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. 2026 Jul 30;6(5):1308–1319. doi: 10.1021/acsphyschemau.6c00096

Computational Investigation of the Effect of Transition Metal Coordination in Bimetallic Uranylpyridine-Dicarboxylate Complexes

Keerthan R Rao †,*, Heather J Culbertson †,‡, Shuxiang Zhou †, Peter C Burns ‡, Tori Z Forbes §, Ashini S Jayasinghe †,*
PMCID: PMC13614035  PMID: 42799153

Abstract

Understanding how secondary metal coordination perturbs uranyl electronic structure is important for developing structure-property relationships in actinide-containing materials relevant to separations, waste management, and environmental chemistry. In this work, periodic and cluster density functional theory calculations were used to investigate the structural, electronic, and vibrational properties of monometallic U-2,6-pyridinedicarboxylic acid (PDC) and heterometallic U-PDC-TM complexes (TM = Fe, Co, Ni, Cu, and Zn). Incorporating transition metal (TM) changes the valence-band electronic structure of the U-PDC-TM framework due to TM d and ligand O 2p contributions near the band edge and reduces the bandgap across most of the series. The nature of these states is supported by the frontier molecular orbital diagrams constructed from the cluster models. Charge redistribution is observed along the TM octahedral unit and the bridging ligand framework rather than direct interaction with the uranyl center. Simulated Raman vibrational modes agree reasonably well with experiments for the ligand ring bending and breathing modes, although subtle TM-dependent changes in the uranyl stretching mode were not consistently captured across the full series. The phonon modes show a blue shift in the uranyl symmetric stretch consistent with experiments when Ni is incorporated into the U-PDC structure, indicating UO bond strengthening. The change in the partial charges on the U center confirms a small electron density accumulation due to the presence of TMs. Our results show that incorporating secondary TM provides a viable strategy for tuning the electronic properties of uranyl coordination complexes through indirect ligand-mediated coupling.

Keywords: uranyl complexes, heterometallic, transition metal, density of state, band structures, Raman, charge density differences and partial charges

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Introduction

Understanding uranium (U) coordination chemistry and redox behavior is critical for controlling speciation and reactivity across natural and engineered environments. Such knowledge underpins uranium separation and processing, informs strategies for nuclear fuel-cycle waste management, and supports environmental monitoring and remediation efforts. − Uranium commonly exhibits oxidation states ranging from +3 to +6, with U­(VI) being the most stable form under oxidizing conditions and soluble in aqueous solutions. U­(VI) typically forms a linear uranyl cation UO2 2+, with two axial oxygen atoms bound to the uranium metal center with average axial UO bond lengths of approximately 1.79 Å. Perpendicular to the uranyl axis, coordination by four, five, or six equatorial ligands gives rise to square-, pentagonal-, or hexagonal-bipyramidal geometries, respectively. Because the uranyl oxo anions are relatively inert, structural diversity in uranyl compounds is governed primarily by equatorial coordination, which leads to isolated clusters, one-dimensional chains, and higher-dimensional architectures.

Uranyl systems may also form three-dimensional frameworks via additional coordination types other than uranyl equatorial bonding. In particular, incorporating a second metal into a uranyl system can increase structural dimensionality by diversifying the coordination environments available within the framework. These heterometallic systems can also affect stability, along with magnetic, spectroscopic, and luminescent behavior, which may influence uranium mobility and bioavailability. − Additionally, the presence of transition metals (TMs) in the second coordination sphere, without directly coordinating to the uranium center, can perturb the electron density at the uranium–metal center.

In the experimentally determined U-PDC-TM chain structures, two PDC ligands coordinate the uranyl center in a tridentate fashion through O and N donors, forming a hexagonal-bipyramidal uranium environment (Figure ). The carboxylate O atom (from the PDC) coordinates with the TM to form a TM–carboxylate–U bridge. In the same study, the U-PDC-TM structures were compared with a previously synthesized monomeric U-PDC reference compound, where two PDC ligands are bonded to the uranium–metal center in a tridentate fashion via the two carbonyl O atoms and the pyridine N atom. Meanwhile, the triethylammonium (TEA) and water molecules stabilized the structure via hydrogen bonding and prevented 1D chain formation as in U-PDC-TM. This study showed that, relative to the monomeric U-PDC species, the heterometallic U-PDC-TM compounds exhibit longer equatorial U–O bond lengths, shorter uranyl U–O axial bonds, and a blue shift in the uranyl symmetric stretching frequency, consistent with the strengthening of the axial uranyl bond when TM was incorporated. These observations have been interpreted in terms of equatorial ligand-mediated electron donation. When the carboxylate O atom coordinates with the TM, it competes for ligand electron density and reduces effective donation to the U­(VI) center, thereby weakening the equatorial U–ligand interactions, which in turn favors stronger axial UO bonding. Computational studies have confirmed that the axial uranyl bond weakening tracks with stronger and more covalent equatorial interactions in similar structures without TMs.

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Local coordination environments of (a) monometallic U-PDC and (b) heterometallic U-PDC-TM units. In U-PDC, the uranyl center is coordinated by two tridentate PDC ligands, whereas in U-PDC-TM the carboxylate oxygen atom also coordinates the transition metal to form a TM–carboxylate–U bridge.

Despite experimental studies of heterometallic actinide-containing coordination compounds, computational insight into those systems containing TMs remains limited. , Specifically, Ejegbavwo and co-workers combined thermochemical measurements and density functional theory (DFT) studies to show that incorporating cobalt (Co) in heterometallic actinide metal–organic frameworks (MOFs) modifies the electronic structure near the Fermi level and alters the bandgap and conductivity. Motivated by the limited number of computational studies that systematically compare coordination by different TMs within a common actinide scaffold, we selected a series of crystal structures with varying TMs (Fe, Co, Ni, Cu, Zn) linked to the uranyl-2,6-pyridinedicarboxylate (U-PDC) scaffold, preserving the one-dimensional (1D) chain topology. This series provides an ideal platform for examining how different TMs perturb uranyl bonding and electronic structure within a common structural framework. In this work, DFT calculations were used to examine how coordination by different TMs influences the electronic properties of the U-PDC structure and perturbs the uranium center. We demonstrate that TM-ligand interactions govern electronic structure modulation through a ligand-mediated pathway, influencing band edge characteristics without significant direct perturbation of the uranyl center.

Computational Methods

Periodic DFT Calculations

Periodic DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) version 6.3.0. , All calculations were performed using the projector augmented wave (PAW) method − with the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation functional. A plane-wave basis set with an energy cutoff of 400 eV was used, where increasing the cutoff to 520 eV produced negligible change in the density of states (DOS). The crystal structures used in this work were originally reported by Jayasinghe and co-workers, where the U-PDC structure crystallizes in the space group C2/c (no. 15), and all U-TM-PDC structures form in the C2/m space group (no. 12). To reduce the computational cost, the crystallographically disordered constituents in the original structures were removed using a combination of Mercury and pymatgen python package, with minimal changes to the lattice parameters. We obtained a primitive cell of U-PDC using the “SpaceGroupAnalyzer” module in pymatgen since the U-PDC crystal structure from the experiment was a C-centered unit cell. The obtained primitive cell of U-PDC has 170 atoms in the C2/c space group (no. 15), while the U-PDC-TM structures have 128 atoms each in the C2/m space group (no. 12). The empirically optimized lattice parameters remained unchanged for the primitive U-PDC and U-PDC-Fe unit cells. To avoid high computational cost, all structures were relaxed with fixed lattice constants from the respective primitive cells, ensuring that forces on all atoms were less than 0.01 eV/Å and a total energy convergence criterion of 1 × 10–6 eV was met. The Brillouin zone was sampled using a 5 × 10 × 7 k-point mesh for U-PDC-TM structures, generated by the Monkhorst–Pack scheme. The Brillouin zone for the U-PDC primitive structure was only sampled at the Γ point to reduce computational cost, and our attempts to obtain convergence at denser k-mesh were not successful. The on-site Coulomb interaction of partially filled d- and f-orbitals was treated with the Hubbard U correction (DFT + U) in the simplified rotationally invariant approach. The Hubbard U parameter was calculated using the linear response method as detailed by Cococcioni and de Gironcoli. The U parameter for the U f-electrons in the U-PDC structure was calculated to be 3.43 eV, consistent with a previously reported study (4.0 eV) on a similar structure. Keeping the U parameter of U f-electrons constant for all the U-PDC-TM structures, we calculated the parameter for Fe d-, Co d-, Ni d-, Cu d-, and Zn d-electrons as 6.34, 6.94, 5.90, 10.38, and 1.84 eV, respectively. All the structures were relaxed again with the Hubbard U applied for these orbitals without spin–orbit coupling (SOC).

For each structure, the self-consistently converged charge density was used to compute the electronic DOS and band structure. The DOS was evaluated using the tetrahedron method with Blöchl corrections without smearing. SOC was explicitly included in all electronic structure calculations to account for relativistic effects, which are particularly important for heavy elements. High-symmetry k-point paths were generated using the pymatgen and SeeK-path libraries. ,, The calculated DOS and band structures were postprocessed and visualized using the sumo-bandplot utility and the pymatgen library.

The charge density difference plots were obtained for U-PDC-TM structures from self-consistent charge densities of (i) the full unit cell containing two TM atoms (Q 2TM ) and (ii) the unit cell with one TM atom removed (Q 1TM )­

ΔQ=Q2TM−Q1TM

The Q 1TM was calculated by removing one of the two TM atoms from the optimized structure and obtaining the self-consistent charge density, keeping the atomic coordinates fixed. To ensure that the charge density difference was determined accurately, the atom was removed from only one equivalent site within the periodic boundary condition. The charge density difference was visualized using VESTA, a 3D visualization software at an isosurface level of 0.005 e/bohr 3.

Phonon calculations were performed using the finite displacement method implemented by the Phonopy package, with a 0.01 Å displacement. The phonon eigenenergy and eigenvectors were computed only at the Γ point for the U-PDC and U-PDC-Ni structures as references. Our phonon calculations yielded 4–6 imaginary modes for U-PDC and U-PDC-Ni, respectively, with small magnitudes (15–56 cm–1). These imaginary modes do not have any bearing on our analysis here and can be ignored.

Cluster DFT Calculations

We performed cluster DFT calculations to obtain partial charges and the Raman-active vibrational modes, which are difficult to obtain in periodic DFT due to computational cost. Periodic structures were truncated to obtain clusters consisting of one uranyl cation equatorially coordinated by two PDC ligands, plus two hydrated TM complexes bonded to the U-PDC unit via a bridging carboxylate oxygen, consistent with the periodic structure. The octahedral coordination environment around each TM was completed by water molecules and one carboxylate O atom from the truncated neighboring PDC ligand, which was protonated to maintain the electroneutrality. All structures were initially computed as singlets (M = 1), and any structure that exhibited form degeneracy was computed with a higher spin state (M = 3). All structures with higher multiplicity reported in this work converged to a lower-energy geometry compared to their singlet counterpart.

Geometries of the truncated structures were optimized using DFT as implemented in Gaussian16, and frequency calculations were performed to ensure that no imaginary modes were present. The local density functional M06-L was employed, and the def2SVP , basis set was used for all of the TMs and light atoms. MWB60 small-core pseudopotential and associated valence basis set were used for uranium. Although larger basis sets could be explored in the future, the methods employed here have proven adequate for modeling actinide-containing systems of similar sizes. , Mulliken partial charges and atomic polar tensor (ATP) charges were computed as part of the optimization and frequency calculations, and natural atomic charges were also computed through natural population analysis (NPA).

Truncating periodic structures introduces more degrees of freedom and less rigidity as compared to a periodic structure, which could lead to deviations from the original crystallographic structure. Specific internal coordinates can be constrained during geometry optimization to preserve structural rigidity and better emulate the parent crystal structure while maintaining the local coordination environment. Given the 1D nature of the U-PDC-TM structures, we attempted to constrain the internal coordinates of atoms aligned along the 1D chain direction. In cases where the structure optimization did not converge to a minimum, we relaxed the structures without any constraints. For each structure, the atoms that were constrained to achieve convergence are shown in Figure S4 in Supporting Information.

Results and Discussion

The structural and electronic effects of introducing TMs (TM = Fe, Co, Ni, Cu, and Zn) into a U-PDC structure were studied using DFT to model and probe the properties of periodic and truncated U-PDC and U-PDC-TM systems. Here, we analyzed the lattice parameters, axial and equatorial U–O bond distances, density of states, band structures, Raman active modes, charge density differences, and partial charges to understand the influence of the TMs in the U-PDC structure. Figure provides a schematic overview of the key structure–property relationships identified in this study.

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Overview of the periodic and cluster DFT approaches used to describe how secondary transition metals influence the electronic properties of U-PDC-TM complexes through ligand-mediated interactions along the TM–O–C–O–U linkage.

Structure Parameters

The optimized primitive cell of the 0D U-PDC structure (Figure S1 in Supporting Information) consists of U-PDC units stabilized by intermolecular H-bonding interactions with solvent water molecules. Attempts to optimize the unit cell in the absence of solvent water molecules did not converge to a ground-state minimum, showing that hydrogen bonding plays an important role in maintaining the structural stability of the isolated U-PDC units.

When TMs are introduced into the U-PDC structure, the O atom of the carboxylate group from the PDC ligand coordinates to the TM center, linking neighboring U-PDC units to form an extended one-dimensional (1D) U-TM-PDC chain structure. This results in the formation of a carboxylate-bridged TM–O–C–O–U linkage (Figure ), which may influence the electronic structure of the uranyl polyhedron through ligand-mediated interactions. ,, The space group changes from C2/c in the U-PDC to C2/m in the U-TM-PDC structures. The lattice parameters of the DFT-optimized structures show small differences when compared to the experimental crystal structures (Table ). However, these changes can be attributed to the disorder removal from the original crystal structure as described earlier as well as the inherent error of the PBE functional.

1. Calculated Lattice Parameters for the Periodic DFT-Optimized Unit Cells with On-Site Coulomb Interactions Showing the Structural Changes upon Introducing the TMs into the 0D U-PDC Structure; Experimental Lattice Parameters Are in Parentheses.

structure a(Å) b(Å) c(Å) α(°) β(°) γ(°) volume(Å3)
U-PDC 14.66 (14.66) 14.66 (14.66) 16.05 (16.05) 71.73 (71.73) 108.27 (108.27) 151.98 (151.98) 1533.11 (1533.11)
U-PDC-Fe 14.68 (14.68) 7.05 (7.05) 10.91 (10.91) 90.00 (90.00) 94.65 (94.65) 90.00 (90.00) 1124.67 (1124.67)
U-PDC-Co 14.79 (14.70) 6.98 (7.05) 10.88 (10.82) 90.00 (90.00) 96.50 (95.04) 90.00 (90.00) 1115.81 (1117.31)
U-PDC-Ni 14.79 (14.71) 6.98 (7.06) 10.88 (10.77) 90.00 (90.00) 96.50 (94.95) 90.00 (90.00) 1115.81 (1114.69)
U-PDC-Cu 14.79 (14.79) 6.98 (6.98) 10.88 (10.88) 90.00 (90.00) 96.50 (96.50) 90.00 (90.00) 1115.81 (1115.81)
U-PDC-Zn 14.68 (14.75) 7.05 (7.15) 10.91 (10.91) 90.00 (90.00) 94.65 (94.82) 90.00 (90.00) 1124.67 (1145.92)
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Truncated clusters were modeled by isolating one UO2(PDC)2 unit with two neighboring triethylammonium for the U-PDC and two hydrolyzed TM complexes for the U-PDC-TM structures (Figure S4 in Supporting Information). Key differences between the experimental crystal structures and corresponding truncated cluster models can be described using the following structural parameters: (i) U-TM distance and TM-U-TM angle, (ii) degree of corrugation or bending of the PDC plane in the direction orthogonal to that of the chain, and (iii) U-axial and U-equatorial bond lengths. For the U-PDC cluster, the N atom of the triethylammonium was used to calculate the structural parameters listed above, in place of a TM.

In the DFT-optimized U-PDC cluster, the U–N distance increases by 0.10 Å, while the N–U–N angle decreases by 2.36° compared to the experimental crystal structure, likely due to the lack of periodic boundary conditions (PBCs), and retains the coordination environment observed in the crystal structure. The optimized clusters of U-PDC-TM showed a decrease in U-TM distances of less than 0.35 Å except for TM = Cu, where the distance increased by 1.28 Å. The TM-U-TM angle (∠) in the crystal structure (180°) was mostly preserved after optimizing the U-PDC-TM (TM = Fe, Co, Ni, Zn) clusters, with bending deviations within 7.24° (Figure S3a in Supporting Information) retaining most of the bonding parameters from the periodic structures. In the optimized U-PDC-Ni and U-PDC-Cu clusters, we observe a rearrangement of the solvent water molecules around the TM due to loss of PBCs, but we do not expect this rearrangement to affect the electronic or vibrational properties of the clusters.

Axial and equatorial bonds of the uranyl group show minimal deviations from experimentally derived structures, with axial U–Oyl bonds in all models decreasing by < 0.05 Å. The U–N and U–OPDC equatorial bond lengths in all clusters are within 0.15 Å of the experimental values. A comprehensive analysis is presented in Tables S1–S3.

Electronic Properties

The DOS for the monometallic U-PDC system exhibits a bandgap (E g ) of 2.60 eV (Figure a). The valence-band maximum (VBM) is composed primarily of O 2p states (Figure ) originating from the equatorial oxygen atoms (Figure ), with a small contribution from U-5f states for the U-PDC structure. In contrast, the conduction-band minimum (CBM) is dominated by U 5f states which are highly localized, as evidenced by the flat, low-dispersion bands. This observation is consistent with prior first-principles studies of U­(VI) oxides, such as CaUO4, where the unoccupied band edge manifold is dominated by uranium 5f states. Similarly, electronic structure analyses of heterometallic actinide frameworks have shown that the states above the Fermi level (E F ) frequently retain strong actinide 5f character; for example, in U/Th–Me2BPDC-8, the calculated DOS above E F is primarily U 5f in nature. ,

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Band structure and density of states for (a) U-PDC, (b) U-PDC-Fe, (c) U-PDC-Co, (d) U-PDC-Ni, (e) U-PDC-Cu, and (f) U-PDC-Zn structures with Fermi level at 0.0 eV. Panel (g) summarizes the effect of TM identity and d character at the VBM on the bandgap across the U-PDC-TM structures.

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Projected band structure (left) and projected DOS (right) for the U-PDC structure, showing contributions from the U1 atom and its bonded (a) equatorial O atoms and (b) axial O atoms near the band edge. The VBM is dominated primarily by O-p states from the equatorial O atoms bonded to the U-metal center.

On introducing the TMs, we observe a calculated E g of 1.74 eV, 2.51 eV, 2.52 eV, 2.66, and 2.39 eV in the U-PDC-TM structures, for TM = Fe, Co, Ni, Cu, Zn, respectively (Figure ). When compared to the monometallic U–PDC system, the E g narrows except for the U-PDC-Cu structure where E g increases slightly (0.06 eV). In each of these compounds, we see the CBM dominated by the U 5f states that remain highly localized as seen in the U-PDC structure. The principal change introduced by TM incorporation therefore occurs at the valence-band edge. Specifically, E g narrowing is observed when the VBM acquires the TM d character in addition to the ligand O 2p character (Figure g). For example, in both U-PDC-Fe and U-PDC-Zn, the VBM has a strong TM-d character, which reduces the E g by 0.86 and 0.21 eV, respectively, when compared to the U-PDC structure. In the case of U-PDC-Co and U-PDC-Ni where the E g narrows by 0.09 and 0.08 eV, respectively, we observe a very small and localized TM-d contribution at the valence-band edge. Even in the case of U-PDC-Cu, a very small TM-d state contribution is still observed near the valence-band edge, but it is much weaker compared to the other U-PDC-TM structures (Figures S11–S15 in Supporting Information). Thus, although Cu d-states would be expected to raise the valence-band edge, this effect is outweighed by accompanying changes in the O 2p states, leading to a small net increase in E g of 0.06 eV. These results indicate that the observed bandgap changes are governed primarily by modifications to the valence-band manifold rather than by significant perturbation of the U-centered conduction-band states.

To further investigate the TM-d contribution at VBM, we closely examined the DOS of all the U-PDC-TM structures. In these compounds, the TM center is coordinated by four water molecules and two carboxylate O donors, giving a distorted octahedral environment rather than an ideal O h geometry. This mixed and anisotropic ligand field lowers the local symmetry of the TM site and lifts the ideal octahedral degeneracy of the d manifold, producing distinct orbital contributions near the valence-band edge. For U-PDC-Fe, we observe several degenerate Fe-d states at the valence-band edge that are highly localized (Figure S11 in Supporting Information). Although the O 2p states below −1.0 eV are similar to those of U-PDC, the additional occupied Fe d states near the VBM raise the valence-band edge and thereby reduce E g (Figures and S11 in the Supporting Information). At the VBM, the orbital projection of the DOS shows that the d yz , d xz, and dx2−y2 orbitals dominate the electronic states, indicating that these orbitals contribute significantly to the states at the VBM and therefore play a key role in narrowing the E g . In the U-PDC-Co structure, we observe that the d xz and dx2−y2 orbitals dominate the electronic states at the VBM (Figure S12b in Supporting Information), but the number of states observed is significantly smaller than the Fe-d states in the U-PDC-Fe. We observe that the d xz and dx2−y2 orbitals dominate the VBM for the U-PDC-Ni and U-PDC-Cu structures as well (Figures S13b and S14b in Supporting Information); however, the number of states is much smaller than the U-PDC-Fe structure. Interestingly, in the case of U-PDC-Zn, we see a significant contribution from the d yz orbital in addition to the d xz and dx2−y2 orbitals at the VBM (Figure S15b in Supporting Information) similar to the U-PDC-Fe structure, and this likely contributes more toward narrowing the E g . Overall, these results show that the degree of TM-derived character at the valence-band edge tracks closely with the magnitude of E g narrowing across the series.

In all of the structures considered here, we observe a loss of degeneracy in the t2g and eg orbitals, likely due to the distorted octahedral environment, indicating a metal-ligand field effect. , The octahedral geometry is stabilized by the t 2g electron pair splitting into two parallel spins and thereby reducing the electrostatic repulsion. ,, For TMs with 4–7 d-electrons in octahedral environments, high-spin configurations are generally favored when the crystal-field splitting is small, consistent with coordination by weak-field ligands. The hydrogen bonding involving water molecules coordinated to the TM may also influence the octahedral distortion and contribute to the lifting of the t 2g and e g degeneracy in first-row TM complexes. , These prior studies suggest that the octahedron is stabilized with parallel spin states in the t2g and eg orbitals.

To understand the metal-ligand interactions, we used the DOS calculations to determine the contribution of the O-p states near the VBM for specific groups of atoms, namely, (i) equatorial O atoms, (ii) axial O atoms bonded to the U-metal center, and (iii) bridging O atoms bonded to the TM (Figures and S6–S10 in Supporting Information). In the absence of the TM in U-PDC (Figure ), we observe the O-p states from the equatorial O atoms to be higher in energy and closer to the VBM compared to the axial O atoms, indicating that the valence-edge states are dominated primarily by the equatorial ligand environment. The same trend is preserved in all U-PDC-TM structures, where the equatorial O atoms remain higher in energy than the axial O atoms (Figures S6b–10b in Supporting Information). These heterometallic systems also show O-p states from the bridging O atom bonded directly to the TM near the VBM, in addition to the TM-d states, clearly depicting the TM–ligand interaction. These states indicate the energetic overlap between the d- and p-orbitals of the TM and the bridging O atoms. Because these bridging O atoms also belong to the PDC ligand coordinated to uranium, the interaction provides a ligand-mediated pathway by which the TM perturbs the electronic structure of the uranyl-containing framework. Thus, the reduction in bandgap is best understood as arising from the introduction of TM d and bridging O 2p states that raise the valence-band edge, while the U 5f-dominated conduction edge remains largely unchanged.

These trends are consistent with prior observations in heterometallic actinide frameworks, where incorporation of cobalt substantially reshapes the electronic structure near the E F . In the U1.23Th4.77-Me2BPDC-8 system, incorporation of Co­(II) into the system resulted in a marked decrease in the electronic bandgap. Specifically, the E g decreased from 3.3 eV in the Th-only framework (Th6–Me2BPDC-8) to 2.5 eV in the mixed U/Th analogue (U1.23Th4.77-Me2BPDC-8) and further to 1.9 eV upon coordination of Co­(II), forming U1.23Th4.77Co3–Me2BPDC-8. Consistent with these works, we observe that incorporating a TM can introduce additional occupied states of TM d and bridging O 2p character near the valence-band maximum. The extent of this valence-edge perturbation determines the narrowing of the E g , with Fe showing the strongest effect, Co and Ni much weaker effects, Zn an intermediate effect, and Cu essentially no change. These results show that TM-ligand interactions modulate the electronic properties of the U-PDC framework primarily through a ligand-mediated mechanism that raises the valence-band edge under distorted octahedral coordination, rather than directly perturbing the U-centered conduction-band states, which can be justified by checking the occupied molecular orbitals directly from cluster DFT calculations.

Frontier Molecular Orbital Diagrams

The frontier molecular orbital (MO) diagrams can reveal the localized orbital contribution at the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) based on cluster DFT calculations. To understand the nature of these orbitals, we first considered the singlet state for all the clusters, and in cases where the HOMO and LUMO orbitals were degenerate (form degenerate), we analyzed the orbitals in the triplet state, if the optimized structure with high multiplicity (i.e., 3) was lower in energy compared to the singlet counterpart.

The MO diagram of the U-PDC cluster (Figure a) reveals the localized nature of HOMO on the PDC ligand, while the LUMO is localized on the U center. The localized nature of these orbitals shows that the U center and PDC ligands are nearly electronically isolated, which is reasonable since they are not connected via π-conjugation similar to well-known TM coordinated complexes. Although we cannot compare the HOMO–LUMO gap (E LUMO‑HOMO ) to the bandgap obtained from the DOS and band structure calculations, as the former pertains to a finite system while the latter describes a periodic solid, the orbital character remains consistent: the VBM dominated by the PDC ligand and the CBM dominated by the U 5f states. When TMs are introduced to the U-PDC clusters, we observe the nature of HOMO to be localized mostly on the TM octahedral, while the LUMO localizes on the U centerconsistent with the DOS and band structure calculations on the periodic structures (Figures and S24–S29 in Supporting Information). The shift in the HOMO from the PDC ligand in the U-PDC cluster to the TM octahedral in the U-PDC-TM clusters describes a clear influence of the TMs on the E LUMO‑HOMO . When TMs are incorporated, the E LUMO‑HOMO decreases, which is characteristic of materials with an increase in reactivity. − The E LUMO‑HOMO in the U-PDC cluster (2.21 eV) decreases the most for the U-PDC-Ni cluster (0.52 eV) followed by U-PDC-Cu (1.25 eV) and U-PDC-Zn (1.65 eV). We should note that the E LUMO‑HOMO for the U-PDC-Cu reported here is calculated at the triplet state since the HOMO and LUMO of the singlet state showed form degeneracy and converged to a minimum with higher energy (+0.83 eV) than the triplet state. Similarly, we observed form degeneracy in the singlet state for U-PDC-TM (TM = Fe, Co) clusters (Figures S25 and S26 in Supporting Information), and calculations modeling higher spin states (M = 3, 5, 7) did not converge. However, the nature of the higher-energy MOs for these two structures localizes on the U center, consistent with the other U-PDC-TM clusters. For the U-PDC-Fe cluster, the HOMO and LUMO are nearly degenerate (E LUMO‑HOMO = 0.02 eV), while the LUMO+1 at higher energy is localized on the U center. In the case of the U-PDC-Co cluster, we see the HOMO as energetically degenerate with the LUMO and LUMO+1 where E LUMO‑HOMO ∼ 0 eV, the higher energy LUMO+2 and LUMO+3 orbitals localize on the U center. These observations from the MO diagrams show the orbital nature of the TM octahedral as the HOMO plays a direct role in influencing the E LUMO‑HOMO of the clusters.

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Frontier molecular orbital diagrams of (a) U-PDC and (b) U-PDC-Ni clusters showing the nature of the HOMO and LUMO orbitals in the two structures. The HOMO is localized on the PDC ligand and the TM octahedral for the U-PDC and the U-PDC-Ni clusters, respectively. The LUMO is localized on the U metal center for both these clusters. Incorporating the TM shows the reduction in the HOMO–LUMO gap.

Charge Density Difference

To further understand how TMs perturb the electronic structure, especially the charge redistribution effect, we calculated the charge density difference on all the periodic U-PDC-TM structures. From the charge density difference plots (Figures and S16–S20 in Supporting Information), we observe the TM perturbs the electronic structure of the PDC ligand rather than the U-metal center directly. The charge accumulation along the conjugated PDC backbone indicates that the TM–O interaction perturbs the ligand π system providing an indirect electronic coupling pathway between the TM and the U-metal center. The observed charge redistribution supports the ligand-mediated effect on the VBM upon introduction of TM into the U-PDC structure. However, the charge density difference plots do not show major qualitative differences among the U-PDC-TM structures and therefore do not, by themselves, explain the variation in the electronic properties across the series. Instead, the bandgap differences observed in the DOS and band structures are more likely governed by differences in the orbital energies and the energetic overlap between TM d- and O p-states near the VBM, rather than distinct charge redistribution pathways.

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Charge density difference plots show the charge redistribution along the U–O equatorial bonds in the U-PDC-Ni structure due to the influence of the Ni-atom at the isosurface level of 0.005 e/bohr. The (a) top view and (b) side view shows charge accumulation (yellow) along the C–C bond of the PDC ligand and the equatorial O atom bonded to the U-metal center. The charge density difference plots were made using VESTA.

Vibrational Modes

Phonon calculations were completed for two periodic structures, the monometallic U-PDC and the bimetallic U-PDC-Ni structures, to understand the change in the uranyl stretching frequency when a TM is incorporated. The U-PDC uranyl symmetric stretching ( νUOyl ) frequency was calculated to be 798 cm–1, which undergoes a blue shift to 852 cm–1 when Ni is incorporated to form the U-PDC-Ni structure. This behavior is consistent with experimental trends, where νUOyl undergoes a blue shift from 825 cm–1 in U-PDC to 859 cm–1 in U-PDC-Ni. The shift here indicates the axial U–O bond strengthening as shown in other U-coordinated complexes. The blue shift in νUOyl is also reflected by a small decrease (0.02 Å) in the bond length of U-PDC-Ni when compared to that of the U-PDC structure. The ring bending and breathing modes from experiments show negligible shift when TMs are introduced into the system, likely due to minimal influence of the TM on the PDC ligand moiety. From the phonon calculations, the ring bending modes of 648 cm–1 and 652 cm–1 for the U-PDC structure show negligible change for the U-PDC-Ni structure with 654 cm–1. Similarly, the ring breathing modes also show minimal change from 1009 cm–1 in the U-PDC structure to 1013 cm–1 in the U-PDC-Ni. These observations are consistent with the experimental values where minimal shifts are observed in the ring bending and breathing modes in the presence of a TM octahedral.

To further understand the effect of TMs on the uranyl symmetric stretching, ring bending, and ring breathing modes, we calculated the frequencies on all of the clusters. For each structure, the observed difference between the calculated and the experimentally determined vibrational frequencies was within the margin of error for DFT calculations.

From experiments, we observe that the uranyl symmetric stretching frequency consistently increases for all of the structures when the TM is introduced. Our cluster calculations do not reflect this trend since the uranyl stretching frequency increases only for the U-PDC-Ni cluster when compared to the monometallic system. We observe a decrease in the stretching frequency for TM = Fe, Co, while this frequency remains similar to the monometallic system for TM = Cu, Zn. While factors like (i) lack of PBCs and (ii) constraining specific atoms during optimization can be attributed to this deviation from experiments, we do observe a linear relationship between the uranyl stretching frequency and the U–O axial bond length (Figure S30 in Supporting Information).

Ring bending modes from our U-PDC-TM cluster calculations suggest TMs have minimal effect on the frequency when compared to the U-PDC cluster and these observations are consistent with experiments. It should be noted that the U-PDC-Ni cluster showed two bending modes (668 cm–1 and 671 cm–1), likely due to loss of energetic degeneracy. The calculated ring bending modes show systematic overestimation between 7–14 cm–1 (Table and Figure S21a). The calculated ring breathing modes (Table ) were similar to the monometallic system without much change when TMs are incorporated into the structure. However, they show an overestimation (between 15 and 21 cm–1) compared to the experiments (Figure S21b). For the U-PDC-Fe cluster, we see two ring breathing modes (1013 cm–1 and 1033 cm–1), which could be due to the weak coupling and loss in symmetry between the two rings as mentioned before. These results show good agreement with experiments except for the uranyl stretching mode, which could be due to other factors mentioned earlier and needs further investigation beyond the scope of this study.

2. Calculated Frequencies for Uranyl Symmetric Stretching, Ring Bending, and Ring Breathing Modes for All U-PDC and U-PDC-TM Clusters; Experimental Values are in Parentheses .

Vibrational Modes U-PDC (cm–1) U-PDC-Fe (cm–1) U-PDC-Co (cm–1) U-PDC-Ni (cm–1) U-PDC-Cu (cm–1) U-PDC-Zn (cm–1)
Uranyl symmetric stretching 848 (825) 828 (857) 783 (858) 866 (859) 849 (864) 851 (860)
Ring bending 669 (660) 667 (660) 666 (659) 668, 671 (660) 670 (660) 670 (658)
Ring breathing 1039 (1019) 1013, 1033 (1015) 1031 (1016) 1037 (1016) 1036 (1017) 1037 (1017)
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Partial Charges

Partial charge analysis using cluster calculations revealed the effect of incorporating TMs on the electronic distribution at the UO2 2+ center. By comparing Mulliken, natural population analysis (NPA), and atomic polar tensor (APT) charges, we evaluated the electron density and polarization at the U center as a function of the TM. Mulliken charge analysis (Table ) shows the lowest charge on the U center for the U-PDC-Co cluster, indicating more electron density on the U atom due to larger delocalization caused by the Co atom. In every cluster with the TM, we observe a decrease in the Mulliken charge on the U center, which indicates an increase in the electron density on this atom. Since Mulliken charge analysis is dependent on the basis set, it would be remiss to not study the NPA, which considers localized bonds and lone pairs.

3. Calculated Partial Charges of the U Center in the Monomer U-PDC and Bimetallic U-PDC-TM Structures Using Mulliken Analysis, NPA, and ATP Analysis Schemes.

Structure Mulliken (a.u.) NPA (a.u.) APT (a.u.)
U-PDC 2.200 2.510 1.592
U-PDC-Fe 1.975 1.841 1.072
U-PDC-Co 1.200 1.800 0.600
U-PDC-Ni 2.135 2.799 1.600
U-PDC-Cu 2.007 2.800 1.174
U-PDC-Zn 1.998 2.123 1.576
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Our NPA analysis (Table ) reveals a similar trend compared to the Mulliken charge analysis where the charge on the U center is the lowest for the U-PDC-Co cluster. However, the TM = Ni,Cu structures show an increased charge compared to the U-PDC cluster, which indicates charge depletion from the U center. The ATP charges (Table ) reveal the polarization of the U center as a function of the TM in the cluster. These results show the lowest charge on the U atom for the U-PDC-Co cluster, which indicates low polarizability of the U center with large delocalization of the electron density and is consistent with stronger electron coupling compared to the other clusters. We compared the charge variation with the calculated uranyl symmetric stretching frequencies to provide insight into changes in the electron density at the U center on the vibrational response (Figure ). Lower charge on the U atom is associated with lower stretching frequency due to electron density accumulation on the U center, which weakens the U–O bonds. These results show a consistent relationship between the charge on the U center and the uranyl stretching frequency.

7.

7

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Partial charges on the U atom from (a) Mulliken, (b) NPA, and (c) APT charge analyses compared with the (d) uranyl symmetric stretching frequency. The result shows that the accumulation of electron density on the U center reduces the U–O bond strength, as indicated by the decrease in the uranyl symmetric stretching frequency.

Conclusions

Uranium coordination chemistry and redox behavior play a key role in separation, processing of nuclear fuel-cycle waste, monitoring, and remediation strategies. This work examined how the incorporation of first-row transition metals (Fe, Co, Ni, Cu, and Zn) influences the structural, electronic, and vibrational properties of heterometallic uranyl PDC complexes. Periodic DFT calculations show that TM incorporation modifies the electronic structure primarily through a ligand-mediated pathway, in which TM d states and bridging O 2p states contribute near the valence-band maximum while the U 5f-dominated conduction-band edge remains largely unchanged. As a result, the bandgap changes observed across the U-PDC-TM series arise mainly from perturbing the valence-band manifold rather than direct alteration of the uranium-centered conduction states. These trends are consistent with the frontier MOs from the cluster calculations in which the HOMO is localized on the PDC ligand in the U-PDC monometallic system and on the TM octahedral unit in the U-PDC-TM clusters. Charge density difference analysis further supports this interpretation by showing charge redistribution along the TM–O–C–O–U linkage, with the principal electronic response occurring through the ligand framework rather than through strong direct perturbation of the uranyl center. The partial charge analysis from the cluster calculations also indicates a small electron density accumulation and delocalization on the U center when TMs are incorporated into the structure.

Vibrational modes from phonon calculations show a blue shift in the uranyl stretching with minimal shift in ring bending and ring breathing modes for the U-PDC-Ni structure compared to the U-PDC structure. Although simulated Raman uranyl stretching modes of the clusters did not consistently reproduce the experimental trend across the full series, the ring bending and breathing modes consistently showed only minimal changes upon TM incorporation, in agreement with the experiment. Overall, these results demonstrated that the electronic properties of uranyl coordination complexes can be tuned through modification of the surrounding coordination environment, with transition metals influencing the system primarily through ligand-mediated electronic coupling rather than strong direct interaction with the uranium center. More broadly, these findings contribute to the emerging view that actinide-containing coordination materials can be designed not only through structural control but also through targeted tuning of their electronic structure. Prior studies have shown that electronic properties in actinide-containing materials are sensitive to metal composition, ligand coordination, and overall material architecture; our results extend this concept to uranyl-transition metal coordination complexes by showing that secondary transition metals can modulate band edge states through a ligand-mediated pathway. This suggests that selecting secondary metals with suitable d-orbital energies and metal-ligand interactions may provide a practical strategy for tuning bandgaps and valence-band character in heterometallic uranyl materials. Such indirect electronic control could guide the future design of actinide-containing optical, electronic, or spectroscopic materials while avoiding strong direct perturbation of the uranyl center.

Supplementary Material

pg6c00096_si_001.pdf (4.7MB, pdf)

Acknowledgments

This work was supported through the INL Laboratory Directed Research & Development (LDRD) Program under DOE Idaho Operations Office Contract DE-AC07-05ID14517. H.C. was supported by the Office of Science Graduate Research fellowship program. This research made use of the Idaho National Laboratory’s High Performance Computing systems located at the Collaborative Computing Center and supported by the Office of Nuclear Energy of the U.S. Department of Energy and the Nuclear Science User Facilities under Contract No. DE-AC07-05ID14517. This research also made use of computational resources provided by the Notre Dame Center for Research Computing.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphyschemau.6c00096.

  • Detailed information on all optimized structures from periodic and cluster DFT calculations; structure parameters from cluster DFT calculations; projected band structure and density of state plots showing contributions from specific atoms; charge density difference plots showing the influence of TM atoms on the charge redistribution; trends in vibrational modes vs U–Oyl bond lengths; and molecular orbital diagrams for all the optimized structures from cluster DFT and correlation between U–Oyl bond length and stretching frequency compared with experimental results (PDF)

The authors declare no competing financial interest.

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pg6c00096_si_001.pdf (4.7MB, pdf)

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