cyclohexene epoxide copolymerization

European Polymer Journal 120 (2019) 109245

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Quinoline-8-olato-chromium catalysts with pseudohalogen effects for the CO2/cyclohexene epoxide copolymerization

T

⁎

Manuel Hartweg , Jörg Sundermeyer Department of Chemistry and Material Science Center, Philipps-Universität Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2/epoxide copolymerization Ring-opening polymerization Chromium catalysts Polymerization catalysis Polycarbonates

Based on our previous studies utilizing CrIII catalysts with trifluoroacetate as labile ligand and an ansa-bisquinoline-olato ligand as a chemically more robust variant compared to known salen-type N2O2 ligands, we synthesized and investigated a series of structurally similar quinoline-8-olato chromium complexes with different active (pseudo-)halogen ligands X in [(babhq)CrX], (babhq = 2,2′-(butyl-azanediyl)bis(quinolin-8-olate), X = Cl, NCO, and N3. We demonstrate a high activity of these CrIII complexes in the solvent free carbon dioxide/ cyclohexene oxide (CHO) copolymerization compared to the state of the art. In much deeper detail than in our previous communication, we altered the labile ligand X and undertook a catalyst screening on the polymerization parameters, such as pressure, temperature, and cocatalyst, i.e. [PPN]N3 (bis(triphenylphosphine) iminiun azide). The activity of the chromium systems was found to much depend on the labile (pseudo-)halogen ligand X and increases in the order Cl < NCO < N3. Especially the azide complex [(babhq)CrN3] displays a high activity, displaying a TOF of 2165 h−1 after 1 h of reaction time along with an induction period of 30 min. At longer reaction times, up to 94% monomer conversion selectively into the respective polycarbonate (pCHC) is observed.

1. Introduction The greenhouse gas carbon dioxide is a chemically friendly reagent [1–3]. Compared to the consumption of rapidly dwindling fossil feedstock, which is not renewed on a human scale of time, the chemical utilization of abundant CO2 has the advantage of being sustainable and potentially very economical [4–8]. Hence, a big challenge for chemists of the 21st century is the investigations of novel technologies for the capture and fixation of carbon dioxide into valuable and cost competitive materials (e.g. plastics) [4,5,9–12]. Amongst others, the synthesis of polymeric materials, such as polycarbonates, from carbon dioxide and epoxides plays an important role nowadays [1,3,4,13–17]. In the pioneering work in 1969, Inoue et al. disclosed the first example of metal catalyzed carbon dioxide/epoxide copolymerization using ZnEt2 as catalyst [18]. Based on their results, in the 1980s Rokicki and Kuran et al. developed further catalytic zinc systems [19]. Nowadays, a variety of discrete complexes are reported as active catalysts in the CO2/epoxide copolymerization using different metal centers, such as aluminum [20], cobalt [16,21], chromium [22–25], zinc [20,26,27], magnesium [28], iron [29], titanium and germanium [30]. Amongst, various (transition) metal complexes with salen (N,Ń -ethylen-bis(salicylimine)) or related structurally and electronically similar N2O2 ligand ⁎

are the most intensively studied systems [1,4,5,14,17,22,27,31,32]. By employing these catalysts, polymers with unique structures, and architectures, as well as properties can be manufactured [15,33–36]. Darensbourg et al. for instance studied the influence of different reaction parameters on the carbon dioxide/epoxide copolymerization in presence of N2O2 salen-type catalysts. Each of these systems owns intrinsically distinctive parameters that need to be identified specifically for each system [1]. First, the effect of temperature was found to be crucial [37], if the formation of cyclic carbonates as byproducts should be avoided. Second, the influence of the pressure was examined, which affected the catalyst activity, e.g. due to dilution effects [38]. Third, also the effect of the cocatalyst on the copolymerization was reported [39]. The hydrophobic PPN+ ((bis(triphenyl-phosphine)iminiun) salts was found to show superior reactivity compared to ammonium salts and neutral nucleophiles as cocatalysts. Thereby, a decreasing catalytic activity in order of N3 > Cl > Br > I > OAc > HCO3 of the anions was observed [40,41]. Last, variation of the active/initiation ligand has a notable impact on the catalyst activity. The azide ligand showed the highest activity in the respective CHO/carbon dioxide copolymerization. As of 2000, chromium catalysts have been reported for the copolymerization of CHO and CO2 in many variations and alternations

Corresponding author. E-mail addresses: [email protected] (M. Hartweg), jsu@staff.uni-marburg.de (J. Sundermeyer).

https://doi.org/10.1016/j.eurpolymj.2019.109245 Received 16 August 2019; Received in revised form 5 September 2019; Accepted 6 September 2019 Available online 07 September 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

European Polymer Journal 120 (2019) 109245

M. Hartweg and J. Sundermeyer

[(babhq)CrNCO(EtOH)] 2c was obtained from AgNCO and 2a in ethanol as a brown powder in 88% yield. Therefore, 2c was characterized as ethanol complex [(babhq)CrNCO(EtOH)] by means of HRESI/MS, elemental analysis, and IR spectroscopy.

[14,42]. In early studies [43,44], a TON values of up to > 2000 have already been reported for the CHO/CO2 copolymerization, indicating the high catalytic activities of N2O2-based chromium catalysts. Additionally, Kozak et al. studied a series of diamine-bis(phenolate) chromium(III) complexes [25,45,46] for the above mentioned copolymerization, yet with a lower turnover of typically < 500. Recently, we reported the utilization of the versatile bis(8-hydroxyquinolinyl)-butylamine (babhq) ligand as promising and highly active catalyst [(babhq)Cr(OAcF)] in order to generate cyclic carbonates from CO2 and propylene oxide, as well as for CO2/CHO copolymerization [47]. Compared to the sterically and electronically equivalent salen ligand, the advantage of babhq is its high chemical robustness of its imine functionality due to incorporation into the aromatic system. Its stereo rigidity is more pronounced than that of salen. Nevertheless, the babhq backbone can distort so that two cis coordination sites preferentially used in intramolecular epoxide ring opening by X in the initial step and by alkyl carbonate in the following steps can be provided. Based on our previous findings with [(babhq)Cr(OAcF)], we here present the preparation of optimized [(babhq)CrX(solv)] (X = Cl, N3, NCO, solv = solvent) catalysts and their application in the carbon dioxide/cyclohexene oxide copolymerization. Moreover, on a small scale, polymerization parameters, namely pressure, temperature and cocatalyst were investigated and the conditions upscaled, before reactions were carried out on a larger sale. Overall, we demonstrate that our [(babhq)CrX(solv)] catalysts were found to be active and robust chromium systems and hence can be used as model system for the investigation of the CHO/CO2 copolymerization.

2.2. Copolymerization of CO2 and cyclohexene oxide using the catalysts 2a-c: catalyst screening Based on the reaction conditions applied by Darensbourg et al. [22] and Elmas and Harrer et al. [47], a catalyst screening for the catalysts 2a-c on the copolymerizaton of CO2/CHO was carried out. Trends on the influence of temperature, pressure and cocatalyst were identified by screening reactions on 1.00 mL scale for 4 h in 2.00 mL glass vials with a slit septum cap in a vial holder as autoclave insert (c.f. Fig. S3). After each screening reaction the percentage of polycarbonate and cyclohexene carbonate formation was determined by 1H NMR spectroscopy. For each catalyst series 2a-c, average conversions into pCHC und cyclohexene carbonate (CHC) were determined. At first, it was screened for the pressure (20, 25, 30, and 35 bar, Fig. 1) at constant temperature (100 °C) in presence of PPNCl. A peak for the formation of pCHC (41% on average) was reached at 25 bar. Higher and lower temperatures led to lower conversions. Surprisingly, the formation of cyclic carbonate was not affected by the variation of the pressure. Almost constant values of 5% to 10% were obtained. Next, the catalysts were screened for the temperature (80, 100 and 120 °C, Fig. 2) at constant pressure (25 bar) in presence of PPNCl. The highest conversion into pCHC was obtained at 100 °C. At lower temperature, a significant decrease of polycarbonate formation was observed, whereas at elevated temperatures only a slightly lower average conversion was achieved. However, the formation of CHC was highly depended on the temperature. With an increasing temperature, the ratio of CHC formation increased in a linear manner. Additionally, a variation of the cocatalyst (PPNCl vs. PPNN3) at constant pressure (25 bar) and temperature (100 °C) was investigated. The conversions on this scale were not significantly different, however, on a larger scale a significant enhancement was observed. This indicates that the weakly solvated “naked” anions delivered by the cocatalyst have the strongest impact on the reaction rate. According to Darensbourg et al. [22] and Elmas and Harrer et al. [47], we expect that the coordinated solvent molecules (solv), can be substituted by CHO to form activated complex [(babhq)CrX(CHO)] and that the mobile nucleophilic anion (Nu) of the cocatalyst attacks the epoxide ring carbon from the back leading to an SN2-type ring-opening reaction followed by CO2 insertion to an O-coordinated cyclohexyl carbonate anion. Cyclohexyl- or later on polymer-cyclohexyl-carbonate anions become the chain-propagating nucleophiles for further CHO ring-opening steps. In competition with this path, the cocatalyst anions Nu might substitute solvent and pre-coordinate to the metal center to form hexacoordinate anionic PPN+[(babhq)CrX(Nu)]− (X = Cl, NCO, N3; Nu = Cl, N3 delivered from PPNX), which would play the role of strongest nucleophile in intermolecular ring-opening reactions. However, it is noteworthy that when PPNN3 is applied as cocatalyst in the subsequent large scale

2. Results and discussion 2.1. Synthesis of [(babh)qCrX(solv)] (X = Cl, N3, NCO) complexes In order to prepare the complex [(babhq)CrCl(H2O)] 2a the procedure reported previously was adopted (Scheme 1) [47]. Metallation of babhq 1 was accomplished by deprotonation with potassium hydride under aprotic conditions, and subsequent introduction of CrIII as [CrCl3(thf)3]. The resulting aqua complex 2a was obtained by precipitation after adding water in 88% yield as yellow powder. Based upon, the pseudohalide derivatives 2b and 2c were synthesized from 2a. The axial azide ligand was chosen with respect to the high activity documented in corresponding salen-derived complexes [22]. However, since the potentially hazardous preparation using explosive AgN3, in handling safer AgNCO was used to generate an isocyanato complex as isoelectronic and steric equivalent to the azide derivative. Complexes 2b and 2c were obtained by exchange of chlorido ligand of 2a via silver azide or isocyanate (Scheme 2). For the synthesis of azide derivative an ethanol/acetonitrile 2/1 mixture was used as solvent in order to overcome solubility issues of AgN3 (caution: highly explosive!) and to increase the yield of the light green product 2b (92%) precipitated from and washed with water. Therefore, 2b was characterized as aqua complex [(babhq)CrN3(H2O)] via elemental analysis, IR spectroscopy and HR-ESI/MS. Isoelectronically related

Scheme 1. Synthesis of complex [(babhq)CrCl(H2O)] 2a. 2

European Polymer Journal 120 (2019) 109245

M. Hartweg and J. Sundermeyer

Scheme 2. Synthesis of the complexes [(babhq)CrX(solv)] (X = N3, NCO) 2b and 2c.

Fig. 1. Results of the catalyst screening for the pressure at 100 °C in presence of PPNCl: conversion into polycarbonate (dark grey) and conversion into cyclohexene carbonate (light grey).

Fig. 3. Results of the screening for the catalysts 2a-c: conversion into polycarbonate (dark grey) and conversion into cyclohexene carbonate (light grey).

After each large scale reaction, the percentage of polycarbonate and cyclohexene carbonate formation was determined by 1H NMR spectroscopy. Initially, 2a was employed (Table 1, Entry 1 and 2) as catalyst. A reaction time of 4 h led to conversion of 58% and a yield of 51%. The obtained polymers exhibited comparable molecular weights and dispersities. After 10 h of reaction time, a CHO conversion of 95% was observed, whereby 9% cyclic side product and 86% of pCHC were formed. However, 82% of copolymer was isolated after work up. In contrary, catalyst 2b formed exclusively pCHC with a very high catalytic activity of 886 turnovers h−1 after 4 h of reaction time. An extension of the reaction time to 10 h led to 94% conversion of the epoxide CHO monomer into copolymer, resulting in 89% isolated yield. Furthermore, GPC analysis showed two maxima with corresponding molecular weights of 23 280 and 7290 g·mol−1 and dispersities of 1.22 and 1.25, respectively. However, when using 2c, conversions of 73% after 4 h with no significant increase after 10 h reaction time were observed. However, a good turnover frequency of 585 h−1 was revealed after 4 h.

Fig. 2. Results of the catalyst screening for the temperature: conversion into polycarbonate (dark grey) and conversion into cyclohexene carbonate (light grey).

2.4. Kinetic investigation of [(babhq)CrN3(H2O)] 2b

reactions, it shows superior reactivity to PPNCl. Last, an overall trend for the activity of the axial (pseudo-)halogen ligands can be observed from the results of the screening (Fig. 3). The azide derivative 2b displayed the highest average conversion (39%) of the co-monomers into pCHC. 2a showed only a slightly lower conversion (35%), whereas 2c formed the lowest amount of copolymer (29%). In terms of undesired CHC formation, all 2a-c displayed similar and continuously low ratios (of about 7%).

Due to the excellent performance of the 2b/PPNN3 system in the CO2/CHO copolymerization a conversion with time profile was taken, the reaction progress was monitored by 1H NMR spectroscopy. Surprisingly, after 30 min of reaction time, no significant conversion into pCHC was observed (Fig. 4, Spectrum 2). Thus, neither the presence of polycarbonate signals was detected nor TON and TOF were determined (see Fig. S4). This observation indicated that the catalyst system needs an induction period, which is apparently longer than 30 min. However, after 60 min the conversion dramatically increased (spectrum 3). The induction period can be explained by the presence of the cocatalyst [17]. The respective cocatalyst anion, which is present abundantly with respect to the catalyst, may compete with the insertion of the epoxide (initiating step) [47], and perhaps forms a more potentially more stable bis-azido trans-CrIII complex. Dissociation of this of the second labile ligand is known to be slow [46], which therefore

2.3. Copolymerization of CO2 and cyclohexene oxide using the catalysts 2a-c: larger scale The reactions described in this section were carried out on 10.0 mL scale under the conditions listed below (Table 1). In contradiction to the results obtained in the screening, the catalyst systems employing PPNN3 as cocatalyst revealed superior activity to PPNCl. Thus, the up scaled reactions were carried out in presence of PPNN3 as cocatalyst. 3

European Polymer Journal 120 (2019) 109245

M. Hartweg and J. Sundermeyer

Table 1 Results

of

the

copolymers

obtained

from

the

copolymerizations

of

CO2/CHO

using

the

catalysts

2a-

c.

Entry

Catalyst (CHO/Cat/Cocat)

t (h)

Conv. into pCHC (CHC) (%)a

Isolated yield pCHC (g)

Mnb (g·mol−1)

Đc

TONd

TOFe (h−1)

1

2a [2415/1/2.5] 2a [2220/1/2.1] 2b [4168/1/3.6] 2b [3404/1/3.5] 2c [3379/1/3.0] 2c [3076/1/3.0]

4

58 (8)

7.17 (51%)

15 000

1.66

1235

308

10

86 (9)

11.5 (82%)

11 600

2.11

1820

182

4

90 (< 1)

11.9 (85%)

16 500

1.57

3543

886

2 3 4 5 6

c

c

10

94 (< 1)

12.4 (89%)

23 300 / 7300

1.22 / 1.25

3016

302

4

73 (13)

9.70 (69%)

11 200

1.55

2341

585

10

74 (13)

10.2 (72%)

12 600

1.98

2241

224

bimodal distribution potentially caused by termination reaction of coordinating solvent or water. a determined by 1H NMR analysis from crude reaction mixture: (A4.62/(A4.62 + A4.00 + A3.10). b Analyzed by GPC. c Đ = Mw/Mn. d Mol epoxide consumed/mol catalyst. e Mol epoxide consumed/mol catalyst · h.

catalysts. Another 120 min led to a conversion of 90% (spectrum 5), a yield of 85% and a TOF of 886 h−1). An almost full conversion of 94% of the cyclohexene monomer was achieved after 10 h of reaction time (spectrum 6). The long recording time is directly related to a further significant decrease of the activity to 302 h−1, however, 89% of copolymer was isolated (spectrum 7). On the basis of this previously not recognized induction period of 30 min, we draw the following

greatly increases the initiation time of the polymerization. For the CO2/CHO copolymerization using a chromium catalyst system A catalyst activity of 2165 h−1 was observed after 1 h . After 120 min, a conversion of 79% was observed (spectrum 4), resulting in 75% isolated yield. As expected, the catalyst activity and TOF value decreases to 1384 h−1 with the increasing reaction time, which still displays a high value for this copolymerization using chromium

Fig. 4. Reaction progress of the CO2/CHO copolymerization using the catalyst system 2b/PPNN3. 4

European Polymer Journal 120 (2019) 109245

M. Hartweg and J. Sundermeyer

conclusions: It is the combination of the pre-coordinated mobile ligand X and the extra mobile anion azide provide by the PPN salt which leads to a much more active catalyst system than reported previously [41]. As after the first catalytic cycle, we are facing essentially the same polymer alkoxido and alkyl carbonato groups for further migratory insertion steps at the [(babhq)Cr] scaffold, we believe, that the higher activity of our catalyst system compared to previously reported is mainly due to the shorter induction periods in the presence of PPNN3 cocatalyst under the reported conditions. In fact, the catalytically active species generated from 2b must have a higher activity than suggested by a TOF of 2165 h−1 after 1 h, since in the first 30 min induction period, no conversion was observed. Further studies on the nature of the active species formed by the precatalyst 2b with one mole of PPNN3 and one mole of CHO would be required, e.g. by means of XRD and theory studies.

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

3. Conclusions

[20]

Based on our previous study investigating [(babhq)Cr(OAcF)] as precatalyst in CO2 and CHO copolymerisation, more active [(babhq)CrX (solv)] precatalysts with the labile ligands (X = N3, NCO) were discovered and synthesised. A considerable improvement to the state of the art was gained by applying PPNN3 as preferred cocatalyst. By investigating the kinetic profile of the most active combination of precatalyst and cocatalyst, an induction period of at least 30 min, which is needed for the formation of the catalytically highly active species, was discovered. Once formed, this species displays a TOF > 2165 h−1 within the first thirty minutes which is a top performance for chromium based catalysts. The isoelectronic isocyanato ligand was proven to be a versatile and safer (if prepared via AgN3) alternative to X = azide, albeit the catalytic performance was not exactly as good as using the azide ligand.

[21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]

Declaration of Competing Interest

[35] [36] [37]

The authors declare no competing financial interest. Acknowledgements

[38]

We thank Dr. Julian Kuttner for his valuable support with the GPC analysis.

[39] [40]

Appendix A. Supplementary material

[41] [42] [43] [44]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.109245.

[45] [46] [47]

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