SYNTHESIS AND MOLECULAR-STRUCTURE OF HETEROCYCLES CONTAINING 2 PHOSPHORUS(V) CENTERS BRIDGED BY 2-COORDINATE PHOSPHORUS AND ARSENIC

1,1-Bis[(diphenylphosphanyl)methyl]ethene (3) reacts with PCl3 or AsCl3 in the presence of SnCl2 as a reducing agent to form cationic heterocycles in which the two newly generated phosphonium centers are bridged by two-coordinate, negatively charged phosphorus (4a) or arsenic (4b) centers. Hexachlorostannate(IV) dianions function as counterions to these heterocycles, in which the exocyclic methylene group is retained. By treatment of 4a with d base a rearrangement is induced which includes a proton migration from an endoto the exocyclic CH2 group to give 5-methyl-1,1,3,3-tetraphenyl-1lambda5,2lambda3,3lambda5-triphosphinine (5), a novel 1,2,3-triphosphabenzene species. The reaction of the arsenic compound 4b with base did not yield the corresponding arsa-diphosphabenzene, but gave only decomposition products. Compound 4b . 0.5 CH3CN has been studied by X-ray crystallography. The six-membered heterocycle shows a chair conformation with an arsenic atom solely coordinated by two phosphorus atoms at roughly equal distances with a sharp angle P-As-P of only 93.0(1)degrees.