Potassium organoaluminate: Synthesis, structure, and catalytic activity for the conversion of CO2 into cyclic carbonates

Article abstract of DOI:10.1016/j.jorganchem.2020.121365

Potassium organoaluminate stabilized by bidentate pyrrolyl ligand [C₄H₃NH(2-CH₂NH^tBu)] was successfully synthesized and characterized. The solid state structure reveals it is a rare example of an alkyl-bridged K/Al complex, which is interlocked by bridging potassium ions to form a monodimensional polymeric array, and featured an unsupported bridging methyl interaction between potassium and aluminum metals lead to ultimate 2D supramolecular network structure. It was proved to an efficient catalyst for the conversion of CO₂ into cyclic carbonates in good yields at 50 °C and 1 atm carbon dioxide pressure.

Full text of DOI:10.1016/j.jorganchem.2020.121365

Journal of Organometallic Chemistry 922 (2020) 121365
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Potassium organoaluminate: Synthesis, structure, and catalytic
activity for the conversion of CO2 into cyclic carbonates
,
, c, **
Zhiqiang Guo a *, Leilei Yan b, Yaoming Liu a, Xiaoqin Wu a, Xuehong Wei a
a Scientific Instrument Center, Shanxi University, Taiyuan, 030006, PR China
b School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006, PR China
c Institute of Applied Chemistry, Shanxi University, Taiyuan, 030006, PR China
a r t i c l e i n f o
a b s t r a c t
Potassium organoaluminate stabilized by bidentate pyrrolyl ligand [C4H3NH(2-CH2NHtBu)] was suc-
cessfully synthesized and characterized. The solid state structure reveals it is a rare example of an alkyl-
bridged K/Al complex, which is interlocked by bridging potassium ions to form a monodimensional
polymeric array, and featured an unsupported bridging methyl interaction between potassium and
aluminum metals lead to ultimate 2D supramolecular network structure. It was proved to an efficient
catalyst for the conversion of CO2 into cyclic carbonates in good yields at 50 ꢀC and 1 atm carbon dioxide
pressure.
Article history:
Received 24 February 2020
Received in revised form
7 May 2020
Accepted 1 June 2020
Available online 20 June 2020
Keywords:
Potassium organoaluminate
Structure
© 2020 Elsevier B.V. All rights reserved.
Catalysis
Carbon dioxide
Cyclic carbonate
1. Introduction
require catalyst loading (>2.5 mol %) and a long reaction time
(24 h), and their catalytic activities are in the range of
Carbon dioxide (CO2) has shown great potential as an important
and ideal one-carbon (C1) source in chemical feed-stock for its
nontoxicity, abundance, availability, and recyclability [1]. Over the
past decade or so, due to the widespread use of cyclic carbonates as
aprotic solvents, synthetic building blocks, electrolytes, monomers
for polymerizations, industrial lubricants, and pharmaceutical in-
termediates so on [2], great progresses in chemical fixation of
carbon dioxide with epoxides to produce the corresponding cyclic
carbonates have been achieved. Among all metal-based catalytic
systems, aluminium is considered an ideal choice for the ring-
opening of epoxides for earth abundance, nontoxicity, and high
Lewis acidity. And the salen, salophen and related ligand-based
monometallic or bimetallic aluminium complexes have been
proved to efficient catalysts [3]. Many examples of efficient cata-
lysts with aluminum metal centers capable of operating at ambient
temperature and 1 atm CO2 are also reported [4]. But for the
moderate reactive styrene oxide, the Al-based catalysts generally
TON ¼ 8.4e39.2 and TOF ¼ 0.63e3.73 hꢁ1 [5]. Thus, the designed
and development of new, more active Al-based catalytic systems
under ambient conditions and pressure is still requisite.
As a part of our ongoing interest in the development of new
reaction systems based on bimetallic compounds supported by
multidentate pyrrole-based ligands, herein we have focused on the
versatile bonding modes of bidentate dianionic pyrrolyl ligand
[C4H3NH(2-CH2NHtBu)] as model proligand [6], and demonstrated
potassium alkyl aluminates with intriguing structures. This new
alkali-metal alkyl aluminate shed a new light on pyrrolyl-based
complexes and could provide useful tools in the field of bimetallic
* Corresponding author.
** Corresponding author. Scientific Instrument Center, Shanxi University, Taiyuan,
030006, PR China.
E-mail addresses: gzq@sxu.edu.cn (Z. Guo), xhwei@sxu.edu.cn (X. Wei).
Scheme 1. Synthetic routes to potassium organoaluminate complex.
https://doi.org/10.1016/j.jorganchem.2020.121365
0022-328X/© 2020 Elsevier B.V. All rights reserved.

2
Z. Guo et al. / Journal of Organometallic Chemistry 922 (2020) 121365
Fig. 1. (a) The ORTEP view of potassium organoaluminate complex, with the atom-numbering scheme and displacement ellipsoids drawn at the 25% probability level; (b) Section of
the extended framework structure showing KeC atom connectivity Hydrogen atoms are omitted for clarity. (c) Two-dimensional network structure extended by bridging methyl
interaction between potassium and aluminum metals.

Z. Guo et al. / Journal of Organometallic Chemistry 922 (2020) 121365
3
Table 1
cooperativity. It is noteworthy that, under the optimal reaction
conditions, this complex in combination with tetrabutylammo-
nium iodide (TBAI) acts as a very efficient catalyst system for the
conversion of monosubstituted epoxides into the corresponding
cyclic carbonates showing broad substrate scope.
Optimization of the synthesis of cyclic carbonate catalyzed by potassium organo-
aluminate.a.
2. Experimental
2.1. General methods and materials
Entry
Cat. (mol %)
Cocat. (mol %)
Solvent.
Yieldb (%)
The syntheses and manipulations of air sensitive materials were
performed using the standard Schlenk techniques without other-
wise noted. Tetrahydrofuran and diethyl ether were distilled from
sodium-benzophenone under nitrogen. Hexane and toluene is
dried using sodium potassium alloy and distilled under nitrogen
prior to use. All epoxides were purchased from commercial sources
(purity > 97%) and used as received unless otherwise indicated.
Carbon dioxide (99.999%) was used as received without further
purification. KH (30% dispersion in mineral oil) was purchased from
Aldrich and washed with hexane (3 ꢂ 10 mL) in a Schlenk tube
before use. 1H NMR (600 MHz), 13C NMR (151 MHz) and 27Al NMR
(156.4 MHz) spectra of the compounds were recorded on a BRUKER
AVANCE III 600 MHz instrument in CDCl3, C6D6 and C5D5N at 298 K.
Elemental analyses were performed on a Vario EL-III instrument.
1
2
3
4
5
6
7
8
2.5
_
2.5
5
5
10
5
5
5
5
5
Bu4NI (2.5)
Bu4NI (2.5)
e
THF
THF
THF
THF
THF
THF
THF
THF
CH3CN
CH2Cl2
Toluene
Solvent-free
THF
57
trace
13
90 (87)
93
91
49
36
32
9
Bu4NI (2.5)
Bu4NI (5.0)
Bu4NI (2.5)
Bu4NBr (2.5)
Bu4NCl (2.5)
Bu4NI (2.5)
Bu4NI (2.5)
Bu4NI (2.5)
Bu4NI (2.5)
Bu4NI (2.5)
Bu4NI (2.5)
9
10
11
12
13c
14d
53
77
95
92
5
5
5
THF
a
Reaction conditions: styrene oxide (2 mmol), CO2 (1 atm), THF (1 mL), 50 ꢀC,
24 h; bYield determined by 1H NMR spectroscopy using trichloroethylene as an
internal standard, isolated yields were given in parentheses;c 36 h;d 75 ꢀC.
2.2. Synthesis of potassium organoaluminate complex
A tetrahydrofuran solution of [C4H3NH(2-CH2NHtBu)] (0.457 g,
3.0 mmol) was added slowly to a suspension of KH (0.40 g,
3.0 mmol) in THF (15 mL) at ꢁ78 ꢀC. The reaction was slowly
warmed to room temperature and stirred for 3 h, then followed by
addition of a solution of Al(CH3)3 (3.0 mmol, 2.0 M in hexane). After
the mixture was stirred for another 3 h at 60 ꢀC, the resulting so-
lution was filtered and concentrated to a small amount and
recrystallized to generate colorless, block X-ray quality crystals
(0.583 g, 74%). 1H NMR (600 MHz, C6D6þC5D5N)
d 7.35 (s, 1H,
C4H3N), 6.35 (s, 1H, C4H3N), 6.19 (s, 1H, C4H3N), 3.99 (d, J ¼ 8.7 Hz,
2H, CH2NBut),1.00 (s, 9H, CH2NBut), ꢁ0.11 (s, 9H, Al(CH3)3). 13C NMR
(151 MHz, C6D6þC5D5N)
d 138.95 (C4H3N), 127.42 (C4H3N), 108.24
(C4H3N), 107.58 (C4H3N), 50.63 (C(CH3)3), 41.47 (CH2NBut), 29.21
(C(CH3)3), ꢁ5.36 (Al(CH3)3). 27Al NMR (156.4 MHz, C6D6þC5D5N)
d
162.85. Anal. Calcd for C12H24AlKN2: C, 54.92; H, 9.22; N, 10.68.
Found: C, 54.83; H, 9.15; N, 10.63.
2.3. General procedure for the synthesis of cyclic carbonates under
ambient conditions
A 50 mL sealed tube with a magnetic stirring bar was evacuated
and backfilled will CO2 for three times, potassium organoaluminate
(0.1 mmol), (n-Bu)4NI (0.05 mmol), epoxide (2.0 mmol), and 1.0 mL
THF were added. The reaction mixture was stirred at 50 ꢀC (heating
mantle temperature) for 24 h. The reaction was then quenched
with H2O (10 mL) and extracted with ethyl acetate (3 ꢂ 15 mL). The
combined organic layers were dried over Na2SO4 and concentrated.
The yields were determined by 1H NMR spectroscopy with
trichloroethylene (90 mL, 1 mmol) as an internal standard. Then, the
Scheme 2. Substrate scope in the coupling of CO2 with various epoxides catalyzed by
(4)/n-Bu4NI.a.
reaction mixture was purified on silica gel column chromatography
(petroleum ether/EtOAc: 3/1). All cyclic carbonates were identified
through comparisons with the corresponding 1H NMR, 13C NMR
data reported in the literatures [7].
a
Reaction conditions: epoxides (2 mmol), CO2 (1 atm), THF (1.0 mL), 50 ꢀC, 24 h
b Yield determined by 1H NMR using trichloroethylene as an internal standard; isolated
yields were given in parentheses.

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Z. Guo et al. / Journal of Organometallic Chemistry 922 (2020) 121365
Scheme 3. 13CO2-Labelling experiment.
3. Results and discussion
3.2. The synthesis of cyclic carbonate catalyzed by potassium
organoaluminate
3.1. Synthesis and crystal structure of potassium organoaluminate
complex
After the successful synthesis and characterization of this
bimetallic complex, it was used as catalysts for the synthesis of
cyclic carbonates under ambient conditions, and the results are
listed in Table 1. We chose styrene oxide as a model substrate, the
reaction was treated in THF solution at atmospheric pressure of CO2
using 2.5 mol% of potassium organoaluminate and 2.5 mol% of TBAI
at 50 ꢀC within 24 h. To our delight, the cyclic product, styrene
carbonate 2a, was obtained in 57% yield (entry 1). Control experi-
ments showed that neither TBAI nor the title complex alone dis-
played significant catalytic activity in the absence of the other
catalyst component under the current reaction conditions (Table 1,
entries 2e3). Pleasingly, that when 5 mol% the title complex and
2.5 mol% TBAI loading was used the reaction afforded the desired
carbonate in 90% yield within 24 h (Table 1, entry 4). Simply
increasing the amount of catalyst or cocatalyst has little effect on
the reaction yield (Table 1, entries 5e6). It was also found that the
order of catalytic activity was I > Br > Cl for tetrabutylammonium
salts (Table 1, entries 7e8). Then, different solvents such as aceto-
nitrile, dichlormethane and toluene were screened, as was the
solvent-free reaction (Table 1, entries 9e12), and poor yields were
observed, which indicated that the solvent also plays an important
role on the reactivity of the reaction. Furthermore, extending the
reaction time to 36 h gave a 95% yield (entry 13). In addition, the
reaction temperature was also examined and the 92% yield of sty-
rene carbonate was obtained at 75 ꢀC (entry 14).
The potassium organoaluminate was readily prepared in good
yield through the addition of a stoichiometric amount of Al(CH3)3 at
room temperature to
a
freshly prepared solution of KL
(L ¼ [C4H3NH(2-CH2NHtBu)]) (Scheme 1). And it was easily purified
by crystallization from a mixture of hexane and diethyl ether and was
characterized by satisfactory C, H, and N microanalysis, 1H, 13C, and
27Al NMR spectra in C6D6þC5D5N at ambient temperature, and single
crystal X-ray structural data. The 1H and 13C spectra revealed a single
set of alkyl signals, suggesting that its polymeric constitution is not
retained in solution.
Crystals of potassium organoaluminate suitable for X-ray quality
was obtained from a concentrated solution in toluene/Et2O at room
temperature. As illustrated in Fig. 1. In the solid state, compound
containing an aluminium atom bridged to an alkali metal by a
methyl group is relatively uncommon, which is interlocked by
bridging potassium ions to form a monodimensional polymeric
array, and featured an unsupported bridging methyl interaction
between potassium and aluminum metals lead to ultimate 2D su-
pramolecular network structure. The
M
ꢁ
C distances of
AleC12 ¼ 1.997 (3) Å and K0-C12 ¼ 3.172 Å indicates the metal
displays a long-range interaction with a methyl carbon. The dis-
tances between the potassium ion K and the pyrrole ring atoms (C1,
C2, C3, C4 and N1) were found to be 3.235(3), 3.340(3), 3.190(3),
2.994(3) and 3.041 (2) Å, respectively, which are quite similar to the
values reported for other pyrrolyl potassium sandwich geometries
[6c,6d]. The KeN2amino bond distance (2.933 (3) Å) also fits well
with the corresponding KeN distances observed in the amino-
pyrrolyl potassium complex.
With the optimized reaction conditions in hand, substrate
scopes for this cyclic carbonates synthesis reaction were investi-
gated using 5 mol% of the title complex and 2.5 mol% of TBAI at
50 ꢀC and 1 bar carbon dioxide pressure. Almost all the substrates
including non-functionalized and functionalized aliphatic epoxides

Z. Guo et al. / Journal of Organometallic Chemistry 922 (2020) 121365
5
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product (Scheme 2), indicating that the catalyst possessed a broad
substrate scope. From our results it was noticeable that the yield of
cyclic carbonates decreases with longer alkyl groups of epoxides,
maybe ascribed to high viscosity of these epoxides with long chains
(2d-2f). Both electron-donating and electron-withdrawing groups
were tolerated, and affording the expected cyclic carbonates in
moderate yields (2b-2c, 2g-2h). Anyway, comparing to other
organoaluminate complexes, this potassium organoaluminate ex-
hibits moderate catalytic activity when using the moderate reactive
styrene oxide as a substrate (TON ¼ 18, TOF ¼ 0.75 hꢁ1). A proposed
mechanism for this cyclic carbonate synthesis catalyzed by title
complex would be similar to the reaction catalyzed by a Lewis acid
and nucleophile combination [3]. Moreover, a13CO2-labeled
experiment was conducted with styrene oxide (Scheme 3), the
corresponding 13C labeled cyclic carbonate was characterized by
13C NMR, with this evidence, It proved that the carbon source was
provided from by CO2.
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4. Conclusions
In this work, a new potassium organoaluminate complex sta-
bilized by bidentate pyrrolyl ligand was synthesized and charac-
terized. The X-ray diffraction analyses of this complex indicated
that it is a rare example of an alkyl-bridged K/Al complex, which is
interlocked by bridging potassium ions to form a monodimensional
polymeric array, and featured an unsupported bridging methyl
interaction between potassium and aluminum metals lead to ulti-
mate 2D supramolecular network structure. It was proved to an
efficient catalyst for the conversion of CO2 into cyclic carbonates in
good yields at 50 ꢀC and 1 bar carbon dioxide pressure.
(c) C.Y. Li, D.C. Liu, B.T. Ko, Synthesis, characterization and reactivity of single-
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A. Otero, Synthesis of cyclic carbonates catalysed by aluminium hetero-
scorpionate complexes, Catal. Sci. Technol. 4 (2014) 1674e1684;
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Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
(i) J.A. Castro-Osma, M. North, X. Wu, Development of a halide-free aluminium-
based catalyst for the synthesis of cyclic carbonates from epoxides and carbon
dioxide, Chem. Eur J. 20 (2014) 15005e15008;
Acknowledgements
Financial supports from the Coal-based Key Scientific and
Technological Project (No. MH2014-07) and the Natural Science
Foundation of Shanxi Province (No. 201801D121041) are gratefully
acknowledged.
(j) J. Martinez, J.A. Castro-Osma, A. Earlam, C. Alonso-Moreno, A. Otero, A. Lara-
ꢀ
ꢀ
Sanchez, M. North, A. Rodríguez-Dieguez, Synthesis of cyclic carbonates cata-
lysed by aluminium heteroscorpionate complexes, Chem. Eur J. 21 (2015)
9850e9862;
(k) S.H. Kim, S.Y. Han, J.H. Kim, Y.Y. Kang, J. Lee, Y. Kim, Monomeric or dimeric
aluminum complexes as catalysts for cycloaddition between CO2 and epoxides,
Eur. J. Inorg. Chem. 2015 (2015) 2323e2329;
(l) J.A. Castro-Osma, M. North, X. Wu, Synthesis of cyclic carbonates catalysed
by chromium and aluminium salphen complexes, Chem. Eur J. 22 (2016)
2100e2107;
(m) J.A. Castro-Osma, M. North, W.K. Offermans, W. Leitner, T.E. Müller, Un-
precedented carbonato intermediates in cyclic carbonate synthesis catalysed by
bimetallic aluminium(salen) complexes, ChemSusChem 9 (2016) 791e794;
(n) X. Wu, M. North, A bimetallic aluminium(salphen) complex for the syn-
thesis of cyclic carbonates from epoxides and carbon dioxide, ChemSusChem 10
(2017) 74e78;
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2020.121365.
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