Quest for Insight into Ultrashort C-H···π Proximities in Molecular "Iron Maidens"

Molecular iron maidens are a strained type of cyclophane in which a methine hydrogen, by the action of the bridges, is placed closer to the center of an aromatic ring. Such constrained molecular frameworks are in fact a noteworthy synthetic challenge. The present study provides a comprehensible theoretical analysis that elucidates unique structural and energetic aspects of this class of molecules, evaluating, in the light of quantum chemistry, both the influence of the aromatic moiety, from π-basic to π-acid, and the nature of the heteroatoms located at the bridges. Our results not only propose the shortest intramolecular centered C-H···π distance to date, which is supported by calculated ¹H chemical shifts, but also shed light on the main factors that rationalize and justify such proximity. QTAIM, NBO, and NCI analyses allow us prematurely to conclude that the ultrashort C-H···π distance is sustained by an interplay between a large stabilizing electrostatic component with a non-negligible covalent character. However, the energetics involving such strained molecular scaffolds, addressed by means of isodesmic reactions, revealed that the C-H···π proximity is modulated mainly by the capacity of the bridges to support the strain imposed by the whole structure, hence compressing the C-H bond against the π-system.