Annealing assisted mechanochemical syntheses of transition-metal coordination compounds and co-crystal formation

This article focuses on heating or annealing process in the mechanochemical preparation of coordination compounds in the solid state. Heating of metal complexes often promotes the liberation of weakly coordinated ligands and results in the complex with its coordination geometry unsaturated. Such "activated" complexes can be used as the building blocks of coordination polymers in the solid state. As one example, our recent work on [M(CNacac)₂]_∞ (CNacac = 3-cyano-pentane-2,4-dionato, M = Mn^II, Fe^II, Co^II, Ni^II, Cu ^II and Zn^II) is presented. Further, co-grinding and annealing treatments of an enantiomeric pair of chiral crystals of [M(phen) ₃](PF₆)₂ (M = Ru^II and Os ^II, phen = 1,10-phenanthoroline) afford diffusion and rearrangement of transition metal complex ions to form co-crystal phases of racemic or quasi-racemic compounds by way of recognizing chirality. Our latest result of co-crystal formation via chirality transfer between optically stable and labile metal complexes is also briefly introduced.