Defect {(WVIO7)WVI4} and Full {(WVIO7)WVI5} Pentagonal Units as Synthons for the Generation of Nanosized Main Group V Heteropolyoxotungstates

We report on the synthesis and characterization of three new nanosized main group V heteropolyoxotungstates KxNay[H2(XWVI9O33)(WVI5O12)(X2WVI29O103)]·nH2O {X3W43} (x = 11, y = 16, and n = 115.5 for X = SbIII; x = 20, y = 7, and n = 68 for X = BiIII) and K8Na15[H16(CoII(H2O)2)0.9(CoII(H2O)3)2(WVI3.1O14)(SbIIIWVI9O33)(SbIII2WVI30O106)(H2O)]·53H2O {Co3Sb3W42}. On the basis of the key parameters for the one-pot synthesis strategy of {Bi3W43}, a rational step-by-step approach was developed using the known Krebs-type polyoxotungstate (POT) K12[SbV2WVI22O74(OH)2]·27H2O {Sb2W22} as a nonlacunary precursor leading to the synthesis and characterization of {Sb3W43} and {Co3Sb3W42}. Solid-state characterization of the three new representatives {Bi3W43}, {Sb3W43}, and {Co3Sb3W42} by single-crystal and powder X-ray diffraction (XRD), IR spectroscopy, thermogravimetric analysis (TGA), energy-dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and elemental analysis, along with characterization in solution by UV/vis spectroscopy shows that {Bi3W43}, {Sb3W43}, and {Co3Sb3W42} represent the first main group V heteropolyoxotungstates encapsulating a defect {(WVIO7)WVI4} ({X3W43}, X = BiIII and SbIII) or full {(WVIO7)WVI5} ({Co3Sb3W42}) pentagonal unit. With 43 tungsten metal centers, {X3W43} (X = BiIII and SbIII) are the largest unsubstituted tungstoantimonate– and bismuthate clusters reported to date. By using time-dependent UV/vis spectroscopy, the isostructural representatives {Sb3W43} and {Bi3W43} were subjected to a comprehensive study on their catalytic properties as homogeneous electron-transfer catalysts for the reduction of K3[FeIII(CN)6] as a model substrate revealing up to 5.8 times higher substrate conversions in the first 240 min (35% for {Sb3W43}, 29% for {Bi3W43}) as compared to the uncatalyzed reaction (<6% without catalyst after 240 min) under otherwise identical conditions.


General Information
All reagents and chemicals were of high-purity grade and were used as purchased without further purification. Na 9 [B-α-SbW 9 O 33 ] and K 12  Elemental analysis: Elemental analysis of W, Bi and Sb contents was performed using a Perkin Elmer Elan 6000 ICP-MS in aqueous solutions containing 2 % ultrapure HNO 3 ({Sb 3 W 43 }, {Bi 3 W 43 }) and a Thermo Scientific Nexsa Photoelectron Spectrometer using Al Kα X-rays as source (X-ray photoelectron spectroscopy XPS) ({Co 3 Sb 3 W 42 }). XPS was performed on a spot size of 400 μm with an energy step size of 1 eV for the survey and 0.1 eV for detailed analysis, respectively. Homogenized powdered samples were etched for 60 s using a low-energy Ar cluster (6000 eV, 1000 atom clusters) prior to analysis to clean the surface of the sample. To assess the reliability of the results obtained from XPS, control measurements were performed on powdered samples of the literature known POT compound K 6  Powder X-ray diffraction was performed on an EMPYREAN diffractometer system using Cu Kα radiation (λ = 1.540598), a PIXcel3D-Medipix3 1 × 1 detector (used as a scanning line detector) and a divergence slit fixed at 0.1 mm. The scan range was from 8° to 50° (2θ).

Unit cell dimensions [Å] and [°]
17.8217 (6) 76.3923 (12  Despite the weakly diffracting nature of the crystals, their quality was sufficient to obtain a simulated PXRD pattern. Note that differences between the simulated and the experimental PXRD patterns may be due to factors such as scanning speed, preferred orientation, and efflorescence of the crystals, which lose solvent molecules further leading to the collapse of the lattice.    Figure S18. UV/Vis-spectrum of an aqueous [1 mM] stock solution containing [Fe III (CN) 6 ] 3displaying a series of absorption bands at 272, 309, 325, and 420 nm, corresponding to a charge transition band and the 2 T 1g → 2 E 1g , 2 T 2u → 2 T 2g , 2 T 2g → 2 A 1g , and 2 T 1g → 2 T 2g transitions, respectively. 28

Post-catalytic POM-precipitation for subsequent analysis with ATR-IR
The catalytic reaction was carried out with 500 μM of Sb 3 W 43 or Bi 3 W 43 , 1 mM [Fe III (CN) 6 ] 3and 8.7 mM Na 2 S 2 O 3 in 1.5 mL H 2 O (pH = 6.8 via HCl [1 M]) to ensure sufficient amounts for post-analysis. After incubation at 55°C for 40 h to ensure complete reduction of the [Fe III (CN) 6 ] 3substrate, solid cesium chloride was added to the reaction mixture resulting in the immediate formation of precipitates. The precipitates were centrifuged at 2500 rpm for 5 min and completeness of the precipitation was insured by adding cesium chloride to the supernatant and by performing a subsequent reloading experiment (see section 9.2). The precipitates were air dried and displayed to IR-spectroscopic analysis (Figures S26, S27).

9.2.
Reloading experiment of the reaction mixture after post-catalytic POMprecipitation Subsequent addition of 1 mM [Fe III (CN) 6 ] 3to the remaining supernatants obtained by the procedure described in section 9.1 and incubation at 55°C to initiate a second reaction cycle showed negligent conversion of [Fe III (CN) 6 ] 3-(~8 % according to the UV/Vis spectra in Figures S28 and S29), which suggests complete removal of the corresponding POT catalyst upon addition of CsCl.

Recyclability of {Sb 3 W 43 } and {Bi 3 W 43 }
To investigate the recyclability of Sb 3 W 43 and Bi 3 W 43 , a stock solution containing 80 μM of the corresponding POT, 1 mM [Fe III (CN) 6 ] 3and 8.7 mM Na 2 S 2 O 3 in 1.5 mL H 2 O was incubated at 55°C for 72 h and the complete conversion (~98% based on the absorption at 420 nm) of the [Fe III (CN) 6 ] 3substrate was shown by UV/Vis spectroscopy. Consecutively, 5 μL of a freshly prepared solution containing [62 mM] [Fe III (CN) 6 ] 3were added to 300 μL of the incubated stock solution followed by addition of 5 μL of a 539.4 mM solution containing Na 2 S 2 O 3 to yield 310 μL of a reloaded reaction mixture with 1 mM [Fe III (CN) 6 ] 3and ~8.7 mM Na 2 S 2 O 3 final concentrations (Figures S30, S31).  Reloading of the reaction mixture with Na 2 S 2 O 3 and [Fe III (CN) 6 ] 3and consecutive incubation at 55°C leads to conversion of the substrate indicated by a decreasing peak at 420 nm thereby highlighting the recyclability of {Bi 3 W 43 }. Note that the slightly higher TOF values are attributed to the presence of additionally added reducing agent in the second cycle. Scheme S1. Schematic representation of the mechanism proposed for the POM catalyzed reduction reaction of [Fe III (CN) 6