C 2-Symmetric ansa-Lanthanidocene Complexes. Synthesis via Silylamine Elimination and β-SiH Agostic Rigidity

• Published in: Journal of the American Chemical Society 2000-04, Vol.122 (13), p.3080-3096
• Main Authors: Eppinger, Jörg, Spiegler, Michael, Hieringer, Wolfgang, Herrmann, Wolfgang A, Anwander, Reiner
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja992786a

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(SiHMe2)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 [Me2Si(C5Me4)2]La[N(SiHMe2)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 (1H, 13C, 29Si, 89Y) 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-[Me2Si(2-Me-Benz-Ind)2]Ln[N(SiHMe2)2] (Ln = Y, Lu) and rac-[Me2Si(2-Me-C9H5)2]Ln[N(SiHMe2)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(SiHMe2)2]3(THF) and partly exchanged [Me2Si(2-Me-4-Ph-Ind)2H]Y[N(SiHMe2)2]2 provides mechanistic details of this peculiar silylamine elimination reaction. Additionally, the pK a values of various protonated ligands including new 9-(SiHMe2)-fluorene, 3-(SiHMe2)-indene, and 3-(SiHMe2)-2-Me-indene determined according to the method of Fraser give evidence of a thermodynamically controlled ligand exchange.