Ancillary ligand effects in organoyttrium chemistry: Synthesis, characterization, and electronic structure of bis(benzamidinato)yttrium compounds

The synthesis of [PhC(NSiMe(3))(2)]Y-2(mu-Cl)(2)Li . 2THF (1) from YCl3 . 3.5THF and [PhC(NSiMe(3))(2)]Li, which is easily transformed into [PhC(NSiMe(3))(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(NSiMe(3))(2)](2)YR (R = BH4 . THF (3), N(SiMe(3))(2) (4), 2,6-(CMe(3))(2)-4-MeOC(6)H(2) (5), (mu-Me)(2)Li . TMEDA (6) (TMEDA = N,N,N',N'-tetramethylethylenediamine), CH(2)Ph . THF (7), CH(SiMe(3))(2) (8)). Similar to 8, [p-MeOC(6)H(4)C(NSiMe(3))(2)]2YCH(SiMe(3))(2) (8(OMe)) could be prepared starting from [p-MeOC(6)H(4)C(NSiMe(3))(2)]2YCl . THF (2(OMe)). Hydrogenolysis (4 atm) of 8 and 8(OMe) affords dimeric hydrides {[p-X-C6H4C(NSiMe(3))(2)]Y-2(mu-H)}(2) (X = H (9), X = MeO (9(OMe))). The alkyl 8(OMe) 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.