Carbon–sulfur bond strength in methanesulfinate and benzenesulfinate ligands directs decomposition of Np(v) and Pu(v) coordination complexes

Abstract

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 O₂²⁻ concomitant with oxidation of U(V) to U(VI) reflects the relatively higher stability of hexavalent uranium versus neptunium and plutonium.