Dimeric uranyl complexes with bridging perrhenates

The reaction between [UO₂(ReO₄)₂·H₂O] and two equivalents of either tri-n-butyl phosphine oxide (TBPO) or tri-iso-butyl phosphate (TiBP) results in the formation of [UO₂(µ₂-ReO₄)(ReO₄)(TBPO)₂]₂ (1) and [UO₂(µ₂-ReO₄)(ReO₄)(TiBP)₂]₂ (2) respectively. Both complexes crystallise as two structurally similar centrosymmetric dimers, the cores containing two uranyl moieties linked by bridging perrhenates. Two PO donor ligands and one monodenatate perrhenate complete the pentagonal bipyramidal coordination sphere at each metal centre. Both complexes have also been characterised in the solid state by vibrational and absorption spectroscopy. Solution spectroscopic characterisation indicates that both perrhenate and phosphine oxide (1) or phosphate (2) remain coordinated, although it is not possible to state conclusively that the dimeric species remain intact. A low resolution structural study of a minor product from the reaction that yielded 2 revealed a monomeric complex with only monodentate perrhenate coordination, [UO₂(ReO₄)₂(H₂O)(TiBP)₂] (2′). These results represent the first structural evidence for the bridging coordination mode of perrhenate on coordination to an actinide and yields further insight into the possible solvent phase pertechnetate complexes that may exist in PUREX process phosphate rich solvent.