Probing the nature of uranium-ligand multiple bonds
In 2005, Los Alamos researchers Trevor Hayton, James Boncella, Brian Scott, Phillip Palmer, Enrique Batista, and P. Jeffrey Hay reported the synthesis and characterization of two different uranium(VI) bis-imido species, U(NtBu)2I2(thf)2 and U(NPh)2I2(thf)3, (1) and (2), respectively, in the illustration on the next page. Density functional theory calculations performed on these early structures displayed striking similarities to the uranyl species, trans-UO2X4, including the presence of a triple bond between the uranium (U) and nitrogen (N) atoms, similar to that between the uranium and oxygen atoms of uranyl.
However, the ordering of the energies for these two sigma (σ) and four pi (π) bonds is altered in the imido species, compared to those of the oxo species. Furthermore, the imido species are predicted to display more covalency in the U–N multiple bond than the analogous uranyl complexes because of the difference in electronegativity values between nitrogen and oxygen. I am exploring the novel reactivity observed with various bis-imido species due to these two factors.
Initially the synthetic route to 1 was explored, as it had only proven to work in this specific case. To test whether this was specific to the starting material, I substituted methylamine in place of tert-butylamine. Instead of yielding the expected bis-methylimido species, a uranium(VI)bis-imido cation (3) was isolated with a tri-iodide counterion, the first positively charged version of this species to be characterized. It was discovered that the tert-butyl version of the cation (4) could also be synthesized from the simple addition of one equivalent elemental I2 to the neutral bis-imido species.
The reactivity of the methyl homologue appears to be rather constrained, being greatly limited by the sensitivity of the tri-iodide anion. However, the ability to replace the associated tetrahydrofuran molecules with other Lewis bases has been demonstrated, as has the replacement of the I3- ions with PF6-, although the final product has proved elusive in the crystalline state. The methyl cation has also been used in the synthesis of the tetrachloro-bis-methylimido dianion (5), the first of the methylimido species to be isolated without an associated I3-.
I am also interested in exploring the properties of the RN=U=NR moieties themselves. To do this, large, multi-dentate organic molecules with various charges are being used to perform two important functions. First, by using a molecule with a 1- or 2- charge, the reactive iodides can be removed from the metal center. Second, by using a large, sometimes bulky organic group, the equatorial region around the metal center can be almost completely occupied, preventing it from reacting with other species that may exist in solution.
In reacting, the protonated version one of these multidentate ligands with the bis-tert-butylimido species, a uranium(V) cationic species was isolated without any imido ligands present (6). This tandem protonolysis/reduction reaction was also observed with the same uranium starting material in the presence of phenol. Not only does this reaction present a unique means of reducing the metal center through the oxidation of iodide, it also allows for an easy entry into uranium(V) chemistry, a rather elusive and unstable oxidation state for this metal center.