The Structures and Energies of Phosphaalkyne Trimers, (HCP) ₃

Abstract

(CH)3P3 structures have been investigated by ab initio (MP4SDTQ/6‐31+G**//MP2(fu)/6‐31G*) and DFT (B3LYP/6‐311+G**) methods. The framework is the main factor determining the realtive energy of isomers; the substitution pattern is less important. The thermodynamic stability order of heteroatom‐substituted benzenes > benzvalenes > Dewar benzenes > prismanes > bicyclopropenyls parallels the situation in the (CH)6 hydrocarbons. The 3 HCP → 1,2,3‐triphosphabenzene trimerization energy is only –84 kcal mol–1; half as large as that for the 3 acetylene → benzene conversion (–164 kcal mol–1). Head‐to‐tail additions are favored for the [4+2] cycloadditions of HCP to 1,2‐ and 1,3‐diphosphete (to give triphospha Dewar benzenes); the computed activation barriers are low: ΔH*298 = 2.2 and 0.8 kcal mol–1, respectively, at MP4/6‐31+G**//MP2(fu)/6‐31G* (2.0 and 1.5 kcal mol–1 at MC‐QDPT2/6‐31G*//CAS‐SCF(6,6)/6‐31G*). Mono‐ as well as ortho‐, meta‐, and para‐diphosphabenzenes and the triphosphabenzenes are found to be as aromatic as benzene according to geometric and energetic criteria. NICS calculations (a magnetic criterion) suggest only slightly reduced aromaticity.

Substituent effect on low coordination phosphorus chemistry

“…1,3,5-Triphosphinine 3 (R: tBu) has been synthesized by a metal catalyzed cycloaddition of 1 [7,8]. The stabilities of the possible isomers of 3 (R: H) have been investigated computationally in the Schleyer group [19]. According to their [19] MP2/6-31G* calculations the most stable isomer was 9 (R: H), followed by 10 (R: H at +8.0 kcal/mol) and 3 (R: H at +11.0 kcal/mol) [19].…”

“…The stabilities of the possible isomers of 3 (R: H) have been investigated computationally in the Schleyer group [19]. According to their [19] MP2/6-31G* calculations the most stable isomer was 9 (R: H), followed by 10 (R: H at +8.0 kcal/mol) and 3 (R: H at +11.0 kcal/mol) [19]. All other possible isomers were by 10-60 kcal/mol less stable than 9 (R: H).…”

“…This energy exceeds the 20 kcal/mol aromatic stabilization in 1,3,5-triphosphabenzene [21], and has also a large impact on the relative stabilities of the possible other isomers. Among the possible triphosphabenzene isomers investigated by Hofmann et al [19] three benzvalene structures (11)(12)(13) were close in energy to 3, the rest of the isomers were less stable by at least 10 kcal/mol. 12 and 13 (the parent compounds being somewhat less stable than 3) exhibit one pair of neighboring tBu groups, while in 11 (which was slightly more stable than 3) two such interactions act destabilizing, while in 3 there is no such destabilizing interaction.…”

Tetraphosphabenzenes Obtained via a Triphosphacyclobutadiene Intermediate

“…[6][7][8][9] Diphosphacyclobutadiene and valence isomers of triphosphabenzene have been accessed through metal-mediated oligomerization of phosphaalkynes, [10][11][12] and two related reactions generate triphosphacyclobutadiene ligands -complexed to reducing metal centers. [13,14] There is also one example of a highly distorted P 4 (C t Bu) 2 ligand complexed between two Rh centers, [15] and a diradical valence isomer of a tetraphosphabenzene has been reported to contain multiple 1e  bonds. [16] Exploiting the chemistry of P 2 -derived cyclo-P 3 complexes, [17] we now describe neutral triphosphacyclobutadiene intermediates and isomers of tetraphosphabenzene that are stabilized only by mild W(CO) 5 substituents.…”

“…Nicholas A. Piro and Christopher C. Cummins* The ability to replace "RC" units in organic motifs with isolobal phosphorus atoms has led to phosphorus being dubbed the "carbon copy." [1] Yet while the all-phosphorus analogues of benzene and cyclobutadiene (the epitomes of aromaticity and antiaromaticity) have been considered theoretically, [2][3][4][5] they remain elusive. What do exist are a few examples of reduced P 6 and P 4 ligands complexed to one or more metal centers.…”