Trigonal Pyramidal {AsO2(OH)} Bridging Tetranuclear Rare-Earth Encapsulated Polyoxotungstate Aggregates

The one-pot assembly reaction of Na2WO4·2H2O, RE­(NO3)3·6H2O, and NaAsO2 in the participation of dimethylamine hydrochloride as an organic solubilizing agent in the acidic aqueous solution led to a class of trigonal pyramidal {AsO2(OH)} bridging rare-earth substituted arsenotungstate (AT) aggregates [H2N­(CH3)2]8Na8­{[W3RE2(H2O)8­AsO8(OH)]­[B-α-AsW9O33]2}2·65H2O [RE = EuIII (1), GdIII (2), TbIII (3), DyIII (4), HoIII (5), YIII (6)], which were structurally characterized by elemental analyses, IR spectra, single-crystal X-ray diffraction, and thermogravimetric (TG) analyses. The common structural feature of 1–6 is that their polyoxoanions consist of a novel tetrameric unit [(W3RE2(H2O)8­AsO8(OH))­(B-α-AsW9O33)2]216– constituted by four trivacant Keggin [α-AsW9O33]9– fragments linked through an unseen elliptical [W6RE4(H2O)16­As2O16(OH)2]20+ moiety. Their polyoxoanionic infrastructures can also be described as a fusion of two equivalent dimeric subunits [(W3RE2(H2O)8O7)­(B-α-AsW9O33)2]8– bridged via two μ2-{AsO2(OH)} linkers. To the best of our knowledge, such a linking mode with trigonal pyramidal {AsO2(OH)} groups as linkers connecting adjacent RE containing polyoxometalate moieties together is very rare. The thermal stability of 1–6 was also investigated on the crystalline samples, and the thermal decomposition processes of 1, 4, and 6 were comparatively deeply studied. The fluorescent properties and decay times of 1, 3, and 4 were measured, and they exhibit the characteristic emissions of RE centers. The lifetimes of 1 and 3 mainly originate from the contribution of RE ions whereas the overall lifetime of 4 is contributed by the synergistic interactions of AT fragments and Dy3+ ions.