Designing the Ideal Uranyl Ligand: a Sterically-Induced Speciation Change in Complexes with Thiophene-Bridged Bis(3-hydroxy-N-methylpyridin-2-one) Page: 3 of 3
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step to convergence. A rotation of 00 corresponds to a co-
planar amide moiety in the conformation seen in the
U02(L)(DMF) structure. This calculation was performed
in the absence and presence of ethylsulfanyl substituents
ortho to the amide moieties; the results for both
calculations are shown in Figure 3. Sharp drops in relative
energy upon incremental amide rotation are a consequence
of significant rearrangement of the neighboring amide,
typically facilitating a new hydrogen-bonding interaction.
- < - H -
C * . -
N a. H
" ' F-.
N C Torsicn Angle deg
N-C Tors on Ang'e leg
Figure 3. Relative energy calculations for rotation of an acetaide
substituent about the Namde-Chophene bond in the presence of an ortho
acetanude in the absence (left) and presence (right) of ethylsulfanyl
In the absence of steric influences, the amide moiety is
expected to prefer conjugation to the thiophene linker (00
and 1800).16 However, in the presence of an ortho amide
(Figure 3, left), 1800 is an energetic maximum due to steric
interference between the two amides. The less than 1
kcal/mol energy difference upon rotations from 00 is a
result of the combination of an energetically costly break in
conjugation combined with favorable inter-amide N-H---O
hydrogen bonding allowed by the free rotation of the
neighboring amide. The small energy differences between
these angles makes the observed 40 and 90 amide torsions
in U02(L')(DMF) reasonable considering the structural
influence of uranyl chelation and increased electronic
conjugation in L' compared to the model compound, with
both factors favoring low amide torsion angles.
In the presence of ethylsulfanyl substituents the energy
profile and energy differences change significantly (Figure
3, right). A 1500 torsion angle is favored due to a
combination of N-H- -S and O- -H-N hydrogen bonding
interactions of one amide to the ethylsulfanyl and ortho
amide substitutents, respectively. This conformation is not
appropriate for mononuclear or dimeric complex
formation, and is thus not observed in the uranyl
complexes with L2. Torsion angles near 00 represent the
highest calculated energies due primarily to a combination
of steric interference between amide oxygen and
ethylsulfanyl sulfur atoms, explaining why ligand L2 does
not form mononuclear uranyl complexes as L' does.
Interestingly, local energy minima occur at 700 and 2350 at
which the amide group is significantly twisted out of
conjugation with the thiophene ring. This conformation
balances the unfavorable effects of steric interference with
the ethylsulfanyl sulfurs and the absence of electronic
conjugation to the thiophene ring, resulting in a
conformation ca. 4 kcal/mol higher than the global
minimum. This ca. 4 kcal/mol torsion cut-off is consistent
with small molecule torsions observed in crystal
structures," and the predicted torsion angles correspond
very well with the Na de-Cthiophene bond torsion angles
exhibited in the [U02(L2)(DMSO)]2 crystal structures:
(65 , 245 ) for the unsolvated structure, and (590, 239 )
and (570, 246 ) for the MeOH-containing structure.
In conclusion, we have demonstrated that relatively
small changes in backbone geometry can significantly
change the coordination behavior of bis-Me-3,2-HOPO
ligands with the uranyl cation which must be taken into
account when designing uranyl-selective ligands. We have
also demonstrated the first instance of uranyl dimer
complex formation using bis-Me-3,2-HOPO ligands.
Future work currently in progress addresses the structural
and solution thermodynamic studies of thiophene- and
other rigidly-linked bis-Me-3,2-HOPO ligands with the
Acknowledgement. This research is supported by the
Director, Office of Science, Office of Basic Energy
Sciences (OBES), and the OBES Division of Chemical
Sciences, Geosciences and Biosciences of the U. S.
Department of Energy at LBNL under Contract No. De-
Supporting Information Available: Experimental procedures,
molecular dynamics data, tables of pertinent bond lengths and
angles, crystallographic refinement details and figures, cif files
for uranyl complexes with Ll and L2. This material is available
free of charge via the internet at http://pubs.acs.org.
* To whom correspondence should be addressed: E-mail:
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Szigethy, Geza & Raymond, Kenneth N. Designing the Ideal Uranyl Ligand: a Sterically-Induced Speciation Change in Complexes with Thiophene-Bridged Bis(3-hydroxy-N-methylpyridin-2-one), article, September 11, 2009; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc1013105/m1/3/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.