Toward a Neutral Single-Component Amidinate Iodide Aluminum Catalyst for the CO2Fixation into Cyclic Carbonates

Catalyst for the CO Fixation into Cyclic Carbonates

Article

Toward a Neutral Single-Component Amidinate Iodide Aluminum

Sebastian

Lutz H. Gade,* and Rene

Saltarini, Nery Villegas-Escobar, Javier Mart í nez, Constantin G. Daniliuc, Ricardo A. Matute,

S. Rojas*

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sı Supporting Information

ABSTRACT: A new iodide aluminum complex ({AlI(κ -naph-

bam)}, 3) supported by a tetradentate amidinate ligand derived

from a naphthalene-1,8-bisamidine precursor (naphbamH, 1) was

obtained in quantitative yield via reaction of the corresponding

methyl aluminum complex ({AlMe(κ -naphbam)}, 2) with 1 equiv

of I in CH Cl at room temperature. Complexes 2 and 3 were

tested and found to be active as catalysts for the cyclic carbonate

formation from epoxides at 80 °C and 1 bar of CO pressure. A

ﬁrst series of experiments were carried out with 1.5 mol % of the

alkyl complex 2 and 1.5 mol % of tetrabutylammonium iodide

(

TBAI) as a cocatalyst; subsequently, the reactions were carried

out with 1.5 mol % of iodide complex 3 as a single-component

catalyst. Compound 3 is one of the ﬁrst examples of a

nonzwitterionic halide single-component aluminum catalyst producing cyclic carbonates. The full catalytic cycle with

characterization of all minima and transition states was characterized by quantum chemistry calculations (QCCs) using density

functional theory. QCCs on the reaction mechanism support a reaction pathway based on the exchange of the iodine contained in

−

the catalyst by 1 equiv of epoxide, with subsequent attack of I to the epoxide moiety producing the ring opening of the epoxide.

QCCs triggered new insights for the design of more active halide catalysts in future explorations of the ﬁeld.

INTRODUCTION

Lewis acids in the activation of the epoxides for their

transformation to cyclic carbonates. The choice of the metal

center is essential, and aluminum, an abundant element in the

earth’s crust, is one of the most suited metals for cyclic

■

The interest in the use and capture of CO has grown

enormously over the past few years

nontoxic, and abundant renewable source which can be

employed as C1 chemical feedstock for the preparation of

−4

as an inexpensive,

33−36

carbonates formation.

Moreover, regarding the use of

ligands, Schiﬀ base ligands have been widely investigated,

high added value products such as methanol, formic acid,

37,38

9−11

promoting increasing reactivity of the catalysts.

Among the

lactams, and cyclic carbonates.

Among these trans-

catalytic systems reported in the literature, the bimetallic

aluminum(salen) complex (Figure 1a) was found to be an

excellent catalyst for the preparation of cyclic carbonates under

formations, the cyclization of epoxides and CO to prepare

cyclic carbonates (Scheme 1) has been extensively explored due

to their role as intermediates in the production of ﬁne

2−14

15,16

mild reaction conditions. Kleij and co-workers reported an

aluminum catalyst supported by a tripodal amino phenolate

ligand (Figure 1b) which displayed remarkable catalytic activity

for a wide range of diﬀerent epoxides toward the selective

chemicals,

as electrolytes in lithium-ion batteries,

and

as green solvents.

A large number of catalytic systems have been developed for

the preparation of cyclic carbonates including organocata-

8−21

22−32

formation of cyclic carbonates. We recently reported a

trinuclear amidinate aluminum complex (Figure 1c) which

lysts

and metal complexes.

These generally act as

was found also to be active for this kind of transformation.

Scheme 1. General Synthesis of Cyclic Carbonates from

Substituted Epoxide and CO₂

Received: November 5, 2020

Published: December 29, 2020

2020 American Chemical Society

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Article

Figure 1. Double-component (a−c) or one-component (d−f) zwitterionic aluminum catalysts for the preparation of cyclic carbonates.

Scheme 2. Synthesis of the Amidinate Ligand Precursor 1

Although all of these species are active catalysts for the

synthesis of cyclic carbonates, they do require a nucleophile

source as a cocatalyst. In the quest for single-component

catalysts, zwitterionic aluminum complexes have been proposed

which already incorporate the nucleophile in their structure.

Interestingly, in some cases, these catalysts can be more eﬀective

than neutral two-component systems due to cooperative eﬀects

among the functional groups which give rise to increasing

behind this work is toward the formation of neutral single-

component catalysts by employing a nucleophile already

contained in the molecular structure of the catalyst. Due to

the relatively long Al−I distance and the pronounced mismatch

in chemical hardness, dissociation of the Al−I bond (for instance

promoted by an exchange reaction) can take place more easily

comparing other Al−X (X = F, Cl, Br) bonds without the need of

a cocatalyst. Indeed, experimental and computational evidence

presented in this work shows that the release of the iodide takes

place even at room temperature.

1−48

selectivity and catalytic activity.

Notably, such one-

component aluminum complexes were reported by North and

46,47

co-workers (Figure 1d), displaying excellent activities,

as has

been the aluminum(salen) complex e in Figure 1 bearing

RESULTS AND DISCUSSION

■

appended quaternary ammonium salt as substituents. Finally,

a bifunctional aluminum complex supported by a hetero-

scorpionate ligand was found to eﬃciently catalyze the

Synthesis and Structural Characterization of Ligand

Precursor 1. The one-step synthesis of the new bis(amidine)

ligand precursor 1 is shown in Scheme 2. The N′,N′′-

generation of bioderived cyclic carbonates (Figure 1f).

(

naphthalene-1,8-diyl)bis(N-(2,6-dimethylphenyl)-

Given these activities aimed at single-component catalytic

systems, we focused on the development of a neutral

acetimidamide) ligand (1) was prepared by the reaction of 1,8-

diaminonaphthalene with 2 equiv of (E)-N-(2,6-

dimethylphenyl)acetimidoyl chloride and Et N in dry toluene

(nonzwitterionic) and bifunctional iodide aluminum catalyst

for the formation of cyclic carbonates from epoxides and CO in

under reﬂux (Scheme 2) according to a modiﬁed protocol with

the absence of a cocatalyst. To the best of our knowledge, there

are only a few examples of neutral aluminum complexes that are

able to catalyze the formation of cyclic carbonates without

respect to that previously reported. After the appropriate

workup, compound 1 was isolated as an orange solid in 76%

yield.

49,50

adding a cocatalyst,

and in particular, only two examples of

The molecular structure of ligand precursor 1 was

characterized by mass spectrometry (MS), nuclear magnetic

nonzwitterionic halide aluminum complexes have been reported

to date that achieve good catalytic transformation and selectively

resonance (NMR) spectroscopy ( H NMR, C{ H} NMR, H

51,52

by themselves in the absence of a cocatalyst.

The concept

NOESY-1D, H− C g-HSQC), and X-ray diﬀraction. In the H

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Inorg. Chem. 2021, 60, 1172−1182

Inorganic Chemistry

of compound 1 shown in Figure 2 , in which the presence of an

Article

NMR spectrum of this compound, two resonances at 11.14 and

crystalline solid in quantitative yield and characterized by NMR

and MS. In the H NMR spectrum of this complex, the

.78 ppm are assigned to the N−H groups as previously

54,55

observed for other bis(amidine) ligands.

The low ﬁeld shift

disappearance of the resonance corresponding to the N−H

group of the ligand precursor 1 con ﬁ rms the formation of the

amidinate complex 2 (see page S5 of the Supporting

of one of these resonances (11.14 ppm) arises due to the

formation of an intramolecular hydrogen bond (HB) between

the N−H group and the nitrogen atom of the amidine moiety.

The presence of the HB is conﬁrmed in the solid-state structure

Information ), while both the H and C{ H} NMR resonance

patterns support the existence of an approximate plane of

symmetry in the molecule. Additionally, the signal at −0.52 ppm

in the H NMR spectrum corresponds to the methyl group

coordinated to the aluminum atom. The spectroscopic evidence

reveals a pentacoordinate coordination geometry for the

aluminum center with the amidinate ligand adopting a κ -

NNNN coordination mode.

With the aim of obtaining a single-component catalytic system

to be active without the need of a cocatalyst, we decided to

replace the methyl group by an iodide ligand at the aluminum

center. This was accomplished by the reaction of complex 2 with

equiv of I in CH Cl (Scheme 4) according to a method

2 2 2

Scheme 4. Synthesis of Iodide Aluminum Complex 3

Figure 2. Molecular structure of ligand 1. Only one molecule (molecule

“A”) is shown. Thermal ellipsoids are set at 30% probability. Selected

bond distances are reported in Å, and angles are reported in deg: N1−

C1 1.286(3), N2−C1 1.373(3), N3−C3 1.356(3), N4−C3 1.298(3),

N2−H2···N4 1.88(3), C3−N4−C39 121.7(2), C3−N3−C21

122.8(2).

recently published by Nembenna and co-workers. Compound

was isolated in high yield (96%) and characterized

intramolecular hydrogen bond N−H···N was found in both

crystallographically independent molecules “A” and “B”

spectroscopically and by X-ray diﬀraction. The clean conversion

of complex 2 to 3 can be readily followed by H NMR,

[

molecule “A”: N2A−H2A···N4A 1.88(3) Å, 144(2)°; molecule

monitoring the disappearance of the signal corresponding to the

methyl group at the aluminum center. Furthermore, the H

NMR resonance pattern of 3 is consistent with an eﬀective C_s

symmetry on the NMR time scale as also found for complex 2

“

B”: N2B−H2B···N4B 1.94(3) Å, 150(2)°]. These two

independent molecules of 1 present similar structural data.

For this reason, only one molecule (“A”) is discussed (see the

Supporting Information for further details).

(The exact point group of both 2 and 3 being C .).

The two NCN amidine fragments of this compound are

notably diﬀerent; whereas the bond length N1−C1 (1.286(3)

Å) is comparable to a normal double CN bond, the distance

N2−C1 is signiﬁcantly longer (1.373(3) Å) and closer to that of

a single C−N bond. In the other NCN fragment, the inverse

situation is observed, the bond N3−C3 (1.356(3) Å) being

longer than N4−C3 (1.298(3) Å).

Compounds 2 and 3 crystallize in a P 2 /c space group (see

Tables S4 and S5 in the Supporting Information ). The crystal

structures of catalysts 2 and 3 are depicted in Figures 3 and 4,

respectively. For pentacoordinate structures, two high-symme-

try molecular geometries are possible, the trigonal bipyramid

57,58

and the square-based pyramid. The τ index

is generally

employed to characterize intermediate coordination geometries:

τ = 0 corresponds to perfect square-pyramidal geometry,

whereas τ = 1 indicates an ideal trigonal-bipyramidal structure.

The observed τ values computed from the X-ray data for

complexes 2 and 3, 0.05 and 0.14, respectively, are indicative of

distorted square-based pyramid geometries for the pentacoordi-

Synthesis and Structural Characterization of Com-

plexes 2 and 3. The preparation of the alkyl aluminum

complex 2 was carried via protonolysis of the ligand precursor 1

with 1 equiv of AlMe in dry CH Cl at room temperature

(Scheme 3). The resulting complex 2 was isolated as a yellow

Scheme 3. Synthesis of Alkyl Aluminum Complex 2

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Inorg. Chem. 2021, 60, 1172−1182

Article

of pentacoordinated amidinate aluminum complexes reported

to date displays distorted trigonal bipyramidal geome-

33,59−62

tries.

In contrast, for complexes 2 and 3, the distorted

square pyramidal geometry appears to be favored by the

orientation of the two amidinate units in the tetradentate ligand

which is enforced by the rigid naphthalene backbone.

Optimized molecular geometries for catalysts 2 and 3 were

computed at the M06-2X/def2-TZVP/LANL2DZ(I) level of

theory, and although optimized DFT minima may diﬀer from

the crystal structures, a good match for both bond lengths and

bond angles was found (the corresponding data are provided in

square brackets in Figures 3 and 4). Notably, the Al1−I1 bond

length in complex 3 obtained by DFT studies 2.541 Å is close to

the observed 2.553(1) Å, with similar distances having been

observed in related compounds containing Al−I bonds.

bond order of 0.87 was found for the Al−I bond, indicating

partial covalency, whereas the relatively large Al−I bond

distance is expected to render the iodide an eﬃcient leaving

“

group/atom” in a single-component catalyst.

Catalytic Performance of 2 and 3 for the Synthesis of

Cyclic Carbonates. Having prepared and characterized

complexes 2 and 3, we explored their potential use as catalysts

for the preparation of a range of cyclic carbonates (5a−h) from

Figure 3. Molecular structure of complex 2. Thermal ellipsoids are

shown with 30% probability. Selected bond distances are reported in Å,

and angles are reported in deg. Structural parameters from DFT

calculations (M06-2X/def2-TZVP/LANL2DZ(I)) are given in

squared brackets: Al1−N2 1.939(1) [1.936], Al1−N3 1.999(1)

their corresponding epoxides (4a−h) and CO . The con-

versions and yields of the catalytic runs are shown in Table 1.

[

1.966], Al1−N4 1.942(1) [1.976], Al1−N1 1.992(11) [2.029],

Al1−C5 1.964(1) [1.959]; N4−Al1−C5 112.2(1) [110.5], N3−

Al1−C5 113.2(1) [116.7], N1−Al1−N4 134.6(1) [140.6], N2−Al1−

N3 131.9(1) [119.0].

Table 1. Conversions and Isolated Yields of Cyclic

Carbonates 5a−h Employing Catalysts 2 and 3

3 conv [%]

conv [%]

entry

epoxide

4a (R = Ph)

4b (R = n-Bu)

(yield, %)

98 (93)

94 (89)

92 (86)

100 (92)

72 (65)

71 (66)

88 (81)

92 (88)

50 (45)

15 (−)

90 (82)

100 (89)

12 (−)

0 (−)

4c (R = CH Cl)

4d (R = CH OPh)

4e (R = 4-ClC H )

4f (R = 4-BrC H )

4g (R = CH OCH CF CHF )

10 (−)

0 (−)

4h (R =

CH OCH (CF ) CHF )

Reactions were carried out at 80 °C and 1 bar of CO pressure for 24

h using 1.5 mol % of complex 2 and 1.5 mol % of TBAI or 1.5 mol %

of complex 3 under solvent-free conditions. Determined by H NMR

spectroscopy of the crude reaction mixture. Yield of pure isolated

cyclic carbonate.

Figure 4. Molecular structure of complex 3. Thermal ellipsoids are

shown with 50% probability. Selected bond distances are reported in Å,

and angles are reported in deg. Structural parameters from DFT

calculations (M06-2X/def2-TZVP/LANL2DZ(I)) are given in

squared brackets: Al1−N2 1.918(2) [1.920], Al1−N3 1.962(2)

The reactions were carried out at 80 °C and 1 bar of CO₂

pressure for 24 h employing a combination of 1.5 mol % of

complex 2 and 1.5 mol % of tetrabutylammonium iodide

(

TBAI) as a cocatalyst, while only 1.5 mol % of the single-

[

1.941], Al1−N4 1.911(2) [1.939], Al1−N1 1.973(2) [1.982], Al1−

I1 2.553(1) [2.542]; N2−Al1−N3 144.4(1) [121.9], N1−Al1−N4

36.1(1) [148.0], N1−Al1−I1 110.7(1) [102], N2−Al1−I1 107.9(1)

121.2].

component catalyst 3 were used under solvent-free conditions.

Interestingly, no polycarbonates were obtained under these

reaction conditions as expected, since these epoxide substrates

generally showed selectivity toward cyclic carbonate forma-

[

18−48

tion.

In general, catalyst 2 achieved moderate to excellent

nate aluminum centers. The axial site is occupied by the methyl

2) or iodide (3) moiety, while the tetradentate amidinate

ligand appears at the base of the pyramid. Notably, the majority

yields for the preparation of cyclic carbonates 5a−h (Table 1,

entries 1−8). It is worth noting that the mixed catalytic system

(2/TBAI) was reactive toward the formation of a wide variety of

(

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Figure 5. (a) H NMR spectrum of 3 + 4a at 25 °C and t = 0 in a 1:1 (3/4a) molar ratio. (b) H NMR spectrum of 4a in CDCl . (c) H NMR spectrum

of 3 in CDCl₃.

Scheme 5. Proposed Reaction Mechanism for the Conversion of 4a into 5a Using Catalyst 3

Main structures and mechanistic events are depicted.

cyclic carbonates functionalized with alkyl, aryl, halide, and ether

groups, which demonstrates that compound 2 has a broad scope

and is selective for cyclic carbonate formation. Among these

cyclic carbonates, the highly ﬂuorinated products 5g and 5h,

which have barely been studied, were isolated in high yields, 81%

and 88%, respectively (Table 1, entries 7 and 8). These latter

compounds are relevant for their potential use as electrolytes in

ion-lithium batteries.

On the other hand, the neutral aluminum 3 catalyst displayed

selectivity toward certain functionalized epoxides. In particular,

it proved to be suitable in the catalytic conversion of the

64−66

epoxides 4c and 4d with CO giving excellent yields (Table 1,

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Figure 6. Gibbs free energy proﬁle for the insertion of CO into epoxide 4a using complex 3. Relevant structures and transition states are shown. Gibbs

−1

free energies are reported in kcal mol .

entries 3 and 4) in the absence of a cocatalyst. Moderate

conversion and yield were observed for the transformation of 4a

38.6 ppm were observed, corresponding to the CH and CH

groups of the opened epoxide, respectively. This also provided

evidence that the iodide anion attack is directed to the carbon

(Table 1, entry 1), whereas low activity was found for the other

epoxide/CO reactions of Table 1 (entries 2 and 5−8) possibly

atom (C ) with the strongest carbocation character (see page

suggesting that electronic properties of the evaluated epoxides

play a decisive role in the initial pre-equilibrium of epoxide

coordination, in the nucleophilic attack of the epoxide by iodide,

and in the ring-closure step, which are apparently key steps of

these catalytic processes (vide infra). Finally, it is worth

highlighting that these results for 3 as a catalyst provide, to

our knowledge, one of the ﬁrst examples of a nonionic single-

component catalytic system.

S24 in the Supporting Information), as reported in previous

20,67

mechanistic studies.

Our results therefore support the

notion that styrene carbonate (5a) is formed starting by an S_N2

reaction of the iodine (exchanged by the epoxide) at the α-

carbon of epoxide.

Based on these results, we suggest a catalytic cycle for the

formation of styrene carbonate 5a from the styrene oxide 4a and

CO in the presence of catalyst 3, a reaction mechanism which is

We then endeavored to obtain insight into the reaction

mechanism by a combination of theoretical modeling and

experimental studies. Initially, a series of NMR experiments

were performed. The reaction between complex 3 and styrene

oxide (4a) in a 1:1 molar ratio was studied spectroscopically in a

particularly interesting since no cocatalyst (e.g., TBAI) is

needed. For catalyst 3, the nucleophile (iodide) is already

contained in the structure, in contrast to many reported systems

33,39,40

requiring a cocatalyst.

Iodide was selected since it has the

larger Al−I bond distance and the lowest dissociation energy

compared to the chloride and bromide analogues (see the

Supporting Information). The proposed catalytic cycle for the

formation of cyclic carbonates catalyzed by 3 is shown in

Scheme 5, and the corresponding computed free energy proﬁle

is depicted in Figure 6. The reaction mechanism in Scheme 5 can

be generalized for other epoxides.

Young valve NMR tube in CDCl (Figure 5). First, the reaction

of complex 3 (Figure 5c) with 4a (Figure 5b) was examined at

room temperature and involved the immediate coordination of

the epoxide upon dissociation of I⁻with subsequent ring-

opening upon the nucleophilic attack of the iodide, giving rise to

a ring-opened intermediate III (Figure 5a). From the NMR

results in Figure 5a, no free styrene oxide was observed, but the

immediate formation of an equilibrium mixture of the

intermediate species II and III takes place, favoring the ring

In a ﬁrst stage, complex 3 forms a reactive complex (I) with 1

−

equiv of epoxide 4a, with a free energy of −12.6 kcal mol .

‡

Adduct I then dissociates an iodine anion (via TS1 with a ΔG

−1

opened species III. The appearance of two new signals in the H

= 19.7 kcal mol ) giving rise to the ion pair II. The cationic

NMR at 5.18 and 4.57 ppm, respectively (Figure 5a and page

S23 in the Supporting Information ), belonging to the CH and

complex in II is stabilized by favorable π-stacking interactions

between the phenyl moiety (in 4a) and the naphthalene-type

moiety of the ligand. Although dispersion and the favorable Al−

O interaction help stabilizing II, strong CH···CH repulsive

interactions within the epoxide moiety and the ligand prevent

this system from being more stable than I. The second step is the

CH protons of the opened epoxide, conﬁrmed the presence of

intermediate alkoxide species III (Figure 5a). Formation of III

was additionally supported by a DEPT-135 NMR experiment, in

which a negative resonance at 70.0 ppm and a positive signal at

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Inorganic Chemistry

Article

nucleophilic attack of the dissociated iodide to the C carbon

decrease the respective span energy, it is necessary to diminish

the oxophilic nature on Al, thus decreasing the stability of key

intermediate III. On the other hand, inclusion of suitable

(strong) electron withdrawing functional groups in the epoxide

helps to decrease the activation barrier for the ring closure step

(TS4), which involves the nucleophilic attack of oxygen to an

atom in the epoxide to give the ring-opened alkoxide structure

−

III via an activation barrier of 6.1 kcal mol . It is worth

mentioning that in this step, the iodide atom can attack either C_α

(

substituted) or C . Butera et al. reported activation energies of

4.6 and 59.7 kcal mol for the C and C attack (for styrene-

α β

−

epoxide and CO reaction), respectively, with the attack at C

thus being kinetically favored. In situ NMR studies (see page

S25 in the Supporting Information ) corroborate the nucleo-

electron deﬁcient C atom.

CONCLUSIONS

■

philic attack at the C atom with the appearance of a signal at

Methyl (2) and iodide (3) aluminum complexes supported by a

tetradentate amidinate ligand have been synthesized. X-ray

diﬀraction, NMR, and MS studies, supported by theoretical

modeling, have established a pentacoordinate structure for the

aluminum atom with a distorted square planar geometry in

.57 ppm in the H NMR and at 38.6 ppm in the C{ H} NMR

spectrum. The ring-opened structure III is highly exergonic,

most likely due to the loss of the strained three-membered

69,70

epoxide ring. On the basis of the energetic span model,

species III corresponds to the TOF-determining intermediate

TDI), being the lowest (free) energy intermediate along the

which the amidinate ligand is binding in a κ -NNNN

coordination mode to the aluminum center (complexes 2 and

(

reaction pathway in the forward direction. In the absence of

temperature, the reaction stops with the formation of III as

shown by NMR experiments (Figure 5). However, in the

Complex 2 was found to be active for the preparation of eight

cyclic carbonates (5a−h) from their corresponding epoxides

(4a−h) and CO employing tetrabutylammonium iodide

^p^r^e^s^eⁿ^t^r^e^a^c^tⁱ^oⁿ^c^oⁿ^dⁱ^tⁱ^oⁿ^s^,ⁱⁿ^t^e^r^m^e^dⁱ^a^t^e^I^I^I^t^h^eⁿⁱⁿ^s^e^r^t^s^C^O2

forming into the Al−O bond giving the alkylcarbonate species

(

TBAI) as a cocatalyst at 80 °C and 1 bar of CO pressure.

IV.

This catalyst system (2/TBAI) was found to be highly reactive

for the cyclic carbonate formation during this catalytic process.

On the other hand, iodide complex 3 displayed elevated catalytic

activity toward some cyclic carbonates without addition of any

cocatalyst, thus representing one of the ﬁrst examples of a

nonzwitterionic aluminum complex reported to date which is

active toward this transformation under the presented working

conditions. Based on quantum chemical modeling, the reaction

This transformation takes place via a four-membered cyclic

‡

−1

transition state, TS3, with an ΔG = 12.5 kcal mol (see Figure

). After the near-isothermic formation of IV, the ring-closure

takes place with formation of the cyclic carbonate moiety still

coordinated to the aluminum center releasing the iodide. The

‡

−1

free activation energy of this process is ΔG = 27.0 kcal mol ,

with TS4 being the TOF-determining transition state (TD-TS).

Therefore, species III and TS4 are thought to determine the

mechanism for the insertion of CO into cyclic carbonates using

turnover frequency for the catalytic ﬁxation of CO to give cyclic

3 was rationalized. In particular, it was found that the TOF-

determining species are the ring-opened alkoxide structure III

and the transition state TS4 of the ring closure step to yield the

cyclic carbonate attached to the catalyst (V). Furthermore, we

conclude that choice between two alternatives reaction

pathways for the release of the cyclic carbonate depends on a

dynamic equilibrium between the concentration of the epoxide

and the formed cyclic carbonates along the reaction. At higher

concentrations of epoxide, a shortcut in the formation of the

epoxide adduct II is favored. On the other hand, near the end of

the conversion (when the concentration of cyclic carbonate

starts to increase), the equilibrium is shifted toward the

restoration of catalyst 3 concomitant to the release of the

reaction product. Computations of the reaction mechanism

triggered new insights for the design of more active halide

catalysts in future explorations of the ﬁeld. Improvements of

catalyst 3 toward a wider range of epoxides are currently under

active investigations in our laboratory.

−

carbonates with a span energy (δE) of 33.2 kcal mol .

For the formation of cyclic carbonate 5a, we explored the

bifurcation of the reaction path from species V. In particular, the

reaction product 5a could be released from V via TS5 and

displacement by the iodide at the Al center with an activation

−

Gibbs free energy of just 0.6 kcal mol . However, since the

solvent in the reaction corresponds to the epoxide itself (4a for

the present case), the coordination of a second equiv of 4a at the

Al center in V was studied. This was found to give a very stable

−1

intermediate (ΔG = −26.5 kcal mol ), and the concerted

exchange between the cyclic carbonate 5a and epoxide 4a

therefore provides a shortcut in the catalytic cycle (Scheme 5)

yielding species II through TS5′ with a relatively low activation

−

barrier (5.6 kcal mol ).

We expect that at high concentration of epoxide the

mechanism through TS5′ should dominate, while toward the

end of the reaction, when the concentration of cyclic carbonate

predominates, the pathway through TS5 may be favored.

In summary, we have demonstrated that a single-component

EXPERIMENTAL SECTION

■

catalyst for the ﬁxation of CO into cyclic carbonates could be

General Procedures. All manipulations were performed under an

inert atmosphere using standard glovebox and Schlenk-line techniques.

Reagent-grade solvents were obtained from E. Merck. Toluene was

distilled from benzophenone ketyl, and dichloromethane and hexane

were dried using an Innovative Technology Pure Solv Model PS-MD-5.

The compounds 1,8-diaminonaphthalene, AlMe (TMA), I , epoxides,

obtained in the form of the amidinate aluminum complex 3

which provides one of the ﬁrst examples of a nonzwitterionic

1,52

single-component catalyst producing cyclic carbonates.

Although the reaction yields are not still superior to the ones

in which TBAI is employed, eﬀorts are being made in our group

to increase the reactivity of these neutral-component catalysts.

These include, inter alia, the modiﬁcation of the ancillary

bis(amidinate) ligand.

Overall, the selectivity toward certain functionalized epoxides

opens the route to a rational design of new catalysts. From the

computational studies, we have characterized species III and

TS4 as the TOF-determining states. Therefore, in order to

and Bu NI were purchased from Aldrich and used as received. The

imidoyl chloride (E)-N-(2,6-dimethylphenyl)acetimidoyl chloride was

prepared according to published procedures. The following instru-

ments were used for the physical characterization of the compounds. H

and C{ H} NMR spectra were recorded on a Bruker Avance-400

spectrometer. Chemical shifts and the coupling constants are reported

in parts per million (SiMe as standard) and Hertz, respectively. Most of

the NMR assignments were supported by additional 2D experiments,

178

Inorg. Chem. 2021, 60, 1172−1182

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

and the numbers of scans used for ¹³C{ H} NMR ranged from 0.5 to 2

K depending on the sample concentration. Mass spectra were acquired

using a Micro Tof (Bruker) or a Clarus SQ 8T GC/MS (PerkinElmer).

Synthesis of N′,N′′-(Naphthalen-1,8-diyl)bis(N-(2,6-

dimethylphenyl)acetimidamide) (naphbamH) (1). (E)-N-(2,6-

Computational Details. Optimization and frequency computa-

tions of fundamental structures along the potential energy surface were

carried out at the density functional theory (DFT) level of theory. For

geometry optimizations, the B3LYP/6-31G**-LANL2DZ(I) level of

theory was used, which has been successfully applied in related

68,75,76

Dimethylphenyl)acetimidoylchloride (1.0 g, 5.51 mmol) was added

to a solution of 1,8-diaminonaphthalene (0.4 g, 2.76 mmol) and

triethylamine (0.8 mL, 5.80 mmol) in 30 mL of toluene. The reaction

mixture was stirred for 12 h under reﬂux. All volatiles were removed

under vacuum. The solid residue was taken up in 30 mL of Et O and

washed with 15 mL of a saturated solution of Na CO . Then the organic

layer was washed with 20 mL of water and dried over MgSO . After that,

solvent was removed, and the crude product was recrystallized with

CH Cl /hexane (1:2) to give orange crystals (0.94 g, 76%). H NMR

400 MHz, [D ]DMSO) δ/ppm = 11.13 (s, 1H, NH ), 8.96 (d, J = 8.0

systems.

Although the B3LYP and M06-2X functionals perform

similarly, comparison of DFT structures with X-ray data, structures 2

and 3 were optimized at the M06-2X/def2-TZVP. All computations

were performed in Gaussian16. For the computation of Gibbs free

energy, all low-frequency vibrations were treated using the quasi rigid-

rotor harmonic oscillator (quasi-RRHO) approximation using 100

−

cm as cutoﬀ. Gibbs free energy diﬀerences (ΔG) were reported as

ΔG = ΔE + ΔG

RRHO

(

where ΔE is the diﬀerence in electronic energies, and ΔG

collects

RRHO

Hz, 1H, H ), 8.77 (s, 1H, NH ), 7.44−6.74 (m, 11H,

the zero-point vibrational energy (ZPVE) and RRHO Gibbs free

energy corrections. Gibbs free energy and ZPVE corrections were

computed using a Python script developed by one of the authors.

), 2.29 (s, 6H, H ), 2.13 (s, 3H, H ), 1.90 (s,

19,29 22

−3,7,8,15−17,25−27

H, H₂₀_,₃₀), 0.83 (s, 3H, H₁₂). ¹³C{ H} NMR (100 MHz, [D ]DMSO)

δ/ppm = 157.01 (C₂₁), 152.33 (C₁₁), 149.05−135.08

Unfortunately, it was not possible to include solvent eﬀects in the

calculations due to the lack of an exact dielectric constant for this

nonconventional solvent (4a).

(

C₄₋₆_,₁₀_,₁₃_,₁₄_,₁₈_,₂₃_,₂₄_,₂₈), 128.31−113.55 (C

), 19.23

1−3,7−9,15−17,25−27

C₁₉_,₂₉), 18.51 (C₂₀_,₃₀), 18.30 (C ), 18.08 (C ). MS (ESI)

C H N [M]: m/z calcd: 448.260; found: 448.262

Synthesis of Complex {AlMe(κ -naphbam)} 2. A solution of

trimethylaluminum (TMA) (24.5 mg, 0.34 mmol) in CH Cl was

quickly added to a solution of 1 (152.5 mg, 0.34 mmol) in CH Cl . The

reaction mixture was stirred for 1 h at room temperature. All volatiles

were removed under vacuum, and the solid was washed with hexane.

Compound 2 was obtained as a light-yellow solid (164.2 mg, 99%).

X-ray Crystal Structure Analyses. Data sets for compounds 1 and

were collected with a Bruker D8 Venture CMOS diﬀractometer. For

compound 3, data sets were collected with a Bruker APEX II CCD

diﬀractometer. The following programs were used: data collection,

APEX3 V2016.10; cell reﬁnement, SAINT V8.37A; data reduction,

SAINT V8.37A; absorption correction, SADABS V2014/7; structure

solution, SHELXT-2015; structure reﬁnement, SHELXL-2015;

Single colorless crystals for X-ray crystallography were grown from cold

and graphics, XP. R-values are given for observed reﬂections, and wR2

values are given for all reﬂections.

hexane at −30 °C. H NMR (400 MHz, CDCl ) δ/ppm = 7.44 (d, J =

.0 Hz, 2H, H ), 7.34 (t, J = 8.0 Hz, 2H, H ), 7.10 (d, J = 8.0 Hz, 2H,

H ), 7.01 (t, J = 4.5 Hz, 2H, H ), 6.94 (d, J = 4.5 Hz, 4H, H₁₁_,₁₃), 2.21

ASSOCIATED CONTENT

Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03290.

(

s, 6H, H ), 2.07 (s, 6H, H ), 1.87 (s, 6H, H ), −0.52 (s, 3H, H ).

■

sı

C{ H} NMR (100 MHz, CDCl ) δ/ppm = 172.61 (C ), 142.08−

34.10 (C₄₋₆_,₉_,₁₀_,₁₄), 128.18 (C ), 127.79 (C ), 125.70 (C ), 124.63

(

C ), 122.45 (C ), 116.26 (C ), 19.15 (C ), 18.84 (C ), 15.44 (C ),

11 3 1 15 16 8

−

5.80 (C ). MS (ESI) C H AlN [M]: m/z calcd: 488.250; found:

17 31 33 4

NMR spectra, noncovalent intermolecular and intra-

molecular interactions, bond lengths, crystallographic

data, NMR data for cyclic carbonates 5a−h, and XYZ

coordinates (PDF)

88.251

Synthesis of Complex {AlI(κ -naphbam)} 3. A solution of

complex 2 (50.0 mg, 0.10 mmol) in CH Cl was quickly added to a

solution of I (26.0 mg, 0.10 mmol) in CH Cl . The reaction mixture

was stirred for 1 h at room temperature. All volatiles were removed

under vacuum, and the solid was washed with hexane. Compound 3 was

obtained as a light brown solid (60.0 mg, 96%). Single colorless crystals

Accession Codes

CCDC 1923607−1923609 contain the supplementary crystallo-

graphic data for this paper. These data can be obtained free of

charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-

ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-

bridge Crystallographic Data Centre, 12 Union Road, Cam-

bridge CB2 1EZ, UK; fax: +44 1223 336033.

for X-ray crystallography were grown from cold hexane at −30 °C. H

NMR (400 MHz, CDCl ) δ/ppm = 7.52 (d, J = 8.24 Hz, 2H, H ), 7.38

(

t, J = 7.7 Hz, 2H, H ), 7.20 (d, J = 7.6 Hz, 2H, H ), 7.06 (d, J = 7.6 Hz,

H, H ), 6.99 (t, J = 7.3 Hz, 2H, H ), 6.94 (d, J = 7.6 Hz, 2H, H ),

11 12 13

.41 (s, 6H, H ), 2.15 (s, 6H, H ), 1.84 (s, 6H, H ). C{ H} NMR

(

100 MHz, CDCl ) δ/ppm = 175.09 (C ), 140.20−134.22

), 128.69 (C ), 127.84 (C ), 125.80 (C ), 125.58 (C ),

4−6,9,10,14

11 13 2 12

AUTHOR INFORMATION

Corresponding Authors

■

23.61 (C ), 116.91 (C ), 20.68 (C ), 18.78 (C ), 15.79 (C ). MS

3 1 15 16 8

(

ESI) C H AlIN [M]: m/z calcd: 600.130; found: 600.132

General Procedure for the Synthesis of Cyclic Carbonates at

bar Pressure. An epoxide 4a−h (1.7 mmol), catalyst 2 (25.5 μmol),

30 30 4

Rene S. Rojas − Laboratorio de Química Inorgánica, Facultad

de Química y Farmacia, Pontiﬁcia Universidad Católica de

Chile, Santiago 6094411, Chile; orcid.org/0000-0002-

5292-0816; Email: rrojasg@uc.cl

and TBAI (25.5 μmol) or catalyst 3 (25.5 μmol) were placed in

individual glass reaction tubes with a magnetic stirrer bar in a multipoint

^r^e^a^c^t^o^r^.^T^h^e^r^e^a^c^tⁱ^oⁿ^mⁱ^x^t^u^r^e^w^a^s^s^tⁱ^r^r^e^d^a^t⁸⁰^°^C^a^t¹^b^a^r^o^f^C^O2

Lutz H. Gade − Anorganisch-Chemisches Institut, Universität

Heidelberg, 69120 Heidelberg, Germany; orcid.org/0000-

pressure for 24 h. The conversion of epoxide to cyclic carbonate was

then determined by analysis of a sample by H NMR spectroscopy. The

0002-7107-8169 ; Email: lutz.gade@uni-heidelberg.de

remaining sample was ﬁltered through a plug of silica, eluting with

CH Cl to remove the catalyst. The eluent was evaporated in vacuo to

Authors

give either the pure cyclic carbonate or a mixture of cyclic carbonate and

unreacted epoxide. In the latter case, the mixture was puriﬁed by ﬂash

chromatography using a solvent system of ﬁrst hexane, then

hexane:EtOAc (9:1), then hexane:EtOAc (6:1), then hexane:EtOAc

Sebastian Saltarini − Laboratorio de Química Inorgánica,

Facultad de Química y Farmacia, Pontiﬁcia Universidad

Católica de Chile, Santiago 6094411, Chile

Nery Villegas-Escobar − Centro Integrativo de Biología y

Química Aplicada (CIBQA), Universidad Bernardo

OHiggins, Santiago 8370854, Chile; orcid.org/0000-

0002-6370-617X

(

3:1), and then EtOAc to give the pure cyclic carbonate. Cyclic

carbonates 5a−h are all known compounds, and the spectroscopic data

for samples prepared using catalysts 2 or 3 were consistent with those

reported in the literature.

33,36,71−74

179

Inorg. Chem. 2021, 60, 1172−1182

Inorganic Chemistry

Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle

Article

Javier Martínez − Laboratorio de Química Inorgánica, Facultad

de Química y Farmacia, Pontiﬁcia Universidad Católica de

Chile, Santiago 6094411, Chile; Instituto de Ciencias

Químicas, Facultad de Ciencias, Isla Teja, Universidad Austral

de Chile, 5090000 Valdivia, Chile; orcid.org/0000-0002-

Assessment. Chem. Rev. 2018, 118, 434−504.

10) Buttner, H.; Longwitz, L.; Steinbauer, J.; Wulf, C.; Werner, T.

Recent Developments in the Synthesis of Cyclic Carbonates from

Epoxides and CO . Top. Curr. Chem. 2017, 375 , 50.

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538-0689

Constantin G. Daniliuc − Organisch-Chemisches Institut der

Universität Münster, 48149 Münster, Germany; orcid.org/

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000-0002-6709-3673

Conversions of Carbon Dioxide. Catal. Sci. Technol. 2014, 4, 1482−

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Ricardo A. Matute − Centro Integrativo de Biología y Química

Aplicada (CIBQA), Universidad Bernardo OHiggins,

Santiago 8370854, Chile; Division of Chemistry and Chemical

Engineering, California Institute of Technology, Pasadena,

California 91125, United States; orcid.org/0000-0002-

metal-based catalysts for the synthesis of cyclic carbonates containing

five-membered rings. Green Chem. 2015, 17, 1966 −1987.

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Stereoselective Formation of Substituted Allylic Thioethers and

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644-3799

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c03290

(15) Chai, J.; Liu, Z.; Zhang, J.; Sun, J.; Tian, Z.; Ji, Y.; Tang, K.; Zhou,

X.; Cui, G. A Superior Polymer Electrolyte with Rigid Cyclic Carbonate

Backbone for Rechargeable Lithium Ion Batteries. ACS Appl. Mater.

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The authors declare no competing ﬁnancial interest.

fluoroethylene carbonate-based electrolyte works better for free-

standing Si-based composite paper anodes for Li-ion batteries. J.

Power Sources 2018, 381, 164−170.

ACKNOWLEDGMENTS

■

This work was supported by FONDECYT, Project Nos.

161091 and 1200748. J.M. acknowledges funding from the

(17) Sarri, F.; Tatini, D.; Ambrosi, M.; Carretti, E.; Ninham, B. W.;

Dei, L.; Lo Nostro, P. The curious effect of potassium fluoride on

glycerol carbonate. How salts can influence the structuredness of

organic solvents. J. Mol. Liq. 2018, 255, 397−405.

CONICYT/FONDECYT Postdoctorate Project (No.

180073). N.V-.E. and R.A.M. acknowledge ﬁnancial support

from FONDECYT grant 1181260. Powered@NLHPC: This

research was partially supported by the supercomputing

infrastructure of the NLHPC (ECM-02). R.S.R. thanks the

Alexander von Humboldt Foundation for support through a

reinvitation fellowship.

18) Sopena, S.; Martin, E.; Escudero-Adan, E. C.; Kleij, A. W.

Pushing the Limits with Squaramide-Based Organocatalysts in Cyclic

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Hydroiodide as a Simple Yet Effective Bifunctional Catalyst for CO ₂

Fixation Reactions with Epoxides under Mild Conditions. ACS

Sustainable Chem. Eng. 2017, 5, 7295−7301.

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