Inorganic Oxoanions Research Articles

Metal-sulfide/polysulfide functionalized layered double hydroxides - recent progress in the removal of heavy metal ions and oxoanionic species from aqueous solutions.

Water constitutes an indispensable resource for global life but remains susceptible to pollution from diverse human activities. To mitigate this issue, researchers are committed to purifying water using a variety of materials to remove harmful chemicals, such as heavy metals. Layered double hydroxides (LDHs), with their intriguing, layered structure and chemical behavior, have attained substantial attention for their effectiveness in removing heavy metal cations and various inorganic oxoanions from water. To enhance the efficiency, considerable endeavors have focused on functionalizing LDHs with different chemical species. Intercalation with metal sulfides has proven to be particularly effective, facilitating heavy metal absorption through multiple mechanisms, including ion-exchange, reductive precipitation, and surface sorption. This review concentrates on the synthesis and performance of polysulfide (Sx, x = 2-5), Mo-S, and Sn-S anion intercalated LDHs for heavy metal cations and inorganic oxoanion sorption, along with their mechanisms. Furthermore, the discussion includes prospects for expanding the chemistry of metal sulfide intercalated LDHs, with existing challenges and future outlooks.

Reaction of Thiosulfate Dehydrogenase with a Substrate Mimic Induces Dissociation of the Cysteine Heme Ligand Giving Insights into the Mechanism of Oxidative Catalysis.

Thiosulfate dehydrogenases are bacterial cytochromes that contribute to the oxidation of inorganic sulfur. The active sites of these enzymes contain low-spin c-type heme with Cys–/His axial ligation. However, the reduction potentials of these hemes are several hundred mV more negative than that of the thiosulfate/tetrathionate couple (Em, +198 mV), making it difficult to rationalize the thiosulfate oxidizing capability. Here, we describe the reaction of Campylobacter jejuni thiosulfate dehydrogenase (TsdA) with sulfite, an analogue of thiosulfate. The reaction leads to stoichiometric conversion of the active site Cys to cysteinyl sulfonate (Cα-CH2-S-SO3–) such that the protein exists in a form closely resembling a proposed intermediate in the pathway for thiosulfate oxidation that carries a cysteinyl thiosulfate (Cα-CH2-S-SSO3–). The active site heme in the stable sulfonated protein displays an Em approximately 200 mV more positive than the Cys–/His-ligated state. This can explain the thiosulfate oxidizing activity of the enzyme and allows us to propose a catalytic mechanism for thiosulfate oxidation. Substrate-driven release of the Cys heme ligand allows that side chain to provide the site of substrate binding and redox transformation; the neighboring heme then simply provides a site for electron relay to an appropriate partner. This chemistry is distinct from that displayed by the Cys-ligated hemes found in gas-sensing hemoproteins and in enzymes such as the cytochromes P450. Thus, a further class of thiolate-ligated hemes is proposed, as exemplified by the TsdA centers that have evolved to catalyze the controlled redox transformations of inorganic oxo anions of sulfur.

Molecular Recognition and Hydration Energy Mismatch Combine To Inform Ion Binding Selectivity at Aqueous Interfaces.

There is a critical need for receptors that are designed to enhance anion binding selectivity at aqueous interfaces in light of the growing importance of separation technologies for environmental sustainability. Here, we conducted the first study of anion binding selectivity across a series of prevalent inorganic oxoanions and halides that bind to a positively charged guanidinium receptor anchored to an aqueous interface. Vibrational sum frequency generation spectroscopy and infrared reflection absorption spectroscopy studies at the water-air interface reveal that the guanidinium receptor binds to an oxoanion series in the order SO42- > H2PO4- > NO3- > NO2- while harboring very weak interactions with the halides in the order I- > Cl- ≈ Br-. In spite of large dehydration penalties for sulfate and phosphate, the more weakly hydrated guanidinium receptor was selective for these oxoanions in contradiction to predictions made from ion partitioning alone, like the Hofmeister series and Collins's rules. Instead, sulfate binding is likely favored by the suppression of dielectric screening at the interface that consequently boosts Coulombic attractions, and thus helps offset the costs of anion dehydration. Geometric factors also favor the oxoanions. Furthermore, the unique placement of iodide in our halide series ahead of the stronger hydrogen-bond acceptors (Cl-, Br-) suggests that the binding interaction also depends upon single-ion surface partitioning from bulk water to the interface. Knowledge of the anion binding preferences displayed by a guanidinium receptor sheds light on the receptor architectures needed within designer interfaces to control selectivity.

Toward Realizing Multifunctionality: Photoactive and Selective Adsorbents for the Removal of Inorganics in Water Treatment.

Persistent and potentially toxic inorganic oxoanions (e.g., arsenic and selenium) are one class of contaminants of concern in drinking water for which treatment technologies must be improved. Effective removal of these oxoanions is made difficult by the varying adsorption affinity of the different oxidation states, as well as the presence of background ions with similar chemical structure and behavior that strongly compete for adsorption sites, greatly reducing removal efficiencies. Recent studies pointing to the negative health effects of inorganic oxoanion contaminants have resulted or are expected to result in new regulations lowering their allowable maximum concentration level (MCL) in drinking water. While these regulations are intended to protect human and environmental health, they must also allow for balanced economic costs. As such, the MCLs are often set at levels that are not as health protective due to high treatment costs that continue to present a significant challenge for small (500-3300 people) to very small (25-500 people) communities. In this Account, we focus on the development of novel cost-effective, sustainable, and efficient multifunctional and selective adsorbents that offer solutions to the above challenges through two platforms: nanoenabled and transition-metal cross-linked chitosan (TMCC) and crystal facet engineered nanometal oxides (NMO). These complementary platforms offer treatment solutions at different scales and flow rates (e.g., in a point-of-use device versus a small-scale community system). Multifunctional adsorbents combine processes that traditionally require multiple steps offering the potential for reducing treatment time and costs. Development of selective adsorbents can greatly increase removal efficiencies of target contaminants by either promoting their adsorption or hindering adsorption of competitive ions. The following sections describe (1) synthesis of novel nanoenabled waste sourced bioadsorbents; (2) development of multifunctional adsorbents to simultaneously photo-oxidize arsenite and adsorb arsenate; (3) development of a selective adsorbent for removal of arsenate and selenite over phosphate; (4) investigations of the conventional wisdom that increased surface area yields increased oxoanion removal using selenium sorption on nanohematite as a case study; and (5) crystal engineering of nanohematite to promote selenite adsorption. The novel technologies developed through these research efforts can serve as templates for the creation of future adsorbents tailored for use targeting other oxoanion contaminants of interest.

Ag48(C≡C tBu)20(CrO4)7]: An Atomically Precise Silver Nanocluster Co-protected by Inorganic and Organic Ligands.

The elaborate selection of capping ligands is of great importance in the synthesis of atomically precise metal nanoclusters. Organic thiolates, alkynyls, phosphines, and/or their combinations are the ligands most widely utilized to protect metal nanoclusters, while inorganic oxo anions have been almost neglected in this field. Herein, the first CrO42-/ tBuC≡C- co-capped Ag48 nanocluster (SD/Ag48, SD = SunDi) was synthesized and structurally characterized by single-crystal X-ray diffraction. The pseudo-5-fold symmetric metal skeleton of SD/Ag48 shows a core-shell structure composed of a Ag23 cylinder encircled by an outer Ag25 shell. Unprecedentedly, coexistence of inorganic (CrO42-) and organic ( tBuC≡C-) ligands was observed on the surface of SD/Ag48. The inorganic CrO42- anion plays three important roles in the construction of silver nanoclusters: (i) passivating the Ag23 kernel; (ii) connecting the core and shell; and (iii) protecting the Ag25 shell. This nanocluster belongs to a 14e superatom system and exhibits successive molecule-like absorption bands from the visible to the ultraviolet region. This work not only establishes a fresh inorganic ligand strategy in the synthesis of silver nanoclusters but also provides a new insight into the important surface coordination chemistry of CrO42- in the shape control of silver nanoclusters.

An Ideal C3-Symmetric Sulfate Complex: Molecular Recognition of Oxoanions by m-Nitrophenyl- and Pentafluorophenyl-Functionalized Hexaurea Receptors.

The anion-binding properties of two tripodal-based hexaureas appended with the m-nitrophenyl (1) and pentafluorophenyl (2) groups have been studied both experimentally and theoretically, showing strong affinities for sulfate over other inorganic oxoanions such as hydrogen sulfate, dihydrogen phosphate, bicarbonate, nitrate, and perchlorate. The structural analysis of the sulfate complex with 1 reveals that the receptor organizes all urea-binding sites toward the cavity at precise orientations around a tetrahedral sulfate anion to form an ideal C3-symmetric sulfate complex that is stabilized by 12 hydrogen-bonding interactions. The receptor and the encapsulated sulfate are located on the threefold axis passing through the bridgehead nitrogen of 1 and the sulfur atom of the anionic guest. The high-level density functional theory calculations support the crystallographic results, demonstrating that the C3-symmetric conformation of the sulfate complex is achieved due to the complementary NH···O between the receptor and sulfate.

(2-Azoniaethyl)guanidinium dichloride – A promising phase-matchable NLO material employing a simple hydrogen bond acceptor in its structure

(2-Azoniaethyl)guanidinium dichloride (AEGCl), a salt of a functional guanidine derivative, was prepared and characterised by single-crystal and powder X-ray diffraction analysis, elemental analysis and by NMR, UV–Vis and vibrational spectroscopy. The crystal structure of AEGCl, which crystallises with the symmetry of the chiral space group P21, is built up via simple and multicentred NH⋯Cl hydrogen bonds and further supported by CH⋯Cl contacts. The compound is thermally robust and exhibits promising NLO properties with powder SHG efficiency better than potassium dihydrogen phosphate. In contrast, (2-azoniaethyl)guanidinium salts with inorganic oxoanions such as (2-azoniaethyl)guanidinium dinitrate and diperchlorate, synthesised for a comparison, form centrosymmetric crystal assemblies having several structurally independent “molecules” in their structures.

Surface functionalization of semiconductor and oxide nanocrystals with small inorganic oxoanions (PO4(3-), MoO4(2-)) and polyoxometalate ligands.

In this work, we study the functionalization of the nanocrystal (NC) surface with inorganic oxo ligands, which bring a new set of functionalities to all-inorganic colloidal nanomaterials. We show that simple inorganic oxoanions, such as PO4(3-) and MoO4(2-), exhibit strong binding affinity to the surface of various II-VI and III-V semiconductor and metal oxide NCs. ζ-Potential titration offered a useful tool to differentiate the binding affinities of inorganic ligands toward different NCs. Direct comparison of the binding affinity of oxo and chalcogenidometallate ligands revealed that the former ligands form a stronger bond with oxide NCs (e.g., Fe2O3, ZnO, and TiO2), while the latter prefer binding to metal chalcogenide NCs (e.g., CdSe). The binding between NCs and oxo ligands strengthens when moving from small oxoanions to polyoxometallates (POMs). We also show that small oxo ligands and POMs make it possible to tailor NC properties. For example, we observed improved stability upon Li(+)-ion intercalation into the films of Fe2O3 hollow NCs when capped with MoO4(2-) ligands. We also observed lower overpotential and enhanced exchange current density for water oxidation using Fe2O3 NCs capped with [P2Mo18O62](6-) ligands and even more so for [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2] with POM as the capping ligand.

Catalysis of CO2 Absorption in Aqueous Solution by Inorganic Oxoanions and their Application to Post Combustion Capture

To reduce CO2 emission into the atmosphere, particularly from coal-fired power stations, post combustion capture (PCC) using amine-based solvents to chemically absorb CO2 has been extensively developed. From an infrastructure viewpoint, the faster the absorption of CO2, the smaller the absorber required. The use of catalysts for this process has been broadly studied. In this manuscript, a study of the catalytic efficiencies of inorganic oxoanions such as arsenite, arsenate, phosphite, phosphate, and borate is described. The kinetics of the accelerated CO2 absorption at 25 °C was investigated using stopped-flow spectrophotometry. The catalytic rate constants of these anions for the reaction of CO2 with H2O were determined to be 137.7(3), 30.3(7), 69(2), 32.7(9), and 13.66(7) M(-1)s(-1), respectively. A new mechanism for the catalytic reaction of oxoanions with CO2 has also been proposed. The applicability of these catalysts to PCC was further studied by simulation of the absorption process under PCC conditions using their experimental catalytic rate constants. Arsenite and phosphite were confirmed to be the best catalysts for CO2 capture. However, considering the toxicological effect of arsenic and the oxidative instability of phosphite, phosphate would be the most promising inorganic catalyst for PCC process from the series of inorganic oxoanions studied.

Citrate-hydrazine hydrogen-bonding driven single-step synthesis of tunable near-IR plasmonic, anisotropic silver nanocrystals: implications for SERS spectroscopy of inorganic oxoanions.

A simplified, single-step aqueous synthesis route to tunable anisotropic silver nanocrystals (NCs) has been developed by tailoring the hydrogen-bonding interactions between a mild stabilizer, sodium citrate, and a mild reductant, hydrazine hydrate. The structure directing ability of the H-bonding interaction was harnessed by keeping a stoichiometric excess of hydrazine under ambient conditions (pH 7, 25 °C). Decreasing the synthesis temperature to 5 °C imparts rigidity to the citrate-hydrazine H-bonding network, and the plasmon peak moves from 500 to 550 nm (using 40 mM hydrazine). On lowering the pH from 7 to 5, the H-bonding is further strengthened due to partial protonation of citrate and the plasmon peak is tuned to 790 nm. Further, we found that, at 5 °C and pH 5, there also exists a sub-stoichiometric regime in which maximum tunability of the plasmon peak (790→1010 nm) is achieved with 1 mM hydrazine. HR-TEM reveals that the near-IR plasmonic NCs are nanopyramids having a pentagonal base with edge length varying from 15 nm to 30 nm. Through second derivative FTIR analysis, a correlation between hydrogen-bonded molecular vibrations and the plasmon tunability has been established. The anisotropic NCs exhibit significant Raman enhancement on the citrate molecules. Further, a solution-phase, non-resonant SERS spectroscopic detection method for an inorganic contaminant of ground water, arsenite, has also been developed.

Positional Isomeric Effect in Nitrophenyl Functionalized Tripodal Urea Receptors toward Binding and Encapsulation of Anions

A set of three positional isomers of nitro-phenyl functionalized tris(urea) scaffold has been extensively studied as the anion coordinating host. The para-isomer, L1, has been structurally authenticated to self-assemble into a dimeric (pseudo)molecular capsule in the presence of oxoanions of varied dimensionality (carbonate and terephthalate anions). Whereas the meta-isomer, L2, has been found to self-assemble into a dimeric molecular capsule in presence of inorganic oxoanions such as hydrogen phosphate and adopts a flat trigonal planar like geometry in the presence of the terephthalate anion where each receptor side arm is in interaction with a carboxylate group of the anion. However, successful crystallization of the ortho-isomer, L3, in the presence of different oxoanions was not fruitful presumably due to the steric effect provided by the nitro group at the ortho-position, which hinders the facile inclusion and coordination of an anion due to electrostatic factor, as confirmed by 2D NOESY NMR analysis...

Determination of Individual Gibbs Energies of Anion Transfer and Excess Gibbs Energies Using an Electrochemical Method Based on Insertion Electrochemistry of Solid Compounds

A method is presented to determine, individually and with minimal extra-thermodynamic assumptions, the Gibbs energy for anion transfer between two solvents using solid state electrochemistry of alkynyldiphosphine dinuclear Au(I) complexes (AuC2R)2PPh2C6H4PPh2 (L1, R = Fc; L2, R = C6H4Fc) and the heterometallic Au(I)–Cu(I) [{Au3Cu2(C2R)6}Au3(PPh2C6H4PPh2)3](PF6)2 (L3, R = Fc; L4, R = C6H4Fc) cluster complexes containing ferrocenyl units. These compounds exhibit a well-defined, essentially reversible solid-state oxidation in contact with different electrolytes, based on ferrocenyl-centered oxidation processes involving anion insertion. Voltammetric data can be used for a direct measurement of the free energy of ion transfer from one solvent to another using midpeak potentials in solutions of suitable salts in each one of the solvents separately or mixtures of the solvents. Excess Gibbs energy of solvation in solvent mixtures can also be directly measured using this procedure. Solvation data for different common inorganic oxoanions in water, MeOH, and MeCN and water–MeOH and water–MeCN mixtures are provided.

A Macrocyclic Ligand as Receptor and ZnII‐Complex Receptor for Anions in Water: Binding Properties and Crystal Structures

Binding properties of 24,29-dimethyl-6,7,15,16-tetraoxotetracyclo[19.5.5.0(5,8).0(14,17)]-1,4,9,13,18,21,24,29-octaazaenatriaconta-Δ(5,8),Δ(14,17)-diene ligand L towards Zn(II) and anions, such as the halide series and inorganic oxoanions (phosphate (Pi), sulfate, pyrophosphate (PPi), and others), were investigated in aqueous solution; in addition, the Zn(II)/L system was tested as a metal-ion-based receptor for the halide series. Ligand L is a cryptand receptor incorporating two squaramide functions in an over-structured chain that connects two opposite nitrogen atoms of the Me(2)[12]aneN(4) polyaza macrocyclic base. It binds Zn(II) to form mononuclear species in which the metal ion, coordinated by the Me(2)[12]aneN(4) moiety, lodges inside the three-dimensional cavity. Zn(II)-containing species are able to bind chloride and fluoride at the physiologically important pH value of 7.4; the anion is coordinated to the metal center but the squaramide units play the key role in stabilizing the anion through a hydrogen-bonding network; two crystal structures reported here clearly show this aspect. Free L is able to bind fluoride, chloride, bromide, sulfate, Pi, and PPi in aqueous solution. The halides are bound at acidic pH, whereas the oxoanions are bound in a wide range of pH values ranging from acidic to basic. The cryptand cavity, abundant in hydrogen-bonding sites at all pH values, allows excellent selectivity towards Pi to be achieved mainly at physiological pH 7.4. By joining amine and squaramide moieties and using this preorganized topology, it was possible, with preservation of the solubility of the receptor, to achieve a very wide pH range in which oxoanions can be bound. The good selectivity towards Pi allows its discrimination in a manner not easily obtainable with nonmetallic systems in aqueous environment.

A new polymeric chain constructed from isopolyanions and lanthanide cations: Synthesis, crystal structure, and properties of (NH 4) 5[Ln(NO 3) 2(H 2O) 3][H 2W 12O 40]· nH 2O (Ln = Ce, Pr, Nd, Gd, Dy, Ho)

A series of 1:1 composite compounds prepared with the isopolyanions and lanthanide cations, (NH 4) 5[Ln(NO 3) 2(H 2O) 3][H 2W 12O 40]· nH 2O (Ln = Ce III ( 1); Pr III ( 2); Nd III ( 3); Gd III, n = 1 ( 4); Dy III ( 5); Ho III, n = 1 ( 6)) were synthesized at room temperature and characterized by single-crystal X-ray structural analysis, elemental analyses, IR spectra, thermogravimetry analyses and cyclic voltammetry. X-ray structural analysis reveals that these structures are isostructural: polyanions are bridged by Ln ions via the coordination of the bridging oxygen atoms and terminal oxygen atoms, forming a W–Ot′–Ln–Ob–W linkage; and through the W–Ot′–Ln–Ob–W linkage a one-dimension infinite linear chain with alternating cations and anions is formed. Compounds 1– 6 represent the first example where two types of inorganic oxoanions (NO 3 − and [H 2W 12O 40] 6−) simultaneously coordinating to Ln III, in which the NO 3 − ions act as the bidentate ligands coordinating to the Ln III, forming a four-membered ring. The room temperature luminescent studies indicate that 3 exhibits strong luminescence in the near-IR region.

Quantitative detection of aqueous arsenic and other oxoanions using attenuated total reflectance infrared spectroscopy utilizing iron oxide coated internal reflection elements to enhance the limits of detection

An attenuated total reflectance Fourier transform infrared spectroscopy technique has been developed utilizing an oxide coated internal reflection element to quantitatively evaluate the concentrations of three inorganic oxoanions, arsenate, sulfate, and selenate, at environmentally significant levels. Two iron oxide coatings, goethite and an iron sol–gel, were used and compared to an uncoated internal reflection element which typically has a limit of detection around 1.0 mM. The goethite coating improved the limits of detection by factors of 45.6 and 137.0 for arsenate and sulfate as compared to an uncoated cell. The iron sol coating improved the limits of detection by factors of 481.2, 156.2, and 114.0 for arsenate, sulfate, and selenate, respectively.

Second-sphere coordination complexes via hydrogen bonding: Synthesis, characterization of [Co(NH 3) 6](XO 3) 3· nH 2O (X=Br, I) and single crystal X-ray structure determination of [Co(NH 3) 6](BrO 3) 3·0.5H 2O

Single crystals of [Co(NH 3) 6](BrO 3) 3·0.5H 2O and peach coloured precipitate of [Co(NH 3) 6](IO 3) 3·3H 2O were obtained by reacting hexaamminecobalt(III) chloride with appropriate silver halate in 1:3 molar ratio in hot aqueous medium. These hexaamminecobalt(III) complexes have been characterized by spectroscopic studies (e.g. UV–vis and IR). Single crystal X-ray structure determination of title complex salt revealed the presence of discrete ions, i.e. cation [Co(NH 3) 6] 3+, three anions, BrO 3 − and half water molecule in the solid state. Crystal lattice is stabilized by electrostatic forces of attractions and hydrogen bonding interactions (N–H⋯O −, O–H⋯O −). The formation of salts of definite composition with halate ions and solubility product data suggest that purely inorganic hexaamminecobalt(III) cation may be used as an anion receptor for these weakly coordinating inorganic oxo-anions.

Binding of inorganic oxo-anions to cationic cobaltammine: Synthesis, characterization, X-ray structures determination and DFT calculations of [Co(NH 3) 6]Cl 2(ClO 3), [Co(NH 3) 6]Br 2(ClO 3) and [Co(NH 3) 6]Cl 2(IO 3) · H 2O

Single crystals of [Co(NH 3) 6]Cl 2(ClO 3) ( 1), [Co(NH 3) 6]Br 2(ClO 3) ( 2) and [Co(NH 3) 6]Cl 2 (IO 3) · H 2O ( 3) suitable for X-ray structure determinations were obtained directly by reacting hexaamminecobalt(III) halide with appropriate metal halates in 1:1 molar ratio in hot aqueous medium. These hexaamminecobalt(III) complexes have been characterized by spectroscopic studies (e.g. UV/Visible and IR). Single crystal X-ray structure determination of title complex salts revealed the presence of discrete ions, i.e., cation [Co(NH 3) 6] 3+ and mixed anions Cl − and ClO 3 - or Br − and ClO 3 - or Cl − and IO 3 - in the solid state. Crystal lattice is stabilized by electrostatic forces of attraction and hydrogen bonding interactions (N–H⋯O −, N–H⋯Cl − or N–H⋯Br −). The formation of definite salts of composition with halate ions suggests that [Co(NH 3) 6] 3+ may be used as purely inorganic anion binding agent for these weakly coordinating oxo-anions. The DFT calculations have been carried out to calculate molecular volumes of the oxoanions and separation between two successive cation layers.

Binding of Inorganic Oxoanions to Macrocyclic Ligands: Effect of the Degree of Protonation on Supramolecular Assemblies Formed by Phosphate and [18]aneN6

Five macrocycle-oxoanion adducts have been isolated from aqueous solutions containing 1,4,7,10,13,16-hexaazacyclooctadecane ([18]aneN(6), L) and phosphoric acid whose pH had been adjusted to selected values in the 1-8 range. Four products, (H(6)L)(H(2)PO(4))(6).2H(3)PO(4) (1), (H(6)L)(H(2)PO(4))(6) (2), (H(4)L)(H(2)PO(4))(4).2H(2)O (4), and (H(4)L)(HPO(4))(2).7H(2)O (5) crystallized from aqueous solutions at pH 1, 3, 6, and 8, respectively, while (H(4)L)(H(2)PO(4))(4) (3) crystallized on diffusion of EtOH into an aqueous reaction mixture at pH 6. Single-crystal X-ray structure determinations enabled an examination of supramolecular interactions between protonated forms of [18]aneN(6), phosphoric acid and its conjugate bases, and water of solvation. The macrocycle adopts a variety of conformations in order to accommodate the supramolecular constructs formed by the oxoanions and solvent molecules as the relative proportions of interacting species are altered. At pH 1 and 3, the fully protonated macrocycle, [LH(6)](6+), is found with six H(2)PO(4)(-) anions. At pH 6 and 8, the tetraprotonated macrocycle, [LH(4)](4+), crystallizes with four H(2)PO(4)(-) and two HPO(4)(2)(-), respectively. Variations in the solute of crystallization are evident, with phosphoric acid being present at the lowest pH and water at pH 6 and 8. In 5, the seven unique water molecules form a string-of-pearls motif within which a new heptameric isomer, consisting of a water pentamer that uses a single water to interact with the other two unique water molecules, is found. Structures 1, 2, 4, and 5 exhibit eta-3 H-bonding of ammonium protons to a single oxygen of the guest phosphates located above and below the macrocyclic ring. In 3, two phosphate oxygens of the cavity anion interact with the macrocycle, one of which participates in eta-2 H-bonding with ammonium groups.

Crystal synthesis of hybrid organometallic-inorganic hydrogen bonded salts of acid oxoanions.

Partially deprotonated inorganic oxoanions derived from sulfuric and phosphoric acids have been used to assemble organometallic cations in inorganic-organometallic hybrid systems. The organometallic sandwich cations [(eta(5)-C(5)H(5))(2)Co]+, [(eta(5)-C(5)Me(5))(2)Co]+ and [(eta(5)-C(5)Me(5))(2)Fe]+ have been used because they do not interfere with hydrogen bonding formation forcing self-assembling of the inorganic acids anions HSO(4)(-) and H(2)PO(4)(-) into hydrogen bonded mono- and bi-dimensional networks.

Binding of inorganic oxoanions to macrocyclic ligands: interactions of sulfate and dithionate with protonated forms of [18]aneN6

Three sulfate-macrocycle adducts [H6L(HSO4)4(SO4)]·H2O (1), [H6L(SO4)3] (2) and [H6L(HSO4)2Cl4]·2H2O (3) (where L = [18]aneN6) were crystallized from solutions containing the macrocycle and the appropriate inorganic acids, and [H4L(S2O6)2]·2H2O (4) from an aqueous solution of the macrocycle and sodium dithionate. All four structures show that the polyammonium groups on the protonated macrocycle participate in ‘co-ordination’ or supramolecular interactions with one or more oxygen atoms from the polyoxoanions (i.e., SO42−, HSO4− or S2O62−). In 3, the protonated macrocyclic cavity holds the two chloride anions in preference to the oxoanions. The sulfate anions show remarkable versatility in their interactions with the macrocycle. Of particular note is a unique η-5 N–H⋯O cluster involving the sole sulfate anion in 1, but there are many common features throughout the structures, such as the inclusion of one anion on either side of the macrocycle and the participation of all ammonium protons in H-bonding interactions.