ONO pincer type ligand complexes of Al(III) as efficient catalyst for chemical fixation of CO2 to epoxides at atmospheric pressure

Article abstract of DOI:10.1016/j.jcat.2019.07.033

Carbon dioxide, the main cause of environmental pollution, its utilization to produce valuable products is of utmost interest. A series ONO pincer hydrazone based most active mono-nuclear Al(III) complexes were successfully synthesized and characterized with the help of NMR, IR, mass spectrometry and only complex 2a was confirmed by single-crystal analysis. The synthesized Al(III) complexes were then employed as capable catalysts for the solvent-free chemical fixation of CO₂ with epoxides at atmospheric pressure and could be reused five times without loss of any catalytic activity. In addition, the catalytic mechanism was investigated by analyzing intermediates via ¹H NMR, ¹³C NMR, and mass MALDI-TOF. The excellent catalytic performance could be due to simultaneous attack and the opening of the epoxide by metal centers to form an alkoxide ion which activates the CO₂ the same time.

Full text of DOI:10.1016/j.jcat.2019.07.033

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Journal of Catalysis 377 (2019) 190–198
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
ONO pincer type ligand complexes of Al(III) as efficient catalyst for
chemical fixation of CO2 to epoxides at atmospheric pressure
e
Habib Ullah a,b, Bibimaryam Mousavi a, , Hussein A. Younus , Zafar A.K. Khattak a,b, Suleman Suleman a,b
,
⇑
Muhammad T. Jan f, Baoyi Yu g, Somboon Chaemchuen a, Francis Verpoort a,b,c,d,
a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, PR China
⇑
b School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, PR China
c Center for Environmental and Energy Research (CEER), Ghent University Global Campus, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon 21985, South Korea
d National Research Tomsk Polytechnic University, Lenin Avenue 30, 634050 Tomsk, Russian Federation
e Department of Chemistry, Faculty of Science, Fayoum University, Fayoum 63514, Egypt
f Department of Chemistry, Islamia College University, Peshawar, KPK, Pakistan
g Key Laboratory of Urban Agriculture (North China), Ministry of Agriculture, College of Biological Sciences Engineering, Beijing University of Agriculture, Beijing 102206, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon dioxide, the main cause of environmental pollution, its utilization to produce valuable products is
of utmost interest. A series ONO pincer hydrazone based most active mono-nuclear Al(III) complexes
were successfully synthesized and characterized with the help of NMR, IR, mass spectrometry and only
complex 2a was confirmed by single-crystal analysis. The synthesized Al(III) complexes were then
employed as capable catalysts for the solvent-free chemical fixation of CO2 with epoxides at atmospheric
pressure and could be reused five times without loss of any catalytic activity. In addition, the catalytic
mechanism was investigated by analyzing intermediates via 1H NMR, 13C NMR, and mass MALDI-TOF.
The excellent catalytic performance could be due to simultaneous attack and the opening of the epoxide
by metal centers to form an alkoxide ion which activates the CO2 the same time.
Received 13 January 2019
Revised 12 July 2019
Accepted 15 July 2019
Keywords:
CO2 fixation
Pincer ligand
Aluminium complexes
Cyclic carbonate
Homogeneous catalysis
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
epoxides, are extensively studied and even well commercialized
[2,4,5]. Environment-friendly and 100% atom efficient, the redox
Worldwide industrialization and rapid urbanization are posing
serious threats to our environment due to the large-scale anthro-
pogenic production of CO2, which is greatly affecting the concept
of green chemistry. Global warming and greenhouse effect are
mainly originating from the CO2 emission1. Nevertheless, the sci-
entific community is taking a great and ever-increasing interest
in the CO2 transformation into valuable chemical products.
Because of its abundance, ready availability, non-toxic and reusa-
ble nature, CO2 is a valuable one-carbon (C1) building block in
organic transformations [1–3]. As a well-known fact, CO2 has a
high thermodynamic stability and kinetic inertness due to the car-
bon is in its highest oxidation state hence making it a challenge to
activate and valorise CO2 to valuable products. Among the various
approaches employed for CO2 utilization, synthesis of cyclic car-
bonates and polycarbonates from the chemical fixation of CO2 to
neutral fixation of CO2 to epoxides to form cyclic carbonates with
no appreciable toxicity [6–9] is specifically investigated in recent
years because of its numerous uses as solvents, electrolytes, syn-
thetic building blocks, fine chemicals, industrial lubricants and as
monomers for polymers [10–14]. Over the past two decades, huge
progress has been made in the chemical fixation of CO2 to epoxides
which can be clearly observed from the increase in the number of
publications since 2000 [15]. Various metal complexes based on
zinc [16–19], chromium [20–22], iron [23–26], cobalt [27–33],
magnesium [17,34], vanadium [35–37], tin [38] etc. have been
reported as catalyst for the CO2 cycloaddition to epoxides. Among
the metal complexes, aluminium has been extensively studied for
the CO2/epoxides cycloaddition. This due to its environment-
friendly nature, earth abundance, nontoxicity, and high Lewis acid-
ity making aluminium the ideal choice for the ring-opening of
epoxides [39]. Various research groups reported the CO2/epoxides
cycloaddition with salen and related ligand-based monometallic
[27,40–44] and bimetallic [39,45–50] aluminium complexes.
Amongst the aluminium complexes, bimetallic aluminium(salen)
complexes [51,52] reported by North and other groups [50] are
⇑
Corresponding authors at: State Key Laboratory of Advanced Technology for
Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, PR
China.
E-mail address: Francis.verpoort@ghent.ac.kr (F. Verpoort).
https://doi.org/10.1016/j.jcat.2019.07.033
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

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H. Ullah et al. / Journal of Catalysis 377 (2019) 190–198
191
efficient catalysts for the CO2 cycloaddition at ambient pressure.
The most active catalyst amongst the mononuclear Al complexes
is the hexachlorinated aluminium(III)–aminetriphenolate complex,
reported by Kleij et al., in combination with tetrabutylammonium
iodide as co-catalyst demonstrating the highest TOF of 24,000 hÀ1
for the cycloaddition of CO2 to 1,2-epoxy-hexane. This high TOF
was obtained at 90 °C under a high pressure of 10 bar along with
a high catalyst to co-catalyst ratio [39]. Similarly, the highest ratio
of cat/co-cat (1:120) was reported by Qin et al. using the porphyrin
ligand-based aluminium complex for synthesis of cyclic carbonates
at a relatively high pressure of 10 bar [53]. Unfortunately, the syn-
thesis of the porphyrin ligand requires a very long and tedious syn-
thetic procedure. Till today, the reported literature suggests that
most of the aluminium complexes studied for cycloaddition of
CO2 to epoxides are based on salen, salophen and phenolate scaf-
folds. Keeping these factors in minds, we hereby report a novel,
highly active monometallic aluminium complexes supported by
an ONO pincer type hydrazone moiety for the chemical fixation
of CO2 at atmospheric pressure. Furthermore, NMR and MALDI-
TOF analyses were used to investigate the mechanistic insight of
the CO2/epoxides coupling reaction catalyzed by these Al
complexes.
firmed the successful coordination of the ligand to the metal ion.
The formation of these complexes was further supported by FT-
IR, which shows the disappearance of the hydroxyl band. Further-
more, the appearance of a new band for the CAO vibration and the
shift of the azomethine C@N stretch vibration towards a lower
wavenumber was observed as well. Moreover, the obtained results
of HR-MS analysis of these complexes exactly matched with the
calculated results. Finally, the exact coordination of the different
ligands to the Al ion was confirmed by single-crystal X-ray
analysis. The single-crystal analysis revealed that the complex 2a
2. Results and discussion
A family of hydrazone-based ligands 1–4 and their respective
aluminum complexes 1a–4a were obtained in good yield by the
reaction of the respective ligand with the metal salt in methanol
as solvent, following the published procedure (Scheme 1) [54].
These complexes were characterized further by nuclear magnetic
resonance (NMRs) (ESI S1-S16), infrared spectroscopy (ESI S17-
S24), High-resolution mass spectrometry (HR-MS) (ESI S25-S31)
and elemental analysis. Complex 2a was successfully analyzed by
single-crystal X-ray analysis (see Fig. 1). For all complexes, the
1H NMR spectroscopic analysis revealed the disappearance of the
OH proton of the ligand due to the binding with the Al ion. In addi-
tion, the position of the NH proton was slightly shifted and the
downfield shifting of the azomethine C@N proton clearly con-
Fig. 1. ORTEP diagram of complex 2a (Labelling of carbon and hydrogen atoms is
omitted for clarity).
Scheme 1. General synthetic procedure of the hydrazone ligands 1–4 and their respective complexes 1a–4a.

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192
H. Ullah et al. / Journal of Catalysis 377 (2019) 190–198
crystalizes in a triclinic system having a space group P-1. The
detailed crystallographic data has been depicted in supporting
information (ESI Table 1). A distorted octahedral coordination of
the Al(III) is obtained by two ONO tridentate ligands having the dif-
ferent protonation states. The two axial positions are occupied by
two imino nitrogen atoms (Al1-N2 2.002 A) and (Al1-N4 1.949
A), while four oxygen atoms construct the equatorial plane of the
distorted octahedron. The distortion is further supported by com-
paring trans bond angles around the Al center of the octahedron.
The trans bond angles between O1AAlAO2, O3AAlAO4, and
N2AAlAN4 are 164.98 (13), 168.64 (14) and 168.78 (14) while
cis bond angles between the O2AAlAO3 and O1AAlAO4 are
91.92 (12) and 89.14 (12) respectively. Similarly, the bond dis-
tances within the coordination sphere also exhibit distortion i.e.
the bond length between the AlAO (phenolate) in the complex
for AlAO2 and AlAO4 are 1.819(3) and 1.811(3), which are shorter
than the AlAO (hydrazonato) bond lengths for AlAO1 2.015(3) and
AlAO3 1.940(3) respectively.
of the reaction conditions. No cyclic carbonate formation was
achieved by using precursors like metal salts and ligands individu-
ally. This reveals the cooperative effect of the metal and their
ligand scaffold in the complexes to precede the cycloaddition of
CO2 to epoxides. Afterwards, all Al-complexes were screened for
the optimization of the reaction parameters. Initially, the reaction
was performed at room temperature for 24 h and only traces of
cyclic carbonate were observed at room temperature (Table 1 entry
1). A steady increase in the cyclic carbonate synthesis was moni-
tored by increasing the temperature up to 120 °C (Table 1 entry
2–5) reflecting an enhancement in the product at an elevated tem-
perature whereby the highest yield was observed at 120 °C and
was chosen as the optimum temperature for further reactions in
this study. Another very important parameter in the chemical fix-
ation of CO2 to epoxides is the carbon dioxide pressure. Most of the
catalytic systems, except a few, are working at high pressure. For-
tunately, the catalyst along with the co-catalyst has shown a satis-
factory catalytic performance at ambient pressure. In order to
confirm the further co-operative effect of catalyst and co-
catalysts (DMAP, TBAB, TBAC, and TBAI), control experiments
under the same reaction conditions were performed. However,
these control experiments revealed a low conversion of the sub-
strate (Table 1 entries 6–13). Thereafter, the effect of these four
co-catalysts (DMAP, TBAB, TBAC, and TBAI) on these Al-
complexes was employed to enhance the formation of cyclic car-
bonates and diminish the time duration (Table 1 entries 5, 7–21)
3. Catalysis
After successful synthesis and characterization of these novel
monometallic Al(III) complexes, these complexes (1a–4a) were
employed as excellent catalysts along with co-catalyst for the
chemical fixation of CO2 under non-solvent conditions. Epichloro-
hydrin (ECH) was used as a model substrate for the optimization
Table 1
Optimization of reaction conditions for the cycloaddition of CO2 to epichlorohydrin as model substrate by complexes (1a–4a).
r
Entry
Catalyst
Co-catalyst
Temp (°C)
Time (Hr)
Conversion%
1a
2a
2a
2a
2a
2a
1a
2a
3a
4a
–
DMAP
DMAP
DMAP
DMAP
DMAP
–
–
–
–
DMAP
TBAB
TBAC
TBAI
TBAB
TBAC
TBAI
DMAP
TBAB
TBAC
TBAI
DMAP
TBAB
TBAC
TBAI
DMAP
TBAB
TBAC
TBAI
DMAP
DMAP
RT
50
75
24
24
24
6
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
–
3
2a
3a
10
50
>99
50
65
40
35
20
17
15
10
70
85
50
85
75
80
60
87
65
85
75
91
80
78
70
30
10
4a
100
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
5a
6a
7a
8a
9a
10a
11a
12a
13a
14a
15a
16a
17a
18a
19a
20a
21a
22a
23a
24a
25a
26a
27a
28a
29b
30c
–
–
–
2a
2a
2a
1a
1a
1a
1a
3a
3a
3a
3a
4a
4a
4a
4a
2a
2a
r
Reaction conditions: Pressure 1 bar, temperature 120 °C, catalyst/substrate ratio (mol%/mol%); a; 0.1 mol%, b; 0.01 mol%, c; 0.002 mol%. Conversion is based on 1H NMR.
Selectivity >99. DMAP: 4-dimethylaminopyridine; TBAB: tetrabutylammonium bromide; TBAC: tetrabutylammonium chloride; TBAI: tetrabutylammonium iodide.

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H. Ullah et al. / Journal of Catalysis 377 (2019) 190–198
193
[55]. After the optimization and comparison of all the results sum-
marized in (Table 1) complex 2a along with co-catalyst DMAP
(Table 1 entry 5) exhibit a superior activity compared to the other
combinations of catalyst/co-catatalyst Interestingly, the co-
catalyst DMAP in combination with all four complexes demon-
strated the highest activity (Table 1 entries 5, 10, 14 and 18). In
addition, the catalyst loading was investigated to obtain the opti-
mum catalyst amount by employing catalyst and substrate ratio
in ranging from 0.01 mol% to 0.002 mol%. A decrease in the yield
of the cyclic carbonate was observed with a lowering of the cata-
lyst loading (Table 1 entries 22, 23). It was found that the catalytic
systems having an aromatic ring like pyridine or naphthalene moi-
ety in the side chain of complex 3a and 4a were found to be the
least reactive compared to 1a and 2a having a furan and thiophene
in the side chain, respectively. This could be explained based on the
low basicity of pyridine and naphthalene due to delocalization
effect.
(>99%) (see ESI S32 for 1H and 13C spectra of all products) were
recorded under ambient pressure for nearly all tested epoxides
using complex 2a (Table 2). Natural epoxy resin like DGEBA was
also explored to observe the effect and conversion towards natural
epoxides (Table 2 entry 11).
As evidenced from Table 2, epichlorohydrin was found to be
the most reactive substrate because of the presence of an
electron-withdrawing group like Cl [38]. The presence of the
electron-withdrawing group in the epoxide will increase the elec-
trophilicity of the secondary carbon, facilitating the ring-opening
step and hence, increase the reaction rate. In contrast, the pres-
ence of an electron-donating group reduced the electrophilicity
resulting in a decline in reactivity which is demonstrated by the
relatively long reaction time, i.e. 20 h for some substrates like
PO and SO.
Most importantly, the investigated complex 2a was highly reac-
tive among reported monometallic aluminum complexes, based on
pressure (Table 3), and comparable to bimetallic aluminum com-
plexes that belong to the extensively studied salen, salphen, por-
phyrin, etc. ligated system.
Finally, applying the optimized reaction conditions to monitor
the flexibility of the complex 2a for the CO2 fixation, different sub-
strates were screened. Excellent conversions and selectivities
Table 2
Cycloaddition of CO2 to tested epoxides by complex 2a.
p
Entry
1
Substrate
Product
Time (h)
2
–Conversion
>99
Yield(%)
–TOF
500
#TON
1000
98
2
3
6
>99
>99
97
95
166
58
1000
930
16
4
6
>99
97
166
1000
5
6
6
8
>99
>99
97
97
166
125
1000
1000
7
20
>99
97
50
1000
8
9
20
8
>99
>99
90
96
50
1000
1000
125
10
11
8
>99
20
97
15
125
16
1000
195
12
Reaction conditions: Pressure 1 bar, temperature 120 °C, catalyst 2a 0.1 mol%, DMAP 0.1 mol%, 13.48 mmoles substrate.
–
Conversion is based on 1H NMR. Selectivity >99.
Isolated yield.
TON = number of mmol of product/number of mmol of catalyst.
TOF = TON/time. Standard deviation 1% for obtained conversions.
p
#
–