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 U⁴⁺ to U⁶⁺ 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^tBu(pip)₂]^- (NPC¹) and [N═P^tBu(pyrr)₂]^- (NPC²) (pip = piperidinyl; pyrr = pyrrolidinyl), is presented, which break pseudo-C₃ axes in the tetravalent complexes, U[NPC¹]₄ and U[NPC²]₄. The reaction of these complexes with two-electron oxygen-atom-transfer reagents (N₂O, trimethylamine N-oxide (TMAO) and 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene (dbabhNO)) yields the U⁶⁺ mono-oxo complexes U(O)[NPC¹]₄ and U(O)[NPC²]₄. This methodology is optimized for direct translation to transuranic elements. Of the two ligands, the NPC² 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 sp² and sp³ configurations describing the slightly shorter P-N_"planar" and slightly longer P-N_"pyramidal" bonds, respectively. In all complexes, the NPC² ligands feature more planar N atoms than NPC¹, in accordance with a higher electron-donating capacity of the former.

catena-phosphorus cations

Recent advances towards a systematic development of catena-phosphorus cations are reviewed. The cations represented in this new and developing chapter in fundamental phosphorus chemistry complement the series of neutral and anionic polyphosphorus compounds.

Acyclic catena-Diphosphinodiphosphonium Dications [R3P-PR‘-PR‘-PR3]2+ or Bisphosphine−Diphosphenium Complexes [R3P→PR‘-PR‘←PR3]2+: Synthesis by Reductive P−P Coupling of [R3P-PR‘Cl]+ and Phosphine Ligand Exchange

The acyclic tetraphosphorus dication [Ph3P-PPh-PPh-PPh3]2+ has been formed by the reductive coupling of [Ph3P-PPhCl]+, providing a new synthetic method for the systematic development of catena-phosphorus cations. Ligand exchange (Ph3P for Me3P) gives [Me3P-PPh-PPh-PMe3]2+, implicating these dications as bisphosphine-diphosphenium complexes.