Gold hydrogen bonds found to be as strong as other similar textbook interactions

‘Whether a metal can truly act as a genuine proton acceptor for a C–H donor has long remained an open and controversial question in chemistry,’ says Jun Chen at the Fujian Institute of Research on the Structure of Matter in China. Looking to answer this question Chen and colleagues conducted spectroscopic analysis of gold complexes and discovered that these species feature hydrogen bonds as strong as those formed by O–H and N–H groups. Understanding these interactions could allow chemists to design better catalysts or host–guest systems that rely on molecular recognition.

Gold is an ideal hydrogen bond acceptor, owing to relativistic effects contracting the 6s orbital, which localises electron density and makes it more directional for a better interaction with a hydrogen bond donor. In fact, gold atoms routinely form hydrogen bonds with conventional donors, such as O–H, N–H and F–H. Yet capturing a true C–H···Au bond is challenging, as the interaction is weaker and, in many cases, these atoms are simply close to each other in space rather than directly forming a bond.

Spectroscopic analysis of gas phase gold anions bound to acetonitrile molecules has now allowed Chen and his team to better probe these interactions. ‘[The cyanide group in acetonitrile] enhances the C–H acidity without reacting with the metal, allowing us to successfully isolate and characterise the complex,’ explains Chen.

Photoelectron spectroscopy and computational analysis revealed that the bond strength of the C–H···Au bond was around 0.50eV, with Chen explaining that this is ‘perfectly comparable to many conventional O–H or N–H anion hydrogen bonds’. ‘This directly challenges the common assumption that only strongly polarised, conventional X–H groups can act as effective hydrogen bond donors towards metal anions,’ he says.

Further analysis showed that electrostatics accounted for around 60% of the interaction, while dispersion and induction effects had small contributions (26% and 16%, respectively).

‘Understanding whether C–H groups can form hydrogen bonds to metals is important because even weak metal–ligand interactions may influence structure, stability and reactivity,’ says Helgard Raubenheimer at Stellenbosch University in South Africa. This includes molecular organisation, conformational preferences and the stability of intermediates, he explains.

Chen adds that such non-covalent interactions are also involved in transition-metal catalysis, for example, and understanding these bonds could help chemists design better catalysts. However, Raubenheimer notes that ‘these systems remain highly idealised and their relevance to typical catalytic conditions is likely to be restricted’.

‘For heavy elements, such as gold, the distinction between hydrogen bonding and other weak interactions is not always clear-cut,’ says Raubenheimer. ‘The main value lies less in assigning a strict label to the interaction, and more in providing reliable quantitative insight into its strength and underlying nature.’