Syntheses and Characterizations of Chiral Tetrahedral Cobalt Phosphates with Zeolite ABW and Related Frameworks

The hydrothermal syntheses, X-ray crystal structures, and magnetic properties of a family of 3-D chiral framework cobalt phosphates are presented. Two different types of framework structures are described:  one has the same topology as the well-known zeolite ABW framework and represents the only pure cobalt phosphate framework with a known zeolite topology; the other exhibits a framework connectivity which can be considered as a hybrid of tridymite and ABW frameworks. These new chiral materials were obtained by systematic chemical variation, accurate control of solution pH values, and use of non-aqueous solvents. Synthesis conditions were found which favored the crystallization of Co²⁺ oxygen tetrahedra in strict alternation with P⁵⁺ oxygen tetrahedra from cobalt phosphate solutions in the presence of Na⁺, K⁺, NH₄⁺, or Rb⁺ ions. The transition from one framework type to the other is affected by the size of these extra-framework cations. The two ABW structures (NH₄CoPO₄-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 framework can be viewed as layers of hexagonal rings directly interconnected by oxygen bridges. The enantiomorphic purity and crystal twinning habits of these materials are correlated with the cation type and synthesis conditions. Magnetic susceptibility measurements at temperatures above 10 K are consistent with pure paramagnetic tetrahedral Co²⁺ displaying the Curie−Weiss behavior. At lower temperatures, a ferromagnetic ordering transition is observed with ordering temperatures at or below 3 K.