Theoretical Study of Coordination and Insertion of Butadiene to Bis(pentamethylcyclopentadienyl)samarium Hydride

The precise mechanism of the reaction of butadiene with Cp* 2 Sm(H) (Cp* = η 5 -C 5 Me 5 ) has been investigated on the basis of DFT calculations. The initial coordination of butadiene to the Cp* 2 Sm(H) moiety was found to be η 2 -fashion, η 4 -coordination being sterically difficult. The experimentally known insertion product, Cp* 2 Sm(η 3 -CH 2 CHCHMe) ( 1a ), where the methyl group is in the syn position, is thermodynamically more stable than the anti -isomer ( 1c ) by 5.7 kcal/mol. The anti → syn transformation, i.e., isomerization of 1c to 1a, is possible via an intermediate with an η 1 -CH(Me)CH CH 2 ligand but requires a rather high energy barrier of 22 kcal/mol. Rotational motion of the allylic ligand in 1a takes place through a [Sm-η 1 -CH 2 CH CHMe] intermediate, i.e., through another η 3 → η 1 change of the allylic coordination or dissociation of the double bond. Complex 1a can be formed directly by the insertion of an η 2 -coordinated trans -butadiene to the Sm−H bond through an early transition state with very small activation energy. In contrast, 1c is derived from a complex with an η 2 -coordinated cis -butadiene via an η 1 -butenyl intermediate complex. The syn -configurated η 3 -butenyl complex 1a is thus the most preferred product in terms of both kinetics and thermodynamics.