Mechanistic studies of the formation of zirconium alkylidene complexes [eta(5)-C5H3-1,3-(SiMe2CH2PPr2i)(2)]Zr=CHR(Cl) (R = Ph, SiMe3)

The reaction of [P2Cp]ZrCl3 (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 alpha-abstraction reaction of dibenzyl 3 which follows a first-order rate, with the rate parameters Delta H double dagger = 19(1) kcal mol(-1) and Delta S double dagger = -22(5) cal(-1) mol K-1. A kinetic isotope effect of 3.0(5) was measured at 70 degrees C for the perdeuterated analogue [P2Cp]ZrCl(CD2C6D5)2. The reaction of 1 with 2 equiv of LiCH2-EMe3 (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 Delta H double dagger = 6(1) kcal mol(-1) and Delta S double dagger = -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 alpha-agnostic interaction and a structure analogous to that of 5.