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 [(Im^RN)­An­(N­{SiMe₃)₂}₃] (3–8) was accomplished by the protonolysis reaction between the respective imidazolin-2-imine (Im^RNH, 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 [(Im^DippN)­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%).