Straightforward Synthesis of Single-Crystalline and Redox-Active Cr(II

Communication pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Straightforward Synthesis of Single-Crystalline and Redox-Active Cr(II)-carboxylate MOFs Michał K. Leszczyński,†,§ Arkadiusz Kornowicz,‡,§ Daniel Prochowicz,† Iwona Justyniak,† Krzysztof Noworyta,† and Janusz Lewiński*,†,‡ †

Institute of Physical Chemistry Polish Academy of Sciences Kasprzaka 44/52, 01-224 Warsaw, Poland Department of Chemistry Warsaw University of Technology Noakowskiego 3, 00-664, Warsaw, Poland

‡

S Supporting Information *

4,4′-(hexafluoroisopropylidene)bisbenzoate), respectively. Remarkably, both 1 and 2 could be easily dehydrated, leading to single-crystalline frameworks Cr3(BTC)2 (1a) and Cr(hfipbb) (2a) with open Cr(II) sites. In addition, redox activity of the resulting MOFs was examined by an electrochemical study. Noteworthy, our effort to the preparation of sizable MOF single crystals nicely fits into a hot topic concerning the large monolithic MOF materials of enhanced application properties.19 Common approaches to the preparation of MOF single crystals often involve reactions in organic solvents and high temperatures, but reported similar strategy aiming at Cr(II)based MOFs yielded polycrystalline materials.7,8 To overcome this problem we decided to explore an alternative approach utilizing slow diffusion of substrates as a tool to control the MOF crystallization. Accordingly, using chromium(II) sulfate and potassium 1,3,5-benzenetricarboxylate in a diffusioncontrolled reaction conducted in water at room temperature we obtained 1 (Scheme 1, path 1) in the form of red octahedral single crystals in size of 50−150 μm after ca. 3−4 weeks (Figure 1a). Notably, by adjusting the diffusion rate the reaction could be conducted in shorter time, which led to the formation of 1 as a microcrystalline powder (for details see Supporting Information). Single crystal X-ray analysis revealed that 1 crystallizes in the cubic space group Fm-3m and forms a porous 3D network isostructural to HKUST-1 (Figure 2a, Tables S1 and S2).20 The framework contains Cr2(O2CR)4 paddlewheel type units with the Cr−Cr distance of 2.344(2) Å and H2O molecules coordinated to the chromium atoms at axial positions, which is comparable to the previously reported H2O-ligated chromium(II) paddlewheel-type clusters.21 The powder X-ray diffraction (PXRD) analysis confirmed the phasepurity of the obtained material (Figure S3). To explore the generality of the diffusion-controlled synthesis of Cr(II)-based MOFs and its potential to accommodate a diversity of carboxylate linkers, a V-shaped 4,4′-(hexafluoroisopropylidene)bisbenzoic acid (H2hfipbb) was chosen. The slow diffusion reaction of potassium salt of the H2hfipbb acid and chromium(II) sulfate yields red cuboid single crystals of Cr(hfipbb)·H2O (2) (50−100 μm in size, Figure 1c) (Scheme 1, path 2). According to PXRD analysis, compound 2 is the only crystalline product formed as the

ABSTRACT: We report on a facile and environmentally friendly synthetic approach for single-crystalline chromium(II) carboxylate metal−organic frameworks (i.e., Cr3(BTC)2·3H2O (1) and Cr(hfipbb)·H2O (2) at room temperature in water. Both MOFs can be easily dehydrated, affording single-crystalline materials with open Cr(II) sites. In addition, the redox activity and porosity of the resulting Cr(II) MOFs were examined.

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etal−organic frameworks (MOFs) continue to receive significant attention related to their numerous applications.1−6 This is reflected by the vast and steadily growing number of papers published every year on the synthesis, structure and properties of new MOFs involving great variety of organic linkers and metal centers. In this plethora of reports, the cases of chromium(II)-based MOFs (Cr(II) MOFs) are extremely rare (only two examples were published by Long et al.),7,8 which is surprising with regard to their interesting redox properties.8,9 Investigation of the redox properties of MOFs is essential for development of the next generation of MOF systems with tailored and adjustable functions for advanced applications, e.g., gas separation and storage, electrocatalysis, electrochemical sensors, and energy storage.10−12 In contrast to a wide family of Cr(III)-MOFs,13−15 the development of redox-active Cr(II)-based MOF systems is essentially hindered by the lack of efficient and straightforward synthetic strategies. The only reported synthetic procedure of the first known carboxylate−Cr(II) MOF utilized the toxic Cr(CO)6 precursor and solvothermal conditions, yielding a polycrystalline product, which required advanced neutron diffraction studies for detailed structural characterizations.7 Only recently has a synthetic approach to redox-active Cr(II) metal−organic polyhedra (MOP) been developed,16 but the general strategy for preparation of Cr(II)−carboxylate MOFs, preferably leading to single-crystalline materials, is still missing, which limits the development of this field. Herein, in the course of our systematic investigations on the development of new synthetic procedures of various inorganic−organic hybrid materials,17,18 we report on a facile aqueous synthetic procedure for Cr(II)−carboxylate MOFs in the form of high-quality single crystals. This approach enables access to both 3D and 2D frameworks based on paddlewheel-type Cr(II) subunits and carboxylate linkers, i.e. Cr3(BTC)2·3H2O (1) (BTC = 1,3,5benzene-tricarboxylate) and Cr(hfipbb)·H2O (2) (hfipbb = © XXXX American Chemical Society

Received: February 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00395 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Scheme 1. Synthesis of Single Crystalline Cr(II)-Based MOFs and Their Dehydration Processes

Figure 1. Visible light microscope images of Cr(II)-MOF single crystals: (a) 1, (b) 1a, (c) 2, and (d) 2a (bar = 200 μm).

obtained diffraction pattern closely matches the data from single crystal diffraction analysis (Figure S4). Single crystal Xray diffraction analysis revealed that 2 crystallizes in monoclinic P2/n space group and forms a 2D framework containing Cr2(O2CR)4 paddlewheel type units with the Cr−Cr distance of 2.3267(12) Å and one water molecule coordinated to each chromium center (Table S5, S6). The 2D networks of 2 form layers involving two independent interpenetrating frameworks (Figure 2c), which assemble into a stacked crystal structure with the stacking distance d = 11.10 Å (Figure 2d). The crystal structure of 2 contains internal cavities connected into 1D channels with relatively narrow bottleneck-type openings (ca. 3.0 Å in diameter) along the crystallographic b-axis (Figure S2a). Notably, 2 represents the first example of structurally characterized 2D Cr(II)-based carboxylate network. In the next step, we investigated the dehydration processes of 1 and 2, which occur readily, as evidenced by the FTIR analysis (Figures S6,S7) and the single crystal X-ray diffraction study. Both materials were heated at 100 °C in vacuum for 8 h, in order to remove the water molecules present in pores and coordinated to the metal centers. The dehydration process was accompanied by a color change from red to orange, and led to the activated frameworks Cr3(BTC)2 (1a) and [Cr(hfipbb)]

Figure 2. Crystal structures of Cr(II)-based MOFs: 3D frameworks of 1 (a) and 1a (b), two interpenetrating 2D frameworks of 2 (shown in red and blue) (c) and stacking of 2D frameworks in 2 (d). Colors (excluding structure c): blue = Cr, gold = F, violet = O, gray = C; H atoms have been omitted for clarity.

(2a) with the coordinatively unsaturated Cr(II) centers (Scheme 1). The single crystal X-ray analysis demonstrated that 1a retained the porous framework structure of 1. The dehydrated material 1a exhibited no axial ligation at Cr2(O2CR)4 paddlewheel-type units with the Cr−Cr distance of 2.077(2) Å which is significantly shorter compared to that observed in 1 (Figure 2b, Table S3, S4). We also note that the B

DOI: 10.1021/acs.inorgchem.8b00395 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

a virial-type fitting to the isotherm data equals 6.2 kJ/mol (Figure S10), and is comparable to the corresponding values reported for HKUST-1 (6.8 kJ/mol)26 and Cr3(BTC)2.27 The adsorption properties of 1a are further exemplified by significant CO2 uptakes at two temperatures: 273 and 283 K up to 1 bar, which reached 31.1 wt % (158 cm3/g, STP) and 24.3 wt % (124 cm3/g, STP), respectively (Figure S11). The isosteric heat of adsorption for CO2 has a zero coverage value of 25.5 kJ/mol. However, experiments involving 2a material revealed a negligible N2 adsorption at 77 K (Figure S8), which indicates that the 1D channels were inaccessible to the N2 molecules possibly due to the narrow bottleneck-type openings. In conclusion, we have developed a straightforward and green strategy for the preparation of redox-active and singlecrystalline Cr(II)-carboxylate MOFs under mild aqueous conditions. These results pave the way for the design and a more detailed exploration of Cr(II)-based porous materials with tunable redox functionalities. Further development of our synthetic strategy and the application-oriented studies are currently underway.

crystal structure of 1a is analogous to that previously reported for Cr3(BTC)2 (albeit prepared quite differently) determined using powder neutron diffraction.7 The dehydration of 2 led to the formation of a new framework Cr(hfipbb) (2a), which contains Cr2(O2CR)4 units with no axial ligation and Cr−Cr distance of 2.014(3) Å, linked by dicarboxylate anions into an extended 2D network (Table S7, S8). Interestingly, the transition from 2 to 2a was accompanied by a change in the stacking geometry of 2D frameworks. The stacking distance dropped from 11.10 to 10.61 Å and the β angle of the unit cell changed from 93.065(4) to 90.297(5) degrees (for an overlay of both structures see Figure S1). Despite the structural change, 2a was obtained in a single crystal form, which indicates flexibility of the chromium carboxylate network, but the void cavities and channels were slightly compressed in comparison to that found in 2 (Figure S2). Moreover, we have also found that the transformation of 2a into pristine 2 readily occurs upon exposure to H2O vapor at room temperature, which was tracked using PXRD (Figure S5). Chromium(II) carboxylates are known to be highly reactive toward dioxygen7,16 which was also observed for materials 1, 2, 1a, and 2a. In order to elucidate the redox properties of the obtained chromium(II)-carboxylate MOFs, we conducted cyclic voltammetry studies of 1, 1a, 2, and 2a using the Me10Fc/Me10Fc+ as a reference electrode. Despite substantial capacitive currents, distinct oxidation and reduction peaks were observed (Figure 3) and the process was fully reversible. All of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00395. Full experimental details containing a detailed description of synthetic work, X-ray crystallography, PXRD analysis, IR spectra, and methodology of CV study (PDF) Accession Codes

CCDC 1568837−1568840 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Figure 3. Cyclic voltammograms of 1 and 1a (a) and 2 and 2a (b).

AUTHOR INFORMATION

Corresponding Author

*(J.L.) E-mail: [email protected].

the examined Cr(II)-based MOFs exhibited a two-step oxidation/reduction pathway showing initial oxidation step in the range from −0.37 to −0.07 V with corresponding reduction peak between −0,63 V and −0,47 V, which could be ascribed to oxidation/reduction of CrII/CrIII.22,23 Interestingly, the examined Cr(II) MOFs exhibited a second oxidation signal observed in the range from 0.51 to 0.56 V (the corresponding reduction signal was weakly resolved). These results are in comparable range to the previously reported CrIII/CrIV 24 and CrIII/CrV 25 redox couples, which indicates that the Cr centers in the studied MOFs can reversibly adopt multiple oxidation states. Finally, the porosity of the dehydrated 1a and 2a frameworks was examined using gas adsorption measurements. The N2 gas adsorption isotherm of 1a measured at 77 K revealed a type I adsorption isotherm typical for microporous material with maximum uptake of 410 cm3/g (at standard temperature and pressure (STP), Figure S8). The (BET) surface area for 1a is calculated to be 1562 m2/g, a value similar to that observed for the microcrystalline Cr3BTC2 material obtained by Long et al.7 Evaluation of H2 adsorption at 77 and 87 K (1 bar) revealed an uptake of 2.0% and 1.3%, respectively (Figure S9). The zerocoverage isosteric heat of adsorption (Qst) for H2 obtained from

ORCID

Michał K. Leszczyński: 0000-0001-9339-101X Daniel Prochowicz: 0000-0002-5003-5637 Janusz Lewiński: 0000-0002-3407-0395 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support by the National Science Centre (project no. 2014/15/N/ST5/ 02919) and the Foundation for Polish Science Team Program cofinanced by the European Union under the European Regional Development Fund TEAM/2016-2/14.

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DOI: 10.1021/acs.inorgchem.8b00395 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00395 Inorg. Chem. XXXX, XXX, XXX−XXX