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

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.