Synthesis and structural trends in a series of simple tetraalkylphosphonium salts

Abstract

A series of sulfates (S) and hydrogen sulfates (HS) containing tetramethylphosphonium (TMP), tetraethylphosphonium (TEP) and tetrabutylphosphonium (TBP) cations have been prepared from the respective bromides using ion metathesis reactions. The products were identified using spectroscopic methods (FTIR, NMR), as well as powder and single-crystal X-ray diffraction. All the hydrogen sulfates contain HSO₄⁻ anions associated via hydrogen bonds, forming either a (HSO₄)₂²⁻ dimer (as for TEPHS and TBPHS) or a zigzag chain, (HSO₄)_nⁿ⁻ (for TMPHS). Such associated anions are surrounded by four TMP and TEP or three TBP cations per one sulfur atom. The reported sulfates reveal much larger separation of the anions, while the double anion charge implies slightly closer distances between the counterion centers and the tendency for forming hydrates. TBPS and TBPHS melt close to 100 °C, at a much lower temperature than the salts containing smaller cations, while thermal decomposition of all these salts begins within ca. 300–400 °C. A mixed-anion salt, [(C₄H₉)₄P]₄[SO₄][HSO₄]₂, is formed in a molten equimolar mixture of TBPS and TBPHS. The obtained salt is composed of separated SO₄²⁻ and (HSO₄)₂²⁻ anions rather than the (SO₄⋯HSO₄)³⁻ mixed-anion dimer.