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

Abstract

Zirconium complexes bearing a series of ortho-substituted aryl diamido ligands [2,6-(RNCH 2) 2NC 5H 3] 2− (R = 2-PhC 6H 4 (BPhP); 2- i PrC 6H 4 (BMPP); 2- i BuC 6H 4 (BMBP), 2- i Pr-6-MeC 6H 3 (MPPP)) have been synthesized. The mixed amide complexes [2,6-(RNCH 2) 2NH 5H 3]Zr(NMe 2) 2 are prepared in high yield from 2,6-(RHNCH 2) 2NC 5H 3 and Zr(NMe 2) 4. The mixed amides react with excess ClSiMe 3 to afford the dichlorides [2,6-(RNCH 2) 2NC 5H 3]ZrCl 2 in nearly quantitative yield. Dimethyl complexes are prepared from [2,6-(RNCH 2) 2NC 5H 3]ZrCl 2 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)ZrX 2 complexes freely rotate at all temperatures (−80 to +80°C) while (BMPP)ZrX 2 (X  NMe 2, Cl, Me) derivatives adopt meso and rac rotamers at low temperatures. In contrast, (BMBP)ZrX 2 and (MPPP)ZrX 2 (X  NMe 2, Cl, Me) compounds are locked at all temperatures. (BMBP)ZrCl 2 is isolated as a single isomer, likely the meso rotamer, while (MPPP)ZrCl 2 is a near statistical mixture of meso and rac isomers.