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

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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
Issue number3
Publication statusPublished - Jan 2013

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