Unexpected intramolecular: N -arylcyano-β-diketiminate cyclization in new aminoquinoline derivative complexes of aluminium for CO2 fixation into cyclic carbonates

Article abstract of DOI:10.1039/c9nj02499a

New 4-amino-3-iminoquinoline derivative ligands (L1-4) were synthesized through an intramolecular exo-dig cyclization of anionic β-diketiminates, containing an N-benzonitrile moiety. The effect of the alkali-metal in this reaction was investigated and an inverse effect between the size of the cation and the cyclization rate was revealed. The effect of the reaction temperature was also studied, in which a direct dependence was observed. Kinetic and theoretical studies were performed in an attempt to clarify the reaction mechanism, obtaining a first-order reaction rate dependence with an activation energy of 20.6 kcal mol^-1, with DFT-based calculations supporting the proposed mechanism. In addition, four new aluminium complexes were isolated in high yields (C1-4), which were evaluated as catalysts for the preparation of cyclic carbonates from epoxides and CO₂, thus becoming the first examples of the use of β-diketiminate ligands in this catalytic process. The reactions were performed at 80 °C and 1 bar CO₂ pressure under solvent-free conditions. We were able to prepare a large variety of cyclic carbonates in excellent selectivities and yields, employing the aluminium complex C3. The L1-4 ligands and C1-4 complexes were characterized using NMR, FT-IR, HRMS, and X-ray diffraction methods.

Full text of DOI:10.1039/c9nj02499a

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b
Unexpected intramolecular N-arylcyano- -diketiminate cyclization in new
aminoquinoline derivatives complexes of aluminium for CO2 fixation into cyclic
carbonates
David Moreno da Costa,1 Luis Borja,1 Camilo Verdugo,1 Javier Martinez,1 Celso
Quintero,1 Pablo Jaque,2 Oleksandra S. Trofymchuk,3 Constantin G. Daniliuc,4 Alan R.
Cabrera*,1 and Rene S. Rojas*,1
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1
Departamento de Química Inorgánica, Facultad de Química y de Farmacia, Pontificia
Universidad Católica de Chile, Vicuña Mackenna 4860, 702843, Santiago, Chile.
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Departamento de Química Orgánica y Fisicoquímica, Facultad de Ciencias Químicas y
Farmacéuticas, Universidad de Chile, Sergio Livingstone 1007, 8380492, Santiago, Chile.
3 Núcleo Científico Multidisciplinario-DI, University of Talca, 747, Talca, Chile.
4 Organisch-Chemisches Institut, Universität Münster, Corrensstrasse 40, 48149, Münster,
Germany.
* Corresponding author: arcabrer@uc.cl, rrojasg@uc.cl
Keywords: cyclic carbonate, intramolecular cyclization, aluminium, aminoquinoline
Abstract
New 4-amino-3-iminoquinolines derivatives ligands (L1-4) were synthesised through
an intramolecular exo-dig cyclization of anionic b-diketiminates containing N-
benzonitrile moiety. The effect of the alkali-metal in this reaction was investigated and
an inversely effect between the size of the cation and the cyclization rate was obtained.
The effect of the reaction temperature was also studied, in which a direct dependence
was observed. Kinetic and theoretical studies were performed in an attempt to clarify
the reaction mechanism, obtaining a first-order reaction rate dependence with a value
of activation energy of 20.6 kcal mol-1 and DFT-based calculations support the proposed
mechanism. In addition, four new aluminium complexes were isolated in high yields
(C1-4), which were evaluated as a catalyst for the preparation of cyclic carbonates from
epoxides and CO2,becoming the first example in which b-diketiminates ligands are used

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in this catalytic process. The reactions were carried out at 80 °C and 1 bar CO2 pressure
under solvent-free conditions. We were able to prepare a large variety of cyclic
carbonates in excellent selectivities and yields employing aluminium complex C3. L1-4
ligands and C1-4 complexes were characterised by NMR, FT-IR, HRMS, and X-Ray
diffraction methods.
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Introduction
Anionic N,N compounds are one of the most used ligands for the synthesis of metal
complexes (see Figure 1).1-12 The V-form of amidinates, containing different sizes of
functional groups (e.g. 113 and 214), and the U-form of the b-diamines (e.g. 315), b-
diketiminates (e.g. 415 and 516) and b-dianilines (e.g. 617) central core, and its relatively
easy modulation by the substituents give to the metal complex the stereoelectronic
environment ensuring spatial position and conferring specific reactivity to the metallic
core. It is important to note that six-member ring metal compounds, such as b-
diketiminates complexes, have been widely studied and interesting applications such
as olefin polymerization, ring-opening polymerization (ROP) of cyclic esters, and small
molecules activation have been developed in recent years.15,
For example,
18-26
compounds 2 and 5 are recognisable for their employment in olefin polymerization.14,
16

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Figure 1. Some anionic N,N ligands used for the synthesis of metal complexes.
The synthesis of metal b-diketiminates complexes is currently carried out by
transmetalation from the lithium, sodium, or potassium b-diketiminate derivatives.
Lithium b-diketiminates are widely used as ligand transfer reagents by their reaction
with various metal chlorides. However, sodium and potassium b-diketiminates
derivatives are more effective due to the ionic nature of the M-N bond (M = Na, K) and
the heavier alkali-metal chloride coproducts (NaCl and KCl) are easier to separate than
LiCl.27, 28
In our research group, the efforts in this area are mainly focused on the preparation of
transition and main-group-metal b-diketiminate complexes for small molecule
activation. In that sense, the formation of new organic molecules used as ligands in
metal complexes that could modulate their stereoelectronic properties have been one
of our principal objectives. For that reason, we decided to focus our attention on the 4-

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aminoquinoline structure, a restricted and bulky b-aminoimines ligand containing an
imine group at 3-position in the pyridyl moiety of the quinoline. In addition, these
ligands are sterically and electronically tunable by typical functional group
transformations, which makes them highly interesting ligands. Although 4-
aminoquinolines derivatives have received plenty of attention in synthetic and
medicinal chemistry,29-33 4-amino-3-iminoquinolines have not been widely studied.
There is only one synthetic procedure that describes the preparation of acridine
analogues, which have a similar heterocyclic core.34 As far as we know, this type of
ligand has never been used for the synthesis of metal complexes.
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It is worth highlighting that we have found a new synthetic methodology to obtain the
aforementioned N-heterocycle, starting from a potassium b-diketiminate. This reaction
was optimised according to the thermal, time, and alkali metal dependence.
Having prepared these 4-amino-3-iminoquinolines ligands and bearing in mind that
this type of ligands has never been used for the ring-opening copolymerization
(ROCOP) of epoxides and CO2, and as our research group has plenty of experience on
the synthesis of cyclic carbonates catalysed by aluminium complexes,35, 36 we decided
to prepare their corresponding alkyl-aluminium complexes before studying their
catalytic activity for the preparation of cyclic carbonates from epoxides and CO2. This
example was the first in which a 4-amino-3-iminoquinolines ligand participated in this
catalytic transformation.
2. Experimental Section
2.1. General Information
All reagents were purchased from commercial sources, unless otherwise specified and
used as received. Ligands L1-2 were prepared as described in literature.37 NMR spectra
were recorded on NMR Bruker AV 400. Chemical shifts are given in parts per million
relative to TMS [1H and 13C, d(SiMe4) = 0] or an external standard. Most NMR
assignments were supported by additional 2D experiments. HRMS-ESI-MS experiments
were carried out using a Thermo Scientific Exactive Plus Orbitrap Spectrometer. FT-IR

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spectra were recorded on a Bruker Vector-22 Spectrophotometer using KBr pellets. For
X-ray crystal structure analysis, data for ligand L3 were collected with a Bruker D8
Venture Dual Source 100 CMOS diffractometer and for ligand L6 and complex C4 with
a Bruker APEX II CCD diffractometer by Dr. Constantin G. Daniliuc; CCDC-1913072 (L3),
CCDC-1913158 (L6) and CCDC-1913073 (C4) contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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2.2 Synthesis of the compounds
2.2.1 General synthetic procedure of ligands L3-4
The corresponding b-diimine ligand (L1-2) (1 eq),37 potassium hydride (1 eq) and 30
mL of anhydrous THF were added in a dried Schlenk under N2 atmosphere and the
mixture was stirred for 2 hours at room temperature. The reaction showed evidence of
hydrogen formation (H2). When the reaction finished, the mixture was filtered through
celite and the solution was transferred and sealed in a round flask fitted with a Teflon
screw cap, heated at 90 °C, and stirred for 3 hours. The orange solution was evaporated
in a vacuum to dryness and the remaining solid was dissolved in 30 mL of
dichloromethane/methanol (3:1) and washed twice with 30 ml of distilled water. The
organic phase was dried with anhydrous Na2SO4 and concentrated to dryness. The
crude product was purified via silica gel chromatography (1:9 Petroleum ether/ethyl
acetate).
Ligand L3. Isolated as a colourless crystals in 80% yield (0.80 g). 1H NMR (400 MHz,
CDCl3, 298 K): δ/ppm = 7.93 (d, J = 8.4 Hz, 1H), 7.80 (d, J = 8.3 Hz, 1H), 7.64 (t, J = 7.6
Hz, 1H), 7.42 (t, J = 7.6 Hz, 1H), 7.23 (d, J = 7.6 Hz, 2H), 7.16 (t, 1H), 6.24 (s, 2H), 3.02
(hept, 2H), 2.75 (s, 3H), 2.09 (s, 3H), 1.29 (d, J = 7.0 Hz, 6H), 1.21 (d, J = 6.8 Hz, 6H).
13C{1H} NMR (100 MHz, CDCl3, 298 K): δ/ppm = 170.52, 156.28, 147.85, 147.32, 145.52,
135.97, 129.96, 128.99, 124.75, 124.34, 123.48, 120.67, 117.63, 115.15, 28.41, 25.88,
24.01, 23.75, 23.21. HRMS (ESI): m/z [M+H]+ for C24H30N3: calc: 360.2440; found:
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Ligand L4. Isolated as a colourless crystals in 82% yield (0.82 g). 1H NMR (400 MHz,
CDCl3, 298 K): δ/ppm = 7.91 (d, J = 8.4 Hz, 1H), 7.76 (d, J = 8.3 Hz, 1H), 7.63 (t, J = 7.6
Hz, 1H), 7.39 (t, J = 7.6 Hz, 1H), 7.11 (d, J = 7.5 Hz, 2H), 6.98 (t, J = 7.5 Hz, 1H), 6.05 (s,
2H), 2.72 (s, 3H), 2.21 (s, 3H), 2.03 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ/ppm
= 170.40, 156.06, 148.34, 147.47, 147.28, 129.94, 128.92, 128.64, 125.25, 124.77,
123.69, 120.65, 117.51, 115.46, 25.60, 23.22, 18.97. HRMS (ESI): m/z [M+H]+ for
C20H22N3: calc: 304.1814; found: 304.1770.
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2.2.2 General synthetic procedure of ligands L5-6
L3 or L4 (1 eq), potassium hydride (1.1 eq), and 50 mL of anhydrous THF were added
in a dried Schlenk under N2 atmosphere. The mixture was stirred for 2 hours at room
temperature. The reaction showed evidence of hydrogen formation (H2). When the
reaction finished, 1-Fluoro-4-nitrobenzene (1.1 eq) was added and the reaction mixture
was stirred for 1 hour at 50 °C. Finally, the solution was evaporated in a vacuum to
dryness and the product was purified via silica gel chromatography (3:2 Petroleum
ether/ethyl acetate).
Ligand L5. Isolated as a light-yellow powder in 76% yield (0.33 g). 1H NMR (400 MHz,
CDCl3, 298 K): δ/ppm = 8.11 (t, J = 8.2 Hz, 3H), 7.72 (t, J = 7.6 Hz, 1H), 7.67 (d, J = 8.4 Hz,
1H), 7.62 (s, 1H), 7.40 (t, J = 7.6 Hz, 1H), 7.22 – 7.12 (m, 3H), 6.71 (d, J = 8.9 Hz, 2H), 2.89
(s, 4H), 2.05 (s, 3H), 1.17 (dd, J = 6.0 Hz, 12H). 13C{1H} NMR (100 MHz, CDCl3, 298 K):
δ/ppm = 169.30, 156.49, 150.83, 148.62, 144.62, 141.01, 140.63, 136.31, 130.53,
129.66, 126.61, 126.11, 124.95, 124.44, 123.76, 122.36, 115.36, 28.40, 25.17, 24.09,
23.53, 23.31. HRMS (ESI): m/z [M-H]- for C30H31N4O2: calc: 479.2447; found: 479.2463.
Ligand L6. Isolated as a light-yellow powder in 79% yield (0.26 g). 1H NMR (400 MHz,
CDCl3, 298 K): δ/ppm = 8.36 (s, 1H), 7.91 (d, J = 9.0 Hz, 3H), 7.74 (t, J = 7.6 Hz, 1H), 7.56
(d, J = 8.3 Hz, 1H), 7.42 (t, J = 7.6 Hz, 1H), 7.13 (d, J = 7.4 Hz, 2H), 7.05 (t, 1H), 6.24 (d, J
= 8.9 Hz, 2H), 2.65 (s, 3H), 2.11 (s, 3H), 1.87 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3, 298
K): δ/ppm = 170.12, 155.55, 151.37, 148.22, 146.58, 140.02, 139.68, 132.04, 130.58,
129.67, 128.86, 126.72, 126.15, 124.74, 123.57, 122.99, 113.39, 24.70, 22.68, 19.06.
HRMS (ESI): m/z [M-H]- for C26H23N4O2: calc: 423.1821; found: 423.1835.

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2.2.3 General synthetic procedure of complexes C1-4
Inside of a glovebox, trimethylaluminum (1 eq) was added dropwise to a solution of L3-
6 (1 eq), as appropriate, in 5 mL of dry dichloromethane. The mixture was stirred for 2
hours at room temperature. The solvent was then evaporated under a vacuum and the
crude product was washed twice with dry and cold hexane. The resulting solid was
filtrated and dried under a vacuum. Complexes C1-4 where obtained as light-yellow
powders (95 %– 99 %).
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Complex C1. Isolated as a light-yellow powder in 97 % yield (0.30 g). H NMR (400
MHz, C6D6, 298 K): δ/ppm = 8.08 (d, J = 8.2 Hz, 1H), 7.33 (t, J = 8.2 Hz, 1H), 7.20 (d, J =
7.4 Hz, 1H), 7.12 – 7.05 (m, 3H), 6.97 (t, J = 8.2 Hz, 1H), 5.85 (s, 1H), 3.06 (m, 2H), 2.58
(s, 3H), 1.80 (s, 3H), 1.28 (d, J = 6.8 Hz, 6H), 0.95 (d, J = 6.9 Hz, 6H), –0.49 (s, 6H). 13C{1H}
NMR (100 MHz, C6D6, 298 K): δ/ppm = 176.14, 160.09, 148.35, 141.65, 140.74, 131.21,
129.50, 124.85, 124.58, 121.46, 120.78, 111.82, 28.52, 28.49, 25.28, 24.76, 24.19, –
11.17.
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Complex C2. Isolated as a light-yellow powder in 99 % yield (0.23 g). H NMR (400
MHz, C6D6) δ/ppm: 8.08 (d, J = 8.5 Hz, 1H), 7.36 (t, J = 7.6 Hz, 2H), 7.02 (t, J = 7.6 Hz, 1H),
6.95 (s, 3H), 5.96 (s, 1H), 2.51 (s, 3H), 2.03 (s, 6H), 1.59 (s, 3H), –0.53 (s, 6H). 13C{1H}
NMR (100 MHz, C6D6) δ/ppm: 176.07, 159.96, 148.42, 143.56, 131.20, 130.84, 129.37,
126.88, 124.56, 121.67, 120.82, 111.51, 28.33, 24.41, 18.35, –10.54.
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Complex C3. Isolated as a light-yellow powder in 95 % yield (0.34 g). H NMR (400
MHz, CDCl3, 253 K): δ/ppm = 8.23 (d, J = 9.1, 2.6 Hz, 1H), 7.94 (d, J = 7.7 Hz, 2H), 7.73
(dd, J = 14.2, 7.9 Hz, 2H), 7.33 (m, 4H), 7.18 (d, J = 6.0 Hz, 1H), 6.37 (d, J = 8.1 Hz, 1H),
2.90 (m, 1H), 2.84 (s, 3H), 2.38 (s, 3H), 2.15 (m, 1H), 1.30 (d, J = 6.8 Hz, 3H), 1.21 (d, J =
6.6 Hz, 3H), 0.80 (d, J = 6.7 Hz, 3H), 0.48 (d, J = 6.6 Hz, 3H), –0.87 (s, 3H), –1.17 (s, 3H).
13C{1H} NMR (100 MHz, CDCl3, 253 K): δ/ppm = 177.92, 157.61, 156.56, 153.80, 148.89,
140.41, 139.46, 138.74, 131.49, 128.27, 127.31, 126.27, 125.83, 125.36, 125.13, 124.66,
123.70, 120.65, 28.57, 28.41, 28.04, 25.72, 24.61, 24.28, 24.15, 23.28, –10.26, –12.14.
1
Complex C4. Isolated as a light-yellow powder in 95 % yield (0.30 g). H NMR (400
MHz, C6D6, 300K) δ/ppm: 8.18 (d, J = 8.3 Hz, 1H), 7.94 (d, J = 9.1 Hz, 2H), 7.79 (d, J = 8.5
Hz, 1H), 7.27 (t, J = 7.6 Hz, 1H), 6.92 – 6.86 (m, 2H), 6.80 (d, 1H), 6.46 (d, J = 7.12 Hz,

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2H), 2.52 (s, 3H), 1.64 (s, 6H), 1.58 (s, 3H), –0.72 (s, 6H). 13C{1H} NMR (100 MHz, C6D6,
300 K) d/ppm: 178.38, 157.55, 156.26, 154.11, 150.15, 142.50, 140.64, 131.37, 130.50,
129.41, 127.55, 125.87, 123.87, 122.39, 120.67, 27.45, 24.09, 18.06, –10.94.
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2.3 Cyclic carbonate catalytic studies
2.3.1 General procedure for catalyst screening
Styrene oxide Xa (1.7 mmol), C1-4 (17.0-25.5 µmol), and TBAI (17.0-25.5 µmol) were
placed in individual glass reaction tubes with a magnetic stirrer bar in a multi-point
reactor Carousel 12 Place Reaction Station. The reaction mixture was stirred at 80 °C
and 1 bar of CO2 pressure for 24 hours. Then the conversion of styrene oxide Xa into
styrene carbonate Ya was determined by 1H NMR spectroscopy analysis.
2.3.2 General procedure for the synthesis of cyclic carbonates
Epoxide Xa-h (1.7 mmol), C3 (25.5 µmol), and TBAI (25.5 µmol) were placed in
individual glass reaction tubes with a magnetic stirrer bar in a multi-point reactor. The
reaction mixture was stirred at 80 °C for 24 hours. The conversion of epoxide to cyclic
1
carbonate was determined by H NMR spectroscopy analysis. The remaining sample
was filtered through a plug of silica, eluting with CH2Cl2 to remove the catalyst. The
eluent was evaporated under a vacuum to give either a pure cyclic carbonate or a
mixture of cyclic carbonate and unreacted epoxide. In the latter case, the mixture was
purified by flash chromatography using a solvent gradient of hexane, hexane:EtOAc
(9:1), hexane:EtOAc (6:1), hexane:EtOAc (3:1), and EtOAc to give the pure cyclic
carbonate. Cyclic carbonates Ya-h are all known compounds and the spectroscopic data
for samples prepared using catalyst C3 were consistent with those reported in the
literature.35, 36, 38-41
3. Results and Discussion
3.1. Aminoquinoline ligand synthesis
In an effort to get insights into a new type of ligands and its reaction mechanism, we
have studied the synthesis of aminoquinoline derivative ligands L3-4 (see Scheme 1).
The bulky synthesis of L3-4 was carried out by an equimolar deprotonation reaction of

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L1-2 with KH in THF, and heated at 90 °C for 3 hours in a sealed round flask. After
reprotonation with CH3OH and silica gel chromatography purification, L3-4 were
afforded as yellow crystalline solids in 80 % and 82 % yields, respectively. These
aminoquinoline ligands were characterised spectroscopically by 1H- and 13C-NMR, FT-
IR, HRMS and, in the case of L3, its molecular structure was determined by X-Ray
diffraction studies (XRD) as shown in Figure 2. Here, L3 showed a planar quinoline core
(dihedral angle: –177.7(1)° C32-C33-C38-C37) with three different substituents, in
positions 2, 3, and 4 of the pyridinic fragment. The imine group attached to the 3-
position is bent out of the quinoline plane with a dihedral angle of 149.5(1)° (C39-C31-
C1-N1). Intramolecular hydrogen bond interaction was observed between the imine
group and the adjacent NH2 group (N3-H3A…N1 2.666(1) Å; 136.0(1)°). This generated
a U-form of the central N,N core deviated from the ideal (–31.3(2)° C32-C31-C1-N1)
(see ESI for further details).
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Scheme 1. Synthetic pathway to obtain aminoquinolines derivative ligands L3-4.
Figure 2. Molecular structure of L3. Thermals ellipsoids are shown with 30 %

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probability.
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Having prepared ligands L3-4, we optimised the reaction conditions and the effect of
the reaction temperature on the cyclization of the potassium salt of the nacnac ligand
(L1-2) was studied. At room temperature, the preparation of the corresponding
potassium salt KL3, starting from KL1, took place in 5 days. From here, the temperature
rose, and the higher the reaction temperature was, less reaction time was required to
afford the synthesis of KL3. Therefore, for KL3 synthesis, the optimal reaction
conditions were found to be 1 hour at 90 °C. On the other hand, the formation of KL4,
starting from KL2, was achieved in 2 days at room temperature and 30 min at 90 °C.
Finally, the addition of CH3OH achieved the protonation of the potassium salt, obtaining
L3-4 in quantitative yields. It is important to note that the heating of L1-2 in THF does
not produce L3-4, respectively, showing the importance of the alkaline salt in this
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1
synthetic process. These reactions were followed by H NMR in THF-d8 as a solvent,
using a sealed NMR tube.
On the other hand, the effect of the alkaline metal size in the cyclization of L1 was
investigated. In this experiment, L1 was deprotonated using n-BuLi in THF at –78 °C.
Once the lithium salt was formed, the reaction from LiL1 to LiL3 was tested at several
temperatures. Herein, we found that, at room temperature, the LiL1 salt does not
proceed to LiL3 even after 10 days of reaction. Whereas, by increasing the reaction
temperature to 90 °C, a very low rate of transformation to LiL3 was observed. This
effect can be explained because Li+ ion has a smaller ionic radius than K+, allowing that
Li+ forms a covalent bond with the k2-N,N coordination site of the nacnac ligand,
blocking any further torsion of the U-type core of the ligand. On the contrary, the bigger
potassium ion forms an ionic pair with the anionic ligand, staying away from the U-type
core and thus conferring flexibility to the ligand. This explanation is suitable with the
reported X-ray diffraction studies about the metal–nitrogen distance (M–N) for lithium,
sodium, and potassium compounds. Lithium–nacnac derivative exhibited a 1.9242 to
1.97 Å43 for M–N distance. In contrast, potassium–nacnac complexes derived from 2-
(2,6-diisopropylphenyl)-aminopent-2-en-4-(2,6-diisopropylphenyl)imine displayed a

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M–N distance between 2,64–2,74 Å.44 On the other hand, 3-sila-b-diketiminates showed
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a close range of bond lengths of M–N (Na: 2.370 Å and K: 2.863 Å).
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These differences mentioned above have an important effect over the reactivity of the
salt and have allowed us to propose an intramolecular exo-dig cyclization mechanism
(see Scheme 2). After deprotonation, the nacnac core of L1 is conjugated (A), which
makes the b-carbon a very strong nucleophile. Additionally, the carbon atom of the
nitrile group has an electrophilic character and, as the aniline bond has free rotation, at
a certain moment, this atom is close enough to suffer the nucleophilic attack by the b-
carbanion (B). The nucleophilic attack produces an intramolecular cyclization to give
C, which suffers a quick proton transfer to generate an anionic structure D, which is an
aromatic specie. Finally, the protonation of D using aqueous methanol affords the final
compound L3.
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iPr
iPr
iPr
K
H
Δ
N
N
KH
N
N
N
N
K
C
iPr
C
iPr
THF
-H2
C
iPr
N
N
H
N
H
H
L1
A
B
iPr
NH2
iPr
iPr
N
NH
H
N
C
H+
C
N
N
iPr
K
iPr
iPr
N
N
N
L3
D
C
Scheme 2. Proposed mechanism for the intramolecular cyclization of L1 to form L3.
With the aim to support the proposed mechanism, we have performed a set of kinetic
and theoretical studies. These studies were conducted for the intramolecular exo-dig
1
cyclization of the nacnac ligand. The kinetic study was followed by H NMR

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spectroscopy and the experiments were carried out in THF-d8 at 25 °C – 80 °C as the
reaction temperature range. The reaction rate was monitored through semi-
logarithmic plots of ln[L1] vs. time (see Figure S42-45 in the ESI). The linear fitting of
these plots and the increasing k1 values with reaction temperature were consistent with
a first-order reaction rates, assuming a trifling back reaction (k−1[D] ≪ k1[L1]). The
Arrhenius plot was used for the calculation of the activation energy (Ea) for this
reaction (Figure S46 in the ESI), obtaining a value of 20.6 kcal mol−1. This value was in
agreement with the behaviour of the reaction, in which the rate was very slow at room
temperature (5 days for completeness) and at 80 °C the reaction finished in less than 2
hours.
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In addition, we performed electronic structure calculations based on the density
functional theory (DFT) frame. Full geometry optimisations for all stationary points
along the minimum energy path computed by intrinsic reaction coordinate (IRC)45, 46
methods were carried out using the Grimme’s dispersion-corrected47 B3LYP48, 49
exchange-correlation functional (B3LYP-D3) combined with the medium-sized
Def2SVP basis set.50 These structures were confirmed as local minima or first-order
saddle-point through the harmonic vibrational analysis in accord with the number of
imaginary frequencies (zero and one for local minima and transition states,
respectively). This type of calculation also provides thermal corrections using the rigid-
rotor-harmonic-oscillator model at a given temperature ( ꢀꢁꢄꢁꢂꢃ ꢅ3ꢆꢇꢈ − ꢉ3/
ꢉꢊꢋ2ꢌꢍꢈ ). More accurate energies were calculated through the triple-z Def2TZVPP50
basis set by single-point energy calculations, and the solvation free energies were
computed by employing the implicit SMD model,51 using THF as the solvent, ꢀꢎꢏꢐꢑ
=
ꢒ + ꢀꢓꢏꢐꢑ ꢅ3ꢆꢇꢈ − ꢉ3/ꢉꢊꢋ2ꢔꢕꢍꢈꢈ//ꢅ3ꢆꢇꢈ − ꢉ3/ꢉꢊꢋ2ꢌꢍꢈ . Notice that the
theoretical study is performed by employing the anionic nacnac bare ligand and the
Gibbs free energies along the IRC computationally support the proposed cyclization
mechanism displayed in Scheme 2. The Gibbs free-energy profile along the IRC,
computed at DFT level, is shown in Figure 3. Our results showed that, on the one hand,
the U-type core reorganization of the anionic nacnac ligand required 19.6 kcal mol-1
and, on the other hand, the cyclization of the distorted nacnac ligand and the proton

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transfer proceed concertedly, which corresponded to the rate-limiting step with an
activation barrier of 29.3 kcal mol-1 and a strongly exergonic process, by releasing an
amount of energy of 35.5 kcal mol-1. The energetics of the rate-limiting step suggests
both a kinetically and thermodynamically feasible process. Note that the computed
activation energy certainly agrees with the experimental data by taking into account
the source of errors, such as implicit solvent effect, nuclear reorganisation in a solvent,
and the absence of K+ in the calculation.
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Figure 3. Gibbs free energy profile for the intramolecular cyclization of anionic nacnac
ligand computed at DFT level.
The development of new ligand derivatives is a great challenge and an attractive goal
for further application in metal complexes. We therefore synthesised 4-nitrophenyl-
aminoquinolines derivatives L5-6 in order to evaluate the effect of an electron-
withdrawing group instead of a more congested environment (see Scheme 3). The
synthetic sequence is composed of two steps, the deprotonation of L3-4 to form KL3-
4, and a nucleophilic aromatic substitution (SNAr) between the later and 4-

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nitrofluorobenzene in THF. Even though the second reaction step was accomplished at
50 °C and 1 hour, the synthesis was also achieved at room temperature in 12 hours.
Ligands L5-6 were obtained as light-yellow powders in 76 % and 79 % yields,
respectively.
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Scheme 3. Synthesis of 4-nitrophenyl-aminoquinolines derivatives ligands L5-6.
The new compounds L5-6 were characterised by spectroscopic and spectrometric
techniques. The 1H NMR data is consistent with the substitution of the amine hydrogen
in L3-4 by the 4-nitrophenyl group. The most characteristic signals correspond with
the doublets in 8.11 / 6.71 ppm for L5 and 7.91 / 6.24 ppm for L6, which agree with the
substitution pattern in the new phenyl ring that was added. Another characteristic
signal is related to the NH group. Here, a down field displacement was observed, in
comparison with the data observed for L3-4, from 6.24 to 7.62 ppm for L5 and 6.05 to
8.36 ppm for L6. It is clear that the electron-withdrawing effect exerted by the NO2
group unshielded the NH proton. However, the differences observed between L5 and
L6, for the same proton, are correlated with the alkyl group in the anilinic portion of
the molecule. The most electron-donor iPr group in L5 shields the NH proton, in
contrast with the effect exerted by the methyl in L6, which explains the 0.74 ppm
difference in the 1H NMR. In addition, molecular structure of L6 was determined by XRD
(see Figure 4). Here, L6 showed a planar quinoline core (dihedral angle: –179.4(0)°
C21-C22-C23-C24). The 4-nitrophenyl substituent attached to the amine is
perpendicular to the quinoline plane (dihedral angle: –83.9(5)° C23-C22-N3-C31),
mainly due to the highly steric congestion exerted by the phenyl group. The imine group

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attached to the 3-position is also rotated out of the quinoline plane with a dihedral angle
of –104.1(3)° (C29-C21-C1-N1). For further information about the characterisation, see
the ESI.
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Figure 4. Molecular structure of L6. Thermals ellipsoids are shown with 30 %
probability.
3.2. Synthesis and characterisation of Aluminum complexes
The preparation of the methyl aluminium complexes C1-4 was carried out via
protonolysis reaction of the ligands precursor L3-6 with one equivalent of AlMe3, in dry
benzene as the solvent, at room temperature, involving elimination of CH4 (see Scheme
4). Complexes C1 and C2 were isolated as yellow crystals in almost quantitative yields,
1
97 % and 99 % respectively, and characterised spectroscopically. In the H NMR of
complexes C1 and C2, the resonance corresponding to the NH2 group of the ligand
precursors L3 at 6.24 ppm and L4 at 6.05 ppm decreased the intensity and shifted to
1
5.85 and 5.96 ppm, respectively. Moreover, H and 13C{1H} NMR spectra of these
complexes exhibited the signals of the aromatic rings, two belonging to the methyl
groups in the imine and pyridine portion of the molecule, and in the case of C2, the
signals corresponding to the methyl groups from the phenyl moieties. The resonances

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at -0.49 and -0.53 ppm were assigned to the methyl groups coordinated to the metal
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centre, confirming the formation of the aluminium complexes.
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Scheme 4. Synthesis of aminoquinolines derivative aluminium complexes C1-4.
The preparation of compounds C3 and C4 was carried out under the same reaction
conditions as those of compounds C1 and C2 described above. After solvent
evaporation, and being washed two times with cold hexane, light-yellow powders were
collected in 97 % and 95 % yields, respectively. In the 1H NMR of these complexes, the
disappearance of the resonances corresponding to the NH group of the ligand
precursors L5 at 7.62 ppm and L6 at 8.36 ppm was observed. In addition, the
appearance of resonances at -0.87 ppm and -1.17 ppm for C3 and -0.72 ppm for C4,
assigned to the methyl groups bonded to the aluminium, settled the formation of the
respective aminoquinolines complexes. It should be notice that the C3 asymmetric
aluminium environment observed in the NMR is due to a highly steric hindrance effect
exerted on the methyl group attached to the metal core by both the diisopropyl group
in the imine on one side of the metal centre and the phenyl substituent in the amine
functional group by the other side.
For unequivocal allocation of most NMR signals, two-dimensional NMR experiments
were performed for complexes C1-4 (see ESI for further details). These data align with
a tetra-coordinated structure for the aluminium centre, in which the aminoquinoline
ligand exhibited a six-member ring in a k2-N,N coordination mode.

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The molecular structure of complex C4 was obtained by singe crystal X-ray diffraction
studies, as shown in Figure 5. The formation of a six-membered metallacycle, similar to
the related b-diketiminato aluminium complexes reported by Ding et al.,52 was
confirmed. Crystallographic data and selected bond angles and distances are given in
Tables S1-4 (ESI). The aluminium centre exhibits a distorted-tetrahedral geometry in
which the aminoquinoline bite angle N3–Al1–N1 is 89.4(1)° and the C3–Al1–C4 angle
is 115.1(1)°. These are not close to the ideal tetrahedral angle of 109.5°. The six-
membered metallacycle adopts an envelope configuration with the N3–Al1–N1 out of
the plane, pushing the phenyl ring bounded to the nitrogen N3 to the opposite side. The
bond distances Al1–N1(imine) and Al1–N3(amino) are 1.996(1) and 1.897(1) Å,
respectively. These bond distances are in good agreement with others b-diketiminato
aluminium complexes reported in the literature.52, 53 The difference is associated with
the nature of the Al-N bond, dative for the imine and covalent for the amino.
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Figure 5. Molecular structure of C4. Thermals ellipsoids are shown with 30 %
probability.
3.3. Catalytic studies for the synthesis of cyclic carbonates
Having prepared these new aminoquinoline aluminium complexes C1-4, we decided to
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CO2. Firstly, the transformation of styrene oxide Xa into styrene carbonate Ya was
carried out to optimise the reaction conditions (Table 1). The reactions were performed
at 80 °C and 1 bar of CO2 pressure for 24 hours using 1 mol% of C1-4 and
tetrabutylammonium iodide (TBAI), which was employed as cocatalyst (Scheme 5).
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O
C1−4 (1.0−1.5 mol%)
TBAI (1.0−1.5 mol%)
O
O
O
+ CO2
Ph
80 ˚C, 1 bar, 24 h
Ph
Xa
Ya
Scheme 5. Synthesis of styrene carbonate Ya catalysed by complexes C1-4.
Table 1. Conversion of styrene oxide Xa into styrene carbonate Ya by using C1-4.a
Entry
1
Catalyst
Cat (mol%)
1.0 mol%
1.0 mol%
1.0 mol%
1.0 mol%
1.5 mol%
1.5 mol%
-
Conversion (%)b
1
2
3
4
3
3
-
75
79
82
66
95
0
2
3
4
5c
6d
7e
5
aReactions were performed at 80 °C and 1 bar CO2 pressure using 1.0-1.5 mol% of
catalysts and 1.0 mol% of TBAI in the absence of a solvent. bDetermined by 1H NMR of
the crude reaction mixture. c1.5 mol% of TBAI were added. dOnly 1.5 mol% of C3 was
added. eOnly 1.5 mol% of TBAI was added.
As shown in Table 1, high conversions were achieved with aluminium catalysts C1-3
(approximately 80 %, Table 1, entries 1-3) while a moderate conversion was observed
with C4 (Table 1, entry 4). This is probably due to the high steric hindrance caused by
isopropyl and aryl groups that complicate the approximation of the epoxide to the

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metallic centre. As C3 was a slightly more active catalyst, we chose it for further
optimisation. Subsequently, in order to increase the conversion, the effect of increasing
the catalyst and cocatalyst loading to 1.5 mol% was studied, keeping the reaction
temperature at 80 °C and the CO2 pressure at 1 bar (Table 1, entry 5). Control
experiments were carried out to demonstrate that both C3 and TBAI are necessary for
the styrene carbonate formation (Table 1, entries 6 and 7). These results allowed us to
conclude that steric effects had a higher influence than electronic ones, owing to the
less steric hindered complexes C1-3 that exhibited more activity than C4 for styrene
carbonate Ya formation. The presence of the 4-nitrophenyl group in C3 did not
influence on its catalytic activity.
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Once the reaction conditions were optimised, we focused our attention on the
preparation of eight cyclic carbonates (Ya-h), from their corresponding epoxides (Xa-
h) and CO2 using C3/TBAI as a catalyst system. As shown in Figure 6, excellent
conversion (83 % - 99 %) and high isolated yields (78 % - 95 %) were obtained for the
preparation of monosubstituted cyclic carbonates. This catalytic system was able to
prepare a large variety of cyclic carbonates with different functionalities, such as alkyl,
aryl, halide, or ether. It is worth highlighting that the synthesis of highly fluorinated
cyclic carbonates Yg and Yh, which have barely been studied and could be used as
electrolytes in lithium-ion batteries,41, 54, 55 were isolated with high yields of 80 % and
87 %, respectively (Figure 6). These results indicated that C3/TBAI is an effective
catalytic system for the preparation of cyclic carbonates.

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O
O
C3 (1.5 mol%) / TBAI (1.5 mol%)
O
O
+ CO2
R
80 ˚C, 1 bar, 24 h
R
Xa−h
Ya−h
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O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
PhO
Ph
Bun
4Cl-C6H4
4Br-C6H4
Yd: Conva: 83 % Ye: Conva: 99 %
Yieldb: 78 %
Yc: Conva: 94 %
Yieldb: 87 %
Ya: Conva: 95 % Yb: Conva: 95 %
Yieldb: 90 %
Yieldb: 95 %
Yieldb: 92 %
O
O
O
O
O
O
O
O
O
F
F F
F
F
F
F
O
F
O
Cl
F
F
F
F
Yf: Conva: 98 %
Yieldb: 91 %
Yg: Conva: 83 %
Yh: Conva: 91 %
Yieldb: 86 %
Yieldb: 80 %
Figure 6. Cyclic carbonates Ya–h were obtained by the catalyst system C3/TBAI in a
molar ratio of 1:1. Reactions were performed at 80 °C and 1 bar CO2 pressure in the
absence of solvent. aConversion was determined by 1H NMR spectroscopy of the crude
reaction mixture. bIsolated yield obtained from purified cyclic carbonate.
-
Results obtained for C3, regarding the synthesis of cyclic carbonates Ya h, can be
compared with others aluminium complexes recently reported in the literature. The
performance data of these complexes are summarized in Table 2. Firstly, related to the
preparation of styrene carbonate Ya (Table 2, entries 1-3), the one-component
aluminium complex56 (Al-I) displayed a greater value of TOF than our catalyst system
(C3/TBAI) (Table 1, entry 1). However, this catalyst required more extreme conditions
(85 °C and 10 bar of CO2 pressure) than our system to achieve it. The aluminium
complex based in porphyrin57 (Al-Por) also exhibited a high catalytic activity for Ya
(Table 1, entry 3). Recently, our research group has prepared a set of mono-, di- and
trimetallic amidinate aluminium complexes36 (Al-ami in Table 2, entry 4) which

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obtained elevated conversions and isolated yields for the synthesis of a large variety of
cyclic carbonates. Moreover, an aluminium(Salen) complex,58 reported by North and
co-workers, performed a good isolated yield for the formation of Yf (Table 2, entry 5),
even though this catalyst required higher temperature (100 °C) than our catalyst
system to afford it. Recently, several aluminium complexes (Al@cage,59 Al(koj)360 and
Al(Mil)61) have been developed for the synthesis of 3-phenoxypropylene carbonate, Yc
(Table 2, entries 7-9). Despite the fact that these catalyst systems exhibited higher
values of TOF than our catalyst, it is also remarkable that they showed some limitations
such as that Al@cage needs 10 mol% of TBAB to achieve a moderate yield (54 %) of Yc
(Table 2, entry 7) and Al(koj)3 and Al(Mil) require 120 °C to afford these catalytic
activities (Table 2, entries 8 and 9). In conclusion, our catalyst system (C3/TBAI)
performed well for the preparation of a large variety of cyclic carbonates Ya-h under
relatively medium temperature conditions of (80 °C) and CO2 pressure (1bar).
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Table 2. Comparison of catalytic activities of different complexes.
Entry
Epoxide
Catalyst
(mol%)
Co-catalyst
(mol%)
Yield (%)a
TOF (h-1)b
1c
2
Xa
Xa
Xa
Xf
-
93
90
91
93
91
63
54
90
94
87
10.3
2.5
Al-I (0.5)
C3 (1.5)
TBAI (1.5)
TBAB (2.0)
TBAI (5.0)
TBAI (1.5)
-
3d
4e
5
Al(Por) (0.25)
Al(ami) (2.5)
C3 (1.5)
60.6
1.5
Xf
2.5
6f
7g
8h
9i
Xf
Al(Salen) (0.25)
Al@cage (0.33)
Al(koj)3 (0.15)
Al(Mil) (0.17)
C3 (1.5)
10.5
3.4
Xc
Xc
Xc
Xc
TBAB (10)
TBAI (0.25)
TBAB (0.14)
TBAI (1.5)
60.0
22.3
2.4
10
a Isolated yield from purified cyclic carbonate. b mol of product/(mol of catalyst x time).
c 85 °C and 10 bar CO2 pressure. d 40 °C and 10 bar CO2 pressure. e 50 °C and 1 bar CO2

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pressure. f 100 °C and 1 bar CO2 pressure. g r.t and 1 bar CO2 pressure. h 120 °C and 10
bar CO2 pressure. i 120 °Cand18 bar CO2 pressure.
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4. Conclusions
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New 4-amino-3-iminoquinolines ligands (L3-6) and four new aluminium complexes
(C1-4) were synthesised and characterised. X-ray single crystal analyses of C4
confirmed the formation of a six-membered metallacycle with an envelope
configuration, in which the ligand displayed a k2-N,N coordination mode and the metal-
centre exhibited a distorted-tetrahedral geometry.
The synthesis of L3 was optimised by studying the effect of alkaline metals and the
reaction temperature. It is worth highlighting that an inverse effect between the cation
size and the cyclization rate was observed. Furthermore, we concluded that the
temperature had a direct influence on the reaction time. In an attempt to clarify the
reaction mechanism, a set of kinetic and theoretical studies were performed on the
cyclization reaction of L3 at 25 °C - 80 °C. From this kinetic study, a first-order reaction
rate dependence and an activation energy of 20.6 kcal mol-1 were found for this
synthetic process. These, in addition to DFT electronic structure calculations, confirmed
the proposed intramolecular exo-dig cyclization reaction mechanism for the formation
of ligands L3 and L4.
Aluminium complexes C1-4 were evaluated as catalysts for the synthesis of cyclic
carbonates from epoxides and CO2. Optimisation of the reaction conditions for the
transformation of styrene oxide into styrene carbonate were performed. Catalysts C1-
3 showed higher conversions (80 %) than C4 (60 %). Among all the complexes, C3 was
found as the most active catalyst for the synthesis of styrene carbonate Ya. These
catalytic results have allowed us to conclude that the steric hindrance effect was more
important than the electronic one. Moreover, the 4-nitrophenyl group did not influence
the catalytic activity. In addition, eight cyclic carbonates (Ya-h) were prepared using
C3/TBAI as a catalyst system. Excellent conversions and high isolated yields were
obtained, using a large variety of epoxides with different functionalities, such as alkyl,
aryl, halide, or ether. Most remarkable, however, was the synthesis of highly fluorinated

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cyclic carbonates Yg (80 %) and Yh (87 %), which had high potential as electrolytes for
lithium-ion batteries. Further efforts in the study of suitable Al(III) complexes synthesis
and its evaluation as effective epoxide/CO2 coupling catalysts are currently ongoing in
our research group.
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Acknowledgements
We gratefully acknowledge the financial support of FONDECYT (projects 1161091,
11160797 and EQM120021). D.M. is grateful for FONDECYT Postdoctoral fellowship
3180563. J.M. is grateful for FONDECYT Postdoctoral fellowship 3180073. L.B.
acknowledges Ph.D. CONICYT fellowship 21161195.
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R1
R1
Al
R1
First-order rate
Intramolecular
cyclization
R2
O
O
N
N
N
Al(CH3)3
NH2
N
R1
R3
O
O
N
HN
R1
Alkali-metal base
Temperature
Ea = 20.6 kcal mol-1
C
R1
1. SNAr
2. Al(CH3)3
CO2
R3
N
N
R1: Me, iPr
R2: H, p-NO2Ph
R1: Me, iPr
R1: Me, iPr
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Exo-dig intramolecular cyclization reaction of -diketiminate ligands allowed to obtain new
4-amino-3-iminoquinolines derivatives ligands through a novel reaction pathway.
Aluminium complexes bearing these ligands where active towards the synthesis of cyclic
carbonates from epoxides and CO2.