Synthesis, Structure, Strain Energy, and Excess Strain of a Phospha[3]triangulane

(7-Phenyl-7-phosphadispiro[]heptane)pentacarbonyltungsten (<b>8</b>), 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 <b>7</b> and phosphirane <b>6</b> 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 <b>11</b>, phosphaspiropentane <b>12</b>, and phospha[3]triangulane <b>13</b> using both G2MP2 theory and ring separation reactions. Our best estimates for the Δ<i>H</i><sub>f</sub> of <b>11</b>, <b>12</b>, and <b>13</b> 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 <b>1</b>, spiropentane <b>2</b>, and [3]triangulane <b>3</b> to give respective Δ<i>H</i><sub>f</sub> 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[<i>n</i>]triangulanes, which is smaller than the 8.6 kcal/mol determined from the heat of combustion measurements for the [<i>n</i>]triangulanes.