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 [(AnVO2)(CH3SO2)2]- or [(AnVO2)(C6H5SO2)2]- where An = Np or Pu. Collision induced dissociation resulted in C-S bond cleavage for methanesulfinate to yield [(AnVO2)(CH3SO2)(SO2)]-, whereas hydrolytic ligand elimination occurred for benzenesulfinate to yield [(AnVO2)(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 [(AnVO2)(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 [(AnVO2)(CH3SO2)(O2)]- for An = Np and Pu, but peroxide [(UVIO2)(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.