Conformational analysis via calculations and NMR spectroscopy for isomers of the mono(imino)pyridine ligand, 2-{(2,6-Me2-C6H 3)NC(i-Pr)}C5H4N

Timothy J. Dudley, Jennifer E. Beck, Earl E.P. Santos, Kathryn A. Johnston, William S. Kassel, William G. Dougherty, Walter J. Boyko, Deanna L. Zubris

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Abstract

Sterically hindered (imino)pyridine 2-{(2,6-Me2-C 6H3)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-EA and 1-EB) and two atropisomers of Z geometry (1-ZA and 1-ZB). A simple carbon-carbon bond rotation to alter the orientation of the isopropyl group provides a fifth, related conformer, 1-ZB′, 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-EB and 1-ZB/1-ZB′ 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.

Original languageEnglish (US)
Pages (from-to)6237-6244
Number of pages8
JournalRSC Advances
Volume2
Issue number15
DOIs
StatePublished - Aug 7 2012
Externally publishedYes

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