Lanthanide Borohydrido Complexes Supported by ansa-Bis(amidinato) Ligands with a Rigid o-Phenylene Linker: Effect of Ligand Tailoring on Catalytic Lactide Polymerization.

A series of lanthanide monoborohydrido complexes {C₆H₄-1,2-[NC(R)NR′]₂}Ln(BH₄)(L) _n (Ln = Y, Nd, Sm; R = tBu, Ph; R′ = 2,6-Me₂C₆H₃, SiMe₃; L = dme = dimethoxyethane, n = 1; L = thf, n = 2), in which lanthanides are coordinated by bulky ansa-bis(amidinato) ligand systems with a conformationally rigid o-phenylene linker ({C₆H₄-1,2-[NC(R)NR′]₂}^2-), were synthesized by the salt metathesis reactions of equimolar amounts of Ln(BH₄)₃(thf)₃ and {C₆H₄-1,2-[NC(R)NR′]₂}X₂(thf) _n (X = Li, Na) in thf. X-ray diffraction studies revealed that the complexes are monomeric. Depending on the denticity of the donor ligand (L = dme or thf), the terminal borohydrido ligand coordinated to the metal ion can be located in either an equatorial (L = thf) or an apical (L = dme) position. All complexes are efficient catalysts for the ring-opening polymerization of rac-lactide, which allows to convert up to 1000 equiv. of monomer into a polymer at room temperature within 10-150 min and affords atactic polylactides with high molecular weights and moderate molecular-weight distributions (1.28-2.16). Yttrium-borohydrido complexes coordinated by the {C₆H₄-1,2-[NC( tBu)N(2,6-Me₂C₆H₃)]₂}^2- ligand system showed enhanced catalytic activity compared to that of the analogue complexes containing the {C₆H₄-1,2-[NC(Ph)NSiMe₃]₂}^2- ligand. The obtained borohydrido complexes catalyze the hydrophosphonylation of benzaldehyde at room temperature with good reaction rates. [ABSTRACT FROM AUTHOR]