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

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.