Synthesis and Hydrolysis of Uranyl, Neptunyl, and Plutonyl Gas-Phase Complexes Exhibiting Discrete Actinide–Carbon Bonds

Gas-phase organoactinyl
complexes possessing discrete An–C
bonds (An = U, Np, Pu) were synthesized in a quadrupole ion trap by
endothermic decarboxylation of [AnO2(O2C–R)3]− anion complexes in which a formally AnO22+ actinyl core is coordinated by three carboxylate
ligands, with R = CH3 (methyl), CH3CC (1-propynyl),
C6H5 (phenyl), C6F5 (pentafluorophenyl).
Decarboxylation and competing ligand loss were studied computationally
by density functional theory complementing experiment. Although decarboxylation
was computed to be the energetically most favorable process in all
cases, reduction from An­(VI) to An­(V) via neutral ligand loss was
often prevalent, particularly for An = Np, Pu, presumably resulting
from barriers associated with decarboxylation. Comparative hydrolysis
rates of the An–C bonds were experimentally determined, and
the chemical properties of these bonds were analyzed by the quantum
theory of atoms in molecules. The measured hydrolysis rates differed
by up to 3 orders of magnitude: the fastest was for [(CH3CC)­UO2(O2C–CCCH3)2]− and the slowest for [(C6F5)­PuO2(O2C–C6F5)2]−. There is a general correlation
between hydrolysis exothermicity and hydrolysis rate. Prototypical
hydrolysis reaction pathways computed for R = CH3 (An =
U, Np) reveal a mechanism in which an outer-sphere water becomes inner-sphere
concomitant with transfer of an H atom to yield an OH ligand and CH4, with a net energy release of 170 kJ mol–1 and a transition state barrier of 45 kJ mol–1 for
An = U. Infrared multiphoton dissociation spectra of selected complexes
were acquired to confirm the predicted structures by agreement between
the computed and observed vibrational frequencies. The experiment
and theory results provide an evaluation of the comparative propensities
for formation of the organoactinyls as a function of actinide and
carboxylate and an assessment of the nature and stability toward hydrolysis
of the primarily ionic An–C bonds.