Basicity of Uranyl Oxo Ligands upon Coordination of Alkoxides

Uranium(VI) alkoxide complexes are prepared via metathesis reactions of [UO2Cl2(THF)2]2 with potassium
alkoxides in nonaqueous media. The dark red compound U[OCH2C(CH3)3]6, 1, results from redistributive exchange
of oxo and neopentoxide ligands between more than one uranium species. Single-crystal X-ray diffraction analysis
of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by six neopentoxide
ligands. Imposition of steric congestion at the metal center prevents oxo−alkoxide ligand exchange in the reactions
using more sterically demanding alkoxides. Simple metathesis between uranyl chloride and alkoxide ligands occurs
in the synthesis of golden yellow-orange UO2(OCHPh2)2(THF)2, 2, and yellow UO2[OCH(tBu)Ph]2(THF)2, 3.
Single-crystal X-ray diffraction analysis of 2 reveals a monomer in which the uranium is coordinated in a
pseudooctahedral fashion by two apical oxo ligands, two diphenylmethoxide ligands occupying trans positions,
and two tetrahydrofuran ligands. Coordination of diisopropylmethoxide allows for synthesis of a more complex
binary alkoxide system. Single-crystal X-ray diffraction analysis of watermelon red [UO2(OCH(iPr)2)2]4, 4, reveals
a tetramer in which each uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, one
terminal alkoxide, two bridging alkoxide ligands, and one bridging oxo ligand from a neighboring uranyl group.
These compounds are characterized by elemental analysis, 1H NMR, infrared spectroscopy, and, for 1, 2, and 4,
single-crystal X-ray diffraction analysis. Luminescence spectroscopy is employed to evaluate the extent of
aggregation of compounds 2−4 in various solvents. Vibrational spectroscopic measurements of 2−4 imply that,
in contrast to the case of uranyl complexes prepared in aqueous environments, coordination of relatively strongly
donating alkoxide ligands allows for enhancement of electron density on the uranyl groups such that the uranyl
UO bonds are weakened. Crystal data are as follows. 1:  monoclinic space group C2/m, a = 10.6192(8) Å, b
= 18.36(1) Å, c = 10.6151(8) Å, β = 109.637(1)°, V = 1949.1(3) Å3, Z = 2, dcalc = 1.297 g cm-3. Refinement
of 2065 reflections gave R1 = 0.045. 2:  monoclinic space group P21/c, a = 6.1796(4) Å, b = 15.669(1) Å, c =
16.169(1) Å, β = 95.380(1)°, V = 1558.7(2) Å3, Z = 2, dcalc = 1.664 g cm-3. Refinement of 3048 reflections
gave R1 = 0.036. 4:  tetragonal space group I4̄, a = 17.8570(6) Å, b = 17.8570(6) Å, c = 11.4489(6) Å, V =
3650.7(3) Å3, Z = 2, dcalc = 1.821 g cm-3. Refinement of 1981 reflections gave R1 = 0.020.