Three Transition-Metal Substituted Polyoxotungstates Containing Keggin Fragments: From Trimer to One-Dimensional Chain to Two-Dimensional Sheet

Three novel transition-metal substituted polyoxotungstates based on Keggin fragments, Cs3K3[Co(H2O)(6)](2)[Co(H2O)(3)(alpha-GeW11CoO38)(3)]center dot 30H(2)O (1), K-18{[Mn(H2O)(3)](2){Mn(H2O)(2)][(B-beta-SiW9O33(OH))Mn-3(H2O)(B-beta-SiW8O30(OH))](2)}center dot 16H(2)O (2), and K-8[Cd(H2O)(3)](2)[Cd-4(H2O)(2)(B-alpha-SiW9O34)(2)]center dot 20H(2)O (3), have been synthesized by reaction of dilacunary Keggin precursors K-8[gamma-GeW10O36]center dot 6H(2)O/K-8[gamma-SiW10O36]center dot 12H(2)O with transition-metal salts at ambient temperature and characterized by inductively coupled plasma (ICP) analyses, IR spectra, UV spectra, and single-crystal X-ray diffraction. The polyoxoanion of 1 is a novel trimer constructed from three mono-Co-II substituted Keggin fragments [alpha-GeW11CoO38](4-) linked by six W-O-Co/W bridges and a capping [Co(H2O)(3)](2+) bridge. 2 displays the one-dimensional chain built by tetrameric {[Mn(H2O)(3)](2)[Mn(H2O)(2)][(B-beta-SiW9O33(OH))Mn-3(H2O)(B-beta-SiW8O30(OH))](2)}(18-) units, which is the first one-dimensional silicotungstate containing asymmetric sandwich-type moieties constructed from [B-beta-SiW9O34](10-) and [B-beta-SiW8O31](10-) fragments. 3 utilizes the two-dimensional sheet established by tetra-Cd-II substituted sandwich-type [Cd-4(H2O)(2)(B-alpha-SiW9O34)(2)](12-) units and [Cd(H2O)(3)](2+) linkers, representing the first two-dimensional (3,6)-topological network with a Schlafli symbol of 3(6)4(6)5(3) built by sandwich-type Keggin units in polyoxometalate chemistry. Magnetic susceptibility measurements indicate antiferromagnetic exchange interactions within Co-II ions in 1 and within Mn-II ions in 2. The best least-squares fitting values for 2 are J = -1.16 cm(-1) and g = 2.13 based on the isostropic spin model. Furthermore, the room-temperature solid-state photoluminescence of 3 displays two emission bands, which are derived from O -> Cd ligand-to-metal charge transfer transitions and O -> W ligand-to-metal charge transfer transitions, respectively.