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This article presents the background of the author's synthesis of phosphabenzene in 1971. It also expands to tell the story behind the syntheses of the heavier group 15 heterobenzenes: arsabenzene, stibabenzene, and bismabenzene, which were prepared nearly at the same time and by a largely similar route.

The potential energy surfaces corresponding to the photochemical reactions of pyridine, phosphinine, and arsabenzene have been investigated by employing the CAS(6,6)/6-311G(d,p) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d,p) methods. The thermal (or dark) reactions of these reactant species have also been examined using the same level of theory. The mechanisms of drastic structural change in the excited- and ground-state reactions of pyridine, phosphinine, and arsabenzene and the differences between them are elucidated. The theoretical investigations suggest that conical intersections play a crucial role in their photoisomerization reactions. The results obtained allow a number of predictions to be made.

International audience; Geometries, aromatic character, Mulliken charge distribution, and MO diagrams of 1,2-aza-, 1,3,2- diaza-, 1,3-aza-, and 1,3,5-diazaphosphinines have been calculated and compared to those of phosphinine and pyridine. This study reveals that the introduction of nitrogen atoms at the position adjacent to phosphorus significantly reduces the aromatic delocalization and induces a [1,4] dipolar character through an increase of the positive charge on the P atom. This phenomenon does not occur in 1,3-aza- and 1,3,5-diazaphosphinines, which exhibit a poor dipolar character. This comparison confirms the high reactivity of 1,3,2-diazaphosphinines toward alkynes. A [4 + 2] cycloaddition reaction between these two types of diazaphosphinines and acetylene has been modelized. Calculated geometries of the resultant [4 + 2] diazabarrelene cycloadducts and that of their respective transition states reveal that, especially with 1,3,2-diaza isomers, the cycloaddition proceeds via a disymmetrical pathway that involves the preliminary formation of the P-C bond.

Phosphabenzene and arsabenzene are analogs of pyridine. Their IR and Raman spectra are reported for the first time. IR spectra are for a polycrystalline solid at about 100 K, and Raman spectra are for the liquid at room temperature. Assignments are suggested for all but five of the 54 fundamentals, and most of them are believed to be reliable. The molecules are known to have aromatic properties. This is demonstrated in the spectra by the regular trends in frequency and intensity in the sequence pyridine-phosphabenzene-arsabenzene.

The He(II) photoelectron spectra of phosphabenzene 2 and arsabenzene 3 yield additional ionization energies in the region 17 to 20 eV. The relative intensity changes of the first three bands, compared to their relative intensities in the He(I) spectra, provide support for the previously proposed sequence of states ˜X = 2B1, ˜A = 2A2, ˜B = 2A1. This and the angular dependence of the photoelectron band intensities in the spectra of 2 and 3 is in excellent agreement with the theoretical predictions made by von Niessen, Diercksen & Cederbaum [9].

The molecular structures of phosphabenzene and arsabenzene partially oriented in the nematic liquid crystal crystal EBBA were studied by NMR. The structures were in excellent agreement with the combined electron diffraction and microwave spectroscopic studies except the angle relating to the β hydrogens. Improved values for this parameter were suggested. The absolute signs of the J PH of phosphabenzene and some information on the 31 P chemical shift anisotropies were also obtained.

THE DIRECTION OF THE DIPOLE MOMENTS OF PHOSPRABENZENE AND ARSABENZENE Arthur J. Ashe, III *1 and Woon-Tunq Chan Department of Chemistrv, Universitv of Michigan Ann Arbor, Wichiqan 48104 (Bcceived in USA 2 June 1975; received rn UK for publication 26 June 1975) Preliminary studies of the spectral properties of phosphabensene (phosphorin) ,? and arsabenzene (arsenin) 2 have emphasized their similarities to 2-6 pyridine . Recently we reported the gas phase dipole moment of phosphabenzene as 1.54D5 and that of arsabenzene as l.10D6. The magnitude is considerably less than that for pyridine ,& (2.215D)7. The direction of the dipole was not determined. Since the difference in electronegativity8 between nitrogen (3.04) and carbon (2.55) relative to phosphorus (2.19) and carbon (2.55) and to arsenic (2.18) and carbon (2.55) changes sign, one might naively expect a change in the dipole moment direction as well. On the other hand both CNDO/2g'10 and ab initio 11 -MO calculations have predicted that all three heterobenzenes have the same direction of polarization. It was felt desirable to test this prediction. A compariscn of the dipole moments of 4-substituted heterobenzenes with those of the parents should unambiguously define the direction of the moments. We now wish to report on the synthesis of 4-methylphosphabenzene 2 and 4-methylarsabenzene

The average ( r z structure of vapor-phase C 5 H 5 As, the arsenic congener of pyridine, was deduced. Vibrational corrections of diffraction data and rotational constants were made via a normal coordinate analysis. The heterocyclic molecule was found to have a rigorously planar, C 2v structure with geometric parameters very close to those inferred from an earlier analysis of the diffraction data alone, but uncertainties were reduced substantially by incorporation of the spectroscopic information in refinements. Amplitudes of vibration were determined from both the diffraction and normal coordinate analyses. Principal structural parameters and uncertainties (2.5σ) were r g (C-As) = 1.850(2) A, r g (C 2 -C 3 ) = 1.392(9) A, r g (C 3 -C 4 = 1.400(10) A, r g (C-C mean) = 1.396(2) A, ∠CAsC = 97.0(3)°, ∠AsCC = 125.3(7)°, and ∠C 3 C 3 = 123.9(13)°. Structural characteristics are those expected for an aromatic molecule.

Durch KOH/[18]Krone-6 phasentransfer-katalysierte Addition von Arylarsanen (Phenylarsan, p-Tolylarsan) an (3-R)-3-Methoxy-1,4-pentadiine 13 werden cis/trans-isomere 1-Aryl-1,4-dihydro-(4-R)-4-methoxyarsenine 14 und 15 erhalten, deren 1H-NMR- und 13C-NMR-Spektren diskutiert werden. - Die 1,4-Dihydroarsenine 14 und 15 lagern sich bei Umsetzung mit BF3-Etherat oder p-Toluolsulfonsaure unter 1,2-Arylverschiebung in die 2-Aryl-(4-R)-arsenine 22 bzw. 24 um. Diese Umlagerung stellt das erste elementorganische Analogon zur Cyclohexadienol Benzol-Umlagerung dar. Die 1H-NMR-, 13C-NMR-, UV- und Massenspektren der dargestellten Arsenine 22 und 24 werden diskutiert. Synthesis of 2-Aryl-(4-R)-arsenines by Cyclohexadienol Arsabenzene Rearrangement KOH/[18]crown-6 phase-transfer catalyzed addition of arylarsines (phenylarsine, p-tolylarsine) to the (3-R)-3-methoxy-1,4-pentadiynes 13 yields the cis/trans isomeric 1-aryl-1,4-dihydro-(4-R)-4-methoxyarsenines, the 1H-NMR- and 13C-NMR spectra of which are discussed. - The addition of BF3-etherate or p-toluenesulfonic acid to the 1,4-dihydroarsenines 14, 15 causes rearrangement under 1,2-aryl migration to the 2-aryl-(4-R)-arsenines 22, 24. This rearrangement represents the first element-organic analogue of the cyclohexadienol benzene rearrangement. The 1H-NMR, 13C-NMR, UV, and mass spectra of the synthesized arsenines 22, 24 are discussed.

Vapor-phase molecules of C 5 H 5 As were found, assuming C 2v symmetry, to have the following structure parameters and uncertainties (2.5σ): r g (C-As)= 1.850 ± 0.003 A, r g (C 2 –C 3 ) = 1.390 ± 0.032 /rA, r g (C 3 –C 4 ) = 1.401 ± 0.032 /rA, r g (C-C ave ) = 1.395 4 ± 0.002 A, ∠CAsC = 97.3 ± 1.7°, ∠AsCC = 125.1 ± 2.8°, and ∠C 3 C 3 C 4 = 124.2 ± 2.9°. Amplitudes of vibration were also determined. Auxiliary information is more restrictive than pure electron diffraction intensities as evidence that the molecule is rigorously planar. Structural characteristics of arsabenzene reinforce prior indications that the heterocyclic molecule is genuinely aromatic.

Vapor-phase molecules of C 5 H 5 As were found, assuming C 2v symmetry, to have the following structure parameters and uncertainties (2.5σ): r g (C-As)= 1.850 ± 0.003 A, r g (C 2 –C 3 ) = 1.390 ± 0.032 /rA, r g (C 3 –C 4 ) = 1.401 ± 0.032 /rA, r g (C-C ave ) = 1.395 4 ± 0.002 A, ∠CAsC = 97.3 ± 1.7°, ∠AsCC = 125.1 ± 2.8°, and ∠C 3 C 3 C 4 = 124.2 ± 2.9°. Amplitudes of vibration were also determined. Auxiliary information is more restrictive than pure electron diffraction intensities as evidence that the molecule is rigorously planar. Structural characteristics of arsabenzene reinforce prior indications that the heterocyclic molecule is genuinely aromatic.

The molecular structures of phosphabenzene and arsabenzene partially oriented in the nematic liquid crystal crystal EBBA were studied by NMR. The structures were in excellent agreement with the combined electron diffraction and microwave spectroscopic studies except the angle relating to the p hydrogens. Improved values for this parameter were suggested. The absolute signs of the J~H of phosphabenzene and some information on the “P chemical shift anisotropies were also obtained.

3 Spectral and Phssieal Properties . . . . , . . . . . . . . . . . . . . . . 132 3.1 Structure Determinat ions . . . . . . . . . . . . . . . . . . . . . 132 3.2 N M R Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . 134 3.3 Mass Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . 136 3.4 Photoelectron (PE) and Electron Transmission (ET) Spectra . . . . . . 137 3.5 Dipole Moments . . . . . . . . . . . . . . . . . . . . . . . . 139