Synthesis and Structures of Plutonyl Nitrate Complexes: Is Plutonium Heptavalent in PuO₃(NO₃)₂⁻ ?

International audienceGas-phase plutonium nitrate anion complexes were produced by electrospray ionization (ESI) of a plutonium nitrate solution. The ESI mass spectrum included species with all four of the common oxidation states of plutonium: Pu(III), Pu(IV), Pu(V) and Pu(VI). Plutonium nitrate complexes were isolated in a quadrupole ion trap and subjected to collision induced dissociation (CID). CID of complexes of the general formula PuOx(NO3)y− resulted in the elimination of NO2 to produce PuOx+1(NO3)y−1−, which in most cases corresponds to an increase in the oxidation state of plutonium. Plutonyl species, PuVO2(NO3)2− and PuVIO2(NO3)3−, were produced from PuIII(NO3)4− and PuIV(NO3)5−, respectively, by the elimination of two NO2 molecules. CID of PuVIO2(NO3)3− resulted in NO2 elimination to yield PuO3(NO3)2−, in which the oxidation state of plutonium could be VII, a known oxidation state in condensed phase but not yet in the gas phase. Density functional theory confirmed the nature of PuVO2(NO3)2− and PuVIO2(NO3)3− as plutonyl(V/VI) cores coordinated by bidentate equatorial nitrate ligands. The computed structure of PuO3(NO3)2− is essentially a plutonyl(VI) core, PuVIO22+, coordinated in the equatorial plane by two nitrate ligands and one radical oxygen atom. The computations indicate that in the ground spin-orbit free state of PuO3(NO3)2−, the unpaired electron of the oxygen atom is antiferromagnetically coupled to the spin-triplet state of the plutonyl core. The results indicate that Pu(VII) is not a readily accessible oxidation state in the gas phase, despite that it is stable in solution and solids, but rather that a Pu(VI)-O• bonding configuration is favored, in which an oxygen radical is involved