π-Delocalization in the vicinal lone pairs of hydrazines: Electronic effects in derivatives of 1-(2-nitrophenyl)-1-phenylhydrazine

• There is a marked electronic interplay between the diaryl system and any substituents on the terminal NH2 group.
• These are nearly unaffected by intermolecular interactions as demonstrated by UV–visible data.
• The balance of the electronic environments in these hydrazines can be quantified in the solid-state by measuring the N–N bond lengths.
• This bond length is surprisingly variable and can vary from a long N–N bond typical of hydrazine itself, to a short N=N double bond.
• Our results, supported by DFT calculations, suggest that the π-contribution to the short bond distances is minimal.

Several highly colored, crystalline derivatives of 1-(2-nitrophenyl)-1-phenylhydrazine have been synthesized and characterized by UV–Vis, 1H- and 13C-NMR, mass and IR spectroscopic methods, as well as by single-crystal X-ray diffraction and combustion elemental analysis. The study indicates that the electronic dependence of one nitrogen atom on the other in these systems becomes more pronounced with a decrease in the difference of their formal hybridization. While 1,1-di-aromatic substitution leads to a formal sp2 hybridization at one nitrogen atom, introduction of a π-bond in the form of a Schiff base or an acetylamide function at the second nitrogen atom leads to different hydrazine conformations and N–N bond lengths. One interpretation of these results is that, in many cases, a greater contribution from σ-bonding from two sp2 hybridized nitrogen atoms leads to stronger and shorter N–N bonds than do the often proposed N–N π-bonding double bonds in the absence of significant intermolecular forces.

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