Probing differences in binding of methylbenzylamine enantiomers to chiral cobalt(ii) salen complexes

Abstract

In this work, we investigate the mode of chiral interactions between the asymmetric Co^II salen complex, (S,S)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamine-Co^II ([Co(1)]), and single enantiomers of methylbenzylamine (MBA) using different continuous-wave and pulsed electron paramagnetic resonance techniques combined with density functional theory computations. While [Co(1)] displays a large affinity for binding a single MBA molecule, it has a much weaker affinity for binding a second MBA molecule. Subtle differences are detected in the EPR spectra of the homochiral (S,S-[Co(1)](S-MBA)) and heterochiral (S,S-[Co(1)](R-MBA)) adducts using low [Co(1)] : MBA ratios. Moreover at high concentrations of racemic MBA, a strong preference (80%) is observed for the formation of the bis-ligated heterochiral adduct (S,S-[Co(1)](R-MBA)₂) compared to the homochiral analogue (20% of S,S-[Co(1)](S-MBA)₂). Differences in the ¹⁴N hyperfine coupling from the diamine backbone in [Co(1)] were also evidenced by hyperfine sublevel correlation (HYSCORE), revealing magnetically equivalent N nuclei for the homochiral adducts and inequivalent N nuclei for the heterochiral adducts. Using DFT, these slight differences were reproduced, and explained based upon the different modes of alignment of the MBA molecule in the adduct. The current findings therefore reveal the appreciable enantiodiscrimination that occurs during the binding of MBA enantiomers to the chiral Co^II salen complex.