Complexes of apatitic layered compound Ca8(HPO4)2(PO4)4·5H2O with dicarboxylates

Octacalcium phosphate(OCP), Ca₈(HPO₄)₂(PO₄)₄·5H₂O, consists of alternative stackings of layers with an apatitic structure and a brushite-like composition. Here we consider whether or not OCP is able to complex with organic substances. The interplanar spacing (d₁₀₀) of OCP prepared in the presence of dicarboxylates (RC₂O₄ ^2–; R=organic group) expanded from the original value of 18.7 Å to 19.2–26.1 Å depending on the length of R. Examples of R were C_nH_2n(n=1–6), CH(CH₃)CH₂, C(CH₃)=CH, CH=CH, CH₂CH=CHCH₂ and C₆H₄. Structural considerations and experimental data suggested that dicarboxylates were incorporated into the OCP structure through the replacement of HPO₄ ^2– by RC₂O₄ ^2–.