The activation of a C-H bond at a metal centre, and its subsequent functionalization, is undeniably one of the most important breakthroughs in organic and synthetic chemistry in the last 15 years. It allows one to access important chemical building blocks for the agrichemical, materials and pharmaceutical sectors. Even though scientists have figured out how to carry out many complex chemical transformations, there remain mysteries around the mechanistic steps that take place, particularly in reactions involving activation and functionalization of a ubiquitous C-H bond.
Researchers from the Department of Chemistry at University of York and Syngenta closely worked with CLF scientists to directly observe the deprotonation step which allows C−H bond activation at a manganese(I) centre. The activation of a C-H bond by formal deprotonation, and hence proton transfer, is extremely difficult to observe in multi-step reactions because of how rapidly the process takes place.
Computational modeling has further facilitated the research by mapping out intermediate steps and likely transition state structures. However, this approach requires experimental validation so that a true and clear picture of the complete reaction pathway may be obtained. This can only be achieved by observing key species from the catalytic reaction over the full timescale (i.e. from pico- to micro-seconds).
Therefore, researchers devised a direct strategy to observe the microscopic reverse of the deprotonation (activation) step, that is the protonation of a photochemically-activated complex, namely [Mn(ppy)(CO)4] (ppy = 2-phenylpyridyl), in the presence of acetic acid. The observation and quantification of the microscopic reverse of the deprotonation processes enables direct translation to the process involved in the forward process of C-H bond activation. The CLF specialist equipment and expertise enabled time-resolved infrared spectroscopy (TRIR) to take snapshots of the catalytic reactions over picosecond to millisecond time scale, allowing the experimental observation of the computationally predicted pathway.
Chemical reactions take place at an alarmingly quick rate, especially so when there are catalysts involved. Testing the previously described reaction, under pseudo catalytic conditions, with TRIR spectroscopy provides mechanistic insight into sequential reaction steps that would have otherwise been missed. Furthermore, using the technique confirmed details derived from many computational analyses.
Learn more about the study here.