Series of Tetravalent Actinide Amidinates: Structure Determination and Bonding Analysis

Two series of isostructural tetravalent actinide amidinates [AnX((S)-PEBA)₃] (An = Th, U, Np; X = Cl, N₃) bearing the chiral (S,S)-N,N'-bis(1-phenylethyl)benzamidinate ((S)-PEBA) ligand have been synthesized and thoroughly characterized in solid and in solution. This study expands the already reported tetravalent neptunium complexes to the lighter actinides thorium and uranium. Furthermore, a rare Ce(IV) amidinate [CeCl((S)-PEBA)₃] was synthesized to compare its properties to those of the analogous tetravalent actinide complexes. All compounds were characterized in the solid state using single-crystal XRD and infrared spectroscopy and in solution using NMR spectroscopy. Quantum chemical bonding analysis including also the isostructural Pa and Pu complexes was used to characterize the covalent contributions to any bond involving the metal cation. Th shows the least covalent character throughout the series, even substantially smaller than for the Ce complex. For U, Np, and Pu, similar covalent bonding contributions are found, but a natural population analysis reveals different origins. The 6d participation is the highest for U and decreases afterward, whereas the 5f participation increases continuously from Pa to Pu.