4-Sulphophenylphosphonic acid: a novel precursor to fabbricate polyfunctional acid materials

A major application of polyfunctional acid materials is the fabrication of solid electrolytes exhibiting proton conductivity in the solid state for operation in fuel cells at medium temperature (< 160 degrees C). Because the advancement of research on solid proton conductors depends on the availability of new precursors, a new acid organosulphur phosphorus compound, i.e., the hydrated 4-sulfophenylphosphonic acid H2O3PC6H4SO3H2 center dot 2 H2O (IA), has been synthesized by the Tavs reaction starting from the 4-bromo(diethyl)sulfonate precursor. This synthetic route is opposite that reported for the synthesis of many other aromatic sulfophosphonic acids, which are obtained by sulfonation of the aromatic phosphonic acid precursor. The reason for the different synthetic route in the former case lies in the fact that sulfonation of benzenephosphonic acid yields only metasubstituted derivatives. Investigation of the solid-state structure at variable temperature indicates that IA yields the anhydrous compound H2O3PC6H4SO3H2 (IB) at 190-210 degrees C. Solid state C-13 and P-31 NMR data and FT-IR measurements are consistent with intermolecular H-bonding between the sulfonic and phosphonic acid groups, and/or protonation of the phosphonic acid by the sulfonic acid group, depending on the presence of water. Ar-SO3-H3O+center dot center dot center dot OP(Ar)(OH)(2)center dot center dot center dot Omicron Eta(2) reversible arrow Ar-SO3-H3O+center dot center dot center dot O=P(Ar)(OH)-O-center dot center dot center dot H3O+; Ar-SO3-H3O+center dot center dot center dot O=P(Ar)(OH)(2)center dot center dot center dot Omicron Eta(2) reversible arrow Ar-SO3-H2O center dot center dot center dot[Eta O-P(Ar)(OH)(2)](+)center dot center dot center dot OH2; Ar-SO3-H2O center dot center dot center dot[Eta O-P(Ar)(OH)(2)](+)center dot center dot center dot center dot Omicron Eta(2) reversible arrow Ar-SO3-H2O center dot center dot center dot(Eta O)(2)P(Ar)O center dot center dot center dot H3O+; 2nH(2)O. IA IB These interactions are very important in determining proton conductivity in relation to the possible use of IA as solid electrolyte component in fuel cells operating at medium temperature. On this basis, 4-sulfophenylphosphonic acid seems an interesting precursor for the fabrication of solid proton conducting electrolytes.