A new Nb₂₈ cluster based on tungstophosphate, [{Nb₄O₆(OH)₄}{Nb₆P₂W₁₂O₆₁}₄]³⁶⁻.

A structurally intriguing polyanion, [{Nb4O6(OH)4}{Nb6P2W12O61}4](36-) (2), was directly formed by controlling the reaction parameters and characterized by single-crystal X-ray diffraction, IR spectroscopy, and elemental analyses. Polyanion 2 is the first niobium-substituted Wells-Dawson-based tetramer and contains the largest Nb28 cluster encapsulated by four hexalacunary Wells-Dawson frameworks. The activities of compounds Na12[H24-2]·24H2O (2a), K12[H2P2W12O48]·24H2O, and K7[HNb6O19]·13H2O against three human tumor cell lines were investigated in vitro. The preliminary results revealed that 2a could efficiently inhibit the growth of the human breast cancer MCF-7 cells. Interestingly, the activity of 2a is significantly better than those of the simple reagents.