Syntheses, characterization, and crystal structures of the tetramethylammonium salts of the novel weakly coordinating anions [MeCO 2{B(C6F5)3}]- and [MeCO2{B(C6F5)<sub

Article abstract of DOI:10.1139/V05-245

The novel compounds [Me₄N][MeCO₂{B(C ₆F₅)₃}] and [Me₄N][MeCO ₂{B(C₆F₅)₃}₂] are prepared and characterized spectroscopically and crysta

Full text of DOI:10.1139/V05-245

225
Syntheses, characterization, and crystal
structures of the tetramethylammonium salts of
the novel weakly coordinating anions
[MeCO2{B(C6F5)3}]– and [MeCO2{B(C6F5)3}2]– 1
Simona Mitu and Michael C. Baird
Abstract: The novel compounds [Me4N][MeCO2{B(C6F5)3}] and [Me4N][MeCO2{B(C6F5)3}2] are prepared and charac-
terized spectroscopically and crystallographically. The compounds are salts of the unisolable acids MeCO2H·B(C6F5)3
and MeCO2H·2B(C6F5)3, respectively, which are sufficiently strong that they can protonate isobutene and initiate its
carbocationic polymerization. The 1:1 adduct contains a conventional, monodentate acetate ion coordinated to the
B(C6F5)3, while the 2:1 adduct contains a bridging acetate ligand.
Key words: weakly coordinating anions, carbocationic polymerization, isobutene, polyisobutene.
Résumé : On a préparé les nouveaux composés [Me4N][MeCO2{B(C6F5)3}] et [Me4N][MeCO2{B(C6F5)3}2] et on les a
caractérisés sur des bases spectroscopiques et cristallographiques. Les composés sont respectivement des acides
MeCO2H·B(C6F5)3 et MeCO2H·2B(C6F5)3 qui ne peuvent pas être isolés, mais qui sont suffisamment forts pour proto-
ner l’isobutène et initier sa polymérisation carbocationique. L’adduit 1 : 1 contient un ion acétate monodentate conven-
tionnel qui est lié par coordination au B(C6F5)3 alors que l’adduit 2 : 1 comporte un ligand acétate qui agit comme
pont.
Mots clés : anions faiblement coordinés, polymérisation carbocationique, isobutène, polyisobutène.
[Traduit par la Rédaction] Mitu and Baird 232
Recent years have seen extraordinary developments in the
once esoteric field of “weakly coordinating anions” (1). For
present purposes, weakly coordinating anions may be de-
fined loosely, if narrowly, as anions that, in solution, bind
sufficiently weakly to cationic species such as H+, carboca-
tions, or metal ions that they are readily displaced by such
normally poor electron pair donors as unsaturated hydrocar-
bons (1). Of great relevance here are the clearly similar, im-
portant properties of weakly coordinating anions in the
complementary fields of the coordination polymerization of
olefins such as ethylene and 1-alkenes (2) and of the car-
bocationic polymerization of, e.g., the more sterically hin-
dered, electron-rich isobutene (3). As well-known examples
of coordination polymerization catalysis, active catalysts are
often generated from neutral zirconocene precursors, as in
Scheme 1 (2).
X– binds too strongly to the metal, it is clearly important
that X– be an exceedingly poor ligand if olefin coordination
and subsequent polymerization are to occur (Scheme 2).
In a similar vein, initiation of carbocationic polymeriza-
tion of an olefin such as isobutene involves interaction of a
molecule of monomer with, e.g., a strong acid, as in
Scheme 3.
The carbocationic center so formed is stabilized by sol-
vent and (or) counteranion X–, and the latter must be a very
poor Lewis base, otherwise the ions will combine to give the
neutral compound Me3CX. Chain propagation involves dis-
placement of X– by monomer and repeated additions of
alkene molecules to the carbocationic center, which migrates
well away from the site of initial attack (Scheme 4).
Weakly coordinating counteranions result in higher mo-
lecular weights (3d, 3e), and thus, in chemistry reminiscent
of coordination polymerization, the C=C bond must be the
most nucleophilic species in the system. Indeed, many
cationic transition metal complexes function as both coordi-
nation catalysts and carbocationic initiators (3f, 4).
Much attention has been paid to the development of cata-
lysts utilizing the chemistry of Scheme 1 (2, 5–8), and many
new weakly coordinating anions X– have been designed on
the premise that delocalization of negative charge over as
many atoms as possible in a large anion should be of advan-
tage (5–8). Thus, there have been reported several very bulky
anions in which the perfluorophenyl groups of
[BMe(C6F5)3]– have been substituted by more complex
perfluoroaryl groups (5g–5p). Alternative, synthetically less
Since monomer access to the active metal site of
[Cp′ZrMe]+ may be effectively blocked if the counteranion
Received 8 June 2005. Published on the NRC Research Press
Web site at http://canjchem.nrc.ca on 14 March 2006.
Dedicated to Dr. Arthur Carty, an excellent scientist, a
visionary, a bon vivant, and a very generous friend.
S. Mitu and M.C. Baird.2 Department of Chemistry,
Queen’s University, Kingston, ON K7L 3N6, Canada.
1This article is part of a Special Issue dedicated to Professor
Arthur Carty.
2Corresponding author (e-mail: bairdmc@chem.queensu.ca).
Can. J. Chem. 84: 225–232 (2006)
doi:10.1139/V05-245
© 2006 NRC Canada

226
Can. J. Chem. Vol. 84, 2006
Scheme 1. Cp′ = substituted cyclopentadienyl, X– = weakly coor-
initiator for both coordination and carbocationic olefin poly-
merization reactions (10a), or they can be used directly to
initiate the carbocationic polymerization of, e.g., isobutene
and similarly polymerizable olefins (eq. [4]) (10b–10d).
dinating anion.
Cp2' ZrMe(µ-Me)B(C6F5)3
Cp2' ZrMe2 + B(C6F5)3
[Cp2' ZrMe]+ + [MeB(C6F5)3]-
[3]
Cp2' ZrMe2 + [Ph3C]X
[Cp2' ZrMe]+ + X- + Ph3CMe
Scheme 2. R = H, alkyl.
B(C6F5)3
+
O
H+
R
+ isobutene
polyisobutene
[4]
-
C
CH3
Zr
X- + olefin
polyolefin
B(C6F5)3
O
R
The species formed when a carboxylic acid is combined
with 2 equiv. of the borane is thus a novel type of Brønsted
acid, sufficiently strong to protonate isobutene; presumably
its conjugate base, [RCO2{B(C6F5)3}2]–, is stabilized by ex-
tensive delocalization of the negative charge. However, to
this point the nature of the species in solution had been in-
ferred only from the chemistry observed, and we have pos-
sessed only spectroscopic information concerning the
structures of the putative products of eqs. [1] and [2].
Complicating our attempts to investigate spectroscopically
the nature of the species in solution, we have found that the
1:1 and 2:1 adducts A and B are sufficiently acidic that
cleavage of the B—C6F5 bonds occurs readily above about
–30 °C to give pentafluorobenzene. We wished, however, to
determine unequivocally the nature of the borane–
carboxylate species in solution, and we have now accom-
plished this by isolating and characterizing salts of the
conjugate bases of the products of eqs. [1] and [2]. Although
the best carbocationic polymerization initiators found previ-
ously have involved long-chain carboxylic acids (10), we de-
cided that we might have better success obtaining crystalline
materials if we were to use short-chain carboxylic acids, and
we have now accomplished the successful syntheses and
characterization of the 1:1 and 2:1 acetato compounds
[Me4N][MeCO2{B(C6F5)3}] and [Me4N][MeCO2{B(C6F5)3}2].
Crystal structures of both tetramethylammonium salts con-
firm their structures. A subsequent paper will consider the
nature of the conjugate acids of these types of anions and
their roles in initiating carbocationic polymerization of iso-
butene.
Scheme 3.
CH2=CMe2 + H+X-
CH3 C+Me2
X-
Scheme 4.
Me
Me
Me
Me
Me
Me
+
C
+
Me
+Me2C=CH2
Me
Me
C
C
+
CH2 CMe2
Me2C=CH2
Polymer
+
+
H
H
Me
Me
C
-
C
-
challenging procedures involve interaction of B(C6F5)3 with
–
various anions Y– (Y– = e.g., OH– (6), CN– (5f, 7a), N3
(7b), OR– (6b–6d, 6j), SR– (6b–6d), imidazolate (7c)) to
form complex anions of the types [YB(C6F5)3]– and [(µ-
Y){B(C6F5)3}2]–. In addition, we have recently reported uti-
lization of [Me3CH2B(C6F5)3]– in which the sterically de-
manding neopentyl group renders the borate anion much less
weakly coordinating than is the above-mentioned methyl an-
alogue, [BMe(C6F5)3]– (8).
Complementing aspects of the above-mentioned research,
we have reported the synthesis and utilization for isobutene
polymerization
of
strong
acids
of
the
types
[ROH{B(C6F5)3}1,2] and [RCO2H{B(C6F5)3}1,2] (R = alkyl,
aryl) (9, 10). Carboxylic acids, RCO2H, do not by them-
selves initiate isobutene polymerization but are apparently
activated via formation of adducts with B(C6F5)3, as in
eqs. [1] and [2].
We note reports elsewhere of isobutene polymerizations
being effected in the presence of other dinuclear counter-
anions (4g, 11), and it seems clear that counteranions, which
are weakly coordinating because of combinations of steric
factors and extensive delocalization of charge, do indeed
play a role similar to those mentioned above for coordina-
tion polymerization catalysts.
OH
RCO2H + B(C6F5)3
R
C
[1]
O
B(C6F5)3
A
Experimental
Most synthetic procedures, unless otherwise noted, were
carried out under an atmosphere of nitrogen or argon puri-
fied by passing through a column of BASF catalyst heated to
140 °C and a column of 5 Å molecular sieves. Manipulation
of air-sensitive materials employed standard Schlenk line
techniques and an Mbraun Labmaster glovebox. Solvents
were dried by passing them through activated alumina col-
B(C6F5)3
B(C6F5)3
O
-
O
H+
R
[2]
[RCO2H·B(C6F5)3] + B(C6F5)3
C
B
These can then react with metallocene-type compounds,
as in eq. [3], generating the cationic [Cp*TiMe2]+, a known
© 2006 NRC Canada

Mitu and Baird
227
Table 1. Crystallographic data for [Me4N][MeCO2{B(C6F5)3}] and [Me4N][MeCO2{B(C6F5)3}2].
[Me4N][MeCO2{B(C6F5)3}]
[Me4N][MeCO2{B(C6F5)3}2]
C44.72H20.43B2Cl5.43F30NO2
Empirical formula
C25H17BCl2F15NO2
(C24H15BF15NO2·CH2Cl2)
([N(CH3)4]{[(C6F5)3B]2(OOCCH3)}·2.72CH2Cl2)
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
730.11
180(2)
0.710 73
Monoclinic
P21/c
1 387.99
150(2)
0.710 73
Triclinic
P1
9.659(2)
14.080(3)
21.244(5)
90
90.137(4)
90
2 889.1(12)
4
1.679
0.349
1 456
0.3 × 0.3 × 0.2
1.74–28.24
–12 ≤ h ≤ 12,
11.682(2)
15.629(3)
17.363(3)
100.132(3)
108.095(3)
108.198(3)
2 728.7(8)
2
1.689
0.429
1 368
0.4 × 0.4 × 0.3
1.44–28.30
–14 ≤ h ≤ 14,
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Density (calcd.) (Mg/m3)
Absorption coefficient (mm–1)
F(000)
Crystal size (mm3)
θ Range for data collection (°)
Index ranges
–17 ≤ k ≤ 17,
–20 ≤ k ≤ 19,
–28 ≤ l ≤ 27
20 046
–22 ≤ l ≤ 21
19 369
Reflections collected
Independent reflections
Completeness to θ = 25.00° (%)
Absorption correction
Max. and min. transmission
Refinement method
6 686 [R(int) = 0.033 1]
100.0
Empirical (Bruker SADABS)
1.000 0 and 0.744 7
Full-matrix least-squares on F2
6 686 / 0 / 474
12 259 (R(int) = 0.016 9]
99.5
Empirical (Bruker SADABS)
1.000 0 and 0.912 1
Full-matrix least-squares on F2
12 259 / 667 / 871
1.103
Data / restraints / parameters
Goodness-of-fit on F2
0.844
Final R indices I > 2σ(I)
R1 = 0.038 1, wR2 = 0.077 4
R1 = 0.062 6, wR2 = 0.194 9
R indices (all data)
Largest diff. peak and hole (e Å–3)
R1 = 0.083 9, wR2 = 0.087 4
R1 = 0.086 5, wR2 = 0.208 4
0.233 and –0.307
1.246 and –0.994
umns (Innovative Technology). All chemicals were pur-
chased from Aldrich and were purified as appropriate before
use. The compound B(C6F5)3 was prepared as described in
the literature (12).
were converted to structure amplitudes and their esds using
the program SAINT-plus, which corrects for Lp and decay.
Absorption corrections were applied using the program
SADABS. The crystals are monoclinic space group P21/c for
[Me4N][MeCO2{B(C6F5)3}] and triclinic space group P1 for
[Me4N][MeCO2{B(C6F5)3}2] based on the systematic ab-
sences, E statistics, and successful refinement of the struc-
ture. The structure was solved by direct methods. Full-
matrix least-square refinements minimizing the function
∑w(Fo – Fc ) were applied to the compound. For the struc-
ture of [Me4N][MeCO2{B(C6F5)3}], all non-hydrogen atoms
in the structure were refined anisotropically, and only the
hydrogen atoms of the solvent molecule were calculated; all
the other hydrogen atoms were located directly from the
difference Fourier map. For the structure of
The 1H, 11B, and 19F NMR spectra were run on an Avance
400 NMR spectrometer, the residual proton resonances of
the deuterated solvents serving as internal references. Mass
spectra were obtained on a Quatro Fisons Pro Quadrupole
mass spectrometer in EI mode with CH2Cl2 as solvent.
Canadian Microanalytical Services of Delta, British Colum-
bia, performed elemental analyses. X-ray crystallographic
structure determinations were carried out using a Siemens
P4 diffractometer with a Bruker SMART CCD 1000 detector
and graphite-monochromated Mo Kα radiation (λ =
0.710 73 Å) controlled with a Crysostream controller 700.
Typically a crystal was mounted on a glass fibre with epoxy
glue. No significant decay was observed during data collec-
tion, and the data were processed on a Pentium PC using the
Bruker AXS Windows NT SHELXTL software package
(version 5.10) (13a). Neutral atom scattering factors were
taken from Cromer and Waber (13b). The raw intensity data
2
2 2
[Me4N][MeCO2{B(C6F5)3}2],
the
solvent
molecules
(2.72CH2Cl2) in the crystal are severely disordered, and the
SHELXL commands SIMU, DFIX, SAME, and PART were
thus used to refine the structure. All non-hydrogen atoms in
the structure (II) (except C47, C49, Cl13, and Cl14) were
refined anisotropically. All hydrogen atoms were calculated,
© 2006 NRC Canada

228
Can. J. Chem. Vol. 84, 2006
Table 2. Important bond lengths and bond angles for [Me4N][MeCO2{B(C6F5)3}] and
[Me4N][MeCO2{B(C6F5)3}2].
Compound
[Me4N][MeCO2{B(C6F5)3}]
[Me4N][MeCO2{B(C6F5)3}2]
Bond lengths (Å)
C(1)—C(2)
C(2)—O(1)
C(2)—O(2)
B(1)—O(1)
B(2)—O(2)
B(1)—C(3)
B(1)—C(9)
B(1)—C(15)
1.499(3)
1.324(2)
1.217(2)
1.514(2)
—
1.654(3)
1.651(3)
1.644(3)
—
1.496(4)
1.279(4)
1.260(4)
1.543(4)
1.568(4)
1.649(5)
1.650(5)
1.628(4)
1.644(4)
1.636(5)
1.645(5)
B(2)—C(21)
B(2)—C(27)
B(2)—C(33)
—
—
Bond angles (°)
C(1)-C(2)-O(1)
C(1)-C(2)-O(2)
O(1)-C(2)-O(2)
C(2)-O(1)-B(1)
C(2)-O(2)-B(2)
O(1)-B(1)-C(15)
O(1)-B(1)-C(9)
O(1)-B(1)-C(3)
O(2)-B(2)-C(27)
O(2)-B(2)-C(33)
O(2)-B(2)-C(21)
C(15)-B(1)-C(9)
C(15)-B(1)-C(3)
C(9)-B(1)-C(3)
C(21)-B(2)-C(27)
C(21)-B(2)-C(33)
C(27)-B(2)-C(33)
112.84(18)
123.32(19)
123.84(17)
119.74(14)
—
112.20(14)
105.93(14)
106.33(14)
—
114.8(3)
124.5(3)
120.7(3)
128.0(2)
127.8(2)
108.8(2)
103.3(2)
109.9(2)
105.9(2)
103.7(2)
112.1(3)
111.3(2)
116.7(2)
105.9(2)
116.3(2)
104.0(2)
114.3(3)
—
—
116.35(15)
104.42(14)
111.28(15)
—
—
—
and their contributions were included in the structure factor
calculations with isotropic thermal parameters 1.2 times that
of the attached carbon atoms (1.5 times for methyl hydro-
gens). Crystallographic data for the two compounds studied
are summarized in Table 1, with selected bond distances and
angles in Table 2. Molecular structures are shown in Figs. 1
and 2, and complete crystallographic data are given in the
supporting information.3
0.49 mmol) and [Me4N][MeCO2] (64.9 mg, 0.49 mmol) in
dry CH2Cl2 (20 mL) was stirred at room temperature for 1 h,
after which the solvent was removed under reduced pres-
sure. The resulting white solid was recrystallized from a
mixture of CH2Cl2 (5 mL) and hexanes (15 mL), filtered off,
washed with hexanes, and dried to give 253 mg (80% yield)
of product. IR (Fluorolube mull, cm–1) ν(CO): 1682, 1491.
1H NMR (CD2Cl2) δ: 3.13 (s, 12H, MeN), 1.94 (s, 3H,
MeC). 11B NMR (CD2Cl2) δ: –5.1 (sharp). 19F NMR
(CD2Cl2) δ: –135.9 (o-F), –163.6 (p-F), –168.2 (m-F). HR-
MS (EI) calcd. for [(CH3)4N]: 74.1460; found: 74.0784. HR-
MS (EI) calcd. for [MeCO2{B(C6F5)3}]: 571.0296; found:
571.0672. Anal. calcd. for C24H15BF15O2·CH2Cl2: C 41.13,
H 2.35, N 1.92; found: C 41.58, H 2.38, N 1.88. Crystallo-
graphic quality crystals were grown by layering a solution of
[Me4N][MeCO2{B(C6F5)3}] in CH2Cl2 with hexanes.
Preparation of [Me4N][MeCO2{B(C6F5)3}]
[Me4N][MeCO2] was prepared by neutralizing
tetramethylammonium hydroxide (6 mL, 25 wt% aqueous
solution) with glacial acetic acid (1.43 mL, 0.0250 mol)
(14). The solution was evaporated to dryness, and the solid
residue was recrystallized from acetone and dried under re-
duced pressure for 12 h at 150 °C to give 2.1 g of
[Me4N][MeCO2] (95% yield). IR (Fluorolube mull, cm–1)
1
Preparation of [Me4N][MeCO2{B(C6F5)3}2]
A mixture of B(C6F5)3 (500 mg, 0.98 mmol) and
[Me4N][MeCO2] (64.9 mg, 0.49 mmol) in CH2Cl2 (30 mL)
ν(CO): 1681, 1491. H NMR (CD2Cl2) δ: 3.43 (s, 12H,
MeN), 1.80 (s, 3H, MeC). To prepare [Me4N][Me-
CO2{B(C6F5)3}],
a
mixture of B(C6F5)3 (250 mg,
3 Supplementary data for this article are available on the journal Web site (http://canjchem.nrc.ca) or may be purchased from the Depository
of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 4085. For more
information on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 275372 and 275373 contain the crystal-
lographic data for this manuscript. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2006 NRC Canada

Mitu and Baird
229
Fig. 1. X-ray structure of [Me4N][MeCO2{B(C6F5)3}].
Fig. 2. X-ray structure of [Me4N][MeCO2{B(C6F5)3}2].
was stirred at room temperature for 1 h, after which the sol-
vent was removed under reduced pressure. The resulting
white solid was recrystallized from a mixture of CH2Cl2
(5 mL) and hexanes (15 mL), filtered off, washed with hex-
anes, and dried to give 398 mg (70% yield) of product. IR
(Fluorolube mull, cm–1) ν(CO): 1566, 1417. 1H NMR
(CD2Cl2) δ: 3.22 (s, 12H, MeN), 2.19 (s, 3H, MeC). 11B
NMR (CD2Cl2) δ: –1.6 (broad). 19F NMR (CD2Cl2) δ:
–135.6 (o-F), –161.0 (p-F), –167.1 (m-F). HR-MS (EI)
calcd. for [(CH3)4N]: 74.1460; found: 74.0832. HR-MS (EI)
calcd. for [MeCO2{B(C6F5)3}2]: 1083.0147; found: 1083.0642.
Anal. calcd. for C42H15B2F30O2: C 43.59, H 1.31, N 1.21;
found: C 43.28, H 1.26, N 1.52. Crystallographic quality
mental analyses and IR and NMR spectroscopy; the crystals
of the 1:1 adduct contained one molecule of CH2Cl2, as
shown by the elemental analyses and X-ray crystallography.
The IR spectra are illustrated in Figs. 3 and 4, and we note
especially that the observed values of ν(CO) of 1682 and
1491 cm–1 for the 1:1 adduct and of 1566 and 1417 cm–1 for
the 2:1 adduct are consistent with the literature for mono-
dentate and bridging acetato ligands as in D and E, respec-
tively (16).
-
-
B(C6F5)3
B(C6F5)3
O
O
B(C6F5)3
O
O
crystals were grown by layering
a
solution of
C
Me
Me
C
[Me4N][MeCO2{B(C6F5)3}2] in CH2Cl2 with hexanes.
D
E
Results and discussion
The combination of carboxylic acids and B(C6F5)3, espe-
cially in a 2:1 B(C6F5)3 – carboxylic acid ratio, results in an
acid system sufficiently strong to protonate isobutene (IB)
and to initiate its carbocationic polymerization (10).
Carboxylic acids themselves are too weak to effect this
chemistry, of course, and furthermore carboxylate anions are
expected to bind very effectively to the carbocation interme-
diates. However, the inductive effects of carboxylic acid co-
ordination to the strongly electrophilic borane in a species
such as B would very likely result in an acid much stronger
than the free carboxylic acid itself (10), as has been ob-
served elsewhere with other potentially acidic Lewis base
donors (6j, 15), and it seemed very likely that the reactions
described in eqs. [1] and [2] would pertain. The purpose of
this research was to prepare, characterize, and determine the
structures of salts of the putative counteranions of species A
and B.
The positions of the symmetric stretches are in both cases
uncertain because absorptions of the B(C6F5)3 (indicated in
the spectra by B) and the [Me4N]+ ion also appear in the re-
gion 1300–1800 cm–1.
The 1H, 19F, and 11B NMR spectra are shown in Figs. 5, 6,
and 7, respectively, where they are compared with the spec-
tra of [Me4N][MeCO2] (1H) and B(C6F5)3 (19F, 11B). The
electron-withdrawing power of the borane is evident in the
1
acetate methyl H resonances, and the changes in the 19F
chemical shifts on going from free to coordinated B(C6F5)3
are characteristic (5–10). The 11B chemical shifts of the ad-
ducts are similar to those of other adducts with O-donors,
and the observed narrowing of the resonances is consistent
with tetrahedral structures (6h, 17b–17d), although it is not
at all clear why the 2:1 complex has a much broader reso-
nance than does the 1:1 complex.
Syntheses of [Me4N][MeCO2{B(C6F5)3}] and
[Me4N][MeCO2{B(C6F5)3}2]
These two compounds were synthesized by reacting
[Me4N][MeCO2] with 1 and 2 molar equiv. of B(C6F5)3, re-
spectively. Both compounds were characterized fully by ele-
Crystal structures of [Me4N][MeCO2{B(C6F5)3}] and
[Me4N][MeCO2{B(C6F5)3}2]
Crystal structures were obtained for both compounds,
with the 1:1 complex containing a molecule of CH2Cl2. Se-
lected bond distances and angles are shown in Table 2, and
© 2006 NRC Canada

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Can. J. Chem. Vol. 84, 2006
Fig. 3. IR spectrum (Fluorolube mull) of [Me4N]{[MeCO2][B(C6F5)3]}: * identifies ν(CO); B identifies peaks assignable to B(C6F5)3.
80
70
60
50
B
40
*
B
30
*
B
20
B
10
4000
3200
2400
1800
1400
1000
600 cm-1
ν
(cm-1)
Fig. 4. IR spectrum (Fluorolube mull) of [Me4N]{[MeCO2][B(C6F5)3] 2}: * identifies ν(CO); B identifies peaks assignable to B(C6F5)3.
80
70
60
B
B
50
*
*
40
30
B
20
10
4000
3200
2400
1800
1400
1000
600
ν
(cm-1)
1
Fig. 5. H NMR spectra of (a) [Me4N][MeCO2], (b) [Me4N][MeCO2{B(C6F5)3}], and (c) [Me4N][MeCO2{B(C6F5)3}2] (CD2Cl2).
c
b
a
5.0
4.5
4.0
3.5
3.0
2.5
2.0
δ
molecular structures are shown in Figs. 1 and 2. As can be
seen, the crystal structures are completely in accord with the
modes of bonding suggested by D and E. Thus the carbon–
oxygen bond lengths of the 1:1 adduct are 1.216(2) and
1.321(2) Å, as expected for a double and a single bond,
respectively. On the other hand, the two carbon–oxygen dis-
tances of the 2:1 adduct are intermediate between these two
values and are very similar to each other. Interestingly, the
boron–oxygen distance in the 1:1 adduct is significantly
shorter than that of the 2:1 adduct, consistent with the
expected stronger B—O bond in the former. However, all
three B—O distances fall in the range established for a vari-
ety of adducts of B(C6F5)3 with a variety of other O-donors
(6h, 6j, 17).
Possibly complementing the stronger donor–acceptor in-
teraction in the 1:1 adduct, the average of the B—C6F5 bond
lengths in this adduct is marginally longer than the average
of the 2:1 adduct, although steric repulsions should be more
© 2006 NRC Canada

Mitu and Baird
231
Fig. 6. 19F NMR spectrum of (a) B(C6F5)3, (b) [Me4N][MeCO2{B(C6F5)3}], and (c) [Me4N][MeCO2{B(C6F5)3}2] in CD2Cl2.
c
b
a
-120
-130
-140
-150
-160
-110
-170
δ
Fig. 7. 11B NMR spectra of (a) B(C6F5)3, (b) [Me4N][MeCO2{B(C6F5)3}], and (c) [Me4N][MeCO2{B(C6F5)3}2] (CD2Cl2).
c
b
a
150
100
50
0
-50
δ
Rieger, and R.M. Waymouth. Angew. Chem. Int. Ed. Engl. 34,
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important in the latter. The acetate ions are essentially planar
in both adducts, but the bond angles vary widely. Thus the
O-C-O bond angle of the 1:1 adduct is significantly less than
120°, the two Me-C-O bond angles both being significantly
greater. In contrast, the O-C-O bond angle of the 2:1 adduct
is 120.7(3)°, while the two (chemically equivalent) Me-C-O
bond angles vary widely. The O-B-C6F5 and C6F5-B-C6F5
bond angles vary considerably, but the latter variations seem
to be normal (18).
Acknowledgements
We thank Queen’s University, the Natural Sciences and
Engineering Research Council of Canada (NSERC), and
Lanxess Inc. for financial support, and Drs. A. Carr and M.
Drewitt for helpful discussions and advice. We also thank
Dr. R. Wang for his crystallographic expertise and help.
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