Carboxylic acids as Lewis bases: Structures and properties of strongly acidic carboxylic acid adducts of B(C6F5)3

Article abstract of DOI:10.1021/om060601k

Carboxylic acids react with B(C₆F₅)₃ in CH₂Cl₂ to form 1:1 adducts, which have been characterized by room-temperature IR and low-temperature (-30 °C) ¹H, ¹⁹F, ¹³C, and ¹¹B NMR spectroscopy. Coordination appears to occur via the carbonyl oxygen atom, and exchange between an adduct and its components is generally sufficiently slow that ¹H, ¹⁹F, ¹³C, and ¹¹B resonances of the adduct are readily distinguished from the ¹H and ¹³C resonances of the corresponding free acid and the ¹⁹F and ¹¹B resonances of free B(C₆F₅)₃. The electron-withdrawing power of the highly electrophilic B(C₆F₅)₃ increases the Bronsted acidity of the coordinated carboxylic acids sufficiently that they are able to protonate isobutene and thus initiate its carbocationic polymerization. The enhanced acidity also results in slow, partial cleavage of B-C₆F₅ bonds, and with benzoic acid the compound [(μ-C₆H₅CO₂)(μ-OH){B(C ₆F₅)₂}₂] has been isolated and characterized crystallographically. Because of slow B-C₆F₅ cleavage, crystallographically useful crystals of a 1:1 adduct of a carboxylic acid have not been obtained, but C₂H₅CO ₂Me·B(C₆F₅)₃, the 1:1 adduct of methyl propionate, has for purposes of comparison been prepared and characterized spectroscopically and crystallographically. In this case coordination occurs unequivocally via the carbonyl oxygen atom.

Full text of DOI:10.1021/om060601k

4888
Organometallics 2006, 25, 4888-4896
Carboxylic Acids as Lewis Bases: Structures and Properties of
Strongly Acidic Carboxylic Acid Adducts of B(C6F5)3
Simona Mitu and Michael C. Baird*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada
ReceiVed July 4, 2006
Carboxylic acids react with B(C6F5)3 in CH2Cl2 to form 1:1 adducts, which have been characterized
1
by room-temperature IR and low-temperature (-30 °C) H, 19F, 13C, and 11B NMR spectroscopy.
Coordination appears to occur via the carbonyl oxygen atom, and exchange between an adduct and its
components is generally sufficiently slow that 1H, 19F, 13C, and 11B resonances of the adduct are readily
distinguished from the 1H and 13C resonances of the corresponding free acid and the 19F and 11B resonances
of free B(C6F5)3. The electron-withdrawing power of the highly electrophilic B(C6F5)3 increases the
Brønsted acidity of the coordinated carboxylic acids sufficiently that they are able to protonate isobutene
and thus initiate its carbocationic polymerization. The enhanced acidity also results in slow, partial cleavage
of B-C6F5 bonds, and with benzoic acid the compound [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2] has been isolated
and characterized crystallographically. Because of slow B-C6F5 cleavage, crystallographically useful
crystals of a 1:1 adduct of a carboxylic acid have not been obtained, but C2H5CO2Me‚B(C6F5)3, the 1:1
adduct of methyl propionate, has for purposes of comparison been prepared and characterized
spectroscopically and crystallographically. In this case coordination occurs unequivocally via the carbonyl
oxygen atom.
A very important commercial application of isobutene (IB)
polymerization is the manufacture of IB homopolymers and IB-
isoprene copolymers via a carbocationic process employing an
AlCl3/water mixture as a protic carbocationic initiator system
in methyl chloride.1 Although the mechanisms of the propaga-
tion and chain transfer steps of IB polymerization are complex,1
it has been found that cryogenic temperatures are normally
necessary if the effects of chain transfer processes are to be
minimized.1 On the other hand, rates of chain transfer are
strongly affected by the nature of the counteranion, and poly-
isobutene (PIB) of unusually high molecular weight can be
obtained at relatively high temperatures by utilizing weakly
coordinating anions such as [B(C6F5)4]-.2 Since commercial
manufacture of high molecular weight PIB is of necessity carried
out at cryogenic temperatures (-100 °C),1 high-temperature
polymerization processes mediated by weakly coordinating
counteranions would be very desirable and, indeed, are of scien-
tific interest in their own right for the insight that they may
give to the chemical properties of weakly coordinating anions.3
The Brønsted acidity of the 1:1 adduct has been studied, and
its pKa has been estimated to be 8.6 in acetonitrile, comparable
to that of HCl in this solvent.4 In accord with this finding, the
adduct has been shown to be capable of inducing initiation of
IB polymerization.5
Seeking other potential acidic initiators of IB polymerization,
we have recently presented evidence that this same borane,
B(C6F5)3, reacts with a variety of carboxylic acids RCO2H (R
) alkyl, aryl) to form 1:1 and/or 2:1 adducts of the types
RCO2H‚B(C6F5)3 and RCO2H‚2B(C6F5)3 (R ) alkyl) as in
Scheme 1.6
Of great interest, adducts of B(C6F5)3 with the series of
carboxylic acids CH3(CH2)nCO2H (n ) 6, 8, 10, 12, 14, 16,
18, 20) have been shown to behave in CH3Cl/CH2Cl2 mixtures
as extremely good carbocationic initiators of isobutene polym-
erization at -50 °C, inducing the formation of polyisobutene
in high conversions and with ultrahigh molecular weights (Mw
∼2 × 106, Mw/Mn ∼1.5), although adducts with n ) 2, 4 were
not as effective.6f Thus this series of adducts behave as strong
acids, with very weakly coordinating counteranions.
In addition to the AlCl3-water adduct mentioned above, other
Brønsted-Lewis acid combinations are also effective initiators
for carbocationic polymerization, an example being the adduct
formed on reacting water with the very electrophilic borane
B(C6F5)3 (eqs 1, 2).
* Corresponding author. Fax: (613) 533-6669. E-mail: bairdmc@
chem.queensu.ca.
(1) (a) Kennedy, P. J.; Mare´chal, E. Carbocationic Polymerization; John
Wiley and Sons: New York, 1982. (b) Kennedy, P. J.; Iva´n, B. Designed
Polymers by Carbocationic Macromolecular Engineering: Theory and
Practice; Hanser Publishers: Munich, 1991. (c) Kennedy, P. J. Cationic
Polymerization of Olefins: A Critical InVentory; Wiley & Sons: New York,
1975.
(2) (a) Shaffer, T. D.; Ashbaugh, J. R. J. Polym. Sci. A 1997, 35, 329.
(b) Pi, Z.; Jacob, S.; Kennedy, J. P. In Ionic Polymerizations and Related
Processes; Puskas, J. R., Ed.; Kluwer: Dordrecht, The Netherlands, 1999;
p 1.
(4) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.;
Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581.
(5) Shaffer, T. D. ACS Symp. Ser. 1997, 665, 96.
(6) (a) Tse, C. K. W.; Kumar, K. R.; Drewitt, M. J.; Baird, M. C.
Macromol. Chem. Phys. 2004, 205, 1439. (b) McInenly, P. J.; Drewitt, M.
J.; Baird, M. C. Macromol. Chem. Phys. 2004, 205, 1707. (c) Cordoneanu,
A.; Baird, M. C. Macromolecules 2004, 37, 6744. (d) Tse, C. K. W.; Penciu,
A.; McInenly, P. J.; Kumar, K. R.; Drewitt, M. J.; Baird, M. C. Eur. Polym.
J. 2004, 40, 2653. (e) Mitu, S.; Baird, M. C. Can. J. Chem. 2006, 84, 225.
(f) Mitu, S.; Baird, M. C., Eur. Polym. J. 2006, 42, in press.
(3) Strauss, S. H. Chem. ReV. 1993, 93, 927.
10.1021/om060601k CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/01/2006

Carboxylic Acids as Lewis Bases
Scheme 1
Organometallics, Vol. 25, No. 20, 2006 4889
While the structures and properties of the adducts have to
this point been inferred only from their solution chemistry,6a-d,f
the 1:1 and 2:1 adduct salts [Me4N][MeCO2{B(C6F5)3}] and
[Me4N][MeCO2{B(C6F5)3}2] have been prepared and character-
ized spectroscopically and crystallographically.6e The 1:1 adduct
contains a monodentate acetate ion coordinated to the B(C6F5)3
via the C-O rather than the CdO oxygen, as in (deprotonated)
A′, while the 2:1 adduct contains a bridging acetate ion bonded
to two molecules of B(C6F5)3, much as in B although the two
C-O bond lengths are marginally different.6e Thus the structures
of the anions in the crystalline solids are similar to the structures
proposed in Scheme 1 for the neutral adducts, and both 1:1 and
2:1 adducts appeared to be feasible solution species.
With a view to identifying and better characterizing the solu-
tion species which initiate carbocationic polymerization, we have
now carried out a spectroscopic investigation (IR, 1H, 19F, 13C,
and 11B NMR spectroscopy) of the species formed in solution
on combining B(C6F5)3 and several carboxylic acids in various
ratios and at various temperatures. In addition to the novelty of
simple carboxylic acids behaving as Lewis bases and being
rendered sufficiently acidic that they can protonate alkenes, this
work may be of more general interest because the strongly Lewis
acidic properties of B(C6F5)3 have been used in a wide variety
of synthetic organic transformations.7
Figure 1. IR spectra (CH2Cl2, 21 °C) of (a) propionic acid, (b)
B(C6F5)3, and (c) propionic acid and B(C6F5)3 in a 1:1 molar ratio.
decanoic acid were also studied by 1H, 11B, and 19F NMR spectros-
copy at 25 °C in CD2Cl2. Solutions were generally prepared as
above, using a range of B(C6F5)3:carboxylic acid ratios, and were
run as quickly as possible (within 10 min).
Low-Temperature NMR Experiments Using Propionic Acid.
Ten solutions, each containing 3 µL (2.98 mg, 0.0402 mmol) of
propionic acid in 0.2 mL of CD2Cl2, were made up in NMR tubes
and cooled to -30 °C. Aliquots of B(C6F5)3 (approximately 0.1,
0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2.0, 2.5, 2.75 equiv) were dis-
solved in 0.3 mL of CD2Cl2 in separate vials and cooled to -30 °C,
and each B(C6F5)3 solution was then injected into one of the NMR
Experimental Section
1
solutions. The samples were quickly mixed by shaking, and a H
NMR spectra were recorded on Bruker Avance 400, 500, and
600 NMR spectrometers; IR spectra, on a Perkin-Elmer Spectrum
One IR spectrometer. All syntheses and initiator manipulations were
carried out under dry nitrogen or argon atmospheres using standard
Schlenk line techniques or an MBraun Labmaster glovebox. Toluene
and methylene chloride were dried by passing through Innovative
Technology solvent purification columns, CD2Cl2 by refluxing over
CaH2. Nitrogen and argon were purified by passage through a heated
column of BASF catalyst followed by a column of dry, activated
3 Å or 4 Å molecular sieves (Linde). The various carboxylic acids
used, propionic (CH3CH2CO2H), n-butanoic (CH3(CH2)2CO2H),
n-decanoic (CH3(CH2)8CO2H), and n-octadecanoic (CH3(CH2)16-
CO2H) acids, were purchased from Aldrich and distilled or
recrystallized from hexanes before use. Propionic acid, 13C-enriched
propionic acid (CH3CH213CO2H), and n-butanoic acid were also
dried by passage through a column of activated alumina and were
stored over molecular sieves. The compound B(C6F5)3 was syn-
thesized as in the literature.8
Ambient-Temperature Experiments. The reaction between
10.3 mg of B(C6F5)3 (0.02 mmol) and 1.5 mg of propionic acid
(0.02 mmol) in 1.0 mL of CH2Cl2 was studied at room temperature
by IR spectroscopy. Solutions were prepared quickly and run within
1 min. The results are shown in Figure 1, where the spectrum of
the adduct formed (Figure 1c) is compared with those of free
propionic acid (Figure 1a) and B(C6F5)3 (Figure 1b). The reactions
between B(C6F5)3 and propionic, n-butanoic acid, and n-octa-
NMR spectrum of each was run at -30 °C (100 scans, recycle
delay time 10 s). Representative spectra in the range δ 1-14 are
shown in Figure 2, and expanded spectra in the range δ 1-3 in
Figure 3.
In complementary experiments, several solutions each containing
∼31 mg of B(C6F5)3 (∼0.06 mmol) in 0.4 mL of CD2Cl2 in an
NMR tube were cooled to -40 °C. Solutions containing various
amounts of propionic acid or propionic acid enriched in 13C at the
carboxylic carbon, also in 0.4 mL of CD2Cl2, were then added
dropwise to the still cooled B(C6F5)3 solutions in NMR tubes such
that B(C6F5)3:CH3CH2CO2H ratios in the range 0.5:1-2.0:1 were
achieved. 1H, 19F, 13C, and 11B NMR spectra of the solutions were
obtained at -30 °C; the 1H NMR spectra were as obtained
previously. The 19F, 13C, and 11B spectra are shown in Figures 4,
5, and 6, respectively.
Low-Temperature NMR Experiments Using n-Decanoic
Acid. Solutions containing B(C6F5)3 and n-decanoic acid in 0.5:1,
1.5:1, and 2:1 ratios were made up in CD2Cl2 at -30 °C as above,
1
1
and H and 19F NMR spectra were run at -30 °C. The H NMR
spectra are shown in Figure 7.
Attempted Syntheses of Compounds of the Type RCO2B-
(C6F5)2; Crystal Structure of [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2].
A solution of 59 mg of recrystallized benzoic acid (0.49 mmol) in
2 mL of CH2Cl2 was treated dropwise with a solution of 205 mg
of B(C6F5)3 (0.49 mmol) in 5 mL of CH2Cl2. The reaction mixture
was stirred for 30 min, and then 20 mL of hexanes was added to
induce precipitation of the product. The supernatant was then
removed, the solid residue was dried in vacuo, and a small number
of crystals of [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2] were obtained by
(7) Erker, G. Dalton Trans. 2005, 1883.
(8) (a) Pohlmann, J. L.; Brinckmann, F. E. Z. Naturforsch. 1965, 20b,
5. (b) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.

4890 Organometallics, Vol. 25, No. 20, 2006
Mitu and Baird
fiber with grease and cooled to -93 °C in a stream of nitrogen gas
controlled with Cryostream Controller 700. Data collection was
performed on a Bruker SMART CCD 1000 X-ray diffractometer
with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å),
operating at 50 kV and 30 mA over 2θ ranges of 3.56-56.36° at
-93 °C controlled with Crysostream Controller 700. No significant
decay was observed during the data collection. Data were processed
on a Pentium PC using the Bruker AXS Windows NT software
package (version 5.10).9a Neutral atom scattering factors were taken
from Cromer and Waber.9b The raw intensity data were converted
to structure amplitudes and their esds using the program SAINT-
Plus. Empirical absorption corrections were applied using the
program SADABS. The structure was solved and refined by using
SHELXTL. The crystal is monoclinic, space group C2/c, based on
the systematic absences, E statistics, and successful refinement of
the structure. The structure was solved by direct methods. Full-
2
matrix least-squares refinements minimizing the function ∑w(Fo
- Fc2)2 were applied to the compound. All non-hydrogen atoms
were refined anisotropically. All the hydrogen atoms were located
gradually from the difference Fourier map, and their contributions
were included in the structure factor calculations.
Convergence to final R1 ) 0.0551 and wR2 ) 0.0651 for 2509
I > 2σ(I) independent reflections and R1 ) 0.1321 and wR2 )
0.0725 for all 5333 (R(int) ) 0.0804) independent reflections by
using 528 parameters were achieved, with the largest residual peak
and hole being 0. 228 and -0.264 e/Å3, respectively. Crystallo-
graphic data are shown in Table 1, the molecular structure is shown
in Figure 8, and important bond lengths and angles are given in
Table 2.
Figure 2. 1H NMR spectra (400 MHz, -30 °C, CD2Cl2) of
solutions of B(C6F5)3 and propionic acid in the ratios (a) 0:1, (b)
0.25:1, (c) 0.5:1, (d) 0.75:1, (e) 1:1, (f) 1.25:1, (g) 1.5:1, (h) 1.75:
1, (i) 2:1, (j) 2.5:1, and (k) 2.75:1.
Synthesis, Characterization, and Crystal Structure of C2H5-
CO2Me‚B(C6F5)3. A solution of 103 mg of methyl propionate (1.17
mmol) in 3 mL of CH2Cl2 was added dropwise to a solution of
400 mg of B(C6F5)3 (0.78 mmol) in 10 mL of CH2Cl2 at room
temperature. The mixture was stirred for 5 min, the solvent was
removed in vacuo, and the resulting yellow oil was washed twice
with 10 mL portions of hexanes to give a white, crystalline solid
(380 mg, 0.63 mmol, 81%), which was recrystallized from a mixture
of 2 mL of CH2Cl2 and 10 mL of hexanes. 1H NMR (CD2Cl2): δ
4.18 (s, 3H, OCH3), 2.37 (q, 2H, CH2CH3), 1.07 (t, 3H, CH2CH3).
19F NMR (CD2Cl2): δ -135.8 (ortho-F), -157.7 (para-F), -165.0
(meta-F). 11B NMR (CD2Cl2): δ 6.28 (sharp). 13C NMR (CD2-
Cl2): δ 188.0 (CdO), 58.8 (OCH3), 28.2 (CH2CH3), 8.3 (CH2CH3).
Anal. Calcd for C22H8O2BF15: C, 44.00; H, 1.34. Found: C, 44.44;
H, 1.54. IR (Fluorolube mull): ν(CdO) ) 1601 cm-1
.
A crystal of C2H5CO2Me‚B(C6F5)3 (colorless, prism-shaped, size
0.25 × 0.10 × 0.10 mm) was mounted on a glass fiber with grease
and cooled to -93 °C in a stream of nitrogen gas controlled with
Cryostream Controller 700. Data collection was performed on a
Bruker SMART CCD 1000 X-ray diffractometer with graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å), operating at
50 kV and 35 mA over 2θ ranges of 3.92-50.00°. No significant
decay was observed during the data collection. Data were processed
as above. The crystal is monoclinic, space group P21/c, based on
the systematic absences, E statistics, and successful refinement of
the structure. Convergence to final R1 ) 0.0328 and wR2 ) 0.0562
for 2594 (I > 2σ(I)) independent reflections and R1 ) 0.0559 and
wR2 ) 0.0596 for all 3874 (R(int) ) 0.0322) independent reflec-
tions, with 393 parameters, were achieved. The largest residual peak
and hole were 0.172 and -0.234 e/Å3, respectively. Crystallographic
data are given in Table 1, the molecular structure is shown in Fig-
ure 9, and important bond lengths and angles are given in Table 3.
Crystallographic data, atomic coordinates, and equivalent isotropic
1
Figure 3. Expanded H NMR spectra (600 MHz, -30 °C, CD2-
Cl2) of solutions of B(C6F5)3 and CH3CH2CO2H in the ratios
(a) 0.25:1 (expanded vertically), (b) 0.5:1, (c) 0.75:1, and (d) 1:1
(* ) impurities).
1
layering a CH2Cl2 solution with hexanes. H NMR (CD2Cl2): δ
8.93 (s, br, 1H, OH), 8.32 (d, J ) 8.1 Hz, 2H, ortho-H), 7.90 (t, J
) 7.6 Hz, 1H, meta-H), 7.66 (t, J ) 7.8 Hz, 2H, para-H). 19F NMR
(CD2Cl2): δ -138.1 (d, J ) 15.7 Hz, 8F, ortho-F), -154.2 (t, J )
20.3 Hz, 4F, para-F), -163.2 (t, J ) 18.8 Hz, 8F, meta-F). A
similar procedure was also carried out with CH3CH2CO2H,
ultimately giving only a small amount of [(µ-C6H5CO2)(µ-OH)-
{B(C6F5)2}2], which could not be obtained pure but which was
1
identified by NMR spectroscopy. H NMR (CD2Cl2): δ 8.19 (s,
br, 1H, OH), 2.94 (q, J ) 7.4 Hz, 2H, CH2), 1.35 (t, J ) 7.4 Hz,
3H, CH3). 19F NMR (CD2Cl2): δ -138.2 (d, J ) 14.5 Hz, 8F,
ortho-F), -154.4 (t, J ) 21.0 Hz, 4F, para-F), -163.4 (t, J )
17.1 Hz, 8F, meta-F).
A crystal of [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2] (colorless, block-
shaped, size 0.35 × 0.30 × 0.28 mm) was mounted on a glass
(9) (a) Bruker AXS Crystal Structure Analysis Package, Version 5.10,
SMART NT (Version 5.053), SAINT-Plus (Version 6.01), SHELXTL (Version
5.1); Bruker AXS Inc.: Madison, WI, 1999. (b) Cromer, D. T.; Waber, J.
T. International Tables for X-ray Crystallography; Kynoch Press: Birming-
ham, UK, 1974; Vol. 4, Table 2.2 A.

Carboxylic Acids as Lewis Bases
Organometallics, Vol. 25, No. 20, 2006 4891
Figure 4. 19F NMR spectra (CD2Cl2, -30 °C) of (a) B(C6F5)3, (b) a mixture of B(C6F5)3 and propionic acid in a 1:1 ratio, and (c) a mixture
of B(C6F5)3 and propionic acid in a 2:1 ratio.
various ratios of C2H5CO2Me‚B(C6F5)3 and propionic acid, as well
as of C2H5CO2H, B(C6F5)3, and PhCOMe.
Crystallographic data for [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2] and
C2H5CO2Me‚B(C6F5)3 have been deposited with the Cambridge
Crystallographic Data Centre. Copies of these data may be obtained
free of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK. The deposition numbers are CCDC
616968 and CCDC 616969, respectively.
Results and Discussion
For purposes of comparison, the relevant spectral data for
[Me4N][MeCO2{B(C6F5)3}] are as follows. 1H NMR (CD2Cl2,
21 °C): δ 1.94 (s, MeC). 19F NMR (CD2Cl2, 21 °C): δ -135.9
(ortho-F), -163.6 (para-F), -168.2 (meta-F). 11B NMR (CD2-
Cl2, 21 °C): δ -5.1 (sharp). IR (Fluorolube mull): ν(CO) 1682,
1491 cm-1 6e
The comparable data for [Me4N][MeCO2-
.
13
Figure 5. 13C NMR spectra (-30 °C, CD2Cl2) of (a) CH3CH2
-
{B(C6F5)3}2] are as follows. 1H NMR (CD2Cl2, 21 °C): δ 2.19
(s, MeC). 19F NMR (CD2Cl2, 21 °C): δ -135.6 (ortho-F),
-161.0 (para-F), -167.1 (meta-F). 11B NMR (CD2Cl2, 21
°C): δ -1.6 (broad). IR (Fluorolube mull): ν(CO) 1566, 1417
CO2H and of solutions of B(C6F5)3 and CH3CH213CO2H in the ratios
(b) 0.75:1, (c) 1:1, (d) 1.5:1, and (e) 2:1.
cm-1 6e
The methyl chemical shift of CH3CO2H is δ 2.07 in
.
CD2Cl2 at 21 °C, falling midway between the methyl resonances
of [MeCO2{B(C6F5)3}]- and [MeCO2{B(C6F5)3}2]-. Thus the
proton appears to have an electron-withdrawing ability ap-
proximately midway between that of a single B(C6F5)3 and those
of two B(C6F5)3.
1
Room-Temperature IR and H NMR Studies. To begin,
we obtained IR and NMR data at room temperature for CH2-
Cl2 solutions containing B(C6F5)3 and propionic acid, n-butanoic
acid, n-decanoic acid, or n-octadecanoic acid in various
B(C6F5)3:acid ratios. The IR spectrum of a solution of propionic
acid exhibits a significantly broadened OH stretching band
(∼2400 to ∼3400 cm-1) because of the hydrogen bonding.10a
This band disappears on the addition of an equimolar amount
of B(C6F5)3, and no new band attributable to ν(OH) appears, a
finding that can probably be interpreted in terms of extreme
broadening in the presumed adduct(s) because of strong hydro-
gen bonding.
We show in Figures 1a,b the IR spectra of 0.04 M CH2Cl2
solutions of propionic acid and B(C6F5)3, respectively, in the
range 1300-1800 cm-1. The peaks at 1749 and 1715 cm-1 in
the spectrum of propionic acid are the CdO stretching modes
of respectively the monomeric and hydrogen-bonded dimeric
Figure 6. 11B NMR spectra (-30 °C, CD2Cl2) for solutions of
B(C6F5)3 and CH3CH213CO2H in the approximate ratios (a) 2:1,
(b) 1.5:1, (c) 1:1, and (d) 0.75:1, (e) 1.0; (e) shows the resonance
of free B(C6F5)3.
displacement parameters, bond lengths and angles, anisotropic dis-
placement parameters, hydrogen coordinates and isotropic displace-
ment parameters, and torsion angles are given in the Supporting
Information.
Exchange Experiments Involving C2H5CO2H, B(C6F5)3 and
Either C2H5CO2Me or PhCOMe. A solution of 24.1 mg of C2H5-
CO2Me‚B(C6F5)3 (0.0402 mmol) in 0.5 mL of CD2Cl2 was reacted
(10) (a) Silverstein, R. M.; Webster, F. X. Spectrometric Identification
of Organic Compounds; Wiley & Sons: New York, 1998; pp 95, 96. (b)
Lascombe, J.; Haurie, M.; Josien, M.-L. J. Chim. Phys. 1962, 59, 1233. (c)
Collings, A. J.; Morgan, K. J. J. Chem. Soc. 1963, 3437.
1
with 2.98 mg of propionic acid (3 µL, 0.0402 mmol), and a H
NMR spectrum was obtained at room temperature. In the same
1
manner, H NMR spectra were obtained for solutions containing

4892 Organometallics, Vol. 25, No. 20, 2006
Mitu and Baird
Figure 7. 1H NMR spectra (-30 °C, CD2Cl2) of solutions of (a) n-decanoic acid, (b) containing B(C6F5)3 and n-decanoic acid in a 0.5:1
ratio, (c) containing B(C6F5)3 and n-decanoic acid in a 1.5:1 ratio, and (d) containing B(C6F5)3 and n-decanoic acid in a 2:1 ratio.
Table 1. Crystal Data and Structure Refinement of [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2] and C2H5CO2Me‚B(C6F5)3
[(µ-C6H5CO2)(µ-OH){B(C6F5)2}2]
C2H5CO2Me‚B(C6F5)3
empirical formula
fw
C31H6B2F20O3
827.98
C22H8BF15O2
600.09
temperature
wavelength
cryst syst
180(2) K
0.71073 Å
monoclinic
180(2) K
0.71073 Å
monoclinic
space group
unit cell dimens
C2/c
P2(1)/c
a ) 21.429(12) Å, R ) 90°
b ) 14.402(8) Å, â ) 118.519(8)°
c ) 22.384(18) Å, γ ) 90°
6070(7) Å3
a ) 10.1619(11) Å, R ) 90°
b ) 14.3044(14) Å, â ) 91.076(2)°
c ) 15.1871(15) Å, γ ) 90°
2207.2(4) Å3
volume
Z
8
4
density (calcd)
absorp coeff
1.812 Mg/m3
1.806 Mg/m3
0.198 mm-1
0.200 mm-1
F(000)
cryst size
θ range for data collection
index ranges
3248
1184
0.35 × 0.30 × 0.28 mm3
0.25 × 0.10 × 0.10 mm3
1.78 to 25.00°
-25 e h e 24, -17 e k e 17, -23 e l e 26
14 201
5333 [R(int) ) 0.1030]
99.8%
empirical (Bruker SADABS)
0.6374 and 0.3808
full-matrix least-squares on F2
5333/0/528
1.96 to 25.00°
-12 e h e 11, -17 e k e 16, -18 e l e 18
12 715
3874 [R(int) ) 0.0322]
99.9%
empirical (Bruker SADABS)
1.0000 and 0.8584
full-matrix least-squares on F2
3874/0/381
no. of reflns collected
no. of indep reflns
completeness to θ ) 28.18°
absorp corr
max. and min. transmn
refinement method
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole
0.922
1.000
R1 ) 0.0551, wR2 ) 0.0651
R1 ) 0.1321, wR2 ) 0.0725
0.228 and -0.264 e/Å-3
R1 ) 0.0329, wR2 ) 0.0562
R1 ) 0.0559, wR2 ) 0.0596
0.172 and -0.234 e/Å-3
forms of the acid and are typical for aliphatic carboxylic
acids.10b,c On the basis of data for 1:1 adducts of B(C6F5)3 with
the carbonyl Lewis bases benzaldehyde, acetophenone, ethyl
benzoate, and N,N-diisopropylbenzamide,11 one would anticipate
that the IR spectrum of a 1:1 adduct of the propionic acid
monomer, with a structure as in A, would exhibit ν(CdO) some
49-83 cm-1 lower than ν(CdO) of the free, monomeric acid,
i.e., in the range 1666-1700 cm-1. On the other hand, a 1:1
adduct with a structure as in A′ might be expected to exhibit a
ν(CdO) perturbed relatively little from that of the free acid.
The IR spectrum of B(C6F5)3 exhibits peaks at 1647, 1523, and
1481 cm-1; these are attributed to ring vibrational modes12a and
would reasonably be expected to shift relatively little on for-
mation of an adduct.
both 0.02 M; a very similar spectrum is observed when a second
molar equivalent of B(C6F5)3 is added. As can be seen, the two
ν(CdO) of the free propionic acid have disappeared completely,
consistent with essentially quantitative formation of one or more
adducts of the acid with B(C6F5)3. However there are no well-
defined new peaks, attributable to species such as A or A′, in
the region 1660-1800 cm-1; one observes only those at 1561,
1533, and 1520 cm-1, the latter two of which are probably
attributed to C6F5 vibrational modes.12b
Room-temperature 1H NMR spectra were obtained for
mixtures of propionic, n-butanoic acid, and n-octadecanoic acids
with B(C6F5)3 in CD2Cl2, and it was found in all cases that reso-
nances of RCH2CO2H and RCH2CH2CO2H groups shifted to
(12) (a) Frankiss, S. G.; Harrison, D. J. Spectrochim. Acta 1975, 31A,
1839. (b) While a peak is also observed at 1533 cm-1 in the spectrum of
the 1:1 mixture, this peak gains in intensity with time and, consistent with
the NMR data, may be assigned to pentafluorobenzene, a product of protic
cleavage of the B-C6F5 bond. The peaks at 1520, 1485, and 1473 cm-1
may all be assigned to vibrational modes of B(C6F5)3.
In Figure 1c we show a spectrum in the carbonyl region of
a solution containing propionic acid and B(C6F5)3 in a 1:1 ratio,
(11) Parks, D. J.; Piers, W. E.; Parvez, M.; Atencio, R.; Zawarotko, M.
J. Organometallics 1998, 17, 1369.

Carboxylic Acids as Lewis Bases
Organometallics, Vol. 25, No. 20, 2006 4893
Figure 9. Molecular structure of C2H5CO2Me‚B(C6F5)3.
Figure 8. Molecular structure of [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2].
Table 3. Important Bond Lengths and Bond Angles of
C2H5CO2Me‚B(C6F5)3
Table 2. Important Bond Lengths and Bond Angles of
Bond Lengths (Å)
[(µ-C6H5CO2)(µ-OH){B(C6F5)2}2]
C(19)-O(2)
C(19)-O(1)
O(2)-C(22)
C(19)-C(20)
C(20)-C(21)
B(1)-O(1)
B(1)-C(1)
1.292(2)
1.248(2)
1.472(2)
1.477(3)
1.509(3)
1.579(2)
1.628(3)
1.631(3)
1.627(3)
Bond Lengths (Å)
C(1)-C(7)
C(7)-O(1)
C(7)-O(2)
B(1)-O(1)
B(2)-O(2)
B(1)-O(3)
B(2)-O(3)
1.463(5)
1.272(4)
1.271(4)
1.509(4)
1.494(5)
1.522(5)
1.527(4)
B(1)-C(7)
B(1)-C(13)
Bond Angles (deg)
Bond Angles (deg)
C(1)-C(7)-O(1)
C(1)-C(7)-O(2)
O(1)-C(7)-O(2)
C(7)-O(1)-B(1)
C(7)-O(2)-B(2)
118.3(4)
118.1(3)
123.6(3)
126.1(3)
125.8(3)
C(19)-C(20)-C(21)
C(20)-C(19)-O(1)
C(20)-C(19)-O(2)
O(1)-C(19)-O(2)
C(19)-O(2)-C(22)
C(19)-O(1)-B(1)
O(1)-B(1)-C(1)
O(1)-B(1)-C(7)
O(1)-B(1)-C(13)
115.61(19)
126.45(18)
115.54(18)
118.00(17)
117.24(16)
132.86(15)
101.61(14)
109.99(15)
105.54(15)
lower field, consistent with coordination. However, within min-
utes these resonances decreased in intensity as others appeared
in the same region; a multiplet at δ 6.96, attributed to C6F5H,
also appeared and is indicative of protic cleavage of B-C bonds.
structure of the benzoic acid product, which showed that the
compound was [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2] (Tables 1, 2;
Figure 8). Thus the compound obtained was not the anticipated
C6H5CO2B(C6F5)2, but rather a secondary product resulting
presumably from hydrolysis by adventitious water. The bond
lengths and angles shown in Table 2 are generally unexceptional
where precedents exist,13 but the structure does serve to confirm
the presence among the products of a species containing a
B(C6F5)2 moiety. We note that, as might be anticipated, the two
B-OH bond lengths (1.516(6), 1.529(6) Å) are significantly
longer than are the two B-O bond lengths (1.496(2), 1.306(2)
Å) of the complex anion [(C6F5)3B(µ-O)B(C6F5)2]-.13d
Low-Temperature NMR studies. In an effort to circumvent
the B-C6F5 cleavage problem, we investigated reactions of
B(C6F5)3 with propionic and n-decanoic acid at lower temper-
atures, down to -50 °C. We found that temperatures of about
-30 °C were optimal, the rates of B-C6F5 cleavage being slow,
while reasonable solubilities and usable resonance line widths
were still generally attainable.
1
The H NMR spectrum of free propionic acid in CD2Cl2 at
-30 °C exhibits methyl, methylene, and OH resonances at δ
1.06 (t), 2.36 (q), and ∼12.7 (s), respectively (Figure 2a). As
can be seen in Figure 2, addition of increasing amounts of
B(C6F5)3 to a solution of propionic acid resulted in initial broad-
ening of the OH resonance and then narrowing as it shifted
from δ ∼12.7 to higher field. Ultimately the resonance leveled
off at δ ∼9.7 in the presence of 1.0-1.5 equiv, and little change
was observed on the further addition of B(C6F5)3. Similar results
were obtained in CDCl3, C6D6, and toluene-d8. Several of the
reactions were also monitored by 19F NMR spectroscopy, and
in all cases several sets of C6F5 resonances were observed to
form, indicating mixtures of products that included C6F5H. Thus
it seems in general that B(C6F5)3 reacts with carboxylic acids
to give adducts that exhibit resonances distinguishable from
those of the free acid, but that rapid protic cleavage of B-C6F5
bonds results in the formation of C6F5H and, presumably,
compounds of the type RCO2B(C6F5)2.
In an attempt to identify the latter, B(C6F5)3 was reacted with
propionic acid and benzoic acid (both 1:1 molar ratio) in CH2-
Cl2 at room temperature for 30 min. Addition of hexanes gave
in both cases small amounts of crude products with very similar
(13) (a) Doerrer, L. H.; Green, M. L. H. J. Chem. Soc., Dalton Trans.
1999, 4325. (b) Vagedes, D.; Fro¨hlich, R.; Erker, G. Angew. Chem., Int.
Ed. 1999, 38, 3362. (c) Barrado, G.; Doerrer, L.; Green, M. L. H.; Leech,
M. A. J. Chem. Soc., Dalton Trans. 1999, 1061. (d) Di Saverio, A.; Focante,
F.; Camurati, I.; Resconi, L.; Beringhelli, T.; D’Alfonso, G.; Donghi, D.;
Maggioni, D.; P.; Mercandelli, P.; Sironi, A. Inorg. Chem. 2005, 44, 5030.
(e) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli,
P.; Sironi, A. Organometallics 2004, 23, 5493.
1
19F NMR spectra and broad, low-field resonances in the H
NMR spectra (see Experimental Section). The latter were
ultimately assigned to µ-OH groups on the basis of a crystal

4894 Organometallics, Vol. 25, No. 20, 2006
Mitu and Baird
1
Propionic Acid. A series of H NMR experiments in CD2-
Note that the 11B chemical shift of the new species is similar to
that of [Me4N][MeCO2{B(C6F5)3}].6e
Cl2 and involving B(C6F5)3 and propionic acid in various
B(C6F5)3:CH3CH2CO2H molar ratios (0.25:1-2.75:1) were
carried out at -30 °C. Representative spectra in the range δ
1-14 are shown in Figure 2, and expanded spectra in the range
δ 1-3 in Figure 3. In view of the errors involved in measuring
the miniscule quantities of reagents involved, the ratios varied
somewhat from the nominal ratios stated in the Experimental
Section and in Figure 2; however, the trends in the ratios are
undoubtedly as stated.
n-Decanoic Acid. On treating a CD2Cl2 solution of n-
decanoic acid with B(C6F5)3 (nominal B(C6F5)3:n-decanoic acid
ratios ∼0.5:1-2:1) at -30 °C, the OH resonance of the free
acid at δ 12.56 first broadened then narrowed and shifted
ultimately to δ ∼9.5, as shown in Figure 7. Interestingly, at a
B(C6F5)3:n-decanoic acid ratio of ∼0.5:1, the CH3(CH2)7CH2-
CO2 resonance at δ 2.32 decreased in intensity and a new
resonance, of comparable intensity and attributable to the CH3-
(CH2)7CH2CO2 resonance of the 1:1 adduct, appeared at δ 2.61
(Figure 7b). Similarly the CH3(CH2)6CH2CH2CO2 resonance of
the free acid at δ 1.55 split into two resonances of comparable
intensity at δ 1.68 and 1.48. The resonance at δ 1.68 is pre-
sumably attributed to the CH3(CH2)6CH2CH2CO2 resonance of
the 1:1 adduct, and that at δ 1.48 possibly to free n-decanoic
acid hydrogen bonded to the adduct since the chemical shift is
not identical to that of the free acid. In Figures 7c,d, we show
the spectra of n-decanoic acid in the presence of 1.5 and 2 equiv
of B(C6F5)3, respectively. There is in both spectra clearly only
single sets of CH3(CH2)7CH2CO2 and CH3(CH2)6CH2CH2CO2
resonances, the chemical shifts of which correspond closely with
the chemical shifts of the pair of downfield resonances in Fig-
ure 7b and strengthen their assignments to the 1:1 adduct.
Changes in the aliphatic region were more subtle but, as can
be seen in Figure 3a, addition of ∼0.25 molar equivalent of
B(C6F5)3 to a solution of propionic acid resulted in the appear-
ance of very broad, weak methyl and methylene resonances at
δ ∼1.18 and ∼2.56, respectively. Both exhibited intensities
approximately one-third those of the corresponding methyl and
methylene resonances of the free acid although the breadth of
the lines made accurate measurements impossible. When the
B(C6F5)3:propionic acid ratio was increased to ∼0.5:1, the new
resonances shifted to δ 1.26 and 2.74 and gained in intensity
such that the ratio of their intensities to those of the free acid
resonances was now ∼0.5:1 (Figure 3b). (If, as is concluded
below, the new species is a 1:1 adduct, then these relative
integrations suggest that the nominal 2:1 ratio of reactants had
not been achieved exactly.)
These results are consistent with those discussed above for
propionic acid, although the chemical shift differences between
the CH3(CH2)nCH2CO2 and CH3(CH2)n-1CH2CH2CO2 reso-
nances of the free and coordinated acids are greater for
n-decanoic acid. 19F NMR spectra of the same B(C6F5)3/n-
decanoic acid solutions (not shown) also exhibited separate
resonances for the three pairs of ortho-F (δ -136.3), para-F
(δ -159.7), and meta-F (δ -166.0) resonances. These chemical
shifts are very similar to those reported above for the 1:1 pro-
pionic acid adduct.
Increasing the ratio to ∼1:1 resulted in disappearance of the
resonances of the free acid and observation of only the new
resonances (Figure 3d), while addition of excess B(C6F5)3
(up to a 2.75:1 ratio) resulted in no other changes in the spec-
trum. Interestingly, when the B(C6F5)3:propionic acid ratio was
∼0.75:1, the pairs of methylene and methyl resonances coa-
lesced (Figure 3c).
These experiments were complemented by 19F, 13C, and 11
B
NMR spectra of solutions (all in CD2Cl2 at -30 °C) containing
B(C6F5)3 and propionic acid in various ratios. On the addition
of approximately 1 equiv of propionic acid to a solution of
B(C6F5)3, the resonances of free B(C6F5)3 at δ -128.6 (ortho-
F), -144.3 (para-F), and -158.7 (meta-F) (Figure 4a) disap-
peared and a new set of somewhat broadened resonances at δ
-136.9 (ortho-F), -158.7 (para-F), and -165.4 (meta-F)
appeared (Figure 4b); these 19F chemical shifts are very similar
to those of the acetate adduct [Me4N][MeCO2{B(C6F5)3}].6e The
spectrum of a 2:1 reaction mixture (Figure 4c) exhibited both
this set of resonances and also those of free B(C6F5)3 at δ
-128.6 (ortho-F), -144.3 (para-F), and -158.7 (meta-F), the
three pairs of ortho-, meta-, and para-resonances being of
comparable intensities and somewhat broadened because of
exchange. Several sharp, weak resonances were also observed,
presumably a result of slow cleavage reactions of the type
discussed above.
The 13C NMR experiments were carried out using propionic
acid enriched in 13C at the carboxylic carbon. On the addition
of ∼0.75 equiv of B(C6F5)3 to a solution of propionic acid, the
resonance of free acid at δ 182 weakened, broadened, and shifted
somewhat while a new, broad resonance at δ ∼189 appeared
(Figure 5b). When the ratio of B(C6F5)3 to propionic acid was
1:1 and higher, only the (now sharp) resonance at δ 189
remained (Figures 5c-e). Similar 11B NMR experiments (Fig-
ure 6) showed that the broad resonance of free B(C6F5)3 at δ
62.0 was replaced completely by a new, somewhat narrower
resonance at δ 0.2 in the presence of g1 equiv of propionic
acid, but that the two resonances were present in comparable
intensities when the B(C6F5)3 to propionic acid ratio was 2:1.
Nature of the Adduct(s) in Solution. On the basis of the
IR spectroscopic data for propionic acid, it seems that a 1:1
adduct, C2H5CO2H‚B(C6F5)3, is formed as in Scheme 1 (A, R
) CH3CH2), although slow, proton-induced B-C6F5 cleavage
even at low temperatures impaired all attempts to grow crys-
tallographically useful crystals. The IR spectral data show clearly
that ν(OH) and ν(CdO) of free propionic acid disappear
completely on the addition of 1 molar equiv of B(C6F5)3 and
that addition of excess B(C6F5)3 results in little further change
in the IR spectrum. It was disturbing initially when we found
that the adduct did not exhibit ν(CdO) in the range 1666-
1700 cm-1 as anticipated on the basis of IR data for 1:1 adducts
of B(C6F5)3 with other carbonyl Lewis bases,11 but we success-
fully prepared and characterized spectroscopically and crystallo-
graphically the corresponding 1:1 adduct of methyl propionate,
C2H5CO2Me‚B(C6F5)3 (see below). The ester in this compound
coordinates to the borane via the carbonyl oxygen atom, as in
the corresponding ethyl benzoate adduct,11 but ν(CdO) shifts
from 1746 cm-1 in the free ester to 1601 cm-1 on coordination,
a ∆ν(CdO) of 145 cm-1, which contrasts markedly with the
∆ν(CdO) of 49 cm-1 reported previously for the 1:1 adduct of
ethyl benzoate.11 The reasons for this apparent discrepancy are
discussed below, but observation of an apparent ν(CdO) of
C2H5CO2H‚B(C6F5)3 at 1561 cm-1 now seems quite reasonable.
Turning now to the 1H, 19F, 11B, and 13C NMR spectroscopic
evidence for the carboxylic acid systems, the data support the
conclusion that 1:1 adducts are formed essentially quantitatively.
For both propionic and n-decanoic acids, addition of ∼0.5 equiv
of B(C6F5)3 to a CD2Cl2 solution of the acid at -30 °C resulted
in spectra that exhibited resonances of free and coordinated acid

Carboxylic Acids as Lewis Bases
Organometallics, Vol. 25, No. 20, 2006 4895
(1H, 13C), but no resonances of free borane (19F, 11B). On the
other hand, similar experiments using 1:1 ratios of B(C6F5)3
and carboxylic acid resulted in NMR spectra (1H, 19F, 11B)
exhibiting only resonances of the adduct, and experiments using
a 2:1 ratio of B(C6F5)3 and carboxylic acid resulted in NMR
spectra (1H, 19F, 11B) exhibiting resonances of free borane and
adduct, but not of free carboxylic acid. Although all spectra
where resonances of adduct and a reactant were observed were
integrated, the results were not always useful. As indicated
above, the fact that very small amounts of reactants were being
measured meant that the nominal ratios of reactants were not
always achieved. In addition resonances were in some cases
quite broad and difficult to integrate accurately.
In no case did addition of excess carboxylic acid or B(C6F5)3
result in perturbations, which may be interpreted in terms of
significant formation of other products, although evidence for
exchange processes was observed in some cases. For instance,
the 1H NMR spectrum of the solution containing B(C6F5)3 and
propionic acid in a 0.75:1 ratio exhibited coalescence of the
pairs of methyl and methylene resonances of the free and
coordinated acid, while several of the 13C and 19F NMR spectra
discussed above exhibited significant line broadening. There
presumably exists exchange between coordinated acid and both
the monomer and the dimer of the free acid, and the equilibria
could in principle also involve hydrogen-bonded dimeric adducts
such as C, although the formation of C perhaps seems unlikely
for steric reasons.
chemical shifts. On this basis, the trends in δ(OH) discussed
here would imply decreased levels of hydrogen bonding on
formation of the adducts, inconsistent with the strongly hydrogen-
bonded structure of C and providing indirect evidence for the
1:1 adducts being monomeric as in A. Indeed, the chemical
shifts (δ ∼9.5) of the 1:1 adducts are very similar to those of
free monomeric carboxylic acids (δ 5.5-8.5). We also note that
autoionization of the 1:1 adducts is possible and may also result
in exchange broadening of the OH resonance, but we have not
attempted conductivity experiments that might shed light on this
issue. The presence of traces of water would also cause com-
plications, but our solvent purification procedures ensure that
any adventitious water is a very minor species. Furthermore
the reproducibility of our results is inconsistent with the presence
of variable amounts of any unrecognized reagent that could
affect the results.
Synthesis, Characterization, and Crystal Structure of
C2H5CO2Me‚B(C6F5)3. Having established the existence and
the NMR and IR spectroscopic properties of 1:1 adducts of
carboxylic acids with B(C6F5)3, we felt it desirable to learn more
about their chemistry. For instance, we were surprised by our
finding that mixtures of adducts RCO2H‚B(C6F5)3 and either
free RCO2H or free B(C6F5)3 do not generally undergo exchange
that is rapid on the NMR time scale, as do the above-mentioned
1:1 adducts of B(C6F5)3 with the carbonyl Lewis bases benz-
aldehyde, acetophenone, and ethyl benzoate.11 While most of
our experiments with carboxylic acid adducts were carried out
at lower temperatures, where exchange would in any case be
1
slower, separate H and 19F resonances were also observed in
room-temperature spectra. It thus seemed worthwhile to make
a direct comparison with an ester complex, and we chose methyl
propanoate rather than the previously studied ethyl benzoate11
in order to minimize steric differences between acid (propionic)
and ester adducts.
The readily isolable 1:1 adduct C2H5CO2Me‚B(C6F5)3 was
prepared and characterized both spectroscopically and crystallo-
graphically (see above). The molecular structure is shown in
Figure 9, and the adduct clearly assumes the expected mode of
coordination via the carbonyl oxygen atom. Important bond
lengths and angles are shown in Table 3, and all are very similar
to those of the analogous ethyl benzoate adduct, although the
B-O bond of the ethyl benzoate adduct is slightly longer (1.594-
(6)11 vs 1.579(2) Å).
Broadening of 19F resonances may also arise from hindered
rotation about the B-C6F5 bonds in some species and perhaps
also from quadrupolar effects of the 10B and 11B. We find, for
instance, that the 19F resonances of B(C6F5)3 are considerably
sharper at -20 °C than at 20 °C, an observation that cannot be
attributed to hindered rotation. Thus the processes responsible
for line broadening in many of the NMR spectra are complicated
and were not studied further.
As with the 1:1 adducts of benzaldehyde, acetophenone, and
ethyl benzoate, for which the order of basicity toward B(C6F5)3
is benzaldehyde > acetophenone > ethyl benzoate,11 there is
also rapid exchange between free and B(C6F5)3-coordinated
methyl propionate. An approximately equimolar solution con-
taining free methyl propionate and the 1:1 adduct in CD2Cl2
exhibited a single, averaged methoxy resonance at δ 3.94, quite
distinct from the chemical shifts observed for the pure adduct
(δ 4.18) and the free ester (δ 3.62). The nonetheless large change
in the chemical shift of the methoxy resonance is consistent
with electron withdrawal from the ester, although the methylene
and methyl resonances shift much less, about 0.1 and 0 ppm,
respectively. As mentioned above, what is particularly interest-
ing is that ν(CdO) shifts from 1746 cm-1 in the free ester to
1601 cm-1 on coordination, a ∆ν(CdO) of 145 cm-1, which
contrasts markedly with the ∆ν(CdO) of 82, 83, and 49 cm-1
reported previously for the 1:1 adducts of benzaldehyde, aceto-
In addition to the above observations, the behavior of the
OH resonances suggests that the acidic protons are at all ratios
undergoing rapid exchange of some kind. As shown in Fig-
ures 2 and 7, addition of ∼0.5 equiv of B(C6F5)3 to a solution
of a carboxylic acid results in distinct broadening and shifting
of the OH resonances from δ ∼12.7 to higher field. As the ratio
of B(C6F5)3 to carboxylic acid increases, there is a smooth
shifting of the OH resonances until they level off as relatively
narrow resonances at δ ∼9.5, some 4 ppm from the chemical
shift of the free acids. The -CH2CO2H resonances shift much
less at B(C6F5)3:carboxylic acid ratios >1:1, from δ 2.36 to
δ 2.75 in the case of propionic acid and from δ 2.32 to δ 2.71
in the case of n-decanoic acid.
It is not clear just how to interpret these observations. It seems
likely that hydrogen-bonded carboxylic acid dimers (δ ∼12.7)
undergo proton exchange with the adducts, which may be
monomeric (A) or dimeric (C). However, it is well established
1
that the H NMR spectra of free monomeric carboxylic acids
(14) (a) Muller, N.; Rose, P. I. J. Phys. Chem. 1965, 69, 2564. (b) Muller,
N.; Hughes, O. R. J. Phys. Chem. 1966, 70, 3975. (c) Jentschura, U.; Lippert,
E. Ber. Bunsen Ges. 1971, 75, 782. (d) Goldman, M. A.; Emerson, M. T.
J. Phys. Chem. 1973, 77, 2295.
exhibit higher field OH chemical shifts, in the range δ 5.5-8.5
depending on the acid and the solvent used,14 and thus for simple
acids strong hydrogen bonding results in relatively low-field

4896 Organometallics, Vol. 25, No. 20, 2006
Mitu and Baird
phenone, and ethyl benzoate, respectively.11 On this basis it
would seem that methyl propionate coordinates much more
strongly to B(C6F5)3 than does for instance ethyl benzoate,
consistent with the relative B-O bond lengths mentioned above.
Interestingly, in competition experiments involving mixtures
of C2H5CO2Me‚B(C6F5)3 and propionic acid, the latter com-
pletely displaced the coordinated ester to form the 1:1 complex
C2H5CO2H‚B(C6F5)3. This result suggests that the carboxylic
acid is a better Lewis base with respect to B(C6F5)3, at least,
than is the corresponding methyl ester, and is consistent with
the greater propensity to dissociation and exchange in the case
of the 1:1 ester adduct than with the corresponding propionic
acid adduct. Interestingly, since methyl propionate has a signif-
icantly higher gas phase proton affinity than does propionic
acid,15 one might anticipate that the acid would be a poorer
Lewis base. However previous work has shown that steric
factors play a major role in determining stabilities of adducts
of B(C6F5)3,11 and it seems likely that a decreased steric contri-
bution is an important factor enhancing the stabilities of B(C6F5)3
adducts of carboxylic acids.
Formation of stable 1:1 adducts of carboxylic acids, as A of
Scheme 1, appears to be without precedent, and it is very inter-
esting that the adducts behave as strong acids. Although similar
carboxylic acid adducts of, for example, BF3 are known, they
have been little studied because of their corrosive nature,16 and
hence study of carboxylic adducts of B(C6F5)3 should shed
considerable light on the chemistry of such species.
Role(s) of Carboxylic Acid Adducts in Carbocationic
Polymerization Processes. As is clear, the electron-withdrawing
power of the highly electrophilic B(C6F5)3 increases the Brønsted
acidity of coordinated carboxylic acids sufficiently that they are
able to protonate isobutene and thereby initiate its carbocationic
polymerization. However, although only the 1:1 adducts are
observed here, initiation of isobutene polymerization proceeds
best when the B(C6F5)3:carboxylic acid ratio is 2:1.6a,d We
interpret this apparent incongruity by positing that the 1:1 adduct
is sufficiently acidic to protonate IB and that the anion formed
then coordinates a second B(C6F5)3 to form the much more
sterically hindered and more weakly coordinating 2:1 anionic
complex. Certainly the 2:1 anions are capable of existing,6e and
molecular modeling6a,f suggests that the oxygen atoms in the
2:1 anions are sufficiently sterically shielded by aliphatic and
B(C6F5)3 groups6a,f that the anions can indeed behave as weakly
coordinating anions.
Summary. Carboxylic acids react with B(C6F5)3 in CH2Cl2
to form 1:1 adducts, which have been characterized by room-
1
temperature IR and low-temperature (-30 °C) H, 19F, 13C,
and 11B NMR spectroscopy. Coordination appears to occur via
the carbonyl oxygen atoms, and exchange between an adduct
1
and its components is generally sufficiently slow that H, 19F,
13C, and 11B resonances of the adduct are readily distinguished
from the 1H and 13C resonances of the free corresponding acid
or the 19F and 11B resonances of free B(C6F5)3. The electron-
withdrawing power of the highly electrophilic B(C6F5)3 increases
the Brønsted acidity of the coordinated acids sufficiently that
they induce slow, partial cleavage of B-C6F5 bonds, and with
benzoic acid the compound [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2]
has been isolated and characterized crystallographically. Because
of B-C6F5 cleavage, crystallographically useful crystals of a
1:1 adduct of a carboxylic acid have not been obtained.
However, C2H5CO2Me‚B(C6F5)3, the 1:1 adduct of methyl pro-
pionate, has for purposes of comparison been prepared and
characterized spectroscopically and crystallographically. In this
case coordination occurs unequivocally via the carbonyl oxygen
atom.
Acknowledgment. We thank Queen’s University, the Natu-
ral Sciences and Engineering Research Council, and Lanxess
Inc. for financial support, and Drs. A. Carr, M. Drewitt, and K.
Kulbaba, all of Lanxess Inc., for helpful discussions and advice.
We also thank Dr. R. Wang for his crystallographic expertise
and help.
Supporting Information Available: Crystallographic data,
atomic coordinates and equivalent isotropic displacement param-
eters, bond lengths and angles, anisotropic displacement parameters,
hydrogen coordinates and isotropic displacement parameters, and
torsion angles for the compounds [(µ-C6H5CO2)(µ-OH){B(C6F5)2}2]
and C2H5CO2Me‚B(C6F5)3 are available free of charge via the
Internet at http://pubs.acs.org.
(15) Hunter, E. P. L.; Lias, S. G. J. Phys. Chem. Ref. Data 1998, 27,
413.
(16) (a) Topchiev, A. V.; Zavgorodnii, S. V.; Paushkin, Y. M. Boron
Fluoride and its Compounds as Catalysts in Organic Chemistry; Pergamon
Press: New York, 1959, p 68. (b) Heaney, H. In Encyclopedia of Reagents
for Organic Synthesis; Paquette, L., Ed.; Wiley & Sons: New York, 1995;
Vol. 1, p 655.
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