Synthesis and Molecular Structure of Heterocycles Containing two Phosphorus(V) Centers Bridged by Two‐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 a base a rearrangement is induced which includes a proton migration from an endo‐to the exocyclic CH₂ group to give 5‐methyl‐1,1,3,3‐tetraphenyl‐1λ⁵,2λ³,3λ⁵1‐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)°.