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<sup>R</sup>N)­An­(N­{SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] (<b>3</b>–<b>8</b>) was accomplished by the protonolysis reaction between the respective imidazolin-2-imine (Im<sup>R</sup>NH, R = <i>t</i>Bu, Mes, Dipp) and the actinide metallacycles [{(Me<sub>3</sub>Si)­N}<sub>2</sub>An­{κ<sup>2</sup><i>C</i>,<i>N</i>-CH<sub>2</sub>SiMe<sub>2</sub>N­(SiMe<sub>3</sub>)}] (<b>1</b>, An = U; <b>2</b>, 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 <b>3</b>–<b>8</b> 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 <b>3</b>–<b>8</b> toward benzaldehyde was studied in the catalytic dimerization of aldehydes (Tishchenko reaction), displaying low to moderate catalytic activities for the uranium complexes <b>3</b>–<b>5</b> and moderate to high catalytic activities for the thorium analogues <b>6</b>–<b>8</b>, among which <b>8</b> 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<sup>Dipp</sup>N)­Th­{N­(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] (<b>8</b>), exhibiting high activity even at room temperature. Moreover, complex <b>8</b> 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%).