Synthesis and structural studies of some [1 4]paracyclo-bis-(1,2)pyrazolium- and (1,3)imidazolium-phanes

Abstract

The crystal and molecular structure of [1 4]paracyclo-bis-(1,2)pyrazoliumphane dibromide ( 1a) has been determined. The compound exists in the solid state in the chair ( C) conformation while both chair ( C) and boat ( B) conformations are present in solution in comparable amounts. The barrier to the C ⇌ B interconversion has been determined by 1H NMR spectroscopy ( ΔG ‡ ≈ 17 kcal mol −1 ). AM1 semiempirical calculations conveniently reproduce the difference in stability between the chair ( C) and the boat ( B) conformations. [1 4]Paracyclo-bis-(1,2)pyrazolium-phane dibromide exists in the solid state in the chair conformation (X-ray crystallography) while both chair and boat conformations are present in solution in comparable amounts. The barrier to the chair boat interconversion has been determined by 1H NMR spectroscopy ( ΔG‡ ≈ 17 kcal mol −1 ).