Introducing dibenzocyclooctatetraene into actinide chemistry: isolation of rare trivalent uranium sandwich complexes

Peerless actinide complexes containing π-conjugated dibenzocyclooctatetraene (dbCOT) dianions were isolated. These three uranium molecules have the tripositive oxidation state of the metal ion in common and vary in ligand arrangement where the difference originates from the presence of coordinating solvent molecules and encapsulating chelating agents, respectively. The first compound [K(DME)₂][U(dbCOT)₂] (where DME = dimethoxyethane), 1, features a potassium ion interacting with one of the dbCOT ligands and was obtained from a salt metathesis reaction employing UI₃(dioxane)_1.5 and K₂dbCOT. The second compound [K(crypt-222)][U(dbCOT)₂], 2, arose from treating 1 with 2.2.2-cryptand, yielding an outer-sphere potassium ion. The third compound [K(crypt-222)][U(dbCOT)₂(THF)], 3, was generated through the exposure of 1 to THF solvent and 2.2.2-cryptand, resulting in the coordination of THF to uranium(III) along with η⁸-ligation of each dbCOT ligand while placing the potassium ion outer sphere, encased in the chelating agent. Notably, the compounds present the largest uranocene derivatives, considering the presence of fused aromatic rings to the central COT framework, which bear a total of 36 π-electrons. The three uranium molecules were thoroughly studied via single-crystal X-ray diffraction and UV-Vis-NIR, IR, and NMR spectroscopy. DFT computations on 1–3 shed light on their divergent electronic structure.