NHC-stabilized Al(III) and Ga(III) cationic alkyls: Synthesis, structure and use in hydrosilylation catalysis

Article abstract of DOI:10.1016/j.poly.2020.114956

Cationic Al(III) and Ga(III) species supported by N-heterocyclic carbene (NHC) ligands, (IDipp)AlMe₂(PhBr)]⁺ ([1]⁺, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and (IDipp)GaMe₂]⁺ ([2]<

Full text of DOI:10.1016/j.poly.2020.114956

Polyhedron 194 (2021) 114956
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
NHC-stabilized Al(III) and Ga(III) cationic alkyls: Synthesis, structure and
use in hydrosilylation catalysis
⇑
Anaëlle Bolley, David Specklin, Samuel Dagorne
Institut de Chimie de Strasbourg, CNRS-Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Cationic Al(III) and Ga(III) species supported by N-heterocyclic carbene (NHC) ligands, (IDipp)
AlMe2(PhBr)]+ ([1]+, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and (IDipp)GaMe2]+
([2]+), were prepared and structurally characterized as B(C6F5)ꢀ4 salts via ionization of the corresponding
neutral precursors (IDipp)MMe3 (M = Al, Ga) with [Ph3C][B(C6F5)4] in PhBr at room temperature. Both [1]
[B(C6F5)4] and [2][B(C6F5)4] salt were isolated in high yield and their solid state structures established
through X-ray crystallographic studies. Cations [1]+ and [2]+, which are rare examples of structurally
characterized tris-organyl Al(III) and Ga(III) cations, stand as potent Lewis acids as experimentally esti-
mated through the Gutmann-Beckett method. These cations were further exploited in hydrosilylation
catalysis of alkynes, benzaldehyde and CO2 using HSiEt3 as an hydrosilane source. Hydrosilylation of 1-
hexyne, 4-phenylbutyne and phenylacetylene led to the formation of the corresponding Z-selective prod-
ucts 3–5, respectively, while benzaldehyde was converted to PhCH2OSiEt3 (6). Cations [1]+ and [2]+ also
slowly catalyze CO2 hydrosilylation with the selective formation of the methanol-equivalent MeOSiEt3.
Ó 2020 Elsevier Ltd. All rights reserved.
Received 30 October 2020
Accepted 27 November 2020
Available online 3 December 2020
Keywords:
N-heterocyclic carbine
Aluminium
Gallium
Hydrosilylation
CO2
1. Introduction
and a poor functional group tolerance, hindering to some extent
their wider use in catalysis.
Ligand-supported well-defined Al(III) species have been widely
studied for their structural interest and as Lewis acidic catalysts for
various organic transformations [1]. In this regard, cationic Al(III)
species are potent electrophilic and Lewis acidic entities with an
enhanced Lewis acidity [2], and are known to activate various
unsaturated/polar substrates [3,4]. For example, we previously
showed that three-/four-coordinate Al alkyl cations bearing N-/O-
based chelating ligands mediate hydride transfer reactions to ole-
fins or ketones and effectively polymerize cyclic ethers/esters [5].
Five-/four-coordinate Al(III) cations were also recently exploited
as catalysts for the cyanosilylation of carbonyl substrates and the
Tishchenko reaction [6]. Strong Lewis acids of the type AlR+2 and
AlBr+2 also catalyze CO2 hydrosilylation and carbonyl-olefin
metathesis, respectively, further illustrating the potential of low-
coordinate Al cations in catalysis [7,8]. Three-coordinate Al cations
of the type (NacNac)Al–R+ (R = H, alkyl) were recently shown to be
highly efficient catalysts in alkene hydrosilylation [9]. Yet, the high
reactivity of low-coordinate (two-/three-coordinate) Al(III) orga-
nometallics comes along with their limited stability in polar media
Due to their exceptional strong r-donation and steric tunabil-
ity, N-heterocyclic carbenes (NHCs) are now ubiquitous ligands
in organometallic and coordination chemistry since they are able
to stabilize various metal/heteroelement complexes [10], including
oxophilic and electropositive centers such as group 13 metal M(III)
salts (M = Al, Ga, In) [11]. Early examples of Al-NHC species of the
type (NHC)AlX3 were reported in the early 19900s by Arduengo
[12], and were found to be more robust and thermally stable than
their phosphine adduct analogues. In contrast, NHC-bearing Al(III)
cations remain little explored. Several mono- and dicationic Al
hydrido species of the type [(NHC)2AlH2]+, [(NHC)AlHI]+, [(NHC)2-
Al-H]2+ and [(NHC)AlH2]22+ were recently characterized [13,14].
We also showed that four-coordinate cations of the type (NHC)
MR2(L)+ (M = Al, Ga, In; L = Et2O, THF) were stable and robust
cations able to initiate the polymerization of cyclic esters [15]. A
few NHC-supported Ga(III) and In(III) halido cations have also been
structurally characterized [16]. For enhanced electrophilicity/reac-
tivity, we have become interested into three-coordinate Al(III) and
Ga(III) alkyl species of the type (NHC)MR+2 (M = Al, Ga) for their
structural interest and for subsequent use in activation/functional-
ization catalysis. As part of these studies, we here report on the
synthesis and structural characterization of Al(III) and Ga(III) catio-
nic alkyls of the types (NHC)AlMe+2 and (NHC)GaMe2+. Such strong
⇑
Corresponding author.
E-mail address: dagorne@unistra.fr (S. Dagorne).
https://doi.org/10.1016/j.poly.2020.114956
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

Anaëlle Bolley, D. Specklin and S. Dagorne
Polyhedron 194 (2021) 114956
electrophiles were exploited in aldehyde, alkyne and CO2 hydrosi-
lylation catalysis, as also discussed herein.
with no cation/anion interactions (Fig. S7, SI). As shown in Fig. 1,
the Al cation [1-PhBr]+ consists of a [AlMe2]+ moiety stabilized
by one IDipp carbene and the coordination of PhBr through the
Br atom. To our knowledge, [1-PhBr]+ is first structurally charac-
terized PhBr adduct of an Al(III) species and its formation clearly
substantiates the strong Lewis acidity of the Al(III) center in 1+.
The Al-Br bond distance (2.663(1) Å) lies in the upper range of
Al-Br-Al bond distances and is a bit shorter than that found in a
vinylic Al(III) species containing a dative Al–Br bond (2.703(8) Å)
[18,19]. The latter is consistent with an effective coordination of
PhBr to the Al(III) center in [1-PhBr]+. PhBr coordination causes a
slight pyramidalization of the geometry around the Al(III) center
in [(IDipp)AlMe2]+, as reflected by the sum of C-Al-C bond angles
(around 351°). The Al–CNHC bond distance in [1-PhBr]+ (2.045(4)
Å) is similar to that in cation [(IMes)Al(OEt2)Me2]+ (2.066(4) Å)
and expectedly shorter than that in neutral precursor (IDipp)AlMe3
(2.103(3) Å) [20].
The molecular structure of the Ga(III) salt [2][B(C6F5)4], crystal-
lized from PhBr/toluene, was also confirmed through X-ray crystal-
lographic analysis, and consists of discrete [2]+ and [B(C6F5)4]ꢀ ions
with no cation/anion interactions (Fig. S8, SI). As depicted in Fig. 2,
cation [2]+ features a central three-coordinate Ga(III) center in a
trigonal geometry. The effective coordination of IDipp to the
[GaMe2]+ group is shown by a much shorter Ga-CNHC bond distance
(2.018(3) Å) than in (IDipp)GaMe3 precursor (2.105(4) Å) [19]. The
absence of PhBr coordination to the Ga(III) center in [2]+ presum-
ably indicates the lower Lewis acidity of Ga(III) vs. Al(III).
2. Results – Discussion
2.1. Synthesis and structure of the Al-NHC and Ga-NHC cations (IDipp)
MMe+2 (M = Al, Ga). [1][B(C6F5)4] and [2][B(C6F5)4]
Cations 1+ and 2+ were prepared via a methide abstraction reac-
tion of the corresponding neutral precursors (IDipp)AlMe3 and
(IDipp)GaMe3 (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-
2-ylidene) and [Ph3C][B(C6F5)4] [15], a methodology well-estab-
lished in the literature [2,3]. Thus, the ionization reaction of
(IDipp)MMe3 (M = Al, Ga) and 1 equiv of [Ph3C][B(C6F5)4] (PhBr,
room temperature) led to the quantitative formation of cations
[(IDipp)AlMe2]+ (1+) and [(IDipp)GaMe2]+ (2+), respectively, as B
(C6F5)ꢀ4 salts (Scheme 1), as deduced from NMR data. Both cations,
[1][B(C6F5)4] and [2][B(C6F5)4], were isolated in high yield as analyt-
ically pure colorless solids. In solution, both salts are unstable in
CH2Cl2 and decomposes within minutes at room temperature to
unknown species, likely reflecting the strong Lewis acidity of cations
1+ and 2+ [17]. In contrast, the earlier reported [(NHC)MR2(L)]+
(M = Al, Ga; L = Et2O, THF) cations are stable for days in CH2Cl2 con-
firming that Lewis bases such as Et2O and THF significantly quench
the Lewis acidity of the (NHC)MR+2 (M = Al, Ga) moiety.
Cations 1+ and 2+ are however stable for days in PhBr under
inert atmosphere. The 1H, 13C and 19F NMR data for [1][B(C6F5)4]
and [2][B(C6F5)4] (C6D5Br, room temperature) agree with the pro-
posed formulation for cations 1+ and 2+ with no evidence of
cation/anion interactions in solution. For instance, in the case of
the Al cation 1+, characteristic 1H NMR resonances include: 1) a
significantly downfield shifted 1H NMR chemical shift for the H-
C4/C5 NHC resonance (d 7.02 ppm) vs. that in neutral precursor
(IDipp)AlMe3 (d 6.58 ppm), in line with NHC coordination with
the more electrophilic [AlR2]+ moiety; 2) an upfield shifted Ccarbene
13C NMR signal (d 163.0 vs. 178.3 ppm in (IDipp)AlMe3) reflecting
an enhanced Lewis acidity of the Al(III) center in cation 1+.
Though no solvent (PhBr) interaction/coordination were
observed in solution for 1+ and 2+ under the studied conditions,
fast coordination/de-coordination of PhBr to the M(III) cation, at
least in the case of the more Lewis acidic Al derivative, seems prob-
able and certainly stabilize these electrophiles: the latter is sug-
gested by the solid-state structure of the Al(III) cation 1+ as an
Al–PhBr adduct. Thus, the molecular structure of salt [1][B
(C6F5)4], crystallized from a PhBr/pentane solution, was confirmed
through X-ray crystallographic analysis. Salt [1][B(C6F5)4] crystal-
lizes as discrete Al-PhBr cationic adducts and [B(C6F5)4]ꢀ anions
Cations [1]+ and [2]+ immediately reacted with 1 equiv Et2O or
THF (C6D5Br, room to form the corresponding four-coordinate
cationic adducts [(NHC)MR2(L)]+ (M = Al, Ga; L = Et2O, THF), as
deduced from comparison of NMR data with literature [15]. Since
[1]+ and [2]+ stand as potentially strong Lewis acids and their Lewis
acidity was thus experimentally estimated using the Gutmann-
Beckett method and compiled in Table 1 [21]. According to these
measurements, the Lewis acidity of the Al cation [1]+ is similar to
that of B(C6F5)3, a landmark Lewis acid (Dd
31P (C6D5Br) = 76.2
and 76.6 ppm for B(C6F5)3 and [1]+, respectively). The Ga cation
[2]+ appears to be slightly less Lewis acidic
(Dd
31P
(C6D5Br) = 73.2 ppm) , in line with other structural data.
Fig. 1. Molecular structure of the Al cation [1]+ (ORTEP view; ellipsoids enclose 50%
electronic density) with selected atom labeling for clarity. The hydrogen atoms are
omitted for clarity.
Scheme 1. Synthesis of the NHC-supported Al(III) and Ga(III) species [1–2][B
(C6F5)4]
2

Anaëlle Bolley, D. Specklin and S. Dagorne
Polyhedron 194 (2021) 114956
2.3. Benzaldehyde and CO2 hydrosilylation catalysis mediated by [1–2]
[B(C6F5)4]
The potential of Al and Ga cations [1]+ and [2]+ as Lewis acidic
catalysts for the hydrosilylation of carbonyl substrates was also
evaluated. Thus, [1]+ (5% mol) is highly active in benzaldehyde
hydrosilylation at room temperature to lead within 50 min to the
quantitative conversion of a 1/1 benzaldehyde/HSiEt3 mixture to
the mono-reduction benzyl silyl ether product 6 (Scheme 3; entry
7, Table 2). Though inactive at room temperature, the less Lewis
acidic Ga cation [2]+ also allowed benzaldehyde reduction to 6
upon heating (entry 8, Table 2). The present benzaldehyde hydrosi-
lylation catalysis likely proceeds through a similar Lewis-acid type
activation to that thoroughly studied for borane-mediated Lewis
acid catalysis [24].
Despite being a challenging carbonyl substrate for activation/-
functionalization, CO2 was recently shown to be reduced to metha-
nol-equivalent MeOSiR3 products and/or methane by Lewis acid-
promoted hydrosilylation, primarily using Al- and Zn-based
cations as strong electrophiles [7,25]. Salt [1–2][B(C6F5)4] were
thus probed as CO2 hydrosilylation catalysts and exhibited moder-
ate performances (entries 9 and 10, Table 2). Thus, in the presence
of CO2 (1.5 atm) and HSiEt3 (10 equiv vs. catalyst), Al cation [1]+
slowly but selectively hydrosilylates CO2 to methanol-equivalent
MeOSiEt3 (C6D5Br, 90 °C, 41 h , 43% conversion to MeOSiEt3), as
deduced from NMR and GC–MS data. In line with the observed
trend for these systems, the less acidic Ga center [2]+ showed a
similar selectivity but displayed a lower activity (C6D5Br, 90 °C,
41 h , 15% conversion to MeOSiEt3). For both catalysts [1]+ and
[2]+, MeOSiEt3 was the only observed product by 1H NMR, thus
with no NMR detection of the first and second hydrosilylation
products, formate HCO2SiEt3 and ketal Et3SiO-CH2-OSiEt3, respec-
tively, as the CO2 hydrosilylation proceeds. The latter is in line with
the initial CO2 activation/functionalization being rate-limiting, as
has been observed for other Lewis acid-type CO2 hydrosilylation
catalysts [7,25], and fast consumption of HCO2SiEt3 to Et3SiO-
CH2-OSiEt3 and eventually MeOSiEt3.
Fig. 2. Molecular structure of the Ga cation [2]+ (ORTEP view; ellipsoids enclose 50%
electronic density) with selected atom labeling for clarity. The hydrogen atoms are
omitted for clarity.
2.2. Alkene/alkyne hydrosilylation catalysis mediated by [1–2][B
(C6F5)4]
The Lewis acidic Al and Ga salts [1–2][B(C6F5)4] were initially
tested in 1-hexene hydrosilylation. As monitored by 1H NMR, no
reaction was observed at room temperature in C6D5Br between
[1–2][B(C6F5)4] (5% mol) and a 1/1 1-hexene/HSiEt3 mixture (20
equiv vs. catalyst). Heating to 65 °C for extended time (days) only
led to 1-hexene polymerization, as deduced from NMR and GPC
data, along with unreacted HSiEt3. The formation of poly(1-hex-
ene) likely occurs via a carbocationic Lewis acid-initiated polymer-
ization mechanism, as is well-known for highly electrophilic
species [22].
In contrast, as compiled in Table 2, the Al cation [1]+ was found
to be an extremely efficient and selective for alkyne hydrosilyla-
tion, allowing the fast and quantitative trans-hydrosilylation of 1-
hexyne, 4-phenylbutyne and phenylacetylene within minutes (5
to 30 min; entries 1, 3 and 5, Table 2) at room temperature using
a low catalyst loading (5% mol) to selectively afford the Z products
3–5, respectively (Scheme 2). Catalyst [1]+ retains its integrity as
the catalysis proceeds, which is consistent with [1]+ solely acting
as a Lewis acid. Al-based alkyne hydrosilylation catalysis have thus
far been restricted to the use of AlCl3 as catalyst, yet requiring a
much higher catalytic loading (typically 20% mol) and an extre-
mely careful AlCl3 purification prior to catalysis [23].
The present Al-catalyzed alkyne hydrosilylation likely occur in a
similar way to that observed with AlCl3 with 1,2 anti-addition of
H–SiEt3 to the CAC triple bond [22a]. As a comparison, the less
Lewis acidic Ga cation [2]+ is significantly less active in alkyne
hydrosilylation (entries 2, 4 and 6, Table 2), and expectedly dis-
plays the same selectivity (trans-hydrosilylation) with the forma-
tion of hydrosilylated products 3–5.
3. Conclusion
The present study showed that NHC-supported Al(III) and Ga
(III) alkyl cations of the type [(NHC)AlMe2(PhBr)]+ and [(NHC)
GaMe2]+ may be readily prepared as thermally stable yet strongly
electrophilic species. Cations [1]+ and [2]+, which are rare struc-
turally characterized tris-organyl Al(III) and Ga(III) cations, are
robust Lewis acidic species that may act as effective and selective
hydrosilylation catalysts of alkynes, aldehydes and CO2. As a com-
parison, it may be noted that robust four-coordinate [(NHC)
MR2(L)]+ (M = Al, Ga; L = Et2O, THF) cations displayed no catalytic
activity in any of the hydrosilylation reactions studied herein. In
line with its stronger Lewis acidity, the Al cation [1]+ is signifi-
cantly more active than its Ga analogue [2]+, while both cations
display identical selectivities across all hydrosilylations. All
hydrosilylation catalysis data agree with Lewis acid-mediated pro-
cesses mediated by [1]+ and [2]+.
Table 1
Lewis acidity assessment of salt [1][B(C6F5)4] and [2][B(C6F5)4] via the Gutmann-Beckett method.a
POEt3
46.8
B(C6F5)3 + 1 equiv POEt3
76.2
[1][B(C6F5)4] + 1 equiv POEt3
[2][B(C6F5)4] + 1equiv POEt3
31P NMR (C6D5Br) d(ppm)
76.6
73.2
a
The 31P NMR chemical shift difference between the Et3P = O Lewis adduct and free Et3P = O allows an estimation of Lewis acidity [21]. The 31P NMR measurements were
done in C6D5Br at RT.
3

Anaëlle Bolley, D. Specklin and S. Dagorne
Polyhedron 194 (2021) 114956
Table 2
Alkyne, benzaldehyde and CO2 hydrosilylation catalysis results using [1–2][B(C6F5)4] as catalysts and HSiEt3 as an hydrosilane source.a
Entry
Catalyst
Substrate
Time/T (°C)
Conv. (%)
Productd
1
2
3
4
5
6
7
8
9
10
[1][B(C6F5)4]
[2][B(C6F5)4]
[1][B(C6F5)4]
[2][B(C6F5)4]
[1][B(C6F5)4]
[2][B(C6F5)4]
[1][B(C6F5)4]
[2][B(C6F5)4]
[1][B(C6F5)4]b
[2][B(C6F5)4]b
1-hexyne
1-hexyne
5 min/RT
7 days/RT
5 min/RT
45 h/RT
30 min/RT
40 h/90 °C
50 min/RT
5 h/70 °C
41 h/90 °C
41 h/90 °C
98%
20%
3
3
4
4
5
5
6
6
4-phenylbutyne
4-Phenylbutyne
phenylacetylene
phenylacetylene
benzaldehyde
benzaldehyde
100%
100%
100%
100%
90%
90%
43%
15%
c
CO2
CO2
MeOSiEt3
MeOSiEt3
c
a
b
c
d
Conditions: NMR-scale reactions, 5% mol of catalyst vs. silane (HSiEt3), solvent = C6D5Br. 10% mol catalyst was used. 1.5 atm of CO2. hydrosilylation products 3–6
(Schemes 2 and 3) and MeOSiEt3 were identified from NMR, GC–MS data and comparison with literature.
(C6F5)4] (199.78 mg, 216,6 lmol) in PhBr (1.5 mL) at 25 °C giving
a colorless solution at the end of the addition. The solvent was then
immediately removed in vacuo and the oily residue triturated with
pentane (3x5 mL) to obtain salt ([1][B(C6F5)4]) as an analytically
pure white powder (96% yield). X-ray quality single crystals of
[1][B(C6F5)4] were grown at ꢀ35 °C by layering pentane over a
PhBr solution of [1][B(C6F5)4]. Anal. Calcd. for C53H43AlBF20N2: N,
2.49; C, 56.55; H, 3.85. Found: N, 2.47; C, 56.55; H, 3.81. 1H NMR
(400 MHz, C6D5Br): d (ppm) 7.38 (t, J = 7.6 Hz, 2H, CH-Ar), 7.16
(d, J = 7.8 Hz, 4H, CH-Ar), 7.02 (s, 2H, NCHCHN), 2.32 (hept,
Scheme 2. Alkyne hydrosilylation catalyzed by [1–2][B(C6F5)4]
i
J = 6.8 Hz, 4H, CH-iPr), 1.20 (d, J = 6.7 Hz, 12H, CH3– Pr), 1.01 (d,
J = 6.9 Hz, 12H, CH3– iPr), ꢀ1.06 (s, 6H, AlMe). 13C NMR
(126 MHz, C6D5Br):
d (ppm) 163.05 (Ccarbene), 148.29 (dm,
JCF = 237.9 Hz, o-C6F5), 144.86 (C-Ar), 138.07 (dm, JCF = 238.3 Hz,
p-C6F5), 136.18 (dm, JCF = 239.1 Hz, m-C6F5), 132.26 (Cipso),
131.44 (CH-Ar), 124.62 (CH-Ar), 121.97 (NCHCHN), 28.62 (CH-iPr),
25.20 (CH3-iPr), 22.01 (CH3-iPr), ꢀ8.45 (Al(Me)2). 19F NMR
(282 MHz, C6D5Br): d (ppm) ꢀ131.41 (d, JFF = 11.7 Hz, 8F, o-F),
ꢀ161.87 (t, JFF = 21.0 Hz, 4F, p-F), ꢀ165.68 (t, JFF = 18.8 Hz, 8F, m-F).
Scheme 3. Hydrosilylation of benzaldehyde and CO2 catalyzed by [1–2][B(C6F5)4]
4. Experimental section
4.1. Material, reagents and experimental methods
All work was performed under N2 atmosphere using standard
glove box techniques. Solvents were stored over 4 Å molecular
sieves and were freshly distilled under argon from sodium-ben-
zophenone or CaH2, or they were dispensed from a commercial sol-
vent purification system. Deuterated solvents were used as
received and stored over 4 Å molecular sieves. NMR spectra were
recorded on Bruker Avance I ꢀ 300 MHz, Bruker Avance III ꢀ
400 MHz, Bruker Avance II ꢀ 500 MHz and Bruker Avance III ꢀ
600 MHz spectrometers. NMR chemical shift values were deter-
mined relative to the residual protons in C6D6, C6D5Br as internal
reference for 1H (d of the most downfield signal = 7.16, 7.30, 5.32,
7.26 ppm) and 13C{1H} (d of the most downfield signal = 128.39,
130.89, 53.84, 77.23 ppm). IR spectra were recorded on an alpha
ATR spectrometer from Brucker Optics and analyzed with OPUS
software. AlMe3, GaMe3, [CPh3][B(C6F5)4] were obtained from
Strem Chemicals Inc. B(C6F5)3 was obtained from TCI Europe and
recrystallized from cold pentane prior to use. The M-NHC adducts
(IDipp)AlMe3, (IDipp)GaMe3 were prepared according to a litera-
ture procedure [15]. All other chemicals were purchased from
Merck Corp. All olefinic, alkyne and carbonyl chemicals were dried
over molecular sieves (4 Å) for a least 24 h prior to use. GC-MS anal-
ysis was conducted on a GC System 7820A (G4320) connected to a
MSD block 5977E (G7036A) using Agilent High Resolution Gas
4.2.2. [(IDipp)GaMe2][B(C6F5)4] ([2][B(C6F5)4])
To a solution of (IDipp)GaMe3 (100 mg, 198.2
(1.5 mL) was added dropwise a stirring solution of [CPh3][B
(C6F5)4] (182.6 mg, 198.2 mol) in PhBr (1.5 mL) at 25 °C giving
lmol) in PhBr
l
a colorless solution. The solvent was then immediately removed
in vacuo and the oily residue triturated with pentane (3x5 mL) to
obtain salt ([2][B(C6F5)4]) as an analytically pure white powder
(98% yield). Anal. Calcd. for C53H43GaBF20N2: N, 2.40; C, 54.48; H,
3.71. Found: N, 2.40; C, 54.53; H, 3.88. 1H NMR (400 MHz,
C6D5Br): d (ppm) 7.36 (t, J = 7.8 Hz, 2H, CH-Ar), 7.16 (d,
J
= 7.7 Hz, 4H, CH-Ar), 6.95 (s, 2H, NCHCHN), 2.25 (hept,
J = 6.9 Hz, 4H, CH-iPr), 1.06 (d, J = 6.6 Hz, 12H, CH3– Pr), 1.02 (d,
J = 6.9 Hz, 12H, CH3– iPr), ꢀ0.59 (s, 6H, Ga(Me)2). 13C NMR
i
(126 MHz, C6D5Br):
d (ppm) 164.45 (Ccarbene), 148.26 (dm,
JCF = 239.9 Hz, o-C6F5), 144.38 (C-Ar), 133.13 (dm, JCF = 230.9 Hz,
p-C6F5), 136.11 (dm, JCF = 239.5 Hz, m-C6F5), 131.30 (Cipso), 131.19
(CH-Ar), 124.96 (CH-Ar), 121.97 (NCHCHN), 28.62 (CH-iPr), 24.54
(CH3-iPr), 22.94 (CH3-iPr), ꢀ1.32 (Ga(Me)2). 19F NMR (282 MHz,
C6D5Br): d (ppm) ꢀ131.38 (d, JFF = 10.7 Hz, 8F, o-F), ꢀ161.87 (t,
JFF = 21.1 Hz, 4F, p-F), ꢀ165.69 (t, JFF = 18.8 Hz, 8F, m-F).
4.3. Alkene/alkyne hydrosilylation catalysis data
Chromatography Column HP-5MS UI, 30 m, 0.25 mm, 0.25 lm.
4.3.1. General procedure
A J-Young NMR tube was charged with a solution of the appro-
4.2. Synthesis and characterisation of the Al and Ga complexes
4.2.1. [(IDipp)AlMe2][B(C6F5)4] ([1][B(C6F5)4])
priate catalyst [1–2][B(C6F5)4] (4.4 lmol) in C6D5Br (0.5 mL), the
alkene/alkyne (20 equiv vs. catalyst) and HSiEt3 (20 equiv vs. cata-
lyst). The reaction was monitored by 1H NMR spectroscopy, allow-
ing the idenfication of products on the basis of literature data.
After completion of the reaction, the volatiles were removed under
To a solution of (IDipp)AlMe3 (100 mg, 216.6
lmol) in PhBr
(1.5 mL) was added dropwise a stirring solution of [CPh3][B
4

Anaëlle Bolley, D. Specklin and S. Dagorne
Polyhedron 194 (2021) 114956
vacuum for 1 h and the residue extracted with pentane. Further
evaporation led to the desired NMR-pure hydrosilated products.
For 1-hexene hydrosilylation attempts, poly(1-hexene) oligomers
were found to be the major reaction products from 1H NMR data
and were further analyzed by GPC and MALDI-TOF spectrometry.
For the alkyne hydrosilylation products, GC–MS analysis confirmed
their identity.
CRediT authorship contribution statement
Anaëlle Bolley: Data curation, Methodology. David Specklin:
Data curation, Methodology. Samuel Dagorne: Supervision, Writ-
ing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
4.3.2. Attempted 1-hexene hydrosilylation by [1–2][B(C6F5)4] (5% mol)
In the presence of 1-hexene and HSiEt3 (20 equiv of each),
cation [1][B(C6F5)4] (5% mol.) is unreactive at room temperature
for 48 h. The oligomerization of 1-hexene occurred upon heating
(65 °C, C6D5Br, quantitative conv. of 1-hexene after 3 and 10 days
for [1][B(C6F5)4] and [2][B(C6F5)4], respectively) as deduced from
1H NMR data.[26] Isolation of the products and subsequent GPC
and MALDI-TOF analysis confirmed the formation of oligomers.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2020.114956.
References
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for 5: 100% conv. within 30 h, 1H NMR data in line with literature
data [29], GC/MS: tR = 5.373 min (100%), m/z = 218.20, (E)-triethyl
(styryl)silane.
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6