A Redox-Innocent Uranium(IV)-Quinoid Metal-Organic Framework

MOFs with 12-Coordinate 5f-Block Metal Centers

We have constructed an unprecedented MOF platform that accommodates a range of 5f-block metal ions (Th⁴⁺, U⁴⁺, Np⁴⁺, Pu⁴⁺) as the primary building block. The isoreticular actinide metal-organic frameworks (An-MOFs) exhibit periodic trends in the 12-coordinate metal environment, ligand configuration, and resulting ultramicroporosity. It holds potential in distinguishing neighboring tetravalent actinides. The metal ionic radius, carboxylate bite angle, anthracene plane twisting, interligand interactions, and countercation templating collectively determine an interplay between solvation, modulation, and complexation, resulting in a coordination saturation of the central actinide, while lanthanide counterparts are stabilized by the formation of a dimer-based motif. Quantum chemical calculations indicate that this large coordination number is only feasible in the high-symmetry environment provided by the An-MOFs. This category of MOFs not only demonstrates autoluminescence (4.16 × 10⁴ counts per second per gram) but also portends a wide-bandgap (2.84 eV) semiconducting property with implications for a multitude of applications such as hard radiation detection.

How the Ancillary Ligand X Drives the Redox Properties of Biscyclopentadienyl Pentavalent Uranium Cp2U(═N-Ar)X Complexes

Relativistic zero order regular approximation (ZORA) density functional theory computations, coupled with the conductor-like screening model for solvation effects, are used to investigate the redox properties of a series of biscyclopentadienyl pentavalent uranium(V) complexes Cp₂U(═N-Ar)X (Ar = 2,6-Me₂-C₆H₃; X = OTf, C₆F₅, SPh, C═CPh, NPh₂, Ph, Me, OPh, N(TMS)₂, N═CPh₂). Regarding the U^V/U^IV and U^VI/U^V couple systems, a linear correlation (R² ∼ 0.99) is obtained at the ZORA/BP86/TZP level, between the calculated ionization energies and the measured experimental E_1/2 half-wave oxidation potentials (U^VI/U^V) and between the electron affinities and the reduction potentials E_1/2 (U^V/U^IV). The study brings to light the importance of solvation effects that are needed in order to achieve a good agreement between the theory and experiment. Introducing spin-orbit coupling corrections slightly improves this agreement. Both the singly occupied molecular orbital and the lowest unoccupied molecular orbital of the neutral U^V complexes exhibit a majority 5f orbital character. The frontier molecular orbitals show a substantial ancillary ligand X σ and/or π character that drives the redox properties. Moreover, our investigations allow estimating the redox potentials of the X = Ph, X = C₆F₅, and N(TMS)₂ U^V complexes for which no experimental electrochemical data exist.