Subtle Interactions and Electron Transfer between U^III, Np^III, or Pu^III and Uranyl Mediated by the Oxo Group

Abstract

A dramatic difference in the ability of the reducing An(III) center in AnCp3 (An=U, Np, Pu; Cp=C5 H5 ) to oxo-bind and reduce the uranyl(VI) dication in the complex [(UO2 )(THF)(H2 L)] (L="Pacman" Schiff-base polypyrrolic macrocycle), is found and explained. These are the first selective functionalizations of the uranyl oxo by another actinide cation. At-first contradictory electronic structural data are explained by combining theory and experiment. Complete one-electron transfer from Cp3 U forms the U(IV) -uranyl(V) compound that behaves as a U(V) -localized single molecule magnet below 4 K. The extent of reduction by the Cp3 Np group upon oxo-coordination is much less, with a Np(III) -uranyl(VI) dative bond assigned. Solution NMR and NIR spectroscopy suggest Np(IV) U(V) but single-crystal X-ray diffraction and SQUID magnetometry suggest a Np(III) -U(VI) assignment. DFT-calculated Hirshfeld charge and spin density analyses suggest half an electron has transferred, and these explain the strongly shifted NMR spectra by spin density contributions at the hydrogen nuclei. The Pu(III) -U(VI) interaction is too weak to be observed in THF solvent, in agreement with calculated predictions.

Transuranic Computational Chemistry

“…In one study,t hey reported the first selectivef unctionalizations of the uranyl O yl by another actinide cation, exploring the ability of the reducing An III centre in AnCp 3 (An = U, Np, Pu) to oxobind and reduce the uranyl VI dicationi nt he complex[ (UO 2 )(THF)(H 2 L)] (L = "Pacman" Schiff-base polypyrrolic macrocycle). [16] The reactions attempted are summarized in Figure 4; clearly,u ranium, neptunium and plutonium behaver ather differently from one another in this context. Arnold et al write "not all of the experimental characterization data agree on the extent of electron transfer from the organometallic actinide to the uranyl group".…”

Future Directions for Transuranic Single Molecule Magnets

“…The effect on the non-Kramers Np III ion could be even worse. For instance, reductive oxo-metalation of uranyl complexes by UCp 3 affords a U IV -U V hetero-valent dimer that behaves as a SMM below 4 K; on the other hand, the same reaction by NpCp 3 gives an isostructural donor-acceptor oxo bridged Np III -U VI dimer with magnetic properties closely matching those of an isolated Np III ion but no slow relaxation [36]. Similarly disappointing results have been reported for a Np III organometallic compound with a bis(arene)-type geometry that exhibits slow relaxation of the magnetization only well below 2 K, despite a favourable ligand arrangement [37].…”

Officer of the Order of the British Empire: P. L. Arnold / Robert T. Koch Gold Medal: C. T. Walsh / Inhoffen Medal: H. Wennemers / OMCOS Award: Rubén Martín

“…Arnold's research is focused on the design and synthesis of highly reactive complexes of f‐block elements that can activate inert small molecules such as carbon oxides and hydrocarbons. She has reported in Angewandte Chemie on uranium C−H agostic bonds, and on the reduction of uranyl oxo species by other actinide cations …”