics 1999, 18, 1724-1735.
Results and Discussion
Synthetic Aspects and Thermal Properties of (CF3)3BCO.
The synthesis of (CF3)3BCO involves the in situ generation of
coordinated CO by the solvolysis of a single trifluoromethyl
group10 of the recently reported salt K[B(CF3)4]11 in concentrated
H2SO4 (96%). The formation reaction may be formulated as
9
15388 J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002

(CF ) BCO−Synthesis
A R T I C L E S
3 3
Table 2. IR Bands of Ar Matrix Isolated Products, Formed by Low Pressure Flash Thermolysis of (CF3)3BCO
band
positiona
band
positiona
band
positiona
I b
assignment
I b
assignment
I b
assignment
2138
1510
1498
1458
1451
1447
1404
1381
1372
1337
0.062
0.050
0.017
0.190
0.025
0.079
0.018
0.015
0.067
0.023
CO
1328
1230
1221
1211
1207
1180
1148
1143
1135
1115
0.090
0.153
0.196
0.129
0.178
0.083
0.085
0.102
0.076
0.072
1102
984
919
755
712
695
676
605
559
525
0.42
CF2
CF310BF2
10BF3
0.065
0.005
0.013
0.004
0.020
0.003
0.021
0.010
0.016
CF2
CF311BF2
11BF3
CF310BF2
CF311BF2
11BF3
C2F4
C2F4
a Most intensive matrix site, cm-1 b Absorbance units.
.
at room temperature, so that the proposed overall decomposition
is formulated as shown in eq 4
(CF3)3BCO h CO + (CF3)3B f
CO + CF3BF2 + BF3 + further products (4)
In contrast, BH3CO is found to decompose in the same IR cell
at 28 °C in a different manner. The initial decomposition rate
(t1/2 ≈ 100 min), which is comparable to that of (CF3)3BCO,
decreases gradually, as equilibrium (5) is approached
2BH3CO h B2H6 + 2CO
(5)
Low pressure flash thermolysis of thermally labile compounds
such as (CF3)3BCO, followed by subsequent quenching of the
products in low-temperature matrixes, allows the detection of
short-lived intermediates by IR spectroscopy.14,33 This technique
is utilized here, to obtain further information, regarding the first
step in the mono-molecular dissociation of (CF3)3BCO. Initial
decomposition products, isolated in an Ar matrix, are observed
by passing highly diluted (CF3)3BCO and Ar, through a heated
spray-on nozzle at 100 °C. At about 180 °C, the initial amount
of (CF3)3BCO is completely depleted. All matrix IR bands of
the decomposition products are simultaneously increased in
intensity. The same experiments were repeated by using N2 as
matrix gas in order to form the isoelectronic (CF3)3BN2
molecule. However, this attempt failed.
The “new” bands observed in an Ar matrix are listed in Table
2. By comparison to reference matrix IR spectra of authentic
samples, the main products CO, BF3, CF2, and minor amounts
of C2F4 are identified. The molar ratio CO/BF3 of 4:1 is
estimated from a reference spectrum of a CO/BF3 mixture,
isolated in an Ar matrix. All additional, unassigned bands in
Table 2 can be attributed to a mixture of CF3BF2, (CF3)2BF
and possibly minor amounts of B(CF3)3. From these observa-
tions, the following reaction pathway shown in eq 6 is proposed
Figure 2. Arrhenius plot of the 13CO exchange reaction with (CF3)3BCO
(1st order).
mol-1, whereas the B-CO bond energy is estimated to be 114
kJ mol-1 (see below).
The thermal decomposition of (CF3)3BCO is in the gas-phase
retarded by the presence of CO and in solution by inert aprotic
solvents (SO2, CH2Cl2). In the latter case, the solvation of the
electrophilic CO carbon atom (see below) by polar solvent
molecules, seems to be responsible for a strengthening of the
B-CO bond. In the presence of CO the primary step in the
decomposition (6) appears to be suppressed, by the backward
reaction. The presence of low concentrations of the free Lewis
acid (CF3)3B in equilibrium with CO is confirmed by the use
of isotopically enriched 13CO and C18O
(CF3)3BCO + 13CO h (CF3)3B13CO + CO
(8)
which permits the syntheses the 13CO and C18O enriched borane
carbonyls.
With 13CO in a large excess, the exchange process (8) is of
first order. The exchange rates at 308.3, 303.2, 298.4, 294.2,
and 284.1 K are quantitatively determined to be k ) 4.025,
1.967, 0.9605, 0.4777, and 0.1005 × 10-4 s-1, respectively, by
slow
13
using the observed absorbance ratio Z ) A(ν CO)/A(νCO) and
(CF3)3BCO y z CO + (CF3)3B
(6)
the equation ln(1 + Z) ) kt + B. The corresponding Arrhenius
plot of these data is displayed in Figure 2. From the mean square
regression slope, ln(k) ) -13423 T-1 + 35.7, the activation
energy is calculated to be EA ) 112 ( 1 kJ mol-1. This value
is in good agreement with the theoretically predicted B-CO
bond dissociation energy of 114 kJ mol-1 (see below). Hence,
the activation energy for the B-CO bond cleavage seems to
be nearly identical with the B-CO bond energy. The high value
of the frequency factor of 3.2 × 1015 s-1 may be due to the
high CO pressure of 2.5 bar in kinetic experiments.
followed by a series of fast reactions as displayed in eq 7
(CF3)3B
8 (CF3)2BF
8 CF3BF2
8 BF3 (7)
-CF2
-CF2
-CF2
This implies that the activation energy for the CF2 elimination
of B(CF3)3 is lower, than the B-CO bond energy. Indeed DFT
calculations predict for each step in eq 7 values of about 80 kJ
(33) Sander, S.; Pernice, H.; Willner, H. Chem.-Eur. J. 2000, 6, 3645.
9
J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002 15389

A R T I C L E S
Finze et al.
Table 3. Thermal and Other Properties of Selected Borane Carbonyl Derivativesa
compd
T
decomp. [°C]b
ref
D (B−C) [kJ mol-1
]
ref
r(B−C) [Å]
ref
ν
CO [cm-1
]
ref
F3BCO
(C6F5)3BCO
(CF3)3BCO
H3BCO
(BF2)3BCO
(BCl2)3BCO
1,10-B10H8(CO)2
1,12-B12H10(CO)2
-200
-120
0
6
7.6
34
2.89
(1.61)
1.62
1.53
1.52
1.54
35
2151
2230
2252
2165
2162
2176
2147
2210
36
c
c
(38)
32
32
c
c
c
c
c
112
90
d
10
7
38
38
37
39
38
40
40
20
20
200
400
38
38
40
40
1.54
41
a In parentheses: calculated values. b Estimated by behavior described in the Literature. c This work. d Best estimate from five experimental values according
to reference.32
Scheme 1. Some Selected Chemical Reactions of (CF3)3BCO
The thermal properties of (CF3)3BCO and selected charac-
teristic features are listed in Table 3 and compared to those of
other borane carbonyls. In this comparison, the properties of
the so far unknown32 (C6F5)3BCO are of interest. Because any
experimental data of this borane carbonyl are unavailable, the
compound is synthesized by isolation of (C6F5)3B molecules in
solid CO, followed by subsequent slow evaporation of excess
CO at 40 K. The solid film of (C6F5)3BCO, formed in this
manner, exhibits ν(CO) at 2230 cm-1 and loses CO at about
-120 °C. As can be seen in Table 3, the thermal stabilities of
borane carbonyls vary widely and range from -200 °C for F3-
BCO to 400 °C for 1,2-B12H10(CO)2.11 As far as appropriate
data are available, the stabilities seem to correlate well with
the D(B-CO) bond energies. The (B-CO) bond lengths for
the less stable species are quite large, but in more stable
compounds, the (B-CO) bond lengths are invariant and do not
appear to reflect the thermal stabilities of the respective species
very well. For ν(CO) stretching wavenumbers, which will be
discussed later, a simple correlation is not immediately obvious.
The bond-forming reaction between Lewis acids of the type
R3B and Lewis base including CO has recently been analyzed
in some detail.32 It is concluded that the energy required to
distort the R3B unit from its planar ground state to a pyramidal
geometry, formed in the R3B-CO complex, is an important
factor. It can hence be argued, that (CF3)3BCO should be more
stable than (C6F5)3BCO. To obtain pyramidal (C6F5)3B, required
for bonding to CO, more energy is needed than for (CF3)3B, on
account of the bulkier C6F5 groups and the possibility, of
π-interactions of the aryl π-system with the central boron atom
in the trigonally coordinated parent compound. However, the
synthetic route to (CF3)3BCO by partial hydrolysis of a CF3
group does not require a change in ground state and the
difference between (CF3)3BCO and (C6F5)3BCO is reflected in
the markedly different thermal stabilities of both compounds.
In the closo-boranes the pyramidal R3B fragments are already
present and hence very strong B-CO bonds result and carbonyl
boranes of unusually high thermal stability.40
Chemical Properties of (CF3)3BCOsAn Overview. Reac-
tions of (CF3)3BCO with nucleophiles can proceed in two
different ways; (i) the attacking nucleophile can react with the
carbon atom in an addition to the carbonyl group, which remains
bound to the boron atom or (ii) the nucleophile can substitute
CO on the central boron atom. Whether nucleophilic addition
(i) or substitution (ii) occurs, depends on the relative stability
of the resulting products and the nucleophilicities of the
attacking reagents. Because, as discussed above, dissociation
of (CF3)3BCO under reaction conditions is slight, only small
amounts of decomposition products are observed and all
reactions studied are fairly clean.
As can be seen from an overview of the reactions we have
studied so far in Scheme 1, addition reactions to the carbon
atom of the CO substituent dominate, where the incoming
nucleophile forms stable bonds with carbon. Examples are the
reactions with methyllithium, methanol, water, KF, or liquid
ammonia. The reactions with CH3CN and [B(CN)4]- result in
substitution of the carbonyl by the stronger nucleophile. The
9
15390 J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002

(CF ) BCO−Synthesis
A R T I C L E S
3 3
Table 4. Rotational Transitions (MHz) in (CF3)3B13CO and
reaction of (CF3)3BCO with NOCl also proceeds by exchange
of the CO-ligand, giving rise to a novel, convenient synthesis
of [(CF3)3BCl]-.42 In reactions, where the CO-ligand is replaced,
(CF3)3BCO can be viewed as a synthon, used in place of the
unknown Lewis acid (CF3)3B.
(CF3)3B12CO
13C
J + 1 r J
12C
8759.40
7 r 6
15 r 14
16 r 15
17 r 16
18 r 17
19 r 18
20 r 19
21 r 20
8795.40
18849.00
20102.23
21358.92
22615.60
23870.00
25126.63
26382.82
18770.63
20021.06
21273.23
22525.20
23777.05
25028.80
26277.18
Most of the reactions shown in Scheme 1 are fast because of
the high reactivity of the borane carbonyl. Hence, the number
of byproducts is usually small and in some cases negligible.
On account of the highly reactive character of (CF3)3BCO, the
number of solvents, suitable as reaction media, is limited.
Especially in SO2 and CH2Cl2, (CF3)3BCO is stable and can be
stored over a period of one week without any notable decom-
position. Unfortunately, reactions with strong bases cannot be
performed in these two solvents. In Et2O, (CF3)3BCO is stable
at -78 °C for a few hours and reactions with organometallic
reagents usually give high yields. THF reacts with (CF3)3BCO
readily at -78 °C in a ring opening reaction, that gives a
complex mixture of anionic esters of the type [(CF3)3BC(O)OR]-
and other not identified decomposition products. Ring opening
reactions of THF with strong Lewis acids are well-known in
the literature.43
thermal behavior. [Pr3NH][(CF3)3BC(O)OH] is a white solid,
that is readily soluble in CH2Cl2. It melts at 146 °C and
decomposes at 220 °C. In the case of the dianion, the potassium
salt K2[(CF3)3BCO2] is obtained as a white solid, that is
insoluble in Et2O, but soluble in CH3CN. The salt does not melt
before decomposition occurs at 220 °C.
Details on the other reactions and the new compounds,
presented in Scheme 1, will be published elsewhere.
Molecular Structure of (CF3)3BCO in the Gas Phase. (I)
Rotational Spectra. Tris(trifluoromethyl)borane carbonyl is a
symmetric rotor with a small rotational constant of B ≈ 0.6
GHz. One therefore expects a spectrum with J + 1 r J
transitions, approximately 1.2 GHz apart. The structures of the
individual transitions depend on centrifugal distortion (K-
structure) and nuclear quadrupole coupling. 10B possesses a spin
of 3 and a quadrupole moment of 0.086 × 1024 cm2 ap-
proximately (the size of the 35Cl quadrupole moment). 11B with
a spin of 3/2 has a quadrupole moment of 0.042 × 1024 cm2
(intermediate between the quadrupole moments of 35Cl and 14N).
A further complication arises, because the B atom is located
in close proximity (∼0.2 Å according to ab initio calculations)
to the center of mass. As a consequence, the rotational transitions
of the 10B- and 11B-isotopomers are still within 1 MHz of one
another even at J ) 20. Hence, it is not possible to resolve any
fine- or hyperfine structure by this technique. It is equally
impossible to distinguish between the 10B- and 11B-isotopomers.
Because the natural abundance ratio of 10B/11B is approximately
20:80, we assume in structural calculations, that the observed
peaks are due solely to the 11B-isotopomer.
The spectra are mainly recorded in K-Band (18.2-20.1 GHz
recorded continuously, narrow ranges recorded in the region of
20.1-26.4 GHz). To check the possibility of resolving the fine
structure only, transitions between 8.7 and 10.4 GHz are also
recorded under the lowest possible pressure. Even under these
conditions, any hyperfine structure remains unresolved. Most
of the lines appear as unstructured peaks with a half width of
approximately 8 MHz.
With a sample enriched (by approximately 50%) in the 13C-
isotopomer, it is possible to measure this isotopomer as well.
There are hardly any distinctions between the transitions of the
12C- and 13C-isotopomers in this sample (except for the line
positions). The recorded transitions are collected in Table 4 and
the resulting rotational constants are found in Table 5. The large
half width of rotational transitions results in an accuracy for
the rotational constants, that is lower than usual. Thus, the
Kraitchman rs coordinate for the C atom of the CO group,
derived from the two B0 constants (rs ) 1.797(48) Å) possesses
a large uncertainty. From the calculated (B3LYP) force field,
the vibrational corrections ∆B ) B0 - Bz are derived. The
An interesting reaction is observed between (CF3)3BCO and
water. Upon dissolving (CF3)3BCO in H2O the dihydroxy
carbene complex, (CF3)3BC(OH)2, is obtained, according to eq
9
(CF3)3BCO + H2O f (CF3)3BC(OH)2
(9)
which will undergo ionic dissociation in water (eq 10)
+
+
(CF3)3BC(OH)2 y-H z [(CF3)3BC(O)OH]- y-H z
[(CF3)3BCO2]2- (10)
+H+
+H+
A potentiometric titration of (CF3)3BCO in water reveals pKa
values to be <1.5 and 7.0 for the loss of the first and the second
proton, respectively. These values are comparable to those
reported for the loss of the first two (4.2) and the second two
protons (9.0) of 1,12-B12H10(CO)2 in aqueous solution.41
A direct comparison with the pKa value of isoelectronic
(CF3)3CC(O)OH is not possible because this compound appears
to be unknown. Because (CF3)3CC(O)OH should have a smaller
pKa value than trifluoroacetic acid, CF3C(O)OH (pKa ) 0.23),
the comparison of the pKa value of CF3C(O)OH with that of
[(CF3)3BC(O)OH]- already shows that there is a significant
difference. This different behavior is due to the overall negative
charge of [(CF3)3BC(O)OH]-, that results in an increase of
electron density at the carbon atom of the acid. Both anions,
[(CF3)3BC(O)OH]- and [(CF3)3BCO2]2-, are isolated as stable
salts, which are characterized by NMR spectroscopy and their
(34) Sluyts, E. J.; van der Veken, B. J. J. Am. Chem. Soc. 1996, 118, 440-445.
(35) Janda, K. C.; Berstein, L. S.; Steed, J. M.; Novick, S. E.; Klemperer, W.
J. Am. Chem. Soc. 1978, 100, 8074-8079.
(36) Gebicki, J.; Liang, J. J. Mol. Struct. 1984, 117, 283-286.
(37) Bethke, G. W.; Wilson, M. K. J. Chem. Phys. 1957, 26, 1118-1130.
(38) Jeffery, J. C.; Norman, N. C.; Pardoe, A. J.; Timms, P. L. Chem. Commun.
2000, 2367-2368.
(39) Timms, P. L. J. Am. Chem. Soc. 1967, 89, 1629.
(40) Knoth, W. H.; Sauer, J. C.; Balthis, J. H.; Miller, H. C.; Muetterties, E. L.
J. Am. Chem. Soc. 1967, 89, 4842-4850.
(41) Fox, M. A.; Howard, J. A. K.; Moloney, J. M.; Wade, K. Chem. Commun.
1998, 2487-2488.
(42) Brauer, D. J.; Bu¨rger, H.; Chebude, Y.; Pawelke, G. Inorg. Chem. 1999,
38, 3972.
(43) Mair, F. S.; Morris, J. H.; Gaines, D. F.; Owell, D. J. Chem. Soc., Dalton
Trans. 1993, 135.
9
J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002 15391

A R T I C L E S
Finze et al.
Table 5. Rotational Constants B0 and Bz of (CF3)3BCO
tions ∆r for bonded distances, if low-frequency-large-amplitude
vibrations occur. According to the B3LYP method (CF3)3BCO
possesses several low-frequency vibrations, such as the CF3
B
0
B (exp)
z
B (calcd)a
z
12CO
13CO
628.20 (9)
625.69 (6)
630.14 (29)
627.63 (29)
629.97
627.53
torsions near 45 cm-1 and C-B-C deformations near 90 cm-1
.
The concept of perpendicular amplitudes (first harmonic
approximation) leads to a correction ∆r ) 0.025 Å for the C-F
bond lengths and such an rz structure is not compatible with
the rotational constants. For this reason, the corrections ∆r are
calculated with the second harmonic approximation proposed
by Sipachev.44,45 This method results in small corrections for
all bond distances (see Table S5). The rz structure obtained with
these corrections, reproduces the experimental rotational con-
stants Bz very well. In the joint least-squares analysis of GED
intensities and rotational constants for the parent isotopic species
and the 13CO substituted species (CO carbon), the B-O distance
is determined very accurately, but the individual B-CO and
C-O bond lengths correlate strongly.
a Derived from joint analysis of GED intensities and rotational constants.
Because the primary interest is the B-CO distance, the C-O
bond length is fixed. The rz value for this bond is derived from
the r0 value in CO (r0 ) 1.1308 Å),46 which is corrected to rz
(1.1321 Å) and then reduced by the calculated (B3LYP)
difference between the re distance in free CO (1.1379 Å) and
that in the complex (1.1298 Å). This leads to rz ) 1.124 Å for
the C-O bond length in (CF3)3BCO. Otherwise, the same
assumptions described for the GED analysis are used in the joint
analysis. The relative weight of the GED intensities and the
rotational constants is adjusted until the Bz values are fitted
within their estimated uncertainties (see Table 5). The results
of the joint analysis are listed in Table 6 (geometric parameters)
and Table S5 (vibrational amplitudes) together with the calcu-
lated values. A molecular model is shown in Figure 4.
Molecular Structure in the Solid State. Crystals, suitable
for a single X-ray diffraction analysis are grown by sublimation
in sealed ampules. Crystallographic data and details on the
structure determination are summarized in Table 1. The low
quality of the structure is due to a phase transition near -70
°C and the formation of twins. Crystals, which are grown at
-20 °C, are disordered and hence not suitable for a single-
crystal X-ray structure analysis. When these crystals are cooled
below -70 °C, they shatter into pieces. The growth of crystals
at temperatures lower than -70 °C is not feasible, because the
vapor pressure of (CF3)3BCO is too low. Therefore, it is
necessary to grow crystals for single-crystal X-ray structure
analysis near -70 °C.
A structural analysis of the crystals, grown at -70 °C,
confirms at the very least the identity of (CF3)3BCO and the
correct connectivity of all atoms in the molecule. The bond
lengths show uncertainties of about 0.02 Å and their discussion
will be limited. The unit cell packing can be described as a
molecular lattice, consisting of alternating layers, which are
stacked along the a-axis. All intermolecular contacts between
individual (CF3)3BCO molecules in the crystal seem to be longer
than the sum of the van der Waals radii.47,48 The internal bond
parameters will be compared below with the gas-phase structure.
Quantum Chemical Calculations. The geometry of (CF3)3-
BCO is optimized with the help of Hatree-Fock and MP2
Figure 3. Experimental radial distribution function and difference curve
of (CF3)3BCO.
uncertainty in Bz constants (Table 5) is estimated to be 15% of
the correction ∆B. The accuracy of these Bz constants is
sufficient for a joint analysis of GED intensities and rotational
constants (see below). The rz coordinate of the C atom of the
CO group derived from this analysis (rz ) 1.768(12) Å),
reproduces the Kraitchman rs value within experimental uncer-
tainties.
Gas Electron Diffraction (GED) and Structure Analysis.
The radial distribution function (RDF) is calculated by Fourier
transformation of the molecular intensities, by applying an
artificial damping function exp(-γs2) (γ ) 0.0019 Å2). A
preliminary structural model which is derived by analysis of
the radial distribution function RDF (Figure 3) is refined by
least-squares fitting of the molecular intensities. On the basis
of quantum chemical calculations, the following assumptions
are applied in this refinement: (i) the structure is constrained
to C3 symmetry; (ii) all F-C-F angles in the CF3 groups are
assumed to be equal. According to calculations, these angles
deviate by less than 0.5° from the mean value; (iii) the
differences between individual C-F bond lengths and the tilt
angle of the CF3 group are set according to the MP2 values;
and (iv) vibrational amplitudes, which cause high correlations
between geometric parameters or which are poorly determined
in the GED experiment, are set equal to calculated (B3LYP/6-
31G*) values.
With these assumptions, seven bond parameters, B-CO,
B-CF3, (C-F)avg, C-B-C, F-C-F, the dihedral angle φ(C-
B-C1-F13), and 10 vibrational amplitudes (l1 to l10) are
refined simultaneously. For the B-CO distance, which for this
compound is the most interesting parameter, a value of 1.643
Å with a very large uncertainty (3σ value) of (0.031 Å is
obtained.
To derive a more accurate value for the B-CO bond length,
a joint analysis of GED intensities and rotational constants is
performed. Such an analysis requires the vibrational corrections
∆r ) ra - rz for GED parameters and ∆B ) B0 - Bz for
rotational constants. It is well-known, that the concept of
perpendicular vibrations results in unrealistically large correc-
(44) Sipachev, V. A. J. Mol. Struct (THEOCHEM) 1985, 121, 143.
(45) Sipachev, V. A. In AdVances in Molecular Structure Research; Hargittai,
I. H., M., Eds.; JAI: Greenwich, 1999; Vol. 5.
(46) Gieliam, O. R.; Johnson, C. M.; Gordy, W. Phys. ReV. 1950, 78, 140.
(47) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(48) Alcock, N. W. AdV. Inorg. Radiochem. 1972, 15, 1.
9
15392 J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002

(CF ) BCO−Synthesis
A R T I C L E S
3 3
Table 6. Experimental and Calculated Bond Parameters for (CF3)3BCOa
b
(CF ) BCO
(CF ) BCO
3 3 (s)
HF/6-311G(2d)
MP2/cc-pVDZ
B3LYP/6-311+G*
3 3
(g)
bond length
C-O
1.124c
1.11(2)
1.69(2)
1.60(2)
1.35(2)
1.35(2)
1.34(2)
1.090
1.700
1.626
1.330
1.323
1.314
1.138
1.610
1.623
1.366
1.356
1.345
1.119
1.589
1.646
1.363
1.355
1.342
B-C
1.617(12)
1.631(4)
1.358(1)d
1.348(1)e
1.337(1)
B-C1
C1-F11
C1-F12
C1-F13
bond angle
C-B-C1
C1-B-C2
F11-C1-F12
103.8(4)
114.5(4)
107.2(1)
104.4(12)
114.0(12)
105.7(19)
104.1
114.3
107.2g
105.0
113.5
107.2g
105.5
113.1
107.3g
torsion angle
tilt(CF3)f
0.8c
11.5(9)
0.8
0.9
0.6
τh
16(3)i
15.5
16.1
13.0
a Values in Å and °. For atom numbering see Figure 4. b rz values with 2σ uncertainties. c Not refined. d Difference (C-F2)-(C-F1) set to 0.021 Å.
e Difference (C1-F12)-(C-F13) set to 0.011 Å. f Tilt angle of CF3 group which makes the B-C1-F12 angle larger than the B-C1-F13 and B-C1-F11
angles. g Mean value. h Deviation from C3V-symmetry. i Average value.
Figure 5. Schematic representation of the molecular structure of (CF3)3-
BCO in the gas phase and in the solid state.
(by electron diffraction and microwave spectroscopy) and the
solid state (by single X-ray diffraction). This is to our knowledge
unprecedented, for carbonyl boranes and allows a comparison
of both sets of structural data for (CF3)3BCO.
Figure 4. Molecular model and numbering of (CF3)3BCO.
approximations and with the hybrid method B3LYP, using
different basis sets. The vibrational frequencies are calculated
with the B3LYP method and 6-31G* basis sets. Vibrational
amplitudes and vibrational corrections for interatomic distances
and rotational constants are derived from the Cartesian force
field (see below). Results for geometric parameters, obtained
from calculations with the largest basis sets, vibrational
amplitudes, and corrections are listed together with the corre-
sponding experimental values in Table 6 and Table S5.
A comparison of internuclear distances (d(B-CF), d(B-CO),
and d(C-O)) and selected bond angles for the C3BCO skeleton
of (CF3)3BCO is shown in Figure 5. Although the bond angles
appear to be identical within stated esd values, the bond lengths
show some interesting variations: on going from the gas phase
to the solid state, d(B-CF) appears to contract very slightly by
about 0.03 Å, just outside the esd limit. More noticeably, the
B-CO bond length increases from 1.617(4) Å in the gas phase
to 1.69(2) Å in the solid state. Finally, the C-O bond length
appears to contract very slightly, best expressed in terms of the
CO-stretching wavenumber ν(CO) which increases from 2252
to 2269 cm-1 (see Table 7). As a consequence of these changes,
all four B-C distances, which are nearly identical in the gas
phase, are now in the solid state for d(B-CF) and d(B-CO)
different by about 0.09 Å.
According to B3LYP/6-31G* calculations, the partial charges
q for the atoms within the BCO group are: qB ) -0.002, qC )
+0.404, and qO ) -0.166 e- respectively. These data support
our view, that the C(O) atom is the electrophilic center in (CF3)3-
BCO. In addition a rather high value of 3.27 D is predicted for
the dipole moment. Finally the differences in the absolute
energies of (CF3)3BCO, CO, (CF3)xBF3-x, x ) 1-3, and CF2
lead to a B-CO bond energy of 114 kJ mol-1 and for the
transformation B-CF3 into B-F + CF2 to an average energy
(49) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; 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.; Gonzales, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzales, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6;
Gaussian, Inc.: Pittsburgh, PA, 1998.
of 80 kJ mol-1
.
All quantum chemical calculations are performed with the
GAUSSIAN98 program package.49
Discussion of the Gas Phase and Solid State Molecular
Structure of (CF3)3BCO. Tris(trifluoromethyl)borane carbonyl,
(CF3)3BCO, has on account of its convenient physical properties
allowed molecular structure determinations in both the gas phase
9
J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002 15393

A R T I C L E S
Finze et al.
Table 7. Observed and Calculated Band Positions and Band Intensities for the Main Isotopomera of (CF3)3BCO
IR
Raman solid
−196 °C
assignments according
to C symmetry
3
cal.b
Ic
gasphase
σd
Ar matrixe
If
3523
2595
2252
1
1
156
3521
2604
2251
1271
1266
1218
1196
1163
1155
1121
1073
919
904
883
856
725
697
554
0.4
1.1
74
ν1 + ν2
ν1 + ν9
A ν1
2279
1278
1255
208
273
492
2269
1270
1266
vs
sh
s
ν(CO)
νs(CF3)
νs(CF3)
154
230
16
9
261
156
141
12
100
4
A ν2
1270
1214
634
40
}
}
E ν14
ν5 + ν22
ν6 + ν8
A ν3
E ν15
E ν16
A ν4
1197
1189
1157
1104
930
116
135
143
6.5
100
1173
1140
1119
1065
891
m
w
m
w
m
νas(CF3)
νas(CF3)
νas(CF3)
νas(CF3)
νas(BC3)
1158
291
1129
1074
916
119
13
221
E ν17
ν7 + ν9
ν8 + ν9
A ν5
5
6
5
29
898
722
694
556
516
514
513
464
366
299
277
254
246
122
117
86
3.0
2.8
29
4.2
0.2
0.01
5.5
12
13
1.3
848
725
698
552
10
12
76
4
867
727
698
554
m
s
w
w
νs(BC3)
δs(CF3)
δs(CF3)
δas(CF3)
δas(CF3)
δas(CF3)
δas(CF3)
δ(BCO)
ν(B-CO)
F(CF3)
A ν6
E ν18
E ν19
A ν7
1.8
523
6
A ν8
}
523
453
349
300
278
5.3
17
10
1.5
525
447
357
303
283
260
m
m
w
s
s
E ν20
E ν21
A ν9
454
343
299
278
21
7
3
E ν22
A ν10
E ν23
F(CF3)
A ν12
E ν24
E ν25
E ν26
A ν13
0.3
1
0.6
F(CF3)
F(CF3)
0.05
0.001
2.5
0.5
0.8
w
A ν11
132
125
95
2.5
0.5
0.5
139
w
δ(CBC)
δ(CBC)
δ(CBC)
τ(CF3)
132
92
5
2
}
41
38
0.03
0.01
τ(CF3)
a (12CF3)311B12C16O. b B3LYP/6-31G*. c Relative band intensity I(ν17) )ˆ 100 ) 153 km mol-1 d Absorption cross section 10-20 cm2. e Band position at
.
most intensive matrix site. f Relative integrated band intensity
Table 8. Comparison of Structural and Spectroscopic Data for (CF3)3BCO with Related Compounds and σ-Metal Carbonyl Cations
species
d (X−C) [Å]
d (C−O) [Å]
ν(CO)avg. [cm-1
]
fCO [N m-1
]
δ(13C) [ppm]
ref
COa
1.1281a
n.o.
n.o.
2143a
2184a
2185
2190
2269
2294
2340
1911
2215
2209
2269
2261
2280
18.6a
21.3a
n.o.
n.o.
20.8
n.o.
184
140
n.o.
n.o.
159
150
122
125
178
147
121
137
169
59-61
62, 63
64
[H-CO]+
n.o.b
n.o.
n.o.
[Li-(CO)]+c
OBe-COd
n.o.
65
e
(CF3)3BCO(s)
[CH3-CO]+f
[N(CO)2]+g
OCO
1.69(2)
1.38(2)
1.250
1.1615
1.911(5)
2.027(5)
2.029(10)
1.982(9)
2.083(10)
1.11(2)
1.116(21)
1.118
1.1615
1.103(5)
1.102(7)
1.090(10)
1.110(9)
1.104(12)
56, 57, 66
58
59, 66
67
2
68, 69
70
71
n.o.
15.5
19.7
19.71
20.8
20.64
21.0
[Fe(CO)6]2+h
[Os(CO)6]2+h
[Ir(CO)6]3+i
[Pt(CO)4]2+h
[Hg(CO)2]2+ h
a Gas phase. b n.o. ) not observed. c Li(zeolithe-ZSM-5). d Ar-matrix. e This work. f [SbF6]- as counteranion. g [Sb3F16]- as counteranion. h [Sb2F11]- as
counteranion. i In [Ir(CO)6][SbF6]3‚4HF.
This lengthening of the B-CO bond from the gas phase to
the solid state observed here, is unusual. Commonly for adducts
of boranes with various Lewis acids the opposite trend is
observed. For example, the B-N bond in H3BNH3 contracts
from 1.656(2) Å in the gas phase50 to 1.564(6) Å in the solid
state,51 which implies tighter, more efficient packing of H3BNH3
in the crystal lattice. Other examples are the adducts of
acetonitrile and HCN with boron trifluoride.52,53 The nature of
bonding in Lewis acid-base adducts such as H3BNH3 or F3-
BNCH has been the subject to several experimental and
theoretical studies,54 and in (CF3)3BCO it is clearly different
than in the “classical” adducts.
In Table 8 spectroscopic and structural properties of some
main group analogues of (CF3)3BCO are listed. In the series
(CF3)3BCO, [CH3-CO]+,55-57 [N(CO)2]+,58 and OCO59
a
(54) Jonas, V.; Frenking, G.; Reetz, M. T. J. Am. Chem. Soc. 1994, 116, 8741.
(55) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; Wiley: New York,
1985.
(50) Thorne, L. R.; Suenram, R. D.; Lovas, F. J. J. Phys. Chem. 1983, 78, 167.
(51) Bu¨hl, M.; Steinke, T.; Schleyer, P. v. R.; Boese, R. Angew. Chem., Int.
Ed. Engl. 1991, 30, 1160.
(56) Olah, G. A.; Kuhn, S. J.; Tolgyesi, W. S.; Baker, E. B. J. Am. Chem. Soc.
1962, 84, 2733.
(52) Dvorak, M. A.; Ford, R. S.; Suenram, R. D.; Lovas, F. J.; Leopold, K. R.
J. Am. Chem. Soc. 1992, 114, 108.
(57) Boer, F. P. J. Am. Chem. Soc. 1966, 88, 1572.
(58) Bernhardi, I.; Drews, T.; Seppelt, K. Angew. Chem. 1999, 111, 2370.
(53) Burns, W. A.; Leopold, K. R. J. Am. Chem. Soc. 1993, 115, 11 622.
9
15394 J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002

(CF ) BCO−Synthesis
A R T I C L E S
3 3
Figure 6. IR (gas phase) and Raman (solid) spectra of (CF3)3BCO.
decrease of d(X-CO) is observed. The strong similarity between
(CF3)3BCO and superelectrophilic σ-metal carbonyl cations
suggested here, is evident from a comparison of structural and
spectroscopic data for solid (CF3)3BCO and the homoleptic σ
carbonyl cations of the 3d and 5d metals Fe, Hg, Pt, Ir, and Os
in Table 8, where fCO and ν(CO) of (CF3)3BCO is comparable
or occasionally higher than the fCO and ν(CO)avg values, which
are the highest so far observed for metal carbonyl cations.1,2
The absence of π-back-bonding is reflected in unusually long
M-C bonds.1,2 This applies also to (CF3)3BCO. All B-CO
bonds of borane carbonyl derivatives of comparable or higher
thermal stability than that of (CF3)3BCO, listed in Table 3, have
d(B-CO) values between 1.52 and 1.54 Å. The average B-C
distance from the Cambridge listing,72 based on 29 examples
has a value of 1.597 Å and an upper quartile qu value of 1.611
Å. Even the d(B-CF) values for (CF3)3BCO are comparable
(in the solid state) or longer (in the gas phase) than the qu value
from the Cambridge index.72 This is consistent with the absence
of π-back-bonding contributions for all C-B bonds. The ease
of variation of d(B-CO) in (CF3)3BCO by packing effects,
indicates a shallow potential curve for this bond in agreement
with the low ν(B-CO) vibration of 343 cm-1
.
For boranes a strong CH3/CF3 substitution effect has been
observed. Because (CF3)3B is unknown, which precludes a direct
comparison with (CH3)3B, we haven chosen to compare the
B-C bonds in (CH3)2BNHMe (1.586(2) Å73) and in (CF3)2-
BNMe2 (1.623(4) Å74). For these two related compounds, a
lengthening of the B-C bond on CH3/CF3 substitution by about
0.04 Å is observed. The B-CF3 bond length in the latter
compound is very similar to that in (CF3)3BCO.
As can be seen in Table 6, the results of various quantum
chemical calculations, discussed above, are in good agreement
with each other, except for the B-CO bond length. This
parameter appears to depend on the state of matter and the
method of calculation, resulting in a spread from 1.589 Å to
∼1.700 Å.
(59) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure.
Constants of Diatomic Molecules; van Nostrand: New York, 1979.
(60) Willner, H.; Schaebs, J.; Hwang, G.; Mistry, F.; Jones, R.; Trotter, J.; Aubke,
F. J. Am. Chem. Soc. 1992, 114, 8972.
(61) King, R. B.; Bisnette, M. B. J. Organomet. Chem. 1967, 8, 287.
(62) de Rege, P. J. F.; Gladysz, J. A.; Horvath, I. T. Science; Washington, DC,
1883 1997, 276, 776.
(63) Hirota, E.; Endo, Y. J. Mol. Spectrosc. 1988, 127, 527.
(64) Katoh, M.; Yamazaki, T.; Ozawa, S. Bull. Chem. Soc. Jpn. 1994, 67, 7,
1246.
(65) Andrews, L.; Tague, T. J., Jr. J. Am. Chem. Soc. 1994, 116, 6856.
(66) Kalinowski, H.-O.; Berger, S.; Braun, S. 13C NMR-Spektroskopie; Georg
Thieme Verlag: Stuttgart, 1984.
(67) Bernhardt, E.; Bley, B.; Wartchow, R.; Willner, H.; Bill, E.; Kuhn, P.;
Sham, I. H. T.; Bodenbinder, M.; Bro¨chler, R.; Aubke, F. J. Am. Chem.
Soc. 1999, 121, 7188.
(68) von Ahsen, B.; Berkei, M.; Henkel, G.; Willner, H.; Aubke, F. J. Am. Chem.
Soc. 2002, 124, in press.
(69) Bach, C.; Willner, H.; Wang, C.; Rettig, S. J.; Trotter, J.; Aubke, F. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1974.
The C-O bond length cannot be determined accurately from
experimental data. Quantum chemical calculations predict the
C-O internuclear distances in (CF3)3BCO to be shorter by 0.008
Å (B3LYP), 0.009 Å (MP2), and 0.013 Å (HF), respectively,
than that calculated for free CO.59 These calculated bond lengths
are in agreement with the observed shift in the vibrational
frequencies from 2143 cm-1 for free CO to 2252 cm-1 for
(CF3)3BCO. The respective calculated (B3LYP-MP2, -HF)
frequencies are 2208 and 2279 cm-1, respectively.
Vibrational Spectra. The IR and Raman spectra of gaseous
and solid (CF3)3BCO, are shown in Figure 6. Vibrational band
positions and intensities are listed in Table 7. They are compared
to vibrational data obtained from DFT calculations.
(70) Willner, H.; Bodenbinder, M.; Bro¨chler, R.; Hwang, G.; Rettig, S. J.; Trotter,
J.; von Ahsen, B.; Westphal, U.; Jonas, V.; Thiel, W.; Aubke, F. J. Am.
Chem. Soc. 2001, 123, 588-602.
(71) Bodenbinder, M.; Balzer-Jo¨llenbeck, G.; Willner, H.; Batchelor, R. J.;
Einstein, F. W. B.; Wang, C.; Aubke, F. Inorg. Chem. 1996, 35, 82.
(72) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.;
Taylor, R. C. J. Chem. Soc., Perkin Trans. 2 1987, S1.
(73) Almenningen, A.; Gundersen, G.; Mangerud, M.; Seip, R. Acta Chem.
Scand., Ser. A 1981, 35, 341.
(74) Hausser, W.; Oberhammer, H.; Bu¨rger, H.; Pawelke, G. J. Chem. Soc.,
Dalton Trans. 1987, 1839.
9
J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002 15395

A R T I C L E S
Finze et al.
(CF3)3BCO possesses C3 symmetry and the irreducible
representations of fundamental vibrations and their activities
are in accordance to eq 11
electropositive nature of boron within the B(R3)3 group, and
(iv) the lack of π(B f CO) back-bonding.
In (CF3)3BCO the electron affinity of the Lewis acidic
fragment, B(CF3)3, is expected to be high. The B-CO bond
length of 1.69(2) Å indicates the proposed CO bond polarization
and the charge of the B atom is increased, due to the high
electronegativity of the CF3 groups. Furthermore, there should
be very little π-back-bonding in the case of CF3 ligands. These
combined contributions explain the extremely high ν(CO) value
of (CF3)3BCO.
Γvib ) 13A (IR, Ra p) + 13 E (IR, Ra dp)
(11)
for a total of 26 IR and Raman active vibrational fundamentals.
Of these possible 26 fundamentals of (CF3)3BCO, the wave-
numbers of 21 fundamentals are obtained experimentally. The
“missing” CF3 torsional modes ν13 and ν26 are expected at ∼38
and 41 cm-1 and judged to be outside the range of our
spectrometer. The other missing bands are, according to
calculations, too weak for detection. Also in the IR gas-phase
spectrum, the bands due to several fundamentals overlap which
each other. To achieve resolution, IR matrix data are included
in Table 7. Because the most intensive band at 1266 cm-1 (ν14)
is not completely resolved, even in Ar matrix, the single strong
band at 919 cm-1 (ν17) is chosen as reference for relative
intensity determinations.
As can be seen in Table 7, the agreement between experi-
mental and calculated band positions and IR band intensities is
reasonable. This allows an assignment of all 26 fundamentals.
In addition, the assignments are supported by experimental and
calculated isotopic shifts listed in Table S6. Isotopic shifts are
obtained from IR measurements on (CF3)3B13CO and (CF3)3-
BC18O isotopically enriched molecules, isolated in Ne, Ar, and
N2 matrixes.
The six C-F stretching vibrations of the symmetry type 3A
+ 3E are observed in the expected range of 1074-1271 cm-1
.
Of these, the in phase νs(CF3) stretch, ν2, exhibits the highest
wavenumber and shows an 10/11B isotopic shift of 2.5 cm-1
,
due to some vibrational mixing with νs(BC3) ) ν5. The out of
phase νs CF3-stretch, ν14, appears to be slightly lower in energy.
The large 10/11B isotopic shift of 14.5 cm-1 is due to strong
vibrational mixing with νas(BC3) ) ν17.
All remaining CF3-vibrations are fairly isolated. They are in
order of decreasing wavenumbers followed by the two ν(BC3)
vibrations ν17 and ν5 at 931 and 880 cm-1 respectively. The
δ(CF3) deformation modes are observed in the region of 730-
510 cm-1 and are assigned according to the calculated symmetry
type and the atom displacement vectors. The next two vibrations
at lower wavenumbers of 454 and 343 cm-1 are sensitive toward
isotopic substitution of the CO group and are best described as
δ(BCO) and ν(B-CO), respectively. The latter vibration is
coupled weakly with ν(CO). All low frequency skeletal vibra-
tions are extensively mixed and a description of these modes is
somewhat arbitrary.
UV Spectrum. The UV spectrum of (CF3)3BCO features
according to Figure S1 in the Supporting Information a single
high energy electronic transition with a maximum <190 nm
(outside the range of our instrument) and an onset at about 220
nm. This is consistent with the noted lack of color for (CF3)3-
BCO.
The three different matrix materials allow us to distinguish
between matrix and isotopic splitting. The average isotopic shifts
of the three matrix spectra are listed in Table S6. The
assignments of all bands are unambiguous, except for that of
ν5. This fundamental is expected to appear near 900 cm-1 with
a
10B satellite at 935 cm-1. However, three bands of similar
intensity are observed in this region. This is only possible if
one or two combination modes are in anharmonic resonance
with ν5. The fundamental should be the most intensive band
(found at 856 cm-1, Ar-matrix). Because the change of this band
pattern is stronger on C18O substitution as on 13CO substitution,
we assume the resonance partners to be (ν7 + ν9) and (ν8 +
ν9).
NMR Spectra. Tris(trifluoromethyl)borane carbonyl, (CF3)3-
BCO, its reaction products with water, the anions [(CF3)3-
BC(O)OH]- and [(CF3)3BCO2]2-, and the adduct (CF3)3-
BNCCD3, are subject of detailed 11B- and 19F-NMR spectro-
scopic investigations. (CF3)3BCO, [(CF3)3BC(O)OH]-, and
(CF3)3BNCCD3 are also investigated by 13C-NMR. Furthermore
(CF3)3BC(OH)2 is observed as an impurity in the mixture of
the reaction of (CF3)3BCO with CD3CN. A spectroscopic
The approximate description of modes, presented in Table
7, is based on the calculated displacement vectors of the atoms
in the respective modes and the observed isotopic shifts. Very
characteristic is the ν(CO) stretching vibration at 2252 cm-1
(gas phase). This mode is found in the solid state with 2269
cm-1 at an even higher wavenumber and correlates well with
the longer B-CO bond in solid (CF3)3BCO as discussed.
Hence, (CF3)3BCO has among borane carbonyls (see Table
3) the highest CO stretching wavenumber reported so far.
1
characterization by H-, 11B-, and 19F-NMR will be given of
all compounds mentioned above. All relevant data are listed in
Table 9. The 19F-NMR spectra of the borane carbonyl,
[(CF3)3BC(O)OH]-, and [(CF3)3BCO2]2- are shown in Figure
7 and the 11B-NMR spectra are displayed in Figure 8.
Furthermore the 13C-NMR spectra of (CF3)3BCO and [(CF3)3-
BC(O)OH]- are shown in Figures 9 and 10, respectively.
In the 11B- and 19F-NMR spectra, the compounds show signals
in the typical range of tetrahedrally coordinated boron species,
with three CF3 groups attached to boron.11,77 In the 19F-NMR
spectra the signal of each compound is split into a quartet, due
to coupling with the 11B-nucleus. Furthermore for [(CF3)3-
BC(O)OH]-, [(CF3)3BCO2]2-, and (CF3)3BC(OH)2 10B satellites
Isotopic shifts for 12/13CO (52.3 cm-1) and C16/18O (50.5 cm-1
)
differ slightly from calculated values, based on the two mass
model (50.1, 54.3 cm-1 respectively). This could be an
indication of some slight vibrational mixing.
A characteristic feature of all borane carbonyls, is the
observation of the CO stretch at higher wavenumbers than for
gaseous CO (2143 cm-1). This is mainly due to a bond
polarization of the CO ligand attached to boron.3,4 The positive
charge on the carbon atom of CO in the borane carbonyl is the
result of (i) the high electron affinity or Lewis acidity of the
R3B fragment, (ii) the length of the (B-CO) bond, (iii) the
(75) Brauer, D. J.; Bu¨rger, H.; Chebude, Y.; Pawelke, G. Eur. J. Inorg. Chem.
1999, 2, 247.
(76) Brauer, D. J.; Pawelke, G. J. Organomet. Chem. 2000, 604, 43.
(77) Pawelke, G.; Bu¨rger, H. Coord. Chem. ReV. 2001, 215, 243.
9
15396 J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002

(CF ) BCO−Synthesis
A R T I C L E S
3 3
Table 9. NMR-Data of (CF3)3BCO and Related Speciesa
δ(11B)
δ(13C) CF
δ(13C)
δ(19F)
1J(11B,13C) 1J(11B,13C) 1J(13C,19F) 2J(11B,19F) 3J(13C,19F) 3J(13C,19F) 4J(19F,19F)
3
compd
[ppm]
[ppm]
[ppm]
[ppm]
CF [Hz]
3
[Hz]
[Hz]
[Hz]
CF [Hz]
3
[Hz]
[Hz]
ref
c
c
(CF3)3BCO, CD2Cl2
(CF3)3BCO, SO2
[(CF3)3B(OH)]-
-17.9
126.2
126.7
134.7
n.o.
159.8 -58.7 80 ( 5
30 ( 5
298 ( 3
36 ( 2
32 ( 2
26.8
27.2
25.8
25.8
25.8
28.0
25.9
n.o.b
n.o.
2.7
n.o.
n.o.
4.0
4.0
n.o.
3.9
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
6.3
n.o.
5.8
n.o.
n.o.
-17,8
158.9 -58.3 75 ( 10 30 ( 10 298 ( 3
-10.6
-18.7
-19.1
-18.9
-19.0
-10.8
-18.9
-22.3
-15.1
-68.0 75.8
-60.3 n.o.
-60.3 n.o.
311.4
n.o.
n.o.
305.5
305.1
305.0
304.3
304.3
∼296
75
d
c
(CF3)3BC(OH)2
n.o.
n.o.
n.o.
n.o.
68.2
67.0
73.7
n.o.
n.o.
3.5
3.4
n.o.
c
c
c
[(CF3)3BCO2]2-
n.o.
[(CF3)3BC(O)OH]-
[(CF3)3BC(O)OD]-, D2O
(CF3)2BNMe3(C(O)OH)
[B(CF3)4]-
133.9
135.0
132.2
132.9
132.5
129.4
186.4 -60.4 72.8
192.5 -60.6 72.8
184.4 -60.3 77.1
-61.6 73.4
127.5 -62.1 75.9
-65.9 n.o.
76
11
[(CF3)3BCN]-
(CF3)3BNCCD3
64.0
29.0
∼24
n.o.
n.o.
n.o.
11
e
29 c
a NMR-solvent if not specified otherwise: CD3CN. b n.o. ) not observed. c This work. d 1H NMR: δ(OH) ) 11.06 ppm. e 1H NMR: δ(CD2H) ) 2.79
2
1
ppm, J(1H,2D) ) 2.06 Hz, 13C NMR: δ(CD3) ) 5.0 ppm, J(2D,13C) ) 21.6 Hz, δ(NC) ) 122.1 ppm.
Figure 9. 13C NMR spectrum (bottom) and 13C{19F} NMR spectrum (top)
of (CF3)3BCO.
Figure 7. 19F NMR spectra of (CF3)3BCO (left), [(CF3)3BC(O)OH]- (right)
and [(CF3)3BCO2]2- (top).
Figure 10. 13C NMR spectrum of [(CF3)3BC(O)OH]-. The expanded
3
sections of the two signals show the J(13C,19F) coupling patterns.
and (CF3)3BC(OH)2 are shifted to lower frequencies, compared
to the signals of (CF3)3BCO. The 11B- and 19F-NMR shifts of
the substances, obtained from the reaction with water, are very
similar and it is not possible, to assign the observed signals to
a specific species, by comparison of the chemical shifts alone.
Figure 8. 11B NMR spectra of (CF3)3BCO (bottom), [(CF3)3BC(O)OH]-
In Figures 9 and 10, the 13C-NMR spectra of (CF3)3BCO and
[(CF3)3BC(O)OH]- are shown, respectively. In both 13C-NMR
(middle) and [(CF3)3BCO2]2- (top).
1
spectra, the signal and the J(13C,19F) coupling of the C atoms
are observed as septets, with intensities according to the relative
abundances of the nuclei.
The 11B-NMR signals are split into decets because of the
coupling with nine equivalent 19F-nuclei. In both 11B- and 19F-
NMR spectra, the signals of [(CF3)3BC(O)OH]-, [(CF3)3BCO2]2-,
of the CF3 groups is in the typical range of CF3 groups attached
to boron (Table 9). The CO carbon atom of (CF3)3BCO shows
a signal at 159 ppm, which is in the range of transition metal
carbonyl cations1 and other related species (Table 8).
9
J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002 15397

A R T I C L E S
Finze et al.
The 19F-NMR spectrum of [(CF3)3BCO2]2- shows the beginning
of a distortion. In the 11B-NMR spectra of the compounds, a
similar distortion effect is not observed and the usual intensity
distribution as well as the true 2J(19F,11B) value is found. Hence,
it can be excluded that exchange of the CO ligand is responsible
for the line broadening and distortion of the signal pattern.
Table 10. Relaxation Times of (CF3)3BCO and Related Species
compd
T a [ms] σ1b [Hz] T c [ms]
σd [Hz]
σ1/σ
∆(σ)e [Hz]
1
2
(CF3)3BNCCD3
(CF3)3BCO
6 53
10 32
52 6
157 2
181 1.8
5-6
11-5
40
80-106 3-4
144
50-70 1-0.8
30-60 1-0.5
0-17
0-28
2
[(CF3)3BCO2]2-
(CF3)3BC(OH)2
[(CF3)3BCN]-
[(CF3)3BC(O)OH]-
[B(CF3)4]-
8
0.8
0.7-0.5 1-2
0.8
2.2
0.4
0.22
0.09
746 0.43 490
39358 0.008 3183
0.65
0.10
0.7
0.08
Summary and Conclusion
Tris(trifluoromethyl)borane carbonyl, (CF3)3BCO, which is
formed in a unique, unprecedented reaction, the partial hydroly-
sis of a single CF3 group of the anion [B(CF3)4]-11 in
concentrated H2SO4,10 has been extensively characterized. Its
thermal stability has been explored and a decomposition pathway
is established. A first glance of its potential as chemical reagent
has been obtained. Its molecular structure has been determined
by a combination of electron diffraction and microwave
spectroscopy in the gas phase and by single-crystal X-ray
diffraction. Its rotational and vibrational spectra are reported.
a Iτ ) I0 + p‚exp(-τ‚T1). b σ1 ) (π‚T1)-1
line width. e σ - σ1.
.
c T2 ) (π‚σ)-1
.
d Measured
The carbon atom of the acid function in [(CF3)3BC(O)OH]-
has a chemical shift of 186 ppm which is typical for the carbon
atoms of carboxylic acids.66 For both species, the signals of
the CF3 groups as well as the signals of the CO ligand or of the
C(O)OH ligand are split into quartets, due to coupling with 11B.
In the case of [(CF3)3BC(O)OH]-, the 10B satellites are shown
3
and furthermore the J(13C,19F) couplings are resolved. In the
Vibrational assignments are made with the help of 13C and 18
O
13C-NMR spectra of (CF3)3BNCCD3, the signals of the carbon
atoms of the coordinated d3-acetonitrile are shifted to higher
frequencies, relative to free d3-acetonitrile. The signal due to
the CF3 groups is in the same range, as the signals of the
compounds described above.
isotopomers and supported by density functional theory (DFT)
calculations. The characterization of (CF3)3BCO is completed
by 11B, 13C and 19F NMR spectroscopy.
Like the synthetic route, the spectroscopic and structural
properties are unique among borane carbonyls. The structural
features, d(B-C) and d(C-O), and the spectroscopic features
ν(CO), fCO and δ(13C) are compared to those of related main
group species and the superelectrophilic σ-metal carbonyl
1
In the H-NMR spectrum of (CF3)3BC(OH)2, the signal of
the hydroxy protons are observed with a chemical shift of δ )
11.1 ppm, which is typical for acidic protons.78 In the case of
[(CF3)3BC(O)OH]-, it is not possible to measure the signal of
the acidic proton, probably due to a fast exchange process.
(CF3)3BCO and (CF3)3BNCCD3 show very broad signals
(Table 10). This behavior is a typical feature of 11B-NMR due
to the quadrupolar moment of the 11B-nucleus.79,80 In Table 10,
the T1 values of 11B of some compounds described herein are
listed. The values are compared to the T2 values, calculated from
the measured line width. For substances with a line width greater
than 0.5-1.0 Hz in the 11B-NMR spectra, the relaxation time
is determined by the quadrupolar relaxation. The 19F-NMR
signals of the carbonyl and the acetonitrile adduct are broadened
as well, because of the coupling with the 11B-nucleus.
cations: linear [Hg(CO)2]2+ 71
,
square planar [Pt(CO)4]2+ 70
[Os(CO)6]2+ 2 and [Ir(CO)6]3+ 68,69
,
octahedral [Fe(CO)6]2+ 67
,
.
This comparison reveals a remarkable similarity because in all
instances the bond between CO and E, E ) B, Hg2+, Pt2+ and
Ir3+, is a pure σ-bond and the C atom(s) of the CO ligand(s) is
(are) the electropositive center(s).
Acknowledgment. Financial support by the Deutsche Fors-
chungsgemeinschaft, DFG, and the Fonds der Chemischen
Industrie is acknowledged. Furthermore, we are grateful to
Merck KGaA, Darmstadt, Germany, for providing financial
support and chemicals used in these studies. We thank Professor
Frohn for a sample of B(C6F5)3, Mr. M. Za¨hres for performing
some NMR measurements, and Dr. Ljudmilla V. Kristenko,
University of Moscow, for calculating vibrational amplitudes
and corrections with the program of Sipachev. Professor
Minkwitz, University of Dortmund, is gratefully acknowledged
for allowing the use of the CCD diffractometer.
In the 13C- and 19F-NMR spectra of (CF3)3BCO (Figures 7
and 9) and (CF3)3BNCCD3 the NMR signals are distorted and
the usual quartet with four signals of equal intensity is not found.
It is well-known that the spectrum of a nucleus A coupled to
another nucleus B with a spin >1/2 differs from the expected
multiplet structure if the inverse spin-lattice relaxation rate σ1
of nucleus B is comparable to the coupling constant J(A,B).80-82
By comparison of the σ1 values (Table 10) with the coupling
constants 1J(13C,11B) and 2J(19F,11B) (Table 9) it can be
understood why the distortion is observed in the cases of (CF3)3-
BCO and (CF3)3BNCCD3 and not in the case of [(CF3)3-
BC(O)OH]-. Especially the comparison of the line shapes of
the two 13C-NMR signals of (CF3)3BCO rationalizes this
observation, since the signal of the CO ligand with the smaller
1J(13C,11B) is much more distorted, than the signal of the CF3
Supporting Information Available: A complete table of
crystallographic data, a table of atomic coordinates in the crystal
structure, a table of selected bond length and angles in the crystal
structure, a table of the anisotropic displacement factors, a table
of interatomic distances, experimental (GED) and calculated
vibrational amplitudes and vibrational corrections, a table of
the observed and calculated isotopic shifts of the fundamental
vibrations of (CF3)3BCO, the UV spectrum, a view of the
molecule in the crystal structure and a view of the unit cell.
This material is free of charge via the Internet at http://
pubs.acs.org. Details of the crystal structure determination can
be obtained from the Fachinformationzentrum Karlsruhe, D-76344
Eggenstein-Leopoldshafen, Germany, (Fax: 49-7247-808-
666; E-mail: crysdata@fiz.karlsruhe.de) on quoting the deposi-
tory number CSD-412578.
groups which shows the larger coupling constant J(13C,11B).
1
(78) Hesse, M.; Meier, H.; Zeeh, B. Spektroskopische Methoden in der
organischen Chemie, 4 ed.; Georg Thieme Verlag: Stuttgart, 1991.
(79) Hubbard, P. S. J. Chem. Phys. 1970, 53, 985.
(80) Halstead, T. K.; Osment, P. A.; Sanctuary, B. C.; Tagenfeldt, J.; Lowe, I.
J. J. Magn. Reson. 1986, 67, 267.
(81) Abragam, A. Principles of Nuclear Magnetism; Clarendon Press: Oxford,
1986.
(82) Bendel, P. J. Magn. Reson. 1995, Ser. A 117, 143.
JA0209924
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15398 J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002