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

Abstract

Previous results obtained for the cis-polymerisation of isoprene with scandium half-sandwich complex Cp*Sc(BH 4 ) 2 (THF) 1a were extended to its neodymium analog. The X-ray structure of the already reported neodymium monomer compound Cp*Nd(BH 4 ) 2 (THF) 2 1b is presented. Cp*Nd(BH 4 ) 2 (THF) 2 /[CPh 3 ]-[B(C 6 F 5 ) 4 ]/Al( i Bu) 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(BH 4 ) 2 (THF) n pre-catalyst (Cp* = η 5 -C 5 Me 5 , Ln = Sc, n = 1, 1a; Nd, n = 2, 1b; Sm, n = 2, 1c) or to a trisborohydride Ln(BH 4 ) 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{(N t BuCH)} 2 ] (L 1 ) and functional N-heterocyclic carbenes, two amino-tethered HNBu t CH 2 CH 2 [1-C{N(CHCH)NR}] (HL 2-R ) (R = t Bu, Mes (Mes = 2,4,6-Me 3 -C 6 H 2 )) and the hydroxyl-tethered HOCMe 2 CH 2 [1-C{N(CHCH)N i Pr}] (HL 3 ). Neodymiumbased complex (L 2-tBu )Nd(BH 4 ) 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 HL 2-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.

Enantiomerically Pure Constrained Geometry Complexes of the Rare‐Earth Metals Featuring a Dianionic N‐Donor Functionalised Pentadienyl Ligand: Synthesis and Characterisation

“…The M−H bond distances in 8‐La lie between 2.51(6) and 2.83(4) Å and do not differ significantly from the values found for 8‐Nd within the standard deviations. They are also in good agreement with other Nd‐BH 4 complexes reported in literature [17b, 21] . The M−B bond distances of both compounds lie in the range of literature‐reported compounds featuring bridging BH 4 groups, [17g, 18, 19a, 22] but are longer than in species with terminal BH 4 ligands [17b‐e, 21, 23] …”

Trans-stereospecific polymerization of butadiene and random copolymerization with styrene using borohydrido neodymium/magnesium dialkyl catalysts

“…Alternatively, ionic metathesis from 1 and following the procedure described in previous works [47], afforded a complex (2') that was isolated and stored in the glove box at room temperature. This compound has two THF molecules per Nd atom when freshly isolated [35], but we observed that it displayed a propensity to desolvate and clusterize [50], affecting the THF amounts and solubility properties. As an example, 1 H NMR integration in C 5 D 5 N gave Cp*Nd(BH 4 ) 1.9 (THF) 0.…”

“…We disclosed in these recent years a very efficient catalytic system for the polymerization of isoprene, based on the combination of the borohydride Nd(BH 4 ) 3 (THF) 3 as pre-catalyst with a magnesium dialkyl, which afforded a polymer with a high degree of trans-stereoregularity [34,35], along with a good control of macromolecular data. To our knowledge, regarding the trans-selective polymerization of butadiene, Mg dialkyl has been scarcely reported as cocatalyst associated with neodymium versatate [36], alkoxide/aryloxide [37], constrained geometry half-sandwich [38], and more recently with a phosphazen derivative of this lanthanide element [39].…”

Lanthanides and actinides: Annual survey of their organometallic chemistry covering the year 2013

“…This result is the only example of highly transpolyisoprene synthesized with a scandium catalyst. Coordination of the carbene moiety to the rare-earth center was confirmed by NMR studies on the paramagnetic neodymium pre-catalysts [131].…”