A structural, spectroscopic and theoretical study of an o-vanillin Schiff base derivative involved in enol-imine and keto-amine tautomerism

Abstract

The potassium salt of the Schiff base derived from o-vanillin and taurine is involved in enol-imine (I) and keto-amine (II) tautomerism. Both tautomeric forms coexist in the crystal and they are stabilized by strong O–H⋯N and O⋯H–N intramolecular hydrogen bonds.

Synthesis, crystal structure and cytotoxicity assays of a copper(II) nitrate complex with a tridentate ONO acylhydrazone ligand. Spectroscopic and theoretical studies of the complex and its ligand

“…Thus, the involvement of the phenolate oxygen (O1) in the coordination is corroborated by the absence of the in-plane and out-of-plane deformation modes of the O1-H phenol group, which appears in the IR spectrum of the ligand at 1368 cm -1 and at 837 cm -1 , respectively. It is interesting to mention that the relatively high frequency of the (O1H) (IR:837 cm -1 , calculated 753 cm -1 ) is in accordance with the predicted correlation between the strength of the hydrogen interaction and the expected frequency shifting of the (OH) and (OH) modes in related compounds [53,54].…”

Solvent Dependent Prototropic Tautomerism in a Schiff Base Derived from o‐Vanillin and 2‐Aminobenzylalcohol

“…Recently a number of works were dedicated to the investigation of the keto‐enol tautomerism in salicylidene type Schiff bases . These studies have revealed that (a) the OH form is in general much more stable than NH form, (b) the NH form can be stabilized at low temperature, proton donor solvents or aggregation forces (strong intermolecular interactions), and (c) the occurrence of a particular OH/NH form can be determined by both internal factors, such as the molecular structure of the SB, especially the nature and position of substituent group(s) on the salicylidene or amine moieties, and external factors, such as temperature, solvent or crystal packing forces established during the crystallization process .…”

“…Recently a number of works were dedicated to the investigation of the keto-enol tautomerism in salicylidene type Schiff bases. [28][29][30][31][32] These studies have revealed that (a) the OH form is in general much more stable than NH form, (b) the NH form can be stabilized at low temperature, proton donor solvents or aggregation forces (strong intermolecular interactions), and (c) the occurrence of a particular OH/NH form can be determined by both internal factors, such as the molecular structure of the SB, especially the nature and position of substituent group(s) on the salicylidene or amine moieties, [29,30] and external factors, such as temperature, solvent or crystal packing forces established during the crystallization process. [31][32][33] As crystallographic criteria for the assignment of the tautomeric form in the salicylidene SBs the following geometrical parameters are commonly used: (1) the C7-O and C8-N bond lengths should have values close to single (1.362 Å) and double (1.222 Å) bonds for the enol and vice versa for the keto form (the corresponding bonds with atoms numbering are presented in Scheme 2); (2) comparison of the C6-C8 and C6-C7 bond lengths: d(C6-C8) > d(C6-C7) in keto and d(C6-C8) < d(C6-C7) in enol-form; (3) for the enol-form in the benzaaldehyde ring all CÀ C bonds have approximately the same values (an indication of aromaticity) while in the keto-form they usually alternate as long-short bonds (deviation from aromaticity); (4) the intramolecular (OH⋅⋅⋅N) hydrogen bond in the enol form is stronger than the (NÀ H⋅⋅⋅O) hydrogen bond in the keto form.…”

Triphenylphosphinopalladium(II) complexes with ONO‐donor ligands: syntheses, structures and catalytic applications in CC cross‐coupling reactions

“…All of the three Schiff bases display a pair of sharp and strong closely spaced bands at ca 1633 and ca 1614 cm −1 . It is known that Schiff bases having the 2‐hydroxy‐1‐arylidene moiety are known to exhibit tautomerism between the keto‐amine and phenol‐imine forms (Scheme ) . It is very likely that these two bands are associated with the CO and the CN stretching frequencies of the equilibrium mixture of the two tautomers of H 2 L n .…”