Abstract
Phosphate is an anion of both environmental and medical significance. The increase in phosphate levels in surface waters due primarily to run-offs from fertilized agricultural fields causes widespread eutrophication and increasingly large dead-zones. Hyperphosphatemia, a condition in which blood phosphate levels are elevated, is a primary cause of increased mortality and morbidity in chronic or advanced kidney disease. Resolving both of these issues require, in part, new technology that could selectively sequester phosphate in water at neutral pH. The high hydration energy of phosphate, which prevents organic receptors from functioning in water with sufficient affinity, can be overcome via coordination to a hard metal ion. The hardness, oxophilicity and lability of lanthanide ions make them excellent candidates for the design of high affinity phosphate receptors. In this perspective, we discuss how the principles of lanthanide coordination chemistry can be exploited to design sensitive and selective receptors for phosphate. Unlike many supramolecular systems, these hosts do not recognize their anionic guests via directed electrostatic and hydrogen bonding interactions. Instead, the selectivity of our fluxional receptors is governed entirely by acid-base chemistry and electrostatic forces. Parameters that affect the affinity and selectivity of the receptors include the basicities of the coordinating ligand and of the targeted anion, the acidity of the lanthanide ion, and the geometry of the ligand. Uniquely, their affinity for phosphate can be readily tuned by orders of magnitude either by peripheral interactions or by the lanthanide ion itself without affecting their exquisite selectivity over competing anions such as bicarbonate and chloride.
Highlights
Lanthanide Receptors for PhosphateKidney diseases. (Sherman, 2016). With time, this accumulation leads to vascular calcification with significant increased risk of mortality and morbidity. (Block et al, 1998; Ganesh et al, 2001; Block et al, 2004)
NEED AND REQUIREMENTS FOR PHOSPHATE RECEPTORSPhosphate, a tetrahedral oxyanion with one of the largest hydration energy (ΔGhydration H2PO4− = -465 kJ mol−1), (Marcus, 1991), has received much attention in supramolecular chemistry due to its significance in both environmental and medical applications
The above discussion highlighted the principles behind the design of lanthanide-based receptors with high affinity for phosphate and high selectivity over other common competing anions in water at neutral pH, notably bicarbonate
Summary
Kidney diseases. (Sherman, 2016). With time, this accumulation leads to vascular calcification with significant increased risk of mortality and morbidity. (Block et al, 1998; Ganesh et al, 2001; Block et al, 2004). Coordination of an additional oxyanion would be thermodynamically favored because it would increase the ΣpKa. we have demonstrated that if the geometry of the ligand and the charge of the complex are constant, and in the absence of steric hindrance at the open coordination sites, the affinity of gadolinium(III) complexes for anions is directly determined by only two factors: the stability of the complex, which is itself a function of the basicity of the ligand, and the basicity of the anionic guest. In the absence of steric hindrance at the coordination site, the affinity of GdIII complexes for anions always follows the trend in the basicity of the anions: phosphate > arsenate > bicarbonate > fluoride. It is evident that this class of fluxional lanthanide hosts does not follow the standard lock-and-key geometric standards of supramolecular chemistry articulated
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