Mechanisms of ligand substitution reactions in ternary dioxouranium(VI) complexes

The activation parameters of various inter- and intramolecular ligand exchange reactions in ternary complexes of the type UO2LF3 (where L is one of the bidentate ligands picolinate, 4-nitropicolinate, 4-(3-pentyl)picolinate, oxalate, or carbonate) and UO2L2F (where L = picolinate or oxalate) have been determined by different NMR techniques. The activation entropies for the reactions have been used to discuss their intimate mechanism, particularly the solvent participation. Additional mechanistic information has been obtained from studies of H+/ D+ catalysis of the various reactions. Through the use of the proton-catalyzed pathway, the rate of carbonate exchange in UO2CO3F33- was varied by a factor of ca, 10. The fact that the rate of fluoride exchange remained constant clearly indicates that the exchanges of carbonate and fluoride follow parallel pathways. The activation entropies of most of the fluoride exchange reactions in UO2LF3 have values close to 10 J K-1 mol(-1), indicating dissociative (D) or dissociative interchange (I-D) mechanisms. The exchange of L takes place in two steps: The first is a chelate ring opening/closing, resulting in a ligand rotation. The second is bond breaking at the carboxylate end and exchange; hence, the activation entropies for the exchange vary with L. In the 4-nitropicolinate and carbonate systems, where the exchange of L is faster than that of F-, the exchange takes place in a dissociative reaction via the intermediate/transition state "UO2F3" which contains a lower number of coordinated water molecules than that of UO2(H2O)(2)F-3(-). Then is no evidence of the presence of unidentate L at equilibrium, indicating that the equilibrium constant for the ring opening, K = k(1)/k(-1), is smaller than 0.05. Hence, the observed rate constant for the ligand rotation is approximately equal to that for the ring opening whereas that of the ring closing is at least 20 times larger. The activation entropies for these reactions indicate a much smaller involvement of water in the activated state in the 4-nitropicolinate complex than those of the other two picolinates, a signature of reactions of I-D mechanisms. The exchange of fluoride and oxalate in UO2(oxalate)(2)F3- is first-order with respect to the concentration of the complex and independent of the concentrations of free fluoride and oxalate. The fact that the exchange of oxalate is independent of the free oxalate concentration but still slower than that of fluoride shows that they take place along separate pathways and not as consecutive reactions along the fluoride exchange pathway. The exchange reactions have nearly the same activation parameters whereas those for the fluoride exchange change significantly between H2O and D2O. This together with the negative activation entropy, -74 J K-1 mol(-1), indicates solvent participation in the activated state, presumably an I-D mechanism with extensive solvation of the leaving fluoride. All ligand exchange reactions are proton-catalyzed with a reverse-isotope effect, indicating the formation of a protonated precursor. The exchange reactions also are catalyzed to a small extent by hydrofluoric and acetic acids, which act as proton donors, without the involvement of the conjugate bases in the exchange reactions. These reactions have normal isotope effects.