Chelate-enforced phosphine coordination enables α-abstraction to give zirconium alkylidenes

The rigid PNP pincer ligand (PNP = deprotonated anion of bis(ortho-diisopropylphoshinoaryl)amine) is shown to stabilize the (Zr=CHR)(2+)- fragment. (PNP)Li(THF) (2-THF) contains P-Li bonds, as evinced by the observation of the Li-P coupling in the solution P-31 NMR spectrum and by the X-ray structural determination in the solid state. 2-THF reacts with ZrCl4(Et2O)(2) to give (PNP)ZrCl3 (3). (PNP)ZrCl3 (3) can be alkylated with RCH2MgCl to give (PNP)Zr(CH2R)(3) (4a-c). (PNP)ZrMe3 (4a) is thermally stable, and its solid-state structure is characterized by severe distortions from the octahedral geometry. The (PNP)Zr(CH2R)(3) (R = phenyl (4b) or p-tolyl (4c)) compounds undergo a-abstraction at ambient temperature to give isolable Zr alkyl/alkylidenes (PNP)Zr(=CHR)(CH2R) (5b,c). The reaction follows a first-order rate law (t(1/2) at 298 K approximate to 2.3 h). The activation parameters were determined from the VT NMR studies (4b --> 5b): DeltaH(double dagger) = 19(1) kcal/mol; DeltaS(double dagger) = -14(3) cal/(mol K); DeltaG(298)(double dagger) = 23(2) kcal/mol. The importance of the enforcement of the phosphine coordination by the rigid PNP ligand is discussed.