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

Abstract

The uranyl(VI) ‘Pacman’ complex [(UO₂)(py)(H₂L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp₂Ti(η²-Me₃SiCCSiMe₃) and [Cp₂TiCl]₂ to oxo-titanated uranyl(V) complexes [(py)(Cp₂Ti^IIIOUO)(py)(H₂L)] 1 and [(ClCp₂Ti^IVOUO)(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 Ae⁰ 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 M₄ 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.