Ligand Control of Oxidation and Crystallographic Disorder in the Isolation of Hexavalent Uranium Mono-Oxo Complexes.

The development of high-valent transuranic chemistry requires robust methodologies to access and fully characterize reactive species. We have recently demonstrated that the reducing nature of imidophosphorane ligands supports the two-electron oxidation of U ⁴⁺ to U ⁶⁺ and established the use of this ligand to evaluate the inverse-trans-influence (ITI) in actinide metal-ligand multiple bond (MLMB) complexes. To extend this methodology and analysis to transuranic complexes, new small-scale synthetic strategies and lower-symmetry ligand derivatives are necessary to improve crystallinity and reduce crystallographic disorder. To this end, the synthesis of two new imidophosphorane ligands, [N═P ^t Bu(pip) ₂ ] ^- ( NPC ¹ ) and [N═P ^t Bu(pyrr) ₂ ] ^- ( NPC ² ) (pip = piperidinyl; pyrr = pyrrolidinyl), is presented, which break pseudo- C ₃ axes in the tetravalent complexes, U[NPC ¹ ] ₄ and U[NPC ² ] ₄ . The reaction of these complexes with two-electron oxygen-atom-transfer reagents (N ₂ O, trimethylamine N -oxide (TMAO) and 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene (dbabhNO)) yields the U ⁶⁺ mono-oxo complexes U(O)[NPC ¹ ] ₄ and U(O)[NPC ² ] ₄ . This methodology is optimized for direct translation to transuranic elements. Of the two ligands, the NPC ² framework is most suitable for facilitating detailed bonding analysis and assessment of the ITI. Theoretical evaluation of the U-(NPC) bonding confirms a substantial difference between axially and equatorially bonded N atoms, revealing markedly more covalent U-N _ax interactions. The U 6d + 5f combined contribution for U-N _ax is nearly double that of U-N _eq , accounting for ITI shortening and increased bond order of the axial bond. Two distinct N-atom hybridizations in the pyrrolidine/piperidine rings are noted across the complexes, with approximate sp ² and sp ³ configurations describing the slightly shorter P-N _"planar" and slightly longer P-N _"pyramidal" bonds, respectively. In all complexes, the NPC ² ligands feature more planar N atoms than NPC ¹ , in accordance with a higher electron-donating capacity of the former.