Mapping C−H⋅⋅⋅M Interactions in Confined Spaces: (α‐ICyD^Me)Au, Ag, Cu Complexes Reveal “Contra‐electrostatic H Bonds” Masquerading as Anagostic Interactions**

Abstract

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 "contraelectrostatic" 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 (a-ICyD Me )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" Hbonding interaction.

Au⋅⋅⋅H−C Hydrogen Bonds as Design Principle in Gold(I) Catalysis

“…Very recent computational studies by Alabugin, Sollogoub and co‐workers demonstrated that these attractive interactions are also present in methane complexes with NHC‐Au‐Cl, thus emphasizing that Au⋅⋅⋅H−C hydrogen bonds are not exclusively the result of steric congestion. [24a] …”

A Zwitterionic Heterobimetallic Gold–Iron Complex Supported by Bis(N‐Heterocyclic Imine)Silyliumylidene

“…[28] In contrast to agostic interactions that are understood as three‐center‐two‐electron bonds between C−H and vacant d ‐orbitals of transition metals with rather short H−M distances and narrow C−H−M angles (≈1.8–2.3 Å; ≈90–140°), the weaker anagostic interactions are associated with filled d ‐orbitals, longer H−M distances and larger C−H−M angles (≈2.3–2.9 Å; ≈110–170°). [29] Consequently, M I (M=Cu, Ag, Au) complexes, that exclusively feature occupied d ‐orbitals, are good candidates to observe such contacts. The 29 Si NMR spectrum of 4 displays a single resonance at δ( 29 Si)=+67.6 ppm.…”

Au⋅⋅⋅H−C Hydrogen Bonds as Design Principle in Gold(I) Catalysis

“…The later term emphasises the relation to classical H‐bonds, which however differ in the charge of the C and H atom upon approximation of the donor (gold versus the H‐bond acceptor). Very recent computational studies by Alabugin, Sollogoub and co‐workers demonstrated that these attractive interactions are also present in methane complexes with NHC‐Au‐Cl, thus emphasizing that Au⋅⋅⋅H−C hydrogen bonds are not exclusively the result of steric congestion [24a] …”