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Acetic acid as a solvent for the synthesis of metal–organic frameworks based on trimesic acid
Polyhedron 170 (2019) 458–462
Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
Acetic acid as a solvent for the synthesis of metal–organic frameworks based on trimesic acid Marta Sanchez-Sala a, Oriol Vallcorba b, Concepción Domingo c, José A. Ayllón a,⇑ a
Departamento de Química, Universidad Autónoma de Barcelona, 08193-Bellaterra, Barcelona, Spain ALBA Synchrotron Light Source, Cerdanyola del Vallés, Barcelona, Spain c Instituto de Ciencia de los Materiales de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain b
a r t i c l e
i n f o
Article history: Received 5 May 2019 Accepted 9 June 2019 Available online 18 June 2019 Keywords: Trimesic acid Green chemistry Acetic acid Cobalt Bismuth
a b s t r a c t Nowadays, there is a great interest in the design of new metal–organic frameworks (MOFs) synthetized by routes that can be transferred to industrial scale with low environmental impact. In this context, this work explores the use of a soft method for the synthesis of MOFs, in which neat acetic acid is used as the reaction media. Trimesic acid (H3BTC) was chosen as the organic linker source and combined with different metal acetates, all these precursor being moderately soluble in hot acetic acid. The metal acetate provides both the metallic cation and a base, the acetate, necessary to deprotonate the H3BTC. Three first row transition metals, Cu, Co and Ni, and one heavy metal, Bi, were chosen for the study. Results denoted that the metal acetate was determinant to direct the nature of the precipitated material. For instance, the precipitation of the well-known porous HKUST-1 was observed from Cu acetate and H3BTC in neat acetic acid, giving systems with hierarchical porosity. Contrarily, for Co and Bi the use of acetic acid as a solvent yielded two 2D non porous coordination polymers, [Co6(BTC)2(Ac)6(HAc)3] (1) and [Bi(HBTC)(Ac)] (3), respectively. These new compounds were structurally elucidated. Moreover, the magnetic behaviour was analysed for the Co compound (1). Finally, an amorphous phase was precipitated for Ni. In the latter case, the use of propionic acid, with a higher boiling point than acetic acid, led to the precipitation of a crystalline material. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Metal–organic frameworks (MOFs) present a huge diversity of structures with tunable properties. These materials found multiple applications in different areas, including gas storage and separation, catalysis and as precursors for nanostructured porous materials [1]. Nowadays, besides searching for new MOF phases with improved properties, there is also a large interest in the design of new green synthetic routes that can be easily transferred to industrial scale, allowing at the same time strong control on the properties of the prepared materials [2,3]. In this regard, the use of acetic acid as a solvent has recently been proposed as an alternative improve the ‘‘greenish” character of MOFs synthesis [4]. This solvent that can be produced from renewable feedstock, hence it has a relatively low environmental impact, together with a reduced processing risk related to flammability and storage [5]. The role of acetic acid as modulator to control de crystal growth and hierarchical porosity of different MOFs has been reported. In these works, ⇑ Corresponding author. E-mail address: [email protected] (J.A. Ayllón). https://doi.org/10.1016/j.poly.2019.06.017 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
acetic acid is used as an additive combined with other solvents [6–11]. The main objective of this work is to design a green synthetic method, based on the use of acetic acid exclusively as a solvent, while avoiding the use of pressurized reactors. In the openatmosphere reactor, the processing temperature is limited by the boiling point of the solution. Trimesic acid (H3BTC) was selected as a linker precursor in this work to explore the preparation of MOFs due to its reasonable solubility in hot acetic acid. H3BTC is a low cost tricarboxylic acid, with a rigid structure, that has been used for the formulation of a large number of MOFs, including HKUST-1 (commercially known as Basolite C300), one of the most thoroughly studied MOFs, also produced at industrial scale. Finally, metal acetates were chosen to provide the metal cation. Acetates are environmentally advantageous, since the anion byproduct generated is less corrosive than the ones produced from other inorganic salts, such as chloride or sulphate [3]. In the designed process, acetate also acts as the base necessary to deprotonate the H3BTC linker, yielding acetic acid, i.e., the used solvent [12]. In this exploratory work, three representative first-row transition metals (Cu, Co and Ni) and one heavy metal (Bi) were studied in the precipitation of MOFs. The nature
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of the precipitated material was characterized by powder and single crystal X-ray diffraction (XRD), elemental analysis (EA) and magnetism. The crystal structures of two new 2D coordination polymers, [Co6(BTC)2(Ac)6(HAc)3] and [Bi(HBTC)(Ac)], were elucidated. 2. Results and discussion In short, solids were precipitated after dissolving the precursors under ambient pressure in hot acetic acid. The solvent was then eliminated by distillation until a solid begun to precipitate. The same procedure was followed for Cu, Co, Ni and Bi.
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The crystal structure of [Co6(BTC)2(Ac)6(HAc)3] (1) contains two types of cobalt atoms, Co(1) and Co(2), arranged in hexanuclear clusters (Fig. 2a). Each cluster includes three cobalt atoms of each type, associated by a C3 symmetry axis parallel to the c axis. Both Co atoms present different coordination number, five for Co(1) and six for Co(2) (Fig. 2b). In each cluster, the six cobalt cations are held together through six tridentate acetate ligands, each having a l3, g1,g2-bridging mode. There are two types of acetate ligands; in both the g2 oxygen connects a Co(1) to a Co(2), but differs on the metal bonded to the g1 oxygen. In each cluster, there are six
2.1. Copper(II) The combination of copper acetate and H3BTC in hot HAc yielded a blue precipitate. Powder XRD characterization denotes that it correspond to HKUST-1 (Fig. 1), with a very small amount of impurities. EA results match the [Cu3(BTC)25H2O HAc] stoichiometry. This sample had a weight loss of 16 wt% at 120 °C under vacuum, which roughly agrees with the loss of H2O and HAc in this composition. The N2 adsorption–desorption isotherm displays first a sharp rise in the low pressure range (P/P0 < 0.01), which indicates the presence of substantial microporosity; and second (P/P0 > 0.4), a hysteresis loop associated with capillary condensation taking place in mesopores. Hence, the isotherm is a combination of Type I and IV according to the IUPAC classification (Fig. S1). The BET specific surface area of the material is 580 m2/g, which is relatively low in comparison to other values reported in the literature that mostly are above 1000 m2/g. This result suggests that the material has either low crystallinity or contains some impurities that partially occlude the pores. It is worth noting that the presence of mesopores is a feature desirable in different adsorption applications by improving diffusion [9,13–15]. 2.2. Cobalt(II) H3BTC and Co(Ac)2 in hot acetic acid yielded a crystalline violet material. This is a new compound (1), with stoichiometry [Co6(BTC)2(Ac)6(HAc)3], elucidated structurally by single crystal X-ray diffraction. EA data was in accordance with this stoichiometry. Phase purity of the bulk sample was confirmed by powder X-ray diffraction (Fig. S2).
Fig. 1. X-ray patterns of [Cu3(BTC)25H2O HAc] (up) measured at room temperature; (down) calculated from the crystal structure solved from single crystal XRD measured at 100 K.
Fig. 2. (up) Crystal structure of [Co6(BTC)2(Ac)6(HAc)3] (down) Detail of the local coordination environment of each types of Co(II) atoms (hydrogen atoms are omitted for clarity).
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additional carboxylate bridges provided by trimesate ligand with a l2,g1,g1 bonding mode, that connects also Co(1)–Co(2) pairs. There are also two kinds of trimesate units, connected by the C3 axis, one shown more symmetric Co–O distances than the other (see Table S2). Focusing in the metal centers, five positions in the coordination sphere of each Co(II) cation are occupied by oxygen atoms included in these bridges: two from oxygen atoms from trimesate carboxylate, situated at the relative cis position, and three from acetate ligands. This completes the coordinating sphere of each Co(1), that presents an intermediate geometry between square pyramidal and trigonal bipyramidal, closer to the latest (s = 0.58) [16]. The coordination of one non dissociated acetic acid to each Co(2), which acts as a monodentate ligand through the oxygen not linked to the proton, yields a roughly octahedral coordination geometry for the other half of metallic cations. Each trimesate anion connects three clusters, and the propagation of these inter-cluster bridges defines sheets parallel to the ab plane. Each cluster is connected thought trimesate bridges to the six closer neighbors in the plane. Layers are stacked yielding a compact structure (Fig. S3). Variable-temperature magnetic susceptibility measurements in the range of 300–5 K were performed on a sample powder of compound (1) [Co6(BTC)2(Ac)6(HAc)3]. A vpT value of 17.2 cm3 K/mol was attained at 300 K (Fig. 3a). This value is in the range of reported data for other hexanuclear Co(II) compounds, but is much higher than the calculated spin-only value of 11.25 cm3 K/mol for six CoII cations (S = 3/2 and g = 2.0), which indicates a significant orbital contribution of the Co(II) ions [17–19]. As the temperature is reduced, vpT smoothly decreases over the entire temperature region, indicating the dominant antiferromagnetic interaction among metal centers through the carboxylate bridges. Above 50 K, the reciprocal molar magnetic susceptibility data obeys
Curie–Weiss law (Fig. 3b), with a very small Weiss constant h = 0.97 K, indicating very weak antiferromagnetic interactions among CoII centers. [Co6(BTC)2(Ac)6(HAc)3] reacts very slowly with the humidity present in the air, thus, it can be easily manipulated. However, it reacts quickly in liquid water, producing first a pink solution, and next a pink precipitate after a few minutes. This solid has been identified by powder XRD (Fig. S4) and EA as the previously described [Co3(BTC)212H2O] hydrogen bonded MOF [20–22]. This is a porous material that has shown affinity for the adsorption of iodine [23] and that also loss and gain water molecules reversibly as a function of the temperature and humidity. It should be said that previously described routes for the synthesis of this compound required hydrothermal conditions (140 °C, 24 h), while in this work the material was obtained at room temperature and atmospheric pressure. Presumably the reaction proceeds following equation (1).
Co6 ðBTCÞ2 ðAcÞ6 ðHAcÞ3 þ 12H2 O ! Co3 ðBTCÞ2 12H2 O ðsÞ þ3 CoðAcÞ2 ðaqÞ þ 3HAcðaqÞ
ð1Þ
2.3. Nickel(II) The reaction between nickel(II) acetate and H3BTC in hot acetic acid produced always an amorphous yellowish precipitate (Fig. S5). To obtain a crystalline compound, it was necessary the use of a solvent with higher boiling point than acetic acid (118 °C). Specifically, propionic acid (HPro), with a boiling point of 141 °C, was essayed to favor the crystallization of the material. From an environmental point of view, propionic acid has similar advantages than acetic acid [5]. Interestingly, the use of propionic acid allowed the preparation of a crystalline MOF involving Ni and the tricarboxylic acid (Fig. S5). The powder XRD pattern was similar to the one corresponding to [Co6(BTC)2(Ac)6(HAc)3] (compare patterns in Figs. S5 and S2) indicating structural similarities between both compounds. EA data also agrees with a compound having a comparable stoichiometry, i.e., [Ni6(BTC)2(Pro)6(H2O)6] (2). Both compounds crystallized with a hexagonal morphology (Fig. S6). The behavior of [Ni6(BTC)2(Pro)6(H2O)6] in regard to water was similar to the one observed for cobalt compound (1). Hence, the nickel compound reacted in liquid water giving place to the precipitation of the trinuclear compound [Ni3(BTC)2(H2O)14]4H2O, according to EA and powder XRD data (Fig. S7) [24]. 2.4. Bismuth(III)
Fig. 3. (up) Thermal variation of vpT for complex [Co6(BTC)2(Ac)6(HAc)3]. (down) Thermal variation of 1/vp; solid red line is the best fit to the Curie–Weiss equation h = 0.97 K.
For Bi(III) acetate, [Bi(HBTC)(Ac)] (2), a compound in which the trimesic acid was only partially deprotonated, has been obtained. Other bismuth compounds containing the HBTC2 ligand have been described previously [25–27]. HBTC2 could be considered as an intermediate in the deprotonation of trimesic acid to give BTC3 ligand, observed in the compounds precipitated previously with Cu, Co and Ni. When HBTC2 intermediate is able to form insoluble compounds in the hot acetic acid medium, its formation will be favored respect the formation of species including BTC3 linker. The crystal structure of [Bi(HBTC)(Ac)] was elucidated by single crystal XRD. It contains only three independent entities, the bismuth atom, the monoprotonated HBTC2 ligand and the acetate ligand. Each Bi(III) cation is heptacoordinated, with Bi-O distances in a wide 2.270(8)–2.687(7) interval (Fig. 4a). Bismuth atoms are arranged into chains, parallel to the b axis (Fig. 4b). Each consecutive pair of cations is linked by a triple bridge. Two bridging groups are carboxylate groups from two different HBTC2 ligands, both showing a l2,g1,g1 bonding mode. The acetate ligand creates the
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ment with water at room temperature. In the case of nickel, the temperature attained with acetic acid was not enough to precipitate a crystalline material and propionic acid was used instead to obtain [Ni6(BTC)2(Pro)6(H2O)6]. Finally, for bismuth a compact coordination polymer was precipitated, containing both acetate ligand and partially deprotonated trimesic acid (HBTC2). 4. Experimental 4.1. Materials and synthesis
Fig. 4. (up) Crystal structure of [Bi(HBTC)(Ac)(HAc)3] (down) Detail of the local packing of the 2D layer by hydrogen bonding.
third link through a l3,g1,g2-briding mode. HBTC2 ligands assemble these chains into sheets, parallel to the (-1 0 1) plane. Inside each layer, the relative position of the aromatic rings of HBTC allows weak p–p interactions (3.699(5) Å). These layers are interconnected trough pairs of hydrogen bonds between the protonated carboxylic acid of HBTC2 ligand from adjacent layers, defining thus a 3D-suparmolecular network (Fig. 4c). Powder XRD confirms the purity of the bulk powder (Fig. S8). 3. Conclusions The use of acetic acid as neat solvent in the synthesis of coordination polymers using trimesic acid and metal acetates has been proved for different metals. Only in the case of copper(II) a known porous product was obtained but, in this case, with a hierarchical meso-microporosity. A 2D compact coordination polymer was obtained for cobalt, [Co6(BTC)2(Ac)6(HAc)3], which incorporates both acetic acid and acetate ligands. This compound can be used as precursor in the synthesis of [Co2(BTC)212H2O] by simply treat-
All reagents were from commercial sources and used without further purification. Reactions and manipulations were carried out in air atmosphere. [Cu3(BTC)25H2O HAc] – A solution of Cu(Ac)2H2O (0.500 g, 2.50 mmol) in HAc (50 mL) was added to a solution of H3BTC (0.323 g, 1.53 mmol) of boiling HAc (50 mL). The obtained blue solution was distillated until a blue precipitate appears. Then, the mixture was allowed to cool down and the solid was filtered and washed twice with HAc (5 mL) Yield: 0.257 g (44%, respect H3BTC). Anal. Calc. for C20H20O19Cu3: C, 31.82; H, 2.67. Found: C, 32.03; H, 2.80. [Co6(BTC)2(Ac)6(HAc)3] (1) – A solution of Co(Ac)24H2O (1.93 g, 7.73 mmol) in HAc (20 mL) was added to a solution of H3BTC (0.813 g, 3.86 mmol) of boiling HAc (80 mL). The violet obtained solution was refluxed for an hour and then allowed to concentrate until a violet precipitate appears. Then, the mixture was allowed to cool down and the solid was filtered and washed twice with HAc (5 mL) Yield: 1.38 g (82%, respect Co(Ac)24H2O). Anal. Calc. for C36H36O30Co6: C, 33.20; H, 2.79. Found: C, 33.30; H, 2.78. [Co2(BTC)212H2O] – [Co6(BTC)2(Ac)6(HAc)3] (0.450 g, 0.35 mmol) was added to 20 mL of water giving a pink solution. After few minutes at room temperature, a pink solid precipitated. The solid was filtered, washed with water and dry at room temperature. Yield: 0.256 g (92%). Anal. Calc. for C18H30O24Co3: C, 26.78; H, 3.75. Found: C, 26.80; H, 3.81. [Ni6(BTC)2(Pro)6(H2O)6] (2) – A solution of Ni(Ac)24H2O (1.95 g, 7.84 mmol) in hot propionic acid (HPro, 40 mL) was added to a solution of H3BTC (1.089 g, 5.18 mmol) of boiling HPro (60 mL). The green obtained solution was refluxed for an hour and then allowed to concentrate until a yellow green precipitate appears. Then, the mixture was allowed to cool down and the solid was filtered and washed with HPro (5 mL) and EtOH (5 mL). Yield: 1.52 g (88%, respect Ni(Ac)24H2O). Anal. Calc. for C36H48O30Ni6: C, 32.93; H, 3.68. Found: C, 32.74; H, 3.82. [Ni3(BTC)2(H2O)14]4H2O – [Ni6(BTC)2(Pro)6(H2O)6] (0.100 g, 0.08 mmol) was added to 20 mL of water giving a light green solution. After twenty minutes at room temperature, a green solid precipitated. The solid was filtered, washed with water and dry at room temperature. Yield: 0.043 g (62%). Anal. Calc. for C18H42O30Ni3: C, 23.64; H, 4.63. Found: C, 23.90; H, 4.50. [Bi(HBTC)(Ac)], (3) – A solution of Bi(Ac)3 (0.216 g, 0.56 mmol) in HAc (10 mL) was added to a solution of H3BTC (0.118 g, 0.56 mmol) of boiling HAc (20 mL). The obtained solution was refluxed for an hour and then allowed to concentrate until a white precipitate appears. Then, the mixture was allowed to cool down and the solid was filtered and washed twice with ethanol (5 mL) Yield: 0.100 g (38%). Anal. Calc. for C11H7O8Bi: C, 27.75; H, 1.48. Found: C, 27.89; H, 1.43. 4.2. Characterization Elemental analyses (C, H, N) were carried out on a Euro Vector 3100 instrument. Powder XRD patterns were measured at room temperature with a Siemens D5000 apparatus using the Cu Ka
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radiation. Patterns were recorded from 2h = 5 to 50°, with a step scan of 0.02° counting for 1 s at each step. Magnetic measurements from 5 to 300 K were performed in a Quantum Design MPMS-5S SQUID susceptometer. The molar susceptibility was corrected for the sample holder and for the diamagnetic contribution of all atoms by means of Pascal’s tables [28]. Adequate prism-like crystals were used for single crystal X-ray diffraction experiments, a violet crystal in the case of [Co6(BTC)2(Ac)6(HAc)3]n and a transparent crystal in the case of [Bi(HBTC)(Ac)]n. Data were collected using Mo Ka radiation (k = 0.71073 Å) in a SMART-APEX diffractometer with a CCD detector. Measurements were performed at 23 ± 1 °C. An empirical absorption correction was applied (SADABS). The structure was solved by direct methods (SHELXNT) and refined by full-matrix least-squares methods on F2 for all reflections (SHELXL-2014/7) [29]. Non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms bonded to carbon atoms were placed in calculated positions with isotropic displacement parameters fixed at 1.2 times the Ueq of the corresponding carbon atoms. Crystal data and further refinement details are presented in Table S1. Molecular graphics were generated with the program Mercury 3.6 [30,31]. Acknowledgements This work was partially financed by the Spanish National Plan of Research CTQ2017-83632-CO2-P1. C.D./ICMAB acknowledges financial support from the Spanish MEC, through the ‘‘Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-20150496). Appendix A. Supplementary data CCDC 1556887–1556888 contains the supplementary crystallographic data for [Co6(BTC)2(Ac)6(HAc)3] and [Bi(HBTC) (Ac)] respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data to this article can be found online at https://doi.org/10.1016/j.poly.2019.06.017.
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Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
Acetic acid as a solvent for the synthesis of metal–organic frameworks based on trimesic acid Marta Sanchez-Sala a, Oriol Vallcorba b, Concepción Domingo c, José A. Ayllón a,⇑ a
Departamento de Química, Universidad Autónoma de Barcelona, 08193-Bellaterra, Barcelona, Spain ALBA Synchrotron Light Source, Cerdanyola del Vallés, Barcelona, Spain c Instituto de Ciencia de los Materiales de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain b
a r t i c l e
i n f o
Article history: Received 5 May 2019 Accepted 9 June 2019 Available online 18 June 2019 Keywords: Trimesic acid Green chemistry Acetic acid Cobalt Bismuth
a b s t r a c t Nowadays, there is a great interest in the design of new metal–organic frameworks (MOFs) synthetized by routes that can be transferred to industrial scale with low environmental impact. In this context, this work explores the use of a soft method for the synthesis of MOFs, in which neat acetic acid is used as the reaction media. Trimesic acid (H3BTC) was chosen as the organic linker source and combined with different metal acetates, all these precursor being moderately soluble in hot acetic acid. The metal acetate provides both the metallic cation and a base, the acetate, necessary to deprotonate the H3BTC. Three first row transition metals, Cu, Co and Ni, and one heavy metal, Bi, were chosen for the study. Results denoted that the metal acetate was determinant to direct the nature of the precipitated material. For instance, the precipitation of the well-known porous HKUST-1 was observed from Cu acetate and H3BTC in neat acetic acid, giving systems with hierarchical porosity. Contrarily, for Co and Bi the use of acetic acid as a solvent yielded two 2D non porous coordination polymers, [Co6(BTC)2(Ac)6(HAc)3] (1) and [Bi(HBTC)(Ac)] (3), respectively. These new compounds were structurally elucidated. Moreover, the magnetic behaviour was analysed for the Co compound (1). Finally, an amorphous phase was precipitated for Ni. In the latter case, the use of propionic acid, with a higher boiling point than acetic acid, led to the precipitation of a crystalline material. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Metal–organic frameworks (MOFs) present a huge diversity of structures with tunable properties. These materials found multiple applications in different areas, including gas storage and separation, catalysis and as precursors for nanostructured porous materials [1]. Nowadays, besides searching for new MOF phases with improved properties, there is also a large interest in the design of new green synthetic routes that can be easily transferred to industrial scale, allowing at the same time strong control on the properties of the prepared materials [2,3]. In this regard, the use of acetic acid as a solvent has recently been proposed as an alternative improve the ‘‘greenish” character of MOFs synthesis [4]. This solvent that can be produced from renewable feedstock, hence it has a relatively low environmental impact, together with a reduced processing risk related to flammability and storage [5]. The role of acetic acid as modulator to control de crystal growth and hierarchical porosity of different MOFs has been reported. In these works, ⇑ Corresponding author. E-mail address: [email protected] (J.A. Ayllón). https://doi.org/10.1016/j.poly.2019.06.017 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
acetic acid is used as an additive combined with other solvents [6–11]. The main objective of this work is to design a green synthetic method, based on the use of acetic acid exclusively as a solvent, while avoiding the use of pressurized reactors. In the openatmosphere reactor, the processing temperature is limited by the boiling point of the solution. Trimesic acid (H3BTC) was selected as a linker precursor in this work to explore the preparation of MOFs due to its reasonable solubility in hot acetic acid. H3BTC is a low cost tricarboxylic acid, with a rigid structure, that has been used for the formulation of a large number of MOFs, including HKUST-1 (commercially known as Basolite C300), one of the most thoroughly studied MOFs, also produced at industrial scale. Finally, metal acetates were chosen to provide the metal cation. Acetates are environmentally advantageous, since the anion byproduct generated is less corrosive than the ones produced from other inorganic salts, such as chloride or sulphate [3]. In the designed process, acetate also acts as the base necessary to deprotonate the H3BTC linker, yielding acetic acid, i.e., the used solvent [12]. In this exploratory work, three representative first-row transition metals (Cu, Co and Ni) and one heavy metal (Bi) were studied in the precipitation of MOFs. The nature
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of the precipitated material was characterized by powder and single crystal X-ray diffraction (XRD), elemental analysis (EA) and magnetism. The crystal structures of two new 2D coordination polymers, [Co6(BTC)2(Ac)6(HAc)3] and [Bi(HBTC)(Ac)], were elucidated. 2. Results and discussion In short, solids were precipitated after dissolving the precursors under ambient pressure in hot acetic acid. The solvent was then eliminated by distillation until a solid begun to precipitate. The same procedure was followed for Cu, Co, Ni and Bi.
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The crystal structure of [Co6(BTC)2(Ac)6(HAc)3] (1) contains two types of cobalt atoms, Co(1) and Co(2), arranged in hexanuclear clusters (Fig. 2a). Each cluster includes three cobalt atoms of each type, associated by a C3 symmetry axis parallel to the c axis. Both Co atoms present different coordination number, five for Co(1) and six for Co(2) (Fig. 2b). In each cluster, the six cobalt cations are held together through six tridentate acetate ligands, each having a l3, g1,g2-bridging mode. There are two types of acetate ligands; in both the g2 oxygen connects a Co(1) to a Co(2), but differs on the metal bonded to the g1 oxygen. In each cluster, there are six
2.1. Copper(II) The combination of copper acetate and H3BTC in hot HAc yielded a blue precipitate. Powder XRD characterization denotes that it correspond to HKUST-1 (Fig. 1), with a very small amount of impurities. EA results match the [Cu3(BTC)25H2O HAc] stoichiometry. This sample had a weight loss of 16 wt% at 120 °C under vacuum, which roughly agrees with the loss of H2O and HAc in this composition. The N2 adsorption–desorption isotherm displays first a sharp rise in the low pressure range (P/P0 < 0.01), which indicates the presence of substantial microporosity; and second (P/P0 > 0.4), a hysteresis loop associated with capillary condensation taking place in mesopores. Hence, the isotherm is a combination of Type I and IV according to the IUPAC classification (Fig. S1). The BET specific surface area of the material is 580 m2/g, which is relatively low in comparison to other values reported in the literature that mostly are above 1000 m2/g. This result suggests that the material has either low crystallinity or contains some impurities that partially occlude the pores. It is worth noting that the presence of mesopores is a feature desirable in different adsorption applications by improving diffusion [9,13–15]. 2.2. Cobalt(II) H3BTC and Co(Ac)2 in hot acetic acid yielded a crystalline violet material. This is a new compound (1), with stoichiometry [Co6(BTC)2(Ac)6(HAc)3], elucidated structurally by single crystal X-ray diffraction. EA data was in accordance with this stoichiometry. Phase purity of the bulk sample was confirmed by powder X-ray diffraction (Fig. S2).
Fig. 1. X-ray patterns of [Cu3(BTC)25H2O HAc] (up) measured at room temperature; (down) calculated from the crystal structure solved from single crystal XRD measured at 100 K.
Fig. 2. (up) Crystal structure of [Co6(BTC)2(Ac)6(HAc)3] (down) Detail of the local coordination environment of each types of Co(II) atoms (hydrogen atoms are omitted for clarity).
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additional carboxylate bridges provided by trimesate ligand with a l2,g1,g1 bonding mode, that connects also Co(1)–Co(2) pairs. There are also two kinds of trimesate units, connected by the C3 axis, one shown more symmetric Co–O distances than the other (see Table S2). Focusing in the metal centers, five positions in the coordination sphere of each Co(II) cation are occupied by oxygen atoms included in these bridges: two from oxygen atoms from trimesate carboxylate, situated at the relative cis position, and three from acetate ligands. This completes the coordinating sphere of each Co(1), that presents an intermediate geometry between square pyramidal and trigonal bipyramidal, closer to the latest (s = 0.58) [16]. The coordination of one non dissociated acetic acid to each Co(2), which acts as a monodentate ligand through the oxygen not linked to the proton, yields a roughly octahedral coordination geometry for the other half of metallic cations. Each trimesate anion connects three clusters, and the propagation of these inter-cluster bridges defines sheets parallel to the ab plane. Each cluster is connected thought trimesate bridges to the six closer neighbors in the plane. Layers are stacked yielding a compact structure (Fig. S3). Variable-temperature magnetic susceptibility measurements in the range of 300–5 K were performed on a sample powder of compound (1) [Co6(BTC)2(Ac)6(HAc)3]. A vpT value of 17.2 cm3 K/mol was attained at 300 K (Fig. 3a). This value is in the range of reported data for other hexanuclear Co(II) compounds, but is much higher than the calculated spin-only value of 11.25 cm3 K/mol for six CoII cations (S = 3/2 and g = 2.0), which indicates a significant orbital contribution of the Co(II) ions [17–19]. As the temperature is reduced, vpT smoothly decreases over the entire temperature region, indicating the dominant antiferromagnetic interaction among metal centers through the carboxylate bridges. Above 50 K, the reciprocal molar magnetic susceptibility data obeys
Curie–Weiss law (Fig. 3b), with a very small Weiss constant h = 0.97 K, indicating very weak antiferromagnetic interactions among CoII centers. [Co6(BTC)2(Ac)6(HAc)3] reacts very slowly with the humidity present in the air, thus, it can be easily manipulated. However, it reacts quickly in liquid water, producing first a pink solution, and next a pink precipitate after a few minutes. This solid has been identified by powder XRD (Fig. S4) and EA as the previously described [Co3(BTC)212H2O] hydrogen bonded MOF [20–22]. This is a porous material that has shown affinity for the adsorption of iodine [23] and that also loss and gain water molecules reversibly as a function of the temperature and humidity. It should be said that previously described routes for the synthesis of this compound required hydrothermal conditions (140 °C, 24 h), while in this work the material was obtained at room temperature and atmospheric pressure. Presumably the reaction proceeds following equation (1).
Co6 ðBTCÞ2 ðAcÞ6 ðHAcÞ3 þ 12H2 O ! Co3 ðBTCÞ2 12H2 O ðsÞ þ3 CoðAcÞ2 ðaqÞ þ 3HAcðaqÞ
ð1Þ
2.3. Nickel(II) The reaction between nickel(II) acetate and H3BTC in hot acetic acid produced always an amorphous yellowish precipitate (Fig. S5). To obtain a crystalline compound, it was necessary the use of a solvent with higher boiling point than acetic acid (118 °C). Specifically, propionic acid (HPro), with a boiling point of 141 °C, was essayed to favor the crystallization of the material. From an environmental point of view, propionic acid has similar advantages than acetic acid [5]. Interestingly, the use of propionic acid allowed the preparation of a crystalline MOF involving Ni and the tricarboxylic acid (Fig. S5). The powder XRD pattern was similar to the one corresponding to [Co6(BTC)2(Ac)6(HAc)3] (compare patterns in Figs. S5 and S2) indicating structural similarities between both compounds. EA data also agrees with a compound having a comparable stoichiometry, i.e., [Ni6(BTC)2(Pro)6(H2O)6] (2). Both compounds crystallized with a hexagonal morphology (Fig. S6). The behavior of [Ni6(BTC)2(Pro)6(H2O)6] in regard to water was similar to the one observed for cobalt compound (1). Hence, the nickel compound reacted in liquid water giving place to the precipitation of the trinuclear compound [Ni3(BTC)2(H2O)14]4H2O, according to EA and powder XRD data (Fig. S7) [24]. 2.4. Bismuth(III)
Fig. 3. (up) Thermal variation of vpT for complex [Co6(BTC)2(Ac)6(HAc)3]. (down) Thermal variation of 1/vp; solid red line is the best fit to the Curie–Weiss equation h = 0.97 K.
For Bi(III) acetate, [Bi(HBTC)(Ac)] (2), a compound in which the trimesic acid was only partially deprotonated, has been obtained. Other bismuth compounds containing the HBTC2 ligand have been described previously [25–27]. HBTC2 could be considered as an intermediate in the deprotonation of trimesic acid to give BTC3 ligand, observed in the compounds precipitated previously with Cu, Co and Ni. When HBTC2 intermediate is able to form insoluble compounds in the hot acetic acid medium, its formation will be favored respect the formation of species including BTC3 linker. The crystal structure of [Bi(HBTC)(Ac)] was elucidated by single crystal XRD. It contains only three independent entities, the bismuth atom, the monoprotonated HBTC2 ligand and the acetate ligand. Each Bi(III) cation is heptacoordinated, with Bi-O distances in a wide 2.270(8)–2.687(7) interval (Fig. 4a). Bismuth atoms are arranged into chains, parallel to the b axis (Fig. 4b). Each consecutive pair of cations is linked by a triple bridge. Two bridging groups are carboxylate groups from two different HBTC2 ligands, both showing a l2,g1,g1 bonding mode. The acetate ligand creates the
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ment with water at room temperature. In the case of nickel, the temperature attained with acetic acid was not enough to precipitate a crystalline material and propionic acid was used instead to obtain [Ni6(BTC)2(Pro)6(H2O)6]. Finally, for bismuth a compact coordination polymer was precipitated, containing both acetate ligand and partially deprotonated trimesic acid (HBTC2). 4. Experimental 4.1. Materials and synthesis
Fig. 4. (up) Crystal structure of [Bi(HBTC)(Ac)(HAc)3] (down) Detail of the local packing of the 2D layer by hydrogen bonding.
third link through a l3,g1,g2-briding mode. HBTC2 ligands assemble these chains into sheets, parallel to the (-1 0 1) plane. Inside each layer, the relative position of the aromatic rings of HBTC allows weak p–p interactions (3.699(5) Å). These layers are interconnected trough pairs of hydrogen bonds between the protonated carboxylic acid of HBTC2 ligand from adjacent layers, defining thus a 3D-suparmolecular network (Fig. 4c). Powder XRD confirms the purity of the bulk powder (Fig. S8). 3. Conclusions The use of acetic acid as neat solvent in the synthesis of coordination polymers using trimesic acid and metal acetates has been proved for different metals. Only in the case of copper(II) a known porous product was obtained but, in this case, with a hierarchical meso-microporosity. A 2D compact coordination polymer was obtained for cobalt, [Co6(BTC)2(Ac)6(HAc)3], which incorporates both acetic acid and acetate ligands. This compound can be used as precursor in the synthesis of [Co2(BTC)212H2O] by simply treat-
All reagents were from commercial sources and used without further purification. Reactions and manipulations were carried out in air atmosphere. [Cu3(BTC)25H2O HAc] – A solution of Cu(Ac)2H2O (0.500 g, 2.50 mmol) in HAc (50 mL) was added to a solution of H3BTC (0.323 g, 1.53 mmol) of boiling HAc (50 mL). The obtained blue solution was distillated until a blue precipitate appears. Then, the mixture was allowed to cool down and the solid was filtered and washed twice with HAc (5 mL) Yield: 0.257 g (44%, respect H3BTC). Anal. Calc. for C20H20O19Cu3: C, 31.82; H, 2.67. Found: C, 32.03; H, 2.80. [Co6(BTC)2(Ac)6(HAc)3] (1) – A solution of Co(Ac)24H2O (1.93 g, 7.73 mmol) in HAc (20 mL) was added to a solution of H3BTC (0.813 g, 3.86 mmol) of boiling HAc (80 mL). The violet obtained solution was refluxed for an hour and then allowed to concentrate until a violet precipitate appears. Then, the mixture was allowed to cool down and the solid was filtered and washed twice with HAc (5 mL) Yield: 1.38 g (82%, respect Co(Ac)24H2O). Anal. Calc. for C36H36O30Co6: C, 33.20; H, 2.79. Found: C, 33.30; H, 2.78. [Co2(BTC)212H2O] – [Co6(BTC)2(Ac)6(HAc)3] (0.450 g, 0.35 mmol) was added to 20 mL of water giving a pink solution. After few minutes at room temperature, a pink solid precipitated. The solid was filtered, washed with water and dry at room temperature. Yield: 0.256 g (92%). Anal. Calc. for C18H30O24Co3: C, 26.78; H, 3.75. Found: C, 26.80; H, 3.81. [Ni6(BTC)2(Pro)6(H2O)6] (2) – A solution of Ni(Ac)24H2O (1.95 g, 7.84 mmol) in hot propionic acid (HPro, 40 mL) was added to a solution of H3BTC (1.089 g, 5.18 mmol) of boiling HPro (60 mL). The green obtained solution was refluxed for an hour and then allowed to concentrate until a yellow green precipitate appears. Then, the mixture was allowed to cool down and the solid was filtered and washed with HPro (5 mL) and EtOH (5 mL). Yield: 1.52 g (88%, respect Ni(Ac)24H2O). Anal. Calc. for C36H48O30Ni6: C, 32.93; H, 3.68. Found: C, 32.74; H, 3.82. [Ni3(BTC)2(H2O)14]4H2O – [Ni6(BTC)2(Pro)6(H2O)6] (0.100 g, 0.08 mmol) was added to 20 mL of water giving a light green solution. After twenty minutes at room temperature, a green solid precipitated. The solid was filtered, washed with water and dry at room temperature. Yield: 0.043 g (62%). Anal. Calc. for C18H42O30Ni3: C, 23.64; H, 4.63. Found: C, 23.90; H, 4.50. [Bi(HBTC)(Ac)], (3) – A solution of Bi(Ac)3 (0.216 g, 0.56 mmol) in HAc (10 mL) was added to a solution of H3BTC (0.118 g, 0.56 mmol) of boiling HAc (20 mL). The obtained solution was refluxed for an hour and then allowed to concentrate until a white precipitate appears. Then, the mixture was allowed to cool down and the solid was filtered and washed twice with ethanol (5 mL) Yield: 0.100 g (38%). Anal. Calc. for C11H7O8Bi: C, 27.75; H, 1.48. Found: C, 27.89; H, 1.43. 4.2. Characterization Elemental analyses (C, H, N) were carried out on a Euro Vector 3100 instrument. Powder XRD patterns were measured at room temperature with a Siemens D5000 apparatus using the Cu Ka
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radiation. Patterns were recorded from 2h = 5 to 50°, with a step scan of 0.02° counting for 1 s at each step. Magnetic measurements from 5 to 300 K were performed in a Quantum Design MPMS-5S SQUID susceptometer. The molar susceptibility was corrected for the sample holder and for the diamagnetic contribution of all atoms by means of Pascal’s tables [28]. Adequate prism-like crystals were used for single crystal X-ray diffraction experiments, a violet crystal in the case of [Co6(BTC)2(Ac)6(HAc)3]n and a transparent crystal in the case of [Bi(HBTC)(Ac)]n. Data were collected using Mo Ka radiation (k = 0.71073 Å) in a SMART-APEX diffractometer with a CCD detector. Measurements were performed at 23 ± 1 °C. An empirical absorption correction was applied (SADABS). The structure was solved by direct methods (SHELXNT) and refined by full-matrix least-squares methods on F2 for all reflections (SHELXL-2014/7) [29]. Non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms bonded to carbon atoms were placed in calculated positions with isotropic displacement parameters fixed at 1.2 times the Ueq of the corresponding carbon atoms. Crystal data and further refinement details are presented in Table S1. Molecular graphics were generated with the program Mercury 3.6 [30,31]. Acknowledgements This work was partially financed by the Spanish National Plan of Research CTQ2017-83632-CO2-P1. C.D./ICMAB acknowledges financial support from the Spanish MEC, through the ‘‘Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-20150496). Appendix A. Supplementary data CCDC 1556887–1556888 contains the supplementary crystallographic data for [Co6(BTC)2(Ac)6(HAc)3] and [Bi(HBTC) (Ac)] respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data to this article can be found online at https://doi.org/10.1016/j.poly.2019.06.017.
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