Ortho-substituted aryl diamido complexes of zirconium: observation of rotameric isomers

Zirconium complexes bearing a series of ortho-substituted aryl diamido ligands [2,6-(RNCH2)(2)NC5H3](2-) (R = 2-PhC6H4 (BPhP); 2-'PrC6H4 (BMPP); 2-'BuC6H4 (BMBP), 2-'Pr-6-MeC6H3 (MPPP)) have been synthesized. The mixed amide complexes [2,6-(RNCH2)(2)NC5H3]Zr(NMe2)(2) are prepared in high yield from 2,6-(RHNCH2)(2)NC5H3 and Zr(NMe2)(4). The mixed amides react with excess ClSiMe3 to afford the dichlorides [2,6-(RNCH2)(2)NC5H3]ZrCl2 in nearly quantitative yield. Dimethyl complexes are prepared from [2,6-(RNCH2)(2)NC5H3]ZrCl2 and 2 equiv. of MeMgCl. NMR spectroscopy has been used to identify rotameric isomers derived from restricted rotation about the N-C-ipso bond of the ligand. The aryl groups in (BPhP)ZrX2 complexes freely rotate at all temperatures (-80 to + 80 degrees C) while (BMPP)ZrX2 (X = NMe2, Cl, Me) derivatives adopt meso and rac rotamers at low temperatures. In contrast, (BMBP)ZrX, and (MPPP)ZrX, (X = NMe2, Cl, Me) compounds are locked at all temperatures. (BMBP)ZrCl2 is isolated as a single isomer, likely the meso rotamer, while (MPPP)ZrCl2 is a near statistical mixture of meso and rac isomers. (C) 1998 Elsevier Science Ltd. All rights reserved.