Mapping C-H....M Interactions in Confined Spaces: (α-ICyDMe)Au, Ag, Cu Complexes Reveal Contra-electrostatic H Bonds Masquerading as Anagostic Interactions**

What happens when a C-H bond is forced to interact with unpaired pairs of electrons at a positively charged metal? Such interactions can be considered as contra-electrostatic H-bonds, which combine the familiar orbital interaction pattern characteristic for the covalent contribution to the conventional H-bonding with an unusual contra-electrostatic component. While electrostatics is strongly stabilizing component in the conventional C-H...X bonds where X is an electronegative main group element, it is destabilizing in the C-H...M contacts when M is Au(I), Ag(I), or Cu(I) of NHC-M-Cl systems. Such remarkable C-H...M interaction became experimentally accessible within (alpha-ICyDMe)MCl, NHC-Metal complexes embedded into cyclodextrins. Computational analysis of the model systems suggests that the overall interaction energies are relatively insensitive to moderate variations in the directionality of interaction between a C-H bond and the metal center, indicating stereoelectronic promiscuity of fully filled set of d-orbitals. A combination of experimental and computational data demonstrates that metal encapsulation inside the cyclodextrin cavity forces the C-H bond to point toward the metal, and reveals a still attractive contra-electrostatic H-bonding interaction.

Automated summary

This study explores the interaction between C-H bonds and positively charged metal unpaired electrons, resulting in contra-electrostatic H-bonding. The research demonstrates that metal encapsulation inside the cyclodextrin cavity forces the C-H bond to point towards the metal, revealing an attractive contra-electrostatic H-bonding interaction.