A multinuclear magnetic resonance study of intramolecular hydrogen bonding in some Schiff and Schiff-Mannich bases in...

Two Schiff bases, N,N'-bis(5-bromosalicylidene)-1,2-diaminoethane, BS, and 7-[(1-[5-bromo-2-hydroxyphenyl] methylidene)amino]-4-methylcoumarin, Sc, and two appropriate Schiff-Mannich bases, N,N'-bis[5-bromo-3-[(diethylamino)methyl]salicylidene]-1,2-diaminoethane, BSM, and 7-[(1-[5-bromo-3-[(diethylamino)methyl]-2-hydroxyphenyl] methylidene)amino]-4-methylcoumarin, SMc, capable of intramolecular hydrogen bonding have been investigated by multinuclear magnetic resonance methods in both solid and liquid phases. In all of the compounds under investigation tautomeric equilibrium involving an intramolecular hydrogen bond has been found. The Schiff-Mannich bases, which can form two different kinds of H bonds at room temperature, form relatively weak H bonds with the imino nitrogen atoms. At low temperatures the tautomeric proton exchange becomes slow on the NMR time scale and both hydrogen-bonded forms can be observed by 1H, 13C, and 15N NMR methods. In the solid state the tautomeric process is frozen and only one H-bonded form is present. On the basis of 13C and 15N CPMAS NMR spectra this is identified as the form with hydrogen bonds involving the imino groups. This conclusion is in good agreement with previous results obtained by X-ray diffraction methods. The investigated Schiff bases (BS and Sc) form relatively weak H bonds. The proton position in the hydrogen bridge, estimated from 15N and 13C chemical shifts, is very similar in both the solution and solid phases. In chloroform solution the observed tautomeric equilibria are almost insensitive to a temperature change within the range 223 to 303 K.