Mechanochemical interconversion between discrete complexes and coordination networks – formal hydration/dehydration by LAG

Abstract

Using a small planetary ball mill, liquid-assisted grinding (LAG) of metal salts or oxides (ZnO, CdO, CdCO₃, Cu(OAc)₂·H₂O, Co(OAc)₂·4H₂O, Mn(OAc)₂·4H₂O, Ni(OAc)₂·4H₂O, FeSO₄·7H₂O) with two equivalents of isonicotinic acid (HINA) and small amounts of water (up to 5.6 molar equivalents) gave discrete aquo complexes trans-[M(INA)₂(OH₂)₄] (M = Zn, Cd, Cu, Fe, Co, Ni, Mn) efficiently within 30 min. For M = Zn, Cd and Cu these complexes readily undergo reversible formal dehydration to the extended network structures [M(INA)₂] (M = Zn, Cu) or [Cd(INA)₂(OH₂)]·DMF by further LAG with non-aqueous liquids such as methanol or DMF. Overall, the mechanochemical dehydrations are more effective than heating or immersion in bulk solvents. The work demonstrates a two-step mechanochemical synthesis of coordination networks via discrete aquo complexes which may be preferable to single step reactions or grinding-annealing procedures in some cases. For example, the two step method was the only way to prepare [Cd(INA)₂(OH₂)]·DMF mechanochemically and the porous network Cu(INA)₂ could not be obtained from the aquo complex by heating.