Reactivity of [3+1+1] Uranyl-DGA Complex as Lewis-Acid Catalyst in Nucleophilic Acyl Substitution of Acid Anhydrides

A UO22+ complex bearing N, N, N′, N′-tetraisopropyldiglycolamide (TiPDGA) and two DMF molecules was prepared to explore the catalytic activities of the Lewis-acidic U centre. The cationic complex, [UO2(TiPDGA)(DMF)2]2+, was obtained as a ClO4− salt under optimised reaction conditions with an appropriate mixing ratio between UO22+ and TiPDGA to maintain 1:1 stoichiometry, a non-coordinating ClO4− counteranion to reserve the coordination sites for substrate activation, and the presence of extra HClO4 to suppress undesired hydrolysis of UO22+ competing with the expected complex formation. This UO22+ complex was characterised by IR, elemental analysis, X-ray crystallography, and 1H NMR to confirm that the desired [3+1+1] equatorial coordination is actually formed in the solid state and is still maintained even after dissolution in CD2Cl2. [UO2(TiPDGA)(DMF)2]2+ was further subjected to nucleophilic acyl substitution reactions of acid anhydrides to assess its activity and capability as a Lewis-acid catalyst there. Although the observed reaction rates were not very rapid, some characteristic aspects to gain reaction- and substrate-selectivity appeared thanks to the equatorial coordination sphere sterically regulated by the tridentate auxiliary TiPDGA ligand and labile monodentate DMF molecules to activate an acid anhydride after ligand substitution.