Hydrothermal synthesis of copper coordination polymers based on molybdates: Chemistry issues

Crystal engineering of metal organic coordination polymers or metal organic frameworks have been attracting unprecedented efforts in the past few years due to potential applications in the area of catalysis, gas separation and storage. Hydrothermal synthesis is a popular soft chemical route employed by a number of researchers to grow suitable single crystals for unambiguous structural characterization. The structural diversity of the various solid frameworks is attributed to the choice of metal, its ability to exist in multiple oxidation states exhibiting different coordination geometry and multidentate nature of organic ligands. Polyoxomolybdate is another interesting class of inorganic materials that are potential catalysts. A contemporary theme is to develop porous solids combining the acidic properties of oligomeric molybdates integrated with first-row transition metal ions that can form coordination polymers with suitable multidentate ligands. A bottleneck to this problem is to develop reliable synthetic protocols that can produce reproducible materials with designed structural characteristics. In this paper, we present our results on the formation of self-assembled metal organic hybrid solids from acidified aqueous molybdate solution containing cupric ions and one of the multidentate organic ligands (pyrazine, 2-pyrazine carboxylic acid or isonicotinic acid). We propose a molecular mechanism to rationalize the formation of solids.