Sterically Controlled Tautomeric Preference in New o ‐Hydroxynaphthaldehyde Schiff Bases: Synthesis, Crystal Structure, Hirshfeld, FMO, MEP, and NLO Analysis
L. C Nimbus, Mohd. Aamir Bin Riyaz, Srivarsha Dilliraj, Prema Raghunandan, Toka SwuABSTRACT
The o ‐hydroxynaphthaldehyde Schiff bases are known to exhibit prototropic tautomerism, which is strongly influenced by the electronic nature of the amine precursor and the steric environment surrounding the proton‐transfer site. In this work, eight new o ‐hydroxynaphthaldehyde (2‐hydroxy‐1‐naphthaldehyde and 1‐hydroxy‐2‐naphthaldehyde) Schiff bases were synthesized in high yield from 2‐aminoacetophenone derivatives and characterized by single‐crystal XRD, 1 H and 13 C NMR spectroscopy, infrared spectroscopy, and mass spectrometry. Steric encumbrance at the 6‐position of 2‐aminoacetophenone plays a significant role in governing the tautomeric preference. 2‐Aminoacetophenones with larger substituents at the 6‐position resulted in Schiff bases in enol–imine form, whereas 2‐aminoacetophenones with small or no substitution at the 6‐position resulted in Schiff bases in keto–amine form. Single‐crystal XRD analysis revealed the loss of coplanarity, extended conjugation, and bifurcated intramolecular hydrogen bond in the enol–imine tautomers. The electronic properties of the synthesized Schiff bases were explored using FMO analysis. DFT further reveals that the synthesized Schiff bases show significant nonlinear optical (NLO) properties, with the Schiff bases in keto–amine form displaying more first‐order hyperpolarizability than the Schiff bases in enol–imine form.