Steric Control of Substituted Phenoxide Ligands on Product Structures of Uranyl Aryloxide Complexes

A series of uranyl aryloxide complexes has been prepared via metathesis reactions between [UO2Cl2(THF)2]2 and
di-ortho-substituted phenoxides. Reaction of 4 equiv of KO-2,6-tBu2C6H3 with [UO2Cl2(THF)2]2 in THF produces the
dark red uranyl compound, UO2(O-2,6-tBu2C6H3)2(THF)2·THF, 1. Single-crystal X-ray diffraction analysis of 1 reveals
a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo groups, two
cis-aryloxides, and two THF ligands. A similar product is prepared by reaction of KO-2,6-Ph2C6H3 with [UO2Cl2(THF)2]2 in THF. Single-crystal X-ray diffraction analysis of this compound reveals it to be the trans-monomer
UO2(O-2,6-Ph2C6H3)2(THF)2, 2. Dimeric structures result from the reactions of [UO2Cl2(THF)2]2 with less sterically
imposing aryloxide salts, KO-2,6-Cl2C6H3 or KO-2,6-Me2C6H3. Single-crystal X-ray diffraction analyses of [UO2(O-2,6-Cl2C6H3)2(THF)2]2, 3, and [UO2Cl(O-2,6-Me2C6H3)(THF)2]2, 4, reveal similar structures in which each U atom is
coordinated by seven ligands in a pseudopentagonal bipyramidal fashion. Coordinated to each uranium are two
apical oxo groups and five equatorial ligands (3, one terminal phenoxide, two bridging phenoxides, and two
nonadjacent terminal THF ligands; 4, one terminal chloride, two bridging phenoxides, and two nonadjacent terminal
THF ligands). Apparently, the phenoxide ligand steric features exert a greater influence on the solid-state structures
than the electronic properties of the substituents. Emission spectroscopy has been utilized to investigate the
molecularity and electronic structure of these compounds. For example, luminescence spectra taken at liquid nitrogen
temperature allow for a determination of the dependence of the molecular aggregation of 3 on the molecular
concentration. Electronic and vibrational spectroscopic measurements have been analyzed to examine trends in
emission energies and stretching frequencies. However, comparison of the data for compounds 1−4 reveals that
the innate electron-donating capacity of phenoxide ligands is only subtly manifest in either the electronic or vibrational
energy distributions within these molecules.