Kinetics and Mechanism of Formation of Yttrium Alkyl Complexes from (Cp* 2 YH) 2 and Alkenes Charles P. Casey,* Jon A. Tunge, Ting-Yu Lee, and Donald W. Carpenetti II Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received September 10, 2001 The dissociation of the dimer (Cp* 2 YH) 2 (2) to the Cp* 2 YH monomer is an important process in reactions of 2 with alkenes. The rate of dissociation of 2 was measured by NMR line- broadening techniques (14 s -1 at 0 °C, ΔG q ) 14.5 kcal mol -1 , ΔH q ) 14.9(8) kcal mol -1 , and ΔS q ) 3(2) eu). A full range of dissociative and associative mechanisms for reaction of alkenes with 2 was found. For the most crowded and least reactive alkenes studied, 2-butene and 2-methylpropene, reaction with 2 occurred slower than dissociation of dimer 2; kinetic studies established reversible dissociation of 2 to monomeric Cp* 2 YH followed by competitive trapping by alkene and recombination to regenerate 2. Kinetic studies of the less crowded alkene 3-methyl-1-butene are consistent with rate-limiting dissociation of dimer 2 followed by efficient trapping of the intermediate Cp* 2 YH by alkene. The least crowded terminal alkenes such as 1-hexene reacted with 2 at a rate faster than dimer dissociation; kinetic studies established a two-component rate law involving a second-order term for direct attack of alkene on the dimer and a first-order term involving rate-determining dimer dissociation followed by rapid alkene reaction with monomeric Cp* 2 YH. The reactions of terminal alkenes with 2 initially gave mixtures of single- and double-alkene-insertion products but no triple- insertion products. The initially formed n-alkyl yttrium complex reacts with terminal alkenes at a rate similar to the reaction of yttrium hydride dimer 2 with terminal alkenes. The more crowded -alkyl yttrium double-insertion product is much less reactive toward terminal alkenes. Introduction The insertion of an alkene into lanthanide and group 3 hydride bonds represents an important elementary step in catalysis of alkene hydrogenation 1 and hydro- silylation. 2 Furthermore, the reaction of alkenes with d 0 metal-hydride bonds is important in the initiation and chain transfer steps of alkene oligomerization and polymerization. 3-5 Bis(cyclopentadienyl)yttrium hy- drides have been shown to be among the most active ethylene polymerization catalysts, and recently Bercaw has designed 1 for the isospecific polymerization of propene (Scheme 1). 5 Alkene polymerization by lan- thanide hydrides and alkyls has received much atten- tion because the catalysts are active and can be studied without the need for a cocatalyst. This provides the basis for the “lanthanide model” 6 of Ziegler-Natta olefin polymerization. We have investigated d 0 yttrium 7 and zirconium 8 metal alkyl-alkene complexes as models of Ziegler- Natta polymerization intermediates. Given the results of these studies, we were interested in preparing mono- meric yttrium alkyl complexes for the study of non- chelated yttrium alkyl-alkene complexes. 7e In addition to the synthesis of bis(pentamethylcyclopentadienyl)- (1) (a) Evans, W. J.; Meadows, J. H.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1984, 106, 1291. (b) Jeske, G.; Lauke, H.; Mauer- mann, H.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8111. (c) Haar, C. M.; Stern, C. L.; Marks, T. J. Organometallics 1996, 15, 1765. (d) Giardello, M. A.; Conticello, V. P.; Brard, L.; Sabat, M.; Rheingold, A. L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10212. (e) Giardello, M. A.; Conticello, V. P.; Brard, L.; Gagne ´, M. R.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241. (f) Molander, G. A.; Hoberg, J. O. J. Org. Chem. 1992, 57, 3266. (g) Molander, G. A.; Winterfeld, J. J. Organomet. Chem. 1996, 524, 275. (2) (a) Molander, G. A.; Julius, M. J. Org. Chem. 1992, 57, 6347. (b) Molander, G. A.; Dowdy, E. D.; Noll, B. C. Organometallics 1998, 17, 3754. (c) Molander, G. A.; Knight, E. E. J. Org. Chem. 1998, 63, 7009. (d) Molander, G. A. Chemtracts 1998, 11, 237. (e) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 7157. (f) Voskoboy- nikov, A. Z.; Parshina, I. N.; Shestakova, A. K.; Butin, K. P.; Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J. A. K. Organometallics 1997, 16, 4041. (3) (a) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091. (b) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8103. (c) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. J. Am. Chem. Soc. 1990, 112, 1566. (d) Evans, W. J.; DeCoster, D. M.; Greaves, J. Organometallics 1996, 15, 3210. (4) (a) Sandig, N.; Dargel, T. K.; Koch, W. Z. Anorg. Allg. Chem. 2000, 626, 392. (b) Margl, P.; Deng, L.; Ziegler, T. J. Am. Chem. Soc. 1998, 120, 5517. (c) Margl, P.; Deng, L.; Ziegler, T. J. Am. Chem. Soc. 1999, 121, 154. (5) (a) Mitchell, J. P.; Hajela, S.; Brookhart, S. K.; Hardcastle, K. I.; Henling, L. M.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 1045. (b) Coughlin, E. B.; Bercaw, J. E. J. Am. Chem. Soc. 1992, 114, 7606. (6) (a) Watson, P. L.; Herskovitz, T. ACS Symp. Ser. 1983, No. 212, 459. (b) Watson, P. L. J. Am. Chem. Soc. 1982, 104, 337. (c) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471. (d) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51. (7) (a) Casey, C. P.; Klein, J. F.; Fagan, M. A. J. Am. Chem. Soc. 2000, 122, 4320. (b) Casey, C. P.; Fagan, M. A.; Hallenbeck, S. L. Organometallics 1998, 17, 287. (c) Casey, C. P.; Hallenbeck, S. L.; Wright, J. M.; Landis, C. R. J. Am. Chem. Soc. 1997, 119, 9680. (d) Casey, C. P.; Hallenbeck, S. L.; Pollock, D. W.; Landis, C. R. J. Am. Chem. Soc. 1995, 117, 9770. (e) Casey, C. P.; Lee, T.-Y.; Tunge, J. A.; Carpenetti, D. W., II J. Am. Chem. Soc. 2001, 123, 10762. (8) (a) Casey, C. P.; Carpenetti, D. W., II. Organometallics 2000, 19, 3970. (b) Casey, C. P.; Carpenetti, D. W., II; Sakurai, H. J. Am. Chem. Soc. 1999, 121, 9483. (c) Casey, C. P.; Carpenetti, D. W., II; Sakurai, H. Organometallics 2001, 20, 4262. 389 Organometallics 2002, 21, 389-396 10.1021/om010817g CCC: $22.00 © 2002 American Chemical Society Publication on Web 12/19/2001