Crystal engineering of molecular to nonmolecular metal malonates in the presence of piperazine: Role of metal ions in tuning architectures

Abstract

Growth of transition metal malonate (mal) based solids from aqueous solution is investigated in the presence of piperazine (pip). Reaction under ambient conditions favored the growth of [{M(mal)₂(H₂O)₂}{H₂pip}] (M: Ni, 1; M: Co, 2), [{Cu(mal)₂(H₂O)₂}{H₂pip}].2H₂O (3), and, [Zn(mal)(pip)(H₂O)] (4). While 1-3 are molecular solids, 4 is a 2-D coordination polymer. In the case of cobalt, higher acidic condition favored the formation of a molecular organic-inorganic salt, [(H₂pip){CoCl₄}] (5). The same reaction under a different crystallization condition, i.e., lesser polar solvent and slightly higher temperature (solvothermal) led to a 2-D coordination polymer [{Co(mal)(H₂O)(Hpip)}]Cl (6). A retrosynthesis approach has been employed to interpret the crystallization reaction and compare the supramolecular reactivity of aggregating metal tectons in engineering the molecules to form either a molecular solid or coordination polymer.