class="head no_bottom_margin" id="sec1title">IntroductionMetal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have rapidly evolved as an important subset of porous materials (, , , , , ). The interest in this domain has expanded in recent years owing to the wide range of applicability exhibited by these materials (, , , , , , , , , href="#bib41" rid="bib41" class=" bibr popnode">Li et al., 2016a, href="#bib1" rid="bib1" class=" bibr popnode">Aguilera-Sigalat and Bradshaw, 2016, href="#bib6" rid="bib6" class=" bibr popnode">Bao et al., 2016, href="#bib50" rid="bib50" class=" bibr popnode">Lustig et al., 2017). MOFs can be broadly segregated into two classes, based on the charge of the coordination network, viz., neutral and ionic MOFs (i-MOFs); i-MOFs are further classified into cationic and anionic (href="#bib37" rid="bib37" class=" bibr popnode">Karmakar et al., 2016, href="#bib42" rid="bib42" class=" bibr popnode">Li et al., 2016b). MOFs afford significant advancement over congener polymeric materials owing to their crystalline nature, which furnishes precise structure-property correlation. Despite several advantages, there remain a few core issues such as hydrolytic and chemical stability that have stalled the progress of MOFs for real-time applications (href="#bib9" rid="bib9" class=" bibr popnode">Canivet et al., 2014, href="#bib8" rid="bib8" class=" bibr popnode">Burtch et al., 2014, href="#bib62" rid="bib62" class=" bibr popnode">Qadir et al., 2015, href="#bib29" rid="bib29" class=" bibr popnode">Hendon et al., 2017). Although a few benchmark MOF compounds having hydrolytic or chemical stability are known, the majority of them are found to be stable predominantly in acidic pH (href="#bib24" rid="bib24" class=" bibr popnode">Ferey et al., 2005, href="#bib60" rid="bib60" class=" bibr popnode">Park et al., 2006, href="#bib10" rid="bib10" class=" bibr popnode">Cavka et al., 2008, href="#bib34" rid="bib34" class=" bibr popnode">Howarth et al., 2016, href="#bib79" rid="bib79" class=" bibr popnode">Wang et al., 2016a, href="#bib80" rid="bib80" class=" bibr popnode">Wang et al., 2016b, href="#bib81" rid="bib81" class=" bibr popnode">Wang et al., 2016c, href="#bib47" rid="bib47" class=" bibr popnode">Liu et al., 2015, href="#bib18" rid="bib18" class=" bibr popnode">Duan et al., 2017, href="#bib4" rid="bib4" class=" bibr popnode">Bai et al., 2016). The infrequent MOFs exhibiting base resistance are typically neutral frameworks built from azolate-based ligands (href="#bib34" rid="bib34" class=" bibr popnode">Howarth et al., 2016). Development of i-MOFs is seeking greater relevance owing to them being potentially viable alternatives to conventional ion exchangers for various applications (href="#bib37" rid="bib37" class=" bibr popnode">Karmakar et al., 2016, href="#bib59" rid="bib59" class=" bibr popnode">Oliver, 2009, href="#bib5" rid="bib5" class=" bibr popnode">Banerjee et al., 2016, href="#bib38" rid="bib38" class=" bibr popnode">Kumar et al., 2017, href="#bib43" rid="bib43" class=" bibr popnode">Li et al., 2017a, href="#bib90" rid="bib90" class=" bibr popnode">Zhu et al., 2017a, href="#bib92" rid="bib92" class=" bibr popnode">Zhu et al., 2017c, href="#bib48" rid="bib48" class=" bibr popnode">Liu et al., 2017). The challenges of stability assume greater relevance for cationic MOFs, which generally are vulnerable to disintegration in aqueous medium or mild acid/basic conditions. To overcome the limitations of weak hydrolytic or chemical stability, design strategies that can provide robust compounds offering resistance are highly sought after. Surveying the literature, some reviews have chalked out broad principles guiding the design of water and chemically stable, porous MOFs. These include strengthening the metal-ligand bond and shielding this bond from the influence of foreign species (href="#bib62" rid="bib62" class=" bibr popnode">Qadir et al., 2015, href="#bib18" rid="bib18" class=" bibr popnode">Duan et al., 2017). The proper choice of the metal ions or the suitable kind of the organic ligand have also been found to play a crucial role in bestowing stability to a compound. This formal outline has generally been derived from stable benchmark compounds, which in most cases are neutral MOFs. The systematic design and development of stable cationic MOFs from the insights gained in literature reports is extremely uncommon (href="#bib37" rid="bib37" class=" bibr popnode">Karmakar et al., 2016).With this background we sought to focus on the development of approaches for designing stable cationic MOFs. Typically, cationic MOFs are fabricated from neutral N-donor ligands, which render cationic frameworks and afford the presence of uncoordinated, substitutable anions (href="#bib22" rid="bib22" class=" bibr popnode">Fei et al., 2010, href="#bib70" rid="bib70" class=" bibr popnode">Schoedel et al., 2011, href="#bib52" rid="bib52" class=" bibr popnode">Ma et al., 2012, href="#bib53" rid="bib53" class=" bibr popnode">Manna et al., 2013, href="#bib54" rid="bib54" class=" bibr popnode">Manna et al., 2016, href="#bib12" rid="bib12" class=" bibr popnode">Chen et al., 2013, href="#bib32" rid="bib32" class=" bibr popnode">Hou et al., 2013, href="#bib73" rid="bib73" class=" bibr popnode">Sheng et al., 2017, href="#bib91" rid="bib91" class=" bibr popnode">Zhu et al., 2017b). For affording stability, ligands with higher pKa have found preference (href="#bib14" rid="bib14" class=" bibr popnode">Colombo et al., 2011, href="#bib85" rid="bib85" class=" bibr popnode">Zhang et al., 2012, href="#bib61" rid="bib61" class=" bibr popnode">Pettinari et al., 2016, href="#bib28" rid="bib28" class=" bibr popnode">He et al., 2016, href="#bib65" rid="bib65" class=" bibr popnode">Rieth et al., 2016), and hence ligands with imidazole/triazole-donating units can be more effective as neutral donor ligands (href="#bib11" rid="bib11" class=" bibr popnode">Chen, 2016). Furthermore, the smaller size of five-membered donating moieties can render greater density of the ligands around the metal nodes by feasibility of hexa-coordination, which can shield the metal nodes from the influence of external species. In general, higher dentate ligands are better suited for generating higher dimensional frameworks. In the present discourse, the additional benefit of such linkers is in affording superior kinetic stability (href="#bib80" rid="bib80" class=" bibr popnode">Wang et al., 2016b). Likewise, the appropriate selection of metal center is vital while fabricating stable systems. The choice of the metal node is directed by its ability to bind to the donor groups of the ligands and the resistance to dissociation of the resulting bonds. Among transition metals that bind equally well with N- and O-donor ligands, Ni(II)-based MOFs have been found to offer remarkable hydrolytic stability and, in certain cases, resistance to varying chemical environments (href="#bib34" rid="bib34" class=" bibr popnode">Howarth et al., 2016, href="#bib80" rid="bib80" class=" bibr popnode">Wang et al., 2016b, href="#bib14" rid="bib14" class=" bibr popnode">Colombo et al., 2011, href="#bib17" rid="bib17" class=" bibr popnode">Desai et al., 2016, href="#bib51" rid="bib51" class=" bibr popnode">Lv et al., 2017). In case of cationic MOFs, although the uncoordinated anions are not a direct part of the framework backbone, their choice can be significant in the preparation. From the existing literature, it is observed that organic sulfonates, which are bulky molecules, are known to bind to metal centers typically at higher temperatures only (href="#bib21" rid="bib21" class=" bibr popnode">Fei and Oliver, 2011, href="#bib23" rid="bib23" class=" bibr popnode">Fei et al., 2013, href="#bib72" rid="bib72" class=" bibr popnode">Sergo et al., 2015, href="#bib82" rid="bib82" class=" bibr popnode">Yang and Fei, 2017). In the current context, such compounds can adapt the function of template anions for the creation of voids and can be an integral part of the framework.Combining the above-mentioned facets, herein we report the synthesis of a two-dimensional (2D) Ni(II)-centered cationic MOF, viz., [{Ni(L)2}·(BPSA)·xG]n (L is the ligand; BPSA is 4,4′-biphenyldisulfonic acid; and G is the guest; it is hereafter referred to as IPM-MOF-201, where IPM stands for IISER Pune Materials). The compound is built from a tridentate ligand having terminal imidazole rings and free organosulfonate anions. The compound was found to exhibit extraordinary base resistance, which is uncommonly observed in porous MOFs and even more infrequently observed among i-MOFs. The stability over wide pH range has been tapped for trapping small organic dye molecules across different pH conditions.
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