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 [Formula: see text] 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.

Gold(I) Hydrides as Proton Acceptors in Dihydrogen Bond Formation.

Wavefunction and DFT calculations indicate that anionic dihydride complexes of AuI form strong to moderate directed Au-H⋅⋅⋅H bonds with one or two HF, H2 O and NH3 prototype proton donor molecules. The largely electrostatic interaction is influenced by relativistic effects which, however, do not increase the binding energy. Very weak Au⋅⋅⋅H associations-exhibiting a corresponding bond path-occur between neutral AuH and HF units, although ultimately F becomes the preferred donor atom in the most stable structure. Increasing the hydridicity of AuH by attachment of an electron donating NHC ligand effects Au-H⋅⋅⋅H bonding of moderate strength only with HF, whereas competing Au⋅⋅⋅H interactions dominate for H2 O and NH3 . Rare η2 coordinated and HX (X=F or OH) associated H2 complexes are produced during interaction with a single ion of stronger acidity, H2 F+ or H3 O+ . Theoretically, reaction of excess [AuH2 ]- as proton acceptor with H3 O+ or NH4+ in 3:1 or 4:1 ionic ratios, respectively, affords H⋅⋅⋅H bonded analogues of Eigen-type adducts. Outstanding analytical relationships between selected bonding parameters support the integrity of the results.

Gold setting the "gold standard" among transition metals as a hydrogen bond acceptor - a theoretical investigation.

The Au(i) atom of dimethylaurate (DMA) is shown to behave as a hydrogen-bond acceptor, providing theoretical evidence that it can act as a Lewis base. Calculations at the MP2/aug-cc-pVTZ-pp level of theory confirm that DMA forms hydrogen bonds decreasing in strength from -16.2 kcal mol-1 to -2.4 kcal mol-1 in the order HCN ≈ HF > H2O > HCCH > NH3 > CH4, i.e. following the trend of decreasing proton acidity of the hydrogen-bond donor. The geometrical and Atoms in Molecules (AIM) parameters of the hydrogen-bonded adducts compare well to those obtained with the auride anion, a known hydrogen-bond acceptor. Relativistic effects are shown to play a dominant role in the formation of the hydrogen bonds with DMA: omission of these effects (confirmed using two different approaches) results in the loss of the hydrogen bond. Instead, the hydrogen-bond donor interacts with the carbon atom on one of the methyl ligands, yielding an adduct that is closely comparable to those found with the Cu and Ag analogues of DMA.

Preparing Gold(I) for Interactions with Proton Donors: The Elusive [Au]⋠⋠⋠HO Hydrogen Bond.

MP2 and DFT calculations with correlation consistent basis sets indicate that isolated linear anionic dialkylgold(I) complexes form moderately strong (ca. 10 kcal mol(-1) ) Au⋅⋅⋅H hydrogen bonds with single H2 O molecules as donors in the absence of sterically demanding substituents. Relativistic effects are critically important in the attraction. Such bonds are significantly weaker in neutral, strong σ-donor N-heterocyclic carbene (NHC) complexes (ca. 5 kcal mol(-1) ). The overall association (>11 kcal mol(-1) ), however, is strengthened by co-operative, synergistic classical hydrogen bonding when the NHC ligands bear NH units. Further manipulation of the interaction by ligands positioned trans to the carbene, is possible.