Rational Ligand Design for U(VI) and Pu(IV) Page: 99 of 322

                                
the similarity in pKai and pKa2 values of these ligands and because metal chelation will
drive deprotonation of the ligand at lower pH. The UV-visible spectra change again
around neutral pH to what was refined as a UO2L(OH) species. This partial hydrolysis
occurs at very mildly basic conditions because it does not require the displacement of a
ligand; the fifth equatorial coordination position on the uranyl is known to be occupied by
solvent in bis-Me-3,2-HOPO complexes, and thus hydroxide coordination need not
overcome a chelate effect of the ligand. The formation of a partial hydrolysis complex
UO2L(OH) is also seen in the aqueous solution thermodynamics of the uranyl-
desferrioxamine B (DFO) complex, which was observed to form a U02(DFO)(OH)
complex starting at ca. pH 7 (log 011-1 =22.8), indicting that the uranyl center is not
sufficiently complexed by the ligand to exclude solvent-dependent coordination.20
Because DFO is a hexadentate ligand designed for Fe(III) chelation, it is unclear if
formation of U02(DFO)(OH) requires a displacement of an otherwise-coordinated ligand,
but with the tetradentate bis-Me-3,2-HOPO ligands this hydrolysis is most assuredly a
coordination of hydroxide (or deprotonation of coordinated water), and not a
displacement of coordinated ligand.
The dimeric crystal structure of the [U02(2-37)]2 complex required the refinement of
dimeric uranyl complexes with 2-21 in solution. Strong acid titration data refinement did
not support a model consistent with the formation of U02(2-21)Ho1 monomer units
preceding dimer formation, so direct dimer formation at low pH was assumed. This
behavior suggests that the substitution on the thiophene ligand does not allow
mononuclear complex formation even at the low concentrations used, which the amide
torsion angle calculations would lead us to expect. In the solid state the uranyl dimer with

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