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

Abstract

A series of pyridine dipyrrolide actinide(IV) complexes, ( Mes PDP Ph )AnCl 2 (THF) and An( Mes PDP Ph ) 2 (An = U, Th, where ( Mes PDP Ph ) is the doubly deprotonated form of 2,6bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine), have been prepared. Characterization of all four complexes has been performed through a combination of solid-and solution-state methods, including elemental analysis, single crystal X-ray diffraction, and electronic absorption and nuclear magnetic resonance spectroscopies. Collectively, these data confirm the formation of the mono-and bis-ligated species. Time-dependent density functional theory has been performed on all four An(IV) complexes, providing insight into the nature of electronic transitions that are observed in the electronic absorption spectra of these compounds. Room temperature, solution-state luminescence of the actinide complexes is presented. Both Th(IV) derivatives exhibit strong photoluminescence; in contrast, the U(IV) species are nonemissive.

Ligand–Metal Complementarity in Rare-Earth and Actinide Chemistry

“…Experimental methods to probe these properties exist, such as X-ray absorption spectroscopy, but luminescence spectroscopy combined with DFT calculations is a convenient laboratory-scale method to probe the nature of ligand-to-metal or metal-to-ligand charge-transfer events and hence begin to deconvolute the nature of the metal–ligand interaction. In their contribution, Milsmann’s and Matson’s groups explored the use of pyridine dipyrrolide ligands on both Th IV and U IV . While both heteroleptic [Th­( Mes PDP Ph )­(Cl) 2 (THF)] ( Mes PDP Ph = {NC 6 H 3 -2,6-(NC 4 H-3-Ph-5-Mes) 2 }) and homoleptic [Th­( Mes PDP Ph ) 2 ] are highly luminescent, the U IV analogues are not, which is attributed to self-quenching due to ligand-to-metal charge transfer and transitions within the 5f manifold by analogy with d-block analogues with this ligand system …”

“…In their contribution, Milsmann’s and Matson’s groups explored the use of pyridine dipyrrolide ligands on both Th IV and U IV . While both heteroleptic [Th­( Mes PDP Ph )­(Cl) 2 (THF)] ( Mes PDP Ph = {NC 6 H 3 -2,6-(NC 4 H-3-Ph-5-Mes) 2 }) and homoleptic [Th­( Mes PDP Ph ) 2 ] are highly luminescent, the U IV analogues are not, which is attributed to self-quenching due to ligand-to-metal charge transfer and transitions within the 5f manifold by analogy with d-block analogues with this ligand system …”