Structures and Uranium-Uranium Multiple Bond of Binuclear Divalent Uranium Complex of Pyrrolic Schiff-base Macrocycle: a Relativistic DFT Probe

Abstract

Although attempts to synthesize divalent uranium molecules were begun three decades ago, molecular U(II) species isolable in solution have been not achieved until recent years. In 2013, Evans and co-workers synthesized the first U(II) complex, [U(Cp')3]·[K(2,2,2-cryptand)] (Cp'=C5H4SiMe3) via flash reduction, that was suitable for X-ray crystal diffraction characterization. A year later, the group of Meyer obtained another divalent uranium complex, [U((Ad,MeArO)3mes)]·[K(2,2,2-cryptand)] employing their particularly interesting tris(aryloxide) arene ligand. The 5f36d1 and 5f4 ground states were assigned to these two complexes, respectively, by the jointed experimental/theoretical studies. It was demonstrated that the ligand significantly affect the nature of the ground state of divalent uranium complex by tuning the energetic separation of the 5f and 6d orbitals. Therefore, careful selection of ligand makes it possible to have access to +II oxidation state of uranium and prepare new UII complex. A flexible octadentate polypyrrollic Schiff-base macrocycle (H4L) has been developed to complex a variety of metals such as actinides, rare earth and transition metals that show a wide range of size and diverse oxidation states. Both mono- and bimetallic complexes featured with an intriguing "Pacman-like" structure were obtained. For example, the reaction of H4L with a trivalent uranium precursor [(UIII)I3(THF)4] yielded a neutral [(UIV)(L)] complex, where the uranium ion was determined by the single crystal X-ray diffraction to be situated inside the ligand mouth and held by eight nitrogen atoms together. The +IV oxidation state was assigned to the uranium by presuming dihydrogen elimination. Considering the flexibility, tetravalent-anion nature as well as capability of accommodating bimetallic ions and stabilizing various oxidation states of uranium (e.g. III~VI complexes have been found so far) that the polypyrrolic ligand has exhibited in previously synthesized complexes, two divalent uranium ions would be likely complexated by the ligand to generate a complex, [(UII)2(L)]. In addition to enriching the coordination chemistry of U(II), it is also a good example to explore electronic structures of the low-valent uranium complex and unravel the uranium-uranium multiple bonding nature. Although many theoretical studies have explored uranium complexes, the study focusing on the divalent diuranium complex of a single macrocyclic ligand remains rare. In the work, a relativistic density functional theory has been employed to investigate [(UII)2(L)]. The structures in electron spin states (singlet, triplet, quintet, septet and nonet) were optimized. Short distances of U-U (2.32~2.67 Å), large bond order (2.95~3.90) and high stretching vibrational frequencies (180~263 cm-1) were calculated. Energetic calculations find that its triplet state is the ground state. It has the electronic configuration of π4σ2δ2, primarily contributed by U(5f) character. Structural and molecular-orbital analyses suggest a slightly weak uranium-uranium quadruple bond, which is confirmed by the quantum theory of atoms in molecule (QTAIM) calculations. Further comparison with analogues [(UIII)2(L)]2+ and [(UIV)2(L)]4+ was also addressed. It is found that the uranium oxidation state is able to tune the energetic matching between the highest-energy occupied orbital of ligand and the adjacent low-energy metal-based orbital, as well as correlates with the electron transfer between metal and ligand and the diuranium multiple bond number.