A series of rare-earth metal complexes attached by an amino-functionalized cyclopentadienyl ligand (C5Me4H-C6H4-o-NMe2) (1) was prepared. The metathesis reaction of the ligand lithium salt [C5Me4-C6H4-o-NMe2]Li with LnCl3(THF)n afforded the dichlorido complexes [(C5Me4-C6H4-o-NMe2)2Ln2Cl4][LiCl(THF)2] (Ln = Y (2a), Lu (2b)), which are trinuclear connected by μ3-Cl and μ2-Cl multiple bridges. The straightforward metathesis reaction of Ln(BH4)3(THF)n with equimolar [C5Me4-C6H4-o-NMe2]Li in THF medium yielded the first linked half sandwich ligand stabilized THF-free rare-earth metal bis(borohydrido) complexes (C5Me4-C6H4-o-NMe2)Ln(BH4)2 (Ln = Sc (3a), Sm (3b)), respectively. The single component borohydrido complex 3a showed high activity towards the bulk polymerization of methyl methacrylate without specific control, which showed high iso-selectivity (mm = 80%) when the polymerization was performed in benzene medium, and switched to syndio-selectivity (rr = 74% at −20 °C) in polar THF medium, whilst the metal chlorido species was inert. The binary catalyst system of 3a/MgnBu2 had similar catalytic performances when compared with 3a in THF medium, but provided enriched syndio-control in benzene solution that was in contrast to the iso-control of 3a. Surprisingly, 3a upon activation with nBuLi displayed an extremely high activity (1.1 × 106 g molSc−1 h−1) and afforded syndiotactic PMMA (rr = 75%) at low polymerization temperature (−20 °C) in THF.
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