1-(Naphthylamino)-1-(p-chlorophenylhydrazono)-2-propanone and 2-(p-tolyldiazenyl)-[1H]-3-methylbenzo[g]indole: Crystallographic and spectroscopic elucidation of the cyclisation of an arylamidrazone

The crystal structure of the organic compound 1-(naphthylamino)-1-(p-chlorophenylhydrazono-2-propanone, 1a, comprises two crystallographically nonequivalent independent molecules in the asymmetric unit with dispositions and relative orientations amenable to inter- and intramolecular hydrogen bonding of the type N-H⋯O. The former arises from interactions between the phenylhydrazono unit of one molecule and the acetyl group of the other in the next asymmetric unit whereas the latter occurs between the naphthylamino fragment and the acetyl group within the same molecule. The molecular framework from the phenyl to the acetyl group which incorporates an imine (CN) bond is essentially planar; however, conspicuously, the naphthyl group is twisted off this plane somewhat orthogonally, causing the formation of π-π stacked aromatic layers from interactions of alternating pairs of these groups between the hydrogen-bonding chains. The molecular structure of 2-(p-tolyldiazenyl)-[1H]-3-methylbenzo[g]indole, 2b, has not only established definitively the cyclisation of the arylamidrazone 1-(naphthylamino)-1-(p-tolylhydrazono-2-propanone, but also revealed an unprecedented oxidation of the phenylhydrazono fragment (PhNHN) to the phenylazo unit (PhNN) with polyphosphoric acid. The overall structures of 1a and 2b contrast sharply in that of the latter is virtually planar in its entirety with greater rigidity. The structures have been corroborated by vibrational and NMR spectroscopic techniques.