Systematic Study on the Structures and Reactivity of Hydrazobenzenes and Azobenzenes Bearing a Chalcogenophosphoryl Group

Hydrazobenzenes 3−5 bearing a chalcogenophosphoryl group were synthesized by palladium-catalyzed cross-coupling reactions. Their X-ray crystallographic analyses and NMR and IR spectra showed the presence of
intramolecular hydrogen bonds between the N−H protons and the chalcogenophosphoryl groups. The intermolecular
hydrogen bonds in phosphine oxide 3 and selenide 5 were observed in the solid state. Phosphine oxide 3, sulfide
4, and selenide 5 constructed a dimeric structure, a discrete monomeric structure, and a chain structure, respectively.
As the chalcogen atom changed, the crystalline structures of the 2-chalcogenophosphorylhydrazobenzenes also
changed. The hydrogen bonds affected the oxidation reactions of the hydrazobenzenes, and oxidation of
hydrazobenzenes bearing a lighter chalcogen atom was more difficult. For azobenzenes bearing a chalcogenophosphoryl group, X-ray crystallographic analyses and NMR spectra showed little interaction between the azo
group and the chalcogenophosphoryl groups. However, in the UV−vis spectra, the red shifts of the absorption
maxima due to the n → π* transitions indicated intramolecular interactions in the excited state, in contrast to the
corresponding 4-substituted azobenzenes. In addition, photoirradiation of phosphine oxide (E)-7 gave (Z)-7, whereas
that of phosphine sulfide (E)-8 and phosphine selenide (E)-9 did not give (Z)-8 and (Z)-9, suggesting that heavy
chalcogen atoms quench excited states by through-space interactions. Introduction of a chalcogenophosphoryl
group at the 2-position had a significant effect on the structure, spectral properties, and reactivity of hydrazobenzenes
and azobenzenes. Although azobenzene (E)-10 bearing a hydroxyphosphoryl group at the 2-position did not show
hydrogen bonding in the crystalline state, its optical properties and photoisomerization ratio were different from
those of (E)-7.