Tris(perfluorotolyl)borane—A Boron Lewis Superacid

Article abstract of DOI:10.1002/anie.201704097

Tris[tetrafluoro-4-(trifluoromethyl)phenyl]borane (BTolF) was prepared by treating boron tribromide with tetrameric F₃CC₆F₄-Cu^I. The F₃CC₆F₄-Cu^I was generated from F

Full text of DOI:10.1002/anie.201704097

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Accepted Article
Title: Tris(perfluorotolyl)borane - a Boron Lewis Superacid
Authors: Norbert Werner Mitzel, Jan Schwabedissen, Leif Körte,
Marcel Soffner, Sebastian Blomeyer, Christian Reuter, Yury
Vishnevskiy, Beate Neumann, and Georg Stammler
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10.1002/anie.201704097
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Tris(perfluorotolyl)borane – a Boron Lewis Superacid
Leif A. Körte, Jan Schwabedissen, Marcel Soffner, Sebastian Blomeyer, Christian G. Reuter, Yury V.
Vishnevskiy, Beate Neumann, Hans-Georg Stammler and Norbert W. Mitzel*
Abstract: Tris(tetrafluoro-4-(trifluoromethyl)phenyl)borane (BTolF)
was prepared by reacting boron tribromide with tetrameric F3CC6F4-
Cu(I). The latter was generated from F3CC6F4MgBr and copper(I)
bromide. Lewis acidities of BTolF evaluated by the Gutmann-Beckett
method and calculated fluoride ion affinities are 9 and 10%, respect-
tively, higher than that of tris(pentafluorophenyl)borane (BCF) and
even higher than that of SbF5. The molecular structures of BTolF
and BCF were determined by gas electron diffraction, that of BTolF
also by single crystal X-ray diffraction.
This ether adduct had to be prepared each time by reacting
boron trichloride with perfluorotolyl lithium, facing the dangerous
thermolability of the latter above −40 °C.
Free BTolF – if it were available – promises to combine steric
crowding at boron comparable to that of BCF while possessing a
higher Lewis acidity. The Hammett constants of a fluorine (σp =
0.06) and a CF3 group (σp = 0.54) in para-position predict the
latter to act as stronger electron-withdrawing group due to its
inability of π-donation.[16] It would also protect this position
against nucleophilic substitution.
Several attempts to obtain BTolF via reactions of perfluorotolyl-
lithium or the respective Grignard reagent with boron trihalides
remained unsuccessful in our hands. However, the synthesis of
BTolF via a donor-free copper(I)-based transfer was successful.
This protocol is related to a variant of the BCF synthesis via
C6F5Cu[17] by Jäkle et al.[18] The precursor p-bromoheptafluoroto-
luene (2) was obtained in 64% yield after distillation by
bromination of neat perfluorotoluene (1) with aluminum tribro-
mide at 130 °C along a modified procedure.[19] (Scheme 1). The
corresponding Grignard reagent was prepared and reacted with
copper(I) bromide. Addition of 1,4-dioxane precipitated the mag-
nesium salts to separate them by filtration. Removal of the sol-
vents in vacuum yields copper compound 3 as its 1,4-dioxane
adduct. The donor-free reagent was obtained by applying high
vacuum while heating the yellow solid gradually from 50 to
100 °C. The heating rate in this step is crucial for obtaining pure
compound 3. Heating too fast or above 110 °C leads to decom-
position under formation of perfluorobitolyl and copper(0). Com-
plete removal of 1,4-dioxane is always in conflict with minor pro-
duct decomposition. In most attempts we accepted minor de-
composition (diagnosed by slightly too low carbon contents).
Anyhow, traces of copper or residual 1,4-dioxane do not disturb
further reactions significantly, as compared to experiments
employing the reagent in higher purity.
Tris(pentafluorophenyl)borane (BCF) is a widely used Lewis acid
in synthetic chemistry. Since its first preparation in 1963[1] it
received over 20000 reaction entries in SciFinder. In contrast to
the volatile and very moisture- and air-sensitive trihaloboranes
BX3 (X = F, Cl, Br), BCF was described as an ideal boron Lewis
acid: it combines high Lewis acidity, thermal stability and
tolerates water and oxygen.[2] It finds applications in Lewis-acid
catalysis[3], activation of olefine polymerization pre-catalysts.[4]
BCF is also the most commonly used Lewis-acid component in
frustrated Lewis pairs chemistry.[5] One of its drawbacks is the
susceptibility of its para-fluorine atom towards nucleophilic sub-
stitution.[6] Trying to remedy that by replacing these fluorine by
hydrogen atoms led to the synthesis of B(p-C6F4H)3, but this was
on the cost of Lewis acidity.[7] A variety of perfluoroarylboranes
has been synthesized[8] and their Lewis acidities were ranked on
the Gutmann-Beckett[9] and Childs Scales.[10] Only few boranes
are more Lewis acidic than BCF: tris(3,5-(trifluoromethyl)phenyl)-
borane and bis((2,4,6-trifluoromethyl)phenyl)borane about 5–6%
more than BCF.[11] Tris(o-perfluorobiphenyl)borane is 14% more
Lewis acidic than BCF by the Gutmann-Beckett test,[12] but the
increased steric shielding of the boron atom by the ortho-C6F5
groups reduces its fluoride ion affinity to less than that of BCF.[13]
Perfluoropentaphenylborole and 9-borafluorene both feature
higher Lewis acidities compared to BCF – a consequence of the
antiaromaticity of the borole and fluorene 4π-systems rings..[14]
Although unknown as a pure substance, an application of tris(p-
perfluorotolyl)borane (BTolF) has been recently reported.[15] In
contrast to the less Lewis acidic BCF, the adducts of BTolF with
a variety of heteroaromatics like quinolines and pyridines appea-
red to be activated for selective trifluoromethylation by a reagent
based on CF3SiMe3 and difluorotriphenylsilicate.[15] More than 30
adducts were obtained using in situ generated BTolF in ether.
[*]
Dr. L. A. Körte, J. Schwabedissen, M. Soffner, Dr. S. Blomeyer, Dr.
C. G. Reuter, B. Neumann, Dr. Y. V. Vishnevskiy, Dr. H.-G.
Stammler, Prof. Dr. N. W. Mitzel
Scheme 1. Synthesis of BTolF using the donor-solvent free copper(I) transfer
reagent 3. i) AlBr3 (neat), 130 °C, 12h. ii) a) Mg, Et2O, 50 °C; b) CuBr, 50 °C;
c) 1,4-dioxane, filtration, removal of solvent d) high vacuum, 100 °C, 20h,
removal of 1,4-dioxane. iii) BBr3, CH2Cl2, −78 °C, 1.5h.
Lehrstuhl für Anorganische Chemie und Strukturchemie and
Centrum für Molekulare Materialien CM2,
Fakultät für Chemie, Universität Bielefeld,
Universitätsstraße 25, 33615 Bielefeld, Germany.
E-mail: mitzel@uni-bielefeld.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201704097
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with that of SbF5 (493 kJ/mol) characterizes BTolF as a Lewis
superacid.[21]
Attempts to prepare bis(2,3,5,6-tetrafluoro-4-trifluoromethyl)bo-
rane, HB(p-C6F4CF3)2, by employing the standard protocol[22] of
(C6F5)2BH synthesis, i.e. treatment of BTolF with triethylsilane,
were not successful. Instead, unselective reactions and formati-
on of fluorotriethylsilane were detected by NMR spectroscopy
3
[δ(19F) = 175.0 ppm (sept, JF,H = 6.3 Hz), lit.[23]]. It is probably
formed by fluoride abstraction from a CF3 group by an
intermediately formed triethylsilyl cation.
There is still no experimental structure for BCF, but crystals of
BTolF for X-ray diffraction experiments were grown from a con-
centrated solution in n-hexane at −30°C. The molecules of
BTolF (Figure 2) lie on twofold axes of rotation passing through
B(1), C(8), C(11) and C(12). This leads to disorder of the fluorine
atoms bonded to C(12).
̅
Figure 1. Molecular structure (P1) of tetramer 3b as its toluene complex.
Displacement ellipsoids are displayed at 50% probability level. F(26) to F(28)
are disordered. Hydrogen atoms and one non-coordinate toluene molecule are
omitted for clarity. Selected bond lengths [Å] and angles [deg]: Cu(1)–Cu(2)
2.488(1), Cu(1)–Cu(3) 2.601(1), Cu(1)–Cu4 2.421(1), Cu(2)–Cu(3) 2.435(1),
Cu(3)–Cu4 2.485(1), Cu(1)–C(1) 1.964(2), Cu(2)–C(1) 2.139(2), Cu(2)–C(29)
2.302(2), Cu(2)–C(30) 2.354(2), centroid [C(8)-C(13)] – centroid [C(29)-C(34)]
3.672(1), centroid [C(22)-C(27)] – centroid [C(36)-C(41)] 3.717(1), Cu(4)-
Cu(1)-Cu(2) 107.8(1), Cu(1)-Cu(2)-Cu(3) 63.8(1), dihedral angle [Cu(1)-Cu(2)-
Cu(3)]-[Cu(1)-Cu(3)-Cu(4)] 36.0(1).
Perfluoroaryl copper 3 was characterized by NMR spectroscopy,
infrared spectroscopy and CHN elemental analysis. Mass spec-
tra (EI, 70 eV) show perfluorobitoluene due to decomposition of
3 when heated for evaporation. Solvent-free crystals of 3 could
not be obtained so far, but crystals grown from a concentrated
toluene solution at −30 °C allowed determining the molecular
structure of a toluene adduct 3b by X-ray diffraction (Figure 1).
The tetrameric aggregation with a butterfly-like structure is simi-
lar to that found for [Cu(C6F5)]4(η2-toluene)2.[20] The dihedral
angle between the two Cu3 triangles [Cu(1)Cu(2)Cu(3) and
Cu(1)Cu(3)Cu(4)] measures 36.0(1)°. Copper atoms Cu(2) and
Cu(4) are η2-coordinated by toluene with short contacts to the
para-carbon atom (Cu(2)–C(29) 2.302(2) Å) and slightly longer
contacts to the meta-carbon atom (Cu(2)–C(30) 2.354(2) Å). The
copper atoms are bridged by perfluoroaryl groups with a copper
contact of 2.601(1) Å between the non-toluene-coordinated
atoms Cu(1) and Cu(3), The toluene molecules undergo intra-
molecular π-stacking with the adjacent perfluoroaryl groups
(distance between centroids: 3.672(1) and 3.717(1) Å)) framing
copper contacts of 2.435(1) (Cu(2)···Cu(3)) and 2.421(1) Å
(Cu(1)···Cu(4)). These contacts are slightly shorter than those
between the copper atoms Cu(1)···Cu(2) and Cu(3)···Cu(4)
(2.488(1) Å, 2.485(1) Å), without additional π-stacking.
Treatment of three equivalents of the perfluorotolyl transfer rea-
gent 3 with BBr3 in dichloromethane at −78 °C afforded tris(p-
perfluorotolyl)borane (BTolF). After removal of the solvent,
BTolF can directly be sublimed from the remaining copper salts.
It was obtained as a colorless solid in 80% yield and characteri-
zed by NMR spectroscopy, mass spectrometry, CHN elemental
analysis and single crystal X-ray diffraction.
The relative Lewis acidity determined by Gutmann and Beckett’s
method[9,10] (details: ESI, Table S1) shows BTolF to be 8–9%
more Lewis acidic than BCF. The fluoride ion affinities[13] (FIA) of
BTolF (499 kJ/mol) and BCF (450 kJ/mol), calculated at the
(RI)BP86/SV(P) level of theory, reveal BTolF to be 10% more
Lewis acidic than BCF. Comparison of the absolute FIA value
Figure 2. Molecular structure (C2/c) of tris(p-perfluorotolyl)borane. Displace-
ment ellipsoids are displayed at the 50% probability level except F(10)-F(14),
which are disordered about the C2 axis. Selected bond lengths [Å] and angles
[deg]: B(1)–C(1): 1.567(2), B(1)–C(8) 1.562(4), C(2)–C(1)–B(1)–C(1’) 32.5(1),
C(9)–C(8)–B(1)–C(1) 46.2(1).
We determined now structures of the free molecules of BTolF
and BCF in the gas phase by electron diffraction using the
diffractometer at Bielefeld University[24,25] (details see ESI).
Models of D3 and C3 symmetry were used for refinement of BCF
and BTolF, respectively. The refined structures (rh1-type, see ESI
and [27]) resulted in Rf factors of 1.8 % for both, BTolF and BCF.
Figure 3 shows radial distribution curves and illustrations of the
structures.
BTolF and BCF have planar BC3 units. The three perfluoroaryl
groups are tilted out of this plane and are arranged in a propel-
ler-like manner. The free electron pairs of the para-fluorine atom
of the aromatic system are in conjugation with boron in BCF, but
the para-CF3 group in BTolF cannot. Consequently, the π-
electron density and thus its interaction with the vacant boron p-
orbital is less in BTolF than in BCF. This makes the
perfluorotolyl groups more flexible regarding their rotation. It also
results in lower vibrational frequencies corresponding to the their
torsional motion. These low frequency vibrations of BTolF are
highly populated at the temperature of the GED experiment
(162 °C). As a consequence the vibrationally averaged torsional
angle is apparently larger for BTolF (56.8(4)°) than for BCF
(40.6(3)°). However, these determined values for the torsional
angle are highly dependent of the distinct dynamics of the
molecule and their least-square errors should not be considered
to represent a limit range of the torsional angles.
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Figure 3. Gas-phase structures of BCF (left) and BTolF (right) and corresponding experimental (circles O) and model (solid red line) radial distribution functions of
BCF and BTolF. The difference curves are shown below. Vertical bars indicate interatomic distances in the modelled geometry.
A suitable correction for vibrational effects was attempted but
failed due to physically insensibly large correction terms calcula-
ted by the usually applied procedures.[26] A modeling of this
molecular dynamics was not attempted, as the three torsions of
the C6F4 groups plus three of the CF3 groups are highly correla-
ted and would thus require at least a six-dimensional treatment,
which is beyond the present resources.
We demonstrated this also for a set of simplified model com-
pounds (p-R1C6F4)BR22 [R1 = F, CF3; R2 = H, F; TPSS/6-
311G(2d) avoiding the complications of correlated aryl torsions]
by investigating the dependence of the B–C distance on the di-
hedral angle, which is fully consistent with the described beha-
vior of BCF and BTolF (for details see ESI).
To further investigate the effect of the conjugation between the
aromatic π-system on the barrier of rotation of the two different
perfluoroaryl groups we calculated the potential energy
(TPSS/6-311G(2d)[27]) for the rotation of the aryl groups for both
species under – a simplifying – maintenance of C3 and D3 sym-
metry of the corresponding minimum structures. These potential
energy curves show a 12 kJ mol–1 lower barrier of rotation at 90°
torsion for the perfluorotolyl substituents compared to the per-
fluorophenyl groups (Figure 4). This supports the experimental
findings of an apparently wider vibrationally averaged torsional
angle found for BTolF. Due to the steric interaction between the
ortho-fluorine atoms when decreasing the torsional angle to-
wards 0°, the energy rises dramatically. This results in a sub-
stantially unsymmetrical potential and therefore in an anharmo-
nic rotation of the perfluoroaryl groups.
In contrast, the quantum-chemically predicted structures
[TPSS/6-311G(2d)], represent minimum structures and do not
take zero-point vibrations into account. They suggest no differ-
rence in the magnitude of the torsional angle C(2)–C(1)–B(1)–
C(1)’ for BCF (38.0°) and BTolF (38.2°). They also predict very
similar B–C distances at 1.566 Å and 1.568 Å, respectively. In
contrast, the B–C distance of BCF (rh1 = 1.546(3) Å) determined
in the electron diffraction experiment is shorter than that determi-
ned for BTolF (rh1 =1.564(2) Å). This reflects again vibrational
averaging of a real system with elongating B–C distances when
the molecule performs its aryl group torsions. At larger torsional
angles the π-interaction between the perfluorotolyl group and
the empty boron p-orbital is decreased and thus the B–C bonds
become elongated.
Figure 4. Calculated potential energy (TPSS/6-311G(2d)) of the D3 symmetric
minima of BCF and C3 symmetric minima of BTolF in dependence of predefi-
ned dihedral angles between the aryl planes and the BC3 plane.
In essence, the synthesis and isolation of tris(perfluorotolyl)bo-
rane (BTolF) gives access to a new boron Lewis acid. It is 9%
stronger Lewis acidic than BCF (Gutmann-Beckett method) and
has a 10% higher calculated fluoride ion affinity (FIA) than BCF.
A higher FIA than SbF5 characterizes BTolF as Lewis superacid,
and to the best of our knowledge the most Lewis acidic single-
site triorganoborane. The clearly higher electron withdrawal of
the perfluorotolyl groups in the absence of an electron donation
ability of the para-fluorine group has observable structural con-
sequences in gas-phase experiments. It makes in particular the
torsional/vibrational behavior of BCF and BTolF different. Being
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scope of application in many fields of chemistry.
Acknowledgement
We thank Klaus-Peter Mester, Gerd Lipinski and Dr. Andreas
Mix for NMR spectra, Brigitte Michel for elemental analyses and
Dr. Jens Sproß, Heinz-Werner Patruck and Sandra Heitkamp for
mass spectra. We acknowledge support from the Deutsche
Forschungsgemeinschaft for the Core Facility GED@Bi (MI-
477/21-1). Calculations were performed on resources provided
by Regionales Rechenzentrum der Universität zu Köln (RRZK)
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10.1002/anie.201704097
Angewandte Chemie International Edition
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FULL PAPER
Superacidic tris(tetrafluoro-4-(trifluoro-
methyl)phenyl)borane (BTolF) was pre-
pared from BBr3 and F3CC6F4-Cu(I). It is
a stonger Lewis acid than tris(pentafluo-
rophenyl)borane (BCF) and has a high-
er fluoride ion affinity than SbF5. The
structures of BTolF and – for the first
time – of the widely used BCF were de-
termined by gas electron diffraction and
reflect the different electronic situations.
L. A. Körte, J. Schwabedissen, M.
Soffner, S. Blomeyer, C. G. Reuter,
Yu. V. Vishnevskiy, B. Neumann, H.-
G. Stammler and N. W. Mitzel*
Page No. – Page No.
Tris(perfluorotolyl)borane – a
Boron Lewis Superacid
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