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 = Eu(III) (1), Gd(III) (2), Tb(III) (3), Dy(III) (4), Ho(III) (5), Y(III) (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]2(16-) 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 Dy(3+) ions.