Silver 2,4'-Bipyridine Coordination Polymer for the High-Capacity Trapping of Perrhenate, A Pertechnetate Surrogate.

Pertechnetate, the most stable form of the radionuclide 99Tc in aerobic aqueous systems, is a hazardous anion present in nuclear waste. Its high mobility in water makes the remediation of the anion challenging. In the past decade, significant effort has been placed into finding materials capable of adsorbing this species. Here, we present the synthesis and high-resolution crystal structure of the coordination polymer [Ag(2,4'-bipyridine)]NO3, which is capable of sequestering perrhenate─a pertechnetate surrogate─through anion exchange to form another new coordination polymer, [Ag(2,4'-bipyridine)]ReO4. Both the beginning and end structures were solved by single-crystal X-ray diffraction and the adsorption reaction was monitored through inductively coupled plasma-optical emission spectroscopy and UV-vis spectroscopy. The exchange reaction follows a pseudo-second-order mechanism and the maximum adsorption capacity is 764 mg ReO4/g [Ag(2,4'-bipyridine)]NO3, one of the highest recorded for a coordination polymer or metal-organic framework. A solvent-mediated recrystallization mechanism was determined by monitoring the ion-exchange reaction by scanning electron microscopy-energy-dispersive spectroscopy and powder X-ray diffraction.

Selective Chromium(VI) Trapping by an Acetate-Releasing Coordination Polymer.

We report the high-capacity and selective uptake of Cr(VI) from water using the coordination polymer silver bipyridine acetate (SBA, [Ag(4,4'-bipy)][CH3CO2]·3H2O). Cr capture involves the release of acetate, and we have structurally characterized two of the product phases that form: silver bipyridine chromate (SBC, SLUG-56, [Ag(4,4'-bipy)][CrO4]0.5·3.5H2O) and silver bipyridine dichromate (SBDC, SLUG-57, [Ag(4,4'-bipy)][Cr2O7]0.5·H2O). SBA maintains a high Cr uptake capacity over a wide range of pH values (2-10), reaching a maximum of 143 mg Cr/g at pH 4. This Cr uptake capacity is one of the highest among coordination polymers. SBA offers the additional benefits of a one-step, room temperature, aqueous synthesis and its release of a non-toxic anion following Cr(VI) capture, acetate. Furthermore, SBA capture of Cr(VI) remains >97% in the presence of a 50-fold molar excess of sulfate, nitrate, or carbonate. We also investigated the Cr(VI) sequestration abilities of silver 1,2-bis(4-pyridyl)ethane nitrate (SEN, [Ag(4,4'-bpe)][NO3]) and structurally characterized the silver 1,2-bis(4-pyridyl)ethane chromate (SEC, SLUG-58, [Ag(4,4'-bpe)][CrO4]0.5) product. SEN was, however, a less effective Cr(VI) sequestering material than SBA.

Exchange Capability of Cationic Silver 4,4'-Bipyrdine Materials for Potential Water Remediation: Structure, Stability, and Anion Exchange Properties.

An in-depth study of the class of cationic materials [Ag(4,4'-bipy)+][X-] (where X- = CH3CO2-, NO3-, BF4-, ClO4-, and MnO4-) has led to key insights on the relationship between anion hydration energy, material structure, solubility, and stability. Since these materials show promise for their potential as water remediation tools, understanding their properties in detail is of significant importance. The structure of the starting and ending materials is the main driving force behind the resultant stability and solubility and can be successfully used to predict the ion exchange capabilities. The solubility trend was determined to be, from most soluble to least soluble, X- = CH3CO2- > NO3- ∼ BF4- > ClO4- > MnO4-. Kinetics and thermal stability also follow predictable trends but involve additional factors. For instance, the kinetics of NO3- to MnO4- exchange was much slower than expected based on that seen for NO3- to ClO4-. Powder X-ray diffraction (PXRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the materials. Solubility was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. Ion exchange was analyzed with ion chromatography (IC) and ultraviolet-visible spectroscopy (UV-vis), and thermal stability was determined with thermogravimetric analysis (TGA).

A Cationic Silver Pyrazine Coordination Polymer with High Capacity Anion Uptake from Water.

We report the first example of linker modification for an N-donor Ag-based cationic material while maintaining and in some cases increasing the anion exchange capacity. Cationic silver(I) pyrazine nitrate selectively traps harmful oxo-anions from water such as permanganate, perrhenate and a variety of α,ω-alkanedicarboxylates. We chose these anions as initial examples of exchange for potential water purification. The host-guest interaction between the cationic layers of π-stacked silver pyrazine polymers and the incoming/outgoing interlamellar anions allows for the exchange. The exchange capacity over 24 h reached 435 and 818 mg/g for permanganate and perrhenate, respectively, a record for a crystalline metal-organic material and over five times the exchange capacity compared to commercial resin. The material also undergoes organic exchange as an analog for pharmaceutical waste, some of which have a carboxylate functionality at the neutral pH range typical of natural water sources. Both the as-synthesized and exchanged materials are characterized by a variety of analytical techniques.

Reversible, Selective Trapping of Perchlorate from Water in Record Capacity by a Cationic Metal-Organic Framework.

We report the capture of ppm-level aqueous perchlorate in record capacity and kinetics via the complete anion exchange of a cationic metal-organic framework. Ambient conditions were used for both the synthesis of silver 4,4'-bipyridine nitrate (SBN) and the exchange, forming silver 4,4'-bipyridine perchlorate (SBP). The exchange was complete within 90 min, and the capacity was 354 mg/g, representing 99% removal. These values are greater than current anion exchangers such as the resins Amberlite IRA-400 (249 mg/g), Purolite A530E (104 mg/g), and layered double hydroxides (28 mg/g). Moreover, unlike resins and layered double hydroxides, SBN is fully reusable and displays 96% regeneration to SBN in nitrate solution, with new crystal formation allowing the indefinite cycling for perchlorate. We show seven cycles as proof of concept. Perchlorate contamination of water represents a serious health threat because it is a thyroid endocrine disruptor. This noncomplexing anionic pollutant is significantly mobile and environmentally persistent. Removal of other anionic pollutants from water such as chromate, pertechnetate, or arsenate may be possible by this methodology.

Erbium hydroxide ethanedisulfonate: a cationic layered material with organic anion exchange capability.

We describe a cationic erbium-based material [Er12(OH)29(H2O)5][O3SCH2CH2SO3]3.5·5H2O. As synthesized, the material is water stable and capable of complete organic anion exchange for a variety of α,ω-alkanedicarboxylates. We chose these anions as initial examples of exchange and as an analog for pharmaceutical waste, some of which have a carboxylate functionality at neutral pH range. Free-floating and partially anchored organosulfonate anions reside between the cationic corrugated layers and allow for exchange. The structure also displays a reversible hydration event above 100 °C. Both the as-synthesized and the exchanged materials are characterized by a variety of analytical techniques.

Anion exchange of the cationic layered material [Pb2F2]2+.

We demonstrate the complete exchange of the interlamellar anions of a 2-D cationic inorganic material. The α,ω-alkanedisulfonates were exchanged for α,ω-alkanedicarboxylates, leading to two new cationic materials with the same [Pb(2)F(2)](2+) layered architecture. Both were solved by single crystal X-ray diffraction and the transformation also followed by in situ optical microscopy and ex situ powder X-ray diffraction. This report represents a rare example of metal-organic framework displaying highly efficient and complete replacement of its anionic organic linker while retaining the original extended inorganic layer. It also opens up further possibilities for introducing other anions or abatement of problematic anions such as pharmaceuticals and their metabolites.

Light-triggered eradication of Acinetobacter baumannii by means of NO delivery from a porous material with an entrapped metal nitrosyl.

A photoactive manganese nitrosyl, namely [Mn(PaPy(3))(NO)](ClO(4)) ({Mn-NO}), has been loaded into the columnar pores of an MCM-41 host. Strong interaction between the polar nitrosyl and the -OH groups on the host wall leads to excellent entrapment of the NO donor within the porous host. With the aluminosilicate-based host (Al-MCM-41), the loading is further enhanced due to electrostatic interaction of the cationic species with the aluminum sites. The extent of loading has been determined via analytical techniques including N(2) adsorption/desorption isometry. Powder X-ray diffraction studies on the loaded materials afford patterns typical of an ordered mesoporous silicate consisting of a hexagonal array of unidimensional channels (with slight loss of crystallinity). Elemental mapping of the loaded particles confirms the incorporation of {Mn-NO} into the porous MCM-41 structure and attests to the homogeneity of the guest molecule distribution throughout individual particles. When suspensions of the loaded materials in saline solution are exposed to low-power (10-100 mW) visible light, rapid release of NO is observed. With continuous exposure, a steady release of 50-80 µM of NO is attained with 5 mg of material/mL buffer within 5 min, and the NO flux is maintained for a period of ~60 min. Rapid bursts of 5-10 µM NO are noted with short light pulses. Loss of either the nitrosyl or its photoproduct(s) from these materials in biological media is minimal over long periods of time. The NO release profiles suggest potential use of these powdery biocompatible materials as NO donors where the delivery of NO (a strong antibiotic) could be controlled via the exposure of light. Such prediction has been confirmed with the successful eradication of both drug-susceptible and drug-resistant Acinetobacter baumannii in a soft-tissue infection model through light-triggered NO delivery.

A cationic antimonite chain templated by sulfate: [Sb6O7(4+)][(SO4(2-))2].

An extended metal oxide possessing a cationic charge on the host has been synthesized by hydrothermal methods. The structure consists of 1D antimony oxide [Sb(6)O(7)](4+) chains with a new structural motif of four Sb atoms wide and unprotonated sulfate anions between the chains. The material was characterized by powder and single-crystal X-ray diffraction. Thermal behavior and chemical resistance in aqueous acidic conditions (pH ~2) indicate a highly stable cationic material. The stability is attributed to the entirely inorganic composition of the structure, where 1D covalently extended chains are electrostatically bound to divalent anions.

A crystal-structural study of Pauling-Corey rippled sheets.

Following the seminal theoretical work on the pleated ß-sheet published by Pauling and Corey in 1951, the rippled ß-sheet was hypothesized by the same authors in 1953. In the pleated ß-sheet the interacting ß-strands have the same chirality, whereas in the rippled ß-sheet the interacting ß-strands are mirror-images. Unlike with the pleated ß-sheet that is now common textbook knowledge, the rippled ß-sheet has been much slower to evolve. Much of the experimental work on rippled sheets came from groups that study aggregating racemic peptide systems over the course of the past decade. This includes MAX1/DMAX hydrogels (Schneider), L/D-KFE8 aggregating systems (Nilsson), and racemic Amyloid ß mixtures (Raskatov). Whether a racemic peptide mixture is "ripple-genic" (i.e., whether it forms a rippled sheet) or "pleat-genic" (i.e., whether it forms a pleated sheet) is likely governed by a complex interplay of thermodynamic and kinetic effects. Structural insights into rippled sheets remain limited to only a very few studies that combined sparse experimental structural constraints with molecular modeling. Crystal structures of rippled sheets are needed so we can rationally design rippled sheet architectures. Here we report a high-resolution crystal structure, in which (l,l,l)-triphenylalanine and (d,d,d)-triphenylalanine form dimeric antiparallel rippled sheets, which pack into herringbone layer structures. The arrangements of the tripeptides and their mirror-images in the individual dimers were in excellent agreement with the theoretical predictions by Pauling and Corey. A subsequent mining of the PDB identified three orphaned rippled sheets among racemic protein crystal structures.

A new paradigm for anion trapping in high capacity and selectivity: crystal-to-crystal transformation of cationic materials.

We describe a new methodology to the selective trapping of priority pollutants that occur inherently as oxo-anions (e.g., perchlorate, chromate, arsenate, pertechnetate, etc.) or organic anions (e.g., salicylate, pharmaceuticals, and their metabolites, which are often chlorinated into potentially more harmful compounds). The typical approach to trapping anions is exchange into cationic hosts such as resins or layered double hydroxides. Both capacity and selectivity are limited by the equilibrium of the process and moreover are often subject to interference, e.g. by carbonate that is always present in water from atmospheric CO(2). Our approach takes advantage of the metastability of our cationically charged materials to instead trap by recrystallization to a new structure. Exceptionally high adsorption capacities for permanganate and perrhenate--studied as models for pertechnetate--were found for a Ag(I)-based cationic extended framework. The exchange capacity reached 292 and 602 mg/g, respectively, over five times the exchange capacity compared to conventional layered double hydroxides. Our cationic material can also selectively trap these and other toxic oxo-anions when nontoxic anions (e.g., nitrate, carbonate) were present in an over 100-fold excess concentration.

Reversible anion exchange and catalytic properties of two cationic metal-organic frameworks based on Cu(I) and Ag(I).

We report the synthesis and characterization of two Ag(I)/Cu(I)-based cationic metal-organic frameworks and their application in both heterogeneous catalysis and anion exchange. The Cu(I)-based material was designed from our previously reported Ag(I) cationic topology. Both structures consist of cationic layers with pi-pi stacked chains of alternating metal and 4,4'-bipyridine. Alpha,omega-alkanedisulfonate serves as an anionic template, electrostatically bonding to the cationic layers. Due to weak interaction between the sulfonate template and cationic extended framework, both materials display reversible anion exchange for a variety of inorganic species. Indeed, the Ag(I)-based material exhibits highly efficient uptake of permanganate and perrhenate anion trapping, a model for pertechnetate trapping. The materials also display heterogeneous Lewis acidity, likely due to the coordinatively unsaturated metal sites which only bind to two bipy nitrogens and a weak interaction with one sulfonate oxygen. A comparative study on the influence of structure versus size selectivity and reusability for both exchange and catalysis is discussed.

Hydrothermal synthesis of two cationic bismuthate clusters: an alkylenedisulfonate bridged hexamer, [Bi6O4(OH)4(H2O)2][(CH2)2(SO3)2]3 and a rare nonamer templated by triflate, [Bi9O8(OH)6][CF3SO3]5.

This paper reports the synthesis, characterization, and application of two cationic bismuthate clusters by anion templating. The compounds were synthesized under mild hydrothermal treatment and characterized by powder and single-crystal X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis. The first material consists of a cationic hexanuclear bismuthate cluster octahedral in geometry and linked by 1,2-ethanedisulfonate molecules. This structure is thermally stable to about 235 degrees C. In the second compound, discrete cationic nonanuclear bismuthate clusters interact electrostatically with trifluoromethanesulfonate anions to pack into a nearly layered assembly. The material undergoes a transformation to Bi(2)O(3) upon loss of the triflate groups at about 385 degrees C. Both materials demonstrate the use of sulfonate groups for the anion-directed assembly of these rare cationic inorganic structures. The application of the 3D octahedral bismuth cluster material toward acidic heterogeneous catalysis is also reported.

Two related gadolinium aquo carbonate 2-D and 3-D structures and their thermal, spectroscopic, and paramagnetic properties.

We report two new extended inorganic materials based on gadolinium. The first, [Gd(CO(3))(2)H(2)O][NH(4)], consists of negatively charged 2-D sheets of gadolinium carbonate with one coordinated water molecule and an ammonium cation between the layers. The coordinated water and one carbonate extend into the interlayer space, connecting the layers via an extensive hydrogen bonding network which includes the ammonium ions. The second, a closely related yet more condensed framework structure, [Gd(2)(CO(3))(3)NH(3)H(2)O], is formed at a higher hydrothermal temperature and was characterized by single crystal X-ray diffraction data collected at a synchrotron. This second structure contains layers that are isostructural to the first, bridged together by carbonate, and coordinated by water and ammonia. The properties of these materials were studied by thermogravimetric analysis-mass spectrometry, photoluminescence, electron paramagnetic resonance, and Raman and infrared spectroscopy. The 2-D [Gd(CO(3))(2)H(2)O][NH(4)] is stable to about 175 degrees C, though water and ammonium loss continues through the entire thermogravimetric analysis trace. The 3-D material remains intact until about 325 degrees C. Both structures exhibit broad luminescence bands in the near-ultraviolet region centered at 354 nm. Electron paramagnetic resonance and magnetic susceptibility show spin-spin coupling between adjacent gadolinium atoms in both structures and confirm that they are paramagnetic. These materials show interesting photoluminescent and paramagnetic properties that could possibly be exploited for chemical sensing or magnetic materials applications.

Antimony oxide hydroxide ethanedisulfonate: a cationic layered metal oxide for Lewis acid applications.

We have discovered a rare example of a cationically charged inorganic material. The layered structure is an example outside the extensively studied isostructural set of anionic clays/layered double hydroxides and our previously reported lead fluoride nitrate. For the present compound, the antimony oxide hydroxide layers are positively charged and are templated by anionic alkylenedisulfonate. The organic molecules are only bonded electrostatically to the layers with sulfonate oxygen to antimony distances beyond the covalent range. The material catalyzes a ketal formation reaction as a Lewis acid without the need for drying the solvent before the reaction or a nonaqueous medium such as toluene. The catalyst is heterogeneous and is completely recovered after the catalysis for reapplication.

Desulfurization of JP-8 jet fuel: challenges and adsorptive materials.

The desulfurization of JP-8 (Jet Propellant 8) fuel is of interest to the U.S. military because of its potential use as a fuel source for solid oxide fuel cells (SOFCs). SOFCs can be used to supply a steady stream of power during military silent watch missions. Adsorptive desulfurization is a promising alternative to hydrodesulfurization, which is unable to remove refractory sulfur compounds and achieve the ultra-low sulfur levels necessary to prevent poisoning of SOFCs. Adsorptive desulfurization could be a portable, on-site process performed on JP-8 stocks already in the field. Within the vast field of fuel processing/reformation, herein we focus on the current status of adsorptive desulfurization performed on JP-8 jet fuel. Currently, the best performing sorbents are those utilizing high surface area porous frameworks with pore sizes large enough to accommodate sulfur contaminants. Additionally, a variety of metals in ionic, metallic, and oxide form serve as promising active sites within these sorbents. Most reports focus on reformation technologies and sorbent materials for gas-phase desulfurization and hydrogen purification of low-sulfur content diesel or light fraction jet fuel. JP-8 is unique to the Army in terms of supply. This review will thus focus on ongoing efforts in the room temperature liquid desulfurization of JP-8 and its higher levels of impurities that are more complex and difficult to remove.

Anion exchange dynamics in the capture of perchlorate by a cationic Ag-based MOF.

We report a detailed study of the host-guest interaction for a cationic metal-organic framework that can reversibly capture perchlorate. The structural transformation and flexibility of silver 4,4'-bipyridine nitrate (SBN) upon formation of silver 4,4'-bipyridine perchlorate (SBP) was evaluated by monitoring the anion exchange dynamics using a combination of powder X-ray diffraction (PXRD) with multinuclear 13C, 15N and 109Ag solid-state NMR spectra at different time intervals of the anion exchange. The structural transformation from SBN to SBP is complete within 70 minutes and was determined to take place by a solvent-mediated process. This pathway is confirmed by the morphological changes of the two crystalline materials observed by SEM. This key understanding may lead to application of this material towards perchlorate capture.

Microfabrication using elastomeric stamp deformation.

Elastomeric stamp deformation has been utilized for the contact printing (CP) of self-assembled monolayers (SAMs) and, more recently, polymers and proteins. Here, we take advantage of this well-studied phenomenon to fabricate a series of new metal thin-film patterns not present on the original stamp. The rounded patterns are of nanoscale thickness, long-range order, and are created from elastomeric stamps with only straight-edged features. The metal was printed onto the surface of an alpha,omega-alkanedithiol self-assembled monolayer (SAM). The new shapes are controlled by a combination of stamp geometry design and the application of external pressure. Previously published rules on stamp deformation for contact printing of SAMs are invalid because the coating is instead a thin-metal film. This method represents a new pathway to micropatterning metal thin films, leading to shapes with higher complexity than the original lithographic masters.

Crystal Structures of a Series of Novel Alkylammonium Phosphates and Their Formation in Aluminophosphate Synthesis Mixtures.

The crystal structures of several novel alkylammonium phosphate salts were determined by single-crystal X-ray methods: cyclopentylammonium monohydrogen phosphate (CMP) ([C(5)H(9)NH(3)(+)](2)[HPO(4)(2)(-)], monoclinic, space group P2(1)/n, Z = 4, a = 14.197(3) Å, b = 5.992(1) Å, c = 16.580(3) Å, beta = 98.57(2) degrees ); octylammonium dihydrogen phosphate (ODP) ([CH(3)(CH(2))(7)NH(3)(+)][H(2)PO(4)(-)], monoclinic, space group P2(1)/c, Z = 4, a = 4.554(1) Å, b = 34.652(3) Å, c = 7.328(1) Å, beta = 90.66(1) degrees ); and decylammonium dihydrogen phosphate (DDP) ([CH(3)(CH(2))(9)NH(3)(+)][H(2)PO(4)(-)], monoclinic, space group P2(1)/n, Z = 4, a = 7.358(2) Å, b = 39.749(12) Å, c = 9.131(4) Å, beta = 90.45(2) degrees ). Each are composed of anionic layers of hydrogen-bonded phosphate molecules, separated by charge-balancing alkylammonium cations. The long-chain alkylammoniums in ODP and DDP are interdigitated, while the cyclopentylammoniums in CMP define a bilayer-like arrangement. Large crystals were grown by recrystallization, but the materials were also observed to form in the corresponding solvothermal aluminophosphate synthesis mixtures. We have isolated phosphate salt compounds in aluminophosphate synthesis mixtures containing a variety of different alkylamines and suggest they are initially present in such experiments whenever the alkylamine is primary or secondary.