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

The synthesis of [PhC(NSiMe 3 ) 2 ] 2 Y(μ-Cl) 2 Li·2THF ( 1 ) from YCl 3 ·3.5THF and [PhC(NSiMe 3 ) 2 ]Li, which is easily transformed into [PhC(NSiMe 3 ) 2 ] 2 YCl·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 = BH 4 ·THF ( 3 ), N(SiMe 3 ) 2 ( 4 ), 2,6-(CMe 3 ) 2 -4-MeOC 6 H 2 ( 5 ), (μ-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 ] 2 YCH(SiMe 3 ) 2 ( 8 OMe ) could be prepared starting from [ p -MeOC 6 H 4 C(NSiMe 3 ) 2 ] 2 YCl·THF ( 2 OMe ). Hydrogenolysis (4 atm) of 8 and 8 OMe affords dimeric hydrides {[ p -X-C 6 H 4 C(NSiMe 3 ) 2 ] 2 Y(μ-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 ] 2 YCH 3 (used as a model for bis(benzamidinato)yttrium alkyl complexes) are rather different from [C 5 H 5 ] 2 YCH 3 . The yttrium atom in [HC(NH) 2 ] 2 YCH 3 is considerably more positively charged than in [C 5 H 5 ] 2 YCH 3 . 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.