Kinetics and mechanisms of anation and exchange reactions in pentakis(N,N-dimethylformamide)dioxouranium(VI) ion in nonaqueous media.

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The exchange reaction of N,N-dimethylformamide (dmf) in [UO2 (dmf)5]2+ ion and the anation reactions by SCN− and N3− have been studied by the NMR method in CD2Cl2 and the stopped-flow method in DMF, respectively. For the dmf exchange reaction, rate=(kd+kIKos[DMF])[UO2(dmf)52+]/(1+Kos[DMF]), where kd, kI, and Kos are (1.30±0.21)×102 s−1, (2.94±0.07)×103 s−1, and 1.59±0.11 mol−1 kg at −50 °C, respectively. For the anation reactions by SCN− and N3−, the pseudo-first-order rate constant (kobsd) is expressed by kobsd={k−1+(k1+k−1)Kos[X]}⁄(1+Kos[X]) (X=SCN− and [UO2(dmf)5]2+ for the anation reactions by SCN− and N3−, respectively). The values of k1, k−1, and Kos at 25 °C are 1.95×102 s−1, 4.27×102 s−1, 1.48×102 M−1 (1 M=1 mol dm−3), and 1.64×103 s−1, 3.72×102 s−1, 1.80×102 M−1 for SCN− and N3−, anation reactions, respectively. These results suggest that the substitution reaction in [UO2(dmf)5]2+ ion proceeds through the associative interchange mechanism. The difference of reactivities for SCN− and N3− toward [UO2(dmf)5]2+ ion seems to be related to the pKa values for acids, HSCN and HN3.
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