SYNTHESES AND CHARACTERIZATIONS OF CHIRAL TETRAHEDRAL COBALT PHOSPHATES WITH ZEOLITE ABW AND RELATED FRAMEWORKS

The hydrothermal syntheses, it-ray crystal structures, and magnetic pr operties of a family of 3-D chiral framework cobalt phosphates are pre sented. Two different types of framework structures are described: one has the same topology as the well-known zeolite ABW framework and rep resents the only pure cobalt phosphate framework with a known zeolite topology; the other exhibits a framework connectivity which can be con sidered as a hybrid of tridymite and ABW frameworks. These new chiral materials were obtained by systematic chemical variation, accurate con trol of solution pH values, and use of non-aqueous solvents. Synthesis conditions were found which favored the crystallization of Co2+ oxyge n tetrahedra in strict alternation with P5+ oxygen tetrahedra from cob alt phosphate solutions in the presence of Na+, K+, NH4+, or Rb+ ions. The transition from one framework type to the other is affected by th e size of these extra-framework cations. The two ABW structures (NH4Co PO4-ABW, RbCoPO4) have eight-ring channels along the monoclinic b axis ; the three hybrid hexagonal structures (NaCoPO4, KCoPO4, NH4CoPO4-HEX ) have six-ring channels along the hexagonal c-axis. The 3-D framework of the hexagonal structures is built from a unique [2.2.2]propellane- like triple four-ring unit. Both the ABW framework and the hexagonal f ramework can be viewed as layers of hexagonal rings directly interconn ected by oxygen bridges. The enantiomorphic purity and crystal twinnin g habits of these materials are correlated with the cation type and sy nthesis conditions. Magnetic susceptibility measurements at temperatur es above 10 K are consistent with pure paramagnetic tetrahedral Co2+ d isplaying the Curie-Weiss behavior. At lower temperatures, a ferromagn etic ordering transition is observed with ordering temperatures at or below 3 K.