10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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 Received: 21 July 2022 Revised: 20 September 2022 Accepted: 24 September 2022 DOI: 10.1002/aoc.6930 RESEARCH ARTICLE One-dimensional metal–organic frameworks built by coordinating 2,4,6-tris(4-pyridyl)-1,3,5-triazine linker with copper nodes: CO2 adsorption properties Núria Portolés-Gil 1 | Oriol Vallcorba 2 | Julio Fraile-Sainz 1 | pez-Periago Ana M. Lo 1 | Concepcio n Domingo 1 n 3 | José A. Ayllo 1 Instituto de Ciencia de Materiales de Barcelona, CSIC, Campus UAB, The reaction between 2,4,6-tris(4-pyridyl)-1,3,5-triazine (4-tpt) and copper(II) Bellaterra, Spain hexafluoroacetylacetone (Cu(hfa)2) yields two different 1D metal–organic 2 ALBA Synchrotron Light Source, CELLS, frameworks (MOFs), [(Cu(hfa)2)2(4-tpt)]n (1) and [Cu(hfa)2(4-tpt)]n (2). The Barcelona, Spain 3 Cu:4-tpt ratio in the new MOFs is determined by the reaction medium, particu- Department of Chemistry, Universitat Autònoma de Barcelona, UAB, Bellaterra, larly, the solvent used. The two compounds have been fully characterized, Spain including crystal structure elucidation. [(Cu(hfa)2)2(4-tpt)]n (1), with a 2:1 Cu:4-tpt ratio, could be precipitated in either 1,1,2-trichloroethane or supercriti- Correspondence José A. Ayll on, Department of cal CO2. In (1), 4-tpt shows a tritopic coordination mode, but only half of the Chemistry,.Universitat Autònoma de Cu(hfa)2 subunits act as a node, thus connecting two 4-tpt and giving a 1D net- Barcelona, UAB, 08193 Bellaterra, Spain. work. The other half of Cu(hfa)2 subunits are connected only to one pyridine Email: joseantonio.ayllon@uab.cat and thus protrude along the chains. The later Cu(hfa)2 fragments show a Funding information labile character and can be dissolved in diethyl ether to give the second MOF Spanish Ministry of Science and [Cu(hfa)2(4-tpt)]n (2), with a 1:1 Cu:4-tpt ratio. The compound (2) has also a 1D Innovation (MICINN), Grant/Award Number: CEX2019-000917-S; Spanish structure, with all the incorporated copper atoms acting as nodes. In this case, National Plan of Research, Grant/Award the packing of the chains defines accessible channels, which are perpendicular Number: PID2020-115631GB-I00 to the chain axis. After activation, N2 adsorption/desorption measurements at 77 K confirm the microporous character of (2) with an apparent surface area of 190 m2 g1. Besides, at 273 K this material clearly shows a significant adsorption of CO2 prompted by noncoordinated nitrogen in the triazine linker. KEYWORDS CO2 adsorption, metal–organic framework, solid-phase transformation, triazine, tritopic linker 1 | INTRODUCTION of enormous importance, which, presumably, would increase in the near future. In this context, the design In the actual technological scenario, processes involving and fabrication of porous materials that facilitates the gasses, as those related to energy, transport, chemical manipulation and storage of these gasses is necessary to synthesis, separation, or purification, among others, are improve future applications.[1] This need has promoted This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. © 2022 The Authors. Applied Organometallic Chemistry published by John Wiley & Sons Ltd. Appl Organomet Chem. 2023;37:e6930. wileyonlinelibrary.com/journal/aoc 1 of 10 https://doi.org/10.1002/aoc.6930 10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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 2 of 10  PORTOLES-GIL ET AL. the development of new porous materials, among which is in the design of synthetic processes that avoid the use stands out the family of metal–organic frameworks of hydrothermal techniques and high boiling point (MOFs), with huge compositional tunability achieved by solvents. the proper combination of metallic nodes and organic It has been established that modifications in the reac- linkers that can yield a material with the desired proper- tion conditions, and, especially, in the solvent, deter- ties.[2] Hence, MOFs are considered emergent options to mines the structure of the framework formed from a replace traditional porous materials, such as zeolites or determined set of precursors. This general observation porous carbons, in most applications. also applied to the synthesis of solids involving triazines. 2,4,6-Tris(4-pyridyl)-1,3,5-triazine (4-tpt) is a ligand For example, the analysis of a 4-tpt closely related linker, specially interesting for the design of new MOFs. It is the 3-tpt (2,4,6-tris(3-pyridyl)-1,3,5-triazine), determines essentially planar and rigid, exhibiting a potential tritopic the existence various coordination modes for it, bidentate coordination mode, although structures with 4-tpt acting and tridentate, resulting in a series of 1D or 2D polymeric as a ditopic linker are often found.[3] 4-tpt is most versa- structures after reaction with [Cu(hfa)2]. The polarity of tile than other 2,4,6-tris(pyridyl)-1,3,5-triazine isomers, the solvent determines the 3-tpt symmetric or asymmet- because the three pyridyl groups are equally accessible ric conformation.[25] In contrast to 3-tpt, the 4-tpt linker for complexation.[4] Consequently, 4-tpt has been exten- can only stand the symmetric conformation. In this case, sively exploited in coordination chemistry coupled to the solvent polarity would not influence the molecular transition metals and lanthanides[3,5] and has already conformation of the reacting linker, because only one is afforded several unusual and highly symmetrical available in all the different situations. However, it has MOFs.[6–13] For instance, it is remarkable the design of been observed that even under these circumstances of tri- the [(ZnI2)3(4-tpt)2]n as a crystalline sponge, a subset of topic symmetric linkers, the choice of solvent can be used MOFs that have been tailored to enhance crystallo- to determine the precipitated structure. Only few exam- graphic investigations by providing a cavity capable of ples of the solvent influence have previously been ordering guest molecules.[14] The ordering of these sub- reported in the synthesis of 4-tpt derivatives, focused on stances inside the MOF pores has allowed the determina- the use of iron thiocyanate, giving compounds with dif- tion of the structure of compounds available in only very ferent Fe:4-tpt ratio.[27] On this basis, the main objective small amounts (nanograms) or difficult to crystalize.[15,16] in this study was to extend the study of the solvent influ- Regarding the use of 4-tpt to produce porous mate- ence for the manipulation of 4-tpt MOF structures, dis- rials, additional benefits arise from the weak basicity of tinctively those involving Cu(II) as the metal node. It is the triazine central ring, which do not usually coordinate demonstrated that the solvent used in the synthesis can to metal centers, and, thus, it is available to interact with determine the denticity (tritopic or ditopic) of the 4-tpt in acid adsorbates in sorption applications.[17–20] This is con- their combination with [Cu(hfa)2]. The two new obtained sidered as a key advantage, because most Lewis basic sites compounds, [(Cu(hfa)2)2(4-tpt)]n (1) and [(Cu(hfa)2) tend to be involved in coordination bonds with metal ions (4-tpt)]n (2), where fully characterized, including crystal and thus are withdrawn for adsorption. The triazine structure determination. In regard of applications, the basicity properties have been used to achieve in particular significant adsorption of CO2 prompted by noncoordi- coordination networks of an improved adsorption for dif- nated nitrogen in the triazine linker was established. ferent gaseous molecules, such as CO2 and CH4.[21] The neutral 4-tpt linker can be combined with noncharged nodes to produce neutral coordination polymers (CPs) 2 | RESULTS A ND DISCUSSION and MOFs with chelated units, which are an interesting topic in the branch of coordination chemistry due to their 2.1 | Synthesis specific properties.[22] In our past research, several MOFs were obtained in our laboratories from the combination Two different MOFs, with a different Cu:4-tpt ratio and of metal(II) hexafluoroacetylacetone (M(hfa)2) complexes crystalline structure, where precipitated in this work: (M = Cu, Zn, Co) with different ditopic and tritopic [(Cu(hfa)2)2(4-tpt)]n (1) and [(Cu(hfa)2)(4-tpt)]n (2). Com- pyridines.[23–25] Particularly, the characteristic high solu- pound (1), with a 2:1 Cu:4-tpt ratio, was straightfor- bility of Cu(hfa)2 in a large variety of solvents used for wardly precipitated from a mixture of the reagents the synthesis of CPs and MOFs, including supercritical dissolved in either hot trichloroethane (TCE) (353 K) or CO2 (scCO2), encouraged its regular use as a metal node in scCO2 (333 K). On the contrary, compound (2), with a in these structures. Actually, most of the described CPs 1:1 Cu:4-tpt ratio, must be precipitated by recrystalliza- and MOFs based on the 4-tpt linker have been hydrother- tion in EtOEt of an intermediate phase consisting in mally synthesized.[26] In contrast, the focus of this work either an amorphous solid obtained in CHCl3 or the 10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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  PORTOLES-GIL ET AL. 3 of 10 F I G U R E 1 Crystal structure of [(Cu (hfa)2)2(4-tpt)]n (1): (a,b) views of the chains with a different angle in respect to the 4-tpt linker and (c) packing in which all the chains are equivalent (neighbor chains are differently colored in red and black to facilitate the understanding) crystalline compound (1). The structure of these com- of the Cu O distances, Cu1a O1a (2.34 Å), is noticeably pounds is described in the next section. larger than the other three (1.92–2.10 Å) (Figure S1), depicting an asymmetric coordination of hfa to Cu, which has been previously reported in a comparable 2.2 | Structures coordination environment.[28–31] The propagation of the Cu1a N6c and Cu1a N14c bonds, which involves two 2.2.1 | Structure of (1) of the three pyridinic nitrogen atoms of each 4-tpt linker, defines chains parallel to the b direction. The remaining Compound [(Cu(hfa)2)2(4-tpt)]n (1) crystallizes in the pyridine groups are attached to the other independent monoclinic P 21 space group. The unit cell parameters Cu(hfa)2 fragments, yielding thus a pentacoordinated are a = 10.5172(2) Å, b = 25.4118(4) Å, and c = 10.0730 Cu(II) center, Cu1b, which presents a distorted square- (2) Å, with β = 112.974(1) (see Table S1 for additional base pyramid coordination, with the nitrogen and three structural information). The asymmetric unit includes oxygen atoms defining the base, and the remaining oxy- two Cu(hfa)2 fragments and one 4-tpt linker. Although gen located at the apex (Figure S1). In these subunits, 4-tpt acts as a tritopic ligand, only half of the one of the main planes defined by one CCCOCuO ring Cu(II) atoms connect two 4-tpt linkers, placed at relative (metal and one hfa) is almost perpendicular to the pyri- trans positions. This distribution of linkers and metal dine ring (88.55 ), while the other is only lightly tilted nodes results in a 1D MOF (Figure 1a,b). The with regards to the plane of the pyridine ring (14.64 ). Cu(II) atoms in this node, Cu1a, present a distorted octa- Although the pyridine adducts of Cu(hfa)2 are most fre- hedral coordination, encompassing four oxygen on the quently hexacoordinated, each Cu(II) atom being usually equatorial plane and two nitrogen at axial positions. One bonded to two pyridine groups, few examples are also 10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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 4 of 10  PORTOLES-GIL ET AL. known in which each copper is linked to only one nitro- are noticeable shorter, around 3.27 Å. The packing of the gen yielding pentacoordinated Cu(II) centers.[29,32] It is chains involves the CuN contact previously mentioned also noticeable that, in the coordination sphere of Cu1b, in (1), as well as the other weak intermolecular FF and all the ligands are positioned in a hemisphere. This Car-HO interactions. In this case, the packing of the arrangement favors a weak interchain interaction chains defines channels parallel to the c axis, which con- between the Cu(II) and one of the nitrogen atoms of the tain residual water molecules. The channels have an central triazine ring of a 4-tpt ligand (Figure S2). 4-tpt elongated section and narrow periodically (Figures 2c,d linker deviates from planarity, showing a slightly and S5). concave shape, in which two of the pyridine rings are slanted with regards to the central triazine ring plane (16–18 ). The packing of the chains involves, besides the 2.3 | Solvent influence CuN contact previously mentioned, weak intermolecu- lar FF and Car-HF contacts, yielding a supramolecu- As for many other coordination compounds, the crystalli- lar compact packing (Figure 1c). The crystal structure of zation experimental parameters, particularly the used sol- [(Cu(hfa)2)2(4-tpt)]n resembles the recently reported one vent, are critical to determine the different coordination for [(Cu(hfa)2)2(3-tpt)]n, although the 3-tpt compound networks that can be produced from the combination of was only obtained in trace amounts as it evolves to the the 4-tpt linker and the [Cu(hfa)2] complex. This study formation of [(Cu(hfa)2)1.5(3-tpt)]n3H2O as the predomi- was initially performed analyzing the system 4-tpt/Cu nant compound, in which all the Cu(II) atoms are linked (hfa)2 in a large number of solvents different from water, to two pyridine groups.[25] Likely, the system with 4-tpt such as 1,1,2-TCE, chloroform (CHCl3), scCO2, diethyl cannot progress to a polymer of similar stoichiometry ether (EtOEt), ethanol, tetrahydrofuran, methoxyethanol, due to the less flexibility of this linker with respect to and mixtures, such as hexafluoroisopropanol and chloro- 3-tpt. form. Most of these solvents produced amorphous com- pounds. Because it was not possible to characterize the complete spectrum of materials, solvents leading to 2.2.2 | Structure of (2) porous compounds with potentially adsorption applica- tions were chosen for analysis in this work. Precipitation Compound [(Cu(hfa)2)(4-tpt)]n (2) crystallizes in the experiments were carried out in solvents in which at least monoclinic P21212 space group. The unit cell parameters one of the two regents was soluble (EtOEt, scCO2), or are a = 20.009(9) Å, b = 26.391(3) Å, and c = 6.3860 even both were soluble (hot TCE, CHCl3). (3) Å (see Table S2 for additional structural information). As mentioned previously, compound (1) can be simply The asymmetric unit includes one Cu(hfa)2 fragment and precipitated from a solution involving the reagents in the one 4-tpt linker. The Cu(II) centers, showing distorting proper ratio. TCE, a solvent that allows the use of high octahedral coordination, constitute connection knots working temperatures in nonpressurized vessels, was the through pyridine groups coordination from two 4-tpt (N- medium of choice to precipitate [(Cu(hfa)2)2(4-tpt)]n (1), Cu-N angle: 178.8(2) ), while the equatorial positions are whose structure was determined by powder X-ray dif- occupied by the oxygen atoms from the two chelating hfa fraction (PXRD) patterns by comparison to the simu- ligands (Cu O distances in the 2.091–2.098 Å range) lated pattern (Figure 3a). Actually, TCE is a suitable (Figure S3). 4-tpt is here acting as a ditopic ligand, bridg- solvent to carry out multiple synthetic procedures with ing pairs of metals through two of its pyridyl moieties. particularly insoluble molecules; however, it is far from This conformation defines 1D zigzag chains, parallel to being considered a green solvent. Thus, a more sustain- the [010] direction (Figure 2a,b). A similar chain has able route for reaching compound (1) was sought. The been described for a MOF involving Cu(hfa)2 and diben- choice was to use green scCO2 as a solvent, previously zoyl chelating instead of hfa.[5] The 4-tpt linker deviates tested in the synthesis of other tritopic triazines.[25] By from planarity, showing a slightly concave shape, but in using this solvent, the target compound (1) could be pre- a less pronounced fashion than in (1). In (2), two of the cipitated, even PXRD analysis indicates that it was pyridine rings are also slanted regarding the central tri- slightly tainted with a trace amount of a nonidentified azine ring plane, but with lower angles (8 to 10 ) than product (Figure 3a). those observed in (1). The interchain interaction in (2) Quite the reverse of (1), [(Cu(hfa)2)(4-tpt)]n (2) could includes an asymmetric π–π contact between the non- not be directly precipitated from solution as a pure com- coordinated pyridine in the ring and the central triazine pound. The best results were obtained by precipitation in ring of neighbor chains (Figure S4). The centroid– EtOEt. However, prolonged reaction times (>120 h) were centroid distance is 3.535 Å, although two C C distances needed, and the final product was contaminated with 10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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  PORTOLES-GIL ET AL. 5 of 10 F I G U R E 2 Crystal structure of [(Cu (hfa)2)(4-tpt)]n (2): (a,b) views of the chains with a different angle in respect to the 4-tpt linker, (c) packing view along c axis, showing the channels (these channels are occupied by water molecules, which are not shown for the clarity of the draw), and (d) void space analysis in the bc plane linker excess. The best synthetic route for (2) was to go The phase transformation of (1) to (2) in EtOEt was through the presence of an intermediate phase, either qualitatively analyzed in this solvent. PXRD data indi- amorphous or crystalline, with an increased percentage cated that the pattern of the sample obtained by treat- of Cu(hfa)2. The intermediate was then recrystallized in ment of compound (1) with this solvent math the one EtOEt, a nonpolar hard Lewis base solvent, which corresponding to compound (2) (Figure S7). EtOEt is a extracted Cu(hfa)2 excess. Both compounds, (1) and (2), low polarity solvent; thus, in principle, it should not dis- only differ in the amount of Cu(hfa)2 units incorporated solve either compound (1) or (2). Actually, the dissolu- to the structure. Consequently, Fourier transformed tion of [(Cu(hfa)2)2(4-tpt)]n was not visually observed, infrared spectroscopy with attenuated total reflection even after prolonged (24 h) treatment, because a solid accessory (FTIR-ATR) spectra denote great similarities was always present in the dispersion, but the analysis of between the two compounds (Figure S6). Elemental anal- the filtered solid indicates the formation of a new crystal- ysis agrees with the crystallographic composition eluci- line phase, [(Cu(hfa)2)(4-tpt)]n, involving only approxi- dated for both compounds (see Section 4). mately half of the Cu(hfa)2 content, with an experimental 10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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 6 of 10  PORTOLES-GIL ET AL. that EtOEt solvent is not strong enough to break the bonds Cu(hfa)2 4-tpt Cu(hfa)2. A crystalline solid (1) ! solution (Cu(hfa)2 + 4-tpt) ! crystalline solid (2) process does not seems feasible in this case, mainly due to the almost null solubility of 4-tpt in this solvent. TGA characterization reveals a moderate thermal stability for both compounds, because they start to degrade at around 470–480 K in N2 flow (Figure S8). A second-stage decom- position is observed at 540–550 K. The weight loss of compound (1) in the first step is more pronounced than for (2), indicating a larger amount of [Cu(hfa)2] units in the structure. For both materials, the by-products subli- mated in the second step, thus giving a solid residue of almost zero. An alternative route, consisting in the transformation of amorphous [(Cu(hfa)2)2(4-tpt)] ! crystalline solid (2) (+Cu(hfa)2 in solution), was designed to avoid the use of carcinogen TCE. Instead, CHCl3 was used to precipitate the amorphous intermediate, which is further recrystal- lized in EtOEt. The different phases were analyzed by PXRD, and the patterns are shown in Figure 3b. The acti- vation of the recrystallized solid under vacuum at 333 K induced a significant weight loss in the sample (4.40%), which clearly suggests that the material contains some residual solvent inside the voids evacuated during the degasification treatment. The material remains crystal- line even after solvent elimination, although some small changes in the PXRD pattern were visible (Figure 3b), especially the fade of the peak at 2θ 8.34 . The pattern of the activated sample reasonably agrees with the simu- lated from the single-crystal structure. F I G U R E 3 PXRD patterns of (a) [(Cu(hfa)2)2(4-tpt)]n (1) simulated from single-crystal data (bottom, red line), precipitated in TCE (middle, black line) and precipitated in scCO2 (up, blue line) 2.4 | Adsorption properties in which * denotes peaks due to traces of unidentified impurity and (b) [(Cu(hfa)2)(4-tpt)]n (2) simulated from single-crystal data For compound (1), low temperature N2 adsorption– (bottom, red line), amorphous precipitate obtained in CHCl3 desorption data indicate that the material behaves as a (middle, green line), crystalline sample obtained after treatment of the amorphous sample with EtOEt (up, blue line), and sample nonporous solid. Nevertheless, structural voids analysis obtained after activation under vacuum of the previous sample (up, performed with Mercury software detects nonintercon- black line) nected pores (2.0%, probe radius 1.2 Å, Figure S9). For the activated compound (2), an isotherm corresponding to a microporous material was obtained (Figure 4a), with Cu:4-tpt ratio of 1.15:1 according to elemental analysis. an apparent surface area of 190 m2 g1 and a micropore The analysis of several aliquots of the supernatant solu- volume of 0.063 cm3 g1. After water elimination from tion taken at different times during the 24 h treatment of the channels, void analysis performed with Mercury soft- [(Cu(hfa)2)2(4-tpt)]n in EtOEt indicates the presence of ware detected a 13.9% void space. Hence, even both com- only Cu(hfa)2, with insignificant amounts of 4-tpt. These pounds have potential voids, only (2) displayed open results are reasoned on the basis of a probable crystalline pores, while the pores in (1) are blind. For compound (2), solid (1) ! crystalline solid (2) (+Cu(hfa)2 in solution) the adsorption of gasses was further analyzed at 273 K, process. This transformation is facilitated by the rela- choosing as probe molecules CO2 and N2. This MOF tively labile character of the Cu(hfa)2 fragments linked to shows a noticeable CO2 adsorption under the used exper- only one pyridine group in (1), which are solubilized in imental conditions and a clear preferential adsorption for EtOEt and eliminated in (2) (Scheme 1). It is suggested CO2 in front of N2 (Figure 4b). The adsorption selectivity 10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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  PORTOLES-GIL ET AL. 7 of 10 S C H E M E 1 Proposed route to the transformation of (1) to (2) by the elimination in EtOEt of [Cu(hfa)2] fragments bonded only to one 4-tpt linker was preliminarily measured just from obtained CO2 and N2 uptakes, and given as the ratio percentage of the uptake, being of near 100% for CO2. High CO2 selectivity versus N2 has been previously related to the presence of the triazine group in the crystalline structure of MOFs[15] and COFs.[33,34] In another recent report of a 4-tpt derived MOF, selectivity was related to a sieving effect.[35] The isosteric heats of adsorption (Qst) for compound (2) were measured in this work as a function of CO2 uptake (Figure S10). A constant Qst value of 20 J mmol1 was estimated, independently of the fractional adsorption coverage, thus indicating an energetically homogeneous adsorbent surface. The measured Qst value, even being below of the energy corresponding to chemical bond for- mation, is quite high due to the presence of triazine rings provoking strong physical adsorption of CO2 molecules on the pore walls of (2). Nevertheless, the Qst can be con- sidered moderated enough to favor regeneration. 3 | CONCLUSIONS This study demonstrates that even with topological sym- metric linkers, such as the 4-tpt molecule, which do not have alternative conformations, different MOFs can be obtained, starting from the same reagents, by manipulat- ing the composition and the structure through the used solvent. Specifically, the same metal–ligand combination, for example, Cu(hfa)2 and 4-tpt, led to two MOFs differ- ent in composition and structure: [(Cu(hfa)2)2(4-tpt)]n (1) F I G U R E 4 Gas adsorption measurements performed for and [(Cu(hfa)2)(4-tpt)]n (2), with a 2:1 and 1:1 compound [(Cu(hfa)2)(4-tpt)]n (2): (a) adsorption isotherm of N2 at Cu(II):4-tpt ratio, respectively. This is a clear example of 77 K and (b) adsorption of N2 and CO2 at 273 K the possibility of having different topologies in symmetric 10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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 8 of 10  PORTOLES-GIL ET AL. linker MOFs, just obtained by employing different syn- depressurized and cooled down to room temperature to thetic solvents. It is demonstrated that the solvent used in recover a green powder. the synthesis can determine the denticity (tritopic or dito- pic) of the 4-tpt in their combination with [Cu(hfa)2]. Moreover, the proper choice of solvent allows the proba- 4.2.2 | Synthesis of [(Cu(hfa)2)(4-tpt)]n (2) ble solid-to-solid transformation of (1) to (2). Both com- pounds are 1D MOFs, although only compound (2) has A solution of 4-tpt (50.0 mg, 0.160 mmol) and an open porous structure that can be accessed by small Cu(hfa)2H2O (85 mg, 0.168 mmol) in CHCl3 was evapo- gas molecules, such as N2 or CO2, which make this com- rated to dryness yielding an amorphous residue. Then, pound a potential candidate for gas adsorption and sepa- the solid was suspended in EtOEt and stirred during ration. Particularly, it shows a high adsorption capacity 48 h. During this period, Cu(hfa)2 excess solubilized. The for CO2 at 273 K and a low partial pressure of the gas, resulting crystalline solid was filtrated and washed with comparable with that of standard Zeolita X13. The EtOEt twice. Samples were dried under vacuum at 333 K. enhanced adsorption is related to the presence of exposed Yield 90%, respect to 4-tpt. Anal. calc. for C28H14O8N6- and noncoordinated N in the triazine rings of the linker. F12Cu (790.0 mol1): C, 42.57; H, 1.79; N, 10.64. Found: C, 42.76; H, 1.65; N, 10.57%. 4 | EXPERIMENTAL SECTION 4.3 | Characterization methods 4.1 | Materials Samples chemical composition was assessed by elemental Precursor reagents Cu(hfa)2xH2O (Aldrich, 94.4% analysis (E.A., Thermo Carlo Erba Flash 2000). PXRD Cu(hfa)2 according to certificate of analysis, abbreviated patterns were recorded at room temperature in a Siemens Cu(hfa)2 throughout the text), 2,4,6-tris(4-pyridyl)- D5000 diffractometer, using the Cu Kα incident radia- 1,3,5-triazine (4-tpt), and solvents 1,1,2-TCE, diethyl tion. Fourier transformed infrared spectroscopy (FTIR) ether, chloroform (CHCl3), ethanol (EtOH, absolute), spectra were recorded on a Tensor 27 (Bruker) spectrom- and compressed CO2 (99.95%) were purchased from com- eter, equipped with an attenuated total reflection (ATR) mercial sources and used as received. accessory model MKII Golden Gate with a diamond win- dow in the range 4000–600 cm1. The textural properties were determined by N2 adsorption/desorption at 77 K 4.2 | Synthesis using an ASAP 2020 Micromeritics Inc. equipment. The micropore apparent surface area and pore volume were 4.2.1 | Synthesis of [(Cu(hfa)2)2(4-tpt)]n (1) determined by the t method. For both, elemental analysis and textural characterization, the samples were first Route A: 4-tpt (100.0 mg, 0.320 mmol) and degassed at 333 K during 24 h to eliminate adsorbed sol- Cu(hfa)2xH2O (325 mg, 0.642 mmol) were dissolved in vent molecules. The same equipment was used to mea- hot TCE (353 K, 80 ml), and the resulting green solution sure the volumetric gas adsorption of N2 and CO2 at was allowed to cool down, thus giving a crystalline green 273 K. The isosteric heat of adsorption (Qst) was calcu- precipitate that was filtered and cleaned with cold TCE, lated from the CO2 isotherms measured at 273, 298, and and finally dried under vacuum at 333 K. Yield 80 wt%. 313 K by using the ClausiusClapeyron equation. Ther- Anal. calc. for C38H16O8N6F24Cu2 (1267.7 g mol1): C, mogravimetric analysis (TGA, NETZSCH-STA 449 F1 36.01; H, 1.27; N, 6.63. Found: C, 36.31; H, 1.03; N, 6.89%. Jupiter) was performed under a N2 atmosphere increas- Route B: reactions in scCO2 were performed in a 5 ml ing the temperature in steps of 10 K min1 up to 1123 K. Pyrex vial placed into a 100 ml high pressure autoclave with two opposite sapphire windows (TharDesign). The vial was charged with Cu(hfa)2H2O (208 mg, 4.4 | Crystal structures 0.411 mmol) and 4-tpt (64.0 mg, 0.205 mmol) organic linker. A small magnetic bar was also added, and the vial Synchrotron X-ray powder diffraction (SR-XRPD) data was finally capped with cellulose filter paper. Experi- for compound (1) were collected at the high-resolution ments were performed at 20 MPa and 333 K. The vial station of the MSPD beamline (BL04) at ALBA synchro- was stirred at 500 rpm. After a running period of 20 h, tron.[36] The sample was introduced in a 0.7 mm glass the product was washed with fresh scCO2 twice to elimi- capillary and measured (10 min) in transmission at an nate any possible residue. The autoclave was then energy of 25 keV (0.49587 Å wavelength, determined 10990739, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aoc.6930 by Jose Antonio Ayllon Esteve - Readcube (Labtiva Inc.) , Wiley Online Library on [08/01/2023]. 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  PORTOLES-GIL ET AL. 9 of 10 with a Si NIST-640d reference) using the Mythen-II DA TA AVAI LA BI LI TY S T ATE ME NT detector (six modules, 1280 channels/module, 50 μm/ The data that supports the findings of this study are avail- channel, sample-to-detector distance 550 mm) at room able in the Supporting Information of this article. temperature. The diffraction pattern was indexed using DICVOL04,[37] and the intensities extracted with DAjust ORCID software[38] were used in the direct-space solution pro- Núria Portolés-Gil https://orcid.org/0000-0002-2580- gram TALP[39] to obtain a crystal structure that was 4641 finally refined with the Rietveld refinement program Oriol Vallcorba https://orcid.org/0000-0001-6499-7688 RIBOLS.[40] The values for the interatomic distance Julio Fraile-Sainz https://orcid.org/0000-0003-2961- restraints and angles were taken from MOGUL.[41] The 7920 crystallographic data and refinement details for com- Ana M. Lopez-Periago https://orcid.org/0000-0002- pound (1) are summarized in Table S1 of the SI. Rietveld 3777-3205 refinement plot is shown in Figure S11. Concepcion Domingo https://orcid.org/0000-0002-6976- For compound (2), single-crystal X-ray diffraction 8283 (SCXRD) experiments were performed in the XALOC José A. Ayllon https://orcid.org/0000-0001-7965-7424 beamline at ALBA synchrotron (Spain).[42] Data were col- lected at 100 K with a 0.72932 Å wavelength using the RE FER EN CES Dectris Pilatus 6 M detector placed at 120 mm from the [1] S. Kitagawa, Acc. Chem. Res. 2017, 50, 514. sample. The ϕ scans were performed from 0 to 360 in [2] Q. Wang, D. Astruc, Chem. Rev. 2020, 120, 1438. steps of 0.5 with a collection time of 0.15 step1. The [3] B. Therrien, J. Organomet. Chem. 2011, 696, 637. scan was repeated at three different κ angles (0 , 45 , and [4] T. J. Podesta, A. G. Orpen, Cryst. Growth Des. 2005, 5, 681. 90 ) and merged afterwards to increase the completeness [5] L. A. Barrios, J. Ribas, G. Aromí, J. Ribas-Ariño, P. Gamez, and redundancy when possible. Data were indexed, inte- O. Roubeau, S. J. Teat, Inorg. Chem. 2007, 46, 7154. grated, and scaled using the XDS software.[43] The crystal [6] H. L. Chen, M.-X. Li, X. He, Z.-X. Wang, M. Shao, S.-R. Zhu, Inorg. Chim. Acta 2010, 363, 3186. structure was solved by intrinsic phasing and refined [7] G. Brunet, D. A. Safin, I. Korobkov, A. Cognigni, with SHELXL (version 2014/7)[44] using Olex2 as graphi- M. Murugesu, Cryst. Growth des. 2016, 16, 4043. cal interface.[45] Crystallographic data are summarized in [8] R. A. Polunin, N. P. Burkovskaya, S. V. Kolotilov, M. A. Table S2. Kiskin, A. S. Bogomyakov, S. A. Sotnik, I. L. Eremenko, Russ. CCDC 2164405 (1) and 2164404 (2) contain the sup- Chem. Bull. 2014, 63, 252. plementary crystallographic data for this paper. These [9] S. R. Batten, B. F. Hoskins, B. Moubaraki, K. S. Murray, data can be obtained free of charge via www.ccdc.cam.ac. R. Robson, Chem. Commun. 2000, 1095. uk/data_request/cif. [10] Y. Zhao, L. Li, Z.-Y. Liu, B. Ding, X.-G. Wang, Y. Luo, X.-J. Zhao, E.-C. Yang, Cryst. Growth Des. 2021, 21, 1218. [11] S. A. Sotnik, R. A. Polunin, M. A. Kiskin, A. M. Kirillov, V. N. A C K N O WL E D G M E N T S Dorofeeva, K. S. Gavrilenko, I. L. Eremenko, V. M. This work was supported by the Spanish Ministry of Novotortsev, S. V. Kolotilov, Inorg. Chem. 2015, 54, 5169. Science and Innovation (MICINN) through the [12] A. Polunin, N. P. Burkovskaya, J. A. Satska, S. V. Kolotilov, Severo Ochoa Program for Centers of Excellence M. A. Kiskin, G. G. Aleksandrov, O. Cador, L. Ouahab, I. L. (CEX2019-000917-S) and the Spanish National Plan of Eremenko, V. V. Pavlishchuk, Inorg. Chem. 2015, 54, 5232. Research with project PID2020-115631GB-I00. The mea- [13] Y.-Y. Zhang, Q. Liu, L.-Y. Zhang, Y.-M. Bao, J.-Y. Tan, N. sures for the elucidation of the crystal structures were Zhang, J.-Y. Zhang, Z.-J. Liu, Dalton Trans. 2021, 50, 647. [14] W. J. Gee, L. E. Hatcher, C. A. Cameron, C. Stubbs, M. R. performed at MSPD and XALOC beamlines at ALBA Warren, A. D. Burrows, P. R. Raithby, Inorg. Chem. 2018, 57, Synchrotron with the collaboration of ALBA staff. 4959. [15] T. R. Ramadhar, S.-L. Zheng, Y.-S. Chen, J. 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