Synthesis and characterization of new barium methylphosphonates

Eight new barium methylphosphonates were prepared and described. In dependence on pH, either barium hydrogen methylphosphonates or barium methylphosphonates can be formed. In the case of barium methylphosphonates, BaCH₃PO₃·3H₂O crystallizes from the solution at room temperature and BaCH₃PO₃·H₂O at a temperature above 65 °C. On heating, these hydrates form two anhydrous barium methylphosphonates (α-BaCH₃PO₃ and β-BaCH₃PO₃) with the same composition but with a different structure. In a basic environment, barium hydrogen methylphosphonate monohydrate, Ba(CH₃PO₃H)₂·H₂O, transforms to BaCH₃PO₃·3H₂O through an intermediate with the formula Ba₂(CH₃PO₃H)₂(CH₃PO₃)·4H₂O. The reverse reaction, that is the reaction of BaCH₃PO₃·3H₂O with methylphosphonic acid, proceeds to the intermediate only and hydrogen methylphosphonate is not formed. Acidic Ba(CH₃PO₃H)₂·H₂O is able to interact with basic amines and form stable intercalates with them. Structures of β-BaCH₃PO₃ (P2₁/c, a = 8.4501(6) Å, b = 7.2555(7) Å, c = 7.4604(8) Å, β = 99.837(8)°, Z = 4) and BaCH₃PO₃·H₂O (P2₁/c, a = 20.5077(5) Å, b = 7.2175(2) Å, c = 7.4909(3) Å, β = 95.522(3)°, Z = 8) were solved from powder X-ray diffraction data. Both compounds are layered, and the layers are formed of two sheets of Ba atoms connected through oxygen atoms of the phosphonate groups. The methyl groups point towards the interlayer space. In the case of BaCH₃PO₃·H₂O, the molecules of water are coordinated to the Ba atoms and are placed in the interlayer space among the methyl groups.