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

Previous results obtained for the cis-polymerisation of isoprene with scandium half-sandwich complex (CpSc)-Sc-star(BH4)(2)(THF) 1a were extended to its neodymium analog. The X-ray structure of the already reported neodymium monomer compound (CpNd)-Nd-star(BH4)(2)(THF)(2) 1b is presented. (CpNd)-Nd-star(BH4)(2)(THF)(2)/[CPh3][B(C6F5)(4)]/Al(Bu-i)(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(star)Ln(BH4)(2)(THF)(n) pre-catalyst (Cp-star = eta(5)-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)] (L-1) and functional N-heterocyclic carbenes, two amino-tethered (HNBuCH2CH2)-C-t[1-C{N(CHCH)NR}] (HL2-R) (R = Bu-t, Mes (Mes = 2,4,6-Me-3-C6H2)) and the hydroxyl-tethered HOCMe2CH2[1-C{N(CHCH)(NPr)-Pr-i}] (HL3). Neodymium-based complex (L2-tBu) Nd(BH4)(2)(THF)(2) 3 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.