Syntheses and catalytic properties of two new multi-manganese-substituted silicotungstates

Transition-metal-substituted polyoxometalates have attracted attention due to their remarkable physicochemical properties and applications in many research areas. Two new multi-manganese-substituted silicotungstates, tetramer [{Mn₃Cl(OH)₃(SiO₄)}₄(SiW₉O₃₄)₄]^36- (1) and banana-shaped [((MnOH₂)_0.5Mn₂SiW₉O₃₄)₂(SiW₆O₂₆)]^22- (2), were prepared under hydrothermal conditions and characterized by single-crystal X-ray diffraction, FT-IR spectroscopy, etc. Compound 1 comprises four {SiW₉O₃₄} fragments, one {Mn₃Cl(OH)₃}₄ cluster and four {SiO₄} groups; it can form a 1 D-chain by connecting [{Mn₃Cl(OH)₃(SiO₄)}₄(SiW₉O₃₄)₄]³⁶⁻ cluster units with Na⁺ cations. Banana-shaped 2 consists of two {(Mn^IIOH₂)_0.5Mn^III₂SiW₉O₃₄} fragments connected by a hexavacant {SiW₆O₂₆} motif, and it displays an interesting 3-D architecture by bridging banana-shaped [((Mn^IIOH₂)_0.5Mn^III₂SiW₉O₃₄)₂(SiW₆O₂₆)]^18- polyanion with four-member Na-cluster. 3-D packing of 2 along the c-axis creates two types of elliptical channels with sizes of 7.86 Å × 13.35 Å and 16.03 Å × 25.28 Å, respectively. Also, while coupling with [Ir(ppy)₂(dtbbpy)]⁺ as photosensitizer and triethanolamine (TEOA) as sacrificial electron donor, both compounds show decent catalytic activity towards hydrogen production under visible LED light irradiation.