15213765, 2022, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202200357 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article Chemistry—A European Journal doi.org/10.1002/chem.202200357 www.chemeurj.org pH-Triggered Removal of Nitrogenous Organic Micropollutants from Water by Using Metal-Organic Polyhedra Laura Hernández-López,[a, b] Alba Cortés-Martínez,[a, b] Teodor Parella,[c] Arnau Carné-Sánchez,*[a, b] and Daniel Maspoch*[a, b, d] Abstract: Water pollution threatens human and environ- exploit the exohedral coordination positions of RhII-MOP to mental health worldwide. Thus, there is a pressing need for coordinatively sequester pollutants bearing N-donor atoms in new approaches to water purification. Herein, we report a aqueous solution, and then harness their exposed surface novel supramolecular strategy based on the use of a metal- carboxyl groups to control their aqueous solubility through organic polyhedron (MOP) as a capture agent to remove acid/base reactions. We validated this approach for removal nitrogenous organic micropollutants from water, even at very of benzotriazole, benzothiazole, isoquinoline, and 1-napthyl- low concentrations (ppm), based exclusively on coordination amine from water. chemistry at the external surface of the MOP. Specifically, we Introduction als such as zeolites, activated carbon, covalent organic frame- works, and metal-organic frameworks, all of which offer large Hazardous organic micropollutants are found in natural water surface areas and whose pores can be chemically modified.[3–6] resources worldwide, posing a threat to human health and to Other candidates are nanomaterials (e. g., nanoparticles, nano- ecosystems.[1,2] Thus, there is a pressing need to develop tubes, graphene, etc.), which boast high surface area-to-volume strategies and materials for water purification. Among the most ratios, given their small size. Additionally, some nanomaterials efficient strategies for removal of organic micropollutants from exhibit highly reactive surfaces that can be functionalised for water is adsorptive removal, which in some cases is followed by catalysing the degradation of the adsorbed pollutants.[7,8] degradation of the pollutant. Effective adsorbents must com- Researchers have recently begun to develop supramolecular bine high surface areas with strong chemical affinity for the strategies based on host-guest chemistry to capture and target pollutants. Promising candidates include porous materi- separate substances of interest.[9–12] In these strategies, discrete molecular compounds can be used in solution to selectively [a] L. Hernández-López, A. Cortés-Martínez, Dr. A. Carné-Sánchez, recognise, adsorb, and entrap the substance of interest inside Prof. D. Maspoch their cavities.[13–16] The resultant host-guest complex is then Catalan Institute of Nanoscience and Nanotechnology (ICN2) isolated in solution by liquid/liquid extraction or phase transfer. CSIC and The Barcelona Institute of Science and Technology Campus UAB, Bellaterra, 08193 Barcelona (Spain) Finally, the guest molecule is liberated from the host upon E-mail: arnau.carne@icn2.cat breakage of the host-guest interaction. For example, metal- daniel.maspoch@icn2.cat organic coordination cages have been used to selectively [b] L. Hernández-López, A. Cortés-Martínez, Dr. A. Carné-Sánchez, separate specific polycyclic aromatic hydrocarbons from a Prof. D. Maspoch mixture of similar molecules by phase-transfer phenomena.[17] Departament de Química, Facultat de Ciències Universitat Autònoma de Barcelona Alternatively, multitopic ion-pair receptors based on 08193 Bellaterra (Spain) calix[4]pyrrole derivatives have been employed to remove [c] Dr. T. Parella inorganics (K +, Li +, and Cs +) from aqueous solution by liquid/ Servei de Ressonància Magnètica Nuclear liquid extraction.[18,19] Universitat Autònoma de Barcelona Our group recently reported an interesting alternative to Campus UAB, Bellaterra, 08193 Barcelona (Spain) the aforementioned host-guest approach to capture species of [d] Prof. D. Maspoch ICREA interest: rather than do coordination chemistry in the pores or Pg. Lluís Companys 23, 08010 Barcelona (Spain) cavities of molecular systems such as cages, we instead focus Supporting information for this article is available on the WWW under on coordination chemistry at the external surface of metal- https://doi.org/10.1002/chem.202200357 organic cages or polyhedra (MOP). As proof-of-concept, we © 2022 The Authors. Chemistry - A European Journal published by Wiley- used the prototypical RhII-based MOP that comprises 12 VCH GmbH. This is an open access article under the terms of the Creative divalent Rh Rh paddlewheel clusters and 24 angular benzene- Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, 1,3-dicarboxylate (bdc) linkers, exhibits a cuboctahedral shape, the use is non-commercial and no modifications or adaptations are made. and has an external diameter of 2.5 nm. Given the nanoscopic Chem. Eur. J. 2022, 28, e202200357 (1 of 8) © 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH 15213765, 2022, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202200357 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article Chemistry—A European Journal doi.org/10.1002/chem.202200357 size and functional outer surface of this MOP, using it to capture COONaRhMOP; where COO-bdc = 1,3,5-benzenetricarboxylate). species resembles the use of nanoparticle to do the same, albeit COONaRhMOP can then be reprotonated upon exposure to an with the benefit of stoichiometric precision. This precision acid (e. g., HCl), which precipitates it out from water such that it stems from the 84 available positions on the outer surface of can be recovered by filtration or centrifugation. We envisioned the MOP, of two types. The first type, of which there are 12, are that this pH-based solubility could serve as a trigger in a located in the 12 Rh Rh paddlewheels. Each of these clusters pollutant-removal system, reasoning that, once a coordinating expose a single exohedral axial coordination site that can be molecule had become anchored to the surface of a MOP, the harnessed to bind coordinating molecules. latter would govern the solubility of the former. The second type, of which there are 72, derive from the 24 Thus, our supramolecular process for pollutant-removal is bdc linkers, each of which can be functionalised at three based on four steps. Firstly, coordinative interactions between positions (4, 5 and 6) of its phenyl ring by conventional organic the organic pollutant and the water-soluble COONaRhMOP are chemistry.[20] Our group previously demonstrated that this established in solution (Figure 1). Secondly, the COONaRhMOP- surface chemistry could be used to separate physicochemically pollutant complex is precipitated out, by lowering the pH using similar molecules that differ in their affinity to the exohedral RhII HCl, and subsequently isolated from water by filtration or axial site (separation by phase-transfer)[21] and in the steric centrifugation. Thirdly, the precipitate is washed with aq. CaCl2, hindrance around their coordinating atom (separation by leading to liberation of the pollutant. Through this washing liquid/liquid extraction).[22] step, CaII ions coordinate strongly to the organic micropollutant, In the work that we report here, we adapted our previous breaking the COOHRhMOP-pollutant coordination interaction surface chemistry approach to develop a new supramolecular and dissolving the pollutant back into an aqueous solution. In strategy that uses the cuboctahedral RhII-MOP as a capture the fourth and final step, the COOHRhMOP is treated with agent to remove organic micropollutants from water by pH- NaOH, causing it to redissolve in the remaining solution, which controlled precipitation (Figure 1). This strategy is based on might contain residual pollutants, thereby enabling its reuse for combining the coordination ability of the RhII sites to capture subsequent cycles of water purification. organic pollutants that bear functional groups, with a simple acid-based reaction performed on the bdc linkers to control the solubility of the MOP in water. Thus, each one of the character- Results and Discussion istic 12 exohedral axial coordination sites of these cuboctahe- dral RhII-MOPs is used to capture and bind coordinating Removal of benzotriazole as a test-case. Step 1: Coordination pollutants from water by coordination chemistry. Moreover, between benzotriazole and COONaRhMOP in water among the different members of these cuboctahedral MOPs, we selected [Rh2(COOH-bdc)2]12 (hereafter named COOHRhMOP; As proof-of-concept, we chose to test our MOP-based strategy where COOH-bdc = 5-carboxy-1,3-benzenedicarboxylate). This by attempting to remove benzotriazole (BT) from water. MOP is functionalised with a carboxylic acid group at the 5- Benzotriazole is broadly used both in industrial and household position of the phenyl ring of each bdc linker, such that its products: for instance, as a corrosion inhibitor; as an ultraviolet external surface is functionalised with a total of 24 carboxylic light-stabilizer in plastics; as an ultraviolet filter; as antifogging acid groups.[23] These groups are essential for our or defogging agent; and in de-icing/anti-icing fluids. The high supramolecular strategy, as they confer the MOP with pH- amount of BT disposed by such activities, the high water- dependent aqueous solubility. Indeed, when COOHRhMOP is solubility of BT (ca. 20 g · L 1), and the slow biodegradation of exposed to a base (e. g., NaOH), it becomes an anionic, water- BT lead to its high persistence in aquatic environments, where, soluble MOP of formula Na24[Rh2(COO-bdc)2]12 (hereafter named at concentrations above about 5 ppm,[24,25] it causes environ- Figure 1. a) Structure of the cuboctahedral RhII-MOP, highlighting the 5-positions in the organic backbone (red balls) and the axial sites of its dirhodium paddlewheels (green balls). b) Schematic of the pH-triggered supramolecular removal strategy. Chem. Eur. J. 2022, 28, e202200357 (2 of 8) © 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH 15213765, 2022, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202200357 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article Chemistry—A European Journal doi.org/10.1002/chem.202200357 mental harmful long-term effects.[26–28] In fact, BT has been resulting solutions (final pH = 8.6 and 7.8, respectively) were proposed as a micropollutant indicator of water contamination incubated for only 10 s and subsequently precipitated by through anthropogenic activities, due to its ubiquity in surface lowering the pH with different amounts of HCl acid. The water and its environmental toxicity.[29] Given that BT contains a precipitation solids were isolated by centrifugation. Once the triazole functional group fused to a benzene ring, we envisaged solids had been isolated, the optimum amount of acid for the that we could anchor the molecule to the surface of precipitation process was determined by analysing the remain- COONaRhMOP, by coordinating the free N-donor atoms in the ing BT in the aqueous solution after the precipitation step by triazole of the former to the exposed axial sites of the Rh Rh means of UV-Vis measurements and establishing its removal paddlewheels in the latter. To confirm this, we added BT to an efficiency (Figure S4). For both mixtures, the best performance aqueous solution of COONaRhMOP, and then monitored their was observed under milder acidic conditions when 10 μL of HCl interaction by naked eye. We found that the addition of BT 1 M (final pH = 2.3) were used to precipitate out the (24 mol equiv, 380 ppm) to an aqueous solution of COONaRh- COOHRhMOP(BT). Based on this amount of acid, the following MOP (0.133 μmol, 1 mL) induced an immediate change in removal efficiency values were determined: 77 % for the colour of the solution, from blue to purple, suggesting a solution initially containing 6 mol equiv of BT; and 53 % for the coordinative interaction between the BT and the Rh Rh one containing 24 mol equiv of BT (see Section S3.5 in the paddlewheel. Next, we monitored this interaction by UV-Vis Supporting Information). In both cases, the amount of MOP spectroscopy, focusing on the bands centred at 500 to 600 nm, remaining in solution was lower than 0.1 % (Table S1). Impor- which correspond to the π*!σ* transitions (λmax) of the Rh Rh tantly, blank experiments (i. e., lacking COONaRhMOP) were bonds. A shift in the Rh Rh bond absorption band (λmax), from performed in solution. These experiments revealed that the 585 to 551 nm, corroborated coordination of the BT to the concentration of BT remained constant throughout the pH Rh Rh paddlewheel (Figure S3 in the Supporting range studied (pH: 5.8 to 1.9), demonstrating the high aqueous Information).[30,31] To further study the coordination of BT to the solubility of BT and indicating that the solubility of the Rh Rh paddlewheel units, we followed the titration of COOHRhMOP(BT) complex is indeed governed by that of COONaRhMOP with BT by UV-Vis spectroscopy. We found that, COOHRhMOP itself (Figure S5). below 10 mol equiv (159 ppm) of BT, the isosbestic point is preserved, indicating that each exohedral dirhodium axial site behaves independently.[22] Finally, we gained additional evi- Influence of the pH, the incubation time, and the dence that coordination of BT to COONaRhMOP proceeds concentration of BT on the removal efficiency through the exposed surface dirhodium axial sites, upon observing a marginal shift in λmax above 12 mol equiv of BT Once we had optimised the pH at which COOHRhMOP(BT) (190 ppm). precipitates, we sought to elucidate the influence of the pH of the polluted water on the coordination of BT to COONaRhMOP and therefore, on the pollutant removal efficiency. This Step 2: pH-triggered precipitation of COONaRhMOP(BT) parameter might influence the removal efficiency of our proposed pH-triggered pollutant removal methodology due to Having demonstrated the coordination of BT to COONaRhMOP, the amphoteric properties of BT (pKa1: 0.42, pKa2: 8.27). To this we reasoned that the captured BT could then be removed from end, we ran new experiments. Thus, three aqueous solutions water through in situ precipitation of the formed complex containing COONaRhMOP (0.133 μmol, 1.03 mL) and BT (24 mo- (hereafter named COONaRhMOP(BT)). Our hypothesis was l equiv; 380 ppm) were prepared. Each of these solutions, were based on the premise that, once the coordination between brought to a different pH: either acidic (pH: 4.3), neutral (pH: COONaRhMOP and BT had occurred, the solubility of the latter 7.6) or basic (pH: 9.9). The UV-Vis spectrum of each aqueous would be dictated by the solubility of the COONaRhMOP. phase revealed that the largest λmax shifts were observed for the Therefore, we expected that protonation of the surface acidic conditions (Figure S10). This result is consistent with the carboxylate groups of COONaRhMOP(BT) would induce its fact that, at basic pH, BT can be deprotonated, which weakens precipitation, as we had already observed for COONaRhMOP its coordination to COONaRhMOP due to electrostatic repulsion. alone (Figures S1 and S2). We expected that the precipitate Contrariwise, acidic conditions lead to the formation of neutral (hereafter named COOHRhMOP(BT)) could then be easily BT, thereby favouring the formation of COONaRhMOP(BT) removed by using simple techniques such as centrifugation or (Figure S9). However, these differences in the coordination of filtration. To this end, we optimised the pH at which a BT to COONaRHMOP did not translate into significative differ- quantitative precipitation of the MOP occurs, while minimising ences in the removal efficiencies after the precipitation step. protonation-induced cleavage of the BT RhII MOP coordination Thus, after addition of the proper amount of diluted HCl (1 M) bond. These experiments were performed by preparing two to each solution (1 μL to the acidic solution; 10 μL to the neutral different mixtures containing COONaRhMOP (0.133 μmol, 1 mL) one; and 10 μL to the basic one) to reach the optimised and 6 mol equiv (95 ppm) or 24 mol equiv (380 ppm) of BT, precipitating pH 2.3, the removal efficiency for BT was found to being these two concentrations representative examples of be around 54 % in all cases (see Section S3.6). Altogether, these defective and excess concentrations with respect to the 12 results highlight that our supramolecular strategy works in exohedral axial sites present in the MOP structure. Both polluted water samples that vary in their initial pH level. Chem. Eur. J. 2022, 28, e202200357 (3 of 8) © 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH 15213765, 2022, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202200357 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article Chemistry—A European Journal doi.org/10.1002/chem.202200357 The experiments that we have described so far indicate that with a linear increase in BT removed per MOP with increasing the interaction between COONaRhMOP and BT is fast, due to initial conc. of BT (Figure 2b). Remarkably, the removal the absence of diffusion barriers, making a rapid method for efficiency was up to 90 % in solutions that had contained BT at pollutant removal feasible. To further confirm the lack of initial concentrations of 16 ppm. In this case, the remaining BT significant diffusion barriers in our proposed method, we was below 1.6 ppm, which is below the level considered to be performed new experiments to assess the impact of the environmentally toxic (ca. 5 ppm).[24,25] Contrariwise, when incubation time prior the precipitation step on the BT removal 24 mol equiv (380 ppm) were initially present in the solution, efficiency. Accordingly, four independent aqueous solutions the final amount of pollutant dropped to 10 mol equiv containing COOHNaRhMOP (0.133 μmol, 1 mL) and 6 mol equiv (158 ppm; removal efficiently: 55 %). These experiments con- of BT (95 ppm) were prepared. Each solution was incubated for firmed that the ratio between pollutant and exohedral axial a different time (either 10 s, 10 min, 30 min, or 60 min), and coordination metal sites in the COONaRhMOP determines the then subjected to the aforementioned precipitation procedure. performance of our pH-triggered coordination-removal strat- The UV-Vis spectra did not indicate any significant differences egy. in the removal efficiencies, all of which were > 70 %, suggesting that the incubation time does not influence the removal efficiency (Figure 2a, green dots, see Section S3.7.1). These Step 3: Regeneration and reusability of COOHRhMOP results further confirmed the rapid capture and binding of BT to COONaRhMOP. Rapid and easy regeneration of the COOHRhMOP is essential for Once we had demonstrated that our pH-triggered, COO- the feasibility of the proposed removal supramolecular strategy. NaRhMOP-based strategy could indeed remove BT from water, Initially, we reasoned that exposing the COOHRhMOP(BT) solid we next evaluated its performance at different concentrations to harsh acidic conditions would promote the formation of the of BT (Figure 2b, see Section S3.8). The initial and final protonated form of BT and consequently, its detachment from concentrations were determined by either UV-Vis spectroscopy COOHRhMOP. However, we found that the detachment of BT (conc. BT: > 16 ppm) or 1H NMR spectroscopy (conc. BT: required incubation under extremely acidic (3 M HCl) condi- < 16 ppm; Figures S21 and S23). The consistency of the results tions, which we rule out, as they endangered the structural obtained using both techniques was corroborated by analysing stability of the RhII-MOP (Figure S25). Alternatively, we sought a samples at an intermediate concentration (6 mol equiv, milder methodology that would entail washing of the precip- 95 ppm), using both techniques (Figures S21c and S22, Ta- itate with an aqueous solution containing a competing metallic bles S6 and S7). The removal efficiency of BT for the solutions centre for the coordination of BT. The formation of a new that had initially contained between 1 mol equiv (16 ppm) and metal-BT complex would thus favour the regeneration of the 10 mol equiv (158 ppm) was found to be > 70 % in all cases, occupied axial coordination sites of COOHRhMOP, whereby Figure 2. a) Left: Three methods for using COOHRhMOP to remove benzothiazole (BT; 6 mol equiv; 95 ppm) from water: as a porous solid under heterogeneous conditions, either with stirring or in a static reaction; or dissolved under homogeneous conditions, in our proposed, pH-triggered, supramolecular strategy. Right: Plot of the influence of reaction/incubation time on removal efficiency for each method: stirring, static, or homogeneous reactions. b) Removal efficiency of COONaRhMOP for BT at different concentrations of BT, ranging from 1 mol equiv (16 ppm) and 24 mol equiv (380 ppm), by using the supramolecular strategy. In both graphs, the data are reported as average values from triplicate experiments. Errors bars indicate standard deviation. Chem. Eur. J. 2022, 28, e202200357 (4 of 8) © 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH 15213765, 2022, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202200357 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article Chemistry—A European Journal doi.org/10.1002/chem.202200357 new uptake cycles could be performed. To this end, CaII was selected as a competing metallic centre, because of its reduced toxicity as well as its low cost. To explore its efficacy, we performed a preliminary washing experiment. To this end, the solid COOHRhMOP(BT) was washed twice by incubating it in a saturated solution of aq. CaCl2 for 30 s. Note that this COOHRhMOP(BT) was initially precipitated from 5 mL solution of BT (6 mol equiv; 95 ppm), to which the removal strategy had earlier been applied using COONaRhMOP (0.67 μmol). After both washing steps, the remaining COOHRhMOP was quantita- tively transformed into COONaRhMOP through basification with NaOH (16.1 μmol). This basification step caused the RhII-MOP to redissolve in the (now basic) aqueous solution. The success of the regeneration procedure was confirmed by UV-Vis analysis, as the initial λmax of 583 nm characteristic of COONaRhMOP was obtained, thus further corroborating the detachment of BT from the outer surface of the RhII-MOP (Figures S26 and S27). Finally, the recovery and reusability of COONaRhMOP was confirmed by comparing the BT removal efficiency for three additional cycles of uptake (using the same conditions as in Cycle 1), precipitation, and regeneration. Remarkably, the recovered COONaRhMOP maintained its removal efficiency for BT for at least three consecutive cycles (Figures S28 and S29, Table S9). Moreover, the integrity of COONaRhMOP was maintained through the whole cycle, as evidenced by UV-Vis, 1H NMR, and mass spectrometry (MS; see Section S3.10). Use of filtration in the pH-triggered, supramolecular strategy Next, we simplified our supramolecular strategy by replacing the centrifugation steps with a unified filtration process in which recovery of COOHRhMOP(BT), detachment of BT, and regeneration of COONaRhMOP occur sequentially. To this end, a test was run on 5 mL of a solution of BT (6 mol equiv; 95 ppm), to which the removal strategy was applied using COONaRhMOP (0.67 μmol). After precipitation with HCl, the aqueous solution was passed through a nylon syringe-filter (0.45 μm). The filter captured the COOHRhMOP(BT), observed as a purple solid, and the aqueous supernatant was recovered and Figure 3. a) Schematic of the partial removal of BT by the pH-triggered, supramolecular strategy using filtration (rather than centrifugation) to isolate analysed by means of UV-Vis spectroscopy (Figure 3a). This the capture agent. b) Comparison of removal efficiency for filtration vs. analysis revealed a comparable removal efficiency to that centrifugation. The data are reported as the average uptake value from previously obtained (73 %; Figures 3b, S33, Table S10). The filter duplicate experiments. Error bars indicate the standard deviation. was then treated with saturated aq. CaCl2, which induced an immediate change in colour of the solid, from purple to blue, representative of cleavage of the Rh-BT coordination bond and Comparison of the removal of benzotriazole using subsequent release of the BT into the CaCl2 solution. Finally, aq. COOHRhMOP as a porous solid NaOH was passed through the filter to redissolve the resultant COOHRhMOP, through formation of COONaRhMOP (Figures 3a To evaluate the possible influence of classical diffusion barriers and S34). Importantly, a blank experiment (i. e., without on the efficiency of COOHRhMOP at removing BT from water, COOHRhMOP) revealed that the nylon syringe-filter alone does we next ran tests under heterogeneous conditions. To this end, not contribute to any removal of BT (Figure S32). Interestingly, COOHRhMOP powder (5 mg, 0.67 mmol) was soaked in a the strategy also proved successful when, instead of Milli-Q solution containing BT (6 mol equiv; 95 ppm; 5 mL) at a pH of water, regular tap water was employed throughout the entire 6.0. This pH was selected to ensure that the MOP remains in its process (Section S3.12). neutral form, and that the BT is predominantly in its more- coordinative state. The heterogeneous removal tests were performed both with stirring and without (hereafter called Chem. Eur. J. 2022, 28, e202200357 (5 of 8) © 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH 15213765, 2022, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202200357 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article Chemistry—A European Journal doi.org/10.1002/chem.202200357 static), and the removal efficiency was quantified after different whereby a balance between the removal efficiency and the incubation times. Interestingly, the removal efficiency was complete precipitation of COOHRHMOP(pollutant) was found. slightly higher in the stirred reactions (46 %) than in the static For all these experiments, UV-Vis and 1H NMR were employed one (33 %; Figure 2b, purple and sky-blue dots, and Section to quantitatively evaluate the removal efficiency. As depicted in S3.7.2). For both conditions, the best performance was observed Figure 4, very high removal efficiencies were found for BTZ (ca. after 1 h of incubation. Altogether, these results evidence the 80 % to 90 %) throughout the tested concentration range, with detrimental effect of diffusion barriers in the coordination of BT values even higher than those for BT. We attributed this to COOHRhMOP under when used as a solid powder (i. e., performance to the greater hydrophobicity of BTZ relative to heterogeneous conditions), for which the kinetics are less BT, which facilitates its removal from water (Section S4.2.4). The favourable – and consequently, the removal efficiency, lower – removal of pollutants containing weak bases/nucleophiles compared to using COONaRhMOP in solution (i. e., homoge- could be limited by the weakness of the coordination bonds. neous conditions). However, despite the weak coordination observed between NA and the dirhodium paddlewheel (Figure S50), the removal efficiency for NA was 80 % at low concentrations (1 mol equiv; Expanding the scope of the pH-triggered 19 ppm; Section S4.3.4). Moreover, although the required pH for coordinative-removal supramolecular strategy the precipitation of either COOHRhMOP(NA) or COOHRhMOP(IQ) was lower (ca. 3.5) than the reported pKa for We reasoned that our strategy might become more challenging either NA or IQ, the removal efficiency for both pollutants was to apply to removal of pollutants that contain coordinating 70 % (Sections S4.3.4 and S4.4.4). These results confirm that the groups that are more pH-sensitive than BT, as protonation of respective coordinative bonds remain intact at these (more these groups preclude the interaction between the pollutant acidic) pH values. Lastly, in all cases, the complexes obtained and the COONaRhMOP. To explore this hypothesis, we tested from precipitation were recoverable again by using CaII as a the performance of our strategy at removing other nitrogenous competing metallic centre for the coordination of the organic organic micropollutants from water, whose coordinating groups coordinating pollutants). Note that both NA and IQ could also are easier to protonate than are the coordinating N-atoms in be recovered by using an acidic (0.3 M HCl) wash, as their easier the triazole ring of BT (pKa1: 0.42). Thus, we chose three polar protonation allowed the complete recovery without endanger- pollutants commonly found in water: i) benzothiazole (BTZ; pKa: ing the MOP structure. Additionally, the recovered materials 2.28), which is used as vulcanisation accelerator in rubber were reusable, as they demonstrated comparable removal manufacture and as an herbicide;[32,33] ii) 1-naphtylamine (NA; efficiency values in subsequent cycles structure (Sections S4.2.5, pKa: 4.25), used both as a fungicide and as a precursor of azo- S4.3.5 and S4.4.5). The integrity of COONaRhMOP was further dyes, which is classified as a carcinogen;[34,35] and iii) isoquinoline evidenced by UV-Vis, 1H NMR, and MS (Sections S4.2.6, S4.3.6 (IQ; pKa: 5.26),[36,37] which is a potentially genotoxic compound and S4.4.6). found in coking wastewaters (Figure 4, right). Firstly, the coordination of BTZ, NA, or IQ to COONaRhMOP was confirmed by UV-Vis spectroscopy (Figures S37, S50 and S66). Then, the Simultaneous removal of multiple nitrogenous organic precipitation conditions for each pollutant were optimised, micropollutants from an aqueous solution using a similar approach to that previously followed for BT, Encouraged by our results, we envisaged that we could use our pH-triggered supramolecular strategy to simultaneously remove multiple organic pollutants from water. To this end, we performed a test in an aqueous solution containing a mixture of BT, BTZ, NA, and IQ. Thus, a 3 mL of an aqueous solution containing COONaRhMOP (0.4 μmol) and equimolar mixture of BT (6 mol equiv; 95 ppm), BTZ (6 mol equiv; 108 ppm), NA (6 mol equiv; 114 ppm), and IQ (6 mol equiv; 102 ppm) was prepared. Then, this mixture was subjected to multiple cycles of pollutant removal/capture-agent regeneration. As indicated by 1 H NMR spectra taken before and after the first cycle, the removal efficiency values were, from highest to lowers: 87 % (BTZ), 85 % (NA), 74 % (IQ) and 66 % (BT; Figure 5). Although a similar value was observed for removal of BTZ from this multi- pollutant solution compared to from the mono-pollutant Figure 4. Efficiency of COONaRhMOP at removing BTZ, NA, and IQ from solution, the corresponding values for the other pollutants did aqueous solutions. *A removal efficiency of > 95 % was assumed for BTZ at differ. Interestingly, the values for removal of NA and IQ were 1 mol equiv (18 ppm) because the residual concentration of BTZ in the remaining aqueous solution was below the limit of quantification (0.9 ppm). higher than from the respective mono-pollutant solutions, All values are reported as an average of triplicate experiments. Errors bars whereas that for BT was slightly lower. Overall, these values indicate standard deviation. confirm that each COONaRhMOP can capture more than 12 Chem. Eur. J. 2022, 28, e202200357 (6 of 8) © 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH 15213765, 2022, 31, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202200357 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article Chemistry—A European Journal doi.org/10.1002/chem.202200357 napthylamine – from water at various concentrations, using both single and multiple-pollutant solutions. In all cases, the COOHRhMOP can be easily regenerated by using readily available and mild reagents (CaCl2 and NaOH), and its perform- ance is maintained through multiple cycles of removal/regener- ation. Our work lays the foundation for the development of pH- induced precipitation of organic pollutants, akin to currently used methods for the removal of inorganic salts. Moreover, we envisage that the wide structural versatility of MOPs will enable our approach to be extended to many other organic pollutants, especially by exploiting other MOP-pollutant interactions than (or in addition to) coordination chemistry, including host-guest, π–π and electrostatic interactions, and combinations thereof. Thus, the results presented here widen the scope of applica- tions for the emerging water-soluble metal-organic cages toward pollutant removal and environmental applications.[38] Acknowledgements This work was supported by the Spanish MINECO (project RTI2018-095622-B I00) and the Catalan AGAUR (project 2017 Figure 5. Top: Schematic of the simultaneous removal of various organic SGR 238). It was also funded by the CERCA Programme/ micropollutant pollutants from an aqueous multipollutant solution by the Generalitat de Catalunya and through a fellowship (LCF/BQ/ pH-triggered, supramolecular strategy. Bottom: Stacked 1H NMR spectra of a D2O solution before (top) and after three subsequent (from high to low) PR20/11770011) from the “la Caixa” Foundation (ID 100010434). removal/regeneration cycles. Left: Legend for both parts. ICN2 is supported by the Severo Ochoa programme from the Spanish MINECO (grant no. SEV-2017-0706). L.H.L. acknowl- edges the support from the Spanish State Research Agency (PRE2019-088056). pollutant molecules (ca. 20). We attributed the results to the contribution of cooperative hydrophobic and Van der Waals interactions, which can enhance the performance of the Conflict of Interest capturing agent once all the specific binding sites have been occupied. Next, the capture agent was regenerated, and the The authors declare no conflict of interest. remaining aqueous solution with the four residual pollutants was subjected to additional removal/regeneration cycles. By the second cycle, all of the BTZ and NA had been fully removed, Data Availability Statement and by the third cycle all of the BT and IQ had been removed (Section S5.3). Again, the stability of the COONaRhMOP was The data that support the findings of this study are available on maintained throughout each cycle, as confirmed by UV-Vis, request from the corresponding author. The data are not 1 H NMR, and MS (Section S5.4). publicly available due to privacy or ethical restrictions. 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Accepted manuscript online: March 29, 2022 Res. Int. 2012, 19, 1781–1790. Version of record online: April 22, 2022 Chem. Eur. J. 2022, 28, e202200357 (8 of 8) © 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH