Zirconium bisamidinate complexes with sterically demanding ligands: structure, solution dynamics, and reactivity

Bisamidinate zirconium dichloride and dimethyl complexes with the sterically demanding amidinate ligands [PhC(NAr)(2))](-) (A) and [PhC(NAr)(NAr')](-) (B) (Ar = 2,6-(Pr2C6H3)-Pr-i; Ar' = 2,6-Me2C6H3) were prepared. The steric demand of ligand A induces the unusual trans geometry in trans-(A)(2)ZrCl2, whereas (A)(2)ZrMe2 and (B)(2)ZrX2 (X = Cl, Me) adopt the more usual cis-X-2 geometry. The dynamic behavior of these complexes in solution was studied by 1D and 2D H-1 NMR spectroscopy. Reaction of (A)(2)ZrMe2 with H-2 leads, after stepwise hydrogenolysis of the Zr-Me bonds, to cyclometalation of the two amidinate ligands. The same product was obtained by thermolysis of the dimethyl complex. The square pyramidal five-coordinate 10 valence electron cationic monomethyl species [(A)(2)ZrMe](+) was generated and structurally characterized. Remarkably, the compound is unreactive toward ethylene, H-2, and CO. The sterically slightly less encumbered species [(B)(2)ZrMe](+) does polymerize ethylene, albeit with modest activity.