Synthesis of linked half sandwich rare-earth metal chlorido and borohydrido complexes and their catalytic behavior towards MMA polymerization

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 [C5Me4H-C6H4-o-NMe2]Li with LnCl(3)(THF)(n) afforded the dichlorido complexes [(C5Me4H-C6H4-o-NMe2)(2)Ln(2)Cl(4)][LiCl(THF)(2)] (Ln = Y (2a), Lu (2b)), which are trinuclear connected by mu(3)-Cl and mu(2)-Cl multiple bridges. The straightforward metathesis reaction of Ln(BH4)(3)(THF)(n) with equimolar [C5Me4H-C6H4-o-NMe2]Li in THF medium yielded the first linked half sandwich ligand stabilized THF-free rare-earth metal bis(borohydrido) complexes (C5Me4H-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 degrees C) in polar THF medium, whilst the metal chlorido species was inert. The binary catalyst system of 3a/(MgBu2)-Bu-n 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 (BuLi)-Bu-n displayed an extremely high activity (1.1 x 10(6) g mol(Sc)(-1) h(-1)) and afforded syndiotactic PMMA (rr = 75%) at low polymerization temperature (-20 degrees C) in THF