A New Nb<sub>28</sub> Cluster Based on Tungstophosphate, [{Nb<sub>4</sub>O<sub>6</sub>(OH)<sub>4</sub>}{Nb<sub>6</sub>P<sub>2</sub>W<sub>12</sub>O<sub>61</sub>}<sub>4</sub>]<sup>36–</sup>

A structurally intriguing polyanion, [{Nb<sub>4</sub>O<sub>6</sub>(OH)<sub>4</sub>}­{Nb<sub>6</sub>P<sub>2</sub>W<sub>12</sub>O<sub>61</sub>}<sub>4</sub>]<sup>36–</sup> (<b>2</b>), was directly formed by controlling the reaction parameters and characterized by single-crystal X-ray diffraction, IR spectroscopy, and elemental analyses. Polyanion <b>2</b> is the first niobium-substituted Wells–Dawson-based tetramer and contains the largest Nb<sub>28</sub> cluster encapsulated by four hexalacunary Wells–Dawson frameworks. The activities of compounds Na<sub>12</sub>[H<sub>24</sub>-<b>2</b>]·24H<sub>2</sub>O (<b>2a</b>), K<sub>12</sub>[H<sub>2</sub>P<sub>2</sub>W<sub>12</sub>O<sub>48</sub>]·24H<sub>2</sub>O, and K<sub>7</sub>[HNb<sub>6</sub>O<sub>19</sub>]·13H<sub>2</sub>O against three human tumor cell lines were investigated in vitro. The preliminary results revealed that <b>2a</b> could efficiently inhibit the growth of the human breast cancer MCF-7 cells. Interestingly, the activity of <b>2a</b> is significantly better than those of the simple reagents.