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

The precise mechanism of the reaction of butadiene with Cp*2Sm(H) (Cp* = η5-C5Me5) has been investigated on the basis of DFT calculations. The initial coordination of butadiene to the Cp*2Sm(H) moiety was found to be η2-fashion, η4-coordination being sterically difficult. The experimentally known insertion product, Cp*2Sm(η3-CH2CHCHMe) (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 η1-CH(Me)CHCH2 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-η1-CH2CHCHMe] intermediate, i.e., through another η3 → η1 change of the allylic coordination or dissociation of the double bond. Complex 1a can be formed directly by the insertion of an η2-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 η2-coordinated cis-butadiene via an η1-butenyl intermediate complex. The syn-configurated η3-butenyl complex 1a is thus the most preferred product in terms of both kinetics and thermodynamics.