Cyclic Carbonates from CO2 and Epoxides Catalyzed by Tetra- and Pentacoordinate Amidinate Aluminum Complexes

Article abstract of DOI:10.1021/acs.organomet.8b00795

A series of mono-, bi-, and trimetallic alkylaluminum complexes ((L₁)AlMe₂ (1), (L₁)Al₂Me₅ (2), (L₂)AlMe₂ (3), (L₂)Al₂Me₅ (4), (L₂)

Full text of DOI:10.1021/acs.organomet.8b00795

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pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Cyclic Carbonates from CO2 and Epoxides Catalyzed by Tetra- and
Pentacoordinate Amidinate Aluminum Complexes
†
†
‡
Yersica Rios Yepes,† Celso Quintero, Danay Osorio Melendez, Constantin G. Daniliuc,
́
Javier Martínez,*,† and Rene S. Rojas*
,†
́
†
́
́
Laboratorio de Química Inorganica, Facultad de Química, Universidad Catolica de Chile, Casilla 306, Santiago 22 6094411, Chile
‡
̈
̈
̈
Chemisches Institut der Universitat Munster, Corrensstrasse 40, 48149 Munster, Germany
S
* Supporting Information
ABSTRACT: A series of mono-, bi-, and trimetallic
alkylaluminum complexes ((L1)AlMe2 (1), (L1)Al2Me5 (2),
(L2)AlMe2 (3), (L2)Al2Me5 (4), (L2)2AlMe (5), (L2)2Al3Me7
(6)) supported by amidine ligand precursors ((E)-N-(2,6-
diisopropylphenyl)-N′-(pyridin-2-yl)acetimidamide (L1H)
and (E)-N-(2,6-diisopropylphenyl)-N′-(pyridin-3-yl)-
acetimidamide (L2H)) were obtained in excellent yields and
characterized by spectroscopic and X-ray diffraction methods.
These complexes were studied as catalysts for the synthesis of
cyclic carbonates from epoxides and carbon dioxide in the
presence of tetrabutylammonium iodide as cocatalyst. The
reactions were carried out at 50 °C and 1 bar CO2 pressure in
the absence of solvent, and we were able to achieve an
extensive variety of terminal epoxides with moderate to excellent yields and high selectivities using aluminum complexes 2 and
6.
systems formed by the combination of Lewis acids and
nucleophiles (most of them operate at 25 °C and 1 bar of CO2
pressure) for the synthesis of cyclic carbonates from epoxides
and carbon dioxide. These systems are principally based on
metal complexes that include mainly aluminum,33−36 chro-
mium,37,38 and cobalt39,40 in combination with a nucleophile as
a cocatalyst, bifunctional systems,41−43 and organocata-
lysts.44−46
Amidinic catalysts have recently been used for several
reactions, such as olefin polymerization47−50 (Figure 1, I48),
ring-opening polymerization of cyclic esters51−54 (Figure 1,
II51), and benzamidinate complexes have been applied in the
transformation of CO2 into cyclic carbonates55 (Figure 1, III).
One of the most interesting properties of the amidinic systems
is their capacity to coordinate to different metallic centers,
which has allowed incorporation of different elements from the
traditional transition metals and expanded the study to
elements of greater abundance such as zinc56−59 (Figure 1,
IV59), magnesium60−62 (Figure 1, V60), and aluminum63−66
(Figure 1, VI66).
INTRODUCTION
■
Carbon dioxide (CO2) as a sustainable C1 feedstock is a
starting material for the synthesis of organic molecules and/or
materials, and its chemical utilization is a challenge for the
scientific community,1−7 even though the CO2 molecule
possesses high thermodynamic stability and its use as a
substrate is limited by kinetic barriers. Given that, it is
necessary to develop new catalytic systems enabling trans-
formation to overcome the associated energy barriers,8−11 even
though the catalytic transformation of CO2 has been widely
studied to obtain a variety of different products, such as
amines, amides, methanol, and formic acid.12−17 One of these
processes is the reaction between epoxides and carbon dioxide
to afford either cyclic18,19 or linear polycarbonates20−25
(Scheme 1), which are highly exothermic processes and have
100% atom economy. Cyclic carbonates stand out for their
promising use as electrolytes in batteries26−29 and generally as
aprotic solvents.30−32 Over the last two decades, several
research groups have been accessed a variety of catalytic
Amidinate units have turned out to be quite versatile
ancillary ligands allowing the tuning of coordinations mode by
their stereoelectronic properties.67 The exploration of various
amidinate aluminum complexes for catalytic purposes has had
a great effect on the development of exceptional catalytic
Scheme 1. Synthesis of Cyclic Carbonates and
Polycarbonates from Epoxides and CO2
Received: October 29, 2018
© XXXX American Chemical Society
A
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Figure 1. Examples of complexes with amidinic ligands.
properties.68−71 Recently, our research group reported the use
of mono- and binuclear amidinate aluminum complexes as
catalysts for polymerizations and copolymerizations of
lactones,72 and later these complexes were examined as
catalysts for the preparation of cyclic carbonates from the
reaction between epoxides and carbon dioxide using TBAI as a
cocatalyst (Figure 1, VI66). On the basis of these types of
amidine complexes, we initiated related studies investigating a
tuning of such systems for the catalytic formation of cyclic
carbonates from epoxides and CO2, changing the coordination
modes of the alkyl aluminum derivatives under the influence of
the size of their polynuclear structures.
Scheme 2. Synthesis of L1H and L2H Amidine Ligand
a
Precursors
a
Reaction i: toluene, Et3N, reflux. Reaction ii: water/CH2Cl2.
allocate most of the 1H NMR resonances, and 1H−13C
heteronuclear correlation (g-HSQC) experiments were used to
assign the signals of the corresponding carbon atoms (see the
Supporting Information).
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of the
Ligand Precursors L1H and L2H. The synthesis of the two
related N-(pyridin-x-yl)acetimidamide (x = 2, 3) L1H and L2H
was investigated with respect to their ability to coordinate on
aluminum centers by reaction with AlMe3, forming catalytically
active organometallic compounds to generate cyclic carbonates
from CO2 and epoxides. When L1H and L2H link to the metal
center, they show the same coordination mode (κ2-NN) and
formation of six-membered (L1H) or four-membered (L2H)
metallacycles.
The solid-state structures of ligand precursors L1H and L2H
were determined by single-crystal X-ray diffraction studies.
Crystal structures, crystallographic data, and selected bond
angles and distances are given in Figures S3 and S6 and Tables
S1 and Table S2 in the Supporting Information, respectively.
Synthesis and Structural Characterization of Com-
plexes 1−6. The mono- and dinuclear alkyl aluminum
complexes 1 and 2 were prepared via the reactions of the N-
(pyridin-2-yl)acetimidamide ligand precursor L1H with 1 or 2
equiv of AlMe3 (Scheme 3). The reactions were carried out in
dry CH2Cl2 for 2 h at room temperature. Complexes 1 and 2
were obtained in high yields (≥95%) as light pink solids.
Alternatively, 2 could be obtained by the addition of 1 equiv of
AlMe3 to 1, under the same reaction conditions. In the
dinuclear complex 2 the second aluminum center is
coordinated through a dative bond to the nitrogen atom of
the imino moiety.
The preparation of the L1H and L2H ligand precursors was
carried out via a reaction between the corresponding
aminopyridine and (E)-N-(2,6-diisopropylphenyl)acetimidoyl
chloride73 in toluene in the presence of Et3N under reflux,
affording L1H and L2H in 67 and 70% yields, respectively
(Scheme 2).
L1H and L2H were characterized spectroscopically by one-
and two-dimensional NMR techniques (1H NMR, 13C{1H}
The imino and pyridine nitrogen atoms have free electron
pairs that cause two isomers, a four-membered (Figure 2a) and
a six-membered (Figure 2b) metallacycle. However, the
reaction between L1H and 1 equiv of AlMe3 afforded only
one complex, which corresponded to the six-membered
metallacycle type. Apparently, this arrangement is thermody-
1
1
NMR, H NOESY-1D and H−13C heteronuclear correlation
(g-HSQC)), FT-IR, mass spectroscopy, and X-ray diffraction
studies. The H NMR spectra of these ligands gave rise to
1
broad singlets at 9.54 ppm (L1H) and 9.26 ppm (L2H)
(Experimental Section), ascribed to the N−H group of the
amidine moiety, in good agreement with signals for other
amidines.74 1H NOESY-1D experiments were carried to
1
namically more stable, which can also be derived from the H
NMR spectrum of 1. The formation of a six-membered
B
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The structure of 5 shows two pyridinic nitrogen atoms
capable of forming an adduct with the Lewis acid AlMe3,
similarly to 1. The reaction of 5 with 2 equiv of AlMe3, in
CH2Cl2 at room temperature for 2 h furnished complex 6 as a
white solid in quantitative yield (Scheme 4). In comparison to
5, 6 bears two additional AlMe3 units, which are bound to the
pyridine nitrogen atoms.
Scheme 3. Preparation of 1 and 2
The solution-state structures of 1−6 were studied by
1
spectroscopic methods. In the H NMR spectra of 1−6 the
signal for the HN proton of the amidine ligand precursor was
missing, thus supporting the formation of amidinate complexes
(Figure 3). Moreover, these compounds exhibited signals for
Figure 2. Structural isomers of 1: (a) four-membered metallacycle;
(b) six-membered metallacycle.
metallacycle became also evident for 1 and 2 from their X-ray
structure determinations.
The protonolysis reaction of N-(pyridin-3-yl)acetimidamide
with the protio ligand L2H in the presence of 1 equiv of AlMe3
in dry CH2Cl2 for 2 h at room temperature afforded the
formation of complex 3 in quantitative yield. The structure of 3
consists of a four-membered metallacycle.72 The addition of
another 1 equiv of AlMe3 to 3, or the direct reaction of L2H
with 2 equiv of AlMe3, resulted in the formation of complex 4
(four-membered metallacyclic AlMe3 adduct), which was
isolated as a white solid in 98% yield (Scheme 4).
Figure 3. 1H NMR of complex 6 in C6D6 (a) and 1H NMR of
complex 2 in C6D6 (b).
the AlMe moieties (Figure 3) as part of a unique set of
resonances that indicated the presence of a single isomer. The
pentacoordinate species 5 and 6 showed a singlet at −0.10
ppm for 5 and at −0.19 ppm for 6 (Figure 3b), respectively,
which belonged to the methyl group.
Scheme 4. Aluminum Complexes 3−6 Bearing the L2H
Ligand Precursor
For unequivocal assignment of most of the NMR resonances
13C{1H} NMR spectra and two-dimensional NMR experi-
ments were carried out for 1−6. These data supported
structural arrangements with tetrahedral coordination geo-
metries for the mononuclear complexes 1 and 3, in which the
amidinate ligand was bound to the aluminum atom in κ2-NN
coordination to the metal center. In the dinuclear aluminum
complexes 2 and 4 the amidinate ligand was found attached in
a κ2-NN-μ-N coordination mode, whereas the amidinate
complexes 5 and 6 displayed pentacoordination.
The molecular geometries and atom-labeling schemes for
the complexes 1 and 2 are shown in Figures 4 and 5,
respectively. The molecular structures of both complexes
confirmed the formation of the six-membered metallacycle,
similarly to the previously reported carbaalumazenes.79−81 The
crystallographic data and selected bond angles and distances
are given in Tables S3 and S4 in the Supporting Information,
respectively. In compound 1, Al1 presents a distorted-
tetrahedral geometry in which the amidinate bite angle N1−
Al1−N3 is 92.8(1)° and the C8−Al1−C9 angle is 116.6(1)°,
which are not close to the ideal tetrahedral angle of 109.5°.
The bond distances Al1−N1(iminic) and Al1−N3(pyridinic)
are 1.904(2) and 1.931(2) Å, respectively.
On the basis of the ability of aluminum centers to form
pentacoordinate complexes,75−78 we pursued the notion to
access metallacyclic complexes of the type (Ln)2AlMe (n = 1,
2), which are expected to possess coordination number 5.
Unfortunately, the reaction of L1H with 0.5 equiv of AlMe3 in
dry CH2Cl2 for 2 h at room temperature did not reveal the
presence of new complexes but resulted in the formation of 1
and free ligand. However, the reaction of L2H with 0.5 equiv of
AlMe3 under the same reaction condition afforded the
(L2)2AlMe complex 5 in 90% yield as a colorless solid
(Scheme 4).
The molecular structure of 2 is shown in Figure 5. The bond
distance Al2−N2 is 0.11 Å longer than the Al1−Ν1 distance,
which confirms that the Al2 center is coordinated through a
C
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Figure 4. Molecular structure of 1. Thermal ellipsoids are shown with
15% probability, and hydrogen atoms has been omitted for clarity.
Selected bond distances (Å) and angles (deg): N1−Al1 1.904(2),
N3−Al1 1.931(2), C3−N2 1.366(2), N2−C1 1.324(2), C1−C2
1.494(2); N1−Al1−N3 92.8(1), C8−Al1−C9 116.6(1), C3−N2−C1
124.7(2).
Figure 6. Molecular structure of 5. Thermal ellipsoids are shown with
30% probability, and hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): N2−Al1 2.019(1),
N1−Al1 1.943(1), C8−Al1 1.968(2), C2−C1 1.498(2); N2−Al1−
N2A 149.9(1), N1−Al1−N1A 126.4(1), C8−Al1−N1 116.8(1),
N1−Al1−N2A 99.5(1), N2−Al1−N1A 99.5(1), N1−Al1−N2
66.4(1).
Figure 5. Molecular structure of 2. Thermal ellipsoids are shown with
30% probability, and hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): N1−Al1 1.914(1),
N2−Al2 2.024(2), N3−Al1 1.930(1), C3−N2 1.393(2), N2−C1
1.349(2), C1−C2 1.494(2); N1−Al1−N3 90.5(1), C8−Al1−C9
122.5(1), C3−N2−C1 122.0(1).
dative bond to the nitrogen atom from the imino moiety,
generating a greater distortion in the ideal tetrahedral
geometry of Al1 in 2. The N1−Al1−N3 bite angle and C8−
Al1−C9 angle are 90.5(1)° and 122.5(1)° respectively, which
are farther from 109.5° than the equivalent angles of 1.
The molecular geometries and atom-labeling schemes for
complexes 5 and 6 are shown in Figures 6 and 7, respectively.
The crystallographic data and selected bond angles and
distances are given in Tables S5 and S6 in the Supporting
Information, respectively. Compound 5 presents a C2/c space
group and roughly C2 symmetry. The angular structure
parameter (τ value)82,83 allows distinguishing between the
geometries of a trigonal bipyramid and a square pyramid in
pentacoordinated structures. A value of 1 shows a perfect
trigonal-bipyramidal geometry, and a value of 0 indicates a
perfect square-pyramidal structure.
The τ values for 5 and 6 (0.55 and 0.61, respectively)
confirm that the geometry for the pentacoordinated aluminum
center is a slightly distorted trigonal bipyramid, in which both
bidentate amidinate ligands occupy an equatorial and an axial
site, similar to what has already been reported for these types
of complexes,84 and the remaining equatorial site is occupied
by the methyl group. In this particular case, in which the ligand
L2H is asymmetric, the nitrogens substituted by a pyridinic
ring occupy the axial positions in the structure and the
Figure 7. Molecular structure of 6. Thermals ellipsoids are shown
with 30% probability, and hydrogen atoms have been omitted for
clarity. Selected bond distances (Å) and angles (deg): N2−Al1
2.015(1), N3−Al2 2.022(1), N6−Al3 2.018(1), N1−Al1 1.935(1),
C8−Al1 1.949(1), C71−Al3 2.002(1), C2−C1 1.489(2); N2−Al1−
N5 154.5(1), N1−Al1−N4 123.3(1), C8−Al1−N1 117.7(1), N1−
Al1−N5 101.1(1), N2−Al1−N4 101.1(1), N1−Al1−N2 66.3(1).
associated exocyclic angle N2−Al1−N2A is equal to
149.9(1)°, which is distorted 30.1° from the ideal structure
of 180°, while the exocyclic angles between the groups at the
equatorial positions N1−Al1−N1A and the two N1−Al1−C8
group are about 126.4(1)° and 116.8(1)°, respectively. As
expected for a trigonal-bipyramidal geometry, the two Al1−N2
bond distances of 2.019(1) Å are longer than the two other
Al1−N1 distances of 1.943(1) Å, which is typical for this type
of moiety.84−86
The molecular structure of 6 is displayed in Figure 7. The
Al2 and Al3 centers of this compound are coordinated to the
ligand by the pyridine nitrogen atom, which has the effect of
approximating the trigonal-bipyramidal structure of 6 to the
ideal trigonal-bipyramidal structure, and the N1−Al1−C8 and
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N1−Al1−N4 angles are 117.7(1)° and 123.3(1)°, respectively,
which are closer to 120° than the equivalent angles of 5.
The Al−Μe distances on the tetracoordinated aluminum
centers are slightly longer (0.012 Å average) than the Al−Me
distances of pentacoordinated aluminum, probably due to the
stronger electron donation of two amidinates in comparison to
pyridine.
Catalytic Studies for the Synthesis of Cyclic Carbo-
nates. Aluminum complexes 1, 2, and 4−6 were initially
tested for the formation of styrene carbonate 8a from styrene
oxide 7a and CO2 at 25 °C and 1 bar of CO2 pressure for 24 h
in the absence of solvent using 5 mol % of monometallic
catalysts 1 and 5, 2.5 mol % of bimetallic catalysts 2 and 4, and
1.7 mol % of trimetallic catalyst 6, with the aluminum
concentration kept constant at 5 mol % and 5 mol % of
tetrabutylammonium iodide (TBAI) added, as can be seen in
Table 1.
basis of these explorations we concluded that 2 and 6 were the
most active catalysts for styrene carbonate production (8a) at
both 25 and 50 °C and that the number of metallic centers in
the catalysts did not have a pronounced effect on the catalytic
activity.
Complexes 2 and 6 were used to investigate the effect of a
cocatalyst addition in the formation of styrene carbonate
(Table 2). The best results were obtained with combinations
Table 2. Influence of Cocatalyst Addition on Styrene
a
Carbonate 8a Catalysis Applying Complexes 2 and 6
b
conversion (%)
entry
cocat.
complex 2
complex 6
1
2
3
4
5
6
Bu4NF
Bu4NCl
Bu4NBr
Bu4NI
0
68
89
92
73
0
0
65
83
90
70
0
c
PPNCl
Table 1. Synthesis of Styrene Carbonate 8a using Catalysts
d
a
DMAP
1, 2, and 4−6
a
Reactions were carried out at 50 °C and 1 bar of CO2 pressure for 24
h using 2.5 mol % of complex 2, 1.7 mol % of complex 6, and 5 mol %
b
conversion (%)
b
1
of cocatalyst. Conversion determined by H NMR spectroscopy of
entry
cat.
amt of cat. (mol %)
25 °C
50 °C
c
d
the reaction mixture. Bis(triphenylphosphine)iminium chloride. 4-
(Dimethylamino)pyridine.
1
2
3
4
5
1
2
4
5
6
2
6
5.0
2.5
2.5
5.0
1.7
2.5
1.7
20
40
15
18
33
0
0
1
9
73
92
65
70
90
0
0
9
20
of 2 and 6 and TBAI (Bu4NI) as the cocatalyst at 50 °C and 1
bar of CO2 pressure, giving almost quantitative conversion
(Table 2, entry 4). Other ammonium salts, such as TBAF and
TBAC, displayed low to moderate conversions in the
formation of styrene carbonate (Table 2, entries 1 and 2),
while TBAB showed a higher conversion in comparison to the
fluoride and chloride salts (Table 2, entry 3). PPNCl exhibited
high catalytic activity under these reaction conditions (Table 2,
entry 5), and DMAP failed to show any cocatalytic effect
(Table 2, entry 6). Thus, TBAI turned out to be the best
cocatalyst for styrene carbonate production (8a). In this way
the optimum reaction conditions for the catalytic production
of styrene carbonate 8a using 2 and 6 were determined.
Therefore, we subsequently explored the chemical potential
for cyclic carbonate formation. The reactions of nine
monosubstituted epoxides 7a−i with carbon dioxide to form
the corresponding cyclic carbonates 8a−i were examined using
2.5 mol % of catalyst 2 and 1.7 mol % of catalyst 6 with 5 mol
% of TBAI as cocatalyst under solvent-free conditions to study
the same reaction parameters as before: for instance, whether
the number of metallic centers would have any effect on the
cyclic carbonate production (Figures 8 and 9).
c
6
c
7
8
d
e
9
AlMe3
5.0
a
Reactions were carried out at 1 bar of CO2 pressure for 24 h using 5
b
1
mol % of Bu4NI. Conversion determined by H NMR spectroscopy
of the reaction mixture. No Bu4NI was added. 5 mol % of Bu4NI
added. 5 mol % of both pyridine and Bu4NI was added.
c
d
e
The monometallic aluminum catalysts 1 and 5 displayed low
catalytic activity for the synthesis of styrene carbonate 8a
under the given reaction conditions (Table 1, entries 1 and 4).
Furthermore, the catalytic activities of the bimetallic aluminum
complexes 2 and 4 were low to moderate (Table 1, entries 2
and 3), although a considerable increase in the catalytic activity
was observed between both complexes 2 (six-membered
metallacycle) and 4 (four-membered metallacycle), showing
the importance of the ligand design, whereas the trimetallic
aluminum complex 6 showed a 33% conversion in styrene
carbonate formation (Table 1, entry 5). According to Table 1
complexes 2 and 6 were the most active catalyst for styrene
carbonate production, probably due to the fact that these
complexes possess higher solubilities in the epoxide.
The influence of the temperature was then tested. As can be
derived from Table 1, at 50 °C almost complete conversion of
styrene oxide into styrene carbonate could be achieved on
applying 2 and 6 (Table 1, entries 2 and 5). The conversion
using 1, 4, and 5 increased at this temperature but their
conversions were slightly lower at around 75% (Table 1,
entries 1, 3, and 4). Control experiments were carried out to
demonstrate that neither TBAI nor complexes 2 and 6 alone
showed significant catalytic activity by themselves (Table 1,
entries 6−8). Furthermore, the adduct pyridine−Al(CH3)3 was
tested and showed a lower catalytic activity in comparison to
aluminum complexes 1, 2, and 4−6 (Table 1, entry 9). On the
The conversion of epoxides 7a−i into the corresponding
1
cyclic carbonates 8a−i were pursued by H NMR spectrosco-
py. Reactions were carried out at 50 °C and 1 bar of carbon
dioxide pressure in a single glass reaction tube of a multipoint
reactor (see the Supporting Information for further details). As
can be seen in Figures 8 and 9, the epoxides could be
converted into their corresponding cyclic carbonates from
moderate to excellent yields by both complexes 2 and 6 under
relatively mild reaction conditions. The formation of
polycarbonates could not be observed under these conditions,
and the selectivities for the cyclic carbonate formation were
higher than 99%.
It is worth mentioning that the epoxides 7a−e were
transformed into their corresponding cyclic carbonates 8a−e
in excellent yields by the catalytic systems 2/TBAI and 6/
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Table 3. Comparation of Catalytic Activities of Different
a
Complexes
c
cat.
cocat.
conversion
(%)
TOF
(h−1
b
entry (amt (mol %)) (amt (mol %))
TON
)
1
2
3
4
5
6
7
8
9
2 (2.5)
6 (1.7)
Al(salen) (2.5)
Cr (2.5)
Al (5.0)
2 (2.5)
TBAI (5.0)
TBAI (5.0)
TBAB (2.5)
TBAB (2.5)
TBAI (5.0)
TBAI (5.0)
TBAI (5.0)
TBAI (5.0)
TBAI (5.0)
TBAB (2.5)
TBAB (2.5)
40
33
62
92
55
76
51
55
39
91
89
16
19
25
37
11
30
30
22
23
36
36
0.67
0.79
8.33
1.54
0.45
1.25
1.25
0.92
0.96
1.5
d
e
e
f
6 (1.7)
2 (2.5)
6 (1.7)
Cr (2.5)
Cr (2.5)
f
e
10
11f
Figure 8. Cyclic carbonates 8a−i from epoxides 7a−i catalyzed by 2
1
1.5
and TBAI. Legend: (a) conversion determined by H NMR of the
reaction mixture; (b) yield determined on purified product.
a
Reactions were carried out by using styrene oxide as the epoxide at
25 °C and 1 bar of CO2 pressure for 24 h unless stated otherwise.
b
c
TON = (mol of product)/(mol of catalyst). TOF = (mol of
d
e
product)/((mol of catalyst) × time). t = 3 h. 2-(4-Chlorophenyl)-
f
oxirane (7h) as epoxide and T = 50 °C. 2-(4-Bromophenyl)oxirane
(7i) as epoxide and T = 50 °C.
aluminum catalyst (Table 3, entry 5), whereas the Al(salen)
catalyst displayed the highest TOF value (8.33 h−1) (Table 3,
entry 3) of all the catalysts studied for the preparation of
styrene carbonate 8a at room temperature and 1 bar pressure
of CO2. On the other hand, the Cr(salophen) complex (Cr,
Table 3) exhibits slightly superior values of TOF in
comparison to complexes 2 and 6 for the synthesis of 4-
chlorostyrene carbonate (8h) and 4-bromostyrene carbonate
(8i) (Table 3, entries 6−11). In conclusion, complexes 2 and 6
displayed an intermediate catalytic activity for the synthesis of
cyclic carbonates at 1 bar of CO2 pressure.
Figure 9. Cyclic carbonates 8a−i from epoxides 7a−i catalyzed by 6
1
and TBAI. Legend: (a) conversion determined by H NMR of the
reaction mixture; (b) yield determined on purified product.
A mechanism for the production of cyclic carbonates
catalyzed by 6 is proposed in Scheme 5. It follows a scheme
TBAI, whereas glycerol carbonate 8f and aryl carbonates 8g−i
were obtained with lower conversions and yields. These results
demonstrated that catalysts 2 and 6 are highly tolerant to alkyl
epoxides, although these catalysts were not as effective with
those epoxides functionalized with aryl, halide, alcohol, and
ether groups, probably due to the lower solubility of these
epoxides in the catalytic solutions (Figures 8 and 9). The
bimetallic catalyst 2 is slightly more active than the trimetallic
system 6, allowing us to conclude that the complexes with the
greatest numbers of metallic centers are not necessarily the
most active catalysts.
Scheme 5. Catalysis of Monosubstituted Cyclic Carbonates
from Terminal Epoxides and CO2 Catalyzed by 6 under
Cocatalysis of Iodide
According to the literature, most catalytic systems reported
to date require a CO2 pressure higher than 1 bar to carry out
the synthesis of cyclic carbonates from epoxides and carbon
dioxide, although there are a few examples of catalysts that
display an excellent catalytic activity at this CO2 pressure. For
that reason, we would like to compare the activity of complexes
2 and 6 with those most active catalysts reported in the
bibliography. Two of them, an Al(salen) complex87 (Al(salen),
Table 3) and a Cr(salophen) complex38 (Cr, Table 3), were
reported by North and co-workers and another excellent
example, a bifunctional heteroscorpionate aluminum com-
plex41 (Al, Table 3), was prepared by Otero and co-workers
(the structures of these complexes are shown in the Supporting
Information).
of Lewis acid catalysis (Lewis acid Al in Scheme 5). The cycle
is consistent with a catalytic scheme previously proposed by
our research group for cyclic carbonates using amidinate
aluminum complexes.66
Complexes 2 and 6 showed low TOF values of 0.67 and 0.79
h−1, respectively (Table 3, entries 1 and 2), which are slightly
higher in comparison to that presented by the bifunctional
F
DOI: 10.1021/acs.organomet.8b00795
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Organometallics
Article
column chromatography using silica gel (ethyl acetate) to give
colorless crystals (0.54 g, 70%). 1H NMR (400 MHz, DMSO-d6, 298
K): δ/ppm 9.26 (br s, 1H, NH), 8.96 (d, J = 2.0 Hz, 1H, H5), 8.35 (d,
J = 8.2 Hz, 1H, H2), 8.14 (d, J = 3.7 Hz, 1H, H4), 7.28 (m, 1H, H3),
7.06 (d, J = 7.6 Hz, 2H, H10), 6.94 (t, J = 7.6 Hz, 1H, H11), 2.87
(hept, J = 6.8 Hz, 2H, H12), 1.76 (s, 3H, H7), 1.13 (d, J = 6.9 Hz, 6H,
H14), 1.05 (d, J = 6.8 Hz, 6H, H13). 13C{1H} NMR (100 MHz,
DMSO-d6, 298 K): δ/ppm 152.80 (C6), 145.08 (C8), 142.02 (C4),
140.35 (C5), 138.11 (C1), 137.54 (C9), 124.93 (C2), 123.28 (C3),
122.59 (C10), 122.34 (C11), 27.57 (C12), 23.39 (C13), 22.61 (C14),
18.00 (C7). IR (KBr): ν/cm−1 3282, 3236, 3175, 3117, 3056, 2959,
2926, 2867, 1664, 1607, 1583, 1547, 1484, 1459, 1436, 1421, 1374,
1311, 1260, 1235, 1200, 1186, 1159, 1101, 1047, 1023, 955, 800, 784,
758, 706. HRMS (ESI) for C19H26N3 [M + H]+: m/z calcd 296.212,
found 296.212.
Synthesis of [(L1)AlMe2] (1). A solution of trimethylaluminum
(AlMe3) (24.51 mg, 0.34 mmol) in dichloromethane was quickly
added to a solution of L1H (100 mg, 0.34 mmol) in dichloromethane.
The reaction mixture was stirred for 2 h at room temperature. All
volatiles were removed under vacuum, and the solid was washed with
hexane. Compound 1 was obtained as a pale pink solid (115 mg,
96%). Single colorless crystals for X-ray crystallography were grown
from cold hexane at −30 °C. 1H NMR (400 MHz, C6D6, 298 K): δ/
ppm 7.53 (d, J = 5.8 Hz, 1H, H5), 7.22 (d, J = 5.1 Hz, 1H, H2), 7.20−
7.13 (m, 3H, H11,10), 6.93 (t, J = 7.8 Hz, 1H, H3), 6.17 (t, J = 6.4, 1H,
H4), 3.42−3.29 (m, 2H, H12), 2.11 (s, 3H, H7), 1.30 (d, J = 6.8 Hz,
6H, H14), 1.13 (d, J = 6.8 Hz, 6H, H13), 0.33 (s, 6H, H15). 13C{1H}
NMR (100 MHz, C6D6, 298 K): δ/ppm 170.01 (C6), 159.36 (C1),
144.83 (C9), 140.90 (C5), 140.28 (C8), 140.18 (C3), 127.34 (C11),
124.78 (C2), 124.63 (C10), 116.20 (C4) 28.28 (C12), 25.53 (C7),
24.89 (C14), 24.64 (C13), −9.16 (C15).
Synthesis of [(L1)Al2Me5] (2). A solution of L1H (200 mg, 0.68
mmol) in dichloromethane was added to a solution of 2 equiv of
AlMe3 (98.04 mg, 1.36 mmol) in dichloromethane. The reaction
mixture was stirred for 2 h at room temperature. All volatiles were
removed under vacuum, and the solid was washed with hexane.
Compound 2 was obtained as a pink solid (285 mg, 99%). Single
colorless crystals for X-ray crystallography were grown from cold
dichloromethane/hexane. 1H NMR (400 MHz, C6D6, 298 K): δ/ppm
7.56 (d, J = 7.7 Hz, 1H, H5), 7.28 (d, J = 5.1 Hz, 1H, H2), 7.08−7.02
(m, 1H, H11), 6.97 (d, J = 7.6 Hz, 2H, H10), 6.86 (t, J = 7.9, 1H, H4),
6.15 (t, J = 6.5 Hz, 1H, H3), 2.82 (br s, 2H, H12), 2.18 (s, 3H, H7),
1.05 (d, J = 6.5 Hz, 6H, H13), 0.97 (d, J = 6.7 Hz, 6H, H14), −0.20 (s,
9H, H16), −0.48 (s, 6H, H15). 13C{1H} NMR (100 MHz, C6D6, 298
K): δ/ppm 175.97 (C6), 157.68 (C1), 143.08 (C9), 140.69 (C4),
140.34 (C2), 138.35 (C8), 128.35 (C11), 124.75 (C10), 124.04 (C5),
119.39 (C3), 28.49 (C12), 24.55 (C7,13), 23.83 (C14), −5.31 (C16),
−11.53 (C15).
Synthesis of [(L2)AlMe2] (3). To a stirred solution of L2H in
C6D6 (12.41 mg, 0.042 mmol) was added a solution of AlMe3 in C6D6
(3 mg, 0.042 mmol). After 30 min the solution was subjected to NMR
spectroscopy, showing a virtually quantitative spectroscopic yield of 3
on the basis of of the 1H NMR spectrum. 1H NMR (400 MHz, C6D6,
298 K): δ/ppm 8.59 (br s, 1H, H5), 8.10 (d, J = 5.0 Hz, 1H, H4), 7.29
(br s, 1H, H2), 7.17−7.10 (m, 3H, H10,11), 6.67 (dd, J = 7.8, 5.4 Hz,
1H, H3), 3.61−3.50 (m, 2H, H12), 1.60 (s, 3H, H7), 1.41 (d, J = 6.7
Hz, 6H, H14), 1.31 (d, J = 6.8 Hz, 6H, H13), 0.06 (s, 6H, H15).
Synthesis of [(L2)Al2Me5] (4). A solution of L2H (200 mg, 0.68
mmol) in dichloromethane was added to a solution of 2 equiv of
AlMe3 (98.04 mg, 1.36 mmol) in dichloromethane. The reaction
mixture was stirred for 2 h at room temperature. All volatiles were
removed under vacuum, and the solid was washed with hexane.
Compound 4 was obtained as a white solid (282 mg, 98%). 1H NMR
(400 MHz, C6D6, 298 K): δ/ppm 8.39 (d, J = 2.5 Hz, 1H, H5), 7.93
(dd, J = 5.2, 0.8 Hz, 1H, H4), 7.12 (dd, J = 8.4, 7.0 Hz, 1H, H11), 7.04
(d, J = 7.4 Hz, 2H, H10), 6.74−6.70 (m, 1H, H2), 6.42 (dd, J = 8.3, 5.3
Hz, 1H, H3), 3.01 (hept, J = 6.9 Hz, 2H, H12), 1.35 (s, 3H, H7), 1.11
(d, J = 6.8 Hz, 6H, H14), 1.05 (d, J = 6.9 Hz, 6H, H13), −0.18 (s, 9H,
H16), −0.26 (s, 6H, H15). 13C{1H} NMR (100 MHz, C6D6, 298 K):
δ/ppm 174.26 (C6), 144.08 (C9), 141.84 (C1), 140.96 (C4), 140.81
CONCLUSIONS
■
New amidine ligand precursors (L1H and L2H) were prepared,
which could be reacted further to give mono-, bi-, and
trimetallic amidinate aluminum complexes. NMR spectroscopy
and X-ray diffraction studies have allowed us to determine the
different coordination modes of these complexes. Complex 1
presents a κ2-NN coordination mode, while the other
mononuclear complex 5 has a pentacoordinated aluminum
center. For the bimetallic aluminum complexes 2 and 4, the
amidinate ligands are attached to the aluminum centers in a κ2-
NN-μ-N coordination mode, while the trimetallic aluminum
complex 6 exhibits one pentacoordinated aluminum center and
the other two aluminum centers present a tetrahedral structure.
Aluminum complexes 1, 2, and 4−6 have carried out the
synthesis of cyclic carbonates from terminal epoxides and CO2
using tetrabutylammonium iodide as a cocatalyst. Among
them, the bimetallic complex 2 and trimetallic complex 6 with
TBAI showed the highest catalytic activity for the cyclic
carbonate formation at 50 °C and 1 bar of CO2 pressure in the
absence of solvent. Complex 2 is slightly more active than
complex 6; thus, it is possible to conclude that the number of
metallic centers present in a compound do not necessarily have
a direct effect on its catalytic activity. However, six-membered
metallacycle (complex 2) generates a considerable increase in
activity in comparison with the four-membered metallacycle
analogue (complex 4).
It is worth noting that these complexes are some of the first
amidinate aluminum complexes which have been used in the
the formation of cyclic carbonates from epoxides and CO2.
These amidinate ligands are easy to use and can be prepared
on a huge scale, which allows us to conclude that these types of
catalysts present a great potential for the synthesis of cyclic
carbonates.
EXPERIMENTAL SECTION
■
Synthesis of (E)-N-(2,6-Diisopropylphenyl)-N′-(pyridin-2-yl)-
acetimidamide (L1H). (E)-N-(2,6-Diisopropylhenyl)acetimidoyl
chloride71 (0.62 g, 2.61 mmol) was added to a solution of 2-
aminopyridine (0.24 g, 2.61 mmol) and triethylamine (0.4 mL, 2.87
mmol) in 30 mL of toluene. The reaction mixture was stirred for 8 h
under reflux. All volatiles were removed under vacuum. The solid
residue was taken up in 30 mL of CH2Cl2 and washed twice with 15
mL of water. After drying and removal of solvent, the crude product
was purified by column chromatography using silica gel (ethyl
acetate) and further recrystallized with ethanol to give light yellow
1
crystals (0.52 g, 67%). H NMR (400 MHz, DMSO-d6, 298 K): δ/
ppm 9.54 (br s, 1H, NH), 8.47 (br s, 1H, H2), 8.26 (d, J = 4.0 Hz, 1H,
H5), 7.68 (t, J = 7.3 Hz, 1H, H3), 7.06 (d, J = 7.5 Hz, 2H, H10), 6.99−
6.90 (m, 2H, H4,11), 2.86 (hept, J = 6.8 Hz, 2H, H12), 1.78 (s, 3H,
H7), 1.12 (d, J = 6.9 Hz, 6H, H14), 1.05 (d, J = 6.8 Hz, 6H, H13).
13C{1H} NMR (100 MHz, DMSO-d6, 298 K): δ/ppm 153.77 (C1),
152.70 (C6), 147.48 (C5), 145.25 (C8), 137.68 (C3), 137.32 (C9),
122.62 (C10), 122.36 (C11), 117.34 (C4), 112.70 (C2), 27.59 (C12),
23.41 (C13), 22.57 (C14), 18.16 (C7). IR (KBr): ν/cm−1 3241, 3183,
3107, 3056, 2960, 2925, 2866, 1676, 1660, 1579, 1540, 1462, 1436,
1372, 1310, 1254, 1233, 1197, 1151, 1101, 1054, 1013, 994, 955, 935,
883, 854, 807, 781, 759, 739, 703, 623, 558, 516, 500. HRMS (ESI)
for C19H26N3 [M + H]+: m/z calcd 296.212, found 296.212.
Synthesis of (E)-N-(2,6-Diisopropylphenyl)-N′-(pyridin-3-yl)-
acetimidamide (L2H). N-(2,6-Diisopropylhenyl)acetimidoyl chlor-
ide (0.62 g, 2.61 mmol) was added to a solution of 3-aminopyridine
(0.24 g, 2.61 mmol) and triethylamine (0.4 mL, 2.87 mmol) in 30 mL
of toluene. The reaction mixture was stirred for 8 h under reflux. All
volatiles were removed under vacuum. The solid residue was taken up
in 30 mL of CH2Cl2 and washed twice with 15 mL of water. After
drying and removal of solvent, the crude product was purified by
G
DOI: 10.1021/acs.organomet.8b00795
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(C5), 136.52 (C8), 132.39 (C2), 127.37 (C11), 125.76 (C3), 124.24
(C10), 28.50 (C12), 24.75 (C14), 23.77 (C13), 14.85 (C7), −7.64
(C16), −9.97 (C15).
AUTHOR INFORMATION
Corresponding Author
*E-mail for R.S.R.: rrojasg@uc.cl.
ORCID
■
Synthesis of [(L2)AlMe] (5). A solution of trimethylaluminum
(AlMe3) (24.51 mg, 0.34 mmol) in dichloromethane was quickly
added to a solution of 2 equiv of L2H (200 mg, 0.68 mmol) in
dichloromethane. The reaction mixture was stirred for 2 h at room
temperature. All volatiles were removed under vacuum, and the solid
was washed with hexane. Compound 5 was obtained as a white solid
(193 mg, 90%). Single colorless crystals for X-ray crystallography were
grown from cold dichloromethane/hexane. 1H NMR (400 MHz,
C6D6, 298 K): δ/ppm 8.19 (br. s, 4H, H5,4), 7.21−7.16 (m, 2H, H11),
7.14−7.08 (m, 4H, H10,12), 6.61−6.56 (m, 2H, H3), 6.48 (d, J = 8.0
Hz, 2H, H2), 3.45−3.33 (m, 4H, H17,14), 1.48 (s, 6H, H7), 1.19 (d, J =
6.6 Hz, 18H, H15,18,19), 1.10 (d, J = 6.8 Hz, 6H, H16), −0.10 (s, 3H,
H20). 13C{1H} NMR (100 MHz, C6D6, 298 K): δ/ppm 173.89 (C6),
145.42 (C5), 145.22 (C13), 144.55 (C9), 144.19 (C4), 140.78 (C1),
138.29 (C8), 129.83 (C2), 126.92 (C11), 124.33 (C10), 123.85 (C12),
123.23 (C3), 28.57 (C17), 28.46 (C14), 24.78 (C18), 24.56 (C19),
24.41 (C15), 24.20 (C16), 14.03 (C7), −10.63 (C20).
Yersica Rios Yepes: 0000-0002-9496-4649
Javier Martínez: 0000-0002-1538-0689
́
Rene S. Rojas: 0000-0002-5292-0816
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by FONDECYT, Project No.
1161091. Y.R.Y. acknowledges funding from Pontificia
■
́
Universidad Catolica de Chile via a VRI 2015−2017 fellowship
and Ph.D. CONICYT 2018 fellowship No. 21182120. J.M.
acknowledges funding from the CONICYT/FONDECYT
Project (No. 3180073).
Synthesis of [(L2)Al3Me7] (6). A solution of 2 equiv of AlMe3
(23.07 mg, 0.32 mmol) in dichloromethane was added to a solution
of complex 5 (100 mg, 0.16 mmol) in dichloromethane. The reaction
mixture was stirred for 2 h at room temperature. All volatiles were
removed under vacuum, and the solid was washed with hexane.
Compound 6 was obtained as a white solid (122.8 mg, 99%). Single
colorless crystals for X-ray crystallography were grown from cold
dichloromethane/hexane. 1H NMR (400 MHz, C6D6, 298 K): δ/ppm
8.16 (s, 2H, H5), 7.82 (d, J = 5.1 Hz, 2H, H4), 7.11−7.05 (m, 2H,
H11), 7.00 (d, J = 7.5 Hz, 2H, H10), 6.95 (d, J = 7.6, 2H, H12), 6.49 (d,
J = 8.1 Hz, 2H, H2), 6.32−6.26 (m, 2H, H3), 3.19−3.04 (m, 4H,
H17,14), 1.38 (s, 6H, H7), 1.10 (d, J = 6.6 Hz, 6H, H15), 1.08−1.02 (m,
18H, H18,19,16), −0.19 (s, 3H, H20), −0.24 (s, 18H, H21). 13C{1H}
NMR (100 MHz, C6D6, 298 K): δ/ppm 174.86 (C6), 144.30 (C13),
144.03 (C9), 142.58 (C1), 140.86 (C5), 140.79 (C4), 136.91 (C8),
133.81 (C2), 127.56 (C11), 125.21 (C3), 124.41 (C10), 124.10 (C12),
28.72 (C17), 28.45 (C14), 24.74 (C15), 24.71 (C16), 24.08 (C18), 24.01
(C19), 14.34 (C7), −7.65 (C21), −10.67 (C20).
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00795.
Experimental section including general descriptions,
synthesis of cyclic carbonates, and X-ray structure
analysis, figures referenced in the text, structural
characterization data (1H and 13C NMR spectra) for
all compounds including cyclic carbonates 8a−i, and X-
ray crystallographic data for L1H, L2H, 1, 2, 5, and 6
(PDF)
Accession Codes
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tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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H
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