Chemistry of half-sandwich compounds of zirconium: Evidence for the formation of the novel ansa cationic-zwitterionic complex [Zr(eta :eta-C5H4CMe2C6H4Me-p)(mu-MeB(C6F5)(3))](+)[MeB(C6F5)(3)](-)

The compound [Zr(eta-C5H4CMe2C6H4Me-p)Cl-3(dme)] (1) has been prepared. Met hylation affords the compound [Zr(eta-C5H4CMe2C6H4Me-p)Me-3] (2), and benzy lation of 1 yields [Zr- (eta-C5H4CMe2C6H4Me-p)(CH2Ph)(3)] (3). Reaction of 2 with [Ph3C](+)[B(C6F5)(4)](-) at -60 degrees C gives the ansa complex [Zr (eta:eta-C5H4CMe2C6H4Me-p)Me-2](+)[B(C6F5)(4)](-) (4a), which has been char acterized by 1D and 2D NMR spectroscopy. Reaction of 2 with an excess (1.5 equiv) of B(C6F5)(3) yields not only the ansa complex [Zr(eta:eta-C5H4CMe2C 6H4Me-p)Me-2](+)[MeB(C6F5)(3)](-) (4b) but also the novel cationic zwitteri onic ansa complex [Zr(eta:eta-C5H4CMe2C6H4Me-p)(mu-MeBC6F5)(3))](+)[MeB(C6F 5)(3)](-) (5) The dynamic behavior of complexes 4b and 5 was extensively st udied by NMR spectroscopy. Reaction of 3 with [Ph3C](+)[B(C6F5)(4)](-) at - 60 degrees C yields the ansa complex [Zr(eta:eta-C5H4CMe2C6H4Me-p)(CH2Ph)(2 )](+)[B(C6F5)(4)](-) (6), containing sigma-bound benzyl ligands.