Mixed Imidazolin-2-iminato–Cp* Thorium(IV) Complexes: Synthesis and Reactivity Toward Oxygen-Containing Substrates

The
mixed pentamethylcyclopentadienyl thorium­(IV) imidazolin-2-iminato
complexes Cp*2Th­(ImDippN)­(Me) (2) and Cp*2Th­(ImMesN)­(Me) (3) were synthesized in quantitative yields via rapid protonolysis
of Cp*2Th­(Me)2 (1) with the respective
neutral imidazolin-2-iminato ligand ImRNH. Cp*2Th­(ImDippN)­(Me) (2) and Cp*2Th­(ImMesN)­(Me) (3) display short Th–N
bond lengths and large Th–N–C angles. The reactivity
of complex 2 and 3 toward oxygen-containing
substrates was studied, and the catalytic activity of 2 was compared to the dimethyl (bispentamethyl–cyclopentadienyl)
thorium complex 1. Complex 2 was applied
in the catalytic Tishchenko reaction with aromatic, heteroaromatic,
and branched aliphatic aldehydes, displaying a higher catalytic activity
than Cp*2Th­(Me)2 and Cp*2Th­(ImMesN)­(Me). Furthermore, 2 was applied as a catalyst
in the crossed Tishchenko reaction and in the oligomerization of bis­(aldehydes),
as well as in the ring-opening polymerization of ε-caprolactone.
In all reactions, the activity of Cp*2Th­(ImDippN)­(Me) (2) and Cp*2Th­(ImMesN)­(Me) (3) was higher than that observed for Cp*2Th­(Me)2 (1), which can be attributed
to the increased electron density introduced by the coordination of
the imidazolin-2-iminato ligand.