Ultrashort optical and X-ray pulses capture elusive intermediate in catalysis

Transforming the greenhouse gas methane and longer-chain alkanes into less harmful chemicals constitutes one of the grand challenges in chemistry. Such transformations necessitate the breaking of carbon-hydrogen (C-H) bonds, one of nature’s strongest chemical linkages. A very attractive way of breaking C-H bonds involves metal catalysts. More than fifty years ago it was found that illuminating such catalysts in alkane solutions with visible light results in broken C-H bonds. However, important details have remained elusive to date: What exactly happens after the catalysts are “switched on” by light? How do the alkane molecules approach the metal catalysts? And then, how do they “get stuck” on it?

An international research team led by scientists from Uppsala University in collaboration with researchers from Hamburg University, Stockholm University, the Max Born Institute and the Helmholtz-Zentrum Berlin has now succeeded in answering these questions with a combination of X-ray and visible light. It is important to understand this process in all detail because once stuck on the metal, the C-H bonds can be broken with the help of the metal in so-called C-H activation reactions.

In two experiments, conducted in the laser lab of the Huse group at Hamburg University and at the BESSY II synchrotron, the team was able to follow how a metal catalyst made of chromium is first switched on by visible light and then “grabs” an alkane from the surrounding solution. By combining ultrashort optical pulses from a laser source and X-ray pulses from the BESSY II synchrotron, the reaction could be followed all the way from the beginning to the end when the alkane gets stuck on the chromium. The measurements revealed the initial “switching on” of the catalyst to happen within less than 100 femtoseconds (0.0000000000001 seconds). In this state, the catalyst is very hot and its parts oscillate around the chromium centre. Only after these oscillations cease, the alkane can approach the catalyst and stick to it in a configuration called 𝜎-complex, a process which takes about 8 picoseconds (0.000000000008 seconds).

“Watching the process with optical and X-ray light allowed us to get the full picture of the process. We packed the light into short pulses to take sequences of short snapshots of the reactions. The optical pulses were so short that we could capture even the fastest atomic motions on the way to the 𝜎-complex. With X-rays, we were then able to sensitively scan in what way the bond between the metal and the alkane forms”, explains Raphael Jay, Researcher at Uppsala University and lead experimentalist of the study.

To model and interpret the complex experimental data, theoreticians from Uppsala University and Stockholm University performed advanced quantum-chemical calculations.

“Our calculations permit to clearly identify all the reaction intermediates before the final 𝜎-complex forms. We can see which parts of the catalyst oscillate in which direction and how these oscillations initially prevent the alkane to approach the catalyst. Once the 𝜎-complex has formed, we can clearly characterize how it impacts the alkane C-H bond in the sticking process,” explains Michael Coates, PhD student in theoretical chemistry at Stockholm University.

The study finally resolves what has been hypothesized for fifty years about how 𝜎-complexes form and how they weaken alkane C-H bonds in C-H bond activation reactions. With their combination of methods, the researchers want to understand next how the structure of the catalyst and the specific metal element in the catalyst’s center influence the way the catalyst is “switched on” and how it interacts with alkanes. This will allow to better direct and tailor catalyst behavior in C-H bond activation reactions.