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

摘要

Gas-phase coordination complexes of actinyl(v) cations, AnO(2)(+), 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(V)O(2))(CH3SO2)(2)](-) or [(An(V)O(2))(C6H5SO2)(2)](-) where An = Np or Pu. Collision induced dissociation resulted in C-S bond cleavage for methanesulfinate to yield [(An(V)O(2))(CH3SO2)(SO2)](-), whereas hydrolytic ligand elimination occurred for benzenesulfinate to yield [(An(V)O(2))(C6H5SO2)(OH)](-). These different fragmentation pathways are attributed to a stronger C6H5-SO(2)(-)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(V)O(2))(CH3SO2)(SO2)](-) complexes produced by CID spontaneously react with O-2 to eliminate SO2, yielding [(AnO(2))(CH3SO2)(O-2)](-), a process previously reported for An = U and found here for An = Np and Pu. Computations confirm that the O-2/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(V)O(2))(CH3SO2)(O-2)](-) for An = Np and Pu, but peroxide [((UO2)-O-VI)(CH3SO2)(O-2)](-). Distinctive reduction of O-2(-) to O-2(2-) concomitant with oxidation of U(v) to U(vi) reflects the relatively higher stability of hexavalent uranium versus neptunium and plutonium.