Quest for Insight into Ultrashort C–H···π Proximities in Molecular “Iron Maidens”

Abstract

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 H 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.

The borderless world of chemical bonding across the van der Waals crust and the valence region

“…In these systems, the interpenetration is forced mechanically, i.e. , by a rigid framework of covalent bonds that keep two non-bonded or weakly bonded atoms at a relatively short distance, as happens in silatranes, tripod ligands and iron maiden compounds, 194 discussed in other sections of this work. A recently reported Ni 0 ·Ni I half bond 195 has a penetration index of 93.5%, remarkably similar to those found in this section for Cu·Al, Cu·Ga and N·B half bonds.…”

“…We think that the attribution of a one-electron bond character to the Cu-M atom pair is an oversimplication of a more sophisticated bonding scheme along the N-M-Cu molecular axis that involves 5 electrons and up to 6 atomic orbitals (s, p z and z 2 of Cu, s and p z of M and the N lone pair orbital). In these systems, the interpenetration is forced mechanically, i.e., by a rigid framework of covalent bonds that keep two non-bonded or weakly bonded atoms at a relatively short distance, as happens in silatranes, tripod ligands and iron maiden compounds, 194 discussed in other sections of this work. A recently reported Ni 0 $Ni I half bond 195 has a penetration index of 93.5%, remarkably similar to those found in this section for Cu$Al, Cu$Ga and N$B half bonds.…”

In Silico Design of Cylindrophanes: The Role of Functional Groups in a Fluoride Selective Host

“…The steric fit in host 6 is ideal for the coordination of F − , and forces the ligand sites at the bridges to be near the coordinated anion. This enforced proximity of functional groups in the cyclophane scaffold is known to provide a unique coordination sphere arising from the balance of NCIs and the conformational stability of the framework . Rational design of the cylindrophane fluoride system takes into account the fact that it posses a defined 3D pocket formed by three bridges with a ligand ( L ) site that can be modified, along with the two aromatic moieties ( R ).…”