Investigation Of The Electronic Ground States For A Reduced Pyridine(Diimine) Uranium Series: Evidence For A Ligand Tetraanion Stabilized By A Uranium Dimer

The electronic structures of a series of highly reduced uranium complexes bearing the redox-active pyridine(diimine) ligand, (PDIMe)-P-Mes ((PDIMe)-P-Mes = 2,6-(2,4,6-Me-3-C6H2-N-CMe)(2)C5H3N) have been investigated. The complexes, ((PDIUI3)-P-Mes-U-Me(THF) (1), ((PDIMe)-P-Mes)UI2(THF)(2) (2), [((PDIMe)-P-Mes)UI](2) (3), and [((PDIMe)-P-Mes)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 [(PDIMe)-P-Mes](center dot/-), [(PDIMe)-P-Mes](2-), [(PDIMe)-P-Mes](3-) and [(PDIMe)-P-Mes](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 eta(5) pyridyl ring.