Theory and application of photoelectron spectroscopy. 60. Phospholes. Electronic structure

Abstract

The phosphole system is investigated both theoretically and by uv photoelectron spectroscopy. The theoretical approach confirms that phospholes despite their pyramidal structure are "aromatic" systems. Their "aromatic" stabilization relative to the conjugatively interrupted subunits ds-butadiene and PR is gained from nx* conjugative and P-C/x* hyperconjugative interactions between both subunits. These interactions appear not to be accompanied by appreciable x-charge transfer (i.e., electron delocalization) from the PR unit to the x part mainly due to strong electron acceptance from the phosphorus d orbitals. In addition, the photoelectron spectroscopic results corroborate strong P-C/x hyperconjugative but only weak n/x conjugative mixing. The results of the present study show that the n orbital energy is not a suitable means to deduce the "aromatic" nature of phospholes as a consequence of the combined nx* and nx interactions. Moreover, the results constitute a first indication of the important role played by P-C hyperconjugation in these systems.

The important role of the phosphorus lone pair in phosphole aromaticity

“…As Schafer et al [5] have pointed out, that both effects are important. In some systems, there is no phosphorus lone pair and hyperconjugation is the main determinator of the cyclic stabilization or destabilization.…”

“…Actually, the work cited in support of this conclusion [5] states that both substituent hyperconjugation and the phosphorus lone pair contribute to the system's aromaticity.…”

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

“…Elements of the third row of the periodic table, however, can have different oxidation state forming hypervalent species. Recently, we have shown [7] that such change at the heteroatom switches the slight aromaticity of phosphole [8][9][10][11][12][13][14][15][16][17] to the slight antiaromaticity of phospholeoxide, resulting even in chemical consequences.…”

“…1), which exhibits a bond angle sum about 290° [16,34]. However, while phosphole is known to be slightly aromatic due to the interaction of the P-H (or P-R) r bond with the p-system [7,9,15], thiophene-oxide has tentatively been suggested to have antiaromatic properties, on the basis of the abovementioned fast cyclodimerization reactions [26].…”

Bis(azidophenyl)phosphole Building Block for Extended π‐Conjugated Systems

“…The phosphole ring, in particular, has potential as a building block in organic materials,2 as exemplified by the organic light‐emitting devices (OLEDs) based on chlorido(dithienylphosphole)gold(I) complex 1 (Figure 1). 3 The electronic structure4 of the phosphole ring combines a low but not negligible degree of aromaticity, originating from hyperconjugation between the carbon‐based π‐system and the exocyclic σ(P–R) orbital,5 with a tunable λ 3 σ 3 ‐phosphorus center 3b. This combination provides a handle to fine‐tune the photophysical properties of phosphole‐containing π‐systems by influencing the extent of conjugation over the diene moiety 6.…”