IRON COMPLEXES AND USES THEREOF GOVERNMENT FUNDING This invention was made with government support under DK124333 awarded by the National Institutes of Health. The government has certain rights in the invention. RELATED APPLICATIONS This application claims priority to United States Provisional Application Number 63/353,332 that was filed on June 17, 2022. The entire content of the application referenced above is hereby incorporated by reference herein. BACKGROUND OF THE INVENTION Phosphate is a crucial component of fertilizers needed to maintain the world’s food supply. Unfortunately, most of the phosphate used as fertilizer leaches out into surface water, causing widespread eutrophication and hazardous algal blooms. Over 65% of US estuaries and coastal waters now have moderate to severe eutrophication, with significant consequences to the ecology and industry relying on those systems ^1,2 . Addressing this issue requires in part facile detection of phosphate in the µ M range ^3,4 . The current protocol of the US Environmental Protection Agency (EPA) for measuring phosphate levels, the molybdenum blue method, relies on the formation of a phosphomolybdate Keggin ion followed by its reduction to yield a blue mixed-valence complex. ⁵ The slow kinetics of these reactions renders this multi-step protocol laborious. Moreover, the strong acidic conditions necessary for the formation of the Keggin ion does not enable distinction between orthophosphate and other polyphosphates such as pyrophosphate that can also be present in large concentration in surface water but have different impact on algae growth. ⁶ As such, although much attention has recently been devoted to developing molecular receptors and fluorescent probes for phosphate, effective probes that can readily distinguish between phosphate and pyrophosphate are still needed. ^7–12 Metal complexes are particularly well-suited for probing phosphates by luminescence. Recognition of the anion can be accomplished either allosterically or via direct coordination. As in the case of the heteroditopic ruthenium(II) bipyridyl complexes, allosteric recognition of phosphate is primarily accomplished by directed hydrogen-bonding interactions. Such probes, however, do not work well with aqueous samples and are rarely selective for phosphate, including over pyrophosphate. ^13–15 Direct coordination of phosphate is better suited for such applications since the metal ions are able to overcome the high hydration enthalpy of phosphate. ^7–11,16,17 The requirements for lability and hardness have limited current studies to copper, zinc, and lanthanide complexes, ^18–23,7 some of which have marked selectivity and affinity for phosphate. Unfortunately, although many of those probes are selective for phosphate over competing anions such as bicarbonate and chloride, selectivity for orthophosphate over polyphosphates such as pyrophosphate has not yet been established. The presence of iron in the active site of many phosphodiesterases and phosphatases suggest that iron could also be used in the design of receptors for phosphate. ^24–26 Yet, despite being the most abundant transition metal, iron is rarely explored in the design of molecular receptors, as evidenced by the paucity of iron complexes for anion recognition. ^27–30 It is believed that no iron-based molecular receptors for any oxyanion that function at neutral pH and that is selective over interfering anions has been reported. ⁷ Despite its hardness appropriate for hard anions, coordinatively unsaturated iron(III) complexes present several challenges for such applications that are not yet fully mastered. In particular, iron(III) complexes with open coordination sites have a propensity to form ^-oxo dimers, ^31,32 which prevents or diminishes further coordination of the targeted anion. ³³ The development of Fe ^III -based receptors for anions thus necessitates a re-engineering of the metal center to prevent such dimerization. In heme- based system, formation of ^-oxo dimers can be prevented by increasing the steric hindrance around the iron center with picket fences ^34,35 or via supramolecular assemblies with cyclodextrins. ³⁶ It was thought that in non-heme iron-based systems, coordination at the open site by a weaker anion could be sufficient to prevent dimerization. Given the propensity of Fe ^III to quench the fluorescence or organic dyes, ³⁷ such metal-based receptors would also function as a fluorescent probe if this weak anion also fluoresces. Other parameters should be taken into consideration in the design of the receptor. First, the affinity of receptors for anions are significantly influenced by the overall charge of the metal complex at the pH of interest. ²¹ Highly negatively charged complexes should be avoided. The Fe ^III complex must also be sufficiently thermodynamically stable to prevent demetallation. The bioinorganic chemistry of siderophores, natural products that are strong iron chelators, ³⁸ suggest that both of these requirements can be met with tetra- or pentadentate ligands comprising all oxygen donor such as 1,2-hydroxypyridinone (HOPO). In corresponding molecular receptors Fe ^III -HOPO-fluo (1) and Fe ^III -HOPO-PhO-fluo (2) (Figure 1), the remaining 1 or 2 open coordination sites are protected by fluorescein, a weaker ligand for Fe ^III than phosphate. It was hypothesized that fluorescein would coordinate sufficiently strongly to iron(III) to prevent formation of ^-oxo dimers, but not too strongly as to enable displacement by phosphate. Accordingly, there is an ongoing need for new metal complexes (e.g., iron complexes) for detecting ions (e.g., anions such as phosphate), for removing ions (e.g., anions such as phosphate)from aqueous solutions or mixtures (e.g., wastewater), and methods for treating hyperphosphatemia. SUMMARY OF THE INVENTION Iron complexes disclosed herein are useful for detecting and sequestering ions (e.g., anions such as phosphate) and may be useful for treating hyperphosphatemia. Accordingly, one embodiment provides an iron complex composition comprising Fe ^II or Fe ^III complexed with two pyridinone ligands, wherein the pyridinone ligands are covalently attached to each other by a linker and wherein each pyridinone is substituted with one hydroxy or -O- and wherein the pyridinone is optionally substituted with one or more (C1-C6)alkyl. One embodiment provides an iron complex comprising a compound of formula I or a salt thereof, wherein Fe is Fe ^II or Fe ^III ; each moiety is independently a pyridinone substituted with one hydroxy or -O-, wherein the pyridinone is optionally substituted with one or more (C ₁ -C ₆ )alkyl, and wherein each dashed bond is independently a single or a double bond; L is a linker, wherein the linker is optionally substituted with -W ^a or -C(=O)(C ₁ - C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b ; X ^a is phenyl substituted with hydroxy or -O-; and W ^a is a linker ^a group; and W ^b is a linker ^b group. One embodiment provides a mixture comprising two or more iron complexes, comprising independently two more compounds of formula I I or salts thereof, wherein Fe is Fe ^II or Fe ^III or a combination thereof; each moiety is independently a pyridinone substituted with one hydroxy or -O-, wherein the pyridinone is optionally substituted with one or more (C ₁ -C ₆ )alkyl, and wherein each dashed bond is independently a single or a double bond; L is a linker, wherein the linker is optionally substituted with -W ^a or -C(=O)(C ₁ - C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b ; X ^a is phenyl substituted with hydroxy or -O-; and W ^a is a linker ^a group; and W ^b is a linker ^b group. One embodiment provides an iron complex comprising a compound of formula I wherein Fe is Fe ^II or Fe ^III ; each moiety is independently a pyridinone substituted with one hydroxy or -O-, wherein the pyridinone is optionally substituted with one or more (C1-C6)alkyl, and wherein each dashed bond is independently a single or a double bond; L is a linker, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with - W ^b -Y; X ^a is phenyl substituted with hydroxy or -O-; and W ^a is a linker ^a group; W ^b is a linker ^b group; and Y is a polymer, hydrogel, membrane, nanoparticle, or material. One embodiment provides an iron complex consisting essentially of a compound of formula I or a salt thereof as described herein. One embodiment provides an iron complex of formula I or a salt thereof as described herein. One embodiment provides a material or device comprising one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) iron complexes or salts thereof as described herein. One embodiment provides a material or device comprising a plurality of iron complexes or salts thereof as described herein. One embodiment provides a material or device comprising one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more) iron complexes or salts thereof as described herein, covalently attached to the material or device (e.g., covalently attached through the linker ^a or linker ^b ). One embodiment provides a material or device comprising a plurality of iron complexes or salts thereof as described herein, covalently attached to the material or device (e.g., covalently attached through the linker ^a or linker ^b ). One embodiment provides a method to detect inorganic phosphate comprising contacting the phosphate with an iron complex as described herein. One embodiment provides a method to remove inorganic phosphate from an aqueous mixture or solution comprising contacting the aqueous mixture or solution with an iron complex as described herein. One embodiment provides a method to treat hyperphosphatemia in a mammal (e.g., a human such as a human patient) in need thereof comprising contacting the blood of the mammal in need thereof, with an iron complex as described herein. In one embodiment the mammal has chronic kidney disease. One embodiment provides processes and intermediates disclosed herein that are useful for preparing an iron complex or a salt thereof comprising a compound of formula I or a salt thereof. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows the chemical structures of iron(III)-based luminescent probes for phosphate and detection mechanism. Sol denotes solvent molecules. Figure 2A and 2B show the spectroscopic analyses of Fe ^III -HOPO-fluo + Pi. Figure 2A shows the ATR-IR spectra of Fe ^III -HOPO-fluo and Fe ^III -HOPO-Pi. Figure 2B shows the ³¹ P NMR of Bu ₄ N·H ₂ PO ₄ * titrated with Fe ^III -HOPO-fluo (DMSO-d ₆ , 162 MHz). Experimental conditions: Samples for ATR-IRanalysis was prepared by isolating, rinsing, and drying the precipitate formed from Fe ^III -HOPO-fluo + 1 eq. Pi. [Bu4N·H2PO4] = 0.11 M in DMSO-d6. External reference: 85% H3PO4 diluted to 4% with DMSO. *Bu4N·H2PO4 was used due to the low solubility of inorganic phosphate in DMSO, and low solubility of Fe ^III -HOPO-Pi in CD ₃ OD. Figures 3A and 3B show the fluorescence titration of Fe ^III -HOPO-fluo and Fe ^III -HOPO- PhO-fluo with phosphate (Pi): Figure 3A shows fluorescence spectra of Fe ^III -HOPO-fluo with phosphate; Figure 3B shows increase in emission intensity. Experimental conditions: [Fe ^III - HOPO-PhO-fluo] and [Fe ^III -HOPO-fluo] = 10 µM in wet ethanol. pH = 7. ^ _excitation = 456 nm, excitation and emission slit widths = 5 nm, voltage: 600 V. T = 25℃. F = integrated fluorescence intensity from 500 nm to 650 nm in the presence of anions, F _o = integrated fluorescence intensity in the absence of anions. Fluorescence spectra were obtained 5 min after mixing to ensure that thermodynamic equilibrium was reached. The pH of all solutions was adjusted to 7 carefully by addition of either HCl or NaOH, as necessary. Figures 4A and 4B show fluorescence response. Figure 4A shows the fluorescence response of Fe ^III -HOPO-fluo and Figure 4A shows the fluorescence response of Fe ^III -HOPO- PhO-fluo to competing anions. White bars represent the relative fluorescence intensity after addition of 1 equivalent of the appropriate anions (NaF, NaCl, NaBr, NaI, Na2SO4, NaNO3, NaHCO3, NaOAc, Na4P2O7, and Na2HAsO4·7H2O). Gray bars represent the relative fluorescence intensity after subsequent addition of 1 equivalent of phosphate (Pi). PPi denotes pyrophosphate: Experimental conditions: [Fe ^III -HOPO-fluo] = 10 µM in wet ethanol, pH 7, ^excitation = 456 nm, excitation and emission slit widths = 5 nm, F = integrated fluorescence intensity from 500 nm to 650 nm in the presence of anions, Fo = integrated fluorescence intensity in the absence of anions. T = 25℃. The pH of all solutions was adjusted to 7 carefully using 0.01 N HCl and 0.01N NaOH Fluorescence spectra were obtained 5 min after mixing to ensure that thermodynamic equilibrium was reached. Control denotes the same volume of water was used in replacement of anions. Figure 5 shows the HPLC chromatogram of Fe ^III -HOPO-fluo (1). Figure 6 shows the ¹ H NMR spectrum of Fe ^III eIII-HOPO-fluo (1), (CD ₃ OD, 400 MHz). Figure 7 shows the experimental (black) and calculated (red) ESI-MS spectrum of Fe ^III - HOPO-fluo (1). Figure 8 shows the HPLC chromatogram of Fe ^III -HOPO-PhO-fluo (2). Figure 9 shows the ¹ H NMR spectrum of Fe ^III -HOPO-Ph-fluo (2), (CD3OD, 400 MHz. Figure 10 shows the experimental (black) and calculated (red) ESI-MS spectrum of Fe ^III - HOPO-PhO-fluo (2). The peak at 906.8265 m/z came is from PPG calibrant for obtaining the high- resolution MS spectrum. Figure 11 shows the ATR-IR spectra of Fe ^III -HOPO-PhO-fluo complex in the presence and absence of 1eq. phosphate. Sample preparations: to the ethanolic Fe ^III complex solution was added equimolar phosphate. The pH was adjusted to pH = 7 using 0.01 N HCl and 0.01N NaOH. The precipitation was centrifuged, rinsed with ethanol, and dried in a vacuum oven. Figure 12 shows the ³¹ P NMR spectrum of Fe ^III -HOPO-Pi (DMSO-d ₆ , 162 MHz). The peak at 0 ppm is the external reference using: 85% H3PO4 diluted to 4% with DMSO. Figure 13 shows the ³¹ P NMR of Bu4N·H2PO4* titrated with Fe ^III -HOPO-Ph-fluo (DMSO-d ₆ , 162 MHz). Experimental conditions: [Bu ₄ N·H ₂ PO ₄ ] = 0.11 M in DMSO-d ₆ . External reference: 85% H3PO4 diluted to 4% with DMSO. *Bu4N·H2PO4 was used due to the low solubility of inorganic phosphate in DMSO, and low solubility of Fe ^III -HOPO-Pi in MeOH. Figure 14 shows the ¹ H NMR of Bu ₄ N·H ₂ PO ₄ * titrated with 1 eq. of Fe ^III -HOPO-fluo+Pi (DMSO-d ₆ , 162 MHz). Experimental conditions: [Bu ₄ N·H ₂ PO ₄ ] = 0.11 M in DMSO-d ₆ . External reference: 85% H3PO4 diluted to 4% with DMSO. *Bu4N·H2PO4 was used due to the low solubility of inorganic phosphate in DMSO, and low solubility of Fe ^III -HOPO-Pi in MeOH.. Figure 15 shows the Job’s plot analysis of Fe ^III -HOPO-fluo with phosphate. F: integrated luminescence from 500 nm to 650 nm. Conditions: total concentration of Fe ^III -HOPO-fluo and phosphate was kept at 10 µM. The pH of all solutions was adjusted to 7 using 0.01 N HCl and 0.01 N NaOH. ^ _ex : 456 nm, excitation and emission slit widths: 5 nm, T = 25℃. The binding ratios of Fe ^III -HOPO-fluo + Pi and Fe ^III -HOPO-PhO-fluo + Pi were estimated by Job’s plot studies. Each data point represents the integrated fluorescence emission change with respect to that of same volume of water added in replacement of Pi. Figure 16 shows the Job’s plot of Fe ^III -HOPO-PhO-fluo with phosphate. F: integrated luminescence from 500 nm to 650 nm. Conditions: total concentration of Fe ^III -HOPO-PhO-fluo and phosphate was kept at 10 µM. The pH of all solutions was adjusted to 7 using 0.01 N HCl and 0.01 N NaOH. ^ex: 456 nm, excitation and emission slit widths: 5 nm, T = 25℃. The binding ratios of Fe ^III -HOPO-fluo + Pi and Fe ^III -HOPO-PhO-fluo + Pi were estimated by Job’s plot studies. Each data point represents the integrated fluorescence emission change with respect to that of same volume of water added in replacement of Pi. Figure 17 shows the kinetics of response of Fe ^III -HOPO-fluo upon addition of phosphate. F: integrated luminescence from 500 nm to 650 nm in the presence of 1 eq. of phosphate. Conditions: [Fe ^III -HOPO-fluo] = 10 µM in wet ethanol. The pH of all solutions was adjusted to 7. ^ex: 456 nm, excitation and emission slit widths: 5 nm Figure 18 shows the kinetics of response of Fe ^III -HOPO-OPh-fluo to phosphate. F: integrated luminescence from 500 nm to 650 nm in the presence of 1 eq. of phosphate. Conditions: [Fe ^III -HOPO-OPh-fluo] = 10 µM in wet ethanol. The pH of all solutions was adjusted to 7. ^ex: 456 nm, excitation and emission slit widths: 5 nm.. Figure 19 shows the UV-Visible and fluorescence titration spectra of Fe ^III complexes with phosphate: a) UV-Visible spectra of Fe ^III -HOPO-fluo with phosphate; b) fluorescence spectra of Fe ^III -HOPO-fluo with phosphate; c) UV-Visible spectra of Fe ^III -HOPO-PhO-fluo with Pi; d) fluorescence spectra of Fe ^III -HOPO-PhO-fluo with phosphate. Conditions: [Fe ^III -HOPO- fluo] and [Fe ^III -HOPO-PhO-fluo] = 10 µM in wet ethanol. pH = 7. ^ex: 456 nm, excitation and emission slit widths: 5 nm Figure 20 shows the ¹ H NMR spectrum of intermediate 3, ( CDCl ₃ , 400 MHz). Figure 21 shows the ¹³ C NMR spectrum of intermediate 3, (CDCl ₃ , 100 MHz). Figure 22 shows the experimental (black) and calculated (red) ESI-MS spectrum of intermediate 3. Figure 23 shows the ¹ H NMR spectrum of intermediate 4 (CDCl ₃ , 400 MHz). Figure 24 shows the ¹³ C NMR spectrum of intermediate 4 (CDCl3, 100 MHz). Figure 25 shows the experimental (black) and calculated (red) ESI-MS spectrum of intermediate 4. Figure 26 shows the ¹ H NMR spectrum of intermediate 5 (CF3COOD, 400 MHz). Figure 27 shows the ¹³ C NMR spectrum of intermediate 5 (CF3COOD, 400 MHz). Figure 28 shows the experimental (black) and calculated (red) ESI-MS spectrum of intermediate 5. Figure 29 shows the ¹ H NMR spectrum of intermediate 6 (CDCl3, 400 MHz). Figure 30 shows the ¹³ C NMR spectrum of intermediate 6 (CDCl3, 100 MHz). Figure 31 shows the experimental (black) and calculated (red) ESI-MS spectrum of intermediate 6. Figure 32 shows the ¹ H NMR spectrum of protected ligand 7 (CDCl3, 400 MHz). Figure 33 shows the ¹ H NMR spectrum of protected ligand 7 (CDCl ₃ , 400 MHz). Figure 34 shows the experimental (black) and calculated (red) ESI-MS spectrum of protected ligand 7. Figure 35 shows the ¹ H NMR spectrum of ligand 8 (CD ₃ OD, 500 MHz). Figure 36 shows the ¹³ C NMR spectrum of ligand 8 (CD3OD, 125 MHz). Figure 37 shows the experimental (black) and calculated (red) ESI-MS spectrum of ligand 8. Figure 38 shows the data fitting results of FeIII-HOPO-fluo+Pi and FeIII-HOPO-OPh- fluo+Pi. DETAILED DESCRIPTION The following definitions are used, unless otherwise described: halo or halogen is fluoro, chloro, bromo, or iodo. Alkyl and alkoxy, etc. denote both straight and branched groups but reference to an individual radical such as propyl embraces only the straight chain radical (a branched chain isomer such as isopropyl being specifically referred to). As used herein, the term "(C _a -C _b )alkyl" wherein a and b are integers refers to a straight or branched chain alkyl radical having from a to b carbon atoms. Thus when a is 1 and b is 6, for example, the term includes methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t- butyl, n-pentyl and n-hexyl. The term “alkoxy” refers to -O(alkyl) and the term “haloalkoxy” refers to an alkoxy that is substituted with one or more (e.g., 1, 2, 3, or 4) halo. Specific values listed below for radicals, substituents, and ranges, are for illustration only; they do not exclude other defined values or other values within defined ranges for the radicals and substituents. Specifically, (C1-C6)alkyl can be methyl, ethyl, propyl, isopropyl, butyl, iso-butyl, sec- butyl, pentyl, 3-pentyl, or hexyl; (C ₁ -C ₆ )alkoxy can be methoxy, ethoxy, propoxy, isopropoxy, butoxy, iso-butoxy, sec-butoxy, pentoxy, 3-pentoxy, or hexyloxy; (C3-C8)cycloalkyl can be cyclopropyl, cyclobutyl, cyclopentyl, or cyclohexyl; and heteroaryl can be furyl, imidazolyl, triazolyl, triazinyl, oxazolyl, isoxazolyl, thiazolyl, isothiazoyl, pyrazolyl, pyrrolyl, pyrazinyl, tetrazolyl, pyridyl, (or its N-oxide), thienyl, pyrimidinyl (or its N-oxide), indolyl, isoquinolyl (or its N-oxide) or quinolyl (or its N-oxide). Moiety (also referred to herein as “moiety A”) As used herein “moiety A” is a pyridinone (e.g., oxo(=O) substituted pyridine) this is substituted with one hydroxy or -O-, wherein the pyridinone is optionally substituted with one or more (C1-C6)alkyl. In general, the hydroxy is substituted on a carbon atom of the pyridinone and the -O- is substituted on a nitrogen atom of the pyridinone. It is to be understood that the oxygen of the hydroxyl, the oxygen of the pyridinone, and the oxygen of the -O- group coordinate to the iron atom of the iron complex. Thus, the hydrogen of the hydroxyl group is not specifically depicted in the compounds of formula I and may or may not be present in the compounds of formula I. Linker (“L”) As used herein, the linker “L” is a molecular moiety that connects the two “moiety A” groups to one another. The linker can be variable provided it functions to connect two “moiety A” groups to one another, so that the two “moiety A” groups can function as a ligand of the iron metal complexes as described herein. The linker can vary in length and atom composition (e.g., C, H, N, O, S) and for example can be branched or non-branched or saturated or unsaturated or a combination thereof. In one embodiment L is a linker that comprises 5-50 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-30 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-20 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-15 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 8-15 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 3-15 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-50 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from halo, C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-30 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from halo, C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-20 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from halo, C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-15 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from halo, C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 8-15 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from halo, C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 3-15 (or 5-10) non-hydrogen atoms, wherein the non-hydrogen atoms are selected from halo, C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ - C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 3 to 15 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from (C1-C4)alkyl, (C1-C6)alkoxy, oxo (=O), and halo, wherein each R ^a is independently H or (C ₁ -C ₆ )alkyl, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ - C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 3 to 30 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from (C1-C4)alkyl, (C1-C6)alkoxy, oxo (=O), and halo, wherein each R ^a is independently H or (C ₁ -C ₆ )alkyl, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ - C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment , wherein the L is optionally substituted optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y Linker ^a (“W ^a ”) and linker ^b (“W ^b ”) (or linker ^a subgroup and linker ^b subgroup) As used herein, the linker ^a (“W ^a ”) and linker ^b (“W ^b ”) are molecular moieties that connect the iron complexes described herein (e.g., the compounds of formula I or salts thereof) to another molecular entity such as a material (e.g., polymer (e.g., synthetic or natural polymers), hydrogel, membrane, nanoparticle, or any other suitable material (e.g., a device)). The linker can be variable provided it functions to connect the compound of formula I to another molecular entity, so that both the compound of formula I (e.g., the iron complex) and the other molecular entity can function as described herein. The linker can vary in length and atom composition (e.g., halo, C, H, N, O, S) and for example can be branched or non-branched or saturated or unsaturated or a combination thereof. In certain embodiments, the material or device includes a membrane has the ligand or the iron complex of the ligand attached thereto. In certain embodiments, the material or device includes a sensor or detector having the ligand or the iron complex of the ligand attached thereto. In certain embodiments, the ligand can be chemically attached to a surface of the material or device (e.g., a surface of the membrane) through covalent and/or ionic bonding using a variety of methods that would be available to one of skill in the art. In certain embodiments, the ligand can include a pendent functional group ( e.g., a N, O, P, and/or S-containing group) that can function as a linker to chemically attach the ligand to a surface of the material or device. As used herein the linker ^a (“W ^a ”) and linker ^b (“W ^b ”) may also include one or more reactive groups (e.g., an amine, hydroxy, thiol, ester, or amide; NR ₂ , OH, SH, CO ₂ R, CONR ₂ wherein each R is independently H or (C1-C6)alkyl). These linkers together with the metal complexes can serve as precursors (e.g., intermediates) to which the other molecular entities such as a materials are covalently bonded. In one embodiment each W ^a and W ^b independently comprises 2-50 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O. In one embodiment each W ^a and W ^b independently comprises 2-50 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from halo, C, N, S, and O. In one embodiment each W ^a and W ^b independently comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 1 to 20 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from (C1-C4)alkyl, (C1-C6)alkoxy, hydroxy, and halo, wherein each R ^a is independently H or (C ₁ -C ₆ )alkyl. In one embodiment each W ^a and W ^b independently comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 1 to 20 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from oxo (C=O), (C1-C4)alkyl, (C1-C6)alkoxy, hydroxy, and halo, wherein each R ^a is independently H or (C ₁ -C ₆ )alkyl. In one embodiment n is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. The term polymer includes any polymer suitable for linking to the metal complex described herein (e.g., synthetic or natural polymer). Examples include polyamides (including star polyamides), polyethyleneglycol, polyethylenemine, polysulfone, and polyethersulfone. The term hydrogel includes any hydrogel suitable for linking to the metal complex described herein. Examples include crosslinked poly(N-isopropylacrylamide), crosslinked polyvinyl alcohol, PMA (polymethacrylate), PMMA (polymethylmethacrylate), PEMA (polyethylmethacrylate), and chitosan. The term membrane includes any membrane suitable for linking to the metal complex described herein. The term nanoparticle includes any nanoparticle suitable for linking to the metal complex described herein (e.g., metal-based, silica-based). Examples include gold nanoparticles, iron oxide nanoparticles, and silica nanoparticles. The term material includes any material suitable for linking to the metal complex (e.g., a solid material). Examples include carbon, porous carbon, gold, carbon nanotubes, CuO nanowires, and WO3 nanowires. Weak binding ligand As used herein the term weak binding ligand is any ligand that can bind to the iron atom of the iron metal complex but then be subsequently displaced by another ligand or ion (e.g., anion such as phosphate). The weak binding ligand also includes any ligand that can prevent the iron complex from forming iron complex dimers (e.g., iron complexes with two iron atoms). It is understood that the embodiments provided below or above are for compounds of formula I and all sub-formulas thereof (e.g., formulas Ia, Ib, Ic, Id). It is to be understood that two or more embodiments may be combined. In one embodiment Fe is Fe ^II . In one embodiment Fe is Fe ^III . In one embodiment Fe is a mixture of Fe ^II or Fe ^III . In one embodiment each moiety is independently selected from the group consisting of: , and wherein each Ra is independently (C1-C6)alkyl. In one embodiment each In one embodiment each In one embodiment each In one embodiment each In one embodiment each In one embodiment L is a linker that comprises 5-50 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-15 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 3 to 15 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from (C ₁ -C ₄ )alkyl, (C ₁ -C ₆ )alkoxy, oxo (=O), and halo, wherein each R ^a is independently H or (C1-C6)alkyl, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C1- C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment , wherein the L is optionally substituted optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y In one embodiment the compound of formula I is a compound of formula Ia is wherein R is H, -W ^a -Y, or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1- C ₆ )alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment R is H. In one embodiment R is -W ^a -Y. In one embodiment R is -C(=O)(C1-C6)alkyl-X ^a , wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b -Y. In one embodiment L is a linker that comprises 5-50 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b . In one embodiment L is a linker that comprises 5-15 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O, wherein the linker is optionally substituted with -W ^a or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b . In one embodiment L is a linker that comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 3 to 15 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from (C1-C4)alkyl, (C1-C6)alkoxy, oxo (=O), and halo, wherein each R ^a is independently H or (C ₁ -C ₆ )alkyl, wherein the linker is optionally substituted with -W ^a or -C(=O)(C ₁ -C ₆ )alkyl- X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b . In one embodiment , wherein the L is optionally substituted with -W ^a or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b . One embodiment provides an iron complex or salt thereof comprising a compound of formula Ia wherein R is H, -W ^a , or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b . In one embodiment R is -W ^a . In one embodiment R is -C(=O)(C ₁ -C ₆ )alkyl-X ^a , wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b . One embodiment provides an iron complex or salt thereof comprising a compound of formula Ib Ib wherein R ¹ is H or -W ^b . In one embodiment each W ^a and W ^b independently comprises 2-50 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O. In one embodiment each W ^a and W ^b independently comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 1 to 20 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from oxo (C=O), (C1-C4)alkyl, (C1-C6)alkoxy, hydroxy, and halo, wherein each R ^a is independently H or (C1-C6)alkyl. In one embodiment each W ^a and W ^b independently comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 1 to 20 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, or -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from oxo (C=O), (C ₁ -C ₄ )alkyl, (C ₁ -C ₆ )alkoxy, hydroxy, and halo, wherein each R ^a is independently H or (C1-C6)alkyl, wherein the chain is substituted with one or more reactive groups. In one embodiment each W ^a and W ^b independently comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 1 to 10 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, or -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from oxo (C=O), (C ₁ -C ₄ )alkyl, (C ₁ -C ₆ )alkoxy, hydroxy, and halo, wherein each R ^a is independently H or (C1-C6)alkyl, wherein the chain is substituted with one or more reactive groups. In one embodiment each reactive group is independently an amine, thiol, hydroxy, amide or ester. In one embodiment ; ; wherein: R is H or (C1-C6)alkyl; and n is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. In one embodiment the compound of formula I is a compound of formula Ib wherein R ¹ is H or -W ^b -Y. In one embodiment each W ^a and W ^b independently comprises 2-50 non-hydrogen atoms, wherein the non-hydrogen atoms are selected from C, N, S, and O. In one embodiment each W ^a and W ^b independently comprises a branched or unbranched, saturated or unsaturated, hydrocarbon chain, having from 1 to 20 carbon atoms wherein one or more of the carbon atoms is optionally replaced independently by -O-, -S, -N(R ^a )-, and wherein the chain is optionally substituted with one or more (e.g.1, 2, 3, 4, 5 or more) substituents independently selected from (C1-C4)alkyl, (C1-C6)alkoxy, hydroxy, and halo, wherein each R ^a is independently H or (C1-C6)alkyl. In one embodiment n is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.19. In one embodiment In one embodiment,

n is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. In one embodiment , wherein the L is optionally substituted with -W ^a or -C(=O)(C ₁ -C ₆ )alkyl-X ^a , and wherein the -C(=O)(C ₁ -C ₆ )alkyl-X ^a is optionally substituted with -W ^b and wherein n is 0 or 1. In one embodiment the compound of formula I is a compound of formula Ia’ or a salt thereof, wherein n is 0 or 1, R is H, -W ^a , or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1- C ₆ )alkyl-X ^a is optionally substituted with -W ^b . In one embodiment the compound of formula I is a compound of formula Ib’

or a salt thereof, wherein n is 0 or 1; R ¹ is H or -W ^b . In one embodiment the compound of formula I is:

. In one embodiment the compound of formula I is: . One embodiment provides a material or device comprising one or more iron complexes or salts thereof as described herein. Ine one embodiment the material or device is attached to the one or more iron complexes or salts thereof at the linker W ^a or W ^b . One embodiment provides a material or device comprising one or more iron complexes selected from

or a salt thereof, wherein the material or device is bonded to W ^a or W ^b as shown by the wavy line. In one embodiment n is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. One embodiment provides a ligand or salt thereof of formula II II wherein: each moiety is independently a pyridinone substituted with one hydroxy or -O- wherein the pyridinone is optionally substituted with one or more (C1-C6)alkyl, and wherein each dashed bond is independently a single or a double bond; L is a linker, wherein the linker is optionally substituted with -W ^a -Y or -C(=O)(C1-C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with - W ^b -Y; X ^a is phenyl substituted with hydroxy or -O-; W ^a is a linker ^a group; W ^b is a linker ^b group; and Y is a polymer, hydrogel, membrane, nanoparticle, or material. One embodiment provides the ligand of formula II II wherein: each moiety is independently a pyridinone substituted with one hydroxy or -O-, wherein the pyridinone is optionally substituted with one or more (C1-C6)alkyl, and wherein each dashed bond is independently a single or a double bond; L is a linker, wherein the linker is optionally substituted with -W ^a or -C(=O)(C ₁ - C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b ; X ^a is phenyl substituted with hydroxy or -O-; W ^a is a linker ^a group; and W ^b is a linker ^b group. One embodiment provides the ligand of formula II, wherein A, L, X ^a, W ^a , W ^b and Y are as defined in any embodiment or claim provided herein. One embodiment provides the compound as described herein that does not include the iron atom. One embodiment provides an iron complex comprising a compound of formula Ic or a salt thereof, wherein Fe is Fe ^II or Fe ^III or a combination thereof; X ¹ is C or N; X ² together with oxygen to which it is attached is N-O, C-O, C=O; X ³ together with oxygen to which it is attached is N-O, C-O, C=O; X ⁴ is C or NR ^a ; X ⁵ is C or NR ^a ; wherein the ring formed by X ¹ , X ² , X3, X ⁴ , X ⁵ and the carbon is a pyridinone, wherein the pyrdinone is substituted with one hydroxy or -O-, wherein the pyridinone is optionally substituted with one or more (C ₁ -C ₆ )alkyl; each R ^a is independently (C ₁ -C ₆ )alkyl. L is a linker, wherein the linker is optionally substituted with -W ^a or -C(=O)(C1- C6)alkyl-X ^a , and wherein the -C(=O)(C1-C6)alkyl-X ^a is optionally substituted with -W ^b ; X ^a is phenyl substituted with hydroxy or -O-; W ^a is a linker ^a group; and W ^b is a linker ^b group. One embodiment provides an iron complex comprising a compound of formula Id Id or a salt thereof. One embodiment provides an iron complex as described herein further comprising a weak binding ligand. In one embodiment the weak binding ligand is fluorescein. One embodiment provides a method to detect inorganic phosphate comprising contacting the phosphate with an iron complex as described herein. In one embodiment the phosphate is selectively detected in the presence of other anions. In one embodiment the anions are selected from the group consisting of carbonate, nitrate, sulfate, halides, arsenate and pyrophosphate. Ine one embodiment the phosphate is contacted with the iron complex as a liquid sample at about neutral pH. In one embodiment the liquid sample is obtained from a body of water. In one embodiment the liquid sample is a eutrophic sample. In one embodiment the phosphate is detected by fluorescence sensing by an indicator displacement assay. One embodiment provides a method to remove inorganic phosphate from an aqueous mixture or solution comprising contacting the aqueous mixture or solution with an iron complex as described herein. One embodiment provides a method to sequester or remove inorganic phosphate from an aqueous mixture or solution comprising contacting the aqueous mixture or solution with an iron complex as described herein. One embodiment provides a method to remove inorganic phosphate from an aqueous mixture or solution comprising contacting the aqueous mixture or solution with an iron complex as described herein, under conditions wherein the phosphate binds to the iron complex and is partially or completely removed from the mixture or solution. One embodiment provides a method to sequester or remove inorganic phosphate from an aqueous mixture or solution comprising contacting the aqueous mixture or solution with an iron complex as described herein, under conditions wherein the phosphate binds to the iron complex and is partially or completely removed from the mixture or solution.. The invention will now be illustrated by the following non-limiting example. Example 1. Experimental Section Unless otherwise stated, all chemicals were purchased from commercial suppliers and used without further purification. Deuterated solvents were obtained from Cambridge Isotope Laboratories (Tewskbury, MA, USA). Distilled water was further purified by a Millipore Simplicity UV system (resistivity 18×10 ⁶ Ω). All organic extracts were dried over anhydrous MgSO4 (s). NaF, NaCl, NaBr, NaI, Na2SO4, NaNO3, NaHCO3, NaOAc, Na4P2O7, and Na ₂ HAsO ₄ ·7H ₂ O were used for anion screen studies. Flash chromatography was performed on Merck Silica Gel. Modified silica gel was prepared by heating the silica gel in 37% HCl (aq) at 50℃ for 6 h, further washing it with deionized water until the pH of the filtrate was neutral and drying it under reduced pressure at 100℃. The collected silica gel was subsequently suspended in toluene with 1% (v/v) hexadecyltrimethoxysilane. The mixture was stirred at 100°C for 6 h, after which the mixture was filtered, rinsed with toluene and ethyl acetate, and dried under reduced pressure at 100°C. ¹ H NMR and ¹³ C NMR spectra were recorded at room temperature on a Bruker Advance III 400 at 400 MHz and 100 MHz, respectively, or a Bruker Advance III AV 500 at 500 MHz and 125 MHz, respectively, at the LeClaire-Dow instrumentation facility of the Department of Chemistry of the University of Minnesota. The residual solvent peaks were used as internal references. Data for ¹ H NMR are recorded as follows: chemical shift (δ, ppm), multiplicity (s, singlet; d, doublet, t, triplet; q, quartet; br, broad; m, multiplet), coupling constant (Hz), integration. Data for ¹³ C NMR are recorded as follows: chemical shift (δ, ppm). Low resolution (LR) and high resolution (HR) electrospray spray ionization time-of-flight mass spectrometry (ESI/TOF-MS) were recorded on a Bruker BioTOF I at the LeClaire-Dow instrumentation facility of the Department of Chemistry of the University of Minnesota. UV- visible spectra were recorded on a Varian Cary 100 Bio Spectrophotometer. Data was collected over the range of 200 – 800 nm. Luminescence data was acquired on a Varian Cary Eclipse Fluorescence Spectrophotometer using a quartz cell with a path length of 1 cm and chamber volume 400 ^L. Sample solutions were allowed to equilibrate for 5 min before measurement of their luminescence spectra, as initial studies demonstrated that this time was sufficient to achieve thermodynamic equilibrium (Figure 17 and Figure 18). All fluorescent titration data were acquired at room temperature (T = 25 ℃ ). Every data point was measured in triplicate from three independently prepared samples. For fluorescent titrations or anion selectivity studies, 5 mM of anions were prepared in water and pH adjusted to 7 using 0.1 N HCl or 0.1 N NaOH. Wet ethanol denotes the water content is ≤ 10 (v/v%) in ethanol. Luminescence data were processed with Scilab 6.0.2 and QtiPlot 0.9.8.9 software. All pH measurements were performed using Thermo Scientific Ag/AgCl refillable probe and a Thermo Orion 3 Benchtop pH meter. High-performance liquid chromatography (HPLC) data was collected on a Varian Prostar Model 210, coupled with an Agilent ZORBAX Eclipse XDB-C18 column, and a Varian ProStar 335 diode array detector. Unless specified otherwise HPLC measurements were performed at a flow rate 1.0 mL min ⁻¹ with the following elution condition: 15% CH ₃ CN/85% water from 0 to 10 minutes, followed with a linear gradient to 85% CH ₃ CN/15% water from 10 to 23 minutes, 85% CH3CN/15% water from 23 to 45 minutes. Synthesis 4-Nitrophenyl 1-(benzyloxy)-6-oxo-1,6-dihydropyridine-2-carboxylate (3). The p- nitrophenol activated ester was prepared in reference to a similar procedure for the preparation of N-hydroxysuccinimidyl ester of the podant (F. Guérard, M. Beyler, Y.-S. Lee, R. Tripier, J.-F. Gestin, M. W. Brechbiel, Dalton Trans.2017, 46, 4749–4758). HOPO(Bn)-OH (1000. mg, 4.080 mmol) was suspended in anhydrous methylene chloride (10 mL) under N2 atmosphere, followed by the injection of oxalyl chloride (400. ^L, 4.90 mmol) and one drop of DMF. The reaction was let stir for 1 hr at room temperature before the removal of solvent, HCl, and excess oxalyl chloride by high vacuum with liquid N ₂ trap. Under N ₂ atmosphere, the residue was dissolved in 10 mL anhydrous methylene chloride. After p-nitrophenol (568 mg, 4.08 mmol) was added to the solution, the mixture was cooled by ice bath, and NEt3 (900 ^L, 6.10 mmol) was injected slowly to the mixture. The mixture was then warmed up to room temperature for 6 hr, after which it was washed with 10% citric acid solution, 10% sodium bicarbonate solution, dried over anhydrous MgSO ₄ (s), and concentrated under vacuum to a viscous liquid. This crude product was purified by flash chromatography over silica eluting with 60% EtOAc / 20% Hex to yield white solid 3 (1.363 g, 91 %). ¹ H NMR (400 MHz, CDCl ₃ ): ^ 8.29 (d, J = 9 Hz, 2H), 7.49 (d, J = 6 Hz, 2H), 7.41-7.26 (m, 6H), 6.94 (d, J = 9 Hz, 1H), 6.79 (d, J = 7 Hz, 1H), 5.42 (s, 2H). ¹³ C NMR (100 MHz, CDCl ₃ ): ^ 158.6, 157.2, 154.4, 145.8, 137.1, 136.9, 133.3, 130.2, 129.4, 128.6, 127.6, 125.3, 122.2, 109.4, 78.8. ESI-HRMS: m/z = 389.0755 ([M+Na] ⁺ ), (Calcd. 389.0744). N,N'-(Azanediylbis(ethane-2,1-diyl))bis(1-(benzyloxy)-6-oxo- 1,6-dihydropyridine-2- carboxamide) (4). p-nitrophenol activated ester 3 (808 mg, 2.207 mmol) was dissolved in 20 mL methylene chloride, followed by slow injection of diethylenetriamine (119 ^L, 1.10 mmol) and NEt3 (300. ^L, 2.21 mmol). The reaction was stirred at room temperature for 1 hr. The mixture was washed with 20 mL of 1N NaOH solution, dried over anhydrous MgSO4 (s), and concentrated under vacuum to a viscous liquid. This crude product was purified by flash chromatography over silica eluting with 12% MeOH / 88% CH ₂ Cl ₂ to yield foaming liquid 4 (1.045 g, 85 %). ¹ H NMR (400 MHz, CDCl ₃ ): ^ 7.59 (br, 2H), 7.35-7.22 (m, 10H), 7.14 (dd, J ₁ = 9 Hz, J2 = 7 Hz, 2H), 6.36 (d, J = 9 Hz, 2H), 6.21 (d, J = 7 Hz, 2H), 5.18 (s, 4H), 3.14 (d, J = 10 Hz, 4H), 2.51-2.48 (m, 4H), 1.62 (br,1H). ¹³ C NMR (100 MHz, CDCl ₃ ): ^ 160.4, 158.3, 143.0, 138.3, 133.3, 129.6, 129.0, 128.3, 123.0, 105.6, 78.9, 47.1, 39.3. ESI-HRMS: m/z = 580.2156 ([M+Na] ⁺ ), (Calcd.580.2167). N,N'-(Azanediylbis(ethane-2,1-diyl))bis(1-hydroxy-6-oxo-1,6- dihydropyridine-2- carboxamide) HBr (5). The protected ligand 4 (200. mg, 0.359 mmol) was dissolved in 1 mL of glacial acetic acid, and 1 mL of 30% HBr was added to the reaction mixture. After 6 hr, acetic acid and HBr were removed under high vacuum to yield a foaming liquid. This crude product was purified by flash chromatography over modified silica eluting with 0% MeOH / 100% H ₂ O gradient to 5% MeOH / 95% H ₂ O to yield white foaming liquid 5 (150 mg, 91%). ¹ H NMR (400 MHz, CF3COOD): ^ 7.80 (t, J = 8 Hz, 2H), 7.52 (d, J = 8 Hz, 2H), 7.30 (d, J = 9 Hz, 2H), 4.00 (br, 4H), 3.65 (br, 4H). ¹³ C NMR (100 MHz, CF3COOD): ^ 165.4, 160.1, 141.4, 138.0, 121.9, 118.6, 51.6, 40.0. ESI-HRMS: m/z = 378.1310 ([M-Br] ⁺ ), (Calcd.378.1408). Fe ^III -HOPO-fluo (1). Ligand 5 (5.0 mg, 0.011 mmol) and fluorescein (3.6 mg, 0.011 mmol) was suspended in anhydrous EtOH (10 mL), followed by injection of 1N NaOH (22 ^L, 0.022 mmol).0.1 N ethanolic FeBr3 (110 ^L, 0.011 mmol) was then added to the reaction mixture. The purity and identity of Fe ^III -HOPO-fluo formed in situ were characterized by HPLC and ESI-HRMS. The solution was used without further purification. ESI-HRMS: m/z = 763.1138 ([M-Br] ⁺ ), (Calcd.763.1208). 3-(2-(Benzyloxy)phenyl)propanoic acid (6). The benzyl protected side arm 6 was synthesized according to the reported procedure ^[2] and characterized by NMR and ESI-HRMS. ¹ H NMR (400 MHz, CDCl ₃ ): ^ 11.05 (br, 1H), 7.46-7.32 (m, 5H), 7.26-7.19 (m, 2H), 6.94-6.90 (m, 2H), 5.12 (s, 2H), 3.04 (t, J = 8 Hz, 2H), 2.72 (t, J = 8 Hz, 2H). ¹³ C NMR (100 MHz, CDCl ₃ ): ^ 179.6, 156.5, 137.2, 130.1, 128.8, 128.6, 127.8, 127.7, 127.0, 120.8, 111.6, 69.8, 34.0, 25.9. ESI-HRMS: m/z = 279.1009 ([M+Na] ⁺ ), (Calcd.279.0992). 1-(Benzyloxy)-N-(2-(N-(2-(1-(benzyloxy)-6-oxo-1,6-dihydropyr idine-2- carboxamido)ethyl)-3-(2-(benzyloxy)phenyl)propanamido)ethyl) -6-oxo-1,6-dihydropyridine-2- carboxamide (7). The protected side arm 6 (316 mg, 1.23 mmol) was suspended in anhydrous methylene chloride (10 mL) under N2 atmosphere, followed by the injection of oxalyl chloride (0.1 mL, 1.4 mmol) and one drop of DMF. The reaction was let stir for 1 hr at room temperature before the removal of solvent, HCl, and excess oxalyl chloride by high vacuum with liquid N2 trap. Under N ₂ atmosphere, the residue was dissolved in 10 mL anhydrous methylene chloride and cooled with ice bath. The protected ligand 4 (687 mg, 1.23 mmol) was dissolved in anhydrous methylene chloride (5 mL) and slowly injected to the cooled reaction mixture. To the mixture was then injected NEt3 (344 ^L, 2.47 mmol). The mixture was then warmed up to room temperature for 1 hr, after which it was washed with 10% citric acid solution, dried over anhydrous MgSO ₄ (s), and concentrated under vacuum to a viscous liquid. This crude product was purified by flash chromatography over silica eluting with 4% MeOH / 96% CH ₂ Cl ₂ to yield white foaming liquid 7 (805 mg, 82%). ¹ H NMR (400 MHz, CDCl3): ^ 7.48-7.46 (m, 2H), 7.41- 7.36 (m, 5H), 7.34-7.27 (m, 9 H), 7.25-7.21 (m, 2H),7.13-7.09 (m, 2H), 6.94 (br, 1H), 6.89-6.82 (m, 3H), 6.68 (dd, J ₁ = 1 Hz, J ₂ = 8 Hz, 2H), 6.24 (t, J = 2 Hz, 1H), 6.23 (t, J = 2 Hz, 1H), 5.32 (s, 2H), 5.24 (s, 2H), 5.02 (s, 2H), 3.31 (br, 4H), 3.06 (br, 4H), 2.88 (t, J = 7 Hz, 2H), 2.48 (t, J = 8 Hz, 2H). ¹³ C NMR (100 MHz, CDCl3): ^ 174.3, 160.7, 160.6, 158.5, 158.4, 156.5, 142.8, 142.2, 138.0, 137.9, 137.0, 133.5, 133.1, 130.3, 130.1, 129.5, 129.1, 128.60, 128.55, 128.50, 128.0, 127.7, 127.3, 124.1, 120.8, 111.6, 105.9, 105.2, 79.3, 79.0, 69.9, 47.40, 45.7, 39.3, 38.7, 32.8, 26.9. ESI-HRMS: m/z = 818.3181 ([M+Na] ⁺ ), (Calcd.818.3160). 1-Hydroxy-N-(2-(N-(2-(1-hydroxy-6-oxo-1,6-dihydropyridine-2- carboxamido)ethyl)-3- (2-hydroxyphenyl)propanamido)ethyl)-6-oxo-1,6-dihydropyridin e-2-carboxamide (8). The protected ligand 7 (450. mg, 0.566 mmol) was dissolved in 1 mL of glacial acetic acid, and 1 mL of 30% HBr was added to the reaction mixture. After 6 hr, acetic acid and HBr were removed under high vacuum to yield a foaming liquid. This crude product was purified by flash chromatography over modified silica eluting with 0% MeOH / 100% H ₂ O gradient to 5% MeOH / 95% H2O to yield white foaming liquid 8 (270 mg, 91%). ¹ H NMR (500 MHz, CD3OD): ^ 7.48-7.42 (m, 2H), 7.04 (d, J = 7 Hz, 1H), 6.97 (dd, J ₁ = 1 Hz, J ₂ = 8 Hz, 2H), 6.75-6.68 (m, 4H), 6.61 (dd, J1 = 1 Hz, J2 = 7 Hz, 1H), 6.56 (dd, J1 = 1 Hz, J2 = 7 Hz, 1H), 3.65-3.61 (m, 4H), 3.56- 3.54 (m, 4H), 2.88 (t, J = 7 Hz, 2H), 2.75 (t, J = 8 Hz, 2H). ¹³ C NMR (125 MHz, CD ₃ OD): ^ 176.5, 162.8, 162.7, 160.2, 160.1, 156.5, 142.0, 141.7, 139.0, 138.8, 131.5, 128.7, 120.94, 120.85, 116.4, 108.7, 108.5, 48.3, 46.3, 39.4, 39.0, 34.7, 27.6. ESI-HRMS: m/z = 524.1775 ([M- H]-), (Calcd.524.1776). Fe ^III -HOPO-PhO-fluo (2). Ligand 8 (5.0 mg, 9.5 ^mol) and fluorescein (3.6 mg, 9.5 ^mol) was suspended in anhydrous EtOH (10 mL), followed by injection of 1N NaOH (29 ^L, 29 ^mol).0.1 N ethanolic FeBr ₃ (95 ^L, 9.5 ^mol) was then added to the reaction mixture. The purity and identity of Fe ^III -HOPO-PhO-fluo formed in situ were characterized by HPLC and ESI-HRMS. The solution was used without further purification. ESI-HRMS: m/z = 911.1830 ([M+3H] ⁺ ), (Calcd.911.1733). Data Fitting Data fitting of Fe ^III -complexes + Pi uses the following equations: where H denotes the host (Fe ^III -complex-fluo), G the guest (Pi), and HG the adduct. Equilibrium constants K is defined as: The luminescence intensity increase can be described as the following: ² F = integrated fluorescence, F0 = initial integrated fluorescence, Δ F _max = F _∞ - F ₀ . Non-linear fitting was performed using QtiPlot 1.0.0 software. Detection limits were calculated according to , where ^^ is the standard deviation of the fitting. The fitting results are shown in Figure 38 and Table S1 Discussion The receptors Fe ^III -HOPO-fluo and Fe ^III -HOPO-PhO-fluo were synthesized according to Schemes 1 and 2, respectively. The p-nitrophenol activated ester of the benzyl-protected HOPO podand 3, previously synthesized following literature precedence, ³⁹ selectively acylate the primary amino groups of the triamine backbone to yield the protected ligand 4. Deprotection under strong acidic conditions yields the final ligand 5, which was further metallated with Fe ^III in the presence of fluorescein to give the final receptor Fe ^III -HOPO-fluo. Scheme 1 Synthesis of FeIII-HOPO-PhO-fluor ^a Fe ^III -HOPO-PhO-fluo employs a pentadentate ligand whose phenolate moiety occupies one more coordination site of the Fe ^III center. Activation of the benzyl-protected phenol podand 6, previously synthesized according to literature reports, ⁴⁰ with oxalyl chloride enabled coupling to the central secondary amine of 4, thereby yielding the protected ligand 7. Deprotection under strong acidic conditions yielded the final ligand 8 that was further metallated with Fe ^III in the presence of fluorescein to give the final receptor Fe ^III -HOPO-PhO-fluo, 2. In both syntheses, formation and purity of the ternary complexes 1 and 2 was confirmed by HPLC and ESI-MS (Figures 5, 7, 8, and 10). No ^-oxo diiron dimer were detected, confirming that coordination of the fluorescein ligand is sufficient to protect the Fe ^III center and prevent the formation of bimetallic species. In contrast, in the absence of fluorescein, the ^-oxo diiron dimer is the predominant species observed by MS. The significant line broadening observed in the ¹ H NMR of the ternary complexes in solution (Figures 6 and 9), which is typical of paramagnetic Fe(III) species, further confirmed coordination of fluorescein to the receptors 1 and 2. Both Fe ^III ·fluorescein complexes were stable as solids and in ethanol for weeks; both can tolerate up to 10 vol% water with pH adjusted to 7 without significant fluorescein dissociation (< 1%) in ethanol. Direct coordination of phosphate to the iron centers of the receptors concomitant with displacement of the fluorescein moiety upon addition of the oxyanion was first confirmed from attenuated total reflection-infrared (ATR-IR) spectroscopic analysis of the precipitate obtained from Fe ^III -HOPO-fluo+Pi and Fe ^III -HOPO-PhO-fluo+Pi. The iron complex Fe ^III -HOPO-Pi displays the characteristic ^(Fe-O) vibrations at 571 and 461 cm ^-1 , ^(P-O) bands at 1088, 1067, 968 cm ^-1 and ^ (O-P-O) bands at (541) cm ^-1 (Figure 2A). ^41–44 Each of those bands was also observed for the Fe ^III -HOPO-PhO-Pi adduct (Figure 11). These observations are in agreement with the formation of the postulated ternary complexes. Formation of a Fe ^III L·Pi ternary complex was also supported by NMR spectroscopy. The ³¹ P NMR spectrum of Fe ^III -HOPO-Pi is nearly featureless (Figure 12), an observation that is attributed to the shortened transverse relaxation times, T2, of the ³¹ P nucleus by the strongly paramagnetic Fe ^III . As is apparent in Figure 2B, when referenced to an external standard of H ₃ PO ₄ , in a titration monitored by NMR, the ³¹ P signal of phosphate progressively shifts downfield from 1.61 to 4.72 ppm upon gradual addition of Fe ^III -HOPO-fluo (1). This shift is accompanied by a significant line broadening corresponding to a decrease in T ₂ of the phosphorus nuclei from 0.11 s (no Fe ^III -HOPO-fluo) to 1.95 ms (1 equivalent of Fe ^III -HOPO- fluo). Both of those observations are attributable to coordination of orthophosphate to the strongly paramagnetic Fe ^III center. ^45,46 Of note, the presence of a single peak in the ³¹ P also suggests the presence of a rapid equilibrium between bound and free phosphate. Fe ^III -HOPO- PhO-fluo (2), which employs a pentadentate ligand, displays similar behavior with the coordination of phosphate to the Fe ^III center confirmed from both the ATR-IR and the ³¹ P NMR spectra (Figures 11 and 12, respectively). Unfortunately, further attempts to characterize the ternary phosphate complexes by mass spectrometry were unsuccessful due to the their low solubility and the known ability of phosphate to suppress ionization. ^47,48 The indicator displacement assays (IDA) was evaluated by both UV-visible and fluorescence spectroscopy. A 20-fold turn-on fluorescence was observed upon gradual addition of 1 equivalent of orthophosphate (Figure 3A). The fluorescence titrations (Figure 3 and Figure 19) of both receptors were best fitted to a 1:1 binding model from which the equilibrium constants were derived (Table 1). This 1:1 stoichiometry was determined first by evaluating the fit of the titrations and subsequently confirmed by Job’s plots (Figures 15 and 16 for Fe ^III - HOPO-fluo and Fe ^III -HOPO-PhO-fluo, respectively). Interestingly, the use of a tetradentate ligand in Fe ^III -HOPO-fluo does not appear to favor coordination of two phosphate anions to the metal center. The two receptors display similar turn-on response (20-fold at 1 equivalent) and similar equilibrium constants for phosphate: 8.8 × 10 ⁵ M ^-1 and 1.1 × 10 ⁶ M ^-1 for Fe ^III -HOPO- fluo and Fe ^III -HOPO-PhO-fluo respectively. This similarity in both turn-on response and apparent equilibrium constants could be attributed to the comparable core structure of both receptors. Interestingly, the extra phenolate podand of 2 does not appear to affect displacement of the fluorescein moiety by phosphate. A likely coordinated solvent molecule appears to have similar effect. The limit of detection (LOD) of phosphate by the two Fe ^III receptors, commonly estimated as three times the standard deviation of measurement (3 ^^), are 3.5 ^M and 4.1 ^M for Fe ^III -HOPO-fluo (1) and Fe ^III -HOPO-PhO-fluo (2), respectively (Table S1). Although not quite as sensitive as prior Eu ^III probes, ^21,22 these iron receptors are sensitive enough to detect problematic phosphate levels in eutrophic samples (2-10 ^M). ^49,50 Table 1. Apparent equilibrium constants of Fe ^III -HOPO-fluo (1) and Fe ^III -HOPO-PhO-fluo (2) with orthophosphate. The selectivity of the two iron receptors for phosphate over competing anions commonly found in environmental samples was also evaluated by fluorescence spectroscopy. As shown in the white bars of Figure 4, the fluorescence intensity of both probes is not affected by the addition of 1 equivalent of common competing anions including halides, sulfate, and nitrate. Subsequent addition of 1 equivalent of phosphate restores the luminescence of the indicator (Figure 4, grey bars) further indicating that these competing anions do not interfere with detection of phosphate. Interestingly, Fe ^III -HOPO-fluo is more selective over bicarbonate and acetate than Fe ^III -HOPO-PhO-fluo. A more sterically hindered recognition site therefore does not appear to generate higher selectivity for the targeted anion. Uniquely, and importantly, both Fe ^III -HOPO-fluo (1) and Fe ^III -HOPO-PhO-fluo (2) are selective for phosphate over pyrophosphate. Whereas numerous probes selective for pyrophosphate over phosphates have been described in the literature, ^39–41 it is believed that complexes 1 and 2 are unique in their reverse selectivity for phosphate over pyrophosphate. This selectivity likely stems from the preferred bidentate binding mode of pyrophosphate and likely steric hindrance at the coordination site. ^51,52 Since only one displaceable fluorescein is present, bidentate binding is disfavored. The slightly softer anion - arsenate, also does not displace fluorescein despite its structurally similarity to phosphate. This is an unusual selectivity given as most metal probe for phosphate also respond to arsenate. ⁷ As such, these fluorescent iron(III) probes offer a distinctive ability to rapidly monitor the level of the most important phosphorus species causing nutrient pollution in surface water: phosphate. Summary Herein is described non-heme iron(III) complexes, including Fe ^III -HOPO-fluo and Fe ^III - HOPO-PhO-fluo, for selective recognition of inorganic phosphate. This is demonstrated via indicator displacement assay. The open coordination sites were sufficiently protected by weakly coordinating fluorescein to prevent dimerization in aerated solutions. Coordination of inorganic phosphate concomitant with displacement of the fluorescein moiety increases the emission of the latter by 20-fold. Uniquely, these probes distinguish themselves from other receptors that function by direct metal coordination in that they are highly selective for phosphate over pyrophosphate. They are also highly selective over common competing endogenous anions such as carbonate, nitrate, sulfate, halides and, unusually, arsenate. The limit of detection of the iron(III) receptors, 3.5 and 4.1 ^M for Fe ^III -HOPO-fluo and Fe ^III -HOPO-PhO-fluo, respectively, enables detection of phosphate typical of eutrophic water samples. On this basis, the two iron(III) probes enable rapid and facile detection of phosphate in eutrophic samples. 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