The reaction of a series of M·Zr heterobimetallic hydride complexes with
dienes and alkynes has been investigated (M = Al, Zn, and Mg). Reaction of M·Zr with
1,5-cyclooctadiene led to diene isomerization to 1,3-cyclooctadiene, but for M = Zn also
result in an on-metal diene-to-alkyne isomerization. The resulting cyclooctyne fragment is
trapped between Zr and Zn metals in a heterobimetallic species that does not form for M
= Mg or Al. The scope of diene isomerization and alkyne trapping has been explored
leading to the isolation of three new heterobimetallic slipped metallocyclopropene
complexes. The mechanism of diene-to-alkyne isomerization was investigated through
kinetics. While the reaction is first-order in Zn·Zr at high diene concentration and
proceeds with ΔH‡ = +33.6 ± 0.7 kcal mol−1
, ΔS‡ = +23.2 ± 1.7 cal mol−1 K−1
, and ΔG⧧
298 K = +26.7 ± 1.2 kcal mol−1
, the rate
is dependent on the nature of the diene. The positive activation entropy is suggestive of involvement of a dissociative step. On
the basis of DFT calculations, a heterobimetallic rebound mechanism for diene-to-alkyne isomerization has been proposed. This
mechanism explains the origin of heterobimetallic control over selectivity: Mg---Zr complexes are too strongly bound to generate
reactive fragments, while Al---Zr complexes are too weakly bound to compensate for the contrathermodynamic isomerization
process. Zn---Zr complexes have favorable energetics for both dissociation and trapping steps.