Toward equatorial planarity about uranyl: Synthesis and structure of tridentate nitrogen-donor {UO₂}²⁺ complexes

The reaction of UO₂Cl₂·3THF with the tridentate nitrogen donor ligand 2,6-bis(2-benzimidazolyl)pyridine (H ₂BBP) in pyridine leads to the formation of three different complexes: [(UO₂)(H₂BBP)Cl₂] (1), [(UO) ₂(HBBP)(Py)Cl] (2), and [(UO₂)(BBP)(Py)₂] (3) after successive deprotonation of H₂BBP with a strong base. Crystallographic determination of 1-3 reveals that increased charge through ligand deprotonation and displacement of chloride leads to equatorial planarity about uranyl as well as a more compact overall coordination geometry. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra of 1-3 at the U-4d edges have been recorded using a soft X-ray Scanning Transmission X-ray Microscope (STXM) and reveal the uranium 4d_5/2 and 4d_3/2 transitions at energies associated with uranium in the hexavalent oxidation state. First-principles Density Functional Theory (DFT) electronic structure calculations for the complexes have been performed to determine and validate the coordination characteristics, which correspond well to the experimental results.