Transition Metal Complexes Containing C2-Symmetric Bis(imidazolin-2-imine) Ligands Derived from a 1-Alkyl-3-arylimidazolin-2-ylidene

The new N- heterocyclic carbene 1-( 2,6- diisopropylphenyl)3,4,5- trimethylimidazolin- 2- ylidene ( IPrMe) ( 3) was prepared in three steps from methyl isothiocyanate and 2,6- diisopropylaniline, affording N- methyl- N - 2,6- diisopropylthiourea ( 1). Reaction of 1 with 3- hydroxybutanone gave the imidazolin- 2- thione ( IPrMe) S ( 2), which was reduced with elemental potassium to the carbene IPrMe ( 3). Treatment of 3 with trimethylsilyl azide ( Me3SiN3) in boiling toluene, followed by desilylation in methanol, gave the imidazolin- 2- imine ( IPrMe) NH ( 4). The ethylene- bridged diimine ligand N, N - bis[ 1-( 2,6- diisoprop- ylphenyl)- 3,4,5- trimethylimidazolin- 2- ylidene]- 1,2- ethanediamine, ( IPrMe) NCH2CH2N( IPrMe) ( 5) was prepared by reaction of 4 with 1,2- ethyleneditosylate and subsequent deprotonation with KOtBu. The reaction of 5 with metal dichlorides furnished the tetrahedral complexes [( 5) MCl2] [ M = Fe ( 6), M = Ni ( 7), M = Zn ( 8)] and the squareplanar complex [( 5) PdCl2] ( 9), as revealed by X- ray diffraction analyses. The X- ray crystal structures of 2, 3, and 4 were also established. Complex 8 and related zinc( II) bis( imidazolin- 2- imine) complexes were tested in the polymerization of d, l- lactide.