C2-Symmetric ansa-Lanthanidocene Complexes. Synthesis via Silylamine Elimination and β-SiH Agostic Rigidity

Abstract

The synthesis as well as the spectroscopic and structural characterization of a new class of C 2 -symmetric, mononuclear metallocene complexes of the lanthanide elements is described. Heteroleptic lanthanidocene silylamide complexes have been obtained ate-complex-free according to silylamine elimination reactions of complexes Ln[N(SiHMe 2 ) 2 ] 3 (THF) x ( x = 1, Ln = Sc; x = 2, Ln = Y, La, Nd, Lu) and linked, substituted cyclopentadiene and indene systems. The molecular structure of [Me 2 Si(C 5 Me 4 ) 2 ]La[N(SiHMe 2 ) 2 ] (95% isolated yield) has been determined by X-ray crystallography. Brintzinger-type, indenyl-derived metallocene complexes have been isolated in racemic yields as high as 72%. IR and multinuclear NMR spectroscopy ( 1 H, 13 C, 29 Si, 89 Y) reveals the presence of an unprecedented strong diagostic interaction between the electron-deficient metal centers and the SiH moiety of the bis(dimethylsilyl)amide ligand: SiH stretching vibrations as low as 1759 cm - 1 and 1 J Si,H coupling constants as low as 133 Hz indicate a distinct weakening of the SiH bonding. X-ray structure analyses of the compounds rac -[Me 2 Si(2-Me-Benz-Ind) 2 ]Ln[N(SiHMe 2 ) 2 ] (Ln = Y, Lu) and rac -[Me 2 Si(2-Me-C 9 H 5 ) 2 ]Ln[N(SiHMe 2 ) 2 ] (Ln = Y, Lu) show that the structural features of both the chelating ancillary ligands and such agostically fused metallacycles depend on the type of the ligand and the size of the metal: bite angles Ω of approximately “U-shaped” ansa- ligands as large as 68.2(2)° and Ln−Si and Ln−H contacts as close as 3.028(1) and 2.37(3) Å, respectively, have been detected, the latter forcing very large Si−N−Si angles up to 160.1(2)°. The isolation of reaction intermediates such as Y[N(SiHMe 2 ) 2 ] 3 (THF) and partly exchanged [Me 2 Si(2-Me-4-Ph-Ind) 2 H]Y[N(SiHMe 2 ) 2 ] 2 provides mechanistic details of this peculiar silylamine elimination reaction. Additionally, the p K a values of various protonated ligands including new 9-(SiHMe 2 )-fluorene, 3-(SiHMe 2 )-indene, and 3-(SiHMe 2 )-2-Me-indene determined according to the method of Fraser give evidence of a thermodynamically controlled ligand exchange.