Chelate-enforced phosphine coordination enables alpha-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 31P NMR spectrum and by the X-ray structural determination in the solid state. 2-THF reacts with ZrCl 4(Et 2O) 2 to give (PNP)ZrCl 3 (3). (PNP)ZrCl 3 (3) can be alkylated with RCH 2MgCl to give (PNP)Zr(CH 2R) 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 2R) 3 (R = phenyl (4b) or p-tolyl (4c)) compounds undergo -abstraction at ambient temperature to give isolable Zr alkyl/alkylidenes (PNP)Zr(=CHR)(CH 2R) (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): H = 19(1) kcal/mol; S = -14(3) cal/(mol K); G 298 = 23(2) kcal/mol. The importance of the enforcement of the phosphine coordination by the rigid PNP ligand is discussed.

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