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

Article abstract of DOI:10.1016/S0277-5387(97)00262-3

Zirconium complexes bearing a series of ortho-substituted aryl diamido ligands [2,6-(RNCH₂)₂NC₅H₃]^2- (R = 2-PhC₆H₄ (BPhP); 2-ⁱPrC₆H₄ (BMPP); 2-^tBuC₆H₄ (BMBP), 2-ⁱPr-O-MeC₆H₃ (MPPP)) have been synthesized. The mixed amide complexes [2,6-(RNCH₂)₂NC₅H₃]Zr(NMe ₂)₂ are prepared in high yield from 2,6-(RHNCH₂)₂NC₅H₃ and Zr(NMe₂)₄. The mixed amides react with excess ClSiMe₃ to afford the dichlorides [2,6-(RNCH₂)₂NC₅H₃]ZrCl₂ in nearly quantitative yield. Dimethyl complexes are prepared from [2,6-(RNCH₂)₂NC₅H₃]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°C) while (BMPP)ZrX₂ (X = NMe₂, Cl, Me) derivatives adopt meso and rac rotamers at low temperatures. In contrast, (BMBP)ZrX₂ and (MPPP)ZrX₂ (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. ° 1998 Elsevier Science Ltd. All rights reserved.