Catalytic Reduction of Carbon Dioxide Using Cationic Organoaluminum and -Gallium Compounds

Article abstract of DOI:10.1021/acs.organomet.7b00716

Ethide abstraction from Et₃M (M = Al and Ga), (2,6-Ph₂C₆H₃)AlEt₂, 1, and (2,6-Dipp₂C₆H₃)GaEt₂, 2 (Dipp = 2,6-iPr₂C₆H₃), usi

Full text of DOI:10.1021/acs.organomet.7b00716

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Catalytic Reduction of Carbon Dioxide Using Cationic
Organoaluminum and -Gallium Compounds
†
‡
,†
Mahmoud Saleh, Douglas R. Powell, and Rudolf J. Wehmschulte*
†Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, United States
‡Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019-5251,
United States
*
S Supporting Information
ABSTRACT: Ethide abstraction from Et M (M = Al and Ga),
3
(
2
2,6-Ph C H )AlEt , 1, and (2,6-Dipp C H )GaEt , 2 (Dipp =
2 6 3 2 2 6 3 2
,6-iPr C H ), using the silylium ion [Et Si][CHB Cl ]
2 6 3 3 11 11
afforded crystalline ion-like compounds [Et M][CHB Cl ]
2
11 11
(
M = Al, 3; Ga, 5) and [(2,6-Ph C H )AlEt][CHB Cl ], 4,
2 6 3 11 11
and the likely solvent-separated ion pair [(2,6-Dipp C H )-
2
6
3
GaEt][CHB Cl ], 6. Crystalline compounds 3−5 feature
11
11
cation···anion contacts in the solid state, and their solubility in low polarity benzene indicates that these contacts are maintained
in solution. All compounds catalyze the reduction of CO with Et SiH, but the activity of the gallium compound 5 is significantly
2
3
lower due to its polymeric structure and the lower Lewis acidity of gallium. Whereas the reduction products from the reactions
catalyzed by compounds 3−5 are mostly methane and toluene (from Friedel−Crafts alkylation of the benzene solvent), catalysis
by 6 led mostly to Et SiOCH .
3
3
INTRODUCTION
3, and [(2,6-Ph C H )AlEt][CHB Cl ], 4, both featuring the
2 6 3 11 11
15
■
−
much less basic [CHB Cl ] counterion. For comparison,
11
11
While the reduction of CO with dihydrogen or hydrosilanes
2
we also synthesized the gallium analogues [Et Ga][CHB Cl ],
2
11 11
catalyzed by transition metal compounds has been known for
1
−4
5, and [(2,6-Dipp C H )GaEt][CHB11Cl11], 6. Here, we report
2 6 3
decades,
the use of main group metal catalysts or even
5
the synthesis and structural characterization of these com-
pounds and their activity as catalysts in the hydrosilylation of
CO with Et SiH.
metal-free catalysts is a rather recent development. Generally,
the latter systems use hydrosilanes as the reducing agent due to
the weaker and slightly polar Si−H and stronger Si−O bonds
compared to H−H and H−O bonds. Furthermore, the
byproduct water would deactivate most of these systems,
whereas siloxane byproducts are usually well-tolerated. Early
attempts include the frustrated Lewis Pair TMP/B(C F )
2
3
RESULTS AND DISCUSSION
■
Synthesis of Terphenylaluminum and -Gallium Com-
pounds. The synthesis of terphenylaluminum (2,6-Ph
AlEt , 1, and terphenylgallium (2,6-Dipp )GaEt , 2 (Dipp
2,6-iPr C H ), was achieved in analogy to reported and
C H )-
6
5 3
2 6 3
6
(
TMP = 2,2,6,6-tetramethylpiperidine), which reduces CO2
2
2
C
6
H
3
2
=
to methane using Et SiH as the reducing agent. This was
2
6
3
3
1
6−18
standard organometallic procedures as shown in eq 1.
new compounds were characterized by H and C NMR
The
followed by the demonstration that strongly basic N-
heterocyclic carbenes were active catalysts to selectively afford
methoxysilanes MeOSiR , although it was later shown that the
active species was the adduct NHC−CO . Other strong
organic bases such as bicyclic guanidines and phosphazenes
can also be used as catalysts. The use of strong Lewis acids such
1
13
7
spectroscopy.
3
8
Synthesis of Cationic Aluminum and Gallium Com-
2
9
10
pounds. Compounds 1 and 2 along with Et Al and Et Ga
3
3
were converted to the corresponding cationic species by ethide
abstraction with the silylium salt [Et Si][CHB Cl ] according
11
as the ion-like [Et Al][CH B I ] was demonstrated in 2012,
3
11 11
2
6 11 6
to eq 2. These reactions were performed in a small grease-free
tube or a Schlenk flask. The use of the trityl salt [Ph C]-
and recently, the tandem system Al(C F ) /B(C F ) was
6
5 3
6 5 3
12
applied in CO hydrosilylation. Interestingly, even simple
3
2
[
CHB Cl ] also afforded the target compounds, but their
salts such as CsF and Cs CO catalyze the CO reduction with
11 11
2
3
2
1
3
purification was more tedious. Despite the small scale of th−e
PhMe SiH, and even the weak Lewis acid BPh when used in
2
3
reactions (due to the limited availability of [CHB11Cl11]
polar solvents was found to catalyze the hydrosilylation of CO2
with several hydrosilanes selectively to the formate esters
compounds) and the high air and moisture sensitivity, all of
these reactions are readily reproducible. The identity of the new
ionic compounds has been established by NMR spectroscopy
1
4
R3SiOCH(O).
While [Et Al][CH B I ] was able to catalyze the CO
2
2
6
11 6
1
1
hydrosilylation, its activity was rather low. We hypothesized
that this may be due to relatively strong cation···anion contacts,
and we set out to prepare the compounds [Et Al][CHB Cl ],
Received: September 19, 2017
2
11 11
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00716
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
1.941 Å (avg.)) and two Al···Cl contacts (2.453 Å (avg.)) to
the anion. These values are close to those observed for the
related ion pairs [Et Al][CHB H Cl ] (Al−C 1.928 Å (avg.),
2
11
5
6
19
Al···Cl 2.435 Å (avg.)) and [Et Al][Me NB Cl ] (Al−C
2
3
12 11
20
1
.923 Å (avg.), Al···Cl 2.430 Å (avg.)), although the slight
lengthening of the Al···Cl contacts in 3 may reflect the lower
−
15,21
basicity of the [CHB Cl ] anion.
Just as in [Et Al]-
2
11
11
[
Me NB Cl ], the Al center in 3 is coordinated to m- and p-
3 12 11
chlorine donors from the anion. The C−Al−C angle of
1
36.44(9)° is significantly wider than the tetrahedral angle but
does not come close to the 180° expected for a true two-
coordinate species. This value is also close to those reported for
[
(
Et Al][CHB H Cl ] (133.8(4)°) and [Et Al][Me NB Cl ]
2
11
5
6
2
3
12 11
127.7(4)°).
[
(2,6-Ph C H )AlEt][CHB Cl ], 4, is significantly less
2 6 3 11 11
soluble than 3, and colorless block-shape crystals suitable for
single crystal X-ray analysis were grown from a hot benzene
solution. Its structure (Figure 2) bears several similarities to
as well as by single crystal X-ray diffraction for compounds 3−
5. Compound 6 was isolated as an oil, and NMR data support
the expected structure.
In contrast to the synthesis of [Et Al][CB H Cl ],
1
5
2
11
6
6
compound 3 was synthesized using the silylium cation to
facilitate the isolation of the product in good yield. The
compound is highly soluble in benzene and the attempted
isolation from this solvent was unsuccessful. The reaction of the
1
6
silylium salt [Et Si][CHB Cl ], prepared in situ, with excess
3
11 11
Et Al in hexane proceeded after sonication for several hours.
3
Compound 3 was obtained as a colorless precipitate that can be
used as is after washing with hexane to remove unreacted Et Al
3
and the side product Et Si. Surprisingly, 3 was sufficiently
4
soluble in hot (105 °C, closed flask) hexane to allow for
crystallization after slow cooling to room temperature. The high
solubility in benzene and even in hot hexanes indicated that 3
maintains its tight ion pair solid state structure (Figure 1) in
solution.
Compound 3 crystallizes in the monoclinic space group P21/
n. The asymmetric unit consists of two independent molecules
with very similar geometrical features so that the data for only
one of those will be used in the discussion here. The Al center
is in a distorted tetrahedral environment with two Al−C bonds
Figure 2. Structure of 4 (50% ellipsoids). Hydrogen atoms except of
H(1A) have been omitted for clarity. Selected bond distances (Å) and
angles (deg): Al(1)−C(1) 1.948(2), Al(1)−C(19) 1.939(2), Al(1)···
Cl(7) 2.4922(6), Al(1)···Cl(12) 2.5153(6), C(1)−Al(1)−C(19)
1
42.24(7), Cl(7)−Al(1)−Cl(12) 88.84(2).
that of 3 such as the distorted tetrahedral Al center and cation−
anion contact involving the Al center and chlorine donors from
the m-belt and the para position of the anion. In contrast, the
Al···Cl contacts are longer than those in 3 (2.504 (avg.) vs
2
1
.453 (avg.) Å), and the C−Al−C angle is wider (142.24(7)° vs
36.44(9)°) possibly indicating a weaker cation−anion
interaction. The structure of 4 is also quite similar to that of
17
[DcpAlEt][CH B Cl ] (Dcp = 2,6-(2,6-Cl C H ) C H ),
6 11 6 2 6 3 2 6 3
which features a slightly bigger and electron-poor substituent
as well as a more basic counterion (Table S1). Furthermore, the
1
H NMR shifts of the ethyl signals in both compounds almost
match (1.13 (t) and 0.65 (q) in 4, 1.02 (t) and 0.64 (q) in
17
[
DcpAlEt][CH B Cl ]), an indication of similar electronic
6 11 6
environments close to the Al center.
The gallium compound [Et Ga][CHB Cl ], 5, was isolated
2
11 11
in good yield (82%) as colorless crystals by layering hexanes on
top of the benzene reaction mixture. Contrary to 3 and 4,
compound 5 does not feature isolated ion pairs in the solid
Figure 1. Structure of one of the two independent molecules of 3
(
50% ellipsoids). Hydrogen atoms except of H(1A) have been omitted
for clarity. Selected bond distances (Å) and angles (deg): Al(1A)−
state, but its structure consists of one-dimensional [Et Ga]-
C(2A) 1.942(2), Al(1A)−C(4A) 1.941(2), Al(1A)···Cl(7A)
2
2
1
.4515(8), Al(1A)···Cl(12) 2.4543(9), C(2A)−Al(1A)−C(4A)
36.44(9), Cl(12)−Al(1A)−Cl(7A) 92.22(3).
[CHB Cl ] chains (Figure 3). The coordination environment
11
11
at the gallium center may be described as distorted octahedral
B
DOI: 10.1021/acs.organomet.7b00716
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
should also be pointed out that the positons of the CH and
2
CH signals are reversed with respect to those from 3 and 4
3
(
see Figure S13).
Dipp*GaEt][CHB Cl ], 6, could only be obtained as a
[
11
11
viscous oil with a low solubility in benzene, but it is readily
soluble in bromobenzene or benzene/bromobenzene mixtures.
This is a common behavior for ionic compounds with large
cations and anions, and such compounds have been described
Figure 3. Structure of 5 depicting the zigzag polymeric motif.
24,25
with two gallium−carbon σ-bonds and four weak gallium···
chlorine coordinative bonds from two anions (Figure 4).
as liquid clathrates.
While not every oil separating from
benzene is a liquid clathrate, removal of the volatiles from the
oil containing 6 results in a significant increase in viscosity
suggesting that solvent (benzene) molecules were incorporated
−
26
in that oil. Many [B(C F ) ] salts show this behavior as well
6
5 4
as [2,6-Mes C H GaBu][CHB Cl ], [Dipp*GaBu]-
2
6
3
11
11
16
[
CHB Cl ], and [Dipp*GaBu][B(C F ) ], and this oil
11 11 6 5 4
formation can be viewed as an indication for the presence of
solvent separated ions. The proposed identity of 6 is supported
1
by its H NMR spectrum, which shows a 1:1 ratio of cation and
anion signals and also displays a high-field shift of the ethyl
signals.
Catalytic Reduction of CO . These experiments were
conducted at the NMR scale using J. Young type NMR tubes
2
Figure 4. Structure of 5 (50% ellipsoids). Hydrogen atoms except of
H(1A) have been omitted for clarity. Selected bond distances (Å) and
angles (deg): Ga(1)−C(1) = 1.927(2), Ga(1)−C(1)# = 1.927(2),
Ga(1)···Cl(6) = 2.824, Ga(1)···Cl(5) = 3.127, C(1)−Ga(1)−C(1)# =
loaded with solutions of catalysts 3−6 in ca. 0.5 mL of C D or
6
6
1
:2 v/v C D Br/C D and ca. 20 equiv of Et SiH. The reaction
6 5 6 6 3
mixture was then exposed to CO (ca. 1.3 atm), heated to 80
1
58.85, Cl(5)−Ga(1)−Cl(6) = 75.99, Cl(6)−Ga(1)−Cl(6)# = 67.88,
2
1
Cl(5)−Ga(1)−Cl(5)# = 140.76. Atoms denoted by # were generated
by the transformation x, 1/2-y, 1-z.
°C, and the progress of the reaction was monitored by H and
C{ H} NMR spectroscopy. Table 1 shows the catalytic
13
1
activities of 3−6 and the product distribution. The reductions
proceed in a stepwise fashion (Scheme 1) similar to those
While the Ga−C bond distance with a value of 1.927(2) Å is
11
+
catalyzed by other Lewis acids including [Et Al][CH B I ]
close to the ones reported for the cation [(2,6-Mes C H ) Ga]
(
2
6 11 6
2
6
3 2
27
22
and [(2,6-Mes C H O) Al][CHB Cl ], and the tandem
system Al(C F ) /B(C F ) . Using strong Lewis acids 3 and
Mes = 2,4,6-Me C H ) with an average value of 1.914 Å, the
2
6
3
2
11 11
3
6
2
12
Ga···Cl contacts are significantly longer (by at least 0.372 Å)
than those in aluminum compounds 3 and 4 with values of
6
5 3
6 5 3
4
as well as weaker Lewis acid 5 in C D solution, methane was
6 6
2
.824 and 3.127 Å, but still well within the sum of the van der
formed in 20−30% yield based on the hydrogen from Et
but the main product was C CH . This product and the
small amounts of (C CH are formed by Lewis acid
catalyzed Friedel−Crafts alkylation of C with the inter-
mediates Et SiOCH and (Et SiO) CH . HD is thought to
arise from the Lewis acid catalyzed coupling of Et SiOD with
Et SiH.
The activities of compounds 3, 4, and 6 in the CO
reactions are comparable in that Et SiH is consumed within 8−
SiH,
3
23
Waals radii (3.62 Å). Some of the lengthening may be
attributed to the higher coordination number in 5 but most is
likely due to the lower Lewis acidity of gallium versus
D
6
5
3
D
6
)
5
2
2
D
6
6
aluminum. Contrary to 3 and 4, the coordination of the
3
3
3
2
2
+
anion to the [Et Ga] fragment involves chlorine donors solely
3
2
from the meta-belt of the anions. The C−Ga−C angle
158.85°) approaches 180°, which may be interpreted either
3
(
reduction
2
as a result of the octahedral coordination or the rather weak
Ga···Cl interactions affording an almost linear coordination
environment for a quasi-two-coordinate gallium center. Its
good solubility in benzene suggests that the chains do not
3
13 h. Contrary to these results, there is still some unreacted
Et SiH present after 1 week at 80 °C in case of the gallium
3
compound 5. Interestingly, while the product distribution in
1
persist in solution, but the broadened H NMR signals for the
the reactions catalyzed by 3−5 with mainly d -toluene
5
ethyl group could be due to dynamic processes in solution. It
(C D CH ) and methane is similar to those reported for the
6
5
3
a
11
27
Table 1. Catalytic Reduction of CO with Et SiH and Catalysts 3−6, [Et Al][CH B I ], and [(ArO) Al][CHB Cl ]
2
3
2
6
11 6
2
11 11
b
c
products (μmol) [%]
d
catalyst
solvent
C D
Et SiH (μmol) time (h) C D CH
(C D ) CH Et SiOCH3
CH4
HD
conversion (%)
3
6
5
3
6
5
2
2
3
3
4
4
5
6
202
219
88
13
28 [42]
24 [33]
9.5 [32]
12 [45]
1.6 [1.6]
0.5 [0.5]
0.8 [1.8]
0.03 [0.1]
0.1 [0.1]
5.6 [6]
11 [21]
11 [21]
6.7 [30]
4.6 [24]
5.8 [11]
33 [69]
4 [16]
36 [18]
21 [9.5]
20 [23]
12 [16]
>99
>99
97
6
6
C D /C D Br
9.5
9.5
181
6
6
6
6
6
5
e
C D
6
f
C D
6
94
82.6
>99
>99
>99
C D /C D Br
218
189
100
8.25
36
0.6 [0.8]
16 [25]
20 [60]
38 [52]
6
6
6
6
6
5
g
[
Et Al][CH B I ]
C D
6
not given
14 [14]
2
6
11 6
h
[
(ArO) Al][CHB Cl ]
C D
6
49
1 [2]
2
11 11
a
b
1
Reactions were performed with ca. 5% catalyst loading at 80 °C. Determined by H NMR integration using the solvent signal as internal standard.
c
d
e
f
Percent calculated per Si−H bond. Based on consumed Si−H. Reaction performed with 7% catalyst loading. Reaction stopped at 181 h.
g
11
h
27
Reaction performed with 8% catalyst loading.
Reaction performed with 3% catalyst loading at 83 °C; Ar = 2,6-Mes C H .
2 6 3
C
DOI: 10.1021/acs.organomet.7b00716
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
2
20
Dipp*Li, and [Ph C][CHB Cl ] were prepared according to
Scheme 1. Important Steps and Side Reactions in the Lewis
Acid Catalyzed CO Reduction with Et SiH
3
11 11
literature procedures. All other reagents were obtained from
commercial suppliers and used as received. NMR spectra were
recorded on a Bruker Avance 400 MHz spectrometer. H NMR
2
3
1
chemical shift values were determined relative to the residual protons
1
3
in C D H in benzene-d as internal reference (δ 7.16 ppm). C NMR
6
5
6
1
1
spectra were referenced to the solvent signal (δ 128.39 ppm).
B
NMR spectra were referenced to an external solution of F B·OEt in
3
2
C D and the chemical shifts are reproducible with an error of less
6
6
than 1 ppm. Given the high sensitivity of the new compounds, the
small scale of their syntheses, the formation of clear solutions for NMR
analysis, the excellent agreement of the NMR spectra with the crystal
structures, and the generally high purity of the compounds based on
NMR spectroscopy, no attempts were made to obtain CHN analysis
data. GC-MS spectra were collected on an HP G1800GCD gas
chromatograph.
Safety Notes. Caution: Et Al and Et Ga are pyrophoric and should
3
3
be handled with care under an inert atmosphere. The NMR tubes for the
CO reduction experiments should not be cooled much below −70 °C to
2
avoid deposition of solid CO .
2
Et Ga. Carefully dried GaF (40.52 g, 319.81 mmol) was slowly
3
3
added through a solids addition funnel to a solution of EtMgBr
(prepared in situ from the reaction of magnesium (26.42 g, 1.09 mol)
and EtBr (82.0 mL, 120.54 g, 1.11 mol) in diethyl ether (350 mL) at 0
°C. The reaction mixture was then refluxed for 2 days. The ether was
distilled off, and the reaction flask heated for 1 h at 120 °C to remove
1
1
Lewis acids [Et Al][CH B I ] and [(2,6-Mes C H O) Al]-
2
6
11 6
2
6
3
2
2
7
[
CHB Cl ], the m-terphenyl gallium species 6 afforded
mostly Et SiOCH . This type of selectivity is less common for
metal catalysts, which was surprising because previous
investigations have shown that strong Lewis acids catalyze the
11 11
3
3
2
8
most of the solvent. The crude Et Ga·etherate was distilled under
3
11
vacuum (oil bath temperature gradually increased up to 145 °C) and
collected into an ice-cooled flask. The product still contained a large
methylation of benzene with Et SiOCH .
3
3
The strong Lewis acidities of 3 and 4 are indicated by the
loss of the J(H−Si−CH ) coupling in Et SiH in the H NMR
spectra as soon as the silane was added. Furthermore, Et SiH
1
3
1
amount of ether based on H NMR analysis. Toluene (ca. 40 mL) was
2
3
added, and the etherate solution was fractionally distilled first under
3
nitrogen to remove the ether and toluene, then Et Ga distilled under
3
was subject to substituent scrambling, and Et Si, Et SiH and
4
2
2
slight vacuum (vacuum valve was partially open) and collected in an
1
13
1
EtSiH were observed in the H and C{ H} NMR spectra.
ice cooled flask (boiling point 35−40 °C). Et Ga was obtained as a
3
3
This effect was also reported when Et SiH was exposed to other
strong Lewis acids such as [(2,6-Mes C H O) Al]-
colorless liquid and contained 0.26 equiv of toluene and 0.02 equiv of
3
Et O based on NMR integration (Figure S1). The isolated product
2
6
3
2
2
2
7
29
was used without further purification. Isolated yield: 13.45 g, 85.72
[
CHB Cl ] and Al(C F ) . Some scrambling also
11 11 6 5 3
1
mmol, 26.8%. H NMR (C D , 400.13 MHz): δ = 1.18 (t, J = 8.06 Hz,
occurred when Et SiH was reacted with compounds 5 and 6,
6
6
3
13
1
CH CH , 9H), 0.46 (q, J = 8.03 Hz, CH CH , 6H). C{ H} NMR
C D , 100.61 MHz): δ = 10.38 (CH CH ), 10.07 (CH CH ).
6 6 2 3 2 3
2
3
2
3
but its rate was significantly slower due to the lower Lewis
acidity of gallium. A similar scrambling was also observed for
the primary siloxane product (Et Si) O, which is rapidly
33
(
(
2,6-Ph C H )AlEt , 1. A solution of Et AlCl (1.05 g, 8.71 mmol)
2 6 3 2 2
3
2
in hexane (20 mL) was added dropwise to a suspension of 2,6-
converted into Et Si and various polysiloxanes denoted here for
4
Ph C H Li (2.00 g, 8.47 mmol) in hexanes (20 mL) at −78 °C. The
2
6
3
simplicity as (Et SiO) .
2
n
reaction mixture was stirred at −78 °C for 1 h, allowed to warm up to
room temperature, and stirred overnight followed by heating at 50 °C
for an additional 5 h. The reaction mixture was filtered through a
medium porosity frit lined with Celite while still warm. Concentration
of the resulting solution to ca. 15 mL and cooling to −20 °C for 4 days
afforded (2,6-Ph C H )AlEt as a colorless crystalline solid. The
CONCLUSION
■
While the crystal structure of [Et Al][CHB Cl ], 3, closely
resembles those of the previously reported compounds
Et Al][CH B X ] (X = Cl and I), its catalytic activity is
about an order of magnitude higher due to the lower basicity of
the undecachlorinated counterion. Introduction of a moder-
ately bulky m-terphenylsubstituent (compound 4) does not
result in significant structural changes and only in a slight
increase in catalytic activity. The lower Lewis acidity of the
polymeric [Et Ga][CHB Cl ] (5) results in a much lower
reactivity, but the likely solvent separated ion pair
Dipp*GaEt][CHB Cl ] (6) is not only very reactive but
also selective with the formation of Et SiOCH as the main
product as opposed to methane or toluene for the other
complexes.
2
11 11
2
6
3
2
[
2
6 11 6
mother liquor was removed by cannula, the crystals were washed with
hexanes (2 mL) and dried under vacuum. Isolated yield: 2.24 g, 7.12
1
mmol, 81.7%. H NMR (C D , 400.13 MHz): δ = 7.44 (d, br, J = 7.4
6
6
Hz, overlapping o-H(Ph) and m-H(C H ), 6H), 7.34 (t, J = 7.7 Hz, p-
6
3
H(C H ), 1H), 7.19 (t, J = 7.6 Hz, partially obscured by C D H, m-
6
3
6
5
H(C H ), 4H), 7.06 (t, J = 7.5 Hz, p-H(C H ), 2H), 0.98 (t, J = 8.1
6
5
6
5
1
Hz, CH
CH
, 6H), 0.04 (q, J = 8.1 Hz, CH
CH
, 4H). 13C{ H} NMR
2
3
2
3
2
11 11
(
C D , 100.61 MHz): δ = 153.06 (i-C), 150.72 (o-C), 148.98 (i-
6 6
C(Ph)), 131.48 (m-C(Ph)), 129.65 (p-C), 128.15 (p-C(Ph), obscured
[
11
11
by C D ), 126.31(o-C(Ph)), 125.45 (m-C), 9.33 (CH CH ), 3.30
6
6
2
3
3
3
(
CH CH ).
2 3
(2,6-Dipp C H )GaEt , 2. A solution of Et GaCl (prepared in situ
2 6 3 2 2
from the reaction of GaCl (0.32 g, 1.82 mmol) with Et Ga (0.56 g,
3
3
3
.57 mmol) in hexanes (20 mL)) was added dropwise to a suspension
EXPERIMENTAL SECTION
General Procedures. All work was performed under anaerobic
and anhydrous conditions by using either modified Schlenk techniques
or a Vacuum Atmospheres drybox. Solvents were freshly distilled
■
of Dipp*Li (2.19 g, 5.41 mmol) in hexanes (20 mL) at −78 °C. The
reaction mixture was stirred at −78 °C for 1 h, allowed to warm to
room temperature, stirred overnight and heated at 40 °C for an
additional hour. The reaction mixture was filtered through a medium
porosity frit lined with Celite. Removal of the solvent under vacuum
yielded an oily phase that solidified upon standing at room
temperature for several hours. The solid was heat-dissolved in a
under N from sodium, potassium, or sodium/potassium alloy and
2
degassed twice prior to use or they were dispensed from a commercial
solvent purification system. The compounds Et GaCl, Ph C H Li,
30
31
2
2
6
3
D
DOI: 10.1021/acs.organomet.7b00716
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
minimum amount of hexanes, ca. 5 mL, and allowed to stand at room
washed with hexanes (2 × 0.5 mL) and dried under vacuum. Isolated
1
temperature for 1 day affording the Dipp*GaEt as a white crystalline
yield: 214 mg, 329 μmol, 82.5% (based on trityl salt). H NMR (C D ,
2
6
6
solid. The mother liquor was pipetted off, and the crystals were washed
with hexanes (2 × 1 mL) and dried under vacuum. Isolated yield: 1.21
g, 2.30 mmol, 43.0%. Contains ca. 3% of the hydrolysis product
400.13 MHz): δ = 2.27 (s, CHB Cl , 1H), 0.73 (s, br, w = 28 Hz,
11 11 1/2
1
3
1
CH CH , 4H), 0.12 (t, J = 7.5 Hz, CH CH , 6H). C{ H} NMR
2
3
2
3
1
1
(C D , 100.61 MHz): δ = 48.25(CHB Cl ), 8.30 (CH CH ).
B
6
6
11 11
2
3
1
Dipp*H. H NMR (C D , 400.13 MHz): δ = 7.30−7.21 (m, p-
NMR (C D , 100.38 MHz): δ = −2.9 (p-B, 1B), −9.8 (o- or m-B, 5B),
6
6
6
6
H(Dipp), m- and p-H(Ph), 5H), 7.15 (d, br, partially obscured by
C D H, J = 4.48 Hz, m-H(Dipp), 4H), 3.07 (sept, J = 6.84 Hz,
−12.2 (o- or m-B, 5B).
[Dipp*GaEt][CHB Cl ], 6. A small grease-free tube equipped
6
5
11 11
CH(CH ) , 4H), 1.24 (d, J = 6.92 Hz, 12 H, CH(CH ) ), 1.06 (d, J =
with a Teflon valve and a small magnetic stirring bar was charged with
3
2
3 2
6
.92 Hz, 12 H, CH(CH ) ), 0.81 (t, J = 8.04 Hz, CH CH , 6H), 0.40
[Ph C][CHB11Cl11] (30 mg, 39 μmol), benzene (1.0 mL) and Et SiH
3
3
3
2
2
3
13
1
(
q, J = 8.08, CH CH , 4H). C{ H} NMR (C D , 100.61 MHz): δ =
(15 mg, 0.13 mmol). The mixture was stirred for 10 h upon which the
orange color of the trityl salt faded giving an oily yellowish phase. The
colorless benzene layer was pipetted off, and the oily layer was washed
with hexanes (2 × 0.5 mL) to yield the silylium salt. To this was added
benzene (0.5 mL) and 3 (23 mg, 44 μmol). The reaction mixture was
stirred for 2 days at room temperature giving a colorless solution
above a colorless dense oily phase. The mother liquor was pipetted off,
and the oily gallium compound (ca. 0.07 mL) was washed with
2
3
6
6
1
56.71 (i-C), 147.45, 146.03, 142.23 (o-C), 129.14 (m-C), 128.05 (p-
C(Dipp)), 127.44 (p-C), 123.76 (m-C(Dipp)), 30.95 (CH(CH ) ),
3
2
2
6.59, 22.99 (CH(CH ) ), 11.11 (CH CH ), 9.29 (CH CH ).
3 2 2 3 2 3
[
Et Al][CHB Cl ], 3. A small grease-free Schlenk tube equipped
2 11 11
with a Teflon valve and a small magnetic stirring bar was charged with
Ph C][CHB Cl ] (150 mg, 196 μmol), benzene (1.0 mL), and
[
3
11 11
Et SiH (52 mg, 0.44 mmol), and the mixture was stirred for 10 h upon
3
which the orange color of the trityl salt faded to give a dense yellowish
oily phase. The colorless benzene upper layer was pipetted off, and the
oily layer was washed with hexanes (2 × 0.5 mL) to afford the silylium
salt. Hexanes (1.5 mL) and Et Al (25 mg, 219 μmol) were added, and
the reaction mixture was sonicated for 2 h to afford [Et Al]-
benzene (3 × 0.3 mL) and dissolved in a C
6
D
6
/C
, 400.13 MHz): δ = 7.38 (t, J = 7.8 Hz, p-
H(Dipp), 2H), 7.31 (t, J = 7.6 Hz, p-H(C ), 1H), 7.16 (d, partially
6 5
D Br 2/1 mixture
1
(0.5 mL). H NMR (C D
6 6
H
3
6
obscured by C
H, p-H(C
Hz, CH(CH
= 6.6 Hz, CH(CH
D
H, p-H(Dipp), 4H, 4H), 7.16 (d, obscured by
H ), 2H), 2.69 (s, CHB11Cl11, 1H), 2.59 (sept, J = 6.7
6 3
3
6
5
C
D
2
6
5
[
CHB Cl ] as a colorless solid. The solids were heat-dissolved in an
3
)
2
, 4H), 1.04 (d, J = 6.8 Hz, CH(CH
3
)
2
, 12H), 0.94 (d, J
, 12H), 0.11 (m, overlapping CH CH , 5H).
D , 100.61 MHz): δ = 148.58 (i-C), 147.62
6
11
11
oil bath at 105 °C. Allowing it to slowly cool down afforded the
product as a crystalline solid containing well-shaped blocks. The
mother liquor was pipetted off, and the crystals were washed with
hexanes (0.5 mL) and dried under vacuum. Isolated yield: 72 mg, 119
3
)
2
2
3
13
1
C{ H} NMR (C
6
(quaternary C), 146.42 (quaternary C), 138.83 (quaternary C), 133.04
(p-C(C H )), 132.15 (p-C(Dipp)), 129.75 (m-C(C H )), 126.08 (p-
6
3
6
3
1
μmol, 60.7% (based on trityl salt). H NMR (C D , 400.13 MHz): δ =
C(Dipp)), 47.53 (CHB Cl ), 31.37 (CH(CH ) ), 25.91 (CH-
6
6
11 11 3 2
1
1
2
.11 (s, CHB Cl , 1H), 0.73 (t, J = 8.2 Hz, CH CH , 6H), 0.12 (q, J
(CH
NMR (C
−12.4 (o- or m-B, 5B).
CO Reduction. The catalytic reduction reactions of CO
catalysts 3−6 were performed in J. Young style NMR tubes in C
1:2 v/v C Br/C solution (ca. 0.5 mL). The reaction mixture
containing the catalyst (usually 4−11 μmol) and ca. 20 equiv of Et SiH
(88−219 μmol, 5% catalyst loading) was degassed by two freeze−
at 1 atm while maintaining
3
)
2
), 24.00 (CH(CH
D
6 6
3 2 2 3 2 3
) ), 15.97 (CH CH ), 8.61 (CH CH ). B
1
1
11
2
3
13
1
=
8.2 Hz, CH CH , 4H). C{ H} NMR (C D , 100.61 MHz): δ =
, 100.38 MHz): δ = −1.6 (p-B, 1B), −9.2 (o- or m-B, 5B),
2
3
6
6
11
4
9.0 (s, CHB Cl , 1H), 7.54 (CH CH ), 5.75 (CH CH ). B NMR
11 11 2 3 2 3
(
C D , 100.38 MHz): δ = −3.6 (p-B, 1B), −10.2 (o- or m-B, 5B),
2
2
using
or
6
6
−
12.2 (o- or m-B, 5B).
6
D
6
[
(2,6-Ph C H )AlEt][CHB Cl ], 4. A small grease-free tube
6
D
5
D
6 6
2
6
3
11 11
equipped with a Teflon valve and a small magnetic stirring bar was
charged with [Ph C][CHB Cl ] (200 mg, 261 μmol), benzene (1.0
mL), and Et SiH (50 mg, 0.43 mmol). The mixture was stirred for 10
3
3
11 11
pump−thaw cycles and filled with CO
3
2
h upon which the orange color of the trityl salt faded giving a dense
oily phase. The colorless benzene top layer was pipetted off, and the
oily layer washed with hexanes (2 × 0.5 mL) to yield the silylium salt.
Benzene (1.5 mL) and 2 (90 mg, 286 μmol) were added at room
temperature. The reaction mixture was stirred for 2 h to afford a white
precipitate. The solids were heat-dissolved in an oil bath at 100 °C and
allowed to slowly cool down to give the product as a colorless
crystalline solid. The mother liquor was pipetted off, and the crystals
were washed with benzene (2 × 0.3 mL) and dried under vacuum.
two-thirds of the NMR tube at −70 °C. The progress of the reaction
1
was monitored by H NMR spectroscopy, and the products were
identified by their NMR chemical shifts and GC-MS data. The results
1
are summarized in Table 1, and Figures S19−S22 show the H NMR
spectra illustrating the progress of selected reactions. The amounts of
CH and HD were estimated using NMR integration and the Henry
4
34 35
constants (0.021 M/atm for CH
a gas volume of 1.5 mL.
, 0.0028 M/atm for H ) assuming
2
4
X-ray Diffraction. Crystals of compounds 3−5 were grown as
described above. The data were collected using a diffractometer with a
Bruker APEX ccd area detector and graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å) at 100(2) K. The data were corrected for
1
Isolated yield: 172 mg, 213 μmol, 81.6%. H NMR (C D , 400.13
6
6
MHz): δ = 7.27−7.22 (m, br, overlapping m- and p-H(C H ), partially
6
3
obscured by o-H(Ph), 3H), 7.22 (d, J = 7.2 Hz, o-H(Ph), 4H), 7.12 (t,
J = 7.5 Hz, m-H(Ph), 4H), 7.00 (t, J = 7.4 Hz, p-H(Ph), 2H), 1.98 (s,
CHB Cl , 1H), 1.13 (t, J = 8.1 Hz, CH CH , 3H), 0.65 (q, J = 8.1
3
6
absorption by the empirical method, and the structures were solved
3
7
and refined using the SHELXS and SHELXL package. Crystal data
and a summary of the data collection and refinement details are
provided in Table S2.
11
11
2
3
13
1
Hz, CH CH , 2H). C{ H} NMR (C D , 100.61 MHz): δ = 151.78
2
3
6
6
(
quaternary C), 147.06 (quaternary C), 142.15 (i-C), 132.07 (p-
C(C H ) partially deuterated), 131.68 (m-C(Ph)), 129.79 (p-C(Ph)),
6
3
1
28.00 (o-C(Ph)), 127.00 (m-C(C H )), 49.15 (CHB Cl ), 10.80
6 3 11 11
ASSOCIATED CONTENT
11
■
(CH CH ), 9.23 (CH CH ). B NMR (C D , 100.38 MHz): δ =
2 3 2 3 6 6
*
S
Supporting Information
−
3.9 (p-B, 1B), −10.4 (o- or m-B, 5B), −12.5 (o- or m-B, 5B).
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00716.
[
Et Ga][CHB Cl ], 5. A small Schlenk tube equipped with a small
2
11 11
magnetic stirring bar was charged with [Ph C][CHB Cl ] (305 mg,
3
reaction mixture was stirred for 10 h upon which the orange color of
the trityl salt faded giving a dark yelllow oily phase. The colorless
benzene layer was pipetted off, and the oily layer washed with hexanes
3
11 11
99 μmol), benzene (3.0 mL) and Et SiH (110 mg, 0.95 mmol). The
3
Multinuclear NMR spectra for Et Ga and compounds 1−
3
6
, reduction of CO catalyzed by compounds 3−6, and
2
(
2 × 1.0 mL) to yield the silylium salt. To that was added benzene (3.0
an X-ray table (PDF)
mL) and Et Ga (72 mg, 459 μmol). The reaction mixture was stirred
3
Accession Codes
overnight affording a clear, colorless solution. The product was
isolated by layering hexanes on top of the reaction mixture (2.0 mL)
and allowing it to stand for 1 day inside the glovebox at room
temperature. The mother liquor was pipetted off, and the crystals
CCDC 1576118−1576120 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
E
DOI: 10.1021/acs.organomet.7b00716
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(24) Atwood, J. L., Anionic and Cationic Organoaluminum
Compounds. In Coordination Chemistry of Aluminum; Robinson, G.
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Eur. J. 2014, 20, 15482−15492.
AUTHOR INFORMATION
Corresponding Author
E-mail: rwehmsch@fit.edu.
■
(26) Chen, E. Y. X.; Lancaster, S. J. Weakly Coordinating Anions:
Highly Fluorinated Borates. In Comprehensive Inorganic Chemistry II,
2nd ed.; Poeppelmeier, K., Ed.; Elsevier: Amsterdam, 2013; pp 707−
*
7
(
2
54 and references therein.
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ORCID
Rudolf J. Wehmschulte: 0000-0003-0719-0620
Notes
The authors declare no competing financial interest.
̈
(
6
(
ACKNOWLEDGMENTS
■
This work was supported in part by the Petroleum Research
Fund administered by the American Chemical Society (PRF
No. 52856-ND3), the 2013 NASA/KSC Chief Technologist
Center Innovation Fund, and the Florida Institute of
Technology.
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DOI: 10.1021/acs.organomet.7b00716
Organometallics XXXX, XXX, XXX−XXX