Ligand intra- and inter-molecular exchange processes in dioxopentakis(trimethylurea)uranium(VI). A proton nuclear magnetic resonance study

Abstract

The complex [UO₂(trmu)₅]²⁺ is shown to be the predominant dioxouranium(VI) species in the presence of trimethylurea (trmu) in CD₃CN solution. The kinetic parameters characterizing rotation about the C–NMe₂ bond in co-ordinated trmu are k_r(298.2 K)= 925 ± 37 s^–1, ΔH^‡= 44.7 ± 0.6 kJ mol^–1, and ΔS^‡=–38.3 ± 2.4 J K^–1 mol^–1. Rotation about the C–NHMe bond is in the fast-exchange limit on the ¹H n.m.r. time-scale. The rate law for intermolecular trmu exchange is rate = 5k_ex[UO₂(trmu)₅²⁺] over a twelve-fold variation of free trmu concentration, consistent with the operation of a D mechanism or, less probably, an I mechanism with a stability constant 200 dm³ mol^–1 for the encounter complex. The exchange parameters are k_ex(298.2 K)= 983 ± 8 s^–1, ΔH^‡= 56.6 ± 0.6 kJ mol^–1, and ΔS^‡= 2.2 ± 1.9 J K^–1 mol^–1. The data are compared with those from other dioxouranium(VI) systems.