Derivatives of a novel pyrrole-containing Schiff base ligand system (called “pyrrophen”) are presented which feature substituted phenylene linkers (R1 = R2 = H (H 2 L 1 ); R1 = R2 = CH3 (H 2 L 2 )) and a binding pocket modeled after macrocyclic species. These ligands bind neutral CH3OH in the solid state through pyrrolic hydrogen-bonding. The interaction of the uranyl cation (UO2 2+) and H 2 L 1–2 yields planar hexagonal bipyramdial uranyl complexes, while the Cu2+ and Zn2+ complexes were found to self-assemble as dinuclear helicate complexes (M2L2) with H 2 L 1 under identical conditions. The favorable binding of UO2 2+ over Zn2+ provides insight into the molecular recognition of uranyl over other metal species. Structural features of these complexes are examined with special attention to features of the UO2 2+ coordination environment which distinguish them from other related salophen and porphyrinoid complexes.

The synthesis and characterization of a new ligand system combining the redox-active backbone of Ar-BIANs (N,N′bis[(aryl)imino]acenaphthenes) and a mixed-donor O−N−N−O salentype binding pocket is reported. Complexes of Co 2+ , Ni 2+ , and UO 2 2+ were prepared and characterized through single-crystal X-ray diffraction and electrochemical studies. The Ni 2+ and Co 2+ complexes have been used as references against which to compare the unique behaviors exhibited by the uranyl (UO 2 2+) complex, as the latter forms two distinct solid-state structures with unusual oxo contacts to CH 2 Cl 2 and CHCl 3 and displays a rich electrochemical profile that indicates a wide range of accessible metal oxidation states through the formation of mixed-valent U(VI)/U(V) and U(V)/U(IV) species in solution.

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.

The study of the redox chemistry of mid‐actinides (U−Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non‐aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=PtBu(pyrr)2]−; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium (1‐M, 2‐M, M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U5+/4+, U6+/5+, and to an unprecedented, well‐behaved Np5+/4+ redox couple. The differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory.

Uranyl complexes of aryl-substituted α-diimine ligands gbha (UO 2 -1a−f) and phen-BIAN (UO 2 -2a-f) [gbha (1) = glyoxal bis(2hydroxyanil); phen-BIAN (2) = N,N′-bis(iminophenol)acenaphthene; R = OMe (a), t-bu (b), H (c), Me (d), F (e), and naphthyl (f)] were designed, prepared, and characterized by X-ray diffraction, FT-IR, NMR, UV−vis, and electrochemical methods. These ligand frameworks contain a salen-type O− N−N−O binding pocket but are redox-noninnocent, leading to unusual metal complex behaviors. Here, we describe three solid-state structures of uranyl complexes UO 2 -1b, UO 2 -1c, and UO 2 -1f and observe manifestations of ligand noninnocence for the U(VI) complexes UO 2 -1b and UO 2 -1c. The impacts of accessible π-systems and ligand substitution on the axial uranium−oxo interactions were evaluated spectroscopically via the intraligand charge-transfer (ILCT) processes that dominate the absorption spectra of these complexes and through changes to the asymmetric (ν 3 ) OUO stretching frequency. This, in combination with electrochemical data, reveals the effects of the inclusion of the conjugated acenaphthene backbone and the importance of ligand electronic structure on uranyl's bonding interactions.