Designing the Ideal Uranyl Ligand: a Sterically-Induced Speciation Change in Complexes with Thiophene-Bridged Bis(3-hydroxy-N-methylpyridin-2-one) Page: 2 of 3
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Figure 2. Top and side views of the crystal structures of UO2(L')(DMF) (left) and [UO2(L2)(DMSO)]2 (right). Only one of the two [UO2(L2)(DMSO)]2
structures are shown due to structural similarity (solvent-free structure shown here). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are
drawn at the 50% level. Carbons are gray, oxygens red, nitrogens blue, sulfurs yellow, and uranium is silver.
linkers are intended to discourage octahedral coordination
modes typical of transition and main-block elements,
imparting selectivity towards the uranyl cation over other
biologically relevant metal ions.
The uranyl complexes with L' and L2 were synthesized
in DMF or DMSO by stoichiometric addition of a
homogeneous ligand solution and Et3N to a stirred solution
of U02(NO3)2-6H20, resulting in the formation of a deep
red, solvated uranyl complex. Crystals of these complexes
were grown from these crude solutions using vapor
diffusion of MeOH at room temperature and layering of
MeOH at 4 C, respectively. These crystals were measured
by single-crystal X-ray diffraction at the UC Berkeley X-
ray facility, and the resultant structures are illustrated in
Figure 2. L' chelates the uranyl cation via four Me-3,2-
HOPO oxygen atoms, leaving a fifth equatorial site
available for DMF coordination, consistent with previous
bis-Me-3,2-HOPO ligand behavior.13 The crystallization of
the uranyl complex with L2 resulted in two crystal
morphologies, one dark red and the other orange. The latter
crystal type suffered from rapid desolvation of the several
methanol inclusions that X-ray diffraction subsequently
revealed. The dark red crystals contained no solvent
inclusions, and the uranyl complexes in both crystal
morphologies exhibited similar molecular geometries: the
uranyl cation is coordinated by L2 at four points of a
pentagonal coordination plane completed by a DMSO
molecule. However, unlike with Li, the uranyl complex
with L2 is a [U02(L2)(DMSO)]2 dimer in which each L2
ligand coordinates to two uranyl cations.
The bite angles of the Me-3,2-HOPO moieties to the
uranyl cation average 65.6(6)0 in U02(L)(DMF) and
66.4(7)0 in the [U02(L2)(DMSO)]2 complexes, which
correspond well to the precedent value of 66.4(4). 13 The
equatorial U-Oande/phenolate bond distances in
U02(L)(DMF) average 2.434(4) A and 2.344(9) A,
respectively, while those in the solvent-containing
[U02(L2)(DMSO)]2 dimer structure are 2.44(3) A and
2.36(2) A. These bond lengths also correspond well to
precedent and are consistent with an expected stronger U-O
bond with the more electronegative phenolate oxygen
compared to the formally neutral amide oxygen. In the
solvent-free [U02(L2)(DMSO)]2 structure, however, one
Me-3,2-HOPO moiety reverses this trend, with the U-Oaide
bond shorter than the U-Ophenolate bond (2.36 A, 2.40 A
respectively). This behavior is assumed to be a solid state
phenomenon that attests to the coordinative flexibility in
these dimeric complexes. The intramolecular Nande-
Ophenolate distances in the uranyl complexes with L' and L2
range between 2.61 A and 2.80 A, attesting to a strong
intramolecular hydrogen bonding interaction characteristic
of Raymond group ligands.14
The equatorial OphenolateU-Ophenolate angle in uranyl
complexes with bis-Me-3,2-HOPO ligands has been shown
to vary significantly with linker length, and can be
considered an overall "ligand bite angle.".13 The ligand bite
angle in U02(L)(DMF) of 65.2 is much smaller than the
72 of the ideal pentagon, and the smallest angle yet
observed with bis-Me-3,2-HOPO ligands. While this
results in a relatively exposed uranyl center, the equatorial
coordination of L' is nearly planar; the Me-3,2-HOPO
rings deviate only 5.8 from co-planarity and only 2.8 and
7.1 degrees from the uranyl coordination plane defined by
the five coordinating oxygen atoms. This planar geometry
is complementary to the equatorial coordination tendencies
of the uranyl cation,4 is the best yet observed with bis-Me-
3,2-HOPO ligands, and is most likely caused by the
extended bond conjugation in 1. The "ligand bite angles"
observed in the [U02(L2)(DMSO)]2 structures are no
longer subject to the short inter-moiety proximity imposed
by the thiophene linker due to the spanning behavior of L2.
As a result, the ligand bite angles in [U02(L2)(DMSO)]2
structures range between 79.0 and 83.2 , which are much
larger than that in U02(L)(DMF) and approach that
observed with the larger 4Li-Me-3,2-HOPO ligand (790).13
The only structural difference between L'H and L2H is
the presence of the ethylsulfanyl substituents on the
thiophene linker, and are thus the most likely cause for the
lack of ligand conjugation that leads to the dimeric
[U02(L2)(DMSO)]2 structures. The Oanmde-Sethylsulfanyl
distances between substituents on the same sides of the
thiophene rings range between 2.90 A and 5.01 A,
depending on the degree of amide twist observed; the
minimum value of 2.90 A is less than the sum of the sulfur
and oxygen Van der Waals radii (3.3 A).
The energetic influence of the ethylsulfanyl substitution
was investigated by molecular dynamics calculations in
which one amide in a simplified thiophene-3,4-bis-amide
was rotated about the Namjde-Cthophene bond through a full
360 rotation at 50 intervals, relaxing the geometry at each
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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/m1/2/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.