Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes

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.