Step by Step Assembly of Polynuclear Lanthanide Complexes with a Phosphonated Bipyridine Ligand Nabila Souri, †,‡ Pingping Tian, † Alexandre Lecointre, † Zoe ́ Lemaire, † Salah Chafaa, ‡ Jean-Marc Strub, § Sarah Cianfe ́ rani, § Mourad Elhabiri, ∥ Carlos Platas-Iglesias, ⊥ and Loïc J. Charbonnie ̀ re* ,† † Laboratoire d’Ingé nierie Mole ́ culaire Applique ́ ea ̀ l’Analyse, Institut Pluridisciplinaire Hubert Curien (IPHC, UMR 7178), CNRS-Universite ́ de Strasbourg,, ECPM, 25, rue Becquerel, 67087 Strasbourg Cedex, France ‡ Laboratoire d’é lectrochimie des Mate ́ riaux Mole ́ culaires et des Complexes, LEMMC, Faculte ́ de Technologie, Universite ́ de Sé tif-1, Sé tif, Algeria § Laboratoire de Spectrome ́ trie de Masse Bio-Organique, IPHC, UMR 7178, CNRS-Universite ́ de Strasbourg, ECPM, 25, rue Becquerel, 67087 Strasbourg, France ∥ Laboratoire de Chimie Bioorganique et Me ́ dicinale, UMR 7509, CNRS-Universite ́ de Strasbourg, ECPM, 25, rue Becquerel, 67087 Strasbourg Cedex 02, France ⊥ Universidade da Coruñ a, Centro de Investigació ns Científicas Avanzadas (CICA) and Departamento de Química Fundamental, Facultade de Ciencias, 15071 A Coruñ a, Galicia, Spain * S Supporting Information ABSTRACT: The synthesis of the octadentate ligand L (LH 8 = ((([2,2 ′ -bipyridine]-6,6 ′ -diylbis(methylene))bis- (azanetriyl))tetrakis(methylene))tetrakis(phosphonic acid)) is reported. The coordination of L with various lanthanide cations was monitored by absorption and luminescence spectrophotometric titration experiments (Ln = Tb, Yb), potentiometry (Ln = La, Eu, Lu), and mass spectrometry (Ln = Tb). It was found that L forms very stable mononuclear (LnL) species in aqueous solutions (log K = 19.80(5), 19.5(2), and 19.56(5) for La, Eu, and Lu, respectively) with no particular trend along the series. Spectroscopic data showed the Ln cations to be enclosed in the cavity formed by the octadentate ligand, thereby shielding the metal from interactions with water molecules in the first coordination sphere. When more than one equivalent of cations is added, the formation of polynuclear [(LnL) 2 Ln x ] complexes (x =1−3) can be observed, the presence of which could be confirmed by electrospray and MALDI mass spectrometry experiments. DFT modeling of the mononuclear (LnL) complexes indicated that the coordination of the cation in the cavity of the ligand results in a very asymmetric charge distribution, with a region of small negative electrostatic potential on the hemisphere composed of the chromophoric bipyridyl moiety and an electron-rich domain at the opposite hemisphere around the four phosphonate functions. DFT further showed that this polarization is most likely at the origin of the strong interactions between the (LnL) complexes and the incoming additional cations, leading to the formation of the polynuclear species. 1 H and 31 P NMR were used to probe the possible exchange of the lanthanide complexed in the cavity of the ligand in D 2 O, revealing no detectable exchange after 4 weeks at 80 °C and neutral pD, therefore pointing out an excellent kinetic inertness. ■ INTRODUCTION Phosphonated ligands have long been shown to be excellent chelators for the coordination of the hard Lewis acid cations of the lanthanide (Ln) series. 1 Their second pK a values are close to neutrality in aqueous solutions, 2 affording strong electro- static interactions with the trivalent Ln cations. In comparison to carboxylate analogues, phosphonates are bulkier and bring larger steric hindrance. 3 For luminescent applications, 4 this property is particularly important with respect to the necessity of shielding the luminescent Ln cations from solvent molecules, particularly those of water, at the origin of important losses in luminescence efficiency by nonradiative vibrational quenching. 5 In polyamino-functionalized ligands, replacement of carbox- ylates by phosphonates generally leads to a decrease in the number of water molecules coordinated in the first sphere of Ln. 6 Although it is beneficial for the luminescence properties, this water displacement has been shown to be detrimental for the corresponding Gd complexes used as contrast agents for nuclear magnetic resonance imaging such as in the case of GdDOTP, lacking water molecules in the first coordination sphere. 7 Phosphonate functions also display a very versatile Received: October 4, 2016 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.6b02414 Inorg. Chem. XXXX, XXX, XXX−XXX