Differential uranyl( v ) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study

The uranyl(vi) 'Pacman' complex [(UO2)(py)(H2L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp2Ti(eta(2)-Me3SiC & xe002;CSiMe3) and [Cp2TiCl](2) to oxo-titanated uranyl(v) complexes [(py)((Cp2TiOUO)-O-III)(py)(H2L)] 1 and [((ClCp2TiOUO)-O-IV)(py)(H2L)] 2. Combination of Zr-II and Zr-IV synthons with A yields the first Zr-IV-uranyl(v) complex, [(ClCp2ZrOUO)(py)(H2L)] 3. Similarly, combinations of Ae(0) and Ae(II) synthons (Ae = alkaline earth) afford the mono-oxo metalated uranyl(v) complexes [(py)(2)(ClMgOUO)(py)(H2L)] 4, [(py)(2)(thf)(2)(ICaOUO)(py) (H2L)] 5; the zinc complexes [(py)(2)(XZnOUO)(py)(H2L)] (X = Cl 6, I 7) are formed in a similar manner. In contrast, the direct reactions of Rb or Cs metal with A generate the first mono-rubidiated and mono-caesiated uranyl(v) complexes; monomeric [(py)(3)(RbOUO)(py)(H2L)] 8 and hexameric [(MOUO)(py)(H2L)](6) (M = Rb 8b or Cs 9). In these uranyl(v) complexes, the pyrrole N-H atoms show strengthened hydrogen-bonding interactions with the endo-oxos, classified computationally as moderate-strength hydrogen bonds. Computational DFT MO (density functional theory molecular orbital) and EDA (energy decomposition analysis), uranium M-4 edge HR-XANES (High Energy Resolution X-ray Absorption Near Edge Structure) and 3d4f RIXS (Resonant Inelastic X-ray Scattering) have been used (the latter two for the first time for uranyl(v) in 7 (ZnI)) to compare the covalent character in the U-V-O and O-M bonds and show the 5f orbitals in uranyl(vi) complex A are unexpectedly more delocalised than in the uranyl(v) 7 (ZnI) complex. The O-exo-Zn bonds have a larger covalent contribution compared to the Mg-O-exo/Ca-O-exo bonds, and more covalency is found in the U-O-exo bond in 7 (ZnI), in agreement with the calculations.