Tailoring the Reactivity of Actinide Complexes Towards Oxygen Containing Substrates

Isabell Sarah Karmel, Chemistry, Schulich Faculty of Chemistry Technion, Haifa, Israel

Recently, several sophisticated heteroatom containing ligand systems have been introduced into the coordination chemistry of the actinides. Among them, ligands with an An-E (E = O, N, C, P) multiple bond character have received special attention. Imidazolin-2-iminato ligands are considered as 2s, 4π electron donors, similar to the ubiquitous cyclopentadienyl moiety, and therefore making them an attractive ligand class for actinide chemistry. A family of uranium (IV) imidazolin-2-iminato complexes was synthesized by an acid-base reaction between the homoleptic [U(NMeEt)₄] and the neutral imidazolin-2-imine Im^RNH, yielding the uranium (IV) complexes [(Im^RN)_nU(NMeEt)_4-n]. The number of ligands coordinated to the metal depends on the steric encumbrance of the imidazolin-2-iminato and not on the stoichiometry of the starting materials, or the reaction conditions, indicating a thermodynamic controlled reaction. Therefore, mono(imidazolin-2-iminato) actinide complexes were prepared by a protonolysis reaction of the actinide metallacycles [{(Me₃Si)N}₂An{k²C,N-CH₂SiMe₂N(SiMe₃)}] with the neutral imidazolin-2-imines Im^RNH. The imidazolin-2-iminato actinide complexes display short An-N bond lengths and large, close to linear An-N-C bond angles, indicating a substantial π-character to the An-N bond. Typically, actinide compounds react due to their high oxophilicity with oxygen containing molecules to form thermodynamically stable actinide-oxo species, impeding their use as active catalysts with oxygen containing substrates. In order to overcome this challenge, we use the strongly nucleophilic imidazolin-2-iminato moiety, which upon coordination to the highly electrophilic metal is expected to slightly increase its electron density, resulting in less oxophilic actinide complexes, which are active catalysts for oxygen containing substrates. The mono(imidazolin-2-iminato) thorium (IV) complex [(Im^DippN)Th{N(SiMe₃)₂}₃] exhibited an unprecedented high activity towards cyclic and branched aliphatic aldehydes in the catalytic Tishchenko reaction, even at room temperature. Moreover, the thorium compound [(Im^DippN)Th{N(SiMe₃)₂}₃] was successfully applied in the crossed Tishchenko reaction yielding selectively the asymmetrically substituted ester in high yields.