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 to U 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═PBu(pip)] () and [N═PBu(pyrr)] () (pip = piperidinyl; pyrr = pyrrolidinyl), is presented, which break pseudo- axes in the tetravalent complexes, and . The reaction of these complexes with two-electron oxygen-atom-transfer reagents (NO, trimethylamine -oxide (TMAO) and 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene (dbabhNO)) yields the U mono-oxo complexes and . This methodology is optimized for direct translation to transuranic elements. Of the two ligands, the 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 interactions. The U 6d + 5f combined contribution for U-N is nearly double that of U-N, 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 and sp configurations describing the slightly shorter P-N and slightly longer P-N bonds, respectively. In all complexes, the ligands feature more planar N atoms than , in accordance with a higher electron-donating capacity of the former.