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 31P 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 α-abstraction at ambient temperature
to give isolable Zr alkyl/alkylidenes (PNP)Zr(CHR)(CH2R) (5b,c). The reaction follows a
first-order rate law (t1/2 at 298 K ≈ 2.3 h). The activation parameters were determined from
the VT NMR studies (4b → 5b):  ΔH⧧ = 19(1) kcal/mol; ΔS⧧ = −14(3) cal/(mol K); ΔG⧧298 =
23(2) kcal/mol. The importance of the enforcement of the phosphine coordination by the rigid
PNP ligand is discussed.