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Abstract
The reaction of anhydrous lanthanide chlorides together with 4,4′-bipyridine yields the MOFs[Ln2Cl6(bipy)3]·2bipy, with Ln = Pr − Yb, bipy = 4,4′-bipyridine, and[La2Cl6(bipy)5]·4bipy. Post-synthetic thermal treatment in combination with different vacuum conditions was successfully used to shape the porosity of the MOFs. In addition to the MOFs microporosity, a tuneable mesoporosity can be implemented depending on the treatment conditions as a surface morphological modification. Furthermore, thermal treatment without vacuum results in several identifiable crystalline high-temperature phases. Instead of collapse of the frameworks upon heating, further aggregation under release of bipy is observed.[LaCl3(bipy)] and[Ln3Cl9(bipy)3], with Ln = La, Pr, Sm, and[Ho2Cl6(bipy)2] were identified and characterized, which can also exhibit luminescence. Besides being released upon heating, the linker 4,4′-bipyridine can undergo activation of C-C bonding in ortho-position leading to the in-situ formation of 4,4′:2′,2′′:4′′,4′′′-quaterpyridine (qtpy). qtpy can thereby function as linker itself, as shown for the formation of the network[Gd2Cl6(qtpy)2(bipy)2]·bipy. Altogether, the manuscript elaborates the influence of thermal treatment beyond the usual activation procedures reported for MOFs.
Highlights
MOFs and coordination polymers [1,2,3,4] are known for their interesting properties such as porosity [5,6], magnetism [7] and luminescence [8,9,10]
The activation conditions vary between the different lanthanides by more than 100 °C all are isotypic, which gave the starting point for a detailed study on the thermal properties of these
Element analysis, mapping and point-ID measurements were carried out using electron dispersive X-ray spectroscopy (EDX) with a silicon drift detector (SDD) X-Max 50 mm2 (Oxford Instruments, Oxfordshire, UK) at 15 keV
Summary
MOFs (metal-organic frameworks) and coordination polymers [1,2,3,4] are known for their interesting properties such as porosity [5,6], magnetism [7] and luminescence [8,9,10]. Knowledge of the behavior of these compounds at elevated temperature is a vital point, as a thermal activation induces high thermal stress on the compounds It can have a direct structural influence on the framework, in the worst case causing decomposition, which is typically addressed to as collapse of the MOF. An alternative to a structural collapse of a framework upon thermal treatment is structural condensation under release of volatile components (including MOF linkers) This is typically observed for complexes as well as 1D coordination polymers transforming into 3D frameworks and MOFs [18,19,20]. We elaborate the influence of deliberate thermal treatment on the MOF system derived from anhydrous lanthanide halides and 4,4′-bipyridine, as it offers both, options for post-synthetic morphology control by surface modification and shaping of the MOF pores as well as it enables various highly aggregated crystalline high-temperature phases. The knowledge of the thermal behavior thereby explains the activation procedure to the microporous MOFs 2∞[Ln2Cl6(bipy)3] [35,36] as well as the inability to activate 3∞[La2Cl6(bipy)5]·4(bipy) [37] to a porous MOF
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