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

Abstract

Schiff bases, obtained by condensation of N, N′-(1 R,2 R)-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)((1 R,2 R)(−)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)((1 R,2 R)(−)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 – HC N–) 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–C N) was observed with the phenyl ring’s hydrogen atoms, a slightly weaker correlation to the cyclohexane’s CH 2 protons, and a much weaker NOE to the cyclohexyl’s CH 2 protons. Results of spectral analyses suggest central ions in a distorted square antiprism geometry with the two basal N 2O 2 chromophore group. The X-ray crystallography study also confirmed antiprismatic distortion from planarity in obtained complexes.