Article pubs.acs.org/IC
A Reaction Cycle for Octahedral Tungsten Iodide Clusters Markus Ströbele and Hans-Jürgen Meyer* Section for Solid State and Theoretical Inorganic Chemistry, Institute of Inorganic Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany S Supporting Information *
ABSTRACT: A reaction cycle is shown for octahedral tungsten iodide compounds. The thermal transformation of W6I16 (W6I12·2I2) via release of iodine proceeds via the new W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) to yield a new modification of W6I12, denoted as αW6I12. When heated, α-W6I12 is converted into the known (β-)W6I12. The reaction of (β)W6I12 with I2 leads to the formation of the starting compound W6I16. The new compounds W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) and α-W6I12 are structurally characterized by powder and single-crystal X-ray diffraction techniques. The thermal decomposition of W6I16 and the monotropic phase transition of α-W6I12 into β-W6I12 are examined by differential scanning calorimetry measurements.
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and placed into a Schlenk flask with two polytetrafluoroethylene (PTFE) valves. The Schlenk flask was then heated to 120 °C for 16 h in a drying cabinet. Afterwards the side product I2 was removed under flowing argon by heating the product in a water bath (95 °C). W3I12 was obtained as a single-phase product, according to the X-ray diffraction pattern.10 W6I16. W6I12·2I2 was prepared by thermal conversion of the cluster compound W3I12. For this purpose, W3I12 (4.5 g, 2.17 mmol) was sealed into a silica tube (l = 4 cm, V ≈ 3 cm3), heated at 450 °C for 48 h (heating rate 2 °C/min), and afterward cooled to 275 °C (cooling rate 2 °C/min). The sample was kept at this temperature for 72 h and finally cooled to 25 °C (cooling rate of 1 °C/min). The reaction product was carefully ground in a mortar under argon atmosphere (glovebox) and placed into a Schlenk flask with two PTFE valves. The side product I2 was removed under flowing argon while heating the product in a water bath (95 °C). W6I16 was obtained as a single-phase product, according to the X-ray diffraction pattern. Yield: 3.2 g of W6I16, 95%. Dark red powder, stable in moist air. Method of Schäfer et al.:6 0.35 g of W6II2 were flame-sealed under vacuum together with 5−10 g of elementary iodine into an ampule (6 × 0.8 cm). The ampule was heated isothermally at 275−300 °C for 8 d. Unreacted iodine was sublimed off under vacuum (1 × 10−3 torr, 8 h, 50−60 °C). Yield: 0.4 g of W6I16, 95%. W6I13. W6I12·xI2 with 0 < x ≤ 1/2 was prepared by thermal decomposition of the cluster compound W6I16. Approximately 0.3 g (0.096 mmol) of W6I16 was sealed into a silica tube (l = 7 cm, V ≈ 3 cm3) and heated in a temperature gradient from 260−25 °C for 36 h (heating and cooling rate 2 °C/min). Orange-red crystals of W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) were obtained as a side phase. Yield ≈ 5%, stable in moist air. This reaction, like other reactions, was performed under nonequilibrium conditions by using a temperature gradient. The starting material was placed in the hot region of the ampule. α-W6I12 was prepared by thermal conversion of W6I16. Approximately 0.75 g (0.239 mmol) W6I16 was sealed into a silica tube (l = 7 cm, V ≈ 3 cm3) and heated in a temperature gradient from 350−25 °C for 36 h (heating and cooling rate 2 °C/min). The ampule was then
INTRODUCTION A diversity of metal chlorides,1,2 bromides,3,4 and iodides5−7 with the [M6X8] (M = Mo, W; X = Cl, Br, I) cluster core has been reported in the literature. Among them, metal iodides appear most impressive, because they reveal a true plethora of binary compounds with this cluster core. During the past, the exploration of molybdenum or tungsten iodides was handicapped by the absence of an appropriate way of synthesis, because preparations of binary molybdenum or tungsten iodides were guided by either the reaction of metal powder or M(CO)6 (M = Mo, W) with elemental iodine.8 Recently, a halide exchange reaction was developed by us, based on a facile reaction of MoCl5 or WCl6 with SiI4. The main products from this type of reaction are MoI3, when starting from MoCl5, and W3I8·2I2, when using WCl6.9 These reduced metal iodide compounds are attractive starting materials to generate more molybdenum or tungsten iodide compounds that have not yet been reported in the literature. The M6X12 structure (M = Mo, W) with the cluster connectivity [(M6X8i)I4/2a−aI2a] (i = innen or inner, and a = außen or outer) represents the archetype structure of some other binary molybdenum or tungsten iodide compounds, such as the polyiodides M6I16 and M6I18, (M = Mo, W). Their connectivity can be described as [(M6I8i)I2aI4/2a−a·2I2] and [(M6I8i)I2aI2/2a−a(I3)2/2a−a·2I2], respectively.10,11 The iodide-rich cluster compound W6I16 is easily accessible by thermal transformation of W3I8·2I2. Starting from W6I16, we report the crystal structures of two new binary tungsten iodides, W6I13 (W6I12·xI2 with 0 < x ≤ 1/2), and a new modification of W6I12. The thermal transformation of W6I16, and the irreversible monotropic phase transition of α-W6I12 into wellknown β-W6I12 are traced by differential scanning calorimetry (DSC).
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EXPERIMENTAL SECTION Received: February 28, 2017 Published: April 28, 2017
W3I12. WCl6 (2.7 g, 6.81 mmol) and SiI4 (5.471 g, 10.21 mmol) were carefully ground in a mortar under argon atmosphere (glovebox) © 2017 American Chemical Society
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DOI: 10.1021/acs.inorgchem.7b00537 Inorg. Chem. 2017, 56, 5880−5884
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Inorganic Chemistry transferred into a glovebox with argon atmosphere. The excess of I2, deposited in the room-temperature region of the tube, was manually removed from the otherwise X-ray pure α-W6I12. Yield: 0.55 g of α-W6I12, 85%. Dark red powder, stable in moist air. Side phase: W6I16 Powder patterns of all reaction products were investigated by powder X-ray diffraction (PXRD) using a StadiP diffractometer (Stoe, Darmstadt) with Ge-monochromated Cu Kα1 radiation, and a Mythen Detector. Additional crystallographic information is available in the Supporting Information. Orange single crystals of W6I12·xI2 (0 < x ≤ 1/2) and of α-W6I12 were measured with a single-crystal X-ray diffractometer (STOEIPDS) at room temperature (T = 293 K) using Mo Kα radiation (λ = 0.710 73 Å). The crystal-structure refinements and solutions were performed with direct methods (SHELXS) and least-squares refinements on F2 (SHELXL).17 Some results and final R values are shown in Table S2 (Supporting Information). Samples were enclosed into gold-plated (5 μm) steel autoclaves (BFT 94, Bächler Feintech AG, Switzerland) under dry argon atmosphere (glovebox). Samples were heated with a rate of 2 °C/ min in a differential scanning calorimeter (DSC 204 F1 Phoenix, Netzsch). Monitored thermal effects were assigned by PXRD patterns from separately prepared samples. The DSC system was calibrated using the sensitivity and temperature calibration software Fa. Netzsch Version 6.1.0 (11.02.2014). As calibration substances, elementary gallium (ABCR 99.9999%), KNO3 (Riedel-de Haen 99.9%), RbNO3 (Aldrich 99.7%), KClO4 (Sigma-Aldrich ≥99%), and CsCl (Merck 99.5%) were used. Measured was the energy of the transformation reaction or phase transition, respectively.
Figure 1. DSC of W6I16, depicting the decomposition of W6I16 into αW6I12 at 392 °C.
PXRD measurements. Note that the formation of the elusive intermediate W6I12·xI2 could not be detected in DSC experiments. Further heating in a temperature gradient from 300 to 25 °C revealed a new modification of W6I12 as main phase, denoted as α-W6I12. Red single crystals of α-W6I12 were obtained in a temperature gradient from 360 to 25 °C and selected for singlecrystal XRD measurements. According to the result of our DSC measurement, the energy of the monotropic phase transition from α-W6I12 into β-W6I12 was determined to be −4.9 kJ/mol (Figure 2).
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RESULTS AND DISCUSSION The thermal transformation of W3I12 allows preparations of some already known, but also a large number of unknown, binary tungsten iodides.12−14 Thereby, the cluster nuclearity increases from three in W3I12 to six. With these studies, we aim to get a better and more complete understanding on the existence of new binary compounds and on phase relationships in the W−I system. Here we depart from the preparation of the octahedral, iodine-rich cluster compound W6I16 and study the decomposition equilibriums related to the release of iodine. The preparation of W6I16 was attempted by two different methods: (1) following the method of Schäfer et al.5 by reacting W6I12 with elemental iodine in a fused silica ampule at 300 °C for 8 d and (2) by thermal conversion of W3I12.10 The identity of W6I16 with the reported structure7 was confirmed by PXRD. All following studies were performed by DSC measurements for the detection of thermal effects combined with reactions in silica tubes for the inspection of crystal structures and compositions of new compounds. The DSC measurement of W6I16, performed in a gold-plated steel autoclave, revealed decomposition at 392 °C with an enthalpy of decomposition of −3.1 kJ/mol (Figure 1). The same reaction stage was studied by separate reactions, where reactants were fused into silica tubing to inspect phases appearing along various decomposition stages via XRD technique. These reactions were conducted by exposing the reaction tube with a temperature gradient in an attempt not to overlook any intermediate phase. This strategy turned out very fortunate, because a new phase, closely related to W6I16, was discovered and identified as W6I13 (better described as W6I12· xI2 with 0 < x ≤ 1/2), when W6I16 was exposed to a temperature gradient from 260 to 25 °C. Single crystals of W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) were isolated from a reaction and used for single-crystal XRD studies. The overall amount of this compound was too small to be detectible in our
Figure 2. DSC of α-W6I12, depicting the phase transition of α-W6I12 into β-W6I12 at 524 °C.
The complete cascade of reactions is summarized in Figure 3. The release of iodine from W6I12·2I2 proceeds via the elusive compound W6I12·xI2 (0 < x ≤ 1/2) to yield α-W6I12, which can be considered as low-temperature modification of β-W6I12. W6I12·2I2 is regained when β-W6I12 is reacted with iodine as described by Schäfer. This cyclic reaction sequence is possible, because all structures are based on a layered [(W6I8)I4/2I2] connectivity with four terminal bridging iodido ligands, shared by two clusters each. This connectivity pattern can be seen in Figure 4. 5881
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Figure 3. Reaction cycle of layered W6I12 cluster frameworks with intercalation and de-intercalation of iodine. Shown are tungsten octahedra, iodido ligands (violet), and I2 red. Inner iodido ligands are omitted for clarity.
Figure 4. Comparison of the crystal structures of W6I16 ((W6I8i)Ia2I4/2a‑a·2l2) (left) and W6I13 (W6I12·xI2 (0 < x ≤ 1/2)) ((W6I8i)Ia2I4/2a‑a·1/2I2) (right). Shown are tungsten octahedra, iodido ligands (violet), and I2 red. Inner iodido ligands are omitted.
Table 1. Comparison of Average Distances
a
empirical formula
W6I187
W6I167
W6I12.735
α-W6I12
β-W6I127
average W−W distances, pm average W−Ia distances, pm average W−Ii distances, pm average W−Ia−a distances, pm torsion angles,a deg
267.3(2) 284.2(3) 279.1(3) 291.6(2)
267.0(2) 281.7(3) 278.6(3) 290.2(3) 40.867 52.443
267.02(8) 278.6(1) 280.0(1) 287.3(1) 39.88(4) 51.57(5)
266.9(1) 279.6(2) 279.8(1) 289.1(1) 38.22(2) 48.57(3)
266.5(2) 279.7(5) 279.5(3) 288.9(4) 0
For definition, see Supporting Information.
and 290.2(3) pm) in W6I16 are slightly elongated in comparison with the average W−Ia and W−Ia−a distances of W6I13 (W6I12· xI2 with 0 < x ≤ 1/2) and W6I12 (Table 1). As W6I18 exhibits even longer W−Ia and W−Ia−a distances (284.2(3), and 291.6(2) pm) this may be related to the additional I2 molecules in the structure. The release of I2 molecules yields the W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) intermediate shown in Figure 4 (right). The refinement of the site occupation factors of iodido ligands (I1−I12) and the I2 molecule (I13−I13) on 2i Wyckoff
A notable feature of the structure of (β-)W6I12 is that one axis of the W6 octahedron coincides with one crystallographic direction. All the other structures of the reaction cascade involving W6 cluster moieties are three-dimensionally rotated relative to each other. The structure of W6I16 (W6I12·2I2) described by Holm et al.7 is shown in Figure 4 (left) and exhibits two I2 molecules (I−I distance 273.2(4) pm) between adjacent cluster layers of the structure. The average W−Ia and W−Ia−a distances (281.7(3), 5882
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Figure 5. Comparison of α-W6I12 (left) and β-W6I12 (right). Shown are tungsten octahedra, iodido ligands (violet). Inner iodido ligands are omitted.
positions revealed only for the position of the I2 a site occupation factor below 100% (Supporting Information Figure S1 and Table S1). Refined compositions, based on two distinct crystals inspected by single-crystal measurements, yielded the compositions W6I12.605(7), and W6I12.737(5), respectively. The refined I−I distance of 272.9(3) pm of the corresponding I2 unit is in good agreement with the intramolecular I−I distance in solid I2 at 25 K (272.1(1) pm).15 This result suggests a variable number of I2 units between adjacent cluster layers of the structure before W6I12 is formed. The low-temperature modification α-W6I12 crystallizes in the monoclinic spacegroup P21/n (Z = 2)16 and can be well-related to the known orthorhombic modification of (β-)W6I12 (Cmca, Z = 4). A comparison of both structures is shown in Figure 5. In (β-)W6I12 the volume per formula unit amounts to 627 Å3 (V = 2508(1) Å3, Z = 4), in α-W6I12 to 649.5 Å3 (V = 1298.9(2) Å3, Z = 2). This suggests that the three-dimensional rotation of clusters relative to each other in α-W6I12, compared to a two-dimensional rotation in (β-)W6I12, involves a slightly bigger volume per formula unit for α-W6I12 (ΔV = 22.5 Å3).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ORCID
CONCLUSION W6I16 was synthesized using W3I12 as starting material. Thermal decomposition of W6I16 yielded the intermediate product W6I13: W6I12·xI2 (0 < x ≤ 1/2) under I2 loss. Further loss of I2 leads to the formation of α-W6I12, a new modification of (β)W6I12 as characterized by single-crystal diffraction methods. The thermal decomposition of W6I16 at 392 °C (−3.1 kJ/mol) and the monotropic transition of α-W6I12 at 524 °C into βW6I12 (−4.9 kJ/mol) were examined by DSC measurements. These studies, with the recovery of W6I16, evidence the pronounced stability of octahedral tungsten iodide clusters containing differently distorted layered W6I12 framework structures.
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Definition of the dihedral angle between the tungsten cluster cores. Picture of the two crystallographically independent tungsten clusters in the structure of W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) including atom numbering (color code: W dark blue, I violet, I2 red). PXRD pattern of W6I16 (black line) superimposed with the simulated powder pattern of W6I16 based on single-crystal data (red lines). PXRD pattern of α-W6I12 superimposed with the simulated powder pattern of α-W6I12 and W6I16 based on single-crystal data. PXRD pattern of W6I12 superimposed with the simulated powder pattern of W6I12 based on single-crystal data. Atomic coordinates (1 × 104) and equivalent isotropic displacement parameters (pm2 1 × 10−1) for W6I12.737(5). Single-crystal refinement data (PDF)
Markus Ströbele: 0000-0002-5147-5677 Hans-Jürgen Meyer: 0000-0003-2450-4011 Notes
The authors declare no competing financial interest. Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49 7247-808-666; E-mail: crysdata@fiz-karlsruhe.de on quoting the depository number CSD-432 005 for α-W6I12, CSD 432 584 for W6I12.73, and CSD 432 585 for W6I12.61.
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ACKNOWLEDGMENTS This research was supported by the Deutsche Forschungsgemeinschaft (Bonn) via Grant No. ME 914/27-1.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00537.
REFERENCES
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X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) 5883
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