Manganese citrate complexes: syntheses, crystal structures and thermal properties

Manganese citrate complexes M2n[Mn2(Hcit)2(H2O)2]n [M = [image omitted] (1); K+ (2)] and [Mn(H2O)2]n[Mn2(Hcit)2(H2O)4]n 4nH2O (3) (H4cit = citric acid) were prepared by reaction of MnCl2 and citric acid when the pH was adjusted to 4.5 by NH4OH, KOH and NaOH, respectively. The isolated complexes were characterized by elemental analyses, IR spectra and X-ray crystal structures. Citrate binds the Mn(II) tridentately via its -hydroxyl, -carboxylate and one of the -carboxylate groups. The carboxylate-bridged di-manganese cores in 1 and 2 were interlocked by -carboxylates of the citrate chains into 2-D layers, forming a 3-D network by ammonium in 1 and K+ in 2. Complex 3, with a tetrahedral water pentamer, forms a 3-D structure through linking the dimeric unit [Mn2(Hcit)2(H2O)4] by uncoordinated oxygens of -carboxylate groups. The structural diversities of these complexes demonstrate that countercations play essential roles in the construction of the different frameworks of manganese(II) citrate complexes. Decompositions of 1 and 3 result in formations of pure Mn3O4 phase at 500C.