Formation of Mono- and Polynuclear Luminescent Lanthanide Complexes based on the Coordination of Preorganized Phosphonated Pyridines

Abstract

A series of polynuclear assemblies based on ligand L (1,4,7-tris[hydrogen (6-methylpyridin-2-yl)phosphonate]-1,4,7-triazacyclononane) has been developed. The coordination properties of ligand L with Ln^III (Ln = La, Eu, Tb, Yb, Lu) have been studied in water (pH = 7.0) and in D₂O (pD = 7.0) by UV-absorption spectrometry, spectrofluorimetry, ¹H and ³¹P NMR, DOSY, ESI-mass spectrometry, and X-ray diffraction. This nonadentate ligand forms highly stable mononuclear complexes in water and provides a very efficient shielding of the Ln cations, as emphasized by the very good luminescence properties of the Yb complex in D₂O, especially regarding its lifetime (τ_D₂O = 10.2 μs) and quantum yield (ϕ_D₂O = 0.42%). In the presence of excess Ln^III cation, polynuclar complexes of [(LnL)₂Ln _x] stoichiometry (x = 1 and x = 2) are observed in solution. In the solid state, a dinuclear complex of La could be isolated and structurally characterized by X-ray diffraction, unraveling the presence of strong hydrogen bonding interactions between a La(H₂O)₉³⁺ cation and the [LaL]^3- complex.