Remarkable Influence of Structures around Rhenium (V) Centers Constructed in Polyoxotungstates for Methanol Dehydrogenation under Visible Light Irradiation

ABSTRACT


Instrumentation/analytical procedures
The elemental analysis was carried out by using a Mikroanalytisches Labor Pascher instrument (Remagen, Germany).The samples were dried overnight at room temperature under 10 -3 -10 -4 Torr vacuum before analysis.The infrared spectra were recorded on a Parkin Elmer Spectrum100 FT-IR spectrometer in KBr disks at room temperature.Thermogravimetric (TG) and differential thermal analyses (DTA) data were obtained using a Rigaku Thermo Plus 2 series TG/DTA TG 8120.The TG/DTA measurements were performed in air with a temperature increase of 4 ºC per min between 20 and 500 ºC.The 31 P-{ 1 H} (242.95MHz) nuclear magnetic resonance (NMR) spectra in solutions were recorded in 5-mm outer diameter tubes on a JEOL ECA-600 NMR spectrometer (Shizuoka University).The 31 P NMR spectra were measured in CH3COOLi-D2O and D2O with reference to an external standard of 85% H3PO4 in a sealed capillary.Chemical shifts were reported as negative on the  scale for resonance upfield of H3PO4 ( 0).Solution UV-vis spectra were recorded on a Perkin-Elmer Spectrum Lambda 650 spectrophotometer.The positions of sharp bands were automatically determined by software of UV-visible spectrometer, and those of broad bands were picked up at the highest values in the ASCII files.Potentiometric titration was carried out with 0.841 mmol/L tetra-n-butylammonium hydroxide as a titrant under argon atmosphere [43].The compound Me2NH2-1 (4.12 mol) was dissolved in acetonitrile (20 mL) at 25 °C, and the solution was stirred for approximately 5 min.The titration data were obtained with a pH meter (Mettler Toledo).Data points were obtained in milivolt.A solution of tetra-n-butylammonium hydroxide (0.841 mmol/L) was syringed into the suspension in 0.20-equivalent intervals.

X-ray crystallography
A black block crystal of Me2NH2-1 (0.180  0.050  0.040 mm 3 ) was mounted in a loop.Data were collected by a Rigaku Marcury70 diffractometer using monochromated Mo K radiation ( = 0.71070 Å) at 113 K. Data were collected and processed using CrystalClear software for Windows.The structural analysis was performed using the CrystalStructure software for Windows.All structures were solved by SHELXS-97 (direct methods) and refined by SHELXL-97 [44].Since one rhenium atom was disordering over twelve tungsten sites (W(4) -W ( 15)) in polyoxoanion 1, the occupancies for the rhenium and tungsten sites were fixed at 1/12 and 11/12 throughout the refinement.The seven dimethylammonium ions and some acetonitrile and ethanol molecules were observed; however, no acetonitrile and ethanol solvent molecules were observed by elemental analysis and 1 H NMR spectroscopy.Thus, the solvent molecules evaporate gradually when crystals are removed from acetonitrile solution.Accordingly, the residual electron density was removed using the SQUEEZE routine in PLATON [45].

Synthesis and molecular structure of [Me2NH2
, was synthesized by the direct reaction of 2 equiv of K2Re IV Cl6 with mono-lacunary 1-Dawson polyoxotungstate, [1-LiP2W17O61] 9-, in a CH3COOH/CH3COOLi aqueous buffer solution (pH = 4.5) under air, at 25 C; this was followed by addition of excess Me2NH2Cl, forming a dark blue precipitate.The formation of polyoxoanion 1 is represented by the ionic balance shown in Eq. 1, in which rhenium(IV) is oxidized to rhenium(V), as observed for compounds K-2 and Me2NH2-3 [39,40].Notably, polyoxoanion 1 was gradually isomerized to the 2-isomer during the reaction of K2Re IV Cl6 with [1-LiP2W17O61] 9-in aqueous solution; the CH3COOH/CH3COOLi aqueous buffer solution is therefore indispensable in inhibiting isomerization, as observed for [1-LiP2W17O61] 9- [41].In addition, the reaction temperature should be kept at ca. 25 C, even in a buffer solution, because heat treatment accelerated the isomerization.An excess of K2Re IV Cl6 was therefore required to complete (accelerate) the coordination of rhenium ion to the mono-vacant site of [1-LiP2W17O61] 9-.For purification, the unreacted K2Re IV Cl6 was completely removed by crystallization via vapor diffusion from acetonitrile/ethanol.
[1-LiP2W17O61] 9-+ Re IV Cl6 2-+ H2O  [1-P2W17Re V O62] 7-+ 2H + + Li + + 6Cl -(1) The sample was dried overnight at room temperature under a vacuum of 10 -3 -10 -4 Torr for elemental analysis.The elemental results for C, H, N, Re, P, and W were in good agreement with the calculated values for the chemical formula of Me2NH2-1 with two hydrated water molecules.The presence of seven dimethylammonium ions suggested that the oxidation state of the rhenium site was 5+; this was also supported by the fact that no protonation was observed in potentiometric titration with tetra-n-butylammonium hydroxide in acetonitrile.The weight loss observed during drying before analysis was 2.54% for Me2NH2-1, corresponding to seven weakly solvated or adsorbed water molecules.However, during TG/DTA under atmospheric conditions, a weight loss of 12.2% was observed below 500 C, corresponding to seven dimethylammonium ions and nine water molecules.The X-ray structural analysis of crystalline [Me2NH2]7[1-P2W17Re V O62]•2H2O revealed that the molecular structure of 1 was identical to that of a monomeric -Dawson polyoxotungstate, [-P2W18O62] 6-, as shown in Figure 1.The bond lengths and bond angles are shown in Appendix.As a result of the high-symmetry space group, 12 tungsten sites [W(4) -W( 15)] were disordered and a mono-rhenium-substituted site was not identified, as observed for [W9ReO32] 5-[47] and [α-PW11Re V O40] 5- [48].Some ethanol and acetonitrile molecules were observed in a single crystal of Me2NH2-1; however, no acetonitrile and ethanol solvent molecules were observed by elemental analysis and 1 H NMR spectroscopy.The solvent molecules therefore evaporate gradually when the crystals are removed from acetonitrile solution.The FTIR spectrum of compound Me2NH2-1, which was obtained using a KBr disk, is shown in Figure 2. The positions of all the bands (1094,1077,1014,959,918,823,776, and 733 cm -1 ) in the polyoxoanion region of this compound are characteristic of polyoxoanions; however, they were different from those for [1-LiP2W17O61] 9-(1122, 1092, 1011, 944, 908,   828, 783, and 744 cm -1 ), K-2 (1091, 1018, 955, 910, and 788 cm -1 ), and [-P2W18O62] 6-  (1091, 1020, 958, 912, 777, and 528 cm -1 ).This suggests coordination of a rhenium(V) ion in the monovacant site of [1-LiP2W17O61] 9-.The 31 P NMR spectrum in CH3COOLi-D2O solution of Me2NH2-1 showed a clear two-line spectrum, with signals at -12.1 ppm and -12.4 ppm, as shown in Figure 3(a).The signals were shifted compared with those of [1-LiP2W17O61] 9-(-8.7 ppm and -13.0 ppm) and [-P2W18O62] 6-(-12.8ppm), indicating complete coordination of rhenium atom to monovacant site of [1-LiP2W17O61] 9-, as shown in Figure 1.No contamination of the sample by polyoxoanion 2 (-11.9 ppm and -12.9 ppm in D2O) was observed; however, polyoxoanion 1 gradually isomerized to polyoxoanion 2 in aqueous solution.The 183 W NMR spectrum of Me2NH2-1 was failed to obtain because of its low solubility in water and acetonitrile.The UV-visible spectrum of Me2NH2-1 in water showed four absorption bands, at263 (4.4  10 4 M -1 cm -1 ), 310 (1.8  10 4 M -1 cm -1 ), 468 (3.7  10 3 M -1 cm -1 ), and 613 nm (5.2  10 3 M - 1 cm -1 ), as shown in Figure 4.The bands at 263 nm and 310 nm were assigned to the charge transfer (CT) band of W-O bonds.The two bands at 468 and 613 nm were assigned to the Re V W VI intervalence charge transfer (IVCT) band and the d-d band of the rhenium(V) atom, respectively [39,40].It was noted that the bands at 468 nm and 613 nm for Me2NH2-1 were significantly broader rather than those for K-2 and Me2NH2-3, and the positions were blue-shifted compared with those for K-2 (496 nm and 737 nm) and Me2NH2-3 (513 nm and 698 nm); this also suggested coordination of the rhenium(V) ion to the monovacant site in [1-LiP2W17O61] 9-.

Catalytic activities in methanol dehydrogenation catalyzed by rhenium(V)coordinated polyoxotungstates in the presence of TiO2 under visible-light irradiation (400 nm)
Methanol dehydrogenation catalyzed by rhenium(V)-coordinated polyoxotungstates at 25 ºC in the presence of TiO2 under light irradiation (400 nm) was investigated; the results are summarized in Table 1.Hydrogen was evolved from methanol catalyzed by Me2NH2-1, K-2, and Me2NH2-3.Formaldehyde was also observed; while, O2, CO, and CH4 were not observed.The three rhenium(V) compounds were hardly soluble in methanol.When a suspension of K-2 (50 mg) and TiO2 (200 mg) in methanol (25 mL) was irradiated under visible light (400 nm) for 6 h, followed by filtration through a membrane filter (JG 0,2 m), UV-vis spectrum of the filtrate showed a small band at around 220 nm due to a charge transfer band of W VI -O; however, this was significantly smaller than that in 30 mM EDTA2Na aqueous solution (25 mL) under the same reaction conditions, as shown in Figure 5.Even when the cesium salts of 1 -3 were used as catalysts, a slight leaching into methanol was observed.These results suggested that the rhenium(V) compounds were predominantly active in the solid state under the present reaction conditions.The initial dehydrogenation rates with Me2NH2-1 and Me2NH2-3 were slow; while, no induction period was observed for K-2, as shown in Figure 6.The colors of these materials changed from white-purple to blue during the reactions; however, the blue color disappeared and the photoreactions stopped when visible light irradiation stopped.In control experiments, hydrogen was not detected when the reaction was catalyzed by TiO2.The rhenium(V)-coordinated polyoxotungstates showed no reaction in the absence of TiO2.K10[-P2W17O61]27H2O also showed no reaction even in the presence of TiO2.A combination of rhenium(V)-coordinated sites in the polyoxotungstates and TiO2 was therefore necessary, as reported for photoreactions using an EDTA•2Na aqueous solution in the presence of TiO2 under light irradiation (400 nm) [39,40].For the three rhenium compounds (1.0 mol of Re), the evolved amounts of H2 after 6 h were 17.7, 206, and 72.4 mol [the turnover numbers (TONs) were 35, 412, and 145, respectively]; these results showed that K-2 had the highest activity among these samples under the present reaction conditions.Even for 0.6 and 2.0 mol of Re, K-2 exhibited the highest activities; however, the TONs decreased with increasing concentration of rhenium(V) atoms.When the cesium salts of 2 and 3, Cs14[O{Re(OH)(2-P2W17O61)}2] (Cs-2) [40] and Cs3.5H0.5[PW11ReO40](Cs-3) [40], were used as catalysts, the activities decreased compared with those of K-2 and Me2NH2-3; however, the activities of Cs-2 were higher than those of Cs-3.These results suggested that the structure dependence was not influenced by the counter i on.2[H2 evolved (mol)] per [Re atoms (mol)] The stabilities of polyoxoanions 1 -3 during methanol dehydrogenation were determined by 31 P NMR spectroscopy, as follows: TiO2 (500 mg) and the rhenium compounds (50 mg) were suspended in 25 mL of methanol.After light irradiation for 6 h, the solids were collected using a membrane filter (JG 0.2 m).The solids were suspended in 10 mL of water, and the filtrates, containing dissolved polyoxoanions 1 -3, were evaporated to dryness at 40 ºC.As shown in Figures 3(b) and 7, the 31 P NMR spectra in CH3COOLi-D2O of 1, and in D2O of 2 and 3, were the same as those of the as-prepared samples.This showed that these compounds did not decompose or isomerize during methanol dehydrogenation under light irradiation.It was clear that the rhenium(V) sites in polyoxometalates significantly affected the photocatalytic activities, and the dirhenium(V)-oxido-bridged site in 2 exhibited the highest activity among these three samples under the present reaction conditions.

CONCLUSION
A rhenium(V) complex composed of mono-lacunary -Dawson polyoxotungstate was presented.We successfully obtained single crystals of dimethylammonium salt [Me2NH2]7[1-P2W17Re V O62]•9H2O (Me2NH2-1) by reacting hexachlororhenate with a monolacunary -Dawson polyoxoanion in CH3COOH/CH3COOLi buffer, followed by crystallization via vapor diffusion from acetonitrile/ethanol. The characterization of compound Me2NH2-1 was accomplished by X-ray structure analysis, elemental analysis, TG/DTA, FTIR, UV-visible, and solution 31 P NMR spectroscopy.For methanol dehydrogenation under visible light irradiation (400 nm) in the presence of TiO2, the molecular structures of three rhenium(V) compounds were stable during the photoreactions, and the dirhenium(V)-oxido-bridged site in 2 exhibited the most effective activities compared with those of the mono-rhenium(V)-substituted sites in 1 and 3.

Fig. 1 .
Fig. 1.(a) The molecular structure (ORTEP drawing) of polyoxoanion 1 with all atom numberings and (b) polyhedral representation of polyoxoanion 1.In (b), WO6 and ReO6 units are represented by the white and purple octahedra, respectively.The internal PO4 units are represented by the red tetrahedra.

Fig. 5 .
Fig. 5. UV-visible spectra of K-2 at 200 -800 nm.K-2 (50 mg) and TiO2 (500 mg) were suspended in (a) 30 mM EDTA2Na aqueous solution (25 mL) and (b) methanol (25 mL), and they were irradiated under the visible light (400 nm) for 6 h at 25 ºC.After a filtration through a membrane filter (JG 0.2 m), UV-vis spectra of the filtrates were observed, respectively.In control experiments, hydrogen was not detected when the reaction was catalyzed by TiO2.The rhenium(V)-coordinated polyoxotungstates showed no reaction in the absence of TiO2.K10[-P2W17O61]27H2O also showed no reaction even in the presence of TiO2.A combination of rhenium(V)-coordinated sites in the polyoxotungstates and TiO2 was therefore necessary, as reported for photoreactions using an EDTA•2Na aqueous solution in