Can Copper(I) and Silver(I) be Hydrogen Bond Acceptors?

Gold(I) centers can form moderately strong (Au⋅⋅⋅H) hydrogen bonds with tertiary ammonium groups, as has been demonstrated in the 3AuCl⁺ (3⁺ =1-(tert-butyl)-3-phenyl-4-(2-((dimethylammonio)methyl)phenyl)-1,2,4-triazol-5-ylidene) complex. However, similar hydrogen bonding interactions with isoelectronic silver(I) or copper(I) centers are unknown. Herein, we first explored whether the Au⋅⋅⋅H bond originally observed in 3AuCl⁺ can be strengthened by replacing Cl with Br or I. Experimental gas-phase IR spectra in the ∼3000 cm^-1 region showed only a small effect of the halogen on the Au⋅⋅⋅H bond. Next, we measured the spectra of 3AgCl⁺ , which exhibited significant differences compared to its 3AuX⁺ counterparts. The difference has been explained by DFT calculations which indicated that the Ag⋅⋅⋅H interaction is only marginal in this complex, and a Cl⋅⋅⋅H hydrogen bond is formed instead. Calculations predicted the same for the 3CuCl⁺ complex. However, we noticed that for Ag and Cu complexes with less flexible ligands, such as dimethyl(2-(dimethylammonio)phenyl)phosphine (L⁷ H⁺ ), the computations predict the presence of the respective Ag⋅⋅⋅H and Cu⋅⋅⋅H hydrogen bonds, with a strength similar to the Au⋅⋅⋅H bond in 3AuCl⁺ . We, therefore, propose possible complexes where the presence of (Ag/Cu)⋅⋅⋅H bonds could be experimentally verified to broaden our understanding of these unusual interactions.