Mechanistic studies of the formation of zirconium alkylidene complexes [η⁵-C₅H₃-1,3-(SiMe₂CH₂PPr(i)₂)₂]Zr=CHR(Cl) (R = Ph, SiMe₃)

The reaction of [P₂Cp]ZrC1₃ (1) with 2 equiv of KCH₂Ph generates an equilibrium mixture of alkyl complexes consisting of [P₂Cp]ZrCl₂(CH₂Ph) (2), [P₂Cp]ZrCl(CH₂Ph)₂ (3), and [P₂Cp]Zr(CH₂Ph)₃ (4). Thermolysis of this mixture yields the alkylidene complex [P₂Cp]Zr=CHPh(Cl) (5) in 85% overall yield. Kinetic studies reveal a composite mechanism that incorporates the above preequilibrium, followed by an intramolecular α-abstraction reaction of dibenzyl 3 which follows a first-order rate, with the rate parameters ΔH((+)) = 19(1) kcal mol^-1 and ΔS((+)) = -22(5) cal^-1 mol K^-1. A kinetic isotope effect of 3.0(5) was measured at 70°C for the perdeuterated analogue [P₂Cp]ZrCl(CD₂C₆D₅)₂. The reaction of 1 with 2 equiv of LiCH₂EMe₃ (E = C, Si) produces a similar equilibrium mixture as observed for the benzyl analogues, consisting of [P₂Cp]ZrCl₂(CH₂EMe₃) (7), [P₂Cp]Zr(CH₂EMe₃)₃ (8), and [P₂Cp]ZrCl(CH₂EMe₃)₂ (9). Thermolysis of this mixture yields [P₂Cp]Zr=CHEMe₃(Cl) (6). A kinetic analysis conducted on 9 (E = Si) indicated a first-order reaction from which the activation parameters ΔH((+)) = 6(1) kcal mol^-1 and ΔS((+)) = -62(5) cal mol^-1 K^-1 were obtained. The results indicate that reaction rates follow the order CH₂Ph > CH₂SiMe₃ > CH₂CMe₃, an exact reversal of the trend for the homoleptic Ta systems Ta(CH₂R)₅. The role of phosphine coordination is discussed to account for this trend. A crystal structure determination obtained for 6b reveals an α-agostic interaction and a structure analogous to that of 5.