Complete assignments of NMR data of salens and their cobalt (III) complexes

Abstract

Salens, derived from 1,2-ethylenediamine and salicylaldehydes, have been widely used as ligands for metal complexes which have been showing enormous potential in chemical properties of asymmetric catalysts as well as biological properties such as anticancer agents. Almost all of the salen-metal complexes with their corresponding metal (II)-complexes show the evidences of chelation of two oxygens in salens. However, several metal (II) complexes, especially cobalt (II) complexes, could not show NMR spectra due to their paramagnetism. Recently, it has been reported that one of the cobalt (III) complexes was used for NMR spectroscopy to evaluate its stereoselectivity as a catalyst. Even though many salen ligands are known, their NMR data are not assigned completely. It was possible that modification in northern part of salen with 2-hydroxyphenyl group afforded another oxygen chelation site in salen ligand. Here we report that synthesis and full NMR assignment of new salen ligands, which form meso 1,2-bis(2-hydroxyphenyl)ethylenediamine) and their cobalt (III) complexes. The assignments of (1)H and (13)C NMR data obtained in this experiment can help us to predict the NMR data of other salen ligands.