Tracking C-H activation with orbital resolution

The reactivity towards C-H bond activation of alkanes with transition metals is determined by the ability of the metal to donate and withdraw electron density in due proportion. Manipulating this reactivity in a controlled way is difficult, because the hypothesized metal-alkane charge-transfer interactions are challenging to access experimentally. Using time-resolved X-ray spectroscopy, we track the charge-transfer interactions in a C-H activating Rh complex and reveal changes in oxidation state as well as valence-orbital energies and character from femtosecond Rh-alkane bond formation to nanosecond C-H bond cleavage. Our X-ray spectroscopic signatures reflect how alkane-to- metal donation determines metal-alkane bond stability and how metal-to-alkane back-donation facilitates C-H bond cleavage by oxidative addition. The ability to dissect charge-transfer interactions on an orbital-level provides new opportunities for manipulating reactivity for C-H activation with transition metals.