Harnessing redox activity for the formation of uranium tris(imido) compounds

Abstract

Classically, late transition-metal organometallic compounds promote multielectron processes solely through the change in oxidation state of the metal centre. In contrast, uranium typically undergoes single-electron chemistry. However, using redox-active ligands can engage multielectron reactivity at this metal in analogy to transition metals. Here we show that a redox-flexible pyridine(diimine) ligand can stabilize a series of highly reduced uranium coordination complexes by storing one, two or three electrons in the ligand. These species reduce organoazides easily to form uranium-nitrogen multiple bonds with the release of dinitrogen. The extent of ligand reduction dictates the formation of uranium mono-, bis- and tris(imido) products. Spectroscopic and structural characterization of these compounds supports the idea that electrons are stored in the ligand framework and used in subsequent reactivity. Computational analyses of the uranium imido products probed their molecular and electronic structures, which facilitated a comparison between the bonding in the tris(imido) structure and its tris(oxo) analogue.

Elucidating bonding preferences in tetrakis(imido)uranate(VI) dianions

“…Thus, this analysis supports that the near-linear nature of the U-N-C angles in 2-Na, 2-Rb and 2-Cs is not reflective of an enhanced π bonding that would contribute to triple-bond formation. These values are lower than those calculated for the uranium tris(imido), ( Mes PDI Me )U(NMes) 3 (2.18 axial and 2.05 equatorial), which are characterized as U-N double bonds with some triple-bond character 17 . The bond orders in 2-Li to 2-Cs are also significantly lower than those in U(NPh) 2 I 2 (THF) 3 , as established by DFT analysis to feature bonding composed of "six orbitals with strong interactions between the uranium centre and the nitrogen ligands, indicating the presence of two triple bonds" 7 .…”

“…This family of hexavalent pyridine(diimine) derivatives, ( Mes PDI Me )U(NR) 3 ( Mes PDI Me =2,6-(2,4,6-Me 3 C 6 H 2 N=CMe) 2 C 5 H 3 N; R=Mes (2,4,6-trimethylphenyl); DIPP=2,6-diisopropylphenyl) 17 , displays a T-shaped arrangement of the imido substituents. In these derivatives, the bond distances along the trans-N ax =U=N ax (ax, axial) axis (1.992(5) Å) are contracted with respect to the corresponding distance for the equatorial imido substituent (2.024(5) Å), which is expected based on the inverse trans influence found in actinide bonding.…”

Thorium–phosphorus triamidoamine complexes containing Th–P single- and multiple-bond interactions

“…This desire stems not only from a need to develop our knowledge and understanding of actinide bonding at a fundamental level45678, but also because of potential applications in nuclear waste clean-up which might utilize extraction methods that exploit covalency differences in metal-ligand bonding9. Because actinide-ligand multiple bonds arguably contain the greatest opportunities to probe covalency101112, in recent years in addition to numerous oxo complexes12 there has been intensive progress in uranium-carbene13141516171819, -imido202122232425262728, -nitride29303132, -phosphinidene/-arsinidene/-arsenido33343536 and -chalcogenido (S, Se, Te) chemistry37383940. This work demonstrates the wide range of multiply bonded ligands that can be stabilised at uranium with appropriate supporting ligands, and also that softer, heavier main group element centres can be stabilised as well as relatively hard first-row elements such as C, N and O.…”

Synthesis, Characterization, and Stoichiometric U–O Bond Scission in Uranyl Species Supported by Pyridine(diimine) Ligand Radicals

“…Refinement of the data revealed 5 to be a seven-coordinate, pentagonal bipyramidal uranium species with trans-trimethylsiloxide ligands, cis-iodides, and a tridentate pyridine(diimine) (Figure 7, structural parameters in Table 1). The U−O distances of 2.091(10) and 2.098(10) Å are on the order of those for a uranium(IV) species with siloxide ligands, similar to [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 UI(OSiMe 3 ) (2.104(4) Å), 37 17 supporting a dative coordination of the chelate. Thus, the ligand radical is no longer present, as confirmed by structural parameters, nor were additional protons noted on the pyridine ring.…”