Mono- And Dinuclear Asymmetric Aluminum Guanidinates for the Catalytic CO2Fixation into Cyclic Carbonates

Article abstract of DOI:10.1021/acs.organomet.1c00319

A set of trisubstituted guanidine ligands L1H2-L4H2 with general formula (iPrHN)2CNR (R = Ph (L1H2), R = 2,4,6-Me3C6H2(L2H2), R = p-BrC6H4(L3H2), R = (C5H4)Fe(C5H5), Fc (L4H2)) was employed to synthesize a family of mono- and dinuclear asymmetric methyl a

Full text of DOI:10.1021/acs.organomet.1c00319

pubs.acs.org/Organometallics
Article
Mono- and Dinuclear Asymmetric Aluminum Guanidinates for the
Catalytic CO2 Fixation into Cyclic Carbonates
́
Yersica Rios Yepes, Angela Mesías-Salazar, Alexandra Becerra, Constantin G. Daniliuc, Alberto Ramos,
̃
Rafael Fernández-Galán, Antonio Rodríguez-Diéguez, Antonio Antinolo, Fernando Carrillo-Hermosilla,*
and René S. Rojas*
Cite This: https://doi.org/10.1021/acs.organomet.1c00319
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: A set of trisubstituted guanidine ligands L1H2−L4H2 with general formula
(iPrHN)2CNR (R = Ph (L1H2), R = 2,4,6-Me3C6H2(L2H2), R = p-BrC6H4(L3H2), R =
(C5H4)Fe(C5H5), Fc (L4H2)) was employed to synthesize a family of mono- and
dinuclear asymmetric methyl aluminum guanidinato compounds ((L2H)AlMe2 (1),
(L4H)AlMe2 (2), (L1)Al2Me4 (3), (L2)Al2Me4 (4), (L3)Al2Me4 (5), (L4)Al2Me4 (6),
(L1H)2AlMe (7), (L2H)2AlMe (8), and (L4H)2AlMe (9)) that were characterized by
NMR spectroscopy (1−9) and single-crystal X-ray diffraction (4 and 8). These
compounds were tested as catalysts for the fixation of carbon dioxide with epoxides to
give cyclic carbonates, using tetrabutylammonium iodide (TBAI) as cocatalyst. The
reactions were performed under solvent-free conditions at 70 °C and 1 bar CO2 pressure.
Complexes 1−9 were more active than their respective free guanidines under the same experimental conditions for the synthesis of
styrene carbonate (11a). The dinuclear complex 6 was the most efficient and active catalyst for the synthesis of several
monosubstituted carbonates (11a−l) with excellent conversions and selectivities. Furthermore, the formation of some disubstituted
cyclic carbonates (13a−c) using this dinuclear aluminum catalyst was also studied.
guanidine derivatives. Since 2010, our group has been
investigating the preparation of substituted guanidines using
cheap ZnEt2 as catalyst (Scheme 2),7 as well as their use as
INTRODUCTION
■
The search for new coordination and organometallic
complexes involved as catalysts in synthetic processes requires
the design of new and ever more complex ligands.
Alternatively, there are some simple organic molecules with
donor atoms that can act as auxiliary ligands for a wide variety
of metals. Guanidinato monoanions of general formula
[(RN)2CNR2]− are related to the widely used amidinato
ligands, bearing an extra amino (R2N) moiety attached to the
ligands’ central carbon, which confers them higher steric and
electronic tunability (Scheme 1).1−4
Scheme 2. Catalytic Synthesis of Trisubstituted Guanidines
Scheme 1. Main Resonance Forms of Guanidinato Ligands
ligands in guanidinato compounds with early and late
transition metals, as well as with elements from the s and p
blocks.8−13 This allows the formation of complexes with
chelate ligands asymmetrically coordinated to the metal center,
regardless of the metals used. Additionally, the presence of two
N−H moieties in the guanidine compounds would allow
further reactivity for these ligands.
Received: June 1, 2021
Although there are stoichiometric synthetic routes, the
catalytic hydroamination of carbodiimides is a 100% atom-
economic procedure that allows the easy preparation of
trisubstituted guanidines.5,6 This offers the steric and electronic
properties of the ligands to be modulated by means of an
appropriate choice of starting reagents in the production of
© XXXX The Authors. Published by
American Chemical Society
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX
A

Organometallics
pubs.acs.org/Organometallics
Article
Figure 1. Previous aluminum amidinates employed as catalysts for the synthesis of cyclic carbonates from terminal epoxides and CO2.39−42
Recent years have seen a renaissance of metal-based catalysts
of the main group, looking for less expensive and more
environmentally benign catalytic systems than traditional
transition metal-based ones. For instance, coordination and
organometallic complexes of group 13 metals have been widely
used in homogeneous catalysis to carry out transformations of
organic compounds, most of which require Lewis acids and
involve polar substrates.14 Aluminum compounds with a well-
defined structure are among the most interesting for that
purpose, because of their higher Lewis acidity and the greater
availability of this metal.15,16
In this context, as a precedent to this work, some of us have
described the use of mono- and dinuclear aluminum
amidinates as efficient catalysts for this coupling process,
from terminal epoxides, at 1 bar of CO2, using Bu4NI or
Bu4NBr as cocatalysts, under solvent-free and mild temper-
ature conditions (Figure 1).39−42
Several aluminum complexes with related guanidinato
ligands can be obtained through various reaction pathways:
(i) carbodiimide insertion in Al−N bonds,43−49 (ii) salt
metathesis reactions between Al-X (X = halide) and lithium
guanidinates,50−55 and (iii) protonolysis of Al−Y bonds (Y =
alkyl or amide) and a neutral guanidine.55−60
On the other hand, as atmospheric levels of carbon dioxide
(CO2) continue to increase daily, it is necessary to make
progress in their reduction and containment. The use of this
dioxide as a raw material to obtain organic compounds means
generating an added value that could participate in subsidizing
the costs of capturing and storing this CO2.17−21 In addition, it
could be an alternative to alleviate our current needs for
chemicals that depend heavily on the availability of fossil fuels.
Among the different described processes, the formation of
cyclic carbonates through the coupling of CO2 to epoxides has
attracted great attention,22−26 as cyclic carbonates are used in
industry as solvents in electrolyte solutions for Li-ion
batteries,27,28 or as polar aprotic solvents.29 This synthesis is
proposed as an industrial alternative to the condensation of the
very poisonous phosgene and diols. Binary catalytic systems,
which include a Lewis acid and a nucleophile, are necessary to
promote the coupling between CO2 and the epoxide. The
Lewis acid activates the epoxide, allowing ring opening by the
nucleophile attacking to the least substituted carbon atom.
This reaction is followed by an insertion of CO2 into the
alkoxide resulting from the previous attack, leading to an alkyl
carbonate which closes to form a new five-membered ring, via
a backbiting mechanism, producing the cyclic carbonate and
releasing the nucleophilic cocatalyst. Among the different
homogeneous catalysts described for this synthesis of cyclic
carbonates, the use of aluminum-based compounds should be
highlighted given that they display some of the best catalytic
performances reported to date.30−38
It should be noted that only in very few cases has their use as
catalysts it been reported, in aldehyde reduction pro-
cesses,49,54,59 ring-opening polymerization,60 or, curiously,
guanylation of aromatic amines,47,50 but never as catalysts for
the formation of cyclic carbonates. Continuing our interest in
the chemistry of guanidines and their derivatives, some of us
have studied the reaction of trimethylaluminum (AlMe3) with
N-phosphinoguanidine ligands, which yields stable phosphini-
mine-amidinato compounds, after the rearrangement of the N-
phosphinoguanidinato intermediates initially detected.61,62
Very recently, we have also described the use of aromatic
guanidines such as those depicted in Scheme 2, as catalysts for
the preparation of cyclic carbonates. In these compounds, the
presence of two N−H groups represented a substantial
improvement in the activity, as hydrogen-bond donor systems,
compared to what was previously found with other types of
guanidines.63
This all prompted us to explore the reactivity of catalytically
obtained trisubstituted guanidines, namely (iPrHN)2CNR (R
= Ph, 2,4,6-Me3C6H2, p-BrC6H4, Fc), toward AlMe3. As a
result of this study, we report herein the preparation of a family
of aluminum guanidinato complexes with different nuclearities
and coordination modes. The catalytic behavior of some of
these complexes in the synthesis of cyclic carbonates is also
discussed and compared with the previous results obtained
with the free ligands as catalysts.
B
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
RESULTS AND DISCUSSION
Synthesis and Structural Characterization of Com-
plexes 1−9. Guanidine ligands L1H2−L4H2 (Scheme 3) were
■
Scheme 3. General Pathway for the Preparation of
Complexes 1−9
1
Figure 2. H NMR spectra in C6D6 of (a) mononuclear complex 2,
(b) dinuclear complex 6, and (c) mononuclear complex 9.
aluminum complexes. Furthermore, these compounds show a
characteristic singlet resonance for the two AlMe2 groups at
−0.94, −0.60, −0.93, and −0.23 ppm, respectively (complexes
3 and 4 in CDCl3, while 5 and 6 in C6D6), all signals
1
integrating for 12 protons (Figure 2b for complex 6). The H
NMR spectra, in C6D6, for complexes 7−9, exhibit a resonance
corresponding to the N−H protons of each guanidine moiety
at 3.48, 3.54, and 5.32 ppm, respectively, jointly with a singlet
at −0.21, 0.09, and −0.15, respectively, attributed to the AlMe
moiety (Figure 2c for complex 9).
synthesized via guanylation of the respective amine with N,N′-
diisopropylcarbodiimide in the presence of ZnEt2 as catalyst, as
previously reported (see Experimental Section).7,10
On the basis of our previous work39−41 and acknowledging
that the presence of two N−H groups in the guanidine ligand
precursor will allow us to obtain mononuclear or dinuclear
complexes, the latter reported to be more active in the
formation of cyclic carbonates from epoxides and CO2,35,39,40
we performed the synthesis of the new alkyl aluminum
guanidinato complexes 1−9 via protonolysis reaction between
the respective guanidine ligands (L1H2−L4H2) with 1.0, 2.0, or
0.5 equiv of AlMe3 to produce the corresponding mono- (1
and 2) or dinuclear (3−6) tetracoordinate aluminum
complexes and the respective mononuclear pentacoordinate
compounds (7−9) (Scheme 3). The reactions were carried out
in dry CH2Cl2 for 2 h at room temperature. After the
appropriate workup, complexes 1−9 were obtained in high
yields (≥95%) as white (1, 3−5, 7, 8) or reddish-brown (9)
solids, and reddish-brown (2 and 6) oils. Of all the latter
species, tetracoordinated complexes proved to be unstable
under air, while the pentacoordinated ones remained
unchanged for several hours of exposure to the atmosphere.
The structural characterization of these mono- and dinuclear
compounds 1−9 was accomplished using standard 1D and 2D
NMR characterization techniques and X-ray diffraction for 4
and 8 (see Experimental Section and Supporting Information).
The 1H NMR spectra of mononuclear complexes 1 and 2, in
CDCl3 and C6D6, respectively, supported the nuclearity of
these compounds due to the signal attributed to one N−H
proton of the guanidinate fragment at 3.88 ppm for 1 and 4.54
ppm for 2 together with a singlet at −0.71 and −0.12 ppm,
respectively, corresponding to 6 protons of the AlMe2 moiety
The 13C NMR spectra of compounds 1−9 evidence the
presence of the diagnostic peak of the central carbon in the
guanidine moiety located around 150 or 160 ppm for all
complexes. Moreover, a characteristic peak for the AlMe2
moiety in complexes 1−6 and the AlMe fragment in complexes
7−9 was detected around −4 or −10 ppm.
Two-dimensional experiments were carried out to assign the
1
1
majority of H NMR signals and H−13C g-HSQC experi-
ments to locate resonances from carbon atoms. The
equivalence of the substituents on the guanidinate ligands
and the equivalence of the alkyl aluminum moieties allowed us
to propose a tetrahedral environment around one or two
aluminum atoms (compounds 1−2 and 3−6, respectively) in
which the guanidinato monoanion acts as bidentate or
tridentate ligand in a κ2-N,N′ or a μ-κ2-N,N′-κ2-N′,N″
coordination mode, respectively. Mononuclear complexes 7−
9 reveal a pentacoordination geometry around the aluminum
atom.
The structures of the binuclear complex 4 and the
mononuclear complex 8 were determined by X-ray single
crystal diffraction. Single crystals of complexes 4 and 8 were
obtained by slow evaporation in cold pentane at −30 °C.
Molecular structures of complexes 4 and 8 are illustrated in
Figures 3 and 4, respectively. A summary of the crystallo-
graphic data and data collection parameters is incorporated in
Tables S1 and S2 in the Supporting Information.
The solid-state structure of complex 4 is shown in Figure 3.
The diffractometric analysis of this compound confirmed the
monomeric structure with two coordinated aluminum centers.
Both aluminum centers present a distorted tetrahedral
geometry and are bridged by the N2 atom which is common
for the two chelating guanidinato units in where each unit
presents a κ2-N,N coordination mode. The amidinate bite
1
(see Figure 2a for complex 2). On the other hand, in the H
NMR spectra for complexes 3−6, the signal ascribed to the
two N−H protons of the guanidine ligand precursor was
missing, therefore supporting the formation of the binuclear
C
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
molecule (molecule “A”) is discussed, as both molecules show
similar structural parameters.
The aluminum metal center presents the expected
pentacoordinated environment. Therefore, the τ value64,65
was calculated to determine how closely this compound
approaches either a perfectly square pyramidal (τ = 0) or a
perfectly trigonal bipyramidal geometry (τ = 1). A value of
0.15 for this parameter confirmed a slight distorted square-
pyramidal structure in which four N atoms belonging to the
two identical guanidinato units occupy the equatorial positions,
while the apical position is occupied by the methyl group. It
should be noted that the few pentacoordinated guanidinate
aluminum complexes that have been published to date exhibit
distorted trigonal bipyramidal geometries,49,52 whereas com-
plex 8 displayed a distorted square-pyramidal geometry
probably favored by the steric and electronic properties of
the asymmetric guanidine ligand employed. The acute bite
angles (66.90°) produce a minor increase of the respective
N1−Al1−N5 (98.4(6)°) and N2−Al1−N4 (99.0(6)°) angles
and a notable increase of the N1−Al1−C8 (115.0(8)°) angle
from the ideal square pyramidal angle of 90°. Furthermore, a
significant decrease of the N1−Al1−N4 (134.7(7)°) and N2−
Al1−N5 (143.6(7)°) angles from the ideal angle of 180° is also
noted. Charge delocalization in the NCN fragments is
supported by the respective C−N bond lengths, which present
values between 1.331(2) and 1.338(2) Å and are comparable
to those C−N distances of other metal guanidinato
complexes.45,49 Furthermore, the Al1−C8 distance 1.972(2)
Å is consistent with values previously reported.51
Figure 3. Molecular structure of 4. Thermal ellipsoids are shown with
30% probability, and hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): N1−Al1 1.946(3),
N2−Al1 1.982(3), N2−Al2 2.023(4), N3−Al2 1.945(4), C1−N3
1.315(4), C1−N1 1.320(4), C1−N2 1.449(4), Al1−C11 1.498(4);
N1−Al1−N2 70.6(1), N2−Al2−N3 69.9(1), Al1−N1−C1 91.5(2),
Al2−N3−C1 90.1(2), N2−C1−N1 110.0(3), N2−C1−N3 110.6(3).
angles N1−Al1−N2 70.6(1)° and N2−Al2−N3 69.9(1)°
together with the C11−Al1−C12 113.3(2)° and C13−Al2−
C14 114.9(2)° angles are far away from the ideal tetrahedral
angle of 109.5°. The C1−N1 and C1−N3 bond distances of
1.320(4) and 1.315(4) Å supported a high degree of electron
delocalization within the guanidinate backbone, like those
previously reported for related Al guanidinato complexes.43,44
The Al1−C bond lengths (range 1.945(6)−1.956(4) Å) of the
Al-bound methyl moieties are according with the expected
values for Al alkyl complexes.51,58
Catalytic Studies for the Synthesis of Cyclic Carbo-
nates. As mentioned before, we reported the use of aromatic
mono- and bisguanidines as very active binary catalyst systems
for the preparation of cyclic carbonates from epoxides and
CO2.63 Thus, having prepared the asymmetric aluminum
guanidinato complexes 1−9, which present different nuclear-
ities and coordination modes, we decided to test them as
The molecular structure of complex 8 is depicted in Figure
4. The X-ray diffraction study indicated that compound 8
crystallized in the triclinic space group P1
independent molecules in the asymmetric unit. Only one
̅
with two
Figure 4. (a) Molecular structure of 8. Thermal ellipsoids are shown with 30% probability, and hydrogen atoms have been omitted for clarity. Only
one of the two molecules (molecule “A”) found in the asymmetric unit is shown. Selected bond distances (Å) and angles (deg): N1−Al1 1.971(2),
N2−Al1 2.011(2), N5−Al1 2.003(2), N4−Al1 1.984(2), C1−N1 1.337(2), C11−N5 1.336(2), Al1−C8 1.972(2), C1−N3 1.363(2); N1−Al1−
N2 66.9(6), N4−Al1−N5 66.9(6), N1−Al1−C8 115.0(8), N1−Al1−N4 134.7(7), N1−Al1−N5 98.4(6), N5−C11−N6 124.8(2). (b) Geometry
around Al.
D
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
catalysts for this catalytic application and compare their activity
with the respective free ligands L1H2−L4H2.
Table 2. Synthesis of Styrene Carbonate 11a Employing
Catalysts 1−9
a
Therefore, the transformation of styrene oxide 10a into
styrene carbonate 11a was initially performed with complexes
1, 2, 4, 6, 8, and 9, which have the same ligands and different
nuclearity, under the same catalytic conditions employed with
the free guanidines,63 and we proceeded to compare their
catalytic activity.
The reactions were performed at 70 °C and 1 bar of CO2
pressure (balloon) for 24 h under solvent-free conditions using
2.0 mol % of free guanidines (L1H2−L4H2), 2.0 mol % of
mononuclear complexes 1, 2, 8, and 9, and 1.0 mol % of
dinuclear complexes 4 and 6, with the aluminum concentration
kept constant at 2.0 mol % and using tetrabutylammonium
iodide (TBAI) as cocatalyst. These results are shown in Table
1.
b
entry
cat.
cat. (mol %)
conversion (%)
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
6
1.0
1.0
0.5
0.5
0.5
0.5
1.0
1.0
1.0
0.5
70
75
90
91
81
92
82
87
88
c
10
11
0
d
8
a
Reactions were carried out at 70 °C and 1 bar of CO2 pressure for 24
h using 1.0 mol % of TBAI and under solvent-free conditions.
b
Conversion was determined by 1H NMR spectroscopy of the
c
Table 1. Synthesis of Styrene Carbonate 11a Employing
reaction mixture relative to starting epoxide. No TBAI was added.
d
Guanidine Ligands L1H2−L4H2 and Catalysts 1, 2, 4, 6, 8,
No catalyst was added.
a
and 9
b
entry
cat.
cat. (mol %)
conversion (%)
compounds, which show higher catalytic activities probably
due to cooperative effects between the two metal centers.39,40
Focusing on the dinuclear complexes, it is noteworthy to see
the electronic influence of the aromatic ring in the catalytic
activity of these systems, as the presence of electron
withdrawing groups (EWGs) such as p-BrC6H4 in 5 resulted
in a slight decrease in the activity (Table 2, entry 5) vs the
unsubstituted phenyl ring in 3 (Table 2, entry 3), whereas
strong electron donating groups (EDGs) such as Fc in 6 and
2,4,6-Me3C6H3 in 4 (Table 2, entries 4 and 6) caused an
increase in the catalytic activity. Nevertheless, the higher
solubility of these complexes in the reagent mixture cannot be
excluded as responsible for this behavior. Out of all the
catalysts tested, dinuclear complex 6 showed the highest
activity toward the preparation of styrene carbonate 11a. For
this reason, control experiments were carried out to determine
that neither complex 6 nor TBAI exhibited significant activity
on their own (Table 2, entries 10 and 11).
With the optimized reactions conditions for the preparation
of 11a with catalyst 6 at hand (see entry 6 in Table 2), we
decided to attempt the synthesis of 12 monosubstituted cyclic
carbonates 11a−l from their respective terminal epoxide
(10a−l) and carbon dioxide (Figure 5). In general, the
catalytic system (6/TBAI) afforded remarkable results with
selectivities > 99% toward cyclic carbonates bearing different
functional groups such as aryl, alkyl, alcohol, halide, and ether
groups under the conditions described about. Particularly, the
alkyl cyclic carbonates (11b−d) together with those bearing
phenyl, ether, and halide groups in their structure (11a, 11f−g,
and 11l) were accomplished in notable yields (82−96%), while
the functionalized aromatic cyclic carbonates (11i−j) were
obtained in good yields (71−77%). A remarkable result was
found for glycerol carbonate (11e), which was obtained in
exceptional yield (93%) despite the tendency of glycidol
epoxide to generate polymer products.66,67 Additionally, the
highly fluorinated cyclic carbonate 11k was prepared in
outstanding yield (85%), this being a cyclic carbonate that
presents a potential application as electrolyte in lithium-ion
batteries.28,68 These results indicate that 6/TBAI is an efficient
system for the synthesis of monosubstituted cyclic carbonates.
As a result of the high activity presented by the catalytic
system (6/TBAI) for the preparation of terminal cyclic
c
1
2
3
4
5
6
7
8
9
L1H2
L2H2
L3H2
L4H2
1
2
4
6
8
9
2.0
2.0
2.0
2.0
2.0
2.0
1.0
1.0
2.0
2.0
70
54
74
93
100
100
100
100
100
100
c
c
10
a
Reactions were carried out at 70 °C and 1 bar of CO2 pressure for 24
h using 2.0 mol % of TBAI and under solvent-free conditions.
b
Conversion was determined by 1H NMR spectroscopy of the
c
reaction mixture relative to starting epoxide. Conversion reported by
Rojas et al.63
It is notable that both, mono- and dinuclear aluminum
complexes (Table 1, entries 5−10), presented excellent
conversions (100%) for the preparation of styrene carbonate
11a compared with the high to moderate conversions (54−
93%) exhibited by the free guanidines (Table 1, entries 1−4).
Therefore, these aluminum guanidinato complexes allowed a
significant improvement in the catalytic activity and turned to
be more active and efficient catalytic systems than free
guanidines This could be explained through the fact that the
Lewis acidity of the Al(III) metal center is more efficient than
the hydrogen-bonding interaction approach ascribed to free
guanidines for the activation of the epoxide. Thus, the Al(III)−
O interaction should facilitate further the epoxide ring
opening.
Furthermore, we focused our attention on the catalytic
behavior of complexes 1−9 for the preparation of styrene
carbonate 11a. For that reason, we opted for decreasing by half
the loadings of the previously studied catalysts. The catalytic
results are illustrated in Table 2.
As can be seen, dinuclear complexes 3−6 (Table 2, entries
3−6) displayed higher catalytic activity than their tetracoordi-
nate (1−2, Table 2, entries 1 and 2) and pentacoordinate (8
and 9, Table 2, entries 8 and 9) mononuclear counterparts.
These results support the importance of the catalyst design and
are in good agreement with previous reports on binuclear
E
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
2.0 mol % of complex 6 and 4.0 mol % of TBAI at 8.5 bar of
CO2 (Table 3, entry 6).
Having optimized the reaction conditions for the synthesis
of cyclopentene oxide 13a, a series of internal epoxides (12a−
c) and CO2 were employed under the same experimental
conditions to prepare other disubstituted cyclic carbonates
(Figure 6).
Figure 5. Cyclic carbonates 11a−l were obtained by the catalyst
system 6/TBAI in a molar ratio of 0.5:1.0. Reactions were performed
at 70 °C and 1 bar of CO2 pressure in the absence of solvent.
aConversion was determined using 1H NMR spectroscopy of the
crude reaction mixture. Isolated yield obtained from purified cyclic
carbonate.
Figure 6. Synthesis of disubstituted cyclic carbonates 13a−c using the
a
1
catalytic system 6/TBAI. Conversion was determined by H NMR
b
b
spectroscopy of the crude reaction mixture. Isolated yield obtained
from purified cyclic carbonate. No polycarbonate was obtained from
stilbene epoxide.
carbonates, we decided to expand the substrate scope by
studying the use of internal or disubstituted epoxides. Initially,
we employed cyclopentene oxide 12a as a model substrate
under 8.5 bar of CO2 due to the lower reactivity of these
internal epoxides. These results are shown in Table 3.
In general, low to high yields (17−92%) were achieved using
the catalytic system 6/TBAI for the preparation of cyclic
carbonates 13a−c. Pleasingly, the cyclic carbonate 13a was
accomplished in good yield, 92%, with 100% retention of
stereochemistry and selectivity. However, for cyclohexene
carbonate 13b, not only was the formation of cyclic carbonate
observed but also the corresponding polycarbonate was
detected in the NMR spectra of the product, as only a 17%
of yield of cis-cyclohexene carbonate 13b was produced,
indicating that cyclohexene oxide 12b was not a good substrate
for this catalytic system (6/TBAI). Finally, when stilbene
epoxide 12c was used, only a 20% yield of its respective cyclic
carbonate 13c was obtained, and no polycarbonate was found.
Table 3. Synthesis of Cyclopentane Carbonate 13a
a
Catalyzed by Complex 6
b
entry
cat. (mol %)
TBAI (mol %)
T (° C)
conversion (%)
1
2
3
4
5
6
7
8
0.5
1.5
2.0
0.5
1.5
2.0
2.0
0
1.0
3.0
4.0
1.0
3.0
4.0
0
70
70
70
90
90
90
90
90
0
15
20
32
64
96
0
CONCLUSIONS
■
In conclusion, mono- and dinuclear aluminum guanidinato
complexes 1−9 were prepared via protonolysis between the
respective trisubstituted guanidine ligands (L1H2−L4H2)
through different molar ratios, and AlMe3. NMR spectroscopy
and X-ray diffraction analysis allowed us to establish a
tetrahedral environment around the mononuclear complexes
1−2 and dinuclear complexes 3−6 in which the guanidinato
monoanion is coordinated to one or two AlMe2 moieties,
respectively, in a κ2-N,N coordination mode, whereas
mononuclear complexes 7−9 present a pentacoordinated
environment around the aluminum metal center, with two
guanidinato ligands chelating the metal center also in a κ2-N,N
coordination mode.
4.0
2
a
Reactions were carried out at 70−90 °C and 8.5 bar of CO2 pressure
for 24 h using 0.5−2 mol % of complex 6 and 1−4 mol % of TBAI
b
and under solvent-free conditions. Conversion was determined by
1H NMR spectroscopy of the reaction mixture relative to starting
epoxide.
As may be noted, none or low conversion to cyclopentene
carbonate 13a was obtained at 70 °C employing different
loadings of catalyst with cocatalyst (6/TBAI) keeping constant
the molar ratio at 1:2 (Table 3, entries 1−3). With the aim to
improve the catalytic activity, we decided to increase the
temperature to 90 °C, and the highest conversion to
cyclopentene carbonate 13a (96%) was achieved by using
Complexes 1, 2, 4, 6, 8, and 9 are very active catalysts for the
transformation of styrene oxide and CO2 into styrene
carbonate. In all cases, the aluminum complexes were more
active and efficient catalysts than their respective free
guanidines (L1H2−L4H2) at 70 °C and 1 bar of CO2 for 24
h employing 2.0 mol % of catalyst and 2.0 mol % of TBAI as
cocatalyst. Consequently, aluminum guanidinato complexes
F
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
product was dissolved with hexane. After the appropriate workup,
provide a significant improvement in the catalytic activity
compared with the respective parent guanidines.
complex 2 was obtained as a viscous orange oil. Yield: 94% (220 mg,
1
0.574 mmol). H NMR (C6D6, 400 MHz, ppm): δ 4.54 (d, J = 8.7
Among complexes 1−9, dinuclear complexes 4 and 6
exhibited higher catalytic activity than their mononuclear
counterparts 1 and 2, and their respective pentacoordinated
analogues 8 and 9 for the preparation of styrene carbonate
using 0.5 mol % of complex and 1 mol % of TBAI at 70 °C and
1 bar of CO2 pressure for 24 h. Dinuclear complex 6 in the
presence of TBAI presented the highest catalytic activity at 70
°C and 1 bar of pressure of CO2 for 24 h and was able to
produce a wide scope of terminal cyclic carbonates from
different functionalized epoxides with aryl, alkyl, alcohol, ether,
and halide moieties, in good to excellent yields (71−96%),
while the respective internal carbonates were obtained in low
to high yields (17−92%) even at higher temperatures and CO2
pressure.
The trisubstituted guanidine pro-ligands described here are
easy to prepare on a large scale and present high tunability, and
equally important, the presence of two N−H groups in these
species allows the future development of new complexes with
different nuclearities and coordination modes that could be
highly active for the preparation of cyclic carbonates.
Hz, 1H, N-H, H5), 4.17 (s, 5H, H10), 4.04 (s, 2H, H8), 3.81 (s, 2H,
H9), 3.48 (m, 1H, H4), 3.27 (m, 1H, H2), 1.08 (d, J = 6.2 Hz, 6H,
H1), 0.74 (d, J = 6.3 Hz, 6H, H3), - 0.12 (s, 6H, H11). 13C{1H} NMR
(C6D6, 100 MHz, ppm): δ 161.11 (C6), 101.46 (C7), 69.24 (C10),
65.07 (C9), 63.81 (d, J = 2.6 Hz, C8), 44.48 (C2), 44.17 (d, J = 11.4
Hz, C4), 24.71 (C1), 23.53 (d, J = 4.8 Hz, C3), −8.62 (C11). Anal.
Calcd. for C19H30AlFeN3: C, 59.54; H, 7.89; N, 10.96. Found: C,
59.72; H, 7.94; N, 11.03.
Synthesis of Complex 3. A dichloromethane solution of 2 equiv
of trimethylaluminum (131.2 mg, 1.82 mmol) was added to a
dichloromethane solution of 1 equiv of 1,3-diisopropyl-2-phenyl-
guanidine (200 mg, 0.912 mmol). The reaction mixture was stirred
for 2 h at room temperature. All volatiles were removed under
vacuum, and the product was dissolved in pentane and kept at −30
°C to precipitate complex 3 as a white solid (0.894 mmol, 296 mg,
1
98%). H NMR (400 MHz, CDCl3, ppm): δ 7.24 (dd, J = 10.8, 5.0
Hz, 1H, H6), 7.00 (t, J = 7.3 Hz, 2H, H7), 6.82 (d, J = 7.6 Hz, 2H,
H5), 3.98 (s, 2H, H2), 1.22 (d, 12H, H1), −0.94 (s, 12H, H8).
13C{1H} NMR (100 MHz, CDCl3, ppm): δ 159.07 (C3), 149.34
(C4), 129.28 (C6), 122.95 (C7), 119.97 (C5), 45.11 (C2), 25.15 (C1),
−9.65 (C8). Anal. Calcd. for C17H31Al2N3: C, 61.61; H, 9.43; N,
12.68. Found: C, 61.52; H, 9.64; N, 12.53.
Synthesis of Complex 4. A dichloromethane solution of 2 equiv
of trimethylaluminum (110.3 mg, 1.53 mmol) was added to a
dichloromethane solution of 1 equiv of 1,3-diisopropyl-2-mesitylgua-
nidine (200 mg, 0.765 mmol. The reaction mixture was stirred for 2 h
at room temperature. All volatiles were removed under vacuum, and
the product was dissolved in pentane and kept at −30 °C to
precipitate complex 4 as a white solid (0.750 mmol, 280 mg, 98%).
Single colorless crystals for X-ray crystallography were grown from
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed under an
inert atmosphere using standard glovebox and Schlenk-line
techniques. Reagent-grade solvents, toluene, dichloromethane, diethyl
ether, pentane, tetrahydrofuran (THF), and hexane, were dried using
an Innovative Technology Pure Solv Model PS-MD-5. Amino-
ferrocene41,69 and guanidine ligands L1H2−L3-H2 and L4H2 were
prepared as previously reported. All other chemical reagents and
solvents were obtained from the usual commercial suppliers and used
7
10
1
cold pentane at −30 °C. H NMR (400 MHz, CDCl3, ppm): δ 6.83
(s, 2H, H8), 3.89 (m, 1H, H4), 3.21 (m, 1H, H2), 2.27 (s, 9H, H10,11),
1.33 (d, J = 6.0 Hz, 6H, H3), 0.85 (s, 6H, H1), −0.60 (s, 12H, H12).
13C{1H} NMR (100 MHz, CDCl3, ppm): δ 161.77 (C5), 137.11
(C6), 135.06 (C9), 133.48 (C7), 128.86 (C8), 54.32 (C4), 44.35 (C2),
24.76 (C1), 22.21 (C3), 20.88 (C10), 19.59 (C11), −6.98 (C12). Anal.
Calcd. for C20H37Al2N3: C, 64.32; H, 9.99; N, 11.25. Found: C, 64.44;
H, 10.09; N, 11.33.
1
as received. H and 13C NMR spectra were recorded on a Bruker
Avance-400 spectrometer. Chemical shifts and coupling constants are
reported in parts per million and hertz, respectively. Most of the NMR
assignments were supported by additional 2D. Microanalyses of solid
samples were carried out with a LECO CHNS-932 analyzer. For X-
ray crystal structure analysis of complex 4, data sets were collected by
́
́
Dr. Antonio Rodriguez-Dieguez, on a Bruker D8 Venture with a
photon detector equipped with monochromated MoKα radiation (λ =
0.71073 Å), and for complex 8, data sets were collected by Dr.
Constantin G. Daniliuc, with a Bruker D8 Venture Photon III
Diffractometer system equipped with a micro focus tube MoKα
radiation (λ = 0.71073 Å) and a mirror monochromator. CCDC
2069446 (4) and 2082070 (8) contain the supplementary crystallo-
graphic data of this paper.
Synthesis of Complex 5. A dichloromethane solution of 2 equiv
of trimethylaluminum (96.6 mg, 1.34 mmol) was added to a
dichloromethane solution of 1 equiv of 2-(4-bromophenyl)-1,3-
diisopropylguanidine (200 mg, 0.671 mmol). The reaction mixture
was stirred for 2 h at room temperature. All volatiles were removed
under vacuum, and the product was dissolved in pentane and kept at
−30 °C to precipitate complex 5 as a white solid (0.651 mmol, 267.5
mg, 97%). 1H NMR (400 MHz, CDCl3, ppm): δ 7.34 (d, J = 8.4 Hz,
2H, H6), 6.70 (d, J = 8.3 Hz, 2H, H5), 3.97 (m, 2H, H2), 1.21 (d, J =
6.3 Hz, 12H, H1), −0.93 (s, 12H, H8). 13C{1H} NMR (100 MHz,
CDCl3, ppm): δ 158.84 (C3), 148.72 (C4), 132.29 (C6), 121.70 (C5),
115.68 (C7), 45.19 (C2), 25.12 (C1), −9.63 (C8). Anal. Calcd. for
C17H30Al2BrN3: C, 49.76; H, 7.37; N, 10.24. Found: C, 49.84; H,
7.44; N, 10.32.
Synthesis of Complex 6. A dichloromethane solution of 2 equiv
of trimethylaluminum (220.6 mg, 3.06 mmol) was added to a
dichloromethane solution of 1 equiv of diisopropyl ferrocenyl
guanidine (500 mg, 1.53 mmol). The reaction mixture was stirred
for 2 h at room temperature. All volatiles were removed under
vacuum, and the product was dissolved with hexane. After the
appropriate workup, complex 6 was obtained as a viscous orange
liquid. Yield: 96% (1.47 mmol, 646 mg). 1H NMR (C6D6, 400 MHz,
ppm): δ 4.14 (s, 5H, H9), 4.08 (s, 2H, H7), 3.73 (s, 2H, H8), 3.62 (m,
2H, H2,4), 0.96 (m, 12H, H1,3), −0.23 (s, 12H, H10). 13C{1H} NMR
(C6D6, 101 MHz, ppm): δ 163.95 (C5), 99.27 (C6), 69.51 (C9), 65.31
(C7,8), 53.87, 44.73 (C4), 23.81, 22.05 (C2), −7.58 (C10). Anal. Calcd.
for C21H35Al2FeN3: C, 57.41; H, 8.03; N, 12.28. Found: C, 57.72; H,
7.94; N, 12.43.
Synthesis of Complex 1. A dichloromethane solution of
trimethylaluminum (55.1 mg, 0.765 mmol) in was added to a
dichloromethane solution of 1,3-diisopropyl-2-mesitylguanidine (200
mg, 0.765 mmol). The reaction mixture was stirred for 2 h at room
temperature, and the volatiles were then removed under vacuum. The
product thus obtained was dissolved in pentane and kept at −30 °C to
precipitate complex 1 as a white solid (0.734 mmol, 233 mg, 96%).
1H NMR (400 MHz, CDCl3, ppm; see Supporting Information for
atom assignment): δ 6.84 (s, 2H, H9), 3.88 (d, J = 9.2 Hz, 1H, N-H,
H5), 3.45 (m, 1H, H2), 3.29 (m, 1H, H4), 2.27 (s, 1H, H11), 2.20 (s,
2H, H12), 1.19 (d, J = 6.3 Hz, 6H, H1), 0.99 (d, J = 6.4 Hz, 6H, H3),
−0.71 (s, 12H, H13). 13C{1H} NMR (100 MHz, CDCl3, ppm):
δ159.68 (C6), 139.07 (C7), 134.28 (C8), 133.19 (C10), 128.86 (C9),
44.03 (C2), 43.93 (C4), 24.59 (C1), 24.20 (C3), 20.91 (C11), 18.78
(C12), −8.94 (C13). Anal. Calcd. for C18H32AlN3: C, 68.10; H, 10.16;
N, 13.24. Found: C, 68.52; H, 9.94; N, 13.33.
Synthesis of Complex 2. A dichloromethane solution of 2 equiv
of trimethylaluminum (44.05 mg, 0.611 mmol) was added to a
dichloromethane solution of diisopropyl ferrocenyl guanidine (200
mg, 0.611 mmol). The reaction mixture was stirred for 2 h at room
temperature. All volatiles were removed under vacuum, and the
G
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Synthesis of Complex 7. A dichloromethane solution of 1 equiv
of trimethylaluminum (32.87 mg, 0.456 mmol) was added to a
dichloromethane solution of 2 equiv of 1,3-diisopropyl-2-phenyl-
guanidine (200 mg, 0.912 mmol). The reaction mixture was stirred
for 2 h at room temperature. All volatiles were removed under
vacuum, and the product was washed with hexane. Complex 7 was
obtained as a white solid (0.451 mmol, 178 mg, 99%). 1H NMR
(C6D6, 400 MHz, ppm): δ7.15 (d, J = 7.2, 4H, H8), 7.11 (d, J = 9.1,
4H, H9), 6.83 (t, J = 6.9, 2H, H10), 3.48 (d, J = 9.6 Hz, 2H, N-H),
3.32 (m, 2H, H5), 3.13 (m, 2H, H2), 1.11 (d, J = 21.1, 12H, H1), 0.59
(d, J = 15.1, 8.6 Hz, 12H, H6), −0.21 (m, 3H, H11). 13C{1H} NMR
(C6D6, 100 MHz, ppm): δ 162.17 (C3), 148.17 (C7), 128.91 (C10),
124.83 (C9), 121.66 (C8), 45.06 (C2), 44.57 (C5), 24.18 (C1), 23.22
(C6), −7.49 (C11). Anal. Calcd. for C27H43AlN6: C, 67.75; H, 9.05; N,
17.56. Found: C, 68.14; H, 8.74; N, 17.72.
Synthesis of Complex 8. A dichloromethane solution of 1 equiv
of trimethylaluminum (27.6 mg, 0.382 mmol) was added to a
dichloromethane solution of 2 equiv of 1,3-diisopropyl-2-mesitylgua-
nidine (200 mg, 0.765 mmol). The reaction mixture was stirred for 2
h at room temperature. All volatiles were removed under vacuum, and
the product was washed with hexane. Complex 8 was obtained as a
white solid (0.378 mmol, 179 mg, 99%). 1H NMR (C6D6, 400 MHz,
ppm): δ6.93 (d, J = 7.2 Hz, 4H, H9), 3.54 (d, J = 9.9 Hz, 2H, N-H),
3.35 (m, 2H, H2), 3.20 (m, 2H, H5), 2.57 (s, 6H, H12a), 2.54 (s, 6H,
H12b), 2.21 (s, 6H, H11), 1.11 (d, J = 6.3 Hz, 6H, H6), 0.75 (d, J = 6.3
Hz, 6H, H1), 0.67 (d, J = 6.4 Hz, 6H, H6), 0.62 (d, J = 6.3 Hz, 6H,
H1), 0.09 (s, 3H, H13). 13C{1H} NMR (C6D6, 100 MHz, ppm): δ
160.28 (C4), 141.34 (C7), 136.17 (C8b), 135.80 (C8a),133.13 (C10),
129.23 (C9), 129.00 (C9), 44.53 (C2), 43.99 (C5), 24.51 (C1), 24.06
(C6), 21.09 (C11), 19.97 (C12a), 19.40 (C12b), −4.12 (C13). Anal.
Calcd. for C33H55AlN6: C, 70.42; H, 9.85; N, 14.93. Found: C, 70.74;
H, 9.64; N, 15.09.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00319.
■
sı
Supplementary figures referenced in the text, structural
characterization data (1H and 13C NMR spectra) for all
compounds including cyclic carbonates 11a−l, 13a−d,
and X-ray crystallographic data for complexes 4 and 8
(PDF)
Accession Codes
CCDC 2069446 and 2082070 contain the supplementary
crystallographic 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.
AUTHOR INFORMATION
Corresponding Authors
■
Fernando Carrillo-Hermosilla − Departamento de Química
Inorgánica, Orgánica y Bioquímica, Centro de Innovación en
Química Avanzada (ORFEO−CINQA), Facultad de
Ciencias y Tecnologías Químicas, Universidad de Castilla-La
Mancha, 13071 Ciudad Real, Spain; orcid.org/0000-
0002-1187-7719; Email: Fernando.Carrillo@uclm.es
René S. Rojas − Laboratorio de Química Inorgánica, Facultad
de Química, Universidad Católica de Chile, Santiago
6094411, Chile; orcid.org/0000-0002-5292-0816;
Email: rrojasg@uc.cl
Synthesis of Complex 9. A dichloromethane solution of 1 equiv
of trimethylaluminum (22.1 mg, 0.306 mmol) was added to a
dichloromethane solution of 2 equiv of diisopropyl ferrocenyl
guanidine (200 mg, 0.612 mmol). The reaction mixture was stirred
for 2 h at room temperature. All volatiles were removed under
vacuum, and the product was washed with hexane. Complex 9 was
obtained as an orange solid. Yield: 95% (202 mg, 0.291 mmol).
Orange single crystals for X-ray crystallography were grown from cold
pentane at −30 °C. 1H NMR (C6D6, 400 MHz, ppm): δ 5.32 (d, J =
8.1 Hz, 2H, N-H, H4), 4.22 (m, 4H, H8), 4.20 (s, 10H, H10), 3.92 (t, J
= 1.92 Hz, 4H, H9), 3.88 (m, 2H, H5), 3.64 (m, 2H, H2), 1.33 (d, J =
6.3 Hz, 12H, H1), 1.03 (d, J = 6.3 Hz, 12H, H6), −0.15 (s, 3H, H11).
13C{1H} NMR (C6D6, 100 MHz, ppm): δ 161.75 (C3), 104.19 (C7),
68.96 (C10), 64.31 (C9), 62.02 (C8), 45.42 (C2), 45.11 (d, J = 10.1
Hz, C5), 24.67 (C1), 23.89 (d, J = 5.1 Hz, C6), −6.83(C11). Anal.
Calcd. for C35H51AlFe2N6: C, 60.53; H, 7.41; N, 12.10. Found: C,
60.84; H, 7.22; N, 12.23.
General Procedure for Catalyst Screening at 1 bar Pressure.
Styrene oxide 10a (1.66 mmol), free guanidines (L1H2−L4H2) (33.2
μmol), or complexes 1−9 (8.3−33.2 μmol), and TBAI (16.6−33.2
μmol) were placed in a vial containing a magnetic stir bar. The vial
was fitted with a rubber stopper pierced by a balloon filled with CO2.
The reaction mixture was stirred at 70 °C and 1 bar of CO2 pressure
for 24 h. Then, the conversion of styrene oxide 10a into styrene
carbonate 11a was determined by 1H NMR analysis of a sample
relative to the starting styrene oxide.
Authors
Yersica Rios Yepes − Laboratorio de Química Inorgánica,
Facultad de Química, Universidad Católica de Chile,
Santiago 6094411, Chile; orcid.org/0000-0002-9496-
4649
́
Angela Mesías-Salazar − Laboratorio de Química Inorgánica,
Facultad de Química, Universidad Católica de Chile,
Santiago 6094411, Chile
Alexandra Becerra − Laboratorio de Química Inorgánica,
Facultad de Química, Universidad Católica de Chile,
Santiago 6094411, Chile
Constantin G. Daniliuc − Organisch-Chemisches Institut der
Universität Munster, 48149 Munster, Germany;
̈ ̈
orcid.org/0000-0002-6709-3673
Alberto Ramos − Departamento de Química Inorgánica,
Orgánica y Bioquímica, Centro de Innovación en Química
Avanzada (ORFEO−CINQA), Facultad de Ciencias y
Tecnologías Químicas, Universidad de Castilla-La Mancha,
13071 Ciudad Real, Spain; orcid.org/0000-0001-7993-
7864
Rafael Fernández-Galán − Departamento de Química
Inorgánica, Orgánica y Bioquímica, Centro de Innovación en
Química Avanzada (ORFEO−CINQA), Facultad de
Ciencias y Tecnologías Químicas, Universidad de Castilla-La
Mancha, 13071 Ciudad Real, Spain
Antonio Rodríguez-Diéguez − Departamento de Química
Inorgánica, Facultad de Ciencias, Universidad de Granada,
18071 Granada, Spain; orcid.org/0000-0003-3198-
5378
Antonio Antinolo − Departamento de Química Inorgánica,
̃
Orgánica y Bioquímica, Centro de Innovación en Química
Avanzada (ORFEO−CINQA), Facultad de Ciencias y
General Procedure for Catalyst Screening at 8.5 bar
Pressure. Cyclopentane oxide 12a (1.66 mmol), complex 6 (8.3−
33.2 μmol), and TBAI (16.6−66.4 μmol) were placed in a stainless-
steel reactor with a magnetic stirrer bar, and it was pressurized to 8.5
bar. The reaction mixture was stirred at 70−90 °C for 24 h; then the
conversion of cyclopentane oxide 12a into cyclopentane carbonate
13a was determined by analysis of a sample by 1H NMR
spectroscopy.
H
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
A.; García-Vivó, D. Insertion Reactions of Small Unsaturated
Molecules in the N-B bonds of Boron Guanidinates. Dalton Trans.
2017, 46, 10281−10299.
Tecnologías Químicas, Universidad de Castilla-La Mancha,
13071 Ciudad Real, Spain; orcid.org/0000-0002-4417-
6417
(13) Moreno, S.; Ramos, A.; Carrillo-Hermosilla, F.; Rodríguez-
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00319
̌
Diéguez, A.; García-Vivó, D.; Fernández-Galán, R.; Antinolo, A.
Selective Three-Component Coupling for CO2 Chemical Fixation to
Boron Guanidinato Compounds. Inorg. Chem. 2018, 57, 8404−8413.
(14) Dagorne, S.; Wehmschulte, R. Recent Developments on the
Use of Group 13 Metal Complexes in Catalysis. ChemCatChem 2018,
10, 2509−2520.
Notes
The authors declare no competing financial interest.
(15) Mason, M. R. Aluminum-Based Catalysis. In Encyclopedia of
Inorganic and Bioinorganic Chemistry; Scott, R. A., Ed.; John Wiley:
Chichester, U.K., 2015.
ACKNOWLEDGMENTS
■
This work was supported by FONDECYT, Project No.
1200748. Y.R.Y. acknowledges funding from Pontificia
Universidad Católica de Chile via a VRI 2015 - 2017
fellowship and Ph.D. CONICYT 2018 fellowship No.
21182120. A.M.-S. acknowledges funding from Ph.D. CON-
ICYT fellowship No. 51150322. The authors also gratefully
acknowledge financial support from the Ministerio de Ciencia
e Innovación, Spain (grant numbers CTQ2016-77614-P and
RED2018-102387-T) and “Plan Propio de I + D + i” of the
Universidad Castilla-La Mancha (grant number 2020-GRIN-
29078).
(16) Nikonov, G. I. New Tricks for an Old Dog: Aluminum
Compounds as Catalysts in Reduction Chemistry. ACS Catal. 2017, 7
(10), 7257−726.
(17) Cokoja, M.; Bruckmeier, C.; Rieger, B.; Herrmann, W. A.;
Kuhn, F. E. Transformation of Carbon Dioxide with Homogeneous
̈
Transition-Metal Catalysts: A Molecular Solution to a Global
Challenge? Angew. Chem., Int. Ed. 2011, 50, 8510−8537.
(18) Omae, I. Recent Developments in Carbon Dioxide Utilization
for the Production of Organic Chemicals. Coord. Chem. Rev. 2012,
256, 1384−1405.
(19) Maeda, C.; Miyazaki, Y.; Ema, T. Recent Progress in Catalytic
Conversions of Carbon Dioxide. Catal. Sci. Technol. 2014, 4, 1482−
1497.
(20) Liu, Q.; Wu, L.; Jackstell, R.; Beller, M. Using Carbon Dioxide
as a Building Block in Organic Synthesis. Nat. Commun. 2015, 6,
5933.
REFERENCES
■
́
̊
̌
̌
(1) Chlupaty, T.; Ruzicka, A. Hybrid amidinates and guanidinates of
main group metals. Coord. Chem. Rev. 2016, 314, 103−113.
(2) Edelmann, F. T. Recent Progress in the Chemistry of Metal
Amidinates and Guanidinates: Syntheses, Catalysis and Materials.
Adv. Organomet. Chem. 2013, 61, 55−374.
(21) Tappe, N. A.; Reich, R. M.; D’Elia, V.; Kuhn, F. E. Current
̈
Advances in the Catalytic Conversion of Carbon Dioxide by
Molecular Catalysts: an Update. Dalton Trans. 2018, 47, 13281−
13313.
(3) Edelmann, F. T. Lanthanide Amidinates and Guanidinates: from
Laboratory Curiosities to Efficient Homogeneous Catalysts and
Precursors for Rare-Earth Oxide Thin Films. Chem. Soc. Rev. 2009, 38,
2253−2268.
(22) Guo, L.; Lamb, K. J.; North, M. Recent developments in
organocatalysed transformations of epoxides and carbon dioxide into
cyclic carbonates. Green Chem. 2021, 23, 77−118.
(23) Sakakura, T.; Kohno, K. The Synthesis of Organic Carbonates
from Carbon Dioxide. Chem. Commun. 2009, 1312−1330.
(24) Martín, C.; Fiorani, G.; Kleij, A. W. Recent Advances in the
Catalytic Preparation of Cyclic Organic Carbonates. ACS Catal. 2015,
5, 1353−1370.
(25) North, M.; Pasquale, R.; Young, C. Synthesis of Cyclic
Carbonates from Epoxides and CO2. Green Chem. 2010, 12, 1514−
1539.
(26) Kamphuis, A. J.; Picchioni, F.; Pescarmona, P.v P. CO2-Fixation
into Cyclic and Polymeric Carbonates: Principles and Applications.
Green Chem. 2019, 21, 406−448.
(27) Clements, J. H. Reactive Applications of Cyclic Alkylene
Carbonates. Ind. Eng. Chem. Res. 2003, 42, 663−674.
(28) Yun, J.; Zhang, L.; Qu, Q.; Liu, H.; Zhang, X.; Shen, M.; Zheng,
H. A Binary Cyclic Carbonates-Based Electrolyte Containing
Propylene Carbonate and Trifluoropropylene Carbonate for 5V
Lithium-Ion Batteries. Electrochim. Acta 2015, 167, 151−159.
(29) Schäffner, B.; Schäffner, F.; Verevkin, S. P.; Börner, A. Organic
Carbonates as Solvents in Synthesis and Catalysis. Chem. Rev. 2010,
110, 4554−4581.
(30) Clegg, W.; Harrington, R. W.; North, M.; Pasquale, R. Cyclic
Carbonate Synthesis Catalysed by Bimetallic Aluminium-Salen
Complexes. Chem. - Eur. J. 2010, 16, 6828−6843.
(31) Tian, D.; Liu, B.; Gan, Q.; Li, H.; Darensbourg, D. J. Formation
of Cyclic Carbonates from Carbon Dioxide and Epoxides Coupling
Reactions Efficiently Catalyzed by Robust, Recyclable One-
Component Aluminum-Salen Complexes. ACS Catal. 2012, 2,
2029−2035.
(32) Whiteoak, C. J.; Kielland, N.; Laserna, V.; Castro-Gomez, F.;
Martin, E.; Escudero-Adan, E. C.; Bo, C.; Kleij, A. W. Highly Active
Aluminium Catalysts for the Formation of Organic Carbonates from
CO2 and Oxiranes. Chem. - Eur. J. 2014, 20, 2264−2275.
(4) Bailey, P. J.; Pace, S. The Coordination Chemistry of Guanidines
and Guanidinates. Coord. Chem. Rev. 2001, 214, 91−141.
̌
(5) Alonso-Moreno, C.; Antinolo, A.; Carrillo-Hermosilla, F.; Otero,
A. Guanidines: from Classical Approaches to Efficient Catalytic
Syntheses. Chem. Soc. Rev. 2014, 43, 3406−3425.
(6) Zhang, W.-X.; Xu, L.; Xi, Z. Recent Development of Synthetic
Preparation Methods for Guanidines via Transition Metal Catalysis.
Chem. Commun. 2015, 51, 254−265.
(7) Alonso-Moreno, C.; Carrillo-Hermosilla, F.; Garcés, A.; Otero,
̌
A.; López-Solera, I.; Rodríguez, A. M.; Antinolo, A. Simple, Versatile,
and Efficient Catalysts for Guanylation of Amines. Organometallics
2010, 29, 2789−2795.
(8) Elorriaga, D.; Carrillo-Hermosilla, F.; Antin
̃
olo, A.; López-Solera,
I.; Menot, B.; Fernández-Galán, R.; Villasen
̃
or, E.; Otero, A. New
Alkylimido Niobium Complexes Supported by Guanidinate Ligands:
Synthesis, Characterization, and Migratory Insertion Reactions.
Organometallics 2012, 31, 1840−1848.
(9) Fernández-Galán, R.; Antin
López-Solera, I.; Otero, A.; Serrano-Laguna, A.; Villasen
̃
olo, A.; Carrillo-Hermosilla, F.;
or, E.
̃
Migratory Insertion Reactions in Asymmetrical Guanidinate-Sup-
ported Zirconium Complexes. Organometallics 2012, 31, 8360−8369.
(10) Nieto, D.; Bruna, S.; González-Vadillo, A. M.; Perles, J.;
̃
Carrillo-Hermosilla, F.; Antinolo, A.; Padrón, J. M.; Plata, G. B.;
̃
Cuadrado, I. Catalytically Generated Ferrocene-Containing Guani-
dines as Efficient Precursors for New Redox-Active Heterometallic
Platinum(II) Complexes with Anticancer Activity. Organometallics
2015, 34, 5407−5417.
(11) De Tullio, M.; Hernán-Gómez, A.; Livingstone, Z.; Clegg, W.;
̃
Kennedy, A. R.; Harrington, R. W.; Antinolo, A.; Martínez, A.;
Carrillo-Hermosilla, F.; Hevia, E. Structural and Mechanistic Insights
into s-Block Bimetallic Catalysis: Sodium Magnesiate-Catalyzed
Guanylation of Amines. Chem. - Eur. J. 2016, 22, 17646−17656.
(12) Ramos, A.; Antin
Galán, R.; Montero-Rama, M. P.; Villasen
̃
olo, A.; Carrillo-Hermosilla, F.; Fernández-
or, E.; Rodríguez-Diéguez,
̃
I
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
(33) North, M.; Quek, S. C. Z.; Pridmore, N. E.; Whitwood, A. C.;
Wu, X. Aluminum(salen) Complexes as Catalysts for the Kinetic
Resolution of Terminal Epoxides via CO2 Coupling. ACS Catal. 2015,
5, 3398−3402.
(34) Martínez, J.; Castro-Osma, J. A.; Earlam, A.; Alonso-Moreno,
C.; Otero, A.; Lara-Sánchez, A.; North, M.; Rodríguez-Diéguez, A.
Synthesis of Cyclic Carbonates Catalysed by Aluminium Hetero-
scorpionate Complexes. Chem. - Eur. J. 2015, 21, 9850−9862.
(35) Cozzolino, M.; Press, K.; Mazzeo, M.; Lamberti, M. Carbon
Dioxide/Epoxide Reactions Catalyzed by Bimetallic Salalen Alumi-
num Complexes. ChemCatChem 2016, 8, 455−460.
(36) Rintjema, J.; Kleij, A. W. Aluminum-Mediated Formation of
Cyclic Carbonates: Benchmarking Catalytic Performance Metrics.
ChemSusChem 2017, 10, 1274−1282.
(37) Martínez, J.; Castro-Osma, J. A.; Alonso-Moreno, C.;
Rodríguez-Diéguez, A.; North, M.; Otero, A.; Lara-Sánchez, A.
One-Component Aluminum(heteroscorpionate) Catalysts for the
Formation of Cyclic Carbonates from Epoxides and Carbon Dioxide.
ChemSusChem 2017, 10, 1175−1185.
(38) Kim, Y.; Hyun, K.; Ahn, D.; Kim, R.; Park, M. H.; Kim, Y.
Efficient Aluminum Catalysts for the Chemical Conversion of CO2
into Cyclic Carbonates at Room Temperature and Atmospheric CO2
Pressure. ChemSusChem 2019, 12, 4211−4220.
(39) Melendez, D. O.; Lara-Sanchez, A.; Martinez, J.; Wu, X.; Otero,
A.; Castro-Osma, J. A.; North, M.; Rojas, R. S. Amidinate Aluminium
Complexes as Catalysts for Carbon Dioxide Fixation into Cyclic
Carbonates. ChemCatChem 2018, 10, 2271−2277.
(40) Rios Yepes, Y.; Quintero, C.; Osorio Meléndez, D.; Daniliuc, C.
G.; Martínez, J.; Rojas, R. S. Cyclic Carbonates from CO2 and
Epoxides Catalyzed by Tetra- and Pentacoordinate Amidinate
Aluminum Complexes. Organometallics 2019, 38, 469−478.
(41) Rios Yepes, Y.; Martínez, J.; Rangel Sánchez, H.; Quintero, C.;
(51) Koller, J.; Bergman, R. G. Synthesis, Characterization, and
Reactivity of Aluminum Alkyl/Amide Complexes Supported by
Guanidinate and Monoanionic OCO-Pincer Ligands. Organometallics
2010, 29, 3350−3356.
(52) Han, H.-F.; Guo, Z.-Q.; Zhang, S.-F.; Li, J.; Wei, X.-H.
Guanidinato Aluminum Complexes: Synthesis, Crystal Structures and
Reactivities. RSC Adv. 2016, 6, 101437−101446.
(53) Riddlestone, I. M.; Urbano, J.; Phillips, N.; Kelly, M. J.; Vidovic,
D.; Bates, J. I.; Taylor, R.; Aldridge, S. Salt Metathesis for the
Synthesis of M-Al and M-H-Al Bonds. Dalton Trans. 2013, 42, 249−
258.
(54) Han, H.-F.; Zhang, S.-F.; Guo, Z.-Q.; Tong, H.-B.; Wei, X.-H.
Three Asymmetric Guanidinato Metal Complexes: Synthesis, Crystal
Structures and their Use as Pre-Catalysts in the Meerwein-Ponndorf-
Verley reduction. Polyhedron 2015, 99, 71−76.
(55) Bonyhady, S.; Collis, D.; Frenking, G.; Holzmann, N.; Jones,
C.; Stasch, A. Synthesis of a Stable Adduct of Dialane(4) (Al2H4) via
Hydrogenation of a Magnesium(I) Dimer. Nat. Chem. 2010, 2, 865−
869.
(56) Noor, A.; Bauer, T.; Todorova, T. K.; Weber, B.; Gagliardi, L.;
Kempe, R. The Ligand Based Quintuple Bond-Shortening Concept
and Some of its Limitations. Chem. - Eur. J. 2013, 19, 9825−9832.
(57) Cole, M. L.; Davies, A. J.; Jones, C.; Junk, P. C.; McKay, A. I.;
Stasch, A. Aluminum and Indium Complexes Derived from
Guanidines, Triazenes, and Amidines. Z. Anorg. Allg. Chem. 2015,
641, 2233−2244.
(58) Peddarao, T.; Baishya, A.; Hota, S. K.; Nembenna, S. Bimetallic
Aluminum Alkyl and Iodide Complexes Stabilized by a Bulky Bis-
Guanidinate Ligand. J. Chem. Sci. 2018, 130, 97−114.
(59) Peddarao, T.; Sarkar, N.; Nembenna, S. Mono- and Bimetallic
Aluminum Alkyl, Alkoxide, Halide and Hydride Complexes of a Bulky
Conjugated Bis-Guanidinate (CBG) Ligand and Aluminum Alkyls as
Precatalysts for Carbonyl Hydroboration. Inorg. Chem. 2020, 59,
4693−4702.
Ortega-Alfaro, M. C.; López-Cortés, J. G.; Daniliuc, C. G.; Antinolo,
̃
A.; Ramos, A.; Rojas, R. S. Aluminum Complexes with New Non-
Symmetric Ferrocenyl Amidine Ligands and their Application in CO2
Transformation into Cyclic Carbonates. Dalton Trans. 2020, 49,
1124−1134.
(60) Rösch, A.; Seifert, F.; Vass, V.; Görls, H.; Kretschmer, R.
Synthesis, Structure, and Catalytic Activity of Dinuclear Aluminium
Bis(amidinate) and Bis(guanidinate) Complexes. New J. Chem. 2021,
45, 972−981.
(42) Saltarini, S.; Villegas-Escobar, N.; Martínez, J.; Daniliuc, C. G.;
Matute, R. A.; Gade, L. H.; Rojas, R. S. Toward a Neutral Single-
Component Amidinate Iodide Aluminum Catalyst for the CO2
Fixation into Cyclic Carbonates. Inorg. Chem. 2021, 60, 1172−1182.
(43) Aeilts, S. L.; Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young,
V. G. Aluminum Alkyl Complexes Containing Guanidinate Ligands.
Organometallics 1998, 17, 3265−3270.
(44) Chang, C.-C.; Hsiung, C.-S.; Su, H.-L.; Srinivas, B.; Chiang, M.
Y.; Lee, G.-H.; Wang, Y. Carbodiimide Insertion into Organo-
aluminum Compounds and Thermal Rearrangement of the Products.
Organometallics 1998, 17, 1595−1601.
(45) Kenney, A. P.; Yap, G. P. A.; Richeson, D. S.; Barry, S. T. The
Insertion of Carbodiimides into Al and Ga Amido Linkages.
Guanidinates and Mixed Amido Guanidinates of Aluminum and
Gallium. Inorg. Chem. 2005, 44, 2926−2933.
(46) Rowley, C. N.; DiLabio, G. A.; Barry, S. T. Theoretical and
Synthetic Investigations of Carbodiimide Insertions into Al-CH3 and
Al-N(CH3)2 Bonds. Inorg. Chem. 2005, 44, 1983−1991.
(47) Ziffle, L. C.; Kenney, A. P.; Barry, S. T.; Muller, J. Thermal
Fragmentation of the Guanidinato Aluminum Amide Precursor
[Me2NC(NiPr)2]Al(NMe2)2: An Investigation of Reactive Species
by Matrix-Isolation FTIR Spectroscopy and Time-of-Flight Mass
Spectrometry. Polyhedron 2008, 27, 1832−1840.
(48) Zhang, W.-X.; Li, D.; Wang, Z.; Xi, Z. Alkyl Aluminum-
Catalyzed Addition of Amines to Carbodiimides: A Highly Efficient
Route to Substituted Guanidines. Organometallics 2009, 28, 882−887.
(49) Han, H.; Guo, Z.; Zhang, S.; Hua, Y.; Wei, X. Synthesis and
Crystal Structures of Guanidinato Aluminum Complexes and
Catalytic Study for MPV reduction. Polyhedron 2017, 126, 214−219.
(50) Koller, J.; Bergman, R. G. Highly Efficient Aluminum-Catalyzed
Hydroamination/-hydrazination of Carbodiimides. Organometallics
2010, 29, 5946−5952.
(61) Huergo, E.; Fernández-Galán, R.; Ramos, A.; Antinolo, A.;
̃
Carrillo-Hermosilla, F.; Rodríguez-Diéguez, A.; García-Vivó, D.
Reactivity of N-Phosphinoguanidines of the Formula (HNR)-
(Ph2PNR)C(NAr) toward Main Group Metal Alkyls: Facile Ligand
Rearrangement from N-Phosphinoguanidinates to Phosphinimine-
Amidinates. Inorg. Chem. 2020, 59, 15262−15275.
(62) Fernández-Galán, R.; Ramos, A.; Huergo, E.; Antinolo, A.;
̃
Carrillo-Hermosilla, F.; Rodríguez-Diéguez, A.; García-Vivó, D.
Unusual Ligand Rearrangement: from N-Phosphinoguanidinato
toPhosphinimine Amidinato Compounds. Chem. Commun. 2019,
55, 2809−2812.
(63) Mesías-Salazar, A.; Martínez, J.; Rojas, R. S.; Carrillo-
̃
Hermosilla, F.; Ramos, A.; Fernández-Galán, R.; Antinolo, A.
Aromatic Guanidines as Highly Active Binary Catalytic Systems for
the Fixation of CO2 into Cyclic Carbonates under Mild Conditions.
Catal. Sci. Technol. 2019, 9, 3879−3886.
(64) Munoz-Hernandez, M. A.; Keizer, T. S.; Wei, P.; Parkin, S.;
̃
Atwood, D. A. Reactivity and Derivatization of Five-Coordinate,
Chelated Aluminum. Inorg. Chem. 2001, 40, 6782−6787.
(65) Muller, J.; Boese, R. Molecular structure of the azidoalane
[Me2N(CH2)3]AltBu(N3). X-ray structural determination and ab
initio calculations. J. Mol. Struct. 2000, 520, 215−219.
(66) Lee, K. W.; Chung, J. W.; Kwak, S.-Y. Highly Branched
Polycaprolactone/Glycidol Copolymeric Green Plasticizer by One-
PotSolvent-Free Polymerization. ACS Sustainable Chem. Eng. 2018, 6,
9006−9017.
(67) Utrata-Wesołek, A.; Zymełka-Miara, I.; Kowalczuk, A.;
Trzebicka, B.; Dworak, A. Photocrosslinking of Polyglycidol and
ItsDerivative: Route to Thermoresponsive Hydrogels. Photochem.
Photobiol. 2018, 94, 52−60.
J
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
(68) Nishikawa, D.; Nakajima, T.; Ohzawa, Y.; Koh, M.; Yamauchi,
A.; Kagawa, M.; Aoyama, H. Thermal and oxidation stability of
organofluorine compound-mixed electrolyte solutions for lithium-ion
batteries. J. Power Sources 2013, 243, 573−580.
(69) Butler, D. C.D; Richards, C. J. Synthesis of 1′-Substituted
Derivatives of 1,2,3,4,5-Pentaphenylferrocene. Organometallics 2002,
21, 5433−5436.
K
https://doi.org/10.1021/acs.organomet.1c00319
Organometallics XXXX, XXX, XXX−XXX