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

• Published in: European journal of inorganic chemistry 2013-12, Vol.2013 (35), p.6009-6018
• Main Authors: Tolpygin, Aleksei O., Skvortsov, Grigorii G., Cherkasov, Anton V., Fukin, Georgy K., Glukhova, Tatyana A., Trifonov, Alexander A.
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Diffraction; Lanthanides; Ligand design; Ligands; Lithium; Monomers; Polymerization; Yttrium
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201300931

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. The replacement of two molecules of monodentate thf by one bidentate dme ligand in the coordination sphere of yttrium results in the migration of a borohydrido ligand from an equatorial position to an apical one.