24 research outputs found
Identification of an XâBand Clock Transition in CpâČ<sub>3</sub>Pr<sup>â</sup> Enabled by a 4f<sup>2</sup>5d<sup>1</sup> Configuration
Molecular qubits
offer an attractive basis for quantum information
processing, but challenges remain with regard to sustained coherence.
Qubits based on clock transitions offer a method to improve the coherence
times. We propose a general strategy for identifying molecules with
high-frequency clock transitions in systems where a d electron is
coupled to a crystal-field singlet state of an f configuration, resulting
in an MJ = ±1/2
ground state with strong hyperfine coupling. Using this approach,
a 9.834 GHz clock transition was identified
in a molecular Pr complex, [K(crypt)][CpâČ3PrII], leading to 3-fold enhancements in T2 relative to other transitions in the spectrum. This result
indicates the promise of the design principles outlined here for the
further development of f-element systems for quantum information applications
Bonding in Uranium(V) Hexafluoride Based on the Experimental Electron Density Distribution Measured at 20 K
The electron density distribution
of [PPh<sub>4</sub>]Â[UF<sub>6</sub>] was obtained from high-resolution
X-ray diffraction data measured at 20 K. The electron density was
modeled with an augmented HansenâCoppens multipolar formalism.
Topological analysis reveals that the UâF bond is of incipient
covalent nature. Theoretical calculations add further support to the
bonding description gleaned from the experimental model. The impact
of the uranium anomalous dispersion terms on the refinement is also
discussed
Macrocyclic 1,2-Hydroxypyridinone-Based Chelators as Potential Ligands for Thorium-227 and Zirconium-89 Radiopharmaceuticals
Thorium-227 (227Th) is an α-emitting
radionuclide
that has shown preclinical and clinical promise for use in targeted
α-therapy (TAT), a type of molecular radiopharmaceutical treatment
that harnesses high energy α particles to eradicate cancerous
lesions. Despite these initial successes, there still exists a need
for bifunctional chelators that can stably bind thorium in vivo. Toward
this goal, we have prepared two macrocyclic chelators bearing 1,2-hydroxypyridinone
groups.
Both chelators can be synthesized in less than six steps from readily
available starting materials, which is an advantage over currently
available platforms. The complex formation constants (log ÎČmlh) of these ligands with Zr4+ and Th4+, measured by spectrophotometric titrations, are greater than 34
for both chelators, indicating the formation of exceedingly stable
complexes. Radiolabeling studies were performed to show that these
ligands can bind [227Th]Th4+ at concentrations
as low as 10â6 M, and serum stability experiments
demonstrate the high kinetic stability of the formed complexes under
biological conditions. Identical experiments with zirconium-89 (89Zr), a positron-emitting radioisotope used for positron emission
tomography (PET) imaging, demonstrate that these chelators can also
effectively bind Zr4+ with high thermodynamic and kinetic
stability. Collectively, the data reported herein highlight the suitability
of these ligands for use in 89Zr/227Th paired
radioimmunotheranostics
Theory and Xâray Absorption Spectroscopy for Aluminum Coordination Complexes â Al KâEdge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR<sub>2</sub>, and (BDI)AlX<sub>2</sub> Complexes
Polarized
aluminum K-edge X-ray absorption near edge structure
(XANES) spectroscopy and first-principles calculations were used to
probe electronic structure in a series of (BDI)ÂAl, (BDI)ÂAlX<sub>2</sub>, and (BDI)ÂAlR<sub>2</sub> coordination compounds (X = F, Cl, I;
R = H, Me; BDI = 2,6-diisopropylphenyl-ÎČ-diketiminate). Spectral
interpretations were guided by examination of the calculated transition
energies and polarization-dependent oscillator strengths, which agreed
well with the XANES spectroscopy measurements. Pre-edge features were
assigned to transitions associated with the Al 3p orbitals involved
in metalâligand bonding. Qualitative trends in Al 1s core energy
and valence orbital occupation were established through a systematic
comparison of excited states derived from Al 3p orbitals with similar
symmetries in a molecular orbital framework. These trends suggested
that the higher transition energies observed for (BDI)ÂAlX<sub>2</sub> systems with more electronegative X<sup>1â</sup> ligands
could be ascribed to a decrease in electron density around the aluminum
atom, which causes an increase in the attractive potential of the
Al nucleus and concomitant increase in the binding energy of the Al
1s core orbitals. For (BDI)Al and (BDI)ÂAlH<sub>2</sub> the experimental
Al K-edge XANES spectra and spectra calculated using the eXcited electron
and CoreâHole (XCH) approach had nearly identical energies
for transitions to final state orbitals of similar composition and
symmetry. These results implied that the charge distributions about
the aluminum atoms in (BDI)Al and (BDI)ÂAlH<sub>2</sub> are similar
relative to the (BDI)ÂAlX<sub>2</sub> and (BDI)ÂAlMe<sub>2</sub> compounds,
despite having different formal oxidation states of +1 and +3, respectively.
However, (BDI)Al was unique in that it exhibited a low-energy feature
that was attributed to transitions into a low-lying p-orbital of b<sub>1</sub> symmetry that is localized on Al and orthogonal to the (BDI)ÂAl
plane. The presence of this low-energy unoccupied molecular orbital
on electron-rich (BDI)Al distinguishes its valence electronic structure
from that of the formally trivalent compounds (BDI)ÂAlX<sub>2</sub> and (BDI)ÂAlR<sub>2</sub>. The work shows that Al K-edge XANES spectroscopy
can be used to provide valuable insight into electronic structure
and reactivity relationships for main-group coordination compounds
Impact of Natural Organic Matter on Uranium Transport through Saturated Geologic Materials: From Molecular to Column Scale
The risk stemming from human exposure to actinides via
the groundwater
track has motivated numerous studies on the transport of radionuclides
within geologic environments; however, the effects of waterborne organic
matter on radionuclide mobility are still poorly understood. In this
study, we compared the abilities of three humic acids (HAs) (obtained
through sequential extraction of a peat soil) to cotransport hexavalent
uranium (U) within water-saturated sand columns. Relative breakthrough
concentrations of U measured upon elution of 18 pore volumes increased
from undetectable levels (<0.001) in an experiment without HAs
to 0.17 to 0.55 in experiments with HAs. The strength of the HA effect
on U mobility was positively correlated with the hydrophobicity of
organic matter and NMR-detected content of alkyl carbon, which indicates
the possible importance of hydrophobic organic matter in facilitating
U transport. Carbon and uranium elemental maps collected with a scanning
transmission X-ray microscope (STXM) revealed uneven microscale distribution
of U. Such molecular- and column-scale data provide evidence for a
critical role of hydrophobic organic matter in the association and
cotransport of U by HAs. Therefore, evaluations of radionuclide transport
within subsurface environments should consider the chemical characteristics
of waterborne organic substances, especially hydrophobic organic matter
Theory and Xâray Absorption Spectroscopy for Aluminum Coordination Complexes â Al KâEdge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR<sub>2</sub>, and (BDI)AlX<sub>2</sub> Complexes
Polarized
aluminum K-edge X-ray absorption near edge structure
(XANES) spectroscopy and first-principles calculations were used to
probe electronic structure in a series of (BDI)ÂAl, (BDI)ÂAlX<sub>2</sub>, and (BDI)ÂAlR<sub>2</sub> coordination compounds (X = F, Cl, I;
R = H, Me; BDI = 2,6-diisopropylphenyl-ÎČ-diketiminate). Spectral
interpretations were guided by examination of the calculated transition
energies and polarization-dependent oscillator strengths, which agreed
well with the XANES spectroscopy measurements. Pre-edge features were
assigned to transitions associated with the Al 3p orbitals involved
in metalâligand bonding. Qualitative trends in Al 1s core energy
and valence orbital occupation were established through a systematic
comparison of excited states derived from Al 3p orbitals with similar
symmetries in a molecular orbital framework. These trends suggested
that the higher transition energies observed for (BDI)ÂAlX<sub>2</sub> systems with more electronegative X<sup>1â</sup> ligands
could be ascribed to a decrease in electron density around the aluminum
atom, which causes an increase in the attractive potential of the
Al nucleus and concomitant increase in the binding energy of the Al
1s core orbitals. For (BDI)Al and (BDI)ÂAlH<sub>2</sub> the experimental
Al K-edge XANES spectra and spectra calculated using the eXcited electron
and CoreâHole (XCH) approach had nearly identical energies
for transitions to final state orbitals of similar composition and
symmetry. These results implied that the charge distributions about
the aluminum atoms in (BDI)Al and (BDI)ÂAlH<sub>2</sub> are similar
relative to the (BDI)ÂAlX<sub>2</sub> and (BDI)ÂAlMe<sub>2</sub> compounds,
despite having different formal oxidation states of +1 and +3, respectively.
However, (BDI)Al was unique in that it exhibited a low-energy feature
that was attributed to transitions into a low-lying p-orbital of b<sub>1</sub> symmetry that is localized on Al and orthogonal to the (BDI)ÂAl
plane. The presence of this low-energy unoccupied molecular orbital
on electron-rich (BDI)Al distinguishes its valence electronic structure
from that of the formally trivalent compounds (BDI)ÂAlX<sub>2</sub> and (BDI)ÂAlR<sub>2</sub>. The work shows that Al K-edge XANES spectroscopy
can be used to provide valuable insight into electronic structure
and reactivity relationships for main-group coordination compounds
Influence of Pyrazolate vs <i>N</i>âHeterocyclic Carbene Ligands on the Slow Magnetic Relaxation of Homoleptic Trischelate Lanthanide(III) and Uranium(III) Complexes
Two
isostructural series of trigonal prismatic complexes, MÂ(Bp<sup>Me</sup>)<sub>3</sub> and MÂ(Bc<sup>Me</sup>)<sub>3</sub> (M = Y,
Tb, Dy, Ho, Er, U; [Bp<sup>Me</sup>]<sup>â</sup> = dihydrobisÂ(methypyrazolyl)Âborate;
[Bc<sup>Me</sup>]<sup>â</sup> = dihydrobisÂ(methylimidazolyl)Âborate)
are synthesized and fully characterized to examine the influence of
ligand donor strength on slow magnetic relaxation. Investigation of
the dynamic magnetic properties reveals that the oblate electron density
distributions of the Tb<sup>3+</sup>, Dy<sup>3+</sup>, and U<sup>3+</sup> metal ions within the axial ligand field lead to slow relaxation
upon application of a small dc magnetic field. Significantly, the
magnetization relaxation is orders of magnitude slower for the <i>N</i>-heterocyclic carbene complexes, MÂ(Bc<sup>Me</sup>)<sub>3</sub>, than for the isomeric pyrazolate complexes, MÂ(Bp<sup>Me</sup>)<sub>3</sub>. Further, investigation of magnetically dilute samples
containing 11â14 mol % of Tb<sup>3+</sup>, Dy<sup>3+</sup>,
or U<sup>3+</sup> within the corresponding Y<sup>3+</sup> complex
matrix reveals thermally activated relaxation is favored for the MÂ(Bc<sup>Me</sup>)<sub>3</sub> complexes, even when dipolar interactions are
largely absent. Notably, the dilute species UÂ(Bc<sup>Me</sup>)<sub>3</sub> exhibits <i>U</i><sub>eff</sub> â 33 cm<sup>â1</sup>, representing the highest barrier yet observed for
a U<sup>3+</sup> molecule demonstrating slow relaxation. Additional
analysis through lanthanide XANES, X-band EPR, and <sup>1</sup>H NMR
spectroscopies provides evidence that the origin of the slower relaxation
derives from the greater magnetic anisotropy enforced within the strongly
donating <i>N-</i>heterocyclic carbene coordination sphere.
These results show that, like molecular symmetry, ligand-donating
ability is a variable that can be controlled to the advantage of the
synthetic chemist in the design of single-molecule magnets with enhanced
relaxation barriers
Vanadium Bisimide Bonding Investigated by Xâray Crystallography, <sup>51</sup>V and <sup>13</sup>C Nuclear Magnetic Resonance Spectroscopy, and V L<sub>3,2</sub>-Edge Xâray Absorption Near-Edge Structure Spectroscopy
Syntheses
of neutral halide and aryl vanadium bisimides are described.
Treatment of VCl<sub>2</sub>(N<i>t</i>Bu)Â[NTMSÂ(N<sup><i>t</i></sup>Bu)], <b>2</b>, with PMe<sub>3</sub>, PEt<sub>3</sub>, PMe<sub>2</sub>Ph, or pyridine gave vanadium bisimides via
TMSCl elimination in good yield: VClÂ(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>3</b>, VClÂ(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>4</b>, VClÂ(PMe<sub>2</sub>Ph)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>5</b>, and VClÂ(Py)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>6</b>. The
halide series (ClâI) was synthesized by use of TMSBr and TMSI
to give VBrÂ(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>7</b> and VIÂ(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>8</b>. The
phenyl derivative was obtained by reaction of <b>3</b> with
MgPh<sub>2</sub> to give VPhÂ(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>9</b>. These neutral complexes
are compared to the previously reported cationic bisimides [VÂ(PMe<sub>3</sub>)<sub>3</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>]Â[AlÂ(PFTB)<sub>4</sub>] <b>10</b>, [VÂ(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>]Â[AlÂ(PFTB)<sub>4</sub>] <b>11</b>, and [VÂ(DMAP)Â(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>]Â[AlÂ(PFTB)<sub>4</sub>] <b>12</b> (DMAP = dimethylaminopyridine, PFTB = perfluoro-<i>tert</i>-butoxide). Characterization of the complexes by X-ray
diffraction, <sup>13</sup>C NMR, <sup>51</sup>V NMR, and V L<sub>3,2</sub>-edge X-ray absorption near-edge structure (XANES) spectroscopy provides
a description of the electronic structure in comparison to group 6
bisimides and the bent metallocene analogues. The electronic structure
is dominated by Ï bonding to the imides, and localization of
electron density at the nitrogen atoms of the imides is dictated by
the cone angle and donating ability of the axial neutral supporting
ligands. This phenomenon is clearly seen in the sensitivity of <sup>51</sup>V NMR shift, <sup>13</sup>C NMR ÎÎŽ<sub>αÎČ</sub>, and L<sub>3</sub>-edge energy to the nature of the supporting phosphine
ligand, which defines the parameters for designing cationic group
5 bisimides that would be capable of breaking stronger Ï bonds.
Conversely, all three methods show little dependence on the variable
equatorial halide ligand. Furthermore, this analysis allows for quantification
of the electronic differences between vanadium bisimides and the structurally
analogous mixed Cp/imide system CpVÂ(N<sup><i>t</i></sup>Bu)ÂX<sub>2</sub> (Cp = C<sub>5</sub>H<sub>5</sub><sup>1â</sup>)
Vanadium Bisimide Bonding Investigated by Xâray Crystallography, <sup>51</sup>V and <sup>13</sup>C Nuclear Magnetic Resonance Spectroscopy, and V L<sub>3,2</sub>-Edge Xâray Absorption Near-Edge Structure Spectroscopy
Syntheses
of neutral halide and aryl vanadium bisimides are described.
Treatment of VCl<sub>2</sub>(N<i>t</i>Bu)Â[NTMSÂ(N<sup><i>t</i></sup>Bu)], <b>2</b>, with PMe<sub>3</sub>, PEt<sub>3</sub>, PMe<sub>2</sub>Ph, or pyridine gave vanadium bisimides via
TMSCl elimination in good yield: VClÂ(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>3</b>, VClÂ(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>4</b>, VClÂ(PMe<sub>2</sub>Ph)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>5</b>, and VClÂ(Py)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>6</b>. The
halide series (ClâI) was synthesized by use of TMSBr and TMSI
to give VBrÂ(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>7</b> and VIÂ(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>8</b>. The
phenyl derivative was obtained by reaction of <b>3</b> with
MgPh<sub>2</sub> to give VPhÂ(PMe<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub> <b>9</b>. These neutral complexes
are compared to the previously reported cationic bisimides [VÂ(PMe<sub>3</sub>)<sub>3</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>]Â[AlÂ(PFTB)<sub>4</sub>] <b>10</b>, [VÂ(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>]Â[AlÂ(PFTB)<sub>4</sub>] <b>11</b>, and [VÂ(DMAP)Â(PEt<sub>3</sub>)<sub>2</sub>(N<sup><i>t</i></sup>Bu)<sub>2</sub>]Â[AlÂ(PFTB)<sub>4</sub>] <b>12</b> (DMAP = dimethylaminopyridine, PFTB = perfluoro-<i>tert</i>-butoxide). Characterization of the complexes by X-ray
diffraction, <sup>13</sup>C NMR, <sup>51</sup>V NMR, and V L<sub>3,2</sub>-edge X-ray absorption near-edge structure (XANES) spectroscopy provides
a description of the electronic structure in comparison to group 6
bisimides and the bent metallocene analogues. The electronic structure
is dominated by Ï bonding to the imides, and localization of
electron density at the nitrogen atoms of the imides is dictated by
the cone angle and donating ability of the axial neutral supporting
ligands. This phenomenon is clearly seen in the sensitivity of <sup>51</sup>V NMR shift, <sup>13</sup>C NMR ÎÎŽ<sub>αÎČ</sub>, and L<sub>3</sub>-edge energy to the nature of the supporting phosphine
ligand, which defines the parameters for designing cationic group
5 bisimides that would be capable of breaking stronger Ï bonds.
Conversely, all three methods show little dependence on the variable
equatorial halide ligand. Furthermore, this analysis allows for quantification
of the electronic differences between vanadium bisimides and the structurally
analogous mixed Cp/imide system CpVÂ(N<sup><i>t</i></sup>Bu)ÂX<sub>2</sub> (Cp = C<sub>5</sub>H<sub>5</sub><sup>1â</sup>)
Synthesis and Characterization of Eight Compounds of the MU<sub>8</sub>Q<sub>17</sub> Family: ScU<sub>8</sub>S<sub>17</sub>, CoU<sub>8</sub>S<sub>17</sub>, NiU<sub>8</sub>S<sub>17</sub>, TiU<sub>8</sub>Se<sub>17</sub>, VU<sub>8</sub>Se<sub>17</sub>, CrU<sub>8</sub>Se<sub>17</sub>, CoU<sub>8</sub>Se<sub>17</sub>, and NiU<sub>8</sub>Se<sub>17</sub>
The solid-state MU<sub>8</sub>Q<sub>17</sub> compounds ScU<sub>8</sub>S<sub>17</sub>, CoU<sub>8</sub>S<sub>17</sub>, NiU<sub>8</sub>S<sub>17</sub>, TiU<sub>8</sub>Se<sub>17</sub>, VU<sub>8</sub>Se<sub>17</sub>, CrU<sub>8</sub>Se<sub>17</sub>, CoU<sub>8</sub>Se<sub>17</sub>, and NiU<sub>8</sub>Se<sub>17</sub> were synthesized from the reactions
of the elements at 1173 or 1123 K. These isostructural compounds crystallize
in space group <i>C</i><sub>2<i>h</i></sub><sup>3</sup> - <i>C</i>2<i>/m</i> of the monoclinic system in the CrU<sub>8</sub>S<sub>17</sub> structure
type. X-ray absorption near-edge structure spectroscopic studies of
ScU<sub>8</sub>S<sub>17</sub> indicate that it contains Sc<sup>3+</sup>, and hence charge balance is achieved with a composition that includes
U<sup>3+</sup> as well as U<sup>4+</sup>. The other compounds charge
balance with M<sup>2+</sup> and U<sup>4+</sup>. Magnetic susceptibility
measurements on ScU<sub>8</sub>S<sub>17</sub> indicate antiferromagnetic
couplings and a highly reduced effective magnetic moment. Ab Initio
calculations find the compound to be metallic. Surprisingly, the ScâS
distances are actually longer than all the other MâS interactions,
even though the ionic radii of Sc<sup>3+</sup>, low-spin Cr<sup>2+</sup>, and Ni<sup>2+</sup> are similar