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Remediation and Control Technologies

A Cationic Silver Pyrazine Coordination Polymer with High Capacity Anion Uptake from Water Eaindar Soe, Beatriz Ehlke, and Scott R. J. Oliver Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01633 • Publication Date (Web): 07 Jun 2019 Downloaded from http://pubs.acs.org on June 9, 2019

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Environmental Science & Technology

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A Cationic Silver Pyrazine Coordination Polymer

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with High Capacity Anion Uptake from Water

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Eaindar Soe, Beatriz Ehlke and Scott R. J. Oliver*

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University of California, Santa Cruz, Department of Chemistry and Biochemistry, 1156 High

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Street, Santa Cruz, California 95064

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Keywords: silver pyrazine, anion exchange, coordination polymers, perrhenate,

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alkanedicarboxylates

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Abstract

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We report the first example of linker modification for an N-donor Ag-based cationic material

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while maintaining and in some cases increasing the anion exchange capacity. Cationic silver(I)

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pyrazine nitrate selectively traps harmful oxo-anions from water such as permanganate,

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perrhenate and a variety of α,ω-alkanedicarboxylates. We chose these anions as initial examples

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of exchange for potential water purification. The host-guest interaction between the cationic

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layers of -stacked silver pyrazine polymers and the incoming/outgoing interlamellar anions

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allows for the exchange. The exchange capacity over 24 h reached 435 and 818 mg/g for

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permanganate and perrhenate, respectively, a record for a crystalline metal-organic material and

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over five times the exchange capacity compared to commercial resin. The material also

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undergoes organic exchange as an analog for pharmaceutical waste, some of which have a

ACS Paragon Plus Environment

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carboxylate functionality at the neutral pH range typical of natural water sources. Both the as-

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synthesized and exchanged materials are characterized by a variety of analytical techniques.

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Introduction

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Open-framework metal-organic polymers, derived from a combination of selected

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transition metal and bridging ligands, have attracted much attention owing to their various

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potential applications including electrocatalysis,1 magnetism,2 luminescence,3 gas storage,4 ion

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exchange5 and heterogeneous catalysis.6,7 Coordination polymers (CPs) based on Ag(I) cations

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are attracting a great deal of attention, in part because they are structurally versatile.8,9 The

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coordination sphere of the d10 metal, Ag(I), is very flexible and can adopt coordination numbers

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two to six and various geometries from linear to trigonal, tetrahedral, trigonal pyramidal and

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octahedral.10 The structural flexibility of these complexes is essential for the investigation of

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non-covalent host-guest interactions. Weak intermolecular forces significantly affect the

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geometry and topology of the Ag(I) coordination polymers in the solid state and in turn the anion

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exchange.11

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There has been much recent interest in the study of linear 1-D silver coordination

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polymers involving bridging N-heterocyclic ligands.12,13 The simplest such ligand is pyrazine,

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which has two nitrogen donors in its six-ring. Using a 1:1 ratio with silver nitrate, silver(I)

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pyrazine structures can be synthesized.14,15 The first silver pyrazine structures, [Ag(NC4H4N)+]n

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and [Ag(NC4H4N)2+]n, were first reported in the 1960s16 and have since been the subject of

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various studies including measurements of stability constants,16 infrared spectroscopy17 and mass

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spectroscopy.18 Both structures displayed a high melting point (250 °C), which led the authors to

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suggest polymeric structures because pyrazine can act as bidentate bridging group. In a 1966


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report, the silver pyrazine crystals were inherently twinned and the structure consisted of kinked

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chains of the type [-Ag-NC4H4N-]n.19 Due to the lability of the Ag-donor atom bond,20 the

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process of coordination polymer formation is highly reversible. As a result, the Ag(I)

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coordination polymers can readily be crystallized with various counter-anions, allowing

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investigation by single crystal X-ray diffraction. Anion exchange was not studied; our group is

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thus interested in developing new anion exchange chemistry using silver pyrazine CPs.

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Many of the priority pollutants in wastewater listed by the U.S. Environmental Protection

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Agency (EPA) are in their oxo-anionic form, for example, pertechnetate (TcO4–), chromate

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(CrO42–) and arsenate (AsO43–).21 These toxic, harmful anions are problematic in the vitrification

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of low activity radioactive waste, weakening the integrity of the waste glass by forming

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spinels.2{Citation}2 Another class of water-borne toxic anion pollutants are organic, generated

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through both industrial processes and personal use. EPA classifies many of these pollutants as

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pharmaceutical and personal care products (PPCPs).23 Many pharmaceuticals are sold in their

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acid or chloride form and consequently form anions when dissolved in water. Pharmaceutical

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anions and their metabolites are a growing problem because of ineffective removal or

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chlorination to toxic derivatives in wastewater treatment facilities.24

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Resins with cationic side groups and exchangeable counter anions are still considered the

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standard ion exchanger.25,26 Due to their organic polymer nature, these resins are of limited

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thermal and chemical stability.27 One alternative to possibly replace resins is layered double

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hydroxides (LDHs), an isostructural group of materials containing cationic brucite type layers

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that are charged balanced by interlayer anions with general formula [M2+1−xM3+x(OH)2][An–

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x/n]·xH2O

where M2+ and M3+ are a range of metals, An− is the intercalated anion, and x is ratio


3


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of M3+/( M2+ + M3+). LDHs generally suffer, however, from low selectivity and re-intercalation

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of common anions such as carbonate or sulfate due to the memory effect.28,29

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Owing to the structural diversity and weak host-guest interaction, cationic coordination

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polymers are gaining considerable attention for anion exchange. Silver-based CPs in particular

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have been previously studied for their ability to selectively trap anions via a crystal-to-crystal

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transition.30,31 We reported silver(I) with 4,4-bipyridine polymers and ethanedisulfonate [EDS, –

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O3S(CH2)2SO3–] as a charge balance anion.32 We also reported oxo-anion pollutants exchange

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materials such as [Cu4(OH)62+][EDS2–]·2H2O33 and [M(H2O)n(4,4’-bipyridine)22+][EDS2–] [M =

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Cu(I), Ag(I), Co(II) or Zn(II)].32,34,35 We subsequently described silver-4,4’-bipyridine nitrate

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(SBN)36,37 and acetate (SBA)38 materials. Both circumvent the need for EDS anions and have

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even higher capacity, rate, selectivity and reversibility for trapping of perchlorate from water.

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Wang and coworkers recently reported a variety of cationic extended materials, denoted as SCU-

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n (Soochow University, structure No. n) with outstanding performance for TcO4–/ReO4–

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removal.39–42 These authors also recently reported organic polymers with similar properties.42,43

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There have also been significant advances with selective crystallization approaches for anion

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removal44 and very recently Zr6O8-based cationic MOFs, though the latter require ligands with

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multiple step syntheses and have limited selectivity.45,46

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In this study, a variety of anionic pollutants including permanganate, perrhenate (as an

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analog for pertechnetate) and α,ω-alkanedicarboxylates (as analogs for anionic pharmaceutical

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pollutants) were exchanged into the cationic CP Ag-pyrazine nitrate. The weak interaction

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between the interlamellar nitrates and the cationic silver pyrazine layers likely allows for the

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high capacity exchange.

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Experimental

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Reagents: Silver nitrate (AgNO3, Alfa Aesar, 99%) and pyrazine (NC4H4N, Acros Organics,

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99%) were used as-received for the synthesis. For the exchange studies, the following were used

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as-purchased: potassium chromate (K2CrO4, Fischer Scientific), potassium permanganate

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(KMnO4, Acros Organics), sodium tetrafluoroborate (NaBF4, Acros Organics), potassium

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chloride (KCl, Fischer Scientific), potassium bromide (KBr, Acros Organics), potassium iodide

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(KI, Spectrum Chemical), sodium perrhenate (NaReO4, Alfa Aesar), disodium malonate ([−O2C-

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CH2-CO2−]Na2, TCI America), disodium succinate ([−O2C-(CH2)2-CO2−]Na2, TCI America),

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disodium glutarate ([−O2C-(CH2)3-CO2−]Na2, TCI America), disodium adipate ([−O2C-(CH2)6-

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CO2−]Na2, TCI America), disodium sebacate ([−O2C-(CH2)8-CO2−]Na2, TCI America), ascorbic

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acid (C6H8O6, Fischer Scientific), salicylic acid [HOC6H4(CO2H), Fischer Scientific],

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terephthalic acid [C8H4(CO2H)2, TCI America], 2,6-naphthalene dicarboxylic acid

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[C10H6(COOH)2, TCI America] and Purolite A530E (Purolite).

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Instrumental Details: Powder X-ray diffraction (PXRD) data were obtained on a Rigaku

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Americas Miniflex Plus powder diffractometer, scanning from 2° to 40° (2θ) at a rate of 3°min-1

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with a 0.02° step size under Cu-Kα radiation (λ = 1.5418 Å). Inductively coupled plasma optical

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emission spectrometry (ICP-OES) data were acquired on a Perkin Elmer Optima 4300DV, run in

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radial mode using yttrium as an internal standard. A standard curve for silver(I) was prepared to

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quantify the amount of free silver in the samples. Fourier Transform Infrared (FT-IR)

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spectroscopy data was collected on a Perkin Elmer Spectrum One FT-IR spectrometer.

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Thermogravimetric analysis (TGA) was performed using a TA Instruments Q500 TGA, heating

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from 25 to 600 °C with a gradient of 10 °C/min and nitrogen flow. In situ variable temperature

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PXRD (VT-PXRD) was performed on a Rigaku SmartLab X-Ray Diffractometer with Cu-


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K radiation ( = 1.57418 Å) from 2° to 70° (2) at a scan rate of 2°/min and 0.02° step size.

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The samples were heated at a rate of 10°C/min and held at the set temperature for 5 min before

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scanning. SEM images were collected with a FEI Quanta 3D Dualbeam microscope.

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Synthesis: Silver nitrate and pyrazine (1:1 molar ratio) were added together in 10 mL of

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Millipore water and the mixture was kept at 70 °C for 1 h. The reaction mixture was then cooled

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in an ice bath and rinsed with cold Millipore water and acetone. The filtration gave rise to white

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needle-like small crystals (yield: 65.8% based on AgNO3). Direct synthesis of silver malonate

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was performed by mixing silver nitrate and sodium malonate (1:1 molar ratio) together in 10 mL

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of Millipore water and the mixture was stirred at room temperature for 1 h. The product was

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filtered and rinsed with Millipore water and acetone (yield: 63.4% based on AgNO3).

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Anion Exchange: The exchange reactions were performed by placing [Ag(pyrazine)+][NO3–]

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(0.2 mmol) and the anion of interest (0.2 mmol) in 10 mL of Millipore water. The resulting

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systems were kept under 300-400 rpm magnetic stirring rate at room temperature 25 °C for 24 h.

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Equimolar quantities were used for all anion exchanges but modified if necessary according to

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the charge of the incoming anion. The selectivity tests were performed by introducing the

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material into a sealed beaker containing 10 mL Millipore water and the anion mixture of interest

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(0.2 mmol for each). The reaction was stirred for 24 h under ambient conditions.

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Silver leaching: To study the leaching of silver ion of exchanged product [Ag(pyrazine)+][ReO4–

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], the sample was placed in water under dark, static conditions for 24 h and the solution was

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analyzed by ICP-OES.

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Results and Discussion

132 133

The Ag(I) pyrazine nitrate (SPN) structure (Scheme 1) was synthesized using a slight modification of the literature method.19 The crystals formed within 1 h under low heat, as


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evidenced by the match of the experimental PXRD pattern to the theoretical pattern simulated

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from single crystal data (Figure 1). The single crystal data and structure refinement indicate a

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much better structure solution (4.84%) that the previous publication (9.10%) (Table S1). AgNO3

137 138

+

N

N

H2O

Ag N

N

n

NO3

Ag(pyrazine)+NO3-

Scheme 1: Reaction scheme of SPN.

139

140 141

Figure 1: PXRD pattern of SPN: (a) theoretical; (b) as-synthesized.


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142 143

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Figure 2: SEM image of a representative needle-shaped crystal of SPN. SPN crystallizes as colorless needle crystals (Figure 2). The crystals are in the Pī space

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group of the triclinic system. Single crystal XRD revealed that the structure contains one

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dimensional silver(I) pyrazine chains, -stacked [3.483(2) to 3.975(4) Å] into layers with charge-

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balancing nitrate anions between the layers (Figure 3). The chains are slightly kinked with Ag-N

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distance ranging from 2.211(7) to 2.229(7) Å (Table S2). Within the polymeric chain, the N–Ag–

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N is significantly bent at an angle of 157.4(2)°, likely due to the presence of weakly coordinating

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oxygen atoms of nearby nitrate anions. The bond length between Ag(I) and the oxygen from

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nitrate was 2.558(6) Å, creating weak covalent Ag-O bonds47 that play a crucial role in the anion

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exchangeability (vide infra).


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Figure 3: Crystallographic views of SPN: (a) a-projection; (b) b-projection;

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(c) c-projection (hydrogen atoms are omitted for clarity; silver = gray, nitrogen = blue, oxygen =

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red, carbon = black).

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The thermal behavior of SPN was investigated by in-situ variable temperature powder X-

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ray diffraction (VT-PXRD) and thermogravimetric analysis (TGA), both under nitrogen flow

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(Figure 4). At 145 °C, an additional unknown crystalline phase appeared [first peak at ~ 12°

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(2)]. Both the original phase and this new phase are gone by 225 °C, replaced by peaks at ~ 19°,

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21°, 24° and 29° (2, peaks indicated with asterisks in Figure 4) and correspond to silver nitrate

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(ICDD PDF #00-043-0649). The TGA trace showed only a single weight loss at ~ 175 °C and is

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likely due to the partial loss of pyrazine (calculated: 20.8%, observed: 18.1%). At 400 °C, the

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VT-PXRD shows a mixed phase of silver nitrate, silver oxide (ICDD PDF #03-065-3289, peaks

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indicated by number signs) and silver metal (ICDD PDF #01-071-5025, peaks indicated by solid

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diamond).


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Figure 4: Thermal analysis of SPN: (left) TGA trace and (right) variable temperature PXRD. (*,

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# and  denote the most intense peaks of silver nitrate, silver oxide and silver metal,

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respectively).

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All of the anion exchange studies of [Ag(pyrazine) +][NO3–] were performed in aqueous

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conditions and pH 7. The first exchange investigated was organic, using various -

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alkanedicarboxylate anions. An equimolar ratio of malonate, succinate, glutarate, adipate or

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sebacate to the CP material was tested in aqueous solution (Figure 5). The principal peak of the

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as-synthesized nitrate-containing material is at 12.5° (2θ) (Figure 5a), corresponding to a basal

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spacing of 7.1 Å (Table 1). Based on the relative carbon chain length and size of carbonate end

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groups [−O2C(CH2)nCO2−], it was expected that succinate (n = 2), glutarate (n = 3), adipate (n =

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4) and sebacate (n = 8) would shift the principal (200) peak to lower 2θ values as the interlayer

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distance increases.


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Figure 5: PXRD of (a) as-synthesized SPN and after uptake of [−O2C(CH2)nCO2−]: (b) n = 1,

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malonate; (c) n = 2, succinate; (d) n = 3, glutarate; (e) n = 4, adipate; (f) n = 8, sebacate.

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Table 1: Principal PXRD peak (2 and d-spacing values) for SPN and the organic-containing

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materials

Anion

2 (°)

d spacing (Å)

As synthesized

12.5

7.1

Malonate

16.6

5.3

Succinate

11.9

7.4

Glutarate

8.5

10.4

Adipate

8.0

11.0

Sebacate

5.7

15.5

185 186 187

Malonate (n = 1, Figure 5b) shifted to higher 2θ value due to its shortest length. Organic uptake was confirmed by directly synthesizing silver malonate from silver nitrate and disodium


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malonate in a 1:1 ratio. The resultant PXRD pattern was identical to Figure 6c, with slight

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variation in intensity due to the fine microcrystalline nature of the directly synthesized material.

190 191

Figure 6: PXRD pattern of: (a) as-synthesized SPN (black); (b) after reaction with sodium

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malonate (red); (c) directly synthesized silver malonate (blue).

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As expected, longer carbon chains shifted the prominent peak to lower angle, with

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sebacate being the longest of the dicarboxylates tested (Figure 5d-f). The principal PXRD peak

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of the [Ag(pyrazine)+][−O2C(CH2)nCO2−] materials and their corresponding d-spacing are

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reported in Table 1 and plotted in Figure 7.

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Figure 7: Principal PXRD peak versus total number of carbons for

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[Ag(pyrazine)+][−O2C(CH2)nCO2−] (n = 1 to 8).

200

As the number of carbons of the dicarboxylate anion increased, the 2θ value of the 100% peak

201

decreased according to Bragg’s law (Figure 7). Attempts to reverse the exchange have been

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unsuccessful to date, implying the organic-intercalated materials are of higher stability. The

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[Ag(pyrazine)+][−O2C(CH2)nCO2−] materials are thermally stable up to 300 °C (Figure S1),

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which further supports the irreversibility. The appearance of carboxylate peaks in the FTIR

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spectra also confirms that organic uptake has taken place (Figure 8). The C=O and C-O stretches

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of carboxylate bands were found at 1570 and 1300 cm−1, respectively.

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Figure 8: FTIR spectra of: (a) as-synthesized SPN; after uptake of [−O2C(CH2)nCO2−]: (b) n = 1,

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malonate; (c) n = 2, succinate; (d) n = 3, glutarate; (e) n = 4, adipate; (f) n = 8, sebacate.

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The selectivity amongst the -alkanedicarboxylates was also tested to see if there is a

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preference between them. The exchange started out with a mixture of even (C2, C4 and C8) or

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odd (C1, C3 and C8) carbon chain length dicarboxylates (Figure 9). In all tests, sebacate


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[−O2C(CH2)8CO2−] was the only phase observed, likely due to its greater amphiphilicity and thus

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stability of the n = 8 phase via van der Waals close-packing.

215 216

Figure 9: PXRD pattern of: (a) as-synthesized [Ag(pyrazine)+][NO3−]; (b) the product after

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malonate, glutarate and sebacate in competition; (c) after succinate, adipate and sebacate in

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competition; (d) exchange with sebacate only.

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In addition to α,ω-alkanedicarboxylates, the material displays high capacity inorganic

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anion exchange for water-borne metal oxo-anion pollutants. Permanganate, perrhenate and

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tetrafluoroborate were studied as initial examples. The latter was chosen as a comparitive

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tetrahedral anion whereas permanganate and perrhenate were selected as group 7 surrogates for

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pertechnetate, which is a problematic radioactive pollutant during the vitrification of nuclear

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waste.48 An equimolar amount of permanganate and as-synthesized SPN were introduced into

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aqueous solution under mild stirring. As monitored by UV-Vis spectroscopy, the permanganate

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concentration rapidly decreased by 32% to 57% with reaction intervals of 0.5 h to 3 h,

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respectively (Figure 10). The uptake of permanganate was confirmed by PXRD (Figure S2) and

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was 92% after 24 h with an adsorption capacity of 435 mg/g (0.91 mol/mol). This uptake


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performance is better than SLUG-21 (292 mg/g)32 and Ag[4,4’-bis(1,2,4-

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triazole)]PF6·0.5CH3CN (342 mg/g).49

231 232

Figure 10: (a) UV-Vis spectra of permanganate solution during anion exchange with SPN at

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increasing time interval; (b) relative intensity of the 524 nm MnO4– maximum versus time.

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Similarly, perrhenate exchanged successfully according to PXRD, with the powder

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pattern corresponding to the directly synthesized [Ag(pyrazine)+][ReO4–] (Figure 11). ICP-OES

236

was used to quantitatively monitor the Re uptake, reaching over 82% removal from solution in

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8 h (Table 2, Figure 12).


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Figure 11: PXRD pattern of: (a) as-synthesized SPN (black); (b) exchanged with ReO4−; (c)

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directly synthesized [Ag(pyrazine)+][ReO4–].

241 242

Figure 12: Concentration of perrhenate vs. time based on ICP-OES for anion exchange by

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[Ag(pyrazine)+] [NO3–] (black) and Purolite A530E (red).


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To understand the change in morphology of SPN during anion exchange with perrhenate,

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in-situ optical microscopy was performed to monitor the transformation between the two

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crystalline phases (Figure 13). As-synthesized SPN crystals were immersed in an aqueous

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solution of sodium perrhenate under ambient conditions, yielding the highly crystalline phase

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[Ag(pyrazine)+][ReO4–]. The formation of [Ag(pyrazine)+][ReO4–] was observed on the surface

249

of the SPN crystals after 40 min. The competitive crystallization allows [Ag(pyrazine)+][ReO4–]

250

to gradually grow in expense of the original SPN crystal to form the distinct new phase and

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crystal morphology after 60 min. Unlike solid-state equilibrium-driven anion exchange process

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for LDHs, this crystal-to-crystal transition process indicates that our material undergoes a

253

solvent-mediated recrystallization mechanism typical of this type of material, including SBN.36,37

254 255

Figure 13: In-situ optical micrographs of a single crystal of SPN in sodium perrhenate solution

256

for 10 min (a), 20 min (b), 30 min (c), 40 min (d), 50 min (e) and 60 min (f). Scale bar is 500

257

m.


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Table 2: Adsorption capacity of SPN and Purolite A530E with an equimolar ratio of perrhenate

259

in 50 mL aqueous solution over a 24 h interval, as monitored by ICP-OES

Time (h)

SPN

Purolite A530E

Adsorption (mg/g)

Removal (%)

Adsorption (mg/g)

Removal (%)

1

476.6

47.8

85.6

8.6

2

355.2

35.6

120.4

12.0

4

764.7

76.6

125.5

12.6

8

812.6

81.4

167.7

16.8

24

818.0

82.0

175.0

17.5

260 261

The adsorption capacity of SPN is 818 mg ReO4−/g of material, higher than that of SBN (786

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mg/g),43 NDTB (720 mg/g),50 , SCU-COF-1 (702.4 mg/g),55 SLUG-21 (602 mg/g),32 SCU-8

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(540 mg/g),39 SCU-100 (541 mg/g)40, SCU-102 (291 mg/g),56 SCU-101 (217 mg/g)42, UiO-66-

264

NH3+ (159 mg/g)51. The rate of uptake is slower than that of SBN, SCU-8, SCU-100, SCU-101

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and SCU-COF-1 nor did it reach as high capacity as the recently reported organic polymer, SCU-

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CPN-1 (999 mg/g).41 Our material is extremely easy to synthesize using water, commerically

267

available reagents and ambient conditions. The higher specific uptake of SPN versus the other

268

CP/MOF type materials is likely due to the shorter and thus lighter organic linker, coupled with a

269

higher cationic charge density and thus anion density. PXRD before and after anion exchange

270

confirmed that SPN retains its crystalline layered character after intercalation, with the d-spacing

271

slightly increasing from 7.0 to 7.4 Å due to the larger anion (Figure 14). The exchange for both

272

permanganate and perrhenate were irreversible after all attempts. Reusability is, in fact,

273

undesirable for a highly problematic pollutant such as the radionuclide pertechnetate.


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274 275

Figure 14: PXRD spectra of as-synthesized SPN (a) and after exchange with MnO4− (b), ReO4−

276

(c) and BF4− (d).

277

Since BF4−, MnO4− and ReO4− are all tetrahedral geometry, we were interested in the

278

selectivity. According to PXRD data, perrhenate outperformed the other anions (Figure 15).

279


19


280

Figure 15: PXRD spectra of SPN after exchange with: (a) ReO4− only; (b) BF4− and ReO4− in

281

competition; (c) BF4−, MnO4− and ReO4− in competition.

Page 20 of 30

282

Given the high adsorption capacity of [Ag(pyrazine)+][ReO4–], we also performed the

283

competition study with chloride (Cl–) which is commonly present in water (Figure 16). The

284

selectivity of SPN for perrhenate over chloride was demonstrated by performing two lower

285

chloride concentration tests containing 100 ppm and 200 ppm, as evidenced by PXRD (Figure

286

16b,c). In both tests it was observed that the chloride did not interfere with the uptake of

287

perrhenate. When SPN is exposed to an equimolar ratio of ReO4– and Cl–, the resulting mixture

288

gave rise to pure-phase AgCl (Figure 16d).

289 290

Figure 16: PXRD pattern of SPN exchange with (a) ReO4– only in nanopore water; (b) ReO4–

291

and 200 ppm Cl– in tap water; (c) ReO4– and 100 ppm Cl– in tap water; (d) ReO4– and Cl– in

292

equimolar ratio.


20

Page 21 of 30

293


We attempted to prepare crystals suitable for single crystal X-ray diffraction in order to

294

have a deeper understanding of the specific binding. Among them, only [Ag(pyrazine)+][ReO4–]

295

successfully yielded colorless needle-type crystals, matching the theoretical pattern based on the

296

crystal data reported by Maggard et. al.52 The ReO4− tetrahedra, which reside in the space

297

between the silver pyrazine layers, are separated from each other by 4.61 Å (O1-O1’). Each

298

ReO4− tetrahedron bonds via three O vertices to three different Ag+ in the layer [Ag–O distance

299

2.603(3) to 2.686(5) Å], creating stronger silver to oxygen bonds than BF4– and MnO4– anions

300

(Figure 17). Indeed, Wang et al. reported density functional theory (DFT) analysis for the

301

bonding of a similar silver(I) polymer, [Ag(4,4’-bipyridine)+][ReO4–], where ReO4– has a

302

stronger electrostatic interaction with the polymers that creates a better match of coordinating

303

environment compared to other anions such as BF4– and MnO4– .43

304 305

Figure 17: Crystallographic views of the [Ag(pyrazine)+][ReO4–] structure (hydrogen atoms

306

omitted for clarity; silver = gray, nitrogen = blue, oxygen = red, carbon = black).

307 308 309

A silver leaching measurement was carried out to verify the stability of the SPN material and to confirm the heterogenous nature of the CP. The material was placed in solution and the


21


Page 22 of 30

310

latter was analyzed after 24 h using ICP-OES in order to measure the leached silver. The

311

concentration of the leached silver in the solution was found to be less than 2% for all the

312

samples (Table 3). This may be due to the strong interaction of silver with nitrogen from

313

pyrazine groups which prevents the leaching of silver from the polymer matrix. Table 3: The

314

concentration of leached silver from SPN was measured by ICP-OES. The data was collected at

315

328.06 wavelength (nm) for silver Silver concentration (ppm)

% silver loss

0.54

0.05

25.54

0.24

48.13

0.45

100.80

0.93

158.22

1.46

316 317

Figure 18: PXRD spectra: (a) after reaction of SPN with salicylic acid; (a-i) directly synthesized

318

silver salicylate; (b) after reaction of SPN with p-terephthalate; (b-i) directly synthesized silver


22

Page 23 of 30


319

terephthalate; (c) after reaction of SPN with 2,6-naphthalene dicarboxylate; (c-i) directly

320

synthesized silver 2,6-naphthalene dicarboxylate.

321

Attempts to intercalate other aromatic dicarboxylates such as salicylic acid, p-

322

terephthalate, and 2,6-naphthalene dicarboxylate were not successful, likely due to the limited

323

aqueous solubility of these analytes at neutral pH. Other reports of the intercalation of

324

terephthalate derivatives into cationic materials occurred in basic aqueous solutions.53,54 PXRD

325

data revealed that the exchange products turned out to be identical to the directly synthesized

326

silver dicarboxylates (Figure 18), confirming that pyrazine in this case had been lost upon the

327

exchange attempt.

328

The ability to modify the N-donor linker for these Ag-based cationic CP materials may

329

allow tuning of the anion propensity for water purification and pollution abatement. The cationic

330

material silver(I) pyrazine shows the highest group 7 oxo-anion uptake (435 and 818 mg/g for

331

permanganate and perrhenate, respectively) to date for a metal-organic extended structure,

332

making it potentially useful to trap pertechnetate. The capacity and kinetics far outperform the

333

anion exchange resin Purolite 530E.57 Unlike some of the recently reported cationic MOFs, all of

334

the synthesis and exchange occur aqueously with commercially available reagents. With the

335

formation of a new crystal structure driving the reaction, the anions become incorporated into a

336

structure. Reuse of the material would, in fact, be undesirable for a highly problematic

337

radionuclide. The material also displays uptake toward organic water-borne pollutants through a

338

series of α,ω-alkanedicarboxylates.

339 340

ASSOCIATED CONTENT


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Page 24 of 30

341

Supporting Information. Crystal data tables of [Ag(pyrazine)+][NO3–] (SPN), TGA of

342

[Ag(pyrazine)+][−O2C(CH2)nCO2−] exchange products and PXRD of the kinetic studies for

343

[Ag(pyrazine)+][MnO4–]. This material is available free of charge via the Internet at

344

http://pubs.acs.org.

345

AUTHOR INFORMATION

346

Corresponding Author

347

*Email: [email protected]

348

Author Contributions

349

The manuscript was written through contributions of all authors. All authors have given approval

350

to the final version of the manuscript.

351

Funding Sources

352

This work was supported by the National Science Foundation under Grant No. 1603754 from the

353

CBET Environmental Engineering GOALI Program.

354

Notes

355

ACKNOWLEDGMENTS

356

This work was supported by the National Science Foundation under Grant No. 1603754 from the

357

CBET Environmental Engineering GOALI Program. We thank Professor Timothy C. Johnstone,

358

University of California, Santa Cruz, Department of Chemistry and Biochemistry for solving the

359

twinned data set. We also thank Professor Allen Oliver of the University of Notre Dame,

360

Department of Chemistry and Biochemistry and the American Crystallographic Association

361

2018 Summer Course for data collection.


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Page 25 of 30


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