CO2 Fixation and Catalytic Reduction by a Neutral Aluminum Double Bond

Article abstract of DOI:10.1002/anie.201905045

CO₂ fixation and reduction to value-added products is of utmost importance in the battle against rising CO₂ levels in the Earth's atmosphere. An organoaluminum complex containing a formal aluminum double bond (dialumene), and thus an alkene equivalent, was used for the fixation and reduction of CO₂. The CO₂ fixation complex undergoes further reactivity in either the absence or presence of additional CO₂, resulting in the first dialuminum carbonyl and carbonate complexes, respectively. Dialumene (1) can also be used in the catalytic reduction of CO₂, providing selective formation of a formic acid equivalent via the dialuminum carbonate complex rather than a conventional aluminum–hydride-based cycle. Not only are the CO₂ reduction products of interest for C₁ added value products, but the organoaluminum complexes isolated represent a significant step forward in the isolation of reactive intermediates proposed in many industrially relevant catalytic processes.

Full text of DOI:10.1002/anie.201905045

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Accepted Article
Title: CO2 Fixation and Catalytic Reduction by a Neutral Aluminium
Double bond
Authors: Shigeyoshi Inoue, Catherine Weetman, Prasenjit Bag, Tibor
Silvási, and Christian Jandl
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201905045
Angew. Chem. 10.1002/ange.201905045
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Angewandte Chemie International Edition
10.1002/anie.201905045
COMMUNICATION
CO
2
Fixation and Catalytic Reduction by a Neutral
Aluminium Double bond
[
a]
[a]
[b]
[a]
[a]
Catherine Weetman, Prasenjit Bag, Tibor Szilvási, Christian Jandl, Shigeyoshi Inoue*
Abstract: CO
2
fixation and reduction to value added products is of
levels in the Earth’s
Donor-stabilised bis(silylene)[17] and a disilicon(0) species[18]
showed that CO binding and subsequent C-O cleavage resulted
in carbonate formation. Whereas, reaction of a digermyne with
CO
,[19] which is also thought to contain some multiple bond
utmost importance in the battle against rising CO
2
2
atmosphere. Herein we present the use of an organoaluminium
complex containing a formal aluminium double bond (dialumene), and
2
thus an alkene equivalent, for the fixation and reduction of CO
CO fixation complex undergoes further reactivity in either the
absence or presence of additional CO resulting in the first
dialuminium carbonyl and carbonate complexes, respectively.
Dialumene (1) can also be used in the catalytic reduction of CO
providing selective formation of a formic acid equivalent via the
dialuminium carbonate complex rather than conventional
aluminium-hydride based cycle. Not only are the CO reduction
products of interest for C added value products, but the novel
2
. The
character, resulted in oxygen abstraction and concomitant
formation of CO. Examples of C-O cleavage has also been
observed in f-element chemistry, in particular for U(III) complexes,
2
2
,
where CO
2
cleavage to CO and O has been noted to occur at
room temperature.[
20,21]
2
,
Whilst the reactivity of group 14[22] and boron multiple
a
bonds[23] is rather well established, aluminium multiple bond
chemistry is still in its infancy.[24] Even though aluminium is the
most abundant metal found within the Earth’s crust, and the high
industrial use of Al(III) related catalysts[25,26] research into low
oxidation state and/or low coordinate Al(I) complexes is
somewhat often overlooked due to the synthetic challenges in
isolating such reactive species. Despite this, recent advances in
Al(I) chemistry have shown that not only can Al react as an
electrophile and undergo oxidative addition reactions[27] but also
as a nucleophile and thus challenging traditional conceptions of
2
1
organoaluminium complexes isolated represent a significant step
forward in the isolation of reactive intermediates proposed in many
industrially relevant catalytic processes.
With worldwide increasing energy demands and
subsequent rising carbon dioxide (CO ) emissions, alternate
2
energy production methods are required as well as the
development of new methods for the sequestration and utilization
[28, 29]
aluminium’s chemical behaviour.
Recently our group succeeded in isolating the first neutral
dialumene (1) species, a compound with a formal Al=Al double
bond.[30] This highly reactive species, with a significantly low
HOMO-LUMO gap (2.24 eV), showed varied reactivity towards C-
C multiple bonds. Thus, highlighting its ability to undergo multiple
reaction pathways, as both [2+2]-cycloaddition and C-H activation
products were observed. With this is mind, our attention turned to
more challenging substrates. Herein, we report the use of 1 for
[
1,2]
of CO
production of value-added products has been a rapidly expanding
area of research.[3] Conversion of CO
to carbon monoxide (CO),
2
.
2 1
In this regard, use of CO to form C feedstocks for the
2
methanol, formic acid, and cyclic/polycarbonates has been shown
to be possible by several homogeneous catalytic systems.[4-6]
Whilst transition metals have dominated this field, the
development of main group complexes that can act as transition
metals[7,8] has begun to emerge as a cheaper and more
the fixation and selective reduction of CO
2
, both of which can be
environmentally benign alternative for CO
Lewis pairs’ (FLPs)[9-11] and polar main group multiple bonds have
proven to be efficient methods for CO
activation.[12,13]
In terms of non-polar multiply-bonded main group
compounds, CO activation has been shown to proceed by an
2
activation. ‘Frustrated
achieved in a stoichiometric and catalytic fashion.
2
2
initial [2+2]-cycloaddition for diborenes[14] and disilenes.[15,16] The
resulting E-E-C-O (E = B or Si) 4-membered rings were found to
be thermally unstable and undergo C-O cleavage-based reactivity.
[
a]
Dr. C. Weetman, Dr. P. Bag, Dr. C. Jandl, Prof. S. Inoue
Department of Chemistry, Catalysis Research Center and Institute of
Silicon Chemistry, Technical University Munich, Lichtenbergstrasse 4,
85748, Garching Bei München, Germany
E-mail: s.inoue@tum.de
[
b]
Dr. T. Szilvási
Department of Chemical and Biological Engineering, University of
Wisconsin-Madison, 1415 Engineering Drive, Madison, WI 53706-1607,
USA.
Supporting information for this article is given via a link at the end of the
document.
Scheme 1. CO
(1).
2
fixation and stoichiometric reduction mediated by dialumene
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Angewandte Chemie International Edition
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COMMUNICATION
Reaction of a toluene solution of dialumene (1) with 1 atm
of CO at −78°C (Scheme 1), and subsequent warming to −30 °C
2
resulted in the loss of the deep purple colour associated with 1
and formation of a yellow solution with concomitant yellow
precipitate. Removal of all solvent and extraction with THF
allowed for isolation of a yellow powder (2) in almost quantitative
(Scheme 1), resulted in a dark red solution within 1 hr after which
subsequent work up and crystallisation provided 3 in 20% yield.
X-ray crystallographic analysis revealed 3 (Figure 1) to contain a
bridging carbonyl and oxo unit.
The central Al-CO-Al-O core in 3 is completely planar, with
the NHC and silyl groups trans-orientated to the ring. Compound
yields. Single crystal X-ray analysis revealed that CO
occurred via formal [2+2]-cycloaddition to the Al=Al double bond
Figure 1).
2
fixation had
3 represents the first dialuminacarbonyl compound and a rare
example of a central M(μ-CO)(μ-O)M bridging motif.[
14,31,32]
The
(
retention of the carbonyl group is confirmed upon comparison of
structural data. The C=O bond length is 1.194(8) Å which fits well
with average μ-CO bridging motifs (1.17-1.22 Å), and the notable
downfield shift in 13C NMR is also similar to those reported for
M(μ-CO)(μ-O)M cores (M = B (δ 278.7 ppm) and Fe (δ 289.8
-1
ppm)). The IR stretch was found to be at 1630 cm , which is lower
-1
than expected with typical μ-CO stretches found >1700 cm and
-1 [33]
2
a matrix isolation of Al CO was reported at 1737 cm . A short
Al-Al interatomic distance was also noted (2.730(1) Å), which is
within range of Al-Al single bonds.[24]
DFT
calculations
were
performed
at
the
B97d/Def2SVP/W06 level of theory on 3. The calculated structure
is in good agreement with experimental parameters, including the
-1
IR stretch for the C=O (1640 cm ). The calculated orbitals
suggest that one of the degenerate π* orbitals of the CO unit is
effectively occupied, with the other being the LUMO (Figure 2).
Thus, the bridging unit can best viewed as a CO2- anion which
also accounts for the lower wavenumber observed. The Al-O
bonds in the bridging oxo unit were found by NBO analysis to be
highly polarized, almost ionic in character ([7%]Al(1)-O(2)[93%]).
Despite the short Al-Al distance observed, no evidence of an Al-
Al bond was found in the orbital analysis.
Figure 1. Molecular structures of compounds 2-4 in the solid state with thermal
ellipsoids set at the 50% probability level. Hydrogen atoms and co-crystallised
solvent molecules are omitted for clarity and NHC ligands are depicted in
wireframe for simplicity. Selected bond lengths (Å) and angles (°): Compound 2
Al(1)-Al(2) 2.5848(16), Al(1)-Si(1) 2.5150(16), Al(2)-Si(2) 2.5039(16);
Compound 3 Al(1)-Al(1’) 2.730(1), Al(1)-O(2) 1.707(5), Al(1)-C(21) 2.166(6),
C(21)-O(1) 1.194(8), Al(1)-C(1) 2.069(2), Al(1)-C(21)-Al(1’) 78.2(2), Al(1)-O(2)-
Al(1’) 106.7(3); Compound 4 Al(1)-O(1) 1.796(3), Al(1)-O(4) 1.720(3) Al(1)-C(1)
2.074(3), C(41)-O(1) 1.315(4), C(41)-O(3) 1.217(4), C(41)-O(2) 1.324(4), Al(2)-
O(2) 1.798(2), Al(2)-O(4) 1.713(3), Al(2)-C(21) 2.068(3), Al(1)-Si(1) 2.4781(15),
Al(2)-Si(2) 2.4971(16), O(1)-C(41)-O(2) 117.4(3), Al(1)-O(4)-Al(2) 120.54(14).
Figure 2. HOMO (−2.17 eV, left) and LUMO (−1.18 eV, right) of 3 (isovalue
0.04)
The resulting planar 4-membered Al-Al-C-O ring is
structurally analogous to the related diboron and disilicon
compounds.[14,16] Retention of the carbonyl moiety exocyclic to the
6 6 2
Heating a C D solution of 1 under an atmosphere of CO
resulted in the initial formation of 2 followed by formation of a
-1
ring, was confirmed by IR spectroscopy (1640 cm ) which is in
colourless solution, rather than the expected red solution for the
-1
good agreement with the calculated value (1658 cm ). As
expected, the Al-Al distance is longer than that of 1 (2.5848(16) Å
vs 2.3943(16) Å, respectively) indicating loss of double bond
13
formation of 3. Inspection of the resulting C NMR spectrum
revealed new signals at δ154.4 ppm and δ184.4 ppm indicating
likely carbonate and CO formation, respectively. Crystallographic
analysis, revealed compound 4 to consist of a six-membered ring
containing a bridging carbonate group and oxygen between the
two aluminium centres, reminiscent of the cyclic silicon structures
reported.[17,18] The C=O bond (1.217(4) Å) is longer than in 2 and
character, and is now in the range of a typical Al-Al single bond
24]
(
(
2.53-2.75 Å).[ Compound 2 is relatively stable in the solid state
mp. 135 °C) and can be stored at room temperature in an inert
atmosphere for at least
decomposition. Monitoring a solution (C
1
month without any signs of
or THF-d ) of 2 at
6
D
6
8
3
, and the C-O single bonds were found to be 1.32 Å (average)
indicating a degree of delocalisation across the CO unit. This
along with the notably long Al-OCO bond lengths (1.797 Å
room temperature, however, resulted in the loss of the yellow
colour of 2 after 18 hrs to form a dark orange solution. The NMR
spectra showed the emergence of a new species, with a downfield
signal in the 13C NMR spectrum at δ 276.0 ppm, similar to that
reported by Braunschweig and co-workers for the carbonyl-
bridged diboron complex.[14] Repetition of this reaction at 50 °C
3
2
average) indicates a high degree of polarisation across the Al-O
bond, and likely contributes to the lower than expected C=O (1670
cm ) IR stretch.
-1
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COMMUNICATION
Mechanistic insights into the formation of compounds 3 and
were sought both experimentally and computationally. As noted
previously both compounds are formed via the initial [2+2]-
cycloaddition product (2). This compound can either undergo
isomerisation, to form carbonyl species 3, or undergo oxygen
abstraction and de-carbonylation to form carbonate species 4 in
Computational studies on the formation of compound 4 via
the two different routes, Route A via compound 2 (Fig. S37) and
Route B via compound 5 (Fig. S38), were undertaken to provide
further insight. Both routes were found to be overall exothermic
processes (−77.3 kcal mol-1 and −36.5 kcal mol-1 for Routes A and
B, respectively). Differences in the observed isomers of
compound 4 can also be accounted for via these mechanisms;
Route A proceeds through a series of 6-and 5-membered rings,
4
2
the absence or presence of further CO , respectively (Scheme 1).
Computational studies indicated an overall exothermic
isomerisation of 2 to 3 (Fig. S36), the C-O cleavage step is
endergonic (+24.9 kcal mol ) but this is offset by the
recombination of CO and formation of the 4-membered ring
2 2 2
Al(CO ) Al (B) and Al(CO )(O)Al (C), respectively. Whereas in
-1
Route B, the 4-memebered ring is broken (Intermediate D, Fig.
S38) allowing for recombination at either face of the Al center,
thus accounting for the 50-50 geometric isomers.
-1
(
−42.4 kcal mol ). Attempts to confirm this mechanism
experimentally focused on the isolation of a Al O 3-membered
ring (Compound A, Fig. S36). Exposure of 1 to approximately 1
atmosphere of either O or N O at −78 °C and gradual warming
2
Our attention turned to the catalytic reduction of CO
2
.
Attempts at hydrosilylation of CO failed, and resulted in
2
2
2
decomposition of 1. Use of pinacol borane (HBpin), however,
1
to room temperature, resulted in the formation of a colourless
solution indicating loss of the double bond character of 1. Upon
inspection of the corresponding 1H NMR spectra, the same
symmetrical single species was observed to have formed in both
cases (Compound 5, Scheme 2). Compound 5 was identified as
resulted in turnover. Close monitoring of the reaction through H
and 11B NMR spectroscopy, identified the formic acid derivative
(OCHOBpin, 86%) after 1 week at room temperature. Use of
higher temperatures (60 and 80 °C) allowed for lower catalyst
loadings (5 mol%) and resulted in 80% consumption of HBpin
within 3-4 hours and selective formation of the formic acid
equivalent in NMR yields of 94% and 95%, respectively.
2
a cyclic dialumanoxane [{(NHC)(Si)Al(μ-O)} ] (Fig. S12), formed
by the oxidative addition of oxygen to dialumene (1).
Compound 5 is a centrosymmetric dimer featuring a planar
Al
obtained from the reactions of heavier group 13 dimetallenes (Ga
and In) towards N
O.[34] Notably, this is a rare example of a
discrete [RAlO] type complex where n=2. Due to the highly
2 2
O square core, these are structurally analogous to those
2
n
polarised Al-O bond self-oligomerisation readily occurs and thus
requires sterically demanding substituents to control aggregation.
With the only other example of a [RAlO] species requiring the use
2
of the chelating β-diketiminate ligand. [35] NBO analysis confirmed
the highly polarised bond ([9%]Al(1)-O(1)[91%]) and despite the
short Al-Al distance (2.4812(12) Å), no such interaction between
the two Al centres was found (Fig. S43).
Whilst compound 5 was not implied as an intermediate in
the formation of carbonate species (4), it is plausible that CO
2
could insert into the Al-O bond to yield such a species. Therefore,
1
exposure of 1 atm of CO
2
to 5 was tested. The resulting H NMR
spectrum identified two similar species in an approximately 50:50
mixture, one of which was identified as compound 4. Further
inspection of the 13C NMR data revealed the formation of both cis
(
4c) and trans (4t) isomers due to the characteristic carbonate
signals (δ 158.45 and δ 154.81 ppm).
Scheme 3. Proposed Catalytic Cycle.
Mechanistic studies indicated the reaction proceeds via
initial [2+2]-cycloaddition (2) as reaction of 1 with HBpin resulted
in no reaction, and after 24 hrs at room temperature
decomposition of 1 was observed. This suggests that formation of
an aluminium hydride complex is unlikely and therefore catalysis
is thought to proceed via the dialuminium carbonate compound
(
4). It was initially thought that that 4 undergoes a σ-bond
metathesis type reaction with HBpin to form OCHOBpin and dioxo
species 5. This non-hydridic route would also account for the
selectivity of the observed products, as OCHOBpin could easily
Scheme 2. Formation of compound 5 and stoichiometric reduction CO
2
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Angewandte Chemie International Edition
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COMMUNICATION
insert back into a Al-Hydride based cycle resulting in hemi-acetal
and methanol derivatives. Addition of 1 eq. of HBpin to 4t resulted
in the consumption of starting materials and a mixture of species,
however no products that were identified in the catalysis were
observed indicating that this stoichiometric reaction is not
representative of the active catalytic cycle. Subsequent
computational analysis found the conversion of 4 to 5 to be
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2
and HBpin, revealed a facile transformation (overall
-1
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-1
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In summary, we report CO fixation by an aluminium-
aluminium double bond which promotes both catalytic and
stoichiometric reduction of CO to value added C products under
2
2
1
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the European Union’s Horizon 2020 research and innovation
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Keywords: Aluminium • Multiple Bonds • Carbon Dioxide
Fixation • Subvalent Compounds • Catalysis
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9
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Angewandte Chemie International Edition
10.1002/anie.201905045
COMMUNICATION
COMMUNICATION
[
a]
CO
aluminium
promotes
2
fixation by an aluminium-
Catherine Weetman, Prasenjit
Bag,[ Tibor Szilvási,[b] Christian
Jandl,[a] Shigeyoshi Inoue*[a]
a]
double
bond
and
catalytic
stoichiometric reduction under
mild conditions.
Page No. – Page No.
CO
Reduction by
Aluminium Double bond
2
Fixation and Catalytic
a
Neutral
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