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 kr(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 = 5kex[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 kex(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.