Explorations of the N-C Atropisomerism of Indigo Diimines and Related Complexes
Date
2022-09-27
Authors
Richard, Nicholas
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
This study focused on the preparation and characterization of new indigo diimine (Nindigo) derivatives as a new atropisomeric scaffold. Trans- and cis- indigo diimines were studied and structure-property relationships were investigated regarding N-C atropisomerism using variable temperature 1H NMR studies and density functional theory calculations.
Neutral trans- and protonated cis-Nindigos were prepared featuring a variety of mono ortho-substituted aryl imine groups with varying levels of steric bulk. The neutral trans-Nindigo derivatives generally have smaller N-C rotational energy barriers than their protonated cis-congeners. This finding is consistent with the latter having closer proximity of the N-aryl groups to each other, leading to steric repulsions between the two groups. The N-C rotational energy barriers are substituent dependent; the N-C rotational energy barriers of mono ortho substituted trans-Nindigos were in the range of 6.0 – 16.4 kcal/mol and can be classified as predominantly “Class 1’ atropisomers as defined by LaPlante, while the mono ortho substituted protonated cis-Nindigo analogs have N-C rotational barriers between 12.3 – 25.5 kcal/mol and are classified as “Class 1” and “Class 2” atropisomers. The introduction of additional substituents onto the other ortho position of the aryl imine subunit has significant consequences for the N-C rotational energy barriers of both the neutral trans- and protonated cis-Nindigos making them stable, or close to being, ‘Class 3’ atropisomers, having N-C rotational energy barriers between 31.5 – 276.9 kcal/mol and 29.3 – 32.6 kcal/mol respectively.
Recognizing that the protonation state induced trans- to cis-isomerization process could have significant consequences regarding the potential applicability of these atropisomeric Nindigo derivatives, cis-Nindigo derivatives were synthesized that contained a tether (oxalyl or palladium (II) acetylacetonate) between the two indole type nitrogens of the Nindigo, which prevent the central -C=C- from isomerizing. The N-C rotational barriers of the tethered cis-Nindigos also displayed substituent dependent N-C rotational energy barriers. The protonation state of the N, N’-oxalyl bridged cis-Nindigos has a significant impact (higher in energy by a minimum of 5.1 kcal/mol) on the N-C rotational barriers; the neutral N, N’-oxalyl bridged cis-Nindigos have N-C rotational energy barriers ranging between 11.8 – 14.9 kcal/mol, classifying them as “Class 1” atropisomers, while their protonated congeners have N-C rotational energy barriers between 16.9 – 19.8 kcal/mol, which classifies them as “Class 1” atropisomers but are on the cusp of being “Class 2” atropisomers. The size of the tether influences the N-C rotational energy barriers of cis-Nindigos; the one-atom bridged palladium (II) acetylacetonate complexes have generally lower N-C rotational energy barriers than their protonated N, N’-oxalyl bridged cis-Nindigo congeners. The palladium acetylacetonate tethered cis-Nindigo complexes displayed substituent N-C rotational energy barrier dependence and the mono ortho substituted analogs have N-C rotational energy barriers between 12.4 – 20.2 kcal/mol and are predominantly “Class 1” atropisomers, while the bulkier analogs are “Class 2” atropisomers. The palladium (II) acetylacetonate cis-Nindigo complexes that have aryl imine groups with a 2,6-disubstitution pattern have N-C rotational energy barriers greater than 19.7 and 20.2 kcal/mol and are presumed to be stable “Class 3” atropisomers like their unbridged neutral trans- and protonated cis-Nindigo counterparts.
Description
Keywords
Physical Organic Chemistry, Dye Chemistry, Computational Chemistry, Atropisomerism, Chirality