{Ta12}/{Ta16} cluster-containing polytantalotungstates with remarkable photocatalytic H2 evolution activity.

Four novel polytantalotungstates K(5)Na(4)[P(2)W(15)O(59)(TaO(2))(3)]·17H(2)O (1), K(8)Na(8)H(4)[P(8)W(60)Ta(12)(H(2)O)(4)(OH)(8)O(236)]·42H(2)O (2), Cs(3)K(3.5)H(0.5)[SiW(9)(TaO(2))(3)O(37)]·9H(2)O (3), and Cs(10.5)K(4)H(5.5)[Ta(4)O(6)(SiW(9)Ta(3)O(40))(4)]·30H(2)O (4) were synthesized. Compounds 1 and 3 are tris-(peroxotantalum)-substituted Dawson- and Keggin-type derivatives, whereas 2 and 4 are tetrameric oligomers containing respectively an unprecedented {Ta(12)} and {Ta(16)} cluster core. The photocatalytic activities of 2 and 4 for H(2) evolution from water were evaluated. The significantly enhanced performance against the control K(6)[P(2)W(18)O(62)] can be attributed to the modulation of the electronic structures of these novel POMs by Ta incorporation. The highest activity observed so far with the use of 2 can be further rationalized by the presence of distorted heptacoordinate Ta atoms in the form of TaO(7) pentagonal bipyramid.