Halide abstraction from halogenated acetate ligands by actinyls: a competition between bond breaking and bond making

Transfer of halogen atoms from halogenated acetate ligands, CX3CO2 (X = F, Cl, Br), to actinyls, AnO2(2+) (An = U, Np, Pu) is stimulated by collision-induced dissociation (CID) in a quadrupole ion trap. CID of [AnO2(CF3CO2)3](-) complexes results exclusively in F atom transfer, concomitant with elimination of CF2CO2, to produce [(CF3CO2)2AnO2F](-), [(CF3CO2)AnO2F2](-), and [AnO2F3](-). This contrasts with CID of transition metal fluoroacetates for which CO2-elimination to produce organometallics is an important pathway, a disparity that can be attributed to the differing bond dissociation energies (BDEs) of the created metal-carbon and metal-fluorine bonds. The dominant pathway for CID of [AnO2(CF3CO2)(CCl3CO2)(CBr3CO2)](-) is Br-atom transfer to produce [(CF3CO2)(CCl3CO2)AnO2Br](-). The preferential formation of bromides, despite that the BDEs of An-F bonds are substantially greater than those of An-Br bonds, is attributed to the offsetting effect of higher BDEs for C-F versus C-Br bonds. The results for the trihaloacetates are similar for uranyl, neptunyl and plutonyl, indicating that for all three the An-X bond dissociation energies are sufficiently high that X atom transfer is overwhelmingly dominant. CID of [UO2(CH2XCO2)2(CX3CO2)](-) (X = F, Cl, Br) resulted in F-transfer only from CH2XCO2, but Cl- and Br-transfer from both CH2XCO2 and CX3CO2, a manifestation of the characteristic increase in BDE[C-F] in CHx-nFn species as n increases; the overall thermochemistry determines the observed CID processes, providing clear distinctions between fluorides and chlorides/bromides. The results of this work reveal the propensity of the actinides to form strong bonds with halogens, and suggest that there is not a large variation in actinyl-halogen BDEs between uranyl, neptunyl, and plutonyl.