Rare earth metal complexes based on β-diketiminato and novel linked bis(β-diketiminato) ligands: Synthesis, structural characterization and catalytic application in epoxide/CO 2-copolymerization

Abstract

Mesityl substituted β-diketiminato lanthanum and yttrium complexes [(BDI)Ln{N(SiRMe 2)} 2] (BDI = ArNC(Me)CHC(Me)NAr, Ar = 2,4,6-Me 3C 6H 2, Ln = La, R = Me ( 1), H ( 2a); Ln = Y, R = H ( 2b)) can be prepared via facile amine elimination starting from [La{N(SiMe 3) 2} 3] and [Ln{N(SiHMe 2) 2} 3(THF) 2] (Ln = Y, La), respectively. The X-ray crystal structure analysis of 1 revealed a distorted tetrahedral geometry around lanthanum with a η 2-bound β-diketiminato ligand. A series of novel ethylene- and cyclohexyl-linked bis(β-diketiminato) ligands [C 2H 4(BDI Ar) 2]H 2 and [Cy(BDI Ar) 2]H 2 [Ar = Mes (=2,4,6-Me 3C 6H 2), DEP (=2,6-Et 2C 6H 3), DIPP (=2,6- i-Pr 2C 6H 3)] were synthesized in a two step condensation procedure. The corresponding bis(β-diketiminato) yttrium and lanthanum complexes were obtained via amine elimination. The X-ray crystal structure analysis of the ethylene-bridged bis(β-diketiminato) complex [{C 2H 4(BDI Mes) 2}YN(SiMe 3) 2] ( 3b) and cyclohexyl-bridged complexes [{Cy(BDI Mes) 2}LaN(SiHMe 2) 2] ( 7) and [{Cy(BDI DEP) 2}LaN(SiMe 3) 2] ( 8) revealed a distorted square pyramidal coordination geometry around the rare earth metal, in which the amido ligand occupies the apical position and the two linked β-diketiminato moieties form the basis. The geometry of the bis(β-diketiminato) ligands depends significantly on the linker unit. While complexes with an ethylene-linked ligand adopt a cisoid arrangement of the two aromatic substituents, complexes with cyclohexyl linker adopt a transoid arrangement. Either one ( 3b) or both ( 7, 8) of the β-diketiminato moieties are tilted out of the η 2 coordination mode, resulting in close Ln⋯C contacts. The β-diketiminato and linked bis(β-diketiminato) complexes were moderately active in the copolymerization of cyclohexene oxide with CO 2. A maximum of 92% carbonate linkages were obtained using the ethylene-bridged bis(β-diketiminato) complex [{C 2H 4(BDI Mes) 2}LaN(SiHMe 2) 2] ( 4).