Layered, network and chain structures of cadmium(II) malonate: Crystal structure and cadmium-113 nuclear magnetic resonance studies

Three structurally non-identical cadmium(II) malonate compounds have been obtained: polymeric tetraaquabis(malonato)dicadmium(II) 1, polymeric monoaqua(malonato)cadmium(II) monohydrate 2 and polymeric triaqua(malonato)cadmium(II) monohydrate 3. The layered-structure compound 1 is the initial product formed at room temperature, and has two cadmium sites in distinct chemical and magnetic environments: six- and eight-co-ordinate cadmium involving oxygen donors including two water molecules, characterized by two Cd-113 NMR resonances at delta 18 and -107 respectively: Compound 2, which has a three-dimensional network structure, is formed irreversibly when 1 is heated at 60 degrees C or exposed to sunlight: its seven-co-ordinate cadmium site involves one six-membered and two four-membered chelate rings and one water molecule (delta-47). Compound 3 has an unusual chain structure and is formed reversibly when 2 is heated at 100 degrees C: a different seven-co-ordinated cadmium site involves two four-membered rings and three water molecules (delta-53). The results of the cross polarization magic angle spinning Cd-113 NMR study of the six-; seven- and eight-co-ordination sites show that cadmium atoms with higher co-ordination numbers tend to be more shielded.