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Dynamic Electromerism: Correlated Rotational and Cross-Conjugation Dynamics in DAMN (DiAminoMaleoNitrile) and DAFN
María-Jesús Aguilera-Roldán1, Pablo Roseiro2, Ferran Esteve1
1Laboratoire de Chimie Supramoléculaire, Institut de Science et d'Ingénierie Supramoléculaires (ISIS), Université de Strasbourg, Strasbourg 67000, France.
Abstract:
Conformational dynamics play key roles in molecular reactivity and the development of functional molecular machines. In this study, we investigate the competitive correlation between internal rotations about the C-NR2 bonds and cross-conjugation dynamics in DAMN, DAFN, and their derivatives. Using a combination of NMR spectroscopy, crystallographic analysis, and computational studies, we demonstrate that conjugation in one of the two push-pull π-systems restricts its own C-N bond rotation while suspending conjugation and facilitating rotation in the other system, leading to synchronized rotation-conjugation oscillation. Variable Temperature (VT)-NMR spectroscopy revealed that the presence of competing cross-conjugated pathways drastically lowers the C-N rotational barriers of the amino groups compared to those of reference compounds presenting a single π-system (11-14 kcal/mol at 298 K). Density functional theory (DFT) studies yielded rotational barriers as low as ∼2 kcal/mol for tetra-methylated DAMN, with a disrotatory mechanism. The introduction of steric effects increased the rotational barriers, yielding measurable NMR values of 7-9 kcal/mol at coalescence in the bulkiest DAMN derivative. Additionally, modulation of the structural desymmetrization of the coupled push-pull systems in monoimine derivatives of DAMN allowed determination of rotational barriers within the range of those of the reference compounds. These effects agreed with the presence of cross-conjugation, which was also supported by X-ray diffraction. The findings provide insight into the structural factors governing rotational barriers in cross-conjugated systems exhibiting correlated molecular motion. Thus, DAMN and DAFN present a dynamic electromerism process where oscillation between two different cross-conjugation pathways is regulated by the C-N rotation frequency.
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