Interplay between steric and electronic factors in determining the strength of intramolecular resonance-assisted NH..O hydrogen bond in a series of beta-ketoarylhydrazones

The crystal structures of six beta-ketoarylhydrazones are reported: 1,(Z)-2-(2-bromophenylhydrazono)-3-oxobutanenitrile; 2 (Z)-2-(2-methylphenylhydrazono)-3-oxobutanenitrile; 3, (E)-methyl-2-(2-methoxyphenylhydrazono)-3-oxobutanoate; 4, E, methyl-2-(2-cyanophenylhydrazono)-3-oxobutanoate;5,(Z)-methyl-2-(4-cyanophenylhydrazono)-3-oxobutanoate; 6, pentane-2,3,4-trione-3-(2-carboxyphenylhydrazone). All of them form intramolecular hydrogen bonds assisted by resonance (RAHB), with N ... O distances in the range 2.541(5)-2.615(3) Angstrom. These hydrogen bonds are differently affected by the substituents at the heterodienic fragment, being strengthened by electronwithdrawing substituents in position 2 (more by 2-COMe than 2-CN substitution), and weakened in beta-esterhydrazones and when the N-H forms a bifurcated hydrogen bond. The role played by the different steric and electronic properties of the substituents in strengthening the H-bond is investigated, besides X-ray crystallography, by IR and H-1 NMR characterization of the NH proton, and quantum mechanical DFT calculations at the B3LYP/6-31 + G(d,p) level of theory on test molecules.