Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes

Abstract

The first actinide complexes of the pyridine dipyrrolide (PDP) ligand class, ( Mes PDP Ph )UO 2 (THF) and ( Cl2Ph PDP Ph )UO 2 (THF), are reported as the U VI uranyl adducts of the bulky aryl substituted pincers ( Mes PDP Ph ) 2– and ( Cl2Ph PDP Ph ) 2– (derived from 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1 H -pyrrol-2-yl)pyridine (H 2 Mes PDP Ph , Mes = 2,4,6-trimethylphenyl), and 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1 H -pyrrol-2-yl)pyridine (H 2 Cl2Ph PDP Ph , Cl 2 Ph = 2,6-dichlorophenyl), respectively). Following the in situ deprotonation of the proligand with lithium hexamethyldisilazide to generate the corresponding dilithium salts (e.g., Li 2 Ar PDP Ph , Ar = Mes of Cl 2 Ph), salt metathesis with [UO 2 Cl 2 (THF) 2 ] 2 afforded both compounds in moderate yields. The characterization of each species has been undertaken by a combination of solid- and solution-state methods, including combustion analysis, infrared, electronic absorption, and NMR spectroscopies. In both complexes, single-crystal X-ray diffraction has revealed a distorted octahedral geometry in the solid state, enforced by the bite angle of the rigid meridional ( Ar PDP Ph ) 2– pincer ligand. The electrochemical analysis of both compounds by cyclic voltammetry in tetrahydrofuran (THF) reveals rich redox profiles, including events assigned as U VI /U V redox couples. A time-dependent density functional theory study has been performed on ( Mes PDP Ph )UO 2 (THF) and provides insight into the nature of the transitions that comprise its electronic absorption spectrum.

Synthesis and Characterization of Isostructural Th(IV) and U(IV) Pyridine Dipyrrolide Complexes

“…A significant increase in bandwidth results in an extension of absorption to ∼600 nm, translating to the dark-red color observed visually for complex 1a . This unique feature is reminiscent of a ligand-to-metal charge transfer (LMCT) transition measured experimentally and confirmed by time-dependent density functional theory (TD-DFT) in ( Mes PDP Ph )­UO 2 (THF) . An analysis of the near-infrared region of the electronic absorbance spectrum of 1a also revealed weak and sharp f–f transitions, consistent with the assignment of a +4 oxidation state (5f 2 valence electron configuration) of uranium.…”

“…Closer inspection of the integrations in the 1 H NMR spectrum of ( Mes PDP Ph )­UCl 2 (THF) reveals a resonance at −4.75 ppm with a relative integration of one and is assigned to the 4-pyridyl proton. This particular signal is typically used as a spectroscopic benchmark for PDP ligands bound to transition metal and metalloid centers. , Moreover, a diagnostic singlet is observed at −2.79 ppm and is attributed to the 4-pyrrolide hydrogens. The highest-intensity signal in the spectrum is located at 9.75 ppm, with a relative integration of 12.…”

Speciation and Photoluminescent Properties of a 2,6‐Bis(pyrrol‐2‐yl)pyridine in Three Protonation States

“…Going beyond transition metal complexes, main group compounds with PDP ligands were first reported by Turner [29] in 2019 and have subsequently been shown to possess unique reactivity and catalytic activity [30–32] . In 2022, further expansion into the f‐block elements was accomplished through the report of the first uranium PDP complexes [33,34] …”

“…[30][31][32] In 2022, further expansion into the f-block elements was accomplished through the report of the first uranium PDP complexes. [33,34] A particular focus of our ongoing studies of transition metal and main group PDP compounds are their remarkable photophysical and photochemical properties. After the initial discovery of long-lived and highly quantum-efficient photoluminescence in Zr(PDP) 2 complexes, [35] we were able to demonstrate that the emission emanates from excited states with significant ligand-to-metal charge transfer (LMCT) character by thermally activated delayed fluorescence (TADF) rather than phosphorescence.…”

Quantitative UO bond activation in uranyl complexes via silyl radical transfer

“…14,15 Previously, our research group has reported the synthesis and characterization of a uranyl complex supported by a sterically encumbered pyridine dipyrrolide (PDP) ligand, ( Mes PDP Ph )UO 2 (THF) (Scheme 1). 16 This compound exhibits a rich electrochemical profile, including a reversible U VI/V couple centred at À1.22 V (vs. Fc +/0 in THF with 0.1 M [ n Bu 4 N][PF 6 ] as supporting electrolyte), suggesting it would make an excellent candidate for the investigation of electron coupled silylium ion transfer for UQO bond activation. Addition of 1 equiv.…”