Synthesis, spectroscopic studies and structural characterisation of some new 2-(phosphonioaryl)imidazolide betaines

The synthesis is described of three series of 2-(phosphonioaryl)(benz)imidazolide betaines, viz (i) the 2-(2-or 3-or 4-tributylphosphoniophenyl)benzimidazoli (ii) the 2-(2-and 4-triorganophosphoniophenyl)-1H-phenanthro[9,10-d]imidazolides, and (iii) the 2-(2-and 4-triorganophosphoniophenyl)-4,5-diphenyl-immidazolides. A detailed H-1, C-13 and P-31 NMR study of the 2-(tributylphosphoniophenyl)benzimidazolides reveals significant differences in the spectroscopic parameters of the betaines compared with those of the phosphonium salts from which they are derived. Full X-ray structural studies are reported for 2-(4-triphenylphosphoniophenyl)-1 H-phenanthro[9, 10-d]imidazol-1-ide and 2-(4-triphenylphosphoniophenyl)-4,5-diphenylimidazol-1-ide, and comparisons made with related pyridiniumimidazolide systems. The phosphonioimidazolide betaines exhibit negative solvatochromism.