Crystallization of phosphomolybdate clusters mediated by copper azole complexes: Influence of pH and temperature

This paper explores reactivity of phosphomolybdate clusters in the presence of in situ generated copper azole complexes under hydrothermal condition using two isomeric azole ligands viz. imidazole (imi) and pyrazole (pz). A systematic analysis of the synthetic variables suggested that with an increase in pH and temperature, the ligand imi favored the reduction of metal center. At room temperature (Himi)3[{CuII(imi)3(H2O)2}{HP2MoVI5O23}]·3H2O (1) was obtained. Under hydrothermal condition, at lower pH and temperature, fully oxidized solids (Himi)[{CuII(imi)4}2(HP2MoVI5O23)]·3H2O (2) and (Himi)5{HP2MoVI5O23}·4H2O (3) were crystallized. An increase in pH and temperature, resulted in the crystallization of (imi)(Himi)2[{CuI(imi)2}2H2P2MoVI5O23] (4), (imi)(Himi)2[{CuI(imi)2}2H2P2MoVI5O23]·H2O (5) and (Himi)6{CuI(imi)2}2{CuI(imi)2(H2O)}2[Na{(HPO4)4MoV6(OH)3O12}2]·7H2O (6). Unlike imi, pz did not cause the reduction of metal centers and only two fully oxidized solids [{CuII(pz)2}P2MoVI2O12(H2O)2] (7) and [{CuII(pz)4}2{H2P2MoVI5O23}]·H2O (8) were obtained. Solids 7 and 8 were found to be stable over a wider pH and temperature range owing to the better complexing ability of pz as compared to imi.