Actinyl species, [AnO2]2+, and their nitrogen analogue, [An(NR)2]2+, are characterized by their two strong trans-An-element multiple bonds. Here we report these robust bonds can be significantly weakened by increasing the number of multiple bonds to uranium, as demonstrated by a family of uranium(VI) dianions bearing four U-N multiple bonds, [M]2[U(NR)4]. The uranium(VI) tetrakis(imido) species, [M]2[U(NDIPP)4], where DIPP = 2,6-diisopropylphenyl, and M = Li, Na, K, Rb, and Cs, have been synthesized and the nature of the bonds in these species has been characterized by quantum chemical calculations. Density functional theory (DFT) calculations have been carried out. Molecular orbital analyses show that the U-N bonds are composed of two covalent bonds of predominantly U 5f, U 6d, and N 2p orbitals, with strong competition for the limited number of uranium-based orbitals due to the sharing among all four strongly π-donating imido ligands, see Fig. 1. Natural bond order (NBO), quantum theory atom in molecule (QTAIM), and complete active space self-consistent field (CASSCF) approaches have been employed to analyze the bonding characters. DFT calculations proved that the most stable structure of [M]2[U(NDIPP)4] family is the observed pseudo-C2v see-saw structures, as compared to the ideal square-planar and tetrahedral structures. To understand the driving force for the see-saw structures, a family of tetrakis(oxo) and tetrakis(imido) dianionic complexes (UO42-, U(NH)42-, U(NMe)42-, U(NPh)42-, and U(NDIPP)42-) and their corresponding neutral complexes (Na2UO4, Na2U(NPh)4) were analyzed. Structural information support that the geometry of [M]2[U(NDIPP)4] family is driven by cation-coordination and sterics rather than electronic factors.

Elucidating bonding preference in tetrakis(imido)uranate (VI) dianions

253rd National Meeting (2017)

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