Temperature-resolved X-ray Powder Diffraction Research Articles

Mechanisms of Reversible Phase Transitions in Molecular Crystals: Case of Ciclopirox

The detailed characterization of several subambient solid state transitions occurring in the pharmaceutical ingredient Ciclopirox between −20 °C and −85 °C was performed by using a combination of DSC, cold-stage optical microscopy, vibrational spectroscopies, solid state NMR at controlled temperature, structural analyses by single crystal X-ray diffraction, and temperature-resolved X-ray powder diffraction. The global analysis of the available data reveals that the mechanisms of these reversible transitions involve a subtle compromise between phenomena related to molecular disorder, cooperative release of strains induced by cooling, and structural reorganization associated with topotactic changes in crystal lattice and symmetry. However, no major change in the main features of crystal packings is observed during the successive single crystal-to-single crystal transitions, which highlights the difficulty to classify such transitions in the frame of conventional theoretical frameworks. The successive therma...

Naproxen–Nicotinamide Cocrystals: Racemic and Conglomerate Structures Generated by CO2 Antisolvent Crystallization

Cocrystallization of naproxen racemic mixture and nicotinamide was investigated in this work, using compressed CO2 as antisolvent. A novel racemic cocrystal structure containing both enantiomers of naproxen linked to nicotinamide has been produced thanks to the CO2 antisolvent batch crystallization process. The structure of the molecular complex and its intermolecular interactions were investigated by single-crystal X-ray diffraction and attenuated total reflectance Fourier transform infrared spectroscopy. The antisolvent feed rate was found to have a direct influence on the cocrystallization outcome. The racemic cocrystal was obtained at slow and moderate CO2 feed rate, while very fast introduction of CO2 resulted in the formation of a mixture of chiral cocrystals (conglomerate). Cross-seedings, thermal analysis, and temperature-resolved X-ray powder diffraction were used to probe the relationship between the different phases. In addition, all powders produced with CO2 technology were obtained as cocryst...

Crystallization kinetics and molecular mobility of an amorphous active pharmaceutical ingredient: A case study with Biclotymol

The present case study focuses on the crystallization kinetics and molecular mobility of an amorphous mouth and throat drug namely Biclotymol, through differential scanning calorimetry (DSC), temperature resolved X-ray powder diffraction (TR-XRPD) and hot stage microscopy (HSM). Kinetics of crystallization above the glass transition through isothermal and non-isothermal cold crystallization were considered. Avrami model was used for isothermal crystallization process. Non-isothermal cold crystallization was investigated through Augis and Bennett model. Differences between crystallization processes have been ascribed to a site-saturated nucleation mechanism of the metastable form, confirmed by optical microscopy images. Regarding molecular mobility, a feature of molecular dynamics in glass-forming liquids as thermodynamic fragility index m was determined through calorimetric measurements. It turned out to be around m=100, describing Biclotymol as a fragile glass-former for Angell’s classification. Relatively long-term stability of amorphous Biclotymol above Tg was analyzed indirectly by calorimetric monitoring to evaluate thermodynamic parameters and crystallization behavior of glassy Biclotymol. Within eight months of storage above Tg (T=Tg+2°C), amorphous Biclotymol does not show a strong inclination to crystallize and forms a relatively stable glass. This case study, involving a multidisciplinary approach, points out the importance of continuing looking for stability predictors.

Solubility Equilibria and Crystallographic Characterization of the l-Threonine/l-allo-Threonine System, Part 2: Crystallographic Characterization of Solid Solutions in the Threonine Diastereomeric System

Features of crystal structures and their thermal deformations in the l-threonine/l-allo-threonine system were investigated by means of single-crystal X-ray diffraction and temperature-resolved X-ray powder diffraction. The l-threonine/l-allo-threonine system belongs to the systems with continuous solid solutions. The comparative analysis of crystal structures of l-threonine (l-Thr), l-allo-threonine (l-aThr), and their mixed crystals was conducted based on literature and our own data. It was found that the distances between methyl groups are more sensitive to the crystal composition than the lengths of the hydrogen bonds. Statistical distribution of l-Thr and l-aThr molecules causes the occurrence of shortened and elongated distances between the methyl groups of the neighbor nonidentical molecules (l-Thr—l-aThr). Thermal deformations of crystal structures of l-Thr, l-aThr, and solid solutions containing 34 and 90% l-Thr were investigated. In the case of diastereomers, the slight negative thermal expansion along the axis a was observed. Correlations between features of crystal structure and thermal behavior of l-Thr, l-aThr, and their solid solutions were identified.

Generation of NiTi Nanoparticles by Femtosecond Laser Ablation in Liquid

NiTi was investigated as a model system for a binary alloy where the properties strongly depend on the relative proportion of the two elements and on the grain size. The NiTi nanoparticles were generated by laser ablation in water. For the analysis of the particle size distribution, we used transmission electron microscopy and dynamic light scattering. Here, we found a broad particle size distribution (10-200 nm). Furthermore, the temperature-resolved x-ray powder diffraction and differential scanning calorimetry (DSC) were used to evaluate the phase transition behavior of the generated NiTi nanoparticles. Here, we found an interesting effect. During the heating by DSC, an austenite phase transition and a weak martensite phase transition in the NiTi nanoparticles appeared. Moreover, the phase transformation temperature was about 40 K lower than that of the bulk target.

Increasing dissolution of trospium chloride by co-crystallization with urea

The search for various solid forms of an active pharmaceutical ingredient (API) is an important step in drug development. Our aim was to prepare co-crystals of trospium chloride, an anticholinergic drug used for the treatment of incontinence, and to investigate if they have advantageous properties for drug formulation. Phase identification was done by powder X-ray diffraction and single-crystal X-ray diffraction. The chemical composition was verified by solution NMR and the dissolution rate of the prepared phases was studied by IDR (intrinsic dissolution rate). For further analysis of phase stability and transitions, combined thermal analysis and temperature-resolved X-ray powder diffraction were used. Urea was selected as a co-crystallization partner. Trospium chloride urea (1:1) co-crystal was prepared by a solvent evaporation. From single-crystal data, the co-crystal structure was solved in a space group P21/c and compared to previously published structures of trospium chloride. Intrinsic dissolution rate revealed that the co-crystal dissolves 32% faster than pure API. However, its low thermal and pressure stability makes it a challenging choice for the final drug formulation.

Investigation of drug polymorphism: Case of artemisinin

Abstract The polymorphism of the anti-malarial compound artemisinin was examined. The phase behavior of solid artemisinin has experimentally been investigated using differential scanning calorimetry and temperature-resolved X-Ray powder diffraction. In addition, complementary solution studies and suspension experiments were performed. The results clearly confirm the existence of two modifications of artemisinin, which are related enantiotropically. The orthorhombic modification is the thermodynamically stable form at low temperatures, while the triclinic form is the stable one at higher temperatures with a transition temperature of ∼130 °C. Problems associated with analysis of the polymorphic phase behavior are comprehensively addressed.

Small Molecule, Big Difference: The Role of Water in the Crystallization of Paclitaxel

Paclitaxel is an important antineoplastic drug, which is used widely in the treatment of many forms of cancer. The crystal structures of the anhydrous form and the hemihydrate were determined from laboratory X-ray powder diffraction data, whereas the dihydrate was solved from single-crystal synchrotron diffraction data. Intermolecular spaces allow for the inclusion of loosely bound water molecules, which are then lost easily upon heating. All three forms were found to crystallize in the orthorhombic spacegroup P2(1)2(1)2(1), with Z' = 2. The unit cell parameters were found to be a = 9.6530(3) Å, b = 28.1196(8) Å, c = 33.5378(14) Å, and V = 9103.5(5) Å for the anhydrous form (363 K); a = 9.6890(2) Å, b = 28.0760(4) Å, c = 33.6166(8) Å, and V = 9144.7(3) Å(3) for the hemihydrate (333 K); and a = 9.512(6) Å, b = 28.064(16) Å, c = 33.08(2) Å, and V = 8829.0(9) Å(3) for the dihydrate (120 K). Water loss occurs in two steps between 120 K ≤ t ≤ 363 K. The thermal stability of the hydrates and accompanying unit cell changes were observed in situ via temperature-resolved X-ray powder diffraction and thermogravimetric analysis.

Chain and layer networks of germanato-polyoxovanadates

Two new members of the {V15E6} (E = As, Sb, Ge, and Si) polyoxovanadate family, [{Mn(tren)(trenH2)}{Mn(tren)}4V15Ge6O48(H2O)0.5]·tren·2H2O (1, tren = tris(2-aminoethyl)amine) and [{Ni(tren)}4(trenH2)2V15Ge6O48(H2O)]·2H2O (2), have been prepared under solvothermal conditions. In 1, the {V15Ge6} cluster is expanded by five in situ formed crystallographically independent Mn2+ complexes via Mn–O(–V/Ge) bonds. Two independent Mn2+ complexes create a rare dinuclear [Mn2O2(tren)2]4+ moiety by bond formation with two terminal O atoms of the Ge2O7 groups of the adjacent {V15Ge6O48(H2O)0.5} clusters, thus connecting the {V15Ge6} cluster shells into a unique charge-neutral helical chain. In the structure of 2, the unusual and hitherto never observed {Ni(tren)(trenH2)Ni(tren)}6+ complex is formed in situ consisting of two tren molecules acting as tetradentate ligands to Ni2+ cations and one additional tren molecule connecting two Ni2+ cations bonded to neighboring cluster anions. The complexes act as linkers joining the {V15Ge6} moieties via Ni–OV bridges, forming a 2D structure. In the coordination polymers of 1 and 2, the spin structure of the {V15Ge6} cluster remains virtually unaffected by the coordinated MnII or NiII spin centers. For both compounds, the magnetic susceptibility can be decomposed into a sum of the contributions of both the {V15Ge6} cluster and the MnII or NiII cations, and a molecular field correction suffices to represent the weak antiferromagnetic coupling between them. Compound 1 is stable up to 350 °C as evidenced by in situ temperature-resolved X-ray powder diffraction studies.

Structural and thermal characterization of zolpidem hemitartrate hemihydrate (form E) and its decomposition products by laboratory x-ray powder diffraction

The crystal structure of zolpidem hemitartrate hemihydrate (I, Form E) has been solved from high-resolution laboratory powder diffraction data. It crystallizes in the orthorhombic P212121 space group with a = 22.4664(6) Å, b = 26.0420(7) Å, and c = 7.4391(1) Å. Protonation of zolpidem molecules could not be unambiguously determined. Thermal stability of Form E has been investigated by TG–DTA and in situ by temperature resolved X-ray powder diffraction. Water loss occurs between 50°C ≤ t ≤ 100°C while structure decomposition commences at approximately 120°C yielding zolpidem tartrate (II) and pure zolpidem base (III) in approximately equimolar amounts. Crystal structures of II and III have been solved simultaneously from a single powder pattern of thermally decomposed I. Zolpidem tartrate crystallizes in the orthorhombic P212121 space group with a = 19.9278(8) Å, b = 15.1345(8) Å, and c = 7.6246(2) Å (at 140°C). Zolpidem base crystallizes in the orthorhombic Pcab space group with a = 9.9296(4) Å, b = 18.4412(9) Å, and c = 18.6807(9) Å (at 140°C). In the reported crystal structures zolpidem molecules form stacks through π–π interaction or dipole–dipole interactions while tartrate moieties, if present, form hydrogen bonded chains. Water molecule in I forms a hydrogen bond to the imidazole nitrogen atom of the zolpidem molecule. Free space in the crystal structure of I could allow for the additional water molecules and thus a variable water content. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:871–878, 2010

Mechanism of Hydration and Dehydration of Ciclopirox Ethanolamine (1:1)

The antifungal agent ciclopirox ethanolamine (1:1), a stoichiometric compound between ethanolamine and 6-cyclohexyl-1-hydroxy-4-methylpyridin-2-(1H)-one, crystallizes as an anhydrate and a monohydrate. The structures of both phases have been characterized by single crystal X-ray diffraction and suggest that, at ambient temperature, ciclopirox ethanolamine (1:1) anhydrate and monohydrate should be classified as salts from the position of the protons in the Fourier difference map. The thermal dehydration process has been elucidated by thermal analyses (TGA/DSC), temperature resolved X-ray powder diffraction and Hot-Stage Microscopy. Moisture sorption and desorption investigations at room temperature under static and dynamic conditions (DVS) reveal that, if the dehydration is not implemented under a sufficient ethanolamine vapor pressure, the dehydration-hydration mechanism is not reversible. This behavior suggests that, at elevated temperature, ciclopirox ethanolamine (1:1) monohydrate could be contemplated...

Thermally induced structural modifications of the high silica zeolite gottardiite: An in situ XRPD study

The thermally induced structural modifications of the natural zeolite gottardiite ((Na 0.78K 0.24Mg 3.51Ca 4.11Sr 0.06Ba 0.05)(Fe 0.14Al 17.47Si 118.70O 272)·92.33H 2O, a = 13.631(1), b = 25.047(3) and c = 22.586(3) Å, V = 7711(1) Å 3, s.g. Cmca, framework type NES) were studied in the temperature range 308–1203 K in a temperature-resolved X-ray powder diffraction experiment, using synchrotron radiation. The unit cell parameters were refined in the whole temperature range, while structural refinements were performed approximately every 100 K, at 308, 371, 475, 580, 674, 779, 873, and 978 K. No signs of phase transitions were observed in the whole investigated T-range and up to the highest temperature the material does not undergo amorphization. Gottardiite shows a high thermal stability, as a consequence of the high Si content, and an extremely rigid structure with a substantial absence of cell volume change. The framework deformation induced by dehydration involves only a slight distortion of the 10-ring and 12-ring channels running in the a and b directions. From room temperature to 580 K gottardiite looses water quite slowly and regularly. The main water loss is observed in the temperature range 580–779 K. Above this temperature gottardiite again dehydrates slowly, and at 978 K some water molecules even remain.

Dehydration dynamics of barrerite: An in situ synchrotron XRPD study

The thermally induced structural modifications of the natural zeolite barrerite [Na 16 Al 16 Si 56 O 144 ·52H 2 O, a = 13.6239(4) Å, b = 18.2033(5) Å, c = 17.8317(7) Å, V = 4422.3(3) Å 3 , space group Amma, framework type STI] were studied in a temperature-resolved X-ray powder diffraction experiment, using synchrotron radiation, in the temperature range 339-973 K. In the initial stage of heating, up to 508 K, barrerite Phase A (space group Amma) is stable, the unit-cell volume decreases by about 4% and a water release of about 66% is observed. Between 521 and 598 K, a phase transition to the collapsed so-called barrerite Phase B (space group Amma) is observed. During the transition, the rotation of the 4 2 5 4 secondary building units causes a large decrease in cell volume and deformation of the channel system. Phase B, at 611 K, shows the statistical breaking of T-O-T bridges in the 4-rings and the migration of the involved tetrahedral atoms to new “face-sharing” tetrahedra, with a consequent reduction of the free volume of the channels parallel to [100]. The new structure is stable up to 741 K and the total volume decrease is about 16%. A new phase appears from 754 K with cell parameters similar to those reported for the highly deformed barrerite Phase D and is stable up to 910 K, which is the temperature at which the total volume decrease is 22.5%. The material does not undergo amorphization up to the highest temperature investigated.

Influence of dehydration kinetics on T-O-T bridge breaking in zeolites with framework type STI: The case of stellerite

The thermally induced structural modifications of the natural zeolite stellerite [Ca 8 Al 16 Si 56 O 144 ·58H 2 O, a = 13.5947(4), b = 18.1823(6), and c = 17.8335(6) Å, V = 4408.1(3) Å 3 , space group Fmmm, framework type STI] were studied in a temperature-resolved X-ray powder diffraction experiment, using synchrotron radiation, in the temperature range RT-976 K. In the initial stage of heating (below 430 K) Stellerite Phase A (space group Fmmm) is stable, and the cell volume decreases only 0.6% to this temperature. Between 430 and 490 K most of the water is released, the symmetry lowers, and a phase transition to the collapsed so-called Phase B (space group Amma) is observed. In this phase rotation of the 4254 Secondary Building Units causes cell volume contraction and deformation of the channel system. This new phase, at 530 K, shows the statistical breaking of T-O-T bridges in the four-rings and the migration of tetrahedral atoms to new .face-sharing. tetrahedra, which partially occlude both the channels parallel to [100] and to [001]. This framework deformation is interpreted as due to the strain induced by calcium atoms on the framework to achieve better coordination after the release of water. The new structure is stable up to 750 K and the total volume decrease is about 8%. The dehydration process causes a similar framework deformation and the transition to a collapsed phase characterized by the statistical breaking of T-O-T bridges in all zeolites with STI-type frameworks. However, comparing the results obtained with different thermal kinetics, it is possible to assume that the experimental conditions play a primary role in the mechanisms of dehydration and of framework bridge breaking.

Synthesis, Crystal Structure and Thermal Decomposition Reaction of the New Copper(I) Cyanide Coordination Polymer Poly[tri-μ2-cyano-C,N)- bis(μ2-2,3-dimethyl-pyrazine-N,N)]tricopper(I)

Abstract The new copper(I) coordination polymer poly[tri-μ2-cyano-C,N)-bis(μ2-2,3-dimethyl-pyrazine- N,N)] tricopper(I) (I) was prepared by the reaction of copper(I) cyanide with 2,3-dimethylpyrazine in acetonitrile. In the crystal structure of Ia novel CuCN substructure is found which is connected by the dimethylpyrazine ligands into a three-dimensional coordination network. The thermal properties of Iwere investigated using simultaneous differential thermoanalysis (DTA), thermogravimetry (TG) and mass spectrometry (MS) as well as temperature resolved X-ray powder diffraction. On heating, compound Ilooses a part of the dimethylpyrazine ligands in an endothermic reaction to form the known ligand poor compound (CuCN)2-(2,3-dimethylpyrazine) IIas an intermediate which decomposes to CuCN on further heating.

On the thermal decomposition pathway of coordination compounds: synthesis, crystal structures and properties of new polymorphic CuI(2-ethylpyrazine) coordination compounds

Five new coordination compounds were prepared at room temperature in solution, under solvothermal conditions at elevated temperatures or by thermal decomposition reactions. In the amine rich 1:2 compound [(CuI) 2(2-ethylpyrazine-N) 4] ( I) discrete molecular complexes are found that consist of (CuI) 2 dimers in which each copper atom is coordinated by two 2-ethylpyrazine ligands. The crystal structure of the 1:1 compound poly[(CuI) 2( μ 2-2-ethylpyrazine-N,N′) 2] ( II) contains (CuI) 2 dimers which are connected by the 2-ethylpyrazine ligands into layers parallel to (010). The second modification of a 1:1 compound poly[(CuI) 2( μ 2-2-ethylpyrazine-N,N′)-(2-ethylpyrazine-N)] ( III) consists of 8-membered CuI rings, which are connected by the 2-ethylpyrazine ligands into layers. In one modification of poly[(CuI) 2( μ 2-2-ethylpyrazine-N,N′)] ( IV) CuI double chains are connected by the ligands into layers parallel to (001). If the preparation is kinetically controlled, crystals of the second modification poly[(CuI) 2( μ 2-2-ethylpyrazine)-N,N′]] ( V) can be obtained. The crystal structure of V is very similar to that of compound IV, with the main difference that the orientation of the ethyl groups of the 2-ethylpyrazine ligands is different. The thermal reactivity of all compounds as well as the thermodynamic stability and the transformation behaviour of the polymorphic modifications were investigated using DTA-TG-MS and DSC measurements, temperature resolved X-ray powder diffraction as well as temperature dependent crystallisation experiments. The occurrence of the different phases is discussed and the results are compared with those obtained on the corresponding CuCl and CuBr compounds on the basis of preparative results and density-functional calculations.

New form of the Cs 2O–B 2O 3 phase diagram

Following the determination of the structures of several anhydrous cesium borates and especially two new compounds with original formula, Cs 3B 7O 12 and Cs 3B 13O 21, a new form of the Cs 2O–B 2O 3 phase diagram is presented. Thermal analyses using DTA and temperature-resolved X-ray powder diffraction were performed to determine the thermal properties of some compounds. CsBO 2, CsB 3O 5, CsB 5O 8 (variety IV) and CsB 9O 14 melt congruently at 720°C, 835°C, 670°C and 597°C, respectively. The other borates melt incongruently at 620°C for Cs 2B 4O 7, 685°C for Cs 3B 7O 12 (B/Cs=2.33) and 663°C for Cs 3B 13O 21 (B/Cs=4.33). These two latter replace Cs 4B 10O 17 (B/Cs=2.5) and Cs 2B 8O 13 (B/Cs=4) in the phase diagram, respectively. CsB 9O 14 and CsB 3O 5 crystallize in a non-centrosymmetric space group that leads to non-linear optical properties, which have already been studied for the latter.

Copper(I) thiocyanate coordination polymers with dimethylpyrazine: synthesis, crystal structures, thermal and luminescence properties

The new copper(I) coordination polymers poly[(di- μ 2-thiocyanato- N, S)-( μ 2-2,5-dimethylpyrazine- N, N)] dicopper(I) ( I) and poly[di- μ 2-thiocyanato- N, S)-( μ 2-2,3-dimethyl-pyrazine- N, N)] dicopper(I) ( II) were prepared by the reaction of copper(I) thiocyanate with 2,3- and 2,5-dimethylpyrazine in acetonitrile. In all compounds different CuSCN sub-structures are found which are connected by the dimethylpyrazine ligands to multi-dimensional coordination networks. The thermal properties of all compounds were investigated using simultaneous differential thermoanalysis (DTA), thermogravimetry (TG) and mass spectrometry (MS) as well as temperature resolved X-ray powder diffraction. On heating, compound I and II loose all of the dimethylpyrazine ligands in an endothermic reaction and transform directly into copper(I) thiocyanate. Optical investigations show two excited states for both compounds in absorption and in luminescence measurements which are both, MC and LMCT in character.

Dimorphism of a New CuI Coordination Polymer:Synthesis, Crystal Structures and Properties of Catena[CuI(2-Iodopyrazine-N)] and Poly[CuI(μ2-2-Iodopyrazine-N,N‘)

Two modifications of the new copper(I) iodide coordination polymer CuI(2-iodopyrazine) were obtained by the reaction of CuI and 2-iodopyrazine in acetonitrile. During this reaction, intensely yellow crystals of form I appear first which transform within several minutes to intensely red crystals of form II which is the thermodynamically most stable form at room temperature. In catena[CuI(2-iodopyrazine-N)] (form I; a = 4.1830 (6) A; b = 10.814 (1) A; c = 17.961 (4) A; V = 812.5 (2) A(3); orthorhombic; P2(1)2(1)2(1); Z = 4), corrugated CuI double chains are found in which each copper atom is coordinated by one additional 2-iodopyrazine ligand. In poly[CuI(mu-2-iodopyrazine-N,N')] (form II; a = 4.2679 (5) A; b = 13.942 (2) A; c = 13.017 (2) A; b = 92.64 (1) degrees; V = 773.76 (2) A(3); monoclinic; P2(1)/c; Z = 4), CuI single chains occur which are connected via mu-N,N' coordination by the 2-iodopyrazine ligands to layers parallel to (010). The thermal behavior of both forms was investigated using simultaneous differential thermoanalysis, thermogravimetry, and mass spectrometry as well as differential scanning calorimetry and temperature resolved X-ray powder diffraction. On heating, both forms decompose to copper(I) iodide, and the decomposition temperature of form I is significantly lower than that of form II. From all experiments, there is no indication of a phase transition of one form into the other or for the formation of a phase with lower amine content.

New Coordination Polymer Changing Its Color upon Reversible Deintercalation and Reintercalation of Water: Synthesis, Structure, and Properties of Poly[Diaqua-(μ2-Squarato-O,O‘)-(μ2-4,4‘-Bipyridine-N,N‘)- Manganese(II)] Trihydrate

The new coordination polymer poly[diaqua-(μ2-squarato-O,O‘)-(μ2-4,4‘-bipyridine-N,N‘)-manganese(II)] trihydrate was prepared by hydrothermal reaction of MnCl2·4 H2O with squaric acid and 4,4‘-bipyridine in water. In the crystal structure the manganese atoms are octahedrally coordinated by two squarate anions, two 4,4‘-bipyridine ligands, and two water molecules. The squarate anions and the 4,4‘-bipyridine ligands connect the metal atoms into interpenetrated sheets and are connected via O−H···O hydrogen bonding. From this arrangement channels are formed in which additional water molecules are intercalated. The channel water molecules can be reversibly de- and intercalated by heating or in a vacuum in a presumably topotactic reaction. The water molecules bound to the manganese atoms can also be removed on heating leading to the new intensely yellow compound Mn(C4O4)(4,4‘-bipyridine). Even this compound incorporates water leading to the formation of the starting material. The reaction is accompanied with a change of the color of the material from yellow to colorless. All reactions were investigated using simultaneous differential thermoanalysis, thermogravimetry and mass spectroscopy, ex-situ X-ray powder diffraction and in-situ temperature-resolved X-ray powder diffraction, as well as optical microscopy and UV−vis spectroscopy.