Tuning the catalytic properties of rare earth borohydrides for the polymerisation of isoprene

Fanny Bonnet, Chloe E. Jones, Sanaa Semlali, Marc Bria, Pascal Roussel, Marc Visseaux, Polly L. Arnold

Research output: Contribution to journalArticlepeer-review

Abstract

Previous results obtained for the cis-polymerisation of isoprene with scandium half-sandwich complex Cp*Sc(BH4)2(THF) 1a were extended to its neodymium analog. The X-ray structure of the already reported neodymium monomer compound Cp*Nd(BH4)2(THF)21b is presented. Cp*Nd(BH4)2(THF)2/[CPh3][B(C6F5)4]/Al(iBu)3 catalytic system was found to be very active and stereoselective towards isoprene polymerisation, leading to highly 1,4-cis polyisoprene up to 92%. The effect on isoprene polymerisation of the addition of a NHC molecule to Cp*Ln(BH4)2(THF)n pre-catalyst (Cp* = η5-C5Me5, Ln = Sc, n = 1, 1a; Nd, n = 2, 1b; Sm, n = 2, 1c) or to a trisborohydride Ln(BH4)3(THF)n (Ln = Sc, n = 1.5, 2a; Nd, n = 3, 2b; Sm, n = 3, 2c) was also studied. Several NHC ligands were assessed: the classical [1-C{(NtBuCH)}2] (L1) and functional N-heterocyclic carbenes, two amino-tethered HNButCH2CH2[1-C{N(CHCH)NR}] (HL2-R) (R = tBu, Mes (Mes = 2,4,6-Me3-C6H2)) and the hydroxyl-tethered HOCMe2CH2[1-C{N(CHCH)NiPr}] (HL3). Neodymium-based complex (L2-tBu)Nd(BH4)2(THF)23 could be isolated and characterized. With some of the catalytic combinations tested, the introduction of the NHC ligand in the coordination sphere of the complex induces a switch of the stereoselectivity of the resulting polymer. Scandium complex 2a, which produces mainly 1,4-cis polyisoprene when associated to a borate activator and aluminum alkyl, leads to 1,4-trans polymer up to 94% regular when HL2-tBu carbene is added to the same reaction mixture. This result is the only example of highly trans-polyisoprene synthesized with a scandium based catalyst. Coordination of the carbene moiety to the rare earth centre is confirmed by NMR studies on paramagnetic neodymium pre-catalysts.

Original languageEnglish
Pages (from-to)790-801
Number of pages12
JournalDalton Transactions
Volume42
Issue number3
DOIs
Publication statusPublished - Jan 2013

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