Hydrogen Evolution from Water under Visible-Light Irradiation Using Keggin-Type Platinum(II)-Coordinated Phospho-, Silico-, and Germanotungstates as Co-Catalysts

The tetramethylammonium salts of diplatinum(II) complexes composed of mono-lacunary Keggin-type silico and germanotungstates, [(CH 3 ) 4 N] 4 [α-SiW 11 O 39 {cis-Pt(NH 3 ) 2 } 2 ]∙13H 2 O (TMA-Si-Pt) and [(CH 3 ) 4 N] 4 [α-GeW 11 O 39 {cis-Pt(NH 3 ) 2 } 2 ]∙11H 2 O (TMA-Ge-Pt), were synthesized and crystallized. Single crystals of a tetramethylammonium salt of Keggin-type diplati-num(II)-coordinated phosphotungstate [(CH 3 ) 4 N] 3 [α-PW 11 O 39 {cis-Pt(NH 3 ) 2 } 2 ] ⋅ 10H 2 O (TMA-P-Pt) were also obtained. The X-ray structural analyses revealed that the two cis-platinum(II) moieties, [cis-Pt(NH 3 ) 2 ] 2+ , were each coordinated to two oxygen atoms in a mono-vacant site of [XW 11 O 39 ] (12−n)− (X n+ = Si 4+ , Ge 4+ , P 5+ ). FTIR spectra of TMA-Si-Pt and TMA-Ge-Pt also suggested that the two platinum(II) moieties were coordinated to the vacant site of [SiW 11 O 39 ] 8− and [GeW 11 O 39 ] 8− . The 1 H NMR spectra in DMSO-d 6 of TMA-Si-Pt and TMA-Ge-Pt showed signals assigned to the two sets of NH 3 ligands coordinated to the platinum sites. These three platinum compounds showed hydrogen evolution from aqueous triethanolamine solution under visible light irradiation (λ ≥ 400 nm) in the presence of eosin Y, α-Keggin mono-aluminum-substituted silicotungstate, and titanium dioxide. The catalytic activities were influenced by the central atoms, and TMA-P-Pt showed the highest activities among the three platinum compounds.


Introduction
The development of photocatalysts, especially those that work under visible light irradiation, for the production of hydrogen from water is a critical issue in establishing clean energy systems [1]- [3]. Among the various possible photocatalysts, platinum is widely used as a co-catalyst to construct efficient photocatalytic systems for producing hydrogen because platinum promotes the separation of photo-generated electrons and holes and improves the efficiency of photocatalysis when it acts as the active center for hydrogen evolution [4]. Therefore, we have synthesized tetramethylammonium and cesium salts of α-Keggin diplatinum-coordinated phosphotungstate, [(CH 3 ) 4 N] 3 [PW 11 O 39 {cis-Pt(NH 3 ) 2 } 2 ]⋅10H 2 O (TMA-P-Pt) [5] and Cs 3 [α-PW 11 O 39 {cis-Pt(NH 3 ) 2 } 2 ]•8H 2 O (Cs-P-Pt) [6], and have constructed a novel photocatalytic system with Cs-P-Pt, eosin Y (EY), α-Keggin mono-aluminum-substituted polyoxotungstate K 5 [α-SiW 11 {Al(OH 2 )}O 39 ]•7H 2 O (K-Si-Al), and titanium dioxide to improve the effective utilization of platinum sites. This system achieved a steady hydrogen production during 12 h of light irradiation with highly effective utilization of the platinum sites for hydrogen production from aqueous triethanolamine (TEA) solutions under visible light irradiation (λ ≥ 400 nm, ≥440 nm, and ≥500 nm) [7]. During the light irradiation, we observed the formation of a reduced species (heteropoly blue species; HPB) of Cs-P-Pt, which was the key to achieving a steady hydrogen evolution during a 12 h of light irradiation. This is because it acted as a photosensitizer in the later stages, and compensated for the decline in hydrogen production caused by the decomposition of EY. Fu and Lu [8] reported that the rate of HPB formation for XW 12 O 40 (8−n)− (X n+ = P 5+ , Ge 4+ , Si 4+ , B 3+ ) is a key factor in determining hydrogen evolution rates under visible light irradiation, which is influenced by the central atoms in the polyoxoanions. In contrast, Wang and co-workers [9] observed no significant effect of the central atom on hydrogen evolution from 20% methanol aqueous solution catalyzed by (HTEA) 2  using a modification of the published method for TMA-P-Pt [5]. The synthesis and characterization of TMA-Si-Pt and TMA-Ge-Pt are described in the supporting information. The crystallization of TMA-P-Pt, TMA-Si-Pt, and TMA-Ge-Pt for X-ray crystallography is also described in the supporting information. All of the reagents and solvents were obtained from commercial sources and were used as received.

X-Ray Crystallography
A yellow block crystal of TMA-P-Pt (0.080 × 0.040 × 0.040 mm), TMA-Si-Pt (space group P2 1 /n) (0.060 × 0.060 × 0.050 mm), TMA-Si-Pt (space group P2 1 /c) (0.080 × 0.040 × 0.040 mm), TMA-Ge-Pt (space group P2 1 /n) (0.030 × 0.030 × 0.020 mm), and TMA-Ge-Pt (space group P2 1 /c) (0.050 × 0.030 × 0.020 mm) was mounted on a loop or MicroMount. The measurement for TMA-Ge-Pt (space group P2 1 /n) were obtained using a Rigaku VariMax with a Saturn diffractometer using multi-layer mirrormonochromated Mo Kα radiation (λ = 0.71075 Å) at 100 ± 1 K. The measurement for TMA-P-Pt, TMA-Si-Pt (space group P2 1 /c), TMA-Si-Pt (space group P2 1 /c), and TMA-Ge-Pt (space group P2 1 /c) was carried out using a Rigaku VariMax with an Xta-LAB P200 diffractometer using multi-layer mirror-monochromated Mo Kα radiation (λ = 0.71075 Å) at 153 ± 1 K. Data were collected and processed using CrystalClear, CrystalClear-SM Expert for Windows, and structural analysis was performed using CrystalStructure for Windows. The structure was solved by SHELXS-2013 and refined by SHELXL-2014 [12]. For the three platinum-coordinated polyoxoanions, 11 tungsten atoms, 2 platinum atoms, single phosphorus atom (or a silicon atom and a germanium atom), 4 nitrogen atoms, and 39 oxygen atoms were clarified. The three tetramethylammonium ions for TMA-P-Pt and four ions for TMA-Si-Pt and TMA-Ge-Pt were also identified; however, the solvated water molecules could not be modeled due to the disorder of the atoms. Accordingly, the residual electron density was removed using the SQUEEZE routine in PLATON [13]. We noticed that at least two polymorphisms with space group of P2 1 /n and P2 1 /c were contained in the crystals of TMA-Si-Pt and TMA-Ge-Pt. The crystal data, molecular structures, and unit cell packings are shown in the supporting information.

Photocatalytic Reaction
Typical

Results and Discussion
To investigate the influence of central atoms in α-Keggin diplatinum(II)-coordinated polyoxotungstates on photocatalytic hydrogen evolution, we synthesized two platinum at 25°C under atmospheric conditions, as previously reported for TMA-P-Pt (see supporting information; Scheme S1 and Scheme S2) [5]. TMA-Si-Pt and TMA-Ge-Pt were finally isolated as analytically pure, yellow crystalline powder at 31% and 44% yield, respectively. A single crystal of TMA-Si-Pt and TMA-Ge-Pt for X-ray crystallography was obtained by vapor diffusion from water/ethanol at 25˚C. The TMA-P-Pt was successfully crystallized by vapor diffusion from water/acetone at 25˚C.
The elemental analyses results were in good agreement with the calculated values for the chemical formula of TMA-Si-Pt and TMA-Ge-Pt with 13 and 11 hydrated water molecules (see supporting information). The weight loss observed during drying before analysis was 4.42% and 4.52% for TMA-Si-Pt and TMA-Ge-Pt, corresponding to nine weakly solvated or adsorbed water molecules. In contrast, during TG/DTA under atmospheric conditions, a weight loss of 6.05% and 5.22% was observed below 259.8˚C and 210.8˚C, corresponding to 13 and 11 water molecules, respectively ( Figure S1 and Figure S2). Therefore, the number of water molecules (13) and (11) (4) and (2) Tables S1-S5. For the all compounds, the three and four tetramethylammonium ions were observed by X-ray crystallography; however, the hydrated water molecules could not be identified because of the disorder. The X-ray crystallography of these platinum compounds revealed that the two cis-platinum(II) moieties, [cis-Pt(NH 3 ) 2 ] 2+ , were coordinated each to the two oxygen atoms in a mono-vacant site of [XW 11 O 39 ] (12−n)− (X n+ = Si 4+ , Ge 4+ , P 5+ ), as previously reported for Cs-P-Pt [6]. The TMA-Si-Pt and TMA-Ge-Pt crystals contained at least two polymorphisms with space group of P2 1 /n and P2 1 /c, while the TMA-P-Pt and Cs-P-Pt crystals did not contain polymorphisms for at least a dozen measurements.
The FTIR spectra of TMA-Si-Pt and TMA-Ge-Pt measured as KBr disks are shown in Figure S13 and Figure S14. The spectral patterns of TMA-Si- Pt (1006, 950, 892, 875, 840, 790, 734, 708, and 524 cm −1 ) and TMA-Ge- Pt (958,950,876,839,816,790,768,717 The 1 H NMR spectra of TMA-Si-Pt in DMSO-d 6 showed two signals at 4.33 ppm and 4.37 ppm with 1:1 intensities ( Figure S15). As previously reported for TMA-P-Pt and Cs-P-Pt, the two 1 H signals were due to the two sets of NH 3 ligands coordinated to the platinum sites [5] [6]. For TMA-Ge-Pt, a broad signal was observed at 4.37 ppm because of an overlap of the two signals of NH 3 ligands ( Figure S16).
The UV-Vis spectra ( Figure S17 and Figure S18) of TMA-Si-Pt and TMA-Ge-Pt in water showed two broad absorption bands at 322 nm (ε 6775 M −1 cm −1 ) and 406 nm (ε 1223 M −1 cm −1 ), and 323 nm (ε 7204 M −1 cm −1 ) and 409 nm (ε 1087 M −1 cm −1 ). The bands at 322 nm and 323 nm were assigned to the charge transfer bands of W-O and a broad band at 406 nm and 409 nm due to the two platinum(II) atoms, as previously reported for TMA-P-Pt and Cs-P-Pt [5] [6].
The photocatalytic activities of TMA-P-Pt, TMA-Si-Pt, and TMA-Ge-Pt at 0.5, 1.0, and 2.0 μmol Pt were determined for the evolution of hydrogen from 100 mM aqueous TEA solution (pH 7.0) under light irradiation (λ ≥ 400 nm) in the presence of EY, K-Si-Al, and TiO 2 . Here, TEA was employed as an electron donor. During the photoreactions, the platinum and aluminum compounds and EY were soluble in the aqueous TEA solution. Hydrogen was formed with 100% selectivity, and O 2 , CO 2 , CO, and CH 4 were not detected under these reaction conditions. For the three platinum compounds, the amount of hydrogen increased with time, as shown in Figure 1 and Figure S19. In the platinum range of 0.5 -2.0 μmol, the order of TONs after 5 h was TMA-P-Pt > TMA-Ge-Pt > TMA-Si-Pt, as shown in Table 1.   Before light irradiation, all spectra were the same as those of EY; thus, the bands of the platinum compounds were not observed because they were hidden by large bands of EY. During the light irradiation, a large band at approximately 520 nm was sifted to approximately 490 nm, which was assigned to a fluorescein-like species, as previously reported [14] [15]. The shift was observed after at least 0.5 h of light irradiation, and the absorbance gradually decreased with time. We noticed that the rates of decrease in absorbance for TMA-Ge-Pt were much slower than those for TMA-P-Pt and TMA-Si-Pt ( Figure 2). Thus, a decomposition of fluorescein-like species was the most restrained in the presence of TMA-Ge-Pt. In contrast, a new broad band due to HPB was observed at around 650, 648, and 654 nm under light irradiation (Figure 3) [8] [16]. During 4 h of light irradiation, the absorbance of HPB formed by reduction of W(VI) sites in the platinum compounds increased with time, decreasing in the order TMA-P-Pt > TMA-Ge-Pt > TMA-Si-Pt, which is consistent with that of hydrogen generation. In contrast, the absorbance of HPB in TMA-P-Pt and TMA-Si-Pt   Further studies into these reaction mechanisms are in progress, and we will report the results in due course.

Summary
Monomeric diplatinum complexes composed of mono-lacunary α-Keggin silico-and germanotungstates were synthesized and characterized by X-ray structure analysis,

Supporting Information
Synthesis     Figure S4. Solid-state packing of TMA-P-Pt.      Figure S10. Solid-state packing of TMA-Ge-Pt (space group: P2 1 /n). dissolved in 150 mL of water at 25˚C. After stirring for 10 days at 25˚C, a yellow precipitate was formed. The yellow precipitate was removed off through a membrane filter (JG 0.2 μm), and solid (CH 3 ) 4 NCl (3.50 g; 16.0 mmol) was added to the filtrate, and stirred for 3 hours in an ice bath. Then, a yellow precipitate was collected by a membrane filter (JG 0.2 μm), and washed with a small amount of ethanol. For purification,  Figure S12. Solid-state packing of TMA-Ge-Pt (space group: P2 1 /c). Table S1. Selected bond lengths and angles around the platinum centers of TMA-P-Pt.