| In our pursuit of developing catalytic porous coordination networks, we designed and prepared two redox-active bis(triester)hexavanadates, [V 6O13{lcub}(CH2O)3CNHCO(4C 5H4N){rcub}2]-2 and [V6O 13{lcub}(CH2O)3CNHCO(4-C6H4COOH){rcub} 2]-2. These two bridging ligands readily react with transition metal ions to yield various redox-active open-framework materials. A pyridine-terminated bis(triester)hexavanadate, [V6O13{lcub}(CH2O) 3CNHCO(4C5H4N){rcub}2] -2, an analogue to 4,4'-bipyridine, reacts with four divalent first-row transition metal ions (MnII, CoII, Ni II or ZnII) to produce one-dimensional (1D) coordination chains connected by hydrogen bonds. The materials can be chemically reduced and re-oxidized by dioxygen. However, the pyridine-terminated hexavanadate does not form stable open-framework structures. The benzoic acid-terminated hexavanadate assembled by d-(CoII) and f-block (GdIII, TbIII, or YbIII) elements form open framework materials. Lanthanides and the benzoic acid functional groups form paddle-wheel cluster moieties to generate two-dimensional layers interconnected by hydrogen bonds between a secondary amine moiety and a bridging oxo group of the POM.{09}Among three f-block elements, only the Yb material maintains framework integrity. Thus, the coordination polymers assembled Yb catalyze aerobic oxidation of PrSH to its oxidized product PrSSPr in a heterogeneous fashion. More robust and sophisticated structures were fabricated by employing mixed-ligand approaches. In these approaches, the benzoic acid-terminated hexavanadate is mixed with conventional organic bridging ligands, such as 4,4'-biphenyldicarboxylic acid, bis(4-carboxyphenyl)acetylene, or 4,4'-bipyridine- N,N'-oxide, and the combined ligand-mixtures are complexed with Co II, or LnIII (Ln = Tb, Gd, or Yb) to yield various open-framework materials. The combination of 4,4'-bipyridine-N,N' -dioxide, the benzoic acid-terminated bis(alkoxo)he xavanadate, and TbIII lead to pillared-layer type, three-dimensional net-work materials. Although the dimensions of channels are significantly diminished in the material, the network material successfully catalyzes two oxidation reactions: PrSH to PrSSPr by air and tetrahydrothiophene (THT) to tetrahydrothiophene oxide (THTO) by tert-butylhydroperoxide. This work show a new way of building up catalytic materials with well-defined catalytically active POM units. |
