X-ray crystal structure and nuclear Overhauser effect studies of cerium(IV) complexes with Schiff bases obtained from N,N′-(1R,2R)(−)-1,2-cyclohexanediamine and benzaldehyde derivatives

Schiff bases, obtained by condensation of N,N′-(1R,2R)-cyclohexanediamine with 2-hydroxy-5-chlorobenzaldehyde or 2-hydroxy-3,5-dichlorobenzaldehyde and of (±)cyclohexanediamine and salicylaldehyde, have been used as ligands for cerium(IV). The complexes were characterized by circular dichroism (CD), 1H NMR,13C NMR and nuclear Overhauser effect (NOE) studies. X-ray crystal structures determined for: [Ce(IV)((1R,2R)(−)chxn(3,5-diClba)2)2] and [Ce(IV)(±)(chxn(salal)2)2] revealed different orientation of a pair of Schiff base ligands in the coordination sphere. In [Ce(IV)((1R,2R)(−)chxn(3,5-diClba)2)2] two ligands form a sandwich with the cerium(IV) ion positioned between two layers of dichlorophenolic moieties. CD spectra in acetonitrile solution revealed exciton coupled π → π∗ and LMCT transitions. CD data are characteristic for distortion of the chelate ring from planarity and λ conformation of the cyclohexane bridge. NOE difference spectroscopy for the Schiff bases (saturated –HCN–) demonstrate correlations of the azomethine protons with the protons of the phenyl ring and the cyclohexane’s CH protons, and a weak correlation with the hydroxyl group. In the case of the cerium(IV) complexes, the strongest NOE (saturated H–CN) was observed with the phenyl ring’s hydrogen atoms, a slightly weaker correlation to the cyclohexane’s CH2 protons, and a much weaker NOE to the cyclohexyl’s CH2 protons. Results of spectral analyses suggest central ions in a distorted square antiprism geometry with the two basal N2O2 chromophore group. The X-ray crystallography study also confirmed antiprismatic distortion from planarity in obtained complexes.

Graphical abstractSchiff bases, obtained by condensation of N,N′-(1R,2R)-cyclohexanediamine with 2-hydroxy-5-chlorobenzaldehyde or 2-hydroxy-3,5-dichlorobenzaldehyde and of (±)cyclohexanediamine and salicylaldehyde, have been used as ligands for cerium(IV). The complexes were characterized by circular dichroism (CD), 1H NMR, 13C NMR and nuclear Overhauser effect (NOE) studies. X-ray crystal structures determined for: [Ce(IV)((1R,2R)(−)chxn(3,5-diClba)2)2] and [Ce(IV)(±)(chxn(salal)2)2] revealed different orientation of a pair of Schiff base ligands in the coordination sphere.Figure optionsDownload full-size imageDownload as PowerPoint slide