A Relativity Enhanced, Medium-Strong Au(I)•••H-N Hydrogen Bond in a Protonated Phenylpyridine-Gold(I) Thiolate

Gold is an electron-rich metal with a high electronegativity comparable to that of sulfur. Hence, hydrogen bonds of the Au(I)center dot center dot center dot H-E (E = electronegative element) type should be possible, but their existence is still under debate. Experimental results are scarce and often contradictory. As guidance for possible preparative work, we have theoretically investigated (ppyH)Au(SPh) (ppy = 2-phenylpyridine) bearing two monoanionic ligands which are not strongly electronegative at the same time to further increase the charge density on the gold(I) atom. The protonated pyridine nitrogen atom in ppy is geometrically ideally suited to place a proton in close proximity to the gold atom in a favorable geometry for a classical hydrogen bond arrangement. Indeed, the results of the calculations indicate that the hydrogen bonded conformation of (ppyH)Au(SPh) represents a minimum geometry with bond metrics in the expected range for medium-strong hydrogen bonds [r(N-H) = 1.043 angstrom, r(H center dot center dot center dot Au) = 2.060 angstrom, a(N-H-A-u) = 141.4 degrees]. The energy difference between the conformer containing the H center dot center dot center dot Au bond and another conformer without a hydrogen bond amounts to 7.8 kcal mol(-1), which might serve as an estimate of the hydrogen bond strength. Spectroscopic properties were calculated, yielding further characteristics of such hydrogen bonded gold species.