Proton magnetic resonance study of inter- and intra-molecular site exchange on the dioxopentakis(tetramethylurea) uranium(VI) ion

Abstract

The species [UO₂(tmu)₅]²⁺ is shown from ¹H n.m.r. studies to be the predominant dioxuranium (VI) species existing in tetramethylurea (tmu) solutions in CD₂Cl₂ and CD₃CN solvents. The kinetic parameters characterizing rotation about the carbonyl carbon–nitrogen bond in the tmu ligand are k(195 K)= 126 ± 7 s^–1, ΔH^‡= 29.0 ± 0.5 kJ mol^–1, and ΔS^‡=–52 ± 3 J K^–1 mol^–1. The rate law for tmu exchange is rate = 5k_ex.[UO₂(tmu)₅²⁺] over a 59.3- and 23.7-fold variation of [tmu] in CD₂Cl₂ and CD₃CN solvents respectively, consistent with a dissociative exchange mechanism. The kinetic parameters in the two solvents show no significant differences, a typical data set being k_ex.(273 K)= 73 ± 7 s^–1, ΔH^‡= 80 ± 2 kJ mol^–1, and ΔS^‡= 85 ± 7 J K^–1 mol^–1 in a solution in which [UO₂(tmu)₅²⁺], [tmu], and [CD₂Cl₂]= 0.130, 0.404, and 12.91 mol dm^–3 respectively. In general, the kinetic parameters for tmu exchange are consistent with the surface charge density of the uranium(VI) centre seen by the equatorial ligands being between that on aluminium(III) and the bivalent first-row transition-metal ions.