Molecular and Electronic Structures of Eight-Coordinate Uranium Bipyridine Complexes: A Rare Example of a Bipy2– Ligand Coordinated to a U4+ Ion

Reaction of trivalent [((Ad,tBuArO)3tacn)­U] (1) with 2,2′-bipyridine (bipy) yields [((Ad,tBuArO)3tacn)­U­(bipy)] (2) and subsequent reduction of 2 with KC8 in the presence of Kryptofix222 furnishes [K­(2.2.2-crypt)]­[((Ad,tBuArO)3tacn)­U­(bipy)] (3). Alternatively, complex 3 can be synthesized from 1 by addition of [K­(bipy)] in the presence of the cryptand. New complexes 2 and 3 are characterized by a variety of spectroscopic, electrochemical, and magnetochemical methods, single-crystal X-ray diffraction, computational methods, and CHN elemental analysis. Structural analyses reveal a bipyridine radical (bipy•–) ligand in 2 and a dianionic (bipy2–) species in 3. Complex 3 represents a rare example of an isolated and unambiguously characterized bipy2– ligand coordinated to a uranium ion. The electronic structure assignments are supported by UV/vis/NIR and EPR spectroscopy, as well as SQUID magnetometry. The results of CASSCF calculations indicate multiconfigurational ground states for complexes 2 and 3. The electronic ground state for 2 consists of an open-shell doublet U4+(bipy•–) state (91%) and a closed-shell doublet U5+(bipy2–) state (9%). The almost degenerate multiconfigurational ground state for 3 was found to be composed of an open-shell singlet and pure triplet state 0.06 eV higher in energy, both resulting from the U4+(5f2) (bipy2–) configuration.