Theoretical Investigations of Isoprene Polymerization Catalyzed by Cationic Half-Sandwich Scandium Complexes Bearing a Coordinative Side Arm

Density
functional theory studies have been conducted for isoprene
polymerization catalyzed by the cationic half-sandwich scandium alkyl
species containing a methoxy side arm [(C5Me4C6H4OMe-o)­Sc­(CH2SiMe3)]+ (1) and that containing
a phosphine oxide side arm [{C5Me4SiMe2CH2P­(O)­Ph2}­Sc­(CH2SiMe3)]+ (2). It has been found that trans-1,4-polymerization of isoprene by species 1 prefers
an insertion–isomerization mechanism: (i) an insertion of cis-isoprene into the metal–alkyl bond to give η3-π-anti-form, (ii) anti-syn isomerization of the resulting 1,2-disubstituted
allyl complex to yield a syn-allyl form, (iii) repetitive
insertion of cis-isoprene into the metal–syn-allyl bond and subsequent anti–syn isomerization. The resulting η3-π-syn-allyl species is suitable for more kinetically favorable cis-monomer insertion. The stability of the key transition
state involved in the most feasible pathway could be ascribed to the
smaller deformation of cis-isoprene and stronger
interaction between the cis-isoprene moiety and the
remaining metal complex. The origin of experimentally observed inertness
of 2 toward isoprene polymerization is that the steric
hindrance derived from the crowding of η3-π-syn-allyl species hampers the insertion of the incoming
isoprene monomer. The modeling of 2-mediated chain propagation
also has a high energy barrier and is endergonic. To corroborate the
steric effect on the kinetic and thermodynamic aspects, various analogue
complexes with smaller hindrance have been computationally modeled
on the basis of 2. Expectedly, lower energy barrier and
favorable thermodynamics are found for the monomer insertion mediated
by these complexes with less steric hindrance around the metal center.