Single-Crystal to Single-Crystal Transformation of a Nonporous Fe(II

Article pubs.acs.org/IC

Single-Crystal to Single-Crystal Transformation of a Nonporous Fe(II) Metal−Organic Framework into a Porous Metal−Organic Framework via a Solid-State Reaction Sebastian Spirkl,† Maciej Grzywa,† Stephan Reschke,‡ Jonas K. H. Fischer,‡ Pit Sippel,‡ Serhiy Demeshko,§ Hans-Albrecht Krug von Nidda,‡ and Dirk Volkmer*,† †

University of Augsburg, Institute of Physics, Chair of Solid State and Materials Chemistry, Universitätsstrasse 1, 86159 Augsburg, Germany ‡ University of Augsburg, Institute of Physics, Experimental Physics V, Center for Electronic Correlations and Magnetism, Universitätsstrasse 1, 86159 Augsburg, Germany § Georg-August-Universität Göttingen, Institute of Inorganic Chemistry, Tammannstraße 4, 37077 Göttingen, Germany S Supporting Information *

ABSTRACT: We report the synthesis of an air-stable nonporous coordination compound based on iron(II) centers, formate anions, and a 4,4′-bipyrazole (H2BPZ) ligand. Upon thermal treatment, a porous metal−organic framework (MOF) formed due to decomposition of the incorporated formate anions. This decomposition step and the following structural changes constituted a single-crystal to single-crystal transformation. The resulting [Fe(BPZ)] framework contained tetrahedrally coordinated iron(II) metal centers. The framework was sensitive toward oxidation by molecular oxygen even at temperatures of 183 K, as followed by oxygen sorption measurements and a color change from colorless to metallic black. The semiconductor properties of the oxidized material were studied by diffuse reflectance UV/ vis/NIR spectroscopy and dielectric spectroscopy.

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INTRODUCTION One of the major challenges in the design of metal−organic frameworks (MOFs) comprises enhancing the thermal and chemical stabilities of these compounds.1−4 MOFs built from N-heterocyclic ligands often feature higher chemical stabilities in comparison to their structurally related carboxylic acid derived counterparts, ascribed to the higher bonding strengths of metal−azolate bonds. Hence, several groups have utilized nitrogen-containing ligands instead of carboxylic ligands.5−9 The bonding strengths between transition metals and ligands depend on their Lewis acidity and basicity. Soft Lewis acids and bases form coordination bonds with strong covalent character and are thus very stable.10,11 Deprotonation of azole ligands leads to N-donor atoms with higher basicities and therefore to stronger bonds with metal centers.12 These features not only enhance the stability against thermal decomposition but also enhance the stability against hydrolysis, which, for example, has been utilized for corrosion resistance of copper surfaces with polymeric metal azolates.13 A considerable difference between azolate and carboxylate ligands is their coordination angles: carboxylate ligands commonly form bonding angles of 180, 120, and 60°, and azolate ligands preferably form coordination angles of approximately 70 and 140°.12 This difference in coordination allows the formation of new framework structures that are not accessible using carboxylate ligands. Nevertheless, strong bases are needed to deprotonate azolate ligands. In the reaction © 2017 American Chemical Society

medium, these strong bases tend to form insoluble metal hydroxides, which are disadvantageous for phase-pure syntheses.14 Furthermore, synthesizing MOFs incorporating exclusively iron(II) centers is challenging due to the extreme sensitivity of iron(II) toward oxidation. On the basis of our previous studies on Fe-CFA-6 (termed coordination framework Augsburg University-6),15 a MIL-53-type framework with the composition [Fe(OH)(BPZ)] (H2BPZ = 4,4′-bipyrazole), we have spent considerable effort synthesizing an analogous iron(II) framework with the composition [Fe(L)(BPZ)] (L = μ2bridging neutral ligand, e.g., dimethylformamide (DMF) or acetonitrile (MeCN)). However, until now, all trials to develop a direct synthesis of such compound have failed, which has been unexpected since Long et al. in 2015 reported the synthesis of [Fe(BDP)], a compound incorporating iron(II) centers and the deprotonated 1,4-benzenedipyrazole (H2BDP) ligand.8 Admittedly, adapting their synthesis conditions did not lead to a crystalline product incorporating iron(II) and the deprotonated BPZ ligand. When the synthesis conditions were varied, the addition of monofunctional ligands (i.e., simple carboxylic acid derivatives) was found to serve as crystallization modulators, providing access to novel (nonporous) Fe(II) coordination frameworks. In the study presented here, formic Received: July 17, 2017 Published: September 29, 2017 12337

DOI: 10.1021/acs.inorgchem.7b01818 Inorg. Chem. 2017, 56, 12337−12347

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Inorganic Chemistry

coordinated. Argon sorption measurements demonstrate that the precursor structure is a nonporous coordination framework, whereas the specific surface area of the porous CFA-10 is ∼700 m2 g−1. One notable advantage of our synthetic strategy is the high overall yield of the porous target compound, CFA-10, requiring only a small amount of solvent for the synthesis of the precursor phase CFA-10-as. The thermal stabilities of the compounds and the phase transition upon thermal treatment were investigated by various techniques. The oxidation states of both compounds were confirmed, and the air sensitivity of CFA-10 was extensively studied by oxygen sorption measurements, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), DR ultraviolet/visible/near-infrared (UV/vis/NIR) spectroscopy, and Mössbauer spectroscopy under oxygen. The metallic appearance of CFA-10 led us to investigate the electric properties by DR UV/vis/NIR and dielectric spectroscopy, revealing an optical band gap below 2 eV.

acid was added to the solvothermal reaction mixture, leading to the formation of the highly crystalline nonporous coordination compound [Fe(OOCH)2(H2BPZ)], denoted CFA-10-as (see Scheme 1). Scheme 1. Synthesis of Nonporous CFA-10-as and the SolidState Reaction of the Porous Framework CFA-10 by Decomposition of the Formate Ligandsa

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RESULTS AND DISCUSSION Syntheses. The organic ligand and mixed-valent precursor complex [Fe3O(O2C2H3)]·3H2O (see Scheme 1) were synthesized according to previous literature procedures and were used without further purification.17,18 The synthesis of CFA-10-as was performed under solvothermal conditions. Single crystals suitable for structure determination were found in the reaction mixture. Microscope images of the crystals are shown in Figure S1 in the Supporting Information. After the crystals were filtered, CFA-10 was synthesized by an irreversible thermal phase transformation. In this procedure, dry crystals of CFA-10-as were heated under an inert gas stream (nitrogen or argon) to 300 °C for at least 10 min. Crystal Structure Determination. CFA-10-as crystallizes in the monoclinic space group P21/c (No. 14). The asymmetric unit consists of one iron(II) ion, two oxygen atoms, two nitrogen atoms, four carbon atoms, and four hydrogen atoms, constituting one formate molecule and half of the bipyrazole ligand. An ORTEP style plot of the asymmetric unit of CFA10-as with atom labels is shown in Figure S2 in the Supporting Information. CFA-10-as features a 3D layered structure built by iron octahedra lying in the (200) plane. The octahedra are interconnected by protonated H2BPZ ligands in the a direction (Figure 2). The central Fe(II) ion, placed at a 1̅ symmetry site (Wyckoff position 2b), is coordinated by four oxygen atoms from four formate molecules and in the apical positions by two nitrogen atoms from two H2BPZ molecules. The Fe−O distances range from 2.0741(6) to 2.1634(7) Å, whereas the Fe−N bond lengths are 2.2062(7) Å. These values are in very good agreement with the values for iron(II) tris(pyrazolyl)methane complexes, mesoporous iron(II) formate [Fe(O 2 CH) 2 ]·1/3HCO 2 H, and porous iron(II) formate [Fe3(O2CH)6].19−22 The crystal-packing motif along the a direction consists of neighboring H2BPZ molecules twisted against each other with an angle of 83.32(2)°. The hydrogenbond distance between the H-donor nitrogen atom of the pyrazole unit and the H-acceptor oxygen atom of the formate group is 2.73 Å (see Figure 1c and Table S1 in the Supporting Information), indicating an interaction that is almost as strong as hydrogen bonds in water.23 Upon thermal treatment of CFA-10-as, an intramolecular proton transfer from the pyrazole ligand to formate might occur, followed by thermal desorption of formic acid. A free binding site at the iron center would then coordinate a deprotonated nitrogen donor atom of the

a The structures of the products are represented as space-filling models. The addition of coordinating solvents might lead to a structure analogous to [Cu(BPZ)]·MeCN7 with iron(II) centers. Oxidation of CFA-10 in the presence of water might lead to the formation of Fe-CFA-6.15

Due to the densely packed ligands in CFA-10-as, the iron(II) centers are protected against oxidation. Since coordinated formate ligands can serve as reducing agents upon thermal decomposition,16 we utilized this property in the solventless synthesis of the iron(II) MOF [Fe(BPZ)] (CFA-10), which forms via a single-crystal to single-crystal (SC to SC) transformation from the nonporous precursor compound CFA-10-as (Scheme 1). CFA-10-as consists of octahedrally coordinated iron(II) centers, bridging formate ligands, and protonated H2BPZ ligands, the last forming interconnecting struts of 1D Fe(II) carboxylate chains. Thermal treatment of CFA-10-as under nitrogen leads to the exclusive formation of the microporous framework CFA-10, a compound which is isostructural with the literature-known [Zn(BPZ)].7 During this solid-state reaction, complete decomposition of the formate ligands occurs, as shown by thermal analysis of CFA-10-as. The unexpectedly selective and complete solid-state reaction led us to examine the putative mechanistic details of the SC to SC transformation of CFA-10-as to CFA-10. To the best of our knowledge, this SC to SC transformation represents a unique example of a solventless preparation of a permanently porous coordination framework from a nonporous reactive solid-state precursor compound, showing complete preservation of the crystalline shape. The formation of CFA-10 involves a structural change, in which the iron(II) centers in CFA-10 are tetrahedrally 12338


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(613.3(4) Å3) for a probe radius of 1.68 Å, corresponding to the approximate van der Waals radius of argon.26 CFA-10 is a novel isoreticular framework to Fe(BDP).8 The structural transformation from the nonporous CFA-10as to the porous CFA-10 is associated with a change of the crystal system from monoclinic (P21/c) to orthorhombic (P42/ mmc). There is no direct group−subgroup relation between the P21/c and P42/mmc space groups. However, 73 possible pathways for the transformation between these groups exist, 12 of which are short pathways (see Table S2 in the Supporting Information).27 A comparison of the powder patterns of CFA10-as and CFA-10 (see Figure 3) indicates that the peaks Figure 1. Packing diagrams of CFA-10-as along the crystallographic (a) c and (b) a directions. (c) The secondary building unit consisting of an octahedrally coordinated iron(II) center with four bridging equatorial formate ligands and two axial pyrazole ligands, forming hydrogen bonds to one oxygen atom of the formate ligands. The bonding distance between the nitrogen H-donors and the oxygen Hacceptors is 2.73 Å.

pyrazolate ligand, leading to an irreversible structural transformation. Heating a crystal of the nonporous CFA-10-as under a nitrogen atmosphere at 300 °C leads to an irreversible SC to SC structural transformation and to the new microporous phase CFA-10 (see Figure S3 in the Supporting Information). CFA-10 crystallizes in the tetragonal space group P42/mmc (No. 131). The asymmetric unit contains one iron(II) ion and two carbon, one nitrogen, and one hydrogen atom, constituting one-fourth of the BPZ ligand. The iron(II) ions, placed at 4̅m2 symmetry sites (Wyckoff position 2f), are tetrahedrally coordinated (ocher tetrahedra in Figure 2) by four nitrogen

Figure 3. Comparison of the powder patterns of CFA-10-as and CFA10 with marked crystallographic planes. Inset: VT-XRD patterns monitoring the transformation of CFA-10-as to CFA-10 with a measurement interval of 5 K. The transition does not consist of an amorphous transition state, on the basis of the (100) peak.

corresponding to the (100) and (200) planes are shifted +0.5 and +1.1° in 2θ, respectively, whereas the (110) peak is shifted by −0.2° in 2θ, which are related to the changes in the unit cell size. The a parameter shortened from 10.18 Å (CFA-10-as) to 9.21 Å (CFA-10), whereas the b and c parameters elongated (b, from 8.51 Å (CFA-10-as) to 9.21 Å (CFA-10); c, from 6.55 Å (CFA-10-as) to 7.24 Å (CFA-10)) (see Figure 4). Additionally, the β-angle value changed from 106.3 to 90° in the CFA-10 phase. Interestingly, the a and b parameters in CFA-10 (9.21 Å) correspond to the nonbonding Fe−Fe distances in the FeBPZ-Fe chains expanding in the a and b directions. In CFA-10as, the Fe−Fe distances in the Fe-BPZ-Fe chains expanding in the a direction are 10.18 Å. A possible explanation for the solidstate structural transformation from the nonporous CFA-10-as to the porous CFA-10 might be as follows: the heating process leads to decomposition of the formate ligands and changes the unit cell size and cell metrics. According to the differences in the powder patterns of CFA-10-as and CFA-10, the peak corresponding to the (011) plane in CFA-10-as disappeared, and a new (101) peak is observed. These differences are related to the reorientation of every second Fe-BPZ-Fe chain (marked with dots in Figure 4c) and the transition of the Fe(II) ions to the former positions occupied by the formate ligands. This

Figure 2. Packing diagram of CFA-10 in the central projection. The iron centers are tetrahedrally coordinated by nitrogen atoms of the BPZ ligand, forming linear chains along the crystallographic c axis.

atoms of four tetradentate BPZ ligands. The deprotonated BPZ ligands bridge adjacent Fe(II) ions, creating 1D linear chains running parallel to the crystallographic c axis. The chains are interconnected by the BPZ ligand along the a and b axes, forming a 3D framework. CFA-10 exhibits a microporous structure with 1D square-shaped channels expanding in the c direction of the crystal lattice (see Figure 2). Taking the van der Waals radius of a carbon atom (1.7 Å) into account, the calculated channel diameter between carbon atoms of the BPZ ligands is 5.66 Å. An estimation with the SQUEEZE/ PLATON24,25 program reveals that the initial solvent-accessible void volume is 295.6 Å3, which is 48.2% of the unit cell volume 12339


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Figure 4. Schematic representations of the selected sections of the crystal structures of (a−c) CFA-10-as and (d) CFA-10 with marked (100), (200), (011) and (101) planes. (c) Simulated model of the unit cell of CFA-10-as after changing the unit cell parameters to the orthorhombic parameters.

process is not accompanied by amorphization of the sample, an experimental observation that was proven by variable-temperature powder X-ray diffraction (VT-PXRD) measurements (see inset in Figure 3). By addition of coordinating solvents, such as MeCN, DMF, and dimethylacetamide, to activated CFA-10, we attempted to initiate a second transformation into a structure more closely related to the structure of [Cu(BPZ)]·MeCN (see Scheme 1).7 For this transformation, the coordinating solvent should occupy the positions between the adjacent iron centers. Nevertheless, all attempts to synthesize a corresponding Fe(II) structure were not successful. Thermal Analysis. Starting from the presented singlecrystal structure (Figure 1), we assumed that the formate anions in CFA-10-as decomposed to form the novel coordination compound. This assumption was based on the small distance between the protonated pyrazole ring and the formate ligands (see Figure 1), implying an interaction between the protons of the pyrazole ligands and the carboxylates. The three following reaction pathways were considered:

Thermogravimetric analysis (TGA) of the nonporous compound CFA-10-as was performed to gain information on the decomposition mechanism. TG mass spectrometry (TGMS) measurements were performed under nitrogen from room temperature to 350 °C. The compound was kept at the final temperature for 10 min before cooling to room temperature. One possible mechanism for this decomposition is as follows: formic acid is formed from the formate anions and the protons of the protonated linker molecules, which are in close proximity. At elevated temperatures, formic acid is unstable and decomposes to carbon dioxide and hydrogen or carbon monoxide and water.28 The results of the TG-MS measurements are shown in Figure S5 in the Supporting Information. Between 200 and 325 °C, a single-step mass loss of 32.83 wt % occurred. The theoretical mass loss of two molecules of formic acid per formula unit would equate to 32.87 wt %. Additionally, the mass spectrometer revealed the release of formic acid (m/z 45 and 46) at temperatures above 200 °C and of its decomposition products CO2 and H2 as well as CO and H2O (m/z 44 and 2 as well as m/z 29 and 18). At these temperatures, formic acid decomposes to CO and CO2 in an approximately 1:1 ratio.29 These findings indicate that decomposition pathway (1) occurs, as all the reaction products are present in the exhaust gas. Furthermore, a color change of the material from colorless to metallic black occurred, attributed to the oxidation of the material after removing the sample from the TGA apparatus.

HCOO− + H+ → HCOOH → CO2 +H 2 +CO + H 2O (1)

HCOO− → CO2 + H− HCOO− → CO + OH−

H− + H + → H 2 OH− + H+ → H 2O

(2) (3) 12340


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Inorganic Chemistry VT-PXRD. VT-PXRD measurements of CFA-10-as were performed under constant nitrogen flow (100 mL min−1) in the temperature range from room temperature to 600 °C with temperature steps of 50 °C. The diffraction patterns of CFA10-as are depicted in Figure S6 in the Supporting Information. Above 250 °C, an irreversible phase transformation occurred due to the decomposition of the formate ligands. Above 300 °C, the transformation was complete, and the diffraction pattern of CFA-10 was present. According to the diffraction data, CFA-10 is highly crystalline; however, broadening of the peaks occurred upon heating, indicating a decrease in the crystallite size of the sample. Thermal treatment of the precursor compound under vacuum did not lead to the formation of highly crystalline CFA-10, as shown in the corresponding VT-PXRD measurements in Figure S7 in the Supporting Information. Therefore, the activation of CFA-10as was performed under inert gas for all further measurements. Sorption Analysis of CFA-10. Crystallographic studies reveal that the thermal activation of CFA-10-as to CFA-10 is accompanied by a structural transformation into a porous framework. To determine the porosity, argon sorption measurements of CFA-10 were performed at 87 K. A fresh sample was prepared by heating the precursor compound at 300 °C under nitrogen for 30 min. The argon adsorption isotherms were used to determine the specific surface area by the Brunauer−Emmett−Teller (BET) method. The specific BET surface area of CFA-10 is ∼700 m2 g−1, which is in good accordance with the reported values of 778 and 926 m2 g−1 for [Zn(BPZ)] and [Co(BPZ)], respectively.7 The gas uptake of the sample at a relative pressure of p/p0 = 0.25 was 214 ccm g−1. Additionally, the pore-size distribution was calculated from the sorption isotherm using a density functional theory (DFT) equilibrium model. The maximum of the pore-size distribution was located at 5.47 Å, which is in good accordance with the value of 5.66 Å determined from the structure model obtained from the crystallographic data. To investigate the sensitivity of the tetrahedral coordination the iron(II) centers toward oxidation, oxygen sorption measurements at low temperatures were performed with the same sample of CFA-10. Oxygen sorption isotherms were measured at temperatures of 183, 193, 203, and 213 K (Figure 5). The isotherm measurement at 183 K was repeated due to an unexpected stepwise uptake of oxygen in the pressure region of p/p0 = 0.1 followed by continuous uptake of oxygen. After this measurement, the sample was evacuated, and the isotherm was recorded under the same conditions, revealing a Langmuir-type oxygen sorption isotherm. Thus, in the first measurement, the sample was clearly oxidized at 183 K even at a low partial pressure of oxygen. The isotherm measurements at 193, 203, and 213 K revealed the typical Langmuir-type isotherms, and hence, the isosteric heat of adsorption of oxygen was determined to be ∼15 kJ mol−1 by applying the Clausius− Clapeyron equation, indicating oxygen physisorption in the oxidized framework.30,31 The sample was removed, and black crystals with a metallic appearance were observed. In a second series of measurements, oxygen sorption was measured under the same conditions followed by argon sorption measurements at 87 K to determine the porosity of the framework after the oxidation of the iron(II) centers. The oxygen sorption isotherms are shown in Figure S8 in the Supporting Information, exhibiting similar oxygen uptake and thus an isosteric heat of adsorption similar to that in the first set of measurements. The evaluation of the argon sorption data

Figure 5. Oxygen sorption isotherms recorded at temperatures between 183 and 213 K. During the first measurement at 183 K, oxidation of CFA-10 occurs, according to the stepwise increase at p/p0 = 0.15.

unveiled a BET surface area of ∼670 m2 g−1, which is only slightly lower than that of CFA-10 prior to oxidation. To determine whether CFA-10 may be transformed into CFA-6 (see Scheme 1) as assumed, further studies were performed to identify the nature of the oxidized framework. The oxidized framework was also a black material with a metallic appearance, similar to the case for Fe-CFA-6.15 The DRIFT spectra of this material are presented in Figures S9 and S10 in the Supporting Information. According to the IR spectra of both materials, the connectivity of the oxidized framework is similar to that of Fe-CFA-6 but possesses slight differences, with almost identical vibrational frequencies (Figure S11 in the Supporting Information). Additionally, the resulting material is less crystalline than Fe-CFA-6, and thus, the powder patterns could not be properly compared. A topological analysis of both frameworks revealed that only a complex oxidation mechanism with a simultaneous rearrangement of the framework would lead to a transformation of CFA-10 to Fe-CFA-6 (see Figure S12 in the Supporting Information). These findings led us to conclude that the transformation from CFA-10 to Fe-CFA-6 does not occur under the applied conditions. DR UV/Vis/NIR Spectroscopy. The transformation and oxidation of CFA-10-as and CFA-10, respectively, are associated with a color change in the crystals. To monitor these changes, temperature-dependent DR spectra of CFA-10as and CFA-10 were recorded under inert gas and air. The UV/ vis/NIR reflectance spectra of the samples were recorded from 2000 to 300 nm at different stages of the reactions. The Kubelka−Munk transformations of the spectra are plotted in Figure S13 in the Supporting Information, revealing no absorption in the visible range for CFA-10-as. Upon heating, the spectrum changed greatly due to the formation of CFA-10. The spectrum of this substance displays a large absorption band covering the entire visible spectrum. Furthermore, a clearly visible absorption edge between 600 and 800 nm is present. Three broad absorption bands with very low intensities were found between 1200 and 2000 nm. The crystals of the resulting material were dark red. By conversion of the wavelengths into wavenumbers, the three bands located at 5454, 5954, and 7041 12341


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Inorganic Chemistry cm−1 are in good agreement with the typical values for tetrahedrally coordinated iron(II) centers, which are located between 3000 and 7000 cm−1.32 Subsequently, the sample was cooled from 300 °C to room temperature, and the measurement chamber was flooded with air. After 1 h, a more intense absorption band covered the entire visible spectrum. Increasing the oxidation time led to a negligible blue shift of the absorption band. The analysis of these two spectra reveals that the sample turned black. Additionally, the broad absorption bands between 1200 and 2000 nm disappeared, suggesting oxidation of the iron(II) centers resulting in either a mixedvalent FeII/FeIII species or finally an iron(III) compound similar to Fe-CFA-6. Due to the black and metallic appearances of the sample and the distinct absorption edges in the spectra, Tauc plots for all four spectra were calculated (see Figure S14 in the Supporting Information). The optical band gaps of the four phases were determined by the method described by Tauc et al.33 For CFA10-as, the optical band gap was approximately 3.66 eV, whereas, after the transformation, the band gap of pure CFA-10 greatly decreased to approximately 1.75 eV. Partial oxidation of CFA-10 led to an even lower band gap of ∼1.55 eV, whereas oxidation of the sample for 16 h resulted in a slight increase of the band gap to 1.64 eV. The absorption edges in this energy range are well-known for organic semiconductor materials, such as indigo (1.7 eV) or Prussian blue (∼1.1 eV).34,35 Hence, CFA-10 might be a promising candidate as a porous semiconductor material. Mössbauer Spectroscopy. During the transformation of CFA-10-as to CFA-10 the crystals turned from colorless to dark red. Therefore, the oxidation states of the iron centers in CFA-10-as and CFA-10 were determined by Mössbauer spectroscopy. The recorded spectra of CFA-10-as and CFA10 are given in Figure 6. At 80 K, the spectrum of CFA-10-as

CFA-10-as to CFA-10 another spectrum was recorded at 80 K, showing a symmetric doublet similar to that in the spectrum of the precursor compound. The isomer shift was reduced (0.86 mm s−1) but was still characteristic of a high-spin iron(II) center. Therefore, the changes after the transformation can be attributed to the lowering of the coordination number: i.e., the spectrum of CFA-10 is assigned to high-spin iron(II) centers in tetrahedral coordination environments, verifying the structure obtained from single-crystal XRD. The oxidation process of CFA-10 was monitored and is described in Figure S15 in the Supporting Information. Magnetic Properties of CFA-10-as and CFA-10. The color change after the formation of CFA-10 is not caused by a partial oxidation of the framework, as determined by Mössbauer spectroscopy. The dark red color of the obtained material may indicate the partial oxidation of a very small number of iron(II) centers. The sensitivity of Mössbauer measurements is lower than 2%, and thus, this technique can detect the presence of a small number of iron(III) centers. Such mixed-valent iron compounds often show black appearances.23 To investigate these findings, the magnetic properties of CFA10-as and CFA-10 were analyzed by SQUID measurements from 2 to 400 K. The SQUID data should clarify the spin states of the materials and therefore the oxidation states of the iron centers. The SQUID measurement of CFA-10-as showed normal paramagnetic behavior (Figure 7), with a linear slope

Figure 7. Temperature-dependent susceptibility curve of CFA-10-as in the range of 2 to 400 K. The Curie−Weiss fit is plotted in red. The inset shows the deviation between the Curie−Weiss fit and the experimental data below 20 K.

for the 1/χ vs temperature curve. This Curie−Weiss behavior χ = C/(T − Θ) characterized by a positive Θ = 4 K indicates that the magnetic interaction between the iron centers in CFA-10as is weakly ferromagnetic. The Curie constant C, determined by the slope of the curve, reveals the effective magnetic moment of the sample. For CFA-10-as, the μeff value equals 5.33 μB, and the Landé g factor is 2.18 for S = 2. These values are reasonable for paramagnetic iron(II) centers in a high-spin state. In the M(H) magnetization curves in Figure S16 in the Supporting Information, soft ferromagnetic behavior is established below 4 K. At 2 K, the magnetization exhibits a steep increase at small fields due to spontaneous magnetization and approaches saturation slightly below 4 μB at 5 T. This reduction with respect to the expected full saturation of 4.4 μB results from thermal excitations of spin waves. The data at 10 K are well described by a Brillouin function with an effective temperature of 5.3 K and a prefactor of 0.6, indicating

Figure 6. Mössbauer spectra of CFA-10-as and CFA-10 measured at 80 K.

showed one doublet with an isomer shift of 1.29 mm s−1 and a quadrupole splitting of 3.27 mm s−1. An isomer shift higher than 1 mm s−1 combined with a large quadrupole splitting is characteristic of a high-spin iron(II) center in an octahedral coordination geometry.36 Thus, the Mössbauer data confirm the findings from the single-crystal structure solution and the superconducting quantum interference device (SQUID) measurements (see below). After the transformation from 12342


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Inorganic Chemistry deviations from the pure mean field behavior due to strong spin fluctuations close to the magnetic order. This behavior is also in agreement with the deviations from the Curie−Weiss law below 20 K, as shown in the inset of Figure 7. For CFA-10, the interpretation of the measurement curve is not simple due to the air sensitivity of the material. This complexity is evident from the two susceptibility curves measured in an external field of 1 kOe. Figure S17 in the Supporting Information shows that the data measured directly after preparation (green squares) are approximately 20% smaller than the values obtained after 4 days (red circles). We ascribe this change to the oxidation of FeII at the sample surface, resulting in some amount of ferrimagnetic iron oxide, which contributes a nearly temperature independent positive offset to the susceptibility. Additionally, such a contribution is confirmed by the M(H) magnetization curves in Figure 8,

Dielectric Spectroscopy. The DR spectra revealed that partial oxidation of CFA-10 leads to a band gap of approximately 1.55 eV. Thus, the semiconducting properties of this compound were studied by dielectric spectroscopy, an essential method to study the charge-transport behavior in many systems. Dielectric spectroscopy has previously been used to reveal the dynamic processes in MOFs: e.g., MFU-1, MFU4l, and MFU-4.37,38 Since most MOFs are good insulators, possessing electrical conductivities below 10−10 S cm−1, twoprobe measurements are often sufficient to determine the electrical conductivity of such samples.39 Nevertheless, for very low electrical conductivities, two-probe measurements are limited or require a sophisticated testing station. In contrast, dielectric spectroscopy is a reliable technique to determine the electrical conductivity, overcoming the limitations of two-probe measurements. The band gap of freshly prepared CFA-10 is approximately 1.75 eV and is therefore slightly higher than the band gap of partial oxidized CFA-10. By in situ dielectric spectroscopy, the conductivity of pure CFA-10 was determined (not shown). Afterward the sample was exposed to air for a short period of time, to form partially oxidized CFA-10. Figure 9 shows the

Figure 8. Field-dependent magnetization curves of CFA-10 measured at different temperatures. The blue curve was plotted for the interpretation of the 10 K magnetization in terms of a small percentage of the starting material that might have remained in the sample.

where a steplike increase at small magnetic fields is observed, indicating spontaneous magnetization even at room temperature. Returning to the temperature dependence of the susceptibility, this field-independent ferrimagnetic contribution also explains why the data measured at 10 kOe are significantly lower than those obtained at 1 kOe. When the temperature is decreased from 400 K, the susceptibility monotonously increases, shows a slight kink at 200 K, and continues to increase down to the lowest temperature. Extrapolation of the slope of the 10 kOe data between 300 and 400 K reveals a Curie−Weiss temperature of −77 K and an effective moment of 6.5 μB. The negative Curie−Weiss temperature indicates antiferromagnetic exchange interactions. The effective moment is enhanced in comparison to pure FeII, but this likely results from contributions from impurities, which have not been subtracted. We interpret the kink at 200 K as the onset of antiferromagnetic order of the main phase. To understand the Curie-like behavior below 200 K, we consider the magnetization curves. Assuming a constant susceptibility of the antiferromagnetic main phase below 200 K, we can evaluate the magnetization at 10 K after subtraction of the magnetization data at 300 K scaled to 200 K with the same Brillouin function as used for CFA-10-as. Indeed, the data are well described with a similar effective temperature of 4.9 K and a prefactor of 2.2%, indicating that a few percent of nonreacted starting material might remain in the sample.

Figure 9. Temperature-dependent dielectric constant ε′ (a) and conductivity σ′ (b) of CFA-10 for selected frequencies. The gray dashed line represents an educated guess for the lower limit of the dc conductivity, while the black dashed line shows the conductivity of the interface. The inset (c) shows the conductivity in an Arrhenius representation.

temperature dependence of the dielectric constant (ε′) and the conductivity (σ′) of the partially oxidized CFA-10, recorded while the sample was heated. The measurements were performed on a pressed powder sample under a nitrogen atmosphere to avoid further oxidation. At temperatures below 180 K, ε′ (Figure 9a) is nearly temperature and frequency independent with a value of less than 4, which typically indicates that the dielectric constant in this region is caused by ionic and electronic polarizability. With increasing temperature, the permittivity increases via several frequency-dependent steps and finally rises to values greater than 1000 for the 1 Hz curve. Such colossal values indicate a nonintrinsic origin that is often 12343


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the result of interface effects.40 Above 400 K, the steep increase to higher values is frequency dependent: i.e., it shifts to higher temperatures. This is the typical signature of a relaxational process. Generally, heterogeneous samples show a relaxational response that is simply generated by heterogeneities, termed Maxwell−Wagner relaxation.40 Measurements on another sample (not shown) did not show this feature at the same temperatures, thus confirming the nonintrinsic origin of this process. In the same regime, the lower frequency curves, which show Maxwell−Wagner relaxation, converge toward a mutual temperature dependence of σ′(T), as displayed in Figure 9b for the 1 and 16.1 Hz curves above 410 K. The temperature dependence of this frequency-independent regime is indicated by the dashed black line. Such an overlap appears as a plateau in the frequency-dependent plot (see Figure S18 in the Supporting Information), which is commonly a signature of dc conductivity. However, in heterogeneous samples, several similar plateaus occur, each caused by a different conductivity within the sample, e.g., by interfaces or grain boundaries. In this work, since the plateau is observed in the regime of the colossal ε′ values, the plateau cannot be caused by dc conductivity but instead is caused by conductivity of the interface, resulting in Maxwell−Wagner relaxation. With decreasing temperature ( 2σ(I)]a wR2 (all data)b largest diff peak and hole/e Å−3 a

FeC8H8N4O4 [Fe(C6N4H6) (HCOO)2] 280.03 297(2) 0.71073 monoclinic P21/c (No. 14) 10.1774(3) 8.5091(3) 6.5474(2) 106.2450(10) 544.37(3) 2 1.708 1.394 284 3.18−42.20 25123 3853 0.0417 1.050 0.0343 0.0850 0.826 and −0.329

CFA-10 C6H4N4Fe [Fe(C6N4H4)] 187.98 100(2) 0.71073 tetragonal P42/mmc (No. 131) 9.205(2) 9.205(2) 7.2374(19) 90 613.3(4) 2 1.018 1.187 188 2.21−25.13 7769 338 0.1234 1.709 0.1431 0.3898 1.617 and −0.860

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2.

sized according to methods described in the literature.17,18 All other chemicals were obtained from commercial sources and used without further purification. The utilized solvents were of analytical grade, so that no purification was necessary. [Fe(O2CH)2(H2BPZ)] (CFA-10-as). Fe3 (30 mg, 0.05 mmol; =0.15 mmol equiv of iron) and 26.8 mg (0.2 mmol) of H2BPZ were mixed in a Pyrex tube. N-Methyl-2-pyrrolidone (NMP, 2 mL) and 0.4 mL of formic acid were subsequently added. The mixture was sonicated for 1 min and then placed into a heating block set to 130 °C. After it reacted for 3 days, the sample was cooled to room temperature, filtered, and washed with NMP and methanol to obtain 39.2 mg of [Fe(O2CH)2(H2BPZ)] as light brown crystals. The crystallinity of the sample was enhanced using surface-modified Pyrex vials. Yield based on the linker: 70%. Anal. Found: C, 34.25; N, 20.92; H, 2.54. Calcd: C, 34.29; N, 20.01; H, 2.88. IR bands (cm−1): 3116 (m), 2950 (vbr), 2849 (m), 1596 (vbr), 1529 (br), 1494 (w), 1380 (s), 1348 (s), 1302 (s), 1265 (w), 1242 (w), 1162 (vs), 1145 (vs), 1045 (vs), 955 (s), 916 (vs), 860 (br), 814 (s), 771 (br), 656 (vs), 620 (vs). [Fe(BPZ)] (CFA-10). CFA-10-as was converted to CFA-10 by heating at 300 °C under a constant inert gas stream (100 mL min−1) for 15 min with a heating rate of 10 K min−1 in a thermal balance equipped with a mass spectrometer. Evolution of 2 equiv of formic acid and its decomposition products was monitored by MS and the mass loss of the compound. After the reaction, the oven was cooled to room temperature, and CFA-10 was obtained as black crystals. Storage 12345


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Inorganic Chemistry scans (one scan every 30 s) were taken. The same procedure was repeated in steps of 10 to 290 °C. Single-Crystal XRD. A crystal of CFA-10-as was taken from the mother liquor and placed in a capillary (Hilgenberg GmbH) made of quartz glass with a diameter of 0.5 mm and a wall thickness of 0.01 mm. The tube was evacuated, filled with nitrogen, and sealed. The Xray data for the single-crystal structure determination of CFA-10-as were collected on a Bruker D8 Venture diffractometer. Intensity measurements were performed using monochromated (doubly curved silicon crystal) Mo Kα radiation (0.71073 Å) from a sealed microfocus tube. The generator settings were 50 kV and 1 mA. The data collection temperature was 297(2) K. After the XRD measurement, the crystal of CFA-10-as was heated in the capillary at 300 °C for 10 h and then mounted on the Bruker D8 Venture diffractometer. The data collection temperature was 100(2) K. APEX3 software was used for the preliminary determinations of the unit cells.42 Determination of the integrated intensities and unit cell refinements were performed using SAINT.43 The structures were solved using the Bruker SHELXTL software package44,45 and refined using SHELXL.46 Selected crystal data and details of the structure refinements for CFA-10-as and CFA-10 are provided in Table 1. The complete crystallographic data for the structures reported in this paper have been deposited in CIF format at the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB21EZ, U.K., as supplementary publication nos. CCDC 1554871 (CFA-10-as) and 1554870 (CFA10). Copies of the data can be obtained free of charge by quoting the depository numbers. (fax, +44-1223-336-033; e-Mail, deposit@ccdc. cam.ac.uk; web, www.ccdc.cam.ac.uk).

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Electronic Correlations to Functionality” (Augsburg, Munich, Stuttgart).

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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.7b01818. Additional crystallographic information, TG-MS, VTPXRD, DRIFT, and UV/vis spectral data, oxygen sorption properties, Mössbauer spectra, and additional SQUID and DES data (PDF) Accession Codes

CCDC 1554870−1554871 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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*D.V.: e-mail, [email protected]; fax, (+49) 821-598-5955. ORCID

Dirk Volkmer: 0000-0002-8105-2157 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Andreas Kalytta-Mewes and Dmytro Denysenko are acknowledged for fruitful academic discussions. This work was partially funded by the DFG priority program 1928 COORNETS (grant holder D.V.). P.S. thanks the BMBF for financial support via ENREKON 03EK3015. P.S. and J.K.H.F. thank Peter Lunkenheimer for fruitful academic discussions. S. R. and H.A. K. v. N., acknowledge partial support by the DFG via the Transregional Collaborative Research Center TRR 80 “From 12346


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Single-Crystal to Single-Crystal Transformation of a Nonporous Fe(II

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