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 similar to 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 ((LH+)-H-7), 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.