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.