Cation Complexes of Uranium and Thorium with Cyclooctatetraene: Photochemistry and Decomposition Products

Ion-molecule complexes of uranium or thorium singly-charged positive ions bound to cyclooctatetraene (COT), i.e., M⁺(COT)_1,2, are produced by laser ablation and studied with UV laser photodissociation. The ions are selected by mass and excited at 355 or 532 nm, and the ionized dissociation products are detected using a reflectron time-of-flight mass spectrometer. The abundant fragments M⁺(C₆H₆), M⁺(C₄H₄), and M⁺(C₂H₂) occur for complexes of both metals, whereas the M⁺(C₄H₂), M⁺(C₃H₃), and M⁺(C₅H₅) fragments are prominent for uranium complexes but not for thorium. Additional experiments investigate the dissociation of M⁺(benzene)_1,2 ions which may be intermediates in the fragmentation of the COT ions. The experiments are complemented by computational quantum chemistry to investigate the structures and energetics of fragment ions. Various cation-π and metallacycle structures are indicated for different fragment ions. The metal ion-ligand bond energies for corresponding complex ions are systematically greater for the thorium species. The computed thermochemistry makes it possible to explain the mechanistic details of the photochemical fragmentation processes and to reveal new actinide organometallic structures.