Manuel
Wilke
ab,
Lisa
Batzdorf
ab,
Franziska
Fischer
ab,
Klaus
Rademann
b and
Franziska
Emmerling
*a
aBAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Strasse 11, 12489 Berlin, Germany. E-mail: franziska.emmerling@bam.de; Fax: +49-30-8104-1137; Tel: +49-30-8104-1133
bDepartment of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany
First published on 24th March 2016
The crystalline cadmium phenylphosphonates Cd(O3PPh)·H2O (1), Cd(HO3PPh)2 (2), and Cd(HO3PPh)2(H2O3PPh) (3) can be synthesized via solid state reactions by grinding together cadmium acetate with the respective equivalents of phenylphosphonic acid. Phosphonates (2) and (3) have not been obtained via any other synthesis method so far. The determination of the crystal structure of the new compounds (2) and (3) based on powder X-ray diffraction (PXRD) data is reported. The milling reactions were investigated using in situ synchrotron PXRD. Based on these data, an identification of intermediates and a detailed analysis of the underlying mechanisms were possible.
Unlike for the related class of metal organic frameworks (MOFs) the use of alternative synthetic routes like mechanochemistry is scarce.18–20 Recently, in situ methods were developed to investigate milling syntheses to provide the necessary data for a thorough mechanistic study.21–24
Here, we present the mechanochemical synthesis of three cadmium phenylphosphonates: the cadmium monophenylphosphonate Cd(O3PPh)·H2O (1) (Ph = C6H5)25 and two novel structures (1:2 and 1:3) (Fig. 1). The structures of the new compounds, Cd(HO3PPh)2 (2) and Cd(HO3PPh)2(H2O3PPh) (3), were determined from powder X-ray diffraction (PXRD) data. Compound (2) contains two monodeprotonated phenylphosphonate ligands. Compound (3) contains a further neutral phenylphosphonic acid ligand. The synthesis of the three compounds was analysed in situ using time resolved synchrotron PXRD. Based on these results, a formation mechanism is proposed.
With respect to the metal atom, the yields of the reactions are 100%. One has to keep in mind that during the removal of the samples, small amounts remain in the grinding jar. About 95% yield is a realistic estimation. For compound (3) 87% yield was obtained with respect to the metal atom after the cleaning step.
The reactions were investigated in situ using synchrotron XRD. The experiments were performed at the μSpot beamline (Bessy II, Helmholtz Centre Berlin for Materials and Energy, Germany).29 The setup has been reported in detail previously.23 A specially adapted vibration ball mill was used. The compounds were synthesized in a 10 mL custom-made Perspex vessel. The starting materials were ground together with two stainless steel balls (10 mm, 4 g). The experimental conditions were similar to those described before, except that a lower frequency (30 Hz) was applied. The XRD patterns were collected using a two dimensional MarMosaic CCD detector (3072 × 3072 pixels and a point spread function width of about 100 μm). The exposure time was 30 s per measurement, with a delay time of 3–4 s between measurements. For the processing of the scattering images, an algorithm of the computer program FIT2D was used.30 The transformation of the scattering vector (q) to the diffraction angle 2θ for Cu Kα1 radiation allows a direct comparison to the results of the laboratory PXRD measurements.
Fig. 2 Comparison of the measured PXRD pattern of (1) (black) and the calculated (red) PXRD pattern for Cd(O3PPh)·H2O.25 |
1 (ref. 25) | 2 | 3 | |
---|---|---|---|
Empirical formula | CdC6H7O4P | CdC12H12O6P2 | CdC18H19O9P3 |
Formula weight/g mol−1 | 286.49 | 426.58 | 584.67 |
Crystal system | Orthorhombic | Triclinic | Monoclinic |
Space group | Pmn21 | P | P21/c |
a/Å | 5.860(4) | 16.1586(16) | 5.732864(91) |
b/Å | 14.459(14) | 5.60337(54) | 24.84899(42) |
c/Å | 5.054(3) | 4.46652(33) | 14.40494(22) |
α/° | 90 | 119.6039(72) | 90 |
β/° | 90 | 94.4432(77) | 91.1232(15) |
γ/° | 90 | 101.7890(80) | 90 |
V/Å3 | 428.2(6) | 336.556(62) | 2051.674(57) |
Z | 2 | 1 | 4 |
D calc/g cm−3 | 2.22 | 2.09479(39) | 1.892851(51) |
R wp | 0.0697 | 0.0416 | |
R Bragg | 0.02694 | 0.02084 | |
GOF | 5.08 | 4.00 |
The structure of (2) is depicted in Fig. 4e along the c-axis. Selected bond lengths and angles are given in Table S1.† The Cd2+ ion is coordinated by six oxygen atoms from six different phenylphosphonate ligands in a distorted octahedron (Fig. 4b). The oxygen atoms of the phenylphosphonate ligands are involved in the coordination of three different Cd2+ ions. Four longer bonds (2.5319(3) Å and 2.5762(3) Å) and two shorter bonds (2.2262(2) Å) can be distinguished. The O–Cd–O angles range from 72.59(1)° to 107.41(1)°. Since all of the oxygen atoms coordinate to only one Cd2+ ion, there is no direct connection between the octahedrons. Here, the connection is established by the phosphonate groups (see Fig. 4b), resulting in a typical layer structure for the inorganic part of the compound including the isolated CdO6-octahedrons. The phenyl rings point into the interlayer space. The phenyl rings are ordered alternately above and below the layer, slightly tilted with respect to the layer plane. The asymmetric unit contains only monodeprotonated phenylphosphonate ligands. The position of the acidic hydrogen atom could not be determined directly. Based on the lengths of the P–O bonds (1.4196(1) Å (O2), 1.5388(1) Å (O3) and 1.5796(1) Å (O1)), O2 can be identified as the double bonded oxygen atom. A strong hydrogen bond connects the oxygen atom O1 with another phenylphosphonate molecule (O2), which additionally stabilizes the structure (dO–O = 2.5569(2) Å).
The structure of (3) is shown in Fig. 4f along the a-axis. Selected bond lengths and angles are given in Table S2.† The Cd2+ ion is also coordinated by six oxygen atoms from six different phenylphosphonates in a distorted octahedron (Fig. 4c). The six Cd–O bonds range from 2.2439(7) Å to 2.5202(5) Å and the O–Cd–O angle ranges from 72.681(16)° to 108.637(26)°. The three phenylphosphonate ligands coordinate differently. The neutral phenylphosphonic acid coordinates monodentately to one Cd2+ ion. The P–O bond length for the coordinating oxygen atom (O7) is 1.5525(6) Å and for the non-coordinating oxygen atoms the bond lengths are 1.5299(7) Å (O9) and 1.5422(4) Å (O8). The second ligand coordinates with two oxygen atoms to two different Cd2+ ions. The third oxygen atom (O5) is non-coordinating and carries the acidic hydrogen atom. The P–O5 bond length amounts to 1.5609(4) Å. For the coordinating oxygen atoms the P–O bond lengths are 1.5220(4) Å (O4) and 1.5708(5) Å (O6). This phenylphosphonate ligand is monodeprotonated. The third phenylphosphonate ligand is also monodeprotonated. Two oxygen atoms are coordinated to three different Cd2+ ions. The oxygen atom O2 is bridging two of them. The P–O2 bond length is 1.5337(4) Å. The non-coordinating oxygen atom (O3) is protonated. The P–O3 bond length is 1.5271(6) Å. The P–O bond length for the non-bridging coordinating oxygen atom (O1) is 1.5333(4) Å. As a result of this coordination, pairs of edge connected CdO6-octahedrons are formed (see Fig. 4c). These pairs are connected to two other pairs in opposite directions via four phosphonate groups. Two neutral phenylphosphonic acid units are located adjacent to this pair. The arrangement results in a double chain built by pairs of CdO6-octahedrons and the inorganic part of the phenylphosphonate. The phenyl rings point into the interchain space. The neutral ligands are located at the smaller edges of the chain and the protonated ligands are located above and below the chain. The chains are connected via π–π-stacking of parallel orientated phenyl rings and build a layered superstructure. The two orientations for the CdO6-octahedron pairs lead to two orientations in the resulting chain and layered structure. The layers are stacked in an alternating manner. Here, the acidic hydrogen atoms are located at the non-coordinating oxygen atoms. The orientation of the hydrogen atoms was estimated based on the direction of the hydrogen bonds. Two strong hydrogen bonds (dO9–O5 = 2.5554(5) Å and dO5–O8 = 2.4135(6) Å) stabilise the structure. The latter value is short, but was also observed for other phosphoric acids.32 Additionally, moderate (dO3–O7 = 2.8242(6) Å) and weak (dO8–O3 = 3.2504(8) Å) hydrogen bonds are formed.
A 2D plot of the XRD data for the synthesis of (1) is shown in Fig. 5a. The data collected at t = 0 s represent the reflections of the starting materials (phase 1). After 30 s a peak at 2θ = 7.1° appears, which can be assigned to (3) (phase 2). 30 s later a reflection at 6.1° indicates the formation of (1) (phase 3). During the next few minutes the intensity of the reflections of the starting materials decreases and it vanishes after 4:30 min (start of phase 4). The intensities of the reflections for (3) also start to decrease, while the number and intensity of the product reflections increase. After 14 min only the reflections of the final product can be detected (phase 5).
Fig. 5 2D plots of synchrotron XRD data for the synthesis of (a) (1), (b) (2) and (c) (3) with a description of the detected compounds; s.m. = starting materials. |
A 2D plot of the XRD data of the synthesis of (2) is depicted in Fig. 5b. In the first phase only the starting materials can be detected. After 1 min of milling, different reflections indicate the formation of (1) and (3) (phase 2). The strongest reflections are at 6.1° and 7.1°, respectively. The reflections for (3) are more pronounced. During this phase, the reflection intensities of the starting materials decrease. After 2:15 min the reflections of the starting materials vanish and only peaks for both of the intermediates can be detected (phase 3). In phase 4 a reflection at 5.7° indicates the formation of the final product. The intensity is very weak compared to the intensity of the reflections at 6.1° and 7.1°. After 6 min of milling, the reflection intensities of the intermediates start to decrease, while at the same time those of the product increase. With increasing milling time, only the strongest reflections of the intermediates can be detected. After 9 min of further milling all the reflections of (3) disappear. Reflections of the final product and a very low contribution of (1) at 6.1° can be detected (phase 5). This additional peak remains unchanged during the observed synthesis. The same phenomenon could be observed in the final PXRD pattern of the product of the laboratory synthesis (see Fig. S1†).
A 2D plot of the XRD data of the synthesis of (3) is shown in Fig. 5c. At the beginning, only the starting materials can be detected. After 1 min of milling a peak at 7.1° indicates the formation of the final product. In the next minute the reflections of cadmium acetate dihydrate disappear (e.g. 12.6°). The ones belonging to phenylphosphonic acid (e.g. 20.4°) persist since the acid is used in excess (phase 3). After 4:30 min a reflection at 6.1° can be detected (phase 4). This reflection indicates the intermediate formation of (1). The peak remains observable until 6:45 min. At 5:30 min a reflection at 5.7° indicates the formation of (2). The reflection can be detected for the next two minutes. In the fifth phase, reflections of the final product and phenylphosphonic acid can be detected. During the last eleven minutes of the reaction, the intensities of the reflections changes. These non-systematic changes are an artifact of the measurement and stem from different amounts of powder in the beam path. A close inspection of Fig. 5c shows that the reflections disappear at a certain time (12 minutes) and reappear again after 4 minutes. Two aspects are important: (i) three reflections of compound (3) persist during this period in a weaker form and (ii) no change in the crystallite size (the widths of the reflections stay the same) was observed before or after this period. We would expect a decrease in the crystallite size before an amorphous product is observed.
In situ measurements of mechanochemical reactions are a new field. Based on recent publications, mechanochemical reactions proceed either directly from the starting materials to the final product or are characterized by the formation of an intermediate phase.21,23,33,34 Beldon et al. also found indications for an Ostwald ripening process for the synthesis of ZIF.35 Summarizing, for all three investigated compounds, a synthesis process involving different intermediate steps could be observed. The formations of (1) and (3) under milling conditions are much faster than the formation of (2). Compound (3) seems kinetically favored. Using a 1:1 and 1:4 starting stoichiometry, reflections of (3) can be detected after 30 s and one minute, respectively. Reflections of (1) can be detected after one minute using a 1:1 starting stoichiometry. Reflections of both compounds can be detected as well after one minute using a 1:2 starting stoichiometry, which finally leads to (2). The reflections of the starting materials vanish prior to the formation of (2). This indicates that (2) is directly formed from intermediates of (1) and (3). The same behavior could be observed for the synthesis of (3). Here, the final product is built first and (1) is formed intermediately. Shortly thereafter, reflections of (2) can be detected and those of (1) start to decrease. Further experiments showed that it is also possible to synthesize (2) by milling (1) and (3) at a molar ratio of 1:1. Acetic acid and water were added to meet the original synthesis conditions. The synthesis is successful but a higher energy is needed (see Fig. S7†). 15 min of milling at 50 Hz results in a mixture of all three compounds. 60 min of milling at the same frequency leads to the pure compound (2).
All compounds involved in the synthesis can be identified as one of the cadmium phenylphosphonates or as starting materials. There are no other intermediates, such as mixed salts or amorphous phases. All reactions can be described using the following equation (with x = 1, 2, 3, and y = 0, 1):
This equation clearly shows that during the reaction acetic acid and water are released. Therefore, the dry milling synthesis is liquid-assisted by its side products. Indeed, all products were wet after milling. Based on these results, a reaction mechanism like the classic hot-spot-theory and the magma-plasma model36 can be ruled out.37 Within the magma-plasma model and its expected high energies, more fragmental intermediates and distinctly amorphous phases would be expected. Ma et al. recently described a milling reaction mechanism based on the thorough stirring of very small solid particles.38 We have observed, in agreement with the literature, that the reaction speed can be easily increased by increasing the applied frequency. Furthermore, the diffusion rate in liquid assisted grinding reactions is higher than in dry ones. It has been shown that acetic acid, as a side product, acts as an assisting liquid in self-sustaining reactions after a short initial activation.39 The decreasing intensity of the reflections of the starting materials and intermediates, together with the increasing reflections of the products, shows that the described diffusion mechanism can be used to explain the formation mechanism of cadmium phenylphosphonates.
Fig. 6 provides a schematic overview of the reaction pathways. The influence of the two directing factors, thermodynamic stability and the stoichiometric ratio of the starting materials, which were identified for the nature of the product, are illustrated. The slurry experiments showed that compound (1) is the most stable one (see Fig. S8†). The thermodynamic stability of metal organic compounds is connected with the density of the framework. For frameworks with the same composition it was shown that a more dense structure (expressed by the metal atom per nm3 ratio) also has a lower total energy.40,41 Beldon et al. showed that the mechanochemical formation of metal organic frameworks can follow Ostwald’s rule of stages. The first types with a lower density of central atoms are formed intermediately.35 The final product is the one with the highest density. This trend is not that clear for compounds with different compositions. Compound (3) has the lowest density, with 2.0 Cd atoms per nm3. Consistently, (3) is formed first in all three reactions. Compound (1) shows the highest value with 4.7 Cd atoms per nm3. This could be the reason why the compound remains stable in a small amount for the whole milling time during the synthesis of (2). This reaction also shows the influence of the second directing factor, the composition of the starting materials. The 1:1 (cadmium acetate dihydrate: phenylphosphonic acid) and the 1:2 syntheses result mainly in a stoichiometric product. The influence of both directing factors can be seen in the PXRD pattern (see Fig. S3†) of the 1:3 synthesis, where (2) (with 2.7 Cd atoms per nm3) and (3) are formed at the same time. Only with an excess of phenylphosphonic acid can (3) be obtained as the final product.
A comparison of the results of the in situ investigations of the mechanochemical syntheses of cadmium phenylphosphonates with the previously reported in situ investigations under hydrothermal and ultrasonic conditions is difficult. In all three investigated systems, the phosphonates carry functionalised organic groups which can also coordinate to the metal ion. The syntheses were carried out from aqueous solutions with the addition of inorganic bases. Nevertheless, in the investigations of the crystallisation of calcium aminoethylphosphonate and copper(II) phosphonoethanesulfonate, crystalline intermediates were also observed. They are not based on the metal:phosphonate ligand ratio, but on the presence of small molecules like water or hydroxide ions in the crystal structure.15,16 A correlation between the crystal density and the thermodynamic stability of the compounds was estimated.
Footnote |
† Electronic supplementary information (ESI) available: Crystallographic information files, tables with selected bond lengths of compounds (2) and (3); PXRD patterns of the starting materials, compounds (1), (2) and (3) (before and after the cleaning step), and of the product mixture of the mechanochemical synthesis with a 1:3 ratio of the starting materials; enlarged 2D plots of the in situ investigations of the syntheses of compounds (1–3); PXRD patterns of the products of the mechanochemical syntheses with compounds (1) and (3) as starting materials; and PXRD patterns of the products of the slurry experiments. CCDC 1402479 and 1402480. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra01080f |
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