Functionalized Triptycene-Derived Tripodal Ligands: Privileged Formation of Tetranuclear Cage Assemblies with Larger Ln(III) Alexandra Vuillamy, † Soumaila Zebret, ‡ Ce ́ line Besnard, § Virginie Placide, † Ste ́ phane Petoud, † and Josef Hamacek* ,† † CBM-CNRS Orlé ans, Rue Charles Sadron, 45071 Orle ́ ans Cedex 2, France ‡ Department of Inorganic and Analytical Chemistry, University of Geneva, 30 quai Ernest-Ansermet, 1211 Geneva 4, Switzerland § Laboratory of Crystallography, University of Geneva, 24 quai Ernest-Ansermet, 1211 Geneva 4, Switzerland * S Supporting Information ABSTRACT: In this Article, we report the self-assembly of lanthanide complexes formed with two new tripodal ligands, L2 and L3, where binding strands are connected to a rigid triptycene anchor. The pyridine moieties are functionalized with methoxy and PEG groups to enhance ligand solubility and to evaluate the effect of these substituents on lanthanide coordination. These ligands were successfully synthesized and characterized, and their coordination properties were examined along the lanthanide series through speciation studies with NMR and ESI-MS. Well-defined tetranuclear complexes are formed with both ligands, but their stabilities with heavier lanthanides are considerably reduced, especially for complexes with L3. This is attributed to a destabilizing effect of pending PEG arms in combination with increased steric hindrance between binding strands upon complexation with smaller cations. The sensitization of lanthanide luminescence in tetranuclear complexes occurs despite one water molecule being coordinated to a metal ion. ■ INTRODUCTION The introduction of supramolecular concepts in the 90s had important consequences on the development of new nanoma- terials for various applications because relatively complex systems can be obtained from simple building blocks by spontaneous self-organization. 1 Behind this new semantics, we often deal with coordination chemistry governed by classical thermodynamic principles, 2 whose control and programming on the molecular level allow self-organized structures with specific microscopic and macroscopic properties to be conceived and prepared. 3 Indeed, metal ions incorporated in such assemblies very often act as building elements responsible for targeted properties. Starting from relatively simple 1D and 2D arrangements of metal ions, the interest of chemists has progressively moved toward more challenging 3D assemblies as precursors for molecular functional devices and nanomachines. 4,5 In this context, transition metal ions such as ruthenium, platinum, palladium, gallium, etc. have been extensively used because of the relative kinetic inertness of the complexes that they form, allowing for the creation of stable self-assembled structures. 6 The same evolution is encountered for lanthanide complexes that are, however, kinetically more labile. After initial investigations of Ln(III) complexes with linear or 2D topologies, 7 the self-assembly of 3D structures has been extensively studied over the past several years. Our research mainly deals with helicoidal assemblies, where the building blocks are tripodal organic receptors. Their design plays a crucial role in programming self-assemblies with Ln(III) in order to match their coordination preferences. Chemical structures of such ligands consist of three metal binding moieties attached to a suitable triamine anchor, which is sufficiently rigid to avoid the complexation of all three strands to the same metal ion. A series of polynuclear complexes has been built with ligands where pyridinedicarboxamide binding moieties were attached to 1,1,1-trisaminomethylethane (TAME), a short aliphatic anchor. The resulting lanthanide complexes are tetra-, 8,9 penta-, 10 and octanuclear, 11 but the central structurally identical tetrametallic cluster represents a common feature of these nanoobjects. Interestingly, the complexes exhibit the same chirality for each metal ion, but they exist as racemic mixtures. In addition, cage tetranuclear complexes were obtained using 1,1,1-trishydroxymethylethane (THME) and undergo host-guest interactions with anions. 12 Highly symmetrical arrangements may also have synergic effects on optical properties, e.g., on emission intensity or energy transfer. 13 In search of a suitable rigid aromatic spacer, we have resorted to triamino-triptycene. Its coupling with dicarbonylpyridine moieties results in ligand L1, and the investigation of its Eu(III) tetranuclear complexes was described in a previous communication. 14 Interestingly, in comparison with ligands derived from aliphatic spacers, the excitation wavelengths is shifted toward the visible and an Received: December 2, 2016 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.6b02900 Inorg. Chem. XXXX, XXX, XXX-XXX