Synthesis of rare earth nitrate complexes with 4′-pyridin-4-yl-terpyridine and their solid-state transformation to coordination polymers

The interaction of the series of trivalent rare earth (RE) nitrates with 4′-(pyridin-4-yl)-2,2′:6′,2″-terpyridine (4-pytpy) was investigated. In direct reaction between RE(NO₃)₃·nH2O with 4-pytpy, both, a series of isotypic complexes [RE(NO₃)₃(4-pytpy)(H₂O)] (1a-RE with CN 10 and 1b-RE with CN 9) and the structurally individual complexes [La(NO₃)₃(4-pytpy)(H₂O)₂] (2) and [Sc(NO₃)₂(4-pytpy)(H₂O)]NO₃ (3) were obtained. The thermal stability of the phases was investigated using simultaneous differential thermal analysis and thermogravimetry (DTA/TG or STA) as well as high-temperature powder X-ray diffraction analysis (HT-PXRD). Coordinated water can be removed from complexes 1–3, resulting in the formation of coordination polymers ¹_∞[RE(NO₃)₃(4-pytpy)] (4-RE) for early and middle lanthanides as well as [RE(NO₃)₃(4-pytpy)] (5-RE) for late lanthanides, yttrium, and scandium. At elevated temperatures (150–370 °C), the polymeric structure 4-RE with a connection via organic linker dissociates, and a series of dimers [RE₂(NO₃)₆(4-pytpy)₂] (6-RE) bridged via nitrates are formed; the temperature of this transition reduces along the lanthanide series. In general, the accessibility of the phases and solid-state transformations depends on the ionic radius of the trivalent rare earth element. Photophysical properties of Eu³⁺ and Tb³⁺ were determined in the solid state. The absolute photoluminescence quantum yield (PLQY) of polymorphs presented here depends on the packing, which determines the ligand energy levels. Highlight is ¹_∞[Eu(NO₃)₃(4-pytpy)] (4-Eu) with the PLQY of 79.3(9) %. In addition to single-crystal X-ray diffraction (SCXRD), 3D electron diffraction analysis (3D-ED) was used for the determination of crystal structures.