Synthesis and characterization of isostructural annulated actinocenes†

An isostructural series of four annulated actinocene complexes, M(hdcCOT)₂ (M = Th, U, Np, Pu), is reported. The syntheses proceed through a trivalent starting material when M = U, Np, Pu with subsequent oxidation or, in the case of M = Th, directly from ThCl₄(DME)₂. X-ray crystallography shows that each actinocene has C_2h molecular point symmetry in the solid state, with the metal atoms symmetrically bonded to two 10π-aromatic [8]annulene dianion rings. UV-Vis spectroscopy shows bands that were assigned as ligand-to-metal charge transfer (LMCT) transitions with the aid of time-dependent density functional theory (TDDFT) calculations. Compared to the M(COT)₂ analogs, the LMCT transitions for M(hdcCOT)₂ are red-shifted by ∼0.1 eV for all four metals. These actinocenes may be readily synthesized and crystallized even on the milligram scale (≤1 mg of metal ion content), which will enable future studies of the organometallic chemistry of transplutonium elements.