Synthesis, Characterization and Theoretical Calculations of Novel Five Membered Rings Containing Phosphorus and Arsenic Atoms

The synthesis of two new arsadiphospholes of formula (AsP2C2Bu t2 )CH(SiMe3)2, from 4-arsa-1,2-diphosphacyclopentadienyl and 2-arsa-1,4-diphosphacyclopentadienyl ring anions are reported together with their characterization by 31P NMR. Ab initio calculations at the MP2/6-31G(d) level of theory were carried out to investigate the energy barrier for pyramidal inversion about the pyramidal P and As atoms, relative stability and aromaticity character of all possible isomers in the gas phase. The thermodynamic isomers distribution is in agreement with the experimental results. The inversion barrier at the sp³ phosphorus compared to the inversion barrier found for the arsenic atom is low. This fact is directly related with the aromaticity of the rings and shows the preference of the side group for the phosphorus atom, which leads to a more aromatic ring structure. The aromaticity character found for three isomers containing a sp³-phosphorus is greater if compared with that of 1,2,4-triphosphole synthesized previously. The effects of bulky R substituents on the aromaticity and on the energy barrier for pyramidal inversion are also discussed.