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 31 P NMR spectrum and by the X-ray structural determination in the solid state. 2-THF reacts with ZrCl 4 (Et 2 O) 2 to give (PNP)ZrCl 3 ( 3 ). (PNP)ZrCl 3 ( 3 ) can be alkylated with RCH 2 MgCl to give (PNP)Zr(CH 2 R) 3 ( 4a − c ). (PNP)ZrMe 3 ( 4a ) is thermally stable, and its solid-state structure is characterized by severe distortions from the octahedral geometry. The (PNP)Zr(CH 2 R) 3 (R = phenyl ( 4b ) or p -tolyl ( 4c )) compounds undergo α-abstraction at ambient temperature to give isolable Zr alkyl/alkylidenes (PNP)Zr( CHR)(CH 2 R) ( 5b, c ). The reaction follows a first-order rate law ( t 1/2 at 298 K ≈ 2.3 h). The activation parameters were determined from the VT NMR studies ( 4b → 5b ): Δ H ⧧ = 19(1) kcal/mol; Δ S ⧧ = −14(3) cal/(mol K); Δ G ⧧ 298 = 23(2) kcal/mol. The importance of the enforcement of the phosphine coordination by the rigid PNP ligand is discussed.