Lawrence Berkeley National Laboratory

Gas-phase coordination complexes of actinyl(v) cations, AnO₂⁺, provide a basis to assess fundamental aspects of actinide chemistry. Electrospray ionization of solutions containing an actinyl cation and sulfonate anion CH₃SO₂^- or C₆H₅SO₂^- generated complexes [(An^VO₂)(CH₃SO₂)₂]^- or [(An^VO₂)(C₆H₅SO₂)₂]^- where An = Np or Pu. Collision induced dissociation resulted in C-S bond cleavage for methanesulfinate to yield [(An^VO₂)(CH₃SO₂)(SO₂)]^-, whereas hydrolytic ligand elimination occurred for benzenesulfinate to yield [(An^VO₂)(C₆H₅SO₂)(OH)]^-. These different fragmentation pathways are attributed to a stronger C₆H₅-SO₂^-versus CH₃-SO₂^- bond, which was confirmed for both the bare and coordinating sulfinate anions by energies computed using a relativistic multireference perturbative approach (XMS-CASPT2 with spin-orbit coupling). The results demonstrate shutting off a ligand fragmentation channel by increasing the strength of a particular bond, here a sulfinate C-S bond. The [(An^VO₂)(CH₃SO₂)(SO₂)]^- complexes produced by CID spontaneously react with O₂ to eliminate SO₂, yielding [(AnO₂)(CH₃SO₂)(O₂)]^-, a process previously reported for An = U and found here for An = Np and Pu. Computations confirm that the O₂/SO₂ displacement reactions should be exothermic or thermoneutral for all three An, as was experimentally established. The computations furthermore reveal that the products are superoxides [(An^VO₂)(CH₃SO₂)(O₂)]^- for An = Np and Pu, but peroxide [(U^VIO₂)(CH₃SO₂)(O₂)]^-. Distinctive reduction of O₂^- to O₂^2- concomitant with oxidation of U(v) to U(vi) reflects the relatively higher stability of hexavalent uranium versus neptunium and plutonium.