Mapping C-H⋅⋅⋅M Interactions in Confined Spaces: (α-ICyD )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 (α-ICyD )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.