Synthesis and crystal structure of a neutral open framework cobalt(II) phosphate Co₆(PO₄)₄·7H₂O with a channel structure

Abstract

Using tri-ethyl phosphate as a phosphate source, the hydrothermal reaction of cobalt(II) oxalate di-hydrate, zinc oxide and 1,8 di-amino octane at 200°C gave purple crystals of Co₆(PO₄)₄ · 7H₂O (1), along with a mixture of open-framework zinc–cobalt phosphates Co–Zn–HPO₄, and Co₃(HPO₄)₂(2OH). Compound 1, has been characterized by thermal analysis, FTIR and single crystal X-ray diffraction. The single crystal structure of Co₆(PO₄)₄ · 7H₂O reveals cobalt in four, five and six-fold coordination with linkages through the bridging water molecules and the oxygen atoms of the phosphate in the subunits. Four subunits are connected together through the oxygen atoms (PO₄), to form the three dimensional open framework structure, with a 20-member ring channel that hosts two uncoordinated water molecules. Thermal removal of the water molecules occurs between 400–600°C, with the collapse of the structure above 600°C.

Keywords:

• Cobalt phosphate
• Zeotype
• Open framework
• Hydrothermal

Acknowledgments

The authors acknowledge support from the Welch Foundation, NSF supported Center for Environmental Beneficial Catalysis, CEBC (funding #EEC 0310689). This work made use of the Cornell Center for Materials Research Facilities supported by the National Science Foundation under Award number DMR-0520404, and the single crystal diffractometer was funded by NSF grant 0087210, by Ohio Board of Regents grant CAP-491, and by YSU.