Cis-1,4-Polymerization of Isoprene by 1,3-Bis(oxazolinymethylidene)isoindoline-Ligated Rare-Earth Metal Dialkyl Complexes

A series of novel chiral nonmetallocene pincer-type rare-earth metal dialkyl complexes bearing the chiral monoanionic tridentate C2-symmetric 1,3-bis(oxazolinymethylidene)isoindoline (BOXMI-H) ligand (BOXMI)Ln(CH2SiMe3)2 1–3 (1: Ln = Sc, yield = 57%; 2: Ln = Lu, yield = 55%; 3: Ln = Y, yield = 62%) have been prepared in moderate yields via the acid-base reaction between the BOXMI ligand and rare-earth metal tri(trimethylsilylmethyl) complexes. The X-ray diffractions show that both of the complexes 1 and 2 contain one BOXMI ligand and two trimethylsilylmethyl ligands, adopting a distorted trigonal bipyramidal configuration. In the presence of a cocatalyst such as borate and AlR3, these complexes 1–3 exhibit high activities of up to 6.8 × 104 (g of polymer)/(molLn h) and high cis-1,4 selectivities of up to 97% in the polymerization of isoprene in toluene, yielding the cis-1,4-polyisoprenes with heavy molecular weights (Mn of up to 710,000 g/mol) and bimodal molecular weight distributions (Mw/Mn = 2.0–4.5).

Recently, we have paid attention to the synthesis of the NNN-ligated rare-earth metal dialkyl complexes and their applications in IP polymerization. In 2013, we reported the synthesis of a series of chiral (S,S)-bis(oxazolinylphenyl)amine ((S,S)-BOPA)-ligated rare-earth metal dialkyl complexes [(S,S)-BOPA]Ln(CH2SiMe3)2 (1-2; 1: Ln = Sc; 2: Ln = Lu). In the presence of an activator with or without a small amount of Al i Bu3, the dialkyl complexes 1 and 2 exhibited very high activities of up to 6.8 × 10 5 (g of polymer)/(molLn h) and trans-1,4 selectivities of up to 100% in the quasi-living polymerization of isoprene, yielding trans-1,4-PIPs with moderate molecular weights (Mn = (0.2-1.0) × 10 5 g/mol) and narrow molecular weight distributions (Mw/Mn = 1.02-2.66) [38]. Recently, we also reported that the 1,3-bis(2-pyridylimino)isoindoline-ligated rare-earth metal dialkyl complexes showed very high activities of up to 1.9 × 10 6 (g of polymer)/(molLn h) and high cis-1,4 selectivities of >99% in the polymerization of isoprene in the presence of an activator and AlR3, affording CPIP with heavy molecular weights (Mn up to 610,000 g/mol) and narrow to moderate molecular weight distributions (Mw/Mn = 1.26-2.08) [39]. These results demonstrated that the effective adjustment of the skeleton of the pincer-type NNN ligand have an important impact on the catalyst performance of these rareearth metal complexes in IP polymerization, which rouses our interests to explore more NNN-ligated rare-earth metal dialkyl complexes and to detect their catalytic performance in the selective polymerization of IP. 1,3-Bis(oxazolinymethylidene)isoindoline (BOXMI-H) ligand, which has Chart 1. The previous pincer-type rare-earth metal catalysts bearing tridentate C 2 -symmetric chelating ligand.
Recently, we have paid attention to the synthesis of the NNN-ligated rare-earth metal dialkyl complexes and their applications in IP polymerization. In 2013, we reported the synthesis of a series of chiral (S,S)-bis(oxazolinylphenyl)amine ((S,S)-BOPA)-ligated rare-earth metal dialkyl complexes [(S,S)-BOPA]Ln(CH 2 SiMe 3 ) 2 (1-2; 1: Ln = Sc; 2: Ln = Lu). In the presence of an activator with or without a small amount of Al i Bu 3 , the dialkyl complexes 1 and 2 exhibited very high activities of up to 6.8 × 10 5 (g of polymer)/(mol Ln h) and trans-1,4 selectivities of up to 100% in the quasi-living polymerization of isoprene, yielding trans-1,4-PIPs with moderate molecular weights (M n = (0.2-1.0) × 10 5 g/mol) and narrow molecular weight distributions (M w /M n = 1.02-2.66) [38]. Recently, we also reported that the 1,3-bis(2-pyridylimino)isoindoline-ligated rare-earth metal dialkyl complexes showed very high activities of up to 1.9 × 10 6 (g of polymer)/(mol Ln h) and high cis-1,4 selectivities of >99% in the polymerization of isoprene in the presence of an activator and AlR 3 , affording CPIP with heavy molecular weights (M n up to 610,000 g/mol) and narrow to moderate molecular weight distributions (M w /M n = 1.26-2.08) [39]. These results demonstrated that the Polymers 2017, 9, 531 3 of 11 effective adjustment of the skeleton of the pincer-type NNN ligand have an important impact on the catalyst performance of these rare-earth metal complexes in IP polymerization, which rouses our interests to explore more NNN-ligated rare-earth metal dialkyl complexes and to detect their catalytic performance in the selective polymerization of IP. 1,3-Bis(oxazolinymethylidene)isoindoline (BOXMI-H) ligand, which has structural characteristics of both the (S,S)-bis(oxazolinylphenyl)amine and 1,3-bis(2-pyridylimino)isoindoline ligands, is an interesting chiral tridentate C 2 -symmetric NNN ligand for the organometallic complex based on transition metals for the asymmetric reaction [40][41][42][43]. Until now, the BOXMI-H-ligated rare-earth metal complexes have never been reported, and their applications for the coordination-insertion polymerization of olefin have never been investigated, as far as we are aware. Herein, we report the synthesis of three pincer-type BOXMI-H-ligated rare-earth metal dialkyl complexes (BOXMI)Ln(CH 2 SiMe 3 ) 2 1-3 (1: Ln = Sc; 2: Ln = Lu; 3: Ln = Y) via the acid-base reaction between the BOXMI-H ligand and rare-earth metal trialkyl complexes. These complexes 1-3 exhibited high activities of up to 6.8 × 10 4 (g of polymer)/(mol Ln h) and high cis-1,4 selectivities of up to 97% in the IP polymerization in toluene, affording cis-1,4-polyisoprenes with heavy molecular weights (M n of up to 710,000 g/mol) and bimodal molecular weight distributions (M w /M n = 2.0-4.5).

Materials and Method
All catalysts and the polymerization procedure were carried out in a nitrogen-filled MBraun glovebox.  [40] and Ln(CH 2 SiMe 3 ) 3 (THF) 2 [44] were prepared according to the literature. Isoprene was purchased from J&K Chemical (Beijing, China), and dried through CaH 2 . Toluene, THF, and hexane were purified by a solvent purification system (SPS-800, Mbraun, Shanghai, China) and dried over Na in the glovebox. The deuterated solvents C 6 D 6 (99.6 atom% D) and CDCl 3 (99.8 atom% D) were purchased from Cambridge Isotope.
Elemental analyses, 1 H NMR and 13 C NMR, of rare-earth metal complexes were performed according to the literature [38]. Polyisoprene samples of gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) measurements were conducted according to the literature [38].
The test method of crystals of complexes 1 and 2 was performed according to the literature [38]. Crystallographic data of complexes 1 and 2 (excluding structure factors) have been deposited to the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-1539945 (1) and 1536645 (2), containing the supplementary crystallographic data for this paper. These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/ from the Cambridge Crystallographic Data Centre.

A Typical Procedure for Isoprene (IP) Polymerization
A detailed polymerization procedure of isoprene is described here as a typical example. In a glovebox at 25 • C, to a toluene solution (8 mL) of Al i Bu 3 (181 µL, 1.1 M, 200 µmol) was added a toluene solution (8 mL) of complex 1 (0.013 g, 20 µmol), a toluene solution (2.5 mL) of [Ph 3 C][B(C 6 F 5 ) 4 ] (0.018 g, 20 µmol), and isoprene (0.27 g, 4 mmol) in succession. In a 50 mL round bottom flask, the reaction mixture then became viscous rapidly. The flask was taken outside after 0.5 h, and then the solution was added to ethanol (50 mL, containing 5% butylhydroxytoluene (BHT) as a stabilizing agent) to quench the reaction mixture. The obtained polymer was washed three times by ethanol and dried under vacuum at 40 • C to a constant weight (0.23 g, yield ca. 85%). The resulting polymer was soluble in THF and chloroform at room temperature. The isomer contents of the polyisoprene products were calculated from the 1 H and 13 C NMR spectra according to the literature [45].
These complexes 1-3 have good solubilities in common organic solvents such as hexane, toluene, and THF. In the 1 H NMR spectra of the complexes 1-3 in C 6 D 6 , all of the proton signals attributed to the NNN ligand except for the proton signal assigned to the N−H group were observed, suggesting the generation of a monoanionic NNN-chelating ligand in these complexes. The molar ratio of the integral areas of the signals for the NNN ligand and trimethylsilylmethyl ligand was 1:2 in each case. No THF molecule was detected in either case. Similarly to the rigid (S,S)-bis(oxazolinylphenyl)amine-ligated rare-earth metal dialkyl complexes, all of the methylene protons of the Ln-CH 2 SiMe 3 groups showed two doublets at a high field for 1 of 0.16 (d, 2H) and −0.56 ppm (d, 2H), for 2 at −0.62 (d, 2H) and −1.44 ppm (d, 2H), and for 3 at −0.41 (d, 2H) and −1.30 ppm (d, 2H) with a germinal H-H coupling constant of 12 Hz, respectively. These results may suggest that these complexes also have a rigid structure and the CH 2 SiMe 3 groups in these complexes are fixed to some extent at the NMR time scale. agent) to quench the reaction mixture. The obtained polymer was washed three times by ethanol and dried under vacuum at 40 °C to a constant weight (0.23 g, yield ca. 85%). The resulting polymer was soluble in THF and chloroform at room temperature. The isomer contents of the polyisoprene products were calculated from the 1 H and 13 C NMR spectra according to the literature [45].

Single Crystals of Complexes 1 and 2
In the glovebox, single crystals of the complexes 1 and 2 suitable for an X-ray determination were grown from a mixed hexane/toluene solution at −30 • C. The ORTEP (Oak Ridge Thermal Ellipsoid Plot) drawings of the complexes 1-2 are shown in Figure 1. The selected bond distances and angles of these complexes 1 and 2 are summarized in Table 1. The X-ray diffraction study revealed that the dialkyl complexes 1 and 2 are isomorphous and isostructural. Both of these complexes contain one These results may suggest that these complexes also have a rigid structure and the CH2SiMe3 groups in these complexes are fixed to some extent at the NMR time scale.

Single Crystals of Complexes 1 and 2
In the glovebox, single crystals of the complexes 1 and 2 suitable for an X-ray determination were grown from a mixed hexane/toluene solution at −30 °C. The ORTEP (Oak Ridge Thermal Ellipsoid Plot) drawings of the complexes 1-2 are shown in Figure 1. The selected bond distances and angles of these complexes 1 and 2 are summarized in Table 1. The X-ray diffraction study revealed that the dialkyl complexes 1 and 2 are isomorphous and isostructural. Both of these complexes contain one C2-symmetric monoanionic tridentate NNN ligand and two trimethylsilylmethyl groups, adopting a distorted trigonal bipyramidal geometry.

C][B(C 6 F 5 ) 4 ] (A), [PhMe 2 NH][B(C 6 F 5 ) 4 ] (B)
and B(C 6 F 5 ) 3 (C)) binary systems showed very low activities in the polymerization of IP. In the presence of both an activator and AlR 3 , the complexes 1-3 could promote the cis-1,4-polymerization of IP similarly to the 1,3-bis(2-pyridylimino)isoindoline-ligated rare-earth metal dialkyl complexes [39], affording the cis-1,4-polyisoprenes (CPIPs) with heavy molecular weights (M n of up to 710,000 g/mol) and moderate molecular weight distributions (M w /M n = 2.0-4.5). Some representative results are summarized in Table 2. As an activator, the trityl borate A and the anilinium borate B generally exhibited similar activities and cis-1,4 selectivities in the IP polymerization, while the neutral borane C was inert under the same conditions ( Table 2, entries 1-5 and 7-10). For the Sc complex 1, the cis-1,4-PIPs obtained by borate A had heavier molecular weights and narrower molecular weight distributions ( Table 2, entries 1-2 and 4-5), while for the Lu and Y complexes 2 and 3, the cis-1,4-PIPs obtained by borate A also had a heavier molecular weight but a broader molecular weight distribution (Table 2, entries 7-10). Similarly to the complex 1/activator/Al i Bu 3 systems, the complex 1/activator/AlEt 3 and the complex 1/activator/AlMe 3 systems also showed moderate activities of around 3 × 10 3 (g of polymer)/(mol Ln h) and cis-1,4 selectivities of around 88% in the IP polymerization, affording the cis-1,4-PIPs with lower molecular weights and broader molecular weight distributions ( Table 2, entries 1-6). When the IP polymerization catalyzed by the complex 3/A/Al i Bu 3 system was carried out at −20 • C, a PIP with higher cis-1,4 selectivity (>95%), a heavier molecular weight (M n = 350,000 g/mol), and a narrower molecular weight distribution (M w /M n = 1.83) could be obtained, as shown by the 1 H and 13 C NMR analysis (Table 2, entry 12). It is noteworthy that the complex 3 exhibited high activities of up to 6.8 × 10 4 (g of polymer)/(mol Ln h) when the temperature increased to 70 • C (Table 2, entry 14). Only 0.5 h was needed to completely convert 500 equiv of monomer, producing a moderate molecular weight for CPIP (cis-1,4 selectivity of 85%, M n = 200,000 g/mol) with a moderate molecular weight distribution (M w /M n = 3.31).    -1,4-t-1,4 Table 2.

Conclusions
In summary, the three pincer-type monoanionic tridentate C 2 -symmetric BOXMI-H-ligated rare-earth metal dialkyl complexes 1-3 have been easily synthesized in moderate yields via one-pot acid-base reaction by using the rare-earth metal tris(trimethylsilylmethyl) complexes with the readily available BOXMI-H ligand. The X-ray diffractions demonstrated that the complexes 1 and 2 are isomorphous and isostructural and that each of them adopt a distorted trigonal bipyramidal configuration. Activated by the activators ([Ph 3 C][B(C 6 F 5 ) 4 ] (A), [PhMe 2 NH][B(C 6 F 5 ) 4 ] (B) and B(C 6 F 5 ) 3 (C)) and AlR 3 (R = Me, Et and i Bu) in toluene, these pincer-type BOXMI-ligated complexes 1-3 exhibited high activities of up to 6.8 × 10 4 (g of polymer)/(mol Ln h) and high cis-1,4 selectivities of up to 97% in the polymerization of isoprene, affording cis-1,4-PIPs with heavy molecular weights (M n of up to 710,000 g/mol) and bimodal molecular weight distributions (M w /M n = 2.0-4.5). In comparison with the trans-1,4-PIPs obtained by the (S,S)-bis(oxazolinylphenyl)amine-ligated rare-earth metal dialkyl complexes [38] and the cis-1,4-PIPs obtained by the 1,3-bis(2-pyridylimino)isoindoline-ligated rare-earth metal dialkyl complexes [39], such results demonstrate that the main body skeleton of the chelating ligand has a more important impact on the catalytic performance of these pincer-type rare-earth metal dialkyl complexes in the IP polymerization. Moreover, the rare-earth metal dialkyl complexes bearing the pincer-type chelating ligand with the rigid skeleton and the bulky substituents are not good for the cis-1,4-polymerization of IP. These findings will benefit the design of the high-efficiency and selective catalysts, as well as the rapid and precise synthesis of natural rubber. Further studies will be focused on the modification of the chelating ligand to improve the selectivity and/or activity of the rare-earth metal catalytic system in the cis-1,4-polymerization of isoprene.