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Bifunctional aluminium catalysts for the chemical fixation of carbon dioxide into cyclic carbonates Felipe de la Cruz-Martinez, Javier Martínez, Miguel A Gaona, Juan Fernandez-Baeza, LUIS F. SÁNCHEZ-BARBA, Ana M. Rodríguez, José A. Castro-Osma, Antonio Otero, and Agustín Lara Sánchez ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00102 • Publication Date (Web): 18 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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ACS Sustainable Chemistry & Engineering

ABSTRACT

Bifunctional aluminium complexes supported by novel zwitterionic NNO-donor scorpionate ligands were found to be efficient bifunctional catalysts cyclic carbonates synthesis from terminal and internal epoxides in good yields and with broad substrate scope. Neutral scorpionate ligands (1−2) were designed and used as precursors to obtain two novel zwitterionic NNOheteroscorpionate ligands (3−4). Reaction of 3 or 4 with [AlX3] (X = Me, Et) in a 1:1 or 1:2 molar ratio afforded the mononuclear and dinuclear cationic aluminium complexes [AlX2{κ2mbpzbdmape}]I2 (X = Me (5), Et (6)), [AlX2{κ2-mbpzbdeape}]I2 (X = Me (7), Et (8)), [{AlX2(κ2-mbpzbdmape)}(µ-O){AlX3}]I2

(X

=

Me

(9),

Et

(10))

and

[{AlX2(κ2-

mbpzbdeape)}(µ-O){AlX3}]I2 (X = Me (11), Et (12)) with elimination of the corresponding alkane. These complexes were investigated as catalysts for cyclic carbonate formation from epoxides and carbon dioxide in the absence of a cocatalyst. Complex 7 was found to be the most active catalyst for cyclic carbonate formation from various epoxides and carbon dioxide.

KEYWORDS. CO2, catalysis, organic carbonates, aluminium complexes, epoxides. INTRODUCTION The use of carbon dioxide (CO2) as a sustainable C1 feedstock remains a challenge for the scientific community due to its intrinsic inertness.1–6 Therefore, a great deal of research is being devoted to the development of chemical processes that use CO2 as a starting material for the synthesis of value-added chemical products such as amines, amides, methanol and formic acid.7– 14

One of the most successful and widely studied processes that uses CO2 as a reactant is the

ACS Paragon Plus Environment

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reaction with epoxides to produce cyclic- or polycarbonates (Scheme 1).15–20 Many catalytic systems have been developed in recent decades and these include metal complexes21–23 and organocatalysts.24–29 The general mechanism for metal-catalysed cyclic carbonate synthesis involves epoxide activation by a Lewis acid centre and ring-opening of the epoxide by the nucleophilic additive to give an alkoxide intermediate that inserts CO2 to form a metallic carbonate, which can undergo ring-closing to afford the cyclic organic carbonate.21–23Amongst the reported catalyst systems, metal complexes that include aluminium,30–35 iron,36–41 cobalt,42,43 chromium,44,45 zinc46–48 and other metals,49–52 in combination with a nucleophile as a cocatalyst, have been extensively studied. The use of bifunctional metal catalysts, which include the cocatalyst within the same molecule, have been less widely studied even though the cooperative effect between the functional groups can improve the catalytic activity and selectivity in reactions.53–55

Scheme 1. Synthesis of cyclic- or polycarbonates.

In the last two decades we have reported the development of heteroscorpionate ligands and a range of metal complexes.56,57 These complexes were found to be highly active catalysts in a range of catalytic applications.56–60 In recent years we have focused our efforts on the development of homogeneous neutral and bifunctional catalysts for CO2 fixation into cyclic carbonates (Chart 1a-b)61,62 and polycarbonates (Chart 1c).63 In our previous work, we developed new zwitterionic NNO-donor scorpionate ligands and used them as precursors for the development of bifunctional aluminium scorpionate complexes containing a quaternised amino


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moiety to incorporate the nucleophile in the same complex (Chart 1b).61 Amongst the catalysts developed, a bimetallic aluminium complex (Chart 1b) was found to be a highly active for cyclic carbonates synthesis from epoxides and CO2.

Chart 1. Neutral and bifunctional scorpionate complexes for CO2 utilisation.

The main drawback was that the zwitterionic scorpionate ligand slowly lost benzyl bromide to reform the neutral ligand precursor. In order to avoid this process, in this work we have synthesised new NNO-donor heteroscorpionate ligands which were quaternised with iodomethane. These compounds were found to be stable both in solution and the solid state. These zwitterionic ligands allowed us to develop new bifunctional aluminium heteroscorpionate catalysts for CO2 fixation into cyclic carbonates. The complexes developed behave as onecomponent catalysts, where the aluminium centre and the iodide cocatalyst are present within the same moiety, and they displayed excellent catalytic activity for cyclic carbonate formation using Earth’s crust abundant metal catalysts.

RESULTS AND DISCUSSION


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Synthesis and structural characterisation. Neutral heteroscorpionate ligand precursors 2,2bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,1-bis(4-(dimethylamino)phenyl)ethan-1-ol, (1),

and

1,1-bis(4-(diethylamino)phenyl)-2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)

bpzbdmapeH ethan-1-ol,

bpzbdeapeH (2) were synthesised by reaction of bis(3,5-dimethylpyrazol-1-yl)methane (bdmpzm)64 with nBuLi, and subsequent reaction with bis(4-(dimethylamino)phenyl)methanone or bis(4-(diethylamino)phenyl)methanone, followed by hydrolysis with ammonium chloride as previously reported (Scheme 2).65,66 Compounds 1 and 2 were isolated as white solids in yields higher than 90%.

Scheme 2. Synthesis of neutral ligand precursors bpzbdmapeH (1) and bpzbdeapeH (2) and zwitterionic ligands (mbpzbdmapeH)I2 (3) and (mbpzbdeapeH)I2 (4).

These neutral compounds were used as precursors for the synthesis of the zwitterionic ligands 4,4'-(2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-hydroxyethane-1,1-diyl)bis(N,N,N-


5

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trimethylbenzenaminium) iodide, (mbpzbdmapeH)I2 (3), and 4,4'-(2,2-bis(3,5-dimethyl-1Hpyrazol-1-yl)-1-hydroxyethane-1,1-diyl)bis(N,N,N-trimethylbenzenaminium)-4,4'-(2,2-bis(3,5dimethyl-1H-pyrazol-1-yl)-1-hydroxyethane-1,1-diyl)bis(N,N-diethyl-N-methylbenzenaminium) iodide, (mbpzbdeapeH)I2 (4), by reaction with excess iodomethane in acetonitrile at 60 ºC for 16 hours. The ammonium salts 3 and 4 were obtained in 85% and 82% yield, respectively as yellow solids (Scheme 2). These ligand precursors were stable in solution −in contrast to the bromide salt reported previously.61 The neutral and zwitterionic heteroscorpionate ligand precursors were characterised spectroscopically (see Experimental Section and Supporting Information). The NMR spectra of compounds 1−4 confirmed that the pyrazole rings are equivalent. In ligand precursors 3 and 4, the NMe resonances are shifted downfield in comparison with the NMe resonances in neutral scorpionate ligands 1 and 2, thus confirming the quarternisation of the amine group (Figure 1).


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NMe3

Me3 OH

Ph

Me5

H4

CH

NMe2

Me5

Me3 OH Ph CH Ph

9.0

8.5

8.0

7.5

7.0

6.5

H4

6.0

5.5

5.0

4.5 ppm

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Figure 1. 1H NMR spectra of bpzbdmapeH (1) and (mbpzbdmapeH)I2 (3) in CD3CN.

1

H NOESY-1D and 1H-13C heteronuclear correlation (g-HSQC) experiments were carried out

to assign most of the proton and carbon resonances (see Experimental Section).


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Scheme 3. Synthesis of aluminium scorpionate complexes 5−12.

The reaction of the corresponding zwitterionic ligand precursors 3 or 4 with one or two equivalents of trialkylaluminium compounds (AlX3) in acetonitrile at 0 °C for 2 hours gave the monometallic and bimetallic cationic aluminium complexes [AlX2{(κ2-mbpzbdmape}]I2 (X = Me (5), Et (6)), [AlX2{(κ2-mbpzbdeape}]I2 (X = Me (7), Et (8)), [{AlX2(κ2-mbpzbdmape)}(µO){AlX3}]I2 (X = Me (9), Et (10)) and [{AlX2(κ2-mbpzbdeape)}(µ-O){AlX3}]I2 (X = Me (11),


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Et (12)) with elimination of the corresponding alkane (Scheme 3) as yellow solids in excellent yields. All complexes were isolated as racemates. In dinuclear complexes 9−12, the second aluminium centre is coordinated through a dative bond to the oxygen atom of the scorpionate ligand (Scheme 3). These complexes displayed high stability in solid state and in solution under N2 and did not decompose to the mixture of the corresponding neutral complex and iodomethane. The structures of complexes 5−12 in solution were determined by spectroscopic methods characterised. The NMR spectra of 5−12 show one singlet for the pyrazole protons and the methine group, , two doublets for the aryl groups of the scorpionate moiety and one set of resonances for the alkyl ligands (Figure 2). Therefore, a fluxional behaviour due to a fast exchange between the two pyrazole rings is proposed in these complexes (See Supporting Information), similarly to previously reported scorpionate aluminium complexes.61,62 NOESY-1D and g-HMQC NMR experiments were carried out to assign most of the NMR resonances (see Experimental Section). The results are consistent with a tetrahedral environment both aluminium centres, with the scorpionate ligand coordinated in a κ2-NO bidentate fashion for complexes 5−8, or a κ2-NO-µ-O coordination mode for complexes 9−12 (Scheme 3).


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NMe3

Me3,3‘ Me5,5‘ AlCH2CH3

Ph

CH

Me Me Me N

NMe3

2I

Me Me N Me

AlCH2CH3

H4,4‘

AlMe3 Ca

O Me Al

HC

N N Me 4' 3` Me5` N N Me Me5

Al(CH3)3

Me3 4

(9)

Me3,3‘ Me5,5‘

Ph

CH

Al(CH3)2

H4,4‘

Figure 2. 1H NMR spectra of [AlEt2{κ2-mbpzbdmape}]I2 (6) and [{AlMe2(κ2mbpzbdmape)}(µ-O){AlMe3}]I2 (9) in CD3CN.


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The strucutres of complexes 6 and 8 were confirmed by X-Ray diffraction analysis (Figure 3). The solid-state structures are consistent with the NMR data and Scheme 3 (See Supporting Information). Due to the κ2-NO coordination fashion of the heteroscorpionate ligands, complexes 6 and 8 are chiral compounds, which crystallise as a racemic mixture of both enantiomers in the unit cell. The geometry at Al1 is distorted tetahedral, with the dihedral angle between the N1−Al1−O1 and C−Al−C planes (88.97, and 86.92° for 6 and 8 respectively). Moreover, substantial deviation from the ideal values is observed for the angles around the Al centre (94.16(10)−115.31(14)° for 6 and 95.44(17)−126.9(5)° for 8). The most acute angle of 94.16(10)° for 6 and 95.44(17)° for 8 is observed for N1−Al1−O1 due to the bite of the heteroscorpionate

ligand.

The

Al−C

distances

(1.870(13)−2.15(1)

Å),

the

Al−N

(1.975(3)−1.970(5) Å) and Al−O distances (1.755(2) −1.747(4) Å) are similar to those previously reported.58,67–69

Figure 3. ORTEP diagrams of complexes 6 (left) and 8 (right).


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Catalytic studies cyclic carbonates synthesis. Given the high catalytic activity displayed by the bifunctional aluminium catalysts, which contained a bromide counterion for cyclic carbonates formation from epoxides and carbon dioxide,61 we investigated the iodide derivatives as catalysts for cyclic carbonate formation (Scheme 4). It is worth noting that iodide ions are better as a cocatalyst for a range of aluminium compounds.31,33,70 An initial catalyst screening was carried out to study the performance of zwitterionic ligands 3 and 4 and complexes 5−12 as catalysts for the reaction of styrene oxide 13a and CO2 into styrene carbonate 14a at 25 oC and one bar CO2 pressure for 24 hours in the absence of solvent using 5 mol% of catalyst loading and the reactions were monitored by 1H NMR spectroscopy (Table 1).

Scheme 4. Cyclic carbonate synthesis catalysed by compounds 3–12.

It can be seen from the results in Table 1 that mononuclear aluminium complexes 5–8 showed reasonable catalytic activity for styrene carbonate 14a formation under these reaction conditions when 5 mol% of complex was used (Table 1, entries 3−6). Bimetallic aluminium complexes 9– 12 displayed much higher catalytic activity than mononuclear ones, with conversions higher than 90% achieved in all cases (Table 1, entries 7−10). Control experiments showed that ligands 3 and 4 had very low catalytic activity (Table 1, entries 1 and 2). Since bimetallic complexes 9– 12 benefit from an aluminium concentration of 10 mol%, experiments using 5 mol% of


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aluminium were carried out in order to perform the catalyst screening while keeping the concentration of aluminium constant at 5 mol% (Table 1, entries 11−14). The results show that monometallic complexes 5–8 are more active than bimetallic complexes 9–12 per aluminium centre at 25 oC and 1 bar CO2 pressure. However, monometallic complexes benefit from an iodide concentration of 10 mol%. Table 1. Styrene carbonate 14a synthesis catalysed by 5–12.a Entry

Catalyst

Conv. 3hb

Conv. 6hb

Conv. 24hb

Conv. 24hb,c

1

3

0

2

6

10

2

4

0

1

5

9

3

5

4

9

52

93

4

6

11

23

52

94

5

7

12

24

55

100

6

8

4

10

48

95

7

9

24

42

95

100

8

10

18

33

92

100

9

11

19

38

94

100

10

12

25

42

99

100

11

d

9

7

13

43

81

12

d

10

6

11

40

79

13

d

11

6

12

40

76

14

d

12

9

15

45

84

15

e

9

9

16

47

89

16

e

10

10

17

50

93

17

e

11

8

13

47

91

18

e

12

14

21

51

95

19

f

2+AlMe3 15

27

58

99

a

Reactions carried out at 25 °C and 1 bar CO2 pressure for 24 hours using 5 mol% of complex 5–12 unless specified otherwise. bDetermined by 1H NMR. cReactions carried out at 35 °C and 1 bar CO2 pressure for 24 h using 5 mol% of complex 5–12 unless specified otherwise. d2.5 mol% of complex. e2.5 mol% of complex + 5 mol% of Bu4NI. f5 mol% of complex resulting from the reaction of neutral ligand 2 and AlMe3 + 10 mol% of Bu4NI.


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Thus, to mantain the concentration of iodide constant at 10 mol%, 5 mol% of Bu4NI was added to the reaction mixture (Table 1, entries 15−18). It can be seen that the addition of external Bu4NI slightly increased the catalytic activity of complexes 9–12. However, monometallic complex 7 displayed the highest catalytic activity for styrene carbonate synthesis. In order to improve the catalytic activity of complexes 5–12, the reaction temperature was increased to 35 °C (Table 1). Nevertheless, the same trend was observed and the mononuclear complex 7 was the most active catalyst. To confirm the bifunctional nature of complex 7, the synthesis of styrene carbonate form 13a and CO2 was investigated using a catalyst system comprised by a combination of 5 mol% of the aluminum complex obtained from the reaction of 2 and AlMe3 and 10 mol% of Bu4NI (Table 2, entry 19). The results showed that the conversion obtained using complex 7 as catalyst was very similar to that obtained using a combination of neutral aluminum complex and Bu4NI, confirming the bifunctional nature of catalyst 7. Having determined the optimal catalyst, the synthesis of a range of monosubstituted cyclic carbonates (14b–l) from their corresponding terminal epoxides (13b–l) was investigated using 5 mol% of complex 7 as catalyst at 35 °C and one bar CO2 pressure for 24 hours (Figure 4). As can be seen in Figure 4, complex 7 displayed excellent catalytic activity under these reaction conditions. As can be seen in Figure 4, the catalyst system is tolerant of alkyl and aryl epoxides and also to compounds functionalised with alcohols, ethers, halides and alkenes, thus demonstrating the high versatility of this catalyst. No polycarbonate was observed under these reaction conditions with a selectivity higher than 99% to the cyclic carbonate. This is probably because the functional unit is too remote from the metal centre to copolymerise terminal epoxides and carbon dioxide.71,72


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Figure 4. Conversion of epoxides 13a−l into cyclic carbonates 14a−l catalysed by complex 7 at 35 oC and one car CO2 pressure for 24 hours using 5 mol% of catalyst loading.

Figure 5. Conversion of epoxides 13a−l into cyclic carbonates 14a−l catalysed by complex 7 at 70 °C and 10 bar CO2 pressure for 18 hours using 0.25−0.5 mol% catalyst loading. In order to reduce the catalyst loading required to achieve complete conversion, the reaction temperature and pressure were increased to 70 °C and 10 bar respectively. It can be seen in


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Figure 5 that this evaluation allowed us to reduce both the catalyst loading and the reaction time to 0.25–0.5 mol% and 18 hours, respectively, under these reaction conditions. The catalyst was more active towards the synthesis of aryl-substituted cyclic carbonates as well as those functionalised with alcohol, halide and aromatic ether groups as only 0.25 mol% of complex 7 was used to obtain excellent yields. On the other hand, 0.5 mol% of complex 7 was needed to achieve good to excellent yields of cyclic carbonates 14b-e,k,l. In an effort to extend further the substrate scope of catalyst 7, the conversion of internal and bio-derived epoxides (15a−h) into their corresponding cyclic carbonates (16a−h) was investigated and the results are shown in Figure 6. O O R' R 15a–h

+ CO2

O O

O

O O O

O 16c Conv: 100% Yield: 94% [7] = 0.25 mol%

16b: Conv: 84% Yield: 54% [7] = 1.5 mol%

O

O

O O

O

O

O

O

O O

O

O

O

O

16f: Conv: 100% Yield: 99% [7] = 0.25 mol% O

O

O

O

O

O

O

O

O

O

O

O

16g: Conv: 100% Yield: 99% [7] = 0.25 mol%

O

O

O

O

O

O

O

O

16e: Conv: 100% Yield: 95% [7] = 0.25 mol% O

O

O 16d: Conv: 100% Yield: 89% [7] = 0.25 mol%

O

O

O

R' 16a–h

O

O O

O

O O

O

O

16a: Conv: 100% Yield: 80% [7] = 2.5 mol%

O

70 oC, 10 bar, 18 h

R

O O

7 (0.25-2.5 mol%) /

16h: Conv: 100% Yield: 98% [7] = 0.25 mol%

Figure 6. Conversion of epoxides 15a−h into cyclic carbonates 16a−h catalysed by complex 7 at 70 oC and 10 bar CO2 pressure for 18 hours.


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As can be seen in Figure 6, complex 7 was an effective bifunctional catalyst for conversion of cyclohexene and cyclopentene oxide into their corresponding cyclic carbonates 16a and 16b in good yields at 70 °C and 10 bar CO2 pressure in 18 hours using 2.5 or 1.5 mol% of compound 7, respectively (Figure 6). When epoxide 15b was used as a substrate, the polyether-polycarbonate was obtained in 25% isolated yield and this confirms the high tendency of this particular epoxide to form polymers.17,15,19,20 We turned our attention to the synthesis of bio-based furan-derived cyclic carbonates 16c–16e. It can be seen in Figure 6 that these cyclic carbonates were obtained in excellent yields using as little as 0.25 mol% of catalyst 7 at 70 °C and 10 bar CO2 pressure. Finally, the synthesis of bio-based diacid-derived cyclic carbonates 16f–16h was undertaken. These cyclic carbonates were obtained in quantitative yield for reactions carried out under the same reaction conditions as used in the furan-derived cyclic carbonate synthesis. It is worth noting that some of these bis(bio-derived cyclic carbonates) are potential building blocks for the synthesis of bio-derived non-isocyante polyurethanes.73,74

Scheme 5. Proposed mechanism for the conversion of epoxides and CO2 into cyclic carbonates catalysed by complex 7.


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Taking into account that complex 7 carries out the synthesis of cyclic carboantes 16a−b with retention of the epoxide stereochemistry, a plausible mechanism for cyclic carbonate synthesis catalysed by the bifunctional aluminium complex 7 is shown in Scheme 5. This mechanism is consistent with that previously proposed for cyclic carbonate formation from epoxides and CO2 catalysed by neutral and one-component scorpionate aluminium complexes.35,61,62,75

CONCLUSIONS Novel bifunctional heteroscorpionate aluminium complexes 5−12 have been synthesised and characterised confirming a κ2-NO or a κ2-NO-µ-O coordination mode for the mononuclear or the dinuclear complexes, respectively. The ligands and complexes that contain iodide as a counterion displayed higher stability than the ligands and complexes that contained a bromide,61 thus highlighting the importance of the counterion. These complexes act as bifunctional catalysts for the conversion of epoxides into their corresponding cyclic carbonates without the need for a cocatalyst. Amongst them, the monometallic aluminium complex 7 exhibited the highest catalytic performance for cyclic carbonate formation from terminal epoxides and CO2 at ambient temperature and pressure. Complex 7 is not only active for the synthesis of monosubstituted cyclic carbonates, but also for the synthesis of disubstituted and bio-derived cyclic carbonates from their corresponding epoxides, displaying a broad substrate scope. It is worth noting that the iodide counterion not only has a positive effect on the stability of the zwitterionic ligands but also on the catalytic activity of the aluminium scorpionate complexes. Thus, complex 7 displayed higher catalytic activity than the corresponding aluminium catalysts containing a bromide counterion both at room temperature and 80 ºC.61


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Although a large number of metal complexes have been developed por the catalytic conversion of epoxides and CO2 into their corresponding cyclic carboantes,17,16,15,18,76,77,19–23 only a few aluminium complexes are active at ambient temperature and pressure. We have shown that bifunctional aluminium complex 7 is an effective catalyst for cyclic carbonate synthesis from a range of terminal, internal and bio-derived epoxides. When compared to other bifunctional aluminium catalyst systems, complex 7 is active even at 25 °C and 1 bar CO2 pressure, i.e., similar to aluminium(salen) complexes.78,79 Moreover, complex 7 exhibits a broader substrate scope and is active under milder reaction conditions than other bifunctional aluminium catalysts for cyclic carbonate formation.80–85

EXPERIMENTAL SECTION Synthesis of bpzbdmapeH (1): In a 250 mL Schlenk tube, bdmpzm (1.00 g, 4.89 mmol) was dissolved in dry THF (70 mL) and cooled to −70 ºC. A solution of nBuLi (1.6 M in hexane, 3.06 mL, 4.89 mmol) was added, and the suspension was stirred for 1 h. The mixture was warmed to −10 ºC, and the resulting yellow suspension was added dropwise to a cooled (−10 ºC) solution of 4,4´-bis(dimethylamino)benzophenone (1.34 g, 4.89 mmol) in dry THF (20 mL). The mixture was stirred for 1 h and was allowed to warm up to ambient temperature. The product was hydrolysed with saturated aqueous NH4Cl (15 mL). The organic layer was extracted, dried over MgSO4 overnight, filtered and the solvent was removed under vacuum to give the product as a white solid. Yield: 90% (2.08 g). 1H NMR (500 MHz, CDCl3, 297 K): δ = 7.73 (s, 1H, OH), 7.10 (d, 3JH-H = 8 Hz, 4H, oH-NPh), 6.79 (s, 1H, CH), 6.55 (d, 3JH-H = 8.5 Hz, 4H, mH-NPh), 5.66 (s, 2H, H4), 2.87 (s, 12H, NMe2), 2.07 (s, 6H, Me3), 2.01 (s, 6H, Me5);

13

C{1H} NMR (125 MHz,

CDCl3, 297 K): δ = 149.5, 147.0, 140.6, 133.1 (Cipso, C3, C5), 127.3 (oC-NPh), 111.9 (mC-NPh),


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106.1 (C4), 81.6 (Ca), 74.3 (CH), 40.7 (NMe2), 13.6 (Me3), 11.3 (Me5); elemental analysis calcd (%) for C28H36N6O (472.63): C 71.16, H 7.68, N 17.78; found: C 71.25, H 7.92, N 17.60. Synthesis of bpzbdeapeH (2): The synthesis of 2 was carried out in an identical manner to 1, using 4,4´-bis(diethylamino)benzophenone (1.59 g, 4.89 mmol). Compound 2 was isolated as a white solid. Yield: 92% (2.38 g). 1H NMR (500 MHz, CDCl3, 297 K): δ = 7.68 (s, 1H, OH), 7.04 (d, 3JH-H = 9 Hz, 4H, oH-NPh), 6.76 (s, 1H, CH), 6.48 (d, 3JH-H = 8.5 Hz, 4H, mH-NPh), 5.66 (s, 2H, H4), 3.27 (m, 8H, NCH2CH3), 2.06 (s, 6H, Me3), 2.00 (s, 6H, Me5), 1.09 (t, 3JH-H = 7.0 Hz, 12H, NCH2CH3); 13C{1H} NMR (125 MHz, CDCl3, 297 K): δ = 146.8, 140.5, 132.0 (Cipso, C3, C5), 127.7 (oC-NPh), 111.4 (mC-NPh), 106.1 (C4), 81.8 (Ca), 74.5 (CH), 44.6 (NCH2CH3), 13.7 (Me3), 12.8 (NCH2CH3), 11.4 (Me5); elemental analysis calcd (%) for C32H44N6O (529.74): C 72.69, H 8.39, N 15.89; found: C 72.80, H 8.63, N 15.67. Synthesis of (mbpzbdmapeH)I2 (3): In a 250 mL Schlenk tube, compound 1 (1.00 g, 2.10 mmol) was dissolved in dry acetonitrile (70 mL). Iodomethane (0.52 mL, 8.40 mmol) was added and the reaction mixture was heated to 60 ºC and stirred for 16 h. The solvent was removed under vacuum, and the crude residue was washed with hexane (3 x 25 mL) to remove the excess iodomethane. The resulting product was dried to give compound 3 as a yellow solid. Yield: 85% (1.35 g). 1H NMR (400 MHz, CD3CN, 297 K): δ = 8.05 (s, 1H, OH), 7.75 (d, 3JH-H = 9.0 Hz, 4H, o

H-NPh), 7.65 (d, 3JH-H = 12.0 Hz, 4H, mH-NPh), 7.01 (s, 1H, CH), 5.73 (s, 2H, H4), 3.59 (s,

18H, NMe3), 2.00 (s, 6H, Me3), 1.96 (s, 6H, Me5); 13C{1H} NMR (100 MHz, CD3CN, 297 K): δ = 149.1, 147.5, 147.2, 142.7 (Cipso, C3, C5), 129.8 (oC-NPh), 121.0 (mC-NPh), 107.4 (C4), 82.5 (Ca), 73.2 (CH), 58.5 (NMe3), 14.0 (Me3'), 11.7 (Me5); elemental analysis calcd (%) for C30H42I2N6O (756.51): C 47.63, H 5.60, N 11.11; found: C 47.70, H 5.72, N 11.03.


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Synthesis of (mbpzbdeapeH)I2 (4): The synthesis of 4 was carried out in an identical manner to 3, using bpzbdeapeH (2) (1.00 g, 1.89 mmol). Compound 4 was isolated as a yellow solid. Yield: 82% (1.26 g). 1H NMR (500 MHz, CD3CN, 297 K): δ = 7.68 (d, 3JH-H = 9.0 Hz, 4H, oHNPh), 7.62 (d, 3JH-H = 9.5 Hz, 4H, mH-NPh), 6.96 (s, 1H, CH), 5.71 (s, 2H, H4), 4.00, 3.87 (m, 8H, NCH2CH3), 3.43 (s, 6H, NMe), 1.98 (s, 6H, Me3), 1.97 (s, 6H, Me5), 0.98 (m, 12H, NCH2CH3); 13C{1H} NMR (125 MHz, CD3CN, 297 K): δ = 148.9, 146.9, 142.6, 141.5 (Cipso, C3, C5), 130.1 (oC-NPh), 123.1 (mC-NPh), 107.2 (C4), 82.5 (Ca), 72.3 (CH), 65.7 (NCH2CH3), 47.8 (NMe), 13.9 (Me3), 11.8 (Me5), 9.4 (NCH2CH3); elemental analysis calcd (%) for C34H50I2N6O (812.61): C 50.25, H 6.20, N 10.34; found: C 50.41, H 6.45, N 10.02. Synthesis of [AlMe2{κ2-mbpzbdmape}]I2 (5): In 100 mL Schlenk tube, (mbpzbdmapeH)I2 (3; 1.0 g, 1.32 mmol) was dissolved in dry acetonitrile (50 mL) and cooled to 0 ºC. A solution of AlMe3 (2 M in toluene, 0.66 mL, 1.32 mmol) was added, and the mixture was allowed to warm up to ambient temperature and was stirred for 2 h. The solvent was removed under reduced pressure to give complex 5 as a yellow solid. Yield: 78% (0.84 g). 1H NMR (500 MHz, CD3CN, 297 K): δ = 7.69 (d, 3JH-H = 8.5 Hz, 4H, oH-NPh), 7.58 (d, 3JH-H = 9.0 Hz, 4H, mH-NPh), 6.93 (s, 1H, CH), 5.85 (s, 2H, H4), 3.56 (s, 12H, NMe3), 2.16 (s, 6H, Me3,3'), 2.05 (s, 6H, Me5,5'), -0.86 (AlMe2); 13C{1H} NMR (125 MHz, CD3CN, 297 K): δ = 150.8, 149.5, 147.5, 144.1 (Cipso, C3,3', C5,5'), 130.7 (oC-NPh), 120.8 (mC-NPh), 107.8 (C4), 82.5 (Cª), 71.0 (CH), 58.5 (NMe3), 13.8 (Me3,3'), 11.9 (Me5,5'), –6.5 (AlMe2); elemental analysis calcd (%) for C32H47AlI2N6O (812.55): C 47.30, H 5.83, N 10.34; found: C 47.60, H 5.98, N 10.09. Synthesis of [AlEt2{κ2-mbpzbdmape}]I2 (6): The synthesis of 6 was carried out in an identical manner to 5, using (mbpzbdmapeH)I2 (3) (1.0 g, 1.32 mmol) and AlEt3 (1 M in hexane, 1.32 mL, 1.32 mmol). Compound 6 was isolated as a yellow solid and was recrystallized from


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acetonitrile at −26 ºC. Yield: 75% (0.83 g). 1H NMR (400 MHz, CD3CN, 297 K): δ = 7.72 (d, 3

JH-H = 9.6 Hz, 4H, oH-NPh), 7.57 (d, 3JH-H = 9.2 Hz, 4H, mH-NPh), 6.92 (s, 1H, CH), 5.84 (s,

2H, H4), 3.59 (s, 12H, NMe3), 2.16 (s, 6H, Me3,3'), 2.06 (s, 6H, Me5,5'), 0.88 (AlCH2CH3), 0.16 (AlCH2CH3); 13C{1H} NMR (125 MHz, CD3CN, 297 K): δ = 150.5, 149.5, 147.1, 143.8 (Cipso, C3,3', C5,5'), 130.2 (oC-NPh), 120.4 (mC-NPh), 107.3 (C4), 81.9 (Cª), 70.3 (CH), 58.1 (NMe3), 13.4 (Me3,3'), 11.2 (Me5,5'), 9.6 (AlCH2CH3), 2.3 (AlCH2CH3); elemental analysis calcd (%) for C34H51AlI2N6O (840.60): C 48.58, H 6.12, N 10.00; found: C 48.89, H 6.44, N 9.62. Synthesis of [AlMe2{κ2-mbpzbdeape}]I2 (7): The synthesis of 7 was carried out in an identical manner to 5, using (mbpzbdeapeH)I2 (4) (1.0 g, 1.23 mmol) and AlMe3 (2 M in toluene, 0.62 mL, 1.23 mmol). Compound 7 was isolated as a yellow solid. Yield: 76% (0.81 g). 1H NMR (500 MHz, CD3CN, 297 K): δ = 7.56 (d, 8H, oH-NPh, mH-NPh), 6.93 (s, 1H, CH), 5.84 (s, 2H, H4), 3.99, 3.85 (m, 8H, NCH2CH3), 3.42 (s, 6H, NMe), 2.15 (s, 6H, Me3,3'), 2.08 (s, 6H, Me5,5'), 0.99 (m, 12H, NCH2CH3), –0.93 (AlMe2); 13C{1H} NMR (125 MHz, CD3CN, 297 K): δ = 150.8, 149.5, 143.9, 141.4 (Cipso, C3,3', C5,5'), 130.8 (oC-NPh), 122.7 (mC-NPh), 107.7 (C4), 82.4 (Ca), 71.0 (CH), 65.8, 65.6 (NCH2CH3), 47.8 (NMe), 13.8 (Me3,3'), 12.1 (Me5,5'), 9.4 (NCH2CH3), –6.7 (AlMe2); elemental analysis calcd (%) for C36H55AlI2N6O (868.65): C 49.78, H 6.38, N 9.67; found: C 49.95, H 6.61, N 9.40. Synthesis of [AlEt2{κ2-mbpzbdeape}]I2 (8): The synthesis of 8 was carried out in an identical manner to 5, using (mbpzbdeapeH)I2 (4) (1.0 g, 1.23 mmol) and AlEt3 (1 M in hexane, 1.23 mL, 1.23 mmol). Compound 8 was isolated as a yellow solid and was recrystallized from acetonitrile at −26 ºC. Yield: 71% (0.78 g). 1H NMR (500 MHz, CD3CN, 297 K): δ = 7.54 (d, 8H, oH-NPh, m

H-NPh), 6.91 (s, 1H, CH), 5.84 (s, 2H, H4), 3.98, 3.84 (m, 8H, NCH2CH3), 3.40 (s, 6H, NMe),

2.17 (s, 6H, Me3,3'), 2.10 (s, 6H, Me5,5'), 1.01 (m, 12H, NCH2CH3), 0.85 (AlCH2CH3), –0.22


22


(AlCH2CH3);

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C{1H} NMR (125 MHz, CD3CN, 297 K): δ = 151.0, 150.0, 141.1 (Cipso, C3,3',

C5,5'), 130.8 (oC-NPh), 122.7 (mC-NPh), 107.8 (C4), 82.4 (Ca), 70.6 (CH), 65.8, 65.6 (NCH2CH3), 47.7 (NMe), 13.8 (Me3,3'), 12.2 (Me5,5'), 10.2 (NCH2CH3), 9.3 (AlCH2CH3), 2.1 (AlCH2CH3); elemental analysis calcd (%) for C38H59AlI2N6O (896.71): C 50.90, H 6.63, N 9.37; found: C 51.22, H 6.95, N 9.02. Synthesis of [{AlMe2(κ2-mbpzbdmape)}(µ-O){AlMe3}]I2 (9): The synthesis of 9 was carried out in an identical manner to 5, using (mbpzbdmapeH)I2 (3) (1.0 g, 1.32 mmol) and AlMe3 (2 M in toluene, 1.32 mL, 2.64 mmol). Compound 9 was isolated as a yellow solid. Yield: 78% (0.91 g). 1H NMR (400 MHz, CD3CN, 297 K): δ = 7.68 (d, 3JH-H = 9.6 Hz, 4H, oH-NPh), 7.57 (d, 3JH-H = 9.2 Hz, 4H, mH-NPh), 6.92 (s, 1H, CH), 5.85 (s, 2H, H4), 3.55 (s, 12H, NMe3), 2.16 (s, 6H, Me3,3'), 2.05 (s, 6H, Me5,5'), –0.86 (AlMe2), –0.98 (AlMe3);

13

C{1H} NMR (100 MHz, CD3CN,

297 K): δ = 150.7, 149.4, 147.5, 144.1 (Cipso, C3,3', C5,5'), 130.7 (oC-NPh), 120.8 (mC-NPh), 107.7 (C4), 82.5 (Cª), 71.0 (CH), 58.5 (NMe3), 13.9 (Me3,3'), 12.0 (Me5,5'), –6.4 (AlMe2), –8.0 (AlMe3); elemental analysis calcd (%) for C35H56Al2I2N6O (884.63): C 47.52, H 6.38, N 9.50; found: C 47.84, H 6.65, N 9.12. Synthesis of [{AlEt2(κ2-mbpzbdmape)}(µ-O){AlEt3}]I2 (10): The synthesis of 10 was carried out in an identical manner to 5, using (mbpzbdmapeH)I2 (3) (1.0 g, 1.32 mmol) and AlEt3 (1 M in hexane, 2.64 mL, 2.64 mmol). Compound 10 was isolated as a yellow solid. Yield: 74% (0.93 g). 1H NMR (400 MHz, CD3CN, 297 K): δ = 7.69 (d, 3JH-H = 9.2 Hz, 4H, oH-NPh), 7.57 (d, 3JH-H = 9.2 Hz, 4H, mH-NPh), 6.91 (s, 1H, CH), 5.84 (s, 2H, H4), 3.56 (s, 12H, NMe3), 2.17 (s, 6H, Me3,3'), 2.06 (s, 6H, Me5,5'), 0.97, 0.88 (AlCH2CH3), 0.31, 0.15 (AlCH2CH3); 13C{1H} NMR (100 MHz, CD3CN, 297 K): δ = 151.0, 150.0, 147.5, 144.2 (Cipso, C3,3', C5,5'), 130.6 (oC-NPh), 120.8 (mC-NPh), 107.9 (C4), 82.3 (Cª), 70.7 (CH), 58.5 (NMe3), 13.8 (Me3,3'), 11.9 (Me5,5'), 10.4, 10.1


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(AlCH2CH3), 4.9–3.0, (AlCH2CH3); elemental analysis calcd (%) for C40H66Al2I2N6O (954.76): C 50.32, H 6.97, N 8.80; found: C 50.67, H 7.32, N 8.61. Synthesis of [{AlMe2(κ2-mbpzbdeape)}(µ-O){AlMe3}]I2 (11): The synthesis of 11 was carried out in an identical manner to 5, using (mbpzbdeapeH)I2 (4) (1.0 g, 1.23 mmol) and AlMe3 (2 M in toluene, 1.23 mL, 2.46 mmol). Compound 11 was isolated as a yellow solid. Yield: 73% (0.84 g). 1H NMR (400 MHz, CD3CN, 297 K): δ = 7.55 (d, 3JH-H = 9.2 Hz, 4H, oH-NPh), 7.51 (d, 3

JH-H = 9.6 Hz, 4H, mH-NPh), 6.91 (s, 1H, CH), 5.85 (s, 2H, H4), 3.93, 3.79 (m, 8H, NCH2CH3),

3.38 (s, 6H, NMe), 2.15 (s, 6H, Me3,3'), 2.08 (s, 6H, Me5,5'), 1.01 (m, 12H, NCH2CH3), –0.92 (AlMe2), –0.98 (AlMe3);

13

C{1H} NMR (100 MHz, CD3CN, 297 K): δ = 150.9, 149.6, 144.0,

141.4 (Cipso, C3,3', C5,5'), 131.0 (oC-NPh), 122.7 (mC-NPh), 107.8 (C4), 82.5 (Ca), 71.1 (CH), 65.9, 65.7 (NCH2CH3), 47.7 (NMe), 13.8 (Me3,3'), 12.1 (Me5,5'), 9.4 (NCH2CH3), -6.0, -6.5 (AlMe); elemental analysis calcd (%) for C39H64Al2I2N6O (940.74): C 49.79, H 6.86, N 8.93; found: C 49.84, H 6.98, N 8.61. Synthesis of [{AlEt2(κ2-mbpzbdeape)}(µ-O){AlEt3}]I2 (12): The synthesis of 12 was carried out in an identical manner to 5, using (mbpzbdeapeH)I2 (4) (1.0 g, 1.23 mmol) and AlEt3 (1 M in hexane, 2.46 mL, 2.46 mmol). Compound 12 was isolated as a yellow solid. Yield: 69% (0.86 g). 1

H NMR (500 MHz, CD3CN, 297 K): δ = 7.55 (d, 3JH-H = 9.5 Hz, 4H, oH-NPh), 7.52 (d, 3JH-H =

9.5 Hz, 4H, mH-NPh), 6.90 (s, 1H, CH), 5.84 (s, 2H, H4), 3.95, 3.81 (m, 8H, NCH2CH3), 3.39 (s, 6H, NMe), 2.17 (s, 6H, Me3,3'), 2.10 (s, 6H, Me5,5'), 1.02 (m, 12H, NCH2CH3), 0.97, 0.87 (AlCH2CH3), –0.20, –0.31 (AlCH2CH3); 13C{1H} NMR (125 MHz, CD3CN, 297 K): δ = 151.0, 150.0, 141.4 (Cipso, C3,3', C5,5'), 130.8 (oC-NPh), 122.7 (mC-NPh), 107.8 (C4), 82.4 (Ca), 70.6 (CH), 65.9, 65.7 (NCH2CH3), 47.7 (NMe), 13.8 (Me3,3'), 12.2 (Me5,5'), 10.4, 10.2 (AlCH2CH3),


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9.3 (NCH2CH3), 4.5-3.0, (AlCH2CH3); elemental analysis calcd (%) for C44H74Al2I2N6O (1010.87): C 52.28, H 7.38, N 8.31; found: C 52.60, H 7.71, N 8.21. ASSOCIATED CONTENT Supporting Information. Supporting information including experimental details, procedures for catalytic reactions, X-ray crystallographic data for complexes 6 and 8, and copies of the 1H and 13

C NMR spectra for cyclic carbonates 14a–l and 16a–h. This material is available free of charge

via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] (A.O.), Tel.;+34926295300. Fax: +34926295318. †Universidad de Castilla-La Mancha. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Ministerio de Economía y Competitividad (MINECO), Spain (Grant Nos. CTQ2017-84131-R, CTQ2014-52899-R and CTQ2014-51912-REDC Programa Redes Consolider). ACKNOWLEDGMENT We gratefully acknowledge financial support from the Ministerio de Economía y Competitividad (MINECO), Spain (Grant Nos. CTQ2017-84131-R, CTQ2014-52899-R and CTQ2014-51912-


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REDC Programa Redes Consolider). Felipe de la Cruz-Martínez acknowledges the Ministerio de Educación, Cultura y Deporte (MECD) for the FPU Fellowship. ABBREVIATIONS CO2,

carbon

dioxide;

bpzbdmapeH,

(dimethylamino)phenyl)ethan-1-ol;

(1),

2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,1-bis(4-

bpzbdeapeH,

1,1-bis(4-(diethylamino)phenyl)-2,2-

bis(3,5-dimethyl-1H-pyrazol-1-yl) ethan-1-ol; bdmpzm, bis(3,5-dimethylpyrazol-1-yl)methane; (mbpzbdmapeH)I2,

4,4'-(2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-hydroxyethane-1,1-

diyl)bis(N,N,N-trimethylbenzenaminium) iodide; (mbpzbdeapeH)I2, 4,4'-(2,2-bis(3,5-dimethyl1H-pyrazol-1-yl)-1-hydroxyethane-1,1-diyl)bis(N,N,N-trimethylbenzenaminium)-4,4'-(2,2bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-hydroxyethane-1,1-diyl)bis(N,N-diethyl-Nmethylbenzenaminium)

iodide;

AlX3,

trialkylaluminium

compounds;

Bu4NI,

tetrabutylammonium iodide; REFERENCES (1)

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GRAPHICAL TABLE OF CONTENTS ENTRY Bifunctional Earth’s crust abundant metal catalysts have been developed for the chemical fixation of carbon dioxide into cyclic carbonates in good to excellent yields.

FOR TABLE OF CONTENTS USE ONLY


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