posted on 2006-12-04, 00:00authored byJohn L. Lewin, Nathaniel L. Woodrum, Christopher J. Cramer
The uni- and bimolecular C−H bond metathesis reactions of ansa-[bis(η5-2-indenyl)methane]ML (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 the 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.