Homoleptic Rare-Earth Aryloxide Based Lewis Pairs for

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Homoleptic rare-earth aryloxide based Lewis pairs for polymerization of conjugated polar alkenes Pengfei Xu, and Xin Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02875 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Homoleptic rare-earth aryloxide based Lewis pairs for polymerization of conjugated polar alkenes Pengfei Xu, Xin Xu* Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China

ABSTRACT: An efficient Lewis pair polymerization of conjugated polar alkenes utilizing simple, homoleptic rare-earth aryloxides with a combination of phosphines and N-heterocyclic carbenes was developed. The polymerizations were found to be active as Lewis acids across the full range of rare-earth metals, and the catalytic activities were observed to be dependent on the ionic radii of the rare-earth metals and the steric and electronic profiles of the Lewis bases. For the methyl methacrylate polymerization, a syndiotactic polymer was produced with an rr value up to 85%. This rare-earth Lewis pair polymerization system was also found to be effective on more challenging acrylates and acrylamide monomers, such as t-butyl methacrylate, furfuryl methacrylate and N,N-dimethylacrylamide. In the case of furfuryl methacrylate, the polymerization proceeded in a controlled manner with a high initiation efficiency. FLP-type addition was confirmed as the initiating step by stoichiometric reactions producing the zwitterionic active species and the end-group analysis.

KEYWORDS: frustrated Lewis pairs, rare-earth, phosphines, polymerization, polar alkenes

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Introduction Polymers with polar functional groups are of considerable interest for their various applications in modern material science.1 Radical2 or coordination polymerization3 of corresponding polar alkenes offers an efficient and straightforward method to obtain these polymers. To date, numerous discrete metal catalysts have been developed for the coordination polymerization of polar alkenes.4 Among them, rare-earth (RE) metal catalysts play a significant role because they could produce polymers with a high activity and stereotacticity under mild conditions.5 These catalysts are typically dominated by complexes containing RE-H,6 RE-C,7 and RE-N8 σ bonds or divalent rare-earth metal complexes9 as they can undergo σ bond insertion or single-electron transfer to initiate the polymerization. However, most of the abovementioned complexes suffer from synthetic challenges and extremely sensitive properties. Recently, inspired by the flourishing frustrated Lewis pairs (FLPs)10 chemistry, the polymerization of polar alkenes by main-group Al- or B-based Lewis pairs (LPs) was realized.11,12 We previously communicated the polymerization of methyl methacrylate and its cyclic analogues utilizing intramolecular cationic RE (Sc, Y and Lu) based LPs initiated by a novel FLP-type addition.13 Subsequently, we found that a neutral scandium mixed diaryloxide/alkoxyl complex featuring a pendant phosphine group was able to undergo stoichiometric FLP-type reactions.14 Compared to the intramolecular analogues, intermolecular systems are advantageous because their electronic and steric profiles are subject to delicate tuning. Based on our previous work on intramolecular rare-earth FLPs, we began to investigate intermolecular RE-based LPs in polymerization studies. Herein, we report the systematic study of the polymerization of a series of polar alkene monomers in the presence of facile Lewis acids homoleptic rare-earth metal tris-aryloxide complexes [RE(OAr)3, RE = Sc, Y, Sm, and La, Ar = 2,6-tBu2-C6H3]15 and Lewis bases


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commercially available phosphines as well as N-heterocyclic carbenes (NHCs) (Scheme 1). The influences of the ionic radii of rare-earth ions and electronic/steric profiles of the Lewis acid/base on the catalytic activities and structures of resulting polymers are mapped.

Scheme 1. Lewis acids, Lewis bases and monomers examined in this work. Results and Discussion We initially investigated intermolecular RE-based Lewis pairs for the polymerization of the widely used acrylic monomer methyl methacrylate (MMA) and representative results are summarized in Table 1. Conducting the polymerization with the Sc complex 1/PPh3 pair only led to a low monomer conversion of 15% after 24 h, producing PMMA with a relatively broad polydispersity (PDI) of 1.70 (Table 1, entry 1). An increased activity was achieved when we switched the Lewis base component to a more basic tris-alkyl phosphine tricyclohexylphosphine (PCy3), which consumed all the monomer in a short time (60 min, Table 1, entry 2). However, the polymer had a much higher molecular weight (Mn = 14.3 × 104 g/mol) than the calculated


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Table 1. Polymerization of MMA with homoleptic rare-earth aryloxide based LPs.a Entry

Lewis acid

Lewis base

[M]/[LB]

t

Conv.

Mn

PDI

rr

mr

mm

I*

[min]

[%]b

[104 g/mol]

[Mw/Mn]

[%]

[%]

[%]

[%]

[LA]

[LB]

1

1

PPh3

200

1440

15

7.61

1.70

75.3

21.9

2.8

27

2

1

PCy3

200

60

100

14.3

1.28

71.0

24.1

4.8

14

3

1

PEt3

200

45

100

3.76

1.21

72.2

25.5

2.3

54

4b

1

PEt3

200

720

100

6.87

1.26

80.2

18.1

1.7

29

5c

1

PEt3

200

4320

100

8.69

1.44

85.0

14.3

0.7

23

6

1

PMe3

200

60

100

3.20

1.22

70.5

25.2

4.3

63

7

1

tBu

200

60

82

17.7

1.05

73.3

24.0

2.7

Y > Sc). For the lanthanum containing pair, even with a very small amount of catalyst loading (0.25 mol%), the polymerization could still achieve 97% monomer conversion in 5 min, producing syndiotactic-rich PMMA with a high molecular weight (Table 1, entry 12). Consequently, these intermolecular RE LPs exhibited much higher activity and stereotacticity than previously reported intramolecular cationic RE/P systems in MMA polymerization.13 It was also noted that the homoleptic rare-earth aryloxide, e.g., 4 alone, was not able to polymerize MMA under our standard conditions up to 24 h (Table S1, entry 16). Finally, the dimeric La complex 523 employed a less sterically demanding aryloxyl ligand exhibited no polymerization


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activity in conjunction with PEt3, presumably due to the high coordination number at the metal center (Table 1, entry 13). Our intermolecular RE/P systems were also found to be active for the polymerization of the sterically encumbered methacrylate, tBuMA, which is an industrially well-known monomer with wide applications.24 This monomer was quantitatively converted to poly(tBuMA) in 90 min by the La complex 4/PEt3 pair (Table 2, entry 1). Switching to a less sterically demanding phosphine (PMe3) did not markedly alter the polymerization activity; however, it yielded a polymer with a much lower Mn of 2.16 × 104 g/mol, resulting in a higher initiator efficiency (I* 66%)17 (Table 2, entry 2). This observation probably indicated that a less sterically hindered phosphine facilitates the initiation process of polymerization. Table 2. Polymerization of polar alkenes with homoleptic rare-earth aryloxide based LPs.a Entry Monomer Lewis acid [M] [LA]

Lewis base

[M]/[LB] t

Conv. Mn

PDI

I*

[min] [%]

[104 g/mol]

[Mw/Mn]

[%]

[LB]

1

t

BuMA

4

PEt3

100

90

100

4.03

1.34

36

2

t

BuMA

4

PMe3

100

90

100

2.16

1.36

66

3

FMA

1

PEt3

100

10

100

1.95

1.36

86

4

FMA

1

PMe3

100

30

100

1.92

1.22

87

5

FMA

4

PEt3

100

1440

0

0

0

-

6

FMA

4

PMe3

100

1440

0

0

0

-

7

DMAA

1

PEt3

100

30

100

2.36

1.73

42

a

Conditions: Polymerizations were conducted at room temperature in toluene (Vmonomer/Vsolvent: 1:2) and a LA/LB ratio of 2, where n[LA] = 40 µmol. Monomer conversions were determined by 1 H NMR spectroscopy and confirmed by gravimetric methods. Mn and PDI were determined by GPC in DMF relative to the PMMA standards.


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The scope of the monomer was subsequently extended to a heterocycle substituted acrylate, furfuryl methacrylate (FMA), which is of great interest in clinical applications, coatings, and adhesives due to its low polymerization shrinkage.25 Remarkably, the reactive pendant furfuryl group makes it possible to modify the polymer through a Diels-Alder reaction26 or to cross-link the polymer via UV irradiation27. However, classical radical polymerization of FMA leads to insoluble and gelled polymers due to chain transfer reactions resulting from the reactive furfuryl group.28 To our delight, polymerization of FMA by the Sc complex 1/PEt3 pair achieved a quantitative monomer consumption in 10 min to produce poly(FMA) with Mn = 1.95 × 104 g/mol and PDI = 1.36 (Table 2, entry 3). Replacement of PEt3 with the less sterically demanding PMe3 led to a narrower PDI of 1.22 (Table 2, entry 4). The polymerization of FMA with the 1/PMe3 pair also showed linear growth of the Mn with the increasing monomer conversion (Figure S10). Together with its high initiator efficiency (87%), the FMA polymerization, promoted by the intermolecular RE/P Lewis pair, proceeded in a living fashion. Surprisingly, when conducting the same polymerization using the La complex 4/PEt3 pair, which exhibited the highest activity in MMA and tBuMA polymerization, no polymer was produced even after 24 h (Table 2, entry 5). Switching to a less sterically demanding phosphine (PMe3) did not improve the polymerization performance (Table 2, entry 6). We assumed that the larger ionic radius of lanthanum allowed the furan oxygen atom to attach to the metal center, leading to a more crowded coordination environment that may hinder the formation of the catalytically active Lewis pair and/or the coordination of the monomer. Poly(acrylamides) are ubiquitous and widely used in many fields.29 Finally, polymerization of N,N-dimethylacrylamide (DMAA) was also investigated in our systems. Preliminary results showed that the Sc complex 1/PEt3 pair was effective for the polymerization, affording a


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quantitative monomer conversion in 30 min (Table 2, entry 7). The polymer produced featured a Mn of 2.36 × 104 g/mol and a PDI of 1.73.

Scheme 2. FLP-type 1,4-addition reactions of the intermolecular Sc/P Lewis pair to MMA and FMA. We proposed a conjugate-addition mechanism and an FLP-type 1,4-addition as the initiating step for current polymerizations (Scheme S3), which is consistent with previous examples of main group11,12 and cationic rare-earth metal13 based Lewis pairs promoted polymerizations. Therefore, we investigated stoichiometric reactions of the Sc complex 1/PEt3 pair with conjugated polar alkenes. First, treatment of the 1/PEt3 pair with MMA in a 1:1 molar ratio in toluene at room temperature yielded the Sc/P FLP-type 1,4-addition complex 6 as a white crystalline solid (Scheme 2, 82% yield), which was comprehensively characterized by multinuclear NMR spectroscopy, elementary analysis, and single-crystal X-ray diffraction (Figure S4). It should be noted that only the E-isomer of 6 was selectively produced during the stoichiometric reaction, while the reactions of the intermolecular main group FLPs with MMA usually afforded 1,4-addition products as a mixture of E- and Z-isomers.11 Based on the loading of LA and LB in the polymerization, we also carried out the stoichiometric reaction of complex 1 with PEt3 and MMA in a 2:1:1 molar ratio, which clearly showed the formation of complex 6 and the unreacted complex 1 in a 1:1 ratio (for details, see the Supporting Information).


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Furthermore, complex 6 itself could catalyze the polymerization of MMA in CH2Cl2 with a low activity, transforming 100 equiv. of MMA in 720 min (Table S1, entry 17, Mn = 2.87 × 104 g/mol, PDI = 1.16). Nevertheless, highly active polymerization could be achieved by adding an additional equivalent of the Lewis acidic Sc complex 1, affording a quantitative monomer consumption in 15 min (Table S1, entry 18, Mn = 3.67 × 104 g/mol, PDI = 1.24). The analogous reaction of the Sc/P pair with FMA also yielded the trans-configurated complex 7 as a white crystalline solid (67% yield, Scheme 2), which represents the first structurally characterized intermediate of the FMA polymerization (Figure 1). Complex 7 exhibited similar structural features and spectroscopy properties to those of the MMA addition product 6 (for details, see the Supporting Information). The polymerization of FMA catalyzed by complex 7 with [FMA]/[7] = 50 in CH2Cl2 led to no monomer conversion up to 24 h (Table S2, entry 9), presumably owing to the steric encumbrance of the zwitterionic propagating species and monomer. This scenario was also confirmed by the stoichiometric reaction of complex 7 with 1 equivalent of FMA, which did not occur at room temperature in 24 h. On the other hand, mixing 7 with 1 equivalent of 1 in CH2Cl2 followed by the addition of FMA (100 equiv.) resulted in 100% monomer conversion in 10 min (Table S2, entry 10, Mn = 2.12 × 104 g/mol, PDI = 1.30), suggesting an activated monomer propagation mechanism.11 Thus, complexes 6 and 7, as potential catalytic active species, provide useful insight into the mechanism of the Lewis pair catalyzed polymerizations of the polar monomers.


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Figure 1. Molecular structure of the FMA addition product 7. Hydrogen atoms are omitted for clarity, ellipsoids are drawn at 30% probability. Selected bond lengths (Å) and angles(o): Sc1-O1 1.9720(15); O1-C1 1.310(3); C1-C2 1.346(3); C2-C3 1.517(3); P1-C3 1.800(2); C1-O1-Sc1 161.35(14); O1-C1-O2 115.48(18); O1-C1-C2 127.90(19); C1-C2-C3 118.94(19); C4-C2-C3 117.17(18); C2-C3-P1 115.46(15). Unfortunately, the stoichiometric reaction of the Sc complex 1/PEt3 pair with acrylamide DMAA failed to isolate the similar 1,4-addition product, which could imply that the initiation is the rate-limiting step in the polymerization process and the active species is consumed as soon as it is generated11c. To support a conjugate-addition mechanism induced by the Lewis basic phosphine, we performed an oligomerization reaction using the 1/PEt3 pair in a [DMAA]/[PEt3] molar ratio of 20. The MALDI-TOF MS spectrum of the resulting oligomers showed a major series of mass ions with a repeat unit of 99.1 (mass of DMAA, Figure S11). A plot of m/z values of this series versus the number of DMAA repeat units yielded a straight line with a slope of 99.09 and an intercept of 119.20 (Figure S12). The intercept is equal to the sum of the masses of H+ and PEt3 moieties, suggesting that the polymer has a structural formula of Et3P+-(DMAA)n-H. Conclusion


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Although polymerization of conjugated polar alkenes by a variety of rare-earth metal catalysts has been widely investigated and well established during last two decades,6-9 it is noteworthy that we developed a unique and efficient Lewis pairs polymerization using simple, homoleptic rare-earth aryloxides with a combination of commercially available phosphines and N-heterocyclic carbenes under mild conditions. The mechanistic study revealed that the polymerization promoted by these intermolecular RE/P(NHC) systems was initiated by an FLPtype addition rather than the traditional and ubiquitous RE covalent bond insertion or singleelectron transfer. Compared with main group or intramolecular cationic RE based Lewis pairs, our systems offer great potential for diverse and delicate tuning of electronic and steric parameters of Lewis acid components, thus producing polymers with tunable key parameters. Remarkably, this rare-earth Lewis pair system can be employed for the polymerization of furfuryl methacrylate, which is difficult to realize by other approaches due to the chain transfer reaction, affording a polymer with a high initiation efficiency in a living fashion. Furthermore, the work presented here represents the first examples of intermolecular rare-earth based Lewis pairs in polymerization, which may open other possibilities for small molecule activation and catalysis.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental procedures, characterizations of the 1,4-addition complexes, and a full listing of the polymerization results (PDF). X-ray crystallographic data (CIF).


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AUTHOR INFORMATION Corresponding Author *E-mail for X.X.: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21502132), the Natural Science Foundation of Jiangsu Province (Grant No. BK20150316), Jiangsu Specially-Appointed Professor Plan, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES (1) (a) Davis, T. P.; Haddleton, D. M.; Richards, S. N. J. Macromol. Sci. Part C. 1994, 34, 243-324. (b) Antoun, S.; Teyssié, Ph.; Jérôme, R. Macromolecules 1997, 30, 1556-1561. (c) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000, 100,1479-1493. (2) Kamigaito, M.; Satoh, K. Macromolecules 2008, 41, 269-276. (3) Cameron, P. A.; Gibson, V.; Graham, A. J. Macromolecules 2000, 33, 4329-4335. (4) Chen, E. Y. -X. Chem. Rev. 2009, 109, 5157-5214. (5) (a) Yasuda, H. J. Organomet. Chem. 2002, 647, 128-138. (b) Yasuda, H.; Ihara, E. Macromol. Chem. Phys. 1995, 196, 2417-2441. (c) Yasuda, H. J. Polym. Sci.; Part A: Polym. Chem. 2001, 39, 1955-1959.


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ACS Catalysis

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Homoleptic Rare-Earth Aryloxide Based Lewis Pairs for

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