Rare-Earth-Incorporated Tellurotungstate Hybrids Functionalized by 2-Picolinic Acid Ligands: Syntheses, Structures, and Properties.

A series of organic-inorganic rare-earth-incorporated tellurotungstate hybrids, Na4[RE2(H2O)4(pica)2W2O5][(RE(H2O)W2(Hpica)2O4)(B-β-TeW8O30H2)2]2·38H2O (RE = LaIII (1), CeIII (2), NdIII (3), SmIII (4), EuIII (5); Hpica = 2-picolinic acid), were prepared via a one-step assembly reaction of Na2WO4·2H2O, RE(NO3)3·6H2O, K2TeO3, Hpica, and triethylamine (tea). Notably, the solubilization of tea toward Hpica and the solubilization of Hpica toward RE cations in the reaction system play an important role in the formation of 1-5. The most significant feature of 1-5 consists of an intriguing tetrameric [RE2(H2O)4(pica)2W2O5][(RE(H2O)W2(Hpica)2O4)(B-β-TeW8O30H2)2]24- polyoxoanion constructed from two tetravacant Keggin sandwich-type [(RE(H2O)W2(Hpica)2O4)(B-β-TeW8O30H2)2]5- entities linked by a RE-W-Hpica {RE2(H2O)4(pica)2W2O5}6+ cluster, in which Hpica ligands not only play a key bridging role in linking RE and W centers by carboxylic groups in an irregular N-O-RE-O-W-O six-membered-ring motif but also can directly chelate with W centers via N and O atoms in a stable N-O-C-O-W five-membered-ring fashion. 1-5 represent rare organic-inorganic hybrid RE-substituted tellurotungstates. Moreover, the solid-state photoluminescence properties of 3-5 have been deeply investigated, and these compounds exhibit the characteristic emission stemming from intra-4f transitions of RE ions. The energy transfer of the O → W transitions sensitizing the emission of SmIII centers in 4 is convincingly proved by time-resolved emission spectra (TRES); the increase in the strongest typical emission of SmIII ions at a decay time of 17 μs is accompanied by the decline of O → W emission, and the CIE 1931 diagram was obtained from the corresponding TRES. Furthermore, a comparison of the luminescence behaviors of 5 in the solid state and in solution reveals the structural skeletal integrity of 5 in solution and a shorter decay lifetime in the solution caused by the high-frequency O-H oscillators.