Phospholes with Reduced Pyramidal Character from Steric Crowding. 2. Photoelectron Spectral Evidence for Some Electron Delocalization in 1-(2,4-Di-tert-butyl-6-methylphenyl)-3-methylphosphole

Abstract

Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus pyramid in a phosphole as caused by a large, sterically demanding P-substituent. Earlier PE spectra had shown no difference in ionization energies (IE) for simple phospholes and their tetrahydro derivatives (both around 8.0-8.45 eV). Calculations of the Koopmans IE at the Hartree-Fock 6-31G level for 1-methylphospholane showed that, as is known for nitrogen, planarization at phosphorus markedly reduced the ionization energy value (8.74 to 6.29 eV). A reduction in IE also occurred on planarizing 1-methylphosphole, but to a lesser extent, being offset by increased electron delocalization (8.93 to 7.16 eV). This suggests that experimental comparison of IE for the unsaturated and saturated systems could be used to detect the presence of electron delocalization in the former. The IE experimentally determined for the crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was 7.9 eV, the lowest ever recorded for a phosphole. The corresponding phospholane had IE 7.55 eV. The difference in the values is attributed to electron delocalization in the phosphole. Calculations performed on the related model 1-(2-tert-butyl-4,6-dimethylphenyl)phosphole showed that the P-substituent adopted an angle of 55.7 degrees (DFT/6-31G level; 57.6 degrees at the HF/6-31 level) with respect to the C(2)-P-C(5) plane (for P-phenyl, 67.1 degrees and 68.3 degrees, respectively).

Analogy between sulfuryl and phosphino groups: the aromaticity of thiophene-oxide

“…Similar difference can be observed between planar transition state structure of selenopheneoxide and arsole. This increased aromaticity can be understood, considering that the stabilization is attributable to the interaction between the empty b 1 (LUMO) orbital of the cisbutadiene fragment, and the lone pair of the heteroatom [43], which is getting more effective, when the energy difference of these two orbitals is getting smaller (see Fig. 7).…”

“…This aromatic stabilization decreases the inversion barrier of phosphole significantly compared to other phosphanes: while the inversion barrier of phosphole was calculated to be 18 kcal mol -1 (at the B3LYP/6-311?G** level) [16], 39.6 kcal mol -1 value can be obtained for its saturated analogue phospholane at the same level. The inversion barrier of phosphole can be even more reduced by the replacement of the ring carbon atoms by phosphorus [16,[38][39][40][41], p-electron withdrawing substituents to the ring atoms [36,37], and by bulky substituents at the heteroatom [42][43][44]. Accordingly, combining all these planarizing effects 1-bis(trimethylsilyl)methyl-3,5-trimethylsilyl-1H-1,2,4-triphosphole [45] is fully planar.…”

The important role of the phosphorus lone pair in phosphole aromaticity

“…It is planar with one phosphorus lone pair in the plane and the other residing in a P pπ orbital resonating with the carbon pπ orbitals, a total of six π electrons forming the very stable 2n + 2, highly aromatic system, similar to that found for pyrrole. Generally speaking, it has been shown that reduced pyramidal character from steric crowding leads to increased aromaticity [19,20]. On the other hand, if one removes the phosphorus hydrogen as an anion, the P+ molecular cation results.…”

The First Delocalized Phosphole Containing a Planar Tricoordinate Phosphorus Atom: 1-[Bis(trimethylsilyl)methyl]-3,5-bis(trimethylsilyl)-1,2,4-triphosphole

“…Both the sum of the bond angles (3428) [9] and the complexing behavior [10, 11] of the recently described 1-[bis(trimethylsilyl)methyl]-3,5-di-tert-butyl-1,2,4-triphosphole (1), indeed show the expected increase in the planarity of the tricoordinate phosphorus atom and the enhanced aromaticity of the ring system. Likewise, the incorporation of bulky substituents [12±15] at the tricoordinate phosphorus atom of a phosphole resulted in characteristic changes both in the photoelectron spectrum [16] and in the chemical reactivity, [15] providing the first example of a phosphole that undergoes electrophilic substitution. Schmidpeter et al recently reported phospholes substituted by electron-withdrawing cationic moieties, [17,18] which exhibited NMR spectroscopic characteristics [17] or structural features (sum of bond angles at the tricoordinate phosphorus atom 3388), [18] which is in accordance with partial flattening of the phosphorus pyramid.…”