INSERTION AND SIGMA-BOND METATHESIS PATHWAYS IN GAS-PHASE REACTIONS OF BIS(ETA-5-CYCLOPENTADIENYL)METHYLZIRCONIUM(1+) WITH DIHYDROGEN AND UNSATURATED-HYDROCARBONS

Reactions of Cp2ZrCH3+ (1; Cp = cyclopentadienyl) and Cp2ZrCD3+ (2) with a number of unsaturated hydrocarbon substrates and dihydrogen have been investigated. Reactions of 2 with hydrocarbon substrates and C2D4 with 1 support two major reaction pathways: insertion/dehydrogenation, which is accompanied by near statistical H/D scrambling, and σ-bond metathesis, which is indicated by the exclusive elimination of CD3H in reactions of 2 with certain substrates. Reactions of 1 and 2 with ethylene and α-alkyl-substituted alkenes result in elimination of dihydrogen and formation of η3-allyl complex cations. For reaction of 1 and 2 with n-alkyl-substituted ethylenes having n-alkyl substituents larger than ethyl (e.g., 1-pentene, 1-hexene) elimination of H2 and 2H2 occurs, and in the current study products formed with H2 elimination are always found in greater abundance. Reactions of 2 with α,α'-substituted alkenes (e.g., isobutene and α-methylstyrene) proceed with elimination of CD3H exclusively, which is consistent with a σ-bond metathesis process. Reaction of 2 with isobutene also yields 1, suggesting an insertion/β-methide shift/isobutene elimination sequence. Cation 1 is also produced in the reaction of 2 with propyne. Reaction of Cp2ZrD+ with ethylene shows formation of Cp2ZrH+, suggesting an insertion/β-hydride shift/ethylene elimination process, presumably because a η3-allyl product cannot be formed by elimination of H2. Reactions of 1 and 2 with cis and trans isomers of 2-butene and 2-pentene have also been investigated, and striking reactivity differences are observed for the two isomers (i.e., cis isomers react predominantly by an insertion/dehydrogenation pathway and trans isomers react almost exclusively by elimination of methane). Products consistent with both insertion/dehydrogenation and σ-bond metathesis are suggested by results of reactions of 1 and 2 with several alkynes. The reaction of 2 with H2 proceeds with exclusive elimination of CD3H, also indicating a σ-bond metathesis process. Rate constants for the reaction of 1 with H2 and D2 yield a kH/kD value of 2.0 ± 0.5. The reactivity of 1 with unsaturated hydrocarbons is compared to the solution reactions of the analogous solvated cation and other d0fn metal complexes. © 1990, American Chemical Society. All rights reserved.