Single crystal-to-single crystal transformations ...

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Cite this: CrystEngComm, 2018, 20, 2907 Received 14th March 2018, Accepted 26th April 2018

Single crystal-to-single crystal transformations induced by ammonia–water equilibrium changes† Marek Daszkiewicz, *a Mariola Puszyńska-Tuszkanow,b Zbigniew Staszak,c Ida Chojnacka,b Hanna Fałtynowicz b and Maria Cieślak-Golonka b

DOI: 10.1039/c8ce00401c rsc.li/crystengcomm

Reversible single crystal-to-single crystal transformations were observed for the NiĲII) complex with 5-methyl-5-phenylhydantoin. Moreover, the symmetry changes from triclinic (P1¯) to monoclinic (P21/c). The change is induced by a disturbance of the ammonia– water equilibrium between the [solid state–solution-gas] phases, which causes an exchange of the solvent in the crystal structure.

Solid-state conversion maintaining single-crystalline nature during transformation can be an invaluable source of knowledge on the life of the solid matter.1 It can be utilized in the design of compounds with desired properties that can be applied as nanoscale catalysts, sensors, and molecular sieves for the separation of solid mixtures and in gas storage.2 The SCSC processes are controlled by factors such as metal coordination preference, covalency,3 hydrogen bonding,4 and metallophilicity.5 Amongst the various physical stimuli inducing conversions in an SCSC manner, those of light,6,7 pressure,8 alcohol vapour,9 heating,10 cooling,11 and solid seeding12 with the mechanism of molecular dominoes and various postsynthetic modifications of a crystal13 belong to the best known external driving forces of SCSC transformation. The chemical factors related to the cooperative movement of atoms in the solid state were found to be the redox and dimerization reactions,14 central metal ion15 and anion exchanges,16 and solvent exposition, release, replacement, and coordination.17 Further, the factors inducing SCSC transfora

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, P.O. Box 1410, 50-950 Wrocław, Poland. E-mail: [email protected] b Faculty of Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-270 Wrocław, Poland c Faculty of Computer Science and Management, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-270 Wrocław, Poland † Electronic supplementary information (ESI) available: Synthesis, X-ray data collection and refinement for ligand mph and for complexes 1 and 2, tables with selected bond lengths and angles, assignments of the IR bands, analysis of the UV-vis-NIR spectra and crystal field parameters. CCDC 784165, 836482 and 836483. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8ce00401c

This journal is © The Royal Society of Chemistry 2018

mation were those showing relatively small variation in the second coordination sphere18 and those breaking and creating new metal–ligand bonds and significantly changing the crystal structure.19 Although a rapid growth of interest in last several years is observed in this field, the complicated nature of the SCSC effect requires new examples. Herein, we report the unprecedented SCSC transformation induced by the changes in the gas-solution–solid state equilibrium of a twocomponent solvent, ammonia and water. Contrary to the thoroughly studied coordination polymers, the isolated compounds belong to a rare series of a discrete mononuclear species showing the SCSC phenomenon.20 Violet crystals of [NiĲmph)2ĲNH3)2ĲH2O)2]·1.23NH3 (1), where mph = 5-methyl-5-phenylhydantoin (mph), were obtained from the system: [NiĲII) – mph – concentrated NH3 in H2O solution] (Fig. 1).‡ The crystals were allowed to stand alone in the mother liquor in air. After few days, violet crystals changed into blue crystals of [NiĲmph)2ĲNH3)2ĲH2O)2] ·1.26H2O (2), retaining their crystalline form. The change was triggered by the evaporation of ammonia in the open system. Blue crystals of compound 2 soaked in the mother liquor of 1 changed again to violet crystals. The resulting crystals were checked using a single-crystal diffractometer, and lattice parameters similar to those of the parent compound 1 were observed. Thus, the reaction is reversible under these conditions. Additionally, similar experiments were carried out for the dried crystals of 2 exposed to gaseous ammonia, and as a result, violet powder was obtained. Moreover, violet powder when removed from the ammonia atmosphere and left in the air was transformed into blue powder of 2 again. The

Fig. 1 The SCSC transformation of the violet crystals (1) to the blue crystals (2).

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physicochemical analysis (FTIR, electronic spectra, thermal analysis with discussion in ESI† and TG/FT-IR) indicates that the transformation of powders 1 and 2 is reversible; thus, these powders have the potential to be used as a sensor.21 The literature data indicate that this is the first example of the SCSC transformation observed in the hydantoin complexes. More importantly, to the best of our knowledge, the SCSC transformation induced by the equilibrium shift amongst the gas, liquid, and solid state in a multicomponent solvent has not been reported to date. In the presented single crystals, all the changes visible with the unaided eye are strongly related to the molecular rearrangements in the crystal structure. The violet crystals of 1 are triclinic, with the space group ¯, whereas the blue crystals of 2 possess monoclinic symmeP1 try, with the space group P21/c. The unit cell volume becomes twice after transformation because the lattice parameter b in 1 corresponds to 0.5c in 2.‡ The symmetry-independent unit of the unit cell contains one mph ligand, ammonia, and water molecules coordinating to the metal centre in both presented crystal structures. Since nickel ion lies on the inversion centre, its coordination sphere is constructed by double number of each type of the ligand, [NiĲmph)2ĲNH3)2ĲH2O)2]. It is worth noting that two mph ligands have opposite chirality (R and S) since they are associated to each other by the inversion centre. Geometry parameters of the coordination sphere differ insignificantly from those of an ideal octahedron (Table S1, ESI†). The coordination sphere of the metal ion in [NiĲmph)2ĲNH3)2ĲH2O)2] is very similar to that found in the previous study reported on [NiĲmhyd)2ĲH2O)4], [NiĲpht)2ĲH2O)4] and [NiĲpht)2ĲNH3)2ĲH2O)2], where mhyd = 1-methylhydantoinate and pht = phenotoinate anions.22–24 Fig. 2 shows that the most significant difference occurring in the [NiĲmph)2ĲNH3)2ĲH2O)2] molecule is that the N1–C5– C21–C26 dihedral angle is related to the position of the phenyl group towards the hydantoinate 5-membered ring. This dihedral angle changes during transformation of the crystal

from 27.27° for 1 to 57.45° for 2. In both crystal structures, molecules of solvent exist between [NiĲmph)2ĲNH3)2ĲH2O)2] entities, and they are located in the channels along the a axis in 1 and along the b axis in 2. When the violet crystal of 1 transforms into the blue crystal of 2, the increase in the Laue symmetry, from ¯ 1 to 2/m, is observed. Thus, additional symmetry elements appear in 2, 21 screw axis, and the glide plane c. To satisfy these symmetry changes, the structure must have changed several parameters. A comparison of the structures reveals that each and every second layer of the [NiĲmph)2ĲNH3)2ĲH2O)2] molecules moves along the arrows depicted in Fig. 3 because in the monoclinic phase, one molecule lies in the middle of the unit cell (Fig. 4). This displacement is assessed to be 1.5 Å along the unit cell axis. Additionally, a comparison of the structures reveals that two mph organic ligands coordinating to nickelĲII) ion have opposite chirality as compared to the case of the neighboring complex molecule (Fig. 4). Thus, the movement of each second layer is correlated with a rotation of each [NiĲmph)2ĲNH3)2ĲH2O)2] molecule around the metal centre about 180 deg. As a result, the translation vector b increases two times, 2b → c, and also two interaxial angles equal 90 deg, leading to the monoclinic crystal symmetry. Thus, simultaneous movement and rotation of the [NiĲmph)2ĲNH3)2ĲH2O)2] molecules result in the appearance of additional symmetry operations, 21 screw axis, and c glide plane associated with the P21/c space group. It is worth noting herein that all the structural changes are connected to the substitution of ammonia by water molecules in the crystal structure. However, this SCSC process is triggered by a decrease in the ammonia concentration in the mother liquor

Fig. 2 Coordination sphere of the nickelĲII) ion (a) in compound 1 [NiĲmph)2ĲNH3)2ĲH2O)2]·1.23NH3 and (b) in [NiĲmph)2ĲNH3)2ĲH2O)2] ·1.26H2O 2.

Fig. 3 Changes of the crystal packing and unit cell during SCSC transformation of (a) violet triclinic crystals to (b) blue monoclinic crystals.

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Fig. 4 Comparison of unit cell packing in (a) compound 1 and (b) compound 2 revealed displacement and rotation of the [NiĲmph)2ĲNH3)2ĲH2O)2] molecule.

over the crystals of 1 due to slow evaporation of ammonia (Le Chatelier's rule). To further characterize the products, thermogravimetric analysis (TG) combined with FTIR spectroscopy was carried out. Herein, TG/FTIR was used particularly to confirm the chemical composition of the second coordination sphere. Complex 1 exhibits a weight loss of 4.5% at 25–100 °C related to ammonia removal from the second coordination sphere (4.79%). Indeed, the three-dimensional image of the infrared spectrum of the evolving gaseous products indicates the presence of ammonia gas (Fig. 5). The bands corresponding to the N–H stretching (3334 cm−1), H–N–H scissoring (1616 cm−1), and N–H wagging (968 cm−1) vibrations are clearly seen above 30 °C (red line in Fig. 5a).25 FTIR spectra of the off-gases are presented in Fig. 5b. In subsequent minutes of heating, an increased intensity of the bands derived from the ammonia molecule vibration indicates the continuation of its

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Fig. 6 (a) 3D FTIR spectrum of the gaseous products obtained during the thermal decomposition process of 2. (b) IR spectra of the gases released during the decomposition process, obtained at different temperatures.

separation from the first coordination sphere. Moreover, the band derived from the vibration of water molecules begins to appear (from 2090 s process → 115 °C) (Fig. 5a – blue line). This is the start of the second stage where disintegration of the first coordination sphere of 1 takes place. In the temperature range from 240 °C to 500 °C (3240 s), decomposition of the remaining part of the complex i.e. organic ligand, takes place. In complex 2, the temperature range from 30 °C to 105 °C exhibits a slow weight loss of 3.69% (calculated for water of crystallisation 3.43%). The 3D FTIR spectrum of gaseous products confirmed that only water molecules were released at this stage (Fig. 6),26 and no bands related to ammonia were seen. Above 115 °C (1750 s), the IR spectrum of 2 shows a continuous increase of band intensity corresponding to the water vibrations, and also a rapid increase of N–H bands is observed due to a release of ammonia gas from the first coordination sphere. In the temperature range from 280 °C to 500 °C, further degradation of [NiĲmph)2ĲNH3)2ĲH2O)2] ·1.26H2O (2) occurs.

Conclusions

Fig. 5 (a) 3D FTIR spectrum of gaseous products obtained during the thermal decomposition process of 1. (b) IR spectra of the gases released during the decomposition process, obtained at different temperatures.


We have found that single crystals of [NiĲmph)2ĲNH3)2ĲH2O)2] ·1.23NH3 (1) are converted over time into a new chemical entity, [NiĲmph)2ĲNH3)2ĲH2O)2]·1.26H2O (2). It takes place without loss of crystallinity despite the observed large molecular movements occurring during the transformation (vide supra). The SCSC transformation is chemically reversible. Moreover, the substantial molecular movements upon exchange of NH3 → H2O in the channels on the SCSC mode resulted in only slight structural and crystal field changes of the [NiĲmph)2ĲNH3)2ĲH2O)2] sphere. However, it was accompanied by distinct colour alteration from violet to blue in 1 and 2.

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This SCSC solid state conversion occurs upon changing the concentration of ammonia in the two component (NH3– H2O) liquid and gaseous phases (the open system). As a result, ammonia from the channels of the violet crystal is transported to the solution, and the voids are filled with water molecules. However, when the blue compound 2 is exposed to gaseous ammonia, the resulting violet compound 1 can be obtained. It can be transformed in the air into a blue powder of compound 2. The blue colour for [NiĲmph)2ĲNH3)2ĲH2O)2] ·1.26H2O (2) changes into violet upon gaseous ammonia, and this indicates that this transformation may have potential application in the sensing devices.

Conflicts of interest There are no conflicts of interest to declare.

Acknowledgements The authors would like to thank U. Połata for her great help in the preparation of the complexes. We gratefully acknowledge the instrumental grant 6221/IA/119/2012 from Polish Ministry of Science and Higher Education which supported our Integrated Laboratory of Research and Engineering of Advanced Materials where the measurements of the Raman and IR spectra were performed. MD would like to thank ILT&SR PAS for financial support by statutory activity subsidy. This work was partly financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Technology (S30042/Z0304).

Notes and references ‡ Respective CCDC 836482, 836483 and 784165 data contain the supplementary crystallographic data for this paper for compound 1, 2 and ligand mph, respectively.

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