Kinetics and mechanism of formation of yttrium alkyl complexes from (Cp*2YH)2 and alkenes

The dissociation of the dimer (Cp*2YH)2 (2) to the Cp*2YH monomer is an important process in reactions of 2 with alkenes. The rate of dissociation of 2 was measured by NMR linebroadening techniques (14 s(-1) at 0 degreesC, DeltaG* = 14.5 kcal mol(-1), DeltaH* = 14.9(8) kcal mol(-1), and DeltaS' = 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*2YH followed by competitive trapping by alkene and recombination to regenerate 2. Kinetic studies of the less crowded alkene 3-methyl-l-butene are consistent with rate-limiting dissociation of dimer 2 followed by efficient trapping of the intermediate Cp*2YH 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*2YH. 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 beta-alkyl yttrium double-insertion product is much less reactive toward terminal alkenes.