Metal-metal multiple bond in low-valent diuranium porphyrazines and its correlation with metal oxidation state: A relativistic DFT study

To explore the uranium-uranium bonding nature, a ligated diuranium complex that could be experimentally possible would show features with no bridging ligand constraints (i.e. discrete or unsupported ligand), rigid ligand skeleton and suitable U-ligand bond. In this respect, we have designed and examined a series of diuranium diporphyrazines (U(2)(m)Pz(2), m = II, III and IV) using relativistic density functional theory. Optimizations on all possible electron-spin isomers find that the triplet, quintet and quintet states are energetically lowest for Um(2)Pz(2) (m = II, III and IV), respectively. They possess bond lengths of U-U at 2.37, 2.46 and 2.91 angstrom, bond orders of 3.48, 3.33 and 2.11, and stretching vibrational frequencies of 239, 172 and 108 cm(-1). Associated with the electronic-structure and QTAIM (quantum theory of atoms in molecules) analyses, a weak quadruple bond is suggested for the triplet state of U(2)(II)Pz(2), and the triple and double bonds for U(2)(III)Pz(2), and U(2)(IV)Pz2, respectively. It shows that the uranium oxidation state approximately correlates with the number of multiple bonds. (C) 2017 Elsevier B.V. All rights reserved.