Cis-1,4-Polymerization of Isoprene Catalyzed by 1,3-Bis(2

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Cis-1,4-Polymerization of Isoprene Catalyzed by 1,3-Bis(2pyridylimino)isoindoline-Ligated Rare-Earth-Metal Dialkyl Complexes Pengfei Zhang,† Hongyun Liao,† Hanghang Wang,† Xiaofang Li,*,† Fanzhi Yang,*,‡ and Shaowen Zhang*,† †

School of Chemistry and Chemical Engineering, Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, People’s Republic of China ‡ Advanced Research Institute for Multidisciplinary Science, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, People’s Republic of China S Supporting Information *

ABSTRACT: The two rare-earth-metal dialkyl complexes 1 and 2 (1, Ln = Sc; 2, Ln = Lu) were obtained via the same acid−base reaction between 1,3-bis(2-pyridylimino)isoindoline (BPI) ligand and rare-earth-metal trialkyl complexes. These complexes 1 and 2 were structurally characterized by X-ray diffraction. Both in the solid state and in the solution state, the mononuclear Sc dialkyl complex 1, containing one monoanionic tridentate C2-symmetric pincer-type BPI ligand, adopts a distorted-trigonal-bipyramidal configuration. When the Sc center of complex 1 was replaced by the larger Lu center, the intramolecular proton transfer from the isoindoline nitrogen atom to one of the imine nitrogen atoms could be observed in the tautomeric BPI ligand, which served as a monoanionic tetradentate ligand bridging two Lu centers and finally afforded the binuclear Lu dialkyl complex 2 with a cage-like symmetrical structure in the solid state. However, this binuclear Lu complex 2 could dissociate into a mononuclear structure in the solution state similar to the case for the scandium complex 1 since the same C2 symmetry was also observed in the 1H and 13C NMR spectra of the lutetium complex 2 in C6D6. In the presence of cocatalyst borate and AliBu3, these complexes 1 and 2 exhibited high activities (up to 1.9 × 106 (g of polymer)/(molLn h)) and high cis-1,4-selectivities (>99%) in the polymerization of isoprene in toluene, affording the cis-1,4-polyisoprenes with high molecular weights (Mn up to 610000 g mol−1) and narrow to moderate molecular weight distributions (Mw/Mn = 1.26−2.08).

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INTRODUCTION The development of highly efficient and regio-/stereoselective rare-earth-metal catalysts for the polymerization of conjugated dienes has brought new opportunities for the preparation of high-performance synthetic rubbers with precisely controlled microstructures and desired properties.1 Among these synthetic rubbers, cis-1,4-polyisoprene (CPIP), which has a structure similar to that of natural rubber, has drawn much attention from both academic and industrial researchers in view of the limited supply of natural rubber and increasing demands for synthetic rubbers. So far, a large number of half-sandwich and nonmetallocene rare-earth-metal catalysts bearing different chelating ligands have been reported for the cis-1,4-polymerization of isoprene.2 In general, the tridentate C2-symmetric pincer-type rare-earth-metal catalysts usually exhibit both high activity and high cis-1,4-selectivity in the polymerization of isoprene (Chart 1).3 In 2007, Hou reported that cationic alkyl rare-earth-metal complexes bearing a bis(phosphinophenyl)© XXXX American Chemical Society

amido (PNP) ligand could serve as effective catalysts in the living cis-1,4-polymerization of isoprene with activity up to 4.9 × 105 (g of polymer)/(molLn h) and cis-1,4-selectivity up to 99%.4 In 2008, Cui developed aryldiimine (NCN)-ligated rareearth-metal dichlorides as catalyst precursors in the cis-1,4polymerization of isoprene (activity up to 4.1 × 105 (g of polymer)/(molLn h), cis-1,4 selectivity ca. 98.8%).5 Subsequently, Cui and co-workers synthesized a series of pincer-type rare-earth-metal complexes as catalyst precursors in the cis-1,4polymerization of isoprene.6−8 The bis(carbene)phenyl (CCC) rare-earth-metal dibromides showed high activity up to 1.3 × 105 (g of polymer)/(molLn h) and cis-1,4-selectivity up to 99.6% in the polymerization of isoprene.6 The bis(phosphino)carbazolide (PNP)-chelated rare-earth-metal complexes exhibited high activity (up to 8.2 × 105 (g of polymer)/(molLn Received: April 27, 2017

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DOI: 10.1021/acs.organomet.7b00322 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Chart 1. Previous Tridentate C2-Symmetric Pincer-Type Rare-Earth-Metal Catalysts

h)), high cis-1,4-selectivity (up to 99%), and living modes in the polymerization of isoprene.7 Moreover, the cis-1,4-selective polymerization of isoprene was achieved using bis(pyrrolidin-1yl)pyrrolyl- or bis(piperidino)methylenepyrrolyl (NNN)-ligated rare-earth-metal complexes as catalysts; however, the activity was only 2.7 × 104 (g of polymer)/(molLn h) and the cis-1,4-selectivity was only 94.1%.8 In 2013, Lv also described that the bis(oxazolinyl)phenyl (NCN)-ligated rare-earth-metal dichlorides served as highly efficient and selective catalyst precursors for the cis-1,4-polymerization of isoprene with activity up to 4.1 × 105 (g of polymer)/(molLn h) and cis-1,4selectivity up to 99.5%.9 Despite the high activity and high selectivity of most tridentate C2-symmetric pincer-type rareearth-metal catalysts exhibited in cis-1,4-selective polymerization of isoprene, the NNN-ligated rare-earth-metal complexes displayed unsatisfactory activity (99 98 98 98 98 98 98 98

4 6 9 6 2 99%), higher molecular weight (Mn = 510000 g/mol), and narrower molecular weight distribution (Mw/Mn = 1.40) could be obtained (Table 2, entry 9). With gradually increasing [IP]:[1] molar ratio from 400:1 to 4800:1, the activity first increased from 3.4 × 105 (g of polymer)/(molLn h) to 1.9 × 106 (g of polymer)/(molLn h)) and then decreased to 1.4 × 105 (g of polymer)/(molLn h), preparing the cis-1,4-PIPs with high molecular weights and narrow molecular weight distributions (cis-1,4-selectivity ca. 98%, Mn = 420000−610000 g/mol, Mw/ Mn = 1.26−1.59) (Table 2, entries 1 and 10−16). The resulting PIPs show good solubility in THF and CHCl3. The 1H NMR spectra of these PIPs in CDCl3 indicate that the resulting PIPs have almost complete 1,4-microstructures. Moreover, the 13C NMR spectra display diagnostic signals at δ 23.4, 26.4, 32.2, 125.0, and 135.2 ppm for an almost pure cis1,4 configuration (Supporting Information). GPC curves reveal that these cis-1,4-PIPs have high molecular weights in the range of 210000−610000 g/mol and narrow to moderate molecular weight distributions (Mw/Mn = 1.26−2.08), suggesting the predominance of a homogeneous single site catalytic species. DSC curves show that these cis-1,4-PIPs have glass transition temperatures in the range of −54 to −67 °C.

Accession Codes

CCDC 1055486 and 1058470 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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*E-mail for X.L.: xfl[email protected]. *E-mail for F.Y.: [email protected]. *E-mail for S.Z.: [email protected]. ORCID

Xiaofang Li: 0000-0003-3788-5431 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (Nos. 21274012, 21322401), the 111 project (B07012), and the Beijing Institute of Technology (No. 1750012331710).

CONCLUSION In summary, the two nonmetallocene rare-earth-metal dialkyl complexes 1 and 2 have been easily synthesized in moderate yields via the same acid−base reaction by using the rare-earthmetal tris(trimethylsilylmethyl) complexes with the readily available 1,3-bis(2-pyridylimino)isoindoline (BPI) ligand. The NMR spectrum and X-ray diffraction demonstrate that the mononuclear Sc dialkyl complex 1 contains a monoanionic tridentate C2-symmetric pincer-type BPI ligand both in the solid state and in the solution state, while the Lu dialkyl complex 2 adopts a binuclear structure bearing two monoanionic tetradentate tautomeric BPI ligand as bridges between two Lu centers in the solid state and a mononuclear structure bearing a monoanionic tridentate C2-symmetric pincer-type BPI ligand in the solution state similar to the scandium complex 1. Activated by activator and AlR3 in toluene, these pincer-type BPI-ligated complexes 1 and 2 exhibited high activities up to 1.9 × 106 (g of polymer)/(molSc h) and high cis-1,4-selectivities up to 99% in the polymerization of isoprene, affording cis-1,4-PIPs with high molecular weights (Mn up to 610000 g/mol) and narrow to moderate molecular weight distributions (Mw/Mn = 1.26−2.08). Such results demonstrate that the BPI-ligated rare-earth-metal complexes can serve as highly efficient catalysts for the cis-1,4-polymerization of isoprene. These findings will benefit the rapid and precise synthesis of natural rubber and its analogues. Further studies will be focused on the modification of the chelating ligand to improve the selectivity and/or activity of the nonmetallocene rare-earth-metal catalytic system in the cis1,4-polymerization of isoprene.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00322. E

DOI: 10.1021/acs.organomet.7b00322 Organometallics XXXX, XXX, XXX−XXX

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DOI 10.5517/cc14f9yt; Hongyun Liao CCDC 1058470, Experimental Crystal Structure Determination, 2015, DOI 10.5517/cc14jf69.

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DOI: 10.1021/acs.organomet.7b00322 Organometallics XXXX, XXX, XXX−XXX