The First Open-Framework Cadmium Phosphate, K4[Cd3(HPO4)4(H2PO4)2], with a Layered Structure

A layered cadmium phosphate of the formula K4[(Cd3(HPO4)4(H2PO4)2], I, containing six-membered apertures, constituting the first example of an open-framework cadmium phosphate, has been synthesized under hydrothermal conditions. Crystal data: monoclinic, P21/n (No. 14), a=9.0698(3), b=6.7341(3), and c=18.2663(7) Å, β=93.092(1)°, V=1114.03 Å3, Z=4. The structure consists of a network of CdO6 octahedra and PO3(OH) and PO2(OH)2 tetrahedra forming a spiro unit, which are connected through Cd2O2 two-membered rings forming one-dimensional chains. The chains are further connected forming two-dimensional macroanionic layers with six-membered apertures possessing hanging phosphate units on either side of the layer. The layers are stabilized by strong hydrogen bond interactions between the terminal P–OH groups forming pseudo channels in which the charge-compensating extraframework K+ ions are located. The presence of the spiro unit and the infinite one-dimensional Cd–O–Cd chains, along with the hanging phosphates are noteworthy structural features.