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High tacticity control in organolanthanide polymerization catalysis: formationof isotactic poly(a-alkenes) with a chiral C3-symmetric thulium complex†‡

Lenka Lukesova,a Benjamin D. Ward,a Stephane Bellemin-Laponnaz,b Hubert Wadepohla and Lutz H. Gade*a,b

Received 8th January 2007, Accepted 10th January 2007First published as an Advance Article on the web 18th January 2007DOI: 10.1039/b700269f

The thulium complexes [Tm(iPr-trisox)(CH2SiMe2R)3] (R =Me 1a, Ph 1b) were synthesized from the thulium trialkylprecursors [Tm(CH2SiMe2R)3(thf)2]; reaction of 1a with twoequivalents of [Ph3C][B(C6F5)4] gave a cationic complex 1c,which was found to polymerize 1-hexene, 1-heptene and 1-octene to give the corresponding polyolefins with moderateto good activities and with minimum isotacticity of 90%, 83%and 95%, respectively.

Introduction

Cationic alkyl-lanthanide complexes have been extensively studiedas catalysts for the polymerization of ethylene.1 While early workfocused on cyclopentadienide complexes as active species,2 theuse of non-Cp supporting ligands has recently gained increasingattention.3 The factors which determine the catalyst activities arenot as yet completely understood, although there is increasingevidence that the ionic radius of the trivalent metal involved mayplay a role.4 Whereas there is previous work on the use of theearly lanthanides in the polymerization of 1,3-dienes, which isbased on a mechanism involving allyl-metal intermediates,5 thereis no example of tacticity controlled polymerization of higher a-olefins. Control of the tacticity, however, is essential, since themolecular microstructure determines the macroscopic properties.Such control must therefore be incorporated into the design ofany prospective catalyst that is to polymerize a-olefins beyondethylene.6

We have recently reported on the use of C3-chiral tris-(oxazolinyl)ethane (trisox) ligands7 as highly versatile supportingligands for a variety of early and late transition metals,8 includingscandium, for which a highly active catalyst was obtained forthe isotactic polymerization of 1-hexene.9 The trialkyl complex[Sc(iPr-trisox)(CH2SiMe3)3] was activated with two equivalents ofthe trityl salt [Ph3C][B(C6F5)4], thus affording a cationic specieswhich we tentatively assigned as [Sc(iPr-trisox)(CH2SiMe3)]2+. Thisdicationic species was found to have a polymerization activity ofsome three orders of magnitude over that of the monocationiccomplex. In light of these results for a group 3 metal, we hereinreport the first examples of the isotactic polymerization of a-olefins

aAnorganisch-Chemisches Institut, Universitat Heidelberg, Im NeuenheimerFeld 270, 69120, Heidelberg, Germany. E-mail: lutz.gade@uni-hd.debInstitut de Chimie, CNRS UMR 7177, Universite Louis Pasteur, 4, rueBlaise Pascal, 67000, Strasbourg, France† Dedicated to Karel Mach on the occasion of his 70th birthday.‡ Electronic supplementary information (ESI) available: 13C NMR spec-tra of: poly(1-hexene), poly(1-heptene) and poly(1-octene). See DOI:10.1039/b700269f

with a lanthanide catalyst in general and, in particular, a C3-chiralthulium(III) complex.

Scheme 1 Synthesis of the trisox-thulium complexes 1a and 1b.

The thulium complexes [Tm(iPr-trisox)(CH2SiMe2R)3] (R =Me 1a, Ph 1b) were synthesized from the thulium trialkyl pre-cursors [Tm(CH2SiMe2R)3(thf)2] (Scheme 1).10 These precursorsproved less stable than the scandium congeners, and were thusprepared in situ at −80 ◦C. The trialkyl species were subse-quently reacted with iPr-trisox in pentane, affording the trisoxsupported complexes as off-white precipitates. Recrystallizationfrom dichloromethane or toluene/hexane respectively affordedthe complexes as white crystalline solids. The structures were con-firmed by means of X-ray crystallographic studies; the molecularstructures of 1a and 1b are shown in Fig. 1 and 2, respectively,along with principal bond lengths and angles.

The molecular structures of both 1a and 1b possess pseudooctahedral geometries at the thulium centres, and the chiral centresC(3), C(9) and C(15) all have the expected S configuration. Thebond lengths to thulium are unremarkable upon comparisonwith examples reported in the Cambridge Structural Database,11

although the angles subtended at the thulium are, as expected,significantly distorted from those of an ideal octahedral complexand are comparable to those found in the scandium complex[Sc(iPr-trisox)(CH2SiMe3)3].

The paramagnetic 1H NMR spectra of 1a and 1b were recorded,and all signals were observed as relatively sharp singlets between+160 and −220 ppm; 13C{1H} NMR spectra displaying sharpresonances were also obtained. Whilst unambiguous assignmentof the spectra was not possible, the resonance patterns are

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Fig. 1 Molecular structure of complex 1a (25% probability). Principalbond lengths (A) and angles (◦):C(21)–Tm(1), 2.3888(19), C(30)–Tm(1)2.393(2), C(39)–Tm(1) 2.397(2), N(1)–Tm(1) 2.5379(18), N(2)–Tm(1)2.5276(18), N(3)–Tm(1) 2.5187(18), C(21)–Tm(1)–C(30) 99.54(7), C(21)–Tm(1)–C(39) 100.47(7), C(30)–Tm(1)–C(39) 103.71(8), N(3)–Tm(1)–N(2)71.48(6), N(3)–Tm(1)–N(1) 73.23(6), N(2)–Tm(1)–N(1) 73.55(6).

Fig. 2 Molecular structure of complex 1b (25% probability). Principalbond lengths (A) and angles (◦):Tm(1)–C(25) 2.393(4), Tm(1)–C(21)2.394(4), Tm(1)–C(29) 2.395(4), Tm(1)–N(3) 2.542(3), Tm(1)–N(1)2.557(3), Tm(1)–N(2) 2.565(3), C(25)–Tm(1)–C(21) 106.69(14), C(25)–Tm(1)–C(29) 106.64(14), C(21)–Tm(1)–C(29) 108.47(13), N(3)–Tm(1)–N(1) 72.58(11), N(3)–Tm(1)–N(2) 71.75(11), N(1)–Tm(1)–N(2) 71.55(10).

consistent with the structures of the molecules as established forthe solid state, which therefore appear to pertain in solution.

Okuda and coworkers recently reported that the thuliumalkyl complex [Tm(CH2SiMe3)3(THF)2],12 when activated with

trityl borate, gave a rather modest polymerization activity forethylene; in addition, the polydispersity was rather high, (4.1)indicating non-single site behaviour. Given the design of ourmolecular catalyst, we concentrated our efforts on the poly-merization of a-olefins, rather than ethylene, since this providedthe probe for the effectiveness of the stereocontrol over thepolymeric microstructure. The neutral pre-catalysts were activatedusing trityl borate in chlorobenzene. Whereas reaction of thephenyl(dimethyl)silylmethyl complex 1b with either one or twoequivalents of trityl failed to produce an active catalyst, reac-tion of the trimethylsilylmethyl-substituted derivative 1a with 2equivalents of [Ph3C][B(C6F5)4] gave a cationic complex 1c, whichwas found to polymerize 1-hexene, 1-heptene and 1-octene. Theactivated catalyst was found to be unstable at room temperature,with only very low activity being observed under these conditions.In order to circumvent the thermal decomposition process, theactivation and subsequent polymerization reactions were carriedout at −5 ◦C (Table 1).

The three a-olefins, 1-hexene, 1-heptene and 1-octene, werepolymerized with in situ generated catalyst 1c to give the cor-responding polyolefins with moderate to good activities. We areunaware of any previous lanthanide catalysis for these reactions.Whereas analysis of the 13C{1H}NMR spectrum of poly(1-hexene)indicated the presence of over 90% of the mmmm pentad, theanalogous poly(1-heptene) was obtained with 83% isotacticity.Finally, the polymerization of 1-octene, whilst occurring withlower catalyst activity gave the corresponding polymer with aminimum of 95% isotacticity.13 For comparison the three 13C{1H}NMR spectra of the polymers are depicted in the ESI.‡ Thepolydispersity indices of between 1.8 and 2.1 for all three polymersindicate the presence of a single site catalyst.

In conclusion, we have shown that thulium complexes supportedby the trisoxazoline ligand are promising candidates for olefinpolymerization catalysis, their activity being dependent on thenature of the precatalyst. Since the polymerization behaviourcan be highly dependent on the ionic radius of the metal centreemployed, we are now focussing our research efforts to cover therange of lanthanides, in an attempt to find a system which canpolymerize with the same high degree of tacticity control, whilstimproving the catalytic activity.

Experimental

[Tm(CH2SiMe3)3(iPr-trisox)] (1a)

Anhydrous thulium trichloride (500 mg, 1.82 mmol) was slurriedin THF (30 ml) and stirred overnight. The resulting suspension wascooled to −80 ◦C for the dropwise addition of a cooled (−30 ◦C)solution of LiCH2SiMe3 (512.2 mg, 5.4 mmol) in pentane (40 ml).

Table 1

Polymera Activity/kg mol−1 h−1 PDI Mn 105/g mol−1 Mw 105/g mol−1 Isotacticity (% mmmm)

Poly(1-hexene) 165 1.95 1.26 2.46 90Poly(1-heptene) 120 2.08 0.99 2.06 83Poly(1-octene) 30 1.80 1.18 2.13 95

a Poly(1-hexene) & poly(1-heptene): cat. 0.02 mmol, 0.04 mmol [Ph3C][B(C6F5)4], 15 min, 1 mL olefin, 1 mL C6H5Cl, −5 ◦C. Poly(1-octene): cat. 0.02 mmol,0.04 mmol [Ph3C][B(C6F5)4], 30 min, 0.5 ml octene, 1 mL C6H5Cl, −5 ◦C.

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The mixture was stirred at −30 ◦C for 15 min, until the solidshad completely dissolved. After this the volatiles were removedunder reduced pressure whilst maintaining the temperature at orbelow 0 ◦C. The colourless solid was extracted with toluene (2 ×20 ml) at −20 ◦C. The toluene solution was cooled to −80 ◦Cfor the slow addition of a cooled (−80◦) solution of iPr-trisox(500 mg, 1.38 mmol) in pentane (50 ml), which caused a partialprecipitation of the product. The reaction was concentrated underreduced pressure to 5 ml, and pentane (50 ml) added in orderto precipitate the remainder of the product, which was isolatedby filtration and dried in vacuo at room temperature. The crudeproduct was extracted with CH2Cl2 (40 ml) and filtered. Thesolution was concentrated to 20 ml and cooled to −30 ◦C overnightto yield the pure product as a white crystalline solid in 79% yield.(powder). 1H NMR (399.9 MHz, CD2Cl2, 293 K): d −228.32,−216.0, −52.71, −15.84, 8.17, 19.80, 23.07, 87.01, 147.6 ppm.13C{1H} NMR (100.6 MHz, CD2Cl2, 293 K): d −81.5, −46.0,−19.0, −12.1, 3.1, 26.2, 102.1, 103.0, 130.1, 212.5 ppm.

[Tm(CH2SiMe2Ph)3(iPr-trisox)] (1b)

Procedure as described for 1a. The crystalline pure product wasisolated in 65% yield from toluene layered with hexanes. 1H NMR(399.9 MHz, toluene-d8, 293 K): d −216.97, −211.81, −115.20,−50.70, −21.09, −11.57, −4.79, −2.49, 7.67, 21.58, 22.14, 59.18,151.10 ppm. 13C{1H} NMR (100.6 MHz, toluene-d8, 293 K): d−76.2, −43.7, −42.8, −14.0, −11.5, 11.9, 36.6, 77.1, 100.5, 103.3,109.7, 112.5, 216.5 ppm.

Polymerization studies

20 lmol of the catalyst 1a was dissolved in chlorobenzene (1 ml)and added to trityl tetrakis(pentafluorophenyl)borate (40 lmol) at−5 ◦C. The appropriate a-olefin (1 ml) was added and the mixturewas stirred for a given period of time. At the end of the reaction,methanol (5 ml) was added to quench the catalyst, and the volatileswere removed under reduced pressure to yield the polyolefin asa waxy solid. The 13C{1H} NMR spectra of the polyolefins areprovided in the ESI.‡

Crystal data for 1a and 1b

Complex 1a: C32H66N3O3Si3Tm, M = 794.08, a = 10.2419(7) A,b = 19.1519(13) A, c = 10.3873(7) A, b = 102.0530(10)◦, V =1992.6(2) A3, T = 100(2) K, space group P21, Z = 2, T =100(2) K, l(Mo-Ka) = 2.348 mm−1, 19 507 reflections measured,10 696 unique (Rint = 0.0356). Final residuals [I > 2r(I)] R1 =0.0324, wR2 = 0.0738. Complex 1b: C47H72N3O3Si3Tm, M =980.28, a = 11.1678(10) A, b = 20.3453(18) A, c = 22.558(2) A,V = 5125.6(8) A3, T = 100(2) K, space group P212121, Z = 4, T =100(2) K, l(Mo-Ka) = 1.840 mm−1, 104 374 reflections measured,17 215 unique (Rint = 0.0485). Final residuals [I > 2r(I)] R1 =0.0247, wR2 = 0.0537.

CCDC reference numbers 627376 and 627377.For crystallographic data in CIF or other electronic format see

DOI: 10.1039/b700269f

Acknowledgements

We thank the Deutsche Forschungsgemeinschaft (SFB 623 andpostdoctoral fellowship to L. L.), the EU (Marie Curie EIFfellowship to B.D.W.) and the Fonds der Chemischen Industriefor financial support.

Notes and references

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6 (a) Y. Okamoto and T. Nakano, Chem. Rev., 1994, 94, 349; (b) G. W.Coates, Chem. Rev., 2000, 100, 1223.

7 (a) S. Bellemin-Laponnaz and L. H. Gade, Chem. Commun., 2002,1286; (b) S. Bellemin-Laponnaz and L. H. Gade, Angew. Chem., Int.Ed., 2002, 41, 3473.

8 (a) C. Dro, S. Bellemin-Laponnaz, R. Welter and L. H. Gade, Angew.Chem., 2004, 116, 4579; C. Dro, S. Bellemin-Laponnaz, R. Welter andL. H. Gade, Angew. Chem., Int. Ed., 2004, 43, 4479; (b) C. Foltz, B.Stecker, G. Marconi, H. Wadepohl, S. Bellemin-Laponnaz and L. H.Gade, Chem. Commun., 2005, 5115.

9 B. D. Ward, S. Bellemin-Laponnaz and L. H. Gade, Angew. Chem.,2005, 117, 1696; B. D. Ward, S. Bellemin-Laponnaz and L. H. Gade,Angew. Chem., Int. Ed., 2005, 44, 1668.

10 The thulium precursors Tm(CH2SiMe2R)(THF)2 were prepared usinganalogous procedures reported for scandium: R = Me: M. F. Lappertand R. Pearce, J. Chem. Soc., Chem. Commun., 1973, 126; R = Ph:D. J. H. Emslie, W. E. Piers and R. MacDonald, J. Chem. Soc., DaltonTrans., 2002, 293.

11 (a) F. H. Allen and O. Kennard, Chem. Des. Automation News, 1993,8, 1 & 31; (b) D. A. Fletcher, R. F. McMeeking and D. J. Parkin, Chem.Inf. Comput. Sci., 1996, 36, 746.

12 S. Arndt, T. P. Spaniol and J. Okuda, Angew. Chem., 2003, 115, 5229;S. Arndt, T. P. Spaniol and J. Okuda, Angew. Chem., Int. Ed., 2003, 42,5075.

13 T. Asakura, M. Demura and Y. Nishiyama, Macromolecules, 1991, 24,2334.

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