Investigation of the Electronic Ground States for a Reduced Pyridine(diimine) Uranium Series: Evidence for a Ligand Tetraanion Stabilized by a Uranium Dimer

Abstract

The electronic structures of a series of highly reduced uranium complexes bearing the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-(2,4,6-Me3-C6H2-N═CMe)2C5H3N) have been investigated. The complexes, ((Mes)PDI(Me))UI3(THF) (1), ((Mes)PDI(Me))UI2(THF)2 (2), [((Mes)PDI(Me))UI]2 (3), and [((Mes)PDI(Me))U(THF)]2 (4), were examined using electronic and X-ray absorption spectroscopies, magnetometry, and computational analyses. Taken together, these studies suggest that all members of the series contain uranium(IV) centers with 5f (2) configurations and reduced ligand frameworks, specifically [(Mes)PDI(Me)](•/-), [(Mes)PDI(Me)](2-), [(Mes)PDI(Me)](3-) and [(Mes)PDI(Me)](4-), respectively. In the cases of 2, 3, and 4 no unpaired spin density was found on the ligands, indicating a singlet diradical ligand in monomeric 2 and ligand electron spin-pairing through dimerization in 3 and 4. Interaction energies, representing enthalpies of dimerization, of -116.0 and -144.4 kcal mol(-1) were calculated using DFT for the monomers of 3 and 4, respectively, showing there is a large stabilization gained by dimerization through uranium-arene bonds. Highlighted in these studies is compound 4, bearing a previously unobserved pyridine(diimine) tetraanion, that was uniquely stabilized by backbonding between uranium cations and the η(5)-pyridyl ring.

Uranium(III)-carbon multiple bonding supported by arene δ-bonding in mixed-valence hexauranium nanometre-scale rings

“…Samples were collected on two separate beam lines to ensure consistency. The sample spectra have strong white lines and are similar in shape to the UO 2 and uranium(III) standards and other uranium(III) and uranium(IV) L 3 -edge XANES described in the literature 67 – 69 . Complexes 3 and 4 lack a shoulder feature on the high energy side of the white line indicating the absence of uranium(V) or uranium(VI), since a shoulder feature would be indicative of multiple scattering along the trans-dioxo moiety 67 .…”

Inner-sphere vs. outer-sphere reduction of uranyl supported by a redox-active, donor-expanded dipyrrin

“…U( iii ) and U( iv ), of classical redox-active ligands such as pyridine di-imines (PDI), bipyridines, amidophenolates, and α-di-imines dominate and display reactivity in which ligand-centred redox processes are implicit. 6 – 15 In contrast, compounds of higher oxidation state uranium e.g. uranyl( vi )/( v ) with redox-active ligands are rare.…”

Spectroscopic and Structural Elucidation of Uranium Dioxophenoxazine Complexes

“…A small amount of bibenzyl was noted by 1 H NMR analysis, but the product was pure enough to use without further purification. Analysis for C 32 Preparation of (DOPO q ) 2 UO 2 (1). A 20 mL scintillation vial was charged with UO 2 [N(SiMe 3 ) 2 ] 2 (THF) 2 (0.050 g, 0.069 mmol) and 5 mL of THF.…”

“…The room temperature moment of 2.87 μ B was consistent with reported magnetic data for uranium(IV) complexes with ligand radicals. 25,32 It is well established that μ ef f decreases monotonically with temperature in the cases of U(IV) cations in low symmetry ligand fields, due to the depopulation of the nine ligand field levels for the ion. 33−35 The data for 2 showed this type of behavior, consistent with its assignment as a uranium(IV) complex ( Figure 2).…”