CO2 activation with bulky neutral and cationic phenoxyalanes

Article abstract of DOI:10.1021/om400628z

The sterically crowded aluminum diphenolate (2,6-Mes₂C ₆H₃O)₂AlEt (2; Mes =2,4,6-Me₃C ₆H₂-) was converted into the ionic species [(2,6-Mes ₂C₆H₃O)₂Al]⁺[CHB ₁₁Cl₁₁]^- (6) by ethide abstraction with the silylium salt [Et₃Si][CHB₁₁Cl₁₁] or by a combination of β-hydride abstraction and concomitant ethylene elimination with the trityl salt [Ph₃C][CHB₁₁Cl₁₁]. Compound 6 consists of solvent-separated ions, and the cation features very short Al···C contacts involving the flanking mesityl groups, leading to an overall distorted-tetrahedral coordination geometry around the aluminum center. Analogous reactions with the diphenolate (2,6-tBu ₂-4-MeC₆H₂O)₂AlEt (1) led to tert-butyl transfer to the benzene solvent to afford tert-butylbenzene as the main product. A 1:1 mixture of the triphenolate (2,6-Ph₂C ₆H₃O)₃Al and tri-tert-butylphosphine forms a frustrated Lewis pair, which reacts with CO₂ at room temperature to give the compound (2,6-Ph₂C₆H₃O) ₃AlOC(O)Pt-Bu₃ (8). The high Lewis acidity of 6 catalyzes the scrambling of the subsituents of Et₃SiH at room temperature to give Et₄Si, Et₂SiH₂, and EtSiH₃ and also the reduction of CO₂ to d₅-toluene (C ₆D₅CH₃) and CH₄ at 82 C in d ₆-benzene solution.

Full text of DOI:10.1021/om400628z

Article
pubs.acs.org/Organometallics
CO Activation with Bulky Neutral and Cationic Phenoxyalanes
2
,†
†
‡
Rudolf J. Wehmschulte,* Mahmoud Saleh, and Douglas R. Powell
†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: The sterically crowded aluminum diphenolate (2,6-
Mes C H O) AlEt (2; Mes =2,4,6-Me C H −) was converted into
2
6
3
2
3
6
2
+
−
the ionic species [(2,6-Mes C H O) Al] [CHB Cl ] (6) by ethide
2
6
3
2
11 11
abstraction with the silylium salt [Et Si][CHB Cl ] or by a
3
11 11
combination of β-hydride abstraction and concomitant ethylene
elimination with the trityl salt [Ph C][CHB Cl ]. Compound 6
3
11 11
consists of solvent-separated ions, and the cation features very short
Al···C contacts involving the flanking mesityl groups, leading to an
overall distorted-tetrahedral coordination geometry around the
aluminum center. Analogous reactions with the diphenolate (2,6-tBu2-
4
-MeC H O) AlEt (1) led to tert-butyl transfer to the benzene solvent
6 2 2
to afford tert-butylbenzene as the main product. A 1:1 mixture of the
triphenolate (2,6-Ph C H O) Al and tri-tert-butylphosphine forms a
2
6
3
3
frustrated Lewis pair, which reacts with CO at room temperature to give the compound (2,6-Ph C H O) AlOC(O)Pt-Bu (8).
2
2
6
3
3
3
The high Lewis acidity of 6 catalyzes the scrambling of the subsituents of Et SiH at room temperature to give Et Si, Et SiH , and
3
4
2
2
EtSiH and also the reduction of CO to d -toluene (C D CH ) and CH at 82 °C in d -benzene solution.
3
2
5
6
5
3
4
6
INTRODUCTION
Aluminum phenolate compounds such as MeAl(OC H -tBu -
6 2 2
■
2
,6-Me-4) (MAD), MeAl(OC H -Ph -2,6) and Al(OC H -
2 6 3 2 2 6 3
In addition to being a problematic greenhouse gas, carbon
dioxide is also the major C feedstock for photosynthetic
organisms, and the chemical industry has been increasing its use
as an inexpensive base chemical. Most applications focus on
redox-neutral substitutions such as the formation of urea from
Ph -2,6) have been used as Lewis acid additives in numerous
2
3
1
organic transformations, including 1,4-addition to α,β-unsatu-
rated carbonyl compounds or exo-selective Diels−Alder
1
,2
1
3,14
reactions.
Several of these compounds have been
characterized by X-ray crystallography, which showed that
NH3 and CO2 or the synthesis of organic carbonates
1
they featured three-coordinate strongly Lewis acidic aluminum
OC(OR) . Catalytic reduction to formic acid or methanol
2
15−17
centers.
Here, we report the synthesis of ethylaluminum
with hydrogen as the reducing agent is also of current interest
as part of the development of alternative fuels.
hydrosilanes as more reactive, easy to handle, and rather benign
3
−5
diphenolate compounds, the first cationic aluminum dipheno-
The use of
late, and the activation and reduction of CO using some of
2
these species. Ethylaluminum compounds were chosen because
methide abstraction is often more difficult in sterically crowded
species than formal ethide or alkide abstraction.
reducing agents has attracted significant interest in recent
6
−10
years.
As part of our investigation of very strong Lewis
acids, we have shown that the tight ion pairs [Et Al]-
2
[
CH B X ] (X = Cl, I) catalyze the reduction of CO to
6 11 6 2
11
RESULTS AND DISCUSSION
toluene and methane with Et SiH as the reducing agent.
■
3
Unfortunately, these systems possess a rather low reactivity,
and they suffer from rapid catalyst deactivation. Although the
identity of the actual catalytic species is not yet known, it is
quite probable that the lability of the Al−C bond contributes to
the facile deactivation, especially in the presence of possible
protic side products. Considering the high strength and polarity
While the synthesis of the diphenolates (2,6-tBu -4-
2
1
5
MeC H O) AlEt (1)
and the new compound (2,6-
6
2
2
Mes C H O) AlEt (2) was readily accomplished according to
2
6
3
2
eq 1, the preparation of (2,6-Ph C H O) AlEt (3) proved more
2
6
3
2
difficult, and a pure product has not yet been obtained.
18
Following Yamomoto’s procedure for the synthesis of the
of the Al−O bond, we hypothesized that low-coordinate
cationic aluminum alkoxides or phenoxides [(RO) Al] could
related methyl analogue in CH Cl led to the isolation of the
2
2
+
2
be more reactive and robust catalysts. In order to prevent
aggregation, large substituents were necessary, and the
commercially available phenols 2,6-tBu -4-MeC H OH and
Special Issue: Applications of Electrophilic Main Group Organo-
metallic Molecules
2
6
2
2
,6-Ph C H OH in addition to the very bulky phenol 2,6-
Mes C H OH (Mes = 2,4,6-Me C H −) were selected.
Received: June 28, 2013
Published: August 20, 2013
2
6
3
12
2
6
3
3
6
2
©
2013 American Chemical Society
6812
dx.doi.org/10.1021/om400628z | Organometallics 2013, 32, 6812−6819

Organometallics
Article
19
triphenolate (2,6-Ph C H O) Al (4), and the mother liquor
Compound 6 is partially soluble in benzene or toluene and
2
6
3
3
contained mostly the dimeric monophenolate (2,6-
tends to form a dense oil when the solubility limit is reached.
Such liquid clathrates are often observed for large solvent-
20
30
Ph C H OAlEt ) (5) and only traces of the target molecule
2
6
3
2 2
3
. Using hexanes or toluene as solvent gave similar results, and
separated ion pairs. A crystalline product has been obtained,
when the silylium salt was employed as the ethyl abstracting
reagent. Its H and C{ H} NMR spectra show broadened
the use of Et O resulted in the isolation of the etherate (2,6-
Ph C H O) Al·OEt . Comproportionation of a 1:1 mixture of
4
NMR tube for 5 days finally afforded compound 3 in
approximately 80% purity. A scale-up of this reaction or
comproportionation without solvents did also not lead to pure
2
1
13
1
2
6
3
3
2
and 5 in C D solution at 80−135 °C in a J. Young type
signals for the o-CH groups and m-H(Mes), indicating a
6
6
3
fluxional behavior at room temperature. The solid-state
structure of 6 consists of separated cations and anions, and
one of the two independent cations is depicted in Figure 1.
2
1,22
3. Interestingly, neither compound 3
nor the related species
) have been isolated
2
2−26
(
2,6-Ph C H O) AlR (R = Me, iBu
2
6
3
2
and characterized completely. Usually, these diphenolates were
generated in situ and used as such. In some instances, partial
NMR data of (2,6-Ph C H O) AlMe (δ (AlCH ) −1.75
2
6
3
2
H
3
1
8
ppm) and (2,6-Ph C H O) AliBu (δ (AlCH ) −1.40
2
6
3
2
H
2
2
5
ppm) and the adduct (2,6-Ph C H O) AlMe·OC(H)tBu
2
6
3
2
1
8,27
(
δ (AlCH ) = −1.32 ppm)
were given, in which the
H
3
presence of the Al−Me and Al−CH signals at high field
2
supports the composition. It is not quite clear why the synthesis
of compound 3 did not work in our hands, but we hypothesize
that during its synthesis compound 3, which is most likely a
monomer with a three-coordinate Lewis acidic aluminum
center, reacts more quickly with the phenol than the primary
reaction product, the dimeric phenolate-bridged 5. Thus, the
major products are compounds 4 and 5.
The two diphenolates 1 and 2 reacted with the trityl and
silylium salts [Ph C][CHB Cl ] and [Et Si][CHB Cl ] (eqs
3
11 11
3
11 11
2
and 3), but only in the reaction with the dimesityl-substituted
Figure 1. Thermal ellipsoid plot (30%) of one of the two independent
cations in 6. Hydrogen atoms have been omitted for clarity. Selected
distances (Å) and angles (deg): Al(1A)−O(1A) = 1.718(3), Al(1A)−
O(2A) = 1.710(3), O(1A)−C(1A) = 1.362(4), O(2A)−C(25A) =
phenolate 2 was the expected product, the ion- pair [(2,6-
Mes C H O) Al][CHB Cl ] (6), obtained.
2
6
3
2
11 11
1
.358(4), Al(1A)···C(7A) = 2.430(3), Al(1A)···C(8A) = 2.273(4),
(
(
ArO) AlEt + [Ph C][CHB Cl ]
2
3
11 11
Al(1A)···C(31A) = 2.356(3), Al(1A)···C(32A) = 2.358(4); O(2A)−
Al(1A)−O(1A) = 119.30(14), C(1A)−O(1A)−Al(1A) = 123.8(2),
C(25A)−O(2A)−Al(1A) = 122.5(2).
8
5 °C
⎯
⎯⎯⎯ →⎯ [(ArO) Al][CHB Cl ] + Ph CH + CH CH
2
11 11
3
2
2
2
days
(
2)
The unit cell contains two independent molecules, whose
geometrical features are very similar, so that only one of the
two molecules will be discussed here. The aluminum center is
four-coordinate in a distorted-tetrahedral fashion by the two
phenolate oxygens and two ortho carbons of the flanking
mesityl groups. The Al−O distances with values of 1.710(3)
and 1.718(3) Å are similar to or slightly longer than those
observed for three-coordinate aluminum phenoxides (see e.g. t-
ArO) AlEt + [Et Si][CHB Cl ]
2
3
11 11
6
5−75 °C
⎯
⎯⎯⎯⎯⎯⎯⎯⎯→ [(ArO) Al][CHB Cl ] + Et Si
2
11 11
4
6 h
6
(3)
Ar = 2,6‐Mes C H ‐
2
6
3
3
1
Bu Al(O-2,6-t-Bu -4-C H ) with d(Al−O) = 1.710(2) Å or
2
2
6
2
17
In contrast to the synthesis of the related species [2,6-
Al(O-2,6-t-Bu -4-C H ) with d(Al−O) = 1.648(7) Å ) and
are only slightly shorter than those observed in the related
2
6
2 3
Mes C H GanBu][CHB Cl ], [2,6-Dipp C H GanBu]-
2
6
3
11
11
2
6
3
2
8
29
[
CHB Cl ], and [2,6-Dipp C H AlEt][CH B Cl ] the
terphenylates 2,6-Trip C H OAlMe ·OEt (Trip = 2,4,6-i-
1
1
11
2
6
3
6
11
6
2
6
3
2
2
3
2
synthesis of 6 through formal ethide abstraction required longer
reaction times and significantly higher temperatures. This may
be a result of the large size of the two phenolate substituents,
which could impede a close contact of the trityl or silylium ion.
Alternatively, the higher polarity of the Al−O bond versus the
Al−C bond could result in a strengthening of the Al−Et bond.
Pr C H −, d(Al−O) = 1.755(2) Å ) and 2,6-
3
6
2
Dipp C H OAlH ·NMe (Dipp = 2,6-i-Pr C H −, d(Al−O)
2
6
3
2
3
2
6
3
33
= 1.745(1) Å ) with four-coordinate aluminum centers. One
of the Al···C distances involving the flanking mesityl groups is
shorter (2.273(4) Å) than those reported for the benzene or
toluene adducts of the strong Lewis acid Al(C F ) with values
6
5 3
6
813
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Organometallics
Article
Scheme 1
3
4
of 2.342(6) and 2.366(2) Å or the intramolecular Al···C
electrophilic uranium(III) complex, in which the bulky ligand
only allows end-on coordination of CO2. As the diphenolate
2, the triphenolate 4, and the ionic species 6 possess similar
44
contact in [(2,6-Mes C H ) Al][B(C F ) ] with a value of
.353(1) Å, whereas the other Al···C contact is very similar to
2
6
3 2
6 5 4
3
5
2
these (2.356(3) Å). As the Al···C distances to the neighboring
characteristics, solutions of these compounds were exposed to
1
carbons are only slightly longer (2.430(3) and 2.358(4) Å), the
an excess of CO . However, no reaction was observed by H
2
2
13
1
interactions may also be viewed as η coordinations. Previously
and C{ H} NMR spectroscopy. Addition of the bulky
2
observed η -Al···C contacts range from 2.485(1) to 2.619(1) Å
phosphine tBu P to these solutions led to the formation of
3
35
in one of the structures of [(2,6-Mes C H ) Al][B(C F ) ]
new sets of signals in the case of 4 and 6, whereas no reaction
was observed for 2. The combination of a bulky Lewis acid and
a bulky Lewis base can lead to the formation of a frustrated
2
6
3 2
6 5 4
and from 2.471(2) to 2.540(2) Å in the cyclohexene adduct
2
36
(
C F ) Al·(η -cyclo-C H ). As a result of the strong Al···C
6 5 3 6 10
45
contacts the O−Al−O angle is reduced to 119.3(1)° from the
expected 180° for a two-coordinate species. Similarly, the same
Al···C contacts are also responsible for the rather long Al−O
distances. There are no cation···anion interactions other than
van der Waals contacts. The shortest H(cation)···Cl(anion)
distance is 2.828 Å (Cl12···H27B), which is close to the sum of
Lewis pair (FLP). Such species can possess remarkable
properties, including the activation of dihydrogen at room
46
temperature.
Addition of CO to a solution of a 1:1 mixture of 4 and the
2
bulky phosphine tBu P afforded the CO2 adduct (2,6-
3
Ph C H O) AlOC(O)PtBu (8) as a colorless crystalline solid
2
6
3
3
37
the van der Waals radii of H and Cl (2.85 Å). The closest Al···
Cl distance is 5.647 Å (Al1B···Cl8C). This is in stark contrast
in 55% yield. Compound 8 is stable up to its melting point of
37 °C in its solid form, but it partially dissociates into its
starting materials upon dissolution into C D or CDCl3.
1
with e.g. [Et Al][CH B Cl ], which features rather short Al···
2
6
11
6
6
6
Cl contacts (2.429 and 2.440 Å) and should be considered a
Increase of the CO partial pressure to about 1 atm increases
2
3
8
tight ion pair. Currently, no other donor-free monomeric
main-group-metal phenolates bearing m-terphenyl groups are
known. The related and isoelectronic alkaline-earth compounds
the amount of dissolved 8 to approximately 50%. The relatively
low yield for 8 is most likely due to the equilibrium detailed in
eq 4. Compound 8 possesses the lowest solubility in the system
and precipitates from the reaction mixture (toluene) in the
presence of excess CO2.
{
(2,6-Ph C H O) M} (M = Mg, Ca, Sr, Ba) are dimeric with
2 6 3 2 2
39
bridging phenolates, ArOM(μ-OAr) MOAr. The large metals
2
Ca, Sr, and Ba also feature secondary M···C(π) interactions
involving the flanking phenyl groups, similar to the case for 6.
Attempts to obtain [(2,6-tBu -4-MeC H O) Al][CHB Cl ]
2
6
2
2
11 11
(7) according to eqs 2 and 3 were unsuccessful. Only a small
amount of Ph CH or Et Si was formed, and the main product
3
4
was d -tert-butylbenzene, t-BuC D . The small amounts of
5
6
5
Ph CH and Et Si suggest that some 7 may have formed, which
3
4
Since the first example of a CO adduct derived from an FLP,
then initiated the tert-butyl transfer from 1 (or 7) to the solvent
2
47
+
+
(C F ) BOC(O)PtBu , a few examples of aluminum-based
(
C H or C D ). The resulting H or D could regenerate 7 to
6 5 3 3
6
6
6
6
adducts have been described, including (C F ) AlOC(O)PtBu
continue the cycle. However, despite numerous attempts we
have not been able to identify the aluminum product(s).
Attempted methyl abstraction from the related compounds
6
5 3
49
3
4
8
(
A),
Me AlCH PMe (μ-CO ) (B),
and PhCH
2
2
2
2
50
CPMes (AltBu )(μ-CO ) (C) and the doubly substituted
2
2
2
5
1,52
Mes PC(OAlX ) (D; X = Cl, Br, I)
(Scheme 1).
In contrast to compound 8, species A−D are stable toward
CO loss in solution, although compound C loses CO when
(
2,6-tBu -4-MeC H O) AlMe and (2,4,6-tBu C H O) AlMe
3
3 2
2
6
2
2
3
6
2
2
with [Ph C][B(C F ) ] also did not lead to the desired
3
6 5 4
4
0
cationic species but to various unidentified products.
2
2
heated at 135 °C under vacuum in the solid state. The crystal
structure of 8 (Figure 2) shows a four-coordinate aluminum
center surrounded by three phenolate ligands in a typical
propeller-like fashion.
Although tert-butyl transfer was not observed in this case, it
has been observed previously that tert-butyl phenols are
27,41
sensitive to strong Lewis acids.
In this context, it should
also be mentioned that the ionic compound 6 experiences slow
H/D exchange in C D solution involving the aromatic
The fourth coordination site is occupied by the tBu
O− group. The Al−Ophenol distances with an average value of
1.710 Å are close to those reported for (2,6-Ph O) Al·L
(L = Lewis base) compounds. The Al−OC(O)− distance
3
PC(O)-
6
6
hydrogens of the phenolate substituents. Residual toluene is
also converted into C D CH . These findings and the short
C
2
H
6
3
3
6
5
3
53−55
intramolecular Al···C contacts in 6 are a testament to the high
Lewis acidity of low-coordinate cationic aluminum dipheno-
lates.
is 1.819(2) Å, the O−C(O) distance is 1.287(4) Å, the CO
distance is 1.206(4) Å, and the (O)C−P distance is 1.906(3) Å.
Despite the bulk of the phenolate substituents these values are
very close to those observed for A (1.828, 1.289, 1.209, and
1.884 Å, respectively) and C (1.859, 1.280, 1.206, and 1.919 Å,
respectively). The CO stretching frequency for 8 (1708
Reaction with CO . Although numerous coordination
2
42
compounds of CO are known, coordination through one
2
1
of the weakly Lewis basic oxygens to form a linear η complex
such as L M−OCO is rare. One example is an
4
3
n
6
814
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Organometallics
Article
Scheme 2
not been observed directly, but it is implied by the formation of
diphenylmethane and toluene. Furthermore, the alcoholysis of
hydrosilanes with Lewis acid catalysis is well-known, and the
Figure 2. Thermal ellipsoid plot (30%) of 8. Hydrogen atoms and the
o-phenyl substituents have been omitted for clarity. Selected distances
56
(
1
Å) and angles (deg): Al(1)−O(1) = 1.716(3), Al(1)−O(2) =
observation of HD can be traced back to the reaction of Et SiH
3
.707(3), Al(1)−O(3) = 1.708(2), Al(1)−O(4) = 1.819(2), O(1)−
with Et SiOD.
3
C(1) = 1.343(4), O(4)−C(55) = 1.287(4), O(5)−C(55) = 1.206(4),
For comparison, the reductions catalyzed by [Et Al]-
2
P(1)−C(55) = 1.906(3), P(1)−C(56) = 1.880(4), P(1)−C(60) =
[
CH B X ] required 10% catalyst loading (vs 3% for 6), 2.5
6 11 6
1
.873(4), P(1)−C(64) = 1.892(4); O(2)−Al(1)−O(1) = 113.37(13),
days (vs 2 days for 6), and CH was the main product (70% of
4
O(3)−Al(1)−O(1) = 109.67(13), O(2)−Al(1)−O(3) = 113.52(13),
O(1)−Al(1)−O(4) = 104.92(12), O(2)−Al(1)−O(4) = 105.42(13),
O(3)−Al(1)−O(4) = 109.45(12), C(55)−O(4)−Al(1) = 129.8(2),
O(5)−C(55)−O(4) = 128.9(3), O(4)−C(55)−P(1) = 112.6(2),
O(5)−C(55)−O(4) = 128.9(3).
the silane hydrogen) followed by C D CH (26%) and
6
5
3
(
C D ) CH (4%).
Whereas mixtures of [Et Al][CH B X ] and Et SiH
6 5 2 2
2
6
11
6
3
remained unchanged at room temperature, mixtures of 6 and
Et SiH (1/24.5) led to scrambling of the silane substituents to
3
−
1
−1
afford Et Si, Et SiH , and even small amounts of EtSiH after 7
4 2 2 3
cm ) is higher than that for A (1686 cm ), which may
days at room temperature. This may be an expression of the
higher Lewis acidity of the cation in 6, which is π coordinated
to arenes but does not display the close cation···anion
account for the more facile CO loss for 8.
Although a reaction was observed when tBu P was added to a
solution of 6 and CO in C D , no product could be isolated,
and this reaction is under current investigation.
Reduction of CO . We have shown previously that the
strong Lewis acids [Et Al][CH B X ] (X = Cl, I) catalyzed the
reduction of CO2 with Et SiH, albeit rather slowly. We
hypothesized that strong Lewis acids with robust Al−O
bonds should be better catalysts. Although no interaction
between the ionic species 6 and CO could be detected,
2
3
2
6
6
3
interactions of [Et Al][CH B X ]. Furthermore, the J H−
2
6 11 6
Si−CH coupling in mixtures of 6 and Et SiH is lost, whereas
the Et SiH H NMR signal is unaffected by the presence of
2
3
2
1
3
2
6 11 6
[
Et Al][CH B X ]. A similar loss of Si−H coupling was
2
6 11 6
3
observed for mixtures of Et SiH and B(C F ) and was
attributed to the equilibrium expressed in eq 5. The
3
6 5 3
57
analogous equilibrium involving 6 and Et SiH is given in eq 6.
3
2
benzene solutions of 6 catalyzed the reduction of CO with
2
Et SiH. As we hoped, the reduction was faster, and the reaction
3
mixture remained homogeneous. A solution of 6 and Et SiH
3
(
1/33) in C D was exposed to excess CO (ca. 1.3 atm), and
6 6 2
1
13
1
the progress of the reaction was monitored by H and C{ H}
NMR spectroscopy. The addition of CO led to a broadening
2
and splitting of the o-Me signals of the phenoxy substituents in
The solution that was obtained after 6 and Et SiH were
3
1
the H NMR spectrum, indicating a chemical change. After 19
reacting with each other for 7 days was exposed to CO as
2
h at room temperature the methyl signals of 6 sharpened, and a
described previously, and broadening and splitting of the o-Me
signals were observed after 21 h at room temperature. Heating
at 82 °C for 1 h afforded some C D CH and CH , and the
small amount of C D CH formed. Subsequent heating at 85
6
5
3
°
C for 4 h resulted in the formation of more toluene as well as
6
5
3
4
the appearance of Et SiH (see below). More than 90% of
reaction was practically complete after 10 h at 82−83 °C to give
2
2
Et SiH was consumed after 25 h at 85 °C, and the reaction was
mainly C D CH (50% of the silane hydrogen), followed by
3
6
5
3
complete after another 24 h at 85 °C. The main product was
CH (ca. 16%) and traces of (C D ) CH (1%) similar to the
4 6 5 2 2
C D CH (60% of the silane hydrogen), followed by CH (ca.
distribution using pure Et SiH as before. The silanes EtSiH
3 3
6
5
3
4
1
8%) and traces of (C D ) CH (ca. 2%). The formation of
and Et SiH were consumed before Et SiH, suggesting a higher
6
5 2
2
2 2 3
HD was also confirmed (ca. 14%). The silane products
consisted of Et Si and various polysiloxanes, similar to the
reductions catalyzed by [Et Al][CH B X ]. The reactions
reactivity for the former. The reduction of a second charge of
CO with Et SiH in this solution required twice as much time
4
2
3
11
(23 h), possibly an indication of catalyst degradation.
2
6 11 6
are summarized in Scheme 2, which is based on previous
Attempts to reduce CO with Et SiH using the triphenolate
2
3
8,11
observations by Piers and us. The formation of Et SiOD has
4, the CO adduct 8, or combinations of 4 with 2,6-di-tert-
3
2
6
815
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Organometallics
Article
butylpyridine or 2,6-lutidine (potential FLP’s) as catalysts were
unsuccessful.
minutes after the end of the alane addition. The mixture was warmed
to room temperature and stirred for another 2 h. Concentration to 5−
1
0 mL under reduced pressure afforded a fine colorless precipitate,
which was redissolved by brief warming with a heat gun. After ca. 30
min at room temperature a crystalline solid began to separate. Cooling
to −20 °C for 1 week afforded the product as a mixture of colorless
CONCLUSIONS
■
Compound 6 is the first example of a donor solvent free
cationic aluminum diphenolate, and the stabilizing intra-
molecular Al···C interactions are among the shortest ones
that have been reported to date. The resulting high Lewis
acidity of the aluminum center leads to the facile catalyzed
very small and several large well-shaped crystals. Yield: 0.42 g, 69%
1
based on 2,6-Mes
C
H
3
OH. Mp: 254−258 °C. H NMR (C
D
6
,
2
6
6
4
6
8
00.13 MHz): δ 6.89 (A part of A B system, J = 7.8 Hz, m-H, 4H),
.84 (B part of A B system, J = 7.8 Hz, p-H, 2H), 7.75 (s, m-H(Mes),
H), 2.24 (s, p-CH , 12 H), 2.05 (s, o-CH , 24 H), 0.47 (t, J = 8.0 Hz,
CH CH , 3H), −1.01 (q, J = 8.0 Hz, CH CH , 2H). C{ H} NMR
C D , 100.61 MHz): δ 153.49 (i-C), 137.80 (o-C(Mes)), 137.53,
2
2
3
3
scrambling of the Et SiH substituents at room temperature, and
3
13
1
2
3
2
3
6
is a more reactive catalyst than the previously reported tight
(
11
6 6
ion pairs [Et Al][CH B X ] for the reduction of CO with
2
6
11
6
2
137.23, 130.59 (o-C), 130.38 (m-C), 130.30 (m-C(Mes)), 120.01 (p-
C), 21.42 (p-CH ), 21.03 (o-CH ), 9.34 (CH CH ), −5.40 (br,
Et SiH to d -toluene and CH . Unfortunately, catalyst 6 also
3
5
4
3
3
2
3
suffers from deactivation during the CO reduction, despite the
CH CH ).
2
2 3
large protecting phenolate substituents and the strong Al−O
Reaction of Et Al with 2 equiv of 2,6-Ph C H OH. Procedure
3 2 6 3
A. A solution of AlEt (0.23 g, 2.0 mmol) in hexanes (6 mL) was
bonds. Potential improvements could be the use of even larger
3
58
added to a solution of 2,6-Ph C H OH (0.99 g, 4.0 mmol) in CH Cl
phenols such as 2,6-Dipp C H OH (Dipp = 2,6-iPr C H -)
2
6
3
2
2
2
6
3
2
6
3
(
20 mL) with cooling in an ice bath. After 30 min the ice bath was
and the employment of a very bulky diprotic bidentate phenol
5
9
removed; the pale yellow solution was warmed to room temperature
and after an additional 2 h concentrated to ca. 10 mL under reduced
pressure. As no crystals formed after 4 days at −20 °C, the solution
was concentrated further (ca. 5 mL), and a small amount of colorless
crystals began to form after 2 h. Cooling at −20 °C for 5 days afforded
a yellow crystalline solid, which was identified by NMR spectroscopy
based on the binaphthol or VAPOL framework. The
formation of the CO adduct 8 is a testament to the high
2
Lewis acidity of the Al−O core in 4. On the other hand the
3
failure of both 4 and 8 to catalyze the reduction of CO with
2
Et SiH may point toward the mechanism of this reaction. In
3
one scenario, the Lewis acid will coordinate CO , and the base
as 4·1.8CH
Cl
2
. Yield 0.77 g, 0.89 mmol, 89% based on eq 7.
2
2
will attack the activated carbon to give the presumed primary
1
product, Et SiOC(O)H. In the other scenario, one which is at
4ArOH + 2AlEt → (ArO) Al + / (Et AlOAr)
3
3
3
2
2
2
work in the B(C F ) -catalyzed hydrosilylation of aldehydes
6
5 3
5
7,60
+ 2EtH↑
(7)
and ketones,
the substrate activates the silane leading to
hydride transfer to the Lewis acid, and this hydride reduces the
substrate. On the basis of the observations that compound 6
Removal of the volatiles from the mother liquor gave a foamy solid,
catalyzes the scrambling of the substituents on Et SiH and that
3
which after crystallization from hot hexanes afforded a 1:9 mixture of 3
triphenolate 4 does not, possibly due to a combination of steric
repulsion and lower Lewis acidity, it seems likely that the
second scenario is at work. In the case of 6, the neutral
aluminum hydride (2,6-Mes C H O) AlH would be the active
20
and 5.
Procedure B. A solution of AlEt (0.24 g, 2.0 mmol) in toluene (5
3
mL) was added to a solution of 2,6-Ph C H OH (1.01 g, 4.1 mmol) in
2
6
3
toluene (10 mL) with cooling in an ice bath. After 30 min the ice bath
was removed; the pale yellow solution was warmed to room
temperature and after an additional 3 h concentrated to ca. 2 mL
under reduced pressure. As no crystals formed overnight, hexanes (4
mL) was added, and pale yellow crystals of 4 began to form within 1 h
at room temperature. Over a period of 4 weeks at room temperature a
second batch of large (2 mm) colorless crystals appeared, which were
2
6
3
2
intermediate during the CO reduction (see eq 6). Future work
2
will focus on the synthesis of this compound and the
investigation of its chemistry.
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
■
2
0
identified as compound 5. Combined yield: 0.83 g.
Comproportionation of 4 and 5. Solutions of 4 (23 mg, 30
μmol) and 5 (11 mg, 16 μmol) in C D (0.5 mL each) were combined
6
6
under N from sodium, potassium, or sodium/potassium alloy and
in a J. Young type NMR tube and heated at 86−87 °C in an oil bath
for a combined 31 h followed by an additional 88 h at 133−136 °C.
The reaction was essentially complete after the first 18 h at 135 °C.
The solution was a mixture of mainly 3 (78%) and small amounts of
2
degassed twice prior to use or they were dispensed from a commercial
1
2
solvent purification system. The compounds 2,6-Mes C H OH,
2
6
3
15
61
(
2,6-tBu -4-MeC H O) AlEt (1), and [Ph C][CHB Cl ] were
2 6 2 2 3 11 11
1
prepared according to literature procedures. All other reagents were
obtained from commercial suppliers and used as received. NMR
unreacted 4 (6%) and 5 (15%) (see the Supporting Information). H
NMR (C D , 400.13 MHz) data for 3: δ 7.66 (d, J = 5.8 Hz, o-H(Ph),
6
6
1
spectra were recorded on a Bruker Avance 400 MHz spectrometer. H
8H), 7.23 (d, J = 7.6 Hz, m-H(C H ), 4H), 6.99 (m, m- and p-H(Ph),
6 3
NMR chemical shift values were determined relative to the residual
12H), 6.90 (t, J = 7.6 Hz, p-H(C H ), 2H), 0.19 (t, J = 8.0 Hz,
6 3
protons in C D or CDCl as internal reference (δ 7.16 or 7.26 ppm).
AlCH CH , 3H), −1.57 (q, J = 8.0 Hz, AlCH CH , 2H).
6
6
3
2
3
2
3
1
3
C NMR spectra were referenced to the solvent signal (δ 128.39 or
[(2,6-Mes C H O) Al][CHB Cl ] (6). Procedure A. [Ph C]-
2 6 3 2 11 11 3
7
7.0 ppm), 11B NMR spectra to a solution of F B·OEt in C D as
[CHB Cl ] (16 mg, 21 μmol) was added to a solution of 2 (15
3
2
6
6
11 11
31
external standard (δ 0 ppm), and P NMR spectra to a solution of
Ph P in C D as external standard (δ −6 ppm). The ATR-FTIR
mg, 21 μmol) in C D (0.5 mL) inside a J. Young type NMR tube.
6
6
The mixture was shaken briefly to afford a yellow-orange emulsion,
3
6
6
spectrum of 8 was collected on a Nicolet IR200 FT-IR spectrometer
with ATR attachment (ATR = attenuated total reflection). GC/MS
spectra were collected on an HP G1800GCD gas chromatograph.
Melting points were determined in Pyrex capillary tubes sealed under
nitrogen with a Mel-Temp apparatus and are uncorrected.
from which a red-orange oil, [Ph C][CHB Cl ], separated within
3
11 11
1
minutes. The H NMR spectrum of the colorless supernatant showed
only traces of the expected side products Ph CH and ethane after 2 h.
3
The reaction mixture was agitated by fastening the NMR tube to a
horizontally aligned overhead stirrer and slowly rotating the tube. As
after 21 h rotation only a very slight increase in the side products was
observed, the reaction mixture was heated at 85 °C for 2 days in an oil
bath. During this time the dense orange phase faded to pale yellow.
The top layer was pipetted off, and the bottom layer was washed with
(
2,6-Mes C H O) AlEt (2). A solution of AlEt (0.10 g, 0.9 mmol)
2 6 3 2 3
in hexanes (10 mL) was added dropwise to a suspension of 2,6-
Mes C H OH (0.56 g, 1.7 mmol) in hexanes (20 mL) with cooling in
2
6
3
an ice bath. A clear pale yellow solution was obtained after a few
6
816
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Organometallics
Article
C D (0.2 mL) and dissolved in a C D /C D Br 2/1 mixture (0.5
(d, br, J = 6.4 Hz, o- or m-H, (Ph), 12H), 7.32 (m, obscured by 4),
7.13 (m, partially obscured by C D H), 6.88 (m, obscured by 4), 0.42
6
6
6
6
6
5
mL).
6
5
13
1
Procedure B. A Teflon-capped Schlenk vial was charged with
(d, JHP = 14.0 Hz, C(CH ) , 27H). C{ H} NMR (C D , 100.61
3 3 6 6
MHz): δ 155.87 (d, J = 92.4 Hz, PCO), 153.86 (i-C), 142.43
1
[
(
Ph C][CHB Cl ] (75 mg, 98 μmol), C D (1.5 mL), and Et SiH
103 mg, 0.88 mmol), and the mixture was stirred for 22 h. During this
3
11 11
6
6
3
CP
(quaternary C), 138.62 (quaternary C), 133.64 (quaternary C),
1
time the yellow-orange trityl salt phase faded to almost colorless. The
colorless top layer was pipetted off, and the oily bottom layer was
132.18, 130.72, 126.31, 119.09, 38.42 (d, J = 20.0 Hz, PC(CH ) ),
CP
3 3
31
1
30.13 (PC(CH
Catalytic Reduction of CO
containing 6 (3 μmol, from procedure A), Ph
(16 μL, 12 mg, 100 μmol), and C (0.6 mL) was degassed by two
at atmospheric
)
3
). P{ H} NMR (C
D
6
, 161.97 MHz): δ 41.3.
3
6
washed with hexanes (1.5 mL) to afford the silylium salt [Et Si]-
2
. A J. Young type NMR tube
3
[
CHB Cl ] as a yellowish sticky solid. A solution of 2 (70 mg, 98
3
CH (5 μmol), Et SiH
3
11
11
μmol) in toluene (1.5 mL) was added to the silylium salt to give a two-
phase system as before. After the system was stirred overnight the
bottom phase turned cloudy yellow-ocher. The mixture was heated at
D
6 6
freeze−pump−thaw cycles and filled with CO
2
pressure, with the bottom half of the NMR tube being cooled at ca.
1
6
5−75 °C for 6 h. The top layer was pipetted off, and the bottom layer
−60 °C. After 19 h at room temperature the H NMR spectrum
showed a change in the appearance of the o-Me signals (one sharp
was washed with C D (0.8 mL). After 3 days at room temperature
6
6
colorless thin plates of 6·2C H formed in both the oily layer and the
signal on top of a broad signal) and a weak signal at 4.85 ppm, which
6
6
8
washing. C D (0.6 mL) was added to the crystals from the oily layer,
could be due to H
C(OSiEt
2
3
)
2
(5.01 ppm in C
6
D
5
Br ). Heating at 83
6
6
and the mixture was heated at 80 °C for 3 h followed by crystallization
for 2 days at room temperature. These crystals were used for the X-ray
structure determination. The crystals obtained from the washing were
used for the NMR spectroscopic analysis. They contained ca. 5.5 equiv
°C for 4 h led to the formation of C
6
D
5
CH
3
(7 μmol) and Et
2
SiH
2
(12
μmol). A small amount of methane (0.15 ppm) and a trace of HD
(4.43 ppm, t, JHD = 42.6 Hz) were also detected. After 25 h at 82−83
°C 90% of Et SiH was consumed, and after an additional 24 h at 82−
3
1
of C D CH . H NMR (C D , 400.13 MHz): δ 6.76 (m), 6.66 (s, m-
83 °C the reaction was judged complete (99% Et
reaction products were C CH (20 μmol, 60% of silane hydrogen),
(C CH (1 μmol, 2% of silane hydrogen), CH (4 μmol, 16% of
3
SiH consumed). The
6
5
3
6
6
H(Mes)), 2.63 (s, CHB Cl , 1H), 2.20 (s, p-CH , 12H), 1.60 (s, br,
6
D
5
3
11
11
3
o-CH , 24H). The aromatic hydrogen signals could not be integrated
6
D
5
)
2
2
4
3
13
1
because the hydrogens were partially replaced by deuterium. C{ H}
silane hydrogen), and HD (14 μmol, 14% of silane hydrogen). The
NMR (C D , 100.61 MHz): δ 149.30 (i-C), 143.38, 39.36, 21.65 (p-
amounts of both gases have been estimated using the Henry constants
6
6
63
11
CH ), 20.66 (br, o-CH ). B NMR (C D , 100.38 MHz): δ −2.21 (p-
for solutions of both gases (k
H
(CH
0.0028 M/atm ) in benzene and a gas volume of approximately 2 mL.
Catalytic Scrambling of Et SiH and Reduction of CO . A J.
4
) = 0.021 M/atm,
k (H ) =
H 2
3
3
6
6
6
4
B, 1B), −9.67 (o- or m-B, 5B), −12.85 (o- or m-B, 5B).
Attempted Synthesis of [(2,6-tBu -4-MeC H O) Al]-
3
2
2
6
2
2
[
CHB Cl ] (7). Procedure A. A J. Young type NMR tube was
Young type NMR tube containing 6 (4 μmol, from procedure B),
Et SiH (16 μL, 12 mg, 100 μmol), and a 2/1 mixture of C and
Br (0.6 mL) was monitored by multinuclear NMR spectroscopy
over a period of 7 days. Within 5 h at room temperature the formation
of Et SiH and Et Si was detected, and after 19 h a small amount of
EtSiH formed. The mixture changed little over the 6 days at room
temperature, and its composition was approximately EtSiH /Et SiH
Et SiH/Et Si = 2.5/18/49/30.5.
CO was added to the degassed sample as before, and the progress
11
11
charged with [Ph C][CHB Cl ] (10 mg, 13 μmol), C D (ca. 0.5
3
D
6 6
3
11 11
6
6
mL), and 1 (7 mg, 14 μmol) inside a drybox. The mixture was shaken
briefly to afford a yellow-orange emulsion, from which a red-orange oil,
C D
6 5
[
Ph C][CHB Cl ], separated within minutes. The reaction mixture
2
2
4
3
11 11
was then sonicated for 2 h in an ultrasound bath. Since no visible
change was observed, the NMR tube was stirred horizontally overnight
with no visible change. NMR and GC-MS spectra showed the
3
/
2
3
2
3
4
62
formation of d -tert-butylbenzene (t-BuC D ) and trace amounts of
2
5
6
5
1
13
1
triphenylmethane. There were no NMR signals from the starting
material, 1.
of the reaction was monitored by H and C{ H} NMR spectroscopy.
The hydrosilanes were consumed (97%) after 11 h at 82−84 °C, and
Procedure B. A J. Young type NMR tube was charged with
the products were C
(C CH
μmol, 10% of silane hydrogen). More Et
μmol) and CO were added, and the mixture was heated at 83−87 °C.
6
D
5
CH
3
(17 μmol, 51% of silane hydrogen),
[
Ph C][CHB Cl ] (20 mg, 26 μmol), benzene (0.4 mL), and Et SiH
6
D
5
)
2
2
(0.2 μmol, 0.5%), CH
4
(4 μmol, 16%), and HD (10
SiH (16 μL, 12 mg, 100
3
11 11
3
(
15 mg, 129 μmol) inside a drybox. The reaction mixture was stirred
3
horizontally for 1 h, giving a colorless oily phase. The mother liquor
was pipetted off, and the silylium compound was washed with hexanes
2
After 23 h 96% of the hydrosilane was consumed, and after an
additional 24 h no more hydrosilane was detectable.
(
2 × 0.3 mL) and benzene (0.5 mL). C D (0.5 mL) and 1 (7 mg, 14
6 6
μmol) were added, and the reaction mixture was rotated horizontally
overnight, forming an insoluble colorless solid. NMR and GC-MS
spectra of the supernatant showed the formation of tert-butylbenzene
and tetraethylsilane. There were no NMR signals from the starting
material, 1. Attempts to dissolve the solid residue in bromobenzene
were not successful.
X-ray Crystallography. Crystals of 6·2C D and 8 were grown as
6 6
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
6
5
absorption by the empirical method, and the structures were solved
66
and refined using the SHELXS and SHELXL packages. There were
two formula units per asymmetric unit of the cell for 6·2C D . The
(
2,6-Ph C H O) AlO(CO)PtBu (8). A solution of 2,6-
6
6
2
6
3
3
3
sample exhibited racemic twinning with a refined twin ratio of 0.41(3),
as shown by the refined Flack parameter. The C−H location in one
anion was disordered. The occupancies of atoms C1D, B4D, and Cl4D
were refined to 0.860(4) and 0.140(4). Restraints on the positional
parameters of the disordered atoms were required. The three tert-butyl
groups on the phosphorus in 8 were disordered and refined with split
occupancies. The occupancies for C57, C58, and C59 refined to
Ph C H OH (0.87 g, 3.5 mmol) in toluene (10 mL) was added
2
6
3
dropwise to a solution of AlEt (0.14 g, 1.2 mmol) in toluene (5 mL)
3
at room temperature, leading to immediate gas evolution and a slight
warming. After 1.5 h the pale yellow solution of 4 was added to solid
tBu P to give a clear pale yellow solution. The reaction flask was
3
evacuated and back-filled with dry CO . After 15 min a fine colorless
2
precipitate formed, and after another 1 h the CO2 uptake was
complete. The stopcock was closed, and the precipitate was dissolved
by brief heating with a heat gun. Within 2 h colorless crystals began to
form (0.52 g). Concentration of the mother liquor to ca. 5 mL,
0
0
0
.684(7) and 0.316(7), those for C61, C62, and C63 refined to
.708(11) and 0.292(11), and those for C65, C66, and C67 refined to
.761(12) and 0.239(12). Restraints on the positional and displace-
ment parameters of the disordered atoms were required.
followed by CO addition and successive dissolution of the precipitate
2
by brief heating, afforded another 0.13 g. Yield: 0.65 g, 55%, based on
2
1
,6-Ph C H OH. Mp: 137−150 °C with gas evolution. ATR-FTIR:
ASSOCIATED CONTENT
2
6
3
■
−
1
708 cm (st, ν
). Dissolution in C D or CDCl led to partial
CO
6 6 3
*
S
Supporting Information
decomposition into 4, tBu P, and CO . Increase of the partial CO
3
2
2
Figures giving NMR data for compounds 2, 3, 6, and 8 and the
reduction of CO catalyzed by 6, X-ray tables, additional images
of the structures of 6·2C D and 8, and CIF files for 6·2C D
6
pressure to ca. 1 atm increased the amount of dissolved 8 to
approximately 50%. The NMR data for 8 are extracted from the
2
1
spectra of the latter solution. H NMR (C D , 400.13 MHz): δ 7.70
6
6
6
6
6
6
817
dx.doi.org/10.1021/om400628z | Organometallics 2013, 32, 6812−6819

Organometallics
Article
and 8. This material is available free of charge via the Internet at
http://pubs.acs.org.
(27) Maruoka, K.; Concepcion, A. B.; Murase, N.; Oishi, M.;
Hirayama, N.; Yamamoto, H. J. Am. Chem. Soc. 1993, 115, 3943−3949.
(28) Young, J. D.; Khan, M. A.; Powell, D. R.; Wehmschulte, R. J.
Eur. J. Inorg. Chem. 2007, 1671−1681.
AUTHOR INFORMATION
■
(
29) Klis, T.; Powell, D. R.; Wojtas, L.; Wehmschulte, R. J.
Corresponding Author
R.J.W.: e-mail, rwehmsch@fit.edu; tel, (+1) 321 725 8085.
Organometallics 2011, 30, 2563−2570.
(30) Atwood, J. L. In Coordination Chemistry of Aluminum; Robinson,
G. H., Ed.; VCH: New York, 1993.
*
Notes
(
31) Petrie, M. A.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc.
991, 113, 8704−8708.
32) Stanciu, C.; Richards, A. F.; Stender, M.; Olmstead, M. M.;
Power, P. P. Polyhedron 2006, 25, 477−483.
33) Shekar, S.; Taylor, M. M.; Twamley, B.; Wehmschulte, R. J.
Dalton Trans. 2009, 9322−9326.
34) Hair, G. S.; Cowley, A. H.; Jones, R. A.; McBurnett, B. G.; Voigt,
A. J. Am. Chem. Soc. 1999, 121, 4922−4923.
35) Young, J. D.; Khan, M. A.; Wehmschulte, R. J. Organometallics
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36) Menard, G.; Stephan, D. W. Angew. Chem., Int. Ed. 2012, 51,
The authors declare no competing financial interest.
1
(
ACKNOWLEDGMENTS
■
(
This paper is dedicated to Prof. Dr. Werner Uhl on the
occasion of his 60th birthday. Financial support from the
National Science Foundation (Grant CHE 0718446) is
gratefully acknowledged.
(
(
2
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