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Plabst, Monika (2009): High Throughput assisted Investigation on Lanthanide (III) Tetrakisphosphonates. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Phosphonic acids and lanthanides are both promising building blocks for the synthesis of new metal organic frameworks (MOFs). However, their chemical and structural flexibility challenges the tailoring of well defined crystalline structures. This work is focused on overcoming this lack of control with the introduction of 1,4-phenylenbis(methylidyne)-tetrakis(phosphonic acid), H8L, a molecule that combines a rigid spacer and two ligands suitable for the chelation of large ions. Using a synthetic high-throughput approach, H8L was investigated as new molecular building block for the formation of coordination frameworks with lanthanides (Ln = La, Nd, Gd, Dy) under hydrothermal conditions. Thereby, 14 new lanthanide phosphonates were discovered, categorized into three structure types and their crystallization fields described. Structure type I comprises six compounds with the general formula Ln(H5L). The examples of Ln[(PO3H)2CH-C6H4-CH(PO3H)(PO3H2)]∙4H2O, where Ln = La, Nd, reveal two-dimensional coordination networks having a layered structure, but differ in the layer stacking. Structure type II comprises 4 compounds of the general formula Ln2(H2L) and is represented by La2[(HO3P)(O3P)-CH-C6H4-CH-(PO3)(PO3H)] ∙8H2O, a layered structure with dimeric lanthanum coordination polyhedra. Four compounds of the type NaLn(H4L) are in stucture type III, which was solved from NaLa[(PO3H)2CH-C6H4-CH(PO3H)2]∙4H2O. This structure consists of a three-dimensional open framework with La3+ coordinated by bisphosphonate units in an exclusively bidentate fashion. It provides one-dimensional rhombic channels which are occupied by sodium ions and water molecules acting as guests. The anionic, open-framework of the new MOF NaLa(H4L) exhibits an exceptional selectivity for monovalent metal cations. The presented work elucidates the relationship between the ion-exchange behavior and the framework flexibility: The exchange of the Na+ ions in NaLa(H4L) with alkaline-earth, alkaline and selected transition metal ions was studied. EDX and ICP-OES elemental analysis revealed that ion exchange was successful with monovalent ions, while higher-valent ions were rejected. An explanation for this charge selectivity could be found in the site-specific role of the guest cation. X-ray diffraction and thermogravimetric studies on the reversible hydration and dehydration behavior demonstrate that NaLa(H4L) has a flexible framework. Contraction of the channels upon dehydration leads to a decrease in the cell volume by 15%. Rietveld refinement of the structure of the dehydrated form NaLa(H4L)dehyd revealed the key role played by the guest cation in the channel-shrinking mechanism. In the hydrated, expanded form, each Na+ ion guest shares three phosphonate oxygens with a La3+ ion in a lanthanum phosphonate chain that defines part of the wall of a rhombic channel. The Na+ ion completes its octahedral coordination sphere with two water molecules and a weaker bond to a fourth phosphonate oxygen. In the dehydrated, contracted form, the Na+ ion loses the two water molecules and moves towards a second La3+ ion, which is located in an adjacent lanthanum phosphonate chain, to share two more phosphonate oxygens, and becomes 5-coordinate. This results in the formation of an -La-O-Na-O-La- chain and a concomitant shrinking of the channels. A comparison of the monovalent metal (M(I)) ion-exchanged compounds, M(I)La(H4L), reveals that both the ionic radius and the enthalpy of hydration of the guest cation affect the equilibrium between the expanded and the contracted forms, and that the framework adapts specifically to the size of the guest cation. Furthermore the influence of the guest ion on the synthesis of NaLa(H4L) was investigated. In a screening of the synthesis parameters, Na+ was identified as a strong structure-directing agent for this framework. In addition, a series of compounds MLa(H4L) with M = Li+, Na+, K+, NH4+ and Rb+ were prepared by synthesis or ion exchange and characterized using X-ray powder diffraction, elemental analysis and scanning electron microscopy. Sorption of N2, Kr and H2O in these compounds revealed a preference of the network for water molecules. Furthermore a strong influence of the guest ions on the shape of the water sorption isotherms could be observed. The isotherms of LiLa(H4L), NaLa(H4L) and KLa(H4L) show one distinctive step of adsorption that can be correlated with the expansion of the flexible framework upon hydration. The required partial pressure for the pore expansion depends on the incorporated guest ion and increases with the decreasing hydration enthalpy form Li+ to K+.