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 m 2 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 U m 2 Pz 2 (m = II, III and IV), respectively. They possess bond lengths of U U at 2.37, 2.46 and 2.91 A, 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 II 2 Pz 2 , and the triple and double bonds for U III 2 Pz 2 , and U IV 2 Pz 2 , respectively. It shows that the uranium oxidation state approximately correlates with the number of multiple bonds.