The “Indenyl Effect” in Zirconocene Dihydride Chemistry

The second-order rate constants for the insertion of cyclohexene into substituted bis(indenyl)zirconocene
dihydrides, (η5-C9H5-1,3-R2)2ZrH2 (R = SiMe3, CHMe2), to yield the cyclohexyl hydride complexes
have been measured. Comparison of these values to the corresponding tetrahydroindenyl derivatives
(η5-C9H9-1,3-R2)2ZrH2 reveals significantly faster insertion reactions for the bis(indenyl) compounds.
Accordingly, primarily σ-donating ligands such as PMe3, PEt3, and tetrahydrothiophene coordinate to
(η5-C9H5-1,3-(CHMe2)2)2ZrH2 to form (η5-C9H5-1,3-(CHMe2)2)2ZrH2(L) compounds, one of which (L =
PMe3) has been structurally characterized. In contrast, the more electron-rich zirconocene tetrahydroindenyl
dihydrides exhibit weaker binding in solution. Taken together, these results establish an “indenyl effect”
in zirconocene hydride chemistry likely arising from increased electrophilicity of the metal center, rather
than a change in hapticity along the reaction coordinate.