Trends in Electronic Structure and Redox Energetics for Early-Actinide Pentamethylcyclopentadienyl Complexes

Detailed cyclic voltammetric and UV−visible−near-infrared electronic absorption spectral data have been obtained for a series of pentamethylcyclopentadienyl complexes of uranium(IV) and thorium(IV) of the general formula (C5Me5)2An(L1)(L2), where L1, L2 = Cl, SO3CF3, CH3, CH2Ph, imido (N-2,4,6-tBu3C6H2), hydrazonato (η2(N,N‘)-RNNCPh2; R = CH3, CH2Ph, Ph), ketimido (−NC(Ph)(R); R = CH3, CH2Ph, Ph) ligands, and for the hexavalent uranium bis(imido) complex (C5Me5)2U(NPh)2. The electrochemical and spectroscopic behavior of the tetravalent uranium complexes falls cleanly into distinct categories based on the nature of L1 and L2. If both ligands are simple σ-donors (Cl, SO3CF3, CH3, CH2Ph), a reversible U(IV)/U(III) voltammetric wave is the only metal-based process observed, and it occurs between ∼−1.8 and −2.6 V vs [(C5H5)2Fe]+/0. If either L1 or L2 is a nitrogen-donor ligand (imido, hydrazonato, ketimido), then both a U(IV)/U(III) reduction wave and a U(V)/U(IV) oxidation wave are observed. The reduction ste...