Computational investigation of Au·H hydrogen bonds involving neutral AuI N-heterocyclic carbene complexes and amphiprotic binary hydrides

In this computational study, we investigate the ability of various neutral R-AuI-NHC (NHC = N-heterocyclic carbene) complexes [R = H, CH3, Cl, OH] to form hydrogen bonds with the amphiprotic binary hydrides NH3, H2O and HF. Optimized geometries of the adducts calculated at various levels of theory all exhibit Au⋯HX hydrogen bonds. In adducts of complexes containing NHC ligands with α(N)H units, (NH)carbene⋯XH interactions also exist, yielding hydrogen-bonded rings with graph-set notation R226 that correspond to pseudo chelates with κ2C,H coordination. AIM analysis at the MP2/aug-cc-pVTZ-pp level of theory indicates that the (NH)carbene⋯XH hydrogen bonds are generally stronger than the Au···HX interactions, except for those involving HF. The Au⋯HX interactions vary with the Lewis basicity of the Au(I) center as a result of the nature of the R ligand, while the (NH)carbene⋯XH hydrogen bonds are unaffected by R. Energy decomposition analysis at the BP86/TZP level of theory identifies the origin of this difference as the greater component of polarization involved in Au⋯HX interactions. Replacing the α(N)Hs with methyl groups prevents formation of a strong (NH)carbene⋯XH interaction, thus reducing the overall stabilization of the adducts. Nevertheless, the Au⋯H interactions remain largely unchanged and are strong enough to sustain the hydrogen-bonded complexes, although weak C–H⋯X interactions are often also present.