Synthesis of heavier alkali metal phospholides. Crystal structure of caesium 2,3,4,5-tetramethylphospholide, an ionic solid

The reaction of rubidium or caesium metals with 2,3,4,5,2′,3′,4′,5′-octamethylbi-1,1′-phospholyl (3) in THF, respectively, afforded rubidium 2,3,4,5-tetramethylphospholide (1) and caesium 2,3,4,5-tetramethylphospholide (2). While the physical properties of 1 are very similar to that of the already described potassium 2,3,4,5-tetramethylphospholide, in particular as far as solubility is concerned, 2 on the contrary is very slightly soluble even in polar solvents such as dimethoxyethane (DME) or THF. Crystals of 2 suitable for X-ray analysis have been obtained by high-temperature recrystallisation in DME at 170 °C. The three-dimensional polymeric structure of 2, where all the phospholyl rings are parallel, has the characteristics of an ionic solid.