Kinetic and mechanistic studies of the transformation of the catalyst, tris(pentafluorophenyl)borane, in the presence of silyl and germyl hydrides

Article abstract of DOI:10.1016/j.jcat.2019.09.023

Tris(pentafluorophenyl)borane catalyzed Si-H bond activation opens the door to numerous transition-metal-free reduction processes and is widely used in organic and polymer chemistry. However, chemical stability of B(C₆F₅)₃ in the presence of silyl hydrides is limited, which can strongly affect its catalytic activity. Transformations of B(C₆F₅)₃ in the presence of phenyldimethylsilane, triethylsilane and triethylgermane were studied by ¹⁹F NMR and UV spectroscopy, GC/MS and quantum-mechanical calculations. The observed exchange of pentafluorophenyl group attached to boron to hydrogen results in the formation of bis(pentafluorophenyl)borane, which has a strongly reduced ability to activate the Si-H bond. The substitution kinetics were studied by following the disappearance of absorption of the B(C₆F₅)₃ charge transfer peak in the UV spectrum. Complementary quantum mechanical calculations allowed us to propose a mechanism of the ligand exchange reaction, which involves electrophilic substitution of the pentafluorophenyl group through a four-center transition state.

Full text of DOI:10.1016/j.jcat.2019.09.023

Journal of Catalysis 379 (2019) 90–99
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Kinetic and mechanistic studies of the transformation of the catalyst, tris
(pentafluorophenyl)borane, in the presence of silyl and germyl hydrides
⇑
Slawomir Rubinsztajn , Julian Chojnowski, Marek Cypryk, Urszula Mizerska, Witold Fortuniak,
Irena I. Bak-Sypien
Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 112 Sienkiewicza, 90-363 Lodz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Tris(pentafluorophenyl)borane catalyzed Si-H bond activation opens the door to numerous transition-
metal-free reduction processes and is widely used in organic and polymer chemistry. However, chemical
stability of B(C6F5)3 in the presence of silyl hydrides is limited, which can strongly affect its catalytic
activity. Transformations of B(C6F5)3 in the presence of phenyldimethylsilane, triethylsilane and triethyl-
germane were studied by 19F NMR and UV spectroscopy, GC/MS and quantum-mechanical calculations.
The observed exchange of pentafluorophenyl group attached to boron to hydrogen results in the forma-
tion of bis(pentafluorophenyl)borane, which has a strongly reduced ability to activate the Si-H bond. The
substitution kinetics were studied by following the disappearance of absorption of the B(C6F5)3 charge
transfer peak in the UV spectrum. Complementary quantum mechanical calculations allowed us to pro-
pose a mechanism of the ligand exchange reaction, which involves electrophilic substitution of the
pentafluorophenyl group through a four-center transition state.
Received 18 July 2019
Revised 10 September 2019
Accepted 12 September 2019
Keywords:
Stability of tris(pentafluorophenyl)borane
B(C6F5)3
Catalyst, Ligand exchange
Silyl hydrides
Kinetics
Dehydrocarbonative condensation
Quantum mechanical calculations
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
mers [21], branched polymers [22], cyclic polymers [23], and in the
modification of polysiloxanes [24]. TPFPB is used as an initiator of
Catalysis by tris(pentafluorophenyl)borane (TPFPB) of various
reactions with the participation of silyl hydrides has been widely
practiced in organic chemistry and organosilicon chemistry. TPFPB
is strong Lewis acid, which effectively activates the Si-H bond in
processes involving hydride ion transfer [1,2]. It is very effective
in promoting the reduction of many functional organic com-
pounds, such as alcohols, ketones, aldehydes, amides, acyl chlo-
rides, imines, carboxylic acids and esters, by hydrosilanes [3–6].
It also promotes rearrangement processes such as reductive carbo-
cyclization [7]. Many silyl derivatives may be obtained by TPFPB
catalyzed hydrosilylation of various organic functions [8–10].
Recent advances in frustrated Lewis pairs open up a new area in
organic synthesis involving Si-H bond activation by TPFBP [2,11].
Silanol and alcohol silylation by SiH functional reagents catalyzed
by this borane are important processes in polysiloxane chemistry
[12,13], which are used for modification of silica, silicon and car-
bon nanotubes surfaces [14–16]. The catalysis of dehydrocarbona-
tive condensation by TPFPB, known as the Piers-Rubinsztajn
reaction, found broad application in polysiloxane chemistry [17–
19]. It is practiced in syntheses of siloxane copolymers [20], oligo-
the ring-opening polymerization of cyclic siloxanes by hydride
transfer mechanism [25], and in oligomerization processes of
hydrosiloxanes [26]. The dehydrocarbonative condensation of
hydrosilanes with alkyl esters of boric acid promoted by TPFBP
was also reported [27].
Reactions involving germanium hydrogen bond activation by B
(C6F5)3 are known, although they are not so common as those with
the SiAH bond activation. Gevorgian et al. reported selective
germylation of alkyne with trialkylgermanes and triphenylger-
mane, which is a promising route to germylated olefins [28]. An
interesting feature of this reaction is that it proceeds in the pres-
ence of carboxyl and ether groups in alkyne reagents leaving them
untouched. This observation proves much lower oxophilicity of
germanium as compared with silicon. Olefin hydrogermylation
has recently been reported using cyclohexa-2,5-dien-1-yl-substitu
ted germanes. These compounds under the action of TPFPB and in
the presence of an olefin, liberated hydrogermanes, which in situ
hydrogermylated the olefin [29]. It was shown that the dehydro-
carbonative condensation between trialkylgermanium hydrides
and alkoxysilanes catalyzed by TPFPB occurs analogously to the
Piers-Rubinsztajn process and leads to products with SiOGe bonds
[30]. The condensation of trialkylsilanes with some alkoxyger-
manes occurred as well, but with very low yields. By contrast, it
⇑
Corresponding author.
E-mail address: srubin@cbmm.lodz.pl (S. Rubinsztajn).
https://doi.org/10.1016/j.jcat.2019.09.023
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
91
has recently been found that the reaction of phenyldimethylsilane
with tetrabutoxygermane in the presence of TPFPB leads exclu-
sively to the reactive group exchange i.e. to the substitution of
alkoxy group at germanium with hydrogen [31].
sample to remove traces of water. The presence of dry borane
was confirmed by recording 19F NMR spectra. The desired amount
of silicon or germanium reagent was then introduced, typically
about a 50-fold molar excess, compared to the borane. The NMR
tube was sealed and 19F NMR spectra were taken at desired time
intervals. The chemical shifts were reported relative to the CFCl3
standard.
The rapid increase in the use of hydrosilanes and hydroger-
manes in combination with catalytic amounts of B(C6F5)3 in syn-
thetic chemistry poses the question about the chemical stability
of these systems. In our experimental works, we observed that
the catalytic ability of TPFPB is decreasing over time in the pres-
ence of reagents containing SiH and GeH moieties. Synthetic che-
mists should be conscious of the reactions of the borane with
hydrosilanes and hydrogermanes and should know to what extent
they affect catalytic activity of TPFPB. The knowledge on the reac-
tions between TPFPB and SiH functional compounds is scarce. It is
known that the reaction leads to the exchange of one of the aro-
matic groups attached to boron to hydrogen. This reaction was
used for the generation of HB(C6F5)2, which was applied further
as hydroboration agent [32]. Theoretical calculation gave the free
energy barrier to this reaction of 27.8 kcal/mol [33]. To the best
of our knowledge, kinetic studies of reactions of B(C6F5)3 with Si-
H reagents and any studies of the interaction of TPFPB with GeH
reagents have not yet been reported. The aim of our research
was to gain more knowledge about reactions of B(C6F5)3 with
hydrosilanes and hydrogermanes, which result in a strong reduc-
tion of its catalytic activity.
2.4. Kinetic studies by UV spectroscopy
A mother solution of tris(pentafluorophenyl)borane in dry
toluene 0.033 mol/L was prepared and stored in dry box. A suitable
amount of this solution was diluted with dry toluene to obtain the
final B(C6F5)3 solution at a concentration of about 5.5 ꢁ 10ꢂ4 mol/L.
0.3 mL of the final solution was transferred to 1 mm thick UV
quartz cuvette. In the case of the germanium reagent, an excess
of dimethylphenylsilane was introduced to remove borane-bound
water. The presence of water-free B(C6F5)3 was confirmed by the
appearance of a strong UV band with a maximum at 305 nm. The
desired amounts of PhMe2SiH, Et3SiH or Et3GeH were then added
using a Hamilton syringe. The cuvette was closed with stopcock,
removed from dry box and placed in the Peltier temperature-
controlled changer set at a constant temperature. UV spectra were
recorded in a desired time interval using a UV–VIS spectrometer,
Specord S600 Zeiss Jena, Analytik Jena AG, Jena Germany, equipped
with Peltier temperature-controlled 8-cell changer.
2. Experimental section
2.5. Theoretical calculations
2.1. Materials
All quantum mechanical calculations were performed using
the Gaussian 16 suite of programs [34]. Geometries of the
reagents and complexes were optimized using the hybrid B3LYP
density functional [35] corrected for dispersion interactions using
Grimme GD3 empirical term [36], with Def2-SVP basis set [37] in
the gas phase. All stationary points were identified as stable min-
ima by frequency calculations. The vibrational analysis provided
thermal enthalpy and entropy corrections at 298 K within the
rigid rotor/harmonic oscillator/ideal gas approximation [34].
Thermochemical corrections were scaled by a factor of 0.985
[38]. More accurate single point electronic energies were
obtained using the B3LYP functional, including Grimme GD3 dis-
persion correction [36], with the larger Def2-TZVP basis set for
the Def2-SVP optimized geometries [39]. This level of theory is
denoted as B3LYP-GD3/Def2TZVP//Def2SVP. Integration grid was
set to ultrafine. The basis set superposition error (BSSE) has been
neglected since it is small (<0.5 kcal/mol) and the method of its
estimation [40] is not accurate enough to precisely calculate weak
interactions.
Triethylsilane and dimethylphenylsilane were products of
Aldrich, declared purities were 99% and ꢀ98% respectively. Tri-
ethylgermane, declared purity 97%, 1,1,3,3-tetramethyldisiloxane,
declared purity 99%, were purchased from ABCR. Their purity
was checked by gas chromatography and were used as received.
Poly(methylhydrosiloxane), Catalog #: AB112087, viscosity 15-
25cSt, molecular weight about 2000 g/mol, was purchased from
ABCR. Tris(pentafluorophenyl)borane was
a product of TCI,
declared purity > 97%. Its purity was confirmed by 19F NMR spec-
troscopy. Toluene HPLC Plus, a product of JT Baker, purity 99.9%,
was additionally dried on column system NBSPS of dry-box
MBRAVN model UNILAB. Toluene d8, a product of Deutero GmbH
of declared purity 99.5%, was stored over 3 Å molecular sieves.
2.2. Analytical methods
19F NMR spectra were registered on a Bruker AV III 500 MHz
spectrometer working at 470.54 MHz. Gas chromatography-mass
spectrometry GC/MS analysis was carried out using a Shimadzu
QP2010 ultra apparatus equipped with Zebron ZB-5MSi Capillary
3. Results and discussions
GC column (30 m ꢁ 0.25 mm ꢁ 0.25
lm). Carrier gas was helium.
3.1. Studies by 19F NMR spectroscopy
Temperature program was: hold at 50 °C for 3 min, heating to
250 °C at a rate 10 °C /min, hold at 250 °C for 20 min. Quadrupole
mass spectrometer, Shimadzu QP2010 Ultra, with electron ioniza-
tion was connected to the GC system. UV spectra were recorded
using a UV–VIS spectrometer, Specord S600 Zeiss Jena, Analytik
Jena AG, Jena Germany, equipped with Peltier temperature-
controlled 8-cell changer. A 1 mm quartz cuvette with a Rotaflo
stopcock was used.
Studies of B(C6F5)3 reaction with silyl and germyl hydrides were
carried out using dimethylphenylsilane, triethylsilane and triethyl-
germane as model reagents and toluene as solvent. 19F NMR spec-
troscopy is an effective method for the investigation of the
chemical transformation of fluoroarylboranes [41–43]. The reac-
tions between borane and PhMe2SiH, Et3SiH and Et3GeH were fol-
lowed directly in an NMR tube at a constant temperature. It is
known that the presence of water strongly reduces the catalytic
activity of TPFPB in various reactions, such as dehydrocarbonate
condensation, ring-opening polymerization of cyclic siloxanes by
a hydride transfer mechanism, and oligomerization of SiH func-
tional siloxanes due to the formation of strong hydrates [44,45].
2.3. Studies by 19F NMR
The solutions of the borane in toluene d8 were prepared in dry-
box and were placed in an NMR tube purged with dry nitrogen.
Small excess of 1,1,3,3-tetramethyldisiloxane was added to the

92
S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
Commercial TPFPB usually contains water in the form of strong
hydrates. Traces of water at the level of 10ꢂ4 – 10ꢂ3 mol/L always
remain even after very careful purification of the solvent and can
form these hydrates. It has been found that both free water and
borane-bound water are efficiently and rapidly removed by a SiH
reagent such as triethylsilane or phenyldimethylhydrosilane, pro-
ducing a small amount of disiloxane and hydrogen in the reaction
[44]. Thus, water is simply removed after mixing the borane with
PhMe2SiH or Et3SiH substrates, which are added in a large excess
to B(C6F5)3. The problem arises in reaction studies with Et3GeH,
which reacts slowly with water and produces additional by-
products revealed by GC–MS. To obtain reproducible results in
experiments with Et3GeH, traces of water were removed by intro-
ducing a small amount of SiH-containing compound into the bor-
ane solution before adding Et3GeH. 19F NMR spectrum of the
mixture of 55 mol eq. of PhMe2SiH with 1 mol eq. of TPFPB in
toluene at 30 °C was collected just after mixing both reagents,
Fig. S1A. Three sets of signals at d = ꢂ128.66 ppm (2Fortho),
nals, and their intensity increases over time, indicating the forma-
tion of two products containing pentafluorophenyl groups. The
signal at d = ꢂ126.61 ppm is attributed to Fortho of PhMe2Si(C6F5).
Corresponding Fpara and Fmeta signals are located at d = ꢂ149.7 ppm
and d = ꢂ161.27 ppm respectively, Fig. S1B. GC/MS analysis of the
final mixture confirmed the presence of PhMe2Si(C6F5) as the main
product containing silicon and pentafluorophenyl group, Figs. S2
and S3. The Fortho signals with chemical shift from d = ꢂ130.8 ppm
to ꢂ132.9 ppm belong to bis(pentafluorophenyl)borane, HB(C6F5)2.
The formation of H2B(C6F5) may also take place in the reaction
mixture as a result of the subsequent exchange process (Eqs. (1)
and (2)). Bis(pentafluorophenyl)borane appears as a mixture of
its monomer, dimer (C6F5)2B(l-H)2B(C6F5)2 [46] and mixed com-
plex (C6F5)2B( -H)2B(H)(C6F5) formed through an interaction of
l
B-H moieties [32]. The formation of an additional mixed complex
of HB(C6F5)2 with an excess of PhMe2SiH can also occur in the reac-
tion mixture, which would explain a larger number of signals in
this region of the spectrum. Quantum-mechanical calculations of
thermodynamics of these processes confirm the possibility of
occurrence of these interactions, Table 1.
Based on these results, it can be concluded that exchange of
pentafluorophenyl group for hydrogen occurs between the TPFPB
and PhMe2SiH according to Eq. (1). The subsequent replacement
of the pentafluorophenyl group in the just formed HB(C6F5)2 with
hydrogen is carried out according to Eq. (2).
d = ꢂ142.67 ppm (Fpara
)
and d = –160.20 ppm (2Fmeta
)
were
observed, which is consistent with the 19F NMR spectrum of pure
TPFPB [42]. The 19F NMR cumulative spectra of the same mixture
collected during the reaction, zoomed to the Fortho region, are
shown in Fig. 1.
The intensity of the Fortho signal of TPFPB at d = ꢂ128.66 ppm
decreases as the reaction proceeds. There are two sets of new sig-
Fig. 1. 19F NMR spectra (Fortho region) of the reaction mixture of Me2PhSiH [2.36 mol/L] and B(C6F5)3 [0.043 mol/L] in deuterated toluene at 30 °C.
Table 1
Enthalpies and Gibbs free energies [kcal/mol], entropies [cal/mol K], and approximate Keq of complex formation at 298 K of model silyl and germyl hydrides with B(C6F5)3 and HB
(C6F5)2 calculated by B3LYP-GD3/Def2TZVP//Def2SVP in the gas phase.
Reaction
D
H
D
G
D
S
Keq
Et3SiH + B(C6F5)3 ? Et3SiHꢃ ꢃ ꢃB(C6F5)3
ꢂ10.2
ꢂ11.2
ꢂ12.2
ꢂ7.9
4.9
3.6
3.5
3.3
ꢂ50.6
ꢂ49.5
ꢂ52.5
ꢂ37.5
ꢂ38.1
ꢂ41.8
ꢂ40.9
2.5 ꢃ 10ꢂ4
2.4 ꢃ 10ꢂ3
3.0 ꢃ 10ꢂ3
4 ꢃ 10ꢂ3
PhMe2SiH + B(C6F5)3 ? PhMe2SiHꢃ ꢃ ꢃB(C6F5)3
Et3GeH + B(C6F5)3 ? Et3GeHꢃ ꢃ ꢃB(C6F5)3
Me3SiH + HB(C6F5)2 ? Me3SiHꢃ ꢃ ꢃHB(C6F5)2
Me3GeH + HB(C6F5)2 ? Me3GeHꢃ ꢃ ꢃHB(C6F5)2
2 HB(C6F5)2 ? (C6F5)2BHꢃ ꢃ ꢃHB(C6F5)2
ꢂ8.9
2.4
1.6 ꢃ 10ꢂ2
3.4 ꢃ 104
5 ꢃ 105
ꢂ19.7
ꢂ20.0
ꢂ6.2
(C6F5)2BH + H2B(C6F5) ? (C6F5)2BHꢃ ꢃ ꢃH2B(C6F5)
ꢂ7.8

S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
93
PhMe2SiH + B(C6F5)3 ! PhMe2Si(C6F5) + HB(C6F5)2
PhMe2SiH + HB(C6F5)2 ! PhMe2Si(C6F5) + H2B(C6F5)
ð1Þ
ð2Þ
Figs. S4 and S5. Conversion of substrate and formation of Et3SiC6F5
are shown in Fig. 4. The reaction proceeds in analogous way to that
with PhMe2SiH, however the rate of the group exchange is much
lower. It was not until after 10 h of maintaining the reaction mix-
ture at 30 °C that distinct signals of the C6F5 group exchange pro-
duct appeared with chemical shifts at d = ꢂ127.3, ꢂ151.98 and
ꢂ161.52 ppm, Fig. 3.
Conversion of the borane and formation of both primary prod-
ucts according to Eq. (1) are shown in Fig. 2. Their concentrations
are calculated from integrations of respective Fortho signals. The
borane substrate is fully converted via exchange of pentafluo-
rophenyl group of the borane with hydrogen at silicon, which
results in the formation of bis(pentafluorophenyl)borane and
PhMe2SiC6F5. The concentration of HB(C6F5)2 reaches a plateau,
suggesting that this compound may undergo the subsequent reac-
tion of pentafluorophenyl group exchange according to Eq. (2).
An analogous reaction between TPFPB and Et3SiH at 60 °C lead-
ing to the formation of Et3SiC6F5 and HB(C6F5)2 was previously
reported [32]. However, kinetics of this reaction have never been
investigated. The course of the reaction between Et3SiH and TPFPB
is illustrated by 19F NMR cumulative spectra, Fig. 3.
The approximate values of the specific rate of the boron sub-
strate disappearance for reactions of PhMe2SiH and Et3SiH with
TPFPB at 30 °C estimated from plots in Figs. 2 and 4, assuming sec-
ond order kinetics, were 6.08ꢃ10ꢂ5 and 1.84ꢃ10ꢂ6 L/molꢃs respec-
tively, Figs. S6 and S7. These results show that the rate of the
group exchange reaction between TPFPB and silyl hydride depends
much on substituents at the silicon atom. In addition, the reaction
of TPFPB with Et3SiH is more complex than the analogous reaction
with PhMe2SiH. Unexpectedly, a sharp multiplet appeared at a very
high field, d = ꢂ175.66 ppm, JSi-F = 290 Hz, Fig. S8. This signal was
assigned to triethylfluorosilane because the observed chemical
shift and spin–spin coupling values fit well with the literature data
for this compound [47,48]. The triethylfluorosilane is formed in
The formation of Et3SiC6F5 as the main product of the pentaflu-
orophenyl group exchange process was confirmed by GC/MS,
Fig. 2. Substrate conversion and products concentrations vs. time for reaction of
Me2PhSiH [2.36 mol/l] with B(C6F5)3 [0.043 mol/L] in deuterated toluene at 30 °C. B
(C6F5)3 – (e), PhMe2SiC6F5 – (s), HB(C6F5)2 – (h).
Fig. 4. Substrate and products concentrations vs. time for reaction of Et3SiH
[2.32 mol/L] with B(C6F5)3 [0.047 mol/L] in deuterated toluene at 30 °C. Substrate –
(e), Et3SiC6F5 – (s); Et3SiF – (h).
Fig. 3. 19F NMR spectra of the reaction mixture of Et3SiH [2.32 mol/L] and B(C6F5)3 [0.047 mol/L] in deuterated toluene at 30 °C.

94
S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
parallel with the pentafluorophenyl group exchange product,
Fig. 4. The competitive cleavage of fluorine from the TPFPB occurs
according to the reaction shown in Eq. (3). This competitive reac-
tion results in a more complex form of the 19F NMR spectrum of
the reaction mixture at high TPFPB conversion, Fig. 3. However,
the fluorine cleavage is significantly slower than the exchange of
pentafluorophenyl group with hydrogen between TPFPB and
Et3SiH, which occurs in an analogous way as shown in Eq. (1).
The intensity of 19F NMR signals associated with TPFPB
decreases as the reaction with Et3GeH progresses and two sets of
new signals appear indicating the formation of two main products
containing pentafluorophenyl groups, Fig. 5. The obtained results
revealed that the predominant germanium containing product
of this reaction giving signals at Fortho d = ꢂ126.8 ppm,
Fpara d = ꢂ153.1 ppm, and Fmeta d = ꢂ161.6 ppm is triethyl(penta
fluorophenyl)germane. The presence of this product was confirmed
by GCMS analysis, Figs. S10 and S11. 19F NMR signals of the other
fluorine product, Fortho d = ꢂ131.5 to ꢂ133.5 ppm, point to the
formation of bis(pentafluorophenyl)borane, Fig. 5. As discussed
earlier, this compound exists as a mixture of its monomer, dimer
and a complex with Et3GeH. At least in the first stage, the exchange
of pentafluorophenyl group for hydrogen between the boron and
germanium reagents is carried out according to Eq. (5).
Et3SiH + B(C6F5)3 ! Et3SiF + B(C6F5)2(C6F4H)
ð3Þ
Comparative studies on the reaction of SiH and GeH reagents
with TPFPB were carried out using triethylsilane and triethylger-
mane as model compounds. The reactions of Et3SiH and Et3GeH
with TPFPB are somewhat similar to each other, however the reac-
tion of the germanium reagent is much faster than the reaction of
the silicon analogue. The cumulative spectra illustrating the reac-
tion of Et3GeH performed at the 50:1 of the Ge/B molar ratio,
together with the spectrum of the borane solution after removal
of water by reaction with 1,1,3,3-tetramethyldisiloxane, are dis-
played in Fig. 5 and Fig. S9. Addition of Et3GeH to the borane solu-
tion in deuterated toluene at 30 °C causes a noticeable upfield shift
of all the three signals of pentafluorophenyl group; Fortho from
d = ꢂ128.7 to ꢂ130.3 ppm, Fpara from d = ꢂ142.0 to ꢂ148 ppm,
Fmeta from d = ꢂ160.1 to ꢂ161.6 ppm. These shifts reflect a strong
interaction between the borane and germanium reagents thus
giving evidence of a partial transfer of hydride ion to boron, Eq. (4).
Et3GeH + B(C6F5)3 ! Et3Ge(C6F5) + HB(C6F5)2
ð5Þ
Quantum mechanical calculation of the thermodynamics of this
reaction confirmed the feasibility of this process, Table 2. The cal-
culated Gibbs free energies values of the exchange reactions sug-
gest only partial conversion of B(C6F5)3 if monomeric HB(C6F5)2
product is assumed. However, it should be remembered that
monomeric HB(C6F5)2 shows a strong tendency to form a dimer
and may also participate in the formation of adducts with an
excess of silyl or germyl hydrides, Table 1. Due to free HB(C6F5)2
consumption, the equilibrium of ligand exchange reaction is con-
tinuously shifting towards the group exchange products.
ꢂ
Et3GeH + B(C6F5)3 ! Et3Gedþ...Hd ...B(C6F5)3
ð4Þ
Based on the calculated free energy barriers, Table 2, it can be
concluded that the reactivity in the exchange process decreases
in order Et3GeH > PhMe2SiH > Et3SiH, which is consistent with
This observation agrees with our theoretical calculations of
thermodynamic parameters for these processes, Table 1.
Fig. 5. 19F NMR spectra (Fortho region) of the reaction mixture of Et3GeH [1.96 mol/L], B(C6F5)3 [0.039 mol/L] in deuterated toluene at 30 °C.
Table 2
Enthalpies [kcal/mol], Gibbs free energies [kcal/mol], entropies [cal/molꢃK], and approximate equilibrium constants Keq at 298 K for the exchange reactions of model silyl and
germyl hydrides with B(C6F5)3 calculated at B3LYP-GD3/Def2TZVP//Def2SVP level in the gas phase.
Reaction
D
H
D
G
D
S
Keq
D
Hà
D
Gà
D
Sà
Et3SiH + B(C6F5)3 ? Et3SiC6F5 + HB(C6F5)2
PhMe2SiH + B(C6F5)3 ? PhMe2SiC6F5 + HB(C6F5)2
Et3GeH + B(C6F5)3 ? Et3GeC6F5 + HB(C6F5)2
0.0
1.3
1.9
ꢂ0.6
0.8
1.3
2.0
1.8
2.1
2.8
0.3
0.1
17.1
13.2
12.7
32.8
29.2
28.4
ꢂ52.6
ꢂ53.5
ꢂ52.5

S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
95
Table 3
Specific rates, k [L/molꢃs] and activation parameters
D
Gà [kcal/mol],
D
Hà[kcal/mol] and
k
D
Sà [cal/molꢃK] at 298 K of the reactions of B(C6F5)3 with Et3SiH, Et3GeH and PhMe2SiH in
toluene determined by UV measurements.
Reaction
D
Gà
D
Hà
D
Sà
Et3SiH + B(C6F5)3 ? Et3SiC6F5 + HB(C6F5)2
PhMe2SiH + B(C6F5)3 ? PhMe2SiC6F5 + HB(C6F5)2
Et3GeH + B(C6F5)3 ? Et3GeC6F5 + HB(C6F5)2
1.36 ꢃ 10ꢂ6
6.98 ꢃ 10ꢂ5
1.97 ꢃ 10ꢂ4
25.4
23.1
22.4
14.4
10.8
12.1
ꢂ36.9
ꢂ41.3
ꢂ34.7
Fig. 6. Substrate conversion and product concentration vs. time for the reaction of B(C6F5)3 [0.039 mol/L] with Et3GeH [1.96 mol/L] in deuterated toluene at 30 °C. Substrate –
(e), (Et3GeC6F5) – (s).
kinetic measurements. The calculated values of entropy of activa-
tion are much lower than the values obtained from kinetic studies,
Table 3, mainly because the calculations were made for the gas
phase and the solvation effects were omitted.
3.2. Studies by UV spectroscopy
The UV spectrum of B(C6F5)3 in a toluene solution shows an
intensive charge transfer band with
a maximum located at
The high intensity of signals of Et3Ge(C6F5) persists after 22 h
while signals of HB(C6F5)2 become much smaller, Fig. S9. This indi-
cates that HB(C6F5)2 is not stable in the reaction system as was pre-
viously observed for silyl hydrides.
Conversion of TPFPB and concentrations of the major germa-
nium product containing C6F5 group calculated based on integra-
tion of Fortho signals of relevant C6F5 group as a function of time
are shown in Fig. 6. The approximate value of the rate constant
for the reaction according to Eq. (5) and assuming its simple sec-
ond order kinetics is 1.46 ꢃ 10ꢂ4 L/molꢃs at 30 °C, Fig. S12.
305 nm resulting from the interaction of pentafluorophenyl
p-
electrons with an empty boron p orbital [44]. The absorbance at
the maximum of this band is directly related to the concentration
of free TPFPB. The kinetics of the TPFPB reactions with PhSiMe2H,
Et3SiH and Et3GeH were studied by following changes in this
absorbance, Fig. S13. A large excess of SiH and GeH reagents over
borane was used to observe the reactions as pseudo first order pro-
cesses. Indeed, for all studied reagents, conversion-time curves
adhere well to the first order kinetics, although in some cases the
Fig. 8. Dependences of the experimental pseudo 1st order rate constant (k’) of
reaction of the pentafluorophenyl group exchange of B(C6F5)3 on the initial
concentration of SiH and GeH reagents (Co) in toluene: Et3SiH – (j) at 80 °C and
Fig. 7. Pseudo 1st order kinetic plots for reaction the pentafluorophenyl group
exchange of B(C6F5)3 with SiH and GeH reagents: Et3SiH – (j), PhMe2SiH – (Ο),
Et3GeH – (
D
), in toluene at 60 °C.
PhMe2SiH – (Ο), Et3GeH – (D) at 60 °C.

96
S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
reactions were followed to over 80% conversion. Representative
first-order kinetic graphs of TPFPB conversion in the presence of
Et3SiH, PhMe2SiH and Et3GeH at 60 °C are shown in Fig. 7.
The dependence of the borane conversion rate on the initial
concentration of SiH and GeH reagents indicates that in these cases
the reaction rate may be well approximated by the second order
kinetics law expressed by Eq. (6), Fig. 8.
d[B(C6F5)3]/dt = k[Mt-H][B(C6F5)3], (k = k1 + k2 + ...)
ð6Þ
where [Mt-H] is concentration of SiH and GeH reagents and k1 is the
rate constant of the pentafluorophenyl group exchange. The specific
rate constant (k) includes also rate constants of competing reac-
tions, such as fluorine cleavage. However, these competitive reac-
tions are much slower than the exchange of ligands between SiH
or GeH and TPFPB and have been neglected in our kinetic analysis.
The measurement of specific rates of the reactions studied at vari-
able temperatures allowed us to create corresponding Arrhenius
plots, Fig. 9, and assess the value of their activation parameters,
Table 3.
Fig. 9. Arrhenius plots of the reaction of the pentafluorophenyl group exchange of B
(C6F5)3 with Et3SiH – (j), PhMe2SiH – (Ο) and Et3GeH – (D) in toluene.
The borane reaction with PhMe2SiH proceeds faster than with
Et3SiH under the same conditions by a factor of about 30. The
observed difference in the rate of TPFPB conversion is due to the
higher activation energy of the borane reaction with of Et3SiH. This
observation shows that the stability of tris(pentafluorophenyl)bo-
rane mixed with the silyl hydride depends to a large extent on
the structure of the hydrosilane reagent. We also measured the ini-
tial conversion rates and half-life of TPFPB in the presence of
selected silyl hydrides and hydrosiloxanes. Currently, these sys-
tems are commonly used in organic and organosilicon chemistry.
The obtained data may be useful for synthetic chemists interested
in using TPFPB / SiH system as a mild reducing reagent in the syn-
Table 4
Specific rates of conversion of TPFPB (k0) and its half-life (t0.5) in the presence of
various SiH reagents in toluene at 60 °C. Initial concentration of SiH was 0.5 mol/L and
[SiH]0/[TPFPB]0 = 7 ꢃ 102.
Reagent
k0/sꢂ1
t0.5/h
(HMe2Si)2O
PhMe2SiH
Me3Si(OSiHMe)nOSiMe3
Et3SiH
5.5 ꢃ 10ꢂ4
2.82 ꢃ 10ꢂ4
1.55 ꢃ 10ꢂ5
1.00 ꢃ 10ꢂ5
0.35
0.68
12.4
19.2
a
a
Poly(hydromethylsiloxane) (PHMS), viscosity = 15-25cSt, molecular weight
about 2000 g/mol.
Fig. 10. Four-center transition state for the reaction of ligand exchange between B(C6F5)3 and Et3GeH; dashed lines mark forming/breaking bonds to germanium. Calculated
at B3LYP-GD3/Def2TZVP//Def2SVP. Interatomic distances: r(Ge-H) = 2.115 Å, r(H-B) = 1.229 Å. For comparison, in Et3Ge-H-B(C6F5)3 complex the corresponding bonding
parameters are r(Ge-H) = 1.637 Å, r(H-B) = 1.514 Å, Fig. S15.

S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
97
thesis, Table 4.
3.3. Reaction mechanism
From the mechanistic point of view, the exchange of ligand
between B(C6F5)3 and SiH or GeH reagents proceeds through elec-
trophilic aromatic substitutions. Support of this statement was
obtained using quantum–mechanical calculations. The proposed
mechanism involves a coordination of the borane to hydrogen,
Table 1, which causes a weakening of the Mt-H bond, where Mt = Si
or Ge, Figs. S14 and S15. As a result of such interaction, the partial
positive charge on the metal atom increases. The proposed borane-
hydrosilane adduct have been recently isolated and characterized
for 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene
and triethylsilane system [49]. At these conditions, the elec-
trophilic substitution in the pentafluorophenyl ring becomes possi-
ble through a four-center transition state, as shown for Et3GeH in
Fig. 10, which leads to the intermediate arenium ion complex,
Fig. 11.
The developing positive charge is delocalized throughout the
aromatic ring. In a situation of high electron deficiency, the
fluorine substituents, which are generally considered electron-
withdrawing, can stabilize the aromatic cation by means of elec-
tron donation via resonance. Decomposition of the intermediate
complex results in the aryl group migration from boron to germa-
nium center. An analogous four-center transition state for the
TPFPB reaction with Et3SiH is shown in Fig. S16. However, the
intermediate complex could not be located in the case of Et3SiH.
It is probably due to a very flat potential energy surface, which
Fig. 11. Intermediate arenium ion complex for the reaction of ligand exchange
between B(C6F5)3 and Et3GeH. Calculated at B3LYP-GD3/Def2TZVP//Def2SVP.
Interatomic distances r(Ge-C) = 2.202 Å, r(C-B) = 1.775 Å (compared to the regular
B-C6F5 bond of 1.633 Å).
Fig. 12. Four-center transition state for ligand exchange between PhMe2SiH and B(C6F5)3. Dashed lines mark forming/breaking bonds to silicon. Phenyl ring and one of C6F5
rings at boron are roughly parallel, which suggests some interaction. Calculated at B3LYP-GD3/Def2TZVP//Def2SVP.
p-p

98
S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
makes the optimization difficult. The rate of Et3GeH reaction with
TPFPB is two orders of magnitude higher than its silicon analogue,
Et3SiH. Such a significant difference in the rate of these reactions
results from the difference of their energy barriers, Table 3. The
quantum mechanical calculations lead to a similar conclusion,
Table 2.
Acknowledgments
The authors are grateful to Wacker Chemie AG for financial sup-
port and permission to publish their results. DFT calculations were
supported by the PL-Grid infrastructure. We thank Dr. Torsten
Gottschalk-Gaudig for valuable discussions.
Since germanium is more electropositive than silicon, the
hydrogen transfer to borane in the complex is more advanced
and therefore the substitution reaction proceeds more easily.
Entropy of activation for TPFPB reactions with ethyl substituted sil-
icon and germanium reagents are similar. The slightly higher
entropy for the germanium substrate results from the less compact
structure of the transition state, which leads to easier rotational
movements, Fig. 10.
The reaction of phenyldimethylsilane with TPFPB shows a lower
enthalpy of activation as compared to the same reaction of tri-
ethylsilane, and a much lower entropy of activation. These results
can be explained by the interaction between the phenyl group and
the pentafluorophenyl ligand in the complex of B(C6F5)3 with
PhMe2SiH, Fig. S17, which lowers the energy barrier for the ligand
exchange reaction, but at the same time reduces the freedom of
movement in the proposed transition state. Indeed, the calculated
optimal transition state structure, Fig. 12 and the intermediate
complex, Fig. S18, for the reaction of TPFPB with PhMe2SiH shows
that one of the pentafluorophenyl groups of the borane adopts
roughly parallel position to the phenyl group of the PhMe2SiH thus
Appendix A. Supplementary material
Supplementary materials, which include 19F NMR spectra, GC/
MS data, kinetic plots, UV Spectra of the reaction mixture B
(C6F5)3 and Et3GeH. Calculated structures of complexes between
B(C6F5)3 and Et3SiH, B(C6F5)3 and Et3GeH, B(C6F5)3 and PhMe2SiH.
Calculated structure of four-center transition state for the reaction
of ligand exchange between B(C6F5)3 and Et3SiH. Calculated struc-
ture of the complex between B(C6F5)3 and PhMe2SiH via Siꢃ ꢃ ꢃHꢃ ꢃ ꢃB
interaction. Cartesian coordinates of the SiH and GeH reagents, and
the Si and Ge adducts with B(C6F5)3 and transition states for the
exchange reactions. Supplementary data to this article can be
found online at https://doi.org/10.1016/j.jcat.2019.09.023.
References
[1] W. Piers, A. Marwitz, L. Mercier, Mechanistic aspects of bond activation with
perfluoroarylboranes, Inorg. Chem. 50 (2011) 12252–12262.
[2] M. Oestreich, J. Hermeke, J. Mohr, A unified survey of Si-H and H-H bond
activation catalysed by electron-deficient boranes, Chem. Soc. Rev. 44 (2015)
2202–2220.
making possible the p-p interaction between both phenyl rings.
[3] V. Gevorgyan, M. Rubin, S. Benson, J. Liu, Y. Yamamoto, A novel B(C6F5)3-
catalyzed reduction of alcohols and cleavage of aryl and alkyl ethers with
hydrosilanes, J. Org. Chem. 65 (2000) 6179–6186.
[4] V. Gevorgyan, M. Rubin, J. Liu, Y. Yamamoto, A direct reduction of aliphatic
aldehyde, acyl chloride, ester, and carboxylic functions into a methyl group, J.
Org. Chem. 66 (2001) 1672–1675.
4. Conclusions
The chemical stability of tris(pentafluorophenyl)borane is
reduced in the presence of Si-H and Ge-H reagents. Undesirable
transformations of TPFPB via metathetic exchange of hydrogen at
silicon or germanium with pentafluorophenyl group of the borane
result in the formation of bis(pentafluorophenyl)borane. Kinetic
studies confirmed by theoretical calculations showed that these
reactions strongly affect catalytic activity of the borane. The TPFPB
decomposition reactions studied for selected silyl hydrides (tri-
ethylsilane and phenyldimethylsilane) and triethylgermane occur
according to the second order kinetic law. Their mechanism
involves electrophilic substitution in the pentafluorophenyl group
via four-centered transition state. The experimentally determined
reactivity order is Et3GeH > PhMe2SiH ꢄ Et3SiH. The same order
is deduced from the theoretical calculations made for these reac-
tions in the gas phase. Substituents at silicon have considerable
impact on the reactivity. Significantly lower entropy of activation
for the TPFPB reaction with PhMe2SiH results from stacking inter-
action between the phenyl group of the silane reagent and the
pentafluorophenyl group of the borane in the transition state. This
interaction lowers the energy barrier and limits the freedom of
movement within the transition state structure. Currently, various
SiH functional silanes in combination with catalytic amounts of
TPFPB are frequently used as transition-metal-free reducing sys-
tems in the organic and polymer chemistry. The obtained rates of
decomposition of TPFPB in the presence of various SiH reagents
may be helpful for synthetic chemists.
[5] R. Chadwick, V. Kardelis, P. Lim, A. Adronov, Metal-free reduction of secondary
and tertiary N-phenyl amides by tris(pentafluorophenyl)boron-catalyzed
hydrosilylation, J. Org. Chem. 79 (2014) 7728–7733.
[6] V. Fasano, J. Radcliffe, M. Ingleson, B(C6F5)3-catalyzed reductive amination
using hydrosilanes, ACS Catal. 6 (2016) 1793–1798.
[7] T.A. Bender, J.A. Dabrowski, H. Zhong, M.R. Gagne, Diastereoselective B(C6F5)3-
catalyzed reductive carbocyclization of unsaturated carbohydrates, Org. Lett.
18 (2016) 4120–4123.
[8] D. Parks, W. Piers, Tris(pentafluorophenyl)boron-catalyzed hydrosilation of
aromatic aldehydes, ketones, and esters, J. Am. Chem. Soc. 118 (1996) 9440–
9441.
[9] D. Parks, J. Blackwell, W. Piers, Studies on the mechanism of B(C6F5)3-catalyzed
hydrosilation of carbonyl functions, J. Org. Chem. 65 (2000) 3090–3098.
[10] Y. Ma, B. Wang, L. Zhang, Z. Hou, Boron-catalyzed aromatic C-H bond silylation
with hydrosilanes, J. Am. Chem. Soc. 138 (2016) 3663–3666.
[11] V. Fasano, J. Radcliffe, L. Curless, M. Ingleson, N-methyl-benzothiazolium salts
as carbon lewis acids for Si-H sigma-bond activation and catalytic (De)
hydrosilylation, Chem. Eur. J. 23 (2017) 187–193.
[12] D. Zhou, Y. Kawakami, Tris(pentafluorophenyl)borane as a superior catalyst in
the synthesis of optically active SiO-containing polymers, Macromolecules 38
(2005) 6902–6908.
[13] J. Cella, S. Rubinsztajn, Preparation of polyaryloxysilanes and
polyaryloxysiloxanes
by
B(C6F5)3
catalyzed
polyetherification
of
dihydrosilanes and bis-phenols, Macromolecules 41 (2008) 6965–6971.
[14] D.H. Flagg, T.J. McCarthy, Rapid and clean covalent attachment of
methylsiloxane polymers and oligomers to silica using B(C6F5)3 catalysis,
Langmuir 33 (2017) 8129–8139.
[15] J. Escorihuela, S. Pujari, H. Zuilhof, Organic monolayers by B(C6F5)3-catalyzed
siloxanation of oxidized silicon surfaces, Langmuir 33 (2017) 2185–2193.
[16] R. Chadwick, J. Grande, M. Brook, A. Adronov, Functionalization of single-
walled carbon nanotubes via the Piers-Rubinsztajn reaction, Macromolecules
47 (2014) 6527–6530.
[17] M.A. Brook, J.B. Grande, F. Ganachaud, New synthetic strategies for structured
silicones using B(C6F5)3, Adv. Polym. Sci. 235 (2010) 161–183.
[18] M. Brook, New control over silicone synthesis using SiH chemistry: the Piers-
Rubinsztajn reaction, Chem. Eur. J. 24 (2018) 8458–8469.
[19] F. Vidal, F. Jäkle, Functional polymeric materials based on main-group
elements, Angew. Chem. Int. Ed. Engl. 58 (2019) 5846–5870.
Declaration of Competing Interest
[20] S. Rubinsztajn, J. Cella, A new polycondensation process for the preparation of
polysiloxane copolymers, Macromolecules 38 (2005) 1061–1063.
[21] D. Thompson, M. Brook, Rapid assembly of complex 3D siloxane architectures,
J. Am. Chem. Soc. 130 (2008) 32–33.
[22] J. Chojnowski, S. Rubinsztajn, W. Fortuniak, J. Kurjata, Synthesis of highly
branched alkoxsiloxane-dimethylsiloxane copolymers by nonhydrolytic
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. The authors declare no competing financial
interest.

S. Rubinsztajn et al. / Journal of Catalysis 379 (2019) 90–99
99
dehydrocarbon polycondensation catalyzed by tris(pentafluorophenyl)borane,
Macromolecules 41 (2008) 7352–7358.
[23] J. Yu, Y. Liu, Cyclic polysiloxanes with linked cyclotetrasiloxane subunits,
Angew. Chem. Int. Ed. Engl. 56 (2017) 8706–8710.
[24] M. Gretton, B. Kamino, M. Brook, T. Bender, The use of Piers-Rubinsztajn
conditions for the placement of triarylamines pendant to silicone polymers,
Macromolecules 45 (2012) 723–728.
[25] J. Chojnowski, J. Kurjata, W. Fortuniak, S. Rubinsztajn, B. Trzebicka, Hydride
transfer ring-opening polymerization of a cyclic oligomethylhydrosiloxane.
Route to a polymer of closed multicyclic structure, Macromolecules 45 (2012)
2654–2661.
[26] J. Chojnowski, W. Fortuniak, J. Kurjata, S. Rubinsztajn, J. Cella, Oligomerization
of hydrosiloxanes in the presence of tris(pentafluorophenyl) borane,
Macromolecules 39 (2006) 3802–3807.
[27] S. Rubinsztajn, New facile process for synthesis of borosiloxane resins, J. Inorg.
Organomet. Polym. Mater. 24 (2014) 1092–1095.
[36] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent and accurate ab initio
parametrization of density functional dispersion correction (DFT-D) for the 94
elements H-Pu, J. Chem. Phys. 132 (2010), 154104/154101-154104/154119.
[37] F. Weigend, R. Ahlrichs, Balanced basis sets of split valence, triple zeta valence
and quadruple zeta valence quality for H to Rn: design and assessment of
accuracy, Phys. Chem. Chem. Phys. 7 (2005) 3297–3305.
[38] J.L. Bao, J. Zheng, I.M. Alecu, B.J. Lynch, Y. Zhao, D.G. Truhlar, Database of
Fequency Scale Factors for Electronic Model Chemistries, https://comp.chem.
umn.edu/freqscale/version3b2.htm.
[39] J. Kreutzer, P. Blaha, U. Schubert, Assessment of different basis sets and DFT
functionals for the calculation of structural parameters, vibrational modes and
ligand binding energies of Zr4O2(carboxylate)12 clusters, Comput. Theor.
Chem. 1084 (2016) 162–168.
[40] S.F. Boys, F. Bernardi, The calculation of small molecular interactions by the
differences of separate total energies. Some procedures with reduced errors,
Mol. Phys. 19 (1970) 553–566.
[28] T. Schwier, V. Gevorgyan, Trans- and cis-selective Lewis acid catalyzed
hydrogermylation of alkynes, Org. Lett. 7 (2005) 5191–5194.
[29] S. Keess, M. Oestreich, Access to fully alkylated germanes by B(C6F5)3-
catalyzed transfer hydrogermylation of alkenes, Org. Lett. 19 (2017) 1898–
1901.
[30] L. Ignatovich, V. Muravenko, S. Grinberga, E. Lukevics, New reactions of the Si-
O-Ge group formation, Khim. Geterotsikl. Soedin. 299–302 (2006).
[31] S. Rubinsztajn, M. Cypryk, J. Chojnowski, W. Fortuniak, U. Mizerska, P.
Pospiech, Reaction of silyl hydrides with tetrabutoxygermanium in the
Presence of B(C6F5)3: difference between silicon and germanium chemistries
and easy route to GeH4, Organometallics 37 (2018) 1585–1590.
[32] D.J. Parks, W.E. Piers, G.P.A. Yap, Synthesis, properties, and hydroboration
activity of the highly electrophilic borane Bis(pentafluorophenyl)borane, HB
(C6F5)2, Organometallics 17 (1998) 5492–5503.
[41] H.J. Frohn, H. Franke, P. Fritzen, V.V. Bardin, (Fluoroorgano)fluoroboranes and -
fluoroborates. I. Synthesis and spectroscopic characterization of potassium
fluoroaryltrifluoroborates and fluoroaryldifluoroboranes, J. Organomet. Chem.
598 (2000) 127–135.
[42] T. Beringhelli, D. Maggioni, G. D’Alfonso, 1H and 19F NMR investigation of the
reaction of B(C6F5)3 with water in toluene solution, Organometallics 20 (2001)
4927–4938.
[43] A. Di Saverio, F. Focante, I. Camurati, L. Resconi, T. Beringhelli, G. D’Alfonso, D.
Donghi, D. Maggioni, P. Mercandelli, A. Sironi, Oxygen-bridged borate anions
from tris(pentafluorophenyl)borane: synthesis, NMR characterization, and
reactivity, Inorg. Chem. 44 (2005) 5030–5041.
[44] J. Chojnowski, S. Rubinsztajn, J. Cella, W. Fortuniak, M. Cypryk, J. Kurjata, K.
Kazmierski, Mechanism of the B(C6F5)3-catalyzed reaction of silyl hydrides
with alkoxysilanes. Kinetic and spectroscopic studies, Organometallics 24
(2005) 6077–6084.
[45] A. Schneider, Y. Chen, M.A. Brook, Trace water affects tris (pentafluorophenyl)
borane catalytic activity in the Piers-Rubinsztajn reaction, Dalton Trans.
(2019), https://doi.org/10.1039/C9DT02756D.
[46] D.J. Parks, R.E. von H. Spence, W.E. Piers, Bis(pentafluorophenyl)borane:
synthesis, properties, and hydroboration chemistry of a highly electrophilic
borane reagent, Angew. Chem., Int. Ed. Engl. 34 (1995) 809–811.
[47] R. Panisch, M. Bolte, T. Mueller, Hydrogen- and fluorine-bridged disilyl cations
and their use in catalytic C-F activation, J. Am. Chem. Soc. 128 (2006) 9676–
9682.
[33] G. Nikonov, S. Vyboishchikov, O. Shirobokov, Facile activation of H-H and Si-H
bonds by boranes, J. Am. Chem. Soc. 134 (2012) 5488–5491.
[34] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, G.A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A.V.
Marenich, J. Bloino, B.G. Janesko, R. Gomperts, B. Mennucci, H.P. Hratchian, J.V.
Ortiz, A.F. Izmaylov, J.L. Sonnenberg, Williams, F. Ding, F. Lipparini, F. Egidi, J.
Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V.G. Zakrzewski, J.
Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K.
Throssell, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M.J. Bearpark, J.J. Heyd, E.
N. Brothers, K.N. Kudin, V.N. Staroverov, T.A. Keith, R. Kobayashi, J. Normand,
K. Raghavachari, A.P. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, J.M.
Millam, M. Klene, C. Adamo, R. Cammi, J.W. Ochterski, R.L. Martin, K.
Morokuma, O. Farkas, J.B. Foresman, D.J. Fox, Gaussian 16 Rev. B.01,
Wallingford, CT, 2016.
[48] W. Piers, T. Chivers, Pentafluorophenylboranes: from obscurity to applications,
Chem. Soc. Rev. 26 (1997) 345–354.
[49] A.Y. Houghton, J. Hurmalainen, A. Mansikkamaki, W.E. Piers, H.M. Tuononen,
Direct observation of a borane-silane complex involved in frustrated Lewis-
pair-mediated hydrosilylations, Nat. Chem. 6 (2014) 983–988.
[35] A.D. Becke, Density-functional thermochemistry. III. The role of exact
exchange, J. Chem. Phys. 98 (1993) 5648–5652.