pH-dependent assembly of saturated to dilacunary Keggin-based silicotungstate: [Cu(en)2(H2O)][H2en]{-SiW10O36[Cu(en)2(H2O)]2}•7.5H2O

By controlling the pH of the reaction system, a dilacunary -Keggin silicotungstate [Cu(en)(2)(H2O)][H(2)en]{-SiW10O36[Cu(en)(2)(H2O)](2)}center dot 7.5H(2)O (1) (en=ethylenediamine) has been hydrothermally synthesized by the reaction of CuCl2 with -H4SiW12O40 and characterized by IR spectra, thermogravimetric analysis, and single-crystal X-ray diffraction. The product is prepared in the pH range 7.9-8.2, which reveals that pH plays a key role in the assembly of saturated -H4SiW12O40 to dilacunary [-SiW10O36](8-) polyoxometalate. The polyoxoanion {-SiW10O36[Cu(en)(2)(H2O)](2)]}(4-) of 1 presents a rare [-SiW10O36](8-)-retaining structure, in which two lacunary sites of [-SiW10O36](8-) are unoccupied, meanwhile, two [Cu(en)(2)(H2O)](2+) groups are grafted on either side of the [-SiW10O36](8-) unit by Cu-O-W bond.