Size Selectivity in Heterolanthanide Molecular Complexes with a Ditopic Ligand.

The similar reactivity of lanthanides generally leads to statistically populated polynuclear complexes, making the rational design of ordered hetero-lanthanide compounds extremely challenging. Here we report on the site selectivity in hetero-lanthanide tetranuclear complexes afforded by the relatively simple ditopic pyterpyNO ligand (4'-(4-pyridil)-2,2':6',2"-terpyridine N-oxide). The sequential room temperature reaction of RE 2 (tta) 6 (pyterpyNO) 2 (where RE=Y, (1); Eu, (2), Dy, (3) Htta=2-thenoyltrifluoroacetone) with La(tta) 3 dme (dme=dimethoxyethane) yielded Y 2 La 2 (tta) 12 (pyterpyNO) 2 (4), Dy 2 La 2 (tta) 12 (pyterpyNO) 2 (5) and Eu 2 La 2 (tta) 12 (pyterpyNO) 2 (6). Single crystals X-ray diffraction studies showed that 4, 5 and 6 are isostructural, featuring a tetranuclear structure with two different metal coordination sites with coordination numbers 8 (CN8) and 9 (CN9). The two smaller cations are mainly bridged by the O-donor atoms of the NO groups of two pyterpyNO ligands (CN8), while the larger lanthanum centres are bound by a terpyridine unit (CN9). Size selectivity has been studied with structural and magnetic studies in the solid state and through 19 F NMR and photoluminescence studies in solution, showing a direct dependence on the difference of ionic radii of the ions and yielding a 91 % selectivity for 4. Furthermore, 19 F NMR, X-ray and PL studies pointed out that the nature of the product is independent from the synthetic route for compound Eu 2 Y 2 (tta) 12 (pyterpyNO) 2 (7), keeping the ion selectivity also for a self-assembly reaction. Unexpectedly, these studies have evidenced that selectivity is not exclusively governed by electrostatic interactions related to size dimensions.