The electronic structures of a series of highly reduced uranium complexes bearing the redox-active pyridine(diimine) ligand, MesPDIMe (MesPDIMe = 2,6-(2,4,6-Me3-C6H2-N=CMe)2C5H3N) have been investigated. The complexes, (MesPDIMe)UI3(THF) (1), (MesPDIMe)UI2(THF)2 (2), [(MesPDIMe)UI]2 (3), and [(MesPDIMe)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 5f2 configurations and reduced ligand frameworks, specifically [MesPDIMe]•/-, [MesPDIMe]2-, [MesPDIMe]3- and [MesPDIMe]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 η5-pyridyl ring. (Graph Presented).
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© 2015 American Chemical Society.