Structural differences of metal biphenylenebisphosphonate with change in the alkali metal

A series of monovalent biphenylenebisphosphonates have been prepared using hydrothermal synthesis resulting in a composition M[HO3PC12H8PO3H2] where M=Li+, Na+, K+, Rb+, and Cs+. Three of the original four phosphonic acid protons are retained, making the compounds Brönsted acids. A synthesis using microwave irradiation results in a new compound, Na2(HO3PC12H8PO3H), where only two protons are retained. Two structural types were found for the three-dimensional compounds where one type has a continuous inorganic layer. In the other structural type, the inorganic layer is formed by hydrogen bonding between neighboring chains. These structural variations arise as the coordination number changes from 4 for lithium to 8 for cesium without change in composition. The dense packing of the biphenylenebisphosphonate restricts the access to the protons, thus these compounds cannot be used as Brönsted acid catalysts. Alternatively, addition of N,N-dimethylformamide to the reaction mixture results in inclusion of ammonium ions in the cavities.

Graphical abstractTwo of five metal biphenylenebisphosphonate structures (lithium (a) and cesium (b)) are presented. Each compound is a potential Bronsted acid catalyst, where three of the original four protons are retained from the biphenylenebisphosphonic acid.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Alkali phosphonates are Brönsted acids with the retention of three protons per alkali cation. ► These compounds are thermally stable to approximately 300 °C. ► A new structure was formed from the in situ synthesis with dimethylformamide as a reactant. ► With lack of access to the protons the alkali phosphonates cannot function as a Bronsted acid catalysts.