Experimental and theoretical studies on isatin-Schiff bases and their copper(II)-complexes: Syntheses, spectroscopy, tautomerism, redox potentials, EPR, PXRD and DFT/TDDFT

The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) react with the copper(II) acetate or nitrate to give the bis[3-(o/p-tolylimino)indole-2-olato-kappa N-2, O]copper(II) (1 and 2) via lactam (L)- to enol (E)-tautomerism in solution. IR spectra show solely the L-tautomer at solid state, while both tautomers in solution. H-1 NMR results indicate a dynamic tautomerization equilibria between the L- and E-tautomers in solution with varying ratios of ca. 83/17, 60/40 and 54/46 (L/E) within 0.5, 5 and 24 h, respectively. Cyclic voltammograms demonstrate two quasi reversible one electron charge transfer processes for the Cu(II)/Cu(I) and Cu(I)/Cu(0) couples in acetonitrile, respectively. PXRD patterns at 5-50 degrees (2 theta) correspond well to the crystalline nature with various degrees of crystallinity for the ligands and complexes. X-band EPR spectrum shows an intense broad band at relatively high field region with isotropic nature. Magnetic moment values (mu(eff). = 1.56-1.63 mu B) indicate paramagnetic nature of the copper(II)-complexes with one unpaired electron in methanol. Optimized structures by DFT show the L-form relatively stable than the E-form by 19.76 kcal/mol, while compounds HL2 and 2 are slightly stable than HL1 and 1 by 0.35 and 0.79 kcal/mol, respectively. Electronic spectra by TDDFT strongly support the experimental results in methanol. (C) 2019 Elsevier B.V. All rights reserved.