Mechanistic Studies of the Formation of Zirconium Alkylidene Complexes [η5-C5H3-1,3-(SiMe2CH2PPri 2)2]ZrCHR(Cl) (R = Ph, SiMe3)

• Published in: Journal of the American Chemical Society 1999-03, Vol.121 (11), p.2478-2487
• Main Authors: Fryzuk, Michael D, Duval, Paul B, Mao, Shane S. S. H, Zaworotko, Michael J, MacGillivray, Leonard R
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja982969h

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 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.