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Density Functional Characterization of Methane Metathesis in ansa-[bis(η5-2-indenyl)methane]ML Complexes [M = Sc, Y, Lu; L = CH3, CH2C(CH3)3]

Lewin, J. L.; Woodrum, N. L.; Cramer, C. J.
Organometallics 2006, 25, 5906.

The uni- and bimolecular C-H bond metathesis reactions of ansa-[bis(η5-2-indenyl)methane]ML complexes [M = Sc, Y, Lu; L = CH3, CH2C(CH3)3] with four ansa-indenyl ligands having various degrees of aromatic ring methylation were modeled with the MPW1K density functional and a relativistic effective core potential basis set. Analysis of theoretical trends as a function of metal and ligand indicates that, in contrast to the situation with analogous (Cp*)2 sandwich complexes, unimolecular metathesis reactions proceeding through tuck-in intermediates have significantly higher enthalpies of activation compared to alternative bimolecular pathways, so that the latter dominate metathesis reactivity in every case under typical experimental conditions; taking account of quantum mechanical tunneling through the reaction coordinate does not change this situation. Narrow bite angles enforced by the ansa methylene bridge on the bis(indenyl) ligands cause steric differences between methyl and neopentyl ligands to have reduced influence on either structural parameters or enthalpic barriers compared to analogous (Cp*)2 sandwich complexes. The degree of methylation of the bis(indenyl) ligand was found to play no significant role in complex structure and reactivity.

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