Three Banana-Shaped Arsenomolybdates Encapsulating a Hexanuclear Transition-Metal Central Magnetic Cluster: [AsIII2FeIII5MMo22O85(H2O)]n− (M = Fe3+, n = 14; M = Ni2+ and Mn2+, n = 15)

Three polyoxometalates encapsulating high-nuclearity magnetic clusters MFe5, [As2MFe5Mo22O85(H2O)]n− (M = Fe3+, n = 14; M = Ni2+ and Mn2+, n = 15), were synthesized and characterized by single-crystal X-ray diffraction, elemental analysis, infrared spectroscopy, thermogravimetric analysis, and magnetism measurements. The polyanion [As2MFe5Mo22O85(H2O)]n− consists of a central MMo7O28 (M = Fe3+, Ni2+, and Mn2+) fragment and two AsMo7O27 fragments linked together by two trimeric clusters, Fe2MoO(μ2–O)2 and Fe3(H2O), to form a banana-shaped structure with C1 symmetry. The MMo7O28 and AsMo7O27 units have a similar structure and can be considered as a monocapped hexavacant α-B-Keggin subunit with a central MO4 group or a central AsIIIO3 group. The polyoxometalates have a low absorption of υ(Mo–Od) (925–913 cm–1) because most of the Mo atoms in the polyanions have at least two longer Mo–Od bonds. The framework of the arsenomolybdates is stable before As2O3 escaping (ca. 300 °C). The analysis of magnetostructural correlations and magnetism measurements indicate the coexistence of ferro- and antiferromagnetic interactions, which give an overall ferromagnetic spin ground state in the compounds.