Synthesis, structure, strain energy, and excess strain of a phospha[3]radialene

(7-Phenyl-7-phosphadispiro[2.0.2.1]heptane)pentacarbonyltungsten (8), a phospha[3]triangulane, was synthesized from bicyclopropylidene. Its single- crystal X-ray structure determination is reported. Comparison of the crystal structure data with those of the related phosphaspiropentane 7 and phosphirane 6 complexes suggests that the phosphirane ring tightens when the number of spiro atoms is increased. This is supported by the B3LYP and MP2/6- 31G* computed geometries of the uncomplexed parent systems. Ab initio calculated heats of formation and strain energies (SE) are reported for the parent phosphirane 11, phosphaspiropentane 12, and phospha[3]triangulane 13 using both G2MP2 theory and ring separation reactions. Our best estimates for the ΔH(f) of 11, 12, and 13 are 18.3, 48.4, and 78.2 kcal/mol, respectively, with corresponding SE values of 21.3, 54.7, and 87.9 kcal/mol. For comparison, the slightly modified G2MP2 method was also applied to cyclopropane 1, spiropentane 2, and [3]triangulane 3 to give respective ΔH(f) values of 12.6, 44.3, and 75.3 kcal/mol, with corresponding SEs of 28.0, 64.6, and 100.5 kcal/mol, all of which are in excellent agreement with reported experimental data. These strain energies suggest that the excess strain per spiro atom is 5.3 kcal/mol for phospha[n]triangulanes, which is smaller than the 8.6 kcal/mol determined from the heat of combustion measurements for the [n]triangulanes.