Ancillary Ligand Effects in Organoyttrium Chemistry: Synthesis, Characterization, and Electronic Structure of Bis(benzamidinato)yttrium Compounds

Abstract

The synthesis of [PhC(NSiMe3)2]2Y(μ-Cl)2Li·2THF (1) from YCl3·3.5THF and [PhC(NSiMe3)2]Li, which is easily transformed into [PhC(NSiMe3)2]2YCl·THF (2), provides a useful entry into the chemistry of several bis(N,N‘-bis(trimethylsilyl)benzamidinato)yttrium complexes. Those prepared from 2 by chloride metathesis include [PhC(NSiMe3)2]2YR (R = BH4·THF (3), N(SiMe3)2 (4), 2,6-(CMe3)2-4-MeOC6H2 (5), (μ-Me)2Li·TMEDA (6) (TMEDA = N,N,N‘,N‘-tetramethylethylenediamine), CH2Ph·THF (7), CH(SiMe3)2 (8)). Similar to 8, [p-MeOC6H4C(NSiMe3)2]2YCH(SiMe3)2 (8OMe) could be prepared starting from [p-MeOC6H4C(NSiMe3)2]2YCl·THF (2OMe). Hydrogenolysis (4 atm) of 8 and 8OMe affords dimeric hydrides {[p-X-C6H4C(NSiMe3)2]2Y(μ-H)}2 (X = H (9), X = MeO (9OMe)). The alkyl 8OMe and the hydride 9 have been characterized by an X-ray diffraction structure determination. Sterically the bis(N,N‘-bis(trimethylsilyl)benzamidinate) ligand system resembles more the bis(pentamethylcyclopentadienyl) than the bis(cyclopentadienyl) ligand set. However, INDO/1 semi-empirical MO studies indicate that the electronic properties of [HC(NH)2]2YCH3 (used as a model for bis(benzamidinato)yttrium alkyl complexes) are rather different from [C5H5]2YCH3. The yttrium atom in [HC(NH)2]2YCH3 is considerably more positively charged than in [C5H5]2YCH3. The resulting strong ionic character of the bis(benzamidinate) system is held responsible for the absence of agostic interactions and H/D exchange and the low hydrogenolysis rate observed.