A stepwise mechanism and the role of water in the liquid-assisted grinding synthesis of metal-organic materials

The mechanism of the room-temperature synthesis of coordination polymers from ZnO by liquid-assisted grinding (LAG) was investigated, and using catalytic amounts of water allowed us to extend the scope of this synthetic method to copper compounds. The mechanochemical synthesis of model compounds zinc fumarate and copper(II) acetate proceeds through a stepwise mechanism which involves the intermediate formation of solvates with water (in the case of zinc fumarate) or acetic acid (in the case of copper(II) acetate) as kinetic products. The course of zinc fumarate LAG synthesis was explored using three different types of grinding liquids: water, aqueous organic solvents and pure organic solvents. With liquid water, the formation of the kinetic product switches the reaction mechanism from LAG to a neat grinding process. As a result, the reaction scope is limited to either the tetrahydrate or the pentahydrate as the major products. In contrast, the use of aqueous organic solvents as grinding liquids allows the selective synthesis as well as screening for different hydrated and anhydrous forms of zinc fumarate. Different polymorphs of the zinc fumarate coordination polymer can be obtained by changing the organic liquid. As a first step towards the quantitative understanding of how the liquid phase directs LAG mechanosynthesis, we demonstrate that product formation is regulated by the mole fraction and activity of water in the grinding liquid.