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

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Abstract
The crystal structures of six β-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) Å. 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 β-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 1H 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.

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