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, [(CH₃)₄N]₃[PW₁₁O₃₉{cis-Pt(NH₃)₂}₂]×10H₂O (TMA-P-Pt) [5] and Cs₃[α-PW₁₁O₃₉{cis-Pt(NH₃)₂}₂]∙8H₂O (Cs-P-Pt) [6] , and have constructed a novel photocatalytic system with Cs-P-Pt, eosin Y (EY), α-Keggin mono-aluminum-substituted polyoxotungstate K₅[α-SiW₁₁{Al(OH₂)}O₃₉]∙7H₂O (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 (l ≥ 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 XW₁₂O₄₀⁽⁸⁻ⁿ⁾⁻ (Xⁿ⁺ = P⁵⁺, Ge⁴⁺, Si⁴⁺, B³⁺) 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{[Na(TEA)₂]H[SiW₁₂O₄₀]}×5H₂O, (HTEA)₂{[Na(TEA)₂][PW₁₂O₄₀]}×4H₂O, and (HTEA)₂{[Na(TEA)₂]H[GeW₁₂O₄₀]}×4H₂O with 1% Pt co-catalyst under a 300 W Xe lamp (without filter).

In this study, we synthesized the tetramethylammonium salts of Keggin-type diplatinum(II)-coordinated silico- and germanotungstates, [(CH₃)₄N]₄[α-SiW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・13H₂O (TMA-Si-Pt) and [(CH₃)₄N]₄[α-GeW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・11H₂O (TMA-Ge-Pt), by the reaction of mono- lacunary Keggin-type silico- or germanotungstate with cis-diamminedichloroplati- num(II) (cisplatin) in an aqueous solution. These compounds were characterized by X-ray crystallography, elemental analysis, thermogravimetric/differential thermal analyses (TG/DTA), Fourier transform infrared (FTIR), cyclic voltammetry, and solution ¹H nuclear magnetic resonance (NMR) spectroscopy. The molecular structure of TMA-P-Pt was also determined by X-ray crystallography. Furthermore, we used TMA- P-Pt, TMA-Si-Pt, and TMA-Ge-Pt as co-catalysts, and investigated their photocatalytic activities for hydrogen evolution from aqueous TEA solutions under light irradiation (l ≥ 400 nm) in the presence of EY, K-Si-Al, and titanium dioxide. The rates of EY (and fluorescein-like species) decomposition and the amounts of HPB anions were also monitored by UV-Vis spectroscopy.

To investigate the influence of central atoms in α-Keggin diplatinum(II)-coordinated polyoxotungstates on photocatalytic hydrogen evolution, we synthesized two platinum compounds, [(CH₃)₄N]₄[α-SiW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・13H₂O (TMA-Si-Pt) and [(CH₃)₄N]₄[α-GeW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・11H₂O (TMA-Ge-Pt), by a 2:1 stoichiometric reaction of cis-diamminedichloroplatinum(II) (cisplatin) with mono-lacunary Keggin silico- and germanotungstates, [SiW₁₁O₃₉]⁸⁻ and [GeW₁₁O₃₉]⁸⁻, in an aqueous solution 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) observed by TG/DTA under atmospheric conditions was consistent with the sum of hydrated water molecules (4) and (2) indicated by elemental analysis and the number of water molecules (9) corresponding to the weight loss observed during drying before analysis. In addition, a weight loss of 10.5% and 9.89% was observed in the temperature range from 259.8˚C to 420˚C and 210.8˚C to 403.4˚C, corresponding to the sum of four tetramethylammonium ions and four ammonia molecules, respectively.

The molecular structures and solid-state packings of TMA-P-Pt, TMA-Si-Pt, and TMA-Ge-Pt are illustrated in Figures S3-S12. Selected bond lengths and angles around the platinum centers are summarized in 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(NH₃)₂]²⁺, were coordinated each to the two oxygen atoms in a mono-vacant site of [XW₁₁O₃₉]^(12−n)− (Xⁿ⁺ = Si⁴⁺, Ge⁴⁺, P⁵⁺), 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 P2₁/n and P2₁/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, 697, and 526 cm⁻¹) were different from those of K₈[SiW₁₁O₃₉]・17H₂O (997, 961, 896, 799, 733, 539, and 515 cm⁻¹) and K₆Na₂[GeW₁₁O₃₉]・12H₂O (957, 880, 848, 799, 720, 518, and 466 cm⁻¹), suggesting that the two platinum(II) moieties were coordinated to the vacant site of [SiW₁₁O₃₉]⁸⁻ and [GeW₁₁O₃₉]⁸⁻.

The ¹H NMR spectra of TMA-Si-Pt in DMSO-d₆ 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 ¹H signals were due to the two sets of NH₃ 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 NH₃ 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 TiO₂. 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 O₂, CO₂, CO, and CH₄ 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.

To determine whether the amounts of HPB anions of TMA-P-Pt, TMA-Si-Pt, and TMA-Ge-Pt were influenced by the central atoms or not, UV-Vis spectra of a mixture of EY (2.5 μmol) and platinum compounds (2.0 μmol Pt) were observed in 100 mM aqueous TEA solution at pH 7 (10 mL) under the light irradiation (λ ≥ 400 nm).

^aReaction conditions: platinum complexes (0.2 - 2.0 μmol of Pt), K-Si-Al (2.5 μmol), EY (2.5 μmol), TiO₂ (anatase:rutile = 80:20) 50 mg, 100 mM TEA aq. soln. (pH 7; 10 mL), 25˚C, light (λ ≥ 400 nm); ^bTurnover number (TON) = 2[H₂ evolved (mol)]/[Pt atoms (mol)]; ^cWithout TiO₂.

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

decreased after 5 h of light irradiation, and the order of absorbance changed to TMA-Ge-Pt > TMA-P-Pt > TMA-Si-Pt. The decrease of absorbance might be due to the lifetime of HPB and/or the remaining amounts of fluorescein-like species. However, the order of photocatalytic activities did not change during 5 h of irradiation even under the same reaction conditions as those of UV-vis measurements (Table 1). Thus, the photocatalytic activities of platinum compounds were not decided by the amounts of sensitizers alone (i.e., fluorescein-like species and HPB anions, under the present reaction conditions).

Here, the cyclic voltammograms of TMA-P-Pt, TMA-Si-Pt, and TMA-Ge-Pt were measured in 0.1 M NaClO₄ aqueous solution (pH 5.5) at a scan rate of 100 mV/s (Figure 4). The TMA-P-Pt and TMA-Ge-Pt had two reduction potentials (values at the maximum of the first and second reduction waves) at (−0.837 and −1.05 V) and (−1.02 and −1.31 V) in the potential range from 0.0 to −1.5 V, and TMA-Si-Pt showed a reduction potential at approximately −1.07 V; these are likely one-electron reduction of W(VI) sites at pH 5.5 [17] . These redox pairs were reproducible in multiple scans at scan rates of 10-300 mV/s, revealing that the platinum compounds were stable during the reversible redox processes. This was also supported by the result that no clear deactivation was observed during 6 h of light irradiation (Figure 1). These results suggest that a slower decrease of absorbance observed in TMA-Ge-Pt after 5 h of light irradiation was not due to the stability of platinum compounds. When mono-lacunary

Keggin-type polyoxotungstates [XW₁₁O₃₉]^(12−n)− (Xⁿ⁺ = Si⁴⁺, Ge⁴⁺, P⁵⁺) were used instead of platinum compounds under the same reaction conditions as those of UV-Vis measurements, a remarkable difference in the UV-Vis spectra of [GeW₁₁O₃₉]⁸⁻ was not observed during 5-h light irradiation. These results suggested that a coordination of cis-platinum(II) moieties to the mono-vacant site in Keggin-type germanotungstate caused a specific influence to a decomposition of fluorescein-like species, and a long-lived fluorescein-like species could cause a slower decrease of absorbance of HPB in TMA-Ge-Pt. Further studies into these reaction mechanisms are in progress, and we will report the results in due course.

Monomeric diplatinum complexes composed of mono-lacunary α-Keggin silico- and germanotungstates were synthesized and characterized by X-ray structure analysis, elemental analysis, TG/DTA, FTIR, cyclic voltammetry, UV-Visible, and ¹H solution NMR spectroscopy. We successfully obtained single crystals of tetramethylammonium salts [(CH₃)₄N]₄[α-SiW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・13H₂O (TMA-Si-Pt) and [(CH₃)₄N]₄[α-GeW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・11H₂O (TMA-Ge-Pt) by reacting cisplatin with mono-lacunary α-Keggin polyoxoanions, followed by vapor diffusion from water/ethanol. The crystals of TMA-Si-Pt and TMA-Ge-Pt contained at least two polymorphisms with space group of P2₁/n and P2₁/c. Furthermore, single crystals of a tetramethylammonium salt of α-Keggin diplatinum-coordinated phosphotungstate [(CH₃)₄N]₃[PW₁₁O₃₉{cis-Pt(NH₃)₂}₂]×10H₂O (TMA-P-Pt) were obtained via vapour diffusion from water/acetone, and polymorphism was not observed under the present conditions. Single-crystal X-ray structure analyses of the three platinum compounds revealed that the two cis-platinum(II) moieties, [cis-Pt(NH₃)₂]²⁺, were coordinated each to two oxygen atoms in a mono-vacant site of [XW₁₁O₃₉]^(12−n)− (Xⁿ⁺ = Si⁴⁺, Ge⁴⁺, P⁵⁺). The three platinum compounds exhibited hydrogen evolution from aqueous TEA solution under visible light irradiation (λ ≥ 400 nm) in the presence of eosin Y, α-Keggin mono-aluminum-substituted silicotungstate, and titanium dioxide. The order of photo- catalytic activities was TMA-P-Pt > TMA-Ge-Pt > TMA-Si-Pt; thus, the central atoms in platinum compounds affected the photocatalytic activities. The rates of fluorescein- like species decomposition and the amounts of HPB anions were also influenced by the central atoms, and TMA-Ge-Pt exhibited a specific influence. However, the order of rates in fluorescein-like species decomposition and amounts of HPB anions did not necessarily accord with that of photocatalytic activities under the present reaction conditions.

This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan. We also acknowledge Mr. Hattori (Shizuoka University) for preliminary experiments regarding the synthesis and crystallization of TMA-P-Pt and TMA-Si-Pt.

Kato, C.N., Suzuki, S., Ihara, Y., Aono, K., Yamashita, R., Kikuchi, K., Okamoto, T. and Uno, H. (2016) Hydrogen Evolution from Water under Visible-Light Irradiation Using Keggin-Type Pla- tinum(II)-Coordinated Phospho-, Silico-, and Germanotungstates as Co-Catalysts. Mo- dern Research in Catalysis, 5, 103-129. http://dx.doi.org/10.4236/mrc.2016.54010

Synthesis and crystallization of [(CH₃)₄N]₄[α-SiW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・13H₂O

(TMA-Si-Pt): K₈[SiW₁₁O₃₉]・17H₂O (0.659 g; 0.20 mmol) was added to a solution of cis-Pt(NH₃)₂Cl₂ (0.120 g; 0.40 mmol) dissolved in 160 mL of water at 70˚C. After stirring for 4 h at 70˚C, a yellow precipitate was formed. The yellow precipitate was removed off through a membrane filter (JG 0.2 μm), and solid (CH₃)₄NCl (13.152 g; 120 mmol) was added to the filtrate, and stirred for two days at 25˚C. Then, a yellow precipitate was collected by a membrane filter (JG 0.2 μm). For purification, the crude product was dissolved in H₂O (5 mL of water per 100 mg of crude product) at 70˚C, followed by crystallization from ethanol at 25˚C. A yellow precipitate was collected by a membrane filter (JG 0.2 μm). The obtained product was dissolved in water (15 mL of water per 100 mg of product) at 70˚C, and crystallized from ethanol at 25˚C again. The obtained product was 0.1145 g (the yield calculated on the basis of [mol of TMA-Si-Pt]/ [mol of K₈[SiW₁₁O₃₉]・17H₂O] ´ 100 was 31%). For the X-ray crystal measurement, the crystals were obtained by vapor diffusion from water (1 - 1.5 mL of water per 10 mg

Scheme S1. Synthetic diagram of TMA-Si-Pt.

Scheme S2. Synthetic diagram of TMA-Ge-Pt.

of product)/ethanol at 25˚C. Elemental analysis results showed: C, 5.21; H, 2.01; N, 3.06; Si, 0.78; Pt, 11.3%. Calculations for [(CH₃)₄N]₄[SiW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・xH₂O (x = 4) = C₁₆H₆₈N₈Pt₂O₄₃Si₁W₁₁: C, 5.49; H, 1.96; N, 3.20; Si, 0.80; Pt, 11.14%. A weight loss of 4.42% was observed during overnight drying at room temperature under 10⁻³ - 10⁻⁴ torr before analysis, suggesting the presence of 9 weakly solvated or adsorbed water molecules

(4.43%). TG/DTA under atmospheric conditions showed a weight loss of 6.05% with- out a clear endothermic point observed below 259.8˚C; calculations showed 6.39% for 13 water molecules (Figure S1). Additionally, a weight loss of 10.5% with three exothermic peaks at 268.7˚C, 291.9˚C, and 401.1˚C was observed in the temperature range

from 259.8˚C to 420˚C; calculations showed four [(CH₃)₄N]⁺ and four NH₃ molecules (total calcd: 10.0%). IR (KBr disk) (Figure S13) results in the 1300 - 400 cm⁻¹ region (polyoxometalate region) showed: 1006 m, 950 s, 892 s, 875 s, 840 s, 790 s, 734 s, 708 s, and 524m cm⁻¹. ¹H NMR (DMSO-d₆, 20.4˚C; Figure S15): 2.53 (DMSO), 3.13 (CH₃), 3.39 (H₂O), 4.33 (NH₃), and 4.37 (NH₃). UV-vis absorption (in H₂O, 7.0 ´ 10⁻⁵ M;

Figure S17) showed: λ 322 nm (ε 6775 M⁻¹cm⁻¹) and λ 406 nm (ε 1123 M⁻¹cm⁻¹).

Synthesis and crystallization of [(CH₃)₄N]₄[α-GeW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・11H₂O (TMA-Ge-Pt): A solution of K₆Na₂[GeW₁₁O₃₉]・12H₂O (0.645 g; 0.20 mmol) dissolved in 40 mL of water was added to a solution of cis-Pt(NH₃)₂Cl₂ (0.120 g; 0.40 mmol)

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 (CH₃)₄NCl (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,

the crude product (0.589 g) was dissolved in 20 mL of water at 70˚C, followed by standing in a refrigerator at around 5˚C. A yellow crystals was collected by a membrane filter (JG 0.2 μm). The obtained product was 0.320 g (the yield calculated on the basis of [mol of TMA-Ge-Pt]/[mol of K₆Na₂[GeW₁₁O₃₉]・12H₂O] ´ 100 was 44%). For the X-ray crystal measurement, the crystals were obtained by vapor diffusion from water (7.0 mL of water per 100 mg of product)/ethanol at 25˚C. Elemental analysis results showed: C, 5.25; H, 2.03; N, 3.05; Ge, 1.98; W, 57.7; Pt, 11.2%. Calculations for

[(CH₃)₄N]₄[GeW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・xH₂O (x = 2) = C₁₆H₆₄N₈Pt₂O₄₁Ge₁W₁₁: C, 5.48; H, 1.84; N, 3.19; Ge, 2.07; W, 57.62; Pt, 11.12%. A weight loss of 4.51% was observed during overnight drying at room temperature under 10⁻³ - 10⁻⁴ torr before analysis, suggesting the presence of 9 weakly solvated or adsorbed water molecules (4.42%). TG/DTA under atmospheric conditions showed a weight loss of 5.22% without a clear endothermic point observed below 210.8˚C; calculations showed 5.4% for 11 water molecules (Figure S2). Additionally, a weight loss of 9.89% with three exothermic

peaks at 268.8, 285.0, and 315.9˚C was observed in the temperature range from 210.8 to 403.4˚C; calculations showed four [(CH₃)₄N]⁺ and four NH₃ molecules (total calcd: 9.9%). IR (KBr disk) (Figure S14) results in the 1300 - 400 cm⁻¹ region (polyoxometalate region) showed: 958 s, 950 s, 876 s, 839 s, 816 s, 790 s, 768 s, 717 s, 697 s, and 526 m cm⁻¹. ¹H NMR (DMSO-d₆, 21.9˚C; Figure S16): 2.53 (DMSO), 3.17 (CH₃), 3.36 (H₂O), and 4.37 (NH₃). UV-vis absorption (in H₂O, 7.1 ´ 10⁻⁵ M; Figure S18) showed: λ 323 nm (ε 7204 M⁻¹cm⁻¹) and λ 409 nm (ε 1087 M⁻¹cm⁻¹).

Crystallization of [(CH₃)₄N]₃[PW₁₁O₃₉{cis-Pt(NH₃)₂}₂]×10H₂O (TMA-P-Pt): A powder sample of TMA-P-Pt (69.3 mg) was dissolved in 23 mL of water at 25˚C. The crystallization was performed by vapor diffusion from acetone at 25˚C for a week. The characterization results obtained by TG/DTA, ¹H and ³¹P NMR, and FT-IR spectroscopy were completely identical with those of the powder sample.

Crystal data for [(CH₃)₄N]₃[PW₁₁O₃₉{cis-Pt(NH₃)₂}₂]×10H₂O (TMA-P-Pt):

C₁₂H₆₈N₇Pt₂O₄₉P₁W₁₁; M = 3538.19, triclinic, space group: P-1 (#2), a = 13.232(3) Å, b =

13.720(3) Å, c = 17.362(3) Å, α = 89.776(4), β = 82.439(5)˚, γ = 84.986(5), V = 3112.5(11) ų, Z = 2, D_c = 3.775 g/cm³, μ (Mo Kα) = 248.425 cm⁻¹, R₁ = 0.0742 [I > 2σ (I)], wR₂ = 0.2076 (for all data). GOF = 0.996 [29787 total reflections and 13856 unique reflections where I > 2σ (I)]. CCDC No. 1472522. The molecular structure and unit cell packings of TMA-P-Pt are shown in Figure S3 and Figure S4.

Crystal data for [(CH₃)₄N]₄[SiW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・13H₂O (TMA-Si-Pt) (space group P2₁/n): C₁₆H₈₆N₈Pt₂O₅₂Si₁W₁₁; M = 3663.49, monoclinic, space group: P2₁/n (#14), a = 13.4189(16) Å, b = 24.808(3) Å, c = 22.351(3) Å, β = 105,820(3)˚, V = 7158.7(16) ų, Z = 4, D_c = 3.399 g/cm³, μ (Mo Kα) = 216.063 cm⁻¹, R₁ = 0.0620 [I > 2σ (I)], wR₂ = 0.1822 (for all data). GOF = 0.901 [68797 total reflections and 16430 unique reflections where I > 2σ(I)]. CCDC No. 1472524. The molecular structure and unit cell packings of TMA-Si-Pt (space group P2₁/n) are shown in Figure S5 and Figure S6.

Crystal data for [(CH₃)₄N]₄[SiW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・13H₂O (TMA-Si-Pt) (space group P2₁/c): C₁₆H₈₆N₈Pt₂O₅₂Si₁W₁₁; M = 3663.49, monoclinic, space group: P2₁/c (#14), a =

13.4261(10) Å, b = 24.8114(17) Å, c = 22.8205(15) Å, β = 106.679(4)˚, V = 7282.1(9) ų, Z = 4, D_c = 3.341 g/cm³, μ (Mo Kα) = 212.402 cm⁻¹, R₁ = 0.0542 [I > 2σ (I)], wR₂ = 0.0979 (for all data). GOF = 0.921 [68063 total reflections and 16679 unique reflections where I > 2σ(I)]. CCDC No. 1472523. The molecular structure and unit cell packings of TMA-Si-Pt (space group P2₁/c) are shown in Figure S7 and Figure S8.

Crystal data for [(CH₃)₄N]₄[α-GeW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・11H₂O (TMA-Ge-Pt) (space group P2₁/n): C₁₆H₈₂N₈Pt₂O₅₀Ge₁W₁₁; M = 3671.97, monoclinic, space group: P2₁/n (#14), a = 12.917(2) Å, b = 25.826(4) Å, c = 20.431(2) Å, β = 100.515(3)˚, V = 6701.1 (18) ų, Z = 4, D_c = 3.639 g/cm³, μ (Mo Kα) = 234.995 cm⁻¹, R₁ = 0.0605 [I > 2σ (I)], wR₂ = 0.1353 (for all data). GOF = 1.162 [109879 total reflections and 15397 unique reflections where I > 2σ (I)]. CCDC No. 1472526. The molecular structure and unit cell packings of TMA-Ge-Pt (space group P2₁/n) are shown in Figure S9 and Figure S10.

Crystal data for [(CH₃)₄N]₄[α-GeW₁₁O₃₉{cis-Pt(NH₃)₂}₂]・11H₂O (TMA-Ge-Pt) (space group P2₁/c): C₁₆H₈₂N₈Pt₂O₅₀Ge₁W₁₁; M = 3671.97, monoclinic, space group: P2₁/c (#14), a = 13.4248(13) Å, b = 24.909(2) Å, c = 22.767(3) Å, β = 106.708(6)˚, V = 7291.7(12) ų, Z = 4, D_c = 3.345 g/cm³, μ (Mo Kα) = 215.962 cm⁻¹, R₁ = 0.0876 [I > 2σ (I)], wR₂ = 0.2403 (for all data). GOF = 1.113 [68383 total reflections and 16705 unique reflections where I > 2σ (I)]. CCDC No. 1472525. The molecular structure and unit cell packings of TMA-Ge-Pt (space group P2₁/c) are shown in Figure S11 and Figure S12.