Motivation behind the research
Neutralization is the formation of salts when mixing an acid and a base, for example: HCl + NaOH –> NaCl + H₂O. Common salt (NaCl) results from the interaction between sodium cations (Na⁺) and chloride anions (Cl⁻). This simple salt is usually unreactive. However, in our system, we observed the reverse process. Testing different gold salts (LAu)X, it is possible to generate a strong gold-gold base capable of removing hydrogen atoms even from small molecules such as acetone. This reaction is called C-H activation and is one of the most studied reactions in chemistry, with numerous applications. Breaking a carbon-hydrogen bond is a challenging task because this bond is very strong. In fact, C-H is the chemical basis of all living organisms. This is why this system is so interesting and useful. We may be able to use this chemical process to create new bonds, which is the key to functionalization, that is, introducing different functional groups to prepare different products.
A very interesting phenomenon is observed when adding water to a gold salt in acetone solution. A base that can directly activate acetone C-H bonds is formed:
Then, we tried to understand how this process actually works.
Mass spectrometry is a good method to study reaction mechanisms because it identifies all the species in a reaction mixture, including intermediates, based on the mass of the ions.
We initially detected the gold-gold base [(LAu)₂OH]⁺. Interestingly, this base degrades at the same time as our target product, the gold acetonyl complex [(LAu)₂(CH₂COCH₃)]⁺, is formed. Moreover, we were able to increase the amount of the gold-gold base by increasing the water content of the acetone solution. So, using this system, we increased the amount of our product simply by adding more water (turning water into gold). We also tested different conditions (gold complexes, solvents) to confirm that the gold-gold base is, indeed, the ion responsible for the formation of the product.
However, how does this base complex actually activate the C-H bond of acetone?
We used a computational chemistry approach to answer this key question. Computer models enable us to identify the most stable structures, simulate chemical reactions and give us information about the energy of the processes. All you need is some powerful processors, chemistry knowledge and a bit of intuition. The output of our calculations showed that the two gold atoms are essential to the whole process and remain linked to each other throughout the reaction. This stabilizes the product and the gold-gold base, which enables the reaction, or as we say in Chemistry, the process is thermodynamically feasible.
Mass spectrometry is not a quantitative method. We use MS to identify different species in reaction, but the intensities of the peaks in a mass spectrum do not give us the concentration of the species in solution, so we always have to use an internal standard. Here, we used acetone (CH₃COCH₃) and deuterated acetone (CD₃COCD₃), which enables us to observe differences in mass, without affecting the structure of the final product, by calculating simple ratios between non-deuterated and deuterated gold acetonyl species.
It would be interesting to the study new systems and reactions with different substrates to test whether this method can have other applications. In other words, future studies may show that we can turn water into many other valuable products.
Research Article: Why can a gold salt react as a base? Org. Biomol. Chem., 2017.