Conformational analysis via calculations and NMR spectroscopy for isomers of the mono(imino)pyridineligand, 2-{(2,6-Me2-C6H3)NC(i-Pr)}C5H4N

Abstract

Sterically hindered (imino)pyridine 2-{(2,6-Me₂-C₆H₃)NC(i-Pr)}C₅H₄N (1) was synthesized via addition of isolated imidoyl chloride to an in situ lithiated pyridine. Room temperature 1-D and 2-D NMR spectroscopy reveals two rapidly equilibrating isomers in solution. Interconversion of these two isomers was verified by 2D-EXSY NMR spectroscopy. Calculations at the B3LYP and MP2 levels of theory reveal four relevant isomers, with two atropisomers of E geometry (1-E_A and 1-E_B) and two atropisomers of Z geometry (1-Z_A and 1-Z_B). A simple carbon–carbon bond rotation to alter the orientation of the isopropyl group provides a fifth, related conformer, 1-Z_B′, that is the most stable species at the MP2 level. The transition states for E/Z isomerization and the isomerization pathways between atropisomers have been characterized. Comparison of experimental and ab initio NMR chemical shifts in combination with NOE analysis suggests that isomers 1-E_B and 1-Z_B/1-Z_B′ are the dominant species in our solution phase NMR studies. Our understanding of the isomerization behavior of 1 will help inform the future design of readily complexed, sterically hindered mono(imine) and bis(imine) ligands.