Inorganic Receptors for Phosphate

Abstract

Although phosphate is one of the essential nutrients for life, excess phosphate can cause negative health and environmental conditions, such as hyperphosphatemia and eutrophication. Phosphate sequestration is thus one of the most effective approaches for medical and environmental remediation. Unfortunately, current treatment is either inefficient or unsustainable. Organic phosphate receptors often fail to satisfy the most important requirement – efficacy and selectivity in water. In contrast, inorganic receptors exploit the strong phosphate-metal interaction to sufficiently overcome the hydration energy of phosphate that impedes organic receptors. In particular, lanthanide-based receptors with open coordination sites are well-suited for phosphate sequestration. Achieving selectivity for phosphate however, remains unmastered. This dissertation seeks to develop a fundamental insight into factors that underpin anion coordination to lanthanide complexes. The host-guest interaction of lanthanide-based receptors is highly dependent on the geometry of the coordination sites or the coordinated water molecules. In the first study, we demonstrated that indeed larger ligand caps favor more water molecules coordinated, while not all them can be displaced by phosphate. However, there is no direct relationship between the number of inner-sphere water molecules and affinity for coordinating anions. Next, in order to tune the binding affinity, we incorporated a charge or hydrogen bonding moiety peripherally to the metal center. The secondary interaction promoted phosphate binding, with the strongest enhancement by Coulombic interaction, followed by hydrogen bonding. Importantly, the selectivity for phosphate remained intact. The performance of the most water-soluble complex in elevated pH was further evaluated. Surprisingly, in basic condition, the europium(III) complex binds cyanide with a 9-fold turn-on luminescence. Unlike most cyanide probes that rely on reaction-based mechanism, the europium(III) complex detects cyanide based on direct coordination. Lastly, we extended our scope to less appreciated iron(III)-based receptors. The presence of a weak coordinating yet highly fluorescent dye effectively prevents ?-oxo dimer formation, and probes phosphate with turn-on fluorescence. Notably, the iron(III)-based receptors are selective over pyrophosphate and arsenate. Collectively, this dissertation demonstrates that the interplay of ligand geometry and peripheral groups can be exploited to develop selective phosphate receptors.