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 (M), H-1 NMR,C-13 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 pi -> pi* and LMCT transitions. CD data are characteristic for distortion of the chelate ring from planarity and A 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 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. (c) 2007 Elsevier Ltd. All rights reserved.