A Water Stable U(IV) Complex Based on a Macrocyclic Ligand Displaying NIR Emission

Complexation of U(IV) by encapsulation into the cavity of a 1,4,7-triazacyclonane (tacn) macrocycle functionalized by phosphonated pyridyl arms affords a water-stable and soluble [U(IV)L] complex. The coordination process was monitored by electronic UV-Vis-NIR absorption spectroscopy and the corresponding complex was characterized by ¹H-, ¹³C- and ³¹P-NMR spectroscopy and by mass spectrometry. Density functional theory (DFT) modeling shows the cation to be embedded into the cavity of the nonadentate ligand, as confirmed by analysis of the paramagnetic contributions of U(IV) on the chemical shifts of the proton nuclei. This effective chelation enabled the observation of a broad NIR emission band centered at 1080 nm observed for the first time in addition to UV-visible emission arising from the U(IV) cation upon electronic excitation in the UV-blue region. The [U(IV)L] complex was shown to be hydrolytically stable for several weeks in aqueous solutions over a broad range of pH values (from 1.6 to 10) under aerobic conditions.