Structures of homoleptic eight- and nine-coordinate uranium(IV) perchlorate complexes with sulfoxide molecules as ligands

Homoleptic eight- and nine-coordinate U(IV) perchlorate complexes with sulfoxide ligands have been characterized crystallographically. Crystals of [U(dmso)8](ClO4)4 · 0.75CH3NO2, [U(dmso)9](ClO4)4 · 4dmso (dmso = dimethyl sulfoxide), and [U(tmso)8](ClO4)4 · 2tmso (tmso = tetramethylene sulfoxide) were found to have dodecahedral, tricapped trigonal prismatic, and square antiprismatic geometries, respectively. Average U–O bond distances in [U(dmso)8](ClO4)4 · 0.75CH3NO2, [U(dmso)9](ClO4)4 · 4dmso, and [U(tmso)8](ClO4)4 · 2tmso are 2.35(3), 2.41 (4), and 2.35(3) Å, respectively. Furthermore, it was found that [U(dmso)8]4+ is in equilibrium with [U(dmso)9]4+ in CH3NO2 solution containing dmso. Thermodynamic parameters for such an equilibrium are as follows: K (25 °C) = 3.4 ± 0.2 dm3 mol−1, ΔH = −54.9 ± 4.5 kJ mol−1, and ΔS = −174 ± 15 J K−1 mol−1.