Article pubs.acs.org/crystal
Indium Imidazolate Frameworks with Differently Distorted ReO3‑Type Structures: Syntheses, Structures, Phase Transitions, and Crystallization Studies Maria E. Schweinefuß,† Igor A. Baburin,‡ Christian A. Schröder,† Christian Naẗ her,§ Stefano Leoni,∥ and Michael Wiebcke*,† †
Institut für Anorganische Chemie, Leibniz Universität Hannover, Callinstrasse 9, 30167 Hannover, Germany Institut für Physikalische Chemie und Elektrochemie, Technische Universität Dresden, Mommsenstrasse 13, 01062 Dresden, Germany § Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Strasse 2, 24118 Kiel, Germany ∥ School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, United Kingdom ‡
S Supporting Information *
ABSTRACT: Three indium imidazolates have been prepared by employing solvothermal, ionothermal, and solventless syntheses. The crystal structures have been determined using laboratory powder XRD methods in combination with DFT calculations. In phases I−III, octahedrally coordinated In(III) ions are bridged by imidazolate (im) ligands into 3D frameworks that are related to distorted filled-ReO3 (ABX3 perovskite) structures with different octahedron tilting. Clathrate-type phases I (cubic space group Im3̅) and II (trigonal R3)̅ may be formulated as [A′A″3In4(im)12] and contain two types of cubic cages, A′ and A″, which may host guest molecules and are occupied by im moieties, respectively. A reversible structural phase transition between low-temperature phase II and high-temperature phase I, occurring on heating at about 90 °C, was studied by DSC and variable-temperature powder XRD experiments. Depending on the synthesis, more specifically on the nature and amount of guests trapped in the A′ cages, the phase transition was suppressible, enabling I to be recovered at room temperature. Phase III (trigonal R3̅) may be formulated as [AIn(im)3]. It is a dense phase in which the cubic A cages are occupied by im moieties. For the first time, the ionothermal formation of a coordination polymer, phase III, was monitored in situ by time-resolved EDXRD experiments. Rate constants and activation energies for both nucleation and crystal growth, as estimated by kinetic analysis of the EDXRD data, are compared to corresponding values reported previously for the solvothermal crystallization of other coordination polymers.
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temperature,11 pressure,12 and guest molecules13 have been investigated. Considering imidazolates of group 13 cations, a binary [Ga2(im)6(Him)] phase with an interrupted octahedral framework, containing terminal neutral Him and anionic im besides bridging im (μ-im) ligands,14 and a ternary [Zn3In2(im)12] phase (ZIF-5) with a fully connected, tetrahedral-octahedral framework related to the garnet structure (gar net)15 have been synthesized and structurally characterized. Additionally, a phase with the approximate composition [Ga(im)3] has been reported, but its structure could not be determined due to poor crystallinity.14 For binary [Ga(im)3] and also [In(im)3] phases, the latter have been unknown so far, it might be expected that they crystallize with ReO3-type structures (pcu
INTRODUCTION
Metal imidazolates form coordination polymers in which metal cations are bridged by ditopic imidazolate (im) and/or substituted imidazolate anions into dense (nonporous) and open (microporous) 3D neutral framework structures. Such coordination polymers are known for a number of metal cations (e.g., Zn(II), Cu(II), Cu(I), Co(II), Fe(II), Cd(II), and Li(I)/ B(III)) mainly with low coordination numbers (2 and 4), and polymorphism is observed for a number of systems.1 The porous phases with zeolite-related tetrahedral frameworks (zeolitic imidazolate frameworks, ZIFs),2 which represent a distinctive subclass of metal organic frameworks (MOFs), are currently under intensive investigations due to their favorable properties with potential applications in a number of fields, such as gas storage,3 separation,4 catalysis,5 and sensing.6 Furthermore, magnetic,7,8 luminescence,9 and mechanical properties,10 as well as structural phase transitions induced by © 2014 American Chemical Society
Received: May 23, 2014 Revised: June 17, 2014 Published: July 31, 2014 4664
dx.doi.org/10.1021/cg5007499 | Cryst. Growth Des. 2014, 14, 4664−4673
Crystal Growth & Design
Article
centrifugation cycles with EtOH were applied, and the product was dried at room temperature. The yield was 88% based on In. Ionothermal Synthesis of III. Crystalline III was synthesized by heating 689.1 mg (10.1 mmol) of Him and 149.1 mg (0.42 mmol) of In(NO3)3·3H2O in 2.0 g (5.1 mmol) of EMim-NTf2 in a tightly closed 7 mL borosilicate glass reaction tube at 160 °C for 7 days in an oven under static conditions. After cooling to room temperature, the beigecolored solid was separated by filtration, thoroughly washed with EtOH, and dried at room temperature. The yield was 83% based on In. Methods of Characterization. PXRD patterns were recorded at room temperature using a Stoe STADI-P transmission diffractometer equipped with a sealed Cu X-ray tube, a curved Ge(111) monochromator (delivering Cu Kα1 radiation, λ = 1.540596 Å), and a linear position-sensitive detector. VT-PXRD experiments were performed on a Stoe STADI-P diffractometer equipped with a Stoe high-temperature oven in Debye−Scherrer geometry (Cu Kα1 radiation). Samples were contained in unsealed, thin-walled borosilicate glass capillaries with an outer diameter of 0.5 mm. TGA and difference thermal analysis (DTA) were performed simultaneously on a Netzsch 429 thermoanalyzer. Samples were filled into alumina crucibles and heated in a flow of air with a ramp of 5 °C·min−1 from room temperature to 1000 °C. DSC experiments were performed using a DSC 1 Star System with STARe Excellence Software from Mettler-Toledo. The measurements were performed at different heating/cooling rates in Al pans with hole in nitrogen atmosphere. The enthalpy of the II−I phase transition was difficult to determine precisely because the transition proceeded over a large temperature range, which depended on the heating rate, and the baseline before and after each thermal event was significantly different. Scanning electron microscopy (SEM) images were taken on a Jeol JSM-7600F instrument with a field emitter as an electron source. A Quantachrome Autosorb1-MP apparatus was used to perform CO2 physisorption experiments at 0 °C. Samples were activated in dynamic vacuum at temperatures up to 150 °C before starting the measurements. Liquidstate 1H NMR spectra were recorded on a Bruker DPX400 Advance spectrometer operated with a frequency of 400 MHz. Crystal Structure Analyses. PXRD data were recorded at room temperature on the above-mentioned STADI-P diffractometer in Debye−Scherrer geometry. Samples of I−III were contained in thinwalled borosilicate glass capillaries with an outer diameter of 0.3 mm. Indexing and Le Bail refinement to obtain accurate lattice constants were performed using TOPAS software.26 The systematic reflection conditions were checked for possible space groups, and, in combination with the formula units per unit cell as estimated from the measured density of II (1.98 mg·mm−3) and the lattice constants of I−III, special positions for the In atoms could be determined in each space group. Simple geometrical considerations taking an expected In···In distance of ∼6.5 Å into account reduced the number of possible space groups. With the In atoms fixed on the derived special positions, the parallel tempering algorism of the direct-space FOX progam27 was used to arrive at approximate locations of the im units. The structures were subsequently refined by the Rietveld method using TOPAS. The intensity profiles were modeled with Thompson−Cox−Hastings pseudo-Voigt peak-shape and Chebychev polynomial background functions. A simple axial model was used to treat peak asymmetry, and an absorption correction for the capillary samples28 was also applied based on an estimated packing factor of 0.5. The im units were treated as rigid body during all refinements. Initially applied restraints on In−N bond lengths were completely released in the final cycles of refinement. An isotropic displacement parameter common to all atoms was refined for each structure. For I and II, the Bravais lattice could be unambiguously determined. The PXRD pattern of III was first cleanly indexed on a cubic Fcentered unit cell, but this allowed only structural models with disordered im units to be deduced, and subsequent Rietveld refinements were not convincing. Therefore, a reduction of symmetry to the trigonal R-centered lattice was considered, which led to a satisfactory structural model and Rietveld refinement. The crystal data and details of the Rietveld refinement for I−III are summarized in Table 1. Atomic parameters, and selected bond length
net) with the metal cations in octahedral coordination, which, because im is a short, nonlinear bridging ligand preferring a metal-im-metal angle of ∼145°,1,2 are distorted variants of the ReO3 aristotype.16,17 Coordination polymers with filled-ReO3 (ABX3 perovskite) structures, which consist of anionic frameworks with short bridging ligands (e.g., formate, azide) and small ammonium guest cations, are currently attracting considerable attention as new materials with potentially tunable ferroic, including multiferroic, properties; examples of this include multiferroic [NMe2H2][M(HCOO)3] (with M = Mn, Fe, Co, and Ni)18 and ferroelastic [NMe4][Cd(N3)3].19 Such perovskite-type coordination polymers typically exhibit structural phase transitions, which are induced by order−disorder motions of the guests and occur below room temperature in most cases. Here, we report for the first time on three indium imidazolate phases, I−III. The materials were prepared by employing solvothermal, ionothermal, and solventless (ligandmelt) syntheses. Crystal structure analyses based on powder Xray diffraction (PXRD) data supported by density functional theory (DFT) calculations revealed that the three phases indeed possess differently distorted ReO3-type structures. We have investigated structural phase transitions and thermal properties by employing variable-temperature (VT) PXRD, differential scanning calorimetry (DSC), and thermogravimetic analysis (TGA). Gas physisorption experiments were also performed to study possible sorption properties. Furthermore, time-resolved in situ energy-dispersive (ED) XRD experiments were performed on ionothermal syntheses of III in order to contribute to recent efforts aimed at improving our understanding of coordination polymer formation20−24 by employing in situ methods.25
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EXPERIMENTAL SECTION
Materials. All reagents were used as commercially supplied by Sigma-Aldrich without further purification: N,N-dimethylacetamide (DMAA), N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), n-propylamine (nPA), 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide (EMim-NTf2), imidazole (Him), indium acetate (In(OAc)3), indium acetylacetonate (In(acac)3), indium nitrate trihydrate (In(NO3)3·xH2O; the water content was determined to be x ≈ 3 by TGA), and ethanol (EtOH). Solvothermal Synthesis of I. Typically, 102.1 mg (1.5 mmol) of Him was dissolved in 3.0 mL of DMAA and placed into a 7 mL borosilicate glass reaction tube. Then, 146.0 mg (0.50 mmol) of In(OAc)3 was suspended in 2.0 mL (24.3 mmol) of nPA and layered on top of the Him solution. The tightly closed reaction tube was placed vertically in an oven and heated at 120 °C for 7 days under static conditions. After cooling to room temperature, the colorless product was separated by filtration, thoroughly washed with EtOH, and dried at room temperature. The yield was 86% based on In. Ionothermal Synthesis of II. The ionothermal synthesis of II was carried out by mixing 689.1 mg (10.1 mmol) of Him and 173.1 mg (0.42 mmol) of In(acac)3 in 2.0 g (5.1 mmol) of EMim-NTf2 in a tightly closed 7 mL borosilicate glass reaction tube and heating the mixture at 150 °C in an oven for 2 days under static conditions. After cooling to room temperature, the gray solid was separated by filtration, thoroughly washed with EtOH, and dried at room temperature. The yield was 67% based on In. Solventless Synthesis of II. For the solventless synthesis of II, 0.5 g (1.2 mmol) of In(acac)3 was mixed with 2.0 g (29.4 mmol) of Him in a 7 mL borosilicate glass reaction tube and heated at 160 °C for 24 h in an oven under static conditions. After cooling to room temperature, the excessive Him was dissolved in EtOH, and the remaining gray powder was separated by centrifugation. Two washing/ 4665
dx.doi.org/10.1021/cg5007499 | Cryst. Growth Des. 2014, 14, 4664−4673
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Crystal Growth & Design
RESULTS AND DISCUSSION Synthesis and Phase Identification. Solvothermal, ionothermal, and solventless (ligand-melt) syntheses led to the discovery of three new indium imidazolate phases. Solvothermal reactions were performed in a common amide solvent, N,N-dimethylacetamide (DMAA), N,N-dimethylformamide (DMF), or N,N-diethylformamide (DEF), without and with addition of n-propylamine (nPA) as a base. For ionothermal syntheses,32 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide (EMim-NTf2) was used as the ionic liquid. Ionothermal conditions have rarely been employed so far for the preparation of metal imidazolates.33 Solventless synthesis in molten imidazole or an imidazole derivative is an old synthetic method34 that has gained renewed interest recently.35−37 The syntheses were performed with Him in excess to the metal precursor (Him/In = 24:1). Phase I was obtained only under solvothermal conditions with In(OAc)3 or In(NO3)3·3H2O as the metal precursor in all the three amides (Figure 1). The addition of nPA was found to
and angles for each structure are listed in Tables S1−S6 of the Supporting Information.
Table 1. Crystal Data and Details of the Rietveld Refinement for I−III formula Z crystal system a (Å) c (Å) V (Å3) space group density (mg·mm−3) Rwp (%) RBragg (%) GoF
I
II
III
C9H9InN6 8 cubic 13.04634(8)
C9H9InN6 12 trigonal 18.8921(2) 10.5452(2) 3259.47(9) R3̅ 1.93 5.82 3.76 1.33
C9H9InN6 6 trigonal 9.0547(2) 22.2869(7) 1582.46(7) R3̅ 1.99 4.07 1.53 1.37
2220.58(4) Im3̅ 1.89 6.62 4.26 1.35
DFT Calculations. The calculations using the rVV10 DFT functional,29 which accounts fairly well for the van der Waals interactions, were performed with the PWscf/Quantum ESPRESSO package.30 The Kohn−Sham equations were solved using the planewave pseudopotential approach applying ultrasoft pseudopotentials without spin polarization. A kinetic energy cutoff of 80 Ry and a density cutoff of 460 Ry were used to achieve fully converged results. During structure relaxation, the atomic positions and lattice constants were optimized. Finally, the residual forces were
