Mono(imidazolin-2-iminato) Actinide Complexes: Synthesis and Application in the Catalytic Dimerization of Aldehydes

The synthesis of
the mono­(imidazolin-2-iminato) actinide­(IV) complexes
[(ImRN)­An­(N­{SiMe3)2}3]
(3–8) was accomplished by the protonolysis
reaction between the respective imidazolin-2-imine (ImRNH, R = tBu, Mes, Dipp) and the actinide metallacycles
[{(Me3Si)­N}2An­{κ2C,N-CH2SiMe2N­(SiMe3)}] (1, An = U; 2, M = Th). The thorium
and uranium complexes were obtained in high yields, and their structures
were established by single-crystal X-ray diffraction analysis. The
mono­(imidazolin-2-iminato) actinide complexes 3–8 display short An–N bonds together with large An–N–C
angles, indicating strong electron donation from the imidazolin-2-iminato
moiety to the metal, corroborating a substantial π-character
to the An–N bond. The reactivity of complexes 3–8 toward benzaldehyde was studied in the catalytic
dimerization of aldehydes (Tishchenko reaction), displaying low to
moderate catalytic activities for the uranium complexes 3–5 and moderate to high catalytic activities
for the thorium analogues 6–8, among
which 8 exhibited the highest catalytic activity. In
addition, actinide coordination compounds showed unprecedented reactivity
toward cyclic and branched aliphatic aldehydes in the catalytic Tishchenko
reaction mediated by the thorium complex [(ImDippN)­Th­{N­(SiMe3)2}3] (8), exhibiting high
activity even at room temperature. Moreover, complex 8 was successfully applied in the crossed Tishchenko reaction between
an aromatic or polyaromatic and an aliphatic cyclic and branched aldehyde,
yielding selectively the asymmetrically substituted ester in high
yields (80–100%).