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 = 5kex.[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 kex.(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.