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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