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

Abstract

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 step occurs in the same potential region as for the σ-donor complexes, and the oxidation wave occurs in the range from ∼+0.2 to −0.7 V vs [(C5H5)2Fe]+/0. This oxidation wave is reversible, indicating that the unusual pentavalent uranium oxidation state is kinetically stable on a voltammetric time scale, and the potential of the oxidation step indicates that the pentavalent state is thermodynamically stabilized from interaction with the nitrogen-donor ligand(s). The separation between reduction and oxidation processes in these nitrogen-donor complexes remains nearly constant over the series of eight complexes, with an average value of 2.09 V. Additional ligand-based redox processes are also observed and assigned on the basis of the existence of corresponding voltammetric waves in the Th(IV) complexes and other cyclopentadiene complexes. The electronic absorption spectra for all U(IV) complexes are comprised of two distinct regions: a lower energy region (E < 15 000 cm-1) containing the narrow f−f transitions arising from within the 5f orbital manifold and a higher energy region (E > 15 000 cm-1) containing broad, unstructured, or poorly structured bands derived from both metal-localized 5f−6d transitions and molecular-based transitions including ligand-localized and metal-to-ligand charge-transfer transitions. A definite trend in intensities of these transitions is observed, depending on the nature of L1 and L2. If L1 and L2 are both simple σ-donor ligands, the f−f transition intensities are relatively weak (molar absorptivity ε ≈ 10−80 M-1 cm-1), consistent with observations for most classical coordination complexes of 5f2 electronic configuration, and the broad, higher energy bands have ε values in the 3000−5000 M-1 cm-1 range. If L1 and/or L2 is a hydrazonato ligand, the f−f transition intensities increase to ∼30−120 M-1 cm-1 and the broad, higher energy bands develop significantly greater intensities (ε ≈ 15 000−20 000 M-1 cm-1). Finally, for the imido and ketimido complexes of U(IV), the f−f transition intensities increase to ∼50−400 M-1 cm-1, and the broad, higher energy bands continue to carry substantial intensity (ε ≈ 10 000−15 000 M-1 cm-1) while also extending to lower energy. This interesting trend in f−f transition intensity is interpreted in the context of an intensity-stealing mechanism from the charge-transfer excited states that reflects an enhanced degree of covalent interaction between the actinide metal center and L1/L2 for the nitrogen-donor ligand systems.