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, H-1 and P-31 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 (tau(D2o) = 10.2 mu s) and quantum yield (Phi(D2O) = 0.42%). In the presence of excess Ln(III) cation, polynuclar complexes of [(LnL)(2)Ln(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)(9)(3+) cation and the [LaL](3)-complex.